Carbon Dioxide Removal (CDR)

Geoengineering II - Carbon Dioxide Removal (CDR) 

Another popular geoengineering method is Carbon Dioxide Removal (CDR), which targets to remove the Greenhouse gases (GHGs) concentrations in the atmosphere. Particularly, CDR aims to remove excess CO2 from atmosphere and various carbon stores in land biosphere, ocean and deep geological reservoirs since it has high possibility of reversing the anthropogenic impacts posed on earth by excessive CO2 emissions. 


Different approaches in CDR (Caldeira et al., 2013) 
1) Afforestation of previously non-forested land and reforestation of land that had been previously altered from forest to other uses. This approach not only aims to change the surface albedo using tree canopies, but also to increase carbon storage as well (Caldeira et al., 2013). 

2) Biomass energy with CO2 sequestration (BECS) refers to capturing CO2 in the atmosphere from electric power plants, fueled with biomass energy, and stores it underground for a long-term storage site such as deep geologic formation or deep ocean (Metz et al., 2005). A diagram below shows example sites than can used to store CO2;

A diagram showing various options for CO2 storage (from Cook, 1999 in Metz et al., 2005). 

First engineered injection of CO2 into the underground geological formation site was launched in Texas, USA, in early 1970s. It was part of the enhanced oil recovery (EOR) projects and by the late 1990s further research projects have been embarked in many other locations (Metz et al., 2005). If such 'permanent' store of carbon underground is successful, some suggests that it is possible to achieve even negative emissions (Caldeira et al., 2013). 


3) Land-based accelerated weathering is the approach to intentionally accelerate the natural chemical weathering reactions, which remove current anthropogenic CO2 emissions from atmosphere or transfer them to the oceans. Normally, natural weathering only removes in the rate of 0.1 Pg C annually from atmosphere but through various intentional methods this process can be occur at faster rate, helping to reduce the existing anthropogenic emissions. One of the methods accelerating the weathering reaction are using silicate minerals like olivine, which could be spread on farmland or forestland extensively to store some of the atmospheric CO2 as a carbonate mineral component (Schuiling and Krijgsman, 2006). 

4) Ocean-based enhanced weathering proposes various methods to enhance the ocean's ability to absorb (additional) CO2. One of the ways involve heating the carbonate minerals such as limestone to produce lime and add them to the oceans to increase the alkaline property, thus improve the ability of ocean to uptake atmospheric CO2 (Caldeira et al., 2013). Also, carbonate minerals can just be directly released into the oceans (Harvey, 2008) or reacted with concentrated CO2 captured at power plants to produce bicarbonate solution, then released to the oceans (Rau, 2008). 

5) Ocean fertilization approach is somewhat similar to the ocean-based weathering, since it proposes to add nutrients to the ocean to increase the planktonic productivity that can positively affect the CO2 removal; greater uptake of atmospheric CO2 by ocean and increase in the downward flux of CO2 out of the near-surface layers of the ocean (Caldeira et al., 2013). 

6) Direct capture from air aims to apply chemical processes to separate CO2 from rest of atmosphere, which then transported and used for commercial purposes (Caldeira et al., 2013) or stored underground as mentioned in BECS section. Three major methods are suggested for CO2 capture, including post-combustion, oxy-fuel combustion and pre-combustion systems. Although it could result in massive uptake of CO2 emissions from atmosphere, challenges of health, safety and environmental risks as well as the legal issues still exist in debate (Metz et al., 2005). 



Next post will explore about the concerns raised in CDR implementation.