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Title
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Eng
An Evaluation of Mineral Carbonation as a Method for Sequestration of Carbon Dioxide
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Date
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2007
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Creator
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Eng
Rock, Robert
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Subject
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Eng
Environmental Studies
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extracted text
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An Evaluation of Mineral
Carbonation as a Method for
Sequestration of Carbon Dioxide.
by
Robert Rock
A Thesis: Essay of Distinction
Submitted in partial fulfillment
of the requirements for the degree
Master of Environmental Studies
The Evergreen State College
June 2007
This Thesis for the Master of Environmental Studies Degree
By
Robert Rock
has been approved for
The Evergreen State College
by
________________________
Paul Butler PhD
Member of the Faculty
_______________________
Date
ABSTRACT
An Evaluation of Mineral
Carbonation as a Method for
Sequestration of Carbon Dioxide.
Robert Rock
Abstract: During a period 55 million years ago, referred to as the Paleocene-Eocene
thermal maximum, Earth’s climate was 2oC warmer than at present. Then an influx of
atmospheric carbon dioxide raised Earth’s temperature by another 5oC. Earth’s own
negative feedback brought the carbon dioxide level and the temperature back to
equilibrium over a period of 100,000 to 200,000 years. Today, we again struggle
with excess carbon dioxide in the atmosphere. The most obvious way to get rid of the
excess is to store it in geological formations, such as empty gas and oil reservoirs, but
they are likely to develop significant leaks. A process has been identified to further
react the carbon atom of the carbon dioxide in order to totally neutralize it, using the
same process as Earth had used. This method has been emulated in the laboratory
and could be applied to carbon dioxide as it is generated by point source emitters such
as power plants. Because the reaction products are solid, benign, common in nature,
and insoluble in water, the carbon dioxide treated in this manner is permanently kept
from entering the atmosphere and ocean, and produces no effect on the climate.
While chemical reaction of the carbon dioxide costs more than pumping it into
geologic formations, the environmental impacts are minimal because the storage is
permanent without the possibility of leakage.
Table of Contents
Page
INTRODUCTION
1
THE PALEOCENE-EOCENE THERMAL MAXIMUM
3
Negative Feedback Resulted in Recovery from the PETM
5
THE MINERAL CARBONATION PROCESS
7
The Reaction Occurs in Three Steps
8
Reactor Types
10
Source of Feedstock Mineral
11
METHODS OF GEOLOGIC SEQUESTRATION
11
COST OF GEOLOGIC AND MINERAL CARBONATION SEQUESTRATION
15
ENVIRONMENTAL IMPACT
19
LOCATING THE CARBONATION REACTION SITE
21
DISCUSSION
22
Taking Earth’s Lead on CO2 Sequestration
22
Feedstock Mineral Availability
22
Comparison of Methods for CO2 Storage
23
The Cost of Mineral Carbonation is Greater than Geologic Storage
25
Relative Environmental Impacts of Geologic Storage and Mineral Carbonation 28
Industrial Size Application of Mineral Carbonation Process
30
PLAN FOR ACTION
31
CONCLUSION
32
REFERENCES
34
LIST OF TABLES
Table 1:
Cost of CO2 per tonne avoided
page 17
Acknowledgment: I recognize the amount of work it took on Paul’s part to guide me
through this process. Without his encouragement and significant assistance, this thesis
would not have been possible.
An Evaluation of Mineral Carbonation as a Method for Sequestration of Carbon Dioxide.
INTRODUCTION
Since the beginning of the industrial revolution the burning of fossil fuels has
been emitting significant amounts of carbon dioxide (CO2), which is now believed to be
affecting Earth’s climate. John Tyndall began working with the heat-absorbing qualities
of gasses such as CO2 in 1859 (Crutzen & Ramanathan, 2000). In 1896 Svante Arrhenius
projected that at some point anthropogenic CO2 emissions might cause global warming
(Uppenbrink, 1996). In the early 1960s, Charles David Keeling was able to measure the
CO2 level in the atmosphere and determined that it was steadily increasing (Quay, 2002).
While the scientific community had become aware of the potential for climate change,
little had been done to address the issue.
The Intergovernmental Panel on Climate Change (IPCC) stated in the Fourth
Assessment Report Executive Summary, published April 6, 2007, that as a result of
climate change, it is likely that the health of millions of people, especially the poor and
those with preexisting conditions, would be affected by decreased food security, extreme
weather events, and increased issues of diarrhoeal, cardio-respiratory and infectious
diseases. Now, even with application of the most stringent measures to reduce green
house gases (GHG), further climate change over the next few decades is inevitable
(IPCC, 2007).
As of July 22, 2007, a new Washington State Law becomes effective. In part, it
requires that all new thermal electric power plants to be built with integrated CO2
separation equipment so that a defined amount of excess CO2 production per unit of
1
power generated can be collected for “permanent geological injection” or permanent
sequestration by “other approved means.” So equipped, the units are designated as
“capture ready.” (Washington State Law, 2007) This law, intended to reduce GHG
emissions, complements those of other states and hopefully precedes many more. While
most of the laws in other states provide for a minimum amount of renewable energy that
is to be produced by certain dates, Washington is looking to handle the emissions
problem in no small way through “disposal” of the CO2 generated by power plants.
The US federal government is moving along similar lines. For example, House of
Representatives Bill 1933, introduced April 18,2007, is intended to amend the Energy
Policy Act of 2005 “…to reauthorize and improve the carbon capture and storage
research, development, and demonstration program of the Department of Energy, and for
other purposes.” (GovTrac, 2007/2008).
A number of methods for reducing CO2 emissions are available, including cutting
back on the amount of energy we consume, increasing the energy efficiency of industrial
and personal activities, using renewable and nuclear energy as possible and
storing/sequestering CO2 produced by point sources, either geologically or through
sequestration by mineral carbonation. This paper recognizes the contributions that each
method can provide, but only investigates CO2 storage/sequestration possibilities,
comparing various methods of geological injection to a method of disposing of the CO2
by converting it into an environmentally benign state through an additional chemical
reaction.
2
THE PALEOCENE EOCENE THERMAL MAXIMUM
The Paleocene-Eocene thermal maximum (PETM) was a period in Earth’s past
demarcating the transition between the end of the Paleocene and the beginning of the
Eocene Epoch. It occurred about 55 million years ago (Ma) and is typified by rising
temperatures superimposed on a world that was already about 2oC warmer than it is now
(Bains et al., 2000). High atmospheric CO2 concentration allows short-wave incoming
solar energy to pass unhindered, but once the light energy has struck molecules of matter,
such as the surface of the earth, it is absorbed and reradiated as long wave heat energy.
High atmospheric concentrations of CO2 trap the heat energy, and prevent it from being
reradiated to space. This process is referred to as the “greenhouse effect.”
The PETM greenhouse Earth condition occurred as a result of a large quantity of
CO2 of uncertain origin that had been released into the atmosphere and ocean. The
carbon atoms in the CO2 had a very low ratio of heavy 13C stable carbon isotope atoms in
relation to 12C (δ13C). The low δ13C occurred as a result of the CO2 influx having been
previously filtered by photosynthesis, which selectively accepts 12C because of its slightly
lower atomic weight and resulting slightly different chemical properties (O’Leary, 1988).
As such it is generally accepted that high 12C sedimentary methane hydrates, formed from
decayed organic matter, situated interstitially within sediments at the ocean’s floor along
continental slopes, dissociated from their stable clathrate conformation en masse in
response to an increased ocean-bottom heat increase of about 4oC. When the carbon
dispersed into the ocean and atmosphere, it resulted in a reduction in ambient δ13C (Kelly
et al., 2005).
3
There is evidence that voluminous flood basalt magmatism, accompanying the
opening of the North Atlantic Ocean, intruded carbon-rich sedimentary rocks that
resulted in the production of metamorphic methane, which acted as a trigger for the
massive release of methane hydrate carbon at the Paleocene-Eocene boundary (Storey et
al., 2007). Methane hydrates appear to have been the source of the carbon for the CO2
GHG that caused the 5oC thermal increase. Anthropogenicly produced CO2 may act as a
trigger for our current inventory of methane hydrates.
The PETM manifest itself through global surface temperature increases of 5-7oC
over a period of about 30,000 years, producing a mean surface air temperature 6.7-8.5oC
warmer than today as a result of the elevated levels of atmospheric greenhouse gases.
The heat persisted for about 60,000 years (Bains et al., 2000). The warming has been
described as “intense” (Bains et al., 2000) and “particularly rapid” with “extreme
magnitude” (Kelly et al., 2005). The PETM is an excellent, perhaps the best, analogy to
our current increasingly warm climate resulting from atmospheric CO2 (Pagani et al.,
2006). Because of the quick warming over 30,000 years and the subsequent gradual
return over a period of 100 to 200 thousand years (ky), the PETM event suggests that the
climate system had been in disequilibrium (Kennett & Stott, 1991) and experienced
negative feedback that reduced greenhouse gas concentration and buffered the ocean’s
acidity (Kelly et al., 2005). Earth’s response to the global warming resulting from the
rapid flux of atmospheric carbon in the lower Tertiary Period may be the best example in
the geologic record of the Earth’s negative feedback to high CO2 (Bains et al., 2000).
4
Negative Feedback Resulted in Recovery from the PETM
As the ocean absorbed vast quantities of CO2 during the PETM, it became more
acidic. Dissolved CO2 in the ocean reacts with carbonate ions (CO3-), to produce alkaline
cations (Mg+, Ca+) and bicarbonate anions (HCO3-), buffering the ocean against the
acidification. This reaction allows the diffusion of replacement CO2 molecules into the
ocean, while sacrificing the ocean’s carbonate sediments. This effect represents the first
stage of Earth’s negative feedback mechanism against excess CO2 levels (Kelly et al.,
2005).
As a result of the high concentration of atmospheric CO2, temperature and
humidity during the PETM were also very high, resulting in a lot of rain that supported
an abundance of land plants. High runoff rates would have carried the decayed organic
matter resulting from plant decomposition to the ocean, and may have supported 60,000
years of sustained blooms of marine phytoplankton. The associated sequestration of
excess CO2 by marine phytoplankton, some of which sank to the bottom when the
organisms died, caused the accumulation of carbon in the sediments (Bains et al., 2000).
Abundance of kaolinite, a clay mineral, found in the stratigraphy at the PETM supports
the idea that continental weathering and runoff were elevated at this time (Kelly et al.,
2005).
A positive feedback system operated during the PETM that worked to maintain
high atmospheric CO2. Biochemical kinetics suggests that nearly all animals experience
a doubled rate of basal metabolism in response to a 10oC increase in temperature. As
abyssal zone (2000-6000 meters deep) temperatures were about 10oC higher than current
day, expected organic-carbon burial was drastically reduced from what would be
5
expected because of the increased consumption required by the organisms for
maintenance of basal metabolism (A. Olivarez Lyle & M.W. Lyle, 2006).
In addition to these feedbacks, the vast majority of permanent sequestration of
CO2 occurred as a result of accelerated weathering reactions of silicate rocks on land.
The chemical reactions would have generated an influx of bicarbonate ions and various
cations in solution into the ocean, further neutralizing the ocean’s acidity and increasing
the bicarbonate content until CO2 equilibrated (Kelly et al., 2005; Zachos et al., 2005).
Silicate rock weathering on land follows the general formula:
CaSiO3 + 2CO2 + H2O Æ 2HCO3– + Ca2+ + SiO2, and subsequent precipitation of calcite:
HCO3– + Ca2+ Æ CaCO3 +CO2 +H2O. Two CO2 molecules are consumed during the
silicate rock weathering, but only one is released during precipitation of calcite for a net
uptake of one molecule. As a result of this reaction, the large amount of carbon dioxide
generated during the PETM was permanently sequestered through weathering of silicate
rock and precipitation of calcite in the form of limestone (Zachos et al., 2005).
A laboratory process for reacting CO2 with solid minerals to produce other solid
minerals has been in development since 1995. The process mimics Earth’s CO2 negative
feedback through weathering of silicate minerals. In nature, high CO2 levels in the
atmosphere result in warm temperature, high humidity and a lot of rain containing
significant carbonic acid. These conditions result in a very slow process of carbonate
mineralization at a rate of about 100 million tons of carbon per year (Seifritz, 1990),
which is less than 2% of global annual anthropogenic carbon emissions. Industrial
6
application of this process could provide CO2 sequestration of nearly all of the CO2
produced at any point source at a greatly accelerated rate, as a highly stable solid, for
geologically significant time.
MINERAL CARBONATION, THE PROCESS
The method of chemically reacting CO2 with another substance for the purpose of
removing it from the atmosphere was first referenced by W. Seifritz of Switzerland in
Science Magazine in 1990. He suggested that a chemical reaction between CO2 and a
common silicate mineral (feedstock) such as olivine (Mg2 SiO4) would react the carbon
atom into a permanently stable, solid carbonate form, magnesite (MgCO3), commonly
found in nature (O’Connor et al., 2000).
The process is referred to as carbonation of silicate minerals. It is different than
other CO2 sequestration schemes in that it emulates Earth’s own CO2 negative feedback
system that operated over a period of 105 years, reducing temperatures at the end of the
PETM (Dickens et al., 1997; Kennett & Stott, 1991).
Various process methods of running the reaction have been tried. The most viable
is referred to as the “aqueous process,” where water is added to pulverized magnesium
(Mg) or calcium (Ca) silicate mineral to form a slurry. Mg and/or Ca cations are
extracted from the mineral as a result of mineral dissolution in the water, and react with
CO2 as it is bubbled through the slurry. This method results in the precipitation of the
carbonate minerals, magnesite or calcite, plus silica gel (SiO2) as end products (Lackner,
Wendt, Butt, Joyce Jr. & Sharp, 1995).
7
The Reaction Occurs in Three Steps
First, the aqueous dissolution of the CO2 produces carbonic acid, which in turn
dissociates to a hydrogen ion (H+) and bicarbonate ion (HCO3-) (aq).
CO2 (g) + H2O (l) Æ H2CO3 (aq) ÆH+ (aq) + HCO3- (aq).
Second, the silicate mineral is hydrolyzed by the H+ causing the magnesium ions
(Mg2+) or calcium ions (Ca2+) to leach out of the rock and react with the HCO3-, forming
a stable carbonate magnesite (MgCO3) or calcite (CaCO3), plus silica (SiO2).
Mg2SiO4 (s) + 4H+ (aq) Æ 2Mg2+ (aq) + SiO2 (s) + H2O (l).
Third, the resulting carbonate mineral precipitates:
Mg2+ (aq) + HCO3- (aq) Æ MgCO3 (s) + H+ (aq).
(O’Connor et al., 2001)
The key to harnessing the carbonation process for controlling fossil fuel CO2
emissions is development of an economically competitive process that mimics natural
weathering, but occurs at a greatly accelerated rate. The reaction occurs slowly at
ambient and elevated temperatures when the pressure is below 7.4 MPa (73
atmospheres), which is the threshold for the supercritical phase of CO2. Above 7.4 MPa
at or above 31oC, the reaction rate increases significantly. Simply raising the temperature
and pressure causes the reaction to occur even more quickly (O’Connor et al., 2000).
There is an optimal temperature, which if exceeded, causes the chemical equilibrium to
shift in favor of free CO2 molecules. Inspection of the free energies of the reactants and
8
products allow computation of the highest temperature possible for the carbonation
reaction, revealing the quickest and least expensive process application (Lackner et al.,
1995).
The carbonation reaction occurs at the surface of the olivine crystal particles,
coating them with carbonate and silica reaction products. The silica-rich “passivating
layer” is thought to be rate-limiting. Observation of crystal surfaces during the
progression of the carbonation process revealed that outside crystal corners routinely
showed erosion, with intergrowth of olivine, silica and magnesite. It was clear that the
erosion and intergrowth occurred during carbonation process, and is illustrative of the
reaction progressing from the surface of the olivine crystal, inward. The layers are
brittle, partially crystalline and partially amorphous. The layers are strained as a result of
reorganization of the molecules. As such, the layers are subject to fracture and
subsequent exfoliation. Adding some larger feedstock particles may promote abrasion at
the surface of the particles, increasing the fresh surface available for reaction and
reducing the cost of grinding. The resulting exposure of a fresh olivine surface enhances
the reaction (Béarat et al., 2006).
Continuous stirring of the reactants in conjunction with good dispersion of
supercritical CO2 resulted in a 90% conversion within 24 hours (O’Connor et al., 2000).
The rate of carbonate precipitation is dependent on the concentration of bicarbonate ion
in the slurry. Experimental results indicate that the silicate dissolution rate is controlled
by surface features, such as the amount of area presented for the reaction of the feedstock
(O’Connor et al., 2001). In order to enhance passivating layer exfoliation, reactions are
stirred causing collisions among the slurry particles, resulting in exfoliation of the silicate
9
surface features on the feedstock crystals (Béarat et al., 2006). Optimal particle size for a
rapid reaction rate requires that some particles are less than 37 microns, though much of
the material may be somewhat larger. The addition of abrasive particles such as quartz
results in significant increase in the rate of carbonation rate increase. In addition,
inclusion of larger feedstock particles may encourage the exfoliation due to abrasion.
Albany Research Center has experimented with natural weathering, particle sizes and
stirring the reactants in order to determine optimal conditions (O’Connor et al., 2001).
Improvements are still being made to the process (Béarat et al., 2006).
Reactor Types
The original carbonation reactions were conducted in an autoclave or other high
pressure vessel with a furnace, and a stirred-tank-reactor, in order to produce continuous
heating and stirring. Records were kept of individual experimental parameters including
pressure, temperature, particle size, rate of stirring, and use of additives (O’Connor et al.,
2000).
Two types of full-sized reactors currently considered for sequestration plants are:
the high temperature and pressure fluidized bed (batch-type), and the continuous pipeline
reactor. The continuous pipeline reactor has provided results better than that provided by
the autoclave system during small scale experiments. It is believed that mixing resulting
from pipeline handling increases the particle-particle abrasion that is responsible for
removing the silicate passivating layer and possibly the carbonated layer, exposing more
reactive fresh surfaces (O’Connor et al., 2000; Béarat et al., 2006; Huijgen & Comans,
2005).
10
Source of Feedstock Mineral
Potential feedstock minerals are available as a result of large sections of ancient
ocean floor that have been thrust onto certain continental margins as a result of
continental-oceanic crustal plate collisions, which left portions of ocean floor at the
surface of continental crust (Voormeij & Simandl, 2004). The western coast of North
America and the eastern United States contain large amounts of this material. Over
periods of millions of years, much of the original rock had been hydrated, creating the
mineral serpentine, which is useful as feedstock, though the chemically bound water
molecules must first be driven off by a costly heating process.
Enormous quantities of potential feedstock minerals are readily accessible
(Lackner et al., 1995). An example of a possible feedstock mineral deposit is the Twin
Sisters dunite massif, situated in northwest Washington State about 40 km east of
Bellingham. The body is 16 km long, 5.5 km wide and about 2 km thick. The top of this
unit is just over 2,000 meters elevation, placing the base of the structure near sea level.
The body is composed entirely of dunite. As the northwest section is virtually
unserpentinised, it could provide ideal feedstock for the process (Voormeij & Simandl,
2004).
METHODS OF GEOLOGIC SEQUESTRATION
In 2005 there were about 15,000 significant point sources of CO2 worldwide.
Approximately 8,000 of those were power plants. The balance included steel mills,
chemical refineries, cement kilns and other industrial plants (Dooley et al., 2005). Just
11
over fifty percent of US electricity is generated by coal fired power plants (David, 2000).
As all of these plants are fixed in location and produce large amounts of CO2, each
presents an opportunity to reduce emissions through capture and storage.
In order to store the CO2, it must first be separated from the rest of the flue gases,
which are mainly nitrogen. A point source such as power plant must be equipped with a
CO2 capture module, which may employ either a membrane or a chemical-reaction for
the separation process. Adding a capture system to an existing plant is complicated and
expensive, but can be more economically built into a new plant (MIT, 2007). Thus
equipped, a plant is designated as “capture ready.” The nearly pure CO2 stream is
compressed into a quasi-liquid referred to as its “super critical” state, and is pumped
through a pipeline to its storage or disposal site (Lackner et al., 1995).
There are several methods of carbon dioxide storage/sequestration that have been
described in the literature. CO2 can be injected into the deep ocean; into oil and gas
reservoirs, regardless of whether the reservoir is depleted of hydrocarbons; into deep
saline aquifers; into unminable coal beds and flood basalt beds; absorbed by natural
sinks, such as forests; or through carbonation of silicate minerals as previously referenced
(Task Force, 2007).
In the deep-ocean-storage scenario, supercritical CO2 would be bubbled into and
dissolved in the deep ocean water. Greater depth of injection provides a longer period
before the CO2 is reintroduced into the atmosphere. Use of the ocean as a CO2 repository
guarantees that storage will be temporary, as it will release much of the dissolved CO2
when thermohaline circulation has brought the injected parcel of water back to the
surface. The method also has negative consequences for marine life. The increase in
12
dissolved CO2 will lower the ocean pH as a result of the formation of carbonic acid,
reducing the availability of carbonate ions necessary for the growth of organisms that
build their shells out of calcium carbonate. As deep ocean ecosystems are not well
understood, monitoring for unexpected consequences must be carried on (Caldeira,
2002). Currently the ocean turnover time is 800 to 1000 years, much too short a
timeframe to be considered as long-term solution (Lackner, 2002).
The process of injection of supercritical CO2 into partially depleted oil and gas
formations can be is used to displace fossil fuel not retrieved through primary recovery
from the reservoir via another borehole. This is referred to as secondary or enhanced oil
recovery (EOR). The CO2 used in this fashion can be left in the reservoir once secondary
recovery has been completed. While this may be appropriate on a small scale for the
purpose of increasing well production, it may not be appropriate for the wholesale storage
of supercritical CO2 because of the possibility of leakage.
As a result of the loss of certain structural support provided by the original fluid
pressure, the previously nonpermeable reservoir walls and/or caprock may become
cracked and incompetent to contain the CO2. Any subsequent repressurization needs to
be evaluated on a case by case basis (Task Force, 2007). As well, there are many
unrecorded abandoned holes that have been bored into reservoirs and not soundly
plugged. Early in US drilling history, there were no regulations requiring resealing of
boreholes. Later, lack of proper materials and enforcement ensured that leakage from
these formations is very likely (Massachusetts Institute of Technology [MIT], 2007).
Major tectonic events such as thrust-fault earthquakes are expected to occur every few
13
hundred years in the Pacific Northwest, and could compromise the integrity of recharged
structures leading to massive outgassing of CO2 (Atwater et al., 2006).
Risk assessments are being developed with respect to various types of geologicstorage failure. Categories of risk include: features such as caprock thickness and
reservoir permeability; events including well blow outs; seismic activity; accidental
penetration of filled reservoirs by new wells; and processes including geochemical stress
changes and chemical reactions within the chamber. Risk mitigation is based on careful
site selection, monitoring, oversight and fixing problems that arise. Remediation for
leakage is to attempt repair of the leak using oil and gas industry techniques. If that
doesn’t fix the problem, the CO2 must be removed and put into a more competent
reservoir (IPCC, 2005a).
A deep saline aquifer is a geologic formation that had been formed by original
deposition of sand and or gravel. It is isolated from the sediments above and beneath it
by an impervious cap and base; the pore-space is filled with saline ocean water. CO2 can
be pumped into such a structure, as has been done commercially since 1996. Located in
the Norwegian North Sea, the Sleipner gas field is operated by Statoil, an integrated oil
and gas company. Statoil separates most of the naturally occurring CO2 from the natural
gas that is extracted from a 3500-meter-deep formation. The CO2 storage project has
been run, in large part, to legitimately avoid paying Norwegian emissions tax (United
States Department of Energy, 2007). Annually, Statoil injects roughly 1 million tonnes
of compressed, supercritical CO2 into the Utsira formation, a high-permeability, saline
filled, unconsolidated sandstone unit that is about 250 meters thick and located at a depth
of about 800 meters beneath the sea floor. The location of the injected CO2 has been
14
followed through time-lapse seismic survey methods. As of 2005, the CO2 bubble
covered about 5 km2 (IPCC, 2005a), occupying the pore-space of about 1.25 km3 of the
unit. Supercritical CO2 has a density of about half that of water (Schrag, 2007), resulting
in a slight decrease to the force of gravity at that location. In 2002, a microgravitychange survey baseline was established. Repeat surveys will provide high accuracy timelapse data (Zumberge & Eiken, 2003).
Enhanced coal bed methane technology is in an early development stage (Task
Force, 2007), as is the use of flood basalt formations for sequestration. If the flood basalt
sequestration process is subsequently shown to be viable, the United States and India
would have such geologic storage available (McGrail et al., 2006). Forest sequestration
has a limited capacity and a lifetime of only a few hundred years because the trees will
die and the sequestered carbon will then be reemitted to the atmosphere as the wood
decays.
COST OF GEOLOGIC VERSES MINERAL CARBONATION SEQUESTRATION
Capital cost of capture equipment is a major factor for any type of
storage/sequestration because CO2 must be first separated from the exhaust stream of the
point source emitter. The new Washington State Law mentioned earlier requires in part
that any new power plant built in the state for electricity generation must be capture ready
(Washington State Laws, 2007). Once a plant has been equipped with a separator, the
capital cost of the equipment can be ignored in absolute cost comparisons between
methods of emission avoidance practiced at that plant.
15
Cost of operating the capture equipment only accrues when it is actually being
used to separate CO2 from the rest of the flue gas. The cost of capture includes
compression of the CO2 into a supercritical state. Using an Integrated Gasification
Combined Cycle (IGCC) power plant to compare the cost of operation of the entire plant
both with and without separation, the cost of capture has been determined to be about
$15/tonne, at 85% capture efficiency (Table 1; Gielen, 2003).
Cost of transporting the CO2 is relatively inexpensive by pipeline, or by ship if
over long distances. By pipeline, cost may range from $1 to $10/tonne. Actual cost
would vary as a result of difference in transportation distance and volume (Gielen, 2003).
Making an assumption that CO2 would need to be piped a similar distance to either a
carbonation plant or to a geologic repository; the same cost of shipping a tonne can be
applied. I’ll use $5/tonne for comparison (Table 1).
Cost of mining, crushing and milling of ore is estimated at $8/tonne based on
mining activity in other industries (Lackner et al., 1998). That value is used in Table 1.
16
TABLE 1. Cost of CO2 per tonne avoided
Geologic
Carbonation
Capital cost capture equipment
fixed
fixed
Cost of operating capture eqpt
15
$15
(Gielen, 2003).
Cost of transportation
5
5
(Gielen, 2003)
Mining and milling
n/a
7
(Lackner, 1998)
Cost of the mineralization
n/a
54
(O’Conner, 2004a)
Cost of geologic sequestration
5
n/a
(IPCC, 2005a)
Cost of mitigating leakage
unknown
0
Ancillary income
unknown
Subtotal
unknown
25
81
Factor
x1.0
x1.3
Total
$25
$105
(sequestered to avoided)
17
Cost of the baseline mineral carbonation process using dunite, the most favorable
feedstock mineral, is approximately $54/tonne for CO2 sequestered and $78/tonne for
CO2 avoided (O’Connor et al., 2004). A tonne of CO2 “avoided” includes a tonne of CO2
produced by the power plant during power generation for the grid plus another .3 tonne
CO2 created as a result of operating the carbonation process. I’ll assume that the first
plants deployed will be near the most favorable bodies of minerals such that the
carbonation cost will be $54/tonne disposed (Table 1).
Studies of the cost of using depleted gas or oil wells, or deep saline aquifers for
storage produce widely disparate figures ranging from $0.20 to $30/tonne of CO2 stored.
A large Australian data set indicates that available onshore storage sites would average
about $5/tonne (Table 1; IPCC, 2005a). Cost of monitoring filled geologic formations is
estimated to be about 3¢/tonne. This small amount is excluded from Table 1. Cost of
mitigating leakage is potentially enormous though unknown (IPCC, 2005a), as such; it is
excluded from Table 1.
Potential ancillary income as a result of mineral carbonation feedstock mining and
processing may result through the recovery of other minerals such as iron, manganese,
copper chromium, nickel and cobalt (Voormeij & Simandl, 2004), with an early estimate
of $10/tonne of CO2 disposed (Lackner et al., 1998). In the case of geologic storage with
EOR application, there could be revenues in excess of the cost of acquiring the reservoir
and handling the CO2 (IPCC, 2005a). Cost of remediation of leaking storage has not
been estimated (IPCC, 2005a). As there are no reasonable estimates that can be used for
either storage or sequestration method, neither potential cost reduction nor contingent
remediation expense for either route is considered in Table 1.
18
ENVIRONMENTAL IMPACT
Environmental impact is inevitable for virtually any large-scale activity associated
with sequestration or carbonation. In the case of ocean storage, CO2 is injected into the
marine environment. The deepest injection, at 3000 meters, provides the longest-lasting
storage. After 1,000 years, 40% of the injected CO2 would have reentered the
atmosphere (Caldeira, 2002). Because this disposal results in a lowering of ocean pH and
the eventual rerelease of CO2 to the atmosphere, this method is simply not a viable
option.
Geologists and engineers have a poor understanding of how abandoned wells will
behave over long periods of time (IPCC, 2005a). Significant leakage is a distinct
possibility. Many natural reservoirs of oil and gas have held pressurized deposits for
millions of years, but there are empty reservoirs that obviously held oil or gas in the past,
which has leaked out. There are reservoirs that currently leak without human
intervention. As the formations best suited for storage have maintained their competency
until their contents were removed, it would appear that they would best serve to store
CO2. This is not necessarily the case. During exploration and extraction, holes are
drilled through the caprock. As contents are withdrawn, pressure is reduced, thus the
chamber may settle, as evidenced by sinking of the ground above a depressurized gas
structure. Settling may cause the caprock to crack. Subsequent repressurization with
CO2 may cause the reservoir to start leaking, not necessarily right away or even in the
immediate vicinity of the bore hole (Gielen, 2003). Potential reservoirs may have
existing wells that penetrate the caprock (MIT, 2007). Even a small rate of leakage
19
would produce a significant effect, like a dripping water faucet, and as there are no data
on leakage rates for underground reservoirs, estimates are highly variable. Thus, the
storage site must be monitored indefinitely.
Saline aquifers are common throughout the world, offering potentially large
volumes of storage. Because of the lack of any existing database, such as exists for gas
and oil reservoirs, saline aquifers have not been well-characterized (Voormeij & Simandl,
2003).
It is important to recognize the potential for ecological damage that could result at
Earth’s surface pursuant to storage/sequestration activity. The possibility exists for both
geologic and mineral carbonation methods. The difference in overall impact lies in the
likelihood of CO2 leakage into the atmosphere from geologic repositories, compared to
localized ecosystem damage resulting from operation of both the fossil fuel and feedstock
mines in the case of mineral carbonation.
When fossil fuel hydrocarbon molecules are oxidized through combustion to
release energy they produce CO2 as a waste product. It is not necessary that the CO2
produced by combusting fossil fuel be released to the atmosphere, rather the carbon can
be further chemically reacted to move it into a stable state as a mineral carbonate
(Lackner et al., 1998). Mineral carbonation provides a solution to the CO2 emission issue
without the potential for even accidental release; the process itself does not generate
harmful by-products (Voormeij & Simandl, 2003). Through mineral carbonation, the
carbon atom in the CO2 is further oxidized, releasing about 1.5 kilojoules of heat energy
per tonne of CO2 released (Zevenhoven, 2004). The more highly oxidized carbon atom
20
cannot revert to CO2 without the addition of heat. It is therefore chemically very stable
and has little environmental impact (Lackner et al., 1998).
LOCATING THE CARBONATION REACTION SITE
In order to process a tonne of CO2, it would take 2.3 to 3 tonnes of feedstock
mineral (Voormeij & Simandl, 2003). Because of the high cost of transportation, the
liquid and less massive CO2 would be moved to the carbonation site for reaction.
Processing will be done at the mineral mine site and the resulting solid reaction products
would be stored in place of the removed feedstock mineral (Lackner et al., 1995).
Alteration of the ecosystem will occur only at the surface of the feedstock mineral
mine site. The mineral would be removed from the mine, processed and the resulting
carbonate returned to the void. As a result of mining activity, the overburden, or other
material removed and subsequently returned to the excavation site becomes more
voluminous. Change to the surface profile at the feedstock mine will occur as a result of
this so-called “swell factor.” Mining regulation CFR §816.105 (b) (3) requires
application of the concept of “approximate original contour” such that replacement of the
“gangue,” or spoil materials follows the original land configuration as closely as possible.
Use of vegetation cover and construction of windbreaks should provide adequate control
of potential soil erosion or landslides at the site, which should comprise the major
potential environmental impacts (IPCC, 2005c). Since mineral carbonation requires
mining activity and attendant surface disruption at both the feedstock mineral mine and
the coal mine, and assuming similar impacts at each mining activity, roughly twice the
costs (both monetary and environmental) would result.
21
DISCUSSION
Taking Earth’s Lead on CO2 Sequestration
Atmospheric CO2 concentrations have been high on Earth before, for example
during the PETM. In response, Earth operated its own negative feedback system to
restore equilibrium. This should provide a valuable example as to whether mineral
carbonation verses geologic storage ought to be deployed as the principal method of
reducing CO2 emissions.
Feedstock Mineral Availability
Potentially useable feedstock minerals are vastly more available than all the fossil
fuel on Earth and are to be found in easily accessible sites (Lackner et al., 1995). If
serpentine is also found to operate well under industrial conditions, there will be no
shortage of feedstock minerals in British Columbia and along the US coasts; however, if
only magnesium olivine (forsterite) is found to be acceptable for the process, supplies of
potential minerals would be both limited and available in only a few areas (Voormeij &
Simandl, 2003). Based on the work done so far, it is likely that if olivine is found to
operate well on an industrial scale, serpentine would also work, though it would be more
expensive because of the extra energy required to activate it. The best feedstock mine
site in the Pacific Northwest is Twin Sisters dunite, located 40 miles east of Bellingham,
Washington.
22
Comparison of Methods for CO2 Storage
Pumping CO2 into the ocean is not a viable option due to the short residence time
until it is re-emitted. The public appears to consider use of the ocean as a CO2 repository
to be the least acceptable method (Voormeij & Simandl, 2003). Pumping supercritical
CO2 into geologic formations is also not a good idea because of its potential mobility and
because of the difficulty of remediation if a problem is detected.
Use of saline aquifers opens large volume for storage of CO2, but concern for
safety and potential leakage may significantly reduce availability. As CO2 is injected,
reservoir pressure would immediately begin to rise, which could lead to fissure widening
and seismic activity providing leakage pathways (Lackner, 2002). As the CO2
underground is buoyant, it would tend to rise to the surface upon escape from immediate
containment. Concerns over safety and leakage would substantially limit the amount of
storage these aquifers might provide (Lackner, 2002). Site-specific data are necessary in
order to accurately evaluate the competence of a particular saline aquifer, and only those
units that have undergone detailed evaluation could be considered for actual storage.
Once a number of aquifers have been evaluated, general guidelines could be developed
(Task Force, 2007), but so far there are only limited data on specific potential reservoirs.
While Statoil is monitoring its Sleipner project for leakage with seismic and
micro-gravity time series surveys (Zumberge & Eiken, 2003), the company won’t be able
to detect loss until it actually happens. Leakage could occur if the CO2 bubble reached a
major breach in the saline formation’s caprock. It may not be possible to remediate such
a high volume reservoir 800 meters beneath the sea floor. Research is continuing as to
whether deep saline aquifers should properly be considered permanent over geologic time
23
or merely temporary storage (McKelvy et al. 2002). Continuous monitoring is necessary
because of this lack of certainty.
Although years of experience with CO2 injection for enhanced oil recovery
projects have accumulated, little data about leakage have been collected. Leakage rates
might rise as reservoirs fill. If several millennia pass before substantial leakage of the
geologically stored CO2 occurs, Earth’s carbon cycle will have time to redistribute it
among natural systems (Schrag, 2007). Alternately, if all Earth’s hydrocarbons were
oxidized and the CO2 stored geologically, the secure storage lifetime would need to be
many thousands or even many tens of thousands of years (Lackner, 2002).
It is unknown whether any geologic reservoirs are competent to contain stored
CO2 without leaking over periods of tens of thousands of years. The manager of Statoil’s
Saline Aquifer Carbon Dioxide Storage Project states that the storage is highly unlikely
to leak for the next several hundred years, and that staying there for the next 5,00010,000 years “must be good enough” (Statoil, 2004). Computing the amount of carbon
that will result from combustion of our remaining 200 years worth the coal world wide
(United States Department of Energy [DOE], 2004), at our current rate of emission of 6
gigatonnes (Gt) carbon/year from combustion of fossil fuels (David, 2000), provides a
figure of 1,200 Gt carbon produced. Assume that all of it is stored geologically and leaks
out during the following 10,000 years. That occurrence would constitute a carbon release
of a similar amount, over a similar period of time, as the approximately 1,500 Gt of
methane carbon that had generated the PETM (Storey et al., 2007; Dickens et al, 1997).
While the PETM endured for a period of 30,000 years, the amount of time that the CO2
remained in the environment, the onset or δ13C excursion segment lasted between
24
approximately 6,000 years (Kennett & Stott, 1991; Kelly et al., 2005) to 10,000 years
(Dickens et al., 1997). Using Statoil’s most conservative estimate of storage life,
significant leakage could conceivably generate a new atmospheric thermal spike similar
to the PETM.
Because of this wide range in predictions for the rate of leakage from geologic
storage sites, the validity of this option for storage of CO2 is uncertain. In order to solve
our emissions problem in part through storage, we need to remove the CO2 permanently
rather than just temporarily storing it. Pumping it into a reservoir with the hope that it
will stay there long enough that sufficient atmospheric dilution will occur as it leaks out
does not constitute adequate planning. It is irresponsible to deal with a major problem
such as high atmospheric CO2 concentration by setting up another in its place because it
is cheaper or easier than fixing the problem. More completely oxidizing the carbon to its
carbonate form is arguably the best way to handle our CO2 problem. Mineral carbonation
sequesters CO2 on a geologic timescale with virtually no chance of release at a later time
(Voormeij & Simandl, 2003). It is virtually certain that after a millennium, 100% of the
carbon sequestered through mineral carbonation will remain in place (IPCC, 2005c).
The Cost of Mineral Carbonation is Greater than Geologic Storage
The new Washington State Law requires in part that new thermal electric power
plants be equipped to store a certain excess amount of the CO2 produced. While it will
take decades for new plants so equipped to replace the present plants, the future will
include CO2 storage/sequestration. The decision to store CO2 in geologic repositories or
25
through mineral carbonation will be largely decided by whether the low cost of the
geologic storage option or the certainty of mineral carbonation prevails.
Using the estimates provided in Table 1, evaluation of the relative cost of
geologic storage verses mineral carbonation provides a total cost of geologic storage of
$25 per tonne of CO2 disposed, plus unknown potential liability if leakage occurs. The
total cost of mineral carbonation is estimated at $105 per tonne of CO2 avoided with no
lingering unknowns. These figures translate roughly into an additional cost of electricity
(COE) of 2.5 ¢ per kilowatt hour (kwh) for geologic storage and 10.5¢/kwh for mineral
carbonation, based on an estimated additional 1¢ per kwh electricity per each $10/tonne
of CO2 stored or sequestered.
The average residential COE in January 2007 over the entire United States was
10.05¢/kwh. Idaho enjoys the cheapest rate at 5.69¢/kwh; Hawaii has the most expensive
at 21.77¢/kwh. Industrial, commercial and agricultural rates are somewhat lower for
each state (Energy Information Administration, 2007). Adding 10.5¢/kwh for mineral
carbonation would double the COE overall. This amount would pay for the entire
program of running mineral carbonation for all power plants in the USA.
The new Washington State Law explicitly provides that the costs associated with
emissions reductions will be internalized by the electric utilities (Washington State Laws,
2007). Ultimately the cost will be paid by the consumers. The COE increase would
occur over a period of decades as old, inefficient power plants are replaced with efficient,
capture-ready plants with carbonation equipment. The additional COE would seem to be
manageable by consumers without serious disruption to the economy. Certain relief
against the additional cost would be provided by the stimulation of the economy that may
26
be expected as a result of developing a clean power system. Over just the last few years
the cost of gasoline in the US has tripled, but the economy has not been crippled by the
increase.
In our current market economy, supply and demand determines price. The
practice eschews lives forfeited and health forgone as a result of ignoring externalities.
As mortality and morbidity are not directly convertible to a dollar amount, an appropriate
dollar value cannot readily be fixed for them. A working estimation of the externalized
cost of a tonne of CO2 emitted has been suggested by the IPCC Working Group II
through evaluation of peer-reviewed estimates for 2005. The values range from $3 to
$130/ tonne with a mean of $12/tonne (IPCC, 2007). If this figure is accurate, it would
appear unwarranted to even spend the money on geologic sequestration at $25/tonne. It
would be informative to see whether and how the cost of deaths, health, hurricane
activity, sea-level rise and destruction of ecosystems has been handled in arriving at the
estimate. In addition, the cost of geologic storage only includes acquisition of the right to
use a particular reservoir for disposal and the preparation and transportation of the CO2; it
does not include any assumed liability for the leakage that may ultimately occur. Legal
issues, such as who owns the disposed-of CO2 and what remedies, both mechanical and
financial, should be pursued in case of loss of integrity of the structure need to be
resolved. Permanent liability may be assumed against the owner (Herzog, Caldeira &
Reilly, 2003). A funded insurance mechanism backed by a government guarantee would
need to be implemented in order to handle the possibility of catastrophic and/or long-term
storage failure (MIT, 2007). While the cost of such potential problems must be added to
the $25/tonne cost of storage for geologic formations to arrive at a more realistic figure
27
for comparison to mineral carbonation, the appropriate dollar amount is unknown (IPCC,
2005a).
The cost of mineral carbonation in excess of geologic storage may provide
commensurate benefits by alleviating the uncertainty of temporary storage. At the crux
of this debate, paying for the externalities of emissions generated, along with the
electrical power by the electric utilities, and ultimately the consumers, is appropriate and
should have been considered long ago. As mineral carbonation does not suffer the
drawback of leakage that is possible with geologic storage, the cost disparity between the
methods is not as large as it appears based on comparing the figures in Table 1, but the
difference in the level of potential ecological damage is enormous.
Our society desires to provide health care for everyone because it is a social rather
than an individual issue. Reduction of communicable disease benefits everyone even
though the cost is not equally borne. In the case of CO2 storage, most of the cost to us
and to future generations will be paid as we go via our power bills, regardless of the
storage method used. Public funds, taxes and subsidies will assist in handling the
transition. The financial cost of doing it right can be managed.
Relative Environmental Impacts of Geologic Storage and Mineral Carbonation
The risks associated with long-term underground storage are not well understood.
Use of geologic reservoirs is considered to be temporary storage due to the paucity of
knowledge regarding the potential for both gradual leakage and accidental release over
long periods of time (IPCC, 2005b). Risk factors include above-ground issues such as
pipeline failure and well-head leakage. Experience in oil and gas industries provides
28
certain expertise useful in making a reasonable determination of such risk and in setting
up contingency plans for handling potential CO2 spill situations. Risks associated with
long-term underground storage itself, however, are not well constrained. Experience
gained during decades of large-scale underground storage of natural gas and in storage of
hazardous nuclear waste can provide some direction in managing underground storage,
but our knowledge of the behavior of repressurized reservoirs over periods of hundreds or
thousands of years is nonexistent. Risk assessment studies are under way to provide
guidelines on how to handle problems that may occur (IPCC, 2005a).
For mineral carbonation, negative environmental impact is limited to the location
of the CO2 pipeline and to the mining sites of both the feedstock minerals and the coal.
As the reaction products are not water-soluble, they will not be spread beyond the
immediate area of containment. Mining the feedstock and storing the reaction products
produces a relatively significant environmental impact to a small area rather than diluting
it over a large area (Lackner, 2002).
The ecological impact of the pipeline for both geologic storage and mineral
carbonation sequestration would be comparable, assuming similar shipping distances.
This leaves a comparison of the potential for leakage of geologic storage over the next
several thousand years against the disruption at the surface ecosystem from mineral
carbonation activity. The difference is significant and it can be argued that mineral
carbonation is superior with respect to environmental issues when compared to geologic
storage.
29
Industrial Sized Application of Mineral Carbonation Process
Although the mineral carbonation method is not currently cost competitive with
temporary geologic storage, it is important to investigate the potential of full-scale
industrial application because of the permanence and stability of the reaction products.
Earth operated the method on much grander scale, but we must increase the reaction rate
by orders of magnitude over that which occurred during the PETM.
Klaus Lackner began working with the mineral carbonation process in 1995
supported by Department of Energy funding. Experimentation by numerous workers
over the last 12 years has shown that the process has no insurmountable obstacles.
Research under industrial conditions is a next step (Huijgen & Comans, 2005). Until
such trials are run, it will not be known whether the process functions well at large scale
and rapid rate. Following through with industrial deployment will present the
opportunity to define relevant issues and obtain operational experience, which could
reduce investment and operating costs (Gielen, 2003). Geologic storage has much less
cost-reduction potential.
While we didn’t suspect that emitting CO2 would cause a problem until a few
decades ago, we do know there could be significant problems with geologic storage. We
don’t know how soon or how bad it will be or whether we’ll even be equipped to handle
them. Use of geologic storage is a recognized gamble by the current population without
knowing the odds against failure; it trades our CO2 concentration liability for some future
CO2 concentration liability.
30
PLAN FOR ACTION
The new Washington State Law Chapter 307, Laws of 2007, “Mitigating the
Impacts of Climate Change” was signed by the governor on 4 May 2007 and becomes
effective 22 July 2007. The law states in part that the act is meant to authorize immediate
action against GHG resulting from the generation of electric power. The law provides a
timetable for reducing overall GHG emissions in the state to the 1990 level by 2020, to
75% of the 1990 level by 2035 and to 50% of the 1990 level by 2050. In addition, the
governor is to develop policy recommendations to the legislature regarding a process for
replacement of the highest GHG-emitting plants that have exceeded their expected useful
life. The law provides that as of July 1, 2008 the GHG “emissions performance
standard” of any new thermal-electric plant shall be 0.5 tonne CO2 emissions per
megawatt-hour, roughly half the current amount, requiring any new Washington plant to
be capture ready. The resulting captured CO2 is expected to be injected into geological
formations or sequestered by other means, which could include mineral carbonation. The
new law states that such activity is expected to spur technology, increase plant efficiency
and result in an economic boost for the state (Washington State Law, 2007). As a result
of the application of the new state law, it is clear that CO2 storage is in Washington’s
immediate future.
If an accident during early deployment of geologic sequestration occurs, or if
there is strong opposition by grassroots organizations, continued use of geologic
sequestration could be as difficult to implement as radioactive waste repositories
(Wellington et al., 2007).
31
CONCLUSION
Atmospheric CO2 concentrations have been high on Earth before, for example,
during the PETM. In response, Earth operated its own negative feedback systems,
bringing the level back to equilibrium (Bains et al., 2000; Kelly et al., 2005). Mineral
carbonation mimics Earth’s system for restoring the balance of CO2 in the environment
through permanent removal of the excess CO2.
CO2 storage is in its infancy; experience, organization and methodology must be
further developed (Task Force, 2007). The most obvious method of disposal is to pump
the CO2 into geologic reservoirs, especially if there is oil or gas that could not be
removed by pumping. The most obvious method, however, is not necessarily a good
solution.
Federal governments around the world; several states including Washington;
voters of the City of Boulder, Colorado; The Evergreen State College; and individuals
who choose to pay for green power from their utility companies are among those who are
working to ameliorate climate issues, while eschewing immediate financial cost as the
one and only consideration. Their perspective of the relative importance of the climate
versus the lowest dollar amount that could be paid supports the contention that if mineral
carbonation is superior to storage of CO2 in geologic formations, we must provide
appropriate consideration in further developing the method, regardless of the resulting
increase of COE.
The new Washington State Law specifically addresses the environmental
externalities of CO2 emissions. It also requires that all new coal-fired power plants
coming on line be CO2 capture ready. Application of the same type of forward reasoning
32
to mineral carbonation would provide support for development and deployment of the
mineral carbonation process.
In order to determine whether mineral carbonation will operate on an industrial
scale, a suitably sized plant must be built. If it works, we could continue to use coal for
our energy needs; it would become almost environmentally friendly. The learning curve
will have started; the process will become cheaper sooner. We will have had a head start
with regard to regaining Earth’s CO2 balance. Our remaining coal deposits have been
estimated to be enough for the next 250 years at our current rate of use (United States
Senate Committee, 2006). Hopefully by the time we’ve exhausted our supply, we’ll have
invented and deployed alternative energy-production methods.
If mineral carbonation is shown to work, it should be deployed prior to having
filled geological repositories to the point where the situation could be dangerous. There’s
a difference between preparing for a thrust-fault earthquake that could happen tomorrow,
and the blow-out of CO2 reservoirs that shouldn’t have been filled in the first place.
Accidents don’t happen, they’re orchestrated. If we move with mineral carbonation as
soon as possible, regardless of price, we’ll have an opportunity to fix the GHG problem.
If the mineral carbonation process doesn’t hold up to full-scale application, it would be
either abandoned or studied further, in which case we’ll need to phase out the use of
hydrocarbons all together. This will be difficult, perhaps not even possible, while
maintaining our civilization.
33
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