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The calcium looping cycle is being developed as a method for capturing CO2 from both flue and fuel gases. It works by using CaO as a CO2 carrier and through repeated cycles of carbonation and calcination can extract CO2 from a gas with a lower partial pressure of CO2 (e.g., exhaust stream from a power station) and provide a pure stream of CO2 suitable for sequestration. A key problem in the development of calcium looping technology is the decrease in reactivity of the sorbent with an increasing number of cycles of carbonation and calcination. The hydration of calcined sorbent has been shown to be a promising way of periodically regenerating the sorbent, so that its reactivity can be recovered, reducing the requirement to purge material from the cycle. In previous work, the reactivity of sorbents after hydration has been mainly studied by thermogravimetric analysis or in a fluidized bed with an unrealistically low calcination temperature. For this work, a laboratory-scale reactor capable of operation under more realistic conditions has been designed, built, and commissioned. It consists of a computer-controlled, resistance-heated, fluidized-bed reactor capable of temperature cycling, allowing the sorbent to be exposed to repeated cycles of carbonation and calcination within the same vessel. The sorbent is “reactivated” by hydration after a number of cycles and then exposed to further cycles of CO2 capture and release. The reactivity of the sorbent is measured from the CO2 uptake and release during successive cycles of carbonation and calcination. Preliminary tests have been completed, and these show that, for limestone reacted under mild calcination conditions, the ultimate uptake of CO2 (the carrying capacity) of cycled Havelock limestone can be more than doubled upon hydration. As the calcination conditions before hydration become harsher (the temperature is increased), the regeneration technique becomes less effective. This is also observed, although to differing extents, with La Blanca and Purbeck limestones. This is shown to be due to mass loss from the fluidized bed because of the increased friability of the hydrated sorbent. A particle breakage model has been developed to describe this phenomenon.

Differences in the development of carbon structures between coal chars and metallurgical cokes during high-temperature reactions have been investigated using Raman spectroscopy. These differences are important for differentiation between different types of carbons present in the dust recovered from the top gas of the blast furnace. Coal chars have been prepared from a typical injectant coal under different heat-treatment conditions. These chars reflected the effect of peak temperature (from 900 to 2400 °C), residence time at peak temperature (from 2 s to 1 h), heating rate (from 1 to 6000 K/s), and pressure (from 3 to 40 bar a ) on the evolution of their carbon structures. The independent effect of gasification on the development of the carbon structure of a representative coal char has also been studied. A similar investigation has also been carried out to study the effect of heat-treatment temperature (from 1300 to 2000 °C) and gasification on the carbon structure of a typical metallurgical coke. Two Raman spectral parameters, the intensity ratio of the D band (1284-1600 cm -1 ) to the G band (ca. 1600 cm -1 ) (I D /I G ) and the intensity ratio of the valley between D and G bands to the G band (I V /I G ), have been found useful in assessing changes in carbon structure. An increase in I D /I G indicates the growth of basic graphene structural units (BSUs), across the temperature range studied. A decrease in I V /I G appears to suggest the elimination of amorphous carbonaceous materials and ordering of the overall carbon structure. The Raman spectral differences observed between coal chars and metallurgical cokes are considered to result from the difference in the time-temperature history between the raw injectant coal and the metallurgical coke. These observed differences may lay the basis for differentiation between metallurgical coke fines and coal char residues present in the dust carried over the top of the blast furnace.

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