# Lesson 6 - CO2 capturing

Carbon dioxide as building block

We must not forget that although the rise of CO2 concentrations in the atmosphere is a threat to the environment, CO2 is also one of the most important building blocks in nature. It is uses by plants and algae during photosynthesis to produce glucose. The glucose is then used by the plant to ‘fuel’ all the other processes taking place inside the plants. Not only plants use carbon dioxide we also use it to produce plastics.

Farmers that use greenhouses understand the photosynthesis process very well and the try to create excellent conditions for plants to grow. They found out that increasing the carbon dioxide concentration in the air helps the plants to grow even better. Nowadays exhaust gasses containing carbon dioxide from fossil fuelled power plants are transported to greenhouses were it is used to help plants grow.

Concentration and speed of reaction

The reason why plants grow better when CO2 concentrations are higher is because the photosynthetic reaction is speeded up. This is not only the case for the photosynthetic reaction but is generally the case for all chemical reactions. If the concentration of reactants is increased the

rate at which the reaction takes place is also increased. This is explained in figure 28 where two molecules of NO2 react to form one molecule of NO and NO3.

To achieve this the two molecules of NO2 have to be near to each other and collide. If the amount (or concentration) of NO2 is increased the chance that molecules will collide will increase and thus the chance of NO and NO3 being formed is increased as well. The same holds for plants; increasing the amount of CO2 available increases the amount of glucose being produced. More glucose means more energy for the plant’s growing process and thus better growth.

In general: increasing the concentration of reactants in a chemical reaction increases the speed (rate) of that chemical reaction.

Capturing CO2

If we want to use CO2 in a chemical reaction effectively we will need it in large and concentrated amounts. One way of getting large amounts of getting if from the exhaust of fossil fuelled power plants. This carbon dioxide can then be converted into methanol. This can be used as a fuel for cars with a combustion engine. This will however not reduce the amount of CO2 greenhouse gas produced.

If we want to reduce the amount of CO2 in the atmosphere effectively we will have to find ways to take away CO2 that is already present in the atmosphere. Different techniques to do so are now being developed. We will discuss two of those techniques here:

- Artificial CO2 capture tree

Plans already exist to use artificial CO2 capture trees just like the artificial solar tree that has been discussed earlier. The idea behind this is to lead air with CO2 through tubes (‘branches’) filled a solution of sodium hydroxide (NaOH). The carbon dioxide in the air will react with the solution to a sodium carbonate (Na2CO3 = soda) solution. This can then be transported and the CO2 can be taken out and stored. Question 3 • Give the reaction of CO2 reacting with a sodium hydroxide solution. The artificial solar trees have to be placed at specific angels to the sun. For the artificial CO2 capture trees this is not important. Explain the difference. • Why will this method be more effective in reducing CO2 concentrations in the atmosphere in comparison with CO2 used from power plants?

- Conventional cooling towers used to capture CO2 and create energy

In figure 29 a conventional cooling tower is sketched that could either provide electricity or CO2 capture. Water pumped to the top cools the air which causes a downdraft inside the tower. The tower has a 10,000 m2 opening. Cooling the air to the degree possible in a desert climate would cause - in the absence of obstructions - a downdraft in excess of 15 m/s generating a flow of nearly 15 km3 of air per day through the tower. The air leaving at the bottom could drive wind turbines or flow over CO2 absorbers. Based on the volumes of air flowing and the potential energy of the cold air generated at the top of the tower, the tower could generate 3 to 4 MW of electricity after pumping water to the top. The same airflow would carry 9,500 tons of CO2 per day through the tower. This CO2 flow equals the output of a 360 MW power plant.