GB2075868A

GB2075868A - Method of treating flue gases

Info

Publication number: GB2075868A
Application number: GB8113537A
Authority: GB
Original assignee: VER SAAR ELEKTRIZITAETS AG
Current assignee: VER SAAR ELEKTRIZITAETS AG
Priority date: 1980-05-16
Filing date: 1981-05-01
Publication date: 1981-11-25
Also published as: DE3018815A1; DE3018815C2; GB2075868B; FR2482477A1; FR2482477B1

Abstract

To ensure that any sulphur dioxide present in flue gases is converted to sulphur trioxide which in turn may form alkali hydrogen sulphate on the surface of dust or other particles, e.g. of alkali silicate, entrained in the gases, an oxygen-supplying compound, such as hydrogen peroxide or an alkali metal peroxide, preferably in aqueous solution, is supplied to the gas. The alkali hydrogen sulphate is more electrically- conductive than the alkali silicate and consequently permits more effective dust separation in a subsequent electrostatic precipitator.

Description

SPECIFICATION Method of treating flue gases Specification This invention concerns a method of treating flue gases, in a process in which dust and particles are separated by means of an electrostatic filter, by oxidising sulphur dioxide present in the gases to sulphur trioxide.

For separation of dust from flue gases in an electrostatic filter, the dust particles must possess a certain electrical conductivity at least over a surface layer thereof, otherwise the particles, negativelycharged by an emitting electrode, cannot give up the charge to a positive precipitation electrode of the filter. If they do not give up their charge, the particles instead of falling offwhen it/they are tapped, remain adherent to the electrode and eventually form a crust which influences the electric field of the filter and thus its effectiveness.

The gas treatment mentioned above creates the necessary conductivity. The sulphur trioxide is converted, by water vapour present in the flue gases, into sulphuric acid and this forms, on the surface of the dust particles which are, for example, of alkali silicate, alkali hydrogen sulphate which is a better conductor than alkali silicate.

Oxidation of sulphur dioxide present in flue gases is known from German Patent Specifications Nos. 1 457 058, 1 457 091, 1 457 093, 1 557 116 and 2 808 199. According to these proposals, vanadium pentoxide or iron oxide are used as catalysts in processes whereby sulphur dioxide is caused to react with residual oxygen which is also present in sufficient quantities in the flue gas. For practical application, however, these processes have always been too expensive. Excessive quantities of the catalysts were consumed; either the catalyst is added at the beginning in the form of dust which is iost during the process, or it is used in the form of a catalyst bed which soon becomes ineffective by being smothered by the flue dust.

Another process for treating the flue gas is also known in which sulphurtrioxide, separately produced or obtained ready in a separate plant from sulphur or sulphur dioxide, is supplied to the flue gas. This process, too, however, is also very expensive.

An object of the present invention is to provide a method of treating flue gases, so as to render them more susceptible to subsequent separation out of dust by means of an electrostatic filter, which is relatively simple but is substantially less expensive than the known processes.

With this object in view, the present invention provides a method of treating flue gases in a process in which dust and particles are separated by means of an electrostatic filter, by oxidising sulphur dioxide present in the gases to sulphurtrioxide, characterised in that an active oxygen-supplying compound is supplied to the gas.

In the method of the invention, therefore, any residual oxygen which might be available in the flue gas for oxidation remains unused, without any disadvantage. This is counter-balanced instead by the advantage that there is no longer any need for a catalyst. Active oxygen combines without a catalyst with sulphur dioxide to form sulphur trioxide. The oxygen-supplying compound, which provides active oxygen, necessary to the method of the invention, requires only a fraction of the expenditure in com parison with-the use of catalysts and also in comparison with the method involving introduction of previously-made sulphur trioxide. Moreover the invention has the additional advantage that it does not pollute the environment with sulphur dioxide which is created when flue dust is converted back by combustion from the alkali hydrogen sulphate on the dust particles.

Preferably in the method of the invention hydrogen peroxide is supplied to the flue gases as the active oxygen-supplying compound. Hydrogen peroxide can be produced as required from raw materials available in the required quantities. If vanadium pentoxide is used as a catalyst in accordance with the known state of the art, on the other hand, a rare element, vanadium is required.

Further in a development of the invention, it is suggested that the hydrogen peroxide should be added in an aqueous solution.

One is then dealing with a somewhat greater volume, which can be correspondingly more easily and more uniformly distributed in the very much greater quantities of flue gas.

There is, in principle, no lower limit to the range of concentration of the hydrogen peroxide in the aqueous solution. The hydrogen peroxide does not take effect whilst in solution, but after evaporation in the flue gas. A practical upper limit arises from the fact that hydrogen peroxide solutions with a hydrogen peroxide concentration of more than 75% are explosive.

According to available sources and market prices, other active oxygen-supplying compounds can be employed. Alkali peroxides are expressly to be mentioned.

The following illustrates a practical example of the performance of the method of the invention: An electrostatic filter unit of a power station is designed for a flue gas capacity of approximately 195,000 m3/h. Flue dust contained in the smoke gas, in accordance with the invention, requires treatment or conditioning with 5 to 25 Vppm SO3, i.e. 975 to 4.875 I/h. To produce this quantity, in accordance with stoicometric reckoning 1,418 to 7,403 gms H202 are necessary, i.e. 4.35 to 21.77 I/h of a 34% aqueous solution obtainable as a commercial product.

The hydrogen peroxide solution is sprayed into the smoke gas duct leading to the electrostatic filter by means of compressed air atomizing jets with a capacity of 1.2 to 1.6 I/h; four to fourteen such sprays are thus required.

Eleven sprays will be employed in a duct whose cross section is 4.5m wide and 1.4m high, those being next to each other and built in, alternately with a clearance of 0.4m and 1 m clearance from the roof of the channel.

Without alteration of the number of sprays there are possibilities for regulating by changing the concentration of the solution and of the quantity of solution sprayed in. Oxygen-supplying compounds, such as alkali peroxides may be used in the place of the hydrogen peroxide.

Claims

1. A method of treating flue gases, in a process in which dust and particles are separated by means of an electrostatic filter, by oxidising sulphur dioxide present in the gases to sulphur trioxide, characterised in that an active oxygen-supplying compound is supplied to the gas.

2. A method as claimed in Claim 1 characterised in that hydrogen peroxide is supplied as the active oxygen-supplying compound.

3. A method as claimed in Claim 2 characterised in that the hydrogen peroxide is supplied in an aqueous solution.

4. A method as claimed in Claim 1 characterised in that an alkali metal peroxide is supplied as the active oxygen-supplying compound.

5. A method as claimed in Claim 4 characterised in that the alkali metal peroxide is supplied in an aqueous solution.

6. A method of treating flue gases as claimed in Claim 1 and substantially as hereinbefore described by way of example.