Cogenerating plant

In the United States, laws passed since the late 1970s have encouraged the development of cogeneration plants and independent power plants developed by nonutiHty power producers. Combustion-turbine-based faciHties have become extremely popular among nonutiHty power plant developers. Among the many reasons for selecting this technology is the abiHty to develop compact, modular, combustion-turbine-based power plants within one or two years. 

Replace old boilers - Older refinery boilers can be a significant source of SO, NO, and particulate emissions. It is possible to replace a large number of old boilers with a single new cogeneration plant with emissions controls. 

Fig. 1.6. Cogeneration plant (combined heat and power plant).

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. 

Fig. 9.2. Cogeneration plants (a) with waste heat recuperator (WHR) and (b) with waste heat boiler (WHB).

A CHP system meeting the same power and heat demands (I, A ) is shown in Fig. 9.3b it is implied that this cogeneration plant is perfectly matched, delivering the required (I, An) precisely, using a WHR. 

Gnu)cg is usually limited by the allowable stack temperature Ts. As a fraction of the heat supplied to the cogeneration plant it remains constant in this application. 

Utility plants primarily employ reheat condensing turbines, whereas cogeneration plants and larger process industries that produce their own electrical power tend to employ extraction turbines. Both types of turbine rely on a surface condenser to receive exhaust steam from the LP turbine stage and condense it to liquid for reuse. 

Many cogeneration plants use PF coal, oil, or gas as a fuel, but if blast furnace (BF) gas, sawdust, bagasse, or other low-grade by-product fuels are available, these normally can be used to reduce overall fuel costs. Often a combination of fossil fuel and by-product fuel is employed. 

The hydrazine reaction with oxygen is slow, but with the advent of organic catalysts such as hydroquinone (which speeds up the reaction 50-fold or so), hydrazine is also the product of choice in medium-pressure (say, over 650 to 950 psig) industrial and cogeneration plants. 

Most of the snow is collected from nearby streets. If necessary, snow guns are used to produce additional snow. In April the snow deposit is thermally insulated by 0.2 m of wood chips. Some meltwater is evaporated through the sawdust, which gives an evaporative cooling effect that corresponds to 25% of the extracted cold. The wood chips in Sundsvall are reused several seasons before being burnt in a nearby cogeneration plant. 

Gonzalez, A. Sala, J.M., Flores, I., and Lopez, L.M., 2003, Application of thermoeconomics to the allocation of environmental loads in the life cycle assessment of cogeneration plants, Energy 28 557—574. 

Pohani, B. and Wen, H., Comparison of Circulating Fluidized Bed Combustion and Pulverized Combustion for Coke Fired Cogeneration Plant. Ninth Annual Energy-Sources Technology Conference and Exhibition, New Orleans, LA, February 1986. 

The cycles considered so far in this chapter are power cycles. However, there are applications in which Rankine cycles are used for the combined supply of power and process heat. The heat may be used as process steam for industrial processes, or steam to heat water for central or district heating. This type of combined heat and power plant is called cogeneration. A schematic cogeneration plant is illustrated in Fig. 5.19. A different schematic cogeneration plant is illustrated in Fig. 5.20. 

Referring to the dairy factory design conditions, except that the compressor efficiency is 80% and the mass flow rate of process steam is 4 kg/sec, determine the power required by the compressor, power produced by turbines 1 and 2, rate of heat added to the combustion chamber, net power produced by the open Brayton gas turbine plant, cycle efficiency of the open Brayton gas turbine plant, rate of heat added to the process steam, rate of heat added in the gas burner, and energy utility of the cogeneration plant. 

Demonstration highlight The world s largest SOFC system, a 100 kW Siemens-Westinghouse SOFC installed at a Dutch cogeneration plant in 1998. 

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