IntroductionIn a combined heat and

Introduction
In a combined heat and power plant (CHP plant or co-generation plant), heat and power are produced simultaneously in the same power plant process. The greatest benefit of a CHP production is a high total efficiency: up to 90% or more of the fuel input is converted to useful energy (heat and power). Due to this an average of 25% less fuel is needed compared to a situation where heat and electricity are produced in separate plants [1]. Heat and power are produced in CHP plants both in industrial and municipal power plants. Industrial CHP plants produce typically middle and back pressure steam for mill processes and municipal plants produce district heat for heating of buildings. In several plants, some fuel is combusted in a boiler and heat is transferred through the walls to the Clausius–Rankine steam cycle where superheated steam expands in a back pressure steam turbine producing electricity. After the turbine the latent heat of steam is recovered by heating some stream (e.g. district heat water). The thermal performance of the CHP plant is usually evaluated by calculating the power to heat ratio which describes how much power/electricity is produced per amount of heat energy produced. For industrial CHP plants the power to heat ratio is typically between 0.2 and 0.3 and for municipal CHP plants from 0.4 to 0.6. CHP plants may also undergo a GTCC (gas turbine combined cycle) process, in which case the power to heat ratio may even be over 1.