| Summary: | A three-stage Pressurized Fluidized Bed Combustor (PFBC) of
principal dimensions, O,4Sm internal diameter by4m high was designed
and fabricated to burn South African coals, with particular reference
to coals unsuitable for burning in conventional boilers.
The combustor which is the first of its kind and probably one of
very few operational PFBCs in the world, was made of three jacketed
sections positioned vertically one above the other and bolted together
at the flanges. Distributor plates were located at the flanges which gave
the combustor a multistage capability. A three, two, or one deep Fluidized
Bed (FB) configurations were possible by removing the interstage distributors.
Interstage solids circulation was made possible by the use of
downcomers transporting solids downwards between the FBs. The solids were
returned to the top FB using a pneumatic conveyor.
The design of the PFBC was a sequence to a series of experimental
and theoretical investigations which were carried out in order to
provide us with the necessary PFBC design parameters. These investigations
dealt with the following areas of research: (a) the development of a new
type of cyclonic tuyere capable of transmitting through it high
quantities of solids with the fluidizing gas, without choking, (b) the
transfer and control of the downward flow of solids through downcomer
pipes, (c) the control of the circulation of solids in a Circulatory
system using a non-mechanical solids flow control valve, (d) the
development of a new type of start up burner which could operate
immersed under the solids, and (e) the combustion of coal in a small FB
under batch conditions and the study of reaction kinetics of South
African coals. On the basis of the results of the investigation in these
research areas and the findings of research of individuals and of
.organizations working in the field of fluidization technology the PFBC
was designed, built, and successfully commissioned. A series of 12
runs, with each run lasting between 2 and 8 days, totalling more than
1500 hours, were carried out on the PFBC. Char and coal with ash
content between 30 and 70 per cent were burnt in the combustor using
various combinations of feeding ports and number of FBs. System
pressures ranged between atmospheric and 6 bar(abs). For some of the
runs the reactor was operated in a counter-current mode with solids
and combustibles descending against the upflowing fluidizing air in
order to study the effect that counter-current flow had on the
efficiency of combustion.
The combustion trials showed that the two-FB combustor, operated
preferably without solids circulation, with the bottom FB acting as
the main combustion cell and the top FB as a smuts burn-out cell,
proved to be the most practical and most suitable combustor for burning
South African high ash coals and fines or, in general, any low-grade
carbonaceous materials of any size. With this configuration combustion
efficiencies of up to 99 per cent, based on the combustibles in the
feed and the ash, were achieved.
The department computer (COC1700) was successfully linked with
the PFBC for real time data logging and data processing.
A mathematical model which was based on our research findings
and the work of T.P. Chen and S.C. Saxena, C. Fryer and O.E. Potter,
and D. Levenspiel was successfully developed and applied to the twoFB
PFBC. The model describes the devolatilization and combustion of
coal particles in the FB in accordance with a shrinking core type
model and uses a population balance over all particles for the overall
mass balance. The results from this model, which was put onto the
computer, compared favourably with the experimental results and the
model can be confidently used to predict the behaviour of the PFBC.
It can also be easily adapted for use on any other single or multifluidized
bed reactors provided that the assumptions made for the
derivation of this mathematical model still hold.
A mathematical model based on the work of H.C. Hottel and
A.F. Sarofim, and L. Wender and G.T. Copper was also developed. This
model describes the transfer of heat from the FB to the cooling coils
using a stepwise heat and mass balance along the length of the cooling
coil. Although this mathematical model was developed specifically for
the cooling coils of our combustor it is strongly believed that it can
also form the basis of a general purpose model. === Thesis (Ph.D.)--University of Natal, 1981.
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