Combustion of residual biosolids from a high solids anaerobic digestion/aerobic composting process

A humus consisting of aerobically stabilized anaerobic digester effluent was burned in two laboratory test facilities to evaluate possible problems in using the humus as fuel in boilers. An atmospheric circulating fluidized-bed combustor (FBC) was used to assess possible bed agglomeration when using alumina-silicate bed media. A multi-fuel capable entrained flow combustor (MFC) with electrically heated walls simulating a boiler was used with air-cooled tubular probes to evaluate potential ash deposition on heat exchangers. Low heating value of the humus led to poor temperature control in the FBC when burned alone. Tests of the humus blended 50% by weight with wood were also carried out in the FBC. No evidence of bed agglomeration was found in the FBC with either humus alone or blended with wood at temperatures up to 900°C; the maximum tested. Bed-drain rate was nearly equal to fuel-ash feed rate to maintain constant bed pressure drop due to the high sand fraction in the humus. Deposits collected on the probes in the MFC experiment were enriched in alkali-sulfate, as were the fine fractions of fly-ash samples. Deposition rates were not established due to particle-induced abrasion of deposits from the heavy sand fraction of the fuel, but at least moderate alkali-sulfate deposition can be anticipated if humus is burned in commercial units.