Introduction
Carbon Capture and Sequestration (CCS) is a group of technologies that received recognition in 21 century. CCS process consists of collection and concentration of CO2 emitted from combustion of fossil fuels with further transportation to special storage locations. The reason for storing carbon dioxide away is related Greenhouse effect and its influence on global warming process. The main goal of carbon capture technologies is to stabilize the levels of greenhouse gas in the atmosphere. It is expected that implementation of CCS technologies will make it possible to reduce the total emission of CO2 up to 54% (to 450 ppmv). (Soundararajan and Gundersen 2013, p.187)
Carbon capture technologies
The majority of coal power plants today are based on conventional PC (pulverized coal) technology. Today, there are three main CCS technologies that may be used in coal power plant projects: post combustion, pre-combustion and oxy-combustion capture.
Figure 1 CO2 capture technologies in coal power systems (Douglas et al. 2007)
The choice of technology used in power plant may depend on various factors, such as: location, elevation, climate conditions and even type of coal.
Pre-combustion technology
This technology is based on use of ‘syngas’ (synthesis gas) that consists mainly of carbon monoxide and hydrogen. ‘Syngas’ is a product of reaction between a fuel and oxygen and water steam. After that, CO is conversed to CO2 in a shift converter (a catalytic reactor). (Kanniche et al. 2010, p.56) This reaction is also called the WGS reaction: CO + H2O→CO2 + H2.
The WGS reaction is a catalytic process. Other important parameters that should be considered are: pressure (up to 30 MPa) and molar ratio of H2O/CO. (Davidson 2011)Then carbon dioxide is separated by absorption resulting in fuel rich with hydrogen. Such fuel can be used in various applications: furnaces, engines, boilers, fuel cells and gas turbines. The partial pressure of CO2 is higher in the fuel gas than in the flue gas. Because of that, pre-combustion technology is great choice to be used in integrated gasification combined cycle (IGCC) plants.
Figure 2 Pre-combustion capture on PC (CCSP 2016)
This technology can be applied to already existing IGCC power plants. Pre-combustion technology is already used in other industrial applications, such as natural gas processing, coal gasification plants and natural gas reforming.
Post-combustion technology
In post-combustion capture (PCC) technology, the exhaust gases are treated by methods of chemical absorption in order to separate CO2. A typical post-combustion process looks like: flue gas passes through an absorber (packed-tower) where carbon dioxide is selectively absorbed by special solvents. After that the solvent that contains CO2 is moved to a stripper (regenerating column) where CO2 is desorbed from the solvent under heat. The solvent is later moved back to the adsorbed, while an almost pure stream of CO2 is compressed. (IEA 2013, p.40)
Figure 3 Post-combustion capture on PC (CCSP 2016)
The most commonly used solvents in PCC are amines (methylethanolamine, diethanolamine, methyldiethanolamine). Large amount of heat is required for regeneration process that is usually supplied from steam. (IEA 2013, p.95) Other technologies, such as membranes, adsorption and cryogenic are not suitable for PCC. (Kanniche et al. 2010, p.54) Carbon capture process runs at almost atmospheric pressure and as such it can be used in both new and existing coal power plants and natural gas power plants. More than that, is it possible to use PCC technology in some other industrial applications with CO2 elimination, such as oil refining and cement and petrochemicals production. (IEA 2013, p.45)
Oxy-combustion technology
The primary reason for oxy-combustion technology development is the most cost-effective and relatively simple method of CO2 separation. The content of CO2 in the resultant flue gas for PC boilers is not higher than 15% that is related to its diluted by nitrogen in the air. This technology removes nitrogen from the air prior to combustion. As result, the flue gas consists of mostly water and CO2 that can be easily removed using various dehydration processes. Then the temperature of the flue gas is reduced in order to eliminate any impurities (nitrogen, oxygen and argon). (IEA 2013, p.40)
Figure 4 Oxy-combustion capture on PC (CCSP 2016)
Coal power plants that use oxy-combustion technology are almost similar in their designs to conventional PC systems. However, there are some major components unique to oxy-combustion systems: (IEA 2013, p.35)
Air Separation Unit (ASU) is a cryogenic equipment that is used to remove nitrogen from air to provide high-quality oxygen. It should be noted that ASU has to be running for at least 48 hours before starting combustion process. It is especially important to have oxygen storage sites in order to reduce costs of generated energy. (IEA 2013, p.95)
Oxy Boiler is a system where coal combustion process with pure oxygen is done. It is almost identical to conventional air-fired boiler, however the main difference is the oxidant.
Gas Quality Control System provides environmental control. Such system is much cheaper than one used in PC systems.
CO2 Purification Unit is responsible for drying and compressing of the flue gas.
The development of oxy-combustion resulted in new application of such technologies other than power generation: hydrogen and steel production. (Perrin et al. 2013, p.1404)
Advantages
Advantages of pre-combustion capture technologies
Acid gas removal processes used for removal of carbon dioxide and the WGS reaction are commercially practiced globally.
Plot space, water use and energy penalty for capturing of the 90% CO2 under pressure (approximately 20%) are less than today’s PCC technologies.
R&D potentially can lead to a step-change reduction of power loss by developing membrane separation technology, higher temperature gas clean-up, more effective catalysts for CO shift. (IEA 2013, p.43)
Advantages of post-combustion capture technologies
It is possible to modify with PCC already existing power plants that already have well-established infrastructures.
PCC enables the continued deployment of conventional PC technology, well-established and familiar around the world.
Various PCC projects have been successfully launched in recent years. (IEA 2013, p.41)
Advantages of oxy-combustion capture technologies
Deployment of conventional and high-efficient steam cycles that requires no substantial amounts of steam to be removed for carbon capture.
Additional equipment needed for oxy-combustion process is conventional and familiar to operators and owners of every power plant: heat exchangers and rotating equipment.
With little or no additional cost at all it may be possible to achieve ultra-low emissions of conventional emissions.
Possibility to achieve an incrementally lower cost for capturing of more than 98% of CO2 in comparison to capturing 90% of CO2 by other CCS technologies.
If purity requirements for CO2 transportation, storage and compression are to be relaxed, the costs of oxy-combustion may be further reduced. (IEA 2013, p.44)
Challenges
Major challenges faced by all carbon capture projects
Figure 5 below shows various CCS projects that were initially planned to construct.
Figure 5 Planned CCS projects (Haszeldine 2009)
Some of this projects were successfully launched, however some of listed projects had to be cancelled for various reason. Major challenges that all CSS projects have to deal with include:
Environmental advocacy opposition to coal. Various environmental organisations oppose CCS projects in their fight against the continued use of coal.
Public opposition to storage. Public opposition in continental Europe notably “NUMBY” movement (not under my backyard) played a part in the cancellation of some European CSS projects (the Vattenfall Jänschwalder and RWE IGCC).
High cost of storage/lack of storage sites. Many countries don’t have simple access to storage sites, because it has to be constructed either undersea or underground. Another related obstacle is many regulatory and technical barriers.
Higher cost estimate for CCS. Majority of CSS projects cost much higher than it was expected by researchers when projects were first initiated.
High power loss from CO2 capture. Using steam and power from the power plant for regeneration of solvent and compression of carbon dioxide results in approximately 30% loss of generated electric power. (IEA 2013, pp.62-67)
Pre-combustion capture challenges
Significant energy loss (around 20%).
Additional purification may be required to vent the CO2.
Capital costs must be reduced in order for pre-combustion technologies to compete more effectively. (IEA 2013, p.43)
Post-combustion capture challenges
Majority of solvent processes are available on market only at relatively small scale.
The process of separation of CO2 from flue gases still has various issues, such as degradation of solvents and requirement for the high regeneration energy.
Carbon capturing with today’s solvent technologies results in 30% loss of net power output and 11% loss of efficiency.
Additional flue gas clean-up is needed for most solvents (in most cases either SO2 or NO2 depending on solvent).
Alternate means of providing steam may be needed.
Significant increase of water needed for post-combustion process. For water-cooled plants the water consumption is almost doubled if carbon capture process is added.
Significant plot space requirements. (IEA 2013, p.41)
Oxy-combustion capture challenges
Development of oxy-combustion requires an integrated plant. It is impossible to create smaller-scale oxy-combustion at already existing power plants.
Auxiliary power in oxy-combustion plant will reduce the net plant output by about 15% in comparison to an air-fired power plant that has same capacity.
Specific flue gas quality controls are required to achieve the very low emissions during air-fired start-up phase.
Significant plot space requirements for both the ASU and auxiliary power unit.
Relatively high water consumption, although less than for PCC. (IEA 2013, pp.44-45)
Future of technologies
Pre-combustion technologies
Improvements in pre-combustion technologies are focused on ‘syngas’ processing techniques that can achieve lower energy penalty. Developing technologies include:
Hydrogen Transport Membranes (HTM). The syngas is not cooled prior to separation and the CO2 product may be produced at relatively high pressure. Thus the HTM technology may avoid thermodynamic penalties that are present in conventional technologies.
Warm-Gas Clean-up (WGCU). Separation systems operate at 260-370°C (that is close to temperatures of the WGS reaction). This technology is able to remove heavy metals and sulphur at high temperatures thus removing the requirement of cooling the syngas. As such, expensive heat recovery systems are not needed.
Improving WGS. The objective is to reduce steam requirements. (IEA 2013, p.43)
Post-combustion technologies
During the past 10 years, researches around the world have been focused on finding most suitable PCC solvents. That is why the today’s PCC technologies are nowhere near the theoretical optimum. In order to capture and compress 90% of carbon dioxide, PCC solvents available on market require about 4 times the theoretical minimum energy. Main objective of scientists is to reduce the energy requirement (to at least 3 times the theoretical minimum) and key areas of researches on post-combustion carbon capture technologies are:
Absorption. Developing more effective solvent processes.
Adsorption. Developing new designs of fluidized beds and new solid sorbent materials (based on metal organic or carbon frameworks).
Membranes. Developing polymer membranes that have both high permeability and selectivity when removing carbon dioxide from flue gas. (IEA 2013, p.42)
Oxy-combustion technologies
There is less R&D going for oxy-combustion in comparison to post-combustion and pre-combustion capture technologies. Two major promising oxy-combustion technologies are:
Chemical looping combustion (CLC) uses a reversible chemical reaction for oxygen separation from air. Suitable solids are used in an “air reactor” and then are transferred to a “fuel reactor”. The solid‐oxygen reaction is reversed in ‘fuel reactor’. The global research community is trying to identify the most suitable solids (metal oxides and calcium sulphide/calcium sulphate are currently leading candidates). (IEA 2013, p.49)
Pressurised oxy‐combustion includes conducting oxy-combustion under ~10-15 bar gas pressure. Recovering of the heat of flue gas moisture condensation can reduce latent heat losses in the flue gas thus improving net efficiency and reducing plant costs. (IEA 2013, p.51)
Conclusion
There are several vital technical problems that need to be solved in order for CCS technologies for coal power plants to become widely used worldwide. The major problems are related to: degradation of amine and hydrogen turbine for pre-combustion technologies, separation of incondensable gases and oxy-coal combustion boilers for oxy-combustion technologies.
Today, it is hard to determine the best technology out of three. However, the analysis of production cost of generated electricity shows that pre-combustion capture is the most suitable for IGCC, post-combustion technology should be used for natural gas power station and oxy combustion – for PC power plants with CO2 capture.
References
CCSP (2016) Carbon Capture and Storage Program [online], available: http://ccspfinalreport.fi/ [accessed.
Davidson, R. (2011) 'Pre-combustion capture of CO2 in IGCC plants', IEA Clean Coal Centre, London, UK, 98.
Douglas, J., Rhudy, R., Wheeldon, J., Parkes, J., Phillips, J., Holt, N., Dillon, D. and Viswanathan, R. (2007) 'The challenge of carbon capture', EPRI Journal, Spring, 14-21.
Haszeldine, R. S. (2009) 'Carbon capture and storage: how green can black be?', Science, 325(5948), 1647-1652.
IEA (2013) 21st Century Coal: Advanced Technologies and Global Energy Solution.
Kanniche, M., Gros-Bonnivard, R., Jaud, P., Valle-Marcos, J., Amann, J.-M. and Bouallou, C. (2010) 'Pre-combustion, post-combustion and oxy-combustion in thermal power plant for CO 2 capture', Applied thermal engineering, 30(1), 53-62.
Perrin, N., Dubettier, R., Lockwood, F., Court, P., Tranier, J.-P., Bourhy-Weber, C. and Devaux, M. (2013) 'Oxycombustion for carbon capture on coal power plants and industrial processes: advantages, innovative solutions and key projects', Energy Procedia, 37, 1389-1404.
Soundararajan, R. and Gundersen, T. (2013) 'Coal based power plants using oxy-combustion for CO 2 capture: Pressurized coal combustion to reduce capture penalty', Applied thermal engineering, 61(1), 115-122.
