Reduction of unburnt gas and nitrous oxide emissions from fluidized bed combustor by use of capacitance effect of porous bed materials

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The objective of this work is reduction of N_20 emission, which is known to be a greenhouse effect gas, and emissions of unburnt hydrocarbons including dioxins from fluidized bed coal combustors. In this work we proposed to employ porous bed material instead of conventional non-porous sand. The present research project clarified that porous bed materials had so-called "capacitance effect", I. e., hydrocarbon capture at high temperatures and retention of carbon within pores, and catalytic activity to decompose N_2O. The present work investigated the effect of such porous bed materials on emissions of hydrocarbons, unburnt carbon, N_2O and Nox during fluidized bed combustion using a bench-scale reactor. For comparison, inert and non-porous sand was also employed as bed material. The porous bed material was found to be effective for reduction of emissions of unbunrt fractions including dioxins.The present work employed two kinds of porous alumina bed materials and compared the results with those for the sand bed. Both alumina particles reduced N_2O emission in comparison to the sand bed. This was explained by the catalytic activity of these bed materials to decompose N_2O. However, the effect of bed material on Nox emission was different between the porous bed materials ; One of the porous alumina bed materials reduced Nox emission whereas the other increased. A kinetic study was conducted for Nox formation from a volatile nitrogen compound (ammonia). The fate of nitrogen compounds in the fluidized bed is discussed.Also the reduction of the unburnt hydrocarbon/carbon emissions by porous bed materials was carried out using a bench-scale reactor. In addition, a kinetic study on hydrocarbon capture and combustion of captured carbon was conducted using a fixed bed reactor. The effect of bed material type and temperature on hydrocarbon capture and carbon burn-up was evaluated.