2.1.3 Operational risks related to CO2 stream composition

It is required that the CO2 stream shall consist overwhelmingly of CO2. This is to ensure that the CO2 stream does not negatively affect the integrity of the storage site or transport facilities and to prevent any significant risk to the environment or human health. The exact composition of the stream is highly dependent of the source and capture processes used. The main issues associated with CO2 stream composition are listed in Tab. 2-1.

Tab. 2-1: Main issues associated with selected incidental substances of a CO2 stream (modified after DNV, 2010).

Component

Health & Safety

Pipeline capacity

Water solubility

Hydrate formations

Materials

Fatigue

Fractures

Corrosion

Operations

Comment

CO2

X

X

X

X

X

X

X

X

X

Non-flammable, colourless, no odour; low toxicity,heavier than air in the gaseous state

H2O

X

X

X

X

X

X

Non-toxic; condensable; forms acids with CO2, NOx and SOx, which have a corrosive impact on transport infrastructure

N2

X

X

Non-toxic; stable

O2

X

X

X

Non-toxic

H2S

X

X

X

X

X

X

Flammable, strong odour, extremely toxic at low concentrations

H2

X

X

X

Flammable, non-condensable at pipeline operating condition; potential impact on transport infrastructure through embrittlement

SO2

X

X

X

Non-flammable, strong odour, toxic; forms sulphuric acid with water

NO2

X

X

X

Non-flammable, toxic; forms nitric acid with water

CO

X

X

Flammable, toxic

CH4

X

X

X

Odourless, flammable

Amines

X

Potential occupational hazard, with corrosive impact

Glycol

X

X

Potential occupational hazard

The three main transportation risks associated with impurities in the CO2 stream are: corrosion, gas hydrate formation, and pipeline flow characteristics. While some minor substances can be safely transported in pipelines, they might have a negative effect on their integrity.

Corrosion of pipelines may occur if there is too much water in the CO2 stream, since it may form acids that corrode the pipelines. The CO2 stream composition could influence the choice of pipeline materials and thickness such as to ensure that safety requirements are met. Consideration of water concentration limits for pipeline corrosion is likely sufficient to address corrosion in other infrastructure components (pumps, valves, injection tubing). CO2 leakage through existing cracks could also lead to the acidification of water outside the pipe causing external corrosion. In addition to corrosion, water in a CO2 stream can also increase the risk for hydrate formation. Hydrates form at temperatures higher than the freezing point of water and its solid like property makes it a danger for pipelines (Carroll, 2003). Hydrates can form in liquids and gases, favourably in low temperatures and high pressure and are therefore mostly a concern for offshore operations. The main strategy for preventing hydrate formation is sufficient dewatering of the CO2 stream (DYNAMIS, 2007).

Oxygen in a CO2 stream can also have corrosive effects in pipelines. In Enchanced Oil Recovery (EOR) another key risk related to oxygen is that it reacts with oil and can cause overheating of injection equipment (IEA GHG, 2004; DYNAMIS, 2007). The DYNAMIS project report (2007) notes that it can be useful to place oxygen sensors in the injection and production wells for EOR to ensure that these wells do not overheat. However, an early report in 1985 indicated that injection of small amounts of O2 in EOR applications should not have significant impacts, and the main issue was corrosion (Taber, 1985). Taber (1985) also suggests that flue gas injection with 1-2% oxygen and air injection for in-situ combustion for EOR takes place without serious corrosion problem, as long as there is sufficient dewatering. However, further research is necessary to assess the impact of O2 in CO2 streams for storage.

Long distance transportation of CO2 is most efficient and economical in the liquid or supercritical states (DNV, 2010). Getting to the supercritical fluid flow is made more difficult by the presence of non-condensable gases such as hydrogen (H2), argon (Ar), nitrogen (N2), oxygen (O2) and methane (CH4), as higher pressure is needed to convert CO2 into the supercritical fluid (DYNAMIS, 2007). Models used need to predict the phase envelopes for the range of mixtures likely to be present. The cost of CO2 purification is important for the total cost of CCS as it affects many other parts such as transportation and storage.

Acid gases can be transported safely in pipelines as long as the stream is sufficiently dehydrated, but interactions will occur with formation water in the storage site. Of particular importance are the potential deterioration of wellbore cement and other geochemical changes from acid interactions (chemical reactions and mineral dissolution and precipitation, along with related permeability enhancements and clogging effects) with the fluids and rocks in the storage formation and heavy metal contamination of deep saline aquifers.

Some of the incidental substances found in the CO2 stream are toxic, such as CO, NO2, SO2 and H2S, and may further influence the potential impacts of a pipeline leak or rupture (IPCC, 2005). Because low levels of H2S are tolerated by the human body quite well, H2S would mainly be a safety concern for the general public living along the pipeline route or workers who would be operating and maintaining the pipeline and pumping stations, where the concentrations of H2S from potential leakages could be higher.

When SO2 is inhaled it can cause immediate irritation in the throat and a sensation of tightness and difficulty in breathing. People with asthma are more sensitive to these health effects and could react to concentrations of SO2 below 1ppm (DYNAMIS, 2007). NO2 is a very toxic gas and exposure at low levels may result in unconsciousness or death. The SOx and NOx produced from air-combustion would be removed in post-combustion capture processes in order to achieve the longevity requirements of acid gas removal and amine solvents (Tzimas et al., 2007). If SOx and NOx are not removed from the CO2 streams from oxyfuel combustion, oxy-fuel combustion will be the source of most of the SOx and NOx.

Amines used in post-combustion CO2 capture can be degraded to different harmful substances such as aldehydes, amides, nitrosamines, and nitramines, some of which have been found to be carcinogenic (Låg et al., 2009). Release of these substances to the air, drinking water or the aquatic ecosystems may need to be limited and several studies are underway to evaluate such effects (da Silva et al., 2013).

Small amounts of tracer substances can be added to the CO2 stream for monitoring and verifying the location and migration of the CO2 plume. Although these substances are no serious health risk in small amounts, the health impacts on operators should be considered (EPA, 2010).