Release characteristics of alkali and alkaline earth metals in nascent char during rapid pyrolysis

The release law of alkali and alkaline earth metals (AAEMs) during coal pyrolysis is crucial for controlling scaling, corrosion and slagging in the utilization of high AAEMs coal. To approximate entrained-flow bed gasification process, a free-falling reactor was used for coal pyrolysis to study the release characteristics of AAEMs in nascent char. Effects of pyrolysis temperature, residence time and atmosphere were investigated. The cumulative release of Na, Ca and Mg at 1100 C was 68.0%, 47.5% and 69.8%, respectively. As pyrolysis temperature increased from 900 C to 1100 C, the release of Na, Ca and Mg increased by 7.0 %, 3.5% and 5.3%, respectively. The cumulative release of AAEMs increased with the pyrolysis residence time, but the release rate decreased significantly in middle and late stages. The promoted thermal decomposition of functional groups by CO2 resulted in a lower char yield. CO2 promoted Na release in early stage and Ca and Mg release in late stage. Furthermore, release of AAEMs was associated with char structure evolution in rapid pyrolysis. The presence of CO2, high temperature and prolonged residence time led to enhanced particle fragmentation, increased surface cracks, and intensified thermal decomposition of functional groups, resulting in the migration of AAEMs in carbon matrix to char surface and the release of AAEMs organically bound to carbon matrix.