Carbon Capture and Sequestration

Carbon Capture and Sequestration

This is an essay I did in 2011 but it provides some well referenced material about carbon capture and sequestration.

 

The Role of Carbon Capture and Sequestration in Mitigating Climate Change

Introduction

Carbon Capture and Sequestration (CCS) is a form of geoengineering, the purpose of which is to: “rectify the Earth’s current radiative imbalance, either by reducing the absorption of incoming solar (shortwave) radiation, or by removing CO2 from the atmosphere (or prevent release) and transferring it to long-lived reservoirs, thus increasing outgoing long wave radiation.”(Lenton and Vaughan, 2009)

Approximately 10 gigatonnes of carbon are annually released into the atmosphere as CO2 by fossil fuel burning and deforestation. More than 50% of which is taken by oceans and the terrestrial biosphere, growing proportionately with our input, which has nearly tripled in the last 50 years.(Baker, 2007)  Remaining atmospheric CO2 lingers for > 100 years and “CCS is essential for addressing greenhouse gas emissions, while simultaneously maintaining a robust and affordable energy supply” (Zenz House, 2010)   and the IPCC estimates CCS could constitute 10-50% of carbon mitigation until 2100. (Mathieu, 2006)

(Herzog and Golomb, 2004) state that in CCS:

a)    Storage should be for hundreds of thousands of years

b)    Cost of storage and transport from source to sequestration site should be minimised

c)    Risk of accident should be eliminated

d)    Environmental impact should be limited

e)    Storage methods should comply with international laws and regulations

CCS is the only available method of mitigating greenhouse gas emissions that allows us to continue burning fossil fuels. (Oh, 2010)  This is important because fossil fuels are so incredibly abundant, supplying 87% of our primary energy (BP, 2012) , and equally cheap that almost all countries can provide power for themselves cheaply, from domestic supplies, and simultaneously limit energy security issues. Although the use of oil has fallen for 12 consecutive years coal remains the fastest growing fossil fuel.(BP, 2012) The goal, therefore, is to continue using this cheap and abundant fuel while minimizing the impact on atmospheric CO2 levels.  This can either be done by capturing atmospheric CO2 or capturing it before it reaches the atmosphere.  Integrated Gasification Combined Cycle (IGCC) power plants can be used for all fossil fuels to produce hydrogen for combustion and CO2 for capture but we are yet to run one on a commercial scale to produce energy and capture carbon simultaneously. 

Methods

Biosequestration

Oceanic-related techniques include

  1. Ocean Fertilisation (OF), with iron sulphate, in regions where it is the limiting nutrient to promote phytoplankton growth which fixes carbon and falls to the deep ocean floor.  Research in this field has targeted high-nutrient-low-chlorophyll oceanic regions for maximum effect but there are many factors surrounding the efficacy of this technique including: rates of supply, advection rates of iron and time of year that affect the efficiency of carbon sequestration (Chever et al., 2010).  Phosphorus has been found to further enhance this process to make it second to air capture and storage of liquid CO2 (in sediments or geologic reservoirs) to offer the next largest long-term potential carbon store.(Lenton and Vaughan, 2009)
  2. Enhanced Oceanic Mixing, another OF method, works by pumping nutrient-rich water up from the deep oceans with large pipes.  This would result in temporary CO2 increases from carbon-rich sub-surface waters outgassing, an unfavourable side effect, but (Zhou and Flynn, 2005) state this method appears ineffective.

Terrestrial techniques include: enhancing peat bogs and reforestation, remembering that dead trees need sequestering when dead to remove their carbon from the system. This can be done through pyrolysis to create Biochar for soils which ameliorates soil field capacity, reduces leaching and increases nitrogen retention (Ding et al., 2010).  This can reduce the necessity of agricultural fertilisers and is estimated to have a capacity of 428 gigatons of carbon (GtC) at 3.25 GtC/y, offsetting 38% of the world’s annual fossil fuel-carbon production of 8.67GtC (Lee et al., 2010)[t1] .

Tilling biomass back into the soil enhances natural nitrification, reducing the need for fertilisers. Further reductions can also be achieved by more accurate fertiliser application and the same applies for irrigation.  This may be economically viable by serving to deintensify the use of fertilisers and water (Inyang et al., 2010) while simultaneously enhancing food security (Lal, 2004)

Physical and Chemical

These two broad classifications are often inextricably linked and a method receiving abundant attention is Subterranean Injection. Here CO2 is injected into the deep ocean, saline aquifers or depleted oil wells.  Some processes rely on gravity anchoring CO2 injected into ocean sediments at >3000m depths which provide stability as CO2 is denser than water at 300 ATM and remains a liquid, even with large geomechanical perturbations (House et al., 2006).   However, uncertainty exists over the ecological impacts in storing large volumes in this way as the CO2 lakes created can have profound effects on biodiversity in the immediate vicinity.(Inagaki et al., 2006)

A more stable alternative is Solubility and Mineral Trapping: reacting CO2 with basalt in deep-ocean to form stable, non-toxic, carbonate minerals that are denser than sea water (eg. MgCO3, FeCO3, CaCO3) and precipitate out to remain as solids at the bottom of the water column. This reduces the likelihood of carbon re-entering the carbon cycle and potential has been identified in the Juan Fuca plate off the Pacific coast of the USA for sequestration of their entire current CO2 production for over a century (Goldberg et al., 2008).  Statoil have done this off the Norwegian coast, storing 1million metric tons of CO2 annually in the Utsira formation: a 250m thick aquifer found 800m below the North Sea bed (Korbol and Kaddour, 1995).  Injecting CO2 into aquifers is also possible and requires impermeable rock, capping the aquifer, to trap CO2 which dissolves in the ground water.  It then mineralises by reacting with rock present to form stable carbonates for permanent sequestration (Herzog and Golomb, 2004)

Magnesia cement is also a CCS possibility as it fixes CO2 during bonding and it is also more easily recycled than traditionally-used Portland cement (Phair, 2006).

Mineralisation can also be used to reduce oceanic acidification by removing hydrochloric acid (HCl(aq)) using silicates. HCl(aq) is primarily responsible for low oceanic pH.  This increases the ocean’s affinity for CO2 absorption by the Solubility Pump (House et al., 2007) by reducing the concentration of hydrogen ions.  This allows dissolution of CO2, creating carbonic acid which is less acidic than the hydrochloric acid that has been removed by reaction with silicates and 50% of the CO2 dissolved in this way will be “permanently” sequestered as calcium carbonate, reducing stress on corals in the process(House et al., 2007).

Unmineable coal seams present excellent and sometimes cost free sequestration opportunities as the coal is porous with an affinity for CO2 over methane by a ratio of 2:1, therefore it can be used in industry to simultaneously sequester CO2 whilst purifying methane from coal beds(Herzog and Golomb, 2004).

Fossil Fuels

COAL

The U.S.A., China and India produce more than half of their electricity from coal, making it difficult to see how cost-effective and politically viable emission reductions can be achieved during the next several decades without coal use (Wilson et al., 2008).  Therefore CCS must play an important part.  The two main methods for Carbon Capture (CC) with coal are:

  1. Oxy-Fuel Combustion which can be retrofitted to existing pulverised coal fired power stations but to date this has not been done on a commercial scale for energy production, only to supply steam to a paper mill in Schwartz Pumpe, Germany, built by Vattenfall.   The coal is burned with enriched oxygen to produce higher temperatures and therefore maximising complete combustion, producing mostly CO2 and H2O with the potential for 100% CO2 capture (Fig1).

Figure 1:  Oxy-Fuel Combustion Process

 

  1. Pre-Combustion Capture is used in Integrated Gasification Combined Cycle (IGCC) coal fired power stations to gasify the coal producing a synthesis gas (Syngas) composed of CO2 and hydrogen (a process also compatible with natural gas) which can then be combusted for energy.  There are 3 running currently in the USA, producing commercially viable energy, but without capturing CO2.  However, the FutureGen Project in Illinois is currently looking for a sequestration site and will be the first of a kind in sequestering all CO2 produced.  (Fig 2)

Figure 2: FutureGen Project

IGCC is cheaper but only oxy-fuel can be retrofitted to existing coal fired power stations so both will have to be employed to reduce our emissions in the case of burning coal.

Extraction processes vary with different fossil fuels, producing an array of commercially valuable by-products.  For CCS to be applicable here the CO2 produced must be transported, to the site of sequestration.  The Wayburn Project in North America uses a pipeline from the Synfuels Plant of Dakota Gasification Companyto Saskatchewan oil fields for use in enhanced oil recovery (EOR).  CO2 is pumped down depleted oil wells to maximise oil extraction, the remaining inaccessible oil absorbs some CO2 and in total they sequester about 95% of the CO2 from the Synfuels Plant.

Safety

The only significant concerns with CCS lie in geological storage, although a wealth of knowledge exists from EOR, concerns exist with contamination of potable aquifers but we may benefit to focus on rectifying failure as this is easier to quantify than probability of failing (Price and Oldenburg, 2009).

Long term stewardship is an issue: companies sequestering at a site will not last the hundreds of years to manage the closed sequestration sites for their lifetime.  Therefore, management should fall to governments, as should proliferating knowledge, harmonising international safety and regulation as accidents could destroy public perception and also future utilisation of CCS (Wilson et al., 2008).  However, leakage of CO2 from geology has been shown to only be significant over geological time scales even in geologies with high structural movement (Ciotoli et al., 2006).

Economic and Logistical Challenges

Not all CC and CCS methods have been mentioned, however, costs is a hurdle in implementation for all of them and as such many are subsidised by governments, with FutureGen for example receiving a 50% subsidy from the US Government’s Department of Energy.

Transport is also costly: in Enhanced Oil Recovery (EOR), for example, to get CO2 from source to sequestration site: either pipelines must be constructed or road tankers (carrying supercritical CO2) are needed.  However, little modification would be needed in EOR to vastly increase capacity for sequestration as much of the necessary technology has been employed for over 40 years (Plasynski and Damiani, 2008). If anthropogenic CO2 is used instead of that from surrounding geology in viable depleted oil wells there is potential for sequestering 100% of the CO2 used in EOR(Plasynski and Damiani, 2008), although the U.S. National Energy Technology laboratory is less optimistic(Fig 2).

Figure 3: CO2 Utilization and Potential in EOR Projects

United States (2006)

Million Tonnes

CO2  used for EOR

48

Naturally Occurring CO2

36

Anthropogenic CO2

12

Estimated CO2 Sequestered by EOR Operations

9

Worldwide

Billion Tonnes

Potential CO2  EOR Sequestration

130

Total CO2 Accumulated in Atmosphere

3-4

Source of U.S. data: National Energy Technology Laboratory, Carbon Sequestration Atlas, 2007

With methods of atmospheric CC such logistical issues of proximity are eliminated between source and sink, making them easier to implement and less of a pressure on land availability.

The proliferation of carbon taxes is stimulating the growth of CCS and this is evident in Norway: with heavy taxation since 1991 (55$/tonne of CO2), Statoil have recouped their $80milion sequestration investment in Sleipner within 1.5 years.  There could be similar outcomes now with India introducing a carbon tax in July this year to generate money for green technology and China announcing one for commencement in 2012 (Young, 2010).

Caution is recommended by (Chadwick, 2009) when choosing sequestration sites to limit leaking and capacity issues but (Ehlig-Economides and Economides, 2010) go further by stating: “underground carbon dioxide sequestration via bulk CO2 injection is not feasible at any cost” and that one hundred times less sequestration can be done per well than previously thought due to lack of careful consideration of aquifer properties such as: rock compressibility, relative permeability and brine salinity.

These claims were later criticised by (Cavanah et al., 2010) disputing their methodology, maintaining that decades of experience from industry injecting gases into geology proved them wrong.

Issues of increased water demand and groundwater contamination still surround EOS. Therefore, the proliferation of EOS is dependent on understanding the geological conditions creating these issues. This enables adaptation in design and the implementation of appropriate monitoring procedures to ensure environmental safety and accountability (Newmark et al., 2010).

Conclusion

CO2 levels are presently at 390ppm (Mauna Loa) and to stabilise them at 550ppm by 2050 we will need a 30-60% reduction in CO2 production by 2050 (Oh, 2010).  This is inconceivable without the use of CCS given our unrelenting dependence on fossil fuels for energy production as to reduce emissions either quality of life (GDP) can fall, efficiency can increase or we can develop cleaner methods of energy production (Pielke, 2010).  But (Pielke, 2010) then suggests cleaner energy as our only option as governments will never cut GDP and efficiency measures will be outstripped by increased energy demand.

The CCS offers a potentially clean means of energy production and atmospheric CCS presents several ways to undo the damage that we have already done.  “With large reductions in CO2 emissions, combined with global-scale air capture and storage: afforestation, and bio-char production might be able to bring CO2 back to its pre-industrial level by 2100” (Lenton and Vaughan, 2009).   As humanity uses no single technology with 100% efficiency it is clear that all CCS technologies will play some part. We are still presented with the need to act immediately and employ these technologies on a grand scale as there are still questions surrounding the missing carbon sink and climate sensitivity which could have massive repercussions even under present day atmospheric CO2 levels.

We should also try to remember that there is associated cost from taking insufficient action as climate change will impact on economies worldwide.  But still the problem remains that: given sufficient temporal or physical distance from an event, people are unlikely to react, even without the inherent uncertainty in an issue like future climate change.

Despite this: there is reason for optimism as attitudes to CCS seem positive: provided explanation was simple, lay people found concepts intuitive and easily understood.  Their primary concern rests with increased cost of energy produced and almost none with safety (Wallquist et al., 2010) indicating there may be little public resistance to proliferation of CCS projects on such grounds and rightly so as CCS carries less risk associated with failure than other geoengineering technologies (Lenton and Vaughan, 2009). Even the Daily Mail can report it without scaremongering (Derbyshire, 2009). This message is also gaining momentum with religious leaders giving moral weight to the motivation to act: “it’s possible for us to act unjustly in relation to future generations” (Williams, 2007).  In addition, the Dali Lama and, more importantly, the Pope have also been vocal on the subject.

Governments are also trying to overcome private sector trepidation to invest in CCS through subsidy and carbon taxes (Giovanni and Richards, 2010).  So if the message that: even a subsidy for the initially non-competitive technology of CO2 sequestration can be a sound economic policy, even in the short term (Keller et al., 2008) can also be heeded then we may finally be on course.

The abundance of fossil fuels and their inextricable link with almost every process that modern humanity indulges in means that even building carbon neutral wind turbines utilises energy generated from burning fossil fuels. Therefore, the challenge is not to excise carbon from energy production, as this will not happen soon enough to avert the 4-5oc (or more) temperature rise that we are likely to see, but to prevent it from entering the atmosphere through carbon capture and sequestration.

 

 

 

 

References

 

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 [t1]How long can this proceed for?

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