CA2109945A1

CA2109945A1 - A method of cooling and cleaning waste gas from an industrial process and apparatus therefor

Info

Publication number: CA2109945A1
Application number: CA002109945A
Authority: CA
Inventors: Keith Russell Mcneill
Original assignee: Individual
Current assignee: Vert Investments Ltd
Priority date: 1991-06-07
Filing date: 1992-06-04
Publication date: 1992-12-10
Also published as: ZA924098B; WO1992021429A1; JPH06510222A; HUT70101A; HU9303472D0; GB9112258D0; EP0641243A1; AU656194B2; BR9206113A; CZ264593A3; AU1798392A

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

WO 9V21429 PCr~GB92/0 t 006 2109~45 A ME~OD OF COOLING AND CLE~ING WaSTE GAS
FRQM AN INDUST~5_~ATUS T~EREFO~ ;

Field of the Inventio~
. ~
This invention rela~es to a method of coolin~ and cleaning waste gas from an $ndustrial process and to app2ratus f or carrying out the method.
Rack o~nd ~o the In~ention Many ~ndustr~al proc~sses result in hot wa~te gas (which ~ay include a mixt~re of gases) which must be discharged.
Fre~uently ~t is desired to utilisP the heat contained in the waste gas to r~ise the temperature of co~d is wh~ch is to be fed as hot air to increase the efficiency of t~e process, and a ~ariety of heat exchangers are known for this purposP.
The waste gas from the industrial processes ~requently contain con~a~inants, ~or example particulates, volat~lised metal compounds, oxi~es of sulphur, oxides of nitrogen and ~ioxi~s, all of which should ~e substantially removed before the wastP gas can be released into the atmosphere.
Offici~l Regulations ar~ becoming progre$sively more sevcre regar~ing the limits placed on the output of these m~terials into the atmosphere.
It is ~n objec- of ~he prese~t invention to pro~ide a me~hod and apparatus i~ which hezt exchange is obtained simultaneously with effective re~o~al cf many contaminants of the waste ~as. That is, the quantities of the :~
contaminants i~ the waste gzs are reduced to le~els which 3g 2 / 01 0 0 2 2 ~ l 5 2 9 ~PR'L 19~3 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 ab~orbent 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 res~lt 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 un~er pressure is caused to pass transversely through a series of horizontally spaced 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 ''' ~ 3~ t Office ~ ~ C ~
~ P~r i .~ ,catioll S~ E ~,HEc l ~

W092/~1~29 2 1 0 ~ 9 d 5 PCT/G~92/01~06 3.
efficiency ~ an indust-ial process. The single method according to the present in~ention com~l~es .hes~
ad~antages in a relia~Le manner.
The pebbles o~ each pebble bed may be static or moving either intermittently or continuously. Preferably the pebbles of each pebble bed are continuou51y moving under ~ravity.
In accordance with a pre~erred feature of the present invention, at least one pebble bed heat exchan~er effects a tempera~ure drop in the waste gas such that a speci~ic compcnent of the waste gas is remo~ed durlng 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 in~idual constituents of a mixture of components in the waste gas may be separated from one another.
The remo~al of a particular c~mponent of the waste gas in one pe~bl~ bed heat exchanger may ~e by deposit of the composition on the pebbles of tha~ pebble bed heat exchan~ger. Such deposit occurs for example ~hen 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.
~he condensad metal ~olatile is then carried out of the pébble bed heat exchanger on the pebbles fram which it ~s remo~ed ~efore the pebbles are returned into the top of the pebble bed heat exchanger.
In a method in accordance with the present invention the temperature droplduring the passage cf waste gas through a specific pebble bed heat exchange_- may be such that a plurality of contaminants is deposited in the pebble bed -~
heat exchanger.
A contaminant of the was~e gas may also be removed i~ a -~
pebble bed heat exchanger as a result of a chemical -~
reaction within the pebble bed be~ween the contaminant and -an appropriate chemical compound applied to the surf aces of ~

WO92/21429 ( 2 1 0 ~ a A ~ PCT/GB92/01006 the pebbles before they enter the pebble bed or by react~on of the c~n~aminant with a material of whioh the pebbles are const-uc_ed.
For example, oxides of nitrogen in the waste. gas may be removed by reaction with ammonia in a pebb}e ~ed heat exchanger as a result of applying ammoniaczl water to the pebbles before they are introduced into the pebble bed heat exchanger. At some temperatures the reaction between oxides of nitrogen (NOX) and ammonia requires a catalyst.
However, it is preferred to introduce ammoniacal water into a pe~ble bed heat exchanger across which the temperature drop is in the range of lO9GC to 870C because the reaction between ammonia and oxides of nitrcgen proceeds ~`~
within this temperature range without the need for a catalyst. If the ammoniacal water is introduoed into z pebble bed heat exchanger the temperature of which is likely to fall outs~de the range of 1090C to 870C, a catalyst may be i~t~oduced into the pebble bed heat exchanger with the ammoniacal water to promote the react~on between NOX and ammon~a.
Lime in the form of calcium hydroxide may be int~oduced into another pebble bed heat exGhanger in order to react with acid gases present in the waste gas and particularly to remove oxides of sulphur from the waste gas.
Alternati~ely, caustic soda or magnesia may be used.
The ~ransfer of he~t from the pebbles to the cooles gas will conveniently be accomplished by passing the cooler gas transversely through the heated pebbles. ~:
Con~eniently, the temperature drop between pebble beds is maintained by spacsng the pebble beds f-om one ano~her by a gas filled region. This arrangement inhibits heat trans~er between adjacent pebble beds to maximise the mechanical fractionin~ potential of the invention and to maintain temperature control.
More specifically in accordance with the present invention there is provided a method of coolin~ and cleaning waste gas resultins from a glass-making process wherein there is provided a se~ies of pebble bed heat WO 9~/2 1 4Z9 210 9 9 !15 PCr/GB92/01 006 exchangers in each of whLch the pe~bles comprising the respective pehble beds may be moving continuously in a substantially vertical direc_~an under gravi~ and wherein the waste gas under pressure is caused to pass h~rizontally through each pebble bed heat exchanger in the saries, heat exchange between the gas and the pe~bles of the pebble bed oc~urring in each pebble bed heat exchanger and speciic contaminants carried in the waste gas being deposited in respecti~e ones of the series of pebble bed heat exchangers through which the waste gas pass2s in succession~
The present inventi~n also includes in another aspect appzrat~s 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 hous~ ng, each pebble bed heat exchanger compr~sing a pair of porous screens each of which physically separates a ~crtlon of the first housins 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 mult~plicity of pebbles filling the parts of the ~rst housing between each pair of porous s~reens, and means for introducing pe~bles into the top cf the part of -~
the first housing between each pair of porous sc~eens 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 f irst first housing and the first pebble bed heat exchanger of the series, and means for permitting was~e gas to escape f-om the por~ion - of the first housi~g between the last pebble bed heat exchanger in the series ~nd the second end o~ the first housing, and means for heating a cooler gas ~y means supplying a cooler gas to one of the hcusings through which h*ated pebbles can pass to heat the cooler gas.
In one embodiment of the present invention which will be :
desc~ibed, apparatus acc~rdins to the presPn. in~ention W092/21429 6. 2 ~ ~ 9 9 a ~, PCT/GB9~/01006 includes a first housin~ and a second housing and the means for heating a cooler gas comprises reversing means for alternately direc~ing waste gases into the first end o the first housing and cold ai- into the second end ~f the second hous~ng or direc~ing cold air into the second end o~
the first housing and waste gas into the first end of the second housing.
In a sacond embodiment o the present in~ention the apparatus comprises three similar housings ha~ing similar series of pebble bed heat exchangers similarly disposed in each housing, the three housings are arranged substant~ally vertically above one another, the means fsr 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 prov~ded for passing hot waste gas in~o the first end of the uppermost housing and further means is provided for passing cold air into the fir~t end of the lowermost housing and directing part~ally heated air from the second end of the lowermost housing $nto the second end ~f the -;~
middle housing. ~-Brief Description of the Drawin~s The present invention will be further understood fsom the following detailed descript~on of embodiments the~eof which is made, ~y way of example, with reference to the accompanying diagrammatic drawings in which:-Figure l is a first e~bodiment of a waste gas cleaner and heat exchanger in accordance with the present invention, the flow through which is period_cally re~ersed for heating cold ais, and Figure 2 is a second embodiment of a waste gas cleaner and heat exchanger in zccordance with the present invention, in whi-h no reversal of flow is necessary.
Descriptian of the Preferred Fmhcdiments Referring to Figure l of the ac_ompanying drawings, a WO9~121-1~9 PCT/G~9~/01006 7 2:~099~
combined h~at exchanger and waste gas cleaner l in ac-~rdance with ~he present in~ention csmprises a housing 2 of alumina refractory material. The housing 2 being of rec-angular c_oss-section and hav~ng end walls 3 and 4 and top and bottom walls 5 and 6. Ducts 7 and 8 are pro~ded in the end wall 3 and 4 respecti~ely for passing gas into or allowing gas to escape from the interior of the housing 2.
The interior of the housing 2 is divi~ed into a series of sections by pairs of porous screens 9 and 10 of which four pairs are shown in F~ gure l. There may however be more than four pairs of porous screens. Each porous screen 9 or lO 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 throu~h the said porous screen g ~:~
~nd 10.
The p~rous scseens 9 and lO are formed from alumina ~;
refractory material to withstand the tempera~ures to which they are subjected. Each porous scre~n 9 and lO has a multiplicity of holes ll through ~t.
Located on the top wall 5 abo~e each pair of porous screens 9 and lO are funnel me ns 12 for directlng pe~bles such zs spherical ~alls 13 into the volume def~ned within the interior of the housing 2 by the porous screens 9 and ~:~
lO so that the most of the ~olume between each pair of - :
porous screens 9 and lO is f~lled with pebbles 13 to form a :~
pebble bed. An extractor means l4 is located ad~acent to the bottom wall 6 of the housing 2 beneath each pair of porous screens 9 and l~0 for remo~ing peb~les 13 f~om the interior of the housing 2 betwe~n each pair of porous screens 9 and lO. ~he extracted pebbles 13 are cleaned by being passed through a sloped rotating drum 15. E~ch extractor means 14 is connected by ducting 54 to 2 respecti~e ~rum l~, only one of which is shown in Figure l. ~-In this way the waste frac~ions can be collected to~ether.
~he motion of the pebbles 13 through the drum l~ knocks of' WO92/21429 f 21 o 9 3 ~ ~ PCT/GB9t~01006 8.
accumulated waste. The pebbles 13 and separated was~e move along the drum lS by gra~ty and are deposited onto a mesh scre~n 16 to which is coupled means 17 for ~ibratins the screen 16. The mesh of the screen 16 is sli5htly smaller than the pebhles 13 so the waste drops through the screen 16 allowing the pebbles 13 to continue of~ the screen 16 onto an endless conveyor 18 for reintroduction through the funnel means 12 into the respect$ve pebble beds de~ined between the porous screens. Graded meshes can be us~d to deposit the pebbles 13 into the correct funnel means 12.
The screens 9 and 10 will occasionally need cleaning as well to remo~e accumulated depcsits. 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 ~or a screen 9 only, but can be used for all screens).
Replacement screens 9A and lOA are mounted in the slide S0. Screens 9A and lOA can be mo~ed to replace the screens - 9 and 10 which are removed and cleaned ready for re- ~-insertion when screens 9A and lOA require cleaning. --There are thus provided with~n the housing 2, four pe~ble beds denotPd respectively by the references 19, 20, 21, and 22.
In operation to treat waste gas from an industr~al process, for example the gasecus emissions from a g~ass -~
melting furnace, the waste gas is passed by a fan 51 into the interior of the house 2 through duct 7 in the direct~on of the arsow 2 The waste gas may be at temperature of t~e order of 14~0C or higher. The wzste gas introduced through the duct 7 oc upy the ~olume ~etween the end wall 3 of the housing 2 and the porous screen 9 of pebble bèd ~19.
The pressure of waste gas de~eloped within this ~alume causes the waste gas to penetrate through the tortuous paths between the pebbles of the pebble bed 19 conta~ned 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 was~e gas through pebble bed 1 4~9 PCT~GB9V01006 9. 210~94~
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 par~ially-cooled and cleaned waste gas sim~larly builds up in the sect~on of the interior of the housing 2 between pebble beds 19 and 20 with the result that the was~e gas similarly passe- through the tortuous paths between the pebbles 13 of pebble bed 20 and then in se~uence 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 betwe~n 1-8m3 (at 20C, 2tmospheric 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 ~s that there must be suff~c~ent interactions between the was~e gas and the ~:
pebbles 1~, ie the surface area of the pebbles 13 encountered by the waste gas must be inc-eased.
The ccmbined 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 ha~ins a thickness of 12 to 15 cms. ~-The pebbles 13 used in pebble bed lg are made of refractory material, preferably alumina, and each pebble 13 is a sphPrical ball approximately 19 mm in diameter. A
pebble bed comprised of such balls is found it effect gocd heat exchange and good removal of particulates which are greater than 2 mlcrons in diameter. ~:
For each 108 Joules of energy that is required to be removed from th2 waste gas as it:passes through the pebble bed 19 about 2.- kg of pebbles 13 are re~uired in the path of ~he waste gas flow.
In order to effect good removal of par._cles of diameter less than 2 microns, one or more of the other pebble beds, for example pebble bed 22, is c~mprised of pebbles which are spherical metal balls of approximately 5 mm in diameter, for example s~ainless steel balls.

WO 9~/tl429 ~ 21 o 9 9 a ~ P~/GB92/01006 10 .
By using sphe-ical balls, the risk of the pebbles 13 becoming clogged in pebble beds 19, ~0, 21 and 22 is minimised.
For the lg mm pebbles 13, the holes 11 in thP screPns 9 and 10 will be slightly smaller a~ 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 concsntrations.
After passage through the four pebble beds 19, 20, 21 and 22 the temperature of the waste gas has been reduc~d ~o z temperature less than 250C, or e~en below 200C.
By appropriate choice of the thickness of the respective peb~le beds 19, 20, 21 and 22 and of the compocition of the pebbles which comprise these pebble beds, the temperature -~
drop across the different pebble beds in the series may be controlled so as ~o facilitate remo~al of different ~mpurities present in the waste gas. For example, the -~
waste gas may contain metal ~olatiles which will condense `
~ut fsom the waste gas as the waste gas is cooled throu~h 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 wh~le passsng -~
throush a particular pebble bed. It is thus ensured that the selected metal ~olatile is condensed in the partscular pebble bed with the result thzt this condensed metal or metal compound can be recovered by cleaning the pe~bles of that particular pebble bed. By this method separat~on of the different metals present a volatiles in the was.e gas can be achie~ed, different metals being deposited in different pebble beds.
Particular metals which can be removed f~om the waste gas in this way are, for example, zinc, iron and lead. Each me~al is deposited on the pebbles cf a dif~erent pebble bed and the apparatus of Figure } a~ts 2S a mechanical ~ractio~ation deYice.
Fre~uently the gaseous emissions con~ain a conside-able W092t~1429 210 9 9 4 ~ PCT/GB92/01006 - 11.
num~er ~f vapor~sed metals or metal compounds, for example there may be nine or ten dif ferent metals present in the gaseous emissions.
When apparatus accord~ng to the present invention and ha~ins only a limited number or pe~ble bads 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 d~fferent me~als~including, for example lead and cadmium, be deposited in a dif~erent pebble bed.
The waste gas from an industrial process will contain oxides of nitrogen (N0x) and oxides o~ sulp~ur tSOx) in guantiti~s such that the waste gas is unsuitable ~or discharge into the atmosphere without substantial removal of such NOX and SOx. Remo~al of both N0x and S0x c~n be ef~ected during passage of the waste gas through the ~-~
combined waste gas cleaner and heat exchanger illustrated in Figure 1. Remo~al of ~x is preferably effected by react~on with ammonia which is pro~ided in the form of ammoniacal water at a reg~on within the housi~g 2 where the w~ste gas is at a temperature in the range of 1090C to - 870C, in which temperature range the reactlon proceeds `~
w~thout the need for a-catalyst. The waste gas may be within th~s temperature range, for example, in the section of the interior of the housing 2 between pebble bed 1~ and ;
pebble bed 20. In those circumstances ammoniacal water is introduced into this section of the interior of the housins 2 by a spray 25.
Alternatively, the waste gas may pass through the temperature range 109~C to 870C within a pebble bed, for example pebble bed 20, in whic~ case the ammoniacal water may be applied,to the spherical balls which constitute the pebbles 13 of pebble bed 20 before these are introduced ~nto the volume ber~een the porous screens 9 and 10 which define pebble bed 20.
Similarly, the SOx content of the was~e gas may be reduced by applying li me in the form of calcium hydroxide to the pebbles 13 to be introduced into a pebble bed at a section o~ the apparatus o' F~gure 1 where the waste gas W092/~l429 ~ 12 2 1 0 9 ~ ~ ~ PCT/GB92/olo~

are at a lower temperature of the order Oc 60~C to 450C. Alternati~ely, caustic soda or magnesia may be used to reduce the S0x 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 remo~e any excess ammonia~
Similarly, alkali ~olutions can be used on the pebbles 1 to ensure mcre complete removal of S0x. Water can then be sprayed into the waste gas in a gap between pebble beds to remove traces of the said alkali solution. A dem~ster can be used to remove any remain~ns 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 Ftgure 1 must be reversed periodic~lly (for example at inter~als of the order or 1 minute to 1.5 minutes), the waste gases from the ~ndustrial process be~ng switched by a valve 53 comprising a reversing means to a second similar wzs~e gas cleanes and heat exchAnger, while the apparatus of Fisure 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 20C (ambient). The heated air is remsved.from housing 2 ~ia duct 7 after the air has been heated by passage through the fou- pebble beds 22, 21, 20 and 19. It is found that using t~e pebble beds in this matter, cold air may be heated f~om 20C to a temperature approaching 90~ of the temperatu_e of the waste gas suppl~ed to the ho~sing 2 when the apparatus 1 is used in the coaling and cleaning mode.
Another, preferred, embodiment of the present invention will now be desc~ibed with reference to Figure 2. In WO97~714Z9 1~. ~ 9 9 4 ~ PCT/GB9Z/01006 Figure 2 there are shown three housings 26, 27 and 28 which ; .
are arranged vertically abo~e one another. Each of the housin~s 26, 27 and 28 is arranged for passag~ of air through the housing longitudinally in a similar manner to :~
that descri~ed for the housing 2 of Figure l. Similarly, each of the housin~s 26, 27 and 28 is ~i~ided into sections - by pe~ble beds through which pebbles are passed vert~cally :
form abo~e the housing to emerse below the housin~. Each of the housings 26, 27 and 2~ has four pebble beds pass5ng ~:.
theret~rough, the pebble beds being similar to pebble beds l~, 20, 21 and 22 of Figure l, or another series o~ pebble ~-~
beds suitable for the treatment of the particular waste gas concerned. Pe~ble 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.
P rts of the deta~l of Fisure l are omitted in Figure 2 rOr clar~ty.
In th$s em~odiment of the present in~e~tion the pebbles - . 13 comprising the pebble beds within housing 26 are fed successively through the corresponding pebbles beds ~n housings 27 an~ 28 before the pebbles 13 are extracted beneath hausing 28, cleaned and returned to the appropriate feeder abo~e housing 26 by means simila~ to that shown in Figure l. In conse~uence the pe~bles comprising pebble bed 2~ are used within that pebble bed in housing 26 to cool and clean the waste gas. The pe~bles 13 are then passed through an appropriate connection 41 into h~using 27 where they constitute pebble bed 3 and then similarly through another connection 42 into housing 28 where the pebbles constitute pebble bed 37. Similarly, the pebbles constituting pe~bie beds 30, 31 and 32 a~e subsequently - passed into housings 27 and 28 where they form part of pebble beds 34, 35 and 36 znd pe~ble beds 38, 39 and 40 respecti~ely.
~ ousing 26 constitutes the waste gas cleaner and heat exchan~er 43 for cooling the was~e gas and is similar in ooeratlon to the apparatus illus.rated in Figure l.
Housings 27 and 28 together ccnstitu~e the heating par~

WO~'/21429 f 210~9d PCT/~B92/01006 the apparatus in which cald ai r is heated and the pebbles 13 are cooled. Cold air at 20~C or below is supplied to the interior af hou5ing 28 through duc- 44 so that the cold air impinges immediately upon pe~ble bed ~7, wh~ch is that one of the four pebble beds in hou5ins 28 which is at the highest temperature, and goo~ coolins Oc pebble bed ~7 is cbtained. The heated air is then passad through pebble beds 38, 39 and 40 at each of progressively lower temperatures with the result that the air emergins from housing 28 through duc- 45 is only ~ery par~ally heated towards the temperature of the used gas. This :~
partially heated air is passed into the cool end of hous~ng 27 through duct 46 so that the partially heated air is progressively heated by pebble beds 36, 35, ~4 and 33 and air at a temperature of the order of 80% - gO% of the temperature of the wzste gas enter~g housing 26 ls ~-~
obtained from hous~ng 27 through duct 47. :~
The cooling of the waste gases in the series o~ pebble beds contained within h3using 2 of Figure 1 or houslng 26 of Flgure 2 ~s very ~uic~. This is impcrtant in ensur~ng the destruction of dioxins. It ~s known that d~oxtns are destroyed on combustion, but the elements of the d~oxin are still present in the sas as it $s cocled in a ccnventional waste gas cooling system. I' the cooling is not swift enough, the elements recombine to re-form the dioxins.
Currently, destruction of the combust~on 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 ~n the action of dest-oyins dioxins. 8y contrast the combined waste,gas cleaner and heat exchanger l and 43 of the present invent~on effects c~aling of the waste sas su~ficiently swiftly without allowin~ sufficient time for the d~oxins to be re-created and also reco~ers the heat from the was.e gas.
The embod~ments of the presen~ in~ention which ha~e been particul2rly desc~ibed are high temperature (140~C -250C) pebble bed heat exchan~e~s which serve 2s both heat recovery beds, waste gas s_~~bbers and as a primary lS. 21~9~345 part~cula~e remo~al system. The embodiment of Figure l of the acc~mpanying drawinss, in which the flow through the pebble bed heat exchangers is switched using reversing ~al~es 53 f-om the cooling and cleaning mode to the inlet air heating mode is sisnificantly m~re ener~y efficient than the flow reversal system which operates on a conventional regenerative furnac~ The emkodimant of Figure 2 of the accompanying drawings is as energy efficient as the Figure l embodiment with the added ad~antage that no re~ersing val~es are re~uired and th~ .
operating difficulties which may arise with such ~alves are totally avoided.
The embodiments of the present in~ention described herein:;
are believe to be capable of operation on gaseous emissions from a glass-making furnace to gi~e ~n o~erall 90% .
reduction for particulates of diameter greater than 2 :~
microns, and an 80% reduction of ~x by non-catalytic remo~al in the 870C to 1090C temperature range without emissions of excess ammonia~ The particul~tes trapped ~nclude the most sisnificant heavy metal contaminant, lead, which is present as the oxide and~or sulphate, and sod~um sulphate. A significant part (probably 50% or m~re) of the sulphur in the emisslons is present as the sulphate, the remainder being sulphur d~oxide which is remo~ed by con~entional treatment with lime, caustic soda or magnes~z in a pebble bed heat exchanger as described.
In 2 specific example of a method using 2pparatus accor~ing to the present invent~on with a press1~re drop of about 3 kilopascals across the pebble beds, it was found that the pebble bedslfiltered out 98~ of all particles - greater than 20 mic-ons diamete~, 95~ of all particles greate- than lO microns dizmete_, gO% of particles in the range of 8-lO mic~ons diameter, and up to about 60% of particles in the range of 2-a microns diameter. The particles of unmelted materials which get car~ied along in the waste gas are particles of ~iameter greater than 2 mic_ons. The part cles of ~iameter less than 2 microns Wo92t21429 ~ ~ PCT/GB92/0l006 21099~5 16.
lnclude ~apourised metals an~ metal compounds, less t~an ~;
50% of wh~ch are separated out by physical entrapment, and the bulk cf which are separated out by condensat~on as hereinbefore described.
It ls to be noted that the present in~ention will operate at a large range of pressure drops across the apparatus.
Unlike systems such as Ventur~ scrubbers, a large pressure ;~
drop ~s unnecessary. Only suffic~ent pressure to keep the waste gas flowing through the pebble beds ~s required. -.;
' ~

,, . ~ ' ,'

Claims

17.
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 horizontally spaced 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, 18.
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 of 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 19.
means for heating a coaled 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 20.
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.