Selection of Fluxing Agent for Coal Ash and Investigation of Fusion Mechanism: A First-Principles Study

Abstract

An approach based on the ab initio quantum chemical modeling [CASTEP, generalized gradient approximation (GGA), and density functional theory (DFT)] was first employed to guide the selection of the appropriate fluxing agent to reduce the coal ash melting temperature. Two kinds of typical Chinese coal ash A and B with a high-melting temperature were chosen as the investigated subjects. Result of the calculation shows that mullite mineral, which is the main component of coal ash, is easier to combine with an electron acceptor than with an electron donor. Because the cations of borax (Na2B4O7·10H2O) and limestone can act as electron acceptors, borax and limestone were selected as the fluxing agents in our experiment. Results of the experiment show that the melting temperatures of coal ash A and B are both decreased by borax and limestone, respectively. Moreover, borax has a better fluxing effect than limestone under the same conditions. The further numerical study on the coal ash fusing mechanism indicates that the Na+ and Ca2+ cations, as acceptors, can enter into the crystal lattice of mullite mainly through O(7) and O(8) and then cause the Al(6)−O(8) and Al(5)−O(7) bonds to rupture in the [AlO6]-octahedron. From this, mullite is forced to transform to feldspar and corundum minerals that have a low binding energy. Because of the phase change of minerals in the coal ash, the coal ash melting temperature is decreased by adding borax and limestone.