AU6465199A

AU6465199A - Filter for separating nitrogen oxides and particles from a gas flow

Info

Publication number: AU6465199A
Application number: AU64651/99A
Authority: AU
Inventors: Paul Kerzel
Original assignee: KARL DICKELS
Current assignee: KARL DICKELS
Priority date: 1998-10-08
Filing date: 1999-10-08
Publication date: 2000-05-01
Also published as: EP1039963A1; DE19846320C1; DK1039963T3; ES2293737T3; KR20010040293A; DE59914489D1; WO2000021645A1; EP1039963B1; ATE372163T1

Abstract

For the purposes of environmental protection, it is increasingly necessary to remove nitrogen oxides from an air flow at an ambient or outer air temperature. While NO2 can be separated from a gas flow by means of methods known per se, whereby the carbon is washed out or absorbed, it is not possible to separate NO using said methods. Hitherto known methods exhibit deficiencies that make it impossible to use them on a widespread scale. The present invention solves this problem in a simple and inexpensive manner. Short pulses lasting up to a few nanoseconds allow for higher electrical voltages inside the ionizer (4) than direct current, without resulting in electrical flashovers. Tests have shown that ozone is formed inside the gas flow to such an extent that nitrogen monoxide present in said gas flow is substantially oxidized. The NO2 formed in the ionizer can subsequently be removed from the gas flow using a known method. A further advantage of the invention is that a pre-ionizer can be arranged upstream from said ionizer, whereby gas molecules and particles can be electrically charged before they enter the ionizer. The filter system to which claim is laid can also be used without a separator for NO2 for the separation of particles only from a gas flow. One particularly advantageous application of the invention is the use thereof in the removal of smoke, especially smoke emanating from fires, from a flow of air or gas.

Description

Description of the Invention Processes for separating Nitrogen oxide from a gas flow at temperatures above 150 'C are known. One such process is the SCR process, in which NOx is converted into nitrogen and water when ammonia is added in the presence of a catalyst. In the context of protection of the environment, increasing requirements are made for the separation of nitrogen oxide from airflows at room temperature or at ambient or outside temperatures. For example it is necessary to clean nitrogen oxides from the air removed from road tunnels via exit portals. The nitrogen oxide arising from vehicle exhaust gases comprises approximately 90 % NO and approximately 10 % NO 2 . While NO 2 can be separated from a gas flow with known processes such as scrubbing, separation of NO is not possible with these processes. Therefore over the last few years various processes have been developed to remove nitrogen monoxide from a gas flow at normal outside temperatures or room temperatures. In one known process nitrogen monoxide is separated using a mixture of ozone, which is produced using an ozone generator, so that it is oxidized to NO 2 . This is then converted to KNO 2 and KNO 3 using potassium hydroxide. In this process the gas containing the nitrogen oxide flows through a honeycomb type support material that has been saturated with potassium hydroxide. After use with potassium hydroxide, the support material is washed out in water, soaked in potassium hydroxide and again placed in the gas flow. In another established process an adsorber adsorbs the nitrogen oxide. The regeneration of the adsorber is achieved using a separate gas circuit. In the circuit an airflow is heated to 200 'C and fed through the adsorber. This frees the nitrogen oxide from the adsorber. The separation of the NOx from the gas circuit is achieved using the known SCR process with a catalyst in a flow of air mixed with ammonia. For a nitrogen oxide concentration of about 4 PPM the adsorber must be regenerated after every 40-hour operating period. Other processes for the microbiological separation of NO from a gas flow have been developed. One process involves passing the gas through a support material dosed with microbes. In another process the microbes are dissolved in an aqueous solution and separated from the gas flow by a gas permeable membrane. The known processes all have drawbacks, which have prevented their wide-scale application up until now. Thus the periodical scrubbing, soaking and rehabilitation of the support material used in the gas flow requires considerable mechanical and control efforts. A further drawback of the processes is the use of an ozone generator. As a consequence of the problematic arrangements required for working with ozone, the process is not widely accepted. In addition the ozone generator also requires an additional, maintenance-intensive, plant component. In the other described processes, the use of ammonia is an obstacle because it is a caustic material and requires special handling precautions. The heating of the regeneration gas circuit to some 200 'C involves heat generation that directly or indirectly involves production of other air pollutants. The heat generation plant with its considerable equipment cost makes the process more expensive. The long reaction times of the microbiological processes mean that a very large building volume is needed for the filter plant, which is an economical factor acting against their employment. This invention solves the problem posed in a simple and economical fashion. Short voltage impulses lasting for only a few nanoseconds allow higher voltages to be applied in an ionizer than with purely D.C. voltage, without the occurrence of arcing. As tests have shown, the formation of ozone in a gas flow is increased so much that any nitrogen monoxide present is largely oxidized. The NO 2 formed in the ionizer can then be separated from the gas flow using a known process. The voltage impulses can be produced using an impulse generator and applied directly to the ionizer, or they can be used superimposed on a base voltage produced using a high voltage generator. Changing the duration and magnitude of the voltage impulses controls ozone formation and therefore oxidation of the nitrogen monoxide. A development of the invention involves measurement of the ozone concentration using an ozone sensor and use of the measurement value as an input value for an electronic controller, with which the magnitude and duration of the voltage impulses can be controlled. In this way it is possible to match the ozone concentration stoichiometrically to the ozone quantity required to remove the amount of NO present in the flow and to prevent generation of any free ozone. A further refinement of the invention is the use of a pre-ionizer in front of the ionizer, so that gas molecules and particles are already charged before they enter the ionizer. The pre-ionizer consists of two or more earthed parallel plates, between which spray electrodes such as spray points or spray wires are arranged perpendicular to, or aligned with the flow direction according to a pre-determined layout, so that a homogenous corona field is produced. To achieve sufficient charging the plates must have a minimum length. If the gas flow is directed between the plates, electrical charging of the gas molecules and particles results, that is then increased in the following ionizer. Use of the pre-ionizer means the obtained electrical charging can be greatly increased without danger of arcing, as demonstrated in tests. The tests have demonstrated that an arrangement of vertical rods with transversely arranged wires is the most advantageous spray electrode design. A further development of the invention involves use of a carbon filter for separation of the NO 2 . As shown by longstanding tests, carbon filters are very efficient at removing nitrogen dioxide. In gas flows containing any residual ozone this is also completely removed in a downstream carbon filter. Tests have shown that for a NO 2 concentration of approximately 4 PPM, as found in the air removed from road tunnels, operating periods of several years are possible. The carbon can be regenerated after it has been saturated and re-used in the filter. Several re-generations are possible. The carbon can finally be burnt to obtain useful thermal energy. Through the combination of a simple NO oxidization process as suggested and described above, a simple and economic nitrogen oxide filter is produced as a solution to the problem described at the outset. The ionizer used in the invention can also be used for the simultaneous electrical charging of particles within the gas flow that can then be removed in a downstream collector. As a separation of particles from the gas flow is usually also required, the invention therefore means that a substantial reduction in the necessary plant components can be made with a resultant reduction of the construction and operating costs of the filter. Alongside the separation of nitrogen oxide the claimed invention also results in improved particle separation. New research shows that small airborne particles have a particularly serious health impairing effect. Therefore there is an increasing requirement for the removal of fine particles from exhaust gas flows. Measurements in road tunnels have shown that the existing electro-filters only remove approximately 50 % of particles smaller than 0.01 gim. Using the filter designed in accordance with the invention, the removal efficiency for fine particles is substantially improved. Using pulsed voltage with the ionizer the particle electrical charge is greatly increased, which results in improved separation of particles from the gas flow. Use of a pre-ionizer also results in improved separation of fine particles. The filter plant, as claimed in the invention, can also be employed without an NO 2 separator purely for the separation of particles from a gas flow. An especially advantageous application for the invention is the separation of smoke, especially combustion smoke, from an air or gas flow. By way of an example the illustration shows one construction design for the invention in which it is used as a tunnel air filter for the separation of soot particles and nitrogen oxide from air removed from the tunnel. Firstly large components such as insects or leaves are retained in a coarse mesh grill (1). Then the air flows though an electro-filter comprising an ionizer (4) and a collector (5) in which up to 85 % of soot and dust particles are removed from the air flow. Both ionizer (4) and collector (5) are connected to a high voltage generator (14). If the filter plates become clogged with soot and dust, they are cleaned using water jet sprayers (3). The washing water that collects in a tank (3) is fed to a separator (10) using a pump (11). The cleaning process starts and runs completely automatically. The filter deposits are separated from the washing water using separator (10) and afterwards the water is again used for filter washing. During the cleaning process the electro-filter is blocked off using an air flap (6). At the same time nitrogen monoxide is largely converted to nitrogen dioxide in the ionizer (4). To achieve this, the high voltage generator (14) of the ionizer (4) is connected to an impulse generator (12) to produce pulsed voltages. An ozone sensor (9) is used for continuous measurement of the ozone concentration in the ionizer (4). The ozone concentration serves as an input for an electronic controller (13), which is used to control the magnitude and duration of the voltage impulses produced by the impulse generator (12).

A pre-ionizer (2) is arranged in front (upstream) of the ionizer in the flow direction and serves to increase the oxidization of the nitrogen monoxide and improve the separation of fine particles. The nitrogen dioxide present in the air flow is removed in a following (downstream) carbon filter (7). Similarly the carbon filter retains up to 90 % of any unburned hydrocarbons and benzene. Air movement through the filter plant in the depicted construction is achieved using blowers (8).

Key 1 Grill 2 Pre-ionizer 3 Jet sprays 4 Ionizer 5 Collector 6 Air flap 7 Carbon filter 8 Blowers 9 Ozone sensor 10 Separator 11 Pump 12 Impulse generator 13 Controller 14 High voltage generator 15 Tank

Claims

1. Device for the separation of nitrogen oxide and particles from a gas flow comprising an ionizer for the electrical charging of gas molecules and particles, a collector and a downstream filter for separation of NO 2 , and characterized in that an impulse generator is used with the ionizer to produce short voltage impulses of sufficiently high voltage that the nitrogen monoxide present in the gas flow is oxidized.

2. Device according to claim 1, characterized in that the voltage impulses are superimposed on a base voltage.

3. Device according to claim 1, characterized in that an ozone sensor is used for measurement of the ozone concentration in the filter and a controller connected to the sensor is used to control the ozone concentration by alteration of the magnitude and duration of the electrical voltage impulses.

4. Device according to claim 1, characterized in that a pre-ionizer is arranged upstream in the gas flow from the ionizer and consists of two or more parallel earthed plates between which spray electrodes are arranged according to a pre-determined plan so that a homogeneous corona field is obtained.

5. Device according to claim 1, characterized in that a carbon filter is used for the separation of nitrogen dioxide. Brief description of the invention In the context of protection of the environment, increasing requirements are made for the separation of nitrogen oxide from airflows at room temperature or at ambient or outside temperatures. For example it is necessary to clean nitrogen oxides from the air removed from road tunnels via exit portals. The nitrogen oxide arising from vehicle exhaust gases comprises approximately 90 % NO and approximately 10 % NO 2 . While NO 2 can be separated from a gas flow with known processes such as scrubbing, separation of NO is not possible with these processes. The known processes all have drawbacks, which have prevented their wide-scale application up until now. This invention solves the problem posed in a simple and economical fashion. Short voltage impulses lasting for only a few nanoseconds allow higher voltages to be applied in an ionizer than with purely D.C. voltage, without the occurrence of arcing. As tests have shown, the formation of ozone in a gas flow is increased so much that any nitrogen monoxide present is largely oxidized. The NO 2 formed in the ionizer can then be separated from the gas flow using a known process. A further refinement of the invention is the use of a pre-ionizer in front of the ionizer, so that gas molecules and particles are already charged before they enter the ionizer. The filter plant, as claimed in the invention, can also be used without an NO 2 separator purely for the separation of particles from a gas flow. An especially advantageous application for the invention is the separation of smoke, especially combustion smoke, from an air or gas flow.