Heat and Structure of Metallurgical Furnaces

Author: Chief Editor: Chen Hongfu
Publisher:
Publish Date: 1999-11-01
Features:
Fragment:
1.1 The production of heat and the scale of heat exchange In the vast metallurgical industry, especially the steel industry, most of the main production processes are carried out at high temperatures, requiring a large amount of fuel and electricity to generate heat to achieve the desired high temperatures. This involves two fundamental issues: the production of heat and its transfer, or heat exchange. Various forms of energy, such as mechanical energy, electrical energy, chemical energy, and atomic energy, can only be converted into heat to create a certain temperature difference before they can transfer heat to the material being processed through heat exchange to reach a certain temperature, ensuring the smooth progress of the process. Various types of heating equipment are the places that ensure the completion of the above processes. Their connection with the production of heat and heat exchange, as well as the conversion relationships between various forms of energy and heat, are shown in Figure 1-1. The production of heat can take various forms (as shown in Figure 1-1), but in the current context, the combustion of fuel remains the primary method of obtaining heat in industrial production. The equipment that accomplishes this heat production is called a burner (or combustion chamber). Heat is transferred from one object to another through radiation, convection, and conduction without the production of heat in the process, such as in heat exchangers and regenerators (chambers), which are purely heat exchange devices. The combination of heat production devices and heat exchange devices in different forms forms different industrial heating devices. Inside the furnace of a flame furnace, the heat generation from fuel combustion and the heat transfer to the material are directly combined, while in a firebox furnace (such as a muffle furnace or a semi-muffle furnace), it is an indirect or partially indirect combination. Blast furnaces, gas generators, or boilers belong to another category. In these devices, coal or water is processed through heat production and exchange to obtain another form of energy. The former produce gas with a certain calorific value, while the latter produces steam with a certain pressure and temperature, both of which can be converted into other forms of energy. Blast furnaces and converter furnaces, in addition to completing the main smelting processes, can utilize the chemical energy of blast furnace gas and converter gas as secondary energy sources. The mechanical energy carried by the residual pressure of the top gas when the blast furnace operates at high pressure can be converted into electrical energy output through a turbine generator set.
1.2 The importance and meaning of furnaces and furnace heat engineering From a historical perspective, the development of the metallurgical industry, especially the steel industry, is closely related to the development of furnaces and furnace heat engineering. For example, in China during the Han Dynasty, the construction of blast furnaces capable of producing about 1 ton of pig iron per day was possible due to the adoption of larger forced-draft equipment—horsepower and water-powered ventilation. In the 1850s and 1860s, the possibility of open-hearth steelmaking was made possible by adopting regenerative preheating of air in the furnace, achieving high temperatures in the furnace, which then propelled the great development of the steel industry. After World War II, the successful application of oxygen in ironmaking, especially in steelmaking, marked a milestone, with the tuyere converter using pure oxygen as a representative. This accelerated the oxidation exothermic reactions, effectively utilized the heat in the smelting process, and significantly improved the technical and economic indicators of furnace production. Today, energy conservation, which is of great strategic importance to China's national economy, is directly related to the performance of furnace heat engineering. This is because industrial furnaces and kilns consume a considerable proportion of the country's total energy consumption. How to effectively and fully utilize this portion of energy while reducing environmental pollution undoubtedly has significant economic and social benefits. Before further elaborating on furnaces and furnace heat engineering, it is necessary to understand the heat processes in metallurgical production, as these are essential for obtaining metallurgical products. Ore is reduced to pig iron in blast furnaces, steel billets are heated and rolled into steel, and steel with certain microstructures and mechanical properties is transformed into steel with different microstructures and properties through heat treatment. In the above cases, from raw materials to products, various different processing methods are involved, all“processes.” These include thermophysical processes, physicochemical and chemical reaction processes, and mechanical-mechanical processes, among others. Since most metallurgical processes are carried out at high temperatures, thermophysical processes (also known as heat processes) play a very important role. They include fluid dynamics, heat and mass transfer processes, as well as transport processes in fuel combustion. For the entire combustion process, of course, it is not solely a physical process, as it also involves chemical reaction kinetics, but the former often plays a decisive role. In industrial production, furnaces are specifically built to enable these heat processing processes to take place. A furnace is a device that ensures the smooth progress of heat processes, isolated from the surrounding environment by its enclosure and obtaining heat from some energy source to maintain a certain temperature, so that heat can be transferred to the workpiece or material being heated inside it according to certain process requirements. Since the processes are various, this determines the diversity of furnace structures. However, they also share some common characteristics, which is also the basis for the classification of furnaces to be discussed later. Here, we mainly study the furnaces used in industrial production, which we call "industrial furnaces." According to statistics from 1990, there are about 500,000 such industrial furnaces in the country, mainly distributed in four sectors: metallurgy, building materials, machinery, and chemicals, accounting for more than 80% of the total number of furnaces. However, due to traditional industrial combinations and divisions, the scope covered by the term "industrial furnace" is not strictly defined. For example, in China's industrial furnaces, blast furnaces, open-hearth furnaces, and converter furnaces (belonging to metallurgical furnaces) are not included, and only rolling heating furnaces, forging furnaces, heat treatment furnaces, kilns, and melting furnaces are referred to. Blast furnaces and power boilers are classified as energy conversion devices due to their complex structures and high investment, falling under the scope of specialized departments. Although gas generators are classified as chemical industry furnaces, they are also managed and designed by specialized departments due to their large systems. Heat is generated inside the furnace using various energy conversion devices and transferred to the workpiece being heated. How to effectively and economically utilize heat inside the furnace to meet the needs of heat processing for materials belongs to the field of furnace heat engineering. From the meaning of the term "heat engineering," it indicates the engineering technology of utilizing heat, naturally including the generation of thermal energy. The scope of research on furnace heat engineering is limited to the furnace system, especially the thermophysical processes inside the furnace. While the broader use of heat falls under the category of "energy conversion and heat utilization."