Accelerated rock weathering
Accelerated rock weathering, known as “enhanced weathering (EW)”, is a chemical approach that makes use of the fact that rock naturally chemically weathers. This requires rainwater,
for example, which always absorbs a certain amount of atmospheric carbon dioxide as it falls to the ground. When carbon dioxide reacts with water, carbonic acid is formed. When rain falls on the surfaces of stones or rocks, this carbonic acid attacks and dissolves the minerals of which they are formed. The dissolved material is washed away with groundwater and surface water. In a further step, acid-binding minerals such as calcium and magnesium react with the carbon dioxide dissolved in the rainwater. Carbonate minerals are formed in the course of this reaction, or, to put it simply, new rock is formed in which parts of the former atmospheric carbon dioxide is firmly bound. In nature, rock weathering is a very slow process. However, accelerated weathering can be achieved by mining, crushing and then field-spreading suitable rocks over large areas to increase the reactive rock surface. Certain types of construction waste and residues from cement production or mining could also be used as source materials. Enhanced weathering processes have as yet only been tested and researched in lab and small-scale field experiments. Knowledge as to potential environmental risks or co-benefits of large-scale applications is therefore still lacking.
It is also still unclear where the required quantities of rock could be mined. According to the German National Academy of Sciences Leopoldina, in order to compensate for the unavoidable emissions in Germany, about 200 million tonnes of rock would have to be mined, ground and landspread annually. This would equate to roughly three quarters of the sand and gravel extraction for construction purposes in Germany in 2019. Experts note that the required logistic effort would likely be very high.
Direct carbon dioxide extraction from the ambient air
According to the Intergovernmental Panel on Climate Change, Direct Air Capture (DAC) methods of carbon dioxide fall into the category of “geochemical CDR methods”. DAC requires technical systems that draw in the ambient air and filter out the carbon dioxide it contains, using a chemical binder medium (liquid or solid). These chemical media are subsequently stripped from the carbon dioxide through the application of heat (up to 900 degrees Celsius) and moisture or under pressure − a generally highly energy-intensive process. This regenerates the chemical media for reuse and the removed carbon dioxide is either stored deep underground (Direct Air Carbon Capture and Storage, DACCS) or used for the production of carbon-containing products (Direct Air Carbon Capture and Utilization, DACCU).
The advantage of the DAC method is that it has a much smaller land footprint than other methods. Moreover, it also lends itself to locations that are not suitable for farming or forestry, such as deserts or inner city areas. However, since air contains very little carbon dioxide, such systems need to filter vast quantities of air, driving up their energy consumption and causing much higher costs than if the carbon dioxide were captured in a power plant or a steel mill.
A sample calculation: according to the German Environment Agency, even with a highly ambitious climate policy, at least five per cent of Germany’s greenhouse gas emissions would still be unavoidable in 2050 and would have to be offset by means of carbon dioxide removal. If one were to attempt to offset these unavoidable emissions through DAC methods, the energy required might amount to more than 100 terawatt hours per year. This would correspond to about one fifth of Germany’s electricity generation in 2021 (518 terawatt hours). However, since DAC processes mainly require heat, waste heat from industrial processes or geothermal energy could also be considered as energy sources.
According to the International Energy Agency, 18 DAC demonstration plants were already in operation in Europe, the USA and Canada in September 2022. Taken together, they were removing about 10,000 tonnes of carbon dioxide from the atmosphere per year. The captured gas was subsequently used mainly in beverage production (carbonic acid) and only a small proportion was injected underground for permanent storage. At that time, a DAC facility capable of capturing one million tonnes of carbon dioxide per annum was under construction in the USA.
Industrial-scale usage of DAC will depend on whether future facilities can be operated with renewable energy and whether sufficient water will be available wherever moisture is needed for the separation of carbon dioxide and binder media. In countries like Germany, the situation is further complicated by the fact that geological storage of captured carbon dioxide is controversial at the societal level and the process currently lacks public support.