Use of heat recovery technology to increase thermal efficiency of rotary furnaces

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The article considers the problem of increasing the energy efficiency of rotary kilns, which are used in various industries for thermal processing of materials, such as metallurgical, chemical, construction, electrode and others. The source of heat for the kilns is gaseous fuel, which is supplied through special burners located at one end of the cylinder. The material for processing is loaded at the opposite end and moves towards the burner due to the force of gravity and rotation of the kiln. One of the important aspects of operating rotary kilns is energy efficiency, which depends on many factors and generally characterizes the degree of use of heat from fuel for heating the material. However, a significant part of the heat from fuel is lost through the kiln shell, which leads to an increase in fuel consumption and a decrease in kiln productivity. Therefore, it is necessary to look for ways to reduce heat losses and use part of the heat for other purposes. In this context, heating secondary air can be one of the important methods for increasing the thermal efficiency of rotary kilns, especially for kilns that operate in a mode for pyrolysis firing of materials such as coal, wood, peat, etc. The aim of the work is to increase the energy efficiency of a rotary kiln using a spiral heat exchanger, using the heat from the external shell of the kiln to heat secondary air. The paper proposes the use of a sectional heat exchanger, which can perform the functions of a cooler, a thermal insulator and a regulator of temperature regimes of the kiln. The heat exchanger consists of several sections that are attached to the outer surface of the kiln shell and have channels for air passage. The air is heated by contact with the hot shell and then supplied to the burner as secondary air. Thus, the heat exchanger allows to reduce the temperature of the shell and provide an additional source of hot air. A mathematical model for numerical calculation of a spiral heat exchanger has been developed, which allows to evaluate its thermal and temperature characteristics. It is shown that by using the heat from the shell it is possible to heat secondary air to a temperature of 270 °C in an amount sufficient to ensure the operation of the burner. An analysis of the location of the heat exchanger and the use of insulation coating has been carried out, which made it possible to significantly reduce its size without changing the total thermal effect compared to a heat exchanger installed along the entire length of the kiln shell. The effectiveness of using thermal insulation has been shown, which makes it possible to increase the temperature of air by 50 °C with its thickness 7 mm. The obtained results can be useful for further development and optimization of projects aimed at improving heat exchange systems and efficient use of thermal resources in rotary kilns.