[Stoves] stoves and credits again

[Andrew Heggie]

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