Final product of the combustion process, CO2 is a component that is thermodynamically very stable. One of the methods to reuse CO2 is to add a reactive component in order to generate a chemical reaction and thus obtain a basic chemical product or an energy product.
Chemical products
Organic synthesis and mineralization are two ways to transform CO2 into a workable chemical product
Organic synthesis
CO2 used as a primary resource can be an alternative to oil for synthesizing fuels and organic chemical products. Using CO2 presents certain advantages: oil consumption is reduced and less harmful products are used. However, the different methods of synthesis are not fully developed for all possible applications:
At an industrial level, synthesized urea is used for fertilizers or plastic production, and salicylic acid is used as drug, food preservative and antiseptic.
Syntheses being developed – well known but not industrialized yet – are polycarbonates, used to produce elastomers for clinical applications; linear organic carbonates, used as solvents in medicine, cosmetics, as chemical intermediaries and diesel additives; and cyclic organic carbonates which become solvents or are used for the synthesis of polymers.
Syntheses in research stage are carboxylic acids, used as solvents for creating adhesive, glue, coating; and carbamates, used to synthesize pharmaceutical products, pesticides or plastics such as polyurethane.
Mineralization/ carbonization
Mineralization consists in converting CO2 into an inert material. This technique was thoroughly studied in the past years, especially for its storage capacities in-situ (see “New Concepts”). What is studied here though is the ex-situ mineralization, considered a CO2 recycling method since it is reused and not stored.
Ex-situ mineralization has the CO2 react with an oxide (calcium, iron magnesium) to transform into a carbonate.
Carbonization is a specific technique of mineralization achieved with lime, which, by Absorption of the carbon dioxide contained in the air, produces limestone and water. Carbonization is the main procedure to reuse CO2 as the limestone is then utilized as a primary resource in the production of cement. The interest of carbonization lies in the use of lime, a waste generated by producing cement.
The main application for CO2 mineralization is limestone production (which can enter the cement production line). Direct production of cement might also be possible, and other minerals (MgCO3) could be used as building materials, incorporated into concrete to build roads, provided they are proved up to standards.
Energy products
Various mechanisms are being studied, but there remain many obstacles to this field (financial viability, energy consumption…)
Hydrogenation
Hydrogenation is a chemical reaction in which a dihydrogen (H2) molecule is added to CO2. Through this technique, a great variety of components is produced such as methanol (both an intermediary for the chemical industry and an energy product – fuel additive), ethanol (easier to handle, convey and combine with gas-oil), methane, and other Hydrocarbons.
Dry reforming (and alternatives)
The most classic reforming way is Steam reforming – or wet reforming, which consists in having methane react with steam. An alternative is to have methane react with oxygen (part oxidation) or with CO2.
Methane reforming generates Synthetic gas – or syngas (a mix of CO and H2) - mainly used to synthesize methanol and its derivatives or synthetic hydrocarbons.
Electrolysis
The electrochemical reduction of CO2 consists in having CO2 react with several electrons and protons in order to produce synthetic hydrocarbons and oxygenate components.
CO2 electroreduction can lead to the formation of several components such as formic acid, carbon monoxide, formaldehyde, methanol and methane.
Photoelectrocatalysis
The concept of photoelectrocatalysis is to have CO2 react with protons and electrons that are produced by photodissociation of water. This photodissociation is achievable using a photocatalyst. The reactions engendered serve to synthesize hydrocarbons or oxygenated components such methanol, hydrogen, methane, carbon monoxide; and other components of the alkene type, paraffin, formic acid, carbon monoxide and formaldehyde.
Thermochemistry
Thermochemistry uses heat as energy to break the CO2 molecule and associate it to water or methane in order to produce synthetic gas. Several oxygenated components can be obtained and then integrated to standard fuels such as methanol, ethanol, olefins (ethylene, propylene, butane) and other hydrocarbons.
