The invention relates to a method for purifying the flue gases of an oil burner from sulphur oxide and heavy metals, wherein lime is fed into the flue gases and the gases are cooled, whereafter the obtained dust-like reaction product is separated from the flue gases. The invention is also concerned with an apparatus for purifying the flue gases of an oil burner from sulphur dioxides and heavy metals, the apparatus comprising means for feeding lime into the flue gases, means for cooling the flue gases, and separating means for separating the obtained dust- like reaction product from the flue gases.

Harmful releases from the burning of heavy fuel oil mainly consist of sulphur and nitrogen oxides. When sulphur contained in fuel oil burns, it forms mainly sulphur dioxide (SO2) when the air coefficient exceeds one and the temperature exceeds 1000°C. Sulphur trioxide (SO3) is always present in small amounts in the burner when the air coefficient exceeds one. Its proportion of the sulphur oxides contained in flue gases from the burning of oil is, however, very small when the temperature of flue gases is reduced rapidly in the burner. To avoid cor¬ rosion problems caused by it, small amounts of alkali earth oxides or carbonates have been injected into the burner. In combustion the nitrogen compounds contained in fuel degrade into nitrogen (N ₂ ) and nitrogen monoxide (NO) already at low temperatures. In addition, nitrogen oxides are formed at high temper¬ atures from oxygen (O2) present in the burning air and nitrogen. A small proportion of nitrogen oxides

is oxidized during combustion into nitrogen dioxide ( O^). The amount of nitrogen oxides is dependent on the amount of excess air used in the combustion and on the combustion temperature. As the amount of burn- ing air decreases and the temperature drops, small amounts of dinitrogen oxide (N2O) may also be formed in the combustion of oil.

Particle releases of heavy oil mainly comprise incombustible carbon compounds, soot and inorganic substances contained in fuel oil, among which certain heavy metal compounds are the most harmful. The more complete the combustion in the burning of oil, the lower is the dust content of flue gases and the higher is the proportion of heavy metal compounds in the dust.

Air protection requirements also apply to the flue gases of oil burners. According to directives issued in Finland, for example, the maximum permissible sulphur concentration of heavy fuel oil is 1.0% by weight S; otherwise the flue gases have to be purified from sulphur dioxide to a level corresponding to the sulphur concentration of the flue gases formed in the combustion of low-sulphur, 1.0% by weight S, heavy oil, that is, about 500 mg/MJ. In the future, the reduction of releases will be necessary, wherefore the removal of sulphur oxides from flue gases will become compulsory in practice. To reduce the release of nitrogen oxides, it has been suggested in Finland that the nitrogen oxide releases of burners using heavy fuel oil below 150 MW should be no more than 130 mg/MJ. Correspondingly, it is re¬ quired that the dust content of flue gases should be below 40 mg/MJ for burners having a fuel capacity of 5-50 MW and using heavy fuel oil. The need to purify flue gases increases

substantially the cost of energy production especial¬ ly for burners intended for the production of heat and the yearly operating time of which during the cold season remains short. In principle, the removal of sulphur dioxide from the flue gases of oil burners can be performed by any of the known methods for the removal of sulphur dioxide presently in use. The application of these methods, however, is limited by the high cost of investment, the proportion of which of the total cost increases with decreasing plant size and decreasing yearly operating time. For this reason, it can be regarded that methods suitable for the removal of sulphur dioxide in oil burners are technically simple and do not have a high cost of investment. Examples of simple prior art techniques presently in use are the LIFAC process, the half-dry DRY PACK and CDAS processes, and sulphur dioxide removal processes based on fluidized bed of lime. An obstacle to the application of all these processes is the excessively high cost of investment.

In the LIFAC process, powdered lime stone is blown into the furnace of a burner, where it burns into lime and binds part of the sulphur dioxide con¬ tained in the flue gases onto the surface of the lime particles. After the burner, water is sprayed into the flue gases in a separate reactor so as to moisten them, and so the sulphur binding capacity of lime is improved and more sulphur dioxide is bound to the lime. The sulphur dioxide removal product so obtained is dry dust. It is usually separated from the flue gases by an electric filter together with the ashes formed from the fuel. In the purification of the flue gases of the oil burner, it is possible to use a mod¬ ification of the LIFAC process, in which lime already burned is blown directly into the moistening reactor

instead of feeding lime stone powder into the burner. In this way the contamination of heat surfaces by the lime stone powder is avoided. The sulphur dioxide removal degree of 60% required from the flue gases of heavy fuel oil is achieved with a lime-sulphur molar ratio 2 when the flue gases are cooled to a temper¬ ature slightly below 70 ^C C by spraying water.

The CDAS process is a simplified modification of the DRYPAC process employed by carbon power plants, where water and dry calcium hydroxide powder are sprayed into the flue gases in a special reactor in which the water dries and acid compounds such as sulphur dioxide are bound to the calcium hydroxide.

Dust contained in the flue gases and containing the calcium hydroxide and the acid compounds bound to it is removed by means of a textile filter. The binding of sulphur dioxide continues in the dust cake accumulating on the surface of the dust filter in the form of a solid bed. The use of calcium hydroxide in the CDAS process is reduced and its efficiency is improved by recycling part of the dust recovered by the textile filter back to the reactor, as such or suspended in water similarly as in the conventional half-dry process.

In the fluidized-bed process, the flue gases are passed through a fluidized bed containing powdery lime or calcium hydroxide. Steam is blown into the lime bed through nozzles, which steam activates the lime and improves its ability to absorb sulphur di¬ oxide. The fluidizing medium in the fluidized bed may be some coarse material, such as quartz sand, which does not follow the powdery material leaving the reactor into the dust separation. Flue gases leaving the fluidized bed are

purified from lime by conventional dust separation means, cyclones, electric filters and textile filters.

Directives concerning the releases of nitrogen oxides in the combustion of oil can be met by adjust¬ ing the burning process. The use of step-wise burning and step-wise introduction of burning air and fuel is technically easier in the burning of oil than in the burning of carbon. In the burning of oil the use of water dispersion decreases the drop size of the oil to be sprayed into the burning process, thus im¬ proving the efficiency of oil burning, which, in turn, makes it possible to decrease the air co¬ efficient in the burning process, and so the result- ing releases of nitrogen oxides are reduced. The effect of small amounts of water on the temperature of the flame is insignificant; on the other hand, nitrogen oxide releases decrease with increasing amounts of dispersed water. As for the chemical composition of the dust particles contained in the flue gases, it is necess¬ ary to pay attention to the element composition of the compounds contained in it so as to obtain information on the harmfulness of the dust. The most important harmful metal aerosols are metals evaporating at high temperatures and disadvantageous to the environment, such as quicksilver, cadmium, vanadium and arsenic. Among conventionally used flue gas purification means, both electric filters and textile filters, in which dust forms a solid layer on the filter texture, can be used for the purification of small particles.

A disadvantage of the known techniques is that their purifying capacity is poor, or at small plants in particular, they are usually unreasonably expens-

ive in view of the size of the plant and thus its energy production. The object of the present inven¬ tion is to provide a method and an apparatus for the purification of the flue gases of oil burners by means of which the purification can be performed in a technically and economically suitable way and suf¬ ficiently efficiently. 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 dust-like material contain¬ ed in the flue gases is separated from the flue gases. The apparatus according to the invention, in turn, is characterized in that the separating means is a rotating grain filter comprising at least one disc filter with several sectors filled with granular bodies, the flue gases being passed to the outer sur¬ face of each disc from where it passes through the grain sectors and is removed from the other side of the grain sectors.

The basic idea of the invention is that the flue gases are passed through a rotating grain filter serving as a dust filter, and so the flue gases pass through a moving dust layer absorbing sulphur di- oxide, which makes the binding of sulphur dioxide to calcium oxide more effective and at the same time improves the degree of utilization of lime. At the same time the heavy metals and other particles con¬ tained in the flue gases remain in the dust filter, from which they are removed with the lime dust so that they will not escape into the surroundings.

The invention will be described in more detail in the attached drawings, in which

Figure 1 shows schematically an apparatus applying the method according to the invention; and

Figures 2a and 2b show schematically a rotating grain filter as viewed in two different directions.

Figure 1 shows schematically an oil burner 1 from which flue gases forming in connection with burning are passed onwards through a duct 2. The flue gases are further passed through the duct towards a dust separator 3, whereby lime oxide from a lime silo 4 is mixed with the flue gases flowing in the duct 2 by means of compressed air supplied by a compressor 5. Further, water is introduced into the flue gases in a cooler 6 from which the flue gases are passed into a rotating grain filter 3. From the grain filter 3 the flue gases are passed through grain separators provided in the filter in a manner to be explained in more detail below, so that dust remains in the separ¬ ators and at the same time sulphur dioxide contained in the flue gases reacts with the lime mixed with the separator material. From the dust separator the flue gases are further passed on through a duct 7 into a chimney 8. The reaction waste separated from the dust separator, that is, the rotating grain filter, and the heavy metals contained in it are removed through a cyclone and passed by means of compressed air supplied by the compressor 9 through a duct 10 into an intermediate container 14 for waste, from which it is further passed on. Part of the separated dust can be recycled through a duct 11 into the duct 2 to make the use of unreacted lime more efficient, whereby water can be added to it in a mixer 12 so as to con- vert lime into hydroxide, thus making it more reactive.

In the method, flue gases containing sulphur oxide, dust and heavy metal particles or dust pass from the burner 1 through the duct 2 onwards, whereby dust-like lime is first mixed with it and it is cool-

δ ed e.g. by introducing water into it so that the tem¬ perature drops preferably below 100°C. Sulphur di¬ oxide contained in the flue gases reacts with calcium oxide to form calcium sulphite, which is gradually oxidized into calcium sulphate.

(1) CaO + S0 ₂ - CaS0 ₃

(2) CaS0 ₃ + 0.5 0 ₂ = CaS0

The formed dust-containing flue gas is further passed into a rotating grain filter 3, in which it passes through grain sectors. Unreacted sulphur dioxide thereby reacts with the lime contained in the sectors to form solid dust-like waste while the rest of the dust, including heavy metals, also remains in the filter, from which the pure flue gases passed through the filter are further passed on. Part of calcium oxide always fails to react, and on the other hand, calcium sulphite is formed on the surface of the cal- cium oxide particles, which prevents or retards the reaction of the encased lime. In certain cases it is thereby advantageous to recycle part of the dust removed in the dust separator and, if required, grind it with a separate grinder 13 so that the particle shell is broken, and unreacted calcium oxide is re¬ vealed, whereafter water is mixed with the dust to be recycled to convert it into a more active hydroxide. Water can be mixed either separately as mentioned above, or it can be mixed with it in the grinder. The sulphur dioxide absorption ability of the material to be recycled can be improved by moistening it under efficient mixing e.g. in the grinder by means of water or steam, whereby the calcium oxide is converted into calcium hydroxide and so it reacts in the dust separator efficiently with sulphur dioxide.

(3) CaO + H ₂ 0 = Ca(0H) ₂

(4) Ca(OH) ₂ + S0 ₂ = CaS0 ₃ + H ₂ 0

Figures 2a and 2b show a rotating grain filter as a sectional viewed in the axial direction of the rotor and as view in the axial direction of the rotor. The grain filter 3 is provided with a filter chamber 31 within which a rotor 32 for the grain filter rotates. The rotor is positioned in the upper portion of the filter chamber 31, into which the flue gases are passed tangentially through a duct 33 ,as shown in Figure 2b. The rotor 32 comprises several disc-like grain filters 34 each having two mutually spaced discs 36 containing sector-shaped chambers 35, the outer edges of the discs being closed so that penetration between the discs 36 in the peripheral direction is not possible. There may be provided one or several discs according to the requirements, even though Figures 2a and 2b show a grain filter com¬ prising several discs. Correspondingly, the space between the discs 36 communicates with a duct 37 po¬ sitioned around the shaft of the rotor 32, the flue gases flown from the outer surface of the disc 36 through it being dischargeable through this duct. To increase the capacity, several such disc-like grain filters are provided side by side on the shaft of the rotor 32, whereby the flue gases are able to enter between adjacent filters and then further through their discs so as to be discharged into a discharge channel 37. Each sector 35 of the disc filter con¬ tains granular material which separates the dust from the flue gases. Since dust is accumulated between the granular material, thus forming a dust layer, the flue gases to be purified have to pass through the

calcium oxide contained in the dust layer with the result that more sulphur dioxide is separated, which further decreases the amount of sulphur dioxide con¬ tained in the flue gases. When the rotor of the grain filter rotates, its sectors reach one at a time a separation chamber 39 separated from a flue gas entrance chamber by partition plates 38, and then the dust accumulated in the sectors can be removed e.g. by means of a pressure impact of compressed air created within each sector through a duct 40, whereby the dust drops into a lower cyclone 41 and can be removed from it at regular intervals, for instance.

The material contained in the sectors of the grain filter may be granular or consist of various balls or the like. As heavy metal waste is primarily dust-like at a low temperature, it can be separated in the dust filter and removed from it together with the rest of the dust so that it does escape into the surroundings. The sulphur removal waste separated from the flue gases in this way can be treated in many different ways, which are principally known per se.

When using the present method, the sulphur di¬ oxide concentration of flue gases produced by heavy fuel oil rich in sulphur can advantageously be de¬ creased to a level corresponding to the sulphur di¬ oxide content of flue gases produced in the burning of fuel oil poor in sulphur. In cases where the absorption agent is lime stone powder injected into the burner and being converted into calcium oxide in the burner, the heat surfaces of the burner are swept e.g. by means of sonic sweepers. Due to the increased amount of ashes, the burner has to be provided with a bottom hopper and an ash discharge. Dispersion of water and additives in oil to improve burning and to

reduce nitrogen oxide releases is not prevented when the invention is applied to the burning of oil. The invention has been explained by way of example in the description above and in the drawings, and it is in no way restricted to it. Instead of a cooler based on spraying water, the cooler may be a cooling unit of any other type, or they can both be used in combina¬ tion. Instead of grains, balls or other similar material suited for the purpose can be used in the grain filter. Instead of feeding the lime separately into the duct for flue gases as shown in Figure 1, it is also possible to feed the lime into the grinder 13 as well, in which it is then mixed with the material to be recycled. Alternatively, instead of part of the lime all of it can be fed through the grinder 13, whereby both the new lime and the lime material to be recycled are fed into the duct at the same point(s).