Introduction to boilers!

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:) Introduction to boilers: A boiler is a heat exchange device that uses the thermal energy of fuel or other energy sources to heat a working fluid (usually water under pressure) to certain parameters (temperature, pressure). Boilers are the source of heat supply. The task of boilers and equipment in boiler rooms is to safely, reliably, and economically efficiently convert the chemical energy of fuel into thermal energy, which is then transferred to water to produce hot water or steam ； Or it can transfer the chemical energy of the fuel to other working fluids, such as heat transfer oil, in order to produce other high-temperature working fluids, such as high-temperature heat transfer oil. Steam is not only used to convert thermal energy into mechanical energy (such as generating electricity with turbines in power plant boilers), but it is also widely used as a heat carrier to provide the heat required in industrial production as well as for heating and ventilation. Generally, we refer to boilers used for power generation and propulsion as power boilers ； Boilers used for industrial purposes and heating are called heat supply boilers, and are typically referred to as industrial boilers. Power station boilers, driven by the need to improve the efficiency of heat cycles, generate steam at high pressure and temperature; they are increasingly developing in the direction of higher temperatures, higher pressures, and larger capacities. For example, the boiler used in domestic 300MW turbine generators has a capacity of 1,025 tons per hour, a steam pressure of 17 MPa (170 atmospheres), and a superheated steam temperature of 555°C. Industrial boilers relevant to us generate steam or hot water that does not require high pressure or temperature, and their capacity is not large either; the pressure is generally below 2.5 MPa (25 atmospheres), while the temperature is usually at the saturation temperature of steam (or it may be superheated, with the superheated steam temperature also remaining low, generally below 400°C). Except where special requirements exist for the production process. 1. Basic structure and working principle of the furnace: A boiler is primarily a combination of two main parts: the pot and the furnace. The fuel burns inside the furnace, converting its chemical energy into thermal energy ； The high-temperature combustion products, namely flue gas, transfer heat to the working fluid in the boiler, such as water; the water is heated, boils, and vaporizes to produce steam. The basic structure of a boiler consists of a drum (also known as a steam vessel), a convection tube bank, a water wall, upper and lower header tanks, and downcomers, which together form a closed steam-water system. The furnace, in the case of chain grate boilers, includes a coal hopper, grate, slag remover, air supply device, etc ； For chamber-type burners, the furnace includes combustion equipment, etc. Furthermore, to ensure the proper operation and safe functioning of the boiler, steam boilers must also be equipped with safety valves, water level gauges, high and low water level alarms, pressure gauges, main steam valves, drain valves, check valves, and other similar devices. 2. Classification of boilers There are numerous types and classification methods for boilers; a general overview of these classifications is shown in the table below: Classification method | Boiler type | Brief description. Classified by purpose: Power station boilers – used for power generation; they are usually large-capacity, high-parameter boilers with combustion taking place in a furnace, resulting in high combustion efficiency. Industrial boilers – used for industrial production and heating; they are generally low-parameter, small-capacity boilers, with combustion either occurring in a furnace or in a bed of fuel. Their thermal efficiency is relatively low. Boilers whose output is steam are called steam boilers, while those whose output is hot water are called hot water boilers. Marine boilers are used as power sources for ships; they generally operate at low to medium parameters and are mostly fueled by oil. It is required that such boilers be small in size and light in weight. Locomotive boilers, on the other hand, are used to power locomotives; they are typically of small capacity with low parameters, use a firebox for combustion, and are mainly fueled by coal. Their design is compact.

**Classification by structure:**
- **Fire-tube boilers:** The flue gases flow inside the fire tubes. These are usually boilers of small capacity with low parameters. Their thermal efficiency is relatively low, but their structure is simple, they require less stringent water quality standards, and they are easy to maintain.
- **Water-tube boilers:** Steam and water flow inside the tubes. There are both low- and high-parameter types, and these boilers require higher standards regarding water quality.

**Classification by circulation method:**
- **Natural circulation boilers:** They have a boiler drum and utilize the density difference of the working fluid in the downcomers and upcomers to create circulation. They can only operate below critical pressure.
- **Multi-stage forced circulation boilers** (also known as auxiliary circulation boilers): They have a boiler drum and a circulation pump, using the density difference of the working fluid in the circulation loop along with the pressure generated by the circulation pump to achieve circulation. They can only operate at critical pressure.
- **Low-ratio circulation boilers:** They have a steam-water separator and a circulation pump; circulation is primarily achieved through the circulation pump. They can be used at subcritical and supercritical pressures, with a circulation ratio of 1.25–2.0.
- **Direct-flow boilers:** They lack a boiler drum; feedwater passes through the heating surfaces under the pressure generated by a pump to produce steam. They are suitable for high-pressure and supercritical applications.
- **Combined-cycle boilers:** They have a circulation pump. When the boiler load is low, they operate in a recirculation mode, while at high loads they operate in direct-flow mode. They are suitable for subcritical and supercritical pressures.

**Classification by the pressure of the working fluid at the boiler outlet:**
- **Low-pressure boilers:** Pressure less than 1.27 MPa (13 atmospheres).
- **Medium-pressure boilers:** Pressure of 3.82 MPa (39 atmospheres).
- **High-pressure boilers:** Pressure of 9.8 MPa (100 atmospheres).
- **Ultra-high-pressure boilers:** Pressure of 13.72 MPa (140 atmospheres).
- **Subcritical-pressure boilers:** Pressure of 13.72 MPa (170 atmospheres).
- **Supercritical-pressure boilers:** Pressure greater than 22.11 MPa (225.65 atmospheres).

**Classification by combustion method:**
- **Firebox combustion boilers:** These are mainly used in industrial boilers, including fixed grate boilers, movable hand-cranked grate boilers, coal-throwing chain boilers, vibrating grate boilers, bottom-fed grate boilers, and reciprocating grate boilers. The fuel burns mainly on the grate.
- **Chamber combustion boilers:** These are mainly used in power station boilers. They can use liquid fuels, gaseous fuels, or coal powder, with the fuel burning in suspension inside the furnace chamber.
- **Cyclone boilers:** There are horizontal and vertical types. They use coarse coal powder or coal chips; the particles burn in suspension at the center of the cyclone tube, while larger particles burn against the walls of the tube.
- **Boilers with liquid slag discharge:**
- **Boilers with bubbling combustion:** The air flow rate to the grate is high, causing the coal to burn in a bubbling manner on the grate bed. These boilers are suitable for using lower-quality coal and are mainly used in industrial boilers. At present, a large number of large-scale circulating fluidized bed boilers have been developed. Classified by the fuel used, there are solid-fuel boilers that burn solid fuels such as coal, liquid-fuel boilers that burn liquid fuels such as heavy oil, and gas-fuel boilers that burn gaseous fuels such as natural gas. There are also waste-heat boilers that use waste heat from industries such as metallurgy and petrochemicals as a heat source, nuclear-energy boilers that utilize the heat released by nuclear reactors as a heat source for steam generation, and waste-burning boilers that use waste materials such as garbage, bark, and waste liquids as fuel. Additionally, there are boilers that make use of energy sources such as geothermal energy and solar energy. Classified by the method of slag discharge, there are solid-slag discharge boilers in which ash and slag are discharged in solid form, and liquid-slag discharge boilers in which ash and slag are discharged in liquid form, where they are then cracked into small particles in the cooling water of a cracking tank before being flushed away via drains. Classified by the arrangement of the boiler drum, there are single-drum vertical arrangements, single-drum horizontal arrangements, and double-drum vertical arrangements. Power station boilers generally use the single-drum design, while industrial boilers can use either single-drum or double-drum designs. Classified by furnace type, there are inverted U-shaped, tower-type, box-type, N-type, D-type, and A-type furnaces. The D-type and A-type furnaces are used in industrial boilers, while the other types are generally used in power station boilers. As for the type of delivery at the factory, there are quick-installation, assembled, and bulk delivery options; small boilers usually come in quick-installation format. 3. The working process of a boiler: The operation of a boiler involves three processes: the combustion of fuel, the transfer of heat from flue gases to water, and the vaporization of water. These three processes take place simultaneously within the boiler. I. The combustion process of fuel: The combustion conditions vary depending on the different combustion methods. Taking a chain grate boiler as an example, its combustion equipment is a chain grate. The fuel falls onto the grate surface in the coal hopper due to its own weight. The grate is driven by a motor that uses a gearbox to reduce speed, and chain sprockets are used to move it; the chain grate acts like a conveyor, continuously feeding fuel into the furnace. The fuel burns on one side of the grate while moving backward on the other side ； The air required for fuel combustion is supplied by a blower, passing upward through the air ducts and the air chambers beneath the grate, reaching the fuel layer where combustion takes place and high-temperature flue gas is generated. The fuel is eventually reduced to ash, which is discharged after passing over the slag removal plate (commonly known as eagle iron) at the end of the grate; this entire process is called the combustion process. Of course, to ensure the continuous burning in the boiler, it is necessary to supply fuel and air continuously, as well as remove flue gas and ash. To meet environmental requirements, the flue gas also needs to be cleaned of dust; for this purpose, blowers, exhaust fans, equipment for transporting coal and ash, as well as dust and smoke removal devices are required. II. Heat transfer from flue gases to the working medium (water, steam, heat transfer oil, etc.) Due to the heat released by fuel combustion, the temperature inside the furnace is very high. Along the surrounding walls of the furnace chamber, there are rows of tubes, commonly known as water wall tubes. High-temperature flue gas undergoes intense radiative heat exchange with the water wall, transferring heat to the fluid inside the tubes. Then, the flue gas flows upward toward the furnace chamber due to the pull of the exhaust fan and chimney. After exiting the flue window (the furnace outlet) and passing over the slag guard tubes, the flue gas flows toward the steam superheater—a set of vertically arranged coiled tube heat exchangers—in which the saturated steam generated in the boiler (vapor drum and water wall) is heated by the flue gas to become superheated. A economizer and an air preheater are arranged in sequence within the tail flue. The flue gas, after multiple stages of heat transfer, is finally discharged from the boiler. III. The vaporization process of water: The vaporization process of water is also the process by which steam is generated, and it mainly includes the water cycle and the separation process between water and steam. The boiler feed water, after being treated with water, is pressurized by a feed pump, preheated first in a economizer, and then enters the drum. When the boiler is in operation, the fluid in the drum is a saturated mixture of steam and water. The convective tube bundles located in the sections with lower flue gas temperatures receive less heat, resulting in a higher specific gravity of the steam-water mixture ； The water wall and convective tube banks located in the high-temperature area of the flue gas are subjected to intense heating, resulting in a lower specific gravity of the steam-water mixture ； As a result, the soda mixture with a higher specific gravity flows downward into the lower drum, while the soda mixture with a lower specific gravity flows upward into the upper drum, thus creating a natural circulation within the boiler. Furthermore, to organize the water cycle more effectively and to facilitate flow distribution, unheated downcomers are typically installed outside the furnace wall; these downcomers are used to guide the working fluid to the lower header of the water wall, while the steam-water mixture is directed to the upper drum through the steam-water outlet pipes on the upper header. The steam-water mixture is separated thanks to the steam-water separation device installed in the upper drum, as well as the gravitational separation effect within the space of the drum itself ； If there is a superheater, the steam is led out at the top of the upper drum and enters the steam superheater, while the separated saturated water returns to the water space in the lower half of the upper drum. The water circulation in the drum also ensures that the metal surfaces in contact with the high-temperature flue gases are cooled and do not get damaged, which is a necessary condition for the long-term safe operation of the boiler. The separation equipment for the soda mixture is essential to ensure the quality of steam and to enable the steam superheater to function properly.