[Stoves] CO2 Max for biomass

[andrew]

On Tuesday 27 May 2008 19:25:29 Crispin Pemberton-Pigott wrote:
> Dear AJH
>
> Those are both figures too far from things quoted by others.

Great, you were asking for agreement; so I don't agree with you 
or "others" ;-) please cite some references.
>
> NO at low flame temps might be 20 to 30 ppm so you can ignore it.

And because "whole" biomass only reaches low temperatures I suggest 
you can generally ignore it unless it is a large proportion of the 
fuel.
>
> Carbon doesn't really exist in a pure form in any fuel.  For coal
> the CO2 max is about 19.4%, maybe 19.2.

This is why I posted a figure for C and not coal, coke or char. As 
much as anything else it was intended to lead readers to the first 
principles which I used for my figures.

The rules strike me as simple

1) a molecule of gas occupies the same volume as any other gas at the 
same temperature and pressure

2) if you are using air as the oxidant then as oxygen molecules (O2) 
form 21% of the atmosphere and nitrogen molecules (N2) the other 
79% , ignoring humidity and trace gases, then for every oxygen 
molecule you use you must give 3.76 nitrogen molecules a ride 
through the system, derived from ratio of nitrogen to oxygen 
molecules 0.79/0.21 

The highest concentration of CO2 that is achievable in a straight 
through combustion system using air as an oxidant is when one carbon 
atom combines with two oxygen atoms

C+O2=>CO2

But don't forget the nitrogen

C+O2+3.76N2=CO2+3.76N2

As these are the only components of the flue gas then the volumetric 
proportion of CO2  is given by

1/(1+3.76) = 21%

Which is hardly surprisingly the proportion of Oxygen in the 
atmosphere.

So this is the top bound of CO2 in the flue gas.

Do the same sum with a simple hydrocarbon methane (CH4) and you get 

CH4+2O2+2*3.76N2=>CO2+2H2O+7.52N2
fraction of CO2=1/(1+2+7.52)= 9%

I'll leave it to you to check my calculation on the idealised wood 
molecule I suggested.

BTW it also checks out quite well with Octane giving 12% which is 
around the 13% figures I've seen cited for car exhausts.
>
> 18.4% is cited from place to place but 'standards' vary. They are
> often found near lists of Siegert numbers.

I've never heard of this!
>
> It would be good to have a sample of the calculation and people
> could input their own fuel's analysis and get an accurate number.

I gave them to you but it would have been easier if you'd said where 
you were coming from!
>
> Ideas?

I've certainly no idea why I'm trying to teach chemistry, something 
I'm certainly not qualified in, when I can't get gainful employment 
in something I do know something about ;-)

AJH