Study on the deposition behaviors of sodium and calcium salts in high-alkali coal under oxy-fuel condition

Previous studies primarily focused on the deposition of whole coal ash instead of specific components, leaving the deposition behaviors of sodium and calcium salts insufficiently understood. In this research, the deposition behaviors of NaCl, Na2SO4, CaSO4, and their blends under oxy-fuel condition were investigated. The influences of mineral species, flue gas temperature, and heat transfer surface temperature on deposition mechanism were evaluated through both experimental study and theoretical calculation. The results show that the deposition efficiency of NaCl (13.9 %) exceeds that of Na2SO4 (10.4 %), which is attributed to the larger mass transfer coefficient of NaCl, leading to a higher condensational deposition rate. The NaCl and Na2SO4 blend possesses a deposition efficiency (10.8 %) that is close to that of pure Na2SO4. As the flue gas temperature rises from 1073 K to 1273 K, the deposition efficiencies of selected minerals increase. The mass transfer coefficient is positively related to the flue gas temperature. Accordingly, the condensational deposition rate is expected to become higher at elevated temperature. As the probe surface temperature rises from 773 K to 873 K, the deposition efficiencies exhibit downward trends. The high probe surface temperature causes low temperature gradient coefficient, so the thermophoretic deposition rate is reduced.