Browsing by Author "Wang, Ke"
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Item Open Access CO2 capture performance of gluconic acid-modified limestone-dolomite mixtures under realistic conditions(ACS, 2019-07-10) Wang, Ke; Gu, Feng; Clough, Peter T.; Zhao, Pengfei; Anthony, Edward J.Calcium Looping (CaL) technology has become one of the most attractive ways to capture CO2 from fossil fuel power plants. However, with increasing numbers of cyclic reactions, the CO2 capture capacity rapidly decreases. To address this shortcoming, limestone-dolomite mixtures modified by gluconic acid were explored to prepare highly effective, MgO-stabilized, CaO sorbents that exhibited a high and stable CO2 capture capacity over multiple cycles. The sorbents were all tested over 10 carbonation-calcination cycles and were performed under realistic CaL conditions (calcination in a high CO2 concentration). The results of this research have demonstrated that the inhomogeneous composition that occurs between CaO and MgO - caused by the small CaO crystallite size, porous texture, nanosheet (~100 nm thick) morphology - provides sufficient void space for the volume expansion during carbonation to mitigate the effects of repeated cycle sintering and retain structural stability. A MgO content as low as 10 mol% was able to ensure a superior CO2 capture performance with a fast carbonation rate, high CO2 carrying capacities and remarkable stability. Furthermore, these sorbents retained a conversion (above 90%) over multiple cycles following a recarbonation stepItem Open Access Developments in calcium/chemical looping and metal oxide redox cycles for high-temperature thermochemical energy storage: A review(Elsevier, 2019-11-27) Yan, Yongliang; Wang, Ke; Clough, Peter T.; Anthony, Edward J.Energy storage is one of the most critical factors for maximising the availability of renewable energy systems while delivering firm capacity on an as- and when-required basis, thus improving the balance of grid energy. Chemical and calcium looping are two technologies, which are promising from both the point of view of minimising greenhouse gas emissions and because of their suitability for integrating with energy storage. A particularly promising route is to combine these technologies with solar heating, thus minimising the use of fossil fuels during the materials regeneration steps. For chemical looping, the development of mixed oxide carrier systems remains the highest impact research and development goal, and for calcium looping, minimising the decay in CO2 carrying capacity with natural sorbents appears to be the most economical option. In particular, sorbent stabilisers such as those based on Mg are particularly promising. In both cases, energy can be stored thermally as hot solids or chemically as unreacted materials, but there is a need to build suitable pilot plant demonstration units if the technology is to advance.Item Open Access Molten shell-activated, high-performance, un-doped Li4SiO4 for high-temperature CO2 capture at low CO2 concentrations(Elsevier, 2020-10-16) Wang, Ke; Gu, Feng; Clough, Peter T.; Zhao, Youwei; Zhao, Pengfei; Anthony, Edward J.Lithium orthosilicate (Li4SiO4) represents a potential class of high-temperature sorbents for CO2 capture in power plants and sorption enhanced methane reforming to produce H2. However, conventional wisdom suggests that pure Li4SiO4 should have extremely slow sorption kinetics at realistic low CO2 concentrations. Here, we report the opposite result: using a simple and cost-effective glucose-based mild combustion procedure, an unusually efficient and pure form of Li4SiO4 (MC-0.6) was synthesized to achieve a maximum uptake capacity of 35.0 wt% at 580 °C for CO2 concentrations under 15 vol% and maintained this capacity over multiple cycles. The characterization results showed that highly porous nano-agglomerate-like (50–100 nm) morphologies were apparent and ensured a rapid surface-sorption of CO2. In this process, a macroporous nano-sized Li2SiO3 cover on the melt layer of Li2CO3 was identified for the first time. This special structure appeared to accelerate the transportation of CO2 and the diffusion of Li+ and O2− through a molten layer enhancing contact with CO2. Thus, the sample MC-0.6 reduced both the surface-sorption and diffusion kinetics dependence on low CO2 concentrations. Rather than use traditional approaches (controlled morphologies combined with doping), we have demonstrated that the slow kinetics can be overcome simply by a controlled morphologies strategy, which opens up a new direction for the synthesis of high-performance Li4SiO4 sorbents.Item Open Access Porous MgO-stabilized CaO-based powders/pellets via a citric acid-based carbon template for thermochemical energy storage in concentrated solar power plants(Elsevier, 2020-01-21) Wang, Ke; Gu, Feng; Clough, Peter T.; Zhao, Pengfei; Anthony, BenThe reversible CaO/CaCO3 carbonation reaction (CaL) is one of the most promising candidates for high-temperature thermochemical energy storage (TCES) in concentrated solar power plants (CSP). Here, a sacrificial citric acid-based carbon template was developed to produce high-performance CaO-based sorbents to mitigate the progressive deactivation with sequential carbonation-calcination cycling. The carbon template was formed through in situ pyrolysis of citric acid in a simple heating process under nitrogen. After a secondary calcination step in air, a stable porous MgO-stabilized nano-CaO powder was generated and achieved high long-term effective conversion due to its resistance to pore plugging and sintering. By dry mixing citric acid with limestone-dolomite mixtures, this procedure can also be applied to synthesize MgO-stabilized CaO pellets via an extrusion–spheronization route, which resulted in comparably stable and effective conversion as the optimized CaO powder. Additionally, the considerable mechanical strength of MgO-stabilized CaO pellets should enable their realistic application in fluidized bed reactors. Thus, this simple, cost-effective and easily-scalable synthesis technique appears to have great potential for CSP-TCES under high temperature operation.Item Open Access Porous MgO-stabilized CaO-based powders/pellets via a citric acid-based carbon template for thermochemical energy storage in concentrated solar power plants(Elsevier, 2020-01-21) Wang, Ke; Gu, Feng; Clough, Peter T.; Zhao, Pengfei; Anthony, Edward J.The reversible CaO/CaCO3 carbonation reaction (CaL) is one of the most promising candidates for high-temperature thermochemical energy storage (TCES) in concentrated solar power plants (CSP). Here, a sacrificial citric acid-based carbon template was developed to produce high-performance CaO-based sorbents to mitigate the progressive deactivation with sequential carbonation-calcination cycling. The carbon template was formed through in situ pyrolysis of citric acid in a simple heating process under nitrogen. After a secondary calcination step in air, a stable porous MgO-stabilized nano-CaO powder was generated and achieved high long-term effective conversion due to its resistance to pore plugging and sintering. By dry mixing citric acid with limestone-dolomite mixtures, this procedure can also be applied to synthesize MgO-stabilized CaO pellets via an extrusion–spheronization route, which resulted in comparably stable and effective conversion as the optimized CaO powder. Additionally, the considerable mechanical strength of MgO-stabilized CaO pellets should enable their realistic application in fluidized bed reactors. Thus, this simple, cost-effective and easily-scalable synthesis technique appears to have great potential for CSP-TCES under high temperature operation.Item Open Access Sorption of CO2 on NaBr co-doped Li4SiO4 ceramics: Structural and kinetic analysis(Elsevier, 2019-07-13) Wang, Ke; Zhao, Youwei; Clough, Peter T.; Zhao, Pengfei; Anthony, Edward J.Structurally modified and improved NaBr-co-doped Li4SiO4 ceramics were developed for CO2 absorption in low-CO2-concentration atmospheres. Pure and NaCl-doped Li4SiO4 ceramics were also prepared for comparison. The samples were analyzed by X-ray diffraction, Scanning Electron Microscopy, N2 adsorption, X-ray photoelectron spectroscopy, differential scanning calorimetry, and thermogravimetric analyses (dynamic and isothermal). The sorption kinetics were obtained using a double exponential model. The results showed that both Na and Br can be introduced into the Li4SiO4 structure and doped on Li and oxygen sites, respectively. The doped sample presented a Li2O-enriched surface, guaranteeing abundant LiO sites and are significantly different from previous anionic (CO3, F and Cl) doping of Li4SiO4, Br doping also generated macroporous features with small particle size, these favorable characteristics promoted the surface chemisorption kinetics. Moreover, DSC analysis confirmed the formation of the molten phases during CO2 absorption, which helps improve the lithium diffusion kinetics. Here, 0.1 mol NaBr doping was used to reach a maximum absorption capacity (>30.0 wt%) in 15 vol% CO2, suggesting that NaBr-doped Li4SiO4 ceramics have great potential for CO2 capture at high temperature.Item Open Access Structural and kinetic analysis of CO2 sorption on NaNO2-promoted MgO at moderate temperatures(Elsevier, 2019-04-12) Wang, Ke; Zhao, Youwei; Clough, Peter T.; Zhao, Pengfei; Anthony, Edward J.(200-500 °C) CO2 capture. However, the structure-performance relationship and kinetic characteristics of NaNO2-promoted MgO remain unclear. Here the effects of physical-chemical properties on the CO2 sorption performance of NaNO2-promoted MgO and the sorption kinetics were comprehensively studied to elucidate the detailed role of NaNO2. Samples were characterized by X-ray diffraction, scanning electron microscopy, N2 adsorption, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The sorption kinetics were obtained by isothermal thermogravimetry and elucidated using a double exponential model. Compared with pure MgO and NaNO3-promoted MgO, NaNO2-modified MgO had a lower initial sorption temperature and a unique bimodal sorption characteristic. Characterizations results revealed that such bimodal sorption was due to the presence of double promoters (mixture of NaNO2 and NaNO3) which implies that some of the nitrite was oxidized to nitrate during the preparation process. Deposition of double promoters further reduced the amounts of hydroxide and carbonate species for pure MgO while still preserving more hydroxide and carbonate species on the surface as compared with NaNO3-promoted MgO. The kinetics analysis demonstrated that the double exponential model can well-describe the sorption process for both NaNO3- and NaNO2-promoted MgO, suggesting that the entire sorption occurs as a double process (surface chemisorption and product layer diffusion). Significantly differences were seen from NaNO3-promoted MgO, and the surface chemisorption process of NaNO2-promoted MgO was independent of temperature, which suggests that an increased presence of hydroxide and carbonate species provide more active sites for greatly facilitating surface chemisorption.Item Open Access Synthesis of highly effective stabilized-CaO sorbents via a sacrificial N-doped carbon nanosheet template(Royal society of Chemistry, 2019-03-18) Wang, Ke; Clough, Peter T.; Zhao, Pengfei; Anthony, Edward J.Calcium looping, a promising high-temperature CO2 capture technique, offers a midterm economic solution to mitigate anthropogenic CO2 emissions. The main challenge for calcium looping is the synthesis of highly efficient CaO-based sorbents that can be used over many reaction cycles. Here, a sacrificial N-doped carbon nanosheet template was developed which produces MgO-stabilized, CaO sorbents with fast adsorption rates, high capacities and remarkable long term performance over many cycles. The characterization results show that such a template was formed through in situ pyrolysis of an organic acid and nitrates in a simple heating process under nitrogen. The presence of a carbonaceous template prevented crystallite growth, featured highly macroporous nanosheet (~60 nm thick) morphologies, ensured homogeneously mixing of Ca and Mg, which is essential to attain minimal diffusion limitations, mitigated sintering, and produced structural stabilization. Thus, 10 mol% MgO acting as an inert stabilizer was sufficient to achieve a CO2 uptake of 0.65 g/g (corresponding to a capacity retention of 89.9%) after 10 cycles in realistic conditions, as confirmed by TGA analyses. This N-doped carbon template can be applied generally to form a wide range of porous and nanostructured stabilized-CaO sorbents with stable CO2 uptakes.