Structural ordering in high temperature coal chars and the effect on reactivity

Fuel 78 (1999) 803–807

Structural ordering in high temperature coal chars and the effect on reactivity Nigel V. Russell a, Jon R. Gibbins b, Jim Williamson a,* a

b

Department of Materials, Imperial college, Prince Consort Road, London SW7 2BP, UK Department of Mechanical Engineering, Imperial College, Exhibition Road, London SW7 2BX, UK

Received 5 August 1998; received in revised form 17 November 1998; accepted 24 November 1998

Abstract This study has been undertaken to determine the extent of structural ordering and thermal deactivation which can occur when coal chars are heated to high temperatures. The chars have been prepared using the captive-sample wire-mesh reactor at temperatures up to 2200⬚C at very short hold times (500 ms), and the microstructures have been examined using high-resolution transmission electron microscopy (HRTEM). The char structures have been compared to those of natural graphites. Regions within the chars have been observed to show a similar degree of ordering to that present in the graphites. The reactivities for oxidation of the high temperature chars and graphites have been determined by TGA methods and compared to the reactivity of chars prepared at lower temperatures. The structural ordering and reactivity data indicates that thermal deactivation may account for the low reactivity of a proportion of the char present in utility boiler fly ash. 䉷 1999 Elsevier Science Ltd. All rights reserved. Keywords: Structural ordering; Char reactivity; Thermal deactivation

1. Introduction Factors affecting the rates of coal char combustion at high overall conversions are extremely important in determining the residual carbon content in pulverised coal boiler fly ash (pfa). Boiler operators look for a high degree of carbon burnout for economic reasons. Greater than 6% carbon in ash [1] represents a loss in efficiency, interferes with precipitators by reducing fly ash resistivity [2], prevents utilisation of ash as a cement replacement in concrete manufacture [3] and lowers the ash bulk density which increases the costs of landfill [4]. Low oxidation reactivities observed in residual carbons from commercial and pilot-scale fly ash indicate that char reactivity is more important under combustion conditions than has been assumed. Thermal deactivation in the high-temperature regions (⬎1500⬚C) of the furnaces has been suggested as a likely mechanism to explain the low reactivities [5–9]. Gibbins and Williamson [10] previously observed that chars from boiler fly ash have a range of reactivities and that reactivity was generally lower for larger particles. At the same time, Hurt and co-workers [11] reported a compar* Corresponding author. Tel.: ⫹44 (0)171 589 5111. Ext: 56758; Fax: ⫹44 (0)171 594 6748. E-mail address: [email protected] (J. Williamson)

ison between laboratory-prepared chars and residual unburnt chars from utility boilers. A joint study concluded that the differences in reaction rates between residual and laboratory chars could be explained by low intrinsic reactivities and that chars were deactivated in the boiler plant [6]. However, the mechanism for deactivation was not clear. Experiments using the high-temperature wire-mesh reactor (HTWM) [12] were able to show that thermal deactivation alone at realistic combustion particle temperatures of 1600⬚C–1800⬚C and heating times of up to 2 s would give chars with intrinsic reactivities as low as those found in utility boiler residual carbons [13]. Complementary studies on eight residual carbons showed that, compared with the laboratory-generated chars, the residual carbons were of similar elemental and petrographic composition and surface area, but higher crystallinity [14], also suggesting that a thermal deactivation process had occurred. The phenomenon of structural ordering within carbons was first suggested [15] in 1949. The pioneering work of Franklin [16,17], who proposed the graphitising, partially graphitising and non-graphitising models for carbon structures based on X-ray diffraction studies, has been substantiated by high-resolution transmission electron microscopy [18,19]. Oberlin proposed a model [19] after experiments carried out on anthracene-based carbons, pitch-based carbons and carbon films heat treated to 2900⬚C at

0016-2361/99/$ - see front matter 䉷 1999 Elsevier Science Ltd. All rights reserved. PII: S0016-236 1(98)00210-5

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Fig. 1. The four stages of texture improvement leading to crystalline order within carbons [19]. Below 500⬚C, basic structural units are present. At temperatures between 800⬚C and 1500⬚C, these associate face-to-face in distorted columns. Between 1600⬚C and 2000⬚C the adjacent columns coalesce into distorted wrinkled layers, while above 2100⬚C, these layers stiffen, become flat and perfect.

20⬚C min ⫺1 all showed similar results. This suggested that as the heat-treatment temperature increased, the successive improvements of texture which lead to those of crystalline order were produced in four discrete stages (Fig. 1). Subsequent deactivation studies [5] using HTWM chars showed that conventional expectations of char reactivity ranking might be reversed when chars were prepared at higher temperatures. Inertinite-rich coal samples gave less reactive chars than vitrinite-rich coals at 1000⬚C, in line with previous studies, but in some cases they deactivated less, compared to vitrinite-rich samples, to give chars with relatively higher reactivities after preparation at 1800⬚C. A high degree of structural ordering was also observed by HRTEM on the high temperature HTWM chars. Hurt and co-workers [20] have shown that more detailed char combustion models incorporating deactivation kinetics based on the HTWM data and statistical distributions of particle properties could account for the persistence of unburnt carbon in the later stages of pulverised coal combustion. Global char combustion kinetics could satisfactorily model the first 90% of burnout, but seriously over predicted conversion rates in the later stages of combustion. A more detailed description of a later version of Hurt’s model has been published recently [21]. In more recent work [22] detailed measurements of deactivation kinetics have been made on two coals of differing rank, with examination of the chars by SEM and HRTEM. The higher-rank coal experienced significantly less deactivation than the other coal. This, together with the earlier observations of differences between coal macerals, confirms the statement [21] that ‘much more work is needed to understand and accurately describe annealing (i.e. deactivation) kinetics under all conditions for a wide variety of parent materials’. The work presented here examines chars prepared with a residence time of 0.5 s at temperatures up to 2200⬚C, above the temperature suggested by Oberlin where carbons

become more graphitic [19], and compares these with natural graphite, using HRTEM, and the reactivity of lower temperature samples. Although these represent severe conditions for combustion processes, they do illustrate the structural ordering process which may occur in high temperature coal/char conversion processes.

2. Experimental 2.1. Samples Pittsburgh #8 coal was taken from the Argonne Premium Coal Sample Program. This is a high volatile bituminous coal [23]; 83.2 wt.% (dmmf) C, 5.3 wt.% (dmmf)H, 9.2 wt.% (db) ash, 37.8 wt.% (db) volatile matter. The graphite sample was an ultra pure American natural graphite powder (99% ⫹ purity, mesh size 200).

2.2. High-temperature wire-mesh reactor (HTWM) Char samples were prepared using the HTWM reactor in which the coal was held between two layers of a hightemperature alloy mesh, which also acted as an electrical resistance heater. A 24 V DC supply was used to deliver up to 2000 A. The mesh was heated at 10 4 K s ⫺1 to the peak temperature and held for a specified time period before uncontrolled, but rapid cooling. The heating programme was controlled using a two-colour infrared pyrometer with a computer using feedback control operating at 1 kHz. A helium gas sweep across the mesh at atmospheric pressure removed volatile products, as well as preventing sample or mesh oxidation. Typically 10 mg of dried sample coal were used, previously ground and sieved to 150–175 mm for compatibility with the mesh material.

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Fig. 2. HRTEM micrograph of natural graphite displaying a highly ordered structure with interlayer spacing of 0.335 nm.

Fig. 4. Pittsburgh #8, 2200⬚C char displaying a highly ordered linear structure similar to Fig. 2.

2.3. High-resolution transmission electron microscopy (HRTEM)

calculated using a standard value of the activation energy for char oxidation [25–27] of 130 kJ mol ⫺1, is the pre-exponential factor in the Arrhenius equation,

Char and graphite samples were prepared by fine-grinding the samples under absolute ethanol in an agate pestle and mortar. The fine particles, held in suspension, were collected on holey-carbon grids and examined in a TEM operating at 200 kV in fringe imaging mode at high magnifications (up to ⬃8 000 000 ×). 2.4. Non-isothermal thermogravimetric analysis (TGA) Oxidation reactivities of the char samples were measured by non-isothermal TGA. Samples (3–3.5 mg) were heated in a thermobalance at 40 K min ⫺1 to 673 K, then at 15 K min ⫺1 to 1173 K in 6.3 vol% oxygen in nitrogen at a flow rate of 25 ml min ⫺1. The sample was held at 1173 K for 5 min to ensure total burnout. Char reactivity was expressed as log(A0) at 50% conversion [24], where A0, which has been

Fig. 3. Natural graphite illustrating the complexity of an ordered carbon with planes emerging from others, a criss-crossing network and a 90⬚ bend showing that regions of order do not always follow the straight line pattern of Fig. 2.

…dW=dt†= ⫺ W ˆ A0 e⫺E=RT :

3. Results and discussion Previous work [5] used HRTEM to examine chars prepared at temperatures between 1000⬚C and 1800⬚C and this showed an increase in structural ordering with temperature. The ordering displayed by the 1800⬚C chars has not, however, been to the extent of the ordering present in graphite. For the present study, the HTWM was used to prepare chars from Pittsburgh #8 at 2200⬚C with 0.5 s hold at peak temperature. The nature of the carbon crystalline structure of the natural graphite and 2200⬚C Pittsburgh #8 chars were examined in the HRTEM. Fig. 2 displays a micrograph of the natural graphite sample featuring a highly ordered

Fig. 5. Pittsburgh #8, 2200⬚C char showing a branching linear region.

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Fig. 6. Pittsburgh #8, 2200⬚C char illustrating a complex region of linear and more circular features.

Fig. 7. Pittsburgh #8, 2200⬚C char with more detailed circular features which may be a cross section through very small nanopores.

structure, with interlayer spacing 0.335 nm. The complexity of an ordered carbon with planes emerging from others, criss-crossing networks and a 90⬚ bend showing that regions of order do not always follow the straight line pattern is illustrated in Fig. 3. The highly ordered linear pattern of the graphite is repeated in the Pittsburgh #8 2200⬚C char (Fig. 4) with a branching feature, possibly around a very small pore, in Fig. 5. A particle edge is illustrated in Fig. 6 showing linear regions terminating with some layers apparently joined to others in a loop and illustrating the complex patterns formed in an ordered system. The circular features displayed in Fig. 7 may be cross-sections through nanopores. The reactivity of the 2200⬚C char has been compared with

a 1000⬚C char (0.5 s hold at temperature) prepared from the Pittsburgh #8 (Fig. 8). The reactivity of the 2200⬚C char and natural graphite are very similar, whilst the reactivity of the 1000⬚C char is much higher. The reactivity decreases by a factor of ⬃ 60 between the 1000⬚C and 2200⬚C chars. There is less than a factor of 2 difference between the reactivities of the 2200⬚C char and the graphite. A comprehensive reactivity study of the deactivation behaviour of the Pittsburgh #8 and a higher rank APC coal is described elsewhere [28]. The structural changes and reactivities observed suggest that thermal deactivation is likely to be as a result of mainly aromatic cluster realignment occurring during heat treatment, producing a more ordered and unreactive carbon matrix.

Fig. 8. Reactivity (thermal deactivation) of Pittsburgh #8 HTWM chars prepared at 1000⬚C and 2200⬚C (0.5 s hold) with the reactivity of the graphite sample for comparison.

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4. Conclusions The effect of heating chars for short times at high temperatures on structural ordering and reactivity has been examined. After high temperature heat treatment, the char structures have been compared to those of a natural graphite and regions within the chars have been observed to show a similar degree of ordering to that present in the graphite. The reactivity for oxidation of the graphite and the high temperature char have been compared to that of a lower temperature char. The structural ordering and reactivity results suggest that structural changes occurring during heat treatment produce a more ordered and unreactive carbon matrix which may account for char thermal deactivation.

[7] [8] [9]

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Acknowledgements

[14]

The authors would like to acknowledge the British Coal Utilisation Research Association and the Department of Trade and Industry for financial and technical support. The authors gratefully note the contribution of Graham Briers, Materials Dept., Imperial College for his assistance with the HRTEM; Dr Chi Man, Mechanical Engineering Dept., for his assistance in the laboratory; and Paul Hemming of Exeter Analytical (UK) Ltd for supplying the sample of natural graphite.

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