Combined Heat and Power Using Biomass– Overview
There are numerous Combined Heat and Power plants throughout Europe and the USA using biomass as a solid or gaseous fuel. Solid biofuels such as chipped or pelleted wood are used to fire the boilers, raising superheated steam to power turbines that drive the power generators.
Wood can also be gasified; the resultant bio-gas used to run gas engines or gas turbines is also used to drive the electricity generators.
We shall examine the operation of a thermal power station which we will presume uses biomass as a boiler fuel to produce superheated steam, in a combined heat and power system.
The superheated steam passes through three stages in the steam turbine; entering the High Pressure (HP) turbine before passing into the Intermediate Pressure (IP) stage, from where it is fed into the Low Pressure (LP) turbine, exiting from here as steam and condensate into a vacuum condenser.
Fresh water is circulated around the condenser which further condenses the steam and condensate before it is pumped into the boiler feed system.
The condenser cooling water would then normally be pumped to a cooling tower from where it is cooled and circulated back through the condenser in a closed circuit.
However, in a CHP plant instead of using a conventional cooling tower to cool the condenser circulating water, it is pumped through a heat exchanger.
The medium used for community heating is also fed through the heat exchanger; extracting heat from the condenser cooling water and raising its temperature to between 80 and 130 °C, before being pumped to the community heating system.
At the same time this heat exchange reduces the temperature of the condenser cooling water before it is pumped back around the steam condenser in a closed circuit.
There can be additional boilers in the community heating system to supplement the hot water output from the power plant at peak times, such as during the winter months.