EP2139583A1

EP2139583A1 - Collection of particulates

Info

Publication number: EP2139583A1
Application number: EP08750456A
Authority: EP
Inventors: Markku Rajala; Kauko Janka; Juha Tikkanen; Lauri Hietaniemi
Original assignee: Beneq Oy
Current assignee: Beneq Oy
Priority date: 2007-04-23
Filing date: 2008-04-22
Publication date: 2010-01-06
Also published as: WO2008129136A1; FI119587B; FI20070319A0; EP2139583A4; FI20070319L

Abstract

The arrangement of the invention for collecting particulates (7) having the size of less than 1000nm, specifically less than 100nm, from a gas flow containing water vapour (8) comprises a collection apparatus including a collection surface (13), the collection surface being arranged in conjunction with the gas flow. According to the invention, the collection apparatus comprises heat adjusting means (14, 19) for adjusting the temperature of the collection surface (13) below the dew point temperature of the gas flow in order to condense the water vapour (8) of the gas flow onto the collection surface and carry the particulates (7) onto the collection surface along with the water vapour drifting thereto.

Description

COLLECTION OF PARTICULATES FIELD OF THE INVENTION

The present invention relates to the collection of particulates, specifically nanoparticles hav- ing the size of less than lOOnm, from a gas flow containing water vapour.

BACKGROUND OF THE INVENTION

Particulates circulating in the air have presently become a matter of great concern due to their effects on health. There are many different kinds of particulates and particulate sources. The particulates suspended in the outdoor air are mostly produced by traffic, emissions from industrial facili- ties and, to a quite considerable extent, from combustion of wood. For example in Finland, 30 - 40% of all particle emissions are caused by small-scale wood combustion. A more recent particulate source is the industrial production and use of nanoparticles, i.e. particles having an aerodynamic diameter of 1-1000 nanometres .

When released into the environment, particulates have various harmful effects on human health. In urban air, particulates have been found to increase immediate mortality and morbidity. Particle concentrations have been found to be associated with mortality in cardiovascular diseases and respiratory disorders as well as with lung cancer.

Even particles having an aerodynamic diameter of less than 10 micrometres penetrate into the human airways. The most harmful ones are small particles of less than 2.5 micrometres in diameter which penetrate deep into the lungs. The effects of even smaller, i.e. ultra small, particles on health are not known pre- cisely. However, it is probable that the harmfulness of the particles is proportional to their area, and therefore especially the smallest particles (having a high specific area) constitute a considerable health hazard. Moreover, other impurities are able to access the airways from the surface of the particles, and the particles may also carry toxic and allergenic substances. According to the present knowledge, combustion-based particulates are more harmful to the health than for example road dust. It has not been possible to define the safe particle concentration.

Due to the above-described effects of the particulates on the environment and further on the human health, it is important to strive to minimize their release into the environment. Various purification methods for controlling the amount of particulates released into the atmosphere from various processes are known. The known methods can be divided for example into dynamic collectors, electric filters, fabric filters and wet separators, i.e. scrubbers.

Dynamic collectors are not efficient for removing particulates.

Electric filters have a moderate separating capacity for even small particles (of less than 1 mi- crometre in diameter) , but the separating capacity decreases as the particle diameter reduces to less than 100 nanometres. Electric filters are also relatively expensive.

Fabric filters are typically used if the emission standards are particularly strict, or if the gas to be purified has a low dust content. Problems relating to fabric filters include wearing, pressure losses and need for maintenance of the equipment. Fabric filters also have limited heat resistance, for ex- ample with fibreglass the maximum temperature is less than 300 °C. Special filters which hold temperatures of more than 1000° C have been manufactured from metal or ceramic fibres.

The problem with all the above-described dry filters is the possible secondary emission of particu- lates, i.e. re-release of the already collected particles into the environment when used filters are being changed or handled for other purposes.

In wet scrubbers, particles in the combustion gases adhere to liquid droplets and remain in the liq- uid which circulates in the scrubber. In order to provide efficient contact between the particles and the droplets, the liquid is atomized in the wet scrubbers into small droplets having the size of 0.1 - lmm. The collection efficiency improves as the droplet size de- creases and the speed difference between the gas and the droplets increases. In wet scrubbers, binding of the particles is based on collisions, direct retention and, with small particles, also on diffusion. If water vapour can be condensed onto the surface of the parti- cle, as with the below-described Venturi scrubber, the collection efficiency can be improved. Wet scrubbers can be classified according to the manner in which the droplets are formed and sprayed. In tower scrubbers, the droplets are sprayed with nozzles into the gas to be washed. In Venturi scrubbers, a high-speed gas flow accelerated in the Venturi tube sprays the liquid. In wet scrubbers, the achieved collection efficiency is typically 80 - 99%, and the smallest well-separable particle size is less than 1 micrometre. The scrubbers are relatively expensive and use much water, producing large amounts of -waste water containing suspended matter.

Patent publication US 4251238 discloses a fabric filter for drying gases and removing particu- lates from them. In the method, the gas flow is led through a filter, the filter porosity being at least 0.985. The publication states that the disclosed solution provides reduced pressure loss in the filter. This is essential for the cost-effectiveness of the filtering, because each increase of 25mmH₂0 in the pressure difference raises the power consumption for 0.07W/hm³. The method disclosed in the publication requires the use of a porous fabric filter.

Patent publication US 4675029 discloses reduction of emissions from a wood burning stove based on the use of an electric filter in the chimney. The disclosed device and apparatus are complex and too expensive for example for small-scale wood combustion.

Patent publication US 5039318 discloses a variant of the electrical filter in which the water in- eluded in the gas is condensed in the same unit in which the electrical filtering of the particles takes place. The device according to the publication allows reduction of the content of harmful gases in the waste gas. The device is expensive and not suitable as such for filtering hot gases.

Patent publication US 6551382 discloses a method and a device for efficiently removing particles having the size of 10 - lOOnm from a gas flow. The method is based on the use of a wet scrubber and re- quires the supply of a relatively large amount of water in the process.

Patent publication US 7160358 discloses a method for reducing the impurities released during drying of wood. In the drying, the gas flow is satu- rated by moisturizing it with droplets which remove the particles from the gas flow. This method, too, has the drawback of requiring the supply of a relatively large amount of water in the process.

Patent publication JP 51019585 describes an apparatus for collecting particles by utilizing iner- tial forces and using impactor disks as collectors. In addition to the relatively complex impactor disk arrangement, the method requires a sufficiently high speed of the filtered aerosol. The collection based on inertial forces also works only for relatively large particles, and particles having the size of about one micrometer or less cannot be efficiently collected by the method without a considerable low pressure.

In summary, one can state that the methods according to the prior art do not disclose any simple, energy-efficient and affordable solution for removing particulates produced in combustion process, which solution would be universal and suitable for small-scale wood combustion for example in private homes, and for example for producing nanoparticles with a flame spraying process.

OBJECTIVE OF THE INVENTION

It is an objective of the invention to provide a simple and energy-efficient solution which is as inexpensive as possible, for removing particulates from a gas flow containing water vapour, which solution is universal and suitable for filtering combustion gases of small-scale wood combustion for example in private homes, and for example for purifying nanoparticles from the waste gases in a flame spraying process which produces nanoparticles.

SUMMARY OF THE INVENTION

The arrangement and method of the invention are characterized by what has been presented respectively in claims 1 and 9.

The invention relates to an arrangement for collecting particulates having the size of less than lOOOnm, specifically less than lOOnm, from a gas flow containing water vapour, the arrangement comprising a collection apparatus including a collection surface, the collection surface being arranged in conjunction with the gas flow. The particle size refers herein to the aerodynamic diameter of the particles, which is defined as the diameter of an imaginary spherical wa- ter droplet (density 1000kg/m³) having the same falling rate as the particle in question, irrespective of the real size, density or geometry of the particle. The collection surface may be, for example, the surface of a plate-like collector or a part of it, but it may also be for example the surface of a cubic object. The essential feature is the connection with the gas flow, which in this context means that the collection surface has an open flow connection with the gas flow, such that the arrangement includes a clear passage for the gases in the gas flow to drift onto the collection surface .

According to the invention, the collection apparatus applied in the arrangement comprises heat adjusting means for adjusting the temperature of the collection surface below the dew point temperature of the gas flow in order to condense the water vapour in the gas flow onto the collection surface and carry the particulates onto the collection surface along with the water vapour drifting thereto. More specifically, the collection of particulates in the invention is based on the diffusion of the water vapour in the manner described below.

When burning a hydrogen-containing substance, such as hydrogen, hydrocarbons, organic matter or the like, water vapour is produced as the end product of the combustion process. For example when the temperature is above 100 "C, about 10% of the combustion gases from a wood-operated boiler is water vapour. On the other hand, for example when producing nanoparticles industrially by flame spraying processes, the water vapour content of the gas flow in the process may be quite high, for example when using a hydrogen/oxygen flame, the content may rise up to 100%, the temperature being as high as over 2000 °C. When this kind of gas flow containing water vapour meets the surface in which the temperature is lower than the dew point of the gas flow, the water vapour included in the gas flow tends to condense on said surface into liquid water. The dew point temperature, or more simply the dew point, of the gas indicates the temperature at which, at the vapour pressure of the gas, the water vapour in the gas condenses into liquid water. For example, at the vapour pressure of 2300Pa, the dew point is at 25°C. The highest possible amount of water in the gas at different temperatures corresponds with the highest possible vapour pressure. The amount of water in the gas, indicated as the mass ratio of a gramme of water vapour per a kilogramme of dry air (g/kg) , is the absolute humidity of the gas. The highest possible absolute humidity also depends on the total pressure of the gas. It is one of the well known physical basic phenomenons that when the temperature drops, the highest ^' possible amount of water in the gas reduces quickly. Therefore, when the collection surface is cooled below the. dew point of the gas flow, the water vapour in the gas flow can be condensed on the collection surface. For example, a collection surface having the temperature of 10 "C operates as an effective collector of water vapour in a gas flow having the temperature of 100°C. As the water vapour which comes into contact with the collection surface condenses on the collection surface, an area in which the water vapour content is very low, close to zero, develops in the immediate proximity to the collection surface. This area of low concentration provides diffusion of the water vapour, i.e. drifting caused by the difference in the concentration from farther away from the collection surface towards the lower concentration and the collection surface. Due to the very low concentration in the immediate proximity to the collection surface, caused by the continuous condensation, the diffusion rate remains continuously high. The higher the temperature of the gas flow, the higher the diffusion rate. For example, at the temperature of 20 °C, the diffusion coefficient of water vapour is about 0.24 x 10^"4m²/s, and at the temperature of 60 °C it is about 30% higher. The diffusion coefficient of water vapour is therefore in the same range as that of an air molecule. On the other hand, the diffusion coefficient of a particulate having the size of 10 nanometres is only about 5.4 x 10^"8m²/s, and that of a particle having the size of lOOnm is about 6.9 x 10^"10rn²/s. Since the diffusion coefficient also represents the transmission flow of the material, the diffusion flow of water molecules onto the collection surface is more than 30 000 fold relative to the diffusion of the particles of lOOnm. The diffusion rate of the actual particulates is insufficient to drive the particulates onto the walls in practical particle collectors, unless the particles are passing very close to the wall (at a distance of not more than a few millimetres), such as in diffusion batteries. However, when the diffusion flow of the water molecules advances towards the cool collection surface, the water molecules which collide with the particulates induce also in the particulates a force which tends to drive them towards the collection surface. Collection of the particulates becomes more efficient also partly by the thermophoretic force induced by the cold collection wall, but this phenomenon is substantially weaker than the effect caused by the diffusion flow of the water vapour. Unlike the gas molecules, the particulates do not bounce off as they hit the collection surface, but instead adhere to it.

The particulates that have adhered to the collection surface can be removed from it with the wa- ter that has condensed onto the collection surface. In one preferred embodiment of the invention, the collection surface is so arranged that the water condensed on the collection surface and with it the particulates accumulated on the collection surface can be removed from the collection surface by the effect of gravity by letting them run off from the collection surface. Besides a suitable slope, the shape of the collection surface may in this case comprise for example a trough-shaped portion for directing the running water as desired.

In one preferred embodiment of the invention, spreading of the film of water on the collection surface and letting it run off from it in a layer which is as even as possible is improved by arranging the collection surface hydrophilic or superhydrophilic. An even spreading of the water on the collection surface ensures an efficient removal of the particulates from the collection surface with the water. A hydrophilic surface can be provided for example by coating the collection surface with a hydrophilic material or by treating the surface by means of radiation or other process in such manner that it becomes hydrophilic. For example by irradiating a surface coated with titanium dioxide with ultraviolet radiation, the surface can be made hydrophilic. In a superhydrophilic surface, the hydrophilicity is the most developed, and on the superhydrophilic surface, the water is spread practically as a perfectly even film, the incident angle of the surface and the water droplet, or rather the film, being almost 0°. A hydrophilic surface can be made superhydrophilic by structuring the surface so as to comprise surface structuring of the order of micrometres and possibly also nanometres. A superhydro- philic surface can also be provided by other means, for example the above-mentioned UV-irradiated titanium dioxide surface may also be superhydrophilic.

The collection apparatus in the arrangement preferably comprises recovering means for recovering the water and along with it the particles removed from the collection surface, and for preventing their re- release and spreading into the environment as carried by the air. The water running off from the collection surface can for example be collected into a receiver or led directly into a discharge duct or the like.

In one embodiment of the invention, the col- lection apparatus is mounted in conjunction with the waste gas flow of a flame spraying process that produces nanoparticles in order to collect the nanoparti- cles, i.e. particles which most typically have the size of less than lOOnm, from the waste gases of the flame spraying process. In this manner it is possible to prevent these particles with potentially harmful effects on health from being released into the environment .

Cooling of the collection surface can be re- alized in many different ways. The simplest alternative is air cooling based on free convection. However, more efficient cooling is achieved for example in the case where the heat adjusting means comprise means for directing a cooling agent which is cooler than the collection surface to pass in the proximity to the collection surface in order to transfer heat from the collection surface to the cooling agent and along with it further away from the proximity to the collection surface. The cooling agent then releases the heat elsewhere. Such, for example water-circulating, cooling system is suitable for use for example in the col- lection apparatuses mounted in conjunction with industrial processes.

In the case of a simple cooling agent circulation, efficient cooling of the collection surface requires a relatively high amount of water or other cooling liquid, so the above-described solution is not suitable for example for a collection apparatus arranged in conjunction with the combustion gases of a heating system based on the combustion of wood or other fuel in a detached house. A more suitable alternative for this is the embodiment of the invention in which the heat adjusting means of the collection apparatus comprise a heat pump evaporator for cooling the collection surface by means of heat transferring from the gas flow to the evaporator via the collection surface .

The heat pump refers herein in a general manner to a system in which a cooling agent is circulated in a closed cycle so that it evaporates in the evapo- rator due to the effect of heat transferring therein, absorbing the heat, and re-liquefies in the condenser, releasing the heat. The refrigerant in the heat pump allows a very low temperature of the collection surface, so the water vapour and with it the particles can be efficiently collected. On the other hand, in a heating system of a building based on the combustion of a fuel, considerable amounts of valuable heat energy exit the building with the warm combustion gases. Therefore, in one particularly preferred embodiment of the invention, the heat pump is connected with the heating system of the building, so that besides an efficient cooling of the collection surface, a recovery of heat from the combustion gases is provided.

The invention also relates to a method for collecting particulates having the size of less jthan lOOOnm, specifically less than lOOnm, from a gas flow containing water vapour onto a collection surface arranged in conjunction with the gas flow. According to the invention, the collection surface is cooled, in the method, below the dew point temperature of the gas flow in order to condense the water vapour in the gas flow onto the collection surface and carry the particulates onto the collection surface along with the water vapour drifting thereto. The method according to the invention is thus based on providing, by means of the cooled collection surface, a diffusion flow in the water vapour drifting thereto, driving with it also the particulates to the collection surface. The advantages provided by the invention and the specific embodiments described above apply, besides the arrange- ment according to the invention, correspondingly to the method according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following section, the invention will be described with reference to the accompanying drawings, in which Fig. 1 schematically illustrates the arrangement and the method according to the invention in conjunction with a flame spraying apparatus and Fig. 2, respectively, in conjunction with a system for small-scale wood combustion.

DETAILED DESCRIPTION OF THE INVENTION

The core of a flame spraying apparatus 1 shown in Fig. 1 is a flame spraying burner 2. Hydrogen and oxygen lines 3, 4 and a source material line 5 are connected with the burner for leading the materials required in the process to the burner. Hydrogen and oxygen are supplied to the burner in the gaseous and the source material in the liquid state. In the bur- ner, hydrogen and oxygen are formed into a hydrogen oxygen flame 6 for producing nanoparticles 7 from the source material. Also water vapour 8 is produced from the combustion of hydrogen and oxygen. The solvent for the liquid source material may be water which also produces water vapour in the burner. The aerosol produced in the burner 5 and comprising particulates 7 and water vapour 8 is led into a process chamber 9 where some of the particulates grow on the surface of a coated product 10. The details of the flame spraying process are not relevant for the invention and are therefore not described herein in detail. The efficiency of the coating process is less than 100%, so some of the particulates and most of the water vapour are led further as an aerosol into a particulate collector 11. In this exemplary embodiment, the particulate collector comprises vertical collector plates 12 in which the surfaces 13 that have contact with the aerosol are coated with a hydrophilic material and structured in such manner that a superhydrophilic surface is provided. A water piping 14 is arranged on the rear surfaces of the collector plates for cooling the collector plates by means of the water circulating in the piping below the dew point of the water vapour 8 in the aerosol, so that the water vapour drifting to the surfaces of the collector plates condenses into liquid water. In this way, an area in which the water vapour content is very low develops in the proximity to each collector plate. This provides a continuous diffusion flow for the water vapour from other parts of the col- lector towards the collector plates. Along with the diffusion flow of the water vapour, also the particulates 7 drift to the collector plates and adhere to their surfaces. The water condensed on the surfaces of the collector plates runs down the collector plates into a collector trough 15 and from the trough further into a receiver, discharge duct or the like (not shown in the figure) . When running down, the water also carries away the particulates adhered to the collector plates .

The small-scale wood combustion system for heating a building 16, presented in Fig. 2, comprises a furnace 17 which may be for example a fireplace, a heating boiler or the like. In the system, the combustion gases produced in the combustion process 6 are led with the particulates 7 and water vapour 8 from the furnace to the chimney 18.

Collector plates 12 are mounted in the upper part of the chimney 18 for collecting the particulates 7 from the combustion gas flow that is exiting the building. In this embodiment as well, the collection surfaces 13 of the collector plates are coated with a hydrophilic, structured coating and are so arranged that the water condensing onto them runs due to the effect of gravity down to the lower edge of the plates and further through a collector trough 15 into a fur- ther treatment system (not shown in the figure) . The collector plates 12 are arranged to be in contact with the evaporator 20 of the heat pump 19. The condenser 21 of the heat pump on the other hand is connected either directly with the interior of the building 16 or with its heating system.

The heat-transfer agent circulating in the piping 22 of the heat pump 19 arrives at the evaporator 20 in the liquid state. In the evaporator, the heat-transfer agent evaporates and warms due to the effect of heat that is transferred from the combustion gases via the collector plates 12. The heat absorbed in the evaporation cools the surfaces of the collector plates at least at certain positions below the dew point of the water vapour in the combustion gases, providing the collection of the water vapour 8 and, along with it, of the particulates 7 as described above in the context of the flame spraying process. The vaporous heat-transfer agent is sucked with low pressure into the compressor 23 of the heat pump, where its pressure rises and it also warms. From the compressor, the evaporated heat-transfer agent is led to the condenser 21 where it cools down and condenses back into liquid, releasing heat into the surroundings of the condenser. Next, the liquid heat-transfer agent flows back to the evaporator through an expansion valve 24, and the cycle goes on as described above. The arrangement according to Fig. 2 combines, in an efficient manner, collection of the particulates and recovery of heat from the combustion gases which are exiting the building. The invention is not limited merely to the examples referred to above; instead, as it is obvious to a person skilled in the art, many variations are possible within the scope of the claims.

Claims

1. An arrangement for collecting particulates (7) having the size of less than lOOOnm, specifically- less than lOOnm, from a gas flow containing water va- pour (8), the arrangement comprising a collection apparatus including a collection surface (13), the collection surface being arranged in conjunction with the gas flow, charac teri z ed in that the collection apparatus includes - heat adjusting means (14, 19) for adjusting the temperature of the collection surface (13) below the dew point temperature of the gas flow in order to condense the water vapour (8) in the gas flow onto the collec- tion surface and carry the particulates (7) onto the collection surface along with the water vapour drifting thereto.

2. The arrangement according to claim 1, wherein the collection surface (13) is so arranged that the water condensed on said collection surface and with it the particulates (7) accumulated onto said collection surface can be removed therefrom due to the effect of gravity by letting them run down the collection surface.

3. The arrangement according to claim 1 or 2 , wherein the collection surface (13) is arranged to be hydrophilic, preferably superhydrophilic, in order to spread the water condensing on the collection surface and to remove it from the collection surface in a layer which is as even as possible.

4. The arrangement according to claim 1, wherein the collection apparatus comprises recovering means (15) for recovering the water and with it the particulates removed from the collection surface.

5. The arrangement according to any one of the previous claims, wherein the collection apparatus is mounted in conjunction with the waste gas flow of a flame spraying apparatus (1) which produces nanoparti- cles (7) for collecting the nanoparticles from the waste gases.

6. The arrangement according to any one of the previous claims, wherein the heat adjusting means comprise means (14) for directing a cooling agent which is cooler than the collection surface (13) from the proximity to the collection surface in order to transfer heat from the collection surface into the cooling agent.

7. The arrangement according to any one of the previous claims, wherein the heat adjusting means comprise an evaporator (20) of a heat pump (19) for cooling the collection surface (13) by means of heat transferring from the gas flow to the evaporator via the collection surface.

8. The arrangement according to claim 7, wherein the gas flow comprises combustion gases from a combustion apparatus (17, 18) disposed in a building

(16), and the heat pump (19) is arranged to transfer the heat recovered from the gas flow into the interior of the building for its heating.

9. A method for collecting particulates (7) having the size of less than lOOOnm, specifically less than lOOnm, from a gas flow containing water vapour (8) onto a collection surface (13) arranged in conjunction with the gas flow, characterized in that the collection surface (13) is cooled below the dew point temperature of the gas flow in order to condense the water vapour (8) of the gas flow onto the collection surface and carry the particulates (7) onto the collection surface along with the water vapour drifting thereto.