Effect Of HCl On Cyclic CO_<sub>2 Capture Characteristics Of Calcium-based Sorbent During Calcination/Carbonation Cycles

Calcium looping technology, i.e. CO2 capture technology of calcium-based sorbents such as limestone during calcination/carbonation cycles, is one of the most promising CO2 capture technology for large-scale industrial applications. Various types of chlorine-containing fuels such as biomass and refuse derived fuel (RDF) produce large amount of HCl in the combustion or gasification. When the cyclic calcination/carbonation reaction of calcium-based sorbents is utilized to capture CO2 after the combustion or gasification of chlorine-containing fuels, HCl can react with calcium-based sorbents and compete with carbonation reaction for active CaO. Therefore, it is necessary to investigate the effects and action mechanism of HCl on cyclic CO2 capture behavior of calcium-based sorbents, which provides theoretical guidance for CO2 capture and HCl removal in the combustion or gasification of chlorine-containing fuels using calcium looping technology.A dual fixed-bed reactor was used to investigate the effect of HCl on cyclic CO2 capture of conventional calcium-based sorbents (e.g. limestone) and typical calcium-based industrial wastes (e.g. carbide slag). The phase and microstructure analytical means such as XRD, SEM and the nitrogen adsorption analyzer were employed to reveal the action mechanism of HCl on cyclic CO2 capture behavior of calcium-based sorbent.The effects of various operating conditions involving carbonation temperature, calcination temperature, HCl volume fraction and particle size on CO2 capture behavior of conventional calcium-based sorbents (e.g. limestone) in the calcination/carbonation cycles was examined. The presence of HCl in the carbonation atmosphere improves CO2 capture capacity of the limestone in the previous a dozen cycles, but sharply decreases its reactivity with further increasing the cycle number above a dozen. As the carbonation process develops to a certain degree, the surface of the sorbent can form compact CaCO3 product layer, which restrains the carbonation of inner CaO. In the previous a dozen cycles, HCl reacts with CaCO3 producing CaClOH. The compact product layer is destructed and CO2 is released in the chlorination. Some pores in the product layer are left, which decreases the diffusion resistance of CO2. It is beneficial to CO2 capture.It may be a reason why the presence of HCl improves the cyclic CO2 capture capacity of the limestone in the previous a dozen cycles. The chlorination product CaClOH accumulates with the number of cycles, which blocks the pores and decreases the CO2 capture capacity of the sorbent.The effects of reaction conditions on CO2 capture characteristics of typical calcium-based industrial wastes (e.g. carbide slag) during calcination/carbonation cycles were investigated. When HCl is present in the carbonation atmosphere, the reaction product of the carbide slag and HCl is CaClOH. The presence of HCl enhances CO2 capture capacity of the carbide slag in the previous cycles, but sharply decreases its reactivity with further increasing the cycle number. As HCl is present in the carbonation atmosphere, the carbonation temperature should be 700℃, and the feasible calcination temperature is 850-900℃. The CO2 capture capacity of the carbide slag achieves the maximum in the presence of 0.3% HCl. The higher CO2 capture capacity of the carbide slag is achieved with smaller particle size in the presence of HCl.As the CO2 capture capacity of limestone and carbide slag decreases with the number of cycles, a new method to enhance the CO2 capture characteristics of calcium-based sorbents with the discontinuous addition of HCl is introduced. The discontinuous addition of HCl into the carbonation atmosphere in the previous cycles obviously enhances the CO2 capture capacity of the limestone and the carbide slag. The CO2 capture capacity of the limestone with the addition of HCl in the first 3 cycles is 2.4 times as much as that of the limestone without HCl. And the CO2 capture capacity of the carbide slag with the addition of HCl in the first 4 cycles is 1.5 times as much as that of the carbide slag without HCl. In order to improve the applicability of the method, a feasible process route in which chlorine-containing fuels is added discontinuously into the carbonation atmosphere is introduced. The HCl produced during the combustion of chlorine-containing fuels can enhance the CO2 capture capacity of calcium-based sorbents.