[Gasification] gas composition

[John Bertl]

On Friday 06 April 2007 11:59 am, Louis Peltier wrote:

>could some one tell me what the gas composition is
>when wood is heated in a CLOSED  container to make charcoal.
>(no air is allowed in the closed container)

The gas composition you are asking for depends on the speed
of temperature rise (speed of pryralysis), the temperature
experience by the wood, the time duration of the gas at
elevated temperatures and if the gas can escape your
closed container to be cooled.

The possible gases produced are:

H2O               =
CO2               =   S
RCOOH           =   L
CO                 =   O     =
RCHO              =  W     =
CH2CO            =          =
ROH                            =   F
CH4                             =  A
C2H6                           =   S
H2                              =    T
C2H4                           =
C2H2                           =
free radicals:
CH, CH2, CH3, CHO    2 to 10
C2

Slow pyrolysis yields charcoal and oxygenated gases
and vapors of low flammability and releases energy.

Fast pyrolysis yields little of no carbon, forms
hydrogenated gases and vapors and consumes
energy

Pyrolysis of wood follows the kinetics of a first
order reaction. It is diffusion controlled rather
than rate controlled, the rate being determined
by the rate of energy transfer within the solid
rather than by the rate of pyrolysis.

Breakdown of wood components, hemicellulose,
cellulose and lignin, is not simultaneous.  The
hemicellulose, particularly its pentosans, are said
to decompose first largely between 200 and
260C followed by the cellulose at 240 to 350 C
and finally by the lignin at 280 to 500 C.

Hemicellulose evolves more gases, less tar, and
about as much aqueous distillate as are formed
from cellulose, but differs from cellulose in that
hemicellulose yields no levolucosan.  Much of the
acetic acid formed is attributed to the hemicellulose.
Scission of a carbon to oxygen bond in a
pentose leads to further splitting to acetic acid,
formaldhyde, carbon monoxide and hydrogen.

Cellulose evolves water in the first stage of
thermal decomposition before any other significant
changes are observable.

The pyrolytic products first formed from hemicellulose,
cellulose and lignin promptly undergo
further reactions, not all of which are pyrolytic.
Polymerizations and condensations reactions to
form more comples molecules such as high boiling tars,
waxes and resinous substances with perhaps
phenolformaldehyde type linkages are also involved.

Hope this helps

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