Fossil power and CHP 

The primary process of fossil power and CHP plants is combustion. Solid fuel (coal) is milled and subsequently burned. The heat is transferred to a heat medium, which turns into steam.  The steam moves a steam turbine, which is connected with an electric generator, converting this energy into electricity. A CHP plant additionally uses the steam as heat source, for example, for district heating. Also the heat from the condensation can be used, but is only available in lower temperatures. 

The main retrofit option for fossil power and CHP plants is changing the feedstock. One option is to completely replace fossil fuels with solid, liquid or gaseous biofuels. Another option is to co-fire solid biofuels together with the current fossil fuels, and this can take place in a number of ways as depicted below. Parallel co-firing uses two separate combustion plants and boilers, with a shared steam turbine as only connection. Indirect or gasification co-firing is gasifying solid biofuel in a separated gasifier, before the product gas is combusted in the steam boiler of the coal power plant. Direct co-firing enables different levels of integration. Solid biofuel can be burned in separate burners, milled in separate mills, or even co-milled. Thermally treated solid biofuel is easiest to integrate with the existing equipment. Such treatments are e.g. torrefaction, steam explosion or hydrothermal carbonization of biomass. 

Source: BIOFIT Project
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