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E.s.r. spectroscopic study on the chemistry of coal oxidation
E.s.r. spectroscopic of coal oxidation M. Rashid
Khan,
R. Usmen,
study on the chemistry
E. Newton,
S. Beer and W. Chisholm
US Department of Energy, Morgantown Energy Technology WV 26505, USA (Received 15 January 1988; revised 18 April 7988)
Center, Morgantown,
A better understanding of the mechanisms ofpre-oxidation could lead to improved methods for prevention of coal weathering, which has a deleterious influence on liquid yield during pyrolysis. Previous Morgantown Energy Technology Center studies demonstrated that the extractable fractions ofcoal are more susceptible to oxidative weathering than the residue. To better understand the chemistry of coal weathering and the influence of pre-oxidation on subsequent devolatilization, in situ electron spin resonance (esr.) spectroscopic studies were performed on coal, weathered coal and on the corresponding untreated and preoxidized fractions (extract and residue). In this technique, pyrolysis is performed in an e.s.r. cavity and the concentration of free radicals is followed as the sample is pyrolysed. Elemental analysis of the pre-oxidized and untreated samples indicated that H/C ratio of the extract and the coal sample was reduced slightly during 48 h of oxidation at 150°C in air whereas the H/C ratio of the residue was essentially unchanged. The influence of pre-oxidation (48 h at 150°C) was to increase the e.s.r. spin concentration of the weathered coal slightly compared to the raw coal when devolatilized at 460°C. The e.s.r. experiments performed at 460°C also showed that pre-oxidation significantly enhanced free radical concentration in the extract but had a lesser effect on the residue. The enhanced free radical formation of the pre-oxidized extract relative to the untreated extract implies that mild pre-oxidation affects hydrogen-rich components (extract) more than the primarily hydrogen-poor components (residue) of coal. (Keywords: oxidation; electron spin resonance; chemical properties)
It is well known that pre-oxidation of coal profoundly influences important coal properties such as pyrolysis liquid yield, thermoplastic properties, elemental composition, heating value, extractability and specific gravity’. The mechanisms of coal oxidation and devolatilization mechanisms of pre-oxidized coal still remain the subject of considerable investigation. Formation of free radicals plays a key step in oxidation mechanisms. The first step in the oxidation of coal is generally considered to be peroxide formation by the oxidation of aliphatic/olel’inic/aromatic structures (R.) via free radical mechanisms: 2R-H +$02+2R.+
Hz0
(1)
R.+ O,+R-0-O.
(2)
R-O-O. + R-H --+R-O-O-H + R. (a peroxide or hydroperoxide)
(3)
Equation (1) can be considered as the initiation of a chain reaction that is further propagated by Equations such as (2) or (3). The peroxide or hydroperoxide formed in Equation (3) can undergo further decomposition producing relatively stable oxygenated products (e.g. alcohols, ketones, aldehydes, esters and ethers) as well as CO, CO, and H,O. The mechanisms and decomposition chemistry of peroxides into other products have been discussed by various authors’-“. The comparatively more stable oxygen functional groups such as hydroxyl, carboxyl, carbonyl and ether have been identified in oxidized coals both chemically” and spectroscopically 12-14. However, relatively little has been reported 00162361/88/12166846$3.00 01988 Butterworth & Co. (Publishers)
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regarding the formation and fate of free radicals in the pre-oxidized coal during subsequent pyrolysis. Yokono et ~1.‘~ studied oxidized samples of strongly coking and very weakly caking coals by e.s.r. (not in situ measurement), infrared (i.r.) and pulsed proton n.m.r. An increase in free radical concentration was reported for samples of strongly coking coal oxidized at 110°C for up to 21 days. A decrease in free radical concentration was noted with further progressive oxidation. However, a progressive decrease in free radical concentration was observed with 42 days of oxidation of very weakly caking coal samples. It was concluded that there are differences in the mechanisms of oxidation of strongly coking and weakly caking coals. Ripmeester et ~1.‘~ studied relaxation times of fresh and oxidized samples of coal and semicoke by ‘H n.m.r. and e.s.r. (not in situ measurement) and indicated that the unpaired electron spin concentration increased on ‘mild’ oxidation, but decreased in the case of ‘severe’ oxidation. However, in this study, the ‘mild’ condition was defined as oxidation at 100°C for 3 12 h, whereas the ‘severe’ oxidation was performed at 190°C for 192 h. Swann and Evans17 studied the oxidation of brown coal at temperatures of 35°C and 70°C by i.r. and gas analysis and concluded that the carbonyl, carboxyl and phenolic groups are formed by oxidation of the aliphatic structures of the coal. Kalema and Gavalas’ * studied the compositional change (by gas analysis) and 13C n.m.r. (data were not corrected for carbonyl and carboxylic carbons) in two bituminous coals and lignite during air oxidation at 200 to 250°C and concluded that both
E.s.r. spectroscopic Table 1
study
on the chemistry
of coal oxidation:
M. R. Khan
et al.
Effect of pre-oxidation on composition of raw coal, pre-oxidized coal and coal fractions (extract and residue) -______ Residue Coal Extract
Ultimate (d.b.) C H N s S + 0 (by difference) H/C (atomic) Proximate (d.b.) Volatile matter Ash Heating value (Btu/lb)
Coal
Pre-oxidized 2 days
Pre-oxidized 6 days
Untreated
71.7 5.0 1.4 2.0 8.6 0.78
71.9 4.2 1.4 15.2 0.71
68.8 3.7 1.4 _ 18.8 0.65
83.7 5.8 2.6 _ 7.9 0.83
37.9 7.3 13 980
Oxidized 2 days
Oxidized 6 days
79.9 5.2 1.6
14.9 4.4 1.6
13.3 0.79
19.1 0.71
Untreated
Oxidized 2 days
Oxidized 6 days
62.5 2.6 2.6 2.0 22.9 0.51
61.8 2.7 2.5 _ 23.6 0.52
60.4 2.4 2.5 _ 25.2 0.47
31.7 9.4
0 _
y Calculated by the following: S + 0 = 100 - (C + H + N + ash), all on dry basis; extractables contained no ash
aliphatic and aromatic types of carbon are consumed at an approximate ratio of 3.5:1. Havens et ~1.‘~ measured aromaticity by ’ 3C cross polarization/magic angle spinning (CP/MAS) n.m.r. and FT-i.r. in samples of vitrinite concentrates oxidized in air at 140°C for up to 16 days and noted preferential attack on aliphatic structures. MacPhee and Nandi” examined the aromaticities of fresh and oxidized samples of bituminous coal by n.m.r. and reported an increase in the ratio of aliphatic to aromatic carbon with oxidation time in air at 105°C (up to 500 h). Based on the n.m.r. data, it was concluded, contrary to some literature data, that the aromatic structures in the coal were being modified more than the aliphatic structures during oxidation. Yun et ~1.~~investigated the products of pyrolysis of two subbituminous and two high volatile bituminous coals oxidized at 100°C for 170 to 212 h by pyrolysis mass spectroscopy and concluded that aliphatic, hydrocarbonrich coal structures (derived from cutinitic or algal sources) show little or no chemical change under low temperature (1OO’C) oxidation. More highly aromatic vitrinite and sporinite show a moderate oxidation tendency. Meuzelaar et ~1.” studied the influence of weathering on structure and pyrolysis of selected Wasatch Plateau coals and proposed that weathering of coal (2 12 h at 1OO’C)is preceded by ‘grafting’ (linking) of ‘mobile phase’ constituents onto the coal ‘network’ chains. However, no extraction work was performed to monitor directly the relative pre-oxidation potential on the extract (i.e. the mobile fraction) and the residue (the network phase). The above literature studies can be classified into two widely conflicting groups: (a) those studies suggesting that pre-oxidation of coal induces changes primarily on the aromatic structures; and (b) the studies proposing that pre-oxidation at least initially changes the aliphatic structures of coal. Little has been reported in the literature regarding the influence of hydrogen content of feedstocks on their pre-oxidation potential. It was reported by Khan 22*23that oxidation causes a reduction in the aliphatic hydrogen content of coals, sales and kerogen as monitored by FT-i.r. It was therefore hypothesized that the relatively hydrogen-rich portion of coal (e.g. extract) was more susceptible to oxidation than the residue (i.e. hydrogen-deficient portion). Assuming that mass transport does not have a controlling influence,
one may hypothesize that the mobile phase extractables, smaller molecules with various side groups, would be more susceptible to oxidation than the more highly condensed structures with a larger number of rings (i.e. residue). To verify this hypothesis, it was necessary to separate the coal into a hydrogen-rich portion (extract) and a hydrogen-poor portion (residue) and perform preoxidation/pyrolysis on each fraction. Pyridine was selected for these extractions because it is known to be a powerful ‘specific’ solvent capable of dissolving a significant portion of coal. van Krevelen24 suggested that the pyridine extraction process removes the low molecular weight (relatively hydrogen-rich) components responsible for agglomerating and swelling properties of coal. It was proposed that pyridine extraction of coal is essentially an ‘unlocking’ process to remove the hydrogen bonded structures from coal. The extractable portion of coal was found to be relatively richer in both aromatic and aliphatic hydrogen25. EXPERIMENTAL Sample preparution
and charucterization
Pittsburgh No. 8 high volatile bituminous coal, freshly obtained from the mine mouth (stored in N,), was used throughout this investigation. The coal was stored in N, until it was ready to be used. It was ground to -200 mesh (-74pm) and sieved under nitrogen. Ultimate and approximate analyses of this coal are provided in Table 1. Simulated weathering of the dried (at 110°C in vacuum for 1 h) coal sample and its fractions to various severities was achieved by treatment at 150°C in dry air for up to 14 days using a procedure described elsewhere22,23. Elemental analysis (H, C, N) of untreated and preoxidized coal as well as its corresponding fractions was performed (see Table I). Thermogravimetric
analyses of coal
Detailed pyrolysis behaviour of the untreated and weathered materials was subsequently determined in He by using a thermogravimetric analysis (TGA) system interfaced with a mass spectrometer, more details of which are presented elsewhere22923. The TGA weight loss data were generally reproducible to within 1% of the initial sample weight utilized.
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E.s.r. spectroscopic study on the chemistry of coal oxidation: M. R. Khan et al.
ro for
2 days
I
1 Pit
Prsoxidizedln
Zeal-200
Mesh
(Store
r
1
8 Air
At
150-c
_
_
lor 6
In N2)
Extraction
Figure
1
Pyridine
Summary
of the extraction/pre-oxidation
procedures
extraction
Pyridine was selected as the extraction solvent since it is capable of removing the ‘mobile’ phase of coal. Pyridine extractions were carried out in a conventional water jacket Soxhlet extractor at or near the boiling point of pyridine. In a typical extraction, about 15g of coal was placed in a predried cellulose extraction thimble. The sample was dried, weighed and placed in the extractor. Fresh solvent (200 ml, obtained from Fisher Scientific) was added to the solvent reservoir. A small flow of nitrogen (10 cm3 min-‘) was continuously bubbled through the reservoir to exclude air from the system. The heating mantle was switched on, and extraction of the sample proceeded as the solvent was vaporized by continuous heating of the reservoir. Heating conditions were set to maintain an extraction rate of one recycle per minute. Extraction was continued until the solution in the extractor appeared clear. When the extraction run was completed, the heating mantle was switched off and the thimble was removed from the extractor and dried in N, for 24 h. The residue was repeatedly washed with water and vacuum dried in attempts to remove the residual pyridine. The residue and extraction thimble were dried in a vacuum oven at 110°C for 16 h, and the extraction yield was 17.4% (82.6 wt % residue), determined by the change in sample weight. The extraction and preoxidation procedures are represented in Figure 1. The influence of small amount of residual pyridine on pyrolysis was considered to be relatively small, as also assumed by others’*25. Em-.
spectroscopy
Spin concentrations were measured on a Varian E 109 spectrometer equipped with a 9-inch magnet and a single
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rectangular cavity. The spectra were recorded at 30second intervals with an IBM 9000 computer and reduced to free radical concentration using a method similar to that of Loveland and Tozen3’. This method included: (a) correction of the raw data for slope and offset, checking to see that areas of positive and negative lobes are equal; (b) single integration; (c) another check on baseline, slope and offset; and (d) final integration. This method works better than one-step integration or integration by the absolute value of the first derivative presentation for broad and noisy signals on a sloping baseline. Dilute dispersion of diphenylpicrylhydrazyl on silica in an evacuated quartz tube was used for calibration. Absolute accuracy is estimated to be +_10% for all experiments reported here. Coal samples were mixed with - 240 mesh silica, in proportion nearly 50 %. Approximately 10 mg of the sample/diluent mixture was used in each experiment. After being placed in a quartz e.s.r. tube, the samples were degassed at 20 ptorr or less and sealed in a vacuum before the experiments. Room temperature spectra were recorded before and after pyrolysis. All pyrolyses were performed in situ isothermally at 460°C. Sample temperature was monitored by a thermocouple located next to the sample. A Wilmad high-temperature Dewar insert and transfer Dewar, controlled by an Omega model proportional controller using nitrogen as a heat transfer gas, provided the heat for pyrolysis. The system was allowed to stabilize at 460°C. The sample reached 460°C within 30 s, and the temperature of the sample settled on 460 f 1°C for the duration of the experiment. Repeating some of the experiments at higher dilution, different sample size and different sample tube size did not significantly alter the results. 13C nmr. CPIMAS CP/MAS spectra were obtained on an IBM NR-80 spectrometer and solids accessory, utilizing a probe and rotors supplied by Doty Scientific (Columbia, SC). The operating field was 1.9 Tesla (1 Tesla= 10 kGauss). It provided a r 3C resonance frequency of 20.1 MHz. Samples were placed in 5 mm ceramic rotors plugged with Kel-f end caps to ensure no background signal would interfere. Spinning speeds were from 2500 to 3500 Hz with the spinning angle set within 0.1” of the ‘magic angle’ of 54.7” on paraditertbutylbenzene (PDTBB). The stator was then reset to within 0.1” of the same angle after the
2.00
Figure 2 Growth in relative e.s.r. spin concentrations (arbitrary units) during pyrolysis (normalized, average). Note: each curve is normalized to its value at time=zero, hence only the relative changes are meaningful; also scatter in the data points that define these curves was always 10.05 units
E.s.r. spectroscopic Table 2
study on the chemistry
of coal oxidation:
M. R. Khan et al.
13C CP/MAS studies of untreated and pre-oxidized samples of Pittsburgh No. 8 coal and its organic fractions”
Sample Untreated coal 2 days oxidized coal 6 days oxidized coal 14 days oxidized coat Extract (untreated) 2 days oxidized extract Residue (untreated) 2 days oxidized residue
Carbon (%)
Aromaticity (f,)
Aromatic carbon (%)
Aliphatic carbon (%)
77.7 71.9 68.8
0.67 0.71 0.75 0.79 0.67 0.69 0.78 0.78
52.0 51.1 51.6 _ 56.1 55.2 48.6 48.2
25.6 20.8 17.2 _ 27.6 24.8 13.8 13.6
8i.7 79.9 62.5 61.8
Carbon consumed (%) Aromatic
Aliphatic
0.9 0.4 _
_ 4.8 8.4 _
0.9 _ 0.4
2.8 0.2
-
_
’ The experimental error is within 2 %
Table 3 Effect of pre-oxidation on pyrolysis weight loss (heating rate, 50°C min-‘) for raw and pre-oxidized coal (performed at 150°C in air) and coal fractions (extract and residue) (Pittsburgh No. 8 coal) Pyrolysis weight loss heated to (wt%), daf Sample
600°C
900°C
Maximum rate dw/dt (%/min), daf
Raw coal
32.6
39.1
14.1
Coal pre-oxidized : 2 days 6 days
26.1 23.9
35.3 34.7
8.7 6.5
Coal extract
45
49
Extract pre-oxidized: 2 days 6 days
32.0 24.0
38.5 31.0
9 6
Coal residue
22.6
35.9
4.4
Residue pre-oxidized : 2 days 6 days
23.2 22.1
35.9 34.8
4.4 _
16
sample was installed in the probe. The methyl carbon signal of PDTBB was used as the chemical shift reference (31.0 ppm relative to tetramethylsilane), and as a means of monitoring the stability of the externally locked magnetic field. This field was stabilized to within 4 ppm. Cross-polarization times were 1 ms and pulse repetition times 1.03 s in all cases. Hartmann-Hahn matching was at radio frequency field strengths of 50 kHz, corresponding to a 90” pulse at 5 ps. Proton decoupling was done at the same level. Aromaticities estimated from integrated spectral intensities include contributions from spinning side bands arising from chemical shift anisotropies 31-33. The partition between the aromatic and aliphatic regions was taken to be the region between the two peaks that was closest to the baseline (e.g. the inflection point of the integral). In general, this partition was midway between the aromatic and aliphatic peaks. RESULTS AND DISCUSSION Influence of pre-oxidation on the elemental composition uromaticity of coal or coal fractions
and
Elemental analysis of the pre-oxidized and untreated samples show that the H/C ratios were significantly reduced during 6 days of pre-oxidation (Table I). The H/C (atomic) ratio of untreated coal decreased from 0.78 to 0.71 in 2 days of pre-oxidation and to 0.65 in 6 days of pre-
oxidation. In duplicate measurements, while the absolute numbers for carbon and hydrogen contents varied, the H/C was reproducible within 50.0 1. The extract had an H/C ratio of 0.83 compared with a 0.78 ratio for raw dried coal, suggesting that the extraction concentrated the slightly hydrogen-rich portion of the coal. While the H/C ratio of the extract was reduced by 14 y0 during 6 days of pre-oxidation, the reduction in this ratio for the residue was 8% when pre-oxidized under the same conditions. The aromaticities estimated from integrated ’ 3C n.m.r. spectral intensities are presented in Table 2. Carbonyl and carboxyl carbon is included in the aromatic region. The aromatic fraction of Pittsburgh No. 8 coal seems to increase as the time of low temperature oxidation is increased from 2 to 14 days. Linear combinations of the spectra suggest that oxidation of aliphatics to carboxylics and carbonyls occurring downfield from 160 ppm is largely responsible for increases in apparent carbon aromaticity. Increases in the 50-100 ppm region were also noted in the 6 and 14 day oxidized samples, indicating the presence of singly-bonded oxygen substituents on aliphatic chains. A slight increase in aromatic fraction was noted when the extract was oxidized for 2 days. No statistically significant change in the aromatic fraction was noted after the residue was oxidized for 2 days. An increase in the aromatic fraction of the pre-oxidized extract, and insignilicant change in the aromatic fraction of the preoxidized residue reflects a difference in susceptibility to weathering between different components of coal. The results show that oxidation of Pittsburgh No. 8 coal at 150°C consumes aliphatic and aromatic carbons approximately at the ratio of4:l (Ref. 18). The ’ 3C n.m.r. results are consistent with the reported finding in the literature for various pre-oxidized coals’ 3,18,25. Influence of pre-oxidation on TGA pyrolysis of coul or coal fractions
weight
loss during
An extraction yield of 19 wt % dry ash free (daf) was determined for the Pittsburgh No. 8 coal. The pyrolysis weight loss at 600°C (Table 3) for the extract was 45 wt ‘x (daf) and that for the residue was 22.6 wt % (daf). Thus, a combined weight loss of 26.8 wt y0 (dafj was achieved for the coal fractions at 600°C. This weight loss at 600°C is considerably lower than the weight loss observed for the raw coal (i.e. 32.6 wt %, daf). The results suggest that the presence of hydrogen-rich portions of the coal (i.e. extract) in the coal structure increases the overall weight loss for coal during pyrolysis. It was hypothesized that the aliphatic34 or the hydroaromatic part of coa13’ serves as
FUEL,
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E.s.r. spectroscopic study on the chemistry of coal oxidation: M. R. Khan et al. Table4 Influence of pre-oxidation on absolute spin concentration (per g of coal) at 460°C (after 60 min of pyrolysis) Per cent increase in spin concentration after 2 days of oxidation (compared with the untreated)
Sample
Spin concentration (per g of coal)
Untreated coal 2 days oxidized coal 6 days oxidized coal 14 days oxidized coal
1.9 x 10’9 2.4 x 1019 8.1 X 10’9 4.1 x 10’9
Extract (untreated) 2 days oxidized extract
9.4 x lo’* 2.0 x 1or9
53%
Residue (untreated) 2 days oxidized residue
2.2 x 10’9 2.6 x lOI
16%
20%
the ‘hydrogen donor’ during coal pyrolysis by supplying hydrogen to the hydrogen deficient moieties that would otherwise undergo cross-linking reactions that form coke. However, verification of this hypothesis is not available in the literature 36. In this study it is noted that the removal of the hydrogen donor source (e.g. extract) from coal, therefore, causes a significant reduction in overall weight loss during pyrolysis. This study confirms the hypothesis proposed by the earlier workers. The data presented in Table 3 provide a comparison of pre-oxidation effects on coal, extract and residue. These data can be used to calculate an ‘oxidation index’ defined by the following equation for various samples: Oxidation index = (W, - w)/W, x 100, where W, = weight loss by the untreated material at 600°C; and W= weight loss by the oxidized (6 days) sample pyrolysed to a temperature of 600°C. The oxidation index for the raw coal was 26.7 % compared with 46.7% for the extract and 2.2% for the residue. When devolatilized up to a temperature of 9OO”C, the oxidation indices noted for coal, extract and residue were 11.3,36.7 and 3.1 ‘A, respectively. The results of this study suggest that the penalty invoked by pre-oxidation was significantly higher for the coal extract compared with that of the residue. Electron spin resonance spectroscopy: comparison of coal extract and residue
The overall shapes of the growth curves showing the spin concentration of coal, oxidized coal and the solvent separated fractions of coal during pyrolysis are presented in Figure 2. Each curve is normalized by its value at time zero, so it is only the relative changes during pyrolysis, not the absolute magnitudes, that are revealed in this figure. The residue shows little sensitivity to oxidation, and the extract shows the most, with coal in between. The absolute spin concentrations after 60 min of pyrolysis, as a function of oxidation time, are reported in Table 4, without temperature corrections. The extract shows the lowest radical concentration of all the samples studied, both initially and after pyrolysis. Two explanations are plausible. First, the extractable components may indeed contain proportionately fewer radicals. Second, there may initially be a much higher radical loading in the extractables, but the chemical stability of these radicals depends on their being fixed rigidly in the coal matrix. During extraction, the radicals could diffusionally encounter other reactive species until eventually a
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FUEL, 1988, Vol 67, December
diamagnetic product is formed. Only the most stable and sterically hindered radicals would remain for detection in the e.s.r. experiment. Based on the room temperature prepyrolysis measurements, the weighted sums of base line radical concentrations in the extract and residue are significantly smaller than the base line radical concentration in coal. Base line radical concentrations of coal, extract and residue are 6.0 x 10i8, 2.0 x lOi and 4.2 x 10i8, respectively. This implies that some radicals are lost in the extraction procedure. The oxidized coal, extract and residue show a somewhat different behaviour. The oxidized extract has slightly less radical concentration at the completion of pyrolysis than oxidized coal. The 2 day oxidized residue contains the highest radical concentration of all 2 day oxidized samples studied. In all cases, oxidation increases the radical concentrations generated during pyrolysis, compared with the unoxidized sample. Table 4 compares each of the oxidized and unoxidized components according to their final radical loading after pyrolysis. The three percentages indicate the relative increase in radical concentration due to 2 days of oxidation. From this data it is apparent that 2 day oxidation strongly affects the extract but influences the residue very little. The effect on coal is, as one may expect, somewhere near the weighted average of the two extremes. The data suggest that oxidation influences the hydrogen-rich extract much more than the relatively hydrogen-poor residue. The effect of mild and severe oxidation on the pyrolysis spin concentration of coal is also shown in Table 4. The e.s.r. spin concentrations show that the unpaired-spin concentration goes through a maximum with time of oxidation. Prior observations by Sprecher and Retcofsky3’ by e.s.r. of free radicals generated in a bituminous coal by constant temperaure in situ pyrolysis revealed a difference in free radical behaviour depending on whether or not volatiles were confined to the region of the sample tube where the coal was, or whether the tube was large enough that the volatiles escaped and condensed on a cold spot of the tube. In their 495°C experiments where volatiles escaped from the region of the pyrolysing coal, a large transient population of free radicals appeared and decayed within a few minutes, in addition to a monotonically, slowly increasing population of more stable free radicals. The same transient population appeared at lower temperatures, but was less visible. These results were interpreted to mean that in the presence of mobile H-donors like the volatiles, the transient free radicals had too short a lifetime to be observed. The 460°C experiments discussed in this work in detail, as well as a series of identical experiments done every 20°C between 400 and 520°C never displayed a similar transient free radical population. Only a monotonically increasing population similar to those shown in Figure 2 was observed. This was true for coal, extract and residue. This indicates that for Pittsburgh No. 8 coal, there are more donor hydrogen available in the non-volatile components than in the case of the West Virginia Ireland Mine coal studied by Sprecher and Retcofsky. Additional work in this area has been reported elsewhere37,38. CONCLUSIONS The coal weight loss during pyrolysis, elemental analyses
E.s.r. spectroscopic
study on the chemistry
and the irr situ measurements of the spin concentration of the coal, preoxidized coal and the corresponding coal fractions (‘mobile’ and the ‘network’ phases) verify that the pre-oxidation of coal influences the hydrogen-rich ‘mobile’ component (extract) more than the hydrogen poor ‘network’ (residue) portion of a bituminous coal. The results of this study also confirm a hypothesis that the extractable portion of coal may serve as the source of internal hydrogen during pyrolysis by supplying hydrogen to hydrogen deficient moieties that would otherwise undergo coking reactions (forming solid residue) rather than desirable volatiles.
10 11 12 13 14 15 16 17 18 19 20 21
ACKNOWLEDGEMENTS Comments by J. Kovach, L. Headley and J. Wachter in this study are appreciated. R. Usmen and E. Newton acknowledge support from the U.S. DOE fossil energy research training programme administered by Oak Ridge Associated Universities for the U.S. Department of Energy.
22 23 24 25 26 27 28 29
REFERENCES Lowry, H. H. (Ed.) in ‘Chemistry of Coal Utilization’, Wiley, New York, USA, 1963 deVries, Von, H. A. W., Bokhoven, C. and Dorman, K. N. M. Brennst. C&n. 1969,50,289 Jones, R. E. and Townend, D. T. A. Chem. Ind. (London) 1949, 68, 197 Chakravarty, S. L. J. Mines, Met. Fuels 1960,8, 1 Studebaker, M. L. in ‘Proceedings Third Biennial Carbon Conference’, Pergamon, Oxford, UK, 1957, p. 289 Yohe, G. R. in ‘Oxidation of Coal’, Illinois State Geol. Survey Report Invest. 1958,207, p. 51 Ignasiak, B. S., Clugston, D. M. and Montgomery, D. S. Fuel 1972,51, 76 Ignasiak, B. S., Szladow, A. J. and Montgomery,D. S. Fuel 1974, 53, 12-15 Chamberlain, E. A. C., Barrass, G. and Therlaway, J. T. Fuel 1976,55, 217
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Volborth, A. in ‘Oxidation of Coal’ Report, 16 June 1976, DOE Contract No. EY-76-S-02-2898; Chem. Abstr. 1977,472OO Liotta, R., Brons, G. and Isaacs, J. Fuel 1983,62, 781 Painter, P. C., Snyder, R. W ., Starsinic, M. et al. Appl. Spectrosc. 1981,35,475 Havens, I. R., Koenig, J. L., Kuehn, D. ef al. Fuel 1983,62,936 Gethner, J. S. Applied Spectroscopy 1987, 41, 50 Yokono, T., Miyazawa, K., Sanada, Y. and Marsh, H. Fuel 1981,60, 598 Ripmeester, J. A., Couture, C., MacPhee, J. A. and Nandi, B. N. Fuel 1984,63, 522 Swarm, P. D. and Evans, D. G. Fuel 1979,58,276 Kalema, W. S. and Gavalas, G. R. Fuel 1987,66, 158 MacPhee, J. A. and Nandi, B. N. Fuel 1981, 60, 169 Yun, Y ., Jakab, E., McClennen, W. H., Hill, G. R. and Meuzelaar, H. L. C. Am. Chem. Sot. Diu. Fuel Chem. Prepr. 1987,32 (l), 129 Meuzelaar, H. L. C., McClennen, W. H., Candy, C. C., Metcalf, G. S. et al. Am. Chem. Sot. Div. Fuel. Chem. Prepr. 1984, 29 (5), 166 Khan, M. R. Energy and Fuels 1987, July/August, pp. 366376 Khan, M. R. Am. Chem. Sot., Dio. Pet. Chew Prepr. 1987,1,49 van Krevelen, D. W. in ‘Coal’, Elsevier, Amsterdam, 1961 Friedel, R. A., Shultz, J. L. and Sharkey, A. G. Jr. Fuel 1968.48, 403 Khan, M. R. and Jenkins, R. G. Fuel 1985,65, 189 Khan, M. R. and Jenkins, R. G. Fuel 1985,65, 1 Given, P. H., Stockman, W., Davis, A., Zoeller, J. et al. Fuel 1984,63, 1655 Khan, M. R. in Proceedings of the First Annual Oil Shale/Tar Sand Contractors Meeting; U.S. Department of Energy, Morgantown Energy Technology Center, July 1985, DOE/METC-85/6026, NTIS,DE85013717, pp. 104-120 Loveland, D. B. and Tozer,T. N. Journal ofPhysics 1972,5,535 Schaefer, J., Stejskal, E. 0. and Buchdahl, R. Macromolecules 1975,8,291 Lowe, I. J. Physical Review Letters 1959, 3, 285 Andrew, E. R., Bradbury, A. and Eades, R. G. Nature 1959,183, 1802 Mazumdar, B. K., Chakrabartty, S. K. and Lahiri, A. Fuel 1962, 41, 129 Neavel, R. C. in ‘Coal Science’ (Eds. M. L. Gorbaty, J. W. Larsen and I. Wender),Vol. 1, Academic Press, New York, 1982,pp. 1-19 Given, P. H., personal communication, 1985 Sprecher, R. F. and Retcofsky, H. Fuel 1983,62,473 Retcofsky, H. L. in ‘Coal Science’ (Eds. J. Larson, M. Gorbaty and I. Wender), Vol. 1, Academic Press, New York, 1982 Petrakis, L. and Gandy, D. W. in ‘Free Radicals in Coals and Synthetic Fuels’, Elsevier, New York, 1983
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1673
Khan,
R. Usmen,
study on the chemistry
E. Newton,
S. Beer and W. Chisholm
US Department of Energy, Morgantown Energy Technology WV 26505, USA (Received 15 January 1988; revised 18 April 7988)
Center, Morgantown,
A better understanding of the mechanisms ofpre-oxidation could lead to improved methods for prevention of coal weathering, which has a deleterious influence on liquid yield during pyrolysis. Previous Morgantown Energy Technology Center studies demonstrated that the extractable fractions ofcoal are more susceptible to oxidative weathering than the residue. To better understand the chemistry of coal weathering and the influence of pre-oxidation on subsequent devolatilization, in situ electron spin resonance (esr.) spectroscopic studies were performed on coal, weathered coal and on the corresponding untreated and preoxidized fractions (extract and residue). In this technique, pyrolysis is performed in an e.s.r. cavity and the concentration of free radicals is followed as the sample is pyrolysed. Elemental analysis of the pre-oxidized and untreated samples indicated that H/C ratio of the extract and the coal sample was reduced slightly during 48 h of oxidation at 150°C in air whereas the H/C ratio of the residue was essentially unchanged. The influence of pre-oxidation (48 h at 150°C) was to increase the e.s.r. spin concentration of the weathered coal slightly compared to the raw coal when devolatilized at 460°C. The e.s.r. experiments performed at 460°C also showed that pre-oxidation significantly enhanced free radical concentration in the extract but had a lesser effect on the residue. The enhanced free radical formation of the pre-oxidized extract relative to the untreated extract implies that mild pre-oxidation affects hydrogen-rich components (extract) more than the primarily hydrogen-poor components (residue) of coal. (Keywords: oxidation; electron spin resonance; chemical properties)
It is well known that pre-oxidation of coal profoundly influences important coal properties such as pyrolysis liquid yield, thermoplastic properties, elemental composition, heating value, extractability and specific gravity’. The mechanisms of coal oxidation and devolatilization mechanisms of pre-oxidized coal still remain the subject of considerable investigation. Formation of free radicals plays a key step in oxidation mechanisms. The first step in the oxidation of coal is generally considered to be peroxide formation by the oxidation of aliphatic/olel’inic/aromatic structures (R.) via free radical mechanisms: 2R-H +$02+2R.+
Hz0
(1)
R.+ O,+R-0-O.
(2)
R-O-O. + R-H --+R-O-O-H + R. (a peroxide or hydroperoxide)
(3)
Equation (1) can be considered as the initiation of a chain reaction that is further propagated by Equations such as (2) or (3). The peroxide or hydroperoxide formed in Equation (3) can undergo further decomposition producing relatively stable oxygenated products (e.g. alcohols, ketones, aldehydes, esters and ethers) as well as CO, CO, and H,O. The mechanisms and decomposition chemistry of peroxides into other products have been discussed by various authors’-“. The comparatively more stable oxygen functional groups such as hydroxyl, carboxyl, carbonyl and ether have been identified in oxidized coals both chemically” and spectroscopically 12-14. However, relatively little has been reported 00162361/88/12166846$3.00 01988 Butterworth & Co. (Publishers)
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1988,
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regarding the formation and fate of free radicals in the pre-oxidized coal during subsequent pyrolysis. Yokono et ~1.‘~ studied oxidized samples of strongly coking and very weakly caking coals by e.s.r. (not in situ measurement), infrared (i.r.) and pulsed proton n.m.r. An increase in free radical concentration was reported for samples of strongly coking coal oxidized at 110°C for up to 21 days. A decrease in free radical concentration was noted with further progressive oxidation. However, a progressive decrease in free radical concentration was observed with 42 days of oxidation of very weakly caking coal samples. It was concluded that there are differences in the mechanisms of oxidation of strongly coking and weakly caking coals. Ripmeester et ~1.‘~ studied relaxation times of fresh and oxidized samples of coal and semicoke by ‘H n.m.r. and e.s.r. (not in situ measurement) and indicated that the unpaired electron spin concentration increased on ‘mild’ oxidation, but decreased in the case of ‘severe’ oxidation. However, in this study, the ‘mild’ condition was defined as oxidation at 100°C for 3 12 h, whereas the ‘severe’ oxidation was performed at 190°C for 192 h. Swann and Evans17 studied the oxidation of brown coal at temperatures of 35°C and 70°C by i.r. and gas analysis and concluded that the carbonyl, carboxyl and phenolic groups are formed by oxidation of the aliphatic structures of the coal. Kalema and Gavalas’ * studied the compositional change (by gas analysis) and 13C n.m.r. (data were not corrected for carbonyl and carboxylic carbons) in two bituminous coals and lignite during air oxidation at 200 to 250°C and concluded that both
E.s.r. spectroscopic Table 1
study
on the chemistry
of coal oxidation:
M. R. Khan
et al.
Effect of pre-oxidation on composition of raw coal, pre-oxidized coal and coal fractions (extract and residue) -______ Residue Coal Extract
Ultimate (d.b.) C H N s S + 0 (by difference) H/C (atomic) Proximate (d.b.) Volatile matter Ash Heating value (Btu/lb)
Coal
Pre-oxidized 2 days
Pre-oxidized 6 days
Untreated
71.7 5.0 1.4 2.0 8.6 0.78
71.9 4.2 1.4 15.2 0.71
68.8 3.7 1.4 _ 18.8 0.65
83.7 5.8 2.6 _ 7.9 0.83
37.9 7.3 13 980
Oxidized 2 days
Oxidized 6 days
79.9 5.2 1.6
14.9 4.4 1.6
13.3 0.79
19.1 0.71
Untreated
Oxidized 2 days
Oxidized 6 days
62.5 2.6 2.6 2.0 22.9 0.51
61.8 2.7 2.5 _ 23.6 0.52
60.4 2.4 2.5 _ 25.2 0.47
31.7 9.4
0 _
y Calculated by the following: S + 0 = 100 - (C + H + N + ash), all on dry basis; extractables contained no ash
aliphatic and aromatic types of carbon are consumed at an approximate ratio of 3.5:1. Havens et ~1.‘~ measured aromaticity by ’ 3C cross polarization/magic angle spinning (CP/MAS) n.m.r. and FT-i.r. in samples of vitrinite concentrates oxidized in air at 140°C for up to 16 days and noted preferential attack on aliphatic structures. MacPhee and Nandi” examined the aromaticities of fresh and oxidized samples of bituminous coal by n.m.r. and reported an increase in the ratio of aliphatic to aromatic carbon with oxidation time in air at 105°C (up to 500 h). Based on the n.m.r. data, it was concluded, contrary to some literature data, that the aromatic structures in the coal were being modified more than the aliphatic structures during oxidation. Yun et ~1.~~investigated the products of pyrolysis of two subbituminous and two high volatile bituminous coals oxidized at 100°C for 170 to 212 h by pyrolysis mass spectroscopy and concluded that aliphatic, hydrocarbonrich coal structures (derived from cutinitic or algal sources) show little or no chemical change under low temperature (1OO’C) oxidation. More highly aromatic vitrinite and sporinite show a moderate oxidation tendency. Meuzelaar et ~1.” studied the influence of weathering on structure and pyrolysis of selected Wasatch Plateau coals and proposed that weathering of coal (2 12 h at 1OO’C)is preceded by ‘grafting’ (linking) of ‘mobile phase’ constituents onto the coal ‘network’ chains. However, no extraction work was performed to monitor directly the relative pre-oxidation potential on the extract (i.e. the mobile fraction) and the residue (the network phase). The above literature studies can be classified into two widely conflicting groups: (a) those studies suggesting that pre-oxidation of coal induces changes primarily on the aromatic structures; and (b) the studies proposing that pre-oxidation at least initially changes the aliphatic structures of coal. Little has been reported in the literature regarding the influence of hydrogen content of feedstocks on their pre-oxidation potential. It was reported by Khan 22*23that oxidation causes a reduction in the aliphatic hydrogen content of coals, sales and kerogen as monitored by FT-i.r. It was therefore hypothesized that the relatively hydrogen-rich portion of coal (e.g. extract) was more susceptible to oxidation than the residue (i.e. hydrogen-deficient portion). Assuming that mass transport does not have a controlling influence,
one may hypothesize that the mobile phase extractables, smaller molecules with various side groups, would be more susceptible to oxidation than the more highly condensed structures with a larger number of rings (i.e. residue). To verify this hypothesis, it was necessary to separate the coal into a hydrogen-rich portion (extract) and a hydrogen-poor portion (residue) and perform preoxidation/pyrolysis on each fraction. Pyridine was selected for these extractions because it is known to be a powerful ‘specific’ solvent capable of dissolving a significant portion of coal. van Krevelen24 suggested that the pyridine extraction process removes the low molecular weight (relatively hydrogen-rich) components responsible for agglomerating and swelling properties of coal. It was proposed that pyridine extraction of coal is essentially an ‘unlocking’ process to remove the hydrogen bonded structures from coal. The extractable portion of coal was found to be relatively richer in both aromatic and aliphatic hydrogen25. EXPERIMENTAL Sample preparution
and charucterization
Pittsburgh No. 8 high volatile bituminous coal, freshly obtained from the mine mouth (stored in N,), was used throughout this investigation. The coal was stored in N, until it was ready to be used. It was ground to -200 mesh (-74pm) and sieved under nitrogen. Ultimate and approximate analyses of this coal are provided in Table 1. Simulated weathering of the dried (at 110°C in vacuum for 1 h) coal sample and its fractions to various severities was achieved by treatment at 150°C in dry air for up to 14 days using a procedure described elsewhere22,23. Elemental analysis (H, C, N) of untreated and preoxidized coal as well as its corresponding fractions was performed (see Table I). Thermogravimetric
analyses of coal
Detailed pyrolysis behaviour of the untreated and weathered materials was subsequently determined in He by using a thermogravimetric analysis (TGA) system interfaced with a mass spectrometer, more details of which are presented elsewhere22923. The TGA weight loss data were generally reproducible to within 1% of the initial sample weight utilized.
FUEL, 1988, Vol 67, December
1669
E.s.r. spectroscopic study on the chemistry of coal oxidation: M. R. Khan et al.
ro for
2 days
I
1 Pit
Prsoxidizedln
Zeal-200
Mesh
(Store
r
1
8 Air
At
150-c
_
_
lor 6
In N2)
Extraction
Figure
1
Pyridine
Summary
of the extraction/pre-oxidation
procedures
extraction
Pyridine was selected as the extraction solvent since it is capable of removing the ‘mobile’ phase of coal. Pyridine extractions were carried out in a conventional water jacket Soxhlet extractor at or near the boiling point of pyridine. In a typical extraction, about 15g of coal was placed in a predried cellulose extraction thimble. The sample was dried, weighed and placed in the extractor. Fresh solvent (200 ml, obtained from Fisher Scientific) was added to the solvent reservoir. A small flow of nitrogen (10 cm3 min-‘) was continuously bubbled through the reservoir to exclude air from the system. The heating mantle was switched on, and extraction of the sample proceeded as the solvent was vaporized by continuous heating of the reservoir. Heating conditions were set to maintain an extraction rate of one recycle per minute. Extraction was continued until the solution in the extractor appeared clear. When the extraction run was completed, the heating mantle was switched off and the thimble was removed from the extractor and dried in N, for 24 h. The residue was repeatedly washed with water and vacuum dried in attempts to remove the residual pyridine. The residue and extraction thimble were dried in a vacuum oven at 110°C for 16 h, and the extraction yield was 17.4% (82.6 wt % residue), determined by the change in sample weight. The extraction and preoxidation procedures are represented in Figure 1. The influence of small amount of residual pyridine on pyrolysis was considered to be relatively small, as also assumed by others’*25. Em-.
spectroscopy
Spin concentrations were measured on a Varian E 109 spectrometer equipped with a 9-inch magnet and a single
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1988,
Vol 67, December
rectangular cavity. The spectra were recorded at 30second intervals with an IBM 9000 computer and reduced to free radical concentration using a method similar to that of Loveland and Tozen3’. This method included: (a) correction of the raw data for slope and offset, checking to see that areas of positive and negative lobes are equal; (b) single integration; (c) another check on baseline, slope and offset; and (d) final integration. This method works better than one-step integration or integration by the absolute value of the first derivative presentation for broad and noisy signals on a sloping baseline. Dilute dispersion of diphenylpicrylhydrazyl on silica in an evacuated quartz tube was used for calibration. Absolute accuracy is estimated to be +_10% for all experiments reported here. Coal samples were mixed with - 240 mesh silica, in proportion nearly 50 %. Approximately 10 mg of the sample/diluent mixture was used in each experiment. After being placed in a quartz e.s.r. tube, the samples were degassed at 20 ptorr or less and sealed in a vacuum before the experiments. Room temperature spectra were recorded before and after pyrolysis. All pyrolyses were performed in situ isothermally at 460°C. Sample temperature was monitored by a thermocouple located next to the sample. A Wilmad high-temperature Dewar insert and transfer Dewar, controlled by an Omega model proportional controller using nitrogen as a heat transfer gas, provided the heat for pyrolysis. The system was allowed to stabilize at 460°C. The sample reached 460°C within 30 s, and the temperature of the sample settled on 460 f 1°C for the duration of the experiment. Repeating some of the experiments at higher dilution, different sample size and different sample tube size did not significantly alter the results. 13C nmr. CPIMAS CP/MAS spectra were obtained on an IBM NR-80 spectrometer and solids accessory, utilizing a probe and rotors supplied by Doty Scientific (Columbia, SC). The operating field was 1.9 Tesla (1 Tesla= 10 kGauss). It provided a r 3C resonance frequency of 20.1 MHz. Samples were placed in 5 mm ceramic rotors plugged with Kel-f end caps to ensure no background signal would interfere. Spinning speeds were from 2500 to 3500 Hz with the spinning angle set within 0.1” of the ‘magic angle’ of 54.7” on paraditertbutylbenzene (PDTBB). The stator was then reset to within 0.1” of the same angle after the
2.00
Figure 2 Growth in relative e.s.r. spin concentrations (arbitrary units) during pyrolysis (normalized, average). Note: each curve is normalized to its value at time=zero, hence only the relative changes are meaningful; also scatter in the data points that define these curves was always 10.05 units
E.s.r. spectroscopic Table 2
study on the chemistry
of coal oxidation:
M. R. Khan et al.
13C CP/MAS studies of untreated and pre-oxidized samples of Pittsburgh No. 8 coal and its organic fractions”
Sample Untreated coal 2 days oxidized coal 6 days oxidized coal 14 days oxidized coat Extract (untreated) 2 days oxidized extract Residue (untreated) 2 days oxidized residue
Carbon (%)
Aromaticity (f,)
Aromatic carbon (%)
Aliphatic carbon (%)
77.7 71.9 68.8
0.67 0.71 0.75 0.79 0.67 0.69 0.78 0.78
52.0 51.1 51.6 _ 56.1 55.2 48.6 48.2
25.6 20.8 17.2 _ 27.6 24.8 13.8 13.6
8i.7 79.9 62.5 61.8
Carbon consumed (%) Aromatic
Aliphatic
0.9 0.4 _
_ 4.8 8.4 _
0.9 _ 0.4
2.8 0.2
-
_
’ The experimental error is within 2 %
Table 3 Effect of pre-oxidation on pyrolysis weight loss (heating rate, 50°C min-‘) for raw and pre-oxidized coal (performed at 150°C in air) and coal fractions (extract and residue) (Pittsburgh No. 8 coal) Pyrolysis weight loss heated to (wt%), daf Sample
600°C
900°C
Maximum rate dw/dt (%/min), daf
Raw coal
32.6
39.1
14.1
Coal pre-oxidized : 2 days 6 days
26.1 23.9
35.3 34.7
8.7 6.5
Coal extract
45
49
Extract pre-oxidized: 2 days 6 days
32.0 24.0
38.5 31.0
9 6
Coal residue
22.6
35.9
4.4
Residue pre-oxidized : 2 days 6 days
23.2 22.1
35.9 34.8
4.4 _
16
sample was installed in the probe. The methyl carbon signal of PDTBB was used as the chemical shift reference (31.0 ppm relative to tetramethylsilane), and as a means of monitoring the stability of the externally locked magnetic field. This field was stabilized to within 4 ppm. Cross-polarization times were 1 ms and pulse repetition times 1.03 s in all cases. Hartmann-Hahn matching was at radio frequency field strengths of 50 kHz, corresponding to a 90” pulse at 5 ps. Proton decoupling was done at the same level. Aromaticities estimated from integrated spectral intensities include contributions from spinning side bands arising from chemical shift anisotropies 31-33. The partition between the aromatic and aliphatic regions was taken to be the region between the two peaks that was closest to the baseline (e.g. the inflection point of the integral). In general, this partition was midway between the aromatic and aliphatic peaks. RESULTS AND DISCUSSION Influence of pre-oxidation on the elemental composition uromaticity of coal or coal fractions
and
Elemental analysis of the pre-oxidized and untreated samples show that the H/C ratios were significantly reduced during 6 days of pre-oxidation (Table I). The H/C (atomic) ratio of untreated coal decreased from 0.78 to 0.71 in 2 days of pre-oxidation and to 0.65 in 6 days of pre-
oxidation. In duplicate measurements, while the absolute numbers for carbon and hydrogen contents varied, the H/C was reproducible within 50.0 1. The extract had an H/C ratio of 0.83 compared with a 0.78 ratio for raw dried coal, suggesting that the extraction concentrated the slightly hydrogen-rich portion of the coal. While the H/C ratio of the extract was reduced by 14 y0 during 6 days of pre-oxidation, the reduction in this ratio for the residue was 8% when pre-oxidized under the same conditions. The aromaticities estimated from integrated ’ 3C n.m.r. spectral intensities are presented in Table 2. Carbonyl and carboxyl carbon is included in the aromatic region. The aromatic fraction of Pittsburgh No. 8 coal seems to increase as the time of low temperature oxidation is increased from 2 to 14 days. Linear combinations of the spectra suggest that oxidation of aliphatics to carboxylics and carbonyls occurring downfield from 160 ppm is largely responsible for increases in apparent carbon aromaticity. Increases in the 50-100 ppm region were also noted in the 6 and 14 day oxidized samples, indicating the presence of singly-bonded oxygen substituents on aliphatic chains. A slight increase in aromatic fraction was noted when the extract was oxidized for 2 days. No statistically significant change in the aromatic fraction was noted after the residue was oxidized for 2 days. An increase in the aromatic fraction of the pre-oxidized extract, and insignilicant change in the aromatic fraction of the preoxidized residue reflects a difference in susceptibility to weathering between different components of coal. The results show that oxidation of Pittsburgh No. 8 coal at 150°C consumes aliphatic and aromatic carbons approximately at the ratio of4:l (Ref. 18). The ’ 3C n.m.r. results are consistent with the reported finding in the literature for various pre-oxidized coals’ 3,18,25. Influence of pre-oxidation on TGA pyrolysis of coul or coal fractions
weight
loss during
An extraction yield of 19 wt % dry ash free (daf) was determined for the Pittsburgh No. 8 coal. The pyrolysis weight loss at 600°C (Table 3) for the extract was 45 wt ‘x (daf) and that for the residue was 22.6 wt % (daf). Thus, a combined weight loss of 26.8 wt y0 (dafj was achieved for the coal fractions at 600°C. This weight loss at 600°C is considerably lower than the weight loss observed for the raw coal (i.e. 32.6 wt %, daf). The results suggest that the presence of hydrogen-rich portions of the coal (i.e. extract) in the coal structure increases the overall weight loss for coal during pyrolysis. It was hypothesized that the aliphatic34 or the hydroaromatic part of coa13’ serves as
FUEL,
1988,
Vol 67, December
1671
E.s.r. spectroscopic study on the chemistry of coal oxidation: M. R. Khan et al. Table4 Influence of pre-oxidation on absolute spin concentration (per g of coal) at 460°C (after 60 min of pyrolysis) Per cent increase in spin concentration after 2 days of oxidation (compared with the untreated)
Sample
Spin concentration (per g of coal)
Untreated coal 2 days oxidized coal 6 days oxidized coal 14 days oxidized coal
1.9 x 10’9 2.4 x 1019 8.1 X 10’9 4.1 x 10’9
Extract (untreated) 2 days oxidized extract
9.4 x lo’* 2.0 x 1or9
53%
Residue (untreated) 2 days oxidized residue
2.2 x 10’9 2.6 x lOI
16%
20%
the ‘hydrogen donor’ during coal pyrolysis by supplying hydrogen to the hydrogen deficient moieties that would otherwise undergo cross-linking reactions that form coke. However, verification of this hypothesis is not available in the literature 36. In this study it is noted that the removal of the hydrogen donor source (e.g. extract) from coal, therefore, causes a significant reduction in overall weight loss during pyrolysis. This study confirms the hypothesis proposed by the earlier workers. The data presented in Table 3 provide a comparison of pre-oxidation effects on coal, extract and residue. These data can be used to calculate an ‘oxidation index’ defined by the following equation for various samples: Oxidation index = (W, - w)/W, x 100, where W, = weight loss by the untreated material at 600°C; and W= weight loss by the oxidized (6 days) sample pyrolysed to a temperature of 600°C. The oxidation index for the raw coal was 26.7 % compared with 46.7% for the extract and 2.2% for the residue. When devolatilized up to a temperature of 9OO”C, the oxidation indices noted for coal, extract and residue were 11.3,36.7 and 3.1 ‘A, respectively. The results of this study suggest that the penalty invoked by pre-oxidation was significantly higher for the coal extract compared with that of the residue. Electron spin resonance spectroscopy: comparison of coal extract and residue
The overall shapes of the growth curves showing the spin concentration of coal, oxidized coal and the solvent separated fractions of coal during pyrolysis are presented in Figure 2. Each curve is normalized by its value at time zero, so it is only the relative changes during pyrolysis, not the absolute magnitudes, that are revealed in this figure. The residue shows little sensitivity to oxidation, and the extract shows the most, with coal in between. The absolute spin concentrations after 60 min of pyrolysis, as a function of oxidation time, are reported in Table 4, without temperature corrections. The extract shows the lowest radical concentration of all the samples studied, both initially and after pyrolysis. Two explanations are plausible. First, the extractable components may indeed contain proportionately fewer radicals. Second, there may initially be a much higher radical loading in the extractables, but the chemical stability of these radicals depends on their being fixed rigidly in the coal matrix. During extraction, the radicals could diffusionally encounter other reactive species until eventually a
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FUEL, 1988, Vol 67, December
diamagnetic product is formed. Only the most stable and sterically hindered radicals would remain for detection in the e.s.r. experiment. Based on the room temperature prepyrolysis measurements, the weighted sums of base line radical concentrations in the extract and residue are significantly smaller than the base line radical concentration in coal. Base line radical concentrations of coal, extract and residue are 6.0 x 10i8, 2.0 x lOi and 4.2 x 10i8, respectively. This implies that some radicals are lost in the extraction procedure. The oxidized coal, extract and residue show a somewhat different behaviour. The oxidized extract has slightly less radical concentration at the completion of pyrolysis than oxidized coal. The 2 day oxidized residue contains the highest radical concentration of all 2 day oxidized samples studied. In all cases, oxidation increases the radical concentrations generated during pyrolysis, compared with the unoxidized sample. Table 4 compares each of the oxidized and unoxidized components according to their final radical loading after pyrolysis. The three percentages indicate the relative increase in radical concentration due to 2 days of oxidation. From this data it is apparent that 2 day oxidation strongly affects the extract but influences the residue very little. The effect on coal is, as one may expect, somewhere near the weighted average of the two extremes. The data suggest that oxidation influences the hydrogen-rich extract much more than the relatively hydrogen-poor residue. The effect of mild and severe oxidation on the pyrolysis spin concentration of coal is also shown in Table 4. The e.s.r. spin concentrations show that the unpaired-spin concentration goes through a maximum with time of oxidation. Prior observations by Sprecher and Retcofsky3’ by e.s.r. of free radicals generated in a bituminous coal by constant temperaure in situ pyrolysis revealed a difference in free radical behaviour depending on whether or not volatiles were confined to the region of the sample tube where the coal was, or whether the tube was large enough that the volatiles escaped and condensed on a cold spot of the tube. In their 495°C experiments where volatiles escaped from the region of the pyrolysing coal, a large transient population of free radicals appeared and decayed within a few minutes, in addition to a monotonically, slowly increasing population of more stable free radicals. The same transient population appeared at lower temperatures, but was less visible. These results were interpreted to mean that in the presence of mobile H-donors like the volatiles, the transient free radicals had too short a lifetime to be observed. The 460°C experiments discussed in this work in detail, as well as a series of identical experiments done every 20°C between 400 and 520°C never displayed a similar transient free radical population. Only a monotonically increasing population similar to those shown in Figure 2 was observed. This was true for coal, extract and residue. This indicates that for Pittsburgh No. 8 coal, there are more donor hydrogen available in the non-volatile components than in the case of the West Virginia Ireland Mine coal studied by Sprecher and Retcofsky. Additional work in this area has been reported elsewhere37,38. CONCLUSIONS The coal weight loss during pyrolysis, elemental analyses
E.s.r. spectroscopic
study on the chemistry
and the irr situ measurements of the spin concentration of the coal, preoxidized coal and the corresponding coal fractions (‘mobile’ and the ‘network’ phases) verify that the pre-oxidation of coal influences the hydrogen-rich ‘mobile’ component (extract) more than the hydrogen poor ‘network’ (residue) portion of a bituminous coal. The results of this study also confirm a hypothesis that the extractable portion of coal may serve as the source of internal hydrogen during pyrolysis by supplying hydrogen to hydrogen deficient moieties that would otherwise undergo coking reactions (forming solid residue) rather than desirable volatiles.
10 11 12 13 14 15 16 17 18 19 20 21
ACKNOWLEDGEMENTS Comments by J. Kovach, L. Headley and J. Wachter in this study are appreciated. R. Usmen and E. Newton acknowledge support from the U.S. DOE fossil energy research training programme administered by Oak Ridge Associated Universities for the U.S. Department of Energy.
22 23 24 25 26 27 28 29
REFERENCES Lowry, H. H. (Ed.) in ‘Chemistry of Coal Utilization’, Wiley, New York, USA, 1963 deVries, Von, H. A. W., Bokhoven, C. and Dorman, K. N. M. Brennst. C&n. 1969,50,289 Jones, R. E. and Townend, D. T. A. Chem. Ind. (London) 1949, 68, 197 Chakravarty, S. L. J. Mines, Met. Fuels 1960,8, 1 Studebaker, M. L. in ‘Proceedings Third Biennial Carbon Conference’, Pergamon, Oxford, UK, 1957, p. 289 Yohe, G. R. in ‘Oxidation of Coal’, Illinois State Geol. Survey Report Invest. 1958,207, p. 51 Ignasiak, B. S., Clugston, D. M. and Montgomery, D. S. Fuel 1972,51, 76 Ignasiak, B. S., Szladow, A. J. and Montgomery,D. S. Fuel 1974, 53, 12-15 Chamberlain, E. A. C., Barrass, G. and Therlaway, J. T. Fuel 1976,55, 217
30 31 32 33 34 35 36 31 38 39
of coal oxidation:
IV. R. Khan et al.
Volborth, A. in ‘Oxidation of Coal’ Report, 16 June 1976, DOE Contract No. EY-76-S-02-2898; Chem. Abstr. 1977,472OO Liotta, R., Brons, G. and Isaacs, J. Fuel 1983,62, 781 Painter, P. C., Snyder, R. W ., Starsinic, M. et al. Appl. Spectrosc. 1981,35,475 Havens, I. R., Koenig, J. L., Kuehn, D. ef al. Fuel 1983,62,936 Gethner, J. S. Applied Spectroscopy 1987, 41, 50 Yokono, T., Miyazawa, K., Sanada, Y. and Marsh, H. Fuel 1981,60, 598 Ripmeester, J. A., Couture, C., MacPhee, J. A. and Nandi, B. N. Fuel 1984,63, 522 Swarm, P. D. and Evans, D. G. Fuel 1979,58,276 Kalema, W. S. and Gavalas, G. R. Fuel 1987,66, 158 MacPhee, J. A. and Nandi, B. N. Fuel 1981, 60, 169 Yun, Y ., Jakab, E., McClennen, W. H., Hill, G. R. and Meuzelaar, H. L. C. Am. Chem. Sot. Diu. Fuel Chem. Prepr. 1987,32 (l), 129 Meuzelaar, H. L. C., McClennen, W. H., Candy, C. C., Metcalf, G. S. et al. Am. Chem. Sot. Div. Fuel. Chem. Prepr. 1984, 29 (5), 166 Khan, M. R. Energy and Fuels 1987, July/August, pp. 366376 Khan, M. R. Am. Chem. Sot., Dio. Pet. Chew Prepr. 1987,1,49 van Krevelen, D. W. in ‘Coal’, Elsevier, Amsterdam, 1961 Friedel, R. A., Shultz, J. L. and Sharkey, A. G. Jr. Fuel 1968.48, 403 Khan, M. R. and Jenkins, R. G. Fuel 1985,65, 189 Khan, M. R. and Jenkins, R. G. Fuel 1985,65, 1 Given, P. H., Stockman, W., Davis, A., Zoeller, J. et al. Fuel 1984,63, 1655 Khan, M. R. in Proceedings of the First Annual Oil Shale/Tar Sand Contractors Meeting; U.S. Department of Energy, Morgantown Energy Technology Center, July 1985, DOE/METC-85/6026, NTIS,DE85013717, pp. 104-120 Loveland, D. B. and Tozer,T. N. Journal ofPhysics 1972,5,535 Schaefer, J., Stejskal, E. 0. and Buchdahl, R. Macromolecules 1975,8,291 Lowe, I. J. Physical Review Letters 1959, 3, 285 Andrew, E. R., Bradbury, A. and Eades, R. G. Nature 1959,183, 1802 Mazumdar, B. K., Chakrabartty, S. K. and Lahiri, A. Fuel 1962, 41, 129 Neavel, R. C. in ‘Coal Science’ (Eds. M. L. Gorbaty, J. W. Larsen and I. Wender),Vol. 1, Academic Press, New York, 1982,pp. 1-19 Given, P. H., personal communication, 1985 Sprecher, R. F. and Retcofsky, H. Fuel 1983,62,473 Retcofsky, H. L. in ‘Coal Science’ (Eds. J. Larson, M. Gorbaty and I. Wender), Vol. 1, Academic Press, New York, 1982 Petrakis, L. and Gandy, D. W. in ‘Free Radicals in Coals and Synthetic Fuels’, Elsevier, New York, 1983
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