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

The effect of HCI on NO x and SO 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 contain varying amounts of chlorine, nitrogen and sulphur. During combustion, the chlorine is mainly released as HCl, the nitrogen as N 2 or NO x and the sulphur as SO 2 . While there has been some work conducted on the interactions between homogeneous reactions involving nitrogen and sulphur, 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 5 H 5 N), 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 between 0 and 2000 ppm. Additionally. 0 to 6000 ppm HCI and 0 to 2300 ppm SO 2 were added to the fluidizing air. The bed material was mainly quartz sand to which batches of limestone or CaCl 2 were added while the concentrations of CO, CO 2 , SO 2 and NO were measured in the freeboard. The fluidized bed studies showed that the presence of CaO, during combustion in the presence of HCI (either added as a gas or as CaCl 2 ), tends to moderate the effect of the HCl (i.e. CO emissions did not increase as dramatically and NO x emissions were not decreased as much as when HCI was present without any limestone). Some of the HCl is consumed by the CaO. Additionally, the partial consumption of CaO by HCI tended to reduce the catalytic effect that CaO has on the formation of NO x . The consumption of HCl by CaO, however, tends to have a buffering effect, since, if the gas phase concentration of HCI is reduced, CaCl 2 is converted back to CaO, releasing the HCI.