The 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 observation has been explained in the literature by the effect of limestone on: (i) the conversion of coal volatile-N to NO/NOx and N2O; (ii) the decomposition of N2O; and (iii) the pool of H, OH and/or O radicals due to sulphur capture. Since char-N is also an important source 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 above observation. The effect of calcined limestone on the conversion of volatile-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 batch-type char combustion tests was carried out in a bench-scale bubbling fluidized bed combustor with different bed materials. These tests reveal that in addition to reducing N2O emissions from char combustion, calcined limestone also enhances the conversion of char-N to NO/NOx similar to the case of volatile-N. Possible reaction mechanisms to explain the experimental results of char combustion tests have been discussed.