The interactions between SO2, NOx, HCl and Ca in a bench-scale fluidized combustor

The effect of HCl on NOx and SO2 emissions from fluidized beds containing C a-based materials was investigated using an electrically healed, laboratory -scale, bubbling fluidized bed. Fluidized beds are used to burn fuels such as coal, biomass and plastics, which contain varying amounts of chlorine, n itrogen and sulphur. During combustion, the chlorine is mainly released as HCl, the nitrogen as N-2 or NOx and the sulphur as SO2. While there has bee n some work conducted on the interactions between homogeneous reactions inv olving nitrogen and sulphur, very little has been done to try and understan d the details of what occurs in the presence of limestone and chlorinated c ompounds. Hence it was the objective of this work to conduct experiments wh ich would yield a better understanding of the effect that HCl has on combus tion systems in the presence of limestone. To accomplish this, pyridine (C5 H5N), a model N-containing compound, was added to a hot bed fluidized by a mixture of N-2 and O-2. The N-content of the fluidizing gas was varied betw een 0 and 2000 ppm. Additionally, 0 to 6000 ppm HCl and 0 to 2300 ppm SO2 w ere added to the fluidizing air. The bed material was mainly quartz sand to which batches of limestone or CaCl2 were added while the concentrations of CO, CO2, SO2 and NOx were measured in the freeboard. The fluidized bed stu dies showed that the presence of CaO, during combustion in the presence of HCl (either added as a gas or as CaCl2), tends to moderate the effect of th e HCl (i.e. CO emissions did not increase as dramatically and NOx emissions were not decreased as much as when HCl was present without any limestone). Some of the HCl is consumed by the CaO. Additionally, the partial consumpt ion of CaO by HCl tended to reduce the catalytic effect that CaO has on the formation of NOx. The consumption of HCl by CaO, however, tends to have a buffering effect, since, if the gas phase concentration of HCl is reduced, CaCl2 is converted back to CaO, releasing the HCl.