2.4 Overview of CCS technologies

This section provides a high level overview of the technologies in the CCS value chain. A more detailed discussion is provided in Report One.

The three general approaches to capturing CO2 generated from fossil fuels (coal, natural gas and oil) or biomass utilisation are post-combustion, pre-combustion, and oxyfuel combustion capture, as shown in Figure 2-3.

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Figure 2-3 A simplified schematic of CO2 capture approaches

This section provides a high level overview of the technologies in the CCS value chain. A more detailed discussion is provided in Report One.

The three general approaches to capturing CO2 generated from fossil fuels (coal, natural gas and oil) or biomass utilisation are post-combustion, pre-combustion, and oxyfuel combustion capture, as shown in Figure 2-3.

Figure 2-3 A simplified schematic of CO2 capture approaches

2.4.1 Post-combustion capture

Post-combustion capture (PCC) of CO2 involves the separation of CO2 from flue gas produced by conventional fossil fuel or biomass combustion in the presence of air. The PCC technology is commercially available to separate CO2 in many applications including the chemical processing industry and in the natural gas industry for CO2 separation to purify natural gas (before it is supplied to customers). However, as many of the existing applications are in a synthetic gas (syngas) or reducing gas environment, a key requirement is to adapt and scale up (particularly chemical solvent based processes) for applications in an oxidising environment where the flue gas volumetric throughputs are also significantly higher. A key benefit of PCC is its ability to be retrofitted to existing large stationary emitters of CO2 such as coal-fired power plants as well as ore smelters, steel production facilities and cement plants.

2.4.2 Pre-combustion capture

Pre-combustion capture involves reacting a fuel with oxygen and/or air and steam in a gasifier or reformer to yield a syngas, mainly consisting of carbon monoxide (CO) and hydrogen. Additional hydrogen and CO2 are produced by reacting the CO with steam in a shift reactor. The CO2 is then separated, typically utilising a physical or chemical solvent scrubbing process, which results in the production of a high purity CO2 stream. A key benefit from the application of the shift reaction and pre-combustion capture is that the resulting fuel is a stream of hydrogen which, when used for power generation, results in a by-product that predominantly consists of oxygen and water. Pre-combustion capture can be applied to integrated gasification combined cycle (IGCC) plants. Technologies for the shift reaction and capture process currently exist in the chemical processing industry, as well as at the scale required to implement the system in power generation plants.

2.4.3 Oxyfuel combustion capture

Oxyfuel combustion for CO2 capture uses near pure oxygen instead of air for combustion of the fuel, resulting in a flue gas that is mainly water vapour and CO2 (more than 60 percent by volume). The water vapour is then removed by cooling and condensation. In oxyfuel combustion, cooled flue gas is recycled back to the combustor to moderate the high flame temperature that results from the combustion in pure oxygen. This process eliminates nitrogen from the flue gas to produce a stream of CO2 for compression, transport and storage. Residual oxygen, nitrogen, noble gases and moisture in the CO2 stream may need further treatment to provide a suitable gas for transport and storage.

2.4.4 Considerations of capture approaches

All three CO2 capture approaches can be utilised for CCS with CO2 capture rates up to 90 percent. The following points highlight the key aspects when assessing an approach for use in CCS projects:

  • Post-combustion: The most appropriate technique to use for CO2 separation in fossil fuel based power generation plants, due to their reliance on fuel burning in air. However, the process requires large capacity equipment to accommodate the high volumetric flow of air used during combustion, which presents energy penalties for CO2 capture, capital expenditure (CAPEX) and land availability issues when retrofitted to existing plants.
  • Pre-combustion: This technique is predominantly used on a commercial scale in the chemical industry. The additional cost of capture appears to be lower than for PCC at the current state of technologies. However, the additional processing steps of air separation, shift conversion and gas clean-up add complexity and cost to pre-combustion operations. It is more likely to be applied to new build plants rather than retrofit.
  • Oxyfuel combustion: A relatively new concept of using a high concentration of oxygen in the combustion process to produce an almost pure stream of CO2 for direct compression and sequestration. Air separation to produce oxygen is energy intensive whilst at the same time the CO2 produced has to be further concentrated (cleaned up) before transport and storage. This may require additional cleaning units in the capture process to meet the specified quality specifications for CO2.

All CO2 capture approaches are technically proven and viable. However, none have been integrated with a coal-fired power plant at commercial scale. Their use depends on the type of application or process being fitted with CO2 capture. The cost of capture (both CAPEX and operation and maintenance expenditure (OPEX)) will be dependent on the type of capture and process application, land availability, environmental factors and prevailing regulations including those which impose an economic value on CO2 emissions. The reduction of capture and compression costs are critical if CCS is to be broadly deployed beyond initial demonstration units. One of the key approaches in reducing costs is through the gaining of experiences in the planning, construction and operation of commercial scale, integrated CCS projects. A fundamental tenet of experience curves is that the more frequently a task is performed, the lower the cost of doing it will be. Implicitly, the G8 goal of launching 20 commercial scale, integrated CCS projects by 2010, for broad deployment by 2020 is an attempt at generating multiple experiences from which operations can be optimised and costs reduced.

2.4.5 Transport of CO2

After CO2 is captured, it can be safely transported for storage by pipelines, land (road or rail) and/or by ship.

  • Transport by Pipeline: Pipeline transportation of CO2 has some industry experience, predominately in the oil and gas sector. This is the most economical method of high quantity CO2 transportation over long distances. Over 6,000 kilometres (km) of dedicated CO2 pipelines are in operation in the USA where such pipelines have operated safely for over 20 years. However, CO2 pipelines operate at much higher pressure than, for example, natural gas pipelines, and CO2 pipeline technology has not developed to the same extent as oil and gas pipelines.
  • Transport by Land: CO2 transport by either rail or truck tankers has been utilised for many years to meet existing industrial needs. This option is attractive for smaller quantities transported over shorter distances when pipeline infrastructure is not cost effective.
  • Transport by Shipping: CO2 transport by ships presents an option when the emission source is within a feasible distance to adequate seaport facilities capable of loading CO2 for injection in offshore fields. Significant interest has emerged from existing shipping businesses to transport CO2 for EOR and/or EGR in the North Sea. Transportation of liquefied natural gas (LNG) has occurred over decades and further research and design work is ongoing in Norway and Japan to adapt this knowledge to transport CO2 by ships.

Over 6,000km of pipelines have been safely transporting CO2 for over 20 years in the USA

Of the three transport options currently available, CO2 transport by pipeline is the most likely option to be used for the deployment of commercial scale CCS projects. There are current initiatives to consider the business case of collecting CO2 from multiple emitters and to transport the CO2 through large diameter trunklines. This is being considered in an attempt to gain economies of scale from transporting large quantities of CO2.

For CCS to be deployed commercially within the timeframe and scale required by the G8, government, industry and community stakeholders will need to work together in strong partnerships. Establishing widespread CO2 transportation infrastructure will require strategic long-term planning, taking into account the potential magnitude of future deployment scenarios for CCS. In order to achieve the CO2 emissions reductions targets using CCS, the scale of infrastructure required for transporting CO2 by pipeline could be comparable to that of the oil and gas infrastructure in existence around the world today.

Information provided by private pipeline developers through undertaking this study indicated that while the business case for capturing and transporting CO2 for EOR is viable, (particularly in the USA), many private developers are unwilling to finance the cost of dedicated CO2 pipelines for geological storage. The uncertainty on existing or proposed regimes for assigning an economic value for CO2 is not conducive to developers of infrastructure with 20-40 year timeframes. This stipulates that it is highly likely that governments will need to initially underwrite the funding of major CO2 pipeline infrastructure.

2.4.6 Storage of CO2

The storage of CO2 can be undertaken through a range of approaches. These include storage through plants such as algae and forests (terrestrial storage), soil organic matter (such as bio char) and geosequestration, which generally refers to the process of safely containing CO2 in the earth’s subsurface, thereby preventing it from being emitted into the atmosphere. The CCS concept refers only to geosequestration and is the focus of this study. As stated above, under the IEA’s BLUE Map scenario, the total CO2 emissions captured, transported and safely stored would need to reach 10.1 Gtpa CO2 by 2050.

A range of potential storage formations have been identified to safely store the CO2. These include geologic formations such as deep saline reservoirs, depleted oil or gas fields, EOR/EGR, enhanced coal bed methane (ECBM) extraction and basalt formations. The various methods of geological storage are illustrated in Figure 2-4.

Figure 2-4 Geological storage options for CO2Source: CO2CRC, 2009

Commercial projects such as Weyburn and Sleipner have been safely storing CO2 in deep saline reservoirs and depleting oil fields for more than a decade

To meet the G8 objectives, the most likely storage sites include deep saline reservoirs and depleted oil or gas fields, as these storage formations are relatively well understood. However, significantly more effort is required to identify and prove sites for the injection and safe storage of large volumes of CO2 for commercial operations. The techniques involved in identifying these storage sites are similar to those employed by the oil and gas sectors when prospecting for hydrocarbons.

Commercial projects such as Weyburn and Sleipner have been safely storing CO2 in deep saline reservoirs and depleting oil fields for more than a decade. However, exploration and appraisal of technically viable and economically feasible storage sites will continue to be a key limiting factor in the deployment of CCS going forward. Whilst CO2 storage through EOR or EGR offers an attractive pathway to commercialise CCS at this stage, it is highly dependent on locations favourable to this practice, reservoir characteristics and the value of oil/gas recovered. The storage of CO2 using EOR and/or EGR is not likely to facilitate the widespread deployment of CCS globally, compared to saline reservoirs.

The selection and characterisation of appropriate storage sites is a key limiting factor for the commercial deployment of CCS. Irrespective of the capture technologies, industries or transport methods, the ability to safely store large volumes of CO2 is the critical issue that underpins the entire CCS value chain.

Given the importance of storage in underpinning the CCS value chain the following section gives an overview of the various storage formations available for geosequestration.

Saline reservoirs

Saline reservoirs are deep underground rock formations containing brine. These are the most abundant and geographically diverse potential sinks for CO2 storage. The ability of this formation to trap and dissolve CO2 within the brine makes it potentially well suited to serve as long-term storage sites.

Depleted oil and gas reservoirs

Depleting and disused oil and gas reservoirs are generally the most well understood storage option for CO2. They have a proven history of containing oil, gas and naturally occurring CO2 for millions of years, and as a result, there is a high degree of confidence that these formations will be able to contain anthropogenic CO2 over time. Furthermore, they represent attractive development opportunities as they have already undergone extensive site analysis during oil and gas production. This information can be directly leveraged to evaluate, model and characterise the formation’s suitability for long-term storage, minimising upfront development costs.

Enhanced oil and gas recovery

There are many EOR activities, primarily in the USA, that currently store large volumes of CO2. For EOR or EGR, CO2 is injected into oil or gas reservoirs to enhance its recovery. Commercial scale, integrated CCS projects for EOR include Rangely in Colorado USA, Weyburn in Saskatchewan Canada and Salt Creek in Wyoming USA.

Enhanced coal bed methane

Storing CO2 in uneconomic coal seams has attracted interest over the last two decades. Coal beds that are either too deep or too thin to be mined could be flooded with CO2 to release coal bed methane which is a valuable source of natural gas.

Basaltic rock formations

Basaltic rock formations have gained increased academic attention as potentially the most stable and robust CO2 storage opportunity in a number of regions. However, to date, there has only been limited laboratory experimentation with storing CO2 in this formation.

Other storage options

Other ideas that have been explored include dissolution in the ocean with iron fertilisation, mineralisation into calcium bearing and magnesium oxide bearing rock formations near the surface, and securing CO2 in liquid pools or clatharites (a form of hydrated CO2 ice) at the sea floor. However, these remain largely conceptual and will not be ready in the timeframes required to meet the G8 objectives.

2.4.7 Conclusions

Ultimately, there will be a need to shift to a new energy economy that involves the use of low or zero emission technologies such as CCS. The development of such technologies will take time and existing reserves of fossil fuels will continue to play an important role in creating the energy needed to drive societies. As such, society will face the prospect of having to deal with continually rising CO2 emission levels from the use of fossil fuels worldwide. Given this, the application of CCS must be accelerated.

Society will face the prospect of having to deal with continually rising CO2 emission levels from the combustion of fossil fuels worldwide