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Flammability characteristics and structure of a pulverized coal, laminar opposed jet diffusion flame
Nineteenth Symposium (International) on Combustion/The Combustion Institute, 1982/pp. 1189-1196
FLAMMABILITY CHARACTERISTICS AND STRUCTURE OF A PULVERIZED COAL, LAMINAR O P P O S E D JET D I F F U S I O N FLAME D. B. GRAVES AND J. O. L. WENDT Department of Chemical Engineering University of Arizona Tucson, Arizona 85721 A novel use of the opposed jet diffusion flame in an experimental study of laminar, pulverized coal flames is described. The opposed jet configuration has previously only been used to study gaseous diffusion flames stabilized either between two jets or off evaporating surfaces. In those cases it yielded information on fuel pyrolysis and so its extension to pulverized coal seems well motivated. It is shown, both theoretically and experimentally, that the p.f. laminar opposed jet flame is one dimensional in species concentration and temperature. Particle size segregation does occur but, provided Stokes drag is applicable, the one dimensionality holds for particle size and number density even when particles do not follow gas streamlines. In general, the opposed jet configuration allowed investigation of the effects of non-premixing of pulverized fuel and oxygen on flame structure and characteristics. Although fundamentally different, this flame possessed some surprising similarities to premixed pulverized coal-air flames, Because of its one dimensionality in temperature and species concentrations and because of apparent similarities in propagation mechanisms, the opposed jet flame can be viewed as the diffusion flame analog to more traditional premixed flame studies.
Introduction Investigations of open atmosphere, laminar, premixed flat flames of pulverized coal in air have revealed important information regarding flame structure and propagation characteristics as well as early time scale phenomena such as coal particle heat up, devolatilization and ignition. 1>'z/'3)'4>Other studies in enclosed furnaces with hot, confining walls5)'6> have generally focused on longer time scale phenomena including char burn out. This paper is concerned with the diffusion flame analog of the aforementioned open atmosphere flames and focuses on theoretical and experimental attributes of the pulverized coal, laminar opposed jet diffusion flame. The opposed jet diffusion flame configuration (see Fig. 1) consists of two vertical, coaxial burners with opposing flows of fuel and oxygen, each diluted by some inert gas, typically nitrogen. The flame is stabilized somewhere midway between the two burners, the actual location of which is a complex function of the coupled transport equations, boundary conditions, and reactant characteristics. This configuration has been used and studied in the past because it offers an opportunity to examine flames in which the reactants are initially unpremixed, and because the flat, one-dimensional nature of the flame has greatly simplified modeling
and analysis. 7>'8) Previous work in this area has been restricted to gaseous fuels or liquid or solid fuel vapors in which the flame was essentially a gas flame. The use of pulverized coal as the fuel in the opposed jet diffusion flame configuration offers advantages similar to those found for gaseous flames: 1) the structure and characteristics of laminar, flat, pulverized coal diffusion flames may be examined experimentally; 2) because of the one-dimensional nature of the flame, these flames can be modeled conveniently. Preliminary results, reported elsewhere, ~ showed that reasonably flat opposed jet, pulverized coal flames could be stabilized in the laboratory with no supplemental gaseous fuel of any kind in the system. This paper is concerned first, with an investigation of the flammability limits of this flame with zero oxygen in the fuel stream; second, with its structure and its appropriateness as a tool for more detailed studies on particle devolatilization and mechanisms of flame propagation under unmixed conditions. Theory For gaseous laminar opposed jet diffusion flames, it has been previously shown 7> that the conservation equations can be solved numerically, with no
1189
1190
COAL COMBUSTION TECHNIQUES Using c as the particle velocity vector for particles of mass Mn and diameter D n and assuming Stokes Law for particle drag, one can write down the steady state particle momentum and mass balance equations:
Pulverized Coal and Transporting Fluid
D Mpj D'--ttcj = Fo i + g
(2)
D
bS (M~j) = - % -- ol
~
Flame Zone
OXIDIZER
FIG. 1. Schematic of coal opposed jet diffusion flame.
restrictive assumptions with respect to constant properties, finite rate elementary reaction rates, or inertia and viscous effects. The only required restrictions were 1) low Mach number, 2) negligible viscous dissipation and 3) negligible burner edge effects over some appreciable region in the neighborhood of the stagnation streamline. The most important result of that work was to show that the gas flame was in fact one dimensional in concentration, axial velocity, and temperature. In this paper we extend the theory to particulate combustion systems, where the particles do not necessarily follow the streamlines, and show that, even then, this diffusion flame is one dimensional in particle density, temperature, concentration and physical properties. For the gas phase, velocities vr and v~ must obey the following self similar solution:
where subscript j refers to initial size class. The possible "jetting" of volatiles is neglected as are all other minor effects on the particle momentum balance. 1~ Since typical values of the particle Reynold's number range between 0.01 to 0.1, Stokes Law is valid for FoI. Restricting ourselves to a single initial particle size class, for simplicity but without loss of generality as far as overall conclusions are concerned, and dropping the subscript j, we can assume, analogously to Equation (1), self similar solutions of the form: c r = br 6(z); cz = cz(z); Mn = Mn(z); n v = nv(Z); D o = Dn(z) (4)
to yield the following ordinary differential equations d b 2 Mp r 2 + bcz M n ~ (k
= - 3~r tx Dp (b~b - aO)
(5)
d c~ M n dz c~ = - Mpg - 3~r Ix Dp (cz - vz)
(6)
d c~ dz Mp = - ,a~
8P r , aP Vr = er ~b(z);v z = vz(z); ~ r = f ( )" ~z = g (z)
(1)
and all other variables are functions of z only. The unknown variables satisfy a set of transformed ordinary differential equations (see ref. 7) which are coupled with the particulate coal phase equations through the mass devolatilization rate, the rates of formation of species, a portion of which results from coal devolatilization, and the reaction rate heat effects. The one dimensionality will now hold only if these coupling terms are also functions of only z, not of r, and so we must specifically show that rhv, the devolatilization rate per particle, and n~, the particle density, are functions of z only, even when particles do not follow gas streamlines.
(31
d
2 b n~ r + -7- (n~ c~) = 0 az
(7)
(8)
Therefore, the one dimensional, similarity solutions (1) and (4) still hold. There will be a velocity field c for each initial particle size class j, and a size segregation along z will occur. We have, for simplicity, neglected the particle heat balance and assumed particles to be at the gas temperature. The constant b in Equation (4) can be evaluated from the cold non-combusting flow, and is a function of initial particle diameter. The important result is that, for the pulverized coal, laminar opposed jet diffusion flame, the flame
STRUCTURE OF PULVERIZED COAL is one dimensional in the z dimension only, even though particle size will vary with z, and segregation occurs, and even though particles do not follow the gas streamlines. The ordinary differential equations which result can be solved numerically for finite rate kinetics together with convection and diffusion, and so the pulverized coal, laminar opposed jet diffusion flame can be modeled in detail. Its usefulness as a laboratory tool to study coal ignition and devolatilization kinetics depends therefore, first, on the conditions under which it can be physically realized in the laboratory, and second, whether it exhibits the appropriate structure. Experimental Facility Fig. 2 is a schematic of the apparatus used in this study. The pulverized coal mass flow rate is controlled using a variable speed auger-type feeder. Transport N~ is added to the coal line and the resulting mixture is introduced into the apex of an inverted cone in the tip of the upper burner. Flow is reversed, providing good mixing, and the relatively gradual expansion of the gas due to the cone eliminates recireulation zones which can cause nonuniform radial dispersion of the coal. The upper burner is 91 cm in length and 11.4 cm in diameter. Near the tip of the burner is a 5 cm segment of aluminum honeycomb with 0.635 cm hexagonal cells which straightens and effectively laminarizes the flow. A pneumatic vibrator, mounted on the top of the upper burner, ensures the continuous and even flow of coal through the matrix. The upper burner design is the key to obtaining
r_~Coal F'eeder~~~Pneumatic Vibrator l'l ~a
U
UpperBurner (91crnx 11.4crn) F-J--]---'I ~clasure I I I To Analysis Honeycornb,I Matrix--~l~l I L"J ' J Probeand meter
Nitrogen Screens-z'l
I
I (20.3crnx 11.4era)
FIG. 2. Schematic of apparatus.
1191
a flat, stable flame and depends primarily upon striking the proper balance between good mixing to enhance coal dispersion followed by flow straightening to laminarize the flow. Flow straightening also involves a trade-off since a matrix with cells which are too large does not straighten the flow, but cells which are too small will plug. Similar comments can be made regarding the length of the matrix. Milne and Beachey1/ report similar considerations with respect to flow straightening in the design of their premixed flat flame burner. The design of the bottom burner is quite straightforward since straightening the gas flow is easily done with 50 and 100 mesh screens. The coal flame is ignited using a natural gas flame until the burners are sufficiently hot. The natural gas is then eliminated and the coal flame sustains itself. Typically, the flame reached a steady state stability about 15 minutes after ignition.
Sampling and Analytical System Gas and solid species were sampled using a 0.635 cm O.D. stainless steel tube with an internal water spray at the tip to immediately quench any reactions that could take place in the probe and to prevent plugging. The sample, consisting of liquid water, water vapor, gas and solid coal particles, passed through a condenser/particulate collection chamber and then proceeded to be analyzed by continuous analyzers for CO, CO~ and Oz. In addition, a Perkin Elmer Sigma 1 chromatograph was utilized to measure H a and CH 4. Limitations in sample flow rates, set by the on-line instruments, thwarted truly isokinetic sampling when gaseous species profiles were being measured. The effect of this was to introduce an error in the spatial resolution and to exaggerate the measured flame thickness as determined from the gaseous species measurements. When solid samples were being withdrawn, sampling velocities were much closer to isokinetic, as determined by estimates of the radial velocity at the point where the sample was extracted. Therefore, the solid composition profiles presented here represent the reaction zone thickness more accurately than do the gas species profiles. The size of the sampling probe (OD = 0.6 cm), however, introduced additional errors in spatial resolution, in regions where there were significant gradients over that distance. Temperature was measured using Type K Inconel shielded and magnesia insulated thermocoupies. Following previous workers, 4)'2/ temperatures are reported as-measured. The coal used throughout this study was a Utah Bituminous, pulverized 75% through 200 mesh with a mass median diameter of 60 microns. Analysis, percent by weight, dry basis, was given by: C = 69.99, H = 5.15, S = 0.98, N = 1.22, Ash = 9.97;
COAL COMBUSTION TECHNIQUES
1192
Proximate Analysis, percent by weight as received, by: Volatile Matter = 40.6, Fixed Carbon = 44.3, Moisture = 5.7, Ash = 9.4.
Results The parameters chosen to characterize the flame are: 1. 2. 3, 4.
Oxygen concentration in the bottom burner. Coal concentration in the top burner. Burner spacing. Gas velocity at the burner exits (in this work velocities were always matched at the burner exits).
The last parameter, gas velocity at the burner exit, is expressed in terms of the cold flow "stretching rate," defined as: ;)z
D where D is the burner spacing, and v z is the velocity of the cold flow at the burner exit, This is not the actual stretching rate applicable to the combusting flow but is utilized here merely as a correlating parameter. The actual stretching rate is affected by gas density variations and becomes an eigenvalue when both jet velocities are specified a p r i o r i , 7)
Fig. 3 shows a close-up photograph of a flat, pulverized coal, laminar opposed jet diffusion flame, stabilized with no supplemental gaseous fuel and no oxygen in the fuel stream. The oxidant stream contained 85% by volume O2. The cold flow stretching rate was 2.15 s-1 and the coal concentration was 240 mg/liter. The burners, spaced 3 cm
apart, are clearly visible and the luminous zone occurs about halfway between them. It is apparent that the larger particles do penetrate the gas stagnation plane, but the flame front is visually flat over one half of the burner diameter. This flatness implies one dimensionality in temperature and species profiles as predicted by the similarity solution. Fig. 4 is a plot of the flammability limits for the apparatus described above. We make no claim that this plot is universal and unaffected by, for example, changes in system adiabaticity. Fig. 4 plots flame extinction points as a function of oxygen concentration in the oxidant jet and stretching rate for an approximately constant coal density. Variations in the coal mass delivery rate caused the differences in density reported in each experiment. Since premixed flames of pulverized coal in air exhibit flammability limits of the order of 100 mg/liter coal concentration as a lean limit to well over 1500 mg/liter as a rich limit, 3) the variation in coal density in Fig, 4 is not excessive. Fig. 4 demonstrates that relatively high oxygen concentrations are required in the oxidant jet as well as very low stretching rates. By way of contrast, a methane opposed jet diffusion flame can be easily stabilized with air at 25 see -1 although a diluted gaseous flame representative of the volatile content of the coal alone exhibits similar low stretching rate extinction limits. Stretching rates below approximately 1.9 sec -1 result in unstable flames because the buoyant edges of the flame appear to become dominant. Then the 90,
P-Cool Density, mcj/liter 85
~=
r I
/
,, 9 / / 9 ( ~ 2 5 0 )
SO 75
~9
o
x2
-~
9
(p:200)
~9"~= g vo
8-
u.
65
~ .
{ o
=E
6O
55
FIG. 3. Close up photograph of pulverized coal, flat, laminar, opposed jet, diffusion flame. Zero supplemental gaseous fuel; zero O2 in gas transporting coal from top, Standard flame: Burner spacing = 3 cm; O2 in oxidant jet (bottom) = 85%; e = 2.15 s-~; p = 240 mg/liter
g
9
~1
9 p=240)
/
/ /~
Extinction
/ ,(,~:23o,
Burner Sl~ocing=3cm
,50
I L5
I 2.0
I
I 2.5
L
30
Stretching Rote, sec-~ (Cold Flow ) FIG. 4. F l a m m a b i l i t y
limits. Burner
spacing 3 era.
1193
STRUCTURE OF PULVERIZED COAL flame, while perfectly capable of sustaining combustion, becomes unsteady and is no longer flat. At a given oxygen concentration, increasing the stretching rate will lead to flame extinction, represented in Fig. 4 by the area to the right of the extinction curve. Thus it can be seen that the extinction curve intersects the minimum stretching rate line at a fairly high oxygen concentration (-45% O2) severely limiting regions of applicability of this flame. However, the area between the two curves is judged adequate for structure analysis. In general, thermal effects appeared to be important for flame stability. For example, increased burner exit temperatures facilitated flame stabilization. On the other hand, introduction of the cold water quenched probe too far into the flame could cause it to become extinguished. However, with = constant, burner spacing does not significantly change the temperature profile within the flame zone, Fig. 5, indicating that the model (which is valid strictly only for infinitely wide burners placed at ---~), may have applicability to phenomena occurring in the neighborhood of the peak temperature zone. One dimensionality in temperature and major species concentration profiles for the standard flame are shown in Fig. 6. These profiles, with points taken at different radial positions demonstrate that temperature and species concentrations depend only upon axial, not radial position in the flame, as predicted by the theory and inferred from the pho-
MOO
- -
II
0 Type K T C
O0 m
IO00
O O
9OO
o []
c~
o
0
~. 8 o c 0
o
700 - - 3 c m ~ Burner Spocing Corresponding to 0 Points
60C
50C
~
~ 4 , 5 c m - Burner Spacing Corresponding to n
__.L - 20
-
--
d I I
Poinls
~ - 1.0
.L_O0
1,0
ZO
Relative Position, cm
FIG. 5. Temperature profile as function of burner spacing. TC at 2.75 cm from axis. Standard flame.
40
=-o~ lz75. m
O-CO ' ~" A - CO21from Ax,s @-CO " / / / c r n
O-Tern0Z65c~1
,>.o
/
0y
'fromAx~, %_ } '~l/O-Temp, 2.75cm I100
/
x
r z5
g
g 5
aoo 20 700
C, ,5
/,02
- -
ilk6 5OO
0 -L5
-LO
--0,5
I
9~
~\
0,0
0.5
I.O
,400 i.5
Rek~tive Position, crn
FIG. 6, Demonstration of one dimensionality of flame. Standard flame; burner spacing 3 cm. tograph on Fig. 3. Exaggerated confidence should not be placed in the absolute values of species concentrations shown because of non-isokinetic sampling referred to above, and so the profiles indicate a broader reaction zone than is probably actually the case. Subsequent data obtained at much lower sampling rates and utilizing the gas chromatograph indicated a reaction zone of about half the width of that inferred from Fig. 6. The data in Fig. 6, however, are useful since they do support (albeit crudely) the one dimensionality of this diffusion flame. The temperature pro{lies are unambiguous, although they report uncorrected thermocouple measurements. Milne and Beachey 1) report that shielding the thermocouple did not change the measured temperature in their premixed p.f. flames and their peak flame temperatures and characteristic distances are similar to those in this study. Fig. 7 is the solid species profile of the same flame. The volatile content and fixed carbon percentages are calculated in the usual way from the proximate analyses using ash in the collected sample as a tracer. The profiles demonstrate there is essentially no pre-flame zone pyrolysis taking place, even though the temperature close to the burner exit is 800 K. Once the particle reaches the flame zone, however, devolatilization occurs very rapidly. Although it is well known that a proximate volatiles test does not yield the true "extent of devolatilization" it should certainly serve to indicate the onset of, and is a relative measure of, the extent of devolatilization. Furthermore, no evidence of hydrocarbon volatiles was observed. The highest levels
1194
COAL COMBUSTION TECHNIQUES
ties for the coal opposed jet configuration were approximately 10-15 cm/sec. This demonstrates the 90, L Fixed fact the premixed and diffusion flames are fundamentally different in character. In other respects, the diffusion flame and the premixed flame are surprisingly similar. For ex80 ample, the experience of Milne and Beachey~) that a water-cooled flameholder-grid would not sustain oo 70 w a pulverized coal flame suggests sensitivity of the Probe Width premixed flames to thermal effects. Smoot and -~0.6 cm Hortona/ note that reported differences in flame speed measurements may be due to the differences Volatile in the adiabaticity of various configurations. The Matter sensitivity of the coal opposed jet diffusion flame "6 50Uncertainty in Analysis ~ • to thermal disturbances has been documented above. Peak flame temperatures in premixed and diffusion =~ 4 0 flames of the same coal concentration are both on the order of 1000 K (measured temperature). Ox~ ygen, carbon monoxide and carbon dioxide profiles ~" 3 0 - Data Normalized behave in a similar manner, and after accounting _ Using Ash Content Appraximate for the considerably higher oxygen concentration -Visible Flame af Collected Sample E Zone used in this study, the peak values are compatible. ,~ 20Both flames appear to be primarily coal volatile flames and both display low concentrations of pyrolysis products, H 2 and CH 4. However, these spe-I.5 -I.(3 -0,5 (3.0 0.5 1.0 L,5 cies reach levels of 1-2% in premixed flames (deRe(Qtive Distance, em pending upon stoiehiometry) and are observed at only trace values in this diffusion flame. The exFIG. 7. Volatile matter and fixed carbon profiles. planation may be the higher oxygen concentration Standard flame; burner spacing 3 cm, used in the diffusion flame. Flame zone thicknesses are both on the order of 5 mm. The extent of devolatilization and solid species of Ha and CH4 found anywhere in the flame were profiles through the flame are also similar3) to pre0.1% and 0.01%, respectively. mixed laminar flames. The diffusion flame gas and solid species profiles show very little pre-flame zone pyrolysis even though the burner exit temConclusions and Discussion perature is 800 K, which is hot enough to initiate pyrolysis in bituminous coal. TM The coal heat-up The pulverized coal, laminar opposed jet diffu- time was approximately 150 milliseconds. Both Milne sion flame was stabilized in the laboratory with no and Beache~ ) and Howard and Essenhigh s) report oxygen in the transporting fluid and with no sup- very little pre-ignition zone pyrolysis. The model plemental gas. Such a flame is flat and is one di- of Smoot et. al. 4) also predicts that, in a premixed mensional in temperature and species concentra- flame, the coal particles do not begin to devolatiltions, and can be modeled by a set of ordinary ize until they enter the flame zone. The model differential equations even though particles do not predicts the major significance of the effects of follow gas streamlines and may even penetrate the molecular diffusion and gas-particle heat conduction stagnation plane. on flame propagation. Our observations on similarThe test of flammability limits reveals that flame ities in flame zone thickness, structure, and stabilstability in the diffusion flame configuration is rel- ity between premixed and diffusion flames suggest atively difficult to achieve. It does not appear pos- that the dominant physical and chemical mechasible, with the apparatus used in this study and in nisms may be similar for both flames. the absence of preheating one or both jets, to staThe absence of pre-flame pyrolysis further inbilize a pulverized coal flame in the opposed jet dicates that the specific laminar opposed jet diffuconfiguration with air. Higher Oz concentrations sion flame presented in this work would not be are required in order to obtain sufficiently high suitable for the study of coal devo|atilization. Fuflame temperatures. In contrast, experiments with ture work will focus on other methods of increasing premixed flames indicate that flames can be main- the peak flame temperature and thus also the diftained at a 20--30 cm/sec cold flow velocity, with fusive heat flux towards the fuel jet, thus decreasair, before blowout. Maximum burner exit veloci- ing the particle heat-up time. I00
STRUCTURE OF PULVERIZED COAL Nomenclature
b
cj Cr
Cz Dn FDj
f(r) g g(r) M~j rh~j
see Equation (4) velocity vector of particle of size class j, cm/s particle radial velocity, cm/s particle axial velocity, crn/s diameter of particle drag force on particle of size class j see Equation (1) gravitational force see Equation (1) mass of particle of size class j, gm devolatilization rate of particle in size class
j, g/s nv
P r 1)r 19z p~
*(z) *(z)
particle number density, 1/cm a pressure radial co-ordinate, cm gas radial velocity, c m / s gas axial velocity, cm/s stretching rate, 1/s viscosity of gas mixture, g/cm-s see Equation (4) similarity function--Equation (1)
1195
3. SMOOT, L. D. AND M. D. HORTON, "Propagation of Laminar Pulverized Coal-Air Flames," Prog. Eng. Comb. Sci., 3:235 (1977). 4. SMOOT, L. D., M. D. HORTONAND G. M. WILLIAMS, "Propagation of Laminar Pulverized CoalAir Flames," Sixteenth Symposium (International) on Combustion, p. 375 Combustion Institute, Pittsburgh, PA (1977). 5. HOWXaD, J. B. AND B. H. ESSENHIGH, "Pyrolysis of Coal Particles in Pulverized Fuel Flames,"
I. and E. C. Process Design and Development, Vo. 6, No. 1, 74 (1967). 6. WENDT, J. O. L., D. W. PERSHING, J. W. LEE, AND J. W. GLASS, "Pulverized Coal Combustion: NO~ Formation Mechanisms Under Fuel Rich and Staged Combustion Conditions,'" Sev-
enteenth Symposium (International) on Combustion, p. 77, The Combustion Institute,
Acknowledgment This work was supported by the U.S. Department of Education through a Domestic Mining, Minerals and Mineral Fuel Conservation Fellowship.
REFERENCES
1. MILNE, T. A. AND J. E. BEACHEY,"The Microstructure of Pulverized Coal-Air Flames: I. Stabilization on Small Burners and Direct Sampiing Techniques," Comb. Sc/. and Tech., 16".123 (1977). 2. M1LNE, T. A. AND J. E. BEACHEY, "The Microstructure of Pulverized Coal-Air Flames: II, Gaseous Species, Particulate and Temperature Profiles," Comb. Sci. and Tech., 16:139 (1977),
Pittsburgh, PA (1979). 7. HAHN, W. A. AND J. O. L, WENDT, "'NO~ Formation in Flat, Laminar, Opposed Jet Methane Diffusion Flames," Eighteenth Symposium (International) on Combustion, p. 121, The Combustion Institute, Pittsburgh, PA (1981). 8. LIU, T. M. AND P. A. LmBY, "Boundary Layer at a Stagnation Point With Hydrogen Injection," Comb. Sci. and Tech., 2:131 (1970). 9. GRaves, D. B. AND J. O. L. WENDT, "Flammability Characteristics of a Pulverized Coal, Opposed Jet, Flat, Laminar Diffusion Flame," paper presented at First Specialists' Meeting (International) of the Combustion Institute, Bordeaux, France, July, 1981 (unpublished). 10. CROWE, C. T. AND L. D. SMOOT, Chapter 2 in
Pulverized Coal Combustion and Gasifwation: Theory and Applications for Continuous Flow Processes, Eds L. D. Smoot and D. T. Pratt. Plenum Press, New York (1979).
11. POHL, J. H. AND A. F. SAROFIM, "'Devolatilization and Oxidation of Coal Nitrogen," Sixteenth Symposium (International) on Combustion, p. 491 The C o m b u s t i o n I n s t i t u t e , Pittsburgh, PA (1977).
COMMENTS Prof. H. S. Mukunda, Indian Institute of Science, Bangalore, lndia. In the photographs I notice that the flame is bending over towards the upper tube. In an earlier study by Dr. Raymond Kushida at JPL, a nitrogen gas shower around the burner was used to obtain a strictly planar flame. Would such a device modification give any problem in your system?
Author's Reply. When we attempted to introduce N 2 gas to quench the flame at the burner edge, we experienced increased flame instabilities, They may have been due to changes in burner cooling or to nonunifurmities in the shield gas velocity profile. We did not pursue this idea further since our interest was on the large flat portion of the flame in the neighborhood of the stagnation
1196
COAL COMBUSTION T E C H N I Q U E S
stream line, over a distance of one half of the burner diameter.
Dr. S. Galant, Societe Bertin, France. Another interesting aspect abunt your experiments is obtaining blow-off limits, under strain rate conditions that are well monitored. What type of strain rates can you cover and have you checked the blow-off value that you obtained against standard opposedjet gas diffusion flames? Author's Reply. Strain rates are very low, approximately 2.0 s L. As mentioned in the paper, a methane opposed jet diffusion flame can be easily stabilized with air at 25 s ~, although diluted gaseuus flames representative of the coal volatile content alone exhibit similar low stretching rate extinction limits. This suggests that our coal flame is essentially a coal volatile flame.
Author's Reply. One additional attribute of the pulverized coal, laminar opposed jet diffusion flame might be that it represents a prototype of the stretched laminar flamelets that may exist between the boundaries of fuel and secondary air streams in turbulent diffusion flames. ~ If, furthermore, it is believed that the stretching rate of the prototype can be related to some mean rate of strain in the turbulent diffusion flame, then it should be possible to use the prototype to determine how a) coal composition b) oxygen level in transporting fluid for fuel stream and c) oxidant prelceat, influence extinction limits with air only as the oxidant stream. In this case the "ignition" or extinction test would be designed to simulate the actual phenomena occurring in a furnace. Thus, we believe that, with considerable additional theoretical and experimental work, our flame has the potential to be a useful laboratory experiment to test the ignitability of coals in furnace flames.
REFERENCE
Prof. T. F. Wall, University of Newcastle, Australia. There is a definite need for a reproducable laboratory experiment to test the ignitability of coals. Could the experiment be used to rank coals, and, if so, what experimental variables would be used (e.g., oxidant preheat, oxygen in oxidant, flame speed)?
1. MARBLE, F. E. AND J. E. BROADWELL, "The Coherent Flame Model for Turbulent Chemical Reactions," Project SQUID, Report 29314-6001RU-00, (1977).
FLAMMABILITY CHARACTERISTICS AND STRUCTURE OF A PULVERIZED COAL, LAMINAR O P P O S E D JET D I F F U S I O N FLAME D. B. GRAVES AND J. O. L. WENDT Department of Chemical Engineering University of Arizona Tucson, Arizona 85721 A novel use of the opposed jet diffusion flame in an experimental study of laminar, pulverized coal flames is described. The opposed jet configuration has previously only been used to study gaseous diffusion flames stabilized either between two jets or off evaporating surfaces. In those cases it yielded information on fuel pyrolysis and so its extension to pulverized coal seems well motivated. It is shown, both theoretically and experimentally, that the p.f. laminar opposed jet flame is one dimensional in species concentration and temperature. Particle size segregation does occur but, provided Stokes drag is applicable, the one dimensionality holds for particle size and number density even when particles do not follow gas streamlines. In general, the opposed jet configuration allowed investigation of the effects of non-premixing of pulverized fuel and oxygen on flame structure and characteristics. Although fundamentally different, this flame possessed some surprising similarities to premixed pulverized coal-air flames, Because of its one dimensionality in temperature and species concentrations and because of apparent similarities in propagation mechanisms, the opposed jet flame can be viewed as the diffusion flame analog to more traditional premixed flame studies.
Introduction Investigations of open atmosphere, laminar, premixed flat flames of pulverized coal in air have revealed important information regarding flame structure and propagation characteristics as well as early time scale phenomena such as coal particle heat up, devolatilization and ignition. 1>'z/'3)'4>Other studies in enclosed furnaces with hot, confining walls5)'6> have generally focused on longer time scale phenomena including char burn out. This paper is concerned with the diffusion flame analog of the aforementioned open atmosphere flames and focuses on theoretical and experimental attributes of the pulverized coal, laminar opposed jet diffusion flame. The opposed jet diffusion flame configuration (see Fig. 1) consists of two vertical, coaxial burners with opposing flows of fuel and oxygen, each diluted by some inert gas, typically nitrogen. The flame is stabilized somewhere midway between the two burners, the actual location of which is a complex function of the coupled transport equations, boundary conditions, and reactant characteristics. This configuration has been used and studied in the past because it offers an opportunity to examine flames in which the reactants are initially unpremixed, and because the flat, one-dimensional nature of the flame has greatly simplified modeling
and analysis. 7>'8) Previous work in this area has been restricted to gaseous fuels or liquid or solid fuel vapors in which the flame was essentially a gas flame. The use of pulverized coal as the fuel in the opposed jet diffusion flame configuration offers advantages similar to those found for gaseous flames: 1) the structure and characteristics of laminar, flat, pulverized coal diffusion flames may be examined experimentally; 2) because of the one-dimensional nature of the flame, these flames can be modeled conveniently. Preliminary results, reported elsewhere, ~ showed that reasonably flat opposed jet, pulverized coal flames could be stabilized in the laboratory with no supplemental gaseous fuel of any kind in the system. This paper is concerned first, with an investigation of the flammability limits of this flame with zero oxygen in the fuel stream; second, with its structure and its appropriateness as a tool for more detailed studies on particle devolatilization and mechanisms of flame propagation under unmixed conditions. Theory For gaseous laminar opposed jet diffusion flames, it has been previously shown 7> that the conservation equations can be solved numerically, with no
1189
1190
COAL COMBUSTION TECHNIQUES Using c as the particle velocity vector for particles of mass Mn and diameter D n and assuming Stokes Law for particle drag, one can write down the steady state particle momentum and mass balance equations:
Pulverized Coal and Transporting Fluid
D Mpj D'--ttcj = Fo i + g
(2)
D
bS (M~j) = - % -- ol
~
Flame Zone
OXIDIZER
FIG. 1. Schematic of coal opposed jet diffusion flame.
restrictive assumptions with respect to constant properties, finite rate elementary reaction rates, or inertia and viscous effects. The only required restrictions were 1) low Mach number, 2) negligible viscous dissipation and 3) negligible burner edge effects over some appreciable region in the neighborhood of the stagnation streamline. The most important result of that work was to show that the gas flame was in fact one dimensional in concentration, axial velocity, and temperature. In this paper we extend the theory to particulate combustion systems, where the particles do not necessarily follow the streamlines, and show that, even then, this diffusion flame is one dimensional in particle density, temperature, concentration and physical properties. For the gas phase, velocities vr and v~ must obey the following self similar solution:
where subscript j refers to initial size class. The possible "jetting" of volatiles is neglected as are all other minor effects on the particle momentum balance. 1~ Since typical values of the particle Reynold's number range between 0.01 to 0.1, Stokes Law is valid for FoI. Restricting ourselves to a single initial particle size class, for simplicity but without loss of generality as far as overall conclusions are concerned, and dropping the subscript j, we can assume, analogously to Equation (1), self similar solutions of the form: c r = br 6(z); cz = cz(z); Mn = Mn(z); n v = nv(Z); D o = Dn(z) (4)
to yield the following ordinary differential equations d b 2 Mp r 2 + bcz M n ~ (k
= - 3~r tx Dp (b~b - aO)
(5)
d c~ M n dz c~ = - Mpg - 3~r Ix Dp (cz - vz)
(6)
d c~ dz Mp = - ,a~
8P r , aP Vr = er ~b(z);v z = vz(z); ~ r = f ( )" ~z = g (z)
(1)
and all other variables are functions of z only. The unknown variables satisfy a set of transformed ordinary differential equations (see ref. 7) which are coupled with the particulate coal phase equations through the mass devolatilization rate, the rates of formation of species, a portion of which results from coal devolatilization, and the reaction rate heat effects. The one dimensionality will now hold only if these coupling terms are also functions of only z, not of r, and so we must specifically show that rhv, the devolatilization rate per particle, and n~, the particle density, are functions of z only, even when particles do not follow gas streamlines.
(31
d
2 b n~ r + -7- (n~ c~) = 0 az
(7)
(8)
Therefore, the one dimensional, similarity solutions (1) and (4) still hold. There will be a velocity field c for each initial particle size class j, and a size segregation along z will occur. We have, for simplicity, neglected the particle heat balance and assumed particles to be at the gas temperature. The constant b in Equation (4) can be evaluated from the cold non-combusting flow, and is a function of initial particle diameter. The important result is that, for the pulverized coal, laminar opposed jet diffusion flame, the flame
STRUCTURE OF PULVERIZED COAL is one dimensional in the z dimension only, even though particle size will vary with z, and segregation occurs, and even though particles do not follow the gas streamlines. The ordinary differential equations which result can be solved numerically for finite rate kinetics together with convection and diffusion, and so the pulverized coal, laminar opposed jet diffusion flame can be modeled in detail. Its usefulness as a laboratory tool to study coal ignition and devolatilization kinetics depends therefore, first, on the conditions under which it can be physically realized in the laboratory, and second, whether it exhibits the appropriate structure. Experimental Facility Fig. 2 is a schematic of the apparatus used in this study. The pulverized coal mass flow rate is controlled using a variable speed auger-type feeder. Transport N~ is added to the coal line and the resulting mixture is introduced into the apex of an inverted cone in the tip of the upper burner. Flow is reversed, providing good mixing, and the relatively gradual expansion of the gas due to the cone eliminates recireulation zones which can cause nonuniform radial dispersion of the coal. The upper burner is 91 cm in length and 11.4 cm in diameter. Near the tip of the burner is a 5 cm segment of aluminum honeycomb with 0.635 cm hexagonal cells which straightens and effectively laminarizes the flow. A pneumatic vibrator, mounted on the top of the upper burner, ensures the continuous and even flow of coal through the matrix. The upper burner design is the key to obtaining
r_~Coal F'eeder~~~Pneumatic Vibrator l'l ~a
U
UpperBurner (91crnx 11.4crn) F-J--]---'I ~clasure I I I To Analysis Honeycornb,I Matrix--~l~l I L"J ' J Probeand meter
Nitrogen Screens-z'l
I
I (20.3crnx 11.4era)
FIG. 2. Schematic of apparatus.
1191
a flat, stable flame and depends primarily upon striking the proper balance between good mixing to enhance coal dispersion followed by flow straightening to laminarize the flow. Flow straightening also involves a trade-off since a matrix with cells which are too large does not straighten the flow, but cells which are too small will plug. Similar comments can be made regarding the length of the matrix. Milne and Beachey1/ report similar considerations with respect to flow straightening in the design of their premixed flat flame burner. The design of the bottom burner is quite straightforward since straightening the gas flow is easily done with 50 and 100 mesh screens. The coal flame is ignited using a natural gas flame until the burners are sufficiently hot. The natural gas is then eliminated and the coal flame sustains itself. Typically, the flame reached a steady state stability about 15 minutes after ignition.
Sampling and Analytical System Gas and solid species were sampled using a 0.635 cm O.D. stainless steel tube with an internal water spray at the tip to immediately quench any reactions that could take place in the probe and to prevent plugging. The sample, consisting of liquid water, water vapor, gas and solid coal particles, passed through a condenser/particulate collection chamber and then proceeded to be analyzed by continuous analyzers for CO, CO~ and Oz. In addition, a Perkin Elmer Sigma 1 chromatograph was utilized to measure H a and CH 4. Limitations in sample flow rates, set by the on-line instruments, thwarted truly isokinetic sampling when gaseous species profiles were being measured. The effect of this was to introduce an error in the spatial resolution and to exaggerate the measured flame thickness as determined from the gaseous species measurements. When solid samples were being withdrawn, sampling velocities were much closer to isokinetic, as determined by estimates of the radial velocity at the point where the sample was extracted. Therefore, the solid composition profiles presented here represent the reaction zone thickness more accurately than do the gas species profiles. The size of the sampling probe (OD = 0.6 cm), however, introduced additional errors in spatial resolution, in regions where there were significant gradients over that distance. Temperature was measured using Type K Inconel shielded and magnesia insulated thermocoupies. Following previous workers, 4)'2/ temperatures are reported as-measured. The coal used throughout this study was a Utah Bituminous, pulverized 75% through 200 mesh with a mass median diameter of 60 microns. Analysis, percent by weight, dry basis, was given by: C = 69.99, H = 5.15, S = 0.98, N = 1.22, Ash = 9.97;
COAL COMBUSTION TECHNIQUES
1192
Proximate Analysis, percent by weight as received, by: Volatile Matter = 40.6, Fixed Carbon = 44.3, Moisture = 5.7, Ash = 9.4.
Results The parameters chosen to characterize the flame are: 1. 2. 3, 4.
Oxygen concentration in the bottom burner. Coal concentration in the top burner. Burner spacing. Gas velocity at the burner exits (in this work velocities were always matched at the burner exits).
The last parameter, gas velocity at the burner exit, is expressed in terms of the cold flow "stretching rate," defined as: ;)z
D where D is the burner spacing, and v z is the velocity of the cold flow at the burner exit, This is not the actual stretching rate applicable to the combusting flow but is utilized here merely as a correlating parameter. The actual stretching rate is affected by gas density variations and becomes an eigenvalue when both jet velocities are specified a p r i o r i , 7)
Fig. 3 shows a close-up photograph of a flat, pulverized coal, laminar opposed jet diffusion flame, stabilized with no supplemental gaseous fuel and no oxygen in the fuel stream. The oxidant stream contained 85% by volume O2. The cold flow stretching rate was 2.15 s-1 and the coal concentration was 240 mg/liter. The burners, spaced 3 cm
apart, are clearly visible and the luminous zone occurs about halfway between them. It is apparent that the larger particles do penetrate the gas stagnation plane, but the flame front is visually flat over one half of the burner diameter. This flatness implies one dimensionality in temperature and species profiles as predicted by the similarity solution. Fig. 4 is a plot of the flammability limits for the apparatus described above. We make no claim that this plot is universal and unaffected by, for example, changes in system adiabaticity. Fig. 4 plots flame extinction points as a function of oxygen concentration in the oxidant jet and stretching rate for an approximately constant coal density. Variations in the coal mass delivery rate caused the differences in density reported in each experiment. Since premixed flames of pulverized coal in air exhibit flammability limits of the order of 100 mg/liter coal concentration as a lean limit to well over 1500 mg/liter as a rich limit, 3) the variation in coal density in Fig, 4 is not excessive. Fig. 4 demonstrates that relatively high oxygen concentrations are required in the oxidant jet as well as very low stretching rates. By way of contrast, a methane opposed jet diffusion flame can be easily stabilized with air at 25 see -1 although a diluted gaseous flame representative of the volatile content of the coal alone exhibits similar low stretching rate extinction limits. Stretching rates below approximately 1.9 sec -1 result in unstable flames because the buoyant edges of the flame appear to become dominant. Then the 90,
P-Cool Density, mcj/liter 85
~=
r I
/
,, 9 / / 9 ( ~ 2 5 0 )
SO 75
~9
o
x2
-~
9
(p:200)
~9"~= g vo
8-
u.
65
~ .
{ o
=E
6O
55
FIG. 3. Close up photograph of pulverized coal, flat, laminar, opposed jet, diffusion flame. Zero supplemental gaseous fuel; zero O2 in gas transporting coal from top, Standard flame: Burner spacing = 3 cm; O2 in oxidant jet (bottom) = 85%; e = 2.15 s-~; p = 240 mg/liter
g
9
~1
9 p=240)
/
/ /~
Extinction
/ ,(,~:23o,
Burner Sl~ocing=3cm
,50
I L5
I 2.0
I
I 2.5
L
30
Stretching Rote, sec-~ (Cold Flow ) FIG. 4. F l a m m a b i l i t y
limits. Burner
spacing 3 era.
1193
STRUCTURE OF PULVERIZED COAL flame, while perfectly capable of sustaining combustion, becomes unsteady and is no longer flat. At a given oxygen concentration, increasing the stretching rate will lead to flame extinction, represented in Fig. 4 by the area to the right of the extinction curve. Thus it can be seen that the extinction curve intersects the minimum stretching rate line at a fairly high oxygen concentration (-45% O2) severely limiting regions of applicability of this flame. However, the area between the two curves is judged adequate for structure analysis. In general, thermal effects appeared to be important for flame stability. For example, increased burner exit temperatures facilitated flame stabilization. On the other hand, introduction of the cold water quenched probe too far into the flame could cause it to become extinguished. However, with = constant, burner spacing does not significantly change the temperature profile within the flame zone, Fig. 5, indicating that the model (which is valid strictly only for infinitely wide burners placed at ---~), may have applicability to phenomena occurring in the neighborhood of the peak temperature zone. One dimensionality in temperature and major species concentration profiles for the standard flame are shown in Fig. 6. These profiles, with points taken at different radial positions demonstrate that temperature and species concentrations depend only upon axial, not radial position in the flame, as predicted by the theory and inferred from the pho-
MOO
- -
II
0 Type K T C
O0 m
IO00
O O
9OO
o []
c~
o
0
~. 8 o c 0
o
700 - - 3 c m ~ Burner Spocing Corresponding to 0 Points
60C
50C
~
~ 4 , 5 c m - Burner Spacing Corresponding to n
__.L - 20
-
--
d I I
Poinls
~ - 1.0
.L_O0
1,0
ZO
Relative Position, cm
FIG. 5. Temperature profile as function of burner spacing. TC at 2.75 cm from axis. Standard flame.
40
=-o~ lz75. m
O-CO ' ~" A - CO21from Ax,s @-CO " / / / c r n
O-Tern0Z65c~1
,>.o
/
0y
'fromAx~, %_ } '~l/O-Temp, 2.75cm I100
/
x
r z5
g
g 5
aoo 20 700
C, ,5
/,02
- -
ilk6 5OO
0 -L5
-LO
--0,5
I
9~
~\
0,0
0.5
I.O
,400 i.5
Rek~tive Position, crn
FIG. 6, Demonstration of one dimensionality of flame. Standard flame; burner spacing 3 cm. tograph on Fig. 3. Exaggerated confidence should not be placed in the absolute values of species concentrations shown because of non-isokinetic sampling referred to above, and so the profiles indicate a broader reaction zone than is probably actually the case. Subsequent data obtained at much lower sampling rates and utilizing the gas chromatograph indicated a reaction zone of about half the width of that inferred from Fig. 6. The data in Fig. 6, however, are useful since they do support (albeit crudely) the one dimensionality of this diffusion flame. The temperature pro{lies are unambiguous, although they report uncorrected thermocouple measurements. Milne and Beachey 1) report that shielding the thermocouple did not change the measured temperature in their premixed p.f. flames and their peak flame temperatures and characteristic distances are similar to those in this study. Fig. 7 is the solid species profile of the same flame. The volatile content and fixed carbon percentages are calculated in the usual way from the proximate analyses using ash in the collected sample as a tracer. The profiles demonstrate there is essentially no pre-flame zone pyrolysis taking place, even though the temperature close to the burner exit is 800 K. Once the particle reaches the flame zone, however, devolatilization occurs very rapidly. Although it is well known that a proximate volatiles test does not yield the true "extent of devolatilization" it should certainly serve to indicate the onset of, and is a relative measure of, the extent of devolatilization. Furthermore, no evidence of hydrocarbon volatiles was observed. The highest levels
1194
COAL COMBUSTION TECHNIQUES
ties for the coal opposed jet configuration were approximately 10-15 cm/sec. This demonstrates the 90, L Fixed fact the premixed and diffusion flames are fundamentally different in character. In other respects, the diffusion flame and the premixed flame are surprisingly similar. For ex80 ample, the experience of Milne and Beachey~) that a water-cooled flameholder-grid would not sustain oo 70 w a pulverized coal flame suggests sensitivity of the Probe Width premixed flames to thermal effects. Smoot and -~0.6 cm Hortona/ note that reported differences in flame speed measurements may be due to the differences Volatile in the adiabaticity of various configurations. The Matter sensitivity of the coal opposed jet diffusion flame "6 50Uncertainty in Analysis ~ • to thermal disturbances has been documented above. Peak flame temperatures in premixed and diffusion =~ 4 0 flames of the same coal concentration are both on the order of 1000 K (measured temperature). Ox~ ygen, carbon monoxide and carbon dioxide profiles ~" 3 0 - Data Normalized behave in a similar manner, and after accounting _ Using Ash Content Appraximate for the considerably higher oxygen concentration -Visible Flame af Collected Sample E Zone used in this study, the peak values are compatible. ,~ 20Both flames appear to be primarily coal volatile flames and both display low concentrations of pyrolysis products, H 2 and CH 4. However, these spe-I.5 -I.(3 -0,5 (3.0 0.5 1.0 L,5 cies reach levels of 1-2% in premixed flames (deRe(Qtive Distance, em pending upon stoiehiometry) and are observed at only trace values in this diffusion flame. The exFIG. 7. Volatile matter and fixed carbon profiles. planation may be the higher oxygen concentration Standard flame; burner spacing 3 cm, used in the diffusion flame. Flame zone thicknesses are both on the order of 5 mm. The extent of devolatilization and solid species of Ha and CH4 found anywhere in the flame were profiles through the flame are also similar3) to pre0.1% and 0.01%, respectively. mixed laminar flames. The diffusion flame gas and solid species profiles show very little pre-flame zone pyrolysis even though the burner exit temConclusions and Discussion perature is 800 K, which is hot enough to initiate pyrolysis in bituminous coal. TM The coal heat-up The pulverized coal, laminar opposed jet diffu- time was approximately 150 milliseconds. Both Milne sion flame was stabilized in the laboratory with no and Beache~ ) and Howard and Essenhigh s) report oxygen in the transporting fluid and with no sup- very little pre-ignition zone pyrolysis. The model plemental gas. Such a flame is flat and is one di- of Smoot et. al. 4) also predicts that, in a premixed mensional in temperature and species concentra- flame, the coal particles do not begin to devolatiltions, and can be modeled by a set of ordinary ize until they enter the flame zone. The model differential equations even though particles do not predicts the major significance of the effects of follow gas streamlines and may even penetrate the molecular diffusion and gas-particle heat conduction stagnation plane. on flame propagation. Our observations on similarThe test of flammability limits reveals that flame ities in flame zone thickness, structure, and stabilstability in the diffusion flame configuration is rel- ity between premixed and diffusion flames suggest atively difficult to achieve. It does not appear pos- that the dominant physical and chemical mechasible, with the apparatus used in this study and in nisms may be similar for both flames. the absence of preheating one or both jets, to staThe absence of pre-flame pyrolysis further inbilize a pulverized coal flame in the opposed jet dicates that the specific laminar opposed jet diffuconfiguration with air. Higher Oz concentrations sion flame presented in this work would not be are required in order to obtain sufficiently high suitable for the study of coal devo|atilization. Fuflame temperatures. In contrast, experiments with ture work will focus on other methods of increasing premixed flames indicate that flames can be main- the peak flame temperature and thus also the diftained at a 20--30 cm/sec cold flow velocity, with fusive heat flux towards the fuel jet, thus decreasair, before blowout. Maximum burner exit veloci- ing the particle heat-up time. I00
STRUCTURE OF PULVERIZED COAL Nomenclature
b
cj Cr
Cz Dn FDj
f(r) g g(r) M~j rh~j
see Equation (4) velocity vector of particle of size class j, cm/s particle radial velocity, cm/s particle axial velocity, crn/s diameter of particle drag force on particle of size class j see Equation (1) gravitational force see Equation (1) mass of particle of size class j, gm devolatilization rate of particle in size class
j, g/s nv
P r 1)r 19z p~
*(z) *(z)
particle number density, 1/cm a pressure radial co-ordinate, cm gas radial velocity, c m / s gas axial velocity, cm/s stretching rate, 1/s viscosity of gas mixture, g/cm-s see Equation (4) similarity function--Equation (1)
1195
3. SMOOT, L. D. AND M. D. HORTON, "Propagation of Laminar Pulverized Coal-Air Flames," Prog. Eng. Comb. Sci., 3:235 (1977). 4. SMOOT, L. D., M. D. HORTONAND G. M. WILLIAMS, "Propagation of Laminar Pulverized CoalAir Flames," Sixteenth Symposium (International) on Combustion, p. 375 Combustion Institute, Pittsburgh, PA (1977). 5. HOWXaD, J. B. AND B. H. ESSENHIGH, "Pyrolysis of Coal Particles in Pulverized Fuel Flames,"
I. and E. C. Process Design and Development, Vo. 6, No. 1, 74 (1967). 6. WENDT, J. O. L., D. W. PERSHING, J. W. LEE, AND J. W. GLASS, "Pulverized Coal Combustion: NO~ Formation Mechanisms Under Fuel Rich and Staged Combustion Conditions,'" Sev-
enteenth Symposium (International) on Combustion, p. 77, The Combustion Institute,
Acknowledgment This work was supported by the U.S. Department of Education through a Domestic Mining, Minerals and Mineral Fuel Conservation Fellowship.
REFERENCES
1. MILNE, T. A. AND J. E. BEACHEY,"The Microstructure of Pulverized Coal-Air Flames: I. Stabilization on Small Burners and Direct Sampiing Techniques," Comb. Sc/. and Tech., 16".123 (1977). 2. M1LNE, T. A. AND J. E. BEACHEY, "The Microstructure of Pulverized Coal-Air Flames: II, Gaseous Species, Particulate and Temperature Profiles," Comb. Sci. and Tech., 16:139 (1977),
Pittsburgh, PA (1979). 7. HAHN, W. A. AND J. O. L, WENDT, "'NO~ Formation in Flat, Laminar, Opposed Jet Methane Diffusion Flames," Eighteenth Symposium (International) on Combustion, p. 121, The Combustion Institute, Pittsburgh, PA (1981). 8. LIU, T. M. AND P. A. LmBY, "Boundary Layer at a Stagnation Point With Hydrogen Injection," Comb. Sci. and Tech., 2:131 (1970). 9. GRaves, D. B. AND J. O. L. WENDT, "Flammability Characteristics of a Pulverized Coal, Opposed Jet, Flat, Laminar Diffusion Flame," paper presented at First Specialists' Meeting (International) of the Combustion Institute, Bordeaux, France, July, 1981 (unpublished). 10. CROWE, C. T. AND L. D. SMOOT, Chapter 2 in
Pulverized Coal Combustion and Gasifwation: Theory and Applications for Continuous Flow Processes, Eds L. D. Smoot and D. T. Pratt. Plenum Press, New York (1979).
11. POHL, J. H. AND A. F. SAROFIM, "'Devolatilization and Oxidation of Coal Nitrogen," Sixteenth Symposium (International) on Combustion, p. 491 The C o m b u s t i o n I n s t i t u t e , Pittsburgh, PA (1977).
COMMENTS Prof. H. S. Mukunda, Indian Institute of Science, Bangalore, lndia. In the photographs I notice that the flame is bending over towards the upper tube. In an earlier study by Dr. Raymond Kushida at JPL, a nitrogen gas shower around the burner was used to obtain a strictly planar flame. Would such a device modification give any problem in your system?
Author's Reply. When we attempted to introduce N 2 gas to quench the flame at the burner edge, we experienced increased flame instabilities, They may have been due to changes in burner cooling or to nonunifurmities in the shield gas velocity profile. We did not pursue this idea further since our interest was on the large flat portion of the flame in the neighborhood of the stagnation
1196
COAL COMBUSTION T E C H N I Q U E S
stream line, over a distance of one half of the burner diameter.
Dr. S. Galant, Societe Bertin, France. Another interesting aspect abunt your experiments is obtaining blow-off limits, under strain rate conditions that are well monitored. What type of strain rates can you cover and have you checked the blow-off value that you obtained against standard opposedjet gas diffusion flames? Author's Reply. Strain rates are very low, approximately 2.0 s L. As mentioned in the paper, a methane opposed jet diffusion flame can be easily stabilized with air at 25 s ~, although diluted gaseuus flames representative of the coal volatile content alone exhibit similar low stretching rate extinction limits. This suggests that our coal flame is essentially a coal volatile flame.
Author's Reply. One additional attribute of the pulverized coal, laminar opposed jet diffusion flame might be that it represents a prototype of the stretched laminar flamelets that may exist between the boundaries of fuel and secondary air streams in turbulent diffusion flames. ~ If, furthermore, it is believed that the stretching rate of the prototype can be related to some mean rate of strain in the turbulent diffusion flame, then it should be possible to use the prototype to determine how a) coal composition b) oxygen level in transporting fluid for fuel stream and c) oxidant prelceat, influence extinction limits with air only as the oxidant stream. In this case the "ignition" or extinction test would be designed to simulate the actual phenomena occurring in a furnace. Thus, we believe that, with considerable additional theoretical and experimental work, our flame has the potential to be a useful laboratory experiment to test the ignitability of coals in furnace flames.
REFERENCE
Prof. T. F. Wall, University of Newcastle, Australia. There is a definite need for a reproducable laboratory experiment to test the ignitability of coals. Could the experiment be used to rank coals, and, if so, what experimental variables would be used (e.g., oxidant preheat, oxygen in oxidant, flame speed)?
1. MARBLE, F. E. AND J. E. BROADWELL, "The Coherent Flame Model for Turbulent Chemical Reactions," Project SQUID, Report 29314-6001RU-00, (1977).