Gaseous Fuels

GASEOUS FUELS It has already been pointed out that before final combustion all fuel is converted to the gaseous state. In this section, however, fuels which when supplied to consuming appliances are already in the gaseous form will he considered separately. These may be grouped as natural gas, coal gas obtained by destructive distilla tion of carbonaceous material at high or low temperatures and gas obtained either by partial combustion or by the action of steam on red hot carbonaceous materials. The latter comprise water gas, producer gas and blast furnace gas or other gas ob tained as a by-product from industrial or chemical processes.

Natural Gas.

Natural gas usually occurs in conjunction with petroleum or coal, being stored in porous rocks with an impervious covering, from which it is obtained by drilling. In the United States very large quantities have been recovered and used for industrial and domestic purposes, as much as 800,000 million cu.ft. per ann. from 1917 to 1922, and in 1926 1,300,000 million cu.ft.; but its utilization in isolated oil-fields is a serious problem. Natural gas consists chiefly of methane (CH,) and other light hydrocarbons, its calorific value varying from 700 B.Th.U. per cu.ft. to 1,5oo B.Th.U. or more. It frequently contains hydro carbons liquid at ordinary temperatures and pressures which can be recovered by washing or other treatment, several million tons of such spirit ("casing-head" gasoline) being obtained annually.

Coal Gas.

Coal gas is obtained by the distillation of coal and was formerly made almost solely for illuminating purposes; but the introduction of the incandescent mantle and the development of gas cooking and heating, have made calorific value a far more important criterion than illuminating power. This change has had a profound effect on the process of manufacture. During the 19th century gas practice tended towards constantly rising temperatures in horizontal retorts. Until about the middle of the century oval or D shape retorts of metal were used, but these were replaced later by retorts of fireclay or other refractory ma terial, about i6in. wide X 24in. broad X 2of t. long, installed in benches one over the other and heated by flues in which is burned producer gas. Such retorts are still largely used, but the vertical retort—which was introduced about the beginning of this century —now carbonizes half of the gas-works coal in Great Britain. Vertical retorts are 20 to 2 5f t. high with a rectangular section of 33 to 43in. by io to 14in. ; they may be either intermittently or continuously operated, and in them the coal travels by gravity.

The quantity and yield of gas vary with the parent coal, the temperature and time of carbonization, the type of retort, etc. A typical town gas has the following percentage volume composi tion :—carbon dioxide 2.2, oxygen 0.5, unsaturated hydrocarbons 2.0, carbon monoxide 14.0, hydrogen 52•o, methane, etc., 23.0, nitrogen 6.3 ; calorific value 500 B.Th.U. per cubic foot.

During recent years the practice of passing steam through the charge in vertical retorts during carbonization has gained ground, since an increased yield can be obtained thereby through the water gas action (see below) . The calorific value of the resultant gas is reduced, but owing to the large volumes produced the aggregate thermal yield is considerably increased. For example : in a test of Yorkshire coal in vertical retorts without steaming, the British Fuel Research Board obtained 13,100 cu.ft. of 544 calorific value gas or 71.3 therms (I therm=loo,000 B.Th.U.). In a parallel test with the same coal, steaming to the extent of 20% by weight of the coal charged increased the yield to 22,58o cu.ft. of 46o calorific value gas or 103.8 therms. Experiments have shown that if a small quantity of oil be introduced into the retort by suitable means, in addition to the steam, both high yields and high calo . rific value may be obtained. Thus a series of tests using 4.75gal. of gas oil per ton of coal and 5% of steam gave 15,450 cu.ft. of calorific value gas or 86.2 therms. An increased measure of elasticity is therefore conferred on the vertical retort. Variations of these methods for increasing the thermal yield or the elasticity of working of vertical retorts are under investigation in the industry.

The purification of coal gas is of great importance especially for domestic purposes. The purification system includes condensers and scrubbers for cooling the gas and extracting such valuable by-products as tar, benzol, naphthalene and ammonia ; but small quantities of impurities as cyanogen and sulphur compounds re main. Sulphuretted hydrogen is removed by passing the gas through layers of hydrated ferric oxide, the spent material being "revivified" by oxidation in the atmosphere, and after repeated use sold for sulphuric acid manufacture. The removal of carbon disulphide is more difficult but processes are now available whereby it may be converted to sulphuretted hydrogen and removed as above.

In Great Britain gas is now usually sold by the therm (I oo,000 B.Th.U.) and gas undertakings are given freedom regarding the calorific value of the gas which they supply. This must be de clared and cannot be altered without due notice, the Company be ing responsible for making the necessary adjustment to consumers' fittings. In Great Britain declared values vary from 64o to 28o B.Th.U., such extremes however being exceptional. Taking 26 of the most important undertakings, the declared value of gas sup plied varied between 450 and 56o B.Th.U., and of these II declared a value of 500 B.Th.U.

Water

Gas.—Large quantities of water gas are used industri ally and are also distributed both in Europe and America for town purposes. This gas is usually enriched by gas from cracked oil and is then termed carburetted water gas.

When steam is passed over highly heated carbonaceous matter a strong reaction occurs. The oxygen in the steam combines with the carbon, the final products of the reaction being formed in definite relative proportions depending upon temperature and con centration of the different gases present and also upon whether there is sufficient time for chemical equilibrium to be established. Two primary reactions occur, viz., and In addition to these there are many others of which perhaps the most important are, (3) CH-CO_,r'2CO, and It can be shown experimentally that above i000° C reaction; (I) and (3) predominate, while reactions (2) and (4) predominat( at temperatures below about 700° C. The percentage of carbon monoxide is high and that of carbon dioxide low at high temper. atures (about i000° C) whilst the reverse is the case below 70o° C. It is desirable then, in order to get the richest possible water gas, that the temperature of the fuel bed should not fall below 1000° C.

The reactions (1) (2) and (3) are strongly endothermic in char acter, that is to say they are accompanied by the absorption of heat and if no heat be applied during the reaction the temperature of the fuel bed will fall. Water gas producers are therefore gen erally worked upon an intermittent system ; that is to say, the gas making period during which the temperature of the fuel bed falls considerably is followed by a period of heat recuperation, in which the steam is replaced by a blast of air, the oxygen of which com bines with the carbon with evolution of heat so that the tempera ture of the fuel bed is again raised to that necessary to produce gas of the required CO content.

Water gas made from coke under commercial conditions usually has a calorific value of about 30o B.Th.U. per cu.ft., the per centage volume composition being approximately :—carbon di oxide 3.5, carbon monoxide 44.o, hydrogen 48.o, methane, etc. 0.5, nitrogen 4.o; calorific value 308 B.Th.U. per cubic feet. While water gas is sometimes used as a diluent for richer gases such as coal gas, it is usually delivered as carburetted water gas "en riched" with oil gas obtained by cracking petroleum oil in "car burettors" incorporated in the plant and heated by waste heat dur ing the blow period. The composition varies but the following may be taken as typical:—carbon dioxide 3.0, unsaturated hydro carbons 8.5, benzene 1•5, carbon monoxide 28.o, hydrogen 37.0, methane, etc. 20.0, nitrogen 2.0.; calorific value 500 B.Th.U. per cubic foot. The calorific intensity of water gas is higher than that of coal gas owing to the heat capacity of the products of com bustion being low. It can for this reason be made to burn with a short flame and is especially valuable for processes such as steel welding, where intense local heat is necessary. It has also been used, especially in Sweden, for small steel melting and re-heating furnaces.

Producer Gas.

For many industrial purposes a cheaply made gas which need not necessarily possess a high calorific value is of importance. Low calorific value gas is produced by the incomplete combustion of the carbon of the raw material, so that the products of combustion are rich in carbon monoxide. Such a gas would contain a proportion of and the whole of the nitrogen from the air used to supply the necessary oxygen. Gas of this type is often referred to as Siemens gas. As we have already shown, the combustion of carbon to carbon monoxide is an exothermic reaction in which large quantities of heat are given off. The bulk of this heat may be utilized if the hot producer gas be immediately consumed in the furnace, but if required for storing, this sensible heat must necessarily be lost. The temperature of producers work ing in this manner will tend to be high and artificial cooling may be necessary.

Alternatively a power gas though not water gas, may be pro duced by combining the above processes with the water gas process ; thus if a mixture of air and steam properly proportioned is admitted to a producer, both water gas and partial combustion gas are made simultaneously and continuously, the heat evolved during the exothermic reaction of C-f --0 = being utilized to maintain the temperature of the fuel bed which is being contin ually cooled by the endothermicity of the water gas reaction. The composition of producer gas varies widely but the following may be taken as a typical gas suitable for a steel melting furnace where a high proportion of CO is considered desirable : carbon dioxide 2.5, carbon monoxide 30.0, hydrogen 12.0, methane, etc. 3.0, nitro gen 52.5; calorific value 164 B.Th.U. per cubic foot. For power gas associated with maximum ammonia recovery a large propor tion of steam must be used. In such a case the composition would be approximately : carbon dioxide 16.0, carbon monoxide 12.0 hydrogen 24.o, methane, etc. 3.0, nitrogen 45.o; calorific value 145 B.Th.U. per cubic foot.

Producer gas forms a good fuel for gas engines. It may be made in positively blown producers or, for comparatively small powers, in those of the suction type where the vacuum induced in the suction stroke of the engine draws the necessary air and vapour through the Producer. It is essential, however, that the gas be cooled down before delivery to the engine, and that it should be thoroughly cleaned from both dust and tarry vapours.

By-product Gas.

The iron and steel industry presents two important cases of large quantities of valuable gas being obtained as a by-product of major operations. These comprise (a) blast furnace gas and (b) coke oven gas.

Blast Furnace Gas.

In blast furnace operations a low grade gas is necessarily evolved. A typical percentage composition for such a gas would be : CO2, io ; CO, 3o ; i ; 59, with a net calorific value of ioo B.Th.U. per cubic foot. From each cwt. of coke consumed in the furnace some 165,000 cu.ft. (at 15° C and 76omm.) of this gas would be obtained at a temperature of about 250° C. In modern steel works it is utilized for many purposes, including the pre-heating of the blast in Cowper stoves, steam raising, gas engines, steel furnaces and soaking pits, etc.

Coke Oven Gas.

The rich gas given off in by-product coking is similar in composition to retort gas made for town purposes but it is usual to extract the benzene. Its composition is approxi mately: carbon dioxide 3.o, unsaturated hydrocarbons 4.o, car bon monoxide 6.o, hydrogen 49.o, methane, etc. 34.o, nitrogen 4.0; calorific value 58o B.Th.U. per cubic foot. Where coke ovens are associated with steel works it is frequently possible to use some of the surplus gas upon the works, but when, as often hap pens, they are situated at a colliery, or are totally independent, difficulties arise in its disposal and it may be burnt to waste. The question of its utilization in greater measure for town purposes over long distances is the subject of experiment and of enquiry at the present time, and there is no doubt that if certain difficulties associated with constancy of supply etc. can be overcome it will find such an outlet in increasing quantities. In both Europe and America long distance pipe-lines are frequent, the gas being trans ported economically under pressures up to about 451b. per sq.in. Recently schemes have been considered for carrying gas long distances at pressures up to 45o1b. per square inch.

Combustion of Gases.

The advantages of gaseous fuels are (a) ease of distribution, (b) high thermal efficiency of utilization, (c) no smoke or ash, (d) ease of control giving either oxidizing or reducing flames and flames of varying temperatures.

The general question of combustion has already been dealt with, but certain special considerations are applicable to the case of gaseous combustion. In the burning of gases both rapidity of combustion and length of flame can be controlled by suitable adjustment of the gas rate and the primary and secondary air supplies.

Gas Burners. Gas burners vary widely but most types can be divided into two main classes according to whether or not air is mixed with the gas prior to its reaching the point of combustion. A relatively long flame, which may be suitable for certain types of furnace work, is produced by burning gas without primary air. On the other hand burners of the well-known Bunsen type can, if desired, be adjusted to give a rapid rate of combustion with rela tively short flames, such as are necessary where the combustion space is limited or maximum flame temperatures are required.

Submerged Combustion. Lately, with a view to obtaining in creased efficiencies in water or steam heating, attempts have been made to develop a type of burner in which gas can be consumed under water. For such submerged combustion it is obviously essential that sufficient primary air for complete combustion should be intimately mixed with the gas.

Surface Combustion.

Sir Humphry Davy in the earlier part of the nineteenth century discovered that a warmed platinum wire when introduced into a non-explosive mixture of air and coal gas became incandescent and continued to glow until practically the whole of the oxygen was consumed. Further experiments, par ticularly by Dulong and Thomson and Dobereiner, showed that many other substances possessed the same power. Before com mencing work on this subject in 1902 W. A. Bone proved (I) "that the power of accelerating gaseous combustion is possessed by all surfaces at temperatures below the ignition point in varying degrees, dependent upon their chemical characters and physical texture" and (2) "that such an accelerated surface combustion is dependent upon an absorption of the combustible gas, and prob ably also of the oxygen by the surface, whereby it becomes `activated' (probably ionized) by association with the surface" and (3) "that the surface itself becomes electrically charged dur ing the process." Bone also pointed out that "there are experi mental grounds for the belief that not only does the accelerating influence of the surfaces rapidly increase with the temperature, but also that the differences between the catalysing powers of various surfaces, which at low temperatures are often considerable, diminish with ascending temperatures until at bright incan descence they practically disappear." This accelerated combustion has been applied by Bone to various industrial heating appliances as a means of increasing the efficiency of heating operations. In these a mixture of air and gas in correct proportions is caused to burn without flame in contact with incandescent surfaces. The advantages are (I) accelerated combustion, (2) concentration of combustion where required, (3) minimum excess air, (4) high temperature, (5) rapid heat transfer by radiation.

Surface combustion has been applied to gas furnaces, gas cookers and heaters, etc., gas and air being forced by slight pres sure through a porous diaphragm of refractory material, the mixture burning on the exit surface and maintaining it at incan descence without flame. Other applications include boiler heating where the gas is burnt in boiler tubes filled with granular fireclay. For further particulars reference should be made to the following articles. Under COAL AND COAL MINING will be found a general survey of the world's coal resources and of the methods of coal mining; cognate articles will be found under ANTHRACITE, LIGNITE, PEAT, COKE and PULVERIZED FUEL. Mineral oil is treated under PETROLEUM, SHALE OIL, GASOLENE and PARAFFIN OIL. See also GAS MANUFACTURE, NATURAL GAS. (C. H. L.)