Combustion of Synthetic Fuels - American Chemical Society

6 Intermediate B t u G a s G l o b a l F l a m e K i n e t i c s

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ARTHUR LEVY, HERBERT A. ARBIB, and E. LEWIS MERRYMAN Battelle, Columbus Laboratories, Columbus, OH 43201

Global rates determined from CO-H -CH flames were effectively applied to Zeldovich, Frank-Kamenetski, Sewenov (ZFKS) burning velocity equations. Four intermediate BTU flame mixtures were probed and analyzed. The fuel mixtures, CO/H /X where X = methane or natural gas, in the mole ratio 1/1/0.22, were burned in 13 percent excess air and slightly rich. With the aid of mole fraction profiles, carbon, hydrogen and oxygen mole balances were obtained and global kinetic rates were calculated. Burning velocity calculations were carried out using the ZFKS theory and global rates validated by our present experiments. Correlations of measured and calculated burning velocities are good. The substitution of natural gas, with its inherent impurities (some four percent of other hydrocarbons), for methane has no measurable influence on the burning behavior of the intermediate BTU fuel mixtures. 2

4

2

The a b i l i t y to develop r e l i a b l e p r e d i c t i o n s f o r s y n t h e t i c n a t u r a l gas mixtures and the a b i l i t y to design burners for these gas mixtures remain a key i s s u e for the uses of low- and i n t e r mediate-Btu gas. Numerous methods f o r assessing interchangea b i l i t y have been used over the y e a r s . For the most p a r t , these are e m p i r i c a l and reasonably u s e f u l methods; i . e . , i n d i c e s f o r l i f t , flashback, y e l l o w - t y p i n g . In an attempt to approach t h i s problem from a more fundamental p o i n t of view, we have been studying for the past few years ways of combining some of the more fundamental combustion p r o p e r t i e s of flames, such as g l o b a l r a t e s of r e a c t i o n , flashback, quenching distance and burning v e l o c i t y . In t h i s paper, we present some of our more recent r e s u l t s where we have developed the g l o b a l k i n e t i c s f o r some ternary f u e l systems CH4 - H2 - CO and n a t u r a l gas - H2O - CO and have c o r r e l a t e d these flame data and other plug flow data with burning v e l o c i t y data.

0097-6156/83/0217-0113$06.00/0 © 1983 American Chemical Society

Bartok; Combustion of Synthetic Fuels ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

114

COMBUSTION OF SYNTHETIC FUELS

Experimental Four f l a t , disc-shaped laminar flow flames were probed and analyzed using standard microprobing techniques. The flames were composed p r i m a r i l y of CO, H 2 , and Ar with small amounts of CH4 or n a t u r a l gas added to simulate intermediate Btu gas mix tures. Gas compositions used i n the probings are presented i n Table 1. Flames A and Β contained excess a i r , a i r / f u e l e q u i valence r a t i o = 1.13; Flames C and D were s l i g h t l y f u e l r i c h , a i r / f u e l equivalence r a t i o = 0.93. Each of the mixtures had a CO/H2/X (X = methane or n a t u r a l gas) mole r a t i o of 1 / 1 / 0 . 2 2 . A l l of the gases were more than 99 percent pure except f o r the n a t u r a l gas which had the f o l l o w i n g composition (furnished by Matheson): Gas Methane Ethane Propane Iso-Butane N-Butane Iso-Pentane

Percent 96.5 1.86 0.36 0.059 0.076 0.035

Gas N-Pentane Hexane Heptane Oxygen Carbon d i o x i d e Nitrogen

Percent 0.028 0.030 0.018 0.0023 0.676 0.312

The flames were s t a b i l i z e d on a s i n t e r e d - c o p p e r - f r i t flame burner at pressures of 110 to 115 t o r r . The copper f r i t was watercooled to prevent or reduce the tendency of H2 to burn i n and/or below the f r i t . An uncooled 1 / 4 - i n . O.D. quartz probe w i t h a 70 micron I . D . t i p was used to withdraw samples from the flames. The pressure d i f f e r e n t i a l between the flame and the i n s i d e of the probe was approximately 70:1. The flame samples were analyzed on a Finnigan Model 1015 quadrupole mass spectrometer. Argon was sub s t i t u t e d f o r n i t r o g e n as a d i l u e n t i n the flames to allow b e t t e r CO a n a l y s i s w i t h the mass spectrometer. Temperature measurements (corrected f o r r a d i a t i o n ) were made w i t h s i l i c a - c o a t e d Pt-Pt/10% Rh thermocouples, about 4 m i l s i n diameter. The temperature and species concentration p r o f i l e as a f u n c t i o n of distance through the flame provided the b a s i c data for the k i n e t i c a n a l y s e s . Results Flame Analyses. Smoothed temperature and mole f r a c t i o n v e r sus distance p l o t s and the associated net r e a c t i o n r a t e versus distance p l o t s f o r a f u e l - l e a n and f u e l - r i c h flame are shown i n Figures 1 and 2, r e s p e c t i v e l y . The computer-generated symbols i n the f i g u r e s i d e n t i f y the species associated with each curve; they do not denote data p o i n t s . Approximately 16 data p o i n t s from each flame were used to generate the curves.


6.

LEVY E T A L .

115

Global Flame Kinetics

TABLE 1.

TERNARY FUEL MIXTURES

Mole F r a c t i o n Flame

Fuel Percent

Β

Gas CO

0.0818

0.0818

0.0838

0.0838

39.8

H

0.0818

0.0818

0.0838

0.0838

39.8

2

CH

8.85

0.0182

4

8.85

0.0183

0.0186

0.0236

0.0236

0.0242

0.0242

2

0.1342

0.1341

0.1129

0.1129

Ar

0.6604

0.6604

0.6767

0.6767

Equivalence R a t i o , Q*

1.13

1.13

0.933

0.933

Natural gas C0 0

2

(A/F)

(A/F) stoich


11.5

116


Figure

la.

Mole f r a c t i o n s

for Flame A.


LEVY E T A L .


.070 ρ

FLAME A .050

Ο

H2

A

+

H20

X

Φ

02

CH4

CO

•

C02

Η

.030

"

o

.010+-

-.010

Ε

h

CVJ

ο -.030

ce -.050

h

-.070 h

J

-.090 0.000

1 1 .400

1 1 .800

I I 1.200

I L 1.600

J 2.000

Ζ (CM) Figure l b . Rate p r o f i l e s for Flame A .


1

118


Figure 2a. Mole f r a c t i o n s

for Flame C.



LEVY E T A L .

NAT

ο

.012

+

FLAME C

A

H2 H20 02

Χ

GAS

CO

C02

.008

.004

to

.000

CO

ι Ε .004

ο r— Χ

.008

en .012

.016

J

-.020 0.000

I I .400

I 1 .800

1 I 1.200

I I 1.600

L 2.000

Ζ (CM) Figure 2b.

Rate p r o f i l e s

for Flame C.


2*400

120


Excess A i r Flames. Concentration P r o f i l e s . Data from Flame A , Figure 1-a, show t y p i c a l mole f r a c t i o n p r o f i l e s obtained from a ternary IBtu mixture burning i n excess a i r . The p r o f i l e s show that the hydrocarbon i s completely consumed i n the flame and that a l l but a few percent of the hydrogen i s a l s o consumed under the excess oxygen c o n d i t i o n s . S l i g h t l y greater consumption of hydrogen appeared to occur when n a t u r a l gas was s u b s t i t u t e d f o r methane i n the 0 2 - r i c h environment. The water p r o f i l e s show e a r l y and r a p i d formation of H 2 O , i n agreement w i t h the e a r l y and near complete consumption of CH4 and H 2 . Somewhat unexpected i s the r e l a t i v e l y low consumption of CO i n the excess oxygen flame. The CO p r o f i l e of F i g u r e 1-a shows only about 60 percent of the CO was o x i d i z e d to C O 2 . Other f u e l lean flames probed a l s o showed incomplete combustion of the CO. E q u i l i b r i u m CO l e v e l s were not a t t a i n e d i n most of the flames. The CO o x i d a t i o n appears to be k i n e t i c a l l y l i m i t e d i n these t e r n ary f u e l mixtures due to the i n h i b i t i n g e f f e c t of methane and the competition of methane f o r the OH r a d i c a l s . Rate Data. F i g u r e 1-b shows t y p i c a l r a t e p r o f i l e s f o r the formation and d e p l e t i o n o f the major species present i n the excess a i r flames as represented by Flame A. The r a t e s are computer generated and r e f l e c t , i n some cases, an o v e r - m a g n i f i c a t i o n of small d e v i a t i o n s i n the o r i g i n a l d a t a , mainly a s s o c i a t e d w i t h the mole f r a c t i o n p r o f i l e s . A d d i t i o n a l smoothing of data would im prove the c o n t i n u i t y of the curves, but o v e r a l l , the trends and r e l a t i v e r a t e values would remain about the same. The r a t e curves of F i g u r e 1-b i n d i c a t e an e a r l y formation of hydrogen i n each flame followed by immediate and r a p i d consump t i o n downstream. O v e r a l l , the r a t e data i n d i c a t e that the maxi mum r a t e s of d e p l e t i o n of reactants are s l i g h t l y l e s s i n the flame with n a t u r a l gas (Flame Β ) , while the maximum r a t e s of formation of CO2 and H 2 O , are about equal i n both flames. Taking i n t o account the e r r o r span i n the mole f r a c t i o n curves and r a t e data f o r each species i n the two excess a i r flames, i t i s concluded that the k i n e t i c processes a r e , w i t h i n experimental e r r o r , e s s e n t i a l l y the same i n both flames, the presence of n a t u r a l gas versus pure methane having no o v e r r i d i n g effects. Table 2 summarizes the maximum r a t e values f o r the excess a i r and the f u e l - r i c h flames. The numbers i n parentheses below each r a t e value i s the temperature at which the maximum r a t e was observed. F u e l - R i c h Flames. Concentration P r o f i l e s . T y p i c a l mole f r a c t i o n curves f o r a substoichiometrie flame are shown i n Figure 2 - a f o r Flame C. Mole f r a c t i o n p r o f i l e s i n Flames C and D were n e a r l y i d e n t i c a l f o r each corresponding species i n the subs t o i c h i o m e t r i c flames i n the presence of methane or n a t u r a l gas. As i n the excess oxygen flames, the hydrocarbons are com p l e t e l y consumed i n the f u e l - r i c h flames. However, as might be



Kelvin

0.028 (1595)

D

*

0.0084 (1765)

0.109 (1605)

C

0.065 (1725)

0.0088 (1815)

0.125 (1715)

0.025 (1735)

0.0099 (1795)

0.032 (680)

Β

0.029 (1725)

0.0027 (1240)

0.044 (1495)

0.019 (1635)

0.012 (1790)

0.053 (970)*

A

0.0065 (1795)

0.011 (1345)

0.081 (1345)

CO

0.014 (1675)

2

0.011 (1560)

2

H

2

4

0.0066 (1725)

0.00060 (1180)

CH

Depletion

MAXIMUM RATE RESULTS, IBTU FLAMES 2 -3 -1 Maximum Rate χ 10 (mol cm s )

Η 0

co

H

Flame

2

Formation

TABLE 2.

2

0.036 (1685) 0.0092 (1685)

0.029 (1680)

0.021 (1680)

0.030 (1470)

0

0.0060 (1385)

Nat Gas

122


expected, l e s s consumed under e a r l y i n these gen-containing

hydrogen and CO ( i n comparison w i t h Flame A) were the f u e l - r i c h c o n d i t i o n s . Water a l s o appears flames r e f l e c t i n g the e a r l y burnout of the hydro species.

Rate Data. The r a t e p r o f i l e s for Flame C are shown i n Figure 2-b. As i n the excess oxygen flames, the p r o f i l e s show m u l t i p l e peaks some of which, as mentioned e a r l i e r , r e s u l t from the computer smoothing r o u t i n e . The r a t e p r o f i l e s , while showing s i m i l a r trends i n each flame, d i f f e r i n the maximum r a t e values (see Table 2 ) . In c o n t r a s t to the oxygen-rich flames, data from the f u e l - r i c h flames g e n e r a l l y show higher maximum r a t e s of de p l e t i o n of reactants and formation of products i n the flame con t a i n i n g n a t u r a l gas, Flame C . However, as i n the o x y g e n - r i c h flames, the r a t e s i n both of the f u e l - r i c h flames agree w i t h i n a f a c t o r of 2, except f o r the H formation r a t e s (and the CO de p l e t i o n r a t e s i n the oxygen-rich flames). The f a c t o r of 2 v a r i a t i o n i s considered w i t h i n the e r r o r range of the k i n e t i c data i n these and the oxygen-rich flames. Maximum r a t e s occur at comparable temperatures i n the f u e l - r i c h flames, a c o n d i t i o n not observed i n the excess oxygen flames. 2

Mole Balances. Mole balances based on the g l o b a l CO + 1/2 0 + C 0 H + 1/2 0 -*· H 0 CH + 2 0 C 0 + 2H 0 each of the four flames are tabulated below. 2

2

2

4

for

2

2

2

2

Hydrogen Balance

Carbon Balance Flame No.

-(ACO + ACH ) 4

A Β C D

0.068 0.046 0.080 0.077

reactions

2

AC0

2

0.077 0.081 0.084 0.082

% Dev

-(ΔΗ

13 76 5 6

2

+ 2ACH ) 4

0.104 0.113 0.122 0.114

ΔΗ 0

% Dev

0.085 0.098 0.095 0.097

-18 -13 -22 -15

2

Oxygen Balance Flame No.

- Ι 1 / 2 ( Δ 0 0 + Δ Η 2 ) + 2ΔΟΙ4]

Δ02

% Dev

A Β C D

16 094 0.109 089 40 0.125 111 22 0.135 106 24 0.131 The carbon balance i s e x c e p t i o n a l l y good for each of the flames, except f o r Flame B. This no doubt i s r e l a t e d to the abnormally low CO values i n that flame (which we suspect are probably r e l a t e d to a c a l i b r a t i o n f a c t o r i n the spectrometer). Hydrogen and oxygen percent d e v i a t i o n s are h i g h , p a r t i c u l a r l y so for the oxygen. Hydrogen d e v i a t i o n can vary up to 20 percent i n hydrocarbon


6.

LEVY E T A L .

123


flames, but deviations should be l e s s i n these flames since hydro gen was added i n i t i a l l y . Oxygen v a r i a t i o n s u s u a l l y are below 10 percent. The high i n t e r c e p t values f o r 0 at zero distance (see mole f r a c t i o n p r o f i l e s ) account f o r most of t h i s d e v i a t i o n . S e n s i t i v i t i e s f o r 0 could be adjusted so that the i n t e r c e p t values agree more c l o s e l y with the c o l d gas values f o r 0 . This would b r i n g the percent d e v i a t i o n down c o n s i d e r a b l y . However, mole f r a c t i o n s were used as derived from the mass spectrometer data. For comparison purposes, as used i n t h i s study, the d e v i a t i o n should not i n t e r f e r e with o v e r a l l c o n c l u s i o n s . 2

2

2

C o r r e l a t i o n of the Burning V e l o c i t y Data One of the o b j e c t i v e s of t h i s program was to assess the p o s s i b i l i t y of c o r r e l a t i n g g l o b a l r a t e data with appropriate burning v e l o c i t y equations. Global r a t e data have for the most part been derived from plug flow d a t a , e . g . , Dryer and G l a s s man (1), and turbulent r e a c t i n g systems, e . g . , Howard et a l (2). Flame-generated rate data have been developed [ e . g . , F r i s t r o m and Westenberg (3)] from flame m i c r o s t r u c t u r e s t u d i e s , but have not been a p p l i e d to burning v e l o c i t y c a l c u l a t i o n s . In t h i s s e c t i o n , we describe some c o r r e l a t i o n data of g l o b a l r a t e s with burning v e l o c i t y . A p p l i c a t i o n of the Thermal Theory to M u l t i p l e Reactions. a flame d r i v e n by a s i n g l e exothermic r e a c t i o n (of the type nA -»· Β + C . . . ) , the laminar burning v e l o c i t y S according to c l a s s i c a l thermal theory of Z e l d o v i c h , Frank-Kamentskii and Semenov (ZFKS) i s given by (4) L

S

L = ι IV r = T - i

1 / 2

w i t h

.

1

Ξ

IT '

T

F

W D T

For the

( 1 )

u ρ f u u Τ 3 where a (mol/cm ) i s the reactant concentration i n the f u e l / o x i d i zer mixture, Τ the temperature, ρ the thermal c o n d u c t i v i t y , c the s p e c i f i c heat, w the r e a c t i o n r a t e , and the s u b s c r i p t s u End f r e f e r to the unburned and the f i n a l s t a t e i n the flame, r e s p e c t i v e l y ( λ / C p i s assumed constant). Expression of w i n Arrhenius form allows the i n t e g r a l I to be approximated. For more than one exothermic r e a c t i o n o c c u r r i n g simultaneously, i t can be shown that Equation 1 remains v a l i d as a p r e d i c t i v e expression for S , except that the r e a c t i o n rate i n t e g r a l I i s now given by U

L

1 =

ϊ-φτ I

A

H

f

i '* u

w

±

d T

(

2

)

where ΔΗ^ ( J / m o l . ) i s the heat of the r e a c t i o n i n which i i s the reactant, a^ (mol./cm^) are the i n i t i a l concentrations i n the f u e l / o x i d i z e r mixture, and w.^ (mol^/cm^ s) i s the molar r a t e of production of species i . Expressing concentrations i n mol f r a c -


124

COMBUSTION O F S Y N T H E T I C F U E L S

x

t i o n s x_£ (a^ = (p = pRT/M),

P /

i

R T

u

) »

u

a

n

d

employing p e r f e c t gas

Equation 1 can be w r i t t e n RT

S

assumption

=

T

,

^ u

1/2

— 1 T--T f u

M c up

(3)

and correspondingly, the i n t e g r a l I ' I

1

=

I[

i s expressed by

T

Σ ΔΗ, / f w.dT] I T u

(4)

with Χ Ξ Σ χ ΔΗ In the Semenov theory (4), i f the r e a c t i o n i s of the order, i . e . , w = kab exp (-E/RT) the r e a c t i o n r a t e i n t e g r a l I can be approximated by 2 m η RT I f - eff eff exp ( - E / R T ) u where ±

±

a

(5) second (6)

b

(7)

f

T

% f

a

2

u

u

RT /E f

u

2 b ., = b u , £ eff u j(-j-Tï) f f u T

R T

/

Ε

(8)

λ

In the f o l l o w i n g , we s h a l l assume that t h i s approximation holds whatever the order of the r e a c t i o n , i . e . , i f w = aV t

h

e

n

. ~

1

1

exp (-E/RT) 2

κ Τ

a

u

(9)

eff

b

eff "

*Tf f ^

(-E/RT^)

An improvement i n Equation 3 can be brought about by

(10) substituting

2 , 3 with X c _ / c , where the bar denotes the mean value Ρ pf/ Ρ The Present Model λ/c

f

£

(5).

The ternary flames considered here have three d r i v i n g exo thermic reactions: CO 4- 1/2 0 •> C 0 (11) 2

2


6.

125


LEVY ET A L .

H CH

+ 1/2

2

+

4

0

2 0

-> H 0

+

2

(12)

2

2

C0

+

2

2H 0

(13)

2

For the computation of the r e a c t i o n rate i n t e g r a l that appears i n the expression f o r S , the f o l l o w i n g two-step model was de v i s e d (confirmed by the measured concentration p r o f i l e s ) : L

1.

In a f i r s t step, CH4 i s depleted to form CO and H 2 O , and H 0 i s formed a l s o by o x i d a t i o n of the H i n the mixture : 2

2

CH

H 2.

+

4

2 0

+ 1/2

2

+

2

0

CO +

1/2

0

2

+

2H 0

(14)

2

(15)

H 0 2

2

In a second s t e p , CO (both from the CH4 r e a c t i o n and that i n i t i a l l y present i n the mixture) r e a c t s to form C0 : 2

(CO)

1

(CO)"

+ 1/2

0

+

0

1/2

-> ( C 0 )

2

F

(16)

2

M

·> ( C 0 )

2

(17)

2

For the r e a c t i o n r a t e s of Equations 14 to 17 g l o b a l expres sions from the l i t e r a t u r e were adopted. For r e a c t i o n s 14, 16, and 17, the o v e r a l l c o r r e l a t i o n s of Dryer and Glassman ( 1 ) were used, expressing r e s p e c t i v e l y the methane disappearance r a t e , the rate of r e a c t i o n of carbon monoxide with oxygen i n the presence of water, and the appearance rate of carbon d i o x i d e i n the methane-oxygen r e a c t i o n : d [ C H

~

4

]

β

_ dMl Î ^ i -

=

i n

1 0

1 0

1 3 . 2[

14.6

R R U

C 1

Ί

0.7

V

Γ Λ

Ο Ί

02 . 8

^Ο· ]

[ C O ] [ H 2 0 ]

0.5

^ - " [ C O J ^ O I

0

[ 0 2 ]

-

5

e x

, 48,000.

P(

0.25

^ ]

0

-

2

ε

5

, 18 . 1 F T

§ϊ—)» ( )

χ

ρ

^ 4 0 ^ ,

^

4

exp ( - - ^ 2 0 )

(20)

For r e a c t i o n 15, the only expression found i n the l i t e r a t u r e f o r the g l o b a l rate of water formation was the one by Fenimore and Jones (6) 2

8[H ]

dt

[CO]

d[H 0]

2

d[C0 ] 2

dt

(21)

and t h i s was used i n conjunction with Equation 19 assuming d[C0 ] f_ = - d[C0] , so dt dt~ o

= 8 χ 10

1 4

that

6

5

· [Η ][Η 0]°· [0 }°· 2

2

2

2 5

βχρ(-

4 0

Q 0 Q

> ) RT


(22)


126

When using Equation 22, a non-zero concentration of water has to be assumed even i f no water i s i n i t i a l l y present i n the r e a c t a n t s . Best r e s u l t s were obtained by taking [H2O] = [ Η ] + 2[CH4], i . e . , the t o t a l water e v e n t u a l l y to be i n the combustion products under o x i d i z i n g c o n d i t i o n s , although part of i t i s produced by d e p l e t i o n of hydrogen. T h i s can be j u s t i f i e d by n o t i n g that Equation 7 was derived f o r r a t e expressions i n which both reactants were depleted during combustion, and assumed that the reactant c o n c e n t r a t i o n i n the combustion zone i s constant and equal to a < a and ^ ef f u b ^ < b^. When one of the species appearing i n the r a t e expres 2

£ £

e

s i o n a c t u a l l y increases that i n t r o d u c t i o n of i t s

during the r e a c t i o n , i t i s

plausible

i n i t i a l value makes Equation 7 i n t o l e r

ably s m a l l . A l t e r n a t i v e expressions f o r the g l o b a l rates of Reactions 14 and 16 were t r i e d while developing the model. For the CO CO2 conversion (Reaction 16) the o v e r a l l c o r r e l a t i o n d e r i v e d by Howard et a l . (2) was i n i t i a l l y used, but w i t h t h i s the c a l c u l a t e d values of S L were considerably lower than the measured ones. For the methane disappearance r a t e (Reaction 14) the c o r r e l a t i o n s proposed by Westbrook and Dryer (7) were t r i e d , and these gave r e s u l t s n e g l i g i b l y d i f f e r e n t from those obtained by Equation 18. V a l i d a t i o n of the G l o b a l Rates E x p r e s s i o n s . In order to v a l i d a t e the g l o b a l r a t e expressions employed i n the model, temperature and concentration p r o f i l e s determined by probing the flames on a f l a t flame burner were s t u d i e d . A t t e n t i o n was con centrated on Flames Β and C . The experimental p r o f i l e s were smoothed, and the s t a b l e species net r e a c t i o n r a t e s were d e t e r mined using the laminar f l a t - f l a m e equation described i n d e t a i l by F r i s t r o m and Westenberg (3) and summarized i n Reference (8). A p l o t of the l o g a r i t h m ^ of Aexp(-E/RT) f o r three of the four rate expressions used i s shown i n Figures 3, 4, and 5 (for Equa t i o n s 18, 19, and 22, r e s p e c t i v e l y ) . I n i t i a l l y , an attempt was made to develop o r i g i n a l g l o b a l r a t e expressions f o r Reactions 14 to 16 from these r a t e d a t a . It soon became c l e a r , however, that the number of experimental p o i n t s was too few to allow the attainment of t h i s g o a l . More over, s i n c e a ternary system was being analyzed, the concentra t i o n p r o f i l e s had an i n t r i c a t e form which made numerical d i f f e r e n t i a t i o n to r e t r i e v e the r a t e s somewhat i n a c c u r a t e . I t was therefore decided to use these r a t e data to check the o v e r a l l r a t e expressions derived by other authors and used i n the present model. I t i s apparent that the adopted c o r r e l a t i o n s represent i n an acceptable way the experimental data at h i g h temperatures. The best agreement i s obtained f o r the CO d e p l e t i o n (Equation 19), while f o r H2O formation and CH4 disappearance, the agreement i s less satisfactory. Given however, the r e l a t i v e l y small number of



Figure 3. Global r a t e constant v s . 1/ t for carbon monoxide o x i d a t i o n r e a c t i o n (Eq. 19).

Figure 4. Global rate constant v s . 1/T for water formation r e a c t i o n (Eq. 22).

128


• Flame Β Ο Flame C

•

Fenimore & Jones (1959) + Dryer & Glassman (1973) l_l I

6

1

1

I

7

8

9 1 0 " 1 . 1 1.2 1.3 1.4 1.5-10

l_J

I

I

I

I

3

l/T

[K" ] 1

Figure 5. Global rate constant v s . formation r e a c t i o n ( E q . 22).

l / T f o r water


6.

LEVY E T AL.


129

experimental data p o i n t s , i t should be deemed s a t i s f a c t o r y that they f a l l around the used c o r r e l a t i o n s (which are p l o t t e d w i t h i n t h e i r claimed range of v a l i d i t y ) . Results and D i s c u s s i o n . The burning v e l o c i t y was c a l c u l a t e d by the model described above f o r a number of d i f f e r e n t gas mix tures burning at s t o i c h i o m e t r i c c o n d i t i o n s . Table 3 presents the compositions of the v a r i o u s gas mixtures s t u d i e d . Each mixture i s c h a r a c t e r i z e d by a mixture number MN and a mixture r a t i o R. The mixture r a t i o R i s a volume concentration of index f u e l (CO + H ) r e l a t i v e to the sum of index and l i m i t f u e l s , where the l i m i t f u e l i s CO + CH4 or H + C H 4 . Mixtures having the same value of MN y i e l d the same composition of combustion p r o ducts and a d i a b a t i c flame temperature when burning s t o i c h i o m e t r i c a l l y with a i r . This choice was made i n order to assess whether a d i a b a t i c flame temperature and f i n a l composition were s i g n i f i c a n t f a c t o r s i n e x p l a i n i n g d i f f e r e n c e s of behavior f o r d i f f e r e n t f u e l compositions. A d d i t i o n a l d e t a i l s on the s e l e c t ion of gas mixtures composition can be found i n Reference (9). The a d i a b a t i c flame temperatures Tf were c a l c u l a t e d by the computer code NASA SP 273. λ and c_ were computed by the c o r r e l a t i o n s of Mansouri and Heywood (10;. The c a l c u l a t e d values of S were compared w i t h the e x p e r i mental ones obtained f o r the same mistures by measurements i n a wedge-shaped flame. A p l o t of c a l c u l a t e d versus measured S L i s shown i n Figure 6. I t i s c l e a r that the model p r e d i c t s c o r r e c t l y the change i n S L that i s to be expected from a change i n mixture composition, w h i l e the c a l c u l a t e d values are s a t i s f a c t o r i l y c l o s e to the measured ones. The b e t t e r f i t between c a l c u l a t e d and p r e d i c t e d values here as compared w i t h the r a t e c o r r e l a t i o n i n Figures 3, 4, and 5 a l s o r e f l e c t s the b e t t e r f i t near s t o i c h i o m e t r i c c o n d i t i o n s and at higher flame temperature (11). 2

2

L

Conclusions Measurements of temperature and c o n c e n t r a t i o n i n C O - H 2 - C H 4 (or n a t u r a l gas) flames were c a r r i e d out. Rate p r o f i l e s were developed f o r two excess a i r and two s l i g h t l y f u e l - r i c h flames as a f u n c t i o n of temperature. S u b s t i t u t i o n of n a t u r a l gas f o r methane does not b r i n g about a marked change i n the o v e r a l l r e a c t i v i t y of these systems. A p p l i c a t i o n of a modified theory a n a l y s i s to these m u l t i p l e - f u e l flame mixtures allows one to s a t i s f a c t o r i l y c o r r e l a t e c a l c u l a t e d values of the burning v e l o c i t y with measured v a l u e s .


130


TABLE 3.

Mixture Number MN

Mixture Ratio R

COMPOSITION OF THE GAS MIXTURES STUDIED (mol f r a c t i o n s )

CO

H

2

CH4

C0

2

0

2

N

2

1

1.0 0.50 0.0

0.590 0.566 0.542

0.295 0.147 0.0

0.0 0.096 0.193

0.115 0.151 0.187

0.0 0.039 0.078

0.0 0.0 0.0

2

1.0 0.50 0.0

0.295 0.147 0.0

0.590 0.373 0.156

0.0 0.205 0.410

0.115 0.191 0.268

0.0 0.083 0.166

0.0 0.0 0.0

3

1.0 0.50 0.0

0.442 0.360 0.277

0.442 0.221 0.0

0.0 0.171 0.342

0.115 0.179 0.243

0.0 0.069 0.138

0.0 0.0 0.0

4

1.0 0.67 0.50 0.33 0.0

0.257 0.249 0.245 0.240 0.232

0.128 0.085 0.064 0.043 0.0

0.0 0.028 0.042 0.055 0.083

0.115 0.131 0.140 0.148 0.164

0.0 0.0 0.0 0.0 0.0

0.500 0.507 0.511 0.514 0.521

5

1.0 0.50 0.0

0.128 0.064 0.0

0.257 0.162 0.067

0.0 0.087 0.173

0.115 0.167 0.218

0.0 0.0 0.0

0.500 0.521 0.541

6

1.0 0.50 0.33

0.192 0.155 0.148

0.192 0.096 0.064

0.0 0.073 0.097

0.115 0.158 0.173

0.0 0.0 0.0

0.500 0.518 0.523

(CO + H ) R =/ "(CO + H ) + ([CO or H ] + C H 4 ) 2

2


6.

LEVY E T A L .


Figure 6. C o r r e l a t i o n of c a l c u l a t e d v s . measured burning v e l o c i t i e s .


131


132 Acknowledgment This of Energy debted to Equation

paper i s based on work conducted under U . S . Department Contract No. DE-AC22-75ET10653. The authors are i n Dr. John R. Overley for working out the expression for 2.

Literature Cited 1.

Dryer, F. L . , and Glassman, I., 14th Symp. (Int.) on Combust. 987 (1973). 2. Howard, J. B., Williams, G. C., and Fine, D. H., 14th Symp. (int.) on Combust., 975 (1973). 3. Fristrom, R. Μ., and Westenberg, Α. Α., Flame Structure, McGraw-Hill (1965). 4. Semenov, Ν. N., NACA TM 1026 (1942). 5. Evans, M. W., Chem. Reviews, 51, 363 (1952). 6. Fenimore, C. P., and Jones, G. W., J. Phys. Chem., 63, 1834 (1959). 7. Westbrook, C. Κ., and Dryer, F. L . , The Combust. Inst. CSS 1981 Spring Meeting; UCRL-84943 Preprint. 8. Levy, Α., Overley, J. R., and Merryman, E. L . , Battelle Topical Report, Contract No. (ERDA) Ε(49-18)-2406, July 26, 1977. 9. Ball, D. Α., Putnam, Α. Α., Radharkrishman, E . , and Levy, Α., Battelle Topical Report, Contract No. (ERDA) E(49-18)-2406, July 26, 1977. 10. Mansouri, S. Η., and Heywood, J. B., Combust. Sci. Technol., 23, 251 (1980). 11. Westbrook, C. Κ., and Dryer, F. L . , Combust. Sci. Technol., 27, 31 (1981). RECEIVED October 25, 1982


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