Experimental study of wood char gasification kinetics in fluidized beds

Abstract

During gasification two steps take place. The first one is pyrolysis and the second one is gasification of the char that remains back after pyrolysis. The second step is slower than the first one, so this step is the limiting factor in designing fluidized beds. Kinetic data for designing fluidized beds are necessary. The paper describes gravimetric measurements directly applied to fluidized bed with large sample sizes. The samples are char of 6 mm wood pellets and 10–40 mm wood cubes in order to directly measure ”apparent kinetics”. The parameters examined in this paper are particle size, product gases (= hydrogen) in the gasification medium, type of wood and differences in CO2/steam gasification. The results are presented as Arrhenius diagrams and half-value period diagrams. The most important parameters are the temperature and product gases (hydrogen) in the gasification agent. The particle size seems to be less important for large wood particles as the measurements do not show significant differences for gasifying char of wood cubes 10–40 mm. The half-value periods for gasification of char from wood cubes (10 mm - 40 mm) with 100% steam at atmospheric pressure lie between 1000 s at 1023 K and 300 s at 1173 K. For char of 6 mm wood pellets the half-value periods lie between 1900 s at 1023 K and 250 s at 1173 K. The reaction is most likely in pore diffusion regime.