(i) Steam turbine co-generation system:
The two types of steam turbines most widely used are the backpressure and the extraction. Another variation of the steam turbine topping cycle cogeneration system is the extraction-back pressure turbine that can be employed where the enduser needs thermal energy at two different temperature levels. The full-condensing steam turbines are usually incorporated at sites where heat rejected from the process is used to generate power. The specific advantage of using steam turbines in comparison with the other prime movers is the option for using a wide variety of conventional as well as alternative fuels such as coal, natural gas, fuel oil and biomass. The power generation efficiency of the demand for electricity is greater than one MW up to a few hundreds of MW. Due to the system inertia, their operation is not suitable for sites with intermittent energy demand.
ii) Gas turbine co-generation system
Gas turbine cogeneration systems can produce all or a part of the energy requirement of the site. The energy released at released at height temperature in the exhaust stack be recovered for various heating cooling applications. The typical range of gas turbines varies from a fraction a MW to around 100 MW. Gas turbine cogeneration has probably experienced the most rapid development in the recent years due to the greater availability of natural gas, rapid progress in the technology, significant reduction in installation costs, & better environment performance. Gas turbine has a low short start up time and provides the flexibility of intermittent operation. Though it has a low heat to power conversion efficiency more heat can be recovered at higher temperatures. If the heat output is less than that required by the user it is possible to have supplementary natural gas firing by mixing additional fuel to the oxygen rich exhaust gas to boost the thermal output more efficiently . Steam generated from the exhaust gas of the gas turbine is passed through a backpressure of extraction condensing steam turbine to generate additional power. The exhaust or the extracted steam from the steam turbine provides the required thermal energy.