[Stoves] Understanding TLUDs, MPF and more. (was Re: Bangladesh TLUD )

Crispin Pemberton-Pigott crispinpigott at outlook.com
Tue Dec 12 19:27:00 MST 2017


Dear Andrew and All

Now that everyone is or is not a chemist, it is time to end the fun.

This is the major point made by you and Paul.

>My point remaining that to release heat from wood you need to add oxygen and the fact the wood already has oxygen containing molecules within it is irrelevant.

I am suggesting that this is incorrect. Whether one can do it in tin cans on a small scale is a different matter.

There are two possible processes that can take place without oxygen (heating fuel without the addition of any air at all). They are

Auto-catalysis
Auto-pyrolysis

The first is what I described using a chemically balanced equation to demonstrate that there is enough oxygen in biomass to combust 93.6% of the hydrogen. This is true, but auto-catalysis (the reformation of all the bonds to provide a re-ordering in a new form to provide H2O and CO2 as the only outputs) does not take place in biomass. It is true that in a fire this could happen, and does, but it is not true that it can happen without a fire, even though all the elements are present to do so.

Correction:
The calculated energy available was using Tom Reed’s formula is for the energy released upon the complete combustion of the fuel. I pointed to the 93.6% of hydrogen combustion which is not the same as 100% combustion. So the actual energy available for the re-ordering of the chemistry of biomass without adding air is 0.5 MJ less than for the full oxidation of the hydrogen. The heat available using only the internal O is 1.7 MJ/kg for dry biomass. That is theoretical and based on the chemical mass balance that would result from auto-catalysis of the elements. As I said, auto-catalysis does not happen below 1000 degrees. It is very likely to happen above 2700 C which would require a reactor beyond our common materials.

On the matter of auto-pyrolysis of biomass, this does happen in the complete absence of air, and this is the key point of the conversation.

Here is a plot of the energy released by the heating of biomass in an inert environment:

[cid:image003.jpg at 01D373EC.09E0A630]
Please note the spike in heat released at 360-400 C. That is the cellulose (etc) breaking down. There is a net release of energy. The charts come from here<http://onlinelibrary.wiley.com/doi/10.1002/pol.1968.150061202/full>. (H/T Philip Lloyd)

“An application of TGA technique to elucidate the chain reaction mechanism of cellulose pyrolysis is discussed.”

The reference to a chain reaction is the way they describe the self-sustaining, auto-pyrolysis of cellulose. There is an investment of energy at 320 C and strong release of energy at 360. The TGA charts above show what happens with other fuel components.

Once it starts, it can continue until there is no raw biomass left to react, provided it is not cooled at a rate greater than the net gain in energy. In an ideal container there would be thermal runaway: the rate of heat release would increase. This reaction (shown above) takes place in the absence of air.

This second phenomenon is not dependent on the chemical balance (which while theoretically possible, does not happen in biomass at a realisable temperature). Auto-pyrolysis actually happens. Additional studies investigating the energy needed/released are here<http://onlinelibrary.wiley.com/doi/10.1002/app.1970.070140518/full> and here<http://onlinelibrary.wiley.com/doi/10.1002/app.1989.070371203/full>.

If readers can accept the results of these experiments (showing the auto-pyrolysis taking place and validation of the theoretical exothermic reactions proven) there are implications for the stove makers. One is that as biomass can pyrolyse without any air at all (as demonstrated) we can drop the claim that it doesn’t.

>…if I am wrong I admit it, I am also not a chemist nor ever claimed to be.

I am wrong several times a day. I don’t see why you shouldn’t be affected by those same experiences. We have on this list a chemist in the shape and likeness of Prof Philip Lloyd, I believe the past president of the South African Institute of Chemical Engineers (I hope I have the name correct).

Regards
Crispin
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