Nickel Catalyzed Air Gasification of Cellulosic Chars Jump in Reactivity

Tj increased with HTT. The jump was accompanied by a transition from a rate-controlled, low reactivity, high activation energy region to a diffusion-c...
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Energy & Fuels 2001, 15, 583-590

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Nickel Catalyzed Air Gasification of Cellulosic CharssJump in Reactivity T. Ganga Devi and M. P. Kannan* Department of Chemistry, University of Calicut, Kerala 673 635, India Received July 12, 2000. Revised Manuscript Received February 5, 2001

Cellulosic chars containing nickel species have been gasified in air in the temperature range 235-400 °C. The chars were prepared from Ni2+-exchanged carboxy methyl cellulose (NiCMC) by pyrolysis in N2 at several heat treatment temperatures (HTTs) ranging from 400 to 900 °C. All the chars displayed a dramatic jump in reactivity when the gasification temperature (GT) was raised to a particular value (jump temperature, Tj). Above Tj, the rate did not vary significantly with a further increase in temperature. Tj increased with HTT. The jump was accompanied by a transition from a rate-controlled, low reactivity, high activation energy region to a diffusion-controlled, high reactivity, low activation energy region. Chars of pure CMC did not show such a jump in gasification rate with increase in GT. XRD studies showed that elemental nickel is present in chars of HTT g 500 °C, in agreement with thermodynamic calculations. A sudden enormous increase in the mobility of the catalyst species in the neighborhood of its Tammann temperature (TTam) appears to be the cause for the jump in char reactivity. This jump strongly influenced the dependence of char gasification rate on HTT, affecting the usual monotonic decrease in rate with an increase in HTT. A monotonic decrease was observed only when the GT lied below the Tj of the char. When GT g Tj, the rate did not decrease with an increase in HTT, but rather increased or remained almost invariant. On the other hand, a monotonic decrease in rate with an increase in HTT was obtained in the absence of nickel species in the char. The study emphasizes the important role of mobility of catalyst species in controlling the char reactivity.

Introduction The catalytic effectiveness of various metal species in the gasification of carbon has been well documented.1-3 Compared with alkali and alkaline earth metals, transition metals have been less extensively studied in this respect, but also function as efficient catalysts.4-14 We15 have reported that cellulosic chars containing copper species exhibited a drastic “jump” (ca. 350-fold) in the rate of gasification in air for a rise of just 5 °C in the gasification temperature (GT). The jump in reactivity * Corresponding author. (1) Walker, P. L., Jr. Shelef, M.; Anderson, R. A. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1968; Vol. 4, p 287. (2) McKee, D. W. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Thrower, P. A., Eds.; Marcel Dekker: New York, 1981; Vol. 16, p 1. (3) Wood, B. J.; Sancier, K. M. Final Report to U.S. Department of Energy, September 1983. Report No. DE-AC21-80MC14593; 1984. (4) McKee, D. W. Carbon 1970, 8, 623. (5) Gallagher, J. T.; Harker, H. Carbon 1964, 2, 163. (6) Marsh, H and Adair, R. R. Carbon 1975, 13, 327. (7) Baker, R. T. K.; Thomas, R. B.; Wells, M. Carbon 1974, 13, 141. (8) Baker, R. T. K.; Chludzinski, J. J., Jr. Carbon 1981, 19, 75. (9) Yang, R. T.; Wong, C. J. Catal. 1984, 85, 154. (10) Figueiredo, J. L.; Rivera-Utrilla, J.; Ferro-Garcia, M. A. Carbon 1987, 25, 703. (11) Moreno-Castilla, C.; Rivera-Utrilla, J.; Lopez-Peinado, A.; Fernandez-Morales, I., and Lopez-Garzon, F. J. Fuel 1985, 64, 1220. (12) Moreno-Castilla, C.; Lopez-Peinado, A.; Rivera-Utrilla, J.; Fernandez-Morales, I.; Lopez-Garzon, F. J. Fuel 1987, 66, 113. (13) DeGroot, W. F.; Richards, G. N. Fuel 1988, 67, 345. (14) DeGroot, W. F.; Richards, G. N. Fuel 1988, 67, 352. (15) Ganga Devi, T.; Kannan, M. P.; Richards, G. N. Fuel, 1990, 69, 1440.

was accompanied by a transition from a high to a low value for the activation energy of the reaction. This was attributed to a physical change occurring in the char during gasification, viz., an increased mobility of the catalyst species (Cu) in the vicinity of its Tammann temperature (TTam).16 A similar jump was reported earlier by Moreno-Castilla et al.12 for copper-catalyzed gasification of a Spanish lignite char in air. We have embarked on a systematic search for other metal systems that are capable of exhibiting such jump phenomenon in cellulosic char gasification. We have found that nickel, another transition metal, also behaves like copper, which we report in this paper. Experimental Section Materials. Carboxy methylcellulose (CMC) (Sigma, 0.71 mequiv H+ per gram) was used as the substrate for supporting nickel as in our previous study with copper.15 About 25 g of CMC was degassed under water at 15 Torr, transferred to a column and percolated slowly with 1.8 L of 0.1 N solution of nickel acetate (Baker analyzed). The column was then washed free of all adsorbed ions using 5 L of water and the product air-dried. The amount of Ni in the nickel-exchanged CMC (NiCMC) thus obtained was calculated to be 18206 ppm (Table 1) from the ash (2.317%) determined by thermogravimetric analysis at 550 °C in oxygen. X-ray Diffraction (XRD) Measurements. The XRD technique was used to determine the nature of the catalyst species (16) Baker, R. T. K. J. Catal. 1982, 78, 473.

10.1021/ef0001540 CCC: $20.00 © 2001 American Chemical Society Published on Web 04/06/2001

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Pyrolysis of Cellulosic Samples. On heating from room temperature to the desired HTT under N2, CMC, and NiCMC undergo pyrolysis producing chars with differential thermogram (DTG) peaks at 385 and 383 °C respectively (Table 1). Thus, nickel did not affect the temperature of peak pyrolysis weight loss unlike the alkaline earth metal calcium (which increased it to 393 °C19) and the alkali metal potassium (which lowered it to 349 °C17). The char yield was enhanced by nickel (Tables 2 and 3), but was not as high as that caused by calcium (e.g., 21.6% at HTT ) 400 °C19) and potassium

(e.g., 31.1% at HTT ) 400 °C17). These effects of nickel were similar to that reported for copper-exchanged CMC (CuCMC).15 XRD Studies. The chars prepared from NiCMC at different HTTs ranging from 400 to 800 °C were studied by XRD to identify the chemical state of the nickel species present in them. The normal residence time used for preparing the chars was 15 min unless otherwise mentioned. The results are presented in Figure 1. Crystalline elemental nickel was present in all chars prepared at HTT 500 °C and above. The oxide and carbide were absent in these chars. No peak was seen at all for the char prepared at HTT 400 °C even with a residence time as high as 120 min. The peaks were assigned according to powder diffraction file.21 Temperature Dependence of Char Gasification RatesJump Phenomenon. Isothermal gasification was studied at different GTs (in the range 230-400 °C) using chars prepared from NiCMC at several HTTs ranging from 400 to 900 °C and the results were compared with the behavior of untreated CMC chars under similar conditions. The values of Rg(max, daf) at several GTs for cmc and NiCMC chars prepared at different HTTs are given in Tables 2 and 3, respectively. The CMC char showed a normal (i.e., exponential) increase in the maximum rate of gasification with an increase in GT. (See the data for CMC chars of HTT 500 and 800 °C in Table 2). However, the NiCMC chars exhibited the normal increase in rate only initially and then at a particular GT the rate suddenly increased enormously to a value that did not change significantly with further increase in GT. (See the data for NiCMC chars of HTT 400, 500, and 600 °C in Table 3). The lowest GT corresponding to this jump in reactivity is referred to as the jump temperature Tj of the char, as adopted earlier.15 The data show that the rate of gasification of NiCMC char of HTT 400 °C, for instance, increased by about 140 times when the GT was raised by just 5 °C from 240 to 245 °C, the Tj of this char. The rate remained almost unaltered above 245 °C. The NiCMC chars of HTTs 500 and 600 °C also showed a similar jump in reactivity at Tj’s approximately equal to 350 and 370 °C, respectively. Tj thus increases with an increase in HTT. In addition, the magnitude of jump decreases with an increase in HTT. Figure 2 illustrates the rate-time profiles for the gasification of the chars at GT e Tj (The profiles for GT > Tj are similar to that for GT ) Tj). The initial negative values for Rg at GT < Tj indicate that the chars gain weight during the first two minutes after the admission of O2 onto the char, which is evidently due to the adsorption of O2 by the char. The chars then begin to lose weight and the gasification proceeds with a low, approximately constant rate causing generally a low fractional conversion (Fg ) 0.1-0.3) of carbon during the entire period of the reaction (i.e., 30 min). However, at GT g Tj, the gasification proceeded very fast reaching the maximum rate in about 2 min and leading to almost complete conversion in just 3 min. The initial gain in weight is apparently absent at GT g Tj. An examination of the weight loss data collected at intervals of 6 s,

(17) Kannan, M. P.; Richards, G. N. Fuel, 1990, 69, 999. (18) Jenkins, R. G.; Nandi, S. P.; Walker, P. L., Jr. Fuel 1973, 52, 288. (19) Ganga Devi, T.; Kannan, M. P. Fuel 1998, 77, 1825.

(20) De Groot, W. F.; Shafizadeh, F. J. Anal. Appl. Pyrolysis 1984, 6, 217. (21) Powder Diffraction File, JCPDS, International Center for Diffraction Data, 1979.

Table 1. Metal Content of the Cellulosic Samples CMC and NiCMC

sample

concentration of metal (ppm)a Na K Mg Ca Fe

CMC 0 NiCMC

0

0

5

Ni

ash (%)

15 0 0 18206 2.317

pyrolysis peak temp (°C) 385 383

a Metal content of CMC was determined by ICAPS and that of NiCMC from the ash.

in chars prepared at different heat treatment temperatures (HTTs). The procedure was described in detail elsewhere.15 Measurement of Char Reactivity. About 10 mg of NiCMC was heated to 120 °C in flowing N2 (flow rate, 80 mL min-1) and held for 10 min to obtain dry weight. It was then heated to the desired HTT in N2 at a heating rate of 100 °C min-1 and held at this temperature for a residence time of 15 min. The char produced was then brought to the desired GT at 100 °C min-1 and held in N2 for 10 min to attain temperature equilibrium. N2 was then replaced by dry air (breathing quality, 22% O2, flow rate: 80 mL min-1), and the char was gasified for 30 min. Air was then cut off and the char maintained under N2 at the GT for 10 min. The gasification time was set to 30 min based on our earlier experience that a significant/measurable extent of gasification of most cellulosic chars occurred during this period. The in situ pyrolysis and gasification processes were carried out in succession in a programmable Perkin-Elmer TGS-2 thermobalance as described elsewhere.15, 17 The data were collected at intervals of 6 s using a Hewlett-Packard data acquisition and control system (model 3497A) interfaced to the TGS-2. The rate of gasification (Rg) at any time t is given by

Rg ) dFg/dt ) -(1/Wo) (dWt/dt) Here Fg is the fractional conversion of the char in time t, Wo the initial weight of the char, and Wt the weight at time t. The maximum rate Rg(max) was then determined from the rate-time data prepared at an interval of 1 min (i.e., dt ) 1 min) and expressed on a dry-ash-free (daf) basis by accounting for the ash in Wo char as shown below:

Rg(max,daf) ) Rg(max) [(% char)/(% char - % ash)] Rg (max, daf) was used as a measure of the reactivity of the chars in this study, as adopted by earlier workers11,12,18 and by us in our earlier papers.15,17,19 The values of Rg(max, daf) reported in this paper are based on single measurements. The experiments were, however, repeated in several cases and the reproducibility was found to be always within 5%.

Results

Gasification of Cellulosic Chars

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Table 2. Maximum Gasification Rate, Rg (max), at Several Gasification Temperatures (GTs) for CMC Chars Prepared at Different Heat Treatment Temperatures (HTTs) HTT (°C)

% char

400 500 600 700 800

14.4 10.7 9.7 8.6 9.0

380 0.93

390

400 4.50 1.46 0.34 0.16 0.11

1.27

Rg(max) x 102 (min-1) at GTs (°C) given below 410 450 480 490 2.03

5.30

500

510

620

1.82

2.72

25.50

10.81 1.17

1.55

Table 3. Maximum Gasification Rate (daf basis), Rg (max, daf), at Several Gasification Temperatures (GTs), and Jump Temperature, Tj, for NiCMC Chars Prepared at Different Heat Treatment Temperatures (HTTs) HTT (°C) % char Tj (°C)

230

240

400 500 600 700 800 900

0.22 0.00 0.00 0.00 0.00 0.00

0.45 0.00 0.00 0.00 0.00 0.00

17.6 14.5 12.7 12.0 11.6 11.6

245 350 370

245

250

Rg(max,daf) x 102 (min-1) at GTs (°C) given below 260 290 300 320 350 360 365 370

63.40 78.74 67.64 65.40 0.00 0.00 0.00 0.00

375

380

390

400

61.38 60.40 53.37 52.30 2.11 50.65 76.10 70.58 0.59 1.28 4.45 7.08 10.02 82.63 61.35 64.14 77.90 75.52 70.49 66.87 6.12 65.39 0.10 0.27

Figure 1. XRD of chars from NiCMC at various HTTs (residence time given in parentheses): (curve a) 400 °C (60 min), (curve b) 400 °C (120 min), (curve c) 500 °C (15 min), (curve d) 600 °C (15 min), (curve e) 700 °C (15 min), and (curve f) 800 °C (15 min).

however, reveals that there is an initial weight gain by the char at GT g Tj. But, unlike at GT < Tj, this weight gain persisted only for a few seconds when fast gasification commenced. The jump in reactivity, accompanied by a rate that is insensitive to a further increase in GT, produced a sharp break in the Arrhenius plots. This indicates a transition from a low reactivity, high activation energy (E1) region to a high reactivity, low activation energy (E2) region. Figure 3 illuminates this phenomenon for NiCMC chars of HTTs 400 and 600 °C. (The plot related to NiCMC char of HTT 500 °C is not shown due to insufficient number of data, but the jump is apparent from Table

3). The estimated values of apparent activation energies E1 and E2 below and above Tj respectively are given in Table 4. Above Tj, the activation energy of the gasification of NiCMC chars is nearly zero, whereas below Tj it is somewhat close to that of the nickel-free CMC chars. Effect of HTT on Gasification Rate. An examination of the data (along columns) in Tables 2 and 3 reveals the dependence of gasification rate on HTT. The results are illustrated in Figure 4. The reactivity of chars from pure CMC decreases monotonically to approximately zero as the HTT increases. The reactivity of chars from nickel-treated samples also decreased with

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Figure 2. Rate-time profiles of gasification of NiCMC chars in air. (A) GT < Tj: (curve a) HTT ) 400 °C, GT ) 240 °C; (curve b) HTT ) 500 °C, GT ) 320 °C; (curve c) HTT ) 600 °C, GT ) 350 °C. (B) GT ) Tj: (curve a) HTT ) 400 °C, GT ) 245 °C; (curve b) HTT ) 500 °C, GT ) 350 °C; (curve c) HTT ) 600 °C, GT ) 370 °C. At GT > Tj the profiles are similar to (B).

Figure 3. Arrhenius plots of chars from (curve a) CMC (HTT ) 500 °C), (curve b). NiCMC (HTT ) 400 °C), and (curve c) NiCMC (HTT ) 600 °C). Table 4. Catalyst Species, Tammann Temperature (TTam) Taken as 0.4 Times the Melting Point in Kelvin of the Catalyst, Jump Temperature (Tj), and Activation Energies of Gasification below (E1) and above (E2) Tj for NiCMC Chars HTT % catalyst in TTam Tj E1 E2 sample (°C) char the char (°C) (°C) (kcal mol-1) (kcal mol-1) CMC NiCMC NiCMC NiCMC

500 400 500 600

10.7 17.6 14.5 12.7

NiO Ni Ni

632 245 417 350 417 375

22.32 36.71

-1.63

31.06

1.72

increasing HTT and became almost zero near HTT 500 °C at GTs 240 and 245 °C. Nevertheless, at all other GTs the char gasification rate increases considerably up to a certain HTT and then decreases sharply to a low value on a further increase in HTT. This complex dependence of char reactivity on HTT is significantly different from the normally reported decrease in rate with an increase in HTT.

Figure 4. Effect of HTT on gasification rate, Rg (max, daf), in air. (A) (curve a) CMC char at GT ) 400 °C and (curve b) NiCMC char at GT 240 °C. (B) NiCMC chars at GTs (curve c) 245, (curve d) 350, (curve e) 375, and (curve f) 400 °C.

Discussion Catalyst Species in NiCMC Chars. Upon pyrolysis, NiCMC undergoes decarboxylation releasing the nickel counterions. The latter might appear at least partly as carbonate and/or hydroxide by reaction with CO2 and/ or H2O produced during the pyrolysis. Differential thermal analysis (DTA)22 of NiCO3 containing some Ni(OH)2 showed two endotherms at 200 and 365 °C corresponding to the decompositions of Ni(OH)2 and NiCO3, respectively, to form NiO. In the presence of carbon, these decompositions may occur even at lower temperatures, as found with CaCO3 by McKee.23 Free energy calculations (not shown) indicate that the reduction of NiO to Ni by carbon occurs in the neighborhood of 500 °C. Thus, the potential gasification catalysts that we would expect in NiCMC chars are NiO below HTT 500 °C and Ni at and above 500 °C. The expectation was borne out to be true with chars of HTT g 500 °C which showed Ni peaks in XRD measurements (Figure 1). However, NiO peaks were not detected in the XRD measurements for the char of HTT 400 °C even when the residence time was increased to 120 min, in contrast to the expectation. We note that XRD fails to detect crystals smaller than about 4 nm.24 By virtue of its origin from ion-exchanged sample, the NiO produced is in a highly dispersed state in the char. Moreover, due to its larger molecular mass and size, NiO is less mobile than Ni. These two factors may not have provided favorable conditions for sufficient crystal growth of NiO at HTT 400 °C to be detected by XRD. We had a similar experience with CaO produced during the pyrolysis of (22) Web, T. L.; Kruger, J. E. In Differential Thermal Analysis; Mackenzie, R. C., Ed.;Academic Press: New York, 1970; Vol. 1 (Fundamental Aspects); p 330. (23) McKee, D. W. Fuel 1980, 59, 308. (24) Klug, H. P.; Alexander, L. E. In X-ray Diffraction Procedures, 2nd ed.; Wiley: New York, 1974; p 687.

Gasification of Cellulosic Chars

calcium exchanged CMC.19 Though formed at a much lower temperature (ca. 500-550 °C), CaO could be detected by XRD only in chars prepared at HTT ) 1000 °C with residence time ) 60 min. The Jump Phenomenon. Not many studies of the present kind have been reported previously on nickelcatalyzed carbon gasification making a direct comparison of our results difficult. However, as already mentioned, chars prepared from copper-impregnated Spanish lignite12 and CMC15 showed a similar jump in reactivity followed by a significant lowering of activation energy. According to the results of this investigation (Tables 3 and 4, Figure 3), upon increasing GT, the maximum rate of gasification of the chars prepared from untreated CMC varies exponentially with absolute temperature obeying the Arrhenius equation. However, the chars of nickel-treated samples showed a sudden increase (jump) in the rate at a particular reaction temperature (Tj). This implicates that the jump in rate is not a temperature effect in the “Arrhenius” sense, but an effect associated with the behavior of the catalyst species. The mobility-mechanism that has been proposed for copper catalysis15 seems to explain the present results. The state of contact of catalyst with carbon is decisive in determining catalytic activity in carbon gasification. In a solid, the particles (atoms/ions/molecules) acquire significant mobility near the Tammann temperature (TTam) of the solid.16 This corresponds to a twodimensional melting of the surface of the solid, or in other words, to a transition from a solid to a liquid-like behavior of the surface. (TTam is equal to a fraction (ca. 0.3 to 0.5) of the melting point in kelvin of the solid.16) The mobility of the catalyst particles in char (here, NiO or Ni) would facilitate the catalyst-carbon interaction and the diffusion of oxygen to the catalyst-carbon interface promoting gasification. In addition, the motion of the catalyst particles is generally accompanied by the formation of irregular channels and/or localized, deep pits in the carbon substrate,4,25-28 generating more number of active sites on the carbon surface, which also would enhance the gasification rate. Upon increasing GT, the nickel species in the char acquires a quasi-liquid state near its TTam. Consequently, the mobility and hence the catalytic effect of nickel species will suddenly develop enormously near the TTam. This leads to a sudden transition from a low reactivity and high activation energy region to a high reactivity and low activation energy region as seen in Figure 3. As HTT increases, the crystallite size of the catalyst species in the char also increases due to increased sintering as shown previously with copper species.15 Mobility of the catalyst species decreases rapidly with increasing crystallite size.4 Thus, larger particles reach the Tammann condition only at a higher temperature than smaller ones. This accounts for the observed increase in the value of Tj as well as the lowering of the magnitude of jump with increase in HTT (Table 3, Figure 3).

Energy & Fuels, Vol. 15, No. 3, 2001 587

The explanation of jump phenomenon in terms of Tammann effect, implies that Tj of a char should be close, if not equal (due to the particle size effect4), to TTam of the catalyst species in the char. By definition, TTam of a solid lies in a wide range between 0.3 and 0.5 times its melting point (mp) in kelvin.16 The jump temperatures of the chars of HTT 500 and 600 °C are 350 and 370 °C, respectively, and they correspond closely to the Tammann temperature taken as 0.4 times the mp of the catalyst species, Ni (mp ) 1726 K), in these chars. However, the Tj value (viz., 245 °C) of the char of HTT 400 °C containing NiO (mp ) 2263 K) lies much lower than even the lowest limit, viz., 0.3 times the mp of NiO (The observed Tj corresponds to about 0.23 times the mp of NiO). We note that NiO species in the char of HTT 400 °C could not be detected by XRD (Figure 1), which indicates its highly dispersed (and hence highly active) state in the char. We also note that Tj increases with crystallite size of the catalyst (Table 3, Figure 3), which implies that a low value of Tj is equally possible for a char containing finely dispersed catalyst as in HTT 400 °C char. We tentatively postulate that the low jump temperature of HTT 400 °C char containing NiO, is due to the extremely small crystallite size of NiO particles. Moreover, that a satisfactory correspondence between jump temperature and Tammann temperature may be expected (only?) if the catalyst crystallites have at least the X-ray detectable size. On the Mechanism of Gasification. A cyclic oxidation and reduction of the catalyst species has been widely suggested as a possible mechanism for catalytic oxidation of carbon.1,2,29-32 Only such metals whose oxides can be reduced by carbon to metal or to a lower oxide at the gasification temperature alone can function as active catalysts under this mechanism, the reduction of the oxide being the rate-determining step. From XRD studies (Figure 1) and thermodynamic calculations we have noticed the following facts: 1. The reduction of NiO to Ni by carbon can occur only in the vicinity of 500 °C and above. 2. Ni is the only species present in NiCMC chars of HTT g 500 °C, but NiO is to be expected in chars of lower HTTs. They indicate that the reduction of NiO to Ni by carbon cannot occur at the low GTs (235-400 °C) used in this study. Therefore, a catalytic cycle involving the reduction of NiO is very unlikely under the gasification conditions used in this investigation. This is in contrast with copper catalysis,15 where the reduction of CuO to Cu by carbon is thermodynamically feasible even at room temperature. Therefore, a catalytic cycle involving the formation of weakly bound oxygen atom32 on the surface of the catalyst species (step-1, fast), which is subsequently transferred to the carbon (step-2, slow and rate-determining), appears to be the possibility in nickel catalysis:

2X + O2 f 2X‚‚‚O

(1)

X‚‚‚O + C f X + CO

(2)

Here, X represents the catalyst species, NiO or Ni and (25) Hennig, G. J. Inorg. Nucl. Chem. 1962, 24, 1129. (26) Thomas J. M. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1965; Vol. 1. (27) Baker, R. T. K. Catal. Rev. Sci. Eng. 1979, 19 (2), 161. (28) Harris, P. S.; Feates, F. S.; Rueben, B. G. Carbon, 1974, 12, 189.

(29) Kroger, C.; Neumann, B.; Fingas, E. Z. Anorg. Chem. 1931, 197, 121. (30) Milner, G.; Spivey, E.; Cobb, G. W. J. Chem. Soc. 1943, 578. (31) Amariglio, H.; Duval, X. Carbon, 1966, 4, 323. (32) Holstein, W. L.; Boudart, M. Fuel 1983, 62, 162.

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Figure 5. Effect of HTT on the gasification rate, Rg (max, daf), in air for (curve a) KCMC char at GT 400 °C, (curve b) CaCMC char at GT 300 °C, and (curve c) CuCMC char at GT 300 °C.

X‚‚‚O represents an oxygen atom dissociatively adsorbed on the catalyst site. The catalyst does not undergo a change in the oxidation state. The jump in gasification reactivity followed by a change in activation energy with increasing GT (Figure 3, Table 4) suggests a change in the rate-determining step near Tj. In the low-temperature region below Tj, the abstraction of oxygen atom by carbon from the catalyst surface (step-2) determines the overall rate leading to an apparent activation energy (E1) of 30-35 kcal mol-1. E1 is slightly higher than the activation energy for untreated CMC char gasification (Table 4). As the reaction temperature approaches the jump temperature, however, the high mobility of the catalyst species leads to a much more efficient contact/interaction of the catalyst with carbon, rendering step-2 very fast and no longer rate-determining. Under this situation, the diffusion of oxygen toward the catalyst-carbon interface (step-1) determines the rate, marking the onset of diffusion control of rate (rather than chemical control), associated with a significant lowering of the activation energy (Figure 3, Table 4). Effect of HTTsInfluence of Jump Phenomenon. Several authors18,33-37 previously studied the effect of HTT on the gasification of carbon and observed a generally monotonic decrease in reactivity with increasing HTT. We earlier studied the individual catalytic effectiveness of potassium,17 calcium,19 and copper species15 in the air gasification of cellulosic chars and obtained significantly different reactivity vs HTT plots, which are included in this paper as Figure 5 for a comparison with the present results shown in Figure 4. With KCMC chars,17 the rate of gasification (GT ) 400 °C) decreased in the HTT range 400-650 °C, above which it dramatically increased in contrast with a monotonic decrease (Figure 5, curve a). This novel reversal of reactivity trend was attributed to the formation of a new and more effective catalyst species in the char by the interaction of elemental potassium (an intermediate formed only above HTT 650 °C) with the carbon matrix. In the case of CaCMC chars,19 the rate (33) Radovic, L. R.; Steczko, K.; Walker, P. L., Jr.; Jenkins, R. G. Fuel 1983, 62, 849-856. (34) Radovic, L. R.; Walker, P. L., Jr.; Jenkins, R. G. J. Catal. 1983, 82, 382-394. (35) Smith, W. R.; Polley, M. H. J. Phys. Chem. 1956, 60, 689-691. (36) Kasaoka, S.; Sakata, Y.; Shimada, M. Fuel 1987, 66, 697-701. (37) McCarthy, D. J. Carbon 1981, 19, 297.

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(GT ) 300 °C) decreased in the HTT range 400-500 °C, changed very little from 500 to 700 °C and then became negligible above HTT 800 °C (Figure 5, curve b). Here, a change in the chemical state of the catalyst from CaCO3 and/or Ca(OH) 2 to more active CaO was noticed at around HTT 500-550 °C, which prevented a monotonic decrease in rate from HTT 500 to 700 °C. A further increase in HTT caused severe deactivation of the catalyst (CaO) due to sintering leading to a sharp decrease in rate. With CuCMC chars,15 which exhibited jump phenomenon, an increase in HTT from 400 to 700 °C caused little change in the rate of gasification (GT ) 300 °C), but the rate decreased significantly above HTT 700 °C (Figure 5, curve c). XRD studies15 have shown that elemental copper is the only species present in chars of HTT 400-900 °C. This indicates that a chemical change, as found with KCMC and CaCMC samples, is not responsible for the above deviation from a monotonic decrease in rate. We have shown that the jump temperatures of the CuCMC chars of HTT up to 700 °C lied much below the GT used (i.e., 300 °C), whereas the jump temperatures of chars above HTT 700 °C lied above the GT. The catalyst particles are exceptionally mobile and hence active near Tj and this physical phenomenon was suggested as the reason for the little change observed in rate in the HTT range 400-700 °C. This hypothesis is verified thoroughly in the present work using nickel catalyst, which also displays the jump phenomenon as seen above. Curve a in Figure 4A shows that the reactivity of the chars of pure CMC (GT ) 400 °C) decreases continuously to approximately zero as HTT increases from 400 to 900 °C. This is due to a progressive decrease, with an increase in HTT, of reactive aliphatic fraction in the char. A similar behavior is normally expected for catalyst-treated chars also because, here, in addition to a decrease in the aliphatic content of the char, the catalyst is progressively deactivated due to an increase in its crystallite size (via sintering).15 This normal monotonic decrease in char reactivity would not be however observed, if the catalyst undergoes a chemical transformation to a more active form on increasing HTT, as found with potassium17and calcium-treated19 CMC samples. Similarly, if it undergoes a physical change such as enhancement in mobility at the gasification temperature, as indicated by the preliminary results obtained with copper-treated15 CMC samples (Figure 5). With NiCMC, the rate of gasification of the char of HTT 400 °C is quite small at GT ) 240 °C and, as found with CMC (Figure 4A, curve a), the rate decreases monotonically to almost zero on increasing the HTT (Figure 4A, curve b). Nevertheless, when GT is raised (by just 5°) to 245 °C, the rate at HTT 400 °C increases significantly or jumps by ca. 140-fold, but falls sharply to zero at HTTs 500 °C and above (Figure 4B, curve c). As already mentioned, XRD studies and thermodynamic calculations showed that Ni is the only species present in NiCMC chars of HTT g 500 °C, whereas NiO is present in chars of lower HTTs. An increase in HTT from 400 to 500 °C is thus accompanied by a change in the chemical state of the catalyst species in the char from NiO to Ni. Therefore, it would appear that the sudden decrease in rate at GT ) 245 °C, upon increasing

Gasification of Cellulosic Chars

HTT from 400 to 500 °C (Figure 4B, curve c), is due to the above change in the chemical state of the catalyst. However, we note that at higher GTs the reactivity of chars of HTTs g 500 °C containing Ni is almost equal to or even higher than the reactivity of HTT 400 °C char containing NiO (see Figure 4B, curves d-f). This fact shows that the catalytic effectiveness of Ni is as good as that of NiO (if not better). Thus, the sharp fall in the rate at 245 °C, on raising HTT from 400 to 500 °C (Figure 4B, curve c), is not due to the above change in the chemical state of the catalyst. We note further that, the HTT range, where the catalyst remains active causing little change in char reactivity on increasing HTT, increases at higher GTs. In other words, as the GT increases, the sharp fall in the rate of gasification on increasing HTT shifts to higher and higher HTTs. For instance, at GTs 350, 375, and 400 °C, the char reactivity is high and approximately invariant in the HTT ranges 400-500, 400-700, and 400-800 °C, respectively. The rate decreases sharply only beyond these ranges, indicating a clear dependence of the HTT curve on the gasification temperature chosen (Figure 4B, curves d-f). A careful analysis of the data would reveal the dominant role of the jump phenomenon behind this complex behavior. The jump temperature Tj of HTT 400 °C char is 245 °C (Table 3). Hence, whenever GT< 245 °C, the catalyst particles (NiO) in this char will be less mobile and consequently less active leading to low char reactivity. Further, the Tj’s of all chars of HTT > 400 °C lie above 245 °C (since Tj increases with HTT; see Table 3). Thus, at GT ) 240 °C, a monotonic decrease in the rate is to be expected on increasing the HTT, due to the progressive deactivation of the catalyst species via sintering (Figure 4A, curve b). However, at GTs 245 and 320 °C, the catalyst species in the HTT 400 °C char will remain highly mobile and thus catalytically active, attributing high reactivity to this char since GT g Tj of this char (see Table 3). However, the above GTs lie much lower than the Tj (350 °C) of HTT 500 °C char and the Tj’s of all other chars of higher HTTs. Consequently, the catalyst particles (Ni) in these chars will remain less mobile and thus catalytically passive, causing a sharp decrease in rate with an increase in HTT beyond 400 °C, as seen in Figure 4B, curve c, for GT ) 245 °C (similarly for GT ) 320 °C; see Table 3). When the GT is raised to 350 °C, the situation changes. Now the GT is higher than the Tj of HTT 400 °C char (i.e., 245 °C) and equal to the Tj of HTT 500 °C char, but lower than the Tj’s of chars of HTT g 600 °C. At this GT, therefore, we expect high reactivity for all chars up to HTT 500 °C and then a sharp decrease beyond this HTT, and we find this reactivity trend experimentally (Figure 4B, curve d). A similar trend is observed at higher GTs, 375 and 400 °C also (Figure 4B, curves e and f). It is thus clear that a monotonic decrease in rate with an increase in HTT need not be observed even if there is no change in the chemical state of the catalyst species in chars. Physical change such as the jump phenomenon can cause inflections in the rate-HTT curves as we found in this study. Tammann effect, which renders high mobility to the catalyst species in char, manifests at and above the jump temperature of the char. For instance, HTT 400

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°C char exhibits Tammann effect (as indicated by the jump in gasification rate) at and above 245 °C. This would mean that the catalyst particles (NiO) in this char show high mobility during the gasification in air at and above 245 °C. At the same time, we note that NiO could not be detected by XRD in the char prepared at HTT 400 °C under N2 atmosphere, even after applying a residence time of 120 min. This implies a poor crystal growth during the heat treatment under N2 at 400 °C due to low mobility of NiO species. These apparently contradicting results lead to the question: Why is Tammann effect silent while preparing the char at 400 °C under N2 atmosphere, but prominent during gasification in air even at as low a temperature as 245 °C? It appears that the mobility of NiO particles is considerably high when heated under oxidizing atmosphere, but low under inert atmosphere. Further investigation is required to resolve this issue. We tentatively postulate that, on heating the char in air, the catalyst particles are “sucked” away by the oxygen molecules forming the catalyst-oxygen-adsorbed species as intermediate. These adsorbed species can reduce the catalyst-catalyst interaction energy and thereby decrease the bond population between the catalyst particles rendering enhanced mobility to the catalyst particles. [It is worth mentioning here that Halachev and Ruckenstein38,39 have shown by quantum mechanical calculations that the adsorbed oxygen atom (electron acceptor species, in general) destabilizes Pt-Pt bonds in platinum clusters.] Such an interaction with catalyst particles does not exist under nitrogen atmosphere. This postulate would imply that the magnitude of the mobility of the catalyst species and hence the jump in char gasification rate would depend on the extent of interaction of O2 with the catalyst species. Consequently, the jump may be silent or less prominent with certain metal systems and prominent with certain others. Investigations in this line are in progress. Summary and Conclusions The dependence of gasification rates of nickel-treated CMC chars in air was examined (1) as a function of GT fixing the HTT of char and (2) as a function of HTT fixing the GT and shown quite complex. The cause of this complex behavior has been probed in detail. The results emphasize the important role of catalyst mobility in the char matrix during the gaisfication process. The mobility increases sharply causing a jump in the rate of gasification at a particular GT called the jump temperature, Tj. Tj normally lies close to the Tammann temperature TTam of the catalyst species in the char indicating a possible connection between jump in reactivity and Tammann effect. Tj of a char increases with HTT due to an increase in the crystallite size of the catalyst species, which implies that Tj can be unusually low if the catalyst particles remain highly dispersed as in HTT 400 °C char. Since Tj of a char depends on its HTT, the jump phenomenon has a dramatic influence on the nature of the rate-HTT curves. The shape of these curves depended strongly on whether the GT is greater than or (38) Halachev, Ruckenstein, E. Surf. Sci. 1981, 108, 252. (39) Halachev, Ruckenstein, E. J. Catal., 1982, 73, 171.

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less than the Tj of the chars involved. If GT < Tj of the chars involved, a monotonic decrease of rate due to the progressive sintering of the catalyst with an increase in HTT may be expected. On the other hand, if GT g Tj of the chars involved, the sintering effect pales into insignificance and Tammann effect comes into prominence leading to a high gasification rate that changes only slightly with a further increase in HTT. Thus, it is not only the chemical transformation of the catalyst

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species that influences the shape of the rate-HTT plots17,19 but also physical phenomena, such as the Tammann effect, can affect the shape significantly. Acknowledgment. The authors are grateful to Prof. G. N. Richards, Director, Wood Chemistry Lab, University of Montana, for providing the experimental facilities during their visit to USA. EF0001540