Changing bioenergy from carbon neutral to carbon negative with carbon capture and storage

In Denmark, 30 percent of all energy consumed comes from renewable sources, and the country intends to increase this percentage to 50 percent by 2030 with the long-term goal of becoming a low-emission country, independent of fossil fuels by 2050. Denmark is far ahead in the process of changing its heat and power production from coal-fired combined heat and power plants (CHP) to renewable energy such as wind, solar and geothermal, supplemented with CO₂ neutral bioenergy (biomass, biofuel and biogas).

With 70 percent of the renewable energy consumption in Denmark being bioenergy-based, this offers the opportunity to change the bioenergy from being CO₂ neutral to being CO₂ negative by implementing carbon capture and storage.

So far technology used for carbon capture and storage has been way too expensive, and previous attempts at implementing the technology at CHP plants have been abandoned.

Now, a techno-economic study of the feasibility of a future integrated CO₂ neutral energy system, where the subsurface is utilised for thermal energy supply and storage, as well as temporary CO₂ storage is being performed as part of a research project carried out by Ramboll, GEUS, DTU and SINTEF. The study is supported by the Danish energy technology development and demonstration research programme EUDP.

“The study looks very promising so far, and the technology used seems to be technically feasible for major CHP plants based on biomass, and with heat recovery for district heating, the economic feasibility has improved dramatically”, says Ramboll engineer Thomas Paarup Pedersen, who is part of the core research team.

The study analyses a generic bio-plant retrofitted with a CO₂ capture. The generic bio-plant is benchmarked against a reference power plant, the biomass-fired Avedøre 1 CHP plant, located in the outskirts of Copenhagen. The plant has a capacity of 640 MWth, with net electric power output of 219 MWel and district heating production of 352 MWth at full-load conditions. The plant operates with 100% wood pellets. The Avedøre plant is considered representative of a state-of-the-art power plant that has been converted from operating with coal to biomass.

In the modelling work, the bio-plant is retrofitted with so-called Monoethanolamine (MEA) capture technology, a well-known carbon capture technique of CO₂ absorption by using amines. In the process, steam from the CHP production is used to regenerate the amine used to capture CO₂. When leaving the capture plant, the CO₂ is compressed to 110 bar, transported by pipeline 3 km towards northeast, and injected into a storage reservoir in 1300 metres depth. 30 years injection of 1 million tonnes/year of CO₂ is feasible from one injection well. The transport and injection case is based on capture from Nordjyllandsværket CHP plant after possible conversion to biomass-firing.

The research results so far show that it is possible to recover a considerable amount of heat from the CO₂ capture process to the DH system for use in the district heating system, significantly improving the thermal efficiency of the bio-plant and consequently reducing the cost of CO₂ capture by roughly 30%, or from €77 to €52 per tonne of CO₂ captured.