Re: BioC: Re: Pyrolysis vs. Gasification

[VHarris001@aol.com]

In a message dated 01/02/2001 1:10:28 PM Eastern Standard Time, 
antonio.hilst@merconet.com.br writes:

> Dear Harry and Harris,
>  Perhaps you both cold elaborate on the gasification/pyrolisis using a "
> severity"
>  parameter such as C/O ratio, temperature, etc.
>  Anonio Hilst

Hi Antonio,

I don't have any formal training in this area so I'm unable to answer your 
question authoritatively.  I have one text which lists five main reactions 
occurring in small-scale gas producers.  They are:

Combustion (C + O2 => CO2 + 393,800 kJ/kg mole)
Water gas (C + H2O => CO + H2 - 131,400 kJ/kg mole)
Water shift reaction (CO + H2O => CO2 + H2 + 41,200 kJ/kg mole)
Boudouard reaction (C + CO2 => 2 CO - 172,600 kJ/kg mole)
Methane reaction (C + 2 H2 => CH4 + 75,000 kJ/kg mole)

The text further states that equilibrium occurs around 700C.

My impression on first reading this was that these reactions could be 
maintained in small-scale, air-suction type gasifiers.  However, other 
reading has lead me to believe the water-gas and Boudouard reactions are too 
endothermic to be sustained in such a system, although I don't off-hand 
remember exactly why (it MIGHT be due to the extra energy requirements of the 
latent heat of vaporization of the constituent parts - but don't quote me).

It seems to me as an outside observer that the actual processes occurring in 
small-scale gas producers has been and continues to be the basis of an 
ongoing disagreement.  I find myself more in agreement with the reasoning of 
the simple starved-air combustion explanation.

It seems a reasonable explanation to me that as the superficial velocity 
increases, the combustion process approaches stoichiometric combustion.  This 
dramatically increases the heat in the combustion zone, which in turn is able 
to crack a greater percentage of the tars prior to them being swept away.  So 
my impression is that when certain configurations of WW2 gas generators 
proved to be successful, they thought the biomass was being gasified (as 
elaborately described above), when in reality it was simply so hot that the 
heat of combustion liberated substantially more permanent VOC gases (and 
correspondingly reducing tar generation).

Of course, I could be wrong.  And if so, I'd like to find out what actually 
IS happening in this type of "gas generator."

Hope to hear from others who are willing to weigh in on this subject.

Best wishes,
Vernon Harris
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