Fuel Processing Technology, 36 (1993) 219-225 219 Elsevier Science Publishers B.V., Amsterdam

Analysis of Local Reactions in a Laboratory Scale Jetting Fluidized Bed Coal Gasifier

T. Kimura, J. C. Bi, S. Uemiya and T. Kojima

Dept. Ind. Chem., Seikei Univ. 3-3-1 Kichijyojikita-machi, Musashino-shi, Tokyo 180, Japan

Abstract A model for coal gasifiers was proposed from the combination of the Kunii-

Levenspiel bubbling bed model and our own grid zone model. Based on the model, local contribution of individual reaction was analyzed and local concentration profile was predicted in the grid and bubbling zones of a laboratory scale fluidized coal gasifier. The effects of lateral temperature and bed height on CO, H2 and CO2 concentration were discussed by comparison with experimental results. The results showed that the n,,merical predictions are corresponding to the experimental ones. It has been verified that combustion occurs mainly nearby the jet region, steam gasification both in the annulus and emulsion regions and C02 gasification mainly in the emulsion region. It is clearly shown that the contribution of the progress of the reactions in the grid zone to total gasification performance is high.

1. INTRODUCTION

The background of this research and the description of our experimental study on coal gasification were presented in a separate paper [1].

In our previous study [2], particle velocity in jet, jet diameter and jet height were measured by a couple of optical fiber sensors in a jetting fluidized bed cold model with a cone-shaped distributor. Axial and radial gas concentration profiles were also measured by gas tracer technique. The effects of operational conditions on gas and solid behaviors were surveyed experimentally as well.

Based on the above mentioned study, a model describing local behaviors of gas and solids in the grid zone was presented [3]. In the model, calculations of jet height and jet diameter according to the expressions of Horio [4] and Tanaka [5] respectively, were employed. Mamuro and Hattori's correlation [6] was modified and was applied to estimate the gas exchange rate between jet and annulus, and partly modified Lapidus's packed bed model [7] was used to evaluate the gas dispersion rate in the annulus region. The results stated that the numerical values of gas concentration in the grid zone and particle velocity in jet were quite well corresponding to the experimental values in the lower part of grid zone. By use of this model that was combined with Kunii-

0378-3820/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

220

Levenspiel Model [8], a new model for coal gasifiers was proposed [9]. In the present study, the model [9] is applied to our experimental results [1].

The analysis on the local contribution of the individual reaction is conducted to elucidate the relative importance. The experimentally observed results about the effects of bed temperature and bed height on the gasifier performance are numerically explained.

I

t A i r + N 2

Figure 1. Outline of the model.

2. MODEL DESCRItrrION

A sketch of the developed model is illustrated in Figure 1. In the model, fluidized bed is supposed to be divided into two zones, bubbling zone and grid zone. Both zones consist respectively of two regions: bubble region and emulsion region in the bubbling zone, jet region and annulus region in the grid zone. The acts of particles and gas were explicated as follows: particles initially entering the jet at velocity of zero are accelerated by jetting gas and then begin to slow down at some height. Part of particles in the annulus region is descending around the jet edge, some of them, entering the jet region. Around the gasifier wall, particles are expected to be flowing down. Gases introduced from the nozzle ascend rapidly along the je t region with lateral gas exchanging at rate of Kja between the jet and annulus. Gases introduced from the distributor ascend uniformly with a velocity of Umf, simultaneously, with a radial dispersion in the annulus region. In the bubbling zone, the Kunii-Levenspiel bubbling bed model [8] for intermediate particles was employed. In this simulation, the following three reactions were concerned: carbon combustion and steam and CO2 gasifications. Detailed correlations were published in the previous paper [9].

The steam gasification rate of carbon employed in this simulation was that by Matsui et al. [11]. The scheme of the steam gasification was given as equation (1).

221

C + aH20 -~ (2-a) CO+(a-1)CO2+aH2 (1) Consider ing t ha t pa r t of CO produced by equat ion (2) C + H20 --~ CO + H 2 (2) reacts again wi th s team by eqation (3). CO + H20 ---> CO2 + H2 (3) The value of a, a f i t t ing parameter , was given to be 1.2 a t 1173 K from the resu l t s on local cencent ra t ion profiles in a s team gasification expe r imen t wi thout a i r [1]. The combustion ra te by Saito et al. [11] was employed. A f i t t ing p a r a m e t e r ~ of combustion defined as equat ion (4) was de te rmined to be 0.7 by compar ing wi th our exper iment wi thout s team [1].

C + ~ O 2 --> ~CO + (1- ~202 (4)

CO~, gasif ication ra te by Matsui et al. [12] was also employed in this model.

Table 1 N ume r i c a l Condi t ions Bed temperature, K 1073 1173 1273 1273 1273 char-g/(char- g+alumina-g),- 0.2089 0.2113 0.2092 0.2122 0.2144 Bed height, m 0.14 0.14 0.14 0.05 0.25 Gas velocity; Air cm 3 s "1 at 298 K 6.69 6.0 6.0 5.19 5.19

; N2 cm 3 s "1 at 298 K 31.7 28.0 28.0 24.8 24.7 ; He cm 3 s -1 at 298 K 4.59 4.0 4.0 3.54 3.54 ; steam m R s -l, water 3.69 3.26 3.81 2.78 2.74

3. R E S U L T S AND DISCUSSION

3.1 L a t e r a l c o n c e n t r a t i o n profi les a n d con t r ibu t ion o f ind iv idua l r e a c t i o n Numer ica l conditions in this s imulat ion are l isted in Table 1. Typical

resu l t s on radial concentra t ion (H2, CO, CO2) profiles f rom the numer ica l calculat ion (lines) compar ing with exper imenta l values (plots) are i l lus t ra ted in F igures 2a, 2b and 2c for t empera tu re 1273 K and distances from gas injector 10, 20, 30 and 40 ram. Concentrat ion profiles for CO come from CO2 gasification, combust ion and s team gasification, and for CO2 from la t t e r two react ions are shown individual ly in Figures 2b and 2e, respectively. The consumpt ion of CO2 by CO2 gasification was assumed to occur using both CO2 produced by combust ion and s team gasification proport ional ly to the i r concen t r a t i ons .

The shape of the numer ica l curves for Concentration of H2 in F igure 2a was expla ined by t ha t char part icles contact with s team main ly in the annu lus region while char concentra t ion is low and gas velocity is high in the jet .

In F igure 2b, i t was confirmed t ha t CO was formed by combust ion n e a r b y the jet , while i t was formed by s team gasification in the annu lus region. The concen t r a t i on profi le of CO produced f rom CO2 gasif icat ion was below 1% because CO2 gasif icat ion is slower t han s team gasification. CO was ma in ly formed by s t eam gasification and combustion in the grid zone. Therefore , the

222

15

1o

E

~'o 5

~: o' 15

~1o

5

i i

.m Gaom~

•

h=20 m m

1 5 I - . Numerical i i i ot I .

0.0 10.0 20.0 (a) r [mm]

~ '151" " ' " " " t ~ 4 t " ' "=40mm t

|1° I ....... .I i jg,* I,.l''''''' ..... 2 ~L [ , ........... /

o & .......... ~L-------~'-e---'---~'; - - - ' r l , • ..........

~ 10~ • • i 3 • h~Omm

15~'[ " Totat CO concentration ' l l ""° ' " ' " ' CO from steam 9u~f~:ation

II . . . . CO, . . . . II I011 ........... co f r o m c o , ~ , i ~ . 11

/I " E.~.m.~,.d J/ ~. / h=lO rnm|

-0.0 10.0 20.0 (b) r [mm]

• ~ , , , " ' " ' , - ,uu, ,

0 • fltllllllJJ ~ o .

. . . . . . . . . . . . . CO'a from steam ~u~ifieaSon - - - ~ from combustion

i & ~ i m e n t a l h- lO m m

k • , .

v '0.0 10.0 2().0 (C) r [mm]

Figure 2. Radial concentration profiles; for H2 (a), for CO (b), for C02 (c).

shape of the total concentration curve was similar to that of H2; however, that of CO in jet was higher than that of H2.

It can be seen that the C02 concentration profile appears quite flat as shown in Figure 2c. The result was explained numerically as follows. It was clearly seen that also CO2 was produced by steam gasification in annulus, but the amount was much less than H2 and CO. Comparable amount of CO2 was produced from combustion in the vicinity of the jet to total steam gasification in the annulus region.

3.2 Effects of t~mperature on gasification performance Concentration profiles of gases in the grid zone for 1173 K, which are not

shown in this paper, had almost the same tendencies with those for 1273 K (Figure 2), while the absolute values were smaller and CO concentration profile made by CO2 gasification was almost disappeared.

Numerical results on the effects of temperature (1073, 1173 and 1273 K) on CO and CO2 concentration in exit stream are drawn in Figures 3a and 3b with experimental data. The numerical prediction well agreed with experimental results for 1173 and 1273 K, while some discrepancy was found at the condition

223

12

Z lO 8

6

8 o 2 o

o

~ f r o m CO2 Gasilicalion ~ , ~ i Total Concentration • E x p e r i m e y 0,,

1073 1173 1273 (a) Bed Temperature [K]

3

Z = 2

o

£ •

...... : , , : : . . - , iiiil|llllll II II *

• i i 1'073 1173 1273

(b) ~ Tem~,~.uro[K]

Figure 3. Effect of temperature on concentration of CO and CO2 at the top of the bed; for CO (a), for CO2 (b).

of 1073 K. The discrepancy would be possibly explained by the problem in the way of determination of a and ~: these values were determined only by using the data at 1173 K.

The contribution of individual reaction is also shown in Figure 3a. CO from combustion was found not to be affected by bed temperature, while it by steam gasification was increased with temperature. CO formed by CO2 gasification would be observed only above 1173 K.

The concentration profile of CO2 from steam gasification has a maximum around 1173 K, as shown in FigurQ 3b. This means that more amount of CO2 is consumed by CO2 gasification than the additionally produced CO2 by steam gasification. This is explained by the higher dependence of CO2 gasification on temperature than that of steam gasification, i.e., the respective apparent activation energies are 268 kJ/mol and 129 kJ/mol. Furthermore, the progress of steam gasification is restricted by the lack of the amount of steam with the progress. The CO2 produced by combustion is also found to be consumed by CO2 gasification.

........ from Steam Gasification from CO2 GssJ•catJorl Total Concentration

• Experimental

r. .o_

8 c O o O o

(a)

15

10

5 ,~,v'.'"".' .........................

100 200 Bed height [ram]

300

2

O

8 r -

8

o 0q

. . . . from Combustion I from Steam Gasification I Total Concentrate1 I

A Expedmen~ J

1;0 (b) Bed height [mm]

300

Figure 4. Effect of static bed height on concentration of CO and CO2 at the top of the bed; for CO (a), for CO2 (b).

224

3~3 Effect of bed height on total gasifier pexformance CO and CO2 concentrations varying with static bed height from 5 cm to 25

cm are shown in Figures 4a and 4b, where contribution of the bubbling zone on each reaction can be observed. It can be seen that CO content ascends along the height by the progress of steam and C02 gasifications while CO2 descends in this zone.

The concentration of C02 from steam gasification was kept almost constant by compensating production with consumption. It is clearly shown in Figure 4 that the contribution of the progress of the reaction in grid zone (<5 cm) to total gasification performance is high; nevertheless, the zone is only one fifth of the 25 cm in bed height and only 10% in volume.

4. CONCLUSIONS

The local gas concentration profiles were well predicted quantitatively by the present model, which evaluated local relative contribution of each reaction. It was confirmed that combustion occurs mainly in the vicinity of the jet region, steam gasification both in the annulus and emulsion regions and C02 gasification mainly in the emulsion region.

5. ACKNOWTJF.nGMENT

T. Kojima gratefully acknowledges for Grant-in Aid for Scientific Research by the Ministry of Education, Science and Culture, Japan. T. Kimura wishes to express his thanks for Sasakawa Scientific Research Grant from the Japan Science Society.

6, REFERENCES

1 M. Matsukata, Y. Honda, H. Yoshitake and T. Kojima, Fuel Processing Tech., in press

2 T. Kimura, K. Horiuchi, T. Watanabe, M. Matsukata and T. Kojima, Proc. 4th China-Japan Symp. Fluidization, (1991) 275

3 T. Kojima and T. Kimura, Proc. 7th Int. Conf. Fluidization, (1992) 151 4 M. Horio, in Ryudoso no Hannokogaku, (I. Muchi, S. Mori and M. Horio)

Chap. 4, Baifukan Tokyo, 1984 5 I. Tanaka, T. Ishikura, S. Hiromasa, Y. Yoshimura and H. Shinohara,

Funtaikogaku Kaishi, 17 (1980) 22 6 T. Mamuro and H. Hattori, J. Chem. Eng. Japan, 1 (1968) 1 7 H.A. Deans and L. Lapidus, AIChE J., 6 (1960) 656 8 D. Kunii and O. Levenspiel, Ind. Eng. Chem. Res., 29 (1990) 1226 9 T. Kimura and T. Kojima, Chem. Eng. Sci., 47 (1992) 2529 10 I. Matsui, D. Kunii and T. Furusawa, J. Chem. Eng. Japan, 18 (1985) 105 11 M. Saito, M. Sadakata and T. Sakai, Combustion Sci. and Tech., 51 (1987)

109 12 I. Matsui, D. Kunii and T. Furusawa, Ind. Eng. Chem. Res., 26 (1987) 88

225

Discussion

Analysis of local reactions in a laboratory scale jetting fluidized bed coal gasifier

T. Kimura, J.C. Bi, S. Uemiya and T. Kojima

Question: A. Bliek

How can one distinguish between CO from gasification and combustion reactions, in view of the coupling by the homogeneous shift reaction?

Answer

We did not include the homogeneous shift reaction in our model. Of course it is better to consider it, but in the first instance we did not because we were mainly focusing on the phenomena in the annulus, especially in the jet, where the oxygen is consumed (please refer to the answer to the question by Mr. Prins with our previous presentation). Instead, we employ the reaction (1) in the manuscript. The fraction of (d-10 of the CO produced is assumed to be converted into CO2, and the value of 0 is derived from the results of other experiments. Anyway, even if the homogenous shift reaction is taken into account, it is easy to distinguish in our model because in a thin block of the bed, the reaction rate of each reaction is individually evaluated. Indeed, in our model, CO is produced from both combustion, CO 2 gasification and steam gasification, and each reaction is evaluated independently.