Chapter 4

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.

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 (CO_{2}+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 [Inline Illustration] 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_.

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A NEW GAS-HEATED BAKER'S OVEN.

During the past few weeks, a highly interesting experiment--and one, moreover, destined to materially influence the development of the uses of gas in a fresh field--has been in progress, under the guidance of Mr. Booer, at a baker's shop in the Blackfriars Road, London. The experiment in question is nothing less than the application of gas for heating bakers' ovens, in a manner not hitherto attempted, and such as to bring the system within the means of the poorest tradesman in all but the smallest towns. It will be remembered that the success of the gas-heated muffles for burning tiles and glass led to the attempted construction of a model baker's oven, heated by the same fuel, which was shown in action at the Smoke Abatement Exhibition at South Kensington in the winter of 1881-82. This model attained considerable success; but its design demanded either a new structure in every case, or considerable alteration of any existing oven. In the proposed system, moreover, the oven was heated wholly from without--a condition supposed to be necessary to meet the objections of the bakers. It is evident, however, that there must be considerable waste of gas in heating a mass of tiles and brickwork, such as go to the construction of a common baker's oven, from the outside; and the objection to handicapping such a costly fuel as gas in this manner becomes more apparent when it is remembered that in the usual way the oven is always heated by an internal coal fire. When it is further considered that the coal commonly used by bakers is of the most ordinary quality, full of dirt that would condemn it in the estimation of a gas manager, the sentimental objection to allowing a purified gas flame to burn in a place which this rubbish is permitted to fill with foul smoke becomes supremely ridiculous. Consequently, when Mr. Booer, whose work in connection with the gas muffle is well known in England and America, seriously addressed himself to construct, upon altogether new lines, a cheap and practical baker's oven, he wisely put the gas inside.

There are many other conditions which Mr. Booer, after consultation with practical bakers and others, set himself to fulfill, the observance of which lends to the present Blackfriars experiment much of its interesting character. Thus it was observed that, while it is not difficult to build an oven in a given spot, and bake bread in it, this cannot truly be called a _baker's_ oven. By this term must be understood in particular an oven in an ordinary bakehouse, set in the usual style and worked by a man with his living to get by it. Before the problem of extending gas to bakers' ovens could be considered solved, it had to be attacked from this aspect. Mr. Booer, to do him full credit, seems to have early appreciated this fact in all its bearings. He not only saw that it was necessary to save gas, as much as possible, by putting it inside the oven; but he was told that, in order to meet with any general success, the cost of converting an oven to the gas system must be rigidly kept down to about ten or twelve guineas. The latter seems a particularly hard condition, when it is remembered that the only improved baker's oven in practical use at the present day is the steam oven invented by Mr. Perkins, which costs two or three hundred pounds to erect. Mr. Booer also had in mind the necessity that everything possible for a coal oven must likewise be performed by a gas oven; and in this respect he set himself to surpass the costly Perkins oven, which will not bake the common "batch" or household bread, generally the principal article of sale, more especially in populous and poor neighborhoods. The peculiar efficacy of the common coal fire in this respect proceeds from the essential principle of action of a brick oven, which is found simply in the fact that the work is done entirely by heat previously imparted to the tile bottom, roof, and sides of the oven, and thence radiated to the bread. No other kind of heat will bake batch-bread--i.e., loaves packed in contact with one another--which requires to be thoroughly soaked by a radiant heat in a close atmosphere of its own steam. Now, as a coal fire is eminently qualified to impart, by radiation and otherwise, this necessary store of heat to the brickwork, it is plainly a difficulty to effect the same purpose with a fuel which, of itself, can scarcely radiate heat at all. The system of the gas cooking-oven--the utilization of the heat of the combustion products as formed--is clearly inapplicable here; for a different kind of heat is needed, under conditions that would not sustain continuous combustion. Therefore, there is nothing for it but to heat the bottom and sides of the brick oven by the direct contact of powerful gas-flames; thus supplanting the coal fire, but leaving the actual work of baking to be done afterward by stored-up heat in the regular way.

Having settled the general principles of a system of this kind, there still remain a number of scarcely less important details, in the dealing with which lies the difference between practical success and failure. Thus it is not merely sufficient to heat an oven for bread baking; it is also necessary to heat it within the times and according to the habits of work to which the baker has been accustomed. Work in town bakeries begins at about midnight, or shortly after, and the condition of the oven must conform to the requirements of the dough, which vary from day to day and from season to season. In order to master all these niceties, as far as a knowledge of them is necessary to his purpose, Mr. Booer has spent many nights in the bakehouse in the Blackfriars Road; and has thereby obtained a command over the technicalities of the work which has served him in good stead, not merely for adjusting his gas heat, but in answering the innumerable objections always raised when a revolution in an immemorial trade is threatened. It is with considerable satisfaction that we are enabled to declare, after duly weighing all the conditions as to first cost and otherwise imposed by himself and others, that Mr. Booer has succeeded, upon these terms, in vindicating the claims of gas to be a cheap, efficient, and cleanly fuel for heating ovens under the control and according to the methods of working of the baker himself.