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

[Andrew Heggie]

On 7 January 2018 at 03:15, Crispin Pemberton-Pigott
<crispinpigott at outlook.com> wrote:

 He has never heard the term 'migrating pyrolysis front' and found it
inexact and not useful. Perhaps that term is known to a circle of
Internet friends.

Which is exactly the point, you applied a the term, up till then only
coined (rightly or wrongly) by users of the TLUD and biochar
fraternity and applied it to other technical devices.



> I doubt this group will succeed at rewriting the gasification and distillation lexicon.

No one is changing any lexicon other than you, you have taken a word
from the term "destructive distillation" an archaic pyrolysis
description and applied it out of context. The distillation refers to
a refining process that happens when distillates are recovered from co
products or pyrolysis. Alex has posted the term "de-volatilization"
which means simply the removal of volatile material, if this volatile
material is condensed then distillation has occurred.

> The useful terms are drying, distillation, and oxidation zones, and emphasizes that what happens in there is very complex. By controlling the process different products can be optimised from the raw material. Which makes sense.

Well actually the terms posted by Alex look more sensible to me.

Whilst oxidation zones exist in a gasifier it is unlikely this happens
in a retort where there is a complete absence of air where the terms
Alex's table shows are more descriptive: Drying-> Depolymerisation-)
Torrefaction->Depolermerisation->Dissociation/carbonistaion


>
> Paul looked up the word distillation and found there was dry distillation and I suppose, wet. ‎I don't see how you can separate them. If I use fractional distillation I might get gases and condensed liquids. How is that different?
>
> If I distill a liquid from biomass and upon further heating it transforms irreversibly into gases, how is that not distillation?

because you have taken the vapour, condensed it to liquid then
reheated it and dissociated it into different compounds.


> ‎It is actually a sensible use of the word. The fact that it is also commonly used to refer to condensing a boiled liquid doesn't rule out other activities. Making wood alcohol is not 'boiling wood' and there is no alcohol in wood, only the ingredients of it. The alcohol is formed during the distillation of biomass, exactly as he describes.

Well I disagree unless it is used in the  archaic term "destructive
distillation"

Yes the alcohol vapour is formed in the depolermerisation phase and
could be condensed when it would be a distilled liquid. Incidentally I
think you only get wood alcohol from broadleaves.
>
> Q2.
> I will continue to look into the energy of reactions because I ‎find this sort of thing interesting. Prof Lloyd already said here that it is possible to get thermal runaway from the 360 degree reaction alone. My interpretation is that the fuel, or at least some of it, should be above 320 first. Frank supports the general idea. Heating internally can happen in a number of ways with wood pellets, creating CO as we discussed. Prof Philip Hopke has proposed a solution to that auto-heating and breakdown leading to the formation of dangerous levels, even fatal, of CO in wood pellet storage chambers.

I don't disagree with any of that, only the implication that oxygen in
the wood is available to react with the hydrogen in the wood.
>
> If there is thermal runaway starting at 360 degrees, we can presume it will drive nearby sub360 fuel to 360+. This would easily lead to the formation of char exothermically if it is insulated, as in a large pile, anaerobically. ‎

Yes we know that if you heated wood uniformly to ~330C and there were
no heat losses the pyrolysis would continue and the temperature would
rise until a point where other endothermic reactions limit the rise.
>
> Once we have the net heat released at 360 C per kg we will be able to calculate the moisture level below which this runaway could happen.

By 330C the moisture in the wood has long been driven off but if you
mean how much initial water could be vaporised  (and heat to raise the
wood initially from ambient temperature) I don't know but I'm sure it
isn't enough.
>
> Regarding Tom's point about exothermic reactions, everything he was discussing was related to gasification in the presence of air, not so? He even provided is some instructions about the superficial air velocity needed to make or consume char on the first pass of the oxidation zone.

This is non sequitur, we are discussing in the absence of air. Once
any air is present we release plenty enough heat to drive the
pyrolysis reaction  and subsequent combustion of the products.

>
> I remind readers that John Davies' gasifier used two passes, one char-making, one char-consuming. Technically the superficial velocity of the air was too low to burn the fuel completely on the first pass, but oxidised in completely on the second in a sub-stoichiometric environment. I have no doubt there were high temperature tars remaining in the coke that were distilled on the second pass, then broken down to H2, CO and perhaps CH4.

Andrew