Dear Crispin,
I appreciate and accept all your arguments.
But I have a few points to make.
Your message and the one from Otto both make me reach for the calculator
because some things are not adding up.
Let's put some numbers on it:
Suppose branches of trees (as one suggested) have an energy content of 16
MJ/kg considering their moisture content. About 40% of that mass is
Carbon.
The proposal is that 25% of the final mass will be char. OK.
The heat content of that 25% will be about 29.5 MJ/kg. So taking 1.0
initial
kg of fuel at 16 MJ will give 29.5/4 = 7.375 MJ of energy not used (in the
form of char at the end).
That leaves 16-7.375 = 8.625 MJ of heat yielded from the fuel. That is 54%
of the energy that used to be yielded.
Before, all 16 MJ was used and the efficiency was perhaps 15% in an open
fire, 30% in an improved stove.
To accomplish the same cooking with only 54% of the heat, the thermal
efficiency would have to be 15/0.54 to 30/0.54 which translates to 28% and
56% respectively.
Are these numbers realistic?
Also, only the pyrolysis gases ( which contain around 2/3rd of the total
energy content of the fuel ) are used for cooking.
This 2/3 number is confusing me. 2/3 of 16 MJ is 10.66. I can't see that
amount of heat coming out of the fuel and still having 25% char.
Biomass is about 50% carbon when it is bone dry. As the moisture is
considered at realistic levels (15-20%) it reduces of course. The problem
for the biochar producer is that all the moisture has to be dealt with
during the pyrolysis burn, leaving dry char at the end. That is just a
fact
of life.
The heat lost getting rid of the moisture is subtracted from the pyrolysis
gas heat, or there will be no gas heat.
If one includes in the pyrolysis gases only the non-carbon energy content,
the heat produced by dry (average) hardwood will be a miserable (18.4 -
(29.5/2)) = 3.95 MJ per kilo 'used'. So it goes without saying that some
of
the carbon is being burned in the process and the char yield will be less
than 50%.
Well, "Burning the charcoal or not" is somebody's choice - and I feel we
should not interfere.
While burning the biomass, a part of the carbon/charcoal definitely gets
burnt - the moisture ( which itself is variable ) in the biomass also plays
a role in reacting with some part of carbon, converting it into gases ( ie.
H2 and CO ). So, I feel it is impossible to arrive at a perfect theorical
calculation towards the end result.
So, I was only trying to give an indicative practical end result.
In case the financial constraint is acute and if it is possible to sell
the
lumpy part of the charcoal ...
Chipped and pelleted fuel and grassy fuels and chopped little branches do
not produce lumpy charcoal to sell. The general proposal is that we burn
crummy fuels that are presently ignored. It is pretty unlikely that anyone
is going to chop wood into sizes that will make lump charcoal. Charcoal is
usually made from giant pieces and broken afterwards. I think the biochar
people are happy to put powder into the ground (please correct me if it
has
to be lumps)
Small lumps may be easier to sell than powder. Powder can always go to the
soil - or pelletised for fuel.
Even then ( assuming the buyer is burning the charcoal ), the whole
process is carbon-neutral ( since only residues are used ). This means
there is no net addition of carbon to the atmosphere.
Biomass is already carbon-neutral.
If so, how are the forests disappearing ?
The real advantage of these stoves is that they can burn a variety of
waste biomass materials in a very efficient way.
Yes it is. And we don't need to say any more than that, do we? If we can
burn, as Otto points out, a much wider variety of fuels that are presently
ignored, there is more fuel available.
I appreciate all these arguments. In fact this makes everybody a little more
sharp thinking.
Best Regards,
Rajan
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