How to Avoid Low-Temperature Slagging During the Start-up Phase of Waste Incinerators
Low-temperature slagging is a common operational risk during the start-up phase of waste incinerators. Essentially, it occurs when moisture, organic matter, and ash in municipal waste undergo adhesion and agglomeration under low-temperature conditions, forming hard clinker deposits. Such slagging not only reduces incineration efficiency but may also trigger a series of operational problems, including equipment shutdowns and pipeline blockages.Drawing on practical cases from domestic and international waste-to-energy plants as well as relevant technical literature, this paper systematically analyzes the formation mechanisms of low-temperature slagging and proposes a comprehensive solution covering the entire operational chain—from fuel pretreatment and combustion control to equipment optimization and intelligent monitoring.
1. Causes and Hazards of Low-Temperature Slagging
1.1 Physicochemical Mechanisms of Low-Temperature Slagging
Low-temperature slagging mainly occurs within the 300–500 °C temperature range. Its primary mechanisms include the following:
Moisture Evaporation and Adhesion: When the moisture content of municipal waste exceeds 50%, the steam generated during evaporation combines with ash particles to form a viscous colloidal substance. Measurements from one waste incineration plant show that for every 10% increase in moisture content, the risk of slagging increases by approximately 35%.
Organic Matter Pyrolysis and Carbonization: Organic materials in waste, such as plastics and paper, undergo pyrolysis at low temperatures, producing viscous substances such as bitumen-like compounds and tar. One study indicates that when the proportion of plastics exceeds 20%, the slagging rate increases by two to three times.
Ash Softening and Deposition: Components in waste ash, including calcium oxide (CaO) and silicon dioxide (SiO₂), may undergo phase changes at 400–500 °C, forming low-melting-point eutectic compounds. Ash analysis from a particular plant shows that when the CaO content exceeds 15%, the slagging temperature decreases to approximately 420 °C.
1.2 Hazards of Low-Temperature Slagging
Reduction in Thermal Efficiency: For every 1 mm increase in slag layer thickness, the furnace heat transfer efficiency decreases by approximately 5–8%.
Equipment Damage: Detached clinker blocks may damage critical components such as the grate and water-cooled walls. In one case, slagging caused the grate to seize, resulting in maintenance costs of up to 500,000 RMB.
Operational Interruptions: Severe slagging requires shutdown for cleaning, and each cleaning cycle typically takes 48–72 hours, directly reducing power generation output.
2. Fuel Pretreatment: Controlling Slagging Risks at the Source
2.1 Waste Classification and Composition Optimization
Reducing High-Moisture Waste: Kitchen waste can be treated separately to reduce the moisture content of waste entering the furnace. After implementing waste classification in one plant, the moisture content of incoming waste decreased from 55% to 45%, resulting in a 40% reduction in slagging incidents.
Controlling Large Particles: Waste particle size can be controlled below 100 mm using crushers to prevent large particles from forming nuclei for slagging. Measurements from one plant show that for every 10% reduction in waste particles larger than 150 mm, the slagging risk decreases by approximately 25%.
2.2 Waste Fermentation and Mixing
Extending Fermentation Time: Waste can be stored in the bunker for 5–7 days to allow microbial activity to reduce moisture content and increase calorific value. In one plant, the calorific value increased from 4000 kJ/kg to 5500 kJ/kg after fermentation, significantly improving combustion stability.
Mixing High- and Low-Calorific Waste: High-calorific industrial waste (such as wood and paper) can be mixed with low-calorific municipal waste at a 1:3 ratio to maintain stable furnace temperatures. After mixing, one plant reduced the furnace temperature fluctuation range from ±100 °C to ±30 °C.
2.3 Application of Additives
Combustion Promoters: Additives such as limestone and dolomite can be used to increase the ash melting point. After adding 5% limestone, one plant observed that the ash melting point increased from 1100 °C to 1250 °C.
Anti-Slagging Agents: Materials such as diatomite and bentonite can be used to reduce ash adhesion. In one plant, the addition of 3% diatomite reduced the slag layer thickness by approximately 60%.
3. Combustion Process Control: Dynamic Optimization of the Furnace Environment
3.1 Temperature Management During the Start-Up Stage
Stepwise temperature increase: The temperature is raised in stages according to the sequence “drying zone (300–400 °C) → combustion zone (500–600 °C) → burnout zone (700–800 °C)”, avoiding sudden temperature fluctuations. After adopting stepwise heating, the slagging rate in one plant decreased by 50%.
Temperature monitoring and feedback: Infrared thermometers are installed in key areas of the furnace to monitor temperature gradients in real time. Data from one plant show that when the temperature gradient exceeds 50 °C/m, the risk of slagging increases threefold.
3.2 Optimization of the Air Distribution Strategy️
Coordination of primary and secondary air: At the early stage of start-up, primary air dominates (about 70% of total air volume) to ensure effective drying of the waste. After stable combustion is achieved, secondary air is gradually increased (about 30%) to enhance turbulence and mixing. After optimizing the air distribution, the CO concentration in flue gas decreased from 800 mg/Nm³ to 200 mg/Nm³ in one plant.
Oxygen concentration control: The oxygen concentration at the furnace outlet should be maintained at 6%–8% to avoid a reducing atmosphere. Measurements from one plant show that when the oxygen level drops below 4%, the slagging rate doubles.
3.3 Fuel Bed Thickness and Pushing Control
Layered waste feeding: The fuel bed thickness should be controlled differently in each furnace zone:
Drying zone: 1200–1300 mm
Combustion zone: 700–800 mm
Burnout zone: 300–400 mm
After implementing layered feeding, one plant improved furnace temperature uniformity by 40%.
Intermittent pushing: A “short stroke, high frequency” pushing strategy is adopted to prevent excessive accumulation of the fuel bed. After increasing the pushing frequency from once every 10 minutes to once every 5 minutes, the slagging risk was reduced by 30%.
4. Equipment Optimization and Maintenance: Enhancing Anti-Slagging Capability
4.1 Furnace Structure Improvement
Flow field optimization: Guide plates are installed at the furnace throat to reduce flue gas dead zones. After modification in one plant, the uniformity of flue gas velocity increased by 25%, and the slag layer thickness decreased by 40%.
Upgrade of refractory materials: Zirconate ceramic coatings are applied to improve slagging resistance. In one plant, the coating service life reached 3–5 years, and the cleaning interval was extended to once per year.
4.2 Auxiliary Equipment Upgrading
Steam soot blowers: Steam soot blowers are installed in slag-prone areas such as the furnace and flue ducts to periodically remove ash deposits. In one plant, the soot blowing frequency increased from once per shift to once every two hours, reducing slag accumulation by 50%.
Mechanical slag removal devices: Equipment such as slag removal shovels and slag breakers are installed to mechanically remove stubborn slag blocks. In one plant, the slag removal efficiency increased by 60%, and manual cleaning time was reduced to 8 hours per operation.
4.3 Regular Maintenance and Inspection
Thermocouple calibration: Thermocouples are calibrated monthly to ensure accurate temperature measurements. After calibration in one plant, the temperature deviation decreased from ±50 °C to ±10 °C.
Air leakage inspection: Quarterly inspections are conducted on the sealing performance of the furnace and flue ducts to repair air leakage points. After repairing leakage points in one plant, the furnace temperature stability improved by 30%.
5. Intelligent Monitoring and Emergency Response: Establishing a Closed-Loop Management System
5.1 Real-Time Monitoring and Early Warning
Multi-parameter integrated monitoring: Data such as temperature, oxygen concentration, pressure, and fuel bed thickness are integrated to establish a slagging risk early-warning model. In one plant, the model achieved a prediction accuracy of 90%, and the warning response time was reduced to 5 minutes.
Image recognition technology: High-definition cameras are installed inside the furnace, and AI algorithms are used to identify slagging patterns. A pilot project in one plant showed that the image recognition accuracy reached 85%, with a false alarm rate below 10%.
5.2 Emergency Response Plans
Rapid cooling and slag removal: When the slagging risk exceeds the threshold, an emergency cooling procedure (such as injecting cooling water) is initiated, followed by mechanical slag removal. After emergency handling in one plant, the shutdown time was reduced from 72 hours to 24 hours.
Fuel switching: In emergency situations, the fuel can be switched to high-calorific-value fuels (e.g., diesel) to quickly increase furnace temperature. After fuel switching in one plant, the slag layer softened and detached within 2 hours.
6. Future Technology Development Directions
6.1 Development of New Anti-Slagging Materials
Self-cleaning coatings: Coating materials with superhydrophobic and superoleophobic properties are being developed to enable automatic slag detachment. Tests by a research institute show that the coating contact angle reaches 160°, reducing slag adhesion by 90%.
High-temperature-resistant alloys: Alloys with melting points above 1500 °C are being developed to enhance the anti-slagging performance of furnace structures.
6.2 Intelligent Combustion Systems
Digital twin technology: A digital twin model of the furnace combustion process is developed to achieve real-time prediction and optimization of slagging risks. A pilot project shows that the prediction accuracy reached 95%, and the optimization response time was reduced to 1 minute.
Adaptive control algorithms: Based on reinforcement learning algorithms, parameters such as air volume, feeding rate, and temperature are dynamically adjusted. After implementation in one plant, the slagging rate decreased by 70%.
6.3 Cross-disciplinary Technology Integration
Plasma technology: Plasma torches are introduced into the furnace to decompose slagging substances using high-temperature plasma. Laboratory tests show that the hardness of the slag layer decreased by 80% after plasma treatment.
Microwave heating: Microwave technology is used to selectively heat the slag layer, enabling rapid softening and detachment. A research project demonstrated that microwave treatment efficiency is five times higher than traditional methods.
Conclusion
The prevention and control of low-temperature slagging during the start-up phase of waste incinerators should follow a coordinated strategy of fuel pretreatment, combustion control, equipment optimization, and intelligent monitoring. Through measures such as waste classification, fermentation and mixing, staged temperature ramp-up, intelligent air distribution, equipment upgrades, and real-time monitoring, the risk of slagging can be effectively controlled.
In the future, with breakthroughs in new anti-slagging materials, intelligent combustion systems, and cross-disciplinary technologies, the start-up efficiency and operational stability of waste incinerators will be further improved, providing more reliable technical support for urban solid waste treatment.
Source:https://mp.weixin.qq.com/s/7yAUDrg7F9K5Hcjfh24wTA