1216 THE COLLIERY GUARDIAN. June 29, 1917. CURRENT SCIENCE AND TECHNOLOGY. Fuel and Power Supply. Mr. E. W. L. Nicol (The Electrician) deals with the mechanical stoking of large capacity steam boilers exclusively with gas coke as fuel, and the lines upon which experiment and research may be directed with the view to improving and standardising steam generating appliances used, as well as to elucidating factors advantageous to the efficient combustion of non-volatile coke as fuel under steam boilers. The sensible heat unavoidably wasted by the quench- ing of gas coke represents a considerable proportion of the total heat used for carbonising the coal, which, if it could be used for the purpose of steam raising, would appear to offer attractive possibilities in a combined fuel and power supply centre, laid out with this end in view. By thus utilising the sensible heat as well as the combustion heat of surplus gas coke in a suitably constructed boiler furnace, a considerable saving in fuel would be effected, as well as economy in transport and in many other directions. One type of furnace suggested by the writer has an inclined grate, and is intended for use with coke exclusively, incandescent or otherwise, so that the boiler tubes, so far as pos- sible, are exposed to the direct radiant effect of the burning fuel, no intervening baffle, arch, or secondary combustion space being considered necessary or desirable. In power plants where normal conditions necessitate the continuous working of boilers at or over their full capacity, as rated on a volatile coal fuel basis, the cutting and grading of coke to the most advantageous sizes will be an important feature. The resistance of Table A. — Coal. Coke. Breeze. System of draught Natural Impelled Impelled System of stoking Mechanical Hand Hand Mechanical Type of boiler Water-tube Lancashire Cornish B.& W. Stirling 60’75 B.& W. Grate surface, square feet 87’88 36 0 .29’0 81’0 50’6 Fuel per grate-foot-hour, lb 26’9 15’0 16 T 25’2 37’1 28’28 CO2, per cent 8’0 14’0 16’0 14’5 14’5 11’7 Excess air, per cent 159’0 48’0 28’0 43’0 43-0 75’0 Fuel loss, per cent 23’0 13’0 11’0 12’5 12’5 Calorific power, B.T.U 12,000 12,000 12,' 00 10,083 9,590 9,018 Evaporation,* lb : 7’5 9’8 9’8 7’84 5’8 6’59 Plant efficiency, per cent 60’3 78’8 78’8 75 Tf 69’6f 71’Olf 10/6 Current rates quoted f.a.s. London, per ton 28/0 28/0 12/10 28/0 10/6 10 6 Fuel cost per 1,000 gals, evaporated 16/8 12/10 6’0 8/0 71 * From and at 212 degs. Fahr, per lb. of fuel as fired. f No economiser or superheater. coke to rapid oxidation varies considerably, and this factor would appear to have some definite relation to its hardness and density. The ratio of surface exposed to the oxidising process to unit of weight also has an important influence upon the economic rate at which coke may be consumed; and the desirability of grading to approximately uniform size will be obvious. . The various standard grades adopted by the London Coke Committee, viz., 0 to |in., Jin. to Jin., Jin. to 2 in., have been so far sufficient to meet the special requirements of steam users for both mechanical and hand firing; but here, again, arise factors having an important bearing on the efficient utilisation and stoking of coke by mechanical means, and also upon the economical rating of coke-fired boilers, which can be determined only by organised and systematic experiment and research conducted on a working scale under competent supervision. The comparative evaporative values of coke and coal as fired under various conditions, and also the rela- tive evaporative capacity of coal- and coke-fired boilers, will be of interest. Where appliances for the positive control of the furnace atmosphere are installed, coke has a decided inherent advantage over bituminous coal, in that it contains practically only one combustible constituent — fixed carbon. Under the best conditions of mechanical stoking, bituminous coal has a relatively low efficiency, as it requires for the complete combustion of its numerous and ever- varying fixed and volatile constituents, which include hydrogen, at least 50 per cent, excess of air. This figure corresponds to some 13 to 14 per cent. CO2 in the flue gases—a desirable result, which is, in the writer’s experience, very rarely obtained in ordinary practice. With all due deference to those in charge of well-conducted steam plants, it is now suggested that 5 to 8 per cent. CO2, indicating 300 to 150 per cent, excess of air, would probably cover the higher limits of working where highly bituminous coal is used at high rates per grate-foot-hour under ordinary condi- tions of draught and supervision. Compared with bituminous coal of equal gross calorific power, coke— which is relatively uniform in character and composi- tion, requires a minimum excess of air and no secondary air supply in order to complete its combus- tion— will have a higher net evaporative value. Table A gives typical results obtained with bitu- minous small coal, gas coke, and coke breeze of good average quality, under various conditions of mechani- cal and hand firing. Rust Prevention in Ferro-Concrete. An interesting contribution to this problem has been published in the Schweizerische Bauzeitung (abstracted in Engineering) by Mr. B. Zschokke, of the Swiss Station for Testing Materials at Zurich. Zschokke uses potassium bichromate in two ways. He either mixes the cement with chromate solution instead of water, or he prepares a paste of Portland cement, sand, and solution of bichromate in different concentrations, and paints one face of iron plates with this paste, the edges and back being painted with asphalt varnish. In the latter case, the moisture should not be less than 50 per cent., else the hardening crust of concrete on the iron will peel off. The speci- mens are sprayed over with water containing carbon dioxide for the first five or six days; afterwards they become so hard that the concrete crust cannot be knocked off with a hammer. These specimens were exposed, together with ordinary cement specimens, for months in partly-covered dishes, in which they rested on dry supports over water. The specimens prepared with ordinary water rusted very badly, in all cases, whilst the chromate plates kept perfectly free of rust and of cracks. In other tests round irons, 18 mm. in diameter, were coated with a layer of cement and calcium or potassium chromate, which hai dened in a day, and then placed in a mould, the empty space of which was filled with concrete. In other tests, again, pieces of sheet iron were coated with ordinary concrete or chromate concrete, and lines were then scratched into the coating, exposing the bare iron ; the rusting spread from these scratches in the ordinary concrete, but not in the chromate concrete, and other experi- ments confirmed the conclusion that the chromate pro- tection will extend over small exposed spots. As the special painting of the iron with chromate might be objectionable, the whole concrete was, in the other series, prepared with a 5 per cent, solution of potas- sium ; the results were equally satisfactory. There remained the question whether the addition of chromate impaired the strength of the concrete or delayed the setting ; there was no delay, though pos- sibly a very slight decrease in the strength; in some cases an increase in the strength was observed. A chromate-concrete would cost from 3 to 5 per cent, more than an ordinary concrete, Zschokke estimates. He has applied for a patent, though the proposal is certainly not novel. The cost might possibly be reduced by diminishing the proportion of chromate; in aqueous solutions less than 1 per cent, of dichromate suffices to render the iron passive. The serious draw- back, to which Zschokke himself directs attention, is that the chromate protection fails in the presence of certain common salts, alkali chlorides, and sulphates, and also of considerable amounts of sulphurous acid vapours; the process could not be recommended, hence, for sea water and acid atmospheres. On the other hand, the painting of iron pipes, etc., with chromates and other suitable materials should be further studied. Hot and Cold Sizes of Firebricks. In an appendix to the report of the Refractory Materials Committee, Dr. J. W. Mellor gives the results of measurements made on hot and cold fire- bricks with the Cop pee apparatus. The bricks are marked with two saw cuts, in each of which a pointed platinum wire is snugly bedded. The distance apart of the tips of the wires is measured while the brick is cold (15 degs. Cent.) by means of a cathetometer pro- vided with an invar scale. The brick is heated in a furnace to the desired temperature, and, by means of suitable apertures in the furnace, the distance apart of the wires is measured again. The results are indi- cated in the following table :—• Distance between platinum wires. Linear coefficient Lab. f J ' s Expan- of thermal mark. Type of brick. Before After expansion. firing. firing. sion. Cm. Cm. Per cent A1... Silica brick 17’95 .. . 18’05 ... 0’56 ... 0’0000048 A2 Firebrick 18 78 .. . 18’87(5) ... 0’51 ... 0’0000044 Bl .. „ 20 31 .. . 20’45 ...0’69 ... 0’0000059 B2... „ 19’43 .. . 19’50 ... 0’36 ... 0’0000031 B3... „ 20’06 .. . 20’25 ... 0’95 ... 0’0000082 Cl Silica brick 20’29 .. . 20’43 ... 0 69 ... 0’0000059 C2 ...Firebrick 19’83 .. . 19’94 ... 0’55 ... 0’0000047 C3... „ 19’80 .. . 19’92 ... 0’61 ... , 0’0000052 D ... „ ......... 19’01 .. . 19’06 ... 0’26 ... 0’1 000022 El... „ 18’35 .. . 18’42 ... 0’38 ... 0’00 0033 E2... „ 18-68 .. . 18’80 ... 0’64 ... 0’0000055 F ... 18’54(5) . . 18’66 ... 0-62 ... 0’0000053 Gl... ,, 18’77 .. . 18’83 ... 0-32 ... 0’0000028 G2... 18’29 .. . 18-41 ... 0-66 ... . 0-0000057 H ... „ 18’71 .. . 18’84 ... 0-70 ... 0’0000060 J ... 19’41 .. . 19*53 ... 0-62 ... 0 0( 00053 K ...Red silica brick 19’29 .. . 19 46 ... 0-88 ... 0’0000076 LI... Silica brick 18 27 .. . 18-41 ... 1 77 ... 0’0000066 L2... 18’52 .. . 18-78 ... 1’40 ... 0’0000121 M ... „ 18’85 .. . 18-94 ... 0’48 ... 0-0000041 Nl... Firebrick 18’53 .. . 18’66 ... 0’70 ... 0’0000060 N2... „ 18’84 .. . 18’98 ... 0’74 ... 0’0000064 O ... „ 18’89 .. . 19-06 ... 0’90 ... . 0’0000177 P ... „■ 19’50 .. . 19-70 ... 1’03 ... 0’0000088 Q ... >, 18’93 .. . 19-07 ... 0’74 ... 0’0000064 R ...Magnesite brick 18’39 .. . 18’66 ... 1’47 0’0000126 If the brick is imperfectly burned, there is super- posed on the effects of thermal expansion an after- contraction or after-expansion. In consequence, in the former case the results are too small, and in the latter case, too large. Still further: (1) The brick does not contract to its original volume on cooling. The difference between the cold sizes of the brick before and after firing shows the magnitude of the after-contraction or after-expansion, when the brick was heated under the conditions of the experiment. (2) In all cases of after-contraction (or mutatis mutandis, after expansion) the brick may show a smaller volume, at, say, 1,180 degs. Cent., than it does at, say, 1,060 degs. Cent. This shows that the effects of thermal expansion are altogether masked by the after-contraction. Thus, distance of wires at 15 degs. Cent., 18-72 cm.; at 1,060 degs. Cent., 18-80 cm.; and at 1,180 degs. Cent., 18-75 cm. It was only when the whole of the cold and hot measurements were under consideration, and were being extended to different temperatures, that the dual character of the pheno- menon was discovered; and it has made the committee suspect these and other measurements of the coefficients of thermal expansion of refractory materials at high temperatures—unless it can be demonstrated that the results are not affected by the error under consider- ation. These measurements also show that the coefficient of expansion of fireclays and of silica bricks decreases with rise of temperature. Thus, in two cases : Ordinary firebricks. silica ( 7 74 ' firebricks. From . 1. 11. 15 degs. to 940 degs. Cent. 0’0000069... 0’0000081... 0’0000051 15 degs. to 1,180 degs. Cent. 0’0000060 .. 0’0000060... 0’0000064 The coefficient of thermal expansion represents the average increase in length which a brick undergoes when unit length is heated 1 deg. Cent. ; so that a 9 in. brick with a coefficient of 0*000005 will be 9 (1 + 0-000005 x 1,200), or 9-054 in. long at 1,200 degs. Cent. Otherwise expressed, a brick 2| in. thick when cold will be 2J (1 + 0-00005 X 1,200) or 2-515 in. thick at 1,200 degs.; or, neglecting the jointing cement, 66 bricks piled one on top of the other would expand 1 in. when heated to 1,200 degs. UTILISING COAL MINE WASTE IN THE PACIFIC NORTH WEST. The following data, obtained by the Puget Sound Traction, Light and Power Company, of Seattle, Washington, in their experimental plant with the burning of pea coal, are given in a report by Mr. Henry Hull in the Puget Sound Electric Journal (the house organ of the company) : — Within 100 miles of Seattle there are numerous coal mines with thousands of tons of fine coal which at pre- sent is unmarketable, and which should be available slightly above the cost of transportation. This coal is a lignite variety, particularly adapted to use in powdered form, due to the high volatile constituent, and the very high fusing point of the ash. The foregoing characteristics are very important, inasmuch as a high carbon coal requires very fine pulverisation and carefully designed furnace to main- tain the high temperature until ignition is complete, and a low fusing ash will, when carried in suspension, cling to the tubes of the boilers, close up the flame space, and make its operation impossible. To prepare this coal for burning, it must first be thoroughly dried, and moisture . content reduced to approximately 1 per cent., before it can be properly pulverised. It must then be pulverised to powder form, where approximately 85 per cent, will pass through a 200-mesh screen, and 95 per cent, through a 100-mesh screen, if the best results are to be obtained. It should then be fed directly to the furnace, or if transportation or storage is necessary, it should be kept airtight so far as possible, to prevent absorption of moisture. The danger from explosion when hand- ling this material is eliminated if it is kept in bulk, and not allowed to become suspended in a mixture of air. In the latter case, a highly explosive atmosphere may be found which will readily ignite if brought in contact with a flame. The coal is dried and pulverised by the Pacific Coast Coal Company at their briquetting plant near Kenton, which is equipped with a Raymond pulverising plant. It is then loaded in a special car equipped for the pur- pose, which consists of a box car, in which is con- structed a metal-lined hopper. The car is spotted at the steam plant over a chute, which is connected to the car by a flexible hose, and which feeds a small con- veyor encased in a metal housing. The coal is elevated and dumped into a bunker, adjoining the power plant, from the bottom of which it is fed by means of two motor-driven screws into the supply pipe. The coal is then, blown through the pipe a distance of 30 ft. to the front of the furnace, where it feeds into specially constructed burners made of sheet iron. The air supply to each burner is furnished by a motor-driven blower, with dampers installed to control the supply. The boiler has been equipped with an extended oven, in order to furnish sufficient space for the proper igni- tion and combustion of the fuel. The following is a record of a test on the equipment run continuously for 12-8 hours, the duration of the test being determined by the limited facilities for storage and handling of the fuel. The coal was weighed in the car as delivered to the plant, and the net weight determined by a subsequent weight of the car after unloading. The test was run until all coal was consumed. The water was measured by a Venturi water meter installed in an individual feed line to the boiler, and all the instruments were checked for accu- racy before starting. The test was made on a 300 horse-power Babcock and Wilcox boiler. The coal analysis is as follows: Moisture, 5-4; volatile, 37’2; fixed carbon, 47; ash, 10-4; sulphur, 0-53; British thermal units, 11,760. The ash analysis is as follows: SiO2, 44 : FeO, 10-45 ; A12O3, 32-88; CaO, 7-75; MgO, 2-40. The screen test gave 5’8 per cent, on 100-mesh, 3-46 per cent, through 100 on 200, and 59-6 per cent, through 200. Results of test: Duration of test, 12-8 hours ; average boiler horse-power developed, 357; total water evap- orated, 143,231 lb. ; average temperature of feed water, 185 degs. Fahr.; average steam pressure, 106-5 lb. gauge ; average temperature of steam, 399 degs. Fahr. ; average flue gas temperature, 528 degs. Fahr. ; average draught at uptake, 0-17 in. water; average flue gas analysis, CO2 17 per cent., oxygen 2 per cent., CO 0 per cent. ; total coal burned, 18,389 lb. ; actual evapora-