Steam-Gasification of Coal in a Fluidized-Bed/Packed-Bed Reactor Exposed to Concentrated Thermal Radiation-Modeling and Experimental Validation

The steam-gasification of coal in a fluidized-bed or a packed-bed chemical reactor is considered using an external source of concentrated thermal radiation for high-temperature process heat. The energy equation that couples heat transfer with the chemical kinetics is solved by means of a numerical model that incorporates the Monte Carlo ray-tracing technique for nonisothermal, nongray, absorbing, emitting, and scattering media. The reaction kinetics are described by LangmuirHinshelwood type rate laws. Validation is accomplished by comparing the numerically computed temperature profiles, product gas composition, and reaction extent with the experimentally measured values using a tubular quartz reactor directly exposed to high-flux irradiation. For the packed bed, the temperature increases monotonically because the internal radiative exchange approaches a conduction-like heat transfer within the bed. For the fluidized bed, the temperature increases rapidly in the first one-quarter of the bed and then reaches a constant value because of the strong fluidization in the upper bed region derived from the 5-fold volumetric growth due to gas formation and thermal expansion. Above 1450 K, the product composition consisted mainly of an equimolar mixture of H2 and CO, a syngas quality that is notably superior than that typically obtained in autothermal gasification reactors (with internal combustion of coal for process heat), besides the additional benefit of the upgraded calorific value.