Two-stage gasification of dried sewage sludge: Effects of gasifying agent, bed material, gas cleaning system, and Ni-coated distributor on product gas quality

Abstract

Dried sewage sludge gasification was conducted using a two–stage gasifier composed of a fluidized bed gasifier and tar–cracking reactor to produce a gas with low levels of tar, NH3, and H2S. In this work, the influence of the type of gasifying agent and bed material and the equivalence ratio on the product gas quality were investigated. Furthermore, the possibility of gasification without a hot filter and electrostatic precipitator, which are usually applied to remove impurities generated during gasification, was examined. Finally, the efficiency of a Ni–coated distributor for tar and NH3 removal was evaluated. Overall, steam/O2 gasification produced H2–rich (38–39 vol%) gases. Further, olivine reduced the tar and NH3 contents in the product gas obtained from steam/O2 gasification to 138 mg/Nm3 and 236 ppmv, respectively. A gas cleaning system without a hot filter and electrostatic precipitator produced a gas with only 8 mg/Nm3 of tar. Moreover, a Ni-coated distributor was found to be very effective in reducing the NH3 content to 60 ppmv. Dried sewage sludge gasification experiments in air were conducted four times for 4 h, producing gases with very low levels of tar (3–10 mg/Nm3) and NH3 (60–90 ppmv).