Post-Combustion Carbon Capture Using Chemical Absorption - Coursework Example

Clean Process Technology Post combustion Carbon capture using chemical absorption Summary The continuing emission of CO2 gas into the atmosphere threatens to cause a climate change disaster. Carbon capture and sequestration provides the method to contain CO2 emissions until alternative clean energy solutions emerge. This paper examines the technology of post combustion carbon capture using amine absorption and compares it with other technologies that are being explored. 1. Introduction Power generation using fossil fuels such as coal, natural gas and petroleum is the largest single source of CO2 emissions into the atmosphere. Clean energy sources such as solar, wind and hydroelectricity can only meet a fraction of the fast growing energy needs of the poorer regions of the world. Nuclear energy provides a carbon free alternative to fossil fuel, but after the Fukushima disaster in 2011, the acceptance of nuclear plants has become very difficult. Most new generating plants will continue to be based on fossil fuels. A study by BP plc., titled “Energy Outlook 2035”, quoted by Mark Webster, shows that over the next 20 years CO2 emissions from the non-OECD region will grow as shown in Figure 1 (Webster). The chart shows that CO2 emissions from the OECD countries would level off at the levels prevailing in 2013 but the emissions from the non-OECD countries would rise rapidly. The red line marked IEA 450 is the target of 450 ppmv (parts per million volume) of CO2 concentration in the atmosphere that the International Energy Agency considers as the absolute upper limit to prevent a temperature rise exceeding 20C. This is considered the upper limit of temperature rise to prevent disastrous climate change (Webster). Figure 1 – CO2 emissions by region (Webster) The vast majority of these new power plants will be coal fired since coal is the most abundant fossil fuel. A coal plant has an operating life of about 50 years and the need is for a technology that can be retrofitted to existing fossil fuel power plants to capture CO2 emissions from the flue gases before they are emitted into the atmosphere. Post-combustion chemical absorption technology is the most promising of the carbon capture technologies presently available. Carbon dioxide emissions also occur from many other industrial processes other than power generation. These include steel mills, cement plants and oil refineries. Post-combustion chemical absorption technology can be readily applied to these industries as well. 2. The post-combustion chemical absorption process The schematic diagram of the post-combustion chemical absorption process is as shown in Figure 2 from a presentation by Dr. Stanley Santos of the International Energy Agency. The flue gases from the power plant are cooled and enter the bottom of an absorption column and come in contact with liquid amine flowing downwards which absorbs the CO2 in the gas stream. A water wash at the top of the absorption column removes the amines and the cleaned exhaust gases are released into the atmosphere. Figure 2 – Chemical absorption of carbon dioxide (Santos) The amine solution that has absorbed CO2 is pumped to the top of a stripper column which has steam flowing upwards. In a coal fired power or combined cycle power plant, the steam can be tapped from the boiler. In other applications, an auxiliary boiler would be needed. The heat from the steam separates CO2 from the amine stream. The CO2 gas is then dried, compressed and filled into containers for transportation to a storage location. The amine stream gets diluted by steam condensation which is reclaimed and pumped back into the absorber column. The chemical name for the amine solvent is Monoethanolamine (MEA) with the chemical formula HOC2H4NH2. MEA is manufactured by various chemical companies and is used in industries such as detergent manufacture, textile finishing and wood treatment. MEA is used as a 30% solution for CO2 removal. The temperature in the absorption column is maintained between 400-600 C and the process efficiency is over 95%. The stripper column is maintained at 1000 – 1200 C and the CO2 recovered is 99% pure (Santos). The major problems with the amine absorption process are: Solvent cost and life. The MEA solvent is quite expensive with up to 15% lost due to evaporation,. The water spray in the absorber column does help reduce evaporation loss. For the solvent action to be effective, the flue gases should contain very low SOx (< 10 ppm) and low NOx (< 20 ppm). Presence of SOx and NOx can cause degradation of the solvent and degradation products such as Ammonia are harmful emissions. Particulate matter such as fly ash need to be removed from the flue gas with an Electrostatic Precipitator or Bag Filter since these can cause foaming in the absorber and stripper columns degrading performance (Wang, et al, p7). The cost of Carbon Capure is estimated by the International Panel on Climate Change to be € 40 – € 60 per ton of CO2, which is targeted to be halved with research into better solvents and processes (Wang, et al, p7). Amine is corrosive, which makes it necessary to use stainless steel in place of carbon steel for the plant components (Santos). Regeneration of solvent is energy intensive. It is estimated that a substantial 8% - 17% of the energy generated by the power plant may be needed for the carbon capture process (Wang, et al) . 3. Other technologies for post combustion carbon capture While Chemical absorption with MEA is the most mature of technologies for the post combustion carbon capture, work is under way on various other technologies as indicated in the chart below (Wang, et al). Figure 3 – Various carbon capture technologies under development (Wang, et al, p2) i. In the chemical absorption process, work is under way to test alternatives to MEA with Potassium Carbonate and Ammonia based solvents that would cost less and require less energy for carbon stripping (Wang, et al, p5-6) . ii. Physical absorption occurs in certain solvents such as Selexol (dimethyl ethers of polyethylene glycol) and Rectisol (methanol) when the flue gas is pressurized. The energy needed for flue gas pressurization is the disadvantage (Wang, et al, p2). iii. Adsorption is the attachment of a gas or liquid to a solid surface and experiments are being conducted with alumina , activated carbon and zeolite beds. The adsorbed CO2 is released by the application of heat or reduction of pressure or washing. The low adsorption capacity of most materials is the major challenge in applying this technology to commercial scale CO2 capture processes (Wang, et al, p2). iv. Cryogenic separation by cooling the flue gases to a temperature of -56.60 C to condense CO2. The high cost of refrigeration is the major deterrent (Wang, et al, p3). v. Membrane separation is by the use of thin polymeric films which cause separation based on size of the molecules using the difference in partial pressure of the components on either side of the membrane (Wang, et al, p3). vi. Microbial and algal systems are still at an early concept stage. In the near term, therefore, there appears to be no alternative to the chemical absorption method for post-combustion CO2 capture with the possibility of the emergence of solvents that are lower in cost that MEA and that use less energy for regeneration. 4. Alternatives to post combustion carbon capture The presentation by Dr. Stanley Santos cited earlier talks of two alternatives to post-combustion carbon capture Oxy-coal combustion where the power plant boiler uses oxygen instead of air. A schematic diagram is shown below. The removal of nitrogen content from air reduces fuel consumption and the volume of flue gases generated. The higher temperature of the flame ensures more complete combustion of fuel. The flue gases also do not contain NOx which contaminates the amine solvent. Figure 4- Oxy-fuel combustion technology (Santos) This technology would need an expensive air separation unit to be installed to supply oxygen to the boiler and the redesign of the boiler and the burners for efficient combustion. This would rule out application of this technology to retrofit operating plants but may be beneficial for new power plants (Santos). Integrated Gasification Combined Cycle (IGCC) technology The block schematic diagram is shown below. In this process, coal and other carbon based fuels such as petroleum refinery waste products are converted into synthesis gas (syngas). CO2 , sulphur and other contaminants are removed from the syngas before it is fired in a a combustion turbine. The technology results in greater efficiency of power generation and the removal of major pollutants from coal before it is combusted. Figure 5 – Integrated Gas Combined Cycle technology (Santos) Pilot plants are operating US, Europe and Japan, but the capital costs are much higher than conventional coal based power plants. This technology also does not permit retrofits to existing power plants. In a briefing paper on Carbon Capture Technology, Dr. N.Fennel and Dr. P. Florin of the Imperial College suggest the following as the possible roadmap for technology development in carbon capture. Figure 6 – Technology roadmap for carbon capture technologies (Florin and Fennel) 5. Conclusion Post combustion carbon capture with amine absorption appears to be technology that is currently ready for deployment in power generating and other industrial plants to capture CO2 gas from flue gas emissions. The technology is high in capital cost and requires significant energy. Alternative technologies are even further behind in readiness for deployment. The high costs of the chemical absorption technology have to be thought of in the context of the astronomical costs that the world could face from climate change. References: Florin, N. and P.Fennel. “Carbon capture technology: future fossil fuel use and mitigating climate change”, Imperial College of London, November 2010. Web. 21 November 2014. Santos, Stanley. “An Overview of CO2 Capture Technologies – What are the Challenges Ahead?”, Workshop on Capture and Sequestration of CO2 (CCS), 10 July 2008, Mexico City. Web. 21 November 2014. . Wang, M., A. Lawal, P. Stephenson,J.Sidders,C.Ranshaw, and H. Yeung . “Post Combustion CO2 Capture with Chemical Absorption : A State-of-the-art Review” , Chemical Engineering Research and Design, Vol. 89, Issue 9, September 2011, Pages 1609-1624. Web. 21 November 2014. Webster, Robin. “No shortage of energy as emissions rise: BP”, The Carbon Brief, 15 Jan 2014. Read More