Date of Award
August 2024
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Energy Engineering
First Advisor
Daniel Laudal
Abstract
Low-rank solid fuels impact heat transfer efficiency and energy conversion in steam generation, with adverse operational and environmental consequences. Despite the attention coal-biomass co-combustion has garnered as a climate-aware power generation alternative, inorganic species in low-rank fuel types can lead to counterproductive outcomes in combustion systems. However, improved combustion technology and fuel quality enhancement can facilitate the safe utilization of these fuels. A review of scientific literature underscores the need for research in fuel quality improvement as a climate change mitigation strategy during the global transition to renewable energy sources. Identifying problematic species in solid fuels and understanding ash formation mechanisms are therefore critical steps toward designing effective strategies to control ash aerosol-induced fouling.
Methods for promoting clean combustion, including fuel pre-treatment and co-combustion include fuel improvement strategies, such as CFD-targeted-in-furnace injection (CFD-TIFI), intelligent soot blowing, and fuel additive technology, focusing on mineral additives. Due to its high efficiency, low cost, and minimal retrofit requirements, additive technology holds promise for large-scale applications, paving the way for fuel flexibility in utility-scale power generation. The current study emphasizes the importance of fuel ash mineral transformation and characterization techniques for evaluating the effectiveness of fuel additives. Assessing the economic viability and feasibility of large-scale demonstration will however require further research.
Emissions of alkali metals present a notable challenge in combustion systems. Combustion of alkali-rich fuel which leads to troublesome fouling of heat-transfer surfaces, diminishing boiler efficiency, and increasing maintenance expenses would have to be targeted for capture to limit the impacts of these problems. Interaction between sulfur-based compounds and alkali elements facilitates ash bonding and subsequent deposit accumulation on surfaces. While current approaches such as fuel blending and combustion monitoring offer partial mitigation, a more targeted strategy is needed to address alkali vapor release during combustion. This study reports laboratory-scale experiments conducted on a 10-kW down-fired combustor to demonstrate the efficacy of cost-effective clay-based sorbents in capturing vaporized alkali species released during high-sodium fuel combustion. The analysis of ash samples yielded important insights into ash chemical composition, size distribution, and the fate of alkali species, crucial for assessing their potential as fouling agents.Technical investigations were carried out in bench and large-scale demonstrations. Locally or commercially sourced sorbents were combusted with lignite coal from local mines to evaluate sorbent performance and respective capture capacity. The study explores the chemisorption and adsorption properties of various sorbents for sodium capture under high-temperature conditions and specific sodium-to-sorbent ratios. Sorbent properties were characterized using X-ray diffraction (XRD) spectroscopy and scanning electron microscopy (SEM) techniques to identify alkali-active components and their utilization for capture. Sorbents were ultimately characterized by key parameters such as performance, active composition, qualitative stickiness (based on AAEM composition), diluent composition, and ease of grinding.
Results from the simulated combustion environments indicate increased alkali (sodium) retention for all sorbents, with kaolin achieving the highest capture efficiency at 86%, significantly outperforming two commercially procured sorbents, which achieved 53% and 68%, respectively. Locally sourced sorbents showed varied performance, ranging from 31% to 55%. Despite lower reactivity compared to kaolin, locally sourced mine clays demonstrated promising potential due to their high bulk sodium numbers, capturing sodium in particles larger than 1 µm. Interestingly, sorbent capture efficiency did not directly correlate with specific surface area or mean particle size. SEM analysis of captured ash revealed the formation of alkali aluminosilicates, indicating a shift towards insoluble forms. A novel analytical method differentiated elemental sodium into soluble and insoluble forms using a size-aggregated Dekati Low-Pressure Impactor (DLPI+). This serves as crucial quality control for validating experimental results. Local sorbents tended to generate more fine particles under high-temperature conditions than the CS1 sorbent, with kaolin favoring larger particles.
Lab-scale coal/sorbent testing showed sorbents significantly capturing sodium-based submicron-size aerosols, achieving removal rates ranging from 40% to 77%. Coarse sorbent particle size did not hinder alkali affinity, and low-cost sorbent additives performed favorably compared to more expensive commercial alternatives. Further analysis revealed the formation of high-melting sodium aluminosilicates in both fine and coarse sorbents. Pre-experimental material characterization suggests sorbent alkali capture efficiency is primarily influenced by its active mineral composition, with potential additional effects from alkali and alkaline earth metals (AAEM).
Large-scale testing of sorbent injection at the power plant successfully demonstrated reductions in fouling factors, enhanced operational efficiency, and mitigated environmental impacts. Substantial decreases in sodium and sulfur concentrations within the sub-micron-sized fraction of entrained ash were observed at the electrostatic precipitator inlet. The medium injection rate exhibited the most effective performance. Improvements in cleanliness were noted across various boiler sections. Sorbent injection proved to optimize boiler operations and meet environmental regulations. The substantial decreases in sodium and sulfur levels demonstrate the sorbent's ability to improve boiler cleanliness and reduce fouling factors. Further research is needed to determine the extent of ash quality enhancement due to silica enrichment, with potential applications in combustion systems and construction materials.
Through laboratory-scale experiments and large-scale demonstrations, the efficacy of sorbent injection was demonstrated in reducing sodium and sulfur concentrations, thereby improving boiler cleanliness and offering environmental benefits. Moving forward, further research is needed to explore the economic viability and feasibility of large-scale implementation and assess the quality enhancement of resultant ash. These insights will pave the way for more sustainable and efficient utilization of low-rank fuels in the global energy landscape.
Recommended Citation
Bankefa, Temitope Folusho, "Mitigation Of Alkali-Induced Ash Aerosol Particles Impacts On Solid Fuel Combustion Systems" (2024). Theses and Dissertations. 6414.
https://commons.und.edu/theses/6414