Carbon Dioxide Capture: An Effective Way to Combat Global Warming

Evidence proves that the climate is changing due to both natural cycles and anthropogenic influences. The increase in global carbon dioxide concentration, along with that of other greenhouse gases, is a major cause of global warming. Studies indicate that total emissions released from fossil fuels are related to the economy, are still on the rise, and can be stabilized through conscious efforts. Improving land management practices, incorporating carbon sequestration techniques, increasing energy efficiency, and using alternative energy sources are current ways that we can decrease our use of fossil fuels. Some alternative energy sources include waste biomass and cellulosic ethanol, along with others. Current research aims to identify ways to provide affordable fuel sources with little environmental impacts.

Until cost-effective alternative energy sources are available, it is necessary to capture harmful waste gases at the source (i.e., smoke stacks). CO₂ gas can be captured prior to combustion, through an oxyfuel combustion stage, or after combustion. Currently, the most common way to separate carbon dioxide gas is through an amine-scrubbing system in post-combustion scenarios; however, this method can be costly and byproducts are harmful to the environment. For this reason, absorbents, adsorbents, membranes, gas hydrates, and chemical looping processes are considered as methods for CO₂ gas capture. Industries such as iron and steel and cement industries can also incorporate CO₂ separation and capture technologies. Once CO₂ is captured, it can be transported through pipelines and stored in geological formations and ocean reservoirs.

Solid adsorbents are promising for applications in post-combustion CO₂ capture scenarios. Some adsorbents include zeolites, activated carbon, carbon nanotubes, zeolites, and silicon-based adsorbents. The materials are characterized by their surface functional groups, porosity, surface area, pore size, metal ligands, and electrostatic interactions to determine their potential as adsorbents for CO₂. Organic adsorbents are promising for low temperature CO₂ adsorption because of their surface properties, such as high surface area, which enables it to be modified by adding additional metals and functional groups. Carbonaceous materials can be physically or chemically activated in order to enhance capture. Biochar, a material known for its benefits to agriculture, can also be used to capture CO₂. Modified Organic Frameworks (MOFs) combine metal ions and organic ligands to produce a crystalline porous network that is capable of selectively binding molecules at high capacity. Other solid adsorbents include zeolites, clays, silica-based adsorbents, and metal oxide-based adsorbents.

CO₂ can be absorbed through either physical or chemical pathways. In order to increase efficiency of capture/separation, decrease cost, and reduce negative environmental impacts, it is necessary that absorbents have high net cyclic capacity, high absorption rate, good chemical stability, low vapor pressure, and low corrosiveness. Aqueous solutions of alkanolamines, including primary amines (monoethanolamine), secondary amines (diethanolamine), and tertiary amines (methyldiethanolamine), and combinations of the three are commonly used to remove acidic gases like CO₂ from flue gas. Each material has its favorable and unfavorable properties; it is important to design a material with an optimal balance, as increased efficiency and bond strength results in the need for higher regeneration energy and therefore money. Ionic liquids are highly adaptable and are thermally stable, which makes them also good candidates to be employed in CO₂ capture systems; however, are still in the research phase of development.

As CO₂ capture technology is employed extensively, the volume of CO₂ that is released through various processes has potential to surpass the limited amount of storage capacity. One sustainable way to deal with CO₂ is to convert it into a usable form, such as for energy. The major obstacle for CO₂ reduction is that the reaction isn’t energetically favorable; therefore, it is necessary to use catalysts to incite this reaction. Photocatalytic reactions are performed in a variety of conditions, such as under UV light exposure or concentrated sunlight in the presence of a catalyst and water. A variety of transition metal complexes, such as TiO₂, SiO₂, and metal decorated nanoparticles have been studied for their potential as photocatalysts. Current research is being funded to expand our knowledge on how to best use or convert captured CO₂.