| Innovative coal technologies are essential for addressing concerns about air pollution and global climate change. A key pathway to advancing these technologies is through developing a thorough understanding of the fundamental physical and chemical processes that occur during coal combustion. Ignition influences many aspects of coal combustion, including flame stability, submicron aerosol evolution, and char burnout. As important as ignition and these associated processes are, they are challenging to study because they depend on many factors, such as the combustion environment, particle size, and particle-particle interactions.;While there have been many studies of coal ignition, none have studied the process in a way that simulates the coal particles going from a reducing to an oxidizing environment, which is characteristic of what coal particles experience in the near-burner region of pulverized coal furnaces. Fundamental studies of the impacts of this "reducing-to-oxidizing" environment on ignition as a function of residence time, gas temperature, and particle size can provide valuable insights for optimizing advanced burner design.;In addition to ignition, ultrafine aerosols are mainly formed in the near-burner region of pulverized coal furnaces where temperature is high, and a better understanding of the formation mechanisms in the reducing-to-oxidizing environment can aid the development of in-flame control of particulate formation. Thus, in this dissertation, a new experimental platform, called a two-stage Hencken flat-flame reactor, is designed and fabricated for evaluating coal ignition and aerosol formation in conditions relevant to pulverized coal furnaces with respect to timescales, gas temperatures, and combustion environments...;First, a proof-of-concept study is performed to evaluate the influence of the "reducing-to-oxidizing" environment (R-O) on single particle ignition. The proof-of-concept study is repeated for single particles experiencing an oxidizing environment only (O). The results show that the R-O affects single particle ignition significantly. Specifically, at a high gas temperature of 1800 K, a hetero-homogeneous mechanism is promoted in the R-O while a homogeneous-to-heterogeneous mechanism prevails in the oxidizing environment. For 1300 K gas temperature environment, it is found that volatiles are released mainly after the particle has transitioned to the oxidizing environment, thus promoting homogeneous ignition. Due to the R-O, average ignition delay times for single particles in nominal 1300 K and 1800 K gas temperatures increase over those of O by 20% and 40% respectively.;Next, the role of different particle sizes on single particle ignition is studied. Unique to the R-O, ignition delay times for particles above 106 microm size are found to be similar in 1800 K gas temperature. The similarity is due to high volatile fluxes from such large particles. Hence, homogeneous ignition occurs as soon as such a particle with its volatile transitions to the oxidizing environment from the reducing environment. In a 1300 K gas temperature environment (lower heating rate), the ignition delay times in O and R-O are similar for the same particle size range because significant volatile release occurs after the reducing zone. For the various particle sizes studied, gas temperature in O has a first order effect on single particle ignition, reducing ignition times in an 1800 K environment by a factor up to 5 times over those in 1300 K environment... |
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