- Email: [email protected]
Upper flammability limits of coal dust-AIR mixtures
Eighteenth Symposium (International) on Combustion
UPPER
FLAMMABILITY
LIMITS
The Combustion Institute, 1981
OF
COAL
DUST-AIR
MIXTURES
~*~
B, D E G U I N G A N D AND S. GALANT Societe Bertin dr Cie Division Energetique Centre De Bayonne 40220 Tamps, France
The present paper is a progress report on a comprehensive study of upper flammability limits of coal dust-air mixtures. The study was undertaken in the context of safety in pneumatic conveying systems. An improved &liter combustion chamber was designed which allows dispersal of combustible particles up to concentrations around 5 k g / k g air. The study includes: i)
use of two narrow size distributions of a high volatile content coal (viz. 13 and 50 ~m) ii) measurement of ignition energy requirements iii) measurement of the extent of flame propagation based on the pressure attained at constant volume, and the residual Oa, CO 2 a n d CO content after combustion. Flame propagation was obtained up to concentrations around 3 k g / k g air (30 times the stoichiometrie concentration), weakly d e p e n d e n t upon particles size. Electrical ignition energies of hundreds of Joules were required. Although no direct dust concentration measurements were carried out at such elevated values, good reproduetibility of pressure traces and residual 0 2 , CO 2 a n d CO concentrations measurements up to 2.5 k g / k g air was obtained. This might be interpreted as a proof of constant coal-air dispersion characteristics around the energy source. A discussion of the underlying physical p h e n o m e n a is presented, with some evidence of a radiation enhanced propagation rate. Direct implications for safety in pneumatic conveying systems are drawn, together with an appraisal for future experimental studies concerning various industrial combustible dusts in a simulated pneumatic conveying environment.
1. Introduction
The questions raised by combustion specialists as well as industrial managers are confirmed by three recent disasters in grain elevator systems in France, Germany and Spain. Although the explosion hazard concern has motivated numerous studies on homogeneous gases and vapors [3], there appears to be no consensus among researchers on the flammability limits of heterogeneous substances. A constructive critical review of the known experimental results is presented in Ref. [4]. The same authors present results on new set of measurements for lean limits in coal dust-air mixtures. This study concentrates on the definition of a novel experiment with spatial and temporal measurements of dust concentration and surface temperature. To our knowledge there are very few systematic studies for rich limits, except for results on coal
An accurate knowledge of the upper flammability limit behavior of various dusty materials is essential for a realistic approach to the explosion hazards involved in their pneumatic transport. More than 70% of processed industrial powders are potentially combustible materials. Standard safety regulations include the use of vents [1] or an inert gas carrier. However, the technology involved as well as extrapolation of standard apparatus lead to economically prohibitive solutions. A recent symposium on "grain elevator explosions" [2] shows the acuteness of the present problem in the agricultural industry.
(*) To be presented at the 18th Symposium on Combustion, Waterloo, Canada (1980) 705
706
COAL FLAMMABILITY
dust laminar flame propagation [5] and some specific studies on the flammability of polyethylene powders [6]. This lack of experimental work is certainly due to difficulties in: assessing mixture homogeneity at the time of the energy input ii) monitoring the significant experimental variables iii) appreciating their respective importance. This paper will describe the results of a set of measurements near the rich limits, as well as the effect of particle size and some influence of ignition energy on the propagation rate:
2. Apparatus and Method A sketch and a functional block diagram of the apparatus is shown in Fig. 1. It is essentially an 8-liter modified form of the Hertzberg bomb [4] to which improvements were made to handle large dust concentrations. Much exploratory research went into the design and optimization of the dispersion system. Essentially, the same apparatus as the one described in Ref. [4] was used. Systematic checks were made of the coal dust collected after dispersion by the cylinder and top walls. An average 20% mass fraction adheres to the wall whereas the remaining 80% falls back into the dispersion cup. No concentration monitoring was therefore implemented. Instead, repeated experiments were carried out, keeping all experimental values constant. Pressure traces as well as measured concentration values were found to be repeatable within experimental error (+ 20%) except near the upper flammability limits. This point is discussed later. A typical ignition experiment is conducted as follows:
pump
Elr
i)
--high volatile content coal (36%) was chosen to provide an easily ignitable mixture to which potential modelling information was already available [7]. Narrow size distributions around 13 and 50 p.m were used --improvements were made to qualify the extent of flame propagation via measurements of pressure at constant volume, and the residual 02, CO~ and CO content after combustion --the dispersion method is an extension of very recent techniques to study lean limits [4] which have proven to give reproducible results for a wide range of dust/air mass ratios.
~VocuqJm
Pmssurt tmnsducer~
anatys~s Electrode
Air dispersion tank ~.5 L.
a measured sample of coal is placed in the cup around the dispersal cone ii) the 8-liter vessel is initially evacuated to 1/41/3 atm iii) a dispersal pulse is turned on and lasts until
r
$
Time~
IPmssunl and/ol, spolk cncroy rtcotding ]
Fie. 1. Sketch and functional block diagram of the BERTIN 8-liter bomb (modified version of Ref. 4 bomb to handle very high dust concentrations)
chamber pressure again reaches 1 atm iv) a concentration-dependentdelay is imposed before the ignition source is energized v) during flame propagation a pressure trace is recorded to characterize combustion mechanisms (if any) vi) after flame propagation has ceased, combustion gases are sampled through a water-cooled nozzle and analyzed in continuous monitoring systems. CO and C02 are analyzed via infra red Sehlumberger systems, whereas 02 is measured by a paramagnetie analyzer (Sehlumberger). The ignition systems consists of four electric match heads whose ratings are 34 mA under 8000 volts. They can be energized separately to make ignition energies vary over a wide range. Effective VAp requirements were measured in separate experiments to yield the following semi-empirical rules: E,f = K (Ax,n) nt where
t
(Joules)
is the time after sparks are turned on
i)
~
(s)
n number of electric match heads K(Ax,n) is a function of the electrode spacing, and the number of electrodes which was determined experimentally [7].
UPPER FLAMMABILITY LIMITS OF COAL DUST-AIR MIXTURES
707
TABLE I Coal properties
3. Coal Dust Characteristics Coal dust was chosen for these first upper limit experiments which show that:
Name : Freyming Volatile content Ash (for 10 and 50 micron) Carbon Oxygen Hydrogen No ash analysis was carried out. Higher heating value
- - h i g h volatile content facilitates ignition mechanisms - - a parallel modelling study was undertaken to elucidate experimental results. Enough information could be gathered to set up a realistic unsteady coal-air flame propagation model [7]. The coal reference is Freyming, 35% volatile content. Proximate analysis was carried out. Results are presented in Table 1. Size distributions for the two diameters considered are presented on Fig. 2 which represents cumulative weight percentage as a function of diameter. Size analysis was carried out with water-coal mixtures passed through a series of meshes. The smaller mesh size was 10 microns. The measured distributions are thought to be quite representative of monodisperse systems around 13 and 50 Ixm.
: 35% : 10% : 77% : 8% : 5% : 31 M J / k g
4. Experimental Results A systematic investigation was made of the influence of coal particle concentration, coal particle diameters and ignition sources requirements on the explosibility of the coabair mixtures. Combustion was characterized by two measurements:
Mass fraction of particles with diameter < d
99.5.
90. 80. 70. 60. 50. 40. 30.
0
d50=13prn
=
20. 10.
3. 2. 1
0.5
Particte diarneter(Fm~) I
'
'
'
'
'
I
10
,
.
.
,
.
,
I|
100
I
I
w
1000
Flc. 2. Mass fraction of particles with diameter less than d. Experimental points obtained for the two coal sizes studied.
708
COAL FLAMMABILITY
--pressure development (rate of pressure rise and peak pressure ratio) --residual O=, CO~ and CO concentrations. 4.1. Pressure Development The explosibility hazard of a dust-air mixture is often characterized by the rate of pressure rise. Some pressure growth curves for the two sizes studied are shown in Fig. 3 as a function of coal concentration. The lowest concentration studied is 0.5 k g / m 3 which is just the upper value studied by Hertzberg [4]. The highest concentration depicted is 3 k g / m 3 near the upper flammability limit for the 13 p.m particle size. Results are as follows: - - b o t h peak pressure and maximum rate of pressure rise decrease monotonically with increasing concentration. They are indicative of an "average" decreasing flame temperature. --pressure development does not depend too markedly on the particle size (except near limits), but is a function of dust concentration. The very same result was obtained by Hertzberg et al. [4] in their study of lower flammability limits. --near the upper limits, a lack of reproducibility was obtained for both coal particle sizes.
d50=13pm ~ - - ~ d50 =50pro =
. Pressure(bors)
0.5kglm3
Pressure traces tend to flatten out with very low rate of pressure rise. The evolution is confirmed on Fig. 4 where rates of pressure rise (two definitions) are plotted as a function of dust concentration. The monotomic decrease from 0.5 k g / m 3 exhibits quite a large spread near extinction (2.5 to 3 kg/m3). - - t h e pressure curves allow the calculation of burning velocities as carried out by Hertzberg [4]. Values around 0.25 m / s at 0.5 k g / m ~ are comparable to the ones obtained for methane-air mixtures in the same apparatus [4] and coal dust-air burner supported flames [5]. 4.2. Residual Gas Concentration Measurements These measurements were conducted to further characterize the completion of the oxidation reactions and to identify possible mechanisms which lead to sustained propagation at concentrations 30 times the stoichiometric ratio. On Fig. 5, CO e and O= volume concentrations (dry basis) are plotted as a function of coal particle concentrations, for two diameters and various ignition energies. The following comments must be made: --residual Oz concentrations are always very low (<1%) even near the upper flammability limits, This is indicative of complete oxygen depletion via volatile oxidations and possibly the beginning of carbon-oxygen surface reactions - - C O 2 maximum concentrations seem to occur near the flammability limits at d = 50 p,m, with a corresponding depletion of CO molecules. This maximum does not occur for smaller particles,
~ . o , . o, 0 . . . . . . . .
(bars/s)
//~m,~,
,.
/ ~ , ~
1kg/m3 0f
o'.1
o'.z
o'.3
o'.~
ZOC
S/
"dso{prn)Z~PM/~t ~Prnl~t
'5;t:l
8,
?7"-
2.
~.
oi(
o'.e
6. 4.
150 T--
:
100
3 kgI m 3 . ~ . ~ . -'-.. . ~ . ~ . S kg/m3
z.
0
0
o13 o2~
2kg/m3
0'A
, 0.Z
~ 0.3
.,~
0.~
9
Timeafter, ~ dispersionpurse(s)
Fie, 3, E v o l u t i o n o f chamber pressure traces as a f u n c t i o n o f t i m e for d i f f e r e n t coal d u s t / a i r concentrations and t w o particle sizes.
FIG. 4. E v o l u t i o n
o
Concentration
of the rate of pressure risk
( b a r / s ) as a f u n c t i o n o f coal eoneentration, T w o
possible definitions of rate of pressure risk are used.
UPPER FLAMMABILITY LIMITS OF COAL DUST-AIR MIXTURES ~ t */* vol CO2 dry
~ */ovol 0 2 dry
1
1
9
0.8 0.6 0.4 0.2 al concentration
6. 4.
o.s
1
1.s
~%vol C02 dry
It. ~
2
2'.5
~
3'.s
~
%vol 02dry 21 7 .6
12.
10.~1 8. 6. t,_
~
.5 /, .3 .it
~-~
o'.s
~kg/m3j
Cs
~
2'.s
~
3'.5
(kg/m3)
FIG. 5. Final gas concentrations as a function of coal concentrations and particle sizes.
also 13 Ixm
viz. "icy" sugar and laurel powder. Such dusts are supposed to produce less volatile products with potential gas phase oxidation mechanisms. In Fig. 6, final CO and CO2 concentrations are plotted as a function of dust concentration. Sharp CO concentration maxima occur in a similar way to coal particles. In other words, no straightforward differences in rate limiting mechanism could be deduced from such experiments. Only complete numerical simulation studies [7] will be able to closely identify the various rate limiting steps which allow for stable flame propagation up to 3 k g / m 3 concentrations, together with more systematic and basic experimental investigations. 4.3. Ignition Energy Requirements
COal concentmtion
~
709
1 Elec- 2 Elec- 4 Electrode trodes trodes 9 A 9 % vol Or dry tl) 9 9 % vol CO~ dry
Ignition energy input was varied by switching off one, two or three of the four sparks. Due to the very small volume between the four electrodes (less than 100 mm3), the respective positioning of the various working electrodes had no strong influence on the ignition characteristics. Limit concentrations as a function of energy input are listed in Table 1 for both particle sizes. A comparison of various pressure traces as a function of the number of electrodes and concentration is presented in Fig. 7. For both particle sizes, increasing the number of electrodes results in a clear
2 Elec- 3 Elec- 4 Electrodes trodes trodes 50 Ixm 9 A 9 % vol 0 2 dry il) ~ 9 % vol CO~ dry
~% CO,CO2 vol. dry basis
~ CO - - - ~ CO2
Cool ~ / ~ / /
which is characteristic of a change in the diffusion-kinetic regimes of carbon oxidation. Further comments are given in Section 6. - - C O concentrations were given special attention. In Fig. 6, we illustrate CO concentration changes for the 10 I~m average size as a function of initial coal concentration. The curves shown are fitted of repeated (3) measurements at identical initial coal concentrations. Surprisingly enough, peak CO concentrations reach 15% (volume fraction, dry basis) near 0.5 k g / m 3 coal concentration. As this concentration increases, there is a net decrease in CO concentration, reaching zero values near the flammability limits. CO 2 concentrations exhibit a shifted peak around 2 k g / m 3. Both evolutions suggest the respective contribution of temperature within the reaction zone, volatile oxidation rates and gas-surface reaction rates. To further characterize possible independence of one or more of these parameters, runs at identical concentrations were carried out with vegetable powders,
\\
Icy SugQr
10~.
Laurel
5-
05
1
FIG. 6. C O
15 2 25 Content r(tt ion, kg/m 3 and
3
CO 2 concentrations
(vol.
dry
basis) as a function of dust loading (kg/m3). Studied dusts are: coal 10 Ix, laurel powder and "'icy" sugar.
710
COAL FLAMMABILITY ignition point, the possibility of particle agglomeration and spark simulation.
Pressure{ bars )
lkg/rn 3
~J/
1.Skg/m 3
//
i / ~ j 0'.1
T ~ aft~ eod d
0'.2
0'.3
~4
dispersion pu|~(s)
1 ELectrode 2 Electrodes
....
Pressure (bars)
O.5kg/m3
1.5kglrn3
9
0
2 0'.1
..... 0.2 0'.3 ....
2kg/m 3
tkg/m3
O',t,
/
after er~d of dispersion putse($ )
2 Electrodes 3 Electrodes
FIG. 7. Evolution of chamber pressure traces as a function of time for different energy input.
tendency to steepen pressure profiles and to somewhat shorten the ignition delay. Therefore not only absolute energy values but also deposition time and volume are of importance in determining upper flammability limits. An independent assessment of the ignition energy characteristics (power per unit volume) should be made before true limit concentrations can accurately be measured. Similar conclusions are reached by Hertzberg et al. in their lower flammability experiments. It is also worthwhile to point out that much development work was done to electrically insulate electrode materials from the coal air mixture. Intense skin effects were noticed at large coal concentrations when electrical insulation was poor. Microdischarges between coal particles could originate at ambient pressure along the electrodes and even on the dusty chamber walls. At present, it is not clear whether microdischarges could influence ignition phenomena at very high concentrations.
Concentration Measurement For the time being, no real answer can be given to the problem of achieving homogeneity near the spark gap. For one thing, concentrations are so high that standard optical techniques [4] cannot work at all. Only beta-ray transmission could be of interest 14[. We tried to resolve the problem by repeating measurements, everything else being held constant. Reproducibility (up to 2.5 kg/m 3 concentration) was good for both sizes studied. The importance of bad dispersion should have been observed at large sizes. Free failing velocities are larger and so are body forces between particles at large concentrations. This point led us to assume that constant dispersion characteristics near the electrodes were obtained for given air pulse conditions. However, this does not prove that average concentrations were reached near the electrodes. In other words, present upper flammability results should be considered on a relative basis.
Particle Agglomeration During their experiments on lean flammability limits, Hertzberg et al. [4] showed the existence of agglomeration phenomena above a 0.3 kg/m 3 concentration, with the agglomeration varying as the coal diameter and origin changed. It is clear that no microscopic dispersion defects (agglomeration) could be monitored at the concentrations studied. However, it is assumed that inefficient microscopic dispersion is due to from the presence of strong surface forces, i.e. at small particle sizes. Since consistent diameter dependence was found during these experiments up to 2.0 kg/m 3 for both particle sizes, it likely that: i) either agglomeration effects would become important only at very high concentrations for the type of coal used ii) or agglomeration effects have reached some asymptotic state, which is not modified further by higher concentrations.
Spark Simulation It should be improved in two ways:
5. Discussion of the Experimental Technique It is justified to question whether the present set of data could realistically answer the problem of explosion safety in pneumatic conveying systems. Although improvements were made to obtain homogeneous dispersion, answers are still needed to the problem of the concentration evaluation near the
--better monitoring of energy disposition to make absolute energy measurements for sustained flame propagation. An apparatus has been designed [8] which is going to be used in subsequent experiments. It allows separate monitoring of voltage, intensity, deposition time and volume of the spark. --simulation of realistic spark generation in pneumatic conveying systems, viz. electrostatic
UPPER FLAMMABILITY LIMITS OF COAL DUST-AIR MIXTURES discharges. It is thought that the apparatus described [8] will be suited for that purpose. 6. Discussion of the Experimental Results The results obtained cannot be explained in terms of equilibrium thermodynamics. We use the classical thermodynamic model of flammability limits [91 to compute upper limit concentrations. For the coal considered, a 0.6 kg / m'~air concentration was found, which is of course much lower than the limit concentrations actually measured. The physics of the results can be discussed in terms of three phenomena: i)
concentration stratification at the time of ignition ii) kinetic versus diffusion controlled propagation iii) radiation enhanced flame propagation.
Concentration Stratification The lack of reproducibility of ignition phenomena near the "upper" limits could originate from concentration stratification near the electrodes. This concentration is of course size dependent: at constant d.~o diameter, it originates from the polydisperse nature of the coal particles. Ignition may then occur in a favorable (i.e. smaller diameter) concentration domain and propagate steadily within richer or leaner regions. Dixon-Lewis [10] has shown numerically that ignition in gaseous mixtures may occur above the upper flammability limits only by radical species diffusion. Similar phenomena may occur in two-phase mixtures. In our experiments, oxygen depletion was always observed to be complete up to very near the concentration limits. Further experiments with changes in electrode positionning are therefore needed to elucidate stratification enhanced flame propagation. Kinetic Versus Diffusion Controlled Propagation Coal-dust / air flame velocities derived from bomb experiments are around 25 cm/s. The agreement with a one-dimensional burner-supported [5] laminar flame is quite good, which suggests that general rules for laminar flame propagation of high volatile coal are applicable in our experiments. The conclusions drawn from Ref. [5] simulations (without radiation modelling) are that: the volatile oxidation (gas phase) and the heterogeneous reaction of the carbon (char) are not rate-limiting; ii) the rate of devolatilization is the limiting kinetic step; iii) the rates of conduction and diffusion in the gas phase, as well as the rates of conduction between the particle and gas phases, are of major importance.
711
In our experiments, high CO concentrations result from a combination of ii) and iii). At high coal concentrations, maximum flame front temperatures decrease with a slowing down of the devolatilization and CO oxidation reactions (mainly CO + OH), However, the size dependency observed in the CO2 change as a function of coal concentration cannot be explained solely in terms of the above conclusions. In fact, known devolatilization rates do not depend upon particle size whereas gas-to-particle conductive energy exchange should decrease as concentration increases (since the maximum temperature declines). A possible explanation is given below. To support such conclusions, a model of unsteady one-dimensional coal-air flame propagation was built along the lines described in Ref. [7]. Although complete results will be reported later, it can be said that preliminary calculations on the size dependency of propagation rates showed increased particle diameter influence as the coal concentration was increased. Competitive effects do occur, including temperature decrease due to lower flame temperature, decreased conductive and convective gas-toparticle heat transfer, but there is increased radiation through a suspension of bigger particles. Special attention should also be given to convective heat transfer modelling between particles at very high concentration, which should notably modify flame quenching mechanisms through particle agglomerates.
Radiative Heat Transfer The dependency of CO and COs concentrations in Fig. 5 may be explained in terms of increased radiative heat transfer. As the number of particle increases (i.e. smaller sizes at constant concentrations), a shielding effect occurs, preventing heat losses from the flame front toward the cold particles. Shield effects are not as prominent for 50 ~xm particles as for 13 I~m particles, Hence, the slow increase in CO 2 concentrations for 50 tLm particles around 2 kg/m 3 concentration could be due to increased devolatilization rates. A more systematic experimental and modelling study is needed to investigate such radiative shield effects, Recent work on coal dust modelling dwelt upon the necessity of two-dimensional radiative energy transfer in coal dust-air flame systems [11].
i)
7. Safety in Pneumatic Conveying Systems The present experimental results support the potential explosion hazards, at least in coal-air pneumatic conveying systems. Standard configurations work at concentrations between i and I0 kg/kg air. Observed combustion phenomena up to 3 kg/m :~ underline risks in stratified configurations such as
712
COAL FLAMMABILITY
=Effective e~r
give an asymptotic coal concentration limit between 4 and 4.5 kg/m3).
(joules) 9 dso=13pm I d50=50 }Jrn ]'his
ii) influence of flow velocity upon ignition charac-
teristics
work
A work similar to the approach taken for gaseous mixtures must be undertaken [12,13]. iii) modifications of flammability limits by proper
inerting iv) risk assessment in current conveying technology, taking into account flammability characteristics in both static (cyclones) and dynamic configurations.
Rcf,(4)
~Z~,-~-0.1
Conc~tration 4 (kg/m 3)
FI(;. 8. Comparison of lower flammability limits (Ref. I4] ) and upper flammability limits obtained from the present work.
Future program work will deal with flowing systems since a full-scale, closed-loop, pneumatic conveying system has been built for experimental simulation purposes. Spark discharges will be released within the flowing system at different locations to study modifications of the so called "static" (i.e. bomb) experiments. Moreover various dusts of industrial importance will be studied. Work has already begun on metallic powders and will be reported later.
8. Conclusions cyclones or sudden changes in direction. The problems that remain to be resolved are: i)
simulation of electrostatic or electric discharges in static configurations and experimental determination of upper limit concentrations In Fig. 8, we compare effective ignition energies (measured as VAp) for lower and upper flammability limits. Lower flammability limits are a fit of horizontal limit propagation experiments carried out by Hertzberg et al. [4]. Our results from Table 2 are reported in the same plane. There is a net trend toward upper flammability limits for the coal studied (a linear plot would
A comprehensive study of the upper flammability limits of coal dust-air mixtures has been presented in the context of safety in pneumatic conveying systems. Original work concerns: i)
improvements on a 8-liter combustion chamber to disperse combustible particles up to concentrations around 5 k g / k g air ii) use of two narrow size distributions of a high volatile content coal (viz. 13 and 50 Izm) iii) measurement of effective ignition energy requirements (in the Joule range) iv) measurements of the extent of flame propagation based on pressure attained at constant volume,
TABLE II
Number of electrodes
Spacing (ram)
Effective <~ power (watts)
Deposition (~ time (s)
Particle size
(~m)
Limit concentration (kg/m a)
2 2 2 2 2 2 2
0.32 0.90 1.80 0.32 0.90 1.35 1.80
0.10 0.15 0.25
13 13 13 50 50 50 50
1.75 2.0 3.25 0 1.25 2.5 3.0
oo
0.10 0.25 0.25
(a) last measurement before no propagation occurence. Effective power is the slope of Vhp as a function of time at t = 0.
UPPER FLAMMABILITY LIMITS O F COAL DUST-AIR MIXTURES and residual 02, CO2 and CO content after combustion v) obtention of sustained flame propagation at concentrations around 3 k g / k g air with weak dependency upon particle size vi) identification of possible radiation-enhanced propagation for large size particles a n d high concentrations. On the basis of the results obtained, it can be said that a basis has been developed for a standard methodology to assess explosion hazards of pneumatic conveying systems. Systematic experimental work should proceed to quantify absolute ignition energy requirements and their modification in flowing system. Supporting model studies using a new unsteady code will help to analyze the experimental results, especially at large concentrations where the nonlinear coupling between heat transfer and kinetics seem to be very important.
Acknowledgements The present study was financed by the F r e n c h "D61bgation Gbnbrale h la Recherche Scientifique et Technique" (D.G.R.S.T.) under contract n u m b e r 78-7-0262. Fruitful discussions with Prof, N. MANSON, Dr. BOURIANNES and Dr. CAMPOS of the "Laboratoire d'Energ6tique et de D6tonique" (Poitiers) are acknowledged. Pieces of information obtained from Dr. M. H E R T Z B E R G of the US Bureau of Mines were also of invaluable help at the start of the study.
713
REFERENCES 1. D. BRADLEYANDA. MITCHESON. Comb. and Flame, 32, 221 (1978) 2. N. MAt~soN. "Compte rendu de m i s s i o n - Symposium on Grain Elevators" January 1979 3. M.G. ZAaETArlS. Bureau of Mines, Bulletin 627 (1965) 4. M. HERTZBERG, K.L CASHDOLLARAND ].F. OPFERMAN. Bureau of Mines, Report of Investigation 8360 (1979) 5. L.D. SMOOT, M.P. Hoa~roN m~o G.A. WlLUAMS. 16th (Int) Symposium Comb. 375 (1977). The Combustion Institute. 6. A. FIUMAa^ AND P. CARDILLO. La Rivista del Comb., 31, p. 115 (1977). 7. J. CRAMBES, B. DEGUINGAND AND S. GALANT, BERTIN Technical Note to appear (March
1980). 8. M. MOREAU, R. BOURIANNES. Technical Note E N S M A / M N 79-2 (1979). 9. K.N. PALMER. "Dust explosions and fires". London, Chapman and Hall (1973). 10. G. Dlxo~-LEWlS AND I.G. SnEPERD. 15th (Int) Symposium Comb., 1483 (1975) T h e Combustion Institute. 11. J.L. KaAZlNSKI, R.O. BVCKINSANO H. KmEa. Prog. Energy Combust. Sci., 5, p. 31 (1979). 12. C.G. DE SOETE. 15th (Int) Syrup. on Comb., p. 735 (1971). The Combustion Institute. 13. D.R. BALLALANDA.H. L~ZVEaVaE.16th (Int) Symp. on Comb., p. 1685 (1977). The Combustion Institute,
COMMENTS C. Huggett, National Bureau of Standards, USA. The increased flammability of the larger particle size sample at low concentrations is surprising at first glance. Do you discuss this in your paper? Is it because in these oxygen-limited systems the total energy available is constant but the smaller particles, because Of their greater surface area per unit mass, exert a greater quenching effect through energy absorption or possibly through surface quenching of reactions? Author's Reply. No thorough discussion on this point is given in the paper. We are still unable to give a satisfactory answer to that question. However this result might originate from two independent reasons: (i~ concentration stratification which is more prominent for large particles: ignition occurs with smaller
particles (low falling velocity) and then deflagration spreads to bigger ones. (ii) large particles do not exert a strong quenching effect, as you suggest, because of their elevated thermal inertia. In the paper we outline the type of further studies that are needed to elucidate such an unexpected behavior.
E. Suuberg, Carnegie-Mellon University, USA. Considering the very high coal loadings studied and the fact that carbon monoxide is often a major product of coal pyrolysis, might not the rather high levels of CO in the product gases be at least partly due to products of pyrolysis of the coal?
714
COAL FLAMMABILITY
Author's Reply. Coal p y r o l y s i s is one possibility to explain s u c h an u n e x p e c t e d behavior. As y o u m a y see in T a b l e 1, the o x y g e n c o n t e n t of t h e coal is not negligible. H o w e v e r t e m p e r a t u r e m o n i t o r i n g of the c o m b u s t i o n w a v e is u s e d to a s s e s s potential p y r o l y s i s routes l e a d i n g to C O formation.
Smoot, Comb. Flame, 28, 1977). 2. D i d y o u d e t e r m i n e the c o m p o s i t i o n (proximate, ultimate) of the residual char in the b o m b f o l l o w i n g the test as a f u n c t i o n of d u s t concentration.
Author's Reply.
F. Tamanini, Factory Mutual Research Corp., USA. I am s o m e w h a t s u r p r i s e d b y the h i g h levels of C O a n d C O 2 that y o u m e a s u r e d . D i d y o u verify that y o u r gas c o n c e n t r a t i o n m e a s u r e m e n t s are consistent, by doing, for example, an o x y g e n balance?
Author's Reply. We are also q u i t e c o n c e n e d a b o u t the h i g h levels of C O a n d C O 2 w h i c h are m e a s u r e d after c o m b u s t i o n s occurred. T h e c o n s i s t e n c y of the gas c o n c e n t r a t i o n m e a s u r e m e n t s w a s c h e c k e d in the f o l l o w i n g way. An o x y g e n b a l a n c e n e e d s the knowledge of H 2 0 c o n c e n t r a t i o n s . In the a b s e n c e of s u c h a m e a s u r e m e n t , a trial a n d error p r o c e d u r e w a s u s e d to f i n d the HzO concentration. It g i v e s g o o d results. W h e n e v e r a c c o u n t is taken of the coal o x y g e n c o n t e n t (see T a b l e 1) the o x y g e n c o m i n g f r o m t h e solid fuel m i g h t be a key p o i n t since low C O c o n c e n t r a t i o n s were o b t a i n e d for icy s u g a r a n d laurel p o w d e r experiments. S u c h organic s u b s t a n c e s are k n o w n to h a v e a h i g h o x y g e n content.
Y. Timnat, Technion-Isreal Institute o f Technology, Israel. S o m e time ago, I d i d a review of the C O ~ C O z ratio r e s u l t i n g from c a r b o n c o m b u s t i o n a c c o r d i n g to the literature. It is clear that this ratio d e p e n d s on the type of coal u s e d a n d on the conditions p r e v a i l i n g d u r i n g c o m b u s t i o n . Moreover, it is clear that initially the C O a n d C O 2 a m o u n t do n o t c o r r e s p o n d to e q u i l i b r i u m c o n d i t i o n s , It is, therefore, h a r d l y s u r p r i s i n g that the a u t h o r s were p u z z l e d b y the r e s u l t s o b t a i n e d f r o m s u c h calculat i o n s w h i c h are n o t applicable at each stages.
Author's Reply. I agree e m p h a t i c a l l y with y o u r statement. O n e of the b a s i c c o n c l u s i o n s of the p r e s e n t paper that n o n - e q u i l i b r i u m c o n d i t i o n s do prevail in s u c h e x p e r i m e n t s . F u r t h e r m o d e l i n g work is u n d e r w a y to q u a n t i f y the c o u p l i n g s w h i c h allow stable flame p r o p a g a t i o n at 30 t i m e s the s t o i c h i o m e trie c o n c e n t r a t i o n of coal in air.
1. W e d i d n o t u s e a n y d i s p e r s i n g a g e n t to m i n i mize particle agglomeration. As d i s c u s s e d in the paper, we t h i n k that a g g l o m e r a t i o n r e a c h e d s o m e a s y m p t o t i c state, since g o o d r e p r o d u c i b i l i t y of the e x p e r i m e n t s w a s o b t a i n e d f r o m 0.5 to 2 kg in 3. S u c h a state s e e m e d to o c c u r in s o m e o f H e r t z b e r g ' s e x p e r i m e n t s on a lower f l a m m a b i l i t y limits [1]. F u r t h e r m o r e , it is i n t e n d e d to c o m p a r e s u c h results w i t h r e s u l t s o b t a i n e d for full scale i n d u s t r i a l p n e u m a t i c c o n v e y i n g of the s a m e mixture. To o u r k n o w l e d g e , no d i s p e r s i n g a g e n t s are u s e d in s u c h a context. 2. We d i d n o t i n t e n d to cover in details all the c h e m i c a l a n a l y s i s n e e d e d to characterize the solid c o m b u s t i b l e materials before a n d after c o m b u s t i o n occurred. Yet we shall definitely d e t e r m i n e t h e c o m p o s i t i o n of the residual char in the next set o f experiments. REFERENCE [lJ M. HERTZBE/~C, K. L. CASI-IDOLLARANDJ. F. OPFERMAN, B u r e a u of Mines, Report of I n v e s t i g a t i o n s 8360 (1979).
H. Schacke, Bayer AG, West Germany. (A) H o w do y o u m a n a g e to o v e r c o m e scaling probl e m s w h i c h arise from explosion limits, maxim u m explosion p r e s s u r e a n d especially for m a x i m u m rate of p r e s s u r e rise w i t h f l a m m a b l e (coal) d u s t / ' a i r m i x t u r e s ? It h a s b e e n very clearly s h o w n (e.g. by Bartknecht, [1] that one c a n reliably extrapolate e x p l o s i o n data from laboratory a p p a r a t u s w i t h s m a l l v o l u m e (<20 litres) to real site p l a n t s a n d m i n e s . (B) W h a t w a s the time delay b e t w e e n the d u s t d i s p e r s i o n p u l s e a n d ignition, a n d did you vary this p a r a m e t e r in y o u r experintmnts? T h i s p a r a m e t e r (Delay time) s t r o n g l y i n f l u e n c e s (e.g. by alteration of t u r b u l e n c e s ) values o f m a x i m u m p r e s s u r e a n d m a x i m u m p r e s s u r e rise a n d h a s to be o p t i m i z e d either for m a x i m u m p r e s s u r e or m a x i m u m p r e s s u r e rise (references also g i v e n in *)
L. D. Smoot, Brigham Young University, USA. 1. W h a t m e t h o d s d i d y o u u s e to m i n i m i z e particle a g g l o m e r a t i o n of the d i s p e r s e d feed coal? We f o u n d it n e c e s s a r y to u s e an SiO 2 d i s p e r s i n g a g e n t for this p u r p o s e . (Horton, G o o d s o n ,
REFERENCES [1] VEREIN DEUTSCHERLUGERR1CURE--Guideline VDI 3673, W. G e r m a n y 1979.
UPPER FLAMMABILITY LIMITS OF COAL DUST-AIR MIXTURES
Author's Reply. (A) We are o f course aware o f the p i o n e e r i n g work o f DR. Bartknecht. T h i s is w h y a full scale c o n v e y i n g s y s t e m was b u i l t by B E R T I N to seek for extrapolation rules, starting from b o m b e x p e r i m e n t s (see c o n c l u s i o n part of the paper). Yet, it m u s t be s t r e s s e d that we are p r i m a r i l y interested into deflagration w a v e s o r i g i n a t i n g in pipe s y s t e m s (at m o s t 4 to 8 i n c h e s ID). I n this context, m u c h w o r k r e m a i n s to be done on the interaction b e t w e e n t w o - p h a s e c o m b u s tion a n d flow g e n e r a t e d - t u r b u l e n c e . (B) We u s e d a t i m i n g s y s t e m that allows t h e u s e of specific time delays b e t w e e n d u s t d i s p e r s i o n p u l s e a n d ignition. H o w e v e r , the p r o c e d u r e followed for the e x p e r i m e n t s we d e s c r i b e d refers to the w e l l - k n o w n H A R T M A N N a p p a r a t u s : the d i s c h a r g e is of a c o n t i n u o u s i n d u c t i v e type. It is t u r n e d on a very s h o r t time (i.e. less t h a n 0.1 sec) after d i s p e r s i o n started. T h i s p r o c e d u r e was u s e d b e c a u s e we are looking for u p p e r f l a m m a b i l i t y limits in real s y s t e m s h e n c e the m o s t efficient ignition s y s t e m w a s used. Finally, it m u s t be s a i d that we just started e x p e r i m e n t s with single, h i g h p o w e r capacitive discharges. P r e l i m i n a r y results s h o w that s t e a d y deflagration waves are o b t a i n e d with coal-air concentration dependent time delays.
A. 1. Saber, Concordia University, Canada. T h i s paper a d d r e s s e s the practical a n d i m p o r t a n t areas of coal a n d ignitability. M y q u e s t i o n s are: W a s air h u m i d i t y controlled? ii) W a s coal dried? iii) W a s the a p p a r a t u s g r o u n d e d ? iiii) Is this a f l a m m a b i l i t y or ignitability s t u d y ? Author's Reply. Air was d r i e d u p prior to introd u c t i o n in the h i g h p r e s s u r e air tank via c h e m i c a l action on m i n e r a l materials. Coal was g r o u n d as received a n d stored w i t h i n two air-tight containers. H o w e v e r , we o b s e r v e d s o m e c h a n g e over the e x p e r i m e n t a l study. Coal particles b e c o m e m u c h stickier. Yet, we c a n n o t infer b a s i c c o n c l u s i o n s on coal h u m i d i t y increase from s u c h visual observations. We h a d m u c h difficulty at the b e g i n n i n g of the s t u d y w i t h electric insulation. T h e a p p a r a t u s w a s g r o u n d e d a n d teflon was u s e d to p r e v e n t electrical leaks. We are c o n c e r n e d with the u p p e r f l a m m a b i l i t y of coal particles in air, w h i c h is d e f i n e d as the a s y m p t o t i c c o n c e n t r a t i o n w h e r e no d e f l a g r a t i o n p r o p a g a t i o n o c c u r s w h a t e v e r the p o w e r d e n s i t y input. H e n c e p o w e r d e n s i t y variation is n e e d e d to look for s u c h an a s y m p t o t i c state.
715
R. H. Essenhigh, Ohio State University, USA. T h e work d e s c r i b e d is i n t e r e s t i n g a n d v a l u a b l e b u t d o e s it really a d d r e s s t h e p r o b l e m o f p n e u m a t i c c o n v e y ing? I h a d occasion to m e a s u r e f l a m m a b i l i t y limits in a 2 " dia. p n e u m a t i c c o n v e y i n g s y s t e m 27 years ago ( s u m m a r y results reported in I n s t i t u t e o f F u e l " S h e f f i e l d C o n f e r e n c e " on Science in the Use o f Coal, 1958). We f o u n d that c o n v e y i n g s p e e d a n d r a d i u s o f c u r v a t u r e o f the t u b e were o v e r - r i d i n g factors c o n t r o l l i n g propagation, w i t h extinction occ u r r i n g at lower a n d lower velocities as coal concentration i n c r e a s e d above t h e s t o c h i o m e t r i c level. W o u l d it n o t be better to f o c u s o n the d y n a m i c rather t h a n the static c l o u d b e h a v i o r ? Author's Reply. First o f all, I t h i n k y o u for pointi n g o u t this reference. As I i n d i c a t e d at the b e g i n n i n g of the talk, o u r s t u d y c o n s i s t s o f a three step program: (i) static e x p e r i m e n t s for w h i c h p r e l i m i n a r y results are p r e s e n t e d here; it deals w i t h f l a m m a b i l i t y limits in n o n f l o w i n g s y s t e m s . T h e objective is to u n d e r s t a n d in the s i m p l e s t p o s s i b l e flow c o n d i t i o n s , t h e b e h a v i o r o f a deflagration wave. We t h i n k that t h e p r e s e n t results delineate s o m e of t h e risks that are faced. (ii) a m o d e l i n g s t u d y w h i c h is c o u p l e d with t h e static experiments. It tries to p o i n t o u t the s e r i o u s limiting factors w h i c h control s u s t a i n e d f l a m e propagation. P r e l i m i n a r y c o m p u t a t i o n a l r e s u l t s s h o u l d be reported soon. (iii) d y n a m i c e x p e r i m e n t s for w h i c h a f u l l scale p n e u m a t i c c o n v e y i n g s y s t e m h a s b e e n built. G i v e n s i m i l a r e x p e r i m e n t a l c o n d i t i o n s as the o n e s u s e d in static e x p e r i m e n t s , deflagration p r o p a g a t i o n is s t u d i e d as a f u n c t i o n of flow velocity a n d p i p i n g g e o m e t r y (straight tubes, twins, cyclones) t h r o u g h s u c h an extensive study, it is h o p e d that a better u n d e r s t a n d i n g o f e x p l o s i o n s risks for i n d u s t r i a l p o w d e r s will be o b t a i n e d together w i t h safety rules to be u s e d by Bertin in d e l i v e r i n g c o n v e y i n g facilities.
B. Vollerin, Battelle, Geneva Research Center, Switzerland. You p r e s e n t e d v a r i o u s results on pressure variation in y o u r a p p a r a t u s v e r s u s time, at two particle size distributions a n d for d i f f e r e n t d u s t c o n c e n t r a t i o n levels. At a b o u t 1 k g / m ~ a n d a b o u t 3 k g / m ~ d u s t c o n c e n t r a t i o n levels t h e peak p r e s s u r e as well as the d p / a t (pressure rise) are e q u i v a l e n t w h a t e v e r is the particle size d i s t r i b u t i o n . H o w e v e r at a b o u t 2 k g / m 3, w h e r e a s the peak p r e s s u r e is a b o u t the same, it a p p e a r s that the 50 Ixm particle d i a m e t e r c l o u d yields a s t e e p e r p r e s s u r e rise t h a n t h e 13 p.m particle d i a m e t e r cloud. It can also be s e e n that in the 50 lxm case, t h e p r o c e s s of c o m b u s t i o n starts earlier than in the 13 ~.m case.
UPPER
FLAMMABILITY
LIMITS
The Combustion Institute, 1981
OF
COAL
DUST-AIR
MIXTURES
~*~
B, D E G U I N G A N D AND S. GALANT Societe Bertin dr Cie Division Energetique Centre De Bayonne 40220 Tamps, France
The present paper is a progress report on a comprehensive study of upper flammability limits of coal dust-air mixtures. The study was undertaken in the context of safety in pneumatic conveying systems. An improved &liter combustion chamber was designed which allows dispersal of combustible particles up to concentrations around 5 k g / k g air. The study includes: i)
use of two narrow size distributions of a high volatile content coal (viz. 13 and 50 ~m) ii) measurement of ignition energy requirements iii) measurement of the extent of flame propagation based on the pressure attained at constant volume, and the residual Oa, CO 2 a n d CO content after combustion. Flame propagation was obtained up to concentrations around 3 k g / k g air (30 times the stoichiometrie concentration), weakly d e p e n d e n t upon particles size. Electrical ignition energies of hundreds of Joules were required. Although no direct dust concentration measurements were carried out at such elevated values, good reproduetibility of pressure traces and residual 0 2 , CO 2 a n d CO concentrations measurements up to 2.5 k g / k g air was obtained. This might be interpreted as a proof of constant coal-air dispersion characteristics around the energy source. A discussion of the underlying physical p h e n o m e n a is presented, with some evidence of a radiation enhanced propagation rate. Direct implications for safety in pneumatic conveying systems are drawn, together with an appraisal for future experimental studies concerning various industrial combustible dusts in a simulated pneumatic conveying environment.
1. Introduction
The questions raised by combustion specialists as well as industrial managers are confirmed by three recent disasters in grain elevator systems in France, Germany and Spain. Although the explosion hazard concern has motivated numerous studies on homogeneous gases and vapors [3], there appears to be no consensus among researchers on the flammability limits of heterogeneous substances. A constructive critical review of the known experimental results is presented in Ref. [4]. The same authors present results on new set of measurements for lean limits in coal dust-air mixtures. This study concentrates on the definition of a novel experiment with spatial and temporal measurements of dust concentration and surface temperature. To our knowledge there are very few systematic studies for rich limits, except for results on coal
An accurate knowledge of the upper flammability limit behavior of various dusty materials is essential for a realistic approach to the explosion hazards involved in their pneumatic transport. More than 70% of processed industrial powders are potentially combustible materials. Standard safety regulations include the use of vents [1] or an inert gas carrier. However, the technology involved as well as extrapolation of standard apparatus lead to economically prohibitive solutions. A recent symposium on "grain elevator explosions" [2] shows the acuteness of the present problem in the agricultural industry.
(*) To be presented at the 18th Symposium on Combustion, Waterloo, Canada (1980) 705
706
COAL FLAMMABILITY
dust laminar flame propagation [5] and some specific studies on the flammability of polyethylene powders [6]. This lack of experimental work is certainly due to difficulties in: assessing mixture homogeneity at the time of the energy input ii) monitoring the significant experimental variables iii) appreciating their respective importance. This paper will describe the results of a set of measurements near the rich limits, as well as the effect of particle size and some influence of ignition energy on the propagation rate:
2. Apparatus and Method A sketch and a functional block diagram of the apparatus is shown in Fig. 1. It is essentially an 8-liter modified form of the Hertzberg bomb [4] to which improvements were made to handle large dust concentrations. Much exploratory research went into the design and optimization of the dispersion system. Essentially, the same apparatus as the one described in Ref. [4] was used. Systematic checks were made of the coal dust collected after dispersion by the cylinder and top walls. An average 20% mass fraction adheres to the wall whereas the remaining 80% falls back into the dispersion cup. No concentration monitoring was therefore implemented. Instead, repeated experiments were carried out, keeping all experimental values constant. Pressure traces as well as measured concentration values were found to be repeatable within experimental error (+ 20%) except near the upper flammability limits. This point is discussed later. A typical ignition experiment is conducted as follows:
pump
Elr
i)
--high volatile content coal (36%) was chosen to provide an easily ignitable mixture to which potential modelling information was already available [7]. Narrow size distributions around 13 and 50 p.m were used --improvements were made to qualify the extent of flame propagation via measurements of pressure at constant volume, and the residual 02, CO~ and CO content after combustion --the dispersion method is an extension of very recent techniques to study lean limits [4] which have proven to give reproducible results for a wide range of dust/air mass ratios.
~VocuqJm
Pmssurt tmnsducer~
anatys~s Electrode
Air dispersion tank ~.5 L.
a measured sample of coal is placed in the cup around the dispersal cone ii) the 8-liter vessel is initially evacuated to 1/41/3 atm iii) a dispersal pulse is turned on and lasts until
r
$
Time~
IPmssunl and/ol, spolk cncroy rtcotding ]
Fie. 1. Sketch and functional block diagram of the BERTIN 8-liter bomb (modified version of Ref. 4 bomb to handle very high dust concentrations)
chamber pressure again reaches 1 atm iv) a concentration-dependentdelay is imposed before the ignition source is energized v) during flame propagation a pressure trace is recorded to characterize combustion mechanisms (if any) vi) after flame propagation has ceased, combustion gases are sampled through a water-cooled nozzle and analyzed in continuous monitoring systems. CO and C02 are analyzed via infra red Sehlumberger systems, whereas 02 is measured by a paramagnetie analyzer (Sehlumberger). The ignition systems consists of four electric match heads whose ratings are 34 mA under 8000 volts. They can be energized separately to make ignition energies vary over a wide range. Effective VAp requirements were measured in separate experiments to yield the following semi-empirical rules: E,f = K (Ax,n) nt where
t
(Joules)
is the time after sparks are turned on
i)
~
(s)
n number of electric match heads K(Ax,n) is a function of the electrode spacing, and the number of electrodes which was determined experimentally [7].
UPPER FLAMMABILITY LIMITS OF COAL DUST-AIR MIXTURES
707
TABLE I Coal properties
3. Coal Dust Characteristics Coal dust was chosen for these first upper limit experiments which show that:
Name : Freyming Volatile content Ash (for 10 and 50 micron) Carbon Oxygen Hydrogen No ash analysis was carried out. Higher heating value
- - h i g h volatile content facilitates ignition mechanisms - - a parallel modelling study was undertaken to elucidate experimental results. Enough information could be gathered to set up a realistic unsteady coal-air flame propagation model [7]. The coal reference is Freyming, 35% volatile content. Proximate analysis was carried out. Results are presented in Table 1. Size distributions for the two diameters considered are presented on Fig. 2 which represents cumulative weight percentage as a function of diameter. Size analysis was carried out with water-coal mixtures passed through a series of meshes. The smaller mesh size was 10 microns. The measured distributions are thought to be quite representative of monodisperse systems around 13 and 50 Ixm.
: 35% : 10% : 77% : 8% : 5% : 31 M J / k g
4. Experimental Results A systematic investigation was made of the influence of coal particle concentration, coal particle diameters and ignition sources requirements on the explosibility of the coabair mixtures. Combustion was characterized by two measurements:
Mass fraction of particles with diameter < d
99.5.
90. 80. 70. 60. 50. 40. 30.
0
d50=13prn
=
20. 10.
3. 2. 1
0.5
Particte diarneter(Fm~) I
'
'
'
'
'
I
10
,
.
.
,
.
,
I|
100
I
I
w
1000
Flc. 2. Mass fraction of particles with diameter less than d. Experimental points obtained for the two coal sizes studied.
708
COAL FLAMMABILITY
--pressure development (rate of pressure rise and peak pressure ratio) --residual O=, CO~ and CO concentrations. 4.1. Pressure Development The explosibility hazard of a dust-air mixture is often characterized by the rate of pressure rise. Some pressure growth curves for the two sizes studied are shown in Fig. 3 as a function of coal concentration. The lowest concentration studied is 0.5 k g / m 3 which is just the upper value studied by Hertzberg [4]. The highest concentration depicted is 3 k g / m 3 near the upper flammability limit for the 13 p.m particle size. Results are as follows: - - b o t h peak pressure and maximum rate of pressure rise decrease monotonically with increasing concentration. They are indicative of an "average" decreasing flame temperature. --pressure development does not depend too markedly on the particle size (except near limits), but is a function of dust concentration. The very same result was obtained by Hertzberg et al. [4] in their study of lower flammability limits. --near the upper limits, a lack of reproducibility was obtained for both coal particle sizes.
d50=13pm ~ - - ~ d50 =50pro =
. Pressure(bors)
0.5kglm3
Pressure traces tend to flatten out with very low rate of pressure rise. The evolution is confirmed on Fig. 4 where rates of pressure rise (two definitions) are plotted as a function of dust concentration. The monotomic decrease from 0.5 k g / m 3 exhibits quite a large spread near extinction (2.5 to 3 kg/m3). - - t h e pressure curves allow the calculation of burning velocities as carried out by Hertzberg [4]. Values around 0.25 m / s at 0.5 k g / m ~ are comparable to the ones obtained for methane-air mixtures in the same apparatus [4] and coal dust-air burner supported flames [5]. 4.2. Residual Gas Concentration Measurements These measurements were conducted to further characterize the completion of the oxidation reactions and to identify possible mechanisms which lead to sustained propagation at concentrations 30 times the stoichiometric ratio. On Fig. 5, CO e and O= volume concentrations (dry basis) are plotted as a function of coal particle concentrations, for two diameters and various ignition energies. The following comments must be made: --residual Oz concentrations are always very low (<1%) even near the upper flammability limits, This is indicative of complete oxygen depletion via volatile oxidations and possibly the beginning of carbon-oxygen surface reactions - - C O 2 maximum concentrations seem to occur near the flammability limits at d = 50 p,m, with a corresponding depletion of CO molecules. This maximum does not occur for smaller particles,
~ . o , . o, 0 . . . . . . . .
(bars/s)
//~m,~,
,.
/ ~ , ~
1kg/m3 0f
o'.1
o'.z
o'.3
o'.~
ZOC
S/
"dso{prn)Z~PM/~t ~Prnl~t
'5;t:l
8,
?7"-
2.
~.
oi(
o'.e
6. 4.
150 T--
:
100
3 kgI m 3 . ~ . ~ . -'-.. . ~ . ~ . S kg/m3
z.
0
0
o13 o2~
2kg/m3
0'A
, 0.Z
~ 0.3
.,~
0.~
9
Timeafter, ~ dispersionpurse(s)
Fie, 3, E v o l u t i o n o f chamber pressure traces as a f u n c t i o n o f t i m e for d i f f e r e n t coal d u s t / a i r concentrations and t w o particle sizes.
FIG. 4. E v o l u t i o n
o
Concentration
of the rate of pressure risk
( b a r / s ) as a f u n c t i o n o f coal eoneentration, T w o
possible definitions of rate of pressure risk are used.
UPPER FLAMMABILITY LIMITS OF COAL DUST-AIR MIXTURES ~ t */* vol CO2 dry
~ */ovol 0 2 dry
1
1
9
0.8 0.6 0.4 0.2 al concentration
6. 4.
o.s
1
1.s
~%vol C02 dry
It. ~
2
2'.5
~
3'.s
~
%vol 02dry 21 7 .6
12.
10.~1 8. 6. t,_
~
.5 /, .3 .it
~-~
o'.s
~kg/m3j
Cs
~
2'.s
~
3'.5
(kg/m3)
FIG. 5. Final gas concentrations as a function of coal concentrations and particle sizes.
also 13 Ixm
viz. "icy" sugar and laurel powder. Such dusts are supposed to produce less volatile products with potential gas phase oxidation mechanisms. In Fig. 6, final CO and CO2 concentrations are plotted as a function of dust concentration. Sharp CO concentration maxima occur in a similar way to coal particles. In other words, no straightforward differences in rate limiting mechanism could be deduced from such experiments. Only complete numerical simulation studies [7] will be able to closely identify the various rate limiting steps which allow for stable flame propagation up to 3 k g / m 3 concentrations, together with more systematic and basic experimental investigations. 4.3. Ignition Energy Requirements
COal concentmtion
~
709
1 Elec- 2 Elec- 4 Electrode trodes trodes 9 A 9 % vol Or dry tl) 9 9 % vol CO~ dry
Ignition energy input was varied by switching off one, two or three of the four sparks. Due to the very small volume between the four electrodes (less than 100 mm3), the respective positioning of the various working electrodes had no strong influence on the ignition characteristics. Limit concentrations as a function of energy input are listed in Table 1 for both particle sizes. A comparison of various pressure traces as a function of the number of electrodes and concentration is presented in Fig. 7. For both particle sizes, increasing the number of electrodes results in a clear
2 Elec- 3 Elec- 4 Electrodes trodes trodes 50 Ixm 9 A 9 % vol 0 2 dry il) ~ 9 % vol CO~ dry
~% CO,CO2 vol. dry basis
~ CO - - - ~ CO2
Cool ~ / ~ / /
which is characteristic of a change in the diffusion-kinetic regimes of carbon oxidation. Further comments are given in Section 6. - - C O concentrations were given special attention. In Fig. 6, we illustrate CO concentration changes for the 10 I~m average size as a function of initial coal concentration. The curves shown are fitted of repeated (3) measurements at identical initial coal concentrations. Surprisingly enough, peak CO concentrations reach 15% (volume fraction, dry basis) near 0.5 k g / m 3 coal concentration. As this concentration increases, there is a net decrease in CO concentration, reaching zero values near the flammability limits. CO 2 concentrations exhibit a shifted peak around 2 k g / m 3. Both evolutions suggest the respective contribution of temperature within the reaction zone, volatile oxidation rates and gas-surface reaction rates. To further characterize possible independence of one or more of these parameters, runs at identical concentrations were carried out with vegetable powders,
\\
Icy SugQr
10~.
Laurel
5-
05
1
FIG. 6. C O
15 2 25 Content r(tt ion, kg/m 3 and
3
CO 2 concentrations
(vol.
dry
basis) as a function of dust loading (kg/m3). Studied dusts are: coal 10 Ix, laurel powder and "'icy" sugar.
710
COAL FLAMMABILITY ignition point, the possibility of particle agglomeration and spark simulation.
Pressure{ bars )
lkg/rn 3
~J/
1.Skg/m 3
//
i / ~ j 0'.1
T ~ aft~ eod d
0'.2
0'.3
~4
dispersion pu|~(s)
1 ELectrode 2 Electrodes
....
Pressure (bars)
O.5kg/m3
1.5kglrn3
9
0
2 0'.1
..... 0.2 0'.3 ....
2kg/m 3
tkg/m3
O',t,
/
after er~d of dispersion putse($ )
2 Electrodes 3 Electrodes
FIG. 7. Evolution of chamber pressure traces as a function of time for different energy input.
tendency to steepen pressure profiles and to somewhat shorten the ignition delay. Therefore not only absolute energy values but also deposition time and volume are of importance in determining upper flammability limits. An independent assessment of the ignition energy characteristics (power per unit volume) should be made before true limit concentrations can accurately be measured. Similar conclusions are reached by Hertzberg et al. in their lower flammability experiments. It is also worthwhile to point out that much development work was done to electrically insulate electrode materials from the coal air mixture. Intense skin effects were noticed at large coal concentrations when electrical insulation was poor. Microdischarges between coal particles could originate at ambient pressure along the electrodes and even on the dusty chamber walls. At present, it is not clear whether microdischarges could influence ignition phenomena at very high concentrations.
Concentration Measurement For the time being, no real answer can be given to the problem of achieving homogeneity near the spark gap. For one thing, concentrations are so high that standard optical techniques [4] cannot work at all. Only beta-ray transmission could be of interest 14[. We tried to resolve the problem by repeating measurements, everything else being held constant. Reproducibility (up to 2.5 kg/m 3 concentration) was good for both sizes studied. The importance of bad dispersion should have been observed at large sizes. Free failing velocities are larger and so are body forces between particles at large concentrations. This point led us to assume that constant dispersion characteristics near the electrodes were obtained for given air pulse conditions. However, this does not prove that average concentrations were reached near the electrodes. In other words, present upper flammability results should be considered on a relative basis.
Particle Agglomeration During their experiments on lean flammability limits, Hertzberg et al. [4] showed the existence of agglomeration phenomena above a 0.3 kg/m 3 concentration, with the agglomeration varying as the coal diameter and origin changed. It is clear that no microscopic dispersion defects (agglomeration) could be monitored at the concentrations studied. However, it is assumed that inefficient microscopic dispersion is due to from the presence of strong surface forces, i.e. at small particle sizes. Since consistent diameter dependence was found during these experiments up to 2.0 kg/m 3 for both particle sizes, it likely that: i) either agglomeration effects would become important only at very high concentrations for the type of coal used ii) or agglomeration effects have reached some asymptotic state, which is not modified further by higher concentrations.
Spark Simulation It should be improved in two ways:
5. Discussion of the Experimental Technique It is justified to question whether the present set of data could realistically answer the problem of explosion safety in pneumatic conveying systems. Although improvements were made to obtain homogeneous dispersion, answers are still needed to the problem of the concentration evaluation near the
--better monitoring of energy disposition to make absolute energy measurements for sustained flame propagation. An apparatus has been designed [8] which is going to be used in subsequent experiments. It allows separate monitoring of voltage, intensity, deposition time and volume of the spark. --simulation of realistic spark generation in pneumatic conveying systems, viz. electrostatic
UPPER FLAMMABILITY LIMITS OF COAL DUST-AIR MIXTURES discharges. It is thought that the apparatus described [8] will be suited for that purpose. 6. Discussion of the Experimental Results The results obtained cannot be explained in terms of equilibrium thermodynamics. We use the classical thermodynamic model of flammability limits [91 to compute upper limit concentrations. For the coal considered, a 0.6 kg / m'~air concentration was found, which is of course much lower than the limit concentrations actually measured. The physics of the results can be discussed in terms of three phenomena: i)
concentration stratification at the time of ignition ii) kinetic versus diffusion controlled propagation iii) radiation enhanced flame propagation.
Concentration Stratification The lack of reproducibility of ignition phenomena near the "upper" limits could originate from concentration stratification near the electrodes. This concentration is of course size dependent: at constant d.~o diameter, it originates from the polydisperse nature of the coal particles. Ignition may then occur in a favorable (i.e. smaller diameter) concentration domain and propagate steadily within richer or leaner regions. Dixon-Lewis [10] has shown numerically that ignition in gaseous mixtures may occur above the upper flammability limits only by radical species diffusion. Similar phenomena may occur in two-phase mixtures. In our experiments, oxygen depletion was always observed to be complete up to very near the concentration limits. Further experiments with changes in electrode positionning are therefore needed to elucidate stratification enhanced flame propagation. Kinetic Versus Diffusion Controlled Propagation Coal-dust / air flame velocities derived from bomb experiments are around 25 cm/s. The agreement with a one-dimensional burner-supported [5] laminar flame is quite good, which suggests that general rules for laminar flame propagation of high volatile coal are applicable in our experiments. The conclusions drawn from Ref. [5] simulations (without radiation modelling) are that: the volatile oxidation (gas phase) and the heterogeneous reaction of the carbon (char) are not rate-limiting; ii) the rate of devolatilization is the limiting kinetic step; iii) the rates of conduction and diffusion in the gas phase, as well as the rates of conduction between the particle and gas phases, are of major importance.
711
In our experiments, high CO concentrations result from a combination of ii) and iii). At high coal concentrations, maximum flame front temperatures decrease with a slowing down of the devolatilization and CO oxidation reactions (mainly CO + OH), However, the size dependency observed in the CO2 change as a function of coal concentration cannot be explained solely in terms of the above conclusions. In fact, known devolatilization rates do not depend upon particle size whereas gas-to-particle conductive energy exchange should decrease as concentration increases (since the maximum temperature declines). A possible explanation is given below. To support such conclusions, a model of unsteady one-dimensional coal-air flame propagation was built along the lines described in Ref. [7]. Although complete results will be reported later, it can be said that preliminary calculations on the size dependency of propagation rates showed increased particle diameter influence as the coal concentration was increased. Competitive effects do occur, including temperature decrease due to lower flame temperature, decreased conductive and convective gas-toparticle heat transfer, but there is increased radiation through a suspension of bigger particles. Special attention should also be given to convective heat transfer modelling between particles at very high concentration, which should notably modify flame quenching mechanisms through particle agglomerates.
Radiative Heat Transfer The dependency of CO and COs concentrations in Fig. 5 may be explained in terms of increased radiative heat transfer. As the number of particle increases (i.e. smaller sizes at constant concentrations), a shielding effect occurs, preventing heat losses from the flame front toward the cold particles. Shield effects are not as prominent for 50 ~xm particles as for 13 I~m particles, Hence, the slow increase in CO 2 concentrations for 50 tLm particles around 2 kg/m 3 concentration could be due to increased devolatilization rates. A more systematic experimental and modelling study is needed to investigate such radiative shield effects, Recent work on coal dust modelling dwelt upon the necessity of two-dimensional radiative energy transfer in coal dust-air flame systems [11].
i)
7. Safety in Pneumatic Conveying Systems The present experimental results support the potential explosion hazards, at least in coal-air pneumatic conveying systems. Standard configurations work at concentrations between i and I0 kg/kg air. Observed combustion phenomena up to 3 kg/m :~ underline risks in stratified configurations such as
712
COAL FLAMMABILITY
=Effective e~r
give an asymptotic coal concentration limit between 4 and 4.5 kg/m3).
(joules) 9 dso=13pm I d50=50 }Jrn ]'his
ii) influence of flow velocity upon ignition charac-
teristics
work
A work similar to the approach taken for gaseous mixtures must be undertaken [12,13]. iii) modifications of flammability limits by proper
inerting iv) risk assessment in current conveying technology, taking into account flammability characteristics in both static (cyclones) and dynamic configurations.
Rcf,(4)
~Z~,-~-0.1
Conc~tration 4 (kg/m 3)
FI(;. 8. Comparison of lower flammability limits (Ref. I4] ) and upper flammability limits obtained from the present work.
Future program work will deal with flowing systems since a full-scale, closed-loop, pneumatic conveying system has been built for experimental simulation purposes. Spark discharges will be released within the flowing system at different locations to study modifications of the so called "static" (i.e. bomb) experiments. Moreover various dusts of industrial importance will be studied. Work has already begun on metallic powders and will be reported later.
8. Conclusions cyclones or sudden changes in direction. The problems that remain to be resolved are: i)
simulation of electrostatic or electric discharges in static configurations and experimental determination of upper limit concentrations In Fig. 8, we compare effective ignition energies (measured as VAp) for lower and upper flammability limits. Lower flammability limits are a fit of horizontal limit propagation experiments carried out by Hertzberg et al. [4]. Our results from Table 2 are reported in the same plane. There is a net trend toward upper flammability limits for the coal studied (a linear plot would
A comprehensive study of the upper flammability limits of coal dust-air mixtures has been presented in the context of safety in pneumatic conveying systems. Original work concerns: i)
improvements on a 8-liter combustion chamber to disperse combustible particles up to concentrations around 5 k g / k g air ii) use of two narrow size distributions of a high volatile content coal (viz. 13 and 50 Izm) iii) measurement of effective ignition energy requirements (in the Joule range) iv) measurements of the extent of flame propagation based on pressure attained at constant volume,
TABLE II
Number of electrodes
Spacing (ram)
Effective <~ power (watts)
Deposition (~ time (s)
Particle size
(~m)
Limit concentration (kg/m a)
2 2 2 2 2 2 2
0.32 0.90 1.80 0.32 0.90 1.35 1.80
0.10 0.15 0.25
13 13 13 50 50 50 50
1.75 2.0 3.25 0 1.25 2.5 3.0
oo
0.10 0.25 0.25
(a) last measurement before no propagation occurence. Effective power is the slope of Vhp as a function of time at t = 0.
UPPER FLAMMABILITY LIMITS O F COAL DUST-AIR MIXTURES and residual 02, CO2 and CO content after combustion v) obtention of sustained flame propagation at concentrations around 3 k g / k g air with weak dependency upon particle size vi) identification of possible radiation-enhanced propagation for large size particles a n d high concentrations. On the basis of the results obtained, it can be said that a basis has been developed for a standard methodology to assess explosion hazards of pneumatic conveying systems. Systematic experimental work should proceed to quantify absolute ignition energy requirements and their modification in flowing system. Supporting model studies using a new unsteady code will help to analyze the experimental results, especially at large concentrations where the nonlinear coupling between heat transfer and kinetics seem to be very important.
Acknowledgements The present study was financed by the F r e n c h "D61bgation Gbnbrale h la Recherche Scientifique et Technique" (D.G.R.S.T.) under contract n u m b e r 78-7-0262. Fruitful discussions with Prof, N. MANSON, Dr. BOURIANNES and Dr. CAMPOS of the "Laboratoire d'Energ6tique et de D6tonique" (Poitiers) are acknowledged. Pieces of information obtained from Dr. M. H E R T Z B E R G of the US Bureau of Mines were also of invaluable help at the start of the study.
713
REFERENCES 1. D. BRADLEYANDA. MITCHESON. Comb. and Flame, 32, 221 (1978) 2. N. MAt~soN. "Compte rendu de m i s s i o n - Symposium on Grain Elevators" January 1979 3. M.G. ZAaETArlS. Bureau of Mines, Bulletin 627 (1965) 4. M. HERTZBERG, K.L CASHDOLLARAND ].F. OPFERMAN. Bureau of Mines, Report of Investigation 8360 (1979) 5. L.D. SMOOT, M.P. Hoa~roN m~o G.A. WlLUAMS. 16th (Int) Symposium Comb. 375 (1977). The Combustion Institute. 6. A. FIUMAa^ AND P. CARDILLO. La Rivista del Comb., 31, p. 115 (1977). 7. J. CRAMBES, B. DEGUINGAND AND S. GALANT, BERTIN Technical Note to appear (March
1980). 8. M. MOREAU, R. BOURIANNES. Technical Note E N S M A / M N 79-2 (1979). 9. K.N. PALMER. "Dust explosions and fires". London, Chapman and Hall (1973). 10. G. Dlxo~-LEWlS AND I.G. SnEPERD. 15th (Int) Symposium Comb., 1483 (1975) T h e Combustion Institute. 11. J.L. KaAZlNSKI, R.O. BVCKINSANO H. KmEa. Prog. Energy Combust. Sci., 5, p. 31 (1979). 12. C.G. DE SOETE. 15th (Int) Syrup. on Comb., p. 735 (1971). The Combustion Institute. 13. D.R. BALLALANDA.H. L~ZVEaVaE.16th (Int) Symp. on Comb., p. 1685 (1977). The Combustion Institute,
COMMENTS C. Huggett, National Bureau of Standards, USA. The increased flammability of the larger particle size sample at low concentrations is surprising at first glance. Do you discuss this in your paper? Is it because in these oxygen-limited systems the total energy available is constant but the smaller particles, because Of their greater surface area per unit mass, exert a greater quenching effect through energy absorption or possibly through surface quenching of reactions? Author's Reply. No thorough discussion on this point is given in the paper. We are still unable to give a satisfactory answer to that question. However this result might originate from two independent reasons: (i~ concentration stratification which is more prominent for large particles: ignition occurs with smaller
particles (low falling velocity) and then deflagration spreads to bigger ones. (ii) large particles do not exert a strong quenching effect, as you suggest, because of their elevated thermal inertia. In the paper we outline the type of further studies that are needed to elucidate such an unexpected behavior.
E. Suuberg, Carnegie-Mellon University, USA. Considering the very high coal loadings studied and the fact that carbon monoxide is often a major product of coal pyrolysis, might not the rather high levels of CO in the product gases be at least partly due to products of pyrolysis of the coal?
714
COAL FLAMMABILITY
Author's Reply. Coal p y r o l y s i s is one possibility to explain s u c h an u n e x p e c t e d behavior. As y o u m a y see in T a b l e 1, the o x y g e n c o n t e n t of t h e coal is not negligible. H o w e v e r t e m p e r a t u r e m o n i t o r i n g of the c o m b u s t i o n w a v e is u s e d to a s s e s s potential p y r o l y s i s routes l e a d i n g to C O formation.
Smoot, Comb. Flame, 28, 1977). 2. D i d y o u d e t e r m i n e the c o m p o s i t i o n (proximate, ultimate) of the residual char in the b o m b f o l l o w i n g the test as a f u n c t i o n of d u s t concentration.
Author's Reply.
F. Tamanini, Factory Mutual Research Corp., USA. I am s o m e w h a t s u r p r i s e d b y the h i g h levels of C O a n d C O 2 that y o u m e a s u r e d . D i d y o u verify that y o u r gas c o n c e n t r a t i o n m e a s u r e m e n t s are consistent, by doing, for example, an o x y g e n balance?
Author's Reply. We are also q u i t e c o n c e n e d a b o u t the h i g h levels of C O a n d C O 2 w h i c h are m e a s u r e d after c o m b u s t i o n s occurred. T h e c o n s i s t e n c y of the gas c o n c e n t r a t i o n m e a s u r e m e n t s w a s c h e c k e d in the f o l l o w i n g way. An o x y g e n b a l a n c e n e e d s the knowledge of H 2 0 c o n c e n t r a t i o n s . In the a b s e n c e of s u c h a m e a s u r e m e n t , a trial a n d error p r o c e d u r e w a s u s e d to f i n d the HzO concentration. It g i v e s g o o d results. W h e n e v e r a c c o u n t is taken of the coal o x y g e n c o n t e n t (see T a b l e 1) the o x y g e n c o m i n g f r o m t h e solid fuel m i g h t be a key p o i n t since low C O c o n c e n t r a t i o n s were o b t a i n e d for icy s u g a r a n d laurel p o w d e r experiments. S u c h organic s u b s t a n c e s are k n o w n to h a v e a h i g h o x y g e n content.
Y. Timnat, Technion-Isreal Institute o f Technology, Israel. S o m e time ago, I d i d a review of the C O ~ C O z ratio r e s u l t i n g from c a r b o n c o m b u s t i o n a c c o r d i n g to the literature. It is clear that this ratio d e p e n d s on the type of coal u s e d a n d on the conditions p r e v a i l i n g d u r i n g c o m b u s t i o n . Moreover, it is clear that initially the C O a n d C O 2 a m o u n t do n o t c o r r e s p o n d to e q u i l i b r i u m c o n d i t i o n s , It is, therefore, h a r d l y s u r p r i s i n g that the a u t h o r s were p u z z l e d b y the r e s u l t s o b t a i n e d f r o m s u c h calculat i o n s w h i c h are n o t applicable at each stages.
Author's Reply. I agree e m p h a t i c a l l y with y o u r statement. O n e of the b a s i c c o n c l u s i o n s of the p r e s e n t paper that n o n - e q u i l i b r i u m c o n d i t i o n s do prevail in s u c h e x p e r i m e n t s . F u r t h e r m o d e l i n g work is u n d e r w a y to q u a n t i f y the c o u p l i n g s w h i c h allow stable flame p r o p a g a t i o n at 30 t i m e s the s t o i c h i o m e trie c o n c e n t r a t i o n of coal in air.
1. W e d i d n o t u s e a n y d i s p e r s i n g a g e n t to m i n i mize particle agglomeration. As d i s c u s s e d in the paper, we t h i n k that a g g l o m e r a t i o n r e a c h e d s o m e a s y m p t o t i c state, since g o o d r e p r o d u c i b i l i t y of the e x p e r i m e n t s w a s o b t a i n e d f r o m 0.5 to 2 kg in 3. S u c h a state s e e m e d to o c c u r in s o m e o f H e r t z b e r g ' s e x p e r i m e n t s on a lower f l a m m a b i l i t y limits [1]. F u r t h e r m o r e , it is i n t e n d e d to c o m p a r e s u c h results w i t h r e s u l t s o b t a i n e d for full scale i n d u s t r i a l p n e u m a t i c c o n v e y i n g of the s a m e mixture. To o u r k n o w l e d g e , no d i s p e r s i n g a g e n t s are u s e d in s u c h a context. 2. We d i d n o t i n t e n d to cover in details all the c h e m i c a l a n a l y s i s n e e d e d to characterize the solid c o m b u s t i b l e materials before a n d after c o m b u s t i o n occurred. Yet we shall definitely d e t e r m i n e t h e c o m p o s i t i o n of the residual char in the next set o f experiments. REFERENCE [lJ M. HERTZBE/~C, K. L. CASI-IDOLLARANDJ. F. OPFERMAN, B u r e a u of Mines, Report of I n v e s t i g a t i o n s 8360 (1979).
H. Schacke, Bayer AG, West Germany. (A) H o w do y o u m a n a g e to o v e r c o m e scaling probl e m s w h i c h arise from explosion limits, maxim u m explosion p r e s s u r e a n d especially for m a x i m u m rate of p r e s s u r e rise w i t h f l a m m a b l e (coal) d u s t / ' a i r m i x t u r e s ? It h a s b e e n very clearly s h o w n (e.g. by Bartknecht, [1] that one c a n reliably extrapolate e x p l o s i o n data from laboratory a p p a r a t u s w i t h s m a l l v o l u m e (<20 litres) to real site p l a n t s a n d m i n e s . (B) W h a t w a s the time delay b e t w e e n the d u s t d i s p e r s i o n p u l s e a n d ignition, a n d did you vary this p a r a m e t e r in y o u r experintmnts? T h i s p a r a m e t e r (Delay time) s t r o n g l y i n f l u e n c e s (e.g. by alteration of t u r b u l e n c e s ) values o f m a x i m u m p r e s s u r e a n d m a x i m u m p r e s s u r e rise a n d h a s to be o p t i m i z e d either for m a x i m u m p r e s s u r e or m a x i m u m p r e s s u r e rise (references also g i v e n in *)
L. D. Smoot, Brigham Young University, USA. 1. W h a t m e t h o d s d i d y o u u s e to m i n i m i z e particle a g g l o m e r a t i o n of the d i s p e r s e d feed coal? We f o u n d it n e c e s s a r y to u s e an SiO 2 d i s p e r s i n g a g e n t for this p u r p o s e . (Horton, G o o d s o n ,
REFERENCES [1] VEREIN DEUTSCHERLUGERR1CURE--Guideline VDI 3673, W. G e r m a n y 1979.
UPPER FLAMMABILITY LIMITS OF COAL DUST-AIR MIXTURES
Author's Reply. (A) We are o f course aware o f the p i o n e e r i n g work o f DR. Bartknecht. T h i s is w h y a full scale c o n v e y i n g s y s t e m was b u i l t by B E R T I N to seek for extrapolation rules, starting from b o m b e x p e r i m e n t s (see c o n c l u s i o n part of the paper). Yet, it m u s t be s t r e s s e d that we are p r i m a r i l y interested into deflagration w a v e s o r i g i n a t i n g in pipe s y s t e m s (at m o s t 4 to 8 i n c h e s ID). I n this context, m u c h w o r k r e m a i n s to be done on the interaction b e t w e e n t w o - p h a s e c o m b u s tion a n d flow g e n e r a t e d - t u r b u l e n c e . (B) We u s e d a t i m i n g s y s t e m that allows t h e u s e of specific time delays b e t w e e n d u s t d i s p e r s i o n p u l s e a n d ignition. H o w e v e r , the p r o c e d u r e followed for the e x p e r i m e n t s we d e s c r i b e d refers to the w e l l - k n o w n H A R T M A N N a p p a r a t u s : the d i s c h a r g e is of a c o n t i n u o u s i n d u c t i v e type. It is t u r n e d on a very s h o r t time (i.e. less t h a n 0.1 sec) after d i s p e r s i o n started. T h i s p r o c e d u r e was u s e d b e c a u s e we are looking for u p p e r f l a m m a b i l i t y limits in real s y s t e m s h e n c e the m o s t efficient ignition s y s t e m w a s used. Finally, it m u s t be s a i d that we just started e x p e r i m e n t s with single, h i g h p o w e r capacitive discharges. P r e l i m i n a r y results s h o w that s t e a d y deflagration waves are o b t a i n e d with coal-air concentration dependent time delays.
A. 1. Saber, Concordia University, Canada. T h i s paper a d d r e s s e s the practical a n d i m p o r t a n t areas of coal a n d ignitability. M y q u e s t i o n s are: W a s air h u m i d i t y controlled? ii) W a s coal dried? iii) W a s the a p p a r a t u s g r o u n d e d ? iiii) Is this a f l a m m a b i l i t y or ignitability s t u d y ? Author's Reply. Air was d r i e d u p prior to introd u c t i o n in the h i g h p r e s s u r e air tank via c h e m i c a l action on m i n e r a l materials. Coal was g r o u n d as received a n d stored w i t h i n two air-tight containers. H o w e v e r , we o b s e r v e d s o m e c h a n g e over the e x p e r i m e n t a l study. Coal particles b e c o m e m u c h stickier. Yet, we c a n n o t infer b a s i c c o n c l u s i o n s on coal h u m i d i t y increase from s u c h visual observations. We h a d m u c h difficulty at the b e g i n n i n g of the s t u d y w i t h electric insulation. T h e a p p a r a t u s w a s g r o u n d e d a n d teflon was u s e d to p r e v e n t electrical leaks. We are c o n c e r n e d with the u p p e r f l a m m a b i l i t y of coal particles in air, w h i c h is d e f i n e d as the a s y m p t o t i c c o n c e n t r a t i o n w h e r e no d e f l a g r a t i o n p r o p a g a t i o n o c c u r s w h a t e v e r the p o w e r d e n s i t y input. H e n c e p o w e r d e n s i t y variation is n e e d e d to look for s u c h an a s y m p t o t i c state.
715
R. H. Essenhigh, Ohio State University, USA. T h e work d e s c r i b e d is i n t e r e s t i n g a n d v a l u a b l e b u t d o e s it really a d d r e s s t h e p r o b l e m o f p n e u m a t i c c o n v e y ing? I h a d occasion to m e a s u r e f l a m m a b i l i t y limits in a 2 " dia. p n e u m a t i c c o n v e y i n g s y s t e m 27 years ago ( s u m m a r y results reported in I n s t i t u t e o f F u e l " S h e f f i e l d C o n f e r e n c e " on Science in the Use o f Coal, 1958). We f o u n d that c o n v e y i n g s p e e d a n d r a d i u s o f c u r v a t u r e o f the t u b e were o v e r - r i d i n g factors c o n t r o l l i n g propagation, w i t h extinction occ u r r i n g at lower a n d lower velocities as coal concentration i n c r e a s e d above t h e s t o c h i o m e t r i c level. W o u l d it n o t be better to f o c u s o n the d y n a m i c rather t h a n the static c l o u d b e h a v i o r ? Author's Reply. First o f all, I t h i n k y o u for pointi n g o u t this reference. As I i n d i c a t e d at the b e g i n n i n g of the talk, o u r s t u d y c o n s i s t s o f a three step program: (i) static e x p e r i m e n t s for w h i c h p r e l i m i n a r y results are p r e s e n t e d here; it deals w i t h f l a m m a b i l i t y limits in n o n f l o w i n g s y s t e m s . T h e objective is to u n d e r s t a n d in the s i m p l e s t p o s s i b l e flow c o n d i t i o n s , t h e b e h a v i o r o f a deflagration wave. We t h i n k that t h e p r e s e n t results delineate s o m e of t h e risks that are faced. (ii) a m o d e l i n g s t u d y w h i c h is c o u p l e d with t h e static experiments. It tries to p o i n t o u t the s e r i o u s limiting factors w h i c h control s u s t a i n e d f l a m e propagation. P r e l i m i n a r y c o m p u t a t i o n a l r e s u l t s s h o u l d be reported soon. (iii) d y n a m i c e x p e r i m e n t s for w h i c h a f u l l scale p n e u m a t i c c o n v e y i n g s y s t e m h a s b e e n built. G i v e n s i m i l a r e x p e r i m e n t a l c o n d i t i o n s as the o n e s u s e d in static e x p e r i m e n t s , deflagration p r o p a g a t i o n is s t u d i e d as a f u n c t i o n of flow velocity a n d p i p i n g g e o m e t r y (straight tubes, twins, cyclones) t h r o u g h s u c h an extensive study, it is h o p e d that a better u n d e r s t a n d i n g o f e x p l o s i o n s risks for i n d u s t r i a l p o w d e r s will be o b t a i n e d together w i t h safety rules to be u s e d by Bertin in d e l i v e r i n g c o n v e y i n g facilities.
B. Vollerin, Battelle, Geneva Research Center, Switzerland. You p r e s e n t e d v a r i o u s results on pressure variation in y o u r a p p a r a t u s v e r s u s time, at two particle size distributions a n d for d i f f e r e n t d u s t c o n c e n t r a t i o n levels. At a b o u t 1 k g / m ~ a n d a b o u t 3 k g / m ~ d u s t c o n c e n t r a t i o n levels t h e peak p r e s s u r e as well as the d p / a t (pressure rise) are e q u i v a l e n t w h a t e v e r is the particle size d i s t r i b u t i o n . H o w e v e r at a b o u t 2 k g / m 3, w h e r e a s the peak p r e s s u r e is a b o u t the same, it a p p e a r s that the 50 Ixm particle d i a m e t e r c l o u d yields a s t e e p e r p r e s s u r e rise t h a n t h e 13 p.m particle d i a m e t e r cloud. It can also be s e e n that in the 50 lxm case, t h e p r o c e s s of c o m b u s t i o n starts earlier than in the 13 ~.m case.