Life Cycle Investigation of CO2
Recovery and Sequestration
H S I E N H . K H O O * A N D
R E G I N A L D B . H . T A N
Institute of Chemical and Engineering Sciences, 1 Pesek Road,
Jurong Island, Singapore 627833
combined with nine CO2sequestration systems, serves
to expand the debate of CO2mitigation methods beyond a
single issuesprevention of global warmingsto a wider
and toxic gases, wastes, etc, so that the overall, and
unexpected, environmental impacts may be revealed.
has spurred worldwide concerns of potential global climate
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
flue gas into the atmosphere. Among those discussed here
are post combustion capture technologies, and ocean and
geological sequestration (4-5).
methods will be presented followed by the introduction of
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
technologies. In Stage 3, LCA will be performed to compare
various CO2sequestration options.
The life cycle impact assessment results and interpreta-
tions/discussions will be presented in Section 7. In Section
8, CO2sequestration 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 CO2is produced
fired power plant can emit up to 6-8 Mt of CO2annually,
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). Accord-
ingly, coal-based electricity is selected as the prime energy
provider for the various types of technologies or systems
investigated throughout the paper.
Basic descriptions of four postcombustion capture tech-
nologies are presented in this section.
4.1. Chemical Absorption. Chemical absorption of CO2
by the use of solvents is the most well-established method
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 monoethanol-
CO2recovery rates of 95-98% can be achieved by using
amines (7). Chemical absorption processes need heat for
340 kWh per ton CO2recovered; these values are for both
solvent is completely recycled in the process, the only
emissions generated in this technique are those caused by
CO2to 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
Typical removal rates are 82-88% of CO2from the power
plants’ flue gases (10-11). The main air pollution generated
from this technology is from energy use.
a cryogenic separation system, CO2is physically separated
from other gases by condensing it at an extremely low
90-95% of the flue gas. The energy requirements are
estimated to be 600-660 kWh per ton of CO2recovered as
a liquid form (9). The main air pollution generated from this
technology is from energy use.
surface areas, such as zeolites and activated carbon, can
separate CO2from gas mixtures by physical adsorption. An
example of this application is pressure swing adsorption
(PSA), which is a commercially available technology for
recovering CO2from power plants (1). The recovery of the
from 160 to 180 kWh/ton CO2 recovered (9, 12). The only
emissions generated in this technique are those caused by
After recovering the CO2gas, it must be stored somewhere
to prevent it from appearing in the atmosphere. One way to
six case studies are presented. They are known as vertical
lift 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 CO2in 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
contains an estimated 40 000 GtC (billion tons of carbon)
compared with 750 GtC in the atmosphere and 2200 GtC in
* Corresponding author phone: (65) 6796-3952; fax: (65) 6873-
4805; e-mail: email@example.com.
Environ. Sci. Technol. 2006, 40, 4016-4024
40169ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 12, 200610.1021/es051882a CCC: $33.50
2006 American Chemical Society
Published on Web 05/06/2006
the terrestrial biosphere. As a result, the amount of carbon
would change the ocean concentration by less than 2% (2).
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 CO2ocean sequestration.
5.1.1. Vertical Injection. In the first option, the injection
of CO2into the ocean depths of 3000 m from a vertical pipe
hanging from a floating platform is introduced (14). First,
CO2gas is recovered from the power plant flue gases and
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 CO2is expected to
be stored for at least 500 years (15).
Technological and engineering challenges faced for
by Nihous (16). The energy required for CO2liquefaction by
the use of a five-stage intercooled compression unit is
estimated to be about 40-50 kWh per ton CO2(17).
5.1.2. Inclined Pipe. In the second option, compressed
CO2is pumped into a depth of 2000 m into the oceans via
a long inclined pipe (18). At this depth, it is estimated that
CO2pipeline 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
25-30 kWh per ton CO2(17).
of 1.5 and will readily sink (13). The process for making dry
ice (sublimation) takes up twice the energy of that required
the estimated travel distance of the tanker is 300 km, where
the CO2blocks 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).
involves the sequestration of low purity CO2gas. After the
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
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.
liquid-CO2is 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).
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 CO2reservoirs (28). EOR is yet to be applied where
the source of CO2is from electricity generation.
A Norwegian case study is investigated to do this. In the
case study, CO2is 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 CO2from
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).
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 CO2injected 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
5.2.2. ECBM. Deep unmineable coal formations provide
an opportunity to both sequester anthropogenic CO2and at
case study is taken from Tamabayashi et al. (35), where the
Chikuhou coalfield in Kyushu, Japan, is identified as a
the injection of CO2, methane or natural gas is recovered. It
requires 100 kWh/ton CO2and 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
5.2.3. Sleipner. In the final case, the Sleipner project in
the North Sea is presented as the world’s first industrial-
scale storage of CO2 in an underground reservoir. In the
Sleipner project CO2gas 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
during the process of sequestering CO2.
of CO2from natural gas via amine scrubbing and ends with
the final injection or disposal of CO2into 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 & TECHNOLOGY94017
about 20% less than the same system used to remove CO2
from power plant flue gases, that is, 240 kWh/ton (8).
and assessment studies pertaining to CO2 sequestration
on costs or economical modeling of CO2removal systems
methods and the design of pipes suitable for deep ocean
and ECBM projects have also been reported (28-29). Most
issues concerning geological sequestration (5, 33).
life cycle assessment study on the nine CO2sequestration
6. Life Cycle Assessment
Due to the different characteristics of all three stages, a
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 per-
spective by offering a system’s point of view. The power of
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 envi-
ronmentally suitable strategy, the best practicable environ-
mental action, or alternative combination of processes/
6.1. LCA Goal and Scope. This work will be the first to
investigate all three stages, thereby linking the “CO2route”
The overall system boundary is illustrated in Figure 1.
First, LCA is performed on the three separate stages as
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).
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 CO2per MWh and
the Sleipner project, the functional unit is taken as 950-kg
CO2generated from the processing of natural gas.
defined system are systematically identified and quantified.
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
(GWP), acidification, human toxicity to air, human toxicity
The EDIP is a problem-oriented (midpoint) method which
is widely used and highly recognized by many LCA experts
In an ideal investigation, the LCIA should include the
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
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 CO2generated from the coal-
fired power plant (per MWh), as well as the Sleipner project,
is taken to be 950 kg. The CO2removal efficiencies of 95%
(chemical absorption), 82% (membrane separation), 90%
(cryogenics), and 85% (PSA), are employed.
7.1. Results for CO2Recovery Methods. Due to the size
for the four CO2recovery technologies will be compiled in
For GWP, the most promising system for CO2postcom-
bustion 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 CO2from the power plant, its large energy
consumption (600 kWh/ton CO2 recovered) resulted in
additional greenhouse gas emissions.
FIGURE 1. LCA Methodology.
40189ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 12, 2006
of air and water emissions that comes along with the
processes. Air emissions containing acidic gases contribute
and mercury, contribute to human toxicity (air). For the
acidification impact category, the results were calculated
according to the regulation of 90% removal of SOxand 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
second bychemical absorption, and followed by PSA.
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
for these types of post combustion recovery systems (11).
7.2. Results for CO2Sequestration Options. The results
options are shown in Figures 2 (GWP), 3 (acidification), 4
(human toxicity, air), 5 (human toxicity, water), 6 (eutrophi-
cation), 7 (ecotoxicity), 8 (wastes), and 9 (resources).
7.2.1. Global Warming Potential. Intuitively, the Sleipner
The safe storage of CO2in the Utsira formation is depicted
in biggest inverted peak in the GWP graph (Figure 2). This
is followed by geological sequestration with ECBM. A
reasonable environmental impacts (positive peaks) is also
displayed by geological sequestration with EOR.
For ocean sequestration, vertical injection appears to be
stored, however, the sublimation process involved imposes
a large energy penalty, which adds unnecessarily to GWP.
For these two options, the final destination for CO2storage
to be trapped for 500 years (15).
Two other viable options are CO2-hydrates and inclined
respectively. For both the pipe towed by ship method and
GLAD, the amount of potential CO2leakage from the ocean
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.
TABLE 1. Impact Assessment Results for CO2Recovery Technologies
membrane separation environmental impact categorieschemical absorptioncryogenics pressure swing adsorption
human toxicity - Air (m3/g)
human toxicity - water (m3/g)
7.87 × 104
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 × 105
7.25 × 101
1.54 × 104
8.53 × 10-5
3.25 × 102
4.66 × 10-3
1.38 × 100
3.51 × 10-4
1.79 × 105
6.21 × 102
1.32 × 105
7.31 × 10-4
2.79 × 103
3.99 × 10-2
1.18 × 101
3.01 × 10-3
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.
VOL. 40, NO. 12, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY94019
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
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
impacts are from the process of CO2liquefaction for vertical
injection, pipe towed by ship, and CO2hydrate formation,
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 signifi-
cantly to the graphs. The CO2 liquefaction process and
pipeline transportation both generate relatively large amounts
demands: 120 kWh/ton CO2for 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 3. Total acidification results for ocean and geological sequestration.
FIGURE 4. Total human toxicity to air results for ocean and geological sequestration.
40209ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 12, 2006
energy is required for dissolution; hence, leading to nearly
negligible environmental impacts.
7.2.5. Wastes and Resources. One of the biggest environ-
displayed in Figure 8. The highest two cases are dry ice and
Sleipner, and the lowest is displayed by GLAD. It must be
of energy for the chain of processes involved in CO2storage
or sequestration is from a coal-fired power plant (45).
The resource results for ocean and geological sequestra-
tion are displayed in Figure 9. The positive peaks exhibit the
energy demands (resource consumed) for the sequestration
the potential amount of resources gained from the EOR and
ECBM geological sequestration technologies. The inverted
the many solutions that contribute toward CO2mitigation,
geological sequestration seems to be a promising path that
presents the advantage of being able to cope with large
volumes of anthropogenic CO2at 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 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
FIGURE 5. Total human toxicity to water results for ocean and geological sequestration.
FIGURE 6. Total eutrophication results for ocean and geological sequestration.
VOL. 40, NO. 12, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY94021
selling natural gas.
7.3. Final Scores. The results for the potential environ-
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 assess-
ment results will have to include the normalization and
weighting stages, which are provided by SimaPro (46).
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,
From Table 2, the “best” negative scores (least environ-
mental burdens) stem predominantly from the three geo-
logical 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
ECBM combined with membrane separation and with PSA.
FIGURE 7. Total ecotoxicity results for ocean and geological sequestration.
FIGURE 8. Total waste results for ocean and geological sequestration.
40229ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 12, 2006
the chemical absorption technology combined with vertical
by any combination of CO2removal with vertical injection.
The second most feasible options are by inclined pipeline
and dry ice disposal, both combined with chemical absorp-
tion for CO2 recovery. The “worst cases” are displayed by
combining any CO2removal methods with pipe towed by
are “suppressed” by the generation of other environmental
burdens. With the exception of using cryogenics to remove
CO2from the power plant, all the final scores for the GLAD
option display rather small environmental benefits.
Supporting Information Available
for the case studies, system boundaries, LCI data, assump-
tions, and estimations. Sequestration effectiveness and
sensitivity analysis for comparing: (i) power plant CO2
between the EDIP and eco-indicator. Further discussions
pertaining to the strengths and weaknesses of CO2seques-
tration is also provided. This material is available free of
charge via the Internet at http://pubs.acs.org.
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40249ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 12, 2006