Theoretical and experimental studies on combustion of alternative fuels in cement kilns
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In this thesis, the utilization of alternative fuels for NOx reduction by means of reburning and advanced reburning is sonsidered. Laboratory experiments, full-scale experiments and computational fluid dynamic (CFD) simulations are the basis of the thesis. The goal of the work was to characterize alternative fuels used in cement kilns, with focus on the processes taking place in the precalciner of the cement kiln. To facilitate testing under controlled process conditons, a lab-scale circulating fluidized bed combustion (CFBC) reactor was designed and constructed. A comprehensive study on the fluidization regime in CFBC reactors and precalciners was required to ensure and verify that the operational regime in the CFBC reactor was simular to the regime in a precalciner. Different alternative fuels, such as refuse derived fuel, animal meal and solid hazardous waste, were tested in the CFBC reactor, which proved well suited for characterization of alternative fuels and investigations of NOx reduction, even though the operation of a CFBC reactor is quite complex and gives a certain variation in stability. Experiments with and without circulating mass n the CFBC reactor demonstraded the importance of executing the laboratory combustion experiments in an environment similar to that in the full-scale process, i.e. in the precalciner. Animal meal is beleived to follow the reduction route of selective non-catalytic reduction or advanced reburning and to have a special capability of reducing NOx during increased NOx concentrations at the reactor inlet. The increased CO emissions during advanced reburning with animal meal are most likely to be due to the competition for the OH radical during oxidation of CO and of NH3. Furthermore, it was shown, for all fuels, that an increased concentration of NOx at the reactor inlet increases the ratio of NOx at the exit and NOx supplied. Full-scale experiments were executed at Norcem's kiln 6 in Brevik, using solid hazardous waste in combination with urea pellets, as well as with animal meal, to obtain advanced reburning. The full-scale experiments showed that advanced reburning. The full-scale experiments showed that advanced reburning and reburning with animal meal are well suited for NOx reduction. NOx reductions of up to 73 and 63% during advanced reburning and reburning (with animal meal), respectively, were obtained. For the CFD simulations a grid of 240,000 cells was generated using GAMBIT. Euler-Euler granular multiphase simulations with FLUENT 4 were executed to verify the assumption of dilute flow and the importance of particle-particle collisons. Euler-Langrangian simulations were executed in FLUENT 6 to investigate the combusting particles, e.g. particles-trjectory, retension time and species concentrations. The CFD simulatons revealed important information about the flow regime e.g. low degree of mixing between the kiln side and tertiary side of the calciner, which is important as far as burnout is concerned.