The science of capturing carbon dioxide

According to many of the world’s governments, we put about 36 billion tons of CO2 into our atmosphere every year. Just for the record, that dwarfs ,US emissions, currently about 5.9 billion tons annually. The total amount of CO2 in our atmosphere is about 2,200 billion tons, or about 0.04%, so we add to it a little bit each year.

It’s a good thing that 100 billion tons of the gas is removed from our air supply every year by photosynthetic processes. But, according to many of the world’s governments, this isn’t enough; so as humans we must step up to the plate and somehow aid the effort too, without producing more in the long run. Green plants are very efficient. They make cellulose as a product of CO2 and H2O, because they grow and produce seeds.

But is there a technique that humans can devise to convert carbon dioxide into useful substances that can actually help mankind?

We must first ask why carbon dioxide gets such a bad rap, and that’s pretty easy to explain. All of the energy from the sun that reaches our Earth arrives in the form of electromagnetic radiation. There is no conduction through empty space and there is no convection, two of the only three methods that heat can be transported. It’s the photons from the sun that make a sidewalk hot on a summer’s day. Nothing else.

On average, 340 watts per square meter of solar energy arrives at the top of the atmosphere, taking into account the fact that not all of it is coming straight in. Looking at the solar spectrum, ultraviolet radiation accounts for less than 4%, while the visible light brings us 43% of this solar energy.

But because things have been pretty steady for thousands of years, our Earth returns an equal amount of energy back into outer space by reflecting some incoming light and by re-radiating heat in the form of thermal infrared energy as governed by the Stephen-Boltzmann Law .

This fourth power equation says anything with a temperature above absolute zero will send out energy too. Not a lot mind you, but just enough to keep our average surface temperature at about 59 degrees F, a nice spring day’s step outside and feel-good greeting.

Unfortunately, not all of the energy baking off the surface heads out into space, some of it is absorbed by our own atmosphere. The main constituents, oxygen and nitrogen make up 99% of our atmosphere, but they are transparent to IR and let the little our Earth emits freely pass through.

Of the remaining 1% of the gases, such as argon and CO2, the latter is the worst at absorbing energy because its atoms, all set in a straight line, can vibrate and spin in just the right way and can suck up outgoing (and incoming) energy, raising the temperature of the atmosphere in the process.

This trapping of heat is what is called the greenhouse effect, because, like the glass structure people use to grow plants, energy comes in but never leaves. Visible light enters through the glass, heats things up and re-radiated at longer wavelengths that is absorbed by the glass. If you could see infrared like a rattlesnake, the walls of a greenhouse would look black to you.

The US government has claimed that the increase in carbon dioxide in the world’s atmosphere from 288 ppm before the industrial revolution to 414 ppm now is caused by burning of fossil fuels and this needs to be stopped because, some have calculated, if this value passes the 500 ppm level, the average global temperature of the Earth will rise between 2 and 5 degrees Celsius, leading to cataclysmic weather events, poor harvests and raising sea levels. This explains the recent push for electric vehicles that can be propelled from solar or wind power.

Recent advances in carbon capture technologies are suddenly becoming a plausible option to help tackle this CO2 increase. Several new ideas have made the news lately for disposing the potent greenhouse gas by converting it into a more valuable product.

One technique from Northwestern University that was published in the May 3 issue of Science, talks about a mechanism to use molybdenum carbide, an extremely hard ceramic material, to convert CO2 into carbon monoxide. Now, CO is a very useful gas and is considered an important building block to produce a variety of useful chemicals.

The article states that the catalyst was made using ordinary granulated household sugar so it seems easy to do. When testing the process, the team’s Dr. Khoshooei was impressed by its success because operating at ambient pressures and high temperatures (300-600 degrees Celsius), the catalyst converted CO2 into CO with 100% selectivity. More importantly, the catalyst also remained stable over time, meaning that it remained active and did not degrade.

But there are others working on the same idea.

MIT offered a different approach. In a paper just published in Cell Reports Physical Science, researchers there have used a glass fiber electrolytic arrangement to convert bicarbonates, easily obtained from CO2 gas, into a fuel that can be stored for use later on. The result of their reaction is the formation of potassium formate, KHCO2, and while itself is not burnable, it can be used in an electrolytic fuel cell. Their mechanism is over 96% efficient and has been tested in a cell putting out 0.3 Watts per square centimeter running continuously for over 200 hours.