SE540530C2 - A combined scrubber design for exhaust gas purification, theuse of it and a method to use it - Google Patents

Abstract

SummaryThe present invention is related to an effective and compact method to clean exhaust gases from particulate matter (PM), sulfur- and nitrogen-oxides. A dynamic or semi-dynamic NaSOscrubber with trays covered with rocker valves, is integrated with a Wet Electro-Static Precipitator (WESP) in a combined tower (2) with a special collecting bottom (23) installed between the scrubber part and the WESP part in the combined tower (2). This type of NaSOscrubber is capable of cleaning 75% of the PM and 98% of the SOx and NOx in exhausted gases from diesel engines and e.g. industrial combustion processes. The WESP integrated above the scrubber in the combined tower (2) effectively collects all remaining PM still in the gas after passing through the scrubber, resulting in a simultaneous 98% cleaning of PM, SOx and NOx.

Description

A combined scrubber design for exhaust gas purification, the use of it and a method to use it.

The present invention is related to an effective and compact method to clean exhaust gases from particulate matter, sulfur- and nitrogen-oxides.

Background In SE1500177-9, a new effective scrubber is presented capable of 98% removal of Sulfur oxide (SOx) from exhaust gases created by diesel engines. This dynamic scrubber type is based on the so-called KEBI-technology, which also is able to collect 75% of the particulate matter, mostly the coarser particles and also 25% of the nitrogen oxides (NOx). In the near future we expect increased environmental requirements regarding particulate matter (PM) and NOx, since they are extremely harmful for the environment and for human beings.

In a follow-up application, PCT/SE2016/050308, the KEBI-technology was complemented with a method to reach 98% NOx reduction, by injecting an oxidizing gas mixture upstream a wet KEBI scrubber, in various industrial processes.

After a review of existing methods to also catch the finest PM (PM2.5), the conclusion was that if a Wet Electrostatic Precipitator (WESP) is integrated into a wet KEBI-scrubber, the ultimate effective flue gas cleaning method is created. With this concept the WESP’s drawback of not being able to handle a large amount of PM is eliminated and also the KEBI-technology’s drawback of only catching 75% of the PM especially the PM2.5 is eliminated. The main reason why this is a great combination between the two technologies, is that the WESP is dependent on an even gas flow over its flow area, which the KEBI can deliver through its special tray design.

Furthermore WESPs have an extremely low pressure drop, which harmonize with the KEBIs low pressure drop, which in principle is constant and not depending on a varying flow rate, due to its unique tray design which comes with a row of important advantages.

To summarize, we have here a great chance to create an outstandingly effective 98% flue gas cleaning method for industrial applications, both regarding PM, SOx and NOx cleaning efficiency and energy efficiency, by combining a WESP and a KEBI-scrubber, integrated into one unit with a small footprint.

State of the art of scrubbers The principle of sulfur oxide purification is based on the basic chemical rule that an acid can be neutralized with an alkali, such as sodium hydroxide (NaOH). In a socalled scrubber the sulfur oxide, which is released by burning fossil fuels containing sulfur, is neutralized when coming in contact with the scrubber liquid, consisting of sodium hydroxide dissolved in water. In this process the sulfur is bonded as sodium sulfite ( Na2SO3) which after aeration is converted to sodium sulfate (Na2SO4), a chemically very stable salt that can safely be discharged into the sea or in a landbased sewage.

How effective a scrubber process is, depends on a variety of factors, primarily related to a creation of large areas where the gas and scrubbing liquid can react with each other. For example the scrubbing liquid can be atomized into droplets in spray nozzles, and /or the scrubber can be filled with packings in order to increase the active contact surface. Another method is to allow the exhaust gases to bubble through a number of so-called trays, flooded with process fluid, on its way up through the scrubber. A prerequisite for the function of the scrubbers known to date, in use for purification of exhaust gases from diesel engines and other industrial processes, is that a majority of enclosed PM is first separated from the gas to be purified. This is for example done by trapping the PM in an initial step, before the exhaust gas is led into the scrubber.

In marine applications, where seawater is salty enough, it is not needed to add NaOH to the scrubber liquid, instead it is enough to wash the flue gas with large quantities of salt water. The scrubber is then operated differently from a so-called closed loop fresh water scrubber, since the scrubbing liquid is continuously discarded into the sea, in a so-called open loop. The requirement for discarding such a liquid is that it satisfies the condition that transparency (turbidity) is less than 25 NTU, otherwise the fluid must be cleaned from PM, i.e. soot particles, before it is released into the sea. In some cases, a marine scrubber is arranged so that it can run both closed loop freshwater and open loop saltwater, a so-called hybrid scrubber.

We will now describe some prior art techniques to obtain large contact area between the liquid and gas and how to deal with the PM problem, and to reach dynamic characteristics in a scrubber process involving a variable flue gas flow.

By filling the scrubber with packings, a larger contact surface between gas and liquid is obtained. This scrubber type is however more limited in the liquid-gas (L/G) mass flow ratio in order to achieve a good contact between the gas and the liquid. The packings used to increase the contact surface can e.g. be so-called Rasching rings or Berl sadels. Important features of these are that they have a large surface area per unit volume and allow the passage of gas and that they are chemically and mechanically durable. The design is however sensitive to blockage by PM in the gas, especially in case of soot particles, since they are both sticky and sharp. The gas flow dynamics of this construction is also limited.

Instead of packings, so-called bottoms or trays can be used. These are very similar to the columns used in conventional absorption and distillation and which may for example be bell trays /or valve tray columns or sieve tray columns, where the contact surface between the gas and the liquid is made as large as possible. Tray columns usually have a wider operating range (L/G) than packing columns, while the valve trays have the widest.

A bell-bottom is a common tray in terms of both distillation and absorption. The principle is to fill each tray with as many bell-like valves as possible. When the bells are closed, scrubber liquid is collected upon the tray, and when the gas pushes from below, the bells open up which gives the dynamics. The problem with this technique is that the scrubber must be large enough to fit in the necessary number of bells, and furthermore also the problem with "dead areas" in the trays where the gas and liquid can’t get in contact with each other. Bell trays are sensitive to PM that slowly but surely will cause the bell valves to jam.

Sieve trays are either sparse, with a small reaction area, or dense causing a high backpressure, if flow is increased, they also tend to be clogged by soot and other PM.

As indicated above the PM in the exhaust gas is an overriding problem when constructing a fail-safe sulfur oxide scrubber for diesel engines and other industrial processes. To get around the soot/ PM problem, flue gas is purified from PM before being led into the scrubber. The prior art is here to introduce a so-called Venturi step before the scrubber, Figure 1. As the name implies, it involves a flow area restriction where liquid in the form of droplets meets the flue gas and the likelihood that the PM is captured by the drops is high. Such a Venturi step is e.g. then complemented with a spray nozzles scrubber, or one with a packing bed, in a second step where the major sulfur oxide reduction takes place with alkali.

A Venturi scrubber operates at a high gas velocity of 30-120 m /sec and a water pressure to the nozzles between 25 and 60 bars. The contact time is short and a high pressure drop in the restriction causes a high energy consumption. Liquid / gas mass flow ratio also becomes limited and rapid variations in the gas flow must be limited, even if the Venturi flow area restriction can be regulated. It should be noted that the Venturi causes substantial additional costs in space and equipment in the form of pumps, pipes and control equipment, and furthermore an increased running cost for driving the liquid and gas flows.

State of the art, Wet Electrostatic Precipitator A Wet Electrostatic Precipitator (WESP) is best suited to catch PM or drops (< 2.5?m) up to, a not too high level of PM/m<3>(about 0.5 g/m<3>) and has up to now not reached a broad usage in industry, except in certain niches for example catching sulfuric acids aerosols . Depending on the amount of PM/m3 to be trapped, a WESP normally must be protected from a high level of PM by some other filter technology, like a cyclone battery, a bag filter, a Venturi scrubber, or a dry electrostatic precipitator, all of which however suffer from either, inefficiency, a high purchase cost or a high energy/service cost.

The WESP technology works as follows.

The flue gases, fully saturated with moisture, pass the wet electrostatic precipitator in parallel flow, in a round, square or honey-comb shaped system of steel or electrically conductive fiber reinforced plastic (FRP) collecting tubes. In the collecting tubes the PM is negatively charged electrically by a corona electrode placed in the middle of each collecting tube. In this way, all the particles and liquid droplets, have an opposite charge to the surrounding wet collecting surfaces which makes them to collect safely there. Basically it is the same effect that makes dust settle on the screen of a TV or a computer screen. The PM stuck to the wall of the collecting tubes is washed down intermittent with fresh water or better with continuously condensed water falling out on the collecting tubes if they are cooled on the outside, to a buffer tank located under the WESP. Compared to a dry ESP-filter, the WESP has a safe and reliable PM catching and transportation system. PM trapped on a wet surface does not return very easily to the gas flow. The risk for remixing of trapped PM is further minimized by letting the gas flow enter from below and the PM be washed away downwards. As said before the most effective WESPs have cooled collecting tubes, creating a continuously liquid layer on the inside of the collecting tubes, so that the high voltage does not need to be shut down during the washing procedure.

The water in the buffer tank is pumped around constantly in the unit and needs not to be replaced. To take care of the separated PM in the buffer tank, a small partial flow is continuously fed to a liquid purification-unit, which separates the PM into a sludge tank at certain intervals.

In order to design a reasonable effective flue gas cleaning plant for industrial applications, regarding PM and SOx cleaning efficiency, a common way is to first clean the gas from PM in a Venturi scrubber and then use a simple spray scrubber to remove the SOx. Up to now the regulations for NOx have been mild, so NOx removing systems have not been in high demand. In the near future this is going to change and many experts recommend the selective catalytic reduction (SCR) method with Urea injection for the NOx reduction. However the SCR method is not an economic choice in general industrial applications with high flue gas flows and there is also a negative side-effect with increased concentrations of NH3and dinitrogen oxide (N2O) in the gas flow.

If a spray scrubber is used in the cleaning plant, the future demands for PM emissions will neither be met, nor will the SOx requirements. Accordingly a more efficient scrubber is needed with a larger wet surface area, e.g. one with packings, which unfortunately is static in flow and comes with a high pressure drop and clogging problems.

A good feature with packing beds, e.g. of glass rings, is that they create an even gas flow over their working area, which makes all tubs in the WESP to have an equal share of the gas flow. Accordingly a scrubber creating an even gas flow is needed, but not with the drawbacks of the packing beds, but preferably a dynamic one with trays and KEBI cassettes.

The present invention can solve the above mentioned problems with PM, SOx and NOx exhaust gas cleaning in industrial processes, by having the features defined in the independent claims.

Technical description A scrubber, of the KEBI-design, disclosed here differs from other scrubber designs in a number of points. Several of these points are listed below together with important advantages. 1. The core feature of the disclosed scrubber is its tray design with its cassettes, consisting of hinge- or blade type rocker valves, which cover the entire tray surface, except for a small frame around each cassette. Thus, there are no dead surfaces, like when bell trays are used. To make the handling of the cassettes smooth, they normally have a surface area around 0.11 m<2>. Thus a scrubber of 1 m<2>cross section in principle has trays consisting of 9 KEBI-cassettes. However the cassettes can of course be made in other sizes if required. The rocker valves may be manufactured in metal, plastic, plastic composite, rubber, or a combination of these or other equivalent materials. The rocker valves may be of a hinged type or of a fixed leaf type, flexible bendable in itself. Minimum pressure drop and the best dynamic properties are obtained if the valves do not have any valve seats on their sides. 2. A scrubber with KEBI-cassettes is capable of keeping the pressure drop over the trays largely constant in a way that the process fluid recirculation flow is increased when the gas flow is reduced or reduced if the gas flow is increased, fig.3. Increased recirculation flow actually leads to accumulation of more process fluid over the trays and vice versa, reduced recirculation flow leads to less process fluid collected over the trays. 3. At rapid variations in the gas flow, e.g. when accelerating a diesel engine, it is protected by all cassette valves immediately opens up fully after which the pressure drop is momentarily reduced. The valves' maximum opening angle is normally mechanically limited to 8°-30°, depending on the process. 4. A scrubber with KEBI-cassettes is self-cleaned from deposit of PM and sodium sulfite crystals due to the continuous valve movements.

. The gas velocity up through the scrubber tower can be varied between 0.5 to 4.5 m/s, which is significantly more than any similar engineering. This denotes a considerably wider operating range and enables a freer choice of the cross section of the scrubber tower. The gas flow is also always evenly distributed over the surface of a tray, which helps all the valves to move with the same frequency and amplitude. 6. By automatically controlling the recirculation flow in relation to the pressure drop over the trays, the pressure drop for the scrubber process can be kept constant, independent of the gas flow, through regulating the amount of process liquid in place over the trays. 7. Due to the frame-type design of the KEBI-cassettes they can slide on rails into the scrubber, and also with additionally easily removable hatches, a very high availability is ensured thus avoiding the need for bypassing the scrubber at inspections (provided the scrubber is under vacuum). 8. In an incidental pump failure of the recirculation flow the scrubber retains its function for a minimum of 2 hours, allowing the pump to be replaced. This phenomena is due to that the foaming process over the trays continues even if the recirculation of process liquid stops. 9. The KEBI-scrubber is designed mostly as rectangular or square, which is an advantage in tight spaces and to bring in as many cassettes as possible.

. The extraction of PM is significantly higher than in any other known scrubber design (except the Venturi type), because the process fluid which accumulates over each tray is whipped to foam due to the rocker valves movements, which makes the probability of PM to get past a number of trays after each other, extremely small. 11. Sulfur oxide capture will also be extra efficient, inter alia, thanks to the rocker valves and their counter weights creating a large wet reaction surface, and their movements leading to the gas being dispersed in the process liquid and the process liquid being dispersed in the gas. 12. The low pressure drop is a great advantage in connection with the purification of exhaust gas from diesel engines, as these engines are sensitive to high back pressure. Pilot runs on different flue gases in marine and industrial processes have shown that the optimal pressure drop across each tray lies between 400 and 600 Pa. 13. The chemical process in a KEBI-scrubber works like in any other scrubber type, with the difference that the installation and operation is more easy and efficient thanks to the above-related differences and advantages.

Figure 1 shows a flow diagram of a Venturi scrubber, representing the prior art.

Figure 2 shows a variant of a rocker valve, a so-called hinged rocker valve with a counterweight and its opening angle (a) in a KEBI-cassette.

Figure 3 shows an exemplary flowchart of a WESP filter integrated into a dynamic scrubber of the KEBI type, in accordance with an industrial embodiment of the invention.

Figure 4 shows an example of a liquid collecting bottom, adaptable to perform separate recirculation loops for a WESP integrated with a scrubber in a combined tower.

Detailed technical description, part 1 In the following the purification of diesel as well as industrial exhaust gases with a combination of a KEBI-scrubber and a WESP filter is described according to an exemplary embodiment.

The exhaust gas to be purified, from PM, sulfur- and nitrogen-oxides, is led directly into the lower part of a combined scrubber/WESP tower, from now on called “combined tower”, which may have different cross-sectional areas and design features along its height. The exhaust gas can advantageously first be cooled down in a gas/ water heat exchanger, as the scrubbing process is most effective at lower temperatures. Accordingly there is no need for a Venturi PM-cleaning step, including its control system and pumping system, in order to prevent too much PM from entering the scrubber process.

In the combined tower the above-mentioned trays with their cassettes and the rocker valves are installed in at least one level. An extra, empty level can be set up to be kept in reserve in case the purification requirements would be increased in the future. A land based industrial scrubber is arranged for a so-called closed loop operation, where fresh water is mixed with sodium hydroxide (NaOH) in a recirculation tank and this process liquid mixture is then pumped up into the combined tower to the top tray of the scrubber, where the process liquid is distributed evenly over the cassettes after which it leaks down to the next tray etc, until the process liquid ends up in the recirculation tank again. As mentioned above, the pressure drop over the scrubber is held relatively constant, by controlling the recirculating flow. If necessary, the recirculating flow is also cooled in a heat exchanger, if heat is building up in the process liquide.

In some cases of purification of various exhaust gases, it becomes necessary to recirculate a scrubber liquid with a high content of suspended sodium sulfite and PM. It has then been found optimal that the top level tray cassettes are provided with rocker valves with a maximum opening angle, which is larger than they in underlying trays. In this way the liquid level at the top tray is lowered and hence the risk that droplets and PM are whipped up against the WESP filter is reduced.

As the sulfur is scrubbed out of the exhaust gas, sodium sulfite Na2SO3is created in the scrubbing liquid, and as the level increases, a portion of the scrubbing liquid is bled out to a treatment plant, where the PM is separated out into a sludge tank. New alkaline NaOH and fresh water replaces the bled out amount. In some industrial processes Na2SO3is already at hand, why NaOH is not needed.

When the scrubbed gas leaves the top tray, it enters and passes through the WESP PM filter stage, with its PM collecting tubes, each equipped with a central corona electrode (cathode) creating a negative charged plasma, which accelerates against the PM collecting tubes inner wall(s) (anode) when colliding with PM particles whilst transferring its negative charges, whereupon the PM is attracted by the collecting tubes inner wall(s) and in the end trapped on their wetted surfaces.

The corona electrodes and the collecting tubes (anode) are electrically isolated from each other and e.g. connected to a switched directed current source of several kV (kilo Volt). Further fresh water is fed into a holding tank by a first valve and at certain intervals the WESP collecting tubes are flushed by opening a second valve to a set of flushing nozzles so all trapped PM is rinsed down to the scrubber’s top tray and further down to the recirculation tank.

Before the scrubbed exhaust gases leave the combined WESP/KEBI unit in the top of the combined tower, they pass a "demister" / mist eliminator, where residual liquid droplets are separated. Downstream of the demister, usually a variable speed gas sucking fan is installed, or a pushing one downstream or a diesel engine downstream pushing the exhaust through the combined tower.

Detailed technical description, part 2 An embodiment of the invention is explained below using the flowchart of Figure 3. The exhaust gas to be cleaned from PM, sulfur- and nitrogen-oxides, is first cooled in a heat exchanger 1 and is then sucked into a combined tower 2 through an inlet 3 located in its lower part. The exhaust gas first passes through a first tray 4 consisting of so-called hinged rocker valve cassettes, over which there is a level of process liquid and foam, which the exhaust gas has to pass, after which it then rises up to a following identical tray and so on until it finally passes a top tray 5 and enters the collecting tubes 6 of the WESP filter above where the particles still in the gas flow are negatively charged by ions emitted from corona electrodes 7 in the center of each collecting tube and thereafter collected on the wetted inner walls of the collecting tubes 6. The cleaned gas then leaves the combined tower 2 through a mist eliminator 8 and further through an outlet 9, e.g. connected to a downstream suction fan.

Further fresh water is fed into a holding tank 19 by a first valve 20 and at certain intervals the WESP collecting tubes are flushed by opening a second valve 21 to a set of flushing nozzles 22 while all trapped PM are fed down to the scrubber’s top tray 5 and further down in the combined tower 2 through all other trays including the bottom tray 4 and is then collected in a lower reservoir 10 which leads to a recirculation tank 11 in which new scrubber liquid is prepared. This occurs at the same rate as old scrubber liquid is bled out to a purification device 12a, where the slurry of PM is separated out into a holding tank 12b, for later disposal. The scrubber liquid now cleaned from PM, can be emptied in a drain, provided the effluent consisting of sodium sulfite was first oxygenized and transformed into sodium sulfate.

After a period of scrubbing the flue gas e.g. from a diesel engine, driven by fuel containing sulfur, the content in the recirculation tank 11 will consist of sodium sulfite and PM dissolved in the process liquide. The concentration of sodium sulfite increases constantly wherein a typical upper limit is in the range 45-50 grams of sodium sulfite / liter of scrubber liquid.

From the recirculation tank 11, a pump 13 pushes the scrubber liquid through a cooler 14 and further to a position 15 over the top tray 5. The pressure drop over the trays 4 to 5 is recorded by a differential pressure gauge 16 which controls the recirculation flow by means of a control valve 17 and thus regulates the amount of scrubbing liquid situated above each tray 4 to 5. With this regulation, the pressure drop is kept largely constant, regardless of the level of exhaust gas flow. For industrial processes not changing the load very often, the control valve 17 can be manually operated to a predetermined pressure drop across the trays and the scrubber is then only semi-dynamic.

The two heat exchangers 1 and 14 are supplied with cooling water from a pump 18. The basis for the present invention is that a scrubber with KEBI-cassettes in combination with a WESP filter, has never previously been tested for the purification of diesel- and industrial exhaust gases, which will now become a novelty in the field. Furthermore, it has been shown that this scrubber type, based on KEBI-cassettes, is not adversely affected by the sharp and sticky PM e.g. soot that diesel engines produce which other scrubber designs have problems with.

If an industrial process has no access to concentrated Na2SO3(from a side process) and has to rely on mixing water and NaOH in order to create the needed scrubber liquid and do not want it diluted by the WESP flush water, the scrubber and the WESP filter need to have separated fluid circulating systems. In order to create this, e.g. a special collecting bottom 23 is installed between the scrubber part and the WESP part in the combined tower 2. Such a collecting bottom 23 collects the down passing WESP flush water and feeds it to a special WESP circulating tank, while the up-flowing scrubbed gas can pass the collecting bottom at the same time. In such a case one or more extra KEBI tray(s) can be placed above the collecting bottom in order to even out the flow over the cross section of the WESP filter.

It has also become clear that a scrubber with KEBI-cassettes, by waiting longer to bleed out and replacing the scrubbing liquid in the recirculation tank 11, can handle 4-6 times higher Na2SO3concentrations than other scrubber types, which saves the amount of alkali and fresh water used and also reduces the size of the particle separation treatment plant 12a, 12b, a feature appreciated not least in the marine context.

The higher sodium content in the recirculation loop brings the added bonus that the nitrogen oxides reduction in the combined tower 2 increases from the previous maximum of 8% to over 25%. Higher NOx reduction is not possible to reach because the scrubbing process only works mainly on nitrogen dioxide (NO2) and not on the nitrogen oxide (NO).

Faced with the new requirements regarding nitrogen oxide purification, assumed to take effect in 2018, the above mentioned NOx reduction is generally not enough. Tests have therefore been conducted with one additional KEBI-scrubber coupled in series before "a KEBI-sulfite scrubber", where an oxidizing water/ chlorine dioxide solution has been injected in the recycle flow in a first scrubber, which in this step converts NO to NO 2 and to some extent even to water-soluble nitrate NO3. The remaining part of the NO2gas is then passed on to the next scrubber with sulfite solution as scrubbing liquid. With this double scrubber approach, a substantially complete purification of both nitrogen oxides and sulfur oxides can be obtained. However, the NOx purification occurs slower than the SOx purification, which reduces the possible gas flow. In addition, the installation and running cost is nearly doubled with two scrubbers. One way to get around these disadvantages is to use only one scrubber of the invention type and for example inject an oxidizing gas mixture of 10% chlorine dioxide gas, upstream of the inlet 3, for example at 24. The oxidation of NO to NO2is now much faster, estimated at less than 1 second, compared with the situation when the chloride ions in the water solution, first must be transferred into a gas phase since the NO gas does not readily dissolve in water. The closest technology to the above is known from e.g. US 4035470, which substantially describes the basics for NOx removal by oxidation of NO to NO2with an oxidizing gas, e.g. chlorine dioxide, hydrogen peroxide or ozone, followed by a scrubber for absorption of formed NO2in a sulfite solution where also SO2is detached (oxidized). Latter patent for example US 9149784 describes various additional devices, such as a reactor vessel and special turbulence vanes, which however do not add any special advantages. A scrubber with KEBI-cassettes is particularly suitable for use as a combined SOx and NOx eliminator, since the tray design together with the overhead foam layer, is optimal for both processes.

A gas mixture of chlorine dioxide should be injected into the flue gas duct upstream of the scrubber so that the residence time in the duct is at least 0.5 second before the mixture reaches the scrubber. Conveniently, a location upstream of an elbow duct is chosen in order to use the turbulence after the bend for the gas to be mixed with the flue gases. The chlorine dioxide gas that not yet has had time to react with NO molecules, continues to the scrubber’s first tray 4, where a uniform flow distribution is obtained, due to the pressure resistance the first tray 4 creates and then moves up in the combined tower and is found in the about 200 mm high foam layer over each tray, leading to a 100% conversion of NO to NO2, which is then converted to nitrate by the sulfite solution in the scrubber. It should also be mentioned that the temperature in the scrubber is kept below 85°C, where the SOx purification is relatively effective, while the chlorine dioxide gas does not dissolve in the scrubbing liquid at this temperature level.

In conclusion, a scrubber with KEBI-cassettes allows a very effective and close to 100% simultaneous cleaning of both NOx and SOx thanks to its large wet reaction area (rocker valves with counter weights) in combination with the foam layer over each tray and not least the ability to create an even gas flow over the cross-sectional area and to handle a high PM content and a high sulfite content without showing clogging problems. These properties, which are unique in the industry, mean that a KEBI-scrubber can be made smaller than other scrubber types and therefore harmonize with the equally compact WESP technology, which is a characteristic that is appreciated in industrial contexts. Furthermore, the possibility of operating at a higher sulfite level saves chemicals.

The bubble and foam process, that is present over the KEBI-cassettes, is an excellent oxy generator if air is supplied e.g. beneath one or more trays. In this manner the conversion of sodium sulfite to sodium sulfate is speeded up. The oxygenation may not be pushed so far that it impacts the sulfur oxide purification. In some embodiments of the here disclosed scrubber, the flow of scrubbing liquid is regulated so that the back pressure is kept at least relatively constant, and within predetermined limit values. This will for example ensure that a process runs as effectively as possible.

Furthermore, in some embodiments, a scrubber liquid with sodium sulfite concentration in the range of 50-200 g / litre is used, which is considerably higher than normal practice. This provides, as mentioned earlier, an enhanced purification of nitrogen oxides, from a normal 8% up to 25-30%.

Likewise in some embodiments, the rocker valves in the cassette or cassettes in the top-tray 5 are set so that they have a maximum opening angle greater than the maximum opening angle of the rocker valves in the cassette or cassettes of the other trays. This enables the level of liquid and foam to be lower over this tray, than over the other trays, which can be advantageous e.g. when you do not want to risk that the level of foam reaches the WESP section in the combined tower. For example, the maximum opening angle of the rocker valves in the top tray 5 cassettes is adjusted to be about 40% -60% greater than the opening angle of the rocker valves in the cassettes of the other trays.

In some embodiments, the rocker valve's maximum opening angle is variable and remotely controllable. This can apply to all rocker valves, or for some, such as the ones in the top-tray. The rocker valves' maximum opening angle can also be controlled separately, e.g. for different trays.

In some embodiments, the maximum rocker valve opening angle can be adjusted in the range of 8-30°, depending on the pre-selected and temperature-dependent gas velocity through the cassettes, where the gas velocity is in the range 0.5-4.5 m/s. In some embodiments the rocker valve’s maximum opening angle is adapted to about 30° at a mass flow ratio of about 10 (liquid) to 1 (gas).

In some embodiments, a control valve 17 is controlled so that the liquid to gas mass flow ratio is about 1 to 1 in the scrubber, when a diesel engine or an industrial process runs at its highest nominal output power rating.

In some embodiments, the control valve 17 is controlled so that the mass flow ratio in the scrubber, in a closed loop case, is about 6 (liquid) tol(gas) at the diesel engine's or an industrial process’ lowest nominal output power rating.

In some embodiments, air is supplied under at least one tray in order to let the sodium sulfite in the scrubbing liquid to be oxygenated to sulfate directly in the purification process.

Claims (20)

Claims

1. A dynamic or semi-dynamic scrubber designed to purify diesel engine and industrial exhaust gas from sulfur-, nitrogen-oxides and partially from particulate matter (PM), consisting of a combined tower (2) with a scrubber in the bottom, having at least one tray (4), consisting of one or more cassettes, on top of which sulfite containing scrubbing liquid can be fed, and where these are provided with one or more rocker valves, covering the entire surface area of a cassette, which open up when the gas to be purified enters an inlet (3), either sucked from a fan downstream or is pushed by one upstream or a diesel engine upstream, whereby the gas bubbles up through the, upon the trays accumulated, scrubbing liquid and foam and is purified by contact with it, directly and on the rocker valves surfaces, CHARACTERIZED in that a Wet Electro-Static Precipitator (WESP), for final removal of the PM that passes the scrubber, is installed above the scrubber in the combined tower (2), where the scrubber and the WESP have separate recirculation loops, made possible by a collecting bottom (23) installed between them collecting the WESPs flush water, while the partially, from PM, cleaned gas can pass upward from below.

2. A scrubber according to claim 1, having an inlet (24) positioned upstream of the gas inlet (3), for injecting of an oxidizing gas and/or an oxidizing liquid, for example a gas mixture containing chlorine dioxide.

3. A scrubber according to claim 2, where one or more extra tray(s), with one or more cassettes, is placed between the collecting bottom (23) and the WESP, in order to even out the gas flow over WESP collecting tubes (6).

4. A scrubber according to one of claims 1 to 3, wherein the cassette rocker valves' maximum opening angle can be adjusted in the range of 8-30° depending on the selected temperature dependent gas velocity through the cassettes, where the gas velocity is in the range 0.5-4.5 m/s.

5. A scrubber according to any one of the above claims, arranged so that the cassette rocker valves' maximum opening angle is adjusted to about 30° at a mass flow ratio in the scrubber at about 10 liquid to 1 gas.

6. A scrubber according to any one of claims 1-5, arranged so that a recirculation liquid flow to the scrubber is automatically adjustable, depending on the pressure drop over the trays by means of a control valve (17).

7. A scrubber according to any one of claims 1-6, arranged in such a way that the control valve (17) is regulated so that a mass flow ratio in the scrubber is about 1 (liquid) to 1(gas), at the diesel engine's or the industrial process maximum nominal output power rating.

8. A scrubber according to any one of claims 1-7, wherein the control valve (17) is regulated to a mass flow ratio in the scrubber, of about 6 (liquid) to 1 (gas) ) for a diesel engine’s or an industrial process<'>lowest specified nominal power rating.

9. Use of a dynamic or semi-dynamic scrubber according to one of claims 1-8 for cleaning of diesel engines or industrial processes exhaust gases from particulate matter, sulfur- and nitrogen-oxides.

10. A method to clean diesel engines and /or industrial process exhaust gases from particulate matter(PM), sulfur- and nitrogen-oxides, where the PM-laden flue gas is supplied to a dynamic or semi dynamic scrubber, comprising a combined tower (2), provided with at least one tray (4), consisting of one or more cassettes, upon which scrubbing liquid is supplied, and where these are provided with one or more rocker valves, covering the entire cassette surface area, which open up when the gas to be purified enters an inlet (3) and is pushed from below or is sucked by a downstream placed fan, whereby the gas bubbles up through the scrubbing liquid and foam, accumulated above the tray(s) and is purified by contact with it, directly and on the rocker valve surfaces, CHARACTERIZED in that a Wet Electro-Static Precipitator (WESP), for final removal of the PM that passes the scrubber, is installed above the scrubber in the combined tower (2), where the scrubber and the WESP have separate recirculation loops, made possible by a collecting bottom (23) installed between them collecting the WESPs flush water, while the partially, from PM, cleaned gas can pass upward from below.

11. A method according to claim 10, where an oxidizing gas and /or an oxidizing liquid, for example a gas mixture containing chlorine dioxide, is injected upstream the scrubber inlet (3).

12. A method of claim 10 where the scrubber is adapted so that the recirculation flow of the scrubbing liquid is automatically adjustable by means of a control valve (17) so that a back pressure of the scrubber is kept constant and within the engine or the industrial process predetermined limit.

13. A method of any of claims 11-12, where the scrubbing liquid has a sulfite content in the range of 50-200 g/l.

14. A method of any of claims 11-13, wherein the rocker valves of the cassettes in the top tray (5) have a maximum opening angle greater than the maximum opening angle for the rocker valves in underlying trays.

15. A method of any of claims of 11-14, where the cassette rocker valves' maximum opening angle is adjusted in the range of 8-30° depending on the temperature dependent maximum gas velocity through the cassettes, where the gas velocity is in the range 0.5-4.5 m /s.

16. A method of any of claims 11-15, where the rocker valve's maximum opening angle is adjusted to about 30° at a mass flow ratio in the scrubber of about 10 (liquid) to 1 (gas).

17. A method of any of claims 11-16, where the control valve (17) is regulated so that the mass flow ratio in the scrubber is about 1 (liquid) tol(gas), at a diesel engine or industrial process' maximum nominal output power rating.

18. A method of any of claims 11-17 wherein the control valve (17) is regulated, to give a mass flow ratio in the scrubber of 6 (liquid) to 1 (gas) for a diesel engine's or an industrial process' lowest specified nominal power rating.

19. A method according to claim 11, wherein the oxidizing gas or liquid contains 1-10% chlorine dioxide

20. A method according to claim 19, where an extra tray, with one or more cassettes, is placed between the collecting bottom (23) and the WESP filter, in order to even out the gas flow over the WESP collecting tubes (6).