The effect of limestone on the conversion of coal-N, volatile-N to NO/NO_x and N_2O under fluidized bed combustion conditions has been extensively investigated and well-documented by many researchers. However, so far few studies have specifically investigated the influence 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 on a bench-scale, electrically heated, bubbling fluidized bed reactor. Char was produced in situ within the reactor. Char combustion tests were conducted with three sets of bed materials, namely silica sand, limestone diluted by silica sand and alumina (>95% γ-Al_2O_3) diluted by silica sand. Comparisons of NO_x and N_2O emissions from char combustion between different bed materials reveal that bed materials containing limestone always results in more NO_x and less N_2O than the pure sand bed under otherwise identical char combustion conditions. Two possible explanations for this observation have been given. One is that limestone/calcium oxide catalyses the decomposition of N_2O and promotes the conversion of char-N to NO/NO_x, as in the case of volatile-N. The other is that limestone reduces the SO_2 level in the combustor and hence results in an increase in the pool of H, OH and/or O radicals, which in turn results in an increase in NO_x and a decrease in N_2O. Experimental results also show that alumina has a similar effect on N_2O emissions as limestone. However, in contrast to limestone, alumina does not promote the conversion of char-N to NO_x, which suggests that partially replacing sand with alumina may reduce the total emissions of N_2O and NO_x from coal-fired fluidized bed combustors.
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