Post Combustion Co2 Capture

Published Date: 02 Nov 2017

Introduction

The Nobel Prize winning Intergovernmental Panel on Climate Change (IPCC) said that if we want to avoid the worst effects of the climate change for us and for our future generation, then the global emission of CO2 from the fossil fuel fired power stations should be reduced from 50-80% within 2050 (project, 2008). CCS (Carbon capture and storage) will play the vital role to ensure our future generation a better place to live. All the major industries which are producing electricity using the fossil fuels are now trying to build CCS plants integrated with the power plants and engineers and scientist around the world are trying to develop CO2 capture, transport and storage technologies to find feasible ways to low emission carbon technologies. Currently, there are 3 (three) major CO2 capture technologies that are well known and are used in the industry scale. These are: post combustion, pre combustion and oxy-fuel combustion.

Below is the brief description of these CO2 capture technologies. (Scottish Carbon Capture and Storage, 2012)

Post Combustion CO2 Capture:

Post-Combustion Carbon Capture - Courtesy of www.co2crc.com.au

Figure: Post combustion CO2 capture process diagram

Source: Scottish Carbon Capture and Storage website (http://www.sccs.org.uk/public/capture/postcombustion.html)

Post combustion CO2 capture technology is a technology where CO2 is removing from the flue gases after the hydrocarbon combustion. Using this method, CO2 is passing through a solvent and the sovent is adsorbing the CO2. Usually amines are used as solvents. Temperature and/or pressure change then releasing the CO2. Another process to capture using post combustion which is still under development is calcium cycle capture of CO2, where quicklime is used to capture CO2 and produce limestone through this process and later to drive off the CO2, limestone is heated off. Though, researchers are looking for new types of solvents which will be much more efficient than the existing ones to reduce the energy penalties to 10%. One promising technology is under research which includes removal of CO2 with an adsorbent solid or by passing it through a membrane.

Advantages:

The most important advantage of post combustion CO2 capture is it is feasible to retrofit in current industrial power stations and with amine solvents the industries has almost 60 years of experience.

Disadvantages:

Because of the high amount of CO2 industries need more absorbers and lot of solvents and for that industries are counting a high running cost. Besides, the industry’s experience is limited to use the technology in a very large scale.

Pre -Combustion CO2 Capture:

Pre-Combustion Carbon Capture - Courtesy of www.co2crc.com.au

Figure: Pre-combustion CO2 capture process diagram

Source: Scottish Carbon Capture and Storage website (http://www.sccs.org.uk/public/capture/precombustion.html)

The name of this technology is "PRE" – combustion, so here the process of capturing CO2 is not happening how it is in the post combustion capture technology. Here the removal of CO2 is happening prior to combustion to produce H2. Because H2 combustion does not produce CO2 as a combustion by product. Here the main by product is water vapor. If it’s possible to store this H2 then it could be a totally CO2 emission free fuel for the future. In this pre-combustion CO2 capture technology, process of capturing CO2 is happening in three stages. Firstly, the fuel is converting into hydrogen and CO to form a synthesis gas and in the second stage by reacting with water CO is becoming CO2. In the final stage the produced CO2 is separated from the H2for a clean combustion. At the end to transport the CO2 to safe storage site it is compressed into liquid.

Advantages:

The main advantage of pre-combustion capture technology is, using this process 90-95% CO2 could be captured. This technology can become the most efficient method of capturing CO2 because of its very low technological risk. This technology can produce H2 which is a transportable energy source. Pre-combustion capture technology is applicable to both natural gas and coal fired IGCC power plants.

Disadvantages:

High investment cost is the main disadvantage of this capture technology because it needs a chemical plant in front of the gas turbine. Again, pre-combustion has high NOx emission and for this industries need expensive gas scrubbers.

Oxy-fuel Combustion CO2 Capture:

Oxy-Fuel Combustion Capture - Courtesy of www.co2crc.com.au

Figure: Pre-combustion CO2 capture process diagram

Source: Scottish Carbon Capture and Storage website

(http://www.sccs.org.uk/public/capture/oxyfuel.html)

Oxy-fuel is the CO2 capture technology where in presence of pure O2 the burning process of fossil fuels is happening. In this process a more complete combustion is happening because of the pure oxygen. After oxy-fuel combustion almost 90% oxygen and water vapor is coming out with the exhaust stream and after that CO2 is separated by condensation.

Advantages:

Because of a more complete combustion in oxy-fuel combustion process, almost 100% CO2 could be captured. And it is possible to retrofit the oxy-fuel burners on to the existing coal fired power plants.

Disadvantages:

The main disadvantage of oxy-fuel combustion capture technology is the method of separating oxygen from the air. It has a high energy penalty because to supply pure oxygen to the combustion tower it requires a lot of electricity that the plant generates (e.g. at least 15% for a 500 MW coal fired power plant). Although, a promising capture technology named "chemical looping combustion (CLC)" is under research which will allow avoiding the energy penalty of oxy-fuel combustion CO2 capture technology.