Bottoming cycles

Institute Shanghai SMS 5 60 coal-fired bottoming cycle test facihty... 

N. R. Johanson and J. W. Muehlhauser, "MHD Bottoming Cycle Operations and Test Results at the CFFF," paper presented at Second International Workshop on Fossil Fuel Fired MHD, Bologna, Italy, 1989. 

The fuel cell must be cooled with either water or air, and the heat can be converted to electricity in a bottoming cycle. The dc electrical output ot the stack is usually converted to ac and stepped up or down in voltage, depending on the application. Analogous to PAFds, M(iF(i stacks are about 1 nr (10.8 ft") in plan area and quite tall. A stack generates 200 to 300 kW. Market entiy is expected in 1999. 

Natural gas requires no fuel treatment however, low-Btu gas, espeeially if derived from various coal gasification processes, requires various types of cleaners for use in a gas turbine. These cycles can get very complex as indicated by a typical system, which utilizes a steam bottoming cycle to achieve high efficiency. Vaporized fuel oil gas is already cleansed of its impurities in the vaporization process. 

An important field of study for power plants is that of the combinedplant [ 1 ]. A broad definition of the combined power plant (Fig. 1.5) is one in which a higher (upper or topping) thermodynamic cycle produces power, but part or all of its heat rejection is used in supplying heat to a lower or bottoming cycle. The upper plant is frequently an open circuit gas turbine while the lower plant is a closed circuit steam turbine together they form a combined cycle gas turbine (CCGT) plant. 

El-Masri, M.A. (1987b), Exergy analysis of combined cycles Part 2, Steam bottoming cycles. ASME J. Engng Power Gas Turbines 109, 237-243,... 

Figure 6.3 provides a simplified block diagram of a fuel cell power plant system. Gasification is used to convert the solid fuel to gas, which is processed to remove sulfur compounds, tars, particulates, and trace contaminants. The clean gas is then converted to electricity in the FC. Waste heat from the FC is used to generate steam, which can be used to run the gasification process and to generate additional power in the bottoming cycle. 

Although a fuel cell produces electricity, a fuel cell power system requires the integration of many components beyond the fuel cell stack itself, for the fuel cell will produce only dc power and utilize only processed fuel. Various system components are incorporated into a power system to allow operation with conventional fuels, to tie into the ac power grid, and often, to utilize rejected heat to achieve high efficiency. In a rudimentary form, fuel cell power systems consist of a fuel processor, fuel cell power section, power conditioner, and potentially a cogeneration or bottoming cycle to utilize the rejected heat. A simple schematic of these basic systems and their interconnections is presented in Figure 9-1. 

The selection of a specific fuel cell pressure will affect numerous design parameters and considerations such as the current collector width, gas flow pattern, pressure vessel size, pipe and insulation size, blower size and design, compressor auxiliary load, and the selection of a bottoming cycle and its operating conditions. 

Although fuel cells are not heat engines, heat is still produced and must be removed in a fuel cell power system. Depending upon the size of the system, the temperature of the available heat, and the requirements of the particular site, this thermal energy can be either rejected, used to produce steam or hot water, or converted to electricity via a gas turbine or steam bottoming cycle or some combination thereof... 

If a steam bottoming cycle is appropriate, many design decisions need to be made, including the selection of the turbine cycle (reheat or non-reheat) and the operating conditions. Usually, steam turbines below 100 MW are non-reheat, while turbines above 150 MW are reheat turbines. This generalization is subject to a few exceptions. In fact, a small (83 MW) modem reheat steam turbine went into operation (June 1990) as a part of a gas turbine combined cycle repowering (43). 

Steam generated in the bottoming cycle is utilized in a reheat turbine to produce 118 MWe, as well as to supply the steam required by the air separation unit (ASU) and the gasifier coal slurry heater. The cycle exhaust exits the heat recovery steam generator at 126°C (259°F) and 0.98 atmospheres. 

Table 9-23. Performance Calculations for a Pressurized, High Temperature Fuel Cell (SOFC) with a Regenerative Brayton Bottoming Cycle Approach Delta T=30F... 

The Rankine cycle diagram placed adjacent the Brayton cycle in Figure 9-15 is indicated as a simple steam cycle with superheat, but no reheat and no multi-pressure steam generation. The thermodynamic advantage of the Rankine bottoming cycle is the lowered temperature of heat rejection, in the steam condenser, from the overall combined cycles. 

The performance of a SOFC system with a Brayton-Rankine bottoming cycle for heat and fuel recovery has been calculated. Gas turbine compressor and expander efficiencies of 83% and 89% and a steam turbine efficiency of 90% have been assumed. 

Two ASPEN (Advanced System for Process Engineering, public version) simulations compare the performance of conventional and networked fuel cell systems having identical recycle schemes and steam bottoming cycles. Each simulated system was composed of three MCFC stacks operating at the same temperature and pressure. The Nemst potential of each MCFC in both systems was reduced by 0.3 volts due to activation, concentration and ohmic voltage... 

When the total fuel utilization of each system was optimized for maximum efficiency, the efficiency of the fuel cell stacks networked in series was nearly 10% greater than that of the stacks arranged in parallel (44.9% vs. 35.4%, LHV). When the power generated by each system s steam bottoming cycle was considered in addition to its fuel cell power, the gap in efficiency narrowed to 5.5%. The efficiency of the total networked system is 56.8%, while that of the total conventional system was 51.3%. 

---