The coking problem in biomass boilers primarily stems from the combined effects of fuel characteristics, combustion conditions, and equipment design. Combustion control technology is the core means of addressing this issue. By optimizing combustion parameters, adjusting air distribution, and controlling temperature distribution, coking can be effectively suppressed, improving boiler operating efficiency and safety.
The ash characteristics of biomass fuel are an inherent cause of coking. Biomass fuel often contains alkali metals such as potassium and sodium, as well as elements such as iron and calcium. These components tend to form low-melting eutectics at high temperatures. When ash contacts the heating surface, if the temperature exceeds its melting point, it adheres and gradually hardens, forming coke slag. Therefore, combustion control must begin with fuel pretreatment. This involves screening to remove impurities, adjusting particle size uniformity, and reducing the risk of fusible components in the ash concentrating. Furthermore, controlling the fuel moisture content within a reasonable range prevents the reaction of water vapor with ash during combustion to form a sticky substance, thereby reducing the tendency to coke.
The rationality of air distribution directly impacts combustion uniformity. Biomass boilers typically utilize a staged distribution system of primary and secondary air: primary air provides the oxygen required for initial fuel combustion, while secondary air supplements the oxygen required for later combustion and enhances the agitation. Insufficient primary air volume prevents complete fuel combustion, and unburned carbon particles easily adhere to the heating surfaces. Uneven secondary air distribution can lead to localized oxygen deficiency or excess, creating a reducing atmosphere and lowering the ash melting point. By optimizing the damper opening and adjusting the air velocity ratio to achieve a uniform oxygen concentration across the combustion area, localized high-temperature zones can be avoided and the potential for coking can be reduced. For example, symmetrically arranged secondary air nozzles, combined with a variable-frequency blower to dynamically adjust the air volume, can significantly improve combustion stability.
Temperature control is crucial for preventing coking. The furnace temperature of a biomass boiler must be strictly controlled below the ash softening temperature, typically between 800°C and 900°C. Excessively high temperatures result in molten ash, which easily adheres to the heating surfaces. Excessively low temperatures lead to incomplete combustion and increase the risk of unburned particle deposition. In actual operation, temperature distribution can be controlled by adjusting the fuel supply, adjusting the primary air preheating temperature, and optimizing the burner layout. For example, staged combustion technology can be used to introduce fuel into the furnace in stages, extending combustion time and preventing local overheating. Alternatively, high-temperature flue gas recirculation can be installed to introduce low-temperature flue gas to lower the flame temperature and inhibit coking.
Burner design has a decisive influence on flame morphology. Traditional burners are prone to flame deflection or wall scouring, leading to localized overheating of the heating surface and exacerbating coking. Modern biomass boilers often use low-NOx burners. By optimizing the nozzle structure and adjusting the swirl intensity, the flame is evenly diffused, avoiding direct scouring of the water-cooled walls. Furthermore, optimizing the burner layout using numerical simulation technology can ensure good flame fill and minimize dead zones, effectively reducing the risk of coking.
The coordinated operation of the sootblowing system is crucial for maintaining clean heating surfaces. Even if coking tendencies are reduced through combustion control, regular ash removal is still necessary. Steam sootblower and sonic sootblower can regularly clean the heating surface to prevent ash accumulation and hardening. Linking sootblowing frequency with combustion parameters, such as increasing sootblowing frequency during load fluctuations or fuel changes, can further enhance coking prevention.
Operator skill is crucial for coking control. Combustion parameters must be adjusted promptly based on fuel characteristics and load changes to avoid parameter mismatches caused by empirical reasoning. For example, when switching fuel types, the air-to-fuel ratio must be recalibrated; during low-load operation, the primary air volume should be appropriately reduced to prevent excessively low flame temperatures. Establishing standardized operating procedures can reduce the risk of coking caused by human factors.
Coking in biomass boilers requires a combination of source control and process management through combustion control technology. From fuel pretreatment, air distribution optimization, temperature control, to equipment improvements and operational management, every step requires meticulous regulation. In the future, with the advancement of intelligent control technology, using sensors to monitor combustion conditions in real time and AI algorithms to dynamically optimize parameters, biomass boiler coking prevention will reach even higher levels.
