Study on the Impact of Municipal Solid Waste Moisture Content Fluctuations on Incinerator Operating Parameters
Introduction
Fluctuations in the moisture content of municipal solid waste (MSW) are one of the key factors affecting the efficiency of incineration treatment. In China, kitchen waste accounts for more than 50% of MSW, with a typical moisture content of 50%–60%, which is significantly higher than the approximately 2% reported in developed countries. This high-moisture characteristic poses multiple challenges during the incineration process, including low calorific value, unstable combustion, and excessive pollutant emissions. Based on operational data from representative domestic waste incineration plants, this paper systematically analyzes the mechanisms by which moisture content fluctuations influence key incinerator parameters such as furnace temperature, flue gas composition, and combustion efficiency, and proposes targeted optimization strategies.
I. Mechanisms by Which Moisture Content Affects the Calorific Value of Waste
1.1 Quantitative Relationship of Calorific Value Degradation
The calorific value of waste exhibits a pronounced negative correlation with its moisture content. Experimental data indicate that for every 10% reduction in waste moisture content, the incineration calorific value can increase by more than 250 kJ/kg. Taking a medium-sized city as an example, the average moisture content of its municipal solid waste is 55.8%, while combustible components account for only 34.5%, resulting in a wet-basis lower heating value of less than 4,000 kJ/kg—far below the benchmark value of 6,000 kJ/kg required for stable incinerator operation. This degradation in calorific value directly weakens the self-sustaining combustion capability of waste as a fuel, forcing power plants to frequently add auxiliary fuels (such as coal) to maintain furnace temperature, thereby significantly increasing operating costs.
1.2 Effects of Calorific Value Fluctuations on the Combustion Stages
Municipal solid waste incineration comprises four main stages: drying, pyrolysis, combustion, and burnout, among which the drying stage accounts for as much as 60%–70% of total energy consumption. High-moisture-content waste requires additional heat input for water evaporation, resulting in a furnace temperature drop of approximately 200–300 °C and a slowdown in the pyrolysis reaction rate. A retrofit case of a 400 t/d circulating fluidized bed boiler showed that when the moisture content of incoming waste increased from 50% to 65%, combustion efficiency decreased by 18%, while the loss on ignition of bottom ash rose from 3.2% to 7.8%, indicating a significant deterioration in combustion completeness.
II. Dynamic Effects of Moisture Content Fluctuations on Incinerator Operating Parameters
2.1 Reconfiguration Effects on the Furnace Temperature Field
Furnace temperature is a core indicator of combustion stability. According to the Standard for Pollution Control on the Municipal Solid Waste Incineration (GB 18485–2014), the temperature in the secondary combustion chamber must be continuously maintained above 850 °C. However, when high-moisture-content waste is fed into the furnace, the heat absorbed by moisture evaporation leads to a reconfiguration of the furnace temperature field:
Changes in temperature gradients: The temperature in the drying zone may decrease by 150–200 °C, causing the pyrolysis zone to shift upstream and the combustion zone to move downstream, thereby forming a “low-temperature, long-flame” combustion mode.
Intensified temperature fluctuations: Operational data from a waste incineration plant indicate that when moisture content fluctuates by ±5%, the furnace temperature fluctuation range expands to ±80 °C, exceeding the adjustment threshold of the automatic control system.
Shortened residence time in high-temperature zones: The steam generated during moisture evaporation accelerates flue gas flow, reducing the residence time above 850 °C by 0.5–1.2 seconds, which directly affects dioxin decomposition efficiency.
2.2 Vicious Cycle in Flue Gas Composition
Moisture content fluctuations alter combustion conditions and trigger a chain reaction in flue gas composition:
Sharp increase in CO concentration: Insufficient drying delays the release of volatiles, and localized oxygen-deficient combustion generates large amounts of CO. Field measurements at a waste-to-energy plant show that when moisture content increased from 50% to 60%, flue gas CO concentration rose from 50 mg/m³ to 200 mg/m³—three times higher than the emission limit.
Excessive emissions of acidic gases: Chlorine and sulfur elements in kitchen waste are more prone to forming HCl and SO₂ under low-temperature combustion conditions. For every 10% increase in moisture content, HCl emission concentrations rise by 15%–20%, accelerating corrosion of heat transfer surfaces in waste heat boilers.
Increased risk of dioxin formation: Under low-temperature combustion conditions, chlorine-containing species more readily form dioxin precursors via the Deacon reaction. Studies indicate that when furnace temperatures fall below 800 °C, dioxin formation increases exponentially.
2.3 Quantified Degradation of Combustion Efficiency
Combustion efficiency (η) can be expressed as:
η=1-(CO₂+CO)/C_total
Moisture content affects combustion efficiency through three main pathways:
Reduced heat transfer efficiency: Steam films formed during moisture evaporation hinder heat transfer, causing the surface temperature of waste to be 50–80 °C lower than theoretical values.
Impeded oxygen diffusion: Steam occupies furnace volume and reduces the effective oxygen concentration. Experiments in a circulating fluidized bed boiler show that at a moisture content of 60%, oxygen utilization efficiency decreases by 22% compared with dry waste.
Increased combustible content in fly ash: High moisture content shifts the combustion zone downstream, allowing partially unburned particles to be entrained by flue gas, increasing the carbon content in fly ash from 3% to 8%.
III. Optimization Strategies for Addressing Moisture Content Fluctuations
3.1 Development of Off-Site Pretreatment Systems
Source reduction:Promote waste segregation and separate collection of kitchen waste. In a pilot zone in Shanghai, implementation of waste classification reduced the moisture content of incoming waste from 58% to 42%, increasing the calorific value by 35%.
Mechanical dewatering: Installing screw presses at transfer stations can reduce waste moisture content by 15%–20%. After adopting this technology, the Tongxing Waste Incineration Plant in Chongqing achieved a 10% increase in power generation per ton of waste.
Sealed transportation: Fully enclosed, compressed transport vehicles prevent rainwater infiltration and leachate leakage. After switching to sealed vehicles, one city reduced the moisture content increase during waste transfer from 8% to 2%.
3.2 In-Plant Process Optimization and Upgrading
Enhanced bunker fermentation: Extending waste residence time in storage bunkers to more than 72 hours, combined with waste turning operations, can increase leachate release by approximately 30%.
Graded feeding systems: Adopting a dual-screw feeder combined with belt scales enables precise control of feed rates (error < 2%). After retrofit, one power plant reduced furnace temperature fluctuations to within ±30 °C.
Intelligent combustion control: Deploying machine-learning-based combustion optimization systems allows real-time adjustment of primary/secondary air ratios. Test results show that such systems can improve combustion efficiency by 5%–8% and reduce NOx emissions by 15%.
3.3 Equipment Adaptability Retrofits
Grate structure optimization: Adopting a segmented grate design with an independent drying zone at the front end, equipped with high-temperature air preheaters (350–400 °C), enhances moisture evaporation efficiency.
Anti-corrosion treatment of heat transfer surfaces: Applying nickel-based alloy coatings to water walls of waste heat boilers can extend equipment service life from 5 years to more than 10 years.
Upgrading auxiliary fuel systems: Retrofitting to a biomass gasification–coupled combustion system enables the use of combustible gases generated from waste itself to replace coal, reducing operating costs by 20%–30%.
Conclusion
Fluctuations in waste moisture content exert systematic effects on incinerator operating parameters through pathways such as calorific value degradation, reconfiguration of the furnace temperature field, and deterioration of flue gas composition. Practical experience demonstrates that establishing a pretreatment framework encompassing “source separation–mechanical dewatering–sealed transportation,” combined with in-plant optimization measures including “enhanced bunker fermentation–intelligent control–equipment retrofitting,” can effectively mitigate the impacts of moisture content variability. Looking ahead, further development of digital-twin-based incineration process simulation platforms is required to achieve dynamic, coupled optimization of moisture content, calorific value, and combustion parameters, thereby promoting the transformation of the waste incineration industry toward higher efficiency and lower carbon emissions.