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On May 31, 2021, according to contract No. _ / X6 for a visual inspection of the state of the existing refractory lining of the furnace of the HURST "HYBRID 1700 HP-250" boiler with a steam capacity of 25 MTH, the specialists of Wellons.RU LLC performed a technical inspection, the results of which are described below.
THE REFRACTORY OF THE BOILER’S CELL:
The boiler has been in operation since February 2017 and over a four-year period of operation, the refractory lin-ing of the boiler furnace has been repeatedly changed. During the operation of the boiler, there was a transition from the original monolithic lining (refractory concrete) recommended by the manufacturer of the boiler (Hurst company) to a refractory lining using molded refractory bricks. With a properly selected refractory, adherence to the technology of its laying, the service life of the refractory, regardless of the type of refractory used (concrete or brick), on such boilers should be at least 5-7 years or more (according to the experience of our company, the service life of the refractory on boilers for biofuel can reach more than 10-15 years, subject to the technological mode of operation of the equipment). The inspection revealed a number of problems related to both the quality of the refractory and the technical solutions for its laying, which lead to the premature failure of the refractory liner as a whole. A more detailed description of defects is shown in the corresponding sections of this report.
Fuel Auger supply area – The working layer of the front end wall lining, including the fuel chute lining and part of the lining up to the first header of the upper front screen, is made of monolithic refractory concrete. Consequently, this section has a service life equal to that of the boiler (~ 4 years); since the rest of the original monolithic lining had been broken off during previous repairs. Taking into account this lining service life, it can be stated that the technical condition of this lining section is in a satisfactory state. The working layer has no significant destruction. The existing surface defects (small chips, cracks) do not have a critical impact on the integrity of the lining as a whole. When carrying out capital repairs, to replace the boiler furnace lining, this section, taking into account its service life, is recommended to be replaced using refractory concrete with appropriate characteristics.
Ash discharge area / rear end wall – multilayer lining, the working layer of which is made of refractory concrete. This lining has not changed since the start of boiler operation, the situation is similar to the one described above for the lining of the fuel auger zone. The technical condition of the lining is satisfactory. The existing expansion joints in the working layer made it possible to ensure the integrity of the lining, the existing surface chips and cracks do not have a critical effect on the lining as a whole. There are significant refractory chips in the area of the rear header of the ceiling the screens formed as a result of many years of operation, as well as chips in the lower part of the wall, in the area of the junction of the side walls, are periodically sealed with chamotte clay / mortar, or a similar material. Unfortunately, this material does not have the required strength and carries only a shortterm solution to restore the integrity of the lining, this material must be constantly updated during routine repairs or during technical inspections. The monolithic lining of the technological manhole zones, as well as the inspection hole and the zone of the tuyere blow holes are in normal technical condition.
IMPORTANT NOTE! Despite the fact that in the indicated zones (the zone of screw loading of fuel and the zone of ash discharge) the temperatures of the technological process of sunflower husk combustion are in lower limits than in the fuel com-bustion zone, the integrity of the monolithic lining in these zones makes it possible to conclude analysis of the reliability of the proposed solutions in terms of the use of monolithic refractory solutions in the installation of the boiler furnace lining. It is safe to say that if all the necessary requirements of the technology for laying monolithic refractory on a given boiler are met, the lining durability can be significant (at least 5–7 years).
Furnace side walls (grates area) – fireclay refractory brick, type ШБ-5, which does not differ in strength characteristics, and density in relation to similar fireclay refractory ША-5, which has a higher class according to GOST R 53406– 2009 "Fireclay and semi-acid products for general use." The design of the walls, unfortunately, does not provide for the presence of the necessary expansion joints in the lining. It can also be assumed that there are no anchors for fastening the lining, by the presence of extrusion of the lining of the walls into the inside of the working space of the furnace. Those the lining of the walls has lost its straightness. The absence of expansion joints, the absence of anchoring is one of the reasons for the emergence of wall extrusion, increased formation of refractory chips, especially in the lower lining belt at a height of up to 600 mm from the grate level. In addition, one of the factors of the destruction of refractory masonry may be the absence of horizontal rows of bricks, which have an angle of inclination equal to the angle of inclination of the bar grating (inclination angle 22 °). In the absence of additional devices (supports, anchors, expansion joints) that distribute the load from the lining more evenly, the lower belt of the lining experiences maximum tangential loads on the brick (the resulting force of gravity is inclined towards the surface of the brick), leading to its destruction. A variable thermal load on the lining enhances this factor during the operation of the boiler. Moreover, taking into account the low strength characteristics of the fire-resistant brick ШБ-5, as well as the design features of the existing lining, or rather their absence, leads to significant destruction of the refractory bricks of the lower belt, in the area of the grate. This destruction is maximally observed in the fuel combustion zone and reaches 150 mm and more. Taking into account the thickness of the lining of the working layer equal to 230 mm, the remaining thickness of the lining is less than 80 mm, which is critical (in relation to the initial one).
We also draw your attention to the presence of ash / slag deposits on the walls of the furnace with pronounced yellow deposits. Having analyzed the chemical composition of sunflower husks, we can say with a high degree of confidence that these deposits are sulfurous, which, in turn, in the presence of a large amount of alkalis in the husk ash (alkali metal oxides), form fusible eutectics that penetrate into the structure of the refractory, destroying him from the inside. This is confirmed by the presence of a large number of superficial small and medium brick chips from 20 to 60 mm thick.
The process of deposition of sulfate salts is most pronounced in the zone of high temperatures / in the zone of fuel combustion and manifests itself along the trajectory of the gas flow. To reduce the negative impact of this factor on the durability of the lining, it is necessary to constantly monitor the temperature in the boiler furnace and avoid prolonged exposure to peak temperatures in the technological process of disposal of waste / sunflower husk. We also draw your attention to the presence of ash / slag deposits on the walls of the furnace with pronounced yellow deposits. Having analyzed the chemical composition of sunflower husks, we can say with a high degree of confidence that these deposits are sulfurous, which, in turn, in the presence of a large amount of alkalis in the husk ash (alkali metal oxides), form fusible eutectics that penetrate into the structure of the refractory, destroying him from the inside. This is confirmed by the presence of a large number of superficial small and medium brick chips from 20 to 60 mm thick.
Important Note! When choosing the concept of a refractory brick lining (made of molded refractories), it is necessary to use refractory bricks that have not only higher strength (for example, the ultimate strength of ШБ-5 according to GOST is not less than 15 MPa, the one recommended by us has an actual strength of at least 55 MPa), but also a higher resistance to chemical aggression, as well as a higher fire resistance (ШБ - content Al2O3=28-30%, proposed refractory – content Al2O3>40%).
Lining of the roof screen arch (front and rear) – two-layer lining, the working layer of which is made of aluminosilicate refractory tiles, the heat-insulating layer - using heat-insulating concrete. The state of the thermal insulation layer is in good condition. The state of the working layer, especially the front screen, is in an unsatisfactory state. The material has a loose structure, the joints of the tiles have significant wear. The presented video report clearly shows that with a small manual impact on the tiles of the front screen, the material crumbles very easily, the structure of the refractory is loose, the refractory has lost its strength and refractory properties. This material is more like a heat-insulating material than a refractory material, which, in principle, is not used as a working layer in boilers of this type. We believe that the material for this zone was selected incorrectly, this material requires replacement due to the loss of its strength and refractory properties.
Lining of ceiling screen collectors – The original lining of the transition sections of the collectors from the ceiling screen to the upper screen / the zone of the inner part of the collectors should be made of monolithic refractory concrete with a maximum ap-plication temperature of up to 1500 ° C. These are concretes with high strength characteristics and high Al2O3 content.
Unfortunately, by indirect indications, it is safe to say that the laid refractory concrete does not correspond to the characteristics laid down by the project from the boiler manufacturer. The state of refractory concrete in this area, in the area of transition collectors, is unsatisfactory. Concrete has many deep, through and open cracks, large chips and low strength. The presented video report shows that even with the slightest impact on the refractory, it collapses very easily, exfoliates and breaks. This concrete has a very low strength. It can be assumed that the laid concrete is heat insulating and this transition area is filled with the same concrete that was used for the installation of the heat-insulating layer of the upper ceiling screens (the zone of heat boxes and a collector for supplying combustion air through the tuyere openings of the collectors of the water screens). We believe that the material for these sections of the collectors was selected incorrectly; this mate-rial requires replacement due to the loss of its strength and refractory properties.
Important Note! We consider it necessary to draw your attention to the condition of the front ceiling water screen of the boiler, namely, to the technical condition of the front and transition collectors and water pipes. We recommend to carry out the diagnostics of the pipes of the water screen for compliance with the design thickness of the pipes, be-fore the start of the lining work.
Lining of the side and end walls of the boiler backup zone (pipe part of the boiler) – the working layer of the refractory lining is made of refractory bricks ШБ-5, the condition is satisfactory. On the walls, traces of significant volumes of ash and slag deposits (after cleaning the boiler) are visible, formed due to the removal of ash particles from the furnace space of the boiler - the question of the possibility of technological regulation of the fuel combustion process (sunflower husk) in accordance with its physical characteristics (humidity, fractional composition, ease of entrainment of particles, etc.).
Due to the fact that we do not know the exact design of the lining, due to the lack of executive documentation for the existing lining scheme, and, accordingly, the presence of supporting devices in the lining (supports, anchors) is unknown, especially in the transition zone furnaces and boiler afterburning zones (pinch zone), we have developed a concept for replacing the lining, including the lining of the side walls as a whole, from the grate level to the collectors of the side screens of the water tube system in the boiler afterburning zone.
The lining of the end walls of the boiler afterburning zone has a normal technical condition and is made independently of the side walls and can be left in case of a planned replacement of the boiler furnace lining. as a result, it is not included in the total volume of replacement of the boiler furnace lining, and is not taken into ac-count in the estimated cost of work in the proposed conceptual solutions and technical and commercial proposals. If you wish to replace these areas, the volume of materials and work will be taken into account in the estimated calculations at the time of the conclusion of the contract, do not have a significant impact on the cost of work as a whole.
Finally: A visual inspection of the refractory lining of the furnace of the HURST "HYBRID 1700 HP-250" boiler with a steam capacity of 25 t / h, showed the need to replace the lining in the furnace zone of the boiler, for the reasons described in this report. I would like to draw your attention to the fact that the service life of the lining in boilers operating on the utilization of various types of biofuel, in your case working on burning high-calorie sunflower husks, should be at least 5-7 years. To achieve this goal, regardless of the adopted concept for the implementa-tion of a monolithic (concrete) or molded (brick) lining, or combined solutions of these refractories, it is neces-sary to fulfill an obligatory number of conditions:
- ensure the selection of an appropriate refractory with an Al2O3 content of at least 42% for refractory bricks, at least 55% for refractory concrete;
- to ensure compliance with the technology of laying refractory with the device of expansion joints in the required amount;
- to provide the necessary systems for heat-resistant fastening of the lining (anchor) and systems of supporting / unloading devices (supports) in the required number to ensure the strength of the structure as a whole.
Based on the results of the technical inspection, we prepared for you two conceptual solutions for replacing the existing lining using a monolithic (preferred, but more expensive solution) and molded refractory, as well as technical and commercial proposals for each of the options. These materials are part of the general documenta-tion for the execution of the contract.
All presented photos, as well as photos that were not included in this report, together with a video report of the technical inspection of the lining, were transferred in electronic form to the specialists of the plant.