Life Cycle Investigation of CO 2 Recovery and Sequestration

Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, Singapore.
Environmental Science and Technology (Impact Factor: 5.33). 07/2006; 40(12):4016-24. DOI: 10.1021/es051882a
Source: PubMed
ABSTRACT
The Life Cycle Assessment of four CO2 recoverytechnologies, combined with nine CO2 sequestration systems, serves to expand the debate of CO2 mitigation methods beyond a single issue-prevention of global warming-to 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.

Full-text

Available from: Hsien H. Khoo, Jul 11, 2014
Life Cycle Investigation of CO
2
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 CO
2
recovery technologies,
combined with nine CO
2
sequestration systems, serves
to expand the debate of CO
2
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 (CO
2
) 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 CO
2
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 CO
2
recovery
methods will be presented followed by the introduction of
CO
2
sequestration. In Section 6, life cycle assessment or LCA
will be used as powerful tool to analyze the CO
2
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 CO
2
recovery
technologies. In Stage 3, LCA will be performed to compare
various CO
2
sequestration options.
The life cycle impact assessment results and interpreta-
tions/discussions will be presented in Section 7. In Section
8, CO
2
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 CO
2
is produced
each year from power plants (2). A 1000 MW pulverized coal-
fired power plant can emit up to 6-8MtofCO
2
annually,
an oil-fired power plant emits about 25% less, and a natural
gas combined cycle power plant emits about half of the CO
2
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.
4. CO
2
Recovery
Basic descriptions of four postcombustion capture tech-
nologies are presented in this section.
4.1. Chemical Absorption. Chemical absorption of CO
2
by the use of solvents is the most well-established method
of CO
2
capture in power plants. In this process, the CO
2
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 monoethanol-
amine (MEA), diethanolamine, and potassium carbonate (6).
CO
2
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 330-
340 kWh per ton CO
2
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
CO
2
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 CO
2
(9).
Typical removal rates are 82-88% of CO
2
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 CO
2
from other gases using pressure and temperature control. In
a cryogenic separation system, CO
2
is physically separated
from other gases by condensing it at an extremely low
temperature. The amount of CO
2
recovered is approximately
90-95% of the flue gas. The energy requirements are
estimated to be 600-660 kWh per ton of CO
2
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 CO
2
from gas mixtures by physical adsorption. An
example of this application is pressure swing adsorption
(PSA), which is a commercially available technology for
recovering CO
2
from power plants (1). The recovery of the
CO
2
gas can be in the range of 85-90% with energy demands
from 160 to 180 kWh/ton CO
2
recovered (9, 12). The only
emissions generated in this technique are those caused by
energy use.
5. CO
2
Sequestration
After recovering the CO
2
gas, it must be stored somewhere
to prevent it from appearing in the atmosphere. One way to
achieve this is by CO
2
sequestration. For ocean sequestration,
six case studies are presented. They are known as vertical
injection, inclined pipeline, pipe towed by ship, dry ice, gas-
lift advanced dissolution (GLAD) system, and CO
2
-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 CO
2
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 CO
2
. 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
* Corresponding author phone: (65) 6796-3952; fax: (65) 6873-
4805; e-mail: khoo_hsien_hui@ices.a-star.edu.sg.
Environ. Sci. Technol.
2006,
40,
4016-4024
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 12, 2006 10.1021/es051882a CCC: $33.50 2006 American Chemical Society
Published on Web 05/06/2006
Page 1
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).
CO
2
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 CO
2
.
Herzog et al. (15) projected, through scientific experiments,
that the amount of time over which the percentage of the
injected CO
2
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 CO
2
respectively will remain stored or completely dissolved in
the ocean for at least 500 years. Several options have been
proposed for the large-scale CO
2
ocean sequestration.
5.1.1. Vertical Injection. In the first option, the injection
of CO
2
into the ocean depths of 3000 m from a vertical pipe
hanging from a floating platform is introduced (14). First,
CO
2
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-CO
2
directly into the
ocean. At depths of 3000 m, 90% of the CO
2
is expected to
be stored for at least 500 years (15).
Technological and engineering challenges faced for
transporting liquid CO
2
to the ocean sites have been discussed
by Nihous (16). The energy required for CO
2
liquefaction by
the use of a five-stage intercooled compression unit is
estimated to be 120 kWh per ton CO
2
(9). Energy requirements
for compression and injection from the floating platform are
estimated to be about 40-50 kWh per ton CO
2
(17).
5.1.2. Inclined Pipe. In the second option, compressed
CO
2
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
CO
2
pipeline transportationsfor distances of 250-500 km
from the power plant to the offshore sitescan be up to 100
kWh/t of CO
2
(19). Re-compression is required for the final
injection, which requires up to 30-40 kWh/ton CO
2
(17).
5.1.3. Pipe Towed by Ship. In the third case, liquefied CO
2
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 CO
2
(17).
5.1.4. Dry Ice. Solid CO
2
or dry ice blocks are disposed into
the ocean from a moving ship. Solid CO
2
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 CO
2
liquefaction (21). In this ocean sequestration system,
the estimated travel distance of the tanker is 300 km, where
the CO
2
blocks are assumed to reach complete dissolution
at depths of 3000 m (22). Preliminary tests have shown that
the CO
2
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 CO
2
gas. After the
CO
2
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 CO
2
into
seawater at a relatively shallow depth of 200-300 m and
then transports CO
2
-rich seawater to depths of 1000-3000
m(24).
An advantage of the GLAD method is that it bypasses the
need to liquefy CO
2
. The energy requirement for the
compression for the GLAD system is 3.7 kWh per ton CO
2
(25). It is assumed for this case that the CO
2
gas reaches
complete dissolution at an average depth of 1500m.
5.1.6. CO
2
-Hydrate. In the last ocean sequestration system,
liquid-CO
2
is transported by pipe to a hydrate reactor and
injected as hydrates into the ocean at depths of 1000-1500
m(26). The CO
2
-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,
CO
2
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 CO
2
sequestration with EOR is a
proven technology (5). Under supercritical conditions, CO
2
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 CO
2
is transported by pipeline from
natural CO
2
reservoirs (28). EOR is yet to be applied where
the source of CO
2
is from electricity generation.
A Norwegian case study is investigated to do this. In the
case study, CO
2
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 CO
2
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 CO
2
(32).
In the proposed CO
2
-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 CO
2
injected is released to the
atmosphere. The recovery of oil is taken to be 0.18 ton of oil
for every ton CO
2
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 CO
2
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 CO
2
sequestration. The recovered
CO
2
gas is transported by pipeline to the injection site. During
the injection of CO
2
, methane or natural gas is recovered. It
was estimated that compression and pipeline transportation
requires 100 kWh/ton CO
2
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 CO
2
-to-gas recovery
is taken as 3:1 (37). The leakage rate which is considered
“safe and acceptable” for the underground storage of CO
2
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 industrial-
scale storage of CO
2
in an underground reservoir. In the
Sleipner project CO
2
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 CO
2
is extracted as a byproduct from natural gas production. As
for EOR and ECBM, the source of CO
2
is from electricity
generation, with the recovery of natural resources taking place
during the process of sequestering CO
2
.
The system for the investigation starts with the extraction
of CO
2
from natural gas via amine scrubbing and ends with
the final injection or disposal of CO
2
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 CO
2
from the natural gas is estimated to be
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about 20% less than the same system used to remove CO
2
from power plant flue gases, that is, 240 kWh/ton (8).
5.3. Investigation of CO
2
Sequestration. Many feasibility
and assessment studies pertaining to CO
2
sequestration
methods have been performed. Initial investigations focused
on costs or economical modeling of CO
2
removal systems
(41-42). Others discussed various types of CO
2
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 CO
2
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 per-
spective 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 envi-
ronmentally suitable strategy, the best practicable environ-
mental 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 “CO
2
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 CO
2
emissions entering
the system due to the generation of 1 MWh (functional unit)
from a coal-fired power plant, and ends with the final CO
2
recovered. Stage 3 begins with the same amount of CO
2
entering the system -estimated as 950-kg CO
2
per MWh and
ends with the final amount of CO
2
sequestered or stored. For
the Sleipner project, the functional unit is taken as 950-kg
CO
2
generated from the processing of natural gas.
6.2. Inventory Analysis. The inputs and outputs of a well-
defined 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 CO
2
. 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 CO
2
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 CO
2
generated from the coal-
fired power plant (per MWh), as well as the Sleipner project,
is taken to be 950 kg. The CO
2
removal efficiencies of 95%
(chemical absorption), 82% (membrane separation), 90%
(cryogenics), and 85% (PSA), are employed.
7.1. Results for CO
2
Recovery Methods. Due to the size
and complexity of the studies, the impact assessment results
for the four CO
2
recovery technologies will be compiled in
Table 1.
For GWP, the most promising system for CO
2
postcom-
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 CO
2
from the power plant, its large energy
consumption (600 kWh/ton CO
2
recovered) resulted in
additional greenhouse gas emissions.
FIGURE 1. LCA Methodology.
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Page 3
While the purpose of the postcombustion technologies is
to reduce CO
2
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 SO
x
and NO
x
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 N
2
O and
NO
x
. 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 CO
2
effectively, while imposing lighter energy and waste penalties
for these types of post combustion recovery systems (11).
7.2. Results for CO
2
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 (eutrophi-
cation), 7 (ecotoxicity), 8 (wastes), and 9 (resources).
7.2.1. Global Warming Potential. Intuitively, the Sleipner
project will offer the highest potential for CO
2
sequestration.
The safe storage of CO
2
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 CO
2
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 CO
2
stored and amount of energy spent in the sequestration
process. Dry ice also offers a high percentage of the final CO
2
stored, however, the sublimation process involved imposes
a large energy penalty, which adds unnecessarily to GWP.
For these two options, the final destination for CO
2
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 CO
2
-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 CO
2
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 CO
2
, 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
TABLE 1. Impact Assessment Results for CO
2
Recovery Technologies
CO
2
recovery technologies
environmental impact categories chemical absorption membrane separation cryogenics pressure swing adsorption
GWP (g-CO2-eq) 7.87 × 10
4
1.86 × 10
5
1.79 × 10
5
1.72 × 10
5
acidification (g-SO2-eq) 3.42 × 10
2
7.25 × 10
1
6.21 × 10
2
1.66 × 10
2
human toxicity - Air (m
3
/g) 7.24 × 10
4
1.54 × 10
4
1.32 × 10
5
3.51 × 10
4
human toxicity - water (m
3
/g) 4.02 × 10
-4
8.53 × 10
-5
7.31 × 10
-4
1.95 × 10
-4
eutrophication (g-NO3-eq) 1.53 × 10
3
3.25 × 10
2
2.79 × 10
3
7.44 × 10
2
ecotoxicity (m
3
/g) 2.20 × 10
-2
4.66 × 10
-3
3.99 × 10
-2
1.06 × 10
-2
wastes ((kg/kg) 6.49 × 10
0
1.38 × 10
0
1.18 × 10
1
3.15 × 10
0
resources (kg/kg) 1.65 × 10
-3
3.51 × 10
-4
3.01 × 10
-3
8.02 × 10
-4
FIGURE 2. Total global warming potential results for ocean and geological sequestration.
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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
and the dry ice ocean sequestration option. Other observable
impacts are from the process of CO
2
liquefaction for vertical
injection, pipe towed by ship, and CO
2
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 CO
2
sublimation process (dry ice) both contribute most signifi-
cantly to the graphs. The CO
2
liquefaction process and
pipeline transportation both generate relatively large amounts
of wastewater from the power plant due to substantial energy
demands: 120 kWh/ton CO
2
for liquefaction and about an
average amount of 122 kWh/ton for long distance pipeline
transportation of CO
2
(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.
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Page 5
energy is required for dissolution; hence, leading to nearly
negligible environmental impacts.
7.2.5. Wastes and Resources. One of the biggest environ-
mental 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 CO
2
storage
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
systems, accumulated from CO
2
liquefaction process, trans-
portation, 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 CO
2
mitigation,
geological sequestration seems to be a promising path that
presents the advantage of being able to cope with large
volumes of anthropogenic CO
2
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 CO
2
themselves generate
the recovery of oil and gas. Whereas in the Sleipner case
study, the LCA system boundary starts with the production
of CO
2
(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 & TECHNOLOGY
9
4021
Page 6
selling natural gas.
7.3. Final Scores. The results for the potential environ-
mental impacts (GWP, acidification, human toxicity, etc) for
the four CO
2
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).
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 CO
2
(per
MWh from the power plant or from Sleipner process), to
necessary processes involved in the sequestration methods,
and ending with the final amount of CO
2
stored in the ocean
or underground.
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 CO
2
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% CO
2
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 7. Total ecotoxicity results for ocean and geological sequestration.
FIGURE 8. Total waste results for ocean and geological sequestration.
4022
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 12, 2006
Page 7
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 CO
2
removal with vertical injection.
The second most feasible options are by inclined pipeline
and dry ice disposal, both combined with chemical absorp-
tion for CO
2
recovery. The “worst cases” are displayed by
combining any CO
2
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
CO
2
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, assump-
tions, and estimations. Sequestration effectiveness and
sensitivity analysis for comparing: (i) power plant CO
2
emissions of 950, 970, and 990 kg-CO
2
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 CO
2
seques-
tration is also provided. This material is available free of
charge via the Internet at http://pubs.acs.org.
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membrane 82% -0.04 -0.03 0.00 -0.03 -0.02 -0.02 -0.06 -0.08 -0.12
separation 88% -0.06 -0.05 -0.01 -0.04 -0.04 -0.03 -0.07 -0.10 -0.12
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cryogenics
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pressure 85% -0.04 -0.03 0.00 -0.03 -0.02 -0.02 -0.06 -0.08 -0.12
swing
adsorption 90% -0.06 -0.04 -0.01 -0.04 -0.03 -0.03 -0.07 -0.09 -0.12
VOL. 40, NO. 12, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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ES051882A
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  • Source
    • "A review of CCS life cycle assessments found that reference distances for pipeline transportation of CO 2 ranged from 50 to 500 km (Corsten et al., 2013). While distances upward of 500 km have been reported for Norwegian projects directing CO 2 to oil and gas fields for enhanced recovery (Khoo and Tan, 2006), it is unlikely that the storage of CO 2 in saline aquifers would warrant such distances since there is no monetary incentive. Hasan et al. (2014) used an upper bound of 200 miles (322 km) in their supply chain network optimization model of CO 2 utilization options , including enhanced oil recovery, arguing that pipeline lengths greater than this were unlikely to be a part of the most economical supply chains in the United States. "
    [Show abstract] [Hide abstract] ABSTRACT: Implementation of carbon capture and sequestration (CCS) will increase water demand due to the cooling water requirements of CO2 capture equipment. If the captured CO2 is injected into saline aquifers for sequestration, brine may be extracted to manage the aquifer pressure, and can be desalinated to provide additional freshwater supply. We conduct a geospatial analysis to determine how CCS may affect local water supply and demand across the contiguous United States. We calculate baseline indices for each county in the year 2005, and project future water supply and demand with and without CCS through 2030. We conduct sensitivity analyses to identify the system parameters that most significantly affect water balance. Water supply changes due to inter-annual variability and projected climate change are overwhelmingly the most significant sources of variation. CCS can have strong local effects on water supply and demand, but overall it has a modest effect on water balances.
    Full-text · Article · Jan 2016 · Environmental Modelling and Software
    • "Thermal energy duty is generally avoided. Furthermore, a relatively low environmental impact has been predicted in the literature (Khoo and Tan, 2006). The technology can be adopted for several industrial applications, including CCS (Abanades et al., 2015; Ebner and Ritter, 2009), and an extensive literature can be found regarding processes (Reynolds et al., 2006) and materials "
    [Show abstract] [Hide abstract] ABSTRACT: The main goal of this paper is to provide a comprehensive overview on the performance of an integrated gasification combined cycle (IGCC) implementing CO2 capture through a pressure swing adsorption (PSA) process. The methodology for integrating a PSA process into the IGCC plant is first defined and then a full-plant model is developed. A reference case is outlined both for the PSA-based plant and for an absorption-based plant. Physical absorption is considered the benchmark technology for the application investigated. The full-plant model allowed an assessment of the potentials of PSA in this framework. The plant performance obtained was evaluated mainly in terms of energy penalty and CO2 capture efficiency. Several process configurations and operating conditions were tested. The results of these simulations demonstrated the influence of the PSA process on the overall performance and the possibility to shape it according to specific requirements. A sensitivity analysis on the adsorbent material was also carried out, aiming to establish the possible performance enhancements connected to advancements in the material. Improving the properties of the adsorbent demonstrated to have a strong impact not only on the CO2 separation process but also on the performance of the entire plant. However, nor modifications in the process or in the material were able to fully close the gap with absorption. In this sense a synergetic approach for addressing further performance enhancements is outlined, based on the close collaboration between process engineering and material science.
    No preview · Article · Dec 2015 · International Journal of Greenhouse Gas Control
  • Source
    • "A 2010 base scenario is also included for comparison (García-Gusano et al., 2013). Similar LCA studies have already been undertaken for power plants with CCS technology, both for gas and coal (Brekke et al., 2012; Khoo and Tan, 2006a, 2006b; Koornneef et al., 2008; Korre et al., 2009, 2012; Odeh and Cockerill, 2008; Singh et al., 2011; Stanley and Dávila-Serrano, 2012; Strazza et al., 2012; Veltman et al., 2010), but there is a lack of environmental studies to explore CCS in cement production. Exceptionally, Volkart et al. (2013) detail the impacts of applying CCS on a cement plant in 2025 using ReCiPe midpoint method. "
    [Show abstract] [Hide abstract] ABSTRACT: Although cement production is a very energy-intensive industry which releases huge amounts of pollutants to the environment, there is a lack of environmental studies focused on applying CO2 capture technologies to mitigate global warming in this industry. Furthermore, other environmental and human health impacts are omitted or underestimated. This paper carries out a detailed Life Cycle Assessment of the Spanish cement production in order to analyse the effect of applying post-combustion CO2 capture technology using monoethanolamine as absorbent. Moreover, the work discusses the pros and cons of CO2 capture within the cement manufacture from an environmental point of view. On the basis of the International Reference Life Cycle Data System (ILCD) 2011 midpoint method, results show improvements in global warming, ozone depletion and abiotic depletion potentials but acidification, photochemical ozone formation, eutrophication, human toxicities, ionising radiation, particulate matter, ecotoxicity, and land use potentials are increased by several times. Besides, the paper shows the decisive contribution of the cogeneration plant required to produce heat. It is necessary to carry out more research concerning how to face the energy penalty. Authors strongly recommend exploring natural gas or biomass CHP plants implementation as well as synergies between cement facilities and power plants.
    Full-text · Article · Oct 2015 · Journal of Cleaner Production
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