A tool for climate change mitigation
The hot topics and most well-known solutions when talking about greenhouse gas emissions related to energy are renewable energies and energy efficiency. However, being realistic and considering how far and how much carbon dioxide (CO2) emissions we already have we decided to write a post about CO2 capturing and sequestration, a technology field which can be as important as the renewable and energy efficiency areas for climate change mitigation.
Recalling some Chemistry lessons, a combustion is an exothermic (heat is released) chemical reaction between a fuel and an oxidant. Usually the fuels are carbon or hydrocarbons and the oxidant is pure oxygen or the one found in the air. For example, the methane (natural gas can be approximated as methane) combustion reaction will take place like this:
CH4 + 2O2 - - > CO2+2H2O
Combustion reactions are used worldwide in transportation, industrial processes and power generation at a large scale continuously producing large amounts of carbon dioxide and contributing to the greenhouse effect and climate change. The concentrations of CO2 are increasing at an accelerating rate and the atmospheric CO2 levels have been higher than safety limit (350 parts per million, a concentration measurement unit) since early 1988. The trend does not seem to change even though countries, associations and inpiduals are increasing their awareness on climate change and the need to act.
The ways to reduce carbon dioxide emissions are mainly:
- Use of renewable energies which are free of carbon fuels
- Increase of energy efficiency to reduce the use of carbon fuels
- And carbon dioxide capturing and storage techniques.
In this post we want to highlight the last and least known approach for carbon dioxide reduction: carbon dioxide capture and sequestration.
One approach is to sequestrate the carbon dioxide at its source (power and industrial plants). There are three main techniques to achieve this:
Post-combustion is the most intuitive capture of CO2 and is performed once the combustion takes place using liquid solvents (a substance able to absorb another substance) in most of the cases. Then the CO2 is separated from the solvent by heating it. It is the most mature technology on this field since it has been used for decades in large industries such as soft drink manufacturers.
Pre-combustion technique is based on using a set of reactions to convert the fuel into a mix of CO2 and hydrogen (H2) without combustion. The CO2 can be captured by solvents or solid adsorbents (materials able to adhere certain substances to its surface). The main target is to produce carbon free or reduced carbon content fuels: it is possible to use the pure hydrogen as a fuel or to use it in refinery processes to obtain cleaner fuels.
The main drawback for both previous techniques is the complexity of integrating them in existing generation plants.
Finally, oxyfiring involves using pure oxygen in the combustion (removing nitrogen for the air), which results on having mainly CO2 and water vapor after the reaction. Cooling and compressing them removes the water vapor and a final purification may be need depending on the level of impurities of the CO2. The main drawback of this technique is the high energy cost of obtaining pure oxygen from air and it has not reached the commercial scale for carbon dioxide capture.
Once the carbon dioxide is captured with one of these three techniques, we have prevented it from going to the atmosphere, but something must be done with it. The usual path is to compress it in order to reduce its volume and transport it by pipelines to a suitable and carefully chosen storage placement: underground or deep on the ocean. The most suitable places are:
- Deep salines reservoirs
- Depleted oil and gas reservoirs
- Unmineable coal seams
Besides, there is the option to use the CO2 for Enhanced Oil Recovery (EOR). EOR are a set of techniques aiming to increase the amount of oil extracted from an oil field. It can use CO2 to flush out the oil and help extracting it while storing some or all the used CO2 in the subsurface.
Ocean storage is under investigation since deep ocean water is unsaturated with respect to CO2: it is estimated that if we stored the surplus of CO2 in the atmosphere into the ocean, its carbon concentration would change by less than 2%. However, the interaction between CO2 and seawater depend on the depth and the consequences of the interaction must be completely understood and clarified.
Another approach for carbon sequestration is the enhancement of natural processes to increase the removal of carbon from the atmosphere. An example is the use of microalgae which can absorb the carbon dioxide in its photosynthesis process and we can afterwards convert the microalgae into biofuel to substitute fossil fuels. We will dedicate another post to talk more deeply about this and other uses of CO2. Keep an eye on our blog if you find this interesting!
To sum up, even though preventive options are better, we definitely need some healing techniques for our atmosphere. Capturing and accumulating CO2 is an option to stabilize the carbon dioxide found on the atmosphere to a safe and acceptable level. In the climate change mitigation all the options must be investigated and taken into account! Everything adds up!