Combustion of Wood Processing Residues in a Circulating Fluidized Bed

The combustion of wood processing residues was tested in the 0.8 MWth CANMET Circulating Fluidized Bed Combustor (CFBC) pilot plant. The specific residues tested were three different types of coniferous tree bark (i.e. from different locations to represent a range of possible fuels and fuel properties). Combustion conditions may be summarized as follows: fuel moisture levels 42–60%, fluidizing velocity 2.1–2.4 m/s; bed temperature 785–910 °C; maximum freeboard temperature 980–1070 °C and excess air levels 20–75%. The CFBC unit was able to burn the high moisture level fuels with no detrimental effect. In all trials the residues burned very well, with combustion efficiency greater than 99% based on overhead carbon loss. Emissions measurements were made of the following pollutant species CO, NO_x, N₂O, SO₂, and dioxins and furans. The emissions levels were: 100–130 ppm NO_x; <1 ppm N₂O; 5–20 ppm SO₂ and 400–1800 ppm CO. These emission levels are well below pollution guidelines for all major pollutants except CO. This however can be traced to the non-homogeneous nature of the coarse feed in the pilot plant. The problem can reasonably be addressed in a full-scale unit by a more stable feed system. A preliminary economic analysis of a new 25 MW FBC power plant firing these fuels was performed. Conservative inputs give a cost of 6 cents/kWh for the electricity produced and a economic wood haulage radius of 70 km.