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I'm no chemist, but much of my work revolves around biochar. I've always understood it with the following rough approximation:

$$ \ce{CH2O (dry plant matter) + heat -> black carbon + H2 + CO + CH4 + hydrocarbons} $$

I've got two questions:

  1. Is the above a decent approximation? In what ways is it misleading (as all approximations always are)?

  2. A colleague recently told me that methane is only formed in this reaction when there is water vapor present -- he said that it mixes with CO at high temperature to form the methane. Is this true? Will one not get any methane from oven-dried biomass that is exposed to heat in the absence of oxygen?

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The presumption here is that the biomass is purely sugars: glucose- and xylose-based starches and cellulose. The essential components of lignin, which make up about 20% of the biomass, is phenols and propane. You may need to rebalance based on those additional components.

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    $\begingroup$ Thanks for the answer! But could you take it down a notch? What about that equation is misleading in terms of resultant products? Under what conditions will methane form? $\endgroup$ – generic_user Jun 17 '13 at 10:32
  • $\begingroup$ The amount of methane you get will depend on moisture, temperature, the original biomass, and other contaminants. There are also some other complications: the stuff left in the bottom of your fireplace consists largely of potassium hydroxide (KOH). 'HCs' might include the phenols and propane, but might also simply be bits of partially combusted product or reactions with nitrogen to create NOx. If carbon starts out bound to 2H and 1O, the extra hydrogen has to come from somewhere. Water would work, but it might also come from H2 or ammonia. $\endgroup$ – Meredith Poor Jun 18 '13 at 20:14
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The equation above is a decent approximation of one group of reactions. Another one given here:

$$ \ce{C_{c}H_{h}O_{o} -> z CO + x H2 + w H2O + v C_{n}H_{m}O_{p} + u C_{(s)}} $$

This is only the pyrolysis part of the process. There's a host of reduction reactions, I only give the ones that produces methane:

$$ \ce{CO + 3 H2 <-> CH4 + H2O} \\ \ce{C_{(s)} + 2 H2 <-> CH4} $$

As you can see, methane is formed when there's hydrogen in the gas phase. Besides the pyrolysis there's two reactions that generate free hydrogen:

$$ \ce{CO + H2O -> CO2 + H2} \\ \ce{C_{(s)} + H2O <-> CH4} $$

So indeed, you need water vapor to generate the hydrogen for your methane. If I recall correctly, methane is only a minor component in pyrolysis gas most of the time. By caloric value, the biggest component is either carbon monoxide or tar (long, complex hydrocarbons).

Last but not least you have a host of oxisation reactions (both in the gas phase and on the solid stuff) that power the pyrolysis - I won't go into them here.

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  • $\begingroup$ my Latexs rusty! how do I get proper equation arrows? $\endgroup$ – mart Nov 23 '13 at 9:02
  • $\begingroup$ See here for information on how to use mhchem to typeset chemical formulas and the like. $\endgroup$ – pentavalentcarbon Jan 27 '18 at 19:01

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