SE541286C2 - A scrubber designed to purify exhaust gas, the use of it and a method - Google Patents

Abstract

SummaryA scrubber with one or more tray levels, provided with rocker valve cassettes, has in pilot tests been proved excellent to clean marine diesel engine exhaust gases from sulfur oxides and nitrogen oxides, since the back pressure is low and by the invention can be kept constant and within permitted limits, and this over the entire engine RPM - and power-rating register, because these engines are particularly sensitive to high back pressures. The invention can also be applied in similar demanding industrial processes such as being sensitive to variations in backpressure or having requirements for 100% SOx purification and / or NOx purification> 8% .In testing, performed with diesel fuel with increased sulfur content, a 100% efficient sulfur oxide purification has been confirmed as well as a simultaneous NOx purification up to 25% and it was also verified that the exhaust gases can be introduced directly into the scrubber, without clogging problems due to the soot in the exhaust. Further advantages are obtained by that a scrubber of the invention operates at high sulfite content, up to at least 200 g / L, where less chemicals and fresh water is spent, whilst the space for holding tanks is reduced.The applications are mainly in the marine sector.

Description

A Scrubber designed to purify exhaust gas, the use of it and a method Background 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 washing liquid/ 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 land-based 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 particulate matter is first separated from the gas to be purified. This is for example done by trapping the particles 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 washing 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 particulate matter, 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 dust 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 be so-called Raschig rings or Berl sadels etc. 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 particulate matter in the gas, especially since diesel soot particles are both sticky and sharp. The 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, washing 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 if the necessary number of bells is to be fitted in, and 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 particulate matter that slowly but surely will get the bell valves to jam.

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

As indicated above the soot particles in the exhaust gas is an overriding problem of constructing a fail-safe sulfur oxide scrubber for diesel engines and other processes. To get around the soot/ particulate matter problem, flue gas is purified from particulate matter 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 soot particles are captured by the drops is great. Such a venturi step is primarily used for particulate matter/ soot separation in a first step and is complemented by a spray nozzles scrubber or a packing one 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.

The present invention can solve the above mentioned problems with exhaust gas from diesel engines, or similar demanding industrial processes, by having the features defined in the independent claims.

The story behind the present invention is that one of the inventors had done some pioneering work with soot cleaning, by cyclone filters on large diesel engines for power generation, in the Middle East and also had a background in liquid sludge separation. The other inventors had done pioneering work in Sweden to provide the pulp and paper industry with special scrubbers for the cleaning of flue gases from furnaces and boilers. One day their paths crossed, and one thing led to another and they realized that the special scrubbers could be adapted to solve the problems of cleaning exhaust gases from ship-based diesel engines, which would be mandatory from 2015, and new regulations that will come in 2018 both for diesel engines and industrial processes regarding nitrogen-oxide emissions.

Technical description A scrubber may be permanently adjusted for a certain process with constant parameters, such as gas flow and pressure drop, and is therefore called a static scrubber. Scrubbers capable of a varying gas flow without creating problems in the process are called dynamic. Diesel engines are typically run with varying speed and load, and the flow of exhaust gases from such an engine will therefore vary over time. Diesel engines are also sensitive to variations in downstream back-pressure, which for example can be caused by long flue gas ducts or heat exchangers connected to an exhaust gas outlet which obstruct the exhaust flow and thereby creates a back pressure. A scrubber with ideal back pressure characteristics for purification of exhaust gas should have a low and constant pressure drop, independent of the flow. In the case of diesel emission control, the requirements of a scrubber is therefore at its peak, both in terms of ability to cope with varying gas flow and in the creation of a low back pressure.

This document describes embodiments of a scrubber for cleaning exhaust gases from one or more diesel engines, or from a similar demanding industrial process. Embodiments of the disclosed scrubber meet the requirements in terms of handling a variable gas flow and to keep the back pressure within an allowable range.

Embodiments of the described scrubber, nicknamed KEBI, are also capable of purifying exhaust gases without a prior cleaning from particulate matter, which is a big advantage.

The cleaning efficiency of particulate matter for a KEBI-scrubber is normally about 75-80%. It is the smallest particles which passes the scrubber. If a “close to 100%” capture of particulate matter is in demand, then for example a wet electric precipitator (WESP) device can be installed, in the scrubber tower, above the top tray and preferably also over the inflow of the scrubbing liquid. This device collects the small particles below 2.5 micron, and is dependent on the extreme even gas flow distribution in the tower, created by the tray(s) with KEBI-cassettes. Indeed a nice symbiosis.

The inventors have realized that a scrubber of the invention with its tray design with a special kind of cassettes with hinged rocker valves, in the following called “KEBI cassettes”, can be adapted to meet the demands of a scrubber for purification of diesel exhaust gas or general industrial flue gases and moreover also of soot and particulate matter. A basic working principle of a "KEBI-scrubber" is described in Patent US 4,557,875 including this type of valve cassettes. These cassettes have been used in the last 25 years, mainly in the pulp and paper industry for its generally good properties, since the requirement of dynamics in this context has been extremely limited. To consider this construction or parts thereof in a process with high demands on dynamics, and also pressure drop, is a novelty in the industry.

The use of KEBI-cassettes, for purification of diesel exhaust gases or flue gases from similar demanding industrial processes, has several surprising advantages, which will be described in this document. "Similar demanding" industrial processes refers to those that are dynamic in flow (amplitude) and at the same time sensitive to variations in back pressure and /or a demand on NOx purification over 8% and/ or demand on an extreme even gas flow distribution in the scrubber tower. The epithet "demanding" may also include a requirement that the scrubber will have great effectiveness in relation to its size and /or have low energy and chemical consumption. Henceforth, the term "industrial processes" apply to those who are "similarly demanding" above. The cassettes in US 4,557,875 are provided with a special kind of hinged rocker valves. However, we would not limit ourselves to just the exact configuration described therein, without using the term KEBI as including also rocker valves of any other (similar) nature, which is described later.

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 a flowchart in accordance with a marine embodiment of the invention.

The inventors have realized that a scrubber with KEBI-cassettes has the following characteristics, from the use in the pulp and paper industry; 1) Very high separation of sulfur-oxides, in principle 100%. 2) Low pressure drop, which can be kept constant. 3) Reliability and simple regulation. 4) Unaffected by particulate matter in the exhaust gas.

) Space-saving and energy-saving.

The inventors have further recognized that these characteristics or advantages are extremely important for the cost-effective nitrogen oxide and sulfur oxide scrubbing of diesel exhaust gases, in particular on ships. More stringent rules for emissions of sulfur oxide and particulate matter from ships (MARPOL 2015) have led to increased need for efficient purification of exhaust gas in marine environments. For diesel engines that work with different speeds and loads, it is important to have a good cleaning efficiency for sulfur oxide and a low pressure drop through the entire register of the motor as the engine performance is adversely affected at 1000-1500 Pa back pressure. For both land based and marine diesel engine installations the demand is focused on purification solutions with a small footprint, reliability and low energy consumption. Low alkali and fresh water consumption is also high on the wishing list, features in which the KEBI- scrubbers excels, which we shall return to.

Against this background, a cleaning solution based on KEBI-cassettes has recently been tested by the inventors in a pilot scale on board a diesel-powered ship, driven by a fuel with increased sulfur content. During the test it was confirmed that the result regarding sulfur oxide separation was as good as before, in the land-based installations of the paper and pulp industry. Soot particles did not create clogging problems either, which before had been in doubt, since competing scrubber designs have been struggling with this issue. In the marine testing of a KEBI-scrubber it was run for a certain period, over the sodium sulfite Na2SO3concentration limit (about 50 g /liter) previously regarded as an upper limit, which other scrubber designs not can exceed, because the washing liquid becomes viscous by soot/ particles and / or crystal formation. At these "beyond the limit" test runs it was surprisingly recorded that the KEBI-scrubber easily can handle a concentration of 200 g of sulfite /liter, which directly reduces the fresh water consumption. Furthermore, a surprising side effect of the use of the high concentration of Na2SO3was namely, that the nitrogen oxide separation increased from a modest 8% to the full 25%. In this document, the terms "process liquid", “process fluid" and "scrubbing liquid" are used as synonyms unless otherwise is explicitly stated.

The scrubber disclosed here differs from other scrubber designs on a number of points. Several of these points are listed below together with important advantages. 1. The heart of the disclosed scrubber is its tray design with its cassettes, consisting of hinge- or blade type rocker valves, which covers 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 0.1 m<2>. Thus a scrubber of 1 m<2>in principle has trays consisting of 10 KEBI- cassettes. 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 if the gas flow is reduced or else 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, especially when accelerating a diesel engine, 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 and the choice of closed or open loop. 4. A scrubber with KEBI-cassettes is self-cleaned from deposit of particles and sodium sulfite crystals by the continuously 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. By the design of the KEBI-cassettes and that they can slide on rails into the scrubber, and 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 hard to explain, but the foam process over the trays does not vanish 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. 10. The extraction of particulate matter is significantly higher than in any known scrubber design, because the process fluid which accumulates over each tray is whipped to foam due to the rocker valves movements, which makes the probability of soot and particulate matter 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 300 and 500 Pa.

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.

Detailed technical description, part 1 In the following the purification of diesel exhaust gas with a KEBI-scrubber is described according to an exemplary embodiment.

The exhaust gas to be purified is passed directly into the lower half of the scrubber tower. It can advantageously 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 step, including a control system and a pumping system, in order to prevent particulate matter from entering the scrubber process.

In the scrubber 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. If the KEBI-scrubber is arranged for closed loop operation, fresh water is mixed with sodium hydroxide NaOH in a recirculation tank and this mixture is then pumped up to the top part of the tower to the top tray, where the liquid is distributed evenly over the cassettes after which it leaks down to the next tray etc., until the liquid ends up in the recirculation tank again. As mentioned above, the pressure drop over the scrubber is at least held relatively constant, by controlling the recirculating flow. If necessary, the recirculating flow is also cooled.

In the case of purification of various exhaust gases, it becomes necessary to recirculate a wash liquid with a high content of suspended sodium sulfite and particles and then it has been found optimal that the top level tray cassettes are provided with rocker valves with a maximum opening angle, which is larger than the underlying trays'. In this way the liquid level at the top tray is lowered and the risk that droplets and dust particles are whipped up against the mist eliminator before they reach the outlet is reduced. A better pressure drop distribution is also obtained over the trays, which means more efficiency and less risk of clogging by dust particles.

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 dust is separated out into a sludge tank, and is thereafter oxygenated into sodium sulfate Na2SO4before it can be poured into a drain or into the sea. New alkaline NaOH and fresh water replaces the bled out amount.

If the KEBI-scrubber runs with sea water, the above-described recirculation loop is replaced by a direct supply of seawater on at least two tray levels in the upper part of the scrubber tower. When the sea water has passed the scrubber, it is led directly back to the sea, through a treatment plant.

Before the scrubbed exhaust gas leaves the scrubber at the top, a "demister" / mist eliminator is passed, where residual liquid droplets are separated out. Downstream of the demister, usually a variable speed sucking fan is installed, which further helps to keep a diesel engine back pressure at a low stable level.

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 particulate matter, sulfur- and nitrogen-oxides, is first cooled in a heat exchanger 1 and is then sucked into a scrubber tower 2 through an inlet 3 located in the lower half. 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 leaves the scrubber tower 2 through a mist eliminator 6 and further through an outlet 7, connected to a downstream suction fan.

The process liquid leaking downwards in the scrubber tower 2 is collected in a lower reservoir 8 which leads to a recirculation tank 9 in which new washing liquid is prepared by mixing sodium hydroxide and freshwater. This occurs at the same rate as old washing liquid is bled out to a purification device 10a, where the slurry of particulate matter is separated out into a holding tank 10b, which, for example, later can be emptied in a harbor, provided the effluent consisting of sodium sulfite was first oxygenized and transformed into sodium sulfate before being discharged into the sea or into a drain.

After a period of scrubbing the flue gas from a diesel engine, driven by fuel containing sulfur, the content in the recirculation tank 9 will consist of sodium sulfite and soot particles dissolved in the process water. The concentration of particles and sodium sulfite increase constantly, until new washing liquid is mixed in and the old is bled out. A typical upper limit is in the range 45-50 grams of sodium sulfite / liter of washing liquid.

From the recirculation tank 9, a pump 11 pushes the washing liquid through a cooler 12 in an upper part 13 of the scrubber tower 2. The pressure drop over the trays 4 to 5 is recorded by a differential pressure gauge 14 which controls the recirculation flow by means of a control valve 15 and thus regulates the amount of washing liquid which is 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 and thus independent of the engine speed and load.

For diesel engines, or similar demanding industrial processes that only runs with a constant load, the control valve 15 can be manually operated to a predetermined pressure drop across the trays and the scrubber is then only semi-dynamic. A faster and more accurate control of the pressure drop over the trays may be obtained if the pressure drop is measured over each individual tray and the washing liquid is regulated to each tray individually.

The two heat exchangers 1 and 12 are supplied with cooling water from a pump 16. A KEBI-scrubber can also be run in a so-called open salt water mode, in contrast to the above described environmentally friendly closed freshwater mode. In the saltwater mode the seawater pump 16 feeds the sea water to a maximum of two levels 17 and 18 in the scrubbing tower 2 wherein the recirculation loop is disabled, after which the scrubbing liquid directly goes to the particle separation treatment plant 10a, 10b for removing of sludge, before being deposited in the sea.

The basis for the present invention is that a scrubber with KEBI-cassettes, in pilot runs with scrubbing diesel exhaust gas, has shown striking, and in this respect, superior characteristics. Such a scrubber has never previously been tested for the purification of diesel exhaust, which will now become a novelty in this industry.

Furthermore, it has been shown that this scrubber type is not adversely affected by the sharp and sticky soot that diesel engines produce which other scrubber designs have problems with.

It became also clear that a scrubber with KEBI-cassettes, by waiting longer to bleed out and replacing the scrubbing liquid in the recirculation tank 9, can handle 4-6 times higher Na2SO3concentrations than other scrubber types can handle, which saves alkali and fresh water and also reduces the size of the particle separation treatment plant 10a, 10b, 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 scrubbing 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, which takes effect in 2018, the above mentioned NOx reduction is not enough. Tests have therefore been conducted with one more KEBI-scrubber coupled in series before "a KEBI-sulfite scrubber", where an oxidizing water/ chlorine dioxide solution has been injected in the recycle flow, which in this step converts NO to NO2and 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 twice as high with two scrubbers. One way to get around these disadvantages is to only use one scrubber of the invention type and for example inject an oxidizing gas mixture of 10% chlorine dioxide gas, upstream of the inlet 3. 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 because the NO gas does not readily dissolve in water.

The closest technology 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 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 creates and then moves up in the scrubber 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 particulate matter content and a high sulfite content without showing clogging problems. These properties, which are unique in the industry, means that a KEBI-scrubber can be made smaller than other scrubber types, which is a characteristic that is appreciated in industrial contexts. Furthermore, the possibility of operating at a higher sulfite level saves chemicals.

Another way to neutralize a portion of the NOx upstream a scrubber, is that instead of an oxidizing substance, a reducing ditto such as ammonia in gas phase or in aqueous solution is injected, which converts the NO to pure nitrogen N2. The problem other scrubbers have in this regard, is that unreacted ammonia slips through and is released into the atmosphere, which is not allowed. A scrubber with KEBI-cassettes does not have this problem, because its construction with trays filled with cassettes with rocker valves over which a foam phase are present, effectively solves the excess ammonia, since the ammonia is readily soluble in water. Although a scrubber with KEBI-cassettes is particularly adapted to use for NOx purification by means of ammonia, the method has a major drawback in that the reaction requires a temperature between 800 and 1000°C, which excludes many processes.

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 the 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 diesel engine run as effective as possibly.

Furthermore, in some embodiments, a scrubber liquid with sodium sulfite concentration in the range of 50-200 g / 1 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 in 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 for example when you do not want to risk that the level of foam reaches the demister or the exit of the scrubber. For example, the maximum opening angle of the rocker valves in the top tray 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 15 is controlled so that the liquid to gas mass flow ratio is about 1 to 1 in the scrubber, when the diesel engine or a similar demanding industrial process runs at its highest nominal output power rating.

In some embodiments, the control valve 15 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 a similar demanding 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 (21)

Claims

1. A dynamic or semi-dynamic scrubber, designed to purify diesel engine exhaust gas from soot, sulfur- and nitrogen-oxides, consisting of a scrubber tower (2), 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) and is pressing from below, 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 the scrubber is adapted so that the flow of the scrubbing liquid is controllable by means of a control valve (15), so that the scrubber keeps a constant back pressure within the diesel engine’s predetermined limit values.

2. A scrubber according to claim 1, wherein the rocker valves of the cassettes in a top tray (5) have a maximum opening angle greater than the maximum opening angle of the rocker valves in the cassettes in the underlying trays.

3. A scrubber according to claim 1 or 2, wherein the rocker valves in the cassettes in the top tray (5) have a maximum opening angle which is 40-60% greater than the possibly opening angle of the rocker valves in the cassettes in the underlying trays.

4. A scrubber according to any one of claims 1-3, wherein the cassette rocker valves' maximum opening angle is variable and remotely controllable.

5. A scrubber according to one of claims 1-4, 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.

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

7. A scrubber according to any one of claims 1-6, 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 (15).

8. A scrubber according to any one of claims 1-7, arranged such that the control valve (15) is regulated so that at a liquid to gas mass flow, in the scrubber, is about 1 to1, at the diesel engine's maximum nominal output power rating.

9. A scrubber according to any one of claims 1-8, wherein the control valve (15) is regulated to a liquid to gas mass flow ratio, in the scrubber, in a closed loop case, is about 6 to 1 for an idling diesel engine.

10. Use of a dynamic or semi-dynamic scrubber according to one of claims 1 -9 for cleaning of diesel engines exhaust gas from soot, sulfur- and nitrogenoxides.

11. A method to clean diesel engines and /or industrial process exhaust gases from particulate matter, sulfur- and nitrogen-oxides, CHARACTERIZED in that the particle-laden flue gas is supplied to a dynamic or semi dynamic scrubber, comprising a scrubber 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 pressing from below or is sucked by an downstream placed fan, whereby the gas bubbles up through the, above the tray(s) accumulated, scrubbing liquid and foam and is purified by contact with it, directly and on the rocker valves surfaces and that an oxidizing gas and /or an oxidizing liquid, for example a gas mixture containing chlorine dioxide, is injected upstream the scrubber.

12. A method of claim 11 where the scrubber is adapted so that the recirculation flow of the scrubbing liquid is automatically adjustable by means of a control valve (15) so that a back pressure of the scrubber is kept constant and within the engine or industrial process predetermined limit.

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

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 11-14, wherein the rocker valves of the cassettes in the top tray (5) have a maximum opening angle 40% -60% greater than the opening angle of the rocker valves in cassettes in the underlying trays.

16. A method of any of claims 11-15 where the cassette rocker valves' maximum opening angle is variable and remotely controllable.

17. A method of any of claims of 11-16, 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 Is.

18. A method of any of claims 11-17, where the rocker valve's maximum opening angle is adjusted to about 30° at a liquid to gas mass flow ratio, in the scrubber, of about 10 to 1.

19. A method of any of claims 11-18, where a control valve (15) is regulated so that the liquid to gas mass flow ratio, in the scrubber, is about 1 to1 at a diesel engine's or an industrial process' maximum nominal output power rating.

20. A method of any of claims 11-19 wherein the control valve (15) is regulated, in a closed loop case, to give a liquid to gas mass flow ratio, in the scrubber, of 6 to 1 for a diesel engine's or an industrial process' lowest specified nominal power rating.

21. A method of any of claims 12-20, wherein the oxidizing gas or liquid contains 1-10% chlorine dioxide.