ARC is working towards making Amager Bakke CO2-neutral. With a full-scale plant for carbon capture, Amager Bakke will be capable of capturing up to 500,000 tonnes of CO2 annually, helping Copenhagen take a big step towards becoming CO2-neutral.
How carbon capture works
CO2 is captured by a chemical process in which the CO2 molecules are washed out of flue gas and bound in a liquid.
1. Your waste becomes electricity, district heating… and CO2
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 smoke from waste incineration has been purified, it’s approximately 40 degrees, and consists almost entirely of CO2 and water vapour. The purified smoke starts by being passed through an absorber. – As the name suggests, this is where the CO2 is absorbed from the smoke.
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 small amount of CO2 are fed out of the absorber and into the plant chimney. 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 105 degrees, causing the CO2 to be released. – It’s bit like fizzy drinks going flat when they get warm. The CO2 can then be collected and transported for storage or use.
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 collects CO2 again. And again. And again.
Due to the large amount of energy it takes to heat the amine liquid to over 100 degrees, it can be difficult to make carbon capture climate-friendly. But Denmark has the big advantage that we always have reused heat in our district heating network. The same applies to carbon capture at Amager Bakke: the energy we use to heat the amine liquid is not wasted, but is circulated into the district heating network. That lets us make carbon capture as close to energy neutral as possible.
4. CO2 storage and Power-to-X
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 carbon dioxide.
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.
5. CO2 negative = Climate positive
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.
Most of the waste at Amager Bakke comes from biogenic sources, but there is still approximately 10% plastic in the waste. If all of the CO2 (from biogenic and fossil sources) in one waste energy facility 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. Active 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.
FAQ about carbon capture
Carbon capture is the capturing of CO2 in gaseous form. It’s achieved through a chemical reaction in which the smoke from, for example, an industrial chimney is passed through a liquid that binds or absorbs the CO2. The liquid can then be heated, so that it releases its CO2 in a closed container, from which it can be collected. Collected CO2 can either be stored or used to produce fuel (Power-to-X).
Carbon capture is an important contribution and above all an available technology to meeting the Danish climate goals of 70% CO2 reduction by 2030. It also supports the Paris Agreement’s goal of reducing global temperature rises to 1.5 degrees. The UN Climate Panel, the European Commission, the International Energy Agency and the Climate Council all state that carbon capture is key to achieving the goals of the Paris Agreement.
During the capturing process, CO2 is compressed and transported to an intermediate storage facility at a nearby port. From there, it’s sailed out into the North Sea, where it is stored in depleted underground oil fields. Over the years, the CO2 will bind permanently to the underground strata.
On 24 June 2021, ARC commissioned a pilot plant able to capture a small amount of the CO2 in flue gas. This will be followed by a demonstration plant, expected to be operational by late summer 2023, and which can capture up to 4 tonnes of CO2/day. The goal is to commission a full scale carbon capture unit and capture up to 500,000 tonnes of CO2 annually.
The pilot facility can capture 850 kg. CO2 per day, and the subsequent demonstration plant will capture 4 tonnes per day. This corresponds to the emissions from 1,185 cars driving 30 km per day in the city. The captured CO2 from the demonstration plant will be sold and used, for example, to soften water or for the production of soft drinks.
A full-scale plant will be able to capture up to 500,000 tonnes of CO2 per year.
The technology behind carbon capture is very energy-intensive, and has therefore been too expensive to use in the past. By integrating carbon capture with the district heating network, ARC will demonstrate that it can be done with neutral energy consumption. This also means that ARC expects a price below DKK 1,000 per tonne CO2. That is, somewhat lower than the socio-economic price for CO2 emissions calculated by the Climate Council in 2020 of DKK 1,500 per tonne CO2.