Comparative study of the MILD combustion characteristics of biomass and brown coal

Abstract

The present paper reports a comparative study on the burning characteristics of pulverized biomass and coal under moderate or intense low-oxygen diluted (MILD) combustion conditions. Two types of biomass fuels—namely, grape marc and almond husk—and a high volatile Victorian brown coal were used as pulverized fuels to burn in a vitiated coflow inside a vertical MILD combustion furnace. The furnace walls, as well as coflow temperature, and local oxygen concentrations were controlled by a secondary swirling burner. Fuels were introduced into the furnace employing CO₂ as a carrier gas with a constant velocity (i.e., bulk jet Reynolds number, Rejet = 20 000) and a fixed range of particle sizes (250–355 μm). Detailed measurements of in-furnace and exhaust temperatures and chemical species (i.e., O₂, CO, CO₂, and NO) are demonstrated and discussed, together with optical images at the top, middle, and bottom sections of the furnace. It was found that MILD combustion was successfully established for all of the fuels investigated without any visible flame inside the furnace. Under similar experimental conditions, biomass volatiles are released earlier leading to a difference of the maximum temperature within the furnace of ∼150 K along the centerline. The largest NO emission was measured to be ∼185 ppmv (db at 3% excess O₂) for grape marc case, because of the higher value of in-fuel N of grape marc and the lowest was ∼125 ppmv (db at 3% excess O₂) for coal case. From the comparison of CO emission, biomass shows more eminent burning characteristics than brown coal under MILD combustion conditions.