[Stoves] Pyrolysis: No Air?

[Paul Anderson]

Dean,

Any solid biomass that is burning is undergoing the same various 
chemical and physical processes.   But in gasifiers the processes are 
somewhat separated (make gases and burn them later in a different 
location, even if the later is only a few milli-seconds and the location 
is only a few centimeters away).   In contrast, the fires in Rocket 
stove and bonfires, etc, have the processes occurring virtually 
simultaneously and intermingled, some at essentially the same location 
(or with mixing).

In a standard fire (Rocket, etc.) that the relatively cool secondary air 
(needed to burn the gases) is entering at or close to the fuel, 
resulting in some cooling of fuel that needs to be hot to create the 
gases.   This is a fundamental difference that allows gasifiers to be 
burning gases separately from where the gases are being created, with 
benefits of cleaner combustion (if done properly).

In somewhat simplistic terms, carbonization and pyrolysis are two sides 
of the same coin (or process).  Carbonization creates char (focus is on 
the resultant char) and Pyrolysis creates gases (focus is on the 
gases).   Both relate to "burning".

Remember also that the char (once it is created) is hot and IF exposed 
to oxygen will itself "char-oxidize" or "char-gasify" (into CO or CO2) 
and therefore will "disappear" in a regular fire.   But in a TLUD the 
char just sits there, being hot but without any O2 (unless fans, etc 
bring in sufficient air with O2 to reach the surface of the char.)

Terminology:  Pyrolysis is by HEAT.   The heat can be with FIRE present 
or not (retort).  There is oxic pyrolysis (oxygen present / coming in) 
and anoxic pyrolysis (no oxygen from outside the biomass).

Paul

Doc  /  Dr TLUD  /  Prof. Paul S. Anderson, PhD
Email:  psanders at ilstu.edu
Skype: paultlud      Phone: +1-309-452-7072
Website:  www.drtlud.com

On 5/16/2015 10:34 PM, kgharris wrote:
> Dean,
> Thank you for bringing this up. I have never really thought about 
> there being a difference between pyrolysis and gasification.  The 
> definition of gasification in Wikipedia, "This is achieved by reacting 
> the material at high temperatures (>700 °C), without combustion, with 
> a controlled amount of oxygenand/or steam", is different from the 
> definition of pyrolysis which is without additional oxygen.  Your 
> statement seems correct according to this.
> As I read further in Wikipedia, there is a section on chemical 
> reactions.  Here there are 5 different processes which the fuel 
> undergoes, pyrolysis being one of those.  This would lead me to 
> believe that pyrolysis is part of the more general process of 
> gasification.  Rather than it being one or the other, a TLUD would 
> include both.  The small amount of added air supports the combustion 
> of a small amount of the gas to produce heat, but that air is soon 
> used up and most of the fuel pyrolyzes without added air.  The 
> combination, including pyrolysis, would be gasification.
> I will look further to see if Christa Roth adresses this question in 
> her Micro-gasification manual.
> Kirk
>
>     ----- Original Message -----
>     *From:* Dean Still <mailto:deankstill at gmail.com>
>     *To:* Discussion of biomass cooking stoves
>     <mailto:stoves at lists.bioenergylists.org>
>     *Sent:* Saturday, May 16, 2015 3:50 PM
>     *Subject:* [Stoves] Pyrolysis: No Air?
>
>     1.
>         Hi All,
>
>     When I look up the word pyrolysis I find the following:
>
>     1.
>         *Pyrolysis*is a thermochemical decomposition of organic
>         material at elevated temperatures in the absence of oxygen (or
>         any halogen). It involves the simultaneous change of chemical
>         composition and physical phase, and is irreversible. The word
>         is coined from the Greek-derived elements pyro "fire" and
>         lysis "separating".
>     2.
>         However, I think that folks use it to describe what happens in
>         a TLUD, etc? Isn't that gasification not pyrolysis because of
>         the presence of some air?
>
>     Best,
>
>     Dean
>
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