AN AMERICAN CHEMICAL SOCIETY JOURNAL VOLUME 8, NUMBER 5
SEPTEMBEWOCTOBER 1994
0Copyright 1994 American Chemical Society
Articles Effects of Clay Minerals on Char Texture and Combustion Rosa Menkndez,* Diego Alvarez, and Antonio B. Fuertes Instituto Nacional del Carbbn, Oviedo, Spain
Gert Hamburg and John Vleeskenst Netherlands Energy Research Foundation ECN, Petten, The Netherlands Received August 19, 1993. Revised Manuscript Received May 16, 1994@
This study reports on the influence of mineral matter in parent coals on the formation and combustion of chars from these coals. Two Spanish high mineral matter coals were used, San Jose (SJ), 1.85%reflectance, and Santa Barbara (SB), 1.04% reflectance. Sizes of coal particles were 63-125 pm (SJ) and 36-75 pm (SB). These coal particles were separated, using a floatsink procedure, into six fractions with densities ranging from 1.35to 1.85 g ~ m - Ash ~ . contents within the particles range from 3.5 to 23 wt %. Combustion chars of intermediate burn-off were prepared from coal particles in a linear flow furnace in an atmosphere of nitrogen and oxygen at 1500 "C. Organic and associated mineral matter were characterized using optical microscopy, XRD, FTIR, and SEM-EDX. These chars were used to study the effect of mineral matter on further conversion. Results show that char reactivities at 500 "C (thermogravimetry) and combustibilities at 1200 "C (entrained flow) increase with increasing mineral matter content of the char, effects being more pronounced at the lower temperature. Surface combustion temperatures of coal particles at 1200 "C did not vary with ash content. The increase in reactivity is not attributed to catalytic effects of mineral matter or t o differences in internal surface areas of the chars as measured from carbon dioxide isotherms. It is considered that the enhancement in rate is associated with an enhanced macroporosity within chars in association with the decomposed clay mineral illite, having a platelike morphology. The presence of the illite facilitates the formation of parallel wide pores within the char matrix and these enhance mass-transfer of oxygen to the carbon and of reactants from the carbon. The development of such macropores is not detected by the adsorption isotherm.
Introduction Mineral matter plays an important role in coal combustion processes. Ash deposition within the fur-
nace, in particular on the heat exchangers, reduces the efficiency of heat transfer by radiation and convection. Also, a shield of mineral particles around the flame
+ Present address: MINEX Consultancy, Breelaan 16 D, Bergen NH, The Netherlands.
@Abstractpublished in Advance ACS Abstracts, July 15, 1994.
0887-0624/94/2508-1007$04.50/0
0 1994 American Chemical Society
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disturbs radiation heat transfer.l Such minerals as chemistry, and its size and extent of dispersion within iron-sulfur compounds and alkali salts are prone to the coal matrix. cause slagging and fouling.2 In recent years, aspects Mineral matter or “ash” content of coal is given as a of ash deposition have received special attention with mean value. [In this paper the term “ash content” is a focus directed to the identification and characterizaused for material derived from the “mineral matter” of tion of mineral matter within the pulverized coaL3 coal, as a result of the ashing process, ie., the complete Mechanisms of fly-ash formation in utilities and the combustion of coal material. The paper describes the mineralogy and phase diagrams associated with the effects of mineral matter on combustion, but amounts chemistry of mineral matter are topics of i n t e r e ~ t . ~ , ~ of total mineral matter have to be reported as ash Further, the chemical composition of dust particles content. The authors hope there is no confusion over emitted into the atmosphere must now be known the use of these two terms.] Further information because of possible toxic effects. concerning composition, size distributions, and heterogeneity within the coal and within the macerals of coal Recent years have seen the development of clean-coal requires dedicated analytical procedures. It has to be technologies, e.g. combined-cycle systems, which involve recognized that all coal particles ( < l o 0 pm) as in the direct impingement of combustion gases, with pulverized coal for utilities will not be identical and that significantly reduced hot mineral matter (ash) onto there could be a range of combustion phenomena turbine blades. However, such technologies are not yet exhibited by these particles. Some may be free of widely implemented, and coal without ultracleaning still mineral matter while others will have high concentrahas to be combusted. Further, coals as coming from tions of mineral matter, with different associations of washeries or cleaning plants have to be studied in terms macerals. Any systematic study of the influence of of relative amounts of mineral matter and its composimineral matter on coal particle combustion will have tion. to take into account effects of this heterogeneity on Literature of mineral matter in coal and its combuscombustion behavior, and to plan the experimental tion is mainly concerned with effects on heat-transfer approach accordingly. Possible approaches include the processes and the toxicity of dust emissions. The following: (a)the study of cleaned coal particles, (b) the influence of mineral matter on the combustion processes study of cleaned coal particles with free mineral matter of the coal particles themselves is beginning to be particles, (c) the study of coal particles with similar appreciated, mainly in terms of gasification and commineral matter contents, and (d) the study of coal bustion reactivity, that is, rates of combustion of the particles exhibiting a wide heterogeneity in terms of coalkhar particles. Several authors,6-8using thermocontent and composition. Although this approach is the gravimetric analysis systems, have reported enhanced industrial approach, a scientific study must initially use reactivities of chars from low-rank coals. Mineral a simplified system. matter in coal can have catalytic effects on combustion Simplification in terms of heterogeneity of mineral reactions. Conversely, it has been reported that chars matter content can be brought about by using the from washed high-rank coals had a higher reactivity approach of density gradient separations of the coal than chars from the unwashed coals.8 The authors particles. In this way a series of homogeneous fractions attributed these effects to an increase in porosity, is obtained that cover the full range of mineral contents surface area, and active site concentration resulting in raw coal particles. The density difference interval from the removal of mineral matter from the matrix of can be controlled t o within narrow limits, so facilitating coal material. Mitchellg has reported that the flame the availability of coal particle samples with welltemperature of combusting uncleaned coal was lower defined mineral matter contents and mineral matter than when the precleaned coal was combusted. The compositions. Such separated coal particles can then author could not precisely establish the influence of be subjected to a two-stage combustion process consistminerals because the particles in his experiment had ing of primary conversion at 1500 “C, followed by a different mineral contents. Reviewing recent studies burn-off step at lower temperature. indicates that little is known about the influence of mineral matter, in the original coal particle, on the The objectives of this study can be defined as follows. behavior of char combustion at high temperatures. One To study the combustion behavior of coal char parreason for this is the complexity of mineral matter ticles: (a) with a range of mineral matter contents as found in coals discharged from cleaning plants, (b) with (1)Wall, T. F.;Lowe, A,; Wibberley, L. J.; Stewart, I. Mc. Prog. special reference to mineral matter contents, (c) with Energy Combust. Sci. 1979,6,1-29. special reference to size and distribution of mineral (2)Raask, E.Mineral Impurities in Coal Combustion; Hemisphere: matter, and (d) with special reference to mineral matter Washington, DC, 1985. ( 3 )Quann, R. J.; Neville, M.; Janghorbani, M.; Sarofim A.Enuiron. composition, taking note of the reactivity of the char Sci. Technol. 1982,16, 776-781. particles and the features and morphologies generated (4)Sadakata, M.; Mochizuki, U.; Sakai, T.; Ono, M. Combust. Flame within the particles during the combustion process. 1988,74, 71-80. ( 5 ) Hebble, J. J.: Srinivasachar. S.; Boni, A. A. Prog. - Energy __ Combust. Sci. 1990,16,267-279. (6)Best, P. J.; Solomon, P. R.; Serio, M. A.; Suuberg, E. M. Prepr. Pup.-Am. Chem. SOC.,Diu. Fuel. Chem. 1987,32(4),138-146. (7)Serargeldin, M. A.;Wang, H. Thermochim. Acta 1990, 171(1), 193-206. (8)Serio, M. A.;Solomon, P. R.; Bassilekis, R. Prepr. Pap.-Am. Chem. SOC., Diu. Fuel. Chem. 1989,34(1),9-21. (9)Mitchell, R.E.The influence of t h e mineral matter content of coal on the temperatures and burning rates of char particles during pulverized coal combustion. I n Proccedings of the Sixth Annual International Pittsburgh Coal Conference, Sept. 25-29, 1989; The Combustion Institute: Pittsburgh, 1989;pp 69-78.
Experimental Section Preparation Methods. Preparation of Coals. Two Spanish high-ash bituminous coals, Santa Barbara (SB) and San were the parent coals of this study. Table 1contains Jose (MI, the proximate, elemental, and petrographic analyses of these coals. Representative samples of the coals were ground under nitrogen and sieved to particle sizes of 63-125 ,um (SJ) and 36-75 ,um (SB).A compromise between particle size and
Effects of Clay Minerals on Char Texture
Energy & Fuels, Vol. 8, No. 5, 1994 1009
Table 1. San Jose and Santa Barbara Coals: Proximate, Elemental, and Petrographic Analyses proximate analysis ash (wt%daf7 (wt% db)
vx
coala San Jose Santa Bhrbara a
14.64 33.13
33.85 48.41
elemental analysis (wt % dmf) C H N So, Odif 1.76 0.80 1.84 0.67
91.73 4.23 86.61 5.18
petrographic analysis vitrinite liptinite inertinite (kcalkg) Ro(%) ( ~ 0 1 %mmf) (vol%mmf) (vol %mmf)
1.48 5.70
cv
8012 7846
1.85 1.04
0.0 3.0
96.6 93.4
3.4 3.6
Samples available from NMI-SBN; FAX no. 31.45.465.653. Table 2. San Jose and Santa Barbara Coals: Density Intervals and Ash Contents of Coal Fractions SJ1-SJ6 and SB1-SB6 San Jose Santa Barbara densitj ash densitr ash fraction (g cm- ) (wt % db) fraction ( g cm- ) (wt % db)
Perceqtage of floated coal
0
SJ1 552 553 554 555 SJ6
90 100
1
1 0 10 20 30 40 50 60 70 80 90 100 Ash content of the heaviest fraction (wt %)
Figure 1. San Jose coal. Washability curve: variation of percentage of floated coal against ash contents of fractions. density ranges is necessary t o obtain sufficient amounts of sample. As this work is focused on mineral matter, it was decided to work on narrow density intervals rather than a narrow size range. The samples of pulverized coal were density-fractionated using a float-sink method.1° Washability curves were generated by plotting weight percentages of floated coal against the densities of the heaviest particles floated. So, these curves inform on the percentage of raw coal that floats at any given density of the washing liquid. This information is necessary to characterize the samples for our experiments. Figure 1 is the washability curve for the SJ coal. The separation range corresponds to the density limits 1.35 and 1.85 g ~ m - ~The . coal floating on the 1.35 g cm-3 liquid is the cleanest coal available by density separation (3.5 wt % mineral matter). The higher density gives a high mineral matter coal (23.0 wt %) that is still acceptable for industrial combustion. Raising the separation density improves fuel recovery by about 13 wt %. The washability curve of SB coal is similar to that of SJ coal. Once the relationship between density and mineral matter content is established by successive analyses, it is possible to calculate the minimum and maximum mineral contents of the particles within a density interval. For each of the two coals, six intervals were selected and the corresponding fractions were numbered SJ 1-6 and SB 1-6. The widths selected for each interval are a compromise between homogeneity within the fraction and a workable amount of the sample. The higher density fractions have wider ranges of density to fulfill the latter requirement. Preparation of Chars. Five grams of each coal fraction underwent partial combustion in a laminar flow furnace (LFF). The partial combustion was controlled such that with the char recovered not only combustion rate data could be obtained but char morphology could be characterized by optical and scanning electron microscopy. The main characteristics of the laboratory LFF include (1) an internal heat shield formed by an air-methane flame, (2) a n upward flow of combustion (10)Osbome, D. G. In Coal Preparation Technology, Graham Trotman: London, 1988;Vol. 2, pp 153-189.
1.370-1.395 1.405-1.450 1.460-1.500 1.505-1.570 1.650-1.720 1.755-1.820
4.8 12.1 16.4 22.4 35.8 43.6
SB1
SB2 SB3 SB4 SB5 SB6
1.290-1.300 1.340-1.350 1.395-1.415 1.495-1.520 1.575-1.610 1.625-1.690
2.8 4.5 12.1 17.8 30.9 40.8
gases, (3) axial sampling at k n o w n heights within the LFF, and (4) sample collection in a cyclone. In the sampling area, the temperature of the heat shield was 1500-1550 "C. The composition of the outlet gas was 95 vol % of nitrogen and 5 vol % of oxygen, using a flow rate of 1 m s-l. The residence time of the coal particle was 60 ms at the sampling point located 60 mm from the coal particle inlet position. Preparation of LTA Concentrates. Three fractions of the S J coal were subjected to low-temperature ashing in a LTA-504 equipment. The resultant particles of mineral matter, subject to a minimum of oxidation change, could then be examined for content, elemental composition, and morphology. Most coal matrix material had been oxidized away. Identification Methods. Identification of the Organic Matrix. Coals and chars were characterized using reflected light microscopy and scanning electron microscopy of polished blocks. With the optical microscope (Leitz MPV2, Zeiss) the use of crossed nicols and a 1 A wave retarder plate generated interference colours so facilitating the distinction between isotropic and anisotropic carbon in the chars. The analyses of the char structure and optical texture were carried out independently by three researchers. A point counter was used t o assess the structures beneath 300 points along a rectangular grid at an overall magnification of 800x. The cross-sectional morphology of the char particle was considered to be the most suitable way to distinguish and characterize, comparatively, the char morphologies observed. The classification used is similar to that of Bailey et and Bend et a1.12 Coulter particle size analysis of the fractions were made to monitor for possible changes caused by the contained mineral matter. Surface areas of selected chars (SJ2,SJ3, and SJ5) were calculated from adsorption isotherms of carbon dioxide at 273 K using the Dubinin-Radushkevich equation of a d ~ o r p t i o n . ~ ~ Identification of Mineral Matter. Mineral matter and its occurrence relative to the maceral components of the coals were determined by optical microscopy. The composition of the mineral matter in the coal, the char and LTA materials, was determined using both optical and scanning electron microscopy. A Jeol JXA-840SEM coupled to a Tracor TN2000 EDX system was used. Data for mineral composition were obtained using the coal minerals analysis (CMA) program (11)Bailey, J. G.; Tate, A,; Diessel; C. F. K.; Wall, T. F. Fuel 1990, 69, 225. (12) Bend, S. L.; Edwards; I. A. S.;Marsh, H. A. Fuel 1992,71,493501. (13) Dubinin, M. M. In Chemistry and Physics of Carbon; Walker, P. L., Ed.; Marcel Dekker: New York, 1966;Vol. 2, 51-119.
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Menkndez et al.
Table 3. San Jose Coal, Fractions SJ2,SJ5, and SJ6: Compositionaof Mineral Matter in Fractions, from SEM-EDX Analysis of LTA Samples mixed LTA kaolinite illite silicates pyrite siderite calcite ankerite others 7.0 5.5 5.0 12.3 SJ2 10.9 29.4 6.6 23.3 10.1 0.7 2.1 1.4 SJ5 8.0 71.8 4.7 1.2 3.9 5.0 0.4 0.5 SJ6 3.2 81.2 3.3 2.5 a
In wt o/o of mineral matter
developed by Huggins et al.14 and marketed by Tracor Northern. Analysis data obtained using CMA are not corrected for matrix effects and thus should be considered as being semiquantitative. Low-temperature ashes were additionally analyzed by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). Reactivity and Burn-OffExperiments. Reactivity (500 "C). Char reactivities at 500 "C in air were measured using a thermogravimetric method (a vacuum electromagnetic balance) as described by Crelling et a1.15 Char Burn-Off (1200 "C). Combustion experiments in entrained flow for chars SJI, SJ4, SJ6, and SB1, SB3, SB5 were performed at ACIRL (Australia). Temperatures of combustion were in the range 1200-1250 "C, some 300 "C below the temperature of formation of the char in the LFF. This difference was selected as being close to the difference between the temperatures of the flame and the cooler zones in an industrial combustion furnace. The inlet gas was 5.9 vol % oxygen at a flow rate of 34.2 L (STP) h-l. The char feed rate was 2.7-3.0 g of combustible matter per hour. The oxygedfuel ratio is 0.09 mol oxygen on 0.22-0.25 mol carbon, which is fuel-rich conditions. Conditions were maintained to generate about 50 wt % burn-out in the char particles. It was essential to prepare sufficient weight of char for subsequent analyses. Particle TemperatureMeasurement during Combustion. In a verification experiment the effects of mineral matter on the particle temperature reached during char combustion were investigated. Char particles were injected axially through a flat methane-air flame. [The flat flame burner for this experiment was not the same as used for char production.] The surface temperatures of the burning char particles were measured using a two-color pyrometer system (at wavelengths of 600 and 800 nm). The actual temperatures were derived from the radiation pulse height ratios. Each data point is an average of 100 experimental data. The temperature of gas around the char particles was measured by a thermocouple and corrected for radiation.
Results and Discussion Mineral Identification. Types of Coal Minerals. The density separation of the coal particles, using t h e float-sink procedure, provided 12 fractions with mineral m a t t e r contents varying from 5 to 44 wt %. Table 2 shows the density intervals and the mineral matter contents of the two coals. The mineral m a t t e r occurs as inclusions, typically 2-5 p m in size, dispersed within the carbonaceous coal matrix. The mineral m a t t e r , obtained from the low-temperature ashing (LTA), w a s characterized by FTIR, XRD, a n d SEM-EDX. XRD identified about twelve different mineral species including clay minerals, carbonates (calcite a n d dolomite) a n d silicates (quartz a n d feldspar). FTIR a n d SEM-EDX (14)Huggins; F. E.; Huffmann, G. P.; Lee, R. J. Scanning electron microscope-basedautomated image analysis (SEM-AIA)andMossbauer spectroscopy: Quantitativecharacterization of coal minerals. In Coal and Coal Products: Analytical Characterization Techniques, Fuller, E. L., Ed.; ACS Symp. Series No. 205;American Chemical Society: Washington, DC, 1982,239-258. (15)Crelling, J. C.; Hippo, E. J.; Woerner, B. A.; Gillespie, E. M. Ironmaking Proc. 1990,49, 211-216.
180 340 300 2 6 0 2 2 0 190 170 150 130 110 90 7 0 50
C M" x 10 Figure 2. San Jose coal. Samples 552, SJ5, SJ6: FTIR spectra of mineral matter from the LTA experiments (K = kaolinite, C = calcite, Q = quartz). confirmed these findings. Figure 2 shows the FTIR spectra of the mineral matter from LTA of San Jose coal. With increasing mineral m a t t e r content within the coal particles, a decrease in carbonates a n d an increase in q u a r t z a n d kaolinite are observed. The spectra of mineral m a t t e r from the SB coals were similar. Table 3 contains the SEM-EDX d a t a for minerals in lowtemperature ashes from t h r e e San Jose coal fractions. Within the coal particles with high mineral m a t t e r contents, pyrite a n d calcium minerals were minor
Effectsof Clay Minerals on Char Texture
Energy & Fuels, Vol. 8, No. 5, 1994 1011 L-
-1 i
I
Figure 3. San Jose coal. Optical micrographs of polished surface of sJ6: (a)clay minerals parallel to coal bedding plane (position A), quartz (position B);(b)clay minerals perpendicular to coal bedding plane; (c) pyrite (seearrow);(d) nodular syngeneic carbonates (see arrow).
components. The percentage of pyrite in the LTA materials was recalculated from the data of Table 3 as percentages within the carbonaceous matrix. The pyrite values decrease from 3.2 wt % in the coal SJ2 to 0.7 and 1.9 wt % for the high mineral matter content particles of SJ5 and SJ6. Calcium is present in amounts comparable to those of the pyritic iron, in the forms of calcite and ankerite. The principal group of minerals in all of the coal particles is clay. In the mineral matter from the LTA experiments using coal particles SJ5 and SJ6, 72 and 81 wt % of the mineral matter is in the form of the clay mineral illite. Optical microscopy of the coal fractions confirms the above results. Figure 3 shows optical micrographs of
polished surfaces of particles of SJ6. The clay minerals exist within the coal matrix as layers parallel to the bedding plane of the coal (Figure 3a,b). Quartz and pyrite are seen in Figure 3, a and c. Isolated nodules with spherical concretions of carbonates are in Figure 3d. Effects of Mineral Matter. Effects on Char Structure. The mineral matter contents of chars from the density separated fractions varied from 5.5 to 51 w t % (San Jose) and from 5.8 to 64 wt % (Santa Barbara). Optical and scanning electron microscopy show mineral matter particles dispersed within the walls of the char particles. A major part of the iron in the char particle, before combustion, exists as decomposed pyrite, FeS, 1
1012 Energy & Fuels, Vol. 8,No.5,1994
Mendndez et al.
Table 4. San Jose and Santa Barbara Chars and Chars after B u r n - m Petrographic Analyses of Polished Sections0 chars combustion residues TYPE SJ1 554 SJ6 SB1 SB3 SB5 SJ1 SJ4 SJ6 SB1 SB3 51.8 18.6 19.6 5.6 0.6 80.2 60.8 2.2 75.7 24.6 9.0 cenosphere 40.2 68.0 3.6 2.0 0.2 16.8 35.2 84.2 19.1 70.0 80.0 network 5.2 6.2 62.6 74.4 73.2 1.2 1.2 6.6 3.2 3.0 6.6 mixed 2.0 2.4 2.2 7.8 4.6 2.2 1.8 1.2 4.4 2.4 1.8 solid and fragment 0.0 0.4 5.0 0.0 2.8 13.8 0.0 1.6 0.0 4.6 0.6 mineroid ~~~
a
SB5 26.2 53.4 0.4 2.0 18.0
In ~ 0 1 % .
I t
V
t,.i
'L
#
lp
I 1
of' chars from density separated particles of coal SJ6: (a-d)
show different shapes of porosity within the char particles.
pm particle size. There are differencesin the shape and size of the macroporosity of char particles with low and high mineral matter contents. In the former, regular shapes are observed (Figure 4a,b) but in the latter the
porosities are elongated with directions parallel to the clay platelets (Figure 4c,d). Cross-sectional areas of the char particles were characterized by optical microscopy for morphological
Effects of Clay Minerals on Char Texture
Energy & Fuels, Vol. 8, No. 5,1994 1013
Figure 6. San Jose coal. SEM micrographs of polished surfaces of chars, after 50 w t % burn-off, showing mineral matter platelets at positionsA (light gray), and residual carbonaceous material at positions B (dark gray). Platelets were identified as (decomposed) illite by SEM-EDX.
details and optical texture. All chars of the study were anisotropic in terms of optical texture. The char morphologies were classified, in decreasing order of porosity into the six main groups of cenosphere,network, mixed, solid, fragment, and mineroid. A problem with this system is that mineral matter is identified, as a mineroid component, only when it constitutes more than 50 vol % of the char particle. Unfortunately then, this system13does not relate to the mineral matter content of most char particles. Networks of both low and high mineral matter contents belong to the same char type. The morphological analyses of SB and SJ chars are in Table 4. It is noted that the proportion of cenospheres decreases and that of the networks increases with increasing mineral matter content. Networks form a major part of all chars. Effectson Structure of Chars after Burn-Ofi Table 4 lists the morphologies of the chars, after burn-off, from SB and SJ coals using the same system.13 The morphologies of the original chars change little during the combustion process. The clearest change is that mineral matter is more recognizable. SEM-EDX of polished sections of the chars, after burn-off, identifies the platelet minerals which are interconnected by the remains of the networks of the char (Figure 5). Much of the platelet has no associated carbonaceous material, as also identified in the optical microscope (Table 4). Decomposed illite constitutes more than 50 w t % of the platelet components. Influence on Char Temperature. Char particles, of the same density within a given fraction, vary little in mineral matter content. There are no significant variations with different samples. Figure 6 gives char particle temperatures, at three gas temperatures, during combustion. Only at a gas temperature of 1500 "C does a small decrease occur.
For particles at a homogeneous temperature, heat losses depend on the total external surface. Reactivity, on the other hand, is a function of the mineral matter free portion of the surface. Hence, for particles of high mineral matter content, an imbalance between heat production and heat loss would result in a decreased temperature. As no significant differences were observed, it is implied that variations in heat-transfer properties have only a marginal role in the following burn-off experiments. Char Reactivity and Burn-Off. Reactivity (so0 "C). Char reactivity at this low temperature, as determined by thermogravimetric analyses, increases with increasing mineral matter content of the char particles. There is a factor of 3-4 in the reactivities of lowest and highest reactivity chars (Figure 7). The small differences in reactivities between the SJ and SB chars are attributable to differences in coal rank, 1.85% reflectance for the higher rank SJ and 1.04% reflectance for the lower rank SB. The surface area measurements, from isotherms of carbon dioxide a t 273 K, indicate that the areas within the char particles do not depend on mineral matter content. For all char particles, micropore surface areas of 130 m2 g-l are reported. Hence, the observed differences in reactivity cannot be attributed to differences in total available surface area. Further, these coals are of too high a rank to possess matrix-bound catalytic elements, e.g. calcium. The decomposed pyrite may have a catalytic influence at 500 "C. But, concentrations of FeS are less in the chars of lowest mineral matter content. Hence, it is not unreasonable to look for other effects associated with the mineral matter contents (see below). Burn-Off (1200 "C). The char burn-off at 1200 "C under fuel-rich conditions was measured by the ashtracer method on selected samples from the SJ and SB
Menendez et al.
1014 Energy & Fuels, Vol. 8, No. 5, 1994
c I
I600
I
02 = 2.5 YO V O I .
1
4-
2-
I 01
3
5
I
1
0-
I
w-
1
0
Tg = 1362'C,
I300
SI I
1
I
p>
1500
REACTIVITY*exp.4 Imglmg
10
10
0
1500
I
20
40
30
ASH IN CHAR
I
50
60
70
Iw~%]
Figure 7. San Jose and Santa Barbara coals, density gradient fractions: variation of char reactivity in nitrogedoxygen, at 500 "C,with ash content of the chars. CARBON BURN-OFF/ OXYGEN [at%]
80 1
I600
I4O0 I300
c
70
t
Tg=1512'C,
I
02 = 1 . 2 5 ' / 0 ~ 0 l .
L
0
10
20
30
40
A S H CONTENT,
50 O/o
I
Figure 6. San Jose coal. Density gradient fractions: variation of particle surface temperature of combustion with ash content for three flame temperatures, in a flat methane-oxygen flame.
series. Flow rates and residence times were reproducible for all samples. The feed rate was adjusted to the carbon content of the chars in order to normalize with respect to the availability of the oxygen. To compensate for small differences in oxygen supply, the burn-off data were recalculated for each experiment as a fraction of the maximum possible burn-out given by the oxygen/ carbon ratio (Figure 8). Results of these experiments are in Figure 8 and show that there is an increase in char burn-off with increasing mineral matter content, the effect being less pronounced for the SJ chars, compared with the SB chars. Increases are 27 and 36%, respectively, going from low to high mineral matter contents. As indicated above, effects of temperature differences between samples of different mineral matter contents can only be marginal. Also, catalysis is less likely to be influential at 1200 "C compared with 500 "C. At these higher temperatures, rates of reaction are more likely to be influenced by mass-transfer effects of oxygen to the surfaces and products of reaction away from surfaces. The positive effects of the mineral matter are attributed to specific structures developed within the chars of high mineral matter content and their burnoff samples. The clay minerals within SJ and SB coals contain '50 wt % illite, occurring as platelets interconnected within the matrix carbon. During the carbonization (pyrolysis) of the coal particles, at high heating rates, the coal becomes plastic and volatile matter is
-
-
40
;
0
10
20
30
40
50
60
ASH IN CHAR [wt%l
Figure 8. San Jose and Santa Barbara coals, density gradient fractions: variation of char burn-off in nitroged5% oxygen, at 1200 "C, with ash content of the chars. Burn-off is determined by ash-tracer method and calculated as carbod oxygen.
released. With high internal gas pressures, pore walls break and a porous coke structure develops. Under these conditions of pyrolysis the illite platelets reinforce the matrix carbon. The illite in the coal retains its platelike structure and remains adhered to the char material during subsequent stages of conversion. The optical and electron microscopy evidence is that large pores are generated running parallel to the plane of the platelet. The net effect caused by these illite platelets is an enlargement of the macropore structure (not seen in the isotherm of carbon dioxide at 273 K) such that mass-transfer effects are enhanced. This positive effect on combustion rate is largest when studied at 500 "C and is still operating a t 1200 " C . Under the fuel-rich conditions applied at 1200 "C, the high-ash samples make a more efficient use of the available oxygen than the low-ash chars.
Conclusions 1. The chars of the study have composite structures of carbonaceous material in association with platelets of decomposed illite, a clay mineral. The mode of distribution of the mineral matter in the coals controls the shape and size of the pores in resultant chars. 2. Reactivities of chars increase with increasing mineral matter content of the original coal. The en-
Effects of Clay Minerals on Char Texture
hancement of conversion rate is more pronounced at 500 "C in a fixed bed than a t 1200 "C using free particle flow procedures. The reactivity increases are about 250%at low temperature and 30% at high temperature. 3. The increase in conversion rate is explained in terms of enhanced macroporosity, and hence enhanced reactant gas accessibility, generated by the presence of the platelets of decomposed illite within the char matrix. This enhanced accessibility of reactant gases is not detected as an increased internal surface area in isotherms of carbon dioxide, which give equal surface areas of 130 m2 g-l. 4. Mineral matter content within the chars has a marginal effect on particle temperature during combustion. Only a t 1500 "C was a slight decrease observed in the surface combustion temperature of the char.
Energy & Fuels, Vol. 8, No. 5, 1994 1015
5. Coal cleaning, important to prevent slagging problems, does not necessarily improve combustion efficiency.
Acknowledgment. This study is part of an EC JOULE-1 program with contract no. JOUF-0050-C(TT). The support of the EC is acknowledged. Further, the authors thank the following colleagues for their supportive cooperation: M. Barrero for advice with coal preparation; D. Phong-anant for the burn-off experiments; C. M. Roos for the coal petrography; and S. Eenkhoorn for the char preparation experiments. The support of Harry Marsh in editing the manuscript is gratefully acknowledged.
