WO1996008677A1

WO1996008677A1 - Process for cleaning superheaters and other heat-transferring surfaces in recovery boilers

Info

Publication number: WO1996008677A1
Application number: PCT/SE1995/000931
Authority: WO
Inventors: Lars Olausson; Lennart Hjalmarsson
Original assignee: Kvaerner Pulping Ab
Priority date: 1994-09-14
Filing date: 1995-08-17
Publication date: 1996-03-21
Also published as: SE9403064L; SE504718C2; SE9403064D0

Abstract

The invention relates to a process for removing a heavy deposit, which is entirely or partially sintered, from superheaters and other heat-transferring surfaces in recovery boilers and other boilers by means of so-called soot blowing during operation which is amplified by thermal shedding. The process is characterized in that the pressure in the soot blower is at most 80 %, preferably 50 % , of the normal pressure (which is 20-25 bar), in that the cooling effect of the soot blowing medium is regulated by means of water injection and in that the blowing time is at least 1 minute for each pipe unit (screen).

Description

TITLE:

Process for cleaning superheaters and other heat- transferring surfaces in recovery boilers.

TECHNICAL FIELD:

The present invention relates to a process for removing a heavy deposit, which is entirely or partially sintered, from heat-transferring surfaces such as superheaters, boiler banks and economizers in recovery boilers and other boilers by means of so- called soot blowing during operation which is amplified by thermal shedding.

STATE OF THE ART:

Recovery boilers are employed within the cellulose industry for combusting so-called black liquor which has been obtained during the cooking of the cellulose and which consists of organic substances together with inorganic chemicals such as sodium sulphate, sodium carbonate, etc. The recovery boiler essentially consists of a shaft-shaped combustion oven of substantial size, approximately 20 - 50 metres in height, having, inter alia, heat-transferring surfaces in the upper part of the shaft for cooling the flue gases while at the same time recovering the heat energy. The black liquor is combusted in concentrated form, and soot and inorganic chemicals are mixed in with the flue gases. As the flue gases pass the tubes in the heat-transferring surfaces in the upper part of the boiler, the inorganic chemicals condense and coat the heat-transferring surfaces, which are cooler than the flue gases. A variety of cleaning systems have been arranged for keeping the heat exchanger surfaces as clean as possible, the foremost of which systems are so-called soot blowers which, in principle, consist of a lance which passes backwards and forwards between the heat exchanger pipes and which blows out steam or another soot blowing medium at high pressure onto the pipes. The pressure of the steam in the soot blowers is usually in the order of size of 20 - 25 bar. A more detailed description of soot blowers and their mode of operation is provided by R.E. Chappell in the 1987 publication Kraft Recovery Operations, pp. 159-163.

Water jets have also been used instead of steam. A process of this nature is described in Swedish Patent 82 06 666-3. In this process, a lance is used which has two nozzles. The surface which is to be treated is first struck by the jet from one of the nozzles and then by a pulsating jet at higher maximum pressure from the other nozzle.

US Patent 3 661 124 describes another process which also makes use of a lance having two nozzles, with a water jet being sprayed through one of the nozzles and a jet of steam being sprayed through the other nozzle.

Yet another method of cleaning heat surfaces which is practised within the pulp industry is to break down the deposit which has accumulated, and which cannot readily be removed, using so-called thermal shedding.

This can take place in such a manner that the heating is switched off or is reduced and as much cold air as possible is drawn through the heat surfaces. If full soot blowing can be in progress at the same time, the deposit is both broken up and blown away with the aid of a powerful jet of steam, a combination which often gives a good result. This method of cleaning is described in more detail by Tran, Martinez, Reeve, Cole, Damon and Clay in the publication Pulp and Paper Canada 94:11 (1993), pp. 49-55, "Removal of recovery boiler fireside deposits by thermal shedding". Finally, simple washing with water also takes place when the plant is out of operation and the recover boiler is being renovated. TECHNICAL PROBLEM:

Superheaters and other heat-transferring surfaces in recovery boilers are normally greatly over¬ sized in order to be able to cope with the performance of the recovery boiler even after having been in operation for some time. The reason why they are greatly oversized in this way is that there is a gradual build-up of a heavy deposit which is entirely or partially sintered and which accumulates on the surfaces despite carrying out normal soot blowing or despite using other measures as described above.

In most cases, the problems with soot blowing are, on the one hand, that the deposit on the tubes is not removed to the extent desired and, on the other, that the tubes become eroded due, for example, to the powerful mechanical effect exerted by the water jets, etc. These drawbacks are also to be found in a process using conventionally executed thermal shedding, which is always combined with soot blowing. In addition, the process using conventionally executed thermal shedding also suffers from the disadvantage that the normal operation has to be interrupted, with it being possible for this interruption to last, for example, for 12 hours. There has, therefore, always been a need to improve the cleaning methods cited above and to solve the problems which are associated with these methods.

SOLUTION: A process has, therefore, been produced, in accordance with the present invention, for removing a heavy deposit, which is entirely or partially sintered, from superheaters and other heat-transferring surfaces in recovery boilers and other boilers by means of so- called soot blowing during operation which is amplified by thermal shedding, which process is characterized in that the pressure in the soot blower is at most 80 % of the normal pressure (which is 20 - 25 bar) , preferably 50 % of the normal pressure, in that the cooling effect of the soot blowing medium is regulated by means of water injection and in that the blowing time is at least 1 minute for each tube unit (screen) .

According to the invention, the addition of water to the soot blowing medium, which normally consists of steam, can amount to 0 - 50 % by weight, preferably 1 - 30 % by weight or, more preferably, 5 - 20 % by weight of the mass flow during a normal soot blowing exercise. According to the invention, the temperature of the steam in the soot blower can be within the range 100 - 300°C.

According to the invention, the cooling effect of the soot blower should be adjusted, by means of admixing water, to a value which is in the order of size of 30 kW/m².

According to the invention, one or several soot blowers can operate at reduced pressure while the others operate in the normal manner or are shut down. The soot blowing during operation at reduced pressure, in accordance with the invention, can expediently be carried out only occasionally, for example every 14 days.

DESCRIPTION OF THE FIGURE:

The invention will be described in more detail below with reference to the attached figure, which shows, in vertical section, a type of recovery boiler with heat-transferring surfaces.

PREFERRED EMBODIMENT:

The figure shows, in vertical section, a recovery boiler having a shaft-shaped furnace 1 which is fed with black liquor, air, etc. in its lower part, where the combustion mainly takes place. The flue gases rise upwards and are turned away, as indicated by the black arrows, through the heat-transferring surfaces 2 at the upper end of the boiler 1. The heat-transferring surfaces 2 are suspended and are normally not supported from below. Soot blowers 3 are arranged between the different sections of the heat-transferring surfaces 2. These soot blowers 3 mainly run horizontally and they are arranged on opposite sides of the boiler if the latter is not a small one. The soot blowers 3 move backwards and forwards between the pipe assemblies and direct a jet of what is preferably steam against the pipes. The boiler, the heat-transferring surfaces and the soot blowers are known per se and are not included in the present invention.

As has been mentioned above, soot blowing is normally carried out using steam at a pressure of 20 - 25 bar and at the temperature which is necessary for keeping the steam in steam form at this high pressure. The time for which each screen is exposed is normally at most 10 seconds. Although the steam which is released through the nozzles is subjected to a powerful cooling effect due to the fact that it is expanding, it is not able, during this short period, to cool the deposit on the pipes down to any very great depth. If it is only its surface which is cooled down, the deposit will not then break up, at least not in the region adjacent to the pipes. The effect of soot blowing in this manner is therefore principally achieved by means of mechanical influence.

In accordance with the invention, however, the heat-transferring surface is subjected to so-called thermal shedding, while the boiler is in full opera¬ tion, by means of greatly increasing the treatment time for each individual screen while simultaneously giving the jet of steam or other gaseous soot-blowing medium an increased cooling effect by admixing water with it. At the same time, the pressure is lowered to less than 80 % of the corresponding pressure during a normal soot-blowing exercise in order to avoid harmful erosion on the heat-transferring surfaces.

In order to ensure that the deposit is cooled down sufficiently in the region adjacent to the pipe in the heat-transferring surface, the aqueous steam jet should be allowed to act for a substantially longer period than is normally the case, namely at least 1 minute for each pipe unit, so-called screen. During this substantially longer period, the deposit has time to cool down to such an extent that it cracks and can, therefore, be more easily removed in a subsequent, normal soot-blowing stage. In order to prevent reheating and the sintering together of cracks which have formed, a subsequent, normal soot-blowing stage should be effected relatively soon after having carried out the cooling.

Normally, the distance between the screens in the heat surfaces of a recovery boiler, transverse to the direction of the flue gas, is between 0.1 and 0.6 of a metre. However, the effective working width of a normal aqueous steam jet is of the order of size of 0.3 of a metre, which value has, therefore, been taken as the basis for calculating a suitable dwell time. This also means that a jet of the soot blowing medium will, at a certain time, be treating more than one screen when the distance between the screens is at its lowest, and also that the jet is less effective in between two screens when the distance between the screens is at its greatest. When water has been admixed, the jet which emerges from the nozzle during the treatment is mingled with water droplets which cool the deposit both directly, by vaporizing on the surface of the deposit, and indirectly, by cooling the gas around the heat- transferring surfaces. According to the invention, the quantity of water which can be used for admixing with the steam is from zero, since even a water-free medium has a certain intrinsic cooling effect, up to approxi¬ mately 50 % of the total mass flow during a normal soot-blowing stage. An appropriate combination should impart a cooling effect of the order of size of 30 kW/m² to the mixture which is being blown out.

In order to eliminate the risk of harmful erosion caused by admixing the water, the pressure is normally reduced to half the normal pressure, a measure which has a very great effect due to the exponential relationship between the speed of discharge from a soot blower and the erosion. However, it is possible to use a pressure amounting to 80 % of the normal without the erosion becoming so great that it is troublesome.

In order to minimize interference with the normal operation of the boiler while carrying out soot blowing during operation which is amplified by thermal shedding, only one or a few screens should be treated during one continuous sequence. After the selected screens have been treated as intended, any water for admixing is shut off, the pressure is increased to its normal value and the soot blower returns to its normal function. There is no need, therefore, for the boiler to be shut down in any way and it can be operated normally.

According to the invention, the temperature of the steam in the soot blower is within the range 100 - 300°C.

When soot blowing during operation which is amplified by thermal shedding is taking place in accordance with the invention, one or a few soot blowers can be in use while the other soot blowers can be shut down or can be operating in the normal manner.

According to the invention, it can be suffi¬ cient to carry out soot blowing at reduced pressure, in accordance with the invention, only occasionally, for example every 14 days. By means of using the present invention, the superheaters in a recovery boiler can be kept clean for a very long period of time and there is then no need for these superheaters to be oversized in order to cope with the performance of the boiler. The invention is not limited to the embodiment shown and can be varied in different ways within the scope of the patent claims.

Claims

Patent claims

1. Process for removing a heavy deposit, which is entirely or partially sintered, from superheaters and other heat-transferring surfaces in recovery boilers and other boilers by means of so-called soot blowing during operation which is amplified by thermal shedding, c h a r a c t e r i z e d i n that the pressure in the soot blower is at most 80 % of the normal pressure (which is 20 - 25 bar) , preferably 50 % of the normal pressure, in that the cooling effect of the soot blowing medium is regulated by means of water injection and in that the blowing time is at least 1 minute for each pipe unit (screen) .

2. Process according to Claim 1, c h a r a c t e r i z e d i n that the addition of water to the soot-blowing medium amounts to 0 - 50 % by weight, preferably 1 - 30 % by weight or, more preferably, 5 - 20 % by weight of the mass flow during a normal soot blowing exercise.

3. Process according to any one of Claims 1 - 2, c h a r a c t e r i z e d i n that the temperature of the soot-blowing medium is within the range 100 300°C.

4. Process according to any one of Claims 1 - 3, c h a r a c t e r i z e d i n that the cooling effect of the soot blower is adjusted, by means of admixing water, to a value which is in the order of size of 30 kW/m².

5. Process according to any one of Claims 1 - 4, c h a r a c t e r i z e d i n that one or several soot blowers operate(s) at reduced pressure while the others operate in the normal manner or are shut down.

6. Process according to any one of Claims 1 - 5, c h a r a c t e r i z e d i n that soot blowing at reduced pressure is carried out only occasionally, for example every 14 days.