A cogeneration system is the sequential or coincident generation of multiple forms of useful energy (usually mechanical and thermal) in a single, integrated system. It consist of a number of individual components – steam generator, heat recovery, Condenser and electrical inter connection. The steam generator are capable of burning a variety of fuels, including bagasse, natural gas, coal, oil, bio mass fuels to produce mechanical energy.
Steam turbines are one of the most resourceful and oldest prime mover technologies still in general production. In the steam cycle, DM water is first pumped to medium to high pressure and then heated to the boiling temperature corresponding to the pressure, boiled and then most frequently superheated. A multistage turbine expands the pressurized steam to lower pressure and the steam is then exhausted either to a condenser at vacuum conditions or into an intermediate temperature steam distribution system that delivers the steam to the process application. The condensate from the condenser or from the steam utilization system returns to the feed water tank for continuation of the cycle.
The two types of steam turbines most widely used are the backpressure and the extraction -condensing types. The choice between backpressure turbine and extraction - condensing turbine depends mainly on the quantities of power and heat, quality of heat, and economic factors. The extraction points of steam from the turbine could be more than one, depending on the temperature levels of heat required by the processes
In such a system, steam for the thermal load is obtained by extraction from one or more intermediate stages at the appropriate pressure and temperature. The remaining steam is exhausted to the pressure of the condenser, which can be as low as 0.05 bar with a corresponding condensing temperature. It is rather improbable that such low temperature heat finds useful applications. Consequently, it is rejected to the environment. In comparison to the back - pressure system, the condensing type turbine has a higher capital cost and, in general, a lower total efficiency. However, to a certain extent, it can control the electrical power independent of the thermal load by proper regulation of the steam flow rate through the turbine.
A back pressure steam turbine is the simplest configuration. Steam exits the turbine at a pressure higher or at least equal to the atmospheric pressure, which depends on the needs of the thermal load. This is why the term back- pressure is used. It is also possible to extract steam from intermediate stages of the steam turbine, at a pressure and temperature appropriate for the thermal load. After the exit from the turbine, the steam is fed to the load, where it releases heat and is condensed. The condensate returns to the system with a flow rate which can be lower than the steam flow rate, if steam mass is used in the process or if there are losses along the piping. Make- up water retains the mass balance.