[Stoves] Collaborative Stove Design Workshop

Thu May 22 11:14:48 CDT 2014

Dear Paul and Kevin

I would like to look at the same question with slightly different numbers:

Biomass 50% Carbon by dry mass.

Assume 15% moisture

1000 g fuel = 850 g dry mass

425 g carbon, 425 g everything else

Final mass of char at 85% carbon = 200 g

Mass of carbon in the char = 170 g

Mass of carbon burned = 425-170 = 255 g

Energy released (mostly burning the volatiles which are partly carbon, don't
forget, and some of the carbon that might have been char)

Available heat = 18,400,000 Joules/kg * 0.85 = 15,640,000 Joules

Heat remaining in the char = 200 g x 28,000,000/kg = 5,600,000 Joules

Heat released = 15,640,000 - 5,600,000 = 10,040,000 Joules

Note that no compensation as made for the fuel moisture. This is not
necessary if the only question you ask is how much heat remained, or how
much was released from the dry fuel. 

If you ask more, the fuel moisture heating and evaporation must be
considered:

Heat released as a fraction of the heat available in the original fuel:

10,040,000/15,640,000 = 64.2%

Heat available for cooking: at this point you have to consider the fuel
moisture:

150 g heated from 20 to 100 C = ΔT = 80

Latent heat of evaporation = 2257 J/g

(80 x 4.186 + 2257) x 150g = 388,782 Joules

Available heat = 10,040,000 - 388,782 = 9,651,218 JoulesNET

Available heat as a % of original heat available in the raw fuel: you have
to consider whether you want to know this answer with respect the net heat
available, or the maximum heat available.

Heat in the moist fuel, being 85% of the LHV of dry fuel = 15,640,000

Net heat available 15,251,218

Cooking heat available / Net heat available (the normal question)

9,651,218/15,251,218 = 63.3%

Energy available in the char as a % of the net heat available in the raw
fuel

5,600,000/15,251,218 = 36.7%

The energy consumption question: Of the energy released by the fuel
(assuming 100% combustion efficiency) was available to heat the pot?

9,651,218 / (15,251,218-5,600,000) = 100% showing that the numbers 'add up'.

The system efficiency is Net heat gained by the pot / net heat available in
the raw fuel.

Regards

Crispin

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of
Paul Anderson
Sent: Thursday, May 22, 2014 10:34 AM
To: Discussion of biomass cooking stoves; biochar at yahoogroups.com
Subject: Re: [Stoves] Collaborative Stove Design Workshop

Jock, Kevin and all,

Jock, please provide info/links etc about the Jerry Whitfield's new heater
that produces biochar [and please use a new Subject line].    Depending on
its price and capabilities, it might have a market for some affluent people.
Perhaps not enough to make it "profitable", but there is more to this
discussion and the needed actions than only about making profits.   [[
Similarly, the cookstoves-for-impoverished-people activities are not driven
only by profit motives  ---  although that is where the money seems to be
going. ]]   What has Whitfield offered?

Kevin, you wrote in your reply to Jock:    

#Given that the wood energy distribution is about 50% in the Char and 50% in
teh pyrolysis gases, a person wanting an efficient, effective and safe stove
would have to consume roughly twice as much fuel for the same stove effect.

one correction:   in biomass, about 70% of the heat energy comes out with
pyrolysis (30 % stays in char).   But about 50% of carbon atoms are in the
char and in the released (usually combusted) pyrolytic gases.    And about
20% of weight is in the char and 80% of weight is gone via the pyrolytic
gases and moisture content.   Could be plus or minus 5 to 10% in all cases
depending on how a device is operated, and it varies with some biomass.

I learned from Tom Reed and others that it is the hydrogen in
carbohydrates    (C+O+H)  that gives the energy difference.   Otherwise you
would be correct that combustion of half of the carbon would give half of
the energy.    The other interesting fact from this that impressed me is
that the Oxygen released during pyrolysis (from within the fuel, not from
the surrounding air) will readily "combust" with the Hydrogen to give H2O,
probably in the instant of pyrolysis that releases those atoms in a hot
environment.   There is some surplus H that comes out with the pyrolytic
gases, but how much is beyond my training and interests in chemistry.

Paul

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