GB2229733A

GB2229733A - Hydrocarbon combustion apparatus and method

Info

Publication number: GB2229733A
Application number: GB9005868A
Authority: GB
Inventors: Sanjay Marc Correa
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1989-03-24
Filing date: 1990-03-15
Publication date: 1990-10-03
Anticipated expiration: 2010-03-15
Also published as: DE4008698A1; FR2644846A1; GB2229733B; GB9005868D0; JPH0317403A; IT9019796A0; IT1241080B; DE4008698C2; IT9019796A1

Description

1. -. _., HYDROCARBON COMBUSTION APPARATUS AND METHOD RD-18, 563 This

invention relates to hydrocarbon fuel burning processes and, more particularly, to such processes which include methodology for reducing NOx combustion products.

Hydrocarbon fuel burning processes are widely used in stationary powergenerating gas-turbine systems. Combustion by-products which pollute the atmosphere are required to be minimized as part of a growing c6ncern about the quality of the earth's atmosphere. Therefore, combustors for stationary power-generating gas-turbine systems are required to produce low levels of nitric oxides (NO, N02, N20, etc., collectively referred to as NOX) and of CO. Such emissions lead to acid rain and other environmental problems. The NO, can result from reactions with atmospheric nitrogen, such reactions being referred to as "thermal" and "prompt" NOx, or witfuel-bound nitrogen (FBN). According to well- supported com bustion theory, NOx produced by the "thermal" mechanism is due to atmospheric nitrogen being fixed by the radicals responsible for flame initiation and propagation, as shown by the following:

N2 + 0 = NO + N N + 02 = NO + 0 N + OH = NO + H with the net reaction rate approximately given by d[NO] = 7.6 x 10'0 [N21 [0] exp (-38000/T) dt in System International (S.I.) units. Because of the large activation energy in the exponential term, the formation rate of NOx is not. significant below about 2780F, accounting for the descriptor "thermal".

- 2 RD-18,568 The concentration of certain radical species is also important, particularly at low (on the order of atmospheric) pressures. The radicals can exist in superequilibrium concentrations as discussed in an article by S.M. Cc-rrea et al., entitled "Prediction and Measurement of a Non- equilibrium Turbulent Diffusion Flame," Twentieth (International) Symposium on Combustion, The Combustion Institute, pp. 337343, 1984, and augment the thermal NOx mechanism. Since radical consumption reactions speed up at the relatively high pressures in power-generating systems, the degree of superequilibrium and the resultant excess radicals are reduced. For a further discussion on the formation of thermal NOx see the following articles: M.C. Drake et al. , 11Superequilibrium and Thermal Nitric Oxide Formation in Turbulent Diffusion Flames", Comb. Flame, 69, pp. 347-365, 1987; "Nitric Oxide Formation from Thermal and Fuel-Bound Nitrogen Sources in a Turbulent NonPremixed Syngas Flame," Twentieth Symposium (Int.) on Combustion, The Combustion Institute, Pittsburgh, PA, 1983-1990, 1984 and S.M. Correa, 11NOx Formation in Lean Premixed Methane Flames", Engineering Systems Laboratory, 89CRD001, January 1989.

The preponderance of thermal NOx in conventional (fuel and air not premixed) combustors, due to the high temperatures in the turbulent mixing interfaces, has led to water or steam injection for NOx control. In this approach, the injected water or steam absorbs heat, reduces the peak temperatures (to below the NOx-forming threshold) and so reduces NOx emission levels. The lower temperatures have the undesirable side effect of quenching CO consumption reactions and so the CO levels increase and combustor life and efficiency are reduced. Thus the water or steam injection technique is not ideal.

Prompt NOx is so named because it is formed very rapidly (in hydrocarbon flames) when atmospheric nitrogen is 1 RD-18,55.068 fixed by alkyl radicals, e.g., CH, CH2, CH3. The latter occur in the hydrocarbon combustion kinetic chain. The nitrogen is fixed as cyanide (HCN, CN) species which lead to NHi species and are eventually oxidized to NOx by oxygen-containing radicals. The mechanism does not require the high temperatures of the thermal mechanism and so prompt NOx is not amenable to control by water or steam injection. FBN NOx is very similar in that the fuel-bound nitrogen species are extracted as NHi species which are oxidized to NOx. FBN occur for example, in coal, and also in so-called "dirty" gas derived from coal. However, prompt NOx is not as much a problem as FBN. In typical applications, FBN NOx can be on the order of 500 ppm or more, while (conventional) combustors with non-FBN fuel have 100-300 ppm thermal NOx and 10-30 ppm prompt NOx. It would be desirable to burn dirty (FBN) fuel with <100 ppm NOx and clean fuel with <10 ppm NOx.

Powerplant constraints dictate that the stability, turndown ratio (i.e. power changes corresponding to power demand reductions) and efficiency be similar to those of cur- rent equipment. NOX control techniques without water or steam injection are referred to as "dry" combustion. Two dry low-NOx combustion techniques have been suggested (i) richlean staged combustion (originally intended for thermal and FBN NOx control but not successful for the reasons discussed below) and (ii) lean premixed combustion (intended for thermal NOx control).

In rich-lean staged combustion, the combustor is divided into a first zone which is rich (equivalence ratio O=-1.3-1.8; note that (D = 1 for stoichiometric conditions, (D>1 being rich and (D<1 being lean) and a second zone which is lean. Because of the off-stoichiometric conditions, temperatures in each zone are too low for NOx (e.g. less than 2780F) to form via the "thermal" mechanism.

RD-1 8, 5 68 However in prior art staged systems, the mixing cf: air with the efflux of the rich zone occurs at finite rates and cannot prevent the formation of hot near-stoichiometric eddies. The attendant high temperatures lead to the copious production of thermal NOx, which is triggered at temperatures above about 2780F. This has been the experience both in the laboratory and in mainframe (100 MW class) gas-turbine equipment. However, rich'combustors are suitable for fuels with a significant fuel-bound nitrogen content because the amount of oxygen available to produce FBN NOx is limited.

Lean premixed combustors, which are useful if the fuel does not contain nitrogen, are fueled by a lean (pre- vaporized, if liquid fuel) premixed fuel-air stream at (M.7. The ensuing temperatures are uniformly too low (e.g., less than 2780F) to activate the thermal NOx mechanism. Detailed chemical kinetic studies of two such combustors by the present inventor have lead to the discovery that most of the NOx is produced by the "prompt" NOx mechanism described above (recall that FBN is not present). This forms a lower limit to the minimum NOx obtainable in current hydrocarbon- fueled combustors. Advanced combustors under development by General Electric have reached an aPparent 30-40 ppm NOx barrier (using clean natural gas which minimizes total NOx production). This barrier can be crossed only with an increase in CO and an unacceptable loss of flame stability. Such (lean) combustors also produce unacceptably high levels of NOx from FBN species in the fuel if FBN species are present. Thus each of the prior art systems has advantages and disadvantages.

According to the present invention, the efflux of a rich combustor is cooled to prevent ignition during mixing to a lean condition. Ignition and flame stabilization occur only after the lean mixture has been established. According to one embodiment, a portion of the air is burned under rich - 5 RD-18, 5 68 conditions (e.g., overall equivalence ratio, (D = 2.5-3.0) in a preburner to produce a partially combusted stream which contains CO and H2, referred sometimes as syngas, and very little CH4 (the original fuel), C02 and H20. The hot gas stream is then cooled by way of example by expansion through a turbine or passage through a heat exchanger. The cooled gas stream is then mixed with the remaining portion of the air stream, without ignition. The lean stream (e.g., 0.5-0.6) is then burned.

The production of NOx is minimized due to the rela- tively cool temperatures in the rich and lean burning cycles, which temperatures are below the established level for the production of thermal NOx. Prompt NOx is also minimized since CH in the lean cycles tends to be negligible. FBN Nox is minimized because the rich combustor runs with too little oxygen for production of NOx.

Illustrative embodiments of the invention will now be described with reference to the drawings, in which:- Figure 1 is a schematic diagram of a combustion cycle in accordance with one embodiment of the present inven tion; and Figure 2 is a schematic diagram of a combustlon cycle in accordance with a second embodiment of the present invention.

In the schematic diagram of Figure 1, a representa tive combustion system in accordance with one embodiment of the present invention is illustrated. In the system shown a main combustion machine 10 comprises a compressor 12, a gas mixer 14 and a primary combustor 16 whose combustion products drive turbine 18. A system comprising compressor 12, mixer 14, combustor 16 and turbine 18 are commercially available mainframe machines, which for example, may be a General Electric Company MS7000 series machine for driving a 100 megawatt class electrical generatori A second combustor 20 is coupled to receive 100% of the fuel at an inlet thereof which fuel RD-18, 56-3 preferably is methane, coal or coal derived gas or a liquid hydrocarbon fuel. The output of combustor 20 is applied to gas cooling stage 22 which may comprise a turbine or expansion nozzles to cool the gas produced by the combustor 20. The inlet of combustor 20 receives x% of air from the compressor 12. The remaining portion of the air 100 - x% is applied to the mixer 14 of the mainframe machine 10.

Combustor 20 may be similar in construction as the combustor in a commercially available gas turbine generator known as a General Electric Company model LM500. The LM500 however, includes a fuel compressor and air compressor for compressing the fuel and air supply to the combustor 20. In the embodiment of the machine 10 however, the air compressor is not included as the air is compressed via compressor 12 and the fuel is compressed and supplied to the input of combustor 20. The cooling stAge 22 may include a turbine as available in the model LM500 gas generator. However, a turbine is not essential for the gas.cooling stage as indicated above.

100% of the fuel is applied to the combustor 20 which burns that fuel in a rich combustion mixture with a relatively low amount of air supplied via the compressor 12. For example, the amount of air supplied to combustor 20 may be 10% of the air supplied to the mixer 14 from compressor 12. The combustion products are applied to the cooling stage 22 while at a relatively hot temperature. The temperature is below about 2780F at which thermal NOx is generated. Because of the rich combustion, little oxygen is available for the combustion process in combustor 20 and the tempera- ture thereof does not exceed the threshold temperature at which thermal Nox is generated. The relatively rich characteristics of the burning process generates little 0, OH and other oxidizing radicals in the burning process minimizing prompt NOx. Also, the rich combustion process favors reformRD-18,55-65 ing chemistry, i.e. tends to avoid the generation of CH gas products;. instead produces gas products comprising primarily CO and H2. The CO and H2 mixture is commonly referred to as syngas or synthetic gas. The FBN species, if present, are 5 converted to N2 (molecular nitrogen).

The combustion efficiency of combustor 20 is believed to be generally about 75% and therefore about 25% of the fuel in the syngas products remains unburned. Combustor 20 because it is relatively rich operates at an equivalence ratio (ER) of approximately 2.5-3. Of course, the equivalence ratio will vary between the head end and the exit of the combustor 20. The exit ER is in the range indicated, the head end being lower, within the rich stability limit. The combustor 20 is illustrative of a more complex system in which a staged combustor may be provided with more fuel added to the products of a rich primary zone having a (D of approximately 2. The added fuel promotes "reforming" chemistry. Because the temperature is below the threshold value for the generation of thermal NOx, such thermal NOx is substantially negligible at the output of combustor 20. The combustor process maximizes CO and H2 and fuel conversion.

The cooling stage 22 may be either a turbine or a heat exchanger to cool the hot syngas produced by the combustor 20 and deliver power or heat as may be required in a given implementation. The output of such a turbine or heat exchanger is such to cool the syngas to a temperature below. ignition temperatures before delivery to the mixer 14 in the system 10. This step is critical.

In accordance with the principles of the present invention, the syngas produced by the rich combustor 20 has negligible total Nox because of the low temperature and the lack of oxidizing species. FBN species are converted to N2. However, in the transition to the mixer 14 it is important that the syngas be reduced to a temperature sufficiently low - 8 RD- 18, 5 6 _z that the temperatures in the process of turbulent mixing in mixer 14 remain below the threshold for the generation of thermal NOx. Without the cooling produced by stage 22 the hot syngas produced by the combustor 20 when mixed with air in the mixer 14 could lead to ignition and flame in mixer 14 and to copious thermal NOx. Because there is little CH component in the syngas product of the combustor 20, there is little prompt NOx in the system 10.

It should be understood that the combustors 16 and 20 include more complex combustion systems including primary burners (head ends) and downstream addition of air in the case of combustor 16, per conventional practice, and downstream addition of fuel in the'case of combustor 20. Assuming a preburner is included in the combustor 20, then the cooling stage 22 may be provided a pressure reducing nozzle which will increase the usual approximately 4% pressure drop available for mixing. Air needed to premix to lean main-combustor conditions is admitted via jets within such a nozzle (not shown). With the use of a nozzle, integration may be accomplished because the cooling and premixing can both occur within the nozzle. In this case the mixer 14 would be combined in such a nozzle with the mixing occurring in the nozzle. Otherwise the mixer 14 mixes the cooled syngas which -'s at a temperature below the 2780F threshold temperature for the generation of thermal NOx, and mixes that air gas at compressor temperature, for example, 600F. The mixing of the syngas with most of the air stream produces a lean premixed stream having an equivalence ratio (D of approximately 0. 5 at the head end of the combustor 30 16 and about 0.3 at the exit. The mixing process of mixer 14 or nozzles (not shown) is at a sufficiently low temperature so that a flame and thermal NOx cannot be formed during dilution.

9 RD-1 8, 5 68 Relatively negligible amounts of hydrocarbons are available-at the mixer 14 since only air from the compressor 12 is added at the mixer 14 to the syngas produced by the combustor 20. Therefore, very little prompt NOx is generated in combustor 16. The particular operating points for the fuel and air mixers and pressures and temperatures can be selected by analysis and experimental variations of the components for a given implementation. In particular, the stoichiometries of the combustors 16 and 20 are optimized for producing maximum power at the turbine 18. Not shown is an electrical generator or other utilization means coupled to the turbine 18 and driven thereby.

Because the generation of hydrocarbons and FBN NOx are minimized in the syngas output or the cooling stage 22 and because the generation of the thermal NOx is minimized by maintaining the temperatures below the threshold, the fuel supplied to combustor 20 may comprise coal gas, liquid fuels and other types of fuels with relatively high fuel bound nitrogen. Employing the process as discussed above in con- nection with Figure 1, the fact that the fuels used in the combustor 20 are rich in nitrogen will not affect the resulting products in the syngas at the output of the cooling stage 22. Nitrogen in FBN species will be converted to N2.

By way of example, combustor 20 may be supplied with approximately 0.5 lbs. per second of methane (CH4) accompanied with 2.5 lbs. per second of air. The combustor 20 as mentioned above has an overall equivalence ratio of about 3. The syngas output of the cooling state 22 comprises approximately a flow rate of 1 lb. per second of carbon monoxide plus hydrogen (CO + H2) the rest being mostly N2 (nitrogen). This is combined with about 15 lbs. per second of air which is applied to the mixer 14. Air for providing dilution and cooling is provided to the combustor 16 at approximately 7.5 lbs. per second to provide a downstream RD-18,506-9exit equivalence ratio (D of approximately 0.3. This process yields an. approximate NOx level of 5 ppm NOx. It should be understood that the combustors 20 and 16 are supplied fuel and air at various inputs at the head end and downstream inputs in accordance with conventional combustors. Combustor 16 uses air for downstream inputs while combustor 20 uses fuel for downstream inputs. In Figure 2, a second embodiment employing two stand alone commercially available combustion machines are 10 employed for implementing the present invention. A compressor 200 compresses all of the hydrocarbon fuel such as methane to a pressure of about 20 atmospheres and applies the compressed fuel to combined combustor mixer 202. The compressor 204 supplies a portion x% of the total air required 15 overall. Compressor 204 supplies the compressed air to the combustor 202. Combustor 202 consists of a head end operated near the rich limit with downstream addition of more fuel to achieve the required stoichiometry. By way of example, x may be 10% of the air required overall. Combustor 202 burns the 20 fuel air mixture and applies the burned combustion products to a turbine 206. The purpose of the turbine 206 is similar to the cooling stage 22 of Figure 1 which provides cooling o. the hot combustion gases to produce a cooled carbon monoxide (C0) and hydrogen (H2) syngas. The cooled syngas is applied to the input of mixer 208. The remaining air required is applied to compressor 210. For example, where 10% of the air is applied to compressor 204, 90% of the air required to burn overall is applied to compressor 210. Compressor 210 provides a pressure of about 10 atmospheres to the air supplied 30 to the mixer 208. Mixer 208 mixes the air from compressor 210 with the cooled syngas from turbine 206. The mixed cooled gas product is applied to combustor 212 whose hot gas products are exhausted to a turbine 214 which drives a generator (not shown).

RD-1 8, 5 68 In one calculation example to verify the process, a 0.5 lbs per second of methane is supplied as the fuel to com pressor 200. This is applied at atmospheric pressure at room temperature (60F). To this is added 0.3% NH3 (ammonia).

The ammonia represents fuel bound nitrogen in a coal derived gaseous fuel. The efficiency of fuel compressor 200 is assumed approximately 0.9. The output of compressor 200 has a temperature of about 677F.

Compressor 204 compresses 2.86 lbs. per second of air supplied at atmospheric pressure at room temperature.

The output of compressor 204 is at approximately 842F with the outputs of both compressors 200 and 204 at 20 atmo spheres. Combustor 202 mixes the fuel and air and burns the combination with (D at about 2.0 at the head end and 3.0 at the exit port. The output of the rich combustor 202 has a temperature of about 2520'F. It is calculated that the prod ucts from the combustor 202 have less than 1 ppm NOx which value increases as the equivalence ratio decreases. It is also estimated that there are about 750 ppm NHi, HCN. The gas products from combustor 202 are applied to turbine 206 which runs at about a 2 to 1 pressure ratio which serves to cool the combustor gas products, producing a cooled syngas on line 207.

Combustor 202 burns a rich fuel air mixture to which more fuel is added in the downstream region of the com bustor. This leads to reduction of the initial products by the fuel added downstream. The process is referred to as "reforming" chemistry such that the products of the syngas on line 207 are primarily CO and H2 rather than fuel and combus tion products. The NOx emissions are low on line 207 due to the relatively low temperatures and lack of oxidizing radi cal species such as 0 and OH in combustor 202. This has been verified by laboratory experiments and kinetic studies. The output pressure of the turbine 206 on line 207 is at about RD-1 8, 5 68 10.5 atmospheres and at a temperature of about 2136'F. If the equivalence ratio in the head end of combustor 202 is made too high, the flame can become unstable in the combustor. There may also be excessive soot because the combination of gas, fuel and air is too rich. Further, there can be excessive production of NOx as the (D is lowered. For this reason, it is preferred that the head end (D of combustor 202 be in the range of 2 to 2.5, with more fuel added downstream..

Compressor 210 receives the remaining air. This air is applied to compressor 210 at a rate of 25.74 lbs. per second, in this example, at room temperature and atmospheric pressure. Compressor 210 operates at an efficiency of 0.9. The output pressure of compressor 210 is 10 atmospheres at a temperature of about 600'F. This air is mixed in mixer 208 with the cooled syngas an& applied to lean combustor 212. The lean combustor 212 has a head end (D of about 0.6 and an exit (D of about 0. 3. Combustion products at the exit of combustor 212 are at about 1860F, and exhibit approximately 58 ppm Nox and less than 1 ppm CO. Recall that the fuel contained FBN (0.3%). Turbine 214 operates with an assumed efficiency of 0.9 and has an output temperature of about 1005F at one atmosphere. The 58 ppm NOx and less than 1 ppm CO products produced by the combustor 212 is considered excellent in view of the combustion of dirty fuel containing 3% ammonia applied to the compressor 200. Normally, such dirty fuel will produce hundreds of ppm of NOx. Of course, different ratios of fuel, air and dirty fuel contaminants such as FBN will produce different values of temperature at the different stages. The 10% air applied to the compressor 204 and 90% air applied to compressor 210 is believed optimum for one implementation. Turbine 214 is then employed to operate an electric generator or other utilization means.

1 L L - RD-18,5065 Turbine 206 causes expansion of the combustor out put gases.and reduces the temperature of the syngas to a level where mixing can be accomplished in mixer 208 without premature ignition. The turbine 206 exit pressure is larger than the operating pressure of the combustor 212 by about 5% (10.5 atm vs 10 atm) to facilitate mixing of the syngas from line 207 and the air from compressor 210 to an overall lean condition. The figures given above with respect to the pro portions of fuel to air, efficiencies of the compressors and turbines and approximate temperatures are based on calcula tions of the various operating points, emissions and overall thermal efficiency. The various assumptions are included in these calculations as indicated.

The total fuel and air flow rate are consistent with combustor cans in current mainframe power generation machines. Calculation of the cycle efficiency of the embodi ment of Figure 2 shows the cycle efficiency of 30.7% to be comparable to a base machine comprising compressor 210, mixer 208 and combustor 212 with the same level accuracy in the calculation, that is a cycle efficiency of 30.5%. Slight improvement in the cycle efficiency is due in part to the straight forward improvement of the Brayton cycle with the designated pressure ratios, since the combustor 202 runs at atmospheres pressure as compared to the 10 atmosphere pressure of combustor 212.

Claims

Claims:-

RD-18, 5 68 1. A dry low NO, hydrocarbon combustion apparatus comprising: first fuel burning means for rich burning hydrocarbon fuel at an equivalence ration (ER) sufficiently greater than one so as to produce hot combustion gas products comprising substantially CO and H2 and negligible NOx; cooling means for cooling said combustion gas products to a temperature below which ignition and thermal NOx occur; and second fuel burning means for burning said cooled combustion gas products at an ER sufficiently below one so as to minimize the production of NOx and CO.
2. The apparatus of claim 1 wherein said cooling means includes turbine means coupled to be operated by said hot combustion gas products.
3._ The apparatus of claim 1 wherein said cooling means includes heat exchange means responsive to said hot gas products for extracting heat from said hot products.
4. The apparatus of claim 3 including a nozzle coupled to receive said hot gas products for adiabatic cooling of said products by causing rapid expansion thereof.
5. The apparatus of claim 1 wherein said first fuel burning means includes a first fuel combustor for combusting said fuel with X% air and said second burning means includes a second fuel combustor for burning said hot gas products with Y % air where X + Y = 100 and X is substantially less than Y.

1 - 15 RD-18, 568
6. The apparatus of claim 1 wherein the exit ER value of the first means is in the range of about 2.0 to 3.0 and the exit ER value of the second means is about 0.3 to 0.4.
7. The apparatus of claim 1 further including utilization means coupled to said second burning means for doing work in response to receipt of said burned cooled gas products.
8. The apparatus of claim 1 further including an air compressor for compressing a given amount of air and means for supplying a portion of said given amount of air to said first burning means and the remaining portion of said given amount of air to said second burning means, said given amount of air corresponding to a given amount of fuel to be burned by said first and second means so as to produce an overall ER of 0.3 to 0. 4.
9. The apparatus of claim 8 further including gas mixing means for mixing said cooled gas products with said remaining portion of air prior to said burning of said cooled gas products.
10. The apparatus of claim 1 further including a first compressor for compressing a first portion of air, a second compressor for compressing said fuel, said first means for burning said compressed fuel with said compressed first portion of air, and a third compressor for compressing a second portion of air greater in amount than the first portion and mixing means for mixing the cooled gas products with said second air portion prior to burning by said second means.

RD-18,565
11. The apparatus of claim 10 wherein the first compressor compresses said first portion of air to a pressure of about 20 atmospheres and the second compressor compresses said second portion of air to a pressure of about 10 atmospheres.
12. A dry low NOx hydrocarbon fuel combustion apparatus comprising: a preburner for rich burning of a hydrocarbon fuel with air to produce said combustion products; cooling means for cooling the hot combustion products of said preburner to a temperature below which ignition occurs and thermal NOx products are formed; mixing means for mixing said cooled combustion products with air to provide a lean mixture of said cooled products and air; and burner means for burning said lean mixture.
1 3. The apparatus of claim 12 further including compressor means for compressing at least said air prior to said burning in said preburner and in said burner means.
14. The apparatus of claim 12 further including compressor means for compressing said air and means for supplying a portion of said compressed air to said preburner and the remaining portion of the compressed air to said burner means.
15. The apparatus of claim 12 further including first compressor means for compressing the fuel and the air supplied to said preburner and second compressor means for compressing air and for supplying this compressed air to said mixing means.

- 17 RD-18, 5 68
16. The apparatus of claim 12 wherein said cooling means includes a turbine for receiving said hot combustion products.
17. The apparatus of claim 12 further including a turbine responsive to said burned lean mixture applied as an input thereto.
18. A method of dry low NOx burning of hydrocarbon fuel comprising: rich burning the fuel with air to produce hot combustion gas products; cooling the hot gas products to a temperature below which ignition occurs and thermal NOx products are formed; mixing the cooled hot gas products with air to produce a lean mixture of products and air; and burning the mixture.
19. The method of claim 18 further including the step of compressing the air prior to rich burning and prio.r to said mixing.
20. The method of claim 19 further including compressing the fiiel prior to said rich burning.
21. The method of claim 20 including compressing a first portion of air prior to said rich burning and compressing a second portion of air prior to said mixing wherein the first portion is compressed to a pressure higher than the second portion and wherein the first and second portions represent 100% of the air needed overall.
22. Combustion apparatus substantially as hereinbefore described with reference to Figure 1 or 2.

1 1 - 1 8- L RD-18, 568
23. A method of burning hydrocarbon fuel substantially as hereinbefore described with reference to Figure 1 or 2.

Published 199D W. The Patent Mce, State House,6671 High Holborn. London WC1R 4TP. Furthercopies maybe obtained from The PatentWice EWes Branch. St MLry Cray. Orpington, Kent BR5 3RD. Printed by Multiplex techniques WL St Mary Cray. Kent. Con 1187