Bioenergy with carbon capture and storage in a future world Ch 14

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C H A P T E R 14 c0014 Bioenergy with carbon capture and storage in a future world Patrick Moriarty1 and Damon Honnery2 1 2 Department of Design, Monash University-Caulfield Campus, Melbourne, VIC, Australia Department of Mechanical and Aerospace Engineering, Monash University-Clayton Campus, Melbourne, VIC, Australia 14.1 Introduction s0010 p0010 p0015 The First Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) was released in 1990 and stressed the dangers of climate change and the urgent need for reductions in greenhouse gas (GHG) emissions to the atmosphere, as did the Fifth Assessment Report, released in 2013. Yet, despite numerous conferences and papers on the need for immediate action, both global fossil fuel energy use and energy-related carbon dioxide (CO2) emissions have continued to rise, and in 2016 emissions were 55% above the 1990 value [1]. So, despite all the rhetoric, the world is yet to take climate change seriously. The 2015 Paris Climate Conference set 2.0 C temperature rise above preindustrial values as a safe limit, with an aspirational target of 1.5 C. According to a report in New Scientist [2], the world could pass the aspirational temperature limit of 1.5 C as early as 2026, and even keeping to the 2.0 C limit looks increasingly unlikely [3]. Yet Xu and Ramanathan [4] have stressed the importance of keeping within these limits: they argued that any rise beyond 1.5 C should be seen as dangerous, and above 3 C as “catastrophic.” In keeping with this view, Hansen et al. [5] have argued that atmospheric CO2 concentrations should be limited to 350 ppm; in their view, we have already overshot by more than 50 ppm. A variety of approaches have been promoted for mounting an effective response to the dangers posed by climate change. One approach is to reduce energy consumption, either through increasing the efficiency of energy-using equipment (including power stations themselves) or by energy conservation. Another is to change our energy mix, by replacing fossil fuels with low- or zero-carbon-emitting renewable energy (RE) or nuclear energy. Bioenergy with Carbon Capture and Storage DOI: https://doi.org/10.1016/B978-0-12-816229-3.00014-4 Pires-1631794 1 978-0-12-816229-3 © 2019 Elsevier Inc. All rights reserved. 00014 AU:1 To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter MPS. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. 2 p0020 p0025 p0030 14. Bioenergy with carbon capture and storage in a future world These approaches themselves are not without serious difficulties [6 9]. Alternatively, with carbon capture and storage (CCS), CO2 emissions to the atmosphere would be reduced by AU:4 capturing CO2 from the exhaust gases of large emitters, such as fossil fuel power stations and oil refineries, compressing the CO2, then piping it to suitable storage sites deep underground. Yet another approach would sequester CO2 after it has been emitted from burning fossil fuels, either by biological or mechanical sequestration, thus allowing the possibility of negative CO2 emissions. An increasing number of researchers believe that negative emissions are now the only way to avoid dangerous climate change (e.g., Refs. [5,10 12]). The advantages of negative emissions are clear: they allow some temporary overshoot of atmospheric CO2 levels, because levels can be later drawn down to more sustainable levels. Without negative emissions, only current emissions can be reduced. Bioenergy with CCS (BECCS) would both produce energy and allow negative emissions by combining the use of RE and conventional CCS. Other researchers (e.g., Refs. [13 16]), however, have pointed out the problems and even risks with such approaches. The CO2 in the atmosphere is only one of several sinks for CO2. Others include soils, biomass, and oceans. Table 14.1 gives rough estimates for each of these sinks today. Larger estimates are available for soils and oceans when inorganic C is included, and for different soil depths [18]. It is clear that oceans dominate, with the carbon stored mainly as sediments in the deep ocean. These reservoirs can be compared with the remaining fossil fuel proven reserves, including nonconventional oil and gas reserves, estimated at 4130 GtC. Over recent centuries the atmospheric sink has grown as the biota and soil sinks have decreased, and fossil fuels have been burnt. The IPCC [10] examined four climate change scenarios, or representative concentration pathways (RCPs): RCP2.6, RCP4.5, RCP6.0, and RCP8.0. The numbers represent the approximate global climate forcing function (in W/m2) in each scenario. Only for the AU:5 RCP2.6 case the world would avoid the risk of dangerous climate change (if geoengineering is ruled out as a solution, because of its potential risks and political problems). The analysis by van Vuuren et al. [19] has shown what the global energy sector would need to look like in the years up to 2100 under RCP2.6. Their modeled results project a base case global primary energy demand rising continuously from about 520 EJ in 2010 to around 1290 EJ in the year 2100. In the RCP2.6 case, energy demand in year 2100 is slightly AU:6 t0010 TABLE 14.1 Environmental carbon reservoirs. AU:11 Carbon reservoir (Gigaton carbon) Biota 610 Atmosphere 860 Soils 1580 Oceans 38,400 Total 41,450 Source: P. Moriarty, D. Honnery, Rise and Fall of the Carbon Civilisation, Springer, London, 2011 [17]. Bioenergy with Carbon Capture and Storage Pires-1631794 978-0-12-816229-3 00014 To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter MPS. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. 3 14.1 Introduction p0035 p0040 p0045 smaller, at roughly 1070 EJ, with the difference from the base case result of energy efficiency and energy conservation improvements totaling 220 EJ in RCP2.6. Their breakdown in year 2100 energy by type is shown in Table 14.2. The results are of course only one of many combinations for achieving RCP2.6; for example, nonbioenergy RE at zero and nuclear energy at 336 EJ (or vice versa) would give the same total energy and roughly the same GHG emissions. The table indicates the low level of GHG emissions in 2100 under RCP2.6—the only emissions are from the 132 EJ of fossil fuels still without CCS, but these are offset by 120 EJ of BECCS with negative emissions. CO2 emissions in RCP2.6 are reduced 95% compared with the baseline emissions. However, van Vuuren et al. have also stressed the difficulties in achieving this lowemission target. First, they pointed out that high carbon prices would be needed to induce the changes from the base case—after 2050, simulated carbon prices need to rise to US dollars (USD) 700 900/t of carbon (or USD 200 250/t CO2) and be maintained at this level. They further indicated that the rate of reduction in GHG intensity has to be several times the historical rate and that it has to start very soon: “In fact, in order to avoid a too large overshoot and/or extremely rapid reduction rate requirements in the second half of the century, stringent emission reductions are already required in the current decade.” It now seems very unlikely that such radical changes will occur before 2020. In this chapter the prospects for BECCS in the light of three competing methods for negative emissions—air capture, enhanced weathering (EW) of minerals, and biological carbon sequestration are evaluated. Like BECCS, none are in operation anywhere (with the partial exception of reforestation/afforestation) and are at best in the pilot stage: their potential lies in the future, perhaps decades away, judging by the slow response to climate mitigation. Since both BECCS and air capture also require carbon sequestration, carbon capture and sequestration (CCS) is also discussed. Other negative emission methods are possible; Fuss et al. [20] have provided a full list of possibilities, including ocean fertilization and ocean liming. t0015 TABLE 14.2 RCP2.6. Primary energy breakdown in year 2100 under Energy type Primary energy (EJ) Fossil fuels (no CCS) 132 Fossil fuels (with CCS) 350 Biomass (no CCS) 132 Biomass (with CCS) 120 Nonbioenergy RE 156 Nuclear energy 180 Total 1070 RE, Renewable energy. Source: D.P. van Vuuren, E. Stehfest, M.G. Elzen, T. Kram, J.V. Vliet, S. Deetman, et al., RCP2.6: exploring the possibility to keep global mean temperature increase below 2  C, Clim. Change 109 (2011) 95 116 [19]. Bioenergy with Carbon Capture and Storage Pires-1631794 978-0-12-816229-3 00014 To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter MPS. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. 4 p0050 p0055 s0015 p0060 p0065 14. Bioenergy with carbon capture and storage in a future world Not only must BECCS compete on cost (calculated as USD/t of carbon avoided) with other the negative emission technologies, but also with the more conventional mitigation methods—energy reductions, more use of alternatives to fossil fuels, and halting further net deforestation. The relative costs of these various carbon mitigation approaches will vary by region. It will vary greatly for each method, depending on the level of carbon reduction required for a particular method. There cannot be just one global price for biosequestration; for example, it may be cheap for selected small areas but costs much more for very large areas. If a comprehensive curve for a given country showing progressive increase in unit cost per ton of carbon against cumulative carbon emissions avoided is constructed, as Harmsen et al. [21] have done for energy, it will typically show a variety of unit costs for each reduction method. Some indications of the scale of resources needed to meet the biomass-derived primary energy listed in Table 14.2 can be attained by assuming typical input values. Assuming a yield of 10 t/ha per annum (Fajardy and Mac Dowell [22] report both lower and higher values) and energy content of 15 MJ/kg for dry woody biomass, per hectare energy production is 150 GJ. Thus around 16.8 million km2 would be required to service the biomass primary energy demand of RCP2.6, slightly more than the 15 million km2 of tilled land. Assuming biomass with CCS is used solely for the production of electricity, around 1000 GW thermal power stations operating carbon capture and storage would be needed by 2100 with each being serviced by around 8000 km2 of land dedicated to energy production. 14.2 Conventional carbon capture and sequestration With mechanical sequestration, in the form of CCS, the CO2 emitted from the smoke stacks of large consumers of fossil fuels, such as power plants, oil refineries, and cement plants, would be collected using amine solutions, which is a proven technology. After collection the CO2 would be compressed, then transported, either by pipeline or perhaps by ship, for burial at 800 m or more depth, in, for example, depleted oil and gas fields or saline aquifers. A similar process could be used for power stations fueled wholly or partly with bioenergy (BECCS). CCS can only capture current CO2 emissions, but it is not suitable for small (and mobile) CO2 sources, such as transport vehicles or domestic furnaces. Some of the captured CO2 could be (and presently is) used for enhanced oil recovery, where it would be sequestered. The irony is that enhanced oil recovery is a process that enables even more fossil fuels to be burnt. CCS could conveniently capture less than half of all fossil fuel emissions—an upper value might be 60% [20]. Furthermore, climate forcing emissions also include the other GHGs (chiefly methane, nitrous oxide, and chlorofluorocarbons), as well as CO2 emitted from net deforestation, and other land use changes. In all, CCS can probably only sequester much less than half of present annual anthropogenic GHG emissions. Since RE will need to account for an ever-rising share of electricity and also nonelectric energy needs, the share of anthropogenic emissions that can be sequestered annually by this method will steadily fall over time [23] and cannot ever be more than a minor solution to climate change mitigation. Bioenergy with Carbon Capture and Storage Pires-1631794 978-0-12-816229-3 00014 To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter MPS. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. 5 14.3 Biosequestration in soils and forests p0070 p0075 p0080 s0020 p0085 Because of the limitations for capturing current emissions, interest has grown in other methods that would directly extract CO2 from the air, or even from the vast amounts of CO2 dissolved in oceans by liming, which would also reduce ocean acidification [20,24]. These methods have the potential to reduce levels of GHGs to safer levels should the AU:7 world overshoot, as now appears likely. Indeed, one prominent scientist believes that 350 ppm should be set as the safe level for CO2, well below even the current level of over 400 ppm [25]. The three most widely discussed methods are treated in turn in the following sections: biosequestration, direct air capture (DAC), and EW of rocks, such as serpentine and olivine [26]. Other methods that have been proposed to have significant potential include biochar, iron fertilization of the ocean [27], and, as mentioned, extraction of CO2 from seawater, which would then cause atmospheric CO2 to enter the now CO2-depleted surface waters [28]. High hopes were held for a dedicated CCS power plant in Kemper County, Mississippi, the United States, which was being constructed at an initial cost of USD 2.4 billion. But after costs rose to USD 7.5 billion, plant construction was suspended. The plant was to run on lignite coal, which would then be gasified, and with CO2 capture, thus produce “clean coal” [29]. As Robert Service has documented [30] how even before this, after initial investments in CCS pilot plants, interest has declined since 2013. Nykvist [31] has stressed the multiple problems facing large-scale CCS. But other problems face CCS, especially those involving very large-scale underground storage [32]. Only about 10 million tons of CO2 are sequestered underground annually, which is insignificant compared with annual global emissions. Although global underground storage may be large, questions remain regarding annual storage capacity, given that some CO2 sequestration processes are rapidly available, while others are slow. Sedimentary basins are the best sites for storage, but fracking in sedimentary basins for unconventional oil and gas renders them unsuitable for CO2 storage [33]. The process of injection, whether of liquid wastes or CO2, can cause microseisms, which could cause loss of capstone integrity [34], as can natural earthquakes. In former gas and oil fields, any unplugged holes, perhaps drilled decades ago, can allow leakage of the injected gas. Also, given that for major CO2 storage, many thousands of holes would be needed, some blowouts would inevitably occur, with loss of CO2 and perhaps risk to the general population. For all these reasons, complex legal problems remain to be solved [35], and public acceptance is far from assured. 14.3 Biosequestration in soils and forests Biological sequestration would draw down atmospheric CO2 by enhancing its uptake in plants or soils. According to one estimate [36], terrestrial net primary production (NPP) today is now 45% lower than it was two millennia ago (NPP is a measure of the annual carbon—in GtC—fixed in living plants each year, net of plant respiration). Since 1850, some 136 Gt of carbon have been lost from soils and plants through agricultural expansion and deforestation [37], so biosequestration would help return the biosphere to its former state. Also, as Field and Mach [15] have pointed out, the ecological cobenefits are high, while both the level of complexity and energy inputs per ton of carbon sequestered are Bioenergy with Carbon Capture and Storage Pires-1631794 978-0-12-816229-3 00014 To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter MPS. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. 6 p0090 p0095 p0100 p0105 p0110 14. Bioenergy with carbon capture and storage in a future world low. At present, CO2 fixation from forest expansion (largely caused by CO2 fertilization) only partly offsets the CO2 lost to the atmosphere through deforestation, which occurs mainly in the tropics [10]. As a result, total CO2 atmospheric concentrations are now rising faster than ever [38]. Researchers disagree on the potential for global biosequestration. Some researchers have considered that it could be a major part of climate mitigation (e.g., Refs. [20,27,39,40]). Marshall [27] has given a list of estimates of the potential (in GtC) for each carbon dioxide removal method, together with costs per ton of carbon. Planting trees is shown as having several GtC/year potential by 2100, at a cost of only USD 20 100, considerably lower than other methods. Griscom et al. [39] have advocated “natural climate solutions” a combination of “conservation, restoration, and improved land management actions that increase carbon storage and/or avoid greenhouse gas emissions across global forests, wetlands, grasslands, and agricultural lands.” Even allowing for food security and biodiversity preservation, they found that the total potential was 23.8 GtCO2-eq per year. They further estimated that one-third of these foregone emissions could be delivered at the very low cost of USD 10/t CO2 (USD 36.7/tC), which fits in with Marshall’s cost AU:8 range. Others (e.g., Refs. [41 45]) have doubted that it can be of more than marginal use. In contrast to Griscom et al. [39], Boysen et al. [42] have concluded: “Furthermore, in the required large-scale applications, these plantations would induce significant trade-offs with food production and biodiversity and exert impacts on forest extent, biogeochemical cycles and biogeophysical properties.” There is also disagreement on the potential for carbon sequestration in soils. Some researchers have argued that the global potential for enhancing carbon storage in soils is large [18,20], with positive agricultural benefits [18]. Lam et al. [46] have reviewed Australian studies and found contradictory results. They concluded, “the potential of these improved practices to store C is limited to the surface 0 10 cm of soil and diminishes with time.” Hansen et al. [5], while promoting the need for negative emissions, nevertheless felt that both soil and forest carbon sequestrations were of limited value, being subject to saturation effects. They gave a global cumulative likely value of 14 GtC, with 35 GtC being an upper limit. If present emission trends continue, it is likely that, globally, existing forests could eventually become net emitters of CO2, because of rising losses from forest fires, pests, and drought. Allen et al. [47] have surveyed the existing literature and found that there are “potential increases in tree mortality associated with climate-induced physiological stress and interactions with other climate-mediated processes such as insect outbreaks and wildfire.” This risk can be expected to rise with further increases in global temperature and may already be having a negative impact on some forest ecosystems. Field and Mach [15] have accordingly claimed that there is a high risk of carbon loss in the future. Even if large-scale reforestation/afforestation could be implemented and climateinduced die-off was not a problem, its climate mitigation benefits are not clear. Forest expansion in boreal areas will not only draw down atmospheric CO2 but also locally increase absorption of insolation (decrease the albedo), at least partly offsetting any climate mitigation gains [48]. The albedo decreases because snow covered ground reflects more insolation than green trees. The analysis by Arora and Montenegro [41] supported Bioenergy with Carbon Capture and Storage Pires-1631794 978-0-12-816229-3 00014 To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter MPS. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. 7 14.4 Carbon capture and storage with bioenergy this conclusion and noted that afforestation in the tropics per unit area is around three times as effective as boreal afforestation, but in any case, afforestation could not substitute for CO2 emission reductions. In summary, afforestation/reforestation in tropical regions would be of greatest climate benefit, but unfortunately large forest losses are still occurring there, partly because of the need for agricultural land for an expanding tropical population. s0025 p0115 p0120 p0125 p0130 14.4 Carbon capture and storage with bioenergy One possible advantage of BECCS compared with biosequestration is that although mature forests can provide ecosystem services, they no longer show any NPP growth— annual carbon fixation is matched by plant respiration. Eventually, their net annual CO2 drawdown from the atmosphere will fall to zero. Biomass energy plantations in contrast would probably focus on fast-growing grasses or trees such as willows, which can be coppiced. BECCS of course, is basically biomass used for energy, and, as such, must compete with the two other main uses of biomass—food and biomaterials. As with biosequestration, there is no agreement on either the global potential for BECCS, or for its costs per ton of avoided carbon. Marshall [27] listed a global potential of perhaps 6 GtC per year, at a cost of between USD 50 and 250. Fuss et al. [20] gave a far smaller annual potential, 0.14 1.4 GtC (0.5 5.0 GtCO2). These values can be compared with total annual emissions from all sources of 11.2 GtC (41 GtCO2) [38]. Schlesinger [49], discussing the production of wood pellets from the United States for shipment to Europe, has even argued, “Unless forests are guaranteed to regrow to carbon parity, production of wood pellets for fuel is likely to result in more CO2 in the atmosphere and fewer species than there are today.” Baik et al. [50] have examined in detail the initial potential for BECCS in the near term for the United States and found that the annual potential was reduced from 370 to 100 MtCO2 when limitations on biofuel and CO2 transport and storage capacity were considered. Nevertheless, a vital advantage of BECCS compared with DAC and EW is that it is a net producer of energy. The longer the world delays in taking effective action to reduce GHG emissions, the greater the need, ceteris paribus, for negative emissions to reduce atmospheric CO2 ppm. If either DAC or EW is the preferred technology, a huge and increasing future energy debt will occur. Paying off this debt, in the form of reduced energy available for running the global economy from a given total primary energy, will be left to the next generation. In effect, they will have to pay for our excess emissions. With BECCS as a major negative emissions technology, this energy debt would not occur. Furthermore, a possible problem with biosequestration—the fact that it may take decades for new plantings to draw down the carbon [51]—is avoided if short rotation bioenergy crops, such as coppiced willow, or annual grasses are used. BECCS—and to a lesser extent, biosequestration—will both have their potential increased if the human population changes toward a more vegetarian diet, with far less animal products. A number of studies have shown that such a diet would free up agricultural land and save on energy and water inputs, and GHG emissions [52]. However, if present trends continue, global diets will be higher in animal products in 2050 [53]. Bioenergy with Carbon Capture and Storage Pires-1631794 978-0-12-816229-3 00014 To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter MPS. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. 8 p0135 s0030 p0140 p0145 p0150 14. Bioenergy with carbon capture and storage in a future world But not all BECCS applications need energy plantations or forest wastes. Pour et al. [54] have examined the global potential for BECCS applied both to directly incinerating municipal solid waste (MSW) for energy, and also combusting landfill gas in a gas turbine. For the more effective direct MSW option the authors found, “around 0.7 kg CO2-eq is removed for each kg of wet MSW incinerated, for the MSW-CCS scenario. This translates to approximately negative 2.8 billion tonnes CO2 if all the available 4 billion tonnes MSW generated per year by 2100 is utilised in a MSW-CCS system.” This direct combustion of MSW would have two further benefits: the avoidance of methane emissions (methane is an effective GHG) from landfill sites, and a reduction in material going to landfill sites. Also Sanchez et al. [55] have argued that BECCS could be deployed on US corn ethanol plants in the near term. 14.5 Direct air capture DAC has been proposed as a means of drawing down atmospheric CO2 to any desired level. One recent modeling study even claimed that meeting the aspirational Paris target of 1.5 C maximum temperature rise above preindustrial values would only be possible with DAC [56]. With DAC, ambient air would pass over CO2 absorbent chemicals, using either natural or forced convection. Several reversible absorbents have been suggested, including calcium, potassium, and sodium hydroxide solutions, solid alkali oxides, and aqueous amine solutions. After CO2 separation the absorbents are regenerated [57]. Some pilot plants have already been built [58], and a Swiss company now sells small modular plants for providing CO2 for greenhouses [59]. The DAC devices would each be capable of absorbing perhaps a few hundred tons of CO2 annually [60], although both Keith et al. [61] and Krekel et al. [62] have performed conceptual analyses on a 1 million ton per year plant. If each plant could remove 400 tCO2 annually, then 10 million such plants would be needed to annually remove 4 GtCO2, which is still only about 10% of estimated 2017 annual global CO2 emissions [38]. They could be preferentially located in the areas of good winds to encourage air flow over the absorbent, and also close to underground storage sites, to avoid the need for piping CO2 long distances. The sequestration process is the same as that for conventional CCS and BECCS and would face the same problems. With DAC, not only could nations collect emissions from point sources such as vehicles, but also emissions from previous years and from other countries. Organization for Economic Cooperation and Development (OECD) nations, for example, could at least partly redress their responsibility for the bulk of past GHG emissions [63]. Compared with other negative emission methods, DAC would not use much land, since the devices, while numerous, would be compatible with other land uses. The real drawback relates to the energy costs of capture, since unlike conventional CCS, which collects from exhaust stacks with CO2 concentrations of up to 10% 15%, ambient air has a CO2 concentration of only 0.04%. Neither the energy costs nor the monetary costs per ton of CO2 sequestered by this method are really known, since the technology is still under development, with no full-scale installations [60]. Indeed, cost estimates span nearly two orders of magnitude. Based on other trace gas removal plants, House et al. [64] estimated Bioenergy with Carbon Capture and Storage Pires-1631794 978-0-12-816229-3 00014 To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter MPS. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. 9 14.6 Enhanced weathering of minerals p0155 s0035 p0160 p0165 p0170 a cost of USD 1000/t CO2 captured. A 2011 report by American Physical Society [28] gave a lower figure (USD 600) but assumed larger plants each capturing 1 Mt/year. A 2008 estimate by Stolaroff et al. [65] using a sodium hydroxide spray calculated a low figure of USD 96/tCO2 (cost range USD 53 127) for a full-scale plant in the base case. Solution recovery costs were not included, and as is the case for other estimates, neither were sequestration costs. The high monetary costs are largely the result of high input energy costs for construction and operation of this negative emissions technology. These energy costs are largely unknown, but could be very high. Considering the case where to draw down the emissions from the combustion of 100 EJ of coal, another 100 EJ must be expended to capture the CO2 emissions [23]. If the second 100 EJ was from noncarbon sources then no further emissions would result. But, of course, using the 100 EJ of noncarbon energy directly would entirely save the need for air capture device construction and operation and CO2 disposal. As long as fossil fuels are still in use, it will be more cost effective to use alternative fuels directly, rather than for powering DAC. Hence DAC should only be considered after fossil fuel use is virtually eliminated, and if its energy and monetary costs per ton of carbon sequestered can be made low enough. 14.6 Enhanced weathering of minerals The weathering process, in which basic minerals react with atmospheric CO2, thus lowering CO2 concentrations already occurs naturally: about 0.25 GtC is fixed annually from “weathering of silicate and carbonate rocks,” with the drawn down CO2 ultimately “locked up in carbonates on the ocean floor” [26]. With EW of minerals, basic igneous rocks would be quarried, finely ground, then—for significant CO2 drawdown—spread over vast areas of the Earth’s vegetated land surface [26]. The technology for mineral application to soils is not new, as already in agriculture such minerals are applied to soils to lower their acidity, improve crop production and soil structure. As with natural weathering of these minerals, such existing applications also fix modest amounts of carbon annually [66]. For maximum effectiveness, igneous rocks such as dunite and harzburgite, both of which are rich in olivine, or basalt, would be used. In the study by Taylor et al. [26], the first two minerals were found to be about twice as effective in fixing CO2 for a given application (kg/m2) rate than basalt. Their study found that an application rate of 5.0 kg/ m2 was about twice as effective in fixing carbon as a 1.0 kg/m2 application rate for the olivine-rich minerals. Effectiveness also rose with assumed depth of natural mixing in the soil from 10 to 30 cm. The tropics would be the preferred location, particularly tropical weathering hotspots, since higher temperatures and intense rainfall there would accelerate chemical weathering. Using these locations would enable the land area required for application to be reduced by about 70% compared with more temperate vegetated regions. The scale would need to be vast. At 5.0 kg/m2, 5000 t would be applied to each km2 of treated land. Taylor et al. [26] calculated that, in order to reduce atmospheric CO2 levels by a total of 50 ppm by year 2100, tens of Gt of such minerals would first need to be ground to fine powder, to increase their reactive surface by orders of magnitude. Next, the powder Bioenergy with Carbon Capture and Storage Pires-1631794 978-0-12-816229-3 00014 To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter MPS. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. 10 p0175 p0180 s0040 p0185 p0190 14. Bioenergy with carbon capture and storage in a future world would need to be spread, probably using aircraft, over anywhere from 20 to 70 million km2 of the Earth’s surface. For comparison the entire land surface of the Earth, including deserts and icecaps, is only 149 million km2, and the cultivated area is about 15 million km2 [67]. The costs of a 50 ppm drawdown of CO2 would likewise be enormous. Depending on assumptions, such as the mineral type and application rate, and the application location, the study estimated total costs as “$60 600 trillion for mining, grinding and transportation, assuming no technological innovation, with similar associated additional costs for distribution” [26]. Such estimates of up to USD 1200 trillion need to be compared with current global product of around USD 100 trillion [68]. The high monetary costs suggest that energy costs would also be high, especially for grinding and spreading. If such energy was supplied by fossil fuels, the resulting emissions would offset much of the carbon drawdown. So costs alone would seem to eliminate mineral weathering as a major negative emissions technology and reduce its use to those cases where agricultural benefits could offset the costs. Serious environmental problems could also result. “Harzburgite, for example, includes asbestos-related minerals that carry health risks to local populations near application sites” [26]. Olivine-rich rocks, which are most effective for CO2 removal, are found to contain “relatively high concentrations of either chromium (Cr), nickel (Ni) or both” [66]. If such material was spread over crop lands, they could enter the human food chain, with possibly harmful consequences. A further problem arises because increasing the reactive surface area of the minerals requires them to be very finely ground. Inhaling such respirable particles during production (mining and grinding) and spreading can increase the risk of silicosis. Washing out of such particles into streams and ultimately the oceans will also likely cause ecological changes, both good and bad [66]. Historically, the OECD countries have been responsible for most cumulative GHG emissions and thus should finance EW. Nevertheless, if the tropics were preferentially targeted for mineral application, the environmental costs would still be borne by the countries that have historically contributed the least to global warming. 14.7 Discussion and conclusions This chapter has argued that all negative carbon emissions approaches, both biological and mechanical, face major challenges for their implementation on a large scale, particularly since it would need to be done rapidly. Given that the technologies are still at an experimental stage, none will make a significant contribution to reversing further growth in atmospheric CO2 levels any time soon. The US Energy Information Administration [69] in their base case has forecast that fossil fuels will still dominate global primary energy use in 2040. Energy-related carbon emissions are also expected to continue their growth. The conclusion is that for some decades to come, emission reductions (or in climate forcing more generally) will need to largely come from reductions in CO2 emissions by conventional approaches: reductions in both energy use and use of low-carbon energy sources, although at scales vastly larger than we have seen to date. Nevertheless, atmospheric levels of CO2 will continue to rise as long as some fossil fuels are still being burnt (and forests cleared), given the longevity of CO2 in the atmosphere [70]. Yet a number of Bioenergy with Carbon Capture and Storage Pires-1631794 978-0-12-816229-3 00014 To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter MPS. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. 11 14.7 Discussion and conclusions p0195 p0200 p0205 p0210 p0215 atmospheric scientists believe that even if the world can level off atmospheric CO2 levels at 450 ppm, the planet will experience increasingly adverse climate effects—particularly in the intensity and frequency of extreme weather events. The human risk will be magnified by continued population increases and consequent loss of ecosystem services [71]. We are therefore faced with a serious dilemma: for any hope of climate stabilization, the world appears to need negative emission technologies, with that need growing in step with rising atmospheric GHG concentrations, yet all negative emission technologies that have been proposed for delivering large carbon drawdown have their own serious problems. Conventional CCS does not allow the drawdown of CO2 atmospheric concentrations, and in any case, even if successfully implemented, it must decline as CO2 emissions from large energy/industrial sources decline. The energy costs per ton of carbon sequestered would also be high for existing nonoptimized power plants [17]. Biosequestration is a low-technology solution that has the potential to give ecosystem cobenefits. The scope for afforestation/reforestation will decrease if forests are cleared for increasing agricultural production for a growing global population, which the UN [72] do not expect to peak before the end of this century. As forests mature, annual CO2 sequestration eventually falls to zero, the reforested area would need to continually rise in decades to come. Furthermore, the sequestration of carbon does not necessarily translate into a reduced climate forcing function, because of albedo and other changes such large-scale plantings induce. BECCS has some problems in common with both conventional CCS and DAC, and also with biosequestration. Large-scale geological storage of CO2 will be needed for BECCS, just as with CCS and DAC, and competition for inputs of land, water, and fertilizer will affect BECCS, just as it does for biosequestration. A global change toward a more vegetarian diet could, however, gradually reduce this competition and so improve the future potential for BECCS. On the other hand, BECCS will not be a drain on future energy production, as would DAC and EW. DAC has one important advantage: atmospheric CO2 levels can be potentially drawn down to any desired level and would seem to have few ecological side effects. It would also allow OECD countries to redress their high cumulative carbon emissions. But it faces several crucial problems. First, the process of capturing CO2 directly from the air is very energy intensive. To the extent that this electrical energy would be supplied by fossil fuels, its net climate mitigation benefit would depend on carbon capture from these power stations. Given the necessary growth in fossil fuels, depletion could then become a serious problem [73]. Its monetary costs would also be very high. And it would share with CCS and BECCS the challenges facing geological CO2 burial, although CO2 transport distances could be greatly shortened. An advantage that EW of minerals shares with biosequestration is that it does not require compression and transport of captured CO2, followed by geological carbon storage. Furthermore, like DAC, the potential for carbon drawdown from the atmosphere is vast. But it also shares the disadvantages of DAC: its energy and monetary costs will be very high. A further serious disadvantage is that while modest global applications of pulverized minerals would on balance and have positive benefits for agriculture, at the very large scale needed for significant climate mitigation, its adverse health effects would outweigh any side benefits and would be unevenly distributed spatially. Bioenergy with Carbon Capture and Storage Pires-1631794 978-0-12-816229-3 00014 To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter MPS. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. 12 p0220 s0045 14. Bioenergy with carbon capture and storage in a future world In summary, CCS is not a negative emissions technology, although the sequestration component is shared with BECCS and DAC. DAC and EW have presently unknown but likely very high energy and monetary costs, with EW also having serious environmental costs. Their deployment is, at best, several decades away. Biosequestration may be cheap in some cases and has already been implemented to some extent. In future, it will face rising competition for suitable land—as will BECCS. However, BECCS carbon sequestration is more permanent than is the case with biosequestration and, further, could be implemented with municipal wastes, which are likely to be a growing resource base, or on ethanol plants. Acknowledgments p0225 Patrick Moriarty would like to thank the Department of Design, Monash University, for providing him with accommodation during the research for, and writing of, this chapter. 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Giurco, Projection of world fossil fuels by country, Fuel 141 (2015) 120 135. Bioenergy with Carbon Capture and Storage Pires-1631794 978-0-12-816229-3 00014 To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter MPS. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. Pires-1631794 978-0-12-816229-3 00014 To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter MPS. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. NON-PRINT ITEM AU:2 Abstract Conventional approaches to climate change mitigation do not appear to be working, since the level of greenhouse gases (GHGs) in the global atmosphere continues to rise. The 2015 Paris Conference set 2.0 C temperature rise above preindustrial values as a safe limit, with an aspirational target of 1.5 C. The world looks to pass the 1.5 C limit as early as AU:3 2026, and the 2.0 C limit some time later. Some scientists argue that any rise beyond 1.5 C should be seen as dangerous. If temperatures do exceed safe levels, the only way atmospheric GHG levels and global temperatures could be reduced is by negative emissions technologies that draw down carbon dioxide from the atmosphere. These technologies include bioenergy with carbon capture and storage (BECCS), biosequestration in plants and soils, and direct air capture and enhanced weathering of minerals. The latter two methods have heavy energy costs, while for biosequestration, the energy costs are much lower. Only BECCS is a positive energy generator. There is considerable uncertainty about the global potential for BECCS in a world with rising population and, consequently, food needs. Nevertheless, in some favored regions, BECCS could be of considerable help for emissions reduction. Keywords: BECCS; biosequestration; carbon capture and storage; climate change mitigation; direct air capture; enhanced weathering of minerals; global climate change Pires-1631794 978-0-12-816229-3 00014