The boiler is the core equipment in a combustion system that converts the chemical energy of the fuel into thermal energy for heating a work mass (e.g. water) and thus producing steam. The function of the boiler in a power plant is not only to heat the work material, but also involves complex combustion processes, heat exchange processes and thermochemical processes. The structure and design of the boiler directly determines the operational efficiency and economy of the power plant.Composition of the boiler body
The body of a modern boiler consists of a number of parts, the main ones being:
- furnace: Fuel is burned here, generating a lot of heat.
- flue: The combustion flue gas is discharged through the flue.
- hot surface: Includes water-cooled walls, superheaters, air preheaters, etc., which are responsible for transferring heat to the work mass.
- transfer case: Pipes that connect different systems to help distribute steam and water flow.
- Piping Accessories for Steam and Water Systems: Includes safety valves, gauges and other equipment.
- Combustion equipment and furnace walls: Responsible for maintaining combustion stability and protecting the furnace structure.
Boiler key parameters
The operating characteristics of the boiler are mainly determined by the following parameters:
- vapour: Indicates the amount of steam that the boiler can produce per hour in tonnes per hour (t/h).
- Steam parameters: Refers to the rated pressure (MPa) and temperature of the steam at the outlet of the superheater.
- feedwater temperature: Refers to the temperature of the feed water at the inlet of the economiser.
- Boiler efficiency: An indicator characterising the economy of a boiler, expressing the ratio of the heat efficiently used by the fuel consumed by the boiler in the production of steam to the total heat input.
Functions of boiler heating surfaces
The heated surface of a boiler is primarily used to transfer the heat released by combustion to water or steam, ensuring that the work mass absorbs sufficient heat. The following are the key components of a boiler's heated surface:
- water cooling wall: The water-cooled wall consists of an upright bundle of tubes laid around the boiler chamber. It absorbs heat mainly through radiant heat transfer, heats water and converts it into steam. The structure of the water-cooled wall ensures the cooling and protection of the furnace wall, and its water circulation is divided into two types: natural circulation and forced circulation. The water-cooled wall in natural circulation drives the flow of the work material through the temperature difference, whereas the forced circulation relies on pumping.
- overheater: The superheater heats saturated steam to the required superheat temperature, absorbing heat from the flue gas mainly by means of convective heat transfer. The superheater is usually located in the horizontal flue of the boiler, while some of the superheaters are located on the inside of the furnace wall or on the top of the furnace. The temperature of the superheater can reach 500°C or more, and a desuperheater is often required to regulate the temperature of the superheated steam to prevent excessive temperature fluctuations.
- economiser: The coal economiser uses the waste heat from the boiler flue gas to heat the feedwater, thus increasing the boiler feedwater temperature, reducing the exhaust temperature, improving the boiler efficiency and saving fuel. The coal economiser is arranged in the flue at the end of the boiler, and the feedwater is heated through the serpentine tube, which produces about 15%-20% of the total evaporation of the boiler.
- air preheater: The air preheater improves combustion efficiency by heating the combustion air. It is located in the flue at the end of the boiler and uses the heat of the flue gas to heat the air. The air preheater can be designed in tube or rotary type. Tubular air preheater works stably, but is larger in size; rotary air preheater has compact structure and good heat transfer effect, suitable for large and medium-sized boilers.
Boiler selection principles
Boiler sizing requires consideration of a number of factors, including:
- fuel suitability: Select the type of boiler that is appropriate for the coal type used and ensure that the combustion characteristics of the coal type are matched to the combustion equipment of the boiler. For example, the ignition characteristics, burnout characteristics, slagging characteristics and sulphur content of the coal all affect the boiler type.
- Steam parameters and operating modes: For different steam parameters, different types of boilers are used. Natural circulation boilers are suitable for steam parameters up to and including sub-critical pressure, while direct current boilers are required for steam at supercritical pressure.
- Boiler structureBoiler selection also needs to consider the mode of operation, for example, boilers required to operate with intermediate loads should be selected control cycle boilers or direct current boilers, while the requirements of fixed-pressure operation can be selected from the vertical tube coil water-cooled wall fixed-pressure boilers.
The application of boiler equipment in power plants is crucial, and its structure, selection and operating efficiency directly affect the energy consumption and economic efficiency of power plants. Through reasonable design and optimisation, boilers can not only increase steam production, but also effectively save fuel and reduce emissions. Understanding the working principle, function of the heating surface and selection principles of the boiler can help to improve the overall performance of the boiler and ensure its efficient operation.
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