Influence Of Steam On The Cyclic CO₂ Capture Performance Of The Carbide Slag In Calcination/Carbonation Cycles

Calcium Looping, namely calcination/carbonation cycles of calcium-based sorbents （for example, limestone） to capture CO₂ in, has with good technical and economic characteristics, which can be both used in post-combustion CO₂ capture （such as coal-fired power plant）, and pre-combustion CO₂ capture （such as reforming of coal gasification） process to achieve CO₂ capture and hydrogen production efficient coupling. Calcium Looping is regarded one of the most promising technologies to achieve large-scale CO₂ capture currently.Carbide slag is an industrial waste formed by acetylene production. Our previous studies have shown that carbide slag has higher CO₂ capture performance in cyclic calcination/carbonation, which means carbide slag has good industrial application prospect in calcium looping. However, the above studies of reaction atmosphere are dry, without considering the effect of steam in real reaction condition （i.e. flue gas and syngas）. The effect of steam on the cyclic CO₂ capture performance of the carbide slag has not been reported. It is necessary to investigate the effects and mechanism of steam on carbonation and calcination of the carbide slag in calcination/carbonation cycles deeply.In this thesis, the effect of steam on the carbonation characteristics of the carbide slag is systematically investigated in dual-fixed-bed and triple-fixed-bed reactor. The phase and microstructure analytical means such as SEM and the nitrogen adsorption analyzer were employed to reveal the action mechanism of steam on cyclic CO₂ capture behaviour of the carbide slag. Theoretical calculation is employed to analyse the effect of steam on the energy consumption of the calciner.Effects of calcination condition, steam content in carbonation atmosphere, carbonation duration and CO₂ concentration in carbonation atmosphere are investigated on dual-fixed-bed reactor. It is shown that the CO₂ capture capacity of the carbide slag is greatly reduced by switching calcination condition from 850℃/100% N2 to 950℃/100% CO₂, and the presence of steam in carbonation atmosphere can ease the detrimental effect of severe calcination condition. On one hand, the carbide slag shows better CO₂ capture performance in the presence of higher concentration of steam, but the effect is more profound when the carbonation duration is 5min than 20min. On the other hand, when the steam concentration is relatively high in carbonation atmosphere, too long carbonation duration damages the microstructure of the carbide, which is harmful for the carbonation reaction.In this thesis, a novel process is proposed, in which O₂/H2O calcination is employed to provide the energy consumption in calciner. Comparing with O₂/CO₂ combustion, steam concentration is higher, which reduces the CO₂ concentration in calcination atmosphere and hence reduces the decomposing temperature of the carbide slag. The cyclic CO₂ capture performance of the carbide slag is investigated by calcining carbide slag in high steam content. It is shown that the carbide slag shows best CO₂ capture performance at 800℃ in 95% H2O/5% CO₂. In this calcination condition, the carbide slag shows better CO₂ capture performance in the presence of higher steam content in carbonation atmosphere. The energy consumption of the calciner is calculated for O₂/H2O combustion and O₂/CO₂ combustion. Lower energy consumption of calciner is achieved for O₂/H2O combustion.The effect of recarbonation on the carbonation performance of the carbide slag is investigated in the calcination condition of high steam concentration. It is shown that the process of recarbonation can re-activate the cycled carbide slag, and hence it can maintain high CO₂ capture capacity of the carbide. Steam concentration in recarbonation atmosphere has little effect on the CO₂ capture performance of the carbide slag. The carbonation and recarbonation duration have more profound effect on the carbonation performance of the carbide slag.