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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