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

Sun Jan 7 17:29:36 CST 2018

Since we’re digging out textbooks I go back to my text from graduate school, “The Chemical Technology of Wood” by Hermann Wenzl, translated from German in 1970. Wenzl credits his mentor, Carl Gustav Schwalbe (1971-1938), who established the Chair for Cellulose Chemistry at the Technical University in Darmstadt in 1906 and who headed Wood Research at the Forestry School in Eberswalde until he retired in 1933.  After describing acid hydrolysis Wenzl describes pyrolysis as “Further Destructive Processing of Wood” along with other processes in which the fiber structure of wood is completely destroyed. At the time (1938) pyrolysis was also known as carbonization or dry distillation. He included pressure oxidation and “hydrogenolysis” as destructive processes.  

Hydrogen formed in a downdraft gasifier is probably consumed in the oxidation that is driving the gasification by heating the biomass. A typical downdraft gasifier will make producer gas with ~20% H2 and ~20% CO. Increase the air and you get 10% H2 and 20% CO. Are we making less H2 or consuming more of it? 

Tom Reed demonstrated his “flaming pyrolysis” in a gold plated transparent gasifier in his lab in 1982. At the time he was developing an oxygen blown down draft gasifier to make synthesis gas. The visible migration of the flame front was heating wood pellets and generating pyrolysis gases as it advanced. In that case suction drew the gases down through the unreacted fuel. “Stratified Downdraft” and “Top Lit Updraft” came later. 

Tom

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Gordon West
Sent: Sunday, January 07, 2018 6:54 AM
To: Discussion of biomass cooking stoves <stoves at lists.bioenergylists.org>
Subject: Re: [Stoves] Distillation and oxidation Re: Understanding TLUDs, MPF and more. (was Re: Bangladesh TLUD )

I’m only weighing in here because I think there is value in agreement on the definition of terms. There can’t be any useful discussion if words have different meanings for different people.

Here are my basic functional understandings of the important words:

Distillation is a physical process of separating the components or substances from a liquid mixture by selective boiling and condensation. 

Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures in the absence of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible. 

TLUDs use pyrolysis in the following way:

- Starter heat is applied to a column of biomass through an external means (lighter fluid and a match, for instance).

- The heat “boils” the more volatile components of the biomass into a mixed gas.

- Heat to drive pyrolysis is generated by burning (oxidizing) a portion of the vaporized volatile biomass. 

- The oxygen supply for oxidation is limited and the condition of absence of oxygen results from the consumption of all of the limited introduced oxygen.

- The temperature remains low to vaporize the carbon in the biomass.

- The remaining vaporized volatiles rise out of the char as syngas, which is processed separately and used directly for combustion fuel or is condensed into liquids, which may or not be distilled.

As designers and fabricators of biochar+energy devices, we are interested in how to optimize the process for the best system performance, which has a lifecycle triple-bottom-line context stretching from acquisition of biomass to the highest and best use of the char and syngas. While our focus is not on ultra cheap and simple cooking devices, I believe that the same interest applies to them, too. I follow this list looking for any discussion leading to better optimization strategies serving practical purposes.

"A mathematician and an engineer are sitting at a table drinking when a very beautiful woman walks in and sits down at the bar.

The mathematician sighs. "I'd like to talk to her, but first I have to cover half the distance between where we are and where she is, then half of the distance that remains, then half of that distance, and so on. The series is infinite. There'll always be some finite distance between us."

The engineer gets up and starts walking. "Ah, well, I figure I can get close enough for all practical purposes."

Gordon West

The Trollworks

On Jan 7, 2018, at 3:21 AM, Andrew Heggie <aj.heggie at gmail.com <mailto:aj.heggie at gmail.com> > wrote:

On 7 January 2018 at 02:31, alex english <aenglish444 at gmail.com <mailto:aenglish444 at gmail.com> > wrote:

Destructive distillation of wood was the term back in the 1930s when my
grandfather wrote a high school text book.
Attached;
is a slide on pyrolysis stages from our national labs senior scientist
responsible for bioenergy and the tar sands pyrolysis.
and two images of google word use search between 1800-2000 for pyrolysis and
destructive distillation.

Good graphs of the change in terminology from destructive distillation
toward pyrolysis. I knew the term but considered it more to do with
the making of simple organic compounds from wood before organic
chemistry really took off with the petrochemical industry.

The wood gas distillation produced acetic acid and methanol amongst
many other chemicals. I have posted in the past about an acetic acid
plant in Germany which pyrolysed beech logs, distilled vinegar and
used the other gasous products in a spark ignition engine to power the
plant. charcoal was a less valuable by product.

There have been a flurry of offlist e-mails which I have replied to
without realising they will not be seen on stoves.

Anyway your slide clearly shows agreement with what Tom Reed said all
those years ago, there is an exothermic stage and here the range is
given a bit wider as 280-500C but the salient point is that the oxygen
bearing compounds given off do not include water beyond the drying
phase. So in the final stages the  gases CO and H2 are favoured as I
have said earlier.

Whilst acknowledging there may be a means for disociating all the
elements in wood and recombining the oxygen with the hydrogen it does
not happen in simple stoves or gasification.

Andrew

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