How could we capture carbon instead of emitting it?
Wouldn’t it be cool if we could capture CO₂ before it is emitted into the atmosphere and then store it somewhere (like underground)? Well, we can! This process is called carbon capture and storage (CCS).
CCS can help remove emissions from many different manufacturing processes; wherever CO₂ is released from burning, high temperatures or chemical reactions, CCS could be used to capture it!
CCS power generation in natural gas and coal power plants is particularly exciting as it allows us to burn some fossil fuels for energy, without releasing any CO₂. But does it work, and is it affordable? Let’s have a look!
There are three ways to capture carbon:
Pre-combustion carbon capture (preCCC): Here, carbon (in the form of CO₂) is removed from fossil fuels in a reactor before being burnt .
Post-combustion carbon capture (postCCC): Here, carbon (in the form of CO₂) is removed after the fossil fuels are burnt .
Oxy-fuel combustion: Here, pure oxygen is used to burn the fossil fuel, resulting in a relatively pure stream of CO₂, which is easier to capture .
You can learn more about CO₂ storage in the Undo Climate Change course. Here we will look at how the carbon is actually captured, starting with preCCC and then going onto postCCC and oxy-fuel combustion in the following chapters.
What is preCCC and why is it useful?
Fossil fuels, such as coal and natural gas, are hydrocarbons - they are made up of hydrogen (H) and carbon (C).
PreCCC turns fossil fuel hydrocarbons into hydrogen (H₂) and carbon dioxide (CO₂). The hydrogen is then used as a cleaner energy source (see our energy course!) while the CO₂ is captured and stored, avoiding its emission into the atmosphere !
How does preCCC work?
Simplified diagram of how preCCC works
Let’s go through this step-by-step! Although the process works for natural gas and coal plants, we’ll only cover natural gas here. Gas is cheaper and it is not economically feasible to add CCS to older coal plants as they have a shorter operating life left .
1. Syngas production
Firstly, we need to break down the hydrocarbon into what is called syngas, a mixture of mostly carbon monoxide (CO) and hydrogen (H₂). The cheapest method of achieving this is steam reforming. So how does it work?
At high temperatures (1050 oC) and low pressures, steam (vaporized H₂O), can be used to split natural gas into CO and H₂ .
Steam reforming natural gas to produce syngas
2. Removing impurities
The fuel needs to have impurities removed to avoid damaging the turbine which generates electricity and to reduce the pollutant emissions . If the initial fossil fuel was coal then even more filtering stages would be needed .
3. Water-gas shift reaction
The CO in syngas is then converted to CO₂ by reacting it with steam. This creates even more H₂ that can be used as fuel :
Creating more H₂ (to use as fuel) by reacting the syngas with steam.
The overall reaction to convert natural gas (mostly CH4) into CO₂ and H₂ with the aid of catalysts to speed up the reaction.
4. Separation and storage
The CO₂ then needs to be separated from the H₂. Absorption into a physical solvent is the most common (and cheapest) method for capturing the CO₂ , which is then, finally, transported into storage.
We are now left with just H₂!
By producing water rather than CO2, hydrogen acts as a much cleaner energy source than the initial fossil fuel!
How feasible is preCCC?
Increased costs and decreased efficiencies of using preCCC
As we can see, installing and running preCCC in a gas power plant increases the cost of the energy output from the plant. It also decreases the efficiency of the plant because of the added energy required to run the preCCC systems .
It is cheaper for companies to pay a tax for their emissions than to install preCCC technologies, and in some places carbon isn’t even taxed at all !
At what stage is the technology right now?
There are currently no test plants for natural gas-fueled preCCC power and the first coal power plant with preCCC faced delays and cost 2.3 times as much as expected .
So, there would need to be a lot of research, development and testing if preCCC is to become more common, all of which require time and money.
What will preCCC be like in the future?
Reducing the energy requirements of preCCC will be needed to make this technology efficient and cost effective in the future.
Energy demands from the process can be reduced in two main ways: by reducing how much steam is used (because producing steam requires lots of energy) or by combining the conversion of CO to CO₂ and the removal of CO₂ into one step .
To understand why, let’s look back at the water-gas shift reaction:
The reversible water-gas shift reaction
This is a reversible reaction that reaches an equilibrium when the forward and reverse reactions have constant rates . Once the reaction reaches an equilibrium, it is possible to cause the reaction to shift by altering the temperatures, concentrations, and pressures under which it occurs . For example, if we change the concentration by adding or removing molecules, the reaction will adjust to counteract the change .
For the water-gas shift reaction, we want to produce as much H2 as possible, so let’s see how removing the products might help us:
This is why removing CO₂ and H₂ from the reaction causes it to shift and produce more CO₂ and H₂
If we continuously remove H₂ and CO₂ during the reaction, we will change the concentrations of these molecules in the reaction, causing the equilibrium to adjust to counteract this change . The CO and H₂O will therefore react together faster to replace these missing products. So, by constantly removing CO₂ and H₂ during the water-gas shift reaction, we get more H₂ and CO₂ produced overall .
Another method that works with the same idea is the use of membranes to remove H₂ as it is produced. This results in the same effect as above !
So that covers the basics of preCCC, let’s now look at postCCC!
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