Patent Application
20100254869
The present invention concerns an improved process and a device for removing carbon dioxide (CO2) from combustion gases after burning of fossil fuels such as natural gas, wood, coke, (wood) coal, wood chips and wood pellets, amongst others, wherein the exhaust gas contains mixtures of dust, water, carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NOx), amongst other components, by using an aqueous solution of carbonates.
The invention concerns a process and a device for purifying smoke gases containing, together or separately, dust, soot, carbon oxides, sulphur dioxides and nitrogen oxides, and being characterized in the device comprising a container in the form of a cylinder with an inlet for the relevant smoke gas and with an outlet for purified gas, wherein the container furthermore comprises an inlet for a washing liquid and an outlet for spent washing liquid, wherein the container internally is not equipped with liquid stream baffles so that the gas and the liquid can rotate freely, but with gas impenetrable discs with a central hole and a stirring device comprising at least one disc over each disc hole, wherein the disc material comprises at least one perforated material, e.g. a netting material, and said disc(s) being rotatable inside the container, and wherein the reactant that is to capture the CO2 gas is added to a stirring device tank containing an aqueous solution including optionally sodium chloride, e.g. sea water, for an improved crystallization of sodium bi-carbonate, and wherein this mixture is pumped into the reactor over the rotating discs in the reactor, and wherein said aqueous solution, after a rotating time spent in the reactor, passes to a sedimentation tank for separation of solids and aqueous solution, whereby the overrun from the sedimentation tank is returned back to the pumping tank, and wherein the sediment of sodium bicarbonate is removed in a solids/liquids separator and wherein the liquid is returned back to either the sediment tank or to the pump tank.
The invention also concerns a device for performing the process disclosed supra, wherein the device comprises a reactor comprising a cylinder without any current baffles with an internally rotating axle with one or more discs of a screen material is mounted, to said cylinder there being supplied water and gas and wherein the discs through rotation rotate the water and disperses the aqueous phase so that there is created s foam by controlling the pH-values of the water so that the absorption of the gases into the water phase happens very quickly. When the water rotates together with the rotating discs this results in a very low energy consumption/m3 gas supplied to the absorption unit.
In a test the carbonate solution that was used to the capturing of CO2 contained 100 grams Na2CO3/litre. When using the embodiment shown in
The water solution that was pumped from a pumping tank through the reactor and back to the pumping tank was added 50 grams NaCl/litre for decreasing the solubility of sodium bicarbonate in the aqueous solution, this being known from the literature. It was further attempted to add grainy sodium bicarbonate in solid form directly to the pumping tank, which then had mounted a stirring device for good mixing of the added sodium carbonate particles, a technique that only is simplifying.
Such an embodiment represents a preferred embodiment of the process according to the invention where the aqueous solution contains at least 30 grams sodium chloride/litre solution, e.g. sea water, and this solution is added sodium carbonate in solid form to a pumping tank having mounted a stirring device for good admixture of carbonate, and wherein said mixture is pumped into the reactor having one or more rotating discs creating a foam inside the reactor, and wherein the aqueous solution from the reactor passes to a sedimentation tank wherein the solids sink to the bottom, and the aqueous solution is returned to the pumping tank for re-use.
The salt sinking to the bottom in the sedimentation tank could consequently easily be filtered from the solution in a filter press, and the filtrate is returned to the sedimentation tank or the pumping tank.
When heating 10 grams of dried salt from the bottom of the sedimentation tank to 120-140° C., it was determined that this salt was >95% pure NaHCO3 by measuring the weight loss being pure CO2 and water vapour (gas).
The present invention is of course not limited to only sodium carbonate, but also to potassium carbonate, ammonium carbonate and other carbonates that through the addition of CO2 form bi-carbonates that may be used. Other chemicals binding CO2 may of course also be used, e.g. magnesium salts as one such salt. Furthermore other chemicals may of course also be used for removing CO2 and NOx indicating other regions of use.
One of the new aspects of the present invention is the technique that is used for promoting the transfer of CO2 into the aqueous solution and further to promote the CO2 gas to react with the carbonate to create bi-carbonate. This happened after only a few seconds duration in the strongly foaming reactor zone in a reactor device as disclosed infra, where an embodiment of the reactor is shown in
The removal of the CO2 fraction from exhaust gases by using old and well-known reactions wherein CO2 is absorbed into an aqueous solution containing a carbonate for creating a bi-carbonate, is an old technique.
After the absorption and the creation of a bi-carbonate, it is a matter of choice to determine how the CO2-gas again is to be liberated and how the CO2-gas is to be used. The gas may e.g. be liberated and used for greenhouses for promoting the growth through increased photosynthesis. For this it is required 800 to 1000 ppm CO2 in the atmosphere of the greenhouse.
The process according to the invention may be used within a temperature interval from the freezing point of the relevant carbonate solution (at lower concentrations about 0° C.) up to the boiling point of the solution (at low concentrations about 100° C.), even if intervals such as 5-80° C., 10-70° C., 15-60° C., 20-50° C. or other intervals also may be used.
It is also possible to mount more discs over each other on the pipe rod (axel), and the diameter of such disc stacks may be varied.
Of course it lies within the present invention to lead the washing water containing one or more carbonates in a mixture into different locations in the reactor tube for obtaining the best possible effect of the purification (
Several inventions mention netting meshes on a rotating axel for an improved mixing of two or more components.
Here it is to be mentioned DE 1 542 450 disclosing a rotating cylinder, not discs. This is completely submerged in the fluid, and admixes a second liquid or a solid. The present invention with rotating disc(s) may not be derived from said invention.
EP 136 394 A2, DE 1 808 593 and WO 01/45830 all disclose different rotating cylinders and do not mention rotating discs as disclosed in the present invention.
GB patent application GB 2121701 A discloses a device for mixing gas into a fluid. The devise have current baffles for preventing rotation of the fluid on account of the rotating disc on the bottom of the absorption device (claim 2). Furthermore, said GB patent application shows that the rotation force is all of 1-2 kilowatts/m3 gas. The present invention uses only 1/1000 of this, and it is not possible to read from said GB patent application that by removing the current baffles and use discs of a netting material, it is possible to obtain 1/1000 part thereof (1-2 watts/m3 gas) used effect/m3 gas by the liquid and the rotor spinning together inside the absorption tower so that there is formed a foam improving the absorption effect to all of 60%.
In U.S. Pat. No. 3,896,212 it is referred to the use of carbonates for capturing acid gases and CO2 through additions of other chemicals such as amines for an improved absorption and oxidizing chemicals for corrosion prevention. Here it is mentioned hydrogen peroxide. Said US patent uses a classic absorption tower. This art is not relevant for the present technique where it is not used any additions in the form of chemicals as mentioned id this US patent.
The present invention with one or more rotating disc(s) located as stated supra, used very little energy for the stirring, only 1-2 watts/m3 gas supplied, probably because the discs rotated in the fine foam and the absorption tower had a fluid level below the lowest disc at a static fluid condition, as shown in
The present invention is to be disclosed further under reference to the enclosed figures wherein
In the following it is referred to
The present invention shall be further disclosed through the following
In the following it is referred to
The present invention shall be further disclosed by the following
In the following disclosure it is referred to
The device (purification column and rotor) according to the invention may be supplied with a number of modifications without this affecting the object of the invention. The rotating discs may e.g. be made completely of a netting material, but the disc may also comprise an impenetrable disc section being between 40% and 95% of the diameter of the disc, while outside of this being equipped with a netting part. These embodiments are alternative since the reaction zone seems to lie at the periphery of the disc where there exists a netting material. Alternatively the gas-impenetrable may comprise small sections in the disc consisting of a netting material.
Such a device may have a rotating disc with a perforation with light openings of between 1 and 10 mm2.
Furthermore, such device according to the invention may have a disc being smooth (containing no flow baffles).
Such a device may also include a stirring device comprising several discs, one disc above each plate separator (
Such a device may also include a washing fluid comprising sodium carbonate and/or potassium carbonate or ammonium carbonate or other substances absorbing CO2 singularly or in mixtures.
Such a device may also include a stirring device with a diameter between 10 and 99% of the internal diameter of the container.
The material of the disc(s)/netting may in a preferred embodiment of the invention be of any metallic material being inert or not reacting to any significant degree with the aqueous solution, e.g. acid-resistant steel and metal alloys, or plastic materials, indicated as examples.
The process may further include that the gas to be purified is brought into a temperature interval of between 10 and 60° C.
The process may further include that the aqueous solution that is to strip the gas, contains between 10 and 300 grams/litre sodium chloride (NaCl) for further promoting crystallization/precipitation of sodium bicarbonate, if this is wanted.
The process may further include that the aqueous solution that is to strip the gas, contains between 1 and 200 grams/litre sodium carbonate and/or between 1 and 1000 grams/litre potassium carbonate.
Furthermore, the invention concerns an apparatus improving the absorption and optionally desorption of gases in fluids by being able to finely disperse gas and fluid so that a large surface in the form of a foam is obtained for rapid reaction. This is achieved by the region where the gas is to be absorbed into the fluid is replaced with one or a multiple of rotating wire netting discs as a replacement for conventional filling bodies. Furthermore, absorption fluid may optionally be pumped continuously in over each rotating disc for a multi-step desorption at temperatures between 90 and 100° C.
Said in another way, the area in gas scrubbers usually being filled with solid filling bodies, is replaced with a number of rotating wire netting discs wherein the washing fluid rotates together with the discs inside the gas scrubber.
The rotating discs may also have a centre disc being impenetrable for gas so that the gas only reacts in the periphery.
It was for the applicant completely unknown that such a stack of rotating discs could replace the region of filling bodies and have such an excellent effect in the different absorption towers that have been tested.
It is of course within the ambit of the present invention to use discs of different formats with or without a gas-impenetrable centre.
It is preferred to use discs that have a wire netting only at the periphery of the discs. This makes the discs stable.
Such a device as disclosed supra may be used for both absorption and desorption of gases depending on the working conditions. For example for carbon dioxide the absorption may be performed within the temperature interval 30-50° C., while the desorption of this gas may be performed with the same device, but by increasing the temperature to about 90-100° C., while when regenerating at temperatures above 100° C. a pressure-boiler has to be used, or a sintering kiln for solids, e.g. sodium bicarbonate.
In tests it is clearly observed that discs with a gas-impenetrable centre and a wire mesh only in the periphery are better than discs consisting purely of wire mesh. The cause of this may be that gas passes unpurified through the centre of the disc, while the water passes along the periphery.
It is also obvious that the absorption effect is improved when increasing the number of discs, but this on the expense of used stirring effect.
It is preferred to use a reactor with several netting discs, one disc/plate separator (
The present invention is to be further disclosed through the following examples.
A pipe of PVC with an internal diameter of 10 centimetres and a height of 30 centimetres had a welded-on and watertight bottom and a loose lid for service of a stirring device. The pipe had no fluid baffles.
The gas was passed into the bottom through a welded-on pipe, whereas the washing fluid was passed inside through the lid from a pump. The pipe had an outlet for spent absorption water 10 centimetres from the bottom, while the lid had an exit for purified gas.
The rotor was an acid-proof rod mounted a disc with a diameter of 9 cm made of acid-proof netting (sieving cloth) with a light opening of 2×2 millimetres. The disc was located 0, 5-10 millimetres below the static fluid surface at the same level as the outlet for used absorption water of 10 cm from the bottom when the rotor was not operative.
The pumping volume of the pump could be adjusted from 10 to 100 litres/hour.
The absorption water was pumped from a 20 litres pumping tank to the absorption column from which the water flowed naturally back to the tank for re-use. The concentration of the carbonate solution of the washing water may lie within the interval 1-200 g/l for Na2CO3, and 1-1000 g/l for K2CO3, more preferred 4-100 g/l, most preferred 10-50 g/l for both compounds. The CO2 content of the supplied smoke gas may vary, but will normally lie within the interval 1-40% by volume, more preferred 4-20% by volume, most preferred 4-10% by volume.
The results from the test are provided infra in Table 1.
The smoke gas came from the burning of propane.
The gas that (after drying) entered the absorption column contained:
10% CO2, 20-21 ppm NOx, varying amounts of CO (30-120 ppm).
Water volume=15 litres/hour continuously supplied to the column.
Concentration of Na2CO3 in the water=90 grams/litre.
The results show that a device with a mesh stirring device worked. The rotations of the stirrer could be adjusted with a frequency transformer between 500-2000 rpm. The results show that a temperature of between 30-40° C. is preferred. The absorption of NO was very small, <10%. The gas analyses were performed continuously with Dräger online instruments for CO2, CO and NO after calibration with standard gases from AGA-Norgass. It was also observed that the device created a foam.
The design of this example was like example 1. In this example the change of pH in the washing water was measured over a period of 4 hours. The pH changed from an initial value of 11.12 to 9.58 showing that CO2 is absorbed by it making the washing water progressively more acid on account of the production of bicarbonate.
The results from Example 1 and Example 2 show that the present invention with rotating mesh discs, work.
To be able to return the washing water for re-use it is advantageous if it is possible to remove the bicarbonate through filtration such as sodium bicarbonate or strip CO2-gas from a potassium bicarbonate-containing fluid through heating to 90-100° C. and using the same apparatus that absorbed the CO2-gas at 30-50° C.
A 100 millimetre in diameter pipe of PVC with a height of 300 millimetres had mounted a demister on top of the pipe. The rotor inside the pipe had a stack of 12 discs with a height of 10 cm and the stack was mounted 10 cm from the bottom, this time 1 cm above the fluid level inside the cylinder.
The stack had a bearing at the bottom and the top and was rotated by a 3-phase motor with a rotational regulation through a frequency transformer.
The washing water comprising a mixture of K2CO3 and KHCO3 was pumped in through the top of the stack, whereas gas was passed inside beneath the stack in a classical counter-current fashion. The gas passed upwards through the device and through the demister and out to a CO2 analyser, whereas the water flowed continuously downwards through the reactor and out through a fluid lock at the bottom. The gas was produced by a propane burner.
The test proves that a mesh+gas-impenetrable centre of the discs is best, and that a number of discs are better than one. Again foam was created, at times flowing out through the top of the absorber.
The same equipment as in the Examples 1-3 was used here.
Carbonate added=Na2CO3 (crystal soda), 4.0 grams/litre water.
A simulation of 3 reactors in a row coupled in series.
The efficacy of 3 steps in a series is obvious even without a gas-tight material that partly covers the discs. The gas flowing into the reactor was each time diluted with air to reduce the CO2-content.
The same absorption unit and working conditions as mentioned in Example 4 were used here. 25 litres of pure tap water was added 100 millilitres sodium hypochlorite (NaClO). No carbonates were added.
The pH was reduced from 9.33 to 7.29 during the first test here. This may be caused by some carbonates remaining in the system resulting in 15.9-17% absorption of CO2.
In test 2 the pH increased to 7.62 after 200 ml had been added to the pumping tank of 25 litres. It is obvious that the addition of NaClO reduces NOx in the gas excellently at pH-values in the washing solution of between 7.2 and 7.7 without carbonates present. This may then be used for a pre-wash of the gas if NOx is to be removed. It is known that also H2S and SO2 are removed by using NaClO.
Norwegian patent NO 303.565 discloses the use of sodium hypochlorite for removing SO2 and quicksilver.
A circular pipe with an internal diameter of 37 cm and a height of 90 cm had a rotatable shaft with a netting disc with a diameter of 22 cm. On the disc there was mounted a gas-tight PVC disc with a diameter of 18 cm. The axel with the discs was rotated by a frequency-controlled motor. A feed solution of sodium carbonate with a concentration of 180 grams/litre dissolved was continuously pumped in onto the top of this one disc by using pH-control of the water exiting the pipe. This kept the pH inn the washing water constant at a pH=10.0. When stopping this feed, the pH dropped very rapidly towards pH 9.5 when bicarbonate was formed. The water was exited at the bottom of the tank to an external pumping tank from which the carbonate water again was pumped to the top of the absorption tank. The gas was introduced underneath the bottom disc and exited at the top of the tank.
By leading the gas under the bottom disc, this disc caused the water and the gas to form a foam with several very fine bubbles/drops for an accelerated reaction. The absorption effect was calculated from the reduction of carbonate being known through analyses of the gas into the absorber. Additionally the used effect was measured by this absorption technique performed in a foam. It appeared that the used effect was very small, only 2-3 Watts/m3 purified gas.
Amount of gas entering the absorber=30 Nm3/hour
Content of CO2=4.05% by volume
CO2 in the entering gas=2.43 kilos/hour
Rotations of disk=700 rpm
Used effect for disk rotation=85 Watts
Used Na2CO3=3.83 kilos/hour
Absorbed CO2 according to used Na2CO3=1.60 kilos
Purifying ratio for captured CO2=65.8%
Used effect=2.83 Watts/Nm3
It is obvious that this system works excellently. This example is only one among many, but it characterizes all of the tests. It provided the same absorption result if a feed solution of K2CO3 was added to the pump tank or directly into the absorption tank.
The same equipment that was used in Example 6, was used here, but now there were added two extra equipment units.
The water from Example 6 was added sodium chloride to a concentration of 200 grams/litre was obtained. Additionally a sedimentation tank was mounted for receiving the water from the reactor. A stirring device was mounted on the pumping tank for good mixing of added sodium bicarbonate in solid form from a screw feeder that was pH-adjusted through the water exiting the reactor. The bottom sediments (NaHCO3) from the sedimentation tank was continuously pumped through a filter press. The filtrate from the press was returned to the sedimentation tank. The overflow from the sedimentation tank was returned to the pumping tank (
It was observed that the bi-carbonate crystallised out as an easily filterable solid at the bottom of the sedimentation tank.
Additionally it was also observed that the temperature of the aqueous solution from this pumping tank was higher than the return water to this pumping tank.
Additionally it was observed that the pH of the water exiting the reactor did not rise above 9.2-9.3 even at a continuous dosing of 3.8 kilos/hour of sodium carbonate to the pumping tank with a control point for the pH dosing established at 9.8. This suggests a rapid and effective reaction. When stopping the dosage, the pH dropped to <9.0.
At a dosage of 1.2 kilos/hour sodium carbonate the pH of the water from the reactor did not reach more than 8.8 pH-units, even at a pH control point of the same=9.8 pH-units.
It is known to the person skilled in the art that the reaction for forming bicarbonate from the expense of sodium bicarbonate, water and CO2 gas is exothermal meaning the liberation of heat.
The same equipment as the one used in Example 7, is used here, but the one rotating disc has been exchanged with a stack of 4 discs mounted on top of 4 plates, as shown in
The washing water was passed into the top of the plates and exited at the bottom of the pipe. The gas was passed into the bottom of the pipe and exited at the top, a classical counter current configuration for gas stripping.
The washing water contained 100 grams of Na2CO3/litre and 50 grams NaCl/litre for an improved precipitation of NaHCO3 crystals. The temperature was 30° C. in the washing water.
Amount of gas in the absorber=30 Nm3/hour
Amount of water=5 litres/minute
CO2-content of gas supplied to the pipe=4.85% by volume
CO2-content of gas exiting the pipe=1.70% by volume
CO2 absorbed=1.89 kilos/hour
Revolutions of the discs=700 rpm
Supplied effect for disc rotation=36 Watts
Effect of water pump=21 Watts
Purification rate=65%
Total used effect=30 kiloWatts/ton captured CO2.
This indicates that the present invention, being disclosed in Example 8, is the preferred device for performing the process.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20072655 | May 2007 | NO | national |
| 200773467 | Jul 2007 | NO | national |
| 20076332 | Dec 2007 | NO | national |
| 20076581 | Dec 2007 | NO | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/NO2008/000183 | 5/26/2008 | WO | 00 | 4/28/2010 |
