[Stoves] Shields E450c as a way to test char-making stoves (attn: GACC testers)

Frank Shields frank at compostlab.com
Fri Oct 11 12:24:16 MDT 2013


Dear Crispin,

 

I missed the phone discussions and was unable to download the most recent
WBT method. 

 

Seems the goal is to 1) determine the energy used and 2)  a measure of the
task completed. That could be the WBT or any other task. But to compare
stoves we use the WBT so the task part is standardized for common like
tasks. 

 

I am just working with the 1) energy used to improve that for less error and
simplify the procedure. The process of dragging out burning fuel, separating
out the char, consider all moisture gone, guess as the energy left in the
woody fraction (if still being done?) is filled with potential error and
really impossible to determine the energy in the mix short of drying,
grinding and testing. I want to eliminate this step. Replace it with a
simpler, more accurate procedure. 

 

If White Oak has a HHV of 19.42kJ/g and we used 1000 grams for the task. We
start with 19420 kJ. With a fixed carbon content of 17.2% DAF meaning we
have 172 g fixed carbon in the sample used. Oak Char (565c) is reported to
have HHV of 23.05kJ/g so we have 3965 kJ energy we remove from the total. So
we report using 15455 kJ energy for the task. All numbers are Look-up values
from Tom Reeds book. But all these values can be determined using TGA and
Calorimeter. The ones in the Look-up is mostly E650c and I suggest E450c so
that would be another look-up table but there is not much difference.  

 

Please see below.

 

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of
Crispin Pemberton-Pigott
Sent: Friday, October 11, 2013 9:15 AM
To: 'Discussion of biomass cooking stoves'
Subject: Re: [Stoves] Shields E450c as a way to test char-making stoves
(attn: GACC testers)

 

Dear Frank

 

This is not really a matter of only determining the heat released from the
fire, unfortunately, though I find you approach inspiring.

 

Before trying to find a novel method of working out the heat that was
extracted from the fuel by deducting what is left from the original, it is
wise to first consider what the point of the exercise is.

 

The whole purpose of working out the energy flow is to be able to say how
much fuel was used to perform a cooking task. If the final result does not
predict energy efficiency (i.e. a mass of fuel used turned into a quantity
of energy)  it has no real purpose for someone buying the stove.[Frank >]
"how much fuel was used" The method I suggest has all the values. If we have
a pile of wood of 2000g and we use 1500g for the task that 1500 is the
weight of the tree that was used. That has moisture so we need to determine
the dry matter from the 1500 used and smarter people help me with the
calculations working with moisture and H values in the biomass used. 

 

In the same way, of the 'rating' of the stove does not tell a programme
manager how much fuel the stove will use, they will not be able to predict
the effect it will have on a forest if large numbers are disseminated.[Frank
>]  All measurements are taken from the tree dragged in. 

 

The number you seek (we all seek) is the energy needed in the form of fuel
to be fed into the stove to accomplish a task - hopefully a task that
represents what people do with the stoves they would eventually use. [Frank
>] You suggested a time back that we need several runs to determine the
amount of fuel used if left over fuel is re-used in a new test. That is more
important with my suggest method. But we do not need total energy - only the
energy fraction used for the task. The other 'energy' is like ash and stones
and thrown out. But if the stove is such a great design this char energy is
used it is still reflected in the efficiency calculation. If some of the
E450c fraction is left over and not used it is reflected in the efficiency
calculation. So all material is accounted for in the testing procedure. 

 

Fuel put into the stove has a certain potential to generate heat. What comes
out of the stove might, as in the case of an open fire, be put into the next
fire the next day.  If the same task is performed, the fuel remaining from
day 1 goes into the fire on day 2. This is real life. Some of the fuel is
burned, some of the fuel is discarded in the ash, some may be used the next
day.[Frank >]  That's why we may have several tasks for a single weight loss
from use of the pile of fuel. And, we may need to extend the time to a
couple days, week or more to cover all the tasks (like heating the home) to
account for all the uses of a unit weight of fuel used. But if we want to
determine efficiency of a single task (WBT) we may need to repeat that task
several times as left over fuel is re-used. 

 

What matters is the energy requirement to complete a daily cycle. [Frank >]
Agree - task, daily, weekly, yearly etc. All tasks monitored and kept track
of and summed up to the E450c used to the E450c in the starting tree used as
fuel. 

 

If you were to find out how much energy was released from the fire, it does
not tell you how much energy was consumed in the process of that cooking
cycle. The energy needed to be fed in each day is the real consumption.
Whether that is burned fully, burned partly to CO, roasted to charcoal,
burned to small pieces of unusable charcoal, does not really matter if there
is nothing the stove can use the next day. In that case, whatever went in
was 'consumed' for all intents and purposes. [Frank >] We should try to keep
the stove going until the secondary flame goes out to get the most from the
fuel. That means all the accounted for E450c has been used for the task.
After that whatever energy we can squeeze out of the char is bonus.  

 

If it is an open fire, there will be some fuel remaining that is useful,
some that is lost. [Frank >] The energy is 'centered around' the E450c
value. We may get more energy from the char or less with left over unburned
biomass. All reflected in the efficiency value calculated.  

 

If we know the amount of heat transferred from the flame to the pot, it is
interesting, but it is not related directly to the fuel that will be drawn
from the forest each day, which is what buyers want to know. They have to
collect and transport it. 

 

As you know the WBT standard formula (which has since been edited)
calculates the energy used by the fire. Then it turns that into a dry mass
equivalent. But that is not the fuel consumption nor the dry fuel
consumption. It is just the energy from the fire turned into a dry mass of
fuel of a certain type.

 

As Jim Jetter tried to explain very carefully in his recent webinar, if the
fuel remaining is not burnable to the stove that created it, it is lost to
that device - even if it is useable for something else. [Frank >] I
disagree. If it is used for anything that results in less biomass removed
from the stock pile it is saving fuel. That's why I suggest we only measure
energy used as weight loss from the pile in the corner. 

 

The calculation should be: the amount of fuel arriving from a previous fire
(heat content unknown) plus any new fuel added, minus the amount of burned
but reusable fuel remaining, noting the additional mass of fuel that is not
usable (including ash). If you repeat the test, the leftover fuel
(presumably about the same because the task is identical) is put into the
next fire. The energy content does not have to be assessed. Just put it into
the next fire, and get more left from that one. The more times you repeat,
the less it matters what the heat content is because the energy put in and
coming out will balance.[Frank >]  If we use my suggested all we need to do
is determine the amount of energy E450c added from the stock pile of fuel.
Keep track of the tasks that energy was used for. . 

 

The thing to know is the new fuel mass required for each replication, [Frank
>] Not this "and the mass of fuel disposed of as unusable[Frank >] ".  You
do not need to know the energy value of that either, only the raw mass of
fuel needed to create it. Simple subtraction reduces this to a surprisingly
simple solution for the total energy needed per cycle. If you did 3 or 4
tests, just divide the total new fuel by the energy content of new fuel. As
the species and moisture content will be known, that is also easy.  No
fiddling with charcoal and half-burned sticks at all. Why? because we are
not, at any point, trying to work out how much heat was created by the fire,
only the heat that got into the pot(s) which is very easy.[Frank >]  If we
have a stock pile of biomass and determine the E450c (look up or tested) and
the moisture -then keep that pile in a plastic bag so moisture doesn't
change its only change in weight as it is used that need monitored. Real
easy.

 

 

Next, the efficiency, meaning the overall thermal efficiency, is the work
done in the pot(s) divided by the energy put into the system each time the
task is replicated. It is the system efficiency from an energy point of
view, and it is predictive of energy use, meaning fuel use.[Frank >]  I am
not sure about this one. I think we need a new term like EffE450c or
something. That because since what I propose is not true efficiency based on
all the energy in the biomass added (we do not include left over char
energy) rather a value centered around E450c.  I think we need to
distinguish that somehow so to not make the definition of 'efficiency'
something it is not. Down the road we could run into problems.   

 

There is one fiddle required that deals with the usable fuel remaining.
Suppose the amount you have left is after 3 or 4 tests is not the same mass
as was put into the first measured run. It would be helpful to know the
difference, and the heat value of that material. This are several ways to
find that including the one you propose. Divide the number by 3 or 4 (for
the number of replications) and you have the correction to be applied to the
raw fuel mass using the ARV (As Received Heat Value of the raw fuel).[Frank
>]  If the stove is continued until the secondary flame goes out (all the
E450c energy used ) it should be fairly accurate I think. 

 

The final result is much easier to find than working with 'charcoal
remaining' and burned sticks, it is much more precise, and it is predictive
of future performance. [Frank >] But not as much fun! My memory of the first
Stove Camp was Getting-out-of-the-way! as Dean carried a shovel of fire from
the stove and placed it into a pot with flames high as he tried to weigh
this burning mass.  

 

Correctly predicting the overall thermal efficiency requires knowing how
much heat got into the pot. That means also considering the mass and
material the pot is made from. The British Standard, the Indian Standard,
the SeTAR and Indonesian Standard use this. I am sure many other do as well.
Again, the result is more precise and gives predictive results even if the
pot material or mass is changed.

 

The only thing left is to make sure that the task performed is culturally
relevant and representative of actual use, including fuels available in the
community.[Frank >]  Completely agree.

 

Lots to think about.

[Frank >] I will not be at the conference in Amherst so am hoping you agree
with what I propose and, because you know all the right people, can explain
this so others can make decisions as to its benefit. 

  

 

 

Regards

Crispin

[Frank >] Thanks

[Frank >] 

Frank

 

 

Frank Shields

Control Laboratories; Inc.

42 Hangar Way

Watsonville, CA  95076

(831) 724-5422 tel

(831) 724-3188 fax

frank at biocharlab.com

www.controllabs.com

 

 

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