Predictive model for countercurrent coal gasifiers

The paper deals with a model able to predict the stationary operating conditions of a moving bed gasifier with upflow of gas. The model was used to simulate coal gasifiers but the proposed approach can also be useful for biomass gasifiers. The model considers the different phenomena that occur along the reactor: drying and pyrolysis take place at the top, while the behaviour of the lower part is dominated by the combustion and gasification reactions between the coal char and the oxidizing gas. The model is structured into two levels: particle level and reactor level. At the particle level, the strong gradients of temperature in the single pellet caused by external heating were taken into account. Literature kinetic models developed with isothermal particles were employed. The calculated results of fixed carbon yield and gas were compared with experimental results obtained in our laboratory.At the whole apparatus level, the reactor was subdivided into two zones: pyrolysis (upper) zone and gasification/combustion (lower) zone. The upper zone was modelled by coupling convective heating of the particles, temperature gradients inside the particles and kinetics. In the lower zone transport of heat by radiation was taken into consideration. The governing equations were solved as a boundary condition problem, leading to a stable and reliable solution. The model was validated by comparison with some literature data of full scale gasifiers.