YOU SHOULD KNOW OUT POWDERED FUEL
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BEFORE Dr. Diesel succeeded in running an engine on oil fuel he
had failed in experiments with powdered coal.. That was over 40 years ago, as is indicated by the following note which appeared in a technical paper dated May 15, 1897: " A German engraver named Diesel has designed and brought out a new pattern motor. In this he has, it is stated, had the financial assistance of the great firm of Krupp. It is elaiined that the engine can run on oil, gas or coal dust." • " Coal dust," being something new in fuels in those early years, was put in italics.
This vIT'as probably the starting of the interest in coal dust, powdered coal or pulverized fuel. its progress was very slow,and did not continue along the first line of application, namely, employment in an internalcombustion engine. Diesel's difficulties were so great that he soon deserted this type of fuel and switched over to heavy oils of the hydrocarbon series, with the successful results of which we ale all aware.
Abrasion of Cylinder Walls His troubles centred on the apparent impossibility of preventing -the ash in the . coal, released by combustion, from causing excessive wear of the cylinder walls. This would surprise few of us, thinking of the amount of ash that falls out of a fire grate, "even with the best of coals.
Further, when we consider that the ash content of oil fuel to-day is a fraction of 1 per cent.—it would not have been much higher in those days —and that it is a good coal that has as little as 4 per cent., most brands being of 10 per cent, and upwards, to 25 per cent. and over, we can visualize his troubles.
The success of Diesel on oil was such that he never returned to his experiments on pulverized fuel. It is regrettable that his early death prevented him from participating to the full in the wonderful developments springing from his genius and energy.
After a lapse of some years, one at least of Diesel's associates took up the dropped experiments and carried them through to a successful issue. Not, however, to the extent that road transport can participate in• the system. It is in the realm of stationary engines that progress has been made.
'Reports have appeared from time to time in the Technical Press of the
developments with an engine referred to as. the " Rupa," made by A German concern under the direction of Mr. Pawlikovvsky, the co-experimenter with Diesel in the early researches, referred to above. Experiments in recent years have been carried out on brown coal, of which so much is available in Germany: The principle of operation of the Rupa engine is that of pre-combustion of the fuel in an ante-chamber, the pressure therein generated injecting the unconsumed fuel into the main cylinder. This system also minimizes the possibilities of ash trouble.
In the pulverizing of the fuel, the ash .content can be loWered to between 2 and 3 per cent. It is passible to bring it down to 1 per cent., but the cost of doing this has to be weighed against the requirements of such fuel, and the operation is usually carried sa far only for certain purposes or processes.
Coal's Advantage on Points With oil at from three to four times the cost of coal, and allowing for the difference of calorific values in favour of oil (oil 19,500 B.Th.U.; coal, 14,000 B.Th.U. per lb.) and the cost of liner renewal as a result of ash wear, pulverized coal appears to hold the possibility of successful competition with oil in oil's sphere of activity. For us in this country it has the extra inducement of being an indigenous product.
There are several enthusiasts working in a quiet way on the job, and the Department of Scientific and Industrial Research has issued much information in the form of technical papers as a result of their investigations. Tile' internal-combustionengine application of pulverized fuel is, however, only a small percentage 'of the development &at has been carried out in connection with the utilization of our enormous coal resources by this particular system.
Paradoxically, America—the petrol country—.is that which has done most towards the development of pulver Ized fuel. All its modern power stations are being equipped to use it, as also are some of our own stations in this country.
We of the petrol-transport section of fuel users are apt to think that petrol is 100 per cent. transport fuel. As a matter of fact, 75 per cent. of the power generated by heat engines iii this world is produced by coal and only 25 per -cent. comes from 'the liquid products of the earth's stores.. In the modern power station, which in this country is almost exclusively steam operated, the thermal efficiency has gone up over 15 years from 18 per cent. to 28 per cent. In America, where pulverized fuel has advanced much beyond the practice of this country, this efficiency has improved at a greater rate. Indeed, increase in efficiency is, in a large degree, tied up with the use of pulverized fuel.
The usual specification of pulverized fuel for steam raising is as follows:— Calorific value not less than 14,000 B.Th.U. per lb.
Volatile matter content not less than 30 per cent.
Moisture Content notexceeding 3 per cent.
Ash content not exceeding 4 per cent.
Fineness: 85 per cent. through 200-mesh B.S. sieve; 98 per cent. through 100-mesh B.S. sieve.
Methods of Pulverizing Coal There are two ways of preparing the fuel for introduction to the boiler. Most power plants to-day pulverize the raw fuel on the site of its utilization, the mills being directly connected with the boiler in which the fuel is to be used. The other method is ;to pulverize the fuel at central stations and, to deliver it to the site of its use as a prepared. powder.
The former method solves many problems encountered by the second process, particularly that of spontaneous ignition, to which powdered fuel is prone. In the powdered form it is subject to the accumulation of moisture. It is this moisture that is the cause of spontaneous combustion. Combustion is oxidation and the moisture is the source of the necessary • oxygen.
There are several types of mill for the breaking doWn of the coal to the pulverized state — ring-roll mills, high-speed impact mills and ball and tube mills. They all do more or less the same job of work. There are certain variations in the class of product from these different mills, but these differences have no effect on our fuel. .
When we speak of " mesh" as being the .measurement of the size Of pulverized fuel, it is as a closed book to most of us, as compared with our well-known thousandths of an inch. It will be useful to make some crosscomparisons, so that we shall know, on both sides of the fence, what each of us means.
When we come to this matter of measurement, we might as well go the whole way, and include the much finer measurements that we 'shall find necessary in our excursions into the at present unknown region of micronized fuel.
The size of fuel used in boilers for steam raising is, at the present time, usually 200 mesh. This is the specification given above. This fuel can be purchased and will be delivered in tank wagons; it is used. under such conditions in London and many large towns.
In our engineering terms 200 mesh means that the • average size of the particle of fuel is 0.003 in., that there are 333 of them lying alongside of each other in 1 in. This is ordinary pulverized fuel.
Coal Micronization
Coming now to micronized fuel, we consider that this .200-mesh stuff is coarse, liming in mina what a degree of finenesrcan actually be produced. Up -to themoment micronization has not been applied to coal to be used as a fuel; it has not been necessary. The process of micronization is an
additional expense superimposed on pulverization, , Two uses to which micronized material has been put are in the production of face 'powder and in the preparation of metalt for processing by powder metallurgy; purposes guile unconnected with dirty, coal.
• Now, road-transport requirements may bring about the need for a development of micronization of already pulverized coal.
Flame length is a factor related to, particle size. In the power-station boiler, using particles of 200-mesh size, the length of flame is in the region of 15 ft.
Considering the possibility of the application of steam to road vehicles, *and of the use of pulverized fuel in those boilers, on account of the speed of reactivity to steam demand unobtainable by solid fuel in a flash boiler, the condition of a 15-ft. flame length is unthinkable. Therefore, independently of technical considerations, this flame length must be cut down to one-third or one-quarter of that of the power-station boiler. Under such conditions a steam boiler fed by micronized fuel may become a possibility, and the technical problem arises as to how to do it.
Size for Vehicle Use First, the size of the particle must be reduced. Is that practicable? Here is where an adaptation of the face-powder practice is likely to help us in our problems of steam raising in a confined space.
It has been calculated that nothing larger than one-thousandth of an inch will give us a.fla.me length with which it will be possible to deal in a boiler of a size that we shall be able to carry on a road-transport vehicle.• It does not follow that such a size is the smallest that will be useful to us in our search 'for a really small portable boiler. Nor is this size, which is represented by a mesh' of 000, the smallest by a long way that it is technically possible to prepare. The accompanying table gives some information on the sizes that are
possible, and their comparison with several standards of measurement :— From mesh 625 to 2,500, .referred to as -the sub-sieve range, it has been technically, but not commercially possible, to produce micronized particles by dry grinding.
The process, which is claimed to make this technical possibility also into 'a commercial one, has been developed in America. It is called the Micronizer " reduction process. Not only doesat convert the technical possibility into a commercial one in the range referred to, •but also it extends that possibly commercial range down to 12,500.
Micronization is not an ordinary grinding operation as is pulirerization. It is a process in which the particles are made to reduce themselves by causing them to travel round a circular chamber by means of jets of superheated steam or compressed air -injected tangentially, the supply of coal being also tangentially injected in the form . of a coarse partially pulverized powder of about 30 mesh (0.02 in.).
Cyclone• Reduction Process The outlet of the finished finely micronized fuel is at the centre of the chamber, the centrifugal force of the larger particles keeping them at the periphery in, the steam stream until they are sufficiently reduced to escape from the action of centrifugal force arid drift to the centre hole.
This process is not yet in commercial operation in this country, but such material is obtainable in quantities sufficient for experimental purposes, and will later be available in commercial quantities, as the demand for itappears. There are numbers of processes using pulverized. fuel in which particle-size limitation prevents further development.
The reduction of particle size will bring the opeiation of combustion from its existing condition of burning a solid fuel into one in which the fineness of the particle becomes analogous to the condition of a 'gaseous fuel. The result will be an enormous increase in the speed of the release of the heat units of the fuel; reactivity is the expression in producer-gas practice.
The effect of such a aevelopraent will be to provide road transport with a monotu.be boiler having the same response to the accelerator as that of , a boiler as used in present-day heavy steamers, at a fifth he weight.
There isa possible answer to the old cry" taxed out of existence." '