- Email: [email protected]
The use of biodiesel blends in domestic vaporising oil burners
Energy 35 (2010) 501–505
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
The use of biodiesel blends in domestic vaporising oil burners C.D. Barnes, D.R. Garwood, T.J. Price* Faculty of Advanced Technology, University of Glamorgan, Pontypridd, UK CF37 1DL, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 January 2009 Received in revised form 2 September 2009 Accepted 12 October 2009 Available online 18 November 2009
Biodiesel is receiving increasing attention as a partial substitute for home heating oil. The properties that make it a suitable fuel for use in diesel engines also make it suitable for heating systems using pressure jet burner technology. In the UK, however, there are a significant number of vaporising burners whose suitability for operation with biodiesel has not been properly studied. The purpose of this study was therefore to investigate the use of different blends of biodiesel and kerosene in a production Aga that employed a sleeve-type vaporising burner. It was found that significant fouling of the burner well would occur within a short period of time, even with a blend as low as 5% by volume of biodiesel in kerosene. Further investigation revealed the build-up to consist of polymerised biodiesel, most likely triggered by heat. Factors contributing to the rate and extent of fouling are thought to include the type of vegetable oil used as feedstock for the biodiesel, as well as the degree of prior utilisation of the feedstock oil. Further investigation is warranted, possibly with the use of suitable fuel additives to inhibit the polymerisation process. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Biodiesel Vaporising burner Polymerisation
1. Introduction Biodiesel and blends of biodiesel with fossil-based fuels have been used with relative success in a number of different stationary and mobile applications. In fact, biodiesel is now a mandated constituent of vehicle diesel fuel in a number of European countries. Because of the similarities between vehicle diesel fuel and the heating fuel commonly used in Europe and the USA (known as no. 2 fuel oil or Mazout), as well as the kerosene used for home heating in the UK, biodiesel has also been studied as a partial replacement for these heating fuels. However, there are differences in certain key properties of biodiesel compared to kerosene, and these differences can be significant depending on the type of domestic heating oil burner in which it is used. In the UK, whilst the majority of oil-fired systems are of the pressure jet type, there are significant numbers of sleeve-type vaporising burners in the domestic sector. In a pressure jet burner, the fuel is pumped under pressure and spray-injected into a combustion chamber, where the resulting fuel mist is ignited. The process of atomisation assists the combustion process, and may allow the use of heavier oils without detriment to overall performance and servicing schedules. This is evidenced by the fact that, even when using fuel oil, which contains higher chain-length hydrocarbons than kerosene, pressure jet burners have continued
to give satisfactory performance [1]. The efficacy of using biodiesel and blends of biodiesel with this type of burner is now well established in research [1,2]. This is not the case with vaporising burners, which operate on a completely different principle. In this type, fuel is fed under gravity to a fuel well, where it is drawn up a series of concentric wicks. At the top of the wicks the fuel vaporises and burns. On start-up this vapour must be ignited, but thereafter combustion continues for as long as fuel is drawn up the wicks and vaporises. Vaporising burners thus rely more on the ability of the fuel to vaporise, which on initial start-up it must do at ambient temperatures. This means that properties which affect vaporisation, such as the fuel flash and fire points, are especially important. As such, vaporising burners are more affected by variations in the properties allowed within the relevant fuel standard, as well as by deviations from the standard and/or the presence of fuel contaminants. Of additional interest in the case of biodiesel is the thermal stability of the blended fuel when contained within the hot well of the burner. However, to date there have been no studies to demonstrate the viability of using biodiesel for this application. The aim of the current tests, therefore, was to investigate the use of blends of biodiesel and kerosene in a domestic vaporising oil burner.
2. Materials and test methods * Corresponding author. Tel.: þ44 (0) 1443 480480; fax: þ44 (0) 1443 4654050. E-mail address: [email protected] (T.J. Price). 0360-5442/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2009.10.014
Blend tests were conducted at the Aga Technical Centre near Telford, Shropshire. The biodiesel for the tests was sourced initially
502
C.D. Barnes et al. / Energy 35 (2010) 501–505
from Sundance Renewables, based near Carmarthen, Wales. The biodiesel supplier was then switched to Argent Energy in Motherwell, Scotland. Both suppliers use waste vegetable oil as their feedstock. However, the change was made because Argent Energy is better able to ensure consistency in fuel quality, and routinely test all fuel batches to ensure compliance with the British and European standard BS EN 14214. Table 1 shows the analysis results for the biodiesel obtained from Argent Energy. Kerosene was sourced initially from Oil Direct Limited. However, a switch was made to using Aga’s own kerosene, which is routinely sourced from CPL Petroleum. The change in kerosene supplier was again made to eliminate the fuel source as a potential issue affecting results. Fuel mixing was achieved by splash-blending the biodiesel on to the surface of the kerosene, the manually stirring. The rationale for this is that biodiesel is denser than kerosene, so must be added to the kerosene. The mixture was to be further agitated during the act of manually pumping the fuel from the mixing tank to the conical supply tank. The test plan required a total of 60 litres of fuel for each test run, sufficient to run the cooker for at least seven days. Depending on the blend, a percentage of the final volume was biodiesel (i.e. for a 5% blend, three litres of biodiesel was mixed with 57 litres of kerosene). Fuel amounts for the blending process were measured using a five litre measuring cylinder, graduated in 0.5 L increments. Assuming an error of half the smallest scale on the measuring cylinder, the error in the blend percentage of 5% is about 1% for all tests. The test installation was a twin-hob, twin-oven Aga cooker. This was gravity-fed with the blended fuel from a 120 litre polypropylene conical tank situated adjacent to the cooker. The test setup is shown in Fig. 1. Fuel temperature was always around room temperature at 20 C 2 C. The fuel flow rate was controlled by a needle valve and float chamber assembly attached to the side of the cooker. The flow control assembly is set at a pre-determined height in order to ensure the correct level of fuel in the burner well. A thermostat situated underneath one of the cooker hobs provides feedback to the control valve, which increases or decreases flow in order to maintain the temperature at around 240 C. Before starting, the fuel lines and float chamber are empty of fuel downstream of the shut-off valve at the base of the fuel tank. When the shut-off
Fig. 1. Test set-up, showing bio-kerosene supply tank and Aga cooker.
valve is opened, fuel flows into the supply lines, flooding the float chamber, and then flows gradually to the burner well. From the well the fuel is drawn up the burner wicks before the fuel vapour that develops above the wicks is manually lighted via an access port in the burner sleeves. The lighting process takes about 5 min to achieve. Once lit, the cooker takes approximately 24 h to reach its target temperature. All the correct procedures pertaining to the commissioning and start-up of the Aga cooker were observed, and were carried out by one of Aga’s own technicians.
3. Results Four test runs were completed before a re-assessment of the test fuel became evident. Each test run used a separately made up batch of bio-kerosene. The test runs are summarised in Table 2. Test 1 appeared to run normally for approximately the first two days. There were no issues during lighting, and when at temperature a normal blue flame was observed above the burner sleeves. The
Table 1 Certificate of analysis for test biodiesel sourced from Argent Energy (ID no. B201500). Test Parameter
Ester content Density at 15 C Viscosity at 40 C Flash Point Total Sulphur Content Cold Filter Plug Point Carbon Residue (CCR) (from 10% distillation residue) Sulphated Ash Content Water Content Total Contamination Oxidation Stability, 110 C Neutralisation Number Iodine Number Content of Linolenic-Methyl Acid Content of Fatty Acid Methyl Ester with more than 3 Double Bonds Methanol Content Monoglycerides Diglycerides Triglycerides Free Glycerine Total Glycerine ND ¼ not detected.
Units
Specification
Result
Test Method
0.3 0.02 500 24 – 0.5 120 12 1
97.4 879 4.4 168 4.92 þ3 0.002 <0.01 45 8.2 9.0 0.29 52 3.1 ND
BS EN 14103 EN ISO 3675 EN ISO 3104 EN ISO 3679 EN ISO 20846 EN ISO 116 ISO 6615 ISO 3987 EN ISO 12937 EN 12662 BS EN 14112 BS EN 14104 Pr EN 14111 BS EN 14103
0.2 0.8 0.2 0.2 0.02 0.02
0.00208 0.0902 0.0034 0.0017 0.0044 0.0359
BS BS BS BS BS BS
Min
Max
% (m/m) kg/m3 mm2/s C mg/kg C % (m/m) % (m/m) mg/kg mg/kg hour mg KOH/g – % (m/m) % (m/m)
96.5 860 3.5 >120 – Report Result – – – – 6.0 – – – –
– 900 5.0 – 10, 10, 50
% % % % % %
– – – – – –
(m/m) (m/m) (m/m) (m/m) (m/m) (m/m)
EN EN EN EN EN EN
14110 14105 14105 14105 14105 14105
C.D. Barnes et al. / Energy 35 (2010) 501–505
503
Table 2 Summary of bio-kerosene tests in a vaporising burner. Test
Fuel Source
Blend
Test Duration
Result
Oil Direct Limited
5%
2½ days
Oil Direct Limited CPL Petroleum CPL Petroleum
5% 5% 5%
6 days 10days 2½ days
Extensive fouling leading to flame-out Extensive fouling Extensive fouling Significant fouling
Biodiesel
Kerosene
1
Sundance Renewables
2a 3 4
Sundance Renewables Argent Energy Argent Energy
a
Test 2 used the same fuel batch as Test 1.
cooker continued to run unmonitored outside of hours of watch, but at some point the flame extinguished. An estimate of the total time before failure was made based on the temperature of the cooker, which was cool to the touch when next examined. The cooker was stripped and the internals examined. An extensive build-up of a black, spongy material was found within the burner well. Small amounts of the same material were also found in the fuel wick channels. There was, however, no evidence of a build-up any where else in the system, including the fuel lines right up to the burner well. The burner well was cleaned and the system was purged with kerosene before being re-started with the same batch of blended fuel (Test 2). This time the cooker ran for six days until the fuel tank was emptied. The system was stripped and examined. There was again an extensive build-up of black material, this time brittle in nature. As before, there was no evidence of a build-up any where else in the system. Test 3 again appeared to run normally, with no issues during lighting. This time the cooker ran until the fuel tank was run dry after about 10 days. On examination, however, the burner well contained a large deposit of a black brittle, carbon-like material, with no evidence of a build-up elsewhere in the system. Test 4 was a repeat of Test 3 in terms of the source of fuels used for blending. However, the kerosene and biodiesel were mixed directly in the tank in order to remove the fuel transfer pump from the preparation process, and only 20 litres of a 5% blend was made up. The cooker was shut down manually after about 2½ days of running and allowed to cool before being inspected. This test further confirmed
the results of test three, with a significant build-up being found in the burner well after just a few days, but no where else in the system. 4. Discussion To understand the mechanism by which debris was being formed, a number of factors were considered. This first of these is the disparity between the flashpoints of biodiesel and kerosene, and the effect this could have on the flashpoint of the blended fuel. The fuel flashpoint is significant for fuel burning equipment such as vaporising burners because it defines the lowest temperature at which a liquid will give off sufficient vapour to ignite in air. Krishnakumar et al., [3] gave a value of 183 C for the flashpoint of pure biodiesel made from rice bran oil, and 128 C for biodiesel made from jatropha oil. These values represent typical extremes for biodiesel made from different feedstocks. The flashpoint of the pure biodiesel obtained from Argent Energy was 168 C. By comparison, the minimum flashpoint of class 2 kerosene is 38 C, according to the British Standard BS 2869:2006. Although the flashpoint for the 5% blended fuels was not determined for these tests, Krishna [1] found that even a 50% blend of soybean methyl ester in kerosene raised the flashpoint by only 7 C compared to neat kerosene. A 5% blend would therefore have had a negligible effect on the flashpoint of the fuel tested in a vaporising burner. Another potential factor that may have contributed to the buildup of debris is contamination of the blended test fuel with metal
Fig. 2. FT-IR of (from top to bottom) raw sample, sample washed in DCM, sunflower oil reference and kerosene.
504
C.D. Barnes et al. / Energy 35 (2010) 501–505
Burner well 241 / 190
Feed pipe
313 / 244 142 / 104
65 / 55
62 / 56
285 / 221
Fig. 3. Average oil feed temperatures ( C) burning kerosene at low fire/high fire (Source: Aga Rayburn).
ions, which are known to catalyse the polymerisation of fuels like kerosene. An analysis of the residue from Test 1 was performed at the University of Glamorgan using gas chromatography (GC) mass spectrometry. The results suggested that contamination by iron may have precipitated the build-up. In fact the hand pump used to transfer the bio-kerosene to the Aga supply tank was found to be contaminated by rust, and an effort was made to drain the contaminated fuel from the lines before the first test began. The hand pump was cleaned prior to preparing the nest fuel batch for the third test run, and was eliminated altogether from the fourth test by blending directly in the supply tank. Metal ion contamination was thus ruled out as a contributory factor in debris formation for these tests. It should be noted that the Aga was run on the pure kerosene obtained from Oil Direct Limited between Tests 2 and 3, with no build-up whatsoever observed after several days of running. Samples of the build-up from Test 4 were analysed, this time by Minton, Treharne and Davies, an independent international fuels testing laboratory in Cardiff, Wales. The results of Fourier Transform – Infra-red spectroscopy are shown in Fig. 2. Of particular significance is the signal at 1740 cm1, present in all but the kerosene trace and thus indicative of debris that is vegetable in nature. This suggests that the biodiesel was polymerising. A profile of the heat distribution in the feed-pipe and across the base plate in a sleeve-type vaporising burner (shown in cross-section in Fig. 3) suggests that heat appeared to be the mechanism by which such
100% 90% 80% 70% 60% 50% 40% 30% 20% 10%
ut
C
C
oc on
Pa lm
d
ot to
La r
ns ee
d
an be
e O liv
or n C
w er flo
er Su n
ow Sa ffl
C
an ol
a
0%
So y
349 / 291
polymerisation was taking place. This theory is made plausible by the fact that debris was found only in the burner well or the channels that hold the burner wicks, where temperatures are greatest. Note the steep temperature gradient between the outer temperature reading in the feed-pipe and the entrance to the burner well, a distance of only about 15 cm. Literature searches have so far not revealed any similar work on the use of biodiesel for vaporising burners, and so a perspective on the current issue has not been found. However, fuel polymerisation is not a new problem. It has been reported in the fuel process industry, where it can lead to losses in production and increased maintenance costs. Biodiesel itself can in fact be more susceptible to oxidation and polymerisation than petroleum-based fuels, and this has led to problems with fuel acidity and corrosion of system components, and the formation of sediments which can block fuel nozzles and filters when used in compression-ignition engines [4,5]. This may be due partly to the fatty acid composition and thus properties of the source oil used as feedstock for the biodiesel, and partly to the degree of prior utilisation of the feedstock oil in cooking operations. Using data from McCance and Widdowson [6], Fig. 4 shows a number of common oils and their fatty acid composition. Those oils with significant levels of unsaturated fatty acids would be expected to oxidise and polymerise more readily than those oils which are largely saturated. Rapeseed and sunflower oils fall into this category. At the other end of this scale are oils such as palm oil and coconut oil, the latter in particular consisting predominantly of saturated fatty acids. It should be noted that the process of transesterification does not significantly alter this composition [7,8]. All the biodiesel used in tests was manufactured from waste cooking oil, though the exact source for any given batch is not recorded. According to government statistics [9], in the UK most cooking oil is derived from either rapeseed oil (canola) or palm oil (see Fig. 5). Fig. 5 indicates annual production output, but an overall breakdown of actual oil use in the UK is not available. However, a study prepared for DEFRA and the Food Standards Agency in 2003 by LMC International [10] used figures from the USA to predict oil use in Europe. These figures indicated that the largest consumption of oil is likely to be in the salad and cooking sector, which in Europe is generally dominated by rapeseed and palm oil, followed by sunflower and olive oils. The waste cooking oil collected for use in
Saturated
Monounsaturated
Polyunsaturated
Fig. 4. Fatty acid composition of common fats and oils (Adapted from McCance and Widdowson, 2002).
C.D. Barnes et al. / Energy 35 (2010) 501–505
5. Conclusions
Palm Kernel 1% Soybean 4%
505
Sunflower 0%
Other 8%
Coconut 11% Rapeseed 45%
Palm 31% Fig. 5. Share of Oils and Fats used in the Production of Refined Vegetable Oil in the UK in 2001 (DEFRA, 2007).
biodiesel production in the UK is thus likely to consist predominantly of the former two oils. The implication is that the waste vegetable oil used as feedstock for the test biodiesel most likely contained a high percentage on unsaturated rapeseed oil. As noted above, the degree of utilisation of the feedstock also plays a part in stability. Ferrari et al [8] looked at the oxidative stability of neutralised, refined and waste soybean oils, and partially hydrogenated frying waste, using the RancimatÒ stability test. Although not measured, the presence of natural anti-oxidants, such as tocopherol, was considered a major factor in the stability of the different quality oils. Thus the neutralised soybean oil presented the greatest stability, owing to the presence of natural anti-oxidants. The biodiesel from the waste soybean oil was the least stable. This was considered to be due to the fact that the used oil had already suffered oxidative and hydrolytic degradation as a result of the cooking process, which effectively destroys the oil’s natural anti-oxidants. The partially hydrogenated waste oil contained a greater percentage of saturated fatty acids than any of the other oils, which should have resulted in a higher oxidative stability of the resulting biodiesel. However, its poor stability was attributed to the greater negative impact of the frying process to which it had been subjected. The oxidation stability result of the biodiesel obtained from Argent Energy was 9 h, compared to the required minimum of 6 h, and was thus well within the requirement set out in BS EN 14214. The oxidative stability result for the test biodiesel would therefore appear to have little significance in the context of this study.
The use of biodiesel blended with kerosene in a vaporising burner, even in concentrations as low as 5% by volume, has been shown to cause fouling and eventual blockage of the burner within a short period of time. Given the circumstances under which fouling of the burner took place during testing, and the short timescales involved, it would appear that the principle mechanism for the build-up of debris was polymerisation of the biodiesel, triggered by heat. However, other sources of biodiesel instability cannot be ruled out with longer term use. Though the use of biodiesel made from unused refined oils composed of saturated fatty acids may mitigate the problem, this is not a sustainable option. A more logical approach to further work in this area is to investigate the use of anti-oxidants and anti-polymerisation additives, which are already extensively used in the petroleum industry. Acknowledgements The authors would like to thank the EPSRC for funding this work via an Industrial CASE studentship, along with Aga Rayburn Ltd who provided the test facilities and gave much technical assistance during testing. References [1] Krishna CR. Biodiesel blends in space heating equipment. National Renewable Energy Laboratory; 2004. NREL-SR-510–33579. [2] Mushrush G, Beal EJ, Spencer G, Wynne JH, Lloyd CL, Hughes JM, et al. An Environmentally benign soybean-derived fuel as a blending stock or replacement for home heating oil. Journal of Environmental Science and Health 2001;36(5):613–22. [3] Krishnakumar J, Venkatochalapathy VSK, Elancheliyan S. Technical aspects of biodiesel production from vegetable oils. Thermal Science 2008;12(2):159–69. [4] Clark SJ, Wagner L, Schrock MD, Plennaar PG. Methyl and ethyl soybean esters as renewable fuels for diesel engines. Journal of the American Oil Chemists’ Society 1984;46(4):945–54. [5] Monyem A, Van Gerpen JH. The effect of biodiesel oxidation on engine performance and emissions. Biomass and Bioenergy 2001;20:317–25. [6] McCance, Widdowson. The composition of foods. Cambridge: The Royal Society of Chemistry; 2002. [7] Darnoko D, Cheryan M. Kinetics of palm oil transesterification in a batch reactor. Journal of the American Oil Chemists’ Society 2000;77(12):1263–7. [8] Ferrari R, Oliveira V, da Silva, Scabio A. Oxidative stability of biodiesel from soybean oil fatty acid ethyl esters. Scientia Agricola (Piracicaba, Braz.) 2005;62(3):291–5. [9] Department for Environment. Food and rural affairs national statistics. Output of refined, deodorised vegetable, marine oils and animal fats by UK processing plants. Downloaded from, http://statistics.defra.gov.uk/esg/datasets/histoils; 2007 [accessed 23.05.08]. [10] Department for Environment. Food and rural Affairs and the Food Standards agency. Supply chain impacts of further regulation of products consisting of, containing, or derived from, genetically modified organisms. LMC International; 2003.
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
The use of biodiesel blends in domestic vaporising oil burners C.D. Barnes, D.R. Garwood, T.J. Price* Faculty of Advanced Technology, University of Glamorgan, Pontypridd, UK CF37 1DL, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 January 2009 Received in revised form 2 September 2009 Accepted 12 October 2009 Available online 18 November 2009
Biodiesel is receiving increasing attention as a partial substitute for home heating oil. The properties that make it a suitable fuel for use in diesel engines also make it suitable for heating systems using pressure jet burner technology. In the UK, however, there are a significant number of vaporising burners whose suitability for operation with biodiesel has not been properly studied. The purpose of this study was therefore to investigate the use of different blends of biodiesel and kerosene in a production Aga that employed a sleeve-type vaporising burner. It was found that significant fouling of the burner well would occur within a short period of time, even with a blend as low as 5% by volume of biodiesel in kerosene. Further investigation revealed the build-up to consist of polymerised biodiesel, most likely triggered by heat. Factors contributing to the rate and extent of fouling are thought to include the type of vegetable oil used as feedstock for the biodiesel, as well as the degree of prior utilisation of the feedstock oil. Further investigation is warranted, possibly with the use of suitable fuel additives to inhibit the polymerisation process. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Biodiesel Vaporising burner Polymerisation
1. Introduction Biodiesel and blends of biodiesel with fossil-based fuels have been used with relative success in a number of different stationary and mobile applications. In fact, biodiesel is now a mandated constituent of vehicle diesel fuel in a number of European countries. Because of the similarities between vehicle diesel fuel and the heating fuel commonly used in Europe and the USA (known as no. 2 fuel oil or Mazout), as well as the kerosene used for home heating in the UK, biodiesel has also been studied as a partial replacement for these heating fuels. However, there are differences in certain key properties of biodiesel compared to kerosene, and these differences can be significant depending on the type of domestic heating oil burner in which it is used. In the UK, whilst the majority of oil-fired systems are of the pressure jet type, there are significant numbers of sleeve-type vaporising burners in the domestic sector. In a pressure jet burner, the fuel is pumped under pressure and spray-injected into a combustion chamber, where the resulting fuel mist is ignited. The process of atomisation assists the combustion process, and may allow the use of heavier oils without detriment to overall performance and servicing schedules. This is evidenced by the fact that, even when using fuel oil, which contains higher chain-length hydrocarbons than kerosene, pressure jet burners have continued
to give satisfactory performance [1]. The efficacy of using biodiesel and blends of biodiesel with this type of burner is now well established in research [1,2]. This is not the case with vaporising burners, which operate on a completely different principle. In this type, fuel is fed under gravity to a fuel well, where it is drawn up a series of concentric wicks. At the top of the wicks the fuel vaporises and burns. On start-up this vapour must be ignited, but thereafter combustion continues for as long as fuel is drawn up the wicks and vaporises. Vaporising burners thus rely more on the ability of the fuel to vaporise, which on initial start-up it must do at ambient temperatures. This means that properties which affect vaporisation, such as the fuel flash and fire points, are especially important. As such, vaporising burners are more affected by variations in the properties allowed within the relevant fuel standard, as well as by deviations from the standard and/or the presence of fuel contaminants. Of additional interest in the case of biodiesel is the thermal stability of the blended fuel when contained within the hot well of the burner. However, to date there have been no studies to demonstrate the viability of using biodiesel for this application. The aim of the current tests, therefore, was to investigate the use of blends of biodiesel and kerosene in a domestic vaporising oil burner.
2. Materials and test methods * Corresponding author. Tel.: þ44 (0) 1443 480480; fax: þ44 (0) 1443 4654050. E-mail address: [email protected] (T.J. Price). 0360-5442/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2009.10.014
Blend tests were conducted at the Aga Technical Centre near Telford, Shropshire. The biodiesel for the tests was sourced initially
502
C.D. Barnes et al. / Energy 35 (2010) 501–505
from Sundance Renewables, based near Carmarthen, Wales. The biodiesel supplier was then switched to Argent Energy in Motherwell, Scotland. Both suppliers use waste vegetable oil as their feedstock. However, the change was made because Argent Energy is better able to ensure consistency in fuel quality, and routinely test all fuel batches to ensure compliance with the British and European standard BS EN 14214. Table 1 shows the analysis results for the biodiesel obtained from Argent Energy. Kerosene was sourced initially from Oil Direct Limited. However, a switch was made to using Aga’s own kerosene, which is routinely sourced from CPL Petroleum. The change in kerosene supplier was again made to eliminate the fuel source as a potential issue affecting results. Fuel mixing was achieved by splash-blending the biodiesel on to the surface of the kerosene, the manually stirring. The rationale for this is that biodiesel is denser than kerosene, so must be added to the kerosene. The mixture was to be further agitated during the act of manually pumping the fuel from the mixing tank to the conical supply tank. The test plan required a total of 60 litres of fuel for each test run, sufficient to run the cooker for at least seven days. Depending on the blend, a percentage of the final volume was biodiesel (i.e. for a 5% blend, three litres of biodiesel was mixed with 57 litres of kerosene). Fuel amounts for the blending process were measured using a five litre measuring cylinder, graduated in 0.5 L increments. Assuming an error of half the smallest scale on the measuring cylinder, the error in the blend percentage of 5% is about 1% for all tests. The test installation was a twin-hob, twin-oven Aga cooker. This was gravity-fed with the blended fuel from a 120 litre polypropylene conical tank situated adjacent to the cooker. The test setup is shown in Fig. 1. Fuel temperature was always around room temperature at 20 C 2 C. The fuel flow rate was controlled by a needle valve and float chamber assembly attached to the side of the cooker. The flow control assembly is set at a pre-determined height in order to ensure the correct level of fuel in the burner well. A thermostat situated underneath one of the cooker hobs provides feedback to the control valve, which increases or decreases flow in order to maintain the temperature at around 240 C. Before starting, the fuel lines and float chamber are empty of fuel downstream of the shut-off valve at the base of the fuel tank. When the shut-off
Fig. 1. Test set-up, showing bio-kerosene supply tank and Aga cooker.
valve is opened, fuel flows into the supply lines, flooding the float chamber, and then flows gradually to the burner well. From the well the fuel is drawn up the burner wicks before the fuel vapour that develops above the wicks is manually lighted via an access port in the burner sleeves. The lighting process takes about 5 min to achieve. Once lit, the cooker takes approximately 24 h to reach its target temperature. All the correct procedures pertaining to the commissioning and start-up of the Aga cooker were observed, and were carried out by one of Aga’s own technicians.
3. Results Four test runs were completed before a re-assessment of the test fuel became evident. Each test run used a separately made up batch of bio-kerosene. The test runs are summarised in Table 2. Test 1 appeared to run normally for approximately the first two days. There were no issues during lighting, and when at temperature a normal blue flame was observed above the burner sleeves. The
Table 1 Certificate of analysis for test biodiesel sourced from Argent Energy (ID no. B201500). Test Parameter
Ester content Density at 15 C Viscosity at 40 C Flash Point Total Sulphur Content Cold Filter Plug Point Carbon Residue (CCR) (from 10% distillation residue) Sulphated Ash Content Water Content Total Contamination Oxidation Stability, 110 C Neutralisation Number Iodine Number Content of Linolenic-Methyl Acid Content of Fatty Acid Methyl Ester with more than 3 Double Bonds Methanol Content Monoglycerides Diglycerides Triglycerides Free Glycerine Total Glycerine ND ¼ not detected.
Units
Specification
Result
Test Method
0.3 0.02 500 24 – 0.5 120 12 1
97.4 879 4.4 168 4.92 þ3 0.002 <0.01 45 8.2 9.0 0.29 52 3.1 ND
BS EN 14103 EN ISO 3675 EN ISO 3104 EN ISO 3679 EN ISO 20846 EN ISO 116 ISO 6615 ISO 3987 EN ISO 12937 EN 12662 BS EN 14112 BS EN 14104 Pr EN 14111 BS EN 14103
0.2 0.8 0.2 0.2 0.02 0.02
0.00208 0.0902 0.0034 0.0017 0.0044 0.0359
BS BS BS BS BS BS
Min
Max
% (m/m) kg/m3 mm2/s C mg/kg C % (m/m) % (m/m) mg/kg mg/kg hour mg KOH/g – % (m/m) % (m/m)
96.5 860 3.5 >120 – Report Result – – – – 6.0 – – – –
– 900 5.0 – 10, 10, 50
% % % % % %
– – – – – –
(m/m) (m/m) (m/m) (m/m) (m/m) (m/m)
EN EN EN EN EN EN
14110 14105 14105 14105 14105 14105
C.D. Barnes et al. / Energy 35 (2010) 501–505
503
Table 2 Summary of bio-kerosene tests in a vaporising burner. Test
Fuel Source
Blend
Test Duration
Result
Oil Direct Limited
5%
2½ days
Oil Direct Limited CPL Petroleum CPL Petroleum
5% 5% 5%
6 days 10days 2½ days
Extensive fouling leading to flame-out Extensive fouling Extensive fouling Significant fouling
Biodiesel
Kerosene
1
Sundance Renewables
2a 3 4
Sundance Renewables Argent Energy Argent Energy
a
Test 2 used the same fuel batch as Test 1.
cooker continued to run unmonitored outside of hours of watch, but at some point the flame extinguished. An estimate of the total time before failure was made based on the temperature of the cooker, which was cool to the touch when next examined. The cooker was stripped and the internals examined. An extensive build-up of a black, spongy material was found within the burner well. Small amounts of the same material were also found in the fuel wick channels. There was, however, no evidence of a build-up any where else in the system, including the fuel lines right up to the burner well. The burner well was cleaned and the system was purged with kerosene before being re-started with the same batch of blended fuel (Test 2). This time the cooker ran for six days until the fuel tank was emptied. The system was stripped and examined. There was again an extensive build-up of black material, this time brittle in nature. As before, there was no evidence of a build-up any where else in the system. Test 3 again appeared to run normally, with no issues during lighting. This time the cooker ran until the fuel tank was run dry after about 10 days. On examination, however, the burner well contained a large deposit of a black brittle, carbon-like material, with no evidence of a build-up elsewhere in the system. Test 4 was a repeat of Test 3 in terms of the source of fuels used for blending. However, the kerosene and biodiesel were mixed directly in the tank in order to remove the fuel transfer pump from the preparation process, and only 20 litres of a 5% blend was made up. The cooker was shut down manually after about 2½ days of running and allowed to cool before being inspected. This test further confirmed
the results of test three, with a significant build-up being found in the burner well after just a few days, but no where else in the system. 4. Discussion To understand the mechanism by which debris was being formed, a number of factors were considered. This first of these is the disparity between the flashpoints of biodiesel and kerosene, and the effect this could have on the flashpoint of the blended fuel. The fuel flashpoint is significant for fuel burning equipment such as vaporising burners because it defines the lowest temperature at which a liquid will give off sufficient vapour to ignite in air. Krishnakumar et al., [3] gave a value of 183 C for the flashpoint of pure biodiesel made from rice bran oil, and 128 C for biodiesel made from jatropha oil. These values represent typical extremes for biodiesel made from different feedstocks. The flashpoint of the pure biodiesel obtained from Argent Energy was 168 C. By comparison, the minimum flashpoint of class 2 kerosene is 38 C, according to the British Standard BS 2869:2006. Although the flashpoint for the 5% blended fuels was not determined for these tests, Krishna [1] found that even a 50% blend of soybean methyl ester in kerosene raised the flashpoint by only 7 C compared to neat kerosene. A 5% blend would therefore have had a negligible effect on the flashpoint of the fuel tested in a vaporising burner. Another potential factor that may have contributed to the buildup of debris is contamination of the blended test fuel with metal
Fig. 2. FT-IR of (from top to bottom) raw sample, sample washed in DCM, sunflower oil reference and kerosene.
504
C.D. Barnes et al. / Energy 35 (2010) 501–505
Burner well 241 / 190
Feed pipe
313 / 244 142 / 104
65 / 55
62 / 56
285 / 221
Fig. 3. Average oil feed temperatures ( C) burning kerosene at low fire/high fire (Source: Aga Rayburn).
ions, which are known to catalyse the polymerisation of fuels like kerosene. An analysis of the residue from Test 1 was performed at the University of Glamorgan using gas chromatography (GC) mass spectrometry. The results suggested that contamination by iron may have precipitated the build-up. In fact the hand pump used to transfer the bio-kerosene to the Aga supply tank was found to be contaminated by rust, and an effort was made to drain the contaminated fuel from the lines before the first test began. The hand pump was cleaned prior to preparing the nest fuel batch for the third test run, and was eliminated altogether from the fourth test by blending directly in the supply tank. Metal ion contamination was thus ruled out as a contributory factor in debris formation for these tests. It should be noted that the Aga was run on the pure kerosene obtained from Oil Direct Limited between Tests 2 and 3, with no build-up whatsoever observed after several days of running. Samples of the build-up from Test 4 were analysed, this time by Minton, Treharne and Davies, an independent international fuels testing laboratory in Cardiff, Wales. The results of Fourier Transform – Infra-red spectroscopy are shown in Fig. 2. Of particular significance is the signal at 1740 cm1, present in all but the kerosene trace and thus indicative of debris that is vegetable in nature. This suggests that the biodiesel was polymerising. A profile of the heat distribution in the feed-pipe and across the base plate in a sleeve-type vaporising burner (shown in cross-section in Fig. 3) suggests that heat appeared to be the mechanism by which such
100% 90% 80% 70% 60% 50% 40% 30% 20% 10%
ut
C
C
oc on
Pa lm
d
ot to
La r
ns ee
d
an be
e O liv
or n C
w er flo
er Su n
ow Sa ffl
C
an ol
a
0%
So y
349 / 291
polymerisation was taking place. This theory is made plausible by the fact that debris was found only in the burner well or the channels that hold the burner wicks, where temperatures are greatest. Note the steep temperature gradient between the outer temperature reading in the feed-pipe and the entrance to the burner well, a distance of only about 15 cm. Literature searches have so far not revealed any similar work on the use of biodiesel for vaporising burners, and so a perspective on the current issue has not been found. However, fuel polymerisation is not a new problem. It has been reported in the fuel process industry, where it can lead to losses in production and increased maintenance costs. Biodiesel itself can in fact be more susceptible to oxidation and polymerisation than petroleum-based fuels, and this has led to problems with fuel acidity and corrosion of system components, and the formation of sediments which can block fuel nozzles and filters when used in compression-ignition engines [4,5]. This may be due partly to the fatty acid composition and thus properties of the source oil used as feedstock for the biodiesel, and partly to the degree of prior utilisation of the feedstock oil in cooking operations. Using data from McCance and Widdowson [6], Fig. 4 shows a number of common oils and their fatty acid composition. Those oils with significant levels of unsaturated fatty acids would be expected to oxidise and polymerise more readily than those oils which are largely saturated. Rapeseed and sunflower oils fall into this category. At the other end of this scale are oils such as palm oil and coconut oil, the latter in particular consisting predominantly of saturated fatty acids. It should be noted that the process of transesterification does not significantly alter this composition [7,8]. All the biodiesel used in tests was manufactured from waste cooking oil, though the exact source for any given batch is not recorded. According to government statistics [9], in the UK most cooking oil is derived from either rapeseed oil (canola) or palm oil (see Fig. 5). Fig. 5 indicates annual production output, but an overall breakdown of actual oil use in the UK is not available. However, a study prepared for DEFRA and the Food Standards Agency in 2003 by LMC International [10] used figures from the USA to predict oil use in Europe. These figures indicated that the largest consumption of oil is likely to be in the salad and cooking sector, which in Europe is generally dominated by rapeseed and palm oil, followed by sunflower and olive oils. The waste cooking oil collected for use in
Saturated
Monounsaturated
Polyunsaturated
Fig. 4. Fatty acid composition of common fats and oils (Adapted from McCance and Widdowson, 2002).
C.D. Barnes et al. / Energy 35 (2010) 501–505
5. Conclusions
Palm Kernel 1% Soybean 4%
505
Sunflower 0%
Other 8%
Coconut 11% Rapeseed 45%
Palm 31% Fig. 5. Share of Oils and Fats used in the Production of Refined Vegetable Oil in the UK in 2001 (DEFRA, 2007).
biodiesel production in the UK is thus likely to consist predominantly of the former two oils. The implication is that the waste vegetable oil used as feedstock for the test biodiesel most likely contained a high percentage on unsaturated rapeseed oil. As noted above, the degree of utilisation of the feedstock also plays a part in stability. Ferrari et al [8] looked at the oxidative stability of neutralised, refined and waste soybean oils, and partially hydrogenated frying waste, using the RancimatÒ stability test. Although not measured, the presence of natural anti-oxidants, such as tocopherol, was considered a major factor in the stability of the different quality oils. Thus the neutralised soybean oil presented the greatest stability, owing to the presence of natural anti-oxidants. The biodiesel from the waste soybean oil was the least stable. This was considered to be due to the fact that the used oil had already suffered oxidative and hydrolytic degradation as a result of the cooking process, which effectively destroys the oil’s natural anti-oxidants. The partially hydrogenated waste oil contained a greater percentage of saturated fatty acids than any of the other oils, which should have resulted in a higher oxidative stability of the resulting biodiesel. However, its poor stability was attributed to the greater negative impact of the frying process to which it had been subjected. The oxidation stability result of the biodiesel obtained from Argent Energy was 9 h, compared to the required minimum of 6 h, and was thus well within the requirement set out in BS EN 14214. The oxidative stability result for the test biodiesel would therefore appear to have little significance in the context of this study.
The use of biodiesel blended with kerosene in a vaporising burner, even in concentrations as low as 5% by volume, has been shown to cause fouling and eventual blockage of the burner within a short period of time. Given the circumstances under which fouling of the burner took place during testing, and the short timescales involved, it would appear that the principle mechanism for the build-up of debris was polymerisation of the biodiesel, triggered by heat. However, other sources of biodiesel instability cannot be ruled out with longer term use. Though the use of biodiesel made from unused refined oils composed of saturated fatty acids may mitigate the problem, this is not a sustainable option. A more logical approach to further work in this area is to investigate the use of anti-oxidants and anti-polymerisation additives, which are already extensively used in the petroleum industry. Acknowledgements The authors would like to thank the EPSRC for funding this work via an Industrial CASE studentship, along with Aga Rayburn Ltd who provided the test facilities and gave much technical assistance during testing. References [1] Krishna CR. Biodiesel blends in space heating equipment. National Renewable Energy Laboratory; 2004. NREL-SR-510–33579. [2] Mushrush G, Beal EJ, Spencer G, Wynne JH, Lloyd CL, Hughes JM, et al. An Environmentally benign soybean-derived fuel as a blending stock or replacement for home heating oil. Journal of Environmental Science and Health 2001;36(5):613–22. [3] Krishnakumar J, Venkatochalapathy VSK, Elancheliyan S. Technical aspects of biodiesel production from vegetable oils. Thermal Science 2008;12(2):159–69. [4] Clark SJ, Wagner L, Schrock MD, Plennaar PG. Methyl and ethyl soybean esters as renewable fuels for diesel engines. Journal of the American Oil Chemists’ Society 1984;46(4):945–54. [5] Monyem A, Van Gerpen JH. The effect of biodiesel oxidation on engine performance and emissions. Biomass and Bioenergy 2001;20:317–25. [6] McCance, Widdowson. The composition of foods. Cambridge: The Royal Society of Chemistry; 2002. [7] Darnoko D, Cheryan M. Kinetics of palm oil transesterification in a batch reactor. Journal of the American Oil Chemists’ Society 2000;77(12):1263–7. [8] Ferrari R, Oliveira V, da Silva, Scabio A. Oxidative stability of biodiesel from soybean oil fatty acid ethyl esters. Scientia Agricola (Piracicaba, Braz.) 2005;62(3):291–5. [9] Department for Environment. Food and rural affairs national statistics. Output of refined, deodorised vegetable, marine oils and animal fats by UK processing plants. Downloaded from, http://statistics.defra.gov.uk/esg/datasets/histoils; 2007 [accessed 23.05.08]. [10] Department for Environment. Food and rural Affairs and the Food Standards agency. Supply chain impacts of further regulation of products consisting of, containing, or derived from, genetically modified organisms. LMC International; 2003.