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A boiler is a mechanical device that uses the thermal energy of fuel or other sources to heat water into hot water or steam. A boiler consists of two main parts: the boiler shell and the furnace. The term \"boiler shell\" originally refers to a container for water that is heated over a fire, while the \"furnace\" is the area where fuel is burned. The hot water or steam generated in the boiler can directly provide the heat energy needed for production and daily life; it can also be converted into mechanical energy through steam power plants, or the mechanical energy can further be converted into electrical energy via generators. A boiler that provides hot water is called a hot water boiler; it is mainly used for domestic purposes, with some application also in industrial production. A boiler that generates steam is called a steam boiler, or steam generator; it is often simply referred to as a boiler. It is an important component of steam power systems and is widely used in thermal power plants, ships, locomotives, and industrial enterprises. Boilers are exposed to high temperatures and pressures, so safety is of great importance. Even small boilers can cause severe consequences in the event of an explosion. Therefore, strict regulations exist for the material selection, design calculations, manufacturing, and inspection of boilers. The development of boilers involves two aspects: the boiler itself and the furnace. In the first half of the 18th century, the steam engines used in British coal mines, including Watt’s early steam engines, operated at a steam pressure equal to atmospheric pressure. In the second half of the 18th century, steam at a pressure higher than atmospheric pressure was used. In the 19th century, the commonly used steam pressure was increased to around 0.8 MPa. In line with this, the earliest steam boilers were large-diameter cylindrical vertical boilers filled with water; later, horizontal boilers were used, with the fire being burned in a brick-built furnace beneath the boiler shell. As boilers became larger, fire tubes were added to the boiler shell in order to increase the heating surface area. Fire is burned at the front end of these tubes, and the smoke exits from the back end; it then passes through brick-lined flues to reach the chimney, thereby heating the outside of the boiler shell. Such boilers are known as fire-tube boilers. Initially, only one fire tube was used, and such boilers were called single-fire-tube boilers or Cornish boilers; later, two fire tubes were added, and they became known as double-fire-tube boilers or Lancashire boilers. Around 1830, fire-tube boilers appeared after the mastery of high-quality steel tube production and tube expansion techniques. Some fire tubes are installed within the boiler shell, forming the main heat-exchanging surface of the boiler, with fire (flue gas) flowing through these tubes. A horizontal external combustion backfire tube boiler is one in which as many fire tubes as possible are installed below the water level line of the boiler shell. It requires less metal, but a large amount of masonry is needed. In the mid-19th century, water-tube boilers appeared. The boiler’s heating surface consists of water tubes outside the boiler shell, replacing both the boiler shell itself as well as the fire tubes and burners inside it. The increase in the heating surface area of the boiler and the steam pressure is no longer limited by the diameter of the boiler shell, which helps to increase the boiler’s evaporation capacity and steam pressure. The cylindrical boiler shell in this type of boiler was then renamed the boiler drum, or steam drum. Early water-tube boilers used only straight tubes, and both the pressure and capacity of such boilers were limited. In the early 20th century, steam turbines began to develop, requiring boilers with higher capacity and steam parameters. Straight-tube boilers can no longer meet the requirements. With the development of manufacturing processes and water treatment technologies, bent-tube boilers came into existence. Initially, a multi-drum type was used. With the use of water wall, superheater, and economizer, as well as improvements in the vapor-water separation elements inside the boiler drum, the number of boiler drums has gradually decreased. This not only saves metal but also helps to increase the pressure, temperature, capacity, and efficiency of the boiler. The earlier fire-tube boilers, tube boilers, and water-tube boilers all belonged to the category of natural circulation boilers, in which steam and water moved naturally due to differences in density resulting from varying heating conditions in the rising and descending pipes. While developing natural circulation boilers, once-through boilers were put into use starting in the 1930s, and auxiliary circulation boilers were introduced in the 1940s. A auxiliary circulation boiler, also known as a forced circulation boiler, was developed on the basis of a natural circulation boiler. A circulation pump is installed in the downcomer system to enhance the water circulation in the evaporation heating surface. A once-through boiler does not have a drum; the feedwater is pumped by a feed pump into the economizer, and after passing through evaporation surfaces such as the water wall and superheater, it turns into superheated steam which is then sent to the turbine. All flow resistance in these components is overcome by the feed pump. After World War II, these two types of boilers developed rapidly, as generator sets at that time required high temperature, high pressure, and large capacity. The purpose of developing these two types of boilers is to reduce or eliminate the use of a boiler drum; small-diameter tubes can be used as the heating surfaces, allowing for a more flexible arrangement of those surfaces. With the advancement of automatic control and water treatment technologies, they have gradually become more mature. At supercritical pressure, the once-through boiler is the only type of boiler that can be used; in the 1970s, the largest single-unit capacity was 27 megapascals with a generator set of 1300 megawatts. Later, a combined cycle boiler was developed, which is a combination of a supplementary cycle boiler and a once-through boiler. In the development of boilers, the type of fuel has a significant impact on the furnace and combustion equipment. Therefore, it is necessary not only to develop various types of furnaces to suit the combustion characteristics of different fuels, but also to improve combustion efficiency in order to save energy. Furthermore, technical improvements in the furnace and combustion equipment also require minimizing pollutants (sulfur oxides and nitrogen oxides) in the boiler exhaust. Early shell boilers used fixed grates, burned high-quality coal and wood, and both coal feeding and slag removal were carried out manually. With the advent of straight-tube boilers, mechanical grates were introduced, among which chain grates found wide application. The air supply under the grate has evolved from a non-segmented \"uniform air supply\" to a segmented air supply. In the early days, the furnaces were low, resulting in low combustion efficiency. Later, people realized the role of the furnace volume and structure in combustion; by making the furnace taller and using a furnace arch as well as secondary air, combustion efficiency was improved. When the power of generator sets exceeds 6 megawatts, the grates of these layer-fired boilers are too large, their structure is complex, and they are difficult to install; therefore, room-fired boilers were introduced starting in the 1920s, as these boilers burn coal powder and oil. After the coal is ground into powder by a coal grinder, it is injected into the furnace through burners for combustion, thereby freeing the capacity of the generator set from being limited by the combustion equipment. Since the beginning of World War II, power station boilers have almost all used room-fired boilers. Coal-fired boilers manufactured in the early years used U-shaped flames. The coal powder stream ejected by the burner first descends in the furnace chamber, and then turns upward. Later, swirl burners arranged on the front wall appeared, with the flame forming an L-shaped torch inside the furnace. As the boiler capacity increases, the number of swirl burners also increases; they can be installed on the side walls or on the front and rear walls. Around 1930, direct-flow burners were introduced, which were placed at the four corners of the furnace and mostly operated in a tangential combustion mode. After World War II, with oil being inexpensive, many **began to widely use fuel boilers. The degree of automation in fuel boilers can be easily increased. After oil prices rose in the 1970s, many **turned back to using coal resources. At this time, the capacity of power plant boilers is also increasing, requiring that the combustion equipment not only burn completely, start up stably, operate reliably, and perform well at low loads, but also reduce the pollutants in the exhaust gases. In power plant boilers that burn coal, especially lignite, the use of staged combustion or low-temperature combustion techniques – such as delaying the mixing of coal powder with air or introducing flue gas into the air to slow down combustion, or dispersing the burners to control the furnace temperature – not only helps to reduce the formation of nitrogen oxides but also decreases slag formation. The boiling combustion method is a type of low-temperature combustion; in addition to solid fuels with very high ash content that can be used for combustion, limestone can also be added to the boiling bed for desulfurization. The operation of a boiler: Boiler parameters are the key indicators that reflect the performance of a boiler, including boiler capacity, steam pressure, steam temperature, feedwater temperature, etc. The boiler capacity can be expressed in terms of the rated evaporation rate or the maximum continuous evaporation rate. The rated evaporation capacity is the amount of steam that can be produced continuously per unit time under specified outlet pressure, temperature, and efficiency conditions. The maximum continuous evaporation rate is the maximum amount of steam that can be produced continuously per unit time at the specified outlet pressure and temperature. Steam parameters include the steam pressure and temperature of the boiler; they usually refer to the superheated steam pressure and temperature at the exit of the superheater and reheater. In the absence of a superheater and reheater, they refer to the saturated steam pressure and temperature at the boiler outlet. Feedwater temperature refers to the temperature of the water entering the economizer; in the absence of an economizer, it refers to the temperature of the water entering the boiler drum. Boilers can be classified in different ways. Boilers can be classified by purpose into industrial boilers, power station boilers, marine boilers, and locomotive boilers, etc ； Boilers can be classified according to their outlet pressure into low-pressure, medium-pressure, high-pressure, ultra-high-pressure, sub-critical pressure, and super-critical pressure boilers, etc ； Boilers can be classified into fire-tube boilers, water-tube boilers, and shell-and-tube boilers based on the flow path of water and smoke; among these, fire-tube boilers and water-tube boilers are collectively referred to as shell boilers ； Based on the circulation method, they can be classified into natural circulation boilers, auxiliary circulation boilers (i.e., forced circulation boilers), once-through boilers, and combined cycle boilers ； Based on the combustion method, boilers are classified into room-type burners, layer-type burners, fluidized bed burners, etc. In terms of the water vapor system, the feed water is heated to a certain temperature in the heater, then enters the economizer through the feed water pipes. After further heating, it is sent to the boiler drum, where it mixes with the boiler water and flows downward along the downcomers to the inlet header of the water wall. Water absorbs the radiant heat from the furnace inside the water wall tubes, forming a steam-water mixture that then moves to the drum through the rising tubes; a steam-water separation device is used to separate the water from the steam. The separated saturated steam flows from the upper part of the boiler drum to the superheater, where it absorbs more heat to become superheated steam at 450°C, and is then sent to the turbine. In terms of the combustion and flue gas systems, the forced draft fan sends air into the air preheater to heat it to a certain temperature. The coal powder, which has been ground to a certain fineness in the coal grinder, is carried by a portion of the hot air from the air preheater and injected into the furnace through the burner. The coal powder mixture ejected by the burner mixes with air in the furnace, where it burns together with the remaining hot air, releasing a large amount of heat. The hot flue gas after combustion flows sequentially through the furnace, slag duct bundle, superheater, economizer, and air preheater, then passes through a dust removal device to remove the fly ash contained in it, and is finally sent to the chimney by an exhaust fan to be discharged into the atmosphere. Structure of the boiler: The overall structure of the boiler consists of two main parts: the boiler body and the auxiliary equipment. The main components in a boiler, such as the furnace, boiler drum, burner, water wall, superheater, economizer, air preheater, framework, and furnace walls, constitute the core part responsible for steam production; this is known as the boiler body. The two most important components of the boiler itself are the furnace and the drum. The furnace, also known as the combustion chamber, is the space where fuel burns. A furnace in which solid fuel is placed on a grate for combustion in a bed of flames is called a stratified combustion furnace, also known as a bed furnace ； A furnace in which liquid, gaseous, or powdered solid fuels are injected into a combustion chamber for burning is called a chamber-type furnace, also known as a combustion-chamber furnace ； A furnace in which air lifts coal particles to keep them burning in a bubbling state, and is suitable for burning low-quality fuels, is called a bubbling furnace, also known as a fluidized bed furnace ； A cylindrical furnace that uses an air flow to spin coal particles at high speed and subject them to intense burning is called a cyclone furnace. The cross-section of the furnace chamber is generally square or rectangular. Fuel burns in the furnace to produce flames and hot flue gases; therefore, the furnace walls surrounding the furnace are made of high-temperature resistant materials and insulating materials. Water wall tubes are commonly installed on the inner surface of the furnace wall; they protect the furnace wall from being damaged and absorb a large amount of radiant heat from the flames and hot flue gases. The furnace design must take full account of the properties of the fuel being used. Each boiler should use the fuel originally designed for it as much as possible. When fuels with significant differences in combustion characteristics are used, the economic efficiency and reliability of boiler operation can both be reduced. The drum is a cylindrical vessel in natural circulation and multi-pass forced circulation boilers that receives the feed water from the economizer, connects the circulation circuit, and transports saturated steam to the superheater. The drum shell is made of high-quality thick steel plates and is one of the heaviest components in a boiler. The main function of the boiler drum is to store water, separate steam from water, and remove salts and sediment from the boiler water during operation, thereby preventing boiler water containing high concentrations of salts and impurities from entering the superheater and turbine along with the steam. The internal equipment of the boiler drum includes steam-water separation and steam cleaning devices, feedwater distribution pipes, sludge removal and chemical dosing equipment, etc. The function of the steam-water separation device is to separate the saturated steam coming from the water wall from the water, and to minimize the amount of tiny water droplets carried in the steam. In medium and low-pressure boilers, baffle plates and slotted baffle plates are commonly used as coarse separation elements ； Boilers above medium pressure, in addition to making extensive use of various types of cyclone separators for preliminary separation, also employ louver screens, wire meshes, or vapor equalizing plates for further separation. The boiler drum is also equipped with monitoring and protection devices such as a water level gauge and a safety valve. To evaluate performance and improve design, boilers are often subjected to heat balance tests. The method of calculating the boiler’s thermal efficiency directly from the efficient use of energy is called positive balance, while the method of determining the efficiency by considering various heat losses is called negative balance. When considering the actual efficiency of a boiler room, it is necessary to take into account not only the thermal efficiency of the boiler but also the energy consumed by the auxiliary equipment associated with it. The amount of air required, as calculated based on chemical reactions, when a unit mass or unit volume of fuel is completely burned, is known as the theoretical air volume. To give the fuel more opportunities to come into contact with oxygen and burn inside the furnace, the actual amount of air supplied to the furnace must always be greater than the theoretical amount of air. Although introducing more air can reduce heat losses due to incomplete combustion, it increases heat losses from flue gases, as well as exacerbating sulfur oxide corrosion and nitrogen oxide formation. Therefore, efforts should be made to improve combustion technology in order to achieve complete combustion in the furnace with the lowest possible excess air coefficient. The dust contained in boiler flue gas (including fly ash and carbon black), as well as sulfur and nitrogen oxides, are all substances that pollute the atmosphere; without purification, their emission levels can be several times to dozens of times higher than the limits set by environmental protection regulations. Measures to control the emission of these substances include pre-combustion treatment, improved combustion technologies, dust removal, desulfurization, and denitrification. Relying on tall chimneys can only reduce the concentration of pollutants in the air near the chimneys. The forces used for dust removal from flue gas include gravity, centrifugal force, inertial force, adhesion, as well as sound waves and static electricity. For coarse particles, gravity sedimentation and separation by inertial forces are generally used; at higher capacities, centrifugal force separation is often employed. Electrostatic dust collectors and bag filters exhibit high dust removal efficiency. In wet and Venturi-water film dust collectors, the water droplets and water film can adhere to fly ash, resulting in high dust removal efficiency as well as the ability to absorb gaseous pollutants. Since the 1950s, efforts have been made to develop comprehensive utilization of ash and slag in order to turn harm into benefit. Materials such as cement, bricks, and concrete aggregates can be produced using ash residues. Starting in the 1970s, hollow microspheres were also extracted from fly ash for use as materials in fire resistance and insulation applications. The future development of boilers will further improve the thermal efficiency of boilers and power plants ； Reduce the equipment cost per unit of power for boilers and power plants ； Improve the operational flexibility and automation level of boiler units ； Develop more types of boilers to accommodate different fuels ； Improve the operational reliability of boiler units and their auxiliary equipment ； Reduce environmental pollution.