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If there is a particularly high concentration of sulphur dioxide in the flue gases, or if especially high removal efficiencies are required, the contact zone of the single loop absorber in the limestone-gypsum process can be divided into two loops. Then the limestone suspension and flue gas are set in a counter-current in two stages. The flue gas passes through an initial suspension loop, which operates like the suspension loop of the single loop absorber; suspension is circulated from the absorber sump. This loop contains approx. 30 % absorbent concentration, in order to achieve good-quality gypsum. In the second suspension loop, on the flue gas side above the first, fresh absorbent is added and a very high concentration is used. This loop has an external sump. The proportion of gypsum suspension, which was removed from the first loop, is replaced by feeding suspension from the second loop. Due to the high excess of absorbent in the second loop, very high levels of separation are achieved, while at the same time the system is well able to manage quick changes in load and changes of the raw gas SO2 concentration. This is especially important for modern, flexible coal-fired power stations. Equally, the height of the absorber is much lower than the corresponding single loop absorber, yet with the same desulphurisation capacity. Thanks to the clearly more efficient counter-current process, operating costs are significantly lower than for single loop absorbers. When deciding on a dual loop absorber, the additional investment costs for the installation (CAPEX) should be compared with the savings in operational costs (OPEX).
Advantages of the dual loop absorber
Best SO2 separation efficiencies