March 2, 1917, THE COLLIERY GUARDIAN. 433 LIQUID FUEL AND ITS COMBUSTION. By Pi\>f. J. S. S. Bra me. In the course of his paper on this subject before the Institution of Petroleum Technologists on February 20, the author said he did not propose to deal with the whole subject of the use of liquid fuel, but to confine himself to the use of oil fuel when burnt free in air for steam raising and other industrial heating operations. The characters which a fuel oil should possess were high calorific value, fluidity at moderately low temperatures, freedom from solid matter which might choke the jets of atomisers or cause trouble through abrasion, and a satisfactorily high flash point. Two typical specifications for petroleum fuel oils were cited. That drawn up for the United States Bureau of Mines (Technical PajJer No. 3, 1911) specifies for a closed flash point of 140 degs. Fahr.; that the oil shall not congeal or become too sluggish to flow at 0 deg. Cent.; it shall not contain more than 2 per cent, of water or 1 per cent, of sulphur, and must not contain more than a trace of sand, clay, or dirt. The standard calorific value is to be 18,450 British thermal units (net), and shall not fall below 18,000 British thermal units per lb. In the Admiralty specification no mention appears of calorific value, but as the oil must be either a shale or petroleum oil, the calorific value will necessarily be satisfactory. Provision is made for testing the viscosity at the freezing point of water by means of the special pattern viscometer designed by Sir Boverton Redwood for this purpose. The flash point of the oil shall not be lower than 175 degs. Fahr, close test, except for oils of low viscosity, such as shale oils, the flash point of which must not be less than 200 degs. Fahr. The quantity of water in the oil shall not exceed 0-5 per cent., and the sulphur 3 per cent. A clause specifies further that the acidity shall not exceed 0-05 per cent., calculated as oleic acid. The calorific value of petroleum fuel oils does not vary over very wide limits, since the ultimate composition of oils from any source is fairly constant. In this respect petroleum oil fuels show to marked advantage in com- parison with coal, the calorific value and composition of which vary over such wide limits, notably in the amount of true combustible matter present in different samples. The following was given as the composition of four typical and well-known fuel oils :— "^(Texas)1^ Californian. Ostatki. Admiralty. * * t I Carbon........... 83*26 ... 81*52 ... 87*5 ... 86*40 Hydrogen ........ 12*41 ... 11*01 ... 11*0 ... 11*55 Sulphur ........... 0*-0 ... 0*55 ... — ... 0*34 Oxygen & nitrogen 3 83 ... 6*92 ... 1*5 ... 1*71 * Used in United States Navy Board trials, f Heck. I W. H. Patterson, J. S. Chem, Ind., vol. xxxii., 218,1913. In the following table the calorific value, etc., of some typical fuel oils is given :— Specific Calorific value. gravity (Bomb cat rimeter.) Source of oil. at 15 degs. , A__ — Cent. Calories. B.Th U. Russia (ostatki) .. .... 0*914 ... 10,990 ... 19,780 Do .... 0*920 ... 10,580 ... 19,040 Texas 0*928 ... 10,750 ... 19,350 Do 0*927 ... 10,730 ... 19,310 Do 0’934 ... 10,900 ... 19,630 Burma .... 0*924 ... 10,520 ... 18,940 Do .... 0*900 ... 10,610 19,100 Borneo .... 0’915 ... 10,780 ... 19,400 Mexico — 10,460 ... 18,800 Rumania — 10,710 ... 19,250 California — 10,400 ... 18,700 Persia — 10,840 ... 19,500 Trinidad . .. — 10,480 ... 18,HO (The last five from Redwood.) In this country, particular interest centres around the shale oils of Scotland and the tar oils, or even crude tars, produced. The higher boiling oils from the Scotch shale are almost ideal oils for fuel purposes when the paraffin has been removed; being distilled oils, they are very clean, and, in addition, have the valuable property of fluidity. Coal tar products are likely to play a far more important part as fuel oils in the future. At the present time there is such a demand for coal tar products as raw material for munitions of war, that tar is too valu- able to burn, and there has been a great extension in the production of tar owing to the replacement of the old wasteful beehive coke ovens by modern recovery plant. So important are tar products, that some enthusiastic advocates of fuel economy demand that no coal should be burnt in the raw state, but that all should be car- bonised and the by-products recovered. In the normal state of things, it is fairly certain that we shall be pro- ducing far more tar than is required to furnish , all the special by-products for which there is a market, and much tar will be available for fuel purposes. Within very recent years the price of crude tar and tar products has been so low that producers have applied it for steam raising and retort heating in gas works. Whilst the gas tar of a few years back was a thick viscous black mess, hardly sometimes worthy of the term “ liquid,” many of the modern tars produced by carbonisation of coal in bulk are truly liquid fuels, and in many cases may be employed directly in Diesel engines, and certainly could be burnt with any good form of atomiser. In addition to the ordinary tar from the carbonisation of coal at the high, temperatures in ordinary gas works, a large supply of liquid fuel may be forthcoming from low-temperature carbonisation, which is finding so many advocates as a practical means of solving the problem of a smokeless fuel for the ordinary domestic grate. The domestic consumption of coal is about 35 million tons annually; if half of this were replaced by low-temperature coke, in the carbonisation for which probably some 18 gals, of tar per ton would result, over 300 million gals. of tar would be available. From this a large quantity of motor spirit would be distilled oft the valuable cresylic acids obtained, but a very large proportion would undoubtedly find its application as liquid fuel. Petroleum oils must always possess a marked superi- ority over tar or tar oils. The latter always contain a certain proportion of oxygen, and have therefore a lower calorific value. Again, tars in burning give off pungent irritating fumes, which, if they escape, are very trouble- some in any stokehold. The average calorific value of a petroleum fuel oil and crude tar are respectively about 19,400 and 15,850 British thermal units. Turning to the methods of combustion, the author mentioned the open Nobel troughs and percolation on a porous bed of non-combustible material, and then the attempts to gasify the oil, and burn the oil gas, such as the system installed by Foote on the United States gun- boat “Palos.” In the Dorsett (1868-69) system improved by Eames (1875), an external gasifying chamber was employed. The oil trickled over a scrips of horizontal shelves in the gasifier, and met an upward current of superheated steam. The gases were sent forward into a combustion chamber placed beneath the boiler or heating furnace. Simm and Barff (1865-67) also used a vaporising system. In all such systems of vaporisation, the natural result of “ cracking ” followed, carbon deposits soon choked the vaporisers, and the heaters also quickly burnt out. A more recent system, in which these' difficulties were less pronounced, was that of Durr. A forward extension of the furnace carried two vaporisers, the larger one nearest the furnace was the main vaporiser; a smaller one situated beyond this served to heat the larger in starting The general failure of such systems as gasification of heaVy oils led naturally to attempts to burn the oil in the next best physical condition, namely, in the form of a fine spray. The nearer such a spray approached to the condition of a “ mist ” of oil globules, the better the chance of perfect combustion, if the supply of air to each little globule could be satisfactorily accomplished, and now these atomisers completely hold the field as far as heavy fuel oils are concerned. In the report of the United States Fuel Board, 1904, the opinions are expressed that “ the most efficient for any purpose is, in general, the simplest piece of mechanism using the least amount of steam or air for atomising purposes,” and “ the actual form of burner of a good pattern has little influence on the results.” The following summarises the main conditions :— Ease of instalment and control of oil and spraying agent; possibility of easy substitution, of rapid inspec- tion, and of quick renewal of parts. Atomisation is com- monly carried out by spraying with (a) steam, (5) air, and (c) direct mechanical breaking up of an oil stream, escaping under pressure. Types of Atomiser. The author then gave an outline of the principal types of atomisers, and a brief description of well-known atomisers characteristic of each of the three main types. In the first type, the simple breaking up of the oil is effected by the impact of a jet of steam and air under pressure, the oil issuing from an upper orifice, and trickling to the end of the pipe carrying the atomising agent. Atomisers of this type are the simplest in design and construction, and have, moreover, given some of the best results obtained in practice, where it has not been neces- sary to force the combustion of a large quantity of oil. The Booth burner (long slot type) has given most excel- lent results on the Santa Fe Railway, the atomisers of these engines being arranged at the forward end of the firedoor, thus enabling the usual brick arch to me dis- pensed with, though at the sacrifice of easy accessibility. The size of the oil orifice is 2| in. by ~ in.; the steam orifice, 2| in. by in. In the W. N. Best atomiser, the spraying medium plays down into the horizontal cup of the oil pipe. Provision is made for altering the position of the jet in relation to the cup. Atomisers of the “ injector ” pattern are the most common type for use with a spraying medium, and most of the well-known atomisers are on these lines. Sub- divisions of the type include the simple jet injector, the slot form which gives an increased capacity, and the concentric form with oil and steam orifices of circular form. To obtain satisfactory combustion, special design of the furnace, with suitable brick baffles, is often neces- sary. In the atomiser used by Mr. Thos. Urquhart, the air is introduced around the jet. Air. James Holden’s atomiser, which has done such good work in the Great Eastern Railway and other railways, is of similar type, with a centrally-introduced air supply. In the latest form of Holden atomiser, the atomised oil enters a for- ward enlarged chamber, from which it issues in several jets on slightly different alignment from the axis. The Carbogen atomiser has a central and surrounding air supply, and this form is well known for its successful results in many heating operations, notably glass melt- ing. The Rusden and Eeles atomiser has oil and steam vents arranged around a central blank spindle, the oil chamber being jacketed on both sides with steam. ■ In direct spray atomisers, no spraying agent is employed, but the oil, being raised sufficiently in tem- perature to lower its viscosity and coherence, is forced under pressure through suitable orifices, in some cases striking against an external baffle. By causing the oil to leave a suitable orifice under high pressure and with a strong rotary motion, very perfect atomisation is accomplished. In one form, the rotation is imparted by means of a thread cut in the central spindle, and in another by three or more jets of oil issuing through suit- able holes cut tangentially to the central axis of the atomiser. In one of the earlier forms, that of Swensson, the oil jet struck a knife-edge sprayer, the sharp point of which was towards the hole. The well-known Korting atomiser is of rotary type, the oil passing through a narrow channel formed by the threads in the screw and the inner wall of the oil tube. Other atomisers of similar type are the Kermode and one form of Thorneycrofts. A variation which exhibits novel features is that made by Messrs. Samuel White and Company, of Cowes, in which, to overcome the difficulty that oil must be heated before it sprays from the usual rotary pattern atomiser, a central spindle carries a small spraying end, which passes through the outer orifice. When outside, the cold oil jet is forced against this end, and steam can be raised in this way. When the heaters are at the proper temperature, the spraying end is drawn back inside a small chamber, and the hot oil as sprayed through the same orifice, the requisite rotary motion being imparted by suitable spiral channels cut in the face of a wide-angled cone. In comparing the relative merits of these different methods of atomising oil, the author claimed for steam the great advantage that, once the pressure in the boiler has been raised, the atomising agent is right to hand in unlimited quantities, and at any pressure likely to be demanded, though usually steam atomisers do not respond so well as air or pressure systems, when boilers have to be forced. Air may be truly termed the natural atomising agent, because not only does it effect the spraying, but in doing so should ensure every little oil globule being carried forward with the requisite air for its combustion. Insuffi- cient attention is sometimes given to the character of the’ flame from atomisers in relation to the combustion space. Atomisers giving a long narrow cone of flame, such as that from many of the steam type, are particu- larly suited to the long, comparatively narrow furnace of the Lancashire type of boiler. On the other hand, with the wide combustion space and smaller depth of the usual water tube type of boiler, the wide cone of flame from the usual pressure spraying burner is most suited. Another point, and one on which the United States Board laid great emphasis, is the tendency to instal too few atomisers. Of course, the design of the furnace in many cases very stringently limits this number, but there can be no doubt of the advantage of using atomisers spraying a reasonable amount of oil, and working under the best conditions, with additional ones which can be put in operation when required, over attempting to get good results from the same atomisers under widely varying oil consumptions. A number of atomisers give a far more uniform heat, which is of great importance in the case of water tube boilers where the proper circulation of water will not take place if there is undue heating in certain parts. A number of atomisers reduces greatly the blowpipe-like action so frequently found. Dealing with the preliminary treatment of the oil before atomisation, Prof. Brame said that oils should be sufficiently fluid at ordinary temperatures to be dealt with by the pumps, but heating might be necessary. It was obviously impracticable to heat the whole of the oil in a large bunker, but a steam coil around the suction end of the pipe would serve to heat the oil sufficiently when necessary. With an oil properly free from water, and carried in well-riveted compartments, no trouble should be experienced from water. If a pocket of water gets carried forward to the atomisers, the flame may be extinguished, and then fresh oil following into the hot furnace, a slight explosion probably results. With oils of density of about 0*9, owing to the viscosity of the oil and slight difference between the gravity of the oil and the water, the latter may never separate at ordinary temperatures if the water globules are small. Heating promotes separation by reducing the viscosity of the oil so that a globule of wafer, of such a size that it remains indefinitely suspended in cold oil, settles rapidly in the heated oil; moreover, heating accentuates the difference in specific gravity between the two fluids, owing to the greater 'expansibility of oil. The approximate coefficients of expansion of heavy oil and water per Fahr. deg. is 0-00039 for oil, and 0*000264 for water. The reduction of the viscosity which results on heat- ing facilitates greatly the proper atomisation of the oil; and heating is absolutely essential with pressure sprayers. There is, however, an easily-reached maxi- mum for good results with any atomiser. On the Santa Fe Railway (Booth atomiser) irregular working was noted at 140degs. Fahr, oil temperature; regular working was attained with an oil temperature of 90 to 95 degs. Fahr. In connection with the use of thick oils, mention may be made of the patent taken out by Air. Arnold Phillip, the Admiralty chemist at Portsmouth (1913, No. 14778), who found that the addition of naphthalene (8 per cent.) greatly reduced the viscosity of thick oils; and this material being plentiful and cheap, the method promises to render available certain thick oils which are not suited for fuel purposes in the untreated condition. Combustion of Oil Fuel. For the proper combustion of oil fuel in a restricted space, such as a boiler furnace, the United States Fuel Board emphasised the necessity of a short intense flame, and specified the conditions for its attainment as : (1) The use of a pure hydrocarbon fuel; (2) initial heating of the air; (3) intimate admixture of sprayed fuel with air; (4) large surface of fuel (i.e., fine atomisation) exposed to impact of this air. It should always be remembered that the spherical form of the globules offers the least surface for combustion, which emphasi'seu the necessity of the globules being as fine as possible. Tunning next to the important question of furnace arrangements, on .which success in burning oil fuel depends almost more than any other factor, experience shows that ample combustion space is necessary. The method of introducing air to complete the combustion (secondary air) initiated by the primary air passing through the atomisers, or front of the furnace, is closely connected with the construction of the furnace.