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

[Crispin Pemberton-Pigott]

Dear Paul

>1.  Yes, there is O (oxygen) in carbohydrates.   But those atoms will not become available during drying or even during torrification.

The volatiles released during torrefaction are hydrocarbons and organic molecules that contain oxygen. I agree that the pyrolysis temperature must be reached to get a self-sustaining reaction. I can’t comment on the amount of O available and when. Torrefied pellets are brown from what – caramelization of sugars? Maybe Tom Miles can comment.

>2.  Inside the retort, even with some ambient O2 present between the pieces of fuel at the start, there is no combustion (chemical combinations releasing heat from burning of O2) if there is no spark or spontaneous combustion.

As soon as the fuel reaches the auto-ignition temperature, there is combustion. There is no need for a spark or flame to start. Long before that, the fuel start decomposing, releasing O, H and N as gases. In theory there could be a *poof* but the flame speed of the wet gas is very slow and you might not notice.

>And I think that if such a spark (or flame) came into an enclosed retort chamber with a mixture of oxygen and combustible gases, that there could be some risk of explosion.

The oxidation takes place within the fuel. There could only be an explosion if there were some free oxygen present in the freed gases.

For those who want to look further into the chemistry of this process, there is a calculation made in the HTP analysis spreadsheet that calculates the air demand of the fuel based on the elemental composition of the fuel and the char remaining at the end. From an assumed need for all oxygen to come from air, the actual requirement for a TLUD making a lot of char is only 20% of the theoretical need for the whole fuel, because 80% is provided from the biomass itself.

EA+1 = λ

where λ is the total air demand. This calculation is incorrect. The real values for a TLUD are

EA+0.2 = λ

You only need to provide 1/6th of the theoretical oxygen requirement of the fuel, provided it is biomass and you want to make char.

Do you remember list member Yuri in Russia talking about explosions in burning charcoal piles cause by dousing it with water? A reaction creates hydrogen and oxygen from the water. Those conditions don’t exist in the Adam Retort where there is no free O2 to speak of.

Regards
Crispin

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