678
Energy & Fuels 1989,3,678-685
Ignition and Combustion Characteristics of a Lignite Philip G. Sweeny Energy and Mineral Research Center, Box 8213, University Station, University of North Dakota, Grand Forks, North Dakota 58202
Dana T. Grow* Department of Chemical Engineering, Box 8101, University Station, University of North Dakota, Grand Forks, North Dakota 58202 Received April 20, 1989. Revised Manuscript Received August 18, 1989 The ignition behavior of Beulah lignite (PSOC-1507) and its char was determined from COz and CO release profiles obtained by introducing 240-pm particles into a preheated oxidant stream. Light emission from these materials was also measured. Decreased volatile matter, increased reaction temperature, increased oxygen concentration, the presence of inorganic constituents, and the fusain lithotype all decreased the ignition delay of the lignite. Moisture had little effect on the ignition delay. The effect of oxygen concentration on the early stages of combustion was dramatic. At high oxygen concentrations, oxidized carbon was produced in a single stage, while at low oxygen concentrations a two-stage process was observed. The similarity of the initial stage observed at low oxygen concentrations with pyrolysis measurements and the large observed effect of inorganic constituents on ignition indicated that, at the conditions examined, heterogeneous reactions were more important in ignition than homogeneous reactions. The measured CO:CO2ratios were correlated by using an Arrhenius form and were assumed to be representative of surface reactions. This assumption was supported by the observation that high concentrations of CO were produced at high oxygen concentrations, which indicated that minimal CO oxidation occurred in the boundary layer. Overall combustion rates were determined and were temperature dependent.
Introduction Ignition characteristics of a coal are important to the prevention of spontaneous ignition and in the optimization of combustion parameters in industrial-scale utility boilers of coal-fired power plants. The ignition of single particles is extremely complex, involving simultaneous heat, mass, and momentum transfer. Ignition of particle clouds is additionally complicated by interparticle radiation effects. When a single particle of coal is exposed to a hot oxidizing environment, ignition is thought to proceed by either (a) homogenous ignition (which proceeds by the following series of steps: (1)heating of the particle, (2) devolatilization, (3) ignition of the volatiles, and finally (4) reaction at the surface) or (b) heterogeneous ignition (in which step 4 occurs prior to steps 2 and 3). Early efforts to measure the ignition of small particles were concerned largely with cloud ignition.'v2 The work of Cassel and Leibman3represents one of the f i t attempts to characterize the combustion of single particles. They examined the combustion of Mg using photographic techniques to determine burn lifetimes. Subsequently, the ignition characteristics of single coal particles have been the focus of much attention. Particle temperatures were computed by Thomas et al.4 to determine whether ignition occurred with temperature jump. Bandyopadhyay and Bhaduri5 measured the ignition temperature of anthracite, bituminous, and lignite
particles in a furnace by observing the flash point. They noted that the ignition temperature fell with increasing particle diameter. Karcz et a1.6 observed the ignition temperature of bituminous and anthracite coal, noted the decrease in ignition temperature with increased particle size, and estimated activation energies from the ignition data. A number of different ignition criteria have been applied by Gomez and V a ~ t o l awho , ~ used CO, C02, and luminosity to detect ignition of a subbituminous coal as a function of time. Huang et ale8measured the temperatures of the gas layer surrounding subbituminous coal particles and concluded that this material ignites homogeneously. Midkiff et al.9 examined the early stages of combustion of bituminous and subbituminous coal and concluded that heterogeneous reactions were favored by small particle size, high oxygen concentration, and decreased material rank. There has been a substantial effort to model the ignition behavior of single particles. These computations generally involve stating the problem in terms of the differential equations of mass, energy, and momentum conservation and then making appropriate approximations to solve the mathematical problem. Early modeling efforts were those of Semenov,lowho developed the thermal theory of ignition, and Van't Hoff." This approach begins with a steady-state energy balance on the particle and solves for the minimum gas temperature needed for ignition. It has been used by Cassel and Liebman3and by Bandyopadhyay
(1) Essenhigh, R. H. Dust Explosion in Factories: Ignition Testing and Design of a N e w Znflammator; Safety in Mines Research Establishment (U.K.) Research Report Number 118, Safety in Mines Research Establishment; Ministry of Power: London, U.K., 1960. (2) Palmer, K. W. Dust Explosions and Fires; Chapman and Hall;
(6) Karcz, H.; Kordylewski, W.; Rybak, W. Fuel 1980, 59, 798-802. (7) Gomez, D. 0.; Vastola, F. J. Fuel 1980, 64, 558-563. (8) Huang, G.; Vastola, F. J.; Scaroni, A. W. Energy Fuels 1988, 2,
London, 1973. (3) Cassel, H. M.; Leibman, I. Combust. Flame 1956, 3, 467-475. (4) Thomas, G. R.; Stevenson, A. J.; Evans, D. G. Combust. Flame 1973, 21, 133-136. (5) Bandyopadhyay, S.; Bhaduri, D. Combust. Flame 1972,18, 411.
385-390. (9) Midkiff,K. C.; Altenkirch,R.A.; Peck, R. E. Combust. Flame 1986,
64,253.
(IO) Semenov, N. N. Chemical Kinetics and Chain Reactions; Clarendon Press: Oxford, U.K., 1935. (11) Van't Hoff, J. H. Studies i n Chemical Thermodynamics; Amsterdam, 1894.
1989 American Chemical Society
Ignition and Combustion of a Lignite
Energy & Fuels, Vol. 3, No. 6,1989 679
in Figure 2. This apparatus is similar to that used by Gomez and Vastola.7 Both gas and wall temperatures were recorded and deviated from each other by no more than 1% . The particle drop tube, shown in Figure 2, consists of a stainless-steel tube with a quartz window glued into a position that coincides with the optical trigger. The steel portion of the tube is electrically grounded. The flow rate of gas through the combustion zone was 200 1 mL/min, producing a linear flow rate of 26 cm/s. The pressure in the combustion zone was 102 kPa. A detailed description of the apparatus is available.22 The operational procedure for the ignition apparatus is as follows. The coal particle is picked up with a suction bulb attached to a capillary tube and sized under a microscope. Due to the ....-. -..... - -...-- ....---_-_.._.__...___._______.___..___ ..._. ......-..... __ _, i asymmetrical nature of coal particles, the lengths of three axes were measured and the volumes were calculated by assuming a Figure 1. Schematic of ignition apparatus. rectangular particle. The average particle had a volume equivalent to that of a 240-gm-diameter sphere. The particle is then placed and Bhaduri5 t o correlate experimental results in t e r m s into a syringe needle and injected into the drop tube. The CO of an activation energy. Essenhigh12 and co-workers have and COz analyses are triggered by the particle traveling past the used this model t o interpret their measurement of gas optical trigger. The particle then falls into the horizontal quartz ignition t e m p e r a t u r e for a series of chars a n d cokes. tube and is blown against the particle stop. Lithotype, moisture content, inorganic constituent content, and A n u m b e r of workers, including T h o m a s e t al.4 a n d degree of charring were systematically varied. Beulah lignite Stevenson et al.,13have integrated the unsteady-state en(PSOC-1507) was obtained from The Pennsylvania State Univergy balance and have calculated a particle t e m p e r a t u r e ersity Sample Bank and separated into three fractions according versus time curve that typically rises to a plateau and then to lithotype. Coal analysis showed a volatile matter content of rapidly increases. In addition, changes in t h e pore 34.9%, a moisture content of 20.4%, a fixed carbon content of structure have been in~orporated.'~Other workers, among 37.3%, and an ash content of 7.5%. The percentages of C, H, t h e m Williams a n d co-workers15 a n d Libby a n d co-workN, S, and 0 were found to be 50.7, 5.5, 0.7, 0.9, and 34.8%, e r ~ applied ~ ~ activation * ~ ~ energy asymptotics t o arrive at respectively. The Beulah lignite, the Beulah lignite char, and the theoretical ignition times, relative contributions of various Beulah lignite demineralized char were burned in air in a Dupont , energy terms, a n d t h e r m a l histories of carbon particles. 1090 TGA to determine reactivity differences. The samples were heated at a rate of 20 OC/min to drive off moisture and then at In addition, Annamali a n d Durbetaki'* examined t h e 100 "C/min until combustion occurred. The lignite was found transition from homogeneous to heterogeneous ignition and to contain 40% volatile matter, while the char contained 24% noted that heterogeneous ignition is favored at small and the demineralized char contained 5%. After the volatile particle sizes. T h i s analysis was extended by Kordylewski matter was removed, the mass remained constant until the onset e t al.19 Arthurm a n d Mitchell and Hardesty21 have deterof combustion. The half-life is inversely proportional to the rate mined t h e CO:CO2 ratio as a function of temperature. of reaction, and the half-lives of the lignite, the char, and the Past work has concentrated o n the characterization and demineralized char are 1.0,0.9,and 1.2 min, respectively, indicating modeling of coal ignition and combustion mechanisms for similar reactivities for the three materials. Vitrain was categorized high-rank materials such as anthracite and bituminous by its hard texture and glassy appearance, attritus by its hard texture and grainy appearance, and fusain by its brittleness and coal. Presently, a t r e n d exists toward t h e utilization of tendency to fracture into needle-like fragment^.^^ The vitrain lower rank materials such as lignites for power generation. lithotype comprised the largest portion of the sample, and it was Because ignition and combustion properties are a function the lithotype used unless otherwise specifically noted. of material rank, lignite ignition mechanisms a n d comThe moisture content of the particles was established by storing bustion properties were characterized t o provide basic them either over water or concentrated sulfuric acid at 30 O C for information on t h e behavior of this fuel. The rates of CO a period of 2 or more weeks. This procedure establishes the and C02 production from lignite/char particles as a relative moisture content as 100% or O%, r e s p e c t i ~ e l y . ~De~ function of time were measured by introducing t h e m into mineralized lignite was prepared by washing with HCl and HF a hot oxidizing gas stream. I n addition, t h e luminosity or according to a previously described procedure.n A portion of the demineralized material was reloaded with calcium by ion-exflash points of t h e particles were monitored in a hot-stage changing with calcium acetate.22 The calcium concentrations of microscope. the original, demineralized, and calcium-reloaded lignite, as determined by atomic absorption, were 3.1%, 0.003%, and 1.1%, Experimental Equipment and Procedure respectively. Chars were prepared from sized fractions in a drop-tube furnace at 1275 K, a residence time of 150 ms, and an CO/COz Experiments. A schematic of the ignition apparatus oxygen concentration of 3%. is shown in Figure 1, and the combustion zone furnace is detailed Product gases flow into CO and COz integral analyzers. The outputs of the CO and COz analyzers were differentiated to obtain rate versus time profiles. The time constants used in the dif(12)Chen, M.; Fan, L.; Essenhigh, R. H. Twentieth Symposium (Znferentiation of the COz and CO analyses were 100 and 400 ms, ternational)on Combustion;The Combustion Institute Pittsburgh, PA, respectively. The larger differentiation time used for the CO 1984;p 1513. analysis was necessitated by the lower sensitivity of this analyzer. (13)Stevenson, A. J.; Thomas, C. R.; Evans, D. G. Fuel 1973,52, The high degree of noise present in the CO analysis is a result 281-287. (14)Srinivas, B.; Amundson, N. R. Can. J. Chem. Eng. 1980,59, of this lower sensitivity. Calibration of the analyzers with standard
*
jw
L..
L.....
728-738. (15)Linan, A.; Williams, F. A. Combust. Flame 1972,18,85. (16)Libby, P. A. Combust. Flame 1980,38,285-300. (17)Kassoy, D. R.; Libby, P. A. Combust. Flame 1982,48,287-301. (18)Annamali, K.; Durbetaki, P. Combust.Flame 1977,29,193-208. (19)Kordylewski, W.; Kruczek, H.; Rybak, W. Combust.Sci. Technol. 1981,26,157-160. (20)Arthur, J. R. Trans. Faraday SOC.1951,47,164-178. (21)Hardesty, D. R. "Coal Combustion Science, Quarterly Progress Report October-December 1987";Submitted to Hickerson, J. D., US.
DOE Pittsburgh Energy Technology Center.
(22)McCollor, D. P.; Sweeny, P. G.; Benson, S. A. "Coal Char
Reactivity"; Final Technical Report for the Period April 1,1987through
March 31,1988,DOE Coowrative Ameement No. DE-FC21-86MC10637. Available from' NTIS: Springfield,-VA. (23)Kleesattel, D. R.In Proceedings from the 1984 Symposium on the Geology of Rocky Mountain Coal; Houghton, R. L., Clausen, E. N., Eds.; North Dakota Geological Survey Publication 84-1,North Dakota Geological Survey: Bismarck, ND, 1984;p 28. (24)Deevi, S.C.; Suuberg, E. M. Fuel 1987,66, 454.
680 Energy & Fuels, Vol. 3, No. 6, 1989
Sweeny and Grow
0 . 5 mm O . D . S y r i n g e N e e d l e
I mm I.D., 2 mm 0.0.316
Water Cooled Shealth
1 mm I.D..
SS T u b i n g
2 mm O.D. Q u a r t z T u b i n g
Screen for Particle Stop
Ignition Furnace
F i g u r e 2. Detailed side view of ignition apparatus combustion zone furnace. 4
000
F i g u r e 3. Effect of dispersion on COz profiles. gas plugs was performed to account for dispersion of the sample during the flow from the furnace to the analyzers (see Figure 3). This figure shows that a combustion event characterized by ignition followed by an infinitely rapid rise to a constant combustion rate, an event equivalent in nature to the sample gas plugs, would produce a bell-shaped curve rather than an infinitely rapid rise followed by a plateau. The CO/CO2 production curves presented, therefore, do not directly represent the event of combustion. Rather, they are the shape of the gas sample entering the gas analyzers, which has been modified by dispersion during its transit from the furnace to the gas analyzers. The data were analyzed by assuming that combustion proceeded via rapid rise to steady-state oxidation, Le., that the C 0 2 release profiles observed during combustion were modified by dispersion in the same manner as the sample gas plugs. As a result, the tabulated values of the combustion durations contain corrections for the increased sample length and decreased maximum CO/CO2 production rates produced by dispersion. The CO/CO2 concentration versus time profiles presented do not contain any corrections for dispersion. However, they can be used to qualitatively characterize the ignition and combustion process. The calibrations set the range of error for the CO and C 0 2 rate calculations as f 5 % , and the standard deviations reported are those obtained from particle-to-particle variations in each group.
The measured CO:CO2 ratios had a standard deviation of approximately 25% of the mean. Ignition delays were obtained by using malonic acid particles to define a single operational constant for the ignition apparatus. Malonic acid particles were injected into the ignition apparatus, and the delay between the trigger ahd initial C 0 2 detection was measured. Since malonic acid decarboxylates at the relatively low temperature of 140 "C, a decarboxylation time of zero was assumed. Subtracting the malonic acid response time from the total delays yields the ignition delay. The ignition delays are calculated from the COz analyses and not the CO because of the 4-fold greater sensitivity of the COP analyzer. The standard deviation of the observed malonic acid ignition delays was approximately *0.050 s. The CO/C02 analyses were monitored every 0.005 s, and the precision of the data acquisition apparatus was limited by the selection of this time constant. The variation observed in the ignition delays for the coal measurements were usually on the same order as those observed for the malonic acid particles. The combustion durations of the coal particles varied over a larger range than the ignition delays. This variation does not indicate a decrease in precision of the measurements but rather that this parameter fluctuates more significantly between particles. For this reason, the number of significant figures presented for the combustion durations was left equivalent to those of the ignition delays. The carbon recovery is the sum of the C 0 2 and CO contents divided by the carbon content in the initial sample. The carbon content of the particle was estimated from the bulk density, the estimated carbon percentage, and the particle volume. The maximum carbon conversion rates reported are the sum of the peak C 0 2 and CO production rates corrected for dispersion. The CO:CO2 product ratio is the ratio of the total amounts of these products produced during combustion. The reaction of C 0 2 with C to form CO is endothermic, and ita contribution to the formation of CO is expected to be small. A carbon conversion rate per unit surface area, p , was calculated from the measured conversion rate. Surface area was calculated from the particle size measurements. Three possible mechanisms exist for the formation of COG (1) initial production at the particle surface, (2) oxidation of CO to COB in the boundary layer of the particle where the gas temperature is influenced by the particle, and (3) CO oxidation in the bulk gas phase during the transport of the gas sample through
Ignition and Combustion of a Lignite
Energy & Fuels, Vol. 3, No. 6, 1989 681 1 3
1';
-I
/
-
Tc=1273 K
CAMERA
SCOPE
I THERMOCOUPLE
Figure 4. Schematic of optical microscopy apparatus. the combustion zone furnace. In order to remove the latter mechanism from consideration, its magnitude was estimated by introducing CO/air mixtures into the combustion furnace at various temperatures. No significant CO oxidation was observed at or below 1098 K. At 1273 K, significant CO conversion was observed, and an overall rate constant was estimated for this reaction. The COz:CO product ratios reported were corrected for conversion of CO to COzin the bulk gas stream when appropriate. Hot-Stage Microscope Experiments. An optical microscope coupled with a heated stage was used to visually monitor ignition and combustion. A schematic diagram of the optical microscopy apparatus is shown in Figure 4. This system consists of three major components: a heated stage, a microscope, and a video camera. The stage was a modified Lietz Westlar Microscope Heating Stage, Model-1750. Since a light background is needed to view the lignite/char particles, a thin layer of AlzO, cement was applied to the heated stage. Further details of this apparatus are availableeZ2 The procedure for the optical microscopy studies consists of introducing the desired atmosphere into the preheated stage and then injecting six to ten particles onto the stage by using a syringe needle. Each event was recorded on videotape. The error in the timing of each event is no more than 0.02 s. The standard deviations reported are the variations produced within each group of particles. For burning particles, the events that were recorded consisted of (1)the ignition delay, the time from particles contact with the stage until observation of light emission; (2) the combustion duration, the time from initial light emission to burnout; and (3) the cloud duration, the time from the appearance of a volatile cloud until its disappearance.
Results and Discussion Ignition. The experiments performed provide details on both the ignition and combustion mechanisms of Beulah lignite. The goals were to discern the effects of oxygen concentration, temperature, volatile matter, and inorganic constituents on ignition delay. The relative contributions of homo- and heterogeneous reactions to the ignition process were of specific interest. The two pieces of data pertinent to the characterization of the ignition mechanism are the ignition delay and the general shape of the CO/CO2 profile. Two complementary techniques were used to obtain the delays: COz analysis and light detection. The delay obtained by using the ignition apparatus was the time elapsed between the injection of a particle into the preheated furnace and the onset of rapid COz production. This value is termed tdc. Since C 0 2 is produced by both pyrolysis and combustion, the tdc values do not always coincide with the onset of combustion. Therefore, an additional ignition delay was obtained by using optical microscopy. This delay, tdl, is the time elapsed between the contact of a particle with the preheated stage and the observation of light emission. The overall C O / C O z release profiles were obtained from the ignition apparatus. An examination of tdc, tdl, and the overall CO/CO2profiles provides a detailed understanding of the ignition process.
0000
0 100
0 200
0 300
0400
Time ( s )
Figure 5. Particle temperature versus time profiles for 240-pm carbon particles in 100% N2.
A knowledge of the particle heating rate in the two experimental apparatuses is essential if comparisons are to be made and if extrapolation of the results to larger, more applied systems is desired. Both surface reactions and physical configuration of the experimental apparatus contribute to particle heating rates. Prior to ignition, the contribution of surface reactions is assumed to be small in comparison to that of the experimental apparatus. The latter source has three components: conduction from the gas phase, radiation from the furnace walls, and conduction via the solid in contact with the particle. In the ignition apparatus, conduction from the surface in contact with the particle is assumed to be minimal, because this surface is composed of Zr02,which has a low thermal conductivity and a high porosity. Both experimentally determined and calculated heating rates support the assumption that the particle heating rate in the ignition furnace is dictated primarily by conduction from the gas with a smaller radiative contribution. The heating rate in the ignition furnace was experimentally determined by comparing the tdc values of malonic acid and calcium carbonate. These materials have decomposition temperatures of 413 and 1172 K, respectively. The results showed that the average heating rate was 3.5 f 2.1 X lo3 K/s at a furnace temperature of 1273 K. The heating rate of a 240-pm carbon particle was also calculated by using the energy balance on the particle Here m is particle mass (g), and A, is surface area (cm2/g). The term on the left-hand side is the rate of heating and C is the heat capacity (cal/g), a function of temperature. &e heat capacity of graphite was used in these calculations. The time is t (9). The first term on the right is the conduction term, while the second is the radiation term. The temperatures (K) of the particle, gas, and wall are Tp, Tg, and T,, respectively. The heat-transfer coefficient is h and is given by h = 2X/d, where X is the thermal conductivity (cal cm-'s-l K-l) and d is the particle diameter (cm). The emissivity is E and is assumed to be unity. The Stefan-Boltzman constant is (1.355 X 10-l2cal s-l cm-2 K-4). T h e calculated particle temperature versus time curves are shown in Figure 5 for gas temperatures of 923 and 1273 K. The average heating rate between 273 and 1172 K is seen to be 5.8 X lo3 K/s at a gas temperature of 1273 K. This value compares favorably with the experimentally determined value, which indicates that the primary mechanism of particle heating is by conduction from the surrounding gas. The t d c values obtained from the Beulah lignite are consistent with the ignition delays observed in systems
682 Energy & Fuels, Vol. 3, No. 6,1989
Sweeny and Grow
Table I. Ignition and Combustion Parameters of Dry Beulah Lignite (PSOC-1507)Vitrain at 923 K: The Effects of Volatiles, Oxygen Concentration.and Mineral Matter’ carbon C0:COp distinct total data 02, av particle recovery, prod CO/C02 combustion set 9a vel, cm3 % ratio stages P ~ . a/(cm2 ~ . s) duration. s tr,. s Volatiles BVLE 5 8.0 f 3.0 X 10” 65 f 22 0.14 2 6.3 f 2.4 X 10“‘ 4.795 f 1.773 0.382 f 0.062 BCLBb 5 1.0 f 0.2 X 63 f 18 0.10 1-2 4.2 f 1.4 X 10“‘ 4.816 f 2.350 0.364 f 0.080 ~~~~~~
BVLC BVLE BVLD BVLA BVLJ BVHAe
20 50 20
BVLE BDLAf BRLAN BCLBb BCLCbJ
5 5 5 5 5
0
5 11
1.3 f 0.5 X 8.0 f 2.0 X 8.7 f 1.5 X 5.4 f 1.3 X 9.6 f 1.6 X 1.1 f 0.5 X
O2 Concentration and Temperature 1.02 1 2.1 f 0.9 X 0.14 2 6.3 f 2.4 X lo4 0.64 1 1.6 f 0.5 X 1.19 1 2.4 f 0.6 X 2.17 1 3.5 f 0.7 X 0.84 1 2.5 f 0.6 X
10” 10” 10” 10”
3 f le 65 f 22 7 6 f 11 90 f 21 82 f 17 75 f 23
8.0 f 3.0 X 10” 6.8 1.1 X lo4 8.7 f 2.7 X lo4 1.0 f 0.2 X 1.2 f 0.4 X
65 f 22 23 f 4 6 8 i 12 63 f 18 31 f 6
Mineral Matter 0.14 2 1.52 1 0.22 2 0.10 1-2 1.33 1
6.3 f 2.4 X 5.1 f 1.0 X 4.2 f 1.1 X 4.2 f 1.4 X 4.0 f 2.3 X
10“‘ 10“‘ lo4
10”’ 10“‘
0.316 f 0.190 4.795 f 1.773 1.551 f 0.615 0.976 f 0.578 0.590 f 0.109 0.751 f 0.140
0.430 f 0.028 0.382 f 0.062 0.363 f 0.076 0.419 f 0.051 0.234 f 0.070 0.137 f 0.038
4.795 k 1.773 0.385 f 0.356 7.002 f 0.975 4.816 f 2.356 0.357 f 0.122
0.382 f 0.062 0.371 f 0.119 0.275 0.059 0.364 f 0.080 0.358 f 0.072
“Based on results of between 6 and 11 particles. bChar. eThe mass recoveries were lower than the calibrated range. dThe pyrolysis reaction duration was too short to accurately measure p . e 1273 K. f Demineralized. #Calcium reloaded.
where the particle heating rate was dictated by gas conduction. Conti and Zlochower26measured ignition delays between 0.67 and 0.092 s for poly(methy1 methacrylate) (PMMA) particles at gas temperatures of 673 and 1173 K, respectively. The reactivity of PMMA is between that of Pittsburgh seam bituminous coal and Beulah lignite.26 These ignition delays are similar to those observed for the 240-pm Beulah lignite particles in the ignition apparatus (see Table I). The trends in the t d l values are in general agreement with those of the tdc values; however, their exact values are usually lower (compare Tables I and 11). It is expected that light emission would be accompanied by CO/CO2 production, and therefore, the shorter t d l times are attributed to a faster heating rate in the optical microscopy apparatus. It has been indicated that the heating rate in the ignition apparatus is dictated by conduction from the surrounding gase. It is reasonable that the heating rate in the optical microscopy experiments is enhanced by a larger contribution of heat conduction from the contacting solid. This solid is a smooth layer of A1203 in the optical microscope experiments, as opposed to porous Zr02 in the ignition apparatus. This smooth A1203 surface would provide a large area of contact for the box-shaped particles. Having defined the relation of the tdc and the td values, it is possible to examine the ignition results in detail. To determine the relative contributions of homo- and heterogeneous reactions to the ignition mechanism, the lignite was compared to its char. As shown in Table I, the particle size is kept approximately constant for all runs. The char must ignite heterogeneously. Similarities in the ignition characteristics of the char and the lignite would indicate heterogeneous ignition of the lignite, while dissimilar behavior would indicate that homogeneous reactions were significant to the ignition process. An examination of all three pieces of evidence (i.e, the tdc, t d l , and the overall combustion profiles) is necessary for an accurate picture of the ignition process. The tdc values were similar for the char and the lignite at 923 K and 5% O2 (see Table I). (25) Conti, R. S.; Zlochower, I. A. Presented at the Eastern Section of the Combustion Institute, 1987. (26) Conti, R. S.; Herzberg, M. Industrial Dust Explosions; ASTM STP 958; Cashdollar,K. L., Martin Hertzberg, M., Eds.; American Society for Testing and Materials: Philadelphia, PA, 1987; p 45.
3 800 3 7C01
S 400
0 300 0 200 0.100
v-
0.000 -0 ’00 3 009
-
~
2000
4 000 6000 Time (seconds)
8000
Figure 6. Rate of C 0 2 production from dry Beulah lignite and its char a t 923 K in 5 % 02.
18
20% Q
10
08 06 04
0.2 DO
__
0 000
2 000
4 000
6 300
Time (seconds)
Figure 7. Effect of O2 concentration on the rate of C 0 2 production from dry Beulah lignite at 923 K. However, the overall combustion profiles of the lignite and its char differed significantly (see Figure 6). The lignite is seen to have an initial period of low reactivity followed by a region of high reactivity, while the char has a uniform reactivity intermediate to the levels observed for the lignite. The td values were smaller for the char than for the lignite under these conditions (see Table 11). The presence of the lignite’s volatile matter, therefore, appears to hinder the initial reactivity of the lignite at low oxygen concentrations. Additional information necessary to interpret these results is the effect of 0 2 concentration on the combustion of the lignite (see Figure 7). As discussed later, the CO:CO2product ratio increased with increased oxygen
Ignition and Combustion of a Lignite
Energy & Fuels, Vol. 3, No. 6,1989 683
Table 11. Optical Microscopy Results from the Combustion of Dry 180-250 pm Beulah Lignite and Beulah Lignite Char Particles' combustion tdlt duration, s Beulah lignite (PSOC-1507) temp, volatiles and mineral matter K 02, % av std dev av std dev vitrain 923 20 0.09 0.03 0.70 0.03 923 20
