Energy & Fuels 2008, 22, 2043–2052
2043
Solar-Driven Coal Gasification in a Thermally Irradiated Packed-Bed Reactor Nicolas Piatkowski† and Aldo Steinfeld*,†,‡ Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland, and Solar Technology Laboratory, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland ReceiVed January 12, 2008. ReVised Manuscript ReceiVed March 5, 2008
Coal gasification for high-quality synthesis gas production is considered using concentrated solar energy as the source of high-temperature process heat. 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 5 kW prototype reactor with an 8 cm depth, 14.3 cm diameter cylindrical bed was fabricated and tested in a high-flux solar furnace, subjected to solar flux concentrations up to 2600 suns and packed-bed temperatures up to 1440 K. The reactor is modeled by formulating the 1D unsteady energy conservation equation that couples conductive-radiative heat transfer with the reaction kinetics and solving it by the finite volume technique for a transient shrinking domain. The overall reaction rate was determined experimentally by thermogravimetry, while the effective thermal conductivity was determined experimentally in a radial heat flow oven. Model validation was accomplished in terms of bed temperatures, gasified mass, and bed shrink rates measured in solar experiments conducted with beech charcoal. Heat transfer through the bed proved to be the rate-controlling mechanism, indicating an ablation regime.
1. Introduction Solar steam gasification of coal makes use of concentrated solar energy to convert a polluting, solid fossil fuel feedstock into high-quality synthesis gas (syngas), mainly H2 and CO, applicable for power generation in efficient combined cycles and fuel cells or for Fischer–Tropsch processing of liquid fuels. Conventional autothermal coal gasification requires a portion of the injected coal mass to be combusted with pure O2 to supply high-temperature process heat for the endothermic gasification reaction. For example, 12 MJ/kg are required to steam gasify a typical bituminous coal of LHV 34 MJ/kg.1 Thus, about 35% of the injected coal mass must be burned uniquely to power the reaction, which inherently decreases coal utilization and contaminates the product gases. In contrast, solar-driven gasification is free of pollutants and combustion byproducts. The product synthesis gas has a lower CO2 intensity because its calorific value is solar-upgraded over that of the original coal feedstock by an amount equal to the enthalpy change of the reaction. A second law analysis indicated that Brayton-Rankine combined power cycles running on solar-made syngas can double the specific electric output per unit mass of coal and, consequently, avoid half of the specific CO2 emissions of conventional coal-fired generation plants.1 Solar gasification is estimated to reduce CO2 emissions at least 30% vis-a-vis conventional autothermal gasifiers.2 In the long term, solar coal gasification has the potential of becoming economically competitive. Its specific costs per unit of produced syngas are * To whom correspondence should be addressed. Telephone: +41-446327929. Fax: +41-44-6321065. E-mail:
[email protected]. † ETH Zurich. ‡ Paul Scherrer Institute. (1) Von Zedwitz, P.; Steinfeld, A. Energy 2003, 28, 441–456. (2) Kodama, T.; Kondoh, T; Tamagawa, T.; Funatoh, A.; Shimizu, K.I.; Kitayama, Y. Energy Fuels 2002, 16, 1264–1270.
estimated to be 13% lower than those for the autothermal process of Lurgi,3 mainly because of the elimination of the energy-intensive oxygen separation unit, as well as 43% lower coal consumption.3 Solar thermochemical gasification of coal is ultimately a means of chemically storing intermittent solar energy in a dispatchable form. Recently, solar gasification of petroleum coke and coal was studied in directly irradiated fluidized-bed and vortex-flow solar reactors.2,4–8 Direct irradiation of these particle suspensions was found to be an effective means of heat transfer directly to the reaction site, leading to extremely fast heating rates (∼1000 K/s) and enhanced kinetics.8 However, the transparent window needed for the optical access of concentrated solar radiation becomes a troublesome, critical component under high pressures and severe atmospheres. The large volume flow rates of inert carrier gases or excess steam necessary for fluidization and protecting the window displace syngas, decreasing production and energy conversion efficiency.9 Additionally, effective fluidization requires small particle sizes (
