Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1058889
The present invention relates to a fluidised bed
comhustion apparatus and is particularly concerned with a
method of operating the fluidised bed and apparatus for
performing the method.
It is known for fluidised bed combustion apparatus to
operate in a fuel rich mode where there is an excess of coal
in the bed of inert material and the rate of heat production
is determined primarily b~ the rate of supply of air, so that
at the top of the bed only a small amount of oxygen remalns
in the gases leaving the bed. Under these conditions, the
combustion is effectively near stoichiometric and there is only
about 10% excess air which is not combusted by the excess fuel in
the form of char or coke in the bed. In this type of apparatus
excess fuel can remain in the bed as a fuel reserve and its
combustion rate and the heat release is controlled by the air
flow. Because the combustion is near stoichiometric, the
temperature rise would approach 2,000C if the heat were not
removed from the bed by some method such as water pipes.
The present invention seeks to provide an alternative
20 -- method of operating a bed of inert materials and particulate
fuel such as coal in which the rate of combustion is controlled
by the fuel. In a fuel rich bed which is oxygen limited, the
fraction of fuel in the inert bed might be of the order 5%-10~,
the fraction of fuel-in a bed operated according to the present
invention might be of the order 0.1% to 1% and all the air is
not burnt.
In accordance with one broad aspect, the invention
relates to a method of operating a fluidised bed combustion
apparatus, in which the bed of materials to be fluidised
includes inert materials and particulate fuel and in which the
heat of combustion is transferred only by direct contact of
the fuel with combustion air, t,he method comprising supplying
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combustion air in su~stantial excess of that required for
stoichiometric combustion and metering a supply o~ particulate
fuel into the bed to maintain the bed at substantially
predetermined temperature.
In one particular form of the present invention about
25% of the combustion air may be burnt and the ratio of inert
materials to particulate fuel may be 100;1 to 1000;1.
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The fluid~sed bed combustion apparatus may be used to
hcat the work~ng fluid ~f a ~as tur~ine engine and in order to
control the heat output of the fluidised bed so that the gas
turbine engine can be controlled, a proportion of the combustion
and fluidising air which is the compressor delivery air from
the engine is arranged to by-pass the ~ed and mix with the
heated air leaving the bed. The by-passing can be obtained by
the use of a by-pass valve arranged across the bed.
When a proportion of compressor delivery air is by-passed,
a part of the flow of fluidising air may be closed off from part
of the bed.
Conveniently, the bed can be divided into a number of
chambers by partition walls, each part having a supply of
fluidising and combustion air controlled by a separate control
valve. The control valve can be operated together or
independently of each other and each chamber may not
necessarily have a control valve.
The present invention will now be more particularly
described with reference to the accompanying drawings in which:
Fig. 1 shows in diagrammatic form, one form of
fluidised bed combustion apparatus according to the present
invention which can be used to put into practice the method of
the present invention, and
Fig. 2 shows in diagrammatic form one chamber of the
fluidised bed combustion apparatus shown in Fig. 1.
Referring to the drawing, a gas turbine engine power
~lant 10 in which the working fluid is heated in a coal burning
fluidised bed comprises, a compressor 12, a fluidised--coal
burning bed-14, a compressor driving turbine 16 and a power
turbine 18~arranged to drive a load 20 which in this case is
shown as a generator.
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The flow path between the compressor 12 and the
turbine 16 includes a by-pass valve 25 and the fluidised bed 14
which is divided into a number of independent chambers 14a
with corresponding air cleaners 14b and control
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valves 14c. Each chamber 14a contains inert m~terials
including coal ash and an amount o~ coal in particulate form
and each chamber is arranged to receive a supply of coal
particles by means shown in Fig. 2 and the fluidising and
- combustion air from the compressor 12 flows into each chamber
through a distributor plate 22. The fraction of coal in the
bed in each chamber 14a is of the order 0.1% to 1% and the bed
will give a temperature rise of about 500C so that only about
one quarter of the available air is burnt and the bed does not
require a heat exchanger e.g. water pipes immersed in the bed
to conduct away heat.
Referr;ng to Fig. 2, the coal supply means to each
chamber 14a which is generally similar to the coal supply
means described in our co-pending application no. 237,748
comprises a duct 24 along which a suspension of coal particles
in air can be blown at intervals or continuously into the
bed. Thè duct 24 has a valve 26 which is controlled by the bed
temperature as will be described below, to induce the fuel to
flow into the bed or to return the fuel to a store (not shown)
along a duct 28.
The bed temperature is sensed by a device 30 which
comprises a stainless steel tube 32 enclosing a quartz rod 34
; immersed in the bed, so that changes in the bed temperature
produce a relative movement between the rod 34 and the tube 32.
This relative movement is used to operate the valve 26 by a
linkage 36 which is shown diagrammatically since any suitable
position transmission system can be used.
In known fluidised bed coal combustion apparatus where
the air to fuel ratio is near-stoichiometric, the bed temperature
is controlled by the rate of air supply, since for a given
supply rate if fuel is added to give a non-stoichiometric air to
fuel ratio, the excess coal will not change the bed temperature
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as there will not be sufficient oxygen to burn the excess coal.
In the present arrangement, the bed temperature is controlled
by the rate at whi:ch fuel is supplied to the lied as there is
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an excess o~ air and thus sufficient oxygen to burn any iuel a~ it is
~upplied.
In operation, coal is blown along the duCt 24 at predetermined
interval~ and if the bed is at the correct temperature, the bed does not
require rurther iuel and the valve 26 i~ in the position to return the
fuel to store. If the bed temperature fall~ below the required value,
the valve 26 is operated by the relative movement between the tube 30
and the quartz rod 34 so that ruel iY added to the bed. The bed
temperature will rise and the signal produced by the temperature sensitive
device will operate the valve 26 to reduce or cut ofi the ilow oi ~uel to
the bed. In this manner~ the bed i8 maintained in a steady state
condition at a substantially constant predetermined temperature.
The power plant described above is analogous to a conventional liquid
oi gas iuelled gas turbine power plant with the diiierence that the
1~ relatively large mass oi coal ash in the bed acts as a heat reservoir and
thereiore the power output Or the engine can only be altered relatively
slowly,
To give rapid control of the power plant, it is proposed to run the
fuel-weak bed 14 at a constant near maximum temperature oi about 900 C, and
to alter the air flow through it (to reduce the net heat output to the
turbine) by by-pasSing a rraction of the incoming air and mixing it with
the hot gas output oi the bed. This is done by a rapid acting by-pass
valve 26 of known type and thi~ valve i9 the immediate short period control
~or the dynamics oi the power plant which are then only limited by the
volumes o~ air and gas in the bed and its connections~ Again, surplus
stored energy in these volumes can be di~sipated by short period blow-o~i
valves (not shown) to maintain the highest rates of response in power
reduction.
If the situation when part Or the air has suddenly been by-passed, e~g~
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50~ is considered, the rate oi reaction oi coal char particles in the
bed may not be signiiicantly aiiected by the reduction of air ilow a~
there i~ still plenty Or surplus oxygen, and heat would continue to be
generated at the original rate, and this would be stored in the inert ash,
producing a temperature rise which would continue $or a considerable
period even ii iurther coal were not added to the bed, since the burnlng
time Or a coal particle in the bed may well be oi the order o~ minutes.
As a result, the bed temperature could rise by the order oi 100-200 C,
which would cau~e a~h melting and clinkering, together with probable
release o~ volatile salt~ oi alkall metals.
While ~uch temperature ri~es could be quenched, e,g. by the addition
oi water (turning it into steam) the extra volumes of steam generated in
thi~ iashion would upset the aerodynamics o~ the turbine and lead to
compre~sor sur~ ing.
This problem is avoided by shutting ofi one or more chambers 14a oi
the iluidi~ed bed through which the air is blown 80 that although there i~
coal still leit in the unblown area Or the ~luidising bed, it would not
burn or generate heat because there would be no oxygen going through lt.
In the case where it i~ required to reduce the net heat output by 50%, up
to 50% oi the area oi the iluidised bed would be blocked out of operation,
and the ash and coal would be allowed to slump.
Interaction between parts oi the bed which are slumped and those which
were iluidised, is Prevented by the walls which separate the chambers oi
the bed associated with the valves 14c controlling the air ilow to the
appropriate sections.
The control valves 14c can be operated together or independently oi
each other to open or close or vary the ilow oi combustion and i'luidising
air to any or all of the chambers 14a.
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In an arrangement not shown~ some of the chambers 14a are not
provided with a control valve and are always in operation while the
engine is running whilst the remaining chambers which do have control
valYes act as the control for modulating the heat output o~ the bed to
suit the reduced power associated with operation of the power control
by-pass valve,
Whilst the invention described utilises coal as the particulate
~uel, other particulate fuels may also be used.
