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Influence of pyrolysis temperature on char optical texture and reactivity
Journal of Analytical and Applied Pyrolysis 58–59 (2001) 887– 909 www.elsevier.com/locate/jaap
Influence of pyrolysis temperature on char optical texture and reactivity M.J.G. Alonso, A.G. Borrego *, D. Alvarez, J.B. Parra, R. Mene´ndez Instituto Nacional del Carbo´n, CSIC, Francisco Pintado Fe 26, Ap. 73. 33080 O6iedo, Spain Received 17 April 2000; accepted 6 October 2000
Abstract In this study a set of 10 coals varying in rank and maceral composition has been devolatilised in a flat flame burner at 1000 and 1300°C in order to investigate the changes in char reactivity and structure as a function of pyrolysis temperature. The intrinsic reactivity of the chars, measured isothermally in a thermobalance at 500°C, has been related to the petrographic characteristics of the chars and their CO2 surface areas. The temperature of pyrolysis has shown to have a strong effect on char reactivity for certain coals. Overall, the increase of temperature provoked an enhanced plasticity and a more extensive consolidation during the metaplast stage of pyrolysis and reduced the amount of unfused material in the chars. This effect was more pronounced in low volatile bituminous vitrinites and low rank inertinites. The presence of inertinite derived materials in high rank coal chars enhanced their reactivities, whereas the opposite was observed for low rank coal chars. This was attributed to the fact that inertinite macerals yield both isotropic and anisotropic materials: inertinite in low rank coals increases the amount of anisotropic, less reactive char, but reduces it in high rank coals. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Pyrolysis temperature; Char optical texture; Char reactivity; Coal; Maceral composition; Char surface area; Structural order
* Corresponding author. Tel.: + 34-985-280800; fax: +34-985-297662. E-mail address: [email protected] (A.G. Borrego). 0165-2370/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 2 3 7 0 ( 0 0 ) 0 0 1 8 6 - 8
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1. Introduction The combustion of pulverised coal particles under the environmental conditions prevailing in an industrial boiler is known to occur as a two-stage process [1]. The first one is the fast pyrolysis of the coal, an essentially thermal effect which consists in the sudden release of volatiles accompanied by drastic changes in the morphology and molecular structure of the fuel particles, and the second one is the chemical reaction between the oxygen and both the gaseous (volatiles) and the solid (char) products of pyrolysis. Different attempts have been made to simulate the conditions prevailing in the near-burner zone of pulverised fuel boilers, and a number of different facilities such as drop tube furnaces (DTF) [2,3], hot-wire meshes [4], flat flame burners [5], etc. are described in the literature. These reactors are able to reproduce the thermal conditions prevailing in the near-burner zone of the boiler, so that coal particles undergo similar structural changes to those occurring during the combustion process at full scale. If the heating of the coal particles is carried out under an oxygen-free atmosphere, the solid product obtained can be readily isolated and recovered for its full characterisation. The importance of this relies in the fact that the isolated material is the final product of the pyrolysis stage and, at the same time, the starting point of the subsequent gasification (combustion) stage, and a full knowledge of this material can greatly simplify the study of the overall combustion process. The difficulties to simulate the conditions prevailing in full scale boilers and the lack of standardised tests for the assessment of coal combustibility may explain to some extent why after so many years of coal combustion research, only very general guidelines can be outlined about the behaviour of coal. Besides, varying coal rank and maceral composition do not make this task any easier. There is nevertheless general agreement in that the increase in coal rank results in a decreased reactivity to oxygen [6,7], although deviations from this trend have also been reported [8,9]. Results are however more contradictory when systematic trends for the behaviour of the different macerals are sought. Thus, chars from inertinite-rich coals, which showed generally lower intrinsic reactivities than vitrinite-rich chars of similar rank in thermogravimetric experiments [10], gave a satisfactory performance in industrial boilers or DTF test [11,12]. Some recent studies carried out on the reactivity of inertinite [13 –15] have also reported higher combustion reactivities of semifusinite compared to vitrinite, and higher reactivities of unfused inertinite chars than those of fused chars derived from either vitrinite or inertinite. In this context, optical microscopy is a most versatile tool for the study of the structural organisation of char material, since this technique permits the analyst to distinguish and quantify the isotropic (disordered) and anisotropic (ordered) char materials formed after the passage of coal through a thermoplastic stage. The type and size of anisotropic domains in chars is strongly related to their reactivities as it will be shown below. The purpose of this paper is to study the influence of flame temperature on the pyrolysis behaviour of pulverised coal particles from coals of variable origin and maceral composition. It particularly focuses on the differences observed in the optical texture and reactivity as a function of the characteristics of the parent coal.
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2. Experimental
2.1. Coal preparation Ten coals varying in rank (high volatile bituminous to anthracite) and maceral composition (vitrinite- to inertinite-rich) were selected for this study. The coal set comprises a wide range of maceral occurrences in terms of abundance and maturity degree. It is intended to take advantage of this feature and come to a better understanding of the thermoplastic behaviour of the different macerals and its variation with rank. Details of coal provenance and geological age are given in Table 1. Representative coal samples were ground and sieved to a narrow size range (36 –75 mm diameter). This interval comprises the medium-size particles within the range typically used in industrial pulverised fuel boilers (\ 70 wt.% of particles B75 mm). Those coals with ash contents significantly higher than 15% (BB2 and FM3) were beneficiated by flotation in dense liquids, thus ensuring that all the coals had similar and reasonably low mineral matter contents. The minimum floating density used was 1.7 g cm − 3, since any further reduction might have resulted in an undesirable effect of segregation of the denser inertinite macerals [16].
2.2. Coal characterisation Ultimate and proximate analyses of coals were carried out following standard procedures. Maceral analyses were performed using a combined maceral/reflectance procedure in which random reflectances were measured on any maceral selected by point-counting (500 points) annotating the corresponding identification. For the estimation of coal rank from petrographic analysis (Rr) only the readings taken on collotelinite [17] (former telocolollinite), the vitrinite maceral following the most homogeneous coalification path, were averaged.
Table 1 Provenance and geological age of coals selected Code
Provenance
Country
Age
BMC BB2 PHA BSE FM3 CRA SMK TAF DAN VCB
Paparoa coal Sydney basin Phalen (Nova Scotia) Brunner coal Coal mountain Cadinal river Smoky river Taff merthyr Ibbenbu¨ren Victoria
NZ AU CA NZ CA CA CA UK DE PE
Cretaceous Permian Caboniferous Eocene Jura/L.cretaceous Lower cretaceous Lower cretaceous Caboniferous Caboniferous Cretaceous
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2.3. Char preparation Representative samples of each coal were pyrolysed at 1000 and 1300°C in an air/methane flame. Coal particles were axially injected by a gas flow at the base of the flame. The flame was always operated under highly sub-stoichiometric oxygen levels (Table 2) so that particles were suddenly pyrolysed and then immediately cooled down when leaving the flame. Details of the experimental device are given elsewhere [18]. This sort of flame has been successfully used to resemble the time/temperature conditions of the pulverised particles in the near burner zone of an industrial boiler [5].
2.4. Petrographic analysis of the chars Chars were embedded in an epoxy resin, left overnight, cut, and polished for the microscopic examination of particle cross-sections. The optical texture of the char material was examined under a polarised reflected light microscope with oil immersion objectives of 50× and 100× magnification, partially crossed pollars, and 1 u retarder plate. Petrographic characterisation of the chars was performed according to the classification system currently used by the International Committee for Coal and Organic Petrology (Inertinite in Combustion W.G.). This classification distinguishes between vitrinite- and inertinite-derived material. The latter is further subdivided according to its optical texture (isotropic/anisotropic) and porosity (unfused, porous, dense).
2.5. Char surface area The surface area of micropores (B2 mm in diameter) was calculated using the Dubinin – Radushkevich (D – R) equation [19] from CO2 adsorption data at 273 K, which were obtained using a Micromeritics Gemini 2375 at the interval of pressure 0.035 –0.0001 torr. Different range of pressure (0.035 –0.0035 torr) was used for the vitrinite-rich VCB anthracite chars, since they both showed activated diffusion at lowest pressures.
2.6. Thermogra6imetric analysis Char combustion was isothermally recorded using a Perkin Elmer TGA7 thermal analysis system. A small quantity of char (13 mg) was homogeneously spread at the bottom of the platinum crucible and then heated up to 500°C under nitrogen flow (50 cm3 min − 1) at a heating rate of 25°C min − 1. After weight stabilisation, nitrogen was replaced by air at the same flow rate and the temperature was maintained until combustion was completed. At this low temperature and using such small sample sizes, bed effects in the thermobalance can be ruled out and the kinetic control of the reaction is ensured. Char reactivity is defined as
Run 1 Run 2
O2
2.0 2.0
1.5 1.5
3.2 3.2
0.0 0.5
30 28
15 20
O2
CH4
Air (flame)
CH4
Air (injector)
Gas composition (vol.%)
Gas flow rate (cm3 min−1 STP)
Table 2 Operating conditions of the methane/air burner
55 52
N2
1000 1300
Measured
1140 1550
Adiabatic
Flame temperature (°C)
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Fig. 1.
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Rx = −
1 mo
dm dt
893
, x
where mo is the initial weight of ash-free sample and x is the conversion value chosen. Both the reactivity at x =50% conversion and the maximum reactivity (x =max) were calculated.
3. Results and discussion Results of the chemical and petrographic analyses of the coals are summarised in Table 3. Coals are listed by increasing rank as determined by random collotelinite reflectance (Rr) and its range is from 0.64 to 5.63%. The selected coals have ash contents from 0.6 to 18.7%. Coals FM3 and BB2, which were both floated, still retained a relatively high ash content (15.9 and 18.7%, respectively). This indicates an intimate association between organic and mineral matter. Data from chemical analysis followed the expected trends with increasing rank, this is: a decrease in volatile matter, hydrogen and oxygen contents and an increase in carbon content and fuel ratio (Table 3), with some deviations which are due to differences in maceral composition. The high fuel ratio and low volatile content of BB2 is attributable to its high inertinite content, as it is known that inertinite contains less volatile matter than vitrinite for a given coal [20]. The relatively high liptinite proportion of PHA (about 10%) might be responsible for its high hydrogen content (Table 3) since liptinite is the H-richest maceral group.
3.1. Petrographic study of the chars Optical micrographs of chars obtained at 1000 and 1300°C are shown in Figs. 1–4. BMC chars prepared at 1000 and 1300°C did not show remarkable differences in their morphologies. Both chars formed preferentially isotropic, single-chambered particles with rather elongated shapes and abundant development of porosity within the walls (Fig. 1). Inertinite-derived char from BB2 (1000°C) was made up of angular particles, most of them showing signs of having passed through a plastic stage as reflected by the abundant spheroidal pores distributed throughout the particle. The char material was either isotropic or anisotropic, but the size of the anisotropic domains was rather small and dispersed within an isotropic matrix. This char also contained a significant amount of partially pyrolysed material (pp in Fig. 1), which was not observed in the vitrinite-derived char from a coal of similar rank Fig. 1. Optical micrographs showing the aspect of the chars obtained at both 1000°C (left) and 1300°C (right) from the high volatile bituminous coals. BMC consists of isotropic cenospheric particles, BB2 of molten and non-molten inertinites, either isotropic or anisotropic. The low temperature BB2 char contains partially pyrolysed material. PHA and BSE chars show incipient anisotropy development. Images taken with a reflected light microscope, oil immersion objective and crossed pollars. (vd;, vitrinite-derived; af, anisotropic fused inertinite; fs, fusinoid; pp, partially pyrolysed material; if, isotropic fused inertinite).
0.64 0.66 0.84 1.03 1.05 1.23 1.53 1.77 3.20 5.63
85.8 1.2 80.4 96.2 55.0 53.0 57.8 71.6 61.8 98.0
V 2.4 97.2 9.4 – 45.0 47.0 42.2 28.4 38.2 2.0
I
Maceral (vol mmf %)
11.8 1.6 10.2 3.8 – – – – – –
L 0.60 18.7 15.3 0.04 15.9 7.3 12.8 5.2 2.3 1.9
Ash (d %)
1.73 2.78 1.71 2.59 2.99 3.68 4.29 6.29 14.48 58.13
FR
46.2 24.8 36.7 32.0 28.9 22.3 18.1 13.6 6.2 1.6
VMa
(daf %)
80.6 80.1 81.3 83.0 86.3 89.5 90.6 90.8 92.1 94.9
Ca 5.9 3.5 5.2 4.8 5.0 4.6 4.4 4.1 3.2 1.2
H
1.4 1.8 1.3 1.1 1.5 1.8 1.1 1.3 1.7 0.9
N
0.3 0.4 2.6 3.2 0.9 0.2 0.4 0.7 0.5 0.2
S
11.8 14.2 9.6 8.0 6.3 3.9 3.5 3.1 2.5 2.8
Odif 0.87 0.52 0.77 0.69 0.69 0.62 0.58 0.54 0.42 0.15
H/C
0.11 0.13 0.09 0.07 0.05 0.03 0.03 0.03 0.02 0.02
O/C
Corrected for carbonates, mmf = mineral matter free; d = dry basis; daf =dry-ash-free basis; dif. =by difference; VM, volatile matter; FR, Fuel ratio (Fixed carbon/volatile matter), Rr, random vitrinite reflectance, V, vitrinite; L, liptinite; I, inertinite.
a
BMC BB2 PHA BSE FM3 CRA SMK TAF DAN VCB
Rr (%)
Table 3 Results of the petrographic and chemical analyses
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Fig. 2. Detail of the char forming material in BSE and PHA. Incipient anisotropy is observed in BSE char (granular appearance of the wall) whereas PHA char shows locally well-developed anisotropic domains (positions indicated by arrows). Images taken with a reflected light microscope, oil immersion objective and crossed pollars.
(BMC), indicating that inertinite of this rank exhibited a more complex devolatilisation behaviour than vitrinite [18]. The 1300°C char from BB2 was more efficiently pyrolysed and showed an enhanced plastic behaviour with less amount of unfused material (Table 4). The difference of 300°C between the low and high temperature runs seem to have been enough to provoke a significant improvement in inertinite devolatilisation. Chars from PHA prepared at both 1000 and 1300°C were made up of thickwalled rounded cenospheric particles exhibiting an optical texture of fine mosaic (size less than 1 mm) and abundant secondary porosity within the walls. The mosaics were more evident than in BSE char (from a coal of higher rank) which indicates a rather peculiar devolatilisation behaviour for coal PHA (Fig. 2). This might be due to the higher liptinite content of PHA coal, which locally raises the volatile matter content of the coal. The development of anisotropic texture and porosity in a char depends both on the volatile matter content and the rate of crosslinking in the structure of the parent coal [21]. These two features display a smooth variation with coal rank, and therefore the textural and porous development of chars follow an accordingly smooth variation with the rank of the parent coals. However, the liptinite particles (sporinite) present in the vitrinite matrix of PHA give an extra supply of volatiles which might locally raise the pressure of volatiles. This, in turn, increases the tensile strength (the driving force of coales-
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Fig. 3.
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Fig. 4. Optical micrographs showing the aspect of both 1000°C (left) and 1300°C (right) the chars obtained from the two anthracites. Chars consist of unfused angular particles exhibiting some cleats (arrows) that are more common in the higher temperature chars. Images taken with a reflected light microscope, oil immersion objective and crossed pollars. (uv, unfused vitrinite, ui, unfused inertinite).
cence) in the char walls, thus giving rise to a more accentuated anisotropic texture than would be expected for the rank of the coal. This effect became more pronounced in the high temperature char, in which anisotropic mosaics of larger size were locally observed (Fig. 2). No signs of anisotropy were found in BMC, the other high liptinite coal, but this might be due to the different nature of this liptinite, mainly diffuse resinite impregnations with reflectance (and therefore volatile matter content) similar to that of the vitrinite matrix and more widely spread than the sporinites of PHA. Except for the presence of incipient anisotropy development, the morphology of BSE chars at both temperatures closely resembled that of BMC chars. Fig. 3. Optical micrographs showing the aspect of the chars obtained at both 1000°C (left) and 1300°C (right) from high to low volatile bituminous coals. Vitrinite-derived material is always anisotropic and its plasticity increases with the rank of the coal as shown by the presence of thinner walls in the higher ranked coal chars. The increase in preparation temperature has a similar effect on the plasticity. Images taken with a reflected light microscope, oil immersion objective and crossed pollars. (vd, vitrinitederived; um, unfused massive inertinite).
94 1 87 100 47 55 50 54 71 100 84 100 51 49 48 46 73 100
96 52 2 25 17 27 43
1 63 3 27 16 24 24
1300
5 3
1000
AD
4 6 4
2
1300
3 1 1 2
2 12 5
1000
IP
2 4 5 2
1 24 8
1300
7 5 5 5
1 3
1000
ID
8 9 6 2
13
1300
10 15 14 10 28
1 16 3
1000
UM
Unfused
6 11 7 3 25
1 9 4
1300
6 3 3 5 1
1 5 2
1000
FS 1300
4 5 3 4 2
2 1 2
VD, Vitrinite-derived; AP, Anisotropic porous; AD, Anisotropic dense; IP, isotropic porous; ID, Isotropic dense; UM, unfused massive; FS, unfused fusinoid.
a
BMC BB2 PHA BSE FM3 CRA SMK TAF DAN VCB
1000
1000
1300
AP
Fused
Inertinite-derived
VD
Vitrinite-derived
Table 4 Petrographic analysis of chars (vol%)a
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Significant differences were observed in the optical texture and morphology of BMC and FM3 chars, both generated from coals of similar rank. The anisotropic texture was more easily distinguishable in char FM3 than in char BSE (Figs. 1 and 3). The chars from FM3, CRA and SMK formed abundant multi-chambered particles containing both vitrinite- and inertinite-derived material (mixed particles) due to the intimate association of both macerals in the parent coals. The size of the anisotropic texture of the vitrinite-derived material in chars FM3, CRA, SMK and TAF increased with the rank of the parent coal. In addition, the 1300°C char of FM3 and CRA were characterised by thinner char walls and larger anisotropic domains than the low temperature char. Vitrinite from SMK and TAF tended to form at 1000°C thick-walled rounded cenospheric particles, whereas at 1300°C yielded more angular particles with a central pore surrounded by a net of extremely thin walls comprising elongated pores (Fig. 3). This behaviour was considered in a previous work [18] to be the result of a temperature gradient in the particle due to a high heating rate. The outermost part would fuse first and re-solidify when the core of the particle was still unfused. This would prevent the formation of spherical particles and, at the same time, the internal volatile pressure would deform the porosity existing within the walls (Fig. 3). DAN and VCB did not show significant changes after the passage through the flame except for a higher reflectance and enhanced bireflectance (difference between maximum and minimum reflectance). Neither inertinite nor vitrinitederived material fused during pyrolysis and both displayed the inherited anisotropic pattern of the parent material (Fig. 4). The differences between coal and char particles were more evident in DAN (lower rank anthracite) than in VCB. Chars from DAN, particularly the 1300°C particles, displayed extensive development of long cleats parallel to each other indicating the occurrence of some contraction in the structure parallel to the bedding planes due to limited devolatilisation. These cleats were not found in the low temperature VCB char and the 1300°C counterpart only displayed cleats in some of the particles (Fig. 4). Overall, the plasticity of inertinite during pyrolysis decreased as coal rank increased, as shown by the fact that the chars from BB2, a coal consisting of nearly pure inertinite only contained a low percentage of unfused material (Table 4), whereas FM3, CRA and SMK chars, all from coals consisting of about 50% vitrinite, exhibited relatively larger amounts of unfused material. It is not only coalification, but also the level of pre-depositional oxidation which determines the reflectance of inertinite [22]. The high reflecting inertinites (fusinites and some massive macrinites) achieve, prior to deposition, a highly cross-linked structure[23] that prevents them from melting during pyrolysis and is responsible for the large amounts of unfused material in the chars. Moderately altered inertinites still retain a certain amount of volatiles which might be lost during sudden heating, generating particles with either isotropic or anisotropic texture but with clear signs of having passed through a plastic stage.
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3.2. Char reacti6ity 3.2.1. The effects of coal rank and maceral composition Two main aspects control the reaction rate of any carbonaceous material at low temperatures, and these are the specific surface area, which determines the surface of the solid accessible to oxygen, and the intrinsic reactivity [24], and the following discussion will mainly focus on these two features. The reactivity parameters (reactivity at 50% conversion and maximum reactivity) and surface areas (Dubinin – Radushkevich) of the chars are shown in Table 5. For all the studied chars, the maximum reactivity was reached at conversion rates of 10–20%. Similar variations in reactivity among the studied chars can be noticed regardless the reactivity parameter selected, and therefore we will only deal hereinafter with one of them, namely the reactivity at 50% conversion. The influence of coal rank and maceral composition on the reactivity of the chars will be considered first. The changes undergone by coals during coalification are well systematized as regards the vitrinite group, the major component of most Northern Hemisphere coals [23]. Low rank coals tend to form, upon rapid devolatilisation, chars exhibiting an isotropic optical texture, with large surface areas and reactivities. This is due to the highly crosslinked structure of these coals, which limits the mobility of the molten material and prevents its macromolecules from coalescing. With the increase of coalification, the crosslinking is greatly reduced and the coals undergo drastic changes in their molecular arrangement during fast pyrolysis, as a consequence of the high fluidity of the molten char material, and this results in the formation of bigger, more ordered domains (anisotropic texture). The surface areas and reactivities would be accordingly lower for these chars [6]. Finally, the large polyaromatic units in the anthracites are highly ordered parallel to the bedding plane and do not melt at all upon heating. This three-dimensional arrangement only leaves a small-sized, hardly accessible to oxygen pore network. The behaviour of the inertinite (major component of most Southern Hemisphere coals) during pyrolysis is more controversial, and little is known about the molecular arrangement of this maceral group and its variation with coal rank. This is due to the inherent heterogeneity of these macerals, which underwent variable alteration degrees prior to deposition. However, the inertinites are known to have a highly crosslinked structure, similar to low rank vitrinites, but more aromatic than these [23]. A substantial part of the inertinite contained in a coal is able to fuse to variable extents, which will be higher the higher the temperature of pyrolysis [15]. The surface area and reactivity of their chars will strongly depend on the plastic properties reached during pyrolysis. We will now analyse the data of Table 5 on the basis of the above explained facts. The coals BMC, PHA, BSE, TAF and VCB can be regarded as a series of vitrinite-rich coals (\70% vol.) of increasing rank (0.64 –5.63% vitrinite Rr). Their chars display a broad trend of decreased reactivity with the increase of rank (see Fig. 5a–b), a well-known fact in coal combustion research [1,6,7]. However, BMC, the coal at the low-rank end showed a much lower reactivity than PHA, the coal
a
1.82 2.14 3.31 0.79 1.94 0.96 0.71 0.84 1.68 0.70
2.88 0.48 3.14 1.00 2.48 1.14 0.80 0.18 0.88 0.54
3.04 4.37 6.39 1.29 3.46 1.45 1.09 1.51 2.47 0.78
5.33 0.78 6.86 2.34 4.80 2.09 1.53 0.37 2.12 0.67
1300°C
1000°C
1000°C
1300°C
Rmax×104 (mg mg−1 s−1)
R0.5×104 (mg mg−1 s−1)
36 27 17 84 37 61 93 74 44 147
1000°C
tR0.5 (min)
21 127 18 53 23 53 76 326 60 166
1300°C
9 8 5 13 15 17 19 17 21 98
1000°C
tRmax (min)
7 35 5 10 8 11 18 39 29 84
1300°C
351 278 260 350 261 280 236 212 383 289
1000°C
357 108 288 357 312 303 258 53 322 212
1300°C
Area (m2 g−1 ash free)
R0.5, reactivity at 50% conversion; Rmax, maximum reactivity; tR0.5, time to reach 50% conversion; tRmax, time to reach maximum reactivity.
BMC BB2 PHA BSE FM3 CRA SMK TAF DAN VCB
Coal
Table 5 Reactivity and CO2 surface areas of the chars obtained from coals varying in rank and maceral composition at 1000 and 1300°Ca
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char immediately following in rank. This unexpected result was attributed to partial burnout of this sample in the flame, resulting in char deactivation [25,26]. In the air –methane flame used in this study, both methane and the coal particles themselves compete for the oxygen, the former being in clear advantage mainly for two reasons: (i) this is a diffusionally unrestricted gas –gas reaction; and (ii) methane is more reactive to oxygen than coal. The importance of the second reason is minimized for the very reactive low rank coals, and thus it is likely that BMC might
Fig. 5. Variation of measured reactivity at 50% conversion (R50%) with the rank of the parent coal (Rr) for the 1000°C (a) and 1300°C (b) chars. The lines illustrate the trends obtained for the vitrinite-rich coal chars.
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have undergone a partial combustion in the flame. Partly burnt chars are known to be less reactive than their ‘fresh’, non-combusted counterparts [3,27]. On the other hand, the VCB anthracite char produced at 1300°C has a higher reactivity than the TAF (lower ranked coal) char obtained at the same temperature. This was attributed to the very high plasticity of TAF coal under the pyrolysis conditions used in this work, which gives rise to the annihilation of the microporosity (and therefore, the surface area available to oxygen) of the carbonaceous material [28]. This coal is in the high-rank end of the interval of coking coals, and thus it might be expected that other lower rank coals (SMK or CRA, for instance) would show much better plastic properties than TAF. However, the interval of high plasticity of coals is shifted towards higher ranks with the increase of temperature and heating rate [29,30], and this is why low volatile bituminous coals submitted to the pyrolysis conditions typical of pulverized fuel combustion show much higher plasticities than in experiments performed under typical coking conditions. Analytical data in support of this will be given below. The maceral effects on the pyrolysis behaviour of coals are mainly related with the inertinite. The small effects due to the normally scarce liptinites have been mentioned above. BB2 is an almost pure inertite, from which valuable information about the pyrolysis behaviour of the inertinite macerals can be obtained. The reactivity of BB2 (Fig. 5) is much higher for the 1000°C char than for the 1300°C char, a similar effect to that observed for TAF, and which can also be explained by the same reasoning: the higher temperature of pyrolysis provokes an increase in the plastic behaviour of the inertinite, followed by a reduction in the surface area (see below). One cannot expect, however, that this behaviour will be maintained throughout the whole coalification scale. In fact, the reflectance of the inertinite macerals broadly increases with the increase of coal rank, as shown in Fig. 6 for the coals used in this study. It is well known [31,7,15] that the increase in the reflectance of the inertinites, either due to coalification or to pre-depositional alteration, reduces their fusibility. Therefore, it can be expected that the role of the inertinite will vary from low to high rank coals, leading to an increase of anisotropic (highly altered) material for the former, but to a reduction for the latter. Once established the behavioural trends of vitrinite and inertinite with rank, we will now analyse the reactivity data obtained from FM3, CRA, SMK and DAN, the coals with significant percentages of both vitrinite and inertinite. The first two of these produced, at the two temperatures used, chars with lower reactivities than expected from their ranks. This can be better noticed when the reactivities of BSE and FM3, both of similar rank, are compared. Thus, the reactivities of the FM3 chars (45% inertinite) are much smaller (about 50%) than those found for the inertinite-free BSE chars, the same as would be expected for an artificial mixture (55 –45) of BSE and BB2. In the case of the anthracite DAN chars, its highly reflecting inertinite did not melt at all at either temperature, and therefore remained isotropic, more reactive than the accompanying vitrinite. The reactivities of these two chars were accordingly higher than expected from the broad trend of variation with rank. In between, the reactivities of the SMK chars, despite the high inertinite
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Fig. 6. Variation of mean random inertinite reflectance (Rinertinite) with the rank of the coal (Rr)
content (42% vol.) of their parent coal, where similar to the values expected for a vitrain of the same rank. Assuming that inertinite exerts a negative effect on reactivity at low ranks and a positive effect at high ranks, then at some intermediate rank the effect of inertinite must be negligible. In other words, at coal ranks around 1.5% vitrinite reflectance, and for the environmental conditions used in this study, the pyrolysis behaviour of the vitrinites and inertinites must be quite similar, as regards the reactivities of the obtained chars.
3.2.2. The effect of pyrolysis conditions Figs. 7 – 9 illustrate the effects of the temperature of pyrolysis on various reactivity parameters of the chars obtained. The most striking features of the reactivity data plotted in Fig. 7 are the high positive deviation, i.e.: the positive effect of the increase in temperature on char reactivity, of BMC chars, and the negative deviations found for BB2, TAF and DAN. As already mentioned, the most important parameters affecting the reactivity of a char are the intrinsic reactivity and the specific surface area. For a more detailed discussion of the data of Fig. 7, they have been split into these two separated parameters. The specific surface areas given in Table 5 are plotted in Fig. 8, and the intrinsic reactivities (measured reactivity per unit surface area) are shown in Fig. 9. The highest differences between the surface areas of the low and high temperature chars were found for BB2, TAF, DAN and VCB, this is, the inertite and the three highest ranked coals. All these coals produced chars with lower surface areas at 1300°C than at 1000°C, especially BB2 and TAF, the effect being less pronounced for the two anthracites. As mentioned above, both BB2 and TAF
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were expected to considerably enhance their plastic properties with the increase in the temperature of pyrolysis, as it seems to have happened. The formation of a very fluid metaplast during pyrolysis brings about the annihilation of the microporous structure in the chars, thus leading to much lower surface areas. In the case of the anthracites, with no plastic properties whatsoever, the reduction in surface area must be due to a different mechanism. Actually, no variations would be expected in the surface areas from the 1000 to the 1300°C chars and, if any, they should run in opposite direction to the results found in this study. Thus, the increase in temperature led to the formation of abundant cracks in the vitrinite particles of DAN and, to a lesser extent, also in VCB, as a consequence of the stronger thermal shock undergone by these higher temperature chars. This, in turn, would facilitate the entrance of CO2 to the whole volume of the particles, therefore raising the figures of specific surface area. More, it is quite likely that not only cracking but even fragmentation of some of the high temperature char particles had occurred, which is known to increase the surface area (or at least the measured value) of anthracites [32]. Only the possibility that, even in such mature coals, some residual short-distance mobility existed in the pyrolysing groundmass, would permit the annihilation of a part of the porous network in the solid. In any case, the small quantitative importance of this effect of surface area reduction, as well as the lack of any experimental evidence or literature data supporting the existence of molecular mobility in anthracites, do not allow to draw any firm conclusion about this phenomenon.
Fig. 7. Relationship between the measured reactivity at 50% conversion of the chars obtained at 1000 and 1300°C.
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Fig. 8. Relationship between the CO2 surface areas of the chars obtained at 1000 and 1300°C.
The plot of Fig. 9 reveals that the temperature of char formation does not have a strong effect on its intrinsic reactivity to oxygen. Only BMC displayed an enhanced reactivity in the 1300°C char compared with its 1000°C counterpart, and the opposite was observed for BB2. The above mentioned possibility that BMC had burnt to some extent during the pyrolysis experiment might be the reason for this peculiar result. In the case of BB2, it has been mentioned that its 1000°C char was not fully devolatilised. This means that some of its carbon skeleton might still contain aliphatic carbons (sp 3-hybridisation), intrinsically more reactive than aromatic carbons (sp 2-hybridisation). Summarising, the increase in the measured reactivity of BMC chars with increasing temperature of pyrolysis is due to the higher intrinsic reactivity of the 1300°C char, the reduction observed in TAF and DAN has to be attributed to a decrease in surface area, and, finally, the decrease of both the surface area and the intrinsic reactivity is responsible for the lower reactivity of the 1300°C char of BB2, compared with the corresponding 1000°C char. One more factor has to be borne in mind in reactivity studies on chars, and this is the possibility that the mineral matter might exert a catalytic effect on char combustion. This was also taken into account in the present work. Fig. 10 plots the variation of intrinsic reactivity with the ash content of chars, where a broad trend of increased reactivity with increasing ash content can only be noticed for the chars with ash contents over 20 wt.%. These are BB2 (1000°C) and the two chars obtained from both PHA and FM3. On the other hand, the reactivity of the 1300°C char from BB2 is comparable to those of lowest ash chars. BB2 and FM3 are both
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floated coals, which ensures that the free mineral matter grains have been removed and thus the mineral matter in these coals is intimately mixed with the organic matter, which supports the idea of a catalytic effect on reactivity. In the case of PHA chars, the low rank of the parent coal justifies itself the high reactivity of these chars, although an additional catalytic help cannot be ruled out. In any case, the discussion of the reactivity data based on the chemical structure of the char material is, in our opinion, sufficient to explain the behaviour of the studied chars and, besides, whenever an apparent contradiction was found, this could not be rationalized on the basis of the ash content of the chars involved.
4. Conclusions The results presented in this study show that the pyrolysis behaviour of coals submitted to the conditions prevailing in pulverised fuel combustion is rather complex and difficult to systematise. However, the use of coal petrographic characterisation data and the microscopic examination of chars can give valuable information about the process. Char reactivity was shown to decrease with increasing rank of the parent coal, although the coals showing high plasticities during pyrolysis (low-volatile bituminous) might be slightly less reactive than the anthracite chars as a consequence of the extensive consolidation of their carbonaceous matter during the metaplast stage.
Fig. 9. Relationship between the intrinsic reactivity (reactivity per unit surface area) of the chars obtained at 1000 and 1300°C.
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Fig. 10. Variation of char reactivity with the ash content of the chars.
The effect of the inertinite content of coals on the reactivity of their chars seems to depend on the melting ability of this maceral, which is, to some extent, a rank-dependent parameter. According to the data shown in this paper, the reactivity of the inertinite chars would be higher the lower the plastic properties of the parent inertinite. Thus, whenever the inertinite reach better plastic properties than the accompanying vitrinite (normally, in low rank coals), a negative effect of inertinite content on char reactivity will be observed, and the opposite will occur in high rank coals. The temperature of pyrolysis can have a strong effect on char reactivity for certain coals. Generally speaking, the increase of temperature provokes an enhanced plasticity and a more extensive consolidation during the metaplast stage of pyrolysis, and this effect is more pronounced in low volatile bituminous vitrinites and low rank inertinites. Coals having either of these components will accordingly display lower reactivities the higher the temperature of pyrolysis. No clear catalytic effects of mineral matter were observed in the combustion of the studied coal chars, but these could not either be ruled out.
Acknowledgements Financial support of the European Union through the project ECSC-7220-D/075 is gratefully acknowledged. W. Kalkreuth, C.F.K. Diessel and M. Steller are thanked for providing the Canadian, Australian and German coals.
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.
Influence of pyrolysis temperature on char optical texture and reactivity M.J.G. Alonso, A.G. Borrego *, D. Alvarez, J.B. Parra, R. Mene´ndez Instituto Nacional del Carbo´n, CSIC, Francisco Pintado Fe 26, Ap. 73. 33080 O6iedo, Spain Received 17 April 2000; accepted 6 October 2000
Abstract In this study a set of 10 coals varying in rank and maceral composition has been devolatilised in a flat flame burner at 1000 and 1300°C in order to investigate the changes in char reactivity and structure as a function of pyrolysis temperature. The intrinsic reactivity of the chars, measured isothermally in a thermobalance at 500°C, has been related to the petrographic characteristics of the chars and their CO2 surface areas. The temperature of pyrolysis has shown to have a strong effect on char reactivity for certain coals. Overall, the increase of temperature provoked an enhanced plasticity and a more extensive consolidation during the metaplast stage of pyrolysis and reduced the amount of unfused material in the chars. This effect was more pronounced in low volatile bituminous vitrinites and low rank inertinites. The presence of inertinite derived materials in high rank coal chars enhanced their reactivities, whereas the opposite was observed for low rank coal chars. This was attributed to the fact that inertinite macerals yield both isotropic and anisotropic materials: inertinite in low rank coals increases the amount of anisotropic, less reactive char, but reduces it in high rank coals. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Pyrolysis temperature; Char optical texture; Char reactivity; Coal; Maceral composition; Char surface area; Structural order
* Corresponding author. Tel.: + 34-985-280800; fax: +34-985-297662. E-mail address: [email protected] (A.G. Borrego). 0165-2370/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 2 3 7 0 ( 0 0 ) 0 0 1 8 6 - 8
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1. Introduction The combustion of pulverised coal particles under the environmental conditions prevailing in an industrial boiler is known to occur as a two-stage process [1]. The first one is the fast pyrolysis of the coal, an essentially thermal effect which consists in the sudden release of volatiles accompanied by drastic changes in the morphology and molecular structure of the fuel particles, and the second one is the chemical reaction between the oxygen and both the gaseous (volatiles) and the solid (char) products of pyrolysis. Different attempts have been made to simulate the conditions prevailing in the near-burner zone of pulverised fuel boilers, and a number of different facilities such as drop tube furnaces (DTF) [2,3], hot-wire meshes [4], flat flame burners [5], etc. are described in the literature. These reactors are able to reproduce the thermal conditions prevailing in the near-burner zone of the boiler, so that coal particles undergo similar structural changes to those occurring during the combustion process at full scale. If the heating of the coal particles is carried out under an oxygen-free atmosphere, the solid product obtained can be readily isolated and recovered for its full characterisation. The importance of this relies in the fact that the isolated material is the final product of the pyrolysis stage and, at the same time, the starting point of the subsequent gasification (combustion) stage, and a full knowledge of this material can greatly simplify the study of the overall combustion process. The difficulties to simulate the conditions prevailing in full scale boilers and the lack of standardised tests for the assessment of coal combustibility may explain to some extent why after so many years of coal combustion research, only very general guidelines can be outlined about the behaviour of coal. Besides, varying coal rank and maceral composition do not make this task any easier. There is nevertheless general agreement in that the increase in coal rank results in a decreased reactivity to oxygen [6,7], although deviations from this trend have also been reported [8,9]. Results are however more contradictory when systematic trends for the behaviour of the different macerals are sought. Thus, chars from inertinite-rich coals, which showed generally lower intrinsic reactivities than vitrinite-rich chars of similar rank in thermogravimetric experiments [10], gave a satisfactory performance in industrial boilers or DTF test [11,12]. Some recent studies carried out on the reactivity of inertinite [13 –15] have also reported higher combustion reactivities of semifusinite compared to vitrinite, and higher reactivities of unfused inertinite chars than those of fused chars derived from either vitrinite or inertinite. In this context, optical microscopy is a most versatile tool for the study of the structural organisation of char material, since this technique permits the analyst to distinguish and quantify the isotropic (disordered) and anisotropic (ordered) char materials formed after the passage of coal through a thermoplastic stage. The type and size of anisotropic domains in chars is strongly related to their reactivities as it will be shown below. The purpose of this paper is to study the influence of flame temperature on the pyrolysis behaviour of pulverised coal particles from coals of variable origin and maceral composition. It particularly focuses on the differences observed in the optical texture and reactivity as a function of the characteristics of the parent coal.
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2. Experimental
2.1. Coal preparation Ten coals varying in rank (high volatile bituminous to anthracite) and maceral composition (vitrinite- to inertinite-rich) were selected for this study. The coal set comprises a wide range of maceral occurrences in terms of abundance and maturity degree. It is intended to take advantage of this feature and come to a better understanding of the thermoplastic behaviour of the different macerals and its variation with rank. Details of coal provenance and geological age are given in Table 1. Representative coal samples were ground and sieved to a narrow size range (36 –75 mm diameter). This interval comprises the medium-size particles within the range typically used in industrial pulverised fuel boilers (\ 70 wt.% of particles B75 mm). Those coals with ash contents significantly higher than 15% (BB2 and FM3) were beneficiated by flotation in dense liquids, thus ensuring that all the coals had similar and reasonably low mineral matter contents. The minimum floating density used was 1.7 g cm − 3, since any further reduction might have resulted in an undesirable effect of segregation of the denser inertinite macerals [16].
2.2. Coal characterisation Ultimate and proximate analyses of coals were carried out following standard procedures. Maceral analyses were performed using a combined maceral/reflectance procedure in which random reflectances were measured on any maceral selected by point-counting (500 points) annotating the corresponding identification. For the estimation of coal rank from petrographic analysis (Rr) only the readings taken on collotelinite [17] (former telocolollinite), the vitrinite maceral following the most homogeneous coalification path, were averaged.
Table 1 Provenance and geological age of coals selected Code
Provenance
Country
Age
BMC BB2 PHA BSE FM3 CRA SMK TAF DAN VCB
Paparoa coal Sydney basin Phalen (Nova Scotia) Brunner coal Coal mountain Cadinal river Smoky river Taff merthyr Ibbenbu¨ren Victoria
NZ AU CA NZ CA CA CA UK DE PE
Cretaceous Permian Caboniferous Eocene Jura/L.cretaceous Lower cretaceous Lower cretaceous Caboniferous Caboniferous Cretaceous
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2.3. Char preparation Representative samples of each coal were pyrolysed at 1000 and 1300°C in an air/methane flame. Coal particles were axially injected by a gas flow at the base of the flame. The flame was always operated under highly sub-stoichiometric oxygen levels (Table 2) so that particles were suddenly pyrolysed and then immediately cooled down when leaving the flame. Details of the experimental device are given elsewhere [18]. This sort of flame has been successfully used to resemble the time/temperature conditions of the pulverised particles in the near burner zone of an industrial boiler [5].
2.4. Petrographic analysis of the chars Chars were embedded in an epoxy resin, left overnight, cut, and polished for the microscopic examination of particle cross-sections. The optical texture of the char material was examined under a polarised reflected light microscope with oil immersion objectives of 50× and 100× magnification, partially crossed pollars, and 1 u retarder plate. Petrographic characterisation of the chars was performed according to the classification system currently used by the International Committee for Coal and Organic Petrology (Inertinite in Combustion W.G.). This classification distinguishes between vitrinite- and inertinite-derived material. The latter is further subdivided according to its optical texture (isotropic/anisotropic) and porosity (unfused, porous, dense).
2.5. Char surface area The surface area of micropores (B2 mm in diameter) was calculated using the Dubinin – Radushkevich (D – R) equation [19] from CO2 adsorption data at 273 K, which were obtained using a Micromeritics Gemini 2375 at the interval of pressure 0.035 –0.0001 torr. Different range of pressure (0.035 –0.0035 torr) was used for the vitrinite-rich VCB anthracite chars, since they both showed activated diffusion at lowest pressures.
2.6. Thermogra6imetric analysis Char combustion was isothermally recorded using a Perkin Elmer TGA7 thermal analysis system. A small quantity of char (13 mg) was homogeneously spread at the bottom of the platinum crucible and then heated up to 500°C under nitrogen flow (50 cm3 min − 1) at a heating rate of 25°C min − 1. After weight stabilisation, nitrogen was replaced by air at the same flow rate and the temperature was maintained until combustion was completed. At this low temperature and using such small sample sizes, bed effects in the thermobalance can be ruled out and the kinetic control of the reaction is ensured. Char reactivity is defined as
Run 1 Run 2
O2
2.0 2.0
1.5 1.5
3.2 3.2
0.0 0.5
30 28
15 20
O2
CH4
Air (flame)
CH4
Air (injector)
Gas composition (vol.%)
Gas flow rate (cm3 min−1 STP)
Table 2 Operating conditions of the methane/air burner
55 52
N2
1000 1300
Measured
1140 1550
Adiabatic
Flame temperature (°C)
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Fig. 1.
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Rx = −
1 mo
dm dt
893
, x
where mo is the initial weight of ash-free sample and x is the conversion value chosen. Both the reactivity at x =50% conversion and the maximum reactivity (x =max) were calculated.
3. Results and discussion Results of the chemical and petrographic analyses of the coals are summarised in Table 3. Coals are listed by increasing rank as determined by random collotelinite reflectance (Rr) and its range is from 0.64 to 5.63%. The selected coals have ash contents from 0.6 to 18.7%. Coals FM3 and BB2, which were both floated, still retained a relatively high ash content (15.9 and 18.7%, respectively). This indicates an intimate association between organic and mineral matter. Data from chemical analysis followed the expected trends with increasing rank, this is: a decrease in volatile matter, hydrogen and oxygen contents and an increase in carbon content and fuel ratio (Table 3), with some deviations which are due to differences in maceral composition. The high fuel ratio and low volatile content of BB2 is attributable to its high inertinite content, as it is known that inertinite contains less volatile matter than vitrinite for a given coal [20]. The relatively high liptinite proportion of PHA (about 10%) might be responsible for its high hydrogen content (Table 3) since liptinite is the H-richest maceral group.
3.1. Petrographic study of the chars Optical micrographs of chars obtained at 1000 and 1300°C are shown in Figs. 1–4. BMC chars prepared at 1000 and 1300°C did not show remarkable differences in their morphologies. Both chars formed preferentially isotropic, single-chambered particles with rather elongated shapes and abundant development of porosity within the walls (Fig. 1). Inertinite-derived char from BB2 (1000°C) was made up of angular particles, most of them showing signs of having passed through a plastic stage as reflected by the abundant spheroidal pores distributed throughout the particle. The char material was either isotropic or anisotropic, but the size of the anisotropic domains was rather small and dispersed within an isotropic matrix. This char also contained a significant amount of partially pyrolysed material (pp in Fig. 1), which was not observed in the vitrinite-derived char from a coal of similar rank Fig. 1. Optical micrographs showing the aspect of the chars obtained at both 1000°C (left) and 1300°C (right) from the high volatile bituminous coals. BMC consists of isotropic cenospheric particles, BB2 of molten and non-molten inertinites, either isotropic or anisotropic. The low temperature BB2 char contains partially pyrolysed material. PHA and BSE chars show incipient anisotropy development. Images taken with a reflected light microscope, oil immersion objective and crossed pollars. (vd;, vitrinite-derived; af, anisotropic fused inertinite; fs, fusinoid; pp, partially pyrolysed material; if, isotropic fused inertinite).
0.64 0.66 0.84 1.03 1.05 1.23 1.53 1.77 3.20 5.63
85.8 1.2 80.4 96.2 55.0 53.0 57.8 71.6 61.8 98.0
V 2.4 97.2 9.4 – 45.0 47.0 42.2 28.4 38.2 2.0
I
Maceral (vol mmf %)
11.8 1.6 10.2 3.8 – – – – – –
L 0.60 18.7 15.3 0.04 15.9 7.3 12.8 5.2 2.3 1.9
Ash (d %)
1.73 2.78 1.71 2.59 2.99 3.68 4.29 6.29 14.48 58.13
FR
46.2 24.8 36.7 32.0 28.9 22.3 18.1 13.6 6.2 1.6
VMa
(daf %)
80.6 80.1 81.3 83.0 86.3 89.5 90.6 90.8 92.1 94.9
Ca 5.9 3.5 5.2 4.8 5.0 4.6 4.4 4.1 3.2 1.2
H
1.4 1.8 1.3 1.1 1.5 1.8 1.1 1.3 1.7 0.9
N
0.3 0.4 2.6 3.2 0.9 0.2 0.4 0.7 0.5 0.2
S
11.8 14.2 9.6 8.0 6.3 3.9 3.5 3.1 2.5 2.8
Odif 0.87 0.52 0.77 0.69 0.69 0.62 0.58 0.54 0.42 0.15
H/C
0.11 0.13 0.09 0.07 0.05 0.03 0.03 0.03 0.02 0.02
O/C
Corrected for carbonates, mmf = mineral matter free; d = dry basis; daf =dry-ash-free basis; dif. =by difference; VM, volatile matter; FR, Fuel ratio (Fixed carbon/volatile matter), Rr, random vitrinite reflectance, V, vitrinite; L, liptinite; I, inertinite.
a
BMC BB2 PHA BSE FM3 CRA SMK TAF DAN VCB
Rr (%)
Table 3 Results of the petrographic and chemical analyses
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Fig. 2. Detail of the char forming material in BSE and PHA. Incipient anisotropy is observed in BSE char (granular appearance of the wall) whereas PHA char shows locally well-developed anisotropic domains (positions indicated by arrows). Images taken with a reflected light microscope, oil immersion objective and crossed pollars.
(BMC), indicating that inertinite of this rank exhibited a more complex devolatilisation behaviour than vitrinite [18]. The 1300°C char from BB2 was more efficiently pyrolysed and showed an enhanced plastic behaviour with less amount of unfused material (Table 4). The difference of 300°C between the low and high temperature runs seem to have been enough to provoke a significant improvement in inertinite devolatilisation. Chars from PHA prepared at both 1000 and 1300°C were made up of thickwalled rounded cenospheric particles exhibiting an optical texture of fine mosaic (size less than 1 mm) and abundant secondary porosity within the walls. The mosaics were more evident than in BSE char (from a coal of higher rank) which indicates a rather peculiar devolatilisation behaviour for coal PHA (Fig. 2). This might be due to the higher liptinite content of PHA coal, which locally raises the volatile matter content of the coal. The development of anisotropic texture and porosity in a char depends both on the volatile matter content and the rate of crosslinking in the structure of the parent coal [21]. These two features display a smooth variation with coal rank, and therefore the textural and porous development of chars follow an accordingly smooth variation with the rank of the parent coals. However, the liptinite particles (sporinite) present in the vitrinite matrix of PHA give an extra supply of volatiles which might locally raise the pressure of volatiles. This, in turn, increases the tensile strength (the driving force of coales-
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Fig. 3.
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Fig. 4. Optical micrographs showing the aspect of both 1000°C (left) and 1300°C (right) the chars obtained from the two anthracites. Chars consist of unfused angular particles exhibiting some cleats (arrows) that are more common in the higher temperature chars. Images taken with a reflected light microscope, oil immersion objective and crossed pollars. (uv, unfused vitrinite, ui, unfused inertinite).
cence) in the char walls, thus giving rise to a more accentuated anisotropic texture than would be expected for the rank of the coal. This effect became more pronounced in the high temperature char, in which anisotropic mosaics of larger size were locally observed (Fig. 2). No signs of anisotropy were found in BMC, the other high liptinite coal, but this might be due to the different nature of this liptinite, mainly diffuse resinite impregnations with reflectance (and therefore volatile matter content) similar to that of the vitrinite matrix and more widely spread than the sporinites of PHA. Except for the presence of incipient anisotropy development, the morphology of BSE chars at both temperatures closely resembled that of BMC chars. Fig. 3. Optical micrographs showing the aspect of the chars obtained at both 1000°C (left) and 1300°C (right) from high to low volatile bituminous coals. Vitrinite-derived material is always anisotropic and its plasticity increases with the rank of the coal as shown by the presence of thinner walls in the higher ranked coal chars. The increase in preparation temperature has a similar effect on the plasticity. Images taken with a reflected light microscope, oil immersion objective and crossed pollars. (vd, vitrinitederived; um, unfused massive inertinite).
94 1 87 100 47 55 50 54 71 100 84 100 51 49 48 46 73 100
96 52 2 25 17 27 43
1 63 3 27 16 24 24
1300
5 3
1000
AD
4 6 4
2
1300
3 1 1 2
2 12 5
1000
IP
2 4 5 2
1 24 8
1300
7 5 5 5
1 3
1000
ID
8 9 6 2
13
1300
10 15 14 10 28
1 16 3
1000
UM
Unfused
6 11 7 3 25
1 9 4
1300
6 3 3 5 1
1 5 2
1000
FS 1300
4 5 3 4 2
2 1 2
VD, Vitrinite-derived; AP, Anisotropic porous; AD, Anisotropic dense; IP, isotropic porous; ID, Isotropic dense; UM, unfused massive; FS, unfused fusinoid.
a
BMC BB2 PHA BSE FM3 CRA SMK TAF DAN VCB
1000
1000
1300
AP
Fused
Inertinite-derived
VD
Vitrinite-derived
Table 4 Petrographic analysis of chars (vol%)a
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Significant differences were observed in the optical texture and morphology of BMC and FM3 chars, both generated from coals of similar rank. The anisotropic texture was more easily distinguishable in char FM3 than in char BSE (Figs. 1 and 3). The chars from FM3, CRA and SMK formed abundant multi-chambered particles containing both vitrinite- and inertinite-derived material (mixed particles) due to the intimate association of both macerals in the parent coals. The size of the anisotropic texture of the vitrinite-derived material in chars FM3, CRA, SMK and TAF increased with the rank of the parent coal. In addition, the 1300°C char of FM3 and CRA were characterised by thinner char walls and larger anisotropic domains than the low temperature char. Vitrinite from SMK and TAF tended to form at 1000°C thick-walled rounded cenospheric particles, whereas at 1300°C yielded more angular particles with a central pore surrounded by a net of extremely thin walls comprising elongated pores (Fig. 3). This behaviour was considered in a previous work [18] to be the result of a temperature gradient in the particle due to a high heating rate. The outermost part would fuse first and re-solidify when the core of the particle was still unfused. This would prevent the formation of spherical particles and, at the same time, the internal volatile pressure would deform the porosity existing within the walls (Fig. 3). DAN and VCB did not show significant changes after the passage through the flame except for a higher reflectance and enhanced bireflectance (difference between maximum and minimum reflectance). Neither inertinite nor vitrinitederived material fused during pyrolysis and both displayed the inherited anisotropic pattern of the parent material (Fig. 4). The differences between coal and char particles were more evident in DAN (lower rank anthracite) than in VCB. Chars from DAN, particularly the 1300°C particles, displayed extensive development of long cleats parallel to each other indicating the occurrence of some contraction in the structure parallel to the bedding planes due to limited devolatilisation. These cleats were not found in the low temperature VCB char and the 1300°C counterpart only displayed cleats in some of the particles (Fig. 4). Overall, the plasticity of inertinite during pyrolysis decreased as coal rank increased, as shown by the fact that the chars from BB2, a coal consisting of nearly pure inertinite only contained a low percentage of unfused material (Table 4), whereas FM3, CRA and SMK chars, all from coals consisting of about 50% vitrinite, exhibited relatively larger amounts of unfused material. It is not only coalification, but also the level of pre-depositional oxidation which determines the reflectance of inertinite [22]. The high reflecting inertinites (fusinites and some massive macrinites) achieve, prior to deposition, a highly cross-linked structure[23] that prevents them from melting during pyrolysis and is responsible for the large amounts of unfused material in the chars. Moderately altered inertinites still retain a certain amount of volatiles which might be lost during sudden heating, generating particles with either isotropic or anisotropic texture but with clear signs of having passed through a plastic stage.
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3.2. Char reacti6ity 3.2.1. The effects of coal rank and maceral composition Two main aspects control the reaction rate of any carbonaceous material at low temperatures, and these are the specific surface area, which determines the surface of the solid accessible to oxygen, and the intrinsic reactivity [24], and the following discussion will mainly focus on these two features. The reactivity parameters (reactivity at 50% conversion and maximum reactivity) and surface areas (Dubinin – Radushkevich) of the chars are shown in Table 5. For all the studied chars, the maximum reactivity was reached at conversion rates of 10–20%. Similar variations in reactivity among the studied chars can be noticed regardless the reactivity parameter selected, and therefore we will only deal hereinafter with one of them, namely the reactivity at 50% conversion. The influence of coal rank and maceral composition on the reactivity of the chars will be considered first. The changes undergone by coals during coalification are well systematized as regards the vitrinite group, the major component of most Northern Hemisphere coals [23]. Low rank coals tend to form, upon rapid devolatilisation, chars exhibiting an isotropic optical texture, with large surface areas and reactivities. This is due to the highly crosslinked structure of these coals, which limits the mobility of the molten material and prevents its macromolecules from coalescing. With the increase of coalification, the crosslinking is greatly reduced and the coals undergo drastic changes in their molecular arrangement during fast pyrolysis, as a consequence of the high fluidity of the molten char material, and this results in the formation of bigger, more ordered domains (anisotropic texture). The surface areas and reactivities would be accordingly lower for these chars [6]. Finally, the large polyaromatic units in the anthracites are highly ordered parallel to the bedding plane and do not melt at all upon heating. This three-dimensional arrangement only leaves a small-sized, hardly accessible to oxygen pore network. The behaviour of the inertinite (major component of most Southern Hemisphere coals) during pyrolysis is more controversial, and little is known about the molecular arrangement of this maceral group and its variation with coal rank. This is due to the inherent heterogeneity of these macerals, which underwent variable alteration degrees prior to deposition. However, the inertinites are known to have a highly crosslinked structure, similar to low rank vitrinites, but more aromatic than these [23]. A substantial part of the inertinite contained in a coal is able to fuse to variable extents, which will be higher the higher the temperature of pyrolysis [15]. The surface area and reactivity of their chars will strongly depend on the plastic properties reached during pyrolysis. We will now analyse the data of Table 5 on the basis of the above explained facts. The coals BMC, PHA, BSE, TAF and VCB can be regarded as a series of vitrinite-rich coals (\70% vol.) of increasing rank (0.64 –5.63% vitrinite Rr). Their chars display a broad trend of decreased reactivity with the increase of rank (see Fig. 5a–b), a well-known fact in coal combustion research [1,6,7]. However, BMC, the coal at the low-rank end showed a much lower reactivity than PHA, the coal
a
1.82 2.14 3.31 0.79 1.94 0.96 0.71 0.84 1.68 0.70
2.88 0.48 3.14 1.00 2.48 1.14 0.80 0.18 0.88 0.54
3.04 4.37 6.39 1.29 3.46 1.45 1.09 1.51 2.47 0.78
5.33 0.78 6.86 2.34 4.80 2.09 1.53 0.37 2.12 0.67
1300°C
1000°C
1000°C
1300°C
Rmax×104 (mg mg−1 s−1)
R0.5×104 (mg mg−1 s−1)
36 27 17 84 37 61 93 74 44 147
1000°C
tR0.5 (min)
21 127 18 53 23 53 76 326 60 166
1300°C
9 8 5 13 15 17 19 17 21 98
1000°C
tRmax (min)
7 35 5 10 8 11 18 39 29 84
1300°C
351 278 260 350 261 280 236 212 383 289
1000°C
357 108 288 357 312 303 258 53 322 212
1300°C
Area (m2 g−1 ash free)
R0.5, reactivity at 50% conversion; Rmax, maximum reactivity; tR0.5, time to reach 50% conversion; tRmax, time to reach maximum reactivity.
BMC BB2 PHA BSE FM3 CRA SMK TAF DAN VCB
Coal
Table 5 Reactivity and CO2 surface areas of the chars obtained from coals varying in rank and maceral composition at 1000 and 1300°Ca
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char immediately following in rank. This unexpected result was attributed to partial burnout of this sample in the flame, resulting in char deactivation [25,26]. In the air –methane flame used in this study, both methane and the coal particles themselves compete for the oxygen, the former being in clear advantage mainly for two reasons: (i) this is a diffusionally unrestricted gas –gas reaction; and (ii) methane is more reactive to oxygen than coal. The importance of the second reason is minimized for the very reactive low rank coals, and thus it is likely that BMC might
Fig. 5. Variation of measured reactivity at 50% conversion (R50%) with the rank of the parent coal (Rr) for the 1000°C (a) and 1300°C (b) chars. The lines illustrate the trends obtained for the vitrinite-rich coal chars.
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have undergone a partial combustion in the flame. Partly burnt chars are known to be less reactive than their ‘fresh’, non-combusted counterparts [3,27]. On the other hand, the VCB anthracite char produced at 1300°C has a higher reactivity than the TAF (lower ranked coal) char obtained at the same temperature. This was attributed to the very high plasticity of TAF coal under the pyrolysis conditions used in this work, which gives rise to the annihilation of the microporosity (and therefore, the surface area available to oxygen) of the carbonaceous material [28]. This coal is in the high-rank end of the interval of coking coals, and thus it might be expected that other lower rank coals (SMK or CRA, for instance) would show much better plastic properties than TAF. However, the interval of high plasticity of coals is shifted towards higher ranks with the increase of temperature and heating rate [29,30], and this is why low volatile bituminous coals submitted to the pyrolysis conditions typical of pulverized fuel combustion show much higher plasticities than in experiments performed under typical coking conditions. Analytical data in support of this will be given below. The maceral effects on the pyrolysis behaviour of coals are mainly related with the inertinite. The small effects due to the normally scarce liptinites have been mentioned above. BB2 is an almost pure inertite, from which valuable information about the pyrolysis behaviour of the inertinite macerals can be obtained. The reactivity of BB2 (Fig. 5) is much higher for the 1000°C char than for the 1300°C char, a similar effect to that observed for TAF, and which can also be explained by the same reasoning: the higher temperature of pyrolysis provokes an increase in the plastic behaviour of the inertinite, followed by a reduction in the surface area (see below). One cannot expect, however, that this behaviour will be maintained throughout the whole coalification scale. In fact, the reflectance of the inertinite macerals broadly increases with the increase of coal rank, as shown in Fig. 6 for the coals used in this study. It is well known [31,7,15] that the increase in the reflectance of the inertinites, either due to coalification or to pre-depositional alteration, reduces their fusibility. Therefore, it can be expected that the role of the inertinite will vary from low to high rank coals, leading to an increase of anisotropic (highly altered) material for the former, but to a reduction for the latter. Once established the behavioural trends of vitrinite and inertinite with rank, we will now analyse the reactivity data obtained from FM3, CRA, SMK and DAN, the coals with significant percentages of both vitrinite and inertinite. The first two of these produced, at the two temperatures used, chars with lower reactivities than expected from their ranks. This can be better noticed when the reactivities of BSE and FM3, both of similar rank, are compared. Thus, the reactivities of the FM3 chars (45% inertinite) are much smaller (about 50%) than those found for the inertinite-free BSE chars, the same as would be expected for an artificial mixture (55 –45) of BSE and BB2. In the case of the anthracite DAN chars, its highly reflecting inertinite did not melt at all at either temperature, and therefore remained isotropic, more reactive than the accompanying vitrinite. The reactivities of these two chars were accordingly higher than expected from the broad trend of variation with rank. In between, the reactivities of the SMK chars, despite the high inertinite
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Fig. 6. Variation of mean random inertinite reflectance (Rinertinite) with the rank of the coal (Rr)
content (42% vol.) of their parent coal, where similar to the values expected for a vitrain of the same rank. Assuming that inertinite exerts a negative effect on reactivity at low ranks and a positive effect at high ranks, then at some intermediate rank the effect of inertinite must be negligible. In other words, at coal ranks around 1.5% vitrinite reflectance, and for the environmental conditions used in this study, the pyrolysis behaviour of the vitrinites and inertinites must be quite similar, as regards the reactivities of the obtained chars.
3.2.2. The effect of pyrolysis conditions Figs. 7 – 9 illustrate the effects of the temperature of pyrolysis on various reactivity parameters of the chars obtained. The most striking features of the reactivity data plotted in Fig. 7 are the high positive deviation, i.e.: the positive effect of the increase in temperature on char reactivity, of BMC chars, and the negative deviations found for BB2, TAF and DAN. As already mentioned, the most important parameters affecting the reactivity of a char are the intrinsic reactivity and the specific surface area. For a more detailed discussion of the data of Fig. 7, they have been split into these two separated parameters. The specific surface areas given in Table 5 are plotted in Fig. 8, and the intrinsic reactivities (measured reactivity per unit surface area) are shown in Fig. 9. The highest differences between the surface areas of the low and high temperature chars were found for BB2, TAF, DAN and VCB, this is, the inertite and the three highest ranked coals. All these coals produced chars with lower surface areas at 1300°C than at 1000°C, especially BB2 and TAF, the effect being less pronounced for the two anthracites. As mentioned above, both BB2 and TAF
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were expected to considerably enhance their plastic properties with the increase in the temperature of pyrolysis, as it seems to have happened. The formation of a very fluid metaplast during pyrolysis brings about the annihilation of the microporous structure in the chars, thus leading to much lower surface areas. In the case of the anthracites, with no plastic properties whatsoever, the reduction in surface area must be due to a different mechanism. Actually, no variations would be expected in the surface areas from the 1000 to the 1300°C chars and, if any, they should run in opposite direction to the results found in this study. Thus, the increase in temperature led to the formation of abundant cracks in the vitrinite particles of DAN and, to a lesser extent, also in VCB, as a consequence of the stronger thermal shock undergone by these higher temperature chars. This, in turn, would facilitate the entrance of CO2 to the whole volume of the particles, therefore raising the figures of specific surface area. More, it is quite likely that not only cracking but even fragmentation of some of the high temperature char particles had occurred, which is known to increase the surface area (or at least the measured value) of anthracites [32]. Only the possibility that, even in such mature coals, some residual short-distance mobility existed in the pyrolysing groundmass, would permit the annihilation of a part of the porous network in the solid. In any case, the small quantitative importance of this effect of surface area reduction, as well as the lack of any experimental evidence or literature data supporting the existence of molecular mobility in anthracites, do not allow to draw any firm conclusion about this phenomenon.
Fig. 7. Relationship between the measured reactivity at 50% conversion of the chars obtained at 1000 and 1300°C.
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Fig. 8. Relationship between the CO2 surface areas of the chars obtained at 1000 and 1300°C.
The plot of Fig. 9 reveals that the temperature of char formation does not have a strong effect on its intrinsic reactivity to oxygen. Only BMC displayed an enhanced reactivity in the 1300°C char compared with its 1000°C counterpart, and the opposite was observed for BB2. The above mentioned possibility that BMC had burnt to some extent during the pyrolysis experiment might be the reason for this peculiar result. In the case of BB2, it has been mentioned that its 1000°C char was not fully devolatilised. This means that some of its carbon skeleton might still contain aliphatic carbons (sp 3-hybridisation), intrinsically more reactive than aromatic carbons (sp 2-hybridisation). Summarising, the increase in the measured reactivity of BMC chars with increasing temperature of pyrolysis is due to the higher intrinsic reactivity of the 1300°C char, the reduction observed in TAF and DAN has to be attributed to a decrease in surface area, and, finally, the decrease of both the surface area and the intrinsic reactivity is responsible for the lower reactivity of the 1300°C char of BB2, compared with the corresponding 1000°C char. One more factor has to be borne in mind in reactivity studies on chars, and this is the possibility that the mineral matter might exert a catalytic effect on char combustion. This was also taken into account in the present work. Fig. 10 plots the variation of intrinsic reactivity with the ash content of chars, where a broad trend of increased reactivity with increasing ash content can only be noticed for the chars with ash contents over 20 wt.%. These are BB2 (1000°C) and the two chars obtained from both PHA and FM3. On the other hand, the reactivity of the 1300°C char from BB2 is comparable to those of lowest ash chars. BB2 and FM3 are both
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floated coals, which ensures that the free mineral matter grains have been removed and thus the mineral matter in these coals is intimately mixed with the organic matter, which supports the idea of a catalytic effect on reactivity. In the case of PHA chars, the low rank of the parent coal justifies itself the high reactivity of these chars, although an additional catalytic help cannot be ruled out. In any case, the discussion of the reactivity data based on the chemical structure of the char material is, in our opinion, sufficient to explain the behaviour of the studied chars and, besides, whenever an apparent contradiction was found, this could not be rationalized on the basis of the ash content of the chars involved.
4. Conclusions The results presented in this study show that the pyrolysis behaviour of coals submitted to the conditions prevailing in pulverised fuel combustion is rather complex and difficult to systematise. However, the use of coal petrographic characterisation data and the microscopic examination of chars can give valuable information about the process. Char reactivity was shown to decrease with increasing rank of the parent coal, although the coals showing high plasticities during pyrolysis (low-volatile bituminous) might be slightly less reactive than the anthracite chars as a consequence of the extensive consolidation of their carbonaceous matter during the metaplast stage.
Fig. 9. Relationship between the intrinsic reactivity (reactivity per unit surface area) of the chars obtained at 1000 and 1300°C.
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Fig. 10. Variation of char reactivity with the ash content of the chars.
The effect of the inertinite content of coals on the reactivity of their chars seems to depend on the melting ability of this maceral, which is, to some extent, a rank-dependent parameter. According to the data shown in this paper, the reactivity of the inertinite chars would be higher the lower the plastic properties of the parent inertinite. Thus, whenever the inertinite reach better plastic properties than the accompanying vitrinite (normally, in low rank coals), a negative effect of inertinite content on char reactivity will be observed, and the opposite will occur in high rank coals. The temperature of pyrolysis can have a strong effect on char reactivity for certain coals. Generally speaking, the increase of temperature provokes an enhanced plasticity and a more extensive consolidation during the metaplast stage of pyrolysis, and this effect is more pronounced in low volatile bituminous vitrinites and low rank inertinites. Coals having either of these components will accordingly display lower reactivities the higher the temperature of pyrolysis. No clear catalytic effects of mineral matter were observed in the combustion of the studied coal chars, but these could not either be ruled out.
Acknowledgements Financial support of the European Union through the project ECSC-7220-D/075 is gratefully acknowledged. W. Kalkreuth, C.F.K. Diessel and M. Steller are thanked for providing the Canadian, Australian and German coals.
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.