1. 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 ClausiuseRankine 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