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Research Papers

The Efficiencies of Internal Reforming Molten Carbonate Fuel Cell Fueled by Natural Gas and Synthetic Natural Gas From Coal OPEN ACCESS

[+] Author and Article Information
Hai-Kyung Seo

Mem. ASME
Clean Power Generation Laboratory,
KEPCO Research Institute,
105, Munji-Ro, Yuseong-Gu,
Daejeon 34056, South Korea
e-mail: seohk@kepco.co.kr

Won-shik Park

Mem. ASME
Clean Power Generation Laboratory,
KEPCO Research Institute,
105, Munji-Ro, Yuseong-Gu,
Daejeon 34056, South Korea
e-mail: wspark@kepco.co.kr

Hee Chun Lim

Mem. ASME
Clean Power Generation Laboratory,
KEPCO Research Institute,
105, Munji-Ro, Yuseong-Gu,
Daejeon 34056, South Korea
e-mail: hclim123@nate.com

1Corresponding author.

Manuscript received January 3, 2016; final manuscript received March 25, 2016; published online April 26, 2016. Assoc. Editor: Kevin Huang.

J. Electrochem. En. Conv. Stor. 13(1), 011005 (Apr 26, 2016) (10 pages) Paper No: JEECS-16-1002; doi: 10.1115/1.4033255 History: Received January 03, 2016; Revised March 25, 2016

When synthetic natural gas (SNG) is produced from coal and used as a fuel in the internal reforming molten carbonate fuel cell (ir-MCFC), electric efficiency can be no greater than 31%. This is because there are several exothermic reactions in the processes of converting coal to SNG, so that a maximum 64% of coal's energy is converted into SNG energy. This results in a lower efficiency than when the ir-MCFC with the electric efficiency of 48% is fueled by natural gas (NG). To increase electric efficiency with SNG, it is necessary to recover the exothermic heat generated from the processes of converting coal to SNG as steam, which can then be used in a steam turbine. When steam produced in the gasification, water gas shift (WGS), and methanation processes is used in a steam turbine, the gross electric efficiency will become 41%. If the steam and auxiliary power for CO2 capture process is consumed more, the net efficiency will be 27%. Use of additional steam from the exhausted gas of fuel cell can increase the total net efficiency to 49%.

FIGURES IN THIS ARTICLE
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Fuel cells have more advantages than conventional power plants: high efficiency even in small size, environmental friendly impact, easy to site, quieter operation. These attracted points lead fuel cells to distributed power generation and many countries to fund the development of fuel cell-based power generation systems [1]. Among types of fuel cells, molten carbonate fuel cell (MCFC) is one of the most advanced candidates as a large scale power plant. Also, the exhaust heat it produces can be used for cogeneration applications such as high-pressure steam, district heating, and air conditioning. The direct fuel cell developed by fuel cell energy (FCE) in the U.S. is one of the MCFC systems which can readily and directly use fuels such as NG and waste water treatment gas. POSCO [2] of South Korea, one of the top steel companies in the world and already a strategic partner of FCE, has been producing MCFC systems since 2007. The largest capacity MCFC system, at 58.8 MW, has been in operation in Hwaseong of South Korea since January 2014.

In most ir-MCFCs, NG is usually used as a fuel. NG is a relatively clean fuel which affects the environment less than other carbon fuels such as coal, and its utilization is gradually increasing. However, not many countries in the world have underground deposits of NG, or NG fields. So, there has been great demand for technologies that can produce SNG, especially from coal, and this has been the focus of research by countries seeking a substitute for NG.

As coal is converted to SNG, a heat or energy loss occurs in the process, and thus the total thermal output of the produced SNG is smaller than that of the original coal, even though the quality of SNG is nearly similar to NG.

In this paper, SNG from coal is used as fuel in the ir-MCFC, and its calculated efficiency is compared with NG fuel. When NG was used in the ir-MCFC, gross electric efficiency reached 48.73%. On the other hand, when the coal was used in the ir-MCFC via gasification, WGS and methanation processes, the efficiency was reduced to 29.13% due to energy losses to the atmosphere, such as exothermic conversion.

In the gasification process, about 80% of coal thermal input is converted into the synthesis gas [3,4]. Then, through the WGS and methanation processes, the synthesis gas is altered to SNG, which also retains around 80% of the thermal energy of the synthesis gas, because 61–63% of conversion efficiencies are shown in systems converting to SNG from coal [5]. Ultimately the electric efficiency of an ir-MCFC fueled by SNG from coal cannot exceed merely 31%. Display Formula

(1)electric efficiency=thermal input of coal×0.8×0.8×0.4873 thermal input of coal=31%max

In making the comparison between SNG and NG, this study also considered the increase in efficiency of gasification process that could be obtained by recovering heat produced from the exothermic conversion as steam which is used in a steam turbine to produce additional electricity.

Some papers show the systems integrated with MCFC. Among others, in the paper of Carapellucci et al. [6], the integrated system with natural gas combined cycle (NGCC) power plant and MCFC was considered. Because fossil-fuels produce CO2, even though NG fuel is converted to less CO2 than coal fuel, they said, CO2 emission would be reduced when NGCC exhaust gas was used at the cathode side of MCFC. But in the above paper, coal gasification was not treated. In the paper of Morita et al. [7], there was the comparison of the system with biomass-gasification/gas turbine and the one with biomass-gasification/MCFC which showed that when MCFC cathode exhaust gas was used at expander, the additional electricity was produced. But the heat recovery from gasification block and CO2 capture were not considered. In the paper of Toonssen et al. [8], the systems integrated with biomass gasification and solid oxide fuel cell/gas turbine (SOFC/GT) hybrid systems were studied through using cycle-tempo program [9]. In the systems, the gases synthesized from biomass gasification were used in anode inlet of SOFC after gas cleaning and then the anode off gas and cathode off gas were used in the gas turbine. As a result of some configurations, the electric efficiencies were compared. The synthesis gases from the gasification were directly used in the anode inlet of SOFC without converting into SNG from synthesis gas. Then the electric efficiencies produced from just gas stream were estimated as 46–49.9%. These efficiencies were higher than our results, because the exothermic energy from converting synthesis gas to SNG was not lost and the additional energy for CO2 capture was not also considered.

In our paper, the electric efficiency of system integrated with a gasification process converting coal to SNG and an MCFC was studied as a new trial. This study was aimed that the feasibility of this integrated system would be comparably verified in comparison with the electric efficiency of conventional thermal power plant.

In this paper, the cycle-tempo program was used for the calculation of heat and mass balances. A fuel cell and steam turbine can be easily combined and simulated by cycle-tempo, which can calculate the stack performance which shows the variation of the output voltage according to the operation conditions such as electrical load, input gas rates, and temperature in several types of fuel cells such as MCFC stack integrated with indirect internal reforming, MCFC stack with direct internal reforming, MCFC stack integrated with external reformer, and other fuel cells [711] as well as thermodynamic evaluation of energy conversion system [11]. And there are some papers using cycle-tempo program on the study of stack reaction of ir-MCFC fuel cell [6].

Figure 1 shows the simulation of the ir-MCFC system fueled by NG. In Fig. 1, apparatus number 1 is the stack of the ir-MCFC having indirect internal reforming and “A” indicates the anode and “C” the cathode. For the basic model of chemical reaction and voltage variation according to the reactions, the papers of Morita et al. [7,10] could be referred. Through the cycle-tempo program, the NG (apparatus number 7) and steam (pipe number 14) are heated by the heat from cathode-off gas, respectively, and then mixed. After that, the mixed gas is heated to 580 °C by the heat from catalytic combustor and then introduced into the anode inlet. Air is first heated by the cathode off-gas and then by the catalytic combustor (apparatus number 9) and is then supplied into the cathode inlet at a temperature of 580 °C. Water is supplied through apparatus number 6, heated by the heat of the cathode off-gas, and then split into the streams of pipe numbers 14 and 15. Stream number 14 is ultimately supplied into the anode inlet at the ratio of steam to NG of 2.5. Even though the fuel cell reaction proceeds and steam is being produced, steam needs to be supplied abundantly. This is because carbon deposition can occur when CH4 rich gas is supplied in high temperature state in the absence of steam [12] into the inlet area of the stack of the ir-MCFC. NG of 1.621 kg/s with the lower heating value of 708.3 kJ/mol (LHV value of the pipe number 11 of Table 1) and the average mole mass of 18.64 kg/kmol, which makes the total thermal input of NG 61,596.66 kW, is supplied through apparatus number 7 into the anode inlet. The pressure, temperature, enthalpy, and the amount of each stream in the cross legend is shown at the tip of each pipe in Fig. 1.

Under these conditions, the fuel cell produced 30,014.65 kW of electricity, which corresponds to 0.4873 of thermal input of NG, at gross electric efficiency of 48.73%. The air blower of the unit consumed 328.99 kW of electricity as auxiliary power. Consequently, the net electric efficiency was 48.19%. Display Formula

(2)electric efficiencynet=(30,014.65328.99)kW61,596.66 kW=48.19%

Pipe number 15 carries additional steam which can be used to recover excess heat from the fuel cell system. The additional steam is produced at 255 bar and 452.64 °C with the thermal output of 59,889.96 kW. When this additional steam is used in a steam turbine having an electric efficiency of 28.71%, the thermal energy of steam, 59,889.96 kWth can be converted into 17,194.41 kWe of electricity. Then the total efficiency can be presented as follows: Display Formula

(3)electric efficiencynet=(30,014.65+17,194.41328.99)kW61,596.66 kW=76.11% 

The Stack of ir-MCFC.

In this paper, the stack (apparatus number 1) is simulated as an indirect internal reforming type MCFC [13]. The voltage of each cell is 0.747 V and the total area of the cells stacked is 26,915.15 m2. The utilization ratio of the fuel is 0.7, that of O2 is 0.4192 and that of CO2 is 0.6989 as shown in Fig. 1. At the stack, the following reactions occur: Display Formula

(4)anodeside:CH4+2H2OCO2+4H2
Display Formula
(5)4H2+CO2+4CO324H2O+5CO2+8e
Display Formula
(6)cathodeside:4CO2+2O2+8e4CO32

The reforming reaction (4) results from the following reactions: Display Formula

(7)anodeside:CH4+H2OCO+3H2,ΔH°=205.81kJ/mol
Display Formula
(8)CO+H2OCO2+H2,ΔH°=41.17kJ/mol

Since reaction (7) is a severely endothermic reaction and reaction (8) is a weak exothermic one, then the total reaction (4) is a slightly strong endothermic one with an enthalpy change of 164.64 kJ/mol.

The total reaction for the anode and cathode sides is as follows: Display Formula

(9)H2+1/2O2H2O(g),ΔH°=241.82kJ/mol

The summed reaction of (5) and (6) above is four times that of reaction (9). Then, the water formation enthalpy is 967.28 kJ/mol4H2. Around 60% among this energy is consumed for conversion into electricity theoretically [14]. And then the residual energy is exhausted into heat. With internal reforming, the reaction heat required by the endothermic reforming reaction is supplied by this residual exhausted heat. In this case, the net heat produced by the stack is 2.5–3 times lower than that of external reforming, thus strongly reducing the cooling requirements for the stack [15]. Even though the heat from the stack is lower than that of external reforming stack, the excess heat should be removed to avoid a problem related with the system's materials, which results in weak durability at high temperature.

The exothermic heat is removed by the excess air. This is the reason the utilization ratio of O2 is so low, at 0.4192. This value means that just 41.92% among the introduced oxygen is used in the reaction of fuel cell. The rest of oxygen and nitrogen are used in cooling the stack of fuel cell. Because the mole fraction of O2 among the air is around 0.21, if the oxygen is used at just 41.92%, the used oxygen among the air is 8.8% and the rest of gas remain at the mole percentage of 91.2%, which is used just in cooling without participating in the electrochemical reaction. Then the outlet temperature of the stack can be reduced to 669.45 °C.

The Catalytic Combustor.

Since the utilization ratio of fuel is 0.7, remaining fuel, that is, 0.3 is exhausted into the outlet of anode. This residual fuel can be used to increase overall efficiency by combustion in the catalytic combustor along with the excess air.

The compositions of the anode and cathode outlets shown in the pipe numbers 5 and 6 of Fig. 1 appear in Table 1. The composition of the anode outlet is shown as CO2 42.17%, H2O 46.18%, CH4 1.32%, CO 2.33%, H2 6.13%, and N2 1.86%. In this stream, CH4, CO, and H2 are still fuel and these gases can be combusted in the catalytic combustor. The levels of CO2 and H2O gases in the anode outlet are rich, as shown in reaction (5). Because the CO2 gas is necessary in the cathode side, as shown in reaction (6), this anode outlet gas can be recycled to the cathode inlet side via the catalytic combustor.

The composition of the cathode inlet, shown as pipe number 19 in Table 1, is as follows: N2 54.25%, O2 12.21%, H2O 18.25%, Ar 0.64%, and CO2 14.65%. The composition of the cathode outlet is N2 64.10%, O2 8.38%, H2O 21.56%, Ar 0.75%, and CO2 5.21%, shown as pipe number 6 in Table 1. The amount of cathode gas gets to be reduced as shown in reaction (6). The N2, H2O, and Ar gases are not participated in reaction of cathode (6). As amounts of N2, H2O, and Ar gases do not change at the cathode inlet and outlet due to their no reaction, increases of mole fractions of these gases in the cathode outlet mean that the volume of the outlet gas was reduced.

As the residual fuel from the anode off-gas burns in the catalytic combustor, the temperature of the outlet gas of the combustor becomes 653.19 °C. This temperature is too high to be introduced into the cathode inlet. So, this outlet stream of the combustor is used for heating the anode inlet gas and producing additional steam, that is, apparatus number 17 of Fig. 1, until the outlet gas of the combustor decreases to 580 °C, which is the acceptable temperature for the cathode inlet.

The Process of Converting Coal to SNG Without Heat Recovery.

The synthetic gas produced from the coal gasifier needs to be changed to SNG, which is a CH4 rich gas, because the ir-MCFC, manufactured by POSCO of Korea, requires a CH4 composition of over 60%. Therefore, the gasification process, including ash removal and desulfurizer, should be designed to include WGS and methanation processes for the quality of SNG.

As a result of the gasification process, the initial gas product has the ratio of 0.8 of the coal thermal input. That synthesis gas from the gasification process is then converted to SNG with a thermal energy of 0.8 of the synthesis gas. Ultimately, the thermal output of SNG has a maximum value of 0.64 of the coal thermal input. As a consequence, when SNG is used in the fuel cell which has a demonstrated electric efficiency of 48.73% using NG, its total efficiency cannot reach over 31%, as mentioned above.

Using the cycle-tempo program, the process of converting coal to SNG can be simulated, as illustrated in Fig. 2. The coal used in the simulation was the Adaro coal from Indonesia, which composition includes 60.08 wt. % of C, 0.09 wt. % of Cl, 4.29 wt. % of H, 12.6 wt. % of H2O, 1.31 wt. % of N, 16.39 wt. % of O, 0.23 wt. % of S, and 5.01 wt. % of ash. These values were changed into mole basis ratio, as indicated in pipe number 1 of Table 2. A calorie basis of 22,993.25 kJ/kg (LHV basis) was used for the simulation. The thermal input energy of the coal is 102,089.81 kW as shown at apparatus 2 in Fig. 2.

In this gasifier, the weight ratio of oxygen to coal is 0.7. Steam (apparatuses 5 and 6 passing the apparatus 1 in Fig. 2) circulating in the cooling water structure surrounding the gasifier (apparatus 1) is introduced to recover the heat produced in the gasifier and lower the temperature of the outlet gas leaving the gasifier to around 1100 °C. As the ratio of oxygen to coal is increased, the temperature of the outlet gas leaving the gasifier becomes higher [3]. Then, more water is needed to match the temperature of the outlet gas of the gasifier to 1100 °C. Therefore, the amount of steam produced by the cooling water structure is dependent on the ratio of oxygen to coal and the setting temperature of the outlet gas of the gasifier.

In the syngas cooler (apparatus number 3), heat was set to be recovered until the temperature of the synthesis gas reaches 400 °C. Then this ash remover was simulated to remove up to 100% of the ash. The composition of the synthesis gas is as follows: H2 27.85%, H2O 7.45%, N2 1.96%, CO 58.74%, CO2 3.87%, COS 0.01%, HCl 0.03%, and H2S 0.08%, as shown in pipe number 7 in Table 2. This composition is not suitable for the following methanation reaction (10) since it contains more CO than H2: Display Formula

(10)CO+3H2CH4+H2O,ΔH°=205.81kJ/mol

In this case, the WGS reaction shown in reaction (8) is necessary. Here, the sour WGS reaction including a sulfur resistant sour WGS catalyst [16] is used for the simulation. In this process, the following reaction also occurs in the WGS reactor: Display Formula

(11)COS+H2OH2S+CO,ΔH°=33.8kJ/mol

Steam needs to be introduced into the WGS reactor until the ratio of CO to H2 is around 1/3. After the WGS reaction, over 30% CO2 gas is produced. It is necessary to remove this rich CO2 and a little H2S through a S and CO2 capture process like the selexol process [17]. In this paper, up to 90% of the CO2 gas was simulated to be removed and 100% of H2S and HCl, respectively. If the CO2 were fully removed, it might result in carbon deposition in the Ni catalyst layer of the anode side, caused by the following Boudourd reaction [18]: Display Formula

(12)2COC+CO2

Next, reaction (10), that is, the methanation reaction occurs in the methanation process. As this reaction is a severe exothermic reaction, the temperature of the outlet gas after the reactor can reach 700 °C or more. As the reaction is proceeding continuously, the reverse reaction of methanation is more dominant due to the change to high temperature gradually. Then methane cannot be produced richly in the state of high temperature, because the yield of methane is inversely proportional to the increase of temperature of the reactor. When the temperature of the reactor is lower, the amount of methane produced grows larger.

It is also necessary to keep the temperature below 700 °C because the materials of the methanation reactor and catalyst cannot endure durably over 700 °C. Therefore, it is necessary to cool the gas after the methanation reaction. To accomplish this, in the Tremp process [19] of Haldor Topsoe, three or four methanation reactors were used in sequence, recycling, and cooling the outlet gas of the first reactor and operating each following reactor at a lower temperature step by step. In this paper, two reactors were used and 70% of the outlet gas of the first reactor was recycled into the first reactor via recycling compressor and cooler, while the other 30% went to the second reactor. The temperature of the produced SNG was cooled to 300 °C. The composition of SNG was the value shown in the pipe number 2 of Table 2, which indicates compositions according to pipes from Fig. 2. The thermal output of this produced SNG from Fig. 2 was 62,844.07 kW, which was calculated as follows: Display Formula

(13)SNGth=LHV(kJmol)×SNG mass(kgs)Avg.molemass(kgkmol)×1000(molkmol)

Here, LHV is lower heating value found in Table 2, which is the calorie value of produced SNG. The Avg. mole mass means average mass value per mole of produced SNG. Then, SNG mass appears in the legend of mass flow (Φm) at the tip of pipe number 2 in Fig. 2.

The conversion efficiency or cold gas efficiency of the coal to SNG process was as follows: Display Formula

(14)conversion efficiency=62,844.07 kW 102,089.81 kW=61.56% 

This value depends on the ratio of oxygen to coal. When the ratio of O2/coal is 0.64, the conversion efficiency will be 63.46%. As the ratio of O2/coal is lower, the amount of CO and H2 becomes larger and results in higher conversion efficiency.

As the heat recovery is performed in the gasification process, the final energy value of SNG is a little changed due to variation of heat taken by the steam as shown in Figs. 3 and 4. The compositions of each pipe produced from Figs. 3 and 4 were shown in Table 3.

The ir-MCFC system fueled by SNG is seen in Fig. 5 and its electric efficiency when fueled by SNG with 62,670.93 kW which is the thermal energy of SNG produced in Fig. 4 is as follows:

Display Formula

(15)electric efficiencygross=30,050.69 kW 102,089.81 kW=29.44%

As the SNG is in a high-pressure state, it should be expanded to the operating pressure of the ir-MCFC, that is, nearly atmospheric pressure. Here, it produces a little power. However, some electricity is consumed in the air blower. Then the net electric efficiency is Display Formula

(16)electric efficiencynet=(30,050.78+111.5328.99)kW 102,089.81 kW=29.22% 

The compositions of each pipe produced from Fig. 5 were shown in Table 4. For more understanding of the systems of Figs. 1 and 5, the comparison was displayed in Table 5. For comparison of Figs. 24, Table 6 was prepared.

The Process of Converting Coal to SNG Including Heat Recovery in WGS and Methanation Process.

When SNG from the coal synthesis gas is used instead of NG fuel in the ir-MCFC, its electric efficiency cannot exceed 31%, as mentioned above. Consequently, it is necessary that the heat from the exothermic reactors be recovered in order to produce additional electricity. In this step, the heat from the WGS reactor and methanation process is recovered as steam and then the steam is used in a steam turbine to produce additional electricity. The process is shown in Fig. 3. The heat is recovered as steam after the WGS reactor and at three points in the methanation process. The steam is converted into 4,865.15 kW of electricity. The gross electric efficiency is presented below: Display Formula

(17)electric efficiencygross=(30,050.69+4,865.15)kW 102,089.81 kW=34.2%

The Process of Converting Coal to SNG Including Heat Recovery in Gasification, WGS, and Methanation Process.

If the heat recovery in the gasification block is used for additional steam production, the steam turbine power will be enhanced. In the gasification block, the heat from the cooling water structure surrounding the gasifier and the syngas cooler after the gasifier can be recovered. The production of totally 11,316.24 kW electricity from the steam produced in the gasification, WGS, and methanation processes is shown in Fig. 4.

All the heat exchangers are used for heat recovery by cooling the streams. The composition of each stream can be referred to Table 3.

The ir-MCFC System Fueled by the SNG.

The SNG gas produced from the process of converting coal to SNG to fuel the ir-MCFC is shown in Fig. 5. Since the stream pressure of the conversion process of coal to SNG is around 27.6 bar, the pressure is reduced to the operating pressure of the ir-MCFC of 1.1 bar. The thermal energy of coal, 102,089.81 kW is converted into the thermal SNG of 62,670.93 kW. Table 4 shows the composition of each pipe of Fig. 5. The SNG gas is introduced through the pipe number 9 and 11.

Unlike fueling the ir-MCFC with NG, the SNG is introduced at a temperature of 181.76 °C, compared to the normal temperature, that is, 20 °C of NG. Therefore, heating the SNG with the exhausted cathode off-gas is not necessary. As a result, that excess heat can produce additional steam more. As shown in Fig. 5, the excess thermal steam, 79,472.05 kWth is produced through apparatus number 17. If the thermal steam is used in a steam turbine with an electric efficiency of 28.71%, the steam can produce 22,816.43 kWe of electricity.   This power would be controlled from the amount of water introduced to the apparatus 6. If the amount of water is increased, the temperature of produced steam in apparatus 17 will be lowered, then it cannot be used in the steam turbine efficiently.

In the stack of the ir-MCFC, 30,050.69 kW of electricity is produced. The total electricity, summed with that (11,316.24 kW) of the steam turbine from the process of converting coal to SNG, is 41,366.93 kW, which corresponds to an electric efficiency of 40.52%. Display Formula

(18)electric efficiencygross=(30,050.69+11,316.24)kW 102,089.81 kW=40.52%

If an additional 22,816.43 kW of electricity can be produced from the exhausted gas of the ir-MCFC and added to the above, the result is as follows: Display Formula

(19)electric efficiencygross=(30,050.78+11,316.24+22,816.43)kW 102,089.81 kW=62.87% 

The Reflection of the Result of the CO2 Capture in the Process of Coal to SNG.

Here, the utilization of heat and the power to compress CO2 in the CO2 capture process have not been considered. If then energy consumed in the CO2 capture process is considered, the net power output is reduced by the high auxiliary load of the CO2 compression and the need to introduce the extraction steam into the WGS reaction and the regeneration tower for CO2 absorbent. It is known that approximately 25% additional coal is necessary to obtain the same net power output as the system without CO2 capture in dry feeding type Shell gasification system [20]. Auxiliary power of 6.56% of coal energy is also necessary in the process of converting coal to SNG, including the air separation unit (ASU), gas cleaning, methanation, and so on [5]. Thus, the overall electric efficiency will be reduced after considering the consumption of superheated steam and the power of compression in the CO2 capture process and the auxiliary power of all other processes.

At first, the ratio of electricity production from the recovered heat of Fig. 4 is calculated to be Display Formula

(20)ratio of electricity production=11,316.24 kW (102,089.8162,670.93)kW=0.2871 

If the total energy of consumption of heat for CO2 capture and CO2 compression power are applied at 25% of coal energy, the output of the steam turbine in Fig. 4 will be Display Formula

(21)output of steam turbine=(102,089.81(102,089.81×0.25)62,670.93)×0.2871=3989.66 kW

If the auxiliary power is considered, the result is Display Formula

(22)electric efficiencynet=((30,050.69+111.5328.99)+3989.66(102,089.81×0.0656))kW 102,089.81 kW=26.57%

If the additional steam in the ir-MCFC is utilized in the steam turbine, the result is Display Formula

(23)electric efficiencynet=(29,833.2+3989.66(102,089.81×0.0656)+22,816.43)kW 102,089.81 kW=48.92% 

This efficiency is comparable to that of the advanced thermal power plant based on coal.

In this paper, the electric efficiency of the ir-MCFC fueled by NG was calculated. The net electric efficiency was 48.19% and when the heat from the exhausted gas of fuel cell is recovered as steam and the steam is used in a steam turbine, the net electric efficiency will be increased to 76.11%.

It should be noted that when SNG produced from coal synthesis gas is used to fuel the ir-MCFC, the net electric efficiency from coal is just 29.22%, because the thermal output of the SNG corresponds to 61.39% of thermal input of coal. As the processes of converting coal to SNG have lots of exothermic reactions, it is useful to recover that heat from the exothermic reactions to enhance overall efficiency. If the heat from the processes of WGS and methanation is recovered and used in the steam turbine, the gross electric efficiency will be increased up to 34.2%. When all the heat from the processes of gasification to methanation is recovered, the gross efficiency will become 40.5%. If the steam produced from the exhausted gas of fuel cell is also used in the steam turbine, the gross efficiency will be 62.87%.

In the CO2 capture process, however, additional energy consumption is required, which corresponds to 25% additional coal energy in the dry feeding gasification process. If auxiliary power is also considered, the reduction of the power corresponding to 6.56% of coal energy will take place. Then, the net electric efficiency will be 48.92% when including the recovery of heat from the exhausted gas of fuel cell. This efficiency is comparable to that of the advanced thermal power plant based on coal.

When the efficiency of the system was being calculated, Mr. Theo Woudstra from Delft University of Technology contributed greatly to the elimination of errors, so the authors appreciate his kindness and technological support.

This paper is one of deliverables of the project of “Preparation of Conceptual Design of Pretreatment Process for Synthetic Gas Fuel Cell (SGFC),” which was solely supported by our company.

  • Afc =

    cell area, m2

  • ASU =

    air separation unit

  • H =

    enthalpy, kJ/kg

  • HHV =

    higher heating value, kJ/mol

  • ir-MCFC =

    internal reforming molten carbonate fuel cell

  • LHV =

    lower heating value, kJ/mol

  • MW =

    power unit, 106 W = 106 J/s

  • NG =

    natural gas

  • p =

    pressure, bar

  • P =

    power, kW

  • Pel =

    electrical power, kW

  • Pel, AC =

    AC power, kW

  • SNG =

    synthetic natural gas

  • T =

    temperature,  °C

  • Uf =

    fuel utilization, %

  • Uo =

    oxidant utilization, %

  • UCO2 =

    CO2 utilization, %

  • Vfc =

    cell voltage, V

  • WGS =

    water gas sift reaction

  • Δ =

    standard enthalpy change of reaction at 298 K

  • ΦE, in =

    energy input, kW

  • Φm =

    mass flow, kg/s

  • ΦE =

    energy loss, kW

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McPhail, S. , Moreno, A. , and Bove, R. , “ International Status of Molten Carbonate Fuel Cell (MCFC) Technology, Report Ricerca Sistema Elettrico/2009/181, Accordo di Programma Ministero dello Sviluppo Economico—ENEA,” Last accessed Mar. 25, 2016, http://www.enea.it/it/Ricerca_sviluppo/documenti/ricerca-di-sistema-elettrico/cell-a-combustibile/rse181.pdf/view
Seo, H.-K. , Park, S. , Lee, J. , Kim, M. , Chung, S.-W. , Chung, J.-H. , and Kim, K. , 2011, “ Effect of Operating Factors in the Coal Gasification Reaction,” Korean J. Chem. Eng., 28(9), pp. 1851–1858. [CrossRef]
Yun, Y. , and Yoo, Y. D. , 2001, “ Performance of a Pilot-Scale Gasifier for Indonesian Baiduri Coal,” Korean J. Chem. Eng., 18(5), pp. 679–685. [CrossRef]
NETL, 2011, “Cost and Performance Baseline for Fossil Energy Plants Volume 2: Coal to Synthetic Natural Gas and Ammonia,” National Energy Technology Laboratory, Washington, DC, accessed Mar. 25, 2016, https://www.netl.doe.gov/File%20Library/Research/Energy%20Analysis/Coal/SNGAmmonia_FR_20110706.pdf
Carapellucci, R. , Saia, R. , and Giordano, L. , 2014, “ Study of Gas-Steam Combined Cycle Power Plants Integrated With MCFC for Carbon Dioxide Capture,” Energy Procedia, 45, pp. 1155–1164. [CrossRef]
Mirita, H. , Yoshiba, F. , Woudstra, N. , Hemmes, K. , and Spliethoff, H. , 2004, “ Feasibility Study of Wood Biomass Gasification/Molten Carbonate Fuel Cell Power System-Comparative Characterization of Fuel Cell and Gas Turbine Systems,” J. Power Sources, 138(1–2), pp. 31–40. [CrossRef]
Toonssen, R. , Sallai, S. , Aravind, P. V. , Woudstra, N. , and Verkooijen, A. H. M. , 2011, “ Alternative System Design of Biomass Gasification SOFC/GT Hybrid Systems,” J. Hydrogen Energy, 36(16), pp. 10414–10425. [CrossRef]
Asimptote, 2016, “cycle-tempo Documentation,” Delft, The Netherlands, accessed Mar. 25, 2016, http://www.asimptote.nl/software/cycle-tempo/cycle-tempo-documentation/
Morita, H. , Komoda, M. , Mugikura, Y. , Izaki, Y. , Watanabe, T. , Masuda, Y. , and Matsuyama, T. , 2002, “ Performance Analysis of Molten Carbonate Fuel Cell Using a Li/Na Electrolyte,” J. Power Sources, 112(2), pp. 509–518. [CrossRef]
Woudstra, N. , van der Stelt, T. P. , and Hemmes, K. , 2006, “ The Thermodynamic Evaluation and Optimization of Fuel Cell Systems,” ASME J. Fuel Cell Sci. Technol., 3(2), pp. 155–164. [CrossRef]
Ahmeda, S. , Aitani, A. , Rahman, F. , Al-Dawood, A. , and Al-Muhaish, F. , 2009, “ Decomposition of Hydrocarbon to Hydrogen and Carbon,” Appl. Catal. A, 359(1–2), pp. 1–24. [CrossRef]
Katikaneni, S. , Yuh, C. , Abens, S. , and Farooque, M. , 2002, “ The Direct Carbonate Fuel Cell Technology: Advances in Multi-Fuel Processing and Internal Reforming,” Catal. Today, 77(1–2), pp. 99–106. [CrossRef]
Breeze, P. , 2014, Power Generation Technologies, 2nd ed., Newnes, Oxford, UK.
Jansen, D. , and Mozaffarian, M. , 1997, “ Advanced Fuel Cell Energy Conversion Systems,” Energy Convers. Manage. 38(10–13), pp. 957–967. [CrossRef]
Haldor Topsøe, 2009, “ Sulphur Resistant/Sour Water-Gas Shift Catalyst,” Haldor Topsøe A/S, Lyngby, Denmark, accessed Mar. 25, 2016, http://www.topsoefuelcell.com/business_areas/gasification_based/Processes/~/media/PDF%20files/SSK/topsoe_SSK%20brochure_aug09.ashx
UOP Honeywell, 2010, “UOP SelexolTM Technology for Acid Gas Removal,” UOP, Des Plaines, IL, accessed Mar. 25, 2016, http://www.uop.com/?document=uop-selexol-technology-for-acid-gas-removal&download=1
Selman, J. R. , Uchida, I. , Wendt, H. , Shores, D. A. , and Fuller, T. F. , eds., 1997, Carbonate Fuel Cell Technology IV, The Electrochemical Society, Pennington, NJ.
Holdor Topsøe, 2009, “ From Solid Fuels to Substitute Natural Gas (SNG) Using TREMP TM,” Haldor Topsøe A/S, Lyngby, Denmark, accessed Mar. 25, 2016, http://www.topsoefuelcell.com/business_areas/gasification_based/Processes/~/media/PDF%20files/SNG/Topsoe_TREMP.ashx
NETL, 2013, “Cost and Performance Baseline for Fossil Energy Plants Volume 1: Bituminous Coal and Natural Gas to Electricity,” National Energy Technology Laboratory, Washington, DC, accessed Mar. 25, 2016, http://www.netl.doe.gov/File%20Library/Research/Energy%20Analysis/OE/BitBase_FinRep_Rev2a-3_20130919_1.pdf
Copyright © 2016 by ASME
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References

Brandon, N. , and Thompsett, D. , eds., 2005, Fuel Cells Compendium, Elsevier, New York.
McPhail, S. , Moreno, A. , and Bove, R. , “ International Status of Molten Carbonate Fuel Cell (MCFC) Technology, Report Ricerca Sistema Elettrico/2009/181, Accordo di Programma Ministero dello Sviluppo Economico—ENEA,” Last accessed Mar. 25, 2016, http://www.enea.it/it/Ricerca_sviluppo/documenti/ricerca-di-sistema-elettrico/cell-a-combustibile/rse181.pdf/view
Seo, H.-K. , Park, S. , Lee, J. , Kim, M. , Chung, S.-W. , Chung, J.-H. , and Kim, K. , 2011, “ Effect of Operating Factors in the Coal Gasification Reaction,” Korean J. Chem. Eng., 28(9), pp. 1851–1858. [CrossRef]
Yun, Y. , and Yoo, Y. D. , 2001, “ Performance of a Pilot-Scale Gasifier for Indonesian Baiduri Coal,” Korean J. Chem. Eng., 18(5), pp. 679–685. [CrossRef]
NETL, 2011, “Cost and Performance Baseline for Fossil Energy Plants Volume 2: Coal to Synthetic Natural Gas and Ammonia,” National Energy Technology Laboratory, Washington, DC, accessed Mar. 25, 2016, https://www.netl.doe.gov/File%20Library/Research/Energy%20Analysis/Coal/SNGAmmonia_FR_20110706.pdf
Carapellucci, R. , Saia, R. , and Giordano, L. , 2014, “ Study of Gas-Steam Combined Cycle Power Plants Integrated With MCFC for Carbon Dioxide Capture,” Energy Procedia, 45, pp. 1155–1164. [CrossRef]
Mirita, H. , Yoshiba, F. , Woudstra, N. , Hemmes, K. , and Spliethoff, H. , 2004, “ Feasibility Study of Wood Biomass Gasification/Molten Carbonate Fuel Cell Power System-Comparative Characterization of Fuel Cell and Gas Turbine Systems,” J. Power Sources, 138(1–2), pp. 31–40. [CrossRef]
Toonssen, R. , Sallai, S. , Aravind, P. V. , Woudstra, N. , and Verkooijen, A. H. M. , 2011, “ Alternative System Design of Biomass Gasification SOFC/GT Hybrid Systems,” J. Hydrogen Energy, 36(16), pp. 10414–10425. [CrossRef]
Asimptote, 2016, “cycle-tempo Documentation,” Delft, The Netherlands, accessed Mar. 25, 2016, http://www.asimptote.nl/software/cycle-tempo/cycle-tempo-documentation/
Morita, H. , Komoda, M. , Mugikura, Y. , Izaki, Y. , Watanabe, T. , Masuda, Y. , and Matsuyama, T. , 2002, “ Performance Analysis of Molten Carbonate Fuel Cell Using a Li/Na Electrolyte,” J. Power Sources, 112(2), pp. 509–518. [CrossRef]
Woudstra, N. , van der Stelt, T. P. , and Hemmes, K. , 2006, “ The Thermodynamic Evaluation and Optimization of Fuel Cell Systems,” ASME J. Fuel Cell Sci. Technol., 3(2), pp. 155–164. [CrossRef]
Ahmeda, S. , Aitani, A. , Rahman, F. , Al-Dawood, A. , and Al-Muhaish, F. , 2009, “ Decomposition of Hydrocarbon to Hydrogen and Carbon,” Appl. Catal. A, 359(1–2), pp. 1–24. [CrossRef]
Katikaneni, S. , Yuh, C. , Abens, S. , and Farooque, M. , 2002, “ The Direct Carbonate Fuel Cell Technology: Advances in Multi-Fuel Processing and Internal Reforming,” Catal. Today, 77(1–2), pp. 99–106. [CrossRef]
Breeze, P. , 2014, Power Generation Technologies, 2nd ed., Newnes, Oxford, UK.
Jansen, D. , and Mozaffarian, M. , 1997, “ Advanced Fuel Cell Energy Conversion Systems,” Energy Convers. Manage. 38(10–13), pp. 957–967. [CrossRef]
Haldor Topsøe, 2009, “ Sulphur Resistant/Sour Water-Gas Shift Catalyst,” Haldor Topsøe A/S, Lyngby, Denmark, accessed Mar. 25, 2016, http://www.topsoefuelcell.com/business_areas/gasification_based/Processes/~/media/PDF%20files/SSK/topsoe_SSK%20brochure_aug09.ashx
UOP Honeywell, 2010, “UOP SelexolTM Technology for Acid Gas Removal,” UOP, Des Plaines, IL, accessed Mar. 25, 2016, http://www.uop.com/?document=uop-selexol-technology-for-acid-gas-removal&download=1
Selman, J. R. , Uchida, I. , Wendt, H. , Shores, D. A. , and Fuller, T. F. , eds., 1997, Carbonate Fuel Cell Technology IV, The Electrochemical Society, Pennington, NJ.
Holdor Topsøe, 2009, “ From Solid Fuels to Substitute Natural Gas (SNG) Using TREMP TM,” Haldor Topsøe A/S, Lyngby, Denmark, accessed Mar. 25, 2016, http://www.topsoefuelcell.com/business_areas/gasification_based/Processes/~/media/PDF%20files/SNG/Topsoe_TREMP.ashx
NETL, 2013, “Cost and Performance Baseline for Fossil Energy Plants Volume 1: Bituminous Coal and Natural Gas to Electricity,” National Energy Technology Laboratory, Washington, DC, accessed Mar. 25, 2016, http://www.netl.doe.gov/File%20Library/Research/Energy%20Analysis/OE/BitBase_FinRep_Rev2a-3_20130919_1.pdf

Figures

Grahic Jump Location
Fig. 1

Flow diagram of a 30 MW class ir-MCFC system fueled by NG. (Pipe 11, 23, 13, 4, 21, 22: NG to anode inlet, Pipe 5: anode outlet to combustor, Pipe 16, 19: combustor to cathode inlet, Pipe 6, 7, 8, 9: cathode exhaust gas, Pipe 1, 2, 3: air to combustor.) Apparatus 1: the stack of ir-MCFC, 2: heat exchanger for air heating, 3: heat exchanger for heating water, 4: heat exchanger for heating NG, 6: water, 7: NG, 8: air blower, 9: catalytic combustor, 10: Flare stack, 11: mixer of NG and steam, 12: heat exchanger for producing the superheated steam, 13: the valve splitting the steam, 14: air, 16: heat exchanger for cooling the catalytic combustor off-gas, 17: the additionally produced steam, 18: mixer of NG and CO2, and 19: CO2.

Grahic Jump Location
Fig. 2

Process of coal to SNG without heat recovery. Apparatus 1: gasifier, 2: coal, 3: syngas cooler, 4: ash remover, 5 and 6: the water and steam passing the water wall in the gasifier, 7: cooler, 8: the mixer, 9: the steam introduced into the WGS, 10: WGS reactor, 11: ash disposal, 12: cooler, 13 and 14: the inlet and outlet of steam recovering the heat from the syngas cooler, 15: 16; the inlet and outlet of steam recovering the heat, 17 and 18: S, CO2 capture and its disposal, 19: methanation first reactor, 20: 95% purified oxygen, 22: mixer, 23: cooler, 24: compressor, 25: cooler, 26: the valve splitting the first methanation gas, 27: the second methanation reactor, 32: cooler, 28, 29, 30, 31, 34, 35, 36, and 37: the steam recovering the heat, and 33: the produced SNG.

Grahic Jump Location
Fig. 3

Process of converting coal to SNG including the heat recovery in WGS and methanation (circles mean heat recovery parts). Apparatus 1–20, 22–27, 32, and 33: the same as in Fig. 2, apparatus 21: the steam drum, 38: the steam turbine, 40: the deaerator, 39: the condenser, 29, 30, and 41: the pumps, 42: water source.

Grahic Jump Location
Fig. 4

Process of converting coal to SNG including the heat recovery in gasification, WGS, and methanation (circles mean heat recovery parts). All apparatus are the same as in Fig. 3.

Grahic Jump Location
Fig. 5

Flow diagram of a 30 MW class ir-MCFC system fueled by SNG. (Pipe 9, 11, 4, 21, 22: SNG to anode inlet, Pipe 5: anode outlet to combustor, Pipe 16, 19: combustor to cathode inlet, Pipe 6, 7, 8: cathode exhaust gas, Pipe 1, 2, 3: air to combustor.) Most of all apparatuses are the same as in Fig. 1, except apparatus 7: SNG, and apparatus 4: the pressure reducer like the expander.

Tables

Table Grahic Jump Location
Table 1 Compositions according to gas streams of the ir-MCFC system related with Fig. 1
Table Grahic Jump Location
Table 2 Compositions of gas streams in the process of converting coal to SNG related with Fig. 2
Table Grahic Jump Location
Table 3 Compositions of gas streams in the process of converting coal to SNG related with Figs. 3 and 4
Table Grahic Jump Location
Table 4 Compositions of gas streams of the ir-MCFC system fueled by SNG related with Fig. 5
Table Grahic Jump Location
Table 5 Comparison of Figs. 1 and 5
Table Grahic Jump Location
Table 6 Comparison of Figs. 24

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