The present invention relates to reactor vessels such as absorbing or stripping columns that are suitable for capturing carbon dioxide from flue gas streams of fossil fuel fired powered stations such as coal fired power stations generating 100 to 500 MW. The side walls of the reactors are made of concrete or structural ceramics that are preferrably steel reinforced. The reactors are on a sufficiently large scale such that a flue gas stream in the order of at least 1,000 t/hr and normally greater than 2,000 or 3,000 t/hr can be scrubbed of carbon dioxide in a single absorption column and then recovered in a stripping column. The absorbing and stripping columns may be free standing structures or, alternatively, the absorbing column may be located at least partially within the stripping column.