GB1577234A - Apparatus for treating fly ash for the production of bricks or tiles - Google Patents

Description

PATENT SPECIFICATION

Application No 37895/79 Divided out of No 1 577 233 ( 11) 1 577 234 ( 22) Filed 15 Mar 1977 ( 31) Convention Application No 2611213 ( 32) Filed 17 Mar 1976 in ( 33) Federal Republic of Germany (DE) ( 44) Complete Specification Published 22 Oct 1980 ( 51) INT CL 3 F 27 D 13/00 ( 52) Index at Acceptance F 4 B 7 V 1 ( 54) APPARATUS FOR TREATING FLY ASH FOR THE PRODUCTION OF BRICKS OR TILES ( 71) We, STEAG AKTENGESELLSCHAFT, a German Body Corporate, of Bismarckstrasse 54, D-4300 Essen, German Federal Republic, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:-

The invention relates to an apparatus for treating fly ash, more particularly that originating from hard-coal fired power stations.

The invention furthermore relates to an improved form of fly ash and to bricks or tiles containing fly ash.

It is known to utilise the combustion residue, i e ash, which is obtained in power stations by mixing it with clay and moulding the mixture to form bricks or tiles which moulded shapes are then fired Such a process is particularly used for the manufacture of klinker bricks or clinkers The combustion residue consists mainly of combustion chamber granulate and fly ash particularly from hard-coal-fired power stations A comparatively large quantity of this raw material is obtained (between 4 and 6 million tonnes per annum in the Federal German Republic) The conversion of this material into bricks or tiles leads to a comparatively higher degree of value enhancement than its use in other building materials.

The conversion of hard-coal ash into bricks is known (West Virginia University, Coal Research Bureau, Technical Reports Nos 11, 16, 29, 34 and 55) Tests were carried out with a mixture containing approximately two-thirds by weight of fly ash obtained from the exhaust gas dust precipitator and water glass as bonding agent Semi-commercial tile-making tests with hard-coal ash and clay as bonding agent are also known (published in Brennstoff-WarmeKraft, Volume 8 ( 1956) pp 584, 587).

It is also known that in some circumstances fly ash contains widely varying quantities of residual carbon These contents can amount to 8 % and more and originate from the unburnt constituents in the raw dust The carbon must be removed because brick products call for a material free from nuclei of oxidisable particles to provide the necessary mechanical strength It was found that the removal of carbon by burning was the most efficient solution but that it was important to incorporate this procedure correctly into the manufacturing process on the one hand to obtain bricks of improved and more uniform quality, and on the other hand to achieve efficient manufacture of such products.

The carbon is removed by oxidation in the course of firing the bricks, but the time required for this procedure amounts to approximately 50 h in the case of brick products of the highest quality with a correspondingly dense structure of the body laboratory tests on specimens of 7 x 4 x 4 cm containing 65 % by weight of fly ash and varying amounts of carbon showed the following results in the temperature range between 600 and 800 C for the heating-up time which corresponds substantially to the time required for burning out the carbon:

C content(%) 0 2 4 6 8 Heating-up time (h) 3 0 6 0 7 5 9 5 11 5 These times are increased by a multiple in a large kiln for bricks of normal size ( 24 x 11 4 x 7 1 cm).

The substantially longer firing time of carbon-containing bricks compared with carbonfree bricks requires a proportional increase in the size of the kiln This greatly increases the investment costs of a brickwords and can render the conversion of hard-coal ash into bricks impossible for economical reasons Conversion into bricks of fly ash is therefore possible in these circumstances only up to a specific maximum carbon content because the firing operation is made extremely difficult by a higher and varying carbon content and can in ( 21) ( 62) I" m cl t_ t_ tn P 1 577 234 fact become uncontrollable There are, however, other reasons why an increase in the size of the brick kiln represents an insurmountable obstacle for converting hard-coal fly ash into bricks For economical reasons it is desirable for the brickworks to be erected directly adjacent to the power station as this saves transporation costs for the ash and breakdowns of transportation means can be minimized The brick kiln should be situated closely adjacent to the boiler house because the exhaust gases of the brick kiln contain sulphur dioxide and volatile fluorine compounds which can be rendered harmless in the interests of environmental protection by means of the power station flue gas purification system and thus without any substantial increase in costs.

However, it is this choice of location in the case of power stations which encounters substantial difficulties in many cases because space in the power station is insufficient to accommodate such large brick kilns.

A varying carbon content in the fly ash also leads to a varying compression density in the blanks This in turn leads to a varying density of the green bricks After firing, it causes varying porosity of the body with substantial quality differences in the end product as a result.

In Application No 10948/77 (Serial No.

1 577 233) out of which the present applications is divided, there is described and claimed a method of producing carbon-free fly ash comprising the steps of burning off the carbon in the raw fly ash with combustion air and then separating the solids from the combustion products Thus a carbon-free fly ash is produced which can be used for making bricks or tiles, thereby avoiding the problems described above which arise when carbon-containing fly-ash is employed for brick or tile making.

This process substantially shortens the time for burning out the carbon in the raw fly ash and the process can be performed in a separate kiln The large specific surface area of the finely divided carbon can be utilised by these means and on the other hand it is possible for the size of the kiln to be reduced and the quality of the end product is simultaneously improved while quality fluctuations are reduced to a minimum It is a consequence of the method in which the carbon is burnt to obtain a carbon-free dust that iron is oxidized into Fe 203 due to a specific excess of air in combustion if there is iron in the mixture.

It is another advantage of the method that it yields a fireproof clay (free of carbon), free of water, low shrinkage and pulverized) which can be used as an ideal semi-finished product for brickworks and other ceramic applications and can be used as filler, for example, for concrete, mortar, plastics, paper and board.

It is advantageous to perform combustion of the carbon at a temperature which is below the softening point of the fly ash This defines an upper limit for the combustion temperature.

The lower limit is the ignition temperature of the carbon In one embodiment of the method according to the invention, the combustion temperature is therefore between approximately 70 600 and 1000 TC To effect instantaneous ignition of the carbon, preferably the material is preheated to approximately 500 TC before combustion of the carbon is performed.

Advantageously, the method performed so 75 that the quantity of raw fly ash and the amount of combustion air supplied thereto is maintained at a constant value in order to simplify the process In this case, additional fuel is supplied to the raw material if the carbon content 80 falls This fuel can be coal dust or a combustible gas.

Using this method, a fly ash which is practically free of carbon can be produced from the raw dust The mixture from which the bricks 85 are to be moulded and fired is therefore practically free of carbon After being fired, these bricks have a lower pore volume, which is a decisive feature for a higher body density and therefore for a higher body quality 90 The present invention is directed to an apparatus for producing carbon-free fly ash.

According to the present invention, apparatus for producing carbon-free fly ash comprises an upright tubular combustion chamber, 95 an outlet at the top of said chamber, injector means for injecting preheated raw fly ash and combustion air into the bottom of said combustion chamber to be carried upwardly by the air stream to said outlet, a bunker surrounding 100 said combustion chamber, means for feeding raw fly ash into said bunker to be preheated therein by heat from the combustion chamber and feed means for feeding the preheated fly ash from the bunker to said injector means 105 The accompanying drawing illustrates, in diagrammatic form, one exemplified form of apparatus according to the invention suitable for producing carbon-free fly ash.

The apparatus illustrated in the drawing is 110 suitable for a capacity of approximately 25 tonnes/h of fly ash containing approximately % of carbon This apparatus is also described in the specification of copending Application

No 10948/77 (Serial No 1 577 233) out of 115 which the present application is divided The fly ash is obtained from the flue gas passing through a dust precipitator plant of a power station powered by hard coal but not shown.

The raw dust or ash from the precipitator 120 flows via a pipeline 1 to a treatment apparatus.

Initially, the dust passes into an annular bunker 2 around a combustion tube 3 The capacity of the bunker 2 is chosen in accordance with the rate of throughput so that the fly ash is held in 125 the bunker for approximately one to two hours During this time it is preheated to at least 5000 C by the hot gases in the combustion tube 3.

Under gravity, the raw ash flows downwardly 130 1 577 234 through the bunker 2 and the heated ash passes into the bunker discharge 4 and from there into a pipeline 5 The pipeline 5 conveys the hoot raw ash into the suction entry of an injector, references in its entirety by the numeral 6.

Preheated combustion air is fed at the rate of 12,000 N 3 N Tp/h in the case of the selected numeral example, into the injector 6 to entrain the raw ash and to blow it into the bottom conically flared part 7 of the combustion tube 3.

The flow velocities are reduced and the pressure is increased as the air passes upwardly through the conical flaring of the part 7 This ensures that coarse carbon particles dwell for a longer period in the combustion tube 3 than the smaller carbon particles, thus resulting in complete combustion of the carbon.

In cross-section, the combustion tube 3 above the conically flared part 7 has a diameter of approximately 2 m and this part is approximately 21/4 m long The dwell time of the dust is therefore approximately 5 seconds.

The preheating of the ash raises the initial temperature of the dust to approximately 500 C This leads to reliable ignition and complete combustion of the carbon in the tube 3.

The combustion temperature is in the range between 600 and 1,0000 C.

These conditions in the combustion tube are maintained constant by maintaining constant amounts of air and dust If the dust contains % of carbon and if approximately 2 kg of dust/m 3 N Tp are blown into the system, the final temperature will be approximately 900 TC.

If the carbon content in the raw dust drops to approximately 3 % C, a temperature of approximately 600 TC will be reached and this will have to be regarded as the bottom limit for the required rapid combustion.

The temperature must be increased as soon as the C content drops below 3 % Carbon dust, held available in a bunker 8 and passing into the injector 6 through a duct 9, is provided to this end The system is automatically controlled.

Valve means 10, incorporated into the coal dust supply duct 9, are therefore controlled through a temperature sensor 11 which is installed in a discharge duct 13 that extends from the combustion tube 3 The said temperature sensor 11 controls the supply of coal dust to the injector 6 so that the temperature at the outlet of the combustion tube is at least 600 TC.

The amount of air is adjusted at 1 m 3 NTP per 2 kg of raw dust at 5 % C so as to ensure complete combustion into CO 2 If the carbon content exceeds 5 %, part of the carbon will merely be burnt into CO so that the final temperature does not exceed 900 TC despite the increased C content.

In the embodiment illustrated in the accompanying drawing the residuel gases containing C-O are oxidized into CO 2 in an afterburning chamber 14 However, a maximum temperature, conveniently also 1000 TC, should not be exceeded The combustion chamber 14 is therefore fed with supplementary air The supplementary air enters at 15 and is controlled by valve means 16 The valve means 16 in turn are controlled by a temperature sensor 17 installed in the exhaust gas duct 18 downstream of the 70 afterburning chamber 14 The valve means 16 therefore regulates the amount of supplementary air so that a temperature of 1,000 TC is not exceeded.

If the raw dust has an extremely large C 75 content, for example in excess of 10 %, the amount of supplementary air at 15 would become excessive The supply of raw dust is therefore substantially reduced in such cases.

To this end, a regulating element 19 is in 80 corporated into the duct 5 and can be embodied, for example in the form of an apertured slider The flow rate of dust can be suddenly reduced by means of such a slider A regulating element 19 with a continuous closing action 85 would not be sufficiently reliable because the hot dust, would have acquired a temperature of the order of 5000 C.

The combustion air fed to the injector 6 is preheated by a heat exchanger 20 incorporated 90 in the exhaust gas duct 18 This con ustion air is supplied at 21 to a compressor 22 for feeding to the heat exchanger 20 A second heat exchanger 23 abstracts heat from the exhaust gases which are discharged from the heat ex 95 changer 20 in duct 18 and this heat can be supplied to a waste heat boiler, not shown Such a boiler will supply saturated steam at the rate of 15 tonnes/h and a pressure of 8 atm abs if the raw dust contains approximately 5 % of 100 carbon The saturated steam can be used for drying the green bricks which are moulded from the mixture as described later.

Downstream of the heat exchanger 23 in the illustrated embodiment the exhaust gas 105 has been cooled to approximately 100 TC It then passes into a cyclone separator 24 in which the major part of dust is separated from the gas stream for transfer to a collecting bin Another dust precipitator 26 in the illu 110 strated embodiment is constructed as a hose filter and delivers the separated fine dust through a duct 27 to the bin 25 while the exhaust gas which has been stripped of dust is discharged from the system at 28 115 To start the system from cold, combustion gas is introduced in place of the coal dust which passes via the regulating element 10 to the injector 6, until the temperature at the outlet of the combustion tube 3 has risen to 120 approximately 800 TC and the dust in the bunker 2 has been preheated to 500 TC The regulating element 19 initially remains closed.

When the stated temperatures are reached, the supply of gas is shut down after the supply of 125 dust has first been released.

In a simplified construction, the supply of coal dust 8-10 is replaced by the gas feed used for starting up the system.

Bricks can be manufactured by the produc 130 1 577 234 tion of a moist mixture from the fly ash at 25, screened combustion chamber granulate, and a bonding agent, from which said mixture the bricks are moulded After drying, these bricks are fired in a kiln.

In the selected example relating to raw dust containing 5 % C the entire firing time for standard format bricks is approximately 60 h.

The method described above results in a reduction to 20 h This means that, given the same output of the brickworks, the brick kiln need only be designed to a third of its normal size.

Claims (13)

WHAT WE CLAIM IS:-

1 Apparatus for producing carbon-free fly ash comprising an upright tubular combustion chamber, an outlet at the top of said chamber, injectormeans for injecting preheated raw fly ash and combustion air into the bottom of said combustion chamber to be carried upwardly by the air stream to said outlet, a bunker surrounding said combustion chamber, means for feeding raw fly ash into said bunker to be preheated therein by heat from the combustion chamber and feed means for feeding the preheated fly ash from the bunker to said injector means.

2 Apparatus as claimed in claim 1 and further having separator means connected to said outlet for separating carbon-free fly ash from the combustion products leaving the combustion chamber.

3 Apparatus as claimed in either claim 1 or claim 2 wherein the combustion chamber is conically flared outwardly in the upward direction at the lower end of said chamber.

4 Apparatus as claimed in any of the preceding claims and having an afterburning chamber connected to said outlet.

Apparatus as claimed in any of the preceding claims and having a waste heat boiler and a heat exchanger connected to said outlet to transfer heat from the combustion products to said waste heat boiler.

6 Apparatus as claimed in any of the preceding claims and having a heat exchanger connected in said outlet for the combustion products and arranged to transfer heat from the combustion products to said combustion air.

7 Apparatus as claimed in claim 6 and having a supplementary air feed to said afterburning chamber, an adjustable regulator for said supplementary air feed, and temperature sensing means responsive to the temperature of the combustion products from said afterburning chamber arranged to control said regulator in response to the sensed temperature.

8 Apparatus as claimed in any of the preceding claims and having control means for regulating the feed of preheated fly ash from said bunker to said injector means.

9 Apparatus as claimed in any of the preceding claims and having an auxiliary fuel supply to said combustion chamber.

Apparatus as claimed in claim 9 and having a temperature sensor in the outlet from said combustion chamber arranged to control the fuel feed from said auxiliary supply.

11 Apparatus as claimed in either claim 9 or claim 10 wherein said auxiliary fuel supply comprises a coal dust bunker and feed duct.

12 Apparatus as claimed in either claim 9 or claim 10 wherein said auxiliary fuel supply comprises a gas supply.

13 Apparatus for producing carbon-free fly ash substantially as hereinbefore described with reference to the accompanying drawing.

BOULT, WADE & TENNANT AGENTS FOR THE APPLICANTS Printed for Her Majesty's Stationery Office by MULTIPLEX techniques ltd, St Mary Cray, Kent 1980 Published at the Patent Office, 25 Southampton Buildings, London WC 2 l AY, from which copies may be obtained.