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Some considerations of the lean flammability limits of mixtures involving hydrogen
lnt. J. Hydrogen Energx,
0360-3199/85 $3(10 + o(l(I Pergamon Press Ltd. (~) 1985 International Association for Hydrogen E n e r g )
Vol. 10. No. 1, pp. 117-123, 1985.
Printed in G r e a t B r i t a i n
SOME C O N S I D E R A T I O N S OF THE LEAN FLAMMABILITY LIMITS OF MIXTURES INVOLVING H Y D R O G E N G. A. KARIM, I. WIERZBA and S. BOON Department of Mechanical Engineering, The University of Calgary, Calgary. Alberta. Canada T2N 1N4 (Received 6 August 1984) Abstract---Consistent data regarding the flammability limits of homogeneous mixtures of hydrogen gas in air at atmospheric pressure are presented for various temperatures extending down to - 130 °C. The retardation of the combustion of lean homogeneous mixtures of hydrogen and air due to the homogeneous mixing of inerts such as CO2, N2 and He is also considered. Moreover, the enhancement of the flammability limits of fuel mixtures involving CO, CI-L, C3H8 and C;H4 in air due to the presence of some hydrogen is also presented. Guidelines are then suggested for predicting the role of the presence of hydrogen in the fuel-air mixtures over a wide range of concentrations and temperatures. INTRODUCTION The lean flammability limit is a well-recognized effective criterion for determining, under any set of operating conditions, the minimum fuel concentration needed in air that can allow the propagation of a flame from an adequate ignition source throughout the homogeneous mixture. The question of the flammability limits of fuel mixtures and its prediction from a knowledge of the fuel mixture composition needs to be carefully assessed in terms of the relevant accuracy and limitations, particularly when substantial concentrations of inerts are present. Reference to the literature indicates that although there have been some excellent reviews of the flammability limits literature, such as those clue to Coward and Jones (1952) and Zabetakis (1965), there are still some nagging uncertainties that need to be considered when using such information, particularly in relation to hydrogen and its role when present in a fuel mixture. There is, for example, a very wide range of apparatus and procedures employed over the years by numerous workers in obtaining such data. It soon becomes obvious that there is a need for consistent data to be obtained carefully in one apparatus in the presence of a wide range of diluents and fuels, particularly those encountered as by-products of various industrial processes. The gaseous fuel systems considered in the investigation with hydrogen were those involving methane, carbon monoxide, propane and ethylene; while the inerts employed were nitrogen, carbon dioxide and helium. Upward flame propagation representing in practice the most hazardous situation was used throughout. The temperature range considered extended on occasions from ambient temperature down to - 1 2 0 °C. APPARATUS AND EXPERIMENTAL PROCEDURE
by the U.S. Bureau of Mines in their flammability limits work. A stainless steel tube of 5 cm diameter of clean smooth surface was employed. The flame tube was approximately one-metre long with adequate repeatable spark ignition at the bottom, as shown in Fig. 1. The flame tube was normally closed at the top but open at the bottom at the instant of ignition and subsequent upward flame propagation at essentially atmospheric pressure throughout. Detection of successful flame arrival at the top of the tube was made through provision of a set of rapid response thermocouples located in the central portion of the tube and their output monitored on both an oscilloscope and a multi-voltmeter chart recorder.
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The apparatus developed for the flammability limit tests was in general similar to that developed and used
~6
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Fig. 1. Schematic diagram of the apparatus. 117
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G. A. KARIM, I. WIERZBA AND S. BOON
118
The test mixture was prepared in a mixing chamber on partial pressure basis of the components and introduced gently into the evacuated flame tube. Provision was made for four fuel and inert gas lines in addition to that of the air. Great care was taken throughout to ensure very precise determination of the concentrations of the components of the test mixture. It was considered essential that the entire flame tube temperature should be maintained as uniform as possible even under very low temperature conditions. Temperature variation along the length of the flame tube, sometimes suspected to have been present in reported information in the literature rendering such data both uncertain and misleading, was eliminated in the present setup. In this investigation, when needed, the cooling of the mixture was provided through elaborate controlled jacketing of the whole tube with boiling liquid nitrogen. This was done in such a way that the temperature along the entire length of the tube was to within better than -+0.5°C even at temperatures as low as - 130°C. T H E D E T E R M I N A T I O N OF T H E L E A N F L A M M A B I L I T Y LIMIT The definition of the flammability limit in the relevant literature is strictly speaking insufficiently precise. Our work emphasized the need to consider the limit, under any set of operating conditions, on a probabilistic basis. When tests for the flammability limit determination were repeated a large number of times, a range of fuel concentrations can be obtained where the probability of flame propagation is seen distinctly to be somewhere between 100 and 0%. This is shown in Fig. 2, where every point represents the result of a very large number of tests, typically 25 times, repeated under otherwise the same operating conditions. Accordingly, it can be suggested that the zero-probability propagation figure needs to be quoted for safety considerations representing no apparent flame propagation irrespective of how many trials are attempted. For the consideration of
adequate fuel utilization by combustion, the 100% probability of flame propagation needs to be quoted. This probabilistic view of the limit values is more realistic and appears to be fundamental in nature, rather than merely a reflection of the inevitable randomness associated with the measurement of the values of the experimental parameters. The zero propagation limits obtained in the described apparatus for hydrogen at initial temperature To = 25°C was 4.13%-by volume. For comparison the corresponding value of the lean limit selected by Coward and Jones [2] from values obtained by different authors using different apparatus was 4.0%-by volume. The influence of ambient temperature on the limit was established using the apparatus and is shown in Fig. 3. The presence of diluents in limiting mixtures affects primarily the effective thermodynamic and transport properties of the mixture, and hence the mode of heat flow from the flame. The reaction kinetics can also be affected, though to a much lesser extent because of the low flame temperatures involved. The diluents employed in this investigation were nitrogen, carbon dioxide and helium having widely different properties. Figure 4 shows the extent of changes of the lean limit for hydrogen with the addition of these diluents in turn. It can be seen, on the whole, that the addition of a diluent tends to increase the limit almost linearly. Expectedly, hydrogen appears very tolerant to diluent addition allowing for example, up to 15 times its volume in nitrogen to be added without loss of combustion. Moreover, the addition of CO,. is more effective than nitrogen in impeding combustion. However, the relative role of helium addition is known to be significantly different depending on the fuel involved, Boon (1982). For hydrogen, the order of effectiveness in reducing the lean limit with these three diluents is helium followed by carbon dioxide and then by nitrogen. The lean upward flame propagation limit was established for a wide range of combinations of gaseous fuel mixtures with hydrogen involving CH4, C3Hs, CzH4 and CO shown in Fig. 5 and Tables 1 and 2. Table 3 lists
H z ~ Qir
8O
•
g ,o
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o ~.
~
;iI/! I_.__
4
42
.ft ~'°
l L 4.61
I Ii ko
I H z in m i x t u r e
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44
i-
7 !
I
5.4
II
.5.6
Ip ~,.o
ko
I
58
62
I
64
( */= b y v 0 L u m e )
Fig. 2. Percentage of trials supporting a flame propagation for a range of hydrogen concentrations and initial mixture temperatures.
119
LEAN FLAMMABILITY LIMITS OF HYDROGEN MIXTURES 14r
I0-
>
7
"~
6
,,\
E
12--m
~
"-
5 .~
TO= 25%
tC
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160
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180
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200
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220
240
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260
I
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280
7
300
I n i t i o l t e m p e r o t u r e of f u e l - oir m i x t u r e ( TO ( K )
E~
Fig. 3. The lean flammability limit of hydrogen in air for a range of initial temperatures.
E
the lean limits of mixtures of hydrogen and methane containing various amounts of carbon dioxide and nitrogen.
0
M E T H O D S O F P R E D I C T I O N OF T H E LEAN LIMIT OF F U E L M I X T U R E S A commonly accepted view is that flames fail to propagate when combustion temperatures become too low for reactions to maintain the burning velocity and overcome the dissipation processes. It has also been suggested in the open literature that the flammability limit can be reasonably judged to be associated with a mixture that yields on combustion a certain constant flame temperature irrespective of the initial mixture temperature or the extent of inert addition. This statement, if accurate, is useful and convenient to use for predicting the extent of changes in flammability limit
8o
E
L
20
30
60
70
Hydrogen in f u e l
40
50
(%)
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2
l
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1
4
L
1
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L
90
BOO
with changes in common operating conditions. However, it soon becomes evident that at best this statement should be used only approximately and certainly not indiscriminately. Our results for hydrogen in air for a range of temperatures are presented in Fig. 6. The flame temperature for limiting hydrogen-air mixtures was remarkably low and changed only little over the whole temperature range. However, it was for hydrogen
0
h
80
Fig. 5. The lean limit of binary fuel mixtures involving hydrogen as a function of the volumetric concentration of the hydrogen in the fuel at 25°C.
= zs°c
70j
O
10
k
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5 6 7 8 9 lO II I n e r t g o s / f l o m r n o b l e gOs ( by volume)
i
12
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13
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15
Fig, 4. The volumetric limiting concentration of hydrogen and a diluent in air as a function of the volumetric ratio of the inert gas to hydrogen.
120
G. A. K A R I M , I. W I E R Z B A A N D S. B O O N Table 1. Lean limits of mixtures of two gaseous fuels (Hz + CH~), (H2 + C3Hs), (H: + CO) at To = 25°C Fuel composition (% by volume) H2
CFL,
23.08 50,00
76-SI2.. 50.00
Tf (K)
Calculated limit (% by volume)
Error (%)
C3H8
CO
Lean limit (% by volume)
---
---
5.00 4.63
1320 1065
5.09 4.71
+ 1.80 + 1.55
28.57 42.85 50.00 62.50 77.78
------
71.43 57.15 50.00 37.50 22.22
------
2.80 3.04 3.10 3.33 3.67
1534 1436 1355 1231 1043
2.73 2.93 3.04 3,26 3,56
-2.5 -3.9 -2.1 -2.4 -3.1
6.24 16.67 24.97 50.00 64.32 83.33
-------
-------
93.76 83.33 75.03 50.00 35.68 16.67
12.03 9.96 8.69 6.40 5.55 4.70
1351 1174 1063 855 775 694
12.01 9,91 8.70 6.35 5.51 4,68
-0.19 -0.50 + 0.07 -0.72 -0.79 -0.57
a r o u n d m e r e l y 640 K as c o m p a r e d to a r o u n d 1585 K f o r methane-air mixtures. T h e a d d i t i o n o f i n e r t s to t h e fuel c h a n g e d t h e f l a m e t e m p e r a t u r e o f t h e limit m i x t u r e d i f f e r e n t l y d e p e n d i n g on the type of the diluent involved. T h e calculated flame t e m p e r a t u r e of limiting mixtures involving diluents was for nitrogen almost invariant f r o m the c o r r e s p o n d i n g v a l u e in air o n l y to w i t h i n 2 - 3 % . Simi-
Table
2.
Lean
larly, w i t h c a r b o n d i o x i d e a d d i t i o n , t h e c a l c u l a t e d flame t e m p e r a t u r e i n c r e a s e d o n l y slightly to a n e x t e n t a little g r e a t e r t h a n t h a t w i t h n i t r o g e n , b u t r e m a i n e d to w i t h i n a f e w p e r c e n t o f t h e c o r r e s p o n d i n g v a l u e w i t h air. H o w e v e r , h e l i u m a d d i t i o n p a r t i c u l a r l y in r e l a t i v e l y large a m o u n t s , c h a n g e d the calculated flame t e m p e r a t u r e of t h e limit m i x t u r e m a r k e d l y to a n e x t e n t t h a t t h e a s s u m p tion of p r o d u c i n g a nearly invariant flame t e m p e r a t u r e
limits of mixtures of three gaseous fuels at To = 25°C (CH4 + CzH4 + C3H8); (CH4 + CzI-h + Hz); (CI-h + C3H8 + H2); (CFL + CO + H2)
Fuel composition (% by volume) TI H
Calculated limit (% by volume)
Error (%)
CtL
C2H4
C3H8
Hz
CO
Lean limit (% by volume)
84.60 40.00 40.00 33.33 33.30 30.00 12.50 10.00
7.70 30.00 20,00 33.33 44.40 30.00 75,00 10,00
---------
7.70 30.00 40.00 33.34 33.30 40.00 12.50 80.00
---------
5.12 4.13 4.25 4.05 3.91 4.05 3.57 4.15
1506 1297 1198 1277 1371 1218 1487 852
5.07 4.20 4.32 4.09 3.98 4.10 3.52 4.12
-0.92 + 1.62 +1.62 + 1.10 + 1.61 + 1.23 - 1.37 -0.70
83.34 50.00 40.00 40.00 33.30 29.40 14.29 10.00
-~ ~ -~ ----
8.33 20.00 20.00 30.00 33.30 19.60 71.42 10.00
8.33 30.00 40.00 30.00 33.40 51.00 14.29 80.00
---------
4.88 4.03 3.98 3.89 3.83 3.97 2.83 3.97
1528 1365 1296 1453 1460 1224 1601 919
4.82 4.05 3.95 3.70 3.57 3.87 2.79 3.94
-1.31 +0.35 -0.80 -4.80 -6.90 -2.54 -1.52 -0.63
85.70 33.33 8.34 3.13
---~
-----
7.15 33.33 83.33 3.13
7.15 33.34 8.33 93.74
5.56 6.07 4.46 12.21
1491 1143 735 1420
5.58 6.03 4.48 12.27
+0.38 -0.74 +0.52 +0.49
121
LEAN FLAMMABILITY LIMITS OF HYDROGEN MIXTURES Table 3. Lean limits of mixture (H: -e CH4) containing various amounts of carbon dioxide and nitrogen Volume percent in mixture (CH4 + H2 + N,, + CO: + air)
CH4
H:
N:
CO2
Lean limit (% by volume)
4.30 4.27 3.97 3.95 3.80 2.37 1.55 0.75
0.75 0.80 1.03 1.07 1.I7 2.37 3.00 3.65
15.80 9.30 10.00 16.15 11.10 13.85 15.80 22.60
---------
20.85 14.37 15.00 21.17 16.07 18.55 20.35 27.00
1390 1387 1340 1339 1313 1084 937 792
14.9 15.8 20.5 21.4 23.6 50.0 65.9 82.9
1408 1395 1350 1340 1315 1065 918 775
2.48 1.00 3.83 3.36 0.77
2.48 3.75 1.37 1.67 3.95
------
11.45 18.00 8.00 10.90 16.87
16.41 22.75 13.20 15.93 21.59
1070 816 1292 1202 798
50.0 78.9 26.3 33.0 83.6
1065 810 1290 1220 775
would have resulted in significant and unacceptable errors. The flame temperature, when mixtures of other fuels are involved with hydrogen, depends on what kind of fuels are involved and their proportions in the mixture. The flame temperature for mixtures involving hydrogen and another fuel changes dramatically due to the very low flame temperature of hydrogen-air mixtures. The calculated flame temperature for different binary fuel mixtures is shown in Fig. 7. The flame temperature does not appear to vary linearly with the extent of the presence of hydrogen in the fuel. However, for a fixed composition of a binary fuel mixture the concept of constant flame temperature can be adequately used for prediction of the lean limit of such a fuel in air with the addition of a diluent, particularly nitrogen. Table 3 shows a comparison of the flame temperature of dif-
Tr (K)
Hydrogen in fuel (% by volume)
(TOm,,
ferent methane-hydrogen mixtures in air and in the presence of diluents. The calculated flame temperature of limiting mixtures involving the diluent N: and CO,, was within 1.7% from the corresponding value in air. Only in the case of a high content of hydrogen in the fuel mixture ( - 8 3 % ) the flame temperature of the mixture involving diluents was about 2.5% higher than its value in air. There is an acute need for predicting reliably the flammability limit of a fuel mixture in air from a knowledge of the corresponding flammability limits of the components making up the fuel mixture. A simple formula, advanced by Le Chatelier, calculates the limits of
1800 F
B t600 k~
,,nap, \.
7oo~
~
650--
° ~ e ~ ° %
1200
\.c Hs+H,
•
• ..........,..~.~.®
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c,
t20
k 140
i t L 160 180 2 0 0 ]nttlaL temperature
i [ t 220 240 260 of ( H 2 * o i r ) m i x t u r e
I 280
L 500
(To(K)
Fig. 0. The calculated adiabatic flame temperature of the lean limit mixture of hydrogen in air for a range of initial mixture temperatures.
600
i 20
, 40
I 60
i 80
, I00
Hydrogen in fuel ( % by volume)
Fig. 7. The calculated adiabatic flame temperature of the lean limit mixtures of binary fuels involving hydrogen for a range of concentrations of hydrogen in the fuel mixture.
122
G. A. KARIM, I. WlERZBA AND S. BOON
any mixture of combustible gases as follows: I=N
1 ~
yi
= E,
where L,, is the overall limit of the fuel mixture by volume. L~ is the corresponding limit of the" individual combustible fuel component, y~ is the volumetric fraction of component 'i' in the fuel mixture. N is the total number of fuel components in the mixture. The accuracy of the formula has been tested carefully for many mixtures containing hydrogen. Measured lean limits and the corresponding calculated limits using Le Chatelier's formula are shown in Tables 1 and 2 for mixtures of two and three combustible gases, respectively. The application of this formula showed remarkably good agreement with experimental values. The error appears surprisingly low ( < 0 . 5 % for mixtures of hydrogen) and probably not much greater than the associated experimental error in measuring the components of the mixture. The above formula can be applied with adequate accuracy to mixtures when its applicability has been proven, but obviously it should not be applied indiscriminately. This approach can also be applied to atmospheres that may not involve pure air. It holds, for example, for mixtures of hydrogen, methane and carbon monoxide, in a wide range of mixtures of air, nitrogen and carbon dioxide except near the point at which the lower and higher limits meet, Karim (1981). The mixture may then be considered to be made up of simpler mixtures that contain the flammable gas and part or all of the inert gas present. Table 4 shows a comparison of measured and pre-
dicted limits for hydrogen-methane mixtures with addition of nitrogen or carbon dioxide in air. Predicted limits were calculated using Le Chatelier's formula in different ways. I n c a s e (1), the diluent was considered to be associated entirely with the methane, i.e. 100 L,, ---y~CH4+diluenQ + YH2 L(CH~ + diluent) LH2
while in case (2) the diluent was considered to be associated entirely with the hydrogen: 100 L,nycH, + y/H2+diluent)" LCH, Z(Hz+diluent) In the third case, it was assumed that the lean flammability limit of the diluent, carbon dioxide or nitrogen. is infinite (Ldiluent= ~o) and therefore: Lrn =
100 yCH4 + YHZ + ydilucnt' LCH, LH2
The only limitation for the breaking up of the complex mixture into simpler mixtures appears to be the accurate determination of the corresponding limits using experimental data such as those in Fig. 4 and those available in the literature. It can be seen, that when nitrogen is 'added' to a fuel mixture slightly better prediction of the lean limits is obtained, based on the assumption LN2 = oo. However, this approach gives slightly worse results when carbon dioxide is added to the fuel mixture which is a reflection of the higher flame extinction ability of carbon dioxide.
Table 4. Comparison of measured and predicted limits for hydrogen-methane mixtures with addition of nitrogen or carbon dioxide in air Percent by volume in mixture (CH4 + Hs + N2 + COs + air)
Calculated limit (% by volume)
CH4
H:
Ns
COs
Measured limit (% by volume)
2.48 1.00 3.83 3.36 0.77
2.48 3.75 1.37 1.67 3.95
------
11.45 18.0 8.0 10.9 16.87
16.41 22.75 13.20 15.93 21.59
--13.93 17.42 --
16.23 22.31 13.13 16.18 20.64
15.57 20.86 12.79 15.63 19.68
4.3 4.27 3.975 3.95 3.8 2.375 1.55 0.75
0.75 0.8 1.025 1.075 1.175 2.375 3.0 3.65
15.8 9.3 10.0 16.15 11.1 13.85 15.8 22.6
-w w ~ -~ ---
20.85 14.37 15.00 21.17 16.07 18.60 20.35 27.00
21.84 14.9 15.49 21.98 16.6 18.73 ---
-14.79 15.44 21.76 16.47 18.56 20.43 26.68
21.53 14.74 15.38 21.54 16.40 18.43 20.16 26.46
Necessary conditions: %CO2 < %N2 %CO~ _ %N2 %CH, 3"4;%~-~4 < 6 . 5 ; ~ < 9.3;--~-~2< 15.3.
(1) CI-L + diluent H2
(2) CH4 Hs + diluent
(3) Ldil~=,~=
LEAN FLAMMABILITY LIMITS OF HYDROGEN MIXTURES CONCLUSIONS (a) The lean flammability limits of fuels such as hydrogen and their mixtures should be considered on probabilistic basis. For safety considerations the zeroprobability figure needs to be quoted, while for fuel utilization by combustion, the 100% probability figure needs to be considered. (b) The concept of constant flame propagation temperature for judging the flammability limit at different initial temperatures and the extent of inert addition should be used only to yield approximate values. The flame temperature when mixtures of fuels are involved depends on the kind of fuels and their proportions in the mixture used. For fixed composition of the fuel mixture the concept of constant flame temperature gives adequate prediction of the lean limit of such a mixture with the addition of nitrogen or carbon dioxide. (c) The lean flammability limits of fuel mixtures containing hydrogen, with the addition of diluents can be predicted accurately on the basis of a knowledge of the
123
corresponding flammability limits of the components under the same conditions using Le Chatelier's formula. Acknowledgement--The financial assistance of Alberta Energy
& Natural Resources, Natural Sciences & Engineering Research Council, Canada, and Imperial Oil Limited is gratefully acknowledged~
REFERENCES 1. S. Boon, M.Sc. Thesis, Department of Mechanical Engineering, The University of Calgary (March 1982). 2. H. R. Coward and G. W. Jones, Limits of flammability of gases and vapours. Bulletin 503. Bureau of Mines (1952). 3. G. A. Karim, S. Boon and I. Wierzba, Some considerations of the lean flammability limit of some gaseous fuels and their mixtures. Proc. Vlhh Int. Syrup. Combustion Processes, Jablonna, Poland (1981). 4. M. G. Zabetakis, Flammability characteristics of combustible gases and vapours. Bulletin 627, Bureau of Mines (1965).
0360-3199/85 $3(10 + o(l(I Pergamon Press Ltd. (~) 1985 International Association for Hydrogen E n e r g )
Vol. 10. No. 1, pp. 117-123, 1985.
Printed in G r e a t B r i t a i n
SOME C O N S I D E R A T I O N S OF THE LEAN FLAMMABILITY LIMITS OF MIXTURES INVOLVING H Y D R O G E N G. A. KARIM, I. WIERZBA and S. BOON Department of Mechanical Engineering, The University of Calgary, Calgary. Alberta. Canada T2N 1N4 (Received 6 August 1984) Abstract---Consistent data regarding the flammability limits of homogeneous mixtures of hydrogen gas in air at atmospheric pressure are presented for various temperatures extending down to - 130 °C. The retardation of the combustion of lean homogeneous mixtures of hydrogen and air due to the homogeneous mixing of inerts such as CO2, N2 and He is also considered. Moreover, the enhancement of the flammability limits of fuel mixtures involving CO, CI-L, C3H8 and C;H4 in air due to the presence of some hydrogen is also presented. Guidelines are then suggested for predicting the role of the presence of hydrogen in the fuel-air mixtures over a wide range of concentrations and temperatures. INTRODUCTION The lean flammability limit is a well-recognized effective criterion for determining, under any set of operating conditions, the minimum fuel concentration needed in air that can allow the propagation of a flame from an adequate ignition source throughout the homogeneous mixture. The question of the flammability limits of fuel mixtures and its prediction from a knowledge of the fuel mixture composition needs to be carefully assessed in terms of the relevant accuracy and limitations, particularly when substantial concentrations of inerts are present. Reference to the literature indicates that although there have been some excellent reviews of the flammability limits literature, such as those clue to Coward and Jones (1952) and Zabetakis (1965), there are still some nagging uncertainties that need to be considered when using such information, particularly in relation to hydrogen and its role when present in a fuel mixture. There is, for example, a very wide range of apparatus and procedures employed over the years by numerous workers in obtaining such data. It soon becomes obvious that there is a need for consistent data to be obtained carefully in one apparatus in the presence of a wide range of diluents and fuels, particularly those encountered as by-products of various industrial processes. The gaseous fuel systems considered in the investigation with hydrogen were those involving methane, carbon monoxide, propane and ethylene; while the inerts employed were nitrogen, carbon dioxide and helium. Upward flame propagation representing in practice the most hazardous situation was used throughout. The temperature range considered extended on occasions from ambient temperature down to - 1 2 0 °C. APPARATUS AND EXPERIMENTAL PROCEDURE
by the U.S. Bureau of Mines in their flammability limits work. A stainless steel tube of 5 cm diameter of clean smooth surface was employed. The flame tube was approximately one-metre long with adequate repeatable spark ignition at the bottom, as shown in Fig. 1. The flame tube was normally closed at the top but open at the bottom at the instant of ignition and subsequent upward flame propagation at essentially atmospheric pressure throughout. Detection of successful flame arrival at the top of the tube was made through provision of a set of rapid response thermocouples located in the central portion of the tube and their output monitored on both an oscilloscope and a multi-voltmeter chart recorder.
j13
/A
6~
X
TO" , ,
• .-
~/I
,J
,c '1
.g8~
• A
..-/1
TClLA /A eA
I FLametube 2 Ignition 3 Mixer 4 Monometer 5 Vocuum 6,7,8 Feed lines 9,10, I I Display units 12 Volve 13 ReLiefdiapbrogm 14 Cooling coils I5 ]nsulotlon 16 Battery 17 Vol't.mete r
The apparatus developed for the flammability limit tests was in general similar to that developed and used
~6
7~3 [!23
Fig. 1. Schematic diagram of the apparatus. 117
'I
I
CB
G. A. KARIM, I. WIERZBA AND S. BOON
118
The test mixture was prepared in a mixing chamber on partial pressure basis of the components and introduced gently into the evacuated flame tube. Provision was made for four fuel and inert gas lines in addition to that of the air. Great care was taken throughout to ensure very precise determination of the concentrations of the components of the test mixture. It was considered essential that the entire flame tube temperature should be maintained as uniform as possible even under very low temperature conditions. Temperature variation along the length of the flame tube, sometimes suspected to have been present in reported information in the literature rendering such data both uncertain and misleading, was eliminated in the present setup. In this investigation, when needed, the cooling of the mixture was provided through elaborate controlled jacketing of the whole tube with boiling liquid nitrogen. This was done in such a way that the temperature along the entire length of the tube was to within better than -+0.5°C even at temperatures as low as - 130°C. T H E D E T E R M I N A T I O N OF T H E L E A N F L A M M A B I L I T Y LIMIT The definition of the flammability limit in the relevant literature is strictly speaking insufficiently precise. Our work emphasized the need to consider the limit, under any set of operating conditions, on a probabilistic basis. When tests for the flammability limit determination were repeated a large number of times, a range of fuel concentrations can be obtained where the probability of flame propagation is seen distinctly to be somewhere between 100 and 0%. This is shown in Fig. 2, where every point represents the result of a very large number of tests, typically 25 times, repeated under otherwise the same operating conditions. Accordingly, it can be suggested that the zero-probability propagation figure needs to be quoted for safety considerations representing no apparent flame propagation irrespective of how many trials are attempted. For the consideration of
adequate fuel utilization by combustion, the 100% probability of flame propagation needs to be quoted. This probabilistic view of the limit values is more realistic and appears to be fundamental in nature, rather than merely a reflection of the inevitable randomness associated with the measurement of the values of the experimental parameters. The zero propagation limits obtained in the described apparatus for hydrogen at initial temperature To = 25°C was 4.13%-by volume. For comparison the corresponding value of the lean limit selected by Coward and Jones [2] from values obtained by different authors using different apparatus was 4.0%-by volume. The influence of ambient temperature on the limit was established using the apparatus and is shown in Fig. 3. The presence of diluents in limiting mixtures affects primarily the effective thermodynamic and transport properties of the mixture, and hence the mode of heat flow from the flame. The reaction kinetics can also be affected, though to a much lesser extent because of the low flame temperatures involved. The diluents employed in this investigation were nitrogen, carbon dioxide and helium having widely different properties. Figure 4 shows the extent of changes of the lean limit for hydrogen with the addition of these diluents in turn. It can be seen, on the whole, that the addition of a diluent tends to increase the limit almost linearly. Expectedly, hydrogen appears very tolerant to diluent addition allowing for example, up to 15 times its volume in nitrogen to be added without loss of combustion. Moreover, the addition of CO,. is more effective than nitrogen in impeding combustion. However, the relative role of helium addition is known to be significantly different depending on the fuel involved, Boon (1982). For hydrogen, the order of effectiveness in reducing the lean limit with these three diluents is helium followed by carbon dioxide and then by nitrogen. The lean upward flame propagation limit was established for a wide range of combinations of gaseous fuel mixtures with hydrogen involving CH4, C3Hs, CzH4 and CO shown in Fig. 5 and Tables 1 and 2. Table 3 lists
H z ~ Qir
8O
•
g ,o
/
o ~.
~
;iI/! I_.__
4
42
.ft ~'°
l L 4.61
I Ii ko
I H z in m i x t u r e
,I
I
J fl ko
44
i-
7 !
I
5.4
II
.5.6
Ip ~,.o
ko
I
58
62
I
64
( */= b y v 0 L u m e )
Fig. 2. Percentage of trials supporting a flame propagation for a range of hydrogen concentrations and initial mixture temperatures.
119
LEAN FLAMMABILITY LIMITS OF HYDROGEN MIXTURES 14r
I0-
>
7
"~
6
,,\
E
12--m
~
"-
5 .~
TO= 25%
tC
4 -3
H2
§ z-J
I --
t40
L
160
h
180
I
200
L
I
220
240
I
260
I
I
280
7
300
I n i t i o l t e m p e r o t u r e of f u e l - oir m i x t u r e ( TO ( K )
E~
Fig. 3. The lean flammability limit of hydrogen in air for a range of initial temperatures.
E
the lean limits of mixtures of hydrogen and methane containing various amounts of carbon dioxide and nitrogen.
0
M E T H O D S O F P R E D I C T I O N OF T H E LEAN LIMIT OF F U E L M I X T U R E S A commonly accepted view is that flames fail to propagate when combustion temperatures become too low for reactions to maintain the burning velocity and overcome the dissipation processes. It has also been suggested in the open literature that the flammability limit can be reasonably judged to be associated with a mixture that yields on combustion a certain constant flame temperature irrespective of the initial mixture temperature or the extent of inert addition. This statement, if accurate, is useful and convenient to use for predicting the extent of changes in flammability limit
8o
E
L
20
30
60
70
Hydrogen in f u e l
40
50
(%)
I
I
2
l
3
1
4
L
1
/ x
L
90
BOO
with changes in common operating conditions. However, it soon becomes evident that at best this statement should be used only approximately and certainly not indiscriminately. Our results for hydrogen in air for a range of temperatures are presented in Fig. 6. The flame temperature for limiting hydrogen-air mixtures was remarkably low and changed only little over the whole temperature range. However, it was for hydrogen
0
h
80
Fig. 5. The lean limit of binary fuel mixtures involving hydrogen as a function of the volumetric concentration of the hydrogen in the fuel at 25°C.
= zs°c
70j
O
10
k
i
I
I
5 6 7 8 9 lO II I n e r t g o s / f l o m r n o b l e gOs ( by volume)
i
12
I
13
I
P4
L
15
Fig, 4. The volumetric limiting concentration of hydrogen and a diluent in air as a function of the volumetric ratio of the inert gas to hydrogen.
120
G. A. K A R I M , I. W I E R Z B A A N D S. B O O N Table 1. Lean limits of mixtures of two gaseous fuels (Hz + CH~), (H2 + C3Hs), (H: + CO) at To = 25°C Fuel composition (% by volume) H2
CFL,
23.08 50,00
76-SI2.. 50.00
Tf (K)
Calculated limit (% by volume)
Error (%)
C3H8
CO
Lean limit (% by volume)
---
---
5.00 4.63
1320 1065
5.09 4.71
+ 1.80 + 1.55
28.57 42.85 50.00 62.50 77.78
------
71.43 57.15 50.00 37.50 22.22
------
2.80 3.04 3.10 3.33 3.67
1534 1436 1355 1231 1043
2.73 2.93 3.04 3,26 3,56
-2.5 -3.9 -2.1 -2.4 -3.1
6.24 16.67 24.97 50.00 64.32 83.33
-------
-------
93.76 83.33 75.03 50.00 35.68 16.67
12.03 9.96 8.69 6.40 5.55 4.70
1351 1174 1063 855 775 694
12.01 9,91 8.70 6.35 5.51 4,68
-0.19 -0.50 + 0.07 -0.72 -0.79 -0.57
a r o u n d m e r e l y 640 K as c o m p a r e d to a r o u n d 1585 K f o r methane-air mixtures. T h e a d d i t i o n o f i n e r t s to t h e fuel c h a n g e d t h e f l a m e t e m p e r a t u r e o f t h e limit m i x t u r e d i f f e r e n t l y d e p e n d i n g on the type of the diluent involved. T h e calculated flame t e m p e r a t u r e of limiting mixtures involving diluents was for nitrogen almost invariant f r o m the c o r r e s p o n d i n g v a l u e in air o n l y to w i t h i n 2 - 3 % . Simi-
Table
2.
Lean
larly, w i t h c a r b o n d i o x i d e a d d i t i o n , t h e c a l c u l a t e d flame t e m p e r a t u r e i n c r e a s e d o n l y slightly to a n e x t e n t a little g r e a t e r t h a n t h a t w i t h n i t r o g e n , b u t r e m a i n e d to w i t h i n a f e w p e r c e n t o f t h e c o r r e s p o n d i n g v a l u e w i t h air. H o w e v e r , h e l i u m a d d i t i o n p a r t i c u l a r l y in r e l a t i v e l y large a m o u n t s , c h a n g e d the calculated flame t e m p e r a t u r e of t h e limit m i x t u r e m a r k e d l y to a n e x t e n t t h a t t h e a s s u m p tion of p r o d u c i n g a nearly invariant flame t e m p e r a t u r e
limits of mixtures of three gaseous fuels at To = 25°C (CH4 + CzH4 + C3H8); (CH4 + CzI-h + Hz); (CI-h + C3H8 + H2); (CFL + CO + H2)
Fuel composition (% by volume) TI H
Calculated limit (% by volume)
Error (%)
CtL
C2H4
C3H8
Hz
CO
Lean limit (% by volume)
84.60 40.00 40.00 33.33 33.30 30.00 12.50 10.00
7.70 30.00 20,00 33.33 44.40 30.00 75,00 10,00
---------
7.70 30.00 40.00 33.34 33.30 40.00 12.50 80.00
---------
5.12 4.13 4.25 4.05 3.91 4.05 3.57 4.15
1506 1297 1198 1277 1371 1218 1487 852
5.07 4.20 4.32 4.09 3.98 4.10 3.52 4.12
-0.92 + 1.62 +1.62 + 1.10 + 1.61 + 1.23 - 1.37 -0.70
83.34 50.00 40.00 40.00 33.30 29.40 14.29 10.00
-~ ~ -~ ----
8.33 20.00 20.00 30.00 33.30 19.60 71.42 10.00
8.33 30.00 40.00 30.00 33.40 51.00 14.29 80.00
---------
4.88 4.03 3.98 3.89 3.83 3.97 2.83 3.97
1528 1365 1296 1453 1460 1224 1601 919
4.82 4.05 3.95 3.70 3.57 3.87 2.79 3.94
-1.31 +0.35 -0.80 -4.80 -6.90 -2.54 -1.52 -0.63
85.70 33.33 8.34 3.13
---~
-----
7.15 33.33 83.33 3.13
7.15 33.34 8.33 93.74
5.56 6.07 4.46 12.21
1491 1143 735 1420
5.58 6.03 4.48 12.27
+0.38 -0.74 +0.52 +0.49
121
LEAN FLAMMABILITY LIMITS OF HYDROGEN MIXTURES Table 3. Lean limits of mixture (H: -e CH4) containing various amounts of carbon dioxide and nitrogen Volume percent in mixture (CH4 + H2 + N,, + CO: + air)
CH4
H:
N:
CO2
Lean limit (% by volume)
4.30 4.27 3.97 3.95 3.80 2.37 1.55 0.75
0.75 0.80 1.03 1.07 1.I7 2.37 3.00 3.65
15.80 9.30 10.00 16.15 11.10 13.85 15.80 22.60
---------
20.85 14.37 15.00 21.17 16.07 18.55 20.35 27.00
1390 1387 1340 1339 1313 1084 937 792
14.9 15.8 20.5 21.4 23.6 50.0 65.9 82.9
1408 1395 1350 1340 1315 1065 918 775
2.48 1.00 3.83 3.36 0.77
2.48 3.75 1.37 1.67 3.95
------
11.45 18.00 8.00 10.90 16.87
16.41 22.75 13.20 15.93 21.59
1070 816 1292 1202 798
50.0 78.9 26.3 33.0 83.6
1065 810 1290 1220 775
would have resulted in significant and unacceptable errors. The flame temperature, when mixtures of other fuels are involved with hydrogen, depends on what kind of fuels are involved and their proportions in the mixture. The flame temperature for mixtures involving hydrogen and another fuel changes dramatically due to the very low flame temperature of hydrogen-air mixtures. The calculated flame temperature for different binary fuel mixtures is shown in Fig. 7. The flame temperature does not appear to vary linearly with the extent of the presence of hydrogen in the fuel. However, for a fixed composition of a binary fuel mixture the concept of constant flame temperature can be adequately used for prediction of the lean limit of such a fuel in air with the addition of a diluent, particularly nitrogen. Table 3 shows a comparison of the flame temperature of dif-
Tr (K)
Hydrogen in fuel (% by volume)
(TOm,,
ferent methane-hydrogen mixtures in air and in the presence of diluents. The calculated flame temperature of limiting mixtures involving the diluent N: and CO,, was within 1.7% from the corresponding value in air. Only in the case of a high content of hydrogen in the fuel mixture ( - 8 3 % ) the flame temperature of the mixture involving diluents was about 2.5% higher than its value in air. There is an acute need for predicting reliably the flammability limit of a fuel mixture in air from a knowledge of the corresponding flammability limits of the components making up the fuel mixture. A simple formula, advanced by Le Chatelier, calculates the limits of
1800 F
B t600 k~
,,nap, \.
7oo~
~
650--
° ~ e ~ ° %
1200
\.c Hs+H,
•
• ..........,..~.~.®
¥ -~ 6o0 N
c,
t20
k 140
i t L 160 180 2 0 0 ]nttlaL temperature
i [ t 220 240 260 of ( H 2 * o i r ) m i x t u r e
I 280
L 500
(To(K)
Fig. 0. The calculated adiabatic flame temperature of the lean limit mixture of hydrogen in air for a range of initial mixture temperatures.
600
i 20
, 40
I 60
i 80
, I00
Hydrogen in fuel ( % by volume)
Fig. 7. The calculated adiabatic flame temperature of the lean limit mixtures of binary fuels involving hydrogen for a range of concentrations of hydrogen in the fuel mixture.
122
G. A. KARIM, I. WlERZBA AND S. BOON
any mixture of combustible gases as follows: I=N
1 ~
yi
= E,
where L,, is the overall limit of the fuel mixture by volume. L~ is the corresponding limit of the" individual combustible fuel component, y~ is the volumetric fraction of component 'i' in the fuel mixture. N is the total number of fuel components in the mixture. The accuracy of the formula has been tested carefully for many mixtures containing hydrogen. Measured lean limits and the corresponding calculated limits using Le Chatelier's formula are shown in Tables 1 and 2 for mixtures of two and three combustible gases, respectively. The application of this formula showed remarkably good agreement with experimental values. The error appears surprisingly low ( < 0 . 5 % for mixtures of hydrogen) and probably not much greater than the associated experimental error in measuring the components of the mixture. The above formula can be applied with adequate accuracy to mixtures when its applicability has been proven, but obviously it should not be applied indiscriminately. This approach can also be applied to atmospheres that may not involve pure air. It holds, for example, for mixtures of hydrogen, methane and carbon monoxide, in a wide range of mixtures of air, nitrogen and carbon dioxide except near the point at which the lower and higher limits meet, Karim (1981). The mixture may then be considered to be made up of simpler mixtures that contain the flammable gas and part or all of the inert gas present. Table 4 shows a comparison of measured and pre-
dicted limits for hydrogen-methane mixtures with addition of nitrogen or carbon dioxide in air. Predicted limits were calculated using Le Chatelier's formula in different ways. I n c a s e (1), the diluent was considered to be associated entirely with the methane, i.e. 100 L,, ---y~CH4+diluenQ + YH2 L(CH~ + diluent) LH2
while in case (2) the diluent was considered to be associated entirely with the hydrogen: 100 L,nycH, + y/H2+diluent)" LCH, Z(Hz+diluent) In the third case, it was assumed that the lean flammability limit of the diluent, carbon dioxide or nitrogen. is infinite (Ldiluent= ~o) and therefore: Lrn =
100 yCH4 + YHZ + ydilucnt' LCH, LH2
The only limitation for the breaking up of the complex mixture into simpler mixtures appears to be the accurate determination of the corresponding limits using experimental data such as those in Fig. 4 and those available in the literature. It can be seen, that when nitrogen is 'added' to a fuel mixture slightly better prediction of the lean limits is obtained, based on the assumption LN2 = oo. However, this approach gives slightly worse results when carbon dioxide is added to the fuel mixture which is a reflection of the higher flame extinction ability of carbon dioxide.
Table 4. Comparison of measured and predicted limits for hydrogen-methane mixtures with addition of nitrogen or carbon dioxide in air Percent by volume in mixture (CH4 + Hs + N2 + COs + air)
Calculated limit (% by volume)
CH4
H:
Ns
COs
Measured limit (% by volume)
2.48 1.00 3.83 3.36 0.77
2.48 3.75 1.37 1.67 3.95
------
11.45 18.0 8.0 10.9 16.87
16.41 22.75 13.20 15.93 21.59
--13.93 17.42 --
16.23 22.31 13.13 16.18 20.64
15.57 20.86 12.79 15.63 19.68
4.3 4.27 3.975 3.95 3.8 2.375 1.55 0.75
0.75 0.8 1.025 1.075 1.175 2.375 3.0 3.65
15.8 9.3 10.0 16.15 11.1 13.85 15.8 22.6
-w w ~ -~ ---
20.85 14.37 15.00 21.17 16.07 18.60 20.35 27.00
21.84 14.9 15.49 21.98 16.6 18.73 ---
-14.79 15.44 21.76 16.47 18.56 20.43 26.68
21.53 14.74 15.38 21.54 16.40 18.43 20.16 26.46
Necessary conditions: %CO2 < %N2 %CO~ _ %N2 %CH, 3"4;%~-~4 < 6 . 5 ; ~ < 9.3;--~-~2< 15.3.
(1) CI-L + diluent H2
(2) CH4 Hs + diluent
(3) Ldil~=,~=
LEAN FLAMMABILITY LIMITS OF HYDROGEN MIXTURES CONCLUSIONS (a) The lean flammability limits of fuels such as hydrogen and their mixtures should be considered on probabilistic basis. For safety considerations the zeroprobability figure needs to be quoted, while for fuel utilization by combustion, the 100% probability figure needs to be considered. (b) The concept of constant flame propagation temperature for judging the flammability limit at different initial temperatures and the extent of inert addition should be used only to yield approximate values. The flame temperature when mixtures of fuels are involved depends on the kind of fuels and their proportions in the mixture used. For fixed composition of the fuel mixture the concept of constant flame temperature gives adequate prediction of the lean limit of such a mixture with the addition of nitrogen or carbon dioxide. (c) The lean flammability limits of fuel mixtures containing hydrogen, with the addition of diluents can be predicted accurately on the basis of a knowledge of the
123
corresponding flammability limits of the components under the same conditions using Le Chatelier's formula. Acknowledgement--The financial assistance of Alberta Energy
& Natural Resources, Natural Sciences & Engineering Research Council, Canada, and Imperial Oil Limited is gratefully acknowledged~
REFERENCES 1. S. Boon, M.Sc. Thesis, Department of Mechanical Engineering, The University of Calgary (March 1982). 2. H. R. Coward and G. W. Jones, Limits of flammability of gases and vapours. Bulletin 503. Bureau of Mines (1952). 3. G. A. Karim, S. Boon and I. Wierzba, Some considerations of the lean flammability limit of some gaseous fuels and their mixtures. Proc. Vlhh Int. Syrup. Combustion Processes, Jablonna, Poland (1981). 4. M. G. Zabetakis, Flammability characteristics of combustible gases and vapours. Bulletin 627, Bureau of Mines (1965).