1. Faculty of Engineering and Information Sciences
Measurement, interpretation and modelling of supercritical carbon dioxide
sorption on various geological media
Joshua William Winfield
"This thesis is presented as part of the requirements for the
award of the Degree of Bachelor of Engineering (Mining)
from
University of Wollongong"
October 2015
2. Abstract
Since the Industrial Revolution, anthropogenic CO2 has been released into the atmosphere in gradu-
ally more significant quantities due to the burning of fossil fuels and other industrial processes. The
ramifications of this rapid increase in CO2 and other greenhouse gases is manifest in notable negative
changes to climate systems. Therefore, it is critical to develop technologies to mitigate this problem.
One very promising approach to reducing greenhouse gas emissions is CO2 capture, transport, and
sequestration in deep subsurface geological layers. However, if the promise of this approach is to
come to fruition, the understanding and estimation of geological CO2 storage capacities needs to be
improved.
In this study CO2 sorption experiments up to 10 MPa at 40◦C were performed on two dry coal and
three dry sandstone samples using a gravimetric apparatus. In all cases an anomaly at the critical point
was observed in the total sorption isotherm data. A new theory to explain the anomaly is proposed and
it is shown, based on a sensitivity analysis of sorption equation variables, to fully explain the cause
of the anomaly. A method for correcting the anomalous behaviour was developed. The isotherms
produced from these adjustments provide CO2 storage capacities well into the supercritical region
that are more accurate than any other modern method can supply.
Coal swelling was integrated into the sorption equations by introducing a swelling function that
predicts the dynamic volume for each coal sample. From the isotherms produced using this new
equation it is clear that the effect of swelling has a non-trivial impact on the measure of total sorption.
Therefore, sorption equations that do not take swelling effects into account cannot produce accurate
supercritical CO2 storage capacity results.
The relationship between sample porosity and the maximum CO2 storage capacity of coal and sand-
stone samples was investigated. A positive correlation between porosity and the maximum CO2
storage capacity was observed for both coal and sandstone. This correlation may be helpful in deter-
mining what types of coal or what types of sandstone are likely to store the most CO2. However, at
this stage the results are too few to conclusively say that higher porosity values always correspond to
higher CO2 storage capacity for the same geological rock type.
The present research extends the modified Dubinin-Radushkevich equation to include contributions
to sorption by the volumetric filling of pore spaces with CO2 in its free state. The extended Dubinin-
Radushkevich equation now has the capability to model the total amount of supercritical CO2 that
can be stored for a wider range of materials. This extension allows material for which the main form
of storage is via the filling of pore space (such as a sandstone layer under a suitable cap rock) to be
accurately represented by the D-R isotherm equation.
i
3. Acknowledgements
First and foremost I offer my sincerest gratitude to my supervisor, Associate Professor Ian Porter,
who has supported me throughout this undergraduate thesis. His patience, guidance and knowledge
have been instrumental for the completion of this study and his willingness to let me approach this
thesis in my own unique way is appreciated.
I would like to profoundly thank Professor Naj Aziz and Dr Ali Mirzaghorbanali not only for their
interest, but unwavering willingness to help me overcome any barriers I encountered.
To all the technical staff, in particular Mr Ritchie McLean and Mr Colin Devinish, thank you. I would
never have been able to complete these comprehensive experiments or obtain meaningful results if it
was not for your continued support.
To my family, thank you for providing me with stability throughout this intensive process. A special
thanks to Denis Whitfield for assistance in proofreading, and to Breeanna Salter for her love and
support.
ii
11. Equation Nomenclature
ρs = bulk density of sample (g/cm3)
ms = mass of sample (g)
Vs = total volume of sample (i.e. Vparticles + Vpores) (cm3)
Vs+pw = volume of sample coated with paraffin wax (cm3)
Vpw = volume of paraffin wax (cm3)
ms+pw = mass of sample coated with paraffin wax (g)
sms+pw = submerged mass of sample coated with paraffin wax (g)
ρpw = bulk density of paraffin wax (g/cm3)
ρw = bulk density of distilled water (g/cm3)
Ds = average diameter of sample (cm)
Ls = average length of sample (cm)
φ = porosity of sample (0−100%)
Vp = volume of pores (cm3)
Vd = volume of dead space (cm3)
Vb = volume of bomb (cm3)
n = moles of helium (mol)
Pabs = absolute pressure (kPa)
mh = mass of helium at Pabs (g)
R = universal gas constant 8314.462 (kPa.cm3.mol−1.K−1)
M = molar mass of helium 4.002602 (g/mol)
T = temperature of bomb (K)
Vh = volume of helium inside the bomb (cm3)
Vw = volume of water inside the bomb (cm3)
mw = mass of water inside the bomb (g)
maw = mass of bomb after water infiltration (g)
x
12. mbw = mass of bomb before water infiltration (g)
ρfree = free phase CO2 density at measured Pabs and set T (g/cm3)
mmeas = mass of CO2 in bomb at measured Pabs and set T (g)
mb = mass of bomb at measured Pabs and set T (g)
mvb = mass of vacuumed bomb (g)
(P,T) = function of absolute pressure and temperature (kPa, K)
mtot = total CO2 sorbed mass (g)
ktot = total sorption of CO2
kg of CO2
m3 of Sample
Qtot = total sorption of CO2
m3 of CO2 at NTP
tonne of Sample
ρfree,NTP = free phase CO2 density at NTP (g/cm3)
mexcess = CO2 excess sorbed mass (g)
kexcess = excess sorption of CO2
kg of CO2
m3 of Sample
Qexcess = excess sorption of CO2
m3 of CO2 at NTP
tonne of Sample
kfree = free phase sorption of CO2
kg of CO2
m3 of Sample
Qfree = free phase sorption of CO2
m3 of CO2 at NTP
tonne of Sample
f(P) = volumetric swelling (%)
P = equilibrium pressure (MPa)
α = argument constant (MPa−1)
Q0 = surface adsorption capacity free parameter m3 of CO2 at NTP
tonne of Sample
ρads = density of the adsorbed CO2 phase (kg/m3)
D = free parameter (unitless)
Ps = saturation pressure (MPa)
k = constant (m6.kg−1.t−1)
VFinal = water volume in cylinder flask post submersion (cm3)
VInitial = water volume in cylinder flask pre submersion (cm3)
VDisplaced = change in water volume due to submersion (cm3)
xi
13. mpw = mass of paraffin wax (g)
A = adsorption long-term security factor (0−100%)
B = volumetric filling long-term security factor (0−100%)
PabsBefore
= equilibrium pressure before anomaly correction (MPa)
∆P = equilibrium pressure correction (MPa)
PabsAfter
= equilibrium pressure after anomaly correction (MPa)
xii
14. 1 Introduction
1.1 Overview
Anthropogenic greenhouse gas (GHG) emissions are higher than they have ever been before. They
have been on the rise since the late 1700’s; solely driven by industrialisation and the energy de-
manded from a rapidly growing global population. Unsurprisingly, the Earth and its natural systems
have responded to these changes in human activity. One of the more notable changes is the increasing
atmospheric concentration of carbon dioxide, methane and nitrous oxide which are at levels unprece-
dented in the last 800,000 years (IPCC, 2014). Carbon dioxide (CO2) is the major anthropogenic
GHG contributor and accounts for 76% of total human emissions (IPCC, 2014). The source of these
emissions is mainly from fossil fuel combustion which accounts for up to 78% of total CO2 human
emissions (IPCC, 2014). The ramifications of this rapid increase in GHGs is manifest in notable
negative changes to climate systems, and is likely to be the primary cause of climate warming and
the increase in total energy stored in the atmosphere, ocean and land (IPCC, 2014).
There is a notable past and ongoing effort to transition away from fossil fuels and into more sustain-
able alternatives such as the use of solar, geo-thermal and wind. However, the world still remains
heavily reliant on fossil fuels and this is likely to be the case for many years to come (Krooss et
al., 2002). For this reason, mitigation strategies that ameliorate any harmful impacts from fossil
fuel usage need to be explored. The desired effect is a continued use of fossil carbons without the
substantial CO2 emissions that typically follow.
Barring an outright ban of fossil carbons, Carbon Capture and Sequestration (CCS) is perhaps the
best emissions mitigation option to date. Carbon Capture and Sequestration involves capturing CO2
from large point emitters and storing it in suitable media for a geological time frame. Currently, the
technology for each phase of CCS exists and has been utilised in multiple small scale CCS operations;
however, a large scale economically viable roll-out is yet to be established.
The capture and separation of CO2 from anthropogenic stationary sources is the first phase in the
CCS process. Current post-combustion and pre-combustion systems for power plants can capture
80% to 90% of the net CO2 that is produced (Metz et al., 2005). Oxy fuel combustion systems are,
in principle, able to capture just over 90% of the net CO2 produced (Metz et al., 2005). Higher
values are theoretically possible but exponential cost increases and a significant decline in incre-
mental efficiency prevent practical application. Although additional energy is needed for plants with
capture technology, which itself contributes to CO2 emissions, the total amount of CO2 emitted is
significantly less than that of a plant without capture technology (Metz et al., 2005).
1
15. Figure 1.1 illustrates the reduced emissions a typical power plant can achieve by utilising capture
technology.
Figure 1.1: CO2 Avoided Through Plant Capture Technology (Metz et al., 2005)
Finding a suitable storage media for captured CO2 is a key element in the last phase of CCS. Four
broad means of sequestration (storage and long-term security) have been subject to significant inves-
tigation: mineral carbonation (Huijgen et al., 2005; Lackner et al., 1995), ocean storage (Saito et al.,
2000; Herzog, 1999), soil storage (Chmura et al., 2003; McCarl et al., 2007) and geological storage
(Reeves, 2001; Bachu, 2000; White et al., 2005; Goodman, 2005). Of these options, subsurface ge-
ological systems (including coalbeds and sandstone layers) represent the best and most likely option
for significantly reducing CO2 emissions (Bachu, 2008). In part, this is due to the significant storage
capacity that geological formations offer (Metz et al., 2005) and because it is one of the few CCS
options available to landlocked energy producers.
While the capture and transportation phases of CCS are limited by economic and efficiency factors,
the key issues for sequestration are estimating the potential storage capacity and integrity of storage
media, and understanding the natural phenomena that govern the interaction between the injected
CO2 and the media.
Laboratory isotherm tests (CO2 storage vs. pressure at a constant temperature) are required to re-
veal the potential CO2 storage capacity of any media. Results obtained from these tests attempt to
reflect the in situ storage capacity for a range of conditions. Presently, the storage capacity of any
media cannot be obtained directly from the characteristics (such as rank, density, moisture content
and porosity) of the sample and so isotherms must be established (Busch et al., 2003b; Reeves et al.,
2
16. 2005). The sorption equations typically used to interpret isotherm tests have fundamental inherent
limitations which result in an inability to accurately reflect what is physically occurring within the
sample. The equations do not take into account non-trivial swelling of the sample, especially under
supercritical conditions, which results in an incorrect estimation of storage capacity (Harpalani and
Chen, 1992). This is only one of the limitation of the commonly used theoretical equations. Cur-
rently, models that describe the results of isotherm tests utilise mathematical functions that are not
directly related to media characteristics, but relate, rather, to the bounds of experimental data and
"best fit" constants (Siemons and Busch, 2007; Day et al., 2008a; Goodman, 2005). The ultimate
goal in sorption science (in the context of CCS) is to develop models that accurately reveal the stor-
age capacity of any media based on sample characteristics alone (Reeves et al., 2005). However, a
starting goal is to remove inherent flaws in the theoretical equations by integrating swelling effects
and establish the foundation for a model that incorporates the modified results.
1.2 Aim
The aim of this study was to measure, interpret and model supercritical CO2 sorption behaviour
of various geological media with the intention to contribute to the understanding of rock storage
capacity for practical use in geosequestration.
1.3 Objectives
• Integrate volumetric swelling into the sorption equations to attempt to overcome some of their
inherent limitations.
• Interpret encountered anomalies in sorption data using volumetric swelling impacts and changes
in free phase CO2 density relative to the density of the adsorbed CO2 layer.
• Extend the Dubinin-Radushkevich isotherm model to incorporate supercritical conditions for
a wider range of materials and fit to measured sorption data.
• Determine the maximum CO2 storage capacity of each geological rock type tested.
• Establish a comparison between the sorption behaviour results obtained and previous results
obtained at the University of Wollongong in 2013.
• Comment on the correlation between sample porosity and CO2 storage values for each type of
geological media tested.
3
17. 1.4 Scope
This study investigated the supercritical CO2 sorption behaviour of various Australian geological
media. It focused on the sorption characteristics of dry coal and sandstone cores. Experiments were
conducted with pressures up to 10 MPa at a constant temperature of 40◦C. These constraints allowed
for the activation of the supercritical phase in CO2.
1.5 Hypotheses
1. Gravimetric analysis will reveal distinct differences in the sorption capacity of all tested media,
even those of the same type.
2. The relative total sorption capacity will be governed, in large part, by the porosity values of
each sample. In the case of coal, micropores are the primary contributor to porosity in a typ-
ical specimen (Harpalani and Chen, 1992). Thus it is likely that greater porosity values will
correspond to a greater number of micropores which positively influence the observed high
CO2 determined surface area (Marsh and Siemieniewska, 1965) and contribute to increased
gas uptake capacity (Shi and Durucan, 2005). Since the main mechanism for sorption in coal
is adsorption, an increase in surface area will translate to an increase in total sorption capac-
ity. Furthermore, Day et al. (2008a) suggest that an increase in coal porosity corresponds to a
greater sorption capacity because of better access to the adsorption sites. In the case of sand-
stone, higher porosity values indicate more internal volume. Since the primary mechanism for
sandstone sorption is the filling of pore spaces, higher porosity values will translate to a greater
total sorption capacity.
3. The integration of volumetric swelling will smooth transitions between adjacent sorption points
and lessen the severity and impact of any anomalies present in the sorption data.
4. The extended Dubinin-Radushkevich isotherm model will accurately fit the measured sorption
isotherm adjusted for anticipated volumetric swelling.
4
18. 2 Literature Review and Theoretical Background
2.1 Climate Change
Since the Industrial Revolution, anthropogenic CO2 has been released into the atmosphere in grad-
ually more significant quantities due to the burning of fossil fuels and other industrial processes
(Normile, 2009; Thorbjörnsson et al., 2014). The ramifications of this rapid increase in CO2 and
other GHGs is manifest in notable negative changes to climate systems, and is likely to be the pri-
mary cause of climate warming and the observed increase in total energy stored in the atmosphere,
ocean and land (IPCC, 2014).
Figure 2.1 illustrates the substantial energy gain within components of the Earth’s climate system,
relative to levels measured in 1971. From that reference year to 2010, it is estimated from indepen-
dent research (Domingues et al., 2008; Purkey and Johnson, 2010; Beltrami, 2002) that 274×1021 J
have been deposited in these systems, at an average rate of 213 TW (IPCC, 2014).
Figure 2.1: Accumulation of Energy within the Earth’s Climate System (IPCC, 2014)
5
19. The oceans provide the main storage for energy in the climate system. Oceans have absorbed more
than 90% of the energy accumulated between 1971 and 2010 (IPCC, 2014). Additionally, the increase
in ocean uptake of CO2 has resulted in ocean acidification (Orr et al., 2005) and the surface water
pH levels have decreased by 0.1 since the Industrial Revolution. This corresponds to an increase in
ocean acidity of almost 30% (IPCC, 2014).
Ocean acidification has the potential to strongly impact marine ecosystems. With an increase in the
uptake of CO2, the percentage of oxygen diminishes and a fundamental ingredient for life comes
under threat. There is potential for species extinction and shifts in marine ecosystems if the level of
CO2 uptake by the oceans is not curbed.
Anthropogenic intervention in the climate is the likely root cause of the frequency and intensity of
daily temperature extremes which have been observed globally since the 1950s (IPCC, 2014). It
is also likely that the rising occurrence of heat waves in a number of global regions is a result of
human interference with natural climate systems (Watson and Albritton, 2001). Intensified weather
conditions have the potential to negatively impact food security, crop yields and water supply on a
global scale. Although some studies have indicated a positive impact on a number of these factors,
the majority of research predicts a negative net outcome (Kang et al., 2009; IPCC, 2014).
From 1901 to 2010, global mean sea level rises have peaked to just over 0.2 m (IPCC, 2014). The
Intergovernmental Panel on Climate Change (2014) reported that the rate of sea level rise since the
mid-19th century has been larger than the mean rate during the previous two millennia. Global mean
sea level rise will continue beyond this century with the true extent being governed by future remedial
actions. The flooding of populated areas and the shifting of habitable locations are some of the flow
on effects of sea level rises.
According to the IPCC (2014), the link between rising CO2 emissions and a changing climate is
well established. Ice core measurements reveal that over the last several thousand years atmospheric
concentration of CO2 has been static (IPCC, 2013). The global carbon cycle was once in a near
equilibrium state; however, the recent rise in CO2 emissions has destabilised this equilibrium and
continues to do so.
The cause and effect of climate change is subject to debate but the majority of the scientific com-
munity believe that the world cannot wait for a definitive answer on the matter (IPCC, 2014). The
repercussions of a delayed response could be catastrophic in the long-term and, therefore, preventa-
tive and mitigating actions have to be taken now.
A significant portion of climate change linked impacts will continue for centuries, regardless of any
successful execution of current mitigation strategies. Even if no additional GHGs were emitted into
the atmosphere from this point in time, effects will continue to be observed well into the 22nd century.
6
20. 2.1.1 Carbon emissions
There is consensus that the salient cause of the observed increase in the stored energy of climate
systems is the increase in atmospheric concentration of GHGs, specifically CO2. Around 2040±310
Gtonnes of anthropogenic CO2 have been added to the atmosphere since 1750, half of which would
have been added in the last 40 years (IPCC, 2014). This deposit of CO2 is likely to be responsible
for about two-thirds of the increase in stored energy of the atmosphere (IPCC, 2014).
The majority of these emissions result from the combustion of fossil fuels used in power generation,
transportation, industrial processes, and residential and commercial buildings (Metz et al., 2005).
Approximately 40% of these emissions are still in the atmosphere, with the rest being stored in
soils, plants and oceans as a result of natural storage processes. Conditions are likely to worsen as
the annual anthropogenic GHG output rises. Annual levels are increasing by about 10 GtCO2-eq
(measurements taken from 2000 to 2010) (IPCC, 2014). The contribution to anthropogenic increases
is dominated by the energy sector, industrial processes and transportation. Figure 2.2 illustrates the
contributions by sector to the increase in GHG emissions between 2000 and 2010 (IPCC, 2014).
Figure 2.2: Sectors Contributing to the Rising Rate of GHG emissions (IPCC, 2014)
7
21. 2.1.2 Atmospheric concentration
Atmospheric concentration of CO2 has risen substantially over the past two centuries. Air and ice
measurements can reveal the estimated concentration changes over significant time periods. As
shown in Figure 2.3, over the last two millennia concentration levels have been stable at around
280 ppm. It is only since the Industrial Revolution that the global average concentration has risen
rapidly (to roughly 400 ppm (CSIRO, 2015)).
Figure 2.3: Atmospheric CO2 Concentration over Two Millennia (CSIRO, 2015)
There are three monitoring sites (two in the northern hemisphere and one in the southern hemisphere)
that actively record changes in CO2 atmospheric concentration. The Earth System Research Labo-
ratory in Hawaii is the primary northern site. In May 2013, it recorded, for the first time in history,
a daily CO2 atmospheric concentration level of 400 ppm (ESRL, 2013). Although seasonal fluctu-
ations do exist and daily levels do not necessarily represent long-term trends, in February 2015 the
average recorded CO2 concentration level also exceeded 400 ppm (Krummel and Fraser, 2015). The
Cape Grim monitoring station in Tasmania has recorded a CO2 concentration rise, from 328 ppm in
1976 to 396 ppm in 2015 (CSIRO, 2015). The consistent increase in atmospheric carbon dioxide in
Tasmania roughly reflects that of any geographical location on Earth. Currently, concentration levels
are increasing at the fastest observed decadal rate of change (2.0 ± 0.1 ppm/yr) (measured from 2002
to 2011) (CSIRO, 2015).
8
22. 2.1.3 The energy trapping mechanism of carbon dioxide
The majority of electromagnetic radiation from the Sun is emitted in wavelengths less than 4 µm.
The heat energy re-emitted from the Earth, however, is on average greater than 4 µm. This longer
wavelength is in the range that can be absorbed by CO2. Thus, CO2 absorbs the heat energy released
from the Earth and returns a portion to the Earth’s surface upon de-excitation (NASA, 2015). Over
time this effect can produce significant results as energy added to a system (in this case by the Sun)
is either used as useful work or stored as potential energy. Figure 2.4 depicts a simplified process of
how energy is trapped as a result of GHGs.
Figure 2.4: CO2 Trapping Mechanism
9
23. 2.2 Mitigation Strategies
A chief challenge in mitigating climate change effects is to reduce anthropogenic CO2 emissions. It
is the intent that successful reduction strategies ultimately lead to a stabilisation of atmospheric CO2
to no more than 550 ppm and prevent further interference with the climate system (IPCC, 2014; Metz
et al., 2005).
There is a notable effort to transition away from fossil fuels to more sustainable alternatives such
as the use of solar, geo-thermal and wind. Even with this transition the world still remains heavily
reliant on fossil fuels and this is likely to be the case for many years to come (Krooss et al., 2002).
For this reason, mitigation strategies that ameliorate any harmful impacts from fossil fuel usage need
to be explored. The desired effect is a continued use of fossil carbons without the substantial CO2
emissions that typically follow.
To curb and, perhaps optimistically, reverse atmospheric CO2 levels a number of promising methods
have been suggested. Solar Radiation Management (SRM) relates to the intentional manipulation
of planetary solar absorption. It has been proposed to limit the direct solar intensity. Although this
strategy does not directly influence atmospheric CO2, it does influence the effect it can have. Carbon
Dioxide Removal (CDR) revolves around the removal of CO2 directly from the atmosphere, mainly
through enhancing and stimulating natural CO2 storage processes. Carbon Capture and Sequestration
involves capturing CO2 from large point emitters and storing it in suitable media for a geological time
frame.
2.2.1 Solar Radiation Management
Solar Radiation Management centres on techniques aimed at reflecting or diverting solar radiation
back into space by increasing the planet’s reflectivity (Ming et al., 2014). Manipulating the Sun’s
energy output impact on the Earth, a fundamental driving force of the climate, will have significant
impacts. Past studies show that solar variability has played a chief role in past climate change events
(IPCC, 2014). For example, NASA (2010) reports that a decrease in solar activity is thought to have
triggered the Little Ice Age between 1650 and 1850.
Even though solar energy output is a key element of the climate, its exact effect is not yet fully
understood. This problem is reinforced by an inability of scientist to integrate solar irradiance directly
into current climate change models. Moreover, models that incorporate solar irradiance fluctuations
cannot reproduce the observed temperature trend over the past century without including a significant
rise in GHGs (IPCC, 2014). The most up-to-date sunspot records analysis suggest that no more than
10% of the 21st century’s climate change can be explained by solar irradiance (Lockwood, 2010).
10
24. Controlling outgoing radiation by reducing solar irradiance has the potential to lower global mean
temperatures; however, this may eventually lead to a less intense global hydrological cycle (Bala et
al., 2008) with regionally diverse climate impacts (Govindasamy et al., 2003; Matthews and Caldeira,
2007). Even optimistically assuming that SRM could influence the energy supply to the climate
system in a way to reduce climate change effects, it will not resolve unwanted biogeochemical effects
of increased CO2 uptake such as ocean acidification (Snyder et al., 2002; Naik et al., 2003).
Solar Radiation Management is untested and involves numerous uncertainties, side effects and risks
(IPCC, 2014). Therefore, SRM is not the optimum mitigation strategy to employ against the rising
impacts of climate change.
2.2.2 Carbon Dioxide Removal
There are a number of CDR techniques, most of which revolve around accelerating natural carbon
cycles and sinks.
Ocean fertilisation is a climate engineering strategy that aims to increase the growth of phytoplankton
by nourishing the upper ocean with additional micro-nutrients. Phytoplankton eat CO2, and the in-
crease in marine food production would theoretically remove more CO2 directly from the atmosphere
(IPCC, 2014). When the plankton dies they sink and transport the CO2 to the deep ocean. Although
this seems beneficial, in practice a noteworthy reduction in atmospheric CO2 would require copious
ocean volume and would probably have detrimental impacts on marine ecosystems and biogeochem-
ical cycles (IPCC, 2013; IPCC, 2014). Additionally, deep ocean circulation would eventually return
the sequestered CO2 to the atmosphere on a time scale that is too short for ocean fertilisation to be a
long-term mitigation strategy.
Photosynthetic Carbon Fixation (PCF) involves the absorption of CO2 into vegetation via photosyn-
thesis. Accelerating the impact of the photosynthesis phenomenon requires a rapid increase in the
amount of undisturbed long-term vegetation areas. Unfortunately this goes against the growing de-
mand for agricultural land use. Furthermore, PCF can only store a small amount of CO2 even if
operated on a large scale. Top estimates are only as high as 350 Gtonnes of CO2; not enough to
unravel the carbon emissions dilemma (Watson and Albritton, 2001).
Exploiting the natural sink in forests by reducing deforestation and increasing afforestation could
potentially aid in the uptake of a significant amount of atmospheric CO2. However, the risk of
emissions due to fires and future changes to the land is high.
11
25. Figure 2.5 summarises the intentionally modified carbon capture processes that are mentioned above.
Additionally Figure 2.5 mentions other CDR techniques (biomass burial, biochar, algae farming etc.),
nearly all of which suffer the same limitations and provide only low CO2 storage capacity.
Figure 2.5: CDR Methods (adapted from IPCC, 2013, p. 547)
Although CDR methods show promise as a solution to the growing concentration of CO2 in the
atmosphere, most methods rely on very slow processes that typically cannot sequester enough carbon
to alter the effects of climate change significantly (IPCC, 2013). While no single CDR technique can
significantly affect climate change, a combination of CDR techniques implemented on a sizeable
scale could have the potential to make a notable impact.
12
26. 2.2.3 Carbon Capture and Sequestration
Metz et al. (2005) identify CCS as the most promising method to reduce rising anthropogenic CO2
emissions. Carbon Capture and Sequestration is a process that consists of:
• separating and capturing CO2 from large stationary point sources (such as power plants, re-
fineries and smelters),
• transporting it to a suitable storage site, and
• isolating it from the atmosphere for a geological time frame.
Figure 2.6 depicts a simplified model of the basic steps involved in CCS. It transitions through the
beginning stages where CO2 is emitted form a stationary source, and finishes with the injection and
storage into a suitable media; in this case a geological rock layer.
Figure 2.6: A Simplified View of CCS (Gibson-Poole et al., 2008)
Presently, four broad means of sequestration have been subject to significant investigation: mineral
carbonation (Huijgen et al., 2005; Lackner et al., 1995), ocean storage (Saito et al., 2000; Herzog,
1999), soil storage (Chmura et al., 2003; McCarl et al., 2007) and geological storage (Reeves, 2001;
Bachu, 2000; White et al., 2005; Goodman, 2005).
13
27. 2.2.3.1 Mineral storage
Mineral carbonation involves using chemical reactions to convert CO2 into solid inorganic carbon-
ates. This process is akin to natural weathering processes but the processes are driven at an accel-
erated rate. The full storage extent depends on the availability of host minerals, such as olivine and
serpentine. Theoretically, there is enough of these minerals in the Earth’s crust to sequester all the
CO2 that would be produced from combustion of all global fossil fuel reserves (Metz et al., 2005).
Furthermore, the storage security of CO2 due to mineral carbonation is very high and there is no
need for costly monitoring post storage. However, the emissions of CO2 from mining, crushing and
milling needed to obtain these minerals far outweighs the amount of CO2 that can become stored
through carbonation (Bachu, 2008). Moreover, carbonation is relatively expensive compared to other
storage options. Carbonation costs 50-100 USD/tCO2 compared to 0.5-8 USD/tCO2 for geological
storage (Metz et al., 2005). Figure 2.7 illustrates the life cycle of mineral carbonation.
Figure 2.7: CO2 Mineral Carbonation Life Cycle (Metz et al., 2005)
2.2.3.2 Ocean storage
It has long been known that large quantities of CO2 can be sequestered in the ocean (Herzog, 1999;
Marchetti, 1977). Metz et al. (2005) report that there are various viable methods to store captured
CO2 in the ocean:
• dissolution – deep ocean injection of CO2 which dissolves into the water and becomes part of
the deep ocean equilibrium,
14
28. • sea floor lakes – deep sea floor injection where the denser CO2 forms a stable lake,
• conversion - converted stable CO2 bicarbonates stored in the ocean, and
• hydrates formation – ocean injection of CO2 forming heavy hydrates that sink to the floor.
Although there is no practical physical limit to the CO2 storage capacity of the ocean, ocean acidifi-
cation is already at levels that threaten marine life and any additional large quantity injection would
only exacerbate this situation (Seibel and Walsh, 2001; Metz et al., 2005).
2.2.3.3 Soil storage
Carbon can be stored in soil by either directly increasing the carbon levels or by converting organic
matter into a more stable form of carbon. For the former, McCarl et al. (2007) suggest that by intro-
ducing plant material to soil the net amount of lost carbon reduces; thus increasing the total amount
of stored carbon. For the latter, biochar charcoal can be utilised in an oxygen depleted environment to
convert the organic matter (McCarl et al., 2007). Even though it is possible to successfully sequester
CO2 in soil it is not possible to guarantee it will be stored for an appropriate time period to make an
impact on rising atmospheric CO2 levels.
2.2.3.4 Geological storage
Geological storage of CO2 as a climate change mitigation technology has been widely investigated
over the past two decades (Reeves, 2001; Bachu, 2000; White et al., 2005; Goodman, 2005). The
storage capacity of geological media is typically a function of both adsorption (adhesion of atoms
onto the rocks internal and external surfaces) and absorption (filling the pore spaces of confined
porous rock layers or integration into the solid matter of the geological material such as the organic
matrix of coal). In the context of this thesis, persorption, the deep penetration of a fluid into a highly
porous solid, will be included in the category of absorption.
In contrast to other CCS media, geological formations currently represent the best and most likely
option for significantly reducing carbon emissions (Bachu, 2008). In part, this is due to the significant
storage capacity that geological formations offer (Metz et al., 2005) and because it is one of the few
CCS options available to landlocked energy producers.
For geological media to be compatible with large scale CCS activity, it must have the following key
features (Bachu, 2008):
• capacity (appropriate storage space),
• injectivity (adequate rate of injection), and
15
29. • confinement (to prevent both migration and leakage).
Of the geological formations that have these characteristics there is general consensus (Li et al., 2010;
Metz et al., 2005; Bachu, 2008) that the most capable for storage are:
• deep saline aquifers (including sandstone layers),
• depleted oil and gas reservoirs, and
• uneconomic coal seams.
Barring an outright ban of fossil carbons, CCS in geological media is likely the best emissions miti-
gation option to date. This is because it is practical for most energy producers and has the potential to
securely store large quantities of CO2 which will significantly reduce the growth rate of atmospheric
CO2 and assist in solving the carbon emissions dilemma.
2.3 Carbon Capture and Geosequestration
2.3.1 Capture
The capture and separation of CO2 from anthropogenic stationary sources is the first phase in the
CCS process. Carbon capture involves separating CO2 from other gases generated during energy
production and industrial activity. High emission producing industries including transportation have
been targeted for carbon capture. Transportation is Australia’s second biggest energy consuming in-
dustry (following electricity generation (BREE, 2014)) and could have potentially significant effects
if its carbon emissions could be lowered. Unfortunately, due to the mobile nature of the industry, no
practical carbon capture solution has been discovered (NETL, 2015).
It is not necessary for the captured stream to be pure CO2, but low CO2 concentration streams are
likely to be impractical for capturing CO2 and provide insignificant mitigating effects on CO2 emis-
sions (Metz et al., 2005).
Coal-fired power plants are the dominant contributor of carbon emissions from stationary sources.
Currently there are three broad capture technologies that can be readily applied (Figueroa et al.,
2008):
• pre-combustion capture,
• post-combustion capture, and
• oxy-combustion capture.
16
30. Direct capture of CO2 straight from ambient air is also a potential capture technology that can be
utilised to produce CO2 streams. However, its viability on scales that will impact the CO2 emissions
dilemma is yet to be proven.
2.3.1.1 Pre-combustion
In pre-combustion capture, CO2 is separated before combustion, typically as part of an integrated
gasification cycle (Thorbjörnsson et al., 2014). The raw combustion fuel is converted into syngas
(mainly H2 and CO) under heat and pressure in the presence of sub-stoichiometric oxygen from
an air separation unit (Figueroa et al., 2008). A shift reaction then recovers CO2 from the syngas
mixture. The H2 stream is burned in a gas turbine to produce energy and the CO2 stream is stored.
Pre-combustion is more complex than the other combustion capture alternatives, and hence more
costly. However, the high pressure conditions encountered in pre-combustion capture can lead to
a greater recovery of CO2 (Thorbjörnsson et al., 2014). Figure 2.8 depicts a simplified integrated
gasification cycle power plant with inbuilt pre-combustion CO2 capture technology.
Figure 2.8: Pre-combustion Capture (Figueroa et al., 2008)
2.3.1.2 Post-combustion
Post-combustion technology separates CO2 from the exhaust gases produced from the primary com-
bustion process. A liquid solvent is typically used to separate CO2 from the combustion exhaust
gas. Post-combustion CO2 capture is mainly applicable to conventional coal-fired, oil-fired or gas-
fired power plants, but can be extended to other energy circuits such as a natural gas combined cycle
(Figueroa et al., 2008). Figure 2.9 depicts a simplified power plant cycle with inbuilt post-combustion
CO2 capture technology.
17
31. Figure 2.9: Post-combustion Capture (Figueroa et al., 2008)
2.3.1.3 Oxy-combustion
Oxy-combustion aims to produce exhaust gases of mainly CO2 and water which can be easily sepa-
rated by using a high concentration of oxygen in the combustion reaction (Yukun et al., 2014; Wall
et al., 2013). A high concentration stream of CO2 (greater than 60%) can be produced from the
oxy-combustion process. The CO2 is separated from water vapour by condensing the water through
compression and cooling. Oxy-combustion technology is easily integrated in both new and existing
coal-fired power plants; however, the energy cost of supplying almost pure O2 (95 to 99%) can easily
outweigh the benefits of CO2 capture. Figure 2.10 depicts a simplified oxy-fuel power plant and its
basic working principles.
Figure 2.10: Oxy-combustion Capture (Figueroa et al., 2008)
2.3.2 Transport
It is more than likely that the storage site for CCS is not located at the site of CO2 point capture
and there is a need for suitable transport. To facilitate transport (typically in pipelines and ships),
CO2 streams are compressed to high pressures. This increases the density of the CO2 and avoids
costly two-phase flow regimes. In contrast, streams can also be transported at conditions well below
ambient temperatures (in liquid phase) in ships, road and rail tankers.
18
32. 2.3.3 Storage options
It has been shown that CO2 can be sequestered in saline aquifers (Kumar et al., 2005a; Kopp et al.,
2009; Bachu et al., 1994), coal (White et al., 2005; Reeves, 2001; Ozdemir et al., 2004; Larsen, 2004;
Krooss et al., 2002), carbon rich shale (Nuttal et al., 2005; Kang et al., 2011) and sandstone (Liu et
al., 2003). These geological formations have been extensively investigated in recent years. In all
cases, sequestration is achieved by injecting dense CO2 directly into the subsurface rock formation.
2.3.3.1 Saline aquifers
Suitable sedimentary layers containing saline aquifers exist in all sedimentary basins. These basins
are abundant throughout the world but suitability for CCS can vary considerably (Bradshaw and
Dance, 2005; Bachu, 2003). Saline aquifers have the biggest storage capacity of all geological media
(Metz et al., 2005). Their nature allows direct storage without the need for significant grooming
preparation. It is estimated that CO2 can be stored in as little as a few percent to well over 30% of
the total rock volume. Estimates for the storage of CO2 in saline aquifers range from 1,000 Gtonnes
to 10,000 Gtonnes (Metz et al., 2005).
Although promising, these structures are not always available in close proximity to the capture source
in which case transportation costs are not feasible (Day et al., 2008a). For example, the majority of
coal-fired power plants in eastern NSW are positioned in areas where suitable storage aquifers are
not present.
2.3.3.2 Depleted oil and gas reservoirs
Oil and gas reservoirs are typically located in porous rock formations. The pores are dominated by
natural gases, oils and brines that are great candidates for CO2 storage. Reservoirs suitable for CO2
storage exist both onshore and offshore and usually 1 km below the surface (Metz et al., 2005). An
added benefit of CO2 reservoir storage is the liberation of valuable hydrocarbons. Liberated hydro-
carbons can be produced in such high quantities that commercial recovery operations using CO2
injection to free the last remains of nearly depleted reservoirs exist. This is referred to as Enhanced
Oil Recovery. Since CO2 has the ability to displace in situ fluids it typically has access to the entire
pore volume for storage. Estimates for the storage of CO2 in depleted reservoirs range from 675
Gtonnes to 900 Gtonnes (Metz et al., 2005).
19
33. 2.3.3.3 Uneconomic coal seams
The storage of CO2 in uneconomic coal seams offers more advantages than other geosequestration
methods (Ohga and Fujioka, 2002). This is because sequestration mainly relies on gas adsorption
onto coal leading to a high degree of storage capacity and security. Additionally, the methane dis-
placed and recovered from the coal during CO2 injection can offset the costs significantly. The
injection of CO2 into coal while simultaneously producing coalbed methane is referred to as En-
hanced Coalbed Methane Recovery. The adsorption of CO2 as opposed to the filling of pore spaces
is also more likely to stand the test of time, being stably secure for a geological significant period
(Krooss et al., 2002). Estimates for the storage of CO2 in coalbeds range from 15 Gtonnes to 200
Gtonnes (Metz et al., 2005).
Figure 2.11 illustrates the varying geosequestration methods utilised for each suitable geological
formation and feature. It includes the promising top media contenders (saline aquifers, depleted
hydrocarbon reservoirs and unmineable coal seams) as well as less thoroughly investigated storage
options (basalts, evaporites and caverns).
Figure 2.11: Overview of Geosequestration Storage Options (Metz et al., 2005)
20
34. 2.4 Trapping Mechanisms
Injecting large quantities of CO2 into suitable geological media is only half the battle; the CO2 must
be trapped and secured within the media and be able to stand the test of time. Carbon dioxide can
be stored in geological media by several methods and through a variety of physical and chemical
trapping mechanisms (Gunter et al., 2004). These mechanisms enable the long-term storage of CO2
with a low likelihood of leakage.
2.4.1 Physical trapping
When CO2 is immobilised as either a free gas or supercritical fluid it is said to be physically trapped.
The contribution of physical trapping to total storage capacity greatly depends on the available stor-
age volume inside or surrounding the chosen storage media (Bachu, 2008). Physical tapping mecha-
nisms control the initial phase of trapping whereas chemical processes tend to be more dominant over
periods of hundreds to thousands of years. There are three branches of physical trapping: structural
and stratigraphic, residual, and hydrodynamic.
2.4.1.1 Structural and stratigraphic
Injected CO2 is held in place by a surrounding low-permeability rock (cap rock) whose presence
prevents the upward and lateral movement of CO2. A low-permeability cap rock seal is essential
in ensuring that CO2 remains trapped underground (Metz et al., 2005). Figure 2.12 depicts a strati-
graphic trap where an overlying cap rock seals in the CO2 (left), two structural traps where a natural
fold holds the CO2 in place (middle), and a sealing fault which prevents the migration of CO2 (right).
Figure 2.12: Structural and Stratigraphic Trapping (Price and Smith, 2008)
21
35. 2.4.1.2 Residual
Capillary pressure from the water between rock grains traps the outer edge of a CO2 plume which
form during free phase CO2 migration. Residual trapping can potentially immobilize substantial
amounts of CO2 (Kumar et al., 2005b). Figure 2.13 shows the pore spaces (in blue) and quartz grain
(in white) of a sandstone specimen. The pore spaces is where CO2 can be stored as a result of residual
trapping.
Figure 2.13: Residual Trapping (Price and Smith, 2008)
2.4.1.3 Hydrodynamic
Carbon dioxide becomes trapped and isolated by the fluid flow of reservoir water. When the hydro-
dynamic force of the water exceeds the buoyant force of the trapped gas, the gas will be restrained
from upward migration.
2.4.2 Chemical trapping
Chemical trapping occurs in a variety of ways: CO2 can adsorb onto organic material, dissolve into
fluids or integrate itself into the rock matrix/grain.
2.4.2.1 Physisorption and chemisorption
Physisorption occurs when CO2 is weakly bounded to a surface by a combination of Van der Waals
forces and electrostatic forces. Chemisorption is less common and a lot stronger, it occurs when CO2
is strongly bound to a surface by a covalent interaction.
22
36. 2.4.2.2 Solubility and mineral trapping
Over a time scale of hundreds to thousands of years CO2 dissolves in the host materials fluid. The
relatively heavier CO2 richer water eventually sinks. This adds another layer of security by preventing
it from rising towards the surface (Metz et al., 2005).
Over a time scale of thousands to millions of years, dissolved CO2 reacts with host rock minerals
forming precipitates and trapping CO2 in its most secure form (Price and Smith, 2008). The fol-
lowing sequence of chemical transformations is an example of simple chemical reactions that create
solubility trapping and mineral carbonation (Gunter et al., 2004; Bachu, 2008). The chemical trans-
formations show how injected CO2 gas dissolves in pore water of a calcium rich rock layer, reacts
with water molecules and ions, and given sufficient time forms stable solid carbonates.
CO2(gaseous) −→ CO2(aqueous)
CO2(aqueous) +H2O −→ H2CO3(aqueous)
H2CO3(aqueous) +OH−
−→ HCO−
3(aqueous) +H2O
HCO−
3(aqueous) +OH−
−→ CO=
3(aqueous) +H2O
CO=
3(aqueous) +Ca2+
−→ CaCO3(solid)
The contribution to long-term CO2 storage from each of the aforementioned physical and chemical
trapping mechanisms is a function of initial storage capacity, time and long-term migration security.
23
37. 2.4.3 Storage security
It is not enough to simply trap large amounts of CO2 underground; it must be kept there for a sig-
nificant time period. It is likely that a measurable amount of initially injected CO2 will not remain
underground long enough to be securely bound by the more time demanding processes such as sol-
ubility and mineral trapping (Price and Smith, 2008; Bachu, 2008). Choosing a location for CO2
storage is not just about storage capacity but also about a high likelihood of long-term security.
Immediately following the injection of CO2 into a deep subsurface geological layer, primary mech-
anisms will work toward keeping the CO2 localised. Given enough time, a number of secondary
trapping mechanisms start operating and, although they do not increase CO2 storage capacity, they
notably increase storage security (Metz et al., 2005). Figure 2.14 summarises these primary and
secondary mechanisms and illustrates their trapping contribution as a function of time (Metz et al.,
2005; Bachu, 2008).
Figure 2.14: Trapping Mechanisms Relationship to Storage Security (Metz et al., 2005)
24
38. 2.5 Properties of Carbon Dioxide
Carbon dioxide has inherent properties that aid its ability to be securely stored underground. Chief
amongst them is its high density supercritical phase, its solubility in water and oil, and its notable
affinity to organic substances such as coal (Day et al., 2012; Dutta et al., 2011).
At Normal Temperature and Pressure (20◦C and 101.325 kPa) CO2 is a gas that is denser (1.842
kg/m3) than air. The critical point for CO2 occurs when the temperature exceeds about 31◦C and
the pressure exceeds about 7.30 MPa (exact numerical values slightly differ according to the source).
Immediately preceding the critical point, CO2 is either a liquid (above the vaporisation line), a gas
(below the vaporisation line) or a combination of both (on the vaporisation line). Beyond the critical
point, CO2 transitions into a supercritical fluid. Figure 2.15 shows the phase diagram for CO2, it
highlights the key phases and transition points as a function of temperature and pressure.
Figure 2.15: CO2 Phase Diagram (Bachu, 2008)
At supercritical conditions CO2 exhibits characteristics of both a liquid and a gas. It harnesses the
high density characteristic of a liquid and, like a gas, the ability to occupy the entire available volume
of a sorbent. This means supercritical CO2 has the capability to access the entire available space of
a material and deposit a substantial quantity of CO2. Both temperature and pressure increase with
subsurface depth, and below about 1,000 m, depending on the geothermal gradient and the density of
the overlaying rock (Bachu, et al., 1994), the supercritical phase of CO2 is activated (Bachu, 2003).
Figure 2.16 illustrates the effect depth has on CO2 density under two different geothermal regimes. It
shows how a steeper geothermal gradient corresponds to smaller CO2 density values. This is because
CO2 density will typically decrease with increasing temperature and thus it is essential to take into
consideration both depth and geothermal gradient when estimating CO2 storage capacities.
25
39. Figure 2.16: Effects of Depth on CO2 Density (Bachu, 2008)
When CO2 is stored in the pore spaces of a subsurface geological layer overpressure conditions,
if reached, can provide additional trapping. Overpressure conditions can cause CO2 to exceed the
density of the surrounding subsurface water (Bachu, 2003). At a density greater than water, CO2
sinks, and the water provides an extra layer of security preventing surface migration (Metz et al.,
2005).
The oxygen ends of the CO2 molecule have a slight negative charge enabling it to react and dissolve
in water and oil (Wang et al., 2013; Hartmann and Ganzera, 2015). Its ability to dissolve in oil
is roughly ten times greater than that of water (Toth, 2011). The solubility of CO2 increases with
increasing pressure and decreases with increasing temperature and water salinity (Toth, 2011).
Carbon dioxide has a greater affinity to coal than do a number of common in situ gases. Figure 2.17
highlights the relative affinity that CO2 has to coal; it is greater than nitrogen, methane and hydrogen,
and less than that of hydrogen sulphide and sulphur dioxide (Bachu, 2008).
Figure 2.17: Relative Affinity to Coal (Bachu, 2008)
26
40. 2.6 Sorption
Sorption refers to the amount of a substance adsorbed and absorbed within a defined volume. There
are numerous definitions of sorption, each differing slightly due to the inclusion or exclusion of
varying terms. With the intention of removing any ambiguity, the following provides a definition of
all relevant technical terms used in this paper.
Absorption is the integration of a substance into the volume of another material. A sponge filling
up with water, the pore spaces of a material filling up with gas, and the organic matrix of coal
being impregnated by CO2 are key examples of this phenomenon. Adsorption is the adhesion of
a substance to the internal and external surfaces of another material. The adhesion, which forms a
denser substance layer relative to its free state, is governed by attractive surface forces. The density
change is often significant enough to alter the state of the adsorbed substance (i.e. gas to liquid).
Attractive forces diminish significantly with increasing distance from the surface; thus adsorption is
a pure surface interaction. Once the attractive forces diminish completely, only free phase particles
are present.
For sorption on a two-dimensional surface, the Gibbs dividing surface (a theoretical surface first
proposed by Gibbs (1928)) separates the free volume into two regions. On one side, gas molecules
are in an adsorbed state, and on the other, gas molecules are in a free state. Figure 2.18 illustrates
how molecules in both the adsorbed state and free state contribute to different sorption metrics.
Figure 2.18: Absolute and Total Sorption (Mason et al., 2013)
27
41. Absolute sorption is the sum of the experimentally measured excess sorption (i.e. additional molecules
above the free phase density) plus the free phase molecules that would have been present in the ad-
sorbed volume if the phenomenon of adsorption did not exist (i.e. free phase molecules within the
adsorption volume). Since it is not experimentally possible to determine the Gibbs dividing surface
or the volume of the adsorbed region (Gumma and Talu, 2010) absolute sorption cannot be directly
determined (Dinc˘a et al., 2006). Instead, it is estimated from excess sorption data, an inferred CO2
adsorption density and the free phase CO2 density. Absolute sorption is a common metric for non-
porous materials or porous material that have been pulverised into a fine powder. If a material is
highly non-porous and has a notable affinity to the adsorbate then absolute sorption can provide
a good estimation of the material’s storage capacity. Figure 2.19 illustrates how the contribution
to absolute sorption from excess sorption diminishes as pressure increases and the free phase CO2
becomes more dense. Dark blue circles represent excess sorption, light blue circles represent the
difference in absolute and excess sorption, grey circles (determined from the free phase CO2 density)
are used to determine the proportion of light blue circles and the green dotted line represents the
Gibbs dividing surface.
Figure 2.19: Changes in Sorption Composition (adapted from Stadie, 2012, p. 35)
Although the volume of adsorption cannot be determined it is known to always be less than the pore
space volume of a material and thus absolute sorption is not fully equipped to reveal the storage
capacity of most materials. Although this seems like a fundamental problem in determining the
storage capacity of some materials, it has no impact on the measure of excess sorption which is the
far more useful metric for highly CO2 adsorbent materials. Moreover, Gross et al. (2011) identify
total sorption as the key metric for gas storage applications, not absolute sorption, since the total
amount that can be stored inside the sample is far more useful when looking at significant storage
areas.
28
42. Total sorption encompasses all molecules within the volume of a material (Gross et al., 2011). To
better interpret the total sorption this study introduces a new sorption metric called the free phase
sorption. The free phase sorption describes the amount of CO2 molecules in a free state that would
occupy all available spaces inside a sorbent if the volume of adsorption was zero (or if the phe-
nomenon of adsorption did not exist). This newly introduced metric allows the total sorption to be
defined as the addition of excess sorption and free phase sorption (assuming absorption into the solid
sorbent matter is negligible). The major benefit of this is the total amount of CO2 in the adsorbed
phase (or free state) can be approximated. This is because the CO2 molecules that contribute to ex-
cess sorption are always in an adsorbed state and CO2 molecules that contribute to free phase sorption
are almost always in a free state. The notable exception to this is when the supercritical phase in CO2
is activated and excess sorption begins to decease. At this point the free phase sorption will consist
of CO2 molecules in both the free and adsorbed state. An estimate of the proportion of adsorbed
CO2 molecules contributing to free phase sorption can be obtained from the difference between the
maximum excess sorption value and the current excess sorption value. Note that absolute sorption
is not used to determined the total amount of CO2 in the adsorbed state since its value is estimated
from an inferred CO2 adsorption density which is likely to contain a moderate degree of uncertainty.
The total sorbed amount of any substance will always increase with increasing pressure. For material
with a relatively high CO2 adsorption affinity (such as coal), at low pressure the free phase density
is considerably less than the adsorption density and thus contributions from free phase CO2 to total
sorption is insignificant. This allows the total sorption to be approximated by the excess sorption
at low pressures. However, if pressure and temperature increases enough such that the supercritical
phase of CO2 is activated then the contribution to total sorption from free molecules rapidly increases.
This increase widens the gap between excess sorption and total sorption. Excess sorption reaches a
maximum value when the increase in adsorption density is equal to the increase in free phase density.
Beyond this point the excess sorption will begin to decline and plateau. If pressure is increased
enough it is theoretically possible for the free phase density to exceed the adsorption density and at
this stage the excess sorption will be zero (Stadie, 2012). More commonly though, the free phase
CO2 will reach a maximum density and contributions to total sorption will remain relatively static.
Figures 2.20 and 2.21 depict simplified scenarios of sorption for material with a relatively high CO2
adsorption affinity. These scenarios assume that the pore volume is fully saturated with CO2 and that
absorption into the solid matter of the material is negligible. If absorption into the solid matter was
not negligible, the reading of excess sorption will act as an upper limit rather than an exact result.
29
43. Figure 2.20: Simplified Sorption A (subcritical) Figure 2.21: Simplified Sorption B (supercritical)
For materials with a relatively low CO2 adsorption affinity (such as sandstone), the total sorption
follows a pattern almost entirely dictated by the density of the free phase CO2. There is a steady
linear increase until supercritical conditions where a sharp rise begins. This rise gradually slows
down and plateaus as the free phase density slows down and plateaus. Figure 2.22 depicts a simplified
scenario of the sorption for materials with low CO2 adsorption affinity. The free phase sorption will
approximately mimic the total sorption over the full range of pressures tested. The excess sorption
will only mildly contribute to the total sorption since the phenomenon of adsorption is weakly acting
in these materials.
Figure 2.22: Simplified Sorption C (supercritical)
The sorption capacity of a substance is typically measured as a function of pressure while the temper-
ature is held constant. These isothermal pressure-sorption curves are called isotherms. The accepted
practice is to express sorption measurements in terms of excess sorption (Purewal, 2010). Excess
sorption is considered the gold standard quantity because it directly relates to the thermodynamic
properties of the substance. However, in the context of storage capacity, total sorption is the metric
which reveals the most about a material’s ability to store large amounts of CO2.
30
44. 2.7 The Sorption Capacity of Dry Coal
The matrix structure of coal is characterised by both microspores (< 2 nm) and macrospores (> 50 nm)
(Aziz et al., 2004) which have an important influence on the sorption of CO2. The storage of CO2 in
coal occurs through three different mechanisms: firstly by adsorption onto the surface of micropores,
macropores and fractures, secondly by absorption into the coal matrix, and lastly by the filling of pore
spaces. It is thought that close to 95% of naturally stored gas in coal is in the adsorbed state (Aziz et
al., 2004); however, at supercritical conditions the denser free phase CO2 will contribute a significant
portion to the overall sorption. Coal has a duel-porosity structure: the larger cleat network, and
the smaller micro and macropores of the matrix. The larger cleat network and macropore structure
provide a pathway for CO2 to reach the many internal surfaces of the microspores, thus enabling
large adsorption storage capacities (Day et al., 2008a).
For subcritical conditions coal has been shown to store up to 37 m3/t (m3 of NTP CO2 per tonne of
coal) (Charrière et al., 2010) and 54 m3/t (Busch et al., 2003a). A common feature of all subcritical
tests listed here is the minute size of the coal particles tested (< 0.15 mm equivalent diameter). Small
particles do not necessarily reflect the total sorption capacity of coal for typical in situ conditions.
This is mainly due to smaller specimens having greater access to coal surfaces per unit mass. Figures
2.23 and 2.24 show two CO2 isotherms obtained at 22◦C (subcritical temperature) by Busch et al.
(2003a). The excess sorption increases with increasing pressure and exhibits a typical Langmuir type
adsorption pattern.
Figure 2.23: Subcritical-A (Busch et al., 2003a) Figure 2.24: Subcritical-B (Busch et al., 2003a)
Over the past decade literature pertaining to the supercritical CO2 sorption in coal has become more
available (Day et al., 2008a; Krooss et al., 2002; Li et al., 2010; Gensterblum et al., 2013; Siemons
and Busch, 2007). For supercritical conditions coal has been shown to store up to 71 m3/t (Krooss
et al., 2002), 55 m3/t (Day et al., 2008a; Siemons and Busch, 2007), 60 m3/t (Li et al., 2010) and 46
m3/t (Gensterblum et al., 2013). Figures 2.25 to 2.32 highlight all the aforementioned supercritical
31
45. CO2 sorption experiments in coal. The figures show a climb in excess sorption capacity up till around
7 to 10 MPa, followed by an anomalous sharp drop/rise or expected slow decline and plateau. Expla-
nations for the anomalous behaviour revolve mostly around failure to take into account volumetric
changes over the pressure range of the test, temperature variability near the critical point and the free
phase CO2 density coming in close proximity to the density of the adsorbed CO2 layer.
Figure 2.25: Supercritical-A (Krooss et al., 2002) Figure 2.26: Supercritical-B (Krooss et al., 2002)
Figure 2.27: Supercritical-C (Krooss et al., 2002)Figure 2.28: Supercritical-D (Siemons and Busch,
2007)
Figure 2.29: Supercritical-E (Day et al., 2008a) Figure 2.30: Supercritical-F (Gensterblum et al.,
2013)
32
46. Figure 2.31: Supercritical-G (Li et al., 2010) Figure 2.32: Supercritical-H (Li et al., 2010)
A large part of the excess sorption increase from subcritical to supercritical conditions can be ex-
plained by coal having a higher adsorption affinity to the supercritical state of CO2 compared to its
subcritical state (Krooss et al., 2002). Furthermore, the contributions to storage capacity from CO2
in its free state become relatively significant at supercritical densities.
Adsorption onto coal has traditionally been interpreted using Langmuir monolayer coverage models.
At high pressures (> 6 MPa) the Langmuir model does not give results with the same accuracy as it
does at low pressures. The more recent Dubinin-Astakhov and Dubinin-Radushkevich models have
been shown to fit the experimental data better than the Langmuir model does, especially at high
pressures (Sakurovs et al., 2007; Ottiger et al., 2006; Ozdemir et al., 2004).
Moreover, the modified Dubinin-Radushkevich model has been used in the past to accurately repre-
sent sorption data well into the supercritical region of CO2 (Sakurovs et al., 2007). However, free
parameters in the equation have not been directly related to the properties of the storage material.
It would be a tremendous innovation in sorption science if by knowing a materials properties (such
as density, porosity, and moisture content) suitable free parameters could be determined that reveal
the sorption behaviour and capacity of any material. Currently, no model correctly incorporates non-
negligible effects from volumetric swelling. This limitation can reduce the accuracy of models by a
notable amount.
Li et al. (2010) are of the view that the total amount of supercritical CO2 that can be deposited
in coal not only depends on the excess sorption capacity but also on the porosity of the coal seam.
This implies that the metric of excess sorption will not reveal accurate storage capacity values. This
was discussed in an earlier section of this thesis and the solution suggested was to use total sorption
instead. Total sorption is capable of including the contribution from adsorption (as excess sorption
does) but also takes into account the contribution from free phase CO2 in the pore spaces of coal.
Presently, there is no literature available that deal with this metric in the context of carbon storage
and coal.
33
47. 2.8 The Sorption Capacity of Sandstone
Sandstone layers that are suitable for carbon sequestration are typically deep saline aquifers overlain
by a cap rock (Holloway and Savage, 1993; Liu et al., 2003). Sequestration simulations conducted by
Barnes et al. (2009) on the Cambrian Mount Simon Sandstone in Michigan, endeavoured to reveal
the potential storage capacity of the Michigan sandstone layer. Over a simulation period of twenty
years, 12 Mtonnes was stored of which about 80% was stored as a free phase supercritical fluid in
the sandstone pores, approximately 5% as capillary-entrapped supercritical CO2, and roughly 15%
was dissolved in brine.
The majority of CO2 that can be sequestered in sandstone is located in the pore spaces as free phase
CO2 trapped by a low permeability roof (Wickstrom et al., 2006). Sandstone is particularly porous
relative to other common deep rock layers and it is this porosity that provides considerable storage
capacity (Bachu, 2008).
For supercritical conditions, dry Kimachi sandstone has been shown to store up to 36 m3/t (m3 of
NTP CO2 per tonne of sandstone) and Berea sandstone up to 28 m3/t (refer to Figures 2.33 and 2.34)
(Fujii et al., 2013).
Figure 2.33: Supercritical-I (Fujii et al., 2013)
34
48. Figure 2.34: Supercritical-J (Fujii et al., 2013)
The above isotherms are potentially inaccurate since the excess sorbed amount was not directly mea-
sured. The excess sorbed amount was inferred from the raw experimental data and predicted values
from pore filling and solubility models.
Presently there is scant literature that focuses specifically on the CO2 storage capacity of dry sand-
stone. In large part, this is because a sandstone layer is more likely to be considered as a means
of sequestration if it is also a deep saline aquifer. Deep saline aquifers offer a number of trapping
mechanisms (such as solubility trapping and residual trapping) that dry sandstone alone does not.
The current leading literature on sandstone in the context of CO2 storage (Bachu et al., 2014; Liu et
al., 2003; Kopp et al., 2009) discusses in detail the storage capacities of deep brine bearing sandstone
but does not mention the capacities of dry sandstone. Most of the current data on the CO2 storage ca-
pacities of dry sandstone is only available to provide a comparison for a saturated sandstone sample
(see above, Fujii et al., 2013).
35
49. 2.9 Conclusion
Since the Industrial Revolution, anthropogenic CO2 has been released into the atmosphere in gradu-
ally more significant quantities due to the burning of fossil fuels and other industrial processes. The
ramifications of this rapid increase in CO2 and other greenhouse gases is manifest in notable negative
changes to climate systems.
The cause and effect of climate change is subject to debate but the majority of the scientific com-
munity believe that the world cannot wait for a definitive answer on the matter (IPCC, 2014). The
repercussions of a delayed response could be catastrophic in the long-term and, therefore, preventa-
tive and mitigating actions have to be taken now.
A chief challenge in mitigating climate change effects is to substantially reduce anthropogenic CO2
emissions. It is the desired outcome that successful reduction strategies will ultimately lead to a
stabilisation of atmospheric CO2 at no more than 550 ppm and prevent further interference with the
climate system (IPCC, 2014; Metz et al., 2005).
There is a notable past and ongoing effort to transition away from fossil fuels to more sustainable
alternatives such as the use of solar, geo-thermal and wind. Even with this transition in motion the
world still remains heavily reliant on fossil fuels and this is likely to be the case for many years to
come (Krooss et al., 2002). For this reason, mitigation strategies that ameliorate any harmful impacts
from fossil fuel usage need to be explored. The desired effect is a continued use of fossil carbons
without the substantial CO2 emissions that typically follow.
Carbon capture and geosequestration is perhaps the best emissions mitigation option to date. This
involves capturing CO2 from large point emitters and storing it in suitable geological media for an
extended period of time. Currently, the technology for each phase of CCS exists and has been utilised
in multiple small scale CCS operations; however, a large scale economically viable roll-out is yet to
be established.
The current key issues for sequestration are estimating the potential storage capacity and integrity
of storage media and understanding the natural phenomena that govern the interaction between the
injected CO2 and the media.
Laboratory isotherm tests are required to reveal the potential CO2 storage capacity of any media. Re-
sults obtained from these tests attempt to reflect the in situ storage capacity for a range of conditions.
At this stage the storage capacity of any media cannot be obtained directly from the characteristics
(such as rank, density, moisture content and porosity) of the sample and so isotherms must be estab-
lished (Busch et al., 2003b; Reeves et al., 2005). The sorption equations typically used to interpret
isotherm tests have fundamental inherent limitations which result in an inability to accurately reflect
36
50. what is physically occurring within the sample. One of the limitation of the current commonly used
theoretical equations is that the equations do not take into account non-trivial swelling, especially
under supercritical conditions, and this results in an estimation of storage capacity that is inaccurate
(Harpalani and Chen, 1992).
Currently, models that describe the results of isotherm tests utilise mathematical functions that are
not directly related to media characteristics, but relate, rather, to the bounds of experimental data and
"best fit" constants (Siemons and Busch, 2007; Day et al., 2008a; Goodman, 2005). The ultimate
goal in sorption science (in the context of CCS) is to develop models that accurately reveal the
storage capacity of any media based on sample characteristics alone (Reeves et al., 2005). However,
a starting goal is to remove inherent flaws in the theoretical equations by integrating swelling effects
and establish the foundation for a model that incorporates the modified results.
There is a need to develop equations that accurately describe and measure supercritical CO2 storage
capacity of a material. The current equations do not have the ability to provide accurate sorption
measurements (they neglect the swelling effects) and do not possess the flexibility to easily break
the total storage capacity into its constituent parts (free phase and excess sorption). This present
research aims to overcome these limitations and begin to develop a foundation from which CO2
storage capacities can be determined with greater accuracy.
This research contributes to the understanding and application of CO2 storage estimation, and poten-
tially makes CCS a more attractive CO2 emissions mitigation strategy.
37
51. 3 Methods and Equipment
The supercritical CO2 sorption behaviour of coal and sandstone cores were investigated. To achieve
this, isotherms were developed for pressures up to 10 MPa at a constant temperature of 40◦C. The
following chapter includes all methods and necessary equipment that was needed to go from sample
procurement to the full development of the isotherms.
3.1 Outline
A chronological listing of experimental activity is as follows (N.B. bomb implies pressure vessel):
1. Sample Preparation
(a) Cores
2. Rock Property Testing
(a) Bulk density
(b) Porosity
3. Pre-testing Investigation
(a) Determining the volume of the bomb
(b) Determining the free phase CO2 density as a function of pressure
4. Sorption Testing
(a) Determining the sorption isotherm
(b) Charging the bomb
(c) Vacuuming the bomb
(d) Leakage testing
38
52. 3.2 Samples
For the determination of the sorption behaviour for various geological media, a total of five Australian
rock samples were tested. Of these five, two were coal, labelled Coal A and Coal B, and three were
sandstone, labelled Sandstone A, Sandstone B and Sandstone C.
Coal A was sourced from Ulan Coal Mines Limited in the western coalfields of NSW. It had been
stored at University of Wollongong since 2013 where it had been subjected to subpar storage condi-
tions. Prior to use in the present study the coal was stored in a container partially filled with water
and wrapped inside a plastic sheet. The water level reached just over a quarter of the height of the
coal and the plastic sheet that insulated the coal was broken and fragmented. It is likely that the coal
was more heavily weathered and oxidised relative to its in situ state.
Coal B was sourced from Appin West Mine in the Macarthur region of NSW. It was a relatively fresh
sample, stored at the University of Wollongong since 2015, only a few months before testing. Prior
to testing, the coal was stored fully submerged by water inside a closed container.
Sandstone A and B was sourced from unknown regions of Australia. Both had been stored in an
open bucket in 54 mm diameter core form. Sandstone C was sourced from Kangaroo Valley in the
Illawarra region of NSW. It was stored as a broken block in an open bucket.
All tested samples are shown below in Figures 3.1 and 3.2. The longer 38 mm diameter cores were
used in porosity and sorption experiments, and the shorter 38 mm diameter cores were used in density
experiments.
Figure 3.1: Coal Samples Figure 3.2: Sandstone Samples
39
53. 3.3 Sample Preparation
To transform the coal and sandstone samples to usable cores for sorption testing the following pro-
cedure was used.
3.3.1 Core samples (38 mm diameter and 70 mm high)
1. If sample is small or lacks stability place in steel framing box, and fill with concrete until the
top of sample is just visible (refer to Figures 3.3 to 3.6).
(a) Cement, sand and aggregate ratio 1:2:3.
(b) Wait 48 hours for the concrete to dry and set.
2. Secure sample or concrete block beneath a drill using straps to secure it directly to the base
(refer to Figure 3.5).
3. Use a core drilling rig to recover as many 40 mm dia cores as possible (refer to Figure 3.8).
(a) Shaving from the rig results in cores roughly 38 to 39 mm.
4. Inspect cores to ensure that they are stable and have no major faults.
5. Use a circular saw to obtain a minimum of:
(a) One 70 mm length core for sorption testing (refer to Figures 3.9 and 3.10).
(b) Five 10 mm length cores for bulk density testing (refer to Figures 3.9 and 3.10).
6. Dry sample at 105 ◦C for 20 hours.
7. Label and store in a dry place.
8. Repeat all steps for all rock samples or combine in one concrete block initially.
Figure 3.3: All Testing Samples Figure 3.4: Steel Framing Box
40
55. 3.4 Rock Property Testing
To calculate the sorption capacity of various samples, certain rock properties must be known. The
bulk density of each sample allows its volume to be determined at Natural Temperature and Pres-
sure (NTP). A subtraction from the measured volume of the bomb then reveals the empty space
inside the bomb. This volume and an interpolated free phase density value directly determines the
expected mass gain due to CO2 between the sample and the interior bomb wall. A comparison with
the measured mass gain at a particular pressure then determines the sorption capacity of each rock
sample.
The porosity of each sample allows the measured total sorption to be broken into its components. By
assuming the pore space volume is fully saturated with free phase CO2, the sorption contribution of
the molecules in the free state can be deducted from total sorption to determine the excess amount
sorbed.
3.4.1 Bulk density
3.4.1.1 Wax immersion
The bulk density was found according to ATSM standard C914-09, Bulk Density and Volume of
Solid Refractories by Wax Immersion. The density of paraffin wax was assumed to be the average
of the densities quoted by the manufacturer. As per the standard, determining sample weights to four
significant figures could not be achieved in this experiment due to a limitation of available equipment.
Instead, the submerged samples were weighed to an accuracy of 0.01 g which corresponds to two or
three significant figures depending on the sample being tested. A summarised and tailored version of
the method detailed in ATSM standard C914-09 follows.
1. Weigh each of the five 10 mm length cores for each sample obtained in sample preparation.
(a) When 10 mm length cores where difficult to obtain small off-cuts can be used.
2. Melt paraffin wax at 60◦C just above its melting point (refer to Figures 3.11 and 3.12).
3. Use long nose pliers to dip just over half the sample in the wax for two seconds.
4. Dry in front of a fan for two seconds and dip the other side ensuring an overlap is created.
(a) Press down any air bubbles that form between the sample and the wax during the coating
process.
5. Dry samples in ambient air for one hour.
42
56. 6. Weigh.
7. Attach a basket to the bottom of a scale which possess the capability to measure submerged
weights (refer to Figures 3.13 and 3.14).
8. Take temperature of distilled water.
9. Fill density testing sink with distilled water fully covering the bulk of the testing basket
10. Place sample underwater on top of the testing basket and measure its submerged weight.
11. Calculate the bulk density of the sample based on the difference between submerged and non-
submerged weights, the density of water, and the known mass and density of paraffin wax (see
Equation 1)
12. Repeat for all rock samples.
Figure 3.11: Paraffin Wax
Figure 3.12: Melting Pot
Figure 3.13: Submerged Basket Figure 3.14: Submerged Weight Set-up
43
