Liquid Fuels

LIQUID FUELS The use of fuel in liquid form is undoubtedly of very ancient origin, vegetable and animal oils being burned for the production of light and, to a limited extent, of heat. The modern oil industry, however, originated about the middle of the 19th century in the distillation of Scottish shales, which was carried out on a consider able scale during the subsequent 5o years, the most important products for fuel purposes being kerosenes and paraffin wax. When, however, natural petroleum was discovered, its phenomenal development gradually affected more and more the shale oil industry and the enormous quantities of natural oil now produced completely overshadow that obtained by distillation.

Petroleum.

Natural petroleum forms the basis of practically all industrial liquid fuels. The development of the internal com bustion engine has increased enormously the consumption of the lighter types of oils; the heavier fractions being utilized as fuel oils. Petroleum supplies also illuminating oil and the mineral lubricating oil which is required in such large quantities for mechanical equipment.

Crude petroleum is a mixture of hydrocarbons and small pro portions of oxygen, nitrogen and sulphur derivatives of hydro carbons. It can be divided into two main classes known respec tively as paraffin base crudes and asphalt base crudes. accordino to whether the residue after distillation consists mainly of paraffin wax or is of an asphaltic nature. In a third type, known as mixed base crudes, paraffins and asphalts co-exist.

The carbon in a crude petroleum varies under analysis from about 81 to 87%, hydrogen from i o to 14%, oxygen and nitrogen from i to 7%. Sulphur is an undesirable constituent; it is usually present only in small quantities but special methods of refining are necessary where it occurs in larger proportions.

Petroleum, which is usually associated with water and natural gas, is found in voids in porous sedimentary rock where the geo logical formation is such as to allow the oil to collect from over a wide area. Impervious rock usually overlays the petroleum con taining strata which is commonly concave downwards (anticline), the dome formed collecting natural gas at the top, under which is oil floating upon water. Occasionally the oil occurs in strata which are concave upwards, collecting in a pool from which it is prevented from escaping downwards by underlying impervious strata. In the former case the oil is obtained by drilling the non porous strata, when the natural gas and oil are forced to the surface by the pressure exerted by the underlying water. Under these conditions the well is termed a "gusher," but sooner or later it usually becomes necessary to pump out the oil.

It has been estimated that not more than 20% or 30% of the oil present in a natural well reaches the pipe line. The porous rock retains a considerable proportion and further waste occurs through infiltration of water and the escape of natural gas, in addition to losses due to improper storage and fires. Possibly some such means as mining may be developed for the recovery of a further percentage, but this has not yet become feasible economically.

Petroleum Resources.

In 1928 the U.S.A. produced about 70% of the world's total annual supply of one million barrels of petroleum ; but important oilfields are being worked also in Russia, Dutch East Indies, Persia, India, Galicia, Mesopotamia and Burma. Various attempts have been made to predict the world's oil resources but such computations have usually proved too conservative. Haslam, however, has recently estimated that the United States and Mexico control some 2o% and Great Britain 70% of the total recoverable supplies. Many petroleum producing districts have passed the peak of their productivity, but supplies have been maintained by the opening up of further fields. It is, however, certain that if oil requirements continue to increase at the present rate, a period will ultimately be reached when other sources, such as shale deposits, which exist in vast quantities, will have to be tapped. For this to become commercially possible the shortage of natural petroleum will have to reach a point sufficiently acute to be marked by considerable advance in prices.

Preparation of Fuel Oils.

Crude petroleums are usually subjected to fractional distillation, the more volatile constituents being used as motor spirit, which is also obtained from natural gas. Other fractions are kerosene, gas oil, fuel oil and lubri cating oil, a heavy viscous residue being left in the still. The great demand for motor spirit has had the effect of increasing the amount of light fractions "topped" from the crude, which may reach 2o% or more of the total. An additional quantity is obtained by ,cracking some of the heavier fractions, e.g., gas oil; the light product is a gasoline with anti-knock qualities. Stand ard specifications for motor and aviation spirit have been laid down by the British Engineering Standards Association. Crude petroleum which has lost its volatile constituents by evaporation during storage is sometimes used in the natural state as fuel oil; distillate and residual fuel oils are also used.

The gross calorific value of fuel oil averages about 19,o00 B.Th.U. per lb., but an oil of high specific gravity and compara tively low calorific value may contain more heat units per gallon than one of low specific gravity and high calorific value.

The flash point of an oil is the temperature at which under defined conditions a momentary flash is produced when a flame is brought near the surface. A minimum flash point is laid down in navy and other specifications. The temperature at which sufficient vapour to support continuous combustion is given off by an oil is known as the fire point; it is usually about F higher than the flash point.

The viscosity of oil, which changes rapidly with temperature, is determined by its resistance to flow as measured by the amount passing through a standard orifice in a prescribed time. The formation of solid paraffins, etc., may also cause difficulty in pumping oil through pipes at low temperatures. Asphalt base crudes as a rule must be preheated to reduce their viscosity to a degree which allows of ready atomization in the burner.

The chief drawback in the use of fuel oil is the presence of foreign matter; for it has been shown that sulphur and asphalt play little part in the difficulties that have been experienced. Con sequently all fuel oil should be strained, filtered or centrifuged.

Artificial Liquid Fuels.

To countries not possessing oil fields the problem of developing means of obtaining fuel oils from available materials, if only for emergency use, is of great importance. Several such processes are feasible, and attempts are being made to bring them to a commercial stage.

From Carbonization Processes.—The tars formed by the de structive distillation of coal, lignite or shale at low temperatures yield under suitable treatment a range of fuel oils. European oil shales yield over 20 gallons of oil per ton, while in the United States there are large shale deposits capable of yielding over 40 gallons per ton. About 15-22 gallons of crude tar per ton of bi tuminous coal can be obtained. After dehydration and removal of suspended matter this is suitable for use in Diesel engines, but its flash point is below the Admiralty specification, while after removal of the light spirit the residue is above the specified viscosity. By fractionation and refining, however, fuel and lubri cating oils can be obtained in addition to a quantity of motor spirit. The tar from high temperature distillation also contains petroleum constituents, but in relatively small quantities. It is frequently used as a fuel, the lighter fractions forming the source of an excellent fuel (benzole) for internal combustion engines.

Synthetic Fuels from Carbon Monoxide.—In the first of these, described initially by the Badische-Anilin and Sodafabrik and de veloped by Fischer in Germany and Patart and Audibert in France, carbon monoxide and hydrogen are produced by the action of steam upon red-hot incandescent carbonaceous matter according to the water-gas reaction, and the recombination of the gaseous molecules is subsequently effected by the action of heat in the presence of a catalyst. By suitable ad justment of pressure and temperature, together with a suitable choice of catalyst, products varying from oxygen-containing compounds such as methyl and ethyl alcohol to liquid and solid hydrocarbons of the paraffin series may be obtained. Patart has developed a method in which water gas made from coal or coke is used for producing alcohols by heating it to about 400° C at 30o atmospheres in catalyst tubes, while in the Fischer process synthetic liquid and solid hydrocarbons of the petroleum series are produced at atmospheric pressure.

It is not easy at the present stage of progress to estimate the extent to which the high thermal losses associated with these processes will affect the commercial position of the production of motor spirit from coal, which can only be decided from the results of working a full-scale plant.

Hydrogenation of Coal.—Another process for the manufacture of oils and motor spirit from coal is that of hydrogenation inves tigated by Bergius. Hydrogen is added to coal by the action of heat and high pressure in such a proportion as to bring the carbon hydrogen ratio to about that of natural oil, the molecules being caused to split up and combine with this hydrogen for the produc tion of an artificial oil material. The coal after mixing with a small quantity of alkaline iron oxide (luxmasse) and suitable oil or tar (usually obtained from previous runs) is forced into a reaction bomb to which hydrogen at 18o-200 atmospheres is pumped, the temperature being raised to some 400° C. Approxi mately 50% of the coal is converted into oil, which on distillation yields 20 to 25% of pitch. Of the remaining 5o% of the coal 2o% is converted into gas, 8% into water, o.5% into ammonia, while I r.5% remains as insoluble organic matter, the ash being An intermediate scale Bergius plant has been installed at the British Fuel Research Station and a large scale unit is being erected in Germany. A modification of the Bergius process is being worked commercially for the production of motor spirit by the Interessen Gemeinschaf t at Leuna in Germany.

Utilization of Liquid Fuels.

Liquid fuels derived from petroleum may be utilized either by combustion in furnaces or to produce energy directly in the cylinders of internal combustion engines. Either light or heavy oils are applicable to the internal combustion engine ; the former can be ignited by a flame or spark while with heavier oils vaporization and ignition are effected by the high temperature developed on the compression stroke.

In furnaces oil must be changed prior to combustion from the liquid to the gaseous state. In general a mixture of heavy and light gaseous hydrocarbons is formed, and for smokeless combus tion these must be brought into contact with a sufficient supply of air at a high temperature to ensure the complex series of changes involved in the successive picking up of molecules of oxygen overtaking the tendency of the heavier molecules to split up into solid carbon and hydrogen under the action of heat.

Oil Burners.

Preliminary vaporization is sometimes effected by the application of heat to light oils, but in general fuel oils are pre-heated only sufficiently to ensure free flow in the pipe line and burners. The main function of the burner is then to break up or atomize the oil, which enters the furnace in the form of fine mist or spray and is volatilized almost immediately. There are three main types of atomizing burners ; firstly the spray type in which a current of steam or air is used to break up the oil into a fine spray, secondly rotary burners where atomizing is effected by a disk or cylinder rotating at high speed, and thirdly mechanical spray burners where the oil is forced by high pressure through fine slots or orifices.

Each type has its advantages and disadvantages and the choice of burner must depend on local conditions. Thus, atomizers are cheap and simple in design and operation, but it is not practicable to use them with small flames and close regulation is difficult, and since they can only be used with dry steam there is a chimney loss in sensible heat of steam. Air atomization is used only when compressed air is already available; short intense flames are readily obtained and closer adjustment is possible than with steam. Rotary burners are often low in efficiency, but they are easy to operate and are very useful for low pressure installations where pressure for other types is not available. Mechanical spray burners require neither high pressure air nor steam, and are efficient at high capacities. Short intense flames can he ob tained and less energy is necessary than is required in either air or steam atomization. The disadvantages include high first cost of installation, lack of flexibility and tendency to clogging of fine orifices.

Advantages of Oil Fuel.

The price of oil on a thermal basis is usually relatively high, but it has many advantages over solid fuels. For the same calorific value it is considerably lighter than coal and is only half the volume. The saving in weight and storage space made possible and the ease of bunkering are of particular significance for steamships, especially since the labour entailed in stoking and trimming coals is reduced and the handling of ash eliminated. Further, the quality of oil is more uniform than that of coal, it is not liable to spontaneous combustion and it is clean and safe. Efficiencies up to 8o%, and high flame temperatures, are easily obtainable with oil fuel. It is interesting to note that in 1927 36% of the tonnage recorded in Lloyd's Register was equipped to burn oil though it does not follow that all these vessels were actually using oil.

Fuels for Light Internal Combustion Engines.

In light internal combustion engines such as are used in aircraft and motor vehicles the fuel used is termed petrol in England, gasolene in America and essence in France. It usually consists of the lighter hydrocarbons obtained from crude oil by distillation, of specific gravity about .725 and net calorific value 18,600 B.Th.U. per lb.

Liquid fuel is vaporized more or less completely in its passage by the suction of the engine through fine orifices (carburettor jets) and subsequently through the induction pipes and manifolds. On reaching the cylinders it consists of a mixture of liquid mist, hydrocarbon vapours and air. The efficiency of combustion de pends largely upon the completeness with which the fuel is vaporized ; if fuels of the kerosene type, which are less volatile than motor spirit, are used, they must be previously converted into vapour by the application of heat in a specially designed vaporizer. The theoretical efficiency obtained in the engine varies with the compression ratio, the highest practicable value of which depends upon the design of the engine and certain characteristics of the fuel used, in particular the latter's tendency to produce de tonation or "knocking." Benzol and alcohol can stand much higher compression ratios than other fuels before this phenomenon occurs. Detonation can be inhibited by the introduction into the spirit of small quantities of tetra ethyl lead; but since this material possesses toxic properties caution is necessary in its use.

Fuels for Heavy Oil Engines.

The Diesel engine, unlike the gas or petrol engine, draws in air during the suction stroke with out admixture of liquid fuel or gas. Oil is injected mechanically in the form of spray during the earlier portion of the working stroke, and burns at the high temperature brought about by the compres sion. A third type of engine, often termed semi-Diesel, is inter mediate between the gas and Diesel engine, the oil being injected into a combustion space incorporating surfaces maintained at a much higher temperature than the remainder of the engine cyl inder. Ignition is effected partly by exposure to these surfaces and partly by the temperature induced by moderate compression of the air drawn in during the suction stroke. The fuels used are intermediate in gravity and volatility between fuel oil and Diesel engine oil. The fuel for semi-Diesel and Diesel engines may be either gas oil or the residuum left in the still after distillation. Tar oils are sometimes used but owing to the difficulty of ignition special precautions have to be taken.

The British Admiralty specify for Diesel engine purposes either shale oil, petroleum, or a distillate or residual product of petro leum, with a flash point not lower than 175° F. The sulphur con tent is limited to 3% and a maximum acidity is also specified. The viscosity must not exceed 2,000 for an outflow of 5o cubic centi metres at a temperature of 32° F as determined in the Admiralty type Redwood viscometer for testing oil fuel.