CA1187047A - Process for removing inorganic impurities from coal - Google Patents

Abstract

ABSTRACT
A process for removing inorganic impurities from coal comprising the steps of transferring pulverized coal to be treated by entraining it on a stream A of anti-oxidation gas, feeding the coal laden gas into a dry-type electric precipitator 2 after suitably adjusting the temperature and himidity of the gas stream during transport, and separating particulate inorganic impurities from coal parti-cles by capturing the former in the precipitator 2 by virtue of difference in specific resistance between the two kinds of particles, whereby highly purified coal B is obtained as entrained on the gas stream at the outlet 3 of the pre-cipitator 2.

Description

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 elim-inating coexistent inorganic impurities therefrom.
Pulverized coal is chiefly composed of carbon par-ticles; however, it also contains particulate inorganic impurities such as SiO2, CaO, A12O3, 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 pul-verized 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 pro-perty of coal. With such a method, however, not only is the working efficiency low but also it is 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 physically removing particulate inorganic impurities from pulverized coal by subjecting the pulverized coal to electrostatic separation based on ~, the difference in specific resistance between said particulate inorganic impurities and the coal particles, characterized in that (a) the pulverized coal is entrained on a stream of non-oxidizing gas for transport and passed through a dry-type electric precipitator to selectively capture the particulate inorganic impurities in said precipitator, (b) the coal laden gas is subjected to temperature and humidity adjustment in separate steps prior to the introduction of the gas into said precipi-tator, said temperature adjustment being effected to enhance said difference in specific resistance, said humidity adjustment being effected to enable a high voltage operation of said precipi~ator, and (c) highly purified coal is obtained at the outlet of said precipitator as entrained on the ~as stream.
The non-oxidizing gas usable in this method com-prises one or any combination of carbon dioxide gas, nitrogen gas and an inert 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. For example, the temperature is set to a range of 100C to 200C, and the humidity to not less than 10%.
The electric precipitator may be any standard dry-type electric precipitator having pairs of opposed electrodes and discharge means for discharging collected particles.

:;

~ ~, ~' The flat electrode type of precipitator is preferredt although not essential.
The characteristic features and effects of the pre-sent invention will become apparent from the following S description of an em~odiment given with reference to ~he accompanying drawing, which schematically illustrates a coal treating system for effectlvely carrying out the method according to the present invention.

- 2a -3~8~ 7 In the drawing, numeral 1 represents a hopper for storlng pulverized coal to be treated; numeral 2 a dry-type electric precipitator disposed downstream from the hopper l; numeral 3 a discharge duct extend.ing from the precipitator 2; numeral 4 a blower provided in the duct 3;
numeral 5 a feed duct connecting the entrance of the pre-cipitator 2 and the outlet of the hopper 1; numeral 6 a discharge ~eans 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 carxier 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 dischar~e means 6 is entrained on the carrier gas and transferred toward the electric ; 20 precipitator 2 by the action of the blower 4. Before enter-ing the precipitator 2, the coal laden gas is heated to a predetermined temperature by the temperature adjusting uni-t 7 and humidified to a humidity of not less than 10%. The gas then enters the precipitator 2.
The specific resistance Pc of carbon particles is :

generally represented by Pc ~104 (Q.cm) and thus is low while the specific resistance Pa of inorganic impurities is medium as indicated by the expression lO (Q.cm~Pa<lOll(Q~cm~. Due to this difference in specific resistance, inorganic impurities alone are captured by unillustrated dust-collecting electrodes in the electric precipita~or 2 so that highly purified coal 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 aEorementioned resist-ivity difference. Thus the electrodes, i.e., the precipitator 29 can reliably provide selective capture of inorganic impurities, which thereafter go down the electrodes to fall into the collector 9 of the precip~
itator 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 60C and increases with temperature up to the maximum value at about lO0 to 200~C, followed by gradual decrease with further rising temperature. In view of this, the temperature adjusting unit 7 must be operated to control the temperature of the carrier gas so 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 high voltage can be applied to the electric precipitator 2 under high humidity, the humidity adjusting unit 8 must be so operated as to enable such high voltage operation of the precipitator 2. Logically stated, a strong electric field produced by the high voltage operation of the pre-cipitator 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.
l~o sum up, since the method ~scribed 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 adjust-ing 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.

Further since the precipitating chamber is filled with a non-oxidizing gas, there is no likelihood, in the chamber, of explosion due to dischargi.ng sparks in spite of inflowing coal particles, thus assuring safe purification of pulverized coal.

Claims (4)

Claims

1. A process for physically removing particulate inorganic impurities from pulverized coal by subjecting the pulverized coal to electrostatic separation based on the difference in specific resistance between said particulate inorganic impurities and the coal particles, characterized in that (a) the pulverized coal is entrained on a stream of non-oxidizing gas for transport and passed through a dry-type electric precipitator to selectively capture the particulate inorganic impurities in said precipitator, (b) the coal laden gas is subjected to temperature and humidity adjustment in separate steps prior to the intro-duction of the gas into said precipitator, said temperature adjustment being effected to enhance said difference in specific resistance, said humidity adjustment being effected to enable a high voltage operation of said precipitator, and (c) highly purified coal is obtained at the outlet of said precipitator as entrained on the gas stream.

2. A process as defined in claim 1 where said non-oxidizing gas comprises one or any combination of carbon dioxide gas, nitrogen gas and an inert gas.

3. A process as defined in claim 1 wherein the humidity of the coal laden gas stream is adjusted to not less than about 10%.

4. A process as defined in any of claims 1 to 3 wherein the coal laden gas stream is heated to a temperature of about 100 to 200°C, for example, at which the inorganic particles have a maximum specific resistance.