Cofiring (or co-combustion) involves utilizing existing power generating plants that are fired with fossil fuel (generally coal), and displacing a small proportion of the fossil fuel with renewable biomass fuels. Cofiring of biomass with coal and other fossil fuels can provide a short-term, low-risk, low-cost option for producing renewable energy while simultaneously reducing the use of fossil fuels.

Biomass can typically provide between 3 and 15 percent of the input energy into the power plant. Cofiring of biomass has the major advantage of avoiding the construction of new, dedicated, biomass power plant. An existing power station is modified to accept the biomass resource and utilize it to produce a minor proportion of its electricity.

Cofiring of biomass may be implemented using different types and percentages of biomass in a range of combustion and gasification technologies. Most forms of biomass are suitable for co-firing. These include dedicated energy crops, urban wood waste and agricultural residues such as rice straw and rice husk.

The fuel preparation requirements, issues associated with combustion such as corrosion and fouling of boiler tubes, and characteristics of residual ash dictate the co-firing configuration appropriate for a particular plant and biomass resource. These configurations may be categorized into direct, indirect and parallel firing.

Direct Co-Firing

This is the most common form of biomass co-firing involving direct co-firing of the biomass fuel and the primary fuel (generally coal) in the combustion chamber of the boiler. The cheapest and simplest form of direct co-firing for a pulverized coal power plant is through mixing prepared biomass and coal in the coal yard or on the coal conveyor belt, before the combined fuel is fed into the power station boiler.

Indirect Co-firing

If the biomass fuel has different attributes to the normal fossil fuel, then it may be prudent to partially segregate the biomass fuel rather than risk damage to the complete station.

For indirect co-firing, the ash of the biomass resource and the main fuel are kept separate from one another as the thermal conversion is partially carried out in separate processing plants. As indirect co-firing requires a separate biomass energy conversion plant, it has a relatively high investment cost compared with direct co-firing.

Parallel Firing

For parallel firing, totally separate combustion plants and boilers are used for the biomass resource and the coal- fired power plants. The steam produced is fed into the main power plant where it is upgraded to higher temperatures and pressures, to give resulting higher energy conversion efficiencies. This allows the use of problematic fuels with high alkali and chlorine contents (such as wheat straw) and the separation of the ashes.