Influence of limestone addition on the behaviour of NO and N_2O during fluidized bed coal combustion

摘要

Atmospheric bubbling fluidised bed coal combustion (ABFBCC) of a bituminous coal and an anthracite with particle diameters in the range 500-4000 mum 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 percent) and bed temperatures in the range 750-900 deg C. Combustion air was staged, with primary air accounting for 100, 80 and 60 percent 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 deg C, whereas for anthracite coal the trend is the opposite. On the other hand, the N_2O 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 N_2O emission depends on the first stage stoichiometry: (1) under first stage fuel lean conditions the NO emission increases, while that of N_2O decreases. (2) under first stage fuel rich conditions (for example, high air staging) the opposite trend is observed.