Thermal plant electricity generation fuels

Thermal power plant is more commonly associated with very large central power stations. The capital cost for thermal power plant, in terms of cost per installed kilowatt of electrical generating capacity, rises sharply for outputs of less than some 15 MW. It is for this reason that thermal power plant is not usually considered for industrial applications unless it is the combined cycle or combined heat and power modes. However, for cases where the fuel is of very low cost (for example, a waste product from a process such as wood waste), then the thermal power plant, depending on output, can offer an excellent choice, as its higher initial capital cost can be offset against lower running costs. This section introduces the thermal power cycle for electrical generation only. 

Stationary power is the most mature application for fuel cells. Stationary fuel cell units are used for backup power, power for remote locations, stand-alone power plants for towns and cities, distributed generation for buildings, and cogeneration where excess thermal energy from electricity generation is used for heat. 

A conventional power plant fired by fossil fuels converts the chemical energy of combustion of the fuel first to heat, which is used to raise steam, which in turn is used to drive the turbines that turn the electrical generators. Quite apart from the mechanical and thermal energy losses in this sequence, the maximum thermodynamic efficiency e for any heat engine is limited by the relative temperatures of the heat source (That) and heat sink (Tcoid) ... 

Nuclear reactors have rather elaborate cooling systems that absorb the heat given off by the nuclear reaction and transfer it outside the reactor core, where it is used to produce enough steam to drive an electric generator. In this respect, a nuclear power plant is similar to a conventional power plant that bums fossil fuel. In both cases, large quantities of cooling water are needed to condense steam for reuse. Thus, most nuclear power plants are built near a river or a lake. Unfortunately this method of cooling causes thermal pollution (see Section 12.4). 

Here 17 is the thermal efficiency of the power plant (ratio of electricity generated to heat produced), U is the total number of metric tons of uranium in the reactor, n is the number of fuel zones, and U/n is the mass of uranium in one lot of fuel. With the dependence of bumup on enrichment and batch fraction given in Fig. 3.11, this equation permits evaluation of the electric energy that can be generated by a fuel batch of enrichment e w/o U, making up/ (= 1/n) fraction of the reactor. Figure 3.12 shows this relationship for the 1060-MWe PWR, with a thermal efficiency n = 0.326. 

Heat production and electricity generation are the most important uses worldwide for biomass fuel so far. Direct combustion devices are widely distributed with thermal capacities ranging from a few kW in household stoves up to heating plants with several tens of MW. The conversion efficiencies vary from 8 to 18% for simple stoves up to approximately 90% and above for modem heating units with high-end technology. Electricity production has been based mainly on the conventional steam cycle with efficiencies around 30% and capacities of several 100 kW and above. 

Figure 18.2 shows the current and projected electricity generation capacity by fuel type. Table 18.1 lists typical power plant parameters for facilities using different fuel types. Current power generation is dominated by thermal processes that use the action of heat generated by either renewable or nonrenewable fuels on a working fluid to drive a turbine. This includes fossil fuel combustion, nuclear fission, biomass combustion, geothermal extraction, or solar-thermal collection. 

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