While calcium oxide has been identified to be the best candidate for capturing CO 2 at high temperature, it suffers a well-known problem of loss-in-capacity; that is, its capacity for carbon capture decays dramatically during cyclic carbonation/ calcination processes. Recently, extensive research work has been conducted on the enhancement of the cyclic performance of calcium oxide through either improving the performance of natural minerals, such as water/steam hydration and pretreatment or modification of calcium oxide sorbents by some techniques such as doping and synthesis. This work summarizes the state-of-theart research in the literature aiming to identify potential solutions to the loss-in-capacity problem. It is found that hydration during or after calcination is effective in recovering the capacity of natural minerals and mixing can produce highly effective synthetic sorbents. Periodic hydration of synthetic sorbents could be a good strategy to overcome the technical issues associated with loss-in-capacity while meeting the requirements of the physical properties of sorbents in many potential applications.

Sorbents for high temperature CO2 capture are under intensive development owing to their potential applications in advanced zero emission power, sorption-enhanced steam methane reforming for hydrogen production and energy storage systems in chemical heat pumps. One of the challenges in the development is the prevention of sintering of the sorbent (normally a calcium oxide derivative) which causes the CO2 capture capacity of the material to deteriorate rapidly after a few cycles of utilization. Here we show that a simple wet mixing method can produce sintering-resistant sorbents from calcium and magnesium salts of d-gluconic acid. It was found that calcium oxide was well distributed in the sorbents with metal oxide nanoparticles on the surface acting as physical barriers, and the CO2 capture capacity of the sorbents was largely maintained over multiple cycles of utilization. This method was also applied to other organometallic salts of calcium and magnesium/aluminum and the produced sorbents showed similarly high reversibility.

A device is presented for efficiently enriching parahydrogen by pulsed injection of ambient hydrogen gas. Hydrogen input to the generator is pulsed at high pressure to a catalyst chamber making thermal contact with the cold head of a closed cycle cryostat maintained between 15 and 20 K. The system enables fast production (0.9 standard liters per minute) and allows for a wide range of production targets. Production rates can be systematically adjusted by varying the actuation sequence of high-pressure solenoid valves, which are controlled via an open source microcontroller to sample all combinations between fast and thorough enrichment by varying duration of hydrogen contact in the catalyst chamber. The entire enrichment cycle from optimization to quantification and storage kinetics are also described. Conversion of the para spin-isomer to orthohydrogen in borosilicate tubes was measured at 8 minute intervals over a period of 64 hours with a 12 Tesla NMR spectrometer. These relaxation curves were then used to extract initial enrichment by exploiting the known equilibrium (relaxed) distribution of spin isomers with linear least squares fitting to a single exponential decay curve with an estimated error less than or equal to 1 %. This procedure is time-consuming, but requires only one sample pressurized to atmosphere. Given that tedious matching to external references are unnecessary with this procedure, we find it to be useful for periodic inspection of generator performance. The equipment and procedures offer a variation in generator design that eliminate the need to meter flow while enabling access to increased rates of production. These tools for enriching and quantifying parahydrogen have been in steady use for 3 years and should be helpful as a template or as reference material for building and operating a parahydrogen production facility.

The global warming issue has resulted in a great demand in zero emission power generation systems. Recently a few new concepts have been proposed that promise to achieve zero emissions while delivering unprecedentedly high efficiency. In these concepts, CO2 adsorbing material (CAM) is a key component. This paper reviews briefly the current development in CO2 adsorbing material, screens the potential materials under the conditions relevant to those in zero emission power generation systems, and identifies the best candidate and the optimum operating conditions for the production of high-purity hydrogen. It is found that CaO is thermodynamically the best candidate among metal oxides for CO2 capture in zero emission power generation systems. There exists a region within which high-purity H2 can be produced in steam methane reforming and carbon gasification.

A screening of potential calcium precursors for the production of CaO sorbents for CO(2) capture at high temperature was conducted in this work. The precursors studied include microsized calcium carbonate (CC-CaO), calcium hydroxide (CH-CaO), nanosized (<70 nm) calcium carbonate (CC70 nm-CaO), nanosized (<160 nm) calcium oxide (CaO160 nm-CaO), calcium acetate hydrate (CA-CaO), calcium l-lactate hydrate (CL-CaO), calcium formate (CF-CaO), calcium citrate tetrahydrate (CCi-CaO), and calcium d-gluconate monohydrate (CG-CaO). The capture capability of these sorbents was investigated using a thermogravimetric analyzer (TGA) for multiple capture cycles. CG-CaO exhibited the best capacity for capturing CO(2) with a 1-min conversion of 65.9% and a 30-min conversion of 83.8% at the ninth cycle. Subsequently, a further parametric study was conducted to examine the effect of reaction conditions such as reaction temperature (550-750 degrees C) and CO(2) gas concentration (1-15%) on the capture capacity of CG-CaO. The sorbent CG-CaO also showed a much lower decomposition temperature and higher predicted residual conversion after prolonged cycles, compared with CC-CaO.

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The reversible reaction of CaO with CO2 can be used for CO2 capture. However, two challenging problems, i.e., loss-in-capacity and high attrition rate for CaO-based sorbents, must be solved before it can be practically applied. In this paper, sorbents with various CaO contents were prepared from calcium hydroxide and cement using a screw extruder and the physical and chemical properties of the sorbents were obtained. The mechanical properties of sorbent particles were tested by friability and compression testers, and the sorption capacity and regenerability were measured in a thermogravimetric analyzer. It appears that the sorbents occupied acceptable attrition resistance and good mechanical strength. In the meantime, the sorbent particles showed higher CO2-capture capacities compared to pure CaO after 18 cycles, which was attributed to the stable phase of Ca12Al14O33 in the sorbents. The sorbent particles would be suitable for the calcium looping processes.