NO Emissions from Anthracite Oxy-Firing in a Fluidized-Bed Combustor: Effect of the Temperature, Limestone, and O2

Along with SO2, NOx emissions are considered the main pollutants from solid-fired combustion systems. In fluidized-bed boilers, injection of limestone pursues SO2 retention, achieving capture efficiencies over 90%. Nevertheless, CaO formed from limestone calcination has been identified as a catalyst of N-compound reactions, increasing NO emissions. This paper investigates the effect of limestone addition on NO emissions under oxy-fired conditions, as well as the influences of bed temperature, O-2 concentration in the fluidizing gas, and excess oxygen. To this purpose, a set of experiments were conducted in a 90 kW(th) bubbling fluidized-bed reactor, testing two different limestones for a variety of operating conditions and using anthracite as the fuel. The limestone with the lower SO2 capture capacity has shown the higher impact on NO emissions for all of the O-2/CO2 atmospheres tested. It has also been observed that the higher the bed temperature and O-2 excess, the higher the NO emissions. If compared to conventional air combustion, oxy-fired tests result in lower values of fuel-N conversion ratios.