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Oxidation studies on coking coal related to weathering. 3. The influence of acidic hydroxyl groups, created during oxidation, on the plasticity and dilatation of the weathered coking coal
Oxidation studies on coking coal related to weathering. 3. The influence of acidic hydroxyl groups, created during oxidation, on the plasticity and dilatation of the weathered coking coal* Boleslaw S. lgnasiak, Adam 3. Szladow and Douglas S. ~ont~meryt Fuel Sciences Division, Research Council of Alberta, Alberta, Canada (Received 17 May 19731
I 137587th
Avenue, Edmonton 7,
A high-volatile bituminous coal possessinga high Gieseler fluidity and large dilatation was subjected to slight oxidation in air under chosen conditions. The resultant ‘oxycoal’was treated with various chemical reagents in order to determine to what extent the original caking properties of this coal, as measured by Gieseler fluidity and dilatation, could be restored. Treatment with aqueous barium hydroxide or barium acetate solutions succeeded in doing so to a considerable degree. tt is tentatively concluded that certain acidic hydroxyl groups formed in the course of oxidation are indirectly responsible for the deterioration of fluidity and dilatation - possibly via condensation reactions involving the hydroxyl groups - during subsequent pyrolysis of oxycoal. The presence of carboxyl groups appears to have no effect,
The distribution of l80 in pyrolysis products of 0xy-~*0coal was the subject of a previous paper’ in which we reported that up to about 85% of labelled oxygen introduced into coal during slight oxidation could be removed by subsequent pyrolyses at temperatures to 450°C. In particular, it was shown that the concentration of l80 in the residual 450°C char did not exceed 17% of the total labelled oxygen initially introduced into the coal, while concentrations of l80 in evolved gas and tar amounted to, respectively, about 23% and 62%. These observations, and the later finding that practically all Hz180 was evolved below 450°C led us to suppose that decreases in (Gieseler) fluidity and dilatation accompanying the oxidation might be caused by condensation reactions during pyrolysis of oxycoal, and by a consequent increase of the average molecular weight of the plasticizer. Considering the thermal stability of the oxygenbearing functional groups created by oxidation, we were inclined to associate such condensation reactions with hydroxyl groups and to suppose that if condensation could be prevented or delayed, it might be possible to restore the (Gieseler) fluidity and ability to dilate of the oxycoal at substantially the same values as those observed for the fresh coal. Proceeding from this hypothesis we report in this paper, which is an enlarged version of a previous report’, attempts to stabilize hydroxyl groups formed in the course of slight oxidation by blocking them through reactions with reagents that convert the hydroxyl into thermally more stable metal salts. * Co~~ution No.646 from the Research Council of Alberta, Edmonton, Alberta, Canada t Head, Fuels Research Centre, Mines Branch, Department of Energy, tines and Resources, Ottawa, Canada
12
FUEL,
1974, Vol 53, January
EXPERIMENTAL Fresh samples of Moss 3 coal (cf. Table I), i.e. the coal used in the earlier work, were (unless otherwise stated) ground in an argon atmosphere to pass a GO-mesh Tyler screen and then oxidized in air at (a) room temperature for periods varying from three days to several months, and (b) 100°C for one to five hours. Both fresh and oxidized samples were then subjected to various treatments with a concentrated aqueous solution of barium hydroxide, an aqueous solution of barium acetate (lN), an aqueous solution of calcium acetate (lN), and a saturated aqueous solution of calcium hydroxide. To a 250 ml round-bottom flask fitted with a groundglass stopper and containing 150 ml of one of these solutions, 1O-l 1 g of weathered coal were added and the unstoppered flask was immersed in boiling water and heated for 10 min. The flask was then tightly stoppered and removed from the bath, vigorously shaken for 30 min, again immersed in boiling water for 10 min, and fmally again shaken for 30 min. After cooling, the suspension was filtered through a fine sintered-glass plate and washed with cold distilled water until the pH of the effluent was that of distilled water. The barium hydroxide-treated coal was then dried in a vacuum desiccator over anhydrone. In addition, samples were also reacted with concentrated barium hydroxide and 1N barium acetate solutions by vigorous shaking in those solutions for up to 24 h. In all suspensions 10 g of coal in 100 ml of solution were used. The consumption of barium hydroxide or barium acetate from solution was determined volumetri~lly3. The concentration of barium in the coal sample after it had been thorou~y washed with cold, distilled water was also determined gravimetrically.
5. S. Ignasiak, A. J. Sxladowand Table f
Oxidation studies on coking coal related
to weathering. 3
Characteristics of Moss 3 coal
Chemical and proximate analysis
f-1 Moisture Ash (dry basis) VM (daf) (%I 3T.f) 64.1 Fixed carbon Sulphur (dry basis) 1 0.5 . Free-swelling index 8 851 C H 5.3 tdaf, %I N I.0 o+s 8.6 ‘I
l
D. S. Montgomery:
maximum
fluidity
(ddpm)
was determined
Petrographic composition Reactive components (% by volume) : 3.8 Resinoid f exinoid Reactive semi-fusinoid 3.6 Total vitrinoid 68.3 Inert components (% by volume): 10.4 Micrinoid 3.1 Fusinoid 7.1 Inert semi-fusinoid 3.7 Mineral matter 1.12% Mean reflectance Distribution of vitrinoid types7 (%I: 6.1 Type: 9 16.4 IO 11 36.3 95 12 here without
application
of washer and was
Plastic and dilatometric
analysis
Gieseler plasticity: Soft. temp. (“Cl Max.-fluid. temp. (“C) Solidif. temp. (“C) Max. fluid. (ddpm)* Ruhr-dilatation: Soft. temp. (“Cl Max.-contr. temp. (“C) Max.-dilat. temp. (“Cl Max. dilatation (56)
396 455 494 9800 391 443 488 180
therefore much higherthan in T&k 2.
The weathered samples were subjected to methylation with diazomethane, Ten-gram samples of weathered coal, some of them demineralized with hydrochloric acid, were methylated. The reaction conditions and substrate/reagent ratios were as specified by Blom4. Fresh and oxidized Moss 3 coal before and after the treatments outlined above were subsequently tested to determine their plastic and dilatometric properties. The fluidity was determined with an automatic Gieseler plastometer, ASTM Standard 1971-D2639. In order to improve the reproducibility of the measurements, a stainlesssteel washer was placed on the top of the pressed samples; this artificially suppresses the fluidity. The dilatometric measurements were made with the Ruhr-d~atometer, DIN 5 1739.
RESULTS AND DISCUSSION The effect on dilatation of barium hydroxide treatment of fresh and oxidized samples of the coal investigated is depicted in Figure 1. It is noteworthy that the dilatation of a sample, after it had been oxidized for 5 h in an oven at 100°C, was not completely restored to the value of fresh coal after treatment; nevertheless, the increase in dilatation was significant. Subsequent experiments revealed that the restoration of (Gieseler) fluidity (F&ure 2), caused by blocking with barium hydroxide, was much greater than was observed in the case of dilatation. It is believed that the barium ion replaces only the proton sites of phenolic and carboxylic groups of coa13. If the mechanism of restoration is really connected only with these protons, the oxidized sample subjected to methylation prior to treatment with barium acetate or barium hydroxide solutions should not show any change of plasticity. Our experiments showed that methylation (which eliminates the acid hydroxyl groups) completely suppresses the restoration of the plasticity by subsequent barium hydroxide treatment. The results of these experiments seem also to suggest that carboxyl groups - converted here into barium salts - do not affect the plasticity. It was also found that the oxycoal treated with hydrochloric acid to remove alkali carbonates
410 Temperature Figure 7 The effect of oxidation on dilatation Particle size: -200 mesh
fresh coal Curve la: barium-trea~d fresh
440 I ‘C I
and barium
hydroxide
treatment
Curve 1:
coal
Curve 2: coal oxidized for 7 days in thin layer at room temperature Curve 2a: barium-treated after 7 days of oxidation in thin layer at room temperature Curve 3: coal oxidized for 2 h in oven at 1 OO°C Curve 3a: barium-treated coal, previously oxidized 1 OOec
for 2 h in oven at
Curve 4: coal stored for 7 months at room temperature Curve 48: barium-treated after storage for 7 months temperature Curve 5: coal oxidized for 5 h in oven at 1 OO°C Curve 5a: barium-treated after previous oxidation at 1OoOC
FUEL, 1974, Vol53,
at room
for 5 h in oven
January
13
Oxidation studies on coking coal related to weathering. 3: 6. S. lgnasiak, A. J. Szladow and D. S. Montgomery
2000
c
a
b
200
100
0
~~ 400
450
500
ratu r e (“Cl
7 :
‘2
2
~~200
- 100
a
450 Temperature
(‘C)
Figure 2 Plastometric and dilatometric curves a: weathered coal b: weathered coal after barium hydroxide treatment weathered coal additionally oxidized for 4 h at IOO’C ;: weathered coal additionally oxidized for 4h at 1OO’C. barium-treated
and
prior to methylation and/or barium hydroxide treatment did not then show any improvement of its caking properties relative to the untreated oxycoal, as measured in terms of Hydrochloric acid is (Gieseler) fluidity and dilatation, irreversibly chemisorbed on coal, as reported previously by It would appear that this chemisorption Radmacher’. affects irreversibly the barium-exchangeable acidic hydroxyl groups of oxycoal. Additional proof that the carboxyl groups do not appreciably affect the plasticity was furnished by the results of the following experiment, in which one of two samples, both oxidized precisely in the same way, was heated in a sealed glass tube in a helium atmosphere for 1 h at about 380°C. Then both samples were treated with barium hydroxide. It was found that the dilatation curves for both samples were identical. This suggested that decrease in the number of carboxyl groups owing to partial decarboxylation had no effect upon the plastic properties of oxycoal. At the same time a second conclusion could be drawn, that the condensation reactions of the acidic hydroxyl groups which are responsible for the reduction of the dilatation of slightly oxidized coal do not proceed below 380°C. In contrast to the barium hydroxide treatment, the treatment of oxycoal with calcium acetate solution, as described under item 3 of the Experimental part, did not show any effect on the plasticity. Nor was the plasticity changed after the coal was treated with a saturated solution of calcium hydroxide. The cause might be either differences in solubility of the calcium salts formed as compared with the respective barium salts, or the generally lower thermal stability of the calcium salts. Owing to the much lower atomic weight of calcium as compared with barium, the
14
FUEL, 1974, Vol 53, January
gravimetric analysis described in the paper was not dependable when applied to calcium-treated coal. Therefore after washing with distilled water some introduced calcium may have been left in the coal, and consequently we cannot indicate which alternative is the right one. In the course of this work some effort was also made to clarify the effect, on the deterioration of the (Gieseler) fluidity and dilatation of oxycoal, of keto, aldehyde and peroxide groups formed during the oxidation. The reduction of these groups by sodium borohydride2 changed neither the fluidity nor the dilatation of the sample, despite a sub stantial increase of hydroxyl groups (about SO%), and consequently no direct evidence of the inert or active nature of keto, aldehyde and peroxide groups, compared with hydroxyl, on plastic properties of oxycoal could be established. At this stage the emphasis was placed on establishing the relation between the amount of barium chemisorbed and the (Gieseler) fluidity. It was found that the loss of barium hydroxide from the solution could not be a measure of the amount of barium chemisorbed, because during subsequent washing with distilled water a considerable part of the barium chemisorbed initialiy was removed from oxycoal. Also, the gravimetric determination of barium chemisorbed, by precipitation of barium sulphate from the ash obtained from barium-blocked oxycoal, could not be considered as a satisfactory analytical technique because of the presence of sulphates in coal. Finally, we accepted the increase in ash level of the samples blocked with barium hydroxide or barium acetate solutions as a measure of barium chemisorbed. The assumption was made here that the barium introduced to the coal existed in the ashed residue in the form of equimolecular quantities of oxide and carbonate (BaO:BaCOj = 1: 1). When the coal sample was blocked with barium acetate, the chemisorbed amount of barium determined by the above method was in good agreement with that determined volumetrically. Table 2 presents the data obtained from the Gieseler plasticity measurements of oxidized samples treated with barium acetate by boiling or shaking. The results are averages from at least two independently conducted chemisorption experiments. The calculated standard deviations of the differences between duplicates for the Gieseler characteristic temperatures and differences between duplicates for the maximum fluidity based on their arithmetical
Tab/e 2 Gieseler plasticity of oxy-Moss 3 coal treated with barium acetate by boiling or shaking Gieseler plasticity Oxy-Moss 3 coal, method of treatment with No.
1 N Ba(CH3C00)2
Softening temp. (*C)
1 2 3 4 5 6 7 a
Not treated Shaking 1 h Shaking 1 h Shaking 4 h Shaking 4 h Shaking 24 h Boiling 2 h Boiling 5 h
397 397 396 392 393 392 394 395
456 456 456 455 456 456 455 456
489 490 490 490 491 490 490 490
960 1060 1430 1580 1560 1600 1480 1380
2.7*c
f .aoc
2.0°c
f 0.4%
Standard deviations for the differences between duplicates
Max.fluid. temp.
Max. fluid.
(W
Solidif. temp. (%I
(ddpm)
B. S. Ignasiak, A. .I. Szladow and D. S. Man tgomery: Oxidation studies on coking coal related to weathering, 3
0.01
0.02 Content
Figure 3 in farm
0.03 of
a.04
0.05
O-06
Oo, (%ofcoal)
Consequently we may estimate? that the concentration of barium-exchangeable 0oI-l in oxyplasticizer, responsible for the substantial change of plasticity, amounts to at least O-1O?&and does not exceed 0-I 4% by weight. It should be emphasized here that although dilatatian and (Gieseler) fluidity are generally considered to be different parameters of coal plasticity6, the barium-treatment procedure gives rise to parallel changes in both these parameters. Bearing in mind that both of them - dilatation and (Gieseler) fIuidity - are functions of plasticizer flow properties (viscosity), we are inchned to interpret the mechanism of restoration of fluidity and dilatation by barium treatment in chemical terms ‘- the prevention of increase of molecular weight caused by condensation reactions in turn prevents an increase in oxy-plasticizer viscosity. It is noteworthy that the effect ou coat fiuidity and dilatation of blocking by barium treatment seems to be a general phenomenon characterizing all coals that show high (Gieseler) fluidity and large dilatation and at the same time extreme susceptibility towards oxidation.
Relation between maximum fluidity, and oxygen content groups that were blocked with barium
of hydroxyl
means are presented in their respective columns. It should be emphasized here that the barium chemisorbed by the oxidized sampIes also causes a noticeabIe downward shift of about 5°C in the softening temperature. Ruth effects are opposite to those caused by oxidation. The relation between the amount of oxygen in acidic hydroxyl groups, linked to barium, and the maximum fluidity is shown in Figure 3. The assumption was made that one oxygen atom corresponds to one atom of barium ehemisorbed (ROSaOH). In order to raise the (Gieseler) fluidity of the particular oxidized sample from ddpm 7=950 to the maximum value of about 1600, we had to chemisorb an amount of barium equivalent to only 0*015%* 001-1 for this degree of oxidation. Larger concentrations of barium chemisorbed on this coal sampIe do not seem to affect the plasticity positively. On the contrary, a slight downward trend was observed. To summarize the results obtained so far, it was shown that in slightly oxidized coal only the more strongly acidic hydroxyl groups, in which hydrogen can be replaced by treatment of coal with either barium acetate or hydroxide solutions, have a marked effect on the RI-as&cityand diIatation. In more extensively oxidized coals, the treatment with barium cannot restore the (Gieseler) fluidity and dilatometric properties completely. We cannot, however, exclude the possibility that other hydroxyl groups formed during oxidation, which are not blocked by barium treatment, do undergo condensation It should be emphasized that after slight oxidation the deterioration of coal (Gieseler) fluidity and dilatation is due not to all the oxygen chemisorbed but only to the small fraction capable of condensation reactions. The relatively negligible amount of oxygen (O,Ol-0.0~) responsible for the substantid change of the (Gieseler) fluidity is particular& remarkable - E@re 3, On the basis of the revious findings’ concerning the distribution of oxygen ;pso in pyroIysis products of ox@80Moss-coal, this amount of oxygen is linked mostly to the part of the organic structure of coal which gives rise to tar formation and determines the plastic properties of coat. * Weight % oxygen fon the dry basis) in the form. of OH
The authors are indebted to Dr N, Berkowitz and Mr J. F. Fryer of the Research Council of Alberta for their encouragement of this work. Thanks are aIso due to Dr 3. N. Nandi and Mr S. E. Nixon of the FueIs Research Centre, Mines Branch, Department of Energy, Mines and Resources, for diiatometric determinations and petrographic analysis.
REFERENCES Ignasi&, 3. S., Clugston, D. M. and Montgomery, D. S. Fuel, Land, 1972,51,76 Ignasiak, B. S., Nandi, B. N. and Montgomery, D. S. Divisional Rep. FRC 69/83-RBS, Dept of Energy, Mines and Resources, Mines Bran&, Ottnwu, Ckmda Ihnatowicz, A. Ko~ni~~ GK, No_iZ5, 1952 Biom, L. ‘~~y~~ Methods in Coal Chemistry’, Drukkerij eu Uitgeverij Job. Luijk C.V. Eindhoven, 1960 Radmacher, W. and Mohrhauer, P. Brennst-Chem. 1956, 37,353 Berkowitz, N. J. Fuel Sot. Japan 1968,47,801 ASTM methods D279949T and D279849T
t The amount of oxygen, equivalent to barium, which was responsible for increase of the maximum @3eselerer)fluidity by 80% WaS
o-01 + 0432 ~% OOH’ 0.015% OOH 2 It follows’ that about 70% of the oxygen chemisorbed during oxidation is linked to tar. The amount of the oxypiasticizer was arbitrarily taken as similar to the tar yield, ml@&. About 70% of GO151 OOH by weight of coal fdaf) = @OX%, all in the oxyplasticizer. @01%X l~/lo= @1% 00~ by Weight Of oxypfastk~er.
FUEL,
1974, Vol53,
January
15
I 137587th
Avenue, Edmonton 7,
A high-volatile bituminous coal possessinga high Gieseler fluidity and large dilatation was subjected to slight oxidation in air under chosen conditions. The resultant ‘oxycoal’was treated with various chemical reagents in order to determine to what extent the original caking properties of this coal, as measured by Gieseler fluidity and dilatation, could be restored. Treatment with aqueous barium hydroxide or barium acetate solutions succeeded in doing so to a considerable degree. tt is tentatively concluded that certain acidic hydroxyl groups formed in the course of oxidation are indirectly responsible for the deterioration of fluidity and dilatation - possibly via condensation reactions involving the hydroxyl groups - during subsequent pyrolysis of oxycoal. The presence of carboxyl groups appears to have no effect,
The distribution of l80 in pyrolysis products of 0xy-~*0coal was the subject of a previous paper’ in which we reported that up to about 85% of labelled oxygen introduced into coal during slight oxidation could be removed by subsequent pyrolyses at temperatures to 450°C. In particular, it was shown that the concentration of l80 in the residual 450°C char did not exceed 17% of the total labelled oxygen initially introduced into the coal, while concentrations of l80 in evolved gas and tar amounted to, respectively, about 23% and 62%. These observations, and the later finding that practically all Hz180 was evolved below 450°C led us to suppose that decreases in (Gieseler) fluidity and dilatation accompanying the oxidation might be caused by condensation reactions during pyrolysis of oxycoal, and by a consequent increase of the average molecular weight of the plasticizer. Considering the thermal stability of the oxygenbearing functional groups created by oxidation, we were inclined to associate such condensation reactions with hydroxyl groups and to suppose that if condensation could be prevented or delayed, it might be possible to restore the (Gieseler) fluidity and ability to dilate of the oxycoal at substantially the same values as those observed for the fresh coal. Proceeding from this hypothesis we report in this paper, which is an enlarged version of a previous report’, attempts to stabilize hydroxyl groups formed in the course of slight oxidation by blocking them through reactions with reagents that convert the hydroxyl into thermally more stable metal salts. * Co~~ution No.646 from the Research Council of Alberta, Edmonton, Alberta, Canada t Head, Fuels Research Centre, Mines Branch, Department of Energy, tines and Resources, Ottawa, Canada
12
FUEL,
1974, Vol 53, January
EXPERIMENTAL Fresh samples of Moss 3 coal (cf. Table I), i.e. the coal used in the earlier work, were (unless otherwise stated) ground in an argon atmosphere to pass a GO-mesh Tyler screen and then oxidized in air at (a) room temperature for periods varying from three days to several months, and (b) 100°C for one to five hours. Both fresh and oxidized samples were then subjected to various treatments with a concentrated aqueous solution of barium hydroxide, an aqueous solution of barium acetate (lN), an aqueous solution of calcium acetate (lN), and a saturated aqueous solution of calcium hydroxide. To a 250 ml round-bottom flask fitted with a groundglass stopper and containing 150 ml of one of these solutions, 1O-l 1 g of weathered coal were added and the unstoppered flask was immersed in boiling water and heated for 10 min. The flask was then tightly stoppered and removed from the bath, vigorously shaken for 30 min, again immersed in boiling water for 10 min, and fmally again shaken for 30 min. After cooling, the suspension was filtered through a fine sintered-glass plate and washed with cold distilled water until the pH of the effluent was that of distilled water. The barium hydroxide-treated coal was then dried in a vacuum desiccator over anhydrone. In addition, samples were also reacted with concentrated barium hydroxide and 1N barium acetate solutions by vigorous shaking in those solutions for up to 24 h. In all suspensions 10 g of coal in 100 ml of solution were used. The consumption of barium hydroxide or barium acetate from solution was determined volumetri~lly3. The concentration of barium in the coal sample after it had been thorou~y washed with cold, distilled water was also determined gravimetrically.
5. S. Ignasiak, A. J. Sxladowand Table f
Oxidation studies on coking coal related
to weathering. 3
Characteristics of Moss 3 coal
Chemical and proximate analysis
f-1 Moisture Ash (dry basis) VM (daf) (%I 3T.f) 64.1 Fixed carbon Sulphur (dry basis) 1 0.5 . Free-swelling index 8 851 C H 5.3 tdaf, %I N I.0 o+s 8.6 ‘I
l
D. S. Montgomery:
maximum
fluidity
(ddpm)
was determined
Petrographic composition Reactive components (% by volume) : 3.8 Resinoid f exinoid Reactive semi-fusinoid 3.6 Total vitrinoid 68.3 Inert components (% by volume): 10.4 Micrinoid 3.1 Fusinoid 7.1 Inert semi-fusinoid 3.7 Mineral matter 1.12% Mean reflectance Distribution of vitrinoid types7 (%I: 6.1 Type: 9 16.4 IO 11 36.3 95 12 here without
application
of washer and was
Plastic and dilatometric
analysis
Gieseler plasticity: Soft. temp. (“Cl Max.-fluid. temp. (“C) Solidif. temp. (“C) Max. fluid. (ddpm)* Ruhr-dilatation: Soft. temp. (“Cl Max.-contr. temp. (“C) Max.-dilat. temp. (“Cl Max. dilatation (56)
396 455 494 9800 391 443 488 180
therefore much higherthan in T&k 2.
The weathered samples were subjected to methylation with diazomethane, Ten-gram samples of weathered coal, some of them demineralized with hydrochloric acid, were methylated. The reaction conditions and substrate/reagent ratios were as specified by Blom4. Fresh and oxidized Moss 3 coal before and after the treatments outlined above were subsequently tested to determine their plastic and dilatometric properties. The fluidity was determined with an automatic Gieseler plastometer, ASTM Standard 1971-D2639. In order to improve the reproducibility of the measurements, a stainlesssteel washer was placed on the top of the pressed samples; this artificially suppresses the fluidity. The dilatometric measurements were made with the Ruhr-d~atometer, DIN 5 1739.
RESULTS AND DISCUSSION The effect on dilatation of barium hydroxide treatment of fresh and oxidized samples of the coal investigated is depicted in Figure 1. It is noteworthy that the dilatation of a sample, after it had been oxidized for 5 h in an oven at 100°C, was not completely restored to the value of fresh coal after treatment; nevertheless, the increase in dilatation was significant. Subsequent experiments revealed that the restoration of (Gieseler) fluidity (F&ure 2), caused by blocking with barium hydroxide, was much greater than was observed in the case of dilatation. It is believed that the barium ion replaces only the proton sites of phenolic and carboxylic groups of coa13. If the mechanism of restoration is really connected only with these protons, the oxidized sample subjected to methylation prior to treatment with barium acetate or barium hydroxide solutions should not show any change of plasticity. Our experiments showed that methylation (which eliminates the acid hydroxyl groups) completely suppresses the restoration of the plasticity by subsequent barium hydroxide treatment. The results of these experiments seem also to suggest that carboxyl groups - converted here into barium salts - do not affect the plasticity. It was also found that the oxycoal treated with hydrochloric acid to remove alkali carbonates
410 Temperature Figure 7 The effect of oxidation on dilatation Particle size: -200 mesh
fresh coal Curve la: barium-trea~d fresh
440 I ‘C I
and barium
hydroxide
treatment
Curve 1:
coal
Curve 2: coal oxidized for 7 days in thin layer at room temperature Curve 2a: barium-treated after 7 days of oxidation in thin layer at room temperature Curve 3: coal oxidized for 2 h in oven at 1 OO°C Curve 3a: barium-treated coal, previously oxidized 1 OOec
for 2 h in oven at
Curve 4: coal stored for 7 months at room temperature Curve 48: barium-treated after storage for 7 months temperature Curve 5: coal oxidized for 5 h in oven at 1 OO°C Curve 5a: barium-treated after previous oxidation at 1OoOC
FUEL, 1974, Vol53,
at room
for 5 h in oven
January
13
Oxidation studies on coking coal related to weathering. 3: 6. S. lgnasiak, A. J. Szladow and D. S. Montgomery
2000
c
a
b
200
100
0
~~ 400
450
500
ratu r e (“Cl
7 :
‘2
2
~~200
- 100
a
450 Temperature
(‘C)
Figure 2 Plastometric and dilatometric curves a: weathered coal b: weathered coal after barium hydroxide treatment weathered coal additionally oxidized for 4 h at IOO’C ;: weathered coal additionally oxidized for 4h at 1OO’C. barium-treated
and
prior to methylation and/or barium hydroxide treatment did not then show any improvement of its caking properties relative to the untreated oxycoal, as measured in terms of Hydrochloric acid is (Gieseler) fluidity and dilatation, irreversibly chemisorbed on coal, as reported previously by It would appear that this chemisorption Radmacher’. affects irreversibly the barium-exchangeable acidic hydroxyl groups of oxycoal. Additional proof that the carboxyl groups do not appreciably affect the plasticity was furnished by the results of the following experiment, in which one of two samples, both oxidized precisely in the same way, was heated in a sealed glass tube in a helium atmosphere for 1 h at about 380°C. Then both samples were treated with barium hydroxide. It was found that the dilatation curves for both samples were identical. This suggested that decrease in the number of carboxyl groups owing to partial decarboxylation had no effect upon the plastic properties of oxycoal. At the same time a second conclusion could be drawn, that the condensation reactions of the acidic hydroxyl groups which are responsible for the reduction of the dilatation of slightly oxidized coal do not proceed below 380°C. In contrast to the barium hydroxide treatment, the treatment of oxycoal with calcium acetate solution, as described under item 3 of the Experimental part, did not show any effect on the plasticity. Nor was the plasticity changed after the coal was treated with a saturated solution of calcium hydroxide. The cause might be either differences in solubility of the calcium salts formed as compared with the respective barium salts, or the generally lower thermal stability of the calcium salts. Owing to the much lower atomic weight of calcium as compared with barium, the
14
FUEL, 1974, Vol 53, January
gravimetric analysis described in the paper was not dependable when applied to calcium-treated coal. Therefore after washing with distilled water some introduced calcium may have been left in the coal, and consequently we cannot indicate which alternative is the right one. In the course of this work some effort was also made to clarify the effect, on the deterioration of the (Gieseler) fluidity and dilatation of oxycoal, of keto, aldehyde and peroxide groups formed during the oxidation. The reduction of these groups by sodium borohydride2 changed neither the fluidity nor the dilatation of the sample, despite a sub stantial increase of hydroxyl groups (about SO%), and consequently no direct evidence of the inert or active nature of keto, aldehyde and peroxide groups, compared with hydroxyl, on plastic properties of oxycoal could be established. At this stage the emphasis was placed on establishing the relation between the amount of barium chemisorbed and the (Gieseler) fluidity. It was found that the loss of barium hydroxide from the solution could not be a measure of the amount of barium chemisorbed, because during subsequent washing with distilled water a considerable part of the barium chemisorbed initialiy was removed from oxycoal. Also, the gravimetric determination of barium chemisorbed, by precipitation of barium sulphate from the ash obtained from barium-blocked oxycoal, could not be considered as a satisfactory analytical technique because of the presence of sulphates in coal. Finally, we accepted the increase in ash level of the samples blocked with barium hydroxide or barium acetate solutions as a measure of barium chemisorbed. The assumption was made here that the barium introduced to the coal existed in the ashed residue in the form of equimolecular quantities of oxide and carbonate (BaO:BaCOj = 1: 1). When the coal sample was blocked with barium acetate, the chemisorbed amount of barium determined by the above method was in good agreement with that determined volumetrically. Table 2 presents the data obtained from the Gieseler plasticity measurements of oxidized samples treated with barium acetate by boiling or shaking. The results are averages from at least two independently conducted chemisorption experiments. The calculated standard deviations of the differences between duplicates for the Gieseler characteristic temperatures and differences between duplicates for the maximum fluidity based on their arithmetical
Tab/e 2 Gieseler plasticity of oxy-Moss 3 coal treated with barium acetate by boiling or shaking Gieseler plasticity Oxy-Moss 3 coal, method of treatment with No.
1 N Ba(CH3C00)2
Softening temp. (*C)
1 2 3 4 5 6 7 a
Not treated Shaking 1 h Shaking 1 h Shaking 4 h Shaking 4 h Shaking 24 h Boiling 2 h Boiling 5 h
397 397 396 392 393 392 394 395
456 456 456 455 456 456 455 456
489 490 490 490 491 490 490 490
960 1060 1430 1580 1560 1600 1480 1380
2.7*c
f .aoc
2.0°c
f 0.4%
Standard deviations for the differences between duplicates
Max.fluid. temp.
Max. fluid.
(W
Solidif. temp. (%I
(ddpm)
B. S. Ignasiak, A. .I. Szladow and D. S. Man tgomery: Oxidation studies on coking coal related to weathering, 3
0.01
0.02 Content
Figure 3 in farm
0.03 of
a.04
0.05
O-06
Oo, (%ofcoal)
Consequently we may estimate? that the concentration of barium-exchangeable 0oI-l in oxyplasticizer, responsible for the substantial change of plasticity, amounts to at least O-1O?&and does not exceed 0-I 4% by weight. It should be emphasized here that although dilatatian and (Gieseler) fluidity are generally considered to be different parameters of coal plasticity6, the barium-treatment procedure gives rise to parallel changes in both these parameters. Bearing in mind that both of them - dilatation and (Gieseler) fIuidity - are functions of plasticizer flow properties (viscosity), we are inchned to interpret the mechanism of restoration of fluidity and dilatation by barium treatment in chemical terms ‘- the prevention of increase of molecular weight caused by condensation reactions in turn prevents an increase in oxy-plasticizer viscosity. It is noteworthy that the effect ou coat fiuidity and dilatation of blocking by barium treatment seems to be a general phenomenon characterizing all coals that show high (Gieseler) fluidity and large dilatation and at the same time extreme susceptibility towards oxidation.
Relation between maximum fluidity, and oxygen content groups that were blocked with barium
of hydroxyl
means are presented in their respective columns. It should be emphasized here that the barium chemisorbed by the oxidized sampIes also causes a noticeabIe downward shift of about 5°C in the softening temperature. Ruth effects are opposite to those caused by oxidation. The relation between the amount of oxygen in acidic hydroxyl groups, linked to barium, and the maximum fluidity is shown in Figure 3. The assumption was made that one oxygen atom corresponds to one atom of barium ehemisorbed (ROSaOH). In order to raise the (Gieseler) fluidity of the particular oxidized sample from ddpm 7=950 to the maximum value of about 1600, we had to chemisorb an amount of barium equivalent to only 0*015%* 001-1 for this degree of oxidation. Larger concentrations of barium chemisorbed on this coal sampIe do not seem to affect the plasticity positively. On the contrary, a slight downward trend was observed. To summarize the results obtained so far, it was shown that in slightly oxidized coal only the more strongly acidic hydroxyl groups, in which hydrogen can be replaced by treatment of coal with either barium acetate or hydroxide solutions, have a marked effect on the RI-as&cityand diIatation. In more extensively oxidized coals, the treatment with barium cannot restore the (Gieseler) fluidity and dilatometric properties completely. We cannot, however, exclude the possibility that other hydroxyl groups formed during oxidation, which are not blocked by barium treatment, do undergo condensation It should be emphasized that after slight oxidation the deterioration of coal (Gieseler) fluidity and dilatation is due not to all the oxygen chemisorbed but only to the small fraction capable of condensation reactions. The relatively negligible amount of oxygen (O,Ol-0.0~) responsible for the substantid change of the (Gieseler) fluidity is particular& remarkable - E@re 3, On the basis of the revious findings’ concerning the distribution of oxygen ;pso in pyroIysis products of ox@80Moss-coal, this amount of oxygen is linked mostly to the part of the organic structure of coal which gives rise to tar formation and determines the plastic properties of coat. * Weight % oxygen fon the dry basis) in the form. of OH
The authors are indebted to Dr N, Berkowitz and Mr J. F. Fryer of the Research Council of Alberta for their encouragement of this work. Thanks are aIso due to Dr 3. N. Nandi and Mr S. E. Nixon of the FueIs Research Centre, Mines Branch, Department of Energy, Mines and Resources, for diiatometric determinations and petrographic analysis.
REFERENCES Ignasi&, 3. S., Clugston, D. M. and Montgomery, D. S. Fuel, Land, 1972,51,76 Ignasiak, B. S., Nandi, B. N. and Montgomery, D. S. Divisional Rep. FRC 69/83-RBS, Dept of Energy, Mines and Resources, Mines Bran&, Ottnwu, Ckmda Ihnatowicz, A. Ko~ni~~ GK, No_iZ5, 1952 Biom, L. ‘~~y~~ Methods in Coal Chemistry’, Drukkerij eu Uitgeverij Job. Luijk C.V. Eindhoven, 1960 Radmacher, W. and Mohrhauer, P. Brennst-Chem. 1956, 37,353 Berkowitz, N. J. Fuel Sot. Japan 1968,47,801 ASTM methods D279949T and D279849T
t The amount of oxygen, equivalent to barium, which was responsible for increase of the maximum @3eselerer)fluidity by 80% WaS
o-01 + 0432 ~% OOH’ 0.015% OOH 2 It follows’ that about 70% of the oxygen chemisorbed during oxidation is linked to tar. The amount of the oxypiasticizer was arbitrarily taken as similar to the tar yield, ml@&. About 70% of GO151 OOH by weight of coal fdaf) = @OX%, all in the oxyplasticizer. @01%X l~/lo= @1% 00~ by Weight Of oxypfastk~er.
FUEL,
1974, Vol53,
January
15