Oxyfuel for beginners

By Prof. Dr. Lars Strömberg, Vice President Group Function Research and Development

Whenever you combust a fossil fuel, be that coal, oil, gas or indeed wood, with the surrounding air, the flue gas consists fundamentally of water vapour, carbon dioxide and nitrogen compounds. This composition occurs in every combustion process – be that your garden grill or in a conventional power plant. If this flue gas is cooled down, the water vapour can be condensed out as water. What remains is a mixture of approximately 80 percent nitrogen and approximately 20 percent carbon dioxide.

If instead you combust the fuel in the power plant with pure oxygen and successively replace the share of nitrogen in the surrounding air by introducing flue gas into it, the CO₂ concentration in the flue gas can increase far above 90 percent. If you then, like with a conventional power plant, condense out the water vapour, the carbon dioxide remaining is in an almost pure form and can be further processed for transport and storage without any difficulty.

This is exactly how the Oxyfuel process works which is being implemented in the power plant with CCS separation at Vattenfall and which will lead to large-scale production. Hence with this method we do not need to utilise any additional energy in a costly process in order to filter out the carbon dioxide from the flue gas.

Regardless of this, corresponding filter and scrubbing technologies have not yet been developed for large-scale operations at present. However, for the Oxyfuel method, the surrounding air has to be fractionised in order to separate out the nitrogen so as to able to use the oxygen alone for the combustion. Unfortunately this air fractation consumes a certain share of the power plant’s own energy. On the other hand the technology required is about 100 years old and reliably available today on an industrial scale.

Furthermore the Oxyfuel method has the additional advantage that environmentally damaging nitrogen compounds which occur in conventional power plant processes and have to be separately filtered out of the flue gas are avoided to a great extent.

Of course just like in modern conventional plants, sulphur oxide, dust particles and heavy metals, etc., are separated out. Hence we will in fact manage to develop a power plant with close to no emissions of CO₂. There will never be an absolute breakeven situation – but we will come very, very close to this target.