A kind of porous alumina was employed as bed material of bubbling fluidized bed coal combustion instead of non-porous silica sand. The effect of the bed material on emissions of N₂O and NO_x was evaluated using a bench-scale combustor. The present porous alumina suppressed N₂O emission. This result is explained by the catalytic activity of porous alumina to decompose N₂O. In addition, NO_x emission with porous bed material was nearly the same as or lower than that for the sand bed. Thus the decrease in N₂O without increasing NO_x was attained.
A modification of desulfurization by limestone was proposed. Fine limestone particles were employed as sorbent in order to conduct SO₂ capture in the freeboard. By employing fine particles, the contact between volatile matter and limestone, which is known to increase the emission of NO_x, was avoided. Thus the increase in NO_x emission during limestone feed was avoided.