AU698257B2

AU698257B2 - Improvements in the combustion and utilisation of fuel gases

Info

Publication number: AU698257B2
Application number: AU37660/95A
Authority: AU
Inventors: Donald Ray Cummings
Original assignee: Isentropic Systems Ltd
Current assignee: Isentropic Systems Ltd
Priority date: 1994-10-27
Filing date: 1995-10-27
Publication date: 1998-10-29
Anticipated expiration: 2015-10-27
Also published as: AU698257C; AU3766095A

Description

HVIPROVEMEiNTS IN THE COMBUSTION AND UTILISAΗON

OF FUEL GASES Technical Field

The present invention relates to the improved combustion and utilisation of fuel gases. In one preferred embodiment, methods and apparatus are described that utilise gases from mines, landfills or the like that have marginal energy value and otherwise are wasted.

In this specification, the term "fuel gas" is to be understood as embracing a combustible gas such as methane or methane-containing gases such as natural gas, coal seam gas, sewage gas, landfill gas and the like, and carbon monoxide-containing gases such as blast furnace gas, steel-making gases, producer gases and the like. These combustible gases also may contain impurities such as nitrogen, carbon dioxide and air. In some instances, such as for mine ventilation air, the gases may be substantially diluted, and the ratio of such combustible gas to air must be either below the lower explosive limit or above the upper explosive limit of the mixture. Furthermore, "fuel gas" can include a mixture of purposefully admixed air with the combustible gas.

In the context of "fuel gas" provided to a combustor embodying the present invention, the term is to be understood as being the case of a mixture of combustible gas(es) and air such that the mixture is below the lower explosive limit of that mixture.

Background of the Invention

Many low quality fuels, such as blast furnace gas, and excessively diluted fuel gases such as methane-containing coal mine ventilation air, are flared or vented and thus not beneficially utilised due primarily to problems associated with the very high cost of established techniques and or the non-availability of suitable techniques and low cost equipment enabling the fuel's commercial utilisation. Very large volumes of these such fuel gases must be compressed to be utilised, and the mass flow rates through the compression and expansion stages of the gas turbine, and the associated combustion problems, are such that standard turbine designs cannot be used. A further problem with the use of gas turbines to utilise diluted fuel gases is that substantially all of the gas must be compressed to above the combustion pressure of the turbine to be fed as fuel to the turbine, and this compression is costly both in equipment and power requirements. In gas-prone underground coal mines, methane extraction may remove part of the total methane present in coal seams affected by underground mining as a methane fuel gas stream contaminated with air and other contaminants, such as carbon dioxide. The ratio of methane extracted by this technique is generally less than 50% of the total quantity of methane emitted during mining, with the greater part being emitted in the mine ventilation air. The methane content in ventilation air in these mines typically is between 0.8 and 1.0% . It has been proposed to utilise this otherwise wasted gas by ingestion in gas turbines as all or part of the air feed to the turbine, although it has been found that modern high efficiency and cost-effective gas turbines are generally not suited to this duty, as the required overall ratio of methane fuel to ingested air ratio for such turbines is generally in excess of 2% and normally closer to 3% by volume, hence even if the ingested methane in the methane-contaminated air could be burnt, and even with 1.0% by volume of methane in the air, only approximately one third of the turbine's fuel requirements would be provided. Such a turbine could not make effective use of the gas flows from such mines even if the combustion problems could be solved and mine drainage gas used as supplementary fuel.

With such dilute fuel gas streams, a further problem exists in that modern high efficiency turbines typically use in excess of 15% of the total air flow for cooling and purging purposes. Consequently a significant part of the methane in the ingested air by¬ passes the combustion stage, hence any fuel in that air can not be utilised and thus is wasted. In addition, it is known that methane by-passing the combustion stage and being added to the combustion gas flow at intermediate temperatures can form active radicals such as methyl-hydroxyl radicals which promote the conversion of nitric oxides formed in the combustion stage to toxic and visible nitrogen dioxide. Some low purity gases such as those from sewerage treatment, "landfill" operations, coal seams and the like may contain high concentrations of carbon dioxide up to an possibly in excess of 70% by volume. Such gases have the problem of being very difficult if not impossible to combust in conventional gas turbines and related combustion systems.

A paper entitled "The Elimination of Coal Mine Methane Emissions" in The Australian Institute of Energy's News Journal of June 1992, and authored by the present inventor, proposed to ingest methane containing coal mine ventilation air into a gas turbines, however this was proposed prior to fall realisation of the problems discussed above. At the time it was proposed that a water seal be used as a flame arresting and mine isolation device between the mine and the turbine combustion system, together with a special control valve for varying an amount of dilution air to ensure the intrinsic safety of operation of the turbine should the methane in the ventilation air exceed the pre-determined upper level of methane in ventilation air. This proposed system also had the problem of being relatively complex and the turbine would, without the safety control device, operate with a methane in air content of about % by volume, or higher if combustion was not complete. This would be unacceptably ose to the lower explosive limit of methane in air.

A yet further problem exists, in that if a mixture of fuel gas in air is to be provided for ingestion in a gas turbine or supplied to a combustor, and the mixture is to be below the lower explosive limit, explosive mixtures will exist transiently at the point of mixing. For gas turbines and large industrial units, low pressure drops in the mixing stage are desirable, and the risk and magnitude of any possible explosion should be minimal despite the large volumes of air and fuel gas required for gas turbines and industrial furnaces.

Despite the benefits of utilising the otherwise wasted gas, until now turbine manufacturers have not proposed a suitable turbine design.

More generally, Nitric Oxide (NOX) formation during the combustion of fuel gases is a general problem for a wide range of heating systems such as commercial and industrial combustors, furnaces, gas turbines and the like. In many applications, the desired temperature of the combustion gases formed by the combustion process is significantly less than the flame temperature of the gas burnt, however the level of NOX produced in the resultant gas is determined by the flame temperature and not the temperature of the final mixed gases. Typical of this problem is the combustion of fael in gas turbines where the combustion gases are required at temperatures in the order of 850°C to 1,200°C, but where NOX levels are deteπnined by flame conditions in the combustor, even with very advanced designs for burners (such as rich burn followed by lean burn) which are expected to reduce NOX in exhaust gases to below 20 ppm, the possibility of reducing NOX emissions to below 10 ppm is generally thought to be achievable only with the use of very special combustion systems such as catalytic devices.

Catalytic combustion systems generally rely on the use of noble metal catalysts such as platinum and palladium which have the problem of being rare and expensive elements, and which are also easily poisoned by impurities in some fael gases, one example being the presence of silanes in landfill gases.

A farther combustion problem exists where mixtures of gaseous fael and air exist and the level of fael in the air is below the lower flammability of the mixture, hence cannot be combusted using currently available non-catalytic combustion technology without requiring the combustion of additional fael to initiate and maintain combustion.

It is known to ingest hydrocarbon contaminated air in gas turbines to combust the contaminant and utilise it as part of the turbine fael, but generally only where the contaminant represents a minor portion of the fael. This technique is used primarily to reduce the emissions of the contaminant, and cannot be readily used to supply a significant portion of the gas turbine's fael requirements using conventional gas turbines due to limitations of conventional combustors.

It is a preferred objective of the present invention to overcome or at least ameliorate one or more of the foregoing problems in the prior art. Disclosure of the Invention

In accordance with a first aspect, there is disclosed a combustor for the combustion of fael gases, the combustor comprising a vessel having an inlet for the supply of fael gas, an array of hollow tubes having one end open to the supplied fael gas and the other end open to a combustion zone, the tubes being spaced-apart so that the outside of the tubes forms an exit path for combusted gas from the combustion zone passing to an outlet from the vessel, the heat of the combusted gas being partially transferred to the inlet gas within the tubes to preheat the inlet gas, the ratio of the supplied fael gas with air being less than the lower explosive limit for that fael gas, and combustion in said combustion zone occurring by reaction or self-combustion of the preheated fael gas.

The combustor can farther comprise baffle means between the tubes to form a tortuous exit path. The tubes can be arranged longitudinally of the vessel and said inlet and said outlet being proximate the same end of the vessel and the combustion zone being at the other end of the vessel. The tubes can be arranged in a hexagonal array by said baffle means formed of hexagonal shaped sleeves fitting over said tubes and interlocking adjacent sleeves. Further there can be a burner located adjacent said combustion zone for preheating said combustion zone to the combustion temperature before the fael gas is introduced to the combustor.

In accordance with another aspect, there is disclosed a method for combusting fael gases comprising the steps of: preheating supplied fael gas by heat given up by previously combusted fael gas; containing said preheated fael gas for a time sufficient for reaction or self- combustion to occur; discharging said combusted fael gas by a tortuous path to exchange heat with said supplied fael gas. Preferably, there is the farther step of disposing a combustion area at a temperature sufficient to cause combustion of said preheated fael gas on its supply.

In accordance with a further aspect there is disclosed a gas turbine system for the utilisation of fael gas to produce useful energy, the system comprising: a compression stage receiving said fael gas and outputting compressed fael gas; a combustor receiving said compressed fael gas, the fael gas having a ratio to air of less than the lower explosive limit for that fael gas, and preheating said compressed fael gas by heat transfer from combusted fael gas thereof before combustion of the preheated fael gas, the combustion occurring by reaction or self- combustion; and an expansion stage mechanically coupled to said compression stage, and receiving said combusted fael gas, the energy of expansion being converted to useful rotational energy. In a preferred form, the exhaust expanded gas from said expansion stage passes to a heat exchanger to give up heat to said compressed fael gas before reaching said combustor. Furthermore, an electrical generator or an air compressor is coupled to said expansion stage to convert said rotational energy to electrical energy. The supplied fael gas can be an admixture of coal mine drainage gas and ventilation air. The admixture can be formed by a mixing stage comprising a duct for said mine ventilation air in the path of which is a plurality of tubes receiving said mine drainage gas, the tubes having a plurality of holes therein to allow said mine drainage gas to be inducted to mix with said mine ventilation air. Most advantageously, the fael gas is methane bearing. The proportion of methane to air preferably is .≤. 2% . The invention yet farther discloses a gas turbine system as defined above incorporating a compressor as farther defined above as the compressor stage.

The invention yet farther discloses a method for utilising fael gas in a gas turbine to produce useful energy, the method comprising the steps of: compressing fael gas; preheating said compressed fael gas prior to combustion by heat given up by previously combusted fael gas; combusting said preheated fael gas by reaction or self-combustion; and expanding said combusted gas to produce useful energy. Advantageously there is the farthei; step of heating the compressed fael gas by heat exchange from exhaust gas following the expansion stage.

Description of the Drawings

Fig. 1 is a cross-sectional side view of a combustion chamber embodying the invention;

Figs. 2a and 2b show further embodiments of the combustion chamber;

Fig. 3a shows detail of a component heat exchanging tube;

Fig. 3b shows an array of the component tube of Fig. 3a;

Fig. 4 is a schematic diagram of apparatus for the utilisation of mine drainage and ventilation gases in generating electricity;

Fig. 5 is the same apparatus of Fig. 4, indicating operational parameters;

Figs. 6a and 6b show detail of the gas mixing device; and

Fig. 7 shows details of the gas mixing zone in the gas mixing device.

Best Mode of Performing the Invention

An embodiment of the invention is now described in more detail with reference to Fig. 1 which shows a simplified form of combustion chamber 2.

Fuel gas (e.g. methane) containing combustion air at a concentration level below the lower explosive level fed to the inlet header 14 of the combustion chamber 2 via a pipeline 20 to pass past a tubesheet 4 to the heat exchanger tube 6 where the gas is to be heated to its ignition temperature before entering the combustion zone 8. The elevated temperature, together with the volume of the high temperature combustion zone 8 provides the necessary residence time for the fael gas to react and self-combust via free radical-induced combustion reactions. The combustion thus is spontaneous in nature, with the methane component gas being converted to carbon dioxide gas and water vapour. The obviation of the need to provide a flame for the combustion occurs means that there will be very low production of NOX compounds, even as low as < 5 ppm. The hot gases leaving the combustion zone 8 pass over the external side of the heat exchanger tubes 6 with the gases being mixed and directed across the external surface tubes by a series of baffles 10 in a tortuous or circuitous path before exiting via duct 22, substantially combusted and at the desired temperature for provision to the expansion stage of a gas turbine. The combustion chamber 2 is suitably lined with insulating material and has a heat resisting inner lining such as ceramic tiles and/or rings. Where the tube 6 is made of non-metallic material, such as silicon carbide, the tube is cemented into the metallic tubesheet 4 and the tubesheet 4 is suitably insulated against the high temperature combustion gases on the side corresponding to the exit duct 22. The combustion chamber 2 is brought up to operating temperature by a burner

12 which is fed separately with fael from a pipeline 24 and air from a separate pipeline 26, both of which can be derived from the same source of the fael and air in supply pipeline 20. Once the operating temperature (for methane approximately > 1000°C) is achieved, the burner 12 is shut down and cooled by a continuing small flow of air only to the burner 12. The burner can also operate at low rating as a pilot burner.

Fig. 2a shows a typical preferred arrangement for a combustor based on the simplified combustor 2' described above. An array of heat exchanging tubes 6 are provided, the lower end of each of which is open to the supply pipeline 20. The baffles 10 are arranged for conventional cross-flow baffling. Figure 2b shows a similar combustor 2" with extension plates 16 arranged to ensure continuous upward flow followed by segregated downward flow to the heat exchanger zone.

Fig. 3a shows a section of a silicon carbide tube having a hexagonal baffle section integral with the tube 6, while Fig. 3b shows the array of tubes 6 where the hexagonal baffle supports 10 are staggeredly arranged to form a tortuous path to the exit duct 22, ensuring effective heat exchange.

Example

In one embodiment, the combustor 2" is used to fael a turbine similar to a Centaur 3000R recuperated gas turbine with the combustor located external to the turbine casing and the turbine fael, as methane, ingested into the turbine together with the inlet air. The turbine has following basic dimensions and characteristics.

Air/methane feed gas flow 17.4 kg/second

Analysis Saturated with water at 24°C at atmospheric pressure

1.6% methane (vol/vol dry basis)

98.4% air (vol/vol dry basis)

Air/methane inlet temperature 400°C

Combustion gas exit temperature 850°C

Combustor internal diameter 1,190 mm

Tube bundle:

No of tubes 1,794

Tube outside diameter 19 mm

(spaced on triangular pitch)

Tube length 2,000 mm

No of baffles 5

Top of tubes to top of combustor 1 ,000 mm

The tubes are 2 mm thick Sandvik 253-MA alloy serrated on the inside by a BSW "fine" tap die and electro-coated with nickel. The tubesheet 4 and baffles 10 are also of Sandvik 253-MA alloy. The external (high temperature) surfaces of the tubes can be coated with a ceramic oxide such as Mullite. A combustor such as described above also has application in gas turbines, refinery furnaces and other forms of process furnaces.

In a yet farther embodiment, the tubes 6 can have installed within them rods of porous ceramic which may be impregnated by known means by known catalysts such as nickel for methane-containing faels, or known zinc oxide based catalysts for carbon monoxide-containing faels.

A farther embodiment of this invention, applied to the use of methane- contaminated coal mine ventilation air and mine drainage gas, now will be described with reference to Fig. 4, that shows a generalised gas turbine 100. The compression stage 120 of the gas turbine is coupled to an expansion stage 104 by a drive shaft 108. In turn, the expansion stage 104 is coupled to an electrical generator 106 by a farther drive shaft 110. The electrical output of the generator 106 is power recovered from the mine ventilation air and drainage gas can be supplied to an electricity grid, thus representing a saleable commodity. Methane-contaminated mine ventilation air passes by an inlet duct 250 to a mixing device 112 (as presently will be described), farther into which a controlled amount of mine drainage gas, supplied by a farther inlet duct 252, is added. The mixture then passes via an outlet duct 254 by the induced suction of the gas turbine compressor 102. Compressed air containing methane then leaves the compressor 102 via an outlet duct 256 and to pass to a recuperator 116, in which it is heated in a heat exchange manner, and passes via a duct 258 to a combustor 114. The combustor 116 is constructed in accordance with the teaching of Figs. l-3b, and for the purpose of this example, in accordance with particular specification of the proceeding Example. The hot (combusted) gases leaving the combustor 114 pass via a duct 262 to the expansion stage 104.

Exhaust gas from the expansion stage 104 leave via an outlet duct 264 to pass through the recuperator 116, and the cooled exhaust gases then leave the recuperator 116 via an outlet duct 266 to pass to atmosphere. A waste heat boiler can be fed with the cooled turbine exhaust gas leaving the recuperator 116 to generate steam for cooling turbine discs, bearings and other components normally cooled by air from the turbine's compressor, thereby forcing or enabling all or substantially all of the methane (fael gas) contaminated air ingested into the turbine to be passed through the combustion system.

Start-up fael for the turbine, in this case distillate fael, is fed to the combustor 114 via a pipeline 260 together air for the burner that is fed via a farther pipeline 268.

Multiple turbines will be utilised for most applications, and operate at close to their rated capacity. Major variations in load can be made by shutting-down individual turbines, and some variation of the target turbine load can be made by varying the temperature set for the gas exiting the combustor 114 in the outlet duct 262. Load variation also can be made by variation of the mass flow rate of fael through the turbine by known means, such as by use of variable inlet vanes incorporated in the compressor stage 102 and expansion stage, In a farther preferred version of this invention, the gas turbine 100 shown in

Fig. 4 has substantially all of its fael requirements supplied by methane in the inlet air and by a controlled injection of the fael gas into the suction stream of the compressor 102.

Turbines such as the Westinghouse CW191PG and the Solar Centaur 400R, both of which are currently not in production, and the General Electric Frame-1,

Frame-2 and Frame-5 turbines which are regarded as old and dated designs, are ideally suited to be used in embodiments of the invention.

The turbine models listed, when modified to include the combustor 114 described, would be inoperable with methane in the ingested air in excess of 2.0% by volume, which is significantly below the lower explosive limit for methane in air. Where the turbine is used to ingest mine ventilation air, the turbine itself becomes an effective monitoring device paralleling, and independent of, any regulatory analytical instruments or other mine safety devices. Fig. 6a and 6b show details of a duct structure where the inlet mine ventilation air and mine drainage gas are mixed in the mixing device 112. Fig. 6a is a cross- sectional view along line A- A as shown in Fig. 6b. The duct 250 includes a plurality of spaced-apart vertically arranged hollow tubes 150 that are interconnected by a top and bottom connecting pipes 152,154. The top pipe 152 receives the supply of mine drainage gas from inlet duct 252 to be fed down the vertical tubes 150. As best can be seen from Fig. 7, each of the vertical tubes 150 has an array of small holes 160 in the perimeter, from which the drainage fael gas exits to be mixed with the mine ventilation air. The space between adjacent tubes acts as a form of venturi mixer, with the gas and air velocities at the point of fael gas injection being such as to prevent any stable flame operation in the unlikely event of a source of ignition being present.

Example

The embodiment shown in Fig. 5 is based on use of a Solar Centaur 3000R turbine unit. In this example, turbine speed control is achieved by control of power drawn from the generator 106 and the rate of gas fed to the turbine system via the inlet pipeline 252 to control gas input to maintain a pre-set temperature range for the combustor outlet temperature in the outlet duct 262. The flow rate of the component parts of the mine ventilation are 13.306 N.m³/s of air, 0.134 N.m³/s of methane and 0.403 N.m³/s of water vapour. The flow rate of the component parts of the mine drainage gas are 0.081 N.m³/s of methane, 0.081 N.m³/s of air and 0.005 N.m³/s of water vapour. The unit N.m³/s refers to "normal cubic metres per second", a well known unit of mass flow rate related to 0°C and standard pressure. The compression stage has a compression ratio less than 14:1 and preferably less than 10: 1. The concentration ratio of compressed fael gas to air must be below the lower explosive level, and in this example is 1.6% (by volume), and preferably in the range 1.5% to 2% . The residence time within the combustor is sufficient to allow combustion of substantially all of the resident fael as it passes through. The exit temperature of the gas products of combustion most preferably should be in the range 850-950°C, although the upper limit is 1,100°C. The resulting temperature of fael gases output from the combustor 114 about 870°C, with the exhaust gas from the compressor 104 being 455°C. The electrical output from the generator 106 is 2,420 kW.

The rate of gas fed to the combustor 114 via the inlet pipeline 260 indicates the amount of methane in the ventilation air in the supply duct 250, and a suitable alarm of low gas flow or high combustion temperature would shut down the turbine and indicate abnormal and unsafe conditions in the mine ventilation system.

In a further embodiment, the gas turbine 100 can be used to combust carbon monoxide-containing steelworks gases by ingestion with air into the suction of d e turbine. Part of the energy generated is used directly or indirectly to compress air for air separation for oxygen for steelworks-related operations and for air separation operations. There further can be fael-free air provided for purging and cooling within the turbine, ensuring that substantially all of the fael and air mixture ingested into the turbine passes to the turbine's combustion system. In a farther embodiment utilising gas turbines, the air and fael gas leaving the compressor may be contacted with water to saturate and farther cool the air and fael mixture, and the water may be waste water such as sewerage or site wastes and the like, which would be reduced in volume and have at least part of their odour producing components transferred to the combustion air for incineration in the combustion chamber.

Claims

CLAIMS:

1. A combustor for the combustion of fael gases, the combustor comprising a vessel having an inlet for the supply of fael gas, an array of hollow tubes having one end open to the supplied fael gas and the other end open to a combustion

5 zone, the tubes being spaced-apart so that the outside of the tubes forms an exit path for combusted gas from the combustion zone passing to an outlet from the vessel, the heat of the combusted gas being partially transferred to the inlet gas within the tubes to preheat the inlet gas, the ratio of the supplied fael gas with air being less than the lower explosive limit for that fael gas, and combustion in said combustion zone occurring by o reaction or self-combustion of the preheated fael gas.

2. A combustor as claimed in claim 1, further comprising baffle means between said tubes to form a tortuous said exit path.

5 3. A combustor as claimed in claim 2, wherein the tubes are arranged longitudinally of the vessel and said inlet and said outlet being proximate the same end of the vessel and the combustion zone being at the other end of the vessel.

4. A combustor as claimed in claim 2, wherein said tubes are arranged in 0 a hexagonal array by said baffle means formed of hexagonal shaped sleeves fitting over said tubes and interlocking adjacent sleeves.

5. A combustor as claimed in any one of claims 1 to 4, farther comprising a burner located adjacent said combustion zone for preheating said 5 combustion zone to the combustion temperature before the fael gas is introduced to the combustor.

6. A method for combusting fael gases comprising the steps of: preheating supplied fael gas by heat given up by previously combusted fael gas; containing said preheated fael gas for a time sufficient for reaction or self- combustion to occur; discharging said combusted fael gas by a tortuous path to exchange heat with said supplied fael gas.

7. A method as claimed in claim 6, comprising the farther step of initially heating a combustion area to a temperature sufficient to cause combustion of said preheated fael gas on its introduction to the combustion area.

8. A gas turbine system for the utilisation of fael gas to produce usefal energy, the system comprising: a compression stage receiving said fael gas and outputting compressed fael gas; a combustor receiving said compressed fael gas, the fael gas having a ratio to air of less than the lower explosive limit for that fael gas, and preheating said compressed fael gas by heat transfer from combusted fael gas thereof before combustion of the preheated fael gas, the combustion occurring by reaction or self- combustion; and an expansion stage mechanically coupled to said compression stage, and receiving said combusted fael gas, the energy of expansion being converted to usefal rotational energy.

9. A gas turbine system as claimed in claim 8, wherein exhaust expanded gas from said expansion stage passes to a heat exchanger to give up heat to said compressed fael gas before reaching said combustor.

10. A gas turbine system as claimed in claim 9, farther comprising an electrical generator coupled to said expansion stage to convert said rotational energy to electrical energy.

5 11. A gas turbine system as claimed in claim 9, farther comprising an air compressor coupled to said expansion stage to utilise said rotational energy to generate a supply of compressed air.

12. A gas turbine system as claimed in claim 11, wherein at least a o portion of said supply of compressed air is returned to cool and/or purge at least said coupling between said compression stage and said expansion stage.

13. A gas turbine system as claimed in any one of claims 9 to 12, farther comprising a booster receiving exhausted gas from said heat exchanger to generate a 5 supply of steam for water provided to said boiler.

14. A gas turbine system as claimed in any one of claims 8 to 13, farther comprising a mixing stage having a duct for either air or fael gas below the lower explosive limit in the path of which is a plurality of tubes receiving fael gas above the 0 upper explosive limit, the tubes having a plurality of holes therein to allow said fael gas above the upper explosive limit to be inducted to mix with said either air or fael gas below the lower explosive limit.

15. A gas turbine system as claimed in claim 14, wherein said fael gas is 5 methane bearing.

16. A gas turbine system as claimed in claim 15, wherein said supply fael gas is an admixture of coal mine drainage gas and ventilation air.

17. A gas turbine system as claimed in any one of claims 8 to 16, wherein the proportion of methane to air of said compressed fael gas is <_ 2% .

18. A gas turbine system as claimed in any one of claims 8 to 17, wherein said compressor stage comprises a compressor as claimed in any one of claims 1 to 4.

19. A method for utilising fael gas in a gas turbine to produce usefal energy, the method comprising the steps of: compressing fael gas; preheating said compressed fael gas prior to combustion by heat given up by previously combusted fael gas; combusting said preheated fael gas by reaction or self-combustion; and expanding said combusted gas to produce usefal energy.

20. A method as claimed in claim 19, comprising the farther step of heating said compressed fael gas by heat exchange from exhaust gas following said expansion step.