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Development of a coupled reactor with a catalytic combustor and steam reformer for a 5 kW solid oxide fuel cell system
Applied Energy 114 (2014) 114–123
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Development of a coupled reactor with a catalytic combustor and steam reformer for a 5 kW solid oxide fuel cell system Sanggyu Kang a,⇑, Kanghun Lee b, Sangseok Yu b, Sang Min Lee a, Kook-Young Ahn a a b
Korea Institute of Machinery and Materials, Jang-dong, Daejeon 305-343, Republic of Korea Chungnam National University, Gung-dong, Daejeon 305-764, Republic of Korea
h i g h l i g h t s Proposes the scale-up strategy to develop a large-scale coupled reactor. Investigation of performance of steam reformer coupled with catalytic combustor. Experimental parameters are inlet temp., air excess ratio, SCR, fuel utilization. Evaluation of the heat transfer distribution along the gas flow direction. The mean value of methane conversion rate is approximately 93.4%.
a r t i c l e
i n f o
Article history: Received 18 May 2013 Received in revised form 2 September 2013 Accepted 17 September 2013 Available online 15 October 2013 Keywords: Scale-up strategy Thermal integration Steam reformer Catalytic combustor Methane conversion rate
a b s t r a c t The methane (CH4) conversion rate of a steam reformer can be increased by thermal integration with a catalytic combustor, called a coupled reactor. In the present study, a 5 kW coupled reactor has been developed based on a 1 kW coupled reactor in previous work. The geometric parameters of the space velocity, diameter and length of the coupled reactor selected from the 1 kW coupled reactor are tuned and applied to the design of the 5 kW coupled reactor. To confirm the scale-up strategy, the performance of 5 kW coupled reactor is experimentally investigated with variations of operating parameters such as the fuel utilization in the solid oxide fuel cell (SOFC) stack, the inlet temperature of the catalytic combustor, the excess air ratio of the catalytic combustor, and the steam to carbon ratio (SCR) in the steam reformer. The temperature distributions of coupled reactors are measured along the gas flow direction. The gas composition at the steam reformer outlet is measured to find the CH4 conversion rate of the coupled reactor. The maximum value of the CH4 conversion rate is approximately 93.4%, which means the proposed scale-up strategy can be utilized to develop a large-scale coupled reactor. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Compared to a low temperature fuel cell, high temperature fuel cells, such as a solid oxide fuel cell (SOFC) and a molten carbonate fuel cell (MCFC), have the advantage of high fuel flexibility [1–4]. That is, hydrocarbon (HC) fuel can be converted into hydrogen (H2) by the heat generated in the stack, called internal reforming. However, when the amount of heat generated in the stack is not enough to fully convert HC into H2, an external reformer is needed in the system to pre-reform HC fuel. Various types of external reforming, such as partial oxidation (POX) reforming, auto-thermal reforming (ATR), and steam reforming (SR), have been utilized to attain the appropriate H2 yield rate. POX reforming is an exothermic reaction in which HC combines with a sub-stoichiometric amount of oxygen [5]. On the other ⇑ Corresponding author. Tel.: +82 42 868 7267; fax: +82 42 868 7284. E-mail address: [email protected] (S. Kang). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.09.046
hand, the SR reaction is an endothermic reaction, combining HC fuel with steam over the catalyst. The ATR reaction combines the POX reaction, SR reaction, and water–gas shift (WGS) reaction, which means that the heat required by the endothermic reaction of SR can be acquired from the exothermic reaction of POX [5]. Although POX reforming and ATR reaction have advantages such as a quick response and the simplification of thermal management, the SR reaction is widely used as external reforming for the SOFC system due to its highest production rate of H2 [5]. To maximize the H2 yield rate of SR, optimal thermal management is very important. Because a SOFC is usually operated with fuel utilization within the range of 60–80%, the exhaust gas at the SOFC anode can be used as a fuel for the combustor, which increases the system efficiency [6,7]. Some researchers have used a conventional burner as an anode-off gas reactor due to its easy operation and low cost [8,9]. However, since the composition of the SOFC anode-off gas is in fuel lean condition, a catalytic combustor is the most suitable anode-off gas reactor for the stability of combustion [10]. To
S. Kang et al. / Applied Energy 114 (2014) 114–123 Table 1 The set value of the space velocity and the corresponding volume of the 5 kW coupled reactor. Catalytic combustor Space velocity (h1) Volume (L)
18,000 4.3
Steam reformer Space velocity (h1) Volume (L)
1200 1.5
Table 2 The mass flow rate and composition of the feed gas for the catalytic combustor and steam reformer (base operating condition). Composition
Flow rate (slpm)
Volume %
Reformer inlet gases CH4 H2O Total
23.2 69.5 92.7
25.0 75.0 100
Combustor inlet gases H2 CO CO2 CH4 H2O Air Total
33.9 4.7 18.4 0.01 81.6 322.46 461.43
7.4 1.0 4.0 – 17.7 69.9 100
ppm
20.5
achieve a high heat transfer rate between the steam reformer and the catalytic combustor, a steam reformer is thermally integrated with the catalytic combustor, called a coupled reactor. Since the coupled reactor is a type of heat exchanger composed of the hot part of the catalytic combustor and the cold part of the steam reformer, many research studies on the optimal configurations of the coupled reactor have been conducted [11–15]. Patel et al. presented a steady-state numerical model for a thermally coupled membrane reactor and analyzed the effect of the key operating variables on the reactor performance [12]. Ramasway et al. analyzed the steady state and the dynamic behavior of directly coupled reactors using the one-dimensional pseudo-homogeneous plug flow model. They proposed a methodology to optimize the exothermic reaction parameters to minimize the hot spot while maintaining the desired endothermic conversion [13]. Kolios et al. experimentally investigated the concept of a coupled reactor
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to overcome the restriction of the overall thermal efficiency of the conventional steam reforming process due to heat transfer limitations and insufficient heat recovery [14]. In our previous work, a 1 kW coupled reactor with a catalytic combustor and a steam reformer was developed, and its performance with various operating parameters was investigated [15]. Although previous researches have contributed to optimizing the design of a coupled reactor, most of them were limited to a fundamental study or small scale power generation. The objective of the study is to develop a coupled reactor with a catalytic combustor and a steam reformer for a 5 kW SOFC system. To establish the scale-up strategy for the coupled reactor, the geometric parameters of the space velocity, diameter, and length were extracted from the previous 1 kW coupled reactor and tuned and applied to the design of the 5 kW coupled reactor. The configuration of the 5 kW coupled reactor was optimized by considering the heat transfer rate with the variations of the tube number of catalytic combustor. To confirm the scale-up strategy developed in the work, the performance of the 5 kW coupled reactor was experimentally estimated by varying the operating parameters, such as the fuel utilization in the SOFC stack, the inlet temperature of the catalytic combustor, the excess air ratio of the catalytic combustor, and the steam to carbon ratio (SCR) in the steam reformer. The distribution of the heat transfer rate along the gas flow direction between the catalytic combustor and the steam reformer was calculated by capturing the temperature distribution of each reactor. The gas composition at the reformer outlet experimentally captured was compared with the predicted value from the maximum temperature of the steam reformer. With the thermodynamic properties of the catalyst and reactors, the thermal energy balance in the coupled reactor was investigated at the reference operating condition. Deep analysis on the thermochemical characteristics of the coupled reactor and the scale-up strategy of the coupled reactor developed in the work can provide the basic insight to establish the optimal design of the large scale coupled reactor. 2. Scale-up strategy for the 5 kW coupled reactor Main considerations for the design of the coupled reactor is high reaction rate in both of catalytic combustor and steam reformer, high heat transfer rate from the catalytic combustor to the steam reformer, and flow uniformity of feed gas of each reactor. The scale-up strategy for the coupled reactor means that total amount of the reactant gases and reaction rates for both reactors
Fig. 1. The schematic diagram of the SOFC system model developed by ASPEN PLUSÒ.
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Table 3 The references of the operating conditions.
Table 5 The specifications of the coupled reactor and catalyst.
Inlet temperature of catalytic combustor (°C)
Excess air ratio (%)
Fuel utilization (%)
SCR
300
350
60
3.0
Table 4 Target value of heat transfer rate and inlet and outlet temperature of catalytic combustor and steam reformer. Target value of heat transfer rate Inlet temperature of catalytic combustor Outlet temperature of catalytic combustor Inlet temperature of steam reformer Outlet temperature of steam reformer
3.1 kW 918 °C 630 °C 200 °C 570 °C
Reactor material Outer tube diameter (mm) Inner tube diameter (mm) Active length of coupled reactor (mm) Wall thickness (mm) Center distance between reactors (mm) Fin height (mm) Fin thickness (mm) Number of fin per single tube (inner, outer) Catalyst diameter (mm) Reforming catalyst Combustion catalyst
Stainless steel 304 175 40 270 2 55 9.5 2 15 (6, 9) 3, Sphericity 1.95 wt% Ru/Al2O3 1 wt% Pt–Pd/Al2O3
The main reactions in the steam reformer are the methane steam reforming (MSR) reaction, the WGS reaction, and the reverse methanation reaction, written as:
CH4 þ H2 O ! CO þ 3H2
CO þ H2 O ! CO2 þ H2
CH4 þ 2H2 O ! CO2 þ H2
DH ¼ þ206:1
DH ¼ 41
kJ mol
kJ mol
DH ¼ þ164
kJ mol
ð3Þ
ð4Þ
ð5Þ
Generally, the reaction rate is represented by the space velocity, written as [16]:
SV ¼
Fig. 2. Schematic diagram of the 5 kW coupled reactor: (a) front view and (b) side view.
are increased, which indicates that the geometric parameters of the reactor should be scaled-up to facilitate the increased reaction kinetics. Since the SOFC anode-off gas is mainly composed of carbon monoxide (CO) and H2, the main reaction in the catalytic combustor is the oxidation reaction of CO and H2, written as:
1 kJ H2 þ O2 ! H2 O DH ¼ 242 2 mol
ð1Þ
1 CO þ O2 ! CO2 2
ð2Þ
DH ¼ 283
kJ mol
Qv V
ð6Þ
where Qv is the volumetric flow rate of the feed gas and V is the reactor volume. The reciprocal of the space velocity is the residence time of the gas, which means that when the space velocity is very high, a significant portion of the reactant gases can be slipped without reaction. On the other hand, a local hot spot can occur at a very low space velocity, which has a detrimental effect on the catalyst and reactor. The optimal range of the space velocity for the reactor depends on the catalyst and the temperature and pressure of the feed gas inlet. Because the catalyst and the temperature and pressure of the feed gas inlet for the 5 kW coupled reactor is the same as those of the 1 kW coupled reactor, the space velocity for the 5 kW coupled reactor is set to be the same as that of the 1 kW coupled reactor. Table 1 shows the set value of the space velocity and the corresponding volumes of the catalytic reactor and steam reformer. For the steam reformer since the MSR reaction is a strong endothermic reaction, a high heat transfer rate from the catalytic combustor should be ensured to maximize the reaction rate. Because the volumetric flow rate of the feed gas for the 1 kW coupled reactor is relatively small, its configuration is designed to be of the annular type. That is, one tube of catalytic combustor is surrounded by the steam reformer, which is enough to attain a high heat transfer rate between the catalytic combustor and the steam reformer. However, the volumetric flow rate of the feed gas for the 5 kW coupled reactor is five times that of the 1 kW coupled reactor, which means that the amount of heat generated by the catalytic combustor and required from the steam reformer is five times as much as that of the 1 kW reactor. This also means that five times the amount of heat should be transferred from the catalytic combustor to the steam reformer. To attain a high heat transfer rate between the steam reformer and the catalytic combustor, the configuration of the 5 kW coupled reactor is designed to be of the shell-and-tube type. That is, several inner tubes of the catalytic combustor are surrounded with the steam reformer. Even though the increase of tube number increased the heat transfer
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Fig. 3. Schematic diagram of the experimental apparatus for the estimation of the performance of the coupled reactor.
and steam reformer, respectively. The inlet temperature of hot fluid is 918 °C, the adiabatic flame temperature of the catalytic combustor. Since the SR reaction depends on the temperature, the outlet temperature of cold fluid was determined as 570 °C by considering the target methane conversion rate, 90%. And the inlet temperature of the cold fluid is 200 °C. Consequently, the outlet temperature of hot fluid is 630 °C and the log mean temperature difference is 387.6 °C. Because the target value of heat transfer rate from the catalytic combustor to the steam reformer at the reference operating condition is calculated as 3.1 kW, the value of U A should be higher than 8.7 e–3 kW/K, which is determined as follows [17]:
Table 6 The variations of the operating conditions. Inlet temperature of catalytic combustor (°C)
Excess air ratio (%)
Fuel utilization (%)
SCR
150, 200, 250, 300, 350 300
60 60
3.0 3.0
300
350 330, 350, 400, 450, 500 350
3.0
300
350
60, 65, 70, 75, 80 60
2.5, 3.0, 3.5
area, which make it difficult to fabricate the reactor and to insert the catalyst into the reactor. In order to determine the minimum number of tubes, the heat transfer rate between catalytic combustor and steam reformer with the variation of the tube number was obtained as follows [17]:
Q_ tr ¼ U A DT LM
ð7Þ
where Q_ tr ; U; A; and DT LM is the heat transfer rate between steam reformer and catalytic combustor, overall heat transfer coefficient, overall heat transfer area, and log mean temperature difference, respectively. To determine the amount of heat which should be transferred to the steam reformer from the catalytic combustor and the feed gas composition for the coupled reactor shown in Table 2, a system analysis for the 5 kW SOFC system composed of the SOFC stack and coupled reactor was conducted using ASPEN PLUSÒ. Fig. 1 shows the schematics of the SOFC system model. The base operating condition of the system model is presented in Table 3. The methane was reformed in the steam reformer with the steam at the SCR of 3.0 and 60% of reformed gas was utilized in the SOFC stack for the electrochemical reaction. Unused reformed gas from the stack, anode-off gas, was flowing into the catalytic combustor with the inlet temperature of 300 °C, then which was fully oxidized with the excess air ratio of 350%. And the heat generated by the catalytic combustor was supplied to the steam reformer. Table 4 shows the target value of heat transfer rate and inlet and outlet temperature of catalytic combustor
ln DD0i 1 1 1 ¼ þ þ UA ðg0 hAÞcc 2pkL ðg0 hAÞSR
ð8Þ
where g0 ; h; D0 ; Di ; k; cc; and SR refer to the overall surface efficiency, convection heat transfer coefficient, outer diameter of catalytic combustor, inner diameter of catalytic combustor, conductivity, catalytic combustor, and steam reformer, respectively. In order to obtain the convection heat transfer coefficient, the characteristics velocity, characteristics diameter, and Nusselt number in the packed bed reactor were determined as follows, respectively [18]:
v¼
_ m N eq p4 D2h
Dh ¼
Nu ¼
e 1e
Dcat
2 1 1 12 Re þ Re3 Pr3 2
h ¼ Nu
k Dh
ð9Þ
ð10Þ
ð11Þ
ð12Þ
_ N; k; Nu; Re, and Pr refer to the characteristics where v ; Dh ; Dcat; e; q; m; velocity, hydraulic diameter, catalyst diameter, porosity, density, mass flow rate of feed gas, tube number of catalytic combustor, thermal conductivity of feed gas, Nusselt number, Reynolds number,
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Fig. 4. Temperature distributions of the coupled reactor along the gas flow direction and the gas compositions at the steam reformer outlet with the inlet temperatures of the catalytic combustor varying from 150 °C to 350 °C at fuel utilization = 60%, SCR = 3.0, and excess air rate = 350%.
and Prandtl number, respectively. When the catalytic combustor was divided into four or five tubes, predicted value of heat transfer rate were 2.804 kW or 3.352 kW, respectively. Eventually, the catalytic combustor was designed to be five tubes of reactors. The distribution of inlet gas is another critical factor for obtaining a high performance and stability for the coupled reactor. When the inlet gas is not well distributed, an un-uniform reaction can occur inside the reactors and decrease the reactor performance or degrade the catalyst due to the formation of a local hot spot [19]. Some previous researches conducted by our group confirmed that the perforated plate is one of an excellent device for achievement of the flow uniformity [19,20]. To obtain the flow uniformity of the feed gas for the catalytic combustor and the steam reformer, a perforated plate was used at the inlet and outlet of the catalytic combustor and steam reformer, respectively. 3. Experimental setup 3.1. 5 kW coupled reactor The schematic of the 5 kW coupled reactor developed in the work is presented in Fig. 2. The coupled reactor has an annular shape, and the five tubes of the catalytic combustor are arranged
Fig. 5. Temperature distributions of the coupled reactor along the gas flow direction and the gas compositions at the steam reformer outlet with the excess air ratios of the catalytic combustor varying from 330% to 500% at fuel utilization = 60%, SCR = 3.0, and inlet temperature of the catalytic combustor = 300 °C.
in the outer tube of the steam reformer. Table 5 shows the specifications of the coupled reactor and catalysts. The whole coupled reactor is made from stainless steel. The diameter of the coupled reactor is 175 mm and that of five inner tubes is 40 mm. All the tubes of the coupled reactor have a thickness of 2 mm, and the active length of the coupled reactor is 270 mm. Two cross-coupled perforated plates are installed at the inlet and outlet of the reactor to ensure flow uniformity of the gas induced by the catalyst bed. For a uniform distribution of the catalyst, the reforming and combustion catalysts are inserted through three longitudinal parts and the front part of the reactor, respectively. The pellet type spherical catalyst of the Pd/Pt-base (1 wt% Pd–Pt/Al2O3) and the Ru-base (1.95 wt% Ru/Al2O3) is used for combustion and the MSR reaction, respectively. Several grooved fins are attached to the inner and outer surfaces of the catalytic combustor to enhance the heat transfer rate. The height and thickness of the fins are approximately 9.5 mm and 2 mm, respectively. To investigate the temperature distributions of the coupled reactor, seven type K thermocouples are installed in the steam reformer and one tube of the catalytic combustors along the gas flow direction. An additional thermocouple is installed to capture the gas temperature at the reactor inlet and outlet. Reforming catalysts was inserted
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Fig. 6. Temperature distributions of the coupled reactor along the gas flow direction and the gas compositions at the steam reformer outlet with the SCR in the steam reformer varying from 2.0 to 3.5 at fuel utilization = 60%, excess air rate = 350%, and inlet temperature of the catalytic combustor = 300 °C.
into the reformer through the three section of catalyst insert entrance for the uniform arrangement of the catalyst. 3.2. Experimental apparatus Fig. 3 shows the schematic diagram of the experimental apparatus used to estimate the coupled reactor performance. The mass flow rate of the reactant gas was controlled by the mass flow controller (Brooks InstrumentÒ 5853S). Pure CH4 (>99.95%) mixed with steam was used as the reactant gas for the steam reformer. In order to simulate the SOFC anode-off gas, H2, CO, CO2, CH4, and steam was uniformly mixed by passing it through the static mixer and supplying it to the catalytic combustor. The electric heater and line heater were used to preheat the inlet gas mixture of the catalytic combustor and the steam reformer to the desired temperature, respectively. The gas compositions at the outlet of the steam reformer and catalytic combustor were measured by gas chromatography (Agilent 7890A). 3.3. Experimental conditions To confirm the scale-up strategy developed in the work, the performance of the 5 kW coupled reactor was experimentally
119
Fig. 7. Temperature distributions of the coupled reactor along the gas flow direction and the gas compositions at the steam reformer outlet with the fuel utilizations varying from 60% to 90% at SCR = 3.0, excess air ratio = 350%, and inlet temperature of the catalytic combustor = 300 °C.
estimated by varying the operating parameters. Tables 3 and 6 presented the references and variations of the operating parameters, respectively. Because the catalyst for catalytic combustion is drastically degraded at a temperature higher than 950 °C, the range of operating conditions were set to values that make the temperature of the catalytic combustor lower than 950 °C. When the excess air ratio and inlet temperature of the reactant gas for the catalytic combustor are lower than 3.3 or higher than 350 °C, respectively, the temperature of the catalytic combustor becomes higher than 950 °C. Thus, the variations of the excess air ratio and the inlet temperature of the catalytic combustor were set to be 330%, 350%, 400%, 450%, and 500% and 150, 200, 250, 300, and 350 °C, respectively. When the SOFC anode-off gas has a fuel rich composition, the maximum temperature in the catalytic combustor can be higher than 950 °C. Thus, the variations of the fuel utilization for the stack were set to 60%, 65%, 70%, 75%, and 80%. When the SOFC system is operated at a high SCR, the system efficiency can be decreased since a higher amount of steam is generated. On the other hand, the performance of the catalyst can be degraded by coke formation on the catalyst surface at a low SCR, suggesting that the SOFC system should be operated within the optimal range of the SCR. Thus, the SCR was set to be 2.5, 3.0, and 3.5.
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Fig. 8. The variations of the remaining heat and the heat transfer rate between the catalytic combustor and the steam reformer for variations of the operating parameters; (a) inlet temperature of the catalytic combustor, (b) excess air ratio, (c) SCR, and (d) fuel utilization.
4. Results and discussions Figs. 4–7 show the gas compositions at the steam reformer outlet and the temperature distributions of the coupled reactor obtained along the gas flow direction by varying the operating parameters, such as the inlet temperature of the catalytic combustor, the excess air ratio of the catalytic combustor, the SCR in the steam reformer, and fuel utilization in the stack, respectively. Increasing the inlet temperature of the catalytic combustor from 150 °C to 350 °C increases the maximum temperature of the catalytic combustor from 781 °C to 941 °C, which increases the heat transfer between the catalytic combustor and the steam reformer. The temperature increase in the steam reformer from 521 °C to 607 °C facilitates the MSR reaction. Eventually, the H2 mole fraction at the steam reformer outlet increases from 69.9% to 74.9%. Even though the excess air ratio increased from 300% to 500%, the reaction rates of the oxidation of H2 and CO did not increase, which decreased the temperatures of the catalytic combustor and steam reformer from 931 °C to 748 °C and from 597 °C to 544 °C, respectively. The decrease of the steam reformer temperature decreased the MSR reaction but increased the WGS reaction. Thus, the species mole fraction of CO decreased from 9.0% to 5.8%. However, because the magnitude of the reaction rate of the WGS was
lower than that of MSR, the H2 mole fraction decreased from 74.8% to 70.5%. As the SCR increased from 2.5 to 3.5, the maximum temperatures of the catalytic combustor and steam reformer decreased from 932 °C to 895 °C and from 618 °C to 572 °C, respectively. When the SCR increased higher than the value that maximized the CH4 conversion rate, the overall temperature of the coupled reactor can be decreased because of the increase of the steam mass, which means an SCR of 2.5 was enough to maximize the conversion rate of CH4 at this temperature. As a result, the species mole fraction of CH4 increased from 7.38% to 8.16%. Since the temperature decrease facilitated the WGS reaction, the species mole fraction of CO decreased from 11.1% to 7.5%. The decrease of the MSR reaction and the increase of the WGS reaction maintained the H2 mole fraction at approximately 74%. As the fuel utilization in the stack increased, the concentration of the H2 and CO in the anode-off gas decreased, which means that the total amount of heat that can be generated in the catalytic combustor decreased. As a result, the maximum temperatures of the catalytic combustor and steam reformer decreased from 916 °C to 764 °C and from 595 °C to 508 °C, respectively. The species mole fraction of the CH4 at the steam reformer outlet drastically increased from 8.1% to 22.6% due to the low temperature of
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Fig. 9. The measured and calculated values of the coupled reactor temperature for variations of the operating parameters; (a) inlet temperature, (b) excess air ratio, (c) SCR, and (d) fuel utilization.
the MSR reaction. Although a low temperature facilitates the WGS reaction, the H2 mole fraction at the steam reformer outlet decreased from 73.8% to 63.3%. Because the coupled reactor is a type of heat exchanger composed of the hot part of the catalytic combustor and the cold part of the steam reformer, obtaining a high heat transfer rate is important. Since the wall is sandwiched between the catalytic combustor and the steam reformer, the heat is transferred from the catalytic combustor to the steam reformer through the wall. Since the temperature distribution along the gas flow direction in both the catalytic combustor and the steam reformer was measured, the heat transfer rate between the catalytic combustor and the wall and the steam reformer and the wall of each local section along the gas flow direction can be obtained as follows:
NC v
X X X dT X _ Q_ tr Q_ reaction ¼ Nin hin N_ out hout dt
ð13Þ
where NC v ; N_ in hin ; N_ out hout ; Q_ tr , and Q_ reaction represent the thermal capacity of the fluid in the reactor, enthalpy flux in and out of the reactor, the heat transfer to the reactor, and the reaction enthalpy in the reactor, respectively. Fig. 8 shows the variations of the remaining heat and the heat transfer rate in the coupled reactor.
The solid and dotted lines of the lower part in graph indicate the heat transfer rate distribution along the gas flow direction between the steam reformer and the wall and the catalytic combustor and the wall, respectively. The distribution of the remaining heat in the catalytic combustor and the wall along the gas flow direction can be obtained by subtracting the heat transfer rate of each local section from the remaining heat of the former local section. The whole catalytic combustion reaction occurs at the inlet of the catalytic combustor. The solid and dotted lines of the upper part in each graph indicate the distribution of the remaining heat in the catalytic combustor and the wall, respectively. Theoretically, the consumed heat in the steam reformer for the SR reaction is supplied from the catalytic combustor. When the measured temperatures of the catalytic combustor and the steam reformer represent the mean temperatures of each reactor, the heat transfer rate calculated from the temperature distribution of the catalytic combustor should be higher than that from the temperature variation of the steam reformer. That is, the heat transfer rate between the catalytic combustor and the wall should be higher than that between the wall and the steam reformer. The heat transfer rate between the catalytic combustor and the wall is almost the same from the first to the fifth local section and then decreases from the sixth local section to the
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Fig. 11. The thermal energy balance of the 5 kW coupled reactor at the base operating condition.
Fig. 10. The measured value and the predicted value of the gas compositions at the steam reformer outlet from the measured temperature and the calculated temperature at the operating parameters; (a) inlet temperature, (b) excess air ratio, and (c) fuel utilization.
outlet of the steam reformer. On the other hand, the minimum and maximum amount of heat transfer between the steam reformer and the wall occur in the second local section and the sixth local section, respectively. At the first, fourth, and fifth local sections, the heat transfer rate between the catalytic combustor and the wall is
almost same as that between the wall and the steam reformer, which means that the heat transfer efficiency at these local sections is high. At the second and third local sections, the heat transfer rate between the steam reformer and the wall is lower than that between the catalytic combustor and the wall, which indicates that some portion of the heat transferred from the catalytic combustor is transferred to the steam reformer. However, at the sixth local section, the heat transfer rate between the steam reformer and the wall is higher than that between the catalytic combustor and the wall. This is because the experimentally measured temperature may not be the mean temperature of each reactor. Specifically since the five catalytic combustors are surrounded in the steam reformer, the temperature in the steam reformer may not be uniform along the radial direction. Furthermore, the location of the thermocouple that detects the temperature of the steam reformer is close to the outside of the reactor, as shown in Fig. 1. When whole amount of the heat transfer rate calculated from the temperature distribution of each reactor is transferred to the other reactor, the theoretical temperature of each reactor can be determined and is shown as a dotted line in Fig. 9. Since the heat transfer efficiency is lower than 100% due to the thermal resistance inside the reactor, the actual temperature of the each reactor cannot be the same as the calculated value. The temperature from the calculation in both reactors is almost the same as the measured temperature at the first and second local sections. However, the calculated temperature of both reactors is different from the measured temperature of the third to sixth local sections because of the thermal resistance of the wall and the catalyst. The composition of the combustion off gas was dependent on the maximum temperature of the catalytic combustor. The temperature at the combustor inlet shown in Fig. 9 means the theoretical value of adiabatic flame temperature of catalytic combustor. In every case, the difference between maximum temperature experimentally captured and adiabatic flame temperature at various operating condition is less than 10 °C. Since the thermocouple is located at the 27 mm away from the combustor inlet, the actual value of the maximum temperature of combustor should be higher than captured value of it. This means the actual maximum temperature was almost same with the adiabatic flame temperature, which means most of the H2 and CO was oxidized. Consequently, most of the combustion off gas can be predicted as H2O and CO2. Because the reforming reaction kinetics highly depends on the temperature of the steam reformer, the gas compositions at the steam reformer outlet can be predicted by the maximum
S. Kang et al. / Applied Energy 114 (2014) 114–123 Table 7 The thermodynamic properties of combustion catalyst, reforming catalyst, and reactor wall.
Combustion catalyst Reforming catalyst Reactor wall
Volume (cc)
Thermal conductivity (W/m K)
Specific heat capacity (J/g °C)
864
30.391
0.020
1933
31.677
0.022
71.3
16.2
0.5
temperature of the steam reformer. Fig. 10 compares the measured and predicted values of the gas compositions at the steam reformer outlet from the maximum temperature of the steam reformer. The solid line is the theoretical value of the gas compositions at the steam reformer outlet for the temperature variations. Several points on the graph mean the measured value of the gas compositions at the steam reformer outlet. The measured temperature detected by the thermocouple installed at the steam reformer outlet may not be the highest temperature in the reactor, and this discrepancy can cause the difference between the points on the graph and the solid line. To investigate the correlation between the measured value of the gas compositions at the steam reformer outlet and the predicted value calculated from the predicted maximum temperature of the steam reformer, the predicted maximum temperature of the steam reformer is calculated by considering the adiabatic flame temperature of the catalytic combustor. The dotted line shows the predicted value of the gas compositions with temperature variations by considering the difference of the maximum temperature between the predicted value and the measured value. In every case, measured value of CH4 is a little bit higher than the theoretical value, which can be resulted from the low MSR reaction. However, high measured value of CO and H2 means the high MSR reaction. Low measured value of CO2 can be caused by low WGS reaction. In WGS reaction, the mole number of consumed CO and produced CO2 is the same. However, the difference between measured value and theoretical value of CO2 is higher than that of CO. This means that some error of the data acquisition can be another reason which causes the discrepancy between measured value and theoretical value of CO2. Although slight error, the several points are relatively in good agreement with the dotted line, which means that the predicted maximum temperature of the steam reformer from the calculation represents the actual maximum temperature. Fig. 11 presented the thermal energy balance of the 5 kW coupled reactor at the base operating condition. The total amount of heat generated in the coupled reactor was about 6.5 kW. The 47.12% and 28.89% of the generated heat was transferred to the steam reformer and was utilized for the steam reforming reaction, respectively. The 49.94% and 18.23% of the total thermal energy used to increase the sensible heat of the gas for the catalytic combustor and steam reformer, respectively. The thermal energy consumed by the combustor catalyst, reformer catalyst, and combustor wall was 0.09%, 0.06% and 2.79%, respectively, which is calculated by considering their thermodynamic properties shown in Table 7.
5. Conclusions A coupled reactor with a catalytic combustor and a steam reformer for a 5 kW SOFC system has been developed. The geometric parameters, such as the space velocity, diameter and length of the coupled reactor, selected from the 1 kW coupled reactor in a previous study are utilized for the design of the 5 kW coupled reactor. The configuration of the 5 kW coupled reactor was opti-
123
mized by considering the heat transfer rate with the variations of the tube number of catalytic combustor. The performance of the 5 kW coupled reactor is experimentally investigated by varying the operating parameters, including the fuel utilization in the SOFC stack, the inlet temperature of the catalytic combustor, the excess air ratio of the catalytic combustor, and the SCR in the steam reformer. The distributions of the heat transfer in the coupled reactor were obtained from the temperature distributions experimentally measured along the gas flow direction. With the thermodynamic properties of the catalyst and reactors, the thermal energy balance of the 5 kW coupled reactor was investigated at the reference operating condition. The gas composition at the reformer outlet experimentally captured was compared with the predicted value from the maximum temperature of the steam reformer. The maximum value of the CH4 conversion rate is approximately 93.4% at the inlet temperature of catalytic combustor of 350 °C, excess air ratio of 350%, SCR of 3.0, and fuel utilization of 60%, which means that the proposed scale-up strategy can be utilized to develop a largescale coupled reactor with a catalytic combustor and a steam reformer. Deep analysis on the thermochemical characteristics of the coupled reactor and the scale-up strategy of the coupled reactor with catalytic combustor and steam reformer developed in the work can provide the basic insight to establish the optimal design of the large scale coupled reactor. References [1] Zhang X, Chan SH, Li G, Ho HK, Li J, Feng Z. A review of integration strategies for solid oxide fuel cells. J Power Sources 2010;195:685–702. [2] Eguchi K, Kojo H, Takeguchi T, Sasaki K. Fuel flexibility in power generation by solid oxide fuel cells. Solid State Ionics 2002;152:411–6. [3] Park SK, Kim TS, Sohn JL, Lee YD. An integrated power generation system combining solid oxide fuel cell and oxy-fuel combustion for high performance and CO2 capture. Appl Energy 2011;88:1187–96. [4] Wee JH. Molten carbonate fuel cell and gas turbine hybrid system as distributed energy resources. Appl Energy 2011;88:4252–63. [5] Laminie J, Dicks A. Fuel cell systems explained. 2nd ed. John Wiley & Sons Ltd; 2003. p. 241–50. [6] Georgis D, Jogwar SS, Almansoori AS, Daoutidis P. Design and control of energy integrated SOFC systems for in situ hydrogen production and power generation. Comput Chem Eng 2011;35:1691–704. [7] Mueller F, Jabbari F, Gaynor R, Brouwer J. Novel solid oxide fuel cell system controller for rapid load following. J Power Source 2007;172:308–23. [8] Braun RJ, Klein SA, Reindl DT. Evaluation of system configurations for solid oxide fuel cell-based micro-combined heat and power generators in residential applications. J Power Source 2006;158:1290–305. [9] Yen TH, Hong WT, Huang WP, Tsai YC, Wang HY, Huang CN, et al. Experimental investigation of 1 kW solid oxide fuel cell system with a natural gas reformer and an exhaust gas burner. J Power Source 2010;195:1454–62. [10] Lai WH, Hsiao CA, Lee CH, Chyou YP, Tsai YC. Experimental simulation on the integration of solid oxide fuel cell and micro-turbine generation system. J Power Source 2007;171:130–9. [11] Rahimpour MR, Dehnavi MR, Allahgholipour F, Iranshahi D, Jokar SM. Assessment and comparison of different catalytic coupling exothermic and endothermic reactions: a review. Appl Energy 2012;99:496–512. [12] Patel KS, Sunol AK. Modeling and simulation of methane steam reforming in a thermally coupled membrane reactor. Int J Hydrogen Energy 2007;32:2344–58. [13] Ramaswamy RC, Ramachandran PA, Dudukovic MP. Coupling exothermic and endothermic reactions in adiabatic reactors. Chem Eng Sci 2008;63:1654–67. [14] Kolios G, Glockler B, Gritsch A, Morillo A, Eigenberger G. Heat-integrated reactor concepts for hydrogen production by methane steam reforming. Fuel Cells 2005;5:52–65. [15] Ghang TG, Lee SM, Ahn KY, Kim Y. An experimental study on the reaction characteristics of a coupled reactor with a catalytic combustor and a steam reformer for SOFC system. Int J Hydrogen Energy 2012;37:3234–41. [16] Hayes RE, Kolacczkowski ST. Introduction to catalytic combustion. Amsterdam: Gordon and Breach Science Publishers; 1997. p. 1– 95. [17] Incropera FP, Dewitt DP. Fundamentals of heat and mass transfer. 5th ed. John Wiley & Sons; 2002. [18] Mills AF. Basic heat and mass transfer. 2nd ed. Pearosn & Prentice hall; 2003. [19] Yu SS, Hong DJ, Lee YD, Lee SM, Ahn KY. Development of a catalytic combustor for a stationary fuel cell power generation system. Renew Energy 2010;35:1083–90. [20] Lee SM, Ahn KY, Lee YD, Han JY, Im SY, Yu SS. Flow uniformity of catalytic burner for off-gas combustion of molten carbonate fuel cell. Trans ASME 2012;9:021006-1–021006-12.
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Development of a coupled reactor with a catalytic combustor and steam reformer for a 5 kW solid oxide fuel cell system Sanggyu Kang a,⇑, Kanghun Lee b, Sangseok Yu b, Sang Min Lee a, Kook-Young Ahn a a b
Korea Institute of Machinery and Materials, Jang-dong, Daejeon 305-343, Republic of Korea Chungnam National University, Gung-dong, Daejeon 305-764, Republic of Korea
h i g h l i g h t s Proposes the scale-up strategy to develop a large-scale coupled reactor. Investigation of performance of steam reformer coupled with catalytic combustor. Experimental parameters are inlet temp., air excess ratio, SCR, fuel utilization. Evaluation of the heat transfer distribution along the gas flow direction. The mean value of methane conversion rate is approximately 93.4%.
a r t i c l e
i n f o
Article history: Received 18 May 2013 Received in revised form 2 September 2013 Accepted 17 September 2013 Available online 15 October 2013 Keywords: Scale-up strategy Thermal integration Steam reformer Catalytic combustor Methane conversion rate
a b s t r a c t The methane (CH4) conversion rate of a steam reformer can be increased by thermal integration with a catalytic combustor, called a coupled reactor. In the present study, a 5 kW coupled reactor has been developed based on a 1 kW coupled reactor in previous work. The geometric parameters of the space velocity, diameter and length of the coupled reactor selected from the 1 kW coupled reactor are tuned and applied to the design of the 5 kW coupled reactor. To confirm the scale-up strategy, the performance of 5 kW coupled reactor is experimentally investigated with variations of operating parameters such as the fuel utilization in the solid oxide fuel cell (SOFC) stack, the inlet temperature of the catalytic combustor, the excess air ratio of the catalytic combustor, and the steam to carbon ratio (SCR) in the steam reformer. The temperature distributions of coupled reactors are measured along the gas flow direction. The gas composition at the steam reformer outlet is measured to find the CH4 conversion rate of the coupled reactor. The maximum value of the CH4 conversion rate is approximately 93.4%, which means the proposed scale-up strategy can be utilized to develop a large-scale coupled reactor. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Compared to a low temperature fuel cell, high temperature fuel cells, such as a solid oxide fuel cell (SOFC) and a molten carbonate fuel cell (MCFC), have the advantage of high fuel flexibility [1–4]. That is, hydrocarbon (HC) fuel can be converted into hydrogen (H2) by the heat generated in the stack, called internal reforming. However, when the amount of heat generated in the stack is not enough to fully convert HC into H2, an external reformer is needed in the system to pre-reform HC fuel. Various types of external reforming, such as partial oxidation (POX) reforming, auto-thermal reforming (ATR), and steam reforming (SR), have been utilized to attain the appropriate H2 yield rate. POX reforming is an exothermic reaction in which HC combines with a sub-stoichiometric amount of oxygen [5]. On the other ⇑ Corresponding author. Tel.: +82 42 868 7267; fax: +82 42 868 7284. E-mail address: [email protected] (S. Kang). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.09.046
hand, the SR reaction is an endothermic reaction, combining HC fuel with steam over the catalyst. The ATR reaction combines the POX reaction, SR reaction, and water–gas shift (WGS) reaction, which means that the heat required by the endothermic reaction of SR can be acquired from the exothermic reaction of POX [5]. Although POX reforming and ATR reaction have advantages such as a quick response and the simplification of thermal management, the SR reaction is widely used as external reforming for the SOFC system due to its highest production rate of H2 [5]. To maximize the H2 yield rate of SR, optimal thermal management is very important. Because a SOFC is usually operated with fuel utilization within the range of 60–80%, the exhaust gas at the SOFC anode can be used as a fuel for the combustor, which increases the system efficiency [6,7]. Some researchers have used a conventional burner as an anode-off gas reactor due to its easy operation and low cost [8,9]. However, since the composition of the SOFC anode-off gas is in fuel lean condition, a catalytic combustor is the most suitable anode-off gas reactor for the stability of combustion [10]. To
S. Kang et al. / Applied Energy 114 (2014) 114–123 Table 1 The set value of the space velocity and the corresponding volume of the 5 kW coupled reactor. Catalytic combustor Space velocity (h1) Volume (L)
18,000 4.3
Steam reformer Space velocity (h1) Volume (L)
1200 1.5
Table 2 The mass flow rate and composition of the feed gas for the catalytic combustor and steam reformer (base operating condition). Composition
Flow rate (slpm)
Volume %
Reformer inlet gases CH4 H2O Total
23.2 69.5 92.7
25.0 75.0 100
Combustor inlet gases H2 CO CO2 CH4 H2O Air Total
33.9 4.7 18.4 0.01 81.6 322.46 461.43
7.4 1.0 4.0 – 17.7 69.9 100
ppm
20.5
achieve a high heat transfer rate between the steam reformer and the catalytic combustor, a steam reformer is thermally integrated with the catalytic combustor, called a coupled reactor. Since the coupled reactor is a type of heat exchanger composed of the hot part of the catalytic combustor and the cold part of the steam reformer, many research studies on the optimal configurations of the coupled reactor have been conducted [11–15]. Patel et al. presented a steady-state numerical model for a thermally coupled membrane reactor and analyzed the effect of the key operating variables on the reactor performance [12]. Ramasway et al. analyzed the steady state and the dynamic behavior of directly coupled reactors using the one-dimensional pseudo-homogeneous plug flow model. They proposed a methodology to optimize the exothermic reaction parameters to minimize the hot spot while maintaining the desired endothermic conversion [13]. Kolios et al. experimentally investigated the concept of a coupled reactor
115
to overcome the restriction of the overall thermal efficiency of the conventional steam reforming process due to heat transfer limitations and insufficient heat recovery [14]. In our previous work, a 1 kW coupled reactor with a catalytic combustor and a steam reformer was developed, and its performance with various operating parameters was investigated [15]. Although previous researches have contributed to optimizing the design of a coupled reactor, most of them were limited to a fundamental study or small scale power generation. The objective of the study is to develop a coupled reactor with a catalytic combustor and a steam reformer for a 5 kW SOFC system. To establish the scale-up strategy for the coupled reactor, the geometric parameters of the space velocity, diameter, and length were extracted from the previous 1 kW coupled reactor and tuned and applied to the design of the 5 kW coupled reactor. The configuration of the 5 kW coupled reactor was optimized by considering the heat transfer rate with the variations of the tube number of catalytic combustor. To confirm the scale-up strategy developed in the work, the performance of the 5 kW coupled reactor was experimentally estimated by varying the operating parameters, such as the fuel utilization in the SOFC stack, the inlet temperature of the catalytic combustor, the excess air ratio of the catalytic combustor, and the steam to carbon ratio (SCR) in the steam reformer. The distribution of the heat transfer rate along the gas flow direction between the catalytic combustor and the steam reformer was calculated by capturing the temperature distribution of each reactor. The gas composition at the reformer outlet experimentally captured was compared with the predicted value from the maximum temperature of the steam reformer. With the thermodynamic properties of the catalyst and reactors, the thermal energy balance in the coupled reactor was investigated at the reference operating condition. Deep analysis on the thermochemical characteristics of the coupled reactor and the scale-up strategy of the coupled reactor developed in the work can provide the basic insight to establish the optimal design of the large scale coupled reactor. 2. Scale-up strategy for the 5 kW coupled reactor Main considerations for the design of the coupled reactor is high reaction rate in both of catalytic combustor and steam reformer, high heat transfer rate from the catalytic combustor to the steam reformer, and flow uniformity of feed gas of each reactor. The scale-up strategy for the coupled reactor means that total amount of the reactant gases and reaction rates for both reactors
Fig. 1. The schematic diagram of the SOFC system model developed by ASPEN PLUSÒ.
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Table 3 The references of the operating conditions.
Table 5 The specifications of the coupled reactor and catalyst.
Inlet temperature of catalytic combustor (°C)
Excess air ratio (%)
Fuel utilization (%)
SCR
300
350
60
3.0
Table 4 Target value of heat transfer rate and inlet and outlet temperature of catalytic combustor and steam reformer. Target value of heat transfer rate Inlet temperature of catalytic combustor Outlet temperature of catalytic combustor Inlet temperature of steam reformer Outlet temperature of steam reformer
3.1 kW 918 °C 630 °C 200 °C 570 °C
Reactor material Outer tube diameter (mm) Inner tube diameter (mm) Active length of coupled reactor (mm) Wall thickness (mm) Center distance between reactors (mm) Fin height (mm) Fin thickness (mm) Number of fin per single tube (inner, outer) Catalyst diameter (mm) Reforming catalyst Combustion catalyst
Stainless steel 304 175 40 270 2 55 9.5 2 15 (6, 9) 3, Sphericity 1.95 wt% Ru/Al2O3 1 wt% Pt–Pd/Al2O3
The main reactions in the steam reformer are the methane steam reforming (MSR) reaction, the WGS reaction, and the reverse methanation reaction, written as:
CH4 þ H2 O ! CO þ 3H2
CO þ H2 O ! CO2 þ H2
CH4 þ 2H2 O ! CO2 þ H2
DH ¼ þ206:1
DH ¼ 41
kJ mol
kJ mol
DH ¼ þ164
kJ mol
ð3Þ
ð4Þ
ð5Þ
Generally, the reaction rate is represented by the space velocity, written as [16]:
SV ¼
Fig. 2. Schematic diagram of the 5 kW coupled reactor: (a) front view and (b) side view.
are increased, which indicates that the geometric parameters of the reactor should be scaled-up to facilitate the increased reaction kinetics. Since the SOFC anode-off gas is mainly composed of carbon monoxide (CO) and H2, the main reaction in the catalytic combustor is the oxidation reaction of CO and H2, written as:
1 kJ H2 þ O2 ! H2 O DH ¼ 242 2 mol
ð1Þ
1 CO þ O2 ! CO2 2
ð2Þ
DH ¼ 283
kJ mol
Qv V
ð6Þ
where Qv is the volumetric flow rate of the feed gas and V is the reactor volume. The reciprocal of the space velocity is the residence time of the gas, which means that when the space velocity is very high, a significant portion of the reactant gases can be slipped without reaction. On the other hand, a local hot spot can occur at a very low space velocity, which has a detrimental effect on the catalyst and reactor. The optimal range of the space velocity for the reactor depends on the catalyst and the temperature and pressure of the feed gas inlet. Because the catalyst and the temperature and pressure of the feed gas inlet for the 5 kW coupled reactor is the same as those of the 1 kW coupled reactor, the space velocity for the 5 kW coupled reactor is set to be the same as that of the 1 kW coupled reactor. Table 1 shows the set value of the space velocity and the corresponding volumes of the catalytic reactor and steam reformer. For the steam reformer since the MSR reaction is a strong endothermic reaction, a high heat transfer rate from the catalytic combustor should be ensured to maximize the reaction rate. Because the volumetric flow rate of the feed gas for the 1 kW coupled reactor is relatively small, its configuration is designed to be of the annular type. That is, one tube of catalytic combustor is surrounded by the steam reformer, which is enough to attain a high heat transfer rate between the catalytic combustor and the steam reformer. However, the volumetric flow rate of the feed gas for the 5 kW coupled reactor is five times that of the 1 kW coupled reactor, which means that the amount of heat generated by the catalytic combustor and required from the steam reformer is five times as much as that of the 1 kW reactor. This also means that five times the amount of heat should be transferred from the catalytic combustor to the steam reformer. To attain a high heat transfer rate between the steam reformer and the catalytic combustor, the configuration of the 5 kW coupled reactor is designed to be of the shell-and-tube type. That is, several inner tubes of the catalytic combustor are surrounded with the steam reformer. Even though the increase of tube number increased the heat transfer
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Fig. 3. Schematic diagram of the experimental apparatus for the estimation of the performance of the coupled reactor.
and steam reformer, respectively. The inlet temperature of hot fluid is 918 °C, the adiabatic flame temperature of the catalytic combustor. Since the SR reaction depends on the temperature, the outlet temperature of cold fluid was determined as 570 °C by considering the target methane conversion rate, 90%. And the inlet temperature of the cold fluid is 200 °C. Consequently, the outlet temperature of hot fluid is 630 °C and the log mean temperature difference is 387.6 °C. Because the target value of heat transfer rate from the catalytic combustor to the steam reformer at the reference operating condition is calculated as 3.1 kW, the value of U A should be higher than 8.7 e–3 kW/K, which is determined as follows [17]:
Table 6 The variations of the operating conditions. Inlet temperature of catalytic combustor (°C)
Excess air ratio (%)
Fuel utilization (%)
SCR
150, 200, 250, 300, 350 300
60 60
3.0 3.0
300
350 330, 350, 400, 450, 500 350
3.0
300
350
60, 65, 70, 75, 80 60
2.5, 3.0, 3.5
area, which make it difficult to fabricate the reactor and to insert the catalyst into the reactor. In order to determine the minimum number of tubes, the heat transfer rate between catalytic combustor and steam reformer with the variation of the tube number was obtained as follows [17]:
Q_ tr ¼ U A DT LM
ð7Þ
where Q_ tr ; U; A; and DT LM is the heat transfer rate between steam reformer and catalytic combustor, overall heat transfer coefficient, overall heat transfer area, and log mean temperature difference, respectively. To determine the amount of heat which should be transferred to the steam reformer from the catalytic combustor and the feed gas composition for the coupled reactor shown in Table 2, a system analysis for the 5 kW SOFC system composed of the SOFC stack and coupled reactor was conducted using ASPEN PLUSÒ. Fig. 1 shows the schematics of the SOFC system model. The base operating condition of the system model is presented in Table 3. The methane was reformed in the steam reformer with the steam at the SCR of 3.0 and 60% of reformed gas was utilized in the SOFC stack for the electrochemical reaction. Unused reformed gas from the stack, anode-off gas, was flowing into the catalytic combustor with the inlet temperature of 300 °C, then which was fully oxidized with the excess air ratio of 350%. And the heat generated by the catalytic combustor was supplied to the steam reformer. Table 4 shows the target value of heat transfer rate and inlet and outlet temperature of catalytic combustor
ln DD0i 1 1 1 ¼ þ þ UA ðg0 hAÞcc 2pkL ðg0 hAÞSR
ð8Þ
where g0 ; h; D0 ; Di ; k; cc; and SR refer to the overall surface efficiency, convection heat transfer coefficient, outer diameter of catalytic combustor, inner diameter of catalytic combustor, conductivity, catalytic combustor, and steam reformer, respectively. In order to obtain the convection heat transfer coefficient, the characteristics velocity, characteristics diameter, and Nusselt number in the packed bed reactor were determined as follows, respectively [18]:
v¼
_ m N eq p4 D2h
Dh ¼
Nu ¼
e 1e
Dcat
2 1 1 12 Re þ Re3 Pr3 2
h ¼ Nu
k Dh
ð9Þ
ð10Þ
ð11Þ
ð12Þ
_ N; k; Nu; Re, and Pr refer to the characteristics where v ; Dh ; Dcat; e; q; m; velocity, hydraulic diameter, catalyst diameter, porosity, density, mass flow rate of feed gas, tube number of catalytic combustor, thermal conductivity of feed gas, Nusselt number, Reynolds number,
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Fig. 4. Temperature distributions of the coupled reactor along the gas flow direction and the gas compositions at the steam reformer outlet with the inlet temperatures of the catalytic combustor varying from 150 °C to 350 °C at fuel utilization = 60%, SCR = 3.0, and excess air rate = 350%.
and Prandtl number, respectively. When the catalytic combustor was divided into four or five tubes, predicted value of heat transfer rate were 2.804 kW or 3.352 kW, respectively. Eventually, the catalytic combustor was designed to be five tubes of reactors. The distribution of inlet gas is another critical factor for obtaining a high performance and stability for the coupled reactor. When the inlet gas is not well distributed, an un-uniform reaction can occur inside the reactors and decrease the reactor performance or degrade the catalyst due to the formation of a local hot spot [19]. Some previous researches conducted by our group confirmed that the perforated plate is one of an excellent device for achievement of the flow uniformity [19,20]. To obtain the flow uniformity of the feed gas for the catalytic combustor and the steam reformer, a perforated plate was used at the inlet and outlet of the catalytic combustor and steam reformer, respectively. 3. Experimental setup 3.1. 5 kW coupled reactor The schematic of the 5 kW coupled reactor developed in the work is presented in Fig. 2. The coupled reactor has an annular shape, and the five tubes of the catalytic combustor are arranged
Fig. 5. Temperature distributions of the coupled reactor along the gas flow direction and the gas compositions at the steam reformer outlet with the excess air ratios of the catalytic combustor varying from 330% to 500% at fuel utilization = 60%, SCR = 3.0, and inlet temperature of the catalytic combustor = 300 °C.
in the outer tube of the steam reformer. Table 5 shows the specifications of the coupled reactor and catalysts. The whole coupled reactor is made from stainless steel. The diameter of the coupled reactor is 175 mm and that of five inner tubes is 40 mm. All the tubes of the coupled reactor have a thickness of 2 mm, and the active length of the coupled reactor is 270 mm. Two cross-coupled perforated plates are installed at the inlet and outlet of the reactor to ensure flow uniformity of the gas induced by the catalyst bed. For a uniform distribution of the catalyst, the reforming and combustion catalysts are inserted through three longitudinal parts and the front part of the reactor, respectively. The pellet type spherical catalyst of the Pd/Pt-base (1 wt% Pd–Pt/Al2O3) and the Ru-base (1.95 wt% Ru/Al2O3) is used for combustion and the MSR reaction, respectively. Several grooved fins are attached to the inner and outer surfaces of the catalytic combustor to enhance the heat transfer rate. The height and thickness of the fins are approximately 9.5 mm and 2 mm, respectively. To investigate the temperature distributions of the coupled reactor, seven type K thermocouples are installed in the steam reformer and one tube of the catalytic combustors along the gas flow direction. An additional thermocouple is installed to capture the gas temperature at the reactor inlet and outlet. Reforming catalysts was inserted
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Fig. 6. Temperature distributions of the coupled reactor along the gas flow direction and the gas compositions at the steam reformer outlet with the SCR in the steam reformer varying from 2.0 to 3.5 at fuel utilization = 60%, excess air rate = 350%, and inlet temperature of the catalytic combustor = 300 °C.
into the reformer through the three section of catalyst insert entrance for the uniform arrangement of the catalyst. 3.2. Experimental apparatus Fig. 3 shows the schematic diagram of the experimental apparatus used to estimate the coupled reactor performance. The mass flow rate of the reactant gas was controlled by the mass flow controller (Brooks InstrumentÒ 5853S). Pure CH4 (>99.95%) mixed with steam was used as the reactant gas for the steam reformer. In order to simulate the SOFC anode-off gas, H2, CO, CO2, CH4, and steam was uniformly mixed by passing it through the static mixer and supplying it to the catalytic combustor. The electric heater and line heater were used to preheat the inlet gas mixture of the catalytic combustor and the steam reformer to the desired temperature, respectively. The gas compositions at the outlet of the steam reformer and catalytic combustor were measured by gas chromatography (Agilent 7890A). 3.3. Experimental conditions To confirm the scale-up strategy developed in the work, the performance of the 5 kW coupled reactor was experimentally
119
Fig. 7. Temperature distributions of the coupled reactor along the gas flow direction and the gas compositions at the steam reformer outlet with the fuel utilizations varying from 60% to 90% at SCR = 3.0, excess air ratio = 350%, and inlet temperature of the catalytic combustor = 300 °C.
estimated by varying the operating parameters. Tables 3 and 6 presented the references and variations of the operating parameters, respectively. Because the catalyst for catalytic combustion is drastically degraded at a temperature higher than 950 °C, the range of operating conditions were set to values that make the temperature of the catalytic combustor lower than 950 °C. When the excess air ratio and inlet temperature of the reactant gas for the catalytic combustor are lower than 3.3 or higher than 350 °C, respectively, the temperature of the catalytic combustor becomes higher than 950 °C. Thus, the variations of the excess air ratio and the inlet temperature of the catalytic combustor were set to be 330%, 350%, 400%, 450%, and 500% and 150, 200, 250, 300, and 350 °C, respectively. When the SOFC anode-off gas has a fuel rich composition, the maximum temperature in the catalytic combustor can be higher than 950 °C. Thus, the variations of the fuel utilization for the stack were set to 60%, 65%, 70%, 75%, and 80%. When the SOFC system is operated at a high SCR, the system efficiency can be decreased since a higher amount of steam is generated. On the other hand, the performance of the catalyst can be degraded by coke formation on the catalyst surface at a low SCR, suggesting that the SOFC system should be operated within the optimal range of the SCR. Thus, the SCR was set to be 2.5, 3.0, and 3.5.
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Fig. 8. The variations of the remaining heat and the heat transfer rate between the catalytic combustor and the steam reformer for variations of the operating parameters; (a) inlet temperature of the catalytic combustor, (b) excess air ratio, (c) SCR, and (d) fuel utilization.
4. Results and discussions Figs. 4–7 show the gas compositions at the steam reformer outlet and the temperature distributions of the coupled reactor obtained along the gas flow direction by varying the operating parameters, such as the inlet temperature of the catalytic combustor, the excess air ratio of the catalytic combustor, the SCR in the steam reformer, and fuel utilization in the stack, respectively. Increasing the inlet temperature of the catalytic combustor from 150 °C to 350 °C increases the maximum temperature of the catalytic combustor from 781 °C to 941 °C, which increases the heat transfer between the catalytic combustor and the steam reformer. The temperature increase in the steam reformer from 521 °C to 607 °C facilitates the MSR reaction. Eventually, the H2 mole fraction at the steam reformer outlet increases from 69.9% to 74.9%. Even though the excess air ratio increased from 300% to 500%, the reaction rates of the oxidation of H2 and CO did not increase, which decreased the temperatures of the catalytic combustor and steam reformer from 931 °C to 748 °C and from 597 °C to 544 °C, respectively. The decrease of the steam reformer temperature decreased the MSR reaction but increased the WGS reaction. Thus, the species mole fraction of CO decreased from 9.0% to 5.8%. However, because the magnitude of the reaction rate of the WGS was
lower than that of MSR, the H2 mole fraction decreased from 74.8% to 70.5%. As the SCR increased from 2.5 to 3.5, the maximum temperatures of the catalytic combustor and steam reformer decreased from 932 °C to 895 °C and from 618 °C to 572 °C, respectively. When the SCR increased higher than the value that maximized the CH4 conversion rate, the overall temperature of the coupled reactor can be decreased because of the increase of the steam mass, which means an SCR of 2.5 was enough to maximize the conversion rate of CH4 at this temperature. As a result, the species mole fraction of CH4 increased from 7.38% to 8.16%. Since the temperature decrease facilitated the WGS reaction, the species mole fraction of CO decreased from 11.1% to 7.5%. The decrease of the MSR reaction and the increase of the WGS reaction maintained the H2 mole fraction at approximately 74%. As the fuel utilization in the stack increased, the concentration of the H2 and CO in the anode-off gas decreased, which means that the total amount of heat that can be generated in the catalytic combustor decreased. As a result, the maximum temperatures of the catalytic combustor and steam reformer decreased from 916 °C to 764 °C and from 595 °C to 508 °C, respectively. The species mole fraction of the CH4 at the steam reformer outlet drastically increased from 8.1% to 22.6% due to the low temperature of
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Fig. 9. The measured and calculated values of the coupled reactor temperature for variations of the operating parameters; (a) inlet temperature, (b) excess air ratio, (c) SCR, and (d) fuel utilization.
the MSR reaction. Although a low temperature facilitates the WGS reaction, the H2 mole fraction at the steam reformer outlet decreased from 73.8% to 63.3%. Because the coupled reactor is a type of heat exchanger composed of the hot part of the catalytic combustor and the cold part of the steam reformer, obtaining a high heat transfer rate is important. Since the wall is sandwiched between the catalytic combustor and the steam reformer, the heat is transferred from the catalytic combustor to the steam reformer through the wall. Since the temperature distribution along the gas flow direction in both the catalytic combustor and the steam reformer was measured, the heat transfer rate between the catalytic combustor and the wall and the steam reformer and the wall of each local section along the gas flow direction can be obtained as follows:
NC v
X X X dT X _ Q_ tr Q_ reaction ¼ Nin hin N_ out hout dt
ð13Þ
where NC v ; N_ in hin ; N_ out hout ; Q_ tr , and Q_ reaction represent the thermal capacity of the fluid in the reactor, enthalpy flux in and out of the reactor, the heat transfer to the reactor, and the reaction enthalpy in the reactor, respectively. Fig. 8 shows the variations of the remaining heat and the heat transfer rate in the coupled reactor.
The solid and dotted lines of the lower part in graph indicate the heat transfer rate distribution along the gas flow direction between the steam reformer and the wall and the catalytic combustor and the wall, respectively. The distribution of the remaining heat in the catalytic combustor and the wall along the gas flow direction can be obtained by subtracting the heat transfer rate of each local section from the remaining heat of the former local section. The whole catalytic combustion reaction occurs at the inlet of the catalytic combustor. The solid and dotted lines of the upper part in each graph indicate the distribution of the remaining heat in the catalytic combustor and the wall, respectively. Theoretically, the consumed heat in the steam reformer for the SR reaction is supplied from the catalytic combustor. When the measured temperatures of the catalytic combustor and the steam reformer represent the mean temperatures of each reactor, the heat transfer rate calculated from the temperature distribution of the catalytic combustor should be higher than that from the temperature variation of the steam reformer. That is, the heat transfer rate between the catalytic combustor and the wall should be higher than that between the wall and the steam reformer. The heat transfer rate between the catalytic combustor and the wall is almost the same from the first to the fifth local section and then decreases from the sixth local section to the
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S. Kang et al. / Applied Energy 114 (2014) 114–123
Fig. 11. The thermal energy balance of the 5 kW coupled reactor at the base operating condition.
Fig. 10. The measured value and the predicted value of the gas compositions at the steam reformer outlet from the measured temperature and the calculated temperature at the operating parameters; (a) inlet temperature, (b) excess air ratio, and (c) fuel utilization.
outlet of the steam reformer. On the other hand, the minimum and maximum amount of heat transfer between the steam reformer and the wall occur in the second local section and the sixth local section, respectively. At the first, fourth, and fifth local sections, the heat transfer rate between the catalytic combustor and the wall is
almost same as that between the wall and the steam reformer, which means that the heat transfer efficiency at these local sections is high. At the second and third local sections, the heat transfer rate between the steam reformer and the wall is lower than that between the catalytic combustor and the wall, which indicates that some portion of the heat transferred from the catalytic combustor is transferred to the steam reformer. However, at the sixth local section, the heat transfer rate between the steam reformer and the wall is higher than that between the catalytic combustor and the wall. This is because the experimentally measured temperature may not be the mean temperature of each reactor. Specifically since the five catalytic combustors are surrounded in the steam reformer, the temperature in the steam reformer may not be uniform along the radial direction. Furthermore, the location of the thermocouple that detects the temperature of the steam reformer is close to the outside of the reactor, as shown in Fig. 1. When whole amount of the heat transfer rate calculated from the temperature distribution of each reactor is transferred to the other reactor, the theoretical temperature of each reactor can be determined and is shown as a dotted line in Fig. 9. Since the heat transfer efficiency is lower than 100% due to the thermal resistance inside the reactor, the actual temperature of the each reactor cannot be the same as the calculated value. The temperature from the calculation in both reactors is almost the same as the measured temperature at the first and second local sections. However, the calculated temperature of both reactors is different from the measured temperature of the third to sixth local sections because of the thermal resistance of the wall and the catalyst. The composition of the combustion off gas was dependent on the maximum temperature of the catalytic combustor. The temperature at the combustor inlet shown in Fig. 9 means the theoretical value of adiabatic flame temperature of catalytic combustor. In every case, the difference between maximum temperature experimentally captured and adiabatic flame temperature at various operating condition is less than 10 °C. Since the thermocouple is located at the 27 mm away from the combustor inlet, the actual value of the maximum temperature of combustor should be higher than captured value of it. This means the actual maximum temperature was almost same with the adiabatic flame temperature, which means most of the H2 and CO was oxidized. Consequently, most of the combustion off gas can be predicted as H2O and CO2. Because the reforming reaction kinetics highly depends on the temperature of the steam reformer, the gas compositions at the steam reformer outlet can be predicted by the maximum
S. Kang et al. / Applied Energy 114 (2014) 114–123 Table 7 The thermodynamic properties of combustion catalyst, reforming catalyst, and reactor wall.
Combustion catalyst Reforming catalyst Reactor wall
Volume (cc)
Thermal conductivity (W/m K)
Specific heat capacity (J/g °C)
864
30.391
0.020
1933
31.677
0.022
71.3
16.2
0.5
temperature of the steam reformer. Fig. 10 compares the measured and predicted values of the gas compositions at the steam reformer outlet from the maximum temperature of the steam reformer. The solid line is the theoretical value of the gas compositions at the steam reformer outlet for the temperature variations. Several points on the graph mean the measured value of the gas compositions at the steam reformer outlet. The measured temperature detected by the thermocouple installed at the steam reformer outlet may not be the highest temperature in the reactor, and this discrepancy can cause the difference between the points on the graph and the solid line. To investigate the correlation between the measured value of the gas compositions at the steam reformer outlet and the predicted value calculated from the predicted maximum temperature of the steam reformer, the predicted maximum temperature of the steam reformer is calculated by considering the adiabatic flame temperature of the catalytic combustor. The dotted line shows the predicted value of the gas compositions with temperature variations by considering the difference of the maximum temperature between the predicted value and the measured value. In every case, measured value of CH4 is a little bit higher than the theoretical value, which can be resulted from the low MSR reaction. However, high measured value of CO and H2 means the high MSR reaction. Low measured value of CO2 can be caused by low WGS reaction. In WGS reaction, the mole number of consumed CO and produced CO2 is the same. However, the difference between measured value and theoretical value of CO2 is higher than that of CO. This means that some error of the data acquisition can be another reason which causes the discrepancy between measured value and theoretical value of CO2. Although slight error, the several points are relatively in good agreement with the dotted line, which means that the predicted maximum temperature of the steam reformer from the calculation represents the actual maximum temperature. Fig. 11 presented the thermal energy balance of the 5 kW coupled reactor at the base operating condition. The total amount of heat generated in the coupled reactor was about 6.5 kW. The 47.12% and 28.89% of the generated heat was transferred to the steam reformer and was utilized for the steam reforming reaction, respectively. The 49.94% and 18.23% of the total thermal energy used to increase the sensible heat of the gas for the catalytic combustor and steam reformer, respectively. The thermal energy consumed by the combustor catalyst, reformer catalyst, and combustor wall was 0.09%, 0.06% and 2.79%, respectively, which is calculated by considering their thermodynamic properties shown in Table 7.
5. Conclusions A coupled reactor with a catalytic combustor and a steam reformer for a 5 kW SOFC system has been developed. The geometric parameters, such as the space velocity, diameter and length of the coupled reactor, selected from the 1 kW coupled reactor in a previous study are utilized for the design of the 5 kW coupled reactor. The configuration of the 5 kW coupled reactor was opti-
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mized by considering the heat transfer rate with the variations of the tube number of catalytic combustor. The performance of the 5 kW coupled reactor is experimentally investigated by varying the operating parameters, including the fuel utilization in the SOFC stack, the inlet temperature of the catalytic combustor, the excess air ratio of the catalytic combustor, and the SCR in the steam reformer. The distributions of the heat transfer in the coupled reactor were obtained from the temperature distributions experimentally measured along the gas flow direction. With the thermodynamic properties of the catalyst and reactors, the thermal energy balance of the 5 kW coupled reactor was investigated at the reference operating condition. The gas composition at the reformer outlet experimentally captured was compared with the predicted value from the maximum temperature of the steam reformer. The maximum value of the CH4 conversion rate is approximately 93.4% at the inlet temperature of catalytic combustor of 350 °C, excess air ratio of 350%, SCR of 3.0, and fuel utilization of 60%, which means that the proposed scale-up strategy can be utilized to develop a largescale coupled reactor with a catalytic combustor and a steam reformer. Deep analysis on the thermochemical characteristics of the coupled reactor and the scale-up strategy of the coupled reactor with catalytic combustor and steam reformer developed in the work can provide the basic insight to establish the optimal design of the large scale coupled reactor. References [1] Zhang X, Chan SH, Li G, Ho HK, Li J, Feng Z. A review of integration strategies for solid oxide fuel cells. J Power Sources 2010;195:685–702. [2] Eguchi K, Kojo H, Takeguchi T, Sasaki K. Fuel flexibility in power generation by solid oxide fuel cells. Solid State Ionics 2002;152:411–6. [3] Park SK, Kim TS, Sohn JL, Lee YD. An integrated power generation system combining solid oxide fuel cell and oxy-fuel combustion for high performance and CO2 capture. Appl Energy 2011;88:1187–96. [4] Wee JH. Molten carbonate fuel cell and gas turbine hybrid system as distributed energy resources. Appl Energy 2011;88:4252–63. [5] Laminie J, Dicks A. Fuel cell systems explained. 2nd ed. John Wiley & Sons Ltd; 2003. p. 241–50. [6] Georgis D, Jogwar SS, Almansoori AS, Daoutidis P. Design and control of energy integrated SOFC systems for in situ hydrogen production and power generation. Comput Chem Eng 2011;35:1691–704. [7] Mueller F, Jabbari F, Gaynor R, Brouwer J. Novel solid oxide fuel cell system controller for rapid load following. J Power Source 2007;172:308–23. [8] Braun RJ, Klein SA, Reindl DT. Evaluation of system configurations for solid oxide fuel cell-based micro-combined heat and power generators in residential applications. J Power Source 2006;158:1290–305. [9] Yen TH, Hong WT, Huang WP, Tsai YC, Wang HY, Huang CN, et al. Experimental investigation of 1 kW solid oxide fuel cell system with a natural gas reformer and an exhaust gas burner. J Power Source 2010;195:1454–62. [10] Lai WH, Hsiao CA, Lee CH, Chyou YP, Tsai YC. Experimental simulation on the integration of solid oxide fuel cell and micro-turbine generation system. J Power Source 2007;171:130–9. [11] Rahimpour MR, Dehnavi MR, Allahgholipour F, Iranshahi D, Jokar SM. Assessment and comparison of different catalytic coupling exothermic and endothermic reactions: a review. Appl Energy 2012;99:496–512. [12] Patel KS, Sunol AK. Modeling and simulation of methane steam reforming in a thermally coupled membrane reactor. Int J Hydrogen Energy 2007;32:2344–58. [13] Ramaswamy RC, Ramachandran PA, Dudukovic MP. Coupling exothermic and endothermic reactions in adiabatic reactors. Chem Eng Sci 2008;63:1654–67. [14] Kolios G, Glockler B, Gritsch A, Morillo A, Eigenberger G. Heat-integrated reactor concepts for hydrogen production by methane steam reforming. Fuel Cells 2005;5:52–65. [15] Ghang TG, Lee SM, Ahn KY, Kim Y. An experimental study on the reaction characteristics of a coupled reactor with a catalytic combustor and a steam reformer for SOFC system. Int J Hydrogen Energy 2012;37:3234–41. [16] Hayes RE, Kolacczkowski ST. Introduction to catalytic combustion. Amsterdam: Gordon and Breach Science Publishers; 1997. p. 1– 95. [17] Incropera FP, Dewitt DP. Fundamentals of heat and mass transfer. 5th ed. John Wiley & Sons; 2002. [18] Mills AF. Basic heat and mass transfer. 2nd ed. Pearosn & Prentice hall; 2003. [19] Yu SS, Hong DJ, Lee YD, Lee SM, Ahn KY. Development of a catalytic combustor for a stationary fuel cell power generation system. Renew Energy 2010;35:1083–90. [20] Lee SM, Ahn KY, Lee YD, Han JY, Im SY, Yu SS. Flow uniformity of catalytic burner for off-gas combustion of molten carbonate fuel cell. Trans ASME 2012;9:021006-1–021006-12.