On the Burning of Sawdust in a MILD Combustion Furnace

The purpose of this work is to extend the applicability of moderate or intense low oxygen dilution (MILD) combustion to solid biomass fuels. A laboratory-scale furnace fitted with a parallel jet burner was operated in conventional nonpremixed flame mode, and in MILD combustion mode, using either natural gas or pine sawdust particles. Sawdust with particle sizes in the range of 212-355 mu m were injected into the furnace using either air, CO2, or N-2 as a carrier gas. Measurements of in-furnace wall temperatures and exhaust gas emissions of O-2, CO2, NOx, and ash are presented, together with visual observations at the burner exit region. It was found, through detailed comparisons. that MILD combustion was established without air preheat for both gaseous and solid fuels, suggesting that the parallel jet burner system is suitable for MILD combustion. A 3-fold reduction in NOx emissions and an increase in CO were recorded during the transition from conventional to MILD combustion using natural gas. The optimal equivalence ratio (phi) to reduce both CO and NOx emissions, when burning sawdust, was determined to be in the range of phi = 0.71-0.75, with CO2 as the carrier gas, and at phi approximate to 0.75, with N-2 as the carrier gas. Ash content analysis showed that the extent of carbon burnout was low, which is thought to be due to the relatively short furnace residence times.