Pressurized fluidized-bed combustion part-load behavior. Volume I. Summary report Page: 90 of 122
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pressurised fluidised bed are complex. The proportion of dolomite
elutriated depends primarily on the proportion of elutriable material
in the feed with a small additional amount being produced by attrition
in the bed. Attrition depends mainly on the fluidising velocity.
Coal ash elutriation arises from both the proportion of elutriable
material in the feed and from attrition in the bed3 which in turn
depends on the fluidising velocity and the nature (hardness) of the
adventitious ash associated with the coal.
Using the information given above it is possible to make an approximate
estimate of the amount of dolomite and coal ash which will be elutriated
(bearing in mind that the cJiariges in weight due to calcination and
sulphation have to be allowed for)3 but it is desirable that pilot-
scale tests should be carried out where reasonably precise design
data are required.
6.5 HEAT TRANSFER
Measurements of heat flux were made at part-load ccnditicns simulated
in different ways:-
(1) at different bed depths and temperatures (steady state conditions)
(2) whilst varying the bed depth (other parameters - except excess air
- being held constant).
Measurements were also made with a constant bed depth of 4 ft and
constant bed temperature whilst varying the amount of cooling in the
freeboard. The increase in heat (coal) input required to maintain
a constant bed temperature was a measure of the heat being abstracted
by the freeboard tubes from the bed itself.
6.5.1 Heat transfer to immersed tubes. Heat fluxes were measured at
irregular intervals during the three tests, attempting to cover as many
ccnditicns of bed temperature and bed depth as possible. Figs. 40 and
4i shew seme of the heat transfer coefficients as a function of height
above the distributor for different temperature bands. Each diagram
shews a characteristic rise in heat transfer coefficient through the
first few rows of the tube bank, followed by a more gradual or negligible
rise in the remainder of the tube bank. This change in coefficient is
due possibly to bubble break-up in the first few rows of the tube bank
with a consequent reduction in particle/tube contact time.
Fig. 40 shews that there was a slight reduction in heat transfer
coefficient with lower bed temperature. This effect is masked in
82
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Roberts, A. G.; Pillai, K. K.; Raven, P. & Wood, P. Pressurized fluidized-bed combustion part-load behavior. Volume I. Summary report, report, September 1, 1981; United States. (https://digital.library.unt.edu/ark:/67531/metadc1088075/m1/90/: accessed July 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.