GB1601009A

GB1601009A - Process and plant for treating flue dust

Info

Publication number: GB1601009A
Application number: GB19171/78A
Authority: GB
Original assignee: Steag GmbH
Current assignee: Steag GmbH
Priority date: 1977-05-14
Filing date: 1978-05-12
Publication date: 1981-10-21
Also published as: DE2721932C2; DE2721932B1; PL110665B1; PL206728A1; NL7805009A; FR2390391B3; FR2390391A1

Description

(54) PROCESS AND PLANT FOR TREATING FLUE DUST (71) We, STEAG AKTIENGESELLSCHAFT, a Germany Body Corporate of Bismarckstrasse 54, 4300 Essen, Federal Republic of Germany, 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 a process for producing from flue dust a substance which is suitable for dumping or for use as an intermediate product, the process being of the kind comprising combusting the carbon content of the flue dust to melt the precipitated flue dust, producing from the melt encrusted globules of melt, and cooling the encrusted globules of melt, to produce a granular material. The invention also relates to a plant for carrying out a process of the kind referred to.

The invention can be applied in particular to coal dust firing, especially to pit-coal dust firing for large boilers, such as are used in power stations. In these boiler plants the ash is in the form of a flour-fine dust. The dumping of this dust, which is soluble to a certain extent in water, presents considerable problems. It is known to mix the flue dust produced in coal dust firings with coal, and to fire this under a separate steam boiler, so that a temperature is attained which lies above the melting point of the ash. The ash is therefore liquefied, and is introduced in a molten steam into a water bath. A granular material then forms which is fragile in its solidified form. A granular material of this type cannot be used as an intermediate product without further processing.A further disadvantage is the plant required, since this is of relatively large construction and can only be operated in conjunction with the main boiler.

Also known are the so-called melting chamber boilers in which a liquid slag is yielded.

This slag flows into a water bath in which the slag solidifies to a brittle silicate glass. In melting chamber boilers of this kind around half of the flue dust is fused in with the slag yielded, and the other half is separated out from the smoke gases with the aid of a filtering plant and re-introduced into the boiler. It is estimated that around 6 percent by weight of carbon is required for melting the dust. A disadvantage here is that the substance produced from the flue dust in the described manner likewise cannot be used as an intermediate product without further processing, due to its tendency to break up into small pieces. In addition, the process is not satisfactory from an economic point of view, since the melting heat of the slag is largely lost. On the other hand, the operation of this type of boiler plant has so far been without problems.The fire-resistant cladding which is required for lining out the furnace, which protects the steam pipes from attack by the aggressive slag, has in fact proved itself satisfactorily in long-term operation, and has then endured through the inspection intervals.

A change has occurred here, arising from considerations concerned with the protection of the environment, the increased price of pit-coal and the fact that owing to the latter circumstance the production of electrical power from pit-coal has been more emphatically displaced into the medium and low load range. This means frequent running-up and running-down of the boiler, and therefore thermal stressing of the cladding, which greatly reduces its working life. In addition, the combustion temperatures of up to 18000 C which have been maintained until now in the melting chamber boiler cause some of the air-nitrogen to be oxidised to form nitric oxide, which contributes to a great extent to air pollution, especially in dense industrial areas.

For this reason, in large new power station complexes, the boiler evaluated in the introduction is again being introduced. The combustion temperature in these boilers is kept down to a level at which the particles of ash remain below their melting temperature and therefore fire-clay cladding is not required. In addition, the formation of nitric oxide is greatly reduced, and the investment required for the boilers is less.

However, a new problem has been created by the disposal of the flue dust, due to its flour-fineness and its ability to dissolve in water, since considerable costs are involved in dumping it in an environmentally acceptable manner. The invention is therefore based on the task of producing from the flue dust in an economically viable manner a product which can be dumped more acceptably than hitherto, or an intermediate product which can be economically further processed.

According to one aspect of the present invention a process of the kind referred to for producing from flue dust a substance which is suitable for dumping or for use as an intermediate product, is characterised in that the rate of removal of heat from the encrusted globules of melt during cooling of the latter is controlled so that a crystalline structure corresponding to, or at least substantially similar to, that of basalt is obtained in the granular material, and in that heat recovered from the cooling of the encrusted globules of melt is used to pre-heat the flue dust.

The pre-heating of the flue dust serves firstly to enable instantaneous ignition of the residual carbon during combustion so that the melting time can be kept to a minimum, and secondly to reduce the amount of carbon required for melting, which can be reduced to the residual content of carbon in the flue dust (e.g. in coal dust firing with pit-coal the level of residual carbon generally lies between 3 and 5%). The removal of the heat from the encrusted globules of melt is a step which is comparable to annealing, and reduces the thermal stresses in the granular material and allows the formation of a crystalline structure which corresponds to that of extrusive or igneous rocks; the reference to basalt is to be taken in this sense.The high strength and other properties of this type of rock, which are important for the dumping and further processing of such granular material, are sufficiently well-known.

Preferably, the flue dust is melted without additional carbon, the amount of carbon which is necessary for this being covered by the residual carbon in the flue dust involved. In this case, there is a considerable saving in fuel compared with the flue dust melting process carried out until now, amounting to around 3%.

Suitably a mineral or mixtures of minerals can be added to the flue dust before the latter is melted; these act as crystallisation aids in the achievement of the desired crystalline structure of the granular material. Lime can be used for this purpose, for example; this has the capacity to reduce the crystallisation or annealing time. A tough, tensile extrusive rock which can be used as a high-grade raw material in the construction industry is produced thereby.

It has generally been found satisfactory to pre-heat the flue dust to around 600C C and to cool slowly (i.e. anneal) the encrusted globules of melt in the temperature range from 1300 1100 C.

The process according to the invention is particularly economical if the waste heat from the melting of the flue dust is used for pre-heating the combustion air.

Particularly when the carbon level in the flue dust varies or changes, according to a further embodiment of the invention the basic temperature of the melt can be used as a guide for the amount of carbon to be added to the flue dust when the level of residual carbon is too low, or for the addition of extra air when the level of residual carbon is too high.

The main advantages obtained with the invention consist primarily in the fact that the flue dust, e.g. from large steam generators fired with solid fossilised fuels, particularly with pit-coal, can be converted to a high-grade intermediate product, such as a construction material, irrespective, for example, of the range in which the boiler plant is operating according to the schedule of the power station, without occasioning increased costs. If there are marketing difficulties, the material produced in this way can also be stored without the problems which normally arise with dust dumping.

According to another aspect of the present invention there is provided plant for carrying out the process according to said one aspect of the invention, comprising a preheater to which the flue dust is fed, a combustion chamber, an injector for feeding the pre-heated flue dust and combustion air together into the combustion chamber, a granulation drum for receiving molten material from the combustion chamber and forming it into encrusted globules of melt, an annealing vessel in which encrusted globules of melt from the drum are cooled at a controlled rate, and heat-exchange means for transferring heat from the annealing vessel to the preheater.

The invention will now be described, by way of example, with particular reference to the accompanying drawing the sole Figure of which shows a plant, for carrying out the process according to the invention, in the form of a block diagram.

The block diagram shows the proportions for a 2 x 700 MW pit-coal power station, where around 300,000 tonnes of flue dust are yielded per annum. Furthermore, the plant shown is laid out with the proportions adopted to avoid thermal stresses in the cladding of the melting tube for an even charge of 20 tonnes/hour with 7,500 hours/annum.

At 1, a feeder 2 supplies the intially cold flue dust to a dust pre-heater 3. This is a container with heating coils 3a consisting of pipes. Heated air flows through the inside of the pipes. The cold flue dust is heated in the pre-heater, and in the construction example shown its temperature is raised to 1000"C.

The dust flows at this temperature under the influence of gravity into an injector 10. The injector is loaded with pre-heated propellant air. The propellant air is also heated to 1000" C in the construction example shown. Corresponding to the level of residual carbon in the flue dust, the amount of propellant air is set at 5380 m3/h so that it is stoichiometrically sufficient for the combustion of the carbon to form CO2. In addition, the injector serves as a mixing nozzle in which, due to the strong turbulence, thorough mixing of the flue dust and the propellant air takes place. Accordingly, there is immediate ignition of the hot carbon contained in the flue dust.

The dust-air mixture flows from the top towards the bottom centrally into a combustion chamber 11, which is constructed as a cylindrical tube with a diameter of around 2 m. In the combustion tube the dust-air speed is reduced to around 3 m/s, so that with approximately 30 m height there is a period of dwell of around 10 s.

This means that a relatively long period of time is involved, which results in all the dust particles being melted. The liquid melt is deposited on a built-in recovery grid 11a at the lower end of the combustion chamber 11 and flows as a continuous stream into a collecting hopper llb.

Hot smoke gas flows over a droplet separator (not shown) at around 1400 C, to a pre-heating unit 9 for heating up combustion air 7. The smoke gas cools down, in the construction example shown, from 1400 C to 565" C. For this reason, in the hot part down to 12000 C the heat-exchanger 9 is constructed of high-temperature resistant material, such as molybdenum, titanium or ceramic, for instance. Below 1200 C the normal commercial heat and scale resistant, high chrome content steels are adequate.

In the construction example shown, the liquid melt flows at 12 at 1400 C out of the hopper 11b and into a rotary drum 13 which is cooled slightly from the outside. The stream arriving on the inner wall of the rotary drum is broken up so that globules or spheres form which harden initially on the surface, but remain soft on the inside (i.e. the globules of melt are encrusted). The thickness of the stream of molten material determines the diameter of these encrusted globules. If the diameter of the globules is reduced, then the period of dwell in the rotary drum is reduced proportionately.These parameters thus make it possible to adapt the size of the granules of the granular material, which are formed subsequently by cooling the encrusted globules, to the particular requirements of the market without the subsequent use of grinding plant.

About 20% of the heat contained in the melt is lost in the rotary drum 13 which acts as the granulating appliance (i.e. for forming the encrusted globules of melt). The surface hardened (i.e. encrusted) globules of melt drop out of the drum at 14 with a mean temperature of around 12500C (corresponding to 320 kcal/kg) in the construction example shown, into an annealing vessel 15. In the annealing vessel the encrusted globules are cooled down slowly, preferably in the range from 1300 - 1100" C, to form granular material.

A crystalline structure is formed during this cooling and centre-hardening process, which structure corresponds to, or is substantially similar to, that of basalt. The aim of the annealing (or slow cooling) process is above all to prevent the formation of a brittle aluminium silicate glass.

The cooling air, which in the construction example shown is supplied in an amount of 14,500 m3/h at 17 and is forced through the annealing vessel from the bottom towards the top with the aid of a blower 16, is finally heated to 1200"C, after it has extracted this amount of heat from the encrusted globules during their formation into granular material. The hot air produced in this way is used in the construction example shown to pre-heat the cold flue dust in the dust pre-heater 3 to the above-mentioned temperature of around 1000" C. After it emerges from the dust pre-heater, the cooling air has a residual temperature of around 300 C.It is combined with the smoke gas at 565 degrees from the air pre-heater 9 in a mixing chamber 21, and emerges at 20 at around 400" C into the open air.

This mixing chamber 21 also has the task of after-burning unburnt carbon monoxide from the melting chamber 11 with an excess amount of air. This is necessary when the carbon content of the flue dust is higher than 2.88 percent by weight. If such a composition occurs frequently during the course of operation, it can be expedient from the point of view of energy conservation for the plant to be extended by the addition of a waste heat boiler connected in front of the waste gas chimney shown at 20. With 5 percent by weight C in the flue dust, around 5 Gcal/h can be obtained additionally in the waste-heat boiler.

Control of the process is effected in a simple manner. The raw dust supply is maintained constant via the feeder indicated at 2. The combustion air supply at 7 and the cooling air supply at 17 are controlled by suitable blowers.

If the level of the carbon content in the flue dust falls below the set value, the final temperature of 1400 C which accords with the layout of the plant is not attained. In this case a control valve 22 is actuated by a melt temperature sensor 24. Additional fuel 23 can then reach the combustion chamber 11 via the mixing nozzle 10, until the required temperature of 1400 C is attained again, which is maintained by the control circuit indicated at 24 - 22.

If the level of the carbon content in the flue dust is too high, owing to the constant amount of air supplied only some of the carbon burns and carbon monoxide is formed, which reduces the temperature. In the following, the anticipated conditions and the operating conditions with too high a proportion of carbon are compared: C + 2 = CO2 - 94 kcal (anticipated conditions) 2C + 2 = 2 CO 52 kcal (too high a proportion of C) Deficiency : 42 kcal With the aid of additional air some of the carbon monoxide is burnt to form carbon dioxide, i.e.

0.62 CO + 0.31 O2 = 0.62 CO2- 0.62 x 68 = - 42 kcal If there is an excess quantity of carbon monoxide, then the control valve 26 is operated by a temperature sensor 25 so that hot air from the annealing vessel 15 flows into the combustion chamber 11 until the required temperature of 1400 C is attained, and the temperature is maintained by the control circuit 25 - 26. Starting up the described plant from the cold condition is carried out as follows: First the dust pre-heater 3 is filled with cold flue dust 1 by the dust feeder 2. The shut-off slide valve 4 is closed during this process. Then the cooling air blower 16 is switched on. The cold cooling air flows via the empty annealing vessel 15 into the start-up combustion chamber 5, where it is heated up slowly to 12000 C by the start-up fuel 6.

After several hours the raw dust has been heated to around 1000" C. During the heating-up period the feeder 2 is shut off. The duration of the starting up process is chosen so that dangerous thermal stresses are avoided. A period of time of around 8 hours can be allotted for this. It will be appreciated also that where possible, the described plant should be operated continuously.

The combustion air blower 8 is switched on and the air is blown via the injector 10, to which additional fuel is supplied and ignited, into the combustion chamber 11. The supply of fuel is controlled so that the melting chamber 11 is heated up to 1400 C over a period of several hours. As in the operating state described initially (continuous operation), the waste gases heat up the fresh air in the heat-exchanger 9. When the required temperatures are attained (dust 1000" C) the shut-off slide valve 4 is opened, so that now dust can be sucked in by the injector 10. At the same time, the feeder 2 is switched on.

The first dust from the tip of the pre-heater 3 is still cold. For this reason the supply of fuel 23 continues until the dust temperature has reached 1000" C. The granulation drum 13 is then set in operation. The cooling air blower 16 is switched off for about 1 hour at the described operating state, and the start-up fuel 6 is shut off for the entire running time.

As soon as the annealing vessel 15 is approximately half full, the cooling air blower 16 is switched on again. After the annealing vessel 15 has been filled to the required level, the slag lock 18 is set in operation. The operating state described initially (continuous operation) now obtains.

The granular material produced can be formed of pieces sized from around 10 - 100 mm, so that additional breaking down can normally be dispensed with. The number of personnel required is relatively low (around 1 man/shift) due to the high degree of mechanisation. The total amount of air required amounts to around 0.5% of that required for the power station of which the entire yield of flue dust is processed in the plant to give the described product.

This means that the dimensions of the plant are relatively small, which results in correspondingly low investment costs.

WHAT WE CLAIM IS: 1. A process for producing from flue dust a substance which is suitable for dumping or for use as an intermediate product, which process involves combusting the carbon content of the flue dust to melt the precipitated flue dust, producing from the melt encrusted globules of melt, and cooling the encrusted globules of melt to produce granular material, characterised in that the rate of removal of heat from the encrusted globules of melt during cooling of the latter is controlled so that a crystalline structure corresponding to, or at least substantially similar to, that of basalt is obtained in the granular material, and in that heat recovered from the cooling of the encrusted globules of melt is used to pre-heat the flue dust.

2. A process according to claim 1, characterised in that the flue dust is melted without addition of carbon.

3. A process according to claim 1 or claim 2, characterised in that minerals which act as crystallisation aids in the achievement of the desired crystalline structure for the granular material are added to the flue dust before it is melted.

4. A process according to any of claims 1 to 3, characterised in that the flue dust is pre-heated to at least the ignition temperature of the residual carbon therein, and the encrusted globules of melt are then slowly cooled, being at all times in the temperature range from 1300 to 11000C, to produce the desired crystalline structure for the granular material.

5. A process according to any of claims 1 to 4, characterised in that heat recovered from waste gases from the chamber in which the melt is formed is used for pre-heating the air in which the carbon content of the flue dust is combusted.

6. A process according to any of claims 1 to 5, characterised in that the ultimate temperature of the melt is used as a control parameter either for the amount of carbon to be added to the flue dust when there is a carbon deficiency in the flue dust, or for the addition of extra air when the carbon content of the flue dust is too high.

7. Plant for carrying out the process according to any one of the preceding claims, comprising a preheater to which the flue dust is fed, a combustion chamber, an injector for feeding the pre-heated flue dust and combustion air together into the combustion chamber, a granulation drum for receiving molten material from the combustion chamber and forming it into encrusted globules of melt, an annealing vessel in which encrusted globules of melt from the drum are cooled at a controlled rate, and heat-exchange means for transferring heat from the annealing vessel to the preheater.

8. A plant according to claim 7, in which the combustion chamber is constructed as a vertically arranged vessel, the injector feeding the dust and air into an upper region thereof and the granulation drum receiving melt from a lower region thereof.

9. A plant according to claim 7 or claim 8, in which means is provided to supply additional air or supplementary fuel to the combustion chamber and thereby to control the temperature in the combustion chamber.

10. A process for producing an improved product from flue dust substantially as hereinbefore described with reference to the accompanying drawing.

11. A flue dust treatment plant substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. WHAT WE CLAIM IS:

1. A process for producing from flue dust a substance which is suitable for dumping or for use as an intermediate product, which process involves combusting the carbon content of the flue dust to melt the precipitated flue dust, producing from the melt encrusted globules of melt, and cooling the encrusted globules of melt to produce granular material, characterised in that the rate of removal of heat from the encrusted globules of melt during cooling of the latter is controlled so that a crystalline structure corresponding to, or at least substantially similar to, that of basalt is obtained in the granular material, and in that heat recovered from the cooling of the encrusted globules of melt is used to pre-heat the flue dust.