Key Takeaways & Executive Findings
- •• • The sludge combustion process is divided into four distinct stages with activation energies of 52.24, 48.81, 192.38, and 102.27 kJ/mol for moisture evaporation, volatile combustion, fixed carbon combustion, and burnout, respectively; the high activation energy for fixed carbon combustion (192.38 kJ/mol) indicates a significant energy barrier, necessitating optimized temperature control to ensure complete combustion. • • The ignition temperature of 220.7 °C and burnout temperature of 605.9 °C, combined with a comprehensive combustion characteristic index of 6.32×10⁻⁸ %²/(min²·K³), demonstrate good ignitability and burnout performance, supporting the feasibility of incineration for energy recovery from high-ash municipal sludge. • • Kinetic analysis validated by two independent methods (CR and ABS) with deviations <15% confirms that moisture evaporation and volatile combustion follow first-order kinetics (F1), while fixed carbon combustion and burnout follow second-order kinetics (F2); this mechanistic distinction is critical for accurate reactor design and residence time estimation. • • Thermodynamic analysis shows negative entropy changes (ΔS: -148.36 to -30.74 J/mol·K) and positive Gibbs free energy (ΔG: 105.76 to 271.04 kJ/mol) across all stages, indicating that the incineration process is non-spontaneous and requires continuous external energy input, with the highest energy demand (ΔG=271.04 kJ/mol) in the burnout stage, which must be addressed to improve energy efficiency.
Abstract
To optimize municipal sludge incineration and enhance disposal efficiency, sludge from the First Sewage Treatment Plant in Chengdu was analyzed via synchronous thermal analysis (TG-DTG-DSC) in air at 20 K/min. The combustion process comprised four stages: moisture evaporation (35–150 °C), volatile combustion (150–400 °C), fixed carbon combustion (400–600 °C), and burnout (600–1000 °C). Ignition and burnout temperatures were 220.7 °C and 605.9 °C, respectively, with a comprehensive combustion characteristic index of 6.32×10⁻⁸ %²/(min²·K³), indicating good stability. Kinetic analysis using Coats-Redfern (CR) integral and Achar-Brindley-Sharp (ABS) differential methods showed deviations below 15%, confirming CR reliability. Moisture evaporation and volatile combustion followed first-order models (F1) with activation energies of 52.24 and 48.81 kJ/mol, while fixed carbon combustion and burnout followed second-order models (F2) with activation energies of 192.38 and 102.27 kJ/mol. Thermodynamic parameters (ΔH: 49.06, 43.59, 185.94, 95.00 kJ/mol; ΔS: -148.36, -211.63, -30.74, -201.38 J/mol·K; ΔG: 105.76, 176.52, 209.74, 271.04 kJ/mol) revealed negative entropy and positive Gibbs free energy across all stages, indicating external energy dependence, with the highest demand in the burnout stage. These findings provide a theoretical basis for optimizing incineration process parameters and energy recovery.
1. Introduction
Municipal sludge management in China faces a critical bottleneck: while wastewater treatment capacity has expanded rapidly, with treatment rates reaching 98.11% by 2022, the harmless disposal rate of sludge remains only 66%. This disparity creates an urgent need for efficient terminal treatment technologies. Incineration offers the most significant volume reduction and energy recovery potential, yet its adoption is hindered by high capital and operational costs, primarily due to energy-intensive drying and combustion processes. The high ash content and complex composition of municipal sludge further complicate combustion, leading to incomplete burnout and elevated energy consumption.
This study addresses these challenges by systematically characterizing the combustion kinetics and thermodynamics of municipal sludge from a full-scale wastewater treatment plant in Chengdu. Using synchronous thermal analysis, the research delineates the four-stage combustion process and quantifies the activation energies and thermodynamic parameters for each stage. By validating the Coats-Redfern integral method against the Achar-Brindley-Sharp differential method, the study provides reliable kinetic data essential for optimizing incinerator design and operation. These findings directly inform temperature control, air distribution, and heat recovery strategies, offering a pathway to enhance combustion efficiency and reduce energy penalties in real-world applications.
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LI Tenghao, WANG Yin, LIU Bo, ZHANG Shan, BAI Xue (2026). Kinetic and Thermodynamic Analysis of Municipal Sludge Combustion Characteristics. Journal of Environmental Engineering Technology. https://doi.org/10.13205/j.hjgc.202607005
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Frequently Asked Questions
What are the specific activation energies for each combustion stage, and how do they influence reactor design?
The activation energies are 52.24 kJ/mol (moisture evaporation), 48.81 kJ/mol (volatile combustion), 192.38 kJ/mol (fixed carbon combustion), and 102.27 kJ/mol (burnout). The high activation energy for fixed carbon combustion indicates a kinetically limited step, requiring higher temperatures or longer residence times to ensure complete burnout. Reactor design must account for this by providing sufficient temperature and residence time in the high-temperature zone.
How does the comprehensive combustion characteristic index (6.32×10⁻⁸ %²/(min²·K³)) compare with typical values for other solid fuels, and what does it imply for process stability?
This index is relatively low compared to coal or biomass, reflecting the high ash content and lower reactivity of municipal sludge. However, the low ignition temperature (220.7 °C) and moderate burnout temperature (605.9 °C) indicate acceptable combustion stability. The index suggests that the sludge can be burned reliably, but process control must be robust to handle variability in feed composition.
What are the implications of the negative entropy changes and positive Gibbs free energy for the energy balance of an incinerator?
Negative entropy changes (ΔS from -30.74 to -211.63 J/mol·K) indicate a decrease in disorder, while positive ΔG values (105.76 to 271.04 kJ/mol) confirm that all stages are non-spontaneous and require external energy. The burnout stage has the highest ΔG (271.04 kJ/mol), meaning it is the most energy-intensive. This necessitates efficient heat recovery and possibly auxiliary fuel support to maintain temperatures above 600 °C for complete burnout.
How reliable are the kinetic parameters obtained from the Coats-Redfern method, and what is the significance of the deviation being less than 15%?
The Coats-Redfern integral method was validated against the Achar-Brindley-Sharp differential method, with deviations in activation energy below 15% for all stages. This cross-validation confirms the reliability of the kinetic models, ensuring that the derived parameters are robust for engineering calculations. The small deviation also indicates that the chosen reaction models (F1 and F2) accurately represent the combustion mechanisms.
What operational parameters should be adjusted to optimize the incineration process based on these findings?
To optimize incineration, operators should maintain temperatures above 600 °C to ensure complete burnout, as indicated by the burnout temperature. The high activation energy for fixed carbon combustion (192.38 kJ/mol) suggests that a longer residence time in the high-temperature zone is beneficial. Additionally, air supply should be staged to match the oxygen demand of volatile and fixed carbon combustion, and heat recovery systems should be designed to capture the significant heat released during volatile and fixed carbon combustion stages.
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