Modeling the Use of Sulfate Additives for Potassium Chloride Destruction in Biomass Combustion

Potassium chloride, KCl, formed from
biomass combustion may lead
to ash deposition and corrosion problems in boilers. Sulfates are
effective additives for converting KCl to the less harmful K2SO4 and HCl. In the present study, the rate constants
for decomposition of ammonium sulfate and aluminum sulfate were obtained
from experiments in a fast heating rate thermogravimetric analyzer.
The yields of SO2 and SO3 from the decomposition
were investigated in a tube reactor at 600–900 °C, revealing
a constant distribution of about 15% SO2 and 85% SO3 from aluminum sulfate decomposition and a temperature-dependent
distribution of SO2 and SO3 from ammonium sulfate
decomposition. On the basis of these data as well as earlier results,
a detailed chemical kinetic model for sulfation of KCl by a range
of sulfate additives was established. Modeling results were compared
to biomass combustion experiments in a bubbling fluidized-bed reactor
using ammonium sulfate, aluminum sulfate, and ferric sulfate as additives.
The simulation results for ammonium sulfate and ferric sulfate addition
compared favorably to the experiments. The predictions for aluminum
sulfate addition were only partly in agreement with the experimental
results, implying a need for further investigations. Predictions for
the effectiveness of the sulfur-based additives indicate that ferric
sulfate and ammonium sulfate have similar effectiveness at temperatures
ranging from approximately 850 to 900 °C, whereas ferric sulfate
is more efficient at higher temperatures and ammonium sulfate is more
effective at lower temperatures.