The
solar thermochemical steam-based gasification of carbonaceous
materials is investigated using concentrated solar energy as the source
of the high-temperature process heat. Vis-à-vis conventional
autothermal gasification, the solar-driven process delivers a higher
syngas output of higher quality and lower CO2 intensity
because no portion of the feedstock is combusted and its energy content
is solar upgraded. The operation of a solar gasification pilot plant
for a 150 kWth solar-radiative power input was experimentally
demonstrated using a packed-bed solar reactor operated in batch mode.
The experimentation was carried out in a solar tower. Six different
carbonaceous waste feedstocks have been successfully processed: industrial
sludge, fluff, tire chips, dried sewage sludge, low-rank coal, and
sugar cane bagasse. The calorific value of the produced syngas was
upgraded by a factor of up to 1.3. The solar-to-fuel energy-conversion
efficiency, defined as the ratio of the heating value of the fuel
produced to the solar and feedstock energy inputs, varied between
22 and 35%.
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 Langmuir-Hinshelwood 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 H 2 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.
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