Requiremnts for charcoal production
Carbon-rich feedstock
- All organic material can be charred, but plant material is to be preferred, because its rich content of carbon.
Counted as dry mass, about 40 % of the plant material is carbon.
Reduced oxygen environment
- If there are too much oxygen around, the carbon in the feedstock will add to the oxygen and turn to carbon dioxide (burning).
- If there is a lack of oxygen, the carbon will remain unchanged in its original lattice.
Electron micrograph of charcoal.
Its inner surface is about 400 m2 per gramme.
Heat
- When the heat reaches at least 150 oC, the pyrolysis phase starts and the volatile gasses in the biomass, as water, methanol, acetaldehyde, formaldehyde, formic and acetic acids, methane and hydrogen are emitted.
These products can be used as raw material in a chemical industry, but they are also flammable and can be used to get more heat and drive the pyrolysis process forward (the retort method).
- As the heat rises during he process (up to 400oC), gasses with higher boling point, as tars and phenols boil off from the biomass feedstock.
- When all the volatile parts of the biomass are emitted, the pure carbon lattice (and some non-volatile ash) is all that remains (400 - 500 oC).
- Adding a limited amount of oxygen to the hot charcoal leads to the formation of carbon monoxide (CO), the gasification phase. The CO can be used a a fuel (syngas, generator gas), but this consumes the newly created chrcoal.
At most, you can get about 40% (more common: 30-35%) of the biomass dry weight as charcoal.
The charring process works well even if the biomass is not absolutely dry, but the water consumes some of the carbon by forming gases, hydrogen, CO and CO2 , thus reducing the potential output of charcoal.
Therefore, it is good to dry the biomass as much as possible before charring.
Because of the difference in atomic weight of carbon (12), constituting roughly 95% of the charcoal, and carbon dioxide (12+16+16=44),
the weight of the catured charcoal should be multiplied with 3.5 ((44/12)*0.95=3.48), when counting how much CO2 is captured from the air.
Thus, if you make one tonne of charcoal and dig it down into the soil, you have removed about 3,5 tonnes of carbon dioxide from the atmosphere.
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