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2 Scientific Background for BECCS
BECCS (Bio-Energy with Carbon Capture and Storage) is a technology that integrates biomass systems with geological carbon storage.2 During combustion, fermentation, putrefaction, biodegradation and other biological processes, large amounts of CO2 are emitted from trees, plants and agricultural crops. These processes are for example found in biomass fuelled power plants, pulp and paper industries, ethanol plants and biogas plants.
As biomass grows, CO2 is absorbed from the atmosphere. Through the photosynthesis, carbon is incorporated into plant fibres, while oxygen from the decomposed CO2 molecule is set free. The energy for the process comes from the sun that induces the photosynthesis.
When biomass is broken down through combustion or any other natural process, the carbon atoms that the plant was composed of are released. Together with the oxygen in the air, they form CO2. In this way, large amounts of biogenic CO2, obtained though natural biodegradation processes, are released back into the atmosphere. The CO2 molecules are then split again through the growth of new biomass, which is captured in the next generation of plants. When applying BECCS, the CO2 previously tied up in biomass is captured from the atmosphere, and the gas flow is diverted to the bedrock for permanent storage.3 In this way, BECCS systems create a flow of CO2 from the atmosphere into the underground (see Figures 3 and 4).
The BECCS technology was first mentioned in scientific publications in the 1990s.4, 5 Since then, the BECCS technology has been discussed as a variant of the CCS technology that is applied to fossil sources. Most interest has been directed towards the fact that BECCS provides an opportunity to create permanent negative carbon emissions, i.e. the removal of CO2 from the atmosphere. Since BECCS is a new and complex technology, it has come to be known by different names depending on the author and context. The IPCC uses the acronym “BECCS” to describe the technology in its fourth assessment report from 2007.6 Other authors use the abbreviations “BECS”,7 “biomass-based CCS”,8 “BCCS”,9 and “biotic CCS”.10 This report uses the acronym BECCS, as applied by the IPCC, throughout.
Figure 3 Bio-energy carbon flow
Figure 4 Bio-Energy with CCS (BECCS) carbon flow
There are many techniques, established ones as well as those under development, which have the potential of radically reducing CO2 emissions. Examples include solar, wind, bio and geothermal energy; decarbonisation of the transport sector; increased energy efficiency and also the application of CCS technology on fossil fuels in power production and in industries. What sets BECCS apart as a climate mitigation measure, is that it may result in permanent net negative carbon emissions. This is achieved as CO2 from the atmosphere, which has been locked into biomass, is stored underground. BECCS combines the natural CO2 capture process in trees and plants, with the benefits of geological carbon storage, CCS.
For an overview of the main flows of carbon and CO2 in different energy systems, see Figure 5. Please note that in addition to these main system flows, we are also including the support systems for construction, fuel extraction and transportation. In other words, we need to consider the fact that all systems currently involve certain fossil emissions at some part of the production chain. Even the design and installation of wind turbines involves carbon emissions, though the quantities are relatively small.
Figure 5 General comparison of carbon flows in different systems
The radical difference between negative carbon emissions and other energy systems becomes evident when looking at Figure 5. Fossil fuels increase the amount of CO2 in the atmosphere in absolute terms. As fossil coal and oil, which are not part of the natural carbon cycle, are extracted and combusted, CO2 is added to the atmosphere. Fossil fuels with CCS also increase the amount of CO2, but not as much as without CCS. Renewable energy generated by wind, solar, geothermic and hydroelectric power plants affects the carbon cycle to a very limited extent, once in operation. Bio-energy emits as much carbon as the biomass previously captured. BECCS however only emits parts of the previously captured CO2, and the rest is permanently removed from the atmosphere.
In contrast to other types of carbon sinks such as oceans and forests, geological storage is not affected by temperature increases, tree logging or other changes that might jeopardize these other forms of carbon sequestration. Other sinks involve the risk of negative feedback loops at increased temperatures, potentially leading to significant releases of stored CO2. For instance, the oceans absorb and store large amounts of CO2. This contributes to reducing the rate at which the amount of CO2 is added to the atmosphere. However, this ability is strongly dependent on temperature and decreases with increasing temperatures. In addition, the oceans have already stored such large amounts of CO2 that the ability to absorb additional amounts is declining; in other words, the oceans begin to reach saturation. This implies that our continued emissions will have a greater impact on atmospheric CO2 levels than they have had until now.11
By contrast, research on natural geological occurrences of CO2 and experiences from ongoing carbon storage projects are showing that the expected duration of storage in geological formations will be very long, probably millions of years.12 For details on retention times and storage security, see further section 3.4.
In summary, the ability of BECCS to create permanent negative CO2 emissions has four important ramifications:
- BECCS can be applied to offset greenhouse gases emitted by other sources. In this regard, BECCS can be compared with the capture of CO2 directly from the atmosphere, as biomass absorbs CO2 from the atmosphere regardless of where it has been emitted.13 This means that BECCS could be used as a tool to restore the greenhouse gas emissions that are the most difficult and expensive to reduce, for example CO2 emissions from gasoline powered cars and air traffic. As the costs of emission mitigation are rising and the most cost-effective alternatives have been exhausted, the importance as well as the economic benefits of applying BECCS will increase.
- BECCS is a mitigation tool that can be added as a supplement to other measures, on top of bio-energy use. The application of BECCS would make it possible to reach agreed climate targets at lower costs, and also involves considerable opportunities to raise the ambitions for the level of emission reductions and the pace of climate mitigation work. With ambitious BECCS implementation schemes, countries such as Brazil and Sweden could reach zero net emissions of greenhouse gases already by 2030, and thereafter provide negative carbon emissions, a product that can be ‘exported’ to other countries.
- BECCS can mitigate carbon emissions that have already taken place. In other words, BECCS can restore the atmosphere from emissions that occurred previously. This has been explained in a number of long-term climate scenarios in which emissions not only reach a peak and then turn downward, but where the absolute levels of CO2 in the atmosphere also decrease.14 In some of these scenarios, such a peak is followed by a stabilization level more than one hundred ppm lower than the peak. The difference between peak and stabilized level is the result of using BECCS for a period stretching over several decades in order to remove CO2 from the atmosphere.
- The possibility to restore the atmosphere turns the BECCS-technology into a risk management tool in the long-term climate mitigation action.15 Regarding the two-degree target, i.e. the earth’s average temperature is to be increased by a maximum of two degrees Celsius above the pre-industrial level as a result of human emissions, one cannot know with certainty what level of GHG in the atmosphere this corresponds to. This depends on the complex climate system and a number of dynamic factors with complex linkages between GHG levels and the resulting atmospheric temperatures. We cannot accurately predict what CO2 concentrations will result from different levels of emissions, given the unpredictability of the buffering systems in the oceans and on land. Therefore, it is important that a long-term global perspective includes BECCS as a technology that can be used to compensate for inaccurate forecasts, as well as delayed political decisions on carbon mitigation policies.16 Otherwise we may not be able to meet the targets that have been agreed upon related to increases in temperature and negative impacts of global climatic change.17,18
In a number of different scenarios, the long-term sustainable capacity of BECCS is assessed to be large in a global perspective.19 In the modelling of climate scenarios, a number of forecasts for the potential magnitude of BECCS assume that the potential to create negative emissions is 5 to 20 billion tonnes of CO2 per year.20 In a forthcoming report by Ecofys in cooperation with the IEA GHG R&D Programme, using novel biomass combustion and conversion technologies, 5-10 billion tonnes of CO2 from biomass could be removed from the atmosphere with BECCS annually in 2050. This value can be compared with the annual greenhouse gas emissions in the world today, which are roughly at 30 billion tonnes CO2e. It can also be compared to the emission levels if we were to reach an 80 % cut in global emissions until 2050, which by then would be only 6 billion tonnes annually. Thus, BECCS could in that case outweigh the total emissions from other sectors, and create a system of global net negative emissions.
Some authors have argued that a massive application of BECCS would be sufficient to within 50 to 60 years21 counteract and compensate for all anthropogenic emissions of greenhouse gases that ever occurred and will occur, see Figure 6. The sustainability of producing biomass at the scale proposed here has however been questioned.22 At the same time, it is widely accepted that BECCS systems can compensate for anthropogenic emissions over long periods of time, 100 years and more.23
The sustainability issue of BECCS is complex in detail, as it involves both biomass as well as CCS systems, both of which involve many sub-systems. Issues span from biomass availability and geologic storage capacities to water use, competition for land and risk of storage leakage.
Figure 6 Levels of CO2 in the atmosphere in different climate mitigation scenarios. Adapted from P. Read and J. Lermit, “Energy”, 2005
Still, the most important aspect of BECCS in a life cycle perspective could be considered to be the underlying use of biomass. All energy systems that are dependent on biomass are facing the same situation. Biomass can be grown in unsustainable manners which may involve negative contributions in several ways, for example in the emission of CO2 from cultivation and transport, unsustainable water use and monoculture dependent biodiversity loss. If the demand for biomass would increase too quickly as a result of the development of BECCS systems, and these potential negative effects are not adequately countered, they could counter the benefits of negative carbon emissions.24 On the other hand, there is already an extensive production of biomass that is sustainable, from a carbon viewpoint. An example is the Swedish forestry sector, which renders a net uptake of CO2 (that is, higher growth than harvesting) equal to 20 % of Sweden’s emissions.25 There are also good opportunities to produce sustainable biomass at a global scale.26
In the scientific literature, BECCS is sometimes described as a system in which biomass is grown primarily in order to achieve negative emissions. However, BECCS systems can be created in easier and cheaper ways by combining existing biomass plants with carbon capture and storage. By introducing CCS to established biomass plants, application of the technology can be initiated in the near future, and to a lower cost than in systems where the biomass is grown only for the sake of the negative emissions. In addition, these add-on systems would not claim any new land or cultivation resources.
Another option to consider is co-firing of biomass with fossil fuels. If coal and biomass are co-fired and CCS is applied, large scale negative emissions may be difficult to achieve, though it is fully feasible to have a small negative emission impact or at least a zero or very low emission profile, including emissions arising throughout the life cycle of fuel production, extraction and transport. The combination of biomass and CCS could in this way provide an interesting strategic alternative for reaching zero or negative CO2 emissions also for predominantly coal fired power plants. This is not possible to achieve with CCS alone or even with a total switch to biomass firing, as the emissions from mining, cultivation and transportation are not possible to capture or in most present day cases replace or avoid.
According to climate change mitigation scenario modelling, BECCS is a cost-effective technology for reducing the concentration of CO2 in the atmosphere and for meeting ambitious climate targets. For ambitious CO2 levels such as 350 ppm and below, alternative options are to be considered inadequate or too expensive.27,28,29,30 It may be necessary to reach these levels in order to avoid severe climate change.31
It is worth noting that according to the scientific studies referenced above, the BECCS technology also reduces the cost of less ambitious climate targets, if included in the total portfolio for climate mitigation measures, see Figure 7. With delayed policy decisions for climate change mitigation, BECCS may be needed to reach higher stabilization levels such as 400 and 450 ppm in an economically attainable way.
Figure 7 Cost of reaching various CO2 concentration targets depending on mitigation portfolio32
The International Energy Agency has published a report on the role of CCS and BECCS in the global energy portfolio, using their BLUE map scenario.33 The report shows that BECCS has a very important role to play, if we want to meet the 450 ppm emission target. Using technical, physical and economic constraints in the optimization model, BECCS is shown to have a profound overall impact. It was found that CCS applied to biomass has more potential than all other industrial applications combined. Of the total CCS deployment called for in the scenario, BECCS accounted for a quarter of the CO2 stored, see Figure 8.
Figure 8 Global deployment of CCS needs to be 10 billion tonnes in 2050 in order to meet the BLUE map climate mitigation scenario. Of this, BECCS represents a fourth of the potential at 2.4 billion tonnes.
While BECCS is not as known a technology, conventional CCS (Carbon Capture and Storage) has been increasingly discussed in recent years, and proposed as a key technology to mitigate CO2 emissions. In most contexts, CCS technology is commonly associated with large coal-fired power plants, but it can also be used to reduce emissions from, for example, gas power plants, steel mills and cement manufacturing plants.
CCS applied to fossil sources cannot generate negative emissions, but it reduces the amount of CO2 emissions. One option available is to co-fire fossil fuels and biomass. Overall, such a combination could either lead to lower, zero or negative emissions, depending on the share of biomass and the efficiency of the CCS system. It should be added that in the same way as biomass can produce emissions during production, the extraction of fossil fuels also involves emissions, for example during mining and transportation. Therefore, thorough life cycle analyses are needed in order to determine the total impact of the systems, in terms of CO2 emissions.
2 Fisher et al., 2007 (IPCC 4th Assessment Report)
3 Obersteiner et al., 2001
4 Williams, 1996
5 Herzog et al., 1996
6 Fisher et al., 2007 (IPCC 4th Assessment Report)
7 Royal Society, 2009; Azar et al., 2006; Metz et al., 2005
8 Metz et al., 2005 (IPCC Special Report on CCS)
9 Bonijoly et al., 2009
10 Gröonkvist et al., 2006b
11 Rockström et al., 2009
12 Stenhouse, 2009
13 Keith, 2005
14 e.g. Fisher et al., 2007
15 Obersteiner et al., 2001
16 Krey, 2009
17 Hare and Meinshausen, 2006
18 Kypreos, 2008
19 Fisher et al., 2007
20 Azar et al., 2006
21 Read et al., 2005
22 Rhodes et al., 2008
23 Royal Society, 2009; Azar et al., 2006; Metz et al., 2005
24 Rhodes et al., 2008
25 Naturvårdsverket (Swedish Environmental Protection Agency), 2010
26 Kraxner et al., 2003
27 Azar et al., 2006
28 Kypreos, 2008
29 Krey et al, 2009
30 Azar et al., 2010
31 Hare och Meinshausen, 2006
32 Azar et al., 2006
33 IEA, 2009