FUEL-STAGED CO-COMBUSTION OF HIGH-ALKALI OIL PALM RESIDUES IN A FLUIDIZED-BED COMBUSTOR USING MIXTURES OF ALUMINA AND SILICA SAND TO PREVENT BED AGGLOMERATION

Oil palm kernel shell (primary fuel) and empty fruit bunch (secondary fuel) with elevated potassium contents were co-combusted in a fluidized-bed combustor using fuel staging with conventional (bottom) air supply. During experiments for variable excess air at fixed fuel feeding, alumina sand (AS) mixed with silica sand (SS) in different proportions was used as bed material to inhibit bed agglomeration in the combustor. The study revealed substantial effects of the empty fruit bunch energy fraction (EF2) and excess air (EA), at a quite weak influence of the bed material type, on combustion and emission performance of the reactor. Under optimal operating conditions (EF2 = 0.15 and EA = 50%), the combustor can be operated at minimal "external" (emission) costs, while reducing the NO emission by about 35% compared to firing pure palm kernel shell. By using AS/SS beds with a sufficient/high proportion of AS, bed agglomeration can be prevented for a relatively long time. However, the AS/SS beds show noticeable time-domain changes in their physical appearance and chemical composition. With increasing the SS content in a AS/SS mixture, the bed diminishes its capability to withstand bed agglomeration. When using a (25% AS + 75% SS) mixture as bed material, bed agglomeration may occur at the reactor bottom within 17 h.