Influence of limestone addition on the behaviour of NO and N2O during fluidised bed coal combustion

Atmospheric bubbling fluidised bed coal combustion (ABFBCC) of a bituminous coal and an anthracite with particle diameters in the range 500–4000 μm was investigated in a pilot-plant facility, with and without limestone addition. The experiments were conducted at steady-state conditions using three excess air levels (10, 25 and 50%) and bed temperatures in the range 750–900 °C. Combustion air was staged, with primary air accounting for 100, 80 and 60% of total combustion air.During limestone addition, in general, the NO emission decreases with the decrease in excess air and the increase in air staging, for both coals (as also observed without limestone). The bed temperature does not influence the NO emission significantly (as also observed without limestone); however, it was observed that during bituminous coal combustion there is a slight trend for a decrease on the NO emission with temperature increase in the range 825–900 °C, whereas for anthracite coal the trend is the opposite. On the other hand, the N₂O emission increases with: the decrease in excess air, the increase in air staging (as opposed to what was observed without limestone), and the decrease in bed temperature (as also observed without limestone).Taking the coal combustion without limestone as reference, it was observed that the effect of limestone addition on the NO and N₂O emission depends on the first stage stoichiometry: (1) under first stage fuel lean conditions the NO emission increases, while that of N₂O decreases, (2) under first stage fuel rich conditions (for example, high air staging) the opposite trend is observed.