A Comparison of Coal Char Reactivity Determined from

Hemisphere coals compared to the Northern Hemisphere coal Cerrejon in the high-temperature combustion regime, where char morphology and thereby mass ...
0 downloads 0 Views 328KB Size
Download PDF
268

Energy & Fuels 1998, 12, 268-276

A Comparison of Coal Char Reactivity Determined from Thermogravimetric and Laminar Flow Reactor Experiments Alfredo Zolin,* Anker Jensen, Lars Storm Pedersen, and Kim Dam-Johansen Department of Chemical Engineering, Technical University of Denmark (DTU), Building 229, DK-2800 Lyngby, Denmark

Peter Tørslev Elsam I/S, Fjordvejen 1-11, DK-7000 Fredericia, Denmark Received June 23, 1997

The reactivity of nine different coals ranking from subbituminous to low-volatile bituminous has been studied by thermogravimetric analysis (TGA). At a standard set of conditions a qualitative fuel reactivity classification (ranking) with respect to one of the coals, Cerrejon, is presented. Particle reaction rates per unit external surface area and a normalized reactivity index based on raw experimental data were used as reactivity parameters to compare the fuels. The TGA chars were prepared at 900 °C with 15 min holding time and then combusted in a 20 mol % O2 environment at several temperatures in the range 450-650 °C. TGA reaction rate data were adequately interpreted by a random pore model. However, at 650 °C it is believed that particle ignition gave rise to a char reaction rate behavior that the model was incapable of describing properly. Except for two Southern Hemisphere coals, the reactivity ranking obtained with the TGA apparatus at a combustion temperature of 550 °C agrees well with a corresponding classification based on experiments carried out in another study with a laminar flow reactor (LFR) at ∼1400 °C. A possible explanation for this is a more dense structure of the Southern Hemisphere coals compared to the Northern Hemisphere coal Cerrejon in the high-temperature combustion regime, where char morphology and thereby mass transfer effects such as internal pore diffusion are reaction rate determining. The maximum difference in reaction rates based on external surface area between the coal chars in the low-temperature TGA experiments was 1 order of magnitude higher than in the high-temperature LFR experiments, due to the increasing effect of pore diffusion and thermal annealing of the coal chars in the LFR tests. The similarity in the reactivity ranking obtained for the Northern Hemisphere coals from both reactor systems indicates that a ranking can be performed by thermogravimetric analysis. This provides a simple means for determining a fuel reactivity ranking that could be applied to full scale suspension fired plants.

Introduction Based on the increasing use of coals of different origin and rank, more emphasis has been given during the past decade to the determination of coal quality in combustion applications. The prediction of the combustion performance of a fuel under the wide range of conditions present in either suspension fired or fluid bed combustion seems at the moment not to be possible, due to the heterogeneous nature of coal and the complexity of the processes involved. Since the experimental results of fuel reactivity are equipment specific, it appears that the tests can rather be applied to provide a relative ranking of the expected combustion behavior of a fuel. Several experimental setups have been used for the determination of char reaction rates, i.e., reactivity, and * Corresponding author. Fax: +45 45 88 22 58. E-mail: az@ olivia.kt.dtu.dk.

the estimation of kinetic parameters at the early and intermediate stages of combustion. Among these experimental setups, the more important are the entrained flow reactors (EFR),1 where particle heating rates and temperatures are close to those found in suspension-fired plants. The tests involved with this reactor type are costly and time consuming. Coal combustion behavior has also been studied by thermogravimetry,2,3 where the mass and temperature history of the fuel sample are continuously monitored throughout the experiment. Full scale conditions cannot be simulated in this type of reactor, but kinetically controlled conditions are easy to obtain. Combustion (1) Hurt, R. H; Mitchell, R. E. 24th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1992; pp 1243-1250. (2) Alvarez, T.; Fuertes, A. B.; Pis, J. J.; Ehrburger P. Fuel 1995, 74, 729-735. (3) Zhang, D. K.; Wall, T. F.; Harris D. J.; Smith I. W.; Chen J.; Stanmore, B. R. Fuel 1992, 71, 1239-1246.

S0887-0624(97)00095-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/05/1998

Coal Char Reactivity

Energy & Fuels, Vol. 12, No. 2, 1998 269 Table 1. Composition of Coals Testeda and TGA Experimental Program

106-125 µm dry basis (wt %)

Cerrejon

Blair Athol

Ulan

volatiles % ash % carbon % hydrogen % oxygen % nitrogen % sulfur % ASTM rank origin (state) TGA experimentsk

37.4 6.8 76.2 4.8 11.1 1.7 0.7 HVB bit. Colombiab 500-650

29.4 8.4 72.8 3.8 9.66 1.9 0.3 HVB bit. Australiac 500-650

31.2 16.7 69.9 4.2 6.0 1.7 0.6 HVB bit. Australiad 500-650

PSOC 1445 PSOC 1451 PSOC 1488 PSOC 1493 PSOC 1502 PSOC1516 3.5 74.4 5.3 13.8 1.4 0.7 subbit. USAe 450-600

10.3 76.8 5.1 7.6 1.7 2.9 HVA bit. USAf 550-650

4.2 71.1 4.8 18.5 1.0 0.3 subbit. USAg 450-600

11.2 69.5 4.5 12.2 1.1 5.5 HVB bit. USAh 450-600

7.1 73.1 4.8 12.3 1.1 0.4 HVC bit. USAi 450-600

29.0 60.3 3.4 5.4 1.0 2.8 LV bit. USAj 550-650

a From ref 12. b La Guajira. c Queensland. d New South Wales. e New Mexico. f Pennsylvania. g Montana. h Illinois. i Utah. j Pennsylvania. Combustion (oxidation) temperature. Temperature steps of 50 °C, end points included. Pyrolysis conditions: HTT ) 900 °C, ht ) 15 min.

k

temperatures of 400-700 °C are normally used depending on the reactivity of the fuel. Since char reactivities are strongly dependent on temperature and time during pyrolysis, the chars used to study combustion are often prepared at conditions close to those found in practical systems. Entrained flow reactors (EFR) or wire-mesh reactors (WMR) are commonly used to produce the chars, while the intrinsic reaction rates are obtained by thermogravimetric analysis.4,5 However, keeping in mind the need to use simple and rapid tests for fuel characterization, the use of a thermogravimetric analysis (TGA) technique, in which both pyrolysis and char combustion are carried out consecutively, seems an appealing possibility for evaluation of fuel reactivity. Several ways of testing fuels by thermogravimetric analysis have been reported. Most of the studies are based on coal burning profiles, i.e., curves where the mass loss and time derivative of the mass loss of a coal are monitored as a function of the temperature at a constant heating rate. Typically, characteristic temperatures in these profiles are defined to express the reactivity of a fuel:6 the initial temperature, i.e., the temperature at which the rate of weight loss exceeds 0.1% min-1; the peak temperature PT, where the burning rate is maximum, and the burnout temperature BT, i.e., the temperature where the rate of weight loss is 1% min-1 and thus the sample oxidation practically complete. In general, a reactive fuel gives lower values of the initial, peak, and burnout temperature. Burning profiles of coal chars produced in entrained flow or wire mesh reactors rather than directly produced from TGA apparatus are often used.7,8 Several comparative studies of burnout performance from coal burning profiles have been published. He Huang et al.9 presented a ranking of eight coals from lignite to low-volatile bituminous. A gradual shift toward higher values of peak temperature, i.e., lower reaction rate with increasing coal rank, was seen. In other investigations10,11 industrial process chars and laboratory-prepared chars have been subjected to (4) Cai, H. Y.; Gu¨ell, A. J, Chatzakis, I. N.; Lim J. Y.; Dugwell, D. R.; Kandiyoti, R. Fuel 1996, 75, 15-24. (5) Hindmarsh, C. J.; Thomas, K. M.; Wang, W. X.; Cai, H. Y.; Gu¨ell, A. J.; Dugwell, D. R.; Kandiyoti, R. Fuel 1995, 74, 1185-1190. (6) Cumming, J. W. Fuel 1984, 63, 1436-1440. (7) Morgan, P. A.; Robertson, S. D.; Unsworth, J. F. Fuel 1987, 66, 210-215. (8) Beeley, T. J, Crelling J. C.; Gibbins, J. R., Hurt, R. H.; Man, C. K.; Williamson, J. Coal Science; Elsevier Science: New York, 1995; pp 615-618. (9) Huang, H.; Wang, K.; Wang, S.; Klein, M. T.; Calkins, W. H. Proc. 211th ACS Natl. Meet. 1996, 1-7.

isothermal thermogravimetric analysis (∼600 K) and the results compared with those obtained from reactors operating at higher temperatures such as a fixed-bed (∼1000 K) and a drop tube reactor (∼1500 K). The reaction rates determined from thermogravimetry showed a greater variance than those tested at higher temperatures, the reaction rates differing by at least 1 order of magnitude between each configuration. The purpose of this study was to investigate whether relatively simple and rapid thermogravimetric analysis tests could be used to establish a fuel reactivity ranking which could be useful in full scale applications. This was done by comparing the reactivity ranking of nine coals obtained by TGA with the reactivity ranking obtained for the same coals in a laminar flow reactor, having conditions close to those found in full scale suspension fired plants. Experimental Section Equipment and Samples. The coals tested in this study correspond to those used in a previous work12,13 at Sandia’s Coal Combustion Laboratory, where char reactivity measurements were performed for two Australian coals, one Colombian coal, and six US coals by laminar flow reactor experiments. The reactor was designed so that postflame gases simulating those found in the upper stages of a full scale PC boiler were produced. A flow of 106-125 µm particles was injected through the center axis of the reactor and the devolatilization and char combustion was followed at different reactor heights in 6 and 12 mol % oxygen environments. The experimental technique allows the measurement of size, temperature and velocity of the particles at different heights in the reactor. A thorough description of the reactor can be found elsewhere.14 Table 1 shows the proximate and ultimate analysis of the different coals as well as the ASTM rank. In our study, a thermogravimetric analyzer Netzsch STA 409C was used in the experiments. Gas cylinders supplied the gases (N2, O2) and their flow rates were regulated by mass flow controllers. The change in sample weight was determined by a highly sensitive analytical balance located in the casing (10) Wells, W. F.; Kramer, S. K.; Smoot, L. D. 20th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1984; pp 1539-1546. (11) Hargrave, G.; Pourkashanian, M.; Williams, A. 21st International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1986; pp 221-230. (12) Tørslev Jensen, P.; Mitchell, R. E. Energy Research Project No. 1323/87-16, 1993. (13) Mitchell, R. E.; Hurt, R. H.; Baxter, L. L.; Hardesty, D. R. Sandia Report SAND92-8208, Sandia National Laboratories, Livermore CA, 1992. (14) Hardesty, D. R.; Pohl, J. H.; Stark, A. H. Sandia Report SAND78-8234, Sandia National Laboratories, Livermore CA, 1978.

270 Energy & Fuels, Vol. 12, No. 2, 1998

Zolin et al.

of the apparatus. The system recorded the weight loss and temperature of the sample. The sample carrier is connected to the balance in the casing by a plug in such a way that the crucibles placed on the top of the carrier are located at the upper part of the reaction chamber. Thermocouples for determination of sample temperature are connected to the top of the carrier and are attached to the crucibles at their bottom part precisely underneath the sample layer. Purging of the apparatus before the experiments was done with nitrogen. After completion of the pyrolysis process, carried out in a nitrogen atmosphere, isothermal char combustion was initiated by introducing the oxidative gas (N2 and O2) directly into the crucible region. Mass transfer effects that could influence the reactivity measurements were minimized by using a thin layer of sample and by spreading it over the bottom of the crucible. A coal sample, typically less than 5 mg, was weighted in the crucible and placed in the reactor (TGA). Particles sieved to a size of 90105 µm were used. During pyrolysis the samples were heated at 10 °C/min up to 110 °C and held at this temperature during 15 min for moisture elimination. The samples were then heated at 45 °C/min up to a heat treatment temperature of 900 °C and held at this temperature for a holding time of 15 min. After this, the chars were cooled to the programmed combustion temperature and the oxidative gas (20 mol % O2, 0.2 atm partial pressure) was injected into the system after stabilization at the desired combustion temperature. An isothermal combustion mode was chosen, since nonisothermal combustion could imply mass transport effects as the temperature increased. Besides, due to the dependence of char reaction rate upon both burnoff and temperature, the influence of these on the reaction rate was decoupled when combustion was carried out at constant temperature. For all the coal chars, the reaction rate constant and activation energies were determined. Because of the differences in the reactivities of the chars, different combustion temperatures were used for each sample in the range 450-650 °C to ensure kinetic control. Table 1 shows the experimental program. In the following, the burnoff (X) refers to the weight loss relative to the pyrolyzed char and is calculated on a dry ashfree basis:

Figure 1. Reaction rate based on initial char weight as a function of burnoff at different combustion (oxidation) temperatures for Cerrejon coal char. Dashed lines: experimental results. Solid lines: random pore model predictions.

The reaction rate Rco is defined as the combustion rate normalized with the initial weight of the char:

reaction model (GRM).17 Of these, the RPM is the only one that can interpret the maximum in the curves of Figure 1, since it accounts for the overlapping of reacting surfaces as they grow, the reaction surface being the interface between the nonoverlapped portion of the cylindrical pores surface and the unreacted carbon.15 Other structural models, like the bifurcated pore size model,18 the overlapped grain model,19 and discrete gas-solid reaction models,20 are also able to predict the maxima in the reaction rate Rco ()dX/dt) but are not discussed here. The random pore model was used, since it is simple and easy to apply, and has proven its wide applicability in the prediction of char reaction rates. The simpler models, the volumetric reaction and grain reaction models, were used here, since they only contain one fitting parameter (either KVRM or KGRM) that could be used directly for a straightforward comparison of the reactivity between the coals. In the random pore model, the burnoff X as a function of time t under kinetic control is expressed by

Rco ) -1/Wo dW/dt ) dX/dt

X ) 1- exp(-KRPMt(1+ ΨKRPMt/4))

X ) 1 - W/Wo

(1)

(2)

where W is the mass of remaining char at time t (dry ash-free basis) and W0 the initial mass of char (dry ash-free basis).

Results and Discussion Kinetic Measurements. Cerrejon coal, having an ASTM rank between that of the other eight coals, was selected as the reference coal. The reaction rates based on initial char mass (Rco) vs burnoff (X) at different combustion (oxidation) temperatures (Tox) for the Cerrejon coal chars are shown in Figure 1. The curves show maxima, suggesting pore growth followed by collapse of the pore structure due to coalescence of neighboring pores. For fitting of the experimental data, several gassolid reaction models were investigated: the random pore model (RPM) of Bhatia and Perlmutter,15 the volumetric reaction model (VRM),16 and the grain (15) Bhatia, S. K.; Perlmutter, D. D. AIChE J. 1980, 26, 379-386. (16) Ishida, M.; C. Y. Wen. Chem. Eng. Sci. 1971, 26, 1031.

(3)

The model contains two parameters, the reaction rate constant KRPM and Ψ the structural parameter based on the initial properties of the char:

Ψ ) 4πL0(1 - 0)/S02

(4)

where L0 is the total length of the pores per unit solid volume (m/m3), S0 the initial char reaction surface area per unit volume (m2/m3), and 0 the initial porosity. The values of KRPM were determined by least-squares fitting, in which KRPM represents the parameter that minimized the difference between the experimental burnoff values and those predicted by the model. In principle, the values of Ψ could be evaluated from eq 4 (17) Szekely, J.; Evans, J. W. Chem. Eng. Sci. 1970, 26, 1091-1107. (18) Tseng, H. P.; Edgar T. F. Fuel 1989, 68, 114-119. (19) Adschiri, T.; Kojima, T.; Furusawa, T. Chem. Eng. Sci. 1987, 42, 1319-1322. (20) Sandmann, C. W. Jr.; Zygourakis, K. Chem. Eng. Sci. 1986, 41, 139.

Coal Char Reactivity

Energy & Fuels, Vol. 12, No. 2, 1998 271

Table 2. Reaction Rate Constants in Random Pore Model (RPM), Volumetric Reaction Model (VRM) and Grain Reaction Model (GRM)a reaction rate constant coal Cerrejon (Ψ ) 3.0) PSOC 1488, Dietz (Ψ ) 19.0) PSOC 1445, Blue No. 1 (Ψ ) 7.7) PSOC 1502, Hiawatha (Ψ ) 7.0) PSOC 1493, Illinois No. 6 (Ψ ) 3.6) PSOC 1451, Pittsburgh No. 8 (Ψ ) 3.0) Ulan (Ψ ) 3.6) Blair Athol (Ψ ) 7.0) PSOC 1516, Lower Kittaning (Ψ ) 2.2)

Tox (°C)

KVRM (min-1)

KGRM (min-1)

KRPM (min-1)

500 550 600 650 450 500 550 600 450 500 550 600 450 500 550 600 450 500 550 600 550 600 650 500 550 600 650 500 550 600 650 550 600 650

0.04 (0.006 60)b 0.13 (0.001 80) 0.27 (0.003 30) 0.83 (0.000 94) 0.02 (0.003 64) 0.23 (0.004 56) 0.71 (0.001 66) 0.87 (0.009 20) 0.01 (0.007 19) 0.07 (0.004 46) 0.22 (0.002 00) 0.46 (0.003 13) 0.01 (0.013 10) 0.08 (0.004 00) 0.34 (0.000 72) 0.55 (0.001 40) 0.019 (0.000 64) 0.07 (0.001 27) 0.21 (0.000 33) 0.46 (0.000 14) 0.08 (0.001 10) 0.16 (0.001 22) 0.48 (0.000 43) 0.06 (0.000 48) 0.22 (0.000 51) 0.46 (0.000 19) 1.40 (0.001 07) 0.05 (0.000 85) 0.18 (0.001 78) 0.45 (0.001 67) 1.62 (0.000 76) 0.016 (0.000 19) 0.04 (0.002 39) 0.14 (0.000 99)

0.03 (0.002 69)b 0.10 (0.000 20) 0.22 (0.000 76) 0.64 (0.002 46) 0.02 (0.003 40) 0.19 (0.001 57) 0.57 (0.000 09) 0.73 (0.004 70) 0.01 (0.002 50) 0.06 (0.001 78) 0.18 (0.000 06) 0.36 (0.000 88) 0.01 (0.005 37) 0.08 (0.001 40) 0.27 (0.000 29) 0.44 (0.000 53) 0.016 (0.00 30) 0.06 (0.000 52) 0.17 (0.000 71) 0.36 (0.000 72) 0.06 (0.000 44) 0.13 (0.000 29) 0.38 (0.001 66) 0.05 (0.001 28) 0.18 (0.002 22) 0.37 (0.000 80) 0.99 (0.004 49) 0.04 (0.000 49) 0.14 (0.000 25) 0.36 (0.000 44) 1.26 (0.000 83) 0.014 (0.000 60) 0.03 (0.000 79) 0.11 (0.003 43)

0.03 (0.002 10)b 0.08 (0.000 05) 0.17 (0.000 18) 0.51 (0.003 40) 0.009 (0.00 01) 0.08 (0.000 15) 0.24 (0.001 41) 0.31 (0.000 48) 0.007 (0.000 24) 0.03 (0.002 17) 0.11 (0.000 75) 0.22 (0.001 43) 0.008 (0.000 41) 0.05 (0.000 44) 0.16 (0.001 20) 0.27 (0.000 66) 0.012 (0.000 86) 0.04 (0.001 21) 0.13 (0.001 30) 0.27 (0.000 13) 0.05 (0.000 19) 0.10 (0.000 19) 0.30 (0.002 57) 0.04 (0.002 04) 0.14 (0.002 80) 0.27 (0.001 26) 0.84 (0.004 64) 0.02 (0.004 60) 0.09 (0.000 73) 0.22 (0.000 39) 0.77 (0.003 95) 0.012 (0.001 20) 0.02 (0.003 30) 0.10 (0.003 95)

a HTT) 900 °C, ht ) 15 min, P b Values of fit in parentheses: the values of the reaction rate O2 ) 0.20 atm. Parameter Ψ in RPM constants are determined by least-squares fitting; the criterion was the minimization of fit, defined as: fit ) 1/nΣ(Xi - Xcalc,i)2, where n is the number of data points, Xi the experimental burnoff values, and Xcalc,i the value of X calculated from the models, eqs 3, 6, and 7.

by measuring the properties of the char prior to combustion. However, since in our experiments, no char collection was done, it was decided to estimate Ψ from the experimental values of Xm, the burnoff value at which the reaction rate Rco (eq 2) is maximum. By differentiation of eq 3, Ψ can be determined in terms of Xm:15

Ψ ) 2/(2 ln(1 - Xm) + 1)

(5)

Table 2 summarizes the random pore model parameters for Cerrejon and the other coal chars. The model predictions and experimental values for Cerrejon at different combustion temperatures (Tox) are depicted in Figure 1. It is seen that RPM describes the reaction rate data well. To interpret the experimental results it was necessary to determine whether gas mixing in the reactor was fast compared to the reaction rate of the char and that the maxima seen in the curves of Figure 1 was not just the result of incomplete mixing of the oxidative gas at the beginning of combustion. For this purpose, separate experiments for different coals were performed in the TGA, revealing that only the data corresponding to the first 15-20 s of combustion had to be left out when fitting of the random pore model to the experimental data. This time period corresponded to less than 10% burnoff even for the most reactive char. Therefore, the maximum in the burnoff rates can be

Figure 2. Reaction rate based on initial char weight as a function of burnoff at a combustion temperature of 650 °C.

attributed to the pore growth followed by collapse of the pore structure. For Cerrejon and four other coals, in which experiments at Tox) 650 °C were carried out, the curves Rco vs X present a complex oxidation behavior, which the random pore model is incapable of describing. The curves show sharp maxima followed by one or two inflection points, as depicted in Figure 2. The simultaneous sample temperature measurements indicated

272 Energy & Fuels, Vol. 12, No. 2, 1998

Zolin et al.

Figure 3. Reaction rate based on initial char weight as a function of burnoff X for all coals investigated at a combustion temperature of 550 °C. Experimental results.

that the sharp initial increase in the reaction rate was a consequence of particle ignition. The characteristic shape and inflection points after the maxima of Figure 2 have been attributed to the inaccesibility of micropore surface area and the maceral content of the parent coals, in particular inertinite, which has a lower reaction rate with respect to oxygen than vitrinite.21 Vitrinite has been related to the higher reaction rate of the first part of the curves.21 The data were also fitted using the volumetric reaction model (VRM) corresponding to Ψ ) 0 in the RPM. This model assumes a homogeneous reaction throughout the particle and a linearly decreasing reaction surface area with burnoff:

X ) 1 - exp(-KVRMt)

(6)

The maximum in reaction rate Rco vs burnoff data cannot be predicted with this simple model. In the grain reaction model (GRM), the burnoff dependency on time in the kinetic control regime is expressed by

X ) 1 - (1 - KGRMt/3)3

(7)

Here it is assumed that the particle consists of equal sized spherical grains, each of which reacts according to a shrinking core model. Similarly to the RPM, the GRM assumes the reaction to occur at the surface of the unreacted char, but as in the case of the VRM, a maximum in reaction rate Rco vs burnoff cannot be predicted. Having only one parameter (KVRM and KGRM), the fit of the VRM and GRM to the experimental data was in general less accurate than the RPM. Nevertheless, the values of the reaction rate constants were significantly different between the coal chars and hence a characteristic reaction rate parameter for each of the samples was obtained. These are presented in Table 2. In the following only the random pore model (RPM) is considered. The validity of the RPM was supported by the fact that the same structural parameter Ψ could be used for all combustion temperatures (Tox). Figure 3 depicts the experimental reaction rates Rco vs X for all the coal chars at a combustion temperature of 550 °C. At this temperature, kinetically controlled conditions prevailed in the experiments.

Figure 4. Arrhenius plot of the reaction rate constant in the random pore model (KRPM) for the coal chars.

The activation energies Ea of the different coal chars were calculated from the Arrhenius plot of the reaction rate constants in the random pore model (Figure 4). To enhance the clarity of the plot, the Arrhenius plots of Ulan and Blair Athol are not included. The transition from the rectilinear region of kinetic control (zone I) to the diffusion-controlled regime (zone II) is seen at approximately 650 °C (1/T ) 1.1 × 10-3) for Cerrejon and between 550 and 600 °C (1/T ) 1.2 × 10-3 - 1.15 × 10-3 K-1) for PSOC 1488. As the pore diffusion regime is reached at high temperatures, the reaction rate constant becomes less dependent on temperature. Full penetration of oxygen does not take place and probably only the macropore region in the outer layers of the particle contributes to the evolution of surface area and thus the reaction rates of the chars. For the other samples the data points appear to be under kinetically controlled conditions. From Figure 4 the activation energies as well as intrinsic preexponential factors of Table 3 were determined. The experimental burnoff data as well as the predictions of the random pore model for all coals at Tox ) 550 °C are plotted in Figure 5. To enhance the clarity of the plot, only the model results of PSOC 1488, Cerrejon, PSOC 1451, and PSOC 1516 are included. From these curves it is seen that Dietz PSOC 1488 represents the most reactive coal followed by Hiawatha PSOC 1502 and Blue No. 1 PSOC 1445, while Pittsburgh No. 8 PSOC 1451 and Lower Kittaning PSOC 1516 are the least reactive. The reference coal, Cerrejon, has an intermediate reactivity and three fuels including the Australian coals (Ulan, Blair Athol) and Illinois No. 6 PSOC 1493 have a middle-high reactivity. The random pore model is seen to interpret the experimental data satisfactory. It is interesting that the Ulan coal at any time has a higher burnoff than PSOC 1445 and PSOC 1493 up to approximately 70% burnoff. Above this value the burnoff values of Ulan decrease significantly compared to the other two coals and even with respect to Blair Athol, which at the last stages of combustion (X > 85%) is observed to reach higher burnoff values than Ulan and PSOC 1493. On the other hand, it can be seen from Figure 3 that the reaction rate of Ulan, defined as the gradient dX/dt ()Rco) is only higher than that of PSOC 1445 and PSOC 1493 at burnoff values lower than approximately 0.4 and 0.6,

Coal Char Reactivity

Energy & Fuels, Vol. 12, No. 2, 1998 273 Table 3. Kinetic Parameters of the Coals from TGA and LFR Experiments LFRd

TGA 10-3

coal Cerrejon PSOC 1488 Dietz PSOC 1445 Blue No. 1 PSOC 1502 Hiawatha PSOC 1493 Illinois No. 6 PSOC 1451 Pittsb. No. 8 Ulan Blair Athol PSOC 1516 Lower Kitt.

ka

(s-1)

2 100000 811 8000 63 10 52 1000 42

Ea (kJ/mol) 97.4 163.3 136.2 148.6 117.7 112.5 116.8 140.5 132.4

Fp

c (g/cm3)

0.93 0.98 0.97 1.05 1.01 0.93 1.14 1.01 1.17

106q (g/cm2 2.19 12.50 3.99 6.08 3.90 1.32 4.64 3.29 0.37

s)

q/qCerrejon 1.00 5.68 1.82 2.77 1.78 0.60 2.11 1.50 0.17

103q

(g/(cm2 4.80 7.40 5.10 7.70 4.80 3.90 3.30 4.50 3.30

s))

q/qCerrejon 1.00 1.54 1.06 1.60 1.00 0.81 0.69 0.94 0.69

a Preexponential factor k in K -Ea/RTox. b Apparent density calculated from 1/F ) x /F + (1 - x )/F RPM ) ke p c c c ash, where xc is the carbon fraction of the char in the TGA experiments, Fash is the ash density ()2.25 g/cm3, ref 12) and Fc the char density in the absence of ash. Fc is determined from Fc ) Fcoal (1 - VM), where Fcoal is the coal density (ref 12) and VM the volatile matter content on a dry basis obtained in the TGA experiments. c Reaction rate, from eq 10 with particle diameter dp ) 1 × 10-4 m. d Values of reaction rate obtained by combining 6 and 12 mol % O2 environment data. Temperature, ∼1400 °C. From ref 12.

Figure 5. Burnoff as a function of time for the coals investigated at a combustion temperature of 550 °C. Dashed lines: experimental results. Solid lines: random pore model predictions.

respectively. The reaction rate of PSOC 1445 becomes higher than that of PSOC 1493 already at a 0.3 burnoff value. Comparative Ranking. TGA and LFR Experiments. The comparison of the reactivities between the low-temperature TGA experiments (450-650 °C) and high-temperature LFR experiments (∼1400 °C) is based on the values of the particle reaction rates per unit external surface area q (mass carbon/external surface area - s) for both configurations: n q ) ksPO 2,s

(8)

in the LFR experiments, and

q ) (Vp/Ap)FpRco

(9)

in the TGA experiments. The random pore model is coupled to eq 9 by differentiation of eq 3 with respect to time:

q ) (Vp/Ap)FpKRPM(1 - X)(1 - Ψ ln(1 - X))0.5

(10)

In the LFR experiments, the apparent chemical reaction rate ks combines the effect of internal burning and pore diffusion. PO2,s is the partial pressure of oxygen at the particle surface and n the apparent

reaction order. In eqs 9 and 10, Vp represents the particle volume, Fp the apparent particle density, and Ap the external surface area. The reaction rates of the LFR tests were determined by applying the single-film model coupled to the mass, momentum, and energy conservation equations of the char particles as they burn in the reactor.22,23 In the single-film model it is assumed that oxygen is transported to the outer surface of the carbon particle by diffusion and CO formed by the heterogeneous reaction leaves the boundary layer before it is oxidized to CO2. The values of q, evaluated at 25% char burnoff for the LFR experiments, were taken from refs 12 and 13. For the case of the TGA tests the reaction rates q were estimated by using eq 10 and by assuming the same mean particle diameter for all coal chars. The results are shown in Table 3. No change of particle size due to swelling was assumed for the TGA tests, due to the low heating rate (45 °C/min) used during the pyrolysis stage. Even for the LFR experiments, if heating rates of the order of 104-105 K/s were applied, the linear swelling factors reported12 showed an increase in particle diameter after pyrolysis of maximum 29% for the coal Lower Kittaning PSOC 1516. The mean value of the linear swelling factors for all the nine coals (including PSOC 1516) in the LFR reactor reveals an increase of less than 10% in particle diameter due to pyrolysis. The use of the reaction rate constants for the volumetric and grain reaction models (KVRM, KGRM) of Table 2 resulted in a similar fuel reactivity ranking as the one obtained from the reaction rates based on external surface area of Table 3. The q values were normalized with respect to the corresponding values of Cerrejon, so that q/qCerrejon ) 1. These are depicted in Figure 6. Qualitatively, a good agreement between the normalized reaction rates is observed. The low-rank Dietz PSOC 1488, having the highest normalized reaction rate, is the most reactive of the group. The high-rank coal Lower Kittaning PSOC 1516 is the least reactive. Cerrejon coal, being the reference fuel, has a normalized value of 1, regardless of the reactor type. Figure 6 shows similar rankings for both types of reactors except for the Southern Hemisphere coals (21) D’Amore, M.; Di Maio, F. P.; Lignola, P. G.; Masi, S. Combust. Sci. Technol., 1993, 89, 71-82. (22) Field, M. A.; Gill, D. W.; Morgan, B. B.; Hawksley, P. G. W. BCURA, Leatherhead, 1967; p 186. (23) Mitchell, R. E. 22nd International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1988; pp 69-79.

274 Energy & Fuels, Vol. 12, No. 2, 1998

Zolin et al.

Figure 7. Char burnoff as a function of reactor height in the LFR experiments. Figure 6. Comparative ranking of coal chars between the TGA and LFR experiments based on normalized reaction rates per external surface area.

Table 4. Reactivity Index Rxreact for the TGA and LFR Experiments TGA

(Australia), Ulan and Blair Athol. Their vertical bars in the figure cross the horizontal dashed line at the normalized rate of one. Both coal chars have a higher reaction rate than Cerrejon when measured in the TGA apparatus. The TGA presents a greater variation in reaction rates compared to the LFR, the band of the former differing by 2 orders of magnitude between PSOC 1488 and PSOC 1516 and 1 order of magnitude between the same coal chars in the LFR. This is a consequence of conducting the TGA experiments under kinetically controlled conditions (zone I). In the LFR tests (high temperature, ∼1400 °C), internal pore diffusion and external gas film resistance becomes more important (zone II) and lower variation in reaction rates among the coal chars is observed. Furthermore, the high-temperature environment of the laminar flow reactor may have annealed (deactivated) the coal chars, giving rise to a reactivity loss that resulted in lower relative differences among their reaction rates. Such a phenomenon is certainly less significant at the low char preparation temperature (900 °C) of the TGA tests. Since the determination of reactivity as char reaction rates per external surface area is based on model predictions, the comparison of the reactivities between the two reactors was also determined from the original experimental burning profiles of both configurations. The char weights were calculated as a function of height in the laminar flow reactor experiments (Figure 7). Hence, direct comparison with the burning profiles (X vs t) obtained in the TGA apparatus was not possible. Instead, from Figure 7, a normalized value of the char burnoff (X ) 1 - m/mz)6.4 cm, z ) 6.4 cm, the height where char combustion begins) with respect to Cerrejon is defined at a reactor height z of 25.4 cm. The figure was constructed from the experimental data given in ref.12 At this height, denoted here as a cutoff height, the burnoff of the reference coal char, Cerrejon, has reached 62% on a dry ash-free basis. Equivalently, a cutoff time is defined in the burnoff vs time curves of the TGA experiments (Figure 5), where the burnoff of the Cerrejon char reaches 62%. A normalized value of the burnoffs with respect to Cerrejon coal is defined at these values of cutoff height and cutoff time in Table 4. In the case of the laminar flow reactor it was assumed

LFR

coal

Xa

Rxreactc

Xb

Rxreactc

Cerrejon PSOC 1488 PSOC 1445 PSOC 1502 PSOC 1493 PSOC 1451 Ulan Blair Athol PSOC 1516

0.62 1.00 0.86 0.95 0.83 0.45 0.82 0.77 0.15

1 1.62 1.39 1.53 1.34 0.73 1.32 1.24 0.24

0.62 1.00 0.94 0.85 0.69 0.56 0.43 0.49 0.33

1 1.61 1.52 1.37 1.11 0.90 0.69 0.79 0.53

a Burnoffs determined from Figure 5 at a cutoff time of 8 min, where X ) 0.62 for Cerrejon coal char. bBurnoffs determined from Figure 7 at a cutoff height of z ) 25.4 cm, where X ) 0.62 for Cerrejon coal char. cFrom eq 11.

that at this point all the PSOC 1488 char has reacted (100%) burnoff. This approach permits a simple evaluation of the extent of combustion in both reactors directly from the experimental data. A normalized reactivity index Rxreact is defined:

Rxreact ) Xcoal/Xcerrejon

(11)

Thus, it is considered here that the higher the values of Rxreact, the more reactive the char. This integral index has to be seen with caution, however, since it only gives directions on what coal char has combusted the most compared to Cerrejon at a determined time or position in the TGA and LFR, but not the actual char burnoff rate as expressed by the differential index of eq 2. Figure 8 shows the comparative ranking for both reactors obtained by using this reactivity index. The LFR data show a lower extent of combustion for the Australian coals Ulan and Blair Athol with respect to Cerrejon, while in the case of the TGA experiments these coals achieve a higher extent of combustion than Cerrejon. Again, discounting the Southern Hemisphere coals (Australia), the qualitative ranking is similar in both reactors. The TGA experiments reveal a higher extent of combustion with respect to Cerrejon for the HVC bituminous Hiawatha PSOC 1502 than the subbituminous Blue No. 1 PSOC 1445. For these coal chars, the TGA rendered ash fractions of more or less equal size. However, the CaO content of PSOC 1502, based on the ash analysis of the coals12 is approximately

Coal Char Reactivity

Figure 8. Comparative ranking of coal chars between the TGA and LFR experiments based on reactivity index Rxreact defined by eq 11.

3 times higher than in PSOC 1445. The increase of reactivity of PSOC 1502 compared to PSOC 1445 at the low-temperature experiments of the TGA apparatus may have its explanation in the catalytic effect of mineral matter and particularly CaO, resulting in high char reactivities. In the LFR experiments the influence of CaO is minor, probably due to the sintering of this mineral at high temperatures. This effect is well-known and has been the subject of extensive research.24,25,26 Nevertheless, the model calculations of the normalized char reaction rates of Figure 6 reveal a lower reactivity of PSOC 1445 with respect to PSOC 1502 in both reactor systems. The influence of CaO cannot explain the higher reactivities of Ulan and Blair Athol at low temperatures (TGA) with respect to Cerrejon. Although the ash fractions of the Australian coals in the TGA experiments were approximately 2 times higher than that of Cerrejon, the CaO content in the raw coal ash (as received basis)12 of Ulan and Blair Athol is 1 order of magnitude lower than that of Cerrejon. Thus, no explanation to this behavior can be given at the moment on the basis of the catalytic effect of minerals. Nevertheless, demineralization of chars before combustion should be considered in future experiments. It is possible that the difference in reactivity ranking between the two reactors for the Australian coals can be explained in terms of the influence of morphology and intrinsic reaction rates of the chars. Southern Hemisphere coals, e.g. Australian, are typically richer in the inertinite maceral group than Northern Hemisphere coals, e.g. Colombian. Northern Hemisphere coals have, in contrast, a high content of the maceral vitrinite. Wall et al.27 compared the char reactivity of inertinite- and vitrinite-rich Australian coals of the same rank. The chars were prepared in a drop tube furnace (DTF) at high temperature (1600 °C). The (24) Gopalakrishnan, R.; Bartholomew, C. H. Energy Fuels 1996, 10, 689-695. (25) Gopalakrishnan, R.; Fullwood, M. J.; Bartholomew, C. H. Energy Fuels 1994, 8, 984-989. (26) Cope, R. F.; Arrington, B. A.; Hecker, W. C. Energy Fuels 1994, 8, 1095-1099. (27) Wall, T.F.; Tate, A. G.; Bailey, J. G.; Jennes, L. G.; Mitchell, R. E.; Hurt, R. H. 24th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1992; pp 1207-1215.

Energy & Fuels, Vol. 12, No. 2, 1998 275

vitrinite-rich samples had a higher reactivity, with char particles consisting mainly of thin-walled highly porous chars. A higher char density was observed for the inertinite-rich samples. Although the study was based on Southern Hemisphere (Australian) coals, the results indicate that coals with a high inertinite content upon heat treatment develop a more dense char structure compared to coals of high vitrinite content, which results in lower reactivities for the inertinite-rich fractions. This may be true both for coals from the Northern and Southern Hemisphere. It is apparent that the lower reactivities of the Australian coals Blair Athol and Ulan (inertinite-rich) with respect to Cerrejon (vitrinite-rich) found in the high-temperature LFR experiments is supported by the observations mentioned above. From a study of morphology of a suite of coal chars,28 among others two Australian and one Colombian coal of the same rank, it was found that the Australian chars were more dense and less porous than the Colombian char, both at low and high temperatures (800, 1000, 1150 °C drop tube furnace, DTF and 1400 °C laminar flow reactor, LFR, respectively). On the basis of the results discussed above, the reactivity of the more dense and less porous Australian coals expressed in terms of for example degree of burnout (dry ash-free basis) should have been lower than that of the Colombian coal at both low (