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Cost is not the only barrier to thewidespread use of carbon capture andstorage, say Tim Fowler and colleagues
M ANY people consider that man-
made global warming is the
cause of extreme weather, be
it too hot, too dry, too wet or too windy,and worldwide there are plenty of recent
examples of extreme weather events.
The long-term response to mitigating
global warming is to reduce emissions of
greenhouse gases, mainly by reducing
consumption of fossil fuels. But in the
short term, carbon capture and storage
(CCS) has been proposed as a means of
limiting the rise of CO2 concentration in
the atmosphere, to help minimise the
frequency and severity of future extreme
weather events. But can it be done safely
and economically?
In the mid-2000s the Norwegian
government committed resources to
building a full-scale demonstration plant
at the Mongstad refinery, located north of
Bergen. The plan was to capture CO2 from
Devil in
the detail
the flue gas of a new combined heat and
power plant, preferably using amine-based
capture technology. The design basis was the
capture of around 1.3m t/y of CO2. The CO2 would then be conditioned and compressed
for pipeline transport to geological storage
under the Norwegian Continental Shelf.
In addition to the relatively well-known
issues of process cost and the risk of
leaks from the geological storage site,
the CO2 capture process poses possible
environmental impact concerns, not least
because local surface water resources
at Mongstad are used for drinking water
supplies. A large amine-based CO2 capture
plant may cause very low but possibly
significant concentrations of amines and
amine degradation products (sometimes
called secondary pollutants) in the
environment. Nitrosamines and nitramines,
emitted by the process or formed in the
atmosphere by oxidation of amines, were of
particular concern at Mongstad since some
of these are acknowledged carcinogens
and very low environmental quality criteria
have consequently been proposed. Ahealth risk due to potential nitrosamine
and nitramine exposure was identified as a
main project risk, so methods to assess this
were developed.
The CO2 capture Mongstad project (CCM
project) initiated a series of academic
and technical research projects1 that
aimed to characterise fundamental
physio-chemical properties of a range
of relevant compounds, including the
model compounds mono-methyl amine
(MMA), di-methyl amine (DMA) and mono
ethanol amine (MEA) and their daughter
nitrosamines and nitramines. Quantifying
parameters such as reaction rates, reaction
branching ratios, Henry’s Law constants,
wet and dry deposition parameters –
amongst others – were all required to
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In the short term, CCS has
been proposed as a meansof helping to limit the riseof CO
2 concentration in the
atmosphere.
understanding the problem Assessing impacts of secondary pollutants
involves evaluating competing processes,
including: the rate of air dispersion of
the emitted primary pollutant (amines in
this case); the rate of reaction of primary
pollutants to form secondary pollutants
(nitrosamines and nitramines) as well as therate of reaction of primary pollutants to form
other species (which reduces the amount of
amine that can be converted to secondary
pollutants); and the rate of dispersion
and destruction of secondary pollutants.
Modelling all processes is a very significant
challenge, so finding the appropriate level of
detail to model is important.
gas phase free radical reactions The research confirmed that nitrosamines
and nitramines are formed from amine by
gas phase free radical chemical processes
(shown in simplified form in Figure 1). If the
free radical (X .) takes a hydrogen atom from
the amine group (-NH or -NH2) nitrosamine
or nitramine can result, but not so if it
abstracts a hydrogen atom from elsewhere.
The relative size of these rate constants
defines the branching ratio, which is typically
about 10–50%. The higher the branching
ratio, the greater the percentage of amine
converted to nitramine/nitrosamine.
The radicals OH. and NO
3
. were confirmed
as the main initiation free radicals. These are
formed by natural photo-chemically driven
processes. The radicals Cl. and Br
. were
considered but were not included in thefinal modelled process since these species
have both low concentration and will react
rapidly with atmospheric moisture to form
OH. radicals.
photochemistry The rate of photochemical reactions will
depend on the intensity of sunlight. This
depends on the time of day, the time of year,
the latitude, and the weather conditions.
The intensity of photochemical processes
controls the concentration of the OH. and
NO3
. radicals. The research results also
showed that nitrosamines can be photo-
chemically dissociated into amine radicals
(eg CH3N
.H) and NO (see reaction k
6 in
Figure 2).
partition with atmospheric moistureMost amines, nitrosamines and nitramines
are very soluble in water. It was found that
Henry’s Law constants, which represent the
position of equilibrium of a gas (or vapour)
between the gas and dissolved liquid phase
(or ‘partition’), were all greater than 10
mol/kg/atmosphere. A value of 10 or more
indicates that over 90% of the species isin the aqueous phase. Thus, over 90% of
the amine would be unavailble to react to
nitrosamine or nitramine – limiting the
amount of carcinogen that can be formed.
Mongstad has no shortage of atmospheric
moisture, be it vapour, invisible aerosols, fog
or precipitation (ie rain, snow, or hail).
aqueous phase chemistry As part of the work, we considered if
nitrosamines or nitramines could be formed
from amines in the aqueous phase3 and
concluded that this was not an important
reaction. Thus the main effect of partition with atmospheric moisture is to buffer the
concentration of the gas phase species
and hence reduce the rate of formation of
nitrosamine and nitramine.
Figure 1: Formation of nitrosamines and nitramines by free radical oxidation (MMA example)
CH3NH
2
.CH
2NH
2
CH3NHNO CH
3NHNO
2
CH3N
. H
Mono-methylamine
Methylnitrosamine Methylnitramine
Other product
X.= OH
., NO
3
.
X.
X.
NO2
NO
All species and reactions are only in the gas phase (subscript ‘g’ omitted)
Mongstad refinery by night. Weather,
location and choice of CO 2 capture
process can influence modelling results
– so every project should make its own
evaluation of the impacts and risks.
H a r a l d P e t t e r s e n / S t a t o i l
predict the impacts on local air and water
quality.
The CCM project1, 2 commissioned Det
Norske Veritas (DNV, now merged withGermanischer Lloyd to form DNV GL),
amongst several other scientific institutes
and entities, to assess the air and water
quality impacts of nitrosamines and
nitramines due to the proposed CO2 capture
process at Mongstad. The first stage was to
evaluate the significance of the research
project’s results to the environmental
assessment process.
Alkyl radicalintermediate
Amino radicalintermediate
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Figure 2: Summary of significant chemical and partition processes modelled (MMA example)
Chemical reaction
Mass transfer between gas and aqueous phase
All species are in the gas phase unless shown as aqueous (a)
CH3NH
2a
CH3NH
2
CH3NHNO
2
CH3N
.H
CH3NHNO
2a
CH3NHNO
kA 3
kA 5
kA 6
kA 1 kA 2k
3NO
2
k1OH k
6hv
k2NO
k4NO
2k
5O
2
k10
NO3
k11
OH
k12
NO3
k14
k13
kA 4
Other product
CH3NHNO a
developing the modelFigure 2 summarises the conceptual model of
the significant processes. Only the relatively
stable species of interest (MMA, methyl
nitrosamine, methylnitramine) can partition
with atmospheric moisture. Some pathways
were implemented in the model but also
‘turned off’ by setting the corresponding rate
constant equal to 0.0 (eg k 14
was always 0.0, k 13
was sometimes 0.0 – depending on whether
the amine was primary or secondary).
Figure 2 shows that five different counter-
species need to be included in the model in
addition to the amine and amine daughter
species that result from process emissions:
• the concentration of OH. is a function of the
angle the sun makes with the local vertical
at the location latitude and time of day,
normalised by the maximum value measured
at Mongstad;
• the concentration of NO3
. is higher at night
than during the day;• the concentration of NO and NO
2 is the sum
of the measured background concentration
plus the concentration emitted by the
proposed CO2 capture process as calculated
by the dispersion model; and
• the concentration of oxygen is constant.
deposition processes Air pollutants can be transferred to the
ground by dry and wet deposition processes.
Dry deposition occurs by adsorption of
pollutants onto vegetation and other surfaces. Wet deposition occurs when precipitation
falls through the pollutant plume and
pollutants transfer to the droplets as they fall.
At Mongstad, wet deposition dominates over
dry deposition.
modelling the water catchmentsPollutants deposited to ground can, amongst
other processes, adhere to soils or other
materials, can be transported and diluted
by water flow, can volatilise back into the
atmosphere, can chemically react, or can
be metabolised by flora, fauna or microbial
activity. It is necessary to represent the mostsignificant of these processes to evaluate the
impact on water quality. The primary inputs
used are the deposition fluxes, parameters to
describe the physical and chemical behaviour
of each pollutant, water catchment data (ie
which areas drain where) and rainfall data.
No data was found to support the
transformation of amines into nitrosamines
or nitramines in soil or surface water, so such
processes were not included.
model implementationThe Calpuff modelling system4 is an open
source modified Gaussian puff air dispersionand deposition model. We used this model
as the starting point and developed a
bespoke chemical and physical processes
model to represent all the processes shown
in Figure 2. To verify the new codes, the
differential equations were first solved using
the commercial software package MathCad .
This showed that the selected numerical
methods were stable. The methods were
then translated into a Fortran box model
(chemistry without dispersion) for testing andfinally added to Calpuff. Inert tracer species
(which disperse but do not react) were added
to the model to perform molar and mass
balance checks.
Calpuff outputs wet and dry deposition
fluxes as a function of location. These were
used as inputs into the water catchment
model5. Two water catchments near
Mongstad were initially considered, but early
results indicated clearly that one was much
more sensitive to pollutants, so this was used
for the remaining water quality estimates.
other inputs Air quality models also require meteorological
input data, data to describe local significant
buildings, local ground cover, topography and
land use. Local data for Mongstad were used.
main resultsThe model developed above was used to
help evaluate the performance of several
CO2 capture process options. While we can’t
provide full details here – due to commercial
confidentiality– we can, however, share some
important high-level results.
First and foremost, the analysis shows thatsome CO
2 capture processes can comfortably
satisfy the proposed environmental quality
criteria for impacts to air and to water.
Second, the detailed design of the CO2
absorber and the use of emission-reducing
technologies have a highly significant effect
on the emissions of amines released to the
atmosphere.
Third, different process release conditions,
such as stack height, tall buildings, released
gas temperature and velocity could
significantly affect dispersion performance.
Fourth, at Mongstad, water quality impacts were more significant than air quality issues.
This observation may be due to the greater
uncertainty in the deposition parameters
used in the model. It also suggests that
greater CO2 capture process flexibility may be
possible in localities where surface water is
not used as a source of drinking water, though
in this case a groundwater quality assessment
may be necessary.
Fifth, environmental and health impacts
can be assessed in a variety of ways. Each
way has both strengths and weaknesses.
We describe a model where chemical
transformation and dispersion processes arerepresented together. The concentrations we
predict are significantly lower compared to
simpler dispersion only models. In effect, we
have shown that the location of the maximum
Pollutants deposited toground can, amongst otherprocesses, adhere to soilsor other materials, can betransported and diluted by water flow, can volatilise back
into the atmosphere, canchemically react, or can bemetabolised by flora, fauna ormicrobial activity.
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ground level concentration obtained from
the dispersion model is not also the location
where maximum conversion of amine to
nitrosamine and nitramine is predicted. But
our model requires knowledge of a wide
range of physio-chemical parameters.
conclusionsIt has been shown that representing relevant
physical and chemical processes results in
lower estimates of environmental impacts of
nitrosamines and nitramines from amine-
based CO2 capture processes compared to
results from less sophisticated dispersion-
only models. In turn, this allows greater
process design flexibility and possible lower
process costs, perhaps by reducing the
required stack height, reducing the gas exit
velocity or reducing the gas exit temperature.
Balanced against this is the need to
estimate a large number of physio-chemicalparameters.
In conclusion, this work has shown how
dispersion models can incorporate amine
chemistry and be used to evaluate the health
and environmental impacts of emissions
from a CO2 capture plant. Different CO
2
capture processes and different plant
locations can also influence the results, so
every project should make its own evaluation
of the impacts and risks. The CCM project
has developed tools to make this possible.
the futureIn October 2013 the Norwegian government
withdrew financial support to the Mongstad
CCS project, citing delays and cost over-
runs6. Norway, however, remains committed
to developing a full-scale CCS process before
2020. A strategy for how to achieve this goal is
under development. tce
Tim Fowler ( tim.fowler@dnvgl.com ) is
senior principal consultant at DNV GL.
Graham Vernon, Stavros Yiannoukas,
Katrina Parry and Mark Purcell, also atDNV GL, all contributed to the work.
Richard Williams at The NERC Centre of
Ecology and Hydrology performed the
water catchment modelling work under
contract to DNV.
further reading1. www.gassnova.no/en
2. bit.ly/1ctTuew
3. Nielsen, CJ et al , “Atmospheric chemistry
and environmental impact of the use of
amines in carbon capture and storage”,
Chemical Society Review , 2012, 1–21, DOI10.1039/c2cs35059a
4. www.src.com/calpuff/calpuff1.htm
5. bit.ly/1gQYJBQ
6. The Chemical Engineer , November 2013, p13
The Mongstad refinery site where the carbon-
capture plant was to be constructed. Water
from lakes in the area is used for potable water
supplies
H el g eH a n s en
In October 2013 theNorwegian government withdrew financial supportto the Mongstad CCS project,citing delays and cost over-
runs. Norway, however,remains committed todeveloping a full-scale CCSprocess before 2020.
Chemical Engineering MattersThe topics discussed in this article refer to thefollowing lines on the vistas of IChemE’s technicalstrategy document Chemical Engineering Matters:
EnergyLines 2, 15, 24
WaterLines 12, 17
Health and wellbeingLines 11,19, 28
Visit www.icheme.org/vistas1 to discover wherethis article and your own activities fit into the myriadof grand challenges facing chemical engineers