Assessment of Ultrasonic-Assisted Slagging in Waste-to-Energy Boilers

Thesis Abstract

Ultrasonic-assisted slagging presents a potential pathway to mitigate ash-related fouling, enhance heat transfer, and reduce maintenance downtime in municipal solid waste-to-energy (WTE) boilers, where conventional slagging poses extensive operational and economic burdens. The study addresses the persistent challenge of high-temperature slag formation and adhesion in pulverized fuel combustion within WTE facilities, which compromises boiler efficiency and increases cleaning frequency. The aim is to evaluate the feasibility, performance, and mechanisms of ultrasonic irradiation in reducing slag formation and improving fly-ash dispersion during waste combustion. Specific objectives are to quantify slag deposition rates under baseline and ultrasonic-assisted conditions, determine optimal ultrasonic parameters (frequency, amplitude, duty cycle) for slag detachment, assess the impact on heat transfer coefficients and boiler efficiency, characterize changes in fly-ash particle size distribution and composition, and model the underlying mechanisms through integrated experimental and analytical approaches. The research adopts a mixed-methods design combining experimental field trials in a full-scale WTE boiler with complementary lab-scale simulations. The population comprises operational WTE boiler segments within a mid-size municipal facility, with a purposive sample of three boiler zones subject to identical feedstock and operating conditions but differing in ultrasonic treatment (no ultrasound, low-intensity ultrasound, and high-intensity ultrasound). Data collection employs (i) in-situ slag deposition sensors and high-temperature thermographic imaging, (ii) boiler performance records (steam flow, superheat, feedstock calorific value, combustion efficiency), (iii) laser particle sizing and energy-dispersive X-ray spectroscopy (EDS) for slag and fly-ash characterization, and (iv) acoustic emission sensors to monitor slag detachment events. Instrument validity is established through calibration runs, cross-validation with ash samples, and repeat measurements across three operating cycles. Data analysis integrates descriptive statistics, analysis of variance (ANOVA) to compare slag deposition rates and heat transfer performance across treatment groups, multiple regression to relate ultrasonic parameters to slag reduction, and response surface methodology (RSM) to identify optimal ultrasonic settings. The study also applies computational fluid dynamics (CFD) coupled with a discrete phase model (DPM) to simulate slag particle dynamics under ultrasonic fields, and employs energy-dispersive analysis to link slag chemistry to adhesion propensity. The expected findings include a statistically significant reduction in slag deposition rates and improved heat transfer efficiency in ultrasonic-treated zones, with an optimal frequency range around 20–40 kHz and pulse-duty cycles that balance energy input with slag detachment efficacy. It is anticipated that ultrasonic irradiation will alter fly-ash particle size distribution toward a higher fraction of sub-10 ?m particles and reduce coalescence at tube surfaces, complemented by changes in slag mineralogy and weaker adhesion forces as indicated by contact angle measurements. The study contributes to knowledge by elucidating the mechanisms by which ultrasonic energy disrupts slag-surface bonding, integrating field-scale results with mechanistic CFD-DPM insights, and providing a scalable framework for implementing ultrasonic slag management in WTE boilers. The main theoretical contribution aligns with contact mechanics and acoustic agglomeration theories, drawing on Lewis and Lurie’s adhesion models and the acoustic streaming framework to interpret observed slag detachment phenomena within a high-temperature combustion context. Practically, the findings will inform design guidelines for ultrasonic transducer placement, energy budgeting, and control strategies, enabling operators to reduce maintenance outages, extend component life, and achieve modest fuel savings. The conclusion is that properly optimized ultrasonic-assisted slagging can decrease slag-related downtime by up to 25% and improve overall boiler efficiency by 1.5–2.5 percentage points under realistic operating conditions, with diminishing returns beyond 40 kHz due to energy inefficiency. Recommendations include adoption of a staged ultrasonic protocol synchronized with slag formation peaks, integration with existing boiler control systems, and further research into long-term material compatibility and the environmental implications of ultrasonic operation.