Low embodied carbon cement using Ca-looping purged sorbent in clinker making process

Calcium looping is a promising post-combustion technology for capturing CO2 from flue gases by using solid CaO-based sorbents (e.g. limestone). It is based on a dual fluidized bed (DFB) system that involves a carbonator - where flue gas is absorbed by CaO - and a regenerator where captured CO2 is enriched to be processed and stored. The potential reuse of spent – from repeated cycles - CaO in cement raw meal is among the advantages of the technology. This could allow substantial CO2 savings in an industry that thrives for decarbonisation. In this study, purged CaO – taken from a 200 KWth DFB pilot facility in the University of Stuttgart - was used in raw meal. The aim was to evaluate if clinker (thus cement and concrete) of acceptable quality can be produced and if any bottlenecks exist. Purged CaO was found chemically compatible with raw meal and can replace all limestone in the mix without altering the quality targets. Its elevated SO3 content (approx. 2%) however allows for approximately. 10% addition or else clogging phenomena can occur in the preheater. Practically to allow complete limestone replacement and avoid undesired process implications – in kiln systems with no by-pass installed - SO3 of the purged sorbent must be kept up to 0.3%. Trace elements were found very low, posing no concern for clinker or emissions. Sulphur was in the form of anhydrite, thus it is not expected to give rise to SOx emissions. Burnability of raw meal was improved and clinker quality was found similar to the reference case. A higher C2S content that was detected was not in expense of C3S thus no detrimental effect on early strength is anticipated. It was estimated that for every 1% of purged CaO replacing limestone, a CO2 save of 3.6 kg/tn of raw meal can be achieved. Equivalently, a plant producing 1 mil. tons of clinker/annum, can save approx. 300.000 tons of CO2 emitted.