Date of Award
Doctor of Science (DSc)
In the past century, coal contributed more to the global electricity supply than any other source. Currently, it alone accounts for over 35% of the global electricity supply. Though coal is globally well-distributed and can provide cheap, stable, and reliable energy on demand, it emits a large amount of carbon dioxide—a greenhouse gas responsible for global warming. Additionally, it is a major source of particulate matter (PM) pollution. Thus, in recent years, to reduce the impact of coal on climate, several policies have been introduced to phase out coal. However, replacing coal with intermittent renewables has led to a reduction in the reliability of the electricity grid. To protect the electricity grid, dispatchable, low-carbon sources are required, which will largely come from sustainable, stable, and reliable fossil-fuel sources. Based on the projections from the International Energy Agency (IEA), coal will continue to be a major source of energy in the long term due to its abundance and competitively low prices. The need of the hour is to focus on addressing the key challenges of coal combustion by developing technologies for sustainable coal utilization. This approach will help in harnessing abundant and cheap coal resources without compromising the environment.
This dissertation investigates two key pathways for sustainable coal combustion: Improvement in the understanding of fundamental coal combustion processes, and the development of new coal combustion technologies.
In the first pathway, the fundamentals of coal combustion processes were investigated at a lab-scale level. The early-stage coal combustion processes such as submicron PM formation, and particle ignition were studied in controlled combustion environments, similar to those experienced by coal particles in near-burner regions of industrial coal combustors. In industrial coal combustors, the coal particles experience either oxidizing environments or reducing-to-oxidizing environments at different oxygen concentrations. Studies that mimic the industrial coal combustors in controlled lab-scale environments are rare. Thus, in order to fill the research gap, a two-stage Hencken burner was used in this study to simulate the practical coal combustion environments. Additionally, the instrumentation for a better understanding of submicron PM formation and particle temperatures were developed and calibrated. These studies provide valuable insights into optimizing advanced burner designs, in-flame control of PM formation, improving combustion efficiency, and heat transfer to boiler tubes.
It was observed that ash evolution, soot formation, and soot oxidation are the governing processes for submicron PM evolution during the early stages of pulverized coal combustion. The submicron total PM and submicron soot PM are the first-order functions of ambient oxygen concentration and gas temperature, whereas submicron ash PM is the first-order function of coal particle temperature. In high-temperature environments, when particles undergo reducing-to-oxidizing transition, the critical factor for ignition is the presence of oxygen, not the amount of oxygen.
In the second pathway, this dissertation presents the development of a dry-feed pressurized oxy-combustion pilot-scale combustor as well as a high pressure-high temperature particle sampler to demonstrate combustion characteristics of a promising future coal combustion technology - Staged Pressurized Oxy-Combustion (SPOC). The SPOC process, developed by Washington University in St. Louis, is an oxy-coal combustion technology that employs coal combustion at elevated pressure. The technology allows for carbon dioxide (CO2) capture rates of 90% or higher with significant improvements in efficiency and costs for sustainable coal utilization. The developed pressurized oxy-combustion pilot-scale combustor was used to examine pulverized coal flame stability and shape, coal particle flame temperature, char burnout, and submicron PM formation at SPOC design conditions. The understanding of these coal combustion characteristics is important for the commercialization of SPOC technology.
The experiments establish that the pressurized oxy-combustion pilot-scale combustor has exceptionally good coal flame stability at SPOC design conditions. The coal particle flame temperatures are highest near the burner and decrease downstream of the combustor. Complete char combustion can be achieved with only 0.8 vol % oxygen mole fraction in the flue gas, compared with a minimal value of 2.5 vol % in conventional atmospheric pressure pulverized coal combustors. The number particle size distribution (PSD) of submicron PM exhibits two distinct modes, ultrafine and intermediate mode; however, with increasing residence time, the ultrafine mode peak decreases, and the intermediate mode peak values decline steadily and become stable.
The experimental observations in this dissertation aim to complement the development of existing and future sustainable coal combustion technologies.
Pratim Biswas, Rajan Chakrabarty, Brent Williams, Xuebin Wang,
Available for download on Tuesday, August 13, 2024