Co-flred combustion involves using biomass to supplement the primary feed, typically coal or natural gas. Two methods of co-firing biomass exist. The first method, direct co­firing, involves biomass being co-fed directly into the boilers with the coal or natural gas. Direct co-fire technology allows up to 20% of the feed to be supplied by biomass without significant changes to the facility [13]. The second, indirect co-firing, involves first gasifying biomass then co-firing the resulting gases in a combustion system. Co-firing is an immediate, low-cost option for efficiently and cleanly converting biomass to electricity by adding biomass as a partial substitute fuel in high-efficiency coal boilers. There is little or no loss in total boiler efficiency after adjusting combustion output for the new mixture. Co-firing with biomass will also help reduce greenhouse gas emissions, primarily SO2, NOx and CO2.

Opportunities for biomass co-firing are great because large-scale coal-powered boilers represent 310 GW of generating capacity [1]. While direct co-firing usually requires less modification to a generation system, it restricts the amount and type of biomass that can be used. Indirect co-firing through gasification allows for removing alkali metals and chlorine, which can cause fouling, slagging and corrosion in a coal or natural gas boiler system, from the feedstock. Indirect co-firing also reduces the effects of feedstock variability. Biomass feedstocks, by their nature, have a high degree of variability in terms of moisture content, heating value and ash profile. By using gasification to convert these fuels to a gaseous form, much of this variability can be reduced or eliminated. This has the effect of making operation of the boiler more stable [14]. There are a lot of issues yet to be completely solved with co-firing biomass with coal; however, biomass co-firing has been a proven opportunity for coal facilities for more than a decade [13].