Influence of calcined limestone on NOx and N2O emissions from char combustion in fluidized bed combustors

Limestone addition to a coal-fired fluidized bed combustor usually results in an increase in NO/NOx emissions and a decrease in N2O emissions. This ob servation has been explained in the Literature by the effect of limestone o n: (i) the conversion of coal volatile-N to NO/NOx and N2O; (ii) the decomp osition of (NO)-O-2; and (iii) the pool of H, OH and/or O radicals due to s ulphur capture. Since char-N is also an important sourer of NO/NOx and N2O under fluidized bed combustion conditions, any effect of calcined Limestone on the conversion of char-N to NO/NOx and N2O can also contribute to the a bove observation. The effect of calcined limestone on the conversion of vol atile-N species, i.e. NH3 and HCN has been well documented. However, so far few studies have studied the effect of limestone on the conversion of char -N under fluidized bed combustion conditions. In this study, a series of ba tch-type char combustion tests was carried out in a bench-scale bubbling fl uidized bed combustor with different bed materials. These tests reveal that in addition to reducing N2O emissions from char combustion, calcined limes tone also enhances the conversion of char-N to NO/NOx similar to the case o f volatile-N. Possible reaction mechanisms to explain the experimental resu lts of char combustion tests have been discussed. (C) 2001 Elsevier Science Ltd. All rights reserved.