When speaking of carbon dioxide, and not working in the gas trade or the chemical industry, often the thoughts surround the daunting images of polar bears drowning, and fears of government taxation needed to implement a massive carbon reduction effort; both are essentially distant, or incomplete in terms of interpretation, and implementation.

Personally, I think there is sufficient evidence to believe in global warming driven by the rise in greenhouse gas content over the years, decades, and centuries. No doubt, true action will have to be taken to reduce the massive amount of CO2 emitted compared to business and politics; however, carbon dioxide emissions reduction will be essential globally.

Outside of the CO2 emissions discussion and context, CO2 is an extremely versatile chemical which has numerous environmentally friendly applications, when compared with alternate chemicals or methods which are either harmful to our health, and/or harmful to our planet.

Ahead are a few takes on the green nature of CO2 when applied in various industries. For those in the CO2 or gases business, it is well known that more applications come along over time, many of which are quite ‘green’ in terms of the environmental impact. To name a few green applications for CO2, consider solvent technologies, insect control, cleaning applications, Ph reduction, agricultural markets, and renewable fuel production.

Water treatment and Ph applications
Some of the older, well established, and fully ‘tried and true’ applications for CO2 with a truly ‘green’ nature include dissolving or diffusing CO2 into water – generally for the purpose of reducing alkalinity where a self limiting and safe acid is needed rather than mineral acids like sulphuric.

The same application can be in places where caustic soda or other alkaline materials are used, thus an alkaline effluent requires Ph adjustment.

In municipal water treatment plants, often lime softening operations use CO2 for Ph reduction, downstream of lime treatment; and other municipal water or waste water plants require Ph reduction as well, outside of lime softening operations. Some time ago, underground gas or coal combustion units were used in municipal water treatment, specific for their flue gas containing a lean CO2 content, thus diffused for Ph reduction. This is inefficient, and requires a lot of maintenance; therefore most municipal water treatment carbon dioxide systems have been replaced with liquid CO2 stored on-site, vaporised, and diffused through the water.

Many other environments including some food plants, chemical facilities, and anywhere a weak acid, which is self eliminating, may be best served. Carbonic acid of course is the product we are speaking of in this case, created by diffusing CO2 in water; and it only exists in a solution – also sensitive to pressure and temperatures.

Therefore, in many industrial environments, where safety and odours are the main problems associated with mineral acids such as hydrochloric and sulphuric, and where CO2 can be dissolved in a water-based solvent, it would be the role of the supplier to investigate and implement a carbonic acid application.

The environmentally friendly nature of this application is a more natural method of reducing CO2, as well as the by-product yield in the form of carbonates and bicarbonates – which are natural and harmless, compared to the difficult-to-manage sulphates, when otherwise applying sulphuric acid.

Insect control
In select environments, particularly where a grain storage facility can be sealed when applying CO2 liquid vaporised to a gas or even in places where dry ice can be sublimated; this application can work quite well.

From an environmentally friendly point of view, some of the potentially carcinogenic and toxic agents can be replaced; thinking in terms of some of the halogenated hydrocarbons, and other agents, which are totally unnatural. The most standardised and successful method of application is via gaseous CO2, vaporised from on-site liquid storage. The CO2 can be applied into the grain storage headspace, thus settling downward; or it can be injected at the base of a bin, and atmospheric displacement occurs, up and out.

In the end, the rule of thumb is to maintain a CO2 atmosphere of about 50%. This application is best suited to warmer climates, over 60°F, and can be fully completed in 4 7 days maintaining this content of CO2 in the atmosphere. If cooler, the timeframe would be longer for results to be achieved. Perhaps the greatest advantage of CO2 instead of other agents is the lack of toxic residues left behind when using CO2.

Agricultural applications
The first agriculturally related CO2 application which is common would be greenhouse atmosphere modification.

Older greenhouses used to burn hydrocarbons, usually natural gas, to yield a lean CO2 by-product in the flue gas; which is inefficient, maintenance intensive, and ecologically harmful.

The clean, simple and reliable method of applying CO2 gas in greenhouse atmospheres is via stored liquid product on site, vaporised, and applied. This can be via carrying the CO2 rich atmosphere with fans through the bedded plants, flowers, and like crops. In this respect, CO2 is as much a nutrient as soil and water; the actual ‘vigour’ of plants would be increased by CO2 atmospheric enrichment in the greenhouse environment setting.

Most greenhouse crops receive increased photosynthesis as the CO2 atmospheric content rises to levels ranging from 350 to 1000 ppm. With CO2 content approaching 1000 ppm, photosynthesis will rise by about 50% over what is found in ambient CO2 content which is about 340 ppm.

The next application may become a great carbon sink, and a means by which an energy-rich form of renewable energy is produced; that being algae production, and extracting the oil for biodiesel, and algae materials as a feedstock for fermentation. In general terms, certain types of algae are under development for commercial operations.

On some of the lab and pilot operations, it is conceivable that merchant CO2 will satisfy this requirement; however long-term planning and models indicate trends and plans to use CO2 sourced directly from power plant flue gas, fermentation by product, and other industrial by-product sources.

Downstream of the raw industrial source, depending upon the constituents in the raw gas, some purification may be required. Therefore, algae projects will become a carbon dioxide sink from industrial raw gas sources at the very least, but may also be markets for certain merchant CO2 sources.

Blast cleaning with carbon dioxide
Dry ice (usually rice sized and shaped ice) is commonly being expanded to include the cleaning and treating of many surface types; including specific delicate surfaces, through heavy, industrial cleaning challenges.

The way we can look at CO2 blast cleaning in green terms, is often eliminating solvents, which leach into the soil and the work environments. When replacing with CO2 for blast cleaning, the results are a much safer and healthier environment; this also applies to blast cleaning with sand and other materials.

The strong advantage in terms of a by-product is mere sublimation of the dry ice, and no large pile of sand, or pool of solvents to handle after cleaning. The subject, CO2 blast cleaning, has been well published and well advertised, and is growing in popularity all over the globe. Availability and location of the source for this dry ice product is one factor in affordability; as well as the cost and availability of the blasting machines.

Solvent technologies and applications
As for CO2 use as a solvent, a very green application today would be the use of CO2 in specially designed dry cleaning machines, where the CO2 acts purely as a safe solvent; thus safe in an environmental friendly context.

This eliminates the venting of VOC to the atmosphere and perhaps even more importantly, eliminating the cancer ridden syndrome which the dry cleaning industry is famous for; that being where ‘perc’ (perchloroethylene) has always been the standard.

There happen to be some phase-out mandates for this harmful chemical, and this will effect a significant number of dry cleaning operations in the US and internationally. Other solvent technologies include extraction; under high pressure super critical extraction of essential oils, and other materials from plant and vegetable matter is something to develop further.

These two applications represent essentially a small market, since in the case of dry cleaning machines using CO2 this is so far a niche industry, however probably destined to grow as the tide turns against ‘perc’.

As for the supercritical extraction application, this is a closed loop system and the usage is probably as a sum total small; however, very environmentally friendly, often replacing hydrocarbon based solvents.