[Stoves] biochar rocket stove

[Crispin Pemberton-Pigott]

Dear Nolbert

 

You have raise a very important point that is relevant not only to your work
and field experience, but it stove evaluation in general and to CREEC's work
in particular plus it is directly relevant to the IWA.

 

>.i have experienced for the last 2+ years how economical in terms of wood
consumption (therefore wood saving) TLUDs are in this region.



That is very good news. I have no problem with that, meaning I see no issues
with TLUD's saving fuel. They should because they operate (usually) with a
low excess air (EA) level and therefore have an inherently better heat
transfer efficiency. There are two main thermal gains from a TLUD: better
combustion efficiency (burning all the fuel) and lower EA levels. The more
EA, the lower the efficiency. It is important to document the actual fuel
use reductions, or fuel switching if that is relevant. As has been pointed
out many times here, fuel switching is often more important than saving
kilos of fuel because there is waste biomass lying around.

>Lab tests at CREEC (Makerere University)  have also proved that TLUD
micro-gasifiers are efficient backed with statistical data, including PEMS
tests and comparisons with other types of stoves like the classic charcoal
burners, rocket stoves and of course the 3-stone.

This is the important part: "Lab tests at CREEC"

They are using the WBT, right? They are stating the following: according to
the WBT's conducted, "the calculated specific fuel consumption of the TLUD
is xxx and the calculated specific fuel consumption of the other stove is
yyy". Is that pretty much the say the result is given?

The WBT makes use of a metric called the Specific Fuel Consumption. As has
been pointed out at the IWA and in several other places and letters and
comments, the SFC is not the fuel consumption. It is a number calculated
from the energy used to calculate the thermal efficiency. I hope you can
follow this because it is well-hidden in the math of the WBT and is why I
make so much noise about this ruinous relic of stove testing's past. The EPA
webinar presented by the SeTAR Centre a couple of weeks ago covers this
point as well:

The WBT calculates first the thermal efficiency. v.4.1.2 does a much better
job in this regard that UCB-WBT 3.0 did. So when the calculation is done,
the energy used to do the cooking is calculated, including deducting for the
energy that remains in the char. What wood gases are burned, and what carbon
was burned, are calculated to be the heat energy given to the pot, some of
which gets into the pot. The efficiency with which that transfer is made is
the heat transfer efficiency. If it is detected using the water temperature
only (as per WBT 4.1.2) or the pot and water temperature (as per the Indian
WBT) you get a net heat transfer efficiency which is always lower than the
actual heat transfer efficiency. No problem.

The energy used during cooking is then divided by the Dry Fuel energy
content, let's say 18 MJ/kg. That yields a Dry Fuel Equivalent of the energy
used to make the thermal efficiency calculation. It is not the mass of fuel
used to do the cooking, nor is it the dry mas equivalent of the fuel used to
do the cooking, it is the dry fuel equivalent of the net energy liberated,
as calculated for the determination of the thermal efficiency.

The point that Kevin raised is very important: if the char produced by the
stove during the cooking is not useable in the that same stove, then it is
lost. It can be lost to a biochar project, it can be lost to a charcoal
stove, it can be lost to the ground around the house, it does not matter.
But it was used by the stove during cooking.

Now consider: suppose a stove produces a lot of char and you use the WBT
4.1.2 to calculate the 'specific fuel consumption'?  That is the dry fuel
equivalent divided by the number of litres of food cooked (well, in the case
of the WBT 4.1.2 it erroneously uses the mass of food remaining in the pot,
not the actual mass but that is a quibble). The point is the stove
comparison is not made using the actual fuel consumed, it uses the dry fuel
equivalent per litre calculated from the energy efficiency formula. If a
stove uses 1 kg of fuel each time it is lit, and it produces (as a
left-over) char with a mass of 100g, and that 100g is not used the next time
the stove is run, then the consumption is 1 kg. Simple as that. You cannot
credit char number to a stove for fuel consumed unless it is used. The most
extreme acceptance would be that the next time the stove is lit, that char
is going to be burned in it. If it cannot, or does not if you are paying
attention to social behaviour, then that stove cannot be credited as 'not
having burned that portion of the fuel'. It did not burn it, but it
consumed. Saving fuel means not consuming fuel.

The implication for CREEC and for anyone else using the WBT 4.1.2 is that
the stove fuel consumption comparisons are not valid measures of the fuel
used (consumed) by the stove each time it is lit. This error is not large
when burning relatively dry fuel and the char mass remaining is small. But
in the case of TLUD's producing char, the WBT number is very misleading.

The fact that the TLUD's you see are saving fuel is valuable information,
provided the measurement is made of fuel being taken from the forest, and
not the result of a misleading interpretation of fuel consumption as is
embedded din all of the UCB-WBT tests and its derivatives. The error is
huge, like 70 or even 80%. The example of the Namibian stove given in the
webinar shows an error of 40% for a modest stove producing 20% char, which
that stove actually does. 

That stove won awards because the 'test result' was 'so good' but in fact it
saves nearly no fuel at all compared with an open fire. That is the problem
with these wretched WBT's with defecting concepts embedded in them.

>These stoves are using less wood, cooking faster which is a major reason
for their acceptance in East Africa. 

Good combustion, low EA.

>Women in the kitchens have tried and experienced how they use less quantity
of wood, are less smoky and also cook quicker than their current
conventional stoves.

Ditto.

>The notion of making char is a plus for TLUDs because they use the char for
other valuable purposes including cooking.

Interesting as long as it does not increase fuel demand, in my view. That
char comes from raw fuel not burned.

>If you think otherwise, then you should try personally using these stoves
for a while to prove what we are experiencing here.

I have built and tested lots of stoves including TLUD's. You can see one in
the stove library called the Grasifier - a $1 stove made from sheet metal,
originally designed for Haiti, burning switchgrass pellets.

Best regards

Crispin

PS Kevin I would appreciate your input on the subject.

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