This invention relates to a structural solution for a smelt spout of a recovery boiler as defined in the preamble of claim 1. The application of the smelt spouts according to the invention is in the chemical wood processing industry, more particularly in boiler plants related to the manufacturing process of cellulose, developed for the re covery of chemicals and production of energy, which are referred to in the technical language as recovery boilers. Smelt spouts are used to direct hot chemical smelt from the bottom of the boiler for further processing and recycling back to the chem ical cellulose manufacturing process. Typically, a recovery boiler has between 5 and 11 smelt spouts.

The invention is characterized by that which has been defined in more detail in the characterizing part of the independent claim. Structural solutions related to the im plementation of the invention and additional details supplementary to the independ ent claim have been defined in the dependent claims.

The smelt spouts of a recovery boiler are critical components for the function of the boiler, and their operating conditions are highly demanding. The temperature of the chemical smelt directed out of the boiler is typically above 800 degrees Celsius, at its highest up to 1000 °C. In addition, the chemical smelt is extremely corrosive, highly alkaline lye. Further, the smelt spout is subjected to heavy abrasive wear due to the flow of the smelt and the need for mechanical cleaning of the spout. Mechan ical cleaning rods and robotized cleaning rods are often used for cleaning the spout and eliminating interruptions in the flow of the smelt, causing, in addition to abra sive effects, an intense mechanical pressure effect on the flow surface of the spout and the entire structure of the spout.

Due to the high heat load, smelt spouts are nowadays in general fitted with a liquid cooling arrangement, in which cooling liquid is circulated in the duct(s) between the surface opposite to the spout surface in contact with the chemical smelt and the body plate in order to maintain the spout temperature at a controllable tempera ture. Water is typically used as a cooling liquid, however, its use is associated to an extremely high risk of a smelt-water explosion, in case water gets in contact with the hot smelt.

Attempts to control the intense corrosion and mechanical wear effect caused by the chemical wear brought on by the smelt as well as the impact of the high tempera ture have been made with different material choices and coating solutions for the flow surface of the spout in contact with the smelt. Typical solutions include differ ent welding coatings and covering structures. Quality issues related particularly to welding coatings and factors brought about by the heavy chemical wear and the high temperature cause cracking, hairline cracking and grain boundary damages. Further contributing to the cracking and hairline cracking of the spouts in contact with the smelt is the vast temperature difference between the opposing sides of the spout plate, resulting from the current structural solutions of the spouts. The above- described damages and hair cracks in the spout in contact with the smelt increase the risk of cooling water getting in contact with the hot smelt, which causes smelt- water explosions. Smelt-water explosions can cause equipment damage, and at worst, serious risk of injury and outages in the use of the boiler.

Due to the demanding operating conditions and risks of damage, the service life of smelt spouts is short, typically around one year. Damages occur almost exclusively in the part of the spout in contact with the smelt. With the current spout solutions, changing the spouts requires a lengthy outage in the use of the boiler, and the spouts are expensive components using the current structural solutions, which are typically welded integral plate structures.

Next, a few typical smelt spout structures appearing in patent literature and in prac tice are described. The published application US 5 437 687 discloses a typical water- cooled spout structure, wherein cooling water flows on the opposite side of the spout plate of spout directing the smelt flow.

The publication JP 2003211287 discloses a spout structure in which the surface fac ing the smelt flow of the spout directing the flow is coated with a weldable coating improving the corrosion resistance in order to extend the service life of the spout. Cooling water flows on the opposite side of the plate of the weld coated spout. The surfaces of the water flow channel are treated with metal spraying in order to im prove the corrosion resistance. The publication JP 2012130998 presents a more ad vanced solution for removing the welding stresses as an overlay welded spout plate, but the basic construction of the spout for the flow of cooling water is similar to that of the publication JP 2003211287.

In addition to the above-described technical and operational safety issues, the maintenance and need for regular replacement associated to the smelt spouts cur rently in use cause significant servicing costs, particularly in the form of boiler out ages, during which the entire cellulose manufacturing process is typically forced to a stop. Moreover, the spout types currently in use, being welded plate structures, are demanding structures from a technical manufacturing point of view and require sig nificant manufacturing efforts.

With the smelt spout structure according to the present invention, the above-pre sented technical problems can be minimized and the risks associated to operational safety reduced, and the direct and indirect costs associated to the servicing of smelt spouts significantly reduced.

The invention is characterized by the smelt spout consisting of two separate struc tural parts, being the body of the smelt spout and a replaceable wear part detacha bly connectable thereto, coming into contact with the smelt during use, i.e. an in sert. Further, the invention is characterized in that the cooling liquid circulation is arranged inside the smelt spout body in such a way that the cooling liquid is not in contact with the insert that is in contact with the smelt, thus preventing the smelt from contacting the cooling liquid in case of damage in the insert. Moreover, the re placement of the insert is a quick maintenance procedure and the costs of the insert are small compared to the costs of replacing the entire spout.

Next, the basic construction and functions of the smelt spout according to the in vention as well as solutions for its maintenance are described with reference to the drawings, in which: Figure 1 shows the cross-section of a smelt spout according to the invention connected to the wall structure of a boiler,

Figure 2 shows the section A - A of figure 1, i.e. the cross-section of the smelt spout

In figure 1, the internal contact surface 1 of the boiler wall structure is represented by dotted lines. The smelt spout body 2 is detachably attached to the lead-through housing 3 of the wall pipe system of the boiler, represented by a dotted line. The smelt spout body comprises a mounting flange structure 4. The liquid cooling circu lation of the smelt spout is arranged in the flow channel 5 inside the body 2 and the body is equipped with necessary connecting members in order to connect it to the cooling liquid pipe system. The insert 6 of the smelt spout, represented by a thick line in the figure, abuts the spout-shaped surface of the body, as shown in figure 2. The insert 6 is a monolithic metallic part comprising a tubular structure along the length of the boiler wall lead-through part and a spout-shaped structure with an open top in the part extending outside the boiler wall.

Figure 2 shows the section A - A of figure 1. The cross-section of the insert 6 is de picted with a thick black line. The cross-section of the body 2 is depicted with sec tion lines. The structure of the body comprises an internal cooling liquid flow chan nel 5.

During maintenance of the smelt spout, only the worn or corroded insert is re placed, making the changeover a quick procedure with low costs.

The structural details of the smelt spout according to the invention are not limited to the above-presented solutions of principle, but can range widely within the scope of the characteristic features of the claims.