Gasification of biomass and waste in a staged fluidized bed gasifier: Modeling and comparison with one-stage units

Staged gasification is an efficient method of converting biomass, producing a gas with a low concentration of tar and with high process efficiency. This is achieved with reasonable simplicity and is cost-effective, making staged gasification ideal for power production at small-to-medium scale. Several staged-gasification systems have been developed based on fixed/moving beds. In the present work, a new staged-gasification system based on a fluidized-bed design is presented and modeled. The process includes three main stages: devolatilization of the fuel, homogeneous gas reforming/oxidation of volatiles, and heterogeneous reforming of gas over in situ generated char. Each thermochemical stage is modeled using kinetics data obtained in dedicated tests in a laboratory-scale fluidized bed or taken from the literature. The fluid-dynamics is characterized using the results from a cold rig constructed to model the gas and solids flows in the new system. The numerical model developed is employed to evaluate the performance of the new system during gasification of dried sewage sludge. The significant improvement achieved in the new staged gasifier compared to a single-stage unit is shown, as well as the potential optimization of the system under different operating conditions. (C) 2012 Elsevier Ltd. All rights reserved.