Fabrication Of Novel Synthetic CaO/MgO Sorbent And Its Cyclic CO₂ Capture Performance In Calcium Looping

The calcium looping process, namely utilizing the calcination/carbonation cycles of calcium-based sorbents to capture CO2 at high temperature, is considered to be the most promising technology for large-scale CO2 capture. Previous research indicated that carbide slag, a kind of chlor-alkali industrial waste, can be used as a sorbent in calcium looping process for CO2 capture, but the CO2 capture capacity of carbide slag decreases with the number of cycles. In order to improve the CO2 capture capacity of carbide slag, two kinds of novel synthetic CaO/MgO sorbents were prepared by the combustion synthesis method using carbide slag as CaO precursor, magnesium nitrate and dolomite as MgO precursor, and by-product of biodiesel-deionized water solution as solvent in this work. The cyclic CO2 capture characteristics of synthetic CaO/MgO sorbents were investigated and the reaction mechanisms were revealed.The cyclic CO2 capture characteristics of the first kind of synthetic CaO/MgO sorbent, which was prepared from carbide slag as CaO precursor and magnesium nitrate as MgO precursor, were investigated and the effects of preparation condition and calcination/carbonation atmosphere on CO2 capture capacity of synthetic sorbent were studied. The results show that a quantity of gas released during the combustion of by-product of biodiesel makes great contribution to the abundant pore structure of the synthetic sorbent. CaO and MgO are the main components of the synthetic sorbent and they’re homogeneous dispersed. When the mass ratio of CaO/MgO is 80:20, the combustion condition is 60 min and 850℃, the synthetic sorbent achieves the highest CO2 capture capacity,0.42 g/g after 20 cycles. The CO2 capture capacity of synthetic sorbent is obviously higher than carbide slag under high-concentration of CO2 or high-concentration of steam.The presence of steam during carbonation promotes the CO2 adsorption of the sorbent. The calcined synthetic CaO/MgO sorbent has the larger surface area and pore volume than the calcined carbide slag and possesses more pores in the range of 20-100 nm, which makes contribute to the higher CO2 capture capacity and better durability of the synthetic sorbent.Despite the well CO2 capture capacity of the first kind of synthetic CaO/MgO sorbent, magnesium nitrate is high-cost and releases NOx during the combustion. The second kind of synthetic CaO/MgO sorbent was prepared using dolomite as the CaO and MgO precursor and carbide slag as the CaO precursor and its cyclic CO2 capture characteristics was investigated. The results show that the synthetic sorbent gets the optimum CO2 capture capacity,0.44 g/g after 10 cycles, when the mass ratio of CaO/MgO is 90:10. The calcinations temperature and atmosphere have great influences on the cyclic CO2 capture capacity of synthetic sorbent. When the synthetic sorbent is calcinated under 100％CO2 and 950 ℃, the CO2 capture capacity of synthetic sorbent is 67％higher than carbide slag after 10 cycles. Calcinated under 100％steam reduces the sintering of sorbent, which is beneficial for CO2 capture. The CO2 capture capacity of sorbent is improved by adding 20％steam during carbonation, synthetic sorbent gets a fast carbonation rate in the first 5 min, the CO2 capture capacity of 5 min is higher than that of 20 min without steam addition.In order to futher improve the CO2 capture capacity of the second kind of synthetic CaO/MgO sorbent, the modification of manganese nitrate was proposed and the CO2 capture characteristics of manganese-modified synthetic sorbent was investigated. The results show that modification of manganese nitrate improves the CO2 capture capacity of synthetic sorbent, and the optimum mole ratio of Mn/Ca is 0.75:100. The CO2 capture capacity of the synthetic sorbent is 0.45 g/g after 20 cycles. The presense of steam during carbonation makes great contributions to the CO2 capture performance of manganese-modified synthetic sorbent, which even can achieve a high CO2 capture capacity of 0.52 g/g after 10 cycles under severe calcination conditions of 950℃ and 100％CO2. Compared with the sorbent without manganese modification, modified sorbent is more porous and maintains well pore structures during the cycles, which is benefitial to the CO2 diffusion and improves the CO2 capture capacity of synthetic sorbent.