CO2 capture by carbide slag calcined under high-concentration steam and energy requirement in calcium looping conditions

In this work, calcination under high-concentration steam that can be implemented by O2/H2O combustion was proposed to replace high-concentration CO2 that usually was implemented by O2/CO2 combustion in calcium looping. CO2 capture performance of the carbide slag, an industrial solid waste from chlor-alkali plants, was investigated in a dual fixed-bed reactor under high-concentration steam calcination condition during the calcium looping cycles. Calcined under high-concentration steam (95%) atmosphere, the carbide slag can be completely and quickly decomposed at 800°C, which is 150°C lower than the calcination temperature under high-concentration CO2 (100%) atmosphere. The carbonation conversions of the carbide slag calcined under high-concentration steam condition after 1 and 10 cycles are about 42% and 36% higher than those calcined under high-concentration CO2 condition, respectively. This is because when the carbide slag is calcined under high-concentration steam, the relatively smaller CaO grains and more porous structure are generated, which are beneficial for CO2 capture by carbide slag. Due to the low calcination temperature and the high CO2 capture capacity of the carbide slag, the energy requirement in the calciner to capture per mole CO2 under high-concentration steam condition is lower than that under high-concentration CO2 condition. High-concentration steam calcination in place of high-concentration CO2 calcination improves the CO2 capture efficiency from 0.68 to 0.88 and saves a quarter of the energy consumption in the calciner for capturing per mole CO2 in the calcium looping system, when the ratio of recycled carbide slag flow rate to CO2 flow rate is 2 and the sorbent make-up ratio is 0.09. Thus, it is reasonable to consider O2/H2O combustion as an alternative energy-supply way of O2/CO2 combustion for the calcium looping of carbide slag.