Re: Pyrolisis and pressure Questions.

[Reedtb2@cs.com]

Dear Dan and All:

I may be the only real "chemist" (actually a physical chemist) on these 
lists, having practiced and written chemistry at Union Carbide, MIT, NREL and 
CSM.

Your question of why pressure helps carbon yield is an excellent one and has 
a simple answer (but I hope Mike Antal reads this and will comment further).

Most organic compounds melt, then boil as they are heated, so a list of 
melting points and boiling points helps to characterize simple compounds.  
Some compounds, such as naphthalene and camphor (and CO2) have a "boiling 
point" below the melting point, so they "sublime" (like mothballs= 
naphthalene) and only melt under pressure.

 Unfortunately, biomass is a mixture of three polymers:  Cellulose, 
hemicellulose and lignin (with minor variations of each), so does not have a 
specific melting and boiling point and yet the analogy is useful.  

Prof. Jaques Lede first discovered that wood does indeed have a "melting 
point" of about 480 C under very high pressure. 

During slow pyrolysis wood "sublimes" and emits the monomers, oligomers 
(larger chunks) and fragments of C,H,L for all to see if they have a mass 
spectrograph or our book on biomass tars.  Some of these fragments then 
decompose further to charcoal on their way out of the biomass matrix giving a 
yield of charcoal typically 20to 30%.  

At a pressure of 1 atmosphere this pyrolytic breakdown of biomass occurs at 
250 to 450 C.  In a vacuum the gases leave the matrix more readily at lower 
temperatures and therefore charcoal yield is reduced.  At a pressure of 10 
atmospheres, the gases leave more slowly and at a higher temperature, thus 
having more time to undergo the charcoal decomposition.  It is my belief that 
this is the primary basis of Antal's patent.

Comments?

TOM REED        BEF


In a message dated 1/4/02 5:53:02 AM Pacific Standard Time, 
Carefreeland@aol.com writes:

<<  Questions for Chemists, 
     What is the change in the physical reaction of pyrolisis which favors 
 higher percentage carbon retention in the biomass/char at higher pressures? 
      Is there any other way to achieve this action at lower pressures? 
      How does it change the way O, N, H&C bonds break or form?  
     What bonds are favored? 
     What happens when H2O is added? 
      What role does temperature play relative to pressure?  
     Please explain this in detail.
       I have taken first semester chemistry, but some on these lists may 
have 
 not.  This seems to be a critical area of combustion chemistry. 
       Thank you in advance, 
                                        Daniel Dimiduk 
  >>

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