Effects of air distribution on pollutant emission and flame characteristics of open buoyant wood combustion.

摘要

Effects of air distribution on emission and flame characteristics of a buoyant wood fire are studied. About 0.275 kg of wood pellets each 1.8x1.8x5 cm in size are burnt in burners constructed from a base around which a mesh wire is attached to hold the fuel. The basic burner (Plain) has a diameter of 15.6 cm and a pellet stack depth of 7cm. The stack is ignited at the bottom and let to burn through a sequence consisting of smoldering, flaming, and glowing phases. Three geometric modifications, each with 10% more bed area, are compared with the Plain burner. In the first (Grate), the extra bed area accommodates sixteen equal-sized holes in the bed. The second modification (Annular) has a 5cm diameter flow-through space in middle to simulate a partially premixed fire. The third configuration (Center Space) is obtained by dosing the center-hole of the Annular burner to leave a central combustion space.;The Center Space fire burns without a smoldering phase, which consumes up to 25% of the fuel burnt in the other fire geometries and constitutes up to 90% and 30% of total PM and CO released from the batch, respectively. Smoldering phase particulate matter (PM) and carbon monoxide (CO) emission factors (g of pollutant/kg of fuel burnt) are in the order: Plain ≈ Grate > Annular. The Annular burner smoldering phase PM (Epm) and CO (Eco) emission factors are several times less than those for the Plain and Grate burners. The flaming phase has a PM emission factor approximately an order of magnitude lower than that for smoldering, a burning rate approximately ten times higher than glowing rate and a CO emission factor more than ten times lower than that for the glowing phase. Whole-batch PM and combined smoldering flaming phase CO emission factors correlate linearly.;Flame length is shortest in the Annular burner. Overall combustion temperatures are in the order: Center Space > Annular Grate > Plain. Maximum burn rate occurs before peak exhaust temperature. Flaming phase axial temperature rises with height to a peak, then decreases in three regions with decreasing temperature gradients.