"Options for Biomass Fuels Utilization in Power Plants"

By Don Koza, PE, Principal Engineer and Shuang Ma, PE , Boiler Technical Specialist, Bechtel Power Corp. Specialist, Bechtel Power Corp.

Excerpts from Power Engineering magazine (May 2011)

The term “biomass” refers to materials derived from growing or recently grown plant matter and can be generally classified using the following categories:

• Wood residues (forestry, pulp and paper)
• Agricultural residues (bagasse, stover, rice hulls)
• Dedicated energy crops (sawgrass, bamboo, eucalyptus)
• Process wastes (furniture manufacturing, construction and demolition).

Biomass does not store well and some types begin to badly decay within a matter of a few weeks.
 
Heating value of raw biomass is typically half that of the bituminous coal.
 
The moisture content of biomass is usually greater than 20 percent and can contain as much as 60 percent.
 
Biomass Combustion Technologies

In the power generation industry, the most common biomass combustion technologies fall into three categories: 1) stoker/grate firing, 2) fluidized bed combustion (FBC)—including circulating fluidized bed (CFB) and bubbling fluidized bed (BFB) boilers—and 3) suspension firing or pulverized fuel (PF) firing.

Stoker/Grate Firing: Grate-type boilers are likely the oldest commercial combustion technology still in use today.

Fluidized Bed Technology: Fluidized bed technology is the most flexible for firing a wide range of fuels and fuels that are variable, including biomass.  Circulating fluidized bed (CFB) boilers technology is generally applied to larger capacity units and more broad range of fuels, with the largest CFB to date at around 460 MWe. One of the largest CFB firing 100 percent biomass is 190 MWe and is under construction.
 
Suspension/Pulverized Fuel Combustion: In suspension fired boilers the pulverized fuel into fine particles burn while suspended in air as opposed to on a grate or in a fluidized bed.
 
Biomass Emissions

One primary driver for firing biomass is CO2 reduction where it is considered a greenhouse-gas neutral fuel. Firing biomass also reduces other emission such as NOX and SO2.

Fuel Handling and Supply

Where biomass is stored in an enclosed structure for long periods of time, significant off gassing of CO, CO₂ and CH4 will occur and will need to be vented for personnel protection and fire prevention.

Dust Hazard Considerations: Biomass can be very dusty when dry. This dust can be an explosion hazard every bit as bad as Powder River Basin (PRB) coal, if not worse. If water spray is used for dust control, it will raise the temperature of the stored biomass and may lead to spontaneous combustion.
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The Center is examing the feasibility of taking conversion of wood to gas for burning in a turbine to produce electricity from bench scale to commercial scale.