[Stoves] Fuel Moisture and Stove Performance

[AJH]

On Fri, 27 Oct 2006 21:59:34 GMT, steve wrote:

>Is wood gas largely CO ?

Steve this depends on what you mean by wood gas. There are stages of
wood gasification and the first stage is drying, the second pyrolysis,
this leaves charcoal that is subsequently partially oxidised to CO.
The products of pyrolysis depend on temperature but initially, as long
as it is done in a retort or with a minimum of primary air, the wood
breaks down into tarry vapours which rapidly condense into a sol of
tarry droplets suspended in true gases. This is why it appears as a
white smoke, much as milk is a sol of fatty droplets in water but
appears white. The energy in this smoke is likely to be concentrated
in these tarry droplets. Once the secondary stage of gasifying the
char is reached then the temperatures ramp up and remaining tars get
cracked to H2 and CO as the char gasifies to CO2 which is then reduced
to CO in the air starved conditions, as long as the char bed is deep
enough. To do this more primary air is required than the small amount
that was necessary to provide the heat for pyrolysis, with this
additional air is nitrogen, so wood gas constitution varies depending
on what stage of the process you are looking at. In the tlud these are
separated into different phases but in a downdraught gasifier it's
dynamic and everything happens at the same time, here the tarry
products of pyrolysis pass through a hot oxidation then less hot
reduction stage where they are cracked, oxides and then reduced to
true gases, H2, CO and attendant N2.