- Email: [email protected]
Removal of calcium from low rank coals by treatment with CO2 dissolved in water
Removal of calcium from low rank coals by treatment with CO2 dissolved in water
Jun-ichiro Shigeharu
Hayashi, Morooka
Kaori Takeuchi,
Katsuki
Kusakabe
and
Department of Applied Chemistry, Kyushu University, 6- lo- 1, Hakozaki, Higashi-ku, Fukuoka 812, Japan (Received 19 October 1990; revised 7 March 1991)
Calcium and magnesium were removed from a brown coal and two subbituminous coals by treating CO, dissolved in water under a pressure of 600kPa. The results were compared with treatment
with
in a hydrochloric acid solution. Removal of calcium was significantly dependent on the chemical form of calcium in the coals. Calcium removal from Wandoan and Taiheiyo coals was about 50% after CO, treatment of 12 h, but only 10% for Morwell brown coal. The effects of pH and coal:water ratio on calcium removal were explained by a new mode1 based on ion exchange equilibrium. When the content of carboxyl groups in Morwell and Wandoan coals was decreased by mild preheating, calcium and magnesium removal was greatly increased. Preheating also improved the oil yield in direct coal liquefaction. (Keywords: carbon dioxide; calcium; ion exchange)
In the course of direct coal liquefaction, fine ash particles sometimes precipitate and cause scale and sludge: the main components of these particles are carbonates. The growth rate of solids in a dissolver increases with increasing calcium content in the feed coal’.2. Many researchers have shown that calcium and magnesium affect the reactivity of coal in pyrolysis3q4, gasification5y6, carbonization7.8 and liquefaction7,9. Therefore the removal of these components prior to coal conversion is very important. The alkaline earth metals in coal are usually removed by treating in an acid solution such as hydrochloric or sulphuric acid. However, chlorine and sulphur residues in the treated coal may cause serious erosion in coal conversion units. Carbon dioxide dissolved in water, on the other hand, does not contain corrosive components, and has been employed to remove mineral matter from oil shales’o,1’ and coals’2.‘3 . Calcite, dolomite and other carbonates were removed in carbonate water at room temperature. Slegeir et ~1.‘~ treated two coal samples at 354K under CO, pressure of 7.5-8.5 MPa. The Ca content in Bruceton Hvb coal was reduced by the CO, treatment, while Ca in North Dakota lignite was hardly removed. The ion exchangeable Ca and Mg, which are abundant in low rank coals, are the target of removal by the CO, treatment. However, the leaching mechanism in the carbonated water is not yet understood. In the present work, two subbituminous coals and a brown coal were treated in CO, dissolved in water, and the effects of the ion exchangeable Ca content in the coal and the treatment conditions on the removal yield were investigated. The results were analysed by a model based on the ion exchange equilibrium. The original and the CO, treated coal were liquefied, and the effect of the CO, treatment on the oil yield was investigated.
001~2361/91/101181-06 0 1991 Butterworth-Heinemann
Ltd
EXPERIMENTAL Coal samples Two subbituminous coals (Wandoan and Taiheiyo) and a brown coal (Morwell) were pulverized to 0.1055 0.210mm and dried at 373 K for 24h in uacuo. The elemental analyses of the samples are shown in Table I. The ion exchangeable Ca and Mg content in the coals was determined by the ammonium acetate leaching method according to Morgan et a1.14. The coal samples were first demineralized by leaching in a 5 mol l- ’ HCl solution for 24 h. Then Ca ions were introduced to carboxyl groups in the coal by dipping the sample in a 0.5 mol 1- ’ calcium chloride and 0.12 mol 1 - 1 triethanolamine solution. To avoid the reaction between Ca and phenolic groups”, the solution was buffered at pH 8.25. After stirring the coal slurry for 18 h at room temperature, the Ca-exchanged coal (Ca-coal, hereafter) was washed repeatedly with distilled water to remove residual CaCl,. Ion-exchangeable Ca in the Ca-coals was then removed completely by acid washing with 5 mall- ’ HCl, and the content of Ca was determined by inductively coupled plasma (ICP) spectroscopy. The total carboxyl content was calculated by assuming that two carboxyl groups were associated with each Ca ion15.
Table 1
Characteristics of coal samples
Coal
H”
c
N”
Cab
Mg*
Wandoan Taiheiyo Morwell
5.77 5.88 4.63
76.2 75.9 68.0
1.03 I .22 0.66
0.285
0.107
0.525 0.350
0.181 0.263
“wt%, daf “mEqg_’ coal, db
FUEL,
1991,
Vol 70, October
1181
Removal
of Ca from low rank coals: J. -i. Hayashi et al.
was determined by the solvent separation method using tetrahydrofuran, benzene and hexane. RESULTS AND DISCUSSION Hydrochloric
acid treatment
Figure 1 shows the time dependent change in the Ca
4 0 30
0
60
Treatment
time
I
I
90
120
,
I min I
Figure 1 Effect of treatment time on Ca and Mg removal yield by HCI treatment at pH 0. Ca: 0, Morwell; 0, Taiheiyo; A, Wandoan. Mg: 0, Morwell; l , Taiheiyo; A, Wandoan
and Mg removal yield by the HCl treatment at pH0. For Morwell brown coal, 90% of Ca and Mg was removed within 30min. The removal yield for the subbituminous coals (Wandoan and Taiheiyo) was lower than that of Morwell coal, especially removal of Mg. Figures 2 and 3 show the effect of pH on the Ca and Mg removal yield after the acid treatment for 120min. The Ca removal yield gradually decreases with an increase in pH. The rapid decrease at pH 3-5 is due to the ion exchange reaction of carboxyl groups. The removal of the Ca-coal is shown in Figure 2. The total Ca content in the Ca-coal was 0.62 and 2.4 mEq g- ‘, db of Wandoan and Morwell coal, respectively. Table 2 summarizes the result of the HCI treatment and the content of ion exchangeable Ca and Mg in the
Morwell and Wandoan coals were pretreated by heating at 573 K for l-l .5 h in nitrogen atmosphere, and were also used for the CO,/water treatment. The dry gases generated were analysed by FID-g.c. Carbon dioxide and carbon monoxide in the gas were hydrogenated to methane prior to g.c. detection. Water was collected with a CaCl, column. Hydrochloric
acid treatment
The mixture of coal (3 g) and HCl solution (150 ml) was stirred in a flask at room temperature or boiling temperature. The pH of the mixture was kept at a constant value between 0 and 4 by changing HCl concentration with a pH controller. The coal sample was filtered and washed with water until the chloride ion disappeared. Carbon dioxide
treatment
The mixture of coal particles (O.l-3.Og) and water (30g) was placed in an autoclave and pressurized by CO, to 600 kPa at 298 K. The slurry was stirred at 1500rev min-’ for a prescribed time and was then depressurized and filtered. Analytical
I
I
I
I
2
3
4
5
PH
C-l yield by HCI treatment. A, Ca-Wandoan; other
1OC
removal yield (%) = cco-c
x 100
(1)
0
where Co is the initial content of Ca or Mg and C is the value in the treated coal.
60 Tij 6 E aJ L 40 g
20
test
The original and treated Morwell coal (1 g) was mixed with 3 g of a hydrogen donor (1,2,3,lOb-tetrahydrofluoranthene (4HFL)) in a tube bomb of 20 ml. The bomb was pressurized with nitrogen to 1 MPa and shaken for 10 min in a tin bath at 523 K. The product distribution
1182
I
I
1
Figure 2 Effect of pH on Ca removal Treatment time, 120min. 0, Ca-Morwell; symbols as in Figurr I
methods
The coal samples were decarbonized with a low temperature oxygen plasma asher. The ash was completely dissolved in an aqueous 5 mol l- ’ HNO, solution at boiling temperature, and the total content of Ca and Mg in the solution was determined by ICP spectroscopy. The Ca or Mg removal is defined as follows:
Liquefaction
0
FUEL, 1991, Vol 70, October
0
0
1
2
pH Figure 3 Treatment
3
4
5
C-I
Effect of pH on Mg removal yield by HCI time, 120 min. Symbols as in Figure f
treatment.
Removal Table 2
Content
of ion exchangeable
Ca and Mg” Taiheiyo
Wandoan
Morwell
Total carboxyl
0.290
0.620
2.40
Total Ca Ion exchangeable Ca Ca removed by HCl at pH 0 12 h treatment at room temp. 4 h treatment at boiling temp.
0.525 0.245
0.285 0.155
0.350 0.320
0.325 0.471
0.205 0.250
0.320 0.340
Total Mg Ion exchangeable Mg Mg removed by HCl at pH 0 12 h treatment at room temp. 4 h treatment at boiling temp.
0.181 0.016
0.107 0.041
0.263 0.247
0.018 0.159
0.039 0.078
0.255 0.255
’ AII values in mEq g-
’ coal, db
501Tl
of Ca from low rank coals: J.-i. Hayashi et al.
equivalent to the value removed by HCl treatment after 12 h. Otaka et ~1.‘~ studied the CO, treatment of several coals. They concluded that Ca and Mg contained as carbonate and suiphate minerals were removed easily, while Ca bound to carboxyl groups was difficult to remove. The coal samples treated for 1 h and 12 h in the present work were further treated in a 1 mol 1-l ammonium acetate solution for 24 h, and the residual content of ion exchangeable Ca was determined. The sum of Ca removed by the CO, treatment and the ammonium acetate treatment was almost equal to that removed by the HCl treatment for 1 h as shown in Table 3. To determine the ability of the CO, treatment to remove ion exchangeable Ca, the Ca-coals and a commercial ion exchange resin (crosslinked polymetacrylic acid, -[CH,CHCOOH],) were treated for 1 h. Table 4 shows the initial Ca content and the Ca removal yield. The Ca removal from Ca-exchanged Wandoan and Morwell coals was much higher than from the original coals. Less Ca was removed from the ion exchange resin than from the coal samples by CO* treatment. These results show that it is possible to remove ion exchangeable Ca by CO, treatment and that the removal yield depends on the content of Ca in coal. Removal modei based on ion exchange reaction The ion exchange in coals is similar to that in weakly acidic ion exchange resins or polyacids’*-*I, e.g. poly-
0
1
Treatment
2
time
II
Cii
Figure 4 Ca removal yield by CO* treatment. Coal:water weight ratio, 1 :SO. A, Wandoan (repeating treatment after 30 min); other symbols as in Figure 1
samples. In the case of Morwell coal, virtually all Ca and Mg exist in the form of ion exchangeable cations”. Therefore the treatment of this sample in a 1 mol 1-l ammonium acetate solution at room temperature showed the same effect as in a 1 mol 1-l HCl solution”. For Wandoan and Taiheiyo coals, however, about 50% of Ca and l&40% of Mg were in the form of ion exchangeable cations. Non-ion exchangeable alkaline earths exist in coals as clay minerals, carbonates and sulphates. The difference in Ca removal yield for treatment by HCl and ammonium acetate solution at room temperature can be attributed to the difference in the solubility of calcium carbonate and sulphate. When the subbituminous coals were treated in HCl solution of pH0 at boiling temperature, the Ca and Mg removal yield increased remarkably. This is thought to be because Ca and Mg associated to clay minerals, which are insoluble at room temperature, are partially dissolved in the boiling HCl solution. Curhon dioxide treatment Fyures 4 and 5 show the Ca and Mg removal yield by CO, treatment as a function of treatment period. The Ca removal yield is in the order Morwell< Wandoan
0
2
1
Treatment
time
Figure 5
ratio,
Mg removal yield by CO, l:50. Symbols as in Fiyure I
treatment.
Table 3 Ca removed by HCl and CO, exchangeable Ca in treated coal”
Taiheiyo Ca removed (I ) Residual ion exchangeable Ca (2) (l)+(2) Wandoan Ca removed Residual ion exchangeable (U+(2)
(I )
12
Ch I Coal:waler
treatment
weight
and residual
CO,
ion
treatmenth
HCI treatment (I mol I ’ ) Ih
Ih
12h
0 325
0.150
0.245
0.325
0.155 0.305
0.060 0.305
0.205
0.062
0.125
0.205
0.143 0.205
0.078 0.203
Ca (2)
“All values in mEqg-’ coal, db “Coal: water weight ratm I :50
FUEL, 1991, Vol 70, October
1183
Removal Table 4
of Ca from low rank coals: J. -i. Ha yashi et al.
Ca content
in Ca-coal
and Ca removalby
Sample
Ca content (mEqg- ’
Wandoan Morwell Taiheiyo Resin
0.620 2.40 0.290 1.75’
CO,
treatment’ Removal (X)
coal, db)
56 30(38)* 59 8.7
___.
“Coal:water weight ratio I:50 “8% of Ca was precipitated as CaCO, ‘Only part of the carboxyl group content carboxyl content was IO mEq g- ’
consumes 1 mole of H+ and produces 1 mole of HCO;, while the ion exchange reaction of 1 mole of Ca(-COO), consumes 2 moles of H+. Then the relationship between the concentration of H+ and HCO; is given by: [H+]=[HCO,]-2C,R[f+(l-f)y]
[Ca(-COO),] was
exchanged.
(13)
Equations (8), (9), (10) and (13) can be expressed as follows by using the overall removal yield Y:
Total
[-COOH]
= C,R( 1 - Y)
(14)
= A,R - 2C,R( 1 - Y)
(15)
[Ca’+]=C,RY
acrylic acid or polymetacrylic acid. To simplify the model, only the ion exchange of Ca with the carboxyl group of coal and the dissolution of CaCO, are considered. These reactions occur in both CO, treatment and HCl treatment at room temperature. The following two assumptions are made. First, the concentration of carbonic acid (H&O,) is equilibrated with the saturated CO, concentration in the solution. H+ ion is produced by: H,O + CO, Z$ H&O,
(2)
H,CO, = H+ +HCO, (3) ‘The dissociation constant of carbonic acid, K,, is defined as: K, = CH’I CHCO,l/CHzCW
(4)
Second, Ca in coal is removed by the following reactions: + 2H + + Ca’+ + 2(-COOH)
Ca(-COO),
CaCO, + H +eCa’++HCO;
(5) (6)
CaCO, is assumed to be dissolved completely at the equilibrium state. The ion exchange constant of reaction (5) is: K, = [Ca(-COO),]
[H+]‘/[Ca’+]
[-COOH]2
(7)
The constant K,, which is introduced by Gregor et ~1.‘~ to describe the association equilibrium of bivalent metal ions and polyacid molecules, is the only fitting parameter of this model. The total carboxyl content, A,, the total calcium content, C,, and the mass fraction of calcium existing as CaCO, in the raw coal,f, are determined experimentally. Then the factors on the right-hand side of Equation (7) are expressed as follows: [Ca(-COO),] [-COOH]
= C,R(l -f)(l
-y)
(8)
= A,R - 2[Ca(-C00)2]
(9)
CCa2+1 = GW+
(1-f)~l
(16)
[H+]=[HCO,]-2C,RY
(17) It is clear that the fraction of CaCO, (f), i.e. the composition of Ca in coal, does not affect the overall removal yield, and the total Ca content (C,) and the total carboxyl content (A,) are the parameters that determine it. Prediction qf Ca removal yield The model accurately predicts the dependence of pH on the removal yield by the HCl treatment as shown in Figure 6. Because the model treats only ion exchangeable Ca and CaCO,, the ordinate is normalized as the relative Ca removal yield at each pH divided by the value at pH 1. The estimated value of K, for Wandoan and Morwell coal is 4 x 10e6 and 5 x 10m5, respectively. It is possible that the dissociation constant of carboxyl group in Morwell coal is widely distributed. This may be the reason why the removal yield of Morwell coal at higher pH is not reproduced well by this model. For precise prediction, the distribution of K, should be considered. Figure 7 shows the Ca removal yield as a function of the water:coal mass ratio in the CO, treatment. The Ca removal significantly decreases with increase in coal concentration in a batchwise operation, because H + is consumed by the dissolution of Ca. Therefore to obtain a higher removal yield, the flow-through operation, which prevents the increase of pH, must be effective as well as the treatment under a higher CO, pressureI and the multiple treatment. The solid symbols in Figure 4 represent the Ca removal yield of Wandoan coal after
(10)
where R is the weight ratio of coal:water, and y is the removal yield of ion exchangeable Ca at equilibrium. The overall removal yield, Y, is expressed as: Y=f+(l-f)Y
(11) .$
During the CO, treatment, the concentration of H+ varies with the extent of Ca removal. In the absence of coal, the concentration of H+ is equal to that of HCO; produced by reaction (3): [H +] = [HCO;]
FUEL, 1991, Vol 70, October
;;i 5
‘1.
I
2
Xo1
3
4
5
6
(12)
However, this equation is not valid when reactions (5) and (6) occur. The dissolution of 1 mole of CaCO,
1184
20-
Figure 6
Prediction of relative Ca removal yield by acid treatment. Morwell; A, Wandoan. Lines are calculated from the
0, IRC-84;0, model
Removal
of Ca from low rank coals: J.-i. Hayashi et al.
Figure 8 predicts that a high Ca removal yield is possible if the concentration of carboxyl group in the coal is reduced. Improvement
3
60-
CoalLCOOH
iz L LO-0 2
,/’
50
10
Water/ coal Figure 7 Taiheiyo; model
yield by preheating
300
100
ratio
[ - I
EtTect of water:coal ratio on relative Ca removal a. Wandoan: 0. Morwell. Lines are calculated
yield. 0, from the
2
3
PH
4
5
6
-+ Coal-H
+ CO2
(18)
The composition of gas evolved during the heat treatment is given in Table 5. The amount of carboxyl group decomposed was estimated from the amount of CO, evolved to be at least 1.Ommol g- ’ db (43% of the total amount) for Morwell and 0.48 mmol g- ’ db (77% of the total amount) for Wandoan coal. Water was the most abundant component in the products. The heat treatment was considered not to damage the coal because traces of methane and tar were present. When the heat treated Morwell and Wandoan coals were treated with CO, dissolved in water for 2 h at room temperature, the Ca removal yield was 57% and 45%, respectively. These values are much higher than those obtained with the original coals (7% for Morwell and 25% for Wandoan coal). The decrease in carboxyl group content led to the drastic increase of Ca removal, and the result supports the model prediction. FTi.r. analysis shows that the absorption peak at 1710cm-‘, originating from the protonated carboxyl groupz2, was decreased by the heat treatment and was increased after the CO,/water treatment, which indicates the formation of protonated carboxyl group by the ion exchange reaction of Ca(-COO), as in Equation (5). Liquqfuction
-1
of removal
Carboxyl groups tend to decompose at relatively lower temperature without the decomposition of other functional groups3.
of’ treated coul
Figure 9 shows the distribution of liquefied products using the raw Morwell coal and the CO, treated Morwell coal with preheating. The yield of oil and asphaltene of
C-l
Figure 8 Effects of the content of Ca and carboxyl group on the predicted relative Ca removal yield. K, = 5 x 10e5. -, Morwell coal: Morwell coal (total ------, Morwell Ca-coal; - -, decarboxylated total Ca=0.32mEqgm1 db coal) carboxyl group=:0,32mEqg-‘,
repeating the CO, treatment for 30min followed by filtration. The Ca removal yield increased to -35% by repeating this procedure four times (total treatment time= 120min), which was higher than the removal obtained by a single treatment of 120min. Figure 8 shows the other important aspects of coal properties and treatment conditions. The solid lines are calculated by using the equilibrium data of the Morwell raw coal. The broken line represents the calculation for the Morwell Ca-coal. The decrease in the Ca removal yield at a higher pH is attributed to the ion exchange of Ca. An increase in the pH value increases the Ca content bound to carboxyl groups of the coal. The order of the Ca removal yield shown in Figure 4 for the CO, treatment is in agreement with that of the ion exchangeable Ca content per unit carboxyl group in the coals. The ratio of carboxyl group content:Ca content is a key factor controlling removal yield. If the carboxyl group content is reduced, the equilibrium of the exchange reaction (Equation (5)) is shifted to the right. The chain line in
Table 5
Composition
of evolved
Temperature (K) Time (h) Water (wt%, db) CO, (wt%>, db) CO (wt%. db) CH, (wt”/,. db) Tar (wt%. db) Total (wt%. db)
Treated
gas during
heat treatment
Morwell
Wandoan
573 1.5 6.1 4.5 0.6
573 1.0 2.8 ‘.I 0.3 0.04
1I.8
Raw I
I
A
0
G
0
A
0
G 1
IGPl
5.2
I
ti
Yield
11
50
11
PA
PA
11
R
R
I 100
c wt %,daf 1
Figure 9 Liquefied product distribution. 0, oil; G, gas; A, Asphaltene; PA, preasphaltene; R, residue; GP, gas evolved during the preheating (conditions as in Table 5)
FUEL,
1991,
Vol 70, October
1185
Removal of Ca from low rank coals: J. -i. Ha yashi et a I.
the treated coal was higher than that of the original coal. Mochida et al. 23 also obtained a hig her yield of oil and asphaltene with thermally treated Morwell coal. The partial decomposition of oxygen-containing groups and removal of Ca release coal molecules from non-covalent hydrogen bond and ionic bond, respectively and affects the solubility of coals with hydrogen-donor solvents23.
2 3
7
Wakely, L. D., Davis, A., Jenkins, G., Mitchell, G. D. and Walker Jr., P. L. Fuel 1979,!%, 379 Franklin, H. D., Cosway, R. G., Peters, W. A. and Howard, J. B. Ind. Eng. Chem. Process Des. Deu. 1983, 22, 39 Tyler, R. J. and Schafer, H. N. S. Fuel 1980, 59, 487 Johnson, H. L. Am. Chem. Sot. Div. Fuel Chem. Prepr. 1975, 20(4), 85 Hippo, E. J., Jenkins, R. D. and Walker Jr., P. L. Fuel 1979, 58, 338 Mochida, I., Tahara, T., Iwamoto, K., Korai, Y., Fujitsu, H. and Takeshita, K. Nippon Kagaku Kaishi 1980, 899
CONCLUSIONS
8
The Ca and Mg removal yields by the CO, treatment were dependent on pH, water:coal ratio and the chemical forms of Ca and Mg in the coals. The model based on the ion exchange equilibrium between Ca and carboxyl groups in the coal provided a good explanation of the experimental results of both the CO, and HCl treatments. A mild thermal pretreatment of Morwell and Wandoan coal at 573 K drastically improved the Ca removal.
Mochida, I., Shimohara, T., Korai, 1984,63,847
9
22
Sakanishi, K., Zao, X. Z., Sakata, R. and Mochida, I. in Proceedings of International Conference on Coal Science, 1989, p. 811 Thomas, R. D. Fuel 1989, 48, 75 Vandegrift, G. F., Winans, R. E. and Horwitz, E. P. Fuel 1980, 59,634 Slegeir, W., Sanchez, J., Coughlan, R. et al. Coal Pretreatment With Carbon Dioxide and Water: Effects on North Dakota Lignite and Utah Coal’, EPRI report AP-5222, USA, 1987 Otaka, Y., Akiyama, M., Hirai, A., Ohnishi, T. and Matsuo, K. in Proceedings of International Conference on Coal Science, 1989, p. 1015 Morgan, M. E., Jenkins, R. G. and Walker Jr., P. L. Fuel 1981, 60, 189 Schafer, H. N. S. Fuel 1970, 49, 197 Durie, R. A. Fuel 1961,40,407 Shimohara, T. and Mochida, J. Fuel Sot. Japan 1987,66, 134 Marinsky, J. A. ‘Mass Transfer and Kinetics of Ion Exchange’, NATO Scientific Affairs Division, Martinus Nijhoff Publishers, 1983 Gregor, H. P., Luttigner, L. B. and Loebl, E. M. J. Phys. Chem. 195s. 59, 34 and 366 Mandel. M. and Levte. J. C. J. Polvm. Sci.. Part A 1964, 2, 2883 and 3771 . Anspach, W. M. and Marinsky, J. A. J. Phys. Chem. 1975, 79, 433 Kister, J., Guikiano, M., Mille, G. and Dou, H. Fuel 1988, 67,
23
Mochida, I., Yufu, J., Sakanishi, K., Korai, Y. and Shimohara, T.
10 11 12
13
14
ACKNOWLEDGEMENT The authors thank Professor I. Mochida and Dr K. Sakanishi of Institute of Advanced Material Study, Kyushu University for their useful discussions. Coal samples were supplied by New Energy and Industrial Technology Development Organization and Kobe Steel Ltd.
15 16 17 18
19 20 21
REFERENCES 1
1186
Ying, D. H. S., Sivasubramanian, R. and Givens, E. N. ‘Gas Slurry Flow in Coal Liquefaction Processes’, EF-14801-3, USA, 1979
FUEL, 1991, Vol 70, October
Y and Fujitsu, H. Fuel
1076 Fuel Sot. Japan 1986,65,
1020
Jun-ichiro Shigeharu
Hayashi, Morooka
Kaori Takeuchi,
Katsuki
Kusakabe
and
Department of Applied Chemistry, Kyushu University, 6- lo- 1, Hakozaki, Higashi-ku, Fukuoka 812, Japan (Received 19 October 1990; revised 7 March 1991)
Calcium and magnesium were removed from a brown coal and two subbituminous coals by treating CO, dissolved in water under a pressure of 600kPa. The results were compared with treatment
with
in a hydrochloric acid solution. Removal of calcium was significantly dependent on the chemical form of calcium in the coals. Calcium removal from Wandoan and Taiheiyo coals was about 50% after CO, treatment of 12 h, but only 10% for Morwell brown coal. The effects of pH and coal:water ratio on calcium removal were explained by a new mode1 based on ion exchange equilibrium. When the content of carboxyl groups in Morwell and Wandoan coals was decreased by mild preheating, calcium and magnesium removal was greatly increased. Preheating also improved the oil yield in direct coal liquefaction. (Keywords: carbon dioxide; calcium; ion exchange)
In the course of direct coal liquefaction, fine ash particles sometimes precipitate and cause scale and sludge: the main components of these particles are carbonates. The growth rate of solids in a dissolver increases with increasing calcium content in the feed coal’.2. Many researchers have shown that calcium and magnesium affect the reactivity of coal in pyrolysis3q4, gasification5y6, carbonization7.8 and liquefaction7,9. Therefore the removal of these components prior to coal conversion is very important. The alkaline earth metals in coal are usually removed by treating in an acid solution such as hydrochloric or sulphuric acid. However, chlorine and sulphur residues in the treated coal may cause serious erosion in coal conversion units. Carbon dioxide dissolved in water, on the other hand, does not contain corrosive components, and has been employed to remove mineral matter from oil shales’o,1’ and coals’2.‘3 . Calcite, dolomite and other carbonates were removed in carbonate water at room temperature. Slegeir et ~1.‘~ treated two coal samples at 354K under CO, pressure of 7.5-8.5 MPa. The Ca content in Bruceton Hvb coal was reduced by the CO, treatment, while Ca in North Dakota lignite was hardly removed. The ion exchangeable Ca and Mg, which are abundant in low rank coals, are the target of removal by the CO, treatment. However, the leaching mechanism in the carbonated water is not yet understood. In the present work, two subbituminous coals and a brown coal were treated in CO, dissolved in water, and the effects of the ion exchangeable Ca content in the coal and the treatment conditions on the removal yield were investigated. The results were analysed by a model based on the ion exchange equilibrium. The original and the CO, treated coal were liquefied, and the effect of the CO, treatment on the oil yield was investigated.
001~2361/91/101181-06 0 1991 Butterworth-Heinemann
Ltd
EXPERIMENTAL Coal samples Two subbituminous coals (Wandoan and Taiheiyo) and a brown coal (Morwell) were pulverized to 0.1055 0.210mm and dried at 373 K for 24h in uacuo. The elemental analyses of the samples are shown in Table I. The ion exchangeable Ca and Mg content in the coals was determined by the ammonium acetate leaching method according to Morgan et a1.14. The coal samples were first demineralized by leaching in a 5 mol l- ’ HCl solution for 24 h. Then Ca ions were introduced to carboxyl groups in the coal by dipping the sample in a 0.5 mol 1- ’ calcium chloride and 0.12 mol 1 - 1 triethanolamine solution. To avoid the reaction between Ca and phenolic groups”, the solution was buffered at pH 8.25. After stirring the coal slurry for 18 h at room temperature, the Ca-exchanged coal (Ca-coal, hereafter) was washed repeatedly with distilled water to remove residual CaCl,. Ion-exchangeable Ca in the Ca-coals was then removed completely by acid washing with 5 mall- ’ HCl, and the content of Ca was determined by inductively coupled plasma (ICP) spectroscopy. The total carboxyl content was calculated by assuming that two carboxyl groups were associated with each Ca ion15.
Table 1
Characteristics of coal samples
Coal
H”
c
N”
Cab
Mg*
Wandoan Taiheiyo Morwell
5.77 5.88 4.63
76.2 75.9 68.0
1.03 I .22 0.66
0.285
0.107
0.525 0.350
0.181 0.263
“wt%, daf “mEqg_’ coal, db
FUEL,
1991,
Vol 70, October
1181
Removal
of Ca from low rank coals: J. -i. Hayashi et al.
was determined by the solvent separation method using tetrahydrofuran, benzene and hexane. RESULTS AND DISCUSSION Hydrochloric
acid treatment
Figure 1 shows the time dependent change in the Ca
4 0 30
0
60
Treatment
time
I
I
90
120
,
I min I
Figure 1 Effect of treatment time on Ca and Mg removal yield by HCI treatment at pH 0. Ca: 0, Morwell; 0, Taiheiyo; A, Wandoan. Mg: 0, Morwell; l , Taiheiyo; A, Wandoan
and Mg removal yield by the HCl treatment at pH0. For Morwell brown coal, 90% of Ca and Mg was removed within 30min. The removal yield for the subbituminous coals (Wandoan and Taiheiyo) was lower than that of Morwell coal, especially removal of Mg. Figures 2 and 3 show the effect of pH on the Ca and Mg removal yield after the acid treatment for 120min. The Ca removal yield gradually decreases with an increase in pH. The rapid decrease at pH 3-5 is due to the ion exchange reaction of carboxyl groups. The removal of the Ca-coal is shown in Figure 2. The total Ca content in the Ca-coal was 0.62 and 2.4 mEq g- ‘, db of Wandoan and Morwell coal, respectively. Table 2 summarizes the result of the HCI treatment and the content of ion exchangeable Ca and Mg in the
Morwell and Wandoan coals were pretreated by heating at 573 K for l-l .5 h in nitrogen atmosphere, and were also used for the CO,/water treatment. The dry gases generated were analysed by FID-g.c. Carbon dioxide and carbon monoxide in the gas were hydrogenated to methane prior to g.c. detection. Water was collected with a CaCl, column. Hydrochloric
acid treatment
The mixture of coal (3 g) and HCl solution (150 ml) was stirred in a flask at room temperature or boiling temperature. The pH of the mixture was kept at a constant value between 0 and 4 by changing HCl concentration with a pH controller. The coal sample was filtered and washed with water until the chloride ion disappeared. Carbon dioxide
treatment
The mixture of coal particles (O.l-3.Og) and water (30g) was placed in an autoclave and pressurized by CO, to 600 kPa at 298 K. The slurry was stirred at 1500rev min-’ for a prescribed time and was then depressurized and filtered. Analytical
I
I
I
I
2
3
4
5
PH
C-l yield by HCI treatment. A, Ca-Wandoan; other
1OC
removal yield (%) = cco-c
x 100
(1)
0
where Co is the initial content of Ca or Mg and C is the value in the treated coal.
60 Tij 6 E aJ L 40 g
20
test
The original and treated Morwell coal (1 g) was mixed with 3 g of a hydrogen donor (1,2,3,lOb-tetrahydrofluoranthene (4HFL)) in a tube bomb of 20 ml. The bomb was pressurized with nitrogen to 1 MPa and shaken for 10 min in a tin bath at 523 K. The product distribution
1182
I
I
1
Figure 2 Effect of pH on Ca removal Treatment time, 120min. 0, Ca-Morwell; symbols as in Figurr I
methods
The coal samples were decarbonized with a low temperature oxygen plasma asher. The ash was completely dissolved in an aqueous 5 mol l- ’ HNO, solution at boiling temperature, and the total content of Ca and Mg in the solution was determined by ICP spectroscopy. The Ca or Mg removal is defined as follows:
Liquefaction
0
FUEL, 1991, Vol 70, October
0
0
1
2
pH Figure 3 Treatment
3
4
5
C-I
Effect of pH on Mg removal yield by HCI time, 120 min. Symbols as in Figure f
treatment.
Removal Table 2
Content
of ion exchangeable
Ca and Mg” Taiheiyo
Wandoan
Morwell
Total carboxyl
0.290
0.620
2.40
Total Ca Ion exchangeable Ca Ca removed by HCl at pH 0 12 h treatment at room temp. 4 h treatment at boiling temp.
0.525 0.245
0.285 0.155
0.350 0.320
0.325 0.471
0.205 0.250
0.320 0.340
Total Mg Ion exchangeable Mg Mg removed by HCl at pH 0 12 h treatment at room temp. 4 h treatment at boiling temp.
0.181 0.016
0.107 0.041
0.263 0.247
0.018 0.159
0.039 0.078
0.255 0.255
’ AII values in mEq g-
’ coal, db
501Tl
of Ca from low rank coals: J.-i. Hayashi et al.
equivalent to the value removed by HCl treatment after 12 h. Otaka et ~1.‘~ studied the CO, treatment of several coals. They concluded that Ca and Mg contained as carbonate and suiphate minerals were removed easily, while Ca bound to carboxyl groups was difficult to remove. The coal samples treated for 1 h and 12 h in the present work were further treated in a 1 mol 1-l ammonium acetate solution for 24 h, and the residual content of ion exchangeable Ca was determined. The sum of Ca removed by the CO, treatment and the ammonium acetate treatment was almost equal to that removed by the HCl treatment for 1 h as shown in Table 3. To determine the ability of the CO, treatment to remove ion exchangeable Ca, the Ca-coals and a commercial ion exchange resin (crosslinked polymetacrylic acid, -[CH,CHCOOH],) were treated for 1 h. Table 4 shows the initial Ca content and the Ca removal yield. The Ca removal from Ca-exchanged Wandoan and Morwell coals was much higher than from the original coals. Less Ca was removed from the ion exchange resin than from the coal samples by CO* treatment. These results show that it is possible to remove ion exchangeable Ca by CO, treatment and that the removal yield depends on the content of Ca in coal. Removal modei based on ion exchange reaction The ion exchange in coals is similar to that in weakly acidic ion exchange resins or polyacids’*-*I, e.g. poly-
0
1
Treatment
2
time
II
Cii
Figure 4 Ca removal yield by CO* treatment. Coal:water weight ratio, 1 :SO. A, Wandoan (repeating treatment after 30 min); other symbols as in Figure 1
samples. In the case of Morwell coal, virtually all Ca and Mg exist in the form of ion exchangeable cations”. Therefore the treatment of this sample in a 1 mol 1-l ammonium acetate solution at room temperature showed the same effect as in a 1 mol 1-l HCl solution”. For Wandoan and Taiheiyo coals, however, about 50% of Ca and l&40% of Mg were in the form of ion exchangeable cations. Non-ion exchangeable alkaline earths exist in coals as clay minerals, carbonates and sulphates. The difference in Ca removal yield for treatment by HCl and ammonium acetate solution at room temperature can be attributed to the difference in the solubility of calcium carbonate and sulphate. When the subbituminous coals were treated in HCl solution of pH0 at boiling temperature, the Ca and Mg removal yield increased remarkably. This is thought to be because Ca and Mg associated to clay minerals, which are insoluble at room temperature, are partially dissolved in the boiling HCl solution. Curhon dioxide treatment Fyures 4 and 5 show the Ca and Mg removal yield by CO, treatment as a function of treatment period. The Ca removal yield is in the order Morwell< Wandoan
0
2
1
Treatment
time
Figure 5
ratio,
Mg removal yield by CO, l:50. Symbols as in Fiyure I
treatment.
Table 3 Ca removed by HCl and CO, exchangeable Ca in treated coal”
Taiheiyo Ca removed (I ) Residual ion exchangeable Ca (2) (l)+(2) Wandoan Ca removed Residual ion exchangeable (U+(2)
(I )
12
Ch I Coal:waler
treatment
weight
and residual
CO,
ion
treatmenth
HCI treatment (I mol I ’ ) Ih
Ih
12h
0 325
0.150
0.245
0.325
0.155 0.305
0.060 0.305
0.205
0.062
0.125
0.205
0.143 0.205
0.078 0.203
Ca (2)
“All values in mEqg-’ coal, db “Coal: water weight ratm I :50
FUEL, 1991, Vol 70, October
1183
Removal Table 4
of Ca from low rank coals: J. -i. Ha yashi et al.
Ca content
in Ca-coal
and Ca removalby
Sample
Ca content (mEqg- ’
Wandoan Morwell Taiheiyo Resin
0.620 2.40 0.290 1.75’
CO,
treatment’ Removal (X)
coal, db)
56 30(38)* 59 8.7
___.
“Coal:water weight ratio I:50 “8% of Ca was precipitated as CaCO, ‘Only part of the carboxyl group content carboxyl content was IO mEq g- ’
consumes 1 mole of H+ and produces 1 mole of HCO;, while the ion exchange reaction of 1 mole of Ca(-COO), consumes 2 moles of H+. Then the relationship between the concentration of H+ and HCO; is given by: [H+]=[HCO,]-2C,R[f+(l-f)y]
[Ca(-COO),] was
exchanged.
(13)
Equations (8), (9), (10) and (13) can be expressed as follows by using the overall removal yield Y:
Total
[-COOH]
= C,R( 1 - Y)
(14)
= A,R - 2C,R( 1 - Y)
(15)
[Ca’+]=C,RY
acrylic acid or polymetacrylic acid. To simplify the model, only the ion exchange of Ca with the carboxyl group of coal and the dissolution of CaCO, are considered. These reactions occur in both CO, treatment and HCl treatment at room temperature. The following two assumptions are made. First, the concentration of carbonic acid (H&O,) is equilibrated with the saturated CO, concentration in the solution. H+ ion is produced by: H,O + CO, Z$ H&O,
(2)
H,CO, = H+ +HCO, (3) ‘The dissociation constant of carbonic acid, K,, is defined as: K, = CH’I CHCO,l/CHzCW
(4)
Second, Ca in coal is removed by the following reactions: + 2H + + Ca’+ + 2(-COOH)
Ca(-COO),
CaCO, + H +eCa’++HCO;
(5) (6)
CaCO, is assumed to be dissolved completely at the equilibrium state. The ion exchange constant of reaction (5) is: K, = [Ca(-COO),]
[H+]‘/[Ca’+]
[-COOH]2
(7)
The constant K,, which is introduced by Gregor et ~1.‘~ to describe the association equilibrium of bivalent metal ions and polyacid molecules, is the only fitting parameter of this model. The total carboxyl content, A,, the total calcium content, C,, and the mass fraction of calcium existing as CaCO, in the raw coal,f, are determined experimentally. Then the factors on the right-hand side of Equation (7) are expressed as follows: [Ca(-COO),] [-COOH]
= C,R(l -f)(l
-y)
(8)
= A,R - 2[Ca(-C00)2]
(9)
CCa2+1 = GW+
(1-f)~l
(16)
[H+]=[HCO,]-2C,RY
(17) It is clear that the fraction of CaCO, (f), i.e. the composition of Ca in coal, does not affect the overall removal yield, and the total Ca content (C,) and the total carboxyl content (A,) are the parameters that determine it. Prediction qf Ca removal yield The model accurately predicts the dependence of pH on the removal yield by the HCl treatment as shown in Figure 6. Because the model treats only ion exchangeable Ca and CaCO,, the ordinate is normalized as the relative Ca removal yield at each pH divided by the value at pH 1. The estimated value of K, for Wandoan and Morwell coal is 4 x 10e6 and 5 x 10m5, respectively. It is possible that the dissociation constant of carboxyl group in Morwell coal is widely distributed. This may be the reason why the removal yield of Morwell coal at higher pH is not reproduced well by this model. For precise prediction, the distribution of K, should be considered. Figure 7 shows the Ca removal yield as a function of the water:coal mass ratio in the CO, treatment. The Ca removal significantly decreases with increase in coal concentration in a batchwise operation, because H + is consumed by the dissolution of Ca. Therefore to obtain a higher removal yield, the flow-through operation, which prevents the increase of pH, must be effective as well as the treatment under a higher CO, pressureI and the multiple treatment. The solid symbols in Figure 4 represent the Ca removal yield of Wandoan coal after
(10)
where R is the weight ratio of coal:water, and y is the removal yield of ion exchangeable Ca at equilibrium. The overall removal yield, Y, is expressed as: Y=f+(l-f)Y
(11) .$
During the CO, treatment, the concentration of H+ varies with the extent of Ca removal. In the absence of coal, the concentration of H+ is equal to that of HCO; produced by reaction (3): [H +] = [HCO;]
FUEL, 1991, Vol 70, October
;;i 5
‘1.
I
2
Xo1
3
4
5
6
(12)
However, this equation is not valid when reactions (5) and (6) occur. The dissolution of 1 mole of CaCO,
1184
20-
Figure 6
Prediction of relative Ca removal yield by acid treatment. Morwell; A, Wandoan. Lines are calculated from the
0, IRC-84;0, model
Removal
of Ca from low rank coals: J.-i. Hayashi et al.
Figure 8 predicts that a high Ca removal yield is possible if the concentration of carboxyl group in the coal is reduced. Improvement
3
60-
CoalLCOOH
iz L LO-0 2
,/’
50
10
Water/ coal Figure 7 Taiheiyo; model
yield by preheating
300
100
ratio
[ - I
EtTect of water:coal ratio on relative Ca removal a. Wandoan: 0. Morwell. Lines are calculated
yield. 0, from the
2
3
PH
4
5
6
-+ Coal-H
+ CO2
(18)
The composition of gas evolved during the heat treatment is given in Table 5. The amount of carboxyl group decomposed was estimated from the amount of CO, evolved to be at least 1.Ommol g- ’ db (43% of the total amount) for Morwell and 0.48 mmol g- ’ db (77% of the total amount) for Wandoan coal. Water was the most abundant component in the products. The heat treatment was considered not to damage the coal because traces of methane and tar were present. When the heat treated Morwell and Wandoan coals were treated with CO, dissolved in water for 2 h at room temperature, the Ca removal yield was 57% and 45%, respectively. These values are much higher than those obtained with the original coals (7% for Morwell and 25% for Wandoan coal). The decrease in carboxyl group content led to the drastic increase of Ca removal, and the result supports the model prediction. FTi.r. analysis shows that the absorption peak at 1710cm-‘, originating from the protonated carboxyl groupz2, was decreased by the heat treatment and was increased after the CO,/water treatment, which indicates the formation of protonated carboxyl group by the ion exchange reaction of Ca(-COO), as in Equation (5). Liquqfuction
-1
of removal
Carboxyl groups tend to decompose at relatively lower temperature without the decomposition of other functional groups3.
of’ treated coul
Figure 9 shows the distribution of liquefied products using the raw Morwell coal and the CO, treated Morwell coal with preheating. The yield of oil and asphaltene of
C-l
Figure 8 Effects of the content of Ca and carboxyl group on the predicted relative Ca removal yield. K, = 5 x 10e5. -, Morwell coal: Morwell coal (total ------, Morwell Ca-coal; - -, decarboxylated total Ca=0.32mEqgm1 db coal) carboxyl group=:0,32mEqg-‘,
repeating the CO, treatment for 30min followed by filtration. The Ca removal yield increased to -35% by repeating this procedure four times (total treatment time= 120min), which was higher than the removal obtained by a single treatment of 120min. Figure 8 shows the other important aspects of coal properties and treatment conditions. The solid lines are calculated by using the equilibrium data of the Morwell raw coal. The broken line represents the calculation for the Morwell Ca-coal. The decrease in the Ca removal yield at a higher pH is attributed to the ion exchange of Ca. An increase in the pH value increases the Ca content bound to carboxyl groups of the coal. The order of the Ca removal yield shown in Figure 4 for the CO, treatment is in agreement with that of the ion exchangeable Ca content per unit carboxyl group in the coals. The ratio of carboxyl group content:Ca content is a key factor controlling removal yield. If the carboxyl group content is reduced, the equilibrium of the exchange reaction (Equation (5)) is shifted to the right. The chain line in
Table 5
Composition
of evolved
Temperature (K) Time (h) Water (wt%, db) CO, (wt%>, db) CO (wt%. db) CH, (wt”/,. db) Tar (wt%. db) Total (wt%. db)
Treated
gas during
heat treatment
Morwell
Wandoan
573 1.5 6.1 4.5 0.6
573 1.0 2.8 ‘.I 0.3 0.04
1I.8
Raw I
I
A
0
G
0
A
0
G 1
IGPl
5.2
I
ti
Yield
11
50
11
PA
PA
11
R
R
I 100
c wt %,daf 1
Figure 9 Liquefied product distribution. 0, oil; G, gas; A, Asphaltene; PA, preasphaltene; R, residue; GP, gas evolved during the preheating (conditions as in Table 5)
FUEL,
1991,
Vol 70, October
1185
Removal of Ca from low rank coals: J. -i. Ha yashi et a I.
the treated coal was higher than that of the original coal. Mochida et al. 23 also obtained a hig her yield of oil and asphaltene with thermally treated Morwell coal. The partial decomposition of oxygen-containing groups and removal of Ca release coal molecules from non-covalent hydrogen bond and ionic bond, respectively and affects the solubility of coals with hydrogen-donor solvents23.
2 3
7
Wakely, L. D., Davis, A., Jenkins, G., Mitchell, G. D. and Walker Jr., P. L. Fuel 1979,!%, 379 Franklin, H. D., Cosway, R. G., Peters, W. A. and Howard, J. B. Ind. Eng. Chem. Process Des. Deu. 1983, 22, 39 Tyler, R. J. and Schafer, H. N. S. Fuel 1980, 59, 487 Johnson, H. L. Am. Chem. Sot. Div. Fuel Chem. Prepr. 1975, 20(4), 85 Hippo, E. J., Jenkins, R. D. and Walker Jr., P. L. Fuel 1979, 58, 338 Mochida, I., Tahara, T., Iwamoto, K., Korai, Y., Fujitsu, H. and Takeshita, K. Nippon Kagaku Kaishi 1980, 899
CONCLUSIONS
8
The Ca and Mg removal yields by the CO, treatment were dependent on pH, water:coal ratio and the chemical forms of Ca and Mg in the coals. The model based on the ion exchange equilibrium between Ca and carboxyl groups in the coal provided a good explanation of the experimental results of both the CO, and HCl treatments. A mild thermal pretreatment of Morwell and Wandoan coal at 573 K drastically improved the Ca removal.
Mochida, I., Shimohara, T., Korai, 1984,63,847
9
22
Sakanishi, K., Zao, X. Z., Sakata, R. and Mochida, I. in Proceedings of International Conference on Coal Science, 1989, p. 811 Thomas, R. D. Fuel 1989, 48, 75 Vandegrift, G. F., Winans, R. E. and Horwitz, E. P. Fuel 1980, 59,634 Slegeir, W., Sanchez, J., Coughlan, R. et al. Coal Pretreatment With Carbon Dioxide and Water: Effects on North Dakota Lignite and Utah Coal’, EPRI report AP-5222, USA, 1987 Otaka, Y., Akiyama, M., Hirai, A., Ohnishi, T. and Matsuo, K. in Proceedings of International Conference on Coal Science, 1989, p. 1015 Morgan, M. E., Jenkins, R. G. and Walker Jr., P. L. Fuel 1981, 60, 189 Schafer, H. N. S. Fuel 1970, 49, 197 Durie, R. A. Fuel 1961,40,407 Shimohara, T. and Mochida, J. Fuel Sot. Japan 1987,66, 134 Marinsky, J. A. ‘Mass Transfer and Kinetics of Ion Exchange’, NATO Scientific Affairs Division, Martinus Nijhoff Publishers, 1983 Gregor, H. P., Luttigner, L. B. and Loebl, E. M. J. Phys. Chem. 195s. 59, 34 and 366 Mandel. M. and Levte. J. C. J. Polvm. Sci.. Part A 1964, 2, 2883 and 3771 . Anspach, W. M. and Marinsky, J. A. J. Phys. Chem. 1975, 79, 433 Kister, J., Guikiano, M., Mille, G. and Dou, H. Fuel 1988, 67,
23
Mochida, I., Yufu, J., Sakanishi, K., Korai, Y. and Shimohara, T.
10 11 12
13
14
ACKNOWLEDGEMENT The authors thank Professor I. Mochida and Dr K. Sakanishi of Institute of Advanced Material Study, Kyushu University for their useful discussions. Coal samples were supplied by New Energy and Industrial Technology Development Organization and Kobe Steel Ltd.
15 16 17 18
19 20 21
REFERENCES 1
1186
Ying, D. H. S., Sivasubramanian, R. and Givens, E. N. ‘Gas Slurry Flow in Coal Liquefaction Processes’, EF-14801-3, USA, 1979
FUEL, 1991, Vol 70, October
Y and Fujitsu, H. Fuel
1076 Fuel Sot. Japan 1986,65,
1020