Co-firing of oil palm residues in a fuel staged fluidized-bed combustor using mixtures of alumina and silica sand as the bed material

Abstract

Oil palm kernel shell (base fuel) and empty fruit bunch (secondary fuel) with elevated/high potassium contents were co-fired in a conical fluidized-bed combustor with bottom air injection using fuel staging to reduce NO emission. During the co-firing tests at a fixed heat input into the combustor and variable excess air, alumina sand (AS), mixed with silica sand (SS) in different proportions, was used as the bed material to inhibit bed agglomeration. The study revealed that the effects of energy fraction of the secondary fuel in the total heat input (EF2) and excess air (EA) on the major gaseous emissions and combustion efficiency of the conical FBC were substantial. Under optimal operating conditions (EF2 ≈ 0.15 and EA ≈ 55%), the combustor with the selected bed material can be co-fired at minimal “external” costs and reduced NO emission, as compared to burning the base fuel alone. By using AS/SS bed mixtures with a prevailing proportion of rather expensive AS, bed agglomeration can be prevented for a relatively long operating time. However, the AS/SS beds exhibited time-related alterations in physiochemical characteristics. With a higher SS content in the AS/SS mixture, the NO emission somewhat increased, mainly because of the lowered catalytic activity of the bed for the NO–CO reaction, whereas the bed material showed a diminished capability to withstand bed agglomeration.