Effects of calcium oxide on the cracking of coal tar in the freeboard of a fluidized bed

The effects of calcium oxide on tar cracking in the freeboard of a fluidized bed under different freeboard conditions have been investigated in order to reduce or eliminate the tar produced in coal pyrolysis and simultaneously to make coal gas of high qualities. Because no bubbles are in the freeboard, the tar would be contacted well with calcium oxide, and the tar cracking is improved. The results show that tar conversion increases apparently with the addition of calcium oxide, but the effect of calcium oxide on the tar conversion decreases gradually with freeboard temperature and is small above 700 degreesC, especially when the percentage of calcium oxide is increased from 6% to 12%. At lower freeboard temperature of 650 degreesC, the effect of calcium oxide on the tar conversion increases with residence time, apparently; however, at 750 degreesC the effect of calcium oxide on the tar conversion is only apparent at short residence time. The rate of tar cracking can be expressed by a simple first-order reaction, and the activation energy is decreased by 30% or 45% when 6% or 12% of calcium oxide is added, which is almost equal to the result of Lv and Yue [Lv, J. F.; Yue, G. X. J. Tsinghua Univ. (Sci. Technol.) 1997,37 (2), 6-10.] in a fixed bed for benzene or toluene, respectively.