Date of Award
May 2024
Document Type
Thesis
Degree Name
Master of Science (MS)
Department
Chemical Engineering
First Advisor
Gautham Krishnamoorthy
Abstract
Coal production has waned in recent years despite its historical significance, driven by mounting concerns over climate change and the escalating adoption of renewable energy sources. With coal production in the U.S. expected to continue its decline and stricter environmental regulations looming due to increasing greenhouse gases and SO2 emissions, there is a critical need to identify an economically viable and environmentally friendly coal type. Embracing carbon capture-based technologies such as oxy-fuel combustion without compromising boiler performance is essential in this endeavor. Sub-bituminous coal varieties like Powder River Basin (PRB) coal boast lower sulfur and nitrogen content compared to bituminous coals, resulting in reduced sulfur and NOX emissions upon combustion. However, transitioning to PRB coal presents a significant challenge in the form of severe slagging and fouling on boilers, which may be exacerbated by temperature and velocity fluctuations associated with oxy-combustion processes.The primary objective of this study was to deepen our understanding of the ash deposition process associated with PRB coal, with the aim of better monitoring, predicting, and mitigating boiler fouling/slagging during practical operations. This endeavor involved investigating four turbulence models (K-epsilon, K-Omega, Detached Eddy Simulation (DES), and Large Eddy Simulation (LES)) to assess their impact on deposition rates on both the top and bottom surfaces of the probe, with a particular focus on the OXY80 case, which has not been extensively studied. To achieve this objective, a computational fluid dynamics (CFD)-based simulation methodology was developed and refined to align with well-controlled ash deposition experimental data collected by researchers at The University of Utah's Department of Chemical Engineering and Institute for Clean and Secure Energy. Four distinct scenarios—AIR, OXY27, OXY50, and OXY80—were examined in this thesis. These scenarios entail varying levels of oxygen enrichment in the oxidizer stream (vol %) and are reflective of conditions encountered in first- and second-generation atmospheric pressure oxy-combustion units. First, combustion simulations were conducted to ensure accurate representation of gas temperatures and velocities within the oxyfuel combustor (OFC) by comparing against experimental estimates. After ascertaining this, decoupled simulations of the ash deposition process on a deposit probe were performed. Results indicated that, alongside gas velocities and temperatures, the particle size distribution (PSD), composition, and concentration of fly ash significantly influenced deposition rates. The capture criterion was based on critical sticking viscosity and particle kinetic energy (PKE). The investigation first focused on how the choice of turbulence models affected deposition rates on the top surface of the probe, attributed to inertial impaction. Notably, overall deposition rates across different scenarios remained unaffected by turbulence models. The dominance of deposits from 50 and 125 μm ash particles, with less than 5% from 10 μm particles, indicated non-homogeneous flow inside the OFC. The OXY80 case exhibited a distinct deposit PSD due to the expected formation of larger particles with the longest residence time. Notably, the total deposition rate, particularly for OXY80, displayed significant sensitivity to temperature variations between 1100 K and 1300 K, suggesting flue gas temperature control as a cost-effective mitigation strategy. The investigation then examined how turbulence models affected deposition rates on the bottom surface of the probe, attributed to eddy impaction. Eddy impaction minimally impacted the overall deposition rate, contributing approximately 1.1% of the total deposition rate, with significant contributions from particles sized 1 and 5 μm. The LES model demonstrated superior accuracy in predicting ash deposition rates, contrasting with earlier results. This implies the necessity of unsteady-state models for accurately tracking the path of smaller particles. Notably, the OXY80 case exhibited the highest deposition rate, underscoring the importance of residence time in ash deposition on the probe's leewardside.
Recommended Citation
Zamzam, Mohamed, "Modeling Ash Deposition During Oxy-Coal Combustion Of A High Alkali Earth Metal (Powder River Basin) Coal: Insights From Turbulence Modeling" (2024). Theses and Dissertations. 6402.
https://commons.und.edu/theses/6402