CO2 Gasification of Sewage Sludge, Plastic Waste, and Municipal Solid Waste in an Internally-Circulated Fluidized Bed Under High-Flux Solar Irradiation

Solar thermal gasification of carbonaceous feedstocks is a promising approach to produce high-quality syngas. Previous work in solar gasification demonstrated merits of fluidized beds such as high residence time of particles and highly-effective solid–gas heat and mass transport. However, large temperature gradients between absorbing surfaces and particles may arise in solar thermal fluidized beds. Internally circulating fluidized beds, i.e. clapboard and draft tube configurations, are attractive for eliminating the stagnation zones and increasing the solid–solid and solid–gas interaction. The most favorable merit is the capability of mitigating the hot spot on the absorber surface by enhancing the interaction and heat transfer of solid particles and the wall. In this work, a novel indirectly irradiated solar gasifier featuring a clapboard-based internally circulating fluidized bed was developed and tested under high-flux irradiation provided by a 28 kW_e solar simulator. The solar gasifier is composed of an internally-circulating fluidized bed and Al₂O₃-SiO₂-insulated cavity. The internally-circulating fluidized bed consists of a 70-mm wide, 100-mm high and 10-mm thick clapboard, a 5-mm thick square bed of 70-mm inner diameter, and a gas distributor at the bottom of the bed. The biomass particles pass through the gap between the clapboard and the gas distributor to form the internal circulation. A SiC absorber plate located at the center of the cavity is attached to the outer surface of the square bed. The biomass feedstock is delivered through a semi-continuous auto feeder. In the present work, dried sewage sludge, plastic waste, and municipal solid waste from Singapore are used as the example carbonaceous feedstocks. The effects of the superficial gas velocity and the CO₂/C ratio on the carbon conversion ratio, the solar upgrade factor, and solar-to-chemical efficiency are investigated.