Generator, bottoming cycle

The hybrid sulphur (HyS) cycle utilises the same H2S04 decomposer and acid concentration section as the S-I plant. The S02 electrolysers are polymer electrolyte membrane (PEM) technology. The hydrogen plant is coupled to two NHSS and produces 4.0 kg/s of product. Oxygen is sold as a by-product. The Rankine bottoming cycle generates 133 MWe for the electrolysis section and an additional 198 MWe is imported. 

Recent studies by Thermo Electron Corporation have shown that certain forms of waste heat utilization (e.g., recuperators or process steam boilers) provide a better return on investment than that obtainable from bottoming cycle generators. The optimum choice of heat recovery strategy depends, in part, upon the temperature of exhaust heat available. 

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. 

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. 

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. 

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. 

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%. 

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