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Assessment of Thermodeactivation during Gasification of a Bituminous Coal Char Piero Salatino,* Osvalda Senneca, and Sabato Masi Dipartimento di Ingegneria Chimica, Universita` degli Studi di Napoli Federico II, Istituto di Ricerche sulla Combustione-C.N.R., P.le Tecchio, 80125 Napoli, Italy Received March 1, 1999
The annealing-induced loss of gasification reactivity of a bituminous coal char with respect to carbon dioxide was studied by means of a combination of experimental techniques. Heat treatment of coal samples under inert atmosphere for different time intervals was accomplished at temperatures as high as 2000 °C by means of thermogravimetric analyzers and of a specially designed heated-strip reactor. Gasification reactivities of untreated and heat-treated coal samples were determined by isothermal thermogravimetric analysis in carbon dioxide atmospheres. The influence of the time-temperature history of the coal sample on the parameters of the apparent gasification kinetics (preexponential factor, activation energy) has been assessed. Results were further analyzed in the framework of a simple annealing kinetic model suitable for implementation in gasification modeling.
Introduction The renewed interest for high-temperature entrainedflow coal gasification, on one hand, the widespreading recourse to reburning as a tool for the reduction of combustion-generated pollutants, on the other, has recently emphasized the importance of char deactivation in the late stage of carbon burnoff. This process is directly related to the unburnt carbon at the combustor/ gasifier outlet and to the loss-on-ignition of fly ashes.1 Along with the renewed interest for annealing of coal chars, a literature has flourished on carbon thermodeactivation during oxygen,2-9 carbon dioxide,10-13 * Corresponding author. Telephone: +39 081 7682258. Fax: +39 081 5936936. E-mail:
[email protected]. (1) Hurt, R. H.; Davis, K. A. Twenty-Fifth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1994; p 561. (2) Jenkins, R. G.; Nandi, S. P.; Walker, P. L., Jr. Fuel 1973, 52, 288. (3) Beeley, T. J.; Crelling, J. C.; Gibbins, J. R.; Hurt, R. H.; Lunden, M.; Man, C. K.; Williamson, J. Twenty-Sixth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1996; p 3103. (4) Beeley, T. J.; Crelling, J. C.; Gibbins, J. R.; Hurt, R. H.; Man, C. K.; Williamson, J. In Coal Science; Pajares, J. A., Tasco`n, J. M. D., Eds.; Elsevier Science B. V.: New York, 1995; pp 615-618. (5) Cai, H.-Y.; Gu¨ell, A. J.; Chatzakis, I. N.; Lim, J.-Y.; Dugwell, D. R.; Kandiyoti, R. Fuel 1996, 75, 15. (6) McCarthy, D. J. Fuel 1982, 61, 298. (7) Radovic, L. R.; Walker, P. L., Jr.; Jenkins, R. G. Fuel 1983, 62, 849. (8) Senneca, O.; Salatino, P.; Masi, S. Twenty-Seventh Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1998; p 2991. (9) Hindmarsh, C. J.; Thomas, K. M.; Wand, W.; Cai, H.-Y.; Gu¨ell, A. J.; Dugwell, D. R.; Kandiyoti, R. Fuel 1995, 74, 1185. (10) Van Heek, K. H.; Mu¨hlen, H. J. In Fundamental Issues in Control of Carbon Gasification Reactivity; Lahaye, J., Ehrburger, P., Eds.; Kluwer Academic: Dordrecht, 1991; pp 1-34. (11) Blake, J. H.; Bopp, G. R.; Jones, J. F.; Miller, M. G.; Tambo, W. Fuel 1967, 46, 115. (12) Senneca, O.; Russo, P.; Salatino, P.; Masi, S. Carbon 1997, 35, 141. (13) Senneca, O.; Salatino, P.; Masi, S. Fuel 1998, 77, 1483.
steam,10,14 and hydrogen15 gasification. It is currently agreed that char deactivation is largely controlled by thermal annealing which coal particles undergo during combustion and gasification. This, in turn, is related to the temperature-time history of coal/char particles over their lifetime in the reactor. The relevance of solid-state transformations of carbon associated with thermal annealing to the kinetics of carbon oxidation has been addressed in references 7, 8, 12, 13, 16-19. Whether turbostratic carbon structure modifications or thermally induced changes of inorganic matter are the prevailing route to loss of gasification activity is still open to some question. Few attempts have been made to model annealing during char combustion/gasification. The pioneering work of Nagle and Strickland-Constable20 first proposed a complex reaction mechanism embodying thermal annealing as one pathway in series/parallel with carbon oxidation. Taking after the theory of Blyholder et al.,21 the NSC model assumed the existence of two carbon sites (a more reactive A and a less reactive B) and of a network of reactions embodying the reactions of sites A and B with oxygen and one annealing reaction, leading from A to B. The pseudo-steady-state ap(14) Kasaoka, S.; Sakata, Y.; Shimada, M. Fuel 1987, 66, 697. (15) Tomita, A.; Mahajan, O. P.; Walker, P. L., Jr. Fuel 1977, 56, 137. (16) Blackwood, J. D.; Cullis, B. D.; McCarthy, D. J. Aust. J. Chem. 1967, 20, 1561. (17) Davis, K. A.; Hurt, R. H.; Yang, N. Y. C.; Headley, T. J. Combust. Flame 1995, 100, 31. (18) Meijer, R. Kinetics and mechanism of the alkali-catalysed gasification of carbon. Ph.D. Thesis, University of Amsterdam, NL, 1992. (19) Rouzaud, J. N.; Duval, B.; Leroy, J. In Fundamental Issues in Control of Carbon Gasification Reactivity; Lahaye, J., Ehrburger, P., Eds.; Kluwer Academic: Dordrecht, 1991; pp 257-265. (20) Nagle, J.; Strickland-Constable, R. F. Proceedings of the Fifth Conference on Carbon, Vol. 1; Macmillan: New York, 1962; p 154. (21) Blyholder, G.; Binford, J. S., Jr.; Eyring, H. J. Phys. Chem. 1958, 62, 263.
10.1021/ef9900334 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/12/1999
Thermodeactivation during Gasification of Coal Char
proximation was made when establishing the balance on sites on the carbon surface. The NSC model was successful in explaining the negative temperature coefficient (i.e., negative apparent activation energy) of carbon gasification by oxygen in the temperature range 1100-1800 °C. More recently, Senneca et al.12 modeled annealing during gasification with carbon dioxide using a triangular reaction network that resembled the NSC scheme once the pseudo-steady-state approximation was relaxed. A notable feature of char thermodeactivation, namely the “memory loss” effect, was underlined: carbons lose memory of the previous time-temperature history along with carbon burnoff. This feature and its relation with rearrangements of the turbostratic carbon structure, on one hand, of inorganic matter, on the other, were addressed in Senneca et al.13 Starting from the consideration of the wide range of solid-state transformations through which annealing takes place, Suuberg22 suggested that annealing should be described by parallel reactions with distributed values of the activation energy. The Suuberg approach was eventually adopted by Hurt et al.,23 who lumped the individual mechanisms of thermodeactivation into a single expression providing the dependency of char gasification reactivity on the temperature history of the particles. Parameters of the proposed expression are the preexponential factor, the mean value, and the standard deviation of the probability density function of the activation energies. The scope of the present work is to provide a simple kinetic expression for annealing to be implemented into coal gasification models. To this end the extent of thermodeactivation has been assessed experimentally by measuring the char-CO2 reactivity of coal samples subjected to different heat treatments in inert atmosphere. A South African bituminous coal has been used as reference material. By using a combination of experimental techniques, among them a specially designed heated-strip reactor, a broad range of heat treatment temperatures (900-2000 °C) and times (0.2 s-300 min) and of heating rates (50-106 °C/min) could be spanned. Experimental Section Materials. The char used in the experiments derived from a South African coal (SA) whose properties are reported in Table 1. This is a bituminous coal from the Southern Hemisphere (Gondwanaland) with a relatively large amount of inertinite and whose ashes are rich in calcium and clay minerals. Coal samples were ground and sieved in the size range of 75-125 µm prior to any processing. Apparatus and Experimental Procedure. The experimental procedure consisted of two steps: (1) Annealing step: Heating of the coal sample in nitrogen to the final temperature THT with heating rate RHT, followed by isothermal heat treatment in nitrogen at THT for a time tHT; (2) Gasification step: Cooling of the sample at the reaction temperature TR followed by gasification in carbon dioxide atmosphere until complete carbon burn out. (22) Suuberg, E. M. In Fundamental Issues in Control of Carbon Gasification Reactivity; Lahaye, J., Ehrburger, P., Eds.; Kluwer Academic: Dordrecht, 1991; pp 269-305. (23) Hurt, R. H.; Sun, J.-K.; Lunden, M. Combust. Flame 1998, 113, 181.
Energy & Fuels, Vol. 13, No. 6, 1999 1155 Table 1: Properties of South African Coal net calorific value (kJ kg-1)
26300
proximate analysis (%), dry basis Volatile matter Fixed carbon Ash
23.1 61.2 15.7
ultimate analysis (%), dry basis Carbon Hydrogen Sulfur Nitrogen Oxygen Ash
68.0 3.8 0.6 1.2 10.7 15.7
free swelling index
1
random vitrinite reflectance (%)
0.72
chemical composition of ashes (%, weight) SiO2 Al2O3 CaO MgO K2O Na2O FeO MnO TiO2 P2O5 SO3 Others
44.1 34.0 8.1 2.2 0.62 0.15 1.53 0.01 1.41 2.35 2.08 3.45
Annealing Step. Step 1, the annealing step, was performed under one of the following sets of conditions: (A) THT ) 900 °C, RHT ) 900 °C/min, tHT ) 1-300 min; (B) THT ) 1050-1400 °C, RHT ) 10000 °C/min, tHT ) 1-30 min; (C) THT ) 1600-2000 °C, RHT ) 106 °C/ min, tHT ) 0.2-80 s. Conditions (A) were established by means of a Rheometrics PL-TG1000M thermobalance characterized by a variable heating rate up to 1000 °C/min and maximum operating temperature of 950 °C. Conditions (B) were established by means of a Rheometrics PL-TG1500 thermobalance purposely modified in order to achieve a fixed heating rate of 10000 °C/min with a maximum operating temperature of 1500 °C. Heat treatment under conditions (C) was accomplished by means of a specially designed heated-strip reactor (HSR) that could be operated at a maximum heating rate of 106 °C/min and maximum temperature of 2200 °C (Figure 1). The latter reactor was characterized by the feature that the usual metal grid was replaced as the sample holder by a pyrolytic carbon strip thermally stabilized for use at temperatures as large as 2500 °C. Typical timetemperature profiles that could be established in the HSR are shown in Figure 2. Gasification Step. Step 2, the gasification step, was carried out at atmospheric pressure in either the Rheometrics PL-TG1000 or the TG1500 thermobalances, depending on the value of TR. Gasification took place in a carbon dioxide atmosphere. TR was varied between 900 and 1200 °C. The extent of gasification was monitored by recording the sample weight as a function of reaction time. The degree of carbon conversion was calculated as:
f)
m0 - m m0
(1)
where m0 and m are the initial and actual carbon mass in the sample, respectively.
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Figure 3. Typical gasification-rate profiles of heat-treated samples.
Figure 1. Heated-strip reactor.
Figure 2. Typical time-temperature profiles in heated-strip reactor.
Results The instantaneous gasification rate has been expressed as the time derivative Ri ) df/dt of the degree of carbon conversion f. Typical gasification rate profiles (Ri vs carbon conversion degree f) relative to coal samples subjected to different heat pretreatment are reported in Figure 3. Apart from the absolute value of the gasification reactivities, it can be appreciated how the patterns of the Ri vs f profiles change with heattreatment conditions. In particular, Senneca et al.13 noted that a commonly (but not always) observed feature of gasification profiles is that curves relative to different heat treatments overlap at large carbon burnoff. This feature, named “memory loss effect”, was
related to the nature of solid-state transformations associated with annealing. Analysis of gasification profiles in Figure 3 indicates that the memory loss effect can be observed, for the coal char at hand, only for heat treatment temperatures below 1200 °C. Beyond this value this feature is no longer observed, regardless of the duration of heat treatment. It can be noted, in particular, that relatively large values of the instantaneous reactivity are observed for samples heat treated at 1600 °C and 2000 °C for less than 1 s in the late stages of burnoff. The parameter R ) 0.5/t0.5, where t0.5 is the time needed to achieve 50% carbon burnoff, was taken as an average reactivity index for the differently heat-treated samples. This choice was a tradeoff between the need to express as much as possible the “initial” sample reactivity and the need to limit inaccuracies associated with sample-weight monitoring in the early carbon burnoff. Table 2 summarizes values of t0.5 for coal samples gasified at different temperatures TR after different heat pretreatments. Analysis of results in Table 2 suggests that, as expected the extent of thermodeactivation increases as heat-treatment time and temperature increase. The maximum thermodeactivation factor, i.e., the ratio of gasification reactivities of the most heavily annealed samples (THT ) 1400 °CtHT ) 300 min; THT ) 2000 °C-tHT ) 80 s) to those of the least annealed ones (THT ) 900 °C-tHT ) 1 min), is in the order of 5. This figure is smaller than thermodeactivation factors reported by other authors4,5,9 for carbons of similar properties with respect to the charO2 reaction. At least two reasons can be given for the observed discrepancies: (a) annealing affects in a different way the kinetics of carbon gasification by different reactants (see, e.g., Senneca et al.8); (b) different choices were made by investigators as to the reference
Thermodeactivation during Gasification of Coal Char
Energy & Fuels, Vol. 13, No. 6, 1999 1157
fit equations were obtained by regression analysis of data points corresponding to different heat treatment conditions:
THT ) TR; tHT ) 1 min: R ) 3.1 × 107 exp THT ) 1200 °C; tHT ) 30 min: R ) 2.2 × 108 exp THT ) 1400 °C; tHT ) 30 min: R ) 6.1 × 106 exp
(
)
(
)
(
)
26600 min-1 TR 29900 min-1 TR 26500 min-1 TR
Quantitative Assessment of Char Thermodeactivation Figure 4. Arrhenius plots of average reactivity for samples subjected to different heat treatments. Table 2: Half-Conversion Times (min) of Heat-Treated Samples heat treatment conditions THT, tHT
gasification temperature, °C 900 1050 1200
Thermogravimetric Analyzer 900 °C, 1 min 120 900 °C, 30 min 160 900 °C, 300 min 180 1050 °C, 1 min 7.6 1200 °C, 1 min 10 1200 °C, 30 min 300 13 1400 °C, 1 min 26 1400 °C, 30 min 630 50 1600 °C, 1 s 1600 °C, 5 s 1600 °C, 9 s 1600 °C, 35 s 2000 °C, 0.2 s 2000 °C, 0.4 s 2000 °C, 2 s 2000 °C, 5 s 2000 °C, 80 s
The average gasification reactivities in Table 2 are hereafter analyzed in the framework of a simple annealing kinetic model. It is based on the assumption that the course of annealing can be quantified by an internal coordinate ξ (ξ ) 0 for the nonannealed char; ξ ) 1 for the fully annealed one). By definition of ξ, char gasification reactivity R is expressed as a linear combination:
R ) R0(1 - ξ) + R∞ξ 1.2 1.7 3 6.2
Heated-Strip Reactor 220 350 350 350 180 280 320 320 440
“nonannealed” material. As regards the latter aspect, the choice of THT ) TR and tHT ) 1 min as the preparation conditions of the “reference” char in the present work was dictated by the need to establish a clear-cut distinction between coal pyrolysis and the further char transformations due to annealing (actually, the distinction between the two stages might be somewhat arbitrary) at the char gasification temperatures. Less severe char preparation conditions might equivalently be considered for the purpose of establishing a reference char, but would correspondingly affect observed thermodeactivation factors. Figure 4 reports Arrhenius plots of R ) 0.5/t0.5 as a function of reciprocal absolute reaction temperature TR. The figure reports data relative to samples annealed at different temperatures for different times. Plots in Figure 4 suggest that the loss of gasification reactivity brought about by thermal annealing considerably affects the preexponential factor, whereas the apparent activation energy remains relatively unchanged, similar to findings of previous investigators.24 The following best-
(2)
of reactivities R0 and R∞ of the nonannealed and of the fully annealed materials, respectively. (“Nonannealed char” refers to the material obtained by stopping the heat treatment as devolatilization is complete; “fully annealed char” refers to the product of prolonged heat treatment whose reactivity can no longer be reduced by further annealing.) It is furthermore assumed that the reactivity of the fully annealed char is much smaller than that of the chars of interest (R∞ , R0) and that its contribution in eq 2 can be therefore neglected. The rate at which the internal annealing coordinate ξ changes upon heat treatment at the temperature THT is expressed by the following nonlinear kinetic equation:
dξ ) k(THT)(1 - ξ)n dt
(3)
where k is an annealing rate equation constant, depending on the annealing temperature THT according to an Arrhenius-type law:
(
k(THT) ) k0 exp -
)
Ea RgTHT
(4)
The assumption is made that eq 3, with k(THT) expressed by eq 4, applies regardless of whether annealing takes place under inert or oxidizing conditions. Upon integration between t ) 0 and the duration of the (24) Chiezzi, C.; Cozzani, V.; Tognotti, L. Proceedings of the 1st European Conference on Chemical Engineering (ECCE 1); Firenze, Italy, 1997; p 2927.
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Figure 5. Regression plot of annealing data according to eq 9. Different symbols refer to different reaction temperatures TR.
Figure 6. Comparison of model and experimental thermodeactivation factors. Different symbols refer to different reaction temperatures TR.
sample heat treatment tHT, eq 3 yields
using samples heat treated at THT ) TR for 1 min were arbitrarily assumed as R0. The value of n was estimated from curve fitting of the linearized model expressed by eq 9 against experimental data. It is n ) 6.5. For given gasification temperature TR and variable annealing time tHT and temperature THT, according to eq 9 with constant values of Ea and n, data points should collapse onto straight lines of slope -Ea/Rg. Figure 5 suggests that application of the annealing model yields a partial success. Data points are brought within a relatively narrow band, although annealing times and temperatures providing the experimental basis span broad ranges. The slope of the best-fit line through data points is in the order of -Ea/Rg = -27500 K. The value of the preexponential constant k0 in eq 4 is 4.4 × 106. The correlation coefficient R2 is 0.87. At given heat treatment temperatures, data points corresponding to different heat-treatment times do not completely overlap, thus pointing at the limit of the present correlation and/or at uncertainties associated with experimental results. Figure 6 compares values of the thermodeactivation factors R/R0 from experiments with those predicted with the use of the above model. Again, data points are rather scattered. If data points circled in Figure 6 are excluded from the regression analysis, the correlation coefficient R2 is 0.8. On the whole the comparison is encouraging in view of the practical implementation of the model in gasification design. It is recalled here that the above development relies on two basic hypotheses: (a) the assumption of a singlevalued activation energy of annealing (eq 4), and (b) the power-law dependence of the annealing rate on its extent, ultimately resulting into a power-law dependence of the extent of annealing on heat treatment time (eqs 7 and 8). Both features are consistent with the approach followed by Murty et al.25 in expressing the time dependence of the degree of graphitization g of a
[
]
1 1 - 1 ) ktHT n - 1 (1 - ξ)n-1
(5)
If the reaction order n is much larger than 1, the following approximation applies:
[
]
1 1 -1 = (1 - ξ)n-1 (1 - ξ)n-1
(6)
and eq 5 modifies into
(1 - ξ) ) [(n - 1)k]-1/(n-1) tHT-1/(n-1)
(7)
Upon substitution and the simplification R∞ = 0, eq 2 yields
R(TR) R0(TR)
) ([(n - 1)k(THT)]-1/(n-1)) tHT-1/(n-1)
(8)
which expresses an (approximated) power law dependence of the gasification reactivity of the annealed chars on the duration of heat-treatment. R(TR)/R0(TR) represents a thermodeactivation factor with respect to an arbitrarily selected reference char whose gasification reactivity is R0(TR). Equation 8 can be further worked out to yield:
-ln
[( ) R R0
(n-1)
]
tHT ) ln[(n - 1)k0] -
Ea RgTHT
(9)
Applicability of eq 8 to the data obtained within the present work is demonstrated in Figure 5. To this end, data points in Table 2 are plotted as linearized forms of eq 8, namely Arrhenius plots or -ln[(R/R0)(n-1) tHT] vs 1/THT. Reaction rates measured at the various TR
(25) Murty, H. N.; Biedermann, D. L.; Heintz, E. A. Carbon 1969, 7, 667.
Thermodeactivation during Gasification of Coal Char
set of cokes by the equation
g ) g0tHT-1/(n-1)
(11)
The similarity between eqs 8 and 11 is evident, as are their differences. The relation between the degree of coke graphitization and the annealing time was established by Murty et al. by looking at the graphene layer stacking parameter g after XRD analysis of untreated/ heat treated cokes. The extent of annealing ξ in eq 8 is assessed through its influence on carbon gasification reactivity. This points at the difference between variables g and ξ as annealing coordinates and prevents from bringing the similarity between eqs 8 and 11 any further. Values of the reaction order n reported by Murty et al. are of the order of 7. Assumptions (a) and (b) were based by the authors on the experimental evidence that both the activation energy and the reaction order were barely dependent on temperature and/or course of graphitization. It must be recalled that different approaches to quantitative assessment of annealing were postulated by other authors who started from the consideration that annealing, much like pyrolysis, consists of a bundle of solid-state transformations with continuous distribution of activation energies.22,23 Whether the single-valued or the distributed activation energy approaches are more appropriate to describe the temperature and course dependence of annealing is still open to question. Regardless of its mechanistic foundation, the kinetic model expressed by eqs 2-8 stands out, for its simplicity, as a convenient quantitative way of expressing char thermodeactivation in practical coal gasification modeling.
Energy & Fuels, Vol. 13, No. 6, 1999 1159
Conclusions The gasification reactivity of a bituminous coal char in carbon dioxide atmosphere has been measured at gasification temperatures ranging from 900 to 1200 °C. Char samples were derived from pyrolysis/annealing pretreatment performed at temperatures ranging from 900 to 2000 °C for time intervals between 0.2 s and 300 min. Such a broad range of heat-treatment conditions could be achieved by the combined use of thermogravimetric analyzers and of a purposely designed heatedstrip reactor. Gasification reactivity is confirmed to be strongly dependent on the time/temperature history of the coal samples. Thermodeactivation by a factor of about 5 is recorded when comparing reactivities of samples annealed at 1400 °C for 30 min or at 2000 °C for 80 s with that of the sample pyrolyzed at 900 °C for 1 min. Annealing significantly affects the preexponential factor of the apparent gasification rate equation. On the other hand, the apparent gasification activation energy turns out to be barely dependent on the time-temperature history of the coal sample and is in the order of 53000 kcal kg mol-1. Experimental results have been analyzed in the light of a simple kinetic model. The model was based on the assumption of a power-law dependence of the rate of annealing on its extent and on a single-valued activation energy. Experimental results obtained within the present work could be satisfactorily reproduced by the kinetic model. In view of its simplicity, the model lends itself for use in quantitative expression of char thermodeactivation in practical gasification modeling. Acknowledgment. The project has been supported by ENEL Polo Termico, Pisa (Italy). EF9900334
