- Email: [email protected]
Oxygen in coal ash: a simplified approach to the analysis of ash and mineral matter in coal
Oxygen in coal ash: a simplified approach to the analysis of ash and mineral matter in coal* Alexis Volbortht,
George E. Miller,
Claudia K. Garner and Paul A. Jerabek
Department of Chemistry, University of California, Irvine, California 92717, USA f Department of Geology, North Dakota State University, Fargo, North Dakota 58102, USA; and Visiting Professor, University of California, Irvine (Received 15 September 1976)
Oxygen was determined accurately in eight U.S. Bureau of Mines coal ash samples A, B, D, F, G, I, and J, NBS coal fly ash 1633 reference material, and two low-temperature ashes (LTA) from Illinois State Geological Survey. The method uses fast-neutron activation (FNA) analysis employing a dual counting and irradiation system which is essentially free from interferences. The stoichiometric balance based on analyses of the ashes performed by the USBM is calculated and summations given in oxide and element percent. Excellent agreement is found with the chemical data obtained by classical silicate analysis methods. Accurate oxygen determination for coal ash and i-T-ash (or mineral matter) is important for calculation of data in the ultimate analysis of coal as such. Knowledge is required for recalculation of the data on a dry and dry-ash-free basis. The routinely used ‘oxygen by difference’ values are inadequate for accurate work. In order to determine the organic oxygen in coal one also has to correct for oxygen in mineral matter and oxygen in the water removed as moisture. The Parr formula and other methods of empirical estimation are inadequate and may be replaced in some cases by the oxygen determination. The complete data provide a quantitative basis for stoichiometric interpretation of coal analyses. It was found that the eight coal-ash samples analysed contained 45.5 f 3% oxygen. Since these ashes represent a large variety of U.S. coals, this figure can be used as an estimate for recalculation and evaluation of the proximate and ultimate coal analyses. It is better, however, to use values actually determined in ash by the rapid fast-neutron activation method. This permits a better estimation of the sum of cations plus sulphates in the ash.
Ash is one of the components determined in the process called the proximate analysis of coal, which also consists of the determination of the moisture and the volatile matter. The fourth component estimated in the proximate analysis is labelled fuced carbon. It is obtained from the sum of the other three components by subtraction from 100. Ash is a product of the decomposition of silicate, sulphide, sulphate, carbonate, and oxide minerals which form the bulk of the inorganic matter in coal. This decomposition is accomplished by burning the coal powder in an oxygen atmosphere at -ISO-850°C. The ash is thus the noncombustible residue that forms when coal is ignited under controlled conditions’*‘. Fundamentally, the sources of ash can be subdivided into ash derived from adventitious mineral matter: the ‘inherent ash’, formed through the decomposition of such minerals as kaolinite, montmorillonite, pyrite, gypsum, jarosite, calcite, and the ‘organic component ash’ derived from the decomposition of plant material contaming bound inorganic elements. The last type of ash is usually relatively insignificant. Once the ashing is completed at the high temperatures used, no distinction cur be made as to the relative quantities of the inherent ash versus the organic ash.
* Supported by Contracts E(O4-3)-34-241 and E(1 l-1)-2898, Energy Research and Development Administration
204
FUEL,
1977, Vol 56, April
U.S
A more serious problem results from the fact that the moisture due to the crystalline and (OH) water in such minerals as kaolinite, montmorillonite,illite, muscovite, chlorite, gypsum, and jarosite is driven off at such temperatures. Since ‘moisture’ that escapes above 105°C is also included in the ‘volatile matter’ data obtained by rapid heating to 950-965’C and recording the weight loss, the summing of the three components of the proximate analysis to determine the fixed carbon is stoichiometrically incorrect. A better way would be to determine the total oxygen in coal before ashing, then in the ash, and then in the volatile matter and coke as well as in the mineral matter estimated by the Parr formula’s or the low-temperature ashing method (LTA) developed by Gluskoter3. The determination of oxygen in coal ash versus LT-ash would then enable the coal chemist to estimate realistically the stoichiometric balance. Block and Dams6 have analysed 20 coal ashes obtained from Belgian coals for oxygen and silicon using a fast-neutron activation technique. The mean oxygen value of these determinations is 46.14% 0 with a total range of 41.52-48.25%. For these ashes one could use an estimated approximate factor for oxygen of 0.46 + O-04. A rough correlation between the ash yield and the oxygen content in coal (as well as between the ash and the silicon content also noted before by Loska and Gorski’) was found by these authors. This means that a determination of total oxygen in coal
A. Volborth, G. E. Miller, C. K, Garner and P. A. Jerabek: Oxygen in coal ash: analysis of ash and mineral matter
may in a limited sense be used to estimate its ash, or vice versa. One of the main uncertainties in the high-temperature ashing procedures (in addition to the behaviour of water) is the behaviour of sulphur. Sulphur occurs in coal as organic sulphur, sulphide or pyrite sulphur, sulphate sulphur, and occasionally elemental sulphur. The organic sulphur may oxidize mostly and evolve as SO;! and SO3 during the ashing. The sulphide sulphur is also oxidized with pyrite, FeS2, transformed ideally to Fe203 with evolution of SO2 and SO3 gas. However, in coals rich in calcium carbonate and if the ashing is so rapid as to decompose the carbonate to calcium oxide while the evolution of SO3 takes place, calcium sulphate may form, which is difficult to decompose even at the high temperatures of ashing. Thus, considerable and varying amounts of sulphur may be retained in coal ash. Calcium-rich ashes usually have considerable sulphur retained (see USBM ash D, B, and G in Table 2). The presence of much calcium and sulphur affects the composition of coal ash somewhat. It seems to diminish the total oxygen content of the ash (see USBM ash D, B, and G in Table 2). This may affect the estimates of oxygen content in ash. In low-temperature ash rich in pyrite, the total oxygen content would be expected to be lower than in the corresponding high-temperature ash. Should the LT-ash be predominantly composed of kaolinite, montmorillonite, quartz, or gypsum, one would expect a relatively high percentage of oxygen. In fact, Hamrin et a1.4 have analysed one ash obtained from Kentucky No.9 coal (3.6% S) and found 42.9 + 0.4% oxygen. If the coal mineral matter should be composed mostly of clayey or shaley components, one might predict oxygen contents as high as those determined by us in the U.S. NBS flint clay, 99a (55.40% 0) and plastic clay, 98a (54.75% 0)‘. The range of oxygen content possible for LT-ash is thus equal to about *lo% relative. In ashrich coals (=15-20% ash) this could mean a 2% error in total oxygen due to ash type if a simple estimate is used. This would cause a serious mistake in the estimation of organic oxygen. Thus we contend that accurate determination of oxygen in coal before ashing, in the coal ash, and in the corresponding LT-ash should provide a better value for estimating the organic oxygen in coal on a moisture- and ashfree (daf) basis. A less accurate method of estimating mineral matter (MM) is the empirical one using the Parr formulas, based on the ash (A) and the total sulphur content (S) in coal: MM = 1.08 A + 0.55 S In this formula the total water of hydration of the inorganic minerals in the coal is assumed to have been 8% and the factor 1.08 is thus derived. Multiplying the ash by this number is then assumed to restore the water of hydration lost during ashing. This was shown to be a good average for 19 Illinois and 11 high-volatile coals from the Eastern United States (Rees’, pp.20-24; Selvig and Gibson”). The factor 0.55 used for sulphur is a combination of two corrections and a compensation for sulphur not in the pyritic form. It takes into account the transformation of 2 FeS2 to Fe203 during ashing, where 3 0 E 4 S, or 48/128 = 318, which means that only 3/8 of the weight of the sulphur in the mineral matter has been replaced by oxygen so that 5/S must be added to the ash to compensate for this loss: A + 5/8 S. It also corrects for the fact that pyrite, FeS2, is not hydrated, by equating the Fe203 con-
tent in ash with its equivalent of sulphur atoms in the coal, or 160/128 = 10/8. Assuming 8% water of hydration, the sulphur correction in mineral matter becomes 0.08 x (A - 10/8 S). Thus the Parr formula becomes: MM = A + 5/8 S + 0.08 (A - 10/8 S) =
To compensate in part for any sulphur in coal not in pyritic form (‘organic’ sulphur) and to further simplify the calculation, Parr proposed to use the fraction 22/40 (0.55) instead of 21/40 (0.525). It may be noted, however, that the water of hydration of the average of nineteen Illinois coals was found by Selvig and Gibson” to be 6*8%, whereas in the eleven Eastern-U.S. high-volatile coals they found 9.87%. While this may not greatly affect the calculated values of calorific value (MM free) using the Parr formula for these coals (14 279 Btu versus 14 317 Btu, p 23, 1) in stoichiometric summation and interpretation of coal analyses where oxygen is determined rather than estimated by difference, an absolute difference of 3% in the water of hydration value would yield a 0.6% difference in the total oxygen content for a coal with 20% of mineral matter. This is significant when estimating the ‘organic oxygen’ in coal on a ‘dry mineral-matter-free’ basis. Therefore, direct determination of oxygen in coal ash (as well as in LT-ash) should be preferable to the Parr mineral matter evaluation as one may then estimate precisely the percentage of cations plus CaS04 in ash. If the total oxygen and sulphur have been determined in the coal, the volatiles, the coke button, and the ash, one can calculate approximately the loss of sulphur to the atmosphere and examine the possible dependence of this loss on the organic content of the coal. A more complex but stoichiometrically more correct formula for mineral matter calculation has been proposed by King et al. 9 : %MM=109A+0.5Spyr
t 0.8 CO, - 1.1 SO, in ash
+ SO, in coal + 0.5 Cl where A = % ash in coal; SPYr= % S, pyritic, in coal; CO2 = % of mineral CO;! in coal; SO3 in ash = % SO3 in ash, SO3 in coal = % SO3 in coal; Cl = % Cl in coal This formula uses similar reasoning to that used in the derivation of the Parr formula, but it also takes into account the CO2 of the carbonates, the SO3 in the ash and in the coal, as well as the chlorine. These two methods of mineral matter estimation use formulae based partially on assumed stoichiometry and make subsequent simplifications. Segments of these formulae are based on accurate and meticulously derived values; other segments are not. For example, in the derivation of the Parr formula, it is assumed that the coal mineral matter is composed solely of kaolinite (Al203 . 2 SiO2 . 2 H20), silica (SiOz), and pyrite (FeS2); it is further assumed that the crystalline water is due solely to kaolinite (water of hydration = Al203 x 0.3535) that the only alumina (Al2O3) in ash is that in kaolinite (kaolinite = Al203 x 2.532) that the excess of silica (SiO2) is equal to the total SiO2 in ash minus the SiO2 of the kaolinite (SiO2 of kaolinite = kaolinite x 0.4655), that all of the Fe203 in ash is due to the pyrite in the coal (FeS2 = Fe203 x 1.503), and further that all the sulphur in coal is pyritic sulphur.
FUEL,
1977, Vol 56, April
205
Oxygen in coal ash: analysis of ash and mineral matter: A. Volborth, G. E. Miller, C. K. Garner and P. A. Jerabek
All these assumptions are approximations acceptable for many coals, however, since coal mineral matter is known to contain other clay minerals and micas such as montmorillonite, illite, muscovite, and chlorite; sulphates, gypsum and jarosite, K2Fe6(OH)12(SO&; carbonates, calcite, siderite, FeC03, and ankerite, FeC03 . MgC03. 2 CaCO3. Since the coal organic matter also contains inorganic elements in varying amounts, any attempts at stoichiometrically accurate calculations cannot be expected to give entirely satisfactory figures for mineral matter in ‘as is’ coal. The main correction factor in these formulae (1.08 or l-09) is due to water of hydration. Out of the fourteen most commonly identified minerals in coal (Spa&man”) seven carry either crystalline water, hydroxyl water, or hydrogen (H+). Dehydration of all these compounds produces water (88.8% 0) of which some evolves at different temperatures over a range of about lOO-600°C. This means that even the low-temperature ash (produced at about 150°C) can be partially dehydrated mineral matter. Oxidation of pyrite may also occur in some cases, affecting the weight loss and thus the moisture determination. Sulphur is a major component in many coals, with concentrations varying between 0.5 and about 6%. Depending on the ashing methods used, a varying portion of the sulphur remains in the ash, probably as calcium sulphate. Most of the organic sulphur, as well as the main portion of the pyritic sulphur, may be expelled in ashing, but the amount is indefinite and varies. In addition, on ashing, the carbonate carbon is expelled as CO2 removing part of the carbon which is useless in terms of the heat value of the coal. Chlorine, in some coals, can be significant; it also may be partially retained in ash. Because of all of these uncertainties, the complexity of the processes involved, and the resultant complications in calculating a material balance for coal ash, in coking and in combustion processes, as well as in relating the ash to the actual ‘as received’ mineral matter in coal, moisture in coal, and oxygen in coal, we believe it important to determine the actual amount of oxygen in coal ash. This information also gives us the stoichiometric value for the summed cations as oxides and silicates plus sulphates. Knowing the oxygen content of the ‘as received’ coal, determined by the same neutron-activation method, as well as the oxygen in the ‘air dried’ coal and the oxygen in the ‘volatiles button’ should permit us to estimate more accurately whether the ‘weight loss’ in moisture determination was due solely to water, to check how much water must have evolved with the volatiles, and, assuming that the nitrogen determination was correct, to estimate more accurately and confirm the data on sulphur in terms of the sulphur loss during ashing and also partially in the volatiles determination, thus giving an estimate for the sulphur that has stayed in the system. The properties of coal ash depend on its composition. Problems with clinkering and boiler-tube slagging, as well as the removal of ash from slag-top furnaces, are related to composition. A better stoichiometric understanding of coal ash in terms of the actual content of oxygen relative to the presence of such fluxing agents as Na20 and K20, and sulphates, may contribute to our understanding of the properties of coal ash. Our belief is that the oxygen determination in coal ash and LT-ash should simplify the analytical calculations and may be able to supplant present empirical and approximation approaches. It may give more relevant estimates of ash characteristics and composition based on one single deter-
206
FUEL,
1977, Vol 56, April
mination of oxygen, rather than on the quite involved calculations which are often very unequal in accuracy. These are reasons to suggest that for some types of coals the oxygen values for high-temperature ash and LT-ash will be very similar, for others quite dissimilar. Retrieval of such information will permit evaluation of the mineral type for the bulk of the minerals present in the coal and the type of decomposition that occurs during ashing. This paper is an attempt to provide preliminary data on the stoichiometry of coal ash and LT-ash. We have used the fast-neutron activation method described in detail elsewhere’3-15. We have suggested in 1974 applying this method to the analysis of coal14.
RESULTS To provide answers to some of the questions raised above we have analysed seven typical ash samples provided to us by Forrest E. Walker of the U.S. Bureau of Mines in Pittsburgh. These ashes represent a variety of types ranging from 0.6 to 21% CaO and O-7 to 14% sulphate sulphur. While six of these ashes were rather dry, containing only O.l-0.3% of hygroscopic water, which is typical for coal and coke ash12 (p 70), the USBM ash D was found to contain 1.25% of minus water (see Table I). For reasons discussed earlier by us5 the oxygen was determined on the ash samples ‘as is’ and recalculated to ‘dry’ basis. The chemical analyses have then been computed to equivalent oxygen and compared to the oxygen determined. We have summed the U.S. Bureau of Mines analyses for these coal ashes including the oxygen data, and listed results in Table 1. From this Table we can see that the chemical analyses do balance very well with the determined oxygen which is, in our opinion, an indication of the good accuracy of the classical silicate analysis methods used. The importance of the minus water determination in recalculation of the oxygen stoichiometry can be seen when one compares the percentages of oxygen determined on the dry basis versus those on the ‘as is’ basis (coal ash D, especially). In this Table the data are reported in oxide percent, since this is the adopted mode of reporting the results of silicate analysis. Stoichiometrically it is more meaningful to use elemental percentages because it is not always known whether all of the sulphur is indeed in the sulphate form; moreover, should considerable fluorine or chlorine be present, an equivalent amount of oxygen should be subtracted from the sum. We have therefore recalculated these analyses into percent of element in Table 2. Certain useful conclusions can be drawn from such data. For example, one may estimate that some sulphur is still present in the sulphide form if the oxide sum calculates considerably higher than 100. Or it may be that, iron being a multivalent element, it is not all in the Fe203 form in the ash. Often, as a matter of fact, in the ignition procedure for iron hydroxide precipitates, iron, in the presence of reducing substances, and if the sample is burned too rapidly, can form some magnetite (Fe304) which, once formed, is not oxidized by further heating. In such cases the calculated stoichiometric sum should tend to be lower than if the iron in the ignited precipitate were completely oxidized. This may be the case in ashes A, G, and D. The recalculation assuming fully oxidized oxides will not then give an accurate result. The differences will depend on the relative percentages of ferric and ferrous oxides in the precipitate. The sum which includes the measured oxygen is thus
A. Volborth, G. E. Miller, C. K. Garner and P. A. Jerabek: Oxygen in coal ash: analysis of ash and mineral matter Table 1
Stoichiometry
SiO2 A1203 Fe203
Ti02 p205
CaO WiO Na20 K20 so3
Sum Odry,cak
0 FNA
(‘as is’) 0 FNA (dry) FNA 0 + metals FNA 0 + metal6
(dry)
Wi
Not corrected for dilution Analysis not available
Tab/e 2
Stoichiometry
Element Si Al Fe Ti P Ca Mg Na K S Odetd
Sumdw Sum,k
0 o$,, !%rna Std dev. for 0 WH
and composition
of coal ash analysed
by the USBM
(in oxide
percent)
A
B
D
F
G
I
J
42.4 27.4 23.1 1.5 0.64 0.6 0.8 0.6 1.8 0.7 9954 44.76 45.55 45.40 100.19 100.37 0.33
39.8 26.2 23.0 1.0 0.80 3.2 0.8 0.8 1.8 2.8 100.20 44.72 45.60 45.46 100.92 101.12 0.33
31.3 12.8 9.4 0.7 0.06 20.8 5.0 5.4 0.4 13.7 99.56 43.43 44.61 44.04 loo*41 100.88 1 a25
42.4 33.9 18.8 1.0 0.10 1.4 0.4 0.5 0.9 0.9 100.30 46.03 46.29 46.23 100.54 100.58 0.15
36.1 21.9 31.1 0.9 0.20 3.2 1.0 0.7 1.1 3.4 99.60 43.07 43.60 43.50 100.02 100.15 0.23
52.0 32.3 6.3 1.5 0.35 2.1 0.6 0.5 1.4 1.5 98.55 47.70 48.30 48.24 99.09 99.17 0.15
b
by percentage
of (Hfl-1
and composition
A
0.28
given
of coal ash analysed
B
4605 45.93
by the USBM
F
D
19.82 1450 16.16 0.90 0.28 0.43 0.48 0.45 1.49 0.28 45.40
18.60 13.86 16.09 0.60 0.35 2.28 048 0.59 1.49 1.12 45.46
14.63 6.77 6.57 0.42 0.26 14.87 3.02 4.01 0.33 5.49 44.04
100.19 99.56 44.76 45.55 100.37 kO.26 0.33 0.04
100.92 100.20 44.72 45.60 101.12 kO.08 0.33 0.04
100.41 99.56 43.43 44.61 100.88 kO.12 1.25 0.14
G 19.82 1794 13.19 0.60 0.04 1 *OO 0.24 0.37 0.75 0.36 46.23
100.54 100.30 46.03 46.29 100.58 0.07 0.15 0.02
(in percent
I
16.87 11.59 21.75 0.54 0.09 2.29 0.60 0.52 0.91 1.36 43.50
24.31 17.09 4.41 0.90 0.15 1.50 0.36 0.37 1.16 1 a60 48.24
100.02 99.60 43.07 43.60 100.15 kO.20 0.23 0.03
99.09 98.55 47.70 48.30 99.17 kO.20 0.15 0.02
Not corrected for dilution by percentage of I-Isgiven, using oxygen by FNA ‘as is’ Analysis not available s Mean oxygen in 8 ashes: 4554 f 3% (dry), 45.70 f 3% (‘as is’) A composite sample of 6 Illinois coals, and a composite sample of 3 Illinois coals, submitted Illinois Geological Survey
of element)
Jb
Coal fly ash NBS 1633
LT-ash C-1567gd
LT-ash C-17215d
4593
45.54c
39.87
46.31
46.05
45.59
+O. 16 0.28 003
kO.10 011 0.01
kO.19 1.56
kO.28 2.68
E
of whether the right assumptions have been made and so provides a test of the quality of both the analytical results and any underlying assumptions. Additional examples can be found; for instance, in samples rich in sulphur some of the sulphur may still be replacing oxygen in the ash, which will result in a high summation if reported on the oxide basis. An accurate total oxygen value may an indicator
by Dr R. R. Ruth
and Or H. J. Gluskoter
of
again bring the sum closer to one hundred. A low sum based on determined oxygen (see ash I, Table 2) may indicate a missing major constituent such as chlorine, which if added in will improve the total analysis. Two analyses of LT-ash are not sufficient to speculate about the variation of oxygen content in these materials; however, the indications are that the range of oxygen con-
FUEL,
1977,
Vol 56, April
207
Oxygen in coal ash: analysis of ash and mineral matter: A. Volborth, G. E. Miller, C. K. Garner and P. A. Jerabek
tent can be quite wide (Table 2). This is not surprising because in LT-ash we have mixtures of clay minerals high in oxygen (about 54%) with the sulphides pyrite and marcasite which contain no oxygen. Also, while quartz has 53% 0, hematite has only 30% 0.
understanding research*.
meaningful. It appears from these limited data that the variation is at least 0.40-0.54. Further work on a larger
number of coal ash matched with proximate and ultimate analyses on the same coal is required in order to explore and interpret fully the stoichiometry of coal ash and coal. Such work may permit considerable simplifications of practical coal analysis and reduce the empirical content of the overall interpretation of coal analysis. This would result in a better
208
FUEL,
1977, Vol 56, April
balance
in solid fuel
REFERENCES Recs, 0. W. ‘Chemistry, Uses, and Limitationsof Coal Analyses’, Illinois State Geol. Survey. Rep. Invest. 220. 1966 Ignasiak, B. S., Ignasiak, T. M. and Berkowitz, N. ‘Advances in Coal Analysis’, Rev. Anal. Chem. 1975, 2, No.3 Gluskoter, H. J. Fuel 1965, 44, 285 Hamrin, C. E., Maa, P. S., Chyi, L. L. and Ehmann, W. D.
CONCLUSIONS From the data retrieved we conclude that it is preferable to determine the oxygen in coal ash as well as in LT-ash from the same coal. Knowing oxygen in both ashes as well as other elemental data will permit a rough estimate of the nature of the mineral matter in coal in terms of whether it is predominantly kaolinitic and clayey, and whether it has much or little sulphide sulphur, iron, and calcium. It is probable that one could roughly estimate the nature of the mineral matter simply from a knowledge only of the oxygen content in the different types of ash. For this more analyses on a wide variety of ashes will be necessary. The sum of the cations, the SO4, S, Cl, F, etc. in ash is also best estimated by the accurate analysis of oxygen. This value may be as reliable as the value derived from the summation of the results of the chemical silicate analyses. The material balances for coal and coke can be estimated better if accurate oxygen data on ash and coal are available rather than using oxygen ‘by difference’ as in coal or oxygen based on assumption of strict stoichiometry in ash when Cl, $ and F are not known. Our data indicate that in approximate recalculations of whole coal analyses where the estimated oxygen in ash and the estimated oxygen due to the crystalline water of the mineral matter are used to calculate the ‘organic’ oxygen, one may use a factor of 0.46 + 0.04 for oxygen in high-temperature ash; the factor for LT-ash, however, may vary too much for a ‘universal factor to be
of the material
Fuel 1975,54,
6 I
IO
Volborth, A., Miller, G. E., Garner, C. K. and Jerabek, P. A., ‘Oxygen Stoichiometry of Some U.S. National Bureau of Standards Standard Reference Materials’, Anal. Chem. 1977 (in press) Block, C. and Dams, R. Anal. Chimica Acta 1974,7 I, 53 Loska, L. and Gorski, L. Radiochem Radioanalyt. Letters 1972,10,315
8 9
Parr, S. W., 77te Analysis of Fuel, Gas, Water, and Lubricants, 4th edn, McGraw-Hill, 1932, pp 49-50 King, J. G., Maries, M. B. and Crossley, H. E. J. Sot. them. Ind. 1936, 55, 217
10 11
12
Selvig, W. A. and Gibson, F. H., ‘Analyses of Ash from United States Coals’, USBM Bull. 567, 1956 Spackman, W., The Nature of Coal and Coal Seams and a Synopsis of North American Coals’, NSF Workshop on the Fundamental Organic Chemistry of Coal, University of Tcnnesseq Knoxville, 1975, pp lo-27 ‘Methods of Analysing and Testing Coal and Coke’, USBM Bull. 638, 1967
13
Volborth. A., Miller, G. E. and Garner.C. K. American 1975, 7, No.10, 87 Volborth. A.. Miller. G. E. and Garner. C. K. Proc. Third Small Accelerator Ckzference, Denton’, Texas, Conf. 14104@P2, 1974, pp 199-208 Volborth, A., Dayal, R., McGhee, P. and Parikh, S., ‘Method for Ultra-Accurate Oxygen Determination for Rare Reference Samples’, @cial Tech. Publ. 539, ASTM. Philadelphia, 1973, pp 120-127 James, W. D., Ehmann, W. D., Hamrin, C. E. and Chyi, L. L. J. Radioanal. Chem. 1976, 32, 195 Laboratory
14 15
16
* The coal ash for this work was received by us in August, 1975, and the analyses performed in August-September, 1975, in order to provide ERDA with preliminary data in our attempt to establish a dedicated facility for analysis of oxygen in coal. Since, similar work by James et al. 16was brought to our attention
George E. Miller,
Claudia K. Garner and Paul A. Jerabek
Department of Chemistry, University of California, Irvine, California 92717, USA f Department of Geology, North Dakota State University, Fargo, North Dakota 58102, USA; and Visiting Professor, University of California, Irvine (Received 15 September 1976)
Oxygen was determined accurately in eight U.S. Bureau of Mines coal ash samples A, B, D, F, G, I, and J, NBS coal fly ash 1633 reference material, and two low-temperature ashes (LTA) from Illinois State Geological Survey. The method uses fast-neutron activation (FNA) analysis employing a dual counting and irradiation system which is essentially free from interferences. The stoichiometric balance based on analyses of the ashes performed by the USBM is calculated and summations given in oxide and element percent. Excellent agreement is found with the chemical data obtained by classical silicate analysis methods. Accurate oxygen determination for coal ash and i-T-ash (or mineral matter) is important for calculation of data in the ultimate analysis of coal as such. Knowledge is required for recalculation of the data on a dry and dry-ash-free basis. The routinely used ‘oxygen by difference’ values are inadequate for accurate work. In order to determine the organic oxygen in coal one also has to correct for oxygen in mineral matter and oxygen in the water removed as moisture. The Parr formula and other methods of empirical estimation are inadequate and may be replaced in some cases by the oxygen determination. The complete data provide a quantitative basis for stoichiometric interpretation of coal analyses. It was found that the eight coal-ash samples analysed contained 45.5 f 3% oxygen. Since these ashes represent a large variety of U.S. coals, this figure can be used as an estimate for recalculation and evaluation of the proximate and ultimate coal analyses. It is better, however, to use values actually determined in ash by the rapid fast-neutron activation method. This permits a better estimation of the sum of cations plus sulphates in the ash.
Ash is one of the components determined in the process called the proximate analysis of coal, which also consists of the determination of the moisture and the volatile matter. The fourth component estimated in the proximate analysis is labelled fuced carbon. It is obtained from the sum of the other three components by subtraction from 100. Ash is a product of the decomposition of silicate, sulphide, sulphate, carbonate, and oxide minerals which form the bulk of the inorganic matter in coal. This decomposition is accomplished by burning the coal powder in an oxygen atmosphere at -ISO-850°C. The ash is thus the noncombustible residue that forms when coal is ignited under controlled conditions’*‘. Fundamentally, the sources of ash can be subdivided into ash derived from adventitious mineral matter: the ‘inherent ash’, formed through the decomposition of such minerals as kaolinite, montmorillonite, pyrite, gypsum, jarosite, calcite, and the ‘organic component ash’ derived from the decomposition of plant material contaming bound inorganic elements. The last type of ash is usually relatively insignificant. Once the ashing is completed at the high temperatures used, no distinction cur be made as to the relative quantities of the inherent ash versus the organic ash.
* Supported by Contracts E(O4-3)-34-241 and E(1 l-1)-2898, Energy Research and Development Administration
204
FUEL,
1977, Vol 56, April
U.S
A more serious problem results from the fact that the moisture due to the crystalline and (OH) water in such minerals as kaolinite, montmorillonite,illite, muscovite, chlorite, gypsum, and jarosite is driven off at such temperatures. Since ‘moisture’ that escapes above 105°C is also included in the ‘volatile matter’ data obtained by rapid heating to 950-965’C and recording the weight loss, the summing of the three components of the proximate analysis to determine the fixed carbon is stoichiometrically incorrect. A better way would be to determine the total oxygen in coal before ashing, then in the ash, and then in the volatile matter and coke as well as in the mineral matter estimated by the Parr formula’s or the low-temperature ashing method (LTA) developed by Gluskoter3. The determination of oxygen in coal ash versus LT-ash would then enable the coal chemist to estimate realistically the stoichiometric balance. Block and Dams6 have analysed 20 coal ashes obtained from Belgian coals for oxygen and silicon using a fast-neutron activation technique. The mean oxygen value of these determinations is 46.14% 0 with a total range of 41.52-48.25%. For these ashes one could use an estimated approximate factor for oxygen of 0.46 + O-04. A rough correlation between the ash yield and the oxygen content in coal (as well as between the ash and the silicon content also noted before by Loska and Gorski’) was found by these authors. This means that a determination of total oxygen in coal
A. Volborth, G. E. Miller, C. K, Garner and P. A. Jerabek: Oxygen in coal ash: analysis of ash and mineral matter
may in a limited sense be used to estimate its ash, or vice versa. One of the main uncertainties in the high-temperature ashing procedures (in addition to the behaviour of water) is the behaviour of sulphur. Sulphur occurs in coal as organic sulphur, sulphide or pyrite sulphur, sulphate sulphur, and occasionally elemental sulphur. The organic sulphur may oxidize mostly and evolve as SO;! and SO3 during the ashing. The sulphide sulphur is also oxidized with pyrite, FeS2, transformed ideally to Fe203 with evolution of SO2 and SO3 gas. However, in coals rich in calcium carbonate and if the ashing is so rapid as to decompose the carbonate to calcium oxide while the evolution of SO3 takes place, calcium sulphate may form, which is difficult to decompose even at the high temperatures of ashing. Thus, considerable and varying amounts of sulphur may be retained in coal ash. Calcium-rich ashes usually have considerable sulphur retained (see USBM ash D, B, and G in Table 2). The presence of much calcium and sulphur affects the composition of coal ash somewhat. It seems to diminish the total oxygen content of the ash (see USBM ash D, B, and G in Table 2). This may affect the estimates of oxygen content in ash. In low-temperature ash rich in pyrite, the total oxygen content would be expected to be lower than in the corresponding high-temperature ash. Should the LT-ash be predominantly composed of kaolinite, montmorillonite, quartz, or gypsum, one would expect a relatively high percentage of oxygen. In fact, Hamrin et a1.4 have analysed one ash obtained from Kentucky No.9 coal (3.6% S) and found 42.9 + 0.4% oxygen. If the coal mineral matter should be composed mostly of clayey or shaley components, one might predict oxygen contents as high as those determined by us in the U.S. NBS flint clay, 99a (55.40% 0) and plastic clay, 98a (54.75% 0)‘. The range of oxygen content possible for LT-ash is thus equal to about *lo% relative. In ashrich coals (=15-20% ash) this could mean a 2% error in total oxygen due to ash type if a simple estimate is used. This would cause a serious mistake in the estimation of organic oxygen. Thus we contend that accurate determination of oxygen in coal before ashing, in the coal ash, and in the corresponding LT-ash should provide a better value for estimating the organic oxygen in coal on a moisture- and ashfree (daf) basis. A less accurate method of estimating mineral matter (MM) is the empirical one using the Parr formulas, based on the ash (A) and the total sulphur content (S) in coal: MM = 1.08 A + 0.55 S In this formula the total water of hydration of the inorganic minerals in the coal is assumed to have been 8% and the factor 1.08 is thus derived. Multiplying the ash by this number is then assumed to restore the water of hydration lost during ashing. This was shown to be a good average for 19 Illinois and 11 high-volatile coals from the Eastern United States (Rees’, pp.20-24; Selvig and Gibson”). The factor 0.55 used for sulphur is a combination of two corrections and a compensation for sulphur not in the pyritic form. It takes into account the transformation of 2 FeS2 to Fe203 during ashing, where 3 0 E 4 S, or 48/128 = 318, which means that only 3/8 of the weight of the sulphur in the mineral matter has been replaced by oxygen so that 5/S must be added to the ash to compensate for this loss: A + 5/8 S. It also corrects for the fact that pyrite, FeS2, is not hydrated, by equating the Fe203 con-
tent in ash with its equivalent of sulphur atoms in the coal, or 160/128 = 10/8. Assuming 8% water of hydration, the sulphur correction in mineral matter becomes 0.08 x (A - 10/8 S). Thus the Parr formula becomes: MM = A + 5/8 S + 0.08 (A - 10/8 S) =
To compensate in part for any sulphur in coal not in pyritic form (‘organic’ sulphur) and to further simplify the calculation, Parr proposed to use the fraction 22/40 (0.55) instead of 21/40 (0.525). It may be noted, however, that the water of hydration of the average of nineteen Illinois coals was found by Selvig and Gibson” to be 6*8%, whereas in the eleven Eastern-U.S. high-volatile coals they found 9.87%. While this may not greatly affect the calculated values of calorific value (MM free) using the Parr formula for these coals (14 279 Btu versus 14 317 Btu, p 23, 1) in stoichiometric summation and interpretation of coal analyses where oxygen is determined rather than estimated by difference, an absolute difference of 3% in the water of hydration value would yield a 0.6% difference in the total oxygen content for a coal with 20% of mineral matter. This is significant when estimating the ‘organic oxygen’ in coal on a ‘dry mineral-matter-free’ basis. Therefore, direct determination of oxygen in coal ash (as well as in LT-ash) should be preferable to the Parr mineral matter evaluation as one may then estimate precisely the percentage of cations plus CaS04 in ash. If the total oxygen and sulphur have been determined in the coal, the volatiles, the coke button, and the ash, one can calculate approximately the loss of sulphur to the atmosphere and examine the possible dependence of this loss on the organic content of the coal. A more complex but stoichiometrically more correct formula for mineral matter calculation has been proposed by King et al. 9 : %MM=109A+0.5Spyr
t 0.8 CO, - 1.1 SO, in ash
+ SO, in coal + 0.5 Cl where A = % ash in coal; SPYr= % S, pyritic, in coal; CO2 = % of mineral CO;! in coal; SO3 in ash = % SO3 in ash, SO3 in coal = % SO3 in coal; Cl = % Cl in coal This formula uses similar reasoning to that used in the derivation of the Parr formula, but it also takes into account the CO2 of the carbonates, the SO3 in the ash and in the coal, as well as the chlorine. These two methods of mineral matter estimation use formulae based partially on assumed stoichiometry and make subsequent simplifications. Segments of these formulae are based on accurate and meticulously derived values; other segments are not. For example, in the derivation of the Parr formula, it is assumed that the coal mineral matter is composed solely of kaolinite (Al203 . 2 SiO2 . 2 H20), silica (SiOz), and pyrite (FeS2); it is further assumed that the crystalline water is due solely to kaolinite (water of hydration = Al203 x 0.3535) that the only alumina (Al2O3) in ash is that in kaolinite (kaolinite = Al203 x 2.532) that the excess of silica (SiO2) is equal to the total SiO2 in ash minus the SiO2 of the kaolinite (SiO2 of kaolinite = kaolinite x 0.4655), that all of the Fe203 in ash is due to the pyrite in the coal (FeS2 = Fe203 x 1.503), and further that all the sulphur in coal is pyritic sulphur.
FUEL,
1977, Vol 56, April
205
Oxygen in coal ash: analysis of ash and mineral matter: A. Volborth, G. E. Miller, C. K. Garner and P. A. Jerabek
All these assumptions are approximations acceptable for many coals, however, since coal mineral matter is known to contain other clay minerals and micas such as montmorillonite, illite, muscovite, and chlorite; sulphates, gypsum and jarosite, K2Fe6(OH)12(SO&; carbonates, calcite, siderite, FeC03, and ankerite, FeC03 . MgC03. 2 CaCO3. Since the coal organic matter also contains inorganic elements in varying amounts, any attempts at stoichiometrically accurate calculations cannot be expected to give entirely satisfactory figures for mineral matter in ‘as is’ coal. The main correction factor in these formulae (1.08 or l-09) is due to water of hydration. Out of the fourteen most commonly identified minerals in coal (Spa&man”) seven carry either crystalline water, hydroxyl water, or hydrogen (H+). Dehydration of all these compounds produces water (88.8% 0) of which some evolves at different temperatures over a range of about lOO-600°C. This means that even the low-temperature ash (produced at about 150°C) can be partially dehydrated mineral matter. Oxidation of pyrite may also occur in some cases, affecting the weight loss and thus the moisture determination. Sulphur is a major component in many coals, with concentrations varying between 0.5 and about 6%. Depending on the ashing methods used, a varying portion of the sulphur remains in the ash, probably as calcium sulphate. Most of the organic sulphur, as well as the main portion of the pyritic sulphur, may be expelled in ashing, but the amount is indefinite and varies. In addition, on ashing, the carbonate carbon is expelled as CO2 removing part of the carbon which is useless in terms of the heat value of the coal. Chlorine, in some coals, can be significant; it also may be partially retained in ash. Because of all of these uncertainties, the complexity of the processes involved, and the resultant complications in calculating a material balance for coal ash, in coking and in combustion processes, as well as in relating the ash to the actual ‘as received’ mineral matter in coal, moisture in coal, and oxygen in coal, we believe it important to determine the actual amount of oxygen in coal ash. This information also gives us the stoichiometric value for the summed cations as oxides and silicates plus sulphates. Knowing the oxygen content of the ‘as received’ coal, determined by the same neutron-activation method, as well as the oxygen in the ‘air dried’ coal and the oxygen in the ‘volatiles button’ should permit us to estimate more accurately whether the ‘weight loss’ in moisture determination was due solely to water, to check how much water must have evolved with the volatiles, and, assuming that the nitrogen determination was correct, to estimate more accurately and confirm the data on sulphur in terms of the sulphur loss during ashing and also partially in the volatiles determination, thus giving an estimate for the sulphur that has stayed in the system. The properties of coal ash depend on its composition. Problems with clinkering and boiler-tube slagging, as well as the removal of ash from slag-top furnaces, are related to composition. A better stoichiometric understanding of coal ash in terms of the actual content of oxygen relative to the presence of such fluxing agents as Na20 and K20, and sulphates, may contribute to our understanding of the properties of coal ash. Our belief is that the oxygen determination in coal ash and LT-ash should simplify the analytical calculations and may be able to supplant present empirical and approximation approaches. It may give more relevant estimates of ash characteristics and composition based on one single deter-
206
FUEL,
1977, Vol 56, April
mination of oxygen, rather than on the quite involved calculations which are often very unequal in accuracy. These are reasons to suggest that for some types of coals the oxygen values for high-temperature ash and LT-ash will be very similar, for others quite dissimilar. Retrieval of such information will permit evaluation of the mineral type for the bulk of the minerals present in the coal and the type of decomposition that occurs during ashing. This paper is an attempt to provide preliminary data on the stoichiometry of coal ash and LT-ash. We have used the fast-neutron activation method described in detail elsewhere’3-15. We have suggested in 1974 applying this method to the analysis of coal14.
RESULTS To provide answers to some of the questions raised above we have analysed seven typical ash samples provided to us by Forrest E. Walker of the U.S. Bureau of Mines in Pittsburgh. These ashes represent a variety of types ranging from 0.6 to 21% CaO and O-7 to 14% sulphate sulphur. While six of these ashes were rather dry, containing only O.l-0.3% of hygroscopic water, which is typical for coal and coke ash12 (p 70), the USBM ash D was found to contain 1.25% of minus water (see Table I). For reasons discussed earlier by us5 the oxygen was determined on the ash samples ‘as is’ and recalculated to ‘dry’ basis. The chemical analyses have then been computed to equivalent oxygen and compared to the oxygen determined. We have summed the U.S. Bureau of Mines analyses for these coal ashes including the oxygen data, and listed results in Table 1. From this Table we can see that the chemical analyses do balance very well with the determined oxygen which is, in our opinion, an indication of the good accuracy of the classical silicate analysis methods used. The importance of the minus water determination in recalculation of the oxygen stoichiometry can be seen when one compares the percentages of oxygen determined on the dry basis versus those on the ‘as is’ basis (coal ash D, especially). In this Table the data are reported in oxide percent, since this is the adopted mode of reporting the results of silicate analysis. Stoichiometrically it is more meaningful to use elemental percentages because it is not always known whether all of the sulphur is indeed in the sulphate form; moreover, should considerable fluorine or chlorine be present, an equivalent amount of oxygen should be subtracted from the sum. We have therefore recalculated these analyses into percent of element in Table 2. Certain useful conclusions can be drawn from such data. For example, one may estimate that some sulphur is still present in the sulphide form if the oxide sum calculates considerably higher than 100. Or it may be that, iron being a multivalent element, it is not all in the Fe203 form in the ash. Often, as a matter of fact, in the ignition procedure for iron hydroxide precipitates, iron, in the presence of reducing substances, and if the sample is burned too rapidly, can form some magnetite (Fe304) which, once formed, is not oxidized by further heating. In such cases the calculated stoichiometric sum should tend to be lower than if the iron in the ignited precipitate were completely oxidized. This may be the case in ashes A, G, and D. The recalculation assuming fully oxidized oxides will not then give an accurate result. The differences will depend on the relative percentages of ferric and ferrous oxides in the precipitate. The sum which includes the measured oxygen is thus
A. Volborth, G. E. Miller, C. K. Garner and P. A. Jerabek: Oxygen in coal ash: analysis of ash and mineral matter Table 1
Stoichiometry
SiO2 A1203 Fe203
Ti02 p205
CaO WiO Na20 K20 so3
Sum Odry,cak
0 FNA
(‘as is’) 0 FNA (dry) FNA 0 + metals FNA 0 + metal6
(dry)
Wi
Not corrected for dilution Analysis not available
Tab/e 2
Stoichiometry
Element Si Al Fe Ti P Ca Mg Na K S Odetd
Sumdw Sum,k
0 o$,, !%rna Std dev. for 0 WH
and composition
of coal ash analysed
by the USBM
(in oxide
percent)
A
B
D
F
G
I
J
42.4 27.4 23.1 1.5 0.64 0.6 0.8 0.6 1.8 0.7 9954 44.76 45.55 45.40 100.19 100.37 0.33
39.8 26.2 23.0 1.0 0.80 3.2 0.8 0.8 1.8 2.8 100.20 44.72 45.60 45.46 100.92 101.12 0.33
31.3 12.8 9.4 0.7 0.06 20.8 5.0 5.4 0.4 13.7 99.56 43.43 44.61 44.04 loo*41 100.88 1 a25
42.4 33.9 18.8 1.0 0.10 1.4 0.4 0.5 0.9 0.9 100.30 46.03 46.29 46.23 100.54 100.58 0.15
36.1 21.9 31.1 0.9 0.20 3.2 1.0 0.7 1.1 3.4 99.60 43.07 43.60 43.50 100.02 100.15 0.23
52.0 32.3 6.3 1.5 0.35 2.1 0.6 0.5 1.4 1.5 98.55 47.70 48.30 48.24 99.09 99.17 0.15
b
by percentage
of (Hfl-1
and composition
A
0.28
given
of coal ash analysed
B
4605 45.93
by the USBM
F
D
19.82 1450 16.16 0.90 0.28 0.43 0.48 0.45 1.49 0.28 45.40
18.60 13.86 16.09 0.60 0.35 2.28 048 0.59 1.49 1.12 45.46
14.63 6.77 6.57 0.42 0.26 14.87 3.02 4.01 0.33 5.49 44.04
100.19 99.56 44.76 45.55 100.37 kO.26 0.33 0.04
100.92 100.20 44.72 45.60 101.12 kO.08 0.33 0.04
100.41 99.56 43.43 44.61 100.88 kO.12 1.25 0.14
G 19.82 1794 13.19 0.60 0.04 1 *OO 0.24 0.37 0.75 0.36 46.23
100.54 100.30 46.03 46.29 100.58 0.07 0.15 0.02
(in percent
I
16.87 11.59 21.75 0.54 0.09 2.29 0.60 0.52 0.91 1.36 43.50
24.31 17.09 4.41 0.90 0.15 1.50 0.36 0.37 1.16 1 a60 48.24
100.02 99.60 43.07 43.60 100.15 kO.20 0.23 0.03
99.09 98.55 47.70 48.30 99.17 kO.20 0.15 0.02
Not corrected for dilution by percentage of I-Isgiven, using oxygen by FNA ‘as is’ Analysis not available s Mean oxygen in 8 ashes: 4554 f 3% (dry), 45.70 f 3% (‘as is’) A composite sample of 6 Illinois coals, and a composite sample of 3 Illinois coals, submitted Illinois Geological Survey
of element)
Jb
Coal fly ash NBS 1633
LT-ash C-1567gd
LT-ash C-17215d
4593
45.54c
39.87
46.31
46.05
45.59
+O. 16 0.28 003
kO.10 011 0.01
kO.19 1.56
kO.28 2.68
E
of whether the right assumptions have been made and so provides a test of the quality of both the analytical results and any underlying assumptions. Additional examples can be found; for instance, in samples rich in sulphur some of the sulphur may still be replacing oxygen in the ash, which will result in a high summation if reported on the oxide basis. An accurate total oxygen value may an indicator
by Dr R. R. Ruth
and Or H. J. Gluskoter
of
again bring the sum closer to one hundred. A low sum based on determined oxygen (see ash I, Table 2) may indicate a missing major constituent such as chlorine, which if added in will improve the total analysis. Two analyses of LT-ash are not sufficient to speculate about the variation of oxygen content in these materials; however, the indications are that the range of oxygen con-
FUEL,
1977,
Vol 56, April
207
Oxygen in coal ash: analysis of ash and mineral matter: A. Volborth, G. E. Miller, C. K. Garner and P. A. Jerabek
tent can be quite wide (Table 2). This is not surprising because in LT-ash we have mixtures of clay minerals high in oxygen (about 54%) with the sulphides pyrite and marcasite which contain no oxygen. Also, while quartz has 53% 0, hematite has only 30% 0.
understanding research*.
meaningful. It appears from these limited data that the variation is at least 0.40-0.54. Further work on a larger
number of coal ash matched with proximate and ultimate analyses on the same coal is required in order to explore and interpret fully the stoichiometry of coal ash and coal. Such work may permit considerable simplifications of practical coal analysis and reduce the empirical content of the overall interpretation of coal analysis. This would result in a better
208
FUEL,
1977, Vol 56, April
balance
in solid fuel
REFERENCES Recs, 0. W. ‘Chemistry, Uses, and Limitationsof Coal Analyses’, Illinois State Geol. Survey. Rep. Invest. 220. 1966 Ignasiak, B. S., Ignasiak, T. M. and Berkowitz, N. ‘Advances in Coal Analysis’, Rev. Anal. Chem. 1975, 2, No.3 Gluskoter, H. J. Fuel 1965, 44, 285 Hamrin, C. E., Maa, P. S., Chyi, L. L. and Ehmann, W. D.
CONCLUSIONS From the data retrieved we conclude that it is preferable to determine the oxygen in coal ash as well as in LT-ash from the same coal. Knowing oxygen in both ashes as well as other elemental data will permit a rough estimate of the nature of the mineral matter in coal in terms of whether it is predominantly kaolinitic and clayey, and whether it has much or little sulphide sulphur, iron, and calcium. It is probable that one could roughly estimate the nature of the mineral matter simply from a knowledge only of the oxygen content in the different types of ash. For this more analyses on a wide variety of ashes will be necessary. The sum of the cations, the SO4, S, Cl, F, etc. in ash is also best estimated by the accurate analysis of oxygen. This value may be as reliable as the value derived from the summation of the results of the chemical silicate analyses. The material balances for coal and coke can be estimated better if accurate oxygen data on ash and coal are available rather than using oxygen ‘by difference’ as in coal or oxygen based on assumption of strict stoichiometry in ash when Cl, $ and F are not known. Our data indicate that in approximate recalculations of whole coal analyses where the estimated oxygen in ash and the estimated oxygen due to the crystalline water of the mineral matter are used to calculate the ‘organic’ oxygen, one may use a factor of 0.46 + 0.04 for oxygen in high-temperature ash; the factor for LT-ash, however, may vary too much for a ‘universal factor to be
of the material
Fuel 1975,54,
6 I
IO
Volborth, A., Miller, G. E., Garner, C. K. and Jerabek, P. A., ‘Oxygen Stoichiometry of Some U.S. National Bureau of Standards Standard Reference Materials’, Anal. Chem. 1977 (in press) Block, C. and Dams, R. Anal. Chimica Acta 1974,7 I, 53 Loska, L. and Gorski, L. Radiochem Radioanalyt. Letters 1972,10,315
8 9
Parr, S. W., 77te Analysis of Fuel, Gas, Water, and Lubricants, 4th edn, McGraw-Hill, 1932, pp 49-50 King, J. G., Maries, M. B. and Crossley, H. E. J. Sot. them. Ind. 1936, 55, 217
10 11
12
Selvig, W. A. and Gibson, F. H., ‘Analyses of Ash from United States Coals’, USBM Bull. 567, 1956 Spackman, W., The Nature of Coal and Coal Seams and a Synopsis of North American Coals’, NSF Workshop on the Fundamental Organic Chemistry of Coal, University of Tcnnesseq Knoxville, 1975, pp lo-27 ‘Methods of Analysing and Testing Coal and Coke’, USBM Bull. 638, 1967
13
Volborth. A., Miller, G. E. and Garner.C. K. American 1975, 7, No.10, 87 Volborth. A.. Miller. G. E. and Garner. C. K. Proc. Third Small Accelerator Ckzference, Denton’, Texas, Conf. 14104@P2, 1974, pp 199-208 Volborth, A., Dayal, R., McGhee, P. and Parikh, S., ‘Method for Ultra-Accurate Oxygen Determination for Rare Reference Samples’, @cial Tech. Publ. 539, ASTM. Philadelphia, 1973, pp 120-127 James, W. D., Ehmann, W. D., Hamrin, C. E. and Chyi, L. L. J. Radioanal. Chem. 1976, 32, 195 Laboratory
14 15
16
* The coal ash for this work was received by us in August, 1975, and the analyses performed in August-September, 1975, in order to provide ERDA with preliminary data in our attempt to establish a dedicated facility for analysis of oxygen in coal. Since, similar work by James et al. 16was brought to our attention