Hop es For Power From Mercer In August

Press, Volume XCVII, Issue 28606, 7 June 1958, Page 6

Hop es For Power From Mercer In August

Expectations are high that the first of six boiler-turbine units at the Mercer steam station will begin turning on August 1, thus feeding a badly-needed 30.000 kilowatts into the electricityhungry North Island grid. Engineers at the site of the power station on the banks of the Waikato river are confident that the target date will be reached. At the present stage the erection of two turbo-alternators has reached an advanced stage and one boiler is nearly completed, with another four at various stages of construction. The coal that will initially fire the boiler will come from the Huntly field. For test purposes it will be more suitable than any other local coal as it has a lower water content.

With an installed capacity of 180.000 kilowatts, Mercer will consume 600,000 to 800,000 tons of coal a year according to the load it carries. Much of this coal will come from the huge Maramarua open-cast mine which lies six miles from the station, the coal being carried on an aerial ropeway. The balance of the coal that the station will need will come from several large open-cast mines in the Huntly coalfield. On the assumption that 21b of coal will provide one unit of electricity, the daily fuel needs for the station will be 2300 tons, and the yearly needs 575,000 tons. For practical purposes this latter figure can be estimated to be about 600,000 tons. The estimated total cost of the station is £l7m, of which £lom has now been spent. Submarine Storage The station will have a large outside storage area for coal capable of holding 25,000 to 30.000 tons, enough to keep the station in fuel for nearly three weeks. This area will be under water to stop spontaneous combustion to which coal is prone when stockpiled. The storage space is exclusive of the six coal bunkers in the station, each of which has a storage capacity of 600 tons. The coal entering the bunkers by conveyor belt will have been previously crushed to one cubic inch in size. From the bunkers the coal chips are fed into a pulverising mill where it is .pounded by heavy steel balls until reduced to the size and consistency of face powder—between 13 thousandths and six thousandths of an inch. A fan will drew heated air into the mill and blow the pulverised coal into the boilerhouses, where it will be ignited. The temperature inside the boilerhouse will be something like 2500 degrees F. There will be 14 miles of boiler 4ubes. The waste gases leaving the boiler will have a .temperature of 625 degrees and will be led to two air-heaters. The air-heaters contain 8979 tubes with a total length of 20 miles over which the hot gases will flow. It is the heated air which is drawn by fans into the pulverising mills. After passing through the air heaters the waste gases are led into a grit collector. The greatest size of the grits entering the collector will be four thousandths of an inch and the greatest size of any able to get through the collector will be only one thou-

sandth of an inch. From the collector the grit will fall into a hopper and from there wil Ibe sluiced into an ash sump before being pumped out into a nearby swamp. The comparatively clean gases are then free to pass up the station chimneys, the tops of which are 150 feet above ground level. In case the gases are not clean enough, even at this stage, the chimneys have been raised above the ground for the possible installation of an electrostatic precipitator to remove the very fine grits. Quenching the Ash Ash from the boilers will fall into hoppers, where it will be quenched by sprays. The capacity of each hopper will be 25 tons and once a day the ash will be removed under the action of highpressure water jets. Steam from the boiler wjll leave a secondary superheater at 865 degrees F. and at a pressure of 6351 b to the square inch, and will enter the turbines, at slightly less than these figures. The boilers will each generate 300.0001 b of superheated steam an hour and the total station output will be I, an hour. A boiler will be brought into operation by the automatic ignition of fuel oil burners bringing the boiler up to one-tenth full load. The pulverising fuel mills and fans will then be started and both kinds of burners will be kept alight until the boiler has reached one-third full load. The fuel oil burners will be stopped and further load carried by the pulverised fuel alone. Six boiler units will supply steam to six turbine alternator units each generating 30,000 kilowatts. Electricity is generated at 11, volts and Tor transmission is stepped up to 110,000 volts by transformers. Although the total output from the station will be 180,000 k.w., only 168,500 k.w- will be fed into the national grid. Station auxiliary services will take 11,500 k.w. Other processes will include a water purifying plant essential to prolonging the life of a valuable equipment, and the condensing process.

The project is on a scale that has not been attempted in New Zealand before. It combines the use of such huge amounts of solid fuel and a technique of generating power that is almost alien to a country used to the thought of electricity flowing from rivers.