CHAPTER FOUR

Bioenergy with Carbon Capture and Sequestration

INTRODUCTION

Combining bioenergy production with carbon capture and sequestration can lead to net negative emissions as carbon stored by photosynthesizing biomass growth is sequestered rather than released to the atmosphere (IEA, 2011). The concept was first developed by Obersteiner et al. (2001) as a backstop climate risk measure, and by Keith (2001) as a potential mitigation tool. Since then, biomass energy with carbon capture and sequestration (BECCS) has come to be viewed as a key carbon dioxide removal approach to keep global atmospheric CO₂ concentrations below 500 ppm and avoid catastrophic climate change. BECCS is largely used by integrated assessment models (IAMs) because its cost is low relative to other low carbon technologies and because the modules to represent other carbon dioxide removal technologies are undeveloped (e.g., direct air capture and soil carbon management). In its Fifth Assessment Report, the Intergovernmental Panel on Climate Change (IPCC) concludes, based on a literature review of mitigation scenarios developed with IAMs, that many scenarios that limit warming to 2°C select BECCS as the lowest cost option to reach the temperature objectives for the second half of the century (high confidence) and that BECCS plays an important role in many low-stabilization scenarios (with limited evidence and medium agreement) (Fisher et al., 2007b). The International Energy Agency climate change models suggest that at least 2 Gt CO₂ per year removal by BECCS should be implemented by 2050 to keep global temperature rise below 2°C (IEA, 2009). To put this in perspective, 1 Gt dry biomass is roughly equivalent to 1.4 Gt CO₂ and 14 exajoules (EJ) primary energy, and the United States annually emits about 6.5 Gt CO₂ and consumes slightly more than 100 EJ of primary energy. Yet many policymakers and academics are not aware of the pervasive and pivotal role that BECCS plays in climate change mitigation pathways despite being in its infancy (Anderson and Peters, 2016).

BECCS typically refers to the integration of trees and crops that extract CO₂ from the atmosphere as they grow, the use of this biomass in power plants, and the application of carbon capture and sequestration via CO₂ injection into geological formations. This

chapter entails a much broader scope of biomass energy–based carbon removal pathways, including (1) biomass combustion to thermal and electrical power with carbon capture and sequestration (traditional BECCS), (2) biomass thermochemical conversion to fuel with biochar soil amendment, and (3) biomass fermentation to fuel with carbon capture and sequestration (Figure 4.1). The scope of this chapter examines biomass energy in the forms of electricity, heat, and fuels, and capture in the forms of CO₂ and biochar. Compression, transportation, and sequestration are covered in Chapter 7.¹

This chapter begins with a review of the various biomass energy–based carbon removal pathways and their commercial status. This is followed by an assessment of their removal and sequestration potential based primarily on biomass supply potential and process economics. The chapter closes with the committee’s proposed research agenda for biomass energy–based carbon dioxide removal technology.

BACKGROUND

Approach Description

This section reviews the various technological pathways for bioenergy with carbon capture and sequestration by dividing the pathways into four steps: (1) biomass production, (2) biomass transportation, (3) biomass conversion, and (4) carbon capture (Figure 4.2).

Biomass Production

Biomass feedstock may come from forest management (e.g., tree stems, branches, bark, logging residues, sawmill waste), agriculture (e.g., purposed-grown feedstock, crop residues), algae cultivation, or collection of municipal organic solid waste. Biomass sequesters atmospheric CO₂ while growing, leading to an initial negative emission. The production and collection of biomass feedstock involve several activities such as seeding, fertilizer and pesticide production and spreading, tilling, logging roads, and tree harvesting. Energy used in these activities is part of the life cycle assessment (LCA).

The total managed land area in the United States potentially available for biomass production is nearly 900 Mha, though much of this land is already used for

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¹ Given that costs for compression, transportation, injection, and storage for CO₂ captured both through BECCS and direct air capture are assumed to be approximately the same, the report discusses them once in Chapter 7 on Geologic Sequestration and again in Appendix F.

commodities such as food, forage, and fiber (Table 4.1). However, the biomass supply could be increased on most of this land without changing land use. An important caveat is that there is only one land base; therefore, in this study, the land base for bioenergy is the same land base potentially available for forestry and agriculture carbon dioxide removal approaches. The discussion of how land use requirements for BECCS and other terrestrial approaches interact is found in Chapter 3.

On forestland, annual biomass production exceeds current harvest by about 70 percent (Smith et al., 2007) or 204 Mt/y dry biomass. Some of this could be harvested for bioenergy, but this would reduce the forest carbon stock and sink strength, which in turn would reduce the carbon removal benefit from forests. However, on forestland that is currently harvested, a significant amount of logging residue is not currently utilized, and some of this is readily available to increase bioenergy supply. Constraints on utilization of existing logging residue include the economic feasibility of removing and transporting the biomass and the potential impacts on ecosystem productivity. Many U.S. states have regulations requiring a certain amount (about 25 percent) of logging residue to be left on site to sustain productivity and wildlife habitat (Janowiak and Webster, 2010; Venier et al., 2014). How the residues have been treated (e.g., left

TABLE 4.1 U.S. Managed Land Area by Land-Use Category in 2015 (mega-hectares)^a

^aData from EPA, 2017.

^bDefined by IPCC and the U.S. EPA.

to decompose, burned, or used in a different wood product) and the rate of on-site decomposition also affect the carbon removal benefit.

Croplands present opportunities to increase use of agricultural residues such as corn stover, much of which is currently not utilized. Settlements produce significant quantities of organic waste, much of which could be used to increase supply of biofuel. One of the more promising and potentially productive options would be to grow energy crops on land that is considered “marginally productive” for crops. There is a significant amount of marginal land in the United States and globally that could be converted to energy crops without affecting production of other commodities (see more details in Chapter 3). A good estimate of such land for the United States is the amount of farmland enrolled annually in the Conservation Reserve Program, which typically exceeded 8 Mha/y before area limits were established (Mercier, 2011).

The productivity of biomass supply alternatives is highly variable by geography and biomass source. Excluding wastes and residues, some categorical, as well as several specific bioenergy crop productivity, estimates are shown in Table 4.2. These data highlight the variability in productivity within and between regions as a result of climate, site factors, and feedstock differences and provide a basis for the increase in biomass cost as the total biomass supply increases.

Biomass Transportation

Biomass must be transported from the source to the conversion facility or end user, where it will be converted to heat, electricity, or other fuels. This fuel must then be distributed to end users. A map of the distribution of biomass resources across the United States (Figure 4.3) shows that the east and west coasts and center of the United

States have the most plentiful resources, while regions in between have far sparser supplies. Thus, biomass would have to be transported over substantial distances for utilization in those regions. Even in the regions of more plentiful biomass supply, where shorter transportation distances can be expected, costs and emissions incurred by transportation can be substantial.

The results of an assessment of transportation costs for densified biomass, summarized in Figure 4.4, show that barge is by far the least expensive transportation mode for long-distance domestic transportation where available (Gonzales et al., 2013). Barge access is quite limited, however, and the remaining options include truck and rail. Truck transportation is less expensive for relatively short distances, and breakeven distances are presented in Figure 4.4. Furthermore, truck transportation can take advantage of the widespread road network in the United States, as compared to the more limited rail network. At longer distances, rail is less expensive than truck transportation.

In addition to cost, emissions associated with biomass transportation can be significant and should be accounted for in assessments of net carbon emissions from biomass utilization. LCAs of greenhouse gas (GHG) emissions in bioenergy production estimate emissions from biomass transportation based on shipment method (road, rail, or sea) and distance. Figure 4.5 presents an example of these estimates for dry biomass transport by truck, train, or sea freight (Beagle and Belmont, 2016). Results

show that truck transportation has significantly higher emissions per kilometer than train and sea freight.

Biomass Conversion

Figure 4.6 provides a detailed illustration of the many potential biomass-to-energy technologies, which are at varying technology readiness levels (TRLs). This section describes two broad approaches to biomass conversion.

Thermochemical. Several thermochemical and biological routes for the conversion of biomass to energy have been demonstrated and implemented. Thermochemical routes broadly include pyrolysis, hydrothermal liquefaction, gasification, and combustion (Goyal et al., 2008). Pyrolysis approaches heat biomass in the absence of air (oxygen-deficient or anoxic) or in the presence of hydrogen (hydropyrolysis) to produce liquids and gases that can be upgraded to fuels or directly combusted and solid

carbon-rich biochars that can be combusted, gasified, or sequestered as a soil amendment.² Pyrolysis may proceed at high heating rates and short residence times to favor liquid yields (fast pyrolysis), or a slow heating rates and long residence times to favor solid carbon production (slow pyrolysis or carbonization).

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² The use of biochar as a soil amendment is discussed in Chapter 3.

Hydrothermal liquefaction converts biomass at elevated temperatures and under high pressure steam to predominantly liquid products. Gasification, in contrast to pyrolysis and liquefaction, uses an oxidant (e.g., steam, air, or CO₂) to partially oxidize biomass to produce synthesis gas, composed of CO and H₂ that can then be converted to liquid fuels via thermocatalytic processes, such as Fischer-Tropsch and methanol-to-gas (MTG); or directly combusted for heat and/or power generation. Finally, combustion uses air or pure oxygen gas to completely oxidize biomass to produce heat for direct use or for power generation.

Biological. In addition to thermochemical biomass-to-fuel conversion routes, several biological pathways produce liquid and gaseous fuels (Antoni et al., 2007). Biological routes harness anaerobic digestion and fermentation to produce hydrogen, methane, and alcohol (e.g., ethanol) fuels. These biologically derived fuels can be burned directly for heat and power or upgraded to other fuels. An additional microbial route to biofuel production is the use of oil-producing microbes to directly generate biofuel precursors, such as the use of algae via photosynthesis.

Carbon Capture and Sequestration

The main carbon capture and sequestration pathways under consideration for biomass energy carbon removal are (1) biomass combustion or fermentation with CO₂ capture, compression, and transportation to a geological site for long-term sequestration and (2) biomass conversion to fuels with biochar (solid carbon) co-production that can be stored long-term as a soil amendment (see Chapter 3).

Carbon Capture. CO₂ capture technologies for biomass thermal and electrical power generation are generally the same as those currently under development for conventional fossil fuel power plant carbon capture and sequestration. Broadly, these technologies fall within four main categories: post-combustion, pre-combustion (or gasification), oxy-combustion, and chemical looping (Figure 4.7). While active research is ongoing in all of these categories for coal power plants, the different approaches vary significantly in technologic maturity. Table 4.3 presents estimated TRLs, carbon capture work, exergy efficiencies, levelized costs of electricity (LCOEs), and carbon capture costs for the different coal power plant carbon capture approaches. The fundamental challenge with carbon capture is achieving a pure stream of CO₂ for sequestration that involves either separating oxygen from air before combustion or CO₂ from power plant exhaust after combustion. The exergy efficiency, calculated from ideal (minimum) work for separating gases over the estimated actual work to separate CO₂, is used to provide a reference point for assessing the overall energy requirement for

carbon capture and benchmarking the state-of-the-art relative to what is theoretically possible. These values are only for CO₂captured cost (energetic and financial), whereas CO₂avoided cost includes the compression, transport, and sequestration of the CO₂ including the extra power and generated CO₂ needed to carry out these operations. Carbon capture from fermentation processes, such as those used to produce ethanol, can utilize the same technology that is being developed for carbon capture in fossil fuel plants. CO₂ is produced as a byproduct of the fermentation process itself, as well as from the power plant that supplies electricity and heat to the fermentation process. Therefore, both sources are candidates for CO₂ capture. In the United States,

TABLE 4.3 Coal Power Plant Carbon Capture Approach and Estimated CO₂ Capture Work, Exergy Efficiency, and Cost for Different Carbon Capture Approaches

NOTES: Exergy efficiency is the minimum work for carbon capture (0.2 GJ/tCO₂) over the process exergy. TRL, technology readiness level; LCOE, levelized cost of electricity; SCPC, supercritical pulverized coal; IGCC, integrated gasification combined cycle; USC, ultra-supercritical; and CDCL; coal direct chemical looping.

SOURCE: Bui et al., 2018.

biorefineries currently emit about 45 Mt/y CO₂ from fermentation, of which 60 percent could be captured and compressed at a cost estimated to be less than $25/t CO₂ (Sanchez et al., 2018).

Biochar Soil Amendment. The thermochemical conversion of biomass to fuels can produce 25-45 percent by mass biochar (solid carbon) as a byproduct, depending on the feedstock and process conditions (temperature, pressure, partial pressures, and residence times). The fraction of biochar production is important because it helps determine whether some biomass-to-fuels pathways are actually carbon negative (Del Paggio, personal communication, 2017). Biochar soil amendment has been proposed as a promising path for long-term carbon removal strategy; however, questions remain about the long-term stability of biochar in soil environments. Proponents claim that biochar application reduces the burden on farmers in several ways: less fertilizer is needed because biochar absorbs, stores, and slowly releases nutrients such as phosphorus to plants and subsequently to the environment; biochar improves soil moisture retention, securing the crops against drought; farmers spend less on seeds as germination rates increase; biochar reduces the methane emissions from paddy

fields and farmyard manures; it increases the soil microbes and other soil-life density; it lessens the hardening of soils; it supports better growth of roots and helps in reclaiming degraded soils (Jeffery et al., 2011). For additional details see Chapter 3.

COMMERCIAL STATUS

Biomass-fueled power generation is commercially deployed across the United States and the world, although no biomass power plants are coupled with carbon capture and sequestration (CCS). Of the 4,000 TWh of electricity generated in the United States in 2016, only 40 TWh was from wood-derived fuels and 22 TWh was from other biomass sources, including municipal solid waste, agricultural byproducts, and other biomass (EIA, 2017d).

Large-scale biological biomass-to-fuel technology has been deployed commercially, most notably in the production of approximately 370 million barrels of ethanol (EIA, 2017c). Table 4.4 presents select biomass-to-fuel pathways and their TRLs and developers. Few of these projects have coupled the fuel production process with CCS. Among these, the largest is the Illinois Industrial Carbon Capture and Storage (IL-ICCS) project, where pure CO₂ gas is formed as a byproduct of fermentation for ethanol production at an Archer Daniels Midland (ADM) plant near Decatur, Illinois, and is collected and injected into the nearby Mt. Simon Sandstone saline formation. This project plans to capture 0.9-1.0 Mt CO₂ annually and began CO₂ injection in 2017. This project follows on the completion of the Illinois Basin-Decatur Project, which captured and injected CO₂ from the ADM plant to the Mt. Simon formation at a lower rate for 3 years. Notably, the ADM emits approximately 5 Mt/y CO₂, making the process net carbon positive because of CO₂ emissions from the power plant. However, techno-economic studies have shown that such processes can be carbon negative if CCS is applied across the entire chemical plant, including the fermenter and power generation unit.

Additionally, biochar production is now a commercial activity with many producers located throughout North America. According to a recent survey commissioned by the U.S. Forest Service (Draper et al., 2018) an estimated 39,000 to 77,000 t/y biochar are produced in the United States, and an additional 1,900 to 7,300 t/y are produced in Canada. Current biochar sales prices are around $1,800/t, with most consumers reportedly using it as a soil amendment to modify texture, increase porosity, improve water management, and increase soil carbon. To increase market size, biochar producers are actively seeking biochar certification as an animal feed supplement, as is done in Europe.

TABLE 4.4 Select Biofuel Processes, Developers, and Technology Readiness Levels (TRLs)

NOTE: CRI, CRI Catalyst Company, Shell Group; GTI, Gas Technology Institute; KIIT, Korea Institute of Industrial Technology; PNNL, Pacific Northwest National Laboratory; RIIHL, Reliance Industrial Investments and Holdings Ltd.

SOURCE: Adapted from Stafford et al., 2017 and IRENA, 2016.

IMPACT POTENTIAL

Availability of biomass feedstock is a critical issue when assessing the potential role of BECCS to mitigate climate change, and a wide range of values are available in the literature with order of magnitude uncertainties (Azar et al., 2010; Slade et al., 2014). For instance, Berndes et al. (2003) estimated the total annual contribution of biomass in the future global energy supply to be between 100 EJ and 400 EJ in 2050 based on a review of 17 published studies. High uncertainties associated with land availability, energy crop yields, and the future availability of waste, forest wood, and residues from forestry and agriculture are the main reasons for this wide range of values (Slade et al., 2014). In this section, the Committee presents an estimate of biomass potential for the United States and the world, associated negative CO₂ flux, potential radiative impacts and costs for bioenergy with carbon capture.

Carbon Flux

Using biomass availability data (summarized in Table 4.5 and assumptions discussed below), the committee estimated the economically feasible (i.e., without significant impacts on current land and biomass use) BECCS CO₂ flux potential for the United States. The economically feasible lower bound is 522 Mt/y CO₂ and is based on the following assumptions:

• No energy crops are utilized for BECCS. Although high levels of BECCS deployment depend on productivity increases of about 1 percent annually, energy crops are eliminated from the lower-bound estimate because energy crop production and utilization is not widespread and significant concerns exist regarding impacts on climate and food security from increasing energy crop production. Indeed, as shown by Heck et al., 2018, the potential for BECCS from dedicated bioenergy plantation is marginal if we use precautionary guardrails to stay within planetary boundaries for nonclimate impacts such as biodiversity or freshwater use.
• Agricultural byproducts are included at an amount equal to the difference between economically feasible production in 2040 and the current utilization level to avoid displacing demand and creating new needs that might lead to land burden (annual flux = 113 Mt/y CO₂).
• Economically recoverable and currently unused forestry logging residues and other wood wastes are included based upon 2040 availability. Whole-tree biomass from thinnings and fuel treatments, and wood currently used for home and industry heating, are not included because barriers to these sources

TABLE 4.5 Estimated U.S. Dry Biomass Potential and Equivalent CO₂ Fluxes (Mt/y)

NOTES: Includes current levels of biomass utilization, lower- and upper-bound dry biomass potentials for different feedstock types, and associated CO₂ flux potentials, assuming all biomass carbon content is captured and sequestered regardless of biomass conversion path.

SOURCES: DOE, 2016; EPA, 2016c; Rose et al., 2015; Seiple et al., 2017; USDA, 2014.

• are expected, such as concerns about effects on intact forests (annual flux = 123 Mt/y CO₂).
• Organic wastes, the majority of which consist of municipal solid waste, are included based upon 2040 availability (annual flux = 286 Mt/y CO₂).

The U.S. economically feasible upper-bound BECCS CO₂ flux potential is estimated to be 1,500 Mty CO₂ based upon all available agricultural byproducts, energy crops, forestry waste and byproducts, and organic waste. This flux corresponds to the total carbon content of available biomass and does not account for losses and other GHG emissions throughout the supply chain. It also does not take into account the land demands or conflicts that might arise at maximum flux values. Therefore, it is not considered as safely achievable.

The global upper-bound BECCS CO₂ flux potential is estimated to be 10-15 Gt/y CO₂by 2050 according to the IPCC (IPCC, 2014b). The lower bound flux is assessed using a reduction factor based upon the maximum versus lower bound U.S. fluxes. This approach assumes that globally available biomass has a similar composition and source distribution as U.S. biomass supply and is subject to similar restrictions. While this assumption is likely flawed, it provides a coarse assessment of global flux potential. The U.S. lower bound range is approximately 35 percent of the maximum potential flux; thus, the global lower bound BECCS CO₂ flux potential is estimated to be 3.5-5.2 Gt/y CO₂ by 2050.

Agricultural Byproducts

Agricultural byproducts include residues and waste streams, as defined and summarized in the Department of Energy’s (DOE’s) 2016 Billion-Ton Report (2016). That report also provided current consumption of all agricultural byproducts for energy production based on 2014 Energy Information Administration (EIA) data, including the byproducts used (9.5 Mt) for annual heat and power production, as well as the substantially larger amounts of agricultural biomass that is used each year for fuel and bio-based chemical production (approximately 115 Mt and 5.3 Mt, respectively). While carbon composition of agricultural residues can vary, an average carbon content of 47.5 percent by mass was used to evaluate CO₂ production.

Agricultural Residues. Agricultural residues include corn stover, wheat straw and sorghum, and oat and barley residues. Technical potential is defined in this report as the total resource available, and availability in the years 2017 and 2040 were estimated from the agricultural residues available at $88 t dry biomass ($80 per short ton), based on DOE (2016) under a base case scenario of 1 percent annual growth in yield.

These quantities were used as reasonable estimates of total availability because the production curves presented in the report show minimal increase in potential production with increase in farmgate price above $88/t and up to $110/t. The economically feasible quantities of agricultural residues presented in Table 4.5 were gathered from DOE (2016) for years 2017 and 2040 under the scenario of $66/t farmgate price and 1 percent annual growth.

Agricultural Waste Streams. Agricultural waste streams include sugarcane bagasse and trash, soybean hulls, rice hulls and straw, grain dust and chaff, orchard and vineyard prunings, cotton gin trash and field residue, and animal manure. Although animal fats and yellow grease were also included in the agricultural waste resources identified in DOE (2016), they were not included in total agricultural waste stream assessment in this report because their likely utilization pathway is the production of biodiesel. Although biodiesel displaces fossil-derived fuels, neither of the carbon negative pathways identified in this report (combustion with CO₂ capture or pyrolysis with biochar sequestration) are achieved via biodiesel. Technical potential and economic feasibility were estimated for years 2017 and 2040 from data in the DOE (2016) report, where economic feasibility under the scenario of $66/t farmgate price and 1 percent annual growth was used in this report.

Energy Crops

Current production for herbaceous energy crops comes from the most recent U.S. Department of Agriculture (USDA) census (USDA, 2014) as presented in DOE (2016). On the one hand, this value may be overestimated because it includes nonenergy uses such as animal bedding. On the other hand, it might be underestimated because producers often do not report planting of unique crops because they are not enrolled in federally subsidized programs or the crops are grown on nonprivate agricultural lands such as public universities. However, this value is so low that a potential under- or overestimation is probably negligible.

Current production for woody energy crops also comes from the most recent USDA census (USDA, 2014). The value available on the USDA website is in acres of crops for short-rotation woody crops. This value has been converted into annual production (Mt/y) by multiplying the acres of crops by the average dry biomass yield per acre (t/acre). The average yields are the mean region-specific yields for poplar and willow crops from DOE (2016).

Theoretically, the technical potential for biomass production from energy crops is very high because all crops in the United States could be converted to energy crops.

However, this would be a very unrealistic scenario. Therefore, a very rough estimate of the current technical potential has been calculated, considering that the difference between current total croplands and harvested croplands according to USDA (2014) could be cultivated as energy crops. In the USDA census, total croplands include “cropland harvested, other pasture and grazing land that could have been used for crops without additional improvements, cropland on which all crops failed or were abandoned, cropland in cultivated summer fallow, and cropland idle or used for cover crops or soil improvement but not harvested and not pastured or grazed.” This area was then converted into dry production per year (Mt/y) by multiplying it by the average yield (t/acre) for all six energy crop types presented by DOE (2016).

Economically feasible biomass productions are detailed for each type of biomass assessed in DOE (2016), that is, switchgrass, miscanthus, biomass sorghum, and energy cane for herbaceous crops and non-coppice (poplar, pine) and coppice (willow, eucalyptus) for woody crops. They are taken from the 2016 Billion-Ton Report for a 1 percent annual yield increase and a biomass price at farm gate of less than $66/dry biomass. Achieving this 1 percent annual yield increase would require research to genetically modify herbaceous crops and develop silvicultural systems that target biomass production rather than wood volume or quality (Dietrich et al., 2014; Lotze-Campen et al., 2010; Robison et al., 2006). The DOE (2016) assumes agricultural lands to stay constant over the years. Therefore, additional energy crops replace other types of current crops such as food crops. In 2040, 8.3 percent of agricultural lands are devoted to energy crops under the 1 percent annual yield increase and the $66/dry biomass scenario. There are no economically feasible production values for 2017 in DOE (2016) because of the constraints included in the model, such as that all agricultural biomass feedstock comes from residues.

The annual CO₂ flux potential was calculated based on a 47.5 percent carbon mass content in dry biomass. Indeed, Schlesinger and Bernhardt (1991) have found that dry biomass carbon content is almost always between 45 percent and 50 percent by weight. Therefore, the mean value was used. This is an estimation of the amount of CO₂ sequestered from the atmosphere during biomass growth that could then be stored using BECCS technologies. However, this is not an estimation of the net potential for CO₂ sequestration from BECCS. Indeed, the specific carbon capture and sequestration process efficiency, as well as other life cycle emissions, including reduction of land carbon stock, significantly affect the CO₂ flux potential.

Forestry

The United States is the largest producer of industrial roundwood products in the world, accounting for 19 percent of the global total. Other countries, particularly in the tropics, use most of harvested roundwood for heating and fuel consumed by households (FAO, 2015a). The large base of industrial timber production in the United States drives fuelwood use, which is mostly associated with timber harvesting for other products such as paper and lumber. The main resource categories are logging residues, increased whole-tree harvest of green or damaged timber, and other wood wastes, which includes unused mill and urban wood residues (Perlack et al., 2005).

Currently, the United States uses about 132 Mt annually of wood and wood waste for thermal and electric power and has the potential to nearly double this amount at a dry biomass price of 66 $/t based on economic modeling that excludes potential additional supplies from lands more than a 0.8 km (0.5 mi) from a road, protected areas, and steep slopes (DOE, 2016). With the exception of whole-tree harvest, potential additional wood for bioenergy is associated with improving utilization of timber and residues associated with current levels of timber harvest for other products; therefore, coupled with supply-area restrictions, there would be limited consequences for other forest values. Increasing whole-tree harvest is restricted by the amount of wood growth that exceeds current harvest, so that the potential increase is sustainable as long as growth is not impacted by factors such as increasing natural disturbances and climate change. However, the area that is harvested is highly variable among the scenarios modeled, indicating that there are potential impacts on net greenhouse gas emissions and other values of impacted forests over time.

Globally, 1,194 Mt/y wood biomass are used for fuel, mostly for household fuel and charcoal, approximately equal to the amount of industrial wood produced annually (FAO, 2016). The potential additional supply of wood for industrial biofuel at the global scale is not well known but could range from 1,316 to 10,532 Mt/y dry biomass (FAO, 2016).

The significant differences between current U.S. and global fuelwood use infrastructure may necessitate different strategies for large-scale deployment of BECCS. In the United States, much of the fuelwood is used to produce paper and other wood products at manufacturing facilities that are concentrated mainly in the south (DOE, 2016). In contrast, the global use of fuelwood is more dispersed, particularly in tropical areas where large wood processing facilities are mostly absent.

Organic Waste

Biogenically derived organic wastes and CO₂ fluxes potentials were estimated from three waste streams: (1) municipal solid waste, (2) construction and demolition, and (3) sewage and wastewater.

Municipal Solid Waste. The U.S. Environmental Protection Agency (EPA) estimated that the United States generated 230.5 Mt dry municipal solid waste in 2013 (4.4 lb/d per person), of which 70.3 percent was biogenic (paper 27.0 percent, food 14.6 percent, yard waste 13.5 percent, wood 6.2 percent, and leather, textiles, and rubber 9.0 percent) (EPA, 2016c). Based on these data and U.S. Census Bureau population estimates (Colby and Ortman, 2015), the dry biomass from municipal solid waste is estimated to be 167 Mt in 2017 and 199 Mt in 2040 (Table 4.5). From this, annual CO₂e flux potentials of 166 Mt in 2017 and 198 Mt in 2040 are estimated using a municipal solid waste CO₂ emissions per dry biomass factor of 0.82 t/t (EPA, 2014) (Table 4.5).

Construction and Demolition. EPA estimated that the United States generated 481 Mt of construction and demolition waste in 2013 (9.2 lb/d per person), of which 7.6 percent was biogenic (wood). Based on these data and U.S. Census Bureau population estimates (Colby and Ortman, 2015), the annual dry biomass from construction and demolition waste is estimated to be 37 Mt in 2017 and 44 Mt in 2040 (Table 4.5). From this, annual CO₂e flux potentials of 68 Mt in 2017 and 81 Mt in 2040 are estimated using a wood waste CO₂ emissions per dry biomass factor of 1.5 t/t (EPA, 2014).

Sewage and Wastewater. Seiple et al., 2017 estimated that the United States produces 12.56 Mt/y dry biomass of wastewater sludge, of which about 50 percent is beneficially used (6.23 Mt/y). Rose et al. (2015) reported a median human stool and urine dry solids generation rates of 29 g/d and 59 g/d, respectively, for a combined median dry biomass generation rate of 88 g/d per person. Based on this median generation rate and U.S. Census Bureau population estimates (Colby and Ortman, 2015), the annual dry biomass from human sewage is estimated to be 10 Mt in 2017 and 12 Mt in 2040 (Table 4.5). Assuming dry human stool and urine have a carbon content of 20 percent and 13 percent by mass (Rose et al., 2015), respectively, then the annual CO₂e flux potentials from human sewage are estimated to be 6 Mt in 2017 and 7 Mt in 2040.

Carbon Capacity

According to the IEA (2016), most climate scenarios that keep average global temperature rise below 2°C include at least 14 Gt CO₂ cumulative removal from BECCS globally by 2050, or about 2 percent cumulative global emissions reduction. If the United

States committed to removing a proportional share of CO₂ emissions (15 percent of global emissions in 2015), then it would need to remove 2.1 Gt CO₂ by 2050.

Supply Capacity Basis

The U.S. lower-bound cumulative CO₂ storage capacity based on biomass supply achievable with minimal impacts on current land and biomass use is 6.0 Gt CO₂ by 2040, if annual CO₂ sequestration is ramped linearly from 0 percent to 100 percent of the lower-bound CO₂ flux potential (522 Mt/y CO₂) from 2018 to 2040. If continued at the same rate, the lower-bound cumulative CO₂ storage capacity is 11 Gt CO₂ by 2050. The U.S. upper-bound CO₂ storage capacity based on biomass supply achievable with minimal impacts on current land and biomass use is 17 Gt CO₂ by 2040, if annual CO₂ sequestration is ramped linearly from 0 percent to 100 percent of the upper-bound CO₂ flux potential (1,500 Mt/y CO₂) from 2018 to 2040. If continued at the same rate, the upper-bound cumulative CO₂ storage capacity is 32 Gt CO₂ by 2050.

The worldwide lower-bound CO₂ capacity is assessed by scaling the total global capacity by the same reduction factor utilized in the United States. A comparison of upper and lower bounds for total U.S. CO₂ capacity shows that lower-bound capacity is 35 percent of upper-bound capacity; thus, the global lower-bound CO₂ storage capacity is estimated to be 57-86 Gt CO₂ by 2050 if the annual CO₂ sequestration rate is increased linearly from 0 percent to 100 percent from 2018 to 2050. Based on the range of upper-bound global CO₂ flux potentials (10-15 Gt/y CO₂), the total global upper-bound cumulative CO₂ capacity is estimated to be 165-248 Gt CO₂ by 2050.

Sequestration Capacity Basis

One constraint on the carbon sequestration potential for biomass energy removal pathways is the availability and economic viability of carbon storage resources. For biomass combustion and fermentation pathways, the limitation is the availability and capacity of geological sequestration (see Chapter 7). For biomass thermochemical conversion to fuels with co-production of biochar, there are no apparent technical or economic limits to the amount of carbon that can be stored, regardless of how it is stored (i.e., as a soil amendment or in landfills).

Radiative Impacts

Creutzig et al. (2015) identified five main sources of radiative (life cycle) impacts for bioenergy systems: (1) GHG emissions from fossil fuels used along the value chain, (2) GHG emissions associated with biomass or biofuel combustion, (3) GHG emissions and uptakes from land disturbances, (4) emissions of short-lived climate forcers (e.g., black carbon) from biomass or biofuel combustion and of non-CO₂ GHGs (e.g., methane [CH₄], nitrous oxide [N₂O]) from land management, and (5) climate forcing resulting from alteration of the land surface (e.g., albedo changes).

A life cycle approach is essential to account for all GHG emissions and uptakes associated with BECCS technologies to determine their net contribution to climate change mitigation. Bioenergy with carbon capture and sequestration involves both fossil and biogenic carbon flows. Biogenic carbon flows consist of the uptake of CO₂ from the atmosphere by growing biomass during photosynthesis, and by CO₂ emissions from biological respiration, degradation, and combustion.

Fossil carbon flows consist of the CO₂ and CH₄ emissions from the combustion of the fossil fuels or materials needed by BECCS technologies. For example, transport of biomass by fossil fuel–powered vehicles or locomotives must be accounted for in estimating net carbon removal. Further, CO₂ is not the only GHG contributing to climate change, and CH₄ and nitrous oxide N₂O are especially relevant for biomass systems. For example, 75.1 percent of U.S. N₂O emissions in 2015 were attributed to agricultural soil management activities such as fertilizer application and other practices that increase nitrogen availability in the soil (EPA, 2017). Methane is also emitted during

biological respiration, degradation, and combustion, and biomass decomposition in anaerobic conditions could lead to high CH₄ emissions. Other GHG emissions can also occur at different stages in the life cycle of a BECCS process. For example, natural gas can be used in biomass conversion processes, leading to CH₄ emissions. Figure 4.8 shows an example of “carbon losses” associated with BECCS for switchgrass burned in an integrated gasification combined cycle power plant with carbon capture and sequestration (data taken from the literature).

Fully accounting for net changes in carbon stocks and fluxes on land associated with bioenergy is an important element contributing to the net effect of BECCS on the atmosphere. The source of biomass, whether live vegetation or waste, determines the essential accounting elements. To calculate the effects on net CO₂ balance, it is first necessary to compare temporal bioenergy production scenarios with a projected business-as-usual baseline scenario to accurately reflect the incremental net change in emissions. The time horizon is important in several ways. Depending on the bioenergy source, it will take different times to recover the utilized biomass (sometimes called “re-paying carbon debt”) and additional “time to carbon sequestration parity,” which refers to the point at which the accumulated net (or “additional”) GHG effect from using the bioenergy equals the net GHG effect of the baseline (often a “no-harvest” scenario) (Ter-Mikaelian et al., 2015). In sharp contrast to harvesting live trees, which may take decades or centuries to recover their original biomass and reach carbon sequestration parity, using wood residues for bioenergy that would otherwise have been left to decompose or slash-burned results in emissions reductions over a shorter term. However, if the wood residues would otherwise have been used in a long-lived product such as particle board, it could take decades for the use of this material for bioenergy to have a positive effect of reducing atmospheric CO₂. Indirect effects, such as broader impacts on land use and supply of other wood products, and their consequent impacts on GHG emissions are important to consider when calculating the overall net CO₂ balance.

Land-cover changes or land-use disturbances (e.g., forest harvesting or conversion of natural lands to crops) can also lead to changes in albedo (Betts and Ball, 1997; Zhao and Jackson, 2014), surface roughness, and evapotranspiration (Swann et al., 2010), influencing the climate system. Albedo changes are the dominant effect, especially in areas with seasonal snow cover (Bathiany et al., 2010), and can possibly be stronger than those of associated biomass carbon sequestration (Bernier et al., 2011; Betts, 2000; Jones et al., 2013b; O’Halloran et al., 2012). Although very important, it is difficult to quantify these biogeophysical climate impacts at large scale because they are site specific and vary in magnitude across geographic regions (Anderson-Teixeira et al., 2012).

ESTIMATED COSTS OF IMPLEMENTING BECCS

The extent to which BECCS is implemented will largely depend on factors such as the costs of biomass supply of competing electricity generation approaches, such as natural gas. These costs are summarized below, followed by specific estimates of carbon costs for CO₂ generated and captured in a power plant and biochar produced by pyrolysis.

Biomass Supply Costs

Cost per ton of biomass supply is affected by many factors: productivity or yields per hectare, transportation (distance from roadside), fertilizer additions, processing, stumpage price or payment to grower, harvest cost, and other feedstock specific

TABLE 4.6 Estimated Levelized Cost of Electricity

NOTE: Based on weighted average of regional values for projected capacity additions from new U.S. power plants entering service in 2022. O&M = operations and maintenance. SOURCE: EIA, 2017b.

factors (DOE, 2011). Figure 4.9 shows the rise in prices for each category of feedstock as the total supply increases, and the relative availability of different feedstocks at different supply levels.

Electricity Costs

CO₂ capture, compression, and transportation to a site for geological sequestration is considered primarily for generating thermal and electrical power through biomass-fueled combustion, as opposed to producing fuels. The primary challenge for biomass electrical power with carbon capture and sequestration is the low efficiency (typically less than 25 percent) of biomass power plants. Low biomass power plant efficiency increases the already high cost of feedstocks ($50-80/t or $3-4/GJ dry biomass) and capital (more than $4,100/kW electricity), which ultimately yields an uncompetitive LCOE of about $100/MWh ($28/GJ) (see Table 4.6) (EIA, 2017b). By contrast, conventional natural gas combined cycle power plants have high efficiency (typically greater than 45 percent), low fuel costs ($2-3/GJ or $2-3/Mcf natural gas), low capital costs (less than $920/kW electricity), which yields an LCOE of about $60/MWh ($17/GJ)—nearly half the cost of biomass electrical power. Therefore, research and development (R&D) in this area should prioritize increasing biomass power plant efficiency over R&D for carbon capture, compression, and transportation research.

Biomass-to-Power with Carbon Capture

Two factors influence the economics of biomass-power generation with carbon capture: the LCOE and the cost of carbon capture. Based on data from the Federal Energy Regulatory Commission (FERC, 2016), the EIA estimated that the LCOE in 2016 for an average coal power plant was $36.1/MWh—$5.1/MWh operations, $5.5/MWh maintenance, and $25.5/MWh fuel costs. Assuming operations and maintenance (O&M) costs do not substantially differ between coal and biomass power plants, the cost of biomass-derived electricity can be estimated by modifying the current cost of fossil fuel–derived electricity to consider biomass feedstock and carbon capture costs. For a simple and direct comparison of fuels, the efficiencies of biomass and coal power plant are assumed to be the same, even though they are largely dependent on firing percentage and biomass pretreatment, such as torrefaction or densification. According to the U.S. EIA (2017a), the average coal power plant efficiency was 32.5 percent in 2016. Assuming a higher heating value for biomass of 17 GJ/t (about half that of coal) and a farmgate cost of $66/t, the farmgate biomass cost contribution to biomass LCOE is $43/MWh—almost 70 percent higher than coal. Biomass transportation contributes another $14/MWh to biomass electricity costs, assuming that transportation costs equal those for coal at $22/t (EIA, 2017a). Combining biomass farmgate and transportation costs, the total fuel contribution power plant electricity cost is $57/MWh—more than twice the $26/MWh for coal. Because fuel costs dominate the LCOE, substituting biomass for coal has a significant impact on electricity costs, as evidenced by the increase in the LCOE from $36/MWh for coal to $67/MWh for biomass.

Inclusion of CO₂ capture increases the plant costs. Based on an estimated post-combustion carbon capture cost of $46/tCO₂ (Rubin et al., 2015), carbon capture adds $52/MWh to electricity generation costs. Thus, the LCOE for biomass power with carbon capture is about $119/MWh. Assuming a carbon content of 47.5 percent by mass across all biomass types, the total cost to generate electricity with carbon capture at a biomass power plant is $105/tCO₂, compared to the total cost to generate electricity without carbon capture at a baseline coal power plant of about $70/tCO₂.

Biomass-to-Fuel with Biochar Carbon Sequestration

Currently, most thermochemical approaches to convert biomass to fuel are optimized for maximum fuel production, where the co-produced biochar is burned to provide process heat. If we assume that the biochar is instead used as a soil amendment to sequester carbon in the soil (or simply buried) and this process heat was instead provided by natural gas at a price of $2-7/GJ ($2-7/Mcf), by proxy the effective biochar

carbon capture cost would be about $37-132/t CO₂. Because biochar is easily separated and, if stored locally, suffers little oxidation to CO₂, the cost to “avoid” CO₂ will be nearly the same as that to “capture” CO₂. This estimate does not assume a cost offset from the sale of biochar to agriculture users because the biochar market is currently small relative to the scale necessary to provide climate benefits. Recently, the USDA commissioned a survey of the U.S. biochar industry and estimated the current biochar market range from 35,000 to 70,000 t/y—roughly equivalent to sequestering 75,000 to 150,000 t/y CO₂e (USDA, 2018). Reported biochar sales prices ranged widely from a low of $600/t and an average price of $1,030/t. Assuming a carbon content of 70 percent by weight, these prices correspond to a carbon price of $230-400/t CO₂e.

SECONDARY IMPACTS

The IPCC has paid little attention to nonclimate impacts on ecosystems and biodiversity from large-scale CO₂ removal technologies such as BECCS (Williamson, 2016). However, several publications in recent years have addressed a set of different types of environmental and societal impacts associated with bioenergy and BECCS. Aside from physical constraints on biomass production, life cycle GHG emissions, and other potential radiative impacts, there are key uncertainties regarding indirect emissions, adverse effects on food security, impacts on biodiversity and land conservation, competition for water resources, and social equity and acceptance issues (Sanchez and Kammen, 2016).

Environmental Impacts

The area of land required per unit mass of carbon removed from the atmosphere is particularly important for BECCS, leading to different potential impacts regarding land-use change, land conservation (e.g., nutrient availability), and biodiversity. Some researchers have provided evidence that suggests some types of BECCS are incompatible with human development within safe operating margins, because they begin to threaten planetary boundaries, such as biosphere integrity and nitrogen flows (Heck et al., 2018). As described above, both U.S. and global lower-bound estimates for BECCS would not require land-use changes because biomass would be sourced from waste and residues from existing land uses. The U.S. upper-bound estimate of 1.5 Gt/y CO₂, based on an average productivity of 18 t/ha CO₂e (Table 4.2) indicates a land area requirement of about 78 Mha. For the global upper-bound estimate of 10-15 Gt/y CO₂, Smith et al. (2016) estimated that the land area required to deliver just 12 Gt/y CO₂e is about 380-700 Mha for all sources, including wastes and residues from existing

land uses, and dedicated energy crops such as willow and poplar or miscanthus on a range of productivity classes. This land area represents 36 percent to 163 percent of land identified as abandoned or marginal³ (Canadell and Schulze, 2014). Similarly, Humpenöder et al. (2014) found the land requirements to range from 300-500 Mha, depending on whether afforestation was also a major part of a carbon dioxide removal program.

Carbon removal through afforestation and reforestation also requires large areas of land (~2,800 Mha), potentially an order of magnitude greater than that required for BECCS (see Figure 3.3) (Humpenöder et al., 2014). This figure shows the simulated time series of global land use for BECCS and forestry NETs. Therefore, large-scale implementation of BECCS will compete with afforestation/reforestation, as well as with food production and delivery of other ecosystem services (e.g. Bustamante et al., 2014).

Nutrient removal associated with biomass harvesting (for energy crops as well as collecting agricultural and forest residues instead of leaving them on the ground as nutrients) differs several-fold among biomass sources. Such nutrient removal prevents further emissions due to decomposition of the biomass, but it could lead to nutrient depletion depending on the vegetation or the land use replaced (Smith et al., 2016). Moreover, enhanced cropping will increase nutrient runoff to the sea and thereby eutrophication, which could lead to reduced coastal fisheries yields and potential negative impacts on coastal blue carbon. The use of bioenergy feedstock with low nutrient concentrations such as residues, forest, and lignocellulosic biomass could help to mitigate nutrient depletion and runoff.

Bioenergy feedstock from food and fiber production waste does not have a direct effect on existing land use. However, establishing new dedicated bioenergy feedstock production capacity will initiate direct competition with other land uses, unless the land is marginally productive and not actively managed. These direct and indirect effects should be considered in the overall accounting for net effects on CO₂, and if significant, can be quantified using an integrated economic land-use modeling approach (Plevin et al., 2010; Searchinger et al., 2008). There may also be effects on commodity supplies and prices for goods that may use the same material, for example, wood products (Ahlgren et al., 2013). Changes in production of other goods may in turn affect CO₂ balance because of differences in production systems and use and disposal patterns of biomass products, and potentially other materials that can be substituted for biomass products. Moreover, the modeling approach selected to predict these

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³ Estimates of marginal land are uncertain due to inconsistencies in the definition of marginal. See Chapter 3 for a detailed discussion of marginal land.

indirect effects should consider the entire world because of globalized food and material markets.

Unirrigated bioenergy crops cause evaporative losses that are higher than that of average short vegetation (Smith et al., 2016). The higher yields of irrigated bioenergy crops can reduce the pressure on land but can increase the pressure on freshwater ecosystems and competition with other users, leading to a trade-off between land and water requirements (Bonsch et al., 2016). Important water withdrawal for energy crops irrigation could lead to freshwater ecosystem degradation and aquatic biodiversity loss. Moreover, the CCS process also requires the use of water. Smith et al. (2016) estimate the amount of water required to deliver 12 Gt/y CO₂e sequestration through BECCS to be approximately 3 percent of the total amount of water currently used by human activities. However, water can also be extracted from CO₂ storage operations so that water use associated with the carbon capture and storage process is case-specific.

In an effort to guide policymaking, researchers are increasingly using IAMs to develop potential mitigation scenarios for different emission pathways. These prospective models integrate representations of human systems (e.g., techno-economic models) and physical processes associated with climate change and/or other environmental impacts (e.g., carbon cycle, water availability). Despite their limitations, IAMs can enhance understanding of how possible technological or policy choices might lead to different outcomes (Edenhofer et al., 2014). Because they capture linkages between regions through trade in energy and agricultural goods, among others, they are suitable for identifying potential indirect impacts from BECCS. However, improvements are still needed to include impacts on biodiversity, ecosystem services, and water resources.

Societal Impacts

Energy crops compete with food crops for available agricultural land. Therefore, the use of BECCS might lead to food security issues. For instance, Powell and Lenton (2012) show that the climate mitigation potential of BECCS highly depends on assumptions regarding future food production efficiency and proportion of meat in diets; the most pessimistic scenarios (low efficiency and high meat proportion) lead to more warming. This competition for land leads to increased food prices. Because food prices are very inelastic, they must rise to ensure that enough land is allocated to food production in high mitigation scenarios (Calvin et al., 2014). Such increase in food prices was observed several years ago. Some authors identify the increase in

first-generation biofuel production as the main cause (Tangermann, 2008; World Bank, 2008). Zhang et al. (2010) also link increased ethanol production to short-run agricultural commodity prices. Crude oil prices and droughts might also influence food prices in the short term (Ajanovic, 2011). Higher food prices reduce low-income populations’ access to food, especially in developing countries, potentially leading to malnourishment and social discord (Rosegrant, 2008).

RESEARCH AGENDA

Scientific and Technical Questions

In developing its research agenda for BECCS, the committee was guided by the following questions:

• What are the limits to biomass resource potential as a carbon negative approach when considering secondary impacts such as food security, competition for water and land use, albedo changes, and biodiversity?
• Can a sufficiently diverse biomass feedstock supply chain be developed to allow for the conversion of existing coal-fired power plants to biomass (1 GW-scale)?
• Is it worth investing in biomass-to-power considering that it would take more than 1Gt dry biomass (~ 15 EJ primary energy) to replace coal in the United States (~ 17 EJ primary energy) alone?
• How does biochar soil amendment affect agricultural productivity, water use, and albedo? And, what is the carbon sequestration limit, if any, for biochar soil amendment?
• What are the techno-economic implications of optimizing current biomass-to-fuel processes for net carbon removal?

Definitions

The proposed BECCS research agenda uses definitions adapted from DOE’s Clean Coal Program (DOE, 2015a) for technology readiness levels (TRLs), bench-scale, pilot-scale, and demonstration-scale, provided in Table 4.7. These definitions assume commercial-scale biomass-to-power or fuel plants have a dry biomass capacity of about 1,000 t/d, roughly equivalent to a fuel heating value of 220 MW at 19 GJ/t dry biomass.

TABLE 4.7 Bioenergy with Carbon Capture and Storage (BECCS) Technology Readiness Levels and Descriptions Based on U.S. Department of Energy Definitions

NOTES: DOE, U.S. Department of Energy; R&D, research and development; TRL, technology readiness level. SOURCE: Adapted from DOE, 2015a.

Components and Tasks

The committee’s research agenda to advance BECCS technology has four main components: (1) crosscutting activities, (2) biomass-to-power with carbon capture, (3) biomass-to-fuel with biochar, and (4) biomass-to-fuel with carbon capture. These research components, specific tasks, and estimated budget are summarized in Table 4.8 and described in detail below.

TABLE 4.8 Bioenergy with Carbon Capture Research Agenda, Budget, and Budget Justification

Component 1. Crosscutting Activities

Task 1.1 Regional Life Cycle Assessments and Integrated Assessment Modeling

The LCA methodology is a mature tool to determine the climate change mitigation potential of BECCS approaches in different contexts. It can therefore be used to estimate the total amount of CO₂ that could be removed from the atmosphere using BECCS in the United States. However, GHG emissions associated with indirect land-use change and other potential indirect effects from competition between uses for biomass and land may occur. IAMs allow for the consideration of these indirect greenhouse gas emissions, because they combine economic and physical models within a coupled framework. However, improvements are still needed. Reducing uncertainty in the outcomes is crucial to increase the robustness of the models that are used as inputs (Popp et al., 2014). More sensitivity analyses should be performed to understand better the implications of various parameters and assumptions. Other carbon dioxide removal approaches such as direct air capture, biochar, or soil carbon sequestration should be incorporated to the models to account for the full portfolio of potential solutions and to improve our understanding of how BECCS and other land use–based mitigation interact in different economic and political contexts (Popp et al., 2014). Most IAMs also exclude important elements such as impacts on ecosystem services, water resources, and biodiversity. Indeed, increasing bioenergy production is likely to lead to losses in ecosystem services and biodiversity, radiative impacts from albedo changes, and water resources depletion (Calvin et al., 2014). There is a need to better represent these secondary impacts in IAMs. Finer scale modeling may be required to project the effects of more local parameters. IAMs should also be updated to reflect the most recent understanding of the responses of consumers to demand-side incentives to reduce meat consumption and waste (Clark and Tilman, 2017; Griscom et al., 2017; Poore and Nemecek, 2018; Stehfest et al., 2009). The committee recognizes the need for additional social sciences research on reducing meat consumption and food waste, but substantial work on these topics, motivated health and economics concerns, is already occurring. Finally, the field requires a better understanding of social consequences, such as the effect of food price increases on food security.

Academic researchers and national laboratories should conduct this research program, because of the large-scale, integrated analyses required and the relevant work that is ongoing in these institutions. National laboratories should be engaged to develop and curate publicly accessible IAM platforms that can be leveraged by academic researchers and to coordinate international IAM efforts. This program would fall within

existing research portfolios at USDA, DOE, and EPA. A coordinated, cross-agency effort to develop IAMs is recommended.

Model Development

The objective of this activity is to improve the robustness of IAMs, including for other carbon dioxide removal approaches, and to develop better estimates of critical parameters. Competition among different land uses will likely increase as global population increases, as well as demand for food, fiber, and other ecosystem services such as biodiversity. Of particular concern for BECCS deployment will be competition for the land needed to produce biomass for bioenergy and the same land needed for other carbon dioxide removal approaches such as afforestation. IAMs are well suited for analyzing how land-use decisions at an aggregate scale are influenced by prices for different commodities as well as policies that affect food, fiber, and bioenergy production; however, some of these demand-driven price factors may not be well represented in current models. Research is needed to improve the estimation of critical parameters such as biomass yields in IAMs and to include other carbon dioxide removal approaches to develop future scenarios that account for the full portfolio of potential climate mitigation techniques.

Secondary Impacts

The field requires improved IAMs of the impact of bioenergy technology deployment on ecosystem services, biodiversity, albedo changes, and water resources. Large-scale bioenergy production might result in negative effects on nonclimate sustainability issues. A multicriteria analysis has shown potential trade-offs between different sustainability issues and possible mitigation solutions (Humpenöder et al., 2018). However, research is still needed to better estimate and quantify these potential environmental impacts. For instance, water availability for energy crops remains a research area of interest. Improved geo-hydrological models and analysis at the regional level are needed to better understand constraints and potential solutions regarding impacts on water (Slade et al., 2014).

Academic researchers and national laboratories should perform this research, which is ongoing in universities and would benefit from additional support to fill knowledge gaps.

Spatial and Temporal Resolution

The objective of this activity is to create IAMs with higher spatial and temporal resolution. Most IAMs function at a global scale and therefore may not accurately represent actual conditions and system responses at the smaller scales needed for effective policy- and decision-making. As a result, researchers should explore ways to nest or link smaller-scale models within global-scale models (similar to what has been done with global climate models) that can be tailored to more local circumstances, but still function adequately within the global context.

Food Security Impacts

The objective of this activity is to improve understanding of the impact of BECCS technology deployment on food prices and food security. The large-scale implementation of land-based carbon dioxide removal approaches might lead to food price increases through competition for land, as has been shown in some studies (Kreidenweis et al., 2016; Smith et al., 2013). However, if the extra land needed is already available or if biomass feedstock does not compete with agricultural land, food price increases may not be that high (Lotze-Campen et al., 2014), and mitigation measures can be implemented to limit impacts on food security (Smith et al., 2013). IAMs estimate food price increases caused by large-scale bioenergy production. However, the potential impacts on food security issues (e.g., malnutrition, food riots) are not yet well understood. Research is needed to better understand these impacts and to develop policy guidelines for the implementation of potential mitigation measures. Research is also needed to design appropriate safeguards to ensure food security. A good deal of social science and policy-focused research has already been conducted on the general safeguards required to ensure food security in the face of large-scale land-use change. This work needs to be reinterpreted and revised for the special case of BECCS.

Technology Assessments

Few LCA studies have assessed the potential environmental impacts associated with carbon dioxide removal technologies. LCA methodology relies heavily on available data, models, and assumptions to quantify CO₂ life cycle emissions. It is therefore critical to follow similar methodological rules when comparing LCA results to ensure valid conclusions. Product category rules allow consistency and comparability of results by providing product- or sector-specific guidance. There is a need to develop such

consensual LCA guidelines specific to biomass energy carbon dioxide removal technologies in the United States.

Component 2. Biomass-to-Power with Carbon Capture

The research agenda for biomass-to-power with carbon capture has two main elements: (1) biomass pretreatment and logistics for the conversion of conventional pulverized coal power plants to biomass and (2) high-efficiency biomass power generation. Pretreated biomass feedstocks for operation in conventional coal plants will, in the near-term, leverage worldwide fixed-capital coal power plant investments, while creating a biomass fuel supply infrastructure to support more efficient biomass power generation. In the long-term, high-efficiency biomass power generation will be essential to the sustainability, scalability, and cost-effectiveness of carbon negative biomass-to-power.

Task 2.1 Biomass Supply and Logistics

The development of a robust biomass feedstock supply and effective supply chain is a key to replacing today’s coal power plants with biomass power plants. This research task aims to establish a coal-compatible biomass feedstock depot capable of delivering enough fuel to completely convert a coal power plant to biomass. The program has two main thrusts: (1) pretreatment technology for converting biomass into a drop-in replacement for coal and (2) logistics research to address biomass supply chain issues (i.e., production, storage, handling, and transportation).

This research program will need to leverage the nation’s entire innovation ecosystem, from academia to private industry. Both the agricultural and electric power industries should be engaged early in this program. Applied research and bench-scale prototype development of pretreatment technology and supply logistics should be performed by university researchers, national laboratories, and R&D organizations. Pilot and demonstration-scale development should leverage public-private partnerships, with private industry and start-up companies taking the lead with support from the national laboratories. In particular, national laboratories should be engaged to operate pilot-scale testing facilities. Program managers should contract third-party engineering design and estimation firms to provide engineering and economic assessments of technologies under consideration for scale-up (stage-gate). The management and operation of the final biomass demonstration depot project should be a public-private partnership, preferably hosted by a large utility with coal power generation assets.

This program would align with the research portfolios and funding priorities of DOE and USDA, and its projects could be governed according to these agencies’ guidelines.

Pretreatment Technology

The objective of this activity is to identify optimal biomass densification, pretreatment, and formation techniques that convert a variety of biomass feedstocks into a standardized drop-in replace for coal. Research is needed to evaluate and develop biomass densification, pretreatment, and formation techniques using a variety of biomass feedstocks (agricultural byproducts, energy crops, wood, and organic waste) into a product that is compatible with coal-fired power plants. Promising technologies should be promoted from applied research to bench-scale prototypes (< 1 t/d biomass) and pilot-scale prototypes (10 t/d biomass). Process designs should be aimed at modualarized solutions with dry biomass capacity of about 100 t/d to enable a distributed pretreated biomass supply chain.

Feedstock Logistics Research

The objective of this activity is to determine the supply chain logistics (i.e., sources, collection, processing, storage, and transporation) needed to deliver pretreated biomass fuel to U.S. coal power plants with the aim of creating a national pretreated biomass demonstration depot capable of supplying enough biomass to convert a conventional coal power plant to biomass. Research projects should evaluate the logistics required to supply an existing U.S. coal power plant less than 20 years old with pretreated biomass.

The proposed research is distinct from the current feedstock supply and logistics research at DOE (BETO, 2017), in that its objective is to pretreat biomass at or near production, that is, distributed biomass pretreatment. This research should consider the availability of geological CO₂ sequestration for implementing biomass-to-power with CCS. Finally, this research should cover biomass production, waste resources, collection, processing, and transportation and carbon LCAs, supply chain economics, and barriers to implementation.

Task 2.2 High Efficiency Biomass Power

The fundamental challenge for biomass-to-power conversion efficiency is the relatively low fire-side boiler temperature in a conventional biomass power plant, which is

typically well below 700°C. At these temperatures, the technology options for thermal-to-electric conversion are limited to conventional steam turbines (Rankine cycle) with efficiencies below 40 percent. Thus, research is needed to develop technology that converts biomass-to-heat and produces working fluid temperatures over 1,100°C and/or biomass-to-power with conversion efficiencies over 60 percent. Some possible, but not exhaustive, transformational research directions include (1) liquid phase (molten glass or salt) combustion; (2) chemical looping combustion; (3) novel reactor designs that are process intensified and leverage additive manufacturing; (4) new high-temperature, corrosion-resistant materials and materials processing; (5) high-efficiency heat exchanger designs that are process intensified and leverage additive manufacturing; (6) gasification pathways; (7) liquification and liquid combustion pathways; (8) in situ high-temperature gas clean-up; (9) biomass pretreatment processes; and (10) small-scale, modular power generation concepts.

This research program will need to leverage the nation’s entire innovation ecosystem, from academia to private industry. Applied research and bench-scale prototype development of high efficiency biomass-to-power concepts should be conducted by university researchers, national laboratories, and R&D organizations. Pilot and demonstration-scale development should be led by private industry and start-up companies with support from universities, national laboratories, and R&D organizations. In particular, national laboratories should be engaged to operate pilot-scale testing facilities. Program managers should contract third-party engineering design and estimation firms to provide engineering and economic assessments of technologies under consideration for scale-up (stage-gates). This program would fit within the research portfolios and funding priorities of DOE. The DOE Office of Fossil Energy’s National Energy Technology Laboratory (NETL) has the most relevant expertise and experience for managing this program because of its long history of developing coal power plant technology, advance power generation, and carbon capture technology—even though biomass power does not exactly fall within the priorities of Fossil Energy.

Component 3. Biomass-to-Fuel with Biochar

Biomass-to-fuel with biochar processes hold tremendous promise as a cost-effective, carbon negative pathway. Despite some recent high-profile failures to commercialize thermochemical biomass-to-fuels technology (Fehrenbacher, 2015), promising new biomass-to-fuel processes continue to emerge. For example, a recent techno-economic analysis of a fast pyrolysis with hydrotreating process suggests this technology can make a gasoline/diesel blendstock with a minimum fuel selling price of $29/GJ ($3.46/gge) from $88/t dry wood and with negative carbon emissions of

-3 kg/GJ CO₂ per fuel produced (NREL, 2015). Another example is a new integrated hydropyrolysis with hydrotreating process (IH²) that makes drop-in gasoline and diesel fuels with a projected minimum fuel selling price of $14/GJ ($1.68/gge) from $79/t dry wood and negative carbon emissions of 0.89 kg/GJ CO₂ per fuel produced (Maleche et al., 2014; Tan et al., 2014). Although these technologies still have net positive life-cycle GHG emissions, the processes themselves can be carbon negative.

The carbon negative biomass-to-fuel with biochar processes can be advanced by determining the value of co-produced biochar and by optimizing existing processes or developing new pathways that maximize carbon removal. To this end, research in two main areas is proposed. First, biochar permanence in soil and impact on crop productivity needs better quantification to determine its long-term value as a soil amendment and viability for carbon sequestration. Second, carbon negative biomass-to-fuel conversion pathways need to be developed that are ideally both profitable from fuel production and carbon negative through the co-production of large quantities of sequestered biochar.

This research program will need to leverage the nation’s entire innovation ecosystem, from academia to private industry. Applied research and bench-scale prototype development of carbon negative biomass-to-fuel processes and enabling subsystems should be conducted by university researchers, national laboratories, and R&D organizations. Pilot and demonstration-scale development should be led by private industry and start-up companies with support from universities, national laboratories, and R&D organizations. National laboratories should be engaged to operate pilot-scale testing facilities. Program managers should contract third-party engineering design and estimation firms to provide engineering and economic assessments of technologies under consideration for scale-up (stage-gates). This program would fit within the research portfolios and funding priorities of the DOE and USDA.

Biochar Soil Amendments

Quantitative assessments are needed of how biochar soil amendments affect agricultural productivity, water use, and albedo. In addition, the carbon sequestration limit and permanence of biochar as a soil amendment needs to be assessed to accurately quantify the carbon storage potential of this technology. Because biochar composition and structure depend on the biomass feedstock and process by which it is produced, quantitative assessments should be coupled with promising biomass-to-fuel conversion processes.

Carbon Negative Pathways

A range of thermochemical conversion pathways for converting biomass to fuels has been developed, including gasification, pyrolysis, hydropyrolysis, and hydrothermal liquefaction. Currently, none of these pathways has net negative life cycle carbon emissions; however, most technology developers optimize processes for maximum fuel production, not carbon emissions—often burning co-produced biochar to provide low-cost process heat. Research is needed to optimize existing biomass-to-fuel processes and to develop new pathways for net carbon negative emissions. Emphasis should be on robust processes and that can utilize a multitude of biomass feedstocks to maximize their long-term commercialization potential as well as enabling subsystem technologies that reduce the overall costs of carbon negative biomass-to-fuel processes.

Component 4. Biomass-to-Fuel with Carbon Capture

Carbon Negative Pathways

At the time of this study, biological pathways for converting lignocellulosic biomass-to-fuels appears to be the most speculative carbon removal approach. The fundamental problem is the recalcitrance of lignin and the inability of organisms to metabolize it. Consequently, most integrated lignocellulosic biorefineries burn their lignin for process heat and power. Because lignin constitutes about 30 percent of the weight and 40 percent of the energy content of all biomass, burning it severely limits the potential for a scalable carbon negative process. Nonetheless, the merits of producing a pure CO₂ stream during fermentation that can be easily captured and stored is sufficient motivation to warrant basic and applied research into net carbon negative biological pathways. Specifically, bioengineering research aimed at designing pathways to break down lignin and convert it into fuels should be conducted. Successful biological lignin valorization could completely transform the economics of biological biomass-to-fuel processes and their potential for carbon removal. Therefore, an ongoing applied research program to develop carbon negative biological conversion of lignocellulosic biomass into fuel, conducted at universities and national laboratories, and managed by DOE, is recommended.

Future Research Considerations

If BECCS technology is to be deployed at scale, additional research considerations will likely arise during its future implementation. Specifically, an efficient and coordinated supply and utilization system will be needed. The committee identified the following systems-level needs:

• The field requires spatially explicit optimization of biomass-to-fuel and power implementation to design optimal BECCS network configuration accounting for plant scale, location relative to biomass supply, competition for biomass supply, CO₂ transport network, and CO₂ and/or char sequestration sites.
• Integration of biomass power plants with an electricity grid that may have a high fraction of various renewable energy technologies (e.g., solar, wind) in the future will present challenges in terms of load following, requiring improved understanding and control over ramping and flexibility.
• Reductions in the capital cost and energy consumption of CCS are needed to improve economic and technical viability of BECCS, with particular attention paid to any challenges that biomass feedstocks may introduce for CCS.
• The quality and variability of CO₂ produced from biomass power plants and the impacts on carbon capture system components will need to be analyzed to understand long-term impacts on pipeline, wellhead, and subsurface equipment.
• Coherent policy and governance, such as emissions accounting when biomass supply is not collocated with consumption (e.g., different countries), is needed to promote biomass markets, and extension and outreach may be needed to encourage landowners to adjust crops and practices.

Implementation of the Research Agenda

Funding

Scale of Funding

The research agenda budgets for prototype development were estimated assuming a 1000 t/d dry biomass commercial-scale plant has a capital cost of about $100 million, which for a 50 percent efficient power plant corresponds to capital cost of about $900/kW electrical capacity—on par with the cost of natural gas combined cycle power plants. Then using the “2/3 law” for economies of unit scale:

TABLE 4.9 Estimated Bench, Pilot, Demonstration-Scale Plant Costs, Dry Biomass Capacities, and Technology Readiness Levels

where k is the plant capital cost ($), c is the plant capacity (t/d), c₀ is the reference unit capacity, and α is the scaling factor of 2/3; an order-of-magnitude cost was estimated for bench, pilot, and demonstration-scale prototypes, Table 4.9.

Sequencing of Funding

The research agenda budgets for development of bioenergy with carbon capture technology are intended to be staggered over a period of 15 years. This approach should reduce technology and financial risk. Figure 4.10 provides an example of this sequence of research funding for each of the three BECCS pathways for the research agenda.

Sources of Funding

Though not explicitly called out in the research agenda, it is generally assumed that most of the funding will come from the federal government. That said, for more mature technology development projects it would not be unreasonable to expect industry participants to provide some or even all project funding. Although the funding of this research agenda falls outside the scope of this study, mechanisms other than traditional federal instruments for funding R&D, such as market-based policy incentives, warrant exploration.

Institutional Structures

Several federal agencies are capable of effectively conducting most of the proposed basic and applied research components and tasks, the most active being USDA, DOE, and EPA. Although programs within these agencies are well suited to conducting basic and applied research, they may not be well equipped to effectively run a technology demonstration program. In the past, public-private partnerships have played a critical role in the demonstration and deployment of new technology. For example, the Electric Power Research Institute (EPRI) and the Gas Research Institute (GRI) received their funds from taxes on interstate transmissions (electricity and gas) until utilities deregulation phased out this funding mechanism in the 1990s. In the absence of such organizations, Deutch (2011) proposed the creation of a new institution that would be responsible for managing and selecting technology demonstration projects and that would be supported by, but separate from, the federal government, which he called “the Energy Technology Corporation.” This organization would house a well-designed

technology demonstration program with the appropriate authority, tools, and expertise to accelerate technology development. The evaluation of existing technology demonstration capabilities with federal agencies and the investigation of institutional structures that will have the necessary capabilities to effectively develop and demonstrate new biomass energy technology is necessary to ensure successful scale-up and deployment of new technologies.

Research Management

The governance of a technology development program that includes demonstration of pilot prototype systems require engineers with industrial experience as well as standards for managing and evaluating new processes. NETL’s CCS program serves as an example of a recent program implemented by the federal government.⁴ NETL developed a standard methodology for assessing the cost of carbon capture from coal power plants and corresponding impacts on electricity costs. In addition, it developed a comprehensive technology development roadmap that spanned basic research to demonstration-scale plants.

The development of pilot and demonstration-scale process evaluation standards is essential for the effective governance of biomass energy technology development. These standards include process design engineering, equipment costing, and techno-economic analysis. Once a standard design basis has been developed, the use of third-party, independent, for-profit process engineering and estimating firms to provide technology assessments and techno-economic analyses is the most cost-effective way to vet new biomass energy processes. These technology evaluations should be made available to the public and how results were obtained transparent.

Carbon Accounting and Monitoring

One practical challenge to implementing any carbon removal technology is the accounting and crediting for carbon removal. By contrast, CO₂ emissions from the combustion of fossil fuels is straightforward and easily accounted for using existing reporting on fossil fuel extraction, imports, and sales. For biomass energy carbon removal approaches, carbon accounting is particularly challenging because the amount of net carbon removal largely depends on the specific pathway chosen (i.e., production,

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⁴ See https://www.netl.doe.gov/research/coal/carbon-capture/carbon-capture-program (accessed January 28, 2019).

transport, conversion, sequestration). Policy research may be necessary to identify methods that provide a simple and fair system to track net carbon removal.

Monitoring the impacts of increasing biofuel production on land area and leakage, as well as CO₂ accumulation rates on land used for biofuel, are essential aspects of program implementation. Research needs that build on existing land monitoring programs are covered in Chapter 3. Likewise, the need to monitor the leakage of sequestered carbon gases from CCS is covered in detail in Chapter 7.

SUMMARY

Developing and deploying carbon negative bioenergy technology that is sustainable, scalable, and commercially viable is a daunting task with many possible approaches, each with its own unique benefits and challenges. The committee has recommended a research agenda that attempts to group and prioritize the most promising of these approaches and to set a realistic path for researching, developing, demonstrating, and deploying commercial BECCS technology. This research agenda has four components: (1) integrated assessment modeling; (2) biomass-to-power with carbon capture; (3) biomass-to-fuel with biochar; and (4) biomass-to-fuel with carbon capture.

Biomass supply is the overarching concern and consideration for large-scale implementation of BECCS technology. Worldwide, full-scale BECCS deployment will require an additional 300-600 Mha of land (roughly equal to the area of Australia) for energy crops. In the United States, assuming 1 billion tons of biomass is needed, a sufficient supply could be achieved with energy crops, forest biomass, organic waste, and agricultural residues, but the associated GHG emissions and environmental impacts remain unclear. To accurately assess the impact of BECCS on net GHG concentrations and climate change, a model is needed with the following essential elements: (1) land-use change impacts, including long-term nutrient and productivity changes; (2) emissions related to biomass harvesting, processing, and transportation (supply-chain emissions); (3) combustion efficiencies and related emissions of different fuels (referred to as “fuel substitution”); (4) indirect impacts, such as changes in land use or reductions in timber product inventories because of increased biomass demand; and (5) emissions related to carbon capture, transport, and storage. In addition, the changes in albedo and other biophysical processes that alter how GHG affect the climate should be explored. Currently, no such comprehensive integrated assessment model exists. To accurately assess the impact of BECCS on GHG concentrations and climate change, research is required to build a holistic integrated assessment platform

that incorporates the essential elements above, as well as albedo and other climate impacts.

Today’s biomass-to-power plants suffer from an inability to sustain a consistent biomass supply, price, and composition and from low power plant efficiency, both of which present barriers to the deployment of carbon negative biomass-to-power with carbon capture. Therefore, the biomass-to-power with carbon capture research agenda focuses on (1) biomass supply and logistics through conversion of conventional pulverized coal power plants to pretreated biomass fuel and (2) next-generation high-efficiency biomass power generation. In the near-term, the development of pretreated biomass as a drop-in replacement for coal in conventional coal power plants will leverage existing fixed-capital investments (coal power plants), while creating a robust, distributed biomass fuel supply infrastructure able to support future, more efficient biomass power generators. In the long term, biomass-to-power conversion must be more efficient, so that this carbon removal approach can be cost-effective, sustainable, and impactful. To accelerate technology deployment, this research agenda calls for the development of bench, pilot, and demonstration-scale prototypes of the most promising biomass pretreatment and biomass-to-power conversion technologies.

Biochar is one of the most promising near-term commercially viable carbon removal approaches. However, emerging commercial thermochemical biomass-to-fuel with biochar processes seek to either (1) maximize the production of liquid fuel and minimize the biochar or (2) maximize biochar production for sale into niche, high-end, home garden markets and minimize production of liquid fuel. Several thermochemical processes may be commercially viable without a price on carbon and may have net negative carbon emissions, but not at the same time. However, if the economics of biochar co-production could be positively changed by definitively demonstrating that biochar soil amendments increase crop yields, technology developers could co-optimize for both fuel and biochar, potentially making their process net carbon negative. As such, one research aim is to quantify biochar permanence in soil and the impact on crop productivity. Another research aim is to optimize existing biomass-to-fuel processes for carbon removal and investigate completely new carbon negative pathways. To accelerate technology deployment, the research agenda calls for the development of bench, pilot, and demonstration-scale prototypes for the most promising carbon negative approaches, scaling the dry biomass capacity from roughly 1 t/d bench-scale to 100 t/d demonstration-scale.

Biomass-to-fuel with carbon capture (or biological conversion) pathways are the last potentially carbon negative biomass technology. However, biological conversion

pathways have been determined to have low carbon negative potential because today’s organism cannot effectively decompose lignin. In integrated biorefineries, lignin derived from biomass is typically burned for heat and power. Given that lignin represents about 30 percent by mass and 40 percent by energy content of all biomass, bioengineering pathways to break down and convert lignin to liquid fuels is specifically recommended. More broadly, only basic and applied research on carbon negative pathways is recommended for biological biomass conversion until a breakthrough is made in lignin valorization.