Life Cycle Investigation of CO2 Recovery and Sequestration

May 6, 2006 - The Life Cycle Assessment of four CO2 recovery technologies, combined with nine CO2 sequestration systems, serves to expand the debate o...
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Environ. Sci. Technol. 2006, 40, 4016-4024

Life Cycle Investigation of CO2 Recovery and Sequestration HSIEN H. KHOO* AND REGINALD B. H. TAN Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, Singapore 627833

The Life Cycle Assessment of four CO2 recovery technologies, combined with nine CO2 sequestration systems, serves to expand the debate of CO2 mitigation methods beyond a single issuesprevention of global warmingsto a wider range of environmental concerns: resource depletion, acidic and toxic gases, wastes, etc, so that the overall, and unexpected, environmental impacts may be revealed.

1. Introduction The increase of carbon dioxide (CO2) levels in the atmosphere has spurred worldwide concerns of potential global climate change among international organizations, governments, and environmental scientists (1). Global warming or climate change is mainly caused by the burning of fossil fuels to meet worldwide energy demands (2). The World Energy Outlook has projected that, given the present trend in industrial development, worldwide energy use will grow by 1.7% annually from 2004 until the year 2030, which is an overwhelming 58% increase (3). Many types of methods are currently being investigated to reduce the amount of CO2 escaping from the power plant’s flue gas into the atmosphere. Among those discussed here are post combustion capture technologies, and ocean and geological sequestration (4-5).

2. Layout In the next three sections, coal-fired power and CO2 recovery methods will be presented followed by the introduction of CO2 sequestration. In Section 6, life cycle assessment or LCA will be used as powerful tool to analyze the CO2 fixation technologies. In Stage 1, LCA is used to study a coal-fired power plant, starting from coal mining, transportation, and ending with the final generation of electricity. In Stage 2, an LCA investigation is carried out for four types of CO2 recovery technologies. In Stage 3, LCA will be performed to compare various CO2 sequestration options. The life cycle impact assessment results and interpretations/discussions will be presented in Section 7. In Section 8, CO2 sequestration effectiveness is carried out. The paper then ends with some further discussions (Section 9).

3. Coal-Fired Electricity Generation In the U.S. alone, over 1.6 billion tons of CO2 is produced each year from power plants (2). A 1000 MW pulverized coalfired power plant can emit up to 6-8 Mt of CO2 annually, an oil-fired power plant emits about 25% less, and a natural gas combined cycle power plant emits about half of the CO2 emissions that come from coal-powered plants (4). Accordingly, coal-based electricity is selected as the prime energy * Corresponding author phone: (65) 6796-3952; fax: (65) 68734805; e-mail: [email protected]. 4016

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provider for the various types of technologies or systems investigated throughout the paper.

4. CO2 Recovery Basic descriptions of four postcombustion capture technologies are presented in this section. 4.1. Chemical Absorption. Chemical absorption of CO2 by the use of solvents is the most well-established method of CO2 capture in power plants. In this process, the CO2 from the flue gas reacts with a chemical solvent to form a compound which is then broken down by the application of heat and regeneration. Typical solvents are monoethanolamine (MEA), diethanolamine, and potassium carbonate (6). CO2 recovery rates of 95-98% can be achieved by using amines (7). Chemical absorption processes need heat for regeneration. The energy demands are estimated to be 330340 kWh per ton CO2 recovered; these values are for both heat requirements and solvent regeneration (8-9). Since the solvent is completely recycled in the process, the only emissions generated in this technique are those caused by energy use. 4.2. Membrane Separation. This physical process allows CO2 to pass through the membrane wall while excluding the other parts of the flue gas (1). Commercially available polymeric gas separation membranes are mostly used with energy demands of 70-75 kWh per ton of recovered CO2 (9). Typical removal rates are 82-88% of CO2 from the power plants’ flue gases (10-11). The main air pollution generated from this technology is from energy use. 4.3. Cryogenics. Cryogenic fractionation can separate CO2 from other gases using pressure and temperature control. In a cryogenic separation system, CO2 is physically separated from other gases by condensing it at an extremely low temperature. The amount of CO2 recovered is approximately 90-95% of the flue gas. The energy requirements are estimated to be 600-660 kWh per ton of CO2 recovered as a liquid form (9). The main air pollution generated from this technology is from energy use. 4.4. Pressure Swing Adsorption. Some materials with high surface areas, such as zeolites and activated carbon, can separate CO2 from gas mixtures by physical adsorption. An example of this application is pressure swing adsorption (PSA), which is a commercially available technology for recovering CO2 from power plants (1). The recovery of the CO2 gas can be in the range of 85-90% with energy demands from 160 to 180 kWh/ton CO2 recovered (9, 12). The only emissions generated in this technique are those caused by energy use.

5. CO2 Sequestration After recovering the CO2 gas, it must be stored somewhere to prevent it from appearing in the atmosphere. One way to achieve this is by CO2 sequestration. For ocean sequestration, six case studies are presented. They are known as vertical injection, inclined pipeline, pipe towed by ship, dry ice, gaslift advanced dissolution (GLAD) system, and CO2-hydrate. Next, geological sequestration with enhanced oil recovery (EOR) and geological sequestration with enhanced coalbed methane (ECBM) recovery will be explored. Finally, the sequestration of CO2 in a saline aquifer will be presented. 5.1. Ocean Sequestration. It was suggested by several scientists (4, 13-14) that the ocean is the largest buffer to “dump” and store CO2. It was estimated that the ocean already contains an estimated 40 000 GtC (billion tons of carbon) compared with 750 GtC in the atmosphere and 2200 GtC in 10.1021/es051882a CCC: $33.50

 2006 American Chemical Society Published on Web 05/06/2006

the terrestrial biosphere. As a result, the amount of carbon that would cause a doubling of the atmospheric concentration would change the ocean concentration by less than 2% (2). CO2 gas is constantly being exchanged between the ocean and the atmosphere, and therefore, questions arise as to how effective the ocean will be as a choice to store CO2. Herzog et al. (15) projected, through scientific experiments, that the amount of time over which the percentage of the injected CO2 would be sequestered permanently would depend largely on the injection or disposal depths. It has been estimated that at depths of 1500 m, 2000 m, 2500 m, and 3000 m, approximately 74%, 81%, 83%, and 90% of CO2 respectively will remain stored or completely dissolved in the ocean for at least 500 years. Several options have been proposed for the large-scale CO2 ocean sequestration. 5.1.1. Vertical Injection. In the first option, the injection of CO2 into the ocean depths of 3000 m from a vertical pipe hanging from a floating platform is introduced (14). First, CO2 gas is recovered from the power plant flue gases and liquefied, after which it is transported by an ocean tanker for a distance of 100 km to a floating platform. From there, a vertical pipe is used to inject liquid-CO2 directly into the ocean. At depths of 3000 m, 90% of the CO2 is expected to be stored for at least 500 years (15). Technological and engineering challenges faced for transporting liquid CO2 to the ocean sites have been discussed by Nihous (16). The energy required for CO2 liquefaction by the use of a five-stage intercooled compression unit is estimated to be 120 kWh per ton CO2 (9). Energy requirements for compression and injection from the floating platform are estimated to be about 40-50 kWh per ton CO2 (17). 5.1.2. Inclined Pipe. In the second option, compressed CO2 is pumped into a depth of 2000 m into the oceans via a long inclined pipe (18). At this depth, it is estimated that 81% of the gas will remain sequestered (15). Compression of CO2 pipeline transportationsfor distances of 250-500 km from the power plant to the offshore sitescan be up to 100 kWh/t of CO2 (19). Re-compression is required for the final injection, which requires up to 30-40 kWh/ton CO2 (17). 5.1.3. Pipe Towed by Ship. In the third case, liquefied CO2 is loaded onto a ship or ocean tanker, transported for an estimated distance of 300 km, and then injected into the ocean via a pipe suspended from the ship (20). The estimated energy for compression and injection from the ship is roughly 25-30 kWh per ton CO2 (17). 5.1.4. Dry Ice. Solid CO2 or dry ice blocks are disposed into the ocean from a moving ship. Solid CO2 has a specific gravity of 1.5 and will readily sink (13). The process for making dry ice (sublimation) takes up twice the energy of that required for CO2 liquefaction (21). In this ocean sequestration system, the estimated travel distance of the tanker is 300 km, where the CO2 blocks are assumed to reach complete dissolution at depths of 3000 m (22). Preliminary tests have shown that the CO2 blocks would fall through the water and slowly dissolve on the sea floor (23). 5.1.5. GLAD. The fifth carbon ocean sequestration method involves the sequestration of low purity CO2 gas. After the CO2 gas is recovered from the power plant, it is passed directly to a gas-lift pump system, named gas lift advanced dissolution or GLAD. The GLAD system first dissolves the CO2 into seawater at a relatively shallow depth of 200-300 m and then transports CO2-rich seawater to depths of 1000-3000 m (24). An advantage of the GLAD method is that it bypasses the need to liquefy CO2. The energy requirement for the compression for the GLAD system is 3.7 kWh per ton CO2 (25). It is assumed for this case that the CO2 gas reaches complete dissolution at an average depth of 1500m. 5.1.6. CO2-Hydrate. In the last ocean sequestration system, liquid-CO2 is transported by pipe to a hydrate reactor and

injected as hydrates into the ocean at depths of 1000-1500 m (26). The CO2-hydrates will sink and is assumed to reach complete dissolution at a depth of 2500m. The estimated energy requirements for the piping, hydrate reactor, and injection system is 30 kWh/ton (17, 27). 5.2. Geological Sequestration. In geologic sequestration, CO2 is injected into underground reservoirs where it is expected to be isolated from the atmosphere for several hundred years (28). Three cases will be presented here. 5.2.1. EOR. Geologic CO2 sequestration with EOR is a proven technology (5). Under supercritical conditions, CO2 acts as a powerful solvent that can be used to increase oil recovery (29). EOR projects are already ongoing in the U.S., where the source of CO2 is transported by pipeline from natural CO2 reservoirs (28). EOR is yet to be applied where the source of CO2 is from electricity generation. A Norwegian case study is investigated to do this. In the case study, CO2 is first captured from the flue gas of existing coal-fired power system and sequestered geologically in conjunction with EOR in the North Sea (30-31). A pipeline, 682 km in length, is used to deliver supercritical CO2 from a coal-fired power plant to the Gullfaks oil field. For this case, it was assumed that steel pipe engineering technology exists to allow the long-distance transportation of CO2 (32). In the proposed CO2-EOR project, the energy requirement for long distance pipeline transportation is estimated to be 130 kWh/ton, and recompression and injection, 7-9 kWh/ ton (31, 33). Stevens et al. (34) estimated that for current EOR projects, up to 10% of CO2 injected is released to the atmosphere. The recovery of oil is taken to be 0.18 ton of oil for every ton CO2 sequestered (29), and the oil recovery process itself requires approximately 94 kWh/ton of oil recovered (33). 5.2.2. ECBM. Deep unmineable coal formations provide an opportunity to both sequester anthropogenic CO2 and at the same time increase the production of methane. The ECBM case study is taken from Tamabayashi et al. (35), where the Chikuhou coalfield in Kyushu, Japan, is identified as a potential area for coalseam CO2 sequestration. The recovered CO2 gas is transported by pipeline to the injection site. During the injection of CO2, methane or natural gas is recovered. It was estimated that compression and pipeline transportation requires 100 kWh/ton CO2 and injection requires 5-6 kWh/ ton (36). These data agree closely with those reported for ECBM studies carried out in the U.S. (33). The production of natural gas requires approximately 38 kWh/ton (33). And the average ratio of CO2-to-gas recovery is taken as 3:1 (37). The leakage rate which is considered “safe and acceptable” for the underground storage of CO2 was estimated to be 0.01% per year (38). This means that for a sequestration period of 500 years, a total of 5% leakage is expected. 5.2.3. Sleipner. In the final case, the Sleipner project in the North Sea is presented as the world’s first industrialscale storage of CO2 in an underground reservoir. In the Sleipner project CO2 gas is being injected 1000 m below sea level into a saline aquifer known as the Utsira Formation (39). The main difference between the Sleipner project and the other two geological sequestration methods is that CO2 is extracted as a byproduct from natural gas production. As for EOR and ECBM, the source of CO2 is from electricity generation, with the recovery of natural resources taking place during the process of sequestering CO2. The system for the investigation starts with the extraction of CO2 from natural gas via amine scrubbing and ends with the final injection or disposal of CO2 into the saline aquifer, where the gas is expected to stay stored for at least 500 years without leakage (40). The amine scrubbing process for the extraction of CO2 from the natural gas is estimated to be VOL. 40, NO. 12, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. LCA Methodology. about 20% less than the same system used to remove CO2 from power plant flue gases, that is, 240 kWh/ton (8). 5.3. Investigation of CO2 Sequestration. Many feasibility and assessment studies pertaining to CO2 sequestration methods have been performed. Initial investigations focused on costs or economical modeling of CO2 removal systems (41-42). Others discussed various types of CO2 transportation methods and the design of pipes suitable for deep ocean injection (16-18). The costs and technology applied for EOR and ECBM projects have also been reported (28-29). Most studies also covered economical feasibility, safety, and social issues concerning geological sequestration (5, 33). This paper will be the first of its kind to perform a complete life cycle assessment study on the nine CO2 sequestration options.

6. Life Cycle Assessment Due to the different characteristics of all three stages, a systematic and holistic approach to investigate and evaluate the pollution generated from each stage is called for. Life cycle assessment or LCA is used for this very purpose. LCA is a scientific and technically oriented assessment tool that can help to broaden the environmental management perspective by offering a system’s point of view. The power of LCA is that it expands the debate on environmental concerns beyond a single issue (global warming) to a broad range of environmental issues (human toxicity, ecotoxicity, wastes, etc.). We (43-44) have successfully applied LCA in various case studies for comparing and identifying the most environmentally suitable strategy, the best practicable environmental action, or alternative combination of processes/ technologies. 6.1. LCA Goal and Scope. This work will be the first to investigate all three stages, thereby linking the “CO2 route” from its source (flue gas) to its final destination (storage area). The overall system boundary is illustrated in Figure 1. First, LCA is performed on the three separate stages as isolated components (sub-systems), and next as an undivided single chain of processes (whole system). Stage 1 starts with coal mining and ends with the final amount of electricity produced. The inventory data was gathered from coal-fired power plants operating in the U.S. (45). Stage 2 begins with the amount of CO2 emissions entering the system due to the generation of 1 MWh (functional unit) from a coal-fired power plant, and ends with the final CO2 recovered. Stage 3 begins with the same amount of CO2 entering the system -estimated as 950-kg CO2 per MWh and 4018

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ends with the final amount of CO2 sequestered or stored. For the Sleipner project, the functional unit is taken as 950-kg CO2 generated from the processing of natural gas. 6.2. Inventory Analysis. The inputs and outputs of a welldefined system are systematically identified and quantified. These input-output flows are then assessed in terms of their potential to contribute to specific environmental impacts. 6.3. Impact Assessment. The SimaPro EDIP 97 method for impact assessment is used to analyze the following eight environmental impact categories: global warming potential (GWP), acidification, human toxicity to air, human toxicity to water, eutrophication, ecotoxicity, wastes, and fossil fuels. The EDIP is a problem-oriented (midpoint) method which is widely used and highly recognized by many LCA experts (46). In an ideal investigation, the LCIA should include the adverse impacts on ocean marine life due to the accumulation of CO2. However, this particular environmental impact category is yet to be developed in the EDIP (46). Impact of marine life or any other types of benthic lifeforms due to the increase of CO2 concentrations in the ocean are not included in the LCA investigation. 6.4. Interpretation. The interpretation of the LCA study can be carried out in various forms. In the next section, the results of the eight impact categories will be presented and discussed. Further interpretations are made based on the generation of the final scores and sensitivity analysis.

7. Results and Discussions In the results, the amount of CO2 generated from the coalfired power plant (per MWh), as well as the Sleipner project, is taken to be 950 kg. The CO2 removal efficiencies of 95% (chemical absorption), 82% (membrane separation), 90% (cryogenics), and 85% (PSA), are employed. 7.1. Results for CO2 Recovery Methods. Due to the size and complexity of the studies, the impact assessment results for the four CO2 recovery technologies will be compiled in Table 1. For GWP, the most promising system for CO2 postcombustion recovery stems from the highest efficiency of the greenhouse gas that can be captured from the power plant, combined with reasonable energy demands. In this case, chemical absorption using MEA, followed by PSA. Although Cryogenics technology is capable of recovering a large amount (90%) of CO2 from the power plant, its large energy consumption (600 kWh/ton CO2 recovered) resulted in additional greenhouse gas emissions.

TABLE 1. Impact Assessment Results for CO2 Recovery Technologies CO2 recovery technologies environmental impact categories GWP (g-CO2-eq) acidification (g-SO2-eq) human toxicity - Air (m3/g) human toxicity - water (m3/g) eutrophication (g-NO3-eq) ecotoxicity (m3/g) wastes ((kg/kg) resources (kg/kg)

chemical absorption

membrane separation

7.87 × 3.42 × 102 7.24 × 104 4.02 × 10-4 1.53 × 103 2.20 × 10-2 6.49 × 100 1.65 × 10-3

1.86 × 7.25 × 101 1.54 × 104 8.53 × 10-5 3.25 × 102 4.66 × 10-3 1.38 × 100 3.51 × 10-4

104

105

cryogenics 1.79 × 6.21 × 102 1.32 × 105 7.31 × 10-4 2.79 × 103 3.99 × 10-2 1.18 × 101 3.01 × 10-3 105

pressure swing adsorption 1.72 × 105 1.66 × 102 3.51 × 104 1.95 × 10-4 7.44 × 102 1.06 × 10-2 3.15 × 100 8.02 × 10-4

FIGURE 2. Total global warming potential results for ocean and geological sequestration. While the purpose of the postcombustion technologies is to reduce CO2 emissions to the atmosphere, there are a series of air and water emissions that comes along with the processes. Air emissions containing acidic gases contribute to acidification; whereas heavy metals, such as arsenic, lead, and mercury, contribute to human toxicity (air). For the acidification impact category, the results were calculated according to the regulation of 90% removal of SOx and NOx from the coal-fired power plant (47). As for human toxicity to air, the impact results were generated after the regulation of the removal of 95% of heavy metals from the power plant flue gas (48). The highest results for the acidification and human toxicity to air impacts are displayed first by cryogenics, second bychemical absorption, and followed by PSA. Eutrophication is caused by the accumulation of nitrates, ammonia, and cyanides, as well as air emissions of N2O and NOx. Wastewater containing acids and sulfides contributes to ecotoxicity (water acute). Wastes and resource depletion are another two environmental concerns caused by the burning of fossil fuels. The rest of the impacts display the same trend: the higher the demand for energy, the higher the impact. Driven by the need to reduce greenhouse gases, further developments will be carried out to capture CO2 effectively, while imposing lighter energy and waste penalties for these types of post combustion recovery systems (11). 7.2. Results for CO2 Sequestration Options. The results for comparing the five ocean and two geological sequestration options are shown in Figures 2 (GWP), 3 (acidification), 4 (human toxicity, air), 5 (human toxicity, water), 6 (eutrophication), 7 (ecotoxicity), 8 (wastes), and 9 (resources).

7.2.1. Global Warming Potential. Intuitively, the Sleipner project will offer the highest potential for CO2 sequestration. The safe storage of CO2 in the Utsira formation is depicted in biggest inverted peak in the GWP graph (Figure 2). This is followed by geological sequestration with ECBM. A significant amount of CO2 sequestered (negative peaks), with reasonable environmental impacts (positive peaks) is also displayed by geological sequestration with EOR. For ocean sequestration, vertical injection appears to be the most promising option in terms of both the final amount of CO2 stored and amount of energy spent in the sequestration process. Dry ice also offers a high percentage of the final CO2 stored, however, the sublimation process involved imposes a large energy penalty, which adds unnecessarily to GWP. For these two options, the final destination for CO2 storage is at depths of 3000 m. At this depth, 90% of the gas is expected to be trapped for 500 years (15). Two other viable options are CO2-hydrates and inclined pipeline, which offers 83% and 81% sequestration potentials, respectively. For both the pipe towed by ship method and GLAD, the amount of potential CO2 leakage from the ocean to the atmosphere is the highest. The disposal depth for these two options is 1500 m, where the leakage rate is about 26% (15). The GLAD system does not have the potential to store large amounts of CO2, however, it offers an advantage of requiring very minimal energy usage (25). Compared to the other four ocean sequestration options, the GLAD system itself hardly poses any environmental damage. 7.2.2. Acidification. The acidifcation results are displayed in Figure 3. The results shown are according to the regulation VOL. 40, NO. 12, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Total acidification results for ocean and geological sequestration.

FIGURE 4. Total human toxicity to air results for ocean and geological sequestration. of 90% removal of acidic gases from the power plant (47). The large environmental impacts caused by ocean tanker transportation can be observed very clearly in the graphs. The acidic gases generated due to pipeline transportation are very small compared to those generated by the ocean tankers. The environmental impacts due to the liquefaction and sublimation processes are moderate in this impact category. 7.2.3. Human Toxicity. The environmental impacts of human toxicity to air and to water are displayed in Figures 4 and 5, respectively. For human toxicity to air, the environmental impact results were generated after the removal of 95% of heavy metals from the power plant (48). The graphs displayed by both human toxicity results exhibit the same trend. Significant environmental impacts are most evidently shown by the amine scrubbing process for Sleipner 4020

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and the dry ice ocean sequestration option. Other observable impacts are from the process of CO2 liquefaction for vertical injection, pipe towed by ship, and CO2 hydrate formation, as well as pipeline transportation for inclined pipeline, EOR, and ECBM. 7.2.4. Eutrophication and Ecotoxicity. The results of eutrophication and ecotoxicity are displayed in Figures 6 and 7, respectively. As expected, the amine scrubbing (Sleipner) and CO2 sublimation process (dry ice) both contribute most significantly to the graphs. The CO2 liquefaction process and pipeline transportation both generate relatively large amounts of wastewater from the power plant due to substantial energy demands: 120 kWh/ton CO2 for liquefaction and about an average amount of 122 kWh/ton for long distance pipeline transportation of CO2 (9, 17). As for the GLAD, much less

FIGURE 5. Total human toxicity to water results for ocean and geological sequestration.

FIGURE 6. Total eutrophication results for ocean and geological sequestration. energy is required for dissolution; hence, leading to nearly negligible environmental impacts. 7.2.5. Wastes and Resources. One of the biggest environmental concerns of coal-fired power plants is the generation of significant amounts of wastes. The solid wastes results are displayed in Figure 8. The highest two cases are dry ice and Sleipner, and the lowest is displayed by GLAD. It must be highlighted that, in all nine cases, it is assumed that the source of energy for the chain of processes involved in CO2 storage or sequestration is from a coal-fired power plant (45). The resource results for ocean and geological sequestration are displayed in Figure 9. The positive peaks exhibit the energy demands (resource consumed) for the sequestration systems, accumulated from CO2 liquefaction process, transportation, compression, etc. The negative peaks demonstrate

the potential amount of resources gained from the EOR and ECBM geological sequestration technologies. The inverted peaks are greater for ECBM due to the higher ratio of methane recovered as compared to the recovery of oil in EOR. Among the many solutions that contribute toward CO2 mitigation, geological sequestration seems to be a promising path that presents the advantage of being able to cope with large volumes of anthropogenic CO2 at stake, while fulfilling the growing energy demands of today’s society. For Sleipner, the results do not include the amount of natural gas produced. This is because for EOR and ECBM, the methods employed to sequester CO2 themselves generate the recovery of oil and gas. Whereas in the Sleipner case study, the LCA system boundary starts with the production of CO2 (as a byproduct) from the process of extracting and VOL. 40, NO. 12, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 7. Total ecotoxicity results for ocean and geological sequestration.

FIGURE 8. Total waste results for ocean and geological sequestration. selling natural gas. 7.3. Final Scores. The results for the potential environmental impacts (GWP, acidification, human toxicity, etc) for the four CO2 recovery technologies and a total of nine sequestration systems were projected individually and separately. In this manner, no “overall verdict” can be reached. To make overall comparisons, a single final score for each combination of options, as an undivided series of processes, must be attained. To do this, the impact assessment results will have to include the normalization and weighting stages, which are provided by SimaPro (46). The final scores are displayed in Table 2. The scores shown are totaled from the accumulation of the eight environmental categories, starting with the generation of 950 kg CO2 (per MWh from the power plant or from Sleipner process), to necessary processes involved in the sequestration methods, 4022

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and ending with the final amount of CO2 stored in the ocean or underground. From Table 2, the “best” negative scores (least environmental burdens) stem predominantly from the three geological sequestration methods, especially Sleipner. For this project, the sequestration of CO2 in the Utsira formation promises zero leakage for at least 500 years (40). The accumulated negative values for both EOR and ECBM methods are not only from the prevention of GWP, but also due to the prevention of resource depletion. The most promising environmental benefit stems from employing ECBM combined with chemical absorption (95%-98% CO2 recovery). The next three highest benefits also stems from geological sequestration, EOR with chemical absorption, and ECBM combined with membrane separation and with PSA.

FIGURE 9. Total resource results for ocean and geological sequestration.

TABLE 2. Final Scores Combining CO2 Recovery and Sequestration Systems ocean CO2 recovery technology and recovery rate chemical absorption membrane separation

vertical injection

inclined pipeline

pipe towed by ship

dry ice

GLAD

CO2 hydrate

with EOR

with ECBM

Sleipner (aquifer storage)

95% 98% 82% 88% 90%

-0.06 -0.07 -0.04 -0.06 -0.03

-0.05 -0.05 -0.03 -0.05 -0.01

-0.01 -0.02 0.00 -0.01 0.01

-0.05 -0.05 -0.03 -0.04 -0.01

-0.04 -0.04 -0.02 -0.04 0.00

-0.03 -0.04 -0.02 -0.03 -0.02

-0.08 -0.09 -0.06 -0.07 -0.04

-0.10 -0.11 -0.08 -0.10 -0.06

-0.13 -0.13 -0.12 -0.12 -0.12

95% 85%

-0.05 -0.04

-0.03 -0.03

0.01 0.00

-0.02 -0.03

-0.01 -0.02

-0.03 -0.02

-0.06 -0.06

-0.08 -0.08

-0.13 -0.12

90%

-0.06

-0.04

-0.01

-0.04

-0.03

-0.03

-0.07

-0.09

-0.12

cryogenics pressure swing adsorption

geological

As for ocean sequestration, the highest benefit comes from the chemical absorption technology combined with vertical injection. Reasonable (negative) scores are also demonstrated by any combination of CO2 removal with vertical injection. The second most feasible options are by inclined pipeline and dry ice disposal, both combined with chemical absorption for CO2 recovery. The “worst cases” are displayed by combining any CO2 removal methods with pipe towed by ship. Most of the efforts taken for preventing global warming are “suppressed” by the generation of other environmental burdens. With the exception of using cryogenics to remove CO2 from the power plant, all the final scores for the GLAD option display rather small environmental benefits.

Supporting Information Available Further details of the LCA study, including the flow diagrams for the case studies, system boundaries, LCI data, assumptions, and estimations. Sequestration effectiveness and sensitivity analysis for comparing: (i) power plant CO2 emissions of 950, 970, and 990 kg-CO2 per MWh; (ii) Different EDIP weights: medium, low, and high, and (iii) Comparison between the EDIP and eco-indicator. Further discussions pertaining to the strengths and weaknesses of CO2 sequestration is also provided. This material is available free of

charge via the Internet at http://pubs.acs.org.

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(22)

(23) (24) (25) (26)

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Received for review September 23, 2005. Revised manuscript received March 29, 2006. Accepted March 30, 2006. ES051882A