Selecting CO2 Sources for CO2 Utilization by Environmental-Merit

Jan 11, 2016 - Capture and utilization of CO2 as alternative carbon feedstock for fuels, chemicals, and materials aims at reducing greenhouse gas emis...
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Selecting CO2 Sources for CO2 Utilization by Environmental-MeritOrder Curves Niklas von der Assen,† Leonard J. Müller,† Annette Steingrube,†,‡ Philip Voll,†,§ and André Bardow*,† †

Chair of Technical Thermodynamics, RWTH Aachen University, Schinkelstrasse 8, 52062 Aachen, Germany S Supporting Information *

ABSTRACT: Capture and utilization of CO2 as alternative carbon feedstock for fuels, chemicals, and materials aims at reducing greenhouse gas emissions and fossil resource use. For capture of CO2, a large variety of CO2 sources exists. Since they emit much more CO2 than the expected demand for CO2 utilization, the environmentally most favorable CO2 sources should be selected. For this purpose, we introduce the environmental-merit-order (EMO) curve to rank CO2 sources according to their environmental impacts over the available CO2 supply. To determine the environmental impacts of CO2 capture, compression and transport, we conducted a comprehensive literature study for the energy demands of CO2 supply, and constructed a database for CO2 sources in Europe. Mapping these CO2 sources reveals that CO2 transport distances are usually small. Thus, neglecting transport in a first step, we find that environmental impacts are minimized by capturing CO2 first from chemical plants and natural gas processing, then from paper mills, power plants, and iron and steel plants. In a second step, we computed regional EMO curves considering transport and country-specific impacts for energy supply. Building upon regional EMO curves, we identify favorable locations for CO2 utilization with lowest environmental impacts of CO2 supply, so-called CO2 oases.

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atmosphere, which currently contains about 3000 Gt CO2.11 Capturing CO2 from the atmosphere (direct air capture, DAC) has therefore also been proposed.12−16 DAC allows to indirectly capture CO2 emissions from mobile CO2 sources. Since the potential CO2 sources emit much more CO2 than the expected demand for CO2 utilization, only the environmentally most favorable CO2 sources should be selected. The selection of CO2 sources for CO2 utilization is a complex task since it depends on a number of important factors:17,18 The energy demand for CO2 capture is the key factor for associated costs and environmental impacts.8,10,19 In Europe, CO2 capture costs were evaluated by the IEA GHG20 for a large variety of CO2 sources. For Austria, Reiter and Lindorfer21 provide a comprehensive evaluation of CO2 sources regarding costs and additional CO2 emissions from CO2 capture. The spatial distribution and the supply amounts of CO2 sources are important factors to determine transport distances to CO2 utilization sites.17,18 Transport distances from high-purity CO2 sources in the United States have recently been nicely illustrated in so-called “CO2 deserts” maps.17 In this paper, we expand the analysis of CO2 sources by an integrated assessment of CO2 capture, compression and transport for all

INTRODUCTION Anthropogenic carbon dioxide (CO2) emissions are a main driver for global warming. To reduce CO2 emissions, the carbon cycle might be partially closed by capture and subsequent utilization of CO2 for fuels, chemicals, and materials.1−3 Since CO2 is emitted and thus replenished at a faster rate than it is currently being utilized, CO2 for utilization can then be regarded as alternative, renewable carbon feedstock ideally leading to carbon-neutral life cycles.4 This feedstock CO2 can be provided by capturing CO2 from existing CO2 sources. It is well-known that capture of CO2 requires substantial amounts of energy and thus causes indirect CO2 emissions and other environmental impacts.5 These environmental impacts from CO2 capture should be minimized. For this purpose, we present and apply an environmentally motivated approach for the selection of CO2 sources for CO2 capture and utilization (CCU). A large variety of potentially promising CO2 sources is available, which can be divided into CO2 point sources and atmospheric CO2.6 CO2 point sources are stationary industrial processes producing CO2 through fermentation, calcination and most notably combustion of carbon fuels.7 Worldwide, large scale (≥0.1 Mt a−1) CO2 point sources emit about 7.6 Gt a−1 CO2-eq of which 78% are emitted by fossil-fueled power plants.7 Other CO2 point sources include production plants for iron and steel, for cement, for pulp and paper, refineries, steam crackers, and chemical plants for, e.g., ammonia, ethylene oxide, and bio ethanol.3,8−10 An alternative CO2 source is the © XXXX American Chemical Society

Received: July 17, 2015 Revised: December 15, 2015 Accepted: January 8, 2016

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DOI: 10.1021/acs.est.5b03474 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Figure 1. Process flow sheet for (a) CO2 point sources with (left) and without (right) CO2 capture; b) direct air capture (DAC). Black arrows indicate mass flows (exception: CO2 transport [tkm]is a service flow); blue arrows indicate energy flows; green flows indicate product flows leaving the system boundaries. DAC units are assumed to be located next to the CO2 utilization plants making CO2 transport negligible. Color online.

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proposed types of CO2 sources with respect to environmental impacts. The goal of this paper is to identify and rank all proposed CO2 sources for CO2 utilization in Europe from a climatemitigation and fossil-resource-preservation point of view. For this purpose, we introduce an environmental-merit-order (EMO) curve: the EMO curve allows to select CO2 sources while minimizing the overall environmental impacts on global warming and fossil resource depletion. To determine the environmental impacts of CO2 capture, we have conducted a literature study for energy demands of CO2 capture from various CO2 sources, and constructed a database for potential CO2 sources from the emissions register E-PRTR for individual facilities in Europe.22 An analysis of the spatial distribution of CO2 sources shows that CO2 transport distances are generally small. Neglecting transport in a first step, we present EMO curves for a European-wide perspective. The EMO curves allow identifying environmentally most favorable CO2 sources in Europe with the CO2 demand as free parameter. In a second step, we refine the European-wide perspective by including impacts from CO2 transport and country-specific impact factors for energy supply. For a 0.1 × 0.1° latitude-longitude grid (about 10 × 10 km), we determine local EMO curves for each grid point. For each grid point, we map the obtained environmental impacts for CO2 supply to identify so-called “CO2 oases”: favorable locations for CO2 utilization with lowest environmental impacts of CO2 supply.

MATERIALS AND METHODS

The Environmental-Merit Order for CO2 Capture. The concept of the environmental-merit-order (EMO) is derived from the classical merit order, which is often used in electricity generation. The classical merit-order curve ranks electricity sources according to their marginal production costs in ascending order over their available supply.23,24 Using the electricity sources with lowest marginal production costs minimizes overall production costs for electricity supply.23 The concept of the merit order to sort by minimal cost is also featured in the industry cost curve.25 In close analogy to the classical merit-order curve, an EMO curve ranks CO2 sources according to their marginal environmental impacts due to CO2 capture in ascending order over the available CO2 supply. Consequently, selecting sources for CO2 capture according to the EMO curve minimizes the overall environmental impact. Assuming that CO2 supply equals CO2 demand, we can consider the EMO curve as a function of the CO2 demand. We then treat the CO2 demand as free parameter. This is particularly convenient since the CO2 demand for CO2 utilization is difficult to predict: In our view, current estimates for the global CO2 utilization potential of 300 Mt a−1 in 201626 and 1.5−2.0 Gt a−1 in the long term27,28 can serve as upper bound for the potential future CO2 demand. Furthermore, treating the CO2 demand as free parameter allows to select CO2 sources without predicting markets for CO2 utilization options. B

DOI: 10.1021/acs.est.5b03474 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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directly through oxy-fuel combustion Q fuel where CO 2 emissions from fuel combustion (mCO2,fuel) are also captured. Compression of CO2 is typically required for the subsequent transport from point sources to CO2 utilization sites. Most data for CO2 capture are obtained from carbon capture and sequestration (CCS) studies, which consider pipeline transport and include compression of CO2 to at least 10 MPa. Thus, we include the compression of CO2 to 10 MPa in the system boundaries to be able to compare across alternative CO2 sources. In this work, the concentrated CO2 provided by CO2 capture is called product CO2. We define the functional unit as provision of 1 tonne (= 1 t = 1000 kg) of product CO2 at 10 MPa. The comparative environmental impacts per tonne of product CO2 represent the marginal environmental impacts required for the EMO. In other words, the marginal environmental impacts describe how capture of 1 tonne of product CO2 changes CO2 emissions and fossil resource depletion compared to the status quo without CO2 capture. The following subsections describe how to obtain marginal environmental impacts for global warming and fossil resource depletion, and present the required data. Marginal Environmental Impacts. CO2 point sources produce a main product such as electricity and provide CO2 as coproduct. As shown in Figure 1a, the marginal CO2 emissions (in t CO2-eq (t CO2,product)−1) are obtained by comparing CO2 emissions of the point source with retro-fitted CO2 capture to the status quo (no CO2 capture), divided by the amount of product CO2. For CO2 point sources, we assume that the same amount of main-product (mmain−product) is produced, e.g. ammonia in ammonia plants.6 For postcombustion capture at power plants, the electricity output is actually reduced. However, we assume a constant electricity output and model the electricity loss as electricity demand (see above). This allows us to evaluate all CO2 point sources with a common approach. For CO2 capture from all CO2 point sources, the marginal CO2 emissions (mCO2−eq,marginal) are determined by

The main motivation for CO2 capture and utilization (CCU) is reducing greenhouse gas (GHG) emissions and substituting fossil resources with CO2.29 To carefully assess this motivation, we evaluate impacts on global warming (as GHG emissions in CO2 equivalents) and fossil resource depletion (as fossil fuel use in oil equivalents). Certainly, other environmental impacts are also very important in the context of CCU and should be assessed. However, detailed emission data are usually not available and many potential environmental effects of CO2 capture are not yet fully understood such as the degradation of unintentional amine solvent losses.30 The considered environmental impacts are determined using comparative life-cycle assessment (LCA).31 Comparative environmental impacts are obtained by comparing the cradleto-gate impacts of the status quo of the CO2 source without CO2 capture to the CO2 source with retro-f itted CO2 capture. We refer to this approach as comparative LCA instead of consequential LCA in order to emphasize that we do not consider typical consequential LCA elements such as market effects.32 The comparative approach avoids ambiguous choices regarding allocation of coproducts. Allocation choices for CO2 capture were shown to lead to strongly differing carbon footprints of captured CO2 and the CO2-based products.6,33−35 The status quo for CO2 point sources is the CO2 source without CO2 capture: the CO2 emissions from the point source are completely emitted to the atmosphere (right part in Figure 1a). The status quo for direct air capture (DAC) of CO is no capture and thus no environmental impacts at all. The system for CO2 capture comprises the CO2 source (point source or atmosphere), the retro-fitted capture process including CO2 compression and corresponding energy supply, and CO2 transport (left parts in Figure 1a+b). The subsequent CO2 utilization options are not included since the goal of this study is to compare alternative CO2 sources for given CO2 utilization options. The resulting cradle-to-gate perspective for CO2 thus allows to compare CO2 sources but it should be kept in mind that end-of-life emissions are neglected where CO2 is usually released again. Thus, negative cradle-to-gate emissions for CO2 capture do not indicate an overall carbon-negative life cycle of CCU; instead, negative values correspond to reduced GHG emissions compared to the status quo. In fact, the whole life cycle of CO2 capture and utilization without permanent CO2 storage can be carbon-neutral at best. To separate CO2 from flue gas (mCO2,fluegas) or atmospheric air (mCO2,air), the CO2 capture process requires different forms of energy: Electricity (Wel) is needed for fans, pumps, compressors etc. For sorption-based CO2 capture, desorption requires heat (Qth), which is typically supplied by steam. For postcombustion capture in power plants, the steam required for solvent regeneration is then no longer available for power generation in the turbines. Power plants with retro-fitted CO2 capture are typically limited by the boiler.34 It is therefore not possible to produce additional steam for solvent regeneration and consequently, the electricity output of the power plant is reduced. Thus, the steam demand for CO2 capture can therefore be expressed as electricity loss for postcombustion capture at power plants.36 We follow a common approach for system-wide LCA for CO2 capture from power plants where the electricity loss is interpreted as electricity demand.5,34,37 For other CO2 sources with heat demand, we assume that heat is provided by steam generated through combustion of natural gas. For cement plants38,39 and DAC,14,40 heating is provided

mCO2 − eq,marginal = (mCO2,el + mCO2,th + mCO2,fuel − supply + mCO2,offgas + mCO2,transport − mCO2,fluegas)/mCO2,product ,

(1)

where mCO2,i are the CO2 flows i as illustrated in Figure 1. Eq 1 can be rewritten as (see Supporting Information, SI): mCO2 − eq,marginal = −1 + WelGWel + Q thGWth + Q fuelGWfuel + GWtransport

(2)

where Wel, Qth, and Qfuel are the energy demands of electricity, heat and fuel per tonne of product CO2, respectively; GWel, GWth, and GWfuel are the GHG emission factors for generation of electricity and heat, and for supply and combustion of fuels, respectively; and GWtransport are the GHG emissions for CO2 transport per tonne of product CO2. The “minus one” in eq 2 comes from the comparison with the status quo (no CO2 capture) and corresponds to a credit for the ‘CO2 otherwise emitted’. The following, positive terms in eq 2 correspond to indirect CO2 emissions: CO2 emissions for electricity, heat supply and transport are directly added. Indirect CO2 emissions from heat supplied by fuel combustion include both the CO2 emissions from supply of the fuel and from fuel combustion. Interestingly, the marginal CO2 emissions do not change whether or not CO2 emissions from fuel combustion are C

DOI: 10.1021/acs.est.5b03474 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Table 1. Energy Demands for CO2 Capture from Alternative CO2 (Point) Sources in Europe Including CO2 Compression to at Least 10 MPa (up to 15 MPa)a average energy demand/(GJ (t CO2,product)−1) type of CO2 source

CO2 concentration

electricity

air NGCC power plant refineries and steam cracker coal power plant integrated pulp and paper mills market pulp mills iron and steel cement IGCC power plant ammonia ethylene oxide gas processing hydrogen

400 ppm 3−4% 3−13% 12−15% 7−20% 7−20% 17−35% 14−33% 1/40%b ≈100% ≈100% ≈100% ≈100%

1.29 1.60 0.91 1.22 0.04 1.03 0.87 0.09 0.61 0.40 0.40 0.40 0.35

heat

natural gas

coal

4.19 3.16 1.57 0.95 3.35 0.81 0.01 0.01 0.01

refs 14−16,40,41,43 20,44,45 8,10,20 20,44,46−49 18,50−52 18,50,51,53 8,10,20 10,20,38,39,54 44,55,56 8,20 8,20 8,20 8,20,44

a

A detailed table with resolved values for the energy demands from the individual literature references is provided in the SI. bCO2 concentration before/after water gas shift reaction.

Figure 2. Distribution of CO2 point sources (>0.1 Mt a−1) in 2011 in a) Europe and b) Germany as exemplary country. The CO2 emissions map has been created using the PowerMap Preview Plugin for Microsoft Excel 2013.59 Color online.

captured: If these CO2 emissions are captured, they reduce the share of flue gas CO2 in the product CO2. Thus, less flue-gas CO2 is captured per product CO2. For example, in some proposed air-capture systems only 50−70% of the total captured CO2 origins from the atmosphere and the remaining 30−50% from combustion of natural gas.15,41 For DAC, the marginal CO2 emissions are the impacts of the DAC system: balancing the CO2 input from ambient air and the direct and indirect CO2 emissions from the capture process also yields eq 2. DAC units are assumed to be located next to the CO2 utilization plants making CO2 transport negligible, i.e., we set GWtransport = 0 for DAC (see SI). The marginal fossil resource depletion (in t oil-eq (t CO2,product)−1) can be calculated in the same way as the marginal CO2 emissions:

For fossil depletion, there is no credit; instead we have to expect an increased fossil demand due to CO2 capture. In the following sections, we present the collected data for energy demands (Wel, Qth/fuel) and environmental impact factors (GWj, FDj). Energy Demand for CO2 Capture. We have collected energy demands for CO2 capture from alternative CO2 sources from literature. We divide the type of energy demands into electricity, heat, natural gas and coal. The CO2 sources considered in this work include the atmosphere, IGCC, NGCC, and coal power plants, refineries and steam crackers, (pulp and) paper mills, production of iron and steel, cement, ammonia (excluding urea production), ethylene oxide, natural gas and hydrogen. Table 1 lists the average energy demand for CO2 capture for each CO2 source. A detailed table with resolved values for the energy demands from the individual literature references is provided in the SI. For details of the considered CO2 sources

mresources,marginal = Wel FDel + Q th FDth + Q fuel FDfuel (+FDtransport )

(3) D

DOI: 10.1021/acs.est.5b03474 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Identification of CO2 Oases. CO2 sources are ideally selected by a detailed sink-source matching18 where CO2 utilization sites (=sinks) must be known. However, since future CO2 utilization sites are unknown, it is first of all desirable to identify which CO2 utilization locations would be environmentally favorable. Environmentally favorable CO2 utilization sites are typically characterized by the availability of “clean” resources for energy supply and for the feedstock CO2. The “CO2 deserts” approach can identify locations with spatially near CO2 sources. Thereby, transport distances and associated impacts are minimized. However, impacts from CO2 capture are not considered, which can vary greatly for alternative CO2 sources. In fact, it might be environmentally more favorable to obtain CO2 from farther but “cleaner” CO2 sources. Therefore, we present an approach to identify environmentally favorable locations for CO2 utilization with minimal environmental impacts of CO2 supply, so-called CO2 oases. The procedure for identifying CO2 oases is similar to the CO2 deserts. Instead of selecting the closest CO2 sources, CO2 sources are selected according to a local EMO curve. Precisely, the following steps are performed for each point within a 0.1 × 0.1° latitude-longitude grid (about 10 × 10 km) in Europe: We compute the actual distances from the considered grid point to all CO2 sources. For all CO2 sources, the transport-associated environmental impacts are computed and added to the environmental impacts of CO2 capture and compression. Together with the potential amounts of CO2 supply, a local EMO curve is obtained. The CO2 sources are selected according to the local EMO curves until a desired CO2 demand is fulfilled. For the selected CO2 sources in each grid point, the average environmental impacts are calculated. Using the MATLAB Mapping Toolbox,61 we illustrate the average environmental impacts for all grid points in the CO2 oases map.

and CO2 capture technologies, the reader is referred to several comprehensive descriptions in the literature3,8−10,36,42 and the references in the SI. Environmental Impacts of Energy Supply. The marginal environmental impacts of CO 2 capture (mCO2−eq,marginal, mresources,marginal) should ideally be determined using marginal environmental impact factors for energy supply (GWj, FDj). For supply of heat and fuels, we believe that marginal environmental impact factors correspond well to the average environmental impact factors. Marginal environmental impact factors for electricity supply, however, are difficult to obtain since a detailed energy systems analysis would be required including a model for the electricity market.23,57 Such detailed assessment of the electricity supply is beyond the scope of this study. We assume average impacts factors also for the electricity grid mix. The grid-mix impact factors would change for large-scale CO2 capture from power plants. However, we assume that only a small number of power plants are retrofitted with CO2 capture. Furthermore, we consider two scenarios as sort of sensitivity analysis: First, we use EUaverage impact factors assuming a common European electricity grid (values given in the SI). Second, we assume country-specific impact factors. For both scenarios, environmental impact factors are obtained from the GaBi LCA database.58 CO2 Emissions Data. We have constructed a database for potential CO2 sources from the E-PRTR v5.1 database for CO2 emissions of individual facilities in Europe.22 The emissions database contains facilities emitting >0.1 Mt a−1 CO2. In total, 1,166 facilities have been classified into the types of CO2 sources shown in Table 1. Figure 2 shows the CO2 emissions by type of CO2 source. Based on the CO2 emissions, we then calculated potential amounts of CO2 supply considering the capture ratio (typically CR = 90%) and additional CO2 emissions from fuel combustion, see Figure 1 and SI for details. The obtained database for potential CO2 sources is in good agreement with a CO2 source database from IEA GHG from 2004.20 Environmental Impacts of CO2 Transport. For CO2 point sources, the environmental impact of CO2 transport depends on the transport distance and the mode of transport. Suggested modes of CO2 transport are pipeline, ship or truck.60 CO2 transport by truck causes GHG emissions of 0.05−0.10 kg CO2-eq/tkm,36,58 by ship 0.01−0.10 kg CO2-eq/tkm,58,60 and by pipeline 0.01−0.02 kg CO2-eq/tkm.58,60 Determining CO2 transport distances requires the locations of CO2 sources and of the CO2 utilization plants. The locations of the CO2 sources are known from the constructed database of potential CO2 sources. However, locations of future CO2 utilization sites are typically unknown. Therefore, we calculated the maximum transport distance for all locations in Europe. For this purpose, we employed the recently presented approach of “CO2 desert” maps by Middleton et al.17 Resulting transport distances are illustrated for various CO2 demands in the SI. For individual locations demanding CO2 of up to 5 Mt a−1, the transport distances for the central part of Europe are mostly below 200 km. The associated CO2 emissions from all kinds of transport are thus mostly below 0.02 t CO2-eq (t CO2,product)−1. Therefore, we first neglect impacts from transport for a first European-wide comparison of CO2 sources. A second regionally refined assessment takes into account actual transport distances and transport by truck as conservative assumption.

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RESULTS AND DISCUSSION Environmental-Merit-Order Curves. First, we present environmental-merit-order curves for a European-wide perspective neglecting impacts from CO2 transport. In this European-wide perspective, two scenarios are considered for the environmental impact factors of energy supply: EU-average and country-specific impact factors. For EU-average impact factors, the EMO curve for impacts on global warming are shown in Figure 3a: the y-axis shows the marginal CO2 emissions for CO2 capture from alternative CO2 sources, whereas the x-axis shows sources’ available CO2 supply amounts. According to eq 2, the marginal CO2 emissions have a baseline at “−1” from the credit for the CO2 otherwise emitted; any other indirect GHG emissions from energy supply are added. The negative values for all cases prove that CO2 capture from all considered sources can reduce GHG emissions at the CO2 source compared to the status quo without capture. However, the CO2 emission reductions vary for the alternative CO2 sources. A general ranking for CO2 reductions from alternative CO2 sources is derived by using EU-average impact factors: the largest reductions arise for CO2 capture from the almost pure sources in hydrogen, ammonia and ethylene oxide production as well as in natural gas processing (0.94 t CO2-eq (t CO2,product)−1). These CO2 savings are in very good agreement with the reported CO2 intensities in Middleton et al.17 CO2 capture in the paper industry reduces CO2 emissions slightly less (0.86 t CO2-eq (t CO2,product)−1. Capturing CO2 from coal, IGCC and NGCC power plants and from iron and steel plants leads to CO2 reductions between 0.78 and 0.83 t E

DOI: 10.1021/acs.est.5b03474 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

Figure 4. Environmental-merit-order curve for marginal fossil resource depletion over the available CO2 supply in Europe using countryspecific impact factors for electricity supply.

CO2 capture from alternative CO2 sources over their available CO2 supply amounts. CO2 capture from alternative CO2 sources leads to an increase of fossil resource depletion between 0.00 and 0.19 t oil-eq (t CO2,product)−1. The EMO for fossil depletion impacts (Figure 4) is very similar to the order regarding impacts on global warming (Figure 3b, Spearman’s rank correlation coefficient of 0.91). Taking into account the uncertainties due to the energy demand (cf. Figure 3a), the EMO for impacts on global warming and fossil resource depletion can be considered as practically nonconflicting. Marginal Fossil Resource Depletion for Avoided CO2 Emissions. Providing CO2 by CO2 capture can reduce CO2 emissions for all considered CO2 sources compared to the status quo (Figure 3). Reducing or, equivalently, avoiding CO2 emissions can also be considered as valuable function from a climate-mitigation point of view. Hence, we define an alternative functional unit as ‘CO2 capture avoiding 1 tonne of CO2 emissions’. The marginal impacts on fossil resource depletion per CO2 avoided over the CO2 supply are shown in the SI. The shown marginal impacts correspond to the marginal fossil resource depletion per product CO2 (Figure 4) divided by the negative marginal CO2 emissions (=CO2 avoided) per product CO2 (Figure 3). Since usually less CO2 is avoided than produced, the marginal fossil resource depletion is larger per avoided CO2 than per product CO2 ranging from 0 to 0.46 t oil-eq (t CO2-eq avoided)−1 for point sources, and from 0.16 to 1.08 t oil-eq (t CO2-eq avoided)−1 for direct air capture (DAC) depending on the country. Selection of CO2 Sources. The selection of CO2 sources according to Figures 3 and 4 depends on the CO2 demand. Currently, about 200 Mt a−1 CO2 are being used worldwide, of which about 110 Mt a−1 are used for production of urea.26 CO2 for urea production is typically obtained from integrated ammonia synthesis. Since the total CO2 demand figure includes urea production, we also include CO2 supply from ammonia synthesis in the following. For the future global CO2 demand, we assume the optimistic CO2 utilization estimate of about 2 Gt a−1 as upper bound as discussed above. Assuming further that about 25% of global CO2 utilization takes place in Europe (Europe’s share of global GDP is 23% and its share of chemical shipments is 29%),63,64 we estimate the European CO2 demand to be about 50 Mt a−1 for today and 500 Mt a−1 at most in the long term.

Figure 3. Environmental-merit-order curve for marginal CO 2 emissions over the available CO2 supply in Europe using a) EUaverage and b) country-specific GHG emission factors for electricity supply. The energy demands for CO2 capture vary between literature sources for each CO2 source (see SI for detailed table) and lead to the error bars shown in a). For country-specific impact factors in b), DAC is not included in the EMO curves for better visualization since the potential amounts of CO2 supply for DAC are generally very large.

CO2-eq (t CO2,product)−1. CO2 capture from refineries and steam crackers is very site-specific and thus, CO2 reductions vary largely between 0.57 and 0.91 t CO2-eq (t CO2,product)−1. Average CO2 reductions for CO2 capture from refineries and steam crackers (0.64 t CO2-eq (t CO2,product)−1) are similar to those in cement production (0.63 t CO2-eq (t CO2,product)−1). Lowest CO2 reductions results from direct air capture (DAC) of CO2 with 0.54 t CO2-eq (t CO2,product)−1. Figure 3b shows the EMO curve for marginal CO2 emissions based on country-specific impact factors for electricity generation. In this case, CO2 capture leads to CO2 reductions from 0.42 to 0.99 t CO2-eq (t CO2,product)−1. Due to the country-specific impact factors, the CO2 reductions vary for identical types of CO2 sources in different countries. Nevertheless, the order of CO2 sources in Figure 3b is similar to Figure 3a (Spearman’s rank correlation coefficient of 0.83). The following discussion is based on the country-specific impact factors. Capture of CO2 from point sources has a total potential for GHG emission reduction of up to 1.1 Gt a−1 CO2-eq. This amount of CO2 emission reductions does not include downstream CO2 emissions from CO2 utilization. Most importantly, CO2 will usually be released at the end of the product life cycle and thus, CO2 utilization is not an overall CO2 sink.6,34,62 For impacts on fossil resource depletion, the EMO curve in Figure 4 illustrates the marginal fossil resources required for F

DOI: 10.1021/acs.est.5b03474 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

Figure 5. CO2 oases maps for Europe showing the maximum average CO2 emission reductions in t CO2-eq (t CO2,product)−1 for CO2 demands of (a) 0.1, (b) 1, (c) 10, (d) 50, (e) 100, and (f) 200 Mt a−1.The line-patterned areas in Scandinavia, France, Spain, and Portugal are regions where direct air capture (DAC) is (partially) employed.

Even for the future demand of up to 500 Mt a−1, only a few CO2 sources are needed to satisfy the demand. Based on average environmental impacts, CO2 should be captured only from the almost pure CO2 sources (hydrogen, gas processing, ethylene oxide, ammonia), and then from the paper industry, coal, IGCC and NGCC power plants, and iron and steel plants. These CO2 sources alone can already supply both today’s and the future CO2 demand. However, the CO2 supply amounts and thus the EMO curves will most likely change over time: On the one hand, renewable energies are expected to substitute a large share of fossil-fueled power plants. Thereby, the potential CO2 supply by power plants is expected to decrease to about 230 Mt a−1 until 2050.65 On the other hand, new large-scale concentrated CO2 point sources such as ethanol plants and biogas fermentation will probably emerge or increase.21,66−68 From this first simple analysis of the European-wide EMO curves, it follows that DAC is not needed in the near to midterm future. DAC has the lowest overall CO2 emission reductions in Figure 3a. For CO2 point sources, the lowest CO2 emission reductions are found for CO2 capture from cement plants which is still favorable compared to DAC. However, CO2 from cement plants has to be transported to the utilization site which causes CO2 emissions. DAC can be beneficial if CO2 from cement plants has to be transported for more than 1000 km by truck or ship or 4000 km by pipeline. Thus, DAC might be environmentally favorable in locations far away from largescale point sources. Whether and where DAC should be implemented can be analyzed in detail by the CO2-oases approach. European CO2 Oases. The European-wide analysis in Figures 3 and 4 can be refined to provide a regional perspective for the selection of CO2 sources for individual CO2 utilization sites. CO2 oases are locations with minimal environmental impacts for CO2 supply. So-called CO2 oases are shown in Figure 5 for global warming impacts and CO2 demands between 0.1 and 200 Mt a−1. The CO2 oases are generally located in the center of Europe: here, CO2 capture leads to the largest CO2 reductions. Although the CO2 oases maps might look similar to the CO2 deserts maps (see SI), the CO2 oases approaches selects environmentally most favorable CO2 sources. Thereby, the CO2 oases approach leads to a larger reduction of CO2 emissions of about 9 to 19% compared to

selecting the nearest CO2 sources in the CO2 deserts approach (see SI for details). The presented approach for identifying CO2 oases does not account for competition of CO 2 : computing the CO 2 reductions for one potential CO2 utilization location is independent from all other CO2 utilization sites. However, even if 50 Mt a−1 CO2 (approximately the European CO2 demand) are used in a single and remote location, CO2 can still be provided with quite large average CO2 reductions of 0.70− 0.90 t CO2-eq (t CO2,product)−1 including emissions from transport. Furthermore, CO2 capture and transport from point sources is still favorable over DAC. Only for large CO2 demands above 50 Mt a−1 in a single location, DAC should be implemented in countries with few CO2 sources and lowimpact electricity generation (Norway, Sweden, Finland, Spain, Portugal and France; line-patterned areas in Figure 5d−f). The CO2 oases approach can identify environmentally favorable locations for CO2 supply and thus CO2 utilization. In summary, the potential CO2 supply of the considered CO2 sources is much larger than the estimated long-term CO2 demand in Europe. Thus, only the environmentally most favorable CO2 sources should be selected. For this purpose, EMO curves have been presented for impacts on global warming and fossil resource depletion. The EMO curves show that CO2 capture from any CO2 source can reduce CO2 emissions at the expense of an increased depletion of fossil resources. Minimal environmental impacts can be achieved by capturing CO2 from almost pure CO2 sources such as chemical plants and natural gas processing, and then from power plants and iron and steel plants. Direct air capture (DAC) of CO2 generally causes highest fossil resource depletion at lowest CO2 emission reductions. Nevertheless, when the CO2 demand will be large enough, DAC should be employed in countries with few CO2 sources and low-impact electricity generation. Favorable locations for CO2 capture and utilization have been identified in the center of Europe using the presented CO2 oases approach. Several LCA studies33,34,62,69−71 have shown that CO2 capture and utilization (CCU) can reduce greenhouse gas emissions and fossil fuel use. Typically important prerequisites for CCU to reduce environmental impacts are availability and use of clean energy and clean feedstocks, including feedstock G

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Environmental Science & Technology CO2.6,62 Regarding the latter, the presented EMO curves and CO2 oases provide a simple, yet effective way of selecting environmentally favorable CO2 sources for CO2 utilization.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b03474. A detailed derivation of the presented formulas for marginal environmental impacts; a detailed list of all collected energy demands for CO2 capture from the literature; EU-average environmental impact factors for energy supply; European “CO2 desert” maps for maximum transport distances; and average CO2 emission reductions for CO2 deserts and oases (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +49 (0)241 80 953 80; fax: +149 (0)241 80 922 55; email: [email protected] . Present Addresses ‡

(A.S.) Fraunhofer-Institut für Solare Energiesysteme (ISE), Freiburg, Germany § (P.V.) Currenta GmbH & Co. OHG, Leverkusen, Germany Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the German Federal Ministry of Education and Research (BMBF) (Grant 033RC1104B). We also thank three anonymous reviewers for their valuable comments and recommendations that significantly helped to improve this work.

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REFERENCES

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