Prediction of CO2 gasification rate of char in fluidized bed gasifier

Prediction of CO, gasification f luidized bed gasif ier Tadafumi Furusawa

Adschiri,

Tohru

Shiraha,

Department of Chemical Engineering, Japan (Received 23 June 1986)

Toshinori

The University

rate of char in

Kojima

and Takehiko

of Tokyo, Bunkyo-ku,

Tokyo,

113,

Chars from 14 different parent coals were produced in fluidized bed and the initial porosity and surface area were correlated with carbon content in the parent coals. Combination of previous results with the present experimental results led to the description of change in carbon dioxide gasification rate with increased conversion. The values calculated by the proposed equation were compared not only with the present data but also with the gasification rates reported by previous investigators. Carbon dioxide gasification rates of chars can be estimated by this equation. (Keywords: gasification; char; fluidized beds)

The rational design and analysis of the coal gasifier require the quantitative evaluation of the gasification rate of coal char. Recently the gasification kinetics of various chars was investigated intensively’-“. The gasification rate of coal char has proved to be largely dependent not only on the rank of the parent coal’ but also on the condition to produce char from coa111y12. Some investigators reported the effect of temperature” and heating rate” of production of char from parent coal on the gasification kinetics of coal char. The study of the gasification kinetics of chars which are produced from coals under conditions similar to those in the gasifier is important for the design and analysis of the gasifier. However, the chars employed for most of the previous experimental studies on the gasification kinetics were produced under conditions which were quite different from those in the actual gasifier. The unified interpretation of the gasification rates of coal chars has proved difficult, because the gasification rate of coal char is dependent on the effects of various physical and chemical factors, such as surface area of the char, chemical structure of char and ash content in coal. Adschiri and Furusawa’ reported that the gasification rate per unit surface area is kept almost constant during gasification and that the difference in gasification rate of carbonaceous materials including graphites and chars produced at various heating rates is interpreted mainly by the difference in surface area under atmospheric pressure over the range of temperature pertinent to the fluidized bed gasifier (85&12Oo”C). dX/dt/S(X) = 1.7 x 10s5 exp( - 140 OOO/R7)Pco2

(1)

The chars in the fluidized bed gasifier have, with continuous feeding of coal, an age distribution attributed to the residence time distribution of the char. Therefore the description of change in gasification rate, or that in the surface area which is proportional to gasification rate, during gasification is also important for the design of 0016-2361/86/121688-006$3.00 01986 Butterworth & Co. (Publishers) Ltd. 1688

FUEL,

1986,

Vol 65, December

gasifier. Adschiri et ~1.‘~ demonstrated that the change in the surface area of char with increased conversion, S(X), can be described from the initial porosity and surface area of the char alone. Thus the relationships between the properties of coal and the properties, S(O), Q,, of char produced under similar conditions to those in fluidized bed gasification, (especially the high heating rate), are necessary for predicting the coal char gasification rate in a fluidized bed gasifier. The first objective of the present study is to obtain the relationships between properties of the present coal and properties, S(O), Ed, of char produced at high heating rate in a fluidized bed. The second objective is to present the formula, which can describe the change in the rate during gasification of these chars by use of the properties of the parent coal alone without any information on the properties of the produced char. The third objective is to compare the initial gasification rates calculated by the proposed equation with the present experimental results and with the results obtained by previous investigators’-“. EXPERIMENTAL Materials

The proximate and ultimate analyses of the coals employed in the present study are given in Table 1. The coal samples were sieved to OS&O.59 mm and the moisture in coal was completely removed in a dryer kept at 390K. The bed materials employed were inert MS (micro-spherical) particles, comprising silica and alumina, with a harmonic mean diameter of 0.61 mm, a density of 0.9 g cm- 3 and a shape factor of 1.0. Apparatus

for char preparation

The flow sheet of the fluidized bed system employed for the char preparation is shown in Figure 1. The 37mm internal diameter stainless-steel reactor was packed with

Prediction of CO2 gasification rate of char: T. Adschiri et al. Table 1

Proximate and ultimate analyses of the coals Ultimate analysis (d.a.f. “4,)

Proximate analysis (dry %) Coal

Ash

Volatile matter

Fixed carbon

C

H

0

Turkish Orhaneli Peat Nakayama Yallourn Beluga Colowyo Tai heiyo Coal Valley Plateau Horonai Ermelo Blair Athol Datung Miike

53.0 29.0 13.4 0.7 14.4 5.6 21.4 10.8 9.8 19.5 13.4 11.1 10.6 16.2

28.5 48.5 68.4 45.4 51.8 39.6 42.7 35.3 42.8 40. I 30.2 21.4 27.8 39.8

18.5 22.6 18.2 53.9 33.8 54.9 35.9 53.9 47.8 40.4 56.3 61.4 61.7 44.0

53.6 58.2 68.7 69.8 70.4 77.3 77.5 79.0 80.0 80.8 81.8 83.4 84.6 84.6

4.7 5.8 4.3 4.2 5.4 5.1 6.4 5.0 5.7 6.3 4.8 4.5 4.6 6.2

35.7 33.8 25.6 25.2 23.0 15.6 14.9 14.8 12.1 11.1 10.4 10.0 9.1 8.6

-

VENT

I

FEED

ICEBATH

L

I

GAS INLEl

Figure 1 Flow sheet of the fluidized bed system employed for the char preparation

MS particles to a static bed height of 12 cm. Details on the reactor are shown in the previous paper12. The fluidizing gas was introduced into the bed through a 1.2 mm thick distributor with eleven holes (OSmm). The bed was heated by an external electrical furnace and bed temperature was controlled and measured with a chromel-alumel thermocouple. Preparation of char employed

The experiments were conducted at atmospheric pressure. The bed was kept at 1273 K with nitrogen flowing through at a rate of five times the minimum fluidizing velocity. The coal sample (3 g) was placed in the inlet tube, then the air around the sample was replaced with nitrogen. The coal sample was introduced into the bed by switching the nitrogen flow from the ‘gas inlet’ line to the ‘feed inlet’ line for an instant. The coal particles introduced were widely and instantaneously spread and were supplied with heat by the surrounding bed particles, thus it was estimated that the temperature of the samples rapidly reached the bed temperature of 1273 K and devolatilization took place instantaneously. After keeping the samples in the bed fluidized by nitrogen for z 5 min, the char particles were withdrawn through the stainless-steel pipe inserted into the bed, by opening the line for particle withdrawal (Valve B) and at the same time closing the gas outlet line (Valve A). The produced char particles were entrained with MS particles

through the pipe, and collected in the catch pot externally cooled with ice. In several experimental runs, the time for devolatilization extended to 40 min; however, remarkable differences in the physical properties and reactivity of the produced chars were not found between the char produced by either 5 or 40 min devolatilization. All oft he particles in the bed were collected in the pot by moving the end of the pipe to the surface of the distributor plate. The char particles collected in the pot were separated from the MS particles by the sink-float separation using the liquids with various densities (water or ethanol). The char particles thus prepared were dried and employed for measuring physical properties and gasification rate as follows. Measurement of surface area and porosity

Surface areas of the chars were determined from CO, adsorption at 298 K using the Polanyi-Dubinin equation. Adsorption isotherms were determined in a conventional thermogravimetric apparatus with a sensitivity of 10e2 mg. The experimental procedure for the measurement of CO, adsorption isotherms was essentially the same as that by Marsh and Siemieniewska’4*‘5. For calculation of the surface area, a molecular area of the carbon dioxide was taken as 0.253 nm2. The initial porosities of these chars were determined from the particle density and the true density of the chars which were measured using a picnometer with water16. Measurement of gasification reaction rate

An isothermal thermogravimetric analysis was carried out at the carbon dioxide partial pressure of 50 kPa to measure the change in gasification rate with time, namely with conversion. Thermogravimetric apparatus employed was a model Shimadzu TG-31 electro-balance. A special sample holder’ ’ was used to eliminate the effect of mass transfer resistance around the sample particles. The details of the experimental apparatus and experimental procedures are found elsewhere’ 3. The time derivative of the extent of conversion (later, referred to as the gasification rate), dX/dt, and the conversion, X, were evaluated on the basis of the weight of the reactive portion of the sample. The conversion was defined as: X = (w, - WWO - w,,)

FUEL, 1986, Vol 65, December

(2) 1689

Prediction

of CO2 gasification

rate of char: T. Adschiri

‘-or---l0

0.8T

,0.6-O

0

0

in outlet concentration was detected continuously by use of the CO sensor after the CO tracer was introduced into the reactor. Since around 2min were required for the constant concentration of the tracer gas, the change in the gasification rate obtained was evaluated in the present investigation.

0

0

0

000 0

RESULTS

k cn

porosity qf chur and dynnmic change in gasification rate during gasificution Initial

aoo.4-

0

2

so.25

-0

et al.

I I I 60 70 80 90 CARBON CONTENT[%)

50

Figure 2

Initial porosity

of chars

produced

in the fluidized

b 100 bed

The initial porosity measured for the different chars prepared as above was plotted against the carbon content in coal in Figure 2. The initial porosity of the char produced in the fluidized bed was relatively high (> 0.5) compared with that of chars produced in a conventional pyrolysis furnace. The present result that the initial porosity of the char is relatively high may be attributed to the high heating rate adopted for the production of char in the fluidized bed12. Previous results13 indicated that the surface area of highly porous chars decreased linearly with increased conversion as : S(X)/S(O)=(l-X)

0q5r----l

Since the gasification surface area during rewritten as follows: dX/dt = (dX/dt),(l

(3) rate is linearly proportional to the gasification, Equation (3) can be -X)

(4)

= k,( 1 - X)

0.1

0

1.0

1.0

Figure 3 Dynamic change in gasification rate measured at the temperature of 1273 K and at the partial pressure of carbon dioxide of 5OkPa: A, Taiheiyo; t, Colowyo; 0, Beluga; 0, Nakayama: A, Turkish Orhaneli

0.8

0

0.2 0.4 Conversion,

0.6 0.8 X C-3

F where W,, W and Washrepresent respectively the initial weight of char, the instantaneous weight of char and the weight of ash. Our previous investigations indicated that the intraparticle diffusion resistance could be ignored for the char whose gasification rate at 1273 K was almost the same as the highest rate among chars employed for the present study”, and whose diameter is larger than that of char employed for the present study. Furthermore, it was also demonstrated that mass transfer resistance around the particle can be ignored by reducing the weight of the sample char, and by increasing gas flow rate” in the TGA apparatus employed for the present study. A preliminary experiment was conducted to evaluate the response time in the apparatus. The dynamic change

1690

(5)

Thus the change in gasification rate of the present chars (except Ermelo char, whose initial porosity is around 0.4), is expected to be expressed by the volume model. In the present experiment, the change in gasification rate for different chars was also measured thermogravimetrically at 1273 K and at carbon dioxide partial pressure of 5OkPa. The results obtained were shown in Figures 3 and 4. These figures suggest that all the gasification rates of these chars are approximately

FUEL, 1986, Vol 65, December

I-

1

0.6

0 xi* uu

0.4

0.2

Oo’0.2

0.4 0.6 0.8 Conversion, X L-1

1.0

Figure 4 Dynamic change in gasification rate measured at the temperature of 1273 K and at the partial pressure of carbon dioxide of 50 kPa: x , Miike; 0, Datung; v, Blair Athol; 0, Ermelo; 0, Plateau; n , Coal Valley

Prediction

1500

of CO2 gasification

rate of char: T. Adschiri

et

al.

Combination of Equation (6) with Equation (7) gives the following equation: dX/dt = (2590-

27.7C)1.7

x 10-5exp(-

140000/RT)Pco2(l -X)

(8)

Thus the change in gasification rate and initial rate over a temperature range favourable to the fluidized bed gasifier under atmospheric pressure for char produced at high heating rate was described only from the carbon content in parent coal. The estimated values of (dX/dt), by Equation (8), using only temperature, carbon dioxide partial pressure and carbon content in parent coal, were compared with the present data and the previously obtained data’ -4 obtained under the conditions similar to those in fluidized atmospheric bed gasification (1130K
‘4’0

50 60 70 80 Carbon Content [%d.a.f.l

90

Figure 5 The relationship between the initial surface area of char produced in the fluidized bed and the carbon content in coal

expressed by the volume model predicted by Equation (5) and these results agree with the expected results derived from Equation (3). Since most of the initial porosities of the chars produced in the fluidized bed were relatively high, the change in gasification rate of the char can be approximately expressed by the volume model for most of the chars. However, special attention should be paid to the application of the volume model for predicting the change in gasification rate of char produced from caking coal, because such char has relatively low initial porosity. Initial surface area oj’the char produced in the experimental

jluidized bed

Some investigators’8-20 reported the relationship between surface area and carbon content in the parent coals for various chars. However the chars employed by these investigators were produced under conditions unlike those in a fluidized bed gasifier. In the present investigation, the initial surface area was measured for various chars produced in the fluidized bed and was correlated with the carbon content in the parent coal as shown in Figure 5. The following equation was obtained from this relationship by a least squares method: S(O)=2590-27.7C

(6)

Prediction ofthe dynamic change in gasification rate ofchar in the atmospheric jluidized bed gasiJier

Combining Equations equation is obtained : dX/dt =

1.7x lo-‘S(O)exp(-

(1) and

1400QO/RT)P,-o,(l

(3), the following -X)

(7)

Figure 6 Comparison of calculated and gasification under the conditions favouring atmospheric fluidized (see legend in 72ble 2) Table 2 Gasilication condition (Legend in Figures6 and 7)

employed

for

rates obtained bed gasification

the

investigation.

Temperature Investigator

(K)

Pressure (k Pa)

Present authors Adschiri and Furusawa’ Dutta, Wen and Belt’ Hashimoto and co-workers3

1273 1073-1273 113&1340 1110 1458 1080-1320 I223 1073 1073 1173 973 1173 1273 I473

50 50 loo 50 50 100 50 800 1600 100 30.3 30.3 25 100

Knight and Sergeant4 Kasaoka and co-workers” Takeda and co-workers’ Hippo and Walker’ DeGroot and Shalizadeh* Beesting and co-workers9 Mochida and co-workers”

FUEL, 1986, Vol 65, December

1691

Prediction

of CO* gasification

rate of char: T. Adschiri

et al.

for the data obtained by Beesting et a1.9 may be attributed to production of chars from coals. The chars employed by Beesting et ~1.~were produced at a relatively low heating rate, but the devolatilization time employed was extremely long (7.5 h). Several mechanisms22-24 have been proposed for the carbon-CO, reaction. Among them, a mechanism proposed by Ergun 24 is widely accepted: c + co,*c(o) C(O)-CO

Figure 7 Comparison of calculated and gasification under different conditions from those in atmospheric gasification (see legent in Table 2)

rates obtained fluidized bed

the value estimated by Equation (8). Thus the carbon dioxide gasification rate of chars can be predicted by Equation (8) using only gasification temperature, carbon dioxide partial pressure and carbon content in the parent coal. DISCUSSION The difference between the surface area evaluated by N, BET method and that by CO, adsorption can be ignored for char with high porosity’. The relations of Equation (1) and (3) were obtained from the results for the relatively high conversion and porosity using N, BET method. In the present study, CO, adsorption was employed for precise measurement, although even for initial char the porosity was high enough. Thus the application of the relation of Equations (1) and (3) will be justified. The results for the data”3-10 obtained under conditions different from those in the atmospheric fluidized bed gasifier are shown in Figure 7. This figure indicates that large discrepancies between the calculated and experimentally measured rates were found, especially for data obtained over the range of lower temperatures (< 1130K)3*4*6,8and pressures greater than atmospheric pressure6 (Figure 7). The deviation from rates calculated from Equation (8) may be attributed to: experimental errors; different pyrolytic conditions; evaluation of the order of reaction with respect to the partial pressure of carbon dioxide under high pressure; catalytic effect of ash content in char; and the chemical structure of char. These discrepancies are discussed below. The gasification rates reported by Mochida et al.” were lower than the estimated values as shown in Figure 7. This result may be attributed to the intraparticle diffusion resistance’ because the gasification rates reported were measured at high temperature (1473 K) for the relatively large char particles. The lower values compared with the estimated values

r,

1692

FUEL,

1986,

Vol 65, December

+ co

(9) (10)

It has been reported that the carbon dioxide gasification rate with respect to carbon dioxide partial pressure is approximately first order at low pressures but approaches zeroth order at high pressures at the same temperature’, and the dependence of rate on carbon dioxide partial pressure approaches from first to zeroth order with reduction of gasification temperature’5-27. These tendencies are found to be specific to high rank coals25. The dependence of the order of reaction on the partial pressure of gaseous reactants and temperature is explained by accounting for the elemental gasification steps based on the mechanism proposed by Ergun”. Such discussion will also explain the discrepancies in Figure 7.

Some of the gasification rates measured over the range of low temperature or high pressure were much lower than the values calculated by Equation (8) as shown in Figure 7. These discrepancies may be explained by the reduced order of reaction with respect to the partial pressure of carbon dioxide at high pressure or low temperature, and by evaluation of the effect of the chemical structure of char and the catalytic effect of ash content in char on the gasification rate. The rate estimated in the present study was assumed to be first order with respect to carbon dioxide partial pressure. However, Takeda et aL6 reported that the order of reaction measured was around 0.25 at relatively low temperature (1073 K) and under high pressure (800 kPa and 1600 kPa). The rates measured at temperatures lower4 than 1130 K and at 1110 K, by Hashimoto et u/.~, are considered to be below the first order, especially for the high rank coals, because the order of reaction with respect to gaseous reactant is reduced at low temperature2’. Furthermore, over the range of low temperature, the effect of the chemical structure of char and the catalytic effect of the ash content in coal on the gasification rate may be significant, so that the differences among gasification rates may not be interpreted by use of surface area alone. Detailed discussion for evaluation of the catalytic effect of ash content in char and the effect of the chemical structure of char upon the gasification rate, as well as the estimation of gasification rate over the range of low temperature and high pressure based on the gasification mechanism, will be made in a forthcoming paper. CONCLUSION Various coals with different nature and origin were devolatilized in an experimental fluidized bed at high heating rate. The porosity and surface area of the produced chars were correlated with the carbon content in the parent coal. The initial porosity of the char produced in the fluidized bed was 3 0.6 and the dynamic

Prediction of CO;, gasification rate of char: 1. Adschiri et al.

change in gasification rate of these chars can be expressed by the volume model. The initial surface areas of these chars were approximately estimated by the carbon content in coals alone. The formulation for the description of the dynamic change in carbon dioxide gasification rate with increased conversion was derived from the combination of the present results with those obtained in a previous investigation”’ 3. The gasification rates obtained over the temperature range pertinent to the fluidized bed gasifier and under atmospheric pressure for the chars produced at high heating rate can be fairly well predicted only from gasification temperature, the partial pressure of carbon dioxide and the carbon content in parent coal. ACKNOWLEDGEMENTS

12 13 14 15 16 17 18 19 20 21 22 23 24

Kojima, T., Furusawa, T. and Kunii, D. Kugaku Kogaku Ronbunshu 1984, 10, 596 Adschiri, T., Kojima, T., Furusawa, T. and Kunii, D., The Institute of Chemical Engineers Symp. Ser., 1984, 87, 345 Marsh, H. and Siemieniewska, T. Fuel 1965.44, 355 Marsh, H. Fuel 1965, 44,253 Karr, C. Jr., Analytical Methods for Coal and Coal Products, 1978, vol. I, p. 128 Matsui, I., Kunii, D. and Furusawa, T. fnd. Eng. Chem. Process Des. Dev. in press Johnson, J. L. Am. Chem. Sot. Div. Fuel Chem. Prepr. 1975,20(4), 85 Torikai, N. and Walker, P. L. Jr., U.S. Oftice of Coal Res. SROCR-3 Report, Washington, D.C., 1968 Hippo, E. J., Ph.D. Thesis, 1977, The Pennsylvania State University, University Park, PA, U.S. Hashimoto, K. et al. 18th Autumn Meeting of the Sot. Chem. Eng. Japan, 1985, SClll, 108 Ergun, S. J. Phys. Chem. 1956,60, 480 Blackwood, J. D. and Ingeme, A. G. Aust. J. Chem. 1960,13,194 Walker, P. L., Jr., Rusinko, F., Jr. and Austin, L. G. Adv. Cataf. 1959, 11, 133 Adshiri, T. et al., Proc. Int. Symp. Coal Science (Sydney, Aust.) 1985, 289 Vulis, L. A. and Vitman, L. A. J. Tech. Phys. (USSR) 1941, 11, 509 Semechlova, A. F. and Frank-Kamenetskii, D. A. Acta Physic0 Chem. (USSR) 1940, 12, 879

T.F. wishes to express his thanks to the Grant-in-Aid for Energy Research (59040033,59045201, 60040034) of the Ministry of Education, Science and Culture, Japan. T.A. wishes to express his thanks to the Grant-in-Aid for Encouragement of Young Scientist (60790052) of the Ministry of Education, Science and Culture, Japan.

26

REFERENCES

NOMENCLATURE

1 2 3

4 5

10 11

Adschiri, T‘. and Furusawa, T. Fuel 1986, 65, 927 Dutta, S., Wen, C. Y. and Belt, R. J. Ind. Eng. Chem. Proc. Des. Dev. 1977, 16, 20 Hashimoto, K. et al. Energy Research of the Ministry of Education, Science and Culture, Report of the study on coal gasitication rate, 1985 Hippo, E. and Walker, P. L., Jr. Fuel 1975, 54, 245 Kasaoka, N. et al. 18th Autumn meeting of Sot. Chem. Eng. Japan, Prepr., 1984, p. 105 Takeda, S. et al. Nenryo Kyoukai-shi 1985,64, 409 Knight, A. T. and Sergeant, G. D. Fuel 1982,61, 145 DeGroot, W. F. and Shatizadeh, F. Fuel 1984.63, 210 Beesting, M., Harwell, R. R. and Wilkinson, H. C. Fuel 1977,56, 319 Mochida, I. et al. Fuel 1984, 63, 136 Gray, J. A., Donatelli, D. J. and Yavorsky, P. M. Am. Chem. Sot. Div. Fuel Chem. Prepr. 1975, 20(4), 103

25

27

C

carbon content in coal (%) activation energy (J mol- ‘) volume reaction rate constant (s- ‘) partial pressure of carbon dioxide (Pa) R gas constant (8.314mol-’ K-’ J) s surface area (m’ g-’ initial carbon) T absolute temperature (K) t time (s) X conversion W instantaneous weight of char (g) initial weight of char (g) w, KS, weight of ash (g) E porosity initial porosity EO

k”

ko,

FUEL, 1986, Vol 65, December

1693