The present invention relates to a method according to the preamble of claim 1 of cleaning the heat-transfer surface which is in connection to the masonry structure of a combustion boiler.

In such a method, the surface is cleaned by blasting solid particles onto it. The present invention also relates to the use according to Claim 15.

The heat-transfer surfaces of power plants are typically cleaned by using sandblasting. In this case, sand (typically screened sand) is used in the sandblasting, and water is used 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.

When burning wood or fossil fuels, also sulphur and organic compounds which are derived from the combustion, are generated in the boiler. By providing water to the ashes which remain on the fire surfaces, various acids and weak acids start to form in the boiler.

However, it is not possible to clean the boiler completely with water. Instead, the residual ash remains wet and acidic. The acids and the weak acids begin to corrode the steel.

Moisture can also reach the inner part and the gaps of the masonry, and at same time, cause corrosion.

When damp ash dries, it becomes almost as hard as concrete. Repeated washes with water gradually accumulate more and more hard surface in the pipes, which reduces the efficiency of the heat production and reduces the flow of gases. Removing the hardened ash from inside the eco- and the superheater packages is very difficult and laborious, in some structural solutions even impossible and, as a result, the only possibility may be to replace part of or all of the structure.

It should also be noted that the masonries absorb moisture. The heating phase of the startup stage is extended because the masonries must be "dried". In such cases, it is quite possible that the masonries will fail. In the prior art, various alternatives are described for using sand in the blasting.

Such materials include steel grains and steel sand, copper slag, glass beads, metal pellets, dry ice, corundum and even ground coconut shells and corn grains. CN Patent Application Publication No. 102313288 describes a solution in which heat- transfer surfaces are cleaned using smooth-surface particles. In particular, the particles described in the publication are metal or non-metal particles or composite particles having a density of 2-8 g/cm ³ . DE Patent Application Publication No. 19723389 describes cleaning of the inner part of the boiler plants where the combustion gases are by using spherical steel particles.

International Application Publication WO 0138815 suggests blast cleaning of the heat- transfer surfaces by using coarse ash which is generated from the incineration of sludge.

JP Patent Application Publication No. 2002098323 describes the use of granulated slag grains to remove, by using blast cleaning, the metal residues which are adhered to the exhaust pipe of an electric furnace. The metal is generated when melting, in an electric furnace, the burning residues which are generated from burning of municipal waste.

Furthermore, the known technology is disclosed in US Patent Specifications Nos.

4 348 340 and 4 666 083.

Also previously known are dry blasting methods. An example of such a method is the solution which is described in the EP Patent Application Publication No. 2 113 339, in which alkali metal carbonates are used for blast cleaning of solid surfaces. According to the publication, alkali metal carbonates provide efficient cleaning of surfaces, without formation of dust or damaging the surfaces. Suitable materials mentioned in the publication are in particular natural carbonates, such as calcium carbonate and dolomite. These may be used, for example, to remove paint, foodstuff and drug residues from the inner

As described above, to date, no solutions have been proposed with which it would be possible to remove stubborn deposits, which are generated at temperatures that are typical for combustion boilers, without damaging the surfaces and without the formation of dust, which is characteristic of sand.

In the present invention, we have unexpectedly discovered that the above objective can be achieved by using, for the blasting, nickel slag or copper grit particles or similar metal slag particles, in essentially dry conditions.

Thus, according to the present invention, the heat-transfer surface of the combustion boiler, especially the heat-transfer surface which is in connection to a masonry structure, is cleaned by blasting onto it, in water-free conditions, metal slag particles having a particle size of approximately 0.3-3.0 mm, and by using a blasting pressure of 8-12 bar.

Thus, the present invention also comprises the use of metal slag particles for cleaning of the heat-transfer surface which is in connection to a masonry structure, by using blasting treatment.

More specifically, the method according to the present invention is mainly characterised by what is stated in the preamble of Claim 1. The use according to the present invention, in turn, is characterised by what is stated in Claim 15.

Considerable advantages are achieved with the present invention. By replacing sand in the blasting cleaning with nickel slag or copper grit particles or similar metal slag particles in dry conditions, it is possible to remove the dirt on the metal surface in objects which are difficult to clean without damaging the surface. At the same time, it is possible to avoid the damaging effect of the water flowing from the surface to be cleaned onto the masonry structures which are in contact with the surface or are near the surface. Furthermore, it has been found that dusting is considerably lower than in sandblasting, even though the operation is carried out in water-free conditions, which contributes to the work performance. The slag particles, which represent the waste fraction, and which otherwise cannot be used in any significant application, form an economically and technically advantageous material which, if needed, can be ground and graded in order to achieve a fraction of a desired fineness.

The 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.

In the following, embodiments will be examined more closely with the aid of the accompanying detailed description.

In the application of the present technology, heat-transfer surfaces of a power plant, in dry conditions, are cleaned using a fine fraction of a waste product generated in a metallurgical process. In one embodiment, especially the heat-transfer surfaces that are connected to the structures of a combustion boiler, in particular the masonry structures of a combustion boiler, are cleaned.

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).

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 the present technology, the operation is carried out in essentially "water-free

conditions". In practice, this means 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.

Typically, the surface to be cleaned is a heat-transfer surface. However, the method can also be used to clean other surfaces of a power plant boiler structure, which surfaces comprise impurities, including ash, coke and/or slag deposits which are generated from the combustion.

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.

The heat-transfer surface may be a metal surface, typically it is a steel surface. The steel may be, for example, a ferritic or an austenitic steel alloy which meets ASTM standards A213 or A213M, respectively. It is also possible to use other types of metal alloys.

However, the surfaces may be of a material other than metal, for example a ceramic.

Examples of surfaces to be cleaned are, in particular, the heat-transfer surface which forms part of a heat boiler, such as the eco- and the superheater packages of grate-fired boilers or fluidised bed boilers.

In one embodiment, the surface to be cleaned, especially a metal surface, typically a steel surface, forms at least part of a superheater or at least part of an Economizer or a Luvo unit.

Preferably, the surface to be cleaned is in the vicinity of the masonry or at least partly on top of it. Typically, the distance to the nearest masonry, i.e. the masonry surface, is at maximum approximately 250 cm, usually at maximum approximately 150 cm, especially at maximum approximately 100 cm, for example at maximum 50 cm. The masonry surface may be in direct contact with the surface to be cleaned.

It is well known that those parts of the wall structures of boilers that are designed for burning moist fuels, and which parts maintain a temperature suitable for the drying and the pyrolysis phases, should be uncooled. An uncooled surface is achieved, for example, by using masonries. For example, ceramic bricks can be used for the masonry. At the fuel layer points, in turn, the furnace walls are cooled, which can reduce the problems that arise when the fuel burns too quickly at the edges of the grate. Most suitably, the blasting is carried out with metal slag, which is in particle form, in which case the particle size of the metal slag is approximately 0.3-3.0 mm.

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. According to a preferred embodiment, metal slag is used which is essentially free of ferritic compounds.

According to a preferred embodiment, metal slag is used, such as nickel or copper slag, the particles of which are non-spherical shaped.

Preferably, the weight of the slag used comprises at least 90 % iron silicate (Fe ₂ Si0 ₄ ) and 1-5 % magnetite (Fe ₃ 0 ₄ ). In addition, it may comprise Al, Ca, Mg and Cr-oxides and - silicates (approximately 0.5-5 %), and minor amounts, typically less than 1 %, in particular less than 0.5 % of, for example, one or more of the following metals: Ni, Cu, Pb, Sn, Sb, Bi, and Cd. According to a preferred embodiment, nickel slag is used. This is a waste product which is generated in association with the recovering of nickel. In another embodiment, the material used in the blasting is copper slag (copper grit).

Experiments have demonstrated that the particle material of the metal slag used must have, besides a particle size which is suitable for the blasting, also a sufficient hardness and weight. Typically, the hardness of the metal slag particles must be greater than

approximately 6°, most suitably even approximately 8°, on the Mohs scale of hardness, and the density must be greater than approximately 3.0 g/cm ³ , for example approximately 3.1- 4.2 g/cm ³ , most suitably approximately 3.3-3.9 g/cm ³ , or 3.5-3.8 g/cm ³ . The hardness can be over 8° on the Mohs scale, but usually a hardness of approximately 8° (± 0.5°) is sufficient to carry out the cleaning.

Besides a sufficient specific gravity, the particles must have a suitable shape. Thus, the value of the bulk density of the particles is in particular greater than 45 %, for example greater than approximately 50 %, or greater than 60 % of the specific gravity of the particles. In one embodiment, the bulk density is greater than 1.8 g/cm ³ , especially 1.85 g/cm ³ or greater. When the hardness, shape, and weight of the material used are within the ranges described above, the dirt can be removed from the object to be cleaned. With excessively light or chip-like slag, the result will not be as desired.

In addition, in preferred applications, the particle size of the slag is within a pre-selected range.

Typically, the particle size of the metal slag used (that is, the "grain size") is approximately 0.3-3 mm. This means that the maximum dimension of at least 80 %, especially at least 90 %, usually at least 95 % of the particles is within the range in question.

The present particles may be individual particles or agglomerates (granules) which are formed of several particles.

More preferably, the average particle size of the nickel slag is within the range of 0.3-2.5 mm, for example 0.5-2.2 mm. Typically, this means that the maximum dimension of least 90 % of the particles, most suitably at least approximately 95 % (by weight) is within the said range.

Similarly, more preferably, typically at least 90 %, especially at least 95 %, (by weight) of the copper slag particles have a particle size within the range of 0.4-2.8 mm, for example 0.425-2.5 mm (largest dimension).

In one embodiment, the size of the copper grit particles ("particle size") is 95 % = 0.425- 2.8 mm. That is, 95 % by weight of the particles are, by their size, within said region. Here, the particle size typically means the screened particle size (that is, grain-size).

It has been found that, with a narrow distribution of particle size, in which at least 90 %, especially at least 95 % of the particles (by weight) have the particle size defined above, in particular with regard to the largest dimension, it is possible to maintain a predetermined pressure in the blasting stream, in which case a uniform effect over the surface is obtained. Typically, the pressure range is 8-12 bar, see below. It has been discovered that the nickel and copper slag particles are not spherical, but they have an irregular shape. This also means that, when the blasted nickel or copper slag particles hit the 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, without damaging the steel surface.

In the present invention, typically the blast nozzle used can be either small or large. The nozzle diameter may be, for example, 0.5-25 mm, usually approximately 1-15 mm, typically approximately 12 mm. These nozzle sizes are particularly suitable for the application described above, in which the metal slag particles have a narrow distribution of particle size. The blasting is carried out by using a blasting pressure of 8-12 bar. More preferably, the pressure used is 9-11 bar. At this pressure, an efficient cleaning of the dirt layers is achieved and, at the same time, damage to the surface is avoided. The consumption of air varies with the nozzle size, but is generally approximately 50-2500 1/min, most suitably approximately 70-1500 1/min, for example approximately 150-1000 1/min.

The blasting can be carried out by using a nozzle which is straight, curved or bent at 45 degrees. The shape of the nozzle is selected according to the object to be cleaned. In connection with the present invention, we have found that after cleaning carried out using the slag particles described, the surface does not corrode as easily as after treatment in which sand is used. In addition, after the present treatment, the surface is not prone to become dirty. One of the reasons for this seems to be that for example nickel slag forms a chromium oxide compound on the metal surface, which compound protects the metal from corrosion, and which, on the other hand, also slows down the adhesion of new dirt to the metal surface.

Citation List

Patent literature

CN 102313288

DE 19723389

WO 0138815

JP 2002098323

EP 2 113 339

US 4 348 340

US 4 666 083