Lean premixed recirculating flow combustion for control of oxides of nitrogen

LEAN PREMIXED RECIRCULATING FLOW COMBUSTION FOR CONTROL OF OXIDES OF NITROGEN R.W. SCHEFER AND R.F. SAWYER

Department of Mechanical Engineering, University of California, Berkeley, California 94720 A study has been made of premixed fuel lean combustion in an opposed reacting jet combustor using propane as a fuel. The objectives were to demonstrate a system in which stable combustion could be maintained under very lean conditions and to investigate pollutant formation characteristics. At elevated inlet temperatures, stable combustion was achieved at equivalence ratios as low as 0.45. Composition was mapped over a range of equivalence ratios from 0.45 to 0.625 and at inlet temperatures from 300 K to 600 K. Measurements included NO, NO 2, C3Ha, CO 2, CO, H20, 02, N~ and temperature. Fuel lean combustion was found to be an effective method of achieving low NO~ emissions and high combustion effieiencies simultaneously. A lower limit on NO x reduction was found to exist due to the tradeoff between reduced NO~ and increased unburned hydrocarbon and CO emissions at low eombustor temperatures and residence times. Under conditions promoting lower flame temperature, NOe constituted up to 100% of the total NO x. The observations appear to be consistent with conversion of NO to NO 2 via the HO 2 radical and rapid quenching of NO 2 destruction reactions. However uncertainties involving the occurrence of sampling probe reactions prevent any definite conclusion to be drawn with regard to NO 2 formation. The governing differential equations were solved for the ORJ flowfield using a numerical finite difference approach. The majority of efforts were directed toward the development of a simplified kinetic model for propane combustion. This approach involved the two step global mechanism 7 Call s + 2 0 2 - - ~ 3CO + 4H20 1 CO + - - 0 2 - - * CO2 2 Agreement between predicted and experimental results was fair. Discrepancies were found to be the result of the simplified kinetic and fluid mechanic approximations used.

Introduction

altitude emissions, particularly the oxides of nitrogen (NO x), on the earth's zone layer. 1 The establishment of suitable and effective methods for the control of NO~ from gas turbines remains unresolved. 2 Of the various advanced concepts b ei n g d ev el o p ed to reduce NO~ emissions perhaps those h o l d i n g the greatest promise involve lean premixed c o m b u s t i o n with fuel prevaporization. 3 Premixing and prevaporization, provide a uniform mixture of fuel and air to the

Aircraft gas turbine engines currently represent only a minor contribution to air pollution. Increases in airport air traffic, the p r o b le m of stratospheric ozone depletion, and an increasing application of gas turbines to stationary power generation create a growing concern for the limitation of pollutant emissions from gas turbines. Perhaps the area of greatest current importance is the possible impact of high 119

120

POWER SYSTEMS

combustion zone. This eliminates locally fuel rich mixtures and low temperature regions associated with soot formation and high levels of unburned hydrocarbons and carbon monoxide. Also eliminated are locally stoichiometric mixtures and the associated high temperatures and nitric oxide production rates. The objectives of the present investigation were to demonstrate a system in which combustion can be maintained under very lean conditions and to investigate the pollutant formation and stability characteristics of this system. An opposed reacting jet (ORJ) model laboratory combustor was chosen for this study, Fig. 1. The ORJ has several advantages which make it adaptable to an investigation of the elementary processes governing pollutant formation. These are primarily due to the simplified geometry and flowfield it provides. In a more complex combustor configuration, such as encountered in many full scale combustors, separation of fluid mechanical and chemical kinetic aspects of the combustion process is extermely difficult. A mode/ combustor such as the ORJ facilitates this process by providing greater control over those parameters which most affect pollutant formation. These include equivalence ratio, inlet temper-

FLAME FRONT

STAGNATI POINT

ature, and recirculation zone size. The recirculating flowfield existing in the ORJ is also characteristic of most conventional gas turbine combustors. Thus a fundamental study of pollutant formation in a simplified recirculation zone such as that provided by the ORJ has direct application to most gas turbine type combustors. The results of an analytical modeling technique based on the computational scheme of Gosman et al. 4 are also presented. Comparisons of analytical predictions with experimental results are made in an effort to determine the usefulness of such modeling techniques in predicting eombustor performance.

Experimental Apparatus and Procedure Figure 1 shows the approximate flowfield in an opposed reacting jet. The ORJ used in the present study consists of a main premixed stream of fuel and air and a smaller stream of premixed fuel and air which was injected at a high velocity along the centerline and in a direction opposite to the main stream. The result was the formation of a stagnation

RECIRCULATION ZONE

JET EXIT

Fro. 1, Opposed reacting jet flowfield.

///~

1 -4-

PROPANE

CONTROL OF OXIDES OF NITROGEN region and zone of strongly recirculating combustion gases. A stable flamefront formed at the upstream end of the recirculation zone which propagated o u t w a r d into the main flow. The combnstor test section consisted of a 58 mm I.D. x 356 m m in length high temperature Vycor liner. The test section was w r a p p e d in 25.4 mm thick asbestos lagging to make the combustor more nearly adiabatic. The stainless steel jet injector extended 63.5 mm into the test section. It had a 6.35 mm O.D. and a 3.97 mm I.D. At the jet exit the inner diameter was r e d u c e d to 1.59 mm through a converging nozzle. T h e injector was of standard triple wall construction to allow water cooling and operation of the ORJ at high combustion temperatures. Air was s u p p l i e d b y the house air system. Main stream air was metered upstream of the combustor using a standard 1.52 cm diameter orifice. The pressure was reduced to approximately atmospheric prior to the combustor inlet. H u m i d i t y of the reactants was measured at each data point and was found to remain relatively constant at 0.00151 gm H 2 0 / g m air. Propane was chosen as a fuel representative of the higher h y d r o c a r b o n constituents found in typical aircraft fuels such as JP-4. The propane was metered t h r o u g h a calibrated rotometer and then mixed with the main stream air in a venturi section. T h e flow then passed d o w n a 1.0 meter long straightening section to assure a fully d e v e l o p e d velocity profile and uniform composition. The inlet velocity profile indicated t u r b u l e n t flow existed under the entire range of experimental conditions. Six 6-Kw resistenee heaters provided control over reactant in|et temperature up to 700 K. Jet air and p r o p a n e were metered separately through rotometers and mixed in a tee before p a s s i n g into the jet injector. C o m p o s i t i o n measurements were taken throughout the flame zone using an aerodynamic quench quartz m i c r o p r o b e designed to m i n i m i z e disturbances to the combustion process in the recirculation zone. The downstream portion of the p r o b e was enclosed in a stainless steel cooling jacket to provide additional cooling of the s a m p l e gases. Traverses were made with the p r o b e inserted through the downstream end of the combustor test section. NO and N O s were m e a s u r e d with a laboratory built c h e m i l u m i n e s c e n t NO analyzer m o d i f i e d for the m e a s u r e m e n t of total NO~. C O was measured using a Beckman model 315A nondispersive infrared analyzer. 02, N2, CO2, H 2 0 , and C a l l 8 were measured using a Bendix model 17-210 time of flight mass

121

spectrometer. Temperatures were measured using a 0.076 mm diameter P t / P t - 1 3 % R h fine wire thermocouple. A yttrium chlorideb e r y l l i u m oxide coating developed b y Kent 5 was used to reduce catalytic reactions on the therrnocouple surface.

Experimental Results

Stability Limits Fuel lean combustion promises to be an effective means of l i m i t i n g pollutant formation. Very lean c o m b u s t i o n however adversely affects the stability of the combustion process. Thus initial efforts were directed toward determining the lean stability limits. The primary variables w h i c h affect the stability limits of the ORJ are jet stream equivalence ratio, jet stream flow rate, and main stream inlet temperature. The m a i n stream equivalence ratio at b l o w o u t can be reduced b y increasing the jet stream equivalence ratio, however the net result is little or no reduction in the equivalence ratio of the reeirculation region. Thus extending the stability limits to lower equivalence ratios through a variation in the jet stream c o m p o s i t i o n did not appear useful to gas t u r b i n e applications. Stable operation with both a lean primary and jet stream can be a c c o m p l i s h e d b y increasing the jet exit velocity. An u p p e r limit of 168.4 m/s was i m p o s e d on the jet velocity in the present investigation due to a desire to avoid c o m p r e s s i b i l i t y effects encountered at M a t h numbers greater than 0.5. Compressibility effects threaten the convergencd of the numerical solution and change the nature of the governing equations from elliptic to hyperbolic. In the present investigation a jet exit velocity of 95.9 m/s was selected. This m i n i m i z e d the n u m b e r of grid points required for the numerical solution while at the same time p r o v i d i n g a flame zone large enough to make possible good spatial resolution in the c o m p o s i t i o n and temperature measurements. The most effective w a y of increasing the lean stability limits of the ORJ was f o u n d to be an increase in p r i m a r y stream inlet temperature, Fig. 2. For a jet exit velocity of 95.9 m / s and a p r i m a r y stream velocity of 7.74 m / s , an inlet temperature of 600 K allowed stable operation to be m a i n t a i n e d at an overall equivalence ratio as low as 0.45. An increase in jet velocity w o u l d of course reduce this stability limit further.

122

POWER SYSTEMS I

I

I

TABLE I Experimental operating conditions

I

17

16 =

Case

Equivalence ratio

Main stream inlet temperature (K)

Main stream velocity (m/see)

Adiabatic flame temperature (K)

1 2 3

0.625 0.625 0.450

300 600 600

7.74 7.74 7.74

1746 1970 1649

15 Tp = 600 ,l~j = 0.4.5

K

14

E

Tp = 300 K ~',I = 0.625

13

~~ w>

0o w 9 z

=E 8

/ J

0.4

r

I

0.5

I

I

0.6

MAIN STREAM EQUIVALENCE RATIO, ,~p

FJ~, 2. Effect of main stream inlet temperature on stability limits of opposed reacting jet combustor. VI = 95,9 m/s; TI = 295 K.

Composition and Temperature Measurements

The experimental conditions considered for composition and temperature measurements are shown in Table I. I n all cases, the jet stream velocity and temperature were m a i n t a i n e d constant at 95.9 m / s and 295 K respectively. Selected composition and temperature profiles are presented. Axial concentration a n d temperature profiles at a radial distance of 11.6 mm from the combustor centerline for experimental case 2 are shown in Fig. 3. The rapid rise in temperature accompanied by a decrease in C a l l 8 and 0 2 indicate the location of the turbulent flamefront at approximately 140 m m upstream from the eombustor exit. The maxim u m temperature measured of 1800 K (uncorrected for radiation) at this radial location compares with an adiabatic flame temperature of 1970 K. A m a x i m u m in CO at the inital flamefront results from a balance between initial CO formation due to C3H s oxidation, followed by the s u b s e q u e n t oxidation of CO to CO 2 via the primary CO oxidation reaction, CO + OH ~ CO 2 + H. Measured mole fractions of H 2 0 , COz, and CO at the combus-

tor exit correspond closely to the e q u i l i b r i u m values of 0.099, 0.074, and 0.0003, respectively. Maximum N O concentrations are obtained at the combustor exit due to the dependence of NO formation on residence time. Radial concentration and temperature profiles at an axial distance of 88.9 mm upstream of the combustor exit are shown in Fig. 4. The low temperature and combustion product concentrations near the outer Vycor wall indicate the flamefront has not yet reached the wall. Moving radially inward a b u i l d u p of H 2 0 and CO s occurs with a corresponding decrease in C a l l s and 0 2. Concentrations of 0 2 and H 2 0 are relatively uniform over the entire recirculation region. The effect of jet injected reactants is apparent as a decrease in CO 2 and temperature and a slight increase in C3H 8 and CO as the combustor centerline is approached. Further upstream this effect disappears and the composition profiles become uniform to the combustor centerline. NO~ reaches a m a x i m u m in the recirculation zone due to the relatively high temperatures and long residence times characteristic of this region. A comparison was made of emission levels obtained in the present investigation with those obtained in typical aircraft combustors. Emission levels are frequently presented in terms of an "emission index" (grams of pollutant per kilogram of fuel burned) to normalize emissions on the basis of fuel flow. The emission index was calculated for cases 1, 2, and 3 of the present investigation based on average UHC, CO, and NO x concentrations at the test section exit. In cases 1 and 3 the flame front did not extend across the entire eombustor. Therefore to make any comparisons more realistic, a local emission index was calculated based on concentration measurements in that portion of the exiting gases which had undergone combustion. The results are shown in

CONTROL OF OXIDES OF NITROGEN 0.50

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Flc. 3. Axial concentration and temperature distributions at a radial location of 11.6 mm from the combustor centerline. Experimental case 3. Tv = 600 K; ~ = 0.625; Vp = 7.74 m/s; Vj = 95.9 m/s. Table II along with calculated combustion efficiencies, "qc' Also s h o w n in the table are emission levels for two typical jet aircraft engines operating at cruise conditions, the J T 9 D for subsonic aircraft and the O ly m p u s

593 turbojet engine for supersonic aircraft, z Th e NO 2 emission index values in parentheses represent values corrected to typical inlet conditions at cruise for the O l y m p u s 593 engine (Tin = 824 K, PIN = 6.5 atm), A

TABLE II Pollutant emission levels

Case 1 Case 2 Case 3 JT9D, cruise Olympus 593, cruise

Elunc (g C H J k g fuel)

Elco (g C O / k g fuel)

EINo 2 (g N O J k g fuel)

"qc

11.4 6.8 1.2 0.1-0.3 <1

76.7 5.2 85.1 0.2-0.8 1-5

0.11 (1.75) 0.58 (3.22) 0.07 (0.34) 16-23 18-19

0.87 0.99 0.98 1.0 1.0

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POWER SYSTEMS 0.30

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RADIAL D I S T A N C E

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FIc. 4. Radial concentration and temperature distributions at a radial location of 88.9 mm upstream of the combustor exit. Experimental case 3. Tp = 600 K; r = 0.625; Vp = 7.74 m/s; Vj = 95.9 m/s. correlation equation d e v e l o p e d b y Niedzwiecki and Jones 6 for a p p l i c a t i o n to swirl can and other lean combustors was used to correct the experimental data of the present investigation. In case 1 the emission index for oxides of nitrogen (presented as NO2) of 0.11 was significantly less than f o u n d in the jet engine combustors. H o w e v e r unacceptably high levels of U H C and C O were obtained due to the low resulting flame temperatures a n d insufficient residence time. Cases 2 and 3 resulted in c o m b u s t i o n efficiencies approaching those attained in the jet engine combustors, with NO 2 emission index values significantly reduced. The d e p e n d e n c e of hydrocarbon oxidation rate on 0 2 concentration is reflected in the improved c o m b u s t i o n efficiency of case

3. An emission index less than 1 kg N O 2 / k g fuel was also attained b y Anderson 7 in a lean premixed p r o p a n e air experimental b u r n e r operating at conditions comparable to turbojet combustors and b y Roffe and Ferri s in a laboratory burner u s i n g premixed-prevaporized JP-5 fuel. The level of oxides of nitrogen formed in lean, premixed c o m b u s t i o n differ substantially from conventional gas t u r b i n e practice. Recent laboratory studies of lean, premixed combustion, i n c l u d i n g the present work, are contrasted to aircraft gas t u r b i n e emissions in Fig. 5. These data are clear evidence of the fundamental difference b e t w e e n current conventional turbulent diffusion flame gas t u r b i n e combustion and fuel lean premixed combustion.

CONTROL OF OXIDES OF NITROGEN 4O

125

ijo

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AIRCRAFT GAS

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tl

/~-

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400

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t.

500 600 700 BOO COMBUSTORINLET TEMPERATURE(*K)

9

900

FIG. 5. Correlation of NO x emissions with combustor inlet temperature, Open symbols denote conventional aircraft gas turbine data3 Solid symbols denote fuel lean premixed model eombustor data. O - Anderson7 (P = 5.5 atm); I' - Roffe and Ferris (P = 4 atm); 9 - present investigation (corrected to P = 6.5 atm).

As noted above, U H C and CO emission levels were slightly higher than those found in aricraft engines operating at cruise conditions. Under conditions encountered in typical gas t u r b i n e eombustors NO~ increases with combustor residence time. Limiting high temperature residence time reduces NO emissions. However, insufficient time may be available to completely oxidize unburned fuel and CO. More favorable U H C and CO emission levels could be obtained in the present investigation, with some increase in NO~ emissions, by p r o v i d i n g slightly longer residence times in the exit portion of the combustor test section. NO and NO 2 Formation Recent controversy has arisen surrounding the formation of NO 2 in in combustion systems. Theoretical studies involving both

chemical e q u i l i b r i u m considerations and chemical kinetic calculations have generally predicted that NO 2 should constitute a negligible fraction of the total oxides of nitrogen emitted from gas t u r b i n e type combustors. 9 However, recent experimental emission studies in both gas turbine exhaust 1~ and model laboratory combustors 11,12 involving gas samp i i n g have y i e l d e d relatively large quantities of NO 2. Experimentally measured N O and NO distributions for the ORJ used in the present study are presented in Figs. 6 and 7 for experimental eases 1 and 2. Also shown are the corres p o n d i n g temperature distributions. NO 2 is taken as the difference b e t w e e n NO~ and NO. It is apparent that in all eases N O s was the p r e d o m i n a n t oxide of nitrogen, constituting from 50 to 100 percent of the total NO~. No NO was measured in ease 3.

POWER SYSTEMS

126

a) NOx c o n c e n t r a t i o n , ppm

"

b) NO concentration, ppm

I JE, ,oBE

c) Temperature, ~

~ 0 1400

0~

"~

~

1I030000

,1300 JET TUBE

FIG. 6. Experimental distributions of NO~, NO, and temperature for case 1. Tp = 300 K; + = 0.625; Vp = 7.74 m/s.

Some general observations can be d r a w n from an examination of these results. Measurable quantities of NO were only found to exist in higher temperature regions. In case 1 N O was measured only in the recirculation zone where the temperature was in a range greater than 1400 K to 1500 K (uncorrected for radiation). In case 2 relatively large quantities of NO were f o u n d in both the recircutation zone and over m u c h of the downstream region of the combustor. Once again however measurable amounts of NO existed only in those regions where the temperature was greater than

approximately 1400 K to 1500 K. I n case 6 the maximum measured temperature was 1450 K and no NO was measured. In a recent experimental investigation by Merryman and Levy la on a flat flame premixed methane-air burner, significant quantities of NO 2 were f o u n d early in the flame zone. Merryman and Levy proposed that rapid conversion of initially formed NO oecured via NO + H O 2 ~--.NO 2 + OH

(1)

Under fuel lean conditions approximately 60

127

CONTROL OF OXIDES OF NITROGEN

a) NOx concentration, ppm

b) NO concentration, ppm

c) Temperature, ~

I

_

.......

I

FIe. 7. Experimental distributions of NO x, NO, and temperature for ease 3. Tp = 600 K; ~b = 0.6"25; V~ = 7.74 m/s.

percent of the NO~ was reconverted to NO via the proposed reaction NO 2 + O--~ NO + 0 2

(2)

I n the above m e c h a n i s m NO 2 exists primarily as a transient species due to the presence of O atoms. It is probably, however, that in t u r b u l e n t systems such as encountered in this investigation rapid mixing between hot and cold regions occurs. Cernansky12 has shown u n d e r fuel lean conditions that if the cooling of hot combustion gases is rapid enough radi-

cals such as H, O, and OH which favor NO 2 destruction fall to low levels and significant conversion of NO to NO 2 can occur via reaction (1). Thus m a x i m u m [NO2] / [NO] ratios would be expected in cooler regions of the combustion zone and near the flame front where rapid mixing between hot and cold gas mixtures occurs. This was precisely where maximum levels of NO 2 were f o u n d in the present investigation. While it appears that the proposed mechanism involving conversion of NO to NO~ by H O s radicals is consistent with the experi-

128

POWER SYSTEMS

mental results, unfortunately the same conditions which can explain the formation of NO z in combustion systems also exist in typical sampling probes due to the rapid decrease in gas temperature resulting from aerodynamic expansion and water cooling, a~Allen ~4has also pointed out the possible importance of probe wall reactions. Thus it appears that additional work is needed, involving perhaps more recently developed optical in situ measurements of NO and NO215 before it can be determined whether NO 2 is formed in the combustion system itself, in the sampling system, or perhaps in both.

was assumed to be adiabatic and impermeable to mass fluxes. Based on detailed radial temperature measurements in the vicinity of the jet, a jet wall temperature of 700 K was assumed. Uncertainties exist in both the mechanism and the rates for the high temperature oxidation of higher hydrocarbons. In addition, consideration of the large number of intermediate species associated with hydrocarbon oxidation would lead to excessive computer time. Thus a simplified kinetic model for propane oxidation was developed. This approach involved the two step global mechanism 7 C3Hs + 2Oe--~ 3C0 + 4H20

Theoretical

(5)

1

Governing Differential Equations

The governing partial differential equations for a turbulent steady flow chemically reacting system are the conservation of mass, the conservation of momentum, the conservation of energy, and a conservation equation for each species. An analytical model for predicting point by point property and concentration distributions based on a numerical finite difference procedure developed by Gosman et al. 4 was adopted in the present investigation. This procedure involves the use of time average quantities in which turbulent transport is accounted for through the use of effective transport properties. Velocity and pressure are eliminated as dependent variables through the introduction of vorticity and stream function. A simplified viscosity model developed by Pun and Spalding a6 was used in the present calculations. This is given by P~e~ = KD2/3 L1/3 P 2/3 [(mY2),,

+ (mv2)~]~/s

(4)

where K is a constant, D and L refer to characteristic chamber dimensions, and m v 2 is the incoming kinetic energy. The subscripts m and j refer to the main or primary stream and the jet stream, respectively. The effective Prandtl and Schmidt numbers were assumed equal to unity. A constant specific heat was assumed and the system was assumed to obey the ideal gas equation of state. The above set of equations are elliptic in nature and therefore require that boundary conditions be speeified for each dependent variable at all points surrounding the flowfield. Inlet conditions corresponded to those of experimental case 2. The outer Vycor wall

CO +--Oz~ 2

CO 2

(6)

Experimental results indicated a region of relatively uniform composition and temperature exists upstream of the jet exit plane once the initial flamefront has been traversed. Experimental propane disappearance rates were calculated based on average propane concentrations and estimated residence times in this region for the experinaental cases studied. The resulting propane disappearance rate was found to correlate quite well with the expression d[C3Hs] dt

- 4.97[C3Hs ] o.~

e x p ( - 9 1 9 5 / R T ) [[gm-m~ 3sec ]

(7)

A global rate expression developed by Howard et al. 17 was used for CO oxidation

d[co]

-

-

- 1.3 x 1014 [ C O ] [ O 2] 0.5 [ H 20] 0.5

dt e x p ( - 3 O O O O / R T ) [ g mc-am ~3 see

(8)

Nitric oxide formation was based on the Zeldovieh mechanism O + N 2 ~--. NO + N N+ 02--~NO+O

(9) (10)

Iverach et al. is has suggested partial equilibrium of the reactions

CONTROL OF OXIDES OF NITROGEN C O + OH ~ CO 2 + H H+O

(11)

2--~OH +O

(12)

The rate of NO formation can then be written as

a[NO]

= 2kglK12Kla

[N~] [co] [02]

function t~, stagnation e n t h a l p y h, c a n 8 mass fraction means , CO 2 mass fraction mco2, and N O mass fraction mNo. T h e solution was considered to have converged when the fractional change in the value of each d e p e n d e n t variable from one iteration to the next is less t h a n 0.005.

(13)

[ C O 2]

dt

Theoretical Results and Discussion

In the present investigation kgf was taken from the rate data of Baulch et al. 19 k9f= 7.6 x 1 0 1 a e x p ( - 3 8 0 0 0 / T ) The e q u i l i b r i u m constants K m and K13 were taken from J A N A F t h e r m o c h e m i e a l data. T h e simplified reaction scheme considers six species: C3H8, 0 2 , CO, H 2 0 , CO 2, and NO. The mass fractions of c a n 8, CO 2, and N O were selected as d e p e n d e n t variables (req u i r i n g three species conservation equations). Mass fractions of 0 2 , H 2 0 , and CO were then c o m p u t e d from O, H, and C atom conservation equations. T h u s the present investigation involved the solution of six equations for six d e p e n d e n t variables: the vorticity variable o~/r, stream-

Predicted and experimental flowfield distributions are presented in Figs. 8 through 10. C 3 H s mole fraction distributions are shown in Fig. 8. Qualitatively agreement is fair. Near the centerline experimental mole fractions of c a n s begin to decrease 15 m m farther upstream than predicted values. In the lower temperature regions near the outer Vyeor wall the predicted c a n 8 mole fractions begin decreasing farther upstream than observed experimentally. The d i s c r e p a n c y can partially be accounted for by the s i m p l i f i e d turbulence model employed. Due to the interaction between fluid mechanics and chemistry however it is difficult to isolate the relative effects of these simplifications. A comparison of the results obtained here with a more sophisticated turbulence model w o u l d be informative in

a) Experimental C3H8 mole fraction

2 9 I0 .2

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I

129

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79

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b) Predicted C~H8 mole fraction

Fic. 8. Experimental and predicted C3H s mole fraction distributions.

POWER SYSTEMS

130

a) Experimental CO mole fraction

///// IIIL

I

I

..... -

b) Predicted CO mole f r a c t i o n

Fro. 9. Experimental and predicted CO mole fraction distributions.

b r i n g i n g out the shortcomings of the p r e s e n t model. Perhaps a more direct cause of the p r e d i c t e d behavior in the lower temperature outer regions would be the use of the experimentally derived rate for C a H 8 oxidation (Eq. (7)). This rate was based on average concentration a n d temperature measurements taken over a limited range of conditions in the high temperature recirculation zone. F o r example, the temperature range used in the derivation extended from 1450 K to 1750 K. Extrapolation of this rate expression to lower temperature regions of the combustor could not be expected to yield accurate predictions throughout the c o m b u s tion zone. T h e trends shown in Fig. 8 indicate a predicted rate w h i c h is too fast at lower temperatures. This is directly attributable to the low activation energy associated with Eq.(7). Predicted C a l l s concentrations are also seen to decrease to m u c h lower values d o w n stream of the initial flamefront. Again this is a result of using the above p r o p a n e oxidation rate over a range of conditions for w h i c h it was not derived. The effect of the kinetic a p p r o x i m a t i o n introduced b y a s s u m i n g a one step oxidation

of CO to CO 2 can be seen through a comparison of p r e d i c t e d a n d experimental C O distributions in Fig. 9. As expected, in the upstream region of the flamefront the p r e d i c t e d initial b u i l d u p occurs farther upstream than is found experimentally. In both cases a maximum in the C O concentration occurs just downstream of the initial flamefront. This maximum c o r r e s p o n d s to a calculated temperature of 1200 K a n d represents the p o i n t at which the oxidation of CO to CO 2 becomes faster than the p r o d u c t i o n of CO from C 3 H 8 oxidation. Subsequently, as one moves d o w n s t r e a m toward the c o m b u s t o r exit, the predicted levels of CO decrease to values considerably b e l o w those found experimentally. As has b e e n pointed out b y H o w a r d e t al. 17 the C O rate expression used to obtain the results of the present section can be expected to give rates up to an order of m a g n i t u d e fast when a p p l i e d late in the p o s t f l a m e region where the back reaction C O 2 + H ~ O H + CO becomes important. This appears to be the case here. The predicted a n d experimental results for NO are shown in Fig. 10. Note that experimental results are presented as total oxides

CONTROL OF OXIDES OF NITROGEN a) Experimental NOx d i s t r i b u t i o n

131

(ppm)

b) Predicted NO distribution (Dnm)

Fic. 10. Experimental and predicted NO, distributions.

of nitrogen ( N O ) whereas predicted values are NO. These concentrations are comparable since all proposed schemes for the production of NO 2 in high temperature flames involve conversion of the initially formed NO. It is immediately apparent that predicted NO levels are significantly lower (up to 50% at the combustor exit). There are several explanations for this discrepancy, mostly related to simplifications used in the kinetic scheme. Malte and Pratt 2~ have proposed that N 2 0 may play an important role as an intermediate in NO production in low temperature fuel lean systems. Thus it appears possible the N 2 0 contribution to NO production could partially account for the low levels of predicted NO. The NO formation rate used in the numerical calculations was given by Eq. (13). An examination of predicted values for N~ and 0 2 indicates good agreement with those found experimentally. Since temperature is a function of the species concentrations, the predicted NO formation rate becomes primarily a function of [ C O ] / [ C O 2 ] . As shown in Fig. 9 the predicted CO concentrations are significantly lower than those found experimentally

in high temperature regions of the combustor where NO formation becomes important. The resulting [CO] / [CO2] ratio is anywhere from a factor of two to an order of magnitude less than the experimental value. As mentioned earlier, the low values of [CO] / [CO 2 ] in high temperature regions of the combustor is a result of neglecting the back reaction CO 2 + H CO + OH. Thus the inability of the 0~ne step CO oxidation scheme to correctly predict [ C O ] / [ C O 2 ] ratios in high temperature regions seems to be the primary cause for the inability of the model to predict NOx concentrations. Finally, all predicted results presented here are subject to the shortcomings inherent in using a time averaged approach. This time averaged approach has been useful 4 in predicting convective patterns in steady flow combustors. However, as more experimental data are obtained, the limitations of this approach are becoming apparent. By dealing with time average properties the entire process by which relatively cold large scale eddies containing unburned fuel-air mixtures break down into smaller scale eddies where molecular

132

POWER SYSTEMS

mixing between reactants and high temperature combustion products occurs is b e i n g substantially simplified.

Conclusions An investigation of pollutant formation under fuel lean premixed conditions in an opposed reacting jet model combustor was undertaken. At elevated inlet temperatures it was possible to m a i n t a i n stable c o m b u s t i o n at equivalence ratios as low as 0.45. Fuel lean premixed combustion appears to be an effective means of achieving low pollutant emission levels, NO, levels significantly lower than those measured in conventional gas turbine combustors were attained. U n b u r n e d hydrocarbon and CO levels were somewhat high due to the limited residence times available in the experimental configuration used. Significant levels of NO 2 were measured. Under conditions promoting lower flame temperatures NO 2 constituted up to 100% of the total NO x' These experimental observations are consistent with a mechanism i n v o l v i n g conversion of NO to N O 2 by the HO 2 radical. Rapid mixing of hot and cold regions of the combustor appears necessary to quench NO 2 destruction reactions involving O, H, and O H radicals. Uncertainties involving s a m p l i n g probe reactions however prevented any definite conclusions to be drawn c o n c e r n i n g the existence of NO v An analytical model or predicting p o i n t properties of the opposed reacting jet flowfield was developed. Agreement with experimental results was fair. Discrepancies were f o u n d to be the result of the simplified kinetic and fluid mechanic approximations.

2, Chapter 3, Department of Transportation Report No. DOT-TST-75-52, 1975. 4. GOSMAN,A. D., PUN, W. M., PUNCHAL, A. K., SPALDING, D, B., AND WOLFSHTEIN, M.: Heat and Mass Transfer in Recirculating Flows, Academic Press, London, 1969. 5. KENT,J. A.: Combust. Flame, 14, 279 (1970). 6. NIEDZWIECKI, R, W. AND JONES, R.: Parametric

7.

8.

9. I0.

11.

12.

13.

14. 15.

Acknowledgments This work was supported by the National Aeronautics and Space Administration under Grant no. NSG-3028. The authors would like to thank Dr. C. T. Bowman and Dr. N. J. Brown for their helpful discussions.

16.

17. REFERENCES 1. JOHNSTON, H. S.: Science, 173, 517 (1971). 2. GROBMAN,J. AND INGEBO,R. D.: Propulsion Effluents in the Stratosphere, CIAP Monograph 2, Chapter 5, Department of Transportation Report No. DOT-TST-75-52, 1975. 3. SAWYER,R. F. AND BLAZOWSKI,W. S.: Propulsion Effluents in the Stratosphere, CIAP Monograph

18.

19.

Test Results of a Swirl Can Combustor, NASA TM-X-68247, 1973. ANDERSON,D. N.: Effects of Equivalence Ratio and Dwell Time on Exhaust Emissions from an Experimental Premixing Prevaporizing Burner, NASA TM-X-71592, 1975. ROFFEE, G. AND FERRI, A.: Prevaporization and Premixing to Obtain Low Oxides of Nitrogen in Gas Turbine Combustors, NASA CR-2495, 1975. MARTENEY,P. J.: Combust. Sci. Technol., 1, 461 (1970). ANON.: Exhaust Emissions Test, AiResearch Aircraft Propulsion and Auxiliary Power Gas Turbine Engines, Report No. GT-8747-R, 10 September 1971. SCHEFER,R. W., MATTHEWS,R. D. , CERNANSKY, N. P., ANOSAWYER,R. F.: Measurement of NO and NO 2 in Combustion Systems, Paper No. WSCI-73-31, Western States Section/The Combustion Institute, E1 Segundo, California, 1973. CERNANSKY,N. P.: Formation of NO and NO z in a Turbulent Propane/Air Diffusion Flame, Ph.D. Dissertation, University of California, Berkeley, California, 1974. MERRYMAN,E. L. ANDLEVY,A.: Fifteenth Symposium (International) on Combustion, p. 1073, The Combustion Institute, Pittsburgh, 1973. ALLEN,J. D.: Combust. Flame 24, 133 (1975). AGRAWAL, Y., HADEISHI, T., AND ROBBEN, F.: Meausrement of NO 2 Concentration in Combustion using Fluorescence Excited by an Argon-Ion Laser, AIAA Paper No. 76-136, AIAA 14th Aerospace Sciences Meeting, Washington, D.C., January, 1976. PUN, W. M. AND SPALDING, D. B.: A Procedure for Predicting the Velocity and Temperature Distributions in Confined, Steady, Turbulent, Gaseous Diffusion Flames, Imperial College, Mechanical Engineering Department S F / T N / l l (1967). HOWARD, J. B., WILLIAMS, G. C., AND FINE, D. H.: Fourteenth Symposium (International) on Combustion, p.975, The Combustion Institute, Pittsburgh, 1973. IVERACH, D., BASDEN, K. S. AND Kmov, N.Y.: Fourteenth Symposium (International) on Combustion, p.767, The Combustion Institute, Pittsburgh, 1973. BAULCH,D. L., DBYSDALE,D. D. AND LLOYD, A. C.: Critical Evaluation of Rate Data for Homo-

CONTROL OF OXIDES O F NITROGEN geneous, Gas Phase Reactions of Interest in High Temperature Systems, Nos. i and 2, Department of Phys. Chem., The University of Leeds, England, 1968. '20. MAI.TE, P. C. AND PRA'I-r, D, C.: Oxides of Nitro-

133

gen Formation for Fuel Lean, Jet Stirred Carbon Monoxide Combustiola, Paper No. WSCI 73-37, Western State Section/Combustion Institute, El Segundo, California, 1973.

COMMENTS E. A. DeZubatr

Westinghouse Research, USA.

What is the basic advantage of counter flow stabilization over conventional bluff body stabilization since additional fuel and air flow systems are required? Do you have any opinion on any effects of using different fuel/air ratios in the two streams? For instance, less than lean lirnit in the main stream and combustible fuel/air ratio in the stabilizing stream.

Authors" Bepl~t. The primary advantage of the opposed jet combustor is the wide range of operating conditions possible with the combustor. Only one unique blowout nmp of approach velocity as a function of mainstream equivalence ratio exists for a given bluff body stabilizer, ttowever with the opposed jet a series of such blowout maps can be generated by varying the jet stream composition. Although the jet mass flow rate is only one percent of the total rnass flow rate, it comprises a large percentage of the total flow entering the recirculation zone, and thus has a great influence on average properties in this zone. It is therefore possible to vary somewhat independently the relative equivalence ratios in the recirculation zone and the main stream, and to investigate pollutant formation characteristics in these combustor regions.

G. S. Samuelsen, University of California, USA. The opposed jet eombustor is an attractive tool for the study of pollutant formation in recirculating flows. Perhaps the most attractive features of the opposed jet are the separation of the reeiretdation zone from solid t)ourldaries, and the sensitivity of the eombustor performance to the jet. In this regard, did the authors explore the effects of enriching tile jet for the lean approach conditions, Flame stability may t)e enhanced while maintaining or further improving the pollutant emission levels desired.

Authors" Repl!r The effects of enriching the jet for lean approach conditions were not explored in the present investigation, ttowever an earlier study by Ftlhs I indicates a unique curve may exist of

blowout velocity as a function of equivalence ratio in the recirculation zone. Thus it appears that stable operation in the opposed jet eombustor may be achieved with a substantially reduced main stream equivalence ratio by maintaining the equivalence ratio in the recircnlation zone at approximately the same value. This can easily be done by varying the composition of the jet stream. Since NO produced in that part of the combustion zone outside of the recirculation zone is a factor of three to ten greater than that produced in the recirculation zone, a decrease in main stream equivalence ratio should indeed make possible a significant reduction in total NO production.

REFERENCES i. Funs, A. E.: ARS Journal, 30, 3 (1960).

Philip C. Mahe, Washington State Universit!], USA. The reaction NO ~- He,2 ---*NO 2

-

Oil

(1)

that is used to tentatively explain combustor-formed NO 2 may also cause N() 2 formation in temp~aturequenched gas-sample probes. In modeling quartz sample probe chemistry at 1/4 atm, ~ we have considered both gas-phase and surface reactions, and have found the above reaction to be an important consequence of rapid temperature quench, i.e., water cooling. Cernansky 1 has also observed this. The rapid temperatnre quench causes forination of the t l O 2 radical from recombination of the O t t / O / H subsystem; and also effectively eliminates the reduction of N() 2 via "The model divides tile probe length into small computational cells. Each cell is treated as a perfectly stirred reactor with a detailed chemical kinetic system (technique due to I). T. Pratt). Temperature variation along the probe is forced 1)y the heat h)ss process. Pressure level is forced by the probe aerodynamics.

134

P O W E R SYSTEMS NO 2 + O ~ N O + O 2

(2)

NO2 s~f. NO

(3)

Reactions (2) and (3) make the presence of NO 2 in the hot, uncooled portion of the sample probe unlikely. Interpretation of the degree of flame- or eombustor-formed NO 2 must be conducted cautiously at present given the competitive, and perhaps dominant, sample probe chemistry.

REFERENCES 1. CERNANS~r N. P.: AIAA Paper 76139 (1976).

Authors" Reply. As was stated in the paper, we agree that NO 2 formation in the sample probe could be an important source of NO 2. Thus it appears that optical in situ measurements of NO and NO 2 will be necessary before the presence of NO 2 in the combustor can be verified.

tions in NOx concentrations from the use of "laboratory tools" in gas turbine engines can be very misleading to those whose responsibility it is to prepare emission-level regulations. Claims for these devices to reduce NOx production should be qualified with an assessment of the amount of development and refinement needed to render these systems to practical application. Would you please comment on the refinement and development that might be needed by the opposed-jet combustor before it might be considered for implementation into gas turbine engines?

Authors" Reply. We are not proposing the opposed-jet geometry as a combustion configuration suitable for engine applications. Rather, it has been employed as a laboratory tool proven effective for the study of premixed lean combustion. The fundamental promise of oxides of nitrogen control through premixed lean combustion has been demonstrated under laboratory conditions. Such demonstrations should guide gas turbine engine developers in the difficult task of translating this approach to "real engines." We are confident that those responsible for establishing emission regulations understand these difficulties.

Stanley A. Mosier, Pratt and Whitney Aircraft, USA. Government regulations establishing upper concentration limits for objectionable exhaust emissions such as NOx, from stationary and aircraft gas turbine engines are precicted in part upon both the demonstrated and the potentially achievable emission levels of " a d v a n c e d " combustion systems. Many combustion schemes have been and continue to be proposed for incorporation into gas turbine engines that purport to be able to control the production of NOx to very low levels. However, m a n y of these devices are simply research tools that have not been operated at engine conditions in an engine environment using the fuels with which the turbine manufacturers must contend. Further, many of these devices have not been examined relative to basic engine performance requirements and operational constraints. Consequently, claims for great reduc-

E. E. Khalil, Imperial College, England. Did you include the various effects of temperature, concentration fluctuations and their correlations in your two steps chemical reaction scheme? Authors" Reply. No attempt was made to account for the effect of turbulent fluctuations on chemical reaction rates. Such effects are however certainly present and could be of great importance in predicting pollutant species levels where chemical reaction rates are strong functions of both concentration and temperature. Due to the approximate empirical nature of the simplified reaction scheme used, any such attempt to include the effect of turbulent fluctuations did not appear to be justified.