Coal fired power plants provide roughly half of the power generated in the United States each year. Since these plants will inevitably keep operating, amine scrubbing to capture CO2 from the plants could be the most effective way to reduce emissions. A technique patented in the 1930’s and currently the most common method for removing sulfur from flue gas at power plants, amine scrubbing has seen technological improvements in the last decade amounting to a cost reduction from $77 to $52/ton of CO2 removed (Rochelle, 2009). Strong legislation in the form of a cap-and-trade system, or a carbon tax could provide the necessary incentives for the first major amine scrubbing project. — Jake Bauch
Rochelle, Gary T., 2009. Amine Scrubbing for CO2 Capture. Science. 325, 1652-1654
Gary Rochelle of the Department of Chemical Engineering at University of Texas at Austin looked at ways that current process and solvent improvements could reduce costs of amine scrubbing. His basis for the economics of amine scrubbing was a 2007 study by the U.S. Department of Energy that showed cost reductions from 2001 to 2006. The process of amine scrubbing is as follows: CO2 is absorbed at a high temperature into an amine solution, water vapor is used to strip the amine from the CO2, the water in the vapor is condensed leaving pure CO2, and then the CO2 is compressed to between 100 and 150 bar for transportation and storage. Costs can be divided into power used (half is steam to heat the vapor and half to compress the CO2), capital costs, and operating and maintenance cost. Improvements in the process such as stripping at multiple pressures could reduce power used but would increase capital costs. Solvents with higher rates of absorption, or greater capacity can reduce power costs.
The theoretical minimum amount of energy used in amine scrubbing is 12% of the power plant capacity. With the improvements in solvents and process, the amount used could get down to 20%. Amine scrubbing technology can be added on to existing plants, and can be shut off during times of peak electricity demand. If the energy demanded is replaced with gas fired power, there will be a projected 74% decrease in CO2 emissions.
Mineral Carbon Dioxide sequestration is a chemical method of sequestering CO2 that produces an environmentally harmless substance with minimal efforts required to monitor or verify the results (Krevor, 2009). The method, which has only been around as a CO2 sequestration technique since the 1990’s consists of adding magnesium silicate or calcium silicate with supercritical carbon dioxide to form a carbonate. Unfortunately, high amounts of energy are required in the process compared to other methods of carbon sequestration. 75% of the energy is from the process of grinding the particles down to a small enough size. Though the addition of sodium citrate, sodium oxalate and sodium EDTA increase dissolution, the costs and energy use preclude the commercial success of this form of carbon sequestration. — Jake Bauch
Krevor, Samuel C., Lackner, Klaus S., 2009. Enhancing Process Kinetics for Mineral Carbon Sequestration. Energy Procedia 1, 4867-4871
Krevor and researchers at Columbia University tested the effect of adding different inorganic salts and sodium salts to the carbonate forming reaction. In all cases the CO2 was supercritical, at 120°C and 20 bars of pressure. Of the salts added, sodium citrate, sodium oxalate and sodium EDTA showed highest initial dissolution, and within 10 to 20 hours, dissolution reached almost 100%. Measurements ended after 24 hours, so the long term dissolution of the other salts was not measured.
The chemical process involves two steps: dissolution and then precipitation. The dissolution is more efficient in more acidic solutions, but precipitation is impossible in an acidic solution. A complicated process could change the pH balance during the process, but has not been developed. Consequently, researchers typically add sodium bicarbonate to the reaction to create a neutral solution so that both dissolution and precipitation can happen simultaneously. Since neutral solutions yield low amounts of carbonate, the dissolution is the limiting part of the reaction. Hence, the particles are ground into finer particles to increase the dissolution.
Carbon capture and storage (CCS) could be responsible for reducing global carbon emissions by up to 20%. To date there are no existing CCS power plants but experiments exist in the form of 1/10th-scale plants with 100% of emissions captured, and full size plants with 0.001% of emissions captured (Haszeldine, 2009). Since commercial CCS plants will not be built until several example plants are built, immediate funding of projects may be necessary if commercial plants are exected to be up and running by 2020.— Jake Bauch
Haszeldine, R. Stuart, 2009. Carbon Capture and Storage: How Green Can Black Be? Science 325, 1647–1652
Stuart Haszeldine reviews the existing literature on CCS to find the issues to be resolved before construction can take place. There are unresolved issues with the capture, transport and storage of carbon. The three capture techniques, postcombustion, precombustion and oxyfuel combustion, are all comparable in terms of cost and efficiency. Barriers to entry for CCS are lack of legal standing in the form of performance standards and lack of economic incentive in the form of carbon being priced. Several other factors are delaying construction even though the technology exists. Technological improvements are expected to increase efficiency by 20 to 60% and pipe sharing by multiple plants could reduce costs. New plants can be designed to easily convert to CCS when it is available. When it leaves the plants, captured carbon can be sent through pipes from power plants to the storage sites in aquifers, oil fields or gas fields.—Jake Bauch