THE HEAT REGENERATIVE SYSTEM OF FIRING GAS RETORTS.
The system of heat regeneration in the firing of gas retorts, in accordance with the principle which Dr. C.W. Siemens has worked out in such a variety of ways in the industrial arts, has lately been applied with very marked success at the Dalmarnock Station of the Glasgow Corporation Gas Works. Notwithstanding the fact that a period of about twenty years has elapsed since Dr. Siemens successfully adapted his system to the firing of retorts at the Paris Gas Works, it seems to have made but little progress up to the present time; for what reasons it is perhaps difficult to explain. It is certain, however, that so-called regenerator furnaces of various forms have, from time to time, been brought into use at gas works for the purpose in question both on the Continent and in this country; and in recent years the subject has received much attention from gas engineers, the general opinion eventually being that the adoption of such a system of working would be certain to result in so great an amount of economy as to put gas as an illuminating agent on a more secure footing to compete successfully with its modern and somewhat aggressive rival, the electric light. Of course, it is now admitted that the mode of adapting the heat regenerative principle at the Paris Gas Works was attended with a degree of complexity in the structural arrangements that was so great and so expensive as to place it practically beyond the reach of gas companies and gas corporations generally, when the expense as well as the scientific beauty and practical efficiency of the new mode of applying and utilizing heat had to be considered. Fortunately, however, Dr. Siemens was enabled two or three years ago to demonstrate that there was no such thing as "finality" in that department of invention which he had made almost exclusively his own. About the time mentioned he placed his most advanced views on gas producers and on the regeneration and utilization of heat before the world, and within that period a most decided step in advance has been made, the structural arrangements now required for gas producers and regenerator furnaces having been immensely simplified and cheapened, while their practical utility has in no way been interfered with.
Scarcely had Dr. Siemens announced his new form of gas producer and regenerator than communication was opened with him by Mr. W. Foulis, the general manager to the Glasgow Corporation Gas Trust, with the view of entering into arrangements for its adoption on an experimental scale at one of the stations under his charge. Encouraged by the hearty co-operation of the gas committee, two or three of whose members were well known engineers, Mr. Foulis very soon came to an understanding with Dr. Siemens to have the regenerative system put to a thorough test at the Dalmarnock Gas Works, situated in the extreme east end of the city, and the largest establishment of the kind in Scotland, the total number of retorts erected being about 750. The system in its most recent shape was applied to four ovens, each of which had seven retorts, but which number has since been increased to eight, owing to the space occupied by the furnace in the ordinary settings being rendered available for an additional retort in the new or "Siemens" setting. For each oven or chamber of eight retorts there was erected a separate gas-producer, so that even one set of eight retorts might alone be used if thought necessary.
GAS RETORTS WITH REGENERATIVE FURNACES .--GLASGOW CORPORATION GAS WORKS.
In Figs. 1 and 2 of our illustrations, the general arrangement and the relationship of the gas producer, the regenerators, and the retorts to each other are clearly shown. It was a sort of sine qua non of the new method of firing the retorts that the producer should be in as close proximity as possible to the place where the gaseous fuel was to be used, and it was concluded that the most convenient situation would be immediately in front of its own set of eight retorts, and with its top on a level with the working floor of the retort house. To place it in such a position meant a good deal of excavation, which was also required, however, for the regenerator flues. The excavation was carried down to a depth of 10 ft. below the level of the retort house floor, and as a matter of course the operation of underpinning had to be resorted to for the purpose of carrying down the foundations of the division walls, which, together with the main arches and the hydraulic main, were in no way otherwise disturbed. As in most new inventions, a good deal of difficulty was experienced at first in connection with these gas producers and heat regenerator furnaces; but by dint of application and by the adoption of modifications made here and there in the arrangements from time to time, as also by a determination not to be beaten, although often disheartened, Mr. Foulis was ultimately rewarded with complete success. The new system of firing being made so simple that there was scarcely any possibility of failure likely to arise in ordinary practice if it was superintended with but a moderate amount of care.
Fig. 3.
The results which were obtained in course of time with four ovens, or a total of 32 retorts, were so exceedingly promising that it was forthwith resolved to extend the new mode of firing to the whole of a double bench of twelve ovens, now containing 96 retorts; and all the improvements which had suggested themselves during the working experiments with the four ovens were adopted from the first in the reconstruction of the remaining eight ovens in the bench. More recently the regenerator system has been applied to other 22 ovens, or 176 additional retorts, being the whole of one of the main divisions of the retort house; and during the very depth of the present winter, when the demand for gas was at its greatest height, all the retorts of the converted or "Siemens" settings, amounting to 272, were in full working activity, in which condition they still remain. It is intended to make another very considerable extension of the heat regenerative system of firing during the ensuing spring and summer. The reconstruction of the present year will extend to the ovens of seven retorts each, giving in this case eighty gas fired retorts; and to twenty ovens of five retorts each, which will become sixteen ovens, each having eight retorts, making 128 retorts in this division, and the total being 208 retorts in place of 170 in the same amount of space. It is confidently anticipated, therefore, that by the month of August of the present year, 480 full sized retorts will be available for working out the new method at the Dalmarnock Gas Works. Furthermore, the confidence which has been inspired in the minds of the members of the Glasgow Corporation Gas Committee and their engineer regarding the actualities and possibilities of the Siemens system of firing gas retorts, in its most improved state, is such that arrangements are being made for starting shortly to apply it throughout at the Dawsholm Station, which is situated in the suburban burgh of Maryhill, and some four or five miles distant from the Dalmarnock Works in a northwestern direction. The station just named, which is also a very large one, will probably require two years for its conversion.
We shall now give some account of the structural arrangements adopted for producing cheap gaseous fuel, and for turning that fuel to the greatest advantage in firing the retorts for the purpose of carbonizing the cannel coal used as the source of the gas.
The gas producer, which is represented in vertical section in Fig. 2, is a cylinder of brickwork inclosed in a casing of malleable iron. It is 7 ft. 6 in. deep, and 3 ft. in diameter, which becomes reduced to 20 in. above, where it is closed by means of a cast-iron lid, which is continuous with the floor of the retort house. There are no firebars at the bottom, so that the fuel rests on a floor of firebrick. At the bottom of the walls of the producer there are several holes about 1 ft. in length by 6 in. in height. By means of these openings any clinker that may form and the ashes of the spent fuel can readily be withdrawn. They also allow of the admission of air to maintain the combustion in the lower portion of the mass of fuel; and at each opening there is a malleable iron tube for delivering a jet of steam direct from a steam boiler. We shall subsequently explain the functions performed by the steam.
The fuel employed is the coke or char resulting from cannel coal when it has yielded up its hydrocarbons and other gases during the process of carbonization in the gas retorts. Being entirely made from Scotch cannel the coke is very poor in quality, as it contains a large percentage of mineral matter or ash relatively to its fixed carbon. The retorts are worked with three-hour charges, but the producer is only charged once in every six hours For each set of eight retorts the charge of raw cannel is about 18 cwt., and it is found in practice that the coke drawn from five of the retorts is quite sufficient to fill up the producer to the top. Formerly a set of seven retorts fired in the ordinary way from a furnace underneath, required from 60 to 75 per cent. of the coke made, but now, with eight retorts in each oven, the quantity has been reduced to about 30 per cent., or less than one-half of what it formerly was. Before the retorts are drawn the lid is removed from the top of the producer, and any fuel still remaining unconsumed is touched up a bit by way of leveling it on the surface, and as soon as it has been filled up to the constricted portion a shovelful of soft luting is spread over the top of the coke, and the lid is laid upon it and driven home, thereby making a perfectly air-tight joint. The contents of the other three retorts, as also the contents of the whole of the retorts at each alternate drawing, are taken to the coke heap in the yard. We have already spoken of a charge of cannel as being about 18 cwt. for each set of eight retorts, but in connection with that matter we should mention that it was formerly about 13 cwt. per oven containing seven retorts, and that there is every prospect of it being increased without increasing the length of time occupied in carbonizing the cannel of each charge.
It may be worth while now to notice briefly what takes place among the mass of coke in the gas producer. The atmospheric air admitted at the several openings previously spoken of ascends through the lower layers of the incandescent coke, the carbon of which burns to carbonic acid gas at the expense of the oxygen of the air. Among the middle and upper layers of the incandescent coke the carbonic acid gas takes up a further quantity of the fixed carbon, and becomes transformed into carbonic oxide gas (CO2+C=2CO), which is an inflammable body, and possesses considerable calorific power. Unless the carbonic acid gas is very completely "baffled" in its ascent through the coke in the producer, a quantity of it passes into the furnace along with the carbonic oxide, the efficiency of which is diminished in proportion as the former increases in quantity. Of course, also, the nitrogen associated with the oxygen in the air admitted to the gas generator passes on with the carbonic oxide gas, this nitrogen acting as a dilutant and being of course absolutely useless as a generator of heat. The steam which we previously spoke of serves two good purposes. In contact with incandescent coke it suffers decomposition, its oxygen uniting with some of the fixed carbon to form carbonic oxide, while the hydrogen which is set free passes onward, and mixes with the other gases to be subsequently consumed with them. The admission of the steam thus causes the absorption of heat in the gas generator where the decomposition takes place, this heat being again evolved on the subsequent combustion of the hydrogen. Then, again, as the steam is delivered in among the coke in a jet, or a series of jets, it has the effect of almost entirely preventing any clinkering or slagging of the earthy and silicious materials, which form such a large portion of the substance of the coke obtained from Scotch cannels, sometimes as much as from 15 to 20 per cent. It is scarcely necessary for the stokers to go down below to the bottom of the producers to remove the ash above once in every six hours. Referring to the composition of the gaseous fuel obtained from cannel coke in one of these gas producers, we give the following typical analysis on the authority of Dr. William Wallace, F.R.S.E., gas examiner, and one of the public analysts for the city of Glasgow:
Per cent.
Hydrogen 8.7
Carbonic oxide 28.1
Carbonic acid 3.5
Oxygen 0.4
Nitrogen 59.3
-----
100.0
By again referring to Fig. 2, it will be observed that an opening is provided for the passage of the gaseous matter as it is formed into the mass of brickwork, the upper half of which is occupied by the retorts of the setting and the lower by the regenerators.
Before following the gas we may first direct attention to the arrangements for dealing with it, and with the air that has to be admitted for the combustion of so much of it as is of a combustible nature. It will be seen by reference to Fig. 1 that the oven proper is occupied by eight
shaped retorts. These are 9 ft. long (set back to back) by 18 in. by 13 in., and they are placed on arches which are 8 ft. 6 in. wide. Underneath the level of the retort oven there are two regenerators or regenerator chambers, which differ very materially in form from the regenerators formerly applied by Dr. Siemens to gas retort ovens, and which are still employed for high temperature furnaces like those used for steel and glass melting. In the case of these latter the regenerators are on the alternating system--that is to say, a mass of brickwork is heated by the waste heat of the effluent gases, and when that is made sufficiently hot, the current of waste gases is turned into a second mass of brickwork, while air is admitted to pass through the brickwork already heated. The system thus briefly described entails a certain amount of attention on the part of the workmen in the altering of the valves or dampers to reverse the currents. The regenerator now adopted consists of an arrangement of six zigzag flues, three on each side of the setting. These flues run the whole length of the setting. As indicated by the arrows pointing downward in Fig. 3, the waste gases on their way to the chimney stack pass to and fro through the side flues, thus giving up a large portion of their contained heat by the process of conduction or contact to the central flue through which the incoming air passes. The air necessary for combustion is first admitted into a large chamber in the center, and then it is divided into two currents, which pass right and left into the central passages of the two regenerators. As the air flue is at a very bright heat for a considerable distance before the air leaves it, the temperature of the air must be equally great, or nearly so. In its most improved form one of these heat regenerative furnaces provides an amount of heating surface extending to 234 square ft., which is exposed to the air on its way to the combustion chamber.
Passing from the producer through the flue provided for it, the gas enters the retort setting underneath the side retorts, where it meets the air coming from the regenerator. It enters the setting, not by a number of small openings, but by one large opening on each side, and meets the air entering also by a large opening, the effect of which is to avoid the localization of intense heat, as all the retorts of the setting become enveloped in an intensely heating flame, due to the combustion of the carbonic oxide and hydrogen gases.
There are various advantages attending this system of firing gas retorts. First of all, there is already a saving of fuel to the extent of one-half, and not unlikely there will soon be a further very decided increase in the saving of fuel to record, inasmuch as it has been experimentally determined within the past two or three weeks that, by increasing its diameter to 3 ft. 4 in., one producer can be made to provide a sufficient amount of gaseous fuel to fire two sets of eight retorts. By the arrangement just hinted at the relative amount of fuel used will be still further reduced. Then, again, an additional retort can well be placed in each oven, as it occupies the position of the fire in ordinary settings. In the third place, by the greater heat which is obtained, the charges can be more rapidly distilled; or heavier charges can be carbonized in a given space of time. When all the gains are put together, the amount of coal carbonized is increased by about 40 per cent. over any specified time. Of course, in the new or regenerator settings there is much greater regularity of heat; and as the gaseous fuel is perfectly free from all solid matter, and burns without any trace of smoke, there is a total absence of deposit on the outside of the retorts. From these two circumstances combined it is but natural to expect that there should be greater durability of the retorts--which is really the case. Another advantage is that, as the fuel used in the furnaces is wholly gaseous, choking of the flues cannot by any possibility arise. It is the confident opinion of Mr. Foulis that the system in question can be applied with advantage to all sizes of gas works, and that it is certainly well adapted for all works where the summer consumption of gas is sufficiently large to give employment to eight retorts.
As this is the first instance of the new form of gas producer and regenerator having been adopted in any gas works, a very great amount of scientific and practical interest attaches to it. Many persons have visited the Dalmarnock Gas Works during their reconstruction, in order to see the system in operation, and doubtless many more will go and do likewise when they learn of the numerous advantages which it possesses, and which are likely to increase rather than diminish.--Engineering.