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A comparative study on combustion characteristics of methane, propane and hydrogen fuels in a micro-combustor
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A comparative study on combustion characteristics of methane, propane and hydrogen fuels in a micro-combustor Aikun Tang*, Yiming Xu, Chunxian Shan, Jianfeng Pan, Yangxian Liu School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, China
article info
abstract
Article history:
The combustion characteristics of three hydrocarbon fuels are investigated so as to study
Received 8 July 2015
the applicability of fuels in a micro-planar combustor. Experiments are performed to
Received in revised form
analyze the differences in flame stabilization, temperature distribution of external wall
19 September 2015
and flammable channel-heights. At different equivalence ratios, it is found that the
Accepted 19 September 2015
mixture of hydrogen/air has a much wider and stable flammable range. The reaction po-
Available online xxx
sitions of methane and propane cases will significantly move downstream along with the increase in mixture flow rate. Under the same chemical energy inputs, the external wall
Keywords:
temperature distribution of methane case is most uniform, and the average wall temper-
Micro combustion
ature is also the highest among the three fuels. When hydrogen is selected as a fuel, the
Flame stabilization
temperature gradient of the combustor wall becomes very large, which is caused by the
Temperature distribution
flame position near the inlet. The flammable channel-heights of the three fuels in the
Channel-height
micro-planar combustor are different. It is observed that stable combustion of methane/air
Fuels
cannot be achieved when the channel-height is less than 2.5 mm. However, the minimum flammable channel-height of propane/air case is 2 mm, and hydrogen/air can be ignited in a 1 mm height micro-combustor.1 Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In recent years, micro-power generators, benefitting from the advantage of a long operating lifetime, non-moving parts and high energy density have caused wide public concern [1,2]. These devices directly convert chemical energy of hydrocarbon fuels into electricity or power, which is supplied for MEMS (such as micro pumps and micro robots), portable electronics, military devices and vehicles. A micro-power system consists of (1) a heat source (micro-combustor), (2) energy-conversion
components, and (3) energy-input parts. However, the micro-combustor is the core component of every micro-power generator. Thus, realizing an efficient and stable combustion process has become the key to improving the performance of the whole system. Various research works on the structure optimization of micro-combustors have been done within the past decades. In order to preheat the fresh incoming reactants with the outgoing combustion products, the reverse flow reactor designs are commonly employed for heat recirculation. Ahn et al. [3] have found that the reactive limits of propane over platinum
* Corresponding author. Tel.: þ86 511 88780214; fax: þ86 511 88780216. E-mail address: [email protected] (A. Tang). http://dx.doi.org/10.1016/j.ijhydene.2015.09.101 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Tang A, et al., A comparative study on combustion characteristics of methane, propane and hydrogen fuels in a micro-combustor, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.101
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are broad and cover the equivalence ratio range of 0.2e40 by using the Swiss-roll design. Yang et al. [4] compared the combustors with and without heat recuperation and found that a higher flow velocity and higher H2/air equivalence ratio will increase the rate of entropy generation. While Federici and Vlachos [5] employed a heat recirculation combustor with two inner parallel plates to increase the blowout limit of propane/air mixture. Yang et al. [6] also proposed the SiC porous media form which was employed in the combustion channel to enhance heat transfer between the hot gas and the wall. Chou et al. [7] compared the performance of a micro cylindrical combustor with and without employing porous media. A micro combustor with a bluff body was developed by Fan et al. [8], which can extend the blow-off limit by 3e5 times. Tang et al. [9] found that compared to a single-passage combustor, the micro planar combustor with parallel separating plates can achieve a higher mean temperature of the radiation wall due to the enhancement of heat transfer. In addition, catalytic method was used to strengthen the micro combustion process. Boyarko et al. [10] tested combustion in platinum micro-tubes with an inner diameter of 0.4e0.8 mm. Chen et al. [11] addressed the combustion characteristics of multi-segment catalysts in a micro-reactor by numerical simulation with detailed heterogeneous and homogeneous chemistries. In addition, Fanaee and Esfahani [12e14] selected hydrogen-air or propane-oxygen as the mixture in a catalytic micro-channel and explored their combustion characteristics. During the research of each micro-power generator, the authors usually chose different kinds of fuels and oxidants in the micro combustion process. Typically, for the microthermophotovoltaic (MTPV) system, Yang et al. [9] used H2/ air as the reactant in a micro-planar combustor, while Pan et al. [15] adopt the mixtures of hydrogen-oxygen. Choi et al. [16] selected methane and air as the mixtures. Park et al. [17] selected C3H8/air as the fuel and oxidizer in a heatrecirculation micro-emitter with an annular-type shield. However, different combustion characteristics will be presented when using different fuels and oxidants, but not many comparative studies have been conducted on this front. Demoulin et al. [18] investigated the catalytic combustion of methane, ethane and propane on a Pd/g-Al2O3 catalyst and
studied the influence of adding ethane or propane in the feed during the catalytic combustion of methane in a U-shape quartz micro-reactor. An experiment on the combustion of methane, methanol, and ethanol in three micro packed bed quartz tubes with the ZSM-5 zeolite supported nanometersized Pt as the catalyst was performed by Deng et al. [19]. In the condition of micro-scale, the combustion process of fuel may present very different characteristics compared with conventional scale combustion, which is subjected to the problems of short residence time and large heat loss. Obviously, most of the researches comparing micro-combustion characteristics among fuels are mainly focused on the catalytic reaction due to the advantages of catalytic combustion. However, the research on gas-phase reaction comparing fuels needs to be further improved, especially when it comes to the comparison on the flammable range, external wall temperature and flammable channel-height. In order to contrast the micro-combustion characteristics of different fuels, a micro-planar combustor is designed. Methane, propane and hydrogen are selected as fuels, and air is chosen as the oxidant. The basic characteristics, combustion limit, and scale effect of different fuels are obtained by an experimental method, which can provide references for further fuel selecting processes of micro-power generators.
Experimental set up Fig. 1 shows a schematic diagram of the experimental platform. It consists of a gas feed system, a mixing chamber, a micro-combustor, and the temperature measurement instruments. At first, the fuel (methane, propane and hydrogen) and oxidant (compressed air) will have their pressure reduced via the two valves and then flow into their respective micro flow meters. Purities of methane, propane and hydrogen in this study are 99.99%, 99.0% and 99.5%, respectively. Following this, the two gases will flow into a mixing chamber so as to realize a uniform mixing, and will then be injected into a micro-combustor. The mixture is ignited at the exit of the micro-combustor, and then the flame retreats into the microcombustor and finally realizes a stable combustion state. The
Flow controller Micro-combustor Flash-back arrestor
Infrared thermographer
Micro flowmeter mixing chamber Reducing valve
Flow controller Fuel
Computer
Air
Fig. 1 e Schematic diagram of micro-combustion test platform. Please cite this article in press as: Tang A, et al., A comparative study on combustion characteristics of methane, propane and hydrogen fuels in a micro-combustor, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.101
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mass flow controllers (DSN-2000B, accuracy of 0.5%, full scale) are used to control and measure the flow rates of air and fuels. The measurements of the temperature distribution on the combustor surface are carried out using an infrared thermal imager (Type: Thermovision™ A40). This apparatus can measure temperatures of up to 2000 C. A close-up lens (dimension of minimum distinguishable body: 200 mm 200 mm; minimum focal length: 150 mm) is equipped on the camera so as to obtain clearer images. In order to measure the temperature accurately, the working condition is a windless and lightless room which avoids the radiation of sunlight and other lighting. Besides, the ambient temperature is kept at about 20 C, which can weaken the atmospheric radiation. Under this experimental condition, the thermal infrared imager can be capable of compensating error and ensures the maximum measuring accuracy is ±2% of the reading. To confirm the flame stability, the K-type thermocouple with a bead diameter of 0.5 mm (measuring accuracy: ±1%, measuring range: 0e1300 C) is adopt to measure the exit temperatures and the data recorder (RX4008B, recording a date per second) is used to monitor the temporal evolution of the exit temperatures. In addition, a digital camera (Type: Nikon S8200) is adopted to illustrate the size and position of high temperature region. In this study, a micro-planar combustor is designed and tested. The total dimensions of this micro-planar combustor are 18 mm 9 mm 4 mm (length width height). The wall thickness of the micro combustor is kept as 0.5 mm, thus, the combustion channel has a width of 8 mm and a height of 3 mm. 316 stainless-steel is used to manufacture the combustor, which can withstand a high temperature of 2000 C and has an emissivity of 0.65.
Results and discussions Comparison of flame location and flammable range In this paper, most of the experimental cases can be regarded as a stable-flame condition, and the flame stability is defined as: under a certain range of flow rates, the fuel/air can keep stable combustion in the micro-combustor without the phenomenon of flame oscillations or repetitive extinction and ignition occurring. Besides, the flame is not apparently asymmetrical. At the same time, the distribution of external wall temperature is relatively steady. Fig. 2 shows the experimental photos of the micro-combustor using different fuels,
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Fig. 3 e Location of maximum wall temperature for the three fuels (equivalence ratio ø ¼ 1). which are all from the digital camera. The equivalence ratio of each case remains as 1, the inlet velocities of the three mixtures are given in the figures, and each experimental case is at a stable-flame condition. It can be seen that all of the external walls become brighter with the increasing of the mixture inlet velocity, and the high temperature regions, which are located at the front area of the combustor, also expand more widely when the mixture's inlet velocity rises. The highest temperature locations of methane, propane and hydrogen cases are depicted in Fig. 3. In our previous study [20], it is found that the location of maximum wall temperature is very close to the location of maximum flame temperature. As a result, it is acceptable that the location of maximum wall temperature is used to indirectly indicate the flame location in this paper. Due to the highly combustible characteristic of hydrogen, the high temperature regions of hydrogen cases consistently remain within very close proximity of the inlet, as such the highest temperature locations nearly have no differences and they are about 1 mm away from the inlet. However, in the cases of methane and propane, the highest temperature locations apparently move towards the outlet with an increase in flow rate. When inlet velocity is from 0.3 m/s to 0.6 m/s, the highest temperature locations of methane cases are 3.5 mm, 4 mm, 5 mm and 6 mm, respectively. For propane, the locations are about 2.6 mm, 3.3 mm and 4.4 mm in the case of 0.3 m/s, 0.5 m/s and 0.7 m/s, respectively. For methane and
Fig. 2 e Experimental photos at different inlet velocities (equivalence ratio ø ¼ 1). Please cite this article in press as: Tang A, et al., A comparative study on combustion characteristics of methane, propane and hydrogen fuels in a micro-combustor, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.101
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propane, the incoming cold feed starts pushing the reaction zone downstream owing to the faster flows. This is similar to the phenomenon observed by Maruta et al. [21], although the external wall is maintained at time-constant, which is a little different with the heat-dissipation condition in this study. However, at the same inlet velocity, the highest temperature location of methane is further away from the inlet along the gas flow direction. The hydrocarbon chain length of methane is 109.3 pm, and the value of propane is 111 pm. In addition, the ignition temperature of methane and propane are 810 K and 745 K respectively [22]. Obviously, it is concluded that the ignition temperature decreases when the length of the hydrocarbon chain increases [18], which leads to the various locations of flame. The conditions of flame stabilization at different equivalence ratios for these fuels are examined and the results are clearly shown in Fig. 4. As for the mixture of methane/air, when the equivalence ratio varies from 0.7 to 1.0, the minimum flammable velocity will be stabilized at 0.2 m/s. It is therefore difficult for methane/air to be ignited when the velocity of mixture is less than this value. Meanwhile, the maximum flammable velocity remains as 0.6 m/s in the lean fuel conditions, and it reaches 0.7 m/s at the stoichiometry ratio case. Although, the flame location is only 6 mm away from the inlet at 0.6 m/s velocity (as seen in Fig. 3). Flame oscillation with a random asymmetry occurs when the velocity of the mixture exceeds 0.7 m/s, and finally leads to blowout. The unstable flame that exhibits dynamic behaviors were also observed by Norton et al. [23] and Maruta et al. [21]. However, in the rich fuel condition, the stable combustion is difficult to achieve in the micro-planar combustor, and the flame is found to be consistently located at the outlet under different velocities. As a result, the rich fuel condition is not favorable for the micro-planar combustor when methane is taken as the fuel. Compared to methane, the minimum flammable velocities of propane/air are higher in lean fuel and stoichiometry ratio conditions. Both the minimum and maximum flammable velocities are 0.5 m/s at equivalence ratio 0.7. When the
equivalence ratio is from 0.9 to 1.2, the minimum flammable velocity remains 0.2 m/s. However, the maximum flammable velocity will be raised gradually along with the increase of the equivalence ratio, and reaches the highest value of 1 m/s at stoichiometry ratio. Similarly, random asymmetry flame can be also observed when the inlet velocity exceeds 1.0 m/s. In addition, a little soft sound will be generated before blowout. As such, the flammable range of propane/air will get an extension in the lean fuel and stoichiometry ratio conditions, which can be seen from Fig. 4. On the other hand, due to heat loss because of unburned propane, the maximum flammable velocity decreases with the increasing of the equivalence ratio in the rich fuel conditions. On the whole, with propane it is relatively easy to realize a stable combustion in the rich fuel condition, and the flammable range is also broader than methane/air under the same condition. In general, propane micro-flames are more robust than methane micro-flames in a burner with two parallel plates. The propane allows a wider range of wall thermal conductivities as well as higher external heat-loss coefficients [24]. In the case of hydrogen and air, the minimum flammable velocity remains basically stable at 0.1 m/s, which is due to the highly combustible characteristic and highest heating value of hydrogen. The minimum flammable velocity of hydrogen is equal to that of methane/air (0.2 m/s) at the equivalence ratio 0.7. As for the maximum flammable velocity of hydrogen/air, limiting to the full scale of mass flow controller (hydrogen: 2000 mL/min), the maximum flammable velocity of hydrogen case can reach about 4.7 m/s (at stoichiometry ratio). Therefore, this value is larger than that of methane (0.7 m/s) and propane (1.0 m/s) in the same condition, which is also much lower than the maximum flammable velocity in a microcombustor. Li et al. [25] pointed out that the blowout limit of hydrogen can reach 8 m/s in a cylindrical tube with 0.8 mm inner diameter.
Temperature distribution at the same chemical energy input The external wall temperature of a micro-combustor is able to decide the energy input of micro-power generators, such as the MTPV system, thus much more focus should be paid on the external wall temperature distribution. In order to evaluate the working performance of different fuels in the same micro-combustor, a comparison of the external wall temperature distribution at different flow rates is made. Firstly, as mentioned above, this study selects that the inlet velocities of methane/air are 0.4 m/s, 0.5 m/s, and 0.6 m/s at the equivalence ratio 1, and the corresponding flow rates of methane are 55 mL/min, 68 mL/min and 82 mL/min, respectively. The highest heating value of methane is 55.5 MJ/kg, hydrogen is
Table 1 e Flow rates of fuels under the same chemical energy input. Chemical energy input Fig. 4 e Flame stabilization at different equivalence ratios for the three fuels (equivalence ratio ø ¼ 1).
CH4 H2 C3H8
36.63 W
45.28 W
54.62 W
55 mL/min 172 mL/min 22 mL/min
68 mL/min 213 mL/min 27 mL/min
82 mL/min 257 mL/min 32 mL/min
Please cite this article in press as: Tang A, et al., A comparative study on combustion characteristics of methane, propane and hydrogen fuels in a micro-combustor, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.101
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Fig. 5 e Experimental photos at different chemical energy inputs (equivalence ratio ø ¼ 1). 141.8 MJ/kg and propane is 50.35 MJ/kg. Based on these methane flow rates, flow rates of other two fuels can be calculated under the same chemical energy input (as seen in Table 1). The experimental photos of the external wall at three chemical energy inputs are shown in Fig. 5. With the increasing of the chemical energy input, it is notable that the external wall of each mixture case becomes more and more bright. The high temperature regions of hydrogen cases are apparently smaller than that of the other two fuels. When the chemical energy input is 36.63 W, the high temperature region of propane case is slightly brighter than that of methane case. However, when the chemical energy input reaches 45.28 W and 54.62 W, the high temperature regions of methane cases broaden significantly, and the whole external walls become much brighter than those in the case of propane. In order to further illustrate the wall temperature distribution of a micro-combustor, the centerline temperature profiles of the external wall along the flow direction at three chemical energy inputs are plotted in Fig. 6. First, the adiabatic flame temperature of hydrogen/air reaches 2390 K, which is higher than that of methane/air (2232 K) and propane/air (2267 K) [26]. In our previous works, it is found that the flame front of hydrogen case is much smoother, which is contributed to its highly reaction rate. Hence, the maximum temperatures of external walls in the case of hydrogen are all larger than that of the other two mixtures. When the chemical energy input is 54.62 W, the maximum temperature of the hydrogen case reaches 1195 K, which is 61 K and 69 K higher than the methane and propane case. It is found that some of the unburned methane and propane will combust near the
outlet at the two higher chemical energy input cases, which is another reason leading to the lower value of maximum temperature. On the other hand, with an increase of the inlet velocity, a corresponding increase in reaction rate resulted in higher heat release and axial heat conduction further acted to spread the absorbed heat in the walls [27] which contributing to the external wall temperature rising of the three fuels. Furthermore, due to the low ignition temperature, the maximum temperature of each hydrogen case apparently remains at the position of 1 mm away from the inlet. Whereas the flame locations of methane and propane cases will move downstream along with an increase in mixture flow rate, which can be noticeably seen in Figs. 3 and 6. However, due to the lower apparent activation energy of propane combustion (126 kJ/mol for propane compared to 203 kJ/mol for methane) [24], the propane is easier to be ignited. As a result, comparing to the methane case, the location of highest wall temperature in the propane case is much closer to the inlet. From Fig. 6(a) we can see that the maximum temperature of propane is 1027 K, which is only 3 K higher than that of the methane case. However, in the high chemical energy input cases, the maximum wall temperature of the methane case turns to be larger than that of the propane case. Starting at the position of 4 mm, the wall temperatures of methane cases are significantly higher than the other two cases. This is consistent with the phenomenon of the external wall being much brighter in the corresponding areas (as seen in Fig. 5). At each chemical energy input case, the flow rates of methane/air are all larger than that of propane/air, and the velocity difference also rises with the increasing of chemical energy input. As mentioned above, the increasing of inlet velocity will bring out a
Fig. 6 e Centerline temperature profiles of external wall along the flow direction at different chemical energy inputs (equivalence ratio ø ¼ 1). Please cite this article in press as: Tang A, et al., A comparative study on combustion characteristics of methane, propane and hydrogen fuels in a micro-combustor, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.101
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corresponding growth of reaction rate, which will result in a higher heat release and axial heat conduction, and finally enhance the heat absorption process of the walls [27]. Hence, at 36.63 W chemical energy input case, although the relatively higher inlet velocity of methane/air do not generate an increase of the maximum wall temperature, the temperature values at most position along the flow direction are all larger than propane case. When it comes to the higher chemical energy input case, the effect of velocity difference has brought out a more obvious influence to the temperature distribution, which also including the maximum wall temperature. When it comes to the whole temperature gradient, the values of methane, propane and hydrogen case at 36.63 W are 148 K, 184 K and 293 K, respectively. Even when the chemical energy input is raised to 54.62 W, the temperature gradient of the methane case only reaches 154 K, which is 24 K and 180 K less than that of propane and hydrogen cases. Due to the monotone decreasing trend of the centerline temperature along the flow direction, the temperature gradients of hydrogen are highest among three fuels. However, as for methane and propane cases, the temperatures will receive a steady growth at the preheating zone, and then gradually decrease at their respective peak-temperature position. This variation trend is bound to improve the uniformity of wall temperature distribution. Especially, in the case of methane the flame location is much closer to the middle of the microcombustor, which leads to the lowest temperature gradient. The mean temperature of the external wall will be affected by the following factors: the maximum wall temperature and the flame position; the maximum temperature gradient and the temperature variation trend. Fig. 7 shows the specific values of the average temperature at selected chemical energy inputs. It is found that the average value of the wall temperature in each case rises gradually with an increase in the flow rate. When the chemical energy inputs are 36.63 W and 45.28 W, the average temperature of methane and propane cases are very close. However, benefiting from the larger maximum temperature and the lower temperature gradient, the mean temperature in the case of methane reaches 1040 K at the chemical energy input of 54.62 W, which is about
Fig. 7 e Average temperature of external wall at different chemical energy inputs (equivalence ratio ø ¼ 1).
12 K larger than that of propane case. The largest temperature gradient finally results in the lowest mean wall temperature at each chemical energy input. In view of the mean wall temperature and gradient, the two hydrocarbon fuels are more suitable for the microthermophotovoltaic system, as the micro-combustor requires greater radiation energy from the external wall. However, as mentioned above, the blow-out inlet velocity of methane/air and propane/air are limited to 0.7 m/s and 1.0 m/ s (at stoichiometry ratio condition). As a result, it is difficult to further improve the output power density and energy conversion efficiency of the MTPV system. Recently, it was found that due to the high combustibility of hydrogen, the microcombustion process of these hydrocarbon fuels could be greatly improved by adding a small amount of hydrogen into the mixture [20,28e30].
Effect of channel height for fuels In the micro-combustion process, the problem of miniaturization limitation cannot be avoided. Take the example of the micro-planar combustor in this paper, the stable combustion will be hardly maintained when the channel-height is lower than a certain value. Hence, combustion characteristics of the three fuels at different channel-heights are compared in the following part. First, the mixture of methane/air is able to keep stable combustion at the channel-height of 2.5 mm. However, when the channel-height decreases to 2 mm, the mixture cannot be ignited in the micro-combustor. Consequently, the quenching distance of methane/air can be defined as 2.5 mm in this micro-planar combustor. Then, the combustion characteristic of this height is compared with the 3 mm channel-height case. In this experiment, when channel-height decreases at the same flow rate, the changing result of external wall temperature is determined by the three factors: Firstly, the separation distance between the flame and the internal wall will be reduced, which enhances the heat absorption process of the internal wall. Secondly, the velocity of mixture will be increased, which brings out an improvement of heat transfer coefficient between the fluid and internal wall. Finally, the residence-time of mixture becomes shorter under the same flow rate, which is likely to result in an incomplete combustion. The experimental photos of the two micro-combustors are shown in Fig. 8 (methane flow rate: 55 mL/min, stoichiometry ratio). It can be seen that the difference of the brightness between the two external walls is very small, and the flame position of the 2.5 mm case is slightly closer to the inlet. Fig. 9 depicts the centerline temperature profiles of the external wall along the flow direction at different flow rates. When the flow rate of methane is 41 mL/min, the trend of temperature distribution and the specific values are nearly the same. However, the differences become more pronounced in 55 mL/min cases. Along with the decrease of channel-height, the separation distance between the flame and the internal wall will be reduced, which enhances the heat absorption process of the internal wall. As a result, at the inlet zone, the wall temperature of the 2.5 mm case is higher than that of the 3 mm case. But the maximum temperature difference is only 4 K. However, beginning at the position of 3 mm away from
Please cite this article in press as: Tang A, et al., A comparative study on combustion characteristics of methane, propane and hydrogen fuels in a micro-combustor, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.101
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Fig. 8 e Experimental photos of methane with different channel-heights (fuel flow rate: 55 mL/min, equivalence ratio ø ¼ 1).
Fig. 10 e Experimental photos of propane with different channel-heights (fuel flow rate: 32 mL/min, equivalence ratio ø ¼ 1).
the inlet, the passive effect of residence-time becomes dominantly and the incomplete combustion in the microchannel will lead to a falling of wall temperature. Consequently, the external wall temperature of the 3 mm case is apparently higher than that of 2.5 mm case. Besides, as mentioned above, the maximum flammable velocity of methane/air is 0.7 m/s in the micro-combustor with 3 mm channel-height. When it comes to 2.5 mm microcombustor, the corresponding value is only 0.5 m/s, which greatly limits the fuel input of the micro-power generators. Therefore, the channel-height of the combustor should be larger than 3 mm when methane and air are selected as the fuel and the oxidant. Zhao et al. [31] pointed out that the quenching distance of propane/air is lower than that of methane/air in the same condition. The same phenomenon is observed in our study, it is found that the flammable channel-height of propane/air case can be minimized to 2 mm. Fig. 10 shows the experimental photos of 2 mm and 3 mm channel-height (at stoichiometry ratio, the propane flow rate is 32 mL/min). And the
centerline temperature profiles of the external wall along the flow direction are plotted in Fig. 11. It can be noted that the external wall of 2 mm case is much brighter than that of 3 mm case, especially at the flame region. Under the two flow rates, the maximum temperatures of 2 mm case reach 1122 K and 1196 K, which are 95 K and 70 K higher than that of 3 mm case. This phenomenon is due to the improvement of heat transfer between the flame and the internal wall. However, the whole wall temperature of the 2 mm case is higher than that of the 3 mm case, which is different with the methane/air experiment. As seen from Fig. 11, the temperature difference of the two combustors decreases evidently along the flow direction. Although, the residencetime of the mixture is shortened with a decrease in channelheight, the two active effects will overcome the adverse influence of residence-time, and finally bring out an evident growth of external wall temperature. In view of test results, the promotional effect of the heat transfer process apparently plays a dominant role. As a result, the average wall temperature of the 2 mm case is about 40 K higher than that of the 3 mm case at 32 mL/min flow rate.
Fig. 9 e Centerline temperature profiles of external wall along the flow direction (fuel: methane; channel-height: 2.5 mm, 3 mm; equivalence ratio ø ¼ 1).
Fig. 11 e Centerline temperature profiles of external wall along the flow direction (fuel: propane; channel-height: 2 mm, 3 mm; equivalence ratio ø ¼ 1).
Please cite this article in press as: Tang A, et al., A comparative study on combustion characteristics of methane, propane and hydrogen fuels in a micro-combustor, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.101
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Moreover, the maximum flammable velocity of a 2 mm channel-height combustor can also reach 1 m/s at stoichiometry ratio. Thus, when propane is selected as fuel, the 2 mm channel-height micro-planar combustor will be more appropriate for the MTPV system. Finally, a comparison of the micro-combustion process with 1 mm, 2 mm and 3 mm channel-height is made, with hydrogen as the selected fuel. The centerline temperature profiles of the external wall along the flow direction are shown in Fig. 12. The flow rates of hydrogen are 200 mL/min and 400 mL/min respectively (at stoichiometry ratio). At the flame zone near the inlet, the external wall temperatures of 1 mm cases are also evidently higher than that of the other two cases. The external wall temperature decreases rapidly after the reaction, and then rises again at the position of 12 mm (200 mL/min) and 13 mm (400 mL/min). It is observed that a phenomenon of secondary combustion appears near the outlet in the 1 mm channel-height case. In order to illustrate this phenomenon, the exit temperature of hydrogen at different channel-heights are measured and depicted in Fig. 13. It is notable that the exit temperatures are relatively steady when the channel-heights are 2 mm and 3 mm, and the temperature amplitude is in a range of 20 K. While the exit temperatures of 1 mm channel-height have an apparent fluctuation, which indicates the flame is unstable. Pizza et al. [32] pointed out that the quenching effect of the wall becomes stronger along with the decrease of channelheight, and it is no longer possible to stabilize flames for narrow enough channels. Although this simulation work is under a forced wall boundary condition, the results are very similar to our study. In this paper, the inlet velocity of hydrogen/air is about 0.7 m/s (hydrogen flow rate: 200 mL/ min) in the 2 mm channel-height case. The flame is closed symmetric steady in the 2 mm channel-height when the inlet velocity is in the range of 0.04 m/s to 0.95 m/s [33]. Hence, Fig. 13 illustrates the flame temperature oscillations are occurred and the flame instability results in the incomplete combustion at the 1 mm channel-height case. Consequently, although the hydrogen/air combustion could be realized in the
Fig. 13 e Variation of the exit temperature with time at different channel-heights (hydrogen flow rate: 200 mL/ min; channel-height: 1 mm, 2 mm, 3 mm; equivalence ratio ø ¼ 1).
1 mm channel-height combustor, the incomplete chemical reaction is unavoidable. As mentioned above, the combustion processes of 2 mm and 3 mm cases will be much more stable and the wall temperature variations of the two cases are also very similar. In general, at a distance of 0e4 mm away from the inlet, the external wall temperatures of 2 mm cases are higher than that of 3 mm cases at both 200 mL/min and 400 mL/min fuel flow rates. However, the situation is different at a distance of 4 mm from the entrance. In the low flow rate condition, the wall temperature of 2 mm case is still higher, but the minimum difference is only 2 K. When it comes to the high flow rate case, the temperature values of the 3 mm case will apparently exceed those of 2 mm case, and the temperature difference near the outlet is about 50 K. It can be concluded that when the channel scale decreases, the comprehensive effect of shortening residence time and raising heat transfer intensity varies with the flow rate of the mixture. Meanwhile, as for the micro-power generators like the MTPV system, it should be noted that under the same radiant energy output, the adopting of micro-combustor with lower channel-height means a larger volume power density could be obtained. From this point of view, the 2 mm channel-height micro-combustor will exhibit a stronger superiority.
Conclusions
Fig. 12 e Centerline temperature profiles of external wall along the flow direction (fuel: hydrogen; channel-height: 1 mm, 2 mm, 3 mm; equivalence ratio ø ¼ 1).
In this paper, the combustion characteristics in a microplanar combustor using three types of fuels are investigated through an experimental method. The effect of inlet velocity, flammable range, external wall temperature and channelheight are analyzed and compared, which are important for the micro-power generators. The main conclusions obtained from this study are summarized below:
Please cite this article in press as: Tang A, et al., A comparative study on combustion characteristics of methane, propane and hydrogen fuels in a micro-combustor, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.101
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(1) As the inlet velocity of mixture increases, the external wall surface becomes brighter and the high temperature region expands more widely. The stable flammable range of hydrogen/air is wider than that of propane/air and methane/air. While the maximum temperature positions of methane and propane cases will obviously move downstream in response to an increase in the mixture's flow rate. (2) Under the same chemical energy input, the maximum wall temperature and temperature gradient of hydrogen are higher than that of methane and propane cases. However, the external wall temperature distribution of methane case is the most uniform, and the average wall temperature is also the highest among the three fuels. Considering the average wall surface temperature, methane and propane are the most suitable gas fuels for the micro-power generators. (3) Methane/air can keep a stable combustion in the microcombustor with a minimum channel-height of only 2.5 mm, while propane/air can achieve combustion at 2 mm channel-height, and hydrogen/air can be ignited in a 1 mm height micro-combustor.
Acknowledgments This work is supported by National Natural Science Foundation of China (No.51206066, No.51376082), China Postdoctoral Science Foundation (No.2014M551514), Natural Science Foundation of Jiangsu Province (No.BK20131253), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Scientific Research Starting Foundation for Advanced Talents of Jiangsu University (No.11JDG139).
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[8] Wan JL, Fan AW, Maruta K, Yao H, Liu W. Experimental and numerical investigation on combustion characteristics of premixed hydrogen/air flame in a micro-combustor with a bluff body. Int J Hydrogen Energy 2012;37:19190e7. [9] Tang AK, Pan JF, Yang WM, Xu YM, Hou ZY. Numerical study of premixed hydrogen/air combustion in a micro planar combustor with parallel separating plates. Int J Hydrogen Energy 2015;40:2396e403. [10] Boyarko GA, Sung C, Schneider SJ. Catalyzed combustion of hydrogen-oxygen in platinum tubes for micro-propulsion applications. Proc Combust Inst 2005;30:2481e8. [11] Chen GB, Chao YC, Chen CP. Enhancement of hydrogen reaction in a micro-channel by catalyst segmentation. Int J Hydrogen Energy 2008;33:2586e95. [12] Esfahani JA, Fanaee SA. The analytical modeling of hydrogen-air mixture in a catalytic micro-channel. J Thermophys Heat Tr 2015;29:274e80. [13] Fanaee SA, Esfahani JA. The analytical modeling of propaneoxygen mixture at catalytic micro-channel. Heat Mass Transf 2014;50:1365e73. [14] Fanaee SA, Esfahani JA. Two-dimensional analytical model of flame characteristic in catalytic micro-combustors for a hydrogeneair mixture. Int J Hydrogen Energy 2014;39:4600e10. [15] Pan JF, Yang WM, Tang AK, Chou SK, Duan L, Li XC, et al. Micro combustion in sub-millimeter channels for novel modular thermophotovoltaic power generators. J Micromech Microeng 2010;20:125021. [16] Choi BI, Han YS, Kim MB, Hwang CH, Bo Oh C. Experimental and numerical studies of mixing and flame stability in a micro-cyclone combustor. Chem Eng Sci 2009;64:5276e86. [17] Park JH, Lee SI, Wua H, Kwon OC. Thermophotovoltaic power conversion from a heat-recirculating micro-emitter. Int J Heat Mass Trans 2012;55:4878e85. [18] Demoulin O, Clef BL, Navez M, Ruiz P. Combustion of methane, ethane and propane and of mixtures of methane with ethane or propane on Pd/g-Al2O3 catalysts. Appl Catal A-Gen 2008;344:1e9. [19] Deng C, Yang WJ, Zhou JH, Liu ZK, Wang Y, Cen KF. Catalytic combustion of methane, methanol, and ethanol in microscale combustors with Pt/ZSW-5 packed beds. Fuel 2015;150:339e46. [20] Tang AK, Xu YM, Pan JF, Yang WM, Jiang DY, Lu QB. Combustion characteristics and performance evaluation of premixed methane/air with hydrogen addition in a microplanar combustor. Chem Eng Sci 2015;131:235e42. [21] Maruta K, Kataoka T, Kim N, Minaev S, Fursenko R. Characteristics of combustion in a narrow channel with a temperature gradient. P Combust Inst 2005;30:2429e36. [22] Werle S, Wilk RK. Ignition of methane and propane in hightemperature oxidizers with various oxygen concentrations. Fuel 2010;89:1833e9. [23] Norton DG, Vlachos DG. Combustion characteristics and flame stability at the microscale: a CFD study of premixed methane/air mixtures. Chem Eng Sci 2003;58:4871e82. [24] Norton DG, Vlachos DG. A CFD study of propane/air microflame stability. Combust Flame 2004;138:97e107. [25] Li J, Chou SK, Yang WM, Li ZW. A numerical study on premixed micro-combustion of CH4-air mixture: effect of combustor size, geometry and boundary conditions on flame temperature. Chem Eng J 2009;150:213e22. [26] Li J, Chou SK, Li ZW, Yang WM. Development of 1D model for the analysis of heat transport in cylindrical micro combustors. Appl Therm Eng 2009;29:1854e63. [27] Jejurkar SY, Mishra DP. Flame stability studies in a hydrogenair premixed flame annular micro combustor. Int J Hydrogen Energy 2011;36:7326e38. [28] Yan YF, Tang WM, Zhang L, Pan WL, Li LY. Thermal and chemical effects of hydrogen addition on catalytic micro-
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Please cite this article in press as: Tang A, et al., A comparative study on combustion characteristics of methane, propane and hydrogen fuels in a micro-combustor, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.101
Available online at www.sciencedirect.com
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A comparative study on combustion characteristics of methane, propane and hydrogen fuels in a micro-combustor Aikun Tang*, Yiming Xu, Chunxian Shan, Jianfeng Pan, Yangxian Liu School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, China
article info
abstract
Article history:
The combustion characteristics of three hydrocarbon fuels are investigated so as to study
Received 8 July 2015
the applicability of fuels in a micro-planar combustor. Experiments are performed to
Received in revised form
analyze the differences in flame stabilization, temperature distribution of external wall
19 September 2015
and flammable channel-heights. At different equivalence ratios, it is found that the
Accepted 19 September 2015
mixture of hydrogen/air has a much wider and stable flammable range. The reaction po-
Available online xxx
sitions of methane and propane cases will significantly move downstream along with the increase in mixture flow rate. Under the same chemical energy inputs, the external wall
Keywords:
temperature distribution of methane case is most uniform, and the average wall temper-
Micro combustion
ature is also the highest among the three fuels. When hydrogen is selected as a fuel, the
Flame stabilization
temperature gradient of the combustor wall becomes very large, which is caused by the
Temperature distribution
flame position near the inlet. The flammable channel-heights of the three fuels in the
Channel-height
micro-planar combustor are different. It is observed that stable combustion of methane/air
Fuels
cannot be achieved when the channel-height is less than 2.5 mm. However, the minimum flammable channel-height of propane/air case is 2 mm, and hydrogen/air can be ignited in a 1 mm height micro-combustor.1 Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In recent years, micro-power generators, benefitting from the advantage of a long operating lifetime, non-moving parts and high energy density have caused wide public concern [1,2]. These devices directly convert chemical energy of hydrocarbon fuels into electricity or power, which is supplied for MEMS (such as micro pumps and micro robots), portable electronics, military devices and vehicles. A micro-power system consists of (1) a heat source (micro-combustor), (2) energy-conversion
components, and (3) energy-input parts. However, the micro-combustor is the core component of every micro-power generator. Thus, realizing an efficient and stable combustion process has become the key to improving the performance of the whole system. Various research works on the structure optimization of micro-combustors have been done within the past decades. In order to preheat the fresh incoming reactants with the outgoing combustion products, the reverse flow reactor designs are commonly employed for heat recirculation. Ahn et al. [3] have found that the reactive limits of propane over platinum
* Corresponding author. Tel.: þ86 511 88780214; fax: þ86 511 88780216. E-mail address: [email protected] (A. Tang). http://dx.doi.org/10.1016/j.ijhydene.2015.09.101 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Tang A, et al., A comparative study on combustion characteristics of methane, propane and hydrogen fuels in a micro-combustor, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.101
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are broad and cover the equivalence ratio range of 0.2e40 by using the Swiss-roll design. Yang et al. [4] compared the combustors with and without heat recuperation and found that a higher flow velocity and higher H2/air equivalence ratio will increase the rate of entropy generation. While Federici and Vlachos [5] employed a heat recirculation combustor with two inner parallel plates to increase the blowout limit of propane/air mixture. Yang et al. [6] also proposed the SiC porous media form which was employed in the combustion channel to enhance heat transfer between the hot gas and the wall. Chou et al. [7] compared the performance of a micro cylindrical combustor with and without employing porous media. A micro combustor with a bluff body was developed by Fan et al. [8], which can extend the blow-off limit by 3e5 times. Tang et al. [9] found that compared to a single-passage combustor, the micro planar combustor with parallel separating plates can achieve a higher mean temperature of the radiation wall due to the enhancement of heat transfer. In addition, catalytic method was used to strengthen the micro combustion process. Boyarko et al. [10] tested combustion in platinum micro-tubes with an inner diameter of 0.4e0.8 mm. Chen et al. [11] addressed the combustion characteristics of multi-segment catalysts in a micro-reactor by numerical simulation with detailed heterogeneous and homogeneous chemistries. In addition, Fanaee and Esfahani [12e14] selected hydrogen-air or propane-oxygen as the mixture in a catalytic micro-channel and explored their combustion characteristics. During the research of each micro-power generator, the authors usually chose different kinds of fuels and oxidants in the micro combustion process. Typically, for the microthermophotovoltaic (MTPV) system, Yang et al. [9] used H2/ air as the reactant in a micro-planar combustor, while Pan et al. [15] adopt the mixtures of hydrogen-oxygen. Choi et al. [16] selected methane and air as the mixtures. Park et al. [17] selected C3H8/air as the fuel and oxidizer in a heatrecirculation micro-emitter with an annular-type shield. However, different combustion characteristics will be presented when using different fuels and oxidants, but not many comparative studies have been conducted on this front. Demoulin et al. [18] investigated the catalytic combustion of methane, ethane and propane on a Pd/g-Al2O3 catalyst and
studied the influence of adding ethane or propane in the feed during the catalytic combustion of methane in a U-shape quartz micro-reactor. An experiment on the combustion of methane, methanol, and ethanol in three micro packed bed quartz tubes with the ZSM-5 zeolite supported nanometersized Pt as the catalyst was performed by Deng et al. [19]. In the condition of micro-scale, the combustion process of fuel may present very different characteristics compared with conventional scale combustion, which is subjected to the problems of short residence time and large heat loss. Obviously, most of the researches comparing micro-combustion characteristics among fuels are mainly focused on the catalytic reaction due to the advantages of catalytic combustion. However, the research on gas-phase reaction comparing fuels needs to be further improved, especially when it comes to the comparison on the flammable range, external wall temperature and flammable channel-height. In order to contrast the micro-combustion characteristics of different fuels, a micro-planar combustor is designed. Methane, propane and hydrogen are selected as fuels, and air is chosen as the oxidant. The basic characteristics, combustion limit, and scale effect of different fuels are obtained by an experimental method, which can provide references for further fuel selecting processes of micro-power generators.
Experimental set up Fig. 1 shows a schematic diagram of the experimental platform. It consists of a gas feed system, a mixing chamber, a micro-combustor, and the temperature measurement instruments. At first, the fuel (methane, propane and hydrogen) and oxidant (compressed air) will have their pressure reduced via the two valves and then flow into their respective micro flow meters. Purities of methane, propane and hydrogen in this study are 99.99%, 99.0% and 99.5%, respectively. Following this, the two gases will flow into a mixing chamber so as to realize a uniform mixing, and will then be injected into a micro-combustor. The mixture is ignited at the exit of the micro-combustor, and then the flame retreats into the microcombustor and finally realizes a stable combustion state. The
Flow controller Micro-combustor Flash-back arrestor
Infrared thermographer
Micro flowmeter mixing chamber Reducing valve
Flow controller Fuel
Computer
Air
Fig. 1 e Schematic diagram of micro-combustion test platform. Please cite this article in press as: Tang A, et al., A comparative study on combustion characteristics of methane, propane and hydrogen fuels in a micro-combustor, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.101
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mass flow controllers (DSN-2000B, accuracy of 0.5%, full scale) are used to control and measure the flow rates of air and fuels. The measurements of the temperature distribution on the combustor surface are carried out using an infrared thermal imager (Type: Thermovision™ A40). This apparatus can measure temperatures of up to 2000 C. A close-up lens (dimension of minimum distinguishable body: 200 mm 200 mm; minimum focal length: 150 mm) is equipped on the camera so as to obtain clearer images. In order to measure the temperature accurately, the working condition is a windless and lightless room which avoids the radiation of sunlight and other lighting. Besides, the ambient temperature is kept at about 20 C, which can weaken the atmospheric radiation. Under this experimental condition, the thermal infrared imager can be capable of compensating error and ensures the maximum measuring accuracy is ±2% of the reading. To confirm the flame stability, the K-type thermocouple with a bead diameter of 0.5 mm (measuring accuracy: ±1%, measuring range: 0e1300 C) is adopt to measure the exit temperatures and the data recorder (RX4008B, recording a date per second) is used to monitor the temporal evolution of the exit temperatures. In addition, a digital camera (Type: Nikon S8200) is adopted to illustrate the size and position of high temperature region. In this study, a micro-planar combustor is designed and tested. The total dimensions of this micro-planar combustor are 18 mm 9 mm 4 mm (length width height). The wall thickness of the micro combustor is kept as 0.5 mm, thus, the combustion channel has a width of 8 mm and a height of 3 mm. 316 stainless-steel is used to manufacture the combustor, which can withstand a high temperature of 2000 C and has an emissivity of 0.65.
Results and discussions Comparison of flame location and flammable range In this paper, most of the experimental cases can be regarded as a stable-flame condition, and the flame stability is defined as: under a certain range of flow rates, the fuel/air can keep stable combustion in the micro-combustor without the phenomenon of flame oscillations or repetitive extinction and ignition occurring. Besides, the flame is not apparently asymmetrical. At the same time, the distribution of external wall temperature is relatively steady. Fig. 2 shows the experimental photos of the micro-combustor using different fuels,
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Fig. 3 e Location of maximum wall temperature for the three fuels (equivalence ratio ø ¼ 1). which are all from the digital camera. The equivalence ratio of each case remains as 1, the inlet velocities of the three mixtures are given in the figures, and each experimental case is at a stable-flame condition. It can be seen that all of the external walls become brighter with the increasing of the mixture inlet velocity, and the high temperature regions, which are located at the front area of the combustor, also expand more widely when the mixture's inlet velocity rises. The highest temperature locations of methane, propane and hydrogen cases are depicted in Fig. 3. In our previous study [20], it is found that the location of maximum wall temperature is very close to the location of maximum flame temperature. As a result, it is acceptable that the location of maximum wall temperature is used to indirectly indicate the flame location in this paper. Due to the highly combustible characteristic of hydrogen, the high temperature regions of hydrogen cases consistently remain within very close proximity of the inlet, as such the highest temperature locations nearly have no differences and they are about 1 mm away from the inlet. However, in the cases of methane and propane, the highest temperature locations apparently move towards the outlet with an increase in flow rate. When inlet velocity is from 0.3 m/s to 0.6 m/s, the highest temperature locations of methane cases are 3.5 mm, 4 mm, 5 mm and 6 mm, respectively. For propane, the locations are about 2.6 mm, 3.3 mm and 4.4 mm in the case of 0.3 m/s, 0.5 m/s and 0.7 m/s, respectively. For methane and
Fig. 2 e Experimental photos at different inlet velocities (equivalence ratio ø ¼ 1). Please cite this article in press as: Tang A, et al., A comparative study on combustion characteristics of methane, propane and hydrogen fuels in a micro-combustor, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.101
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propane, the incoming cold feed starts pushing the reaction zone downstream owing to the faster flows. This is similar to the phenomenon observed by Maruta et al. [21], although the external wall is maintained at time-constant, which is a little different with the heat-dissipation condition in this study. However, at the same inlet velocity, the highest temperature location of methane is further away from the inlet along the gas flow direction. The hydrocarbon chain length of methane is 109.3 pm, and the value of propane is 111 pm. In addition, the ignition temperature of methane and propane are 810 K and 745 K respectively [22]. Obviously, it is concluded that the ignition temperature decreases when the length of the hydrocarbon chain increases [18], which leads to the various locations of flame. The conditions of flame stabilization at different equivalence ratios for these fuels are examined and the results are clearly shown in Fig. 4. As for the mixture of methane/air, when the equivalence ratio varies from 0.7 to 1.0, the minimum flammable velocity will be stabilized at 0.2 m/s. It is therefore difficult for methane/air to be ignited when the velocity of mixture is less than this value. Meanwhile, the maximum flammable velocity remains as 0.6 m/s in the lean fuel conditions, and it reaches 0.7 m/s at the stoichiometry ratio case. Although, the flame location is only 6 mm away from the inlet at 0.6 m/s velocity (as seen in Fig. 3). Flame oscillation with a random asymmetry occurs when the velocity of the mixture exceeds 0.7 m/s, and finally leads to blowout. The unstable flame that exhibits dynamic behaviors were also observed by Norton et al. [23] and Maruta et al. [21]. However, in the rich fuel condition, the stable combustion is difficult to achieve in the micro-planar combustor, and the flame is found to be consistently located at the outlet under different velocities. As a result, the rich fuel condition is not favorable for the micro-planar combustor when methane is taken as the fuel. Compared to methane, the minimum flammable velocities of propane/air are higher in lean fuel and stoichiometry ratio conditions. Both the minimum and maximum flammable velocities are 0.5 m/s at equivalence ratio 0.7. When the
equivalence ratio is from 0.9 to 1.2, the minimum flammable velocity remains 0.2 m/s. However, the maximum flammable velocity will be raised gradually along with the increase of the equivalence ratio, and reaches the highest value of 1 m/s at stoichiometry ratio. Similarly, random asymmetry flame can be also observed when the inlet velocity exceeds 1.0 m/s. In addition, a little soft sound will be generated before blowout. As such, the flammable range of propane/air will get an extension in the lean fuel and stoichiometry ratio conditions, which can be seen from Fig. 4. On the other hand, due to heat loss because of unburned propane, the maximum flammable velocity decreases with the increasing of the equivalence ratio in the rich fuel conditions. On the whole, with propane it is relatively easy to realize a stable combustion in the rich fuel condition, and the flammable range is also broader than methane/air under the same condition. In general, propane micro-flames are more robust than methane micro-flames in a burner with two parallel plates. The propane allows a wider range of wall thermal conductivities as well as higher external heat-loss coefficients [24]. In the case of hydrogen and air, the minimum flammable velocity remains basically stable at 0.1 m/s, which is due to the highly combustible characteristic and highest heating value of hydrogen. The minimum flammable velocity of hydrogen is equal to that of methane/air (0.2 m/s) at the equivalence ratio 0.7. As for the maximum flammable velocity of hydrogen/air, limiting to the full scale of mass flow controller (hydrogen: 2000 mL/min), the maximum flammable velocity of hydrogen case can reach about 4.7 m/s (at stoichiometry ratio). Therefore, this value is larger than that of methane (0.7 m/s) and propane (1.0 m/s) in the same condition, which is also much lower than the maximum flammable velocity in a microcombustor. Li et al. [25] pointed out that the blowout limit of hydrogen can reach 8 m/s in a cylindrical tube with 0.8 mm inner diameter.
Temperature distribution at the same chemical energy input The external wall temperature of a micro-combustor is able to decide the energy input of micro-power generators, such as the MTPV system, thus much more focus should be paid on the external wall temperature distribution. In order to evaluate the working performance of different fuels in the same micro-combustor, a comparison of the external wall temperature distribution at different flow rates is made. Firstly, as mentioned above, this study selects that the inlet velocities of methane/air are 0.4 m/s, 0.5 m/s, and 0.6 m/s at the equivalence ratio 1, and the corresponding flow rates of methane are 55 mL/min, 68 mL/min and 82 mL/min, respectively. The highest heating value of methane is 55.5 MJ/kg, hydrogen is
Table 1 e Flow rates of fuels under the same chemical energy input. Chemical energy input Fig. 4 e Flame stabilization at different equivalence ratios for the three fuels (equivalence ratio ø ¼ 1).
CH4 H2 C3H8
36.63 W
45.28 W
54.62 W
55 mL/min 172 mL/min 22 mL/min
68 mL/min 213 mL/min 27 mL/min
82 mL/min 257 mL/min 32 mL/min
Please cite this article in press as: Tang A, et al., A comparative study on combustion characteristics of methane, propane and hydrogen fuels in a micro-combustor, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.101
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Fig. 5 e Experimental photos at different chemical energy inputs (equivalence ratio ø ¼ 1). 141.8 MJ/kg and propane is 50.35 MJ/kg. Based on these methane flow rates, flow rates of other two fuels can be calculated under the same chemical energy input (as seen in Table 1). The experimental photos of the external wall at three chemical energy inputs are shown in Fig. 5. With the increasing of the chemical energy input, it is notable that the external wall of each mixture case becomes more and more bright. The high temperature regions of hydrogen cases are apparently smaller than that of the other two fuels. When the chemical energy input is 36.63 W, the high temperature region of propane case is slightly brighter than that of methane case. However, when the chemical energy input reaches 45.28 W and 54.62 W, the high temperature regions of methane cases broaden significantly, and the whole external walls become much brighter than those in the case of propane. In order to further illustrate the wall temperature distribution of a micro-combustor, the centerline temperature profiles of the external wall along the flow direction at three chemical energy inputs are plotted in Fig. 6. First, the adiabatic flame temperature of hydrogen/air reaches 2390 K, which is higher than that of methane/air (2232 K) and propane/air (2267 K) [26]. In our previous works, it is found that the flame front of hydrogen case is much smoother, which is contributed to its highly reaction rate. Hence, the maximum temperatures of external walls in the case of hydrogen are all larger than that of the other two mixtures. When the chemical energy input is 54.62 W, the maximum temperature of the hydrogen case reaches 1195 K, which is 61 K and 69 K higher than the methane and propane case. It is found that some of the unburned methane and propane will combust near the
outlet at the two higher chemical energy input cases, which is another reason leading to the lower value of maximum temperature. On the other hand, with an increase of the inlet velocity, a corresponding increase in reaction rate resulted in higher heat release and axial heat conduction further acted to spread the absorbed heat in the walls [27] which contributing to the external wall temperature rising of the three fuels. Furthermore, due to the low ignition temperature, the maximum temperature of each hydrogen case apparently remains at the position of 1 mm away from the inlet. Whereas the flame locations of methane and propane cases will move downstream along with an increase in mixture flow rate, which can be noticeably seen in Figs. 3 and 6. However, due to the lower apparent activation energy of propane combustion (126 kJ/mol for propane compared to 203 kJ/mol for methane) [24], the propane is easier to be ignited. As a result, comparing to the methane case, the location of highest wall temperature in the propane case is much closer to the inlet. From Fig. 6(a) we can see that the maximum temperature of propane is 1027 K, which is only 3 K higher than that of the methane case. However, in the high chemical energy input cases, the maximum wall temperature of the methane case turns to be larger than that of the propane case. Starting at the position of 4 mm, the wall temperatures of methane cases are significantly higher than the other two cases. This is consistent with the phenomenon of the external wall being much brighter in the corresponding areas (as seen in Fig. 5). At each chemical energy input case, the flow rates of methane/air are all larger than that of propane/air, and the velocity difference also rises with the increasing of chemical energy input. As mentioned above, the increasing of inlet velocity will bring out a
Fig. 6 e Centerline temperature profiles of external wall along the flow direction at different chemical energy inputs (equivalence ratio ø ¼ 1). Please cite this article in press as: Tang A, et al., A comparative study on combustion characteristics of methane, propane and hydrogen fuels in a micro-combustor, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.101
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corresponding growth of reaction rate, which will result in a higher heat release and axial heat conduction, and finally enhance the heat absorption process of the walls [27]. Hence, at 36.63 W chemical energy input case, although the relatively higher inlet velocity of methane/air do not generate an increase of the maximum wall temperature, the temperature values at most position along the flow direction are all larger than propane case. When it comes to the higher chemical energy input case, the effect of velocity difference has brought out a more obvious influence to the temperature distribution, which also including the maximum wall temperature. When it comes to the whole temperature gradient, the values of methane, propane and hydrogen case at 36.63 W are 148 K, 184 K and 293 K, respectively. Even when the chemical energy input is raised to 54.62 W, the temperature gradient of the methane case only reaches 154 K, which is 24 K and 180 K less than that of propane and hydrogen cases. Due to the monotone decreasing trend of the centerline temperature along the flow direction, the temperature gradients of hydrogen are highest among three fuels. However, as for methane and propane cases, the temperatures will receive a steady growth at the preheating zone, and then gradually decrease at their respective peak-temperature position. This variation trend is bound to improve the uniformity of wall temperature distribution. Especially, in the case of methane the flame location is much closer to the middle of the microcombustor, which leads to the lowest temperature gradient. The mean temperature of the external wall will be affected by the following factors: the maximum wall temperature and the flame position; the maximum temperature gradient and the temperature variation trend. Fig. 7 shows the specific values of the average temperature at selected chemical energy inputs. It is found that the average value of the wall temperature in each case rises gradually with an increase in the flow rate. When the chemical energy inputs are 36.63 W and 45.28 W, the average temperature of methane and propane cases are very close. However, benefiting from the larger maximum temperature and the lower temperature gradient, the mean temperature in the case of methane reaches 1040 K at the chemical energy input of 54.62 W, which is about
Fig. 7 e Average temperature of external wall at different chemical energy inputs (equivalence ratio ø ¼ 1).
12 K larger than that of propane case. The largest temperature gradient finally results in the lowest mean wall temperature at each chemical energy input. In view of the mean wall temperature and gradient, the two hydrocarbon fuels are more suitable for the microthermophotovoltaic system, as the micro-combustor requires greater radiation energy from the external wall. However, as mentioned above, the blow-out inlet velocity of methane/air and propane/air are limited to 0.7 m/s and 1.0 m/ s (at stoichiometry ratio condition). As a result, it is difficult to further improve the output power density and energy conversion efficiency of the MTPV system. Recently, it was found that due to the high combustibility of hydrogen, the microcombustion process of these hydrocarbon fuels could be greatly improved by adding a small amount of hydrogen into the mixture [20,28e30].
Effect of channel height for fuels In the micro-combustion process, the problem of miniaturization limitation cannot be avoided. Take the example of the micro-planar combustor in this paper, the stable combustion will be hardly maintained when the channel-height is lower than a certain value. Hence, combustion characteristics of the three fuels at different channel-heights are compared in the following part. First, the mixture of methane/air is able to keep stable combustion at the channel-height of 2.5 mm. However, when the channel-height decreases to 2 mm, the mixture cannot be ignited in the micro-combustor. Consequently, the quenching distance of methane/air can be defined as 2.5 mm in this micro-planar combustor. Then, the combustion characteristic of this height is compared with the 3 mm channel-height case. In this experiment, when channel-height decreases at the same flow rate, the changing result of external wall temperature is determined by the three factors: Firstly, the separation distance between the flame and the internal wall will be reduced, which enhances the heat absorption process of the internal wall. Secondly, the velocity of mixture will be increased, which brings out an improvement of heat transfer coefficient between the fluid and internal wall. Finally, the residence-time of mixture becomes shorter under the same flow rate, which is likely to result in an incomplete combustion. The experimental photos of the two micro-combustors are shown in Fig. 8 (methane flow rate: 55 mL/min, stoichiometry ratio). It can be seen that the difference of the brightness between the two external walls is very small, and the flame position of the 2.5 mm case is slightly closer to the inlet. Fig. 9 depicts the centerline temperature profiles of the external wall along the flow direction at different flow rates. When the flow rate of methane is 41 mL/min, the trend of temperature distribution and the specific values are nearly the same. However, the differences become more pronounced in 55 mL/min cases. Along with the decrease of channel-height, the separation distance between the flame and the internal wall will be reduced, which enhances the heat absorption process of the internal wall. As a result, at the inlet zone, the wall temperature of the 2.5 mm case is higher than that of the 3 mm case. But the maximum temperature difference is only 4 K. However, beginning at the position of 3 mm away from
Please cite this article in press as: Tang A, et al., A comparative study on combustion characteristics of methane, propane and hydrogen fuels in a micro-combustor, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.101
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Fig. 8 e Experimental photos of methane with different channel-heights (fuel flow rate: 55 mL/min, equivalence ratio ø ¼ 1).
Fig. 10 e Experimental photos of propane with different channel-heights (fuel flow rate: 32 mL/min, equivalence ratio ø ¼ 1).
the inlet, the passive effect of residence-time becomes dominantly and the incomplete combustion in the microchannel will lead to a falling of wall temperature. Consequently, the external wall temperature of the 3 mm case is apparently higher than that of 2.5 mm case. Besides, as mentioned above, the maximum flammable velocity of methane/air is 0.7 m/s in the micro-combustor with 3 mm channel-height. When it comes to 2.5 mm microcombustor, the corresponding value is only 0.5 m/s, which greatly limits the fuel input of the micro-power generators. Therefore, the channel-height of the combustor should be larger than 3 mm when methane and air are selected as the fuel and the oxidant. Zhao et al. [31] pointed out that the quenching distance of propane/air is lower than that of methane/air in the same condition. The same phenomenon is observed in our study, it is found that the flammable channel-height of propane/air case can be minimized to 2 mm. Fig. 10 shows the experimental photos of 2 mm and 3 mm channel-height (at stoichiometry ratio, the propane flow rate is 32 mL/min). And the
centerline temperature profiles of the external wall along the flow direction are plotted in Fig. 11. It can be noted that the external wall of 2 mm case is much brighter than that of 3 mm case, especially at the flame region. Under the two flow rates, the maximum temperatures of 2 mm case reach 1122 K and 1196 K, which are 95 K and 70 K higher than that of 3 mm case. This phenomenon is due to the improvement of heat transfer between the flame and the internal wall. However, the whole wall temperature of the 2 mm case is higher than that of the 3 mm case, which is different with the methane/air experiment. As seen from Fig. 11, the temperature difference of the two combustors decreases evidently along the flow direction. Although, the residencetime of the mixture is shortened with a decrease in channelheight, the two active effects will overcome the adverse influence of residence-time, and finally bring out an evident growth of external wall temperature. In view of test results, the promotional effect of the heat transfer process apparently plays a dominant role. As a result, the average wall temperature of the 2 mm case is about 40 K higher than that of the 3 mm case at 32 mL/min flow rate.
Fig. 9 e Centerline temperature profiles of external wall along the flow direction (fuel: methane; channel-height: 2.5 mm, 3 mm; equivalence ratio ø ¼ 1).
Fig. 11 e Centerline temperature profiles of external wall along the flow direction (fuel: propane; channel-height: 2 mm, 3 mm; equivalence ratio ø ¼ 1).
Please cite this article in press as: Tang A, et al., A comparative study on combustion characteristics of methane, propane and hydrogen fuels in a micro-combustor, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.101
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Moreover, the maximum flammable velocity of a 2 mm channel-height combustor can also reach 1 m/s at stoichiometry ratio. Thus, when propane is selected as fuel, the 2 mm channel-height micro-planar combustor will be more appropriate for the MTPV system. Finally, a comparison of the micro-combustion process with 1 mm, 2 mm and 3 mm channel-height is made, with hydrogen as the selected fuel. The centerline temperature profiles of the external wall along the flow direction are shown in Fig. 12. The flow rates of hydrogen are 200 mL/min and 400 mL/min respectively (at stoichiometry ratio). At the flame zone near the inlet, the external wall temperatures of 1 mm cases are also evidently higher than that of the other two cases. The external wall temperature decreases rapidly after the reaction, and then rises again at the position of 12 mm (200 mL/min) and 13 mm (400 mL/min). It is observed that a phenomenon of secondary combustion appears near the outlet in the 1 mm channel-height case. In order to illustrate this phenomenon, the exit temperature of hydrogen at different channel-heights are measured and depicted in Fig. 13. It is notable that the exit temperatures are relatively steady when the channel-heights are 2 mm and 3 mm, and the temperature amplitude is in a range of 20 K. While the exit temperatures of 1 mm channel-height have an apparent fluctuation, which indicates the flame is unstable. Pizza et al. [32] pointed out that the quenching effect of the wall becomes stronger along with the decrease of channelheight, and it is no longer possible to stabilize flames for narrow enough channels. Although this simulation work is under a forced wall boundary condition, the results are very similar to our study. In this paper, the inlet velocity of hydrogen/air is about 0.7 m/s (hydrogen flow rate: 200 mL/ min) in the 2 mm channel-height case. The flame is closed symmetric steady in the 2 mm channel-height when the inlet velocity is in the range of 0.04 m/s to 0.95 m/s [33]. Hence, Fig. 13 illustrates the flame temperature oscillations are occurred and the flame instability results in the incomplete combustion at the 1 mm channel-height case. Consequently, although the hydrogen/air combustion could be realized in the
Fig. 13 e Variation of the exit temperature with time at different channel-heights (hydrogen flow rate: 200 mL/ min; channel-height: 1 mm, 2 mm, 3 mm; equivalence ratio ø ¼ 1).
1 mm channel-height combustor, the incomplete chemical reaction is unavoidable. As mentioned above, the combustion processes of 2 mm and 3 mm cases will be much more stable and the wall temperature variations of the two cases are also very similar. In general, at a distance of 0e4 mm away from the inlet, the external wall temperatures of 2 mm cases are higher than that of 3 mm cases at both 200 mL/min and 400 mL/min fuel flow rates. However, the situation is different at a distance of 4 mm from the entrance. In the low flow rate condition, the wall temperature of 2 mm case is still higher, but the minimum difference is only 2 K. When it comes to the high flow rate case, the temperature values of the 3 mm case will apparently exceed those of 2 mm case, and the temperature difference near the outlet is about 50 K. It can be concluded that when the channel scale decreases, the comprehensive effect of shortening residence time and raising heat transfer intensity varies with the flow rate of the mixture. Meanwhile, as for the micro-power generators like the MTPV system, it should be noted that under the same radiant energy output, the adopting of micro-combustor with lower channel-height means a larger volume power density could be obtained. From this point of view, the 2 mm channel-height micro-combustor will exhibit a stronger superiority.
Conclusions
Fig. 12 e Centerline temperature profiles of external wall along the flow direction (fuel: hydrogen; channel-height: 1 mm, 2 mm, 3 mm; equivalence ratio ø ¼ 1).
In this paper, the combustion characteristics in a microplanar combustor using three types of fuels are investigated through an experimental method. The effect of inlet velocity, flammable range, external wall temperature and channelheight are analyzed and compared, which are important for the micro-power generators. The main conclusions obtained from this study are summarized below:
Please cite this article in press as: Tang A, et al., A comparative study on combustion characteristics of methane, propane and hydrogen fuels in a micro-combustor, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.101
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(1) As the inlet velocity of mixture increases, the external wall surface becomes brighter and the high temperature region expands more widely. The stable flammable range of hydrogen/air is wider than that of propane/air and methane/air. While the maximum temperature positions of methane and propane cases will obviously move downstream in response to an increase in the mixture's flow rate. (2) Under the same chemical energy input, the maximum wall temperature and temperature gradient of hydrogen are higher than that of methane and propane cases. However, the external wall temperature distribution of methane case is the most uniform, and the average wall temperature is also the highest among the three fuels. Considering the average wall surface temperature, methane and propane are the most suitable gas fuels for the micro-power generators. (3) Methane/air can keep a stable combustion in the microcombustor with a minimum channel-height of only 2.5 mm, while propane/air can achieve combustion at 2 mm channel-height, and hydrogen/air can be ignited in a 1 mm height micro-combustor.
Acknowledgments This work is supported by National Natural Science Foundation of China (No.51206066, No.51376082), China Postdoctoral Science Foundation (No.2014M551514), Natural Science Foundation of Jiangsu Province (No.BK20131253), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Scientific Research Starting Foundation for Advanced Talents of Jiangsu University (No.11JDG139).
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Please cite this article in press as: Tang A, et al., A comparative study on combustion characteristics of methane, propane and hydrogen fuels in a micro-combustor, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.101