Catalytic reduction of nitrous oxide by hydrocarbons over a Fe-zeolite monolith under fluidised bed combustion conditions

Abstract

Fluidised bed combustion (FBC) is an important source of nitrous oxide emissions. The catalytic reduction of nitrous oxide with different hydrocarbons was carried out over the catalytic system Fe-ZSM-5 supported on ceramic monoliths, under conditions representative of the off-gases of FBC. The selective catalytic reduction (SCR) of nitrous oxide by propane and propene is activated in the presence of oxygen, whereas its conversion is not affected by the increase of oxygen concentration in the feed gas. The SCR of nitrous oxide by methane is inhibited in the presence of oxygen and the increment of oxygen content in the feed gas leads to a decrease in the catalyst activity and selectivity. Water strongly inhibits the nitrous oxide catalytic reduction independently of the type of hydrocarbon. This effect is more pronounced as water concentration in the process gas increases. Propane resulted to be the most appropriate hydrocarbon of those easily available for the SCR of nitrous oxide over the catalytic system. Methane is little active, whereas propane is slightly more active and selective than propene and results in lower hydrocarbon residual emissions, at oxygen and water levels similar to those found in real combustion off-gases. Furthermore, with respect to methane, propane allows lower reaction temperatures in addition to higher reduction efficiency. Moreover, the emission of unreacted hydrocarbon is higher in the case of methane, whose emission is unwanted, because methane is a greenhouse gas itself.