[Stoves] char information book by

[rongretlarson at comcast.net]

Crispin etal 

I guess you are correct - although my brief excursion into Wiki material leaves me somewhat confused on oxygen content. (As you said) hydrogen content in the fuels also makes a big difference. 
One wiki source on this is: 
http://www.engineeringtoolbox.com/adiabatic-flame-temperature-d_996.html 

I learned there are big difference when calculatng with air (numbers Rehder reported) vs oxygen as the oxidizer. Smaller differences when the value is measured at constant pressure or constant volume. 

I think a more important parameter for stove work is the heat of combustion. The kJ/kg value for methane (CH4) is more than twice that for methanol (CH3OH). The extra oxygen for methanol is doing nothing for cookstove fuel - as measured by weight. But methanol has the (slightly) higher AFT . 

I should have also mentioned that this book does a pretty good job of explaining why the AFT is never realized in practice in real hardware - showing in several examples where the energy (and temperature) is lost. Bigger systems can reach higher temperatures. 

Re computation, I don't think this is in the book at all - but my guess is that he knew how to do it. You can find free computer programs to calculate these, using Google. 

Ron 

----- Original Message -----
From: "Crispin Pemberton-Pigott" <crispinpigott at gmail.com> 
To: "Stoves" <stoves at lists.bioenergylists.org> 
Sent: Monday, August 13, 2012 7:45:29 PM 
Subject: Re: [Stoves] char information book by 

Dear Ron 

It sounds like a really good library addition. 

Re the top flame temperature, I am pretty sure that is from the hydrogen content and the fact that wood contains so much Oxygen. Air contains a lot of dilution (and cooling) Nitrogen. 

Again re the heat from wood v.s. Char, the Oxygen is also probably responsible. When putting in air, and then calculating the excess air, one should consider the O2 in all the fuel and the resulting combustion products because to get a true measure of them all the O2 must be considered. 

Think about the O2 buried chemically in rocket fuel. It burns with excess 'air' using no air at all. So when deciding on the amount of air to put into a wood fire, the O2 requirement is not worked out by the C and H2 content, it must also consider that wood has O2 in it (about 42%). 

By being able to supple less air and still get stoichiometric combustion, standard EA calculation gives an 'impossibly low' number and a very high flame temperature. so I am left wondering if the author did this calculation theoretically or not. It would be interesting to see how he handled the chemical balances. 

I bumped into this as an issue in Mongolian coal stoves when at high EA the sum of the O2 was higher than the % of O2 in air, and there was a fire going! The wet coal was apparently creating a water gas shift reaction. Henrick Wallman feels that was impossible but we got as much as 26% Oxygen in the stack. This led to the development of the unusual EA calculation formula in the HTP that tracks all the Oxygen. 

The difference with the char is that all the oxygen has to be supplied by the fans and there is 'dilution' by the Nitrogen in the air. This probably explains the lower temperature. One cannot add O2 without adding the unwanted N2. It is as if pure O2 was being supplied to the wood fire. A higher temperature results. 

Regards 
Crispin 
-----Original Message----- 
From: rongretlarson at comcast.net 
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Date: Tue, 14 Aug 2012 01:04:45 
To: Discussion of biomass<stoves at lists.bioenergylists.org>; biochar-policy<biochar-policy at yahoogroups.com> 
Reply-To: Discussion of biomass cooking stoves 
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Subject: [Stoves] char information book by 

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