Heat recovery systems

Heat recovery systems Cite as: AIP Conference Proceedings 2118, 030029 (2019); https://doi.org/10.1063/1.5114757 Published Online: 27 June 2019 Milan Malcho, Richard Lenhard, Katarína Kaduchová, Dávid Hečko, and Stanislav Gavlas AIP Conference Proceedings 2118, 030029 (2019); https://doi.org/10.1063/1.5114757 © 2019 Author(s). 2118, 030029 Heat Recovery Systems Milan Malcho1, a), Richard Lenhard1, b), Katarína Kaduchová1, c), Dávid Hečko1, d) and Stanislav Gavlas1, e) University of Žilina, Faculty of Mechanical Engineering, Department of Power Engineering, Univerzitná 8215/1, 010 26 Žilina, Slovak republic. 1 a) [email protected] [email protected] c) [email protected] d) [email protected] e) [email protected] b) Abstract. Nowadays, more and more issues are being raised to address the need for sufficient energy security by minimizing its carbon footprint, which is also tied to the use and conversion of thermal energy. Until now, almost all of the energy produced has been obtained by direct use of fossil fuels through thermodynamic conversion of heat into mechanical work. There is a general lack of energy resources in the world, and it is for this reason that it is necessary to look for new alternative sources of energy, but also to rationalize consumption for example heat. The use of alternative sources is sometimes very costly and it is therefore necessary to carefully consider whether the realization of such an energy source will bring the expected economic savings. In order to further develop technologies and to ensure an adequate quality of the environment, it is also necessary to increase the energy efficiency of technological processes by using low- and medium-potential energy sources of waste heat. There has been a trend in the intensified use of renewable, environmentally friendly energy sources and rationalization of energy consumption, especially in the sectors of industry, agriculture and the municipal sector, which are characterized by high energy intensity (metallurgical and ceramic technologies, drying processes, heating and cooling, etc.). Although high-temperature sources with the possibility of utilizing sensible heat appear to be most economically advantageous, there is a high energy potential even with low-temperature mainly drying technologies with latent heat recovery potential. Industrial waste heat generated in various technological processes of industrial and energy production is the most important secondary energy source. Reducing energy consumption and its efficient use is currently one of the main energy policy priorities in our country and in the EU. As energy prices rise, it is becoming more economical to recover heat from low-potential waste air. Of the various options, recovering heat from waste air and flue gas using recuperative heat exchangers is currently the most convenient for us. A significant limitation of their applications in technological processes is the contamination of the waste air with sticky dust or condensate, which can significantly reduce thermal efficiency or even disable heat recovery devices. Based on their own research activities, the authors point to limitations on the deployment of standard heat exchangers and the specificities of some technologies. INTRODUCTION The development of energy carrier prices leads to a re-evaluation of consumers' relationship to the efficient use of all kinds of energy, rationalization of their production, reduction of energy consumption in technological processes and use of secondary energy sources (SES). In the same way, it can help to improve the relationship between energy and the environment. A relatively underutilized group is also currently energy-utilized wastes, which are produced as by-products mainly in industrial, agricultural and other sectors. These are industrial waste heat, chemically bound energy of industrial, municipal and residential waste, which have the character of fuels, chemically bound energy in waste of plant and animal origin - the character of biomass, other energy wastes, e.g. potential energy of compressed gases, mechanical energy. 38th Meeting of Departments of Fluid Mechanics and Thermodynamics AIP Conf. Proc. 2118, 030029-1–030029-8; https://doi.org/10.1063/1.5114757 Published by AIP Publishing. 978-0-7354-1858-5/$30.00 030029-1 Industrial waste heat generated in various technological processes of industrial and energy production is the most important secondary energy source. Reducing energy consumption and its efficient use is currently one of the main priorities of energy policy. As energy prices rise, it is becoming more economical to recover heat from low-potential waste air. Of the various options, recovering heat from waste air and flue gas using recuperative heat exchangers is currently the most convenient for us. A significant limitation of their applications in technological processes is the contamination of the waste air with sticky dust or condensate, which can significantly reduce thermal efficiency or even disable heat recovery devices. WASTE HEAT SOURCES IN TECHNOLOGICAL PROCESSES Secondary – a secondary source of energy arises as a by-product of production technology or other human activity. Unlike primary energy sources (PES), which are derived from nature, its downstream, though partial use, determines the level of production technology and increases its overall efficiency. In industrial technologies, there are often massive flows of low- and medium-potential waste heat. These sources of thermal energy can be reused in secondary utilization facilities as secondary energy sources (SES) most advantageously in the technologies that produce it. In general, secondary energy sources contribute to reducing fuel and energy consumption. But sometimes in the technological processes that produce waste heat, they are not used because their parameters are unsuitable for the original technology. However, these may be the source of energy for other equipment in which they can replace, in whole or in part, fuel or energy. One way to reduce the cost of thermal energy is to recover it from waste heat by means of heat exchangers of different designs, depending on the application. SES are important because:    their use reduces fuel production intensity, replace the consumption of PES and the use of SES does not burn the environment with harmful emissions, the use of SES often also represents significant economic savings. The technical solution of SES is in most cases practically always possible, but in practical solution their use is often hampered by the economic demands of the solution. The amount of energy in SES is given by the product of the qualitative and quantitative indicator. The quantitative indicator is the amount of energy carrier; the quality indicator is the concentration of energy in the unit of quantity (weight or volume). In terms of the ratio between qualitative and quantitative indicator, we distinguish:    low-potential energies, which are characterized by a low quality indicator, ie. low energy concentration in the unit of quantity, as a rule a large amount of energy carrier – e.g. waste heat of cooling water of technological equipment, moist air from drying processes, etc., medium potential energy, e.g. sensible waste heat of flue gas of industrial furnaces – flue gas temperature is 300 – 1 000 °C, high potential energy, e.g. chemically bound heat of waste gases from the process; various types of furnace gases, waste gases in chemical production, biogas from sewage treatment plants, landfill gas, biogas for agricultural waste treatment, waste oils, etc.). In the technical-economic assessment of the use of SES, the following indicators should be considered:  energy level it is characterized by a sensible waste heat temperature and pressure at potential energy. In principle, the use of appreciable waste heat would not have the temperature of the exhaust gas used e.g. for steam production to fall below 450 °C. Usability possibilities are limited at low potential heat due to low energy concentration in the energy carrier. For a certain amount of usable energy, a large amount of substance is needed, and thus the waste energy utilization equipment is large. Therefore, mainly the investment costs and often the operating costs are higher. Experience to date has shown that, with increasing energy potential, the potential for its use is increasing, 030029-2   regularity of occurrence of SES, this indicator is very important because it is difficult to find a suitable place for sampling or irregular occurrence heat recovery. The variation in heat input in the waste heat is also unfavorable, but easier to overcome than the intermittent occurrence, usability of acquired energy, it is a typical problem in most of our industrial plants because SESs are basically larger than their options for their use in manufacturing or in the plant. Therefore, for a part of the heat, it is often necessary to look for application in other areas, respectively. it is necessary to waste part of the heat in a relatively expensive way. It is therefore understandable to seek heat with parameters suitable for generating electricity, which is generally applicable. In addition to the above mentioned indicators for the successful application of the heat recovery system, other conditions:   technical feasibility of the proposed solution, economic indicators during equipment use and environmental requirements. When using SES, the economic principle should be that the energy obtained, taking into account all cost items, must be more economically efficient than energy from another source. The return on investment in our economic environment is interesting if it is two to three times shorter than in Europe's more advanced countries. Possibilities of heat recovery from the production process and its subsequent use is a problem with a wide range of tasks that need to be solved comprehensively for the successful design of SES. In heat recovery from industrial technologies, in the vast majority of cases, this is a heat transfer medium more or less contaminated by different kinds of solid or liquid drift with a tendency to settle on heat exchange surfaces. In such cases, the use of regenerative heat exchangers is particularly problematic for operational reasons. Therefore, in heat recovery applications as well as in gas-liquid exchangers, mainly heat recovery heat exchangers of different designs are used. The use of heat in the recuperators is limited by the wall temperature, which is the case with plastic recuperators max. 120 °C, for metal max. 700 to 950 °C (for higher temperatures it is necessary to use alloy refractory steels), at ceramic temperatures up to 1 000 °C and higher. This also gives the maximum gas temperature before the recuperator, respectively. heated air or gas behind the recuperator. In metal recuperators, the flue gas temperature is at most 700 to 950 °C, air heating is then 300 to 500 °C, in flue gas the maximum flue gas temperature is 1 200 to 1 400 °C, air temperature is 850 to 950 °C. These values are, of course, dependent on the heat transfer parameters, in particular on the heat transfer coefficient of the flue gas into the wall and from the wall into the air, respectively gas. HEAT INTENSIFICATION OPTIONS In order to make the most effective use of the heat recovery devices, it is necessary, among other things, to design the device so as to maximize the heat transfer from the waste medium to the heated medium. From the point of view of heat transfer, it is therefore necessary to know how they influence the different parameters of the heat transfer media that flow in the heat exchanger, especially the heat transfer in the heat exchanger. The heat transfer theory implies that the heat transfer in the heat exchanger is most affected by the boundary layer formed by the heat exchange surface. Therefore, in order to increase the heat flux density and hence increase the efficiency of heat recovery heat exchangers, mainly from low potential air and flue gas, it is appropriate to try to form this boundary layer so that its thermal resistance is minimal. Therefore, it is necessary to examine the process of forming a boundary layer on a heat exchange surface, the dependence of its properties on the nature of the fluid flow, the flow rate or the conditions under which the medium flows and to look for ways to influence the boundary layer properties. By finding ways to influence the boundary layer and knowing these dependencies, it is then possible to quantify these effects, or to experimentally verify the possibilities of increasing the heat transfer efficiency in the heat exchanger. APPLICATIONS HEAT RECOVERY FROM THE HIGH POTENTIAL HEAT High potential waste heat is typical for metallurgy and ceramic industry. Although ferroalloy is not the most typical representative of metallurgical production, ferroalloy production is highly energy intensive. Ferroalloys are 030029-3 produced in electric arc furnaces (EAF), where the most noble form of energy is consumed – electrical energy. In the production of ferro-silicon (FeSi) in EAF, silicon wastes are formed as by-products, which are formed by the condensation of silica vapors emanating from the electric arc area of EAF. The drift is a very fine particle of a regular spherical shape and a smooth surface. Silica drift has excellent thermal insulation properties (λ ≤ 0.04 W·m-1·K-1) and strong settling on the wall of pipes and heat exchangers. These flue gas properties are complicated by a significant way of dealing with the use of SES from EAF flue gas. The feed produces heat mainly by emitting flue gases into the environment, especially in the suction hat, non-stationary and inhomogeneous. The furnace walls are thus exposed to the thermal radiation of the burning charge and the escaping flue gas. Heat gains from a combustion charge can be determined by measurements, computer simulation, or mathematical modeling of heat transfer. The device is designed and constructed for use in the plant. It consists of a furnace hat, door curtains and a ring Fig. 1. FIGURE 1. Design of heat recovery system from EAF – 3D hat The furnace hat is designed as a pressure vessel divided into three independent segments. The segments are formed by a set of plate heat exchangers and a tubular structure. Each of the three EAF segments consists of heat sink plates and tubular segments. Each segment is a separate group of pressure vessels. The radiator panels are large-area plate heat exchangers. The radiator panels are installed in three groups around the perimeter of the lower hat. Each group of panels placed in one tubular segment forms a separate heat transfer system, whereby panels and tubular segment receive radiated heat from the furnace and pass it through the hot water in the heat exchanger to the hot water circuits of the plant. Numerical simulations of heat transport in EAF with simplified geometry of the uncoated furnace casing showed that the total heat output of the EAF23 furnace can be transferred to the heat transfer medium – warm water – with a total heat output of about 2.4 MW. As the previous experience has shown, the cardinal problem of achieving optimum heat sink performance is the settling of EAF silicon drift on the inner walls of the heat exchanger plates in the furnace hat and the cooler tubes. In order to quantify the thermal resistance of the coating, a measurement of its thermal conductivity was performed by the method of coil alignment. Silica drift was placed between the heated inner cylinder of 1.2 m and the outer cylinder of copper. The inner cylinder was heated by an electric spiral and the temperatures on the inner and outer cylinder surfaces were measured by NiCr-Ni thermocouples in two locations and the thermal conductivity coefficient of the EAF drift from FeSi was determined from the thermal gradient and specific heat flux at three heating spiral outputs. The average coefficient of thermal conductivity is 0.0405 W·m-1·K-1. USE OF ENERGY POTENTIAL WASTE HEAT OF MIDDLE POTENTIAL FOR TIMECURRENT PROCESSES The production of electro-porcelain is a technologically demanding process which requires above-standard requirements for technological temperatures. The products are sensitive to processing temperatures when the internal temperature is prescribed to 25 °C and the relative humidity φ = 55 % ± 5 %. One of the final technological operations is the firing of glazed insulators, which is realized in the ventricular resp. tunnel furnace. The firing curve 030029-4 is complex and depends on the type of baked products, glazes and mass. The maximum firing temperature is about 1 300 °C. The curve must be followed consistently because the quality and confusion of the products mainly depend on the method of burning. FIGURE 2. The internal temperature in the chamber furnace On the basis of the technological process data, a heat exchanger was designed, starting from a mathematical model that was created using the similarity theory. The exchanger is composed of a tube and shell, which is formed by membrane walls, whereby the heat transfer from the flue gas to the walls is mainly solved and the cooling of the walls is also solved. Part of the exchanger is also its bypass, through which the flue gas after their cooling through the flap connected to the fluorine absorber. FIGURE 3. Heat exchanger with a membrane walls FIGURE 4. Heat exchangers on the melting unit secondary aluminium The control of the furnace – heat exchanger – bypass technological complex is very complicated for the different dynamics of the individual systems. The dominant technology is a chamber furnace that operates in overpressure. Its maintenance is ensured by pulling the chimney fans. The system is very sensitive to overpressure. A sudden change in furnace pressure conditions causes it to be blocked immediately. 030029-5 FIGURE 5. Implementation of heat recovery system from JUNKER heating furnaces The temperature of the flue gases leaving the furnace, after their partial cooling in the supporting duct by the suction of cold air, is about 700 °C. If the flue gas passes through the bypass of the exchanger, it must be cooled at its inlet by adding more cold air to a temperature of 170 °C, if the flue gas is sucked into the fluorine absorber, respectively. at a temperature of 250 – 300 °C when the fluorine release phase is already overcome. To recover the heat from the flue gas of the secondary aluminium melting furnace, a system of heat exchangers (Fig. 4) was designed to preheat the combustion air and to heat the hot water for the heating system of the company. For the recovery of heat from flue gas furnaces for the heating of metal rods, due to the low content of solid flue, pipe flue gas – water exchangers are suitable. For the calculated heat output contained in the flue gas (615 kW), the flue gas volume (10 430 m3·h-1) and the flue gas temperature of 350 °C, a flue gas-water heat exchanger – 430 kW was designed based on a mathematical model. The exchanger consists of the tube-tube segments, the supporting skeleton, the sheathing and the connection to the flue gas duct and the flue to the chimney. A great advantage of this system is the possibility of placing the heat obtained in the technology that produces it (heating of working fluids). HEAT EXCHANGER APPLICATION OPTIONS FOR LOW POTENTIAL HEAT RECOVERY Some other analyzes of thermal technology processes point to a further possibility of using massive sources, mainly low potential waste heat in drying processes and in ventilation of industrial halls. A typical case is drying technology. The economic losses resulting from the wasteful discharge of waste heat are considerable and the need for economical use of low-potential waste heat will increase with ever-increasing energy prices. Of the relatively small difference in temperature between waste and fresh air, which, even in extreme cases, does not exceed 30 – 40 K and is significantly lower per year on average, there are specific requirements for the design of heat exchangers that are useful for recovering low potential heat. Regenerative exchangers are not applicable in this area, since pollutants and moisture would also enter the intake air, reducing drying efficiency. The heat pipes prevent this effect of fresh air contamination, since the intake and exhaust air can be isolated from each other. However, their disadvantage is the unsuitability of using the tubes in a dirty environment. The rate of contamination of the heat exchange surface, and hence the frequency of its cleaning or replacement, depends on the degree and nature of the waste air pollution. In the case of workspaces, the practical exchange period may be e.g. half a year or a year. For some technological processes, e.g. in rubber vulcanization, pollution is very intense and clogging of the heat exchange surface is very rapid and therefore requires replacement of several days. In these cases, there are usually concentrated resources with relatively small flow rates. The design of the recuperate intended for such conditions must first of all ensure the possibility of continuous exchange of the heat exchange surface. From the point of view of the clean ability of the heat exchange surfaces and the temperature range of the heat transfer media (20 °C to 120 °C), polycarbonate in the form of polycarbonate plates has proved to be very suitable. 030029-6 The heated, unpolluted air is conducted in the ribbed channel and the warm exhaust air, often loaded with a variety of dirt and moisture, in the smooth channel. One of the possible configurations of air flows in the air-to-air heat exchanger is shown in Fig. 6. FIGURE 6. Heat exchangers air to air with polycarbonate inserts The use of heat recovery systems from low potential waste heat in the ventilation air or air used to dry different materials is already economically interesting with regard to the current price of heat. CONCLUSION The most important factors affecting the efficiency of heat recovery are the process of clogging the heat exchanger surfaces, as the exhaust air or flue gas is almost always contaminated by various types of drift. If the clogging problem is solved by maintaining and cleaning the system in sufficiently long periods, then this system is highly efficient and allows a significant reduction in the energy performance of the technological process. The pursuit of the compactness of the heat exchanger design leads to the design of exchangers with narrow channels, to the sensitivity to capture and settling of waste contained in the exhaust air at the heat exchange surfaces and to their complicated cleaning process. Therefore, it seems necessary to find some compromise between the geometrical parameters of the heat exchanger and the technological needs of cleaning. The results also showed that in the implementation of heat exchangers as well as whole heat recovery systems, it will be necessary to use reliable theoretical and / or reliable heat exchangers. numerical models of transmission phenomena (FLUENT, ProEngineering, ...) with various design and spatial constraints given by technological process. The input media temperatures to the heat exchanger and the volumetric flow rates must be verified by measuring and performing an input energy audit. Theoretical and experimental work has clearly shown that the use of such designed heat exchanger systems in technical practice, where the potential heat of polluted air and flue gas is real, is possible and economically advantageous. ACKNOWLEDGMENTS This work is supported by the projects: APVV-15-0778 „Limits of Radiative and Convective Cooling through the Phase Changes of Working Fluid in Loop Thermosyphon”, APVV-17-0311 „Research and development of zero waste technology for the decomposition and selection of undesirable components from process gas generated by the gasifier”, VEGA 1/0738/18 „Optimization of energy inputs for the rapid generation of natural gas and biomethane hydrates for the accumulation of high potential primary energy”. 030029-7 REFERENCES 1. 2. 3. 4. 5. M. Malcho, S. Gavlas, Š. Papučík and K. Kaduchová, Spätné získavanie tepla z technologických procesov. EDIS, 2018. M. Malcho, J. Jandačka, R. Lenhard, S. Gavlas and T. Ochodek, “Numerical simulation of heat transport from the electric arc furnace to the heat recovery system,” in Communications: scientific letters of the University of Žilina, (2016), 18 (1A), pp.115–121. T. Brestovič, M. Čarnogurská, R. Pyszko, M. Kubík, “Effect of Radiation on Heat Exchange in Finned Heat Transfer Surfaces,” in XVIII Intern. scientific conference The application of experimental and numerical methods in fluid mechanics and energy. (EDIS: University of Zilina, 2012), pp. 19-26. B. Skocilasova, J. Skocilas, “Determination of Pressure Drop Coefficient by CFD Simulation,” AIP Conference Proceedings 1608, (American Institute of Physics, Melville, NY, 2014). D. Jasikova, M. Kotek and V. Kopecky, “An Effect of Entrance Length on Development of Velocity Profile in Channel of Millimeter Dimensions,” AIP Conference Proceedings 1745, (American Institute of Physics, Melville, NY, 2016). 030029-8