Mechanisms and kinetic modelling of steam gasification of brown coal in the presence of volatile–char interactions

Fuel 103 (2013) 7–13

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Mechanisms and kinetic modelling of steam gasification of brown coal in the presence of volatile–char interactions Shiro Kajitani a,b, Hui-Ling Tay a, Shu Zhang a, Chun-Zhu Li a,⇑ a b

Fuels and Energy Technology Institute, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia Energy Engineering Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), Yokosuka 240-0196, Japan

a r t i c l e

i n f o

Article history: Received 10 January 2011 Received in revised form 22 September 2011 Accepted 24 September 2011 Available online 15 November 2011 Keywords: Victorian brown coal Gasification Volatile–char interactions Kinetics

a b s t r a c t It is known that Victorian brown coal has higher reactivity for gasification because of catalysis of inherent AAEM species than high-rank coal. However, the experimental results of steam gasification of brown coal in a fluidised-bed/fixed-bed reactor at 800 °C have suggested that the inhibitory effects of the volatile–char interactions on char gasification are not negligible. The mechanisms and kinetics model of the char gasification and volatile–char interactions were discussed to describe quantitatively the inhibition of char gasification by volatiles, in this study. The elementary reactions of the char gasification and volatile–char interactions, which are consist of the adsorption of free radicals from volatiles, the volatilisation of catalyst and the evolution of char structure affected by radicals from volatiles, were proposed. The L-H type reaction rate equations for brown coal gasification were determined, and the kinetics model was verified by the comparison with several series of experiments. The proposed kinetics model described the experimental results of the coal conversions and the concentrations of Na in char during steam gasification very well. This kinetics model would be useful in designing any industrial fluidised-bed gasifier for low-rank fuels and estimating their performance. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Gasification has been considered as one of the promising clean coal technologies to provide energy and feedstock. Gasification technology should be particularly suitable for the utilisation of low-rank coal, such as Victorian brown coal, because of its higher reactivity of gasification than high-rank coal. For example, a high temperature Winkler (HTW) gasifier PDU for Victorian brown coal has been developed [1]. It is known that inherent alkali and alkaline earth metallic (AAEM) species have catalytic activity for gasification. The gasification reactivity was decreased greatly after the Victorian brown coal was demineralized [2] and sodium was the major element of AAEM species in Victorian brown coal [3]. Catalytic gasification of Victorian brown coal has been also discussed in many literatures [4–7]. It was, however, found recently that volatiles of Victorian brown coal have also high activity, and the inhibitory effects of the volatile–char interactions on char gasification are not also negligible [8–10]. It was shown that the volatile–char interactions in a bubbling fluidised-bed reactor could practically inhibit the char gasification in steam and terminate the char gasification at a conversion level ranging from 62% to 85% on the coal carbon basis at 850–950 °C [9]. Another paper also reported the significant ⇑ Corresponding author. Tel.: +61 8 9266 1131; fax: +61 8 9266 1138. E-mail address: [email protected] (C.-Z. Li). 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.09.059

inhibition of steam gasification in the presence of the volatile–char interactions in a fluidised-bed/fixed-bed reactor at 800 °C [10]. The volatile–char interactions are most likely the reactions between char and free radicals, especially H radicals, formed by the thermal cracking and/or reforming of volatiles. Volatiles could be adsorbed onto the char surface dissociatively and/or thermally cracked in the gas phase, donating hydrogen to free carbon sites much more significantly than H2 [9]. Volatiles could also enhance the volatilisation of the monovalent AAEM species, mainly Na, to decrease the overall char gasification rate [11–15]. The volatilisation of AAEM species during pyrolysis in a fluidised-bed/fixed-bed reactor was significant [11,12]. The CM–Na bond, which was the bond between char matrix (CM) and Na, could be formed during pyrolysis, and generated radicals including H radicals could react with char particles and might displace Na bonded to char, leading to the volatilisation of Na [13]. Volatiles could, furthermore, affect the evolution of char structure characterised with FT-Raman spectroscopy [16–18]. H radicals could penetrate deep into the char matrix to induce the ring condensation reactions to convert the smaller aromatics ring systems into the bigger ones, leading to the reduction of the char reactivity [17]. It is also known that coke could be deposited on carbon surface by the thermal cracking and reforming of volatiles. The reforming of nascent tar from rapid pyrolysis of brown coal was very rapid in a char bed in a two-stage reactor. Although the gasification took place faster than the coke formation at 900 °C, carbon

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Nomenclature C FH ij kc kloss knc Mc Mc nt

carbon density, mol m3 mol fraction of H in fed coal against supplied gas, molH in coal molgas1 rate constant for elementary reaction j, s1, s1 atm1, etc. overall rate constant for catalytic gasification of char, s1 overall rate constant for loss of catalytic activity, s1 overall rate constant for non-catalytic gasification of char, s1 density of catalyst, mol m3 average density of catalyst considering residence time distribution, mol m3 density of total active sites of carbon, mol m3

deposit (coke) in char bed was a major primary product from nascent tar at 750–900 °C [19]. Thus the volatile–char interactions show the significant features of Victorian brown coal gasification, but these features should be common to low-rank fuels which are affected by the catalytic activities of inherent AAEM species, not only for Victorian brown coal. Woody biomass has high reactivity, owing to rich inherent sodium, potassium and calcium, with the catalytic effect appearing to be in the order K > Na > Ca during the steam gasification reaction of biochars [20]. The volatilisation of AAEM species and changes of char structure during pyrolysis or gasification of biomass were reported in several literatures [16,20–22]. On the ground of these recent studies, two-stage gasifiers, which separate pyrolysis of the fuel and steam reforming/gasification of volatiles/char from partial or full combustion of the residual char, are considered to be the best for converting low-rank fuels [23]. For example, a triple-bed combined circulating fluidized bed gasifier for advanced IGCC system is now being developed [24]. However, the volatile–char interactions have not been considered in recent modelling for fluidising-bed gasifiers [25]. In this study, the mechanisms and kinetics model of the char gasification and volatile–char interactions were discussed to describe quantitatively the inhibition of char gasification by volatiles. Several series of experiments were conducted to determine some kinetics parameters, and the kinetics model was adopted to describe the previous experiments [10] of gasification in a fluidised-bed/fixed-bed reactor. 2. Experimental 2.1. Coal Samples Loy Yang (Victoria, Australia) brown coal was used in this study. The coal sample was partially dried at the temperature lower (<35 °C), then pulverised and sieved to obtain a sample of particle size between 63 and 150 lm. The properties of the coal sample are: C, 70.4; H, 5.4; N, 0.62; S, 0.28; Cl, 0.10; O, 23.2 and VM, 52.2 wt.% (daf) together with ash, 1.1 wt.% (db) and moisture, 13 wt.% (ad). The concentration of Na, which was the major element of AAEM species in the coal sample, was 0.10 wt.% (db). A ‘H-form’ coal sample was also prepared by washing the above-mentioned coal sample (‘raw’ coal sample) with sulphuric acid to remove inherent AAEM species by the same procedure described elsewhere [11].

ntM PH P H2 P H2 O t Xchar X char Xcoal

density of total active sites of catalyst, mol m3 partial pressure of hydrogen radicals surrounded char particles, atm partial pressure of hydrogen, atm partial pressure of steam, atm time, s char conversion on a char carbon basis, – average char conversion considering residence time distribution on a char carbon basis, – coal conversion on a coal basis, –

Subscript 0 nascent char formed by primary pyrolysis of coal

2.2. Steam gasification A novel fluidised-bed/fixed-bed quartz reactor [3] was used for gasification of coal samples. Two frits were installed in the reactor body. Silica sand was put onto the lower frit and was fluidised by the gasifying agent. Coal samples were gasified with steam in the reactor by the same procedure as the previous experiments [10]. The reactor was heated up with an external electrical furnace and kept at 800 °C in this study. A HPLC pump was used to deliver water directly into the reactor to generate steam as the gasifying agent. The concentration of steam in total supplied gas was 15% balanced by argon. The total gas flow was 2 l/min at the standard ambient temperature and pressure, and the gas velocity at reaction temperature was approximately 5.6 m/min. Coal particles entrained in a coal feeder were fed through a water-cooled injection probe into the heated sand bed at certain feeding rates to achieve rapid particle heating rates. Char was formed in the sand bed and was elutriated out of the sand bed due to its lightness. The char formed a fixed bed underneath the top frit. The configuration of the reactor allowed the volatile–char interactions during char gasification. After coal feeding was stopped, char was gasified without the volatile–char interactions. The reactor was finally lifted out of the furnace and cooled down. The coal conversion was determined by the difference in the weights of reactor and coal/char before and after each experiment. Char was collected after the gasification experiment and the concentrations of AAEM species in char were quantified by the same procedure described elsewhere [12]. TGA (TG-DTA2000S, MAC Science) was used anew for steam gasification of char samples collected from the fluidised-bed/fixed-bed quartz reactor. The flow gas was switched from pure argon to mixture of steam and argon when char was heated up to 800 °C in the TGA. The temperature was kept stable at 800 °C and the weight loss caused by the gasification reaction was measured with the isothermal reaction technique [26]. 3. Results and discussion 3.1. Kinetics Model 3.1.1. Mechanisms of steam gasification It has been known that non-catalytic gasification and catalytic gasification take place in parallel during steam gasification of Victorian brown coal and biomass char [27,28]. It was assumed that the observed char gasification rate dXchar/dt was described as a sum of the overall rate constant for non-catalytic gasification knc and that for catalytic gasification kc in this study.

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ð1Þ

where Xchar is the char conversion and t is time. The mechanisms and reaction rate equations would be discussed as follows. Considering previous studies, mechanisms shown in Table 1 should be most reasonable for steam gasification of Victorian brown coal. The principal mechanistic studies of the carbon-steam reaction have been reported by many authors and mostly two mechanisms have been postulated. One is the oxygen-exchange reaction [29,30]: Cf þ H2 O $ CðOÞ þ H2 , and the other is the dissociation of steam [31,32]: Cf þ H2 O $ CðHÞ þ CðOHÞ. Although the evidences of the oxygen-exchange mechanism dissociation were shown [30], our group have found the evidence of dissociation of steam to H and OH at char surface during steam gasification of Victorian brown coal [18]. Nascent char was gasified with CO2 and O2 and/or steam, and the evolution of char structure was characterised by the Raman spectroscopy. The growth of aromatic rings to bigger ones was enhanced due to H radicals, and O-containing groups in char also greatly increased only in the presence of steam [18]. Therefore, the dissociative adsorption model of steam in Table 1 (i) was adopted for both of non-catalytic gasification and catalytic gasification in this study. But the dissociation mechanism of steam shows the details of the oxygen-exchange reaction. These should not be different mechanisms essentially. The mechanisms of adsorption of H2 on char surface have been discussed elsewhere [9]. Comparing associative chemisorption [29]: Cf þ H2 $ CðH2 Þ, with dissociative chemisorption [33,34]: Cf þ ð1=2ÞH2 $ CðHÞ, the dissociative chemisorption mechanism gave good correlation between the hydrogen concentration and the non-catalytic gasification rate of Victorian brown coal [9]. Therefore, the dissociative adsorption model of hydrogen in Table 1 (ii) was also adopted for the inhibition of steam gasification by hydrogen. The adsorption step of free radicals from volatiles, especially H radicals [13], in Table 1 (iv) was proposed to describe the inhibition of char gasification by volatiles. The partial pressure of H radicals should be unknown, but proportional to that of volatiles and also coal feeding rates (PH / PVM / Feeding Rate/Gas Flow Rate). This step describes one of the major volatile–char interactions. Two mechanisms of the loss of the catalytic activity in Table 1 (v) were also proposed in the volatilisation step of catalyst. One is the volatilisation due to the volatile–char interactions, and the other is the evaporation unrelated with the volatile–char interactions. Although the behaviours of AAEM species are complicated, the successive reaction of the adsorption of free radicals and the first mechanism of volatilisation step of catalyst should describe the major interaction between volatiles and char, and is the same mechanism of the displacement of Na by H radical which is described elsewhere [11,12]: CM  Na þ H ! CM  H þ Na. In the case of biomass gasification, the potassium should be volatilised in the same mechanism as sodium. But calcium and magnesium were not volatilised easily [21]. Condensation reactions to convert the smaller aromatics ring systems into the bigger ones [17,18] and deposition of coke by the thermal cracking and reforming of volatiles [19], such as Table 1 (III) and (IV), are independent steps form gasification reaction, but they should be also considered important parts of the volatile–char interactions. 3.1.2. Non-catalytic gasification Mechanisms for non-catalytic gasification were shown in Table 1 (I) with the rate constants inc1, inc3, inc5, inc6, inc7 and inc8 for each elementary reaction. Assuming steady-state concentrations of C(H), C(OH) and C (O), the overall rate constant knc is given by:

knc ¼

K nc1 PH2 O pffiffiffiffiffiffiffiffi 1 þ K nc2 PH2 O þ K nc3 PH2 þ K nc4 PH

ð2Þ

 nt 1 2 1 where K nc1 ¼ inc1 , K nc2 ¼ inc1 inc3 þ inc5 þ inc7 , K nc4 ¼ iinc8 , , K nc3 ¼ iinc6 C0 nc5 nc5 and nt is the density of the total active sites of carbon on an nascent char basis (m3-nascent char). C0 is the carbon density in nascent char formed by primary pyrolysis of coal. Therefore, Knc1 should change with the density of the active sites during gasification, though Knc2, Knc3 and Knc4 should have the constant values in the isothermal condition. H-form coal was gasified with steam in the fluidised-bed/fixed-bed reactor as shown in Fig. 1, and this should show the non-catalytic gasification because most of all AAEM has been removed from raw coal by acid washing [11]. An approximate amount of 1.5 g H-form coal was fed into the reactor for 17 min (88 mg/min). Coal feeding was finished at 0 holding time in the Fig. 1 and nascent char was gasified in the absence of the volatile–char interactions for each holding time. PH2 and PH were negligible in the experimental condition. The coal conversion Xcoal was determined by the weights of char and fed coal. The char conversion Xchar can be converted with the coal conversion by the correlation: X X coal;0 X char ¼ coal . Xcoal,0 is the initial coal conversion by primary pyrolysis 1X coal;0 of coal. The value of Xcoal,0 was unable to be determined directly but the suitable value could be estimated from these experiments. Eq. (2) described exactly the experimental results as the solid line in Fig. 1 when nt ¼ nt;0 ð1  X char Þ. knc is therefore given by the following first order reaction equation:

knc ¼ knc;0 ð1  X char Þ where knc;0 ¼ 1þK

ð3Þ K nc1;0 P H

2O

nc2 P H2 O þK nc3

pffiffiffiffiffiffi

PH þK nc4 P H 2

, K nc1;0 ¼

inc1 nt;0 , C0

and subscript ‘,0’

denotes the value for nascent char. The char sample collected from the fluidised-bed/fixed-bed reactor at 0 holding time was also gasified in the TGA. The correlation between reaction time and conversion in 15% steam at 800 °C agreed exactly with the results in the fluidised-bed/fixed-bed reactor shown in Fig. 1. The same char sample was therefore gasified also in 30% and 60% steam at 800 °C. The changes of the values of kinetics parameters Knc1, Knc2 during char gasification were analysed from the series of experiments as shown in Fig. 2. The results that Knc1 was reduced linearly with the char conversion and Knc2 can be regard as constant agreed with Eqs. (2) and (3). The values of knc in the absence of the volatile–char interactions at 800 °C were determined from these analyses. 3.1.3. Catalytic gasification Mechanisms for catalytic gasification were also shown in Table 1 (II) with the rate constants ic1, ic3, ic5, ic6, ic7 and ic8 for each elementary reaction. M denote the active site of the inherent catalyst, especially Na or K. Assuming steady-state concentrations of M(H), M(OH) and C(O), the overall rate constant for catalytic gasification kc and the overall rate constant for the loss of the catalytic activity kloss are given by: 100

Coal Conversion [wt%db]

dX char =dt ¼ knc þ kc



90 80 70 60 50 0

120

240

360

Holding Time, t-t F [min] Fig. 1. Coal conversions during char gasification of H-form coal after feeding in the fluidised-bed/fixed-bed reactor in 15% steam at 800 °C. Coal feeding time tF = 17 min. Dots: experimental results. Line: kinetics model.

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0.0005

0.0003

3

Knc2 0.0002

2

K nc1 PH2 O dX = dt 1+ K nc 2 PH2 O

0.0001

0

0.1

Knc2 [atm-1 ]

K nc1 [s-1 atm-1]

4

1

0.2

0.3

0.4

0.5

Coal Conversion [wt%db]

Knc1

0.0004

0

100

5

0

90

80

70

60

0

Fig. 2. Kinetics parameters during steam gasification of H-form coal char in the TGA at 800 °C. Char sample: coal feeding time tF = 17 min and holding time tH = 0 min in the fluidised-bed/fixed-bed reactor.

kc ¼

K c1 PH2 O pffiffiffiffiffiffiffiffi 1 þ K c2 PH2 O þ K c3 PH2 þ K c4 PH

kloss ¼  ¼

pffiffiffiffiffiffiffiffi PH2 þ K c4 PH Þ

ð5Þ

ð6Þ

When it can be assumed that Kc2 has the constant value during char gasification, kloss,0 and kc,0 should also have the constant values. ntM and kc are therefore given by:

ntM ¼ ntM;0 ekloss;0 t

ð7Þ

kc ¼ kc;0 ekloss;0 t

ð8Þ K c1;0 P H

Fig. 3. Coal conversions during char gasification of raw coal after feeding in the fluidised-bed/fixed-bed reactor in 15% steam at 800 °C. Coal feeding time tF = 17.5 min. Dots: experimental results. Line: kinetics model.

3.2. Steam gasification without volatile–char Interactions

  2 c6 c8 , K c3 ¼ ic5iþi where K c1 ¼ ic1Cn0tM , K c2 ¼ ic1 ic31nt þ ic5 þi , K c4 ¼ ic5iþi , c9 c9 c9 ic10 ntM ic9 ntM 2ic1 K loss1 ¼ ntM0 , K loss2 ¼ ntM0 , K c21 ¼ ic5 þic9 , and ntM is the density of the total active sites of catalyst on an nascent char basis (m3-nascent char). Therefore, Kc1, Kloss1 and Kloss2 should change with the density of the active sites during gasification, though Kc21, Kc3 and Kc4 should have the constant values in the isothermal condition. Eqs. (4) and (5) suggest that kloss should be proportional to kc when gas composition is steady. This correlation described by the following equation agrees qualitatively with the experimental results in previous studies [9,27].

kloss K loss1 þ K loss2 ðK c21 PH2 O þ K c3 ¼ K c1 PH2 O kc

Raw coal was gasified with steam in the fluidised-bed/fixed-bed reactor as shown in Fig. 3. The procedure was the same as the above-mentioned experiments for H-form coal. An approximate amount of 1.5 g raw coal was fed into the reactor for 17.5 min (86 mg/min). Nascent char was gasified in the absence of the volatile–char interactions for each holding time after the coal feeding. PH2 and PH were negligible in the experimental condition. Fig. 4 shows Na concentration in char samples. Assuming ntM was proportional to Na concentration in char, analysed concentration Na was normalised as Na (1  Xcoal) on a coal basis to compare with the coal conversion. The normalised Na concentration seemed to decrease linearly with gasification. The char sample collected from the fluidised-bed/fixed-bed reactor at 0 holding time was also gasified in the TGA. The correlation between reaction time and conversion in 15% steam at 800 °C agreed with the results in the fluidised-bed/fixed-bed reactor in Fig. 3. The evolution of kinetics parameters during char gasification were analysed from the experiments in 15%, 30% and 60% steam as shown in Fig. 5. The results suggest that Kc2 has the constant value and overall reaction rate agreed with Eq. (9) before 0.85 of the conversion. These analyses showed that Eqs. (1)–(8) described exactly the experimental results of coal conversions and Na concentrations as the solid lines in Figs. 3 and 4 during char gasification. The values of kc and kloss in the absence of the volatile–char interactions and H2 inhibition at 800 °C were determined.

kc;0 ¼ 1þK P þK p2 ffiffiffiffiffiffi , K c1;0 ¼ ic1Cn0tM0 , kloss;0 ¼ P H þK c4 P H c2 H O c3 2 pffiffiffiffiffiffi2 K loss1;0 þK loss2;0 ðK c21 P H O þK c3 P H þK c4 P H Þ 2 2 pffiffiffiffiffiffi , K loss1;0 ¼ ic10 and K loss2;0 ¼ ic9 . Eqs. þK P þK P 1þK P c2 H2 O

c3

H2

O

c4 H

(7) and (8) suggests that catalytic gasification is the zeroth order reaction and the catalytic gasification rate decreases with volatilisation of catalyst which is the first order reaction. The catalytic gasification rate is much higher than the non-catalytic gasification rate, especially at early stage of steam gasification. When the non-catalytic gasification is negligible, correlations between dXchar/dt, Xchar and t are given by:

dX char kloss;0 X char Þ ¼ kc;0 ð1  dt kc;0 X char ¼

kc;0 ð1  ekloss;0 t Þ kloss;0

These simple equations should be useful for practical use.

ð9Þ

0.06

Na (1-Xcoal) [wt%]

where

60

ð4Þ

1 dntM ntM0 dt

pffiffiffiffiffiffiffiffi K loss1 þ K loss2 ðK c21 PH2 O þ K c3 PH2 þ K c4 PH Þ pffiffiffiffiffiffiffiffi 1 þ K c2 PH2 O þ K c3 P H2 þ K c4 PH

30

Holding Time, t-t F [min]

Conversion of char [-]

0.04

0.02

0 60

70

80

90

100

Coal Conversion, Xcoal [wt%db]

ð10Þ

Fig. 4. Normalised Na concentrations Na (1  Xcoal) in char collected from the fluidised-bed/fixed-bed reactor in char gasification after feeding in 15% steam at 800 °C. Dots: experimental results. Line: kinetics model.

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S. Kajitani et al. / Fuel 103 (2013) 7–13

100

0.0008

Coal Conversion [wt%db]

60% steam

dX/dt [s-1]

0.0006 30% steam 0.0004

15% steam

0.0002

0

0

0.2

0.4

0.6

0.8

w/o volatile-char interactions

80

70

60

1

100 mg/min 50 mg/min 15 mg/min

90

0

30

Conversion of char [-]

60

90

120

Feeding Time, t [min]

(a) Steam gasification rate in 15, 30 and 60 % steam.

(a) Experimental results (dots) [10] and predictions without ring condensation (solid lines).

5

0.005

100

-1

2

0.002

K1 PH2 O

dX = dt 1+ K 2 PH2 O

0.001 0

0

0.2

0.4

-1

3

K2 [atm ]

-1

K1 [s atm ]

K1

0.003

w/o volatile-char interactions

4

1

0.6

0.8

1

0

Coal Conversion [wt%db]

K2

0.004

90

80

60

Conversion of char [-]

100 mg/min 50 mg/min 15 mg/min

70

0

60

120

(b) Kinetics parameters during steam gasification.

ð11Þ

X coal ¼ 1  ð1  X coal;0 Þð1  X char Þ

ð12Þ

0

The normalised concentration of catalyst in char M c / ðntM Þ is also given with Eq. (7) by:

1 t

Z 0

0.06 100 mg/min 50 mg/min 15 mg/min 0.04

w/o volatile-char interactions

0.02

0 60

70

80

90

100

Coal Conversion, Xcoal [wt%db] Fig. 7. Normalised Na concentrations Na (1-Xcoal) in char collected from the fluidised-bed/fixed-bed reactor in gasification with continuous coal feeding feeding in 15% steam at 800 °C. Dots: experimental results. Lines: kinetics model.

t

X char dt

Mc ¼

360

(b) Experimental results (dots) [10] and predictions considering ring condensation (solid lines).

Na (1-Xcoal) [wt%]

Char gasification is affected by the volatile–char interactions during continuous coal feeding in the fluidised-bed/fixed-bed reactor. The experimental results have been reported in the previous paper [10]. Raw coal was continuously fed into the fluidised-bed/fixedbed reactor at a coal feeding rate of 100, 50 or 15 mg/min for each feeding time with no holding time in 15% steam at 800 °C. Coal char was gasified in the presence of the volatile-char interactions, but PH2 should be negligible in the experimental condition. The observed coal conversion decreased with increasing the coal feeding rate at the same feeding time as shown in Fig. 6. The normalised Na concentration also decreased with feeding time and coal conversion as shown in Fig. 7. They are the evidences that the volatile–char interactions would inhibit the adsorption of steam and enhance the volatilisation of Na to decrease the overall char gasification rate. In this section, the proposed kinetics model was also used for simulating these continuous coal feeding experiments. Char has a distribution of residence time because of continuous coal feeding. The average char conversion X char is given by:

Z

300

Fig. 6. Coal conversions during coal gasification with continuous coal feeding in the fluidised-bed/fixed-bed reactor in 15% steam at 800 °C.

3.3. Steam gasification with volatile–char Interactions

1 t

240

Feeding Time, t [min]

Fig. 5. Steam gasification of raw coal char in the TGA at 800 °C. Char sample: coal feeding time tF = 17.5 min and holding time tH = 0 min in the fluidised-bed/fixedbed reactor.

X char ¼

180

t

Mc dt ¼

1  ekloss;0 t M c;0 kloss;0 t

ð13Þ

When suitable values for Knc4 and Kc4 were estimated, the correlations between coal feeding time and coal conversions roughly agreed with the experimental results as the solid lines in Fig. 6a. The overall gasification rate, however, seems much lower than prediction at long feeding time. The reason should be the condensation of aromatics ring systems due to H radicals from volatiles in Table 1 (III). The evolution of char structure during continuous coal feeding and after feeding in the fluidised-bed/fixed-bed reactor was discussed elsewhere

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Table 1 Key mechanisms for steam gasification of low-rank fuels. (I) Non-catalytic gasification C þ H2 O ! CO þ H2 (i) Dissociative adsorption of steam

(II) Catalytic gasification C þ H2 O ! CO þ H2 CMAM þ H ! CMAH þ M

inc1

ic1

Cf þ H2 O ! CðHÞ þ CðOHÞ

M þ H2 O ! MðHÞ þ MðOHÞ

inc3

ic3

Cf þ CðOHÞ ! CðOÞ þ CðHÞ (ii) Dissociative adsorption of Hydrogen

inc5

CðHÞ ¢ Cf þ inc6

(iii) Desorption of carbon monoxide (iv) Adsorption of free radicals from volatiles

C þ MðOHÞ ! CðOÞ þ MðHÞ ic5

1 2 H2

MðHÞ ¢ þM þ 12 H2 ic6

inc7

ic7

CðOÞ ! CO

CðOÞ ! CO

inc8

ic8

Cf þ H ! CðHÞ

M þ H ! MðHÞ

(v) Volatilisation of catalyst (Na)

ic9

MðHÞ ! CMAH þ Mgas ic10

M ! Mgas (III) Condensation of aromatics ring systems Smaller aromatics ring systems þ nH ! Bigger aromatics ring system ðP 6fused ringsÞ (IV) Deposition of coke from volatiles Volatiles ! C þ Gases Cf, M and CM denote the active site of carbon in char, the active site of the catalyst, which is AAEM species (especially Na or K), and char matrix respectively. inc and ic are the rate constants for each elementary reaction for non-catalytic gasification and non-catalytic gasification respectively.

[17]. H radicals could penetrate deep into the char matrix to induce the ring condensation reactions to convert the smaller aromatics ring systems into the bigger ones. Although thermal annealing was another cause of the growth of aromatic ring systems, existence of the volatile–char interactions significantly enhanced the ring condensation reactions. The supply of H radicals was no longer the rate-limiting factor even at a coal feeding rate as low as 15 mg/ min. The reaction rate of ring condensation was depending on coal feeding time, not on feeding rate. The concentration of H radicals could be sufficient to induce the dramatic growth of aromatic ring systems. The ring condensation due to the volatile–char interactions could lead to the reduction of char reactivity. Although the specific reactivity of char was accelerated with holding time and char conversion after coal feeding, the specific reactivity of char was reduced with feeding time and char conversion during continuous coal feeding. A correlation of Na concentration with specific reactivity was found for char samples with different holding time after the same coal feeding time on one hand and the different correlations of Na concentration with specific reactivity were found for char samples with different feeding time [10]. Therefore, it was assumed that the char gasification rate was reduced due to the condensation of aromatics ring systems only in the presence of the volatile–char interactions and Eqs. (3) and (8) were replaced with the following correlations:

knc ¼ knc;0 ð1  X char Þ in the absence of the volatile—char interactions

ð3Þ

knc ¼ knc;0 ð1  X char Þekring t in the presence of the volatile—char interactions

ð14Þ

kc ¼ kc;0 ekloss;0 t in the absence of the volatile —char interactions

ð8Þ

kc ¼ kc;0 eðkloss;0 þkring Þt in the presence of the volatile—char interactions

ð15Þ

where kring is the overall rate constant for condensation of aromatics ring systems. The large difference between kloss,0 and kring is that kloss,0 is the function of the concentration of H radicals but kring is constant not depending on the concentration of H radicals at the condition in this study. When the suitable value for kring was estimated, the correlations between coal feeding time, coal conversions and Na concentrations agreed very well with the experimental

Table 2 Kinetic parameters for steam gasification of raw coal at 800 °C. Non-catalytic gasification

Catalytic Gasification

Condensation reactions

Knc1,0 Knc2 Knc4PH/FH

4.3  104 2.8 200 3

s1 atm1 atm1 molgas molH 1

in coal

1

1

Kc1,0 Kc2 Kc4PH/FH Kloss1,0 Kloss2,0

4.5  10 4.5 45 5.0  105 2.0  104

s atm atm1 molgas molH s1 s1

kring

2.7  104

s1

in coal

1

FH represents the mol fraction of H in fed coal against supplied gas. The partial pressure of H radicals around char particles should be directly proportional to that of volatiles, and then FH. In other words, it is assumed that PH/FH has a constant value. The values of FH were 0.052, 0.026 and 0.0078 mol/mol for the experiments with the feeding rate of 100, 50 and 15 mg/min, respectively.

results as the solid lines in Figs. 6b and 7. The analysed and/or estimated values of kinetics parameters are listed in Table 2. It is known that coke is deposited on carbon surface by the thermal cracking of volatiles [19]. Because the contact of volatiles with char particles was enhanced in the fluidised-bed/fixed-bed reactor, the rate of coke deposition in Table 1 (IV) could be proportional to the amount of carbon in the reactor and also the concentration of H radicals. Although the effects of coke deposition were also estimated in the proposed model, its contribution on coal conversion seemed little in this study because it should be mostly included in the contributions of Knc4 and Kc4. If no volatile–char interaction during continuous coal feeding (PH = 0) was found, coal conversions and Na concentrations would be on the broken lines in Figs. 6 and 7, regardless of the feeding rate. Therefore it is obvious that the differences between the coal feeding rates in coal conversions depend on the volatile–char interactions which are mainly caused by free radicals formed by the thermal cracking and/or reforming of volatiles. It can be said that the volatile–char interactions were described quantitatively in the proposed kinetics model.

4. Conclusion The mechanisms and kinetics model of the catalytic gasification, the non-catalytic gasification and the volatile–char interactions were discussed to describe quantitatively the inhibition of char gasification by volatiles.

S. Kajitani et al. / Fuel 103 (2013) 7–13

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