TECHNICAL FIELD

The present invention generally relates to recovery boilers. The invention relates particularly, though not exclusively, to protecting recovery boiler floor tubes.

BACKGROUND ART

This section illustrates useful background information without admission of any technique described herein representative of the state of the art.

Recovery boilers are fueled with waste liquor (black liquor) generated in connection with pulp manufacturing. Black liquor is a highly corrosive substance which is combusted in a furnace area of the boiler. The floor of the recovery boiler furnace is made of tubes that are filled with water. If the floor tubes are directly exposed to black liquor, this may lead in unfavorable conditions that promote local corrosion or cracking of the floor tubes. During recovery boiler start up, after recovery boiler outage, the floor tubes may additionally be exposed to an excessive heat load due to start-up burner flame impingement if not protected.

In order to protect the floor tubes, a protective layer of a protecting chemical, such as sodium sulfate, may be spread onto the furnace floor during recovery boiler outage after the floor tubes have been inspected. The spreading of the chemical, however, is laborious requiring manual labor in terms of carrying bags into the furnace and spreading the bags/powder to cover the floor. SUMMARY

According to a first aspect of the invention, there is provided a method for protecting a furnace floor of a black liquor recovery boiler, comprising:

mixing protective material with a fluid to form a mixture comprising said protective material; and

covering the furnace floor by said mixture by flowing the mixture onto the furnace floor.

In certain embodiments, the material is or comprises a recovery boiler process chemical, such as sodium sulfate, sodium carbonate or another inorganic sodium salt. In certain embodiments, the material is sodium containing material. In certain embodiments, the sodium containing material is a sodium salt, such as sodium sulfate or sodium carbonate. In certain embodiments, the mixture is free of organic material. In certain embodiments, the mixing consists of blending the protective material with water.

In certain embodiments, the said covering is to form a protective layer. In certain embodiments, the protective layer is to protect the floor against direct exposure of black liquor. In certain embodiments, the protective layer is to protect the floor against start up burner flame impingement. In certain embodiments, an empty or emptied furnace floor is covered, the term empty or emptied referring to a furnace floor that is not covered by hot smelt. In certain embodiments, this means a washed or otherwise cleaned furnace floor.

In certain embodiments, the said mixing is performed by mixing the material forming the protective layer with fluid or water. In certain embodiments, the mixing is performed without a chemical reaction. Accordingly, the material forming the protective layer merely dissolves in the fluid or water. In certain embodiments, the method comprises covering the furnace floor by said mixture by flowing the formed mixture onto the floor from the outside of the furnace.

In certain embodiments, the method comprises causing the mixture to flow onto the furnace floor from the outside of the furnace via an opening in a wall of the black liquor boiler.

In certain embodiments, the method comprises pumping the mixture onto the furnace floor from the outside of the furnace.

In certain embodiments, the mixing is performed in connection with pumping the mixture onto the furnace floor. In certain embodiments, said mixing is performed during said pumping or prior to said pumping.

In certain embodiments, the method comprises providing the mixture as an aqueous solution.

In certain embodiments the said mixing material with a fluid to form a mixture (or providing the mixture as an aqueous solution) is an in-situ or on-site process in contrast to any off-site process in which the mixture or aqueous solution would be formed elsewhere, e.g., another factory location, and transferred to the recovery boiler facility (or building) therefrom.

In certain embodiments, the method comprises:

performing the act of covering the furnace floor with said mixture simultaneously with a removal of a furnace safety roof during outage.

In certain embodiments, the method comprises settling the flown mixture on the floor by gravity alone.

In certain embodiments, the method comprises forming a salt lake onto the floor extending over the floor from side to side during recovery boiler outage. In certain embodiments, the method comprises allowing the salt lake to solidify forming a protective layer to protect floor tubes of the furnace from direct exposure of black liquor and flame impingement.

In certain embodiments, the material comprises at least two different salts.

In certain embodiments, the material comprises at least two different salts selected from a group consisting of: sodium carbonate, sodium sulfate, sodium sulfide, sodium chloride, potassium carbonate, and potassium sulfate.

In certain embodiments, the method comprises using a mixture whose melting point, after solidification, is lower than 850 °C.

According to a second aspect of the invention, there is provided an apparatus for protecting a furnace floor of a black liquor recovery boiler, comprising means for performing the method of the first aspect or any of its embodiments.

Accordingly, in accordance with the second aspect, there is provided an apparatus for protecting a furnace floor of a black liquor recovery boiler, comprising:

mixing means for mixing material with a fluid to form a mixture; and

covering means to cover the furnace floor by said mixture by flowing the mixture onto the furnace floor.

In certain embodiments, the mixing means comprises a container. In certain embodiments, the mixing means comprises a mixer. In certain embodiments, the covering means comprises a pipe and an optional pump to feed the mixture onto the furnace floor.

Different non-binding example aspects and embodiments of the present invention have been presented in the foregoing. The embodiments in the foregoing are used merely to explain selected aspects or steps that may be utilized in implementations of the present invention. Some embodiments may be presented only with reference to certain aspects of the invention. It should be appreciated that corresponding embodiments may apply to other aspects as well, and any appropriate combinations may be formed.

BRIEF DESCRIPTION OF THE DRAWINGS

Some example embodiments of the invention will be described with reference to the accompanying drawings, in which:

Fig. 1 depicts a conventional method for protecting a recovery boiler floor;

Fig. 2 shows a schematic drawing of floor protection in accordance with an embodiment of the present invention;

Fig. 3 shows a schematic drawing of an arrangement for providing recovery boiler furnace floor tube protection in accordance with an embodiment;

Fig. 4 shows a schematic drawing of an arrangement for providing recovery boiler furnace floor tube protection in accordance with another embodiment; and

Fig. 5 shows a flow chart of a method in accordance with an embodiment.

DETAILED DESCRIPTION

In the following description, like reference signs denote like elements or steps.

Fig. 1 depicts a conventional method for protecting a furnace floor of a black liquor recovery boiler. The furnace 100 is bounded by furnace walls 101 and the furnace floor 102 made of water tubes. Since Fig. 1 depicts the situation during a late phase of a recovery boiler maintenance break, i.e., recovery boiler outage, the furnace floor 102 has already been cleaned and inspected for cracks, and there are typically scaffoldings 103within the furnace 100 at this moment. Also a safety roof has been installed into an upper part of the furnace 100 to ensure that any manual labor on the furnace floor 102 can be performed safely. A pile of sodium sulfate bags 107 has been brought onto the floor 102 for spreading. Once spread onto floor tubes, the sodium sulfate serves to protect the floor 102 from direct exposure of forthcoming black liquor and start-up burner flame impingement. The floor protecting method continues as follows: The spreading of the sodium sulfate is performed by manual labor, and the safety roof is removed thereafter.

It has been observed that especially in large boilers the conventional method of providing the floor with the protecting material is laborious and time-consuming. The outage time could be shortened if the protecting material could be provided onto the furnace floor more quickly.

Fig. 2 shows an obtained result of floor protection in accordance with an embodiment of the present invention. The reference numeral 210 depicts a solidified lake of protective material on the furnace floor 102 forming a protective layer that covers the floor tubes of which the floor 102 is made.

The protective layer of protective material is provided by a method comprising mixing appropriate material with a fluid, for example water, to form a mixture, and covering the furnace floor 102 by said mixture by flowing the formed mixture onto the floor 102 from the outside of the furnace 100.

In certain embodiments, the method comprises causing the mixture to flow onto the furnace floor 102 from the outside of the furnace 100 via an opening in the wall of the black liquor recovery boiler, or furnace wall 101. Fig. 2 shows several openings in the furnace wall 101, such as, smelt spout opening(s) 250, primary air openings 260, secondary air openings 270, and start-up burner openings 280.

Fig. 2 also shows black liquor nozzles 230 used to spray black liquor into the furnace, via respective black liquor nozzle openings, during normal operation of the boiler, as well as the smelt spout(s) 255 pouring an overflow of smelt from the floor 102 into a dissolving tank 290 during normal operation. In certain embodiments, the mixture is caused to flow via at least one smelt spout opening 250. In certain embodiments, the mixture is caused flow via at least one primary air opening 260. In certain embodiments, the mixture is caused flow via at least one secondary air opening 270. In addition or instead, a man door opening residing in the wall 101 and/or at least one start-up burner opening 280 and/or at least one black liquor nozzle opening may be used.

In certain embodiments, the formed lake is allowed to solidify forming a protective layer to protect floor tubes of the furnace 100 from direct exposure of black liquor and flame impingement.

In certain embodiments, the method comprises pumping the mixture onto the furnace floor 102 from the outside of the furnace 100. Fig. 3 shows such an arrangement or apparatus in which material 321 and fluid (or water) 322 is mixed in a container 330 or similar on the outside of the furnace 100. The mixing may involve agitation caused by a mixer 331. In an embodiment, the mixer 331 is operated by at least one motor. The formed mixture is pumped along an in-feed line 335 by a pump 332 via an opening 350 (which may be any suitable opening as discussed in the preceding) in the furnace wall 101 onto the floor 102. In an alternative embodiment, the mixture flows along the in-feed line 335 merely based on gravity or based on fluid (or water) pressure.

In certain embodiments, the mixture flown onto the floor settles on the floor by gravity alone forming a lake 210 extending over the whole area of the floor 102. The lake 210 is allowed to solidify forming a protective layer. In certain embodiments, the fluid/water in the lake evaporates, which evaporation may be enhanced by firing oil or gas using start-up burners, and a solid layer of protective material is formed.

In certain embodiments said mixing is performed prior to said pumping such as presented in connection with Fig. 3. In certain other embodiments, mixing is performed during said pumping (or simultaneously with flowing the mixture onto the furnace floor 102). This is shown in Fig. 4, in which material from a container 421 is mixed with incoming fluid (or water) 322 in a dosing device 430, and the mixture is flown along an in-feed line 435 via the opening 350 onto the floor 102. Alternatively, the mixing may occur on the furnace side of the opening 350. For example, the dosing device 430 may reside on the furnace side of the opening 350. The mixture flows along the in-feed line 435 driven by a pump, or merely based on gravity, and/or based on fluid (or water) pressure.

In certain embodiments, the method comprises performing the act of covering the furnace floor with said mixture simultaneously with a removal of the furnace safety roof 105 during outage. Since the presented method does not require workers inside of the furnace 100, the safety roof 105 can be removed simultaneously with flowing the mixture onto the floor 102 and spreading it by gravitation.

Fig. 5 shows a flow chart of a method in accordance with an embodiment. In the first step 801, material is mixed with a fluid to form a mixture (the material not yet being on the furnace floor). And, in the second step 802, the furnace floor is covered by the mixture.

In certain embodiments, the material that is mixed with fluid comprises at least two different salts.

In certain embodiments, the material comprises at least two different salts selected from a group consisting of: sodium carbonate, sodium sulfate, sodium sulfide, sodium chloride, potassium carbonate, and potassium sulfate.

In certain embodiments, the method comprises using a mixture whose melting point, after solidification, is lower than 850 °C. Examples of such mixtures are the mixture of sodium sulfate and sodium carbonate, the mixture of sodium sulfate, sodium carbonate and sodium sulfide, the mixture of sodium sulfate, sodium carbonate, potassium sulfate and potassium carbonate. In certain other embodiments, conventional protective materials, such as mere sodium sulfate or mere sodium carbonate is used.

Without limiting the scope and interpretation of the patent claims, certain technical effects of one or more of the example embodiments of this disclosure are listed in the following. A technical effect is that the protective material can be transferred onto the furnace floor and it spreads evenly without the need of any worker being inside of the furnace during the transfer and spreading. Another technical effect is faster transfer and spreading of the protective material. Another technical effect is a shortened recovery boiler outage time due to the fact that the transfer and spreading of the protective material can be performed simultaneously with the removal of the safety roof in an upper portion of the furnace. Another technical effect is easier removal of the protective layer when needed due to using material mixtures having lower melting temperature.

Various embodiments have been presented. It should be appreciated that in this document, words comprise, include and contain are each used as open-ended expressions with no intended exclusivity. The foregoing description has provided by way of non-limiting examples of particular implementations and embodiments of the invention a full and informative description of the best mode presently contemplated by the inventors for carrying out the invention. It is however clear to a person skilled in the art that the invention is not restricted to details of the embodiments presented in the foregoing, but that it can be implemented in other embodiments using equivalent means or in different combinations of embodiments without deviating from the characteristics of the invention.

Furthermore, some of the features of the afore-disclosed embodiments of this invention may be used to advantage without the corresponding use of other features. As such, the foregoing description shall be considered as merely illustrative of the principles of the present invention, and not in limitation thereof. Hence, the scope of the invention is only restricted by the appended patent claims.