Catalytic gasification of Pakistani Lakhra and Thar lignite chars in steam gasification

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 37, Issue 1, February 2009 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2009, 37(1), 1119

RESEARCH PAPER

Catalytic gasification of Pakistani Lakhra and Thar lignite chars in steam gasification JAFFRI Gul-e-Rana, ZHANG Ji-yu* Institute of Chemical Engineering and Technology, Fuzhou University, Fujian 350002, China

Abstract: The catalytic effects of different catalysts, i.e., 3%Ca, 5%Na-BL, and 3%Ca+5%Na-BL catalyst, on carbon conversion, gasification reaction rate constant, activation energy, and relative amount of harmful sulfur containing gases, were investigated by thermogravimetry in steam gasification under temperature 700°C to 900°C at ambient pressure for two Pakistani Lakhra (LKH) and Thar (THR) lignite chars. High carbon conversion can be obtained by direct gasification of both LKH and THR chars, but the gasification rate became much fast using BL catalyst. THR char with high ash content was easy to form some complex silicates during BL catalytic gasification, leading to a lower conversion than that of LKH char with low ash content. SO2 and H2S as sulfur-containing gases produced by char and BL itself in steam gasification can be captured by the existence of Ca mixed with BL, which is more effective at temperatures less than 900°C. The shrinking core model (SCM) can be considered as a better choice to correlate the relations between conversion and time and to estimate the reaction rate constant (k) under different temperatures. The reaction activation energy (Ea) and pre-exponential factor (A) were predicted based on Arrhenius equation. The reaction activation energy of 44.7 kJ/mol and 59.6 kJ/mol for LKH chars with BL+Ca and BL catalysts were much lower than 114.6 kJ/mol and 100.8 kJ/mol for THR chars with the same catalysts, respectively. They were also lower than 161.2 kJ/mol for LKH char and 124.8 kJ/mol for THR char without catalyst. Keywords: lignite char; black liquor; catalyst; thermogravimetry; steam gasification; kinetics

Coal is a major and viable source of energy. Its conversion to an efficient, cost competitive, cleaner, and environmental friendly fuel is of importance[1,2]. Pakistan is abundant in natural coal reserves and has vast resources in Sindh, Punjab, Balochistan, and north west frontier province. These resources can generate 100,000 MW electricity and might use extensively for the next thirty years[3]. Pakistan has emerged as one of the leading country - seventh in the top 20 countries of the world after the discovery of huge lignite coal resources in Sindh[4]. Sindh coal located at Lakhra, Sonda, Jherruck, and Indus east fields is classified as a “Lignite” with calorific value ranging from 5,219 to 13,555 BTU/lb. The total estimated amount of 184.6 billion tonnes, especially 175.5 billion tonnes in Thar area, can be used for power generation. Both Thar (THR) and Lakhra (LKH) lignites contain low ash content but high moisture, volatility, and sulfur[3]. Because of these characteristics, they require special techniques regarding mining, power generation, and gasification. The high moisture content in lignite can reduce the efficiency of power generation and increase the boiler

loading and coal feeding price. The high volatility can produce more methane and coal tar which needs primarily to be used before further conversion. The presence of ash is responsible for the slugging and fouling problem in the conventional boiler. Because the high price of gas and oil in Pakistan severely affects the generation cost of electricity and enhances the pressure on industry, an efficient utilization of coal conversion, i.e., the catalytic gasification to improve the thermal efficient, reduce environmental pollution, and minimize the processing cost[5,6], needs to be developed. As pointed by our previous studies[7–10], the black liquor (BL) containing a plenty of alkali metal, lignin, and degradation compound, and the Ca2+ ion in CaCO3 reacted with COOH group in coal to form ion exchange calcium can be mixed as a perfect catalyst in the anthracite gasification. In the present work, steam gasification of Pakistani lignite using above BL+Ca mixed catalyst was studied in detail.

1 1.1

Experimental Apparatus

Received: 16-Jul-2008; Revised: 19-Oct-2008 * Corresponding author. E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation of China (20376014); Fujian Science and Technology Council Grant (HG99-01). Copyright ” 2009, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

JAFFRI Gul-e-Rana et al. / Journal of Fuel Chemistry and Technology, 2009, 37(1): 1119

Fig. 1

Flow sheme of experimental apparatus

The flow scheme of experimental apparatus is shown in Fig. 1. Gasification were carried out by NETZSCH thermal analyzer STA 409, associated with mass spectrometer QMS 403 Aeolos, water vapor furnace (Tmax = 1250°C), steam evaporator DV2MK, compact rotary van pump PK 4D, gas control device, system controller TASC 414/4, and computer installed with NETZSCH and Aeolos software[9.10]. The mass spectrometer was basically consisted of ion separation in rod system of quadrupole and the ions separated by mass/charge ratio in the rod system were electrically detected by various types of detectors as Faraday cup, and continuous and discrete dynode secondary electron multipliers (C-SEM and SEM). The steam inside the evaporator DV2MK was kept at 160°C and then introduced into the furnace through the heat preservation tube (150°C). The heat preservation was also set at the inlet, inside and outlet of the furnace to avoid steam condensation. 1.2 1.2.1

Sample preparation Materials

Two kinds of Pakistani THR and LKH lignite provided by Fuel Research Center, PCSIR, Karachi, Pakistan, were used. Their proximate and ultimate analyses are given in Table 1. Prior to sample preparation, char was made by the following procedure. Firstly, 60 g raw THR coal or 76.06 g LKH coal was loaded twice with some ceramic beads in a fixed bed reactor inserted in an electrically heated furnace for carbonization in nitrogen atmosphere under temperature 750°C and ambient pressure with a heating rate of 15°C/min, and a flow rate of 664.675 L/h for 1 h. Then, these chars Table 1 Coal type

removed from ceramic beads were pulverized to particle size of 0–0.154 mm (<100 mesh). The compositions of above chars and ashes are listed in Tables 1 and 2, respectively. The BL from wood pulp, provided by the Naping Paper Mill, Fujian, China, was used as the catalyst and the Na content in BL was 2.073% measured directly by a flame photometer using an equipment model HG-3, 2400318, made by Beijing measuring instrument plant in China. The calcium as a CaCO3 powder (99.0% purity, Xi Long Chemical Plant, Shantou, Guangdong Province) ground and sieved to 0–0.154 mm (<100 mesh) was used to make the mixed catalyst. The preparation of mixed BL+Ca catalyst was already reported in the previous paper[7,8]. 1.2.2

Impregnation of catalyst

The 5%Na-BL loading, and 3%Ca in the char sample were the weight ratio to the raw char. The amount of Na was only from the additive BL and of Ca occupied in CaCO3. The mixed catalysts of 3%Ca plus 5%Na-BL were separately loaded into 100 g raw char, and these mixed char samples with a uniform impregnation were dried at 110°C for 24 h, then ground and sieved to 0–0.154 mm, and stored in the desiccators for use. 1.3

Experimental procedure

All connections of NETZCH thermal analyzer were opened and purged firstly with argon gas of 50 mL/min for 1.5 h. About 50 mg char sample was placed in the Al2O3 crucible, inserted into TG sample carrier, and then evacuated to 1.0×10–5 Pa in the furnace twice for releasing any adsorptive gas. The temperature in the furnace was risen to the required value with a heating rate of 20°C/min and kept constant, and then the steam gasification reaction was started for 60 min. During steam gasification, the TGA and mass profiles were continuously monitored by a computer using the NETZCH and Aeolos software. In all cases of BL, Ca and BL+Ca catalysts the weight changes of char sample were measured under 700°C, 750°C, 800°C, 850°C, and 900°C at ambient pressure with a steam flow rate of 4 g/h at ambient pressure.

Proximate and ultimate analyses of Pakistani LKH and THR Lignite and char Proximate analysis wad /%

Ultimate analysis wad /%

M

A

V

FC

St

H

C

N

O

LKH lignite

10.24

14.66

44.54

30.56

5.15

3.07

52.44

0.85

13.59

THR lignite

8.50

19.37

48.52

23.66

4.02

2.94

49.93

0.98

14.26

LKH lignite char

6.07

28.19

4.82

60.92

5.62

1.71

54.77

0.89

2.75

THR lignite char

3.88

36.25

6.18

53.69

3.46

1.36

51.78

0.74

2.53

Oad%=100–Aad%–St,ad%–Cad%–Had%–Nad%–Mad%

JAFFRI Gul-e-Rana et al. / Journal of Fuel Chemistry and Technology, 2009, 37(1): 1119 Table 2 Ash composition and fusion temperature of Pakistan chars Char

Aad

Ash

/%

SiO2/Al2O3

Composition of ash w/% SiO2

Al2O3

Fe2O3

CaO

MgO

TiO2

Fusion temperature t/°C SO3

k2O

Na2O

P2O5

DT

ST

FD

LKH char

28.19

1.32

27.98

21.21

24.85

9.44

4.77

1.53

8.08

0.15

1.16

0.53

1228

1234

1239

THR char

36.25

2.13

39.01

18.31

22.46

9.82

2.66

1.95

4.10

0.05

0.96

032

1188

1216

1226

rate of 4 g/h H2O was considered as the best condition to achieve an optimum char conversion with elimination of the diffusion effect, and selected for the further study in all cases of BL, Ca, and BL+Ca catalysts, and temperatures ranging from 700°C to 900°C at ambient pressure. 1.5

Evaluation criteria

The carbon conversion (x%) for all char samples is calculated by using the following expression.

x Fig. 2

Effect of steam flow rate on carbon conversion for LKH char at 850ºC

: 2 g/h H2O; : 4 g/h H2O; : 5 g/h H2O; 0: 6 g/h H2O;

1.4

Experimental conditions

As pointed out by the previous works[9–11], in order to eliminate the effect of the external and internal diffusion in char particles on gasification kinetics, the appropriate steam flow-rate should be determined. Therefore, the effects of four steam flow-rates, i.e., 2 g/h, 4 g/h, 5 g/h, and 6 g/h H2O, on char conversion were examined at 850°C by using LKH char sample as shown in Fig. 2. It is clearly elucidated that under the flow-rate of 2 g/h H2O the char conversion curves versus time have lower char conversion, while for 4 g/h, 5 g/h, and 6g/h they are merge together, exhibiting higher reactivity and optimum char conversion. Because both 5 g/h and 6 g/h H2O flow rates were too high in view of energy saving, the flow

Fig. 3

Carbon conversion vs time at different conditions for

'w%  Į u ȕ Ȗ u C ad %

(1)

where 'w: weight change in percentage of char sample during 't time interval, %; Cad: Carbon content in char, %;D: BL(solid weight)/[char weight + BL (solid weight) + calcium (solid weight) ]; E: Weight change of pure BL during 't time interval corresponding to 'w weight loss, %;J: char weight/ [char weight + BL (solid weight) + calcium (solid weight) ]. It should be emphasized that the BL (solid weight), i.e., solid BL, is the residual mass after drying the liquid BL in an oven-evaporated at 120ºC for overnight and is used for the determination of x and its components in pure coal sample as mentioned in equation 1.

2

Results and discussion

2.1

Effect of catalyst concentration on char conversion

2.1.1

Reactivity of single and mixed catalyst

Fig. 4

Carbon conversion vs time at different conditions for

LKH char

THR char

(a): LKH Char; (b): 3% Ca; (c): 5% Na-BL; (d): 5% Na-BL+3% Ca

(a): THR Char; (b): 3% Ca; (c): 5% Na-BL; (d): 5% Na-BL+3% Ca

: 700°C; : 750°C; : 800°C; : 850°C; : 900°C

: 700°C; : 750°C; : 800°C; : 850°C; : 900°C

JAFFRI Gul-e-Rana et al. / Journal of Fuel Chemistry and Technology, 2009, 37(1): 1119 Table 3 Temperature

CT

t/°C

t/min

700

Carbon conversions at CT for different coals and temperatures Conversion x/% none

3%Ca

5%Na-BL

5%Na-BL+3%Ca

LKH

THR

LKH

THR

LKH

THR

LKH

14

9.8

15.8

9.1

15.2

77.3

27.4

84.6

23.1

750

9.5

15.7

21.6

15.1

26.8

78.5

42.0

77.3

29.4

800

7

26.1

34.4

29.2

40.0

85.5

58.1

74.1

46.3

850

6

33.9

46.9

45.5

50.0

91.6

68.0

75.4

49.7

900

4.5

43.7

48.4

55.9

52.3

73.5

66.1

81.1

60.3

Using the best catalyst loadings of 3%Ca, 5%Na-BL, and 5%Na-BL+3%Ca as shown in previous study[8,9] and under temperature of 700°C, 750°C, 800°C, 850°C, and 900°C, the variations of x with t are displayed in Figs. 3 and 4 for LKH and THR char, respectively. In Figs. 3 and 4 each (a), (b), (c) and (d) corresponds to pure char, 3%Ca, 5%Na-BL, and 5%Na-BL+3%Ca, respectively. Both figures indicate that x is gradually enhanced by increasing temperature, x at 3%Ca is close with that of pure LKH and THR chars and the catalytic effect of 5%Na-BL is higher than 5%BL+3%Ca and 3%Ca. Also, the different behaviors of x with gasification temperature and catalyst loading are observed for both chars. These differences can be summarized as follows: (1) In all temperature range, the pure THR char has higher x shown in Fig. 4(a) than LKH char in Fig. 3(a), which might be caused by the more volatile in the former (Vad=6.2%) than the later (Vad=4.8%). Both LKH and THR chars can produce high carbon conversion with x=99.6% at 30 minute and x=86.9% at 25 min by direct gasification at 850ºC, respectively. However, for LKH, char carbon conversion reaches 91.58 % only at 6 min and 88.9% for THR char at 10.5 min by catalytic gasification at 5% Na-BL catalyst. (2) The 5%Na-BL catalyst loading shows a more effective catalytic function. In 5%Na-BL and 5%Na-BL +3%Ca cases, LKH char shows more obvious reactivity as shown in Figs. 3(c) and 3(d) than THR char in Figs. 4(c) and 4(d), which is probably because the later one has more ash content (36.3%) than the former (28.2%) as indicated in Table 2. It implies that the alkali oxides in catalyst can not only enhance the gasification rate but also directly react with the inorganic matters in ash to form some complex silicates[12,13]. Therefore, the more ash content in char, the more formation of complex silicate will be during the catalytic gasification, leading to the lower conversion of THR char than that of LKH char.

THR

catalyst or pure char condition, x is a function of both time and temperature. The variation rate of x with time is faster with the increase of gasification temperature. Under the same catalyst and temperature condition the highest conversion obtained from two chars in this study was related to the shortest gasification time defined as the contrastive time (CT). Therefore, if considering the shortest gasification time (i.e. CT), corresponding to each highest x point at 5%Na-BL from 700°C to 900°C for LKH chars as the comparison base, another highest x of THR chars at different temperatures and catalyst loadings has their corresponding CT at each catalyst concentration (Cc). Based on this CT, the effects of different catalyst loadings on x for both chars are listed in Table 3, which evidently shows the 5%Na-BL catalyst loading has more effective catalytic action. Provide further by taking the x at without catalyst case as a base unity, the ratios of x at different catalyst are listed in Table 4, which indicates that for LKH chars both 5%Na-BL+3%Ca and 5%Na-BL catalyst loadings can distinctly increase the char conversion at lower temperature of 700°C–750°C, and the ratio increments of x is markedly reduced at higher temperature of 800°C–900°C. However, for THR char, the ratio increments of x are higher only at 5% Na-BL with lower temperature of 700°C–750°C, and it is reduced at higher temperature of 800°C–900°C. As pointed before, THR char contained high ash content of 36.2%, which can also react with alkali oxides in BL and Ca catalyst that counteracts some catalytic function from additive catalyst, therefore, the catalyst on THR char becomes less effective. But LKH char with ash content of 28.2% undergoes more obvious catalytic action and has high ratio of x as listed in Table 4. Table 4

Ratio of carbon conversions at CT for LKH and THR char at different temperatures

2.1.2

Effect of catalyst loading on conversion

As indicated above, at 5% Na-BL catalyst loading, both chars had more reactivity and faster conversion. For more clearly comparing the effects of catalyst and temperature on char conversion, we cited a definition of the contrastive time (CT) as mentioned by previous articles[9,10]. At a certain

Temp.

CT

t/°C

t/min

700

Ratio of conversion 3%Ca

5%Na-BL

5%Na-BL+3%Ca

LKH

THR

LKH

THR

LKH

THR

14

0.93

0.96

7.89

1.73

8.63

1.46

750

9.5

0.96

1.24

5.00

1.94

4.92

1.36

800

7.0

1.12

1.16

3.28

1.69

2.84

1.35

850

6.0

1.34

1.07

2.70

1.45

2.22

1.06

900

4.5

1.28

1.08

1.68

1.37

1.86

1.25

JAFFRI Gul-e-Rana et al. / Journal of Fuel Chemistry and Technology, 2009, 37(1): 1119 Table 5 Coal

LKH THR LKH THR LKH THR LKH THR LKH THR

Fig. 5

Ion current I/A

Temp t/°C

700 750 800 850 900

Effect of catalyst on sulphur containing gases at 700°C–900°C

none

3%Ca

5%Na-BL

5%Na-BL+3%Ca

SO2

H2S

SO2

H2S

SO2

H2S

SO2

H2S

16.3×10–12

9.2×10–12

16.8×10–12

3.0×10–12

4.9×10–12

5.3×10–12

5.5×10–12

10.6×10–12

19.8×10–12

9.5×10–12

11.9×10–12

2.6×10–12

12.4×10–12

4.1×10–12

6.9×10–12

4.3×10–12

18.1×10–12

17.6×10–12

15.9×10–12

3.4×10–12

7.9×10–12

104×10–12

10.9×10–12

16.5×10–12

20.2×10

–12

12.4×10

–12

12.4×10

–12

3.5×10

–12

21.0×10

–12

19.0×10

–12

15.6×10

–12

3.2×10

–12

15.3×10

–12

10.7×10

–12

20.3×10–12

13.7×10–12

10.1×10–12

3.0×10–12

16.8×10–12

14.5×10

–12

34.0×10

–12

28.0×10

–12

4.4×10

–12

–12

16.6×10

–12

23.9×10

–12

12.4×10

–12

3.0×10

–12

172×10–12

33.7×10–12

15.6×10–12

–12

–12

–12

22.2×10

65.1×10

19.4×10

13.1×10 9.9×10

3.4×10–12 36.5×10

MS curves for SO2 and H2S in the range of 700°C to 900°C

–12

22.8×10–12

–12

14.0×10

Fig. 6

–12

4.0×10

–12

12.6×10

–12

2.9×10–12 24.4×10

–12

11.9×10

–12

244.5×10–12 36.5×10

–12

6.8×10

–12

14.6×10

–12

18.2×10–12

3.7×10–12 19.0×10–12 5.0×10–12

20.0×10

–12

22.7×10–12

26.4×10

–12

18.2×10–12

17.3×10–12

60.0×10–12

–12

84.9×10–12

22.8×10

MS curves for SO2 and H2S in the range of 700°C to 900°C

for LKH Char

for THR Char

a: SO2 5% Na-BL; b: H2S 5% Na-BL; c: SO2 5% Na-BL+3% Ca;

a: SO2 5% Na-BL; b: H2S 5% Na-BL; c: SO2 5% Na-BL+3% Ca;

d: H2S 5% Na-BL+3% Ca

d: H2S 5% Na-BL+3% Ca

(a): 700°C; (b): 750°C; (c): 800°C; (d): 850°C; (e): 900°C

(a): 700°C; (b): 750°C; (c): 800°C; (d): 850°C; (e): 900°C

2.1.3

Effect of catalyst on sulphur containing gases

It is known that using BL as catalyst in gasification process some sulfur gases, like hydrogen sulphide (H2S) and

carbonyl sulphide (COS), and traces of organic gases, like CH3SH, (CH3)2S and (CH3)2S2, can be produced from BL, which often infract the environmental regulations[14]. In order to achieve the emission control regulation of sulfur gases, the

JAFFRI Gul-e-Rana et al. / Journal of Fuel Chemistry and Technology, 2009, 37(1): 1119

mixed catalyst of CaCO3 plus BL was used in char gasification, in which CaCO3 not only captures an amount of sulphur containing gases[15] but also accelerates the carbon conversion reaction effectively[16]. Therefore, during THR and LKH char gasification at 5%Na-BL and 5%Na-BL+Ca catalysts, some sulfur containing gases, such as SO2 and H2S are qualitatively measured by the amount of ion intensities of evolved gases using a mass spectrometer connected to the thermal analyzer. The relative changes in ion current intensities of SO2 and H2S with catalyst concentrations and temperature are listed in Table 5. At 3%Ca catalyst, it is found that the ion current(IC) of SO2 and H2S is a little less than that at none-catalyst. At 5%Na-BL, in the LKH char, only H2S has higher IC values and a jump increase at 900ºC, whereas SO2 is lower. However, for THR char both SO2 and H2S, exhibit less IC than that from the pure char. In the addition of 5%Na-BL+3%Ca, both SO2 and H2S emission are reduced and captured by the existence of Ca or well dispersed Ca inside char matrix and this action is more effective especially at 900ºC. Normally, the decomposing reaction of CaCO3 to CaO occurs at 825ºC, and then CaO as a capture sulphur medium will continuously react with SO2 and H2S produced in gasification[17] to form CaSO3(s) and CaS(s) to complete the desulphurization process. Because the amount of sulphur contained gases is enhanced with increasing carbon conversion and gasification temperature, at 900°C, the pore of CaCO3 catalyst can be easily agglomerated and the aperture path becomes thinner in the produced CaO. The CaO re-crystallization might also occur[18]. All of these lead to destroy the porosity configuration and surface area of CaO, restrict the diffusion of sulphur containing gases into the inner aperture of CaO, and restrain the desulphurization reaction, which is corresponding to an obvious jump at 900°C.

Fig. 7

Validity of SCM and HM at 5%Na-BL+3%Ca for LKH char

As a typical example, the mass curves of SO2 and H2S, produced in gasification of LKH and THR chars at both 5%Na-BL and 8%(5%Na-BL+3%Ca) catalysts in the range of 700ºC to 900ºC, are represented individually in Figs 5(a) to 5(e) and Figs 6(a) to 6(e). They clearly designate that in the gasification of chars the BL produces more sulphur containing gases especially at higher temperature 900ºC. However, these harmful sulphur gases were greatly reduced by addition of some CaCO3 in BL catalyst. The above observations reveal that the addition of a certain quantity of CaCO3 into catalyst in gasification can improve the catalyst function and desulphurization, and enhance the coal conversion. This action is more effective especially at temperature less than 900ºC. 2.2

Catalytic gasification kinetics

2.2.1 Kinetic expression for catalytic gasification of char In general, the equation of a chemical–reaction-controlled gas-solid reaction rate can be represented by dx/dt = k(1–x)n (2) where, n is in a range from 0 to 1. Normally, the index n can be estimated by substituting the experimental x into above equation. When n is equal to unity, it expresses that the homogeneous first order kinetics with the homogeneous model (HM) is valid. When n is equal to 2/3, the reaction proceeds in accordance with the shrinking-core model (SCM). Both SCM and HM models have been commonly used to evaluate the kinetic analysis of coal gasification as pointed by previous work in both CO2[8] and steam gasification[9]. Therefore, both SCM and HM models are firstly used to examine the gasification of LKH and THR char obtained in this study.

Fig. 8

Validity of SCM and HM at 5%Na-BL+3%Ca for THR char

(a): SCM; (b): HM

(a): SCM; (b): HM

: 700°C; : 750°C; : 800°C; : 850°C; : 900°C

: 700°C; : 750°C; : 800°C; : 850°C; : 900°C

JAFFRI Gul-e-Rana et al. / Journal of Fuel Chemistry and Technology, 2009, 37(1): 1119 Table 6 Temperature t/°C

700 750 800 850 900

Rate constant calculated by SCM and HM models at BL, BL +Ca and Ca catalyst Rate constant k/min–1 Model

none

3%Ca

5%Na-BL

5%BL+3%Ca

LKH

THR

LKH

THR

LKH

THR

LKH

THR

SCM

0.005

0.008

0.005

0.009

LKH

THR

LKH

THR

HM

0.005

0.009

0.006

0.009

0.071

0.025

0.096

0.012 0.014

SCM

0.011

0.020

0.012

0.021

0.083

0.039

0.119

HM

0.015

0.023

0.014

0.024

0.106

0.049

0.124

0.030

SCM

0.033

0.040

0.034

0.049

0.126

0.064

0.149

0.064

HM

0.053

0.057

0.050

0.057

0.167

0.117

0.147

0.064

SCM

0.072

0.068

0.057

0.079

0.205

0.177

0.177

0.072

HM

0.111

0.089

0.073

0.092

0.212

0.156

0.194

0.094

SCM

0.144

0.121

0.169

0.122

0.274

0.206

0.234

0.105

HM

0.223

0.194

0.273

0.145

0.238

0.190

0.250

0.174

Table 7 Activation energy Ea, pre exponential factor A and correlation coefficient U2 in 700°C –900°C Catalyst loading w/% None 3%Ca 5%Na-BL 5%Na-BL+ 3%Ca

Coal

Ea /kJ·mol–1

U2

A /min–1

SCM

HM

SCM

SCM

HM

LKH

161.2

176.6

2.21×106

1.76×107

HM

0.980

0.915

THR

124.8

140.4

4.51×104

3.41×105

0.960

0.960

6

6

LKH

164.8

179.1

3.27×10

0.972

0.972

THR

124.8

129.1

4.92×104

2.17×10

9.12×104

0.950

0.967

LKH

59.6

63.4

1.19×102

2.22×102

0.960

0.908

3

4

THR

100.8

91.4

7.15×10

0.990

0.963

LKH

44.7

46.4

2.35×101

3.51×101

0.983

0.974

THR

114.6

111.4

2.19×104

1.87×104

0.950

0.966

On considering the 5%Na-BL+3%Ca and 5%Na-BL catalyst as an example in steam gasification for LKH and THR char, the validity was evaluated by plotting 3[1–(1–x)1/3] vs t using SCM model and [–ln(1–x)] vs t using HM model in Figs 7(a), 7(b), 8(a) and 8(b). It is obvious that the relationship 3[1–(1–x)1/3] vs t is nearly linear, but [–ln(1–x)] vs t is almost non-linear at temperature from 700°C to 900°C. It implies that HM are not applicable but SCM can be used in steam gasification process of both LKH and THR char. In order to differentiate the applicability of SCM and HM, the reaction rate constant k was obtained by the slope of each straight line fitting through the origin at different temperature. The same behaviors and procedures were also performed at pure char, 3%Ca, 5%Na-BL, and 5%Na-BL+3%Ca catalyst and 700°C–900°C for LKH and THR chars. All of k values predicted, respectively, by SCM and HM under above conditions are listed in Table 6 At pure char, 3%Ca, 5%Na-BL, and 5%Na-BL+3%Ca for LKH and THR chars all k values predicted by SCM are very well and their average correlation coefficients is 0.968 in the temperature range of 700°C–900°C, which is better than that of 0.953 by HM except of only at the cases of 3%Ca and 5%Na-BL+3%Ca for THR char with the correlation coefficient little lower as 0.950 as listed in Table 7.

3.43×10

Therefore, SCM can be considered as a better choice in steam gasification for both chars. It definitely indicates that k is much higher at either 5%Na-BL or 5%Na-BL+3%Ca than at 3%Ca, and pure char in both LKH and THR chars. These results elaborate that the catalytic effect of adding BL catalyst displays the distinct catalytic ability. 2.2.2

Effect of catalyst on activation energies

The pre-exponential factor (A) and activation energy (Ea) at with and without catalyst for LKH and THR chars were calculated by using Arrhenius equation k A u exp[  Ea / RT ] and listed in Table 7. Both Ea and A values predicted by the selected SCM model can be taken as a comparison base for different catalysts and chars. The Ea and A values at 5%Na-BL and 5%Na-BL+3%Ca catalysts were lower than 3%Ca and pure char for both LKH and THR chars. The Ea of 44.7 kJ/mol and A of 2.35×101 min–1 at 5%Na-BL+3%Ca, and Ea of 59.6 kJ/mol and A of 1.19×102 min–1 at 5%Na-BL for LKH char were much lower than 114.6 kJ/mol and 2.19×104 min–1 at 5%Na-BL+3%Ca, and 100.8 kJ/mol and 7.15×103 min–1 at 5%Na-BL for THR char.

JAFFRI Gul-e-Rana et al. / Journal of Fuel Chemistry and Technology, 2009, 37(1): 1119

especially at temperature less than 900°C. (4) SCM model can be considered as a better choice in steam gasification for both chars. LKH char with BL and BL+Ca catalysts is more reactive than THR char. The activation energy of 44.7 kJ/mol with 5%Na-BL+3%Ca and 59.6 kJ/mole with 5%Na-BL for LKH char are much lower than 114.6 kJ/mol with 5%Na-BL+3%Ca and 100.8 kJ/mol with 5%Na-BL for THR char. They are also lower than 161.2 kJ/mol for LKH char and 124.8 kJ/mol for THR char without catalyst.

References Fig. 9

Arrhenius plot of LKH and THR char at different catalyst loading (a): LKH SCM; (b): THR SCM : 3% Ca; : 5% Na-BL; : 5% Na-BL+3% Ca

Therefore, LKH char with BL and BL+Ca catalysts is more reactive than THR char. The activation energies values of two lignite chars in steam gasification in Table 7 are close to the values of 92–102 kJ/mol reported by Everson[19]. Figure 9 illustrates the Arrhenius plots of LKH and THR chars at 3%Ca, 5%Na-BL, and 5%Na-BL+3%Ca catalyst loadings by using SCM. It is observed that for LKH char the catalytic effect at 5%Na-BL+3%Ca is slightly higher than 5%Na-BL at lower temperature but at higher temperature becomes gradually lower as shown in Fig. 9(a). For THR char the catalytic effect of 5%Na-BL is higher than 5%Na-BL+3%Ca in whole temperature range in Fig. 9(b). Figs. 9 (a) and (b) indicates that the slope of 3%Ca is much lower than above two catalysts especially at lower temperature and it changes quickly with gasification temperature. It implies that only adding 3%Ca almost does not have the catalytic action and in this case the temperature has stronger effect than catalytic action.

3

Conclusions

(1) Both LKH and THR lignite chars can produce high carbon conversion by direct gasification. The conversion of 99.6% can be reached at 850°C in 30 min for LKH char and 86.9% in 25 min for THR char. However, 91.58% for LKH char only at 6 min and 88.9% for THR char at 10.5 min can be achieved by 5%Na-BL catalytic gasification. The 5%Na-BL catalyst loading shows a more effective catalytic function and strongly enhances the gasification reaction rate. (2) THR char with high ash content indicated a lower conversion than that of LKH char contained low ash in steam gasification using BL catalyst. (3) Sulfur gases, such as SO2 and H2S, produced by char and BL catalyst in steam gasification are greatly reduced by adding CaCO3 in BL catalyst. This action is more effective

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