WO1992021429A1

WO1992021429A1 - A method of cooling and cleaning waste gas from an industrial process and apparatus therefor

Info

Publication number: WO1992021429A1
Application number: PCT/GB1992/001006
Authority: WO
Inventors: Keith Russell Mcneill
Original assignee: Keith Russell Mcneill
Priority date: 1991-06-07
Filing date: 1992-06-04
Publication date: 1992-12-10
Also published as: EP0641243A1; JPH06510222A; HUT70101A; CZ264593A3; ZA924098B; AU1798392A; CA2109945A1; BR9206113A; AU656194B2; GB9112258D0; HU9303472D0

Abstract

The invention relates to a method of cooling and cleaning waste gas from an industrial process wherein the waste gas under pressure is caused to pass transversely through a series of pebble bed heat exchangers, the passage of the waste gas through a pebble bed being effective to cause both heat exchange between the gases and the pebbles of the pebble bed, and also removal of a contaminant carried in the waste gas, and wherein the heated pebbles are subsequently used to transfer heat to a cooler gas. The invention also relates to apparatus for operating this method, including apparatus for cooling and cleaning waste gas from a glass melting furnace. The invention helps reduce levels of metal volatiles, oxides of nitrogen, oxides of sulphur, dioxins and particulates in the waste gas. It also makes use of the heat from the waste gas.

Description

A METHOD OF COOLING AND CLEANING WASTE GAS FROM AN INDUSTRIAL PROCESS AND APPARATUS THEREFOR

Field of the Invention

This invention relates to a method of cooling and cleaning waste gas from an industrial process and to apparatus for carrying out the method.

Background to the Invention

Many industrial processes result in hot waste gas (which may include a mixture of gases) which must be discharged. Frequently it is desired to utilise the heat contained in the waste gas to raise the temperature of cold air which is to be fed as hot air to increase the efficiency of the process, and a variety of heat exchangers are known for this purpose.

The waste gas from the industrial processes frequently contain contaminants, for example particulates, volatilised metal compounds, oxides of sulphur, oxides of nitrogen and dioxins, all of which should be substantially removed before the waste gas can be released into the atmosphere. Official Regulations are becoming progressively more severe regarding the limits placed on the output of these materials into the atmosphere.

It is an object of the present invention to provide a method and apparatus in which heat exchange is obtained simultaneously with effective removal of many contaminants of the waste gas. That is, the quantities of the contaminants in the waste gas are reduced to levels which are environmentally acceptable so that the cooled and cleaned gas may be discharged into the atmosphere.

The method in accordance with the present invention is based on the use of a pebble bed heat exchanger such as that described by C L Norton Jnr in the Journal of The American Ceramic Society, Volume 29 (1946) No 7 Pages 187- 193. In my published UK Patent Application No 2,225,002 I have described my earlier proposal to use a pebble bed heat exchanger to filter sulphur and particulates from waste gas by passing the waste gas upwardly through a chamber through which pebbles or spherical balls are being moved downwardly. The waste gas was then further passed downwardly through a similar chamber in which spherical balls or pebbles are also being moved downwardly.

In its broadest aspect, a pebble bed heat exchanger can be regarded as a chamber containing a number of heat absorbent pebbles through which a gas can be passed.

Experience has shown that efficient heat exchange does not occur in pebble bed heat exchangers when gas is passed vertically through the pebble bed chambers, because the gas tends to follow the path of least resistance at the circumferential edge of the pebble bed with the result that the chamber wall and the pebbles near the chamber wall become very hot and relatively little heat is exchanged at the centre of the pebble bed chamber. Summary of the Invention

According to the present invention there is provided a method of cooling and cleaning waste gas from an industrial process wherein the waste gas under pressure is caused to pass transversely through a series of pebble bed heat exchangers, the passage of the waste gas through a pebble bed being effective to cause both heat exchange between the gas and the pebbles of the pebble bed, and also removal of a contaminant carried in the waste gas, and wherein the heated pebbles are subsequently used to transfer heat to a cooler gas.

This method provides the combination of waste gas cleaning with regenerative heat exchange to increase efficiency of an industrial process. The single method according to the present invention combines these advantages in a reliable manner.

The pebbles of each pebble bed may be static or moving either intermittently or continuously. Preferably the pebbles of each pebble bed are continuously moving under gravity.

In accordance with a preferred feature of the present invention, at least one pebble bed heat exchanger effects a temperature drop in the waste gas such that a specific component of the waste gas is removed during passage of the waste gas through the said one pebble bed heat exchanger. By providing the series of pebble bed heat exchangers in each of which a specific component of the waste gas is removed, the method in accordance with the present invention is a mechanical fractionating process by which the individual constituents of a mixture of components in the waste gas may be separated from one another.

The removal of a particular component of the waste gas in one pebble bed heat exchanger may be by deposit of the composition on the pebbles of that pebble bed heat exchanger. Such deposit occurs for example when the temperature range through which the waste gas is cooled in the pebble bed heat exchanger includes the condensation temperature of a metal volatile present in the waste gas. The condensed metal volatile is then carried out of the pebble bed heat exchanger on the pebbles from which it is removed before the pebbles are returned into the top of the pebble bed heat exchanger.

In a method in accordance with the present invention the temperature drop during the passage of waste gas through a specific pebble bed heat exchanger may be such that a plurality of contaminants is deposited in the pebble bed heat exchanger.

A contaminant of the waste gas may also be removed in a pebble bed heat exchanger as a result of a chemical reaction within the pebble bed between the contaminant and an appropriate chemical compound applied to the surfaces of the pebbles before they enter the pebble bed or by reaction of the contaminant with a material of which the pebbles are constructed.

For example, oxides of nitrogen in the waste gas may be removed by reaction with ammonia in a pebble bed heat exchanger as a result of applying ammoniacal water to the pebbles before they are introduced into the pebble bed heat exchanger. At some temperatures the reaction between oxides of nitrogen (N0_X) and ammonia requires a catalyst. However, it is preferred to introduce ammoniacal water into a pebble bed heat exchanger across which the temperature drop is in the range of 1090°C to 870°C because the reaction between ammonia and oxides of nitrogen proceeds within this temperature range without the need for a catalyst. If the ammoniacal water is introduced into a pebble bed heat exchanger the temperature of which is likely to fall outside the range of 1090°C to 870°C, a catalyst may be introduced into the pebble bed heat exchanger with the ammoniacal water to promote the reaction between N0_X and ammonia.

Lime in the form of calcium hydroxide may be introduced into another pebble bed heat exchanger in order to react with acid gases present in the waste gas and particularly to remove oxides of sulphur from the waste gas. Alternatively, caustic soda or magnesia may be-used.

The transfer of heat from the pebbles to the cooler gas will conveniently be accomplished by passing the .cooler gas transversely through the heated pebbles.

Conveniently, the temperature drop between pebble beds is maintained by spacing the pebble beds from one another by a gas filled region. This arrangement inhibits heat transfer between adjacent pebble beds to maximise the mechanical fractioning potential of the invention and to maintain temperature control.

More specifically in accordance with the present invention there is provided a method of cooling and cleaning waste gas resulting from a glass-making process wherein there is provided a series of pebble bed heat exchangers in each of which the pebbles comprising the respective pebble beds may be moving continuously in a substantially vertical direction under gravity and wherein the waste gas under pressure is caused to pass horizontally through each pebble bed heat exchanger in the series, heat exchange between the gas and the pebbles of the pebble bed occurring in each pebble bed heat exchanger and specific contaminants carried in the waste gas being deposited in respective ones of the series of pebble bed heat exchangers through which the waste gas passes in succession.

The present invention also includes in another aspect apparatus for cooling and cleaning waste gases from an industrial process, the apparatus comprising a first housing and a second housing, each housing having first and second ends, a series of pebble bed heat exchangers horizontally separated from one another within the first housing, each pebble bed heat exchanger comprising a pair of porous screens each of which physically separates a portion of the first housing which is nearer to the first end of the first housing from another portion of the first housing which is nearer to the second end of the first housing, a multiplicity of pebbles filling the parts of the first housing between each pair of porous screens, and means for introducing pebbles into the top of the part of the first housing between each pair of porous screens and removing pebbles from the bottom of the said part of the first housing between each pair or porous screens,, means for introducing waste gas into the portion of the first housing between the first end of the first first housing and the first pebble bed heat exchanger of the series, and means for permitting waste gas to escape from the portion of the first housing between the last pebble bed heat exchanger in the series and the second end of the first housing, and means for heating a cooler gas by means supplying a cooler gas to one of the housings through which heated pebbles can pass to heat the cooler gas.

In one embodiment of the present invention which will be described, apparatus according to the present invention includes a first housing and a second housing and the means for heating a cooler gas comprises reversing means for alternately directing waste gases into the first end of the first housing and cold air into the second end of the second housing or directing cold air into the second end of the first housing and waste gas into the first end of the second housing.

In a second embodiment of the present invention the apparatus comprises three similar housings having similar series of pebble bed heat exchangers similarly disposed in each housing, the three housings are arranged substantially vertically above one another, the means for heating a cooler gas comprising a means for passing pebbles comprising pebbles for a particular pebble bed in the series successively through the corresponding pebble beds in the uppermost, the middle and the lowermost housing before cleaning of the pebbles and returning for feeding into the relevant pebble bed of the uppermost housing, means is provided for passing hot waste gas into the first end of the uppermost housing and further means is provided for passing cold air into the first end of the lowermost housing and directing partially heated air from the second end of the lowermost housing into the second end of the middle housing.

Brief Description of the Drawings

The present invention will be further understood from the following detailed description of embodiments thereof which is made, by way of example, with reference to the accompanying diagrammatic drawings in which:- Figure 1 is a first embodiment of a waste gas cleaner and heat exchanger in accordance with the present invention, the flow through which is periodically reversed for heating cold air, and

Figure 2 is a second embodiment of a waste gas cleaner and heat exchanger in accordance with the present invention, in which no reversal of flow is necessary. Description of the Preferred Embodiments Referring to Figure 1 of the accompanying drawings, a combined heat exchanger and waste gas cleaner 1 in accordance with the present invention comprises a housing 2 of alumina refractory material. The housing 2 being of rectangular cross-section and having end walls 3 and 4 and top and bottom walls 5 and 6. Ducts 7 and 8 are provided in the end wall 3 and 4 respectively for passing gas into or allowing gas to escape from the interior of the housing 2.

The interior of the housing 2 is divided into a series of sections by pairs of porous screens 9 and 10 of which four pairs are shown in Figure 1. There may however be more than four pairs of porous screens. Each porous screen 9 or 10 fills the whole cross-sectional area of the interior of the housing 2 at its own locality so that gas can only pass from an interior part of the housing 2 on one side of the screen to an interior part of the housing on the other side of the screen by passing through the said porous screen 9 and 10.

The porous screens 9 and 10 are formed from alumina refractory material to withstand the temperatures to which they are subjected. Each porous screen 9 and 10 has a multiplicity of holes 11 through it.

Located on the top wall 5 above each pair of porous screens 9 and 10 are funnel means 12 for directing pebbles such as spherical balls 13 into the volume defined within the interior of the housing 2 by the porous screens 9 and 10 so that the most of the volume between each pair of porous screens 9 and 10 is filled with pebbles 13 to form a pebble bed. An extractor means 14 is located adjacent to the bottom wall 6 of the housing 2 beneath each pair of porous screens 9 and 10 for removing pebbles 13 from the interior of the housing 2 between each pair of porous screens 9 and 10. The extracted pebbles 13 are cleaned by being passed through a sloped rotating drum 15. Each extractor means 14 is connected by ducting 54 to a respective drum 15, only one of which is shown in Figure 1. In this way the waste fractions can be collected together. The motion of the pebbles 13 through the drum 15 knocks off accumulated waste. The pebbles 13 and separated waste move along the drum 15 by gravity and are deposited onto a mesh screen 16 to which is coupled means 17 for vibrating the screen 16. The mesh of the screen 16 is slightly smaller than the pebbles 13 so the waste drops through the screen 16 allowing the pebbles 13 to continue off the screen 16 onto an endless conveyor 18 for reintroduction through the funnel means 12 into the respective pebble beds defined between the porous screens. Graded meshes can be used to deposit the pebbles 13 into the correct funnel means 12.

The screens 9 and 10 will occasionally need cleaning as well to remove accumulated deposits. This can be accomplished by the removal of the screens 9 and 10.

Alternatively, the screens 9 an 10 can be mounted on a slide mechanism 50 (shown for a screen 9 only, but can be used for all screens) .

Replacement screens 9A and 10A are mounted in the slide 50. Screens 9 and 10A can be moved to replace the screens 9 and 10 which are removed and cleaned ready for re¬ insertion when screens 9A and 10A require cleaning.

There are thus provided within the housing 2, four pebble beds denoted respectively by the references 19, 20, 21, and 22.

In operation to treat waste gas from an industrial process, for example the gaseous emissions from a glass melting furnace, the waste gas is passed by a fan 51 into the interior of the house 2 through duct 7 in the. direction of the arrow 23. The waste gas may be at temperature of the order of 1400°C or higher. The waste gas introduced through the duct 7 occupy the volume between the end wall 3 of the housing 2 and the porous screen 9 of pebble bed 19. The pressure of waste gas developed within this volume causes the waste gas to penetrate through the tortuous paths between the pebbles of the pebble bed 19 contained between the porous screens 9 and 10 and to emerge into the section of the interior of the housing 2 between the pebble beds 19 and 20.

During the passage of the waste gas through pebble bed 19 there is heat exchange between the waste gas and the pebbles 13 comprising pebble bed 19 and also deposition of particulate material on the surface of the pebbles 13 comprising pebble bed 19.

The partially-cooled and cleaned waste gas similarly builds up in the section of the interior of the housing 2 between pebble beds 19 and 20 with the result that the waste gas similarly passes through the tortuous paths between the pebbles 13 of pebble bed 20 and then in sequence through pebble beds 21 and 22, after which the cooled and cleaned waste gas is discharged from the housing. 2 by fan 52 through duct 8 as shown by arrow 24.

Typically between l-8m.3 (at 20°C, atmospheric pressure) of waste gas can be cleaned in this combined heat exchanger and pebble bed cleaner, although higher volumes can be dealt with using larger pebble beds, more or smaller pebbles. The limiting factor is that there must be sufficient interactions between the waste gas and the pebbles 13, ie the surface area of the pebbles 13 encountered by the waste gas must be increased.

The combined thickness of the four pebble beds 19, 20, 21 and 22 is of the order of 50 to 60 cms, each pebble bed 19, 20, 21 or 22 having a thickness of 12 to 15 cms.

The pebbles 13 used in pebble bed 19 are made of refractory material, preferably alumina, and each pebble 13 is a spherical ball approximately 19 mm in diameter. A pebble bed comprised of such balls is found it effect good heat exchange and good removal of particulates which are greater than 2 microns in diameter.

For each 10^ Joules of energy that is required to be removed from the waste gas as it passes through the pebble bed 19 about 2.7 kg of pebbles 13 are required in the path of the waste gas flow.

In order to effect good removal of particles of diameter less than 2 microns, one or more of the other pebble beds, for example pebble bed 22, is comprised of pebbles which are spherical metal balls of approximately 5 mm in diameter, for example stainless steel balls. By using spherical balls, the risk of the pebbles 13 becoming clogged in pebble beds 19, 20, 21 and 22 is minimised.

For the 19 mm pebbles 13, the holes 11 in the screens 9 and 10 will be slightly smaller at 17 mm to maximise the flow of waste gas while preventing the escape of pebbles 13. The holes 11 also help to provide even waste gas flow across the pebble beds 19, 20, 21 and 22, minimising local waste gas concentrations.

After passage through the four pebble beds 19, 20, 21 and 22 the temperature of the waste gas has been reduced to a temperature less than 250°C, or even below 200°C.

By appropriate choice of the thickness of the respective pebble beds 19, 20, 21 and 22 and of the composition of the pebbles which comprise these pebble beds, the temperature drop across the different pebble beds in the series may be controlled so as to facilitate removal of different impurities present in the waste gas. For example, the waste gas may contain metal volatiles which will condense out from the waste gas as the waste gas is cooled through the condensation temperature of the particular metal volatile. By designing the pebble beds suitably, it can be arranged that the gas cools through the condensation temperature of a selected metal volatile while passing through a particular pebble bed. It is thus ensured that the selected metal volatile is condensed in the particular pebble bed with the result that this condensed metal or metal compound can be recovered by cleaning the pebbles of that particular pebble bed. By this method separation of the different metals present a volatiles in the waste gas can be achieved, different metals being deposited in different pebble beds.

Particular metals which can be removed from the waste gas in this way are, for example, zinc, iron and lead. Each metal is deposited on the pebbles of a different pebble bed and the apparatus of Figure 1 acts as a mechanical fractionation device.

Frequently the gaseous emissions contain a considerable number of vaporised metals or metal compounds, for example there may be nine or ten different metals present in the gaseous emissions.

When apparatus according to the present invention and having only a limited number of pebble beds is used in such a case, a first group of metals, for example chromium, copper and zinc, may be deposited in one pebble bed and a group of different metals including, for example lead and cadmium, be deposited in a different pebble bed.

The waste gas from an industrial process will contain oxides of nitrogen (N0_X) and oxides of sulphur (S0_X) in quantities such that the waste gas is unsuitable for discharge into the atmosphere without substantial removal of such N0_X and S0_X. Removal of both N0_X and S0_X can be effected during passage of the waste gas through the combined waste gas cleaner and heat exchanger illustrated in Figure 1. Removal of N0_X is preferably effected by reaction with ammonia which is provided in the form of ammoniacal water at a region within the housing 2 where the waste gas is at a temperature in the range of 1090°C to 870°C, in which temperature range the reaction proceeds without the need for a catalyst. The waste gas may be within this temperature range, for example, in the section of the interior of the housing 2 between pebble bed 19 and pebble bed 20. In those circumstances ammoniacal water is introduced into this section of the interior of the housing 2 by a- spray 25.

Alternatively, the waste gas may pass through the temperature range 1090°C to 870°C within a pebble bed, for example pebble bed 20, in which case the ammoniacal water may be applied to the spherical balls which constitute the pebbles 13 of pebble bed 20 before these are introduced into the volume between the porous screens 9 and 10 which define pebble bed 20.

Similarly, the S0_X content of the waste gas may be reduced by applying lime in the form of calcium hydroxide to the pebbles 13 to be introduced into a pebble bed at a section of the apparatus of Figure 1 where the waste gas are at a lower temperature of the order of 600°C to 450°C. Alternatively, caustic soda or magnesia may be used to reduce the S0_X content.

Further pebble beds can be used to apply additional treatments to the waste gas. For instance, stainless steel pebbles 13 can be used coated with a mild acid solution such as hydrochloric acid to remove any excess ammonia. Similarly, alkali solutions can be used on the pebbles 13 to ensure more complete removal of S0_X. Water can then be sprayed into the waste gas in a gap between pebble beds to remove traces of the said alkali solution. A demister can be used to remove any remaining water aerosols.

The addition of these, or similar cleaning sections provide a final waste gas treatment that enables the apparatus described herein to comply with current legislation.

As indicated, the spherical balls constituting the pebbles 13 of each pebble bed separator are moved through the pebble beds at a slow rate of the order of 8-20 lbs (about 4 - about 9 kg) per hour.

The combined waste gas cleaner and heat exchanger 1 of Figure 1 must be reversed periodically (for example at intervals of the order or 1 minute to 1.5 minutes), the waste gases from the industrial process being switched by a valve 53 comprising a reversing means to a second similar waste gas cleaner and heat exchanger, while the apparatus of Figure 1 is used to heat cold air passed into the interior of the housing 1 through duct 8 at a temperature of the order of 20°C (ambient). The heated air is removed from housing 2 via duct 7 after the air has been heated by passage through the four pebble beds 22, 21, 20 and 19. It is found that using the pebble beds in this matter, cold air may be heated from 20°C to a temperature approaching 90% of the temperature of the waste gas supplied to the housing 2 when the apparatus 1 is used in the cooling and cleaning mode.

Another, preferred, embodiment of the present invention will now be described with reference to Figure 2. In Figure 2 there are shown three housings 26, 27 and 28 which are arranged vertically above one another. Each of the housings 26, 27 and 28 is arranged for passage of air through the housing longitudinally in a similar manner to that described for the housing 2 of Figure 1. Similarly, each of the housings 26, 27 and 28 is divided into sections by pebble beds through which pebbles are passed vertically form above the housing to emerge below the housing. Each of the housings 26, 27 and 28 has four pebble beds passing therethrough, the pebble beds being similar to pebble beds 19, 20, 21 and 22 of Figure 1, or another series of pebble beds suitable for the treatment of the particular waste gas concerned. Pebble beds 29, 30, 31 and 32 are present in housing 26, pebble beds 33, 34, 35 and 36 in housing 27, and pebble beds 37, 38, 39 and 40 in housing 28.

Parts of the detail of Figure 1 are omitted in Figure 2 for clarity.

In this embodiment of the present invention the pebbles 13 comprising the pebble beds within housing 26 are fed successively through the corresponding pebbles beds in housings 27 and 28 before the pebbles 13 are extracted beneath housing 28, cleaned and returned to the appropriate feeder above housing 26 by means similar to that shown in Figure 1. In consequence the pebbles comprising pebble bed 29 are used within that pebble bed in housing 26 to cool and clean the waste gas. The pebbles 13 are then passed through an appropriate connection 41 into housing -27 where they constitute pebble bed 33 and then similarly through another connection 42 into housing 28 where the pebbles constitute pebble bed 37. Similarly, the pebbles constituting pebble beds 30, 31 and 32 are subsequently passed into housings 27 and 28 where they form part of pebble beds 34, 35 and 36 and pebble beds 38, 39 and 40 respectively.

Housing 26 constitutes the waste gas cleaner and heat exchanger 43 for cooling the waste gas and is similar in operation to the apparatus illustrated in Figure 1.

Housings 27 and 28 together constitute the heating part of the apparatus in which cold air is heated and the pebbles 13 are cooled. Cold air at 20°C or below is supplied to the interior of housing 28 through duct 44 so that the cold air impinges immediately upon pebble bed 37, which is that one of the four pebble beds in housing 28 which is at the highest temperature, and good cooling of pebble bed 37 is obtained. The heated air is then passed through pebble beds 38, 39 and 40 at each of progressively lower temperatures with the result that the air emerging from housing 28 through duct 45 is only very partially heated towards the temperature of the used gas. This partially heated air is passed into the cool end of housing 27 through duct 46 so that the partially heated air is progressively heated by pebble beds 36, 35, 34 and 33 and air at a temperature of the order of 80% - 90% of the temperature of the waste gas entering housing 26 is obtained from housing 27 through duct 47.

The cooling of the waste gases in the series of pebble beds contained within housing 2 of Figure 1 or housing 26 of Figure 2 is very quick. This is important in ensuring the destruction of dioxins. It is known that dioxins are destroyed on combustion, but the elements of the dioxin are still present in the gas as it is cooled in a conventional waste gas cooling system. If the cooling is not swift enough, the elements recombine to re-form the dioxins. Currently, destruction of the combustion products of dioxins is ensured by quenching the gas in water,- but this operation means that the useful heat from the waste gas is lost in the action of destroying dioxins. By contrast the combined waste gas cleaner and heat exchanger 1 and 43 of the present invention effects cooling of the waste gas sufficiently swiftly without allowing sufficient time for the dioxins to be re-created and also recovers the heat from the waste gas.

The embodiments of the present invention which have been particularly described are high temperature (1400°C - 250°C) pebble bed heat exchangers which serve as both heat recovery beds, waste gas scrubbers and as a primary particulate removal system. The embodiment of Figure 1 of the accompanying drawings, in which the flow through the pebble bed heat exchangers is switched using reversing valves 53 from the cooling and cleaning mode to the inlet air heating mode is significantly more energy efficient than the flow reversal system which operates on a conventional regenerative furnace. The embodiment of Figure 2 of the accompanying drawings is as energy efficient as the Figure 1 embodiment with the added advantage that no reversing valves are required and the operating difficulties which may arise with such valves are totally avoided.

The embodiments of the present invention described herein are believe to be capable of operation on gaseous emissions from a glass-making furnace to give an overall 90% reduction for particulates of diameter greater than 2 microns, and an 80% reduction of N0_X by non-catalytic removal in the 870°C to 1090°C temperature range without emissions of excess ammonia. The particulates trapped include the most significant heavy metal contaminant, lead, which is present as the oxide and/or sulphate, and sodium sulphate. A significant part (probably 50% or more) of the sulphur in the emissions is present as the sulphate, the remainder being sulphur dioxide which is removed by conventional treatment with lime, caustic soda or magnesia in a pebble bed heat exchanger as described.

In a specific example of a method using apparatus according to the present invention with a pressure drop of about 3 kilopascals across the pebble beds, it was found that the pebble beds filtered out 98% of all particles greater than 20 microns diameter, 95% of all particles greater than 10 microns diameter, 90% of particles in the range of 8-10 microns diameter, and up to about 60% of particles in the range of 2-8 microns diameter. The particles of unmelted materials which get carried along in the waste gas are particles of diameter greater than 2 microns. The particles of diameter less than 2 microns include vapourised metals and metal compounds, less than 50% of which are separated out by physical entrapment, and the bulk of which are separated out by condensation as hereinbefore described.

It is to be noted that the present invention will operate at a large range of pressure drops across the apparatus. Unlike systems such as Venturi scrubbers, a large pressure drop is unnecessary. Only sufficient pressure to keep the waste gas flowing through the pebble beds is required.

Claims

1. A method of cooling and cleaning waste gas from an industrial process wherein the waste gas under pressure is caused to pass transversely through a series of pebble bed heat exchangers, the passage of the waste gas through a pebble bed being effective to cause both heat exchange between the gas and the pebbles of the pebble bed, and also removal of a contaminant carried in the waste gas, and wherein the heated pebbles are subsequently used to transfer heat to a cooler gas.

2. A method according to Claim 1 wherein the pebbles of each pebble bed are continuously moving under gravity.

3. A method according to Claim 1 or Claim 2 wherein at least one pebble bed heat exchanger effects a temperature drop in the waste gas such that a specific component of the waste gas is removed during passage of the waste gas through the said one pebble bed heat exchanger.

4. A method according to Claim 3 wherein the component of the waste gas removed during the passage through the said one pebble bed heat exchanger is a metal volatile having a condensation temperature falling within the temperature range through which the waste gas is cooled in the said one pebble bed heat exchanger, whereby the metal volatile is deposited in the said one pebble bed heat exchanger.

5. A method according to Claim 3 wherein the pebbles of the said one pebble bed heat exchanger introduce into the said one pebble bed heat exchanger a chemical promoting removal of the specific component of the waste gas.

6. A method according to Claim 5 wherein the pebbles of the said one pebble bed heat exchanger introduce ammoniacal water into the said one pebble bed heat exchanger.

7. A method according to Claim 6 wherein the temperature drop in the waste gas passing through the said one pebble bed heat exchanger is in the range of 1090°C to 870°C.

8. A method according to Claim 5 wherein the pebbles of the said one pebble bed heat exchanger introduce lime. caustic soda or magnesia into the said one pebble bed heat exchanger for reacting with acid gases.

9. A method of cooling and cleaning waste gas resulting from a glass-making process wherein there is provided a series of pebble bed heat exchangers in each of which the pebbles of the respective pebble beds may be moving continuously in a substantial vertical direction under gravity and wherein the waste gas under pressure is caused to pass horizontally through each pebble bed heat exchanger in the series, heat exchange between the gas and the pebbles comprising the pebble bed occurring in each pebble bed heat exchanger and specific contaminants carried in the waste gas being deposited in respective ones of the series of pebble bed heat exchangers through which the waste gas passes in succession, and wherein the heated pebbles are subsequently used to transfer heat to a cooler gas.

10. Apparatus for cooling and cleaning waste gas from an industrial process, the apparatus comprising a first housing and a second housing, each housing having first and second ends, a series of pebble bed heat exchangers horizontally separated from one another within the first housing, each pebble bed heat exchanger comprising a pair of porous screens each of which physically separates a portion of the first housing which is nearer to the first end of the first housing from another portion of the first housing which is nearer to the second end of the first housing, a multiplicity of pebbles filling the part of the first housing between each pair of porous screens, and means for introducing pebbles into the top of the parts of the first housing between each pair of porous screens and removing pebbles from the bottom of the said part of the first housing between each pair of porous screens, means for introducing waste gas into the portion of the first housing between the first end of the first housing and the first pebble bed heat exchanger of the series, means for permitting waste gas to escape from the portion of the first housing between the last pebble bed heat exchanger in the series and the second end of the first housing, and means for heating a cooler gas by means supplying a cooler gas to one of the housings through which heated pebbles can pass to heat the cooler gas.

11. Apparatus according to Claim 10 in which there is first housing and a second housing, and means for heating a cooler gas comprises reversing means for alternately directing waste gas into the first end of the first housing and cold air into the second end of the second housing or directing cold air into the second end of the first housing and waste gas into the first end of the second housing.

12. Apparatus according to Claim 10 wherein the first housing is one of three similar housings having a similar series of pebble bed heat exchangers similarly disposed in each housing, the three housings are arranged substantially vertically above one another, the means for heating a cooler gas comprises means for passing pebbles comprising a particular pebble bed in the series successively through the corresponding pebble beds in the uppermost, the middle and the lowermost housings before cleaning of the pebbles and returning for feeding into the relevant pebble bed of the uppermost housing, means is provided for passing hot waste gas into the first end of the uppermost housing and further means is provided for passing cold air into the first end of the lowermost housing and directing partially heated air from the second end of the lowermost housing into the second end of the middle housing.

13. Apparatus according to any one of Claims 10 to 12 wherein the pebbles forming the first pebble bed heat exchanger are comprised of refractory material.

14. Apparatus according to anyone of Claims 10 to 13 wherein at least one pebble bed heat exchanger is formed of pebbles of smaller diameter than the pebbles of another pebble bed heat exchangers.

15. Apparatus according to any one of Claims 10 to 14 wherein the pebbles of at least one pebble bed heat exchanger are comprised of a material for reacting with a contaminant of the waste gas to remove the contaminant from the waste gas within the pebble bed heat exchanger formed by the pebbles of the said material.

16. A method of cooling and cleaning waste gases from an industrial process substantially as hereinbefore described with reference to either Figure 1 or Figure 2 of the accompanying drawings.

17. An apparatus for cooling and cleaning waste gases from an industrial process constructed and arranged to operate substantially as hereinbefore described with reference to either Figure 1 or Figure 2 of the accompanying drawings.