System Design and Optimization

Solid Oxide Fuel Cell (SOFC) System Designs and Performance

SOFCs have been described as one of the most promising energy conversion systems due to their many advantages such as high-energy conversion efficiency, low pollution, and versatile fuel input. In the SOFC system, fuel and air are the stack inputs, while electricity, exhaust gas, and hot water or steam exit the system. Such systems include atmospheric SOFC combined heat and power generation (CHP) systems, pressurized SOFC/turbine hybrid systems, atmospheric SOFC residential and auxiliary power systems, and oxygen separating systems [I]. In order to maximize the use of heat generated from the high-temperature exhaust gas of the typical SOFCs, numerous studies have been focused on atmospheric SOFC-CFIP systems [2-4] as well as SOFC tri-generation systems [5-7] and SOFC poly-generation systems [8,9]. The difference between a SOFC system and a SOFC stack is defined as the balance- of-plant (BOP), which can differ for each application witli regard to the operating pressure, size of the system, and fuel type [1]. A typical SOFC-CHP uses BOP modules to enable specific electrochemical processes and produce electrical and thermal powers with efficiency as high as 90%, electrical and thermal combined. The SOFC tri-generation systems produce electrical, thermal, and cooling powers.

Atmospheric SOFC-CHP Systems

A simplified schematic of the main BOP equipment for an atmospheric SOFC system built by Siemens Westinghouse and demonstrated in Westervoort, Netherlands is shown in Figure 5.1 [10]. Here, the SOFC stack is integrated with a catalytic burner into a single SOFC generator module and a pre-reformer to reform higher hydrocarbons in the natural gas. Moreover, the fuel is desulfurized before it enters the SOFC stack. Sulfur can poison the nickel-containing anode and deteriorates the stack performance, as discussed in Chapter 2. In general, in order to avoid any performance loss, it is suggested to keep the sulfur level below 0.1 ppm. Natural gas contains far less sulfur compared to the oil-delivered liquid fuels. To remove sulfur compounds, the most common way is to hydrogenate them into hydrogen sulfide (H₂S) using a catalytic reactor and then adsorb the generated H₂S in a separate zinc oxide-containing reactor at an elevated temperature up to 450°C. Using activated carbon to adsorb sulfur compounds is relatively easier since it works at room temperature; however, it is more costly [1].

In order to avoid carbon formation in the SOFC stack, it is needed to pre-reform heavy hydrocarbons to methane, hydrogen, and carbon monoxide. In general, steam

FIGURE 5.1 The main components of a typical atmospheric SOFC system. (Reprinted with permission from Ref. [1].)

reforming is used. To supply steam to the reformer, the recirculating part of the SOFC exhaust stream is used, as shown in Figure 5.1. This also results in increasing the overall fuel utilization.

Since the concentration of fuel decreases toward the exhaust side of the stack, causing the cell voltage drop, only a certain amount of fuel can be electrochemically converted to electricity and heat. The maximum practical amount considered for fuel utilization is 85%-90%. To burn the sulfur air from the cathode side as well as the remaining fuel from the anode side to avoid nickel oxidation, a catalytic burner is utilized.

In order to avoid thermal shock and the subsequent irreversible damage to the stack, the exhaust gas is led through a recuperator to heat the air up to at least 500°C before it enters the stack. An additional exchanger is used to produce hot water. The process steam can be produced instead of hot water by placing the heat exchanger between the two recuperators, as shown in Figure 5.1.

An inverter is used to convert the direct current (DC) produced by the stack to alternating current (AC). Generally, a two-step converter is used to transform the stack voltage to a stable DC voltage in the first step and then convert the DC to grid quality AC in the second step. The efficiency of the inverter is usually between 94% and 98%.

A SOFC system also has additional components including a blower to supply air to the system, air filters to clean the air, an air heater to start the SOFC generator module and to run at low' load when the stack temperature is low, control equipment and user interface, purge gas systems to avoid stack damage during start-up and shutdown, and start-up steam generator to provide steam for the pre-reformer [1].

The electrical efficiency for such SOFC-CHP systems is considered about 45%-50% based on lower heating value of the fuel [10,11], w'hile it is feasible to reach a total thermal and electrical efficiency of 85%-90%. In such SOFC systems using natural gas as the fuel, emissions are negligible except for CO, due to the clean fuel used and lower operating temperature of the stack compared to a conventional burner. However, the desulfurizer used to remove sulfur from the natural gas fuel produces a low' amount of SO^ and particulates [1].

Residential, Auxiliary Power, and Other Atmospheric SOFC Systems

The general layout of residential and auxiliary power units (APUs) is almost similar to the atmospheric system described above. A 5 kW gasoline-fueled APU along with its main subsystems are shown in Figure 5.2. Electrical efficiency of up to 30% is expected for such an APU system. This system is a close thermal integration where the heat losses are dependent of size and the use of partial oxidation fuel reformers. Partial oxidation reformer is used to preprocess the gasoline and is normally less efficient than a steam reformer. A battery unit is also used in the APU unit, as presented in Figure 5.2, to provide power and level the SOFC stack load. For residential systems operating with natural gas, the electrical efficiency of up to 40% is expected to be achieved.

It is possible to use SOFC technology for the combined production of power and syngas. This syngas, which is a mixture of CO and H₂ generated by the SOFC, can be used as a raw material in different chemical applications [12]. Additionally, it is relatively simple to separate CO, from water in the anode exhaust stream of the SOFC system, thus the possibility for carbon sequestration. A CO, separating SOFC system based on a pressurized SOFC generator combined with a gas turbine can maintain the electrical efficiency while capturing CO, for other applications. By capturing CO, from the SOFC, there is no need to use the conventional after-burner section. Instead, the anode exhaust gas is electrochemically oxidized in a separate special after-burner section using a suitable oxygen-selective ceramic membrane. The water vapor is also separated from CO, by cooling the exhaust gas, as illustrated in Figure 5.3 [1].

Pressurized SOFC/Turbine Hybrid Systems

In order to achieve a very high electrical efficiency of 60%-75%, a pressurized SOFC stack can be combined with a gas turbine. This results in high efficiency even at a very small 1 MW scale with a very low amount of harmful emissions except for CO,. The simplest design for a gas turbine is shown in Figure 5.4a. The fuel is burned with the compressed air coming from a compressor, and exhaust gas with a temperature of 800°C-1300°C is expanded in a turbine mechanically coupled to the compressor and a generator. The temperature of exhaust gas is reduced to 250°C-600°C due to the expansion in the turbine. The exhaust temperature decreases with increasing the

FIGURE 5.3 Simplified configuration for CO, capture from an SOFC system. (Reprinted with permission from Ref. [1].)

FIGURE 5.4 Possible gas turbine configurations, (a) the simplest design for a gas turbine. By replacing the conventional burner in a turbine with a pressurized SOFC generator, configurations (b-f) can be used for hybrid systems. (Reprinted with permission from Ref. [1].)

pressure ratio between the exhaust and turbine inlet. For small size turbines, the electrical efficiency can be about 20%, while it can increase up to 35% for large industrial turbines. Figure 5.4b-f represents different configurations for conventional gas turbines, which can also be used for hybrid SOFC/turbine systems. In principle, there is a need to replace the conventional combustion chamber in a turbine with a pressurized SOFC generator. In some configurations, a recuperator is used to increase electrical efficiency by decreasing the amount of natural gas required to heat the air [1].

To avoid thermal shock and subsequent damage to the SOFC stack, the air inlet temperature of an SOFC generator should be at least 500°C-650°C. Hence, using heat recuperation seems to be necessary. On the other hand, since the exhaust gas temperature becomes very low to heat the air inlet to the required temperature at high-pressure ratios between the gas turbine inlet and outlet, this pressure ratio is also limited (the ratio needs to be 2-4 unless there is additional heat from conventional burners). The SOFC stack can deliver 65%-80% of the total electrical power output of the hybrid system without additional heat from the conventional burners.

The electrical efficiency of a SOFC/turbine hybrid system is dependent of system size, system configuration (Figure 5.4b-f), the additional heat from the conventional burners, and the performances of the SOFC and the turbine used. This efficiency may vary from 55% to 60% for a small and simple SOFC/turbine hybrid system with the power of 250-1,OOOkW to 68% for a 5-10 MW system with intercooler and reheat from a separate SOFC generator. Combining thermal and electrical efficiencies, total efficiency of 85%-90% can be expected [1].

SOFC Tri-Generation Systems

Among the various types of SOFC systems, SOFC tri-generation systems have been the most attractive ones in the market for distributed generation and residential applications [13,14]. Different system configurations, such as coupled designs and decoupled designs, can be utilized for the SOFC tri-generation systems. In the decoupled designs, the waste heat from the SOFC-CHP subsystem is used as a steam generator of absorption refrigeration subsystems, and the heat output of the SOFC tri-generation system is the exhaust heat of the steam generator [15,16]. In the coupled designs, either the SOFC-CHP and refrigeration subsystems can share some components, or the piping of one subsystem goes through the other subsystem components [5]. For instance, Silveira et al. [17] and Shariatzadeh et al. [7] proposed a system in which the exhaust gas from the SOFC stack directly goes to the absorption refrigeration subsystem components and turns back to the BOP components of the SOFC-CHP subsystem.

Residential power is one of the most promising applications for SOFC tri-generation systems. The demand for electricity, cooling power, and heat can frequently vary in this application. Therefore, it is important to investigate the energy outputs of the tri-generation systems and develop strategies to regulate the energy outputs. Recently, Wu and Chen [5] proposed a SOFC tri-generation system that mainly consists of a SOFC-CHP subsystem, an adsorption refrigeration subsystem, and a cooling device between the two subsystems, as shown in Figure 5.5. The SOFC-CHP subsystem consists of a SOFC stack, a reformer, heat exchangers, a burner, and other components. The adsorption refrigeration subsystem consists of two sorption beds, an evaporator, a condenser, and other components. The results indicated that the proposed SOFC tri-generation system provides 4.35 kW electrical power, 2.448 kW exhaust heat power, and 1.348 kW cooling power. Moreover, the energy efficiency of the system was 64.9%. The results also showed that the exhaust heat power varies with the variation of the electrical output power and the cooling power, but the electrical power variations are independent of the cooling power variations.

FIGURE 5.5 Proposed SOFC tri-generation system consisting of three subsystems: SOFC- CHP system, adsorption refrigeration system, and water tank equipped with a temperature controller. (Reprinted with permission from Ref. [5].)

In the other study, Moussawi et al. [6] designed a SOFC tri-generation system for domestic applications. Figure 5.6 shows the SOFC-BOP and the recovery system of the tri-generation design. The results indicated that the tri-generation system is energetically and economically superior. The maximum energy and energy efficiency of 65.2% and 45.77%, respectively, and the minimum system cost rate of 22.2 cents kWh⁴ were obtained under on-grid baseload operation.

Shariatzadeh et al. [7] also proposed a new configuration of the SOFC tri-generation system fed by biogas produced from hospital waste. The system consists of a 50 kW tubular SOFC combined with an absorption chiller, a heat recovery steam generator (HRSG), a combustion chamber, a compressor, and other components, as shown in Figure 5.7. The system recovers and uses waste heat as well as generating electricity. This heat results in steam generation in the HRSG, and the steam generated within the absorption chiller produces cooling load. In the proposed system, air enters the compressor, and the flow rate required by the SOFC initially enters the recuperator and finally enters the SOFC cathode. The compressed fuel enters the SOFC anode. However, before the fuel enters, natural gas is preheated and enters the anode side, passing through the SOFC internal reformer. The DC electricity produced by the SOFC stack is converted to AC using a DC/AC inverter. The SOFC outlet flow (superheated water vapor) enters the combustion chamber until its enthalpy becomes higher than before. Thereafter, the flow enters the HRSG, where the required water vapor for the absorption chiller is supplied. The water vapor is sent to the chiller, and then, the cooling load is supplied in the chiller. The results indicated that the proposed system could be economically affordable in the long term.

FIGURE 5.6 Schematic of (a) SOFC-BOP system and (b) recovery system of the proposed tri-generation design. (Reprinted with permission from Ref. [6].)