GB2331128A

GB2331128A - Gas-fuelled gas turbine power generation apparatus

Info

Publication number: GB2331128A
Application number: GB9723291A
Authority: GB
Inventors: John Andrew Gordon Brown
Original assignee: MAGNOX ELECTRIC PLC
Current assignee: MAGNOX ELECTRIC PLC
Priority date: 1997-11-04
Filing date: 1997-11-04
Publication date: 1999-05-12
Anticipated expiration: 2017-11-04
Also published as: GB2331128B; GB9723291D0

Abstract

Power generation apparatus comprises a generator 10, a gas turbine 1, arranged to drive the generator, fuel gas supply means, including storage means 3, arranged to store a buffer reservoir of fuel gas, and a control valve 4, which is operable to control the flow of fuel gas from the storage means 3 to the gas turbine, in response to transient generator power output demand. A gasification combined cycle power generating apparatus is also disclosed (figures 6 and 7) in which a gasification vessel (in which fuel gas is produced from carboniferous feed stock) is connected to a saturator vessel which is connected to a control valve at an inlet of a gas turbine engine unit. A controllable gas-expanding turbine, driving a generator, is disposed between the gasification vessel and the saturator vessel. A control computer, responsive to power demand on the gas turbine engine unit, controls the control valve and may also control the gas-expanding turbine.

Description

POWER GENERATION APPARATUS The present invention relates to power generation apparatus and facilities incorporating gas turbines arranged to drive electrical generators.

Gas turbines fuelled with liquid fuels are able to provide rapid response to throttle changes. This is because the fuel is almost incompressible, and flow changes can therefore be accommodated rapidly.

In the past, gas turbines fuelled with gaseous fuels have not been able to provide the same level of response, because flow increase driven by pressure rise from a distant upstream reservoir cannot travel faster than the speed of sound in the gas.

In the case of electrical power generation, a known system comprising a gas turbine is shown in Fig.

1. The gas turbine 1 is arranged to drive an electrical generator 10. The hot exhaust gases from the turbine 1 are used to raise steam, which in turn is used to generate more electrical power, thereby increasing the overall efficiency of the process.

This is an example of Combined Cycle (CC) electrical power generation.

A control computer 2 receives various input signals and operates the control valve (or valves) accordingly. These input signals may include: load signals indicative of the overall electrical power output of the generator; a grid frequency signal, which can act effectively as a demand signal; set point signals indicating a desire to run at, say, fixed power output, or fixed gas turbine inlet temperature; and temperature signals from the gas turbine corresponding to the temperature of the flame, exhaust gases, the blades, or other parts of the machinery.

The control valve is situated very close to the fuel gas inlet of the turbine (typically there is a pipeline of less than 3m in length connecting the control valve(s) to the combustor) and controls the flow of fuel gas from the supply into the turbine.

In this example, the fuel gas is natural gas and the control valve is connected directly to the mains supply.

In response to a demand for increased power output, the control computer 2 opens the control valve 4 to increase the flow rate of fuel gas into the turbine. Slow increases in demand may be tolerated, but if the increase is too rapid, then rapidly opening the control valve(s) causes a pressure drop in the pipeline which starves the gas turbine of fuel.

This pressure drop travels back along the main supply pipework at the speed of sound in the gas to an upstream source of gas at higher pressure, which may be a considerable distance away. A pressure rise then travels from the upstream source to the control valve at a speed limited by the speed of sound in the gas.

Until this increased pressure arrives at the gas turbine control valve 4, gas flow at the demanded increased rate, and hence increased electrical power generation, cannot be sustained. The load may actually fall at a time when demand is rising.

Such delays in response are unacceptable in certain applications. For example in the UK, in order for an electrical power generation facility to be connected to the National Grid it must meet the "Grid Code for Flexibility". This means that the facility must be able to meet the output response curve shown in Fig. 2, subject of course to any demands on power output being within the facility's capacity. Up to time t = 0, the electrical power output of the facility is constant at a level shown in the figure as 1 (in arbitrary units). At time = 0, a 10% increase in load (ie. power output) is required within five seconds, and then must be maintained at 5% from ten seconds from the initiating event for up to twenty minutes. In order to provide this response, the system must be able to provide effective energy storage equal to the area shown as shaded on the graph, until the flow can be re-established at a higher level as the system further back responds (see Fig. 3). Such energy is not usually available local to the gas turbine, which normally has a pipeline of significant distance to a point of higher pressure.

It is also known to supply a gas turbine with fuel gas obtained from a gasification process. In this process a carboniferous feedstock (eg. coal, waste oil, biomass etc.) is reacted in a gasification vessel or chamber with insufficient oxygen for complete combustion. The gas produced contains principally carbon monoxide and hydrogen (together known as Syngas or town gas) and, typically, hydrogen sulphide. In response to a demand for increased power output from the gas turbine, one can, therefore, increase gas production by introducing more feedstock and oxygen into the gasifier vessel. However, because of the time constants involved, the response of such a system is relatively slow. For example, in Combined Cycle power generation applications incorporating gasifiers (which shall be referred to as Gasification Combined Cycle processes, GCC) a typical response to increased demand is 20% per minute. This is not fast enough to meet the requirements of the Grid Code for Flexibility.

Therefore, it is an object of the present invention to provide power generation apparatus with improved response to demands for increased power output.

According to a first aspect of the present invention there is provided power generation apparatus comprising: a generator: a gas turbine arranged to drive the generator; and fuel gas supply means including: storage means arranged to store a buffer reservoir of fuel gas and to provide fuel gas from the buffer reservoir to the gas turbine in response to transient increases in demand on the power output from the generator; a control valve at an inlet of the gas turbine, operable to control the rate of flow of fuel gas from the gas storage means into the gas turbine; and a gas supply line interconnecting the control valve and the storage means.

By incorporating a buffer reservoir of fuel gas the apparatus can respond to changes in demand more rapidly than prior art arrangements. The buffer reservoir enables the response of the gas turbine not to be limited by the inability of upstream gas reserves or fuel gas generation means to provide the additional mass flow rate of fuel gas necessary to meet demands for increased power output. The buffer may supply the additional energy needed to meet transient increases in demand.

The buffer reservoir may be in-line, or, alternatively, in parallel with a source of fuel gas.

The buffer reservoir may comprise fuel gas generation means, such as a gasifier.

The buffer reservoir may comprise the pressure vessels and pipework associated with the fuel gas generation means, such as a gasifier or sulphur removal plant.

Advantageously, the storage means may store the buffer reservoir at high pressure, which may exceed 60 bar.

The gas supply line may have a length of less than 300 metres, and in general, the shorter the supply line the faster the response of the turbine as the pressure rises needed to drive increased fuel gas flow rates have shorter distances to travel.

The storage means may have the capacity to store a buffer reservoir comprising a sufficient mass of fuel gas at sufficient pressure to enable the storage means to supply the gas turbine with fuel gas at a mass flow rate corresponding to the maximum rated power output of the gas turbine for at least 60 seconds. Thus the buffer may contain sufficient fuel gas reserves to keep the turbine running in the event of a temporary interruption in the supply of fuel gas or feedstock to the apparatus, or to provide additional fuel gas to meet transient increases in demand for power output.

Thus, the gas storage means may be capacious enough to provide increased fuel gas flow rate to the turbine for a substantial period of time, of course this time being dependent on the magnitude of the increase in flow rate. For example, if the reservoir contains a mass of gas equal to ten times the flow rate at a particular instant, then this reserve will be able to provide for a 10% increase in power output of the turbine for a period of approximately 100 seconds ie. just over one and a half minutes.

Advantageously, the fuel gas supply means may be adapted to provide the gas turbine with fuel gas sufficient to meet a demand for a 10% increase in power output over a nominal initial power output within 5 seconds, and then to sustain a 5 increase in power output over the nominal initial power output for at least a further 20 minutes, subject to these increases being within the maximum rated power output of the gas turbine. Thus the apparatus may be able to meet the United Kingdom Grid Code for Flexibility.

In systems where the fuel gas is supplied from an external source, such as the mains, the buffer reservoir in the gas storage means enables the system to cope with a demand for a rapid increase in output, and sustain that output for a period of time until the external supply is able to respond and provide fuel gas at the necessary increased rate.

In arrangements where the fuel gas is provided from a Gasifier, for example in Gasification Combined Cycle, the gas storage means can provide the fuel necessary to sustain increased power output until the fuel gas output of the Gasifier can be increased.

In addition to providing fuel gas necessary to sustain increased power output, the gas storage means may also provide fuel gas necessary to keep the turbine running if the fuel supply to the system is interrupted, for example owing to a fault, or as a result of a changeover to a different fuel supply (eg. in Gasifier system, changing to a different feedstock).

The gas storage means may be in-line between the source of fuel gas (eg. the mains or the gasifier vessel) and the gas turbine, or, alternatively, in parallel.

Advantageously, the power generation apparatus may further comprise a controllable pressure drop device between the storage means and the control valve, and a controller responsive to power demand for controlling the gas turbine inlet control valve and arranged to adjust the controllable pressure drop device to control the pressure of fuel gas supplied to the control valve.

This may improve the controllability of the gas turbine and may give even faster response times. The controllable pressure drop device may be adjusted as soon as the controller receives a demand for increased output to increase the pressure of the fuel gas supplied to the control valve. Thus the pressure increase from the gas storage means may begin travelling to the control valve immediately, rather than in response to receiving a low pressure wave from the control valve as a result of it being opened rapidly.

The controllable pressure drop device controls the supply pressure of bulk gas to the control valve, which in turn controls the more sensitive admission of gas to the gas turbine.

The controllable pressure drop device, which shall also be referred to as a variable throttle means, may comprise a gas expander, by which it is meant a gas expanding turbine. The flow of fuel gas through the gas expander may be controlled by varying the load on its rotor. Advantageously, the gas expander may be arranged so that work may be extracted from the fuel gas during the expansion (i.e. pressure reduction, or throttling) process. For example, the gas expander may be arranged to drive an electrical generator, and the pressure of the fuel gas supplied to the control valve may be controlled by varying the load on that generator.

Advantageously the controllable pressure drop device may already be present in the apparatus design for other purposes. For example, in Clean Power Generation systems (CPG), the variable throttle means may comprise the gas expander used to reduce the pressure of fuel gas from the gasifier (at high pressure in order to improve the efficiency of the gasification process) before it enters the saturator.

CPG, invented by Nurse, Arundale and Griffiths, is the subject of Patent Nos. EP-B-0384781 and GB-B-2234984 granted to Jacobs Engineering. The gas expander may be arranged to drive a generator already present in the system to increase its overall efficiency.

The high pressure of the fuel gas stored in the storage means may be achieved using a compressor, which may already be present in the design or system for other purposes. For example, where the fuel gas is produced in a gasification process, the compressor may be that used to compress the oxygen before it is introduced into the gasifier vessel, in order to increase the efficiency of the process.

Advantageously, the apparatus may be incorporated in a Combined Cycle (CC) electrical power generation facility, where the hot exhaust gases from the gas turbine are used to raise steam which is then used to drive steam turbines.

The Combined Cycle facility may be a Gasification Combined Cycle (GCC) electrical power generation facility, and the storage means may comprise the gasification vessel.

Advantageously, the Gasification Combined Cycle facility may be a Clean Power Generation (CPG) facility, and the gas expander may be arranged in-line between the gasification vessel and the saturator vessel.

According to a second aspect of the present invention there is provided power generation apparatus comprising: a generator: a gas turbine arranged to drive the generator; and fuel gas supply means including: a gasification vessel in which fuel gas is produced from a reaction between oxygen and a carboniferous feedstock; a control valve at an inlet of the gas turbine, operable to control the rate of flow of the fuel gas into the gas turbine; and a gas supply line interconnecting the control valve and the gasification vessel, said gas supply line including a saturator vessel in which the fuel gas is saturated with water vapour before reaching the control valve, a controllable pressure drop device between the saturator and gasification vessels for reducing the pressure in the saturator, and a controller responsive to power demand for controlling said gas turbine inlet control valve and arranged to adjust said controllable pressure drop device to control the pressure of fuel gas supplied to said control valve.

Advantageously the gasification vessel may operate at a pressure of at least 60 bar, in order to increase the efficiency of the gasification process.

The controllable pressure drop device may comprises a gas expander, which may be arranged to drive an electrical generator.

Embodiments of the present invention will now be described with reference to the accompanying drawings in which: Fig. 1 is a schematic diagram of a known Combined Cycle Gas Turbine electrical power generation system; Fig. 2 is a schematic diagram of a response curve which may be required for certain gas turbine system applications, in particular for electricity generation; Fig. 3 is another schematic diagram of a typical desirable response curve; Fig. 4 is a schematic diagram of a combined cycle gas turbine (CCGT) electrical power generation facility in accordance with an embodiment of the present invention; Fig. 5 is a schematic diagram of a Clean Power Generation system (CPG) in accordance with the prior art; and Fig. 6 is a schematic diagram of a CPG system in accordance with an embodiment of the present invention.

Referring now to Fig. 4, in a first embodiment of the present invention, a CCGT system is supplied with fuel gas from the mains at low pressure (typically in the range 15 to 35 bar). This fuel gas is compressed by a compressor 11 and a reservoir of compressed fuel gas is stored in gas storage means 3. A control computer 2 monitors the pressure of the reservoir and controls the compressor 11 accordingly. The normal control functions, i.e. the supply of fuel gas to the gas turbine 1, are provided by the control valve 4 in response to signals from the control computer 2, which also receives signals indicative of current power output of the electrical generator 10 (load signal), the temperatures of various parts of the gas turbine 1, the demand (grid frequency signal) and desired operating modes (the set point signals, which may correspond, eg, to constant power output operation, constant frequency, constant turbine inlet temperature or other factors).

The control computer 2 also sends signals to control the variable throttle device 5 at the outlet of the gas storage means 3, in order to provide gas at a suitable pressure to the gas turbine control valve.

As soon as the control computer receives a demand for increased power output, it opens the variable throttle device (allowing an increased mass flow rate of fuel gas through the device) to increase the pressure of gas supplied to the control valve 4. The control valve is also opened to allow more gas into the turbine.

As there is only a short pipe length between the variable throttle device 5 and the control valve 4, the pressure of the fuel gas at the control valve can be increased very rapidly, and so the system can quickly respond to increases in demand.

Air at atmospheric pressure is supplied to the compressor stage 101 of the gas turbine 1, and the pressure of this air as it enters the combustor 102 is determined by the design of the compressor stage 101 and the turbine speed.

The pressure of fuel gas supplied to the combustor 102 must always be greater than the air pressure. Thus, the pressure of air supplied to the combustor 102 when the turbine is running at its maximum rated speed represents the minimum pressure necessary to inject fuel gas into the combustor at this speed. According to the present embodiment, the storage means can store fuel gas at at least twice this pressure.

Hot exhaust gases from the expander stage 103 of the turbine are used to raise steam which is also used to generate electrical power.

The present invention is particularly advantageous, improving flexibility, when applied in conjunction with the Clean Power Generation system (CPG) (the subject of Patent Nos. EP-B-0384781 and GB B-2234984) to Gasification Combined Cycle power production (GCC).

The gasification process has an efficiency of about 80%. Some of the 20% loss is in the form of heat in the gasses produced, which exit the gasifier at about 8000C. Some of this lost energy can be recovered by using a radiant boiler situated next to the gasifier outlet. Steam thus generated is sent to the conventional part of the cycle, and the energy extracted at Rankine efficiencies. It is current practice to cool the gases in any case, in order to remove the sulphur. However, the radiant boiler is an expensive item, and results in a Gasification Combined Cycle (GCC) process which is efficient (about 45% for oil-based fuels, 39% for coal), but expensive (typically > f730/kW turnkey capital cost).

An alternative is to quench the gasses at the gasifier exhaust, using a water spray. This reduces the temperature to about 3000C, and the energy remaining can be recovered into the feedheating section of the conventional part of the cycle. The cooled gas is cleaned of sulphur in the same way as for the slagging gasifiers. For a loss in efficiency of about three percentage points, the capital cost is reduced by about 15%. Other advantages of Quench Gasifiers include increased fuel flexibility, reduced corrosion (no boiler),reduced land requirement, and the ability to operate at higher pressures (as there is no boiler shell) which further improves the efficiency of the process. However, GCC plants are not able to meet the requirements of the UK Grid Code for Flexibility because of the thermal inertia of the process.

Most of the heat lost in the quench can, however, be recovered by first cooling the gas (recovering the heat into the feedheating section of the process), then saturating the gas with water, having reduced the pressure through a Gas Expander from which power can be extracted by using a generator. This process, known as CPG (Clean Power Generation), recovers all but about 1/2% lost in the Quench, thus giving GCC with CPG an efficiency of about 44.5 on oil-based fuels, and 38. 5W on coal. A schematic diagram of the CPG process is shown in Fig. 5. Saturating the gas in this way has the additional advantage that the amount of power extracted from the Gas Turbine is greater for a given amount of gas burned, due to the increased mass of gasses passed through. Operation of a large proportion of the process at reduced temperatures allows less expensive materials in the pipework to be used, thus lower capital costs. Since the flame in the Gas Turbine is cooled by the addition of the water, CPG also assists in the reduction of NOX emissions from the Gas Turbine, without the need specifically to develop low NOX burners suitable for use with high hydrogen content, low CV (calorific value) gasification gases.

Gasifying at high pressure (typically 62 bar) improves the efficiency of this part of the process and also improves heat transfer in the downstream heat exchangers (as the fuel is more dense). However, the pressure of the fuel gas must be reduced before it is introduced into the saturator which is not a pressure vessel (typically it operates at 30 bar). Therefore, the CPG system incorporates a gas expander. To improve the overall efficiency of the plant, the expander (a gas expanding turbine) is arranged to drive an electrical generator.

Fig. 6 shows a schematic diagram of the CPG process adapted in accordance with an embodiment of the present invention. The control computer 2 now controls the pressure of gas supplied to the control valve via the saturator by varying the load on the generator, ie. the existence of a Gas Expander in the CPG process is used in the Gas Turbine gas control circuit to provide fuel gas at the required lower pressure; reducing the load on the expander can allow more gas (if the load on the gas turbine, i.e. the power output of the generation facility, is to be increased) to the Gas Turbine, the mass of gas upstream acting as a buffer. Conversely more load on the expander will allow the Gas Turbine load to be reduced, and the upstream inertia can be controlled independently.

In other words, the variable throttle functionality is provided by reducing or increasing the load on the expander electrical generator, thus increasing or decreasing the pressure in the downstream pipework respectively. In Fig. 6 the high pressure storage of gas is provided by the volumes contained in the heat exchanger, sulphur removal and gasifier pressure vessels.

This embodiment, which represents the Magnox Gasification Combined cycle process currently being evaluated, uses CPG with gas fuel volumes to the Gas Turbine controlled by the Gas Expander. Even if the gasifier were to cease operation, about four minutes' worth of gas remains in the system to permit change to an alternative fuel (eg. distillate). This feature, coupled with other cost-saving measures, gives the process what is currently believed to be the lowest unit cost and highest flexibility of any GCC process.

The process takes advantage of the benefits of being able to use coal or oil-based feedstocks, with high efficiency, low capital cost, good environmental performance and operational flexibility to meet the full requirements of the UK Grid Code.

Claims

CLAIMS 1. Power generation apparatus comprising: a generator: a gas turbine arranged to drive the generator; and fuel gas supply means including: storage means arranged to store a buffer reservoir of fuel gas and to provide fuel gas from the buffer reservoir to the gas turbine in response to transient increases in demand on the power output from the generator; a control valve at an inlet of the gas turbine, operable to control the rate of flow of fuel gas from the gas storage means into the gas turbine; and a gas supply line interconnecting the control valve and the storage means.
2. Apparatus in accordance with claim 1, wherein said gas supply line has a length of less than 300 metres.
3. Apparatus in accordance with claim 1 or claim 2, wherein said storage means has the capacity to store a buffer reservoir comprising a sufficient mass of fuel gas at sufficient pressure to enable the storage means to supply the gas turbine with fuel gas at a mass flow rate corresponding to the maximum rated power output of the gas turbine for at least 60 seconds.
4. Apparatus in accordance with any preceding claim wherein the fuel gas supply means is adapted to provide the gas turbine with fuel gas sufficient to meet a demand for a 10% increase in power output over a nominal initial power output within 5 seconds, and then to sustain a 5% increase in power output over the nominal initial power output for at least a further 20 minutes, subject to said increases being within the maximum rated power output of the gas turbine.
5. Apparatus in accordance with any preceding claim, further comprising a controllable pressure drop device between the storage means and the control valve, and a controller responsive to power demand for controlling said gas turbine inlet control valve and arranged to adjust said controllable pressure drop device to control the pressure of fuel gas supplied to said control valve.
6. Apparatus in accordance with claim 5, wherein said controllable pressure drop device comprises a gas expander.
7. Apparatus in accordance with claim 6 wherein said gas expander is arranged to drive an electrical generator.
8. Apparatus in accordance with claim 7 wherein the pressure of fuel gas supplied to the control valve is controlled by varying the load on said electrical generator.
9. A Combined Cycle (CC) electrical power generation facility comprising apparatus in accordance with any preceding claim.
10. A Gasification Combined Cycle (GCC) electrical power generation facility comprising apparatus in accordance with any one of claims 1 to 8.
11. A Gasification Combined Cycle electrical power generation facility in accordance with claim 10 wherein said storage means comprises a gasification vessel.
12. A Clean Power Generation (CPG) facility comprising a Gasification Combined Cycle electrical power generation facility in accordance with claim 10 or claim 11 as dependent from claim 6, wherein said gas expander is arranged in-line between the gasification vessel and the saturator vessel.
13. Power generation apparatus comprising: a generator: a gas turbine arranged to drive the generator; and fuel gas supply means including: a gasification vessel in which fuel gas is produced from a reaction between oxygen and a carboniferous feedstock; a control valve at an inlet of the gas turbine, operable to control the rate of flow of the fuel gas into the gas turbine; and a gas supply line interconnecting the control valve and the gasification vessel, said gas supply line including a saturator vessel in which the fuel gas is saturated with water vapour before reaching the control valve, a controllable pressure drop device between the saturator and gasification vessels for reducing the pressure in the saturator, and a controller responsive to power demand for controlling said gas turbine inlet control valve and arranged to adjust said controllable pressure drop device to control the pressure of fuel gas supplied to said control valve.
14. Apparatus in accordance with claim 13, wherein the gasification vessel is operable at a pressure of at least 60 bar.
15. Apparatus in accordance with claim 13 or claim 14, wherein said controllable pressure drop device comprises a gas expander.
16. Apparatus in accordance with claim 15, wherein said gas expander is arranged to drive an electrical generator.
17. Apparatus in accordance with claim 16, wherein the pressure of fuel gas supplied to the control valve is controlled by varying the load on said electrical generator.
18. A Combined Cycle (CC) electrical power generation facility comprising apparatus in accordance with any one of claims 13 to 17.
19. A Clean Power Generation (CPG) facility comprising apparatus in accordance with any one of claims 13 to 17.
20. Power generation apparatus substantially as hereinbefore described with reference to the accompanying drawings.
21. A Combined Cycle electrical power generation facility substantially as hereinbefore described with reference to the accompanying drawings.
22. A Gasification Combined Cycle electrical power generation facility substantially as hereinbefore described with reference to the accompanying drawings
23. A Clean Power Generation facility substantially as hereinbefore described with reference to the accompanying drawings.