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Removing CO2 using amines
Separating CO2 using amines is the most widespread method for post-combustion capture. Amine technology has already been used for decades to capture CO2 from both flue gas and natural gas. Several installations exist, of which many are operative. The first gas power plant using this technology was built in 1980 in Lubbock, Texas.
Contents
1. The process of removing CO2 using amines
2. Potential for improving the technology
The process of removing CO2 using amines
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Representation of the amine process. (Source: Vattenfall) |
In this process the flue gas is led through a large tower called absorber or scrubber. In this so called absorption process the gas comes into contact with the absorption fluid (amines mixed with water) fixing the CO2 in a weak chemical binding. The bound CO2 is transported to another tower, called a stripper, where the solvent is heated. This separates the CO2 from the amines in a process called regeneration. The amines are then reused to absorb more CO2.
Flue gas from gas power plants has a temperature of about 80 – 100 °C. To achieve the most favorable conditions for binding CO2 to the solvent, the flue gas should usually be cooled to about 40 degrees before absorption.
A 400 MW gas power plant produces a flue gas stream of approximately 2.5 Nm³/hour. To treat this amount of gas an absorption tower with a diameter of 20-30 meters and a height of 30-40 meters is required. This is the largest single unit in the separation facility.
The scrubber is filled with a packing to provide the largest possible surface for contact between the upward-rising gas and downward-running amine solution. On the top of the tower a washing tower removes any amine residue that has followed the flue gas stream, before the low CO2 (approximately 15 percent remains) flue gas is released into the atmosphere. In the bottom of the tower the CO2 rich amine solution is collected.
To compensate for loss in pressure in the absorption tower and to pump the amine solution and the cooling water, fans are applied that consume energy. However, the heating required to separate CO2 from the amine solution requires the largest amount of energy. This heat is provided by steam holding 120-140°C. Steam could be provided by the steam turbine in the power plant or by a separate boiler. Also, the CO2 must be compressed before transport; this alone requires the equivalent of 2 per cent of the generation efficiency of the power plant. If energy for the separation is provided from the power plant without added fuel, the power generation from a large gas power plant will be reduced by 7-11 percentage points, and it will yield an electrical generation efficiency of 47-51 per cent.
Choosing the right amine for separating CO2 depends on several conditions. The most used amine compound for flue gas CO2 removal at atmospheric pressure is MEA (monoethanolamine). To remove CO2 at higher pressures MDEA (methyldiethanolamine) is often used. Many suppliers of amine separation plants supply their own mixture of amines and additives, adapted to the intended use. Best known is KS-1 from Mitsubishi HI. Considerable improvements in energy efficiency have been achieved over the decades of commercial application of amine technology. More efficient system designs and heat integration in the process plants are two areas where further development is taking place.
Potential for improving the technology
Different propositions have been launched in order to make the processes of waste gas removal more energy efficient and economic.
The power process can be modified by recycling the waste gas back into the gas turbine. Recycled exhaust is then replacing fresh air and also making the combustion air and waste gas having less oxygen and increased CO2 concentration in the waste gas. Increased concentration of CO2 will then improve the conditions for absorption. Increased concentration will enable less waste gas volume, which increases the potential for cost reductions.
The great challenge is the demand for changes in the gas turbine due to the recycling of the waste gas. A limitation to the amount that can be recycled, is that the supply of oxygen is reduced, which might make it harder to have a stable flame in the combustion chamber. Recycled waste gas will also have a higher temperature than fresh air, increasing energy demand in the compressor.
Reports and studies
- BIT (Best Integrated Technology, C02 Capture Project) - 2005
See also
