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When using biomass for energy, two popular options are reactors and gasifiers. The first uses anaerobic micro organisms to convert the biomass to fertilizer and a methane rich gas mixture. The second uses heat to pyrolyze the biomass into ash and a carbon monoxide and hyrdogen rich gas mixture.

In terms of the overall potential energy produced by their resulting gas mixtures, is there an efficiency difference between the two?

For example, if you left one unit of wood in a digestor/reactor and let it sit until it was completely consumed by the system would the resulting amount of methane rich gas have more potential energy than if we ran that same unit of wood through a gasifier and captured the resulting amount of carbon monoxide and hyrdogen rich gas.

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  • I was able to find two different research articles on this here and here. Full text of the first one is available, but I couldn't find it for the other, so not quite enough info to put a complete answer together.
    – LShaver
    Dec 30 '19 at 17:38
  • I see you are aware the pyrolytiic method produces odorless ,highly toxic carbon monoxide . It is sort of playing with fire ;pun intended. Jan 31 '20 at 20:52
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Generally speaking, there is an efficiency difference related to the latent heat of the moisture of the feedstock. Moisture in the feedstock needs to be evaporated before thermal degradation occurs; however, biological degradation does not require the moisture to be evaporated. Because of the need to evaporate moisture gasifiers, in general, tend to be less efficient than anaerobic digesters.

There is research into hydrothermal gasification which overcomes this issue by gasifying materials underwater at supercritical temperatures.

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  • All other things being equal, I suspect that you're right. I'm just not sure that they are equal. Which helpfully begs an evolution of the question: In a gasification system what percentage of dry material is not convertible to gas and does it vary from wood to wood, and likewise, in an anaerobic digestion system what percentage of dry material is not convertible to gas and does it vary from wood to wood. I'll see what I can find. I suspect that little if any of the mass won't ultimately be converted to gas by the microbes. Oct 15 '20 at 13:42

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