POWDERED COAL

Evening Post, Volume CVI, Issue 30, 4 August 1923, Page 24

POWDERED COAL

RECENT APPLICATIONS

SOME BIG POWER UNITS.

The use of pulverised fuel a- step in fuel economy and the attainment of high boiler efficiencies has received considerable attention of late, though developments in Britain and on the Continent have not so far realised the anticipations of pioneers. Tho idea of burning coal in a very finely divided state is, of course, not new. As a matter of fact, writes the engineering correspondent of the London "Daily Telegraph," much publicity has during the past few years been accorded it, but it was not until three years ago that substantial developments have occurred in tho United States, and more recntly the progress has been little short of remarkable.

The essential principles of pulverised fuel consist in crushing the coal, passing it over a magnetic separator to get rid of any iron particles, drying it to about 1 to 2 per cent, moisture, arid then pulverising it in ball mill pulverisers to a fineness which depends to some extent upon circumstances, but which in modern practice is generally such that 75 per cent, is capable of passing through a 200-mesh filter (that is, with 200 holes per linear inch, or 40,000 per square inch), and 90 per cent, through 100----mesh. This very finely divided fuel is then burnt like gas with a current of air in special burners. We now appear to be on the eve of further remarkable developments with this typo of fuel, for an order was placed but a few weeks ago by the Union d'Electncite m.conection with their important power station at Vitry, Paris, for a complete installation, which will be the first large equipment in Europe. Before the advent of this modern system in 1920, the advantages of pulverised fuel could' be summed up as (1) greatly increased efficiency because of the more intimate mature of the fuol and air, so that only about 20 per cent, excess of air- over the theoretical was necessary; (2) a high carbonic acid gas content in the flue gas (from 16 to 18 per.cent); (3) absence of smoke; (4) no combustible matena m the ash; (5) no necessity for clmkering and cleaning out; (6) the use of low-grade fuels, which can in the powdered form be efficiently burnt- (7) the easy conveyance of the pulverised luel through long and narrow. pipe-cir-cuits because of its fluid properties: (8) reduced labour and attendance; (9) no stand-by losses'; (10) the ability to start up and shut dow n suddenly; and (11) the capacity to take an enormous overload, as much as 100 per cent., for short periods.

There were, however, concomitant disadvantages. These comprised the danger of explosion and fire from spontaneous combustion, the excessive wear and tear on tho firebrick lmings on account of the intense heat.of combustion, the cost of amil ttf* t yin?> andu grindins th* «»£ and the obnoxious character of the ophs™,\idXa^ blrf

SOME LARGE PLANTS. , These disadvantages, .whilst retaining the a bove substantial advantages, have Po^C^rofSe^chi^ running at 40,000 k.w., and now bring doubled in Blie. The new Cahokia Sta tion for St. Louis, to be 240,000 k w v adopting pulverised fuel only, and there is also the. famous Ford River Rouge plant at Detroit, with enormous boilers of 26,400 square feet heatine surface and 275,0001b evaporation pc? hour fired with mixed pulverised fuel and blast furnace gas, which has now been eclipsed by the Detroit Edison Marywl c boilers of 29,800 square feet, and by the Cleveland Electricity Com?n™ arV nstalling four boilers of 30 500 square feet heating surface each, and a normal evaporation of 300 0001b per hour. Further, it is rumoured that the New Trenton, Channel Station projected by the Detroit Edison Company which will be the largest in the world will also adopt this system of pulverised

The new plant at Vitry is to comprise four very large water-tube boilers, constructed by Messrs. Delaunay-Belleville to the designs of the well-known American 'Ladd ri boiler of 16,685 square feet Seating surface each, corresponding to a normal duty of 160,0001b evaporation from and at 212deg. Fahrenheit, and a prolonged overload of 185,0001b. These boilers will be easily the largest in Europe as compared with, say the 100,0001b "Babcock and Wilcox" boilers at Barton, Manchester, and the "Stirling W. type" boilers of 132,2751b evap°r T atlon,, at Gennevilliers, although Ladd boilers are being constructed an the United States of nearly twice thin latter size.

The Vitry equipment includes, in addition to the latest pulverised plant complete, Delaunay-Belleville superheaters, and steel-tube economises of the hi"h velocity positive circulation small tube type of 9687.6 square feet heating surface, the working pressure being 2701b gauge. These, are big advances in the i use of pulverised fuel, and the adoptionof this form of fuel by some of the largest and most efficient power-stations in the world may be trace/1 to several reasons. In the first place, the efficiency is extraordinary, the standard guarantee as given at Vitry being 84 to 85 per cent, efficiency, and there seems to be little doubt that this very high guarantee will be easily attained in view of I the. fact that the Lakeside installation (40,000 k.w.) at Milwaukee, for example is running on 85 to 86 per cent, efficiency all the year round, and it is calculated that the new Cahokia station at St. Louis (240,000 k.w.) will attain 88 per cent. All the usual advantages of puvlerised fuel are to be reckoned in the system no%y to' be installed at Vitry, but the recently embodied improvements do away with many of the disadvantages, • and are therefore of the greatest' interest. latest Advances. Apart from the triplex burners, the improved coal-fesd mechanism to the burners, a multiple zone furnace of very large volume, and hollow fire-brick walls cooled by the secondary air passing to „the burners, the most striking improvements are the water screen and the new small vertical self-contained coal-drier heated only by exit gases of the boiler plant. The water screen is an arrangement of 4in steel tubes under boiler pressure at the back of the furnace, through which the boiler-feed water circulates. This keeps the furnace so cool that the serious trouble of slagging— that is, the fusion of the ash powderis now entirely eliminated. ' The new drier has superseded entirely the large horizontal revolving directured driers formerly employed, which/ absorbed the equivalent of coal of 1 per cent, of the steam production of the p ant. It consists of a vertical cylinder placed under the crushed coal hoppers •and directly over the pulverisers. The coal falls through the drier automatically as taken by the pulverisers, and there is passed through the drier—by means of a fan—about 10 per cent, of the final

chimney gases mixed with air, so that the temperature does not exceed 215deg. F. This results in no loss of volatile matter, and simple purely mechanical drying of the coal at a cost which is infinitesimal.

Finally, it may be stated that pulverised fuel has now been so perfected that the cost of labour and handling complete (conveying, crushing, drying, and pulverising), together with wear and tear and depreciation, is no greater than mechanical stoking, whilst the excess air over theoretical is, without difficulty, only 20 per cent, as compared with 35 to 40 per cent, for the most unusually efficient mechanical stoker installation. The most elaborate tests at Milwaukee show a less cost of labour as compared with mechanical stoking, about equal wear and tear on plant and brickwork, and a total of 1.75 per cent, of the steam production of the plant complete, using horizontal revolving driers, and, with the new driers, the figure will be less than 1 per cent. The danger from fire and explosion is negligible, and as regards the ash, this is in such a fine state of division that the 25 per cent, or so blown out of the chimney-top is never seen again.

It needs hardly, therefore, to be added that for steam-raising purposes finely powdered fuel has established it-self as a commercially efficient system, and a desperate struggle for supremacy is now being waged in the United States between it and mechanical stoking. There is every prospect that this will soon Bpread to Europe to the advantage of everyone interest in fuel economy.