CO2 Capture and Storage

With climate change mitigation remaining high on the agenda of key decision makers across the world, the portfolio of technological solutions to cut carbon dioxide (CO2) emissions is ever expanding.

One medium-term solution is CO2 capture and storage (CCS), a technique through which carbon emitted from fossil fuel combustion is captured from the combustion process, transported and then permanently stored, usually underground either terrestrially or at sea.

This technology provides an interesting example of how two different environmental impacts – climate change and air quality – interact, and raises questions for operators, investors and regulators to consider.

The benefits

Almost all of the major point source greenhouse gas emissions that contribute to climate change arise either directly or indirectly as a result of energy use, predominantly for power generation and in transport.

This energy use is intrinsically linked to the consumption of fossil fuels, which we are undoubtedly reliant upon. But it is now possible to envisage a world where we are freed from this dependence, although in the short and medium term it is still not practical and will require substantial investments, planning and time to transition to a non-fossil fuel dependant world.

CO2 capture and storage is in this respect a bridging technology that could allow the continued use of large-scale, reliable and cheap fossil fuel plants with fewer emissions of greenhouse gases. CO2 capture and storage would therefore complement increased deployment of other renewable and low carbon technologies providing the diversity and resilience required in any power system. It can also be used for other types of process including, most notably, cement production which the world is similarly dependant upon. Retrofitting of this technology to existing large emitters presents an attractive option for decarbonising existing grid electricity generation.

A further application is to use the captured CO2 to recover otherwise uneconomical reserves of oil and gas (known as Enhanced Oil Recovery).

To this end, the Department for Energy and Climate Change in the UK is currently running a series of competitions to construct demonstration plants in order to advance the understanding and commercialisation of such technology on large-scale power generation. CO2 capture could be an important part of any integrated energy system, as aside from nuclear power or hydro power, fossil fuels plants still offer the most reliable and well understood large-scale baseline generation.

Technology overview

Carbon, capture and storage entail three key processes. First the capture stage, where the CO2 present in exhaust gases is removed using various techniques. Secondly, the captured CO2 must be transported from the point of capture, typically using pipelines or shipping under certain circumstances.

Finally, the CO2 must be stored permanently in a suitable underground location. Each of these stages has specific environmental impacts which must be carefully managed. In terms of air quality, however, the initial capture stage is where any potential air quality effects would be likely to arise.

A range of carbon-capture technologies exists which can be broadly categorised into three groups: pre-combustion, oxy-combustion and post-combustion.

Pre-combustion capture involves reacting the feedstock fuel to produce hydrogen rich fuel (syngas) which is then combusted. For this reason the primary use case for pre-combustion technologies is typically for combined cycle gas turbines (where the H2 can be directly used as the fuel) including integrated gasification combined cycle processes where solid fuel (for example, coal) is gasified to produce syngas for combustion but affords an opportunity to remove CO2 before it is combusted.

Oxy-combustion involves combustion of fuels in nearly pure oxygen together with recycled flue gases. In this case the resulting exhaust gas stream has a high proportion of CO2 and low nitrogen content in the stream, making CO2 capture easier. The result of this process is that there is the potential for much less generation of nitrogen based compounds which would benefit air quality by substantially reducing mass emissions of these pollutants from the process.

Post-combustion capture involves an abatement plant in the same mould as traditional sulphur or nitrogen capture after the combustion process, meaning it can be easily retrofitted to existing plants. The CO2 in the flue gas stream is scrubbed using either a physical or chemical solvent by introducing the flue gas (which is slightly acidic) to the solvent in absorber towers. Accordingly, it has the potential for wide deployment.

To date most applications of post-combustion capture have been based on amine-based chemical solvents (the most prominent being 2-aminoethanol also known as monoethanolamine or MEA and commercial derivatives) as these can be regenerated in the process increasing system efficiencies. This process requires the exhaust gas to be at temperatures typically in the range of 30-60°C, which means that cooling of the exhaust gas would be required in most combustion applications. The solvent with CO2 now absorbed is then passed to a stripper which operates at a higher temperature where the solvent is desorbed and then regenerated into the process. Meanwhile the CO2 is removed ready to be conditioned and compressed for transport in either dense or vapour phase.

Depending on the solvent, other pollutants can be removed from the process too (see Table 1) but performance varies between applications. The way this occurs is dependent on the compounds involved; in chemical solvents, the reaction may instead lead to the production of salts which prohibit solvent regeneration and instead are lost from the process and emitted. While this may then lead to reductions in normal combustion products (particularly for SOx), it leads to the emission of other products and means more solvent is needed, hence reducing the process efficiency. However, the performance of individual applications is likely to vary. Therefore in post-combustion applications it is generally preferable to reduce the incoming concentrations of traditional pollutants such as particles and SOx which may already be the case for large combustion plants.

Potential effects

Some of the potential air quality effects from the post-combustion capture of CO2 for combustion plants differ between retrofitted plants (e.g. abatement added to currently operating plant) and those which are new-build, fully integrated systems. For retrofit plants, these potential effects relate to the changes to properties of the exhaust gases that occur during the process, namely to temperature and flow volumes. Potential effects are also modified depending on whether all or only part of the exhaust gases are treated for CO2 removal. Other impacts may occur regardless of the type of plant, namely the introduction of new pollutants from the solvents used and the potential reductions of traditional pollutants such as particles and SOx. The exhaust gases still need to be adequately dispersed as they still contain pollutants.

Changes to the way in which exhaust gases are released from sites is important because planning and permitting of processes is determined based on the potential impacts a process may have on nearby populations and ecosystems. Changing the conditions of exhaust gases can have detrimental or positive impacts on dispersion; higher temperatures and volumes generally lead to better dispersion through increased plume buoyancy and momentum. This is true of most abatement technologies and CCS shares some of the issues that need to be considered with Flue Gas Desulphurisation or Selective Catalytic

Reduction, for example. However, there are also some other considerations unique to CCS that need to be considered in the design of such plants.

Temperature

When using chemical absorbents, the change in temperature of the exhaust gas means that unless reheated its thermal buoyancy is reduced which could lead to detrimental changes in air quality (see Figure 1). Reduced temperatures at the stack exit mean that plume rise is lessened, which could result in earlier grounding of plumes; this could potentially lead to elevated concentrations of pollutants at ground level. The implication of earlier grounding is that any local receptors may become more exposed to pollutants or that the areas where effects may be evident compared to pre-CCS operation change (see Table 2 and Figure 2).

While abatement technologies for other pollutants also generally reduce exhaust gas temperatures, the operating temperatures in the absorber columns are particularly low and because the absorption solvent is sensitive to other pollutants in the exhaust gas stream, the absorber needs to be at the end of the treatment chain. This means that plants using CO2 capture may have to consider the potential implications of reheating the exhaust gas once it has left the absorbers.

If the impacts on receptors are not acceptable, exhaust gas temperatures can be managed but at additional cost. One option is to redirect heat from the main combustion plant which would affect the overall plant thermal efficiency. A second option is to provide this heat from a supplementary source for which an additional plant would be required. This plant itself may be a substantial installation, therefore leading to its own emissions and potentially its own emission point.

Volume

The removal of CO2 from the exhaust gas reduces the overall flow volume. For retrofit systems this could lead to problems for dispersion of pollutants as the flue through which the gas is exhausted was designed based on pre-CCS flows. The important parameter for defining the momentum of the release is the gas exit velocity which is dependant on the volume of gas emitted and the cross-sectional area of the release point (the stack). As the cross-sectional area is fixed, reduced exit velocities are inevitable, thus decreasing dispersion momentum and affecting the overall dispersion of the plume with the consequential potential impacts on air quality if local receptors are affected. This is, however, not a problem for new-build plants as the volume change can be accounted for in the original design.

The issue for retrofit cases is not without a solution. Depending on the specification of the CO2 capture plant and the reduction in flow volume, the exit velocity may still be adequate to ensure adequate dispersion with limited effects on air quality. It is also possible that the flue could be modified to have a smaller cross-section in order to better accommodate the new flow. However, consideration is needed of the potential bypass modes of any capture process, and the possible impacts a fixed flue diameter might have for cases when CO2 is not removed from the exhaust gas.

Potential changes in the exhaust gas temperature will also affect the volume of the exhaust gas. If the exhaust gases are much cooler than normal, this could have a profound impact on actual volumetric flows which would be compounded by the removal of the CO2 fraction.

In order to manage this issue, it may be possible to resize the existing stack to accommodate the post-capture flue gas volumes; however, the feasibility of this option will be highly site specific. Alternatively, the use of fans might be considered along with the introduction of air, but this would lead to increased process energy demands and would further reduce the temperature of the exhaust gases unless reheat was also adopted. Finally, consideration might be given to constructing a new emission point (stack) which better meets the requirements of the site.

Site constraints

There are a number of issues where changes to the exhaust gas properties may make the use of existing stacks problematic due to either changes in exhaust gas volume or site logistics (available space at the site). In retrofit plants, it could be difficult to introduce bypass routes for the capture plant where the existing stack may be used for both normal and bypass operations. The layout of the site could mean that the ducting requirements for the capture plant might become very expensive, particularly where other abatement technologies have been previously retrofitted to the site. Finally, if a dedicated heating plant is required, additional flue gas volumes may also need to be dispersed and space may not be available in existing stacks. In the UK, the new Carbon

Capture Readiness requirements for new combustion plants means that some of these issues can be incorporated into new plant designs.

In these cases, consideration might be given to introducing a new emission point where it is an economically feasible option. This is an important consideration for air quality because it has the potential to profoundly change the magnitude and distribution of pollutants on local receptors (see Figure 2 for a simple illustration of this). Depending on the location and plant, this could potentially benefit air quality by offering better distribution of pollutants for plants where only partial CO2 capture is undertaken (thus the mass emissions would be split between two emission points – one for the non-captured fraction and one for the captured fraction).

Increases in concentrations post-CCP

In the case of a retrofitted CO2 capture plant, the removal of CO2 from the exhaust gas stream has consequences on where monitoring of pollutants should be undertaken. For plants with regulated emission levels, actual emissions are often determined as concentration in the exhaust gas and a Continuous

Emissions Monitoring System is often used at various points to ensure that the plant is compliant. However, removing CO2 from the flue gas stream could make any pollutants more concentrated in the gas and consequently would increase reported pollutant levels. For retrofit plants, if changes to the nature of the pollutant release occur that affect dispersion, then ambient monitoring sites that have been located to monitor those plants may not reflect the post carbon-capture plant emissions.

Given the uncertainty in this unconventional problem, early consultation with the regulator is prudent. Two possible solutions for a post-combustion CO2 capture could be to either apply the emission concentration limits before the CO2 capture plant or apply mass-based limits rather than concentrations.

However, both of these assume that the capture process does not contribute to or remove pollutants from the flue gas which may or may not be true depending on the technologies deployed.

Monitoring of new pollutants

For post-combustion capture, the regeneration of the solvent used is efficient, but limited slip of the solvent is possible. The nature of chemical solvents and their reception with other contaminants in the exhaust gas means that CO2 capture plants might introduce new pollutants.

There is some considerable uncertainty and debate in the types of pollutants that might be emitted from these processes and research is ongoing, with a leading effort being undertaken by the ADA (Atmospheric Degradation of Amines) project in Spain. It is important to note that the use of amines for acid gas removal does have a historical track record through the abatement of H2S emissions. Increased deployment of CCS could mean some background monitoring of amine-based compounds could be required.

The fate of amines in the atmosphere adds another layer of complexity. Amines are thought to degrade into a range of other pollutants when in the atmosphere, and ADA has found these to be typically short-lived pollutants; these include principally formaldehyde and formamide which usually have EALs, but also smaller amounts of more complex compounds such as nitrosamines and nitramines, some of which are thought to have human health impacts.

All of this poses some regulatory issues as for any new technology. Close consultation is required between developers and regulators to both define and achieve specific limits for those pollutants which may lead to health effects.

The current CCS demonstration projects across the world will provide test cases for resolving several of these issues and knowledge sharing will be important in supporting the deployment of CO2 capture technology on a wider scale, with the optimal result of this leading to the formulation of industry wide guidance.

Summary

CO2 capture and storage technologies will likely have a role to play in allowing the use of fossil fuel technologies to continue in supporting power generation in the medium-term, contributing to low carbon emission targets and climate change policies.

While in some respects CO2 capture is simply another part of the increasingly complex suite of abatement plant that thermal power generation requires, it brings with it a set of issues that could lead to changes in air quality and releases of pollutants that must be further managed. These include the release of new types of pollutants, issues with dispersion for retrofit applications of CCS technology and some potential changes to the way sites may need to be regulated and monitored in the future.

With the number of CCS demonstration projects currently in development increasing, it will be crucial to share knowledge of these processes. Engagement with regulators will be essential in order to facilitate further deployment while avoiding potential air quality issues and to minimise any risk of adverse impacts from this potentially beneficial technology.

References and further reading

International Energy Agency Greenhouse Gas R&D Programme (IEAGHG), http://www.ieaghg.org/

Oke, T. R. (1987), Boundary Layer Climates, Methuen: London

HMIP (1993), Guidelines on Discharge Stack heights for Polluting emission – Technical Guidance Note D1 (Dispersion), HMSO

US EPA (1985), Guideline for Determination of Good Engineering Practice Stack Height (Technical Support Document For the Stack Height Regulations), US GPO

Department for Energy and Climate Change (UK) (2009), Guidance on Carbon Capture Readiness and Applications under Section 36 of the Electricity Act 1989, HMSO.

IEA GHG (2004). Improvement in Power Generation with Post-Combustion Capture of CO2. Report Number PH4/33; November 2004.

IEA GHG (2006). CO2 Capture as a factor in Power Station Investment Decisions. Report Number 2006/8; May 2006, p. 3-65.

Norwegian Institute for Air Research, CCS Programme: http://co2.nilu.no/

Atmospheric Degradation of Amines (ADA) Summary Report: Gas phase photo-oxidation of 2-aminoethanol (MEA) CLIMIT project no. 193438, http://ada.nilu.no/

Department for Energy and Climate Change (UK) (2010 Further Information Document on the UK Carbon Capture and Storage Demonstration Programme, HMSO

Author

Terry Ellis is an MSc qualified environmental scientist and a member of the Institute of Air Quality Management, specialised in air quality and carbon assessment. Terry works in Mott MacDonald’s Power Unit as part of a dedicated Environment team with experience across a range of sectors including power, transport, and development. This has involved projects around the world, including the Middle East, Singapore and Caribbean. Along with the air quality team, Terry also contributes to the University of Birmingham’s Air Pollution Management and Control course, delivering lectures and workshops on air quality and planning issues. Mott MacDonald is an employee-owned engineering, management and development consultancy serving the public and private sector around the world.

For more information on Mott MacDonald’s air quality services contact +44(0)1273 365 038 or visit www.environment.mottmac.com www.osedirectory.com/environmental.php

Published: 10th Mar 2011 in AWE International