A study of the combined S, N, HCl and Ca chemistry in a bench-scale fluidized combustor

The effect of HCl on N{sub x} and SO{sub 2} emissions from fluidized beds containing Ca-based materials was investigated using an electrically heated, laboratory-scale, bubbling fluidized bed. Fluidized beds are used to burn fuels such as coal, biomass and plastics, which can contain varying amounts of chlorine, nitrogen and sulfur. During combustion, the chlorine is mainly released as HCl, the nitrogen as N{sub 2} or NO{sub x} and the sulfur as SO{sub 2}. While there has been some work conducted on the interactions between homogeneous reactions involving nitrogen and sulfur, very little has been done to try and understand the details of what occurs in the presence of limestone and chlorinated compounds. Hence it was the objective of this work to conduct experiments which would yield a better understanding of the effect that HCl has on combustion systems in the presence of limestone. To accomplish this pyridine (C{sub 5}H{sub 5}N), a model N-containing compound, was added to a fluidized bed. The simulated N-content of the fuel was varied between 0 and 2,000 ppm. Additionally, 0 to 6,000 ppm HCl and 0 to 2,300 ppm SO{sub 2} were added to the fluidizing gas. The bed material was mainly quartz sand tomore » which batches of limestone, CaO or CaCl{sub 2} were added while the concentrations of CO, CO{sub 2}, SO{sub 2} and NO{sub x} were measured in the freeboard. The fluidized bed studies showed that the presence of CaO, during combustion in the presence of HCl (either added as a gas or as CaCl{sub 2}), tends to moderate the effect of the HCl (i.e., CO emissions did not increase as dramatically and NO{sub x} 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 consumption of CaO by HCl tended to reduce the catalytic effect that CaO has on the formation of NO{sub x}. The consumption of HCl by CaO, however, tends to be a buffering effect, since, if the gas phase concentration of HCl is reduced, CaCl{sub 2} is converted back to CaO, releasing the HCl. Sulfur capture is improved by the presence of HCl.« less