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 °C. Next, the ash deposition
rates were online measured under different surface temperatures (1000–1400
°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 N2 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.