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Experimental study of the combustion of natural gas and high-hydrogen content syngases in a radiant porous media burner
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 2
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Experimental study of the combustion of natural gas and high-hydrogen content syngases in a radiant porous media burner Carlos E. Arrieta, Alex M. Garcı´a, Andres A. Amell* Grupo de Ciencia y Tecnologı´a del Gas y Uso Racional de la Energı´a, Facultad de Ingenierı´a, Universidad de Antioquia, Calle 67 N 53-108, Bloque, 19-000, Medellı´n, Colombia
article info
abstract
Article history:
The primary objective of this work is to study the blending of natural gas in equimolar
Received 23 January 2017
proportions with three high hydrogen content syngases in a radiant porous media burner.
Received in revised form
We examined the effects of the composition of the syngases, the fuel-to-air ratio and the
11 March 2017
thermal input on the flame stability, the radiation efficiency and the pollutant emissions
Accepted 11 March 2017
(CO and NOx). In this study, we emulated the syngases with H2eCO mixtures, in which the
Available online xxx
H2 to CO ratio was varied between 1.5 and 3. Additionally, pure natural gas was also used as a base fuel for comparison. The thermal inputs evaluated in this study correspond to two
Keywords:
values (300 and 500 kW/m2) found in practical applications. The results indicate that the
High-hydrogen content syngases
thermal input and the fuel-to-air ratio significantly influenced the temperature profile in
Natural gas
the radiant porous media burner, the radiation efficiency, and the pollutant emissions. On
Combustion in a radiant porous
the other hand, contrary to what was observed in other studies for lower hydrogen con-
media burner
centrations, we found that substituting natural gas with high hydrogen content syngases
Fuel interchangeability
(up to 50%) affected the flame stability limits. Significant differences were also observed for the radiation efficiencies and pollutant emissions. © 2017 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
Introduction During past decades, the growing awareness of environmental issues and the strong dependence of industrial activities on fossil fuels have motivated investigations of renewable, environmentally friendly and secure fuel supplies [1e3]. According to many authors, Syngas (synthetic gas, SG) is one of the most promising alternative fuels in many countries [4e8] because its use not only contributes to energy saving and emission mitigation but also facilitates the transition to the
hydrogen economy [9,10]. SG is a gas mixture that contains varying amounts of H2, CO and some inert components, such as N2 and CO2. SG can be obtained from steel production, in the thermal gasification of biomass/coal, or in municipal waste landfills. One example where the use of syngas has shown an enormous potential as an alternative way to reduce pollutants is in the electrical and power generation process in an Integrated Gasification Combined Cycle (IGCC) [11,12]. However, depending on the type of reactor and the gasifying agent, syngas generally has lower heating values between 1.0 and ~2.6 kWh/m3 and Wobbe index values between 1.5 and
* Corresponding author. E-mail addresses: [email protected], [email protected] (C.E. Arrieta), [email protected] (A.M. Garcı´a), [email protected] (A.A. Amell). http://dx.doi.org/10.1016/j.ijhydene.2017.03.078 0360-3199/© 2017 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. Please cite this article in press as: Arrieta CE, et al., Experimental study of the combustion of natural gas and high-hydrogen content syngases in a radiant porous media burner, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.078
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Nomenclature NG SG f PO SL CO2
Natural gas Synthesis gas or syngas Fuel-to-air ratio (equivalence ratio) Thermal load [kW/m2] Adiabatic laminar burning velocity [m/s] max Maximum CO2 fraction at stoichiometric conditions ðCO=CO2 Þsample Ratio of CO to CO2, as measured by the analyzer Radiation efficiency nR ε Emissivity of the burner surface s StefaneBoltzmann constant, s ¼ 5:6704 108 W=m2 K4 Tsup Temperature at the surface of the burner [K]
~4 kWh/m3, which are very low compared to the typical values for natural gas (9.425 kWh/m3 and 14.09 kWh/m3, respectively). From a technical point of view, this indicates that the use of pure syngases in conventional technologies may present difficulties associated with flame stability, combustion efficiency, and reliability. Thus, there is a strong global effort to burn mixtures of SGs and conventional fuels, such as natural gas, NG [13,14]. Systems that include preheating of the reactant mixture have been shown to increase the stability range and facilitate effective combustion in these fuel mixtures [15,16]. One way of designing a burner with internal heat recirculation is by stabilizing a premixed flame inside an inert porous solid matrix, a technique that is known as submerged combustion. A widely accepted name for the burners that implement this technique is radiant porous media burners, RPMBs [16e18]. In a RPMB, the combustion takes place through all the cavities of a porous solid matrix, which makes the combustion process differ substantially from the homogeneous oxidation that occurs in a free flame. In the latter, there is a low heat transfer from the reaction zone to the upstream fuel-air mixture, which is caused by the poor heat transfer properties that characterize gaseous mixtures, especially at lean conditions [19]. As a consequence, the oxidation process in a free flame takes place in a thin reaction zone. By including a porous solid material in the reaction zone, the overall heat transfer coefficient is improved due to the high thermal properties of solid materials compared with gases [19,20]. Thus, part of the heat released in the combustion process is conducted and radiated through the solid phase of the porous matrix to the upstream fuel-air mixture. Which improves the stability of the oxidation process. A series of experimental studies has been performed to promote the use of RPMBs in conjunction with SGs. For example, Keramiotis et al. [21] evaluated a rectangular twolayer porous burner over a range of thermal loads from 200 to 800 kW/m2 under lean combustion regimes. They studied an equimolar blend of NG and SG, where the H2/CO ratio was 0.7. They found that the burner delivered wide power flexibility at lean combustion regimes with low total pollutant
emissions (NOx of less than 50 ppm). In addition, they found that the thermal load and equivalence ratio primarily determine the temperature levels in the porous media, whereas, the fuel variation systematically, but rather unimportantly, influences the thermal performance of the burner. Very recently, Huang et al. [22] numerically and experimentally investigated the performance of an RPMB that was operating with three low calorific fuels, which include one low and one high hydrogen content fuel with H2/CO ratios of 0.2 and 0.7, respectively. These authors also found that the burner performance was not affected by fuel variations and the pollutant emission were very low (NO ~2.0 mg/Nm3, and CO ~150 mg/ Nm3). Francisco Jr. et al. [23] evaluated several blends of CH4 and SG in a RPMB, where the adiabatic flame temperature for all fuel mixtures was kept constant. Their experimental setup considered a SG with a H2/CO ratio of 0.3 and the CH4 composition varied from 0 to 100%. It was found that the stability limits do not vary significantly for SG concentrations lower than 60% in the fuel mixture, while higher concentrations enlarge the stability range of the burner. Additionally, these authors reported that the macroscopic flame shape of the reacting mixtures did not vary significantly as compared to CH4. Pollutant emissions, on the other hand, were found to be significantly decreased with the increasing hydrogen enrichment level. Later, in an experimental setup that considered a RPMB in a confined heated environment, Francisco Jr. et al. [24] found the same trend as with an open environment [23]. Alavandi et al. [25] investigated the combustion of syngas/methane fuels in a two-section porous burner. The CH4 content in the fuel was varied from 100% to 0%, with the remaining amount split equally between H2 and CO (H2/CO of 1). The results showed that RPMBs are effective in burning syngas fuels, and additionally lower CO and NOx emissions were obtained in mixtures containing H2/CO as compared to pure CH4. These authors also reported that the reduction in pollutant emissions in mixtures containing H2/ CO was more significant at higher temperature levels (higher equivalence ratios). These results in RPMB seems to be in agreement with those obtained in other advanced combustion technologies and more fundamental studies, where it has been also demonstrated that the H2/CO ratio has great influence on NOx emissions. For example, Samiran et al. [26] evaluated the combustion performance of several SGs with H2/CO ratios of 1.2 and 3 in a premixed swirl flame combustor. They found that increasing the hydrogen fraction in the fuel leads to lower NOx emissions, which are, at the same time, highly dependent on the equivalence ratio. Similarly, Ouimette et al. [27] showed that the use of higher H2 concentration SGs with H2/ CO ratios higher than 1.3 compared to SGs with a ratio of 0.8 resulted in a significant reduce in NOx emissions. However, on the other hand, Lee et al. [28] reported that fuels with high hydrogen content (up to 20%COe80% H2) emit more NOx. They evaluated a 60 kW turbine combustor at lean conditions (f ¼ 0.481 to 0.616) using methane and four different SGs with H2/CO ratios ranging from 0.25 to 4. Although it was found that high hydrogen content SGs improve CO emissions and do not generate combustion pulsation unlike methane, the NOx emissions were reported to increase with both the thermal load and H2/CO ratio.
Please cite this article in press as: Arrieta CE, et al., Experimental study of the combustion of natural gas and high-hydrogen content syngases in a radiant porous media burner, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.078
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It is worth mentioning that all the studies on RPMBs have been performed using H2/CO ratios lower than 1, as described above. However, some studies show that it is possible to obtain SGs with hydrogen contents that exceed this value. For example, Hermann et al. [29] demonstrated a combined heat and power (CHP) plant using wood chips as fuel. They reported that after the gas cleaning system, the producer gas, which was obtained in a dual fluidized bed steam gasifier, had H2/CO ratios in the range of 1.5e1.75. Ouyang et al. [30] introduced a new process for SGs by coal gasification coupled with natural gas reforming. They implemented a moving bed as the reactor, coke as the raw material and investigated the effect of blowing H2O/CH4/O2 in the flame zone. It was found that with this process the content of H2 and CO is more than 90%, while the H2/CO ratios can be adjusted in the range of 1e2. Yoon et al. [31] implemented microwave plasma gasification using steam and air plasma torch. They found that when pure steam is used as the plasma-forming gas, it is possible to produce SG with high H2 content exceeding 60% with an H2/CO ratio greater than 3. On the basis of the current state-of-the-art applications in the field of SG production, this work attempts to characterize high hydrogen content SGs and the blending of SGs with natural gas in equimolar proportions in a typical RPMB configuration. In this paper, we examine the effects of the SGs composition, the fuel-to-air ratio and the thermal input on the flame stability, the radiation efficiency and the pollutant emissions (CO and NOx). The SGs were emulated wit H2-CO mixtures, in which the H2 to CO ratio was varied between 1.5 and 3 (60% H2 to 75% H2, respectively). The thermal inputs evaluated in this study correspond to two values (300 and 500 kW/m2), which are found in practical applications and the measurements were performed for fuel-lean mixtures due to RPMBs relevance to lean-burning applications [19].
Experimental setup and methodology Fig. 1 is a schematic of the experimental configuration that was used to measure the temperature profiles inside the
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porous media and the concentrations of the pollutant emissions. The experimental setup consisted of six components: rotameters (a), a mixing chamber (b), the burner structure (d), thermocouples (e), the porous media (f) and a sampling probe for continuous monitoring of the emission (g). The air was supplied by an air compressor and dried using two inline water traps. Moreover, high-purity gases (99% purity) were used to simulate the SGs (H2 þ CO) and natural gas was obtained from a local distributor. Table 1 lists the volumetric compositions of these gases. The blends are denoted by the substitution percentage followed by the SG used in each blend. The substitution percentage corresponds to a natural gas substitution of 50% (equimolar blend). The air-to-fuel ratios and thermal input were controlled using the rotameters, which were specifically calibrated for each gas component in a similar manner as those used in Ref. [14]. The fuels and air were mixed in the mixing chamber and the errors in the final composition were estimated to be lower than 2%. The burner design corresponds to a typical two-layer porous media burner, which is similar to those used by Ref. [18]. Specifically, the burners consist of two sections: a small-pored upstream section where the incoming gas mixture is preheated (the preheat section) and a large-pored downstream section where combustion takes place (the combustion section). The preheat section also serves as a flame holder. In this paper, we describe only the geometric properties of the porous media that were used in the present work, and a more detailed explanation of the combustion and heat transfer process inside an RPMB can be found in Ref. [20]. The combustion section is a piece of silicon carbide (SiC) foam with a pore size of 10 ppi and a porosity of 90%. The preheat section is a piece of alumina (Al2O3) ceramic with a porous diameter of 1 mm and a porosity of 3%. The porous media are cylindrical with a diameter of 70 mm. The thickness of the combustion and the preheat sections are 20 mm and 19 mm, respectively. A detailed description of the thermal properties of the material can be found in Ref. [32]. In Fig. 1, the preheating and combustion sections are schematically represented with blue and red colors, respectively. The porous media were insulated using a ceramic mat.
Fig. 1 e Schematic of experimental configuration used to measure temperature profiles inside porous media and pollutant emission concentrations: a-rotameters, b-mixing chamber, c-air-fuel mixture, d-burner, e-thermocouples, f-porous media, g-probe, h-combustion products, i-image of the burner, and j-insulating mat. Please cite this article in press as: Arrieta CE, et al., Experimental study of the combustion of natural gas and high-hydrogen content syngases in a radiant porous media burner, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.078
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Table 1 e Volumetric compositions. NG
SG1
SG3
50SG1
50SG2
50SG3
CH4 C2H6 C3H8 n-CxHy i-CxHy CO2 N2
94.68 2.37 0.81 0.15 0.16 0.56 1.27
e e e e e e e
SG2 e e e e e e e
e e e e e e e
47.34 1.19 0.40 0.08 0.08 0.28 0.63
47.34 1.19 0.40 0.08 0.08 0.28 0.63
47.34 1.19 0.40 0.08 0.08 0.28 0.63
H2 CO
e e
60 40
66.7 33.7
75 25
30.00 20.00
33.35 16.85
37.50 12.50
The temperature profiles within the porous media were measured using three 33-mm wire-diameter type-S thermocouples for the combustion zone, three type-K thermocouples for the preheating zone and one 33-mm wire-diameter type-S thermocouple for the interface between the combustion and preheating zones. The thermocouples were positioned along the centerline of the burner, as shown in Fig. 1. From the preheated section inlet, the thermocouples were located at 3 mm, 10 mm, 14 mm, 19 mm, 25 mm, 30 mm, and 36 mm. Thermal equilibrium was established among the thermocouple hot junction, the gas, and the solid phase; thus, the measurements provided by these sensors should be interpreted as the mean temperatures of the gas and solid phases with a relative measurement error of 1%. With respect to the radiation efficiency, the different radiation sources that are present in an RPMB are the flame, the porous media, and the exhaust gases. Although radiation from the exhaust gases is generally important, we did not consider it because no measurements were performed downstream from the burner due to the lack of a confinement system. Additionally, since it has been shown that the SiC porous structure radiates in a similar manner as a black body [21], for the purpose of this work, it is more practical and less cumbersome to consider only the radiation from the porous media. To accomplish this, we measured the surface temperature of the porous media using an S-type thermocouple, as performed by Keramiotis et al. [33], who found that the thermocouple reading is in agreement with the surface temperature measured by a radiation thermometer. The radiation efficiency values were calculated using Equation (1):
nR ¼
εs T4sup T4∞ Po
(THERMO SCIENTIFIC 42i-HL analyzer). The relative measurement errors were 6% for CO, 3% for CO2, 3% for O2 and approximately 5% for NOx. Air dilution of the samples required the adjustment of the CO emissions to yield air-free measurements. This adjustment was performed by substituting the measured CO2 emissions into the following equation. COairfree ¼ CO2 max ðCO=CO2 Þsample
(2)
All the pollutant and temperature measurements presented in the results section were obtained under stable conditions, i.e., conditions where we did not obtain flashback or flame blowout. For every operating point, we started the system at room temperature and waited until the system reached a steady state, which normally occurred after 60e75 min. Every operating point was repeated three times and the mean values of pollutants and temperatures were calculated at each point. Finally, this study was performed at atmospheric conditions of 298 K and 0.849 bar, which correspond to the local environmental conditions of the city of Medellin, Colombia.
Results and discussion In this section, the experimental results are presented and discussed in three subsections, which correspond to the main subjects of this work, i.e., flame stability, radiation efficiency, and CO and NOx emissions.
Flame stability Fig. 2 shows the flame stability limits, which correspond to NG and the NG-SGs blends. The figure represents several operating conditions that were evaluated. To obtain this operation diagram, we also include operating conditions at 400 kW/m2. Three types of operating conditions can be observed in the
(1)
In Equation (1), ε is the emissivity of the burner surface, which is assumed to be equal to that of the unit [23]. s is the StefaneBoltzmann constant, and Tsup was measured by the thermocouple placed on the burner surface. A water-cooled sampling probe was placed along the centerline 5 cm above the burner (as observed in Fig. 1) to measure the O2, CO, CO2, and NOx concentrations in the combustion products. The sample was cleaned and dried before it reached the analyzers. The analytical instrumentation included a paramagnetic analyzer for the O2 measurements, non-dispersive infrared gas analyzers (SICK MAIHAK S710 analyzers) for the CO and CO2 measurements and a chemiluminescent analyzer for the NOx measurements
Fig. 2 e Flame stability diagram. BL: blowout limit. FL: flashback limit.
Please cite this article in press as: Arrieta CE, et al., Experimental study of the combustion of natural gas and high-hydrogen content syngases in a radiant porous media burner, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.078
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figures: the blowout region (to the left of the blowout limit, BL), the flashback region (to the right of the flashback limit, FL) and the stable region (between the BL and FL). In this work, NG was considered the base case, and everything related to it is represented in black color. We also included dashed arrows to show how the BL and FL change with respect to the base case when the fuel is varied. Although we labeled the right limit of the NG as though it were a flashback region above f ¼ 1, we actually did not obtain flashback operation with NG. f ¼ 1 corresponds to the higher equivalence ratio that was evaluated. However, we decided to use this label to avoid the introduction of more abbreviations. As is shown in Fig. 2, the operation of the burner at f near unity was characterized by the occurrence of flashback when the NG was substituted by SG, especially under the highest thermal loads and for the highest hydrogen concentrations. At 500 kW/m2, flashback occurred from f ¼ 0.9 for SG1 and f ¼ 0.85 for SG2 and SG3, while operating with NG did not result in flashback, as was previously mentioned. On the other hand, it was observed that the BL increased at the lowest equivalence ratios. This indicates that the operating conditions change from fuel to fuel when NG is substituted by the high hydrogen content SG. However, considering that the range of stable operating conditions is very similar for all the fuels, it is also possible to conclude that if slight modifications of the combustion air flow are allowed, the burner exhibits a similar stability diagram for all the reactive mixtures. In particular, no variation was found between SG2 and SG3. Very recently it was shown that, generally, the turbulence in an RPMB is not enough to affect the laminar burning velocity, temperature distribution, and convective heat transfer [34]. Therefore, to understand the trends shown in Fig. 2, we characterized the flows considering them to be in the laminar flow regime, also taking into account the laminar burning velocity (SL). The value SL, as indicated by Law [35], contains the physico-chemical information of the mixture and therefore many premixed flame phenomena, such as flashback and blowout, can be characterized using SL as a reference parameter. NG (100% CH4) is a well-known fuel and there are many studies that report its laminar burning velocity, both numerically and experimentally. However, experimentally derived laminar burning velocities of the SGs-air mixture compositions evaluated in this work were not available, therefore it was necessary to calculate them. Fig. 3 shows the numerical results for SL for NG and the blending with the SGs. These numerical results were obtained using the same numerical methodology implemented in previous studies [36e39], using the GriMech 3.0 reaction mechanism [40], and the PREMIX code from the CHEMKIN-PRO package at ambient temperature (298 K) and pressure (0.849 bar). To verify that the numerical results for the blending with the SGs were acceptable for comparison with NG, we included experimental results reported for similar mixture compositions. The experimental results include those reported by Park et al. [41] (CH4 @ 1.013 bar), Cardona et al. [42] (CH4 @ 0.849 bar), Garcı´aArmingol et al. [43] (50CH4e50H2), Hermanns [44] (60CH4e40H2), and Boushaki et al. [45] (70CH4e30H2). It can be observed that the results obtained for NG are in good agreement with the experimental data. Furthermore, the
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Fig. 3 e Laminar burning velocities for NG and the blending with SGs.
results obtained for the blending with the SGs seem to be acceptable for the comparative purposes of this work. Now, with SL, another way to present the flame stability diagram is as it is shown in Fig. 4, where the ratio of SL to the velocity of the unburned mixture (VU) is plotted on the ordinate axis. One fact that is clear from this figure is that the BL and blowout regions for all the mixtures appear at values of SL/VU below 1. Moreover, from experimental observation, it seems that based on the value of this ratio, the flame propagation and stabilization after the ignition above the burner surface can be divided into up to 3 stages, as illustrated in Fig. 5. First, in those cases where Vu is slightly higher than SL, the flame propagates until it reaches the surface of the combustion zone, as is shown in Fig. 5-1. This is the case for all operation conditions above the BLs and below SL/VU ¼ 1. After this first stage, the flame heats up the porous media and the unburned mixture is preheated upstream [46]. After some
Fig. 4 e Flame stability diagram. The 50SG2 blend is not shown to improve the readability of the data.
Please cite this article in press as: Arrieta CE, et al., Experimental study of the combustion of natural gas and high-hydrogen content syngases in a radiant porous media burner, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.078
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time, which is longer for operation conditions near the BL, it seems that the laminar burning velocity increases (stage 2) and at some point the flame is able to penetrate the porous media (stage 3). For operation conditions where 1 < SL/VU < 3, the stabilization process started at stage 2 and for those cases where SL/VU was greater than 3, the flame penetrated the porous media immediately following ignition. On the other hand, in the blowout regime, the flames were not able to complete stage 1 and blew out after ignition. In Fig. 4, for the blending with SGs, we also included the increase in the convective heat transfer coefficient (percentage) when, at certain operation conditions (thermal load and equivalence ratio), the NG was substituted by a SG. The convective heat transfer coefficients (h) were calculated following the methodology presented in Ref. [47]. It is observed that for a given operation condition, the SG concentration in the fuel mixture not only increases the SL to VU ratio but the convective heat transfer coefficients also. As a result, the SGs with higher hydrogen content exhibit a higher flame stability under lean conditions. The 50SG2 blend is not shown to improve the readability of the data. On the other hand, at conditions near f ¼ 1, the increase in SL has a negative effect, and this effect is higher for the SGs with higher hydrogen content (SG2 and SG3).
Temperature and radiation efficiency Fig. 6 shows the temperature profiles obtained in the RPMB for f ¼ 0.7 and f ¼ 1.0 for the operating conditions at 300 kW/m2 and 500 kW/m2 using NG. The distances shown in Fig. 6 were measured across the porous media, where 0 mm corresponds to the inlet of the preheat section and 39 mm corresponds to the outlet surface. Generally, as expected, the temperature increased significantly as the mixture passed through the RPMB. The temperature profiles and the behavior of the profiles when the equivalence ratio and thermal load were modified are in good agreement with results reported in other works for NG [33,48]. When either f or the thermal load is increased, the temperature level in the RPMB increases. The increase in the temperature level as f increases is due to the increase of the adiabatic flame temperature (Tad), while the
Fig. 6 e Temperature profiles within RPMB for f ¼ 0.7 and f ¼ 1.0 for the operating conditions at 300 kW/m2 and 500 kW/m2 using NG.
increase in the temperature level as the thermal load increases is due to the increase in the released energy. Fig. 7 shows the temperature profiles obtained for the different fuels in the stable region. The behavior of the temperature profile when NG was substituted with the high hydrogen SGs in the stable regions, or more precisely, under the same operating conditions, is similar to the behavior observed in other studies for SGs with lower hydrogen content [23]. The addition of high hydrogen content SGs to NG increases the Tad, as shown in Fig. 8, which includes the results obtained with the PREMIX and EQUIL code from the CHEMKINPRO package. This results in the slight difference in the temperature levels observed in Fig. 7. Additionally, because the maximum temperature always occurs in the thermocouple located 30 mm from the burner inlet, the position of the flame front of the blend fuels presumably does not change with respect to NG. As observed in Fig. 2, there is no clear difference between SG2 and SG3. Figs. 9 and 10 show the temperature profiles obtained for the different fuels at the BL and the FL. From Fig. 9, although
Fig. 5 e Flame propagation and stabilization after the ignition above the burner surface. Please cite this article in press as: Arrieta CE, et al., Experimental study of the combustion of natural gas and high-hydrogen content syngases in a radiant porous media burner, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.078
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Fig. 7 e Temperature profiles obtained for the different fuels at operating conditions in the stable region.
Fig. 8 e Adiabatic flame temperature for NG and the blending with SGs.
Fig. 9 e Temperature profiles obtained for the different fuels at the blowout limit.
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Fig. 10 e Temperature profiles obtained for the different fuels at the flashback limit.
the operating conditions are not the same, no significant difference in the temperature profiles was observed. At 500 kW/ m2 (solid lines), the temperature levels obtained with NG are slightly higher because the BL for NG was at a higher f than that for the SGs. At 300 kW/m2 (dashed lines), the reaction zone operating with NG presumably occurs near the outlet surface of the burner; however, during the experiments, it was not possible to observe any difference in the location of the reaction zone compared with that for the SGs. Similar results were obtained at the FL, as shown in Fig. 10. Fig. 11 shows the radiation efficiency that was obtained from the procedure described in section 2. Adding SG to NG increased the Tad, which simultaneously increased the temperature level of the solid phase and thereby, the radiation. When the hydrogen concentration was increased, the radiation efficiency increased. The highest radiation efficiencies were obtained for SG3 (~14%e~30%). Generally, it was observed that when f increased, the radiation efficiency increased. Additionally, the differences in radiation among the fuels also increased when f was increased. This is due to
Fig. 11 e Radiation efficiencies for the fuels at the operating conditions evaluated in this work.
Please cite this article in press as: Arrieta CE, et al., Experimental study of the combustion of natural gas and high-hydrogen content syngases in a radiant porous media burner, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.078
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the Tad behavior with respect to f and the hydrogen concentration, which is a non-linear relationship [39,49]. On the other hand, when the thermal load was increased, the radiation efficiency (the portion of the thermal input that is transferred to the load by radiation from the solid phase) decreased; this occurred although the temperature of the solid phase on the surface increased. This can be attributed to the increase in the gaseous phase velocity, which reduces the residence time of the gases in the RPMB. This makes it impossible to obtain temperature levels on the burner surface higher enough to increase radiation more than what the thermal load was increased. It is noteworthy that this effect is generally lower for SG2 and SG3, which are the fuels with higher hydrogen concentrations.
Fig. 13 e NOx emissions for the fuels at the operating conditions evaluated in this work.
Pollutant emissions Fig. 12 and 13 show the experimental pollutant emissions (COfree and NOx) for NG and the NG blended with the high hydrogen content SGs. Additionally, numerical results for NG are included as reference values. The simulations were performed using the same methodology described above for SL. To consider the heat transfer process from the combustion zone to the preheating zone, the simulations were initiated using the mean temperature of the preheating zone for each operation condition. Moreover, a heat loss condition by convection was applied to emulate the heat transfer from the gas to the solid phase in the combustion zone. The convective heat transfer coefficients were calculated as suggested by Younis et al. [50]. Generally, the CO emissions were less than 50 ppm. However, this value increased when the equivalence ratio was near either the FL or BL. Moreover, increasing the hydrogen concentration resulted in no significant change in the CO emissions for the different SGs. Generally, the CO emissions were between 13 and 79 ppm for the operating conditions with the SGs. However, two exceptions occurred for SG2 and SG3 at 300 kW/m2 at the BL, where the CO emission levels were ~136 and ~190 ppm, respectively. Considerable differences
(~150e~400 ppm) were observed only between the NG and the NG blended with SG when operating at 300 kW/m2 and f ¼ 0:6 or f ¼ 1. These trends are difficult to explain, however, some observations can be drawn by focusing on the set of chain branching and chain propagating reactions shown in Table 2, which are included in Gri-Mech 3.0. Considering only the experimental results, the trend of CO emissions near the BL can be explained considering the lower temperature registered for these operation conditions, which are shown in Fig. 9 and affect R3 and R4. It has been reported that these two reactions practically stop at low temperatures [51]. However, a different trend was observed in the numerical results. This numerical trend suggests that presumably, not only the temperature influenced the CO emissions, but also the residence time in the combustion zone, which is not consider by the numerical model. For a given fuel composition and thermal load, a decrease in the equivalence ratio implies an increase in VU. Therefore, it is possible that a short postcombustion zone is necessary, similar as reported by Alavandi et al. [25]. The complete conversion of CO to CO2 also depends on the availability of oxygen, as indicated by R4 and R5. However, as it has been also highlighted by Huang et al. [22], it is worth mentioning that the activation energy (Ea) of reactions involving OH (R3) are much lower than those with O or O2 (R4 and R3), which is valid not only for CO, but also for CH4 (R2) and H2 (R7). This observation implies that the OH is a determining factor in the consumption of the main fuel molecules considered in this work. Therefore, for a given operation condition, increasing the concentration of H2 (which is a common way to increase the pool of OH) improve the
Table 2 e Some elementary reactions involved in the consumption of CH4, CO and H2 [33]. Specie involved CH4 CO
Fig. 12 e CO emissions for the fuels at the operating conditions evaluated in this work.
H2
Elementary reaction
Ea (cal mol1)
CH4 þ O$CH3 þ OH (R1) CH4 þ OH$CH3 þ H2 O (R2) CO þ OH$CO2 þ H (R3) CO þ O2 $CO2 þ O (R4) CO þ O þ M$CO2 þ M (R5) H2 þ O$H þ OH (R6) H2 þ OH$H þ H2 O (R7)
8600 3120 70 47,800 2385 6260 3430
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combustion process. This would explain the difference between the NG and the blends with high hydrogen content SGs when either the temperature level in the burner is lower (near the BL) or the oxygen concentration decreases (near the FL). To illustrate what have been discussed above, Fig. 14 shows the net reaction rate of R1, R3, R4, R6 and R7 for NG, 50SG1 and 50SG3 at f ¼ 0:9. Additionally, the chain branching step H þ O2 $ OH þ O (R8) have been included due to its importance in OH production. It is observed, then, that as the H2/CO ratio increases, the net reaction rate increases significantly in those elementary reactions that include OH. With respect the thermal load, presumably, the CO emissions decrease when the thermal load increases, which can be attributed to the increase in the temperature levels in the RPMB. This trend is clear for the operating conditions with NG both numerically and experimentally. However, the thermal load shows no significant effect for the NG blended with SGs.
9
Fig. 13 shows that the NOx emissions were typically less than 1 ppm, which is in agreement with the results in Refs. [52] and [25]. Increasing both the equivalence ratio and the thermal load increases the NOx emissions because the Tad increases, as previously discussed. This trend is also observed in the numerical results. Surprisingly, at the highest thermal load, towards the BL the NOx emissions are lower for the NG, which is in agreement with other works performed at lean conditions [28]. On the other hand, near the FL the result indicate an opposite trend, which is also in agreement with other results obtained near stoichiometric conditions [27]. Generally, near the BL the NOx emissions increase linearly with increases in equivalence ratio, however in some point emissions increase exponentially. It is interesting that as the H2/CO ratio is increased this exponential trend decreases. This could be due to the increase of H radical and decrease of C radical, which reduce the
Fig. 14 e Net reaction rate of R1, R3, R4, R6, R7 and R8 for NG, 50SG1 and 50SG3 at f ¼ 0:9.
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formation of CH and CH2, and therefore retard the production of HCN and NOx [26].
Conclusions In this work, the performance of a typical radiant porous media burner operating with natural gas and the blending of natural gas with three high hydrogen content syngases in equimolar proportions was studied. We examined the effects of the composition of the syngases, the fuel-to-air ratio and the thermal input on the flame stability, the radiation efficiency and the pollutant emissions (CO). The H2 to CO ratio for the syngases was varied between 1.5 and 3. This study complements previous works that studied syngases with H2 to CO ratios of less than 1 [21e23]. The following conclusions can be drawn from the results obtained in this work: The results obtained for the flame stability indicates that the operating conditions change from fuel to fuel when NG is substituted by high hydrogen content SG. However, considering that the range of stable operating conditions is very similar for all the fuels, it is also possible to conclude that, if slight modifications of the combustion air flow are allowed, this burner exhibits considerable potential for the interchangeable use of the gases studied in this work. Compared to the use of pure natural gas, the addition of high hydrogen content syngases did not significantly affect the temperature profile inside the porous media and the flame stability. However, significant differences were observed in the radiation efficiencies and pollutant emissions. The highest radiation efficiency obtained in this work was 30% for the blends that contained the syngas with higher hydrogen concentration; the highest radiation efficiency obtained with natural gas was 18%. This can be attributed to the increase in both Tad and the convective heat transfer coefficient when the hydrogen concentration increased, which resulted in an increase in the temperature level of the solid phase and therefore, an increase in the radiation. In some cases, especially near the flashback limit and the blowout limit, adding high hydrogen content syngases to natural gas in equimolar proportions reduced the CO emission to 400 ppm. The following conclusions can be drawn from the variation in the equivalence ratio and thermal load. The equivalence ratio had a considerable effect on the flame stability, temperature profile, radiation efficiency and the pollutant emissions for all the fuels studied in this work. This is attributed to its effect on variables like adiabatic flame temperature, laminar flame speed, and mean flow velocity, which affect the burner performance. For the blends that contained syngases, the pollutant emissions were not significantly influenced by the thermal load. Generally, CO emissions between 13 and 79 ppm and NOx emissions lower than ~1.3 ppm were obtained with these fuels.
Acknowledgements n The authors thank COLCIENCIAS for financing the Unio n e Innovacio n en Combustio n Temporal de Investigacio Avanzada de Uso Industrial (UT. INCOMBUSTION), the project n de un Sistema de Combustio n “Desarrollo y Evaluacio Sumergida ” and the doctoral program 567. The authors also thank the Universidad de Antioquia for the valuable economic contribution to the development of this research through the program “Sostenibilidad 2014e2015 de la Vicerrectorı´a de n”. Investigacio
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Experimental study of the combustion of natural gas and high-hydrogen content syngases in a radiant porous media burner Carlos E. Arrieta, Alex M. Garcı´a, Andres A. Amell* Grupo de Ciencia y Tecnologı´a del Gas y Uso Racional de la Energı´a, Facultad de Ingenierı´a, Universidad de Antioquia, Calle 67 N 53-108, Bloque, 19-000, Medellı´n, Colombia
article info
abstract
Article history:
The primary objective of this work is to study the blending of natural gas in equimolar
Received 23 January 2017
proportions with three high hydrogen content syngases in a radiant porous media burner.
Received in revised form
We examined the effects of the composition of the syngases, the fuel-to-air ratio and the
11 March 2017
thermal input on the flame stability, the radiation efficiency and the pollutant emissions
Accepted 11 March 2017
(CO and NOx). In this study, we emulated the syngases with H2eCO mixtures, in which the
Available online xxx
H2 to CO ratio was varied between 1.5 and 3. Additionally, pure natural gas was also used as a base fuel for comparison. The thermal inputs evaluated in this study correspond to two
Keywords:
values (300 and 500 kW/m2) found in practical applications. The results indicate that the
High-hydrogen content syngases
thermal input and the fuel-to-air ratio significantly influenced the temperature profile in
Natural gas
the radiant porous media burner, the radiation efficiency, and the pollutant emissions. On
Combustion in a radiant porous
the other hand, contrary to what was observed in other studies for lower hydrogen con-
media burner
centrations, we found that substituting natural gas with high hydrogen content syngases
Fuel interchangeability
(up to 50%) affected the flame stability limits. Significant differences were also observed for the radiation efficiencies and pollutant emissions. © 2017 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
Introduction During past decades, the growing awareness of environmental issues and the strong dependence of industrial activities on fossil fuels have motivated investigations of renewable, environmentally friendly and secure fuel supplies [1e3]. According to many authors, Syngas (synthetic gas, SG) is one of the most promising alternative fuels in many countries [4e8] because its use not only contributes to energy saving and emission mitigation but also facilitates the transition to the
hydrogen economy [9,10]. SG is a gas mixture that contains varying amounts of H2, CO and some inert components, such as N2 and CO2. SG can be obtained from steel production, in the thermal gasification of biomass/coal, or in municipal waste landfills. One example where the use of syngas has shown an enormous potential as an alternative way to reduce pollutants is in the electrical and power generation process in an Integrated Gasification Combined Cycle (IGCC) [11,12]. However, depending on the type of reactor and the gasifying agent, syngas generally has lower heating values between 1.0 and ~2.6 kWh/m3 and Wobbe index values between 1.5 and
* Corresponding author. E-mail addresses: [email protected], [email protected] (C.E. Arrieta), [email protected] (A.M. Garcı´a), [email protected] (A.A. Amell). http://dx.doi.org/10.1016/j.ijhydene.2017.03.078 0360-3199/© 2017 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. Please cite this article in press as: Arrieta CE, et al., Experimental study of the combustion of natural gas and high-hydrogen content syngases in a radiant porous media burner, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.078
2
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Nomenclature NG SG f PO SL CO2
Natural gas Synthesis gas or syngas Fuel-to-air ratio (equivalence ratio) Thermal load [kW/m2] Adiabatic laminar burning velocity [m/s] max Maximum CO2 fraction at stoichiometric conditions ðCO=CO2 Þsample Ratio of CO to CO2, as measured by the analyzer Radiation efficiency nR ε Emissivity of the burner surface s StefaneBoltzmann constant, s ¼ 5:6704 108 W=m2 K4 Tsup Temperature at the surface of the burner [K]
~4 kWh/m3, which are very low compared to the typical values for natural gas (9.425 kWh/m3 and 14.09 kWh/m3, respectively). From a technical point of view, this indicates that the use of pure syngases in conventional technologies may present difficulties associated with flame stability, combustion efficiency, and reliability. Thus, there is a strong global effort to burn mixtures of SGs and conventional fuels, such as natural gas, NG [13,14]. Systems that include preheating of the reactant mixture have been shown to increase the stability range and facilitate effective combustion in these fuel mixtures [15,16]. One way of designing a burner with internal heat recirculation is by stabilizing a premixed flame inside an inert porous solid matrix, a technique that is known as submerged combustion. A widely accepted name for the burners that implement this technique is radiant porous media burners, RPMBs [16e18]. In a RPMB, the combustion takes place through all the cavities of a porous solid matrix, which makes the combustion process differ substantially from the homogeneous oxidation that occurs in a free flame. In the latter, there is a low heat transfer from the reaction zone to the upstream fuel-air mixture, which is caused by the poor heat transfer properties that characterize gaseous mixtures, especially at lean conditions [19]. As a consequence, the oxidation process in a free flame takes place in a thin reaction zone. By including a porous solid material in the reaction zone, the overall heat transfer coefficient is improved due to the high thermal properties of solid materials compared with gases [19,20]. Thus, part of the heat released in the combustion process is conducted and radiated through the solid phase of the porous matrix to the upstream fuel-air mixture. Which improves the stability of the oxidation process. A series of experimental studies has been performed to promote the use of RPMBs in conjunction with SGs. For example, Keramiotis et al. [21] evaluated a rectangular twolayer porous burner over a range of thermal loads from 200 to 800 kW/m2 under lean combustion regimes. They studied an equimolar blend of NG and SG, where the H2/CO ratio was 0.7. They found that the burner delivered wide power flexibility at lean combustion regimes with low total pollutant
emissions (NOx of less than 50 ppm). In addition, they found that the thermal load and equivalence ratio primarily determine the temperature levels in the porous media, whereas, the fuel variation systematically, but rather unimportantly, influences the thermal performance of the burner. Very recently, Huang et al. [22] numerically and experimentally investigated the performance of an RPMB that was operating with three low calorific fuels, which include one low and one high hydrogen content fuel with H2/CO ratios of 0.2 and 0.7, respectively. These authors also found that the burner performance was not affected by fuel variations and the pollutant emission were very low (NO ~2.0 mg/Nm3, and CO ~150 mg/ Nm3). Francisco Jr. et al. [23] evaluated several blends of CH4 and SG in a RPMB, where the adiabatic flame temperature for all fuel mixtures was kept constant. Their experimental setup considered a SG with a H2/CO ratio of 0.3 and the CH4 composition varied from 0 to 100%. It was found that the stability limits do not vary significantly for SG concentrations lower than 60% in the fuel mixture, while higher concentrations enlarge the stability range of the burner. Additionally, these authors reported that the macroscopic flame shape of the reacting mixtures did not vary significantly as compared to CH4. Pollutant emissions, on the other hand, were found to be significantly decreased with the increasing hydrogen enrichment level. Later, in an experimental setup that considered a RPMB in a confined heated environment, Francisco Jr. et al. [24] found the same trend as with an open environment [23]. Alavandi et al. [25] investigated the combustion of syngas/methane fuels in a two-section porous burner. The CH4 content in the fuel was varied from 100% to 0%, with the remaining amount split equally between H2 and CO (H2/CO of 1). The results showed that RPMBs are effective in burning syngas fuels, and additionally lower CO and NOx emissions were obtained in mixtures containing H2/CO as compared to pure CH4. These authors also reported that the reduction in pollutant emissions in mixtures containing H2/ CO was more significant at higher temperature levels (higher equivalence ratios). These results in RPMB seems to be in agreement with those obtained in other advanced combustion technologies and more fundamental studies, where it has been also demonstrated that the H2/CO ratio has great influence on NOx emissions. For example, Samiran et al. [26] evaluated the combustion performance of several SGs with H2/CO ratios of 1.2 and 3 in a premixed swirl flame combustor. They found that increasing the hydrogen fraction in the fuel leads to lower NOx emissions, which are, at the same time, highly dependent on the equivalence ratio. Similarly, Ouimette et al. [27] showed that the use of higher H2 concentration SGs with H2/ CO ratios higher than 1.3 compared to SGs with a ratio of 0.8 resulted in a significant reduce in NOx emissions. However, on the other hand, Lee et al. [28] reported that fuels with high hydrogen content (up to 20%COe80% H2) emit more NOx. They evaluated a 60 kW turbine combustor at lean conditions (f ¼ 0.481 to 0.616) using methane and four different SGs with H2/CO ratios ranging from 0.25 to 4. Although it was found that high hydrogen content SGs improve CO emissions and do not generate combustion pulsation unlike methane, the NOx emissions were reported to increase with both the thermal load and H2/CO ratio.
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It is worth mentioning that all the studies on RPMBs have been performed using H2/CO ratios lower than 1, as described above. However, some studies show that it is possible to obtain SGs with hydrogen contents that exceed this value. For example, Hermann et al. [29] demonstrated a combined heat and power (CHP) plant using wood chips as fuel. They reported that after the gas cleaning system, the producer gas, which was obtained in a dual fluidized bed steam gasifier, had H2/CO ratios in the range of 1.5e1.75. Ouyang et al. [30] introduced a new process for SGs by coal gasification coupled with natural gas reforming. They implemented a moving bed as the reactor, coke as the raw material and investigated the effect of blowing H2O/CH4/O2 in the flame zone. It was found that with this process the content of H2 and CO is more than 90%, while the H2/CO ratios can be adjusted in the range of 1e2. Yoon et al. [31] implemented microwave plasma gasification using steam and air plasma torch. They found that when pure steam is used as the plasma-forming gas, it is possible to produce SG with high H2 content exceeding 60% with an H2/CO ratio greater than 3. On the basis of the current state-of-the-art applications in the field of SG production, this work attempts to characterize high hydrogen content SGs and the blending of SGs with natural gas in equimolar proportions in a typical RPMB configuration. In this paper, we examine the effects of the SGs composition, the fuel-to-air ratio and the thermal input on the flame stability, the radiation efficiency and the pollutant emissions (CO and NOx). The SGs were emulated wit H2-CO mixtures, in which the H2 to CO ratio was varied between 1.5 and 3 (60% H2 to 75% H2, respectively). The thermal inputs evaluated in this study correspond to two values (300 and 500 kW/m2), which are found in practical applications and the measurements were performed for fuel-lean mixtures due to RPMBs relevance to lean-burning applications [19].
Experimental setup and methodology Fig. 1 is a schematic of the experimental configuration that was used to measure the temperature profiles inside the
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porous media and the concentrations of the pollutant emissions. The experimental setup consisted of six components: rotameters (a), a mixing chamber (b), the burner structure (d), thermocouples (e), the porous media (f) and a sampling probe for continuous monitoring of the emission (g). The air was supplied by an air compressor and dried using two inline water traps. Moreover, high-purity gases (99% purity) were used to simulate the SGs (H2 þ CO) and natural gas was obtained from a local distributor. Table 1 lists the volumetric compositions of these gases. The blends are denoted by the substitution percentage followed by the SG used in each blend. The substitution percentage corresponds to a natural gas substitution of 50% (equimolar blend). The air-to-fuel ratios and thermal input were controlled using the rotameters, which were specifically calibrated for each gas component in a similar manner as those used in Ref. [14]. The fuels and air were mixed in the mixing chamber and the errors in the final composition were estimated to be lower than 2%. The burner design corresponds to a typical two-layer porous media burner, which is similar to those used by Ref. [18]. Specifically, the burners consist of two sections: a small-pored upstream section where the incoming gas mixture is preheated (the preheat section) and a large-pored downstream section where combustion takes place (the combustion section). The preheat section also serves as a flame holder. In this paper, we describe only the geometric properties of the porous media that were used in the present work, and a more detailed explanation of the combustion and heat transfer process inside an RPMB can be found in Ref. [20]. The combustion section is a piece of silicon carbide (SiC) foam with a pore size of 10 ppi and a porosity of 90%. The preheat section is a piece of alumina (Al2O3) ceramic with a porous diameter of 1 mm and a porosity of 3%. The porous media are cylindrical with a diameter of 70 mm. The thickness of the combustion and the preheat sections are 20 mm and 19 mm, respectively. A detailed description of the thermal properties of the material can be found in Ref. [32]. In Fig. 1, the preheating and combustion sections are schematically represented with blue and red colors, respectively. The porous media were insulated using a ceramic mat.
Fig. 1 e Schematic of experimental configuration used to measure temperature profiles inside porous media and pollutant emission concentrations: a-rotameters, b-mixing chamber, c-air-fuel mixture, d-burner, e-thermocouples, f-porous media, g-probe, h-combustion products, i-image of the burner, and j-insulating mat. Please cite this article in press as: Arrieta CE, et al., Experimental study of the combustion of natural gas and high-hydrogen content syngases in a radiant porous media burner, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.078
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Table 1 e Volumetric compositions. NG
SG1
SG3
50SG1
50SG2
50SG3
CH4 C2H6 C3H8 n-CxHy i-CxHy CO2 N2
94.68 2.37 0.81 0.15 0.16 0.56 1.27
e e e e e e e
SG2 e e e e e e e
e e e e e e e
47.34 1.19 0.40 0.08 0.08 0.28 0.63
47.34 1.19 0.40 0.08 0.08 0.28 0.63
47.34 1.19 0.40 0.08 0.08 0.28 0.63
H2 CO
e e
60 40
66.7 33.7
75 25
30.00 20.00
33.35 16.85
37.50 12.50
The temperature profiles within the porous media were measured using three 33-mm wire-diameter type-S thermocouples for the combustion zone, three type-K thermocouples for the preheating zone and one 33-mm wire-diameter type-S thermocouple for the interface between the combustion and preheating zones. The thermocouples were positioned along the centerline of the burner, as shown in Fig. 1. From the preheated section inlet, the thermocouples were located at 3 mm, 10 mm, 14 mm, 19 mm, 25 mm, 30 mm, and 36 mm. Thermal equilibrium was established among the thermocouple hot junction, the gas, and the solid phase; thus, the measurements provided by these sensors should be interpreted as the mean temperatures of the gas and solid phases with a relative measurement error of 1%. With respect to the radiation efficiency, the different radiation sources that are present in an RPMB are the flame, the porous media, and the exhaust gases. Although radiation from the exhaust gases is generally important, we did not consider it because no measurements were performed downstream from the burner due to the lack of a confinement system. Additionally, since it has been shown that the SiC porous structure radiates in a similar manner as a black body [21], for the purpose of this work, it is more practical and less cumbersome to consider only the radiation from the porous media. To accomplish this, we measured the surface temperature of the porous media using an S-type thermocouple, as performed by Keramiotis et al. [33], who found that the thermocouple reading is in agreement with the surface temperature measured by a radiation thermometer. The radiation efficiency values were calculated using Equation (1):
nR ¼
εs T4sup T4∞ Po
(THERMO SCIENTIFIC 42i-HL analyzer). The relative measurement errors were 6% for CO, 3% for CO2, 3% for O2 and approximately 5% for NOx. Air dilution of the samples required the adjustment of the CO emissions to yield air-free measurements. This adjustment was performed by substituting the measured CO2 emissions into the following equation. COairfree ¼ CO2 max ðCO=CO2 Þsample
(2)
All the pollutant and temperature measurements presented in the results section were obtained under stable conditions, i.e., conditions where we did not obtain flashback or flame blowout. For every operating point, we started the system at room temperature and waited until the system reached a steady state, which normally occurred after 60e75 min. Every operating point was repeated three times and the mean values of pollutants and temperatures were calculated at each point. Finally, this study was performed at atmospheric conditions of 298 K and 0.849 bar, which correspond to the local environmental conditions of the city of Medellin, Colombia.
Results and discussion In this section, the experimental results are presented and discussed in three subsections, which correspond to the main subjects of this work, i.e., flame stability, radiation efficiency, and CO and NOx emissions.
Flame stability Fig. 2 shows the flame stability limits, which correspond to NG and the NG-SGs blends. The figure represents several operating conditions that were evaluated. To obtain this operation diagram, we also include operating conditions at 400 kW/m2. Three types of operating conditions can be observed in the
(1)
In Equation (1), ε is the emissivity of the burner surface, which is assumed to be equal to that of the unit [23]. s is the StefaneBoltzmann constant, and Tsup was measured by the thermocouple placed on the burner surface. A water-cooled sampling probe was placed along the centerline 5 cm above the burner (as observed in Fig. 1) to measure the O2, CO, CO2, and NOx concentrations in the combustion products. The sample was cleaned and dried before it reached the analyzers. The analytical instrumentation included a paramagnetic analyzer for the O2 measurements, non-dispersive infrared gas analyzers (SICK MAIHAK S710 analyzers) for the CO and CO2 measurements and a chemiluminescent analyzer for the NOx measurements
Fig. 2 e Flame stability diagram. BL: blowout limit. FL: flashback limit.
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figures: the blowout region (to the left of the blowout limit, BL), the flashback region (to the right of the flashback limit, FL) and the stable region (between the BL and FL). In this work, NG was considered the base case, and everything related to it is represented in black color. We also included dashed arrows to show how the BL and FL change with respect to the base case when the fuel is varied. Although we labeled the right limit of the NG as though it were a flashback region above f ¼ 1, we actually did not obtain flashback operation with NG. f ¼ 1 corresponds to the higher equivalence ratio that was evaluated. However, we decided to use this label to avoid the introduction of more abbreviations. As is shown in Fig. 2, the operation of the burner at f near unity was characterized by the occurrence of flashback when the NG was substituted by SG, especially under the highest thermal loads and for the highest hydrogen concentrations. At 500 kW/m2, flashback occurred from f ¼ 0.9 for SG1 and f ¼ 0.85 for SG2 and SG3, while operating with NG did not result in flashback, as was previously mentioned. On the other hand, it was observed that the BL increased at the lowest equivalence ratios. This indicates that the operating conditions change from fuel to fuel when NG is substituted by the high hydrogen content SG. However, considering that the range of stable operating conditions is very similar for all the fuels, it is also possible to conclude that if slight modifications of the combustion air flow are allowed, the burner exhibits a similar stability diagram for all the reactive mixtures. In particular, no variation was found between SG2 and SG3. Very recently it was shown that, generally, the turbulence in an RPMB is not enough to affect the laminar burning velocity, temperature distribution, and convective heat transfer [34]. Therefore, to understand the trends shown in Fig. 2, we characterized the flows considering them to be in the laminar flow regime, also taking into account the laminar burning velocity (SL). The value SL, as indicated by Law [35], contains the physico-chemical information of the mixture and therefore many premixed flame phenomena, such as flashback and blowout, can be characterized using SL as a reference parameter. NG (100% CH4) is a well-known fuel and there are many studies that report its laminar burning velocity, both numerically and experimentally. However, experimentally derived laminar burning velocities of the SGs-air mixture compositions evaluated in this work were not available, therefore it was necessary to calculate them. Fig. 3 shows the numerical results for SL for NG and the blending with the SGs. These numerical results were obtained using the same numerical methodology implemented in previous studies [36e39], using the GriMech 3.0 reaction mechanism [40], and the PREMIX code from the CHEMKIN-PRO package at ambient temperature (298 K) and pressure (0.849 bar). To verify that the numerical results for the blending with the SGs were acceptable for comparison with NG, we included experimental results reported for similar mixture compositions. The experimental results include those reported by Park et al. [41] (CH4 @ 1.013 bar), Cardona et al. [42] (CH4 @ 0.849 bar), Garcı´aArmingol et al. [43] (50CH4e50H2), Hermanns [44] (60CH4e40H2), and Boushaki et al. [45] (70CH4e30H2). It can be observed that the results obtained for NG are in good agreement with the experimental data. Furthermore, the
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Fig. 3 e Laminar burning velocities for NG and the blending with SGs.
results obtained for the blending with the SGs seem to be acceptable for the comparative purposes of this work. Now, with SL, another way to present the flame stability diagram is as it is shown in Fig. 4, where the ratio of SL to the velocity of the unburned mixture (VU) is plotted on the ordinate axis. One fact that is clear from this figure is that the BL and blowout regions for all the mixtures appear at values of SL/VU below 1. Moreover, from experimental observation, it seems that based on the value of this ratio, the flame propagation and stabilization after the ignition above the burner surface can be divided into up to 3 stages, as illustrated in Fig. 5. First, in those cases where Vu is slightly higher than SL, the flame propagates until it reaches the surface of the combustion zone, as is shown in Fig. 5-1. This is the case for all operation conditions above the BLs and below SL/VU ¼ 1. After this first stage, the flame heats up the porous media and the unburned mixture is preheated upstream [46]. After some
Fig. 4 e Flame stability diagram. The 50SG2 blend is not shown to improve the readability of the data.
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time, which is longer for operation conditions near the BL, it seems that the laminar burning velocity increases (stage 2) and at some point the flame is able to penetrate the porous media (stage 3). For operation conditions where 1 < SL/VU < 3, the stabilization process started at stage 2 and for those cases where SL/VU was greater than 3, the flame penetrated the porous media immediately following ignition. On the other hand, in the blowout regime, the flames were not able to complete stage 1 and blew out after ignition. In Fig. 4, for the blending with SGs, we also included the increase in the convective heat transfer coefficient (percentage) when, at certain operation conditions (thermal load and equivalence ratio), the NG was substituted by a SG. The convective heat transfer coefficients (h) were calculated following the methodology presented in Ref. [47]. It is observed that for a given operation condition, the SG concentration in the fuel mixture not only increases the SL to VU ratio but the convective heat transfer coefficients also. As a result, the SGs with higher hydrogen content exhibit a higher flame stability under lean conditions. The 50SG2 blend is not shown to improve the readability of the data. On the other hand, at conditions near f ¼ 1, the increase in SL has a negative effect, and this effect is higher for the SGs with higher hydrogen content (SG2 and SG3).
Temperature and radiation efficiency Fig. 6 shows the temperature profiles obtained in the RPMB for f ¼ 0.7 and f ¼ 1.0 for the operating conditions at 300 kW/m2 and 500 kW/m2 using NG. The distances shown in Fig. 6 were measured across the porous media, where 0 mm corresponds to the inlet of the preheat section and 39 mm corresponds to the outlet surface. Generally, as expected, the temperature increased significantly as the mixture passed through the RPMB. The temperature profiles and the behavior of the profiles when the equivalence ratio and thermal load were modified are in good agreement with results reported in other works for NG [33,48]. When either f or the thermal load is increased, the temperature level in the RPMB increases. The increase in the temperature level as f increases is due to the increase of the adiabatic flame temperature (Tad), while the
Fig. 6 e Temperature profiles within RPMB for f ¼ 0.7 and f ¼ 1.0 for the operating conditions at 300 kW/m2 and 500 kW/m2 using NG.
increase in the temperature level as the thermal load increases is due to the increase in the released energy. Fig. 7 shows the temperature profiles obtained for the different fuels in the stable region. The behavior of the temperature profile when NG was substituted with the high hydrogen SGs in the stable regions, or more precisely, under the same operating conditions, is similar to the behavior observed in other studies for SGs with lower hydrogen content [23]. The addition of high hydrogen content SGs to NG increases the Tad, as shown in Fig. 8, which includes the results obtained with the PREMIX and EQUIL code from the CHEMKINPRO package. This results in the slight difference in the temperature levels observed in Fig. 7. Additionally, because the maximum temperature always occurs in the thermocouple located 30 mm from the burner inlet, the position of the flame front of the blend fuels presumably does not change with respect to NG. As observed in Fig. 2, there is no clear difference between SG2 and SG3. Figs. 9 and 10 show the temperature profiles obtained for the different fuels at the BL and the FL. From Fig. 9, although
Fig. 5 e Flame propagation and stabilization after the ignition above the burner surface. Please cite this article in press as: Arrieta CE, et al., Experimental study of the combustion of natural gas and high-hydrogen content syngases in a radiant porous media burner, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.078
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Fig. 7 e Temperature profiles obtained for the different fuels at operating conditions in the stable region.
Fig. 8 e Adiabatic flame temperature for NG and the blending with SGs.
Fig. 9 e Temperature profiles obtained for the different fuels at the blowout limit.
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Fig. 10 e Temperature profiles obtained for the different fuels at the flashback limit.
the operating conditions are not the same, no significant difference in the temperature profiles was observed. At 500 kW/ m2 (solid lines), the temperature levels obtained with NG are slightly higher because the BL for NG was at a higher f than that for the SGs. At 300 kW/m2 (dashed lines), the reaction zone operating with NG presumably occurs near the outlet surface of the burner; however, during the experiments, it was not possible to observe any difference in the location of the reaction zone compared with that for the SGs. Similar results were obtained at the FL, as shown in Fig. 10. Fig. 11 shows the radiation efficiency that was obtained from the procedure described in section 2. Adding SG to NG increased the Tad, which simultaneously increased the temperature level of the solid phase and thereby, the radiation. When the hydrogen concentration was increased, the radiation efficiency increased. The highest radiation efficiencies were obtained for SG3 (~14%e~30%). Generally, it was observed that when f increased, the radiation efficiency increased. Additionally, the differences in radiation among the fuels also increased when f was increased. This is due to
Fig. 11 e Radiation efficiencies for the fuels at the operating conditions evaluated in this work.
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the Tad behavior with respect to f and the hydrogen concentration, which is a non-linear relationship [39,49]. On the other hand, when the thermal load was increased, the radiation efficiency (the portion of the thermal input that is transferred to the load by radiation from the solid phase) decreased; this occurred although the temperature of the solid phase on the surface increased. This can be attributed to the increase in the gaseous phase velocity, which reduces the residence time of the gases in the RPMB. This makes it impossible to obtain temperature levels on the burner surface higher enough to increase radiation more than what the thermal load was increased. It is noteworthy that this effect is generally lower for SG2 and SG3, which are the fuels with higher hydrogen concentrations.
Fig. 13 e NOx emissions for the fuels at the operating conditions evaluated in this work.
Pollutant emissions Fig. 12 and 13 show the experimental pollutant emissions (COfree and NOx) for NG and the NG blended with the high hydrogen content SGs. Additionally, numerical results for NG are included as reference values. The simulations were performed using the same methodology described above for SL. To consider the heat transfer process from the combustion zone to the preheating zone, the simulations were initiated using the mean temperature of the preheating zone for each operation condition. Moreover, a heat loss condition by convection was applied to emulate the heat transfer from the gas to the solid phase in the combustion zone. The convective heat transfer coefficients were calculated as suggested by Younis et al. [50]. Generally, the CO emissions were less than 50 ppm. However, this value increased when the equivalence ratio was near either the FL or BL. Moreover, increasing the hydrogen concentration resulted in no significant change in the CO emissions for the different SGs. Generally, the CO emissions were between 13 and 79 ppm for the operating conditions with the SGs. However, two exceptions occurred for SG2 and SG3 at 300 kW/m2 at the BL, where the CO emission levels were ~136 and ~190 ppm, respectively. Considerable differences
(~150e~400 ppm) were observed only between the NG and the NG blended with SG when operating at 300 kW/m2 and f ¼ 0:6 or f ¼ 1. These trends are difficult to explain, however, some observations can be drawn by focusing on the set of chain branching and chain propagating reactions shown in Table 2, which are included in Gri-Mech 3.0. Considering only the experimental results, the trend of CO emissions near the BL can be explained considering the lower temperature registered for these operation conditions, which are shown in Fig. 9 and affect R3 and R4. It has been reported that these two reactions practically stop at low temperatures [51]. However, a different trend was observed in the numerical results. This numerical trend suggests that presumably, not only the temperature influenced the CO emissions, but also the residence time in the combustion zone, which is not consider by the numerical model. For a given fuel composition and thermal load, a decrease in the equivalence ratio implies an increase in VU. Therefore, it is possible that a short postcombustion zone is necessary, similar as reported by Alavandi et al. [25]. The complete conversion of CO to CO2 also depends on the availability of oxygen, as indicated by R4 and R5. However, as it has been also highlighted by Huang et al. [22], it is worth mentioning that the activation energy (Ea) of reactions involving OH (R3) are much lower than those with O or O2 (R4 and R3), which is valid not only for CO, but also for CH4 (R2) and H2 (R7). This observation implies that the OH is a determining factor in the consumption of the main fuel molecules considered in this work. Therefore, for a given operation condition, increasing the concentration of H2 (which is a common way to increase the pool of OH) improve the
Table 2 e Some elementary reactions involved in the consumption of CH4, CO and H2 [33]. Specie involved CH4 CO
Fig. 12 e CO emissions for the fuels at the operating conditions evaluated in this work.
H2
Elementary reaction
Ea (cal mol1)
CH4 þ O$CH3 þ OH (R1) CH4 þ OH$CH3 þ H2 O (R2) CO þ OH$CO2 þ H (R3) CO þ O2 $CO2 þ O (R4) CO þ O þ M$CO2 þ M (R5) H2 þ O$H þ OH (R6) H2 þ OH$H þ H2 O (R7)
8600 3120 70 47,800 2385 6260 3430
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combustion process. This would explain the difference between the NG and the blends with high hydrogen content SGs when either the temperature level in the burner is lower (near the BL) or the oxygen concentration decreases (near the FL). To illustrate what have been discussed above, Fig. 14 shows the net reaction rate of R1, R3, R4, R6 and R7 for NG, 50SG1 and 50SG3 at f ¼ 0:9. Additionally, the chain branching step H þ O2 $ OH þ O (R8) have been included due to its importance in OH production. It is observed, then, that as the H2/CO ratio increases, the net reaction rate increases significantly in those elementary reactions that include OH. With respect the thermal load, presumably, the CO emissions decrease when the thermal load increases, which can be attributed to the increase in the temperature levels in the RPMB. This trend is clear for the operating conditions with NG both numerically and experimentally. However, the thermal load shows no significant effect for the NG blended with SGs.
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Fig. 13 shows that the NOx emissions were typically less than 1 ppm, which is in agreement with the results in Refs. [52] and [25]. Increasing both the equivalence ratio and the thermal load increases the NOx emissions because the Tad increases, as previously discussed. This trend is also observed in the numerical results. Surprisingly, at the highest thermal load, towards the BL the NOx emissions are lower for the NG, which is in agreement with other works performed at lean conditions [28]. On the other hand, near the FL the result indicate an opposite trend, which is also in agreement with other results obtained near stoichiometric conditions [27]. Generally, near the BL the NOx emissions increase linearly with increases in equivalence ratio, however in some point emissions increase exponentially. It is interesting that as the H2/CO ratio is increased this exponential trend decreases. This could be due to the increase of H radical and decrease of C radical, which reduce the
Fig. 14 e Net reaction rate of R1, R3, R4, R6, R7 and R8 for NG, 50SG1 and 50SG3 at f ¼ 0:9.
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formation of CH and CH2, and therefore retard the production of HCN and NOx [26].
Conclusions In this work, the performance of a typical radiant porous media burner operating with natural gas and the blending of natural gas with three high hydrogen content syngases in equimolar proportions was studied. We examined the effects of the composition of the syngases, the fuel-to-air ratio and the thermal input on the flame stability, the radiation efficiency and the pollutant emissions (CO). The H2 to CO ratio for the syngases was varied between 1.5 and 3. This study complements previous works that studied syngases with H2 to CO ratios of less than 1 [21e23]. The following conclusions can be drawn from the results obtained in this work: The results obtained for the flame stability indicates that the operating conditions change from fuel to fuel when NG is substituted by high hydrogen content SG. However, considering that the range of stable operating conditions is very similar for all the fuels, it is also possible to conclude that, if slight modifications of the combustion air flow are allowed, this burner exhibits considerable potential for the interchangeable use of the gases studied in this work. Compared to the use of pure natural gas, the addition of high hydrogen content syngases did not significantly affect the temperature profile inside the porous media and the flame stability. However, significant differences were observed in the radiation efficiencies and pollutant emissions. The highest radiation efficiency obtained in this work was 30% for the blends that contained the syngas with higher hydrogen concentration; the highest radiation efficiency obtained with natural gas was 18%. This can be attributed to the increase in both Tad and the convective heat transfer coefficient when the hydrogen concentration increased, which resulted in an increase in the temperature level of the solid phase and therefore, an increase in the radiation. In some cases, especially near the flashback limit and the blowout limit, adding high hydrogen content syngases to natural gas in equimolar proportions reduced the CO emission to 400 ppm. The following conclusions can be drawn from the variation in the equivalence ratio and thermal load. The equivalence ratio had a considerable effect on the flame stability, temperature profile, radiation efficiency and the pollutant emissions for all the fuels studied in this work. This is attributed to its effect on variables like adiabatic flame temperature, laminar flame speed, and mean flow velocity, which affect the burner performance. For the blends that contained syngases, the pollutant emissions were not significantly influenced by the thermal load. Generally, CO emissions between 13 and 79 ppm and NOx emissions lower than ~1.3 ppm were obtained with these fuels.
Acknowledgements n The authors thank COLCIENCIAS for financing the Unio n e Innovacio n en Combustio n Temporal de Investigacio Avanzada de Uso Industrial (UT. INCOMBUSTION), the project n de un Sistema de Combustio n “Desarrollo y Evaluacio Sumergida ” and the doctoral program 567. The authors also thank the Universidad de Antioquia for the valuable economic contribution to the development of this research through the program “Sostenibilidad 2014e2015 de la Vicerrectorı´a de n”. Investigacio
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