The aspects of the disclosed embodiments are related to (soda) recovery boiler floor washing during outage.
Recovery boilers are fueled with waste liquor generated in connection with pulp manufacture containing various sodium salts, mainly sodium carbonate and sodium sulfate, in addition to organics and water. These salts form a salt bed on a furnace floor during boiler operation. The bed is at least partly molten so that molten salt flows continuously from the bed to smelt spouts through smelt spout openings located on a lower part of the furnace, and further to a dissolving tank.
The smelt spout openings are typically placed at a level higher than the furnace floor so that the salt bed on the floor is at least 200-250 mm thick.
Recovery boilers have become gradually larger so that modern recovery boilers have a floor area of 150-300 m2. In addition, modern recovery boilers are typically equipped with a so called decanting floor, with floor tubes sloping downwards from a back ball, where the smelt spout openings typically are, towards the centerline of the floor so that the lowest elevation extends several meters, even over eight meters, from the back wall. The salt bed residing on the floor is hence considerably thicker at the centerline than in the proximity of the back wall.
These factors the increase in recovery boiler size, the use of decanting floor as a standard design instead of flat floors or floors sloping towards the back wall—have together led to the current situation in which the remaining salt bed during the outage in a modern recovery boiler is large both in thickness and volume; in the largest boilers the salt bed thickness at the floor centerline is 0.5-0.7 m and the total volume of the bed is over 100 m3.
When the furnace floor is inspected, the floor needs first to be made free from salt (smelt) remaining on the floor. Conventionally, the recovery boiler furnace floor is cleaned as follows:
The used method is labor-intensive, time consuming, expensive and risky. The outcome of the washing is often so poor that a large part of the salt remaining on the floor has to be removed mechanically. The floor cleaning after the washing may thus take even several days. The time spent on floor cleaning often extends the whole mill outage and accordingly reduces the mill production, so the costs due to this loss of production can be several millions of euros. Mechanical cleaning with chopping the salt may also damage the floor tubes, in which case additional outage time is needed for repairing the damages.
Due to the risks associated with mechanical cleaning the floor washing is performed in certain mills by using high pressure washers which are installed in smelt spout openings or burner openings. The high pressure washers employ considerably high pressure, 80-100 MPa (800-1000 bar) or even 250-300 MPa. The pressures used are of the same order of magnitude as those used in steel cutting machines, so also the use of high pressure washers can damage the floor or wall tubes.
According to a first aspect of the disclosed embodiments there is provided a method in recovery boiler outage, comprising:
The mixing is performed during the furnace floor washing stage, that is, after hot molten salt (smelt) removal stage and before the furnace floor emptying stage in which the floor is emptied from wash water.
In certain example embodiments, the sucked wash water is discharged back into amongst the remaining wash water through discharge openings of the mixing devices for implementing said mixing.
In certain example embodiments, sucked wash water is replaced by guiding fresh wash water onto furnace floor for implementing said mixing.
In certain example embodiments, said mixing is implemented by mixing devices placed on the floor of the recovery boiler.
In certain example embodiments, wash water on the furnace floor is mixed by mixing devices, operated by pressurized air, which are installed on the floor through smelt spout openings.
In certain example embodiments, said mixing is implemented by ejectors with gas or fluid as pressure medium.
In certain example embodiments, wash water is sucked from a wash water pool bottom.
In certain example embodiments, the wash water pool top layer is mixed with the wash water pool bottom layer.
In certain example embodiments, a wash water circulation is formed on the furnace floor by directing the mixing devices in accordance with a desired direction of circulation.
In certain example embodiments, the method comprises removing during recovery boiler outage before washing the floor of the recovery boiler a part of the salt from the furnace in a molten form.
In certain example embodiments, the method comprises guiding wash water into recovery boiler furnace via water passages arranged in the recovery boiler for washing the floor of the recovery boiler. In certain example embodiments, wash water is pumped onto furnace floor with the aid of wash sprays installed in primary air openings on recovery boiler walls. The water pumped into the furnace may be for example feed water, condensate, firewater or water from dissolving tank. Hot (70-80° C.) water will normally be used in order to expedite the dissolving of salt.
In certain example embodiments, a major part of salt remaining on the recovery boiler floor in outage is removed in a molten form and the pumping of wash water onto furnace floor is started as soon as the boiler pressure has been reduced, and the wash water on the floor is mixed with the aid of mixing devices set on the floor.
In certain example embodiments, a major part of salt is removed in a molten form with the aid of suction devices. Smelt may be removed by suction devices from the middle of the floor, whereupon only a thin layer of salt remains on the floor which salt can be removed relatively easily by washing.
In certain example embodiments, the pumping of wash water is started as soon as the boiler pressure has been reduced so that the temperature of the floor tubes has been adequately decreased (that is, the temperature typically is less than 150° C.) or the temperature difference between the wash water and floor tubes is small enough.
According to a second aspect of the disclosed embodiments there is provided an apparatus comprising:
In the following, the disclosed embodiments will be described by way of example with reference to the appended drawings, in which:
The figures shown are not entirely to scale, and they primarily serve to illustrate the embodiments of the present disclosure.
A smelt pool formed on the floor of the furnace 1 is emptied with a smelt removal device 5 installed in the smelt spout 2. The device 5 is selected so that the pool can be emptied as completely as possible so that only a thin layer of salt remains on the floor. Examples of applicable smelt removal devices has been presented for example in patent applications FI20065668 and FI20086166 (smelt ejectors operated by pressurized gas). Alternatively, another removal device, such as a spiral pump, may be used. In the figure, the dashed line shows the surface level of the smelt pool before the commencement of emptying and the double line shows the surface level of the smelt pool at a late stage during emptying.
Smelt removal is continued as long as the pool has been emptied as completely as possible. After this the devices are removed, the firing of auxiliary fuel and black liquor, if used, is stopped and the cooling of the boiler and pressure reduction is commenced. The pressure reduction and cooling is continued until the floor tube temperature is sufficiently low. The salt remaining on the floor cools down simultaneously so that the floor washing can begin safely.
The mixing devices 7 can be placed into the furnace so that the mixing of wash water pool is as efficient as possible or, for example, so that mixing is most efficient in areas with the highest concentration of salt. The figure shows with arrows the wash water flow direction during mixing.
The mixing device 7 can be operated by pressurized air. It can then take the pressurized air needed from the pressurized air system of the mill (not shown).
For example, both “pulling” and “pushing” mixing devices can be used as mixing devices 7 so that on one side of the furnace floor water is “pushed” away from the back wall and on the other side “pulled” towards the back wall so that water on the floor is circulated as well as possible. The direction is determined by the air tube direction inside the suction pipe. If in
Alternatively, the pressure medium in the mixing device can be other pressurized gas or liquid instead of pressurized air, for example water. In the latter case the mixing device may be kind of a liquid-liquid ejector wherein the pressure of water used as pressure medium can be for example 10 bar. Instead of or in addition to the mixing device(s) shown in
The mixing device 7 on the left sucks wash water from the wash water pool bottom into the suction opening and discharges the sucked wash water through the discharge opening back into amongst the remaining wash water also onto the wash water pool bottom. The mixing device 7 in the middle sucks wash water from the wash water pool surface into the suction opening and discharges the sucked wash water through the discharge opening back into amongst the remaining wash water onto the wash water pool bottom. The mixing device 7 on the right sucks wash water from the wash water pool bottom into the suction opening and discharges the sucked wash water through the discharge opening back into amongst the remaining wash water on the wash water pool surface. The wash water pool top layer is mixed with the bottom layer. The presented alternatives can be used in the embodiments presented in the foregoing. The wash water flow patterns are illustrated using arrows.
The foregoing description provides non-limiting examples of certain embodiments of the invention. It is clear to a person skilled in the art that features which have been described in connection with only one or some of the embodiments can be used also in other embodiments. The presented placement of for example parts of the recovery boiler, wash sprays and mixing devices depend on the implementation. It is clear to a person skilled in the art that the invention is not restricted to details presented above, but that the invention can also be implemented in other equivalent ways. In this document, the terms comprise and include are open-ended expressions with no intended exclusivity.
Some of the features of the presented embodiments may be used to advantage without the corresponding use of other features.
In certain embodiments of the present disclosure, a part of the salt is removed from the furnace in a molten form during recovery boiler outage before washing the floor of the recovery boiler. For example, this is not an indispensable requisite for the use of the method in accordance with the first aspect of the disclosed embodiments, but the method can be used without salt being removed in a molten form before washing. As such, the foregoing description should be considered as merely illustrative of the principles of the present disclosure, and not in limitation thereof. Hence, the scope of the disclosed embodiments are only restricted by the appended patent claims.
Number | Date | Country | Kind |
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20155558 | Jul 2015 | FI | national |
Filing Document | Filing Date | Country | Kind |
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PCT/FI2015/050762 | 11/5/2015 | WO | 00 |