Storing the Captured Carbon Dioxide

Whether the carbon dioxide is captured by photosynthesis or abiotic sorption, it is necessary that it stays out of the atmosphere, preferably for at least several centimes. Clearly, there is no benefit to the climate in capturing carbon dioxide which is quickly released back into the atmosphere. Moreover, both the capture and storage should be feasible on the scale needed (Figure 1), and should not cause significant damage to the environment or society. The two questions, “how do we capture the CO,?” and “what do we do with the CO,?” are very closely linked. Table 1 lists a variety of possible locations for storing captured

Table 1. Schemes for removing CO, from the air and stonng it.

CO, captured from air by photosynthesis

C02 captured from air by abiotic sorbent

C-storage location

Plants on land

  • (la) Afforestation and forest management
  • (lb) Wetlands, peatlands and coastal ecosystem restoration and management

(2) DAC, using regenerable solid or liquid sorbent to produce C02-enriched air for improving agricultural yield - e.g. tomatoes

Soil

  • (3) Increasing soil carbon by land management: Soil carbon sequestration
  • (4) Biochar (from biomass pyrolysis)

(5) Enhanced Weathering (EW) of rock particles scattered on soil

Above ground as mineral

(6) Biomass combustion to release energy with C02 captured from flue gas and reacted with mined mineral or suitable waste

(7) Mineral Carbonation reacting either air, or C02-enriched air (flue gas or DAC) with mined (processed) mineral or suitable waste

Below ground as mineral

(8) Following biomass combustion to release energy, with flue gas treating and injecting C02 to react in rock such as silicate or basalt

(9) Following DAC, injecting C02 to react in rock such as silicate or basalt

Below ground, as compressed CO,

(10) Biomass combustion to release energy, with flue gas treating to produce high purity C02 for underground storage (BECCS)

(11) DAC to produce high purity C02for underground storage (DACCS)

Ocean

  • (12) As (10), with C02 dispersed into the ocean, or injected at a depth where it is not buoyant
  • (13) Ocean Fertilization (OF) to stimulate growth of marine biota
  • (14) As (11), with ocean storage of C02
  • (15) Ocean Liming. Calcination of carbonate rock with CCS. Lime distributed at sea

Human

environment

  • (16) Buildings made with wood and similar biomaterials
  • (17) Chemicals and products from biomass

(18) DAC, then utilizing produced C02 as feedstock for chemical products (CCU)

CO,. together with the capture methods commonly suggested for those locations. They comprise some 18 different schemes for removing carbon dioxide from the air and storing it to mitigate climate change, though other suggestions and variations will also be found in the literature.

Plants on land (schemes la, lb, 2)

Forests, wetlands, peatlands and coastal habitats provide significant storage of terrestrial biological carbon, but they are under threat from exploitation, degradation and land-use change. It is estimated that restoration and improved management of these ecologies might yield mitigation of up to 8 Gt/y CO, by 2030 (Griscom et al.. 2017), as well as other benefits to biodiversity, water resources and so on. Planting new forests makes a significant contribution, but as they approach maturity, after some decades, net CO, removal declines. Thus, a long-term strategy for forest management is needed in order to maintain ecosystem health and ensure that captured CO, is not released prematurely. The use of carbon dioxide from DAC to promote growth in greenhouses of cash crops like tomatoes and aubergines has received much publicity. However, the storage lifetime of the carbon in a tomato is very short indeed, and the yearly production is only about 100 million tons. Some 93-95% of a tomato is water, so this is not a method of storage or carbon utilisation that will make any significant impact on climate change!

Soil (schemes 3, 4, 5)

There are many ways in which agricultural land, much of which has lost significant soil organic carbon (SOC), can be managed in order to both increase fertility and store more carbon. For example, the variety and rotation of crops can be improved, as can the use of manures, composts and fertilisers. Carbon (and water) loss can be reduced through a “no-till” policy that avoids ploughing. The potential benefits of restorative land use and the adoption of recommended management practice have been reviewed by Lai

(2011). The storage promoted by these policies is known as soil carbon sequestration.

Biocliar is a carbon-rich (65-90%) porous solid product of the pyrolysis of biomass (and also of animal waste). It can be added to soil which provides a stable storage environment. Biocliar acts as a conditioner which increases SOC, water retention and fertility, and it can also reduce the emission from soil of the greenhouse gases methane and nitrous oxide. The production, properties and utilisation of biochar have been reviewed by Qambrani et al. (2017).

The weathering and mineralisation reactions (equations (4)-(7)) are part of the natural carbon cycle. They yield a stable (i.e., long-lasting) form of carbon dioxide storage as bicarbonate and carbonate ions, which are very common in soils and oceans. The reactions are spontaneous and remove about 1.1 Gt CO, from the atmosphere annually (Ciais et al.. 2014). To capture extra carbon dioxide by these means at a rate sufficiently fast to help meet climate policy targets, the exposed rock needs a high surface area—that is, a small particle size. Mining and crushing rock require significant energy input, leading to higher costs of course. Various terrestrial EW schemes har e been suggested, including spreading suitable mineral particles on agricultural land (which can also improve soil quality). For example, Strefler et al. (2018) estimate that spreading fine basalt particles on croplands might potentially capture 4.9 Gt/y CO,, a rate facilitated by plant and root activity that accelerates the chemical reactions. Beerling et al. (2018) discuss the potential for biogeocliemical improvement of croplands by amending soils with crushed fast-reacting silicate rocks as a strategy to address the threats of climate, food and soil security.

Above ground as mineral (schemes 6, 7)

Ex situ methods of reacting ah' or captured CO, with mineral might involve mining, transport and crashing of virgin rock, but attention has also been given to the estimated 7 billion tons of alkaline materials currently produced annually by industry as product or by-product. Such material—slag, mud and waste from steel, aluminium or cement manufacture, combustion ash, ultrabasic mine tailings—is often available as small particles, a suitable form for capturing carbon dioxide from the atmosphere. As materials for EW schemes, Renforth (2019) estimates their potential storage capacity at 2.9-8.5 Gt/y CO, by the year 2100.

Below ground as mineral (schemes 8, 9)

In situ methods envisage injection of carbon dioxide into permeable rock strata, where higher temperature and pressure will accelerate reaction with water and minerals. Once mineralised, there is a much smaller chance of the carbon dioxide escaping from the reservoir. Matter et al. (2016) report that in an experiment in which CO, was injected into a reservoir of basaltic rock, over 95% was mineralised to carbonate minerals within two years. This indicated a much higher rate of mineralisation than expected. Successful results were also obtained with a less pure stream of CO„ an interesting result since secure storage of less pure carbon dioxide might enable the cost of separating CO, to be reduced.

Below ground as compressed carbon dioxide (schemes 10,11)

BECCS (section 2.1) with CO, compressed for storage underground is the mitigation scheme most commonly assumed in climate modelling work. For example, Rogelj et al. (2018) analysed scenarios which would meet the Paris target limit of 1.5 °C global mean temperature rise this century using Integr ated Assessment Models. In this study, capture of carbon fr om the atmosphere was mainly achieved through BECCS and afforestation, omitting other technologies. However, producing biofuel at the scale required would create serious competition for laud that currently supports food production or diverse natural ecologies (Boysen et al., 2017).

Alternatively, CO, could be removed from the air by DAC (section 2.2.1), using sorption taking place in some more or less conventional processing equipment. The logistics are quite different to BECCS, as plantations of crops are replaced by regenerable sorbents which capture CO, from the air. The combined system (i.e., DAC with geological storage) is sometimes called DACCS.

Both BECCS and DACCS envisage carbon dioxide stored in a compressed state (super-critical if the pressure exceeds 74 bar) underground in geological formations where it would remain stable for a long period of tune. This kind of storage is also expected to be used for storing the carbon dioxide captured fr om flue gas.

Ocean (schemes 12,13,14,15)

An IPCC review of the potential of ocean storage of CO, pointed out that of the 1300 Gt anthropogenic emissions of CO, in the last 200 years, some 500 Gt have been taken up by the oceans as they approach chemical equilibrium with raised CO, concentrations in the atmosphere (Metz et al.. 2005, Chapter 6, Ocean Storage). Compressed CO„ captured on laud by BECCS or DAC plants, could be deliberately stored in the ocean —pumped by pipeline or dispersed by ship, both of which have been considered. It is known however that acidification of the oceans by anthropogenic CO, poses a serious problem to marine ecosystems, as discussed in the IPCC review, so ocean storage has received little support. It remains a controversial topic (Goldthorpe, 2017).

Ocean Liming is a scheme that addresses both climate change and ocean acidification directly. Renforth et al. (2013) describe a typical case. The production of slaked lime follows the chemistry outlined in equations (8) and (9). The lime is then distributed at sea, facilitating further draw-down of carbon dioxide from the atmosphere, as in equation (10), though there will also be an equilibration with existing levels of carbonate and bicarbonate present in the oceans, according to equation (7).

Ocean Fertilisation (OF) was the first negative emission technology to receive much serious attention, from the early 1990s (Minx et al.. 2018). The idea is to promote the growth of marine biota to draw-down carbon dioxide by the photosynthesis reaction, equation (1). A fraction of these marine plants would sink deeper, providing ocean-based sequestration. This is a natural process which could be enhanced by adding the nutrients nitrogen and phosphorus to surface waters in areas where then low availability currently limits plant growth. Alternatively, in about one third of ocean surface waters, where nitrogen and phosphorus are sufficiently present, the micronutrient iron is in limited supply, so fertilisation with this component would enhance plant growth. Williamson et al. (2012) reviewed the benefits and risks of ocean fertilisation and concluded that its contribution would be relatively modest, and extremely challenging to quantify on a long-term basis. In situ and far-field monitoring would be needed to verify the change in carbon flux caused by fertilisation and to check for possible rebound effects that might offset the initial change. There are also concerns about unintended side-effects, and the governance of a process involving the addition of such chemicals to open ocean waters.

Human environment (schemes 16,17,18)

Using plant products to replace materials made from fossil fuels or with the expenditure of much fossil energy will lead to a reduction in atmospheric carbon dioxide. Wood, for example, can be used in construction, and serves to store carbon for some decades at least. There is a similar benefit in replacing petrochemical products with ones made from biomass, when this leads to a net draw-down of CO:. There is scope for the development of CO,-based chemistry to replace petrochemistry, but again, careful accounting is needed in order to ensure that the net effect is beneficial when all processing and energy consumption, direct and indirect, is included. The direct use of biomass, or carbon captured from air or flue gas falls under the general description of Carbon Capture and Utilisation (CCU). Turning the captured carbon dioxide back into fuel by a chemical conversion consumes a lot of energy of course, and only results in minimal storage before the fuel is brunt again and CO, is returned to the atmosphere: This is not CDR. The desire to find a profitable use for the “unwanted” carbon dioxide in the atmosphere is understandable. However, except for the energy market itself, it is difficult to see a sufficiently large market for any carbon-based products which could take up even a small fraction of the 100-1000 Gt of CO, that we are looking to capture and store in this century. To illustrate this point, the current world plastics production is about 0.35 Gt per year (www.plasticseurope.org). which suggests that a very limited capacity of CO, utilisation would be offered by replacing materials currently derived from petrochemicals, a point made by Metz et al. (2005). Mac Dowell et al. (2017) concluded that “it is highly improbable the chemical conversion of CO, will account for more than 1% of the mitigation challenge”.

Storage longevity, monitoring and verification

For the purpose of long-term climate stabilisation, CO, removed from the atmosphere needs to be stored securely, preferably for a duration of at least several centuries (Royal Society and Royal Academy of Engineering, 2018), The storage options listed in Table 1 exhibit varying degrees of permanence and certainty. Those options with biomass or soil organic matter as the stored form of carbon are considered to have the lowest (possibly sub-centennial) permanence; then storage longevity is subject to human interventions, like changes in laud use and utilisation of biomass, and natural processes such as the occurrence of fire. As the RS&RAEug report states “...pathways that store carbon as impermanent organic materials with sub- centennial and uncertain lifetimes may prove ineffective without appropriate management.”

Biocliar and terrestrial weathering probably offer more stable storage, yet the generation of firm evidence and understanding remains a subject of research. At the other end of the longevity spectrum, mineral carbonatiou and ocean liming have the potential to store carbon for millennia or even longer. Storing CO, in deep sedimentary geological formations, which can potentially handle CO, captured in high purity by a range of schemes, is also considered to be among the most stable methods, though there may be leakage, depending on the geological characteristics of the storage site. This method is discussed in Chapter 7 of the report by the National Academies of Sciences, Engineering, and Medicine (2019).

Given the high reversibility of some of the stored forms of carbon and the uncertainty in permanence which is associated with virtually all options to some extent, the implementation of CDR will require careful monitoring and verification. The effectiveness and costs of monitoring and verification and their trade-offs also vary significantly between different CDR schemes. Bellasen et al. (2015) wrote in a review of existing regulatory systems that “The monitoring, reporting and verification (MRV) of greenhouse- gas emissions is the cornerstone of carbon pricing and management mechanisms”. Establishing good MRV practice for CDR could utilise experience in the field of emissions control, in order to improve reliability and cost effectiveness. For example, as pointed out by the National Academies of Sciences, Engineering, and Medicine (2019), in the US, each year more than 2.5 Gt of brines are injected into deep underground formations for disposal. This experience helped underpin technical, administrative and regulatory approaches developed in the U.S. for sequestration of captured CO, in sedimentary basins.

 
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