Solar Driven Steam Gasification of Carbonaceous Waste in An Indirectly Irradiated Packed Bed Reactor

Wednesday, November 10, 2010: 12:55 PM
Grand Ballroom H (Marriott Downtown)
Nic Piatkowski and Aldo Steinfeld, Department of Mechanical and Process Engineering, ETH Zurich, Zurich, Switzerland

Steam gasification of scrap tire powders for high-quality synthesis gas production is considered using concentrated solar energy as the source of high-temperature process heat. By supplying the endothermic reaction energy with concentrated sunlight, feedstock utilization, product quality and the overall energy balance of gasification is improved against auto-thermal processes. The solar reactor consists of two cavities separated by a radiant emitter plate, with the upper one serving as the solar absorber and the lower one containing the reacting packed bed that shrinks as the reaction progresses. A 7 kW batch prototype reactor with a 5.2 cm-depth, 14.3 cm-diameter cylindrical bed in the beam down configuration was fabricated and tested in a high-flux solar furnace, subjected to solar flux concentrations up to 2560 suns and packed-bed temperatures up to 1520 K. The reactor is modeled by formulating the 1D unsteady energy conservation equation that couples conduction-radiation heat transfer with the reaction kinetics and solving it with the finite volume technique for a transient shrinking domain. Heat transfer in the lower cavity is dominated by radiative heat transfer between the bed surface, the emitter plate and the exposed SiC walls lining the lower cavity. Heat transfer in the highly conductive SiC walls is an important transport mechanism for heat to the deeper regions of the bed. The effective thermal conductivity in the packed bed was described in terms of parallel modes of heat transfer in a porous packed bed with the Yagi and Kunni model previously applied to biomass solar gasification. Changes in the particle size, bed density, porosity and active area were analyzed during devolatilization however the results proved too inconclusive to be utilized reliably in the model. Changes in packed bed properties were also investigated in the gasification regime and applied to the model. The reaction rate was determined with a combined pseudo-component model considering three parallel 1st order Arrehnius type reactions fit to dynamic thermogravimetric experimental runs conducted in the temperature range 473-1476 K with 8.5% steam atmospheres in argon. The volatile content of the scrap tire powder was 67.5 wt.% with a 27 wt.% fixed carbon fraction giving it good gasification potential. The temperature ranges of component conversion and apparent activation energies agree well with the results of previous authors. Devolatilization was complete upon reaching 800 K with a clear onset of gasification at 1175 K. The specific enthalpy change of the gasification reaction was determined assuming reactants and products both at temperature T, and the products having the equilibrium composition at T and 1 bar. The specific enthalpy of devolatilization reactions was not considered. Experimental investigation proved complete devolatilization through the emission of higher hydrogen gases ending once the entire bed exceeded 900 K as had been seen in the TG experiments. The strong endothermic heat sink associated with gasification was observed at the depth of the bed above 1100 K and replicated by the model. A high quality synthesis gas was produced with H2/CO ratios of 1.33 and CO2/CO ratios of 0.16. Variations in water concentration showed the expected changes in terms of H2 and CO2 production. Model validation was accomplished in terms of bed temperatures and reacted mass. Heat transfer through the bed proved to be the rate controlling mechanism, as is typical of an ablation regime.

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