Catalytic gasification of gas-coal char in CO2

Catalytic gasification of gas-coal char in CO2

Fuel Vol 74 No. 3, pp. 456458, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved OOl6-2361/95/$10.00+0.00 Cata...

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Fuel Vol 74 No. 3, pp. 456458, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved OOl6-2361/95/$10.00+0.00

Catalytic Shufen

gasification

Li and Yuanlin

of gas-coal

char in CO2

Cheng

Department of Chemical Engineering, Tianjin University, Tianjin 300072, (Received 2 March 1994)

P.R. China

CO, gasification of a gas-coal char catalysed by Na,C03 and K&O, was studied using a fixed-bed reactor at 79S102o”C and 0.2 MPa. The gasification rate increased with increasing temperature. With increasing addition of Na2C03 in the range 9-25 wt% and K&O, in the range 5-20 wt% the rate increased sharply. The catalytic effect of K&O, was greater than that of Na,C03. The effect of catalyst loading on rate showed saturation above 25 wt% for Na,C03 and 20 wt% for K&O,. The reaction rate constants and activation energies were measured in the ranges 850-960°C and catalyst loading 1-16 wt%. In this catalytic gasification reaction a compensation effect appeared; the isokinetic temperatures were 1289°C for Na2C03 and 1466°C for K&O,. (Keywords char; gasification; catalysis)

The catalytic gasification of carbonaceous materials has been studied extensively in recent yearsle3. Various

studies have shown that alkali metals are very effective in H,O and CO2 gasification of carbon. The factors that influence the reaction rate are temperature, pressure of CO and CO,, type of carbon, ash content, catalyst concentration and space velocity. The objectives of the present study were to investigate the effects of Na,CO, and K&O3 loading over a wide range on the rate of the char-CO, reaction and to determine the change in rate during gasification. EXPERIMENTAL Char samples were prepared from Wu Tai gas coal by pyrolysis at 900°C for 1 h in nitrogen. The char analysis is given in Table 1. The char was sieved and the 2-4mm fraction was retained and stored under vacuum. The catalyst (Na,CO, or K&O,) was introduced by impregnation, then the char sample was slowly dried under vacuum in a desiccator. The dried samples were stored under vacuum until needed. The gasification apparatus is shown in Figure 2. It consisted of a fixed-bed tubular reactor (stainless steel, 2.0 cm i.d., 78 cm long) and a furnace, a gas feed system and a product gas purification and analysis system. The experimental conditions under which physical effects (film diffusion and internal diffusion, and mass and particle size of sample) had no influence on the reaction rate were determined, and each gasification experiment was carried out using these conditions: CO, feed flow rate 48 1h- ‘; sample particle size 2-4 mm; sample mass 15 g. RESULTS

AND DISCUSSION

Reaction rate and reactivity for carbon gasification have been defined in several different ways in the literature.

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Fuel 1995 Volume 74 Number 3

In this study the fractional carbon conversion X and rate r (min- ‘) were expressed as X=AW/W, r= l/W.dW/dt where W, is the mass of carbon in the initial char sample (g), A W is the change in mass of the sample at a given time (mg), W is the carbon mass at any time t, and dW/dt is the instantaneous rate of mass change at that time. A set of results for carbon conversion against time is shown in Figure 2. It can be seen that the conversion increases with increasing temperature, and that the catalytic effect of K&O, is better than that ofNa,CO,.

Figure 3 shows the effect of loading on rate for the two catalysts at 880°C. High rates are obtained with loadings of 25 wt% for Na,CO, and 20 wt% for K&O,. Above these values, a decrease in rate is observed, i.e. there is a saturation effect in this reaction system. It is suggested that the fall in rate is due to blocking of pores in the carbon by the catalyst435, restricting access of CO, to the surface of the micropores. At the other end of the scale, a smaller change of rate with increasing loading is observed below 9 wt% for Na,CO, and 5 wt% for K,C03. This can be explained by loss of catalyst by irreversible reaction with

Table 1 Char analysis 33.1 Ash(wt% db) Ultimate analysis (wt% db) C 56.2 H 3.6 s 1.6 N 1.3 0 (diff.) 4.1

Ash analysis (wt%) SiO Al,& CaO K,O Na,O MgO Fe,O, Ti (ppmw)

51.96 36.74 4.29 0.45 0.28 0.25 5.1 8226

Catalytic gasification: Shufen Li and Yuanlin Cheng

the Al&O, and SO, of the char ash, producing K,O.Al,O,.xSiO, and Na,O.Al,O,.xSiOz compounds. The effect of carbon conversion on the rate is shown in Figure 4. The rate increases with increasing conversion initially and then decreases, the manner of the decrease depending on the catalyst concentration, because the rate is a complex function of catalyst concentration and internal surface area of the char, both of which vary with conversion. From the experimental data in the temperature region 85C~96O”C,the rate constants k and activation energies E shown in Tables 24 were obtained. The rate constants for catalytic reaction are presented as Arrhenius plots in Figure 5. For both Na,CO, and K,CO,, the parameters E and A decrease with increase in catalyst concentration, and the lines in Figure 5 are seen to merge at one point, corresponding to a temperature of 1466°C for K&O, and 1289°C for Na,CO,. Thus the values of E and A for these experiments exhibit a so-called compensation effecP. The relation between E and A can be represented by InA=mE+C

as follows: InA=lnk+E/RT (2) Insertion of the value of the isokinetic temperature and Ink at this temperature into Equation (2) gives for

0

I



15

I

1

I

I

I

9

16

20

25

32

Catalyst loading (wt 5%)

Effect of catalyst loading on reaction rate at 880°C for K,CO, (A) and Na,CO, (0) Figure 3

4

(1)

This temperature, 1289°C for Na,CO, and 1466°C for K&O,, is the so-called isokinetic temperature. It indicates the point at which there is no effect of catalyst concentration. The Arrhenius equation can be expressed

3

7 .9 E p! 0

2

c

1

0

Schematic diagram of apparatus. 1, Flow controller; 2, purger; 3, pressure regulator; 4, reactor and furnace; 5, condenser; 6, gas meter; 7, combustor; 8, gas chromatograph; 9, temperature controller Figure 1

I

I

I

I

0.2

0.4

0.6

0.8

1

Conversion, X

Variation of reaction rate with degree of conversion at 880°C. loading (wt%): ?? , 32; A, 25; 0, 20; 0, 16; 0, 9; 0, 5

Figure 4

Na,CO,

0.8

0.6

0.6

0-

20

40

60

80 100

120 1

0

20

40

60

80

100 1

Time (mitt) Figure 2 Carbon conversion versus time of reaction at temperatures of (a) 850, (b) 900, (c) 960°C. K,CO, A, 1. Na,CO, loading (wt%): 0, 16; +, 9; A, 5; 0, 1

Fuel 1995

loading (wt%): ?? , 16; x, 9; ?? , 5;

Volume

74 Number

3

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Catalytic gasification:

Table 2

Shufen Li and Yuanlin Chew

Rate constants (10-s mint)

with Na,CO,

catalyst

Reaction temperature (“C)

Catalyst loading (wt%)

850

880

900

930

960

0 1 5 9 16 20 25 32

5.06 5.83 5.80 8.20 12.7 _ _ _

6.66 6.05 7.00 8.56 20.1 21.4 30.9 28.8

6.75 7.13 7.88 8.30 15.6 _ -

8.32 8.60 9.82 12.5 23.6 _ _ _

10.8 9.1 13.1 16.9 23.8

Table 3

Rate constants (10m3mint)

-

with K,CO, catalyst

Catalyst loading (wt%)

850

880

900

930

960

1 5 9 16 20 25

6.09 6.25 17.3 36.6 _ _

6.44 7.67 16.7 38.1 35.1 26.2

7.02 10.8 18.3 33.6 -

12.2 11.9 16.9 49.7 _ _

11.4 14.7 30.7 73.7 _ _

Reaction temperature (“C) 5.4 7

Table 4 Activation energies E and pre-exponential factors A for the range 85&96OC

Catalyst none K&G,

Na,CO,

Loading (wt%)

:Jmof-‘)

&in-t)

0 1 5 9 16 1 5 9 16

122.0 112.3 91.9 75.3 52.0 118.2 101.8 80.3 75.8

1480 866 116 18.4 4.04 1313 260 75.1 24.3

Na,CO, In A, =7.713 x 10-5E, -2.30

(3)

and for K&O, In A, = 6.928 x 10e5E, - 1.55

(4)

Equations (3) and (4) are the compensation equation for Na,CO, and K&O, respectively. The compensation effect has been observed previously for catalytic gasification of carbon with CO, and H,07. There are various explanations for the compensation effect in different reaction systems, probably because of different mechanisms. Anita et ~1.’ suggested the possibility of loss of the alkali catalyst by evaporation, so the concentration of catalyst has no effect above the isokinetic temperature. Xue’ proposed a mechanism including a catalytic redox cycle and a non-catalytic reaction to explain the catalytic action of the alkali metal

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Fuel 1995 Volume 74 Number 3

8

9

IO

5.8

7

8

9

l/T(lO-4 K-‘) Figure 5 Arrhenius plots for (a) Na,CO, and (b) K&O, Loading (wt%): 0,O; V, 5; A, 9; 0, 16

as catalyst.

and the role of surface oxygen in the carbon-CO, reaction; at higher temperature, the non-catalytic reaction plays a major role in determining the rate, with the result that the catalytic reaction loses much of its significance. In this study, the two catalysts showed different values of isokinetic temperature, because of their different chemical nature. Study of the compensation effect and isokinetic temperature will provide useful guidance for the choice of catalyst and reaction temperature. CONCLUSIONS The rate of CO* gasification at 790-1020°C and 0.2 MPa increases with the catalyst loading from 9 to 25 wt% for Na,CO, and from 5 to 20 wt% for K&O,. The catalytic effect of KJO, is greater than that of Na,CO,. The effect of catalyst loading on rate shows saturation above 25 wt% for Na,CO, and 20 wt% for K&O,. Below 9 wt% for Na,CO, and 5 wt% for K,C03 the loading has little effect. The values of E and A for these experiments exhibit the compensation effect given by Equations (3) and (4). The isokinetic temperatures are 1289°C for Na,CO, and 1466°C for K&O,. REFERENCES Kapteijn, F. and Moulijn, J. A. Fuel 1983, 62, 221 Freund, H. Fuel 1985,64,657 McKee, D. W. Carbon 1982,20, 59 Guzman, G. L. and Wolf, E. E. Ind. Eng. Chem. Process. Des. Dev. 1982, 21, 25 David, A. S. and Farhang, S. Fuel 1983,62, 880 Cremer, E. Adv. Catal. 1959, 7, 75 Kwon, T. W., Jung, R. K. and Sang, D. K. Fuel 1989,68,416 Anti, P. D., Gokarn, A. N. and Doraiswamy, L. K. Fuel 1991,70,839 Xue, J., Miura, K. and Hashimoto, K. .I. Fuel Chem. Tech. (Chinese) 1991,19, 156