Wood Waste Boilers
According to a recent Ecology survey there are currently 85 wood-waste boilers in Washington State. These boilers combust about 3.3 million tons of wood annually, which accounts for 64% of all the fuel combusted in wood-waste boilers. The other fuels burned include refuse-derived fuel, old corrugated cardboard, tire-derived fuel, pulp mill sludges, natural gas, coal, and oil. Of those that combust fuels other than wood (therefore, excluding those that are burn wood-waste only) about one-half of the fuel combusted are other fuels. (DeMay, p. 2-7)
Description of Process
Waste wood in the form of chips, sawdust, shavings, sander dust, hogged wood or bark is fired in a boiler to obtain heat. Depending on the boiler type, the fuel may also include other waste materials that get entrained in the fuel conveyor system such as soil, rocks, rags, and metal parts and pieces. The operator may also burn oil along with the wood waste, especially when the wood is wet, to supplement the heat content of the fuel.
The predominant species of wood burned in the western part of the state are Douglas fir, spruce, western red cedar, cottonwood and hemlock. In eastern Washington the mix of species is markedly different: ponderosa pine, Douglas fir, white fir and a small (1-5%) quantity of western red cedar.
The moisture content of the fuel is typically between 40-50% by weight. Facilities which store the fuel outside have a problem with fuel moisture in the winter, in which moisture content may be up to 60-70%. Wood that has been transported or stored in salty or brackish water creates salt-laden wood. Combustion of salt-laden wood increases fine particulate loading and plume opacity as well as air toxics such as dioxin. About 24% of the wood waste boilers in Washington combust some saltladen wood. (DeMay, p. 7)
