A Mini-review of the Capability of Calcium-Based Binary Fluxing in Adjusting Coal Ash Flowability

Coal ash flowability determines whether the coal ash slag can be successfully discharged from entrained flow gasifiers, to a large extent. Traditional single CaO fluxing cannot perform well enough to meet the requirement of slagging gasifiers for the flowability of coal ash with extremely high SiO2 and Al2O3. Thus, this work reviewed the effect of Ca-bearing binary (CaO-Fe2O3, CaO-Na2O, and CaO-MgO) fluxing on ash flowability. The effect of CaO-Fe2O3 binary fluxing on ash flowability has been systemically investigated, while studies concerning CaO-Na2O and CaO/MgO on coal ash flowability were limited. CaO-Fe2O3 binary fluxing performs well in adjusting the AFTs and viscosity behavior. The CaO/Fe2O3 fluxing mass ratio of 1/1 was recommended within the compromise of AFTs and viscosity at operating temperature. CaO-Na2O also displayed good performance to decrease AFTs, but strong volatilization of Na species brings difficulty to quantify the influence of CaO-Na2O compositional fluxing. CaO-MgO compositional fluxing showed a synergistic effect at controlling both AFTs and viscosity, while the addition of CaO-MgO fluxing easily led to a crystallization slag pattern. Besides, the value of CaO/MgO corresponding to the best fluxing performance and fluidity performance was unrevealed, as well as CaO/Na2O. The exploration of CaO-Na2O and CaO/MgO in the industry needs further testimony in coal ash with bad flowability. In a comprehensive comparison, CaO/Fe2O3 binary fluxing is the most promising compositional flux in adjusting the coal ash flowability. The review can give guiding information for selecting high-efficiency fluxing compositional fluxing.