Development of an Online Ash-Deposition Thermogravimetric Analyzer for Pulverized Coal Combustion

A novel system consisting of an online ash-deposition thermogravimetric analyzer coupled with an electrically heated down-fired furnace was developed for pulverized coal combustion. A steady high-temperature pulverized coal combustion was achieved in the down-fired furnace. Ash deposition weights were online measured without interference of the deposition process. The developed setup was calibrated to quantify the system error and subsequently used to investigate the ash deposition behavior. First, the gas species, coal conversions, and particle size distributions were quantitatively measured under excess air of 1.2 (oxidizing atmosphere) and 0.7 (reducing atmosphere) under furnace temperatures of 1100-1400 degrees C. Next, the ash deposition rates were online measured under different surface temperatures (1000-1400 degrees C), oxidizing and reducing atmospheres (corresponding to excess air of 1.2 and 0.7, respectively), and gas velocities (0.89 to 2.37 m/s,). The deposition samples were collected through the N-2 protection method. The morphology and structural property of the sample were characterized by scanning electron microscope. From a low to a high temperature, it was found that the deposition transformed from a porous layer composed of loosely bond particles to a slag layer. The transition was found to be tightly related to the ash fusion temperature and the unburnt carbon fraction of the deposited particle. Finally, the reaction rate of unburnt carbon in the deposition was measured using a specially designed experiment. A deposition model was proposed to model the deposition process and derive the reaction rate. It was found that both the oxidation and the gasification reaction rates were a few orders of magnitude slower than those in pulverized coal combustion.