Carbon capture and storage – a commercial conundrum (Part 1)

Scientists have advised governments to focus on carbon removal to achieve COP21 Paris agreements. Although the UK pledged for net zero carbon emissions by 2050-2100, climate advisors have warned this is simply not enough. Realistically, carbon emissions from agriculture and aviation will not reduce to net zero. It is becoming increasingly unlikely that we will fall below the 400ppm carbon dioxide threshold. 

Thus, radical CDR technologies are required to reduce atmospheric carbon concentration.
Although I have discussed afforestation as a potential solution, I will now explore CDR in an industrial manner.

Carbon capture & storage (CCS) is a CDR technology to reduce atmospheric emissions. CCS technologies are comprised of 3 components:

• ‘Capture' CO₂ by separating carbon emissions from other gases produced in industrial processes – typically via pre-combustion capture, post-combustion capture and oxyfuel combustion. Seperation can be achieved via membranes, adsorption and cryogenic distillation (Leung et al, 2014)
• Carbon is transported safely for storage, via pipelines or ships
• It is deposited and stored long-term where it is confined from atmospheric contact, generally below the Earth’s surface (CCSA, 2011).

This video effectively sums up the reasoning behind the establishment of CCS technologies. The UK's CCS storage capacity has been estimated at 70 billion tonnes. Approximately 2-5 billion tonnes of carbon need to be stored to meet the UK's decarbonisation target - which could potentially be met (DECC, 2012). 

Carbon can be stored in:

     1)  Deep geological formations (e.g. depleted oil and gas reservoirs, saline aquifers and unmineable coal beds)

     2)   Deep ocean

     3)   Mineral carbonates (IPCC, 2005). 

Figure 1: Carbon capture and storage process

Although oceans are considered the Earth’s largest carbon sink, there are concerns that injecting CO₂ can enhance acidification and prove detrimental to marine life (Pires et al, 2011). Mineralisation of carbon into solid inorganic carbonates, using chemical reactions, can provide safe, long-term storage (Allen and Brent, 2010), however its application is limited by its costliness (Sanna et al, 2014).

Geological storage is considered the most viable due to its potential, low-risk, safe storage and relative cost. One site can hold several million tonnes of CO_2, with saline aquifers storing 400-10,000 Gigatonnes. This can effectively reduce global atmospheric carbon concentrations if undertaken on a large scale (Leung et al, 2014).

Requirements for geological storage

(i)           Suitable porosity and volume (reflecting storage capacity)

(ii)         Suitable permeability (eases injection of carbon)

(iii)       A layer of hard rock to seal the carbon and prevent atmospheric contact

(iv)       A safe environment to maintain the structural integrity of the storage site (Global CCS Institute)

Risks and feasibility

Like all other geoengineering options I have explored, CCS does come with its risks. One of the most important issues is potential leakage into the atmosphere, groundwater sources and ocean which would be catastrophic to marine and terrestrial ecosystems (Widdicombe et al, 2013). 

In earthquake-prone regions, CCS would prove particularly hazardous (Zoback and Gorelick, 2012). Contamination of groundwater aquifers in developing countries can compromise rural livelihoods. Lastly, high concentrations of CO₂ can cause immediate human and animal asphyxiation – a well-known example being the 1986 Lake Nyos Disaster. The lethal outgassing of CO₂ killed over 1,700 people in a 15-mile radius (BBC, 2011). Thus, if the integrity of CCS storage sites is undermined, the consequences can be fatal. 

Ocean fertilisation

After exploring terrestrial geoengineering techniques, I thought it would be interesting to gain a marine perspective.

Ocean fertilisation is a CDR technique that causes shifts in marine biological processes; termed the “biological carbon pump” (Figure 1). This is done by ‘fertilising’ the upper-oceans with limiting nutrients, such as iron (Fe), nitrates (N) and phosphorus (P). The intention is to cause a phytoplankton bloom large enough to sequester substantial atmospheric CO₂ (IPCC, 2011).

Figure 1: 'Biological pump' - Marine carbon cycle processes in upper layer and downward flux (Source: NASA Earth Observatory)

Nutrient limitations

Marine phytoplankton require light, CO₂ and nutrients such as N, P and Fe to photosynthesise. Whilst such nutrients are readily available, the slow-recycling of ¼ of total nutrients means <1% reaches the deep benthic (bottom-dwelling) zone (Williamson et al, 2012) (Figure 1). Shortages of Fe, in particular, have been found to limit biological production of phytoplankton in many oceans (Moore et al, 2013).

Fertilisation process

• Fe is added into the upper ocean
• An increase in photosynthetic activity enhances phytoplankton biomass
• Carbon is sequestered via gas exchange through the air-sea interface
• Carbon moves further down into the ocean column via processes such as decomposition, excretion and water mixing.

Carbon sequestration potential

Below is a map of chlorophyll concentration in the world’s oceans; an indicator of phytoplankton biomass via photosynthetic rates and thus carbon intake. Although this shows the large extent of carbon sequestration potential, it is not spatially uniform. Whilst blooms are evident in the Northern Hemisphere Oceans (Arctic, North Atlantic and parts of the North Pacific Oceans), it is not so in tropical and mid-latitudes.

Figure 2: Chlorophyll concentration in March-June 2006, using NASA's Aqua Satellite (Source: NASA)

It would seem that equatorial and mid-latitude regions are severely nutrient-limited due to low productivity, and therefore would benefit from ocean fertilisation. In theory, this could cause a phytoplankton bloom: however it is not so straightforward. A palaeoclimatological study suggests that although iron dust-fertilisation was 2-3 times greater in the Last Glacial Maximum than in the Holocene, phytoplankton productivity in the equatorial Pacific Ocean remained the same (Costa et al, 2016).

There is considerable scientific interest in fertilising the Southern Ocean. Modelling suggests it has the greatest carbon sequestration potential. In the Late Pleistocene, periods of high-dust influx in the Southern Ocean have been associated with the onset of cold glacial periods, with a possible reduction of 30 ppmv of CO₂. This has also been found to correlate with extensive diatom mats: indicating intensive phytoplankton productivity (Martinez-Garcia,2011).

Many experiments suggest iron-fertilisation in the Fe-limited Southern Ocean could be a potential geoengineering strategy (Salter, 2015), however some regions associated with the South Georgia Islands are naturally Fe-rich (Robinson et al, 2016). It is evident then that some parts of ocean systems may not benefit from iron-fertilisation, as was once suggested.

Unintended consequences

Ocean acidification: It has been suggested that Fe-fertilisation can enhance deep-sea acidification (Cao and Caldeira, 2010). This is due to the downward flux of carbon in oceans (Figure 1), such as decomposing phytoplankton sinking to the ocean floor and releasing carbon (Miquel et al, 2015). Benthic species which have calcium carbonate shells, such as molluscs, can be subject to dissolution. In turn, this can disrupt the functioning of the marine food web (Gazeau et al, 2013). 

Toxic phytoplankton blooms: There is a potential for toxic phytoplankton to capitalise on nutrients and cause HAB's (harmful algal blooms) (Trick et al, 2010), which could alter phytoplankton community composition by favouring larger sized phytoplankton (Subramaniam et al, 2016). HAB's can kill fish, birds and mammals and cause anoxic conditions by depleting oxygen.