Cogeneration combined-cycle operation

The exhaust temperature from a gas turbine ranges from 753 to 800 K. In cogeneration/combined-cycle operation the gas is cooled down to the operating temperatures of SCR (500-673 K). The SCR system downstream from a gas turbine may result in an increase of pressure resulting in a decreasing power output of the gas turbine. If carbon monoxide has to be removed, the oxidation catalyst should be installed before the SCR reactor. Problems arise if the flue gas contains SO2. 

The envelope defined by A, B, C, and D in Fig. 6.70 represents the most thermally optimized use of a gas turbine in a cogeneration application (i.e., provides the lowest FCP). Operation along the line CE, DF, or any intermediate point to the left of line CD represents the nse of condensing steam turbine power generation with the E and F points appUcable for combined-cycle operation without any heat supplied to process.Thus, the cycles along line EF are combined cycles providing power alone. 

However, if the major source of hydrogen is reformed natural gas, the cost of generating electricity with a low-temperature fuel cell would be about 0.20 per kilowatt-hour. This is more than double the average price for electricity. It would also produce 50% more carbon dioxide emissions than the most efficient natural gas plants which are combined cycle natural gas turbines. Low-temperature fuel cells operating on natural gas are not as efficient at generating electricity. A stationary fuel cell system achieves high efficiency by cogeneration. 

Unlike molten carbonates, solid oxides use a hard ceramic electrolyte instead of a liquid. That means the fuel cell can be cast into a variety of useful shapes, such as tubes. With higher temperatures, sofcs may be able to cogenerate steam at temperatures as high as i,ooo°f. The Siemens Westinghouse Power Corporation has built the first advanced hybrid system, which combines a gas turbine with a tubular sofc. As of 2003, the 220 kW hybrid system has operated in California for more than 2,000 hours with a respectable 53 percent efficiency, comparable to current combined cycle gas turbines. The ultimate goal is an efficiency of 70 percent or more. 

In California (US), a 385 MW cogeneration facility comprising four gas turbine combined cycle trains is in operation [137]. Separate fuels such as natural gas, butane, and refinery fuel gas may be used. The refinery gas contains 800 ppm (by volume) sulfur. Sulfur was removed from the fuel before it was burnt in the gas turbine. The gas cleanup system includes a CO oxidation catalyst and downstream from this catalyst a SCR system. 

For cogeneration, a 60 MW(th) helium cooled pebble bed reactor is coupled with a secondary nitrogen cycle through a He/N2 heat exchanger. If the application is electricity production only, a combined cycle of a gas turbine and a steam turbine is used. Major design and operating characteristics of the ACACIA plant are summarized in Tables XIX-1 and XIX-2. 

Table 3.8 compares the estimated costs of potable water production through seawater desalination cogeneration with conventional and VHTR power plants (Sato et al., 2014). The conventional plant is based on a modem gas turbine combined cycle (GTCC) power plant at 55% power generation efficiency. The VHTR cogeneration system is that described in Section 3.4.2.2. The costs were evaluated by an original equipment manufacturer (OEM) vendor active in the Middle East desalination plant constmction. The vendor carried out the plant equipment design and evaluated the required operation and maintenance. The cost estimation was then developed based on the vendor construction and operation know-how of comparable-scale MSF plants. 

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