Zonal separation and solids circulation in a draft tube fluidized bed applied to coal gasification.
In this thesis a fluidized bed containing a draft tube has been studied with the aim of developing the apparatus for coal gasification. The process has the capability of producing synthesis quality gas using air for combustion, and of being able to accomodate poor quality coal feeds containing heavy fines loads. These advantages arise from two special features of a draft tube fluidized bed. In the first place, the bed may be operated as two separate and independent reaction zones, one contained within the draft tube and the other in the annulus region surrounding it. As a result, the gasification reactions may be carried out in one compartment and the combustion reactions in the other, allowing the useful gasification products to be taken off separately and undiluted with the combustion flue gases. Secondly, the fluidized material in the bed may be induced to circulate up the draft tube and down the annulus. These circulating solids provide the heat carrier from the combustion to the gasification zones within the bed. Furthermore, circulation of the bed in this way leads to a much longer residence time of fine particles within the bed and results in a high fine coal utilization efficiency. In order to achieve these benefits in practice, it is necessary to separate the gases supplied to and emitted from the draft tube from those of the annulus, but at the same time allowing free movement of solids between these regions. The thesis deals with how this may be accomplished in three parts: Firstly, the principles underlying division of a fluidized bed with a draft tube into discrete reaction zones are formulated, and strategies for achieving zonal separation, based on these arguments, are experimentally tested. As a result a reactor configuration and operating conditions suitable for coal gasification have been empirically identified. Secondly, a model describing the bulk circulation of solid material in the bed is presented, for the draft tube operating in the slugging mode. This model allows the average solids residence time and the particle velocities in the annulus and draft tube to be predicted, provided that slug velocities and spacings are known. The necessary correlations between hydrodynamic behaviour and the system properties are available in the literature for round nosed and wall slugs, but not for square nosed slugs, which appear to be characteristic in the apparatus used here. The third part consequently examines the square nosed slugging regime, and a theory to describe this behaviour, based on interparticle stress analysis, is presented. This regime is identified as having significant advantage over other bubbling modes because of the high dense phase gas flow rates which are sustained, and the resulting improved gas-solid contacting. The three models together mathematically describe the operation of the draft tube fluidized bed, allowing gas partition between the annulus and the draft tube regions as well as solids circulation to be predicted, for different bed configurations and operating conditions. The predictions compare well with experimental results. The last part of the thesis deals with the application of the system to coal gasification on a one ton coal per day pilot plant. A high quality gas, containing up to 80% CO + H2, (balance CO2), has been produced by steam gasification in the draft tube, using air for the combustion reaction in the annulus. The H2/CO ratio can be varied from about 1 to 3, by changing the operating temperature of the reactor.