US2677434A - Fly ash recovery - Google Patents

Description

C. W. J. HEDBERG ET AL FLY ASH RECOVERY Filed Dec. 27, 1950 Us: umJw mote :Omd &

May 4, 1954 INVENTORS CARLMJ. HEDBERG HARRY A. WINTERMUTE A'ITUKNEYS Patented May 4, 1954 FLY ASH RECOVERY Carl W. J. Hedberg, Bound Brook, and Harry A.

Wintermute, Plainfield, N. J assignors to Research Corporation, New York, N. Y., a. corporation of New York Application December 27, 1950, Serial No. 202,974

2 Claims. 1

This invention relates to a method and apparatus for the recovery of fly ash produced in the burning of pulverized coal.

Although electrical precipitators provide the most satisfactory means for the recovery of fly ash from combustion gases from boiler plants burning powdered coal, the efficiency of the recovery frequently is less than desirable, particularly when anthracite coals are being burned or when a high efficiency of combustion is attained in the boiler.

It has now been found that the efficiency of collection of fly ash can be substantially increased and the electrical operation of the precipitators improved by adding to the fly ash containing combustion gases before they enter the electrical precipitators very finely divided coal particles. Preferably the major portion of the coal particles are less than 10 microns in size. The finely divided coal is added to the combustion gases in an amount of preferably from about 10 to about 20% based on the fly ash content of the gases. This amount may be reduced in proportion to the amount of residual unburned coal present in the fly ash. It is desirable to add the coal to the gases at such a point that the coal will be uniformly distributed in the gases at the time they enter the precipitators and preferably at a point where the temperature is below the temperature of substantial thermal decomposition of the coal. A convenient source of the finely divided coal particles is the powdered coal stream coming from the coal pulverizer of a powdered coal boiler, although it is desirable to separate out the more finely divided portion of the powdered coal for the purpose of the invention.

The method and apparatus of the invention will be more particularly described with reference to the accompanyin drawing which is a diagrammatic representation or" a powdered coal boiler system embodying the principles of the invention.

In the drawing, E is a powdered coal boiler supplied with powdered coal in suspension in air from pulverizer it through blower l2. Combustion gases from the boiler pass through an air preheater i3 and thence to a fly ash collecting system shown by way of illustration as comprising a mechanical collector [4 followed by electrical precipitators i5. Incoming air for combustion passes from exchanger l3 to the boiler. In adapting the principles of the invention to this typical pulverized coal boiler system, there is provided a flue H3 leading a portion of the powdered coal suspension from the blower to a centrifugal separator ll wherein most of the coal particles over microns in size are separated and returned to the blower through star valve l8 and conduit H9, or to the pulverizer H by a suitable conduit to the pulverizer feed.

The finely divided coal particles are carried from the separator by a ilue 20 which is shown opening into the combustion gas fiue between the mechanical collector and the electrical precipitators. The suction on the outlet of the separator ll due to the induced draft of the boiler system and the pressure on the inlet of the separator from blower I2 provides a pressure head adequate to carry the finely divided coal particles to any desired point in the system. The finely divided coal may be supplied to the combustion gases at any other convenient place ahead of the precipitators as between the heat exchanger and the mechanical collector or ahead of the heat exchanger. It is, of course, necessary, in the latter case, that the major portion of the coal particles be small enough to pass through the mechanical collector.

It is advantageous to provide a fiue 2| leading from the combustion air conduit to the pulverized coal take-off system to provide additional air for carrying the finely divided coal to the combustion gas fiue. Suitable dampers for regulation of the fiow of air through flue 2! are also advantageously provided.

The arrangement of apparatus and lines shown in the drawing is purely illustrative and other arrangements may be used without departing from the principles of the invention.

\Ve claim:

1. In a powdered coal boiler system including a coal pulverizer supplying powdered coal to the boiler and an electrical precipitator through which the combustion gases are passed for collection of fiy ash, means for transferring a portion of the powdered coal from the pulverizer to the combustion gas flue ahead of the electrical precipitator.

2. In a powdered coal boiler system including a coal pulverizer supplying powdered coal to the boiler and an electrical precipitator through which the combustion gases are passed for collection of fly ash, a separating device for separating a portion of the powdered coal from the pulverizer into a fraction containing a major portion of coal particles less than 10 microns in size and means for supplying said fraction of finely divided coal to the combustion gas flue ahead of the electrical precipitator.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,329,237 Frisbie Jan. 27, 1920 1,329,817 Wolcott Feb. 3, 1920 1,331,225 Wolcott Feb. 17, 1920 1,343,482 Schmidt et a1. June 15, 1920 1,446,778 Witte Feb. 27, 1923 1,774,604 Lawrence Sept. 2, 1930 1,883,372 Hedberg Oct. 18, 1932

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