Boiler Terminology Explanation (Part 16)

151. Forced Draft Fan (FDF):
A fan that delivers air into the boiler furnace to meet the combustion requirements of the fuel. It is generally positioned before the air preheater inlet. The process works as follows: the fan draws in cold air and sends it to the air preheater, where it is heated to the design temperature. A portion of this heated air is then directly supplied to the furnace through the burners (as secondary air), while the other portion enters the pulverizing system as a drying agent and to transport pulverized coal, then is also introduced into the furnace (as primary air). Therefore, the selection of the forced draft fan must consider not only the quantity of air needed for combustion but also the resistance of the entire combustion system that must be overcome. Typically, two forced draft fans are configured per boiler. A single unit in operation must meet at least 70% of the boiler's load requirement. For large capacities, axial flow fans are commonly used. In industrial setups where components like the inconel 625 globe valve are employed to handle high-temperature, corrosive gases, ensuring proper air delivery via FDF is critical to system performance.

152. Muffler:
A device installed at the inlet or outlet of a gas (or steam) flow pipeline or equipment to reduce aerodynamic noise. According to their working principles, mufflers can be categorized into resistive mufflers, reactive mufflers, impedance composite mufflers, micro-perforated plate mufflers, and small-hole jet mufflers.

153. Chimney:
A structure that emits boiler flue gases into the upper atmosphere for dilution and dispersion, mainly to reduce environmental pollution. In thermal power plants, chimneys are typically standalone structures. Based on construction materials, they can be categorized into brick chimneys, reinforced concrete chimneys, and steel chimneys.

154. Induced Draft Fan (IDF):
Also known as a suction fan, it extracts flue gases from the rear of the boiler and discharges them into the chimney. To reduce wear caused by fly ash in the flue gas, induced draft fans are often installed downstream of the dust collector. Each boiler usually has two ID fans, with one being able to handle at least 70% of the load. Power plant boilers typically use either centrifugal or axial fans. Common operational issues include wear and ash accumulation, which may cause imbalance and vibration due to uneven erosion or deposits. Axial fans, with smaller clearances and higher impeller speeds, are more sensitive to wear than centrifugal fans. Although centrifugal fans are more resistant to wear, they are more affected by ash buildup. Our plant uses variable-pitch axial flow fans with good wear resistance and easily replaceable stator blades.

155. Fuel Burning Equipment:
Devices that introduce air and fuel into a combustion space in various ways to form a continuous and stable flame. Different fuels and combustion methods require different equipment. Typically, burning equipment includes the furnace and burners, which are the core of the boiler combustion system. Besides providing sufficient space for fuel residence time and for arranging heating surfaces, the furnace should have a proper shape and size to coordinate with the burner layout, ensuring good airflow, facilitating ignition and complete combustion, while avoiding flame impingement on walls, ensuring full combustion, and maintaining even thermal load.

156. Boiler Furnace:
The space within the boiler where fuel combustion is organized, also known as the combustion chamber, forming a key part of the fuel-burning equipment. Modern boiler furnaces not only convert the chemical energy of the fuel into thermal energy of the combustion products but also play a role in heat exchange. Therefore, the furnace must be structured to ensure complete combustion and that flue gases are sufficiently cooled at the outlet to protect the downstream heating surfaces.

157. Tangential Pulverized Coal Burner:
A device using multiple interacting straight jets to combust pulverized coal. It is commonly used in boilers and consists of a set of round and/or rectangular nozzles arranged in an interleaved order. Primary and secondary air are injected through different nozzles into the furnace, and the flame rises and rotates in the center. These burners are often arranged at the four corners of the furnace, with their axes tangential to an imaginary circle at the furnace center. High-velocity primary and secondary air is injected tangentially, creating a strong swirling flow. Coal ignition is aided by entrainment of high-temperature flue gas and torch ignition from adjacent corners. This design improves mixing and burnout. Variants include primary air on the tangential circle and secondary air on the counter-tangential circle. A square or near-square furnace cross-section is preferred for optimal flow patterns.

158. Steam Temperature Control by Tilting Burner:
A method that adjusts the tilt angle of the burner to shift the flame center and thereby regulate flue gas temperature at the furnace outlet, which controls steam temperature. This method is widely used in tangentially fired coal-fired boilers. Burners typically tilt within a range of 20°–30°, resulting in a flue gas temperature change of 110–140°C and steam temperature adjustment of 40–60°C. Though primarily used for reheated steam, it also affects superheated steam. The heat exchange of heating surfaces near the furnace outlet varies significantly with burner tilt, making this an effective way to regulate reheater temperatures. This method is responsive and does not require additional heat transfer surfaces or energy consumption, making it a common solution for reheater temperature control.

159. Low-NOx Pulverized-Coal Burner:
A burner that minimizes and suppresses the formation of nitrogen oxides (NOx) during coal combustion. NOx refers to a group of nitrogen oxides including NO, NO₂, NO₃, N₂O, and N₂O₃, with NO and NO₂ being the most common in power plant boilers. NOx forms either from nitrogen in the fuel or from atmospheric nitrogen reacting with oxygen at high temperatures. Factors affecting NOx formation include: (1) flame temperature, (2) oxygen concentration in the combustion zone, (3) residence time of combustion products in the high-temperature region, and (4) coal properties (fixed carbon to volatile matter ratio).

160. SGR Burner (Separate Gas Recirculation):
This burner injects recirculated flue gas above and below the primary air nozzle, while the secondary air nozzle is positioned further away. This setup creates a reducing atmosphere near the primary air, lowering the combustion center temperature and thus suppressing NOx formation.