Carbon Capture

CO₂ is captured by a chemical process in which the CO₂ molecules are washed out of flue gas and captured in a liquid.

ARC receives combustible waste every day from homes and companies at Amager Bakke. ARC utilises the energy in the waste for electricity and district heating.

Like all other combustion processes, waste energy also emits CO₂. But unlike air exhaled from our lungs, it’s easier to capture. Amager Bakke is perfect for capturing CO₂, because the plant is in operation 24/7/365. That provides a large, stable supply of CO₂ which is necessary for efficient carbon capture.

When the flue gas from the waste incineration has been purified, it’s approximately 40 degrees, and consists almost entirely of CO₂ and water vapour. The purified flue gas starts by passing through an absorber. – As the name suggests, this is where the CO₂ is absorbed from the flue gas.

The absorber is shaped like a tower, with smoke fed in from the bottom so that it rises upwards, while being flushed from above with an alkaline amine liquid. CO₂ is bound by and into the amine liquid. Only the water vapour and a the left over amount of CO₂ is exiting the chimney at the plant. The CO₂ containing amine liquid continues on to the desorber.

From the absorber, the CO₂ containing amine liquid continues on to the desorber, which is the most energy-intensive part of the process. In the desorber, the liquid is heated up to approximately 105 degrees. This releases the CO₂ from the amine liquid. – It’s a bit like fizzy drinks going flat when they get warm. The CO₂ can then be collected and transported for storage or utilization.

Once the amine liquid has released the CO₂ into the desorber, the liquid can easily be used again. Once cooled, it’s led back to the absorber, where it captures CO₂ again. And again. And again.

Due to the high energy consumption, it can be difficult to make carbon capture climate-friendly. But Denmark has the big advantage of our widespread district heating network where excess heat can be reused. This also applies to carbon capture at Amager Bakke: We have a goal of harvesting the surplus heat and utilize it in the district heating network when capturing CO₂ at a large scale in the future. This will enable net-zero energy consumption.

The CO₂ collected from the desorber is pressurised and stored in tanks, just like the CO₂ cartridges used for homemade soda: when CO₂ is mixed with water, it turns into carbonated water.

The captured CO₂ can either be stored below ground, or eventually converted into green fuels
(Power-to-X). According to GEUS, the Danish underground is particularly suitable for storing CO₂, and it will be possible to store 500-1000 years of total Danish emissions at the current level.

In CO₂ accounting, a distinction is made between CO₂ from fossil sources, such as plastic, petrol and gas, and CO₂ from biogenic sources such as wood, straw and potato peelings. Even though CO₂ is CO₂, a distinction is made between what is already in rotation between plants and the air (biogenic), and all the extra CO₂ we add to the atmosphere by digging or pumping it up from underground. Something is only considered CO₂-neutral if it only emits CO₂ from biogenic sources, e.g. if energy comes from burning straw or wood pellets.

A lot of the residual waste at Amager Bakke comes from biogenic sources. But there is still fossil residual waste ending up at Amager Bakke (and any other waste-to-energy plant). If all of the CO₂ (from biogenic and fossil sources) in one waste-to-energy plant could be captured, the plant would become CO₂-negative – It’s not just the extra CO₂ that’s removed, but also the CO₂ that was already in circulation. Biogenic CO₂ is thus removed from the atmosphere instead of being recycled.

Because carbon capture can lower the total amount of CO₂ in the atmosphere, the technology is considered one of the tools that can help effectively limit global warming in the short term.