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Investigation of an Integrated Autothermal Reforming (ATR) and SOFC Micro Cogeneration System for Power Generation Atilla ERSOZ a, Alper SARIOGLAN a Sibel OZDOGAN b a
TUBITAK Marmara Research Center Institute of Energy, 41470, Gebze KOCAELI, TURKIYE Tel. +90.262.6412300, Fax. +90.262.6412309
[email protected] b
Marmara University, Faculty of Engineering, Department of Chemical Engineering Goztepe – Istanbul
[email protected]
ABSTRACT: In today's era of cogeneration (and trigeneration) power and energy plants with higher efficiencies and lower costs and emissions, in the "micro" sizes are becoming more prevalent. We define the term "microcogeneration" as the sizes under 1.0 MW. Cogeneration technologies are conventional power generation systems making use of the energy remaining in exhaust gases, cooling systems, or other energy waste stream. Typical cogeneration prime movers include, combustion turbines, reciprocating engines, boilers with steam turbines, micro turbines and fuel cells (SOFC, MCFC and PEM). There are several methods of producing hydrogen from fossil resources such as natural gas or naphtha, for example steam reforming, partial oxidation and auto-thermal reforming. KEYWORDS: Fuel cells, hydrogen, micro cogeneration, reforming
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WHEC 16 / 13-16 June 2006 – Lyon France Introduction Fuel cell systems are being developed for supplying primary power (heat and electricity) for residential applications of the near future. Residential power generation systems can be operated to provide primary or backup power for the home. They can run independently or in parallel to an existing power grid. There are several companies currently working on residential fuel cells. Several demonstration units are being tested by the fuel cell companies around the countries in cooperation with local governments and/or utilities. The pre-commercial units are expected to enter into the market in the 2005-2010 timeframe. Many fuel cell manufacturers are considering the use of natural gas as a hydrogen-carrier fuel for rural and remote residential fuel cell applications. Fuel cells combine hydrogen and oxygen without combustion to produce electricity. Water and heat are the only byproducts of this reaction. The process combines oxygen from the air and hydrogen extracted from any one of a number of suitable hydrogen containing fuels. The result is DC electrical power produced with greater efficiency than most of the other conventional power generation methods, such as internal combustion engine generators. The net electric efficiency of low temperature fuel cell systems is approximately 35 to 45 %. Fuel cells generate electrical power from hydrogen or a hydrogen rich gas and air via electrochemical reactions [1–6]. There is continuous interest in converting current hydrocarbon fuels such as natural gas, propane, gasoline and diesel into hydrogen rich gases acceptable by PEM and SOFC fuel cells [2-10]. Commercial application of fuel cell technologies will require selection of appropriate operating conditions. By varying operating conditions, a wide range of power and efficiency may be derived from an SOFC system. The operating parameters may be selected based on requirements for power, efficiency, or a function of both variables such as cost of electricity [11]. Fuel reforming systems can extract hydrogen from a variety of conventional fuel sources for the residential micro cogeneration application with fuel cell. Existing infrastructures such as natural gas pipelines and propane distribution systems can be used for these systems. Hydrogen rich gases can be produced via various fuel processing technologies. Steam reforming, partial oxidation and autothermal reforming (ATR) are the three major fuel-processing technologies [5-8].
Methodology The use of chemical flow sheeting software has become an integral part of the evaluation of the performance of fuel cell systems (Petterson, 2001, Kivisaari et all., 2001). In this study, the Aspen-HYSYS 3.2 simulation software has been used to evaluate several fuel-reforming technologies for residential fuel cell micro cogeneration systems. This paper presents the results of a study for a 5 kWe DC electrical power residential SOFC fuel cell system utilizing several reforming technologies fed by natural gas fuel. The process simulation code “Aspen-HYSYS 3.2” has been used for residential fuel cell system calculations. Natural gas has been simulated as three different sources for hydrogen production. The chemical compositions of the natural gas fuel are summarized in Table 1. The average molecular weight of natural gas is around 16.6 kg/kmol. All simulation studies are performed based on this composition. Table 1. Natural gas composition Component
Mass Fractions (w %)
Molar fractions (mol % )
Methane
0.9693
0.9375
Ethane
0.0227
0.0412
Propane
0.0080
0.0212
100
100
Total
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The simulated residential SOFC fuel cell system consists of the following sections and their components:
Fuel processing section, Fuel cell section, Auxiliary units,
The overall residential fuel cell micro cogeneration system can be seen in Figure 1. The fuel processing is one of the sections of this whole system.
Figure 1. Schematic diagram of a residential fuel cell micro cogeneration system The aim of this study is to convert as much as the hydrogen in the fuel into hydrogen gas while decreasing CO and CH4 formation. Process parameters of fuel preparation steps have been determined considering the limitations set by the catalysts and hydrocarbons involved. Lower S/C ratios favor soot and coke formation, which is not desired in catalytic steam and auto thermal reforming processes. A considerably wide S/C ratio range has been selected to see the effect on hydrogen yield and CO formation. The mole fractions of air have been simulated as 0.21 and 0.79 for O2 and N2 respectively. The two parameters, S/C (steam to carbon), O/C (oxygen to carbon) ratios have been used to analyze the reforming reactors effectively. These two relationships can be written as follows: O/C = 0.5 * (F O2 / F CH4) S/C = F H2O (v) / F CH4 The catalytic properties limit applicable operational parameter ranges such as S/C, O/C and operation temperatures and pressures. The durability of the reformer is governed by the thermal durability of the reforming catalysts by coke formation. In reality, a certain catalyst for a reforming system might be able to be used at higher temperatures, but most commercially available catalysts have been operated at less than 800 °C to secure their thermal durability. Therefore, it is necessary to determine favorable operating conditions for each of the reforming reactors [13]. The thermodynamic equilibrium system calculations are based on minimizing the Gibbs free energy. The equilibrium temperature and outlet compositions of each reactor have been calculated with simulation studies. For all cases, reactor simulation calculations have been performed under adiabatic conditions keeping “Treactor” almost constant taking reaction heats into account. Within this frame selected S/C and/or O/C ratio(s) are studied parametrically to achieve finally acceptable hydrogen production yield along with low CO formation. The ranges of operating conditions investigated in the simulation studies are given in Table 2.
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Table 2. The investigated ranges of ATR reactor operating conditions Reforming reactor ATR
Temperature (°C) 400-800
Pressure (bar) 3
S/C
O/C
0.3-3
0.35-0.6
SOFC section of the overall system has been simulated in the study for all reforming process options. Fuel cell system and the auxiliary units consist of the following components: ¾ SOFC section SOFC stack DC/AC converter Table 3 summarizes the data of different auxiliary system components utilized in the simulation studies. Table 3. Auxiliary system component data Component Water pump Cooling water pump Compressor Heat exchangers Combustor Chimney DC/AC Converter
Parameter Adiabatic efficiency (%) Adiabatic efficiency (%) Adiabatic efficiency (%) Minimum temperature approach (°C) Outlet temperature (°C) Outlet temperature (°C) Conversion efficiency (%)
Value 75 75 75 25 720 90 98
Fuel processing (ηFP), SOFC (ηFC) and overall system efficiencies (ηnet.el) are calculated as follows: ηFP = η6 ηFC = (UH2) x (ηstack voltage ) x (ηDC/AC) ηnet.el = η8 = η FP x ηFC x ηAux. η : Efficiency
Results and Discussion The efficiencies of three different fuel processing options and overall system efficiencies of natural gas for the investigated fuel-reforming options are presented in Table 4. The simulation results (Table 4) indicate that the fuel processing efficiencies decrease in the order of steam reforming > autothermal reforming > partial oxidation for both gasoline and diesel fuels. The promising and efficient reforming options are the steam reforming and autothermal reforming processes, as can be seen in Table 4. We can compare these two efficient systems in order to observe the equilibrium behavior in the reforming section of the whole micro CHP system. Here, the results of the most efficient options, namely natural gas with steam reforming and autothermal reforming. Table 4. Fuel processing and overall system efficiencies of natural gas fuel Parameter P, bar T, °C ηFP ηnet.el
SREF 3 750 98 41
Natural gas ATR POX 3 3 750 750 93 76 38 29
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WHEC 16 / 13-16 June 2006 – Lyon France The major units of the Aspen-HYSYS simulation for natural gas steam reforming based fuel cell system are presented in Figure 2.
Figure 2. Schematic of Aspen-HYSYS simulation model Autothermal reforming reactor (ATR) is maintained under adiabatic conditions. There is no heat transfer from or to the reactor section during the reaction. The effect of S/C and O/C ratios on the net electric efficiency of the system with fuel cell has been calculated. The results are illustrated for different inlet temperatures (700 ° and 400 °C) in Figure 3 and 4. A decrease of the S/C ratio decreases the efficiency. On the other hand, an increase of the O/C ratio increases the efficiency in general. The operating parameters of the ATR reactor are selected as the inlet temperature of 800°C, S/C=1.5 and O/C=0.45 within the optimum efficiency range.
Net electric efficiency (%)
45
40
O/C = 0.35 O/C = 0.40
35
O/C = 0.45 O/C = 0.50 O/C = 0.55 O/C = 0.60
30 0.3
0.5
0.8
1.0
1.3
1.5
1.8
2.0
2.3
2.5
2.8
3.0
S/C
Figure 3. S/C effect on the net electric efficiency with different O/C (T ATR inlet = 700 °C) 5/9
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Net electric efficiency (%)
40
35
30
O/C=0.35 O/C=0.40 O/C=0.45 O/C=0.50
25
O/C=0.55 O/C=0.60
20 0.0
0.5
1.0
1.5
2.0
2.5
3.0
S/C
Figure 4. S/C effect on the net electric efficiency with different O/C (T ATR inlet = 400 °C) The evaluation of the effect for different S/C and O/C ratios on the temperature difference between the reactor inlet and the outlet has been shown in Figure 5 and 6. The O/C ratio significantly effects the delta T as seen in related figures. The S/C ratio also affects the temperature difference. A decrease of the S/C increases the difference between inlet and outlet reactor temperatures. Higher S/C means lower delta T. The most promising inlet reactor temperature is selected as 700 °C according to the catalysts thermal durability limitations. The value of the delta T is between 50 °C and 100 °C with selected O/C=0.45. The optimum operating value is calculated as around 60 °C (outlet temperature is around 760 °C) for S/C=1.5 and O/C=0.45. 400 350 300
Delta T, °C
250 200 150
O/C = 0.35 O/C = 0.40
optimum
100
O/C = 0.45 O/C = 0.50
50
O/C = 0.55 0
O/C = 0.60 0.3
0.5
0.8
1.0
1.3
1.5
1.8
2.0
2.3
2.5
2.8
3.0
-50 S/C
Figure 5. S/C effect on Delta T (TATR out – T ATR in) with different O/C (T ATR inlet = 700 °C )
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450
400
Delta T, °C
350 O/C = 0.35
300
O/C = 0.40 O/C = 0.45
250
O/C = 0.50 O/C = 0.55
200
O/C = 0.60
150 0.3
0.5
0.8
1.0
1.3
1.5
1.8
2.0
2.3
2.5
2.8
3.0
S/C
Figure 6. S/C effect on Delta T (TATR out – T ATR in) with different O/C (T ATR inlet = 400 °C )
45
0.16
40
0.14
35
0.12
30
0.1
25
0.08
20
0.06
15
0.04
10
0.02
Flow, kmol/h.
Net electric efficiency, %
The outlet concentrations and the net electrical efficiency of the system have been similarly observed for the different reactor inlet temperatures for the selected operating parameters (S/C=1.5 and O/C=0.45) (Figure 7 and 8). The net electric efficiency value is around 41 % for the selected inlet temperature (700 °C). The outlet molar flow of hydrogen and carbon monoxide are 0.12 and 0.034 kmol/h. respectively. The mole fraction of hydrogen decreases after O/C=0.45 as seen in Figure 12. The mole fraction of carbon monoxide increases with increasing O/C ratios at selected S/C ratio of 1.5. Methane, carbon dioxide and water concentrations are also decreasing with increasing O/C ratios.
O/C= 0.45, S/C=1.5 ATR outlet H2,kmol/h.
5 300
400
500
600
700
ATR outlet CO,kmol/h.
0 800
ATR inlet temperature,°C
Figure 7. ATR inlet temperature effect on the net electric efficiency with selected O/C and S/C
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0.50
Mole fraction (%)
0.40
0.30
CH4 CO
0.20
CO2 H2
0.10
H2O
0.00 0.150
0.250
0.350
0.450
O/C
Figure 8. O/C vs Comp. mol fractions at S/C=1.5 (T ATR inlet = 700 °C)
0.40
Mole fraction (%)
0.30
0.20
CH4 CO CO2
0.10
H2 H2O
0.00 300
400
500
600
700
800
ATR İnlet Temperature (°C)
Figure 9. Effect of ATR inlet temp on ATR outlet compositions at S/C=1.5 O/C=0.45 The effect of ATR inlet temperature on the outlet reactor compositions for the selected operating conditions (S/C=1.5 O/C=0.45) is given in Figure 9. The maximum hydrogen yield can be observed with the selected inlet reactor temperature of 700 °C. The value is hydrogen mole fraction is 0.39 for this operating condition.
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Concluding remarks The thermodynamic characteristics of the several reforming options namely, steam reforming, autothermal reforming and partial oxidation have been investigated. All these reforming process options have been investigated with appropriate simulation conditions for 5 kW fuel cell power. The most promising operating conditions has been selected for each process. The material and energy balance calculations have also been performed by using ASPEN-HYSYS simulation code. Natural gas appears as the best fuel for hydrogen rich gas production due to its favorable composition from lower molecular weight compounds. Steam reforming and autothermal reforming appear as the most competitive fuel processing options in terms of fuel processing efficiencies. POX shows the lowest fuel processing efficiency level. Among the options studied the highest fuel processing efficiency is achieved with natural gas steam reforming at about 98%. Several operating conditions have been found which satisfy the requirements for no coke formation. The optimum S/C ratio at 3.5 appears to fulfill the requirements for temperatures around 800 °C for steam reforming process. The optimum O/C and S/C ratios are found 0.45 and 1.5 respectively for ATR reactor simulations at the inlet temperature of 700 °C. High system efficiency levels can be achieved only with intensive heat integration within the fuel cell micro CHP systems. Hence, heat integration system studies are of utmost importance along with the development of novel reforming catalysts, SOFC fuel cell components if on-site hydrogen production is desired for micro CHP applications. References 1. Ersoz, A.; Olgun, H.; Ozdogan, S.; Gungor, C.; Akgun, F.; Tiris M. Autothermal Reforming As A Hydrocarbon Fuel Processing Option For PEM Fuel Cell. Journal of Power Sources, 118, (2003), (1-2), 384392. 2. Anonymous. Fuel Cell Handbook (Fifth Edition), By EG&G Services Parsons, Inc. Science Applications International Corporation, U.S. Department of Energy. Office of Fossil Energy National Energy Technology Laboratory. October (2000). 3. Danial Doss, E.; Kumar, R.; Ahluwalia, R. K.; Krumpelt, M. Fuel processor for automotive fuel cell system: a parametric analysis. Journal of Power Sources 102, (2001), 1-15. 4. Dong Ju Moon; Sreekumar K.; Lee, S. D.; Gwon, B.; Kim, H. S. Studies on gasoline fuel processor for fuel cell powered vehicle application. Applied Catalyst, 215, (2001), 1-9. 5. Pereira, C.; Wilkenhoener R.; Ahmed, S.; Krumpelt, M. Liquid Fuel Reformer Development. Electrochemical Technology Program Chemical Technology Division, Argonne National Laboratory, Illinois, USA, July 2, (1999). 6. Docter, A.; Lamm, A. , Gasoline fuel cell systems. Journal of Power Sources, 84, (1999), 194- 200. 7. Megede D.zur. Fuel processors for fuel cell vehicles. Journal of Power Sources, 106,(2002), 35-41. 8. Pereira, C.; Bae, J-M.; Ahmed, S.; Krumpelt, M. Liquid Fuel Reformer Development: Autothermal Reforming of Diesel Fuel. Electrochemical Technology Program Chemical Technology Division Argonne National Laboratory U.S. Department of Energy, (2000) Hydrogen Program Technical Review San Ramon, California. 9. Bernaya, C.; Marchanda, M.; Cassir, M. Prospects of different fuel cell technologies for vehicle applications. Journal of Power Sources, 108, (2002), 139–152. 10. Pettersson, L. J.; Westerholm, R. State of the art of multi-fuel reformers for fuel cell vehicles: problem identification and research needs. International Journal of Hydrogen Energy, 26, (2001), 243-264. 11. Development of an SOFC Stack Performance Map for Natural Gas Operation J. Hartvigsen, A. Khandkar, S. Elangovan, SOFCo, Salt Lake City, Utah Presented at the Sixth International Symposium on Solid Oxide Fuel Cells MTI October 17 – 22, 1999, Honolulu, Hawaii. 12. Kivisaari, T. Van der Laag P.C. Ramsköld A. Benchmarking of chemical flow sheeting software in fuel cell application, J. of Power Sources, 94, (2001), 112-121. 13. Seo Y.S., Shirley A., Kolaczkowski S.T., Evaluation of thermodynamically favourable operating conditions for production of hydrogen in three different reforming technologies, Journal of Power Sources, 108, (2002), 213-225.
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