GAS TURBINE ENGINE WITH FLUIDISED BED COMBUSTION

The present invention relates to a fluidised bed com'jistion 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 by the rate of supply of air, so that at the top of the bed only a small amount of oxygen remains 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 2,000oC if the heat were not removed from the bed by some method such as water pipes.

The present invention seeks to provide an alternative 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, the method comprising supplying r c combustion air in substantial excess of that required for stoichiometxic combustion and metering a supply of 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.

-2A- The fluidised bed combustion apparatus may be used to heat the working fluid of a gas turbine 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 bed 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 plant 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.

A, 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 -3A- valves 14c. Each chamber 14a contains inert materials including coal ash and an amount of 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 500oC 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.

Referring to Fig. 2, the coal supply means to each chamber 14a which is generally similar to the coal supply comprises a duct 24 along which a suspension of coal particles in air can be blown at intervals or continuously into the bed. The 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 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 which fuel is supplied to the bed as there is -4A"- an excess of air and thus sufficient oxygen to burn any fuel as it is supplied.

In operation, coal is blown along the duct 24 at predetermined intervals and if the bed is at the correct temperature, the bed does not require further fuel and the valve 26 is in the position to return the fuel to store. If the bed temperature falls below the required value, the valve 26 is operated by the relative movement between the tube and the quartz rod 34 so that fuel is 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 off the flow of fuel to the bed. In this manner, the bed is maintained in a steady state condition at a substantially constant predetermined temperature.

The power plant described above is analogous to a conventional liquid of gas fuelled gas turbine power plant with the difference that the relatively large mass of coal ash in the bed acts as a heat reservoir and therefore the power output of 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 of about 900oCf and to alter the air flow through it (to reduce the net heat output to the turbine) by by-passing a fraction of the incoming air and mixing it with the hot gas output of the bed. This is done by a rapid acting by-pass valve 26 of known type and this valve is the immediate short period control for the dynamics of the power plant which are then only limited by the volumes of air and gas in the bed and its connections. Again, surplus stored energy in these volumes can be dissipated by short period blow-off valves (not shown) to maintain the highest rates of response in power reduction.

If the situation when part of the air has suddenly been by-passed, e.g.

507c is considered, the rate of reaction of coal char particles in the bed may not be significantly affected by the reduction of air flow as there is still plenty of surplus oxygren, 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 for a considerable period even if further coal were not added to the bed, since the burning time of a coal particle in the bed may well be of the order of minutes.

As a result, the bed temperature could rise by the order of 100-200oC, which would cause ash melting and clinkering, together with probable release of volatile salts of alkali metals.

While such temperature rises could be quenched, e.g. by the addition of water (turning it into steam) the extra volumes of steam generated in this fashion would upset the aerodynamics of the turbine and lead to compressor surging.

This problem is avoided by shutting off one or more chambers 14a of the fluidised bed through which the air is blown so that although there is coal still left in the unblown area of the fluidising bed, it would not burn or generate heat because there would be no oxygen going through it.

In the case where it is required to reduce the net heat output by 50%, up to 50% of the area of the fluidised bed would be blocked out of operation, and the ash and coal would be allowed to slump.

Interaction between parts of the bed which are slumped and those which were fluidised, is prevented by the walls which separate the chambers of the bed associated with the valves 14c controlling the air flow to the appropriate sections.

The control valves 14c can be operated together or independently of each other to open or close or vary the flow of combustion and fluidising air to any or all of the chambers 14a.

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 valves act as the control for modulating the heat output of 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 fuel, other particulate fuels may also be used.

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS? 1. 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, the method comprising supplying combustion air in substantial excess' of that required for stoichiometric combustion and metering a supply of particulate fuel into the bed to maintain the bed at substantially predetermined temperature.

2. A method as claimed in claim 1 in which the ratio of inert materials to particulate fuel lies within the range 100:1 to 1000:1.

3. A method as claimed in claim 1 in which a variable proportion of the fluidising and combustion air is by-passed around the fluidised bed combustion apparatus and mixed with the air which has been heated in the combustion apparatus.

4. A fluidised bed combustion apparatus comprising a vessel containing a fluidisable bed of inert materials and particulate fuel, the vessel having a distributor plate for the inflow of fluidising and combustion air from a source of compressed air, a duct for the inflow of particulate fuel, and fuel flow control apparatus, the inflow of fluidising and combustion air, being substantially in excess of that required for stoichiometric combustion, the heat of combustion being transferred only by direct contact of the fuel with the combustion air, the fuel being metered by the fuel flow control apparatus in dependence of the required temperature of the fluidised bed.

5. A fluidised bed combustion apparatus as claimed in claim 4 has a by-pass duct connected between the inflow of fluidising and combustion air and an outlet duct from the bed carrying the