CO₂ Separation and Liquefaction

From phase equilibria measurements it is possible to predict the compositions downstream of the exhaust gas treatment train of an Oxyfuel power station, if it can be assumed that in practice equilibrium conditions are in fact achieved there. To investigate the real behaviour and kinetics of the CO2 liquefaction process, a CO2 separation test rig has been built.

A part of the exhaust stream from the Drop Tube Furnace is taken, cooled-down and cleaned with a cyclone. In a first condenser step the water is condensed out of the stream. A compressor, equipped with two inter-coolers, raises stream pressure to up to 60 bar. In a downstream high-pressure heat exchanger the remaining water condenses out, before complete drying is achieved with silica gel. Central part of the liquefaction installation is the CO2 condenser, which is held at a temperature of -50 °C by means of a cryostat. The two phases, i. e., the liquefied CO2 and the non-condensing gases, are sampled independently for subsequent chemical analysis. The two streams are at the end mixed once more, re-warmed and throttled back to atmospheric pressure.

The research capabilities of the experimental CO2 liquefaction installation encompass three key areas:

a)        Water Condensation from the Oxyfuel Exhaust Gas

The chemical composition of the water condensate at various pressure and temperature levels in the process is to be investigated. This will enable comparison of the water solubility of, for instance SO2, as given in the literature with an industrially realistic condensation process. Moreover, it will make possible to study the behaviour of the silica gel under the influence of real Oxyfuel exhaust gas conditions.

b)        CO2 Purity

After the liquefaction of the CO2 both stream phases above will be sampled and analysed through chromatography. This will yield the exact stream compositions, which from the previous equilibrium studies could be estimated only approximately. Especially for the liquid phase this knowledge is of particular importance, as this stream contains the CO2 which afterwards will be stored through sequestration.

c)        Kinetics and Process Behaviour

With the experimental CO2 liquefaction installation it is possible to determine the interaction between the individual process components. Especially the kinetics of the water condensation and of the cryogenic CO2 separation can be studied experimentally, to gain first experiences for the design and subsequent operation of an Oxyfuel CO2 separation train at industrial scale.



Dr Eur-Ing S Rafailidis