A comparative global assessment of potential negative emissions technologies

Process Safety and Environmental Protection 9 0 ( 2 0 1 2 ) 489–500

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A comparative global assessment of potential negative emissions technologies Duncan McLaren a,b,∗ a b

Lancaster Environment Centre, Lancaster University, UK McLaren Environmental, Rågetsvägen 16, 72242 Västerås, Sweden

a b s t r a c t The paper summarises a global assessment of around 30 prospective negative emissions techniques (NETs) found in the literature. Fourteen techniques including direct air capture, BECCS, biochar, and ocean alkalinity enhancement are considered in more detail. The novel functional categorisation of NETs developed in the course of the assessment is set out and a comparative quantitative summary of the results is presented, focusing on the relative readiness, global capacity, costs and side-effects of the prospective NETs. Both technology specific and more generic potential limitations are discussed, notably those arising from energy requirements, from availability of geological storage capacity and from sustainable supply of biomass. Conclusions are drawn regarding the overall scope of NETs to contribute to safe carbon budgets, and challenges arising in the future governance of NETs, with particular reference to the potential role of carbon markets. © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Negative emissions; Direct air capture; Bioenergy with CCS; Carbon storage

1.

Introduction

International climate policy has focused on mitigation for several decades. Arguably progress on mitigation has been so slow that significant, and potentially dangerous, climate change is now unavoidable under most conceivable scenarios (Lowe et al., 2010; Allen et al., 2009; Moss et al., 2010). There is also reason to fear that tipping points in the climate system may be exceeded, leading to more severe impacts (Lenton et al., 2008). Many scenarios for the future management of climate change involve the achievement of globally negative rates of greenhouse gas (GHG)1 emissions in the foreseeable future. So far, investigations of the potential techniques for delivering negative emissions have typically focused on individual technologies, or on ‘carbon dioxide removal’ (CDR) as a category of geo-engineering.

∗

This paper presents a novel typology and a comparative assessment of negative emissions techniques (NETs), with particular emphasis on the overall potential of the techniques, the constraints they face, and the implications of NETs for climate governance.2

1.1.

Defining and categorising NETs

Negative emissions techniques (or technologies) are means of withdrawing GHGs from the environment such that atmospheric concentrations are reduced below the level that would have resulted without their deployment. NETs encompass a wide variety of techniques. Almost all target CO2 as the most abundant anthropogenic GHG, but gases with higher unit global warming potential, such as methane, could also conceivably be targeted. Most existing or

Correspondence address: Rågetsvägen 16, 72242 Västerås, Sweden. Tel.: +46 707 577 594. E-mail address: [email protected] Received 30 April 2012; Received in revised form 10 September 2012; Accepted 3 October 2012 1 Abbreviations: BECCS – bioenergy with CCS; CDR – carbon dioxide removal; CCS – carbon capture and storage; DAC – direct 2 This paper draws heavily on reports published by the present air capture; GHG – greenhouse gas; NET – negative emissions author (McLaren, 2011a) and by Friends of the Earth (McLaren, technique; TDM – thermodynamic minimum; TRL – technology 2011b) in 2011. readiness level. 0957-5820/$ – see front matter © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.psep.2012.10.005

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proposed techniques either make use of a chemical reaction to extract CO2 from the air (or to bind CO2 dissolved in sea water); or else exploit the ability of plants to extract CO2 from the air to form biomass (and then capturing that carbon in some form). Here these two main routes of chemical and biological drawdown are described as ‘direct’ and ‘indirect’ capture respectively. Fig. 1 illustrates how direct and indirect routes could be applied to all the major carbon sinks in the natural carbon cycle, with the exception of the biotic sink, where NETs increase the rate of flow of carbon into, or the proportion retained in, biomass (in this schema biotic sink NETs would all be termed ‘indirect’). Researchers have considered ways to increase carbon flows or retention in most of these areas. In addition to the direct/indirect categorisation, Table 1 also categorises NETs according to the storage mechanism (partly following McGlashan et al., 2010). Mineral storage binds CO2 in a mineral form into rocks or soil; pressurised storage compresses captured CO2 and injects it into a geological storage reservoir; oceanic storage binds the carbon with chemicals naturally occurring in the ocean, and biotic storage holds the carbon in a relatively stable organic form such as soil organic matter, construction timber, or buried biomass.

2.

Methodology and criteria for assessment

The required scale of NETs’ contribution can be roughly estimated by comparing cumulative emissions budgets and outcome concentrations under realistic scenarios (e.g. Lowe et al., 2010) and calculated safe cumulative budgets or concentrations. For the purposes of assessment a rough estimate of 1200Gt-CO2 (24 Gt-CO2 pa over 50 years) to achieve a final level of 350 ppm (Hansen et al., 2008), is used as a central benchmark.3 Further relevant criteria were identified in the course of a review of approximately 30 different NETs found in the literature, which could contribute to meeting these requirements,4 based on their practical (albeit not necessary commercial) potential capacity. Data was gathered from iterated web searches, mainly using Google Scholar, providing a large literature list, primarily from peer-reviewed or official sources. An initial scoping search was analysed to establish the categorisation of NETs, which was then populated with data from subsequent searches. Semi-structured telephone or face-to-face interviews were conducted with researchers and policy experts in the field to explore emerging conclusions about particular technologies, rankings and policy approaches.

3

First-order approximations of the minimum requirement have been derived as follows: if mitigation rates follow a scenario of delivery of the Copenhagen pledges followed by 80–95% reductions in Annex 1 countries and around 55% reductions elsewhere by 2050 (Lowe et al., 2010) concentrations might be restricted to around 500 ppm by 2100, requiring a total negative emission of 1200GtCO2 (24Gt pa over 50 years) to achieve 350 ppm (assuming a rough benchmark of 8Gt-CO2 /ppm (Socolow et al. 2011). Even with very aggressive mitigation including a phase out of unabated coal use by 2030 (Hansen et al. 2008) there is still a residual negative emission required in the order of 400Gt-CO2 (50 ppm, or 8 Gt pa over 50 years). If mitigation is less successful than Lowe et al suggest, the required negative emissions grow in proportion. 4 The full report and detailed spreadsheet of the assessment can be found at https://sites.google.com/site/mclarenerc/ research/negative-emissions-technologies.

Box 1: Assessment criteria, and comments Status: The technique should be at least theoretically proven, and ideally well advanced into laboratory or field trials to give a reasonable degree of confidence regarding its performance on the other criteria identified. Technical capacity and scalability: The technique can make a significant contribution to the overall level of negative emission needed, and does not run into technical limits or become increasingly difficult or expensive at large scale. Ideally the technique should show economies of scale. Controllability: Use of the technique can be controlled, in particular such that it can be halted if unforeseen negative impacts arise. Moreover, choices about deployment should be under the control of democratic institutions rather than being proprietary or corporate controlled. Accountability: The impacts of the technique should be capable of being adequately measured and accounted for. Side effects: The technique should not have unacceptable or unsustainable negative impacts. Ideally it will offer social, environmental or economic co-benefits. Energy requirement: The technique should be relatively energy efficient, and have the potential to be run using renewable energy sources or low grade (waste) heat. Cost: Ideally, predicted costs should be low enough that widespread deployment might be considered practical.

Seven broad criteria – identified in the process of the scoping search – were used to evaluate the techniques (see Box 1). These were applied consistently to fourteen techniques selected according to apparent capacity and information availability. It proved possible to quantify estimates regarding capacity, energy requirement, status and cost for many proposed NETs, although major uncertainties and potential inconsistencies remain with respect to all these factors, due primarily to incomparable approaches and potential promoter bias. The other criteria were evaluated only on a qualitative basis. Other techniques were reviewed but are not included in Table 2 because for various reasons their potential appears more limited, or very limited information was available on them (McLaren, 2011a). These included: mineral carbonation in autoclaves (Schuiling and Krijgsman, 2006); sea-water injection into basalts (Kelemen and Matter, 2008); accelerated carbonation of mineral slag (Eloneva et al., 2007); direct air capture by use of fluidised beds (Nikulshina et al., 2009), electrodialysis (Eisaman et al., 2009) or metal organic frameworks (Banerjee et al., 2008); BECCS on biogas upgrading (Karlsson et al., 2010) or biohydrogen production; ocean liming via electrochemical splitting (Rau, 2008) or removal of HCl (House et al., 2007); ocean fertilisation with iron (Shepherd et al., 2009); use of genetically modifed crops for soil carbon management (Jansson et al., 2010); cellulose aggregate concrete (Hemcrete, undated); and regenerative grazing (Lovell and Ward, undated). Before outlining the assessment it is important to note some significant limitations to the analysis. Very few of the techniques are technically or commercially proven at present. There are important uncertainties regarding all the factors listed above, and in particular forecasts of future costs are for the most part hypothetical, based in the main on preliminary engineering assessments or laboratory stage

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Fig. 1 – Carbon cycle schematic showing potential NETs. Table 1 – Classification of NETs, with examples. Store

Direct capture methods

Indirect capture methods

Mineralised Pressurised Oceanic (including sediment) Biotic (including soil)

Enhanced weathering Air capture Liming N/A

Biochar Bioenergy with CCS Fertilisation Habitat restoration

designs and prototypes. In many cases there are only single estimates of capacities or costs, based on highly simplified assumptions. As a result, even the comparative ranking of techniques cannot be certain. However, for the most promising techniques, where multiple papers have been published by different authors, it has proved possible to interrogate the assumptions in more detail, and thus to derive relative values with greater confidence.

3.

Results of the comparative assessment

Table 3 and Fig. 25 summarise the findings of the assessment. Below, the principal results of the assessment are set out, first readiness, then capacity and finally cost.

5 In Fig. 2 the size of each bubble is proportionate to the estimated capacity. The readiness and costs figures are central values taken from Table 3. The global requirement bubble is included only for scale, not to indicate any specific requirement for readiness or cost.

3.1.

Readiness

Widely used technology readiness level (TRL) definitions (Table 4) have been applied to the different technologies (as listed in Table 3). This is partly an exercise in judgement, particularly with respect to scale. One recent assessment of geo-engineering technologies (USGAO, 2011) rated all the CDR technologies it considered at TRL 2, with the exception of DAC which was rated TRL 3. However it appears that the benchmark applied was of commercial readiness to completely offset the effects of current emissions as a standalone technology. Here a lower benchmark of technological development to demonstrated functioning, is applied to a much wider set of technologies. Nonetheless, even in small, non-commercial forms, few techniques have reached TRL 6 (prototype demonstration in the ‘relevant’ real world environment) and many have not yet reached component validation (TRL 4). Broadly the techniques can be split into three groups based on readiness:

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Table 2 – The negative emissions techniques subjected to detailed assessment. Category

Technique

Description

Mineral

Soil mineralisation

The addition of silicate minerals to soils (or surface water) to accelerate the natural process of carbonation of such minerals through dissolution both in the soil and in runoff (Schuiling and Krijgsman, 2006; Köhler et al., 2010) The replacement of carbonate in cement with a magnesium oxide from magnesium silicate, which combines with atmospheric CO2 while setting (Vlasopolous, 2010) Storing partially combusted organic matter (char) in soil by burial, preventing the return of biotic carbon to the atmosphere via decomposition. The char can be created from organic matter by pyrolysis or gasification, either of which provides some bioenergy (Shackley and Sohi, 2011)

Magnesium cement Biochar

Pressurised

Direct air capture (supported amines)

Direct air capture (wet calcination)

BECCS (bioenergy with CCS) (combustion)

BECCS (ethanol/BLG)

Oceanic

Ocean liming

Ocean fertilisation

Biotic

Forest management

Wetland restoration

Soil management

Timber use in construction Biomass burial

Adsorption of CO2 directly from the atmosphere using amines in a solid form, suspended on a branched framework, through which air is pumped, or circulated by wind. The CO2 is recovered by washing in vacuum, pressurised and injected into geological storage (Lackner, 2009; Eisenberger et al., 2009) Capture of CO2 directly from the atmosphere using wet scrubbing systems based on calcium or sodium cycling technology. The CO2 is recovered by calcining, pressurised and injected into geological storage (Keith et al., 2006; Baciocchi et al., 2006; Socolow et al., 2011) Carbon dioxide is captured from the emissions from combustion of bioenergy sources. Because bioenergy is nominally carbon neutral, capture of the majority of combustion emissions creates a net negative emission across the bioenergy cycle (from plant growth to energy utilisation) (Karlsson et al., 2010) As for BECCS (combustion), but the CO2 is captured at an earlier, fuel conversion stage such as ethanol fermentation or black liquor (wood pulp effluent) gasification (Karlsson et al., 2010) The addition of calcium hydroxide (Kheshgi, 1995) or calcium bicarbonate solution (Rau and Caldeira, 1999; Rau, 2011) to ocean surface waters, accelerating uptake of CO2 from the atmosphere, and enabling the ocean to hold a higher total CO2 store (Jenkins et al., 2010) Increasing ocean productivity through addition of limiting nutrients such as iron, phosphate or nitrogen, with the expectation that dead biological matter will sink into the deep ocean and add to stored carbon there. There is great uncertainty over the effectiveness of these proposed techniques (Shepherd et al., 2009) Increasing forest area by planting new forest or extending agro-forestry on suitable land, and/or enhancing management of existing natural and plantation forests to maximise the carbon sink (Shepherd et al., 2009) Rewetting and restoration of peatlands, tidal salt marshes and mangrove swamps to enhance anaerobic storage of dead organic matter (Parish et al., 2008; Chmura et al., 2003). ‘No-till’ agriculture to reduce the loss of carbon through oxidation when ploughing, thus enhancing the natural soil sink (Lal et al., 2004); or organic soil management, using manures and composts to increase the levels of soil organic content (Azeez, 2009) Increased use of harvested timber in long-life construction applications, thus increasing the store of carbon in the built environment Burial of harvested biomass in anaerobic conditions (on land) or in the deep ocean (Zeng et al., in press; Strand and Benford, 2009)

• The most ready techniques are forest and habitat restoration, and soil management, although all these are subject to fairly large uncertainties regarding the actual net rates of negative emission achievable, as well as potential conflicts relating to land use. The use of timber in construction, cellulose in concrete and the replacement of calcium-based cement with magnesium silicate based cement are also at or close to practical deployment. All these could be realistically deployed within the coming decade. • Biochar, BECCS and direct air capture fall in the mid-range of readiness, attracting substantial levels of academic or privately-funded research, and having various components under trial. It seems unlikely that these could be deployed at scale before the 2020s or 2030s. • The oceanic and mineralisation approaches tend to fall in the lowest readiness group, with primarily theoretical or laboratory work, and indeed, some which have not even been properly theoretically validated yet. Some novel direct air capture technologies such as the use of metal organic

frameworks also fall into this ‘least ready’ group. At best these techniques might reach deployment at scale by 2050. These broad time lines are derived from experience with energy technologies such as carbon capture and storage and flue-gas desulphurisation. These are taking, or have demonstrated multi-decade timescales from initial concept to early demonstration at commercial scale, and further decades to move from first demonstrations to widescale deployment (Watson et al., 2012).

3.2.

Capacity

Table 3 includes estimates for annual potential of the NETs assessed. These are forecast estimates for the period 2030–2050 (allowing for a period of technical development and rollout over the next 20–40 years. They take account of the constraints to deployment noted in the table and described later. Because of the variety of sources and uncertainties, they

Table 3 – Summary of assessment. Technique

Technical status

Potential capacity

Limiting factors

Costs estimates

Soil mineralisation with olivine

TRL1-5

1 Gt-CO2 pa

$20–40/t-CO2

Magnesium silicate cement Biochar – pyrolysis or gasificatiom

TRL 6-7 TRL 4-6

0.4 Gt-CO2 pa 0.9–3.0 Gt-CO2 pa

Supported amines for direct air capture Sodium or calcium scrubbers (wet calcination DAC) BECCS – combustion/co-firing

TRL 3-5.

10 Gt-CO2 pa (plus)

Finite solubility of silicic acid; availability of suitable land Demand for cement Sustainable biomass supply; suitable soils for storage. Energy supply: storage capacity

TRL 4-6

10 Gt-CO2 pa (plus)

TRL 4-6.

2.4-10 Gt-CO2 pa

BECCS – ethanol fermentation

TRL 5-6.

BECCS – black liquor/pulp

TRL 3-5

Currently no more than 48 Mt-CO2 pa 250–375 Mt-CO2 pa

Ocean liming (calcination) (e.g. C-questrate) Ocean liming (electrochemical splitting) Ocean fertilisation (iron)

TRL 3-4.

Multiple Gt-CO2 pa

TRL 2-3.

1 Gt-CO2 pa

TRL 1-4

0–1 Gt-CO2 pa

Ocean fertilisation (macro-nutrients – e.g. phosphate) Ocean ‘burial’ of biomass

TRL 2-3

0.2–0.5Gt-CO2 pa N; 0.5 Gt-CO2 pa for P

TRL 2-3

2.2 Gt-CO2 pa

Forest restoration/creation and enhanced management Habitat restoration: peatlands and other wetlands No-till or organic agriculture practices Timber use in construction

TRL 6-7

$40-300/t-CO2 $165-600/t-CO2 $70-250/t-CO2 As little as $45/t in 2020. As little as $45/t in 2020. $51-64/t-CO2 $100-180/t-CO2 n/a n/a

$74/t-CO2

1.5–3 Gt-CO2 pa

Supply of crop residues. Impacts on ocean biology. Land availability; climate

TRL 5-6

‘Several hundred’ Mt pa

Land availability; climate

$10-20/t-CO2

TRL 2-7

2.3 Gt-CO2 pa max

Land availability

n/a

TRL 8-9

0.5–1 Gt-CO2 pa

Negligible in most applications

TRL 2-3

1–3 Gt-CO2 pa

Construction demand. Sustainable supply of timber. Sustainable supply, suitable sites, logistics.

$20-100/t-CO2

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Tree burial in anaerobic conditions

Energy for calcination, storage capacity. Storage capacity; sustainable supply of biomass Storage; sustainable biomass supply, suitable facilities Storage; sustainable supply of biomass, suitable facilities. Energy for calcination. Vessels and port facilities. Supply of CaCO3, application rates of bicarbonate. Impacts on ocean biology. Suitable locations. Sustainable supply of nutrients.

‘Parity’ with Portland cement. $8–300/t-CO2

$7-50/t-CO2

Note: The key references for each technology are given in Table 2 above. Most of the figures for costs and capacity are derived from those sources. In respect of the costs of DAC and BECCS, the literature is diverse and the costs debated. The costs of these two technologies are considered in more detail in Section 3.3 below.

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Fig. 2 – Capacity, readiness and cost of prospective NETs. are no more than a broad guide to the relative scale of potential. They are also typically technical, rather than commercial, estimates. Below the potential in several scale categories is summarised. Gt-CO2 pa plus: the highest practical potential is likely for direct air capture techniques. Both supported amines, and wet calcination methods would appear capable of reaching levels in the order of, or even above 10 Gt-CO2 pa. Ocean liming might also achieve such rates although this would require optimistic logistical assumptions. 2–10 Gt-CO2 pa: BECCS has been estimated to be capable of delivering 2.4–10 Gt-CO2 pa. This assessment suggests that the likely practical figure would be towards the lower end of this range. Electrochemical splitting of seawater might also deliver rates of this order of magnitude (House et al., 2007). And if proven, regenerative grazing might also fall into this category.6 Use of genetic modification in agriculture and forestry has also been estimated to offer potential of this scale (Jansson et al., 2010): however the assumptions required regarding global uptake of modified crops to achieve such levels appear unrealistic. 1–3 Gt-CO2 pa: several technologies could deliver NETs in the order of 1–3 Gt-CO2 pa, including afforestation and forest management, changes in agricultural practice such as notill or organic cultivation, ocean burial of crop residues, tree burial, biochar, and ocean liming by electrochemical splitting (Rau, 2008). Soil mineralisation with silicates might also achieve this scale.

6

Lovell and Ward (undated) make a high level estimate of a maximum potential of 500 Gt-CO2 over 5 billion ha of rangeland, but at realistic rates of agricultural extension, at most a few gigatonnes of CO2 pa would seem plausible.

Table 4 – Technology readiness level (TRL) descriptions. Level Description 1 2 3 4 5 6 7 8 9

Basic principles observed and reported Technology concept or application formulated Analytical and experimental critical function demonstrated Component validation in laboratory environment Component validation in relevant environment System model or prototype demonstration in relevant environment Prototype demonstration in operational environment Actual system ‘qualified’ through test and demonstration Actual system proven in deployment

Source: US Department of Defence (2008).

Less than 1 Gt-CO2 pa: several options including magnesium silicate cement, timber use in construction, wetland restoration and creation. Ocean fertilisation also falls into this order of magnitude, but with additional uncertainties and side effects that suggest it should be discounted from further consideration at this stage.7 Several techniques are considered to be worth pursuing at a basic level as ‘no-regrets’ strategies. These include soil carbon management in organic agriculture, timber use in construction, wetland restoration, sensitive afforestation and potentially, regenerative grazing. However alone these will not deliver the levels suggested in the scenario set out above. A wider package will be needed. In constructing a package to meet carbon budget needs, it is

7

These include significant uncertainties about both the net drawdown rate from the atmosphere, and the transfer rate into the deep ocean, exacerbated by practical difficulties in measuring and validating results in the open ocean (Shepherd et al., 2009).

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important to recognise that some of the techniques are exclusive options: the same biomass cannot both be retained in forests, and turned into biochar; magnesium silicate cement and construction grade timber cannot both build the same house; nor can the same geologic storage reservoir be used twice. But there are also some possible synergies between different techniques which might help increase the total benefit of a package of NETs: for instance, insofar as biochar or soil mineralisation do deliver benefits for productivity, then the potential sustainable yields of biomass may be raised.

3.3.

Costs

The estimates of the costs of NETs given in Table 3 are early hypothetical estimates, based on figures cited in the literature adjusted where possible to make them more comparable as forecast costs per tonne CO2 stored (at current prices) at the stage of early practical deployment.8 Overall, costs estimates described in the literature range from as little as $8 to more than $1000 per tonne of CO2 abated. Many estimates fall within a range of $40–250/t-CO2 . This is unsurprising, as at an effective carbon price of $250 or more per t-CO2 , economically viable levels of mitigation are capable of drastically reducing global emissions while a level around $40–50/t-CO2 is foreseeable in carbon markets sometime after 2020. Higher costs estimates would be typically unlikely to be seen as worth pursuing or even publishing. Several techniques have been reviewed critically (McGlashan et al., 2010; Socolow et al., 2011), as have estimates of the costs of CCS (Hansson, 2012; Watson et al., 2012). McGlashan et al. (2010) suggest there may be “technical potential for CO2 abatement at prices below $200/t CO2 , and . . . [even] below $100/t CO2 ” but also warn that “these numbers carry a strong health warning . . . more research is needed on barriers and costs from development to implementation.” (p8). Moreover, many estimates in the literature are made by advocates or based on data provided by them, and not all are peer reviewed. Given the strong information asymmetries in such a technical area, optimistic estimates designed to obtain funding are likely to appear as a form of promoter bias. Uncertainties and adverse cost trends are typically ignored in the advocacy of new technologies (Borup et al., 2006; Hansson, 2012), leading to underestimates of costs and overestimates of technical viability. For example, CCS development forecasts typically apply the best real world learning rate9 for coal-power technology (13%) ignoring the range of values downwards to 0–0.5% (Hansson, 2012). Such considerations have therefore been applied to adjust published cost estimates for DAC and BECCS technologies, as follows. With some exceptions, the range cited by promoters of DAC is typically between $40 and $250/t CO2 .10 Solid amine systems are expected to be lower cost ($40–200/t CO2 Lackner, 2009; McGlashan et al., 2010) than wet calcination systems ($140-250/t CO2 , Stolaroff, 2006; Keith et al., 2006; Strand and Benford, 2009). However, the latter more closely resemble existing industrial processes, with consequentially more

8

For more detail see McLaren (2011a). CCS is a useful analogue for many NETs. The ‘learning rate’ is the cost reduction percentage for each doubling of installed capacity. 10 This excludes Eisenberger et al. (2009) estimate of $25/t CO2 , as it is based on an unpublished further reference and unexplained assumptions.

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accurate cost estimates. But even these may be highly optimistic. Socolow et al. (2011) conclude that even their current system reference costs for wet calcination DAC, of $600–800/t CO2 “may well be underestimates. Cost estimates in industry are almost always higher after the completion of pilot plant operations than when the principal guide to the estimate is laboratory results.” (p30). Even allowing for small positive learning rates and novel technological breakthroughs, it would not be wise to count on the availability of direct air capture at costs significantly below $250/t-COt-CO2 in the coming decades. A similar approach to BECCS costs leads to the suggestion that $100/t CO2 would be a fairly optimistic cost level for 2020 or even 2030, especially if development of CCS generally remains slow, and a value of $150/t CO2 seems more likely. Keith et al., 2006 suggest that BECCS would cost around $70/t-CO2 captured from the air (about half the figure the same authors estimate for direct air capture by wet calcination). IEAGHG (2011) cite costs of $70–110/t-CO2 avoided for BECCS. Karlsson et al. (2010) suggest, more optimistically, costs in Sweden of under $75–95/t-CO2 stored by 2020, and significantly lower by 2030. The 2020 figure is equivalent to $90–142/t-CO2 negative emission.11 McGlashan et al. (2010) cite much higher costs estimated at a DECC workshop, in the order of $200–250/t-CO2 captured (equivalent to $250–375/tCO2 negative emission).

3.3.1.

Potential income

In considering likely commercial deployment of NETs, possible income flows besides any form of subsidy for the climate benefit, should also be considered. This potential mainly arises through the sale of energy byproducts or captured CO2 . All the bioenergy techniques including biochar generate energy byproducts, but unless regulation requires CCS, bioenergy production can be conducted at lower cost without it. Commercial uses for CO2 captured by DAC or BECCS vary. In locations where enhanced oil recovery (EOR) is already practised using injection of CO2 , there is a market worth up to $50 per tonne. The EOR market in the USA alone has been estimated at 250Mt-CO2 pa (7.5Gt- CO2 over 30 years) (Ferguson et al., 2008). CO2 for chemical and food industry uses such as carbonation of drinks can be worth significantly more but the global market is just 102–227Mt pa for all industrial uses (IME, 2011) and the majority of the CO2 in such uses is re-released within a year.

4.

Discussion of constraints

Table 3 briefly notes the primary constraints faced by each technology. Here three factors with wide application to a range of techniques (energy consumption, use of geological CO2 storage, and diversion of biological productivity) are considered in more detail.

4.1.

Energy consumption

Energy data was not found for all techniques, there are a range of estimates for direct air capture techniques and a smaller number for oceanic options. These are shown in Table 5.

9

11

Assuming capture and sequestration of 1.25–1.5 tonnes for every 1 tonne negative emission, to allow for the energy penalty of CCS, and the additional inefficiency of biomass in comparison with a fossil reference fuel.

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Table 5 – Energy demand estimates for NETS. Technique

Efficiency (g/kWh)

TWh needed to deliver 24 Gt-CO2 pa

Proportion of 2008 world energy use

Gas/seawater injection (Kelemen et al., 2009) Supported amines (Lackner, 2009) Supported aziridine (Eisenberger et al., 2009) Fluidised bed DAC (Nikulshina et al., 2009) Wet calcination (Zeman, 2007) Wet calcination (Keith et al., 2006) Wet calcination (Baciocchi et al., 2006) Flue gas (BE)CCS (Herzog et al., 2009) Flue gas (BE)CCS (Socolow et al., 2011) Flue gas (BE)CCS (2050) (McGlashan et al., 2010) Ocean liming (Jenkins 2010) Electrochemical HCl removal (Rau, 2008) Electrochemical ocean liming (House et al., 2007)

2779 3166 2000 64 470 384 365 2083 2702 4478 827 827 662

8636 7581 12,000 375,000 51,064 62,500 65,753 11,522 8882 5360 29,021 29,021 36,254

9% 8% 12% 383% 52% 64% 67% 12% 9% 5% 30% 30% 37%

Roughly comparative figures for BECCS are provided by fossil CCS analogues. The figures in column 2 can be compared with current average electricity generation emissions (550 g/kWh) and rough forecast 2050 emissions (50 g/kWh) to reveal that almost all techniques are or will be energetically viable. While biotic NETs rely mainly on direct solar energy, most other NETs would require a mix of heat and electrical energy. In column 4, the energy requirement is therefore compared with total world energy consumption. The total energy demand for delivering 24 Gt-CO2 pa globally is equivalent to 5–67% of all 2008 end use energy consumption depending on the technology deployed. For those techniques requiring primarily electricity, it should be noted that in 2008 8% of world energy consumption was equivalent to 45% of world electricity consumption. Thus, while 24 Gt-CO2 pa may be possible, achieving even a fraction of it would impose a very substantial additional burden on energy production. Moreover, many of the energy estimates here may be optimistic. The best figures shown for air capture are about 2.5 times the thermodynamic minimum (TDM).12 But this would suggest much superior performance to current CCS technologies which might achieve capture and compression at 3.5 times the TDM with perfect conversion of primary energy, or at 9 times the TDM given use of coal or gas to generate electricity (Socolow et al., 2011). Socolow et al. (2011) calculate that direct air capture using wet calcination would deliver only around 15–20 times the TDM. The energy requirements of NETs might be met primarily through use of currently ‘stranded’ energy, such desert solar, but even if such resources were still economically stranded for other purposes in future decades, the material and financial costs of utilising them would add substantially to the costs and impacts of the NET. Alternatively the difficulty of providing required energy might be eased by running the NET intermittently to help with grid balancing of intermittent renewables such as wind, but this would raise unit costs by requiring a higher capital spend to achieve the same overall rate of capture. In conclusion, maximising the sustainable deployment and availability of low and zero carbon energy through aggressive energy conservation and support for renewables would

12 The thermodynamic minimum energy required to remove CO2 from air and compress it is roughly 32 kJ/mole – or approximately 5000 g CO2 per kWh.

be a necessary (but not sufficient) precursor to any subsequent implementation of NETs.

4.2.

Storage

Geological storage for compressed liquid CO2 is required for several categories of NET, including direct air capture techniques and BECCS. Geological storage has been demonstrated for several decades as a result of the use of reinjected CO2 for enhanced oil recovery. The technique also underlies the development of carbon capture and storage technologies for fossil fuel combustion, and has therefore attracted intensive research. In general it is considered that estimates of storage capacity in disused oil and gas fields can be treated as relatively robust, whereas estimates of capacity in saline aquifers are much more uncertain. In a major review the IPCC (2005) concluded that 200 Gt-CO2 capacity is virtually certain, 2000 Gt-CO2 likely and 11,000 Gt-CO2 possible. Experience characterising storage capacity suggests that it is wise to be cautious, as revised figures in the order of 10–20% of initial estimated capacity are not unusual after detailed study (Hazseldine, pers comm). This might suggest a global figure in the order of 400–2000 Gt-CO2 . More practice of CO2 storage will help further improve estimates. The estimates presented above suggest a total need for NETs in the order of 1200Gt. It may be practical to deliver around half of this with techniques that do not rely on geological storage. The remaining half would however require more than 600 Gt-CO2 of storage. Dedicated biomass BECCS would require a factor of 1.25–1.5 more storage because of the energy penalty involved on energy production with CCS, and if BECCS was achieved by co-firing with fossil fuels the factor could be as high as 5.0 (with only 30% biomass) falling to 2.33 (with 50% biomass). DAC might also demand additional storage. For example, if fossil energy were used to power wet calcination, a factor of 2.0 might need to be applied (Socolow et al., 2011). Similarly with ocean liming: if the lime were obtained from fossil fired calcination with CCS (one option proposed in Jenkins et al., 2010) an additional geological storage factor of 0.5 would be required. Overall therefore, even with a portfolio of NETs, achieving 1200 Gt-CO2 negative emission might require over 1000 Gt-CO2 of storage, and at worst, if BECCS alone were used, cofired at 30%, then 6000 Gt-CO2 of storage capacity would be required. If alternative storage routes – such as basaltic injection (Kelemen and Matter, 2008) can be proven at reasonable cost, then this potential limit may be avoided. Otherwise, likely

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storage and likely need will be of the same order of magnitude this century. As a result: efforts should be made to develop the governance and allocation of storage for NETs; unrestrained use of storage for fossil CCS should not be permitted for any prolonged period; and NETs should not be considered as a long-term offset mechanism, but as a multi-decadal transition technology.

4.3.

Biological productivity

Storing carbon from biological productivity will have consequences, either for other species, or for current human use of bio-productivity for food, fuel or fibre. All the biotic technologies (e.g. tree burial), biochar and BECCS would be of concern in this respect, including the oceanic biotic techniques. Estimates of the sustainable supply of feedstock for bioenergy are highly variable. Cautious estimates would suggest a maximum sequestration of biomass carbon through BECCS of 3.3–7.5 Gt-CO2 pa globally (based on Woolf et al., 2010).13 More optimistic forecasts, of 15-50Gt-CO2 sequestered14 require unrealistic assumptions of substantial new areas of production (Bauen et al., 2009) or conversion of ‘abandoned cropland’ (Lenton, 2010) or unsustainable increases in crop yields (Ladanai and Vinterbäck, 2009). Widespread adoption of more sustainable, lower meat, diets might permit some sustainable diversion of land from fodder crops to bioenergy. But water availability is likely to further limit sustainable biomass supply. In ambitious scenarios for increased biomass global evapotranspiration from crops would double (Berndes, 2008). Moreover, in the short term increasing the overall level of biomass use would create a carbon debt which may last for decades (Bird et al., 2010; Walker, 2010). On balance, neither future increases in land availability for bioenergy nor very large increases in productivity should be assumed.15 Given limited productivity, it is useful to briefly consider what might constitute ‘best use of biomass supply’ be. Strand and Benford (2009) contrast ocean biomass burial (92% of carbon stored) with use of crop residues for ethanol production with CCS (in which 46% of original biomass carbon is stored and 38% offsets fossil carbon). For BECCS on centralised power generation, based on the averages presented in McGlashan et al. (2010), the overall net sequestration efficiency would be 78%,16 and once the benefits of avoided fossil fuels are included, BECCS would appear preferable to burial. Matthews et al. (2011) calculate figures for timber which suggest the combined benefits of stored carbon and avoided

13 The feedstuffs considered sustainable included up to 80% of sawmill, yard and other wood wastes; rice husks, up to 20% of cereal straw, up to 25% of cattle manure, the product of up to 175 mha of tropical grassland converted to agroforestry; and the product of degraded or disused cropland not in other uses. 14 The net negative emission is lower at 10–33 Gt-CO2 because of the higher direct emissions per unit energy generated from biomass compared with the fuel replaced. 15 While new processing technologies might extract more energy from bioenergy feedstocks, this will make no significant difference to the carbon content and thus to the level of negative emission achievable from limited productivity. 16 For every 100 tonnes of carbon in biomass, 83 would be sequestered, and 17 emitted in, or as a result of the processing. And an additional 7 tonnes would be emitted as a result of the lower energy density of biomass compared to the fuels it replaces, reducing net sequestration efficiency to 78%.

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emissions (from reduced use of concrete and uPVC) in construction uses are around three times greater than combustion in BECCS. However the maximum potential here is small and the store shorter-lived. In comparison to biochar, BECCS offers both higher energy recovery and higher sequestration, but relies on access to geological storage, and is most likely commercial within a developed energy grid. Biochar may be more appropriate for decentralised energy supply, and where geological stores or CO2 pipelines are distant, or exhausted. While burial might create a larger absolute negative emission from the limited biomass supply, while there is a scarcity of accessible renewable energy, both biochar and particularly BECCS are likely to be preferable, as they utilise the biomass also for low carbon power (and potentially also heat17 ), which can help mitigate existing emissions.

5.

Discussion and conclusions

There are four groups of techniques with substantial technical potential, and all are likely to be limited in practical deployment: DAC by storage availability (and wet calcination DAC also by energy supply); BECCS by bio-productivity (and potentially by storage availability); and oceanic liming by energy supply. Costs and other logistical constraints may further reduce capacities. While the deployment of any one technique might first enjoy economies of scale, most are likely to subsequently face diseconomies of scale as the cheapest and most suitable sites, carbon stores or transhipment facilities are fully occupied, and constraints such as resource exhaustion or environmental impacts take effect. Even utilising a broad portfolio of techniques, achieving adequate rates of negative emission for climate safety will therefore be technically and practically challenging. A rapid move to practical testing and deployment would allow a similar cumulative negative emission to be achieved at lower annual rates, and through enhanced learning, potentially enable higher cumulative totals. But very few techniques are ready for deployment, still less cost effectively. Indeed, this assessment strongly supports the view that NETs cannot be expected to offer an economically viable alternative to mitigation in the coming decades. On the other hand, limited deployment of NETs – in the order of perhaps 10–20 Gt-CO2 pa to complement much accelerated mitigation may become technically and economically plausible by 2030–2050. Significant further technological development and early and appropriate policy intervention would be necessary to release such potential, and the uncertainties involved mean serious moral hazard remains insofar as climate policy decisions rely on the future availability of NETs. At present the most ready techniques are typically small scale, such as timber use in construction, other biotic techniques such as forest management and extension and wetland habitat restoration, and magnesium silicate cements. In so far as they deliver co-benefits such as enhanced water management or benefits for biodiversity, these can be seen largely as small-scale no-regrets options and should be deployed as soon as appropriate governance arrangements are in place. Other no regrets options might include regenerative

17

However, configuring BECCS plants for combined heat and power would add further technical complexity and cost.

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grazing and organic agriculture practices to enhance soil carbon stores. In supporting development of early-stage NETs it is too early to rule out any but the most inefficient. But it is clear that the potential side effects of some will require their deployment to be actively limited (e.g. those requiring additional biomass) or prevented (e.g. ocean fertilisation with iron). On the other hand, those with more promise which merit particular research and development support include various forms of DAC and ocean liming. Advocates of NETs typically suggest that the best means to support development and deployment would be to incorporate them into emerging carbon markets. The assessment suggests several reasons why this would be inappropriate and might exacerbate moral hazard in climate policy. First and most importantly, in carbon markets NETs would be deployed as offsets – enabling reduced mitigation effort. Given the limits to capacity identified, this would sacrifice the unique benefit of NETs – the capacity to achieve absolute reductions in atmospheric concentrations. Secondly, the accounting uncertainties and other characteristics of NETs could add to damaging volatility in carbon markets. Third, NETs share the character of emerging energy technologies such as renewables and CCS, which have (so far) proven difficult to stimulate with carbon market measures – requiring instead targeted support such as feed-in tariffs. Accountability raises further challenges for the deployment (and incentivisation) of most NETs. None are entirely free from uncertainties in measuring or accounting for the actual net negative emission, although DAC comes closest. All the biotic techniques are beset with difficulties relating to the effects of land-use change, and for techniques such as soil mineralisation and ocean liming it is difficult to conceive of fully accurate accounting methods ever being developed. It would therefore be practically difficult to incorporate these techniques into carbon markets, or even into national accounting for UNFCCC purposes which relies on measures of carbon flows. In the assessment, the criterion of controllability proved difficult to measure. But considering the issue highlighted governance questions, such as the degree to which NETs might further raise concern about the role of private corporations in climate policy. While some techniques – like tree planting or peatland rewetting – are simple and widely available, and some others (notably Cquestrate (Jenkins et al., 2010)) are being developed in a deliberately open fashion, the majority of NETs are likely to be subject to multiple patents. Most of the technological development – especially around direct air capture – is being undertaken by venture capital funded university spin-offs in the USA. There is limited effort to claim a public share in the intellectual property arising even from publicly funded research. A better defined measure of controllability in this respect would be valuable for future assessment. All these issues raise important governance questions which must be resolved before even technically ready NETs can be deployed, while substantial further technical research and development will be necessary before any large-scale deployment of direct NETs. The complexity, novelty and potential side effects of the technologies concerned mean that researchers, funders and governments should initiate and accelerate efforts to ensure responsible governance of the research process also.

Acknowledgement The initial assessment of negative emission techniques, subsequently developed here, was commissioned and funded by Friends of the Earth (England Wales and Northern Ireland).

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