Think twice, think green

Capturing CO2 from the air

Detaljer

Publisert mandag 10. oktober 2011 15:24

- In the search for potential approaches to tackle climate change policy makers have to-date largely ignored the contribution that could be made from the implementation of methods that directly extract greenhouse gases (GHGs), particularly carbon dioxide (CO2), from the atmosphere.

This article examines the role that such an approach, known as air capture, can play in tackling the challenge of global warming and makes recommendations for climate change policy developments in this area.

Dr Tim Fox, Institution of Mechanical Engineers At the core of international climate change mitigation policy is the notion that a global legally binding emissions reduction agree­ment can be reached, which in the case of CO2 would drive worldwide carbon pricing to incentivise investment in low-carbon tech­nologies and behaviours. Through this approach it is hoped that global mean temperature rise will be kept be­low the 2°C threshold that many in the sci­ence community tell us is necessary to avoid dangerous climate change.

There is however growing recognition that this approach is not producing the nec­essary action on the required timescale and in the meantime global emissions continue to rise. It is therefore important that policy makers seek ways of accelerating emissions reduction while simultaneously utilizing the full range of mitigation approaches available to them.

Climate change mitigation policy worldwide in relation to CO2 is based on three commonly accepted methods for re­ducing the accumulation of emissions in the atmosphere. These are: 1. Reduce demand for CO2 emitting energy and processes through energy conser­vation, increased energy efficiency and be­havioural change; 2. Substitute technologies characterised by lower CO2 emission levels in place of carbon-intensive industrial processes and en­ergy sources; 3. Capture the CO2 emitted from pow­er generation and other industrial processes utilising fossil fuels and sequestrate the gas by storing it in suitable underground geolo­gies; carbon capture and storage (CCS). These approaches are however missing a mitigation opportunity, as they do not al­low for the contribution that could be made by the removal of CO2 directly from the at­mosphere. This method, known as air capture, can be achieved through a number of technolo­gies (McGlashan, Shah & Workman 2010) including air capture machines.

Air capture for mitigation Air capture offers two mitigation possibili­ties:

1. ‘negative emissions’ through capture and sequestration;

2. ‘carbon recycling’ through the cap­ture and processing of CO2 for onward use in industrial or energy applications that re­sult in ‘closed loop’ carbon cycles.

Both approaches take advantage of the fact that direct capture of CO2 from the at­mosphere can take place at any geographical location regardless of the point at which the gas is emitted. This would enable difficult to tackle CO2 sources to be accounted for in the mitigation process, including non-sta­tionary and dispersed sources such as air­craft, ships and industrial processes that are not amenable to CCS. It could also facilitate the participation of countries with low GHG emissions in the carbon economy and global mitigation ef­fort. This would be achieved for example through the development of a direct CO2 capture and sequestration activity trading in the world’s carbon markets, or a carbon re­cycling industry driving localized fuel man­ufacture for energy security.

These develop­ment routes could be particularly attractive for those countries with low industrial devel­opment costs and abundant ‘stranded’ or ‘ex­cess’ sources of renewable energy, which can be used to drive the machines cost-effective­ly. From an international policy perspec­tive, it is the fact that a negative emissions approach can be used to establish a rational ‘ceiling’ (or cap) price on CO2 emissions globally that is potentially of most signifi­cant benefit. In this regard, if all CO2 emit­ters were subject to a mandatory internation­al requirement to apply an alternative abate­ment method or pay for the negative emis­sions necessary to balance their CO2 emis­sions account, the cost of air capture with onward sequestration would represent the ul­timate limit on the price to the polluter for putting the CO2 in the atmosphere. This simple approach would drive worldwide carbon pricing to encourage cost reduction in competing low-carbon tech­nologies and incentivize both deployment and large-scale behavioral change, thereby removing the need for a complex global legally binding emissions reduction agree­ment. If ultimately required, legacy emissions can be removed from the atmosphere using a negative emissions approach, thereby en­abling CO2 concentrations to be returned to acceptable levels (as defined by the climate science community). Air capture machines Machines that will enable air capture are at an advanced stage of engineering design, reaching pilot demonstration potential, and represent a promising technology for early deployment of this approach.

The two prin­cipal proponents of these machines are Prof Klaus Lackner (Lackner 2009) and Prof David Keith (Keith, Ha-Dong & Stolaroff 2006) both of whom have active air capture R&D programmes with designs based on the use of some form of chemical scrubbing to extract CO2 from air passing through the de­vice. However, though broad descriptions of the technologies are available, the early stage proprietary nature of the work means that many details are not in the public do­main. It must therefore be noted that al­though the technique appears feasible from an engineering perspective (IMechE 2009) there is considerable uncertainty as to future cost levels. For artificial trees Lackner (Lackner 2009) states that in the process of moving beyond prototypes to mass production and operation, the price of air capture could drop from around US$200/tCO2 to as low as US$30/tCO2 for machines delivering 1­3tCO2 /day. It should however be acknowledged that although these ‘target’ air capture costs have been shown to be plausible (Mc-Glashan, Shah & Workman 2010) they are considered by some to be overly optimistic and that, based on current technology in the public domain, a starting point might be nearer US$430/tCO2 (APS 2011). Though it would not be sensible to use air capture to account for emissions from large stationary sources that are amenable to CCS, these figures compare with recent esti­mates for CCS costs in the range US$30­90/tCO2 (Florin & Fennell 2010), including the transportation and storage component of around US$1-12/tCO2. In the context of the uncertainty of fu­ture technical developments and localized prices for equipment, finance, maintenance and energy, the cost of air capture and CCS emissions capture appear to be potentially of broadly similar magnitude. However, re­gardless of any difference between the two, both CCS and air capture machines with on­ward sequestration represent technologies at the most expensive end of the abatement cost curve (McKinsey 2009). It is therefore clear that the development and deployment of air capture machines alongside CCS will define the ‘ceiling’ carbon price. Application of approach To apply a negative emissions approach it will be necessary to engineer CO2 trans­portation and sequestration infrastructure in a similar way to that required for CCS. On the other hand, the application of carbon re­cycling would use the directly captured CO2 in industrial processes to avoid the ‘new’ CO2 emissions that would otherwise result from those processes. In this regard the ap­proach provides mitigation through stabiliza­tion with the potential added benefit of buy­ing us time while we transition to a low car­bon economy. CO2 already has a number of industrial uses in which the gas is supplied as a chem­ical feedstock to the product manufacturing process and ultimately released to the atmos­phere; examples include Urea, Inorganic Carbonates, Polyurethanes and food and drink applications. Currently these uses are largely supplied by the well established com­mercial gas handlers from sources in which the CO2 is a by-product, or waste stream. Taken overall, they are cumulatively small in comparison with annual global CO2 emis­sions (100-200Mt versus 28,000Mt), but nevertheless the substitution of the one-way waste streams by ‘recycled’ CO2 obtained using air capture would prevent further ac­cumulation in the atmosphere of CO2 from these products. Ideally, in such cases the air capture plant would be located adjacent to the CO2 demand, thereby potentially saving pipeline or surface transport costs and GHG emis­sions. The displaced CO2 by-product or waste stream previously utilized as feedstock by the product manufacturer would need to be abated by the source owner in-line with the principle of polluter pays. However, in a future carbon constrained world the manu­facturer would no longer need to be liable for the carbon cost of the CO2 emitted by their product when it later enters the atmos­phere, as a closed loop carbon cycle will have been established. In addition to existing processes, major new industrial uses for CO2 are emerging through R&D which have the potential to provide wider sustainability benefits, as well as consume large volumes of the gas. One such innovation is the use of CO2 to manu­facture synthetic fuels. The basic process; obtaining hydrogen from water via electrol­ysis and combining this hydrogen with CO2 to create methanol or other hydrocarbons, is well known and has been shown to be amenable to a closed loop carbon recycling approach based on the use of air capture technology (Pearson & Turner 2011).

De­spite concerns raised regarding the efficien­cy of the process, which will require careful engineering to address, synthetic fuels have many benefits, including:

• provide high energy densities compa­rable with conventional hydrocarbons;

• do not require a change in the current liquid fuel infrastructure or consumer behav­iour;

• can buy time in the low-carbon tran­sition of ground transportation;

• are not subject to the land-use issues affecting biofuels (they can be used in com­bination with biofuels where appropriate);

• help tackle the difficult challenges of mitigating air and ship transport emissions and have the potential to enhance fuel secu­rity.

Summary Given the slow progress to-date on climate change mitigation using current policy ap­proaches, it is critical that we avoid wasting time and urgently assess alternatives that may help in meeting the challenge of decar­bonisation. Air capture technology is one such alternative in an early stage of devel­opment and, as with CCS, needs interven­tion to drive pilot testing, demonstration at scale and detailed cost assessment. Govern­ments should therefore provide development and assessment support through research budgets and the addition of air capture to ex­isting CCS strategy and policy. In common with many generally ac­cepted technology based methods for miti­gating GHG emissions, negative emissions and carbon recycling will result in an addi­tional cost to society and are unlikely to make economic sense without market inter­vention.

Appropriate changes in policies and mechanisms will therefore be needed to drive their adoption, deployment and utiliza­tion. In this regard Governments should recognise the important contribution that these approaches can make to climate change mitigation and engage in developing national and international policy framework models for their adoption.

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