Combustion of large solid fuels in cement rotary kilns

The cement industry has a significant interest in replacing fossil fuels with alternative fuels in order to minimize production costs and reduce CO2 emissions. These new alternative fuels are in particular solid fuels such as refuse derived fuel (RDF), tire-derived fuel (TDF), meat and bone meal (MBM), waste wood, sewage sludge, paper and plastics. The alternative fuel share of the total energy varies significantly from region to region, but the general trend is towards increased alternative fuel utilization. Solid alternative fuels typically have physical and chemical properties that differ from traditional solid fossil fuels. This creates a need for new combustion equipment or modification of existing kiln systems, because alternative fuels may influence process stability and product quality. Process stability is mainly influenced by exposing the raw material bed in the rotary kiln to reducing conditions, which increases the tendency for deposit formations in the rotary kiln material inlet end, kiln riser duct and lower cyclone stages. Clinker quality may also be affected by minor compounds from the fuel ashes or from unburned carbon leaving the rotary kiln with the clinker.