Catalytic Activity of the Black Liquor and Calcium Mixture in CO2 Gasification of Fujian Anthracite*

Chin. J. Chem. Eng., 15(5) 670-679 (2007)

Catalytic Activity of the Black Liquor and Calcium Mixture in C 0 2 Gasification of Fujian Anthracite* Gul-e-Rana JAFFRI and ZHANG Jiyu( %%-?)** Institute of Chemical Engineering and Technology, Fuzhou University, Fuzhou 350002, China

Abstract C 0 2 gasification of Fuijian high-metamorphous anthracite with black liquor (BL) andor mixture of BL and calcium stuff (BL+Ca) as catalyst was studied by using a thermogravimetry under 7 5 W 5 O " C at ambient pressure. When the coal was impregnated with an appropriate quantity of Ca and BL mixture, the catalytic activity of C 0 2 gasification was enhanced obviously. With a loading of 8%Na-BL+2%Ca, the carbon conversion of three coal samples tested reaches up to 92.9%-99.3% at 950°C within 30min. The continuous formation of alkali surface compounds such as ([-COM], [-COzM]) and the presence of exchanged Ca, such as calcium phenolate and calcium carboxylates (C00)2Ca, contribute to the increase in catalytic efficiency, and using BL+Ca is more efficient than that adding BL only. The homogeneous model and shrinking-core model were applied to correlate the data of conversion with time and to estimate the reaction rate constants under different temperature. The corresponding reaction activation energy (E,) and pre-exponential factor of three anthracites were estimated. It is found that E, is in the range from 73.6 to 121.4kT~mol-'in the case of BL+Ca, and 74.3 to 104.2M~mol-'when only BL was used as the catalyst, both of which are much less than that from 143.5 to 181.4kJ~mol~' if no catalyst used. It is clearly demonstrated that both of BL+Ca mixture and BL could be the source of cheap and effective catalyst for coal gasification. Keywords thermogravimetry, black liquor, calcium, catalytic gasification, high-metamorphous anthracite, reaction kinetics

1 INTRODUCTION The catalytic activity of alkali and alkaline earth metal compounds in the gasification process of carbonaceous material has been known for many years. The catalytic gasification technique has been already used as an efficient and economic method for conversion of coal to clean fuel gas[l-31. The carbonates, oxides and hydroxides and other active salts of Na, K, Ca are the most effective catalyst in CO2 and steam gasification to enhance the reaction rate, promote the gas quality and reduce the reaction temperature[4,5]. However, the alkali and alkaline earth metal catalyst are expensive and economically unfeasible in gasification, and its recovery and treatment after gasification is still a serious problem. So it is essential to choose a cheaper, cost competitive and effective catalyst, such as black liquor (BL) containing Na and K, to replace the pure alkali, some lignin and organic compounds being gasified in coal gasification. Unfortunately in gasification the BL can produce sulfur-containing gases, such as hydrogen sulphide (H2S) and carbonyl sulphide (COS), and some trace organic sulfur-containing gases, such as CH3SH, (CH3)2S, and (CH3)2S2, all of which have crucial influence on the re-utilization of Na2S in kraft pulping and gas turbine combined by gasification process, which often infract the environmental regulations[6]. For conversion of H2S and COS into CaS in sulfur capture processes, calcium carbonate and oxide are most attractive[6,7] and also CaCO3 can react with COOH group to form ion-exchangeable Ca which exhibits high catalytic activity to enhance the gasification rate[8]. Therefore, combining two advantages of BL and Ca catalyst to Received 2007-01-02, accepted 2007-06-25.

make the BL+Ca mixture as catalyst might be a better choice. It not only can implement high effective gasification, but also reduce the catalyst cost since abundant BL and dolomite resource are much cheaper. In the previous study, the BL catalyst was shown to have a distinct catalytic efficiency in carbon dioxide gasification[9] and suitable addition of Na species enhanced the catalytic efficeincy[ lo]. The present investigation will be focused on the effect of BL+Ca concentration on carbon conversion, reaction rate, reaction kinetics on three Fujian high-metamorphous anthracites in C02 atmosphere. Suitable conditions such as temperature and BL+Ca concentration for gasification are presented. 2 EXPERIMENTAL Three Fujian high-metamorphous anthracites, Longyan (LOY), Fenghai (FEH) and Youxi (YOX) coals, were selected and their proximate and ultimate analysis is given in Table 1. The BL from wood pulp, provided by the Naping Paper Mill, Fujian, China, was used as the catalyst and the Na content in BL is 2.07% (by mass), measured directly by a flame photometer using an instrument of HG-3, 2400318 model made by Beijing Measuring Instrument Co., China. In the original BL the solid and H2O content determined on dry basis is respectively 10.61% and 89.39% and its density is 1.077kg.L-'. In the case of BL as the catalyst, the concentration of 3%, 5%, 8%, 10% of Na-BL loading in the coal sample is the mass ratio of Na amount only came from the additive BL to the coal, respectively. For example, when preparing 5g coal

* Supported by the National-Natural Science Foundation of China (No.20376014) and Fujian Science and Technology Council Grant (HG99-01). ** To whom correspondence should be addressed. E-mail: [email protected]

Catalytic Activity of the Black Liquor and Calcium Mixture in C02 Gasification of Fujian Anthracite

671

Table 1 Proximate analysis and ultimate analysis of Fujian anthracite Proximate analysis

Coal type Mad%

LOY FEH YOX NOk:o a d % =

Aad%

Ultimate analysis

vad%

CFadW

&,ad%

Had%

cad%

5.74 2 1.39 71.51 1.36 0.91 25.17 4.46 69.46 1.48 28.66 5.45 64.41 100%-A,d%-St,ad%-C,d%-Had%-Na~%Mad%.

1.23 1.20 1.09

2.18 2.19 2.00

69.37 66.79 63.57

back

oad%

3.70 3.10 2.50

ameter, i.e., 100 mesh, and again oven-evaporated at 120°C to fully dry; Finally these dry samples were stored in desiccators and then used for their gasification. The gasification of BL/coal and (BL+Ca)/coal samples were performed by using a CAHN TG-2151 thermogravimetric system as shown in Fig. 1[9,10]. Under the ambient pressure three gases enter this thermogravimetric system separately. The purge gas is used to protect the thermo-balance from high temperature. The reaction gas enters the reaction furnace from the bottom and passes through the surface of BLfcoal and (BL+Ca)/coal samples placed in the sample container, then merges with the purge gas and exits from the top of the reaction furnace. The furnace gas serves to prevent the furnace from being overheated and isolate it from being corroded by the reaction gas. In order to eliminate the effect of variation in external and internal diffusion in the coal particles on the gasification kinetics, the operation conditions, such as coal sample mass of about 50mg, COz flow-rate of 200cm3.minp' and coal particle size of below 0.154mm, should be followed exactly[lO]. The sample was placed in the quartz crucible (sample container) and heated in N2 stream with the flow-rate of 200cm3min-' at the speed of 20"C.min-' to the desired temperature ranging

sample with 3%Na-BL loading, the amount of BL is (5 X 0.0312.07)X 100=7.24g, i.e. 5g coal was soaked sufficiently in 7.248 of original BL diluted by an appropriate distilled water. In the mixed catalyst the concentration of 3%Na-BL+4%Ca, S%Na-BL+3%Ca, 8%Na-BL+2%Ca, 1O%Na-BL+1%Ca in the coal sample is the mass ratio of both Na amount came from the additive BL and the Ca amount occupied in CaC03 to the coal, respectively. The CaCO3 powder with 99.0% purity is produced from Xilong Chemical Plant, Shantou, Guangdong Province. Similarly for preparation of 5g coal sample with 3%Na-BL+4%Ca, the amount of Ca is (5 X 0.04 X 100.08/40.08)/0.99 = 0.504g of CaC03, i.e. adding 0.504g of CaC03 powder into 7.248 original BL diluted by an appropriate distilled water to soak 5g coal. The coal samples were prepared by the following procedure: First, the resultant BLfcoal mixture was made by adding only BL, but the mixture of BL+Ca was prepared by adding the fine CaC03 powder with a certain quantity of Ca in the dilute BL with the addition of 5ml distilled water and then mixed with the coal sample powder. The mixed samples were stirred at room temperature until homogeneous slurry formed and then the slurry was oven-evaporated at 120°C for overnight; After that the dried samples were ground by mortar with pestle, sieved to below 0.154mm dibalance

Nd% 0.77 0.64 0.70

-u

gas

input

I

l

l

I

flow controller

accumulator

coil

reaction gas

input

accumulator

controller

coil

furnace gas

gas input

Figure 1 The flow sheet of TG2151

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from 750 to 950F, and, then switched quickly to CO2 stream of 200cm .min- preheated to the set temperature for 30min for the gasification. Since both of lignin and organic compounds contained in BL and the coal samples mixed with BL and BL+Ca can be gasified simultaneously, the calculation of carbon conversion (X) for pure coal sample in gasification requires a special correction to delete the carbon conversion contributed from the BL organics. X can be calculated by AW - W G,$ X= (1) Wo YCFd where A W mass change of coal sample during At time interval, mg; Wo: initial sample mass, mg; CFad:fixed carbon in sample coal, %; a: [BL solid mass]/[coal mass+(BL solid mass)+(calcium solid mass)]; p: mass change percent of pure BL during At time interval

0.8

0.6

corresponding to AW mass loss, %; y: (coal mass)/ [(coal mass)+(BL solid mass)+(calcium solid mass)] It should be emphasized that the BL solid weight used in Eq.(l) is the residual mass after drying the liquid BL in oven-evaporated at 120°C for overnight.

3 RESULTS AND DISCUSSION 3.1 Effect of operation conditions To determine the catalytic activity of BL and BL+Ca mixture in gasification, different concentrations were loaded on three Fujian anthracites and their carbon conversion in the range of 750-950°C in 30min is listed in Table 2. It is clear that the carbon conversion of X in the presence of catalysts is much higher than that in the absence of catalysts. In the case of only BL was used as catalyst, taking LOY coal as an example, the carbon conversion against gasification time (s) is shown in Fig.2. It can

0.8

0.8

0.6

0.6

s Y

s Y

s

Y 0.4

0.4

0.4

0.2

0.2

0

10

20

30

20

10

0

1, min

0

30

(a) LOY pure coal

1,

(b) 3% Na-BL

1.0

1.o

0.8

0.8

0.6

0.6

s

20

10

t, min

min

( c ) 5% Na-BL

s Y

Y

0.4

0.4

0.2

0.2

0

I

I

I

10

20

30

t, min

0

20

10

30

1, min

(d) 8% Na-BL (e) 10% Na-BL Figure 2 Carbon conversion versus time at different temperature and BLconcentrationsfor LOY coal 0750°C; 0800°C;

October, 2007

X

850°C; ~ 9 0 0 ° C ;*950"C

30

CatalyticActivity of the Black Liquor and Calcium Mixture in COz Gasification of Fujian Anthracite

673

Table 2 Carbon conversion ( X ) in the range of 75W50'(: in 30min Catalyst

Coal

Loading, % (by mass)

BL

LOY

BL+Ca

BL+Ca

LOY

x,% 800"C

0

750'C 0.5

3%Na-BL S%Na-BL 8%Na-BL lO%Na-BL

14.56 35.49 44.42 41.65

3%Na-BL+4%Ca 5%Na-BL+3%Ca

22.42 30.90 40.66 42.66 0.64

18.43 44.38 67.80 61.91 25.30

8%Na-BL+2%Ca lO%Na-BL+l%Ca 0

FEH

8%Na-BL 8%Na-BL+2%Ca 0 8%Na-BL 8%Na-BL+ 2%Ca

YOX

be seen that for any concentration of Na the variation of X against t exhibits obvious increase with the increase of gasification temperature. The sodium loading in BL, ranging from 0%, 3%, 5%, 8%to lO%Na-BL, influences much strongly on the magnitude of X, as compared to cases with no sodium loading in BL [Fig.2(a)] in which X is very low. The highest X was attained at 8%Na loading on BL [see Fig.2(d)], indicating the saturation effect of sodium loading in BL catalyst, beyond 8%Na loading on BL, probably because the pores in the coal were blocked[5,11] and the access of C 0 2in micro-pores was hindered, resulted in the fall of X [Fig.2(e)]. 100 I

1.5

51.63 62.43 60.75 0.95

17.25 25.88

41.28 5 1.09

0.36 39.70 42.74

0.66 65.94 67.53

850°C 2.0 29.58 78.10 86.03 76.94 35.32 7 1.70 78.31

900 'C 4.3 46.50 80.70 93.55 82.68 63.23 80.12 91.82

80.03 2.10 49.87 70.28 1.69 82.80 85.0

950'C 8.2 71.38 84.85 95.5 1 80.85 73.32 78.46 92.91

9 1.88 4.50

95.41 15.01

61.61 87.64

68.35 95.43 10.48 96.52 99.29

4.55 91.63 94.86

For comparison of catalytic efficiency between BL and mixed BL+Ca catalysts based on the data on LOY coal examples in Table 2, it is found that X in 3Omin changed distinctly with the catalyst concentrations at all tested temperature. If considering the percentage of sodium in both BL and BL+Ca catalyst as an abscissa, i.e. 3%Na-BL, S%Na-BL, 8%Na-BL, lO%Na-BL in Fig.3(a) and 3%Na-BL+4%Ca, 5%Na-BL+3%Ca, S%Na-BL+ 2%Ca, lO%Na-BL+l%Ca in Fig.3(b), the variation of X with Na concentration in BL or BL+Ca catalysts shows quite different patterns. The former exhibits a maximum value of X , corresponding to the saturation catalyst concentration S%Na-BL, under different 100 I

I

80

60

60

s Y

.s

ss

40

40

20

20

0

2 4 6 8 1 catalyst concentration, %

0

0

2 4 6 8 1 catalyst concentration, %

0

(b) Mixed BL+Ca (a) Only BL Figure 3 Effect of catalyst concentrationon X in BL and BL+Ca for LOY coal 0750°C; 0800'C; ~ 8 5 0 ° C ~; 9 0 0 ° C 0950°C ;

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temperature, but the latter displays the gradual rise of

X with the increase in the total BL+Ca concentration and beyond the 8%Na-BL+2%Ca concentration the increment of X becomes slower. Also, beyond 5% Na-EL at 850°C the former reveals the similar and high values of X in the range of 850°C to 950°C as shown in Fig.3(a) since the sodium salts in BL may be converted into the alkali metal carbonates during pyrolysis below 675 "C in C02 gasification[l2] and Na2C03 decomposes more easily near and above its melting point (851°C) that leads to the formation of Na2O. However, beyond 5%Na-BL at 900°C the latter indicates the very close and high values of X in the range of 900°C to 950°C as obviously depicted in Fig.3(b) because CaC03, contained in the mixture of BL+Ca, starts to decompose near 898.6"C and bring out the formation of CaO. Both above metal oxides will react with C02 to reform carbonates that cause

more close results of X as mentioned above. Based on the concept of saturation BL catalyst concentration [see Figs.2 and 3(a)], 8% Na-BL is a better selection and used as the base in making the mixture of BL+Ca for different coal samples. Under 8%Na+2%Ca condition except for LOY coal, both of FEH and YOX coals have much higher X in comparison with using only 8%Na-BL as catalyst as indicated by Table 2. The more clear presentation for FEH and YOX is in Fig.4. In fact the variation of X is not only a complex function of the catalyst concentration, but also the internal surface area of char, coal lithofacies structure and ash composition etc. Therefore, both BL and mixed BL+Ca catalysts offered quite different enhancement on coal conversion: the value of X by using 8%Na-BL+2%Ca is about 1.4 times that using 8%Na-BL for FEH coal, about 1.03 times that for YOX coal, but about 0 . 9 1 3 . 9 8 times that for LOY coal.

1.o

1.o

0.8

0.8

0.6

0.6

s

s Y

Y

0.4

0.4

0.2

0.2

I

W

0

10

20 /,

30

0

I

I

10

20

min

II

30

t, min

(b) 8%Na+2%Ca in FEH

(a) 8%Na-BL in FEH I .0

1.o

0.8

0.8

0.6

0.6

s

s

Y

Y 0.4

0.4

0.2

0.2

0

20

10

t, rnin

30

0

20

10

30

t, min

(c) 8%Na-BLinYOX (d) 8%Na+2%Ca in YOX Figure 4 Carbon conversion versus time at different temperature with BL and mixture of BL+Ca 0750°C; + 800°C; X 850°C; ~ 9 0 0 ° C ; 950°C

October, 2007

Catalytic Activity of the Black Liquor and Calcium Mixture in C 0 2 Gasification of Fujian Anthracite

3.2 Catalytic mechanism of mixed BL+Ca catalyst For the case of using BL catalyst only, the catalytic activity and the mass loss of samples increase significantly with temperature for it contains a certain quantity of alkali metal Na and K[9]. As pointed out by Gea et a1.[12] the sodium salts in BL can be converted into the alkali metal carbonates during pyrolysis below 675°C in C 0 2 gasification. At temperature above 675 "C the alkali carbonates decompose and alkali metals in alkaline BL char form the intermediate alkali-surface compounds, such as [-C02M], [-COM] and [-CM], i.e. M2C03(s)+ C(S)+ [-COM](s) + [-CO~M](S) (2) Then these intermediate alkali-surface compounds will react with the carbon present in the carbonaceous matrix to produce CO gas[12-141. The successive formation of alkali metal carbonates (M2CO3) converted from BL and of alkali-surface compounds ([-COM] and [-C02M]) decomposed from alkali carbonates provides sufficiently catalytic action and much higher conversion. For the case of using CaC03 catalyst only, CaC03 would initially dissolve in deionized water, because the direct reaction between CaC03 and COOH groups in coal is unlikely. Then, Ca2+ions undergo exchange reactions with COOH groups. The ion exchange reaction may proceed according to the following scheme[ 151: CaCO, = Ca2++ (3)

COS-

Ca2++ 2(-COOH) = (-COO)2 Ca + 2Hf

(4)

2H' + C0:- = H 2 0 + CO, (5) When the CaC03 initially dissolved in BL liquid and also in added deionized water, a hydroxyl group and a carboxyl group attached to a benzene ring in coal will react and bind with the ionized calcium to form calcium phenolate and calcium carboxylate (-COO),Ca . When the temperature is raised to about 400"C, C 0 2 will break off from the carboxylate group and the attached benzene ring will exhibit negative charge. In the C02 atmosphere the carbon atom, inside the benzene ring located next to the carbon atom with a calcium phenolate group, can now react with a C02 molecule because of the extra negative 2",harge existed in this benzene ring. Here the role of Ca ion can hold one oxygen end of the C02 molecule, so that another oxygen atom in C 0 2 can react with the benzene ring. A net result is the formation of two CO molecules and destruct one corner of the benzene ring as pointed out in the Ref. [161. Therefore, in the mixed BL+Ca catalytic gasification, the continuous formations of alkali surface compounds ([-COM], [-C02M]) from BL and the presence of exchanged Ca, such as calcium phenolate and calcium carboxylates (-COO),Ca , will enhance the gasification rate and carbon conversion by increased amount of CO than that in the case without catalyst.

3.3 Kinetic modeling A number of kinetic models have been proposed

675

for describing the gas-solid reactions, in which both homogeneous and shrinking-core models are based on the first order reaction as the rate-controlling factor[ 17J and usually accepted in the gasification of coal particles. The homogeneous model (HM) can be described as

dX l d t = k(1- X )

(6)

and its integrated form is -ln(l- X ) = kt (7) The shrinlung-ore model (SCM) can be expressed as dXldt=k(l-X)2/3 Integrating Eq.(8) it is gotten

(8)

3[1- (1 - X ) ' / 3 ]= kt

(9) In both model equations, k is the first order reaction rate constant. Based on the experimental data of X versus t at different catalyst concentration and temperature and by respective1 re ressing -ln(l -X) vs. t for HM and 3[1 -(I -X)x] fs. t for SCM, the reaction rate constant k, represented by the slope of each straight line through the origin, can be given as shown in Table 3 for LOY coal. It is clear that the k value is raised with the increase of temperature. In cases of lower 3%Na-BL+4%Ca and higher 10%Na-BL+l%Ca mixture, k are higher than that at only 3%Na-BL and 10%-BL due to the combined catalytic action of BL+Ca. But for 8%Na-BL+2%Ca and 5%Na-BL+3%Ca, k are less than that for 8%-BL and 5%-BL only, which implies that each mixed BL+Ca catalyst needs an appropriate ratio of Na-BL to Ca, at which a better X can be achieved as indicated in Table 2 and higher k values as shown in Table 3 by comparing with using BL only. However, as mentioned before, when using the saturation BL catalyst concentration 8%Na-BL as a base in making the mixture of BL+Ca, except for LOY coal, both of FEH and YOX coals at 8%Na-BL+2%Ca have much higher X in comparison with using only 8%Na-BL. Therefore, this mixture of 8%Na-BL+2%Ca is still used as a better selection to perform the kinetic investigation for three coal samples. Also, in Table 3 it is clear that the k values estimated by SCM and HM are close when X is lower, without catalyst in the whole temperature range or with catalyst at lower temperature ( d 800°C), but when X is higher using catalyst in the higher temperature range (>8OO0C) the k values calculated by HM are higher than that by SCM, the higher the temperature, the greater the difference between HM and SCM. Considering the 8%Na-BL+2%Ca mixed catalyst as an example, Fig.S(b) demonstrates that the relationship of [ -ln(l -X)] vs. t is linear and the reaction order is very close to 1 in the whole temperature range for all three coals, suggesting that the gasification proceeds uniformly throughout the coal particles. Howvs. t according to ever, the plots of 3[1-(1-X)'"] SCM in Fig.S(a) appear almost linear at lower temperature ( d 800 "C ), while at higher temperature Chin. J. Ch. E. lS(5) 670 (2007)

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Table 3 Rate constant k (min-') calculated respectively by SCM and HM for both BL and BL+Ca catalyst Catalyst loading in Longyan coal, % (by mass) 0 3%Na-BL 5%Na-BL 8%Na-BL lO%Na-BL 3%Na-BL+4%Ca 5%Na-BL+3%Ca 8%Na-BL+2%Ca lO%Na-BL+l%Ca

750 SCM 0.0002 0.0056 0.0141 0.0184 0.0174 0.0082 0.012 0.0163 0.0178

Gasification temperature, 'C 850 900 HM SCM HM SCM HM 0.0005 0.0007 0.0007 0.0014 0.0015 0.0069 0.0110 0,0115 0.0182 0.0196 0.0194 0.0372 0.0441 0.0465 0.0571 0.0386 0.05 15 0.0652 0.0681 0.0953 0.0352 0.046 1 0.0560 0.0537 0.0678 0.0096 0.0129 0.0136 0.0267 0.0301 0.0248 0.0365 0.0428 0.0475 0.0586 0.0350 0.0432 0.0521 0.0634 0.0857 0.0338 0.0471 0.0576 0.0686 0.0947

800

HM 0.0002 0.0057 0.0149 0.0198 0.0126 0.0085 0.0126 0.0174 0.0191

SCM 0.0005 0.0068 0.0181 0.0335 0.03 11 0.0093 0.0226 0.0309 0.03

2 0

$5- t byox d

v

_ I

FEH

I

I / !

Y

m

0

4 2

0

5

10

15 20 t, min

25

30

(a)SCM

-c 1

4 FEH

?

I

.d I

" L n

t, min

(b) HM Figure 5 Comparison of linear relationship using SCM and HM for three coals at 8%Na+2%Camixed catalyst 0750'C; 0800°C; +850"C; X 900°C; r950"C

(>8OO"C) they are nonlinear. Since the same experimental data of X vs. t are analyzed by HM or SCM October, 2007

'

950 SCM HM 0.0028 0.0029 0.0319 0.0368 0.05 18 0.0654 0.0789 0.1171 0.0539 0.0677 0.0355 0.0415 0.0493 0.0609 0.0653 0.0895 0.0779 0.1145

model, it is believed that HM is better than SCM in illustrating the catalytic gasification lunetics when using BL and BL+Ca as catalysts. With 8%Na-BL as the base to examine the additive catalytic action of CaC03 in the mixed BL+Ca catalyst, the reaction rate constants calculated by SCM and HM for three coals are listed in Table 4. Similar to the results in Table 2, both of FEH and YOX coals at 8%Na-BL+2%Ca have higher k values in comparison with using only 8%Na-BL, but LOY coal has an opposite tendency. A probable and rough explanation at least is that a series of higher values, such as SiO2/A1203=2.15, FezO3= 10.04%, total alkali oxides (Fe203+CaO+MgO+KzO+Na20)= 18%, exist in the ash composition of LOY coal, but the lower values of 1.45 and 2.07, 7.13% and 7.56%, 14.95% and 15.38% represent in FEH and YOX coals, respectively. Na2O and CaO in the mixed BL+Ca catalyst accelerate the coal gasification, but also react easily with SiOz and A1203 in ash to form some different complex silicates including Na, K, Ca, Fe, Mg elements[l8], that reduces somewhat the concentration of NazO and CaO for catalyzing the gasification reaction. Therefore, LOY coal at 8%Na-BL+2%Ca to have lower, but FEH and YOX coals have higher X and k values in comparison with using only 8%Na-BL as indicated in Tables 2 and 4. A better explanation requires more detailed examination of coal surface properties and lithofacies components by X-ray diffraction (XRD), scanning electron microscope (SEM), EDS analyses and comparison of coal samples before and after catalytic gasification, that will appear in the further reports. For YOX, FEH and LOY coals at the same concentration of 8%Na-BL+2%Ca the reaction rate constants at 950'C are shown in Fig.6. In the comparison with non-catalytic gasification the very low k values are quit obvious. The catalytic action of 8%Na-BL+2%Ca is much evident and the very high catalytic efficiency occurs in YOX coal, then in FEH and LOY coal. By comparing k values in Tables 3 and 4 it can be found that k estimated by SCM is mostly lower that by HM at temperature 75(+95O"C, especially at beyond 850°C. Based on the k values estimated by HM or SCM, the order of reaction rate of experimental coals

Catalytic Activity of the Black Liquor and Calcium Mixture in C 0 2 Gasification of Fujian Anthracite

677

Table 4 Rate constant k (min-') calculated by SCM and HM for BL+Ca mixtures with three coals Catalyst loading, 9% (by

Reaction temperature, "C Coal

750 SCM

mass)

8% NaBL 8% NaBL+ 2%Ca

800 HM

SCM

850

900

HM

SCM

HM

SCM

950 HM

SCM

HM

LOY

0.0184

0.0198

0.0335

0.0386

0.0515

0.0652

0.0681

0.0953

0.0789

0.1171

FEH

0.0066

0.0068

0.0176

0.0188

0.0231

0.0252

0.0326

0.0371

0.0406

0.0478

YOX

0.0161

0.0172

0.0328

0.0375

0.0506

0.0632

0.0666

0.0909

0.0858

0.1327

LOY FEH YOX

0.0163

0.0174

0.0309

0.0350

0.0432

0.0521

0.0634

0.0856

0.0653

0.0895

0.0095

0.0099

0.0225

0.0246

0.0359

0.0418

0.0564

0.0727

0.0725

0.1037

0.0176

0.0188

0.0335

0.0386

0.0517

0.0652

0.0721

0.1028

0.0939

0.1605

FEH coal >LOY coal at temperature 950°C.

3.4 Reaction activation energy The first order reaction rate constant k for both SCM and HM can be represented by using the well known Arrhenius equation, i.e. k = Aexp[-E, I R T ] , from which the pre-exponential factor (A) and the activation energy (E,) can be determined as given in Table 5 for all non-catalytic, BL only and mixed BL+Ca catalyst cases. It is clear in Table 5 that E, and A values at 3%Na-BL+4%Ca and 8%Na-BL+2%Ca in LOY coal are less than that at only 3%Na-BL and 8%Na-BL, but at 5%Na-BL+3%Ca and lO%Na-BL+l%Ca these E, and A are larger than that at only S%Na-BL and 10%Na-BL. The above discrepant results illustrate that the influence of mixed catalyst on gasification activity is complex and under some appropriate ratios of Na to Ca in mixed catalyst the catalytic reaction can be enhanced.

Figure 6 Effect of catalyst loading 8%Na-BL+2%Ca on k at 950C for LOY, FEH and YOX coals non-cat. by SCM; non-cat. by HM; with cat. by SCM; with cat. by HM

at only 8%Na-BL loading is LOY coal>YOX coal> FEH coal at temperature 7 5 H O O " C and YOX coal > LOY coal > FEH coal at temperature 950"C, at 8%Na-BL+2%Ca loading is YOX coal>LOY coal> FEH coal at temperature 75WOO"C and YOX coal >

Table 5 Activation energy E , and pre-exponential factor A for range of 750-950C Catalyst

Coal

none

LOY

BL

BL+Ca

Loading, %

E ~H.moI-' ,

(by mass)

SCM

FEH

0 0

YOX

0

LOY

A, min-l

Correlation coefficient p'

HM

SCM

HM

SCM

HM

143.51

144.02

3.64 X lo3

0.9965

159.96

2.2s x lo4

3 . 8 6 lo3 ~ 2.47 x lo4

0.9967

159.14

0.9916

0.9914

180.77

181.42

1.70x lo5

I .84 x lo5

0.9981

0.9981

4.9 x 10'

0.9956

0.9935

3.01 X 10'

0.9925

0.9955

3%Na-BL

92.12

98.2

2.4 X 10'

5%Na-BL

74.30

84.42

8.70 X 10'

8%Na-BL

76.16

93.52

1.59X 10'

1.31X lo3

0.9754

0.9923

10%Na-BL

75.93

87.11

5.68 X 10'

FEH

8%Na-BL

89.71

0.9804 0.9614

0.9894 0.9724

YOX LOY

8%Na-BL 3%Na-BL+4%Ca

85.22

96.70 104.22

1.43X 10' 3.16X 10' 4.14 X 10'

0.9586

0.9855

82.04

88.83

1.06X 10'

2.4X 10'

0.9982

5%Na-BL+3%Ca

75.50

84.83

9.9 x 10'

3.13 X 10'

0.9767

0.9957 0.9864

FEH YOX

7.36X 10' 4.03 x lo3

8%Na-BL+2%Ca

73.61

87.7

1.06X 10'

5.91 X lo2

0.9852

0.9970

lO%Na-BL+l%Ca

79.34

96.52

2.14 X 10'

1.69x lo3

0.9646

0.9886

8%Na-BL+2%Ca

104.73 86.34

121.4

2.46 x lo3

1.75x lo4

0.9867

0.9981

109.96

4.95 x 10'

8 . 2 0 lo3 ~

0.9793

0.9983

8%Na-BL+2%Ca

Chin. J. Ch. E. 15(5) 670 (2007)

678

Chin. J. Ch. E. (Vol. 15, No.5)

In the case at the same concentration of mixed BL+Ca catalyst, i.e. at 8%Na-BL+2%Ca, both of E, and A values calculated by SCM or HM for experimental three coals are in order of FEH coal>YOX coal>LOY coal which is not exactly opposite to the above k order of YOX coal>LOY coal>FEH coal, however at only 8%Na-BL catalyst E, computed by SCM is in following order: FEH coal>YOX coal> LOY coal and E, computed by HM is in order of YOX coal>FEH coal>LOY coal. In addition, the quit different orders appear in the pre-exponential factors. It implies that using E, and A to distinguish the reaction activity of coak is somewhat ambiguous, the more reliable criteria judging the reaction activation of coal is based on the magnitude of k value as pointed by early reports[9,10], since the k value is directly given by regressing the relation between X and t and the E, and A are secondly obtained by regressing the Arrhenius equation k = Aexp[-E, / RT] . For LOY coal these values E,= 76.16 and 93.52kJ.rnol-' at 8%Na-BL from SCM and HM respectively are very close to 75.3kJ.mol-' and 80.3kJ.mol-' in case of 9% KzCO~ and NaZCO3loading measured by Li and Cheng[5] and 8% mixing alkali salt Ea=76.03kJ~rnol-' in fixed bed by Zhang et a1.[19] which indicates the comparability between this work and reference results obtained from the same kmd of LOY coal. In case of mixture of BL+Ca the E, predicte? by SCM and HFf is in a range from 73.61kJ.mol- to 121.4kJ~mol- and at only BL case it is in a range from 74.30kJ-mol-' to 104.22kJ~mol-' for experimental three coals, which are more less than that in a range of 143.51kJ~rn01-~-181.42kJ~mol-'for pure coal gasification at without catalyst. Here as mentioned above at the best mixed catalyst concentration, i.e. at 8%Na-BL+2%Ca, for ex-1 perimental three coals the Ink vs. 1/T is calculated respectively as shown in Fig.7(a) for SCM and Fig.7(b) for HM. The perfect fitting with the liner and very high confidence coefficients (seen in Table 5 ) provide a number of valuable and believable values of E, and A in distinguishing and ordering the activation of coal samples. 4 CONCLUSIONS (1) Both BL and BL+Ca catalysts exhibit more distinct catalytic action in gasification of Fujian high-metamorphous anthracite. This is due to the continuous formation of alkali metal carbonates (MzC03) converted from BL and of alkali-surface compounds ([-COM] and [-COzM]) decomposed from alkali carbonates, the presence of exchanged Ca such as calcium phenolate and calcium carboxylates (-COO)zCa. (2) At appropriate catalyst concentrations of 8%Na-BL and 8%Na-BL+2%Ca under 950°C the X in COz reaches respectively 95.51% and 92.91% for LOY coal, 96.52% and 99.29% for YOX coal, 68.35% and 95.43% for FEH coal, but without catalyst above three coals only have very low X less about 15%. (3) Both SCM and HM can be used to simulate the relationship between X and t for their catalytic gasification kinetics in COz atmosphere and to predict October, 2007

-2

1

9.0X

8.0X

I.0X

IIT, K-' (a) Calculated by SCM

-2

2 -3

-4

-5

8.ox

9.ox

LOX

m3

lIT, K-'

~

(b) Calculated by HM Figure 7 Arrhenius plots of LOY, FEH and YOX coal at 8%Na-BL+2%Ca 0 Longyan coal; Fenghai coal; A Youxia coal fit for LOX fit for FEH; - - - - - fit for YOX - - - -

the values of k , E, and A. In comparison by fitting the same experimental data at BL and BL+Ca catalysts HM is better than SCM and has a better agreement and correlation coefficient. (4) At all concentrations of BL+Ca catalyst in the temperature range 75W950"C, the E, predicted by SCM and HM is in a range 73.61-121.4kJ~mol-' ant at only BL case it is in a range 74.30-104.22kJ~rnolfor experimental three coals, which are more less than that in a range 143.51-181.42kJ.mol-' for pure coal gasification at without catalyst. ( 5 ) The reliable criteria judging the gasification activation is based on the comparison of k and not on E, since k is directly given by regressing the relation between X and t and E, is secondly obtained by

CatalyticActivity of the Black Liquor and Calcium Mixture in C 0 2 Gasification of Fujian Anthracite

regressing the Arrhenius equation k = Aexp[-E, I R T ] . Following the magnitude of k in the range 750--900”C the gasification activation order at only 8%Na-BL is as LOY coal > YOX coal > FEH coal and at 8%Na-BL+2%Ca is as YOX coal>LOY coal>FEH coal.

REFERENCES 1

2 3 4 5 6

7 8 9 10

Calahorro, V.C., Pan, Y.G., Garcia, B.A., Serrano, GV., “Thermogravimetric study of anthracite gasification in C02 catalyzed by black liquor”, Energy Fuels, 8, 348-354( 1994). Sams, A.D., Talverdian, T., Shadman, F., “Kinetics of catalyst loss during potassium catalyzed COz gasification of carbon”, Fuel, 64(12), 1208-1214(1985). Lee, J.W., Kim, D.S., “Catalytic activity of alkali and transition metal salt mixture for steam-char gasification”, Fuel, 74(9), 1387-1393( 1995). Mckee, W.D., “Mechanisms of the alkali metal catalyzed gasification of carbon”, Fuel, 62(2), 170-175( 1983). Li, S., Cheng, Y., “Catalytic gasification of gas-coal char in CO;’, Fuel, 74(3), 456458(1995). Zeng, L., Vemll, L.C., Heiningen, V.P.R.A., “Thermodynamic study of calcium based sulphur recovery for Kraft black liquor gasification”, Pulp Paper Can., 101(12), 111-1 17(2000). Yumura, M., Furimsky, E., “Comparison of CaO, ZnO and Fez03 as H2S adsorbents at temperature”, Ind. Eng. Chem. Proc. Des. Dev., 2, 1165-1168(1985). Ohtsuka, Y.,Asami, K., “Highly active catalysts from inexpensive raw materials for coal gasification”, Catal. Today, 39, 111-125(1997). Jaffri, G.R., Zhang, J.Y., “Catalytic gasification of Fujian anthracite in COz with black liquor by thermogravimetry”, J. Fuel Chem. Tech. (China),35(2), 129-135(2007). Lin, R., Zhang, J.Y., Tatalytic gasification kinetics of

679

low active anthracites by carbon dioxide in thermogravity-isothermal thermo-gravimetric analysis”, J. Chem. I d . Eng. (China),56(12), 2332-2341(2005). (inChinese) 11 Zhang, J.Y., Wang, X., Kuang, G., Lin, J., Chen Y., “The BL catalytic gasification of high-metamorphous anthracite in scaled-up and continuous fluidized beds”, In: The 6th International Conference on Fluidization , Beijing, 269-278(2006). 12 Gea, G., Maria, B., Arauzo, J., “Thermal degradation of alkaline black liquor from straw thermogravimetric study”, Ind. Eng. Chem. Res., 41,4714- 4721(2002). 13 Gea, G, Sanchez, J.L., Murillo, M.B., Arauzo, J., “Kinetics of C 0 2 gasification of alkaline black liquor from wheat straw: Influence of CO and COz concentrations on the gasification rate”, Ind. Eng. Chem. Res., 43, 32333241(2004). 14 Grace, T.M., Frederick, W.J., Lisa, K., Wag, K., “New black liquor drop burning model”, In: International Conference on Chemical Recovery, Tampa, 257 (1998). 15 Ohtsuka, Y., Asami, K., “Ion exchanged calcium from calcium carbonate and low rank coals: High catalytic activity in steam gasification”, Energy Fueis, 10, 43 143X1996). 16 Wen, Y.W., “Mechanism of alkali metal catalysis in the gasification of coal, char or graphite”, C a t d Rev. Sci. Enp.. 22U). 1- 28(1980). 17 Leienspiel,’ O., Interpretation of Batch Reactor Data, John Wiley and Sons, New York, 4447(1979). 18 Zhou, C.X., “The study on a new integrated technology of catalytic gasification and manufacturing silicate fertilizer from alkali coal cinder calcinations using high-ash anthracite as a source”, Master Thesis, Fuzhou University, China (2007). (in Chinese) 19 Zhang, J.Y., Lin, J., Huang, W., Zeng, C., “The green engineering on catalytic gasification of low rank anthracite”, In: Proceedings of the Australia-China Joint Workshop on Clean Power from Coal with Maximized Efficiency, Taiyuan, China, 77--85(2001).

Chin. J. Ch. E. 15(5) 670 (2007)