(513o) Reduction in Tar Yield by Temperature Profiles of Gas and Solid Phases during Pyrolysis of Wastes Under Countercurrent Flow
AIChE Annual Meeting
Wednesday, November 7, 2007 - 4:30pm to 6:30pm
Shaft kiln is considered as one of promising pyrolysis devices to decompose efficiently domestic wastes. In the process the temperature distributions of gas phase and solid phase can be controlled separately at quite different profiles. This enables us to design a suitable temperature profile of gas phase for decomposing the tar evolved from solid phase. Based on this concept, we conducted an experimental apparatus that consisted of two-stage reactors kept at different temperatures. As waste samples, we used wood biomass, low-density polyethylene (PE), polypropylene (PP) and polystyrene (PS). The waste was pyrolyzed under a constant heating rate in the lower reactor, and the tar evolved was fed to the upper reactor kept at high temperature. We examined the product distribution at the exit of upper reactor under various combinations of temperature profiles of upper and lower reactors. The tar decreased with the increase in the temperature of upper reactor. Steam reforming of the volatiles was also conducted by feeding steam into the upper reactor. Lumped kinetic model is one of the popular ways to express pyrolysis rate of organic substances. Here, our pyrolysis model was developed by taking into account 12 components: aliphatic tar from polyolefin, aromatic tar from PS, biomass tar, BTX, CH4, C2H4, hHCG (heavier hydrocarbon gases, ethane and C3-C5), H2, H2O, CO, CO2 and coke. The aliphatic tar is represented by C12H24, the aromatic tar is by C8H8, the biomass tar is by C6H4O2, BTX is by C6H6, and hHCG including ethane is by C3H6, respectively, for simplicity. Applying experimental data to this model, kinetic analysis for the secondary gas phase reaction was performed. The kinetic parameters were estimated so as to give best fit between the experimental data and the calculated ones. The experimental data and the calculated ones were in close agreement for most products. Estimated activation energies of each reaction in the model seemed to be very reasonable. Finally, the simulation of pilot shaft kiln was conducted under various temperature profiles in the kiln using the kinetic model determined. We confirmed that the above concept was appropriate to reduce significantly the tar yield from the experimental data and calculated results.
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