A study of the design of fluidized bed reactors for biomass gasification
The present study is in two parts, the first part describes an investigation that was undertaken to determine the feasibility and hydrodynamic behaviour of a cold model circulating fluidized bed system proposed for the continuous combustion-gasification of biomass. The design is based on the principle that the char produced in the gasifier is circulated with the bed material and combusted in a separate reactor to generate the heat required for the gasification process. While high solid circulation rates are required to maintain the heat balance, product and flue gas mixing between the two units must be minimised or eliminated. The design of the circulating bed simply consists of two fluid bed units connected two together via a riser and a downcomer fitted with a non-mechanical valve. Various aspects such as solid circulation rate, gas mixing, solids mixing, and pressure component around the circulating loop were studied. Results show that the solid circulation and gas mixing are strongly influenced by the riser gas velocity, total solids inventory, and position of the riser from the riser gas jet. Solid circulation fluxes of up to 1 l5kg/m 2s were attained and easily controlled. The flue gas crossflow was less than 4% over the range studied. By analysing the experimental data, a series of mathematical correlations were obtained which successfully predict the exponential relationship that exists between the solid circulation rate, gas mixing and the operating parameters. The potential of this system for its purpose is highlighted. The second part of the study focuses on the design and development of a heated fluidized bed reactor with an on-line gas and solids sampling technique to study the steam gasification of biomass (almond shells). Experiments were conducted at temperatures of up to 800°C to investigate the gasification rates of biomass char under different operating conditions. Understanding the gasification and combustion rates of biomass char is an important step towards the proper designing of biomass gasifiers. In the heated fluid bed, the extent of the gasification of biomass increased with increasing temperature as indicated by an increase in the quantity of gaseous products. A hydrogen content of up to 43% by volume was obtained. The sampling technique adopted allows the collection of the bed sample at gasification conditions which can be used to predict the composition of the bed. The gasification rate of biomass (almond shell) char was found to be affected by the gasification temperature. In order to evaluate the char gasification rate constants, the shrinking core model with reaction controlling step (SCM) was applied to the char gasification data. The rate constant values obtained from the SCM were 5.14E-5, 7.67E- 5, and 1.26E-4 s' for temperatures of 712, 753, and 806 °C respectively. The activation energy was evaluated as 89 kJ/mol. These values were in close agreement with those published in the literature. SEM pictures of the surface of the chars shows that at high temperatures, the char formed is very porous and hence very reactive. With regards to practical gasifiers, the results obtained have been used to evaluate an --mum volume for a typical biomass gasifier operating at 850 °C. The aim of the present work were satisfactorily achieved.