CO2 current uses
While some uses of CO2 are well-known (the fizz in your soda or the gas in the fire extinguisher), many others are not. In fact, the largest use of CO2 is in the production of urea and the second largest is for enhanced recovery of oil from depleted oil wells.
Inputs on many other interesting uses of CO2 are provided in this section. We hope that some of these could be scaled a lot more in future, thereby resulting in better utilization and/or sequestration of CO2 emissions.
CO2 has a long history of applications. It is currently being used in over a dozen sectors.
Some of the conventional uses of CO2 are well known - these include its use to add fizz to beverages, in fire extinguishers, as dry ice, or for growth enhancement in greenhouses.
Some other conventional uses are not so well-known. For instance, CO2 is also used in the animal and meat industry to make the animals unconscious before they are slaughtered. CO2 is also used in the entertainment industry for creating special effects through CO2 jets that use the sublimation property of CO2 - when it goes from dry ice, a solid, to gas without becoming a liquid in between.
Interestingly, the largest and most prominent use of CO2 is something many do not know about - it is in the manufacture of urea.
Plants have been making foods from CO2 for hundreds of millions of years. Surely then, it should be possible for us to make foods from CO2 too?
Until now, there was no real business case for companies and innovators to convert CO2 into food. But with the increasing importance attached to CO2 emissions reductions, startups as well established firms in the food industry are keen to make foods from CO2 captured from emissions.
The route to make food from CO2 is to convert CO2 into sugars or into proteins.
Many of the current attempts that convert CO2 to food use biological pathways - in which they use micro-organisms or enzymes to undertake chemical reactions that convert CO2, along with sources of hydrogen & nitrogen (water and air) into sugar, proteins, etc.
Plants absorb carbon dioxide from the air, combine it with water and light, and make carbohydrates (sugars) through photosynthesis. They release oxygen in this process.
Because plants often receive more carbon dioxide and water than they need to sustain their own lives, they end up producing extra food. While some plants store the extra food or energy in their leaves, in others, this extra food is stored in fruits and vegetables.
Thus, we can say that the products we use from plants - fruits, vegetables, seeds, leaves, stems - are all from the CO2 that the plants had captured during their growth.
Carbon dioxide gas does not support combustion. Owing to its higher density, it displaces oxygen and thus prevents it from coming in contact with fire, and effectively extinguishing it.
Dry ice has been used as a cooling agent and for preserving frozen foods for decades. In addition, it is also used for special effects in entertainment and theater in fog machines.
To make dry ice, CO2 in gas form is first pressurised and cooled to form liquid CO2. In the next stage, the pressure is reduced leading to the vaporization of some liquid CO2 and a rapid lowering of temperature. This extreme cold causes the liquid to solidify into a snow-like material at about –78ºC. This material is compressed into blocks of dry ice.
Interestingly, when dry ice is exposed to normal temperature and pressure, it sublimes - that is, it turns from a solid directly to gas, without bothering to pass through the liquid phase in the middle.
CO2 is available in plenty and is low-priced. These should endear it to many applications, and especially in refrigeration where the alternatives could be far more expensive.
But the attractiveness of CO2 as a refrigerant goes beyond these reasons. Compared to many other refrigerants (say, HFCs), it has a lower greenhouse potential.
Also, the critical temperature of CO2 is quite low, just 31.1°C. This allows the possibility to operate not only in subcritical conditions but also in transcritical environments.
For years, agriculturalists have used carbon dioxide to kill insects in vegetable, grain and fruit storage containers.
The benefits are obvious: Because CO2 is not a chemical insecticide, it does not leach into the food, making it totally safe for human consumption.
Carbon dioxide is used in meat packaging due to its ability to inhibit the growth of a wide range of microorganisms.
Urea (NH2-CO-NH2) is produced from ammonia (NH3) and carbon dioxide at high pressure and relatively high temperature.
The easy availability of ammonia through the Haber process led to the development of ammonia/carbon dioxide process for urea production. Urea production is a two step process where the ammonia and carbon dioxide react to form ammonium carbamate which is then dehydrated to urea.
Urea production is currently the largest use - by far - of CO2 in any application worldwide.
CO2 Enhanced Oil Recovery (CO2 EOR) is a type of tertiary oil recovery that can recover even more oil from existing wells and reservoirs.
In CO2 EOR, carbon dioxide is pumped into the oil-bearing rock formation to recover additional oil. CO2 EOR has the potential to recover an additional 15% to 20% of the original oil.
The injection of CO2 into oil reservoirs to enhance oil recovery has a second desirable benefit in that it simultaneously sequesters CO2 in the reservoir.
Enhanced coal bed methane recovery is an approach to produce additional methane from coal beds, similar to the way enhanced oil recovery is applied to oil fields.
CO2 that is injected into a coal bed would occupy pore space and also adsorb onto the carbon in the coal at approximately twice the rate of methane. This results in ejection of methane from the coalbed and an enhanced methane gas recovery.
In theory, higher concentrations of CO2 should make plants yield more because photosynthesis uses sunlight to synthesise sugar (the main energy source) out of water and CO2. So, the more the CO2, there should be more sugars produced assuming there's enough sunlight and water available.
In practice as well, various large scale studies in open farms, as well as real life yield results from greenhouses have confirmed this hypothesis.
Studies have shown that higher concentrations of atmospheric CO2 influence crops in two important ways: they boost crop yields by increasing the rate of photosynthesis, and they reduce the amount of water crops lose through transpiration. Plants transpire through their leaves through tiny pores called stomata that open and collect CO2 molecules for photosynthesis. During the transpiration process, plants also release water vapor. With higher CO2 concentrations, the pores don’t open as wide, resulting in lower levels of transpiration by plants and thus increased water-use efficiency. Studies have shown that a doubling of CO2 concentration could reduce transpiration by about 35%, which combined with the yield increase, indicates that water use efficiency may also double along with higher yield.
The quantum of increase in biomass yield from higher CO2 concentration will however depend upon a variety of factors - the type of plants, the region where it is grown etc.
How much yield increase is produced by the use of enhanced CO2 in greenhouses?
Greenhouses around the world have been practising the use of increased concentration of CO2 in the closed environment to increase plant growth and yields, so they are an excellent platform to obtain real life data points.
How much are the yield increases reported by greenhouses from enhanced use of CO2?
While specific yields vary depending on a number of other inputs and variables, in general, the following inferences and estimations have been made:
Downsides for plants of higher CO2 levels
But higher CO2 concentration is not without its downsides. There are some key reasons why higher CO2 concentrations is not an unqualified boon for plants:
CO2 levels are the main influence for respiration. Oxygen levels only affect breathing when its levels dangerously low.
Isn't that surprising?
If CO2 levels increase, the respiratory center (medulla and pons) is stimulated to increase the rate and depth of breathing. This increases the rate of CO2 removal and returns concentrations to normal resting levels.
CO2 is effective in wastewater treatment as it can effectively control and bring down the pH.
Industry users are keen on alternatives to sulphuric and hydrochloric acids traditionally used in wastewater treatment plants, due to the significant health risks associated with these aggressive acids.
CO2 is an attractive alternative. It has a superior capacity to neutralize wastewater acidity compared to the alternatives. It does not increase health risks, it is easier to use, more sustainable and more economical.
Most of the CO2 used for wastewater treatment are sourced from emissions from industries such as breweries so there is actually no additional CO2 released to the atmosphere.
Supercritical carbon dioxide is a fluid state of carbon dioxide where it is held at or above its critical temperature and critical pressure.
Carbon dioxide usually behaves as a gas in air at standard temperature and pressure, or as a solid called dry ice when cooled and/or pressurised sufficiently.
Supercritical CO2 is a unique solvent that has the characteristics of variable density, low viscosity, and high diffusivity. The ability to manipulate these characteristics has led to numerous applications of this green solvent in diverse areas including extractions, impregnations, particle formation, and cleaning.
As it is not toxic, its supercritical phase is used in solvent extraction for food items as in the case of removal of caffeine from green coffee beans while leaving intact the many chemicals responsible for the aroma and taste of coffee.
Supercritical carbon dioxide is also used to remove pesticides and toxins from crops.
CO2 is generally used to inflate life rafts and inflatable life jackets because it is an inert, non-flammable, low-cost gas easily produced and inexpensive to manufacture worldwide.
Liquid carbon dioxide cleaning is a method that uses pressurized liquid CO2 in combination with other cleaning agents. CO2 is a nonflammable and nontoxic gas that becomes a liquid solvent under high pressure, and thus can be an effective solvent for dry cleaning.
The foam is produced when bubbles of carbon dioxide resulting from the reaction of sodium bicarbonate are trapped in the batter. As the cake bakes, the batter dries, and the trapped bubbles of carbon dioxide form the holes in the cake.
No evidence suggests that carbonated or sparkling water is bad for you. It's not that harmful to dental health, and it seems to have no effect on bone health.
Interestingly, a carbonated drink may even enhance digestion by improving swallowing ability and reducing constipation.