Coal Char Thermal Deactivation under Pulverized Fuel Combustion


A comparison of partially burnt coal chars and the implications of their properties on the blast furnace process. J.M. Steer , R. Marsh , D. Sexton , ...
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Energy & Fuels 2000, 14, 883-888

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Coal Char Thermal Deactivation under Pulverized Fuel Combustion Conditions N. V. Russell,*,† J. R. Gibbins,‡ C. K. Man,‡ and J. Williamson† Department of Materials, Imperial College, Prince Consort Road, London SW7 2BP, U.K., and Department of Mechanical Engineering, Imperial College, London SW7 2BX, U.K. Received November 17, 1999

To examine the potential for thermal deactivation during pulverized coal combustion a hightemperature wire-mesh reactor has been used to prepare chars at heating rates of 104 K s-1, temperatures up to 1800 °C, and hold times of 0-5 s, from two Argonne Premium coals: Pittsburgh No. 8, a high-volatile bituminous coal, and Pocahontas No. 3, a low-volatile high rank coal. Residual char reactivities to oxygen were determined using a nonisothermal TGA method. A comprehensive examination of the effect of preparation conditions on char reactivity is reported. Chars from the higher rank coal were relatively less reactive than chars from the lower rank coal prepared at lower temperatures, in line with accepted trends. At the higher temperatures (up to 1800 °C), more typical of full-scale pulverized coal combustion, the trend was reversed. Due to the greater propensity of the lower rank coal for thermal deactivation, high-temperature chars from the higher rank coal were found to be relatively more reactive than chars from the lower rank coal. High-temperature chars were also found to have reactivities comparable to utility fly ash carbons. Volatile yield and elemental release measurements suggested that thermal deactivation is likely to be a result of structural changes within the char matrix, rather than loss of heteroatoms from the structure.

Introduction Factors affecting the rates of coal char combustion at high overall conversions are extremely important in determining the residual carbon content of fly ash (pfa) in pulverized fuel boilers. Boiler operators look for a high degree of carbon burnout for economic reasons. Greater than 6 wt % carbon in ash1 represents a significant loss in thermal efficiency, interferes with the performance of electrostatic precipitators by reducing fly ash resistivity,2 prevents utilization of the ash as a cement replacement in concrete manufacture,3 and lowers the bulk density of the ash which increases the costs of landfill disposal.4 Deactivation is significant for carbon burnout because the low intrinsic chemical reactivities observed in fly ash carbons cause mixed diffusion/kinetic control under PF combustions conditions rather than pure diffusion control, as has often been assumed.5 It has been proposed that low oxidation reactivities observed in residual carbons from commercial and pilot-scale plants may be due to thermal deactivation in the high-temperature regions of the boilers.5-9 * Corresponding author. Tel.: +44 (0)20 7589 5111 ext. 56758. Fax: +44 (0)20 7594 6748. E-mail: [email protected] † Department of Materials. ‡ Department of Mechanical Engineering. (1) ASTM Designation C618-89a. Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 1989; p 289. (2) Vesma, V. Industrial coal handbook; Energy Publications, Cambridge Information and Research Services Limited: Cambridge, England, 1985. (3) Freeman, E.; Gao, Y. M.; Hurt, R.; Suuberg, E. Fuel 1997, 76, 761. (4) Clarke, L. B. IEA Report No. IEACR/50, 1992. (5) Beeley, T.; Crelling, J.; Gibbins, J.; Hurt, R.; Lunden, M.; Man, C.; Williamson, J.; Yang, N. Twenty-Sixth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1996; pp 3103-3110.

Gibbins and Williamson10 observed that chars from boiler fly ash have a range of reactivities and that reactivity was generally lower for larger char particles. At the same time, Hurt and co-workers11 reported a comparison between laboratory-prepared chars and residual unburnt chars from utility boilers. A joint study between Imperial College and Sandia National Laboratories (SNL) concluded that the differences in reaction rates between residual utility boiler and laboratory chars could be explained by low intrinsic reactivities and that chars were deactivated in the boiler plant.7 However, the mechanisms for deactivation were not clear. Experiments using a high-temperature wire-mesh reactor (HTWM)12 were able to show that thermal deactivation alone at realistic combustion particle temperatures of 1600-1800 °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 by SNL on residual carbons showed that, (6) Hurt, R. H.; Davis, K. A. Twenty-Fifth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1994; pp 561-568. (7) Hurt, R. H.; Gibbins, J. R. Fuel 1995, 74, 471. (8) Davis, K. A.; Hurt, R. H.; Yang, N. Y. C.; Headley, T. J. Combust. Flame 1995, 100, 31. (9) Russell, N. V. Thermal deactivation and reactivity of hightemperature coal chars. Ph.D. Thesis, University of London, 1999. (10) Gibbins, J. R.; Williamson, J. In Proceedings of the Seventh International Conference on Coal Science, Banff, Canada, 1993; pp 3538. (11) Hurt, R. H.; Davis, K. A.; Yang, N. Y. C.; Headley, T. J.; Gibbins, J. R. In Proceedings of the Seventh International Conference on Coal Science; Banff, Canada, 1993; pp 241-244. (12) Gibbins, J. R.; Lockwood, F. C.; Man, C. K.; Williamson, J.; Hesselmann, G.; Downer, B. M.; Skorupska, N. M. Coal selection for NOx reduction in pulverised fuel combustion. Second International Conference on Combustion and Emissions Control; Institute of Energy: London, Dec 1995.

10.1021/ef990241w CCC: $19.00 © 2000 American Chemical Society Published on Web 05/27/2000

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Russell et al. Table 1. Analysis of the Two Argonne Premium Coals23

Figure 1. Four stages of structural rearrangements leading to crystalline order within carbons.19 Below 500 °C, basic structural units are present. At temperatures between 800 and 1500 °C these associate face-to-face in distorted columns. Between 1600 and 2000 °C, adjacent columns coalesce into distorted wrinkled layers, while, above 2100 °C, these layers stiffen, become flat and perfect.

compared with the laboratory-generated chars, the residual carbons were of similar elemental, petrographic composition, and surface area but of higher crystallinity,14 also suggesting that a thermal deactivation process had occurred. The phenomenon of structural ordering within carbons was first suggested in 1949.15 The pioneering work of Rosalind Franklin, who proposed the graphitizing, partially graphitizing, and nongraphitizing models for carbon structures16,17 on the basis of X-ray diffraction studies, has in the 1980s been substantiated18,19 by highresolution transmission electron microscopy. Oberlin19 proposed a model after experiments with anthracenebased carbons, pitch-based carbons, and carbon films which had been heated to 2900 °C at 20 °C min-1 all showed similar results. The study suggested that as the heat treatment temperature increased, the successive improvements in texture which led to those of crystalline order were produced in four discrete stages, Figure 1. Subsequent deactivation studies5 using HTWM chars showed that conventional expectations of char reactivity ranking might be reversed when chars were prepared at high temperatures. Inertinite-rich coal samples gave relatively less reactive chars than vitrinite-rich coals at 1000 °C, in line with previous studies, but in some cases deactivation was less, compared to the vitrinite-rich samples, after preparation at 1800 °C. A high degree of structural ordering was observed by high-resolution transmission electron microscopy (HRTEM) on the less reactive HTWM chars. Hurt and co-workers20 have since 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 pulverized coal combustion. Global char combustion kinetics can satisfactorily model the first 90% of burnout but seriously overpredict conversion (13) Beeley, T. J.; Gibbins, J. R.; Hurt, R. H.; Man, C. K.; Pendlebury, K. J.; Williamson, J. Am. Chem. Soc., Div. Fuel Chem., Prepr. Pap. 1994, 39, 564. (14) Hurt, R. H.; Davis, K. A.; Yang.; N. Y. C.; Headley, T. J.; Mitchell, G. D. Fuel 1995, 74, 1297. (15) Bangham, D. H.; Franklin, R. E.; Hirst, W.; Maggs, F. A. P. Fuel 1949, 28, 231. (16) Franklin, R. E. J. Chim. Phys. 1950, 47, 573. (17) Franklin, R. E. Proc. R. Soc. London A 1951, 209, 196. (18) Oberlin, A.; Oberlin, M. J. Microsc. 1983, 132, 353. (19) Oberlin, A. Carbon 1984, 22, 521. (20) Hurt, R. H.; Lunden, M.; Brehob, E. G.; Maloney, D. J. TwentySixth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1996; pp 3169-3177.

carbon (% db) hydrogen (% db) nitrogen (% db) volatile matter (% db) ash (% db) mean-max Vitrinite reflectance plasticity (dial divisions/min) rank

Pittsburgh No. 8

Pocahontas No. 3

75.50 4.83 1.49 37.82 9.25 0.81 29 267 Hvb

86.71 4.23 1.27 18.60 4.77 1.68 44 Lvb

rates in the later stages of combustion. A detailed description of a later version of the Hurt model has recently been published.21 Subsequent studies carried out on coal chars prepared in an inert atmosphere with the high heating rates and short hold times experienced by coal particles in utility boilers suggest that the loss in reactivity (thermal deactivation) observed in fly ash carbons is due to structural ordering of the carbon in the char.5,9 Fringeimaging HRTEM of several coal chars and maceral-rich samples prepared at temperatures up to 1800 °C show that some graphitization has taken place.5 Chars prepared at 2200 °C were compared with a natural graphite using HRTEM and TGA char reactivity.22 The chars showed a high degree of structural ordering similar to the graphite and had a similar, low, reactivity. This study used a comprehensive set of preparative conditions to investigate the effect of heating time and temperature on the progress of char deactivation at heating rates and temperatures representative of pulverized coal combustion conditions, for which data have not previously been available. The significantly different trends for the Pittsburgh No. 8 and Pocahontas No. 3 coal samples used indicate the sensitivity of char deactivation behavior to coal rank. Experimental Section Samples. Coals were taken from the Argonne Premium Coal Sample Program: Pittsburgh No. 8 and Pocahontas No. 3. The former is a high-volatile bituminous coal; the latter a low-volatile, higher rank bituminous coal. These coals have been fully characterized by the APC program,23 Table 1. High-Temperature Wire-Mesh Reactor. Char samples were prepared using a high-temperature wire-mesh reactor in which the coal particles were held between two layers of molybdenum mesh, which also acted as an electrical resistance heater. A 24 V dc power supply was used to deliver heating currents of up to 2000 A. Temperatures were measured using a two-color infrared pyrometer, with a microcomputer for feedback temperature control. The particles were heated at 104 K s-1 to the peak temperature and held for a specified time period before uncontrolled, but rapid, cooling. 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 was used, previously ground and sieved to give a 125-150 µm size fraction for compatibility with the mesh material. Samples were weighed before and after pyrolysis to determine the volatile yield. The residual char from the HTWM runs was removed from the mesh for elemental microanalysis and reactivity determination by thermogravimetric analysis (TGA). Nonisothermal Thermogravimetric Analysis. Oxidation reactivities of the char samples were measured by (21) Hurt, R. H.; Sun, J. K.; Lunden, M. Combust. Flame 1998, 113, 181. (22) Russell, N. V.; Gibbins, J. R.; Williamson, J. Fuel 1999, 78, 803. (23) Vorres, K. S. Energy Fuels 1990, 4, 420.

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Figure 3. Evolution of volatile matter with time for Pittsburgh No. 8 and Pocahontas No. 3 coals. Mathcad software. The amount of volatile carbon, hydrogen, and nitrogen (elemental loss) was calculated from the CHN results and HTWM volatile yields.

Results and Discussion

Figure 2. Char reactivity versus hold time for (a) Pittsburgh No. 8 and (b) Pocahontas No. 3. The solid line and points are experimental; the dotted lines are modeled (see text). nonisothermal thermogravimetric analysis.24 Samples (3.03.5 mg) were heated in a thermobalance at 40 K min-1 to 673 K and then at 15 K min-1 to 1173 K in 6.3% 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. The results were expressed as an Arrhenius plot (ln[reactivity] versus inverse absolute temperature) and normalized weight loss versus inverse absolute temperature. Char reactivity was expressed as log(A0) at 50% conversion,24 where A0, which has been calculated using a standard value of activation energy for char oxidation25-27 of 130 kJ mol-1, is the pre-exponential (frequency) factor in the Arrhenius equation, (dW/dt)/-W ) A0e-E/RT. Elemental Microanalysis (CHN). The carbon, hydrogen, and nitrogen contents of the chars prepared in the HTWM were determined with a Carlo Erba EA1106 elemental analyzer. The raw data collected were processed using in-house (24) Russell, N. V.; Beeley, T. J.; Man, C. K.; Gibbins, J. R.; Williamson, J. Fuel Proc. Technol. 1998, 57, 113. (25) Charpenay, S.; Serio, M. A.; Solomon, P. R. Twenty-Fourth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1992; pp 1189-1197. (26) Smith, I. W. Fuel 1978, 57, 409. (27) Suuberg, E. M.; Wo´jtowicz, M.; Calo, J. M. Twenty-Second Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1988; pp 79-87.

Char samples were prepared from the two coals at peak temperatures of 1000, 1400, 1600, and 1800 °C, at a heating rate of 104 K s-1, with hold times at the peak temperature of 0.15, 0.5, 2, and, except at 1800 °C, 5 s. Hold times at 1800 °C were restricted to 2 s to limit the thermal load on the apparatus. This peak temperature and hold time were, in any case, considered to be sufficiently close to the most severe conditions likely to be applied to coal particles in a full-size power plant.28 Previous studies have shown the reactivity of pulverized coal fly ash char from front-wall fired boilers to be similar to that of the chars prepared in the laboratory at 1600-1800 °C with 2 s hold.13 For example, the reactivity of Herrin No. 6 coal fly ash char5 was found to give a log(A0) ) 4.53, typical of residual carbons. Figure 2, parts a and b, present the results of reactivity (log(A0) at 50% conversion) versus hold time for Pittsburgh No. 8 and Pocahontas No. 3, respectively. It can be seen for both coals that as the severity of heating was increased, char reactivity decreased monotonically but that the reactivity decreased more for the Pittsburgh No. 8 coal chars than for Pocahontas No. 3 chars over the range of conditions studied. Thus, although Pocahontas No. 3 gave less reactive chars than Pittsburgh No. 8 at the lower temperatures and hold times commonly applied in laboratory studies, under conditions which more closely represent the more severe conditions encountered in utility PF combustion the trend was reversed. This obviously highlights the need to characterize char properties at appropriately severe conditions. (28) Hesselmann, G. Fuel 1997, 76, 1269.

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Figure 4. Changes in char hydrogen and nitrogen content for (a,c) Pittsburgh No. 8 and (b,d) Pocahontas No. 3 coals.

Figure 3 displays the volatile yield for both Pittsburgh No. 8 and Pocahontas No. 3 coals. For the Pittsburgh No. 8 coal, as peak temperature increased, more volatiles were released up to a maximum at 2 s hold for all temperatures studied. For the Pocahontas No. 3 coal, apart from an increase between 0.15 and 0.5 s hold, there was no further volatile loss for all peak temperatures and hold times tested. Changes in the hydrogen and nitrogen contents of the chars (expressed as H/C and N/C atom ratios) are shown in Figure 4. Clear differences exist in heteroatom release patterns between the coals. Hydrogen release from the char was similar at lower temperatures and hold times (Figures 4a,b), but only the Pittsburgh No. 8 chars released additional hydrogen under the most severe conditions: 2s and above at 1600 °C and 0.5 s and above at 1800 °C (Figure 4a). The contrast is even more striking for nitrogen release, with continuing release of nitrogen from the Pittsburgh No. 8 chars with time at 1400 °C and above (Figure 4c) but essentially no release from Pocahontas No. 3 chars after an initial, slight, reduction (Figure 4d). Reactivity and heteroatom release trends are combined in Figure 5.

The following explanation is offered to account for the observed reactivity and char heteroatom content changes. Char reactivity is a function of the concentration of reactive sites, which can be due to both heteroatoms and/or edges and imperfections in the carbon structure itself. Hydrogen contained in labile structures within the char is rapidly depleted, until some minimum proportion, corresponding to an H/C ratio of around 15 at. %, remains attached to (relatively) thermally stable polyaromatic structures. Only if these structures can be rearranged and possibly further condensed will additional hydrogen be released. Nitrogen remaining in the char after primary devolatilization is all contained in aromatic structures which must be disrupted before it is released. A combination of factors then accounts for the lower reactivities of the less-mature chars from the Pocahontas No. 3 coal compared to chars from the Pittsburgh No. 8 coal prepared under the same conditions. Heteroatom (H and N) contents are both lower in the former. The larger aromatic cluster size in the higher rank coal, persisting essentially unchanged in immature chars, will give fewer edge sites for oxidation. At around 1600

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Figure 6. Abundance of low reactivity char components as a function of activation energy for Pittsburgh No. 8 (solid line) and Pocahontas No. 3 (dotted line) coals.

Figure 5. Char reactivity versus hydrogen content for Pittsburgh No. 8 chars (solid lines) and Pocahontas No. 3 (dotted lines). The numbers represent the hold time in seconds at the indicated temperatures.

°C and above, however, as observed by HRTEM in a previous study,22 the Pittsburgh No. 8 char begins to undergo disruption and reorganization of the carbon structures at an appreciable rate. Reactivity, H/C and N/C measurements for the chars initially converge, while, for 2 and 5 s hold times at 1600 °C and at 0.5 and 2 s at 1800 °C, the Pittsburgh No. 8 chars have undergone significant additional heteroatom loss and exhibit lower reactivity. The differences in behavior between the two coals may be explained by the differences between the structures of the two parent coals. Pocahontas No. 3 is a higher rank coal than Pittsburgh No. 8 with an extensive aromatic ring structure. A total of 86% of the carbon atoms in Pocahontas No. 3 are aromatic compared to 71% in the Pittsburgh No. 8,23 with the aliphatic (chain) hydrogen/aromatic (ring) hydrogen ratio being 1.1 for Pocahontas No. 3 and 5.1 for Pittsburgh No. 8,29 indicating that Pittsburgh No. 8 has many more aliphatic chains which are lost rapidly on heating. Pittsburgh No. 8 also has a more plastic nature (Table 1), going through a more fluid stage, known as the “mesophase”, during the loss of aliphatic chains and which facilitates subsequent realignment of the aromatic clusters in the char under the temperature regime proposed by Oberlin.19 Pocahontas No. 3, which already has a more ordered carbon network and less side chains to “flux” a mesophase stage, does not go through a significant plastic stage. While it is not possible to identify clearly the respective contributions of heteroatom loss and carbon structure changes at all stages in deactivation, there are clear indications that structural changes alone have a causative role for some of the deactivation of the Pittsburgh (29) Martin, K. A.; Chao, S. S. Am. Chem. Soc., Div. Fuel Chem., Prepr. 1988, 33, 17.

No. 8 char at elevated temperatures. Reactivities for 1800 °C chars are clearly lower than for higher-temperature chars with the same H/C and N/C ratios. At lower temperatures, since reactivity varies monotonically with H/C and N/C ratios (for chars from both coals), heteroatom loss could either itself be a causative factor in deactivation or a coincidental consequence of structural changes. Char thermal deactivation kinetics for both coals have been modeled (using Mathcad) assuming Arrhenius kinetics for a first-order “decomposition” (i.e. removal of heteroatoms or annealing of structural discontinuities) of “reactivity” (i.e. notional active sites) with distributed activation energies. A satisfactory fit could be obtained for deactivation processes at 1400-1800 °C (see Figure 2 dotted curves), but the same modeling approach underestimated rates of deactivation at 1000 °C. This is tentatively attributed to a change in dominant deactivation mechanism from loss of heteroatoms at low temperatures to structural reordering at higher temperatures. Since low-temperature deactivation kinetics would obviously not be important for pulverized coal combustion modeling, this area has not received further attention. Heteroatom loss is probably still also a secondary factor in the high-temperature deactivation observations, as well as structural changes, particularly for shorter hold times. Activation energies and other parameters used in the model may therefore represent a “composite” effect of different chemical processes. This would seriously reduce confidence in deactivation model predictions if trends had to be extrapolated to other conditions, but since the HTWM can apply an appropriate range of conditions for PF combustion the model is required only to interpolate within the matrix of experimental data. In line with previous studies21 a (truncated) γ distribution has been used for activation energies. The initial abundances of notional active sites as a function of activation energy are displayed in Figure 6, which shows that while similar ranges of activation energies for reactive site destruction are exhibited by chars from both coals, chars from the Pittsburgh No. 8 coal contain a higher proportion of material which is capable of hightemperature reordering than chars from the Pocahontas No. 3 coal. For calculation purposes the abundance/

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Table 2. Values and Definitions of Parameters Used in Eqs 1 and 2 Pittsburgh No. 8

symbol

definition

Efrac abundancefrac.initial A50(t) A50.initial A50.final K

γ distribution shape factor min activation energy in γ distribn max activation energy in γ distribn activation energy for sample subfraction initial fraction of reactive sites in subfraction (initial abundances sum to unity) A50 value at time t (s) for isothermal deactivation at temp T (K) A50 value for char before deactivation asymptotic min A50 value for char at highest temp common preexponential factor for Arrhenius deactivation eqs

activation energy distribution was discretized into 20 subfractions; tests showed that predictions were insensitive to further subdivision. For each fraction, the instantaneous abundance of notional active sites is assumed to change as follows:

d abundancefrac ) -abundancefrac‚K‚e-Efrac/RT (1) dt Integrating for isothermal deactivation and summing for all sample subfractions gives the following formula for predicted A50 values:

A50(t) ) A50.final - (A50.initial 20

abundancefrac.initial‚e-tKe ∑ frac)1

A50.final)[

-Efrac

] (2)

Values and definitions of parameters are given in Table 2. Parameters for Pittsburgh No. 8 were selected automatically to give the best fit to experimental data using the Levenberg-Marquardt method (from Mathcad library subroutines). Changes in reactivity for Pocahontas No. 3 were less, with more experimental scatter, and thus the parameters had to be selected manually. Thus predicted changes in reactivity of chars prepared in the range 1400-1800 °C could be made with confidence.

Pocahontas No. 3

1.089 224 kJ/mol 376 kJ/mol

0.5 200 kJ/mol 400 kJ/mol

105.4 s-1 104.0 s-1 6.9 × 109 s-1

105.2 s-1 104.3 s-1 6.9 × 109 s-1

Conclusions The thermal deactivation behavior of chars prepared from two coals under realistic heating conditions for pulverized coal combustion in a utility boiler has been examined. The results confirm that thermal deactivation can account for the low reactivities of unburned carbon in fly ash and is a consequence of structural ordering taking place at high temperatures within the carbon matrix and possibly also of heteratom loss. Deactivation (and heteroatom loss) trends differed between the Pittsburgh No. 8 and Pocahontas No. 3 coals, indicating the need for characterization tests under appropriate conditions to establish coal properties during pulverized coal combustion. A first-order Arrhenius model with distributed activation energies gave a good fit to the experimental data for thermal deactivation. Since the model is probably an approximation for a complex deactivation process, significant extrapolation is not recommended. The HTWM can, however, cover a wide enough range of temperatures and hold times so that only interpolation is required for pulverized coal combustion modeling. Acknowledgment. The authors are grateful to the U.K. Department of Trade and Industry through ETSU and the British Coal Utilization Research Association for financial and technical support. EF990241W