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When plants grow they absorb carbon-dioxide from air by photosynthesis, but when they die and decompose, then eventually all the captured carbon dioxide in them returns back to the atmosphere. The question is: can we utilize this aerobic decomposition of plant material to capture the carbon-dioxide that gets released in the process?

It's autumn here, and fallen leaves are all over the place on the sidewalks and roads, I wonder if it makes sense collect them and put them into tanks and pass air through it to promote aerobic decomposition and use a gas separation technique to separate the carbon-dioxide produced by decomposition which we can sequester or use it for making synthetic fuels using renewable energy. The residue will be humus which needs to be redistributed to fertilize the soil.

Assume all the energy needed for this process are coming from renewable energy.

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If you were to decompose it anaerobically, you'd get methane, not CO2 and wouldn't need to process it to get fuel.

Composting, on the other hand, should be aerobic, and does produce CO2, as well as humus to add to the soil. If I put my leaves in the garden waste bin (we have those in my area), they're composted at a municipal facility and returned to the soil (farmland) but plenty of diesel is used to run the lorries that collect the waste and the machinery to manipulate, transport, and spread the compost. My garden has 3 mature oaks overhanging it, so I get a lot of leaves.

At home, instead, leaves can be buried, returning organic matter and carbon to the soil, and making rather good soil in the process, or decomposed aerobically in nets or sacks, damp or dry rather than wet. Both of these methods bury some carbon and release some carbon, but can be done with small amounts of manual labour in a modest garden. They're effectively tidier forms of the natural processes in a forest.

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When organic waste decomposes, it produces a mix of methane and carbon dioxide. This is called biogas.

Any of these two are gases containing carbon and they can be converted to each other.

You convert methane into carbon dioxide by burning it. The benefit of this process is that it gives energy.

You can convert carbon dioxide into methane by combining it with hydrogen produced using electrolysis, for example powered by renewable energy. Presumably you won't want to store this methane underground (or if you store it, only temporarily for energy storage) but you want to burn it for energy and store the produced carbon dioxide underground to store carbon.

Also, it is possible to produce biochar from wood for example instead of letting it decompose. This biochar production process is exothermic so it releases energy. Some of the carbon in the wood will stay in the biochar in solid form. The main benefit of this process is that it creates solid carbon, so the volume it takes is very small and it's easy to store therefore.

I presume these processes will be used a lot, because they are the most mature and cheapest atmospheric carbon capture technology. By creating biogas and biochar en masse, it is possible to slowly reverse climate change. We will not reach the ambitious lower goal of Paris agreement (1.5 degrees Celsius warming) solely by reducing fossil fuel use, but we will exceed it. However, by atmospheric carbon capture it's possible to slowly start to reverse climate change and bring back the total warming to an acceptable level, 1.5 degrees.

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