Method and a device for cleaning a carbon dioxide rich flue gas

1. A method of cleaning a carbon dioxide rich flue gas containing sulphur dioxide generated in a boiler combusting a fuel in the presence of a gas containing oxygen gas, the method comprising:

removing at least 80% of the sulphur dioxide content of the flue gas generated in the boiler in a first gas cleaning device to generate a partly cleaned carbon dioxide rich flue gas,

forwarding at least a portion of the partly cleaned carbon dioxide rich flue gas to a second gas cleaning device, the second gas cleaning device being separate from the first gas cleaning device,

cooling, in said second gas cleaning device, the partly cleaned carbon dioxide rich flue gas to condense water therefrom, thereby generating a cleaned carbon dioxide rich flue gas having a lower concentration of water vapour than the partly cleaned carbon dioxide rich flue gas, and

forwarding at least a portion of the condensed water that is formed in the second gas cleaning device to the first gas cleaning device as make-up water,

wherein 50-90% by volume of the partly cleaned carbon dioxide rich flue gas leaving the first gas cleaning device is recirculated to the boiler without such portion being first passed through the second gas cleaning device.

2. The method according to claim 1 , wherein said step of cooling, in the second gas cleaning device, the partly cleaned carbon dioxide rich flue gas to condense water therefrom, comprises bringing the partly cleaned carbon dioxide rich flue gas into direct contact with a cooling liquid.

3. The method according to claim 2 , wherein the pH-value of the cooling liquid is controlled to be in the range of pH 4-6.5 by means of supplying an alkaline substance having a solubility in water at 20° C. of at least 50 gram/liter to said cooling liquid.

4. The method according to claim 3 , wherein said alkaline substance is chosen from the group of alkaline substances comprising sodium hydroxide, potassium hydroxide, and sodium carbonate.

5. The method according to claim 2 , wherein said cooling liquid comprises less than 10 gram/liter of solids.

6. The method according to claim 1 , wherein said step of cooling, in the second gas cleaning device, the partly cleaned carbon dioxide rich flue gas to condense water therefrom comprises cooling the partly cleaned carbon dioxide rich flue gas to such a temperature that the cleaned carbon dioxide rich flue gas comprises 0.5 to 8% by volume of water vapour.

7. The method according to claim 1 , wherein the at least a portion of the condensed water that is forwarded to the first gas cleaning device as make-up water is passed through at least one pipe connected between the first gas cleaning device and the second gas cleaning device.

8. The method according to claim 1 , wherein said step of removing at least 80% of the sulphur dioxide content of the flue gas generated in the boiler in the first gas cleaning device, comprises bringing the flue gas generated in the boiler into direct contact with a calcium based absorbent.

9. The method according to claim 1 , wherein said second gas cleaning device is controlled to remove at least 70% of the sulphur dioxide content of the partly cleaned carbon dioxide rich flue gas.

10. The method of claim 1 , wherein:

the first gas cleaning device is connected to the boiler, and the second gas cleaning device is connected to the first gas cleaning device,

the first gas cleaning device is operative to receive the carbon dioxide rich flue gas generated in the boiler to remove at least a portion of the sulphur dioxide content of the flue gas generated in the boiler, and

the second gas cleaning device, being separate from the first gas cleaning device, is operative to receive at least a portion of the partly cleaned carbon dioxide rich flue gas that has passed through the first gas cleaning device.