WO1993014364A1

WO1993014364A1 - Method and apparatus for removing a deposit from the inlet duct wall of a gas cooler

Info

Publication number: WO1993014364A1
Application number: PCT/FI1993/000008
Authority: WO
Inventors: Matti Hiltunen; Pertti Metso; Kurt Westerlund
Original assignee: A. Ahlstrom Corporation
Priority date: 1992-01-15
Filing date: 1993-01-14
Publication date: 1993-07-22
Also published as: AU3353493A; FI91192C; FI920167A; FI920167A0; FI91192B

Abstract

Method and apparatus for removing a deposit (22) of metallic substance or salt accumulating on the wall (18, 32) of the inlet duct (14) of a gas cooler while cooling hot process or flue gases in the gas cooler. Means (24, 30, 34, 44) are provided in combination with the inlet duct wall for raising the temperature of the deposit accumulating on the wall so as to melt at least a part of the deposit. The deposit may be melted according to the induction principle, by means of an electric resistor or by means of hot gas.

Description

METHOD AND APPARATUS FOR REMOVING A DEPOSIT FROM THE INLET DUCT WALL OF A GAS COOLER

Technical Field

The present invention relates to a method and an apparatus for removing deposits of metallic substance or salt accumulating on the wall of an inlet duct of a gas cooler while cooling hot process or flue gases in the gas cooler.

Background Art

Hot process or flue gases generally contain fouling components, such as fine dust, and molten or vapourized components which when cooling and condensing become sticky and adhere to each other and to the surfaces contacting the gases. Gases containing these kinds of fouling components are, for example, exhaust gases from metal smelting plants and flue gases from combustion of salt- containing substances such as lignite or black liguor.

Deposits of fouling components seem to accumulate most easily in the boundary areas between the hot and the cooled surfaces. For example, deposits tend to accumulate in a waste heat boiler close to the inlet opening of the inlet duct with the result that the opening may be totally clogged if it is not swept from time to time. Sweeping as such may be difficult to arrange in such conditions.

Deposits accumulated in a hot inlet are usually difficult to remove as the deposits adhering to hot surfaces are hard and compact. The inlet ducts are mostly of refractory-lined constructions or made of a ceramic material and the surface is non-smooth, possibly even porous, which contributes to the adhesion of the deposits to the surfaces. Sweeping on the other hand may damage the refractory lining. There have been attempts to avoid formation of deposits by for example blowing gas, such as inert, recycled, cooled and purified flue gas, to the inlet opening. This prevents to some extent the sticky components from attaching to the walls close to the inlet opening. However, the volume of the recycled protective gas must be remarkably large to keep the opening free which decreases the temperature of the gas supplied to the gas cooler and is thus not advantageous for heat recovery. At the same time, the total gas volume increases and thus larger gas purification eguipment is reguired in the process which increases the costs.

It has also been suggested to detach the deposits accumulated in the inlet duct by indirect cooling of the surface the inlet duct wall which is not in contact with the gas by means of a cooling medium, e.g. water. This is done in. order to cool the deposits and to make them brittle. Due to the cooling the deposits are at least partly detached from the wall and are thus easier to remove from the inlet duct. However, the cooling is not always adequate to keep the wall surfaces clean. Further, detaching the deposits also requires mechanical force. Brittle deposits are fairly easily detachable by causing the inlet duct to vibrate.

However, it is not always possible to cool the inlet duct, for example because it is cumbersome to arrange the supply of a cooling medium to the duct, or it may be difficult to achieve an adequate cooling effect. Further, the mechanical sweeping of the inlet duct may be difficult to provide for.

Disclosure of the Invention An object of the present invention is to provide an improvement in the methods and apparatus described above for introducing contaminated process and flue gases to a cooling chamber.

An object of the invention is in particular to provide a method and an apparatus by means of which the deposits already accumulated in the inlet duct for hot gases are

' easily removed or by means of which the tendency to build up deposits is reduced.

A further object of the invention is to provide a method and an apparatus by means of which the accumulated deposits have properties which cause the deposits to be easily detachable or cause the deposits to detach by themselves from the wall of the inlet duct.

The method of removing deposits from the inlet duct of a gas cooler according to the invention is for achieving the objects described above characterized in that the deposit accumulating on the inlet duct wall is heated so as to melt at least a part of the deposit.

The apparatus according to the invention for removing deposits from the wall of the inlet duct of a gas cooler is characterized in that there are means provided in connection with the inlet duct wall for raising the temperature of the deposit accumulating on the wall so high that at least part of the deposit melts.

According to the invention, when cooling the exhaust gases from metal smelting plants, the deposits of metallic substances on the inlet duct wall of a cooling chamber can be, for example, heated and at least partly melted according to the induction principle. This is achieved by supplying alternating current to a copper coil provided around the inlet duct which current creates a magnetic field penetrating into the deposit to be melted and causing eddy currents in the deposit. Because of the electric resistance the metal causes the eddy current, the metal is at first heated and, when the temperature rises enough, in the end melted. The melting efficiency of the eddy currents is at the maximum close to the copper coil, in other words in the part of the deposit which is closest to the inlet duct wall. The efficiency of the eddy current also depends on the size, electrical conductivity and magnetic permeability of the metal deposit to be melted and on the frequency of the current. Once melting has started the molten metal connects different parts of the metal deposit to each other and the eddy currents can freely circulate in the whole deposit. The induction method is applicable, for example, in the treatment of substances containing copper, iron or zinc.

By using frequences higher than the network freguency, for example 500 - 2000 Hz, also small deposits can be melted in the inlet duct wall without molten metal at the beginning of the melting process. Thus, an induction coil operating at the medium or high frequency is a very flexible means of melting deposits of the type discussed here.

The copper coil can be made of a hollow copper rail where- by it can be cooled with water if necessary.

The deposits accumulated on the walls of the inlet duct can be melted, or partly melted, by heating the wall of the duct by means of an electric resistor to a desired temperature, for example to a temperature higher than the melting temperature of the components accumulating in the deposits. This can be done, for example, by providing resistors, e.g. Kanthal resistors, in the inlet duct wall close to the inner surface of the wall lined with a ceramic material. By supplying electricity through the resistors the inner surface of the wall is heated as desired in order to detach the deposits. The lining of the inlet duct can be protected against wearing, if necessary, by a coating with good heat conductivity, for example a silicon carbide coating. This method is applicable, for example, in the treatment of hot gases containing alkaline salts or process gases containing lead. An alkaline salt deposit may in many cases require a temperature of 800 - 900 °C to detach from the wall. Lead, lead oxide or lead sulfate melts at a remarkably lower temperature and already a temperature of approx. 600°C may be adequate to melt enough material in the deposit to detach the whole deposit.

The deposits may be heated periodically; thus heating for a few minutes, depending on the electric power, may suffice to provide a molten a layer between the deposit and the inlet duct wall. The deposits are thus removed with low electric power consumption when needed.

On the other hand, if desired, the temperature of the deposit may be constantly maintained at a level where part of the components accumulating in the deposits are in a molten state. The molten components keep the surfaces of the inlet duct clean and the solid components of the gases do not adhere to the walls of the duct.

In some cases the temperature of the inlet duct can be raised high enough by supplying hot gas to the duct. For example in smelting processes of metals which melt at a low temperature, such as lead, a relatively small raise in the temperature may be adequate to detach the accumulated deposits.

In order to supply gas into the inlet duct, the duct or a part of it may be made of a porous material, such as porous ceramic substance, sinter. A gas chamber is provided outside and around the inlet duct from which gas is introduced through the porous material to the inlet duct at a higher pressure than the pressure in the inlet duct.

If the required raise in the temperature is fairly small, the hot gas may be for example superheated pressurized steam.

On the other hand, gas which causes combustion in the duct and thus raises the temperature may be introduced into the inlet duct through the porous wall. For example,, if the process gas flowing in the duct contains combustible components, oxygen may be introduced through the porous wall to burn the process gas in the duct. The reaction of the oxygen gas and the process gas in the duct, i.e. combustion, begins in the duct at the wall and thus it raises the temperature at first in the vicinity of the wall surface. By increasing the oxygen, amount supplied the combustion may be extended further towards the center of the duct.

If gases containing oxygen are cooled in the gas cooler, combustible gas, such as natural gas or liquid gas, can be introduced into the duct through the porous wall. Also in this case the combustion takes place primarily close to the porous wall surface and, only when the gas volume is increased, in the center of the duct.

The efficiency of the gas supplied through the porous wall in removing the deposits is increased by its simultaneous function as a protective layer between the inlet duct wall and the hot process gas flow whereby the fouling components in the process gas cannot adhere to the wall surfaces as easily as without the protective gas.

Brief Description of Drawings

The invention is described below, by way of example, with reference to the accompanying drawing of which:- Fig. 1 illustrates schematically an inlet duct according to the invention;

Fig. 2 illustrates schematically another inlet duct according to the invention; and Fig. 3 illustrates schematically a third inlet duct according to the invention.

Detailed Description of Preferred Embodiments

Figure 1 illustrates an inlet duct 14 disposed between the top 10 a melting furnace for waste iron and the bottom 12 of a gas cooler operating according to the circulating fluidized bed principle. Gases exhausted from an iron melting plant usually contain at the outlet of the furnace iron splashes and other solid molten and vaporized substances. The temperature of the gas at the inlet of the inlet duct 14 is approx. 1500 - 2000°C.

The velocity of the hot gases in the duct varies depending among other things on the size of the inlet duct and the fluidized bed of the cooling reactor and it can be 5 - 150 m/s. The inlet duct is often tubular and its diameter may be 15 cm - 2 m and the height for example 15 cm - 2 m, depending on the gas volume.

In the bottom portion of a cooling reactor operating on the circulating fluidized bed principle the gases cool almost immediately down to a temperature of approx. 600 - 1000°C when they contact the cold circulating particles. The cooling effect of the cooling reactor extends also to a portion of the inlet duct and the fluidized bed in the cooling reactor cools the gases at the outlet of the inlet duct to a temperature of approx. 650 - 1200°C. Although the gas flow in the inlet duct is adequate to prevent the bed material of the fluidized bed from flowing down from the cooling reactor to the melting furnace a small amount of bed material particles may run down a distance in the inlet duct before the gas flow again transports them up to the bed. Thus, the bed material has a cooling effect also in the inlet duct and deposits 22 originating from the components of the process gas accumulate on the wall surfaces 20 of the inlet duct walls 18 contacting the process gas. The accumulation of the deposits is heaviest in the upper portion 16 of the duct.

In order to remove the deposits a part of the material in the deposits is melted by raising the temperature of the deposit to approx. 1000 - 1600°C which melts an adequate portion of the components of the deposit to detach the deposit from the wall. In the embodiment illustrated in Fig. 1 the temperature is raised according to the induction principle.

The inlet duct is surrounded by a copper coil 24 wound around the duct in a spiral manner and connected to an electricity source (not illustrated in the Figure) . Yokes 26 formed by transformer sheets are provided outside the coil to reduce the resistance of the magnetic circuit outside the coil and thus to improve the efficiency of the coil.

When alternating current is supplied to the coil a magnetic field is created which penetrates into the deposit 22 and causes eddy currents in the metallic material of the deposit. The resistance thus created in the metal raises the temperature of the metal which in turn raises the temperature of the other material surrounding the metal. The component of the deposit having the lowest melting temperature melts first. A deposit which has melted only partly usually detaches from the walls. For example, a material with 75 % of its components in a molten state often behaves like a fully molten material and a material having only 10 - 40 % of its components in a molten state may detach from the walls of the duct. Figure 2 illustrates another embodiment of heating the inlet duct in which hot gases from a copper melting plant 10 are introduced into a gas cooler 12 operating according to the circulating fluidized bed principle. The temperature of the gas in the inlet portion of the inlet duct is approx. 1200 - 1400°C and in the outlet portion 16 of the inlet duct approx. 450 - 700°C. In the gas cooler the gases are cooled down to approx. 400 - 500°C. The inlet duct 18 is made of a ceramic paste and resistors 30 are embeded in its upper portion close to the inner surface 28 of the inlet duct which resistors are heated by supplying electric current through them. By supplying the electric current either continuously or intermittently, as required, the inner surface of the inlet duct can be heated to a temperature of > 1000°C, for example 1000 - 1200°C, whereby the copper and copper oxide in the deposit 22 accumulated on the surface of the duct begin to melt and detach.

If necessary the inner surface 28 of the duct 18 can be protected by a wear-resistant protective layer or a protective coating 32 the heat conducting ability of which is so good that it does not substantially slow down the transfer of heat from the resistors 30 through the lining and the protective layer 32 to the deposit 22. The wear- resisting layer may be made, for example, of silicon carbide.

Figure 3 illustrates an embodiment of the invention according to which flue gases from, for example, a combustion process 10 are introduced via an inlet duct 14 to a circulating fluidized bed cooler 12. The wall 18 of the inlet duct is made of a porous material permeable to gas. A gas chamber 34 is provided outside the wall into which chamber gas is introduced via a duct 38 from a gas source 36. The duct 38 is provided with a pump 40 for increasing pressure, and a valve 42. Gas flows from the gas chamber through pores 44 of the wall 18 to the inlet duct 14 and contacts there both the deposit 22 accumulated on the wall and the hot flue gas flowing in the vicinity of the duct wall surfaces 20.

Deposits may accumulate on the walls of the inlet duct from the flue gases, for example chlorides, carbonates and sulfates from alkaline salts. Their melting temperature is approx. 750 - 900°C. Flue gases arrive from the combustion process at a temperature of approx. 800 - 1000°C. When the temperature in the cooling reactor is 350 - 500°C the flue gases cool down already in the inlet duct partly to a temperature below the melting point of the alkaline salts and deposits begin to accumulate on the walls 20 of the duct.

In order to raise the temperature of the duct walls and the accumulated deposits, oxygen-containing or combustible gas, depending on the composition of the flue gases, is introduced into the gas chamber so as to achieve incineration when the gas flowing to the duct through the pores mixes with the flue gases.

The invention is not intended to be limeted by the embodiments described above by way of example only, but it may be modified and applied within the scope of protection defined by the patent claims.

Claims

We claim:

1. Method of removing a deposit of metallic substance or salt accumulating on the inlet duct wall of a gas cooler when cooling hot process or flue gases in a gas cooler, characterized in that

- the deposit accumulating on the inlet duct wall is heated in such a manner that at least a portion of the deposit is caused to melt.

2. Method according to claim 1, characterized in that the deposit is heated in such a manner that at least a partly molten layer is formed between the deposit and the wall.

3. Method according to claim 1 or 2, characterized in that the inlet duct is subjected to a mechanical force to cause vibration whereby the deposit is detached from the inlet duct wall.

4. Method according to claim 1 or 2, characterized in that the deposit is melted by induction heating.

5. Method according to claim 4, characterized in that electric current of medium frequency is supplied through the copper coil surrounding the inlet duct in order to create a magnetic field in the deposit.

6. Method according to claim 5, characterized in that the freguency of the electric current is 500 - 2000 Hz.

7. Method according to claim 1 or 2, characterized in that the deposit is heated and melted by supplying electric current through a resistor, for example a Kanthal resistor, disposed in the wall of the inlet duct or in the vicinity of said wall.

8. Method according to claim 7, characterized in that electricity is conducted through a resistor in order to achieve a predetermined temperature in the deposit.

9. Method according to claim 1 or 2 , characterized in that hot gas or steam is supplied through the porous wall of the inlet duct to contact the deposit in order to raise the temperature of the deposit.

10. Method according to claim 9, characterized in that superheated high pressure steam is supplied through the wall to melt the deposit which melts at a low temperature.

11. Method according to claim 1 or 2, characterized in that gas which burns when it contacts the gas flowing in the inlet duct is supplied through the porous inlet duct wall in order to achieve combustion and to raise the temperature in the inlet duct.

12. Method according to claim 11, characterized in that oxygen is supplied through the porous wall of the inlet duct in order to burn the gas flowing in the inlet duct, to raise the temperature in the duct and to melt the deposit accumulated on the wall.

13. Method according to any of the preceding claims, characterized in that the temperature in the inlet duct is raised intermittently in order to remove deposits.

14. Method according to any of the preceding claims, characterized in that a temperature is continuously maintained in the inlet duct which temperature is able to melt at least a part of the deposits accumulated on the wall.

15. Apparatus in connection with the inlet duct (14) wall (18) of a gas cooler, for removing a deposit (22) of metallic substance or salt accumulating on the inlet duct i (14) wall, when cooling hot process or flue gases in a gas 5 cooler, characterized in that the inlet duct wall is provided with means (24, 30, 34, 44) for raising the temparature of the deposit accumulating on the wall so that at least a portion of the deposit melts.

10 16. Apparatus according to claim 15, characterized in that the means for raising the temperature comprises a coil (24) at least partly surrounding the inlet duct through which coil electric current can be supplied in order to create a magnetic field in the deposit.

15

17. Apparatus according to claim 16, characterized in that a water-cooled copper coil (24) surrounds the upper portion of the inlet duct.

20 18. Apparatus according to claim 15, characterized in that the means for raising the temperature comprise a resistor (30), for example a Kanthal resistor, disposed in connection with the inlet duct by which the deposit accumulating on the wall can be heated and partly melted.

25

19. Apparatus according to claim 18, characterized in that the coil is disposed in a heating member formed of a ceramic paste disposed tightly in connection with the inlet duct wall or serving as a part of the inlet duct

30 wall.

20. Apparatus according to claim 15, characterized in that the means for raising the temperature comprise at least a part of the inlet duct wall made of a porous

35 material (44) and communicating with a gas space (34) provided for introducing hot gas to the inlet duct to melt the deposit or gas causing combustion in the inlet duct.