Effects of different heat carriers on pyrolysis products of high-sodium low-rank coal and optimization of pyrolysis process

In this study, the effects of quartz sand, coal ash and phosphorite on Na/Cl migration and char characteristics during the pyrolysis of high-sodium coal were investigated in a fluidized bed. The results showed that the Na/Cl release was the largest when quartz sand was used as a heat carrier (20.28 % Na and 88.60 % Cl at 900 °C), while coal ash and phosphorite reduced the Na release ratio to 2.74 % and 13.24 % respectively by forming stable compounds (such as Na-Al-Si and Ca-P-Cl complexes). When phosphorite and coal ash were used as heat carriers, the char yield at 500 °C was increased by 10.72 % and 12.47 % respectively compared with that when quartz sand was used as a heat carrier. Meanwhile, phosphorite and coal ash also improved the reactivity of char. Molecular dynamics simulations show that phosphorite has the strongest adsorption capacity for NaCl (binding energy of −1400 to −2400 kJ∙mol−1), followed by coal ash (−300 to −1800 kJ∙mol−1), and quartz sand is the weakest (−300 to −500 kJ∙mol−1). The innovation of this study is to explore the effects of different heat carriers on the pyrolysis of high-sodium coal, analyze its influence mechanism through molecular dynamics, and propose an optimized process for a double circulating fluidized bed for the utilization of high-sodium low-rank coal. This can provide new ideas for the efficient utilization of high-sodium low-rank coal, reduce the corrosion caused by Na/Cl in industrial applications, improve the quality of char and promote the recycling of resources.