PART VI.
Temperature best adapted for working Cylindrical Retorts.
There is perhaps no subject in the art of manufacturing coal gas, on which practical men are less agreed, than they are on the temperature most economically to be employed for the production of coal gas in the large way. It must be sufficiently evident, that cast-iron retorts, when worked at a low temperature, will last longer, than when exposed to higher degrees of heat.[23] Hence, according to some operators, the economy of the process consists in saving the retorts, at the expense of a diminution, even though considerable, in the quantity of gas obtained; whilst, according to others, it is more economical to obtain the largest possible quantity of gas at the expence of any consequent injury to the distillatory vessel.
[23] It is essential that the retorts should be kept in constant action night and day, or at least never allowed to go below a red heat. The first portion of oxide which forms upon the surface, when allowed to cool, cracks and falls off, leaving a new surface to be acted upon the next time it is heated. By thus being every day heated and cooled, a retort will be speedily destroyed.
The truth appears to be wholly with neither of these extremes, nor indeed in any absolute general rule which can be ventured on the subject.
The degree of temperature proper to be adopted in gas works, where the method of decomposing coal in masses, or layers from four to eight inches in thickness, and upwards, is practised by means of the cast-iron retorts, of which a description has been given, [p. 53], chiefly depends on circumstances of a local nature, with regard to the price of coal and labour, so that where in one place it may be more profitable to employ a very high temperature for the production of the gas, it may be in others quite the reverse.
The utmost therefore that can be done on this head, is to state what these circumstances are, and to shew the value which belongs to them under every supposable situation.
In this metropolis, and in all other places where coal and labour bear a higher price than probably elsewhere in this country, and where saving of time is also an object of primary importance, it is clearly established, that the manufacturer who pursues the method of decomposing coal in masses from five to eight inches and upwards in thickness, by means of cast-iron retorts,[24] will consult his interest best, by employing such a high temperature for the decomposition of the coal, as will produce in the shortest time the greatest possible quantity of gas, from a given quantity of coal, without regarding the unavoidable deterioration of the retorts. But in places where coal and labour is cheap, it will be his interest to save the retorts at the expence of the coal. But that this fact may not rest on mere general assertion, I shall subjoin for the satisfaction of the reader a few statements of experiments made upon a large scale for the purpose of ascertaining these facts.
[24] The Retorts should be manufactured of what is called in commerce, iron of the second process. The best cast-iron of this kind, is of a light grey colour, its fracture is granulated and dull, it receives a dent from the blow of a hammer. The cast-iron which exhibits a dark grey or black colour inclining to blue, and presents granular concretions, readily friable, and therefore unfit for vessels intended to stand a long continued heat.
Annual Creditor and Debtor Account of manufacturing daily from 50,000 to 102,000 cubic feet of gas at the price which coal bears in the metropolis.—The operation being commenced with new Retorts, and the retorts being left in a fit working state.
The first of the following processes was conducted on the principle that coal and labour, being of an high price, as in London, it is most economical to obtain the greatest possible quantity of gas from a given quantity of coal in the least possible time, without any regard to the injury done to the distillatory vessel.
The second process is intended to illustrate the correctness of that principle, by shewing that where coal and labour are at the high prices stated in the first process, it is a losing system to work the retorts at a lower temperature, in order to make them last longer.
In some respects a similarity will be observed between these experiments, and those already given in [page 85], but besides their having reference to the separate and distinct circumstance of the high prices of coal and labour in London, it will be found that they also differ from the former statements, in exhibiting, not merely the expence of working, but the original cost of erecting the retorts, as well as the expence of replacing them.
Process I.
The quantity of gas to be supplied each night, was 50,000 cubic feet.
In order to produce this quantity, thirty cylindrical retorts, each containing two bushels of Newcastle coal, were put in action. The temperature at which the retorts were worked, was a bright cherry redness, at which they produced at the rate of ten thousand cubic feet of gas, from a chaldron of Newcastle coal.
To work the retorts, three workmen by day and three by night, were required.
The retorts were charged three times every twenty-four hours. The first total expence of erecting the retorts, was £. 23 each, and it was found, that when worked night and day, they could not, with the utmost care, be made to continue fit for use for more than from five to six months; hence, a double set of the original number of retorts was requisite each year.
The whole annual operation pursued on this plan stood as follows:
| Cost of sixty retorts, thirty at work and thirty to spare, with brick-work foundation, iron cokehearth, perpendicular pipe connected with hydraulic main, see P, fig. 2, [plate IV.], at £ 23. each. | £. 1380 | 0 | 0 | |||
| Six workmen, three during day-time, and three at night, at £ 1. 6s. each the week | 405 | 12 | 0 | |||
| Coals, 1825 chaldron, requisite for producing the gas, at £ 2. 8s. the chaldron | 4380 | 0 | 0 | |||
| Wear and tear of grate bars, fire-shovels, tongs and rackers | 42 | 0 | 0 | |||
| 4561⁄4 chaldron of Coal for fuel, £. 2 1s. the chaldron | £. 935 | 6 | 3 | |||
| Total expence, | £. 7142 | 18 | 3 | |||
| Subtract the market price of saleableCoke[25] produced by the process, viz. 1825 chaldron,at £ 1. 3s. the chaldron | £. 2098 | 15 | 0 | |||
| 4561⁄4 chaldron of small Coke or Breeze, at ten shillings the chaldron | 228 | 2 | 6 | |||
| 2326 | 17 | 6 | ||||
| There remains | £. 4816 | 0 | 9 | |||
for the annual expence of maintaining the apparatus on this construction.
[25] The tar and ammoniacal liquor afforded by the process, not being always saleable articles, are omitted to be charged in the estimates.
Process II.
The next experiment made was, to ascertain the contrary practice of operating, namely the mode of working the retorts, on the principle which holds out, that it is more economical to be satisfied with a less quantity of gas than what the coal is capable of furnishing, because by so doing the retorts become less deteriorated and remain for a longer time in a state fit for use.
The quantity of gas to be supplied each night, was, as in the preceding process, fifty thousand cubic feet.
The number of retorts required to produce that quantity, was forty-two, and to make them last twelve months instead of six months, as in the preceding process, it was necessary to work them at a temperature, at which a chaldron of coal produces from seven thousand, to eight thousand cubic feet of gas.
The result of this operation was as follows:
| Cost of forty-two retorts, with brick-work foundation, cast-iron coke hearth, perpendicular dip pipe,connected with the hydraulic main, at £. 23 each | £. 966 | 0 | 0 | |||
| Eight workmen, four by day and four by night, at £ 1. 6s. each the week | £. 540 | 16 | 0 | |||
| 2555 chaldron of Coal, requisite for producing the gas, at£. 2 8s. the chaldron | £. 6123 | 0 | 0 | |||
| Wear and tear of grate bars, fire shovels, tongs and rackers | 42 | 0 | 0 | |||
| 638 chaldron of Coal for fuel, at £ 2. 1s. the chaldron | 1307 | 18 | 0 | |||
| £. 8979 | 14 | 0 | ||||
| Deduct the market price of 2555 chaldron of coke, produced by the process, at £ 1. 3s. thechaldron. | £. 2938 | 5 | 0 | |||
| 6383⁄4 chaldron of small coke, or breeze, at 10s. the chaldron | 319 | 7 | 6 | |||
| £. 3257 | 12 | 6 | ||||
| There remains for the annual expence of maintaining the apparatus | £. 5722 | 1 | 6 | |||
| Subtract the annual expence of Process I. | 4816 | 0 | 9 | |||
| The balance in favour of Process I. is | £. 906 | 0 | 9 | |||
Process A.
In the following additional processes, the retorts when begun to be worked, were also new, and were left in a fit working state. The quantity of gas required to be produced daily, was 102,000 cubic feet.[26]
[26] These Experiments were made at the Westminster Gas Works, under the superintendance of Mr. Clegg, to whom I am indebted for this communication.
The retorts were worked at a temperature at which they produced 10,000 cubic feet of gas from the chaldron, (27 Cwt.) of Newcastle coal.
| To sixty-eight retorts, twice replaced, at £ 15. each | £. 2040 | 0 | 0 | |||
| Deterioration of grate bars, fire shovels, tongs, and rackers | 91 | 16 | 0 | |||
| 3723 chaldron of coal for obtaining the gas, at £ 2. 8s. the chaldron | 8935 | 4 | 0 | |||
| 930 chaldron, 27 bushels of Coal, for fuel, at £ 2. 1s. the chaldron | 1908 | 0 | 9 | |||
| 14 Men at £ 1. 6s. each, the week,being 7 for the day, and 7 for the night | 946 | 8 | 0 | |||
| £. 13,921 | 8 | 9 | ||||
| Deduct the market price of 3723 chaldron of saleable coke, at £ 1. 3s. the chaldron | £. 4281 | 9 | 0 | |||
| 9303⁄4 chaldron of small coke, or breeze, at 10s. the chaldron | 465 | 7 | 6 | |||
| £. 4746 | 16 | 6 | ||||
| Cost of obtaining, 37,230,000 cubic feet of gas | £. 9174 | 12 | 3 | |||
Process B.
Producing 8000 cubic feet of gas, from the chaldron of Newcastle coal.
| Eighty-five retorts, once replaced at £. 15 each | £. 1275 | 0 | 0 | |||
| Deterioration of grate bars, fire shovels, tongs and rackers | 117 | 16 | 0 | |||
| 4653 chaldron of coals for obtaining the gas, at £ 2. 8s. the chaldron | 11,167 | 4 | 0 | |||
| 11633⁄4 chaldron of coal for fuel, at £ 2. 1s. the chaldron | 2385 | 13 | 9 | |||
| Eighteen men at £ 1. 6s. each manthe week, being nine for the day, and nine for the night | 1216 | 16 | 0 | |||
| £. 16,162 | 9 | 9 | ||||
| From which deduct 4653 chaldron of saleable Coke, at £. 1 3s. the chaldron | £. 5350 | 19 | 0 | |||
| 11633⁄4 chaldron of small coke, or breeze, at 10s. the chaldron | 581 | 17 | 6 | |||
| £. 5932 | 16 | 6 | ||||
| Cost of obtaining 37,230,000 cubic feet of gas, according to process B, | £. 10,229 | 13 | 3 | |||
| Deduct the cost of Process A, | 9174 | 12 | 3 | |||
| Balance in favour of Process A. | £. 1055 | 1 | 0 | |||
The reader will have no difficulty in calculating from the preceding experiments, every variation which can possibly take place, as to the degree of temperature most economically to be employed in consequence of a variation in the prices of coal, coke and labour.[27]
[27] The average cost at which coal gas can be manufactured on a large scale in London, is seven shillings the thousand cubic feet, deducting not only the interest of the capital sunk in erecting the establishment, rent and taxes, the cost of the coal, labour, wear and tear of the machinery, and superintendence, but all other necessary and incidental expences that may occur.
Comparative facility with which the decomposition of different species of Coal is effected.
The temperature necessary for the decomposition of different kinds of coal, varies. Some species of coal are more readily decomposed, and require a less portion of fuel than others; they yield up their maximum quantity of gas, in an almost equal stream from beginning to end, and no extraordinary increase of temperature is required to terminate the distillatory process. Other kinds of coal require a different treatment; the temperature necessary to complete their decomposition requires that the heat should be considerably increased as the process advances; and without this condition the evolution of the gas would cease altogether.
A striking proof of this statement may be seen when Newcastle or Sunderland coal are attempted to be decomposed at a temperature which is sufficient for the decomposition of Scotch Splent coal, or Lancashire Wiggan coal.
The decomposition of the latter, will be fully effected when the distillatory vessel exhibits to the eye a dull cherry redness, and the evolution of the gas at such a temperature will take place in torrents from beginning to end. In order, on the other hand, to complete the decomposition of Newcastle and Sunderland coal, the heat must be increased as the process proceeds, and the production of the gas will be extended far beyond the time required for decomposing a like quantity of Scotch, or Lancashire Wiggan coal, when exposed to the same degree of heat.
It must be allowed, however, that few experiments have been yet made on this subject. I have reason to believe that all those varieties of coal which afford an incoherent friable coke, are decomposed at a much lower degree of heat, than such as produce, when treated under like circumstances, a ponderous compact coke. And if we give credit to the assertion of those workmen, whose business it is to manufacture a given quantity of gas by means of a certain quantity of coal delivered to them, it would appear that coal which affords gas abounding in sulphuretted hydrogen, is the kind of coal most easily to be decomposed. This, as far as it regards the decomposition of Scotch Splent or cannel coal, is certainly true. No species of coal affords gas at a lower temperature, and of none is the gaseous product more loaded with sulphuretted hydrogen gas. The subject is important and deserves to be pursued; particularly in places where coke is not, as it is in the metropolis, and all places where coal bears a high price, next to gas, the primary article to which the attention of the manufacturer of coal gas ought to be directed.
The following are the result of a series of experiments on the subject made at the Westminster Gas Works,[28] the same temperature being employed throughout the process.
[28] Communicated by Mr. T. S. Peckston.
| Varieties of Coal. | Ratio of time in Decimals. |
|---|---|
| Scotch Splent or Cannel coal | 1,00 |
| Newcastle coal, (Nesham) | 1,04 |
| Gloucestershire coal | |
| Forest of Dean first variety (Low Delph) | 1,08 |
| Newcastle coal, | |
| Second variety, (Middle Delph) | 1,09 |
| Third variety, (Heaton Main) | 1,15 |
| Fourth variety, (Brown’s Wall’s End) | 1,18 |
| Fifth variety, (Hutton’s Low main) | 1,30 |
| Sixth variety, (Tyne Main) | 1,54 |
| Warwickshire coal, | |
| First variety, | 1,60 |
| Second variety, | 1,65 |
| Third variety, | 1,68 |