WASHERY SLUDGE BURNED IN PULVERIZED FORM AT SEATTLE* BY CHARLES H. BROMLEY The Puget Sound Power and Light Co. at Seattle, Washington, includes, among its public utilities, a district steam-heating plant of 4,100 boiler-horsepower, which for nearly two years has been fired with pulverized high-grade lignite. The results shown have been so favorable that all those connected with its operation are enthusiastic advocates of this form of combustion. When the United States entered the war, it early seemed probable that the Federal Government would place restrictions upon the use of fuel oil on the Pacific Coast and that consumers remote from the oil fields would be forced to look to other forms of fuel. Faced with a shortage of oil, Stone & Webster, managers and engineers for the company, undertook a careful study of the possibilities of utilizing the local grades of coal in such of its Seattle plants as were then dependent upon oil. Early in the spring of 1917 one of the smaller boilers at the steam-heating station referred to was set aside for experimental purposes and an extensive series of tests conducted, using practically all grades of coal locally obtainable, in which the thorough practicability of combustion of these coals in the pulverized form was fully demonstrated. Pulverization was finally decided upon as most favorably meeting the local conditions. It was particularly advantageous at Seattle as affording means for efficient utilization of a 200,000ton sludge dump at the Renton mine just outside the city limits. The adoption of pulverized fuel had also the important advantage of leaving the existing oil-burning equipment immediately available in efficient form. Conversion to pulverized fuel was accordingly made by Stone & Webster the following year. The station referred to, which is known as the Western Avenue Station, faces on Western Avenue and extends through to Railroad Avenue on the harbor front. It is about two blocks from the center of the retail shopping district. Originally, the station was designed as an ice plant. The boilers, many of which were installed twenty years ago, were set with low furnaces and with aisle space so narrow as to make a satisfactory stoker installation impracticable. Although the small furnaces would *This article by the Associate Editor of Power is reprinted from the magazine. undoubtedly have curtailed efficiency for any fuel, while burning oil the lack of aisle space involved no serious interference with operation. A change to coal, however, in any other form than pulverized, without rebuilding the entire plant, would have called for hand firing, which not only would have been most difficult from a labor standpoint, but would have restricted the consumption to high-priced coal. The present boiler plant totals 4,100 hp. in rated capacity, made up of four 300-hp., three 400-hp., one 500-hp. and two 600-hp. boilers, all of the B. & W. type. One of the 600-hp. units was installed at the same time as the pulverized-fuel preparation plant and was accordingly set with front headers eleven feet above the boiler floor. All the other boilers, as already referred to, have low furnaces, with headers set only about seven feet above the floor. The coals mined in the vicinity of Seattle are sub-bituminous in nature, closely bordering upon lignite, and require washing in preparation of all the smaller sizes. At the Renton mine, washery sludge, regarded as a total waste, had been accumulating for years. This material carries about 25 per cent moisture and has an average heat value of about 7,300 B.t.u. A typical analysis on the dry basis is approximately as follows: Volatile matter, 37 per cent; fixed carbon, 38 per cent; ash, 25 per cent; sulphur, 1 per cent. This washery sludge is also small in size, about 50 per cent passing through a 10-mesh screen, rendering it totally unfit for use on any existing form of mechanical stoker. As originally reclaimed from the dump, the sludge carries such a high content of clay that its adhering qualities would render it practically impossible of handling in any ordinary form of conveying apparatus. By combining some washing with the sluicing process used in carrying the sludge from the dump, a large part of this clay is removed. Regular washing tables are now being installed on which the clay will be completely removed. The Coal-Preparation Plant The plant that converts the raw coal into pulverized coal consists of three main divisions: (1) The raw-coal handling system, comprising raw-coal storage bunkers, crusher, crushedcoal bunkers and interconnecting conveyors; (2) the drying plant, comprising the driers and dry-coal bunkers with conveyors feeding the driers and delivering the dried coal to the bunkers; (3) the pulverizing plant, comprising the pulverizing mills with their feeders and the necessary conveying system delivering to the pulverized-coal bunkers. The Raw-Coal Handling System Limitations of space made it impracticable to provide sufficient fuel storage within the station building. The desired additional raw-coal storage was accordingly installed on a vacant lot adjoining one of the company's electrical substations and immediately across Western Avenue from the steamheating plant. The three raw-coal bunkers erected for this purpose are of heavy wood construction, lined with galvanized iron and have a total capacity of about 700 tons. The bunkers are served by a standard-gage double track on an overhead trestle from which hopper-bottomed cars discharge. From the bunkers the coal is fed through double rack-andpinion gates to a 20 in. feeder belt, which conveys the coal to a single roll-type crusher where, when lump or run-of-mine coal is used, it is crushed to about one-half inch size. A bypass chute is provided around the crusher, so that the smaller-sized coal can be fed directly from the feeder belt to the conveyor belt leading from the crusher to the raw-coal elevator. The conveyor belt from the crusher is a combination incline-horizontal belt, 20 in. wide, which, carried through a tunnel under Western Avenue, delivers the crushed coal to a double-strand, fixed, continuous-bucket elevator, steel incased, which elevates the coal 75 ft. and discharges it onto a double-strand steel flight conveyor for distribution to the crushed-coal bunker. All the raw-coal handling equipment has a capacity of 75 tons per hour; with buckwheat coal 90 tons has been handled. The crushed-coal bunker, which is of reinforced concrete, has a capacity of about 350 tons. The Drying Plant To convey the coal from the crushed-coal bunker to the two driers, two apron feeders are provided, each served by a 5 ft. by 10 in. opening at the bottom of each of the two V-section hoppers formed beneath the crushed-coal bunker. Each feeder is provided with a vertical sliding rack-and-pinion gate to regulate the flow of coal. Both feeders discharge into a common chute, which is equipped with a flopper gate and steel-plate diaphragm so that either feeder may discharge coal into either or both of the driers. A screw feeder immediately beneath each apron feeder handles the drippings from the return side of the feeder and delivers these drippings into the feeder discharge chute which branches to feed the two driers. Mechanical agitators are installed in the branch chutes to prevent clogging. On account of the high silt content, the sludge in the wet state is of particularly sticky consistency and requires positive mechanical movement at every point. The two driers are of the indirect-fired, rotary type, with settings especially designed for maximum heat utilization. The cylinder dimensions are respectively 5 ft. diameter by 52 ft. long and 6 ft. diameter by 52 ft. long. Pulverized coal is used for the drier fuel. The heat absorbed through that part of the drier cylinder within the heating chamber reduces the hot-gas temperature to about 500 degrees F. The gases finally leave the drier at a temperature of about 270 degrees F. The coal is discharged from the driers at a temperature of about 240 degrees F. Induced draft for the driers is supplied by two turbinedriven fans. These fans discharge into insulated cyclone separators located above the roof of the preparation plant proper, where all but the finest of dust drawn from the drier with the exhaust gases is collected for return to the system. This collected dust is delivered by screw conveyor to the drycoal elevator into which the main drier is discharged. The gases from the cyclone separator pass to an exhaust chamber, where, before being finally allowed to escape to the atmosphere, they are washed for removal of the small amount of dust remaining. Salt water from the harbor adjacent to the plant is used in this washer. The proximity of the plant to the business district of the city necessitates special provisions for dust prevention. From the driers the coal is discharged over magnetic separators for removing tramp iron such as spikes, bolts, nails, and into the dry-coal elevator. The dry-coal elevator is of the single-strand, fixed-bucket type, steel incased, 62 ft. between centers, and is provided with weighted take-ups to compensate for temperature changes met in such service. The dry coal discharged at the top of the elevator is distributed by screw conveyor to the dry-coal bunkers, which are of reinforced concrete and have a capacity of about 150 tons. The coal as it reaches the dry-coal bunkers still carries a small percentage of moisture which for a time continues to be given off as steam. To provide for the escape of this steam atmospheric vents, suitably drained, are installed. It is found in practice that approximately one per cent additional moisture is given off by the coal in the bunkers and removed by this means. Other vents are also provided in the dry-coal elevator casing to remove such moisture as may be liberated between the driers and bunkers. From the dry-coal bunkers the coal is fed to the pulverizing mills. The Pulverizing Plant The pulverizing equipment consists of three 42 in. FullerLehigh mills of a capacity about 31⁄2 tons per hour each, pulverizing to 80 to 85 per cent through a 200-mesh screen and one five-roll, high-side Raymond mill adjusted to a capacity of about four tons per hour for delivery of a slightly coarser product. The pulverized coal from these mills is delivered by screw conveyor into a continuous-bucket, single-strand, centrifugal discharge elevator with reinforced-concrete casing. This elevator discharges into a screw conveyor distributing the coal to the pulverized-coal bunkers which supply the boiler and drier furnaces. The bunkers supplying the boilers provide capacity for about one-half day's normal operation and are of reinforced concrete, except in the case of two bins where it was impracticable to employ this construction. All pulverized-coal elevator and conveyor casings are of dustproof construction as are the connections from both Fuller and Raymond mills. Further, to insure absolute dustlessness within the preparation plant that part of the pulverized-coal system, including the Fuller mills, is operated under a slight vacuum. The small exhauster employed for this purpose is arranged to discharge directly to certain of the boiler furnaces. The vent from the Raymond mill separator is similarly disposed of. The Combustion Equipment The pulverized-coal feeders are of the screw-conveyor type, each operated by a variable-speed direct-current motor, thus providing complete control of the coal feed by simple rheostat adjustment. From the feeders the pulverized coal is picked up by air under a pressure of about five to six ounces and delivered to the burners. The air supply is furnished by turbine-driven blowers delivering to a central blast duct beneath the pulverized-coal bins, from which branches are taken off to the individual feeders and burners. Air regulation is had by means of valves in the air-feed lines. The burners, which were developed by the Puget Sound Power and Light Co.'s steam superintendent, are extremely simple in construction, consisting merely of a special mixing |