Zonal separation and solids circulation in a draft tube fluidized bed applied to coal gasification.
Loading...
Date
1984
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
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.
Description
Thesis (Ph.D.)-University of Natal, Durban, 1984.
Keywords
Coal gasification., Distillation, Destructive., Fluidization., Draft Tubes., Theses--Chemical engineering.