WATER ADDITION TO PRACTICAL COMBUSTION ... - Science Direct

WATER A D D I T I O N TO PRACTICAL C O M B U S T I O N S Y S T E M S m C O N C E P T S A N D APPLICATIONS F. L. DRYER Princeton University, Princeton, New Jersey 08540 The use of water as an ancillary combustion control technique has recently received renewed interest. This paper historically reviews the applications of water addition to practical combustion systems and discusses in detail the fundamental aspects of combustion which are affected. Combustion properties of water-in-fuel emulsions are elaborated upon, and several potentially favorable applications which require additional research are identified.

Introduction In light of the recent resurgence of attention that water addition to combustion is receiving, it is interesting to recall h o w frequently this concept has been suggested during the history of combustion energy conversion development. Reference to water as an ancillary combustion control technique can be found as early as 1791 in one of the first recorded patents of gas turbine principlesJ Water injection into combustion products was advocated for the protection of the turbine section, a principle later employed in early gas turbine development. ~ As liquid fuels began to replace coal in the mid-nineteenth century, the use of steam was introduced as a means of improving fuel atomization. However, few published works exist on the development of twin fluid steam atomizers. Steam atomization was frequently employed during the early part of the current century, and interest has recurred recently due to frequent shortages of natural gas and the difficulty experienced in cold start of large oil-fueled boilers which employ mechanical atomization. 3 However, the most sustained interest in water addition has been with regard to the internal combustion engine. In 1913, Hopkinson noted that "the idea of introducing water into internal combustion engines is not new."4 So successful was the use of water to eliminate detonation and internally cool early gas engines that Hopkinson designed power plants with no other means of cooling. Until shortly after World War I, the aspira279

tion of water or m e t h y l / e t h y l alcohol/water mixtures played an integral role in increasing the operating efficiency of the otto enginesP However, the discovery of fuel soluble antiknock agents in 1922, 6 and the complexity and cost of required control and storage systems discouraged further use of water and alcohol addition to automotive engines. In the early 1900's, attempts were made to formulate stable dispersions of water, alcohols, and fuels to alleviate these difficulties, 7 and with the impending prohibition of tetra-ethyl lead in automotive fuels, such dispersions have again received some attention. 8 Until the development of micro-emulsions, 9 the severe influence of ambient temperature on mixture stability prohibited commercialization of such dispersions. While micro-emulsions have soIved the stability problem, the concentrations of water and alcohols which can be stabilized are severely restricted and require large amounts of stabilizing agent. 1~ The development of supercharged aircraft engines previous to and during World War II rejuvenated interest in water and alcohol aspiration into internal combustion engines (e.g. see References 13-15). Severe detonation and over-heat imposed unacceptable limits on supercharge and compression ratio at high load, and water or water/alcohol aspiration, the latter to prevent additive icing, were frequently employed to attain maximum power at take-off and during combat maneuvers. Water addition was also instituted on early turbo-prop and jet engines to extend the high power limit, and in fact the technique survived

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as an optional feature on the Pratt and Whitney JT9D commercial aircraft engine. 16 Following the war, scarcity and cost of automotive fuel grades compatible with existing engines, and continued efforts to extend engine compression ratio sustained interest in water and alcohol addition. Numerous articles were published from 1944 to 195917 on internal cooling and the relief of otto engine knocking characteristics, and several devices for metering water and alcohol appeared on the commercial market. 17-21 In fact, in 1962, Oldsmobile Division of General Motors introduced a supercharged V-8 engine (compression ratio of 10.25) which employed small amounts of a methanol/water mixture to prevent knock at high load. 22 Additive utilization was the order of 10 -3 gallons per mile on the open road, and special features were included to limit supercharging when additive supply was exhausted. It appears that the lower compression ratio engines required by existing emission standards and the reduced fuel utilization efficiency associated with production of nolead high octane fuels may inspire a new commercial interest in supercharging, but it is doubtful that the use of additive systems will be revived. The work of Zeldovich et al.23 on nitric oxide formation from molecular nitrogen and the discovery of the relation of NO~ to chemical smog formation 24 prompted Kopa 25,26 to suggest a new motivation for water addition: the reduction of combustion-generated NO~ emissions. Numerous investigations of this concept as applied to compression ignition engines, 12,27-37 spark ignition engines, 1~ gas turbines, 16'47-61 and external combustion systems (boilers, etc.) 62,6a have appeared over the last few years. Effects on other pollutant emissions, performance, energy utilization and various methods of water addition were often simultaneously investigated in these works. However, with few exceptions additive techniques have been considered for retrofit to existing hardware rather than in actual design of new systems. Fundamental Effects on Combustion Properties

It is surprising that with such extended study of practical applications, little attention has been devoted to fundamental considerations. The lack of such information is probably the principal cause for the "mystique" surrounding water addition and the unpredictable resuits which have occurred for the various

evaluation studies referenced above. It is the intent of the current article to assemble what is known and what may be be reasonably hypothesized about the fundamental effects of water addition on combustion properties, thus providing suitable background for several papers which follow in this Symposium. Based upon this information, an extensive critical review of available applications data with particular emphasis on the use of water-in-fuel emulsions appears elsewhere. 64 The addition of water may have both physical and chemical kinetic effects on combustion phenomena, the magnitudes of which will depend on how water is introduced to the combustion environment. Let us first consider some of the chemical kinetic effects produced by water addition.

Chemical Kinetic Effects of Water Addition It has been noted in spark ignition engine studies that pre-ignition and detonation are significantly reduced by water addition and this has been theorized to result primarily from internal charge coolingP5 Yet simple calculations show that the addition of 30% water (with respect to fuel volume) to a stoichiometric charge represents less than two percent of the total charge mass, and not more than four percent of the available combustion energy can be consumed in water vaporization and heating. 42 This amount of water addition can produce nearly a unit increase in knock limited compression ratio, 11 and thus water vapor probably exerts some chemical inhibition on hydrocarbon autoignition kinetics other than through decreasing the system temperature. A potential mechanism is suggested by the pronounced effectiveness of water as a promoter for hydrogen-oxygen radical recombination reactions. 66 Lower hydrogen atom concentrations would reduce the rate of chain branching through H + 02--~OH + 0 and longer characteristic times for auto-ignition would result. Similar effects on H atom concentration have been argued as the reason for anti-knock characteristics of tetra-ethyl lead (TEL). Yet unlike TEL addition, the use of water leads to reductions in peak cylinder pressure. Thus, it has been inferred that the addition of water lowers the rate of flame propagation. 3s,65 With the exception of carbon monoxide 67 and hydrogen flames, 6s very little information about the effect of water vapor addition on

WATER ADDITION TO PRACTICAL COMBUSTION SYSTEMS flame velocity exist. Increases in absolute humidity drastically accelerates "dry" stoichiometric carbon monoxide flames. However, laminar hydrogen/air flames are inhibited by water vapor addition, but only by about 1/3 of the amount predicted from inert dilution. The catalytic effect of water-vapor on carbonmonoxide combustion has been well documented to occur from an enhanced production of hydroxyl radicals, 69 primarily through H 2 0 + H--* H2 + OH H20+O---~OH+OH H2 + O---~ OH + H resulting in an increased reaction rate for CO + OH --* CO 2 + H. Kueh168 has reasoned that hydrogen flames are not inhibited to the extent expected because resonant radiant transfer from hot products to the seeded water vapor increases the mixture temperature immediately preceding the flame. However, the argument does not appear to closely predict experimental observations. Effects of water addition on combustion chemistry other than those from lowering the resultant flame temperature were neglected. Limited data on stoichiometric methane and butane flames 7~ suggests about two percent increase in absolute humidity lowers the observed laminar flame speed by about ten percent. Simple one-dimensional flame theory and determined temperature dependences for these flames 7t suggest this inhibition is about three times that predicted Joy increased heat capacity effects. The relative importance of hydrogen abstraction from hydrocarbons by hydroxyl radicals has led some to suggest that catalytic effects on oxidation chemistry similar to those observed for carbon-monoxide should occur. 72 However, experimental results at one atmosphere pressure and in the temperature range 1100-1400 K 7a show the rate of oxidation of lean pre-mixed methane/air is independent of initial water vapor concentration. Using an existing elementary kinetic mechanism for methane oxidation, TM isothermal and adiabatic, constant pressure calculations with an initial temperature of 1400 K and a methane/air equivalence ratio of 0.5 have been performed. Although the gross overall oxidation rate was found to be only slightly decreased, maximum radical concentrations of oxygen atoms and hydroxyl radicals were observed to be significantly modified. For ex-

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ample, the replacement of air-nitrogen by 10 volume percent water vapor in both cases yielded about a 30 percent increase and a 22 percent decrease in maximum hydroxyl radical and oxygen atom concentrations. Maximum hydrogen atom concentration was found to be only weakly affected. These calculations suggest reductions in NO~ formation would occur even at constant combustion temperature. However, the flame inhibition mechanism for methane and butane flames remains incompletely determined. In fuel-rich combustion, it is very probable that similar changes in radical concentrations by water vapor addition may produce inhibition of gas phase soot formation. It has been suggested that hydroxyl radicals are very effective in oxidizing soot precursor species, thus reducing polymerization and carbon nucleation rates. 75 However hydroxyl radical concentrations in fuel rich combustion are generally low since the lack of oxygen results in negligible rates of production through H + O2--~ OH + H RH + O ~

R + OH.

Thus the reaction H 2 0 + H---~ H 2 + OH is the principal source of hydroxyl radicals. Soot inhibition by various metal additives, M, has thus been argued to result from increased production of hydroxyl radicals through M + H20---~ M O H + H MOH+H 20+M(OH) z+H H + H 2 0 - - ~ O H + Hz

i.e. through increasing the concentr~ition of H atoms. Likewise, the rate of the last reaction could be enhanced by adding water to the fuel-rich soot formation region. One might argue that water vapor is already present from the hydrocarbon combustion itself. However, water vapor is not produced until relatively late in the combustion history of premixed, fuel-rich mixtures. As in the case of water addition effects on auto-ignition and flame velocity, little research has been conducted on gas phase soot inhibition, even in pre-mixed reaction systems.* One might expect a more

*A paper on this subject has appeared shortly after this Symposium (see Combustion & Flame 27, p. 205, 1976).

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pronounced effect where water is added to the fuel-rich region of diffusion (mixing) limited combustion, and recent simple spray combustion experiments discussed later in this paper corroborate this argument.

Physical Effects of Water Addition In addition to the lower system temperatures which result from water dilution of the combustion charge and the resulting effects on combustion chemistry, the addition of water as a dispersion in the liquid fuel itself (i.e. as an emulsion) appears to have potential for improving combustion properties associated with fuel atomization. Many investigations have considered a stable dispersion of water droplets within fuel only as a more convenient means of water addition. However, the recent interest in water-in-oil emulsions as alternatives to conventional fuels 36,37,76-85 is primarily associated with exploitation of a seldom considered phenomenon termed "micro-explosions" by those who first discovered it nearly twenty years ago. 86,87 For water-in-fuel emulsions of high boiling point, Ivanov and Nefedov postulated that when an emulsion droplet was introduced into a high temperature environment the small internal droplets of dispersed water would undergo vaporization early in the droplet lifetime. This process was argued to be so disruptive as to shatter the primary emulsion droplet into many smaller fragments. Ivanov and Nefedov attempted to substantiate these concepts by comparing the combustion characteristics of 800 to 3000 micron droplets of conventional and emulsified fuels. Droplets were suspended on either a quartz filament or small thermocouple and thrust into pre-heated air at one atmosphere, and combustion was photographed using cinematography at 300 to 400 frames per second. The resulting data showed that emulsified fuel droplets ignited earlier, underwent a more disruptive combustion history, and were consumed in a shorter period of time. For an 1100 micron droplet of 30% water-in-mazut (residual oil) emulsion and a surrounding air temperature of 740~ the droplet lifetime was 60% of a comparable droplet of neat fuel. Carbonaceous residue remaining on the suspending filament was found to be significantly reduced by emulsification. Ivanov and Nefedov extended the investigation to arrays of suspended droplets and summarized their findings by stating: "1) Emulsified fuels burn faster than anhydrous ones.

2) Water in emulsified fuels does not impair, but improves the combustion process, owing to the addition simultaneous breaking of the droplets, to the increase in evaporation surface of the droplets, and to a better mixing of the burning substances in air. 3) The reduction of the combustion time of emulsified fuels has a favorable influence on the burning of sooty residue, thus improving the completeness of combustion and reducing the deposition of soot (scale) on the working surfaces." Only within the last few years has the application of these principles to practical combustion systems been initiated. 88,89 The potential importance of internal phase ratio, internal droplet size and distribution, ambient pressure and temperature were not investigated by Ivanov and Nefedov. However, recent experiments have confirmed earlier observations and suggest all of these variables must be considered if the process is to be optimized. Dryer et al. 89 have studied the combustion of residual fuel emulsion droplets suspended on quartz filaments. Emulsions were formed using a high speed counter rotating blade mixer, and dispersed phase droplet size distribution was estimated using photomicrography. Droplets were ignited in an open air environment by momentary exposure to a small butane/air pilot flame, and the combustion event was photographed using backlighting and a Fastax high speed camera. While backlighting precluded the observation of the actual flame structure, the droplet surface itself could be viewed. Figures 1-4, and 7 represent a period of time after 20 to 50 percent of the initial droplet volumes were consumed. Figure 1 adequately demonstrates the combustion characteristics of a neat, wide-boilingpoint-range, heavy fuel. Such fuels are generally difficult to finely atomize, and thus heterogenous combustion is significant. While gas phase soot formation occurs in the fuel-rich regions of the diffusion (mixing) limited combustion, multicomponent heavy fuels also produce soot through liquid phase heterogeneous reactions. As the droplet burns, the lower boiling point compounds vaporize first, concentrating the heavier fuel components. Droplet temperature must increase with time in order to sustain vaporization, and surface temperatures are reached which are high enough to produce liquid-phase pyrolysis (coking). TM Even the recovery temperatures for distillate fuels may range high enough for these processes to occur, particularly if the fuel contains

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Fxc. 1. Sequential 16 m m high speed cinematography sequence of a burning droplet of bunker C fuel. Quartz filament (40 Ix) suspended droplet ignited with a small butane/air pilot flame. Initial droplet size, 450 microns, Camera framing rate, 4000 frames per second. Performed at Guggenheim Laboratories, Princeton University, April 1975. a) Char remaining on fiilament.

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FIG. 2. 16 mm high speed cinematography sequence of a burning droplet of 25% water-in-fuel bunker C emulsion. 2.1 percent surfactant added. Internal phase mass mean droplet size, approximately 2 microns. Quartz filament diameter, 45 microns. Suspended droplet ignited with a small butane/air flame. Initial droplet size, 425 microns. Camera framing rate, 4000 frames per second. Performed at Guggenheim Laboratories, Princeton University, April 1975.

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Fie. 3. Enlargement of last four frames of Figure 2. high boiling point aromatics or aspha]tenes. In the experiment shown in Figure 1, the carbon/'hydrogen ratio of the remaining liquid increases with time, slow heterogenous surface burning ensues, and finally combustion terminates with a char remaining on the suspending filament (see "a", Figure 1). In practical combustion systems operating on residual fuels, similar processes occur for arrays of droplets and lead to the formation of large carbon-containing particulates (>5 microns). These car-

Fie. 4. 16 mm high speed cinematography sequence of a burning droplet of 22% water-in-bunker C fuel emulsion (no added surfaetant). Quartz filament (18 Ix) suspended droplet ignited with a small butane/air pilot flame. Initial droplet size, 350 microns. Internal phase mass mean droplet size, approximately 10 microns. Camera framing rate, 5000 ~rames per second. Performed at Guggenheim Laboratories, Princeton University, July 1975.

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bon particles may later be consumed (at least partially) through continued surface combustion. Even if the better atomization achieved through micro-explosions only produced the same mass of coked material distributed as smaller particles, the increased surface area should lead to more rapid consumption of the soot through surface combustion. Figure 2 confirms the existence of microexplosions both for suspended and free droplets of emulsified fuel. In the last frames of the sequence (enlarged in Figure 3), three smaller free droplet fragments are formed, two of which are the size of the suspending filament. While one of these droplets proceeds to leave the field of view, the small droplet near the suspending filament is totally disintegrated within the time between the last two frames of the sequence. This disappearance is too rapid to be accounted for by droplet vaporization/combustion and can only be attributed to "micro-explosive" atomization. It might be suspected that both the total amount of water and the way it is distributed within the fuel droplets (internal phase size distribution) would be important to optimizing the micro-explosion phenomenon. The rates of primary droplet heating and of internal water droplet phase change, the total volume changed during internal vaporization, and the

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internal droplet number density may all be important phenomena in selecting the optimal internal phase properties. Figure 4 shows the combustion of a droplet of water-in-fuel emulsion with nearly the same water content as that displayed in Figures 2 and 3. By reducing the level of stabilizing surfactant, the internal phase droplet size distribution was increased from around 1 to 2 microns to the order of 10 microns. For initial emulsion droplet sizes the order of 400 to 500 microns, the effects of the micro-explosion phenomenon appear to be intensified by the increase in internal droplet size. The sequence shown in Figure 4 also occurred earlier in the droplet lifetime than that shown in Figures 2 and 3. A recent study of emulsified fuel combustion in an experimental gas turbine combustor 7s corroborates that both the water content and internal phase size distribution of the emulsion have substantial effects on emissions reduction (Figures 5 and 6). Emulsions were formed using a Gaulin Model 100 Laboratory homogenizer operating at a constant pressure of 3000

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F16. 6. Summary of effects of emulsion characteristics on the flame radiation at full power conditions for an Allison T-63 gas turbine combustor. Personal communication, Dr. C. A. Moses, U.S. Army Fuels and Lubricants Laboratory Southwest Research Institute, San Antonio, Texas.


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FIG. 7. 16 mm high speed cinematography sequence of a burning droplet of 20% water-in-N-dodeeane emulsion. 2.0 percent surfactant added. Internal phase mass mean droplet size, approximately 2 microns. Quartz filament diameter, 13 microns. Suspended droplet ignited with a small butane/air flame. Initial droplet size, '250 ix microns. Camera framing rate, 5000 frames per second. Performed at Guggenheim Laboratories, Princeton University, May 1975. a) Location of water coalesced on suspending filament.

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psi, and it was confirmed that addition of emulsifier alone did not affect combustion emissions. At low surfactant concentrations, the stabilized internal dispersed phase size distribution reaching the combustor was determined by emulsifier concentration because there was an insufficient amount to support the total interracial area of the internal phase size distribution produced by the homogenizer. As emulsifier concentration was increased, larger interracial areas could be supported and smaller internal phase size particles were produced. The curves of emissions (Figure 5) approach asymptotic values at increased emulsifier loading because the internal phase size distribution was then governed by the homogenization pressure. If homogenization pressure had been increased, internal size distribution might have been further reduced. However, it is reasonable to suggest that at very small internal particle sizes, the material would behave as a multicomponent solution, and the micro-explosion phenomena would be insufficient to rupture the primary fuel-droplet structure. Figure 6 suggests total flame radiation is reduced by using emulsified fuels, and similar results have been reported by Toussaint and Heap 79 for external combustion research of residual emulsions. Reduction in radiation is not surprising, since in diffusion (mixing) limited combustion, continuum radiation from particulates is far greater than spectral radiation from hot combustion gases. Indeed, the addition of water vapor to diffusion flames has been shown to have little effect on spectral radiation. 9~ The experiments of Dryer et al. 89 discussed above are too crude to be used for more than qualitative interpretation, and others have attempted to obtain quantitative data from more sophisticated suspended droplet research, s3,84 However, several aspects of this experimental technique probably limit the practical utility of such data. Figure 7 is a high speed cinematography sequence of a suspended burning droplet of 20% water-in-n-dodecane emulsion. A most interesting feature of this sequence is that for this single component hydrocarbon emulsion, backlighting was sufficient to view internal regions of the droplet. What is to be noted is that the presence of the suspended filament enhances coalescence of the internal phase droplets of water, and eventually leads to water droplet formation on the filament itself. Indeed, filament promotion of internal phase coalescence has been commercially developed as a means of breaking water-in-oil emulsions. 91 Thus, the suspending filament

has a strong influence on the internal phase structure of the emulsified fuel droplet, a property which has just been shown to be of importance in optimizing the micro-explosion phenomena. An even more tacit problem is how the presence of the filament affects the temperature at which the internal phase droplets of water vaporize. Vaporization of a pure liquid requires the growth of a collection of vapor molecules to form a critical size bubble beyond which further growth rate of the bubble is spontaneous. This "nucleation rate" (i.e. the rate at which the number of critical size bubbles form) can be shown to be propoitional to the exponential of the work, W, necessary to form the bubble of critical size (e.g., see Reference 92), and the nucleation rate must exceed the rate of decay of bubbles of critical size for spontaneous vaporization to occur. The exact formulation of this problem is beyond the scope of the current article. However, the temperature at which homogeneous nucleation of a pure liquid occurs, T~, is much greater than the boiling point, Tb. For example, the experimentally determined homogenous nucleate vaporization temperatures (at one atmosphere) of water and methanol are 270~ and 186~ respectively. 93 Liquids generally boil at much lower temperatures than T] because the presence of surfaces, surface irregularities, etc. significantly lowers the work required for bubble nucleation. A critical density of irregularly shaped particles will also produce this effect, hence the purpose of adding "boiling chips" to distillation systems. If Wlies somewhere between that for normal boiling and homogeneous nucleation, vaporization will occur at Tb < T~ < T*~, the limit of superheat. The problem of super heating and vaporization of internal phase droplets of an emulsion TABLE I Nucleate vaporization temperatures (T,) at atmospheric pressure for the internal phase of several water-in-fuel emulsions: comparison of theory and experiment. From reference 94. Emulsion* n-decane n-dodecane n tetradecane n hexadecane

Tb 174.1 216.2

252.2 287.5

Temperatures (~ T~Th.... T~E*p. 230.8 252.5 262.2 269.7

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* 15% water, 83% fuel, 2% emulsifier (60% SPAN 85, 40% TWEEN 85, ICI America Inc.), by volume.


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is more complex than homogeneous nucleation of a pure substance because of the multiplicity of potential nucleation sights introduced by the interface between phases. Research has only recently been initiated on this problem. 94 It has been found that depending on the chemical structure of the emulsion components, the vaporization temperature of internal

water droplets of the emulsion may approach the homogeneous superheat limit of pure water. Table I summarizes observed and predicted maximum nucleation temperatures for a number of water-in-fuel emulsions. Thus, the super heat limit of the internal droplets of water within an emulsified fuel may be lowered by the presence of the suspending

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filament, particularly if the water has coalesced on the filament surface. This effect would modify not only the time in the droplet life at which micro-explosions are initiated, but the intensity of the micro-explosions themselves. Similar arguments may be of relevance to the observation of droplet disruptions ,s during suspended droplet combustion of neat multicomponent fuels. Other complications such as radiative heating of locations within the droplet, or heat transfer from the suspending filament have often been suggested and must also be considered. 95 However, it has been noted in the author's laboratory and elsewhere 96 that disruptive combustion produced by internal vaporization of light fractions of the droplet is generally not observed (at least not as early) in the combustion lifetime of free droplets of multi-component mixtures. Efforts are currently underway to adapt a free droplet combustion technique to the study of water-in-oil emulsion combustion. 97 Whether micro-explosions occur before or after ignition of the primary droplet will also depend on the mode of droplet ignition, the droplet heating rate, and unsteady effects (including internal recirculation) within the droplet itself. For example, Jacques et al. s4 observed disruptions to occur preceding droplet ignition when a suspended emulsion droplet was ignited by immersion in high temperature (-1000 K), quiescent air. In contrast, Dryer et al. s9 observed micro-explosions only after a significant period of combustion when the emulsion droplet was ignited with a pilot flame. Furthermore, emulsification may alter atomization through the process described above as well as by modifying the neat-fuel physical characteristics. For example, fluid viscosity can be increased substantially and non-newtonian characteristics can result from the additional internal phase structure. 9s In addition, the use of emulsions may al~aplify the chemical effects produced by water addition to diffusion (mixing limited) flames. Figure 8 qualitatively compares soot production in pulsed spray combustion of neat and 25 percent water in n-dodecane emulsion. The sprays were formed with a low-pressure, pulsed atomizer and ignited with a propane-air pilot flame. The notable difference in soot formation might be thought to result from secondary atomization. However, Table I shows that the nucleation temperature for the internal water droplets of this emulsion is larger than the saturation temperature of the fuel itself. Thus, the observation must be attributed to chemical soot inhibition. In addition to the mechanism described earlier, mass

and thermal dilution of the fuel rich region within the diffusion flames may reduce the rate of production of soot precursors and nucleation of polyaeetelynic species. Soot inhibition was observed to be even more distinct in the case of neat and emulsified distillate fuel. The distillation range of this fuel exceeded the emulsion nucleation temperature, and thus secondary atomization may have occurred. Water addition effects on NO r formation and other fuel chemistry may also be enhanced by the use of emulsions, and fundamental research on these topics have only recently been initiated. 99 Summary and Overview The use of water as an aneilliary combustion control technique has been a subject of passing interest for nearly two hundred years. Although practical combustion applications have been frequent, fundamental concepts discussed earlier demonstrate that the various ways that water addition affects combustion phenomena are far from clear. The lack of such understanding is probably responsible for many of the inconsistencies often found in practical combustion applications. Both facts and cons~idered speculation on the effects of water addition have been presented with the purpose of stimulating additional fundamental research and guiding future practical evaluation. However, a sufficiently convincing technical base currently exists for dispelling the alchemy inferred by some of the published literature. Critical analysis of the data can identify several favorable aspects of water addition which should be further explored, 64 and these areas of potential opportunity are summarized below.

Internal Combustion Engines Water addition to internal combustion engines has received little commercial interest primarily because of its reflex-association with transport applications. However, a significant portion of the United States stationary NO x emission inventory is produced by internal combustion systems used for power generation) ~176 Addition of water at the levels required to meet current and future NO x regulations does not affect energy utilization to the extent of many other control techniques, and this is particularly true for diesel applications. Furthermore, the use of water-in-fuel emulsions formed in-situ 37 offers a very effective means of controlling diesel particulate emissions.

WATER ADDITION TO PRACTICAL COMBUSTION SYSTEMS Corrosion of engine parts and contamination of the engine lubrication systems have often been argued as insurmountable technical problems, but published data do not support this conte ntion.2~ Although stabilized macroand micro-emulsions are currently not an economically competitive means of controlling emissions or fuel octane rating, scientific advances and energy economics may precipitate a more favorable consideration of mobile applications. Even the technical difficulties of dual-fuel systems have certainly been shown to be surmountable. 22 While the micro-explosion phenomenon can not be of benefit to carbureted engines, it may be of considerable significance to future direct injection spark and compression ignition systems and thus further research on internal combustion applications is warranted.

Gas Turbines Water addition to gas turbines is currently the only available means of reducing NO x emissions from existing stationary systems to future regulatory levels. However, water injection into the primary combustor zone is presently very expensive because of the required volume of high purity water (approximately 30 weight percent of the fuel flow)2 ~176 While the same purity of water is required for steam injection, steam generation from exhaust gases improves the energy utilization of the overall system by twenty to thirty percent. 6~ Insufficient exhaust energy is available to produce the amount of saturated steam required for NO~ control. However, combined steam addition and direct water addition should permit both increased cycle efficiency and NO~ control. Water-in-fuel emulsions are a simple method for implementing direct water addition to existing stationary gas turbine systems for this purpose. In addition, the use of emulsions has shown promise as a means of significantly reducing gas turbine particulate emissions, vs The concept of secondary (micro-explosive) atomization has also recently been found to be a successful approach for relieving the stringent fuel requirements of short-length combustor systems, a~ and may be very important to residual gas turbine combustion, s2

External Combustion Since the discovery of the micro-explosion phenomenon, the use of water-in-fuel emulsions has probably received most attention with regard to external combustion applica-

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tions. Because of the limited attention shown to first understanding which emulsion properties are important to optimizing effects on combustion, the data which has been generated thus far fall short of quantitatively documenting the optimal combustion modifications achievable through the use of water-in-fuel emulsions. Therefore, emulsion technology can not be properly ranked in comparison to other external combustion control techniques. However, it appears that the combustion of water-in-residual oils can lower particulate and thermal N O emissions while simultaneously reducing excess air requirements. In many cases, thermal efficiency may be improved several percent, particularly if the required water concentration is small. A recent survey of industrial boilers, ~~ suggests improvements in operating efficiency may be possible even for these larger systems. Perhaps the more interesting prospect lies in simultaneous application of emulsion technology and other combustion control methods to alleviate both fuel and thermally produced NO ~, as well as particulates, without increase in excess air requirements or degradation in thermal operating efficiency. While some discouraging data already exist on this question, 62 it is possible failure to optimize emulsion structure may have been at fault. Although some practical applications data exist on the combustion of water-in-distillate fuel emulsions, 77 controlling emulsion structure of water-in-distillate emulsions is generally much more difficult than for residual emulsions. Because of the very low viscosity of distillate fuels, emulsion coalescence occurs more rapidly and distillates do not contain sufficient natural surfactants to stabilize the required internal phase droplet size distribution. Thus, there is an economic penalty required by the addition of artificial emtllsifiers. While micro-gas dispersion techniques 1~ appear to stabilize a dispersed gas/water/oil structure with very small amounts of emulsifier, it is difficult to envision the structural decay which must occur during combustion and the relation of micro-gas dispersion structure to the micro-explosion phenomenon.

Acknowledgments The author wishes to acknowledge the assistance of Messrs. J. Sivo, D. Peoples, M. Morgan and G. Rambach in obtaining the experimental suspended droplet combustion data, and the many helpful discussions with Prof. I. Glassman, Dr. R. J. Santoro and Mr. R. Cohen. Research reported in this paper was sponsored by the National Science Foundation,

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POWER SYSTEMS

(Research Applied to National Need), Division of Advanced Energy Research and Technology, under Grant No. G144215.

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WATER ADDITION TO PRACTICAL COMBUSTION SYSTEMS 36. VICHNIEVSKY,R., MURAT, M., PAROIS, A. AND DUJEV, M.: Employment of Fuel-Water Emulsions in Compression Ignition Engines. Paper presented at the CIMAC Conference, Barcelona, Spain, April 28-May 3, 1975. 37. GREEVES,G., KHAN,I. M. ANDONION, G.: Effects of Water Introduction on Diesel Engine Combustion and Emissions. Paper No. 25, this Symposium. 38. NICHOLLS,J. E., EL-MEssIRI, I. A. ANDNEWHALL, H. K.: SAE Trans. 78, 167 (1969). 39. WEATHERFORD,JR., W. D., AND QUILLIAN,JR., B. D.: Total Cooling of Piston Engines by Direct Water Injection. SAC Paper 700886 presented at the SAC National Combined Fuels and Lubricants and Transportation Meeting, Philadelphia, Pa., November 1970. 40. MODAK,A. ANDCARETTO, L. S.: Engine Cooling by Direct Injection of Cooling Water. SAE Paper 700887 presented at the SAE National Combined Fuels Lubricants and Transportation Meeting, Philadelphia, Pa., November 1970. 41. ROBISON,S. A.: Humidity Effects on Engine Nitric Oxide Emissions at Steady State Conditions. SAC Paper 700467 presented at SAC Meeting, May 1970. 42. ZEILINGER,K.: Inst. Mech. Eng. Conference on Air Pollution Control in Transport Engines, p. 7, 1971. 43. QUADER,A. A.: SAE Trans. 80, 20, (1971). 44. LESTZ,S. S., MEYER,W. E. AND COLONY, C. M.: Emissions from a Direct-Cylinder Water-Injected Spark-Ignition Engine. SAC Paper 720113 presented at the SAC Automotive Engineering Congress, Detroit, Michigan, January 1972. 45. MANOS,M. J., BOZEr, J. W. AND HULS, T. A.: Effect of Laboratory Ambient Conditions on Exhaust Emissions. SAE Paper 720124 presented at the SAE Automotive Engineering Congress, Detroit, Michigan, January 1972. 46. LESTZ, S. S. AND MEYER, W. E.: Proceedings 14th FISTA Congress, p. 2/34, 1972. 47. KLApATCH,R. D. AND KOBLISH,T. R.: Nitrogen Oxide Control With Water Injection in Gas Turbines. ASME Paper 71 WA/GT-9 presented at the ASME Winter Annual Meeting, Washington, D.C. November 28-December 2, 1971. 48. DIBELIUS,N. R., HILT, M. B., ANDJOHNSON,B. H.: Reduction of Nitrogen Oxides from Gas Turbines by Steam Injection. ASME Paper No. 71-GT-58 presented at the ASME Gas Turbine Conference and Products Show, Houston, Texas, March 1971. 49. CORNELIUS,W., AND AGNEW, W. G.: Emissions from Continuous Combustion, pp. 279, 345, Plenum Press, 1972.

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50. HILT, M. B. ANDJOHNSON,R. H.: Nitric Oxide Abatement in Heavy Duty Gas Turbine Combustors by Means of Aerodynamics and Water Injection. ASME Paper 72-GT-53 presented at the ASME Gas Turbine and Fluids Engineering Conference and Products Show, San Francisco, Ca., March 1972. 51. SINGH, P. P., YOUNG,W. E. AND AMBROSE,M. J., Formation and Control cf Oxides of Nitrogen Emissions from Gas Turbine Combustion Systems. ASME Paper No. 72-GT-22 presented at the ASME Gas Turbine and Fluids Engineering and Products Show, San Francisco, Ca., March 1972. 52. DAY,W. H. ANDKYDD,P. H.: Maximum Steam Injection in Gas Turbines. ASME Paper 72JPG-GT-1 presented at the ASME Joint Power Generation Conference, Boston, Mass., September 1972. 53. AMBROSE,M. J. AND OBIDINSKI, E. S.: Recent Field Tests for Control of Exhaust Emissions from a 35-MW Gas Turbine. ASME Paper 72-JPG-GT-2 presented at the ASME Joint Power Generation Conference, Boston, Mass., September 1972. 54. KocH, H.: Proceedings of the Tenth International Congress on Combustion Engineering, p. 1247, ASME, 1973. 55. DAVIS, L. B., MURAD,R. J. AND WILHELM, C. F.: Emission and Control of NO in Industrial Gas Turbine Combustors: Experimental Results. Paper presented at the 66th Annual AICHE Meeting, Philadelphia, Pa., 1973. 56. MARCHIONNA,N. R.: Effect of Inlet-Air Humidity on the Formations of Oxides of Nitrogen in a Gas Turbine Combustor. NASA TMX68209, March 1973. 57. INGEBO,R. D. AND NORGREN,C. T.: Effect of Primary-Zone Water Injection on Pollutants from a Combustor Burning Liquid ASTM-A-1 and Vaporized Propane Fuels. NASA TN D7293, May 1973. 58. MABCHIONNA,N. R., DIEHL, L. A., AND TROUT, A. M.: The Effect of Water Injection on Nitric Oxide Emissions of a Gas Turbine Combustor Burning ASTM Jet-A Fuel. NASA TM X-2958, December 1973. 59. SHAW, H.: Trans. ASME, J. Eng. Power 96, 240 (1974). 60. SHAW,H.: The Effect of Water on Nitric Oxide Production in Gas Turbine Combustors. ASME Paper 75-GT-70 presented at the Gas Turbine Conference and Products Show, Houston, Texas, March 1975. 61. WASSER,J. H.: Proceedings of the Stationary Source Combustion Symposium, Vol. 1, p. 227, EPA Document 600/2-76-152a, June 1976. 62. TURNER,D. W. AND SIEGMUND,C. W.: Control of NO x From Oil Combustion: Water in Oil

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Institute, Palo Alto, Ca., October 1975. 79. TOUSSAXNT,M. AND HEAP, M. P.: Formation of Oxides of Nitrogen and of Particles in the Flame of an Emulsion of Heavy Fuel and Water. Paper presented to the Chemistry Commission of the International Flame Research Foundation, Ijmuiden, October 1974. (In French). Translation: NERC Library, TR75227, November 1975. 80. BONNE,U.: External Combustion of Fuel Oil Emulsions. Paper presented to the American Society of Heating, Refrigerating and Air Conditioning Engineers Session on Efficient Use of Fuels II, Semi-Annual Meeting, Dallas, Texas, February 1976. 81. HALL,R. E.: Trans. ASME, J. Eng. Power 98, 425 (1976). 82. SPADACCINI,L. J. ANDPELMES, R.: ACS Preprints, Vol. 21, No. 4, p. 741, Division of Petroleum Chemistry Inc., 1976. 83. JACQUES, M. T., JORDAN, J. B., AND WILLIAMS, A.: Second European Combustion Symposium, p. 397, The Combustion Institute, 1975. 84. JACQUES,M. T., JORDAN,J. B., WILLIAMS,A., AND HADLEY-COATES,L.: The Combustion of Waterin-Oil Emulsions and the Influence of Asphaltene Content. Paper No. 24, this Symposium. 85. SJOGREN, A.: Burning of Water-in-Oil Emulsions. Paper No. 23, this Symposium. 86. IVANOV,V. M., KANTROVICH, B. V., RAPIOVETS, L. S., ANDKHOTUNTSEV,L. L.: J. Acad. Sci. USSR, 56-59 (1957). 87. IVANOV,V. M. AND NEFEDOV,P. I.: Trudy Instituta Goryachikh Iskopayemykh 19 (1962). (Russian) See also NASA TTF-258, January, 1965. 88. DRYER, F. L.: Fundamental Concepts on the Use of Emulsions as Fuels. Paper presented at the Joint Central and Western Section Meetings of the Combustion Institute, San Antonio, Texas, April 1975; also Aerospace and Mechanical Sciences Report No. 1224, Princeton University, 1975~ 89. DRYER, F. L., RAMBACH,G. D., AND GLASSMAN, I.: Some Preliminary Observations on the Combustion of Heavy Fuels and Water-in-Fuel Emulsions. Paper presented at the Central Section Meeting of the Combustion Institute, Columbus, Ohio, April 1976, also Aerospace and Mechanical Sciences Report 1271, Princeton University, 1976. 90. KING,N. K.: Combustion Sci. Technol. 6, 247 (1973). 91. BITTEN,J. F.: J. Colloid Interface Sci. 33, 265 (1970). 92. FISHER,J. C.: J. Appl. Physics 19, 1062 (1948). 93. EBERHART,J. G., HATHAWAY,E. J., ANDBLANDER, M.: J. Colloid and Interface Sci. 44, 389 (1973). 94. AVEDESIAN,T.: Nucleation in Emulsified Fuels.


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Meeting of the Combustion Institute, Drexel University, November 1976. SAROFIM,A. F., AND FI~CA."~, R. C.: Progress in Energy and Combustion Science 2, 1 (1976). KINNEY, R. R., Gaulin Corporation, Garden Street, Everett, Mass. personal communication, 1976. CATO,G. A. AnD HALL,R. E.: Field Measurement of Pollutant Emissions from Industrial Boilers. ASME Paper No. 75-WA/APC-7 presented at the ASME Winter Annual Meeting, Houston, Texas, November 30-December 4, 1975. ESSENHIGH,R. H., KOVAL,A., SHALER,A., SLUPEK, S., KOKKINOS,A. AND MISKOVSKY,N. M.: Combustion of Oil/Water and Coal Emulsions. Paper presented at the Central Section Meeting of the Combustion Institute, Columbus, Ohio, April 1976.

COMMENTS Clifford Moses, Southwest Research Institute, USA. Referring to the earlier comment on the changes in the viscosity of the fuel when it is emulsified, I would like to make mention of the viscosities in my experiments which were used by Dr. Dryer in his presentation. The base fuel was a JP-5 type fuel (kerosene) with a viscosity of about 1.5 centistokes, when emulsified with 10 percent water (by volume) and stabilized with 2 percent surfactant the viscosity increased to about 2.5 centistokes. More recent work has used emulsions of up to 33 percent water at six different operating conditions

in the Allison T-63 combustion representing engine idle through full power. Effects on exhaust chemistry, smoke, and combustion efficiency have been determined. Significant reduction in smoke and NOx can be obtained at the full power condition where their concentrations are the greatest; the effect on combustion efficiency and temperature rise is minimal. At lower power conditions, the reductions in smoke and NOx are much less and the penalty becomes significant. Full scale engine tests on a 379 engine are scheduled for the winter to determine the plume visibility from the exhaust stack of an engine test cell.