Development of the boiler industry and future trends

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The overall structure of the boiler consists of two main parts: the boiler body and the auxiliary equipment. The main components of 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. ①Furnace chamber: Also known as the combustion chamber, it is the space where the 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-of-flames 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 the coal particles to enable them to burn in a bubbling manner, making it suitable for burning low-quality fuel, 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 it are made of heat-resistant materials and insulation 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 flame and hot flue gases. The structure, shape, volume, and height of the furnace must all be such that the fuel burns fully, and that the temperature of the flue gases at the furnace outlet is reduced below the temperature at which the slag begins to solidify. When the temperature inside the furnace exceeds the ash melting point, the ash becomes molten. Molten slag particles stick to the water-cooled wall tubes or other components inside the furnace when they come into contact with them. As more ash particles adhere together, slag lumps are formed, a phenomenon known as slagging. Slagging reduces the heat transfer efficiency of the boiler’s heating surfaces. In severe cases, it can block the pathways for smoke flow, affecting the safe and efficient operation of the boiler. The intensity of combustion is generally expressed in terms of the heat load per volume of the furnace chamber, or the heat load per cross-sectional area of the furnace chamber, or the grate heat load. The volumetric heat load of the furnace is the amount of heat released per unit time per unit volume of the furnace. In boiler technology, the heat load per volume of the furnace is commonly used to determine whether the size of the furnace is appropriate. An excessive volumetric heat load indicates that the furnace volume is too small, resulting in a short residence time for the fuel inside the furnace; this prevents complete combustion of the fuel and reduces the combustion efficiency ； At the same time, this also indicates that the furnace wall area is too small to accommodate enough water-cooled wall tubes; as a result, the flue gas temperature inside the furnace and at the furnace outlet is too high, leading to slag formation on the heated surfaces. The heat load per unit cross-sectional area of the furnace in a chamber-type furnace is the amount of heat released by fuel combustion per unit time per unit cross-sectional area of the furnace. Once the furnace volume is determined, an excessive heat load on the furnace cross-section can cause the wall temperatures in certain areas to become too high, leading to slag formation. The grate heat load of a layer-fired furnace is the ratio of the heat released by fuel combustion per unit time to the area of the grate. An excessive grate heat load will increase fly ash**. The furnace design must take full account of the characteristics of the fuel 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. ②Steam drum: It is a cylindrical vessel in natural circulation and multi-stage forced circulation boilers that receives the feed water from the economizer, connects the circulation circuit, and delivers saturated steam to the superheater. The drum body is made of high-quality thick steel plates and is one of the heaviest components in the 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, in order to prevent boiler water containing high concentrations of salts and impurities from entering the superheater and turbine along with the steam. If these salts and impurities cause scaling, salt deposition, and corrosion on the superheater tubes and turbine passages, it will affect the economic and safe operation of the equipment. The steam at the boiler outlet generally meets certain quality standards. The internal equipment of the boiler drum includes steam-water separation and steam cleaning devices, feedwater distribution pipes, sludge discharge 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 fine water droplets carried in the steam. Baffles and slotted baffles are commonly used as coarse separation elements in medium and low-pressure boilers. 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. With the advancement of water treatment technology, steam separation devices tend to become simpler and more standardized. Waste discharge systems (including continuous and periodic discharge) can remove a portion of the boiler water containing high levels of salts and sludge during boiler operation. The boiler drum is also equipped with monitoring and protection devices such as a water level gauge and a safety valve. In addition to the boiler itself, power station boilers are equipped with many auxiliary devices: ① Coal powder preparation system, including coal mills, powder exhaust fans, coarse powder separators, and coal powder pipelines, etc ； ②Air supply and exhaust systems, including air supply fans, exhaust fans, and flue ducts, etc ； ③Water supply system, including water pump, valves, pipes, etc ； ④Water treatment system (see boiler water treatment) ； ⑤Ash removal system, including slag discharge machines, dust collectors, etc ； ⑥Automatic control and monitoring systems (see Boiler Automatic Control, Boiler Steam Temperature Regulation). Thermal equilibrium is a method for calculating the thermal efficiency of a boiler (referred to as boiler efficiency). The thermal efficiency of a boiler refers to the percentage of the heat from the fuel fed into the boiler that is effectively utilized. The efficiency of modern power station boilers can exceed 90% ； The efficiency of industrial boilers can exceed 75%. Of the heat from the fuel fed into the boiler, aside from the portion that is utilized effectively, the rest is lost in various forms, including heat loss carried away by the exhaust gases ； Heat loss due to incomplete combustion of gases such as unburned carbon monoxide, hydrogen, and methane in the exhaust fumes ； Heat losses due to incomplete combustion of solid materials such as unburned carbon contained in fly ash, slag, and coal leakage from the grate, as well as heat dissipation losses. 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 for the complete combustion of a unit mass or unit volume of fuel, as calculated based on the chemical reaction equation, 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. The ratio of the actual amount of air fed into the furnace to the theoretical amount of air is called the excess air coefficient. The actual excess air coefficient at the furnace outlet mainly depends on the fuel properties and combustion method, generally ranging from 1.05 to 1.5. 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. For example, the excess air coefficient of fuel boilers may already be less than 1.03. This combustion technology that uses a low excess air coefficient is called low-oxygen combustion. Circulation mode: The boiler circulation mode refers to the way in which water vapor flows within the boiler’s evaporation system, and it can be divided into natural circulation, forced circulation, direct flow, and combined circulation. Flue gas purification and ash treatment: The dust contained in boiler flue gas (including fly ash and soot), as well as sulfur and nitrogen oxides, are all substances that pollute the atmosphere. If not purified, 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 technology, 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 force, as well as sound waves and static electricity. For coarse particles, gravity sedimentation and separation by inertial forces are generally used, while centrifugal force separation is often employed for dust removal at higher capacities. Electrostatic precipitators and bag filters have 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; they can also absorb gaseous pollutants. Flue gas desulfurization includes absorption and catalytic oxidation methods. For dry absorption, alkaline alumina, semi-coke, activated carbon, etc. are used ； For wet absorption, ammonia, sodium carbonate, lime slurry, etc. are used. Catalysts such as vanadium pentoxide can oxidize most of the sulfur dioxide to sulfur trioxide at a certain temperature, thereby facilitating desulfurization. Due to the high costs of flue gas desulfurization equipment and operation, most companies prefer to use low-sulfur fuels to reduce sulfur oxide emissions. The nitrogen oxides in flue gas are mainly nitric oxide. Flue gas denitration methods include catalytic decomposition, selective catalytic reduction, as well as denitration through the absorption using high-temperature activated carbon. During operation, coal-fired boilers inevitably produce large amounts of slag and fly ash collected by dust collectors, which are generally removed using hydraulic or mechanical methods and sent to slag dumps. Since the 1950s, efforts have been made to develop comprehensive utilization of ash and slag in order to turn hazards into advantages. 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. Development trends: The main trends in the development of boilers are: ① Further improving the thermal efficiency of boilers and power plants ； ②Reduce the cost of equipment per unit of power for boilers and power plants; ③ Improve the operational flexibility and automation level of boiler units; ④ Develop more types of boilers to accommodate different fuels ； ⑤Improve the operational reliability of boiler units and their auxiliary equipment; ⑥ Reduce environmental pollution.