Carbon Capture & Storage
EG
Solvents for improved
postcombustion CO2 capture
If post-combustion capture (PCC) is to be a viable and practical solution for the reduction of
CO2 emissions in a carbon-constrained world, all challenges to accelerate the implementation of
the technology while maintaining a realistic approach to the economics of the process and its
environmental impacts should be evaluated.
By Moetaz I. Attalla, CSIRO Energy Technology, Newcastle
P
ostcombustion capture (PCC) of
carbon dioxide by amine solvents is
widely recognised as the most viable
and near-ready technology to reduce CO2
emissions at stationary point sources, such
as coal-fired power stations, in the mid
term. The selective removal of CO2 from
gas mixtures using an aqueous amine liquid
is proven on a smaller commercial scale
(typically 400 tonnes CO2 per day), and
has been in use for decades, for example,
the sweetening of natural gas. There are,
however, many challenges that researchers
and governments must face – and overcome
– prior to the roll-out of PCC on the larger
scale needed to reduce greenhouse gas
(GHG) emissions to future mandated levels.
Many of these challenges are self-evident:
• scale-up and plant cost
• energy efficiency penalty for solvent
regeneration/CO2 compression
• the identification of suitable storage sites
• the potential environmental impacts
of solvents and solvent degradation
products (through oxidative and thermal
processes) released to the environment.
The CSIRO’s Novel Solvents group is
undertaking research into cost reductions
through
fast-reacting,
energy-efficient
solvents that are stable and environmentally
safe.
CYCLICAL ABSORPTION-desorption
A schematic representation of a typical
carbon dioxide capture plant utilising
solvents is shown in Figure 1. The
technology
utilises
the
reversibility
of the chemical reaction between an
alkanolamine such as monoethanolamine
(MEA) and CO2 in water. Briefly, a waterbased amine absorbent (liquid) is contacted
with the flue gas stream in the absorber
column at temperatures in the range of
40–60°C. This, in an Australian context,
means cooling the flue gas before it enters
the absorber, and offers an opportunity for
heat scavenging. The CO2 -lean flue gas
exits at the top of the absorber column to
the atmosphere, while the CO2 molecules
remain in the absorber in liquid phase,
chemically bound to the nitrogen atom of
the alkanolamine molecule.
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CO2 Free
Gas
Cool
Lean Amine
Primed
CO 2 Rich Amine
ABSORBER
CO2 absorbed
into solution
Relatively
Pure CO2
STRIPPER
CO2 released
from solution
Cross
Heat
Exchanger
100 °C
40 °C
Flue Gas
10-15% CO2 @ 1 atm
Cool
CO 2 Rich Amine
Hot
CO 2 Lean Amine
Figure 1 PCC plant schematic
The CO2 -rich or ‘loaded’ amine solvent is
then pumped to a stripper column (RHS) via
a heat exchanger. In the stripper column,
the solvent is contacted with steam (heat
regeneration) which releases the CO2 from
the amine solution, and the solvent is ready
to capture more CO2. A relatively pure CO2
stream exits the top of the stripper column,
and is ready for drying, compression,
transportation and finally sequestration.
The solution at the bottom of the
stripper column – now CO2-lean – exits at
temperatures of 100–120°C, passes through
a cross-heat exchanger to transfer heat to
the CO2 -rich solvent entering the top of the
stripper column, and passes on to the top
of the absorber column, finally completing
a single pass. This process is repeated
continuously.
The state-of-the-art
Monoethanolamine, otherwise known as
MEA, is the alkanolamine absorbent against
which the performance of new capture
formulations and blends are measured.
The MEA molecule is deceptively simple,
consisting of an alcohol group (-OH) and an
amine group (-NH2) at opposite ends of an
ethylene (C2H4) molecule, as illustrated in
Figure 2.
The amine group is the part of the MEA
molecule that reacts with carbon dioxide in
Energy Generation
Figure 2 MEA molecule. N is a nitrogen atom;
C are carbon atoms; O is an oxygen atom; and H
are hydrogen atoms.
aqueous solution. Aspects of MEA which
impart a distinct capture advantage relative
to other amines include:
• a high boiling temperature (160 °C)
• a fast reaction rate
• high water solubility
• easy to synthesise on a large scale
A high boiling temperature means very
little solvent is lost to the atmosphere
during the flue gas scrubbing process,
thus minimising potential environmental
impacts. For comparison, patented ammonia
(NH3) processes may require the extensive
use of chillers to avoid solvent losses (i.e.
solvent slip) due to ammonia’s low boiling
temperature and high vapour pressure.
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Carbon Capture & Storage
Continued from page 27
low temperature dilute ammonia
process
The pilot plant operated by CSIRO at
Munmorah power station (on the Central
Coast of NSW) trialled a low temperature
dilute ammonia process for CO2 capture
from flue gas. However, to avoid NH3 slip,
the process was designed to operate at 15°C.
The advantage of MEA over ammonia as a
sorbent is that it is not easily slipped, even
at the temperatures required to regenerate
the amine and release the captured CO2
(typically 100–120°C).
A high reaction rate is of primary
importance when designing absorption
columns. Succinctly, the higher the mass
transfer or reaction rate, the smaller the
absorber column required. This translates
to an overall saving in plant capital costs.
While there are amines that react with CO2
more rapidly than MEA, these are generally
either niche amines which are expensive
to synthesise on a large scale, or there are
other deleterious drawbacks; for example,
ethylene diamine is extremely corrosive to
plant material.
suffer from solubility problems
MEA is also very miscible with water. In
contrast, processes based on other amines
such as piperazine, a cyclic molecule with
two nitrogen atoms, suffer from solubility
problems particularly as the solution CO2
levels increase. Particulates form which can
cause scale and plant corrosion, leading to
significant maintenance costs.
Commercially, MEA is produced in large
quantities via the reaction of ethylene oxide
and ammonia, two chemicals produced on
massive industrial scales for various purposes.
Hence, it is always available and inexpensive.
However there are some drawbacks of MEA
and other organic amines which include:
• a susceptibility to degrade in the
presence of oxygen and trace metals
as typically found in flue gas streams at
absorber temperatures (40–60°C)
• high corrosiveness of CO2 -loaded and
cycled solutions
• a large heat requirement for the
regeneration of the solvent (the energy
penalty), which reduces available
generating power and power plant
efficiency to less than 30%.
Process chemistry
The chemistry involved in the absorption
process
is
reversible
at
moderate
temperatures; and it is more reversible for
some alkanolamines than others. When
CO2 reacts with alkanolamines in solution,
two chemical products may form: these are
known as “carbamate” and “bicarbonate”.
The degree of carbamate versus bicarbonate
formation will impact on the capture
capacity and regeneration energy of the
amine. Primary alkanolamines such as
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Figure 3 The reaction of primary amine MEA with CO2 to form “carbamate”
MEA, and secondary alkanolamines such as
diethanolamine (DEA) and piperazine (PZ),
react with CO2 rapidly to form “carbamates”,
as illustrated in Figure 3. The implication is
that two molecules of MEA are consumed for
each molecule of CO2 captured. In addition,
the heat required to regenerate the CO2
from carbamates is much higher than if
bicarbonate forms. This energy requirement is
a substantial cost penalty for PCC. Bicarbonate
formation has the advantage of only
consuming a single molecule of amine per
molecule of CO2. Unfortunately, this reaction
is much slower than carbamate formation
for MEA. Hence this would translate to
larger reaction vessels and increased cost in
capital. The CSIRO solvent development team
is looking at additives which may speed up
the bicarbonate formation reaction under
normal plant conditions, so that the capture
step still involves a carbamate, that is, it is still
a fast reaction but this product is short-lived
in solution and hence requires a lower energy
to regenerate.
Currently being investigated
Certain amines known as “hindered” amines,
for example 2-amino-2-methyl-1-propanol
(AMP), are receiving greater attention due
to certain advantages in capture capacity
and regeneration energy. In solution, these
chemicals favour bicarbonate formation,
but are much slower to react than primary
amines. Hindered amines are currently being
investigated as additives for ammonia, and
as rate promoters for slower tertiary amines.
Secondary and tertiary amine sorbent
molecules have also been in use since the
inception of gas sweetening technology.
These amines differ from primary amines
in that the reactive nitrogen centre has
only single hydrogen attached directly
(secondary) or none at all (tertiary). Examples
of secondary amines are diethanolamine,
piperidine and piperazine.
Tertiary amines are the slowest of all the
capture solvents used, chiefly because they
cannot form a carbamate. This means that
the rate of reaction of a tertiary amine is
almost reduced to the rate at which caustic
mineral solutions react with CO2 to form
bicarbonate.
In summary, carbamate formation is a
fast reaction with a large heat of solvent
Energy Generation
regeneration,
whereas
bicarbonate
formation is slower with a smaller heat of
regeneration.
Secondary and tertiary amines
One approach to reduce the energy
requirement and increase reaction rate
is to use amine blends – an active area of
research at CSIRO. Blends can be tailored
for the flue gas composition and for the
capture plant footprint. In principle, a small
amount of primary or secondary amine, or
even an enzyme, is added to a secondary or
tertiary amine to enhance the rate of CO2
absorption. The rate of reaction of CO2 with
the amine blend is greater than the rate
of either of the separate components. The
mechanism by which the promoter functions
to enhance CO2 absorption is still unclear
and is currently being studied.
Reduce oxidative degradation
MEA is the most widely studied CO2
scrubbing molecule. The use of MEAbased solvents for flue gas scrubbing at the
pilot scale have demonstrated a decrease
in solvent performance over a time scale
of months, particularly in the presence of
high oxygen concentrations. At typical
capture temperatures, with trace levels of
iron, iron oxide or other catalytic metals in
solution, molecular oxygen reacts to form
highly reactive radical species. The radical
species go on to react with and degrade the
alkanolamine solvent, forming by-products
such as ammonia and small organic acids
(oxalic, glycolic, oxoacetic, acetic and
formic). This leads to the acidification of
the capture solvent and the formation of
heat stable salts, which cause scale and
plant corrosion. The end result of oxidative
degradation is a loss in capture solvent
performance .
Methods of circumventing oxidative
degradation – which do not increase the
plant footprint – include addition of trace
quantities of anti-oxidants. The additives
are either sacrificial reductants or radical
scavengers, molecules which mop up the
oxygen species responsible for solvent
damage. There is an active program in place
at the CSIRO laboratories with purposebuilt equipment (see Figure 4) for oxidative
solvent degradation studies.
January-March 2010
EG
acid, ethylamine, acetone etc. which can
be entrained with the flue gas to the
atmosphere. In particular, NH3 can affect
the formation of particulate matter in
the atmosphere
• Nitrosamines formation as a result of
reaction between an amine and nitrogen
oxide
• The mounting evidence of the presence
of amines in particulate phase.
To ensure the viability of this technology, it
is crucial at this early stage of development
that a thorough understanding of the
potential environmental impacts of emissions
on terrestrial, aquatic and atmospheric
environments is investigated. However, at
present, there is little understanding of the
environmental impact of PCC technology.
Mitigation methods are under review, as
well as the nitrosamine chemistry of amine
degradation products. There is potential
for these chemicals to entrain in released
flue-gas and consequently end up in the
atmosphere; hence there is a need to identify
solution-based preventative measures.
Figure 4 Absorber-stripper photo/LC-MS picture
Metrics of solvent performance
To date, research into solvent performance
has been conducted in an ad hoc fashion,
with mass transfer rates and solvent
capacity being the lead (and often the only)
parameters measured. These ‘mass transfer
rate’ figures are undoubtedly of great
utility, but identifying the unifying chemical
principles of desirable capture solvent
candidates should be the focus of any longterm research plan. A fundamental campaign
is underway at CSIRO involving various
techniques including infrared spectroscopy,
viscosity measurements, rate determination,
vapour-liquid equilibria measurements,
mass spectrometry and ab initio quantum
chemistry modelling to examine shortlived reaction products, preferred reaction
pathways and solvent degradation chemical
mechanisms for a variety of solvents and
blends. The efficacy of various additives
that improve solvent performance and/or
reduce solvent degradation is also being
examined using these methods. The ab initio
computations are particularly promising as
a first-pass screening method for assessing
potential sorbents, as the computations are
inexpensive and fast (on high performance
computing nodes). Radical chemical
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reactions can also be investigated using this
technology, as can mechanisms of reaction
over longer time frames.
An important aspect of CSIRO’s current
research activity that is gaining attention
in the wider research community and
industry is the propensity for certain amines
to form deleterious degradation products
such as nitrosamine derivatives in solution.
Nitrosamines are hazardous chemicals found
in, among other things, cigarette smoke.
There are obvious local environmental
implications from solvent spills and slip. If
solvent-based PCC technology is deployed
at a commercial scale, potentially millions of
tonnes of solvent will be used per annum.
Hence, there is a likelihood of emissions of
solvents and solvent degradation products
(through oxidative and thermal degradation)
to the environment. Interest associated with
these emissions includes:
• Entrainment of the amine/ammonia with
the treated flue gas and their associated
atmospheric chemical reaction pathways
(reactions with nitrous oxide, carbon
monoxide and oxygen)
• Formation of ammonia and other amine
degradation products such as formic
Energy Generation
SUMMARY AND FUTURE SCOPE
The aim of the CSIRO postcombustion CO2
capture absorbent development program
is to identify and develop enhanced
absorbents and additives which improve
the rate of CO2 capture by aqueous
ammonia and alkanolamine. Through the
application of advanced chemistry and
engineering CSIRO will be able to decrease
the degradation rates of alkanolamines,
as well as reduce amine losses through
absorbent slip. Once enhanced amines and
additives with desirable characteristics are
identified, a multi-step testing program for
evaluation purposes will be undertaken.
This will include: testing additive
performance with common industrial
amines; testing the performance of new
formulations with combustion gases from
different coal types with a view to tailoring
the performance to local fuels; determining
degradation rates and whether degradation
products can interact with nitrous oxide to
produce harmful chemicals; and pilot-scale
testing under realistic flue-gas scrubbing
conditions.
Outstanding
enhanced
amines and additives will be used in a
new alkanolamine/ammonia formulation
which will proceed to demonstration and
commercialisation.
In developing PCC as a viable and practical
solution for the reduction of CO2 emissions
in a carbon-constrained world, researchers
are addressing all challenges to accelerate
the implementation of the technology
while maintaining a realistic approach
to the economics of the process and its
environmental impacts.
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