K2CO3-catalytic supercritical water gasification of coal with NaAlO2 addition to inhibit ash agglomeration and decrease the volatility of alkali metals

Abstract

Supercritical water gasification (SCWG) technology is one of the most promising ways to realize efficient and clean utilization of coal because of the special physical and chemical properties of supercritical water. K2CO3 is the most preferable catalyst to improve gasification efficiency. However, K2CO3 leads to ash agglomeration and massive volatility of alkali metals. The accumulation of agglomerated ash prevents the continuous operation of the system, and the volatility of alkali metals weakens the coupling of high-grade H2/CO2/H2O mixture with downstream process. In this study, K2CO3-catalytic SCWG of coal with NaAlO2 loading was firstly investigated in a laboratory-scale continuous reaction system to check whether NaAlO2 can inhibit ash agglomeration and lower the volatility of alkali metals without decreasing the gasification efficiency. The concentration of coal slurry was 20 wt%-35 wt% and the loading content of NaAlO2 was 0 wt%-7.5 wt% of dry coal. By doing GC, PSD, XRD, SEM and ICP-OES analysis for three-phase products collecting from the experiment, it was found that, when NaAlO2 loading increased from 0 wt% to 7.5 wt%, the carbon gasification efficiency of 20 wt%, 30 wt% and 35 wt% coal slurry improved from 90.7% to 93.9%, from 83.8% to 88.6% and from 72.6% to 80.5%, the percentage of H2 increased and the percentage of CO2, CH4 and CO decreased. The addition of NaAlO2 transformed K and Si of coal into K(Na,K)3Al4Si4O16 instead of K2Si2O5, which contributes to the distribution of ash particles into smaller size. 5 wt% NaAlO2 loading decreased the total volatility of alkali metals, and 7.5 wt% NaAlO2 loading will increase the total volatility of alkali metals.