Auto-thermal operation and optimization of coal-fueled separated gasification chemical looping combustion in a pilot-scale unit

•The auto-thermal operation of a pilot-scale SG-CLC unit was achieved.•Carbon capture efficiency was higher than 96.56% during continuous operation.•The SG-CLC performances were satisfying under variable conditions.•Some measures were put forward to optimize the auto-thermal operation. As one of the chemical looping combustion (CLC) technologies, separated gasification-CLC (SG-CLC) can realize the carbon separation during coal combustion process with low degradation of oxygen carrier and high performances. In order to realize the auto-thermal operation of the constructed pilot-scale SG-CLC unit, some oxygen was added into the gasification agent to compensate the endothermic gasification of coal using steam. The combustion performances of SG-CLC unit were investigated under different conditions with variables of reaction temperature, coal feed rate, steam mass flow, oxygen flow and type of loosen gas. Ten conditions were designed and conducted to study the effects of variables on the CLC performances. According to the temperature curves, auto-thermal operation was achieved in seven of these tests. Carbon capture efficiencies under auto-thermal operation were almost higher than 97% and the oxygen demands in these test were less than 4.70%. The peak value of solid fuel conversion was 93.91% at 930 °C while the minimum was 87.36% at 830 °C under auto-thermal conditions. The performances demonstrated that the SG-CLC system could offer great possibility for its commercial application under auto-thermal conditions. Furthermore, some measures were put forward to optimize the operation of this SG-CLC system.