THE COLLIERY GUARDIAN AN® JOURNAL OF THE COAL AND IRON TRADES. Vol. CXVI. FRIDAY, OCTOBER 11, 1918, No. 3015. Calorific Value and Ash Yield of Coal Samples from the same Seam.* By T. J. DRAKELEY. In a former research on coal washing by the author (Transactions Inst.M.E. 1918, vol. 54, pages 418-459), a number of calorific value estimations were made. At any one of the coal washing plants visited in connection with the above research a sample of raw coal, samples of each class of washed coal, and a sample of the slime were collected. These samples may be assumed to have formed a series of mixtures of the same pure coal with varying quantities of impurities, . and as the calorific value and the ash content of each sample were determined, an admirable opportunity was offered for studying the influence of the impurities on the calorific value. At that time it was noted (loc cit. p. 442) that for each set of samples from a washer the calorific value was almost proportionate to the quantity of ash-free material in the coal, that is to say:— Calorific value X 100 An a-------z---------t = constant. 100—percentage ot ash This statement obviously conveys the idea that zero calorific value is only reached when the percentage of ash becomes 100; but when the calorific values and the corresponding ash contents were plotted, it was evident that the above equation was only an approximation. It was observed that the points lay fairly evenly in .a straight band across the diagram, and that if an average straight line were drawn amongst the points, it cut the horizontal axis at 89 instead of at the 100 point. This indicated that zero calorific value was associated with an average ash content of about 89 per cent. Hence a correction in the approximate conclusion drawn in the paper was necessary. Th§ chief impurity in the raw coa^ is shale, and its presence reduces the calorific value of the sample, and increases the yield of ash. The shale — as distinct from carbonaceous shale — is incombustible, and therefore possesses zero calorific value. Hence as the quantity of shale increases, the calorific value decreases until it reaches zero for a sample of pure shale. The percentage of ash obtained by igniting shale never reaches 100 per cent., although the value approximates to that number. Therefore, it is to be concluded that any line drawn to show the relation between the calorific value and the ash content should cut the horizontal axis at a point representing the percentage of ash yielded by the shale. To render the results mentioned in the previous paper more complete, it was necessary to determine, if possible, the ash obtained by igniting samples of shale from each of the washing plants. Samples of shale were carefully selected so as to exclude any carbonaceous matter, and therefore could be assumed to possess zero calorific value. 6 The results of the estimations of the percentages of ash in the shale samples are included in Table I., which also gives the calorific value and ash estimations made in connection with the recent research on coal washing, together with many determinations not included in the first named paper. On plotting the values given in Table I.—it was found that the average ash content which gave zero calorific value was 88’4 per cent., whereas the average line cut the horizontal axis at 89 per cent. This difference, which is really unusually small, may be accounted for in several ways. First, an equal number of points were not plotted for each of the 15 coals, and the preponderance is in favour of those giving a high ash content for zero calorific value. Indeed, if in determining the average, allowance be made for the number of points plotted for each sample, the value of the ash content corresponding to zero calorific value is 88’6 per cent. Secondly, as pointed out previously (Trans. Institution of Mining Engineers, 1918, vol. 84, pp. 441, 457), the determinations of the calorific values in connection with that paper were not carried out as precisely as the author desired. The calorific values were sufficiently accurate for the purposes of the research then in hand, and the results, with the addition of the other values, have proved of the greatest interest as indicating that the probable relation between calorific value and ash content may be represented by a straight line graph. It must not be concluded that the author considers that a more accurate method of determining the calorific values would have resulted in all the points arranging themselves on the average line. Such an effect would most certainly not have been produced. On the other hand, there seems to be no doubt that more accurate results would have indicated tha*t the points for each set of samples were on a separate straight line. Parr ^(Illinois State Geological Survey, 1909, Bulletin No. 16, p. 212 and (1914, Bulletin No. 29, p. 40) has shown that, after making allowances for impurities such as ash and moisture, the calorific value is remarkably constant for samples of coal from a particular seam over a considerable area. Therefore, for each seam of coal there is a definite line on the diagram from which * Paper read before the Manchester Geological and Mining Society held on Tuesday last. the calorific value may be read when the ash content is known. This, however, is only feasible so long as pre- cautions are taken to avoid any errors being introduced by the presence of moisture in varying quantities. In order to prove whether the relation between calorific value and ash content could be represented by a straight line graph, the following experiments were made:— A large sample of coal was crushed so as to pass through a sieve of a mesh of 120 to the inch and was mixed thoroughly. The powder was air dried in the chemical laboratory of the Wigan and District Mining and Technical College. In the laboratory the tempera- ture’ was maintained fairly evenly at 60 degs. Fahr., whilst the hygrometric state of the atmosphere was moderately constant (relative humidity' — 60-70 per cent). In this way the influence on the calorific value of the varying quantities of moisture was eliminated to a considerable extent. This was an important detail of the experiments, as it was found that, unless parti- cular care was taken in regard to the air-drying, the results failed to show any uniformity. This is due to the fact that the presence of moisture not only reduces the calorific value, but also reduces the percentage of ash yielded by the coal. * Table No. 1.—Calorific Values and Ash Contents of Coal Samples. Refer - Ash Calorific ence per value. No. cent. B.Th.U. 1. 2'61 . .. 14,404 3'91 .. 14,128 3'93 . . .. 14,114 6'71 13,639 11'63 .. 13,034 22'20 .. 11,366 80'02 — 2. 3'51 .. 14,366 6'61 .. ,. 13,883 6'93 .. 13,858 7'18 .. .. 13,822 8'35 13,775 9’62 .. 13,2 1 21'72 .. 11,448 28'33 10,294 90'17 — 3. 3'92 .. .. 13,689 8'89 .. 12,902 13'55 ,. 12,422 14'16 12,352 23'90 .. 11,181 24'02 11,160 90'88 — 4. 3'28 . .. 14,116 8'65 .. .. 13,214 8'66 .. 13,190 9'75 .. . 13,019 15'73 11,983 25'04 .. 10,356 91'96 .. — 5. 3'27 .. 14,107 8'64 . 13,433 8'73 .. . 13 396 11'32 .. . 12,794 15'39 .. . 12,094 26'49 .. 10,188 91'05 .. — 6. 4'35 . 13,883 8'58 .. . 13,299 8'97 .. . 13,234 10'90 .. . 12,935 17'37 11,695 26'35 .. 10,306 89'42 .. — 7. ■ 4'63 .. 14,170 10 07 .. 13,397 18'03 .. 12,004 31'22 .. 9,922 90'45 .. — Refer- Ash , Calorific ence pec value. No. cent. B.Th.U. 8. 5'01 .. . 14,236 7'53 .. 13,853 9'21 ,. 13,622 14'23 12,739 16'51 12,141 28'37 . 10,387 90'47 — 9. 4'13 .. . 13,928 11'46 . 12,832 16'74 .. 12,008 20'06 .. . 11,500 86'61 — 10. 4'36 14,042 7'19 .. . 13,565 7'70 13,478 10'49 . 13,052 12'48 12,490 23'36 . 10,807, 88'82 — 11. 3'35 13,826 8'86 13,023 17'67 . 11,606 34'36 .. 8,947 87'47 .. - 12. 2'63 .. 14,386 4'73 14,042 9'13 .. 13,252 15'99 12,209 89'12 .. — 13. *3’12 .. 14,373, 4'47 14,22» 11'35 12,910 13'71 .. 12,708 26'36 .. " 10,495 86'54 — 14. 3'64 .. 14,008 9'93 .. . 13,057 19'98 11,576 45'73 .. 7,512 81'14 — 15. 3'47 .. . 13,934 7'78 . 13,291 8'03 13,216 10'76 .. 12,909 19'62 .. 11,444 25'71 . 10^361 91'38 ... — After the powdered coal had been air-dried satis- factorily, portions of it were mixed with different quantities of pure precipitated calcium carbonate (chalk). By this means, samples of the same coal yielding varying proportions of ash were obtained. The calorific value of each sample was determined, using the Mahler-Cook bomb calorimeter, and pre- cautions were taken to ensure the utmost accuracy. In order to effect the complete combustion of those mixtures containing only small quantities of coal, sufficient pure naphthalene (calorific value 17,415 B.Th.U.) was admixed to give approximately the same rise in temperature as with the coal samples. The heat from the combustion of the naphthalene was deducted afterwards from the total. A radiation correction, calculated from a graph of the temperature readings taken at equal intervals of time, was made for each determination. The percentage of ash obtaining from each sample was estimated by incineration in an open muffle furnace. Tiie results are tabulated in Table II.:— Table II.—Calorific Values and Ash Yields of Mixtures of Coals with Calcium Carbonate. sample No. Ash. Calorific value. Per cent. B.Th.U. 1 3’38 14,191 2 13'89 11,346 3 24'44 8,520 4 34'95 5,675 5 45'46 2,821 6 56'00 — t > If in this particular case the question of heat is con- sidered, it is obvious that, theoretically, zero calorific Viilue should be reached wheii the heat evolved by the combustion of the coal is just sufficient to decompose the calcium carbonate to form lime. According to Kaye and Laby (Tables of Physical and 'Chemical Constants, 1916 2nd edition, p. 62), the molecular heats of formation of the substances under consideration are as follow :— Calories per gramme molecule. Calcium carbonate...... 270 x 103 Calcium oxide ......... 131 x 103 Carbon dioxide........... 97 x 103 Hence the heat absorbed by the decomposition of calcium carbonate to form calcium oxide' and calcium dioxide is 42 x 103 calories per gramme molecule. That is to say, to decompose one pound of calcium carbonate 756 B.Th.U. are required. Supposing a pound weight of the mixture of coal and calcium carbonate to contain x lb. of calcium carbonate, the heat evolved by the combustion of the coal is 14,191 X (1—a?) B.Th.U.; whilst the heat absorbed by the decomposition of the calcium carbonate is 756 X x B.Th.U. For zero calorific value : 14,191 x (1-x) = 756 k x. x = 0 95 lb. ' Since 1 lb. of calcium carbonate decomposes to give 0'56 lb. of calcium oxide, the percentage of ash obtained from such a mixture as that determined above would be approximately 53 per cent. It would be observed that zero calorific value is not reached at 53 per cent., but at 56 per cent., the yield of ash from pure chalk. That indicates that in the bomb calorimeter decomposition of the calcium carbonate had been prevented. Although this had been considered possible, it was thought that, owing to the high temperature of the combustion in the bomb—sufficient to melt platinum wire (m. pt = 3,180 degs. Fahr.)—some evidence of the decomposition of the calcium carbonate would be forth- coming. Actually, the oalcium carbonate appeared to have sintered into a' rather non-coherent mass, possibly with partial decomposition (compare Bocke, Zeitsch. anorg. Chem. 1906, vol. 50, p. 247). No doubt, while the mass cooled, the resulting lime re-absorbed carbon dioxide to re-form almost completely the full quantity of calcium carbonate with evolution of the same quantity of heat which had been necessary to bring about the decomposition. A consideration of the partial pressure of the carbon dioxide within the bomb also provides evidence in favour of the non-decomposition of the calcium carbonate. It may be assumed that, in each calorific value determina- tion, the weight of carbon burnt in the bomb was about one gramme. From this weight of carbon 1,860 c.c. of carbon dioxide would be produced at normal tempera- ture and pressure. Within the bomb the pressure is about 25 atmospheres, and consequently the carbon dioxide would occupy a volume of about 7b c.c. The volume of the bomb is approximately 65 c.c. and, there- fore, the partial pressure due to the carbon dioxide is 78 -— x 25, or 3 atmospheres. At. this pressure the 650 temperature required to liberate any carbon dioxide from the calcium carbonate is exceptionally high. Even for a pressure of carbon dioxide equal to 1 atmosphere, the corresponding equilibrium temperature is 1,517 degs. Fahr. (Le Chatelier, Compt. Bend., 1886, vol. 102, p. 1,234). Consequently, as.the final temperature of the bomb is only about 7 degs. Fahr, above room temperature, no decomposition of the carbonate is likely to occur during the calorific value determinations. This explains satis- factorily the fact that in the graphical representation, the horizontal axis is cut at a point representing an ash content of 56 per cent., instead of at the calculated value, 53 per cent. ’ When coal is burnt in the ordinary ffi&egrate the partial pressure of the carbon dioxide is||p sufficient to prevent the decomposition of any calcium carbonate in the fuel. Consequently as soon as the temperature rises above the decomposition temperature 1,022 Fahr, the carbonate commences to decompose, and as the draught sweeps the gaseous products away, the reaction proceeds to give calcium oxide. Howevei;, when the ashes fall from the grate they are cooled, and the presence of calcium carbonate in the ashes indicates that some of t» e lime has reabsorbed carbon dioxide to form calcium carbonate. The process is not complete, and the author found that, after chalk had been thrown upon an ordinary hot household fire, the resulting ashes contained about 80 per cent, of lime. In the bomb calorimeter the decomposition of the carbonate is prevented entirely, and therefore no