1. Introduction
Bioenergy with carbon capture and storage (BECCS) combines bioenergy with geologic carbon capture and storage to produce power (typically electricity or potentially liquid fuels) while removing CO
2 from the atmosphere [
1,
2]. As demand for atmospheric CO
2 drawdown increases, negative-emissions technologies (NETs) for CO
2 such as BECCS may be an important component of overall strategies to reduce atmospheric CO
2 concentrations [
3,
4]. The extent to which the US bioeconomy can be employed to meet potential future carbon management goals through NETs will depend on the potential quantity and cost of CO
2 sequestration. To understand the national potential for BECCS in the US to contribute to these goals, here we quantify the potential cost and quantity of BECCS as influenced by biomass resources, supply chain and power generation configurations, and proximity to geological formations suitable for BECCS. The main output of this analysis is supply curves illustrating the potential supply and associated cost of CO
2 sequestration under a range of biomass resource, logistics, generation, and carbon accounting scenarios.
The Intergovernmental Panel on Climate Change (IPCC) [
5] finds that “rapid and far-reaching” transitions in land management, energy production, and civil infrastructure are required to limit global warming to 1.5 °C. These can include reduced CO
2 emission from fossil energy, and increased CO
2 sequestration through afforestation, agricultural practices, and NETs, such as BECCS. The best approach to manage atmospheric CO
2 concentrations is subject to broad uncertainties (e.g., technology, economics, and the course of the global COVID-19 pandemic) and is unknown. Because of these uncertainties, the IPCC [
5] presents multiple potential pathways to limit global warming to 1.5 °C or less. Three of these four pathways rely on BECCS to meet this goal. Of the four IPCC illustrative pathways to limit climate change, Pathways 2, 3, and 4 include 151, 414, and 1191 cumulative GtCO
2 removed via BECCS globally by 2100 ([
5], Figure SPM.3b). The present analysis aims to understand the US potential (supply and cost) to contribute to carbon dioxide removal through BECCS.
Potential supply is a first key criterion of the feasibility of BECCS. The US has an untapped potential of about 750 to 1050 million tonnes of biomass per year, depending on offered price and future yields [
6]. Employing these resources in the bioeconomy can reduce CO
2 emissions and contribute nearly
$259 billion and 1.1 million jobs in the US [
7]. Previous efforts have explored the potential to use US biomass resources for BECCS. Baik et al. [
8] find that about 25% of these biomass resources are likely to be found over geological formations suitable for BECCS, and they estimate that the US has the potential to remove up to 110 and 630 million tonnes CO
2 per year, after accounting for biomass colocation with storage basins and injectivity (see
Section 2.2). Other research has found that western North America has the potential to sequester ~150 million tonnes per year by 2050 [
9], and that BECCS can capture 38 million tonnes CO
2 from current ethanol plants at costs <
$90 per tonne CO
2 [
10]. Building on these studies, the present analysis explores potential supplies of CO
2 sequestration through BECCS in the US.
A second key criterion in assessing the potential feasibility of BECCS is cost. Because of opportunity costs of capital, the cost of reducing atmospheric CO
2 is a challenge to meeting climate change targets. Costs of carbon capture, utilization, and storage (CCUS) are perceived to be high, and a holistic valuation of the supply chain is needed to understand the economic potential of CCS [
11]. BECCS is broadly estimated to cost between
$50 and
$250 per tonne CO
2 sequestered in an assessment of thirty-two studies referenced by the IPCC ([
4], Table 4.SM.3). Fuss et al. [
12] report a more narrow range of
$100 to
$200 per tonne CO
2. The cost range of BECCS is generally more expensive than the range of costs of afforestation, reforestation, biochar, and soil carbon sequestration (less than
$100 per tonne sequestered), but within the low range of direct-air capture (
$100–
$300 per tonne CO
2 sequestered) ([
4], Figure 4.2). For BECCS from existing corn ethanol biorefineries, costs may begin at
$30/t CO
2, assuming optimized transportation networks within 50 miles of an injection point [
10]. A 2020 review of CO
2 reduction strategies in California, US, identified costs of
$52–
$71/t CO
2 for BECCS converting waste biomass resources to hydrogen fuels with CCS, and
$106/t CO
2 for waste biomass to electricity with CCS [
13]. True costs of sequestration by BECCS will depend on scenario-specific factors such as biomass type, logistics, conversion and capture efficiency, and technical costs (e.g., [
14]), which are uncertain. Building on these studies, the present analysis explores costs of CO
2 sequestration through BECCS in the US.
In addition to potential supply and cost, a third key criterion of BECCS feasibility is sustainability effects. Though the IPCC Special Report on Global Warming of 1.5 °C [
4] includes BECCS in potential pathways toward climate change targets, the report also identifies potential negative side effects of BECCS including losses of biodiversity and food security. Concerns have been raised that bioenergy in general can have unintended environmental effects or can cause land competition with food production [
15,
16,
17]. An objective position can acknowledge that, as with many types of agricultural or forestry land uses, cellulosic biomass feedstocks can be produced in ways that are environmentally or socially detrimental or beneficial, depending on practices in the field and system-specific contexts [
18,
19,
20,
21,
22,
23,
24,
25]. Deep-rooted perennial biomass feedstocks offer strategies to reduce economic risk in the face of climate change and extreme weather [
26,
27]. Forest management can benefit from price supports for harvesting small-diameter trees to reduce threats of forest fires, mitigate pine beetle infestations, and realize desired future stand conditions. From a food security perspective, inflation-adjusted commodity crop prices in the US are near historic lows [
28], US farm bankruptcies have been rising since 2015 [
29], and billions of dollars are spent annually on US farm subsidies [
30], suggesting there are opportunities for perennial cellulosic biomass feedstocks as an alternative revenue stream for US farmers while meeting food production goals. Of the approximately 1 billion tonnes of potential biomass in the US reported in the US Department of Energy’s Billion-Ton Report [
6], about half is from wastes, agricultural residues, and forestland resources, which do not displace food crops; the remaining portion, in the form of energy crops, is reported to be produced on about 8% of US cropland, with less than 3% change in commodity crop prices and less than 1% impact on retail food prices. In sum, environmental and socioeconomic effects should not be generalized across disparate biomass resources and production practices, but rather the sustainability attributes of each biomass resource under specified production systems should be considered.
Volume 2 of the US Department of Energy’s 2016 Billion-Ton Report, titled “Environmental Sustainability Effects of Select Scenarios from Volume 1” [
31], explores environmental sustainability indicators of the resources used in this analysis. These indicators include quantitative changes in soil organic carbon, water quality effects (nitrate, total phosphorus, and sediment concentrations), water use and yield, greenhouse gas emissions, biodiversity effects, and air quality effects (carbon monoxide, particulate matter, volatile organic carbons, particulate matter, and sulfur and nitrogen oxides). The feedstocks in this study are limited to those that can have neutral or beneficial environmental and socioeconomic effects if applied with strategies such as best management practices, and allocation on the landscape where perennial energy crops can reduce erosion and improve water quality relative to other land uses as described by the US Department of Energy (USDOE) [
31]. If done correctly [
32], the biomass resources used in this analysis have the potential to contribute to United Nations Sustainable Development Goals such as life on land, life below water, affordable and clean energy, decent work and economic growth, sustainable cities and communities, no poverty, and climate action without compromising the other Sustainable Development Goals [
33]. We cannot say with certainty that the biomass resources used in this analysis will be produced with neutral or positive environmental effects, but results from USDOE [
31] suggest that they can be. Other resources not included in this analysis may offer other advantages. For example, forest thinnings in the wildland–urban interface can reduce fire risk [
34,
35], the use of hurricane (e.g., [
36]) and storm [
37] debris can reduce wastes, use of invasive exotic species could aid in their control, and sourcing biomass from agroforestry systems can provide multiple agronomic benefits [
38]. Resources such as these could be explored for initial applications. Long-term monitoring and evaluation of environmental [
39] and socioeconomic [
40] effects is recommended to ensure that potential negative effects are avoided and potential positive effects are enhanced.
The issue of carbon neutrality of bioenergy has been debated in the literature. Biomass resources included in this analysis can cause above-ground carbon stores to increase (e.g., woody biomass crops established on marginal cropland) or decrease (e.g., a forest thinning, until the stand regrows to previous levels). In some studies, it is argued that bioenergy can increase CO
2 emissions from forest or agricultural lands and from biomass combustion [
41,
42,
43]. In the case of bioenergy without CCS from forest biomass, changes in above-ground vegetation may require consideration of a carbon debt repayment period, depending on local conditions and management practices [
44]. However, these issues are raised in the context of bioenergy without CCS, in which case CO
2 emissions from biomass combusted for energy are released to the atmosphere. Conversely, in the case of BECCS in the present analysis, CO
2 emissions from biomass combusted for energy are captured and stored below ground. Thus, carbon accounting of bioenergy without CCS is necessarily different from carbon accounting of BECCS, as considered in the Discussion section. Sustainability effects other than CO
2 emissions, e.g., changes in water quality, biodiversity, soil productivity, food security, and socioeconomic effects, are not evaluated here, but resources in this analysis have the potential for largely neutral or beneficial effects as described in [
31], depending on agricultural and forestry practices. CO
2 emissions from indirect land use change are not included in this analysis, but this effect is expected to be small relative to net CO
2 sequestration because projected demands for food production are endogenous to the modeling by USDOE [
6], with generally small or even negative crop price effects as compared to the baseline projection for the biomass resources used in this analysis ([
6], Tables C-9 and C-10). Biomass resource categories, potential supplies, assessment and modeling sources, and associated sustainability constraints are shown in
Table 1.
To aid in understanding the potential role of BECCS among other NET strategies in the US, this analysis explores the potential cost and quantity of carbon sequestration through BECCS in the forty-eight contiguous US states under various feedstock, logistic, and power generation configurations.
4. Discussion
The maximum 90% CO
2 sequestration scenarios in
Figure 9 and
Figure 10 show capture of up 181 million tonnes CO
2 per year in 2020 and up to 737 million tonnes CO
2 per year in 2040. These results are within the range of CO
2 sequestration potential in the US that is constrained for colocation, storage, and injectivity (100–110 million tonnes CO
2 and 360–630 million tonnes CO
2 in 2020 and 2040, respectively) and the total potential (370–400 million tonnes CO
2 and 1040–1780 million tonnes CO
2 in 2020 and 2040, respectively) reported by Baik, Sanchez, Turner, Mach, Field, and Benson [
8]. Differences are to be expected, as Baik et al. constrain biomass resources to those within (i.e., over) sequestration basins, whereas the present analysis allows for use of biomass resources from outside sequestration basins to the extent that CO
2 emissions from transporting biomass do not exceed sequestration benefits, and associated transportation costs are incurred. Costs shown above expressed as CAC range from
$42 to
$137 per tonne CO
2, which are within or below the
$100 to
$200 per tonne CO
2 reported by the IPCC ([
4], Figure 4.2).
The results illustrate cost trends. Firstly, feedstock pelletization in the IGCC 2040 scenario results in a CAC about 6% cheaper. This is due to cost savings in transportation of the feedstock and higher energy density of the fuel because of lower moisture content of the feedstock. Secondly, as we increase the percentage of maximum CO
2 that can be sequestered through BECCS, the CAC increases. This makes sense because increasing demand for biomass requires the use of increasingly expensive and/or distant feedstock. However, the CACs reported in
Figure 9 and
Figure 10 are not all monotonically increasing as might be expected for supply curves, particularly for the IGCC 2020 conventional logistics scenario. These decreasing costs can be explained by increases in the energy intensity of the feedstock. In the long-term cases, the energy intensity of the feedstock (in MJ per dollar) is seen to decrease. This can be explained by the increase in the cost of fuel and the associated decrease in energy intensity of fuel with increasing demand for BECCS. The energy intensity of the fuel blend decreases once the higher energy-dense fuels, e.g., pine and switchgrass, have been consumed. In the IGCC 2020 case, however, due to the variance in feedstock availability in the near term, the energy intensity of the fuel initially increases sharply with increasing CO
2 sequestration levels, thus decreasing the cost of BECCS. Future work could account for this anomaly in the optimization.
The scenarios illustrated by the IPCC as alternative pathways to meet the Paris Agreement target estimate a requirement of 151, 414, and 1191 cumulative billion tonnes of CO
2 sequestration by BECCS by 2100 in Pathways 2, 3, and 4, respectively ([
5], Figure SPM.3b). To compare results presented in the present analysis with these IPCC estimates, assuming the total technical potential of 181 million tonnes CO
2 per year sequestered by BECCS in the US from 2030 to 2040, and 730 million tonnes CO
2 per year sequestered by BECCS from 2040 to 2100, this results in a cumulative estimated total technical potential of 46 billion tonnes CO
2 sequestered by BECCS in the US by 2100. This is equivalent to about 30%, 11%, and 4% of the targeted sequestration by BECCS by 2100 in Pathways 2, 3, and 4, respectively, in the IPCC. It is unknown how much of this total technical potential may be available. For example, competing demand for bioproducts could reduce the supply (or increase the price) of biomass available for BECCS. Alternatively, increased demand for bioenergy could provide opportunities for BECCS without competing for biomass resources. Of the results shown in
Figure 9 and
Figure 10, the 181 million tonnes CO
2 per year simulated in the near-term scenario used 206 million tonnes of the estimated 223 million tonnes of biomass reported available in 2020; the 730–811 million tonnes CO
2 sequestered in the future scenarios used 731–738 of the 823 million tonnes of biomass reported available in 2040 (
Table 1,
Figure 2,
Tables S2 and S3).
It should also be taken into consideration that while costs shown here as CAC account for avoided emissions from reference scenarios as described in Equation (1), the calculations for the quantity of CO2 sequestered do not take into consideration avoided emissions from the reference scenario. That is, if power from BECCS reduces demand for generation from pulverized coal or NGCC generation, then the CO2 reduction benefit from BECCS would increase.
As discussed in the Introduction and Methods, it has been argued that some bioenergy systems in the absence of CCS may cause carbon emissions because of changes in the amount of biomass on the landscape [
42,
43], which may require some carbon-debt repayment period [
44] before bioenergy can be considered carbon neutral as compared to a business-as-usual scenario. However, in the case of BECCS, 90% of the CO
2 produced from power generation is not emitted, but rather is sequestered below ground. This is unlike the case of bioenergy without CCS, in which case CO
2 produced from biomass is re-emitted to the atmosphere. Thus, with BECCs, because most of the carbon in harvested biomass is not emitted to the atmosphere, the carbon payback time (i.e., the time until plant growth and avoided fossil fuel emissions balance the CO
2 emitted, including supply chain emissions) will be much reduced compared with bioenergy without CCS. All woody resources used in this analysis have short payback periods, i.e., 4 years for willow, 8 years for poplars, and approximately 10–14 years for whole trees (<28 cm diameter at breast height). Of these, only the whole trees from forestlands, comprising 2–39% and 0–9% in the 2020 and 2040 scenarios, respectively, could have some carbon payback period depending on site-specific conditions. Site-specific carbon accounting would depend on previous land use, the expected business-as-usual scenario, and stand-specific silvicultural conditions, and is out of the scope of this analysis. In all scenarios, more than 80% of CO
2 from biomass used for BECCS is sequestered net of emissions from power generation, CCS, transportation, harvest, and production (
Figure 13). Thus, in instances of whole trees from forestlands where a carbon debt repayment period may be applicable, any carbon debt repayment period would be reduced by more than 80%. Resources that incur a carbon payback period could be avoided, and feedstocks that provide carbon sequestration and other environmental services on the landscape could be incentivized.
5. Conclusions
BECCS and other NETs can reduce atmospheric CO
2. This analysis explores the potential supply and cost of BECCS under a range of feedstock, logistics, and power generation scenarios. Results of this study suggest using BECCS in the US has a total technical potential to sequester about 181 to 737 million tonnes of CO
2 annually in the near term and in 2040, respectively. In round estimates of cumulative potential, the US has a technical potential to sequester up to 46 billion tonnes CO
2 by BECCS by 2100. This is equivalent to about 30%, 11%, and 4% of the global sequestration by BECCS by 2100 in Pathways 2, 3, and 4, respectively, in the IPCC [
5]. This US potential can be greatly reduced by future competing demands for biomass resources (e.g., cellulosic biofuels without BECCS) or potentially enabled by synergistic uses (e.g., demand for renewable power using BECCS). Scenario-specific average prices range from
$42 to
$137 per tonne CO
2 depending on cost accounting, power generation system, and biomass logistics system. Interactive visualization of results and output details are available at
https://doi.org/10.11578/1647453.
These results use up to 92% of potential cellulosic biomass resources above current uses (after accounting for constraints for soil organic carbon, erosion, and competing conventional demands for food, feed, fiber, and exports), and thus represent an upper level of CO
2 sequestration potential. However, this study is not exhaustive of scenarios or opportunities to increase biomass supplies in the US. For example, cellulosic biomass energy crops used in this analysis are from the base case reported by USDOE (2016), resulting in about 370 million tonnes biomass per year, though future energy crop production increases to almost 670 million tonnes per year under the high-yield scenario (USDOE 2016), which is not used in this analysis. Over 100 million tonnes per year of waste resources and over 10 million tonnes per year of woody biomass from federally owned timberlands [
6] were also excluded from this analysis and could be explored further. Results from USDOE (2016) do not necessarily represent the spatial distribution of demand for biomass that might respond to demand for BECCS, meaning biomass production could be better tailored to support BECCS in the future than the supplies used in this study. Resources in this analysis were limited to those deemed to have the potential to provide a range of positive environmental effects. If future scrutiny suggests that these environmental effects are in question, other resources such as wastes, hurricane and storm debris, thinnings in the wildland–urban interface to reduce fire risk, biomass from agroforestry systems, and removal of invasive exotic species, which are not included in this analysis, could be alternatively explored.
The authors would be remiss to not acknowledge questions raised in the literature (e.g., [
41]) regarding the carbon neutrality of some bioenergy sources, particularly that woody biomass requires a regrowth or payback period before it is carbon neutral after being used for bioenergy. However, energy from resources that may have a significant carbon payback period if used without CCS would have a shortened payback period if used for BECCS, and would subsequently be net negative after the payback period. Of the biomass resources used for BECCS in this analysis, 2–39% and 0–9% in the 2020 and 2040 scenarios, respectively, are derived from small-diameter trees from forests that could have some carbon payback period depending on site-specific conditions if used for bioenergy without CCS. The large supply and diversity of potential biomass resources explored here provides the opportunity to focus BECCS on the fraction of resources that offer the most beneficial attributes based on locally and regionally determined sustainability criteria.
As an incipient concept, future research is needed to explore tradeoffs and opportunities related to BECCS in the US. As mentioned above, region- and site-specific conditions and practices can influence in situ carbon changes and should be explored on a case-by-case basis. Alternative biomass uses, e.g., biofuels, biochar, and other bioproducts, can compete with or provide synergies with BECCS. Given that feedstock quality specifications vary by application, approaches to feedstock fractionation can enable highest-value use of feedstock streams. Rail and barge transportation of biomass feedstocks was not included in this analysis but could provide low-emissions transportation and should be explored further. This analysis explored transporting biomass to sequestration basins, but an alternative approach is to transport CO2 from power generation sites to sequestration basins, which could provide economic and logistical advantages and should also be explored. “Liability resources” such as biomass from storm and hurricane debris, wastes, and operations to control pine beetle infestations or invasive species were not included in this analysis and should be explored for maximum-benefit BECCS applications. Not included in this analysis is potential power demand as influenced by regional population expansion or climate change. For example, it may be anticipated that population growth in the US South coupled with climate change could increase demand for power in this region, which contains an abundance of biomass resources as well as potential sequestration basins with potential for BECCS. Potential cost-reduction strategies such as torrefaction, co-firing with coal, and implementation of existing biopower and infrastructure should also be explored.