Mechanism of Ca Additive Acting as a Deterrent to Na2CO3 Deactivation during Catalytic Coal Gasification

The deactivation of Na2CO3, caused by the reaction between Na and Si and Al minerals in coal, is one of the key technical bottlenecks for the large-scale application of catalytic gasification. Ca can inhibit the deactivation of Na, and the binary Na–Ca catalyst shows strong synergistic effects during catalytic gasification. In this study, the deactivation behaviors of Na2CO3 and the inhibition mechanism of Ca were systematically investigated by thermogravimetric, X-ray diffraction, Fourier transform infrared (FT-IR), model compounds, and FactSage thermodynamic equilibrium. The results show that the Na–Ca catalyst shows synergistic effects and more Na remains active as water-dissolved Na after gasification with Na–Ca addition. The mechanism of the inhibiting effect of Ca on Na2CO3 deactivation at low temperatures (<1000 °C) is different from that at high temperatures (>1000 °C). At temperatures lower than 1000 °C, Ca inhibits the reaction between Na and Si and Al minerals; as a result, more Na remains active during catalytic gasification, which is validated by FT-IR. Meanwhile, the FactSage thermodynamic equilibrium shows that more Na2CO3 exists with increasing Ca addition. With Ca/Si = 0.1, the disappearing temperature of Na2CO3 is 940 °C. When Ca/Si increases to 1, the disappearing temperature of Na2CO3 further increases to 1166 °C, indicating that more Na survives with Ca addition. At high temperatures (>1000 °C), Ca captures Si and a part of Na in priority to form sodium calcium silicate (Na2Ca3SiO8) and calcium silicate (Ca2SiO4). As a result, the remaining Na and Al form NaAlO2, which has excellent catalytic performance and contributes to the synergistically promoted catalytic performance. At low and high temperatures, Ca can act as a good deterrent to the Na deactivation.