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Project titleLaser modification of the structure and characteristics of the heating surfaces of metals and alloys to improve resistance to low-temperature corrosion and reducing slagging

Research area 03 - CHEMISTRY AND MATERIAL SCIENCES, 03-401 - Non-equilibrium processes – ignition, combustion, detonation, explosion

KeywordsMetals and alloys, laser radiation, texturing, surface modification, boiler, heat exchangers, low-temperature corrosion, slagging, composite fuels

Annotation
One of the most widespread industrial methods in the world for the generation of heat and electric energy is the fuel combustion in the boilers furnaces at thermal power plants and heating boilers. Typical problems in the combustion of solid, liquid and composite fuels (a mixture of several components) are corrosion and slagging of heating surfaces. The most urgent task in the field of thermal power engineering is to reduce greenhouse gas emissions into the atmosphere, most by involving composite fuels based on low-grade coals, liquid combustible waste and biomass in the energy cycle. In the case of using such fuels in practice (at operating power generating facilities), low-temperature corrosion and slagging of heating surfaces are inevitable due to a change in the temperature regime inside the boiler furnace. These negative consequences arise due to the properties of the non-combustible mineral part of the composite fuel (both liquid and solid). At relatively high temperatures inside the boiler furnace, the mineral part of the fuel passes from a solid state to a plastic one. Therefore, the ash, moving along the gas-air duct of the boiler furnace, colliding with the heating surfaces located both in the furnace itself and in its convective part, settles on the metal surfaces of the wall tubes, the superheater and other heating surfaces, forming an ash-slag layer and thereby deteriorating the characteristics of heat transfer, which in turn leads to a decrease in all basic parameters of the boiler and to its subsequent emergency shutdown. Along with this, relatively high concentrations of moisture in the composite fuel, which has a positive effect on the ecological characteristics of flue gases, lead to the development of low-temperature corrosion in the convective part of the boiler furnace. In world practice, there are different methods of dealing with these problems, but the most widely known methods, as a rule, only reduce the intensity of the processes. Therefore, the problems of low-temperature corrosion and slagging of heating surfaces remain relevant today. The acquisition of new knowledge in the field of chemical physics and materials science contributes not only to the development of fundamental science, but also to the development of the theoretical foundations of promising industrial technologies. The development of the theory of modification of the characteristics of the surfaces of metals and alloys by laser radiation, as well as a comprehensive study of the combustion processes of composite fuels within the framework of this project will be aimed at increasing the energy and resource efficiency of long-term operating thermal power facilities (steam and hot water boilers), chemical industry (continuous reactors). The relevance of fundamental nature tasks is determined by modern requirements for increasing energy efficiency and resource conservation, reducing the negative environmental impact in the industrial energy sectors. The development of energy-efficient methods for the combustion of composite fuels and promising methods for reducing the intensity of low-temperature corrosion and slagging of heating surfaces are promising directions for the development of combustion processes of condensed substances and the subsequent conversion of thermal energy. Such measures allow to increase the productivity of power generating facilities, to reduce the negative impact of gaseous products on the environment. The results already obtained to date allow us to conclude that the use of new types of composite fuels (a mixture of low-grade solid fuel, liquid combustible waste, biomass) and promising methods for modifying the characteristics of the surfaces of metals and alloys (laser radiation) are out of competition for solving a group of urgent problems. The scientific novelty of the proposed project consists in the implementation of an integrated approach to laboratory and field research of a group of interrelated processes occurring in boiler furnaces during the combustion of composite fuel and in the gas-air duct when a stream of solid and gaseous combustion products moves in it. It should be noted that currently leading research teams of the international level are working in the framework of the development of promising composite fuels and effective ways to reduce the intensity of slagging and low-temperature corrosion of heating surfaces for various purposes. But the approaches and methods developed by the members of the scientific group of the proposed project, the setting of tasks and the results obtained have no analogues in the world scientific community.

Expected results
1. It will be performed an analytical studies of characteristics (technical, elemental, thermogravimetric, calorimetric analyzes) of a group (at least 10 items) of solid and liquid fuels, their mixtures, as well as texture characteristics (three-dimensional roughness parameters), surface properties (wetting) and elemental composition near-surface layer of a group of samples of typical heating surfaces (at least 2 items of metals and alloys) in the form of fragments of boiler furnace heat exchangers. 2. A promising method will be developed for surface treatment of metals and alloys by modifying them with laser radiation to improve functional properties, in particular, resistance to slag deposits and corrosion when interacting with drops of composite fuels and solid products of their combustion. 3. An experimental setup will be mounted and laboratory studies will be carried out, as a result of which the mechanism of the formation of slag deposits and corrosion defects on a group of heating surfaces (including those modified by laser radiation) and characteristics of the intensity of these processes under conditions of burning solid and liquid fuels will be established, and also their mixtures when varying in wide ranges of parameters of gas-air and steam-water paths. 4. Field studies (on coal-fired and fuel oil boilers) will be carried out to establish the characteristics of the intensity of the formation of slag deposits and corrosion defects on a group of model heating surfaces (including those modified by laser radiation) under real conditions of burning solid and liquid fuels. 5. A mathematical model of heat transfer through the heating surface from heating source to the coolant under the conditions of the formation of slag deposits and corrosion defects will be developed to assess the effect of these processes on the change in the dynamics and efficiency of heat transfer. An algorithm for a numerical solution will be developed, an open program code will be drawn up, a numerical simulation of a group of processes corresponding to real processes in the furnaces of boilers during fuel combustion will be performed. The model will be verified and the reliability of the results obtained will be assessed. 6. Based on the results of analytical, experimental and theoretical studies, a database with recorded characteristics of the processes under study will be developed. The results will be presented in the form of approximation dependences of the main characteristics of the processes under study (thickness of the layer of slag deposits and corrosion defects) on the group of the most significant factors characterizing the intensity of heat supply from the source (fuel flame, gas flow), and the interaction of fly ash with the heating surfaces of the heat exchanger. The expected results of the project will be of high scientific and social significance: 1. In the field of science and technology, they will contribute to the development of modern provisions in the field of materials science and combustion theory of condensed substances through the development of approaches to a detailed study of a group of interrelated physical and chemical processes occurring in the condensed phase and gaseous medium during the combustion of fuels under conditions corresponding to real conditions in boiler furnaces. In the field of materials science and metallurgy, new fundamental knowledge will be obtained, which will provide the basis for the development of promising ways to improve the quality of finished products, energy and resource efficiency, and the operational reliability of long-term operating heat exchangers. 2. Theoretical implications will be applicable in teaching students and undergraduates of universities in metalworking technologies, fuel preparation and combustion, industrial safety and energy efficiency, as well as in the preparation of graduate students in the field of chemical physics, thermal power engineering, materials science, metallurgy. 3. In the social direction, the research results will help to attract the attention of civil society to the high potential of composite fuels, the prospects for energy efficient and environmentally safe operation of existing coal power generation facilities. The development and implementation of new technologies will provide scientific and technical groundwork and the development of materials science in the Russian Federation by obtaining high-quality products that are in demand at the international level. The expected results will correspond to the international level of research, which will be confirmed by publications in high-ranking journals indexed by the Web of Science Core Collection databases, as well as by testing the main results at specialized all-Russian and international conferences, including those held abroad of Russia.

Annotation of the results obtained in 2023
During the second year of the project, the full scope of planned work was completed, including laboratory research, field testing and mathematical modeling. The work being carried out is characterized by novelty, since the effect of texturing a metal surface on the characteristics of its slagging, heat transfer and corrosion processes has not previously been comprehensively studied. In addition, tests have not previously been carried out on real boiler installations and the corresponding results have not been presented. 1. A promising method has been developed for treating the surfaces of metals and alloys by modifying them with laser radiation to improve functional properties, in particular resistance to slag deposits and corrosion when interacting with combustion products of various fuels and aggressive environments. Studies of texture characteristics (three-dimensional roughness parameters), surface properties (wetting) and elemental composition of the near-surface layer of a group of samples of typical heating surfaces (heat-resistant steel) in the form of fragments of boiler furnace heat exchangers were carried out. It is shown that the weight composition of chemical elements on textures in the form of microchannels varies quite significantly in the region of the lower surface of the microchannel, on the side edge and on the non-textured part near the microchannel. Under the condition of deposition of metal vapors generated during laser processing onto the non-textured part near the microchannel, in such a near-surface layer the Cr content increases and the Ni and Fe content decreases, which leads to an increase in magnetic forces and strength while maintaining ductility, but reducing impact strength and thermal conductivity, heat resistance and elasticity. 2. Laboratory studies were carried out to establish the characteristics of the intensity of the formation of slag deposits and corrosion defects on a group of heating surfaces. Steel samples with a non-textured surface after corrosion exposure were characterized by a fairly high polarization resistance, which indicates a deterioration in the characteristics of the material and its higher ability to oxidize. Steel samples with a surface texture in the form of microchannels (of two types) were characterized by high polarization resistance (3173 Ohm/cm2) before corrosion. Corrosive action in a sulfuric acid solution caused an increase in the polarization resistance of samples with a textured surface to 25.52 kOhm/cm2, which indicates improved resistance of the material to corrosion. Steel samples with a surface texture in the form of microchannels were characterized by the lowest corrosion rate - 0.064 mm/year. The anisotropic type of texturing enhanced corrosion processes - the recorded corrosion rate was 17.25 mm/year for steel and 2.093 mm/year for aluminum alloy. Modification of the surface of the aluminum alloy sample led to deterioration in corrosion resistance. Laboratory experimental studies of slagging processes on modified steel surfaces have been carried out. Three spreading fronts were recorded on surfaces treated with traditional grinding and polishing. On surfaces modified by nanosecond laser radiation with a texture in the form of microchannels, two spreading fronts are identified; on an anisotropic texture – one spreading front. When steel interacted with the ash composition at high temperatures (about 1400 °C), slag was formed and the weight elemental composition of the steel changed. In addition to oxidation, a significant increase in chromium content (up to 47%) and a transformation of texture with the formation of grains up to 2.5 microns in size were detected. Laser surface treatment significantly increased the resistance of steel to such changes, and also contributed to a reduction in grain size to 1.5 microns. It has been established that as a result of laser modification of surfaces, conditions are created for shifting the slag formation temperature to higher values (by 60–75 °C compared to a non-textured surface). In general, the “anisotropic” and “parallel microchannels” textures provided the best conditions for reducing slagging intensity. The anisotropic type of texturing contributed not only to a reduction in the size of the slag propagation front, but also created conditions for more efficient cleaning of the heating surface from the mineral melt, all other things being equal. 3. Field tests were carried out (on boilers and a water treatment plant) to establish the characteristics of the intensity of the formation of slag deposits and corrosion defects on a group of heating surfaces (modified by laser radiation). The results of a set of studies (SEM, energy dispersive spectrometry) indicate improved resistance to slag when using a textured surface (“parallel microchannels”) compared to polished steel. Studies have shown that the surface of polished steel after field tests in a boiler was covered with a dense layer of chromium oxides, on top of which there was a layer of distorted crystals of zinc ferrochromites, along which continuous volumetric dendritic structures of fused spherical elements 3–5 μm in size develop. Slag dendritic structures were represented mainly by calcium phosphate globules with a small admixture of lamellar calcium sulfate (3–5% sulfur). The inlay of calcium phosphate slag globules is represented mainly by metallic zinc (up to 80 wt.%). Also, individual microspheres (up to 30 μm), consisting mainly of iron (up to 45%) with an admixture of zinc (up to 25%), coated with a thin layer of calcium sulfate, were recorded on the surface of the polished sample. The surface of the textured sample (microchannels) after field tests was covered with regular extended parallel slag rollers. It has been established that per unit area of a polished surface there are an order of magnitude more slag build-ups than on a textured one. After field testing, the metal surface was uniformly coated with a thin layer of micron-sized calcium sulfate particles, through which heat-resistant steel, characterized by a high content of iron, chromium and nickel, appears. Large (up to 100 µm) globules on the polished surface were represented mainly by metallic iron without traces of chromium or nickel, i.e. These growths were not the result of the release of the melt of the substrate material onto its surface, but were formed during the melting of the mineral part of coal ash. During further operation, these areas can serve as initiators of corrosion and destruction of the material. 4. The development of a mathematical model developed during the first year of project implementation was carried out to study the influence of laser processing of heat exchange surfaces on the characteristics of their slagging when burning different types of fuels. It has been shown that modification of heat exchange surfaces by laser radiation can reduce the time to reach the critical slag layer requiring mechanical cleaning by 64%. It has been shown that laser modification of heat transfer surfaces will potentially increase the efficiency of heat transfer equipment by increasing the absorbed heat per unit area by 1.6–2.2 times. The effectiveness of modifying a steel surface to obtain anisotropic (cauliflower texture) and microchannel textures to reduce the intensity of slagging has been experimentally proven. The trends identified in the study, established quantitative characteristics and recommendations will be useful in the modernization of existing heat and power and water treatment plants and in the design of new equipment. The new knowledge gained about laser modification is the scientific basis for applied technologies aimed at effectively preventing slagging, improving the operating conditions of equipment (boilers, tanks, etc.) and increasing its service life.

Publications

1. Glushkov D.O., Paushkina K.K., Vershinina K.Yu., Vysokomornaya O.V. Slagging Characteristics of a Steam Boiler Furnace with Flare Combustion of Solid Fuel When Switching to Composite Slurry Fuel Applied Sciences, Vol. 13(1), article number 434 (year - 2023) https://doi.org/10.3390/app13010434

2. Vershinina K.Yu., Orlova E.G., Abedtazehabadi A., Laga E.Yu., Rudmin M.A., Feoktistov D.V. Laser modification of heating surfaces: a new approach to reduce boiler slagging   Process Safety and Environmental Protection, - (year - 2023) https://doi.org/10.1016/j.psep.2023.12.010

3. - Расчет характеристик шлакования поверхностей нагрева, обработанных лазерным излучением -, 2023684563 (year - )

Annotation of the results obtained in 2022
Experimental studies of the combustion of a group of fuel mixtures and their individual components from among low-grade fuels, as well as solid and liquid combustible wastes, were carried out to establish the mechanisms and characteristics of the formation of slag deposits and corrosion defects on heating surfaces, to assess their effect on the characteristics of heat transfer from the source to the coolant. Experimental and theoretical studies have been carried out, the results obtained within the framework of the project have been summarized, predictive models and recommendations have been developed for applying the results in practice. The following scientific results have been achieved: 1. The characteristics of co-combustion of a group of solid low-grade fuels, solid and liquid combustible wastes have been obtained. The susceptibility of individual fuels and fuel mixtures to slagging of heating surfaces of boiler furnaces was assessed. It has been established that the highest relative tendency to form strong ferruginous deposits (indicator 0.54) is typical for coal. When adding sawdust to coal and increasing their share in the mixture, this indicator will decrease. The tendency to form pollution based on active alkalis has a minimum value (0.24) for coal and increases 1.12–1.57 times with an increase in the proportion of sawdust in the mixture from 25% to 75%. A high tendency to form sulfate-calcium deposits (indicator 0.64–0.77) was registered for hard coal and mixtures with a share of sawdust of 25–50%. The average values for this indicator are characteristic of sawdust and mixtures with a biomass share of 60–80%. The slagging index has high values (0.78 and 0.76) for hard coal and a fuel mixture with a sawdust fraction of 25%. With an increase in the proportion of sawdust to 75%, the slagging index decreases and has a minimum value of 0.63 for biomass (without additives). With regard to the modification of the characteristics of heating surfaces, it has been established that the surface layer of samples modified by polishing and grinding is characterized by a carbon content of about 55%, oxygen about 45%. Based on the results of the analysis of the carbon spectrum, it was found that carbon is represented by the oxide form (286 and 288.7 eV) of about 40%. The latter characterizes the hydrophilic wettability properties when the contact angle is 85–92 degrees. The surface layer of samples (fragments of heating surfaces) modified by laser radiation is characterized by a carbon content of about 40%, oxygen - about 60%. The latter is the reason that carbon is predominantly in an oxidized state, like all other elements that make up the surface layer of steels. It was also found that steel samples modified by laser radiation are characterized by a large number of oxygen peaks (286, 287.5, 288.5 eV) corresponding to C-O type bonds; C=O; O-C=O. The high oxygen content on laser-treated steel surfaces corresponds to super-hydrophilic wettability properties. 2. A new approach has been developed to study the formation of slag deposits and corrosion defects. The method is based on a combination of methods of profilometry, microscopy, adhesive strength, potential and current sweep, measurement of geometric dimensions by the "sessile drop" method, as well as X-ray photoelectron spectroscopy. A significant role in the adhesion of slag and ash deposits is exerted by the roughness, texture configuration and elemental composition of the near-surface layer of steel. A hypothesis is formulated that the main factor affecting the adhesion force is the oxide layer, and the adhesion force between the oxide layer and slag is stronger than between the oxide layer and metal. It has been established that the adhesion force of slag on the surface treated with laser radiation is less than the similar characteristic on the surface treated mechanically (grinding and polishing). When removing slag deposits from steel surfaces treated by grinding and polishing, pitting corrosion was revealed. On the steel surfaces textured by laser radiation, no manifestations of pitting corrosion were registered after the removal of slag deposits. A fundamentally new result is the obtained assessment of the corrosion characteristics of heating surfaces (alloys and metals) when interacting with fuel mixtures containing liquid hydrocarbons and their evaporation products. It has been established that the corrosion of structural materials proceeds less intensively under the conditions of preliminary nanosecond laser treatment of their surfaces, which creates a group of textures with a modification of the roughness characteristics and elemental composition of the surface layer. The surface of substrates made of structural alloys and metals with an anisotropic texture and superhydrophilic/superoliophilic properties is characterized by the highest resistance to corrosion processes compared to a polished surface not textured by a laser. 3. The developed mathematical models made it possible to analyze the possible tendencies of slagging of the furnace at the micro- and macrolevels during the combustion of solid fuel (lignite), coal-water slurry (CWS) and coal-water slurry with petrochemicals (CWSP). The simulation results showed that relatively high temperatures are achieved when dry coal dust is burned. In this case, in different areas of the furnace, the temperature varied in the widest range of 300–1800 °C. The combustion of CWS and CWSP instead of dry coal dust is characterized by a decrease in the temperature in the boiler furnace by 450–600 °C. The calculation results showed that the zones of high slagging rates are the largest in the case of coal dust combustion and are concentrated in the lower and middle parts of the furnace. During the combustion of CWS and CWSP, pronounced areas of slagging are characterized by lower values of the rate of this process and are localized on small areas of heating surfaces. The use of CWS instead of coal makes it possible to reduce the average rate of slagging of furnace surfaces by almost 12 times. The simulation results showed that, in addition to an increase in peak temperatures in the furnace by 13–38%, the average deposit growth rate is reduced by 5% for CWSP compared to CWS. Modeling the dynamics of the formation of ash deposits and its temperature characteristics at the micro level showed that when coal dust is burned, negative processes proceed at a faster rate. It has been established that when burning CWF and CWF, an ash layer of critical thickness is formed 13 times slower than when burning dry coal dust. The results of numerical simulations made it possible to conclude that the replacement of coal with slurry fuel will probably improve the stability and uninterrupted operation of the boiler in the context of the effect of slagging on these indicators. 4. A database has been developed with the characteristics of both individual fuel components and fuel compositions, as well as the studied physical and chemical processes. Within the framework of the first year of the project implementation, the development of mathematical models required the presentation of a group of results (for example, the thickness of a layer of slag deposits) in the form of approximation dependences of the main characteristics of processes on a group of the most significant factors characterizing the intensity of heat supply from a source (fuel flame, gas flow), fly ash to the surface of the heat exchanger for rapid data exchange between mathematical models of micro- and macrolevels in the implementation of the computational algorithm. 5. The analysis is carried out and the results of theoretical and experimental studies are summarized, theoretical consequences are formed. The results obtained were presented by the members of the scientific team in the form of 3 oral and 1 poster presentations at 4 Russian and international conferences. 6. Two articles were published in journals indexed by the Web of Science Core Collection and Scopus databases.

Publications

1. Zhuikov A.V., Glushkov D.O. Характеристики совместного сжигания каменного угля и лесной биомассы в условиях неизотермического нагрева с оценкой склонности топливной смеси к шлакованию поверхностей нагрева Кокс и химия, № 8. С. 7-15 (year - 2022) https://doi.org/10.52351/00232815_2022_08_7

2. Zhuikov A.V., Glushkov D.O. Характеристики совместного горения бурого угля и сухих остатков сточных вод в условиях неизотермического нагрева Химия твердого топлива, No. 5. С. 45-51. (year - 2022) https://doi.org/10.31857/S0023117722050115

3. - Сибирские ученые нашли способ повысить эффективность топливных смесей РИА Новости, - (year - )