Bioenergy with Carbon Capture and Storage Approaches for Carbon Dioxide Removal and Reliable Sequestration
Proceedings of a Workshop—in Brief
Carbon dioxide removal (CDR) techniques, which aim to remove and sequester excess carbon from the atmosphere, have been identified as an important part of the possible responses to climate change and have been garnering increased attention.1 The Committee on Developing a Research Agenda for Carbon Dioxide Removal and Sequestration was convened to develop a detailed research and development agenda to assess the benefits, risks, and sustainable scale potential for CDR and sequestration approaches, as well as increase their commercial viability. The CDR approaches under consideration by the committee are coastal and land ecosystem management, accelerated weathering, bioenergy with carbon capture, direct air capture, and geologic sequestration. To aid the development of the research agenda, each approach is being examined by the committee through a series of information-gathering workshops and webinars for open discussions with relevant communities about the current state of knowledge, along with the research needs for understanding the potential of each approach and for deploying them at large scales.
Bioenergy with carbon capture and storage (BECCS) is a technology that integrates biomass conversion to heat, electricity, or liquid or gas fuels with carbon capture and sequestration (including biochar) (see Figure 1). BECCS could provide a significant portion of the global energy supply if deployed to its theoretical maximum feasible amount. The future role of BECCS is a subject that divides researchers as estimates of potential future biomass supply vary widely (from tens of EJ/yr to several hundred EJ/yr) due to differences in approaches used to consider factors such as population development, consumption patterns (e.g., diet), economic and technological development, climate change, and societal priorities concerning conservation versus production objectives. Nevertheless, many integrated assessment models use large-scale deployment of BECCS in scenarios that limit climate change to below 2°C.2,3
The committee convened its third workshop on October 23, 2017, in Irvine, California, to explore the state of knowledge and research needs related to the potential of BECCS as a CDR approach. Invited speakers provided the committee with an overview of biomass production pathways and capacities, implications of various feedstocks, advanced conversion technologies, and capture and storage strategies. Presenters at the workshop also discussed cross-cutting issues that include life cycle impacts of large-scale BECCS deployment, policies and incentives for the implementation of these approaches, and social acceptability barriers. The workshop was preceded by an introductory webinar on October 16, 2017, where invited speakers provided a primer on the prospects of BECCS for negative emissions capacity; the capacity for biomass to meet stationary generation and transportation fuel needs; and the
1 NRC (National Research Council). 2015. Climate Intervention: Carbon Dioxide Removal and Reliable Sequestration. Washington, DC: The National Academies Press.
2 NRC. 2015.
3 Fuss, S., et al. 2014. Betting on negative emissions. Nature Climate Change 4(10):850–853.
SOURCE: NRC, 2015, p. 35.
status, challenges, and costs of implemented bioenergy and biofuels. This Proceedings of a Workshop—in Brief summarizes the presentations from both the webinar and workshop.
INTRODUCTION TO BIOENERGY WITH CARBON CAPTURE AND STORAGE
BECCS consists of multiple components and stages: biomass feedstock and collection; conversion of the biomass feedstock into energy; production of heat, electricity, or fuels; and capture and sequestration of the carbon resulting from using that energy (see Table 1). A number of biomass sources and potential feedstocks were addressed during the webinar and at the workshop, including woody biomass, corn grain, agricultural residues, waste biomass, and energy crops (e.g., switchgrass and miscanthus). The discussions focused on thermochemical approaches for energy conversion, including combustion, pyrolysis, and gasification. Workshop participants also discussed the uses of this energy for heat and electricity applications and the production of liquid fuels. Sequestration in geologic formations is the subject of a subsequent workshop and was not discussed in detail. Although, it was noted that biological carbon sequestration can be achieved by applying biochar, a byproduct of pyrolysis, to soils.4
Being a multi-stage supply chain technology, both biomass supply capacities and feedstock strategies are essential components of BECCS. Laurence Eaton and Matthew Langholtz of Oak Ridge National Laboratory (ORNL) addressed these issues of capacity and supply by reviewing the current state of biomass consumption for energy and the relevant contributors and methods. The United States currently consumes 10 quadrillion Btus of renewable energy (10.6 EJ), comprising about 10% of total U.S. energy consumption (biomass from wood and wood wastes constitutes the largest source of domestic renewable energy). The transportation sector is the primary consumer of biomass for energy in the United States (128 million dry tons/year),5 followed by the industrial sector (94 million dry tons/year). Eaton highlighted the enormous biomass potential within the United States as described in the 2005 and 2011 Billion Ton Reports, which identified a potential annual biomass supply of more than 1 billion tons. The most recent Billion Ton Report,6 published in 2016, utilizes an economic supply curve approach and a resource assumption that considers the currently used biomass supply as a constant quantity, with the potential supply as additional amounts, projecting a base case scenario of 0.8 billion dry tons of biomass available annually in 2040 at a feedstock price of $60 per dry ton. The major potential biomass sources in 2040 are simulated to be herbaceous energy crops, such as corn stover, switchgrass, and miscanthus, although wood and waste sources are estimated to be of equal importance to add robustness to the supply. The total forest biomass supply is estimated to be 94 million tons (one-third of which is from forest residues). The estimated agricultural resource supply largely resembles cropland distribution, with the primary growth feedstock being energy crops and one-third of the supply coming from agricultural residues. In addition, for the first time, estimates of the potential biomass supply from microalgae were included and were projected at 80 million tons.
4 NASEM (National Academies of Sciences, Engineering, and Medicine). 2018. Land Management Practices for Carbon Dioxide Removal and Reliable Sequestration: Proceedings of a Workshop—in Brief. Washington, DC: The National Academies Press.
5 “Tons” throughout the text refers to U.S. tons.
6 DOE (U.S. Department of Energy). 2016. 2016 Billion-Ton Report: Advancing Domestic Resources for a Thriving Bioeconomy, Volume 1: Economic Availability of Feedstocks. Oak Ridge, TN: Oak Ridge National Laboratory. 448 pp. doi: 10.2172/1271651. http://energy.gov/eere/bioenergy/2016-billion-ton-report.
|Potential Biomass Feedstocks||Conversion Pathway||Energy||Sequestration|
Woody biomass, such as
Agricultural residues, such as
Energy crops, such as
Combustion, in which the biomass is oxidized completely for power and/or heat production
Pyrolysis, in which biomass is heated in the absence of oxygen, producing liquids and/ or biochar
Gasification, in which biomass is partially oxidized under oxygen-starved conditions for the production of syngas
Liquid fuels, such as
Geologic storage,7 such as
Biochar, which can be used as a soil amendment
Langholtz highlighted near- and long-term strategies to meet capacity generation. There are 365 million tons of biomass currently in use. The addition of currently available but unused waste resources (using the base case scenario) yields more than 100 million tons of potential additional biomass, and forestland resources and agricultural residues yield an additional 100 million tons each. He emphasized that these categories of currently available and unused resources alone would approximately double the current biomass supply with no additional land impacts. The addition of energy crops, at the expense of some land impacts, produces a total biomass potential of more than 1 billion tons (see Figure 2). Moreover, energy crops could be grown using landscape design to reduce soil erosion, improve water quality, and complement current agricultural practices. Overall, there is a significant amount of national biomass supply potential, with one-third of the available resources currently being used, one-third currently available but unused, and one-third potentially available from energy crops, although Langholtz noted that supply varies with price, as well as spatially and temporally. In addition, an analysis of potential supplies for BECCS would require specific consideration regarding feedstock preference, willingness to pay, and spatial distribution of resources. Langholtz identified additional areas of research, including crop improvement, precision agriculture, externalities associated with BECCS, optimized fuels, and conversion processes.
Alan Del Paggio from CRI Catalyst Company remarked that biomass is being generated in ever-increasing amounts as both agricultural productivity and consumer wastes increase. In addition to rising per capita energy demands, he emphasized the importance of understanding which technologies can be adopted now and in the future to move toward carbon negative energy, focusing primarily on technologies that convert biomass to biofuels and biochar. A number of technology pathways exist to convert these residuals into a variety of fuels; however, there is no single universal technology that will provide fuel that meets the requirements of all engines. He pointed out recent trends in biomass utilization in the United States: the decreasing usage of woody biomass in electricity and the increasing usage of waste biomass, with additional increases in the utilization of solar power, wind power, liquid biofuels, and fluctuating trends in hydropower. Del Paggio also noted that the significant advent of advanced biofuel production is still lacking.8
Numerous biofuel processes have been implemented and vary by input feedstock, conversion technology, end products, and applications for the fuels produced. These processes also vary in technological readiness level.9 For example, processes utilizing municipal solid waste and lingo-cellulose for the production of long chain hydrocarbons via hydropyrolysis are at a 5–9 technology readiness level, while several sugar/starch and lingo-cellulose fermentation pro-
7 NRC. 2015.
8 U.S. EIA (Energy Information Agency). 2017. Short-Term Energy Outlook (STEO). See https://www.eia.gov/outlooks/steo.
9 Technological readiness level (TRL) refers to the maturity level of a particular technology, ranked from 1–9, where TRL 1 is the lowest and TRL 9 is the highest. See https://www.nasa.gov/directorates/heo/scan/engineering/technology/txt_accordion1.html.
FIGURE 2 Current and potential U.S. biomass supply, base case at $60/dry ton.
SOURCE: Presentation by Laurence Eaton and Matthew Langholtz, Oak Ridge National Laboratory.
cesses that produce ethanol are already commercially established, noted Del Paggio. Other processes that utilize municipal solid waste for ethanol production through gasification are at an 8–9 technological readiness level. Economic viability constitutes a challenge to biofuel production and utilization depending on the fuel produced, types and availability of biomass, complexity of operation (cost), and feedstock specificity. Del Paggio stressed that achieving a profitable negative carbon biofuel pathway necessitates cost-effective fuel economics, reasonable carbon efficient processes (e.g., hydropyrolysis), capture and recycling of CO2, and to the extent possible, biochar production and sequestration. Lastly, Del Paggio identified potential future projects for utilizing captured carbon from biomass for biofuel/biochar conversion processes, such as their use as a feedstock for various aqua-cultural and agricultural applications, carbon capture and storage (CCS), mineralization, conversion to construction material, and other uses as a chemical intermediate.
BIOMASS SUPPLY CAPACITY: TECHNICAL AND ECONOMIC CONSIDERATIONS
The available supply of biomass will be an important determinant of the potential capacity of BECCS. Several speakers addressed factors related to how to determine how much biomass is currently available, ways to augment supply, and economic considerations that influence biomass availability.
Carolyn Smyth of the Canadian Forest Service (CFS) discussed the carbon mitigation potential of bioenergy feedstocks. The CFS defines mitigation as reduced emissions relative to business as usual based on incremental activity (e.g., change in behavior or technology). She provided an overview of the systems approach to estimating emissions reductions used by CFS, which analyzes carbon dynamics in forest ecosystems with consideration of harvested wood products, biogenic emissions, and biofuels. An initial analysis of mitigation options, including increasing harvests to shift commodities toward bioenergy, found no overall projected decreases in 2050 of emissions at the national level when harvesting live trees (from clear-cut harvest, commercial thinning harvest, and pre-commercial thinning harvest) for bioenergy. These results held despite including substitution benefits (using wood products led to fewer emissions than other emissions-intensive building products and energy fuels).10
However, other analyses specific to the use of harvest residues indicated that the capture of these residues would be effective for mitigation. Overall, the results from various harvest residue scenario analyses showed an estimated 22 million tons of biomass per year that could be captured with the highest mitigation potential in areas that co-locate fiber and the population, such as the provinces of Alberta and Saskatchewan, which highlight the mitigation potential of harvest residues for local bioenergy. Smyth noted that reduction in greenhouse gas (GHG) emissions is sensitive to assumptions for future energy emissions, particularly for the electricity grid, bioenergy conversion efficiency, and remote community fuel mixes. Estimates of total biomass availability in Canada include 14–20 Mt of available forest harvest residues, of which 8–10 Mt could be retrieved annually at a cost of less than $80 per ton. Higher amounts of residues are available from saw mills (i.e., wood chips, sawdust, shavings) estimated at 28–32 Mt per year; however, the majority of these residues are already used in other industries (i.e., pulp and paper). Additionally, there is a potential 6–50 Mt of available forest biomass that has been damaged by natural processes (predominantly wildfire), but accessibility is an issue.
10 Smyth, C. E., G. Stinson, E. Neilson, T. C. Lemprière, M. Hafer, G. J. Rampley, and W. A. Kurz. 2014. Quantifying the biophysical climate change mitigation potential of Canada’s forest sector. Biogeosciences 11:3515–3529.
Recent advances in plant genomics and genetics present opportunities to augment the capacity of biomass supply. Wellington Muchero of ORNL gave an overview of advances in understanding genetic variation that can inform biomass recalcitrance,11 which can improve biomass growth densities and yields, biomass degradation efficiency, and bioenergy enabling technologies. ORNL compared distant organisms for similar gene function and applied the results to bioenergy feedstocks. The experiment was designed with the hypothesis that discoveries made in Populus (a perennial species used for its natural genetic variability characteristics) could be directly tied to prominent biomass feedstocks such as miscanthus (see Figure 3), switchgrass, and corn stover. Thus, knowledge of genome sequences can be utilized to investigate the association of mutations to specific phenotypes, such as cellulose or lignin content, which can then be used to modulate how much lignin is produced in a plant.
FIGURE 3 Miscanthus.
SOURCE: Oak Ridge National Laboratory.
Muchero discussed several examples of targeting traits associated with biomass yield. Differences in poplar tree growth were tied to a nucleotide deletion in the poplar gene that controls cell division rates. Genes regulating protein transcription were identified by comparing human genes and the Arabidopsis plant, which, when deleted from Arabidopsis, resulted in mutants with higher biomass. This revealed the universality of these transcriptional regulators, which could be applied to drive the creation of bioenergy feedstocks with higher biomass yields. Density is another phenotype of interest due to implications for transportation costs (compact, high-density feedstocks reduce costs). Lastly, Muchero identified current knowledge gaps, including landscape scale models incorporating individual traits into carbon sequestration estimates and economic models assessing the value of carbon capture at the trait level.
Robert Abt of North Carolina State University said that the economics of biomass supply and the resulting land use and carbon sequestration implications depend largely on local conditions. Abt provided a depiction of bioenergy in a market-driven forest economy, with a specific focus on the southern United States. From a forestry resource context, the southern United States is characterized by a mixture of timberland, predominately upland hardwood (38%) and planted pine plantations (24%). The market context for southern forests can largely be characterized by analyzing trends in the pulpwood markets of both pinewood and hardwood.12 Abt indicated that an increasing demand for wood pellets during a time of increasing prices in both pulp markets has little impact under typical scenarios (increasing demand of a low-value product), but high prices in the pulpwood markets may have important implications for the cost of wood to pellet mills, and thus, carbon consequences.13 Southern forest market dynamics illustrate that depending on local conditions, land use and carbon respond to increases in commodity demand (through higher prices). However, according to Abt, in a carbon market (as opposed to a commodity market), standing trees would have value. This would have forest management implications that result in changes in carbon stock and sequestration interactions depending on the accounting framework used.14,15 Abt identified the role and tradeoffs associated with soil carbon in response to intensive forest management, as well as integrated bio-economic modeling that helps drive policy development as it identifies questions for further exploration.
11 A natural resistance of plant cell walls to microbial and enzymatic deconstruction, and thus responsible for the high cost of lignocellulosic conversion (for energy). Himmel, M. E., S. Y. Ding, D. K. Johnson, W. S. Adney, M. R. Nimlos, J. W. Brady, and T. D. Foust. 2007. Biomass Recalcitrance: Engineering Plants and Enzymes for Biofuels Production. Science 315(5813):804–807.
12 Wear, D. N., and J. G. Greis, eds. 2013. The Southern Forest Futures Project: Technical report. Gen. Tech. Rep. SRS-GTR-178. Asheville, NC: U.S. Department of Agriculture Forest Service, Southern Research Station. 542 pp.
13 Abt, K., R. C. Abt, C. S. Galik, and K. E. Skog. 2014. Effect of policies on pellet production and forests in the U.S. South: A technical document supporting the Forest Service update of the 2010 RPA assessment. Gen. Tech. Rep. SRS-202, Asheville, NC: U.S. Department of Agriculture Forest Service, Southern Research Station. 33 pp.
14 Galik, C. S., R. Abt, and Y. Wu. 2009. Forest biomass supply in the southeastern United States—implications for industrial roundwood and bioenergy production. Journal of Forestry 107(2):69–77.
15 Abt, K. L., R. C. Abt, and C. Galik. 2012. Effect of bioenergy demands and supply response on markets, carbon, and land use. Forest Science 58(5):523–539.
BIOMASS CONVERSION TECHNOLOGIES (COMBUSTION, PYROLYSIS, GASIFICATION)
After the biomass feedstock is collected, biomass is converted into energy via thermochemical or biological conversion. Three main pathways for this conversion were addressed at the workshop: (1) combustion, in which the biomass is oxidized completely for power and/or heat production; (2) pyrolysis, in which biomass is heated in the absence of oxygen, producing liquids and/or biochar; and (3) gasification, which entails partially oxidizing biomass under oxygen-starved conditions for the production of syngas. It is also possible to use enzymes or microorganisms to convert biomass for carbon removal, but these approaches were not discussed at this workshop. While most of these biomass conversion processes focus on producing energy, agricultural and commercial uses for the captured carbon were also discussed.
Robert Brown from Iowa State University described thermochemical approaches to biomass energy carbon removal, defining thermal chemical conversion as the use of heat and/or a catalyst to convert biomass for carbon removal. Brown focused his remarks on gasification and pyrolysis conversion pathways for use in CDR (see Figure 4). Gasification-based conversion largely produces syngas, a gaseous mixture of carbon monoxide and hydrogen, which if used in electric power generation has the potential to sequester all CO2 from the process by burning hydrogen and depositing CO2 in geological formations. When syngas is used to produce transportation fuels, the CO2 from conversion is emitted into the atmosphere. The produced syngas contains a number of contaminants, which often require additional capital cost for removal. On the other hand, gasification may have lower capital costs when using bio-catalytic approaches, which require less gas cleaning and pressurization. Brown identified a number of advantages to gasification: it accepts diverse biomass feedstocks, produces a uniform intermediate product, and is commercially proven. In general, it is also easier to produce electricity from biomass by gasification than it is to produce liquid fuels.
Pyrolysis-based conversion involves the thermal decomposition of organic compounds in the absence of oxygen. Pyrolysis differs from gasification in that reactions take place at lower temperatures, which maximizes liquid production instead of gas production and results in bio-oil. Pyrolysis also produces biochar, a charcoal product that can be used in agricultural soils. Slow pyrolysis occurs over several minutes or days to produce predominately charcoal (biochar). Fast pyrolysis occurs over a few seconds to produce predominately liquids (bio-oil). The latter can be used as renewable fuel oil, provide a source of bio-based chemicals, and serve as feedstock for refining into transportation fuels. Biochar has been suggested as a sequestration agent for agricultural soils as it is recalcitrant to biological degradation and has potential positive impacts on soil quality, including increased fertilizer and water retention and reduction in nitrous oxide emissions from soils.16,17 Despite the advantages of pyrolysis, there remain challenges in scaling up reactors due to heat transfer bottlenecks and poor stability of fast pyrolysis bio-oils. However, recent research indicates lower capital and operating costs associated with pyrolysis in comparison to gasification.
Vann Bush from the Gas Technology Institute presented two examples of power plants utilizing biomass gasification technologies. The plants are Lahti CFB, Finland, and Skive BFB, Denmark, respectively. These two plants show total conversion efficiency rates of more than 80% for combined heat and power applications. They are highly efficient, but are not equipped to achieve negative carbon emissions. These two plants also illustrate different gasification processes. The Lahti plant utilizes a circulating fluidized bed with sorted municipal waste and woody biomass as feedstock. The Skive plant uses gasification of pelletized wood for combined heat and power application.
Bush emphasized that there are many choices of technologies for gasification and pyrolysis alone. One example is the IH2 process, which utilizes integrated hydropyrolysis and hydroconversion. He noted that the variety of technologies and potential products is one of the confounding features for bioenergy applications—a recent survey of gasification technologies identified 50 vendors for biomass gasification and 8 different gasification categories. Similarly, there are numerous biomass pyrolysis technologies available, such as auger, ablative processes, fluidized beds, and catalytic pyrolysis systems. Another challenging feature of these conversion technologies is economic sustainability—many require substantial feed-in tariffs (payments to energy users for generating electricity from renewable energy to stay in operation). Carbon intensity is rarely a valued economic component, Bush remarked. He noted that additional research on process integration and the integration of biochemical and thermochemical processes, as well as carbon capture recycling solutions, would be useful.
Raghubir Gupta from RTI International discussed carbon capture technology and applications that could be integrated into biomass systems using examples from coal and natural gas applications. A large share of carbon capture research has focused on post-combustion CO2 capture from coal-fired power plants. A significant challenge associated
16 Woolf, D., J. E. Amonette, F. A. Street-Perrott, J. Lehmann, and S. Joseph. 2010. Sustainable biochar to mitigate global climate change. Nature Communications 1:56.
17 Smith, P. 2016. Soil carbon sequestration and biochar as negative emission technologies. Global Change Biology 22(3):1315–1324.
FIGURE 4 Examples of thermochemical approaches to biomass energy carbon removal. NOTE: Not represented is direct combustion, which is another thermochemical approach that can be used in BECCS.
SOURCE: Presentation by Robert Brown, Iowa State University.
with this technology is the large energy penalty of CO2 capture and the thermodynamic efficiency rate for capture. Gupta described new developments in carbon capture technologies using non-aqueous solvents and solid sorbents, which have several technical benefits, such as having faster absorption kinetics, being non-corrosive, and having higher chemical and thermal stabilities. He identified opportunities for the utilization of the CO2 captured in biomass conversion, such as recycling for fluidization and purges and chemical looping. Moreover, according to Gupta, there are additional options for CO2 capture and utilization within biomass conversion systems; for example, carbon capture from fermentation at first and second generation ethanol plants. The cement industry, paper and pulp industries, and iron and steel industries provide further opportunities for the synergetic use of biomass with other fuels (i.e., co-firing coal with biomass to capture CO2).
BECCS ACTIVITIES AT COMMERCIAL SCALES
One bioenergy technology being tested at commercial scales is torrefaction,18 primarily as a solid fuel approach to power generation. Wayne Lei from Oregon Torrefaction provided examples of facilities testing the use of torrefied biomass (largely from forest restoration residuals) for renewable energy. Oregon Torrefaction creates a product that is easily ground (reducing capital costs), has a higher energy density than raw biomass, is resistant to moisture, and can be co-fired or fully fired in existing pulverized coal power plants. Moreover, torrefied biomass can perform similarly or superior to coal in existing coal power plants that utilize pulverized coal. Combustion tests of five torrefied biomass samples compared with Powder River Basin coal conducted at the Western Research Institute revealed differences in grinding, combustion, ash behavior, and a need for modifications for fuel feed and particulate controls. Overall, these tests indicated that biomass could successfully be used as a substitute for pulverized coal.
Lei further described examples of power plants that tested the use of torrefied biomass, including Southern Company’s (49 MW) Plant Scholz in Florida in 2013, Minnesota Power’s (15 MW) Taconite Harbor in 2016, and most recently Portland General Electric’s (600 MW) Boardman Plant (see Figure 5) throughout 2016 and 2017. The Boardman Plant committed to ceasing coal operations by December 2020 and is considering 100% biomass as an option. The latest test conducted at the Boardman Plant by PGE in February 2017 used 100% torrefied biomass at 43% of a full load (255 MW) for 5 hours, which resulted in little ash generation and a majority of the emissions were below detection thresholds. Observations from the three power plants and the Western Research Institute are positive regarding the combustion of torrified biomass, said Lei. However, more information on emissions through larger and longer combustion tests is needed. Lei indicated that next steps should include tests conducted at a full load of power generation with 100% torrefied biomass for a timeframe of 1 to 2 weeks. According to Lei, additional areas for re-
18 Torrefaction is a thermo-chemical process that converts biomass to a high-grade, renewable, solid biofuel by heating it between 200 and 300 degrees Celsius in a controlled environment. During this treatment, moisture and volatile organics are removed and much of the hemicellulose is decomposed. See http://www.oregontorrefaction.com/renewable-fuel-from-forest-restoration.html.
search include (1) region specific biomass resource inventories, including agricultural residues, energy grasses, and wood; (2) increased acceptance of torrefied biomass by utility pulverized coal plants; and (3) improvement or optimization of flue gas CDR and collection processes.
FIGURE 5 PGE Boardman Plant.
SOURCE: Presentation by Wayne Lei, Oregon Torrefaction.
INTEGRATED ASSESSMENT MODELING OF BIOENERGY
Structured integrated assessment models are designed to explore possible socioeconomic systems consistent with climate futures. Steven Rose from the Electric Power Research Institute provided an overview of bioenergy-related modeling exercises. For the case of bioenergy, these models suggest that bioenergy is a potentially valuable long-run climate management option.
Rose described a large inter-model comparison of 15 models that characterized and analyzed future dependence on bioenergy in achieving potential long-term climate objectives. In this study, model scenarios project annual bioenergy growth rates of 1–10% by 2050 (1–35% of global primary energy), with bioenergy accounting for 10–50% of global primary energy by 2100. Moreover, BECCS affects the cost-effective global emissions trajectory for climate management by accommodating prolonged near-term use of fossil fuels, despite land use change and land management emissions. Models trade off land carbon and increased nitrous oxide emissions for long run climate benefits. Rose indicated that many models project the majority of biomass supply and bioenergy consumption in non-OECD countries, with potentially large but uncertain regional roles (e.g., bioenergy is projected to produce 0–35% of regional electricity in 2050 and 0–70% of regional liquid fuel use). Rose highlighted the role of understanding the modeled transition of using BECCS technologies at scales to obtain a better understanding of current integrated assessment modeling. In addition, further characterization and evaluation of specific prospective BECCS systems and constraints, finer resolution energy and economic modeling of large-scale deployment integration, exploration of realistic potential policies for implementation, and the characterization of societal tradeoffs under different conditions is desirable, said Rose.
An understanding of the assumptions and challenges associated with BECCS is important to discern the feasibility of negative emissions from biomass, said Clair Gough of the Tyndall Centre for Climate Change Research. Traditional BECCS supply chains consist of some form of biomass being sent to an energy conversion facility, along with carbon capture and transportation to an underground storage facility. She highlighted recent research exploring the deployment of BECCS at scales assumed in integrated assessment models’ negative emissions scenarios. Nine key assumptions were analyzed (land area available for biomass, future yield, proportion of energy supply from biomass, maximum CO2 storage capacity, technology uptake, capture rate, policy framework, social acceptability, and net negative emissions) and broadly grouped into three categories (bioenergy, CCS, and cross-cutting). These assumptions were evaluated against five criteria: influence on results, agreement among peers, availability of data and information, plausibility, and expediency. The exercise identified several critical issues, including integrated assessment model (IAM) scenarios using unrealistic assumptions regarding the extent of possible bioenergy deployment, the uncertainty of the magnitude of CDR achieved by BECCS due to associated direct and indirect land use change, and a dependence on policy and governance assumptions.
Gough further identified a number of broad policy challenges for delivering BECCS technologies at scale. The implementation of BECCS could be useful in offsetting emissions from sectors such as agriculture and aviation where emissions are difficult to abate, as well as allowing for a certain amount of flexibility within carbon budgets; although the magnitude and duration of these budgets is uncertain. In addition, questions remain as to the quantification of
negative carbon emissions that can be delivered by BECCS. This is due to the multiple-stage supply chain nature of BECCS technologies, as there is no standardized methodology for accurately accounting for life cycle GHG emissions associated with all stages. Social acceptability, global biomass resource capacity and impacts, and the compatibility and alignment with existing bioenergy regulations were identified as additional areas for research. Gough concluded by emphasizing that the wide range of policy and governance issues related to BECCS would benefit from systems approaches to understanding the role of this approach within the wider socioeconomic landscape in the context of mitigation options.
CROSS-CUTTING IMPACTS OF BECCS
Scaling up BECCS may have cross-cutting impacts on multiple processes and systems. Life cycle analysis (LCA) is a comprehensive method for assessing a range of impacts across the entire life cycle of a product system.19 Michael Wang of Argonne National Laboratory described a life cycle perspective for BECCS using Argonne’s GREET (Greenhouse gases, Regulated Emissions, and Energy use in Transportation) model.20 He outlined specific examples of LCAs for first and second generation biofuels with a focus on GHG emissions from each stage of the life cycle. This analysis included elements of both direct (i.e., energy inputs, CO2 emissions from combustion) and indirect (i.e., land use change) effects throughout the biofuel life cycle, revealing that the feedstock growth and biomass conversion stages yield the highest GHG emissions.
Wang additionally highlighted research conducted using the GREET model that investigated GHG emissions (primarily CO2, methane, and nitrous oxide) from the co-production of biofuel and biochar and changes in soil carbon from biofuel feedstock growth. In the co-production process of biofuel and biochar, the source of hydrogen and application of biochar (for internal steam/power generation or as a soil amendment) are key factors for pyrolysis-based fuel GHG emissions results, although the levels of GHG emissions vary among technology options. For example, fuel gas/natural gas reforming for hydrogen results in a 60% GHG reduction relative to gasoline, reforming pyrolysis oil for hydrogen results in an 88% reduction, and the application of biochar as a soil amendment further reduces emissions by 112%. To assess changes in soil carbon, a module was developed for GREET to simulate changes in soil’s organic carbon from land use change associated with bioenergy feedstock production. In these results, the trend of estimated land use change GHG emissions for corn ethanol production was 10 gCO2e/MJ,21 reflecting a downward trend from previous studies. Land use change GHG emissions were found to be largely impacted by changes in factors such as land intensification, price elasticities in response to crop yields and food demand, and land management.
Bioenergy production includes a number of ecological considerations. Margaret Torn of Lawrence Berkeley National Laboratory highlighted natural resource requirements, costs and tradeoffs, and timing of implementation as three potentially problematic aspects of biomass production for BECCS. Land, water, and nutrient requirements constitute the primary ecological constraints and impacts of growing biomass for bioenergy. Water consumption by evapotranspiration is the most water intensive component of biomass usage for energy, consuming about 100 times as much water as the industrial component of energy capture from biomass. Biomass production also requires large amounts of nitrogen and phosphorus, both of which have environmental implications such as air and water pollution and eutrophication of rivers and lakes, respectively. Research conducted by Torn and Lydia J. Smith of the University of California, Berkeley, estimated the natural resource requirements for BECCS to be implemented at a scale of one Gt C/year. Accounting for embedded fossil fuel use for farm transportation, throughout the industrial process, and carbon leakage, 1.3 Gt of biomass carbon is required for the sequestration of one Gt of carbon in the geologic reservoir. This results in a resource intensity of 3.3M km2 of land, 25 Tg of nitrogen fertilizer, and 1,830 km3 of water per year (total evapotranspiration) when using switchgrass as the biomass source. While Torn acknowledged uncertainty in these estimates, she noted that it is difficult to imagine scenarios in which resource needs for BECCS would not have implications for global commodity markets or food production. The expansion of bioenergy production includes tradeoffs between land for feedstock growth in competition with other high-value uses of land (e.g., biodiversity), as well as tradeoffs of biomass itself for BECCS in competition with other uses in a decarbonized energy system (such as diesel for long-haul trucking).
Finally, Torn indicated that many scenarios depict BECCS ramping up at the end of the century. However, from an ecological perspective, that is the point at which demand for natural resources will be higher. Thus, she posed an
19 See https://www.epa.gov/saferchoice/design-environment-life-cycle-assessments.
20 GREET is a publicly available process-based LCA model that includes various biomass feedstocks, conversion technologies, and fuels as inputs; and energy use, GHG emissions, criteria pollutants, and water consumption for vehicle and energy systems as outputs.
21 Grams CO2 emitted per unit of energy.
alternative scenario for consideration in which BECCS was implemented early in this century and phased out as the demand for natural resources intensified. Torn identified several areas for further research, including estimates of biomass production potential that consider ecological constraints and competition for resources and tools for projecting potential biomass production under future scenarios for climate and land use.
BECCS POLICY, GOVERNANCE, AND SOCIAL ACCEPTABILITY ISSUES
Christopher Galik from North Carolina State University presented an overview of the most prominent public policy and governance issues related to the large-scale deployment of BECCS. He said that the monitoring and evaluation of GHG emissions of bioenergy is a primary consideration due to feedstock choice and geographic targeting implications. Another important consideration for policy decisions is spatial allocation: how are facilities, infrastructure, and harvests allocated across the landscape? In addition, the placement of injection sites and pipeline infrastructure for storage, both of which are receiving increasing public scrutiny, are other complicating factors for BECCS. Galik noted that there are important lessons learned from pipeline development and other deployment of new technologies, particularly where there have been spatial disparities and disagreements over the appropriateness or necessity of a particular technology. Incentives are another important element for broad implementation of any new technology.
Regarding social acceptability, Galik noted that increased technical complexity involves increased uncertainty, which in principle calls for a precautionary-type approach or adaptive management. However, different projects present varying obstacles and opportunities based on local resource conditions, supply constraints, and politics, suggesting that a localized approach would be preferable. Moreover, the deployment of CCS and BECCS, at any significant scale, includes demonstrable logistical challenges that would require significant policy signals to overcome. He noted that there appears to be a divergence among different types of stakeholders in accepting BECCS and a general lack of investment support as a whole for the technologies. Finally, Galik highlighted a number of areas for additional research, including (1) agreement on the real (and accounted for) GHG emissions implications of bioenergy, as well as policies to address these issues; (2) ownership and liability for CCS; and (3) societal acceptability of these technologies.
DISCLAIMER: This Proceedings of a Workshop—in Brief was prepared by Yasmin Romitti as a factual summary of what occurred at the meeting. The statements made are those of the rapporteur or individual meeting participants and do not necessarily represent the views of all meeting participants; the planning committee; or the National Academies of Sciences, Engineering, and Medicine.
REVIEWERS: To ensure that it meets institutional standards for quality and objectivity, this Proceedings of a Workshop—in Brief was reviewed by Bruce Babcock, University of California, Riverside; Erica Belmont, University of Wyoming; Göran Berndes, Chalmers University of Technology; Niall Mac Dowell, Imperial College London.
Committee for Developing a Research Agenda for Carbon Dioxide Removal and Reliable Sequestration: Stephen Pacala (NAS, Chair), Princeton University; Mahdi Al-Kaisi, Iowa State University, Mark Barteau (NAE), Texas A&M University; Erica Belmont, University of Wyoming; Sally Benson, Stanford University; Richard Birdsey, Woods Hole Research Center; Dane Boysen, Modular Chemical Inc.; Riley Duren, Jet Propulsion Laboratory; Charles Hopkinson, University of Georgia; Christopher Jones, Georgia Institute of Technology; Peter Kelemen (NAS), Columbia University; Annie Levasseur, École de Technologie Supérieure; Keith Paustian, Colorado State University; Jianwu (Jim) Tang, Marine Biological Laboratory; Tiffany Troxler, Florida International University; Michael Wara, Stanford Law School; Jennifer Wilcox, Colorado School of Mines.
SPONSORS: This activity was supported by Incite Labs, National Oceanic and Atmospheric Administration, U.S. Department of Energy, U.S. Environmental Protection Agency, U.S. Geological Survey, and V. Kann Rasmussen Foundation, with support from the National Academy of Sciences’ Arthur L. Day Fund.
For additional information regarding the workshop, visit http://nas-sites.org/dels/studies/cdr.
Suggested citation: National Academies of Sciences, Engineering, and Medicine. 2018. Bioenergy with Carbon Capture and Storage Approaches for Carbon Dioxide Removal and Reliable Sequestration: Proceedings of a Workshop—in Brief. Washington, DC: The National Academies Press. doi: https://doi.org/10.17226/25170.
Division on Earth and Life Studies
Copyright 2018 by the National Academy of Sciences. All rights reserved.