A METHOD OF REMOVING DEPOSITS FROM THE WALLS OF A GAS COOLER INLET DUCT, AND A GAS COOLER INLET DUCT HAVING A COOLED ELASTIC METAL STRUCTURE

Abstract: A method and apparatus for introducing hot process gases via a gas inlet duct (14) into a gas cooler (12). Deposits (62) formed of particles contained in hot gases on the wall surfaces (60) of the inlet duct are removed by cooling the wall surfaces (60) indirectly. The inlet duct comprises, e.g., a cylinder (20) of an elastic double-casing structure or a spirally wound tube (72). Cooling medium is conducted through the space (25) between the cylinders or through the tube (72) to cool the inlet duct wall.

Description: A method of removing deposits from the walls of a gas cooler inlet duct, and a gas cooler inlet duct having a cooled elastic metal structure. The present invention relates to a method and apparatus for introducing hot process or flue gases through an inlet duct into a gas cooler . The method and apparatus according to the invention are especially suitable for feeding hot gases as fluidizing gas into a gas cooler provided with a fluidized bed. Hot process gases usually contain fouling components, such as fine dust and molten or evaporated components, which turn sticky when they cool and condense, thereby adhering to each other and to surfaces in contact with the gases. In this way, these fouling components may very fast grow harmful deposits on the wall surfaces in contact with the process gases. Usually, the deposits seem to accumulate most easily in the border area between the hot and the cooled surfaces. For example, gas inlets of waste heat boilers are places where such deposits usually accumulate. Consequently, the inlet becomes easily clogged unless swept at times. Sweeping as such may be difficult in those hot conditions. Furthermore, it is normally difficult to disengage the deposits accumulated in the hot inlet opening because the deposits accumulating on hot surfaces are hard and compact. In most cases, the inlet ducts are of refractory-lined construction or of ceramic material, having a slightly uneven and possibly even porous surface, which contributes to the adhesion of deposits to the surfaces. Sweeping of a refractory-lined surface may in turn damage the refractory lining. The formation of deposits has been attempted to prevent, e.g., by blowing gas which is, for example, recirculated, cooled and purified process gas, into the inlet. This prevents, to some extent, sticky compounds from adhering to the walls in the vicinity of the inlet. However, the volume of the recirculated gas has to be considerably large in order to keep the inlet clear. This enlarges the overall gas volume entering the gas cooler, which grows the dimensions of the gas cooler and subsequent gas cooling means, in other words, increases the costs. Furthermore, the efficiency of heat recovery from the gases is lowered by mixing of cooled gas with hot process gases prior to heat recovery units. An object of the present invention is to provide an improved method and apparatus for introducing hot process gases into a gas cooler in comparison with those described hereinabove. An object is especially to provide a method and apparatus by which the deposits accumulated in the hot gas inlet duct are readily removable. A still further object is to provide a method and apparatus by which the properties of the deposits accumulated in the inlet duct allow such deposits to be readily disengaged from the duct walls. A characteristic feature of the method according to the invention for introducing hot process or flue gases into a cooling chamber is that the inlet duct wall is indirectly cooled with a cooling medium by bringing the wall surface opposite to the gas side surface into contact with the cooling medium, whereby the deposits formed on the wall surface on the inlet duct gas side embrittle and become readily removable. For disengaging the deposits from the inlet duct walls, these walls are subjected to a sudden mechanical force, which causes a temporary deformation or vibration of the wall, thereby loosening the deposits accumulated on the wall surface. A characteristic feature of the apparatus according to the invention for introducing hot process or flue gases into a gas cooler is that the inlet duct of the gas cooler is formed of a cooled, elastic structure, in which the inlet duct walls are formed of cooled surfaces made of metal. The inlet duct is preferably provided with an apparatus by which the inlet duct walls may be subjected to a sudden mechanical force, which causes a temporary deformation and/or vibration of the walls. The invention is especially suitable for plants where hot process gases are cooled in a cooling chamber provided with a fluidized bed and where the hot process gas simul- taneously serves as a fluidizing gas. In this case, the inlet duct is arranged in the bottom of the cooling chamber and hot gases are introduced into the fluidized bed via an inlet arranged in the bottom of the cooling chamber. Cooling is most preferably effected in a gas cooler provided with a circulating fluidized bed, where hot gases are introcuded into a mixing chamber and mixed with recirculated, cooled particles, whereby the gases cool very fast. If the inlet duct is too short, particles may flow from the fluidized bed of the cooling chamber downwardly to the inlet duct with harmful results. Some turbulence is formed in the inlet, between the inlet duct and the cooling chamber, when the particles flowing downwardly along the cooling chamber walls meet the hot gases. The particles may thus flow downwardly into the inlet duct. From the inlet duct the particles are, however, carried away by the hot gases back to the cooling chamber provided that the inlet duct is of a certain minimum length. The ratio of the inlet duct length to the inlet duct diameter L/D has to be at least 0.5, preferably 1 to 2. For example, plants with the gas flow of 1000 - 200,000 Nm 3 /h which are equipped with an approximately 5 to 30 m high gas cooling reactor provided with a fluidized bed and having a mixing chamber with an approximately 70 cm to 6 m diameter, may have an inlet duct with a diameter of approximately 15 cm to 2 m and height of 15 cm to 2 m. The inlet duct is preferably made of such a material that provides the duct structure with a certain flexibility or elasticity. The duct structure itself may also be flexible. In accordance with a preferred embodiment of the invention, the inlet duct is formed of two metal cyliners, which are arranged one within the other and which together form a cylindrical double-casing. Between the cylinders is formed an annular slot wherethrough cooling medium is applied. The slot between the cylinders may be either undivided or divided into a plurality of separate sections. The space between the cylinders may, for example, be divided by means of vertical ribs extending from one cylinder to the other, whereby, depending on the quantity of the ribs, two or more separate vertical sections are formed between the cylinders for the cooling medium. Cooling medium may be conducted axially downstream or upstream with respect to the gas flow. As regards to its structure and material, the inlet duct comprising metal cylinders is elastic. A sudden blow of a hammer on the outer surface of the duct causes a deformation of the duct wall, and the deposits accumulated on the inner surfaces of the duct are disengaged. As it is a cooled duct, the deposits formed on its wall are brittle as such and readily disengageable. Neither do deposits attach to smooth metal surfaces as firmly as to, e.g., refractory- lined surfaces. A stiff, refractory-lined or ceramic duct construction cannot be cleaned with sudden blows of a hammer because the material itself may not be resistant to blows and because a stiff structure does not deform, which would contribute to loosening of the deposit. A blow might also cause the stiff inlet duct to come loose from either end thereof. An elastic and cooled inlet duct construction may, according to a second embodiment of the invention, be provided by employing a tube which is bended into a spiral or a snail, wherethrough cooling medium is then conducted. The various layers of the tube bended into a spiral are not fixedly attached to one another, but allow at least some movement of the layers with respect to one another. Removal of the deposits from the inner surface of the inlet duct is effected by, e.g., a blow of a hammer, which is directed to one or more layers of the tube. Consequently, this layer will move with respect to adjacent tube layers, whereby the inner surface of the inlet duct is deformed. As a result of this, the deposits attached to the duct wall come loose. The hammerblow simultaneously causes vibration of the tube, which reflects both ways along the tube in the longitudinal direction. Vibration also loosens the deposits. Water, steam, air or some other appropriate gas or liquid may be used as a cooling medium in cooled inlet ducts. In that case, also purified and cooled process gas may be used because, in itself, it does not add to the gas load. The most preferable cooling medium is, however, water e.g., because the cooling of the inlet duct may then be in connection with the water/steam circulation of the actual cooling chamber. The cooling medium may be pressur¬ ized gas or steam, in which case its heat transfer capacity is better. In that case, the inlet duct is preferably formed of a spirally wound tube, the pressure resistance whereof is higher. A cooled inlet duct according to the invention has, e.g., the following advantages: - cooling in itself embrittles the deposits accumulating on the duct walls, so they are readily removable by vibra¬ tion or deformation of the duct; - a metal duct is capable of vibrating and deforming due to a mechanical blow; - an inlet duct of metal is solid and resistant to sudden mechanical force needed for cleaning, and extra particles do not come loose of its walls unlike, for example, of refractory-lined walls; - deposits do not adhere to smooth metal surfaces as easily as to refractory-lined or ceramic surfaces; - a metal duct is light and easy to connect to the cooling chamber and the process itself; - heat may be recovered from a cooled duct. The present invention is suitable for a great variety of processes. The temperature of the gases issuing from metallurgical processes is normally 700 to 1800°C before they are conducted to the heat recovery stage, i.e., cooling, where they are normally cooled to a temperature of 350 to 1000°C, even to 100°C. The radiation chamber of metallurgical furnaces produces gases of appr. 550 to 1200°C, which are also cooled to appr. 350 to 1000°C. Limestone burning and cement kilns produce gases of appr. 800 to 1000°C, which are cooled to 300 to 500°C. Flue gases from waste incineration furnaces have a relatively low temperature; it may be as low as 300 to 700°C. Still they may contain most different fouling components, which cause trouble until they are cooled to a temperature of appr. 200 to 250°C. Some metallurgical processes also produce gases which have a relatively low temperature but which nevertheless are fouling. Such gases may contain, for example, Pb or Zn compounds melting at a low temperature, and the gases have to be cooled to a relatively low temperature until the formation of deposits is avoided. The temperature of the inlet duct cooling medium has to be always clearly lower than the eutectic temperature of the molten or vaporizing components contained in the hot gases from the process. This is inevitable for fast cooling of the fouling components which come into contact with the wall surfaces. For example, if water of 20 to 50°C is used as a cooling medium, the temperature of this water may rise to about 100°C. The lower the inlet temperature of the cooling medium, the more porous the deposits in the gas duct will be. The temperature of the cooling medium normally rises by about 20-100°C in the inlet duct. Often, however, the rise in the temperature is not more than about 20-30°C. It takes a longer time to cool the deposits in the gas duct by steam, the temperature of which is > 200°C and, consequently, the deposits in the duct become tougher than when using a cooler cooling medium. The gas temperature does not change very much in the inlet duct, usually not more than about 0.5-25°C. In the cooling chamber, cooling is effected by a circulating fluidized bed where cold particles are mixed with the gas, thereby lowering the gas temperature immediately below the eutectic temperature of the molten or vaporizing components contained in the gas. Deposits cannot therefore be accumulated on the walls of the cooling chamber. The invention will be described in greater detail in the following, by way of example, with reference to the enclosed drawings, in which Fig. 1 illustrates an inlet duct arrangement according to the invention; Fig. 2 is a sectional view of Fig. 1 taken along line A-A; Fig. 3 is a sectional view along line A-A of a second inlet duct arrangement according to the invention; Fig. 4 illustrates the second inlet duct arrangement according to the invention; and Fig. 5 is " a sectional view of Fig. 4 along line B-B. Figures 1 and 2 illustrate a cooled inlet duct 14 arranged between a process furnace 10 and a cooling chamber 12. The inlet duct is connected to an opening 16 in the roof 18 of the process furnace. The inlet duct incorporates a cylinder 20 of an elastic double-casing structure, which is composed of metal cylinders 22 and 24 arranged one within the other. The cylinders may be made from a conventional, 3 to 7 mm thick steel plate. If the cooling medium is pressurized, the cylinders have to be made from a thicker plate. An annular space 25, wherethrough cooling medium is led, is formed between the cylinders. The cooling medium is conducted into the annular space 25 via conduit 40 and is discharged therefrom via conduit 50. The gap between the cylinders is, for example, about 5 to 25 mm, preferably 10 to 15 mm wide if water is used as a cooling medium. A gaseous cooling medium calls for a larger space, in which case the slot may be as wide as 50 mm. In the annular space are preferably disposed flow control means, not shown in the Figs. Fig. 2 is a cross-sectional view of the inlet duct 14 taken along line A-A. In this embodiment, the annular space 25 is a single, undivided space for liquid, which space is preferably provided with flow control means. As shown in Fig. 1, the annular space 25 is sealed with packings 54 and 56 against the roof of the process furnace and the bottom 58 of the cooling chamber. Deposits 62 possibly formed on the wall surface 60 of the inlet duct are removed with blow means 64. The blow means comprises a hammer 68 disposed at the end of an arm 66. A blow of the hammer causes a deformation and/or vibration of the inlet duct wall. On the other hand, as shown in Fig. 3, the space for the cooling medium may be formed of separate segments. The inner side of the. double-casing structure 20 of the inlet duct incorporates, as shown in the above described Figs, a cylinder 22, whereas the outer side of the casing is composed of separate, vertical plates 26, the edges whereof are bent towards the cylinder 22 so as to form watertight segment spaces 27 between the cylinder 22 and the plate 26. Each segment has an inlet duct 28 and an outlet duct (not shown) of its own. Figs 4 and 5 show an inlet duct 14 arranged between the process furnace 10 and the cooling chamber 12, the walls 70 of the inlet duct being formed of a tube 72 bent in the shape of a spiral or a snail. The tube spiral is partly surrounded with a cylindrical pressure-tight enclosure 74. The outer diameter of the tube 72 is typically 25 to 100 mm, preferably 38 or 52 mm. The cooling medium is fed into the tube from the upper end thereof via in inlet conduit 76 and is discharged from the lower end of thereof via an outlet conduit 78. The tube 72 is so wound that it forms a flexible tube wall 80, where tubes arranged one on top of the other are not stiffly united, e.g., by welding. Various tube parts are movable with respect to adjacent tubes. Thus, small slots 82, 84 and 86 accessible to gas may be formed between the tubes, between the lowermost tube spiral and the roof of the process furnace and between the topmost tube spiral and the bottom of the cooling chamber. Hot process gas is prevented from leaking through the wall by enclosing the tube wall inside a pressure-tight enclosure or casing 74. A gas space 87 is formed between the casing and the tube construction, into which space interspace or slit gas or extrusion gas is introduced via conduit 88, the pressure of the extrusion gas being higher than that of the hot process gas, thereby preventing leakage of hot process gas. For example, purified and cooled, recirculated process gas. of e.g., 20 to 200°C or some other inert gas or air may be used as a slit gas. It is advisable to pay attention to the composition of the hot gases when the slit gas is selected. Oxygenous slit gas may be used if final combustion, if any, does not cause any trouble. In most cases, some inert gas is, however, the most appropriate choice. The volume of the slit gas is very small, and is therefore of no essential significance as to the total gas volume. The slit gas keeps the slots between the tube layers clean and may, in larger volumes, form a cool gas coat on the inner surface of the inlet duct, preventing small drops from flowing towards the wall. The slit gas thereby forms a border layer on the inner surface of the duct. If a more compact structure is desired, the ducts may be partly attached to one another with bars without binding them tightly to form a totally stiff structure. The bars may, e.g., be welded on to the lowermost and the uppermost tube, whereby the tube spiral structure will have a limited allowance in the vertical direction. The tube spiral wall may also be made of a special tube, the cross section of the outer surface of which is not circular but approaches a square. Therefore, when bended into a spiral, it provides a larger sealing surface between the tube layers and, consequently, a more tight coupling structure than a circular tube. A hammer may also be used in the arrangement according to Figs. 4 and 5 to bring about a sudden deformation of the duct wall. At the point of the hammerblow, between the enclosure 74 and the tube wall 80 is disposed a piece 90, which transmits the blow on the enclosure to a tube layer on the corresponding level. Blow hammers may be arranged opposite to each other or in several places in the duct. As a result of a blow, a spring type deformation of the duct occurs. It loosens deposits from the duct wall very effectively. Vibration reflecting in both directions of the duct contributes to loosening of the deposits. The blow hammer may be arranged inside the gas space 87, whereby the blow of the hammer directly hits the wall formed of a spirally wound tube. Sweeping may also be effected by instantaneously and in a pulse-like manner changing the pressure of the cooling medium in the duct, whereby the tube spiral tends to straighten out and vibrate, thus loosening the deposits from the duct. In some cases, it is also possible to provide a deformation of the inlet duct by heat expansion, whereby the flow of the cooling medium is temporarily slowed down, and the duct is allowed to heat, whereafter it is rapidly cooled by returning the flow rate of the cooling medium to normal.

Claims: Claims 1. A method of introducing hot process or flue gases into a gas cooler in an inlet duct, characterized in that the inlet duct wall is indirectly cooled with a cooling medium by bringing the wall surface opposite to the gas side surface into contact with the cooling medium, whereby the deposits formed on the wall surface on the inlet duct gas side embrittle and become readily disengageable. 2. A method as recited in claim 1, characterized in that the inlet duct wall is subjected to a sudden mechanical force, which causes a temporary deformation and/or vibration of the wall, disengaging the deposits formed on the wall surface. 3. A method as recited in claim 1, characterized in that the gas cooler is arranged with a fluidized bed formed of cooling particles and that the hot process or flue gases are introduced into the gas cooler as fluidizing gas via an inlet arranged in the bottom of the gas cooler. 4. A method as recited in claim 3, characterized in that the gas cooler is provided with a circulating fluidized bed. 5. A method as recited in claim 1, characterized in that the cooling medium is conveyed in the form of a jacket flow along the outer surfaces of the inlet duct walls. 6. A method as recited in claim 1, characterized in that the wall surface of the inlet duct is cooled with a cooling medium which is conducted via a spirally bended tube forming the inlet duct. 7. A method as recited in claim 6, characterized in that deposits are disengaged .from the inlet duct walls by changing the pressure of the cooling medium in a pulse¬ like manner in the spirally bended tube. 8. A method as recited in claim 6, characterized in that deposits are disengaged from the inlet duct walls by changing the temperature of the cooling medium in a pulse- like manner in the spirally bended tube. 9. An apparatus for leading hot process and flue gases into a gas cooler, comprising an inlet duct for leading gas into the gas cooler, characterized in that the inlet duct of the gas cooler is formed of a cooled, elastic structure, in which the inlet duct walls are formed of cooled surfaces made of metal. 10. An apparatus as recited in claim 9, characterized in that the inlet duct is provided with means for subjecting the inlet duct walls to a sudden mechanical force, which force effects temporary deformation and/or vibration of the walls. 11. An apparatus as recited in claim 9, characterized in that the gas cooler comprises a fluidized bed reactor and that the inlet duct serves as a fluidizing gas inlet duct leading to the fluidized bed reactor. 12. An apparatus as recited in claim 11, characterized in that the gas cooler comprises a circulating fluidized bed reactor. 13. An apparatus as recited in claim 9, characterized in tht the inlet duct is formed of two metal cylinders arranged one within the other, the annular slot therebetween forming a space for cooling medium. 14. An apparatus as recited in claim 13, characterized in that the inlet duct is formed of a metal cylinder around which vertical metal plates are fixed gas-tightly to form separate spaces in the form of a segment for cooling medium. 15. An apparatus as recited in claim 9, characterized in that the inlet duct walls are formed of a spirally wound metal tube, which defines a substantially cylindrical inlet duct, allowing the inlet duct to be cooled by leading cooling medium through the tube. 16. An apparatus as recited in claim 15, characterized in that a cylindrical duct defined by a metal tube is surrounded with a cylindrical enclosure, which forms a gas space for the slit gas around the inlet duct.

Inventors: HILTUNEN MATTI, IKONEN OSSI, HILTUNEN, MATTI, IKONEN, OSSI
Applicants: AHLSTROEM OY, A. AHLSTROM CORPORATION
Application year: 1992
Main classification: F23J3
IPC codes: F23J3, F28G5, F28G7, F28G13