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

Sat Jan 6 21:15:53 CST 2018

Dear andrew

Two disparate questions:

The term distillation
Heat of reactions

Q1. I am quoting verbatim one of the world's authorities on the subject of gasification of coal and biomass. He does  not refer to his own published works, he recommends the book cited earlier ‎for anyone who wants to know what happens in these processes. 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. I doubt this group will succeed at rewriting the gasification and distillation lexicon. 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. 

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? ‎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. 

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. 

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. ‎The ratio of gases in the output such as 35 or 45% CO2 indicates the type of reactions that are happening. 

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. 

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. 

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. 

Who is the guy from South Africa who made a charcoal gasifier for his Masters degree? He achieved the same gasification from above, not below. Quite interesting. 

Crispin 

‎
On 6 January 2018 at 21:39, Crispin Pemberton-Pigott
<crispinpigott at outlook.com> wrote:

>
> The evidence from the distillation of biomass to produce liquids and ‎gases shows that there is a conversion of biomass to CO2. Paul asks if the C is completely separated from the OH. It probably does now and then but it is not material. If it happens the heat is lost then regained, lost again and regained as the chemistry rapidly changes. What we do know is that in the end, there is only a net heat released from the biomass at 360C, and that the rest of the time the chemical changes leading to the gaseous products are energy-neutral. This of course is in the absence of air.

Crispin as you are the only one to use the term "distillation" in this
way I wonder if your gasifier expert is a native English speaker?

Too my mind distillation applies to the creation of a vapour and its
subsequent condensation, either in a simple condenser or a
fractionating column, to a purified liquid. In TLUDs we produce a
pyrolysis offgas that is a mixture of gases and vapours with maybe a n
aerosol of solid particles. This is then subsequently burned in a
secondary flame so no condensate is recovered.

We have known of the exothermic part of pyrolysis for years, Tom Reed
mentioned it 20 years ago, but does the heat at 360C amount to less or
more than that necessary to raise the dry wood from ambient to 360C?

Andrew

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