DK175803B1 - Method for treating fly ash and device for use therein - Google Patents

Description

DK 175803 B1 i

The present invention relates to a process for granulating fly ash containing calcium oxide and optionally calcium sulfate by admixing with water, whereby the ash is granulated to an easily manageable and disposable form while grinding the calcium oxide. According to the invention, the mixture is effected by free-fall mixture 5 with the addition of water relative to the amount of ash. The invention also relates to a device for use in carrying out the method, which comprises means for sensing the consistency of the mixture and means for controlling the water supply in dependence thereof.

10 FB type boilers, ie. those which use a fluidizing bed are used in e.g. CHP. The combustion takes place in these with the fuel particles in a bed of fine sand, which is kept floating by the air being blown at high speed. The bearing thereby has liquid-like properties.

15 One of the great advantages of combustion in a fluidizing bed is that you can fire with fuels with different properties in the same boiler. In FB boilers, one thus fires with e.g. coal, wood, peat and other solid fuels such as waste either individually or in different combinations.

Another great advantage of combustion in a fluidising bed is that the emission of sulfur and nitrogen oxides can be reduced in a simpler way than in other types of combustion plants. Particularly coal and peat contain sulfur, whereby the sulfur can be bonded by adding ground limestone to the bed. The limestone reacts with the sulfur dioxide formed to form calcium sulfate.

25

The oxidation of the nitrogen of the air to nitric oxide is temperature dependent. The relatively low combustion temperature in FB boilers, 800 ° - 900 ° C, means that the emission of nitric oxide can be kept at a low level. The normal combustion temperature in other types of boilers is above 1000 ° C.

30 '

However, with large boilers, large quantities of fly ash are obtained, which must be taken care of, which leads to practical problems as well as working environment problems.

Thus, when fired with sulfur-containing fuels, the fly ash will contain 35 calcium sulfate, an excess of added lime in the form of CaCO 3 and lime burnt in the fireplace,

I DK 175803 B1 I

CaO. As mentioned, the limestone is added to minimize sulfur emissions. Limestone I

the amount is determined by the sulfur content of the fuel and the ratio of Ca / S, which usually I

is 1.5 - 3. If the fuel or fuel mixture itself contains a large I

proportion of lime, the addition of lime can be reduced. Also required

5 Minor or no lime addition for low sulfur fuels. It adds

set lime is constituted by CaCO3. When CaCO 3 is heated to temperatures above 800 ° C, I

it undergoes a calcination process. The reaction proceeds according to: -

CaCO3 (s) .......> CaO (s) + CO2 (g). IN

The calcium oxide formed during the calcination can be reacted in the presence of oxygen

sulfur dioxide forming calcium sulphate: I

CaO (s) + SO2 (g) + y2 O2 ......> CaSO4 (s). IN

This reaction is called sulfation. IN

Many parameters are part of the desulphurisation process. This results in the supply of I

Limestone cannot be utilized 100%, but usually only 20-60%. One of the most important control parameters is the temperature of the fireplace, which can be varied between 800

and 900 ° C. IN

The degree of desulfurization can be regarded as a clash between two competing pro- I

cesses, namely the above I

oxygen reduction process which gives calcium sulfate and the following reaction which is in I

equilibrium with it: I

2 CaSO4 (s) + CO / H2 (g) ......> 2 CaO (s) + 2SO2 + CO2 / H2O (g). IN

Both processes can be studied in a stability diagram. IN

If the utilization rate of the applied limestone is low, a large I is excreted

amount of burnt lime from the flue gases in the boiler's electric filter and thus recovered in I

35 fly ash. This may also contain CaO derived from the fuel naturally I

DK 175803 B1 3 occurring calcium. The burnt lime is later crushed by the removal of the ash by wetting the ash. The chemical reaction of crushed lime is:

CaO + H2O .......> Ca (OH) 2.

5

This reaction takes place during strong heat development, which causes both handling and working environment problems.

Thus, the amount of CaO in the fly ash should be limited. On the other hand, 10 extends desulphurisation, which means that one usually overdoses the addition of limestone, which also gives an excess of CaO.

Today, the fly ash separated in the electro filter is usually transported, which constitutes approx. 85% of the total ash volume, via a pneumatic conveyor system to one or 15 more silos: From there they are often loaded via a cell feeder and a humidifier on trucks. The humidifier may consist of an elongated trough with a paddle, which mixes the ashes with water injected with nozzles and transports the mixture to a sludge, from which it is loaded onto a load sheet in the form of a sticky mass.

20 The requirement for the unloaded product is that it must not be dusty or too "soft" for work environment reasons, as this will cause major problems as a result of adhesion to the surface of the drawer or as a result of ash mixed water leaking from loading area. Ashes from different fuels have completely different ability to bind water, causing difficulties in obtaining masses with the "right" consistency.

Due to the water mixing required to reduce the dust formation, the fly ash content of burnt lime and calcium sulfate causes chemical reactions such as leaching of the burnt lime with vigorous heat generation and gypsum formation, thereby seeking to bind water molecules crystalline to the CaSCV molecules.

The limescale and gypsum formation causes the pulp to collapse and adhere to the ladle, causing major problems with the tipping, as well as depositing on the mixing trough and the padding introduced therein. This leads to the need for a large manual _ _

I DK 175803 B1 I

IN

In work effort in difficult working environment conditions, as heavy tools must be hand- I

Iteres to remove the adhesive while causing strong I

In dusting during work.

I 5 The heat generated during the curing process, which reaches its maximum I

For 5-8 minutes after the addition of water, in some cases at the current use I

In tea methods, in the soft mass of the charge, steam bubbles are formed which rise upwards

Outbreaks occur and cause large parts of the cargo to be thrown out from the cargo image. IN

I This poses a serious risk of accidents and also environmental problems. IN

I 10 I

The main object of the present invention is to provide a technique for use

In the act of fly ash of the current type in such a way that the undesirable con- I

In sequences of the chemical reactions are eliminated and that instead of on an advantageous day

This method is used to transform the ash into a simple, manageable and landfill

In removable form, such as removes the environmental risks mentioned above. IN

In accordance with the invention lies the insight that this object can be achieved by machining

In the fly ash before it is transported further to form a dry granulate such as you

You do not stick to the surfaces with which it comes in contact, and on which you were

In the 20 meevelopment can not cause entrapped bubbles, which may burst. I stand- I

In it, hot vapor streams will be able to pass through the granules without

In problems. The larger contact surface of the granulate with the surrounding air results in I

more efficient cooling of the granulate, further reducing the pro- I

In the blisters. IN

I I

For the formation of the granulate, the reaction may be used in conjunction with leaching

In one of the lime by mixing the fly ash with water and the gypsum-forming effect, I

You are thereby obtained when the ash contains calcium sulphate. IN

I No granulation can be obtained from the previously used mixture of aircraft I

In ash and water in a trough form or similar. According to the present invention, this is required

In the invention, the fly ash is subjected to a so-called free fall mixture during addition I

Of water in proportion to the amount of ash so that the contained calcium oxide is quenched and there

gypsum formation occurs upon the occurrence of calcium sulphate, which reactions as follows

In 35 g of the free-fall mixture gives rise to granulation of the ash. IN

5 DK 175803 B1

Thus, by means of an accurate regulation of the amount of water, a granulation of the ash can be effected by means of the method, which granules can then be easily transported and deposited. Depending on the chemical composition of the ash in general, the granulate may possibly be returned to nature or used in, for example, building materials.

The mixing of fly ash and water can be done in portions, whereby a consistency of the consistency of the mixture can be sensed and the water supply is controlled accordingly.

10

The particular characteristic of a device for use in carrying out the method is apparent from the following claims.

The invention will be described in more detail below with reference to the exemplary embodiment of a fly ash processing device shown in the accompanying drawing.

Numbers 1 and 2 denote two silos for receiving the fly ash from a large solid fuel fired boiler, which may not be CFB-type, e.g. working with a circulating fluidizing bed. The fly ash has a degree of fineness which roughly corresponds to cement with a surface weight of 400 m2 / kg. in addition to the minerals contained in the fuels, the ash also contains sand from the fluidizing bed as well as Cao and CaSO4 when adding lime to desulphurisation. The composition of the ash will vary considerably depending on which fuel or combinations of fuel are used. When using wood fuel containing small amounts of sulfur, little or no addition of lime is needed. However, the fuel contains relatively large amounts of alkali metals, which is why wood ash will also contain calcium.

30 When burning with peat, which has a high sulfur content, a lot of limestone is added, which is why the amount of calcium in the ash becomes large, since the fuel itself contains calcium. The ash will also contain CaSO 4 as a desulfurization product. This also applies to coal which has a high sulfur content, which requires a large amount of limestone. \ 35

I DK 175803 B1 I

I 6 I

The fly ash is passed from the silos via an associated conveyor 3, 4 to a weight 5 which I

You weigh a predetermined portion of fly ash. This is conducted via an additional I

conveyor 6 to a mixing machine 7 connected to a water supply line 8. I

5 The mixing machine 7 can in principle be designed as one used for concrete production

In free-fall mixing machine, ie. include a rotating drum with vanes which lift I

In the mass up and down it falls freely down. A known concrete mixer can be modified

To be used for this purpose, remove the diagonal blades and coat I

Inside the drum with a synthetic material. The vanes are thus designed as follows: I

In that the mass is mixed by rotation in one direction, while in rotation in that I

In the other direction, the pulp feeds at a discharge bucket located at an outlet opening. IN

I For the transformation of the pulp wetted mass into granular form I into the water mixer

You are required to regulate the water supply precisely depending on the actual flight I

In 15 ash properties. IN

In addition, the mixing machine can be started after adding a certain amount of I

fly ash, which is moistened with an initial water addition. Then, water is added very slowly, observing the change in mass of the pulp. This

I 20 can advantageously be made during recording of the current torque applied to I

In the mixer, which is a measure of the actual consistency of the pulp. When the mass I

You begin to granulate to gradually completely turn into a dry granule, note it

In total water supply. IN

After such a course, the future portions can be processed automatically by

In the ring from a microcomputer which has been programmed with those of the first device, I

Obtained values in race. The material derived from the mixer thus consists of I

In dry fly ash granules, which can be tipped directly onto a truck and after trans-

In port for the desired deposit can easily be tipped off the charge. When loading .. I

At 30, the driver can load the desired load weight into the process control unit

I turned the microcomputer, after which it weighed an appropriate portion of fly ash and I

You supply water depending on the consistency of the mass, ie. I read it in the usual way

In torque. When the portion is turned into a dry granule, this is automatically tipped

You tic down the drawer, after which the next truck can drive forward and program a new one

In 35 cargo load. IN

Claims (5)

Although the process described above using a batch mixer, it can of course also be carried out in a continuous free-fall mixing process, provided the consistency of the pulp can be sensed to the necessary proportion of the addition water. A process for granulating fly ash containing calcium oxide and possibly calcium sulfate by mixing with water, characterized in that the mixture occurs as a free-fall mixture with the addition of water to the ash volume. Process according to claim 1, characterized in that the mixture of fly ash and water takes place in portions. I DK 175803 B1 I Process according to claim 1 or 2, characterized in that the consistency of the mixture is sensed and that the water supply is controlled accordingly. , I Device for granulating fly ash containing calcium oxide and optionally calcium sulfate by mixing with water in a free-fall mixing machine, characterized in that it comprises means for sensing the consistency of the mixture and means for controlling the water supply as a result. I I ! Device according to claim 4, characterized in that the consistency sensing means comprise 11 means for sensing the driving torque acting on the mixer. I H i I