- Email: [email protected]
Development of a catalytic combustor for a stationary fuel cell power generation system
Renewable Energy 35 (2010) 1083–1090
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Development of a catalytic combustor for a stationary fuel cell power generation system Sangseok Yu a, Dongjin Hong b, Youngduk Lee c, Sangmin Lee c, Kookyoung Ahn c, * a
Chung Nam National University, Daejeon 305-764, Republic of Korea Hyosung Corporation, Anyang 431-080, Republic of Korea c Korea Institute of Machinery and Materials, Daejeon 305-343, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 March 2009 Accepted 12 October 2009 Available online 7 November 2009
The anode off-gas of high temperature stationary fuel cell stacks still includes fuel components such as hydrogen, carbon monoxide, and hydrocarbon due to the innate characteristics of the fuel cell operation. Even though the anode off-gas has fuel contents, the flammability is very limited due to the vapor concentration of the anode off-gas. A catalytic combustor is applied as an off-gas combustor so as to utilize the waste energy of anode off-gas by stimulating a chemical reaction over selected operating conditions. Temperature and flow uniformity in the radial direction are very significant factors of the durability, because the catalytic combustion is carried out on the surface of the catalyst site. On the other hand, the catalyst selection is also very important due to the composition of the anode off-gas. In this study, the flow uniformity is presented prior to a catalyst screening test. From the results of the screening test, where three commercially available catalysts are tested, KIMM-I and KIMM-II are selected as candidates for a catalytic combustor of anode off-gas. Ó 2009 Published by Elsevier Ltd.
Keywords: High temperature stationary fuel cell Anode off-gas Catalytic combustor Flow uniformity
1. Introduction The global greenhouse effect and the long term shortage of fossil fuel have raised demand for the development of novel conceptual power propulsion systems, which should offer zero emissions, high efficiency, and high performance. Stationary power propulsion fuel cells are a very attractive candidate in this regard in that they are capable of meeting the stringent requirements of future power propulsion systems. A fuel cell is an electrochemical energy conversion device that produces heat and electricity by chemical reaction. Different from conventional power plants, the fuel cell is free from mechanical loss in producing electricity and thus is operated at high efficiency. Furthermore, when the fuel cell is operated by hydrogen fuel and air, the final product is also free from air pollution and does not contribute to the greenhouse effect. When the fuel for the stationary fuel cell is reformed from HC fuels, the exhaust gas of a stationary power plant fuel cell is composed of carbon dioxide and water. However, since the efficiency of fuel cells is not very high, but, higher than the efficiency of conventional combustion engines and thus results in the improved fuel consumption, less
* Corresponding author. Tel.: þ82 42 868 7324; fax: þ82 42 868 7284. E-mail address: [email protected] (K. Ahn). 0960-1481/$ – see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.renene.2009.10.015
carbon dioxide is exhausted from the power plant. Among such fuel cell systems, the solid oxide fuel cell (SOFC) and the molten carbonate fuel cell (MCFC) can feasibly achieve high efficiency, largely because the thermal energy of the exhaust gas can be recovered. Williams et al. [1] reported that the SOFC and the MCFC are currently the most advanced and available technologies for distributed power generation. Some market leading companies have already commercialized fuel cell systems for stationary power generation and their commercialized products have begun to penetrate the stationary power plant market [2]. The fuel cell of a stationary power generation system consists of an electric power generation module (stack) and a fuel supply module (reformer), a thermal energy recovery module (combustor and heat exchanger), and an air supply module (compressor and turbo-charger), among others. Balance of plant (BOP) denotes auxiliary components excluding the fuel cell stack that allows organic operation of the system. While the development of the fuel cell stack should be the core technology of the fuel cell system, system efficiency is usually achieved by finely controlling the BOP operations. Additionally, the development of BOP is another technical barrier for fuel cell commercialization due to durability and operating ranges of the fuel cell system. For stable operation and durability of a high temperature stationary fuel cell system, fuel utilization should be less than
1084
S. Yu et al. / Renewable Energy 35 (2010) 1083–1090
one, which implies that the exhaust gas from the anode side contains fuel contents such as hydrogen, carbon monoxide, and hydrocarbon. In order to improve the efficiency, the fuel contents from the exhaust should be burned out and the thermal energy should be recovered. Unfortunately, since a conventional combustor will meet flammability limits at some operating conditions, it is not easily applied for burning out the exhaust gas of the fuel cell. On the other hand, catalytic combustion is generally accepted as a combustion method for the anode off-gas from the fuel cell stack, because it burns efficiently in concentrations outside flammability limits and at low operating temperature. Furthermore, the catalytic combustor burns out the exhaust gas from the fuel cell without any undesired by-products, such as UHC, CO, or NOx [3]. Due to the low emission characteristics of the catalytic combustor, it has been developed for application in gas turbines [4–6]. However, there has been little reported research on catalytic combustors for high temperature stationary fuel cell systems. Even though the catalytic combustor is beneficial to the combustion of fuel cell off-gas, its commercial application has been delayed due to the stability and durability of the system. In particular, since the stability and the durability of the catalytic combustor depend on the type of catalyst and operating characteristics, selection of the catalyst and the design technology to achieve a uniform flow distribution prior to the catalyst bed are very important. The objective of the present study is to develop a catalytic combustor for off-gas combustion of a high temperature stationary fuel cell system and to investigate through experiments the flow and combustion characteristics of catalytic combustors so that an optimal catalytic combustor can be designed. Various factors affecting the performance of the catalytic combustor are examined such as inlet flow condition, preheating temperature, and space velocity.
2. Design considerations of a catalytic combustor for a high temperature stationary fuel cell system High temperature stationary fuel cell systems can be categorized as high pressure cylindrical systems and atmospheric planar systems. In this study, the catalytic combustor is applied to a planar type fuel cell system, shown in Fig. 1. This system is fed by natural gas and air. The system components are the stack, desulfurizer, preprocessor, reformer, heat exchangers, catalytic combustor, T/G module, and HRSG. In this study, sufficient air flow rate required for the catalytic combustor is supplied by a turbo-generator. Furthermore, the required parasitic power is also supplied through electricity generated by the turbo-generator. In general, it is difficult to consume entire fuel induced into the stack due to the durability. The fuel utilization is the ratio of fuel reacted to the fuel supplied, which describes both fuel consumption and ejection from the fuel cell. Since the anode off-gas contains fuel contents, it should be treated before it is exhausted into the atmosphere. Otherwise, it may explode or combust in the exhaust pipes. Even though the anode off-gas contains fuel in its mixture, the high concentration of water vapor in the fuel mixture limits operation of the conventional burner to within limited ranges. Even if a conventional combustor can burn out the fuel mixture from the fuel cell, the product gas of the conventional combustor is usually composed of NOx product, which should be eliminated at the exit. Compared with a conventional combustor, a catalytic combustor can be operated over wide ranges and no NOx product is observed at the exit of the combustor due to the low operating temperature. When the catalytic combustion is completed, the product is only composed of water vapor, CO2, and nitrogen. The low operating temperature of the catalytic combustor effectively eliminates NOx production in the exhausted gas. Additionally, proper selection of catalysts can enhance the role of the catalytic combustor, and thus the catalytic combustor preferentially burns out the target fuel component in the mixture.
Fig. 1. Schematic diagram of high temperature stationary fuel cell power generation system.
S. Yu et al. / Renewable Energy 35 (2010) 1083–1090
1085
AIR-TBN AIR
CCOUT5 AIR3
HTREF
CATALBNR
HX2
HX1 CCOUT0
ANOD-OUT
CCOUT1
HX3
CCOUT2
FCINIDE
NGWAT2
AIRBLOW
EXTRFO1
ANOD-IN
EXTRFO2
SOFC FCINSID3
FCINSID2
CCOUT4
AIR2
INT-REF
SEP
HX7
CCOUT3
MIX
FCINSID1
W-PUMP
EXT-REF
ARTBN5
HTLOSS
Q
RSTOIC
HEATLOSS
HXADD
NGWAT3 HTTRANS1
MULT
CATH-IN
WATER2
AIR1
HTTRANS2
SEP
EXTREFIN
ARTBN3
WATER1
NG2
HX4
ARTBN6
HX5
CAT-OUT1
CAT-OUT2
HX6 CAT-OUT3
CAT-OUT4
NGPUMP
ARTBN2
NG1 AIR-COMP
ARTBN1
Fig. 2. High temperature stationary fuel cell system configuration for estimation of the design point of catalytic combustor with Aspen PlusÒ.
The anode off-gas of the high temperature stationary fuel cell does not only include a high concentration of water vapor but it also includes a small portion of methane (CH4), and some portions of CO and H2. In the fuel mixture at the anode exit of the high temperature stationary fuel cell, combustion of residual hydrogen is straightforward but the combustion of CO and methane needs to be delicately treated due to vapor concentration. In practical terms, the most difficult component of the fuel mixture to be combusted is methane whereas the hydrogen is completely combusted. Accordingly, in selecting a catalyst for the catalytic combustor, the proper catalyst for methane in the mixture at given operating conditions should be chosen. In this study, the performance of three catalysts is evaluated to select the proper catalyst for methane combustion under poor combustible conditions. Prior to the evaluation of the catalysts, the operating temperature of the catalyst combustor and the gas inlet conditions should Table 1 Fuel cell system parameters for Aspen Plus. Components
Input
FC stack
Press. Temp. Rate potential Uf Uair
Atmosphere 750 C 0.75 V@500 mA/cm2 65–80% 20–30%
Reformer: ER/IR
Method Press. Temp. S/C
Steam reforming 1.2–1.3 bara 500–750 C 3.0
<750 C 4
Catalytic combustor
TCC,out Excess air ratio
Turbo-generator
Comp
h
Turb
h
Pr Pr Air blower
h
70% 3.5 bara 80% 1.17 bara 70%
be defined. In this study, the gas temperature at the exit of the catalyst bed is defined as the operating temperature of the catalytic combustor and is set at 750 C. Additionally, the marginal operating temperature of off-design operation is very important for the longevity of the catalytic combustor, because the catalyst and substrate can be damaged at temperatures in excess of 850 C. The inlet temperature of the catalytic combustor is the temperature before the perforated plates and the temperature varies between 150 and 350 C depending on the evaluation conditions. When the inlet gas temperature is 150 C or lower, combustibility of the low temperature gas is very sensitive to catalysts. Thus, as a prior step to designing the catalytic combustor, it is necessary to screen the catalysts for the catalytic combustor. Meanwhile, the distribution of the inlet gas is also an important design factor. In general, the catalytic reaction is a surface reaction, which becomes diffusion limited inside the catalyst site. At the catalyst site, the substrate temperature approaches the adiabatic flame temperature of the fuel/air mixture in the local area. Since the adiabatic flame temperature is very high, locally hot spots result in thermal sintering of the support surface area, thermal sintering and vaporization of the noble metals, and thermal shock fracturing of ceramic supports. Since a locally dense mixture results in local hot spots [3], the gas uniformity should be ensured for stable operation and durability. Furthermore, the reaction is also affected by the mixing quality of the fuel and the air streams. When the fuel and
Table 2 Calculated simulation gas composition and flow rate of anode off-gas for 1 kW high temperature stationary fuel cell system. Gas component
Flow rate (slpm)
Proportion (%)
H2 CO CH4 CO2 H2O
5.71 1.03 0.05 3.19 15.52
22.4 4.0 0.2 12.5 60.9
1086
S. Yu et al. / Renewable Energy 35 (2010) 1083–1090
Catalyst P.P. Ring P.P.
Fig. 3. Flow uniformity of inlet gas with perforated plates.
the air streams are well mixed and supplied uniformly prior to the combustion region, high performance combustion can be expected without any locally defected areas. The space velocity (SV) is also an important design parameter of the catalytic combustor. A physical parameter, SV, is commonly used for the measure of the reactor throughput:
i h SV h1 ¼ Q =V where Q is the volumetric flow rate of the feed and V is the reactor volume. Since the space velocity represents the residence time of the feed to the catalytic combustor, a hot spot may occur at very low space velocity, which can lead to subsequent damage of the catalyst and substrate. When the SV is very high, a significant portion of the fuel mixture might be slipped without any reaction. Accordingly, during the screening of the catalyst, the performance of catalysts is evaluated over various space velocities. However, since the volumetric flow rate is given at the beginning of the design process, the space velocities are changed with geometric modifications. 2.1. Estimation of operating conditions of catalytic combustor The flow rate and composition of the fuel mixture at the anode exit are very important parameters in designing the size and capacity of the catalytic combustor. When the fuel cell in Fig. 1 is operated, the reformer supplies hydrogen and CO to the fuel cell stack and the fuel cell stack consumes some portion thereof to produce electricity. Since the stability and durability of the stack limit the fuel utilization factor to less than one, the off-gas at the anode side unavoidably includes fuel components, and the residual fuel mixture should be burnt out in the catalytic combustor to improve the system efficiency.
The configuration of the fuel cell system in Fig. 1 is analyzed with Aspen PlusÒ, which is shown in Fig. 2. Calculation parameters are shown in Table 1. In this study, the high temperature stationary fuel cell stack is an atmospheric planar fuel cell that is operated at 750 C. The fuel is partially supplied by an external steam reformer even though the exact fraction varies with the system configuration. Since the internal reformer is very important for the heat rejection of a large scale system, an internal steam reformer is also employed. In this study, the fuel cell system is a co-generation system with a turbo-charger to maximize the system efficiency. Computation of operating processes of the fuel cell system provides information on the heat and mass balance of the catalytic combustor, and the system efficiency, essential components, and design criteria of components are also estimated. As a result of the process calculation, the mass and energy balance of the system can be estimated. The gas composition and volumetric fraction of the anode off-gas are also determined, as shown in Table 2. The calculation data is then provided for the design of the catalytic combustor. Additionally, the gas composition and flow rate into the catalytic combustor are controlled by a mass flow controller when evaluating the performance of the catalysts. 3. Performance evaluation of catalytic combustor 3.1. Evaluation of flow uniformity The perforated plate used in this study has a penetration area of 10% and thickness of 2 mm. A hot wire anemometer is applied to measure the velocity of each mesh and thereby evaluate the effects of the perforated plate on the flow distribution. When one perforated plate is installed, it is difficult to achieve a uniform flow. Fig. 3 shows two perforated plates installed below the catalysts. Flow uniformity at the catalyst inlet was achieved by installing two crossing perforated plates between the catalyst and the mixing chamber with a minimal pressure drop. Since the gas mixing order can result in the flashback of hydrogen, we mixed the air and vapor before the gases are mixed with fuel gas (CH4 þ H2 þ CO), as shown in Fig. 4. Furthermore, it is important that the fuel and air streams be well mixed and supplied uniformly prior to the combustion region, because the catalytic combustion is a surface reaction. Therefore, a static mixer is installed to enhance gas mixing. Perforated plates are located at the upstream of the catalyst bed to guarantee a uniform velocity distribution over the cross-section. The combustion chamber is an insulated monolith reactor. The mixing and flow uniformity of gas involve different technologies, as shown in Fig. 4. Even though gas mixing can be
T
Sampling probe H 2O (steam)
Catalyst
CH4+H2+CO
Perforated plate T
T
Air+CO2 Electric heater
Static mixer
Fig. 4. Gas mixing order, sensor locations, and methodology to ensure flow uniformity.
S. Yu et al. / Renewable Energy 35 (2010) 1083–1090
b
3
Measured Velocity [m/s]
Measured Velocity [m/s]
a
2
1
1087
3
2
1
0 24
0 24
12
12
24 0
Y (mm)
12 0
-12 -24
-12
X (mm)
-2 -24
24 0
Y (mm)
12 0
-12 -24
-12 -24
X (mm)
Fig. 5. Velocity distribution at the honeycomb monolith outlet for the case (a) without and (b) with perforated plate: Q ¼ 50 lpm.
guaranteed by a static mixer, the flow uniformity below the catalyst bed is difficult to be achieved with the geometric structure given in Fig. 4. In this study, various types of extra units such as wire meshes, a dummy honeycomb monolith, and baffle plates are installed between the exit of static mixer and the catalyst bed to adjust the geometric eccentricity of the flow, but the flow uniformity was not guaranteed. Finally, the geometrical eccentricity of the gas mixture was adjusted by the perforated plates. Since one perforated plate operates in the same manner as the wire meshes, dummy honeycomb monolith, or baffle plates, two perforated plates were stacked in cross paths. Fig. 5 presents a comparison of the velocity distribution by using the perforated plates at the honeycomb monolith outlet. Additionally, the pressure drop through the perforated plates is 0.434 mbar at the noted perforation rate. Thus, it is difficult to change the blower capacity and system efficiency when the perforated plates are installed. As observed in Fig. 5, the velocity profile of the catalytic combustor inlet with perforated plates is evenly distributed but eccentricity is observed in the catalytic combustor without perforated plates. This eccentricity can result in local hot spots at lower velocity. Since the local hot spots are problematic for the durability of the catalytic combustor in long term operation, this problem should be resolved.
3.2. Selection of catalyst In this study, the catalyst of the combustor should burn out the fuel components in the anode off-gas, such as CH4, H2, and CO. The performance evaluation system shown in Fig. 6 is designed to investigate the combustibility of various catalysts. In the evaluation system, the air is supplied from a compressor and controlled by an MFC (mass flow controller). An electrical heater is placed between the MFC and the static mixer so that the inlet mixture temperature can be actively controlled. Air and CO2 are then electrically preheated through the heater. The flow rate of the gas species except vapor is controlled by the MFC. Vapor supply is controlled by a voltage regulator with a steam generator. The inlet and outlet temperatures are measured by K-type thermocouples at the center of the upstream and the exit plane to the catalyst bed. Gas concentrations at the outlet of the combustor are measured by a HORIBA VA-3000 multi-gas analyzer unit that uses NDIR (non-dispersive infrared absorptiometry) and MPA (magnetic pressure analysis) as measuring principles. Species such as CH4, CO, CO2, and O2 are measured by conventional suction probe analyzers on a dry basis and with a suction probe placed at the exit of the combustion chamber.
Fig. 6. Schematic diagram of test equipment for the performance analysis of catalytic combustor.
1088
S. Yu et al. / Renewable Energy 35 (2010) 1083–1090
800
Table 3 Specifications of catalyst for the performance evaluation.
700
Components
KIMM-I KIMM-II KIMM-III
Ceramic honeycomb monolith Ceramic honeycomb monolith Ceramic pellet
Pd, Ni, Ce/Al2O3 Pt, Pd/Al2O3 Pd/Al2O3
Chemical reaction in the catalyst bed begins when the temperature of the mixture reaches the required temperature of the surface reaction. The heating temperature at which the surface reaction ignites is called the light-off temperature (LOT). The LOT is the temperature when the exit temperature of the catalyst bed begins increasing rapidly apart from the preheating temperature of induced gas. The LOT varies with the composition of the catalyst. Since CO and H2 of the anode off-gas have high activity, the surface reaction in the catalyst bed can be initiated at low temperature. On the other hand, since the LOT of CH4 is higher than that of CO or H2, CH4 begins to combust at higher temperature. Therefore, the combustion of CH4 is the key to reaching complete combustion of the anode off-gas. For the screening test of the catalysts, three commercially available catalysts were selected and the specifications of these catalysts are given in Table 3. Even though these catalysts are known to enhance the chemical reaction of CH4 by Pd/Pt catalyst, the combustibility of the catalysts should be investigated according to the fuel mixture from the anode off-gas, which contains a considerable portion of vapor. In practice, the fuel mixture of the anode off-gas is induced into the catalytic combustor simultaneously, but the mixture components should be induced one at a time for the performance evaluation of the catalysts. In this study, a 100% volumetric flow rate means the volumetric flow rate at the design operation point. When the performance evaluation was carried out, gases are supplied sequentially such that CH4 gases are supplied first and then CO gases are supplied. As pointed out, since hydrogen has high activity, adding up the hydrogen results in activation of the entire reaction. Accordingly, H2 is supplied at a low flow rate to the design operation point so that the entire reaction can be sequentially activated. The outlet temperature and CH4 concentration are measured for the comparison and catalyst A is used as a reference catalyst. In this study, the conversion rate of CO was not evaluated, because activity of CO was enough to remove residual CO and it was proved that the concentration of CO at the outlet is less than the error order of the gas analyzer. As a reference condition, the reaction characteristics of the fuel mixture through the reference catalyst should be determined. Fig. 7 shows the outlet temperature and CH4 conversion with respect to H2 2.5 Tout
700
CH4
Temperature (°C)
2.0 H2 100%
500 400
H2 20%
300 200
1.0
H2 10%
100 0
1.5
H2 30%
Gas Concentration (%)
600
0.5
CO CH4
0
5
10
15
20
25
30
35
40
45
0.0
Time (min.) Fig. 7. Gas outlet temperature and CH4 concentrations of reference catalyst with respect to fuel composition: SV ¼ 18,000 h1, Tin ¼ 150 C.
CH4
600 Temperature (°C)
Support type
2.0
500
H2 20%
H2 30%
H2 100%
1.5
400 H2 10%
300 200
1.0
CO
Gas Concentration (%)
Catalyst
800
2.5 Tout
0.5
100 0
CH4
0
5
10
15
20
25
30
35
40
45
0.0
Time (min.) Fig. 8. Gas outlet temperature and CH4 concentration of reference catalyst with respect to fuel composition: SV ¼ 18,000 h1, Tin ¼ 250 C.
supply at SV ¼ 18,000 h1 when the inlet gas is preheated at 150 C. As observed in Fig. 7, the surface reaction on the catalyst bed does not readily commence with 100% CH4 supply, but activation of the entire reaction occurs with CO supply. H2 supply is a principal controller to activate the entire reaction, because CO and H2 of the anode off-gas have higher activity than does CH4. As the hydrogen supply is increased, the reaction of hydrogen results in an increase of the bed temperature. Since the bed temperature depends on the reaction of the hydrogen supply, the LOT of the CH4 reaction is also determined according to the hydrogen supply. At given conditions, it is observed that the CH4 concentration starts to decrease at 20% H2 supply and is drastically reduced at 30% H2 supply. Based on the measured concentration of CH4 at the outlet of the catalyst bed, most CH4 is completely burnt out as the hydrogen supply approaches 30% of the design operation point. As a result, the temperature of the catalyst bed is raised to the LOT of CH4 at around 30% of the H2 supply. As the preheating temperature is increased, the reaction of CH4 is enhanced at a lower hydrogen flow rate. Fig. 8 shows that the reaction of CH4 is very sensitive to the mixture inlet temperature. Since an increase of the inlet temperature results in an increase of the outlet temperature, the outlet temperature at 250 C preheating temperature is higher. As the preheating temperature is increased, the reaction of CH4 is activated with a lower hydrogen flow rate. Furthermore, as shown in Figs. 7 and 8, the reference catalyst shows excellent combustibility at the design operation point but the LOT of CH4 varies with the preheating temperature. The performance of each catalyst is compared in terms of the gas outlet temperature and CH4 conversion rate. As shown in Fig. 9, since higher preheating temperature can enhance the conversion of CH4, the performance of the catalyst at lower preheating temperature is compared with the case at higher preheating temperature. At 150 C preheating temperature, the temperature of the outlet gas and of the three catalyst beds is increased with higher hydrogen supply rates, because more hydrogen is reacted at a higher hydrogen flow rate. However, when the CH4 conversion rate is compared, each catalyst shows different performance. In the case of KIMM-I and KIMM-II, the CH4 reaction starts at 20% H2 supply, while the CH4 reaction of the KIMM-III catalyst is not yet started at 30% H2 supply. Furthermore, at the design point, 99% or more of CH4 conversion of KIMM-I and KIMM-II was observed while that of KIMM-III is only about 40%. As a result, KIMM-I presents comparable performance with that of KIMM-II, but the performance of KIMM-III is not acceptable for the design operation point of the catalytic combustor. As the preheating temperature is increased to 250 C, the LOT at lower hydrogen supply is observed. The CH4 conversion in KIMM-I
S. Yu et al. / Renewable Energy 35 (2010) 1083–1090
Temperature (°C)
80
CH4 (KIMM-I) CH4 (KIMM-II) CH4 (KIMM-III)
500
60
400 40
300 200
20
600
10
20
30
500
40
300 200
20 0 0
100
b 100
800
100
500
60
400 40
CH4 (KIMM-I) CH4 (KIMM-II) CH4 (KIMM-III)
300 200
20
100
Temperature (°C)
80
80
600 500
60
400
Tout (KIMM-I) t Tout (KIMM-II) Tout (KIMM-III)
300 200
10
20
30
40 20
100 0
0 0
20
30
100
H 2 rate (%)
0 0
CH 4 (KIMM-I) CH 4 (KIMM-II) CH 4 (KIMM-III)
CH4 Conversion rate(%)
Tout (KIMM-I) Tout (KIMM-II) Tout (KIMM-III)
CH4 Conversion rate(%)
Temperature (°C)
100
700
600
0
30
Preheating Temperature, Tin=150 °C
Preheating temperature, Tin=150 °C
700
20 H 2 rate (%)
H2 rate (%)
b 800
60
400
0
0 0
80
CH4 (KIMM-I ) CH4 (KIMM- II) CH4 (KIMM- III)
100
100 0
100
T out (KIMM-I) T out (KIMM-II) T out (KIMM-III)
700
CH4 Conversion rate(%)
600
800
CH4 Conversion rate(%)
Tout (KIMM-I) Tout (KIMM-II) Tout (KIMM-III)
700
a
100
Temperature (°C)
a 800
1089
Preheating Temperature, Tin=250 °C
100
H2 rate (%)
Fig. 10. Outlet temperature and CH4 concentration with respect to H2 supply: SV ¼ 36,000 h1.
Preheating temperature, Tin=250 °C Fig. 9. Outlet temperature and CH4 concentration with respect to H2 supply: SV ¼ 18,000 h1.
and KIMM-II is increased to more than 99% at 20% H2 supply, and consequently the outlet temperature approaches 500 C at 20% H2 supply. Even though the inlet mixture temperature affects the reactivity of methane in two catalysts, it is still difficult to burn out the entire methane component in a range of H2 supply from 20% to 100% with KIMM-III. This appears to be attributable to the activities of each catalyst at the aimed design point. Since the operating temperature of the catalytic combustor should be about 750 C, KIMM-I and KIMM-II are adequate while KIMM-III is not. The space velocity is also an important parameter to measure the reactivity of a catalyst. In this study, since the flow rate is fixed at the design point, the space velocity is varied with geometrical changes. Since higher space velocity implies that the mixture remains for less time in the catalyst bed, higher activity of the catalyst is required for operation at higher space velocity. On the other hand, the effect of space velocity should be investigated due to the prediction of a transient response of the catalytic combustor. While the effect of the space velocity is investigated with geometric variations, the performance of the catalytic combustor can be predicted based on the experimental results. Fig. 10 shows the outlet temperature and CH4 conversion with respect to H2 supply at 36,000 h1. As the space velocity is increased to 36,000 h1, the CH4 conversion rate is
significantly decreased in all catalysts. At a preheating temperature of 150 C and at 36,000 h1, KIMM-I is the only catalyst to show satisfactory result. In contrast, KIMM-II shows a lower conversion rate for CH4 as the space velocity is increased. As observed in Fig. 10, the performance of KIMM-III worsens at higher space velocity. As the preheating temperature is increased, the performance trend is very similar to that at 18,000 h1, and the performance of KIMM-I is comparable to that of KIMM-II. When the transient response is also considered as a design parameter, KIMM-I is the most preferable candidate for the catalytic combustor. Meanwhile, KIMM-III showed unacceptable results in all cases.
4. Conclusions The flow and combustion characteristics of a catalytic combustor for a high temperature stationary fuel cell system are investigated in terms of performance and stability over various operating parameters. From the experimental analysis of the performance of the catalytic combustor, the following conclusions have been reached: 1. When perforated plates are used, the flow uniformity prior to the catalytic bed is enhanced. 2. KIMM-I and KIMM-II catalysts have a higher catalytic activity as compared with KIMM-III catalyst.
1090
S. Yu et al. / Renewable Energy 35 (2010) 1083–1090
3. At a space velocity of 18,000 h1, KIMM-I and KIMM-II show satisfactory performance while the exhaust gas of KIMM-III still contains a higher portion of unburned methane. 4. As the space velocity is increased, KIMM-I shows better performance than KIMM-II at low preheating temperature. However, these two candidates show acceptable performance at higher preheating temperature. Acknowledgement The authors appreciate financial support from the Korea Institute of Machinery and Materials.
References [1] Williams MC, Strakey JP, Singhal Subhash C. U.S. distributed generation fuel cell program. Journal of Power Sources 2004;131:79–85. [2] Bischoff M. Large stationary fuel cell systems: status and dynamic requirements. Journal of Power Sources 2006;154:461–6. [3] Hayes RE, Kolaczkowski ST. Introduction to catalytic combustion. Gordon and Breach Science Publishers; 1997. p. 1–4. [4] Cocchi S, Nutini G, Spencer MJ, Nickolas SG. Catalytic combustion system for a 10 MW class power generation gas turbine. Catalysis Today 2006;117: 419–26. [5] Betta RAD, Rostrup-Nielsen T. Application of catalytic combustion to a 1.5 MW industrial gas turbine. Catalysis Today 1999;47:369–75. [6] Carroni R, Schmidt V, Griffin T. Catalytic combustion for power generation. Catalysis Today 2002;75:287–95.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Development of a catalytic combustor for a stationary fuel cell power generation system Sangseok Yu a, Dongjin Hong b, Youngduk Lee c, Sangmin Lee c, Kookyoung Ahn c, * a
Chung Nam National University, Daejeon 305-764, Republic of Korea Hyosung Corporation, Anyang 431-080, Republic of Korea c Korea Institute of Machinery and Materials, Daejeon 305-343, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 March 2009 Accepted 12 October 2009 Available online 7 November 2009
The anode off-gas of high temperature stationary fuel cell stacks still includes fuel components such as hydrogen, carbon monoxide, and hydrocarbon due to the innate characteristics of the fuel cell operation. Even though the anode off-gas has fuel contents, the flammability is very limited due to the vapor concentration of the anode off-gas. A catalytic combustor is applied as an off-gas combustor so as to utilize the waste energy of anode off-gas by stimulating a chemical reaction over selected operating conditions. Temperature and flow uniformity in the radial direction are very significant factors of the durability, because the catalytic combustion is carried out on the surface of the catalyst site. On the other hand, the catalyst selection is also very important due to the composition of the anode off-gas. In this study, the flow uniformity is presented prior to a catalyst screening test. From the results of the screening test, where three commercially available catalysts are tested, KIMM-I and KIMM-II are selected as candidates for a catalytic combustor of anode off-gas. Ó 2009 Published by Elsevier Ltd.
Keywords: High temperature stationary fuel cell Anode off-gas Catalytic combustor Flow uniformity
1. Introduction The global greenhouse effect and the long term shortage of fossil fuel have raised demand for the development of novel conceptual power propulsion systems, which should offer zero emissions, high efficiency, and high performance. Stationary power propulsion fuel cells are a very attractive candidate in this regard in that they are capable of meeting the stringent requirements of future power propulsion systems. A fuel cell is an electrochemical energy conversion device that produces heat and electricity by chemical reaction. Different from conventional power plants, the fuel cell is free from mechanical loss in producing electricity and thus is operated at high efficiency. Furthermore, when the fuel cell is operated by hydrogen fuel and air, the final product is also free from air pollution and does not contribute to the greenhouse effect. When the fuel for the stationary fuel cell is reformed from HC fuels, the exhaust gas of a stationary power plant fuel cell is composed of carbon dioxide and water. However, since the efficiency of fuel cells is not very high, but, higher than the efficiency of conventional combustion engines and thus results in the improved fuel consumption, less
* Corresponding author. Tel.: þ82 42 868 7324; fax: þ82 42 868 7284. E-mail address: [email protected] (K. Ahn). 0960-1481/$ – see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.renene.2009.10.015
carbon dioxide is exhausted from the power plant. Among such fuel cell systems, the solid oxide fuel cell (SOFC) and the molten carbonate fuel cell (MCFC) can feasibly achieve high efficiency, largely because the thermal energy of the exhaust gas can be recovered. Williams et al. [1] reported that the SOFC and the MCFC are currently the most advanced and available technologies for distributed power generation. Some market leading companies have already commercialized fuel cell systems for stationary power generation and their commercialized products have begun to penetrate the stationary power plant market [2]. The fuel cell of a stationary power generation system consists of an electric power generation module (stack) and a fuel supply module (reformer), a thermal energy recovery module (combustor and heat exchanger), and an air supply module (compressor and turbo-charger), among others. Balance of plant (BOP) denotes auxiliary components excluding the fuel cell stack that allows organic operation of the system. While the development of the fuel cell stack should be the core technology of the fuel cell system, system efficiency is usually achieved by finely controlling the BOP operations. Additionally, the development of BOP is another technical barrier for fuel cell commercialization due to durability and operating ranges of the fuel cell system. For stable operation and durability of a high temperature stationary fuel cell system, fuel utilization should be less than
1084
S. Yu et al. / Renewable Energy 35 (2010) 1083–1090
one, which implies that the exhaust gas from the anode side contains fuel contents such as hydrogen, carbon monoxide, and hydrocarbon. In order to improve the efficiency, the fuel contents from the exhaust should be burned out and the thermal energy should be recovered. Unfortunately, since a conventional combustor will meet flammability limits at some operating conditions, it is not easily applied for burning out the exhaust gas of the fuel cell. On the other hand, catalytic combustion is generally accepted as a combustion method for the anode off-gas from the fuel cell stack, because it burns efficiently in concentrations outside flammability limits and at low operating temperature. Furthermore, the catalytic combustor burns out the exhaust gas from the fuel cell without any undesired by-products, such as UHC, CO, or NOx [3]. Due to the low emission characteristics of the catalytic combustor, it has been developed for application in gas turbines [4–6]. However, there has been little reported research on catalytic combustors for high temperature stationary fuel cell systems. Even though the catalytic combustor is beneficial to the combustion of fuel cell off-gas, its commercial application has been delayed due to the stability and durability of the system. In particular, since the stability and the durability of the catalytic combustor depend on the type of catalyst and operating characteristics, selection of the catalyst and the design technology to achieve a uniform flow distribution prior to the catalyst bed are very important. The objective of the present study is to develop a catalytic combustor for off-gas combustion of a high temperature stationary fuel cell system and to investigate through experiments the flow and combustion characteristics of catalytic combustors so that an optimal catalytic combustor can be designed. Various factors affecting the performance of the catalytic combustor are examined such as inlet flow condition, preheating temperature, and space velocity.
2. Design considerations of a catalytic combustor for a high temperature stationary fuel cell system High temperature stationary fuel cell systems can be categorized as high pressure cylindrical systems and atmospheric planar systems. In this study, the catalytic combustor is applied to a planar type fuel cell system, shown in Fig. 1. This system is fed by natural gas and air. The system components are the stack, desulfurizer, preprocessor, reformer, heat exchangers, catalytic combustor, T/G module, and HRSG. In this study, sufficient air flow rate required for the catalytic combustor is supplied by a turbo-generator. Furthermore, the required parasitic power is also supplied through electricity generated by the turbo-generator. In general, it is difficult to consume entire fuel induced into the stack due to the durability. The fuel utilization is the ratio of fuel reacted to the fuel supplied, which describes both fuel consumption and ejection from the fuel cell. Since the anode off-gas contains fuel contents, it should be treated before it is exhausted into the atmosphere. Otherwise, it may explode or combust in the exhaust pipes. Even though the anode off-gas contains fuel in its mixture, the high concentration of water vapor in the fuel mixture limits operation of the conventional burner to within limited ranges. Even if a conventional combustor can burn out the fuel mixture from the fuel cell, the product gas of the conventional combustor is usually composed of NOx product, which should be eliminated at the exit. Compared with a conventional combustor, a catalytic combustor can be operated over wide ranges and no NOx product is observed at the exit of the combustor due to the low operating temperature. When the catalytic combustion is completed, the product is only composed of water vapor, CO2, and nitrogen. The low operating temperature of the catalytic combustor effectively eliminates NOx production in the exhausted gas. Additionally, proper selection of catalysts can enhance the role of the catalytic combustor, and thus the catalytic combustor preferentially burns out the target fuel component in the mixture.
Fig. 1. Schematic diagram of high temperature stationary fuel cell power generation system.
S. Yu et al. / Renewable Energy 35 (2010) 1083–1090
1085
AIR-TBN AIR
CCOUT5 AIR3
HTREF
CATALBNR
HX2
HX1 CCOUT0
ANOD-OUT
CCOUT1
HX3
CCOUT2
FCINIDE
NGWAT2
AIRBLOW
EXTRFO1
ANOD-IN
EXTRFO2
SOFC FCINSID3
FCINSID2
CCOUT4
AIR2
INT-REF
SEP
HX7
CCOUT3
MIX
FCINSID1
W-PUMP
EXT-REF
ARTBN5
HTLOSS
Q
RSTOIC
HEATLOSS
HXADD
NGWAT3 HTTRANS1
MULT
CATH-IN
WATER2
AIR1
HTTRANS2
SEP
EXTREFIN
ARTBN3
WATER1
NG2
HX4
ARTBN6
HX5
CAT-OUT1
CAT-OUT2
HX6 CAT-OUT3
CAT-OUT4
NGPUMP
ARTBN2
NG1 AIR-COMP
ARTBN1
Fig. 2. High temperature stationary fuel cell system configuration for estimation of the design point of catalytic combustor with Aspen PlusÒ.
The anode off-gas of the high temperature stationary fuel cell does not only include a high concentration of water vapor but it also includes a small portion of methane (CH4), and some portions of CO and H2. In the fuel mixture at the anode exit of the high temperature stationary fuel cell, combustion of residual hydrogen is straightforward but the combustion of CO and methane needs to be delicately treated due to vapor concentration. In practical terms, the most difficult component of the fuel mixture to be combusted is methane whereas the hydrogen is completely combusted. Accordingly, in selecting a catalyst for the catalytic combustor, the proper catalyst for methane in the mixture at given operating conditions should be chosen. In this study, the performance of three catalysts is evaluated to select the proper catalyst for methane combustion under poor combustible conditions. Prior to the evaluation of the catalysts, the operating temperature of the catalyst combustor and the gas inlet conditions should Table 1 Fuel cell system parameters for Aspen Plus. Components
Input
FC stack
Press. Temp. Rate potential Uf Uair
Atmosphere 750 C 0.75 V@500 mA/cm2 65–80% 20–30%
Reformer: ER/IR
Method Press. Temp. S/C
Steam reforming 1.2–1.3 bara 500–750 C 3.0
<750 C 4
Catalytic combustor
TCC,out Excess air ratio
Turbo-generator
Comp
h
Turb
h
Pr Pr Air blower
h
70% 3.5 bara 80% 1.17 bara 70%
be defined. In this study, the gas temperature at the exit of the catalyst bed is defined as the operating temperature of the catalytic combustor and is set at 750 C. Additionally, the marginal operating temperature of off-design operation is very important for the longevity of the catalytic combustor, because the catalyst and substrate can be damaged at temperatures in excess of 850 C. The inlet temperature of the catalytic combustor is the temperature before the perforated plates and the temperature varies between 150 and 350 C depending on the evaluation conditions. When the inlet gas temperature is 150 C or lower, combustibility of the low temperature gas is very sensitive to catalysts. Thus, as a prior step to designing the catalytic combustor, it is necessary to screen the catalysts for the catalytic combustor. Meanwhile, the distribution of the inlet gas is also an important design factor. In general, the catalytic reaction is a surface reaction, which becomes diffusion limited inside the catalyst site. At the catalyst site, the substrate temperature approaches the adiabatic flame temperature of the fuel/air mixture in the local area. Since the adiabatic flame temperature is very high, locally hot spots result in thermal sintering of the support surface area, thermal sintering and vaporization of the noble metals, and thermal shock fracturing of ceramic supports. Since a locally dense mixture results in local hot spots [3], the gas uniformity should be ensured for stable operation and durability. Furthermore, the reaction is also affected by the mixing quality of the fuel and the air streams. When the fuel and
Table 2 Calculated simulation gas composition and flow rate of anode off-gas for 1 kW high temperature stationary fuel cell system. Gas component
Flow rate (slpm)
Proportion (%)
H2 CO CH4 CO2 H2O
5.71 1.03 0.05 3.19 15.52
22.4 4.0 0.2 12.5 60.9
1086
S. Yu et al. / Renewable Energy 35 (2010) 1083–1090
Catalyst P.P. Ring P.P.
Fig. 3. Flow uniformity of inlet gas with perforated plates.
the air streams are well mixed and supplied uniformly prior to the combustion region, high performance combustion can be expected without any locally defected areas. The space velocity (SV) is also an important design parameter of the catalytic combustor. A physical parameter, SV, is commonly used for the measure of the reactor throughput:
i h SV h1 ¼ Q =V where Q is the volumetric flow rate of the feed and V is the reactor volume. Since the space velocity represents the residence time of the feed to the catalytic combustor, a hot spot may occur at very low space velocity, which can lead to subsequent damage of the catalyst and substrate. When the SV is very high, a significant portion of the fuel mixture might be slipped without any reaction. Accordingly, during the screening of the catalyst, the performance of catalysts is evaluated over various space velocities. However, since the volumetric flow rate is given at the beginning of the design process, the space velocities are changed with geometric modifications. 2.1. Estimation of operating conditions of catalytic combustor The flow rate and composition of the fuel mixture at the anode exit are very important parameters in designing the size and capacity of the catalytic combustor. When the fuel cell in Fig. 1 is operated, the reformer supplies hydrogen and CO to the fuel cell stack and the fuel cell stack consumes some portion thereof to produce electricity. Since the stability and durability of the stack limit the fuel utilization factor to less than one, the off-gas at the anode side unavoidably includes fuel components, and the residual fuel mixture should be burnt out in the catalytic combustor to improve the system efficiency.
The configuration of the fuel cell system in Fig. 1 is analyzed with Aspen PlusÒ, which is shown in Fig. 2. Calculation parameters are shown in Table 1. In this study, the high temperature stationary fuel cell stack is an atmospheric planar fuel cell that is operated at 750 C. The fuel is partially supplied by an external steam reformer even though the exact fraction varies with the system configuration. Since the internal reformer is very important for the heat rejection of a large scale system, an internal steam reformer is also employed. In this study, the fuel cell system is a co-generation system with a turbo-charger to maximize the system efficiency. Computation of operating processes of the fuel cell system provides information on the heat and mass balance of the catalytic combustor, and the system efficiency, essential components, and design criteria of components are also estimated. As a result of the process calculation, the mass and energy balance of the system can be estimated. The gas composition and volumetric fraction of the anode off-gas are also determined, as shown in Table 2. The calculation data is then provided for the design of the catalytic combustor. Additionally, the gas composition and flow rate into the catalytic combustor are controlled by a mass flow controller when evaluating the performance of the catalysts. 3. Performance evaluation of catalytic combustor 3.1. Evaluation of flow uniformity The perforated plate used in this study has a penetration area of 10% and thickness of 2 mm. A hot wire anemometer is applied to measure the velocity of each mesh and thereby evaluate the effects of the perforated plate on the flow distribution. When one perforated plate is installed, it is difficult to achieve a uniform flow. Fig. 3 shows two perforated plates installed below the catalysts. Flow uniformity at the catalyst inlet was achieved by installing two crossing perforated plates between the catalyst and the mixing chamber with a minimal pressure drop. Since the gas mixing order can result in the flashback of hydrogen, we mixed the air and vapor before the gases are mixed with fuel gas (CH4 þ H2 þ CO), as shown in Fig. 4. Furthermore, it is important that the fuel and air streams be well mixed and supplied uniformly prior to the combustion region, because the catalytic combustion is a surface reaction. Therefore, a static mixer is installed to enhance gas mixing. Perforated plates are located at the upstream of the catalyst bed to guarantee a uniform velocity distribution over the cross-section. The combustion chamber is an insulated monolith reactor. The mixing and flow uniformity of gas involve different technologies, as shown in Fig. 4. Even though gas mixing can be
T
Sampling probe H 2O (steam)
Catalyst
CH4+H2+CO
Perforated plate T
T
Air+CO2 Electric heater
Static mixer
Fig. 4. Gas mixing order, sensor locations, and methodology to ensure flow uniformity.
S. Yu et al. / Renewable Energy 35 (2010) 1083–1090
b
3
Measured Velocity [m/s]
Measured Velocity [m/s]
a
2
1
1087
3
2
1
0 24
0 24
12
12
24 0
Y (mm)
12 0
-12 -24
-12
X (mm)
-2 -24
24 0
Y (mm)
12 0
-12 -24
-12 -24
X (mm)
Fig. 5. Velocity distribution at the honeycomb monolith outlet for the case (a) without and (b) with perforated plate: Q ¼ 50 lpm.
guaranteed by a static mixer, the flow uniformity below the catalyst bed is difficult to be achieved with the geometric structure given in Fig. 4. In this study, various types of extra units such as wire meshes, a dummy honeycomb monolith, and baffle plates are installed between the exit of static mixer and the catalyst bed to adjust the geometric eccentricity of the flow, but the flow uniformity was not guaranteed. Finally, the geometrical eccentricity of the gas mixture was adjusted by the perforated plates. Since one perforated plate operates in the same manner as the wire meshes, dummy honeycomb monolith, or baffle plates, two perforated plates were stacked in cross paths. Fig. 5 presents a comparison of the velocity distribution by using the perforated plates at the honeycomb monolith outlet. Additionally, the pressure drop through the perforated plates is 0.434 mbar at the noted perforation rate. Thus, it is difficult to change the blower capacity and system efficiency when the perforated plates are installed. As observed in Fig. 5, the velocity profile of the catalytic combustor inlet with perforated plates is evenly distributed but eccentricity is observed in the catalytic combustor without perforated plates. This eccentricity can result in local hot spots at lower velocity. Since the local hot spots are problematic for the durability of the catalytic combustor in long term operation, this problem should be resolved.
3.2. Selection of catalyst In this study, the catalyst of the combustor should burn out the fuel components in the anode off-gas, such as CH4, H2, and CO. The performance evaluation system shown in Fig. 6 is designed to investigate the combustibility of various catalysts. In the evaluation system, the air is supplied from a compressor and controlled by an MFC (mass flow controller). An electrical heater is placed between the MFC and the static mixer so that the inlet mixture temperature can be actively controlled. Air and CO2 are then electrically preheated through the heater. The flow rate of the gas species except vapor is controlled by the MFC. Vapor supply is controlled by a voltage regulator with a steam generator. The inlet and outlet temperatures are measured by K-type thermocouples at the center of the upstream and the exit plane to the catalyst bed. Gas concentrations at the outlet of the combustor are measured by a HORIBA VA-3000 multi-gas analyzer unit that uses NDIR (non-dispersive infrared absorptiometry) and MPA (magnetic pressure analysis) as measuring principles. Species such as CH4, CO, CO2, and O2 are measured by conventional suction probe analyzers on a dry basis and with a suction probe placed at the exit of the combustion chamber.
Fig. 6. Schematic diagram of test equipment for the performance analysis of catalytic combustor.
1088
S. Yu et al. / Renewable Energy 35 (2010) 1083–1090
800
Table 3 Specifications of catalyst for the performance evaluation.
700
Components
KIMM-I KIMM-II KIMM-III
Ceramic honeycomb monolith Ceramic honeycomb monolith Ceramic pellet
Pd, Ni, Ce/Al2O3 Pt, Pd/Al2O3 Pd/Al2O3
Chemical reaction in the catalyst bed begins when the temperature of the mixture reaches the required temperature of the surface reaction. The heating temperature at which the surface reaction ignites is called the light-off temperature (LOT). The LOT is the temperature when the exit temperature of the catalyst bed begins increasing rapidly apart from the preheating temperature of induced gas. The LOT varies with the composition of the catalyst. Since CO and H2 of the anode off-gas have high activity, the surface reaction in the catalyst bed can be initiated at low temperature. On the other hand, since the LOT of CH4 is higher than that of CO or H2, CH4 begins to combust at higher temperature. Therefore, the combustion of CH4 is the key to reaching complete combustion of the anode off-gas. For the screening test of the catalysts, three commercially available catalysts were selected and the specifications of these catalysts are given in Table 3. Even though these catalysts are known to enhance the chemical reaction of CH4 by Pd/Pt catalyst, the combustibility of the catalysts should be investigated according to the fuel mixture from the anode off-gas, which contains a considerable portion of vapor. In practice, the fuel mixture of the anode off-gas is induced into the catalytic combustor simultaneously, but the mixture components should be induced one at a time for the performance evaluation of the catalysts. In this study, a 100% volumetric flow rate means the volumetric flow rate at the design operation point. When the performance evaluation was carried out, gases are supplied sequentially such that CH4 gases are supplied first and then CO gases are supplied. As pointed out, since hydrogen has high activity, adding up the hydrogen results in activation of the entire reaction. Accordingly, H2 is supplied at a low flow rate to the design operation point so that the entire reaction can be sequentially activated. The outlet temperature and CH4 concentration are measured for the comparison and catalyst A is used as a reference catalyst. In this study, the conversion rate of CO was not evaluated, because activity of CO was enough to remove residual CO and it was proved that the concentration of CO at the outlet is less than the error order of the gas analyzer. As a reference condition, the reaction characteristics of the fuel mixture through the reference catalyst should be determined. Fig. 7 shows the outlet temperature and CH4 conversion with respect to H2 2.5 Tout
700
CH4
Temperature (°C)
2.0 H2 100%
500 400
H2 20%
300 200
1.0
H2 10%
100 0
1.5
H2 30%
Gas Concentration (%)
600
0.5
CO CH4
0
5
10
15
20
25
30
35
40
45
0.0
Time (min.) Fig. 7. Gas outlet temperature and CH4 concentrations of reference catalyst with respect to fuel composition: SV ¼ 18,000 h1, Tin ¼ 150 C.
CH4
600 Temperature (°C)
Support type
2.0
500
H2 20%
H2 30%
H2 100%
1.5
400 H2 10%
300 200
1.0
CO
Gas Concentration (%)
Catalyst
800
2.5 Tout
0.5
100 0
CH4
0
5
10
15
20
25
30
35
40
45
0.0
Time (min.) Fig. 8. Gas outlet temperature and CH4 concentration of reference catalyst with respect to fuel composition: SV ¼ 18,000 h1, Tin ¼ 250 C.
supply at SV ¼ 18,000 h1 when the inlet gas is preheated at 150 C. As observed in Fig. 7, the surface reaction on the catalyst bed does not readily commence with 100% CH4 supply, but activation of the entire reaction occurs with CO supply. H2 supply is a principal controller to activate the entire reaction, because CO and H2 of the anode off-gas have higher activity than does CH4. As the hydrogen supply is increased, the reaction of hydrogen results in an increase of the bed temperature. Since the bed temperature depends on the reaction of the hydrogen supply, the LOT of the CH4 reaction is also determined according to the hydrogen supply. At given conditions, it is observed that the CH4 concentration starts to decrease at 20% H2 supply and is drastically reduced at 30% H2 supply. Based on the measured concentration of CH4 at the outlet of the catalyst bed, most CH4 is completely burnt out as the hydrogen supply approaches 30% of the design operation point. As a result, the temperature of the catalyst bed is raised to the LOT of CH4 at around 30% of the H2 supply. As the preheating temperature is increased, the reaction of CH4 is enhanced at a lower hydrogen flow rate. Fig. 8 shows that the reaction of CH4 is very sensitive to the mixture inlet temperature. Since an increase of the inlet temperature results in an increase of the outlet temperature, the outlet temperature at 250 C preheating temperature is higher. As the preheating temperature is increased, the reaction of CH4 is activated with a lower hydrogen flow rate. Furthermore, as shown in Figs. 7 and 8, the reference catalyst shows excellent combustibility at the design operation point but the LOT of CH4 varies with the preheating temperature. The performance of each catalyst is compared in terms of the gas outlet temperature and CH4 conversion rate. As shown in Fig. 9, since higher preheating temperature can enhance the conversion of CH4, the performance of the catalyst at lower preheating temperature is compared with the case at higher preheating temperature. At 150 C preheating temperature, the temperature of the outlet gas and of the three catalyst beds is increased with higher hydrogen supply rates, because more hydrogen is reacted at a higher hydrogen flow rate. However, when the CH4 conversion rate is compared, each catalyst shows different performance. In the case of KIMM-I and KIMM-II, the CH4 reaction starts at 20% H2 supply, while the CH4 reaction of the KIMM-III catalyst is not yet started at 30% H2 supply. Furthermore, at the design point, 99% or more of CH4 conversion of KIMM-I and KIMM-II was observed while that of KIMM-III is only about 40%. As a result, KIMM-I presents comparable performance with that of KIMM-II, but the performance of KIMM-III is not acceptable for the design operation point of the catalytic combustor. As the preheating temperature is increased to 250 C, the LOT at lower hydrogen supply is observed. The CH4 conversion in KIMM-I
S. Yu et al. / Renewable Energy 35 (2010) 1083–1090
Temperature (°C)
80
CH4 (KIMM-I) CH4 (KIMM-II) CH4 (KIMM-III)
500
60
400 40
300 200
20
600
10
20
30
500
40
300 200
20 0 0
100
b 100
800
100
500
60
400 40
CH4 (KIMM-I) CH4 (KIMM-II) CH4 (KIMM-III)
300 200
20
100
Temperature (°C)
80
80
600 500
60
400
Tout (KIMM-I) t Tout (KIMM-II) Tout (KIMM-III)
300 200
10
20
30
40 20
100 0
0 0
20
30
100
H 2 rate (%)
0 0
CH 4 (KIMM-I) CH 4 (KIMM-II) CH 4 (KIMM-III)
CH4 Conversion rate(%)
Tout (KIMM-I) Tout (KIMM-II) Tout (KIMM-III)
CH4 Conversion rate(%)
Temperature (°C)
100
700
600
0
30
Preheating Temperature, Tin=150 °C
Preheating temperature, Tin=150 °C
700
20 H 2 rate (%)
H2 rate (%)
b 800
60
400
0
0 0
80
CH4 (KIMM-I ) CH4 (KIMM- II) CH4 (KIMM- III)
100
100 0
100
T out (KIMM-I) T out (KIMM-II) T out (KIMM-III)
700
CH4 Conversion rate(%)
600
800
CH4 Conversion rate(%)
Tout (KIMM-I) Tout (KIMM-II) Tout (KIMM-III)
700
a
100
Temperature (°C)
a 800
1089
Preheating Temperature, Tin=250 °C
100
H2 rate (%)
Fig. 10. Outlet temperature and CH4 concentration with respect to H2 supply: SV ¼ 36,000 h1.
Preheating temperature, Tin=250 °C Fig. 9. Outlet temperature and CH4 concentration with respect to H2 supply: SV ¼ 18,000 h1.
and KIMM-II is increased to more than 99% at 20% H2 supply, and consequently the outlet temperature approaches 500 C at 20% H2 supply. Even though the inlet mixture temperature affects the reactivity of methane in two catalysts, it is still difficult to burn out the entire methane component in a range of H2 supply from 20% to 100% with KIMM-III. This appears to be attributable to the activities of each catalyst at the aimed design point. Since the operating temperature of the catalytic combustor should be about 750 C, KIMM-I and KIMM-II are adequate while KIMM-III is not. The space velocity is also an important parameter to measure the reactivity of a catalyst. In this study, since the flow rate is fixed at the design point, the space velocity is varied with geometrical changes. Since higher space velocity implies that the mixture remains for less time in the catalyst bed, higher activity of the catalyst is required for operation at higher space velocity. On the other hand, the effect of space velocity should be investigated due to the prediction of a transient response of the catalytic combustor. While the effect of the space velocity is investigated with geometric variations, the performance of the catalytic combustor can be predicted based on the experimental results. Fig. 10 shows the outlet temperature and CH4 conversion with respect to H2 supply at 36,000 h1. As the space velocity is increased to 36,000 h1, the CH4 conversion rate is
significantly decreased in all catalysts. At a preheating temperature of 150 C and at 36,000 h1, KIMM-I is the only catalyst to show satisfactory result. In contrast, KIMM-II shows a lower conversion rate for CH4 as the space velocity is increased. As observed in Fig. 10, the performance of KIMM-III worsens at higher space velocity. As the preheating temperature is increased, the performance trend is very similar to that at 18,000 h1, and the performance of KIMM-I is comparable to that of KIMM-II. When the transient response is also considered as a design parameter, KIMM-I is the most preferable candidate for the catalytic combustor. Meanwhile, KIMM-III showed unacceptable results in all cases.
4. Conclusions The flow and combustion characteristics of a catalytic combustor for a high temperature stationary fuel cell system are investigated in terms of performance and stability over various operating parameters. From the experimental analysis of the performance of the catalytic combustor, the following conclusions have been reached: 1. When perforated plates are used, the flow uniformity prior to the catalytic bed is enhanced. 2. KIMM-I and KIMM-II catalysts have a higher catalytic activity as compared with KIMM-III catalyst.
1090
S. Yu et al. / Renewable Energy 35 (2010) 1083–1090
3. At a space velocity of 18,000 h1, KIMM-I and KIMM-II show satisfactory performance while the exhaust gas of KIMM-III still contains a higher portion of unburned methane. 4. As the space velocity is increased, KIMM-I shows better performance than KIMM-II at low preheating temperature. However, these two candidates show acceptable performance at higher preheating temperature. Acknowledgement The authors appreciate financial support from the Korea Institute of Machinery and Materials.
References [1] Williams MC, Strakey JP, Singhal Subhash C. U.S. distributed generation fuel cell program. Journal of Power Sources 2004;131:79–85. [2] Bischoff M. Large stationary fuel cell systems: status and dynamic requirements. Journal of Power Sources 2006;154:461–6. [3] Hayes RE, Kolaczkowski ST. Introduction to catalytic combustion. Gordon and Breach Science Publishers; 1997. p. 1–4. [4] Cocchi S, Nutini G, Spencer MJ, Nickolas SG. Catalytic combustion system for a 10 MW class power generation gas turbine. Catalysis Today 2006;117: 419–26. [5] Betta RAD, Rostrup-Nielsen T. Application of catalytic combustion to a 1.5 MW industrial gas turbine. Catalysis Today 1999;47:369–75. [6] Carroni R, Schmidt V, Griffin T. Catalytic combustion for power generation. Catalysis Today 2002;75:287–95.