- Email: [email protected]
diesel blends in a small scale furnace
Accepted Manuscript Combustion of jojoba-oil/diesel blends in a small scale furnace
Salah A.B. Al Omari, Mohammad O. Hamdan, Mohamed YE. Selim, Emad Elnajjar PII:
S0960-1481(18)30794-8
DOI:
10.1016/j.renene.2018.07.009
Reference:
RENE 10283
To appear in:
Renewable Energy
Received Date:
25 March 2017
Accepted Date:
02 July 2018
Please cite this article as: Salah A.B. Al Omari, Mohammad O. Hamdan, Mohamed YE. Selim, Emad Elnajjar, Combustion of jojoba-oil/diesel blends in a small scale furnace, Renewable Energy (2018), doi: 10.1016/j.renene.2018.07.009
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ACCEPTED MANUSCRIPT
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Combustion of jojoba-oil/diesel blends in a small scale furnace
2
Salah A.B. Al Omari*1, Mohammad O Hamdan2, Mohamed YE Selim1, and Emad Elnajjar1
3
1Department
of Mechanical Engineering, UAE University, Al Ain City, UAE
4
2Department
of Mechanical Engineering, American University of Sharja, Sharja, UAE
5
*Email: [email protected]
6 7
Highlights:
8
Raw jojoba-oil/diesel blends are used as fuel for small furnace.
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0 to 35% by weight of jojoba in blends is used with input fuel supply of 8.3 kg/h.
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Stable flame was possible with 60 % jojoba share if fuel input is 10 kg/ or higher.
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Jojoba oil in the blend reduces NOx and unburned hydrocarbon emissions.
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Jojoba indirectly impacts CO emissions through its effect on the spray processes.
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Jojoba oil in the blends has adverse impact on thermal radiation to furnace walls.
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Abstract:
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This experimental study investigates the combustion and pollutants emissions from a small scale
16
furnace burning diesel fuel blended with raw jojoba oil. Jojoba oil to diesel proportions in the
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blends (on mass basis) ranging from 0 to 35 percent are considered for total blended fuel flow
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rate of about 8 kg/h. Higher fuel supply rates of about 10 kg/h were needed in order to allow for
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reaching higher jojoba share in the blends up to about 60 percent. This allows for securing
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sufficient amount of the higher volatility component (diesel) whose combustion would support
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the vaporization and subsequent ignition and combustion of the heavier jojoba oil. The presence
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of jojoba in the blends leads to a clear reduction in NOx and hydrocarbon (UBHC) emissions but
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it showed less impact on CO levels. Due to its high viscosity, jojoba in the blends impacts spray
24
formation hence seems to have an indirect detrimental effect on CO emissions. Moreover, jojoba
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oil in the blends adversely impact thermal radiation to furnace walls due to less sooting tendency 1/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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of the flame when jojoba is present. To some extent, this is also attributed to the way jojoba
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influences spray processes.
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Key Words: raw jojoba oil; diesel blends; biofuels, pollutants emissions; NOx; CO.
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1. Introduction
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It is needless is to mention the importance and the need to search for alternative renewable
31
energy resources that are environment friendly and at the same time help sustaining our energy
32
security for the future. Towards these goals, the present research group worked so far on
33
different projects on renewable energy and on the efficient utilization of energy resources in
34
different energy systems (see for example [1-9]). The work reported herein is a continuation to
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our earlier pursuits to achieve the above goals and to meet the ever-increasing requirements and
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stringent regulations imposed on pollutants emissions form combustion systems.
37
In their attempt and search for energy bio-resources to replace petroleum-based fuels, different
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researchers investigated the use of biofuels of different kinds and the biodiesel derived from
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them, but their pursuits were mainly focused on internal combustion engines particularly dual
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fuel diesel engines and diesel engines fueled by diesel blended with liquid biofuels (see for
41
example [10-16]).
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There is limited research available in the literature on the utilization of liquid biofuels as sole
43
fuel or as supplementary fuel for open combustion systems; e.g. furnaces. For example,
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Tashtoush et al. [17] investigated a water cooled furnace fueled with ethyl ester of waste
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vegetable oil. Therein ethyl ester of used palm oil was burned in a water cooled furnace and the
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results were compared for two nominal input energy values with the results based on diesel
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combustion in the same furnace. The Biofuel showed better emission characteristics under the
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conditions of the lower thermal energy input to the furnace, while diesel performance surpassed
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the biofuel at the higher thermal input condition. Still, the results are not conclusive and in that
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study a chemically treated used vegetable oil was used; i.e. not in its straight vegetable oil (SVO)
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form.
2/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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Cavarzerea et al. [18] addressed micro gas turbine fueled with blends of straight vegetable oils
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(SVOs) from different origins with diesel. Different proportions of the biofuel in the blends were
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considered. The results, were so scattered and lead to uncertainty such that they may not lend
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themselves much amenable for drawing conclusive conclusions from the experimental findings.
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Yet, blends with up to 30 percent vegetable oils were found to lead to reduced NOx emissions
57
and lower exhaust temperatures and lower pressure ratio and rotational speeds for the same
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nominal fuel mass input. This was attributed to the lower heating value of the used vegetable
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oils, as compared with diesel. For all cases tested, CO emissions were higher than when only
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diesel is used as a fuel.
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Khalil and Gupta [19] studied the atomization characteristics of two kinds of bio-liquid fuels and
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compared them with spray characteristics attained when a fossil fuel is used. Then, they studied
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the combustion and pollutants emissions from these fuels under gas turbine combustors
64
conditions. One of these two biofuels used is Butyl Nonanoate; one that had been newly
65
processed to be used as a new biofuel and that had been tested to assess its potential for use in
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gas turbines.
67
Jiru et al. [20] considered the use of blends of SVO from soybean and No. 2 fuel oil, as fuel for
68
residential furnaces. The different thermophysical characteristics of the used blends and the
69
pollutants emissions from their burning in residential furnace were reported. Jiang et al. [21]
70
implemented the concept of flow-blurring liquid fuel injection to atomize straight vegetable oil
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of soybean, biodiesel, and a baseline diesel fuel. They studied the combustion and pollutants
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emissions characteristics from these fuels.
73
Huang et al. [22] investigated an oil-fired furnace fueled with blends of crude castor oil and
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diesel. They examined the feasibility of using these blends as a substitute for pure diesel in
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industrial applications. Crude castor oil fractions of 5% up to 30% have been investigated. They
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found that with increasing the castor oil content in the blends, NO and CO emissions only
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slightly decreased and increased, respectively. In respect to furnace temperatures, they verified
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that the use of 5–30% castor oil in the blends produces similar furnace temperature distributions
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and comparable emission levels of CO, NO and SO2 when compared to pristine diesel 3/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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In [23] Enagia et al. gave a review of the utilization of liquid biofuels in gas turbines. The
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authors included discussions on the effect of different input parameters associated with the clean
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combustion systems that have influence on the attainment of ultra-low emissions of NOx and CO
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under premixed and non-premixed modes. They also highlighted that using biofuels as
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supplement with 20 percent share in fuel blends is currently economically viable, with an
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expectation to reach higher share for biofuels in the near future.
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Further studies addressed the use of biofuels either as supplements in blended main fuel or as
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sole fuels. For example, in [24] Hashimoto et al. considered the combustion of Jatropha oil (JPO)
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and Jatropha methyl ester (JME) in a furnace under atmospheric pressure although the study was
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highlighting potential use for gas turbines. They studied emissions and radiation characteristics
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based on Jatropha as a fuel and compared the obtained results with results they obtained when a
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diesel fuel was used to fuel the furnace. They also considered blends of diesel and Jatropha with
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different mixing ratios. Moreover, they measured the flame radiation intensity and concluded
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that flame radiation intensity and the soot emission decrease with increasing mixing ratio of
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Jatropha oil or Jatropha methyl ester to diesel fuel. The authors also studied the effect of
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viscosity of Jatropha on spray formation and the subsequent processes in the furnace. The effect
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of the equivalence ratio was also investigated in [24] by changing it one time by varying the fuel
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flow rate and fixing the air flow rate and vice versa. The results indicated that CO emissions
98
increase when Jatropha is burned under lean conditions (low flame temperatures), as compared
99
with the case of diesel and JME. NOx results were found to be sensitive in the case when JPO is
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used, to the change in the equivalence ratio when it is increased by changing the airflow rate
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while fixing the fuel flow rate. In reference to the work reported by Hashimoto et al. in [24], it is
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interesting to note that Jatropha oil they used in their study shares a number of similar features
103
and thermophysical characteristics with the liquid biofuel used in this study namely Jojoba oil.
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This would consequently make it possible to investigate the results obtained in this study in light
105
of those reported in [24].
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Hashimoto et al. [25] recently investigated the effect of Jatropha oil blending with heavy fuel oil
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on soot emissions in boilers. Their results showed reduction in soot emissions from flames when
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crude Jatropha oil is used in the fuel blends. They attributed this to the low sooting tendency of 4/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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crude Jatropha oil where Jatropha oil’s molecules have no aromatic ring and also due to their
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high oxygen content. This consequently impacted the level of radiation from the flame to the
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furnace walls. Furthermore, due to the low nitrogen and sulfur content in Jatropha oil, the NOx
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and SO2 emissions were reduced as well.
113
From the above presentation, one may conclude that there is still a need to research further and
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validate the use of different kinds of bio-oils for furnaces and boilers applications. Hence, more
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investigation of bio fuels combustion is needed to come up with efficient designs and optimum
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operation maps for furnaces that optimally can run on such biofuels in their straight raw liquid
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form. Achieving this target, would mean to us further justification for our call to increase the
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production of such biofuels by planting more land areas with jojoba shrubs (which is the plant
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subject of this study) at a wider scale for energy production purposes, beside the already-existing
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call to increase their production to serve as environment-friendly fuels for car engines.
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In line with the above arguments, and to the best of the knowledge of the present authors, raw
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jojoba oil as straight vegetable oil (SVO) or the biodiesel based on it, have not been tested
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sufficiently in open combustion systems such as furnaces and boilers. Therefore, this study
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pursues research to highlight the possible use of straight jojoba oil as a sole fuel or at least as a
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fuel supplement for furnaces and highlights emission characteristics from their combustion in a
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small-scale furnace under different air supply conditions.
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Using chemically treated bio liquid fuels for furnaces (i.e. biodiesel), may replace traditional
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fossil fuels, with possibly no or just minimal alterations to existing furnaces and fuel injection
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systems. Nevertheless, this option might be associated with some negative consequences and
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adverse economic and environmental impacts. This is due to the fact that the so far adopted
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conventional methods of transesterification to produce biodiesel require large amounts of energy
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and lead to byproduct (crude glycerol) with large quantities. If these large quantities of crude
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glycerol were to be utilized further in another heat generation system as a fuel, this would still
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require certain arrangements that need to be justified and in most cases of small to medium-sized
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plants is out of the range of economic feasibility to warrant its adoption at a wide scale [26].
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Furthermore, the subsequent direct use of crude glycerol as a fuel in furnaces can lead to
137
emissions that include undesirable chemicals that are rooted in the chemicals used to produce the 5/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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biodiesel in the first place. However Pawlak-Kruczek et al. [26] examined the co-firing of the
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biodiesel-production byproduct crude glycerol with coal in utility boiler and conclusions drawn
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therein were in favor and supportive to that practice. Based on the above arguments, the direct
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use of straight vegetable oils (SVOs) as a fuel or fuel supplement replacing a portion of the
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typically used fossil fuels, would be an attractive option. This would alleviate much of the
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obstacles and concerns highlighted above regarding using biodiesels, especially if the use of
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SVO revealed to be more environment friendly than the traditional petroleum-based fuels, as
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already found by some preliminary research studies [17-21]. This represents one of the
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motivations to conduct the research reported in this study in which crude jojoba oil is utilized.
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The attractiveness of jojoba shrub as a plant is its ability to tolerate extreme different
148
environments [27], which makes it easily and conveniently grown in arid regions in the world.
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Such abilities of jojoba plants allow multifold benefits starting from making the desert green and
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thereby limiting sand migration and the adverse effects of sand storms, and at the same time
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increasing the diversity of our renewable energy resources. In addition, this can also open the
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door for new energy market options. The unique features of raw jojoba oil’s make it a very
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attractive raw material for different industries such as in cosmetics, pharmaceuticals, high-
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pressure lubricants, and plasticizers [27]. Thus, considering it as alternative energy source or as a
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supplement to existing fossil fuels, might look at this moment as economically inviable.
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However, considering the merits of using jojoba oil as a fuel or fuel supplement; (as reported in
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different places in the literature; which what this current work is also trying to explore further),
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the outlook might take a very favorable direction towards considering expanding the jojoba oil
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production at a much wider scale, worldwide. This would even be specifically the case in arid
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regions where preventing desertification is becoming a national priority, aside from any other
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economical or environmental advantages.
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Two of the present authors considered the option of using the jojoba fruit to the max by
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considering the shell of the jojoba nut as a solid fuel for furnaces, after oil extraction [28]. The
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results showed some promising features of that kind of solid biomass fuel but still further
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investigations are needed to come up with more detailed aspects of the used systems and the
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environmental impact of such biofuel. 6/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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The studies that addressed the use of jojoba (whether in its raw oil form or as Methyl ester after
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treatment), as fuel in internal combustion engines (for example [10-15]), revealed that this
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practice has favorable effects on pollutants emissions. On the short run, using such biofuel in
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engines was found not to cause any significant loss in the performance of engines. On the long
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run, however, due to the higher viscosity of oils of biological origin in general, previous research
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on engines revealed some technical problems related to the injection and fuel storage systems.
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On the injection process itself, the higher viscosity of bio-oils (as compared with typical
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petroleum-based liquid fuels) leads to poorer atomization of the spray and consequently can have
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adverse effects on the hydrocarbon and other combustible species emissions from combustion
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systems. To alleviate these adverse effects, modified injection systems with more powerful
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pumping capacity to overcome the large viscosity, would (although will be requiring extra
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expenditures) be a possible solution. To get more details about the needs of improvements in this
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direction, some testing of the performance is required, and this is what had been accomplished to
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a good extent so far for internal combustion engines, but still lacking for the case of furnaces and
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open direct burning systems. One of the main goals of the present study is to come up with
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results that aid achieving the above target and come up with relevant and sound
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recommendations in that direction.
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The study conducted by Shehata and Abdel Razek [13] furnishes an interesting material for
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comparison with the present study, where therein raw jojoba oil/diesel blends were used as fuel
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in diesel engine, while in the present work this research team uses such blends as a fuel for open
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combustion in a small scale furnace. The resulting performance trends in their work can be
188
usefully compared with the results reported here and more comprehensive conclusions with a
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wider breadth and enlightening scope regarding the physics of the underlying combustion and
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heat transfer processes can be drawn accordingly. For example, as found in [13], performance
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and emissions characteristics of an engine fueled with blends of diesel and jojoba oil (80%
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diesel) have been compared with their counter parts when only conventional diesel is used. The
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results showed that the Jojoba oil/diesel blend reduced the brake thermal efficiency, the brake
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power and the break effective pressure. However, the blend led to increase in CO and CO2
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production but to decrease in NOx emissions. Similar conclusions regarding NOx emissions
196
have been made by Al-Widyan and Al-Muhtaseb [15] when diesel was blended with jojoba oil. 7/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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Jojoba oil is miscible in petroleum diesel fuel which means that the two can be mixed in any
198
proportion. However, jojoba oil has much higher viscosity than a typical diesel fuel. This study
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tested jojoba oil-diesel fuel mixtures with three different jojoba oil proportions starting from 0%
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to 35% on mass basis for a nominal total blended fuel energy input to the furnace of about 99 to
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103 kW. The nominal energy input is calculated based on the overall heating value of the blend
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and the total mass of the blend supplied to the furnace. Due to the furnace limitation in the
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present study such as fuel injection pump, atomizer and flame stability, it was difficult to reach
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higher jojoba percentages in the injected blends beyond 35 percent. This is possibly due to the
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large viscosity of raw jojoba oil when large quantities are used in the blends that precludes fine
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enough spray formation. Consequently, the vaporization rates of the formed large droplets of the
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blended fuel with high jojoba content would not be sufficiently high to support the needed
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combustion for a stable flame over a range of air supply conditions. Yet, to be able to include a
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larger share of jojoba oil in the blends beyond the 35 percent level mentioned above, some
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actions can still be taken for example preheating the blends to reasonably high temperatures to
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decrease their viscosity before injection. Altering the injection system by employing a more
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powerful pump and using injectors with smaller orifices or with more features and spray
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atomization techniques that facilitate producing fine enough droplets, are also other options that
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clearly are associated with some additional expenditure requirements, as highlighted above.
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Using higher proportions of jojoba oil in the used blends beyond 35 percent might still be
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possible without considering the pre-treatment or remedial actions mentioned above. However,
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this might be achievable only for furnaces that operate on comparatively large total thermal
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capacity conditions. In other words, in this case there is a need to increase the overall absolute
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mass of the more volatile component in the blend (in our case the diesel fuel) that is being
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supplied to the furnace when the percentage of Jojoba in the blend becomes excessively high.
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For example, to overcome the issues related to the presence of excessive jojoba oil content in the
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blends and the consequent formation of large droplets that vaporize relatively slowly, higher total
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supply rates of such blends can be considered. In this case, the presence of large quantities of
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jojoba oil will be countered by the availability of sufficient amounts of the other more volatile
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component namely the diesel, which will vaporize and burn fast enough to eventually support the
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vaporization and subsequent combustion of its companion the jojoba oil, regardless of the slow 8/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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rates of vaporization of the later. Preliminary testing of the above option has been implemented
228
in this study where the total amount of the injected blended fuel was increased such that the
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nominal fuel energy supply rate to the furnace becomes about 117 kW. In this case, it was
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possible to achieve stable burning of blended fuel with up to about 60 percent jojoba under fairly
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reasonable range of air to fuel ratios. Yet, more testing and runs are required for various high
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jojoba oil blending proportions under different levels of total fuel supply rates and different fuel
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injection temperatures. As such, the results obtained from the last case conducted in this work
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with about 60 percent jojoba oil content in the blends, would only be presented here to highlight
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the feasibility of the above approach, with no further due discussions or comparisons with other
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cases under lower nominal input fueling conditions.
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2. Experimental Setup
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The furnace studied in this work is a small horizontal cylindrical research furnace, with overall
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furnace length of 150 cm and diameter of 60cm. The furnace is equipped with advanced data
240
acquisition system which is used to collect all flow parameters and testing measurements. The
241
schematic diagram for the experimental test rig is shown in Figure 1. The furnace is fitted with a
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water jacket surrounding the furnace so that heat transfer from the flame and the hot combustion
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gases can be quantified. Injection of the liquid fuel is facilitated by an injector that has a full cone
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wide angle nozzle that produces a uniform wide angle 120º cone spray. The full cone nozzle has a
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spray pattern that is round and completely filled with droplets. This pattern is formed by an internal
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vane, which imparts controlled turbulence to the liquid prior to the orifice. The droplets size in the
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case of the diesel fuel are in the range around 40-400 micron. The injector operates with a
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maximum nominal thermal capacity of about 116 to 125 kW. According to manufacturer’s details,
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recommended liquid fuel flow rates that ensure fine enough spray, as described above, is from 5
250
to about 12 kg/h. The different temperature measurements; (for the hot combustion gases, the
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combustion air inlet, the water circulating through the water jacket) are made using K- type
252
thermocouples. A gas analyzer made by VARIO Plus Industry is used to measure gas emissions
253
from the furnace. All measured signals are acquired using data acquisition and stored using a
254
LabVIEW interface program written to collect the data at rate of 1 sample per second. The flow
255
rates of the liquid fuel, the combustion air, and the water circulating through the water jacket were 9/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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measured to facilitate the performance analysis of the furnace. The uncertainties of the used
257
instrumentation in the setup and their measurement range are listed in Table 1.
258 259 260 261 262
Table 2 below summarizes the main fueling conditions considered in the different experimental tests conducted.
Table 1: the uncertainty of measurements Table (1) Measurement CO in ppm O2 in Vol.% NOx in ppm CO2 in Vol.% K-Type thermocouple Air Flow rate Fuel Flow rate
RANGE 502 10.03 80.8 14.14 1250oC 240 kg/h 15 kg/h
TOLERANCE ± 25 ppm ± 0.2% ± 4 ppm ± 0.75 % ± 0.5o C ± 15 kg/h ± 0.5 kg/h
263 264
(17)
(1) Inlet water temperature (2) water volume flow rate (3) water flow controller (4) Inlet air temperature (5) Air volume flow rate (6) Air flow controller
265
(7)Inlet fuel temperature (8)Fuel volume flow rate (9)Fuel flow controller (10)Water Jacket (11) fuel injector (12)Furnace
(13)Furnace temperature (14) Outlet water temperature (15) Exhaust temperature (16) Gas Analyzer (17) Mid Furnace Temperature
Fig. 1: Schematic of the Experimental Setup
266 267
Table 2: Fueling conditions of the experimental tests conducted. 10/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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Table (2): Case Study Test Diesel Fuel % No. Test 1 100% Test 2 80% Test 3 65% Test 4 40%
Jojoba Fuel %
m’fuel (kg/h)
0% 20% 35% 60%
~8.26 kg/h ~8.26 kg/h ~8.26 kg/h ~10 kg/h
Nominal thermal energy input to furnace (kW) 103 101 99 117
268 269 270 271 272 273 274 275 276 277 278
3. Thermo-physical properties of used fuels The raw jojoba oil used in this study is extracted from jojoba seeds which were obtained from jojoba plants grown in Egypt. It has only been refined but not treated by any additives nor chemicals. Shehata and Abdel Razek [13] report thermophysical properties of raw jojoba oil originating from the same geographical region namely Egypt and these are given in Table 3. In their measurements, Shehata and Abdel Razek [13] did not indicate the presence of any water in the jojoba oil they used and they did not report any ash content.
Table 3: Thermophysical properties of raw jojoba oil [13] Property Raw Jojoba Oil Heating Value, MJ/kg 39.862 o Viscosity at 40 C, cSt 52 o 3 Density at 15 C, kg/m 920 Flash point, oC 186 Molecular Weight 560.98 Molecular Structure C38H72O2 Aromatic Content (%w) 0 C/H ratio (by weight) 6.29 Elemental Analysis (%w) C 81.36 H 12.94 O 5.7 N 0 S 0.02
279 280
The heating value of jojoba oil is somewhat lower than that of typical diesel fuel. This might be
281
expected to play some role regarding the combustion and emissions characteristics in cases
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including jojoba, as compared with the baseline case with only diesel. Carbon to hydrogen mass 11/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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ratio of jojoba is slightly higher for a typical diesel than that of jojoba oil. This may have some
284
significance regarding CO2 emissions in the runs utilizing both types of fuel. The flash point of
285
raw jojoba oil indicates a low volatility level, as compared with a typical diesel. Also, the
286
absence of aromatic fuel content in jojoba, while existing in a typical diesel, and also the
287
presence of oxygen in jojoba oil, may reflect on the nature of the flame established based on both
288
fuels, in particular when sooting abilities of the flame and the consequent thermal radiation from
289
it, is taken into account. Future studies dedicated to explore in details the above aspects are
290
recommended to be conducted.
291 292
4. Results and discussions
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The results of the experimental tests listed in Table 2 are presented in Figs. 2 to 6. The discussion
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is divided into two sections. In the first, the discussion focuses on the combustion of only-diesel
295
fuel (baseline test; Test 1) while in the second it focuses on the results of the tests with jojoba-
296
diesel blends and comparisons with the baseline test results, Test 1. It should be recalled that
297
Tests 2 and 3 are under the same total input fuel condition as in Test 1. Some brief highlights on
298
the results of Test 4 with more total fuel supply and higher jojoba content in the injected blends
299
(as indicated earlier), are also presented.
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1) Test 1: Baseline test of only diesel combustion:
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In Test 1, the furnace was fueled only with pure diesel (not blended with raw jojoba oil) at a rate
302
of 8.26 kg/h. The temperature measurement at the midpoint of the exit and middle cross section
303
of the furnace (points 15 and 17 in Fig.1) are given in Fig. 2 (a-d) for all studied cases vs air to
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fuel ratio. For Test 1 these temperature measurements are given in Fig. 2a.
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12/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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Fig. 2a: Temperature measurements at the middle and at the furnace exit for Test 1; 100% diesel at rate of 8.26 kg/hr. 306
Fig. 2b: Temperature measurements at the middle and at the furnace exit for Test 2; 80% diesel and 20% jojoba oil with total fuel supply of 8.26 kg/hr. 13/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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Fig. 2c: Temperature measurements at the middle and at the furnace exit for Test 3; 65% diesel and 35% jojoba oil with total fuel supply of 8.26 kg/hr. 307
Fig. 2d: Temperature measurements at the middle and at the furnace exit for Test 4; 40% diesel and 60% jojoba oil with total fuel supply of 10 kg/hr. 14/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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308 309
As seen from Fig. 2a, at any air to fuel ratio value, the gap between the two temperature
310
measurements at the middle and exit cross section of the furnace gives indication on the level of
311
completeness of the combustion reactions within the furnace space starting from the injector up
312
to the middle of the furnace; the closer that gap between the two temperature measurements, the
313
more complete is the combustion within the first half of the furnace. The quantitative difference
314
between the mid-furnace and the exit temperature is an indication of the surplus part of the
315
sensible heat release by the combustion reactions in the second half of the furnace, up to the exit.
316
Part of this released heat by combustion in the second half of the furnace is conveyed to the
317
furnace walls by radiation and convection. If the exhaust temperature became lower than the
318
mid-furnace temperature, this might be an indication that most of the combustion reaction
319
activity was completed within the first section of the furnace, and beyond that point up to the
320
furnace exit the hot gases were mainly losing part of their heat to the furnace walls, without any
321
significant further reactions, or possibly with reactions that do not lead to heat release that can
322
cope with the heat transfer rate from the hot gas to the furnace walls. This later possibility was
323
not observed in any of the runs conducted, indicating that the length to achieve consumption of
324
all the injected fuel is longer than the length from the injector tip to the middle of the furnace.
325
Based on this, more flame length is attained, especially (as will be seen later) in the tests that
326
included jojoba oil. As expected, the gap between the two temperature measurements decreases
327
with the increase in air/fuel ratio. Of course, with the increase of the overall air to fuel ratio
328
towards lean fuel levels, one cannot exclude the possibility of having some unburned fuel or
329
partially burned species such as carbon monoxide (CO) leaving the furnace exit (regardless of
330
the abundance of air) due to the cooling and quenching effects of the excessive amounts of
331
supplied cool air under these high air supply scenarios. The measurements of unburned species
332
that are reported at the furnace exit (to be presented later) for high air to fuel ratios, would be
333
supportive to the above arguments, especially when CO emissions are considered (Fig. 5).
334
In this respect, it is known that at high temperatures, carbon dioxide (CO2) may go through
335
effective dissociation reactions that may lead to excessive equilibrated amounts of CO
336
concentrations in the products. In this study, the furnace is not adiabatic and hence the 15/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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temperatures are not high enough to support this route of CO formation by thermal dissociation.
338
However, other routes are prevailing in the present furnace when air supply rates become
339
excessive namely the formation of overly lean fuel mixtures and the relatively cool gas
340
temperatures due to the excessive amounts of cool air supplied. The quenching of CO in furnaces
341
environment is attributed to the excessive air supply rates, while in internal combustion engines
342
this is attributed to cold cylinder wall temperatures. It was reported that fuel and CO reactions
343
decay at an exponential rate with temperature (see for example [29] and the references cited
344
therein related to this point). Further to this point, recent research on CO emission from furnaces
345
fueled with liquid biofuels that have characteristics similar to the fuel addressed in this study
346
(crude Jojoba oil) support the above effect on CO emission of low temperatures that result when
347
air supply is increased (see for example [24]). Another significant parameter, which is of clear
348
significance in this regard, is the residence time inside the furnace available for reaction
349
completion. The residence time becomes shorter when flow rate increases. The adverse impact of
350
both the lower temperatures and the shorter residence time, under excessive air supply
351
conditions, may offset the enhanced mixing between the combustible species and oxidizer
352
species.
353
One further point to highlight related to hydrocarbon fuels flames is that CO oxidation becomes
354
effective in the presence of sufficient amounts of the hydroxyl radical OH; not Oxygen
355
molecules. Oxidation by Oxygen is comparatively slow (especially at low temperatures) and it
356
serves mainly as chain reaction sequence initiator (reference to related combustion literature can
357
be made for further details; see for example [30, 31]). The concentrations of OH radical would
358
be expected to decrease significantly under excessive airflow conditions due to the dilution
359
caused by extra air and the reduced flame temperatures. These effects result in slow Oxygen-
360
based reaction that is responsible for initiating the CO oxidation chain reaction sequence. All the
361
above arguments would collectively be expected to lead to drop in the CO oxidation rates at
362
overly high air supply rates.
363
At fuel-rich condition of air to fuel ratio around 11, the relatively big difference between the two
364
temperature measurements seen in Fig. 2a indicates that much of the injected fuel requires longer
365
residence time and a bigger space to vaporize, mix with air, and subsequently combust. 16/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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Obviously, this is due to the relative shortage of the oxygen at that low air supply condition.
367
With more air supply, the required distance to completely combust the fuel becomes less, as
368
indicated by the drop in the gap between the two temperature measurements (cf. Fig. 2a). At
369
about Air/Fuel of 16.5 the minimum gap between the two temperature measurements is attained.
370
With further increase in air supply with up to Air/Fuel of about 23 in our experiments, the two
371
temperature measurements tend to only slightly diverge from each other; a trend that is
372
attributable to the cooling effects of extra air supplied under such high Air/Fuel conditions, as
373
highlighted above.
374
Measurements at the mid-section point in the middle of the furnace might not be an accurate
375
representation of the mean temperature neither it is meant here to be considered as the flame
376
temperature. However, it should be recalled that the investigated experimental furnace was not
377
equipped with the possibility to trace the temperature measurements at different points in the
378
same cross section, but just the middle point of the furnace. Yet the obtained temperature
379
measurement of the middle section reported in Fig. 2 (supported by the other measurements and
380
the calculated heat transfer quantities from energy balance) is to be considered here as a
381
reasonable approximation of the mean value at that section from which some useful insights and
382
conclusions might reasonably be drawn.
383
The distribution of the air-fuel combustible mixture throughout the combustion domain is
384
governed by the spray pattern which is dictated by the used fuel injector. This mixture
385
distribution decides the scenario of the flow and the distribution of other combustion-related
386
quantities such as temperature and the different gaseous species concentrations. This will also
387
highly impact the pollutants emissions from the furnace. In this respect, the liquid fuel viscosity
388
and its level of volatility would have significant role to play. Diesel has much lower viscosity
389
and clearly higher volatility than the liquid Jojoba oil. These two major characteristics would
390
impact the spray formation, fuel distribution and combustible mixture preparation, and chemical
391
reactions within the furnace. Hence these would help us understand and interpret the subsequent
392
results presented below.
393
Fig. 3 presents the corresponding results of heat transferred to the water jacket for all tests
394
studied (1 to 4) as a function of the air to fuel ratio. As can be seen from heat transfer results of 17/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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Test 1, the water jacket captures heat from the hot gases (by radiation and convection) at a rate
396
that varies from about 37 kW to 41 kW. An average value of about 39 kW is attained. The peak
397
of 41 kW is attained at Air/Fuel ratio of about 12 that corresponds to the fuel-rich side of the
398
tested Air/Fuel range and is corresponding to almost highest gas temperatures attained (cf. Fig.
399
2a). The levels of heat transfer to the water jacket attained over the wide range of air flow rates
400
considered matches closely and are in accordance with the gas temperatures measurements
401
reported in Fig. 2a. For the low air flow rate runs radiation is expected to be the dominant mode,
402
particularly in the first half of the furnace, while for the runs with much abundance of air
403
supplied convection is expected to be the dominant mode, especially in the second half of the
404
furnace when the gases temperature decline noticeably due to the loss of heat to the water jacket.
405
In this regard, the high sooting tendency of the diesel fuel would help us interpret the higher
406
radiation to furnace walls in the case of diesel fueled furnace, as compared to the cases where
407
diesel is blended with crude Jojoba oil. This point is confirmed by other researchers for other
408
liquid biofuels conditions where the bio fuels do not include aromatic fuel component in their
409
molecular structure and at the same time they include much oxygen content. These two aspects
410
explain the lower tendency to particulate matter formation when diesel fuel is blended with a
411
liquid biofuel such as Jatropha oil (as in [24,25]) and Jojoba oil in our current study.
412
Fig 4 presents the NOx measurements obtained for all Tests (1 to 4). The measurements are
413
taken at the point in the middle of the exit cross section of the furnace (see Fig. 1). As can be
414
seen from Fig. 4, NOx results of Test 1 are consistent with the temperature measurements
415
reported in Fig. 2a. NOx results lie within a range from about 60 ppm at the highest temperatures
416
attained at air to fuel ratio of about 12, to 50 ppm at the lower temperatures attained at air to fuel
417
ratios of around 22. It should be recalled once more that NOx is effectively formed at high
418
temperatures (roughly above 1800 K) and that the temperatures reported in Fig. 2 for the
419
different runs conducted are to be interpreted as somehow averages at these sections rather than
420
the flame temperatures at which NOx is effectively formed. Yet, as mentioned earlier, these
421
reported temperatures (with the help of other reported measurements and quantities) can be
422
helpfully used to get understanding and draw reasonable conclusions about the combustion and
423
pollutants emissions from the studied furnace. It is already known that different mechanisms
424
dominate NOx formation; thermal NOx, prompt NOx, and fuel NOx formation. The first 18/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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mechanism is the one governing the majority of the NOx formed in the hottest flame sites, and
426
the second is governing NOx formation in fuel rich environment, but the last one is believed not
427
to be so important for the current study since used fuels do not contain Nitrogen to any effective
428
extent. Temperature measurements of the different tests studied will be very helpful to explain
429
the trends of thermal NOx results presented.
430
The measurements of the other gaseous species emissions that reflect on the quality of
431
combustion reactions completeness, namely CO and the unburned hydrocarbons (UBHC), are
432
presented in Figs. 5 and 6, respectively. As can be seen from Fig. 5, CO measurements for Test 1
433
show relatively low emission levels (around 50 ppm) over air to fuel ratio levels up to about 17.
434
Beyond that air flow level, more CO emissions start showing up due to the more pronounced
435
cooling effect of the excess air, as explained earlier.
436
Fig. 3: Heat transferred to the water jacket for Tests 1 to 4. 437
19/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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Fig. 4: NOx emissions at the furnace exit for Test 1 to 4. 438
Fig. 5: CO emissions at the furnace exit for Test 1 to 4. 439
20/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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Fig. 6: UBHC emissions at the furnace exit for Tests 1s to 4. 440 441
The trend of the UBHC emissions in Test 1 shows slightly declining levels with the increase in
442
Air/Fuel ratio from about 200 (mg/m3) at Air/Fuel of around 11 to lowest levels somewhat near
443
about 170 (mg/m3) before a tendency to start increasing upon further increase in air supply is
444
observed at about Air/Fuel of 22. The above scenario is a little bit different form the one
445
observed in Test 1 for CO emissions. In the case of CO, the CO levels remained almost constant
446
at about 40 to 50 ppm, over all the Air/Fuel range up to around 20, before a clear rise due to the
447
reasons mentioned earlier including air cooling effects, is noticed when Air/Fuel is increased
448
beyond 20.
449
While CO levels show clear variations in magnitude with air to fuel ratio as we go from the fuel-
450
rich to lean fuel conditions, for the UBHC the variations on magnitude-basis are marginal over
451
all Air/Fuel range considered; (almost a constant stable UBHC level of about 180 mg/m3 was
452
recorded). For the case of UBHC, the effect of enhanced mixing as we increase air supply seems
453
to be more pronounced and is not offset easily by the cooling caused by the larger air flow rates 21/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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up to Air/Fuel of about 22. Upon increasing the air flow rate beyond this level, we may
455
anticipate seeing more clearly a trend of UBHC emissions increase; a scenario that happened at
456
Air/Fuel ratio of around 19 in the case of CO emissions.
457
2) Tests with Jojoba-Diesel Blends
458
Below is a discussion on the effect of the presence of raw liquid jojoba oil in the injected liquid
459
fuel, on the performance of the studied furnace.
460
2.1) Test 2: Diesel/Jojoba-oil blend with 20 percent jojoba oil:
461
The temperature results of Test 2 that are the counter part of those presented in Fig. 2a for Test 1,
462
are shown in Fig. 2b. The same qualitative trend observed in Fig. 2a for Test 1, is also observed
463
in Fig. 2b of Test 2. Even quantitatively, the temperature results reported in Fig. 2b are
464
comparable to those of Test 1 presented in Fig. 2a. The major clear difference between the two
465
cases is the gap between the two temperature measurements (mid-furnace and exit temperatures)
466
at any of the Air/Fuel conditions tested, being more evident in Test 2. This bigger gap observed
467
in Test 2 indicates that more pronounced combustion activity is still taking place in the second
468
half of the furnace in Test 2, than what is observed in Test 1, indicating thereby that more fuel in
469
Test 2 is being deferred in its reaction, to be completed in the second half of the furnace.
470
The viscosity of crude Jojoba oil is very high compared to that of diesel. This will have a
471
significant impact on spray pattern and the droplet sizes formed when diesel is blended with
472
Jojoba oil, leading to larger droplets that vaporize at a slower rate in this later case. This would
473
also lead to having liquid droplets that can penetrate to longer distances in the furnace and
474
consequently can lead to a distribution of the resulting fuel vapors over a longer and wider space
475
in the combustion domain. All these aspects would be expected to affect the combustion, heat
476
transfer, and pollutant formation and emission form the furnace.
477
Given the nature of the jojoba oil and its lower volatility level, as compared to the diesel fuel,
478
one may infer from the above that most of the liquid fuel droplets that can survive and reach the
479
downstream portions of the furnace combustion space towards the exit have higher jojoba oil
480
concentrations than upon injection at the injector tip. Accordingly, most of the heat liberation 22/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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coming from fuel combustion in the first half of the furnace may be attributed to the higher
482
volatility constituent in the injected fuel namely the diesel, which vaporizes faster. Besides, as
483
highlighted above, the lower volatility and higher viscosity of jojoba oil makes it possible for the
484
droplets to penetrate further and to spread and distribute fuel and hence accomplish the
485
combustion of all the fuel sprayed over a bigger length and a larger space in the furnace. This,
486
under the given turbulent combustion environment, may lead to locally less intense heat
487
liberation by combustion reactions hence lower local temperatures in the flame sites due to the
488
locally lower availability of the fuel vapor when distributed over a bigger space. Consequently,
489
this would possibly contribute to reduction in thermal NOx formation activity. Moreover, the
490
distribution of fuel over a bigger space in the furnace would give the chance for more dilution of
491
the resulting fuel vapor with other gases (mainly air in the early stages of the furnace). This will
492
reduce the level of fuel-richness of the gas which will reduce formation of prompt NOx as well
493
as soot and will give more chance for CO reduction. Consequently, the above arguments may
494
support interpreting the somewhat lower NOx emissions in Test 2, as compared to NOx levels
495
attained in Test 1 (cf. Fig. 4). With the above being said, it is already clear that the interrelated
496
phenomena at hand are so complex and each influences the others in a way or another. Hence,
497
there is still further need to conduct more research studies to come up with a more detailed
498
understanding of the phenomena taking place under furnaces’ environments and for liquid
499
biofuels like the one considered in this study.
500
Quantitatively, NOx emissions levels in Test 2, as given in Fig. 4, are almost uniform over the
501
entire tested air flow range and have an average value around 39 ppm. This average level is
502
around 15 ppm lower than the average attained in Test 1 (cf. Fig. 4). It should be recalled that the
503
temperature results presented for Tests 1 and 2 in Figs. 2a and 2b, are a representation, more or
504
less, of an average temperature at the cross sections where they are measured, and they are not
505
local temperatures at hottest sites of the flame where thermal NOx forms effectively. From the
506
NOx results of both tests and based on the discussions and arguments given above, these hottest
507
sites seem to be more pronounced and effective in producing NOx in Test 1. However, prompt
508
NOx formation may be more pronounced for flames when only diesel is used (as is the case in
509
Test 1) and this may also contribute to the higher levels of NOx in Test 1 than in Test 2, where in
510
Test 1 there is more chance to have fuel-rich sites in the flame that support prompt NOx 23/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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formation. Further future experimental and/or theoretical research may be needed to give more
512
insight in respect to these arguments.
513
Local temperatures measured at the middle section of the furnace in Test 2 are somewhat similar
514
to those attained in Test 1. However, the amount of heat captured by the water jacket in Test 1 is
515
somewhat higher by about 1 to 2 kW. The higher heat transferred to the water Jacket in Test 1
516
might be attributed to the tendency of the only-diesel flame in Test 1 to radiate more heat than
517
that from the flame based on the blended fuel in Test 2. One may anticipate here more soot
518
formation in the only-diesel flame in Test 1 that can be responsible for more thermal radiation to
519
furnace walls. With more thermal radiation to furnace walls in Test 1 than in Test 2 due to the
520
above reasons related to soot formation, we still see comparable temperatures at the middle
521
section in both Tests 1 and 2 (cf. Figs. 2a and 2b). This would then hint to a more complete
522
combustion and reaction activity within the first half of the furnace in Test 1, when only diesel is
523
used as fuel. This means that in Test 1 the additional reactivity of the fuel; hence the associated
524
heat release, would compensate for the more heat loss from the flame by radiation to the walls
525
and hence would maintain temperatures in the first half of the furnace at levels comparable to
526
those attained under the less radiation conditions in Test 2. Again, the arguments related to the
527
lower sooting tendency when such a biofuel as the one used in this study is used, are supported
528
by recent studies [24,25] as highlighted earlier.
529
Although the above arguments might be valid in principle but a more rigorous experiments and
530
analysis and validation of the above may still be needed. This is however beyond the scope of the
531
present work and yet would require more detailed measurements and quantification for (for
532
example) soot formation and concentrations as well as more detailed temperature measurements
533
at various spatial locations within the combustion domain. These, unfortunately, are limitations
534
that were not possible for us to overcome at this stage due to limitations dictated by the furnace
535
tested in this study where no such measurements were possible for us to do at this point.
536
Nonetheless, the above arguments may be considered as an invitation for further more detailed
537
research in the direction of the above thoughts which we believe would be very insightful and
538
would shed light on much of the physics and the chemical reactions activities.
24/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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The higher volatility of the diesel as compared to raw jojoba oil may also lead us to think that the
540
more vapor content in the flame of only-diesel in Test 1 would render the only-diesel flame to
541
behave more closely like a gaseous diffusion flame when compared to the jojoba-based flame,
542
which might be associated with more heterogeneity especially when the percentage of jojoba
543
share in the blends gets higher. On the other hand, the jojoba oil in the fuel blend, leading to
544
larger droplets in the spray, would require longer distance inside the furnace to complete
545
vaporization and subsequent mixing with the air. This may suggest partially premixed flame
546
characteristics in the furnace for the cases with jojoba blended in the fuel (Tests 2-4). The
547
diffusion-based combustion is more likely to prevail when the fuel injected has higher volatility,
548
which is the case of the only-diesel flame in Test 1.
549
Since jojoba oil is an oxygenated fuel, this would be expected to help more effective oxidation of
550
soot and CO that might have been formed in the jojoba blends-based flames, and also would tend
551
to dilute the level of fuel-richness of the fuel-rich sites of the flame that basically represent the
552
sites where soot formation effectively may take place. This argument is in-line with the lower
553
heat transfer by radiation to furnace walls observed in Test 2, as compared with Test 1 (cf. Fig.
554
3), where soot particles are known to be an effective promoter for thermal radiation from flames.
555
As can be further seen from Fig. 3, the heat transferred to the water jacket in Test 2 shows almost
556
uniform distribution over all of the tested Air/Fuel range with a mean value of about 37 kW;
557
compare this with around 39 kW average attained in Test 1.This difference in heat transferred to
558
the water jacket between the two tests (by applying simple energy balance) yields the ultimate
559
difference in the exhaust temperature of both cases of about 10 to 15 degrees over most of the
560
Air/Fuel ratio range tested (see Figs. 2a and 2b).
561
With higher Air/Fuel ratio beyond about 15, the decline in temperature (both in the middle and at
562
the furnace exit), become evident due to the cooling effect of the excess cool air supplied to the
563
furnace. This cooling effect has clear influence on the unburned hydrocarbon species emissions
564
in Test 2, but little on Test 1, where in Test 2 the UBHC emission drops from about 165 mg/m3
565
to about 120 mg/m3. More stable level with less declining tendency is seen in Test 1 as Air/Fuel
566
ratio is decreased; (compare about 27% reduction in Test 2 with only about 10% when the
567
Air/Fuel ratio is increased the same amount in both tests from about 11 to 22 (cf. Fig. 6)). With 25/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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further excessive increase in the air supply, due to the cooling effect, an increasing trend would
569
be expected to start showing in the unburned hydrocarbon emissions. The onset of this increase
570
seems to have started in Test 1 when the Air/Fuel reached about 22. For Test 2, this increase is
571
less evident at Air/Fuel of 22 where the turning point was not reached in the tests conducted.
572
Nevertheless, based on the slope of the data points given in Fig. 6 for Test 2, it is expected to see
573
this happening at Air/Fuel levels roughly above 24.
574
NOx emissions in Test 1 are more responsive to increasing the air flow rate where NOx levels
575
drop from about 60 ppm at air/fuel ratio of about 12 to about 50 ppm at Air/Fuel of about 22.
576
The drop in NOx emissions in Test 2 (which already are lower than in Test 1; cf. Fig. 4) with the
577
increase in air flow, is less noticeable though. In regards to CO emissions, both tests show a clear
578
increasing CO-emission trend upon exceeding air to fuel ratio beyond about 16, with peak values
579
in the range from 200 to 300 ppm attained at the fuel lean-side of the Air/Fuel range considered
580
in the conducted experiments.
581 582
2.2) Test 3: Diesel/Jojoba-oil blends with 35 percent jojoba oil:
583
In Test 3 the results depart more and more from Test 1 conditions by incorporating more jojoba
584
oil in the liquid fuel sprayed into the furnace. Fig. 2c shows the temperatures attained in Test 3.
585
The temperature levels in Test 3, generally, are a bit higher than those attained in the previous
586
two tests and this is particularly the case for the exit temperature (cf. Fig. 2a-c). In addition, with
587
the increase in the Air/Fuel beyond 15, the temperatures in Test 3 do not drop to the levels
588
observed in Tests 1, but are a bit higher. The middle section temperature in Test 3 is comparable
589
to those observed in the previous tests for almost all air flow rates, rendering the discussions and
590
arguments made while discussing Tests 1 and 2 results, valid.
591
The difference in exit temperature between the different tests may impact and reflect on the
592
levels of unburned species emissions in these tests. This impact becomes even more pronounced
593
in the range of Air/Fuel above about 15. For example, the CO emissions of Test 3 maintain low
594
level at about 30 ppm over all Air/Fuel range up to about 20 whereas in Tests 1 and 2, the
595
runaway in CO emissions due to excessive air cooling effect starts at somewhat lower Air/Fuel 26/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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ratio levels (above about 16). Also the hydrocarbons emissions in Test 3 maintain a constant low
597
level around 110 ppm over all Air/Fuel values tested; compare this with the much higher values
598
reported in Tests 1 and 2 (see Fig. 6). The lower CO and hydrocarbons emissions observed in
599
Test 3 may be attributed to the somewhat higher temperatures in the furnace that still support
600
more effective oxidation of the unburned species under high air flow conditions, to a more clear
601
extent than what has been observed in the other two tests.
602
The higher jojoba content in Test 3 than in Test 2 was thought of at the beginning to be an
603
obstacle to having high level of combustion intensity in the flame due to the lower jojoba
604
volatility and higher viscosity (as highlighted above) which would lead to large droplets in the
605
spray. However, what the attained results reveal here is that this is not exactly the case in Test 3.
606
Perhaps the somewhat lower radiation to the walls in Test 3, as compared with the test with only
607
diesel, is behind the somewhat higher temperatures reported when jojoba content in the injected
608
fuel reached up to 35 percent. Another point that might also be of relevance in this regards is the
609
inherent presence of oxygen in the jojoba oil which would then lead to more abundance in
610
oxygen concentrations and hence more combustion activity at different locations in the flame
611
that hence lead to more heat release. In addition, having higher temperatures in the exhaust of
612
Test 3 is still expectable especially when considering the possibility of having larger fuel
613
droplets due to the larger viscosity of the liquid fuel injected in Test 3. These larger droplets have
614
the ability to survive and continue burning in the later stages of the combustion chamber domain
615
up to the furnace exit.
616
Although it was mentioned above that the more volatile component of the injected fuel (i.e.
617
diesel in our case) would have more chance to vaporize and burn first in the first half of the
618
furnace, leaving more room for the jojoba oil to burn subsequently in later stages of the furnace,
619
the larger droplets formed due to the higher viscosity of the blended fuel in this case would have
620
sufficient inertia to reach far in the combustion domain up to the furnace exit and so they act as
621
carriers for the more volatile diesel and with that they give the chance for some more diesel to
622
bypass the first half of the furnace and survive as liquid, to vaporize and burn later in regions
623
near the furnace exit.
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NOx emissions of Test 3 are only little less than those reported in Test 2. The difference between
625
the two tests (in the average) is about 5 ppm. As implied above by the presented temperature
626
results, the local temperatures in the hottest sites of the flame which are the places of the
627
effective NOx generation are not amenable to us in this study for measurement hence hindering
628
more elaboration on this matter related to NOx formation in the different tests conducted.
629
Nevertheless, this would not hinder us here from making the reasonable conclusions in regards to
630
NOx emissions in the light of the attained temperature results namely the effect of spreading the
631
fuel over a bigger combustion envelope that would lessen the intensity of reactions, consequently
632
the local temperatures that may influence NOx formation, as we have highlighted earlier. Some
633
more elaborated future experimental and/or theoretical investigations on NOx formation from
634
jojoba oil-based flames may shed more light on the phenomena behind NOx results obtained in
635
this work.
636
The slightly higher temperatures reported in Test 3, as compared to those of Test 2 (cf. Figs. 3a
637
and 4a) result in only slightly more heat transfer to the water jacket in Test 3 than in Test 2 (cf.
638
Fig. 3). As can be seen from the Fig. 3, both tests show almost a uniform stable level of heat
639
transfer to the furnace walls over almost all air flow conditions considered in the experiments.
640
Once again, having jojoba content in the fuel in Test 3 leads to less sooting tendency hence heat
641
loss from the flame. Radiation is dependent however not only on emissivity due to the presence
642
of soot, but very much on the temperatures which are showing higher levels in Test 3 due to the
643
reasons and arguments given above. Hence this explains the slightly higher radiation to the wall
644
in the case of Test 3 as compared to Test 2. All the above discussion for Test 2 on the spray
645
evolution and vaporization and combustion when having jojoba oil in the injected blended liquid,
646
are believed to still be valid for Test 3 as well.
647
2.3) Test 4: Diesel/Jojoba mix with 60 percent jojoba:
648
Attempting to have more jojoba content in the injected jojoba/diesel blends beyond the levels
649
used in Test 3 (namely beyond 35 percent) using the same injection system setup used in Tests 2
650
and 3 without alteration, was not easy to attain. With as large as 60 percent jojoba content in the
651
blend, no stable flame was possible to be established. The difficulty to achieve this is attributed
652
to the low volatility of jojoba oil and the high viscosity of the injected liquid fuel under such high 28/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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jojoba content conditions that would lead to large and slowly vaporizing sprays hence little
654
ability to secure sufficient heat generation to sustain steady and stable flame. To overcome these
655
obstacles, it was found to be necessary to inject larger amounts of the blended fuel than the
656
amount used in Tests 1 and 2. These larger amounts are necessary in order ensure the presence of
657
sufficient volatile fuel content in the flame that basically originates from the diesel part in the
658
blended droplets in this binary liquid fuel spray. The burning of the vapor resulting from the
659
diesel will then generate enough heat to support and sustain the vaporization and ignition of the
660
rest of the liquid originating from the less volatile constituent namely jojoba oil. It was found
661
that there is a need in the case of 60 percent jojoba in the injected fuel in Test 4 to inject fuel at a
662
higher flow rate than 8 kg/h. No stable self-sustaining flame was possible at a total flow rate of
663
about 8 kg/h, when the jojoba share in the blend was increased up to 60 percent. Therefore, in
664
Test 4 the blended fuel injection rate was increased to about 10 kg/h and this was suitable to have
665
a self-sustaining stable flame over the range air/fuel ratio combustion conditions reported (up to
666
Air/Fuel levels of around 20). It should be emphasized here that conducting Test 4 in this work
667
was for the sole purpose mainly to show that such high raw jojoba oil content of 60 percent was
668
possible to be used to produce stable conditions if total fuel injection rate was sufficiently high.
669
As such, it was not a major target at this stage to compare the results of Test 4 with the previous
670
tests’ results. Further future runs are recommended under high jojoba proportions in the injected
671
blends and under different total fuel supply and initial injection temperature conditions.
672
The higher viscosity of jojoba in the injected blends would tend to damp out any possible
673
disintegration and breakup of the injected droplets and thereby add to the difficulties outlined
674
above regarding attaining small droplets with sufficiently rapid vaporization.
675
Temperature results of Test 4 are presented in Fig. 2d. Higher temperatures are realized than
676
what has been attained in Tests 1-3 mainly due to the overall higher amounts of injected fuel.
677
Moreover, for any given Air/Fuel ratio, this is also attributed to the higher turbulence and mixing
678
intensity between the fuel vapors and the oxidizing species, where to burn a larger amount of
679
fuel at a fixed Air/Fuel, higher air flow rate would be needed. This later effect may possibly
680
explain the smaller gap between the middle and exhaust temperatures, especially in the lean part
681
of the Air/Fuel range. Here in Test 4 also the possibility to have larger droplets that can spread 29/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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682
the fuel over a bigger space of the furnace to complete vaporization and combustion over a
683
bigger area, supported by the more intense turbulent mixing and combustion, may also lead to
684
less difference in temperature between the exhaust and the middle section temperature. Other
685
parameters that may also affect the enhanced fuel reactivity in the first half of the furnace in Test
686
4 is the larger oxygen content accompanying the additional amounts of liquid jojoba in the
687
injected fuel.
688
The heat radiated to the water jacket in Test 5 is at an average of around 41 kW over all air flow
689
rate values considered (cf. Fig. 3); compare this with around 37 to 39 in the previous tests with
690
less overall total fuel injected. In the fuel-rich range of the Air/Fuel range, the rate of heat
691
transfer reaches around 43 kW. The higher heat transfer in Test 4 can be mainly attributed to the
692
more overall total fuel injected and the consequent higher temperatures attained. Less likely is
693
that to be attributed to any more particulate emissions from the flame in Test 4. This is due to the
694
fact that jojoba is abundant (with a high share; 60 percent), in the injected liquid fuel. Given that
695
jojoba is an oxygenated fuel with no aromatic fuel content, it will not support soot formation.
696
Moreover, the inherent oxygen content in jojoba will be even expected to retard in the fuel
697
vapor-rich regions soot formation that may result from the diesel that accompanies jojoba in the
698
injected fuel. In addition, the higher viscosity of jojoba while using the same injector and
699
injection system of the present furnace for all runs without alterations is expected to yield a spray
700
with coarser droplets that (as highlighted above) would promote more for having premixed
701
flame-like with less level of fuel richness within the flame envelop. As mentioned earlier, pure
702
diffusion flame is more closely attained with higher availability of the volatiles content in the
703
injected liquid blend. This may give us another reason for not having extensive soot formation
704
activity in the flames formed in Test 4 leaving us with the higher temperature alone as the main
705
reason for the enhanced heat transfer to the furnace walls.
706
NOx emissions in Test 4 shown in Fig. 4 are less than in the previous tests although overall
707
average temperatures reported in Test 4 are a bit higher than those of the previous tests. Once
708
more, NOx formation is dependent on the local phenomena in the hot sites of the flame and these
709
seem to be less encouraging to NOx formation in Test 4, regardless of the apparently somewhat
710
higher representative average temperatures recorded. The arguments and statements laid down in 30/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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711
the above discussions on NOx for the previous tests are also valid for Test 4. It should be
712
recalled that thermal NOx formation takes place effectively in the hot flame spots, at much
713
higher temperatures than the average temperatures reported here for all tests. These flame sites
714
have apparently lower temperatures in Test 4, which may explain the lower level of NOx
715
emissions in Test 4. The effect of jojoba leading to larger droplets and hence causing spreading
716
of the combustion zone over a bigger space and thereby leading to less local heat release
717
intensity and lower local flame temperatures as well as the more possibility to have diluted fuel-
718
air mixture regions that do not support prompt NOx formation, might be reasonable arguments
719
that may well explain the trends and measurements reported hereby.
720
The unburned species emissions (CO and the unburned hydrocarbons) show lower levels than
721
what has been observed in Tests 1-3. This is possibly attributable to factors like: the higher
722
overall temperatures attained in Test 4, the more availability of oxygen by the virtue of the
723
nature of jojoba oil being an oxygenated fuel, the higher tendency to form a premixed-like
724
combustion due to the expected larger droplets that vaporize at a slower rate, and the higher
725
turbulence mixing levels associated with higher air flow rates for any given Air/Fuel in Test 4.
726
Unburned hydrocarbon emissions in Test 4 show very uniform level at about 180 mg/m3 over a
727
wide range of air flow rates (see Fig. 6). These levels are comparable to the levels attained for
728
Test 1 with only diesel with less total fuel supply rate. This highlights once more the relatively
729
rapid mixing rate due to the higher air flow rates and the corresponding higher turbulence
730
intensity. Also this can be to some extent attributed to the oxygen content in jojoba. CO
731
emissions (Fig. 5) also show clearly low levels in Test 4 for Air/Fuel below about 14 for the
732
reasons explained above. However, for higher Air/Fuel ratios, CO emissions clearly increase to
733
reach about 220 ppm at Air/Fuel of around 20, which might be expected due to the excessive air
734
flow cooling effects and the short residence time when the flow speeds increase.
735
5. Summary and Conclusions
736
The combustion, heat transfer, and pollutants emissions from a small furnace fueled with blends
737
of diesel and raw jojoba oil are investigated experimentally. Different percentages of the jojoba
738
oil in the injected fuel blends are considered ranging from zero (only diesel) to a max of 35
739
percent on mass basis. Runs with total blended fuel input to the furnace of about 8.3 kg/h, which 31/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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740
corresponds to about 99 to 103 kW nominal energy input (which also corresponds to range of
741
Jojoba oil in the blends from 35% to 0%), are conducted. By comparing the emissions from the
742
comparing of blends of raw jojoba oil and diesel (with 35% jojoba oil content) with their
743
counterparts based on the combustion of pure diesel, the results show a significant reduction in
744
NOx emissions (around 30 to 40 percent). In the case of UBHC, the reduction is about 40
745
percent in the average. Regarding CO emissions, 35 percent jojoba in the fuel blends has slightly
746
reduced CO emissions as compared with the baseline case of only diesel. Running the furnace
747
under lean fuel conditions, would mostly affect the CO emissions causing them to increase while
748
other pollutants (NOx and unburned hydrocarbons) showed little to no changes over a wide
749
range of air supply conditions including the lean fuel side.
750
It was found further that attaining stable combustion while including more jojoba in the
751
diesel/jojoba-oil blends beyond about 35 percent was not easy, under the tested fuel injection
752
temperatures at typical room conditions, without increasing the total amount of blended fuel
753
injected. The additional amounts of the blended fuel injected in this case would ensure the
754
availability of sufficient amount of the volatile combustible matter originating from the diesel
755
that would contribute to having a self-sustaining flame to support heating and vaporizing the less
756
volatile jojoba oil. The more the proportion of raw jojoba oil in the blends, the higher the overall
757
viscosity of the injected fuel blends. This would affect the performance of the atomization
758
process and the spray evolution and the related subsequent processes in the furnace. Preheating
759
the blends before injection would possibly allow higher jojoba oil content in the runs that are
760
associated with relatively low nominal fuel energy input, without the need for significant
761
alterations in the injection system.
762
A Jojoba oil content as high as 60 percent in the blend was possible under higher total fuel
763
injection rate of about 10 kg/h. The fact that raw jojoba oil has a lower heating value than diesel
764
might be expected to have an adverse effect on the local measured gas temperatures but the
765
measurements were just to the contrary to that, where the runs with jojoba attained slightly
766
higher local gas temperatures at the middle section and at the furnace exit. Different parameters
767
may contribute to this results among which the low tendency to sooting when high jojoba content
768
is present in the fuel, hence less thermal radiation transfer to the furnace walls. 32/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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References
770
1. Al Omari, S. B., Evaluation of the Biomass “Date Stones” as a Fuel in Furnaces: A
771
Comparison with Coal Combustion, Int. Communications in Heat and Mass Transfer, 36 (9),
772
pp. 956-961, 2009.
773
2. Al Omari, S. B., Experimental investigation of combustion and heat transfer characteristics in
774
a furnace fueled with unconventional biomass fuels (date stones and palm stalks), Energy
775
Conversion and Management, 47, pp. 778-790, 2006.
776
3. M. Y. E. Selim, Y. Haik, S.-A. B. Al Omari, and E. Elnajjar, Combustion of Waste
777
Chocolate Oil Methyl ester in a Diesel Engine, Int. J. of Ambient Energy; 2013; DOI:
778
10.1080/01430750.2013.770795).
779
4. M. O. Hamdan, M. Y. E. Selim, SAB Al-Omari, M Ghannam, Effect of Stabilized Water-
780
Diesel emulsion on External Combustion characteristics, Proceedings of the ASME 2013
781
Summer Heat Transfer Conference, 2013.
782
5. Al Omari S. B., Abu-Jdayil, Some Considerations of the Performance of Small Dual Fuel
783
Furnaces Fueled with a Gaseous Fuel and a Liquid Fuel Mix Containing Used-Engine-Lube-
784
Oil, Renewable Energy, 56, pp.117-122, 2013.
785 786
6. Al Omari S. B., Used engine lubrication oil as a renewable supplementary fuel for furnaces, Energy Conversion and Management, 49 (12), pp. 3648-3653, 2008.
787
7. Mohamed Y. E. Selim, Y Haik, S. A. B. Al-Omari, H Abdulrahman, Biodiesel Oil Derived
788
from Biomass Solid Waste, Proceedings of the World Congress on Engineering 3, 2011.
789
8. M. O. Hamdan, P Martin, E Elnajjar, M. Y. E. Selim, S Al-Omari, Diesel engine
790
performance and emission under hydrogen supplement, ICREGA’14-Renewable Energy:
791
Generation and Applications, 329-337, Springer International Publishing, 2014.
792 793
9. M. O. Hamdan, M. Y. E. Selim, S. A. B. Al-Omari, E. Elnajjar, Hydrogen supplement cocombustion with diesel in compression ignition engine, Renewable Energy 82, 54-60, 2015.
33/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
ACCEPTED MANUSCRIPT
794 795 796
10. M.Y.E. Selim, M.S. Radwan, S.M.S. Elfeky, Combustion of jojoba methyl ester in an indirect injection diesel engine, Renewable Energy, 28, Issue 9, Pages 1401–1420, 2003. 11. A.S. Huzayyin, A.H. Bawady, M.A. Rady, A. Dawood, Experimental evaluation of Diesel
797
engine performance and emission using blends of jojoba oil and Diesel fuel, Energy
798
Conversion and Management 45, 2093–2112, 2004.
799
12. Mohamed Y.E. Selim, M.S. Radwan, H.E. Saleh, Improving the performance of dual fuel
800
engines running on natural gas/LPG by using pilot fuel derived from jojoba seeds,
801
Renewable Energy 33, 1173–1185, 2008.
802
13. M. S. Shehata, S. M. Abdel Razek, Experimental investigation of diesel engine performance
803
and emission characteristics using jojoba/diesel blend and sunflower oil, Fuel 90, 886-897,
804
2011.
805 806 807 808 809
14. H. E. Saleh, Experimental study on diesel engine nitrogen oxide reduction running with jojoba methy ester by exhaust gas recirculation, Fuel 88, 1357-1364, 2009. 15. M. Al-Widyan, M. A. Al-Muhtaseb, Experimental investigation of jojoba as renewable energy source, Energy Conversion and Management, 51, 1702-1707, 2010. 16. F. Millo, B. K. Debnath, T. Vlachos, C. Ciaravino, L. Postrioti, G. Buitoni, Effects of
810
different biofuels blends on performance and emissions of an automotive diesel engine, Fuel
811
159, 614–627, 2015.
812
17. G. Tashtoush, M. Al-Widyan, A. O. Al-Shyoukh, Combustion performance and emissions of
813
ethyl ester of a waste vegetable oil in a water-cooled furnace, Applied Thermal Engineering
814
23, 285-293, 2003.
815
18. A. Cavarzerea, M. Morinib, M. Pinellic, P. R. Spinac, A. Vaccaria, M. Venturinic,
816
Experimental Analysis of a Micro Gas Turbine Fuelled with Vegetable Oils from Energy
817
Crops, 68th Conference of the Italian Thermal Machines Engineering Association, ATI2013,
818
Energy Procedia 45, 91 – 100, 2014.
34/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
ACCEPTED MANUSCRIPT
819 820 821
19. A. E.E. Khalil, A.K. Gupta, Clean combustion in gas turbine engines using Butyl Nonanoate biofuel, Fuel 116, 522–528, 2014. 20. T. E. Jiru, B. G. Kaufman, K. E. Ileleji, D. R. Ess, H. G. Gibson, D. E. Maier, Testing the
822
performance and compatibility of degummed soybean heating oil blends for use in residential
823
furnaces, Fuel 89, 105–113, 2010.
824 825 826 827 828 829
21. L. Jiang, A. K. Agrawal, R. P. Taylor, Clean combustion of different liquid fuels using a novel injector, Experimental Thermal and Fluid Science 57, 275–284, 2014. 22. W-C Huang, S.-S. Hou, Ta-H. Lin, Combustion characteristics of a 300 kWth oil-fired furnace using castor oil/diesel blended fuels, Fuel 208, 71-81, 2017. 23. I. I. Enagia, K.A. Al-attabb, Z.A. Zainal, Liquid biofuels utilization for gas turbines: A review, Renewable and Sustainable Energy Reviews, 90, 43-55, 2018.
830 831 832 833 834 835 836
24. N. Hashimoto, H. Nishida, Y. Ozawa, Fundamental combustion characteristics of Jatropha oil as alternative fuel for gas turbines, Fuel 126, 2014, 194-201, 2014.
837
26. H. Pawlak-Kruczek, M.Ostrycharczyk, J. Zgo´ra, Co-combustion of liquid biofuels in PC
25. N. Hashimoto, H. Nishida, M. Kimoto, K. Tainaka, A. Ikeda, S. Umemoto, Effects of Jatropha oil blending with C-heavy oil on soot emissions and heat absorption balance characteristics for boiler combustion, (accepted in Renewable Energy on April 5, 2018: DOI: 10.1016/j.renene.2018.04.018
838
boilers of 200 MW utility unit, Proceedings of the Combustion Institute 34, 2769–2777,
839
2013.
840 841 842
27. M. Sánchez, M. R. Avhad, J. M. Marchetti, M. Martínez, J. Aracil, Jojoba oil: A state of the art review and future prospects, Energy Conversion and Management 129, 293–304, 2016. 28. M. Y. E. Selim, S.-A. B. Al-Omari, S. M. S. Elfeky and M. S. Radwan, Utilization of
843
extracted jojoba fruit as a fuel, International Journal of Sustainable Energy, 30, Supplement 1
844
pp. S106-S117, 2011.
35/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
ACCEPTED MANUSCRIPT
845 846 847 848 849 850 851 852
29. J. H. Wasser, R. P. Hangebrauck & A. J. Schwartz, Effects of air-fuel stoichiometry on air pollutant emissions from an oil-fired test furnace, Journal of the Air Pollution Control Association 18, No. 5, 332-337, 1968. 30. S. R. Turns, An Introduction to Combustion: Concepts and Applications, 2nd Edition, McGraw-Hill International Editions, 2000. 31. I Glassman, Combustion, 2nd Edition, Academic Press, Orlando, FL, 1987.
36/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
Salah A.B. Al Omari, Mohammad O. Hamdan, Mohamed YE. Selim, Emad Elnajjar PII:
S0960-1481(18)30794-8
DOI:
10.1016/j.renene.2018.07.009
Reference:
RENE 10283
To appear in:
Renewable Energy
Received Date:
25 March 2017
Accepted Date:
02 July 2018
Please cite this article as: Salah A.B. Al Omari, Mohammad O. Hamdan, Mohamed YE. Selim, Emad Elnajjar, Combustion of jojoba-oil/diesel blends in a small scale furnace, Renewable Energy (2018), doi: 10.1016/j.renene.2018.07.009
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
1
Combustion of jojoba-oil/diesel blends in a small scale furnace
2
Salah A.B. Al Omari*1, Mohammad O Hamdan2, Mohamed YE Selim1, and Emad Elnajjar1
3
1Department
of Mechanical Engineering, UAE University, Al Ain City, UAE
4
2Department
of Mechanical Engineering, American University of Sharja, Sharja, UAE
5
*Email: [email protected]
6 7
Highlights:
8
Raw jojoba-oil/diesel blends are used as fuel for small furnace.
9
0 to 35% by weight of jojoba in blends is used with input fuel supply of 8.3 kg/h.
10
Stable flame was possible with 60 % jojoba share if fuel input is 10 kg/ or higher.
11
Jojoba oil in the blend reduces NOx and unburned hydrocarbon emissions.
12
Jojoba indirectly impacts CO emissions through its effect on the spray processes.
13
Jojoba oil in the blends has adverse impact on thermal radiation to furnace walls.
14
Abstract:
15
This experimental study investigates the combustion and pollutants emissions from a small scale
16
furnace burning diesel fuel blended with raw jojoba oil. Jojoba oil to diesel proportions in the
17
blends (on mass basis) ranging from 0 to 35 percent are considered for total blended fuel flow
18
rate of about 8 kg/h. Higher fuel supply rates of about 10 kg/h were needed in order to allow for
19
reaching higher jojoba share in the blends up to about 60 percent. This allows for securing
20
sufficient amount of the higher volatility component (diesel) whose combustion would support
21
the vaporization and subsequent ignition and combustion of the heavier jojoba oil. The presence
22
of jojoba in the blends leads to a clear reduction in NOx and hydrocarbon (UBHC) emissions but
23
it showed less impact on CO levels. Due to its high viscosity, jojoba in the blends impacts spray
24
formation hence seems to have an indirect detrimental effect on CO emissions. Moreover, jojoba
25
oil in the blends adversely impact thermal radiation to furnace walls due to less sooting tendency 1/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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of the flame when jojoba is present. To some extent, this is also attributed to the way jojoba
27
influences spray processes.
28
Key Words: raw jojoba oil; diesel blends; biofuels, pollutants emissions; NOx; CO.
29
1. Introduction
30
It is needless is to mention the importance and the need to search for alternative renewable
31
energy resources that are environment friendly and at the same time help sustaining our energy
32
security for the future. Towards these goals, the present research group worked so far on
33
different projects on renewable energy and on the efficient utilization of energy resources in
34
different energy systems (see for example [1-9]). The work reported herein is a continuation to
35
our earlier pursuits to achieve the above goals and to meet the ever-increasing requirements and
36
stringent regulations imposed on pollutants emissions form combustion systems.
37
In their attempt and search for energy bio-resources to replace petroleum-based fuels, different
38
researchers investigated the use of biofuels of different kinds and the biodiesel derived from
39
them, but their pursuits were mainly focused on internal combustion engines particularly dual
40
fuel diesel engines and diesel engines fueled by diesel blended with liquid biofuels (see for
41
example [10-16]).
42
There is limited research available in the literature on the utilization of liquid biofuels as sole
43
fuel or as supplementary fuel for open combustion systems; e.g. furnaces. For example,
44
Tashtoush et al. [17] investigated a water cooled furnace fueled with ethyl ester of waste
45
vegetable oil. Therein ethyl ester of used palm oil was burned in a water cooled furnace and the
46
results were compared for two nominal input energy values with the results based on diesel
47
combustion in the same furnace. The Biofuel showed better emission characteristics under the
48
conditions of the lower thermal energy input to the furnace, while diesel performance surpassed
49
the biofuel at the higher thermal input condition. Still, the results are not conclusive and in that
50
study a chemically treated used vegetable oil was used; i.e. not in its straight vegetable oil (SVO)
51
form.
2/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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Cavarzerea et al. [18] addressed micro gas turbine fueled with blends of straight vegetable oils
53
(SVOs) from different origins with diesel. Different proportions of the biofuel in the blends were
54
considered. The results, were so scattered and lead to uncertainty such that they may not lend
55
themselves much amenable for drawing conclusive conclusions from the experimental findings.
56
Yet, blends with up to 30 percent vegetable oils were found to lead to reduced NOx emissions
57
and lower exhaust temperatures and lower pressure ratio and rotational speeds for the same
58
nominal fuel mass input. This was attributed to the lower heating value of the used vegetable
59
oils, as compared with diesel. For all cases tested, CO emissions were higher than when only
60
diesel is used as a fuel.
61
Khalil and Gupta [19] studied the atomization characteristics of two kinds of bio-liquid fuels and
62
compared them with spray characteristics attained when a fossil fuel is used. Then, they studied
63
the combustion and pollutants emissions from these fuels under gas turbine combustors
64
conditions. One of these two biofuels used is Butyl Nonanoate; one that had been newly
65
processed to be used as a new biofuel and that had been tested to assess its potential for use in
66
gas turbines.
67
Jiru et al. [20] considered the use of blends of SVO from soybean and No. 2 fuel oil, as fuel for
68
residential furnaces. The different thermophysical characteristics of the used blends and the
69
pollutants emissions from their burning in residential furnace were reported. Jiang et al. [21]
70
implemented the concept of flow-blurring liquid fuel injection to atomize straight vegetable oil
71
of soybean, biodiesel, and a baseline diesel fuel. They studied the combustion and pollutants
72
emissions characteristics from these fuels.
73
Huang et al. [22] investigated an oil-fired furnace fueled with blends of crude castor oil and
74
diesel. They examined the feasibility of using these blends as a substitute for pure diesel in
75
industrial applications. Crude castor oil fractions of 5% up to 30% have been investigated. They
76
found that with increasing the castor oil content in the blends, NO and CO emissions only
77
slightly decreased and increased, respectively. In respect to furnace temperatures, they verified
78
that the use of 5–30% castor oil in the blends produces similar furnace temperature distributions
79
and comparable emission levels of CO, NO and SO2 when compared to pristine diesel 3/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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In [23] Enagia et al. gave a review of the utilization of liquid biofuels in gas turbines. The
81
authors included discussions on the effect of different input parameters associated with the clean
82
combustion systems that have influence on the attainment of ultra-low emissions of NOx and CO
83
under premixed and non-premixed modes. They also highlighted that using biofuels as
84
supplement with 20 percent share in fuel blends is currently economically viable, with an
85
expectation to reach higher share for biofuels in the near future.
86
Further studies addressed the use of biofuels either as supplements in blended main fuel or as
87
sole fuels. For example, in [24] Hashimoto et al. considered the combustion of Jatropha oil (JPO)
88
and Jatropha methyl ester (JME) in a furnace under atmospheric pressure although the study was
89
highlighting potential use for gas turbines. They studied emissions and radiation characteristics
90
based on Jatropha as a fuel and compared the obtained results with results they obtained when a
91
diesel fuel was used to fuel the furnace. They also considered blends of diesel and Jatropha with
92
different mixing ratios. Moreover, they measured the flame radiation intensity and concluded
93
that flame radiation intensity and the soot emission decrease with increasing mixing ratio of
94
Jatropha oil or Jatropha methyl ester to diesel fuel. The authors also studied the effect of
95
viscosity of Jatropha on spray formation and the subsequent processes in the furnace. The effect
96
of the equivalence ratio was also investigated in [24] by changing it one time by varying the fuel
97
flow rate and fixing the air flow rate and vice versa. The results indicated that CO emissions
98
increase when Jatropha is burned under lean conditions (low flame temperatures), as compared
99
with the case of diesel and JME. NOx results were found to be sensitive in the case when JPO is
100
used, to the change in the equivalence ratio when it is increased by changing the airflow rate
101
while fixing the fuel flow rate. In reference to the work reported by Hashimoto et al. in [24], it is
102
interesting to note that Jatropha oil they used in their study shares a number of similar features
103
and thermophysical characteristics with the liquid biofuel used in this study namely Jojoba oil.
104
This would consequently make it possible to investigate the results obtained in this study in light
105
of those reported in [24].
106
Hashimoto et al. [25] recently investigated the effect of Jatropha oil blending with heavy fuel oil
107
on soot emissions in boilers. Their results showed reduction in soot emissions from flames when
108
crude Jatropha oil is used in the fuel blends. They attributed this to the low sooting tendency of 4/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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crude Jatropha oil where Jatropha oil’s molecules have no aromatic ring and also due to their
110
high oxygen content. This consequently impacted the level of radiation from the flame to the
111
furnace walls. Furthermore, due to the low nitrogen and sulfur content in Jatropha oil, the NOx
112
and SO2 emissions were reduced as well.
113
From the above presentation, one may conclude that there is still a need to research further and
114
validate the use of different kinds of bio-oils for furnaces and boilers applications. Hence, more
115
investigation of bio fuels combustion is needed to come up with efficient designs and optimum
116
operation maps for furnaces that optimally can run on such biofuels in their straight raw liquid
117
form. Achieving this target, would mean to us further justification for our call to increase the
118
production of such biofuels by planting more land areas with jojoba shrubs (which is the plant
119
subject of this study) at a wider scale for energy production purposes, beside the already-existing
120
call to increase their production to serve as environment-friendly fuels for car engines.
121
In line with the above arguments, and to the best of the knowledge of the present authors, raw
122
jojoba oil as straight vegetable oil (SVO) or the biodiesel based on it, have not been tested
123
sufficiently in open combustion systems such as furnaces and boilers. Therefore, this study
124
pursues research to highlight the possible use of straight jojoba oil as a sole fuel or at least as a
125
fuel supplement for furnaces and highlights emission characteristics from their combustion in a
126
small-scale furnace under different air supply conditions.
127
Using chemically treated bio liquid fuels for furnaces (i.e. biodiesel), may replace traditional
128
fossil fuels, with possibly no or just minimal alterations to existing furnaces and fuel injection
129
systems. Nevertheless, this option might be associated with some negative consequences and
130
adverse economic and environmental impacts. This is due to the fact that the so far adopted
131
conventional methods of transesterification to produce biodiesel require large amounts of energy
132
and lead to byproduct (crude glycerol) with large quantities. If these large quantities of crude
133
glycerol were to be utilized further in another heat generation system as a fuel, this would still
134
require certain arrangements that need to be justified and in most cases of small to medium-sized
135
plants is out of the range of economic feasibility to warrant its adoption at a wide scale [26].
136
Furthermore, the subsequent direct use of crude glycerol as a fuel in furnaces can lead to
137
emissions that include undesirable chemicals that are rooted in the chemicals used to produce the 5/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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biodiesel in the first place. However Pawlak-Kruczek et al. [26] examined the co-firing of the
139
biodiesel-production byproduct crude glycerol with coal in utility boiler and conclusions drawn
140
therein were in favor and supportive to that practice. Based on the above arguments, the direct
141
use of straight vegetable oils (SVOs) as a fuel or fuel supplement replacing a portion of the
142
typically used fossil fuels, would be an attractive option. This would alleviate much of the
143
obstacles and concerns highlighted above regarding using biodiesels, especially if the use of
144
SVO revealed to be more environment friendly than the traditional petroleum-based fuels, as
145
already found by some preliminary research studies [17-21]. This represents one of the
146
motivations to conduct the research reported in this study in which crude jojoba oil is utilized.
147
The attractiveness of jojoba shrub as a plant is its ability to tolerate extreme different
148
environments [27], which makes it easily and conveniently grown in arid regions in the world.
149
Such abilities of jojoba plants allow multifold benefits starting from making the desert green and
150
thereby limiting sand migration and the adverse effects of sand storms, and at the same time
151
increasing the diversity of our renewable energy resources. In addition, this can also open the
152
door for new energy market options. The unique features of raw jojoba oil’s make it a very
153
attractive raw material for different industries such as in cosmetics, pharmaceuticals, high-
154
pressure lubricants, and plasticizers [27]. Thus, considering it as alternative energy source or as a
155
supplement to existing fossil fuels, might look at this moment as economically inviable.
156
However, considering the merits of using jojoba oil as a fuel or fuel supplement; (as reported in
157
different places in the literature; which what this current work is also trying to explore further),
158
the outlook might take a very favorable direction towards considering expanding the jojoba oil
159
production at a much wider scale, worldwide. This would even be specifically the case in arid
160
regions where preventing desertification is becoming a national priority, aside from any other
161
economical or environmental advantages.
162
Two of the present authors considered the option of using the jojoba fruit to the max by
163
considering the shell of the jojoba nut as a solid fuel for furnaces, after oil extraction [28]. The
164
results showed some promising features of that kind of solid biomass fuel but still further
165
investigations are needed to come up with more detailed aspects of the used systems and the
166
environmental impact of such biofuel. 6/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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The studies that addressed the use of jojoba (whether in its raw oil form or as Methyl ester after
168
treatment), as fuel in internal combustion engines (for example [10-15]), revealed that this
169
practice has favorable effects on pollutants emissions. On the short run, using such biofuel in
170
engines was found not to cause any significant loss in the performance of engines. On the long
171
run, however, due to the higher viscosity of oils of biological origin in general, previous research
172
on engines revealed some technical problems related to the injection and fuel storage systems.
173
On the injection process itself, the higher viscosity of bio-oils (as compared with typical
174
petroleum-based liquid fuels) leads to poorer atomization of the spray and consequently can have
175
adverse effects on the hydrocarbon and other combustible species emissions from combustion
176
systems. To alleviate these adverse effects, modified injection systems with more powerful
177
pumping capacity to overcome the large viscosity, would (although will be requiring extra
178
expenditures) be a possible solution. To get more details about the needs of improvements in this
179
direction, some testing of the performance is required, and this is what had been accomplished to
180
a good extent so far for internal combustion engines, but still lacking for the case of furnaces and
181
open direct burning systems. One of the main goals of the present study is to come up with
182
results that aid achieving the above target and come up with relevant and sound
183
recommendations in that direction.
184
The study conducted by Shehata and Abdel Razek [13] furnishes an interesting material for
185
comparison with the present study, where therein raw jojoba oil/diesel blends were used as fuel
186
in diesel engine, while in the present work this research team uses such blends as a fuel for open
187
combustion in a small scale furnace. The resulting performance trends in their work can be
188
usefully compared with the results reported here and more comprehensive conclusions with a
189
wider breadth and enlightening scope regarding the physics of the underlying combustion and
190
heat transfer processes can be drawn accordingly. For example, as found in [13], performance
191
and emissions characteristics of an engine fueled with blends of diesel and jojoba oil (80%
192
diesel) have been compared with their counter parts when only conventional diesel is used. The
193
results showed that the Jojoba oil/diesel blend reduced the brake thermal efficiency, the brake
194
power and the break effective pressure. However, the blend led to increase in CO and CO2
195
production but to decrease in NOx emissions. Similar conclusions regarding NOx emissions
196
have been made by Al-Widyan and Al-Muhtaseb [15] when diesel was blended with jojoba oil. 7/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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Jojoba oil is miscible in petroleum diesel fuel which means that the two can be mixed in any
198
proportion. However, jojoba oil has much higher viscosity than a typical diesel fuel. This study
199
tested jojoba oil-diesel fuel mixtures with three different jojoba oil proportions starting from 0%
200
to 35% on mass basis for a nominal total blended fuel energy input to the furnace of about 99 to
201
103 kW. The nominal energy input is calculated based on the overall heating value of the blend
202
and the total mass of the blend supplied to the furnace. Due to the furnace limitation in the
203
present study such as fuel injection pump, atomizer and flame stability, it was difficult to reach
204
higher jojoba percentages in the injected blends beyond 35 percent. This is possibly due to the
205
large viscosity of raw jojoba oil when large quantities are used in the blends that precludes fine
206
enough spray formation. Consequently, the vaporization rates of the formed large droplets of the
207
blended fuel with high jojoba content would not be sufficiently high to support the needed
208
combustion for a stable flame over a range of air supply conditions. Yet, to be able to include a
209
larger share of jojoba oil in the blends beyond the 35 percent level mentioned above, some
210
actions can still be taken for example preheating the blends to reasonably high temperatures to
211
decrease their viscosity before injection. Altering the injection system by employing a more
212
powerful pump and using injectors with smaller orifices or with more features and spray
213
atomization techniques that facilitate producing fine enough droplets, are also other options that
214
clearly are associated with some additional expenditure requirements, as highlighted above.
215
Using higher proportions of jojoba oil in the used blends beyond 35 percent might still be
216
possible without considering the pre-treatment or remedial actions mentioned above. However,
217
this might be achievable only for furnaces that operate on comparatively large total thermal
218
capacity conditions. In other words, in this case there is a need to increase the overall absolute
219
mass of the more volatile component in the blend (in our case the diesel fuel) that is being
220
supplied to the furnace when the percentage of Jojoba in the blend becomes excessively high.
221
For example, to overcome the issues related to the presence of excessive jojoba oil content in the
222
blends and the consequent formation of large droplets that vaporize relatively slowly, higher total
223
supply rates of such blends can be considered. In this case, the presence of large quantities of
224
jojoba oil will be countered by the availability of sufficient amounts of the other more volatile
225
component namely the diesel, which will vaporize and burn fast enough to eventually support the
226
vaporization and subsequent combustion of its companion the jojoba oil, regardless of the slow 8/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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rates of vaporization of the later. Preliminary testing of the above option has been implemented
228
in this study where the total amount of the injected blended fuel was increased such that the
229
nominal fuel energy supply rate to the furnace becomes about 117 kW. In this case, it was
230
possible to achieve stable burning of blended fuel with up to about 60 percent jojoba under fairly
231
reasonable range of air to fuel ratios. Yet, more testing and runs are required for various high
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jojoba oil blending proportions under different levels of total fuel supply rates and different fuel
233
injection temperatures. As such, the results obtained from the last case conducted in this work
234
with about 60 percent jojoba oil content in the blends, would only be presented here to highlight
235
the feasibility of the above approach, with no further due discussions or comparisons with other
236
cases under lower nominal input fueling conditions.
237
2. Experimental Setup
238
The furnace studied in this work is a small horizontal cylindrical research furnace, with overall
239
furnace length of 150 cm and diameter of 60cm. The furnace is equipped with advanced data
240
acquisition system which is used to collect all flow parameters and testing measurements. The
241
schematic diagram for the experimental test rig is shown in Figure 1. The furnace is fitted with a
242
water jacket surrounding the furnace so that heat transfer from the flame and the hot combustion
243
gases can be quantified. Injection of the liquid fuel is facilitated by an injector that has a full cone
244
wide angle nozzle that produces a uniform wide angle 120º cone spray. The full cone nozzle has a
245
spray pattern that is round and completely filled with droplets. This pattern is formed by an internal
246
vane, which imparts controlled turbulence to the liquid prior to the orifice. The droplets size in the
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case of the diesel fuel are in the range around 40-400 micron. The injector operates with a
248
maximum nominal thermal capacity of about 116 to 125 kW. According to manufacturer’s details,
249
recommended liquid fuel flow rates that ensure fine enough spray, as described above, is from 5
250
to about 12 kg/h. The different temperature measurements; (for the hot combustion gases, the
251
combustion air inlet, the water circulating through the water jacket) are made using K- type
252
thermocouples. A gas analyzer made by VARIO Plus Industry is used to measure gas emissions
253
from the furnace. All measured signals are acquired using data acquisition and stored using a
254
LabVIEW interface program written to collect the data at rate of 1 sample per second. The flow
255
rates of the liquid fuel, the combustion air, and the water circulating through the water jacket were 9/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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measured to facilitate the performance analysis of the furnace. The uncertainties of the used
257
instrumentation in the setup and their measurement range are listed in Table 1.
258 259 260 261 262
Table 2 below summarizes the main fueling conditions considered in the different experimental tests conducted.
Table 1: the uncertainty of measurements Table (1) Measurement CO in ppm O2 in Vol.% NOx in ppm CO2 in Vol.% K-Type thermocouple Air Flow rate Fuel Flow rate
RANGE 502 10.03 80.8 14.14 1250oC 240 kg/h 15 kg/h
TOLERANCE ± 25 ppm ± 0.2% ± 4 ppm ± 0.75 % ± 0.5o C ± 15 kg/h ± 0.5 kg/h
263 264
(17)
(1) Inlet water temperature (2) water volume flow rate (3) water flow controller (4) Inlet air temperature (5) Air volume flow rate (6) Air flow controller
265
(7)Inlet fuel temperature (8)Fuel volume flow rate (9)Fuel flow controller (10)Water Jacket (11) fuel injector (12)Furnace
(13)Furnace temperature (14) Outlet water temperature (15) Exhaust temperature (16) Gas Analyzer (17) Mid Furnace Temperature
Fig. 1: Schematic of the Experimental Setup
266 267
Table 2: Fueling conditions of the experimental tests conducted. 10/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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Table (2): Case Study Test Diesel Fuel % No. Test 1 100% Test 2 80% Test 3 65% Test 4 40%
Jojoba Fuel %
m’fuel (kg/h)
0% 20% 35% 60%
~8.26 kg/h ~8.26 kg/h ~8.26 kg/h ~10 kg/h
Nominal thermal energy input to furnace (kW) 103 101 99 117
268 269 270 271 272 273 274 275 276 277 278
3. Thermo-physical properties of used fuels The raw jojoba oil used in this study is extracted from jojoba seeds which were obtained from jojoba plants grown in Egypt. It has only been refined but not treated by any additives nor chemicals. Shehata and Abdel Razek [13] report thermophysical properties of raw jojoba oil originating from the same geographical region namely Egypt and these are given in Table 3. In their measurements, Shehata and Abdel Razek [13] did not indicate the presence of any water in the jojoba oil they used and they did not report any ash content.
Table 3: Thermophysical properties of raw jojoba oil [13] Property Raw Jojoba Oil Heating Value, MJ/kg 39.862 o Viscosity at 40 C, cSt 52 o 3 Density at 15 C, kg/m 920 Flash point, oC 186 Molecular Weight 560.98 Molecular Structure C38H72O2 Aromatic Content (%w) 0 C/H ratio (by weight) 6.29 Elemental Analysis (%w) C 81.36 H 12.94 O 5.7 N 0 S 0.02
279 280
The heating value of jojoba oil is somewhat lower than that of typical diesel fuel. This might be
281
expected to play some role regarding the combustion and emissions characteristics in cases
282
including jojoba, as compared with the baseline case with only diesel. Carbon to hydrogen mass 11/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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ratio of jojoba is slightly higher for a typical diesel than that of jojoba oil. This may have some
284
significance regarding CO2 emissions in the runs utilizing both types of fuel. The flash point of
285
raw jojoba oil indicates a low volatility level, as compared with a typical diesel. Also, the
286
absence of aromatic fuel content in jojoba, while existing in a typical diesel, and also the
287
presence of oxygen in jojoba oil, may reflect on the nature of the flame established based on both
288
fuels, in particular when sooting abilities of the flame and the consequent thermal radiation from
289
it, is taken into account. Future studies dedicated to explore in details the above aspects are
290
recommended to be conducted.
291 292
4. Results and discussions
293
The results of the experimental tests listed in Table 2 are presented in Figs. 2 to 6. The discussion
294
is divided into two sections. In the first, the discussion focuses on the combustion of only-diesel
295
fuel (baseline test; Test 1) while in the second it focuses on the results of the tests with jojoba-
296
diesel blends and comparisons with the baseline test results, Test 1. It should be recalled that
297
Tests 2 and 3 are under the same total input fuel condition as in Test 1. Some brief highlights on
298
the results of Test 4 with more total fuel supply and higher jojoba content in the injected blends
299
(as indicated earlier), are also presented.
300
1) Test 1: Baseline test of only diesel combustion:
301
In Test 1, the furnace was fueled only with pure diesel (not blended with raw jojoba oil) at a rate
302
of 8.26 kg/h. The temperature measurement at the midpoint of the exit and middle cross section
303
of the furnace (points 15 and 17 in Fig.1) are given in Fig. 2 (a-d) for all studied cases vs air to
304
fuel ratio. For Test 1 these temperature measurements are given in Fig. 2a.
305
12/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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Fig. 2a: Temperature measurements at the middle and at the furnace exit for Test 1; 100% diesel at rate of 8.26 kg/hr. 306
Fig. 2b: Temperature measurements at the middle and at the furnace exit for Test 2; 80% diesel and 20% jojoba oil with total fuel supply of 8.26 kg/hr. 13/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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Fig. 2c: Temperature measurements at the middle and at the furnace exit for Test 3; 65% diesel and 35% jojoba oil with total fuel supply of 8.26 kg/hr. 307
Fig. 2d: Temperature measurements at the middle and at the furnace exit for Test 4; 40% diesel and 60% jojoba oil with total fuel supply of 10 kg/hr. 14/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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308 309
As seen from Fig. 2a, at any air to fuel ratio value, the gap between the two temperature
310
measurements at the middle and exit cross section of the furnace gives indication on the level of
311
completeness of the combustion reactions within the furnace space starting from the injector up
312
to the middle of the furnace; the closer that gap between the two temperature measurements, the
313
more complete is the combustion within the first half of the furnace. The quantitative difference
314
between the mid-furnace and the exit temperature is an indication of the surplus part of the
315
sensible heat release by the combustion reactions in the second half of the furnace, up to the exit.
316
Part of this released heat by combustion in the second half of the furnace is conveyed to the
317
furnace walls by radiation and convection. If the exhaust temperature became lower than the
318
mid-furnace temperature, this might be an indication that most of the combustion reaction
319
activity was completed within the first section of the furnace, and beyond that point up to the
320
furnace exit the hot gases were mainly losing part of their heat to the furnace walls, without any
321
significant further reactions, or possibly with reactions that do not lead to heat release that can
322
cope with the heat transfer rate from the hot gas to the furnace walls. This later possibility was
323
not observed in any of the runs conducted, indicating that the length to achieve consumption of
324
all the injected fuel is longer than the length from the injector tip to the middle of the furnace.
325
Based on this, more flame length is attained, especially (as will be seen later) in the tests that
326
included jojoba oil. As expected, the gap between the two temperature measurements decreases
327
with the increase in air/fuel ratio. Of course, with the increase of the overall air to fuel ratio
328
towards lean fuel levels, one cannot exclude the possibility of having some unburned fuel or
329
partially burned species such as carbon monoxide (CO) leaving the furnace exit (regardless of
330
the abundance of air) due to the cooling and quenching effects of the excessive amounts of
331
supplied cool air under these high air supply scenarios. The measurements of unburned species
332
that are reported at the furnace exit (to be presented later) for high air to fuel ratios, would be
333
supportive to the above arguments, especially when CO emissions are considered (Fig. 5).
334
In this respect, it is known that at high temperatures, carbon dioxide (CO2) may go through
335
effective dissociation reactions that may lead to excessive equilibrated amounts of CO
336
concentrations in the products. In this study, the furnace is not adiabatic and hence the 15/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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temperatures are not high enough to support this route of CO formation by thermal dissociation.
338
However, other routes are prevailing in the present furnace when air supply rates become
339
excessive namely the formation of overly lean fuel mixtures and the relatively cool gas
340
temperatures due to the excessive amounts of cool air supplied. The quenching of CO in furnaces
341
environment is attributed to the excessive air supply rates, while in internal combustion engines
342
this is attributed to cold cylinder wall temperatures. It was reported that fuel and CO reactions
343
decay at an exponential rate with temperature (see for example [29] and the references cited
344
therein related to this point). Further to this point, recent research on CO emission from furnaces
345
fueled with liquid biofuels that have characteristics similar to the fuel addressed in this study
346
(crude Jojoba oil) support the above effect on CO emission of low temperatures that result when
347
air supply is increased (see for example [24]). Another significant parameter, which is of clear
348
significance in this regard, is the residence time inside the furnace available for reaction
349
completion. The residence time becomes shorter when flow rate increases. The adverse impact of
350
both the lower temperatures and the shorter residence time, under excessive air supply
351
conditions, may offset the enhanced mixing between the combustible species and oxidizer
352
species.
353
One further point to highlight related to hydrocarbon fuels flames is that CO oxidation becomes
354
effective in the presence of sufficient amounts of the hydroxyl radical OH; not Oxygen
355
molecules. Oxidation by Oxygen is comparatively slow (especially at low temperatures) and it
356
serves mainly as chain reaction sequence initiator (reference to related combustion literature can
357
be made for further details; see for example [30, 31]). The concentrations of OH radical would
358
be expected to decrease significantly under excessive airflow conditions due to the dilution
359
caused by extra air and the reduced flame temperatures. These effects result in slow Oxygen-
360
based reaction that is responsible for initiating the CO oxidation chain reaction sequence. All the
361
above arguments would collectively be expected to lead to drop in the CO oxidation rates at
362
overly high air supply rates.
363
At fuel-rich condition of air to fuel ratio around 11, the relatively big difference between the two
364
temperature measurements seen in Fig. 2a indicates that much of the injected fuel requires longer
365
residence time and a bigger space to vaporize, mix with air, and subsequently combust. 16/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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Obviously, this is due to the relative shortage of the oxygen at that low air supply condition.
367
With more air supply, the required distance to completely combust the fuel becomes less, as
368
indicated by the drop in the gap between the two temperature measurements (cf. Fig. 2a). At
369
about Air/Fuel of 16.5 the minimum gap between the two temperature measurements is attained.
370
With further increase in air supply with up to Air/Fuel of about 23 in our experiments, the two
371
temperature measurements tend to only slightly diverge from each other; a trend that is
372
attributable to the cooling effects of extra air supplied under such high Air/Fuel conditions, as
373
highlighted above.
374
Measurements at the mid-section point in the middle of the furnace might not be an accurate
375
representation of the mean temperature neither it is meant here to be considered as the flame
376
temperature. However, it should be recalled that the investigated experimental furnace was not
377
equipped with the possibility to trace the temperature measurements at different points in the
378
same cross section, but just the middle point of the furnace. Yet the obtained temperature
379
measurement of the middle section reported in Fig. 2 (supported by the other measurements and
380
the calculated heat transfer quantities from energy balance) is to be considered here as a
381
reasonable approximation of the mean value at that section from which some useful insights and
382
conclusions might reasonably be drawn.
383
The distribution of the air-fuel combustible mixture throughout the combustion domain is
384
governed by the spray pattern which is dictated by the used fuel injector. This mixture
385
distribution decides the scenario of the flow and the distribution of other combustion-related
386
quantities such as temperature and the different gaseous species concentrations. This will also
387
highly impact the pollutants emissions from the furnace. In this respect, the liquid fuel viscosity
388
and its level of volatility would have significant role to play. Diesel has much lower viscosity
389
and clearly higher volatility than the liquid Jojoba oil. These two major characteristics would
390
impact the spray formation, fuel distribution and combustible mixture preparation, and chemical
391
reactions within the furnace. Hence these would help us understand and interpret the subsequent
392
results presented below.
393
Fig. 3 presents the corresponding results of heat transferred to the water jacket for all tests
394
studied (1 to 4) as a function of the air to fuel ratio. As can be seen from heat transfer results of 17/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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Test 1, the water jacket captures heat from the hot gases (by radiation and convection) at a rate
396
that varies from about 37 kW to 41 kW. An average value of about 39 kW is attained. The peak
397
of 41 kW is attained at Air/Fuel ratio of about 12 that corresponds to the fuel-rich side of the
398
tested Air/Fuel range and is corresponding to almost highest gas temperatures attained (cf. Fig.
399
2a). The levels of heat transfer to the water jacket attained over the wide range of air flow rates
400
considered matches closely and are in accordance with the gas temperatures measurements
401
reported in Fig. 2a. For the low air flow rate runs radiation is expected to be the dominant mode,
402
particularly in the first half of the furnace, while for the runs with much abundance of air
403
supplied convection is expected to be the dominant mode, especially in the second half of the
404
furnace when the gases temperature decline noticeably due to the loss of heat to the water jacket.
405
In this regard, the high sooting tendency of the diesel fuel would help us interpret the higher
406
radiation to furnace walls in the case of diesel fueled furnace, as compared to the cases where
407
diesel is blended with crude Jojoba oil. This point is confirmed by other researchers for other
408
liquid biofuels conditions where the bio fuels do not include aromatic fuel component in their
409
molecular structure and at the same time they include much oxygen content. These two aspects
410
explain the lower tendency to particulate matter formation when diesel fuel is blended with a
411
liquid biofuel such as Jatropha oil (as in [24,25]) and Jojoba oil in our current study.
412
Fig 4 presents the NOx measurements obtained for all Tests (1 to 4). The measurements are
413
taken at the point in the middle of the exit cross section of the furnace (see Fig. 1). As can be
414
seen from Fig. 4, NOx results of Test 1 are consistent with the temperature measurements
415
reported in Fig. 2a. NOx results lie within a range from about 60 ppm at the highest temperatures
416
attained at air to fuel ratio of about 12, to 50 ppm at the lower temperatures attained at air to fuel
417
ratios of around 22. It should be recalled once more that NOx is effectively formed at high
418
temperatures (roughly above 1800 K) and that the temperatures reported in Fig. 2 for the
419
different runs conducted are to be interpreted as somehow averages at these sections rather than
420
the flame temperatures at which NOx is effectively formed. Yet, as mentioned earlier, these
421
reported temperatures (with the help of other reported measurements and quantities) can be
422
helpfully used to get understanding and draw reasonable conclusions about the combustion and
423
pollutants emissions from the studied furnace. It is already known that different mechanisms
424
dominate NOx formation; thermal NOx, prompt NOx, and fuel NOx formation. The first 18/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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mechanism is the one governing the majority of the NOx formed in the hottest flame sites, and
426
the second is governing NOx formation in fuel rich environment, but the last one is believed not
427
to be so important for the current study since used fuels do not contain Nitrogen to any effective
428
extent. Temperature measurements of the different tests studied will be very helpful to explain
429
the trends of thermal NOx results presented.
430
The measurements of the other gaseous species emissions that reflect on the quality of
431
combustion reactions completeness, namely CO and the unburned hydrocarbons (UBHC), are
432
presented in Figs. 5 and 6, respectively. As can be seen from Fig. 5, CO measurements for Test 1
433
show relatively low emission levels (around 50 ppm) over air to fuel ratio levels up to about 17.
434
Beyond that air flow level, more CO emissions start showing up due to the more pronounced
435
cooling effect of the excess air, as explained earlier.
436
Fig. 3: Heat transferred to the water jacket for Tests 1 to 4. 437
19/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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Fig. 4: NOx emissions at the furnace exit for Test 1 to 4. 438
Fig. 5: CO emissions at the furnace exit for Test 1 to 4. 439
20/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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Fig. 6: UBHC emissions at the furnace exit for Tests 1s to 4. 440 441
The trend of the UBHC emissions in Test 1 shows slightly declining levels with the increase in
442
Air/Fuel ratio from about 200 (mg/m3) at Air/Fuel of around 11 to lowest levels somewhat near
443
about 170 (mg/m3) before a tendency to start increasing upon further increase in air supply is
444
observed at about Air/Fuel of 22. The above scenario is a little bit different form the one
445
observed in Test 1 for CO emissions. In the case of CO, the CO levels remained almost constant
446
at about 40 to 50 ppm, over all the Air/Fuel range up to around 20, before a clear rise due to the
447
reasons mentioned earlier including air cooling effects, is noticed when Air/Fuel is increased
448
beyond 20.
449
While CO levels show clear variations in magnitude with air to fuel ratio as we go from the fuel-
450
rich to lean fuel conditions, for the UBHC the variations on magnitude-basis are marginal over
451
all Air/Fuel range considered; (almost a constant stable UBHC level of about 180 mg/m3 was
452
recorded). For the case of UBHC, the effect of enhanced mixing as we increase air supply seems
453
to be more pronounced and is not offset easily by the cooling caused by the larger air flow rates 21/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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up to Air/Fuel of about 22. Upon increasing the air flow rate beyond this level, we may
455
anticipate seeing more clearly a trend of UBHC emissions increase; a scenario that happened at
456
Air/Fuel ratio of around 19 in the case of CO emissions.
457
2) Tests with Jojoba-Diesel Blends
458
Below is a discussion on the effect of the presence of raw liquid jojoba oil in the injected liquid
459
fuel, on the performance of the studied furnace.
460
2.1) Test 2: Diesel/Jojoba-oil blend with 20 percent jojoba oil:
461
The temperature results of Test 2 that are the counter part of those presented in Fig. 2a for Test 1,
462
are shown in Fig. 2b. The same qualitative trend observed in Fig. 2a for Test 1, is also observed
463
in Fig. 2b of Test 2. Even quantitatively, the temperature results reported in Fig. 2b are
464
comparable to those of Test 1 presented in Fig. 2a. The major clear difference between the two
465
cases is the gap between the two temperature measurements (mid-furnace and exit temperatures)
466
at any of the Air/Fuel conditions tested, being more evident in Test 2. This bigger gap observed
467
in Test 2 indicates that more pronounced combustion activity is still taking place in the second
468
half of the furnace in Test 2, than what is observed in Test 1, indicating thereby that more fuel in
469
Test 2 is being deferred in its reaction, to be completed in the second half of the furnace.
470
The viscosity of crude Jojoba oil is very high compared to that of diesel. This will have a
471
significant impact on spray pattern and the droplet sizes formed when diesel is blended with
472
Jojoba oil, leading to larger droplets that vaporize at a slower rate in this later case. This would
473
also lead to having liquid droplets that can penetrate to longer distances in the furnace and
474
consequently can lead to a distribution of the resulting fuel vapors over a longer and wider space
475
in the combustion domain. All these aspects would be expected to affect the combustion, heat
476
transfer, and pollutant formation and emission form the furnace.
477
Given the nature of the jojoba oil and its lower volatility level, as compared to the diesel fuel,
478
one may infer from the above that most of the liquid fuel droplets that can survive and reach the
479
downstream portions of the furnace combustion space towards the exit have higher jojoba oil
480
concentrations than upon injection at the injector tip. Accordingly, most of the heat liberation 22/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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coming from fuel combustion in the first half of the furnace may be attributed to the higher
482
volatility constituent in the injected fuel namely the diesel, which vaporizes faster. Besides, as
483
highlighted above, the lower volatility and higher viscosity of jojoba oil makes it possible for the
484
droplets to penetrate further and to spread and distribute fuel and hence accomplish the
485
combustion of all the fuel sprayed over a bigger length and a larger space in the furnace. This,
486
under the given turbulent combustion environment, may lead to locally less intense heat
487
liberation by combustion reactions hence lower local temperatures in the flame sites due to the
488
locally lower availability of the fuel vapor when distributed over a bigger space. Consequently,
489
this would possibly contribute to reduction in thermal NOx formation activity. Moreover, the
490
distribution of fuel over a bigger space in the furnace would give the chance for more dilution of
491
the resulting fuel vapor with other gases (mainly air in the early stages of the furnace). This will
492
reduce the level of fuel-richness of the gas which will reduce formation of prompt NOx as well
493
as soot and will give more chance for CO reduction. Consequently, the above arguments may
494
support interpreting the somewhat lower NOx emissions in Test 2, as compared to NOx levels
495
attained in Test 1 (cf. Fig. 4). With the above being said, it is already clear that the interrelated
496
phenomena at hand are so complex and each influences the others in a way or another. Hence,
497
there is still further need to conduct more research studies to come up with a more detailed
498
understanding of the phenomena taking place under furnaces’ environments and for liquid
499
biofuels like the one considered in this study.
500
Quantitatively, NOx emissions levels in Test 2, as given in Fig. 4, are almost uniform over the
501
entire tested air flow range and have an average value around 39 ppm. This average level is
502
around 15 ppm lower than the average attained in Test 1 (cf. Fig. 4). It should be recalled that the
503
temperature results presented for Tests 1 and 2 in Figs. 2a and 2b, are a representation, more or
504
less, of an average temperature at the cross sections where they are measured, and they are not
505
local temperatures at hottest sites of the flame where thermal NOx forms effectively. From the
506
NOx results of both tests and based on the discussions and arguments given above, these hottest
507
sites seem to be more pronounced and effective in producing NOx in Test 1. However, prompt
508
NOx formation may be more pronounced for flames when only diesel is used (as is the case in
509
Test 1) and this may also contribute to the higher levels of NOx in Test 1 than in Test 2, where in
510
Test 1 there is more chance to have fuel-rich sites in the flame that support prompt NOx 23/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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formation. Further future experimental and/or theoretical research may be needed to give more
512
insight in respect to these arguments.
513
Local temperatures measured at the middle section of the furnace in Test 2 are somewhat similar
514
to those attained in Test 1. However, the amount of heat captured by the water jacket in Test 1 is
515
somewhat higher by about 1 to 2 kW. The higher heat transferred to the water Jacket in Test 1
516
might be attributed to the tendency of the only-diesel flame in Test 1 to radiate more heat than
517
that from the flame based on the blended fuel in Test 2. One may anticipate here more soot
518
formation in the only-diesel flame in Test 1 that can be responsible for more thermal radiation to
519
furnace walls. With more thermal radiation to furnace walls in Test 1 than in Test 2 due to the
520
above reasons related to soot formation, we still see comparable temperatures at the middle
521
section in both Tests 1 and 2 (cf. Figs. 2a and 2b). This would then hint to a more complete
522
combustion and reaction activity within the first half of the furnace in Test 1, when only diesel is
523
used as fuel. This means that in Test 1 the additional reactivity of the fuel; hence the associated
524
heat release, would compensate for the more heat loss from the flame by radiation to the walls
525
and hence would maintain temperatures in the first half of the furnace at levels comparable to
526
those attained under the less radiation conditions in Test 2. Again, the arguments related to the
527
lower sooting tendency when such a biofuel as the one used in this study is used, are supported
528
by recent studies [24,25] as highlighted earlier.
529
Although the above arguments might be valid in principle but a more rigorous experiments and
530
analysis and validation of the above may still be needed. This is however beyond the scope of the
531
present work and yet would require more detailed measurements and quantification for (for
532
example) soot formation and concentrations as well as more detailed temperature measurements
533
at various spatial locations within the combustion domain. These, unfortunately, are limitations
534
that were not possible for us to overcome at this stage due to limitations dictated by the furnace
535
tested in this study where no such measurements were possible for us to do at this point.
536
Nonetheless, the above arguments may be considered as an invitation for further more detailed
537
research in the direction of the above thoughts which we believe would be very insightful and
538
would shed light on much of the physics and the chemical reactions activities.
24/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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The higher volatility of the diesel as compared to raw jojoba oil may also lead us to think that the
540
more vapor content in the flame of only-diesel in Test 1 would render the only-diesel flame to
541
behave more closely like a gaseous diffusion flame when compared to the jojoba-based flame,
542
which might be associated with more heterogeneity especially when the percentage of jojoba
543
share in the blends gets higher. On the other hand, the jojoba oil in the fuel blend, leading to
544
larger droplets in the spray, would require longer distance inside the furnace to complete
545
vaporization and subsequent mixing with the air. This may suggest partially premixed flame
546
characteristics in the furnace for the cases with jojoba blended in the fuel (Tests 2-4). The
547
diffusion-based combustion is more likely to prevail when the fuel injected has higher volatility,
548
which is the case of the only-diesel flame in Test 1.
549
Since jojoba oil is an oxygenated fuel, this would be expected to help more effective oxidation of
550
soot and CO that might have been formed in the jojoba blends-based flames, and also would tend
551
to dilute the level of fuel-richness of the fuel-rich sites of the flame that basically represent the
552
sites where soot formation effectively may take place. This argument is in-line with the lower
553
heat transfer by radiation to furnace walls observed in Test 2, as compared with Test 1 (cf. Fig.
554
3), where soot particles are known to be an effective promoter for thermal radiation from flames.
555
As can be further seen from Fig. 3, the heat transferred to the water jacket in Test 2 shows almost
556
uniform distribution over all of the tested Air/Fuel range with a mean value of about 37 kW;
557
compare this with around 39 kW average attained in Test 1.This difference in heat transferred to
558
the water jacket between the two tests (by applying simple energy balance) yields the ultimate
559
difference in the exhaust temperature of both cases of about 10 to 15 degrees over most of the
560
Air/Fuel ratio range tested (see Figs. 2a and 2b).
561
With higher Air/Fuel ratio beyond about 15, the decline in temperature (both in the middle and at
562
the furnace exit), become evident due to the cooling effect of the excess cool air supplied to the
563
furnace. This cooling effect has clear influence on the unburned hydrocarbon species emissions
564
in Test 2, but little on Test 1, where in Test 2 the UBHC emission drops from about 165 mg/m3
565
to about 120 mg/m3. More stable level with less declining tendency is seen in Test 1 as Air/Fuel
566
ratio is decreased; (compare about 27% reduction in Test 2 with only about 10% when the
567
Air/Fuel ratio is increased the same amount in both tests from about 11 to 22 (cf. Fig. 6)). With 25/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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further excessive increase in the air supply, due to the cooling effect, an increasing trend would
569
be expected to start showing in the unburned hydrocarbon emissions. The onset of this increase
570
seems to have started in Test 1 when the Air/Fuel reached about 22. For Test 2, this increase is
571
less evident at Air/Fuel of 22 where the turning point was not reached in the tests conducted.
572
Nevertheless, based on the slope of the data points given in Fig. 6 for Test 2, it is expected to see
573
this happening at Air/Fuel levels roughly above 24.
574
NOx emissions in Test 1 are more responsive to increasing the air flow rate where NOx levels
575
drop from about 60 ppm at air/fuel ratio of about 12 to about 50 ppm at Air/Fuel of about 22.
576
The drop in NOx emissions in Test 2 (which already are lower than in Test 1; cf. Fig. 4) with the
577
increase in air flow, is less noticeable though. In regards to CO emissions, both tests show a clear
578
increasing CO-emission trend upon exceeding air to fuel ratio beyond about 16, with peak values
579
in the range from 200 to 300 ppm attained at the fuel lean-side of the Air/Fuel range considered
580
in the conducted experiments.
581 582
2.2) Test 3: Diesel/Jojoba-oil blends with 35 percent jojoba oil:
583
In Test 3 the results depart more and more from Test 1 conditions by incorporating more jojoba
584
oil in the liquid fuel sprayed into the furnace. Fig. 2c shows the temperatures attained in Test 3.
585
The temperature levels in Test 3, generally, are a bit higher than those attained in the previous
586
two tests and this is particularly the case for the exit temperature (cf. Fig. 2a-c). In addition, with
587
the increase in the Air/Fuel beyond 15, the temperatures in Test 3 do not drop to the levels
588
observed in Tests 1, but are a bit higher. The middle section temperature in Test 3 is comparable
589
to those observed in the previous tests for almost all air flow rates, rendering the discussions and
590
arguments made while discussing Tests 1 and 2 results, valid.
591
The difference in exit temperature between the different tests may impact and reflect on the
592
levels of unburned species emissions in these tests. This impact becomes even more pronounced
593
in the range of Air/Fuel above about 15. For example, the CO emissions of Test 3 maintain low
594
level at about 30 ppm over all Air/Fuel range up to about 20 whereas in Tests 1 and 2, the
595
runaway in CO emissions due to excessive air cooling effect starts at somewhat lower Air/Fuel 26/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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ratio levels (above about 16). Also the hydrocarbons emissions in Test 3 maintain a constant low
597
level around 110 ppm over all Air/Fuel values tested; compare this with the much higher values
598
reported in Tests 1 and 2 (see Fig. 6). The lower CO and hydrocarbons emissions observed in
599
Test 3 may be attributed to the somewhat higher temperatures in the furnace that still support
600
more effective oxidation of the unburned species under high air flow conditions, to a more clear
601
extent than what has been observed in the other two tests.
602
The higher jojoba content in Test 3 than in Test 2 was thought of at the beginning to be an
603
obstacle to having high level of combustion intensity in the flame due to the lower jojoba
604
volatility and higher viscosity (as highlighted above) which would lead to large droplets in the
605
spray. However, what the attained results reveal here is that this is not exactly the case in Test 3.
606
Perhaps the somewhat lower radiation to the walls in Test 3, as compared with the test with only
607
diesel, is behind the somewhat higher temperatures reported when jojoba content in the injected
608
fuel reached up to 35 percent. Another point that might also be of relevance in this regards is the
609
inherent presence of oxygen in the jojoba oil which would then lead to more abundance in
610
oxygen concentrations and hence more combustion activity at different locations in the flame
611
that hence lead to more heat release. In addition, having higher temperatures in the exhaust of
612
Test 3 is still expectable especially when considering the possibility of having larger fuel
613
droplets due to the larger viscosity of the liquid fuel injected in Test 3. These larger droplets have
614
the ability to survive and continue burning in the later stages of the combustion chamber domain
615
up to the furnace exit.
616
Although it was mentioned above that the more volatile component of the injected fuel (i.e.
617
diesel in our case) would have more chance to vaporize and burn first in the first half of the
618
furnace, leaving more room for the jojoba oil to burn subsequently in later stages of the furnace,
619
the larger droplets formed due to the higher viscosity of the blended fuel in this case would have
620
sufficient inertia to reach far in the combustion domain up to the furnace exit and so they act as
621
carriers for the more volatile diesel and with that they give the chance for some more diesel to
622
bypass the first half of the furnace and survive as liquid, to vaporize and burn later in regions
623
near the furnace exit.
27/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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NOx emissions of Test 3 are only little less than those reported in Test 2. The difference between
625
the two tests (in the average) is about 5 ppm. As implied above by the presented temperature
626
results, the local temperatures in the hottest sites of the flame which are the places of the
627
effective NOx generation are not amenable to us in this study for measurement hence hindering
628
more elaboration on this matter related to NOx formation in the different tests conducted.
629
Nevertheless, this would not hinder us here from making the reasonable conclusions in regards to
630
NOx emissions in the light of the attained temperature results namely the effect of spreading the
631
fuel over a bigger combustion envelope that would lessen the intensity of reactions, consequently
632
the local temperatures that may influence NOx formation, as we have highlighted earlier. Some
633
more elaborated future experimental and/or theoretical investigations on NOx formation from
634
jojoba oil-based flames may shed more light on the phenomena behind NOx results obtained in
635
this work.
636
The slightly higher temperatures reported in Test 3, as compared to those of Test 2 (cf. Figs. 3a
637
and 4a) result in only slightly more heat transfer to the water jacket in Test 3 than in Test 2 (cf.
638
Fig. 3). As can be seen from the Fig. 3, both tests show almost a uniform stable level of heat
639
transfer to the furnace walls over almost all air flow conditions considered in the experiments.
640
Once again, having jojoba content in the fuel in Test 3 leads to less sooting tendency hence heat
641
loss from the flame. Radiation is dependent however not only on emissivity due to the presence
642
of soot, but very much on the temperatures which are showing higher levels in Test 3 due to the
643
reasons and arguments given above. Hence this explains the slightly higher radiation to the wall
644
in the case of Test 3 as compared to Test 2. All the above discussion for Test 2 on the spray
645
evolution and vaporization and combustion when having jojoba oil in the injected blended liquid,
646
are believed to still be valid for Test 3 as well.
647
2.3) Test 4: Diesel/Jojoba mix with 60 percent jojoba:
648
Attempting to have more jojoba content in the injected jojoba/diesel blends beyond the levels
649
used in Test 3 (namely beyond 35 percent) using the same injection system setup used in Tests 2
650
and 3 without alteration, was not easy to attain. With as large as 60 percent jojoba content in the
651
blend, no stable flame was possible to be established. The difficulty to achieve this is attributed
652
to the low volatility of jojoba oil and the high viscosity of the injected liquid fuel under such high 28/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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jojoba content conditions that would lead to large and slowly vaporizing sprays hence little
654
ability to secure sufficient heat generation to sustain steady and stable flame. To overcome these
655
obstacles, it was found to be necessary to inject larger amounts of the blended fuel than the
656
amount used in Tests 1 and 2. These larger amounts are necessary in order ensure the presence of
657
sufficient volatile fuel content in the flame that basically originates from the diesel part in the
658
blended droplets in this binary liquid fuel spray. The burning of the vapor resulting from the
659
diesel will then generate enough heat to support and sustain the vaporization and ignition of the
660
rest of the liquid originating from the less volatile constituent namely jojoba oil. It was found
661
that there is a need in the case of 60 percent jojoba in the injected fuel in Test 4 to inject fuel at a
662
higher flow rate than 8 kg/h. No stable self-sustaining flame was possible at a total flow rate of
663
about 8 kg/h, when the jojoba share in the blend was increased up to 60 percent. Therefore, in
664
Test 4 the blended fuel injection rate was increased to about 10 kg/h and this was suitable to have
665
a self-sustaining stable flame over the range air/fuel ratio combustion conditions reported (up to
666
Air/Fuel levels of around 20). It should be emphasized here that conducting Test 4 in this work
667
was for the sole purpose mainly to show that such high raw jojoba oil content of 60 percent was
668
possible to be used to produce stable conditions if total fuel injection rate was sufficiently high.
669
As such, it was not a major target at this stage to compare the results of Test 4 with the previous
670
tests’ results. Further future runs are recommended under high jojoba proportions in the injected
671
blends and under different total fuel supply and initial injection temperature conditions.
672
The higher viscosity of jojoba in the injected blends would tend to damp out any possible
673
disintegration and breakup of the injected droplets and thereby add to the difficulties outlined
674
above regarding attaining small droplets with sufficiently rapid vaporization.
675
Temperature results of Test 4 are presented in Fig. 2d. Higher temperatures are realized than
676
what has been attained in Tests 1-3 mainly due to the overall higher amounts of injected fuel.
677
Moreover, for any given Air/Fuel ratio, this is also attributed to the higher turbulence and mixing
678
intensity between the fuel vapors and the oxidizing species, where to burn a larger amount of
679
fuel at a fixed Air/Fuel, higher air flow rate would be needed. This later effect may possibly
680
explain the smaller gap between the middle and exhaust temperatures, especially in the lean part
681
of the Air/Fuel range. Here in Test 4 also the possibility to have larger droplets that can spread 29/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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the fuel over a bigger space of the furnace to complete vaporization and combustion over a
683
bigger area, supported by the more intense turbulent mixing and combustion, may also lead to
684
less difference in temperature between the exhaust and the middle section temperature. Other
685
parameters that may also affect the enhanced fuel reactivity in the first half of the furnace in Test
686
4 is the larger oxygen content accompanying the additional amounts of liquid jojoba in the
687
injected fuel.
688
The heat radiated to the water jacket in Test 5 is at an average of around 41 kW over all air flow
689
rate values considered (cf. Fig. 3); compare this with around 37 to 39 in the previous tests with
690
less overall total fuel injected. In the fuel-rich range of the Air/Fuel range, the rate of heat
691
transfer reaches around 43 kW. The higher heat transfer in Test 4 can be mainly attributed to the
692
more overall total fuel injected and the consequent higher temperatures attained. Less likely is
693
that to be attributed to any more particulate emissions from the flame in Test 4. This is due to the
694
fact that jojoba is abundant (with a high share; 60 percent), in the injected liquid fuel. Given that
695
jojoba is an oxygenated fuel with no aromatic fuel content, it will not support soot formation.
696
Moreover, the inherent oxygen content in jojoba will be even expected to retard in the fuel
697
vapor-rich regions soot formation that may result from the diesel that accompanies jojoba in the
698
injected fuel. In addition, the higher viscosity of jojoba while using the same injector and
699
injection system of the present furnace for all runs without alterations is expected to yield a spray
700
with coarser droplets that (as highlighted above) would promote more for having premixed
701
flame-like with less level of fuel richness within the flame envelop. As mentioned earlier, pure
702
diffusion flame is more closely attained with higher availability of the volatiles content in the
703
injected liquid blend. This may give us another reason for not having extensive soot formation
704
activity in the flames formed in Test 4 leaving us with the higher temperature alone as the main
705
reason for the enhanced heat transfer to the furnace walls.
706
NOx emissions in Test 4 shown in Fig. 4 are less than in the previous tests although overall
707
average temperatures reported in Test 4 are a bit higher than those of the previous tests. Once
708
more, NOx formation is dependent on the local phenomena in the hot sites of the flame and these
709
seem to be less encouraging to NOx formation in Test 4, regardless of the apparently somewhat
710
higher representative average temperatures recorded. The arguments and statements laid down in 30/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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the above discussions on NOx for the previous tests are also valid for Test 4. It should be
712
recalled that thermal NOx formation takes place effectively in the hot flame spots, at much
713
higher temperatures than the average temperatures reported here for all tests. These flame sites
714
have apparently lower temperatures in Test 4, which may explain the lower level of NOx
715
emissions in Test 4. The effect of jojoba leading to larger droplets and hence causing spreading
716
of the combustion zone over a bigger space and thereby leading to less local heat release
717
intensity and lower local flame temperatures as well as the more possibility to have diluted fuel-
718
air mixture regions that do not support prompt NOx formation, might be reasonable arguments
719
that may well explain the trends and measurements reported hereby.
720
The unburned species emissions (CO and the unburned hydrocarbons) show lower levels than
721
what has been observed in Tests 1-3. This is possibly attributable to factors like: the higher
722
overall temperatures attained in Test 4, the more availability of oxygen by the virtue of the
723
nature of jojoba oil being an oxygenated fuel, the higher tendency to form a premixed-like
724
combustion due to the expected larger droplets that vaporize at a slower rate, and the higher
725
turbulence mixing levels associated with higher air flow rates for any given Air/Fuel in Test 4.
726
Unburned hydrocarbon emissions in Test 4 show very uniform level at about 180 mg/m3 over a
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wide range of air flow rates (see Fig. 6). These levels are comparable to the levels attained for
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Test 1 with only diesel with less total fuel supply rate. This highlights once more the relatively
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rapid mixing rate due to the higher air flow rates and the corresponding higher turbulence
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intensity. Also this can be to some extent attributed to the oxygen content in jojoba. CO
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emissions (Fig. 5) also show clearly low levels in Test 4 for Air/Fuel below about 14 for the
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reasons explained above. However, for higher Air/Fuel ratios, CO emissions clearly increase to
733
reach about 220 ppm at Air/Fuel of around 20, which might be expected due to the excessive air
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flow cooling effects and the short residence time when the flow speeds increase.
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5. Summary and Conclusions
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The combustion, heat transfer, and pollutants emissions from a small furnace fueled with blends
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of diesel and raw jojoba oil are investigated experimentally. Different percentages of the jojoba
738
oil in the injected fuel blends are considered ranging from zero (only diesel) to a max of 35
739
percent on mass basis. Runs with total blended fuel input to the furnace of about 8.3 kg/h, which 31/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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corresponds to about 99 to 103 kW nominal energy input (which also corresponds to range of
741
Jojoba oil in the blends from 35% to 0%), are conducted. By comparing the emissions from the
742
comparing of blends of raw jojoba oil and diesel (with 35% jojoba oil content) with their
743
counterparts based on the combustion of pure diesel, the results show a significant reduction in
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NOx emissions (around 30 to 40 percent). In the case of UBHC, the reduction is about 40
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percent in the average. Regarding CO emissions, 35 percent jojoba in the fuel blends has slightly
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reduced CO emissions as compared with the baseline case of only diesel. Running the furnace
747
under lean fuel conditions, would mostly affect the CO emissions causing them to increase while
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other pollutants (NOx and unburned hydrocarbons) showed little to no changes over a wide
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range of air supply conditions including the lean fuel side.
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It was found further that attaining stable combustion while including more jojoba in the
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diesel/jojoba-oil blends beyond about 35 percent was not easy, under the tested fuel injection
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temperatures at typical room conditions, without increasing the total amount of blended fuel
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injected. The additional amounts of the blended fuel injected in this case would ensure the
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availability of sufficient amount of the volatile combustible matter originating from the diesel
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that would contribute to having a self-sustaining flame to support heating and vaporizing the less
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volatile jojoba oil. The more the proportion of raw jojoba oil in the blends, the higher the overall
757
viscosity of the injected fuel blends. This would affect the performance of the atomization
758
process and the spray evolution and the related subsequent processes in the furnace. Preheating
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the blends before injection would possibly allow higher jojoba oil content in the runs that are
760
associated with relatively low nominal fuel energy input, without the need for significant
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alterations in the injection system.
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A Jojoba oil content as high as 60 percent in the blend was possible under higher total fuel
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injection rate of about 10 kg/h. The fact that raw jojoba oil has a lower heating value than diesel
764
might be expected to have an adverse effect on the local measured gas temperatures but the
765
measurements were just to the contrary to that, where the runs with jojoba attained slightly
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higher local gas temperatures at the middle section and at the furnace exit. Different parameters
767
may contribute to this results among which the low tendency to sooting when high jojoba content
768
is present in the fuel, hence less thermal radiation transfer to the furnace walls. 32/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
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References
770
1. Al Omari, S. B., Evaluation of the Biomass “Date Stones” as a Fuel in Furnaces: A
771
Comparison with Coal Combustion, Int. Communications in Heat and Mass Transfer, 36 (9),
772
pp. 956-961, 2009.
773
2. Al Omari, S. B., Experimental investigation of combustion and heat transfer characteristics in
774
a furnace fueled with unconventional biomass fuels (date stones and palm stalks), Energy
775
Conversion and Management, 47, pp. 778-790, 2006.
776
3. M. Y. E. Selim, Y. Haik, S.-A. B. Al Omari, and E. Elnajjar, Combustion of Waste
777
Chocolate Oil Methyl ester in a Diesel Engine, Int. J. of Ambient Energy; 2013; DOI:
778
10.1080/01430750.2013.770795).
779
4. M. O. Hamdan, M. Y. E. Selim, SAB Al-Omari, M Ghannam, Effect of Stabilized Water-
780
Diesel emulsion on External Combustion characteristics, Proceedings of the ASME 2013
781
Summer Heat Transfer Conference, 2013.
782
5. Al Omari S. B., Abu-Jdayil, Some Considerations of the Performance of Small Dual Fuel
783
Furnaces Fueled with a Gaseous Fuel and a Liquid Fuel Mix Containing Used-Engine-Lube-
784
Oil, Renewable Energy, 56, pp.117-122, 2013.
785 786
6. Al Omari S. B., Used engine lubrication oil as a renewable supplementary fuel for furnaces, Energy Conversion and Management, 49 (12), pp. 3648-3653, 2008.
787
7. Mohamed Y. E. Selim, Y Haik, S. A. B. Al-Omari, H Abdulrahman, Biodiesel Oil Derived
788
from Biomass Solid Waste, Proceedings of the World Congress on Engineering 3, 2011.
789
8. M. O. Hamdan, P Martin, E Elnajjar, M. Y. E. Selim, S Al-Omari, Diesel engine
790
performance and emission under hydrogen supplement, ICREGA’14-Renewable Energy:
791
Generation and Applications, 329-337, Springer International Publishing, 2014.
792 793
9. M. O. Hamdan, M. Y. E. Selim, S. A. B. Al-Omari, E. Elnajjar, Hydrogen supplement cocombustion with diesel in compression ignition engine, Renewable Energy 82, 54-60, 2015.
33/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
ACCEPTED MANUSCRIPT
794 795 796
10. M.Y.E. Selim, M.S. Radwan, S.M.S. Elfeky, Combustion of jojoba methyl ester in an indirect injection diesel engine, Renewable Energy, 28, Issue 9, Pages 1401–1420, 2003. 11. A.S. Huzayyin, A.H. Bawady, M.A. Rady, A. Dawood, Experimental evaluation of Diesel
797
engine performance and emission using blends of jojoba oil and Diesel fuel, Energy
798
Conversion and Management 45, 2093–2112, 2004.
799
12. Mohamed Y.E. Selim, M.S. Radwan, H.E. Saleh, Improving the performance of dual fuel
800
engines running on natural gas/LPG by using pilot fuel derived from jojoba seeds,
801
Renewable Energy 33, 1173–1185, 2008.
802
13. M. S. Shehata, S. M. Abdel Razek, Experimental investigation of diesel engine performance
803
and emission characteristics using jojoba/diesel blend and sunflower oil, Fuel 90, 886-897,
804
2011.
805 806 807 808 809
14. H. E. Saleh, Experimental study on diesel engine nitrogen oxide reduction running with jojoba methy ester by exhaust gas recirculation, Fuel 88, 1357-1364, 2009. 15. M. Al-Widyan, M. A. Al-Muhtaseb, Experimental investigation of jojoba as renewable energy source, Energy Conversion and Management, 51, 1702-1707, 2010. 16. F. Millo, B. K. Debnath, T. Vlachos, C. Ciaravino, L. Postrioti, G. Buitoni, Effects of
810
different biofuels blends on performance and emissions of an automotive diesel engine, Fuel
811
159, 614–627, 2015.
812
17. G. Tashtoush, M. Al-Widyan, A. O. Al-Shyoukh, Combustion performance and emissions of
813
ethyl ester of a waste vegetable oil in a water-cooled furnace, Applied Thermal Engineering
814
23, 285-293, 2003.
815
18. A. Cavarzerea, M. Morinib, M. Pinellic, P. R. Spinac, A. Vaccaria, M. Venturinic,
816
Experimental Analysis of a Micro Gas Turbine Fuelled with Vegetable Oils from Energy
817
Crops, 68th Conference of the Italian Thermal Machines Engineering Association, ATI2013,
818
Energy Procedia 45, 91 – 100, 2014.
34/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
ACCEPTED MANUSCRIPT
819 820 821
19. A. E.E. Khalil, A.K. Gupta, Clean combustion in gas turbine engines using Butyl Nonanoate biofuel, Fuel 116, 522–528, 2014. 20. T. E. Jiru, B. G. Kaufman, K. E. Ileleji, D. R. Ess, H. G. Gibson, D. E. Maier, Testing the
822
performance and compatibility of degummed soybean heating oil blends for use in residential
823
furnaces, Fuel 89, 105–113, 2010.
824 825 826 827 828 829
21. L. Jiang, A. K. Agrawal, R. P. Taylor, Clean combustion of different liquid fuels using a novel injector, Experimental Thermal and Fluid Science 57, 275–284, 2014. 22. W-C Huang, S.-S. Hou, Ta-H. Lin, Combustion characteristics of a 300 kWth oil-fired furnace using castor oil/diesel blended fuels, Fuel 208, 71-81, 2017. 23. I. I. Enagia, K.A. Al-attabb, Z.A. Zainal, Liquid biofuels utilization for gas turbines: A review, Renewable and Sustainable Energy Reviews, 90, 43-55, 2018.
830 831 832 833 834 835 836
24. N. Hashimoto, H. Nishida, Y. Ozawa, Fundamental combustion characteristics of Jatropha oil as alternative fuel for gas turbines, Fuel 126, 2014, 194-201, 2014.
837
26. H. Pawlak-Kruczek, M.Ostrycharczyk, J. Zgo´ra, Co-combustion of liquid biofuels in PC
25. N. Hashimoto, H. Nishida, M. Kimoto, K. Tainaka, A. Ikeda, S. Umemoto, Effects of Jatropha oil blending with C-heavy oil on soot emissions and heat absorption balance characteristics for boiler combustion, (accepted in Renewable Energy on April 5, 2018: DOI: 10.1016/j.renene.2018.04.018
838
boilers of 200 MW utility unit, Proceedings of the Combustion Institute 34, 2769–2777,
839
2013.
840 841 842
27. M. Sánchez, M. R. Avhad, J. M. Marchetti, M. Martínez, J. Aracil, Jojoba oil: A state of the art review and future prospects, Energy Conversion and Management 129, 293–304, 2016. 28. M. Y. E. Selim, S.-A. B. Al-Omari, S. M. S. Elfeky and M. S. Radwan, Utilization of
843
extracted jojoba fruit as a fuel, International Journal of Sustainable Energy, 30, Supplement 1
844
pp. S106-S117, 2011.
35/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]
ACCEPTED MANUSCRIPT
845 846 847 848 849 850 851 852
29. J. H. Wasser, R. P. Hangebrauck & A. J. Schwartz, Effects of air-fuel stoichiometry on air pollutant emissions from an oil-fired test furnace, Journal of the Air Pollution Control Association 18, No. 5, 332-337, 1968. 30. S. R. Turns, An Introduction to Combustion: Concepts and Applications, 2nd Edition, McGraw-Hill International Editions, 2000. 31. I Glassman, Combustion, 2nd Edition, Academic Press, Orlando, FL, 1987.
36/30 *Corresponding Author: S.-A. B. Al Omari, UAE University, Mech. Eng. Dept., Email: [email protected]