Carbon capture: a key technology to fight global warming.

Since 1865, our CO2 emissions into the atmosphere have increased from 1,600 million tonnes to 36,000 million tonnes per year. Despite our many efforts, this number is still not decreasing because the reduction efforts put in place can not compensate for the growth and increase in the population. There are now many large-scale projects that aim to reduce our impact on the environment, such as the planting of trees in the Sahara Desert or the use of aerosols at altitude to block some of the sun’s rays.

However these projects present great ecological and economic risks and will only be achievable after years of R & D. The problem is that climate change is not waiting! So instead of carrying out risky projects and using risky engineering tactics to limit the greenhouse effect, why not simply remove CO2 from the atmosphere and repair the damage we’ve done?

In a way, it is already a bit of what is happening. Carbon Capture and Storage or CCS technologies have been used for years. There are different types of carbon capture, but the vast majority are set up in power generation sites such as coal-fired plants by capturing CO2 directly from the chimneys.

Post-combustion carbon capture

With this method, CO2 is captured after burning a fossil fuel. CO2 is separated from the flue gas which contains CO2, water vapor, sulfur dioxide and nitrogen oxides or NOx (Nitrogen Oxides). By bubbling the gas through an absorption column that contains a liquid solvent that may be ammonia. When the chemical compounds in the absorption column become saturated with CO2, a stream of superheated steam (about 120 ° C) is released and comes through the solvent which releases the CO2 which can be stored elsewhere.

Pre-combustion carbon capture

In pre-combustion capture, CO2 is captured before being diluted in a flue gas. In a gasifier, the fossil fuel is mixed with pure oxygen to form a mixture of carbon monoxide and hydrogen. The carbon monoxide is then mixed with water to convert it to CO2 which is captured as well as hydrogen. Hydrogen can be used to produce electricity through a fuel cell and CO2 is stored elsewhere.

These two capture methods can limit the CO2 emissions of production sites by 80 to 90%. The IPCC (Intergovernmental Panel on Climate Change) estimates that carbon capture and storage (CCS) methods can contribute to reducing CO2 emissions by 10 to 55 percent in 2100.

So it looks good, where is the problem?

The problem is that CO2 has to be stored somewhere. In most cases, CO2 is stored underground thanks to geological sequestration. This involves injecting CO2 into underground rock formations. The CO2 is thus stored in the form of a supercritical fluid, which means that it has properties situated between its form in the gaseous state and in the liquid state. CO2 is injected into underground reservoirs that previously contained fossil fuels. These areas have natural rock formations that help contain CO2 in its supercritical form (at 31.1 ° C and 72 atmospheres). This solution seems appropriate but no one really knows what would happen if the supercritical CO2 would disperse in large quantities in the environment due to an underground leak. In addition, such underground storage is known to increase plant mortality and damage local ecosystems. On the other hand, for this solution to be viable, CO2 should remain underground for several hundred years to be exploitable again.

Another method of storing CO2 involves sending it deep into the ocean at a depth of over 3500 meters or it will turn into a material that will sink to reach the ocean floor thanks to high pressures. But this method is not approved and tested at all. No one knows exactly what it will imply for marine life and CO2 could eventually reach the surface in one way or another with the weather thus spreading new ones into the atmosphere.

It is in Iceland that a more promising storage method is being tested. The researchers found that injecting CO2 into underground volcanic rocks could accelerate the natural process or basalt reacts with the gas to form carbonate minerals that form limestone. This method is very encouraging but it also has its limits. Indeed, this process requires 25 tons of water per tonne of buried CO2. This means that this process would be limited to seaside sites. Another limitation is that underground microbes can break carbon into methane, a gas that boosts the greenhouse effect 25 times more than CO2!

Let’s take a step back

Although it is theoretically possible to capture 80-90% of emissions from power generation sites, this represents only a fraction of anthropogenic emission sources. Indeed, only 25% of global CO2 emissions come from the production of electricity and heat at the production sites. Transport, industry in general and agriculture together emit about 60% of CO2 emissions. Is there a way to capture CO2 from these sources?

Direct CO2 capture in the atmosphere has been, until recently, a theoretical method where CO2 is captured directly in the air. A theoretical method because to achieve such a large-scale capture system was too expensive and therefore unprofitable for a company wanting to design. Some experts believe that such a system would cost about $ 600 per tonne of CO2. As a reference, a passenger in a medium-sized car emits around 5 tonnes of CO2 per year. Imagine the costs to make an impact!

Recently a team of researchers at Harvard founded the company Carbon Engineering. They announced that they have found a way to capture CO2 directly in the air at a lower cost. Their catch price will be between $ 94 and $ 232 per tonne of CO2 captured. But Carbon Engineering does not stop there and values ​​the CO2 captured in carbon-neutral fuel. This may sound too good to be true but their methods are, in fact, not very different from those used for decades. The air is sucked by fans and circulates in filters soaked with a hydroxide solution that retains CO2 and converts it into carbonate. The hydroxide solution reacts with CO2 to form ions. These reactions occur in a structure identical to the cooling towers used in industry. The carbonate then reacts with calcium to form calcium carbonate in the form of dry pellets. Then a circulating fluid heats the calcium carbonate pellets to their decomposition temperature. This breaks the pellets, which releases CO2 in the form of gas and leaves calcium oxide. The CO2 is then mixed with hydrogen and converted into liquid fuels that can be gasoline, diesel or kerosene using the Fischer Tropsch process already used for a long time by the oil industry. During this process, a mixture of carbon monoxide and hydrogen is converted into liquid hydrocarbon. These reactions take place in the presence of a metal catalyst and a temperature of between 150 and 300 ° C.

This means that it is possible to produce carbon neutral fuels. If you use these fuels with your car, you will emit CO2 into the atmosphere. But because these fuels are produced from CO2 captured in the air, these emissions will not increase the amount of greenhouse gases in the atmosphere.

This method is also very interesting because the fact of being able to sell fuels makes this technology viable. But to achieve interesting costs through a significant economy of scale, it would require several facilities whose production capacity would exceed hundreds of thousands of barrels. This scenario could take place in the same way as photovoltaics and wind power, which has seen its costs fall in recent years thanks in particular to economies of scale.

Only, to counter climate change, carbon neutral fuels are not enough. Negative Carbon Fuels must be produced by storing some of the CO2 in Icelandic volcanic rocks, for example.

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