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Along with SO2, NO x 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, O2 concentration in the fluidizing gas, and excess oxygen. To this purpose, a set of experiments were conducted in a 90 kWth 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 O2/CO2 atmospheres tested. It has also been observed that the higher the bed temperature and O2 excess, the higher the NO emissions. If compared to conventional air combustion, oxy-fired tests result in lower values of fuel–N conversion ratios.

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