BECCS – Bio Energy and Carbon Capture Storage

The Stockholm Exergi KVV8 facility is Europe’s largest biomass-based Combined Heat and Power plant. By capturing the plant’s biogenic carbon dioxide emissions and permanently storing it in the geosphere, the carbon flow is reversed, and a carbon sink is formed. CO2 that was previously circulating between the atmosphere and trees will now be withdrawn from the atmosphere, directly contributing to cooling of the atmosphere.

Flow-diagrams showing the natural cycle for a bio-fuelled CHP-plant (left) with net zero emissions of carbon dioxide, and the natural cycle when completed with capture and storage (right) leading to negative emission of carbon dioxide

To accomplish this, the CO2 in the flue gases of the bio-energy CHP must be captured. Based on careful assessment and previous experience of carbon capture, Stockholm Exergi has chosen to work with the well-proven Hot Potassium Carbonate technology.

The flue gas with the carbon dioxide meets a solvent liquid, which mostly consists of water and potassium carbonate. Potassium carbonate is a harmless ionic compound (salt) that is also used as a pH adjuster in food.

 

 

Pressurized potassium carbonate reacts with CO2 and H2O, which forms potassium bicarbonate  (i.e. baking powder). When the potassium bicarbonate pressure is lowered, the reaction goes in the opposite direction, releasing CO2 and H2O. In this way, the process works continuously and the CO2 can be separated and captured.

The KVV8 plant, the HPC installation and shipment to geological storage.

After the capture process, the biogenic CO2 will be purified and then liquefied. The purpose of the liquefication is to make the transportation more efficient, since liquified CO2 requires much less volume than CO2 in gas form.  The pressure can, typically, be 7 bar, with the temperature 50 degrees Celsius below zero.  After liquefaction the carbon dioxide is stored in buffer-tanks, before being shipped to the final storage site.

In the capture and liquefaction processes, excess heat is released, which we reuse in Stockholm’s district heating network.

The final storage site is where the CO2 will reside for thousands of years. Already in 2005 did IPCC conclude that geologically stored CO2 had a probability of 99% or more to remain in storage after 1 000 years*.

To achieve this level of permanence, the site must fulfil the following criteria:

  1. The carbon dioxide will be stored in sub-water sedimentary bedrock (saline aquifer) at depths greater than 800 meters, which is the requirement to ensure that the carbon dioxide is liquid or in a so-called super critical phase
  2. The bedrock must have sufficient porosity (micropores).
  3. There must be contact between the pores so that the carbon dioxide can fill the space.
  4. A dense rock must be present above the aquifer, which can act as a ”roof”, so that the carbon dioxide remains in the bedrock.
  5. Geologically stable area.
  6. Satisfactory technical solutions for injection, decommissioning or sealing of wells.
  7. Continuous monitoring.

Based on a long experience of working with CO2 geological storage since 1995, there are several sites around the North Sea which can fullfill these criteria. More information on geological storage is available at the European research network CO2 GeoNet.

 

BioChar

Every year more than 50,000 tonnes of biomass waste material (sticks and branches) are collected and incinerated in the Stockholm area. Although incineration facilitates recover energy and landfilling is avoided, valuable materials are not recycled, and nutrients are lost permanently from the biological cycle.

Instead, these nutrients could be recycled to improve soil quality in gardens, forests and agricultural fields. At the same time, biogenic carbon could be stored for more than 100 years, representing negative emissions similar to BECCS, however with a shorter time horizon.

The process to achieve this is called pyrolysis and the end result is called Biochar. This technology is fundamentally the same as was used previously in history with the help of a charcoal pile, however with a higher efficiency. In the early industrialization, the resulting charchoal was used for instance by the iron industry since it was easier to transport than timber and produced more heat during incineration.

Technically, pyrolysis is a process where a material, in this case wood, is decomposed by exposing it to high heat in the absence of oxygen, preventing it to catch fire (incinerate). The end product, biochar, is a stable solid rich in nutrients and carbon that can be stored in soil for more than hundred years.  In addition to being a carbon sink and increasing soil fertility, it can also, for instance, be used in animal feed.

By deploying the biochar to enrich soils in the city, a positive spiral is created where the growth of trees and other biomass increase, which in turn capture more CO2. Healthy vegetation provides additional ecosystem services in urban areas, such as removal of particulate pollutants, reduction of heat island effects, water drainage during storm rains, and wind/noise reduction.

In the process deployed by Stockholm Exergi, the excess heat of the pyrolysis process is also recovered and used in the district heating system of Stockholm.

 

* IPCC (2005): IPCC Special Report on Carbon Dioxide Capture and Storage. Prepared by Working Group III of the Intergovernmental Panel on Climate Change. Metz, Davidson, de Coninck, Loos, and Meyer (eds.). Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 442 pp. In SR15 Chapter 2.3.4.2, it is noted that newer studies show that 70% would still be retained after 10 000 years.