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The sorbent conversion has been calculated as a function of the gas–solid residence time in each reactor element. However, Ca(OH) 2 particles can achieve carbonation conversions of around 0.7 at temperatures of 550–650 ☌ and CO 2 concentrations in the range of 5–25% v in less than 6 s of gas–solid contact times, (19) therefore facilitating the design of more compact reactors. (25) For post-combustion CO 2 capture using CaO as a sorbent, long entrained bed reactors with several tens of meters are needed to ensure sufficient gas–solid contact times (typically around 20 s) and sorbent conversion. (24) Other reactor configurations such as multiple cyclonic reactors connected in a tower to achieve a countercurrent gas–solid contact, similar to pre-calciners of cement and lime industries, could also be used. Indeed, these types of reactors have been proposed for CO 2 capture using fine CaO particles as a sorbent in cement plants (21−23) and energy storage applications. Entrained bed gas–solid reactor configurations are better suited to handle this kind of particle.

When capturing from a flue gas with a higher CO 2 concentration (14% v CO 2), the sorbent needs to be fed at higher temperatures to effectively capture CO 2 in short contact times (i.e., 6 s).Ĭa(OH) 2 produced by hydration of CaO presents poor fluidization properties, making the use of circulating fluidized bed carbonators challenging. CO 2 capture efficiencies above 90% can be achieved for flue gases with low CO 2 concentrations (4% v CO 2) when they are fed into the carbonator at temperatures of around 500–600 ☌ while maintaining low F Ca/ F CO2 ratios (<2) and feeding the sorbent at ambient temperature. In this work, a basic entrained reactor with recent experimental reaction kinetics has been used to determine suitable operational windows for this kind of carbonator. However, the low enthalpy of the carbonation reaction of Ca(OH) 2 makes it challenging to find the optimum conditions which maximize the CO 2 capture efficiency. To take advantage of these fast reaction kinetics, compact carbonator reactors with short gas–solid contact times (i.e., a few seconds) can be designed. The use of Ca(OH) 2 instead of CaO is better suited to these applications due to the faster reaction kinetics and higher carbonation conversions as Ca(OH) 2 in powder form can achieve conversions of up to 0.7 in just a few seconds at temperatures of 550–650 ☌.

Flexible calcium looping configurations based on the storage of solids are a promising alternative to capture the CO 2 produced in such backup combustion systems. The share of renewables in the energy sector is increasing, and energy storage and backup power combustion systems to cover the periods of time with low renewable energy production are becoming increasingly needed.

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