Biopower is the use of biomass resources to produce, either singly or in combination, electricity, heat/steam, and cooling. A number of technologies can be used to produce biopower including direct fire applications, gasification, and pyrolysis.
Direct fire applications include the dedicated combustion of biomass resources in utilities, the combustion of biomass resources produced as a by-product of the manufacturing process in industrial settings, and co-firing which involves the partial substitution of fossil fuels with biomass resources. Pile burners (consisting of a two-stage combustion chamber with a lower, primary combustion area and an upper, secondary combustion area) and stoker combustors (which utilize a moving grate that allows continuous ash collection and operation) are the most commonly used direct fire technologies.
Co-firing generally involves burning the fossil fuel (usually coal) and the biomass together (more common in the U.S.), or combusting the biomass and the fossil fuel separately and then integrating either the heat, steam, or product gases from the biomass operation into the fossil fuel operation (more common in Europe). Stoker and fluidized bed units are generally more flexible in the handling of fuels other than coal and typically require less modification to permit the use of biomass.
Pulverized coal and cyclone boilers require smaller, more uniform sized particles, requiring the biomass particles to be reduced in size. Pulverized coal units are the most prevalent technology globally, and thus offer the greatest opportunity to expand the use of co-firing due to the large number of facilities.
Gasification involves the thermal decomposition of biomass resources in an oxygen starved environment to produce combustible gases (syngas), tars, liquids and charcoal and is intermediate to combustion (thermal degradation with excess oxygen) and pyrolysis (thermal degradation in the absence of oxygen). Major gasifier types include updraft, downdraft, and fluidized bed reactors. Gasifiers may be sized from those small enough to heat a home, to those large enough to drive major manufacturing or processing applications. Updraft reactors are preferred for applications rated below 1 MW thermal power while the fluidized bed gasifiers are appropriate for ranges above this. Most commercial gasifiers currently in operation are used to produce heat.
Pyrolysis is the thermal decomposition of organic fuels such as biomass resources into volatile compounds (gases and bio-oil) and solids (chars) in the absence of oxygen and usually water. Pyrolysis types (slow, flash, or fast) are differentiated by the temperature, pressure, and processing time of the fuel which determine the types of reactions that occur and the product mix. Current interest focuses on fast pyrolysis because the products formed are similar to fossil fuels currently used, particularly bio-oil which can be used for heating and to produce transportation fuels and organic chemicals.
Historically, wood industry wastes have been the major sources of biomass feedstocks for biopower generation, but recently, other biomass resources have been used including agricultural processing wastes and urban wastes. Dedicated herbaceous crops and short rotation wood crops are being evaluated. Technical issues associated with the use of biomass for biopower applications include adapting the biomass feed system and the burn technology to the biomass feedstock(s). Biomass feedstocks typically have lower energy and weight densities than fossil fuels currently used, and the system may need to be altered to accommodate these characteristics and maintain the efficiency of the unit.