[19]Average proportion of gas from a chaldron of coal.
[19]Average proportion of gas from a chaldron of coal.
Expenditure of Process B.
Products of Process B.
From the result of this process it appears, that at the expence of 265l.17s.61⁄2; the value of the products obtained is £ 828. 19s.
By comparing the two preceding processes, A and B, it will be observed that the same quantity of gas was generated each day, notwithstanding there were fewer retorts in use, and less coal decomposed by process B, than by Report A, and that the expence of fuel, when the distillatory process was continued for a term of eight hours, was considerably less. Also, that the proportion of gas obtained from a chaldron of coals, was greater than when the process was continued for only six hours.
Hence, if from the products of
which, being subtracted from the difference between the expenditure, as specified in the process alluded to, namely
in favour ofworking eight hours charges, for one week, and producing a like quantity of gas, as had been obtained by workingsix hours charges.
Thus, having compared the quantity of coals actually used when working six hours charges, with what was used to produce a like quantity of gas from eight hours charges, I shall next point out, in process C, the quantity of gas obtained when working the same number of retorts for a period of eight hours which had been worked at the process of six hours.
[20]Worked at six hours charges in process A,page 85, but here worked at eight hours charges.[21]Average proportion of gas from a chaldron of coal.
[20]Worked at six hours charges in process A,page 85, but here worked at eight hours charges.
[21]Average proportion of gas from a chaldron of coal.
Expenditure of process C.
Products of process C.
RECAPITULATION.
From the above recapitulation it will appear, that by working equal numbers of retorts, at six and at eight hours charges, the balance is considerably in favour of the latter method; for, from the foregoing statement, there appears to be onthe practice of the latter method an increase of saleable products amounting to
in favour of working the retorts, as stated in process C, over that method shewn in process A; and in such proportion,let the number of retorts worked be what it may, an advantage will always be gained in this mode of manufacturing coal gas, by working the retorts at eight hours charges, as the workmen call it, in preference to adopting the shorter process.
From a series of operations made[22]with twenty parallelopipedal and with twenty cylindrical retorts, worked for one month, it has been ascertained that the decomposition of coal is most economically conducted when each retort is charged with 100 pounds of coal, and the distillatory process be continued for eight hours. Two men, one by day and one by night, can attend nine or ten retorts.
[22]By H. Morrison, Esq. and Self; the coal used, was Newcastle (Bewick and Craister’s Walls End) coal.
[22]By H. Morrison, Esq. and Self; the coal used, was Newcastle (Bewick and Craister’s Walls End) coal.
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 adiminution, 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.
[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 probablyelsewhere 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.
[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.
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 giveninpage 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.
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 couldnot, 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:
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.
[25]The tar and ammoniacal liquor afforded by the process, not being always saleable articles, are omitted to be charged in the estimates.
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 capableof 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:
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.
[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.
Producing 8000 cubic feet of gas, from the chaldron of Newcastle coal.
The reader will have no difficulty in calculating from the preceding experiments, every variation which can possibly take place, as to the degree oftemperature 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.
[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.
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 tocomplete 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 reasonto 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 WestminsterGas Works,[28]the same temperature being employed throughout the process.
[28]Communicated by Mr. T. S. Peckston.
[28]Communicated by Mr. T. S. Peckston.
The many disadvantages attendant on the plan of decomposing coal in masses from five to ten inches in thickness, as already sufficiently exposed in the preceding parts, had naturally the effect of developing a principle of manufacturing coal gas, which practice has now fully established, namely: that to decompose coal, in thin layers from two to four inches in thickness, is to obtain the greatest quantity of gas from a given quantity of coal at the least expence.
Mr. Clegg was the first person who pointed out to the public the advantages that must accrue from this mode of operating, and to him we are indebted forthe construction of an apparatus, the great ingenuity and superiority of which, entitles what is called the horizontal rotary retort, to all the merit and praise that belongs to the character of an original invention.
The numerous and great advantages of this distillatory apparatus, the rapidly increasing adoption of it,[29]and the almost certain prospect which exists of their ultimately superseding all former methods of decomposing coal, make it proper that I should lay before the reader, as full an account as my limits will permit, of the construction and operation of this retort, and the mode of applying it; and this becomes the more necessary on account of the many important improvements which the apparatus has undergone since its first adoption,[30]and of which no description has yet been laid before the public.
[29]Retorts of this description have been lately adopted, in the Gas Works at Bristol, Birmingham, Chester, Kidderminster, and at many other provincial Gas Establishments.[30]An account of the original construction of the rotary retort may be seen in the Repository of Arts, No. CLXXVI, 1816, page I. and also in the Journal of Science, Vol. II. page 133.
[29]Retorts of this description have been lately adopted, in the Gas Works at Bristol, Birmingham, Chester, Kidderminster, and at many other provincial Gas Establishments.
[30]An account of the original construction of the rotary retort may be seen in the Repository of Arts, No. CLXXVI, 1816, page I. and also in the Journal of Science, Vol. II. page 133.
The following account will render the construction of this retort sufficiently obvious:
[31]The retorts lately erected at the Gas Works, at Birmingham, Chester, Bristol, &c. are similar to those at the mint.
[31]The retorts lately erected at the Gas Works, at Birmingham, Chester, Bristol, &c. are similar to those at the mint.
The horizontal rotary Retorts at the Royal Mint, are hollow cylinders, eight feet six inches in diameter and 15 inches high, arched a little at the top. They are made of wrought-iron plates, half an inch thick, rivetted together in the manner of a steam-engine boiler; A, A, A, fig. 2,plate III.exhibits a perpendicular section of the rotary retort. In fig. 1,plate II.the retort is seen fixed in the brick-work;a, fig. 1,plate II.shews the mouth of the retort, through which the coals are introduced, and from whence the coke is withdrawn. It is also shown in perspective at B, B, B. fig. 2.plate VII.The mouth is closed with a cast-iron door fitted on air-tight by grinding.
The door is connected at its upper and lower extremities, with a frame and adjusting rod, see B, B, fig. 1,plate II., and alsoplate VII., by means of which it may readily be slided down below themouth of the retort, when the coals are to be introduced, or coke is to be withdrawn. To the upper extremity of the rod B, fig. 1,plate II., is fixed a lever, loaded with a counterpoise weight C, to balance the door, and to render the opening and closing of it easy and expeditious.
The mouth-piece and its door is three feet long, and nine inches wide; it projects nine inches beyond the brick-work or furnace in which the retort is fixed, as may be seen at fig. 1,plate II.
The fire-place, which is on the opposite side to that of the mouth of the retort, heats only one-third part of the whole capacity of the retort to that degree which is proper for the complete and rapid decomposition of the coal, while the remaining parts, which are not over the fire-place, and to which the fire flues do not extend, are kept at a lower temperature.
The flues are directed under about one-third of the area of the bottom of the retort, and after having passed over one-third part of the area of the top of the retort, they pass into the chimney. Fig. 1,plate VI., exhibits the direction of the flues; A, A, theflues, and the fire-place. The whole retort is guarded from the contact of the fire, which would soon destroy it, by fire-bricks; it notwithstanding speedily receives the full effect of the heat, and retains its temperature when once heated for a long time. Fig. 1,plate II., exhibits one of the retorts fixed in its furnace. A perspective view of three retorts may be seen in fig. 2,plate VII.
Through the centre of the retort, passes perpendicularly, an iron shaft D, as shown in the section of the retort, fig. 2,plate III., and also in fig. 1,plate II.The lower extremity of the shaft revolves upon the bottom of the retort, in a cup-shaped cavity, while its upper extremity passes through the roof of the retort, where the latter is made air-tight by means of a pipe E, fig. 1,plate II., and E, fig. 2,plate III., closed at the top and surrounding the shaft, and hence the shaft must always preserve its centre.
To the lower extremity of the shaft is keyed a box or centre piece, (technically called a rose centre,) F, fig. 2,plate III.It is also seen in the perpendicular section of the retort, fig. 1,plate II.From this shaft radiate twelve wrought-iron arms,G, G, fig. 2,plate III.,[32]fixed in sockets made in the box. These arms are elevated three inches above the bottom of the retort, and extend to nearly within its whole inner circumference. They are wedge-shaped, and their greatest diameter is at right angles to the base of the retort, so that the weight of the arms rests on the axis. They are intersected by two concentric rings, as will be seen on inspecting fig. 5,plate III., which exhibits the plan of the retort, together with the iron arms, G, G, and concentric rings. The centre of figure 5, shows also the plan of the rose centre F, fig. 2,plate III., into which the arms are keyed.