Introduction
Coal is a very cheap and abundant provision of energ. In fact, it is the most abundant fuel in the United States; U.S. has the highest coal reserve in the world, with up to 25% of the known world coal present in America and being mined in 26 out of the U.S. 50 states (fe.doe.gov, 2013). However, this abundant energy is seldom used because it is a very dirty energy. According to Hong & Slatick (1994), up to 204.3 pounds of carbon dioxide per million Btu is emitted when coal having 78% carbon content is completely burned to attain a 14,000 Btu per pound. Moreover, the ills of global warming and climatic effects in the world today associated with carbon dioxide emission cannot be overemphasized. Global policy makers are largely considering how to achieve zero carbon emission level in the near future, but this cannot be achieved in coal-fired power plants if adequate precautions are not taken to minimize or drastically reduce carbon dioxide emission from coal. This is where carbon capture technologies come to play. As the name implies, carbon capture technologies are technologies that can be used in capturing carbon dioxide and therefore sequestrating it in order to prevent its release into the atmosphere. These technologies including post-combustion, pre-combustion and oxy-fuel technology are primary in achieving zero carbon emission level. Although there are a few challenge associated with these technologies, these challenges would be largely addressed by future advances in the technology. This paper takes a look at the various carbon capture technologies used in coal-fired power plants, their problems, and challenges as well as the possible solutions in the future.
Post-Combustion Technology
The post-combustion process of capturing carbon dioxide involves the removal of the carbon dioxide after burning the fossil fuel by primarily extracting or scrubbing it from the flue gas. This technology is already being used in some industrial applications, and it is generally safe. After extraction, the carbon dioxide might be transported by piping it to a suitable storage (British Geological Survey, 2016). The removal of carbon dioxide from the flue gas in this technique is usually accomplished with regenerative solvents, monoethanolamine (abbreviated as MEA) is mostly used. Carbon dioxide is removed during the scrubbing process by passing the flue gas through the solvent in the absorber chamber. The temperature often maintained at this stage having the value between 40oC and 60oC. The solvent binds carbon dioxide present in the flue gas and can be removed (Davidson, 2007). Superheated steam at 120oC is then passed through the saturated solution in order to release the trapped carbon dioxide, which can be transported and stored (Jha, 2008). MEA is often used as the solvent in this process because of its low cost, high reactivity and absorbing capacity on the mass basis, thermal degradation rate and good thermal stability but its corrosive effects greatly limit its usage. However, high energy requirements of the scrubbing process make the cost high but retrofitting the plant with Advanced Supercritical Boiler/Turbine (termed as ASC BT) can boost the efficiency significantly. Improving the process would not just require retrofitting with ASC BT technology but also using alternative and better solvent, especially those with small heat duty for the carbon dioxide scrubbing process, than MEA. Such solvents would help to achieve a higher capacity of carbon dioxide capture, lower generation of energy, reduced corrosivity and degradation, improved stability and lower volatility and so forth (Davidson, 2007). Moreover, the penalty of carbon capture in this process can be reduced by enhancing the internal heat recovery of the plant (Stover et al. 2011). The post-combustion technique comes very handy when there is a need to trap carbon dioxide from the flue gas using chemicals. However, if greater efficiency in carbon dioxide trapping is required, then it may be necessary to consider alternative technologies such as the pre-combustion method.
Pre-Combustion Technology
Unlike post-combustion process, the pre-combustion process involves the removal of carbon dioxide from the fuel before the completion of the combustion process. Coal, natural gas, and some other fuels could be used as the feedstock. The feedstock is first oxidized partially in steam and air or oxygen. The process is maintained at high pressure and temperature, and it produces synthesis gas. The gas so formed comprises CO, CO2, H2, methane and so forth. The gasification process involves a partial oxidation process that would convert hydrocarbons present in the mixture of carbon monoxide and hydrogen. The chemical equation is shown below.
(CH)n+ O2→ H2+ CO
CxHy+H2O+ O2 → aH2+ xCO
Usually, the carbon monoxide and water present in the synthesis gas may be converted to carbon dioxide and hydrogen by carrying out a water-gas shift reaction process on the synthesis gas. This makes the mixture rich in carbon dioxide, ranging from 15 to 50% (Edgar, 2014; energy.gov, n.d; Lupion, 2010). The carbon dioxide produced is then captured and the hydrogen generated can also be separated and used in producing electricity (Jha, 2008). However, much of the installed base of fossil fuel power is comprised of pulverized coal power plants, which is an older technology to the pre-combustion system. The pre-combustion system falls short here because it cannot be modified to suite such older technology. Moreover, the pre-combustion process does not have a clear cut economic edge over the post-combustion process.
The pre-combustion system has some levels of advantages over post-combustion system (energy.gov, n.d). The post combustion system operates at low pressure and removes dilute carbon dioxide but the pre-combustion operates at higher pressure and is rich in carbon dioxide. Thus, it is easier to remove greater volumes of carbon dioxide using this technique than the former. Although it is an efficient process of removing carbon dioxide from flue gas, the pre-combustion process involves more capital cost than the former process. For instance, the cost of capturing carbon dioxide generated through an IGCC power plant in the pre-combustion process would be about $60 per ton. The high cost of the pre-combustion process could be reduced by an advance in technology. The integration of pre-combustion capture technologies into an IGCC facility is a major step in this regard, and further advance here could be able to successfully reduce the resulting cost to about $40 per ton (energy.gov, n.d).
As against post-combustion process, the pre-combustion process is mostly applicable when there is need to remove a greater proportion of carbon dioxide from the flue gas. Additionally, if there is a need to produce both carbon dioxide and hydrogen from the process, pre-combustion is the precise process to leverage. The carbon dioxide separation technology is proven to be effective, and it is already used in several applications (Lupion, 2010).
Oxy-fuel Technology
The waste gas generated when coal is burned in the normal (atmospheric) air usually comprises between 3 and 15% of carbon dioxide and a large amount of flue gas that contains a high percentage of nitrogen. It would take more energy and expensive process to further remove the carbon dioxide from the mixture; thus, it is often better to resort to a different carbon capturing technique in order to ensure increased efficiency (Jha, 2008; The Linde Group, 2017). This would involve burning coal in pure oxygen rather than atmospheric or normal air. The oxy-fuel process has a great edge over other systems because it increases the partial pressure of carbon dioxide in the exhaust gas to a significant level since it prevents the exhaust gas from being diluted with atmospheric nitrogen (Kanniche et al. 2010). Pure oxygen can be obtained by passing atmospheric air through an air separation plant at very low temperatures. In this condition, the atmospheric air would be separated into constituent parts and nitrogen would be removed during this early stage. The oxygen obtained is then fed into the furnace to burn coal. The flue gas obtained in this process is reintroduced into the furnace in order to regulate the flame temperature. Subsequently, this reduces the volume of the flue gas produced and improved combustion efficiency. Another important benefit of the oxy-fuel process is that the carbon dioxide produced is comparatively pure and by cooling the steam, the carbon dioxide can be removed (The Linde Group, 2017).
The major challenge in using this technique is the difficulty in meeting the energy demand. The output of power stations that make use of the oxy-fuel technology usually fluctuates with the demand in energy. Conventional power plants as discussed above do not have this limitation because they are fed with atmospheric air. The difficulty in oxy-fuel system arises from the fact that the stream of oxygen has to be customized in such a way as to match with the power cycle so as to prevent wasting the oxygen generated. Efficiency in this regard is necessary to create room for rapid changes in load. Energy consumption is also an issue especially during the cryogenic air separation stage of the oxy-fuel system. Improvement in the system may efficiently reduce the energy consumption of the system by 25 percent when compared with the conventional method.
Brief Comparison of the Processes
Post-combustion process has the highest efficiency of all the processes, with an efficiency of 50% in natural gas combined cycle. Oxy-fuel has an efficiency of 35% in pulverized coal while pre-combustion with IGCC-Puertollano has an efficiency of 33.5%. Moreover, each of the processes has their limitations, both technical and economic. Technical issues with the oxy-fuel process include purification of carbon dioxide, parasite air entries, electrical consumption of auxiliaries and so forth. Post-combustion technical issues include removal of degraded absorbent, degradation of absorbent and so forth while the technical issues in the pre-combustion process include carbon deposit on the reformer and so forth. Economic limitation is an issue to consider but varies considerably with the systems. The cost of metals, fuel price, and tension on the equipment market are some of the economic factors to consider (Kanniche et al. 2010). The oxy-fuel and pre-combustion processes are pricier than the post-combustion process primarily because of the processes involved such as the cryogenic process in the former and gasification in the latter.
Conclusion
Carbon capturing is necessary in achieving the zero carbon dioxide emission goal in the near future. Coal-fired power plants make use of technologies such as pre-combustion, post-combustion and oxy-fuel process. Each of the techniques has their advantages and limitations as well as challenges, however, the oxy-fuel technique has a great edge of increasing the partial pressure of the exhaust gas and the pre-combustion technique can also be used in producing hydrogen. Adequate use of the various processes can yield untold benefits and help in sequestrating a large amount of carbon dioxide and with further advances in the technologies; their limitations would be largely rectified.
References
British Geological Survey, B. 2016. Post-combustion coal fired power plant. [Online] Bgs.ac.uk. Available at: http://www.bgs.ac.uk/discoveringGeology/climateChange/CCS/PostCombustionCoalFiredPowerPlant.html [Accessed 13 Jan. 2017].
Davidson, R. 2007. Post-combustion carbon capture from coal fired plants – solvent scrubbing. [Online] Iea-coal.org.uk. Available at: http://www.iea-coal.org.uk/documents/81793/6448/Carbon-capture [Accessed 13 Jan. 2017].
Edgar, T. 2014. Pre-combustion technology for coal-fired power plants. Available at: http://www.ieaghg.org/docs/General_Docs/Summer_School_2014/1_Pre_combustion_TF_Edgar_v2SEC.pdf [Accessed 13 Jan. 2017].
Energy.gov. n.d.. Pre-Combustion Carbon Capture Research. [Online] Available at: https://energy.gov/fe/science-innovation/carbon-capture-and-storage-research/carbon-capture-rd/pre-combustion-carbon [Accessed 13 Jan. 2017].
Fe.doe.gov. 2013. Coal Our Most Abundant Fuel. [Online] Available at: http://www.fe.doe.gov/education/energylessons/coal/gen_coal.html [Accessed 13 Jan. 2017].
Hong, B. & Slatick, E. 1994. Carbon Dioxide Emission Factors for Coal. Quarterly Coal Report, DOE/EIA-0121(94/Q1). [Online] pp.1-8. Available at: https://www.eia.gov/coal/production/quarterly/co2_article/co2.html [Accessed 13 Jan. 2017].
Jha, A. 2008. Explainer: How carbon is captured and stored. [Online] the Guardian. Available at: https://www.theguardian.com/environment/2008/sep/05/carboncapturestorage.carbonemissions1 [Accessed 13 Jan. 2017].
Kanniche, M., Gros-Bonnivard, R., Jaud, P., Valle-Marcos, J., Amann, J. & Bouallou, C. 2009. Pre-combustion, post-combustion and oxy-combustion in thermal power plant for CO2 capture. Applied Thermal Engineering, 30(1), pp.53-62.
Lupion, M. 2010. Precombustion technology for Coal Fired Power Plant. Available at: http://www.ieaghg.org/docs/General_Docs/Summer_School/LUPION_-_Pre-combustion_secured.pdf [Accessed 13 Jan. 2017].
Stöver, B., Bergins, C. & Klebes, J. 2011. Optimized post combustion carbon capturing on coal fired power plants. Energy Procedia, 4, pp.1637-1643.
The Linde Group, 2017. CO2 Separation with pure oxygen. [Online] The Linde Group. Available at: http://www.the-linde-group.com/en/clean_technology/clean_technology_portfolio/carbon_capture_storage/oxyfuel_technology/index.html [Accessed 13 Jan. 2017].
