GB2027006A

GB2027006A - Electrostatic Precipitation of Exhaust Gases from Solid Fuel Fired Rotary Kilns

Info

Publication number: GB2027006A
Application number: GB7925880A
Authority: GB
Original assignee: FLSmidth and Co AS
Current assignee: FLSmidth and Co AS
Priority date: 1978-07-25
Filing date: 1979-07-25
Publication date: 1980-02-13
Also published as: GB2027006B

Abstract

A rotary kiln plant which includes an electrostatic precipitator is fired, at least partly, with solid fuel, to which fuel, in order to reduce the resistivity of the dust-laden exhaust gases from the kiln, an alkali-metal compound or compounds is added before firing in the kiln. The alkali metal compound may be added as a solid or as a solution.

Description

SPECIFICATION Improvements Relating to Electrostatic Precipitation of Exhaust Gases from Coal Fired Rotary Kilns The present invention is concerned with a method of operating a rotary kiln plant having an electrostatic precipitator, to condition the exhaust gases of a burning.process in the rotary kiln in which mineral materials are treated by burning a fuel at least part of which is solid pulverized fuel.

It is a well known fact that in the process of precipitation the dust from hot dust-laden gases from rotary kiln plants there are a number of difficulties.

Usually electrostatic precipitators are used, and in order to obtain the best possible cleaning of the gases various agents are used to condition the gases and to reduce the resistivity of the dust, so that an improved performance of the electrostatic precipitator with cleaning efficiencies above 99% can be reached. It is well known to condition the gases by injecting into them water and steam with or without various chemicals, with the result that the resistivity of the dust is reduced to an acceptable level.

From the prior art it is known to condition exhaust gases from rotary kiins for the burning of mineral materials, especially cement raw materials to cement clinker, by dosing or injecting into the gases, before they are passed to the electrostatic precipitator, one or more watersoluble alkali-metal compounds as a conditioning agent This conditioning reduces the resistivity thereby making it possible to increase the filter current in and thus the efficiency of the precipitator.

When using solid fuel in rotary kilns great amounts of fly ash are created and carried with the exhaust to the electrostatic precipitator. The particle size distribution of the fly ash in the exhaust gases is often such that the mean diameters of the particles are between 10 and 20 microns. These particles are difficult to precipitate and their resistivity is often in the order of 1012 to 1013 ohm cm., especially when using low sulphur content coals as preferred in recent years in order to avoid air pollution by sulphur oxides.

According to recent investigations it has been found that the presence of sodium in the ash in amounts of about 2% as NaO2 reduces the resistivity of the fly ash from low sulphur coals below critical levels.

It is well known to inject of water-soluble alkali-metal compounds as a conditioning agent into the exhaust gases before they are passed to the precipitator and this serves to reduce the resistivity of any fly ash in the exhaust gases.

However, by injecting the conditioning agents in the exhaust gases it is not always possible to obtain a regular distribution of the agents because the fly ash particles are extremely small and tend to remain in suspension as they pass through the precipitator. In order to obtain good conditioning results multiple injection nozzles and injection chambers have to be used and this equipment thus complicates the precipitator installation.

According to the present invention, in a method of operating a rotary kiln plant having an electrostatic precipitator and fired at least partly with solid fuel, an alkali-metal compound is added to the solid fuel before firing in the kiln to reduce the resistivity of the dust-laden exhaust gases from the kiln.

Solid alkali-metal compounds may simply be added to the solid fuel and fired together with the solid fuel thereby obtaining the required distribution in the gases and reactions with the fly ash particles.

Preferably, the conditioning agent (i.e. the alkali-metal compound or compounds) is added to the solid fuel before it is ground. An intimate mixing of the fuel and the conditioning agent is thus achieved so that the chemical composition of the fly ash may be infuenced by the conditioning agent to obtain a resistivity below the above mentioned critical value.

An alternative method is to add the alkalimetal compound by sprinkling the solid-fuel with a solution of the compound. Thus a sprinkling of the fuel with sea water or other water containing water-soluble alkali-metal compounds results in an adequate addition of a conditioning agent.

It is preferred that the conditioning agent is added in amounts equivalent to a content of Na2O in the dust in the exhaust gases corresponding to up to 2% by weight of sodium.

A preferred conditioning agent is solid NaCI added to the solid fuel in a ratio up to about 1:200 by weight and corresponding to a sodium content in the exhaust gases equivalent to about 2% Na2O.

The method has been tested and has given remarkable results. In a lime burning kiln plant having a daily throughput of 260 tons an improvement in the electrostatic precipitator efficiency from 99.1% to about 99.9% was obtained.

The kiln was oil/coal fired in the proportion 40/60 oil/coal calculated on the basis of the relative calorific values, and corresponding to a fuel throughput of 40 tons per day of coal.

Sodium chloride was dosed directly to the coal during its transport to a grinding mill on a belt conveyor, the sodium chloride content comprising 0.43% by weight of the coal. The results showed that the Na2O content in the dust precipitated by the precipitator was raised from 0.21% to 0.30%.

A close relationship was found to exist between the dosing of the sodium chloride and the precipitator performance. The resistivity of the dust decreased from 3.5x 1012 ohm cm. to 1 .5x 1011 ohm cm. which resulted in a decrease in stack dust concentration from 517 mg/normal m3 dry gas to 64 mg/normal m3 dry gas and the migration velocity of the particles within the electrostatic precipitator improved from 8.47 cm/sec. to 12.73 cm/sec.

Claims

1. A method of operating a rotary kiln plant having an electrostatic precipitator fired, at least partly, with solid fuel, wherein one or more alkalimetal compounds are added to the solid fuel before firing in the kiln to reduce the resistivity of the dust-laden exhaust gases from the kiln.

2. A method according to claim 1, in which the alkali-metal compound or compounds are added before the solid fuel is ground.

3. A method according to claim 1 or claim 2, wherein the alkali-metal compound or compounds are added by sprinkling the solid fuel with a solution of the compound or compounds.

4. A method according to any of claims 1 to 3, in which the alkali-metal compound or compounds are added in amounts equivalent to an Na2O content in the dust in the exhaust gas of up to 2%.

5. A method according to any of claims 1 to 4, in which the alkali-metal compound is solid NaCI added to the fuel in an amount of up to 0.5% by weight.

6. A method according to claim 1, substantially as described.