FI88681C - Foerfarande och anordning Foer rening av roekgaser vid oljepannor - Google Patents

Description

1 88681

Method and apparatus for cleaning flue gases from oil boilers

The invention relates to a process for purifying oil-boiler flue gases from sulfur oxide and heavy metals, in which lime gases are fed to the flue gases and cooled, after which the formed dusty reaction result is separated from the flue gases. The invention further relates to an apparatus for purifying oil boiler flue gases from sulfur dioxide and heavy metals, which apparatus comprises means for supplying lime to the flue gases, means for cooling the flue gases and a separator apparatus for separating the formed dusty reaction result from the flue gases.

15 Harmful emissions from the combustion of heavy fuel oil are primarily oxides of sulfur and nitrogen. Sulfur in fuel oil burns to form mainly sulfur dioxide (SO2) with an air coefficient of more than one at temperatures above 1000 ° C. Sulfur trioxide (SO3) is present in 20 boilers always in small amounts with an air factor of more than one. However, its relative share of sulfur oxides in the flue gases of oil combustion is usually very small when the temperature of the flue gases in the boiler drops rapidly. The resulting corrosion damage has been counteracted by injecting small amounts of alkaline earth metal oxides or carbonates into the cathode.

The nitrogen compounds contained in the fuel decompose into nitrogen (N2) and nitrogen monoxide (NO) even at low temperatures during combustion. In addition, nitrogen oxides are formed at high temperatures from the oxygen (02) and nitrogen in the combustion air. A small proportion of nitrogen oxides are oxidized during combustion to nitrogen dioxide (NO2). The amount of nitrogen oxides depends on the amount of air used in the combustion and the combustion temperature. As the amount and temperature of the combustion air decrease, small amounts of nitrous oxide (N2O) may also be present in the oil combustion.

2 88681

Particulate emissions from heavy oil: consist of non-combustible carbon compounds, soot and inorganic substances in fuel oil, of which certain heavy metal compounds are more harmful. The more completely the combustion 5 takes place in oil combustion, the lower the dust content of the flue gases and the higher the proportion of heavy metal compounds in the dust.

Air protection requirements also apply to flue gases from oil boiler rooms. In Finland, for example, 10 guideline values have been issued, according to which the sulfur content of heavy fuel oil must not exceed 1.0 wt% S or otherwise the flue gases must be cleaned of sulfur dioxide to a level corresponding to the sulfur content of low sulfur flue gas from 1.0 g%, ie about 500 15 mg / MJ. In the future, it will be necessary to reduce emissions, which is why the removal of sulfur oxides from flue gases will become mandatory in practice. In order to limit nitrogen oxide emissions, a proposal has been made in Finland according to which nitrogen oxide emissions from boilers burning heavy fuel oil below 150 MW may not exceed 130 mg / MJ. The dust content of the gas, on the other hand, is required to be less than 40 mg / MJ in heavy fuel oil boilers with a fuel efficiency of 5-50 MW.

The need for flue gas cleaning substantially increases the cost of energy production 25, especially for boilers for heat production, which have a short annual operating time during the cold season. In principle, any of the known sulfur dioxide removal methods in use is suitable for the removal of sulfur dioxide from the flue gases of oil boilers. However, the introduction of these methods is limited by high investment costs, the share of which in total costs increases as the plant size decreases and the annual operating time shortens. Therefore, methods which are technically simple 3 88681 and which do not involve high investment costs can be considered as suitable methods for removing sulfur dioxide from oil boilers. The known simple technology in use is represented by the LIFAC process, the semi-dry DRY PACK and CDAS processes, and the desulfurization-based sulfur dioxide removal process. The high investment costs of lila are an obstacle to the implementation of all these processes as well.

In the LIFAC process, finely ground limestone is blown into the boiler firebox, where it burns to lime and binds some of the sulfur dioxide contained in the flue gases 10 to the surface of the lime particles. After the boiler, water is injected into the flue gases in a separate reactor to humidify the flue gases, whereby the sulfur-binding capacity of the lime is improved and more sulfur dioxide is bound to the lime. The resulting desulfurization product is dry dust. It is removed from the flue gases usually with an electrostatic precipitator together with the ash generated from the fuel, it is possible to use the LIFAC process application to clean the flue gases of the oil boiler, where no limestone powder is fed to the boiler but the burnt lime is blown directly into the humidification reactor. This avoids contamination of the heating surfaces caused by the limestone powder. The 60% sulfur dioxide removal rate required for heavy fuel oil flue gas cleaning is achieved with a lime-sulfur molar ratio of two when the flue gases are cooled by water-25 injection to slightly below 70 ° C.

The CDAS process is a simplified application of the DRYPAC process used by coal-fired power plants, in which water and dry calcium hydroxide powder are sprayed into the flue gases in a special reactor where the water dries and hap-30 compounds such as sulfur dioxide bind to calcium hydroxide. The dust in the flue gases, which contains kahmium hydroxide and the acidic compounds bound to it, is removed by means of a textile filter. Sulfur dioxide binding continues in the solid mattress-forming dust cake 35 formed on the surface of the dust filter.

4,8681 The use of calcium hydroxide in the CDAS process is reduced and its efficiency is improved by returning some of the dust collected by the textile filter, as such or by slurrying in water as in the conventional semi-dry process, back to the reactor.

In the fluidized bed process, the flue gases are passed through a fluidized bed with fine lime or calcium hydroxide. Steam is blown through the nozzles into the lime mattress, which activates the lime and improves its absorption capacity for sulfur dioxide-10 din. In the fluidized bed, a coarse material can be used as the fluidizing medium, such as quartz sand, which does not follow the fine material leaving the reactor for dust separation.

The flue gases leaving the fluidized bed are cleaned of lime with conventional dust separators, cyclones, electrostatic precipitators and textile filters.

it is possible to achieve the NOx emission guidelines for oil combustion by adjusting the combustion process. The use of phased combustion, combustion air and fuel phasing in oil combustion is technically easier than in coal combustion. in oil combustion, the use of an aqueous dispersion reduces the droplet size of the oil injected into the combustion and thus improves the efficiency of oil combustion, which in turn makes it possible to reduce the air factor in combustion 25 and reduce the resulting nitrogen oxide emissions. The effect of small amounts of water on the flame temperature is small, but as the amount of dispersed water increases, it also has a reducing effect on nitric oxide emissions.

In the chemical composition of the dust particles in the flue gases, it is necessary to pay attention to the elemental composition of the compounds therein, which indicates the harmfulness of the dust. The main harmful metal aerosols are environmentally harmful metals that evaporate at high temperatures, for example mercury, cadmium, vanadium-35 ni and arsenic. Conventionally used flue gas cleaning devices for cleaning small particles are both electrostatic precipitators and textile filters, in which a dust layer is formed on the surface of the filter cloth.

5 The disadvantage of the prior art is that their cleaning capacity is either inefficient or, in most cases, especially in smaller plants, they are unreasonably expensive in relation to the size of the plant and thus its energy production. The object of the present invention is to provide a method 10 and an apparatus for cleaning the flue gases of oil boilers, with which the cleaning is made economically and technically suitable and sufficiently efficient. The method according to the invention is characterized in that the flue gases are cooled to a low temperature, preferably below 100 ° C, that the flue gases are passed through a rotating grain filter, in which the dusty material in the flue gases is separated from the flue gases. The apparatus according to the invention, in turn, is characterized in that the separator apparatus is a rotary granule filter with at least one disc filter with a plurality of sectors filled with granular bodies, the flue gas being led to the outer surface of each disc and passing through the granules.

The essential idea of the invention is that the flue gases are passed through a rotating granular filter acting as a dust filter, whereby the flue gas passes through a moving layer of sulfur dioxide absorbing dust, which enhances the binding of sulfur dioxide to calcium oxide and at the same time improves lime utilization. At the same time, the heavy metals and other particles in the smoke-30 gases remain in the dust filter, from where they are removed together with the lime dust and thus cannot spread into the environment.

The invention is described in more detail in the accompanying drawings, in which Fig. 1 schematically shows an apparatus applying the method 6 88681 according to the invention and Figs. 2a and 2b schematically show a rotating granular filter seen from two different directions.

Fig. 1 schematically shows an oil boiler 1, 5 from which the flue gases formed during combustion are led forward along the duct 2. The flue gases are further led along the flue duct to the dust separator 3, whereby lime oxide from the lime silo 4 is mixed with the flue gas flowing in the flue duct 2 by means of the compressed air supplied by the compressor 5. Further, water is fed to the flue gases in a cooler 6, from which the flue gases are passed to a rotating granule filter 3. From the granule filter 3, the flue gases are passed through the filter grain separators as described below, leaving dust in the separators and reacting with sulfur dioxide in the flue gas. From the dust separator, the flue gases are again passed along the duct 7 to the chimney 8. The reaction waste separated from the dust separator, i.e. the rotating grain filter, and the heavy metals therein are removed via the cyclone 20 and transferred by the compressed air 9 to the waste storage 14. Some of the separated dust can be returned through the channel 11 back to the channel 2 to enhance the use of unreacted lime, whereby water can be added to the mixer 12 to convert the lime to hydroxide and thus more actively reactive.

In the method, flue gas containing sulfur oxide, dust and heavy metal particles or dust passes from the boiler 1 along the flue 2, whereby the pulverulent lime is first mixed and cooled, for example by feeding water so that the temperature drops low, preferably below 100 ° C. The sulfur dioxide in the flue gases reacts with the calcium oxide to form calcium sulfite which gradually converts to oxygen sulfate.

i 7 88681 (1) CaO + SO 2 »CaSO 4 (2) CaSO 3 + 0.5 02« CaSO 4

The formed flue-containing flue gas is further transferred to a rotating granular filter 3, where it passes through the granular sectors.

In this case, the as yet unreacted sulfur dioxide reacts with the lime in the sectors to form solid dusty waste and other dust, including heavy metals, also remains in the filter, from which the clean flue gas passed through the filter is passed on. Some of the calcium oxide always remains unreacted and on the other hand calcium sulfite is formed on the surface of the calcium oxide particles, which prevents or slows down the reaction of the remaining lime. In this case, it is advantageous in some cases to recycle some of the dust removed in the dust separator and, if necessary, grind it with a separate powder 15 so that the particle shell breaks to reveal unreacted calcium oxide, after which the recovered dust is mixed with water to become a more active hydroxide. The water can be mixed either separately as previously mentioned or it can be mixed with it in a grinder.

The sulfur dioxide absorption capacity of the recoverable material can be improved by wetting it under efficient mixing, for example in a grinder with either water or steam, whereby the calcium oxide is converted to calcium hydroxide and reacts efficiently with the sulfur dioxide in a dust separator.

(3) CaO + H 2 O = Ca (OH) 2

(4) Ca (OH) 2 + SO 2 = CaSO 3 + H 2 O.

Figures 2a and 2b show a rotating grain filter 30 in the direction of the rotor axis, cut open and seen in the direction of the rotor axis. The granular filter 3 has a filter chamber 31, inside which the rotor 32 of the granular filter rotates. The rotor is located at the top of the filter chamber 31, to which the flue gases are introduced tangentially through the duct 33 as shown in Fig. 2b. The rotor 32 is formed by a plurality of disc-shaped grain filters 34 having two disc-shaped discs 36 having sector-like chamber chambers 35 spaced apart from each other so that the outer edges of the discs 36 do not penetrate from the circumferential direction. There may be one or more discs as needed, although Figures 2a and 2b show a particle filter formed of several discs. Correspondingly, the space between the discs 36 is connected to a duct 37 located around the axis of the rotor 32, through which the flue gas flowing through it from the outer surface of the disc 36 10 can escape. There are a plurality of such disc-shaped grain filters on the shaft of the rotor 32 in parallel to increase its capacity so that the flue gas enters the outlet duct 37 between adjacent filters and further through the discs. As dust accumulates between the granular material, forming a dust layer, the flue gas to be cleaned has to pass through the calcium oxide 20 contained in the dust layer and thus results in further separation of sulfur dioxide, which further reduces the sulfur dioxide present in the flue gases. As the rotor of the particulate filter rotates, its sectors are brought one by one from the flue gas inlet chamber to the separation chamber 39 separated by spacers 38, whereby dust accumulated in 25 sectors can be removed, e.g.

The material in the sectors of the particle filter may be a granular material or various balls or the like. Because the heavy metal waste is generally dusty at low temperatures, it is caused to separate in the dust filter and leave it with other dust so that it does not spread to the environment. The desulphurisation waste separated from the flue gases in this way can be treated in many different ways, which are generally known per se.

By this method, the sulfur dioxide content of the flue gases of the high-sulfur heavy fuel oil can be advantageously reduced to a level corresponding to the sulfur dioxide content of the flue gases produced in the combustion of the low-sulfur fuel oil. If limestone powder injected into the boiler is used as the absorbent, which is converted to calcium oxide in the boiler, the heating surfaces of the boiler are sooted, for example, with sonicators. Due to the increase in the amount of ash, the boiler must be equipped with a bottom hopper and ash removal. The application of the method according to the invention to oil combustion does not prevent the dispersion of water and additives used in the oil to enhance combustion and reduce nitrogen oxide emissions. The invention has been described by way of example in the foregoing description and drawings and is in no way limited thereto. As the condenser, another condenser unit may be used instead of a water-based condenser, or both may be used together. Instead of granules, spheres or other similar suitable material can be used in the particulate filter. In addition to the lime being fed separately into the flue as shown in Figure 1, the lime can also be fed to a refiner 13, where it is thus mixed with the recyclable material. Alternatively, not only some of the lime but also all of the lime can be fed through the refiner 13, whereby both the new lime and the recyclable lime material are fed into the flue from the same or the same places.

Claims (12)

1. A process for the purification of flue gases from a furnace from sulfur oxide and heavy metals, in which process lime is measured to the flue gases and cooled, whereupon the resulting dusty reaction result is separated from the flue gases, characterized in that the flue gases are cooled off. , preferably below 100 ° C, and the flue gases are passed through a continuously rotating particle filter (3), the dust-like material present in the flue gases being separated from the flue gases and remaining in the particle filter for some of the time and being removed while the rotation is continued. that the lime-containing dust material remaining in the particle filter 15 acts as an additional filter, causing a further reaction with the nitrogen oxide remaining in the flue gases, so that the rotating particle filter also acts as a reactor, where the flue gases are forced through the lime . 20 2. A process according to claim 1, characterized in that the flue gases are cooled by adding water to them. 3. A process according to claim 1 or 2, characterized in that the flue gases are cooled with a separate cooler. 4. A process according to any of the preceding claims, characterized in that part of the dusty material which is separated from the flue gases is returned to the flue gases. 30 5. A method according to claim 4, characterized in that the material to be returned is ground so that the surface of the lime particles is decomposed to obtain the unreacted quay. 6. A process according to claim 4 or 5, characterized in that in the material to be returned water is mixed either separately or in connection with the grinding. 7. Plant for the purification of flue gases from an oil boiler from sulfur dioxide and heavy metals, in which plant there are means (4, 5) for supplying lime to the flue gases, means for cooling the flue gases and a separator (3). ) to separate the formed dusty reaction result from the flue gases, characterized in that the separator (3) is a continuously rotating particle filter (3) having at least one disc filter (34) having multiple sectors (35) filled with particulate pieces, wherein the flue gas is conveyed to the outer surface of each disc (34), from which it passes through the particle sectors (35) and is removed on the other side of the particle sectors (35), and the dusty material present in the flue gases is separated from the flue gases. a portion of the time remains in the particle filter and is removed while the rotation is constantly continuing and the calcareous dust material remaining in the particle filter acts as an additional filter; causes a further reaction with the nitric oxide left in the flue gases so that the rotating particle filter also acts as a reactor where the flue gases are forced to pass through the lime material. Installation according to Claim 7, characterized in that, as the disk filter (34) rotates, each particle sector (35) in turn falls adjacent a separating chamber (39) located below the disk filter (34), wherein compressed air stroke can be directed to the same to trap the dust collected in the sector (34) into the separation chamber (39). 9. An installation according to claim 7 or 8, characterized in that it includes means (11) for returning three of the material separated from the flue gases back to the flue gases in front of the particulate filter (3). Plant according to claim 8, characterized in that it includes a grinding apparatus (13) for grinding the material to be returned. 11. Plant according to claim 10, characterized in that it includes means (12) for supplying water to the material to be returned, either separately or in connection with the grinding.