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Combustion mechanism in pulverized coal flames
92
L e t t e ~ to the Editom
Combustion Mechanism in Pulvedzed Coal Flames I~ a recent communication 1 we reported that ,.aeasurements performed on a one-dimensional pulverized bituminous coal flame indicated that ignition occurs, not in volatile pyrolysis products as was commonly assumed, but on the solid surfaces of the particles. We have now extended the analysis of the data beyond ignition to consider the combustion process, and we have again reached some unexp.ected conclusions. In spite of our first finding that ignition started on the solid particles, we still continued to assume, initially, that the heterogeneous reaction would be quenched at the onset of devolatilization due to the volatiles sweeping away, and using up, the oxygen diffusing to the particle surface. In practice, however, we are now forced to conclude that this classical picture is true only of the larger particles; and, since the smaller particles (less than 65 microns) are evidently incapable of generating a sufficiently concentrated volatiles screen, only a fraction of the oxygen is used up by the volatiles, the rest migrating right on to the solid surface to maintain the heterogeneous reaction. The proportion of volatiles 'reaction to heterogeneous reaction decreases as the ,article size drops, and at 15 microns or less, the reaction is apparently all heterogeneous, with no discreet volatiles flamelet round the particle. The evidence for this picture is partly based on a detailed analysis of all possible (and some improbable) mechanisms, and the progressive elimination of all but one by the method of disproof, by the failure of inapplicable mechanisms to agree with experimental results [the method of multiple hypotheses2]. The argument developed '~ is currently being prepared for full evaluation by publication but is too long to be summarized completely; however, the salient points may be presented here. The principal point is that, if heterogeneous reaction was completely quenched at the onset of devolatization, then there would be no loss of fixed carbon until the pyrolysis stopped. This would show up on a VM/fixed carbon plot (e.g. Figure 2 of ref. 1) as a vertical line. In fact, however, the fixed carbon (percentage of
Vol. 10
original in coal) did not remain constant but dropped, during the pyrolysis, from 90 per cent to 50 or 60 per cent. To establish next what particle size fraction was being affected by the continued heterogeneous reaction, appeal was made to information from a range of sources summarized elsewhere ~.~ indicating that the pyrolysis should be a first-order volumetric process. It follows from this, as is easily shown, that the devolatilization tin., is independent of particle size (below 100 micr~ as), so the surface flux of volatiles must increase with particle size. Borrowing then from oil drop theory, it follows that volatile generation rates above some critical value can maintain a volatiles flamelet round the particle at a finite, di:;creet distance from the particle surface. At the flamelet surface, all the oxygen is assumed to be consumed by the reaction, so the flamelet forms a protective screen round such particles, thus quenching the heterogeneous reaction. At the critical generation rate, the flamelet radius is assumed to equal the particle radius and, below that, there is then an excess of oxygen available for continuation of theheterogeneous reaction, the magnitude of the excess increasing with decreasing particle size. Knowing the volatiles generation rate for different particle sizes, and using appropriately selected diffusion rates and other coefficients, the calculated critical radius required to give an effectively uninhibited surface reaction was found to be 29 microns. This calculated value for the critical diameter can be compared with the experimentally determined diameter of 15 microns, which is about half the calculated value. This experimental value was obtained from the split of total volatile loss into two parts" that due to pyrolysis and gas phase combustion; and that due to combustion of the 'volatiles' by heterogeneous combustion of the whole coal in the particle before pyrolysis. This latter quantity is a measure of the coal contained in particles less than the critical size and this measure, employed in conjunction with the size distribution of the coal, then enabled us to determine the critical diameter experimentally. In view of the approximations and assumptions involved, agreement to within a factor of two is thought to be good. In addition to the main pattern outlined
March 1966
Letters to the Editors
above, the data also provided information on the activation energies of the heterogeneous reaction. In agreement with others e.7, we found that the combustion rate of the solid residue in the tail of the flame was chemically controlled with an activation energy of 60kcal/mole. In the pyrolysis zone the reaction was evidently still chemically controlled since the activation energy was 12kcal/mole. This is far too high to approximate to ,~ diffusion controlled process, but also too low to be a desorption activation energy. We postulate therefore that this is an adsorption activation energy, this being thought possible because of the greatly reduced oxygen partial pressure at the sGlid surface s following the incomplete volatiles reaction.
9~
A Note, on the Stability of Adiabatic Flames I s A paper by Y. B. ZELDOVITCH and G. I. BARENBLATT [Combustion & Flame, 1959, 25, 61] this problem is discussed within the limitations of (a) r + a = 1 during the perturbation; (b) the perturbation is a small function allowing linearization of the equation in the neighbourho~d of the steady state solutions r and a. Although the previous authors were aware that o~sumption (a) can be removed without great difficulty they did not present the proof. Hence we present here a proof which requires neither (a) nor (b). Consider"
02r Ox
This w~,:k was pe','formed in the Combustion Laboratory, Department of Fuel Science, The Pennsylvania State University. We wish to acknowledge joint financial support, and permission to publish, lrom the Babcock and Wilcox Company (Alliance Research Statign) and the Slate Funds of the Commonwealth of Pennsylvania for Coal Research. J. B. HOWARD
Department o[ Chemical En#neering, Massachusetts Institute of Technology, Cambridge, Mass. 02139 and R. H. ESSENHIGH
Combustion Laboratory, Department of Fuel Science, Pennsylvania State University (Received January 1966)
G Or -
02°l
iw + 'b ( r a ) = 7 f f
G Oot
Oa
-4,(ra)=/7i-
Ox----v.-
2]
....
[
and let us define r(xt)=r(x)+~(xt); a ( x t ) = a(x)+~(xt) where ~ and lq do not have to be small, but are assumed to vanish at the boundaries (x > _+c~). r+a-r+ a + e+ ~-/= 1 + ~+~,/ Let ¢+ ~/=:r+ a - 1 = y By adding equations 1 and 2 we see that y satisfies the linear differential equation
02y G Oy Ox~Ox
Oy Ot
. . . . [3 ]
a n d y > 0 a t x > _+oo. Thegeneralsolution of equation 3 with these boundary conditions is O~
y ( x , t ) = ~ y ( x , 0) U(~:,x,t)d~
References J. B. and ESSENHIGH, R. H. Combustion (,~, Flame, 1965, 9, 337 .'2 _OLATT ' j. R. Science, 1964, 146, 347 :~ HOWARD, J. B. Ph.D. Thesis, The Pennsylvania State University, J u n e 1965 t HOWARD, J. B. and ESSF.NmGI-I, R. H. Accepted paper for presentation at Symposium on 'Pyrolysis Reactions of Fossil Fuels', Fuels Division, American Chemical Society, Pittsburgh, March 1966 .', ESSENHIGH, R. H. and t{OWARD, J. B. lndustr. Engng Chem. ([ndustr.), 1966, $8, 14 c, HC,TTEL, H. C. and STEWART, I. M. lndustr. Engng Chem. (lndustr.), 1940, 32, 719 : BEi:R, J. M. and THRI.~G, M. W. Bull. Pa Miner. Industr. exp. Sta. No. 75 (1961) 25 ESSV.','mGH, R. H., FPOe, WR~, R. W. and HOWARD, ]. B. lndustr. Engng Chem. ([ndustr.), 1965, 57, 3,q 1 HOWARD,
. . . [41
-- X
where
U(~, x, t ) -
1 exp [_ ( x - G t - ~ ) °-] v/ 4~rt 4t
which is a decreasing function of time everywhere regardless of the size of the perturbation y(x, 0). Another solution of equation 3 s~ti~lying the same boundary conditiens is the null solution implying that a + 7 = l at all time~ during th~. perturbation. This i.~ the case discussed by the previous authors. Hence we have shown that for arbitrary perturbations, not
L e t t e ~ to the Editom
Combustion Mechanism in Pulvedzed Coal Flames I~ a recent communication 1 we reported that ,.aeasurements performed on a one-dimensional pulverized bituminous coal flame indicated that ignition occurs, not in volatile pyrolysis products as was commonly assumed, but on the solid surfaces of the particles. We have now extended the analysis of the data beyond ignition to consider the combustion process, and we have again reached some unexp.ected conclusions. In spite of our first finding that ignition started on the solid particles, we still continued to assume, initially, that the heterogeneous reaction would be quenched at the onset of devolatilization due to the volatiles sweeping away, and using up, the oxygen diffusing to the particle surface. In practice, however, we are now forced to conclude that this classical picture is true only of the larger particles; and, since the smaller particles (less than 65 microns) are evidently incapable of generating a sufficiently concentrated volatiles screen, only a fraction of the oxygen is used up by the volatiles, the rest migrating right on to the solid surface to maintain the heterogeneous reaction. The proportion of volatiles 'reaction to heterogeneous reaction decreases as the ,article size drops, and at 15 microns or less, the reaction is apparently all heterogeneous, with no discreet volatiles flamelet round the particle. The evidence for this picture is partly based on a detailed analysis of all possible (and some improbable) mechanisms, and the progressive elimination of all but one by the method of disproof, by the failure of inapplicable mechanisms to agree with experimental results [the method of multiple hypotheses2]. The argument developed '~ is currently being prepared for full evaluation by publication but is too long to be summarized completely; however, the salient points may be presented here. The principal point is that, if heterogeneous reaction was completely quenched at the onset of devolatization, then there would be no loss of fixed carbon until the pyrolysis stopped. This would show up on a VM/fixed carbon plot (e.g. Figure 2 of ref. 1) as a vertical line. In fact, however, the fixed carbon (percentage of
Vol. 10
original in coal) did not remain constant but dropped, during the pyrolysis, from 90 per cent to 50 or 60 per cent. To establish next what particle size fraction was being affected by the continued heterogeneous reaction, appeal was made to information from a range of sources summarized elsewhere ~.~ indicating that the pyrolysis should be a first-order volumetric process. It follows from this, as is easily shown, that the devolatilization tin., is independent of particle size (below 100 micr~ as), so the surface flux of volatiles must increase with particle size. Borrowing then from oil drop theory, it follows that volatile generation rates above some critical value can maintain a volatiles flamelet round the particle at a finite, di:;creet distance from the particle surface. At the flamelet surface, all the oxygen is assumed to be consumed by the reaction, so the flamelet forms a protective screen round such particles, thus quenching the heterogeneous reaction. At the critical generation rate, the flamelet radius is assumed to equal the particle radius and, below that, there is then an excess of oxygen available for continuation of theheterogeneous reaction, the magnitude of the excess increasing with decreasing particle size. Knowing the volatiles generation rate for different particle sizes, and using appropriately selected diffusion rates and other coefficients, the calculated critical radius required to give an effectively uninhibited surface reaction was found to be 29 microns. This calculated value for the critical diameter can be compared with the experimentally determined diameter of 15 microns, which is about half the calculated value. This experimental value was obtained from the split of total volatile loss into two parts" that due to pyrolysis and gas phase combustion; and that due to combustion of the 'volatiles' by heterogeneous combustion of the whole coal in the particle before pyrolysis. This latter quantity is a measure of the coal contained in particles less than the critical size and this measure, employed in conjunction with the size distribution of the coal, then enabled us to determine the critical diameter experimentally. In view of the approximations and assumptions involved, agreement to within a factor of two is thought to be good. In addition to the main pattern outlined
March 1966
Letters to the Editors
above, the data also provided information on the activation energies of the heterogeneous reaction. In agreement with others e.7, we found that the combustion rate of the solid residue in the tail of the flame was chemically controlled with an activation energy of 60kcal/mole. In the pyrolysis zone the reaction was evidently still chemically controlled since the activation energy was 12kcal/mole. This is far too high to approximate to ,~ diffusion controlled process, but also too low to be a desorption activation energy. We postulate therefore that this is an adsorption activation energy, this being thought possible because of the greatly reduced oxygen partial pressure at the sGlid surface s following the incomplete volatiles reaction.
9~
A Note, on the Stability of Adiabatic Flames I s A paper by Y. B. ZELDOVITCH and G. I. BARENBLATT [Combustion & Flame, 1959, 25, 61] this problem is discussed within the limitations of (a) r + a = 1 during the perturbation; (b) the perturbation is a small function allowing linearization of the equation in the neighbourho~d of the steady state solutions r and a. Although the previous authors were aware that o~sumption (a) can be removed without great difficulty they did not present the proof. Hence we present here a proof which requires neither (a) nor (b). Consider"
02r Ox
This w~,:k was pe','formed in the Combustion Laboratory, Department of Fuel Science, The Pennsylvania State University. We wish to acknowledge joint financial support, and permission to publish, lrom the Babcock and Wilcox Company (Alliance Research Statign) and the Slate Funds of the Commonwealth of Pennsylvania for Coal Research. J. B. HOWARD
Department o[ Chemical En#neering, Massachusetts Institute of Technology, Cambridge, Mass. 02139 and R. H. ESSENHIGH
Combustion Laboratory, Department of Fuel Science, Pennsylvania State University (Received January 1966)
G Or -
02°l
iw + 'b ( r a ) = 7 f f
G Oot
Oa
-4,(ra)=/7i-
Ox----v.-
2]
....
[
and let us define r(xt)=r(x)+~(xt); a ( x t ) = a(x)+~(xt) where ~ and lq do not have to be small, but are assumed to vanish at the boundaries (x > _+c~). r+a-r+ a + e+ ~-/= 1 + ~+~,/ Let ¢+ ~/=:r+ a - 1 = y By adding equations 1 and 2 we see that y satisfies the linear differential equation
02y G Oy Ox~Ox
Oy Ot
. . . . [3 ]
a n d y > 0 a t x > _+oo. Thegeneralsolution of equation 3 with these boundary conditions is O~
y ( x , t ) = ~ y ( x , 0) U(~:,x,t)d~
References J. B. and ESSENHIGH, R. H. Combustion (,~, Flame, 1965, 9, 337 .'2 _OLATT ' j. R. Science, 1964, 146, 347 :~ HOWARD, J. B. Ph.D. Thesis, The Pennsylvania State University, J u n e 1965 t HOWARD, J. B. and ESSF.NmGI-I, R. H. Accepted paper for presentation at Symposium on 'Pyrolysis Reactions of Fossil Fuels', Fuels Division, American Chemical Society, Pittsburgh, March 1966 .', ESSENHIGH, R. H. and t{OWARD, J. B. lndustr. Engng Chem. ([ndustr.), 1966, $8, 14 c, HC,TTEL, H. C. and STEWART, I. M. lndustr. Engng Chem. (lndustr.), 1940, 32, 719 : BEi:R, J. M. and THRI.~G, M. W. Bull. Pa Miner. Industr. exp. Sta. No. 75 (1961) 25 ESSV.','mGH, R. H., FPOe, WR~, R. W. and HOWARD, ]. B. lndustr. Engng Chem. ([ndustr.), 1965, 57, 3,q 1 HOWARD,
. . . [41
-- X
where
U(~, x, t ) -
1 exp [_ ( x - G t - ~ ) °-] v/ 4~rt 4t
which is a decreasing function of time everywhere regardless of the size of the perturbation y(x, 0). Another solution of equation 3 s~ti~lying the same boundary conditiens is the null solution implying that a + 7 = l at all time~ during th~. perturbation. This i.~ the case discussed by the previous authors. Hence we have shown that for arbitrary perturbations, not