Summary of Cofiring Guidelines
We believe the following guidelines are generally valid, but there are specific instances where each of them is not valid. The discussions in the literature and web site provide the background to determine when such instances arise.
Fuel should generally be prepared and transported using equipment designed specifically for that purpose rather than mixed with coal and simultaneously processed.
Fuels with low nitrogen contents and high volatile yields (most woody fuels) generally are more effective in reducing NOx emissions than high-nitrogen fuels. Most biomass fuels will reduce SOx emissions.
Fuels with low ash contents and, especially, minimal alkali or chlorine in the ash should present minimal ash deposition problems and represent the fuels of choice for cofiring. Fuels with high alkali and chlorine contents can substantially increase deposition problems.
Complete conversion of the carbon in biomass fuels requires that the fuel be processed to small particle sizes and be moderately dry. Particles generally need to be less than 3 mm (1/8 inch) to completely combust. Fuels that pass through a quarter-inch screen are generally dominated by particles less than 1/8 inch. High moisture contents (greater than 40%) and high particle density both significantly increase the time required to completely combust the particles and may increase fly ash carbon content.
Fuel chlorine and alkali concentrations should be limited to less than one fifth of the total fuel sulfur on a molar basis to avoid corrosion problems. This limit should be applied to the fuel composition as fired through any single burner except in the rare case of rapid and complete mixing of in the furnace.
Fly ash from wood-coal cofiring generally does not significantly degrade fly ash performance as a concrete additive. However, strict interpretation of current standards for inclusion of fly ash in concrete preclude mixed ashes, including biomass-coal ashes. Fly ash from many herbaceous fuels may negatively impact concrete properties.
SCR systems may experience significant deactivation when exposed to alkali or alkaline earth constituents, such as are common in some biomass fuels and low-rank coals. Means of avoiding or managing such poisoning remain under investigation.