CO2 Balance in a Compression and Purification Unit (CPU)

Abstract

The next step for many CO2 capture technologies is to move to commercial scale [1]. Without the experience that can only be gained through pilot plants, CCS will not become a commercially viable proposition due to unresolved technical challenges and uncertain cost estimations. Individual demonstration projects need to be at a scale that is sufficiently large to be representative of commercial operation.One of these individual projects was carried out by Fundación Ciudad de la Energía (CIUDEN), who successfully completed the full CO2 capture process in its Technology Development Centre for CO2 Capture and Transport using oxycombustion in Circulating Fluidized Bed (CFB) boiler and a cryogenic CO2 Compression and Purification Unit (CPU).Focused on the CIUDEŃs CPU, this paper describes the unit designed by Air Liquide, their sub-units in which the CO2 composition is increased to achieve the design value of >99.0% v in the liquefied product stream and a final case study based on an empirical mass balance; considering this data, in addition to the CO2 purity, a mass balance provides the recovery rate value; this last parameter was lower than the expected value. Studying the specific behaviour of each subunit, it is concluded that the warm-part of the CPU presents a recovery rate higher than 99% whereas the cold part of the unit (expansion and distillation columns to remove NOx and incondensable gases) obtained an average value of 60%, generated mainly because the CO2 composition in the inlet of the CPU was lower than 70% v w/b (design value). This was due to higher N2 content in the flue gas.Considering these results, the conclusions are very clear that air infiltration must be minimised. It also demonstrates the necessity of increasing the quantity of operating hours on demonstration plants such as CIUDEN in order to further optimise the technology with the end goal of improving the business case for CCS.