[Stoves] MUST CHARCOAL BE A CAUSE FOR CONCERN?

[Crispin Pemberton-Pigott]

Dear Kevin

As always, the devil is in the detail. It shows that monomaniacal intent to
'save fuel' is not always the best solution to anything. As longer term
readers of this will have noted, little is said about increasing the supply
of fuel, rather it is about saving it (cooking a meal with less). If you
increase the fuel supply, you don't necessarily need to save it.  Getting
people to understand that fuel supply is not a problem everywhere, it is a
serious problem in some places. Combining wood gas stoves with charcoal
burning is a way of increasing the fuel supply as I will show below.

It would probably not be a stretch to say that stove development has
focussed on fuel efficiency above all else for the past 40 years. There are
very good reasons for having done so. Fuel supply is an obvious one. Having
to buy it instead of getting it free is another.

Charcoal is invariably mentioned in old documents and some new ones as
'wasteful' in its use of energy, even though a reasonable assessment of the
system efficiency would show that it compares reasonably with an open fire.
This fact of life is unpalatable to some. The reason I say that is my own
efforts to show what Rogerio said is true are not greeted with enthusiasm
because it opens new agendas which may not be well understood.

I want again to go over Rogerio's recent post:

>Making a simple calculation (see table below), I found the following
>numbers based on 1 kg of Eucalyptus grandis, with 4650 kcal/kg as
>fuelwood, or 7600 kcal/kg as charcoal.

>Considering the worst case scenarios, with woodstoves efficiency of 10
>to 15% one would get 450 to 698 kcal of energy into the pot, and to
>have the same amount of energy from charcoal into the pot, one would
>need to have either a relative medium charcoaling efficiency of 20%
>but with a charcoal stove with 30% efficiency, or charcoal stoves of
>normal efficiency around 20% but with higher charcoaling efficiencies
>of  30 or 40-%.

I would not take a worst case example to make the demonstration because it
is misleading. So is the best case example, though a best case could be a
development target for a project.

Open fires are about 15% thermally efficient, based on measurements of
thermal efficiency during heating. Please note that one cannot easily state
the thermal efficiency of 'simmering' because no work is done. If the pot is
as hot at the end as in the beginning of the simmer, the efficiency is
zero%. So let's talk about the net heat transfer efficiency. This can be
measured by heating water in a pot. It is not difficult. It can be done at a
low or high power. Usually the thermal efficiency on low power is higher
than on high power, but not always.

1 kg of Eucalyptus grandis could give 4650 kCal/kg (19.465 MJ/kg) but not
with normal moisture content at say 15%. In real terms it would be about
16.156 MJ/kg

Turning that 1 kg of 15% moisture Eucalyptus grandis into charcoal would
yield about 250 g in a reasonable low tech kiln and 33 g in a good one. The
heat content is perhaps 29.5 MJ/kg so it has 7.4 up to 9.7 MJ of heat
available, about 60% as much as the wood from which it came but in a very
different form. I am not including high yield modern kilns in this
comparison.

A 33% yield means that 9.7/166.156 = 60% of the original wood heat is
available from the charcoal. This is said in opposition to those who claim
that 3/4 (etc) of the heat is lost/wasted when making charcoal. There are
very poor operators of charcoal kiln operations and the cure is not ban
their trade, it is to help them improve their yield. We don't tell poor
farmers to stop farming because they are inefficient! We teach them.

>Based on that, shouldn't be logical to assume that under "primitive"
>existing  general conditions as seeing in the field today, that
>cooking with wood is more energy efficient than cooking with charcoal?

Let's have a good look:

16.2 MJ delivered at a 15% efficiency = 2.423 MJ delivered to the pot. What
you do with it, heating of boiling or simmering is up to you, but that is
how much heat will get there. 

The thermal efficiency of a charcoal stove varies a great deal because at a
high power it is low and at low power they are usually very efficient. A
traditional stove like the ones in Lusaka (see attached) are really poor
with a thermal efficiency of about 20% on high. They would deliver about 1.5
to 2.0 MJ to the pot from all the charcoal made from a kg of wood. However
on low power they are much better - about 40% if it is not windy. This
increase in efficiency changes things a lot. It is also the condition that
applies when cooking on low fuel. That would deliver 3.0 to 4.0 MJ to the
pot - much better than an open fire. So how much average cooking is done on
low power? Depends where you are.

As has been shown elsewhere, an open fire can also be operated well and the
efficiency doubled at the extreme. So a highly skilled and patient cook
might even get 4.8 MJ into a pot using wood. As you can see, this is a test
of the cook, not only of the fuel or the 'stove'. The advantage for the
charcoal stove is that it takes nearly no skill at all to get a high
efficiency at the low power stage. Not so with an open fire. Open fires are
extremely susceptible to wind.

Let's take improved stoves for both: a 35% efficient wood stove and a 55%
efficient charcoal stove. What then?

35% x 16.156 = 5.7 MJ delivered by the wood stove
If the charcoal was made at 25% yield, the improved charcoal stove delivers
4.0 MJ.
If there is more charcoal (33% yield) the same stove delivers 5.35 MJ to the
pot by the time the whole kg has disappeared.

So there is not much difference between good charcoal making, and good
stoves, is there? Not a lot. Operator variability will certainly cover the
small difference. If the wood stove cook leaves some of the wood burning
doing nothing nothing, say 5%, it is quite believable that the consumption
will be the same.

The point being made by the charcoal-making wood-gasifying stoves promoters
is that the wood gas can be used to cook and therefore the overall equation
is hard to beat: the wood gas stove cooking with a thermal efficiency of 40%
(quite possible) would deliver additional heat to pots. The equation looks
like this:

Original heat minus leftover charcoal heat = heat available to 'cook with
gas'.

That is, 16,156 - 7.38 MJ (assuming a 25% yield from the stove) = 8.78 MJ is
available to cook with. At a cooking efficiency of 40% (improved wood
gasifying stove) 8.78 x 0.4 = 3.51 MJ delivered to the pot while leaving the
charcoal behind.

If the charcoal thus produced were to be burned in a charcoal stove (this is
theoretical - I have never heard of someone doing this) the total heat into
the pots would be, best case:

8.78 x 40% gas stove efficiency + 7.38 x 55% charcoal stove efficiency =
7.57 MJ delivered to the pot for a system efficiency of 7.57/16.156 = 47%.

>Assuming that woodstoves can easily achieve 20% efficiency which
>delivers 900 kcal into the pot, and to do the same job with charcoal
delivers 3.52 MJ
>one would need a 30% energy efficiency charcoal stove using charcoal
>produced at 40% charcoaling process,  what is nearly impossible to
>achieve.

Well, not quite impossible. Charcoal stoves can easily be more than 40%
efficient and 35% charcoal production is not difficult at all. Charcoal
stoves designed to run at low power are more than 60% efficient. The reason
for this is that charcoal burning is inherently more efficient than wood
burning. The reason is that the Hydrogen content of the charcoal is very
low. The air demand for wood is higher per MJ than for high carbon fuels.
That is why coal is inherently more efficient than biomass. High energy coal
needs even less.  Less air is required when there is less H2 so the cooling
effect is reduced. For any given quantity of excess air, carbon fires
outperform Hydrogen fires.

For this reason charcoal is always more efficient than wood, just as diesel
engines are always more efficient that gasoline ones (because the higher the
compression ratio, the higher the efficiency).

>Unless charcoal stoves of 40% are possible, so to use with charcoaling
>kilns of high 30% efficiency?

If a charcoal stove is 45% efficient on High and 60% efficient on Low
(average 52.5%), combined with charcoal made at 35% efficiency, the
delivered heat is 5.42 MJ (33.6%). That is about twice as good as an open
fire. It is about the same as a good, improved wood stove. The gas-charcoal
combination would be 40% better (47% total).

One problem is that charcoal produce stove promoters have gone strarry-eyed
over producing char and burying it. In my view there are two camps calling
for this: those promoting agriculture (with spotty, in encouraging results)
and those with a prospect for plunder (carbon credits paying for
unaffordable stoves). The math then looks very different. Taking the char
and burying it and to do the same job with charcoal delivers 3.52 MJ
(assuming a really good gasifier at 40% efficiency). That is 22% - not as
good as any rubbishy improved stove or even a well-run open fire. It means
marginal feedstock savings, plus the inconvenience of managing the fuel,
ignition, and char.

You can see why people are confused about the subject.

Regards
Crispin

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