AU637686B2

AU637686B2 - Process of purifying exhaust gas

Info

Publication number: AU637686B2
Application number: AU75044/91A
Authority: AU
Inventors: Rudolf Dipl.-Ing. Brockl; Hans-Joachim Dr.-Chem. Schmidt; Bertold Dipl.-Ing. Stegemann; Dieter Dipl.-Ing. Stein
Original assignee: Metallgesellschaft AG
Current assignee: GEA Group AG
Priority date: 1990-04-18
Filing date: 1991-04-17
Publication date: 1993-06-03
Anticipated expiration: 2011-04-17
Also published as: AU7504491A; JPH04227818A; ES2067135T3; CA2037839A1; EP0453005B1; DE59104357D1; KR910018065A; DE4012320C1; EP0453005A1

Description

637686 Form COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952-69 COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: 75044/91 17.4.1991 Complete Specification Lodged: Accepted: Published: Priority *Felated Art

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Name of Applicant: Address of Applicant Actual Inventor f Address for Service METALLGESELLSCHAFT AKTIENGESELLSCHAFT Reuterweg 14, D-6000 Frankfurt/Main, Federal Republic of Germany HANS JOACHIM SCHMIDT, BERTOLD STEGEMANN, DIETER STEIN and RUDOLF BROCKL WATERMARK PATENT TRADEMARK ATTORNEYS.

LOCKED BAG NO. 5, HAWTHORN, VICTORIA 3122, AUSTRALIA mplete Specification for the invention entitled: a a PROCESS OF PURIFYING EXHAUST GAS The following statement is a full description of this invention, including the best method of performing it known to PRQOCESS OF PURIFYING EXHAUST GAS

DESCRIPTION

This invention relates to a process of purifying exhaust gases, which in addition to fly ash contain gaseous pollutants, such as $02, HCI and HF, as well as volatile heavy metals, particularly mercury compounds, by a dry separation of solids and a scrubbing with a circulated scrubbing liquid in two stages.

Such process are used, inter alia, for the purification of the exhaust gases which are formed in garbage-incinerating plants (GIP) and which in addition to varying rates of fly dust may contain up to 10,000 mg HCI, up to 2,000 mg SO 2 up to 1,000 mg HF and up to 100 mg heavy metals per sm 3 (sm 3 standard cubic meter). Said exhaust gases must be purified in accordance with the regulations which are applicable in any given case. In Germany the limits for GIP in accordance with the TA-Luft (technical instruction for air pollution control) of 1986 are: HCI 50, SO 2 100, HF s cgo• heavy metals (of Classes I II, II 1) 0.1/1.0/5.0, in mg/sm 3 S1 5 On principle a wet purification adopted to avoid a pollution by exhaust gas will *see always involve a pollution by waste water. For this reason a pollution by exhaust gas cannot be avoided unless the pollution by waste water can satisfactorily be avoided and a satisfactory disposal of the residual materials which are inevitably obtained is assured.

With the exception of those cases in which a dry purification of exhaust gas may be and is adopted as an alternative, relatively complicated plants and processes will be required for a wet purification of exhaust gas and the chemical reactions involved therein will not fully be understood by many operators. In addition to the general desire to solve the problem in a manner which can be understood as clearly as possible, it has always been endeavoured, for the reasons stated, to purify exhaust gases to specified 25 pollutant contents and prescribed pure gas compositions in a manner which is as simple as possible.

From the aspect of the safety of operation, the availability of the equipment for G o. purifying exhaust gas must primarily be taken into account because in the case of a S" defect it will be necessary to interrupt the "production of exhaust gas" so that the power plant, the garbage-incinerating plant or the chemical production plant must be shut down until the exhaust gas purifying plant is operable again. To ensure the safety of operation it is also necessary that the exhaust gas-purifying equipment can respond in such a manner to widely changing operating conditions, such as may arise owing to the changing quality of fuel, particularly in garbage-incinerating plants, that a pure gas 3 having a permissible composition will always be achieved and that this will not give rise to a new sewage disposal or dumping problem.

It will also be understood that the exhaust gas must be purified with the highest possible economy as far as the overall concept is concerned. A simple plant involving a low capital expenditure will not be economical if it involves relatively high costs for reactants and power and for the disposal of residual substances during the total operating life of such plant. On the other hand, a highly effective plant will not be very useful if its capita! expenditure is extremely high or if it is so complicated that it has only a low availability.

1 0 The invention is based on a process which is of the kind described first hereinbefore and in which the low partial pressure of the gaseous pollutants over the scrubbing liquid, which low partial pressure is required for a separation of such pollutants to a high degree, is maintained in that the scrubbing liquid is withdrawn at a o• high rate. In other words, the pollutant concentrations in the scrubbing liquid must not 15 exceed certain upper limits because otherwise the partial pressure of such pollutants will rise excessively and the degree of separation will exhibit an over-proportional e g.

decrease. For this reason a high degree of separation cannot be achieved in such process unless the wast vater is treated and evaporated in a plant which is designed for a withdrawal of waste water at a high rate and such plant will involve a correspondingly high expenditure for supplies. For this reason the requirement for high degrees of separation in processes of that kind will necessarily result in high capital expenditures and operating costs. Besides, the necessary reconditioning of the scrubbing liquid will o result in a production of unusable residual materials at relatively high rates and their final dumping will also involve high costs.

It is an object of the irnvention to avoid in the known process the disadvantages which have been pointed out, to propose a process by which the purification of exhaust gases in its entirety can be carried out more economically without a need for a more complicated plant and processing and without a decrease of the safety of operation.

coo• To accomplish that object it is proposed that the process described first S 30 hereinbefore should be carried out in such a manner that a) the exhaust gases are treated with an acid scrubbing liquid in a first scrubbing stage, particularly to decrease the concentration of HCI, HF and heavy metals and to adjust a temperature of 50 to 700C, and in which a pH value of 0 to 3 is maintained by a controlled addition of water and bases and by a withdrawal of scrubbing liquid, b) the exhaust gases are treated with a basic scrubbing liquidin a second scrubbing stage, which particularly serves to decrease the S0 2 concentration and in which the same temperature is maintained as in the first scrubbing stage and in which a pH value of 6 to 9 is adjusted by a controlled addition of water and bases and by a withdrawal of scrubbing liquid and a purified exhaust gas is obtained, c) the scrubbing liquid withdrawn from the first scrubbing stage is treated with a precipitant and a flocculant and is subjected to a separating process, in which (1 a sludge that contains heavy metals and fluorine compounds and constitutes a first residual product to be dumped and 2) a solution which contains alkali chloride are obtained, and said solution is evaporated and dewatered to recover alkali chloride salts, which can be re-used, as well as a residual solution and condensate, which can be recycled in the process, and d) the scrubbing liquid which has been withdrawn from the second scrubbing stage is S1 5 treated in that it is evaporated and dewatered to provide another residual product which is to be dumped, which essentially contains alkali "sulfate, and 0o a residual solution and condensate, which can be recycled in the process.

A pH value of about 2 is preferably adjusted in the first scrubbing stage and a pH value of about 7.5 in the second scrubbing stage. The base employed suitably consists of an alkali hydroxide, particularly sodium hydroxide. It may be desirable to add the than into the circulation system associated with the first scrubbing stage.

In the process in accordance with the invention the acid pollutant gases HCI and HF which have been removed from the exhaust gas in the first scrubbing stage are 4 substantially converted to their salts in that bases are admixed in less than stoichiometric amounts. This will correspondingly decrease the partial pressures of Ssuch gases so that not only an effective removal of heavy metals but also a highly 0000 effective removal of HCI and HF can be achieved in that first scrubbing stage, in which *0 30 acid conditions are maintained, whereas it is not necessary to continuously withdraw scrubbing liquid from circulation at a high rate. As a result, the plant for regenerating the scrubbing liquid may be distinctly smaller than in conventional processes so that the capital expenditure will be reduced. Another great advantage is afforded by the fact that the scrubbing liquids which have been withdrawn from circulation are separately reconditioned so that the chloride-containing salts formed in the first scrubbing stage can separately be recovered and can be re-used whereas in conventional processes they must be finally dumped together with the residual products. The economic advantages which are offered by the process in accordance with the invention can be summarized as follows: The capital investment required for the reconditioning and evaporation of the scrubbing liquid is decreased by about the steam requirement for the evaporation is decreased by about the electric power required for the reconditioning and evaporation is decreased by about 1 0 the total amount of residual substances obtained is decreased by about 30% and the chloride-containing salts can be re-used and need not be dumped; the amount of residual substances to be dumped is decreased by 30 to The improvement of the known process in accordance with the teaching of the invention does not render the plant concept or its mode of operation more complicated.

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15 There are also no disadvantages involved as regards the availability and the safety of °o o.o operation of a plant conceived for carrying out the process in accordance with the invention. For this reason a purification of exhaust gases which in addition to fly ash o. contain gaseous pollutants, such as SO 2 HCI, and HF, and volatile heavy metals, particularly mercury compounds, can be effected much more economically.

Further details of the invention will be explained more specifically with reference to the diagram of connections shown in Figure 1.

The exhaust gas is first supplied through line 11 to a unit 3 for separating rooo° solids, fly dust and the like, which are removed from the system through line 21.

The exhaust gas is then passed through the consecutive scrubbers 1 and 2 and in a purified state is discharged into the environment through line 12. The two scrubbers 1 and 2 are supplied with circulated scrubbing liquid, which is handled by the pumps 7 and 8. The scrubbing circuits are supplied with fresh water and fresh reactant bases through lines 9 and Part of the scrubbing liquid is withdrawn from the circuit associated with the S 30 first scrubber 1 and after an addition of precipitant and flocculant through line 13 is supplied to a unit 4, in which a sludge which contains the heavy metals and fluorine compounds is separated. The separated sludge is dewatered and is then withdrawn from the process through line 16 as the iirst residual product that is to be dumped. After that removal of sludge from the scrubbing liquid, which consists of a solution that contains alkali chloride, that liquid is evaporated in a unit 5 so that re-usaeable alkali chloride salts and a residual solution and condensate are obtained. The alkali chloride salts are withdrawn through line 17. The residual solution is removed through line 18 and the condensate through line 14. The residual solution and the condensate may be recycled in the process to a desirable point so that there is no need for a separate disposal of waste water.

Part of the washing liquid is also withdrawn from the circuit associated with the second scrubber 2 and is evaporated in a unit 6, in which a second residual product is obtained, which substantially comprises sodium sulfate and is withdrawn from the process through line 19. Residual solution and condensate are also obtained in the unit 1 0 6 and flow off through lines 20 and 15 and can be recycled in the process.

If the condensate and residual solutions obtained in the evaporators 5 and 6 are entirely recycled in the process there will be no need for a disposal of waste water, and in that case the "product" of the process of purifying exhaust gas in accordance with the invention will consist only of purified exhaust gas (in line 12), of a sludge (in line 16) 15 which contains the heavy metals and fluorine compounds and constitutes the first

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residual product that is to be dumped, of re-useable alkali chloride salts (in line 17), and of a second residual product to be dumped, which essentially contains sodium sulfate (in line 19).

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Claims

1. A process of purifying exhaust gases, which in addition to fly ash contain gaseous pollutants, such as SO 2 HCI and HF, as well as volatile heavy metals, particularly mercury compounds, by scrubbing with circulated scrubbing liquid in two stages, characterized in that a) the exhaust gases are treated with an acidic scrubbing liquid at an adjusted temperature of 50 to 700C, in a first scrubbing stage, in which a pH value of 0 to 3 is maintained by a controlled addition of water and bases and by withdrawal of the acidic scrubbing liquid, b) the exhaust gases are treated with a basic scrubbing liquid, which contains alkali hydroxide, in a second scrubbing stage, in which the same temperature is maintained as in the first scrubbing stage and in which a pH value of 6 to 9 is adjusted by a controlled addition of water and bases and by withdrawal of the basic scrubbing liquid and a purified exhaust gas is obtained, c) the acidic scrubbing liquid withdrawn from the first scrubbing stage is treated with a precipitant and a flocculant and is subjected to a separating process, in which a sludge that contains heavy metals and fluorine compounds and constitutes a first residual product to be dumped and a solution which contains alkali chloride are obtained, and said solution is evaporated and dewatered to recover alkali chloride salts, which can be re-used, as well as a residual solution and condensate, which can be recycled in the process, and d) the basic scrubbing liquid which has been withdrawn from the second scrubbing stage is treated in that it is evaporated and dewatered to provide another residual product which are to be dumped, which essentially contains alkali sulfate, and a residual solution and condensate, which can be recycled in the process.

2. A process according to claim 1, characterized in that a pH value of about 2 is adjusted in the first scrubbing stage.

3. A process according to claim 1, characterized in that a pH value of about is adjusted in the second scrubbing stage.

4. A process according to any one of claims 1 to 3, characterized in that sodium hydroxide is used as an alkali hydroxide.

A process according to any claims 1, characterized in that (as an alternative to feature c) the precipitant and flocculant are supplied to the circulated scrubbing liquid for the first stage.

6. A process according to claim 1 further characterized by a dry separation of solids.

7. A process of purifying exhaust gases substantially as hereinbefore described with referenc to Figure 1. DATED this 16th day of March 1992 METALLGESELLSCHAFT AKTIENGESELLSCHAFT WATERMARK PATENT TRADEMARK ATTORNEYS THE ATRIUM 290 BURWOOD ROAD HAWTHORN, VICTORIA 3122 Doc 04 S AU007504491.WPC e o