CO2 Deserts: Implications of Existing CO2 Supply Limitations for

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CO2 Deserts: Implications of Existing CO2 Supply Limitations for Carbon Management Richard S. Middleton,† Andres F. Clarens,*,‡ Xiaowei Liu,‡ Jeffrey M. Bielicki,§,∥ and Jonathan S. Levine⊥ †

Earth and Environmental Sciences, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States Civil and Environmental Engineering, University of Virginia, Thornton Hall, 351 McCormick Road, Charlottesville, Virginia 22904, United States § Civil, Environmental, and Geodetic Engineering, The Ohio State University, Hitchcock Hall, 2070 Neil Avenue, Columbus, Ohio 43210, United States ∥ John Glenn School of Public Affairs, The Ohio State University, Page Hall, 1810 College Road, Columbus, Ohio 43210, United States ⊥ Independent Consultant, 1 Ilium Way, Pittsburgh, Pennsylvania 15217, United States ‡

S Supporting Information *

ABSTRACT: Efforts to mitigate the impacts of climate change will require deep reductions in anthropogenic CO2 emissions on the scale of gigatonnes per year. CO2 capture and utilization and/ or storage technologies are a class of approaches that can substantially reduce CO2 emissions. Even though examples of this approach, such as CO2-enhanced oil recovery, are already being practiced on a scale >0.05 Gt/year, little attention has been focused on the supply of CO2 for these projects. Here, facilityscale data newly collected by the U.S. Environmental Protection Agency was processed to produce the first comprehensive map of CO2 sources from industrial sectors currently supplying CO2 in the United States. Collectively these sources produce 0.16 Gt/ year, but the data reveal the presence of large areas without access to CO2 at an industrially relevant scale (>25 kt/year). Even though some facilities with the capability to capture CO2 are not doing so and in some regions pipeline networks are being built to link CO2 sources and sinks, much of the country exists in “CO2 deserts”. A life cycle analysis of the sources reveals that the predominant source of CO2, dedicated wells, has the largest carbon footprint further confounding prospects for rational carbon management strategies.

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INTRODUCTION Anthropogenic emissions of carbon dioxide (CO2) and other greenhouse gases (GHG) and radiative forcers are driving changes in the global climate system.1 The Intergovernmental Panel on Climate Change (IPCC) and other sources characterize these changes as an unequivocal risk to the present organization of societies relative to the environment upon which they rely.2 These CO2 emissions largely result from stationary sources dominated by fossil fuel combustion for generating electricity.3 The electric power sector in the United States emits over 2 Gt (1 Gt = 1012 kg) of CO2/year, and only a few technologies have been identified that could be scaled sufficiently to manage these emissions. In the short to medium term, CO2 capture, utilization, and/or storage (CCUS) could divert these emissions from the atmosphere. Part of the potential of CCUS arises from the fact that the necessary technologies already exist and thus they could be deployed at a large scale in a relatively short time period.4 In general, CCUS refers to any technology in which CO2 emissions are captured before being emitted to the atmosphere and used or disposed © 2014 American Chemical Society

in such a way that some fraction of the CO2 is permanently kept out of the atmosphere.5 The typical conception of CCUS involves injecting CO2 into the deep subsurface where it would be isolated from the atmosphere, but CCUS can also pertain to sequestration in soils or incorporation into other materials that permanently encapsulate the CO2.6 The most commonly practiced CCUS approach, in terms of CO2 sequestered, is CO2-enhanced oil recovery (CO2-EOR).7,8 In CO2-EOR, CO2 is injected into oil fields that have become uneconomical to produce using primary (e.g., pumping) or secondary (e.g., repressurizing with water injection) methods.9 This CO2 can be used as pressure support and to modify the physicochemical properties of the remaining oil to stimulate additional oil production. Approximately 30−40% of the CO2 that is injected during a single injection typically remains Received: Revised: Accepted: Published: 11713

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mental Protection Agency (EPA) have made facility-scale data available for the United States.25 The EPA database provides two key elements that, for the first time, make this study possible: the EPA database (1) is a source of legally-mandated emissions reports by each facility and (2) provides process-level CO2 emissions for each facility. The legal requirement ensures that the data are comprehensive, while the process-level data enable differentiation of CO2 sources within facilities, specifically distinguishing between dilute and dispersed CO2 sources such as small engines and processes producing concentrated CO2 streams such as acid gas removal or hydrogen production. Process-level data are also required for detailed life cycle analysis (LCA) of individual emissions streams, without which it is impossible to understand the CO2 intensity of capturable CO2. At present, five industrial sectors are supplying the vast majority of CO2 in the United States.11 Unlike the power plants and other CO2 emitters that are currently envisioned by many to be the primary suppliers of CO2, only industries with low marginal operational impacts, and the associated low marginal costs, are providing CO2. Four of these are industries that produce a nearly pure CO2 stream requiring minimal processing to supply CO2 as input to other production processes, typically only dehydration and compression. These industrial sources of CO2 are ammonia production facilities, ethanol refineries, hydrogen production units within oil refineries, and acid gas removal units either in hydrocarbon production/transportation infrastructure or in processing plants.25,26 Dedicated wells drilled into natural CO2 deposits are the fifth type of CO2 supplier. Unlike the other industrial sectors supplying CO2, these dedicated wells produce CO2 as the primary product of the operation. We refer to this CO2 as extracted CO2, to distinguish it from the byproduct supplied by the other industrial sectors. Over 70% of the CO2 supplied in the United States came from dedicated wells, primarily for EOR.23 Of the roughly 30% of the CO2 supplied in the United States in 2012 that was not extracted CO2, acid gas processing plantswhich remove sour gases from natural gas and oil provided 25% of the total supply. Hydrogen production and ammonia production generate CO2 via the same water gas shift reaction in which natural gas is re-formed to produce a hydrogen stream and a CO2 stream. In the case of hydrogen, this reaction occurs at refineries that are processing hydrocarbons; in ammonia plants, the hydrogen is reacted with nitrogen to produce ammonia. Ethanol refineries are an increasingly ubiquitous source of CO2 that comes from the fermentation of plant sugars to ethanol and CO2.27 We compiled a detailed picture of the supply side capacity for CO2 in the United States in order to understand where these facilities that could supply CO2 are located and to interpret these findings in light of the CO2 footprint of their industrial sectors. We hypothesized that CO2 as an industrial feedstock for carbon management activities, i.e., excluding CO2 from dedicated wells, is quite limited. We conducted a geospatial analysis of the energy and CO2 intensities of supplying CO2 using life cycle data of individual facilities according to their specific industrial sectors. The results are interpreted qualitatively through two case studies, one focused on CO2EOR in Texas and the other on algae production in Louisiana. These results can help to refine regulatory frameworks and develop deployment pathways for managing CO2 emissions in the United States that can be used as a blueprint for application elsewhere.

trapped in the reservoir. The CO2 that is not trapped is produced with the oil and often separated and reinjected to further produce oil such that all of the CO2 will ultimately be stored except for any fugitive emissions that occur in handling.10 The United States has 4 decades of industrialscale experience with CO2-EOR.11−13 At present, more than 50 Mt of CO2/year (1 Mt = 109 kg) are being injected.11 Many of the same operational principles and pore-scale physics used in CO2-EOR also apply to geologic carbon sequestration (GCS), where CO2 is injected into deep saline aquifers or other formations. However, no economic commodities are produced with GCS; the only goal is to permanently dispose of CO2.14,15 Consequently, the business case for GCS depends mostly on the establishment of mechanisms that make it costly to emit CO2, e.g., an emissions tax.16 This cost must be high enough to stimulate large CO2 emitters to embrace technologies such as CO2 capture and GCS that avoid these emissions. A large number of engineering approaches have been explored to sequester CO2.17 For example, algae cultivation in terrestrial shallow ponds to produce biofuels is an emerging technology that is increasingly being deployed at large scale.18 Algae are grown using added CO2 to produce biomass that can be converted to fuels.19 Some fraction of the biomass will be recalcitrant and not capable of transforming into a biofuel20 and could be used as a soil amendment or buried as a means of keeping the organic matter out of the atmosphere.21 The exact fraction that would be sequestered would be determined by the growth conditions, species, and conversion process. But, in general, the process has a significant potential to reduce the net amount of CO2 entering the atmosphere because of the fast growth rate of aquatic species relative to terrestrial plants. Often overlooked in discussions related to these and other carbon management strategies is the location of sequestration facilities relative to the sources of CO2, the large-scale supply chains needed to connect these, and the environmental footprint of operations at large scales. Many studies assume that CO2 could be supplied by nearby power plants. At present, only a limited number of power plants in the United States are capturing their CO2, either as a government-subsidized demonstration project or to provide small quantities of CO2 for other industrial processes, e.g., food and beverage industry and chemicals production.2,22 The cost of applying CO2 capture technologies to power plants is presently too high relative to other suppliers of CO2 such as ammonia production or acid gas processing plants. Consequently, existing analysis of the CO2 market suggests it is constrained, which is seemingly counterintuitive, given the ubiquity of CO2 emitters in the United States. However, the lack of a price on CO2 emissions coupled with technical constraints, market drivers, and regulatory uncertainty are impeding more wide-scale capture. For most emitters, the exhaust stream is too small, too far from possible sinks to warrant capture, or too dilute and therefore requires expensive separation equipment, e.g., power plants. Until recently, the characteristics of CO2 suppliers were only described in coarse terms, at a state, national, and industry scale, or were complied on a voluntary basis by external efforts.23 The National Carbon Sequestration Database and Geographic Information System (NATCARB) has collated CO2 emissions data for individual facilities.24 However, the NATCARB data were collected by seven quasi-independent regional partnerships using a combination of nonunified and often voluntary data sources across multiple reporting years. Newly enacted reporting requirements by the U.S. Environ11714

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Figure 1. Sources of high-purity CO2 in the United States by industry type.25 Solid circles indicate that the source is presently supplying its CO2 to other industries, including enhanced oil recovery, industrial gas supply, and food applications. Hollow circles indicate that the source does not supply CO2 or that the disposition of the CO2 is unknown. The size of the circles is proportional to the amount of CO2 being produced.

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MATERIALS AND METHODS Data Sources. High-level summaries of CO2 emissions in the United States have been provided by the EPA to the IPCC for over a decade as part of the National GHG Inventory process.28 However, detailed facility-scale data have only recently become available,25 following a 2008 mandate from the U.S. Congress (H.R. 2764), and EPA’s resulting Mandatory Reporting of Greenhouse Gases rule (74 FR 56260, generally referred to as 40 CFR Part 98; Public Law 110-161-EPA issued the mandatory reporting of greenhouse gases rule). In 2012, the EPA began releasing annual GHG emissions data for facilities emitting over 25 kt of CO2 or the GHG equivalent (CO2e) per year through its web site.29 The most recent data that are available are for 2012. Even with these data, the relevant CO2 supply numbers were not readily available for the five sectors described here from a single data source because of methodological differences in reporting requirements between industries. For example, emissions from acid gas removal units are a small fraction of the total emissions of natural gas transmission infrastructure or processing facilities reported at the facility level. However, process-level data, such as emissions from individual acid gas removal units reported under Subpart W, are available from the EPA Envirofacts database. We developed tools for processing the data (see the Supporting Information (SI)) to produce a list of 597 CO2 sources across the five sectors for the continental United States. This list is the most comprehensive evaluation of CO2 sources from the five industries currently supplying CO2 in the United States to date. A complete spreadsheet of these facilities, after our processing, is available in the SI. We made substantial effort to ensure that the data quality was high and cross-referenced but acknowledge that quality assurance issues may persist because facilities selfreport data to EPA and there could be differences in interpretation. Figure 1 maps the data for the 597 sources as reported to the EPA in 2012. The facilities that capture their CO2 and sell it downstream are plotted as filled circles while facilities that emit the CO2 are plotted as hollow circles. Eighteen of the 22 ammonia manufacturers supply CO2, as do 22 of the 102 hydrogen facilities, 17 of the 211 ethanol facilities, and 11 of the

248 acid gas facilities. There are also 14 dedicated wells supplying CO2 into the market. The size of the circles is proportional to the amount of CO2 produced, and the larger facilities tend to be more likely to provide CO2 than smaller facilities. We note that currently supplied CO2 may not be available for other purposes, and that the CO2 extraction operations could possibly expand their scale and provide more CO2 than indicated in the data. Figure 1 also shows large portions of the country where CO2 sources are relatively scarce. These observations of the raw data, among others, provided the impetus for this work. Even though the EPA does not require producers generating less than 25 kt of CO2 to report their emissions, the data indicate that there were very few facilities producing and capturing their CO2 at such small scales. The number of facilities and their sizes provide some insight into the industrial organization of the CO2 produced in the United States. Figure 2 presents the rank-ordered maximum CO2 production at the facility level. Extracted CO2 operations are the least numerous but produce the most CO2. In contrast, ethanol facilities are the most numerous and also the most consistently sized. Acid gas, ammonia, and hydrogen facilities have a wide variability in facility size. Note that the data are plotted on a log scale on the y-axis indicating that there are a small number of large suppliers. Life Cycle Model. Life cycle assessment is a quantitative accounting tool for evaluating the environmental burdens of a product or process including all relevant material acquisition and end-of-life processes.30 For each industrial sector, we compiled a life cycle model to quantify the environmental burdens (energy and CO2 intensity) associated with capturing and supplying CO2. We report the marginal burdens of capturing the CO2, that is, the life cycle energy and GHG emissions that arise from the specific activity of capturing CO2 from these processes, and preparing it to be supplied as an input to other industrial processes. This approach is unlike previous studies which have sought to perform an allocation between a primary product (e.g., ammonia) and a co-product (e.g., CO2).26 The term “allocation” implies that the burdens must be split between products and co-products using an economic or material balance rationale with little basis in physical processes or policy contexts. Here we assume that 11715

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The burdens of the four byproduct streams are all similar because modeling estimates were not found to be sensitive to the pressure, temperature, and composition differences that may exist between processes. These marginal life cycle burdens were also relatively small in light of the credit that could be assigned to those processes that were avoiding emissions. As a result, the life cycle data fall into two broad categories: dedicated CO2 wells, with a high CO2 burden, and other industrial sources claiming an emissions avoidance credit, which results in a lower CO2 burden. The Supporting Information contains additional information about these calculations. These estimates are consistent with and partially based on those reported in the comprehensive life cycle inventory of CO2 sources published by Overcash et al.26 The burdens associated with CO2 transport via pipeline and utilization (e.g., pumping at an EOR facility) are not included here since the focus was on supply side impacts. Geospatial Analysis. The 2D and 3D maps were made using ESRI’s ArcGIS v10.1 software. Individual sources were aggregated into 5 km grids (CO2 desert maps) and 50 km grids (3D maps) using VBA code in Microsoft Excel 2010. The carbon desert grids were calculated using custom code written in Visual Basic v6.0. The code (1) calculated the distance between the centroid of 311,154 5 km grid cells and the precise location of the 597 CO2 sources, (2) sorted the 597 distances in ascending order, and (3) calculated the distance from each grid cell required to reach a CO2 supply of 1, 5, 10, and 50 Mt of CO2/year. The resultant distances were then binned into 100−500 km bands for visualization purposes.

Figure 2. Individual CO2 source in the United States plotted in terms of CO2 production potential relative to the number of facilities with this capacity.

without a customer for the CO2 it would be vented to the atmosphere, so the burdens of capturing CO2 should be the marginal impacts of capturing the gas versus not capturing it (see the SI for additional information). For the five sectors currently supplying CO2, the life cycle energy and CO2 burdens of capturing CO2 come primarily from dehydration and compression for pipeline transportation. Separation was generally not necessary or already occurs as part of the existing processing/production; i.e., it is not included as part of the marginal change in life cycle impacts. In addition, a credit of 1 kg CO2/kg CO2 was applied for those processes that capture CO2 generated as a byproduct because capturing the CO2 amounts to an emission avoidance. Only dedicated wells extracting CO2 from natural sources do not divert CO2 from being emitted to the atmosphere and thus should not be eligible to receive a credit if they are associated with an endeavor to limit such emissions. Consequently, the burdens of the five different sources are as follows: natural wells, 1.74 MJ and 0.21 kg CO2/kg CO2; natural gas processing, 0.43 MJ and −0.96 kg CO2/kg CO2; ethanol plants, 0.86 MJ and −0.93 kg CO2/kg CO2; ammonia plants, 0.86 MJ and −0.93 kg CO2/kg CO2; hydrogen plants, 0.86 MJ and −0.93 kg CO2/kg CO2.

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RESULTS AND DISCUSSION CO2 availability in the United States from the five industrial sectors currently supplying CO2 is shown in Figure 3. The facility-scale CO2 emissions data were aggregated into 50 km × 50 km grid cells, elevated in proportion to CO2 emissions available for supply for visualization purposes. Individual facilities are shown as black dots. White grid cells have no commercially available CO2, and colored cells have CO2 proportional to the height of the bar. Blue bars indicate CO2 with lower life cycle impacts (−0.9 kg CO2/kg CO2), while red bars are for higher impact CO2 (0.2 kg CO2/kg CO2).

Figure 3. Major commercial sources of CO2 in the United States. Individual facilities are indicated by black dots while the bars represent the total mass of CO2 available in each 50 km × 50 km grid cell. The color of the bars indicates the average environmental impact (kilograms of CO2 per kilogram of CO2 supplied) including credits associated with the CO2 that can be sourced in that cell. Generally speaking, there is much more CO2 on the market (0.2 kgCO2/kgCO2 supplied), tall red bars) from extraction wells, which produce higher impact CO2 (0.2 kg CO2/kg CO2 supplied), red bars) relative to the lower impact CO2 available as a byproduct from industrial sectors (−0.9 kg CO2/kg CO2 supplied), blue bars). 11716

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Figure 4. Impact of demand scale on the presence of CO2 deserts is pronounced. Here the supply amount was varied (Mt of CO2/year) (a) 1; (b) 5; (c) 10; (d) 50.

Figure 5. Regional pipeline networks. Networks have the capacity to alleviate some of the constraints associated with CO2 deserts, but their reach tends to be limited as shown in this detail of the south central United States. Here the thickness of the lines is proportional to their capacity, and the dark gray regions indicate the location of oil and gas fields.

poses both opportunities and challenges. Operations using CO2 could substantially improve their life cycle CO2 emissions profile by using byproduct CO2 instead of extracted CO2. However, transporting sufficient CO2 may be a logistical challenge because of the distributed nature of byproduct CO2 facilities. As in Figure 1, Figure 3 also reveals that large portions of the United States lack sources of CO2. These “CO2 deserts” are the regions between the bars and are concentrated on the East Coast, desert Southwest, and Pacific Northwest. The nonuniform distribution of CO2 is important in the context of CCUS projects because many of the large sources are not co-located with present CO2 demand such as CO2-EOR facilities. Thus, the scale of a utilization or sequestration effort will depend greatly on its location. It is important to note that Figure 3 may not fully represent the amount of CO2 produced at a few facilities that are not captured by the EPA reporting mechanisms. For example, the Dakota Gasification facility in

Several important trends are apparent in Figure 3. The supply of CO2 is dominated by 14 dedicated wells clustered in only three grid cells, shown in red. The contrast between the dedicated wells, which produce very large amounts of relatively “dirty” CO2 at a few select locations, and all other industrial sources is stark. A much larger number of other industrial sources, those that would receive credits for avoiding CO2 emissions, are shown in blue. These sources are geographically dispersed, and each source generally produces much less CO2 than a dedicated well. There are a number of regions with high concentrations of industrial sources, largely refineries on the California and Gulf coasts, that produce substantial quantities of byproduct CO2. In general, however, the other industrial sources each produce relatively small quantities of CO2 requiring regional CO2 sourcing to achieve comparable quantities, e.g., Midwestern ammonia and ethanol (compare Figure 2 and Figure 4). From a practical perspective, the distribution of “clean” versus “dirty” CO2 in the United States 11717

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satisfying the demand (34.4 Mt of CO2/year; see the SI) may require lengthy regional pipeline networks connecting to many sources. As discussed earlier, a pipeline network brings dirty CO2 from western Colorado to the Permian basin, which would otherwise be a CO2 desert owing to CO2 demand of 53.8 Mt of CO2/year. Looking at the United States more broadly, the Department of Energy estimates that CO2-EOR in the United States can utilize 200 Mt of CO2/year.8 This is more than is currently being captured, so meeting this demand would require increases in supply and would require significant pipeline infrastructure to alleviate regional CO2 supply limitations.8 Similarly, large-scale cultivation of algae for biofuels will require key inputs and growing conditions that narrow the number of suitable regions in the United States. Access to water, nutrient supplies, sunlight, and CO2 will all determine the viability of a growth operation, yet almost no attention has been paid to CO2 sources and availability in the literature. For example, two of the large algae-to-energy producers have pilot facilities in New Mexico and Florida, both carbon deserts as shown in Figure 4.20,31 By comparison, the Gulf Coast has previously been shown to be one of the best places in the United States to grow algae in terms of sunlight and water availability.32 Figure 4 shows it also has ample CO2 availability. As shown in Figure 5, Louisiana has a number of large and currently untapped CO2 resources that are tied to the significant refining capacity of the Louisiana and East Texas coast. Meeting the demand for CO2 that the CO2-EOR and algae cultivation markets represent will be a challenge. Meeting it in a way that minimizes overall life cycle emissions by using the lowest burden sources adds another critical constraint that we believe is being overlooked and that the CO2 deserts analysis should bring to light. CO2 from natural wells has a large environmental footprint and represents a net addition of GHGs to the atmosphere. Using byproduct CO2 from other industries and preventing that gas from being emitted to the atmosphere could substantially reduce the United States’ CO2 footprint. The Permian Basin in Texas provides a good example. Many of the acid gas removal units in the region do not capture their CO2 for supply to the nearby EOR fields, likely because most of these facilities are too small or the the CO2 concentration is too dilute in the natural gas stream. Co-location of CO2 sources and CO2 demands/sinks is beneficial from the perspective of each end of the CO2 supply chain. From the perspective of the integrated CO2 supply chain, it can also be beneficial to acquire and transport CO2 from distant sources.33 Either way, there is a large environmental opportunity cost that is lost when releasing byproduct CO2 that is locally sourced and importing, from large distances, extracted CO2 from dedicated wells. The Supporting Information contains additional information on the scenarios.

Beulah, ND only represents CO2 generated from ammonia production and stationary combustion. Considerably more CO2 is produced and captured from a gasification plant located at the same facility for delivery to the Weyburn CO2-EOR facility in Canada. This process is listed as confidential in the EPA data. Issues such as these are rare, however, and overall Figure 3 represents a complete map of the most significant sources in the United States that could viably supply CO2 in the near to medium term. Figure 4 shows the CO2 deserts in the United States for four different levels of supply. We kept track of the distance between each point in the United States (based on a 5 km × 5 km grid) and the 597 CO2 sources. From this, we calculated the radial distance necessary in order to reach a CO2 supply at different levels of CO2 each year (megatonnes of CO2 per year). Figure 4a shows the regions that do not have 1 Mt of CO2/year available, Figure 4b is for 5 Mt of CO2/year, 4c is for 10 Mt of CO2/year, and 4d is for 50 Mt of CO2/year. The final distance information was then converted into five distance categories ranging from 100 to 500 km for visualization purposes. Consequently, a single point in the United States is considered to be in a 5 Mt of CO2/year−500 km carbon desert if that point does not have 5 Mt of CO2/year supply within a 500 km radius. It is important to note that, with the construction of large-scale pipeline networks, these supply/demand limitations can be addressed. The extent of the CO2 deserts grows as the demand for CO2 increases. For small operations needing ∼1 Mt of CO2/year, there are few places in the United States where that supply would be a challenge. The contrast becomes more apparent in Figure 4b,c as the scale of the demand increases to 5 and 10 Mt of CO2/year, respectively. To put this demand in context, a 5 Mt of CO2/year facility would look like a 650 MW coal-fired power plant. And at the scale of 5 Mt of CO2/year, roughly 400 similarly sized operations are necessary to capture the >3 Gt of CO2/year that is currently emitted from all stationary sources in the United States each year.11 In these maps, the darkest red regions indicate that CO2 would have to be transported 500 km or more to supply a demand. At the scale of 50 Mt of CO2/year (Figure 4d), there are no regions with enough CO2 supply currently available without transporting the CO2 from many distant (>500 km) sources. The existence of CO2 deserts is an existing problem in the CO2-EOR industry, requiring large-scale movement of CO2 via pipeline networks. Figure 5 shows a detailed map of the south central United States with CO2 pipelines in blue. The width of the pipelines is proportional to the CO2 capacity of the pipeline: the largest move tens of megatons of CO2 each year. The existing pipeline networks are, unsurprisingly, dominated by transportation of CO2 from the largest sources, dedicated wells, to high-demand regions: Gulf Coast and west Texas Permian Basin oil fields. Like the water transportation projects of many U.S. western states, these CO2 pipelines alter the landscape of CO2 deserts. Pipelines would need to have a large capacity and span large distances to have a discernible impact on the maps presented in Figure 4. Regional Scenarios. To demonstrate CO2 supply limitations and relative life cycle intensities, we considered two regional demand scenarios: CO2-EOR in Texas and algae biofuels in Louisiana. Carbon dioxide supply limitations for CO2-EOR is best demonstrated by comparing the present availability of CO2 (Figure 4) to the demand. The East and Central Texas basins have significant CO2 available, though

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ENVIRONMENTAL IMPLICATIONS Our analysis suggests that the existence of CO2 deserts will likely have important ramifications for developing plans to limit net CO2 emissions at the regional and national scales through CCUS. A full appreciation of their importance requires a description of some elements that were not fully captured in this analysis. CO2 deserts result from a combination of technical, economic, policy, and social reasons. Efforts to address CO2 deserts must address all of these factors. For example, of all the sources included here, ethanol refineries are 11718

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Notes

the only ones where the CO2 is biogenic. That is, the CO2 emitted from ethanol refineries arises from the fermentation of corn, which grew by acquiring CO2 from the atmosphere through photosynthesis. As a consequence, the ethanol sector is the only one that could reduce atmospheric CO2, though the net reduction may be moderate.34 For all of the other industrial sectors modeled here, the CO2 flows are fossil CO2 emissions avoided. Policy objectives will need to be consistent and, ideally, based on a life cycle framework that appropriately embodies consideration of the CO2 intensity of different CO2 sources and the net reductions that these facilities can really provide. Integral to the discussion of CO2 deserts is the importance of scale, both in terms of the quantity of CO2 supplied and demanded as well as the spatial extent associated with matching sources and sinks. The algae cultivation and CO2-EOR scenarios are intended to illustrate the fact that efforts to achieve deep reductions in CO2 emissions require technologies capable of being scaled rapidly. The EPA data provide for a new perspective on the current landscape for CO2 supply, reveal constraints, and identify opportunities for managing CO2 across scales. A number of opportunities could emerge from this understanding including optimized pairings between CO2 sources and sinks to minimize life cycle emissions, a more integrated CO2 distribution pipeline network, and better decision making tools for individual sources.35 In this analysis, we only addressed the industrial sectors that currently provide CO2 to other industries; we intentionally did not address the possibility that coal or natural gas fired power plants or other large-scale emitters (e.g., cement manufacturers) could capture and market their CO2. CO2 capture from these facilities is presently not economically viable, but from a life cycle CO2 standpoint the deployment of CCUS using these facilities provides beneficial opportunities. The CO2 intensity of CO2 capture from power plants (assuming a credit for emissions avoided) is −0.8 to −0.65 kg CO2/kg CO2), which is better than the dedicated wells currently being used to supply CO2 (0.21 kg CO2/kg CO2) but worse than byproduct CO2 produced from natural gas processing (−0.96 kg CO2/kg CO2).36 The fact that thermoelectric power plants are abundant, geographically dispersed, and are responsible for the majority of CO2 emissions from stationary sources in the United States means that if CO2 capture on these facilities could be cost-effective and viable, many of the availability issues described here would be alleviated.

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The authors declare no competing financial interest.

ACKNOWLEDGMENTS

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REFERENCES

This work is supported by the U.S. National Science Foundation to A.F.C. (Grant CBET-1134397) and J.M.B. (Grant SEP-1230691) and by the U.S.-China Advanced Coal Technology Consortium (under management of West Virginia University).

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ASSOCIATED CONTENT

S Supporting Information *

Text describing additional information about our modeling approach, a figure showing the processes of CO2 pretreatment, and tables listing material and energy flow values, a summary of marginal life cycle burdens for capturing CO2, CO2 basin-scale demand and resulting oil production, annual CO2 demand for biofuel production and CO2-EOR, and our raw and processed data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 1-434-924-7966; fax: 1-434-982-2951; e-mail: [email protected]. 11719

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