Understanding effect of phosphorus-based additive on ash deposition characteristics during high-sodium and high-calcium Zhundong coal combustion in drop-tube furnace

Zhundong coal as a high quality steam coal has not been widely utilized due to severe fouling and slagging problems during combustion. To promote clean utilization of Zhundong coal, combustion experiments of Zhundong coal were carried out in the drop tube furnace (DTF). The influence of novel phosphorus-based additive on ash deposition characteristics of Zhundong coal was studied. The appearance and phase composition of the raw coal and mixed coal ash deposits at different flue gas temperatures were researched respectively. The new prediction index of slagging tendency containing phosphorus was proposed, and the balance calculation were performed by FactSage thermodynamic software in different flue gas temperatures, including experimental temperatures selected. The results showed that the values of new indexes for predicting slagging tendency had obvious changes, and the phosphorus-based additive had a significant effect on the ash deposition behavior during the combustion process of Zhundong coal in DTF. Compared with the raw coal, the mixed coal with additive had higher ash fusion points and were in the form of loose, crisp, and porous powder, which was beneficial for soot blowing. When the flue gas temperature was 1000 degrees C or 1150 degrees C, there was much phosphate complex salt as the skeleton in the ash deposits of the mixed coal, such as Na2CaMg7(PO4)(6). Phosphate captured Na and Ca, and effectively inhibited the low temperature eutectic mixtures of Ca with Si and Al, indicating that phosphorus-based additive had good effect on improving ash deposition characteristics of Zhundong coal.