Plants for CO2 extraction from flue gases

CO2 extraction plants are based onimporting flue gas from boilers fired withvarious fossil fuels such as diesel oil, heavyfuel oil, kerosene, natural gas, LPG or LNG. Ifsteam and flue gas supply are continuouslyavailable significant energy savings up to 45 % can be achieved with an CO2 extraction plantcompared with traditional combustion basedplants.

Through appropriate scrubbing, stripping andseparation technology, the CO2 generatingplants meet the strictest CO2 quality requirements regardless of the fuel type.

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The plant is based on drawing flue gas fromboiler exhaust systems. The flue gas will have a CO2 content of 10-14 % v/v and is directed toa flue gas scrubber, in which the gas is cooledand water condensed. Any SO2 present in the flue gas will be removed by means of a chemical reaction with sodium carbonate (soda ash). The soda ash is automatically added to the scrubbing water by means of pH control.

After cooling and scrubbing, the gas is led via an exhauster through an absorber, in which the gas flows counter-current with a MEA solution flow. By chemical reaction, the MEA solution absorbs the CO2 from the flue gas. The MEA solution containing the absorbed CO2 (referred to as rich MEA solution) is first pressurized and heated in the heat exchanger and then led to the flash column. Here most of the contaminants are removed from the rich MEA solution by flashing to the absorber pressure. Further heating is added to the bottom of the flash column for further reduction of the contaminants in the MEA solution. This optimizes the process yield to the best possible CO2 product without any use of expensive chemicals.

Afterwards, the rich MEA solution is pumpedto a stripper, where the CO2 is released fromthe MEA solution by means of the steamheated MEA reboiler. The CO2 depleted MEA solution (referred to as lean MEA solution) isrecycled to the absorber. After exiting the top of the stripper, the CO2 rich gas is cooled in a gas cooler and washed in an after-scrubber. The gas is then compressed in two stages to approx. 15-18 bar(g) by the CO2 compressor. Prior to liquefaction, the gas is dried to a dewpoint of approx. - 60 °C in the dehydrator. Regeneration is done automatically by electrical heating and use of dry purge gas from the CO2 condenser. Traces of acetaldehyde are also removed in the dehydrator.

The CO2 gas then passes through a carbonfilter for removal of any odour substances. To remove the last non-condensable gases,the CO2 gas first passes a distillation processin the purification column. It is then condensed at a temperature of approx. - 27 /- 23 °C in a CO2 condenser, where the non-condensed gases are purged off. Finally, the liquefied CO2 is led to an insulated storage tank.

A refrigeration unit, controlled by the CO2 pressure in the CO2 condenser, supplies therequired refrigeration capacity. The liquid CO2 is stored under a pressure of approx. 15-18 bar(g) and a corresponding temperature ofapprox. - 27 /- 23 °C. During a non CO2 production period, the refrigeration unit is able to operate independently of the rest of the CO2 plant in order to maintain the correct CO2 storage tank temperature/pressure. 

The CO2 produced has a minimum purity of 99.998% (v/v) at oxygen residual content < 5 ppm and fulfills quality standards for brewing industry.

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