Novel synthetic calcium oxide based sorbent for carbon dioxide capture

• Main Author: Wang, Yan
• Other Authors: Heng, Jerry; Williams, Daryl
• Published: Imperial College London 2015
• Subjects: 660
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.659542

This thesis focuses on studying the synthesis of calcium oxide- (CaO-) based sorbents for carbon dioxide (CO2) capture in the post-combustion process. Calcium oxide has been regarded as one the most promising candidates for carbon capture in the last decade due to its high capturing efficiency, low running cost, and abundance in the natural world. However, the main drawback of this category of sorbents (natural limestone and modified CaO) is the rapid decay of the CO2 uptake capacity during the cycles of carbonation and decarbonation reactions. Therefore the target of this research is to enhance sorbent sustainable performance in long-term carbon capture utilisation, for the purpose of reducing the total budget of carbon capture in fossil-fuel power industries. To obtain the optimal CaO-based sorbent, different sacrificial particles were used in the sorbent modification experiment, including hydrophobic polymers and non-ionic surfactants. Among the combinations, modified CaO sorbents prepared with polyethylene glycol (PEG) and Tween80 (also called Polysorbate 80) delivered the best performance. Using sacrificial particles resulted in changing the properties of CaO particles, both physically and chemically: particle size, morphology, surface area and porosity were carefully controlled under specific synthesis conditions, and positively affected the sorbents' reactivity. A more important factor, which has been ignored by most researchers, the polymorph of sorbent precursor, was also investigated in this thesis. Repeatable results proved that of the sorbents derived from all the three polymorphs of calcium carbonate (calcite, aragonite and vaterite) vaterite-derived sorbent has the best CO2 capture capacity and reversibility. We found that the fraction of vaterite would influence the sorbent particles' reactivity proportionally in the first-cycle carbonation process. In order to study the sorbent's physical/chemical properties and its CO2 uptake performance, standard laboratory characterisation methods and a thermal-gravimetric analyser were employed, respectively. A combined gas sorption experiment under controlled conditions using a self-built high pressure reactor also revealed the mechanism of CO2 sorption on different types of porous materials. The existing equipment used to achieve this purpose usually requires a very large scale and involves a rather complicated micro-mechanical structure. A novel measurement methodology based on micro-cantilever design and laser detector was introduced in order to reduce this complexity. Physisorption of CO2 gas molecules by porous materials, such as Zeolite and MCM41, was measured kinetically with the help of this setup. A comparison between chemisorption and physisorption provides useful insights in regard to the search for the best solution for CO2 capture.