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A waste heat boiler is a type of boiler that uses the waste heat contained in exhaust gases, waste materials, or waste liquids from various industrial processes, as well as the heat generated by the combustion of combustible substances, to heat water to a certain temperature. The waste heat boiler consists of a drum, a movable smoke hood, a feeding pipe (discharge chute), an oxygen lance nozzle, a nitrogen sealing device along with a nitrogen seal plug, manholes, a differential pressure measurement device, as well as supports and hangers for the flue ducts. The waste heat boiler is divided into six circulation loops, with each loop consisting of a downcomer and an upcomer. The water supplied to each flue passes from the drum through the downcomer to the lower header of that flue, from where it enters the various heating surfaces. After passing through these heating surfaces, water is converted into steam, which then enters the inlet header before being carried back to the drum via the upcomer. The flues are connected to each other using flanges. Classification of waste heat boilers: Waste heat is the heat that is released in industrial production without being fully utilized; it constitutes a secondary energy source, being a product of the conversion of primary energy sources and combustible materials. (1) Based on the nature of the waste heat, it can be classified into the following main categories: 1. Waste heat from high-temperature flue gas: This is a common form of waste heat, characterized by large amounts generated, concentrated sources, and continuous flow, which makes it easy to recover and utilize. The heat carried away by this type of waste heat accounts for 40–50% of the total heat generated. Waste heat boilers used to recover this heat can be employed for heating in industrial or domestic applications as well as for power generation. 2. Waste heat from high-temperature slag: such as blast furnace slag, converter slag, electric furnace slag, etc. The temperature of this slag is above 1000°C, and the heat it carries away accounts for 20% of the total heat. 3. Residual heat from high-temperature products: such as coke from coke ovens, steel ingots and billets, high-temperature forgings, etc. These generally have very high temperatures and contain a large amount of residual heat. 4. Waste heat from combustible exhaust gases and liquids: such as blast furnace gas, catalytic cracking regeneration exhaust gases from oil refineries, and black liquor from paper mills – all of these can be utilized. 5. Waste heat from chemical reactions: Industrial sectors such as metallurgy, sulfuric acid production, phosphate production, fertilizer manufacturing, chemical fiber production, and paint manufacturing all generate large amounts of waste heat from chemical reactions. 6. Waste heat from cooling media: Large amounts of cooling water discharged by cooling systems such as the water jackets in industrial furnaces, as well as the steam generated by various vaporization cooling devices, all contain significant amounts of waste heat, which can be utilized effectively. 7. Waste heat from condensate water: The sensible heat contained in the steam used in various industrial processes, which is released when this steam condenses after the industrial process is completed. (II) Since waste heat is closely related to other production equipment and processes, its utilization possesses the following characteristics: 1. The heat load is unstable, primarily determined by the production process. 2. The composition, concentration, and particle size of smoke and dust vary greatly. This affects the layout of the boiler’s heating surfaces, and considerations must be given to wear prevention, ash blockage, and dust removal. 3. The diversity of flue gas components makes some of it corrosive. Elements such as SO2 in flue gas, dust, or various metallic and non-metallic elements in slag can all cause low-temperature or high-temperature corrosion as well as ash accumulation in waste heat recovery equipment. 4. Restricted by the inherent conditions of the installed item. In the event of any restrictions on the elevation of the boiler’s smoke inlets and outlets ； Some impose restrictions on the boiler’s exhaust temperature to ensure it meets the requirements of the production process. (III) Due to the differences in the properties of the waste heat flue gas, the types and structural designs of waste heat boilers vary. Based on their structural characteristics, they can be divided into two main categories: shell-and-tube waste heat boilers and flue-type waste heat boilers. Based on the dust content in the flue gas at the inlet of the waste heat boiler and the characteristics of the flue gas, they can be further divided into the following five categories: 1. Waste heat boilers with a dust content in the flue gas of no more than 20 g/Nm3 belong to the first category of waste heat boilers ； 2. Waste heat boilers with a dust content in the flue gas of more than 20 g/Nm3 but not exceeding 70 g/Nm3 are classified as category II waste heat boilers ； 3. Waste heat boilers with a dust content in the flue gas of more than 70 g/Nm3 are classified as third-type waste heat boilers ； 4. Waste heat boilers containing sticky soot fall under the fourth category of waste heat boilers ； 5. Waste heat boilers whose flue gas contains highly corrosive components or toxic fumes are classified as category five waste heat boilers. Classification of waste heat boilers: They are classified according to the pressure level of the steam produced by them. Currently, waste heat boilers utilize five types of steam-water systems: single-pressure, double-pressure, double-pressure with reheat, triple-pressure, and triple-pressure with reheat. 1. Single-pressure waste heat boiler: The waste heat boiler produces steam at only one pressure for supply to the steam turbine. 2. Double-pressure or multi-pressure stage waste heat boilers: Waste heat boilers are capable of producing steam at two different pressures or multiple different pressures to supply the turbine. Classified by the arrangement of the heating surfaces: 1. Horizontal-mounted waste heat boilers; 2. Vertical-mounted waste heat boilers. Classified by the flow characteristics (working principle) of the working fluid within the evaporation heating surfaces: 1. Natural-circulation waste heat boilers: The flue gas flows horizontally through vertically installed tube arrays. The density difference between the water vapor mixture in the tube bundle and the cold water in the downcomer is what drives the natural circulation of the vapor-water mixture within the evaporator. 2. Forced-circulation waste heat boilers: The flue gas usually flows vertically through the horizontally arranged tube banks. A circulation pump is used to maintain a constant circulation flow rate inside the evaporator. 3. Direct-flow waste heat boiler: A direct-flow waste heat boiler uses the head pressure of the feed water pump to pass the feed water through the various heating surfaces once, thereby converting it into superheated steam. In the evaporator heating surface, the flow of the working fluid is achieved by the head pressure of the feed water pump. Features and differences of waste heat boilers: Generally speaking, a “waste heat boiler” has only a “vessel” but no “furnace”. “A “waste heat boiler” usually does not have a burner; if high-pressure and high-temperature steam is required, an additional burner can be installed inside the “waste heat boiler”. By burning fuel to raise the temperature of the entire flue gas, high-parameter steam can be generated. For example, when a certain waste heat boiler has no burner installed, the inlet flue gas temperature is 500°C; after installing an additional burner, the inlet flue gas temperature can be raised to 756°C. The pressure of the steam can be increased from 4 MPa to 10 MPa, and its temperature can be raised from 450°C to 510°C. The steam can be used in high-temperature and high-pressure steam turbines, thereby increasing the electric power output. A waste heat boiler primarily uses exhaust gas as a heat source, therefore it does not require a combustion system (unless supplementary combustion is needed). Difference between waste heat boilers and conventional boilers: Waste heat boilers utilize the waste heat from existing exhaust gases or the sensible heat of solids. Waste heat boilers can generate steam under multiple pressure conditions, improving heat recovery efficiency. A waste heat boiler does not have a fuel combustion device. As a device for utilizing waste heat, the waste heat boiler must meet the requirements of the main process and must not affect its operation. There are various arrangements for the heating surfaces in waste heat boilers. Composition and operating process of the waste heat boiler: The gas discharged from a gas turbine after performing work still has a relatively high temperature, typically around 540°C. By utilizing the thermal energy of this gas, the thermal efficiency of the entire system can be improved. Usually, this heat is used to heat water, turning it into steam. Steam can be used to drive steam turbines and generators, as well as for heating in production processes or for domestic heating. A device that uses the heat from gas turbine exhaust to generate steam is called a \"heat recovery steam generator,\" indicating that the heat from the exhaust is recovered. In our country, it is commonly referred to as a \"waste heat boiler,\" and the exhaust gas from gas turbines is simply called \"flue gas.\" The high-temperature flue gas exiting the gas turbine has two outlets: one leads to the waste heat boiler and is discharged into the atmosphere through the main chimney, while the other goes to the bypass chimney and is also released into the atmosphere. Each flue is equipped with a baffle; there are three in total. The baffle on the main flue is called the \"main baffle\", the one on the bypass flue is called the \"bypass baffle\", and the baffle at the main chimney is called the \"chimney baffle\". All these baffles are used in conjunction with each other. Typically, a waste heat boiler consists of heat exchange tube bundles and vessels such as an economizer, evaporator, superheater, headers, and steam drums. In a steam cycle equipped with a reheater, a reheater can also be added. In the economizer, the boiler’s feed water is preheated, raising its temperature to a level close to the saturation temperature ； In the evaporator, the water phase transforms into saturated steam ； In the superheater, saturated steam is heated to become superheated steam ； In the reheater, the steam is heated to the set reheating temperature. When the gas turbine is operating but the waste heat boiler is not, the bypass valve should be opened and the main valve should be closed. When the gas turbine and the waste heat boiler operate simultaneously, the bypass valve should be closed and the main valve opened. On the other hand, to regulate the steam generation volume of the waste heat boiler, the main damper and the bypass damper can be partially opened or partially closed. When the waste heat boiler is in operation, the chimney damper should be opened. When the waste heat boiler is shut down for a short period of time, the chimney damper can be closed to prevent heat loss from within the waste heat boiler. Since the temperature inside the waste heat boiler is relatively high, cold air from the surroundings can enter the boiler, resulting in natural convection that carries away the heat. By closing the chimney dampers, external air currents can be prevented from entering the waste heat boiler, thus preserving the heat and enabling it to be used at any time to start the boiler. If the waste heat boiler needs to be shut down for maintenance and a faster cooling rate is desired, the chimney dampers can be opened. Application areas of waste heat boilers: Cogeneration provides process steam for factories ； Combined cycle power plants supply steam to steam turbines. Conditions to be met in the design of a waste heat boiler: 1. Flue gas side parameters: flue gas volume (under standard conditions or actual operating conditions), flue gas inlet temperature, flue gas composition, dust content in the flue gas, pressure on the flue gas side (positive or negative pressure), and resistance in the boiler’s flue gas system. 2. Boiler parameters: rated evaporation capacity of the boiler, rated steam pressure, rated steam temperature, and exhaust gas temperature of the boiler. 3. Other parameters: industry to which the main flue gas system belongs, description of the system’s operation process, installation method of the boiler, structural design of the boiler, meteorological conditions at the location of installation, or any other special requirements. The role of waste heat boilers in production systems: Due to the specific characteristics of production systems and waste heat power generation thermal systems, boilers play the following main roles in the entire system: a) Boilers help reduce the temperature of the exhaust gases, which ensures the proper operation of dust removal equipment in the system and enables the emission of exhaust gases at low temperatures in an environmentally friendly manner ； b) The boiler generates high-temperature, high-pressure steam by utilizing waste heat; in conjunction with the make-up combustion boiler, it drives the turbine and generator to produce electricity, thereby supplying power to the equipment in the entire cement production system ； c) The boiler has a certain self-dust removal function; the separated dust can be recycled, enabling it to serve multiple purposes. d) Helps reduce the energy consumption of the entire system, enabling full utilization of energy. Types of waste heat boilers: The main types of waste heat boilers include those used in sintering plants, boilers for burning mixed waste materials along with combustion gases, cement kiln waste heat boilers, steel industry waste heat boilers, waste from hazardous materials processing waste heat boilers, waste incineration waste heat boilers, coking plant waste heat boilers, waste heat boilers used in non-ferrous metal smelting, sulfuric acid production waste heat boilers, waste heat boilers used in electric arc furnace industries, glass kiln waste heat boilers, waste heat boilers for dry quenching of coke, carbon furnace waste heat boilers, waste heat boilers for gas-fired steam turbines, and waste heat boilers used in steel manufacturing. Cement kiln waste heat boilers are high-efficiency boilers that primarily utilize the low-temperature waste heat gas at 250°C to 400°C, emitted from the preheater, the clinker cooler at the kiln inlet, and the preheater at the kiln outlet, to generate slightly superheated steam. This slightly superheated steam is then used to drive low-parameter turbines to generate electricity. The overall structure of this type of boiler adopts a vertical layout, offering significant advantages such as flexible arrangement, small floor space requirement, low capital investment, low air leakage, and reduced steel consumption, which makes it very popular among users. The waste heat from these low-temperature exhaust gases is used for power generation, which represents an effective way to save energy, reduce consumption, and achieve comprehensive utilization of resources. This approach not only helps to lower the production costs of cement and improve the economic efficiency of enterprises but also enhances their competitiveness. It also helps to alleviate electricity shortages. Additionally, it reduces the temperature of exhaust gases and dust emissions, as well as CO2 emissions, thereby reducing air pollution and the greenhouse effect. This enables cement companies to achieve resource recycling and clean production. Mixed-waste incineration furnaces and waste heat boilers for purge gas: Mixed-waste incineration furnace waste heat boilers and waste heat boilers for purge gas are ideal, highly efficient, and energy-saving equipment for use in the fertilizer and chemical industries (especially in the production of methanol, ethanol, formaldehyde, and synthetic ammonia). Based on the characteristics of the waste heat flue gases in this industry, the waste heat boilers developed by our company fall into two main categories: new vertical types and traditional horizontal tunnel kiln types, both of which are natural-circulation waste heat boilers. These boilers represent typical energy-saving devices in the context of chemical industry’s circular economy. When used in combination with waste gas-combustion furnaces and blowing furnaces, they enable manufacturing enterprises to achieve the conversion of \"two types of coal into one type\" as well as the replacement of \"two furnaces with one furnace\", thereby generating significant economic and environmental benefits. They truly ensure safety, environmental protection, increased production, improved efficiency, and cost savings. Furnace types include vertical, horizontal, and “L”-shaped arrangements, among others. This type of waste heat boiler is composed of main water-cooled screens, superheaters, convective tube bundles, economizers, air preheaters, and other heat-exchanging surfaces. It has the following advantages: the overall heat-exchanging surfaces of the boiler adopt a suspended structure, which facilitates absorption of thermal expansion and ensures high operational safety ； The boiler is equipped with a special sealing device, which reduces air leakage and improves the boiler’s efficiency ； The heating surfaces of the boiler are designed with appropriate flue gas velocities to help prevent ash accumulation ； The boiler features a straight-through layout that allows it to utilize the weight of the dust for self-cleaning, thereby keeping the heating surfaces clean and improving the boiler’s heat exchange efficiency ； The boiler features a properly organized flue gas flow field, which ensures a steady and uniform flow of flue gas and prevents uneven currents. Additionally, it incorporates specialized anti-wear structures that help to prevent wear on the boiler, thereby extending its lifespan ； The boiler’s convective tube bundle features an assembled structure, resulting in a short installation period and low costs. Waste heat boiler for sinter coolers: The waste heat boiler for sinter machines is primarily a type of waste heat boiler that utilizes the waste heat generated during the sintering process in steel plants to generate electricity. The recovery of waste heat from sintering mainly involves two parts: one is the waste heat from the exhaust gases at the tail end of the sinter machine; the other is the waste heat from the exhaust gases generated when hot sinter undergoes air cooling in the front section of the cooler. Currently, there are four main types of waste heat power generation projects in this industry: single-pressure waste heat power generation, dual-pressure waste heat power generation, flash steam waste heat power generation, and supplementary combustion waste heat power generation ； There are two types of power generation systems: saturated steam power generation systems and superheated steam power generation systems. Waste heat boilers for carbon kilns mainly consist of two categories of naturally circulating waste heat boilers: those for carbon rotary kilns and those of the tank type. It adopts a vertical inverted-π layout and is divided into two major series: double-drum horizontal type and single-drum horizontal type. The dust-containing flue gas is discharged outside the furnace through the slag tube, superheater, convective tube bank, and steel tube economizer. The boiler slag duct is formed by stretching the front wall water wall. The superheater adopts a suspended structure, divided into high-temperature and low-temperature sections, with a temperature reduction device installed in the middle. The convective tube bundles are arranged in series, with baffles placed in between to enhance the heat exchange capacity of the bundle. The economizer uses coiled tubes. The middle part of the water wall is composed of membrane wall partitions, which not only increase the heating surface area but also enhance the boiler’s sealing performance and safety. Both the front and rear walls adopt a membrane wall structure. Waste heat boiler for glass furnaces. The glass industry is a major consumer of energy. China’s current float glass production technology is on par with international standards, but the average energy consumption per unit of glass produced in China is about 20% higher than that in other countries. The flue gas waste heat recovery power generation system for float glass furnaces can further increase the waste heat utilization rate to over 70%, enabling full recovery of waste heat resources. Superheated steam is generated by installing efficient vertical water-tube waste heat boilers, while the flue gas temperature is reduced to around 160 degrees before being discharged. Superheated steam is fed into the turbine to generate electricity. In this industry, the inlet flue gas temperature of waste heat boilers is generally between 400 and 500 degrees. The ash has a low melting point (around 1 g/Nm3) and high cohesion; therefore, the main challenge for waste heat boilers in this industry is to address the issue of ash cleaning. Waste heat boiler for dry quenching of coke: Dry quenching involves using inert gases to cool coke; the heat-absorbing inert gases are then transferred to a boiler, from where they are sent back into the dry quenching furnace via a circulation fan for reuse. The dry quenching device offers many advantages, including energy savings and reduced consumption, lower costs, less environmental pollution, and improved coke quality. The waste heat boiler for dry quenching of coke is the main energy-saving device used in dry quenching systems; its function is to cool the circulating gas and generate steam. Coking waste heat boiler: In the coking process, coking plants generate large amounts of coking exhaust gas at temperatures of 550–950°C. Combustion-supporting boilers use automatic coke oven gas burners; the coke oven gas is burned either in a waste heat boiler or outside the boiler, and then mixed with coking exhaust gases to form high-temperature flue gas that enters the boiler to generate the required amount of steam through heat exchange. The exhaust gas exits the furnace through the slag tube bank, superheater, convection tube bank, and steel tube economizer, then enters the tail desulfurization and dust removal device, and is finally discharged into the atmosphere via a smoke extractor through the chimney. The waste heat boiler for coking exhaust gas overcomes the shortcomings of traditional horizontal single-chamber or multi-chamber waste heat boilers. It enables the various main components of the boiler to be integrated with each other while maintaining their original functions, allows the heating surfaces to be arranged in a concentrated manner, simplifies the internal structure of the furnace, and facilitates the multifunctional combination of its components. The waste heat boiler for coking exhaust gas features efficient heat transfer and two-stage dust separation within the furnace, and boasts advantages such as a compact and innovative design, low cost, small footprint, high efficiency, environmental friendliness, and energy savings. For sulfuric acid waste heat boilers, depending on the different processes used in sulfuric acid production, those employed in waste heat recovery and power generation projects are mainly sulfur-based sulfuric acid waste heat boilers and pyrite-based sulfuric acid waste heat boilers. The structural types of these boilers include fire-tube and water-tube designs. The water circulation methods in boilers are mainly natural circulation and forced circulation. Waste heat boilers for the incineration of hazardous waste, medical waste, and other types of waste: In this industry, waste heat boilers refer to those that are used in conjunction with equipment for incinerating hazardous waste, medical waste, and other such wastes, and that utilize the heat from the flue gases generated by these incineration devices to produce steam according to the specified industrial parameters. Given the characteristics of waste treatment processes, waste heat boilers play a crucial role in preventing the formation of harmful substances such as dioxins; moreover, they are well-suited to handle the flue gases generated from waste incineration. Waste heat boilers in the electric arc furnace industry: Electric arc furnaces are primarily used for the reduction and smelting of ores, as well as of raw materials such as carbon-based reducing agents and solvents. They are used to produce ferroalloys such as ferrosilicon, ferromanganese, ferrochrome, ferrotungsten, and silicomanganese alloys. These ferroalloys serve as important raw materials in the metallurgical industry, as well as for chemical products like calcium carbide. This waste heat boiler effectively solves the problems such as ash cleaning and ash blockage that plague waste heat boilers in this industry, laying the foundation for the efficient operation of the boiler. The developed waste heat boiler is suitable for various waste heat power generation thermal systems, including single-pressure waste heat power generation, double-pressure waste heat power generation, flash vaporization waste heat power generation, medium-temperature and medium-pressure waste heat power generation, and combined cycle waste heat power generation. Waste heat boilers for non-ferrous metal smelting: In the non-ferrous metal smelting industry, the energy-saving method of using waste heat boilers has been widely adopted. Waste heat boilers not only enable the recovery of secondary energy from the high-temperature flue gases in smelting plants and help reduce the temperature of these gases, but more importantly, they capture the dust present in the high-temperature flue gases, recover precious metals, reduce the dust content in the gases, and ensure the smooth operation of the process systems – thereby creating favorable conditions for the subsequent processes. They are therefore crucial and essential equipment in modern metallurgical plants. Considering the production process of metallurgical furnaces and the operating characteristics of waste heat boilers, a single drum layout, horizontal (vertical) configuration, straight-through type, or fully membrane wall arrangement is primarily used. These are water tube boilers that operate in forced or natural circulation mode; their heating surfaces consist of three main parts: water-cooled flues (flue casings), radiation cooling chambers, and convection areas. The ash hoppers and the heating surfaces on the roof of the boiler are both constructed using membrane water-cooled walls. The methods used for cleaning the boiler include shaking and shock wave cleaning, which are simple, effective, and easy to operate. Gas steam turbine waste heat boiler. The gas-steam combined cycle power generation is a highly efficient and low-pollution power generation technology that is developing at a rapid pace worldwide. It has become the preferred system for newly built thermal power plants, offering significant social benefits and broad market prospects. Heat recovery boilers developed based on the characteristics of gas turbine flue gases come in various structural forms such as single-pressure, multi-pressure, vertical, horizontal, natural circulation, and forced circulation, to meet the requirements of different processes. The waste heat boilers in this series have the following main features: 1. They feature a modular design with a rational layout and advanced performance ； 2. The boiler can be adapted to gas turbines using different fuels, meeting various requirements for waste heat utilization ； 3. The boiler is capable of meeting the requirements for frequent start-up and shutdown of gas turbines, has strong peak-shaving capability, and can start up quickly ； 4. The boiler features a multi-layer insulation structure with protective panels, offering strong airtightness and a simple design ； 5. The boiler components are assembled and shipped ready for use, enabling easy installation and a short installation period. Calcium carbide furnace waste heat boiler: A calcium carbide furnace is a device that relies on the high temperature of an arc to melt lime and carbon-containing materials (coke, anthracite, or petroleum coke), thereby enabling a reaction that produces calcium carbide.