Environmental Impact Evaluation of Conventional Fossil Fuel

1914

Energy & Fuels 2006, 20, 1914-1924

Environmental Impact Evaluation of Conventional Fossil Fuel Production (Oil and Natural Gas) and Enhanced Resource Recovery with Potential CO2 Sequestration Hsien H. Khoo* and Reginald B. H. Tan Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, Singapore 627833 ReceiVed February 20, 2006. ReVised Manuscript ReceiVed June 19, 2006

Conventional oil and natural gas production were compared with two case studies of enhanced resource recovery along with the potential for CO2 sequestration applications. The first case study is a Norwegian enhanced oil recovery (EOR) project, and the second focuses on enhanced coal-bed methane (ECBM) recovery in Japan. Both cases or systems involved the recovery of CO2 gases from a coal-fired power plant, followed by compression, transportation, and final injection of the greenhouse gas into geological formations as a solution to mitigate global warming. A life-cycle assessment (LCA) method was applied to measure how each system, conventional as well as enhanced recovery methods, impacts the environment. The first set of results presented were the inventory of air emissions (CO, CO2, CH4, SOx, NOx, NH3, Pb, Hg, etc.), wastewater-containing acids and sulfides, and solid wastes released because of both fossil fuel production and energy usage from the power plant. The impact assessment results because of the accumulated pollutants from all of the systems were calculated for the following set of common impact measures: global warming potential, acidification, human toxicity, eutrophication, wastes, and resources. The final (combined) scores of the entire system were also generated. These final scores, which included the normalization and weighting steps, allowed for overall comparisons for verifying the final benefits or drawbacks of a system. For the proposed EOR project, the greatest two environmental benefits (total impacts prevented) were calculated to be -9.8 × 10-2 and -9.7 × 10-2. As for ECBM, the best scores were projected to be -1.0 × 10-1 followed by -8.70 × 10-2.

Introduction The atmospheric concentration of carbon dioxide (CO2) is now higher than what was experienced on Earth for at least the last 40 000 years and is expected to continue to rise, leading to a significant global temperature increase by the end of this century. From the 18th century to the present, these concentrations have increased from 280 to about 360 parts per million by volume (ppmv).1 A total of 50% of this increase has occurred during the last 40 years and is mainly due to human activities.2 The source of these greenhouse emissions mostly comes from the burning of fuels to produce energy. A 1000 MW pulverized coal-fired power plant can emit up to 6-8 megatons of CO2 annually; an oil-fired power plant can emit about 25% less; and a natural gas combined cycle power plant can emit about half of the CO2 emissions that come from coal-powered plants.3 The availability of CO2 mitigation strategies would serve as a means to prevent global warming and at the same time allow for worldwide energy demands to be fulfilled by conventional energy systems. Many methods have been proposed to capture CO2 from the flue gases of power plants and store it in suitable reservoirs. For example, the deep ocean has been suggested as * To whom correspondence should be addressed. E-mail: [email protected]. Telephone: (65) 6796-3952. Fax: (65) 6873-4805. (1) McKee, B. Solutions for the 21st Century: Zero Emissions Technology for Fossil Fuels; Technology Status Report, International Energy Agency, Committee for Energy Research Technology, OECD/IEA: France, 2002. (2) Yegulalp, T. M.; Lackner, K. S.; Ziock, H. J. Int. J. Surf. Min., Reclam. EnViron. 2001, 15, 52-68. (3) Herzog, H. J.; Golomb, D. Carbon Capture and Storage from Fossil Fuel Use; Encyclopedia of Energy, 2004; Vol. 1.

a potential sequestration site for CO2 as a long-term solution to prevent global warming.4 Another study for CO2 sequestration was directed toward carbonation processes to bind CO2 as mineral rocks.5 Objective and Scope The objective of this paper is to first present two case studies of enhanced resource recovery methods, for oil and natural gas, alongside CO2 sequestration. The second intention is to employ a life-cycle assessment (LCA) method to predict the potential impacts on the environment for the CO2 mitigation case studies. The impact assessment results will be compared against the environmental burdens generated from conventional oil and natural gas production. Case Studies of CO2 Sequestration CO2 Recovery by Chemical Absorption. Before being able to utilize carbon dioxide gas for enhanced recovery purposes, the CO2 has to be captured and separated from the flue of the power plant. Chemical absorption of CO2 by the use of an amine is the most well-established method of CO2 capture in many commercial power-generating plants. Prior to CO2 removal, the flue gas is cooled and then treated to reduce particulates and other impurities. Next, the gas is then passed into an absorption tower, where it comes in contact with the absorption solution. The gas reacts with the chemical solvent to form a compound, (4) Khoo, H. H.; Tan, R. B. H. EnViron. Sci. Technol. 2006, 40, 40164024. (5) Khoo, H. H.; Tan, R. B. H. EnViron. Prog. 2006, in print.

10.1021/ef060075+ CCC: $33.50 © 2006 American Chemical Society Published on Web 07/26/2006

EnVironmental Impact EValuation of Fossil Fuel Production

Energy & Fuels, Vol. 20, No. 5, 2006 1915

Table 1. Cases for Oil Production/Recovery for LCA Investigation fossil fuel production/resource recovery method conventional oil production EOR with CO2 sequestration a

cases

oil production/ recovery

total CO2 sequestered (for about 500 years) (%)

I II

0.18 0.40a

0 0

1 2 3 4

0.18a 0.40a 0.18a 0.40a

95 90

Per ton of CO2 injection into a geological reservoir.

Table 2. Cases for Natural Gas Production/Recovery for LCA Investigation fossil fuel production/resource recovery method

cases

natural gas production/ recovery

total CO2 sequestered (for about 500 years) (%)

conventional natural gas production

III IV

0.33 0.67

0 0

ECBM with CO2 sequestration

5 6 7 8

0.33a 0.67a 0.33a 0.67a

a

95 90

Per ton of CO2 injection into underground coal seams.

which is then broken down by the application of heat, regenerating the original solvent and producing the pure CO2 stream.1 This is an energy-intensive process. Next, the recovered CO2 gas must be stored in an appropriate reservoir to prevent it from entering into the atmosphere. In terms of CO2 sequestration and utilization, geologic storage offers value-added benefits, such as using CO2 in enhanced oil recovery (EOR) operations and in enhanced coal-bed methane (ECBM) production. Enhanced Oil Recovery (EOR). Geologic CO2 sequestration with EOR is a proven technology. Under supercritical conditions, CO2 acts as a powerful solvent that can be used to reduce the viscosity of oil and therefore increase oil recovery.4 EOR projects are already ongoing in the U.S., such as in the Permian Basin of Texas. Typically, the source of CO2 for this type of project is transported by pipeline from natural CO2 reservoirs in Colorado, New Mexico, and Wyoming.6 EOR is yet to be applied where the source of CO2 is from electricity generation. A Norwegian case study proposes to do this. In this case study, CO2 is first captured from the flue gas of the existing coal-fired power system and sequestered geologically in conjunction with EOR in the North Sea.6-7 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 convinced that steel-pipe-engineering technology exists to allow for the long-distance pipeline to be produced for CO2 transportation.8-9 In the proposed CO2-EOR project, the energy requirement for the long-distance pipeline transportation is estimated to be 130 kWh/ton and recompression and injection are estimated to be 7-9 kWh/ton.4-7 The entire process results in two benefits: the underground storage of CO2 and the extraction of a useful resource, oil. (6) Agustsson, H.; Statoil, A. S. A. An EValuation of EOR by CO2 Injection in the Gullfaks Field; Proceedings of SPE/DOE 14th Symposium on Improved Oil Recovery: Tulsa, OK, 2004. (7) Solli, C. Scientific Assistant, LCA-lab, Industrial Ecology Programme, Norwegian University of Science and Technology, Trondheim, Norway; E-mail correspondence, 2005. (8) Skovholt, O. Energy ConVers. Manage. 1993, 34, 1095-1103. (9) Svensson, R.; Odenberger, M.; Johnsson, F.; Stro¨mberg, L. Energy ConVers. Manage. 2004, 45, 2343-2353.

Figure 1. LCA system boundary for comparing conventional oil production and EOR.

Figure 2. LCA system boundary for comparing conventional natural gas production and ECBM.

Enhanced Coal-Bed Methane (ECBM) Recovery. Another attractive option for disposal of CO2 is sequestration in deep, unmineable coal seams. Deep unmineable coal formations provide an opportunity to both sequester anthropogenic CO2 and, at the same time, increase the production of methane or natural gas. In this type of method, the adsorption of CO2 causes the desorption of the gas. Accordingly, ECBM recovery is a promising technology for mitigating greenhouse gas emissions from coal-fired power plants while providing significant economic benefit.10 The ECBM case study is taken from Tamabayashi at al.,11 where the Chikuhou coal field in Kyushu, Japan, is identified (10) Klara, S. M.; Srivastava, R. D.; McIlvried, H. G. Energy ConVers. Manage. 2003, 44, 2699-2712. (11) Tamabayashi, K.; Sagisaka, M.; Moro, T. EnVironmental and Economical Study on CO2 Sequestration and CH4 RecoVery by Coal Seam in Japan; Proceedings of the 6th International Conference on EcoBalance: Tsukuba, Japan, 2004.

1916 Energy & Fuels, Vol. 20, No. 5, 2006

Khoo and Tan

Table 3. Energy and Emissions Associated with Conventional Oil Production for the conventional production of 1 kg of oil raw material required (kg/kg)

pollutants (g/kg)

CO2 38.90

CH4

hydrocarbons

0.20

10-4

0.90 ×

natural gas

coal

oil

0.0044

0.0097

0.696

N2O 5.60 ×

PM10

10-4

3.00 ×

SO2

10-5

2.62 ×

10-1

NOx 9.85 ×

metals

10-2

3.48 × 10-4

Table 4. Energy and Emissions Associated with Conventional Natural Gas Production for the conventional production of 1 kg of natural gas raw material required (kg/kg)

pollutants (g/kg)

natural gas

coal

oil

0.56

0.054

0.0018

CO2

CH4

hydrocarbons

N2O

PM10

SOx

NOx

metals

89.50

1.12

6.00 × 10-1

na

na

1.12 × 10-1

2.26 × 10-1

na

to be a potential area for coal seam CO2 sequestration. After CO2 recovery, the 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 of CO2 and injection requires 5-6 kWh/ton.4 It has been estimated that for the underground storage of CO2, approximately 5-10% of the gas will “leak” back into the atmosphere after 500 years.12-13 For every ton of CO2 injected, the recovery rate of oil can range from 0.18 to 0.4 tons for EOR14 and from about 0.33 to 0.67 tons of gas for ECBM.15 Life-Cycle Assessment (LCA) A measuring tool must be in place to ensure that the very systems employed to mitigate global warming do not generate other (larger) types of environmental burdens. Measuring how environmental benefits or drawbacks of the entire system can be a complex task. The release of other types of substances to the surrounding must be taken into account. Additional pollution generated from the EOR and ECBM processes themselves, as well as, potential CO2 leakage, i.e., the gradual migration of CO2 back to the atmosphere from the storage site, must be thoroughly evaluated. LCA Goal, Scope, and System Boundary. Conventional oil and natural gas production will be tested against both the EOR and ECBM case studies, respectively. Conventional oil and gas exploration, extraction, and processing operations have existed for centuries. Because various reports featuring oil and gas production are widely available, this paper will not elaborate on the technical details of the two methods. The “cases” that are investigated are compiled in Table 1 (for oil) and Table 2 (for natural gas). The cases are based on the estimated recovery of oil, 0.18-0.4 ton for every ton of CO2 injected,14 as well as, an estimated 0.33-0.67 ton/ton of injected CO2 into the coal seams.15 Because this paper features the comparison between fossil fuel extraction with and without CO2 sequestration, the functional unit for all of the cases is selected as 1 ton of CO2 (from the coal-fired power plant) entering each system boundary. (12) von Goerne, G. The EnVironmental Impacts of CO2 Leakage from Storage; Proceedings of CAN Workshop on Carbon Capture and Storage: Brussels, Belgium, 2004. (13) Stevens, S. H.; Kuuskraa, V. A.; Taber, J. J. Sequestration of CO2 in Depleted Oil and Gas Fields: Barriers To OVercome in Implementation of CO2 Capture and Storage; IEA Greenhouse Gas R&D Program report number PH3/22, U.K., 2000. (14) Aycaguer, A.; Lev-On, M.; Winer, A. M. Energy Fuels 2001, 15, 303-308. (15) Reeves, S. Oil Gas J. 2003, 101, 48-49.

Table 5. Data for Energy Requirements CO2 utilization with EOR

processes involved CO2 recovery (by chemical absorption) compression and pipeline transportation recompression and injection oil recovery

CO2 utilization with EOR

processes involved CO2 recovery (by chemical absorption) compression and pipeline transportation recompression and injection oil recovery

energy requirements kWh/ton kWh/ton of CO2 of oil 330 130 7-9 94 energy requirements kWh/ton kWh/ton of CO2 of oil 330 100 5-6 38

The LCA systems are illustrated in Figure 1 for oil and Figure 2 for natural gas. Life-Cycle Inventory. The raw materials required and generic air emissions data for conventional oil and natural gas production were extracted from reports.16-17 The pollutants for oil and natural gas production (pumping and extraction) are compiled in Tables 3 and 4, respectively. The estimated energy required for the pumping/extraction of the fossil fuels from the ground are calculated to be 138 kWh/ton of oil18 and 40 kWh/ton of natural gas. Solid waste (nonhazardous) was reported as 4.79 kg/ton of oil16 production and 1.34 kg/ton of gas.19 Energy-related details of CO2 recovery, compression, and pipeline transportation, as well as final injection and fossil fuel recovery, have been extracted from the latest reports and from personal communications with other researchers.6-7,10,20 The data are compiled in Table 5. (16) National Renewable Energy Laboratory. Life Cycle InVentory of Biodiesel and Petroleum Diesel; publication number NREL/SR-580-24089, National Renewable Energy Laboratory: Washington, D.C., 2004. (17) Lewis, C. A. Fuel and Energy Production Emission Factors; ETSU report number R112, AEA Technology: U.K., 1997. (18) National Bureau of Statistics of China. http://www.stats.gov.cn/ english/ (accessed June 19, 2006). (19) Spath, P. L.; Mann, M. K. Life Cycle Assessment of a Natural Gas Combined-Cycle Power Generation System; publication number NREL/ TP-570-27715, National Renewable Energy Laboratory: Washington, D.C., 2000. (20) Heddle, G.; Herzog, H. J.; Klett, M. The Economics of CO2 Storage; publication number LFEE 2003-003 RP, MIT: Cambridge, MA, 2003.


Energy & Fuels, Vol. 20, No. 5, 2006 1917 Table 6. Results for Main-Resource Consumption for the Generation of 1 MWh main resources (kg)

coal mining

transportation

electricity generation

coal natural gas oil

3.57 4.82 × 10-1 3.24 × 10-1

4.76 × 10-2 2.15 × 10-2 5.53

4.72 × 10+2 7.47 × 10-1 5.64

Table 7. Results for Main-Air Emissions Because of the Generation of 1 MWh

Figure 3. Impact assessment method is used to assess the potential impacts of the environment (for the selected environmental impact categories).

Impact Assessment The life-cycle impact assessment methodology is employed to calculate the potential effects of a well-defined system on the environment. On the basis of a life-cycle “cradle-to-grave” concept, this methodology can be used to evaluate the environmental burdens of any type of products or processes.4-5 In theory, the impact assessment method converts the pollution results to a set of common impact measures, such as global warming, acidification, eutrophication, etc., that can be used to evaluate the total effect of the system in question. The method is conceptually illustrated in Figure 3. SimaPro’s environment design of industrial products (EDIP) method for impact assessment will be used to calculate the results for the following environmental impact categories: global warming potential (GWP), acidification, human toxicity, eutrophication, wastes, and resources.21 SimaPro’s EDIP method for impact assessment was jointly developed by the Danish EPA, the Technical University of Denmark, and Confederation of Danish Industries. This impact assessment method is developed in line with the ISO standards.22-23 LCIA can be viewed as a five-step process: (i) selection of impact categories, (ii) classification of resources and releases, (iii) characterization of resources and releases to estimate the potential resulting human health and environmental impacts, (iv) normalization, and (v) weighting.21 Results and Discussions The pollution results will be presented in three manners: (i) preliminary results for power generation, (ii) impact assessment results (for whole system), and finally, (iii) final accumulated environmental impacts. Preliminary Results: Power Generation. Prior to calculating the potential impact assessment results for the whole system, the emissions to air, wastewater, solid wastes, and resources consumed have to be generated first. The coal-fired power plant (21) SimaPro. Impact Assessment; http://www.pre.nl/life_cycle_ assessment/impact_assessment.htm (accessed June 19, 2006). (22) ISO. EnVironmental Management: Life Cycle AssessmentsPrinciple and Framework; EN ISO 14040, 1997. (23) ISO. EnVironmental Management: Life Cycle AssessmentsGoal and Scope Definition and InVentory Analysis; EN ISO 14041, 1998.

main-air emissions (kg)

coal mining

transportation


CO2 CO CH4 N2O SOx NOx NH3 PM VOCs HCl HF H2S

9.59 9.21 × 10-3 9.04 × 10-1 1.00 × 10-3 7.10 × 10-2 4.76 × 10-2 9.86 × 10-2 1.29 × 10-2 8.13 × 10-2 5.49 × 10-9 9.71 × 10-9 2.79 × 10-9

1.75 × 10+1 1.01 × 10-1 9.13 × 10-4 2.46 × 10-4 9.51 × 10-2 1.85 × 10-1 9.88 × 10-5 1.84 × 10-2 5.89 × 10-2 5.84 × 10-8 7.34 × 10-9 3.07 × 10-10

9.50 × 10+2 1.56 × 10-1 8.49 × 10-3 3.18 × 10-3 6.53 3.12 1.09 × 10-4 9.18 7.28 × 10-2 1.77 × 10-6 1.53 × 10-7 8.90 × 10-9

Table 8. Results for Heavy-Metal Emissions Because of the Generation of 1 MWh heavy-metal emissions (g)

coal mining

transportation


antimony arsenic boron cadmium chromium cobalt copper lead manganese mercury nickel vanadium

0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

4.10 × 10-3 4.95 × 10-2 1.70 × 10+1 4.10 × 10-3 5.92 × 10-2 6.90 × 10-3 2.34 × 10-2 3.00 × 10-2 4.30 × 10-3 3.66 × 10-2 5.79 × 10-2 8.80 × 10-2

Table 9. Results for Wastewater Discharge Because of the Generation of 1 MWh water emissions (kg)

coal mining

transportation


acids (H+) ammonia chlorides (Cl-) cyanides (CN-) fluorides (F-) metals nitrates (NO3-) phenol sodium (Na+) sulfates (SO4-) sulfides (S-)

2.56 × 10-5 2.53 × 10-2 9.80 × 10-8 2.26 × 10-9 6.93 × 10-6 8.35 × 10-8 1.57 × 10-6 7.06 × 10-9 1.09 × 10-6 1.40 × 10-6 4.53 × 10-9

2.57 × 10-9 0.000003229 1.63 × 10-7 1.90 × 10-10 3.20 × 10-8 9.58 × 10-9 1.46 × 10-9 6.00 × 10-10 2.39 × 10-8 2.31 × 10-8 3.80 × 10-10

8.22 × 10-8 1.5 × 10-8 3.18 × 10-6 6.10 × 10-9 1.05 × 10-6 2.74 × 10-7 5.10 × 10-8 1.91 × 10-8 4.79 × 10-7 4.67 × 10-7 1.22 × 10-8

operating in the U.S. is used as a representative for power generation throughout this paper. These results are produced with the help from the National Renewable Energy Laboratory.24 The pollution outcome because of the energy requirements for the case studies will be based on these data. The results will focus on (i) resources consumed for coal mining, transportation, and electricity generation, (ii) wastes from coal mining, transportation, and electricity generation, (iii) main-air emissions from coal mining, transportation, and electricity generation, and (iv) water emissions from coal mining and electricity generation. (24) Spath, P. L.; Mann, M. K.; Kerr, D. R. LCA of Coal-Fired Power Production; National Renewable Energy Laboratory (NREL), National Technology Information Service (NTIS), U.S. Department of Commerce: Washington, D.C., 1999.


Khoo and Tan

Figure 4. Impact assessment results for GWP for conventional oil production and EOR (with CO2 sequestration).

Figure 5. Impact assessment results for acidification for conventional oil production and EOR (with CO2 sequestration).

These results, in terms of 1 megawatt hour (MWh) generated from the coal-fired power plant, are compiled in Tables 6-9. With the implementation of a LEBS (low emission boiler system) for the power plant, the total solid waste from mining, transportation, and electricity generation is estimated to be 20.7 kg for every generation of 1 MWh.24 Impact Assessment Results (for Oil). The results for GWP, acidification, human toxicity to air, eutrophication, wastes, and resources, for comparing conventional oil production with EOR, are displayed in Figures 4-9, respectively. Many scientists have postulated that the world is undoubtedly warming up because of emissions of carbon dioxide and other greenhouse gases from human activities including fossil fuel combustion.2-3 The global warming results, displayed in Figure 4, illustrate the effects of sequestered as well as unsequestered CO2. All six cases involve the production or recovery of oil from 0.18 (cases I, 1, and 3) to 0.4 tons (cases II, 2, and 4).

Positive peaks display the contribution to global warming, whereas inverted (negative) peaks demonstrate the potential to prevent global warming. For cases I and II, 100% of the CO2 gases from the power plant goes unsequestered. The inverted peaks displayed in cases 1-4 indicate the amount of potential global warming prevented. The small peaks from the same four cases come from CO2 emissions released from the power plant because of energy usage, mostly from the CO2 recovery process (by chemical absoprtion) and unsequestered CO2 gases. Acid deposition is mainly caused by sulfur dioxide (SO2), nitrogen oxides (NOx), and ammonia (NH3). For the acidification impact category, SO2 and its potential for acid formation is suggested as the reference substance. The acidification results (because of energy usage) shown in Figure 5 were calculated according to the regulation of 90% removal of SOx and NOx



Figure 6. Impact assessment results for human toxicity to air for conventional oil production and EOR (with CO2 sequestration).

Figure 7. Impact assessment results for eutrophication for conventional oil production and EOR (with CO2 sequestration).

from the coal-fired power plant.25 Unlike the GWP graphs (Figure 4), the acidification graphs (Figure 5) all display positive peaks, indicating the release of substances that can potentially add to environmental damage. Large contributions to acidification for cases 1-4 (EOR) are mostly from the CO2 recovery stage, which is a highly energy-intensive process,4 as well as from CO2 compression and pipeline transportation to the sequestration site. Gases of SO2 and NOx are reported to pollute the air because of conventional oil production activities,16 but these contributions, as displayed by cases I and II, are less compared to the accumulated impacts coming from the CO2 sequestration chain. Air emissions containing heavy metals such as arsenic, cadmium, lead, and mercury contribute to human toxicity (air).

As displayed in Figure 6, air emissions containing toxic metals are especially high from conventional oil production activities. These emissions from conventional onshore crude oil extraction come from the combustion of natural gas used in crude oil separators, venting and flaring operations, and volatilization (fugitive) emissions of crude oil.16 The contributions to human toxicity for cases 1-4 are relatively less significant. For all six human toxicity results, the environmental impacts were generated after the regulation of the removal of 95% of heavy metals from the power plant flue gas.26 This includes the reduction of Hg and Pb emissions, which can be detrimental to human health. Eutrophication is caused by the accumulation of nitrates, ammonia, and cyanides, as well as air emissions of N2O and

(25) USEPA. Reducing Power Plant Emissions: Interstate Air Quality Rule; Office of Air and Radiation EPA, USEPA: Washington, D.C., 2003.

(26) Offen, G. R. Mercury Control for Power Plants; Subcommittee for Environment Technology Standards, EPRI: Palo Alto, CA, 2003.


Khoo and Tan

Figure 8. Impact assessment results for wastes for conventional oil production and EOR (with CO2 sequestration).

Figure 9. Impact assessment results for resources for conventional oil production and EOR (with CO2 sequestration).

NOx.21 The highest damages for this environmental impact category, displayed in Figure 7, are caused by the CO2 recovery and pipeline transportation processes that are required for the EOR operations (cases 1-4). The processes involved in conventional oil production (cases I and II) result in considerably less significant environmental damage. The waste results, displayed in Figure 8, display the accumulation of wastes because of high-energy consumption required by the CO2 sequestration activities. To reduce the overall damages to the environment, soon all coal-fired power generation technologies will be expected to have a similar type of LEBS equipment in place.24 The oil recovery processes cause less solid waste impacts, as compared to the chain of processes involving CO2 sequestration. The total collective waste generated for all four EOR cases ranges from about 3.19 to 3.61 kg. Resources are consumed in the activities involved in the extraction and recovery of oil, as well as in the generation of

electrical power to support these activities. The resources results are displayed in Figure 9. High peaks because of raw material consumption for cases I and II can be clearly observed. In comparison to the first two graphs, the resources consumed for cases 1-4 are considerably less. Considerable amounts of natural gas, oil, and coal are spent during conventional oil production operations.16 Impact Assessment Results (for Natural Gas). The results for GWP, acidification, human toxicity to air, eutrophication, wastes, and resources, for comparing conventional natural gas production with ECBM, are displayed in Figures 10-15, respectively. The results for GWP for natural gas production and recovery are displayed in Figure 10. Cases III and IV, 0.33 and 0.67 tons of natural gas production by conventional methods, respectively, contribute significantly to the GWP results. Apart from the unsequestered CO2, additional greenhouse gases, including methane, are emitted during conventional natural gas



Figure 10. Impact assessment results for GWP for conventional natural gas production and ECBM (with CO2 sequestration).

Figure 11. Impact assessment results for acidification for conventional natural gas production and ECBM (with CO2 sequestration).

production operations.17,19 The inverted peaks from cases 5-8 (for ECBM) exhibit more or less the same pattern as those shown in the GWP results for EOR (Figure 4). Among the variety of technological opportunities to mitigate global warming, enhanced recovery methods with CO2 utilization and sequestration have been proven to be one of the most reliable solutions.27 The results for acidification are displayed in Figure 11. The total accumulative acidic gases for cases 5 and 6 are about 4 times greater than case III and about double the results of case IV. The conventional production of natural gas contributes more substantially to acidification as compared to the recovery of the gas via enhanced recovery methods. However, the down(27) Holloway, S. Energy 2005, 30, 2318-2333.

stream stages of extracting CO2 from the power plant, in addition to transporting the gas to the sequestration location, potentially cause high environmental impacts. The contribution to human toxicity to air (Figure 12) is less significant for conventional natural gas production as compared to the four ECBM methods. The same goes with eutrophication (Figure 13). A significant portion of the accumulated impacts for eutrophication come from ECBM, that is, cases 5-8. The last two impacts, waste and resources, are displayed in Figures 14 and 15, respectively. It can be observed from Figure 14 that the ECBM methods (cases 5-8) result in even greater impacts than conventional gas production systems (cases III and IV). Life-cycle investigation for the case studies are carried out


Khoo and Tan

Figure 12. Impact assessment results for human toxicity to air for conventional natural gas production and ECBM (with CO2 sequestration).

Figure 13. Impact assessment results for eutrophication for conventional natural gas production and ECBM (with CO2 sequestration).

Figure 14. Impact assessment results for wastes for conventional natural gas production and ECBM (with CO2 sequestration).

for this very purpose, to determine if the very methods employed to reduce CO2 will themselves create any (more severe) types of environmental burdens. However, in Figure 15, the resource

impact category demonstrates the exact opposite outcome. High raw material consumption is required for the conventional production of natural gas.18



Figure 15. Impact assessment results for resources for conventional natural gas production and ECBM (with CO2 sequestration). Table 10. EDIP Normalization and Weighting Parameters21

Figure 16. Normalization and weighting steps in LCIA.

Final Accumulated Environmental Impacts. To make overall comparisons, a single final score, from the total accumulated impacts of GWP, acidification, human toxicity, eutrophication, wastes, and resources, for each of the LCA cases must be attained. The calculations for obtaining the final scores include two additional steps, known as normalization and weighting. These additional steps are illustrated in Figure 16. In the LCIA methodology, weighting allows one to see the environmental tradeoffs when comparing different activities within the same life-cycle study, especially in the setting of priorities for pollution mitigation.4-5 The purpose of weighting

Figure 17. Final scores for conventional oil production and EOR.

environmental impact category

normalization

weights

GWP acidfication human toxicity eutrophication wastes resources

1.15 × 10-7 8.06 × 10-6 1.09 × 10-10 3.36 × 10-3 7.04 × 10-4 1.00

1.3 1.3 2.8 1.2 1.1 1.0

is to further illuminate decision situations and verify the final benefits or drawbacks of a system. Table 10 displays the EDIP normalization and weighting parameters used to reach the final score. The final scores are displayed in Figure 17 (for oil) and Figure 18 (for natural gas). It is obvious from both figures that enhanced resource recovery methods, both with the potential to sequester CO2, result in significant environmental benefits. The final inverted peaks, for cases 1-4 as well as for cases 5-8, illustrate that the total sum of the environmental burdens generated are outweighed by the potential environmental benefit of preventing global warming. For EOR, the most promising option is case 1 (total score -9.8 × 10-2), followed by case 2 (-9.7 × 10-2). In the two cases, the recovery of oil is estimated as 0.18 and 0.4 tons, respectively, for every ton of CO2 injected into the


Khoo and Tan

Figure 18. Final scores for conventional natural gas production and ECBM.

geological reservoir, where 100% of the CO2 remains sequestered permanently (assume zero leakage). As for ECBM, the largest inverted peaks, i.e., greatest environmental benefit, are cases 5 and 6. The total impacts were calculated to be -1.0 × 10-1 and -8.70 × 10-2, respectively. The amount of natural gas recovered in these two cases are 0.33 and 0.76 ton/ton of CO2, where the greenhouse gas is also considered to be 100% sequestered. 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. Driven by the need to mitigate global warming, further developments will be carried out to capture and sequester CO2 effectively while imposing lighter energy and waste penalties for these types of postcombustion recovery systems.27

Although the science of CO2 sequestration is relatively new, numerous renowned scientists, engineers, and researchers have regarded them as promising solutions to mitigate global warming.3,11,20 In this area, geological sequestration efforts alongside the utilization of CO2 offers relatively safe storage of the greenhouse gas, as well as the economical and environmental benefits of the recovery of useful resources (oil and natural gas). The injection of CO2 underground with oil recovery is reported to be a proven technology,6 where pipeline engineering knowledge exists to allow for the transfer of compressed CO2 from its source to the storage site.9 In fact, petroleum industries in the U.S. have for many years been injecting CO2 in geological formations to improve oil recovery.13 A second advantage for the underground storage of CO2 lies in its large storage capacity. The IEA Greenhouse R&D Program estimated that the global storage capacity of oil and gas reservoirs is at least 660 gigatons of CO2.28

Conclusions

Nomenclature

The field of CO2 sequestration is still an immature area of scientific inquiry, and future knowledge or discoveries may change how the processes involved may be carried out. Although not a universally accepted idea, carbon sequestration is being viewed as a solution to prevent global warming. However, for any CO2 mitigation projects, it is crucial that the very activities for capturing and storing the greenhouse gas do not themselves produce other bigger types of environmental damages. To ensure the integrity of this approach, LCA was used to measure the likely impacts or damages on the environment.

LCA ) life-cycle assessment EOR ) enhanced oil recovery ECBM ) enhanced coal-bed methane GWP ) global warming potential EDIP ) environment design of industrial products MWh ) megawatt hour EF060075+ (28) Riemer, P. Energy ConVers. Manage. 1996, 665-670.

Environmental Impact Evaluation of Conventional Fossil Fuel

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