Catalytic gasification of chars

Catalytic

gasification

Josh L. Figueiredo, Faculdade

of chars*

Jose J. M. Orf%o and Maria

de Engenharia,

C. A. Ferraz

Porto, Portugal

The gasification of pure and cobalt-doped chars obtained by carbonization of wood sawdust is compared. The catalvtic action of cobalt affects both the kinetics of char gasification and the texture of the resulting porous.carbons. (Keywords:

coal; gasification;

catalysis)

The manufacture of active carbons usually involves two steps, namely: carbonization of suitable organic material under inert atmosphere; followed by partial gasification of the resulting char with an oxidizing agent such as air, stream or carbon dioxide’. Group VIII metals are known catalysts for carbon gasification*, and the texture of the active carbons is expected to be affected by the catalytic action of the meta13. In this work, the gasification of pure and cobalt-doped chars obtained by carbonization of wood sawdust were compared. The effect of cobalt on the kinetics of char gasification and on the texture of the resulting porous carbons has been ascertained. EXPERIMENTAL The cobalt-doped char was prepared from pinewood sawdust with an average particle size of 0.6 mm by washing with 10% H,S04, impregnation with 0.1 M cobalt nitrate solution and carbonization under nitrogen for 1 h at 850°C. The metal content of the material obtained was 1.6 wt%. To prepare pure char the impregnation step was omitted. Textural parameters were obtained from nitrogen adsorption isotherms determined by the conventional volumetric method. Textural studies of these chars are described in detail elsewhere4. Gasification of pure and doped chars was studied by means of a C. I. Electronics microbalance with suitable flow reactor, so that the sample weight, w, was continuously recorded. The rate of gasification, r, was calculated from the slope of the curve w =f(t): r=(-

conditions and results pertinent to these runs are shown in Table 1.

Both the initial and final rates of gasification of doped chars were determined, and these are plotted in Arrhenius coordinates in Figure 2. Activation energies of 35 and 180 kJ mole-’ were calculated for the initial and final rates, respectively. In some cases, when doped chars were gasified at low temperatures, only the slow regime was observed. The corresponding gasification rates fall on the same line as the final rates plotted in Figure 2. By comparison, the gasification of pure chars occurs in only one stage. A linear plot of carbon conversion versus time is obtained, the rate of gasification being similar to the rate determined in the final stage of cobalt-doped chars. In addition to the isothermal gasification runs, thermogravimetric curves were obtained in flowing carbon dioxide (7 x 10m5 mole s-‘) by increasing the temperature at a constant rate (4 K min-‘) The TG curves obtained show the pure and cobalt-doped chars nearly parallel, except for a slight weight uptake with the cobaltdoped char in the temperature range 500-700 K. Above 850 K, both TG curves had similar slopes. After gasification, important textural differences were observed in the carbons, summarized in Table 2. The pure char produces a microporous carbon, whilst the cobaltdoped char produces a carbon with well-developed mesoporosity.

l/w,)dw/dt=dX/dt

where: X = (w,, - w)/wOis the conversion in time t and w0 the initial weight of the dry sample. Pure carbon dioxide was used as the gasification agent at 1 atm and in the temperature range 1000-l 180 K. The flow rate was either 2 x 10m4 or 4 x 1O-4 mole s-l. RESULTS As shown in Figure 1, there are two kinetic regimes in the gasification of cobalt-doped chars: an initial stage of very fast gasification followed by a slow stage. Experimental *

This paper was presented at the conference held in London, 19-20 December 1983

OOM-2361/84/081059-02$3.00 @ 1984 Butterworth & Co. (Publishers)

Ltd

‘Carbons

and Catalysis’

0

20

40

time(min)

Figure 1 Gasification of chars. Run numbers correspond to those listed in Tab/e 1

FUEL,

1984, Vol 63, August

1059

Catalytic

gasification

Table 1 Experimental

et al.

of chars: J. L. Figueiredo

conditions

and results for the gasification

of

cobalt-doped chars with carbon dioxide at a flow rate of 2 x 1 OF4 mole s-1, except where noted. ro and ff (min-1) are the initial and final gasification rates, respectively

Table 2 Surface areas SBET

(m* g-1)

of the original and activated

carbons

and pore volumes Y (cm3 g-r)

After

20% burnoff

Original Run

T(K)

wo

1

1022 1039

8.1 7.9

0.151 0.177

1053 1062 1067 1078

8.0 8.4 8.3 7.9

0.192 0.209 0.194 0.181

1089

0.185 0.226 0.235 0.270

2.06 2.74 3.23 3.55

0.215 0.300 0.252 0.761 _ -

6.37 11.2 2.74 6.19 0.513 0.463 1.45

2 3 4

5 6 7

8 9 10

1114 1089 1128

8.1 8.3 8.1 7.4

11 12 i3a i4a 15 16 1Jb

1155 1180 1096 1138 1013 1004 1099

8.0 8.1 3.9 3.7 8.4 8.3 6.9

(mg)

ro

10-S

x ‘f

0.665 0.843 0.974

Char

SEIET

SBET

“meso

Vmicro

vmacro

Pure Co-doped

98 210

1084 287

0.03 0.34

0.44 0.14

0.55 1.87

1.19 1.32 1.77

each stage is indicative of pore diffusion resistances. In fact, for pure sawdust chars activation energies of 414 and 238 kJ mole- ’ were found in the chemical and kinetic diffusional regimes, respectively’. Pore diffusion limitations may also be expected to occur during the faster, catalytic stage. The onset of the slow regime may be explained by the loss of activity of the catalyst, since there is a potential for its oxidation according to: co +co2-Coo

a Carbon dioxide b Pure char

flow rate = 4 x lo-’

mole s-r

1C2

+co

This explanation is supported by X-ray diffraction results, showing that the cobalt is present as the metal in the doped char, and as Co0 after gasification with CO*. The TG curves obtained in flowing CO2 show only the slow gasification stage. This is understandable, the weight increase observed in the temperature range 500-700 K reflecting the conversion of Co into COO. Thus, when a temperature is reached where gasification might start, only the uncatalysed route is possible. As a consequence of these different gasification mechanisms the carbons produced exhibit different textures6. There is considerable evidence suggesting that certain metal catalysts promote the gasification of carbon by channelling through the solid2,3,7 which will result in the formation of large pores. When the catalyst particles become inactive, gasification proceeds by the uncatalysed route only, and micropores are formed predominantly3. Therefore, it may be possible to control the texture of active carbons by impregnation with catalysts prior to the activation step. ACKNOWLEDGEMENTS

1o-3 8-6 Figure 2 Arrhenius of gasification

9-O 9.4 9.8 104/T (K-') plot for the initial (0)

and final (0)

This work was carried out with support from Instituto National de Investigacao Cientifica (INIC) and Junta National de Investigacao Cientifica e Tecnologica (JNICT). A grant given by the Rector of the University of Oporto allowed presentation of this work at the meeting ‘Carbons and Catalysis’.

rates

REFERENCES DISCUSSION The results presented above show that there are two distinct kinetic regimes in the gasification of cobalt-doped chars. The first stage of very fast gasification is ascribed to the catalytic action of the cobalt. The rates of gasification measured in the second stage are of the same order of magnitude as those measured with the pure chars, and so they correspond to the uncatalysed reaction. However, the magnitude of the activation energies determined for

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FUEL, 1984, Vol63,

August

Jiintgen, H. Carbon 1977,15,273 McKee, D. W. ‘Chemistry and Physics of Carbon’(Eds. P. L. Walker, Jr. and P. A. Thrower), Vol. 16, Marcel Dekker, New Nork, 1980,p. I Marsh, H. and Rand, 8. Carbon 1971,9,63 Ferraz, M. C. A. Thesis, University ofoporto, 1983 Figueiredo, J. L., Ferraz, M. C. A. and Orfgo, J. J. M. ‘Preparation of Catalysts III’ (Eds. G. Poncelet, P. Grange and P. A. Jacobs), Elsevier, Amsterdam, 1983, p. 571 Figueiredo, J. L. and Ferraz, M. C. A. ‘Adsorption at the Gas-Solid and Liquid-Solid Interface’ (Eds. J. Rouquerol and K. S. W. Sing), Elsevier, Amsterdam, 1982, p. 239 Baker, R. T. K. J. Catal. 1982, 78, 473