Gas turbines in cogeneration

FIG. C-60 System flowchart GT 7 gas turbine in cogeneration application. (Source Aistom.)... 

Example case history 5. Power augmentation for a gas turbine in cogeneration... 

Site composite curves can be used to represent the site heating and cooling requirements thermodynamically. This allows the analysis of thermal loads and levels on site. Using the models for steam turbines and gas turbines allows cogeneration targets for the site to be established. Steam levels can be optimized to minimize fuel consumption or maximize cogeneration. A cost trade-off needs to be carried out in order to establish the optimum trade-off between fuel requirements and cogeneration. 

The SCON Ox process has been used for the removal of NO in gas turbines in a few installations in the USA (32 MW Sunlaw Federal cogeneration facility, 5 MW Wyeth Biopharma plant, 15 MW University of California cogeneration facility in San Diego). However, the high cost of the SCONOx process, primarily due to the cost of the noble metal in the catalysts, represents a serious limitation on its extensive use. 

A variety of technologies have been pursuing on-site CHP for years, including turbines, reciprocating engines, and steam turbines. Gas turbines in the 500-kW to 250-MW produce electricity and heat in a thermodynamic cycle known as the Brayton cycle. They produce about 40,000-MW of the total CHP in the United States. The electric efficiency for units of less than 10-MW, is above 30%, with overall efficiencies reaching 80% when the cogenerated heat is used. 

One method of displaying the many options available by using a gas turbine in a cogeneration application is shown in Fig. 6.70. This diagram has been developed for the GE MS7001EA gas turbine-generator. 

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. 

Gas turbine-based power plants, particularly natural gas-fired cogeneration and combined-cycle faciUties, have proven to be highly rehable, efficient, and environmentally attractive. Advances in machine design, more efficient plant integration, and optimistic forecasts for the availabiUty of affordable natural gas worldwide have boosted the appeal of these systems for both base-load and peaking service. 

Catalyst contamination from sources such as turbine lubricant and boiler feed water additives is usuaUy much more severe than deactivation by sulfur compounds in the turbine exhaust. Catalyst formulation can be adjusted to improve poison tolerance, but no catalyst is immune to a contaminant that coats its surface and prevents access of CO to the active sites. Between 1986 and 1990 over 25 commercial CO oxidation catalyst systems operated on gas turbine cogeneration systems, meeting both CO conversion (40 to 90%) and pressure drop requirements. 

Gas Turbine Cycle in the Combined Cycle or Cogeneration Mode... 

Fig. 9.2 shows how a simple open circuit gas turbine can be used as a cogeneration plant (a) with a waste heat recuperator (WHR) and (b) with a waste heat boiler (WHB). Since the products from combustion have excess air, supplementary fuel may be burnt downstream of the turbine in the second case. In these illustrations, the overall efficiency of the gas turbine is taken to be quite low ((tjo)cg = ccJf ca 0.25), where the subscript CG indicates that the gas turbine is used as a recuperative cogeneration plant. 

Cogeneration is the production of two useful forms of energy in a single energy conversion process. For example, a gas turbine may produce both rotational energy for an electric generator and heat for a building. 

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