EP0735321A2 - Process and device for purification of noxious exhaust gases by chemical purification - Google Patents

Abstract

The device cleans or purifies waste gas containing toxic substances esp. with fluoride carbonated water etc. The method uses chemical conversion in a combustion chamber with a burner for generating a burning gas flame. By spatial separation of the feeding in of gases of different combinations and quantities into the flame a flame region of reduced chemical strength is created. The gas to be cleaned is fed into this region. A strongly oxidised flame region is also formed. The gas is put into this region and then it is passed through the reduced region. The appts. for carrying out this method has a combustion chamber, a ring burner for the burning gas or gas mixture, and a central supply for the toxic waste gas into the burner. One or more inlet channels are arranged in the wall of the chamber near the burner to additionally feed in air or acid.

Description

The invention relates to a method and devices for cleaning pollutant-containing exhaust gases by chemical reaction in a combustion chamber.

In technical plants, in particular in plants for chemical vapor phase separation and for the removal or separation of material by means of plasma processes, waste gases containing pollutants are produced. These exhaust gases include fluorine-containing hydrocarbons or other fluorine compounds. In general, inert gases such as argon or nitrogen are major components of the exhaust gases. Because of the high proportion of such gases, these exhaust gases themselves are usually not combustible. The pollutants or their reaction products have a toxic effect or, due to their harmful effects in the atmosphere, promote ozone depletion and the greenhouse effect.

A whole series of methods are known for exhaust gas purification. The cleaning is very often carried out by sorption of the pollutants from the exhaust gas, for example in oxidizing aqueous solution (DE 3342816).
Pollutants that are not sorbed in this way or are not efficiently treated can be converted by chemical conversion processes, for example thermal decomposition (EU 0384803) by heating or by burning in an oxygen-containing environment (US Pat. No. 5,183,646). If this is done by burning, exhaust gases with a high proportion of inert gas have to be introduced into a combustion gas flame, for example from a natural gas or hydrogen / oxygen mixture. Harmful secondary products of the conversion are then removed from the exhaust gas, for example by sorption or washing processes.

Exhaust gas cleaning is generally a multi-stage process in which partial processes such as thermal decomposition or oxidation, cooling, sorption, hydrolysis and washing out of solid reaction products take place (EP 89 110 875). For this purpose, the exhaust gas is passed in succession, for example, through a device with a combustion chamber and at least one further device, for example one which operates on the washing principle. Devices for purifying exhaust gas have also been proposed, in which the exhaust gas is passed successively through a combustion chamber for combusting the pollutants and a washing chamber, which are structurally combined to form a unit (EP 89 110 875).

A disadvantage of such methods and devices with thermal decomposition of the pollutants in a combustion chamber and with a washing chamber are the thermal and electrolytic corrosion of the wall surfaces and other parts, in particular the combustion chamber, under the action of gaseous reaction products from the combustion. This corrosion is critically increased by the high temperature of the burned fuel gas mixture and exhaust gases, especially if these e.g. Hydrogen halides, e.g. HF, and water vapor included.

This disadvantage can be eliminated if partial cleaning processes are combined in a single reaction chamber, in which the burned exhaust gas is passed through a finely divided liquid (sorbent or coolant) or in contact with such a liquid film on the wall surfaces of the combustion chamber brought (DE 43 200 44). However, the last two solutions mentioned have a low efficiency in their cleaning effect when used for exhaust gases with fluorinated hydrocarbons and with other fluorine compounds. With justifiable consumption of fuel gas, the cleaned exhaust gases still contain critically high proportions of pollutants. The cooling of the reactor walls reduces their corrosion, but rather leads to a deterioration in the efficiency of the cleaning. To a certain extent, an improvement in the efficiency of the cleaning in the direction of a low pollutant content in the cleaned exhaust gas can be achieved by increasing the amount of fuel gas relative to the amount of the supplied exhaust gas, but this route is associated with a critical deterioration in the economy of the exhaust gas cleaning because of the increased fuel gas consumption .

Since several reactions generally take place in the fuel gas flame with exhaust gas supply, the most important results of which are the combustion of the fuel gas (e.g. natural gas or hydrogen) under the influence of the oxygen supplied for the purpose of thermal activation of the harmful gases and the chemical conversion of the harmful gases into hydrolyzable and absorbable or Because of the reaction kinetics, harmless compounds are not to be expected that the desired conversion of the harmful gas takes place completely. This also applies in particular if all reactants (fuel gas, oxygen and harmful gas) are supplied in a stoichiometric ratio. As a result of the proportion of inert gas in the harmful gas, the reaction kinetics are adversely affected and the proportion of the conversion of the harmful gas is further reduced.

An increase in the proportion of fuel gas in the fuel gas mixture (EP-A-0347 753) compared to the stoichiometric ratio for all reactants supplied does improve the conversion of pollutants, in particular in the case of fluorine-containing hydrocarbons, but leads to the emission of unburned, even harmful, fuel gases from the cleaning system. An increase in the oxygen content in the fuel gas mixture compared to the stoichiometric ratio leads on the other hand to the critical deterioration of the pollutant conversion for fluorine-containing exhaust gases and thus to an unacceptably high residual pollutant content in the cleaned exhaust gas.

The invention has for its object to improve the effectiveness of the purification of exhaust gases by chemical conversion in a fuel gas flame, ie to drastically reduce the percentage of residual pollutants in the cleaned exhaust gas. There is also the task of ensuring that intermediate products formed during the reaction in the fuel gas flame are effectively burned. Furthermore, no unburned constituents of the fuel gas should be contained in the cleaned exhaust gas. In particular, it can be achieved that no secondary pollutants arise during combustion and are released with the cleaned exhaust gas.

According to the invention the object is achieved by a method according to claim 1, 2 and 3 and a device according to claim 4 or 5 .

The method assumes that a fuel gas mixture is used in a combustion chamber with a burner to generate a fuel gas flame and that the exhaust gas, which contains the pollutants, in particular fluorine-containing hydrocarbons or other fluorine compounds, is fed into this flame. The chemical conversion of the pollutant components of the exhaust gas takes place in the flame. The burned exhaust gas contains gaseous reaction products of the pollutants.

bzw. $${\text{2H}}_{\text{2}}{\text{+ O}}_{\text{2}}{\text{→ 2H}}_{\text{2}}\text{O)}$$

und die Reaktionen zur Umsetzung des Schadstoffes (eine mögliche Reaktion 1: $${\text{2C}}_{\text{2}}{\text{F}}_{\text{6}}{\text{+ 14H}}_{\text{2}}{\text{→ 4CH}}_{\text{4}}\text{+ 12HF}$$

und als sekundäre Reaktion 2: $${\text{4CH}}_{\text{4}}{\text{+ 8O}}_{\text{2}}{\text{→ 4CO}}_{\text{2}}{\text{+8H}}_{\text{2}}\text{O)}$$

parallel ab. According to the invention , by spatially separating the feed of gases of different composition and quantity into the fuel gas flame, on the one hand a strongly reducing fame area is created, into which the exhaust gases to be cleaned are fed with the pollutants, on the other hand a strongly oxidizing flame area is created. First, a strongly reducing flame is created in which the fuel gas mixture of fuel gas and oxygen, for example town gas, natural gas or hydrogen and oxygen, is fed into the burner with an excess of the fuel gas compared to the portion required for stoichiometric combustion and brought to combustion. Compared to the stoichiometric ratio of the fuel gas mixture, for example CH ₄ : O ₂ = 1: 2 or H ₂ : O ₂ = 2: 1, which can burn without residue of fuel gas (CH ₄ or H ₂ ), a reducing flame area is created by a Excess hydrogen in the fuel gas flame is reached by increasing the ratio of the proportions of hydrogen-containing components in the fuel gas mixture, in the example changing the ratios in the direction of CH ₄ : O ₂ = 1: 1 or H ₂ : O ₂ = 4: 1 Flame operated with an excess of fuel gas generates the energy required to activate the reactants and break the chemical bonds. The exhaust gas, usually a mixture of an inert gas (argon or nitrogen) and the fluorine-containing pollutant, such as C ₂ F ₆ , CHF ₃ , SF ₆ , NF ₃ , is introduced into this flame. The combustion reactions of the fuel gas run in the flame $${\text{(CH}}_{\text{4th}}{\text{+ 2O}}_{\text{2nd}}{\text{→ CO}}_{\text{2nd}}{\text{+ 2H}}_{\text{2nd}}\text{O}$$

respectively. $${\text{2H}}_{\text{2nd}}{\text{+ O}}_{\text{2nd}}{\text{→ 2H}}_{\text{2nd}}\text{O)}$$

and the reactions to Implementation of the pollutant (a possible reaction 1: $${\text{2C}}_{\text{<figure-callout id="6" label="2nd F" filenames="imgf0001.png" state="{{state}}">2nd </figure-callout>}}{\text{F}}_{}{}_{\text{6}}{\text{+ 14H}}_{\text{2nd}}{\text{→ 4CH}}_{\text{4th}}\text{+ 12HF}$$

and as secondary reaction 2: $${\text{4CH}}_{\text{4th}}{\text{+ 8O}}_{\text{2nd}}{\text{→ 4CO}}_{\text{2nd}}{\text{+ 8H}}_{\text{2nd}}\text{O)}$$

in parallel.

As a result of the composition of the fuel gas mixture, not only is its combustion incomplete, but the reaction equilibrium in the combustion gas flame is shifted from the reactions to the conversion of the harmful gas in the direction of the reduction (in the assumed example reaction 1). The reason for this is the excess of hydrogen in the reaction space, so that the law of mass action acts in the direction of the complete conversion of the pollutant (in the example C ₂ F ₆ in HF). The most complete implementation of the pollutant is the goal of cleaning in order to achieve the lowest possible percentage of pollutant in the exhaust gas.

However, the reducing flame now contains unused fuel gas (CH ₄ and H ₂ in the example) and secondary pollutants (CH ₄ and HF in the example).

The spatially separate feeding of a fuel gas mixture of a different composition into the flame creates, in addition to the reducing flame area primarily created in the burner, a second oxidizing flame area. For this purpose, fuel gases and oxygen with a clear excess of oxygen, compared to said conditions, which can burn without residue, air can be supplied as an oxygen supplier or pure oxygen at all.

The second oxidizing flame area is characterized by an excess of oxygen. In addition to gases from the combusted fuel gas mixture (CO ₂ , H ₂ O), unused fuel gas components (in the example CH ₄ and H ₂ and secondary pollutants (in the example HF, CH ₄ ), which now completely oxidize or The complete combustion is also given according to the law of mass action, since an excess of oxygen is set in this flame area.

The heated inert components of the exhaust gas (e.g. N ₂ ), the inert combustion products (predominantly CO ₂ and water vapor) and the secondary pollutants (in the example HF) flow out of the oxidizing flame area. The secondary pollutants are removed in a manner known per se by sorption or hydrolysis. In the simplest case, the combustion products are sucked out of the combustion chamber and passed through an aqueous solution, for example in a spray washing device.

The method is carried out with a device which essentially consists of a preferably rotationally symmetrical combustion chamber with an annular burner arranged in an end face of the combustion chamber. The fuel and the ring burner separately supplied oxygen or the fuel gas mixture (natural gas / oxygen or hydrogen / oxygen). The fuel gas flame forms on the burner. A feed, preferably in the center of the ring burner, supplies the exhaust gas to be cleaned with the pollutants contained therein. Thanks to the central feed, the flue gas is enclosed on all sides by the fuel gas flame, an important prerequisite for effective conversion of pollutants.

According to the invention , the additional supply of fuel gas mixture with an excess of oxygen, air or pure oxygen into a ring burner takes place via one or more bores or channels of a second concentric ring. When the burner is operated with the said two concentric rings for supplying fuel gas mixtures with an oxygen deficit or with an excess of oxygen, a flame is formed above the burner, with a strongly reducing flame area as the core of the flame above the inner ring of the burner and with a strong one oxidizing flame area as the jacket of the flame over the outer ring of the burner.

In another embodiment of the device according to the invention with a ring burner known per se with a ring for supplying the fuel gas mixture with an increased fuel gas content and with a central supply for the pollutant-containing exhaust gas, the additional supply of air or oxygen takes place via one or more inlet channels in the wall the combustion chamber in the area of the ring burner. A flame forms on the ring burner, the core of which is strongly reducing, since a fuel gas mixture with a deficit of oxygen is supplied to the ring burner in the sense mentioned. The flame burns in the combustion chamber in a "jacket" of air or pure oxygen, so that a flame area is also formed in this way around the reducing flame area, which has a strong oxidizing effect.

The extraction on the combustion chamber, through which the combustion products are extracted, is dimensioned such that it passes through the holes or channels in the wall. ensures the air supply required for combustion.

The additional supply of air or oxygen into the flame area under positive pressure can be metered more precisely through the inlet channels in the wall of the combustion chamber.

The invention is explained in more detail below with the aid of a method example and with reference to the drawing Fig. 1 of a preferred embodiment of the device . Fig. 1 shows the device in a schematic longitudinal section.

The device according to the invention essentially consists of a cylindrical combustion chamber (1) made of a corrosion-resistant material. It has a diameter of 18 cm and is 60 cm high. In the area of an end face (2) of the combustion chamber (1) there is an annular burner (3) to which the fuel gas mixture of fuel gas and oxygen is fed via a feed (4). In the example, the position of the combustion chamber is vertical (other positions, also inclined, are also possible). The ring burner (3) has a diameter of 25 mm. The fuel gas flame (6) forms above the ring channel (5). The exhaust gas with the toxic pollutants is fed to the burner (3) via the feed (7). It enters the fuel gas flame (6) centrally through the bore (8).

The burner (3) has an additional feed (9) for a fuel gas mixture of a different composition than that which is let into the feed (4).

At the combustion chamber is the tubular connection for the suction (14) through which the combustion gases are either transferred into the exhaust air duct or into devices for carrying out further sub-processes of a multi-stage exhaust gas cleaning.

The device is completed by an ignition electrode for the fuel gas mixture and by a monitor (15) which provides sensor signals for optimal control of the device for exhaust gas purification.

Example of the execution of the procedure:

In a plasma etching system for removing silicon oxide layers on silicon wafers, 60 l / min of exhaust gas are produced. The exhaust gas consists of approx. 58.5 l / min nitrogen and approx. 1.5 l / min C ₂ F ₆ as a pollutant. The exhaust gas is fed to the cleaning device through an exhaust pipe. In the combustion chamber (1) 9 l / min H ₂ and 9 l / min O _{2 are} fed to the inner ring (5) of the ring burner (3). 3 l / min CH ₄ and 15 l / min O ₂ are additionally fed to the outer ring (10) of the ring burner (3). The flame area (11) of the flame (6) which forms over the annular gap (5) is strongly reducing. In this strongly reducing flame area (11), the exhaust gas to be cleaned, which contains fluorinated hydrocarbon C ₂ F ₆ as the main pollutant, passes through the bore (8) In this reducing flame area (11) the pollutant is converted almost completely into CH ₄ and HF. The flame temperature is in the range from 1200 to 1400 ° C.

The flame region (12) which forms at the annular gap (10) and which envelops the flame region (11) like a jacket is strongly oxidizing. The reactions that preferably take place here are the combustion of CH ₄ to CO ₂ and water vapor and of H ₂ to water vapor. The approximately 1000 ° C hot burned gas mixtures over the flame flow in the direction of arrow (13) perpendicularly from the burner (3) in the direction of the suction (14), from there they are fed to a washing device known per se, in which the gas is below 50 ° C is cooled. The HF part is hydrolyzed. The gas is cleaned, ie practically free of pollutants, released into the environment. The pollutant content is reduced to less than 1% by means of the exhaust gas purification method according to the invention, as a result of which a gas with less than 15 ppm of pollutant enters the exhaust air.

Claims (5)

Process for the purification of pollutant-containing exhaust gases, in particular with fluorine-containing hydrocarbons and other fluorine compounds, by chemical reaction in a combustion chamber with a burner for producing a fuel gas flame, characterized in that, on the one hand, a chemically strong gas is separated by spatially separating the feed of gases of different compositions and amounts into the combustion gas flame reducing flame area is created, into which the exhaust gases to be cleaned are fed and, on the other hand, a strongly oxidizing flame area is created, into which the exhaust gases enter after they have flowed through the reducing flame area. A method according to claim 1, characterized in that the reducing flame area is generated by supplying a fuel gas and oxygen or a fuel gas mixture with an excess of hydrogen, and in that the oxidizing flame area is produced by additional supply of a fuel gas mixture with an excess of oxygen, pure oxygen or Air is created in the flame. Process according to claims 1 and 2, characterized in that harmful reaction products of the combustion in said two flame areas are eliminated by subsequent sorption and / or hydrolysis. Device for carrying out the method according to claims 1 to 3 with a combustion chamber, a ring burner for the fuel gas or the fuel gas mixture and a central feed for the pollutant-containing exhaust gas in the ring burner, characterized in that for an additional supply of a fuel gas or fuel gas mixture with an excess of oxygen on the ring burner, or for an additional supply of pure oxygen, a second concentric ring with one or more holes or with a channel on the ring burner is arranged. Device for carrying out the method according to claim 1 to 3, with a combustion chamber, a ring burner for the fuel gas or the fuel gas mixture and a central feed for the pollutant-containing exhaust gas in the ring burner, characterized in that for the additional supply of air or oxygen in the wall one or more inlet channels are arranged in the combustion chamber in the area of the ring burner.

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