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Role of carboxyl groups in the disintegration of brown coal briquettes by water sorption
FUEL PROCESSING TECHNOLOGY ELSEVIER
Fuel Processing Technology 50 ( 1997) 57-68
Role of carboxyl groups in the disintegration of brown coal briquettes by water sorption Jun-ichi Ozaki a1*, Yoshiyuki Nishiyama a, Peter J. Guy b, Geoffrey J. Perry b, David J. Allardice ’ aInstitute for Chemical Reaction Science, Tohoku University, Katahira, Aobaku, Send& 980-77. Japan b HRL Technology Pty Ltd, Private Bag No. 1, Morwell, Vie. 3840, Australia ’ HRL Technology Pty Ltd. 677 Springvale Road, Mulgrave. Vie. 3170, Australia
Received 20 September 1995; accepted 3 July 1996
Abstract The role of carboxyl groups in the disintegration of brown coal briquettes was investigated by means of water sorption on model briquettes made from acid-washed, cation-exchanged, water vapour-conditioned and heat-treated coals. Cracks were observed on the surfaces of briquettes with initial rates of sorption (Tini) of greater than 0.06 g-H,0 g-coal-’ min- ‘. Results from acid-washed and cation-exchanged coals revealed that the carboxylate cations act as hydration sites to promote the disintegration, whilst carboxyl groups act as hydrogen bonding sites to prevent it. The water vapour-conditioned sample (having 1.2 times the carboxyl group content of the original coal) showed good resistance to the disintegration. The effectiveness of carboxyl groups in preventing the disintegration was demonstrated by a negative correlation between rini and the amount of carboxyl groups present. Heat treated coals showed a rapid decrease in rini up to 200°C in which region there is no significant change in the carboxyl group content. This was explained in terms of suppression of swelling. Keywords:
Brown coal; Water sorption
1. Introduction Binderless briquetting is a technique widely applicable to low rank coals such as lignites, brown coals and peats, and facilitates the effective utilization of these resources. Briquettes made from some coals show disintegration in stockpiles and water sorption
* Corresponding author. Tel.: +81 22 217 5629; fax: +81 22 217 5631. 0378-3820/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PII SO378-3820(96>01059-4
58
J. Ozaki et al./ Fuel Processing Technology 50 (1997) 57-68
by briquettes is believed to cause this disintegration. Disintegration by water sorption is a serious problem for the briquetting industry. Briquettes made from Morwell coal mined in Victoria, Australia are known to have poorer resistance to disintegration than those from Yallourn (Victoria) coal [l]. This difference in water sorption has been attributed to the difference in the concentration of carboxylate cations; Morwell coal contains more cations, particularly calcium and magnesium, which act as hydration sites to swell coal particles leading to disintegration [2]. It is also important to consider the nature of the binding forces which serve to hold the coal particles together. The authors consider disintegration to result from competition between the tendency of coal particles to swell and the binding force between particles to maintain the briquettes’ integrity. The nature of this binding force has been investigated by many researchers as reviewed by Kurtz [3]. Iyengar et al. [4] concluded that hydrogen bonding among carboxyl groups and hydroxyl groups incorporated by water is an essential force. Allardice [5] found that the monolayer water on coal acts as a ‘glue’ to bind the particles together. This hydrogen bond theory remains the most promising theory to explain the nature of the binding force in brown coal briquettes. The importance of carboxyl groups among the hydrogen bond forming groups was demonstrated by the increased briquettability of air-oxidized higher rank coals (SO-90% C) in which carboxyl groups were formed by prolonged aerial oxidation [61. To overcome disintegration of briquettes caused by water sorption several approaches have been proposed to modify the interparticle binding force, such as hot press briquetting [7] or extrusion of a brown coal-water mixture to obtain a dense, dry and hard material from brown coal [8]. In the present investigation an attempt has been made to solve this problem by modifying the nature of the binding force in briquettes by altering the surface chemistry of the functional groups in the coal. The roles of oxygen functional groups in brown coals can be divided into two groups, namely, hydration sites and hydrogen bonding sites. The former includes carboxylate cations [9] and the latter, carboxyl groups. Although the classification of the functional groups has been discussed, the roles of each site in disintegration of brown coal briquettes is not clear. It is important to know the effect of carboxyl groups on the resistance to the disintegration of brown coal briquettes. In this study, Morwell coal was modified by acid washing, cation exchange, water vapour conditioning and heat treatment, and the modified coals were briquetted to examine the water sorption properties.
2. Experimental 2. I. Material preparation
The coal employed in this study was mined and dried at the Morwell Briquette Factory in 1988 and stored in sealed drums by the Coal Corporation of Victoria (now HRL Technology Pty Ltd). This dried coal (12 wt% moisture) is referred to as D-Sample, and was subjected to the treatments described below.
J. Ozaki et al./ Fuel Processing Technology 50 (1997) 57-68
59
D-Sample was ‘conditioned’ with water vapour for 30 days by exposing a 2 cm layer of the sample in an oven (held at 80°C) containing a beaker of distilled water. The coal was then air-dried. This sample is referred to as W-Sample. Acid-washed coals were prepared by shaking D-Sample overnight in hydrochloric acid solutions with different pH levels of between 0 and 4. The coals were filtered and rinsed with fresh water, and then air-dried. These coals are referred to as AW-Samples. Cation exchange was preceded by acid washing of D-Sample at pH 0 to minimize the effect of the intrinsic cations. After filtration, and rinsing with water, the acid-washed sample was mixed with each cation solution without drying. The concentration of the cation solutions was 1% by cation weight basis without pH adjustment. Sodium, magnesium and calcium were supplied as the acetates, whilst iron was supplied as ferrous chloride. After 24 h shaking, the coals were filtered and dried as for the AW-Samples. The cation-exchanged coals are referred to as EX-Samples discriminated by their exchanged cations. Heat treatment was performed in a helium stream at various temperatures (HIT) between 100” and 300°C for 1 h. These are referred to as HT-Samples discriminated by their HTT. 2.2. Characterization Cation contents were determined by atomic absorption spectrometry of the extracts after soaking coal samples in concentrated hydrochloric acid overnight. Diffuse reflectance FAIR spectrophotometry was employed to determine the carboxyl group content of powdered samples prior to briquetting. The integrated intensity of absorption of carboxyl groups (1890-1680 cm-‘) normalized by that of the aliphatic C-H stretching peak of the same sample (A,,,,) was used for quantitative comparison. The concentrations of cations and A,,,, of D-, W-, AW- and EX-Samples are presented in Table 1.
Table I Cation and carboxyl Sample
D W AW AW AW AW
(pH 3.6) (pH 2.6) (pH I .6) (pH 0.6)
AW (pH 0) EX (Na) EX (Mg) EX (Ca) EX (Fe) a not determined.
group contents of treated coal samples Concentration Na 0.10 0.11 0.10 0.10 0.07 0.07 0.09 0.63 0.09 0.08 0.08
of cations (mmol g-
’dry basis)
Mg
Ca
Fe
0.15 0.15 0.15 0.15 0.12 0.02 0.0 1 0.02 0.66 0.01 0.01
0.22 0.22 0.22 0.22 0.21 0.02 0.01 0.01 0.02 0.53 0.01
0.05 0.05 0.05 0.05 0.05 0.03 0.01 0.01 0.02 0.01 0.2
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J. Ozaki et d/Fuel 0.15
Processing Technology 50 (1997157-68
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Fig. 2 Relation between the initial rate of water sorption (rini) and the water sorption at 120 min (q&. Each symbol corresponds to the results for briquettes made from differently treated coals. 0, D-Sample; ?? , W-Sample; A, AW-Samples; v , HT-Samples.
EX(Ca)-Sample could be measured, as almost all of the disintegrated briquette could be collected successfully. Fig. 2 shows a linear correlation between q,20 and rini, indicating that the initial rate of water sorption provides a good gauge for the eventual saturation water uptake. Thus rini is a more useful parameter than q,20 because some samples have not stabilized within 120 min. The formation of cracks on the surfaces of the briquettes was taken as an early indication of disintegration. D-Sample showed many cracks on its flat surfaces after the water sorption test, whilst W-Sample did not. Inspection of the surfaces of other samples revealed that samples with rini greater than 0.06 g-H,0 g-coal-’ min-’ showed cracks on their surfaces after water sorption tests. This indicates that the disintegration is related to the initial rate of water sorption. 3.2. Acid washing and cation exchange As shown in Table 1, the concentrations of magnesium, calcium and iron decrease with decreasing pH of hydrochloric acid used for acid washing, whilst A,,,, increases. This confirms that acid wash treatment transforms the carboxylates to carboxylic acid form. Fig. 3 shows the relation between rini and the sum of the amounts of cations, including the plot for the EX(Ca)-Sample.
J. Ozaki et al./ Fuel Processing Technology 50 (1997) 57-68
63
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350
(H’IT). This plot includes D- and Closed symbols correspond to rini
64
J. Omki et d/Fuel
Processing Technology 50 (1997) 57-68
3.3. Heat treatment
The dependence of rini on HIT is given in Fig. 4. Heat treatment up to 200°C shows a decrease in rini of about 90%. A slight increase in rini can be seen for HITS higher than 200°C. The HIT dependence of A,,,, is also shown in Fig. 4. A,,,, does not show much change in the low-temperature region and then shows a decrease at temperatures higher than 200°C.
4. Discussion 4.1. Roles of car-boxy1group
In this study, diffuse reflectance FTIR was employed to measure carboxyl group contents. The diffuse reflectance method collects information when the incident infrared beam is reflected at the surfaces of each coal particle, so that the spectra obtained contain information on the surface chemical states rather than the bulk states. Fig. 5 shows the relationship between rini and ACOO”. The data, except for the HT
0.20
I
I
2.5
I
3.0
AcooIl (arbi.
3.5
unit)
Fig. 5. Relation between the initial rate of water sorption (rhi) and the relative amount of carboxy groups detected by diffuse reflectance FTIR (A coo”). Each symbol corresponds to the results for briquettes made from differently treated coals. 0, D-Sample; ?? , W-Sample; A, AW-Samples; 0, EX(Ca)-Sample; v, HT-Samples.
J. Ozuki et al./Fuel
Processing Technology 50 (1997) 57-68
65
series, show a trend of decreasing rini with the increase in A,,,,. The effect of heat treatment will be discussed later in this section. The trend shown in Fig. 5 illustrates that the surface carboxyl groups have a role in suppressing the swelling of individual coal particles, probably by increasing the density of hydrogen bonding. Now the roles of the two species can be delineated, the role of carboxylate cations is as hydration sites, in agreement with conventional explanations, whilst that of surface carboxyl groups is as hydrogen bonding sites. 4.2. Mechanism of disintegration Previous explanations for the disintegration of briquettes have basically considered the swelling behaviour of brown coal particles as being due to hydration of cations. This is understandable, as the major variable between the coals briquetted on a commercial scale has been the composition and quantity of exchangeable cations. It has been demonstrated that the degree of the disintegration is dependent on the rate of swelling and not just the overall swelling factor [lo]. It is clear from the present study that the disintegration of briquettes is dependent not only on the concentrations of cations but also on the amount of carboxyl groups present. As the effects of cations and carboxyl groups on rini are opposite to each other, it seems clear that the disintegration occurs as a result of the competition between the tendency to swell the coal particles and the tendency to maintain the briquetted state against the swelling. The basic mechanism of the disintegration is as follows: when the briquette is immersed in water, the outer shell of the briquette quickly swells and cracks because of stresses generated on the interface between the shell and the unwetted core of the briquette. Each crack exposes more coal to be swollen. In this case the time required for water to reach the centre of the briquette should be short, so the sorption curve for this briquette will have a rapid increase in the earlier stages of immersion with saturation in the later stages. Possible treatments which improve the resistance to the disintegration are to increase the carboxyl group content or to decrease the concentration of cations. Water vapour treatment (W-Sample) gives a great decrease in rini as shown in Fig. l(a). In this case the concentration of cations is not influenced, whilst accessible A,,, increases to 1.2 times that of D-Sample as can be seen in Table 1. This may be due to a reversal of the pore collapse which occurs during drying. Water vapour treatment appears to increase the surface carboxyl groups without changing the concentration of cations, whilst acid washing converts carboxylate salts to the acidform carboxyl groups. A positive correlation between rini and the concentration of cations can be seen in Fig. 3, confirming that carboxylate cations are one of the determining factors in the disintegration process. Fig. 3 also supports the work of Schafer [9] who showed a trend of increasing equilibrium moisture content with greater concentrations of exchanged cations on Yalloum coal. Both water vapour conditioning and acid washing essentially reduce the contribution of swelling to the water sorption process. This acts to minimize the formation of cracks on the surface, which retards the penetration of water into the briquette.
66
J. Ozaki et al./Fuel
Processing Technology 50 (1997) 57-68
4.3. Effect of heat treatment As can be seen in Fig. l(c), water sorption behaviour varies with HTT. The curve for I-IT100 shows a slightly gentler slope and more rounded shoulder in the earlier stages of sorption compared with the curve for the D-Sample (Fig. la). Increasing the HIT enhances this tendency and also results in non-saturating behaviour (lack of plateau on curve) even in the later stages of soaking. In this study, rini of acid-washed, cation-exchanged and water vapour-conditioned samples were found to be governed by A,,,, (Fig. 5). On the other hand, rini of heat-treated samples, especially those treated at low temperatures (< 2OO”C),changed without an accompanying change in A,,,, (Fig. 4). Recently we have reported the effect of heat treatment on cation exchange capacity [l 11. It was shown that the exchange capacity decreases more rapidly with the heat treatment temperature than the amount of carboxyl groups does. Furthermore, the reduced capacity by low temperature heat treatment could be retrieved at least by 50% of the original brown coal by the addition of ethanol to the solutions used for cation exchange. This means that the heat treatment up to 200°C induces partly reversible changes. Suppression of swelling of coals by heat treatment is well known and is usually explained in terms of the formation of covalent cross-links between functional groups in coals after treatment at elevated temperatures [12,13]. The present authors consider that this explanation can not be applied to the case of low-temperature heat treatment because of the following respects. First, as the cross-links are essentially covalent bonds, they seem unlikely to be cleaved by solvolysis, simply by immersing the sample into ethanol solutions at room temperature. Secondly, if the formation of cross-links are the main result of low-temperature heat treatment, absorption shifts in the IR spectra can be expected and this should be reflected in A,,,,. However, Acoo,, does not vary in this temperature region as shown in Fig. 4. The water in brown coals has been variously categorized as bulk water, capillary water, multilayer sorbed water and monolayer sorbed water [5], and each type of water has a different effect on the contraction upon drying. In particular, the desorption of multilayer sorbed water induces a larger volume contraction than the volume of desorbed water. Distortion and collapse of the open structure of brown coals and increased cross-linking are considered to be the reason for the large contraction via the formation of shorter and stronger hydrogen bonds because of the desorption of lyosphere water [ 143. The authors believe that what happens during the heat treatment at low temperature is mainly desorption of water rather than formation of cross-links between functional groups. Hydrogen bonding networks are probably reinforced during low-temperature heat treatment because of the ease of reconfiguration of molecular segments caused by desorption of water and thermal activation of molecular movement. Thus the heat treatment yields two advantages in resisting the disintegration of briquettes: (1) preventing the percolation of water by collapsing micropores of which expansion causes swelling [ 101, and (2) reinforcing the hydrogen bonding network which is responsible for maintaining the shapes of briquettes. Taking into account that the desorption of water
J. Ozuki et al./Fuel
Processing Technology 50 (1997) 57-68
67
does not accompany the chemical changes in carboxyl groups, we obtained the decrease in the case of low-temperature heat treatments. in rini without changes in A,,,,
5. Conclusions
In this study the role of carboxyl groups in the disintegration of brown coal briquettes has been investigated using surface-modified Morwell coals. The conclusions listed below have been reached. 1. Carboxylate cations within the coal act as hydration sites to promote the disintegration of briquettes. 2. Carboxyl groups, as measured by diffuse reflectance FTIR, appear to be responsible for resistance to the disintegration process. This probably is due to the role of the group as a hydrogen bonding site to form briquettes. 3. Disintegration of briquettes can be considered as a result of the competition between the tendency to swell owing to the uptake of water, and the action to resist swelling by maintaining the hydrogen bonding networks. Acid washing and water vapour conditioning were found to enhance the latter effect. These treatments, which basically lead to an increase in surface carboxyl group content, were effective in reducing swelling. 4. Heat treatment up to 200°C gave good resistance to the disintegration process though the amount of measured carboxyl groups had not changed. This was considered to be due to suppression of swelling by means of partially reversible structural changes in the coal-pore collapse and reinforcement of hydrogen bonding networks by dewatering. Heat treatment provides an alternative approach for increasing the resistance of briquettes to disintegration.
Acknowledgements
This work was financially supported by a grant for Japan-Australia Joint Research Program aided by the Ministry of Education, Science and Culture, Japan.
References [I] D.J. Allardice and B.S. Newell, in R.A. Durie (Ed.), The Science of Victorian Brown Coal: Structure, Properties and Consequences for Utilization, Butterworth Heinemann, Oxford, 1991, p. 651. [2] D.J. Brockway, A.L. Ottrey and R.S. Higgins, in R.A. Durie (Ed.), The Science of Victorian Brown Coal: Structure, Properties and Consequences for Utilization, Butterworth Heinemamr, Oxford, 1991, p. 597. [3] R. Kurtz, Braunkohle, 32 (1990) 368 and 443. [4] M.S. Iyengar, D.N. Sibal and A. Lahiri, Fuel, 36 (1957) 76. [5] D.J. Allardice, in R.A. Durie (Ed.), The Science of Victorian Brown Coal: Structure, Properties and Consequences for Utilization, Butterworth Heinemann, Oxford, 1991, p. 103. [6] M.S. Iyengar and R.S. Ghosh, Fuel, 39 (1960) 113. [7] G. Ellison and B.R. Stanmore, Fuel Proc. Technol., 4 (1981) 227 and 291.
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[8] R.B Johns, A.L. Chaffee,
[9]
[lo]
[l l] [12] [13] [ 141
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50 (1997)
57-68
K.F. Harvey, AS. Buchanan and G.A. Thiele, Fuel Prcc. Technol., 21 (1989) 209. H.N.S. Schafer, Fuel, 51 (1972) 4. F. Woskoboensko, W.O. Stacy and D. Raisbeck, D., in R.A. Durie (Ed.), The Science of Victorian Brown Coal: Structure, Properties and Consequences for Utilization, Butterworth Heinemann, Oxford, 1991, p. 152. K. Murakami, J. O&i and Y. Nishiyama, Fuel Proc. Technol., 43 (1995) 95. E.M. Suuberg, D. Lee and J.W. Larsen, Fuel, 64 (1985) 1668. J.V. Ibarra, R. Moliner and M.P. Gavilan, Fuel, 70 (1991) 408. D.G. Evans, Fuel, 52 (1973) 186.
Fuel Processing Technology 50 ( 1997) 57-68
Role of carboxyl groups in the disintegration of brown coal briquettes by water sorption Jun-ichi Ozaki a1*, Yoshiyuki Nishiyama a, Peter J. Guy b, Geoffrey J. Perry b, David J. Allardice ’ aInstitute for Chemical Reaction Science, Tohoku University, Katahira, Aobaku, Send& 980-77. Japan b HRL Technology Pty Ltd, Private Bag No. 1, Morwell, Vie. 3840, Australia ’ HRL Technology Pty Ltd. 677 Springvale Road, Mulgrave. Vie. 3170, Australia
Received 20 September 1995; accepted 3 July 1996
Abstract The role of carboxyl groups in the disintegration of brown coal briquettes was investigated by means of water sorption on model briquettes made from acid-washed, cation-exchanged, water vapour-conditioned and heat-treated coals. Cracks were observed on the surfaces of briquettes with initial rates of sorption (Tini) of greater than 0.06 g-H,0 g-coal-’ min- ‘. Results from acid-washed and cation-exchanged coals revealed that the carboxylate cations act as hydration sites to promote the disintegration, whilst carboxyl groups act as hydrogen bonding sites to prevent it. The water vapour-conditioned sample (having 1.2 times the carboxyl group content of the original coal) showed good resistance to the disintegration. The effectiveness of carboxyl groups in preventing the disintegration was demonstrated by a negative correlation between rini and the amount of carboxyl groups present. Heat treated coals showed a rapid decrease in rini up to 200°C in which region there is no significant change in the carboxyl group content. This was explained in terms of suppression of swelling. Keywords:
Brown coal; Water sorption
1. Introduction Binderless briquetting is a technique widely applicable to low rank coals such as lignites, brown coals and peats, and facilitates the effective utilization of these resources. Briquettes made from some coals show disintegration in stockpiles and water sorption
* Corresponding author. Tel.: +81 22 217 5629; fax: +81 22 217 5631. 0378-3820/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PII SO378-3820(96>01059-4
58
J. Ozaki et al./ Fuel Processing Technology 50 (1997) 57-68
by briquettes is believed to cause this disintegration. Disintegration by water sorption is a serious problem for the briquetting industry. Briquettes made from Morwell coal mined in Victoria, Australia are known to have poorer resistance to disintegration than those from Yallourn (Victoria) coal [l]. This difference in water sorption has been attributed to the difference in the concentration of carboxylate cations; Morwell coal contains more cations, particularly calcium and magnesium, which act as hydration sites to swell coal particles leading to disintegration [2]. It is also important to consider the nature of the binding forces which serve to hold the coal particles together. The authors consider disintegration to result from competition between the tendency of coal particles to swell and the binding force between particles to maintain the briquettes’ integrity. The nature of this binding force has been investigated by many researchers as reviewed by Kurtz [3]. Iyengar et al. [4] concluded that hydrogen bonding among carboxyl groups and hydroxyl groups incorporated by water is an essential force. Allardice [5] found that the monolayer water on coal acts as a ‘glue’ to bind the particles together. This hydrogen bond theory remains the most promising theory to explain the nature of the binding force in brown coal briquettes. The importance of carboxyl groups among the hydrogen bond forming groups was demonstrated by the increased briquettability of air-oxidized higher rank coals (SO-90% C) in which carboxyl groups were formed by prolonged aerial oxidation [61. To overcome disintegration of briquettes caused by water sorption several approaches have been proposed to modify the interparticle binding force, such as hot press briquetting [7] or extrusion of a brown coal-water mixture to obtain a dense, dry and hard material from brown coal [8]. In the present investigation an attempt has been made to solve this problem by modifying the nature of the binding force in briquettes by altering the surface chemistry of the functional groups in the coal. The roles of oxygen functional groups in brown coals can be divided into two groups, namely, hydration sites and hydrogen bonding sites. The former includes carboxylate cations [9] and the latter, carboxyl groups. Although the classification of the functional groups has been discussed, the roles of each site in disintegration of brown coal briquettes is not clear. It is important to know the effect of carboxyl groups on the resistance to the disintegration of brown coal briquettes. In this study, Morwell coal was modified by acid washing, cation exchange, water vapour conditioning and heat treatment, and the modified coals were briquetted to examine the water sorption properties.
2. Experimental 2. I. Material preparation
The coal employed in this study was mined and dried at the Morwell Briquette Factory in 1988 and stored in sealed drums by the Coal Corporation of Victoria (now HRL Technology Pty Ltd). This dried coal (12 wt% moisture) is referred to as D-Sample, and was subjected to the treatments described below.
J. Ozaki et al./ Fuel Processing Technology 50 (1997) 57-68
59
D-Sample was ‘conditioned’ with water vapour for 30 days by exposing a 2 cm layer of the sample in an oven (held at 80°C) containing a beaker of distilled water. The coal was then air-dried. This sample is referred to as W-Sample. Acid-washed coals were prepared by shaking D-Sample overnight in hydrochloric acid solutions with different pH levels of between 0 and 4. The coals were filtered and rinsed with fresh water, and then air-dried. These coals are referred to as AW-Samples. Cation exchange was preceded by acid washing of D-Sample at pH 0 to minimize the effect of the intrinsic cations. After filtration, and rinsing with water, the acid-washed sample was mixed with each cation solution without drying. The concentration of the cation solutions was 1% by cation weight basis without pH adjustment. Sodium, magnesium and calcium were supplied as the acetates, whilst iron was supplied as ferrous chloride. After 24 h shaking, the coals were filtered and dried as for the AW-Samples. The cation-exchanged coals are referred to as EX-Samples discriminated by their exchanged cations. Heat treatment was performed in a helium stream at various temperatures (HIT) between 100” and 300°C for 1 h. These are referred to as HT-Samples discriminated by their HTT. 2.2. Characterization Cation contents were determined by atomic absorption spectrometry of the extracts after soaking coal samples in concentrated hydrochloric acid overnight. Diffuse reflectance FAIR spectrophotometry was employed to determine the carboxyl group content of powdered samples prior to briquetting. The integrated intensity of absorption of carboxyl groups (1890-1680 cm-‘) normalized by that of the aliphatic C-H stretching peak of the same sample (A,,,,) was used for quantitative comparison. The concentrations of cations and A,,,, of D-, W-, AW- and EX-Samples are presented in Table 1.
Table I Cation and carboxyl Sample
D W AW AW AW AW
(pH 3.6) (pH 2.6) (pH I .6) (pH 0.6)
AW (pH 0) EX (Na) EX (Mg) EX (Ca) EX (Fe) a not determined.
group contents of treated coal samples Concentration Na 0.10 0.11 0.10 0.10 0.07 0.07 0.09 0.63 0.09 0.08 0.08
of cations (mmol g-
’dry basis)
Mg
Ca
Fe
0.15 0.15 0.15 0.15 0.12 0.02 0.0 1 0.02 0.66 0.01 0.01
0.22 0.22 0.22 0.22 0.21 0.02 0.01 0.01 0.02 0.53 0.01
0.05 0.05 0.05 0.05 0.05 0.03 0.01 0.01 0.02 0.01 0.2
A coOH (arbitrary
3.0 3.7 2.8 3.0 3.1 3.5 3.9 _a _a 2.5 _a
unit)
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J. Ozuki et al. / Fuel Processing Technology 50 (1997) 57-68 0.5-
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61
62
J. Ozaki et d/Fuel 0.15
Processing Technology 50 (1997157-68
r
Fig. 2 Relation between the initial rate of water sorption (rini) and the water sorption at 120 min (q&. Each symbol corresponds to the results for briquettes made from differently treated coals. 0, D-Sample; ?? , W-Sample; A, AW-Samples; v , HT-Samples.
EX(Ca)-Sample could be measured, as almost all of the disintegrated briquette could be collected successfully. Fig. 2 shows a linear correlation between q,20 and rini, indicating that the initial rate of water sorption provides a good gauge for the eventual saturation water uptake. Thus rini is a more useful parameter than q,20 because some samples have not stabilized within 120 min. The formation of cracks on the surfaces of the briquettes was taken as an early indication of disintegration. D-Sample showed many cracks on its flat surfaces after the water sorption test, whilst W-Sample did not. Inspection of the surfaces of other samples revealed that samples with rini greater than 0.06 g-H,0 g-coal-’ min-’ showed cracks on their surfaces after water sorption tests. This indicates that the disintegration is related to the initial rate of water sorption. 3.2. Acid washing and cation exchange As shown in Table 1, the concentrations of magnesium, calcium and iron decrease with decreasing pH of hydrochloric acid used for acid washing, whilst A,,,, increases. This confirms that acid wash treatment transforms the carboxylates to carboxylic acid form. Fig. 3 shows the relation between rini and the sum of the amounts of cations, including the plot for the EX(Ca)-Sample.
J. Ozaki et al./ Fuel Processing Technology 50 (1997) 57-68
63
0.20
z
0
0.15
E \ I : I g
0.10
f 2 '2 i- 0.05
0.00 0.2
0.0
Cone.
of
0.6
0.4
cations
(mmol/g-coal)
Fig. 3. Relation between the initial rate of water sorption (rini) and the total concentrations of cations, Na, Mg, Ca and Fe. Each symbol corresponds to the results for briquettes made from differently treated coals. 0, D-Sample; A, AW-Samples; 0, EX(Ca)-Sample. 0.10
r
4.0
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.z .a &I .- 0.02
2.0
0.00
1.5 0
50
100
150
200
HTT
(‘C)
250
300
on the heat treatment temperature Fig. 4. Dependence of rini and A,,,, W-Samples by tentatively taking their HlT as 50°C and 80°C. respectively. data and open symbols to A,,,, .
350
(H’IT). This plot includes D- and Closed symbols correspond to rini
64
J. Omki et d/Fuel
Processing Technology 50 (1997) 57-68
3.3. Heat treatment
The dependence of rini on HIT is given in Fig. 4. Heat treatment up to 200°C shows a decrease in rini of about 90%. A slight increase in rini can be seen for HITS higher than 200°C. The HIT dependence of A,,,, is also shown in Fig. 4. A,,,, does not show much change in the low-temperature region and then shows a decrease at temperatures higher than 200°C.
4. Discussion 4.1. Roles of car-boxy1group
In this study, diffuse reflectance FTIR was employed to measure carboxyl group contents. The diffuse reflectance method collects information when the incident infrared beam is reflected at the surfaces of each coal particle, so that the spectra obtained contain information on the surface chemical states rather than the bulk states. Fig. 5 shows the relationship between rini and ACOO”. The data, except for the HT
0.20
I
I
2.5
I
3.0
AcooIl (arbi.
3.5
unit)
Fig. 5. Relation between the initial rate of water sorption (rhi) and the relative amount of carboxy groups detected by diffuse reflectance FTIR (A coo”). Each symbol corresponds to the results for briquettes made from differently treated coals. 0, D-Sample; ?? , W-Sample; A, AW-Samples; 0, EX(Ca)-Sample; v, HT-Samples.
J. Ozuki et al./Fuel
Processing Technology 50 (1997) 57-68
65
series, show a trend of decreasing rini with the increase in A,,,,. The effect of heat treatment will be discussed later in this section. The trend shown in Fig. 5 illustrates that the surface carboxyl groups have a role in suppressing the swelling of individual coal particles, probably by increasing the density of hydrogen bonding. Now the roles of the two species can be delineated, the role of carboxylate cations is as hydration sites, in agreement with conventional explanations, whilst that of surface carboxyl groups is as hydrogen bonding sites. 4.2. Mechanism of disintegration Previous explanations for the disintegration of briquettes have basically considered the swelling behaviour of brown coal particles as being due to hydration of cations. This is understandable, as the major variable between the coals briquetted on a commercial scale has been the composition and quantity of exchangeable cations. It has been demonstrated that the degree of the disintegration is dependent on the rate of swelling and not just the overall swelling factor [lo]. It is clear from the present study that the disintegration of briquettes is dependent not only on the concentrations of cations but also on the amount of carboxyl groups present. As the effects of cations and carboxyl groups on rini are opposite to each other, it seems clear that the disintegration occurs as a result of the competition between the tendency to swell the coal particles and the tendency to maintain the briquetted state against the swelling. The basic mechanism of the disintegration is as follows: when the briquette is immersed in water, the outer shell of the briquette quickly swells and cracks because of stresses generated on the interface between the shell and the unwetted core of the briquette. Each crack exposes more coal to be swollen. In this case the time required for water to reach the centre of the briquette should be short, so the sorption curve for this briquette will have a rapid increase in the earlier stages of immersion with saturation in the later stages. Possible treatments which improve the resistance to the disintegration are to increase the carboxyl group content or to decrease the concentration of cations. Water vapour treatment (W-Sample) gives a great decrease in rini as shown in Fig. l(a). In this case the concentration of cations is not influenced, whilst accessible A,,, increases to 1.2 times that of D-Sample as can be seen in Table 1. This may be due to a reversal of the pore collapse which occurs during drying. Water vapour treatment appears to increase the surface carboxyl groups without changing the concentration of cations, whilst acid washing converts carboxylate salts to the acidform carboxyl groups. A positive correlation between rini and the concentration of cations can be seen in Fig. 3, confirming that carboxylate cations are one of the determining factors in the disintegration process. Fig. 3 also supports the work of Schafer [9] who showed a trend of increasing equilibrium moisture content with greater concentrations of exchanged cations on Yalloum coal. Both water vapour conditioning and acid washing essentially reduce the contribution of swelling to the water sorption process. This acts to minimize the formation of cracks on the surface, which retards the penetration of water into the briquette.
66
J. Ozaki et al./Fuel
Processing Technology 50 (1997) 57-68
4.3. Effect of heat treatment As can be seen in Fig. l(c), water sorption behaviour varies with HTT. The curve for I-IT100 shows a slightly gentler slope and more rounded shoulder in the earlier stages of sorption compared with the curve for the D-Sample (Fig. la). Increasing the HIT enhances this tendency and also results in non-saturating behaviour (lack of plateau on curve) even in the later stages of soaking. In this study, rini of acid-washed, cation-exchanged and water vapour-conditioned samples were found to be governed by A,,,, (Fig. 5). On the other hand, rini of heat-treated samples, especially those treated at low temperatures (< 2OO”C),changed without an accompanying change in A,,,, (Fig. 4). Recently we have reported the effect of heat treatment on cation exchange capacity [l 11. It was shown that the exchange capacity decreases more rapidly with the heat treatment temperature than the amount of carboxyl groups does. Furthermore, the reduced capacity by low temperature heat treatment could be retrieved at least by 50% of the original brown coal by the addition of ethanol to the solutions used for cation exchange. This means that the heat treatment up to 200°C induces partly reversible changes. Suppression of swelling of coals by heat treatment is well known and is usually explained in terms of the formation of covalent cross-links between functional groups in coals after treatment at elevated temperatures [12,13]. The present authors consider that this explanation can not be applied to the case of low-temperature heat treatment because of the following respects. First, as the cross-links are essentially covalent bonds, they seem unlikely to be cleaved by solvolysis, simply by immersing the sample into ethanol solutions at room temperature. Secondly, if the formation of cross-links are the main result of low-temperature heat treatment, absorption shifts in the IR spectra can be expected and this should be reflected in A,,,,. However, Acoo,, does not vary in this temperature region as shown in Fig. 4. The water in brown coals has been variously categorized as bulk water, capillary water, multilayer sorbed water and monolayer sorbed water [5], and each type of water has a different effect on the contraction upon drying. In particular, the desorption of multilayer sorbed water induces a larger volume contraction than the volume of desorbed water. Distortion and collapse of the open structure of brown coals and increased cross-linking are considered to be the reason for the large contraction via the formation of shorter and stronger hydrogen bonds because of the desorption of lyosphere water [ 143. The authors believe that what happens during the heat treatment at low temperature is mainly desorption of water rather than formation of cross-links between functional groups. Hydrogen bonding networks are probably reinforced during low-temperature heat treatment because of the ease of reconfiguration of molecular segments caused by desorption of water and thermal activation of molecular movement. Thus the heat treatment yields two advantages in resisting the disintegration of briquettes: (1) preventing the percolation of water by collapsing micropores of which expansion causes swelling [ 101, and (2) reinforcing the hydrogen bonding network which is responsible for maintaining the shapes of briquettes. Taking into account that the desorption of water
J. Ozuki et al./Fuel
Processing Technology 50 (1997) 57-68
67
does not accompany the chemical changes in carboxyl groups, we obtained the decrease in the case of low-temperature heat treatments. in rini without changes in A,,,,
5. Conclusions
In this study the role of carboxyl groups in the disintegration of brown coal briquettes has been investigated using surface-modified Morwell coals. The conclusions listed below have been reached. 1. Carboxylate cations within the coal act as hydration sites to promote the disintegration of briquettes. 2. Carboxyl groups, as measured by diffuse reflectance FTIR, appear to be responsible for resistance to the disintegration process. This probably is due to the role of the group as a hydrogen bonding site to form briquettes. 3. Disintegration of briquettes can be considered as a result of the competition between the tendency to swell owing to the uptake of water, and the action to resist swelling by maintaining the hydrogen bonding networks. Acid washing and water vapour conditioning were found to enhance the latter effect. These treatments, which basically lead to an increase in surface carboxyl group content, were effective in reducing swelling. 4. Heat treatment up to 200°C gave good resistance to the disintegration process though the amount of measured carboxyl groups had not changed. This was considered to be due to suppression of swelling by means of partially reversible structural changes in the coal-pore collapse and reinforcement of hydrogen bonding networks by dewatering. Heat treatment provides an alternative approach for increasing the resistance of briquettes to disintegration.
Acknowledgements
This work was financially supported by a grant for Japan-Australia Joint Research Program aided by the Ministry of Education, Science and Culture, Japan.
References [I] D.J. Allardice and B.S. Newell, in R.A. Durie (Ed.), The Science of Victorian Brown Coal: Structure, Properties and Consequences for Utilization, Butterworth Heinemann, Oxford, 1991, p. 651. [2] D.J. Brockway, A.L. Ottrey and R.S. Higgins, in R.A. Durie (Ed.), The Science of Victorian Brown Coal: Structure, Properties and Consequences for Utilization, Butterworth Heinemamr, Oxford, 1991, p. 597. [3] R. Kurtz, Braunkohle, 32 (1990) 368 and 443. [4] M.S. Iyengar, D.N. Sibal and A. Lahiri, Fuel, 36 (1957) 76. [5] D.J. Allardice, in R.A. Durie (Ed.), The Science of Victorian Brown Coal: Structure, Properties and Consequences for Utilization, Butterworth Heinemann, Oxford, 1991, p. 103. [6] M.S. Iyengar and R.S. Ghosh, Fuel, 39 (1960) 113. [7] G. Ellison and B.R. Stanmore, Fuel Proc. Technol., 4 (1981) 227 and 291.
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J. Ozuki et d/Fuel
[8] R.B Johns, A.L. Chaffee,
[9]
[lo]
[l l] [12] [13] [ 141
Processing
Technology
50 (1997)
57-68
K.F. Harvey, AS. Buchanan and G.A. Thiele, Fuel Prcc. Technol., 21 (1989) 209. H.N.S. Schafer, Fuel, 51 (1972) 4. F. Woskoboensko, W.O. Stacy and D. Raisbeck, D., in R.A. Durie (Ed.), The Science of Victorian Brown Coal: Structure, Properties and Consequences for Utilization, Butterworth Heinemann, Oxford, 1991, p. 152. K. Murakami, J. O&i and Y. Nishiyama, Fuel Proc. Technol., 43 (1995) 95. E.M. Suuberg, D. Lee and J.W. Larsen, Fuel, 64 (1985) 1668. J.V. Ibarra, R. Moliner and M.P. Gavilan, Fuel, 70 (1991) 408. D.G. Evans, Fuel, 52 (1973) 186.