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

Sun Jan 7 06:30:48 CST 2018

Dear Andrew

I want to see what Paul Medwell has to say, but you make my point about the burning of hydrogen: the absence of air does not means the oxygen is not available for reaction with N, H, S or C.

Hydrogen does not need 'a flame' to ignite below the auto-ignition point. It is highly reactive and the fuel bed is full of hot surfaces.

Hydrogen will ignite at room temperature in the presence of platinum powder.

Most interesting, I didn't know, compressed hydrogen at room temperature heats up when decompressed. This has been a busy week.

Regards
Crispin

On 7 January 2018 at 11:27, Crispin Pemberton-Pigott
<crispinpigott at outlook.com> wrote:
> Dear Andrew
>
> I have been looking into the combustion of hydrogen and I think the idea that hydrogen does not make water vapour in a small gasifier at all is not going to hold up. Let's keep looking.

There is no doubt steam  is made in a gasifier and in fact can be
further reduced back to hydrogen if the temperature goes high enough
but the oxygen comes from the air introduced.
>
> According to the hydrogen promoting guys the ignition energy is SO LOW just about anything will make it react. In other words the gas doesn't have to reach 570 C to auto-ignite‎. It will accept any input above 0.05 MJ.

Yes hydrogen (H2) burns over a wide range of concentrations but its
auto ignition point is quite high (over 500C) in the absence of a
spark.
>
> The TLUD experiments from Julien report temperatures above 600 and I suspect the thermocouples are all under-reporting.
>
> I see now that the two different approaches described by Hirendra will probably give different results: the ‎TLUD with a glowing bed through which the distilled gases immediately under it have to pass in one type, and the continuous updraft where the oxidation layer is always under the distillation zone is the other.  In the latter case the hydrogen will not pass through a 'fire' in any sense and if it remains under 570 it will not auto-ignite.

Well they are volatilised gases  but TLUD is a specialise case so
there is not an absence of oxygen until above the pyrolysis front and
even then only if primary air is restricted. If there is free oxygen
present then the hydrogen will ignite below its auto ignition
temperature if there is a flame or spark present.
>
> Some gasifiers are run up to 1100 C so I am not including them. Just the ones we have been calling 'pyrolysers' for some reason that is not nearly as clear now that we have discussed it more.

Yes as I said TLUD is a special case of gasifier and it's temperature
is relatively low at around 700C so a number of the reactions like the
producer gas and water gas reactions don't take place. The upside is
because only enough oxygen is supplied in the air to create enough
heat to pyrolyse the remaining fuel the resulting offgas is not much
diluted by combustion products or nitrogen.
>
> Back in the land of real and small gasifiers, the reactions in the little TLUD's take place in the presence of air so the availability of oxygen is not the limitation, right? At the moment, from what I have read, that if I fed hydrogen into the primary air supply of a TLUD it would all emerge in the gas stream above. If you try it, stand well back.

If you did that the flame would leap back down the air inlet with a
loud retort. Don't ask me how I surmise this as my ears are still
ringing from a similar incident with another gas mixture 20 years ago.

It has been an interesting exchange Crispin but my finger is becoming
worn out and I cannot type as fast as you.

Andrew

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