• Energy Research

Abstract The re-carbonation was proposed to improve the CO 2 capture performance of the carbide slag, an industrial solid waste, calcined under high concentration of steam during the calcium looping process. The effects of re-carbonation conditions, including duration and CO 2 concentration on CO 2 capture by the cycled carbide slag were studied. The results show that the re-carbonation significantly improves the CO 2 capture capacity of the carbide slag. Notably, the re-carbonation dramatically reactivates the carbide slag which has experienced 11 cycles, increasing its carbonation conversion from 0.19 to about 0.6. In addition, the carbide slag shows higher re-carbonation conversions than the limestones reported by other researchers. A porous structure of the carbide slag is formed after the calcination from the extra CaCO 3 generated in the re-carbonation step, which improves its CO 2 capture capacity. In addition, the impacts of the re-carbonation on the CO 2 capture ratio of the calcium looping system and the energy consumption in the calciner were discussed. The re-carbonation improves the CO 2 capture ratio from 0.46 to 0.88 and reduces the energy consumption in the calciner from 321 kJ/mol CO 2 to 261 kJ/mol CO 2 , when the molar ratio of sorbent to CO 2 is 2 and the make-up ratio of the sorbent is 0.16 in the calcium looping system using the carbide slag. Thus, the re-carbonation and high concentration of steam in the calcination is an effective way to improve the CO 2 capture capacity of the carbide slag and reduce the energy consumption in the calciner during the calcium looping process.

Abstract Cu can improve NO removal efficiency of char and CO in carbonation stage of calcium looping, but the mechanism of NO reduction by char and CO in the presence of Cu has been rarely reported. In this work, the density functional theory was utilized to investigate the effect of Cu on NO reduction by both char and CO in carbonation stage of calcium looping for CO2 capture. Density of state result proves that Cu decreases the possibility of char deactivation and retains stable promoting effect for NO reduction. Adsorption energies and structural parameters were used to determine adsorption sites of reactant molecules (CO, NO and O2) and Cu atom on the basic configuration of zigzag graphite structure with six benzene rings. Adsorption energies of reactant molecules on char surface in the presence of Cu follow the order: CO

Abstract Calcium looping is regarded as an effective and viable way to address CO2 emissions. To overcome the loss-in-capacity problems of calcium-based sorbents with the number of calcium looping cycles, a novel CaO/Ca12Al14O33 sorbent with a microtube-like structure was prepared from carbide slag and Al(NO3)3·9H2O using paper fibre as a biotemplate. The CO2 capture performance and microstructure of the novel synthetic sorbent under calcium looping conditions were investigated. The results show that the utilization of the biotemplate is good to retain the high cyclic CO2 capture reactivity of the synthetic sorbent. Due to the unique hollow porous structure, the CO2 capture capacity of the synthetic sorbent containing 7.5 wt% Al2O3 retains 0.56 and 0.33 g/g after 30 cycles under mild and severe calcination conditions, respectively, which are 2.56 and 2.11 times higher than those of carbide slag under the same respective calcination conditions. With the presence of 10% steam in the carbonation atmosphere, the CO2 capture capacity of the synthetic sorbent retains 0.55 g/g under the severe calcination conditions after 10 cycles. The native hierarchical biostructure of paper fibre is preserved in the synthetic sorbent. CaO and Ca12Al14O33 are uniformly distributed in the synthetic sorbent, resulting in a high sintering resistance during multiple CO2 capture cycles. CO2 can penetrate through the microtube-like structure of the synthetic sorbent from two directions, i.e., from the outer surface and inner surface. This phenomenon effectively enlarges the contact area between CO2 and CaO. The CaO/Ca12Al14O33 sorbent with a hollow porous structure by means of a biotemplate appears promising in the calcium looping technology for CO2 capture.

Abstract A novel magnesia-stabilized carbide slag (MSCS) was synthesized with carbide slag, magnesium nitrate hydrate and by-product of biodiesel by combustion, which was used as a CO 2 sorbent during the calcium looping process. The effects of preparation condition (combustion temperature, combustion duration, by-product of biodiesel addition and magnesia addition) and CO 2 capture condition (carbonation and calcination atmosphere) on CO 2 capture capacity of MSCS were investigated during the calcium looping cycles. The main compositions of MSCS are CaO and MgO. The addition of by-product of biodiesel in the preparation of the sorbent leads to the uniform mix of MgO and CaO grains in MSCS, which shows an obviously positive effect on its CO 2 capture capacity. Only on the condition of the addition of by-product of biodiesel, MgO derived from magnesium nitrate hydrate improves the cyclic CO 2 capture capacity and durability of MSCS during the multiple cycles. MSCS with a mass ratio of CaO to MgO of 80:20 combusted at 850 °C for 60 min exhibits higher CO 2 capture capacity and greater durability. The CO 2 capture capacity of MSCS can retain 0.42 g/g after 20 cycles, which is 60% higher than that of carbide slag. MSCS calcined under the high concentration of steam displays much higher CO 2 capture capacity and better sintering resistance during the cycles, compared to MSCS calcined under the high concentration of CO 2 . The addition of steam in the carbonation enhances CO 2 capture capacities of MSCS and carbide slag. MSCS consists of CaO–MgO grain groups and the support of MgO sustains the high sintering resistance of the sorbent. MSCS remains much larger surface area and pore volume than carbide slag during the cycles, compared to carbide slag. MSCS appears promising as an effective and low-cost CO 2 sorbent during the calcium looping.

Abstract Sorption-enhanced reaction process (SEPR) can produce high yield of hydrogen with in-situ CO2 removal using CaO. A key requirement for the process is the integration of effective Ca-based sorbents into catalytic schemes. In this work, the research progress of Ca-based CO2 sorbents in SEPR is summarized. The positive effects by Ca-based sorbents are analyzed. The methods to improve the reactivity of Ca-based sorbents are discussed. A good prospect of compound materials of catalysts/sorbents in SERP is pointed out, for which the relationship between the performance and structure or reaction condition should be further studied. The optimization of process parameters is critically reviewed to provide recommendations for industrial operation. The comprehensiveness of this work extends to the discussion about the applications of Density Functional Theory study in SERP. Process economics requires the integrated H2 production with by-product conversion, renewable resources utilization, waste recycling and heat coupling optimization for the future research.

Abstract A novel calcium-based synthetic CO2 sorbent with hollow core-shell structure was prepared by a carbon microsphere template route where carbide slag, alumina cement and glucose were employed as the low-cost calcium precursor, support and carbon source, respectively. The effects of the alumina cement addition, the pre-calcination temperature during the preparation process, the carbon template addition and calcination conditions on CO2 capture performances of the calcium-based synthetic sorbents were studied during calcium looping cycles. The synthetic sorbent containing 5 wt.% alumina cement possesses the highest CO2 capture capacity during calcium looping cycles, which is mainly composed of CaO and Ca12Al14O33. The CO2 capture capacities of the synthetic sorbent under mild and severe calcination conditions can retain 0.37 and 0.29 g/g after 20 cycles, which are 57% and 99% higher than those of carbide slag under the same conditions, respectively. The synthetic sorbent possesses a hollow micro-sphere morphology with a nano-structured shell and meso-porous structure, which decreases the diffusion resistance of CO2. Periodic density functional theory (DFT) calculations are used to explain why Ca12Al14O33 can effectively retard both agglomeration and sintering of the synthetic sorbent. The hollow core-shell model is proposed to explain the CO2 capture mechanism of the synthetic sorbent. For the same CO2 capture efficiency, the energy consumption in the calciner using the synthetic sorbent is much lower than those using carbide slag and natural limestone. This work designs a good method to prepare the hollow sphere-structured synthetic sorbents with high CO2 capture capacity and provides a promising way to integrate efficient CO2 capture with the utilization of industrial waste.

The high-temperature sintering of CaO-based materials leads to the serious decay of energy storage performance during the calcination/carbonation cycle. To overcome the loss in porosity problem, an efficient CaO-based material for thermal energy storage was synthesized using bamboo fiber as the biotemplate. The synthesis parameters (bamboo fiber addition, pyrolysis, Al2O3 loading) and the energy storage reaction characteristics of CaO-based energy storage material were optimized on the basis of cyclic calcination/carbonation experiments. The results show that the sacrificed biotemplate enhances the porosity of the synthetic material, denoting improved energy storage density. The cumulative energy storage density of the templated material over 50 cycles is 24,131.44 kJ/kg higher than that of limestone. The carbonation conversion and energy storage density of the templated CaO-based material doped with 5 wt.% Al2O3 and 0.5 g bamboo fiber reach 0.75 mol/mol and 2368.82 kJ/kg after 10 cycles, respectively, which is 2.7 times as high as that of original limestone. The maximum apparent carbonation rate of the templated CaO-based materials in the 1st cycle corresponds to a 240% increment compared to limestone. The maximum calcination rate of the synthetic CaO-based material in the 12th cycle remains 93%, as compared with the initial cycle. The microstructure analysis reveals that the hierarchically-stable structure during the cycle is beneficial for a more effective exposure of surface reactive sites for CaO and inward/outward diffusion for CO2 molecules through CaO. The method using the sacrificed biological template provides an advanced approach to fabricate porous materials, and the composite CaO-based material provides high-return solar energy storage for a potential application in industrial scale.

Abstract Loss-in-capacity of carbide slag in CO2 capture restricts the development of industrial wastes in calcium looping technology. In this work, a novel Mn/Mg-copromoted carbide slag was prepared using carbide slag, dolomite and trace Mn(NO3)2 additive. Experimental tests were carried out in the fixed-bed reactor to evaluate how the preparation and the reaction conditions influenced the CO2 capture performance of Mn/Mg-copromoted carbide slag during calcination/carbonation cycles. Results show that MgO diminishes the sintering of synthetic sorbents. The optimal Mn/Mg-copromoted carbide slag (mass ratio of CaO:MgO:MnO2 = 89:10:1) exhibits the highest CO2 capture capacity of 0.52 g/g after 10 cycles under the severe calcination condition (100 % CO2, 950 °C) and the wet carbonation condition (15 % CO2/20 % steam/N2), which is 1.7 times as high as that of untreated carbide slag. MnO2 positively affects the slow carbonation stage by enhancing the electron transfer between CaO and CO2. Observations of the morphology of Mn/Mg-copromoted carbide slag indicate that the stabilized CO2 capture performance is mainly attributed to porous structure, MgO as the skeleton and MnO2 as an electron-transfer promoter.

Calcium looping is a promising technology to capture CO2 from the process of coal-fired power generation and gasification of coal/biomass for hydrogen production. The decay of CO2 capture activities of calcium-based sorbents is one of the main problems holding back the development of the technology. Taking carbide slag as a main raw material and Ca12Al14O33 as a support, highly active CO2 sorbents were prepared using the hydrothermal template method in this work. The effects of support ratio, cycle number, and reaction conditions were evaluated. The results show that Ca12Al14O33 generated effectively improves the cyclic stability of CO2 capture by synthetic sorbents. When the Al2O3 addition is 5%, or the Ca12Al14O33 content is 10%, the synthetic sorbent possesses the highest cyclic CO2 capture performance. Under harsh calcination conditions, the CO2 capture capacity of the synthetic sorbent after 30 cycles is 0.29 g/g, which is 80% higher than that of carbide slag. The superiority of the synthetic sorbent on the CO2 capture kinetics mainly reflects at the diffusion-controlled stage. The cumulative pore volume of the synthetic sorbent within the range of 10–100 nm is 2.4 times as high as that of calcined carbide slag. The structure of the synthetic sorbent reduces the CO2 diffusion resistance, and thus leads to better CO2 capture performance and reaction rate.