NZ200426A - A process for removing inorganic impurities from coal with a dry-type electric precipitator - Google Patents

Description

200426 Priority Oate(s): Complete Specification Filed: Class: ...^.....

Publication Date: .... ^ P .

P.O. Journal, j\jo: .

N-. Z . NO.

NEW ZEALAND Patents Act 1953 COMPLETE SPECIFICATION HZ. PATENT OFFICE 2 7 APR 1982 f- r« "PROCESS FOR REMOVING INORGANIC IMPURITIES FROM COAL." We, HITACHI SHIPBUILDING & ENGINEERING COMPANY LIMITED, i ' • a company incorporated under the laws of Japan of 6-14 Edobori 1-chome, Nishi-ku, Osaka, Japan \ do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- 1 ~ (Followed by 1A.) 2.00426 PROCESS FOR REMOVING INORGANIC IMPURITIES FROM COAL The present invention relates to a process for removing inorganic impurities from coal, and more particularly to a process for purifying pulverized coal by effectively eliminating coexistent inorganic impurities therefrom.

Pulverized coal is chiefly composed of carbon particles; however, it also contains as impurities particulate inorganic impurities such as Si02» CaO, AI2O3, etc. When put into use as fuel, pulverized coal should advantageously contain no or at least reduced inorganic impurities. A conventional method heretofore employed to purify pulverized coal comprises adding pulverized coal to water followed by stirring, and subsequently adding to the mixture oil as a separating agent again followed by agitating. Inorganic impurities can be separated and removed from the coal by utilizing the lipophilic property of coal. With such a method, however, not only is the working efficiency low but also is it difficult to control the degree of separation (separation accuracy).

It is therefore an object of the present invention to provide a process for removing inorganic impurities from coal which is excellent in working efficiency and which provides easy control of separation accuracy.

To fulfil this object, the present invention provides a process for removing inorganic impurities from coal comprising the steps of entraining pulverized coal to be 1 A \ 200426 treated on a stream of non-oxidizing gas for transport, and passing the coal laden gas through a dry-type electric precipitator after adjusting the temperature and humidity to respective predetermined values during transport to selectively capture the particulate inorganic impurities in the precipitator by virtue of the difference in specific resistance between the particulate impurities and the coal particles, whereby purified coal is obtained as entrained on the gas stream at the outlet of the precipitator.

The resulting coal is highly purified relative to the starting coal and preferably has a purity of at least 95%.

The non-oxidizing gas usable in this method comprises one or any combination of carbon dioxide gas, nitrogen gas and a noble gas.

The temperature and humidity of the non-oxidizing gas stream should preferably be so adjusted that the degree of separation of inorganic impurities from carbon particles is increased to the greatest extent. The optimal temperature and humidity of the gas stream may vary depending on the kind and nature of impurities. Generally, however, the temperature is set to a range of 100°C to 200°C, and the humidity to not less than 10%.

The characteristic features and effects of the present invention will become apparent from the following description of an embodiment given with reference to the accompanying drawing, which schematically illustrates a coal treating system for effectively carrying out the method according to the present invention. ^sssss^ 200426 In the drawing, numeral 1 represents a hopper for storing pulverized coal to be treated; numeral 2 a dry-type electric precipitator disposed downstream from the hopper 1; numeral 3 a discharge duct extending from the precipitator 2; numeral 4 a blower provided in the duct 3; numeral 5 a feed duct connecting the entrance of the precipitator 2 and the outlet of the hopper 1; numeral 6 a discharge means provided in the outlet of the hopper 1 for discharging constant amounts of pulverized coal; and numerals 7 and 8 are a temperature adjusting unit and a humidity adjusting unit, respectively, disposed in or on the feed duct 5.

A non-oxidizing carrier gas comprising one or any combination of carbon dioxide gas, nitrogen gas, an inert gas or the like is supplied through one end of the feed duct 5 close to the hopper 1 in the direction of the arrow A. Pulverized coal continually discharged, in a constant amount, from the hopper 1 by the discharge means 6 is entrained on the carrier gas and transferred toward the electric precipitator 2 by the action of the blower 4. Before entering the precipitator 2, the coal laden gas is heated to a predetermined temperature by the temperature adjusting unit 7 and humidified to a predetermined humidity which is not less than 10%. The gas then enters the precipitator 2.

The specific resistance Pc of carbon particles is ..y ^DECWM^ 3flO40S generally represented by Pc <10^ (ft. cm) and thus is low while the specific resistance Pa of inorganic impurities is medium as indicated by the expression 104 (ft.cm)<Pa<1011 (ft.Cm) . Due to this difference in specific resistance, inorganic impurities alone are captured by unillustrated dust-collecting electrodes in the electric precipitator 2 so that highly purified coal (having a purity in this preferred embodiment of at least 95%) as entrained on the carrier gas stream can be obtained in the discharge duct 3 communicating the precipitator outlet. The arrow B represents the flow of the carrier entraining pulverized coal of high purity. Although carbon particles are in effect captured similarly by the dust-collecting electrodes once, they are forced back to scatter away due to the aforementioned resistivity difference. Thus the electrodes, i.e., the precipitator 2, can reliably provide selective capture of inorganic impurities, which thereafter go down the electrodes to fall into the collector 9 of the precipitator 2. The collected impurities are then discharged as indicated by the arrow C.

The specific resistance of particulate inorganic impurities is minimum at about 50 to 6 0°C and increases with temperature up to the maximum value at about 100 to 20 0°C, followed by gradual decrease with further rising temperature. In view of this, the temperature adjusting unit 7 must be 2004.2. £■ that inorganic impurity particles have a maximally high resistivity to assure effective capture of the impurity particles in the precipitator 2. On the other hand, since a higher voltage which is usually from 3000 to 20000V can be applied to the electric precipitator 2 under higher humidity, the humidity adjusting unit 8 must be so operated as to enable the precipitator 2 to run under the highest possible voltage that is allowed ; characteristics of the precipitator 2 and in view of the temperature adjustment of the carrier gas. Logically stated, a stronger electric field produced by a higher voltage operation of the precipitator 2 affords improved dust collecting efficiency, hence enhanced impurity separating efficiency.

Needless to say, it is possible to increase the degree of impurity separation by suitably increasing the number of dust-collecting units in the precipitator 2.

To sum up, since the method discribed above comprises the steps of entraining pulverized coal on a stream of non-oxidizing gas for transfer, feeding the coal laden gas into a dry-type electric precipitator after suitably adjusting the temperature and humidity of the gas stream during transport, separating inorganic impurity particles from coal particles by capturing the former in the chamber by virtue of difference in specific resistance between the two kinds of particles, and obtaining, at the outlet of the precipitator highly purified coal as entrained on the gas stream, it can provide improved working efficiency of impurity separation as well as readier control over the degree of separation. v - 5 - 14DEC1984 *• ;X • Further since the precipitating chamber is filled with a non-oxidizing gas, there is no likelihood, in the chamber, of explosion due to discharging sparks in spite of inflowing coal particles, thus assuring safe purification of pulverized coal. 20042S

Claims (7)

WHAT WE CLAIM IS;

1. A process for removing inorganic impurities from coal comprising the steps of entraining pulverized coal to be treated on a stream of non-oxidizing gas for transport, and passing the coal laden gas through a dry-type electric precipitator after adjusting the temperature and humidity to respective predetermined values during transport to selectively capture the particulate inorganic impurities in the precipitator by virtue of the difference in specific resistance between the particulate impurities and coal particles, whereby highly purified coal is obtained as entrained on the gas stream at the outlet of the precipitator.

2. A process as defined in claim J wherein the highly purified coal has a purity of at least 95%.

3. A process as defined in claim 1 wherein said non-oxidizing gas comprises one or any combination of carbon dioxide gas, nitrogen gas and a noble gas.

4. A process as defined in claim 1, 2 or 3 wherein the humidity of the coal laden gas stream is adjusted to not less than 10%. 7 'XOO (Jjf "Lb

5. A process as defined in any one of claims 1 to 4 wherein the coal laden gas stream is heated to a temperature at which the inorganic particles have a maximum specific resistance .

6. A process according to claim 5 wherein the temperature is between 100 to 200°C.

7. A process according to claim 1 substantially as herein described. HITACHI SHIPBUILDING & By Their Attorneys HENRY HUGHES LIMITED By: ENGINEERING COMPANY LIMITED