Co-firing should not be confused with the use of multiple fuels in boilers designed to burn multiple fuels. Multiple fuel boilers, such as those commonly installed and used as “power boilers” at pulp mills, are designed to burn a wide range of fuels such as coal, fuel oil, natural gas, whole tree chips, mill residues, bark, and paper sludge. The difference between a multiple fuel boiler and co-firing is that co-firing is done in a boiler originally designed to burn only (or primarily) a specific type of coal.
Despite the success with co-firing biomass and coal at utility power plants, co-firing is rarely practiced at industrial, institutional, and other non-utility coal-fired boilers. Although these boilers tend to be smaller than utility-scale boilers, they may be more numerous and have a geographic distribution that results in significant opportunities for co-firing. Similar to utility boilers, co-firing at non-utility boilers can:

• Reduce SOx emissions. Since woodfuel contains only trace amounts of sulfur, co-firing reduces SOx emissions by an amount which is proportional to the co-firing rate.
• Reduce emissions of the greenhouse gas CO2 and help address global warming. Since wood is considered by many to be “CO2 neutral,” co-firing woodfuels also reduces CO2 emissions by an amount proportional to the co-firing rate. When woodfuels are grown on a long-term sustainable basis, the amount of CO2 emitted during combustion is equivalent to that absorbed during growth. This is very different than coal, which is considered to contribute CO2 on a long-term basis since the geological process from which coal is produced takes place over millions of years.
• Create markets for locally-grown, indigenous, and renewable fuels.
• Diversify the types of fuel used and the companies used to supply fuel.
• Provide additional markets for fuel suppliers.
• Help industries and other facilities meet or exceed goals for environmental stewardship.