Field of the Invention

The invention relates to cleaning of industrial spaces, such as combustion boilers. In particular, the invention relates to a method of cleaning boiler surfaces, an apparatus for such use and a boiler provided with such cleaning apparatus. Background of the Invention

Heat-transfer surfaces of power plants are traditionally cleaned by using sandblasting. Typically, water is used is the process to bind the fines, in order to prevent dusting caused by the sand particles. This wet blasting method has proven to be problematic in several respects. For example, when burning wood or fossil fuels, also sulphur and organic compounds which are derived from the combustion, are generated in the boiler, and by providing water to the ashes which remain on the fire surfaces, various acids and weak acids start to form in the boiler. The residual ash remains wet and acidic, which causes corrosion of steel surfaces. Moisture can also reach the inner part and the gaps of the masonry, and at same time, cause corrosion. In addition, when damp ash dries, it becomes almost as hard as concrete, making the cleaning process even more difficult and shortening the lifetime of plant structures. These wet methods are also not usable while the boiler is running, which causes significant accumulation of deposits and service delays while cleaning.

Alternatives to sandblasting have also been proposed. CN 102313288 A describes a solution in which heat-transfer surfaces are cleaned using smooth-surface particles by mixing the particles in a gas flow and spraying them using a blowing gun inside a boiler. WO 2017/103345 Al discloses an alternative method, in which the surfaces are cleaned using slag particles. While the dry cleaning method suggested and currently used in power plants are more efficient against stubborn deposits, they are still used during shutdown periods of the boiler. Currently available run-time cleaning methods, on the other hand are based on the use if steam or sound impulses. These have, however, been found to be relatively limited in their cleaning efficiency. Steam has also negative effects regarding corrosion and formation of sticky substances inside the boiler.

There is a need for improved cleaning solutions.

Summary of the Invention It is an aim of the invention to overcome at least some of the problems of prior art and to provide a cleaning method and related apparatus that allows for increasing the overall utilization efficiency of boilers.

The invention is based on the observation that metal slag particles are suitable to be used for cleaning while the boiler is running, i.e. operated normally for combustion of raw materials to heat water or other heat transfer medium circulated therein. A dry cleaning process involving particulate metal slag, has been found to be particularly efficient in keeping the heat transfer surfaces of the boiler clean and effective without interfering with the combustion or heat transfer process in a negative way.

The invention also provides a novel cleaning apparatus suitable for use during operation of the boiler, and a boiler comprising such apparatus.

In particular, the invention is characterized by what is stated in the independent claims.

The invention offers significant benefits. Thus, when cleaning is carried out

simultaneously to combustion, there is no need to shut down the boiler. This allows for increasing the usage rate of the boiler. Further, as the cleaning can be carried out continuously or at least more frequently, as often as desired, the accumulation of ash, soot and other deposits is much less than if the boiler would be run for a considerable period between the cleaning periods. This has two positive effects. First the overall heat transfer rate remains high. Second, the deposits do not transform into sticky deposits that require a large amount of work to be removed. This allows for reduction of total cleaning time and slag material required. Metal slag particles with size of less than 3 mm, in particular copper and nickel slag particles, have been found be surprisingly effective to clean hot surfaces without damaging them, even when sprayed at very high speed. During operation of the boilers, surface temperatures therein may reach hundreds of degrees Celsius, which makes them more prone to wear. For surfaces that have been fully cleaned already, and therefore do not have a "deposit shield", it is crucial that the cleaning agent does not cause erosion which would shorthen the lifetime of the boiler. This aspect is emphasized in run-time use as the aim is to keep the surfaces continuously relatively clean. As a dry cleaning method, the present method does not suffer from the same drawbacks as the traditional wet sandblasting methods or steam- or sound-cleaning methods. Slag particles do not interfere with the combustion process and do not form stubborn compounds with ash or soot particles. In particular, the drawbacks caused by moisture and in particular the co-presence of moisture and combustion products are avoided.

The proposed apparatus can be easily installed to different kinds of boilers and boiler wall structures.

In some embodiments, the blasting pipe is connected to a blasting unit located outside the boiler and supported by a rail. Further, the blasting unit is arranged to move along the rail in order to cause linear motion of the blasting pipe within the boiler. This allows for reaching large surface areas within the boiler, even the whole heat transfer circulation pipe structure, with one apparatus, and is suitable to be used while the boiler is operative. In some embodiments, the blasting unit comprises a blower for generating the pressurized gas stream. The blower may be based on an electric motor that is able to generate a pressure of 2 - 12 bar, in particular 8 - 12 bar. The blower is arranged to mix the slag particles into the gas stream. In some embodiments, the blasting pipe has a double-wall structure, which makes is strong and allow for circulation of cooling fluid therein. Indeed, in some embodiments, the blasting pipe is actively cooled during blasting in order to increase its lifetime. In some embodiments, blasting pipe comprises a nozzle, which is capable of blowing the slag particles sideways into at least two opposite angles with respect to the axial direction of the blasting pipe. The nozzle can be e.g. a rotary nozzle or an omnidirectional nozzle. In some embodiments, the blasting pipe is arranged to move inside the heat transfer cell structure of the boiler, i.e. between at least some heat recovery surfaces of the boiler. This provides for maximum efficiency combined with wide-angle stream spreading.

The metal slag can be e.g. nickel or copper slag with a particle size of 0.1 - 2.5 mm, in particular 0.1 - 1.2 mm or 0.3 - 2.5 mm. The particles preferably have a melting point higher than the internal temperature of the boiler.

Next, embodiments of the invention and advantages thereof are discussed in more details with reference to the attached drawings. Brief Description of the Drawings

Figs. 1 A - ID show in perspective views a deposit cleaning apparatus arranged in a boiler at four different stages of the cleaning process, respectively.

Fig. 2 shows a zoomed view of a surface to be cleaned.

Detailed Description of Embodiments

In the following, a cleaning apparatus which is based on the utilization of dry metal slag and which can be installed to a boiler and then used during the normal use of a boiler is described.

Here, "metal slag" means the by-product which is generated in the production or cleaning of the metal in question, i.e. in general "material", which typically is primarily silicate- based. The silicate material is, for example, iron silicate, and it comprises, besides the main component, also for example metals which are derived from the metal raw material, and alkaline earth metals, and their compounds, such as oxides, sulphates, sulphides and silicates. "Normal use" or "operation" of a boiler or the boiler being "running" means that there is a combustion process going on in the boiler space, whereby the blasting is carried out at elevated temperature and in the presence of fly ash, flue gases and other substances specific to the combustion process concerned.

In one embodiment, the internal temperature in the boiler which is subjected to blasting (during operation) is greater than 400 °C, in particular 500 °C or more, for example 800 °C or more. Generally, in embodiments, a method is provided for cleaning deposits from internal surfaces of an industrial boiler, which method comprises blasting solid particles with pressurized gas stream against said internal surfaces in order to remove the deposits. The blasting is carried out using metal slag particles while the boiler is running, i.e. during the operation of the boiler. The conditions of the boiler are, during the blasting, therefore at least essentially the same as during operation of the boiler.

According to a first aspect, the blasting is carried out using a blasting pipe which is arranged through an outer wall of the boiler, and wherein the pipe is moving linearly along its axial direction during said blasting.

According to a second aspect, there is provided an apparatus for cleaning heat transfer surfaces of an industrial boiler, the apparatus comprising a container for dry particle-form metal slag, a blasting pipe having an axial direction, and a blasting unit connected to the container for receiving the particle-form slag from the container and to the blasting pipe for forming a pressurized slag stream along the blasting pipe towards the heat transfer surfaces. The blasting unit is movably supported so as to allow linear movement the blasting pipe along its axial direction within the boiler.

According to a third aspect, there is provided a boiler comprising internal heat transfer surfaces and an outer wall, the boiler comprising an apparatus as herein described. The rail and the blasting unit are arranged outside the wall such that the blasting pipe extends through an orifice at said outer wall and is linearly movable along its axial direction between said heat transfer surfaces so as to direct the slag stream towards the heat transfer surfaces. The boiler can be e.g. a biomass boiler, combustion boiler, or recovery boiler, such as a pulping recovery boiler.

Turning now to the drawings, it can be noted that Fig. 1 A shows a cleaning apparatus arranged in a boiler environment for cleaning heat transfer surfaces 8 of the boiler. The outer wall of the boiler is denoted with reference number 7. The apparatus comprises a slag container 1, which can be filled with particular matter having a particle size of 0.1 - 3 mm. Connected to the bottom portion 2 of the container 1, there is transfer tube 3, which transports the particulate slag from the container to a blasting unit 4. In the bottom portion 2, there may also be a mixer (not shown), which prevents aggregation of the slag, ensuring its fluidity and suitability for blasting.

The blasting unit is arranged movably on an elongated rail 5, which is positioned perpendicular to the wall 7 of the boiler. The blasting unit 4 may be arranged to slide or roll on the rail 5 closer to and farther from the wall 7. The wall 7 contains an orifice which is provided with a tight and heat-resistant collar 9. Connected to the blasting unit 4, on the side thereof that faces the wall 7, there is a blasting pipe 6, which extends through the wall, via the collar 9, into the boiler space. The blasting unit comprises a motor, preferably an electric motor, which is capable of creating a blasting pressure into the blasting pipe in particular by generating a gas stream, typically an air stream, into the boiler. Slag particles from the container 1 via the transfer tube 3 are mixed with the gas flow so that a slag particle-containing gas stream is formed. Thus, with reference to Fig. IB, when the blasting unit 4 is switched on, slag particles start to flow from the container 1, via the transfer tube 3, the blasting unit 4 and the blasting pipe 6, gaining a high speed before their exit into the boiler space towards the heat transfer surfaces 8. In the example shown, the blasting pipe 4 is arranged to spread the particles sideways essentially omni-directionally into the boiler space (potentially apart from a specific sector in the downwards direction, which in this example does not contain a surface to be cleaned).

Further, with reference to Fig. 1C, while the blasting unit 4 is operative, is moved closer to the wall 7, whereby the blasting pipe 6 protrudes further into the boiler space, whereby the slag particles reach new areas of the heat transfer surfaces 8. The collar 9 may support the blasting pipe 6 or it may be essentially entirely supported by the blasting unit 4. In the preferred mode of operation, the blasting unit 4 may be moved back and forth along the rail 5 while the blasting function is on.

For moving the blasting unit 4 along the rail 5, there may be provided an electrically controlled actuator, such as a linear motor, wheel motor or the like. The actuator may be manually controlled and/or programmable to perform a cleaning action continuously or at predefined intervals and for predefined durations.

Finally, Fig. ID shows the situation where the blasting unit 4 has been returned to its home position and the heat transfer surfaces 8 within the reach of the apparatus have been cleaned from deposits. In the home position, the blasting pipe 6 is preferably retracted away from the main combustion zone where the ash and soot amounts are lower, so that the pipe 6 lasts longer and does not itself accumulate deposits and become blocked.

Fig. 2 shows in detail the slag stream 10 hitting the surface to be cleaned. The deposits 11 are first and most efficiently first removed from surfaces 12 that the slag hits directly, but the impact has been found to be high also on other areas. The deposits start to crumble on a wider area and eventually each region that are reachable by the particles, either directly or via bouncing, will be free from deposits. As shown in Fig. 2, the deposits start to crumble and even the whole surfaces may be cleaned.

The blasting pipe 4 is preferably a straight rigid pipe, typically made from metal, such as steel. Straightness and rigidity allow for the linear movement of the pipe 4 through the collar 9 along its axial direction, while metal materials may provide for high heat- resistance and durability in high-temperature conditions. They are also erosion-resistant on the internal surface of the pipe against the wear action of the metallic slag particles.

Preferably, the hardness of the metal is higher than that of the slag particles.

The rail 5 may be any elongated element that is capable of guiding the blasting unit 4 along a straight route. The blasting unit 4 is engaged with the rail 5 so that movement sideways is prevented and the blasting pipe 6 remains aligned with the collar 9. It should be noted that the term "rail" herein covers also more complex structures and arrangements, which are capable of the keeping the blasting unit 4 laterally positioned while allowing its movement in one dimension.

To ensure safe operation of the boiler and the cleaner, the collar must be gas-tightly filled to the boiler wall and, respectively, to the blasting pipe 4.

At the distal end of the blasting pipe 6, there is a nozzle (not shown) that directs the slag particles away from the axial direction of the pipe. The redirection may take place simultaneously in several directions sideways (omnidirectional nozzle) or successively (rotating nozzle). The nozzle may change the propagation direction of the stream by at least 40 degrees, such as 35 - 75 degrees, in particular about 45 degrees from the axial direction of the pipe. In one embodiment, the nozzle is adapted to form the stream essentially transverse to their initial direction, i.e. at an angle 75 - 105 degrees therefrom. In some embodiments, the streaming angle of the nozzle can be varied during blasting so as to maximize the cleaning action on surfaces with different orientations.

The nozzle may be integral with the blasting pipe, e.g. welded thereto, or removably attached thereto.

In one embodiment, the nozzle is rotatable and the rotation is powered by the stream itself, whereby no active rotation mechanisms are needed. This has advantages in terms of heat- resistance and service intervals of the apparatus. In one embodiment, there is a separate actuator for rotating the nozzle at the end of an otherwise non-rotating blasting pipe 6. In one embodiment, the whole blasting pipe 6 with the nozzle is rotatably mounted on the blasting unit 4.

In one embodiment, the blasting pipe 6 comprises a double wall structure in at least portion thereof. This improves the durability even more. In particular, the double wall structure may be arranged such that cooling fluid, such as air or water can be circulated inside the wall of the pipe 6 so as to control its temperature during blasting. In particular, the cooling fluid may be provided to circulate from the portion of the pipe that remains outside of the boiler during blasting to the portion that extends in to the boiler and back. There is typically a heat exchanger outside the boiler to lower the temperature of the cooling fluid, which may then be recirculated. With active fluid cooling, a significant temperature drop of 50 °C or more, typically at least 100 °C or more in the pipe material can be provided in typical boiler conditions, in which the temperature can easily be 800 °C or more.

Also the nozzle of the blasting pipe 6 can be cooled.

Temperature control of the pipe 6, with the cooling fluid arrangement or some other active cooling method, increases the lifetime of the pipe due to reduced wear, and broadens the scope of suitable materials.

The blasting is preferably carried out in water-free conditions. The terms "water-free" or "dry" herein mean that water or aqueous solutions are not fed, either together with or separately from the blasted particles, onto the object to be cleaned. Typically, no water or aqueous solutions at all are fed. However, if together with the particles, moisture ends up in the object to be cleaned, operating in water- free conditions means that, due to the effect of particle blasting, condensed water does not flow from the surface to be cleaned into the adjacent structure, which, for example, is masonry.

In particular, the slag stream may be essentially comprised of the slag particles and carrier gas, preferably air, which has a moisture content corresponding to that of ambient air or being less than that.

The heat transfer surfaces of the combustion boiler to be cleaned may be in connection to a masonry structure.

Copper and nickel slag particles are particularly well suited for the cleaning of steel heat- transfer surfaces, because the nickel and copper slag do not comprise significant amounts of ferrite compounds. This, in turn, means that when cleaning steel surfaces with nickel or copper slag, no corrosion problems appear, which is not the case when blasting with metal slags that comprise ferrite compounds, or when blasting with, for example, steel particles that also comprise ferrite compounds.

Thus, the slag particles of the present technology are preferably "ferrite-free". Typically, the surface to be cleaned comprises sulphur or silicate-bearing compounds which are generated when burning wood or fossil fuels or mixtures thereof, and possibly ash, coke or slag which comprise organic compounds (such as tar-like compounds).

In some embodiments, the slag particles are not spherical, but they have an irregular shape. This also means that, when particles hit a surface, both the surface pressure generated and the interaction between the particles and the surface are different from the case where spherical particles are used. The width of the contact area of individual slag particles varies depending on which part of the particle hits the surface. Thus, by using slag particles, the ash layer remaining on the surface can be loosened more efficiently than with steel balls, for example, without damaging the steel surface.

Especially, in an embodiment, deposits and similar dirt layers which are generated during the combustion process are removed from the metal surfaces without substantially damaging these.

In one embodiment, the structure to be cleaned is part of a power boiler, such as a heat boiler, or part of a recovery boiler, such as a kiln for reburning lime sludge or a soda recovery unit.

In more detail, suitable particles and boiler environments, where cleaning is required, are described in WO 2017/103345 Al, which is, concerning those parts thereof, incorporated herein by reference

A single boiler may be provided with several cleaning apparatuses positioned such that their blasting pipes reach different parts of the internal surfaces of the boiler. In one embodiment, two or more cleaning apparatuses are simultaneously operated in a single boiler during operation thereof.

It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and example of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below. The verbs "to comprise" and "to include" are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", i.e. a singular form, throughout this document does not exclude a plurality.

List of reference numbers

1 slag particle container

2 container bottom portion

3 transfer tube

4 blasting unit

5 rail

6 blasting pipe

7 boiler wall

8 heat transfer surface

9 collar

10 slag stream

11 deposit

12 clean surface

Citations list

Patent literature

CN 102313288 A

WO 2017/103345 Al