[Stoves] Fuel Testing

[AJH]

On Sat, 7 Oct 2006 20:04:27 -0700, frank at compostlab.com wrote:

> Also, the
>Hydrogen being so important to fuel is new concept to me. I was always looking
>at the carbon. 

The burning of wood is complex because it is not a single compound but
a mixture of many. For energy calculation purposes we can make many
simplifications that are "near enough" for most purposes. One of these
simplifications is to assume that the wood is modeled as Cx(H2O)y, as
the H2O is in the exhaust we can ignore its heats of reaction. We know
from energy conservation laws that the total energy remains the same
and we know the calorific value of the wood so we can assume that all
that calorific value is derived from the oxidation of the carbon
element less a bit to allow for the breaking of any carbon bonds in
the wood. So although hydrogen is a component of the fuel overall its
oxidation energy has no impact. In fact of course wood burns by first
breaking up into simpler components (pyrolysis) and then these
pyrolysis products being oxidised as a gas, so in the complex
reactions there will be a circumstance where the offgas flame is hot
enough tarry molecules to crack and for hydrogen atoms to exist before
they recombine to form steam.

NB I'm fairly confidence this does not happen in the pyrolysis zone of
the tlud stove as I only measure temperatures ~500C there.

Paul has raised an interesting question about where the fixed carbon
comes from, I'll see if I can provoke some further thought on this.

AJH