Pressurized fluidized-bed combustion part-load behavior. Volume I. Summary report Page: 14 of 122
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1. BACKGROUND
This report is concerned with the supercharged boiler cabined cycle.
In such a cycle, illustrated in Fig. 1, hot gases fron the PFB are
expanded, after cleaning, through a gas turbine. Steam generated and
superheated in trbes immersed in the bed drives a steam turbine.
Although such a utility plant will be nominally base load plant, at
least in the early part of its life, it will be required to operate
for significant periods at part' load and to be ±>le to accarmodate load
changes of up to 3 or 4% per minute.
The problems of changing load in a catbined cycle are best discussed
by describing the stage-by-stage steps which are required. Details will
vary according to the particular gas turbine, steam conditions, and PFBC
conditions being used, but the following describes a philosophy for
reducing load.
(1) The first step is to reduce the bed temperature to a minimum
operating value (one of the objectives of the tests reported
here is to determine what this minimum value mightobe;
traditionally it has been assumed to be about 1380°F). This
has the effect of reducing the gas turbine inlet temperature
and hence the gas turbine power output. Steam output is also
reduced because the heat transfer to the trbes immersed in the
fluidised bed is reduced.
(2) To reduce the gas turbine output to its minimum it is neoessaryo
to reduce the turbine inlet temperature ccnsiderbly below 1380 F
and this can be dene to seme extent by by-passing seme of the
compressed air around the cab us tor and so diluting the combustion
products. However in doing this, two limitations must be borne
in mind: (i) the fluidising velocity must not decrease too much,
otherwise segregation could occur in the bed; and (ii) the excess
air must not fall below some minimum value. With most gas
turbines the first limitation does not normally arise. Although
single-shaft and multi-shaft gas turbines have vastly, different
part-load characteristics, both types operate with a volumetric
flow from the fluidised bed which is approximately constant or
actually increases with reducing load.
The second limitation may apply because of the relatively inflexible
nature of the amount of heat transferred to the steam circuit.
Cnee a minimum bed temperature of, say, 1380°F has been reached,
heat flux to the steam circuit remains constant as the mass flew
through the bed is further reduced. As a consequence the excess
air tends to decrease as load is reduced by air bypass. This is
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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/m1/14/: accessed July 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.