Fixed-bed staged gasification of biomass: Rate-controlled equilibrium modelling

Biomass gasification is a technology suitable for small-scale power plants. This is a well-known and environmentally friendly way to convert biomass (such as forest and agricultural waste) into combustible fuel gas. However there are several disadvantages to overcome. Tarry gases call for a costly gas cleaning system, and the process is often unstable. These drawbacks are typical for widely used single-stage gasification processes. Staged gasification as distinct from the single-stage one allows allothermal and autothermal conversion processes to be separated within the same plant. Separation of the devolatilization and char gasification stages makes tarry volatiles to burn and produce the gasification agent. Such process organization provides both low-tar gas production and better process heat utilization. The plant being studied consists of two fixed-bed reactors and a gas combustion chamber. It has a number of feedbacks that cause a change in parameters of all reactors at a variation in any of them. A mathematical model is developed to predict staged process behaviour and to find stable operation modes. The model consists of blocks describing equilibrium gas combustion constrained by diffusion and kinetics. The results outline the control limits within which a stable and efficient gasification can be performed.