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The essential elementary constituents of a coal are carbon, hydrogen and oxygen, with small quantities of nitrogen and sulphur and some incombustible matter. On heating out of con tact with air, the coal fuses more or less and partially decom poses. Gaseous products of decomposition force their way through the plastic mass and give it a honeycombed structure. By the process of decomposition, however, the coal becomes less fusible, until it becomes a porous solid known as coke. Further heating drives off more gas and results in a shrinkage and harden ing of the coke. The volatile matter coming away in the early stages is rich in easily condensible tarry matter and gaseous hydrocarbons. At a temperature above 800° C, the volatile mat ter is principally hydrogen gas.
The following analyses are given to exemplify the composition of coal-gas, tar and liquor.
Now considering the volatile matter, this is driven off in gaseous form owing to the high temperature, but on cooling down there is a gradual process of condensation. In gas manufacture, the cooling is hastened by washing with water, which also re moves ammonia, formed from the nitrogen compounds in the coal. The consequence is that the volatile matter is divided into three portions, two of them liquid but not mixing, because one is oily, the tar, while the other is the so-called ammonia liquor. The third portion is the gas which, after purification from sul phuretted hydrogen, is ready for distribution.
It will be readily understood that both the quantities and the compositions of the tar, liquor and gas vary according to the nature of the original coal and the conditions of manufacture, particularly the temperature. The low temperature products are those resulting from the first processes of break-down in the coal. The high temperature products contain many of the sub stances formed by the secondary decomposition of the primary products, brought about by subjecting them to a higher tempera ture. The difference shows itself very plainly in the gas yield, which is much higher for high temperature working, and in the nature of the gas which contains much more hydrogen and less of the easily decomposable compounds of carbon and hydrogen. The tar is usually smaller in amount for high temperature work ing and it is characterized by the presence of the so-called aro matic hydrocarbons of the benzene type, which are products of secondary decomposition and are absent from low temperature tars. The increased yield of gas in high temperature working is partly due to the secondary decomposition of some of the more decomposable tar constituents, although it is mainly ac counted for by an extensive formation of hydrogen peculiar to high temperature working.
Fire-clay
Retorts.—The,volume of gas obtainable by working in iron retorts was limited by the properties of this material.An important advance was made when fire-clay was substituted for iron in the construction of the retorts, because a higher temperature was permissible and further improvement of a radical character followed when, in the heating of these retorts, gas firing and the recuperative principle could be employed.
Recuperative Retorts.
This will be understood from fig. 2, which shows a setting of /-shaped horizontal fire-clay retorts in a setting. They are heated by a gas made by passing air through a deep layer of red hot coke. This gas, meeting hot air immediately under the retorts, burns around them and carbonizes the coal contained therein. The waste gases, after heating the retorts, do not, however, pass away directly to a chimney, as in the old "direct" firing (fig. 1), but are turned downwards into the recuperator, where they pass along channels in which they are only separated by a thin fire-clay partition from air travelling upwards to meet the gas. By this plan, some of the heat is abstracted from the waste gases and restored to the setting in the air used for combustion. Consequently, less heat leaves the setting, and a higher temperature can be attained accompanied by a fuel economy. This system of carbonization in horizontal recuperative fire-clay retorts rapidly became standard practice, and remains so to a considerable extent. It enabled an average gas yield of i o,000 cu.ft. of gas per ton of coal to be obtained, and lowered the expenditure of fuel required for heating the setting from 25-3o% of the weight of coal carbonized to 15-20%.Although excellent in many ways, the horizontal retort setting as so far described had the disadvantage of requiring heavy labour for hand charging. This drawback has been to some extent neutralized by the use of mechanical charging machines. Other methods were, however, coming forward by which the aid of gravity could be invoked for the moving of the coal and coke during carbonization, and some other advantages secured.
Vertical Retorts.
The simplest form of the vertical retort was one in which the retorts were all set vertically instead of hori zontally, as in the past, and, being filled with coal, were heated until the whole of the charge had been carbonized, after which it was withdrawn. This so-called Intermittent Vertical System was patented in England by Bueb in 1904, after previous trial at the Dessau Gas Works. It had the advantage, as compared with the hand-charged horizontal retort setting, of lessening labour and requiring less ground space for a given output. It had also the new characteristic, that the retort could be fully charged, thereby lessening that contact of the volatile matter with red hot coke and the walls of the retort, which make for secondary decomposition. A further advance was made almost at once by the introduction of continuous working into the vertical retort system by which, instead of completing the carbonization of the whole charge before withdrawing any portion of the coke resi due, a continuous feed of coal was made to the top of the retort and coke was continuously withdrawn from the bottom by an extracting mechanism.The principal names associated with this system are Woodall Duckham, Glover-West and Robert Dempster and Sons, and it has been widely adopted. The idea had been applied with limited success previously by Settle and Padfield. Fig. 3 illustrates a setting of Glover-West retorts which can be taken as typical. The heating gas from the producer passes through apertures, which can be regulated, into heating channels surrounding the retorts. The upper sections are heated by waste gases alone.
The heating quality of the gas is now of paramount importance. As a consequence, the heating value of a gas per cubic foot has become recognized as of more consequence than its illuminating power in "standard candles" and has become the statutory method of defining its quality. The British Gas Regulation Act of 192o introduced the sale of gas by the therm, a therm being ioo,000 B.T.U., and allowed gas companies and authorities to specify the standard quality in B.T.U.'s per cubic foot of the gas they would supply, insistence, however, being rightly made upon the maintenance of that standard as all-important. These alterations in the conditions of manufacture and use, and in legislation, have permitted and encouraged such developments in gas manufacture as make for more complete gasification of coal, i.e., obtaining a larger proportion of its potential heat in the gas made.
Steaming.
One mode of obtaining this result, to which refer ence has already been made, has been working at a higher tem perature. That has demanded special attention to the quality of the refractory materials used in the construction of the retorts and their settings and has led to an increase in the use of silica instead of fire-clay in vital parts subject to the higher tempera tures. By such means, higher yields of gas per ton (i 3,000 ft. per ton) have become common. The gas so made is rich in hydrogen and poorer in illuminating constituents than was com monly supplied previously for lighting purposes and is lower in calorific value, say Soo as against 600 B.T.U. per cubic foot.Another method of increasing the yield in volume and thermal units has come into use, known as the "steaming" of vertical gas retorts, which is carried out by introducing steam at the base of the continuous vertical gas retort where it can react with the red hot coke. By this means an addition is made to the volume of gas by the interaction of carbon and steam which generates water gas (see below).
An investigation by the Joint Research Committee of the University of Leeds and the Institution of Gas Engineers, carried out on a Glover-West setting of continuous vertical retorts, showed that a lean coal which, without steam, gave 10,400 cu.ft. of gas per ton of a calorific value of 544 B.T.U. per cu.ft. (gross), or 56.5 therms in gas per ton, gave when steaming was applied to the extent of 26.4% of steam on the weight of coal used, a yield increased to 16,900 cu.ft. of gas with a calorific value of 447 B.T.U. per cu.ft., or 75.7 therms in the gas made per ton of coal.
Thermal Efficiency.
The development of gas practice, as traced above, has resulted not only in a greater yield of gas but in an increased thermal efficiency for the whole process of carbonization, that is the total number of heat units obtainable by the combustion of the products, gas, coke and tar, has come gradually to form a higher proportion of the heat units con tained in the coal carbonized. This has been effected by improved design of the setting and the use of the recuperative principle, resulting in a lowered consumption of coke for the heating of the retorts. Moreover, by the use of higher temperatures and steaming, the proportion of the heating value of the coal ob tained in the gas has been increased, as compared with that left in the coke. This is of primary importance, because in the con sideration of a carbonization process it is necessary to bear in mind that, owing to the efficiency of gas in use as compared with that of a solid fuel, the thermal value of a heat unit carried by gas is much greater than that of a heat unit in coal or coke, usually two to four times as great and the comparative mone tary value is correspondingly increased. In normal horizontal retort practice it may be taken that for every loo heat units con tained in the coal carbonized, 24 will appear in the gas, 42 in the coke available for sale after the heating of the retorts has been provided for and 5.6 in tar, which means that 71.6 of the original heat units have been obtained in the available useful products of carbonization. Otherwise expressed the thermal efficiency of the carbonization process so conducted is 71.6%, 28.4% having been used and lost in the manufacture. In a more modern installation a higher value would be attained, for example, in the investiga tion of steaming reported above, the efficiencies of carbonization varied from 8o-83%.Purification of Gas.
As indicated above, the volatile matter driven off from the coal and leaving the retort contains per manent gas and constituents more or less easily condensed or washed from it. Treatment for this purpose is made in a train of apparatus which varies widely in detail but not much in principle. The succession of parts is indicated diagrammatically in fig. 4.In fig. I a
so-called "ascension" pipe (H) is shown leading up wards from a horizontal retort, then bending over and dipping be low the liquid seal in the so-called hydraulic main, the seal being used to prevent access of air to the main when the retort is opened for charging and discharging. Some cooling occurs in the ascension pipe and condensing of tarry matter. Collecting together of the gas from a number of retorts into a hydraulic main is common practice. Easily condensible constituents come down there and in the following foul main which leads to the condensers. These are vertical pipes of considerable area and the cooling effect of air upon them causes such a lowering of tem perature in the gas as to bring down both tar and water, which are gathered from the bases of the condenser pipes. The pipes themselves may be circular or may be made annular, so as to make a sort of chimney up which air will pass the more rapidly because it is warmed by the enclosing gas in the annular space. Water is sometimes used with success instead of air as a con densing medium.The next stage is the washing or scrubbing of the gas in which more complete cooling of the gas can be secured, and at the same time the dissolving out of soluble constituents carried by the gas. The construction of washers and scrubbers has called for many designs but intimacy of contact between gas and liquid in the scrubber is sought in every case. In the Livesey Washer for example, the gas stream is repeatedly broken up and forced through water by an ingenious device. In the scrubber, as shown in fig. 4, the gas passes up a tower packed with boards, coke, rings or other filling, so arranged as to give a large surface of contact, with the descending current of water or weak liquor which is relied upon to complete the removal of ammonia from the gas.
Another type of scrubber contains slowly revolving disc brushes, the fibres of which are alternately moistened by liquid in the bottom of the scrubber and exposed to the gas current which they are called upon to purify (Holmes, "Standard"). Some use is made in the gas industry of centrifugal washers con sisting of a number of superposed chambers in each of which the gas passes through a spray of liquid thrown out centrifugally from a revolving cone or cage (K. H. C. Feld). The condensing and washing described will remove excess of moisture from the gas, ammonia and the more easily condensable tarry constituents. If, however, it is required to remove such volatile tar constitu ents as benzene and toluene with any degree of completeness, a further scrubbing with creosote oil or gas oil is found to be necessary.
As shown in fig. 4, the tar and liquor condensed at different points of the system are led away to a common well, but there is room for much discretion and modification in this respect. One constituent of coal gas which by law has to be completely eliminated if the gas is to be used for public supply, is sulphuretted hydrogen, and the final process of purification in ordinary prac tice is to pass the gas through iron oxide purifiers, and thence to gas-holders (see GAS HOLDERS). The purifiers contain hydrated oxide of iron, or similar material spread on grids. The oxide ab sorbs sulphuretted hydrogen rapidly, becoming converted into sulphide. If, owing to this conversion, the sulphide material no longer operative in absorbing sulphuretted hydrogen, is removed and exposed to air, it is re-oxidized with the formation of free sulphur. If a small amount of air is admitted along with the gas to the purifiers, this re-oxidation will take place in situ, and this is usually done. When the sulphur content of the fouled oxide has reached some 5o%, the material is sold for the making of sulphuric acid. Most of the sulphur in the gas is contained as sulphuretted hydrogen and is removed by this process. There remains, however, a small quantity occurring as carbon bisul phide and not removed by oxide of iron. It has been shown by Carpenter and Evans that by thermal treatment in the presence of a nickel catalyst the carbon bisulphide can be converted into sulphuretted hydrogen, subsequently removed.
The sulphuretted hydrogen to be removed from the gas is dependent upon the composition of the coal and other factors, but is of the order of I% and the carbon bisulphide about one twentieth of this amount or less. Of the same small order of magnitude are cyanogen and naphthalene.
Ammonia.
Liquor containing the ammonia washed out of the gas is either sold as such or used at gas works for the production of ammonium sulphate. When distilled with lime, ammonia is driven off from it and being absorbed in sulphuric acid, forms the sulphate which constitutes a valuable manure. The quantity ob tained at gas works usually lies between 20 and 3o lb. of am monium sulphate per ton. The ammonia yield can be increased by steaming the retorts, but the liquor obtained is usually weaker because of the passage of undecomposed steam from the top of the gas retort into the gas. A weaker liquor has a lower com mercial value if it has to be sent away for treatment and has the further disadvantage that after distillation for ammonia the residual liquor is greater in amount.The direct method of ammonia recovery in which the gas is passed through sulphuric acid for the absorption of ammonia, instead of effecting a separation of the ammonia liquor and dis tilling it has found little application in gas works.
Tar.
The tar made at gasworks is subjected to a complicated process of distillation, resolving it into fractions which boil over in different temperature ranges, the fractions being afterwards refined. These operations are usually carried out at separate tar distilleries. The average yield of tar by the ordinary gasworks process can be taken as 5% of the weight of coal carbonized. At lower temperatures, more tar is produced and the light oil frac tion coming over on distillation is usually greater in volume.Water Gas.
A limited gasification of coke in steam can be effected in the continuous vertical retort as described above, but the complete gasification of the carbon in coke is carried out in an entirely different type of apparatus, known as a water gas plant. At high temperatures, carbon decomposes steam into hydrogen and carbon monoxide, but with an absorption of heat according to the equation 29,000 calories. When the temperature of the carbon has been brought down by this absorption of heat, the reaction is altered with the production of carbon dioxide. An equilibrium tends to be established by the catalytic action of the solid carbon (and inorganic ash constitu ents) so that a ratio CO X H20 may be established among the gas X H2 constituents, the ratio being constant for any one temperature but lowering with the temperature. The reversible reaction occurring makes for a higher carbon dioxide content of the gas as the temperature is lowered, and moreover, since the velocity of gasification is rapidly lowered with falling temperature, the gas made with the same rate of steam supply comes to contain more undecomposed steam. Carbon dioxide lowers the calorific value of the gas and the steam requires condensation. The high temperature of the carbon can, however, be restored by stopping the steam and blowing with air which raises the temperature of the fuel bed, generating a producer gas. The industrial process based upon this principle of alternately blowing a bed of coke with steam and air was made by Gillard (1849), Tessie du Motay and Lowe (1873) and called the water-gas process. The plant as illustrated in fig. 5, is that of Messrs. Humphreys and Glasgow. The coke bed is enclosed in a steel casing, lined with fire-brick, and may be blown through the grate below by either air or steam. An arrangement of valves also enables the steam to be introduced above the coke for a "down-run." The exact arrangement and time in the up-run with steam, down-run with steam and blowing with air is varied to suit the fuel and other conditions and constitutes a cycle of operations which is carried out systematically. The coke is blown with steam until, by lowering temperature, the carbon dioxide produced in the water gas is lowering its quality too far. During the steam blow, the water gas made is carried forward to a scrubber, down which water is running and then goes forward to joint the main gas stream of the works for purification from sul phuretted hydrogen. This water gas should have a calorific value of 30o B.T.U. per cu.ft. When the steam blast is replaced by air, in order to restore the high temperature in the fuel bed, the pro ducer gas generated being heavily charged with nitrogen is not allowed to go forward to the scrubber, but is turned to waste up the stack. That continues until a satisfactory high temperature has been re-established in the fuel bed, when steam is again em ployed. The heaviest thermal loss in the process is that of the potential and thermal heat in the producer gas, but this is being considerably lessened in the most modern plants by the installation of a waste heat boiler. Another means of lessening the same loss is adopted in the Dellwik Fleischer plant, in which, by the use of a thin bed of fuel and a powerful blast of air, regeneration of heat in the fuel bed can be carried out more quickly, more of the carbon being burned to carbon dioxide instead of carbon monoxide, so that more heat is generated in the fuel bed and less leaves the generator as potential heat of combustion in the blow gas. (The steam using stage is known as the "run" and the air using stage as the "blow.") Carburetted Water Gas.—It has been noted that the water gas made by the process as described above has a calorific value approximating to 30o B.T.U. per foot. It is known as "blue" water-gas and is definitely lower in grade than the coal gas made from retorts. There is a means, however, ready to hand, of in creasing the calorific value by utilizing some of the heat in the gases leaving the generator for the purpose of cracking oil, that is converting it into permanent gas, rich in hydrocarbons, and so obtaining a "carburetted" water-gas of enhanced calorific value. Fig. 6 illustrates a Humphreys and Glasgow plant used for this process. The gas from the generator passes through two chambers, a carburettor and a superheater packed with brickwork, which are raised to redness, some air being admitted for the combustion of the "blow" gas therein. The oil is run in from the top of the car burettor and should be such as can be efficiently cracked under the conditions of the process. (In early stages of the development of the plant the oil was run directly upon the coke in the generator, but this was unsatisfactory for various reasons.) In this plant, blue water-gas leaves the generator with a calorific value of 300 B.T.U. per cu.ft., but leaves the superheater enriched by the car buretting to an extent determined by the amount of oil used. The thermal efficiency of the oil cracking in the plant is high, amount ing to something like go% and consequently the thermal efficiency of the carburetted water-gas process is higher than that of the blue water-gas process and increases with the amount of oil used. The extent of carburetting employed is influenced by this factor, by the price of oil and the quality of gas desired. In England, carburetting is usually carried out so far as to bring the car buretted water-gas up to something like the same calorific value as the coal-gas made at the same works, say soo B.T.U., but in America it has in the past been usually carried much further. It is plain too that blue water-gas, enriched by carburetting to the extent desired, can be used as a means of modifying the calorific value of the mixture of coal-gas and water-gas supplied from a works. The extent to which the coke made in a gas works may be economically gasified and water-gas supplied depends on relative capital and operating costs and the prices of coal, coke and oil. A water-gas plant has the advantage, however, of being able to be put rapidly into full operation and the yield of gas per ton of fuel is high. In tests under working conditions, a blue water-gas plant gave the following results : The thermal efficiency of gas production in the carburetted water-gas plant with waste heat boiler, i.e., heat of combustion of gas divided by heat of combustion+heat lost and utilized in carry ing out the process, was 67.1 in this test, using 1.85 gals. of oil and 35.32 lb. of coke (dry) for each i,000 cu.ft. carburetted water-gas made.
In the latest designs of carburetted water-gas plant, the blast steam and hot gas valves are operated automatically. There is a mechanical coke feed, an annular boiler round the generator and a mechanical grate. A so-called back run has been introduced into the working during which steam is passed for a time down the superheater, up the carburettor and finally down the generator.
Complete Gasification and Other Processes.—The proc esses so far described are those which have come into general use. Attempts are being made to make gas for public supply by completing the gasification of coal in one process instead of car bonizing it first in retorts and gasifying the coke residue, so far as may be desired, in steam in a separate generator and process. The water-gas process and generator is in some form embodied in all these plants. In the United Gas Improvement Company's plant some constructional modifications of the water-gas genera tor allow of the use of some coals in it in place of coke. In other plants, a retort is in effect superposed upon a water-gas generator (Tully). In another (Robinson) the working of horizontal gas retorts and water-gas generator is combined so as to discharge coke from the retorts into the generator and to utilize heat from the latter for carbonization. In plant by M. W. Travers, gas is recirculated to the generator after passing through a recuperator (heated by blow-gas) with the object of restoring sufficient heat to complete the carbonization process, both that and the gasifica tion being carried out in the generator. Use is being made in gas works of coke ovens (q.v.) heated by producer gas; they are tend ing to be made narrow with reduced carbonization times. Vertical chambers too are meeting with some success (Otto Pintsch). There is a large amount of gas of approximately the same compo sition as that made in horizontal retorts available from by product coke-oven plants, even when the heating of the ovens has been carried out by coke-oven gas. Such gas is being brought into use for public supply where economically advantageous, transmission over long distances being made by pumping.
The newer systems of low temperature carbonization (q.v.) are still in the experimental stage and cannot be said to contribute to public supply.
The development of gas manufacture is proceeding largely from a systematic study of the quality of different coals for carbonization and gasification at different temperatures and rates. The influence of the composition and physical properties of coal, such as fusibility and size of particle, the possibilities arising from blending different coals or pre-treating them by washing or preliminary gentle heating are receiving attention, while systematic studies of thermal efficiencies are stimulating the design of appa ratus for carbonization and gasification so as to secure higher thermal and chemical yields.
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