Pollution control in continuous combustion engines

POLLUTION CONTROL IN CONTINUOUS COMBUSTION ENGINES A. H. LEFEBVRE

School of Mechanical Engineering, Cranfield Institute of Technology, Cranfield, Bedford, England Brief consideration is given to the formation of smoke, carbon monoxide, unburned hydrocarbons and nitric oxides in continuous combustion systems, and to the manner and extent to which their exhaust concentrations are influenced by changes in engine operating conditions. The main objective, however, is to review the various techniques that have been used or advocated for poUution control. It is shown that the development of low emission combustors is proceeding along two main lines. The simplest and most direct approach is through various minor modifications to established hardware, e.g., by changes in liner geometry and airflow distribution and by the adoption of more sophisticated methods of fuel injection. These modifications may be supplemented, where feasible, by compressor air bleed at low power operation and water injection at high power conditions. The other approach is towards radically new concepts which involve major combustor redesign. Of these the raost promising appear to be variable geometry and staged combustors, and also "prevap/premix" systems in which the fuel is vaporized and thoroughly mixed with all the air required for combustion upstream of the combustion zone. How far the potential of these advanced concepts will be developed nmst clearly depend on the severity of future legislation on emissions control.

Introduction Efforts to produce low-emission eombustors are hampered by the wide range of conditions over which they must operate. With gas turbines, not only may combustion pressure vary over a wide range, especially in aircraft systems, but also the large differences in fuel and air flows between idle and full power conditions can produce substantial variations in primary-zone fuel/air ratio. Thus although the key pollutants have now been identified and the mechanism of their formation fairly well established, at least from an engineering standpoint, the translation of the knowledge acquired into combustion hardware that "..ill meet the proposed emission regulations and, at the same time, satisfy all the other formidable performance requirements, has yet to be demonstrated. In recent years the automotive gas turbine has been brought to an advanced state of development, while work on steam and Stirling engines has ~dso been actively pursued. 1 As the combustion units employed in all three types of engine are basically similar, it has been found convenient to center the discussion on gas turbine

combustion, although examples have been drawn from other continuous combustion systems, including stationary boilers. In the following sections brief consideration is given both to the formation of pollutant emissions and to the manner and extent to which their exhaust concentrations are influenced by changes in operating conditions. However, the main objective is to review the various techniques that have been used or advocated for reducing pollutants, and it is on this aspect of emissions technology that attention is primarily focussed.

Pollutant Formation Of the pollutants generated in combustion those which have created most concern, and for which emission standards have been laid down, are smoke, carbon monoxide (CO), onburned hydrocarbons (UHC), and oxides of nitrogen (NO,). Their formation is highly complex and only a much simplified vemion, designed to elucidate the various proposed techniques for emissions reduction, is presented below.

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COMBUSTION SYSTEMS

Exhaust Smoke Exhaust smoke results from the production of finely-divided soot particles in fuel-rich regions of the flame.~-~ With pressure atomizers the main soot-forming region lies inside the fuel spray at the center of the liner. ~,4 This is the region in which local pockets of fuel and fuel vapour are enveloped in recirculating burned products moving upstream towards the fuel spray. In these high temperature, fuel-rich regions soot may be produced in considerable quantities.

chiometric value, or reduction in reference velocity, will raise the level of NO, emissions.~,7.~'-~7 From these considerations it is clear that the nature of pollutant formation is such that, for gas turbine engines, CO and UHC concentrations are highest at low power conditions and diminish with increase in power. In contrast, NO~ and smoke are fairly insignificant at low power settings and attain maximum values at the highest power condition. Methods of Pollutant Reduction

Carbon Monoxide

Smoke

Reastion kinetics dictate that appreciable amounts of C O are formed as an intermediate product of hydrocarbon oxidation, but the presence of C O in the exhaust gas is usually caused by inadequate burning rates in the primary zone combined with quenching of the post-flame products at the linerwalls and in the coolerzones downstream. Minlinum concentrations tend to occur at primary-zone air-fuel ratios slightly weaker tha~ stoichiometric.5,e Increase in inlet air temperature and pressure both accelerate burning rates and thereby reduce CO.SJ

In the primary zones of combustors featuring spray fuel injection the pattern of burning is highly complex. Since soot is not an equilibrium product of combustion, its formation is influenced as much by the physical processes of atomization, evaporation and fuel/air mixing as by reaction kinetics. During the first portion of its trajectory each individual fuel drop suffers loss by evaporation due to heat transmitted from the flame. If the liberated fuel vapour then mixes with air it will burn in the manner of a premixed flame. As drag forces gradually deplete its momentum the fuel drop eventually reaches a certain critical velocity, which depends on its size, below which it becomes completely surrounded by an attached diffusion flame.TM This diffusion mode of combustion is often a prime cause of the high rates of mot formation and smoke that characterize spray combustion. The proportion of fuel burned in the diffusion mode may be reduced by increasing the relative velocity between the fuel drops and the surrounding gas and by reduction in fuel drop size. TM ]fowever, if improved atomization is accompanied by lower spray penetration the smoke output may actually increase.~9 This is what happens with pressure atomizers and is the main reason for the high smoking tendencies of duplex and dual-orifice atomizers when operating at high pressures) Even when the overall fuel/air ratio in the primary zone indicates sufficient oxygen for complete combustion, imperfections in mixing can give rise to local regions in which pockets of fuel vapour are enveloped in oxygen-deficient gases at high temperatures. Under these conditions increasing the flow of air into this zone is usually very beneficial.~b,~~ If this additional air is accomplished by an increase in liner pressure drop, the combined effects of more oxygen, lower temperature and improved mixing can

Unburned Hydrocarbons Unburned hydrocarbons are normally the result of poor fuel atomization, inadequate btrrning rates, the chilling effects of film-eooli~ air, or any combination of these, b As these same factors also govern the level of CO emissions it might be expected t h a t CO and UHC emissions should follow the same general trends. This has, in fact, been confirmed by Verkamp, et al., s who analysed experimental data from several different types of eombnstor and showed t h a t U H C emissions parallel CO emissions, but at a much lower level.

Oxides of Nitrogen Of prime importance to NO~ formation is the primary-sone flame temperature, s-l~ Other operating parameters affect NO~ emissions almost solely through their influence on flame temperature. Various workers have reported an increase in NO~ with pressure bar this probably stems from the effects of pressure in suppsessing chemical dissociation and accelerating burning rates, both of which combine to raise the flame temperature. Thus a n y change in operating conditions that elevates the flame temperature, such as increase in inlet air temperature or pressure, variation in fuel/air ratio towards the stoi-

POLLUTION CONTROL drastically reduce soot formation and smoke. Unfortunately, this approach is somewhat limited in scope due to the adverse effect of increase in primary-zone air on ignition and stability limits and on CO and UHC emissions at idle. Fuel composition affects smoke through the influence of viscosity and volatility on fuel atomieatioa and evaporation, and also through its effect on soot formation under the prevailing conditions of high temperature and low oxygen concentration. The soot-forming propensity of a fuel increases with aromatic content, boiling point and carbon/hydrogen ratio. 2~'-~9Fuel additives, usually organo-metallie compounds based on barium or manganese, have been used to reduce smoke with varying degrees of success.~Z,a~ The temperature dependence of the mechanisms involved is very different for these two compounds, which suggests t h a t a combination of the two might prove very effective, and this has been confirmed by tests, z' The main drawback to fuel additives, apart from their cost and the logistics problem, is their tendency to produce deposits on turbine blades. The risk of introducing a toxic hazard is also a cause for concern.

Carbon Monoxide Carbon monoxide is produced as an equilibrium product of combustion in the primary zone. However, if a high temperature, slightly fuel-lean primary zone is followed by an intermediate zone in which chemical equilibrium is maintained during air addition, and sufficient mixing is employed to transport CO from the cool regions at the wall to the hotter central zones, then it should be possible to reduce the exhaust concentration of CO to negligible proportions. In practice, however, this ideal situation can rarely be achieved. Since the presence of CO in the exhaust gas is a manifestation of combustion inefficiency, it follows t h a t any combustor modification which improves combustion efficielmy will automatically reduce CO. The various approaches that have yielded worthwhile reductions in CO include the following: (l) Improved fuel atomization. Normally this implies the use of an airblast atomizer 5,8..4 but, if a separate supply of compressed ahr is available, an alternative approach is to employ air assistance to improve atomization at low fuel flows.~.7,14.~'~,34 (2) Redistribution of the air flow to bring the l)rimary-zone equivalence ratio closer to an optimum value of around 0.85. (3) Increase in primary-zone volume or residence time.

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(4) Reduction of film-cooling air. This air issuing from the primary zone normally contains high concentrations of CO and UHC. Unless these species are subsequently entrained into the hot central core with sufficient time to react to completion they will appear in the exhaust gas. Thus reduction in fihu-cooling air, especially in the primary zone, is often effective in reducing CO. (5) Compressor air bleed. This consists of bleeding air from the compressor during low power operation.~,~d It reduces CO emissions by virtue of an increase in primary zone fuel/air ratio, which in turn improves combustion efficiency. (6) Fuel staging. This technique is based on cutting off the supply of fuel to some nozzles and diverting it to the remainder. It reduces enfissions at low power conditions by improving atomization quality and raising the local fuel/air ratio in the burning zones.~ Three basic types of fuel staging have been considered:

(a) Circumferential. Usually this entails disconnecting alternately located nozzles from the fuel supply. It is ideally suited to tuboammlar systems but on annular chambers its advantages are largely offset by the quenching effects of the surrounding cold air on the localized burning zones. 41 (b) Radial. The simplest application of this technique is to double-banked annular combustors where, at low fuel flows, it is a relatively simple matter to inject all the foe] into the inner or outer combustion zone. Snbstantial reductions in CO and U H C at idle conditions seem feasible with this approach.~,7,[~ (c) Axial. By designing the primary zone for optimum performance at low power settings, and then injecting the extra fuel needed at higher power levels at one or more locatious dowustream, substantial reductions of CO seem practicable but have yet to be demonstrated. This system is discussed in more detail below. Unburned Hydrocarbons The problem of UHC emissions requires the same treatment as for CO, with slightly more emphasis on reductions in film-cooling air and improvements in fuel atomization. ~'~,2~ In this respect airblast and air-assist nozzles have proved very beneficial.~-s,~ ta,34

Oxides of Nitrogen The main factor governing NOx formation is temperature. In fact, it can be shown that NO~

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COMBUSTION SYSTEMS

mere "emissiol~ trades." For any given combustor the CO/NO= emission characteristic remains sensibly constant, with the upper and lower ] extremities of the curve corresponding to operation at idle and full power respectively. The main advantage to the designer in most of the emission reduction techniques described below is in allow: v ing movements along the curve over and above those dictated by changes in engine power setting. IIowever, real progress in emissions technology is achieved only by displacement of the CO/NO~ ~'/~'~/CONCEPTS ( / N / /~ characteristic nearer to the origin. The development of "low NO=" combustors is proceeding along two main lines. The most direct approach is through various nfinor modifications ~/~ POWI:R to conventional designs, e.g., by changes in liner geometry and airflow distribntion, by the adoption of more sophisticated methods of fnel injectkm, and by the practical exploitation of new wall-cooling techniqnes t h a t ale more economical in their use of cooling air. The merit of this o.V4 6 ioo approach is t h a t the combustor retains its existing general size and configuration, and improveNOx EMISSIONS.grn NOx/ kg F'UEL Fro. 1. Emission characteristics of conventional ~nd meats can be made without trespassing far outside the bounds of established technology. Its advanced combustors. (After Verkamp, et al. s) main drawback is t h a t the end product must inevitably be a compromise of some kind, both emissions increase exponentially with flame in regard to emissions and other aspects of comtemperature according to the relationship N0,r162 bustion performance. exp(0.009T) where T is the reaction temperature The other approach is essentially a rejection in ~ For most practical purposes it is sufficient of the present design philosophy which is based to regard all obher oombustor parameters as on heterogeneous diffusion /lames and is fairly significant only insofar as they affect flame eouservative in its distribution of fuel and air. Of temperature. Thus in any attempt to reduce the various advanced concepts now being actively NO, the prime goal nmst be to lower the reaction studied the three most promising appear to be temperature. variable geometry, staged combustion and "preThe second objective should be the elimination yap/premix" systems. If the present demand of "hot spots" from the reaction zone. There is for ultra-clean engine exhausts is sustained it little point in achieving a satisfactorily low seems likely t h a t future combustors will embody average temperature if the reaction zone con- one o1" more of these concepts. tains locahzed regions of high temperature in which the rate of NO, formation remains high. Finally, the time available for the formation of (at Modified Conventional Combu~tars NO= should be kept to a minimum. Reductions in both flame temperature and 1. Lean primary zone. Operating with a residence time are readily accomplished by lean primary zone effectively lowers NO= by increa.sing the flow of air into the primary zone, reducing the flame temperature. It also aN but this also produces an increase h~ CO and leviates smoke, but the output of CO and UHC UHC. In fact a basic feature of nearly all methods is increased.1.1.Lt~,3a.4o.4i. of emissions reduction is that they represent 2. Rich primary zone. Decreasing the amount "trade-offs" between CO and U H C on one hand of primary-zone air is usually beneficial to CO and NO, on the other. This point may be illus- and UIIC and, in theory, should also reduce trated by plotting CO emissions versus NO, NO,. However, to achieve NO, reduction the emissions for a typical gas turbine combustor, as fuel-rich combustion products issuing from the shown in Fig. 1. This method of presenting emis- primary zone must be rapidly quenched to a low sions data, as advocated by Verkamp, et al., s is temperature (<1700~ As yet no practical instructive because it enables true advances in means of achieving the abaost instantaneous emission technology to be distinguished from mixing needed has been devised, although some 1D(Xj

/-TYPICALCOI.,IBUSTOR EMISSIONCHARACTERISTIC IDLE/

POLLUTION CONTROL small reductions in NO= using this technique have been reported.U~.lT.a4 3. Reduced residence time. This technique, sometimes referred to as "early quench," can be applied in various ways, but usually by advancing the admission of dilution air into the primary combustion products. Ll~,~ Results obtained using this method suggest t h a t NO~ diminishes in a linear manner with residence time. e,~,s However, as might be expected, this reduction in NO~ is obtained at the expense of increases in CO and UHC, while "delayed quench" acts in the opposite manner. Increase in combustor reference velocity also lowem NO= by reducing the time available for its formation.U4.'3 4. Fuel preparation. There is little doubt t h a t the processes of fael preparation offer considerable scope for the reduction of all types of ponutant emissions as well as a means for improving many other aspects of combustion performance. Airblast atomizers have already demonstrated up to 30 percent reduction in NO~ compared with conventional pressure nozzles, presumably due to reductiou in local flame temperatures achieved by improved fuel/air premixing. In general the most successful fuel injection devices are those which fully premix the fuel with the maximum amount of combustion air. 5. Water injection. Since NO= formation is highly dependent on temperature this suggests that dilution of the fuel/air mixture with an inert or non-combustible substance should reduce the output of NO~. Water injection has already proved successful in reducing smoke, 4 and tests have shown that substantial reductions in NO= can also be achieved by direct injection of wellatomized water, at flow rates of 0.5 to 2.0 times the fuel flow, into the primacy combustion z o u e . 7, 8, ~4),M, 43, 44, 45

Whether or not the NO= reduction is accompanled by increase in CO and UHC depends largely on the prevailing chemical reaction rates in the primary zone. If the overall reaction rate is far in excess of that needed to ensure 100 percent combustion efficiency then the injection of water should produce only very slighf, increuse in the output of CO and UHC. If, however, the reaction rate is barely sufficient to provide complete combustion, then the reduction in flame temperature caused b y water injection could appreciably raise the output of these species. The main drawback to water injection is the problems involved in pumping, storing and handling large quantities of demineralized water. For this reason the must suitable application of water (or steam) is to stationary industrial units located near an ample supply of water. 6. Exhaust gas recirculation. Another inert

1173

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DO 1500 1500 1700 1800 1900 2C PRIMARYZONETEMPERATURE,OK Fro. 2. Influence of primary-zone temperature on CO and NO, emissions. substance t h a t is available in abundance is combnstion products. However, to be effective in reducing NO= these must first be cooled before being returned to the primary zone. Application of this technique has realized appreciable reductions in NO=, but usually at the expense of large increases in CO.as,*9,~,~ Another drawback to exhaust gas recirculation is the attendant increase in size, weight and complexity of the combustion eqUipment.

(b) Advanced Combustor Concepts 1. Variable geometry. The notion of variablegeometry combustors is by no means new. Long before the emissions problem was recognized many and various schemes were proposed for introducing variable geometry into combustion systems, usually as a means of improving the altitude relighting performance of aircraft engines. These schemes almost invariably came to naught because engine designers, while appreciative of the advantages to be gained, were reluctant to accept the mechanical complexities involved. However, the considerable potential t h a t variable geometry holds for emission reduction has led to a renewal of interest in its application. 7.s.~,~3,~.39,4G,4s The manner in which variable geometry may be employed to reduce emissions may be illustrated by reference to Fig. 2 in which the emissions of CO and NO~ are plotted against prhnaxy-zone temperature for a hypothetical combnstur. The emission limits quoted correspond to automotive 1977 standards for a fuel consumption of 10 mpg. This figure shows that too much CO is formed at temperatures below

COMBUSTION SYSTEMS

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Fro. 3. Application of variable georaetry principle to different combustor types.

1600~ while excessive amounts of NO: are produced at temperatures higher than 1730~ Only iu the fairly narrow band of temperatures between 1600~ and 1730~ is the level of both CO and NO= below the required values. The advantage of variable geometry is that it offers a practical means of maintaining the mean primary-zone temperature within this narrow band throughout the entire range of engine operation. To be fully effective variable geometry eombustors should be used in conjunction with premix/prevaporimtion fuel injection systems. Only in this way is it possible to avoid the "hot spots" that promote NO= formation and the "cold spots" that give rise to CO. Many practical forms of variable geometry combustor have been designed. A fully variable system is one in which at maximum power eonditions large quantities of air are admitted at the upstrean~ end of the combustor to minimize soot and NO~ formation and provide adequate filmcooling air. With reduction in engine power an increasing proportion of this air is diverted to the dilution zone in order to maintain the primary-zone temperature within the "low emission band" with diminishing fuel flow. The degree of mechanical complexity involved in the application of variable geometry varies

with individual combustors, This point may be illustrated by reference to Fig. 3, which shows the emission characteristics of three hypothetical combustors, denoted as A, B, and C. The line representing combustor A lies completely outside the emissions "box," which means that variable geometry alone would not suffice to meet the emission requiremeuts. A small part of line B lies just inside the box and therefore it is just possible for this combustor to meet the emission standards using a highly sophisticated, fullyvariable system. Most of line C lies inside tlm box so that a simple, two or three position device would suffice to maintain the flame temperature within the low-emlssion range. More development is needed on the mechanical design and control aspects of variable geometry combusr but its potential for improving many aspects of combustion performance, especially emissions reduction, has now been fully confirmed. 2. Staged combustion. With this technique the combustion process is arranged to occur in a number of discrete stages.8da,~,~,47 In theory, either circumferential, radial or axial staging may be employed, but in practice circumferential fuel staging actually increases NO= because, instead of the fuel being distributed uniformly

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OVERALL FUEL/AIR RATIO

Fro. 4. Diagrams illustrating the prhlciples of multistaged combustion for low emissions.

POLLUTION CONTROL

1175

around the liner, it is injected at a small number until fairly recently a high combustnr inlet of points where it produces regions of high tem- temperature was rega~'led as a disadvantage perature) 1 This highlights the fact t h a t fuel from a NO~ viewpoint, and it was often argued staging, per se, is inadequate, and that both fuel t h a t for this reason the regenerative engine must and air must be staged for effective emissions inevitably emit more NO~ t h a n the simple cycle engine. However, it is now widely appreciated reduction. An interesting combustor that lends itself to that the vital parameter governing emissions is radial staging is the NACA swirl-can system. 6.7 not combustor inlet temperature but flame This annular comliustor contains 120 individual temperature. The main virtue of a high inlet swirl-can modules which are arranged in three temperature is that it considerably simplifies concentric rows with fuel flow independently the processes of fuel evaporation and fuel/air controlled to each row. Each module is designed mixing upstream of the combustion zone. a6 To be fully effective premix/prevaporization specifically for low NO~ emissions so that, at full power conditions, when fuel is supplied to all systems should be used in conjunction with modules, the total output of NO~ is relatively variable geometry. Even with this provision they small. At low power conditions the levels of CO may at times stray pcrilonsly close to the weak and UHC emissions would normally rise to quite extinction limit. This problem may necessitate high values, but are readily reduced by confining some form of piloting device to facilitate ignition and sustain combustion at arduous operating all the fuel to jnst one or two module rows. Axial staging may take any of several forms conditions. The emissions reduction potential of premix/ but a typical approach would involve the followprevaporization combnstors has now been amply ing features 4~,4~: (i) A lightly loaded primary zone employing demonstrated. However, substaetial developwell-mixed fuel injection. This primary zone ment efforts will be required before the knowledge provides all the temperature rise needed at idle gained on small-scale tubular combustors can be and acts as a pilot source of heat for the other successfully applied to large annular combustors operating at relatively low inlet-air temperatures. combustion zones downstream. (it) One or more additional burning zones each with its own separate supply of well-mixed fuel Conclusions and air. In this approach the emphasis is on optimizing Co,miderable reductions have already been the fuel distribution whereas with variable geometry the distribution of air is highlighted. made in the full-power smoke emission levels of The common aim of both methods is the regula- engines using distillate fuels, mainly through tion of combustion temperature to achieve improvements in fuel preparation and increased minimum emissions at all operating conditions. aeration of the soot-forming regions of the flame. The control of combustion temperature attainable The gaseous pollutant emissions from gas turis illustrated in Fig. 4 for a three-stage combus- bines can be reduced by modifications to liner tion system. geometry and fuel injection system, supplemented 3. Premix-prevapurization combustors. Per- where feasible by compressor air bleed at low haps the most promising system at the present power settings and water injection at high power time is the "premix/prevaporization" eombustor conditions. The US Federal emission standards proposed now being actively developed in the USA by both Ford and General Motom. s,~2,3g A key for continuous combustion engines pose formifeature of this concept is the attainment of dable problems in regard to NO~. However, complete evaporation of the fuel and complete sufficient progress has been made to demonstrate mixing of fuel and air prior to combustion. By that combustors featuring premixing of vaporavoiding droplet combustion, and by operating ized fuel and air, in conjunction with variable the primary zone at a lean fuel/air ratio, N0~ geometry or staged combustion, could provide emissions are drastically reduced due to the low a practical solution to this problem. The extent reaction temperature and the elimination of 'hot to which these and other advanced concepts wifl spots' in the cmnbustion zone. 8'l~.la,a4~'~,41.4sThis be developed for widespread engine application premix/prevaporization approach is, of course, must clearly depend on the severity of future ideally suited to the regenerative gas turbine legislation on emissions. The main features of the various proposed on account of its high combustor inlet temperature. methods of pollutant reduction are summarized In this context it is of interest to note that in Table I.

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] 178 REFERENCES

I. WADE, W. E. AND CORNELIUS, W.: Emission Characteristics of Continuous Combustion Systerns of Vehicular Power Plants--Gas Turbine, Steam, Stifling, Symposium on Emissions from Continuous Combustion Systems, General Motors Research Lab, Warren, Mich, p. 375, 1971. 2. TOONE, B.: Cranfield International Symposium Series, Vol. 10, p. 271, Pergamon Press, 1968. 3. ttoLnERN~SS, F. H. A~n MacFxaLANE, J. J.: Soot Formation in Rich Kerosine Flames at High Pressure, AGARD Conf. Proc. No. 125 on Atmospheric Pollution by Aircraft Engines, Section 18, 1973. 4. LEFEBVRE, A. H. Any DUERANT, T.: Design Characteristics Affecting Gas Turbine Combustion Performance, Esso Air World, Vol. 13, No. 3, November/December 1960. 5. NORSTER, E. R. ANn LE~nvm~, A. H.: Effect of Fuel Injection Method on Gas Turbine Combustor Emissions, Symposium on Emissions from Continuous Combustion SystemsGeneral Motors Research Lab., Warren, Mich., p. 255, 1971. 6. N1NDZWINCKI,R. W. AND JoNas, R. E.: Parametric Test Results of a Swirl-Can Combuster, NASA Technical Memorandum, NASA TM X-68247, 1973. 7. JONES, R. E. AND GROEMAN, J.: Design and Evaluation of Combustors for Reducing Aircraft Engine Pollution, AGARD Conf. Proc. No, 125 on ALmuspheric Pollution by Aircraft Engines, Section 31, 1973. 8. VNRKAMP, F. J., Vmmouw, A. J., AND TOMLr~'SON, J. G.: Impact of Emission Regulations on Future Gas Turbine Engine Combusters, AIAA Paper No. 73-1277, 1973. 9. Saw~n, R. F.: Experimental Studies of Chemical Processes in a Model Gas Turbine Combustor, Symposium on Emissions from Continuous Combustion Systems, General Motors Research Lab., Warren, Mich., p. 243, 1971. 1O. S.r R. F., T~XEmA, D. F., AND STANK~aN, E. S.: Trans. Am. Soc. Mech. Eng. of Eng. for Power 91.4, 290 (1969). Ii. LNF~EVRE. A. H.: Contribution to discussion on Modeling Continuous Combustion, Symposinm on Emissions frmn Continuous Combustion Systems, General Motors Research Lab., Warren, Mich., p. 89, 1971. 12. WanE, W. R., SnEN, P. J., OWENS, G. W., ANn McLNAN, A. F.: Low Emissions Combustion for the Regenerative Gas Tnrbine, Am. Suc. Mech. Eng. Paper No. 73-GT-11, 1973. 13. Onosns, J.: Can. Aeronaut. Space J., 339 (October 1970). 14. GEOEMaN,J.: Effect of Operating Variables on

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36. 37. 38.

39.

Oriented Toward Minimum Effect on Engine Performance, AGARD Conf. Proc. No. 125 on Atmospheric Pollution by Aircraft Engines, Section 33, 1973. I~ISWELL, N. J.: Emissions from Gas Turbine Type Combustors, Symposium on Emissions from Continuous Combustion Systems, General Motors Research Lab., Warren, Mich., p. 161, 1971. PAGNI, P. J., HUGHES,L., AND NOVAXOV,T. : Smoke Suppressant Additive Effects on Particulate Emissions from Gas Turbine Combtmtors, AGARD Conf. Proc. No. 125 on At~ mospberic Pollution by Aircraft Engines, Section 28, 1973. C~AMPA(~NE, D. L.: Progress toward a Fully Acceptable Smoke Abatement 1;~uel Additive, to be pnblished. BANE, D. W.: Technology for the Reduction of Aircraft Turbine Engine Exhaust Emissions, AGARD Conf. Proc. No. 125 on Atmospheric Pollution by Aircraft Engines, Section 29, 1973. SAWYER, R. F., CEENANSKY, N. P., AND OPPENHRIM, A. K.: Factors ControUiag Pollutant Emissions from Gas Turbine Engines, AGARD Conf. Proe. No. 125 on Atmospheric Pollution by Aircraft Engines, Section 22, 1973. VRRKAMP, F. J.: Contribution to discussion, AGARD Conf. Prec. No. 125 on Atmospheric Pollution by Aircraft Engines, Section 22, 1973. M~:us, T. AND HEYWOOV, J. B.: Combust. Sci. Technol. ~, 149 (1971). BELL,A. W., BAYaI~oDE VOLO,N., ANDBRERN, B. P.: Nitric Oxide Reduction by Controlled Combustion Processes. Western Statos Section/ Combustion Institute, April 1970. NAnRY, T. F., MYKOLENKO, P.~ NAYLOR, M. E., AND VERKA~P, F. J.: The Low Emission (]-as Turbine Car--What Does the Future Hold, Am. Soc. Mech. Eng. Paper No. 73-GT49, 1973.

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40. BREEN, B. P., BELL, A. W., BAYARDDE VOLO, N., BAGW:ELL,F. A., ANn I~OSRNTHAL, K*: Thirteenth SympositLm(International) on Combustion, p. 391, The Cmnbustion Institute, 1971. 41. Mosmu, S. A. ANn ROBERTS, R.: Low-Power Turboprepulsion Combustor Exhat~t Emis. sions, Technical Report AFAPL-TR-73-36, Vol. I, June 1973. 42. LEF~DV~, A. H. AND FLETCHER, R. S.: A Preliminary Study on the Influence of Fuel Staging on Nitric Oxide Emissions from Gas Turbine Combustors, AGARD Conf. Proc. No. 125 on Atmospheric Pollution by Aircraft Engines, Section 30, 1973. 43. SINGH, P. P., YO,JNO, W. E., AND AMBROSE, M. J.: Formation and Control of Oxides of Nitrogen Emissions from Gas Turbine Combustion Systems, Am. Soe. Mech. Eng. Paper No. 72-GT-22, 1972. 44. H~T, M. B. AND JOHNSON,R. H.: Nitric Oxide Abatement in Heavy Duty Gas Turbine Combu~tors by means of Aerodynamics and Water Injection, Am. Soe. Mech. Eng. Paper No, 72-GT-53, 1972. 45. KLAYPATCH, R. D. AND KOBLISH, T. R. : Nitric Oxide Control with Water Injection in Gas Turbines, Am. Soe. Mech. Eng. Paper No. 71-WA/GT-9, 1971. 46. L ~ n v a ~ , A. It.: Current Problems and Trends in Gas Turbine Combustion, First International Symposium on Air Breathing Engines, Marseifle, Jutm 1972. 47. LEFEnvnE, A. H.: Contribution to Discussion, Symposimn on Emissions from Continuous Combttstion Systems, General Motors Research Lab., Warren, Mich., p. 321, 1971. 48. Wni~, D. J., ROBERTS, P. B., AND COMPTON, W. A. : Low Emission Variable Area Combustor for Vehicular Gas Turbines, Am. Soc. Mech. Eng. Paper No. 73-GT-19~ 1973.

COMMENTS N. A. Chigier, University of Sheffield, England. The NASA-Lewis annular combustor divides the combustion chamber into a large number of separate modules. Earlier designs of these modules with swirler plates did not pay much attention to fuel atomization and mixing within the individual modules. More attention is, however, now being devoted to control of mixing and improvement of fuel atomization in each individual module since it is recognized that oxides

of nitrogen, soot and unburnt hydrocarbons formed in the small reaction zones can be frozen and emitted from the exhaust. I should like to ask Prof. Lefebvre's opinion of the NASA-Lewis annular multi-modular combustion chamber concept as a means of reducing emission of pollutants.

Author's Reply. The NASA modular comliustor has already demonstrated low emission of nitric

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oxides. However, as I have tried to stress in the paper, the emissions performance of this or any other combustor can only be assessed on the basis of both NO| and CO emission levels. It is hoped that sufficient d a t a will soon be available for a proper evaluation of this interesting concept.

J. M. Be~r, University of She~eld, England. You have mentioned t h a t by varying the angle of the liquid spray--typically by widening---it is possible to reduce soot emission from gas turbine combustors. This is in agreement with some results obtained by the International Flame Research Foundation on swirling, liquid fuel flames. The explanation was given tlmt by widening the spray angle the fuel droplets were introduced into regions of the swirling flow, at the boundary of the central recirculation zone, where the turbulent shear stresses axe high. This in turn resulted in high combustion intensities and in a considerable shortening of the flame with improved combustion efficiency. Experimental tools are now available for determining both turbulence characteristics of swirling flows and droplet trajectories of sprays optically, and could be used for improving the emission characteristics of combustors b y satisfying the fuel concentration-turbulent shear matching criterion.

Takashi Tamaru, National Aerospace Lab., Japan. Considering conditions of the air after the cmnpressor, it seems pretty difficult to have enough vaporization of the fuel. (1) Do you think it is possible to "pre-vaporice" enough fuel to control ponutien? (2) Do you think the pre-vaporization i s feasible without a n y special heating device to evaporate the fuel? If such is required, how can it cope with transient conditions?

Author's Reply. Pre-vaporization systems are ideally suited to regenerative gas turbines where the combustor inlet air temperature is sufficiently high to vaporize the fuel in a reasonably short distance.

With non-regenerative engines the combustor in]et air temperature is usually too low for fuel evaporation purposes, except perhaps at the highest power conditions, depending on the engine compression ratio. However, bearing in mind the substantial quantities of heat liberated in combustion, it is hopefully not beyond the capability of the combustion engineer to devise practical methods of utilizing this heat source in fuel evaporation. The problems posed by operation at transient conditions could, in extreme cases, necessitate the use of a fairly sophisticated, variable geometry combustor. Another alternative might be to incorporate some form of "pilot" combustion zone, preferably one supplied with a premixed fuel/air mixture.

A. M. MeUor, Purdue Univers~y, USA. Prolessor Lefebvre has correctly identified the major combustor or engine design parameters which affect the emissions of unburned hydmcaxbons, soot, and CO. However, his comments on NO are perhaps misleading. Prenfixed, prevaporizing combnstors can exhibit exactly similar NO~ emission indices to those of conventional eombustors if they operate in a "diffusion flame" mode, where large recircuiation zones are used to ignite the incoming fluid. Since mixing rates can control in such cases, the emissions of NO are not determined just by temperature, either t h a t computed b y assuming the primary zone homogeneous, or some average associated with the burning (and mixing) eddies. Author's Reply. For premixed/prevaporizing systems the overriding factor controlling NO~ emissions is the flame temperature. This is largely determined by the inlet air temperature and fuel/air ratio, and also by pressure (through its influence on the chemical dissociation of CO~ and H~O). Under these conditions the type of flow recireulation employed and its associated mixing rates are irrelevant to NO~ emissions, although they can clearly govern the burning rate and hence also the size of the combustion zone.