Modeling Wood Gasification in a Countercurrent Fixed-Bed Reactor

A one-dimensional, unsteady mathematical model is presented of fixed-bed countercurrent wood gasifiers, which couples heat and mass transport with wood drying and devolatilization, char gasification, and combustion of both char and gas-phase species. The model is used to simulate the structure of the reaction fronts and the gasification behavior of a laboratory-scale plant as the reactor throughput and the air-to-wood (or char) weight ratio are varied. It is observed that a wide zone, acting essentially as a countercurrent heat exchanger, separates combustion/gasification from devolatilization/ drying. Moreover, the former zone presents interesting dynamic patterns driven by the highly variable solid/gas heat transfer rates. For a constant air-to-wood weight ratio, the gasification process is improved by increasing the reactor throughput as a result of higher temperatures, in spite of the simultaneous reduction in the amount of char generated from wood devolatilization. In fact, an increase in the air-to-wood (or char) ratio always lowers the efficiency of the gasification process. Finally, good agreement is obtained between predictions and experiments for the axial temperature profiles and the composition of the producer gas. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2306–2319, 2004