A Diffusion Limited Model That Accurately Predicts the NOx Emissions From Gas Turbine Combustors Including the Use of Nitrogen Containing Fuels

W. S. Y. Hung Advanced Engineer, Gas Turbine Engine

Division, Westinghouse Electric Corporation, Philadelphia, Pa.

Mem. ASME

A Diffusion Limited Model That Accurately Predicts the Nox

Emissions From Gas Turbine Combustors Including the Use of Nitrogen Containing Fuels A diffusion limited model has been described previously to simulate accurately the thermal NOx emission processes in various gas turbine combustors for fuels containing negligible amounts of fuel bound nitrogen. The application of this model to simulate accurately the water injection process has also been demonstrated. It is currently proposed that any bound nitrogen in fuel is completely reacted to form nitric oxide during the hydrocarbon combustion process; the ultimate net conversion is determined subsequently based on the Zeldovich mechanisms. With this additional assumption, this model has been generalized to include the use of fuels containing significant amounts of bound nitrogen, such as crude or residual oils. The predicted NOx emissions from these nitrogen containing fuels are in excellent agreement with laboratory and field data including the effect of water injection. Comprehensive understanding of the NOx formation processes has been gained from the current analytical study.

Introduction

In recent years, it has been recognized that significant amounts of chemically bound nitrogen in fuel will increase the total emissions of nitrogen oxides (NO*) by a substantial amount [1-4].1 Fla-gan, Galant, and Appleton [5] have recently proposed mathematical models for the formation of nitric oxide (NO) from organic fuel nitrogen with only limited success. It is clear that there is incomplete understanding of the conversion mechanisms of fuel bound nitrogen to NOx. In this paper, a simplistic mathematical modeling approach is utilized to provide better understanding of this subject.

1 Numbers in brackets designate References at end of paper. Contributed by Gas Turbine Division and presented at the Joint Power

Generation Conference, Portland, Oregon, September 28-October 1,1975, of THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS. Manuscript received at ASME Headquarters June 30,1975. Paper No. 75-Pwr-ll.

The development of an analytical model simulating the thermal NO* emission processes in various gas turbine combustors for a variety of fuels containing negligible amounts of bound nitrogen has been reported [6]. The modification of this model to simulate the process of water injection, coupled with the ambient humidity influence in the control of thermal NO* emissions, has also been described [7]. The modification of this model to simulate the thermal NO* emission characteristics of a laboratory premixed combustor has also been reported [8]. Its application to simulate the combustion of gaseous fuels from coal or oil gasification processes has been described briefly. The predicted low NOx emissions from these low flame temperature gaseous fuels have subsequently been verified by laboratory test data. This subject will be described in more detail in a separate paper.

In this paper, the development of a generalized model to allow the assessment of NOx emissions from fuels containing any amount of bound nitrogen, including such fuel types as crude or residual oils, is described.

The rationale and the assumptions made in the modeling of thermal NO* emissions have been described in detail in reference [6]. Following this modeling approach, a single one-step overall

320 / JULY 1976 Transactions of the ASME Copyright © 1976 by ASME

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Fig. 3 Quasl-one-dlmenslonal air-flow distribution model

1~(I+UtV'W)g( I.UtV)g

----f---+---"------!----!---f-----+---L &2~REGION I -+REGION lI+REGION m+REGION Jl[-+REGION Y ~

YH20 = specific humidity/(l + specific humidity) (2)

Y02 = 0.233 (I - YH20) (3)

YN2 = 0.767 (1 - YH20) (4)

tion to the effective area of the openings at these various locations.For any given swirler used, the core radius of the reverse flow, isdetermined from the semiempirical correlation of Kind and Youssef [10]. This empirically determined radius defines the boundarybetween regions I and II (Fig. 2).

The quantity of air flow reversing upstream into the recirculating region is determined based on the same empirical equationused by Roberts, et al. [11]. With reference to the air-flow model ofFig. 3, the fraction of air, k, flowing upstream into the recirculationzone is calculated as follows:

It is further assumed that this reverse air flow, kg, entrains anequal quantity of flow, jg, from region II into region 1.

Composition of Ambient Air. Recognizing that humidity hasa significant influence on NO. emissions, the composition of theambient air is modeled by assuming that it is a mixture containingonly three components, water vapor, oxygen and nitrogen. At anygiven ambient condition, the specific humidity (g H20/g dry air) isfirst determined. With this specific humidity as an input, the massfractions of chemical species, Yi, in the a.rribient air is determinedas follows:

Fuel-Flow Distribution. During gas turbine operation, fuel isinjected from the head end into the combustor as shown in Fig. 2.In a dual fuel machine, liquid fuel is injected through the use of apressure atomizing nozzle and gaseous fuel through gas nozzles.The distribution of fuel into the primary combustion zone for either oil-fired or gas-fired operation is, therefore, different.

(a) Liquid Fuel. The flow distribution of liquid fuel in the primary combustion zone is modeled as shown in Fig. 4. In the model,it is assumed that a single value of the Sauter mean diameter issufficient to characterize the fuel spray and that all the dropletsenter region I at a known velocity characterized by the dischargevelocity in a direction parallel to the axis of the combustor. Withthe nozzles used, the initial droplet diameter is calculated from theempirical expression recommended by the nozzle manufacturer.

In region I, each droplet vaporizes as it is being slowed down by

(1)k = 0.5 (Tair/Tname)1/2

Fig. 1 Production combustor

reaction is assumed to be adequate to model the reaction mechanism of any bound nitrogen in fuel to form nitric oxide. The ultimate net conversion is subsequently calculated based on the Zeldovich mechanisms as described in reference [6]. With this additional provision, this previously developed model has been generalized to allow a priori prediction of NO. emissions from fuels containing any amount of chemically bound nitrogen.

The .essential assumptions made in the modeling are first described. The predictive calculations are then compared with theactual field and laboratory data. Excellent agreement is observed.

Mathematical ModelAn experimentally verified thermal NO. emission model, as de

scribed in reference [6], has been developed for a specific family ofgas turbine combustors. The production combustor configuration

.as shown in Fig. 1 is one of these combustors. The approach, however, should be applicable to most combustion systems similar tothat of gas turbine combustors. The original analyses include afive-region combustor internal flow-field model, as illustrated inFig. 2; a method to account for the influence of ambient humidity;fuel distribution models for liquid and gaseous fuels; a single overall hydrocarbon combustion model; a nitric oxide formation modelusing finite-rate reaction kinetics based on the Zeldovich mechanisms; and, a diffusion limited complete mixing'model. Currently,a model for the conversion of fuel bound nitrogen is added to allowthe a priori prediction of NO. emissions from fuels with anyamount of chemically bound nitrogen. The major assumptions andbases used in the modeling are described.

Air-Flow Distribution. The recirculating air flow in the primary combustion zone is characterized by a simplified flow patternas described by Clarke [9] such that the three-dimensional effectdue to recirculation can be accounted for through the use of aquasi-one-dimensional model as illustrated in Fig. 3. At a giventotal combustor air flow, the mass flow rates of air entering thecombustion chamber at the various holes and openings are calculated based on the assumption that they are distributed in propor-

1rW

@ ~rhf,23I

rh l2 rhl12 } ,GASEOU~ __'_'_c -I

FUEL \ m'CD t,r,. I

mt,I'

LIQUIDFUELSPRAY

Fig. 4 Fuel-flow distribution in primary combustion zone,Fig. 2 Combustor model

SECONDARY --------T---lCOOLING AIR : I

i :I II I

j[PRIMARY ::COMBUSTION AIR I I

I II I

• •FUEL--

Journal of Engineering for Power JULY 1976 / 321


viscous drag force. The solution of the droplet momentum equation coupled with the droplet vaporization equation allows the determination of the amount of fuel being vaporized in region I. The remaining liquid droplet entering region III, if any, is assumed to be completely vaporized in region III.

Instantaneous mixing of the vaporized fuel with the surrounding gas phase mixture in each region is assumed. The combustion of the vaporized fuel in region I takes place. The unburned fuel, if any, enters region II as ro/,12 and further combustion takes place. The remaining fuel, if any, partially recirculates into region I as rhfj and partially enters region III as rhf^a in the ratio of the recirculating air flow, jg, to that air flow, (u + k)g, entering region III from region II. Iterations are performed until steady-state conditions in both regions I and II are reached. Combustion of fuel entering region III from both regions I and II, if any, takes place. Any of the fuel that remains unburned enters region IV and further combustion with the air in region IV terminates the fuel-air reaction in the primary combustion zone.

(b) Gaseous Fuel. The flow distribution of gaseous fuel in the primary combustion zone is also modeled as shown in Pig. 4. Since no rigorous method appears to exist allowing the calculation of the percentage of fuel injected into regions I and II, the current fuel-flow distribution model for gaseous fuel assumes that all the fuel is being injected into region II as an approximation. Mixing and combustion of gaseous fuel with air take place in the same manner as

. described in the previous paragraph for the vaporized liquid fuel.

Mixing Process. Prom the air-flow distribution model as illustrated in Pig. 3, there is always a certain quantity of inlet air and a certain quantity of mixture of reaction products from another region entering each of the five regions in the combustor. Mixing of these two quantities is assumed to be instantaneous and complete. The mass flow rate of the t'th chemical species in the resulting mixture is calculated based on the principle of mass conservation. The temperature of the mixture is calculated from the energy conservation principle as described later.

Chemical Reactions. In modeling the chemical reactions, it has been recognized that the combustion of hydrocarbon takes place in the order of microseconds [12]. It is also reasonable to assume that the conversion of any chemically bound nitrogen in fuel to nitric oxide takes place in a comparable short period of time. During the post-combustion and prequenching period, the Zeldo-vich mechanisms [13] determine the ultimate amount of nitric oxide formed. Since the residence time in gas turbine combustors is in the order of milliseconds, it is reasonable to decouple the slow from the fast reactions in the modeling process.

(a) Hydrocarbon Combustion. From the air-flow distribution model, there is always air flow in each of the five regions in the combustor. When there is fuel-flow in a region as calculated by the fuel-flow distribution model, it is assumed that the hydrocarbon fuel and oxygen react instantaneously to form carbon dioxide and water vapor until either oxygen or the hydrocarbon fuel is completely depleted depending on whether there is a fuel rich or fuel lean condition. The products of combustion and the flame temperature are calculated in accordance with the combustion reaction equation based on the mass and energy conservation principles.

(b) Combustion Process Reactions. During the combustion process, it is assumed that any amount of chemically bound nitrogen in fuel is 100 percent reacted to form nitric oxide. In other words, it is not necessary to model the detail of the reaction mechanisms.

(c) Post-Combustion Nitric Oxide Formation Reactions. During the post-combustion period, if both oxygen and nitrogen are present in a region, the reaction mechanisms originally proposed by Zeldovich [13] are assumed to take place at finite rate as follows:

0 + N2 — NO + N (5)

N + 0 2 ^ N O + 0 (6)

0 2 + M ^ 2 0 + M (equilibrated) (7)

where the symbol, M, denotes any molecule present. Using recent values of the reaction rate constants reported by

Wolfrum [14] and calculating the equilibrium constants from the JANAF Thermochemical Tables [15], the rate equation in ppm/ms can be shown to be

dXN0 = pl/2 r a X N 2 X o 2 V 2 - f e X N 0 2 / X o 2 i / n

dt L 1 + c XNO/Xo2 J where P is the total pressure (in atm) and

a = 4.6 X 1018 T - 1 exp(-136,000/i?T) (9)

b = 2.1 X 1017 T-1 exp(-92,000/i?T) (10)

c = 8.5 exp(7400/#T) (11)

where R = 1.98 cal/mol-K, and, T is temperature (in K). Temperature and Residence Time. The temperature in any

region with or without combustion is calculated based on the energy conservation principle for a constant pressure system as outlined by Obert [16]. The residence time of chemical species in each region is determined based on the assumption of uniform velocity profile, i.e., plug flow.

Diffusion Process. In a conventional combustor, fuel and air are injected separately into the system. The process of diffusion between fuel and air plays a significant role in determining the amount of nitric oxide formed. It was empirically found that the complete mixing assumption is no longer valid when there is more than 30 percent excess air in any fuel lean region [6]. Hence, for a conventional combustion system, it is necessary to further assume that, insofar as NO* emission modeling is concerned, the complete mixing of fuel with air in any fuel lean combustion region is limited to 125 percent theoretical air [6]. For a combustion system where fuel is allowed to premix with the primary combustion air prior to injection into the combustion chamber, this diffusion-limited restriction can be removed [8].

Water Injection. During the water-injection operation of gas turbines for the purpose of reducing the NO* emissions, water is injected into the combustors from the head end as shown in Fig. 2. The word water is used here to mean the injected water only and does not include the water vapor entering the combustor in the form of humidity. The effect of water injection on NO* emissions is assumed to be a pure thermal phenomenon (i.e., no chemical or catalytic reaction is involved).

The water-injection process is simulated by assuming that all the water is being injected from the head end into region I (Fig. 2) and is completely vaporized and mixed with the combustion products in region I, resulting in a lower temperature. As a result, the temperatures in regions II, III, IV, and V of the combustor model (Fig. 2) are, in turn, lowered. Fuel flow has to be increased to maintain the combustor exit temperature, which is also the turbine inlet temperature. Hence, in simulating the water injection process, at a given water/fuel mass flow ratio, iterations are performed on the fuel flow rate such that the combustor exit gas temperature is maintained within ±1.11°C (2°F) of the designed value.

Calculation Selection of Thermo-Physical and Transport Properties.

The actual dimensions of the modeled combustor were selected for the calculations. The property values for No. 2 fuel oil were selected from references [16-17]. From the chemical composition of natural gas [18], it is reasonable to treat natural gas as if it consists of 100 percent methane. From the measured specific gravity of any crude or residual oil at 15.6/15.6°C (60/60°F), the hydrogen/carbon ratio and the lower heating value can be determined from reference [19]. The transport properties as a function of temperature were calculated based on references [16, 20].

Calculation Procedure. The necessary computations as prescribed by the mathematical modei described in the previous section were performed through the use of a digital computer. To sim-

322 / JULY 1976 Transactions of the ASME


ulate dry operation, the fuel/air mass flow ratio was allowed to increase from below the idle load condition to slightly above the peak load value. At any fuel/air mass flow ratio of interest, the amount of nitric oxide (NO) at the exit of the combustor (laboratory test) or at the exhaust of the gas turbine (field test) is calculated. Since the other oxide of nitrogen (NO2) has been measured in both laboratory and field tests, it is necessary to further assume that NO2 is formed only through oxidation of nitric oxide at sufficiently low temperature. Then, the calculated amount of nitric oxide in ppm volume is equivalent to the measured NO* (sum of NO and NO2) emissions. The NO* emissions were plotted as a function of the fuel/air mass flow ratio, or, the load of the turbine (MW).

To simulate the water injection operation, the water/fuel mass flow ratio was allowed to increase in equal incremental amount from zero to a maximum prescribed value at the loading conditions to be simulated. The ratio, NO* with water/NO* without water, was plotted as a function of the water/fuel mass flow ratio.

Field Test. Based on reference [21], the nitrogen contents in distillate oils can be as much as 0.365 percent by weight. From chemical analyses, the No. 2 distillate oils used in our laboratory and field tests generally contain less than 0.01 percent N by weight. In the thermal NO* model [6], the effect due to fuel bound nitrogen was assumed to be negligible when the fuel contained 0.01 percent N or less. In order to checkout this assumption, calculations simulating the field test for a 60-MW gas turbine were performed assuming that the fuel contained 0.01 percent N by weight. These newly predicted values were compared with the actual field data as shown in Fig. 5. These values are for all practical purposes the same as those predicted by the thermal NO* model. Hence, for fuels containing 0.01 percent N or less, there is no practical difference between values calculated by the present model and that calculated by the thermal NO* model.

It should be pointed out here that the data plotted are the average of three independent measurement techniques used. They are the Phenoldisulfonic Acid (PDS) method (ASTM-D-1608), a modification of the Saltzman method (ASTM-D-1607) and a on-line gas analyzer. Description of these measurement techniques has been documented in reference [22]. The spead of data is generally ±10 percent in field tests and ±15 percent in laboratory tests.

Calculations simulating a field test using Es Sider Libyan crude oil as fuel were performed. From chemical analyses, this particular batch of crude oil was found to contain 0.14 percent N by weight. The predicted NO* emissions from this 25-MW gas turbine were

FUEL: ES SIDER LIBYAN CRUDE 1.147.N)

PRODUCTION COMBUSTDR

SPECIFIC HUMIDITY = .017

X PREDICTED VALUES

O FIELD DRTfl

LORD M l

Fig. 6 NO, Emissions from a 25-MW gas turbine

compared with the actual field data as shown in Fig. 6. With the exception of the data at idle load (2.5 MW) where the data spread was unusually high indicating possible error in one of the measurement techniques, the rest of the data are in good agreement with the predictions.

Labora tory Tests. NO* emissions were calculated for tests conducted in the laboratory using crude or residual oil. In each of these laboratory tests, the pressure and inlet air temperature were held constant and the ambient humidity generally did not change significantly during the tests. The NO* emissions from a combustor using the Es Sider Libyan crude oil as a fuel were predicted and compared with the measured laboratory data as shown in Fig. 7. This batch of crude oil contains 0.13 percent N by weight as indicated by chemical analyses. With this same combustor and the same batch of fuel, the reduction of NO* emissions through water injection was also evaluated. The predicted NO* reduction through water injection for the Es Sider Libyan crude oil, verified by laboratory data as shown in Fig. 8, is less than that for fuels containing 0.01 percent N or less [7].

FUEL: N0.2 DISTILLATE OIL

PRODUCTION COMBUSTOR


X PREDICTED VALUES

D FIELD DRTH

Tt.OO .040 .0B0 .120 .160 .200 .24

FUEL/HIR MASS FLOW RATIO (XIO"'I

Fig. 5 NOx emissions from a 60-MW gas turbine

FUEL: ES SIDER LIBYAN CRUDE 1.13ZN)



X PREDICTED VALUES

O LABORATORY DATA

FUEL/HIR MASS FLOW RATIO IXIO"" 1

Fig. 7 NO* Emissions from a production combustor during a laboratory test—crude oil



X PREDICTED VALUES

O LABORATORY DATA



ES SIDER LIBYAN CRUDE (. 13 7. N)

HATER/FUEL MASS FLOW RATIO

Fig. 8 The reduction of NO* emissions through water Injection during a laboratory test—crude oil

The NOx emissions from a laboratory combustor using residual oil (0.16 percent N) as fuel were predicted and compared with the measured laboratory data as shown in Pig. 9. Excellent agreement is again observed. There are no in-house data for fuel containing more than 0.16 percent N. However, since calculations for a variety of laboratory and field testing conditions (pressure ranging from 3-10 atm, inlet air temperature ranging from 250 to 450° C and humidity ranging from 0.001 to 0.02), for a number of combustors using gaseous and liquid fuels containing 0-0.16 percent N by weight correlated well with the measured laboratory and field data, we have considerable confidence in the predicted NO* levels by the present approach for fuels containing bound nitrogen beyond the 0.16 percent level.

Discussion N O x Emissions as Influenced by Various Fuels. It has been

inferred from the present generalized model that when the amount of fuel bound nitrogen is 0.01 percent by weight or less, the NO* emissions resulting from the firing of these fuels are, for all practical purposes, not affected by the bound nitrogen contents. The conclusions made from the thermal NOx model are, therefore, valid for these fuels.

Since the heating values of various fuels are different, in order to compare the NO* emission levels of various fuels, it ia necessary to plot the results on the same basis, e.g., in terms of the load of the turbine in MW rather than the fuel/air mass flow ratio. It has been

FUEL: RESIDUAL D I L I . 16 7. Nl

LABORATORY COMBUSTOR


X PREDICTED VALUES

(D LABORATORY DATA



X LIBYAN CflUOEI. 14 7. Nl

0 NO. 2 FUEL OIL

A NATURAL GAS

FUEL/BIR MASS FLOW RATIO (XICr1 1

Fig. 9 NO, emissions from a laboratory combustor during a laboratory test—residual oil

Fig. 10 The effect of fuels (Libyan crude, No. 2 fuel oil and natural gas) on NOx emissions from a 25-MW gas turbine

concluded in reference [6] that the lower NO* emission level from the firing of natural gas, as compared to the firing of distillate oil containing 0.01 percent N or less, has been attributed to the lower flame temperature resulting from the combustion of natural gas as compared to distillate oil. The predicted NO* emissions from a 25-MW gas turbine using natural gas or distillate oil with 0.01 percent N or less as fuel have been confirmed by experimental data [6] and are compared with the confirmed emission level from the same turbine at the same ambient and operating conditions using the Libyan crude oil (0.14 percent N) as fuel as shown in Fig. 10. Although the calculated flame temperature for the Libyan crude is slightly less than that for the No. 2 fuel oil, it can be observed from Fig. 10 that the NO* emission level for the Libyan crude is significantly higher.

If one neglects the fuel bound nitrogen effect, analyses have shown [8] that 15-20 percent differences in the thermal NO* emission level can result from the combustion of crude or residual oils due to differences in flame temperature resulting from differences in hydrogen/carbon ratio and heating value. Hence, it is inaccurate to assess the net conversion of fuel bound nitrogen by assuming that the NO* emissions from a crude or residual oil in excess of that from any fuel oil containing a negligible amount of bound ni - ' trogen are the amount converted. This could be one of the reasons that explains why the reported net conversion of fuel bound nitrogen ranges anywhere from 0 to 100 percent [1-5]. The net conversion is not a fixed number. It is primarily a function of the flame temperature and the bound nitrogen contents. For the conventional gas turbine combustors, calculations using fuel oils containing up to 1 percent N and simulating operating conditions from 0 to 100 load showed that the net conversion of fuel bound nitrogen is anywhere between 20-60 percent.

Reference [4] gives the typical average nitrogen contents for crude and residual oils as 0.65 and 1.25 percent, respectively. Hence, the crude (0.14 percent N) and residual (0.16 percent N) oils tested here are relatively low nitrogen content oils. The effect of crude oil on NO* emissions relative to No. 2 fuel oil and natural gas as shown in Fig. 10 is minimal. From the present calculations, the total NO* emissions from conventional gas turbine combustors using crude or residual oil containing 1 percent N are at least twice that from the use of fuel oil containing 0.01 percent N or less.

It has been shown in reference [8] that significant reduction in NO* emissions can be achieved through the use of fuels from oil or coal gasification processes with calculated flame temperature less or equal to 1800°C. Since the modeling of NO* emissions using fuels from oil or coal gasification processes is a subject in itself, it

324 J JULY 1976 Transactions of the ASME


-20

0

40

60

nn

ion

^ \

x ^

\

——

\

2.0 %N

I.O%N

0.5% N

0.1% N

0.0 %N

Table 1 The effect of temperature on the rate constants, a, b, and c

WATER/FUEL MASS FLOW RATIO

Fig. 11 Predicted decrease in the reduction of NO* emissions through water injection with increasing amounts of bound nitrogen in fuel oil

will be discussed in a separate paper. Effect of Bound Nitrogen During Water Injection. For

fuels containing 0.01 percent N or less, the injection of water into gas turbine combustors is a proven NO* reduction method [7, 22]. With 0.13 percent N in crude oil, both the prediction and the laboratory data showed a decrease in the percent reduction achievable through water injection (Fig. 8). It is, therefore, of interest to assess the effect of fuel bound nitrogen on NO* emissions during the water injection process.

NO* emission calculations for the water injection process at the peak load operating conditions of a 60-MW gas turbine using fuel oils containing 0.00, 0.10, 0.50, 1.00, and 2.00 percent N were performed. The predicted percent NOx reduction as a function of the water/fuel mass flow ratio for various amounts of fuel bound nitrogen are summarized as shown in Fig. 11. With increasing amounts of bound nitrogen, the present analysis predicts a decrease in the percent NOx reduction achievable through water injection as bound nitrogen increases. In fact, with 2.00 percent N in fuel oil and injecting water beyond the one to one water/fuel mass flow ratio, calculations showed an increase in NOx emissions. This suggests that for fuels containing 1 percent N or more, NO* reduction methods other than the use of water injection have to be developed.

Also, after a 40 percent reduction is accomplished through the use of water injection for fuel oils containing 0.5 percent N, the NO* emission level is about the same as the dry level for fuel oils containing 0.01 percent N or less. Hence, unless the emission regulations give allowance for the fuel bound nitrogen effect, the gas turbine industry will be forced to use low nitrogen content fuels. Above the 0.16 percent N level, these predictions have not been verified experimentally. Based on the analytical match with other data thus far, there is strong indication, however, that these predictions will turn out to be accurate.

In a recent paper, Wilkes and Johnson [23] reported that the net conversion of fuel bound nitrogen was increased when water was injected into their gas turbine combustors. Their experimentally observed phenomenon is in agreement with the present predictions and is consistent with the mathematical model described herein.

The fuel bound nitrogen is reacted 100 percent to form nitric oxide during the combustion process which takes place in the order of microseconds. During the post-combustion and pre-quenching period, the formation or depletion of nitric oxide is governed by equation (8). When there are sufficient amounts of nitric oxide, the negative term in this equation is no longer negligible compared to the positive term. The amount of nitric oxide formed during this period is reduced. In comparing with the NO* emissions from fuel with negligible amount of bound nitrogen, the re-

T (K)

2400. 2300. 2200, 2100. 2000,

& 713.25 214.45

57.69 13.67 2.80

b

341,940. 153,768.

64,176.

24.592. 8 ,541 . .

c

40.4 43.2

46 .5 50.4

55.1

b /a

479. 717.

1.112. 1,800.

3.055.

duction of nitric oxide formed during this period has been interpreted by many as the amount of fuel bound nitrogen not converted.

As water is being injected, the flame temperature is lowered. The rate constants, a, b, and c in equation (8) as a function of temperature are tabulated as shown in Table 1. The ratio, b/a, which is a measure of the relative importance of the backward reactions as compared to the forward reactions represented by equations (5) and (6), is also tabulated. It can be observed from Table 1 that as the flame temperature decreases, although the ratio b/a increases, the rate constants, a and b, decrease so rapidly that less and less of the nitric oxide formed from fuel nitrogen will be converted back to nitrogen and/or oxygen during the post combustion and pre-quenching period. Consequently, the resulting NO* emissions might be interpreted as an increase in the net conversion of fuel bound nitrogen.

Application and Limitation. A mathematical model has been developed for a specific family of gas turbine combustors based on the assumptions already described. For combustion systems operating at conditions similar to that of gas turbine combustors, adoption of the present modeling principles to calculate the NO* emissions from such systems is considered highly favorable. The extent of its application or limitation can only be judged by future experimental verification.

For combustion systems where the physical and/or chemical processes are definitely different from that of gas turbine combustors, modifications are obviously necessary. For the purpose of modeling NO* emissions, however, the present approach should still be applicable.

Conclusions The major contributions resulting from the present study can be

summarized as follows: (a) A mathematical model has been developed to predict a

priori the NO* emissions from gas turbine combustors under a variety of ambient and operating conditions including the use of nitrogen containing fuels.

(b) In modeling the fuel bound nitrogen effect, it is sufficient to assume that any amount of fuel bound nitrogen is 100 percent reacted to form nitric oxide during the combustion process, and, any subsequent formation or depletion of nitric oxide is determined through the Zeldovich mechanisms.

(c) For fuels containing 0.01 percent N by weight or less, the thermal NO* emissions are for all practical purposes the total NO* emissions, i.e., the bound nitrogen effect is negligible.

(d) For fuels containing more than 0.1 percent N by weight, e.g., most crude or residual oils, the effect of fuel bound nitrogen on NO* emissions can no longer be neglected.

(e) For a fixed bound nitrogen content, the net conversion of fuel bound nitrogen is not a fixed number; it is primarily a function of the flame temperature and the bound nitrogen contents.

(f) As a general rule, the net conversion of fuel bound nitrogen increases as the flame temperature decreases.

(g) For fuels containing more than 1 percent N by weight, water injection is no longer a practical method to reduce the NO* emissions.

Acknowledgment The author would like to thank Mr. C. E. Seglem and Mr. S. M.

DeCorso for their encouragements that resulted in the present de-



ve lopment ; Mr . M . J . A m b r o s e , M r . J . E m o r y , Mr . S. S. L in , Mr . R.

S y m o n d s a n d Mr . G. V e r m e s for conduc t ing t h e l abo ra to ry a n d

field tes t s ; Mr . E . S. Obid insk i , Mr . J . M a r l o w a n d t h e C h e m L a b

pe r sonne l for pe r fo rming t h e fuel ana lyses a n d t h e emiss ion mea

su remen t s ; a n d , m a n y o the r s who h a s h e l p e d d i rec t ly or ind i rec t ly

in m a k i n g th i s work possible .

References 1 Martin, G. B., and Berkau, E. E., "An Investigation of the Conversion

of Various Fuel Nitrogen Compounds to Nitrogen Oxides in Oil Combustion," Presented at Symposium on Combustion Processes and Air Pollution Control, AIChE National Meeting, Atlantic City, Aug. 30,1971.

2 Sarofim, A. F., Williams, G. C , Modell, M., and Slater, S. M., "Cover-sion of Fuel Nitrogen to Nitric Oxide in Premixed and Diffusion Flames," Presented at the AIChE 66th Annual Meeting, Philadelphia, Pa., Nov. 1973.

3 Appleton, J. P., and Heywood, J. B., "The Effects of Imperfect Fuel-Air Mixing in a Burner on NO Formation From Nitrogen in the Air and the Fuel," Fourteenth Symposium (Int.) on Combustion, Combustion Institute, Pittsburgh, Pa., 1973, p. 777.

4 Dilmore, J. A., and Rohrer, W., "Nitric Oxide Formation in the Combustion of Fuels Containing Nitrogen in a Gas Turbine Combustor," ASME Paper No. 74-GT-37.

5 Flagan, R. C , Galant, S., and Appleton, J. P., "Rate Constrained Partial Equilibrium Models for the Formation of Nitric Oxide From Organic Fuel Nitrogen," Combustion and Flame, Vol. 22,1974, pp. 299-311.

6 Hung, W. S. Y., "An Experimentally Verified NOx Emission Model for Gas Turbine Combustors," ASME Paper No. 75-GT-71.

7 Hung, W. S. Y., "Accurate Method of Predicting the Effect of Humidity or Injected Water on NOx Emissions From Industrial Gas Turbines," ASME Paper No. 74-WA/GT-6.

8 Hung, W. S. Y., "The Reduction of NOx Emissions From Industrial Gas Turbines," The Eleventh International Congress on Combustion Engines, Vol. 3, CIMAC, Barcelona, Spain, 1975, pp. 161-181.

9 Clarke, J. S., "The Relation of Specific Heat Release to Pressure

Drop in Aero-Gas-Turbine Combustion Chambers," Proceedings of the 1955 IME-ASME Conference on Combustion, 1955, pp. 354-361.

10 Kind, C , and Youssef, T., "Calculating the Flow in the Combustion Chamber of Gas Turbines," Brown Bdberi Review, Vol. 51, 1964, pp. 808-816.

11 Roberts, R., Aceto, L. D., Kollrack, R., Teixeira, D. P., and Bonnell, J. M., "An Analytical Model for Nitric Oxide Formation in a Gas Turbine Combustor," AIAA Journal, Vol. 10, No. 6, June 1972, pp. 820-826.

12 Marteney, P. J., "Analytical Study of the Kinetics of Formation of Nitrogen Oxide in Hydrocarbon-Air Combustion," Combustion Science and Technology, Vol. 1,1970, pp. 461-469.

13 Zeldovich, Ya. B., Sadovnikov, P. Ya., and Frank-Kamenetskii, D. A., "Oxidation of Nitrogen in Combustion," Academy of Sciences of USSR, Institute of Chemical Physics, Moscow-Leningrad, trans, by M. Shelef, 1947.

14 Wolfrum, J., Chemie Ing. Technik, Vol. 44,1972, p 656. 15 "JANAF Thermochemical Tables," Midland, Mich., Dow Chemical

Company, June 1971. 16 Obert, E. F., Concepts of Thermodynamics, McGraw-Hill, New York,

1960. 17 Griswold, J., Fuels, Combustion, and Furnaces, McGraw-Hill, New

York, 1946. 18 Gas Engineers' Handbook, Graf, S. H., ed., McGraw-Hill, New York,

1934. 19 Cragoe, C. S., "Thermal Properties of Petroleum Products," U. S.

National Bureau of Standards Publication No. 97, U. S. Government Printing Office, Washington, 1929.

20 Bird, R. B., Stewart, W. E., and Lightfoot, E. N., Transport Phenomena, Wiley, New York, 1965.

21 Ward, L. F., Jr., Moore, R. T., and Ball, J. S., "Nitrogen Compounds in Distillate Fuels," Analytcal Chemistry, Vol. 25, No. 7, July 1953, pp. 1070-1072.

22 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.

23 Wilkes, C , and Johnson, R. H., "Fuel Property Effects on Gas Turbine Emission Control," Presented at the Joint Power Generation Conference, Miami, Fla., Sept. 1974.

326 / JULY 1976 Transactions of the ASME


Documents

A Diffusion Limited Model That Accurately Predicts the NOx Emissions From Gas Turbine Combustors Including the Use of Nitrogen Containing Fuels