- Email: [email protected]
The effect of inert dust on the combustion limits of lycopodium spores dispersed in air
March 1962
Letters to the Editors
things as c o m p u t a t i o n a l convenience or o n e ' s e x p e r i m e n t a l c a p a b i l i t y to determine or neglect certain of the K s with satisfactory precision. W . E. KASKAN General Electric Research Laboratory, Schenectady, N e w Y o r k and G. L. SCHOTT University of California, Los Alamos Scientific Laboratory, Los Alamos, N e w Mexico (Received N o v e m b e r 1961)
References chem. Phys. 1960, 32, 710 .2 WHITE, D. R. Physics of Fluids, 1961, 4, 465 :~ BOLEWlCZ, E. M., JAMES, C. G. and SUGI)EN, T. M. Proc. Roy. Soc. A, 1956, 235~ 89 .l KASKA,'~,W. E. Combustion 6~ Flame, 1958, 2, 229 .~ KASKAN,XV. E. Combustion 4_-,Flame, 1958, 2, 286; 1959, 3, 49 6 HIRSCHFZLDER, J. O., CORTISS, C. F. and BIRD, R. B. Molecular Theory of Gases and Liquids, p 496. Wiley : New York, 1954 7 HIRSCHFELDER, J. O., CURTISS, C. F. and BIRD, R. ]3. Molecular Theory of Gases and Liquids, pp 748-749. Wiley: New York, 1954 s 13RINKLI.:Y,S. R., Jr. J. chem. Phys. 1946, 14, 563 t SCI-IOTT, G. L. J.
T h e Effect of I n e r t Dust on the C o m b u s t i o n Limits of L y c o p o d i u m Spores Dispersed in Air IN a s t u d y of the combustion of dust falling through a vertical tube, some interesting results concerning explosion limits were recently
~-- ~ r--x~
5
75
o b t a i n e d ' . The work, although carried out with l y c o p o d i u m spores, is p a r t of a more extensive investigation into the m e c h a n i s m of coal-dust explosions. The a p p a r a t u s is s o m e w h a t similar to t h a t used p r e v i o u s l y b y E. JoNEs and A. G. WHITE z, b y H. M. CASSEL et al3, b y R. H. ESSENHIGH a n d D. W . WOODHEAD4 a n d b y K. C. BROWN and R. H. ESSENHIGH ~. Dust was fed from a h o p p e r through an electromagnetically v i b r a t e d sieve over the whole cross section of a 7 c m d i a m e t e r vertical P e r s p e x tube, 4 m in length, open at the lower end. Various m e t h o d s of feeding the dust were tried; the best results were obtained with a sieve tightly stretched on a circular frame. The dust concentration was measured b y collecting the dust in a given length of tube, a n d is w h a t J . H. BURGOYNE6 calls the 'volume concentration'. The short term variation of the concentration with time was measured with a photoelectric extinction method, and the coefficient of variation was a b o u t 12 per cent. Since the particles sediment with a velocity relative to the air, if there were an airflow, the resultant velocity profile would be expected to introduce a concentration profile. F o r this reason, no airflow was used. The dust cloud was ignited b y means of an electrically heated coil, 36 cm from the open end. F l a m e was judged to have p r o p a g a t e d when it travelled more than 1 m u p the tube from the igniter.
5% p r o b a b i l i t y of p r o p a g a t i o n of f l a m e (with 95% c o n f i d e n c e hmits 50*/,
~-~,-- = 95*/,
c
0-64 ~o 3
2
C
I
10
50 100 Lycopodium concentration: g / m 3 air
30
I000
Figure I. Effect of limestone dust on the combustion limits of lycopodium in air
76
Letters to the Editors
A series of experiments to determine the effect of an inert dust on the combustion limits of lycopodium was made. The inert dust was limestone, ground so that 57 per cent by weight passed a 240 B.S. sieve (64 microns) and had a specific surface of 0.340 m 2/ g. The results are shown in Figure I, where the ratio of inert dust to lycopodium necessary to give a 5, 50 and 95 per cent probability of propagation of flame is plotted against lycopodium concentration in g / m '~. The curve shows 10 ~g8
/
Safety in Mines Research Establishment, Ministry o[ Power, Harpur Hill, Buxton, Derbyshire
"X \
I
-
"/
2
I wish to thank Mr B. W. Quince [or carrying out the statistical analysis o[ the results. The illustrations are Crown Copyright, and are reproduced by permission. F. POWELL
/
.oL
4:1 mixtures, in which propagations and nonpropagations occurred almost equally over a wide range of concentrations, making analysis impracticable. H. F. COWARD and F. J. HARTWELL7 who investigated the effect of addition of carbon dioxide on the limits of inflammability of methane in air, obtained a similar curve. For comparison, their results have been redrawn in Figure 2.
x
/ s
Vol. 6
(Received January 1962)
X
\ References
2
X X %
/
t ! I×~
1
I
E
I X,
40 60 80 100 Methane concentration: g/m3air ( CO2 free )
Figure 2. Limits of inflammability of methane in atmosphere containing carbon dioxide the change in the lower limit of inflammability with added inert dust and, what is perhaps more interesting, clearly shows definite upper limits. It appears that small changes in the dust mixture at ratios of about 4:1 produce large changes in the probability of propagation. This would account for the results obtained with nominally
J Safety in Mines Research E s t a b l i s h m e n t . Thirtyeighth Annual Report on Safety in Mines Research 1959, p 16. Her Majesty's Stationery Office: London, 1960 '-' JoNEs, E. and WHITE, A. G. Dust in Industry (Conference Leeds, September 1948) p 129. Society of Chemical I n d u s t r y : London, 1948 3 CASSEL, H. M. et al. Third Symposium (International) on Combustion and Flame, and Explosion Phenomena (Wisconsin 1948), p 185. Williams and \Vilkins: Baltimore, 1949 .l ESSENHIGH, R. H. and WOODHEAD, D. W . Combustion ~ Flame, 1958, 2, 365 ." BROWN, K. C. and ESSENHIGH, R. H. Saf.-i.-Mines Res. Rep. No. 165, 1959 s BURGOYNE, J. H. Chem. Engng Progr. 1957, 53, 121-M = COWARD, H. F. a n d HARTWELL, F. J. Saf.-i.-Mines Res. B d Pap. No. 19, 1926
Letters to the Editors
things as c o m p u t a t i o n a l convenience or o n e ' s e x p e r i m e n t a l c a p a b i l i t y to determine or neglect certain of the K s with satisfactory precision. W . E. KASKAN General Electric Research Laboratory, Schenectady, N e w Y o r k and G. L. SCHOTT University of California, Los Alamos Scientific Laboratory, Los Alamos, N e w Mexico (Received N o v e m b e r 1961)
References chem. Phys. 1960, 32, 710 .2 WHITE, D. R. Physics of Fluids, 1961, 4, 465 :~ BOLEWlCZ, E. M., JAMES, C. G. and SUGI)EN, T. M. Proc. Roy. Soc. A, 1956, 235~ 89 .l KASKA,'~,W. E. Combustion 6~ Flame, 1958, 2, 229 .~ KASKAN,XV. E. Combustion 4_-,Flame, 1958, 2, 286; 1959, 3, 49 6 HIRSCHFZLDER, J. O., CORTISS, C. F. and BIRD, R. B. Molecular Theory of Gases and Liquids, p 496. Wiley : New York, 1954 7 HIRSCHFELDER, J. O., CURTISS, C. F. and BIRD, R. ]3. Molecular Theory of Gases and Liquids, pp 748-749. Wiley: New York, 1954 s 13RINKLI.:Y,S. R., Jr. J. chem. Phys. 1946, 14, 563 t SCI-IOTT, G. L. J.
T h e Effect of I n e r t Dust on the C o m b u s t i o n Limits of L y c o p o d i u m Spores Dispersed in Air IN a s t u d y of the combustion of dust falling through a vertical tube, some interesting results concerning explosion limits were recently
~-- ~ r--x~
5
75
o b t a i n e d ' . The work, although carried out with l y c o p o d i u m spores, is p a r t of a more extensive investigation into the m e c h a n i s m of coal-dust explosions. The a p p a r a t u s is s o m e w h a t similar to t h a t used p r e v i o u s l y b y E. JoNEs and A. G. WHITE z, b y H. M. CASSEL et al3, b y R. H. ESSENHIGH a n d D. W . WOODHEAD4 a n d b y K. C. BROWN and R. H. ESSENHIGH ~. Dust was fed from a h o p p e r through an electromagnetically v i b r a t e d sieve over the whole cross section of a 7 c m d i a m e t e r vertical P e r s p e x tube, 4 m in length, open at the lower end. Various m e t h o d s of feeding the dust were tried; the best results were obtained with a sieve tightly stretched on a circular frame. The dust concentration was measured b y collecting the dust in a given length of tube, a n d is w h a t J . H. BURGOYNE6 calls the 'volume concentration'. The short term variation of the concentration with time was measured with a photoelectric extinction method, and the coefficient of variation was a b o u t 12 per cent. Since the particles sediment with a velocity relative to the air, if there were an airflow, the resultant velocity profile would be expected to introduce a concentration profile. F o r this reason, no airflow was used. The dust cloud was ignited b y means of an electrically heated coil, 36 cm from the open end. F l a m e was judged to have p r o p a g a t e d when it travelled more than 1 m u p the tube from the igniter.
5% p r o b a b i l i t y of p r o p a g a t i o n of f l a m e (with 95% c o n f i d e n c e hmits 50*/,
~-~,-- = 95*/,
c
0-64 ~o 3
2
C
I
10
50 100 Lycopodium concentration: g / m 3 air
30
I000
Figure I. Effect of limestone dust on the combustion limits of lycopodium in air
76
Letters to the Editors
A series of experiments to determine the effect of an inert dust on the combustion limits of lycopodium was made. The inert dust was limestone, ground so that 57 per cent by weight passed a 240 B.S. sieve (64 microns) and had a specific surface of 0.340 m 2/ g. The results are shown in Figure I, where the ratio of inert dust to lycopodium necessary to give a 5, 50 and 95 per cent probability of propagation of flame is plotted against lycopodium concentration in g / m '~. The curve shows 10 ~g8
/
Safety in Mines Research Establishment, Ministry o[ Power, Harpur Hill, Buxton, Derbyshire
"X \
I
-
"/
2
I wish to thank Mr B. W. Quince [or carrying out the statistical analysis o[ the results. The illustrations are Crown Copyright, and are reproduced by permission. F. POWELL
/
.oL
4:1 mixtures, in which propagations and nonpropagations occurred almost equally over a wide range of concentrations, making analysis impracticable. H. F. COWARD and F. J. HARTWELL7 who investigated the effect of addition of carbon dioxide on the limits of inflammability of methane in air, obtained a similar curve. For comparison, their results have been redrawn in Figure 2.
x
/ s
Vol. 6
(Received January 1962)
X
\ References
2
X X %
/
t ! I×~
1
I
E
I X,
40 60 80 100 Methane concentration: g/m3air ( CO2 free )
Figure 2. Limits of inflammability of methane in atmosphere containing carbon dioxide the change in the lower limit of inflammability with added inert dust and, what is perhaps more interesting, clearly shows definite upper limits. It appears that small changes in the dust mixture at ratios of about 4:1 produce large changes in the probability of propagation. This would account for the results obtained with nominally
J Safety in Mines Research E s t a b l i s h m e n t . Thirtyeighth Annual Report on Safety in Mines Research 1959, p 16. Her Majesty's Stationery Office: London, 1960 '-' JoNEs, E. and WHITE, A. G. Dust in Industry (Conference Leeds, September 1948) p 129. Society of Chemical I n d u s t r y : London, 1948 3 CASSEL, H. M. et al. Third Symposium (International) on Combustion and Flame, and Explosion Phenomena (Wisconsin 1948), p 185. Williams and \Vilkins: Baltimore, 1949 .l ESSENHIGH, R. H. and WOODHEAD, D. W . Combustion ~ Flame, 1958, 2, 365 ." BROWN, K. C. and ESSENHIGH, R. H. Saf.-i.-Mines Res. Rep. No. 165, 1959 s BURGOYNE, J. H. Chem. Engng Progr. 1957, 53, 121-M = COWARD, H. F. a n d HARTWELL, F. J. Saf.-i.-Mines Res. B d Pap. No. 19, 1926