Coal char gasification under pressurized CO2 atmosphere

Coal char gasification under pressurized CO2 atmosphere

Letter Coal char gasification (Received 25 June under pressurized 7991; revised 8 August Dear Sir Table 1 Rational design and analyses of a press...

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Letter Coal char gasification (Received 25 June

under pressurized

7991; revised 8 August

Dear Sir

Table 1

Rational design and analyses of a pressurized gasifier require a sound understanding of gas-solid contact and a description of gasification rate under pressure. The effect of CO2 pressure on the rate of coal char gasification has been extensively investigated by many researchers’. Most of them reported that the gasification rate is first order with respect to CO, pressure below atmospheric pressure and zeroth order at elevated pressure’. It is widely accepted that this pressure dependence of the gasification rate can be explained by Ergun’s mechanism (steps (i) and (ii))‘, and expressed by a Langmuir-Hinshelwood (L-H) type equation, regardless of catalytic3 or non-catalytic gasification’. c+co,~c(o)+co kl

(i)

co

C(0) +

Rate = -

(ii) k,NPcoz

I+ WW’,,+

WM’cx,~

(1)

where N is the total number of active sites (mol carbon/mo1initialcarbon)2. However, some exceptions have been reported. Blackwood and Ingeme measured the gasification rate under elevated pressure and proposed a different type of equation4 based on reactions (iiit(vii).

co,

2 co + (0)

(iii)

co;(CO)

(iv)

CO2 atmosphere

7997)

Ultimate and proximate analyses of parent coals

Coal

Ash”

Volatile matter’

Yallourn Baiduri

0.7 5.3 21.4 7.1

45.4 42.8 42.7 6.6

Taiheiyo Hongei

C*

Hb

Nb ~_____

Sb

0*

53.9 51.9 35.9 86.3

69.8 71.3 77.0 92.7

4.2 4.9 6.2 3.7

0.6 1.4 1.2 1.0

0.2 1.1 0.4

25.2 21.4 15.6 2.3

’ Wt% dry basis b Wt% daf shown and the reaction mechanism which can explain the results is discussed. Chars from four parent coals were produced in a N, atmosphere at 1173K by a fluidized bed pyrolyser at a heating rate of 1OOOK min- I. The details of the char preparation have been given previously’j. The properties of the coals are given in Table I. Experiments were conducted with a high pressure apparatus. A quartz tube (3Smm i.d., 5 mm o.d.) inserted in a stainless steel or Incolloy H tube (6mm i.d., 8mm o.d.) was employed as a reactor. The details of the reactor design have been presented elsewhere’. The reactor was loaded with l&100 mg of chars (0.5&0.59mm) and heated up to the reaction temperature (1123K) in a N, stream at a heating rate of 10K mini *. The N, stream was switched over to a CO, stream and gasification was initiated. Effluent gas was sampled intermittently at intervals of several minutes and the CO concentration, C(t), during gasification was analysed by a gas chromatograph equipped with methanator and FID. Carbon conversion, X, and gasification rate, dX/dt, were calculated from C(t) by Equations (3) and (4), respectively. (3)

co,

+ (CO) 4 2co + (0)

1

c3

10

-4

1o-5

t

I1111

I

1 o-*

IIll

I

10-l

I

1

PC0

(MPa) 2

Figure 1 Coal char gasification rate at 1123K: 0, Yallourn; 0, Baiduri; 0, Taiheiyo; V, Hongei. Solid lines are predicted by Equation (8)

8 - 1.5

z m 0 =.

-

v,

1.0

m 0

4 i-

-I $15

-

0.5

-

3 -

(vi) dX

co+(co)ko,+c

(vii)

(2) where X is the carbon conversion level, i, = k4, i, = k,kJk,, i, = (k,/k, + k9/k6) and i4 = (k,/k, + k8/k6). In this study, we measured the CO, gasification rate of four chars over a wide range of Pco2 (0.022SMPa) by a conventional method’ using a fixed bed reactor and found that the dependence of gasification rate on Pco2 cannot be described by Equation (1). In this paper the experimental results are

001~2361/92/010349~2 c> 1992 Butterworth-Heinemann

~__

Fixed carbon”

Ltd.

dt

C(r) -p - s? C(t)dt

(4)

Figure I shows the pressure dependence of the COZ gasification rate. The gasification rate, dX/dt, was evaluated at a carbon conversion level of 0.5. The variation of pressure dependence on the gasification rate is as follows: the reaction order with respect to Pco2 was -0.7th order in the pressure range from 0.02 to 0.05 MPa, which reduced to -0.5th order in the pressure range from 0.05 to 1.5 MPa and then increased again to -0.7th order above 1.5 MPa. The reciprocal of the gasification rates are plotted against the reciprocal of P,,, in

0

20

40

I’&

(MPa-‘1 2

Figure 2 Relationship between the reciprocal of the gasification rates and the reciprocal of pco* at 1123K. Symbols as in Figure I

Figure 2. If Equation (1) can describe the pressure dependence of the CO, gasification rate, a linear relationship between (dX/dt)-’ and P& should be obtained in Figure 2. However, the gasification rate under a pressurized CO, atmosphere cannot be explained by Ergun’s equation. A gasification rate above 1.0 MPa was

FUEL,

1992,

Vol 71, March

349

Letter equation:

Rate constants in Equation (8)

Table 2

Coal

(10-l

Yallourn Baiduri Taiheiyo Hongei

2.5 3.0 3.8 12

MPa-‘)

larger than the value predicted from the rates under lower pressure by using an L-H type equation. This means that the Ergun’s mechanism is not adequate to describe the gasification rate over a wide range of Pco2. There are two ideas which can explain our results: one is the collision between CO, and C(0); the other is the variation of chemical bond strength of C-C(O). (a) Under higher pressure, there are more chances for surface oxide complexes to be attacked by COz molecules. Thus the desorption of the surface oxide complexes is accelerated by the collisions with CO, molecules. This effect is so weak at lower PCo2that the pressure dependence of the gasification rate can be described by an L-H type equation under atmospheric pressure. Under elevated pressure, however, the gasification rate is higher than the predicted value by using an L-H equation due to the effect described. According to this mechanism, the gasification rate is expressed as:

(9

kz

350

1:co

FUEL,

(ii)

1992,

Vol 71,

1.5 3.8 1.0 52

42 98 44 190

c(o)+co, dX -= dt

March

(viii)

“: co+co,

k,N(k3+kIoPco*)Pco~ __ (k,+k,o)Pco,+k,Pco+k,

(5)

(b) The chemical bond strength of C-C(O) is weakened by the increase in the surface oxide complexes. The desorption rate of the surface oxide complexes is accelerated by the variation in the chemical bond strength of C -C(O). On this assumption, the desorption rate constant of C(0) is described as a function of PCO,. c+co

,+(o)+co

(i)

C(0) : co

(ii)

k,=a+bP;,

(6)

dX

2 Pco2 +tjIpcoz

dt-

J’2Pc02

If n= 1 then the gasification expressed as:

rate can be

(8)

+j3

Figure 1 shows good agreement between experimental data and calculated values using Equation (8). The rate constants ji, which were used for simulating the lines, are given in Table 2. Hence, mechanism (a) or (b) may describe COz gasification exactly. A detailed investigation is required to clarify the CO* gasification mechanism. Takao Nozaki,

Tadafimi Adschit? and Kaoru Fujimoto Department of Synthetic Chemistry, The University of Tokyo, 7-3-l Hongo, Bunkyo-ku, Tokyo 113, Japan *Department of Biochemistry and Chemical Engineering, Tohoku University, Aoba-ku, Sendai 980, Japan

REFERENCES Dutta, S., Wen, C. Y. and Belt, R. J. Ind. Eny. Chem., Proc. Des. Dev. 1977, 16(l),

20

Ergun, S. .I. Phys. Chem. 1956, 60,480 Kapteijn, F. and Moulijn, J. A. Fuel 1983, 62, 221 Blackwood, J. D. and Ingeme,

A. J.

Aust. J. Chem. 1960, 13, 194

dX dt

c+co ,~c(o)+co C(0)

(103 s)

j3 (MPa s)

Jz

jI

k, N(a + bPco#‘coz

(7)

(k, +~U’,o,+k,P,o+a

Equations (5) and (7), which are in fact the same, can describe the pressure dependence of our result shown in Figure 1 by solid lines by using the following

Zhu, Z.-B., Furusawa, T., Adschiri, T. et al. ‘Proc. 197th ACS National Meeting, Div. Fuel Chem’, 1989, pp. 87-93 Adschiri, T., Shiraha, T., Kojima, T. et al. Fuel 1986, 65, 1688 Asami, K., Omata, K., Fujimoto, K. et al. Energy & Fuels 1988, 2, 574