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.

Contact:

Dr Eur-Ing S Rafailidis