[Stoves] stoves and credits again

[Philip Lloyd]

My arithmetic may be dicey, but you have not identified any flaw. It was not
necessary for me to calculate the moisture content,  Think about it.
Philip


-----Original Message-----
From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of
Andrew Heggie
Sent: Wednesday, September 27, 2017 8:57 PM
To: Discussion of biomass cooking stoves
Subject: Re: [Stoves] stoves and credits again

On 25 September 2017 at 13:41, Paul Anderson <psanders at ilstu.edu> wrote:
> Philip
>
> I suspect something is not correct,

You may well be right but we have already been admonished by Tami for
attempting to compare apples with oranges.

My original wish when starting this thread was to understand how carbon
credits might be paid to a stoves user. So the fact that charcoal for
cooking is higher in the hierarchy of desirable fuels for cooking or it's
logistics aren't relevant.

Also when I asked if the stove user was also likely to be a grower I was
mostly asking about whether they were likely to cultivate a plot of land for
growing food, rather than growing fuel-wood, as then they also have the
ability to utilise the char produced as a soil amendment and  benefit from
that, both in returning the mineral element, possible soil structure element
but mostly to receive payment for it in excess of its foregone fuel value.

Plainly the discussion about the carbon content of the char is relevant but,
at this level energy efficiency is not, just the cost of the fuel and
cooking task or it's opportunity cost if fuel is not purchased.

Energy efficiency  is about getting stoves accepted by funding bodies or
part of marketing.


>
> 1.  In your example, the 0.395 kg wood contains 5930 kJ of total energy.
> 2.  You calculate that the char produced would contain3163 kJ.   That
would
> be 53.3% of the total energy is maintained in the char.
> 3.  Later you write that "the efficiency of char production would have 
> been ... 36.4%.


We all visualise things slightly differently but the overriding thing is
that energy is conserved, so there will be minor differences between a TLUD
producing char and a conventional stove  e.g. the char at the end of burn
will have it's potential chemical energy but also the product of its mass
times specific heat times its sensible heat whereas the conventional stove
will only have the mass of ash to consider in addition to the heat remaining
in the mass of the stove.

Given Philip's 100% efficient two stoves bring water to the boil from
15 degrees C The energy in the water as it reaches temperature is 1779Kj,
the energy released in the stove to achieve this (with no losses, all the
heat reaching the pot but only 30% being transferred into the water) is
5930kJ.

Philip assumes the energy in the wood to be 15MJ but doesn't mention that
the wood has an associated moisture content plus more hydrogen containing
compounds and the water from oxidation of these is lost.
The charcoal at the end is mostly charcoal with no moisture (it reabsorbs
some on exposure to atmosphere) and little  hydrogen compounds so there are
no losses due to the latent heat of water as as it is burned, i.e its Higher
Heating Values is nearly the same as its Lower Heating Value.

So continuing with Philip's numbers and making the assumption his wood has
17% moisture content on a wet basis and allowing 2.3MJ for the latent heat
of vaporisation of water that is then rejected at a temperature somewhat
higher than boiling. Also that we achieve 20% yield of charcoal  with a
calorific value of 30MJ ( probably slightly low as pure carbon would be
33MJ/kg) as a fraction of the oven dry weight of the wood input. Then with
heat losses between the two stoves the same and heat transfer to the pot
being 30% of heat released in the fire  558  grams of wood does the same
task and leaves 82 grams of char.

My question is a carbon credit available to the stove user to cover the
additional cost  of using 161 grams of extra wood fuel to boil 5 litres of
water?

You will see in my scenario the extra 41% of wood is smaller than Philip's
47% and without actually testing the energy contents of the char and the
wood I don't think we can reconcile them.


Andrew

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