METHOD FOR CLEANING OF COMBUSTION GAS CONTAINING IMPURITIES

Abstract: The present invention relates to a method for cleaning of combustion gas containing impurities in a reactor plant, wherein in the first phase of the cleaning process the combustion gas is wetted with a wetting agent in the wetting zone (1b) of the reactor vault (1) and in the second phase of the cleaning of combustion gas to the combustion gas wetted with a wetting agent is supplied at least one additive reacting with impurities, particularly sulphur compounds. In accordance with the invention, in the third phase of the invention, the speed of the upwards directed flow of the combustion gas that is treated with a wetting agent and additive is retarded in the retardation zone (1b) of the reactor vault (1) by enlarging the horizontal cross section area of the reactor vault in the flow direction of the combustion gas. Further, in the fourth phase of the cleaning process of the combustion gases, the medium flowing speed of the combustion gas is standardised to correspond to the end speed of the combustion gas flow in the third phase of the process by forming in the reactor vault (1), after the retardation zone (1b) as an extension to it, a retention zone (1c) having a substantially standardised horizontal cross section. Subsequent to the above-mentioned phases, the combustion gases, wherefrom wetting medium drops have left, are removed from the reactor vault (1) to after treatment, particularly dust removal.

Description: Method for cleaning of combustion gas containing impurities The present invention relates to a method for cleaning of combustion gas containing impurities in a reactor plant, wherein the combustion gas to be cleaned is supplied to a lower part of a reactor vault, wherefrom it is directed to flow substantially vertically upwards, and to exit at the upper part of the reactor vault to an after-treat- ment or the like, wherein the combustion gas is wetted with a wetting agent in the first phase of the cleaning process of the combustion gas, in the second phase of the cleaning process of the combustion gas at least one additive reacting with impurities of the combustion gas, particularly with sulphur compounds and sulphur oxides, is supplied to the combustion gas that is wetted with a wetting agent, and in the third phase of the process, the speed of the upwards directed flow of the combustion gas that has been treated with a wetting agent and an additive is retarded in the retardation zone of the reactor vault by enlarging the horizontal cross-section area of the reactor vault in the flow direction of the combustion gas. A method of the above-mentioned kind is known from publication EP- 121431. In the method of the publication the supply of combustion gases is carried out to the lower part of the reactor vault, whereafter wetting of combustion gases is performed and alkali used as an additive and reacting with sulphur compounds of combustion gas is added. The combustion gas exits at the upper part of the reactor vault. However, publication EP 121431 presents no disclosure in what manner it is attended to in the process and the construction of the reactor vault that the combustion gas handled with the wetting agent and the additive reacting with impurities would have a sufficient retention time in the reactor vault. A sufficient retention time ensures for the first that the wetting of the combustion gas will be succeeded. Second, mixing of the wetted combustion gas and the additive as well as chemical reactions require a certain time. Further, sufficient drying of exiting combustion gases (removal of wetting agent drops) should be secured before the combustion gas is removed from the reactor vault. It is an aim of the present invention to present a method by which it is possible to a great extent to eliminate the drawbacks of prior art in a manner that the impurities of the combustion gas exiting the reactor vault of the reactor plant, particularly sulphur oxides that have reacted with water in connection with wetting, have reacted with the additive, and the combustion gas is free from wetting agent drops to the extent that it is possible to avoid harmful chemical additional reactions from taking place in the after-treatment of combustion gases, particularly in a filter. For attaining this purpose, the method of the present invention is primarily characterised in that in the fourth phase of the cleaning process of the combustion gases, the average flowing speed of the combustion gas flow is standardised to correspond substantially to the end speed of the combustion gas flow of the third phase of the process by forming in the reactor vault a retention zone having a substantially standardised horizontal cross section after the retardation zone to form an extension to the same, and that the combustion gases are removed from the reactor vault after the retention zone. By using the above-described solution, advantageously in combination with a filter, preferably a fibre filter, a method is obtained by which it is possible to economically resolve simultaneous removal of sulphur compounds and particles particularly, but not exclusively, in small and medium sized combustion plants (1 to 50 MW). Treating combustion gas containing sulphur oxides (SO x ) with alkali compounds, particularly with calcium hydroxides is a generally known method for binding harmful sulphur to an alkali compound. However, the reactions require that the sulphur oxides and alkali oxides, in particular calcium hydroxide, are in an aqueous solution. The structure of the reactor vault has to be designed in a manner that there is sufficient time also for these reactions to take place. On the other hand, it is not sensible to discharge combustion gas containing wetting agent drops from the reactor vault, since it is advantageous in combustion plants to add to the treatment of combustion gas a filter positioned after the reactor plant and intended for dust removal, in which filter the occurrence of acidic sulphur compounds dissolved in water is very harmful. The method according to the present invention resolves the above-described prob- lem complex in a manner that in the retardation zone which enlarges upwards in the flow direction of the combustion gases brings about a sufficient retention time for the combustion gas and thus, at the same time, a sufficient reaction time, since with an appropriate selection for the drop size the retention of the wetting agent drops is longer than the retention of the combustion gas. The retention zone forming an extension to the retardation zone ensures that the combustion gases leaving the upper part of the reactor vault do not contain wetting agent drops, so harmful sulphur compounds dissolved in water exist neither after the reactor vault in the combustion gas nor thus in the filter intended for dust removal of the combustion gas. The accompanying dependent claims describe some advantageous embodiments of the method in accordance with the present invention. The invention will be illustrated in more detail in the following specification with reference made to the enclosed drawing presenting schematically a vertical cross section of an advantageous embodiment of a reactor plant employing the method of the invention. A reactor plant employing a method of the invention comprises as its main parts a reactor vault 1 having in the lower part thereof an input channel 2 for the combustion gas and in the upper part an outlet channel 3 for the combustion gas. Further, the reactor plant comprises a supply arrangement 4 for wetting agent of the combustion gas, said arrangement comprising a wetting agent line 5 and a supply line 6 for compressed air, these having a connection with nozzles 7 in the reactor vault. Further, the lower part of the reactor vault has a rinsing water aggregate 8, a mixer 9 with an actuator 9a thereto, as well as an outlet aggregate 10 for removal of a liquid phase situated on the bottom of the reactor vault. The reactor device includes further a supply unit 11 for the additive reacting with the wetted combustion gas, said unit comprising a supply tube 12 in combination with a storage container 13 for the additive. To the supply tube 12 is connected a supply arrangement for the additive, e.g. an input for compressed air 14. The reactor vault 1 is divided into three main parts, the lowermost of which being formed by a wetting zone 1a having a connection with the input channel 2 for the combustion gas, the middle one being formed by a retardation zone 1b and the uppermost by a retention zone 1c connected to the outlet channel 3 related to a conical top 15 of the reactor vault 1. The wetting zone 1a is formed by means of the walls of the reactor vault to have a substantially cylindrical form, wherein the wetting nozzles 7 are placed in the upper part of the wetting part to surround a horizontal aperture 16 having a contact with the retardation zone 1b. The retardation zone 1b starts at the level of said aperture 16 and continues upwards as a conically enlarging part. As can be discovered in the drawing, the horizontal cross-section area of the aperture 16 is smaller than the horizontal cross-section area of the wetting vault 1a, wherein in the upper part of the wetting zone 1a, around the aperture 16 is formed an annular cam structure 16a, having said wetting nozzles 7 placed in its lower surface, wherein they are positioned covered in a manner that layers K possibly draining down the walls of the retardation zone 1b pass the wetting nozzles 7 without damaging them when falling to the bottom of the wetting zone 1b to be removed via the outlet aggregate 10. In the described embodiment, the retardation zone 1b is formed to open upwards in a conical manner, but it is substantial to the general structure of the retardation zone 1b in the purpose of the invention that the horizontal cross-section area of the retardation zone 1b enlarges in the flow direction of the combustion gas upwards. In the combustion gas wetted with wetting agent, the additive is supplied centrally at the level of the aperture 16 or directly thereabove substantially downstream in the flow direction of the combustion gas, i.e. upwards in a manner that the additive mixes with the wetted combustion gas as evenly as possible. As an extension to the retardation zone in the upper part of the reactor vault there is a retention zone 1c, having a substantially constant horizontal cross-section area. The retention zone 1c is advantageously formed to be a cylindrical part having a diameter corresponding to the upper diameter of the conical form piece of the retardation zone 1b, i.e. the size and form of the aperture 17. Subsequent to the retention zone 1c, the top 15 of the reactor vault 1 is formed by a gathering cone in which the outlet channel 3 for the combustion gases is coupled in a manner that it is directed preferably upwards and centrally with the centre line of the retardation zone 1b and retention zone 1c. The method operates according to the following manner. In the first phase of the cleaning process the combustion gas that comes to the reactor vault 1 via the input channel is wetted with wetting agent par- ticularly with water brought about into shower form by compressed air. The size range of the water drops is medium, which means that the drops cannot be too small, wherein the wetting (absorption) of the combustion gases is of too brief duration the combustion gas dries too fast in the retardation zone 1b. On the other hand, the drop size cannot be too large, wherein the drops remain in the wetting zone and drain or fall to its bottom not finding their way either to the aperture 16 or upwards therefrom. By appropriate selection of drop size such composition for the combustion gas is obtained in the entire area of the retardation zone 1b of the reactor vault that humidity conditions necessary for attaining chemical reactions exist in the whole humidity measure h-, of the retardation zone 1b. In the lower part of the retardation zone, the second phase of the cleaning process takes place, wherein to the combustion gas wetted with the wetting agent a powder like additive is supplied from the additive-supply unit 11 , e.g. by means of compressed air. The additive is usually a suitable alkali compound, a particularly suitable compound is calcium hydroxide reacting with sulphur oxides and hydrocloric acid and other acidifying compounds. In the retardation zone, the flowing speed of the combustion gas is retarded in a manner that the ratio V SA = the initial average speed of combustion gas / V SL = the final average speed is 10 to 40. This lower- ing of flowing speed of the combustion gas in the retardation zone results, in most practical embodiments where the retardation zone 1b is conical, in the fact that an upwards opening conical angle of the conical form piece of the retardation zone is 10° to 20°, preferably about 15°, wherein the definition formula used for the angle and/or the height h- i of the retardation zone 1 b is wherein in the formula = upwards opening conical angle, h.) = height of the conical retardation zone 1 b, d 2 = diameter of the aperture 16 in the lower part of the retardation zone 1 b, and d 3 = diameter of the aperture 17 in the upper part of the retardation zone 1b. The retarding flow of the wetted combustion gas that has been treated with an additive takes place in the retardation zone 1b, upwards, wherein a long retention time is ensured for the combustion gases and thus, at the same time, a long reaction time for the reactions taking place in moist conditions required between the additive and the impurities, mainly sulphur oxides and possibly hydrochloric acid. At the same time the size of the drops is decreasing due to drying and in the retardation zone 1b, in the upwards moving combustion gas flow, the drop size is decreasing. The final drying of wetting agent drops in the combustion gas takes place in the retention zone 1c, wherein the combustion gas leaving the outlet channel 3 contains no wetting agent drops. Significant layers K are not generated in the walls of the retention zone. In case these are generated, they drain downwards along the walls of the retardation zone 1b and fall on the bottom of the reactor vault. Further, the draining slows down the downwards directing speed of the layers to the extent that they do not cause harm when falling on the bottom of the reactor vault 1. The diameters of the apertures 16, 17 in the bottom part and upper part of the retardation zone 1b are defined in the formulas: wherein d 2 = diameter of the aperture (16) in the lower part of the retardation zone (1b), d 3 = diameter of the aperture (17) in the upper part of the retardation zone (1b), m = mass flow (kg/s) of the combustion gas, p = total pressure (bar) of the combustion gas, θ-| = initial temperature (°C) of the combustion gas, θ 2 = end temperature (°C) of the combustion gas, D 50 = median diameter (μm) of the drops. The height of the retention zone 1c is defined by using the formula , 4 -G2 - G3 . .. π-d 3 wherein the terms G2 and G3 are defined in the following manner: F2 g, - 0ι ln (6), W. F3 = — 2,858 - (273+ 2) - 10 "3 (7), Gl = Fl / F3 (8), G2 = 5,0- G1 (9), G3 = F3 (273 + ?2) (10), (273 + E2) wherein in the formulas h-| = height of the conical retardation zone (1b), d 2 = diameter of the aperture (16) in the lower part of the retardation zone (1b), d 3 = diameter of the aperture (17) in the upper part of the retardation zone (1b), m = mass flow (kg/s) of the combustion gas, p = total pressure (bar) of the combustion gas, Θ-) = initial temperature (°C) of the combustion gas, θ 2 = end temperature (°C) of the combustion gas, D 50 = median diameter (μm) of the drops. The cleaned combustion gas is directed to a filter arranged for dust removal of the combustion gas, particularly to a fibre filter (not shown), where the filtering of particulate dust of the combustion gas, which does not contain wetting agent in drop form, takes place. Thus, the method contains an operational entirety fulfilling the following operational requirements: I) Large wetting agent drops do not reach the retardation zone 1b, π) Medium size wetting agent drops join the combustion gas flow, wet the combustion gas and confront the additive, HI) The combustion gas in the upper part of the reactor vault contains no wetting agent drops. As to the requirements I to III, the requirement I is fulfilled by the measuring formula (2) for the aperture 16 in the lower part of the retardation zone 1b. In a corresponding manner, II is fulfilled by the measuring formula (3) for the aperture 17 in the upper part of the retardation zone 1b. The requirements I and II involve also the formula (1), on the basis of which, after the measuring of the apertures (i.e. d 2 and d 3 ), the facients of the product h-, • tan , i.e. h 1 and are defined. Further, the definition formula for the height of the retention zone 1c fulfils the requirement III.

Claims: Claim: 1. Method for cleaning of combustion gas containing impurities in a reactor plant, wherein the combustion gas to be cleaned is supplied to a lower part of a reactor vault (1), wherefrom it is directed to flow substantially vertically upwards, and - to exit at the upper part of the reactor vault (1) to an after- treatment or the like, wherein the combustion gas is wetted with a wetting agent in a wetting zone (1a) of the reactor vault (1) in the first phase of the cleaning process of the combustion gas, in the second phase of the cleaning process of the combustion gas at least one additive reacting with impurities of the combustion gas, particularly with sulphur compounds and sulphur oxides, is supplied to the combustion gas that is wetted with a wetting agent, and - in the third phase of the process, the speed of the upwards directed flow of the combustion gas that has been treated with a wetting agent and an additive is retarded in the retardation zone (1b) of the reactor vault (1 ) by enlarging the horizontal cross-section area of the reactor vault in the flow direction of the combustion gas, characterised in that in the fourth phase of the cleaning process of the combustion gases, the average flowing speed of the combustion gas flow is standardised to correspond substantially to the end speed of the combustion gas flow of the third phase of the process by forming in the reactor vault (1 ) a retention zone (1c) having a substantially standardised horizontal cross section after the retardation zone (1b) to form an extension to the same, and that the combustion gases are removed from the reactor vault (1) after the retention zone (1c). 2. A method according to claim 1 , characterised in that the flowing speed of the combustion gas is retarded in the retardation zone (1b) in a manner that the ration V SA /V SL * 10 to 40, wherein V SA = the initial medium speed of the combustion gas and V SL = the end medium speed of the combustion gas. 3. A method according to claim 1 , characterised in that in the first phase of the cleaning process the wetting of combustion gas is performed in a mainly cylindrical wetting vault (1a) situated in the lower part of the reactor vault (1), the wetted combustion gas is directed out of the wetting vault (1b) via an aperture (16) having a horizontal cross-section area smaller than the horizontal cross-section area of the wetting vault (1a), wherein the area of the plane surface of said aperture forms a bottom part of the retardation zone (1b). 4. A method according to claim 1 or 3, characterised in that to the combustion gas wetted with a wetting agent is supplied at least one additive that reacts with impurities of the combustion gas, particularly sulphur compounds in the bottom part of the retardation zone (1b) substantially in the flow direction of the combustion gas. 5. A method according to claim 1 , characterised in that the diameters of the apertures (16, 17) in the bottom part and the upper part of the retardation zone (1b) is formulated preferably wherein d 2 = diameter of the aperture (16) in the lower part of the retardation zone (1b), d 3 = diameter of the aperture (17) in the upper part of the retardation zone (1b), m = mass flow (kg/s) of the combustion gas, p = total pressure (bar) of the combustion gas, θ = initial temperature (°C) of the combustion gas, θ 2 = end temperature (°C) of the combustion gas, D 50 = median diameter (μm) of the drops. 6. A method according to claim 1 , characterised in that the retardation zone is preferably formed to be conical, wherein the upwards opening cone angle is 10° to 20°, preferably about 15°, wherein the definition formula used for the angle and/or the height h 1 of the retardation zone (1b) is preferably tanc- = ( ^ 3 dl (1 ), wherein in the formula α = upwards opening conical angle, hi = height of the conical retardation zone h 1 , d 2 = diameter of the aperture 16 in the lower part of the retardation zone 1b, and d 3 = diameter of the aperture 17 in the upper part of the retardation zone 1b. 7. A method according to claim 1 or 5, characterised in that the retention zone (1c) is substantially formed to be a cylindrical part whose diameter corresponds substantially to the diameter of the aperture (17) in the upper part of the conical form piece of the retardation zone (1 ). 8. A method according to claim 1 , characterised in that the height of the retention zone (1c) is preferably defined by using the formula 4-G2- G3 = π-d\ (4), wherein the terms G2 and G3 are defined in the following manner: _ 0, -0 2 E2 = (6), E3 = : -• 2,858 •(2734 F2) 10 ~3 (7), G1 = F1 / 3 (8), G2 = 5,0-G1 (9), G3 = F3- (273+ ^ 2) (10), (273 + F2) wherein in the formulas \ - height of the conical retardation zone (1b), d 2 = diameter of the aperture (16) in the lower part of the retardation zone (1b), d 3 = diameter of the aperture (17) in the upper part of the retardation zone (1b), m = mass flow (kg/s) of the combustion gas, p = total pressure (bar) of the combustion gas, θ-| = initial temperature (°C) of the combustion gas, θ 2 = end temperature (°C) of the combustion gas, D 50 = median diameter (μm) of the drops. 9. A method according to claim 1 , characterised in that a gathering cone forming the top (15) of the reactor vault is formed after the reten- tion zone, to which cone a removal channel (3) of the combustion gas is joined, preferably to direct upwards and centrally to the centre line of the retardation zone (1b) and retention zone (1c). 10. A method according to claim 1 , characterised in that the com- bustion gas is directed after the reactor vault to a filter for dust removal, preferably a fibre filter.

Inventors: AHLBECK JARL, BJONDAHL FREJ, HUOTARI JUHA, ISAKSSON JAN, MUSTONEN ERKKI, VIHMA MAIJA, VIRTANEN TARJA, AHLBECK, JARL, BJONDAHL, FREJ, HUOTARI, JUHA, ISAKSSON, JAN, MUSTONEN, ERKKI, VIHMA, MAIJA, VIRTANEN, TARJA
Applicants: AHLBECK JARL, BJONDAHL FREJ, HUOTARI JUHA, ISAKSSON JAN, MUSTONEN ERKKI, PARTEK NORDKALK OY AB, SERMET OY, TAMFELT OYJ ABP, VIHMA MAIJA, VIRTANEN TARJA, AHLBECK, JARL, BJONDAHL, FREJ, HUOTARI, JUHA, ISAKSSON, JAN, MUSTONEN, ERKKI, VIHMA, MAIJA, VIRTANEN, TARJA
Application year: 1998
Main classification: B01D53
IPC codes: B01D53, F23J15