[Stoves] Purpose of testing and value given to char left.

Frank Shields frank at compostlab.com
Mon Feb 24 12:43:51 CST 2014


<subject changed>

 

 

Dear Crispin, Ron and all,

 

I think we are all in agreement with Crispin the final purpose is to save
trees and to Ron it's important to place a value on the char left over. The
question is: How do we determine this?

 

To measure the trees saved:

You suggest we determine the wood used for each burn and after several burns
we have the average wet wood used. That will work BUT this is an on-site
test. It will take many Runs (I suggest  ~ten statistically?) for a stove to
determine the amount of wood used. And then we go to the next house and how
many more houses, all using the same stove (the variable), to come up with a
statistical value.  Then we give all these same houses a different stove for
testing. Is this your plan?  

 

A lab test using fuel Energy as a proxy reduces the variables such that a
lot of tests can be done in a short time at a reasonable cost. Like was done
before. The biggest variable is the left over energy after the WBT ended and
this can be accurately done using a calorimeter on a dried and ground sample
-as done at Aprovecho OR my past suggested E450c method. 

 

As for the value to be placed on the char left over: The quality of this
char ranges from toxic to plants -to- sucking up nitrogen -to- adding heavy
metals -to- producing a large plant and doubling production. To place a
value one needs to collect a quantity and determine its value -if any for a
purpose. This can all be done per stove following the lab tests above. Lots
of tests - I do them all the time. Once the char is separated from the
torrefied wood and biomass we need to determine its quality. We need H/C
ratio, ash, plant toxicity and the like. A value can then be placed as a
separate subject unrelated to the value of the stove. For marketing we may
be able to state that stove such-and-such consistently puts out a char of
quality to improve soils, sequester carbon or can be used for char cookers
-after collecting a composite of the char from a stove and testing it.  To
add this value to the stove efficiency formula is just adding another big
variable and we have too many as it is (un-burned fuel). We need to reduce
and control variable. So any value given to char left should be restricted
to marketing a stove -as I see it. 

 

Thanks

 

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

 

 

 

 

 

 

 

 

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of
Crispin Pembert-Pigott
Sent: Monday, February 24, 2014 8:10 AM
To: 'Discussion of biomass cooking stoves'
Subject: Re: [Stoves] Request for technology proposals - Clean Stove
Initiative, Indonesia

 

Dear Ron

 

This is getting complicated and the definitions are not completely in
alignment so rather than bore people with endlessly inserting them into
statements, I will try to re-organise everything for plainer reading. If any
misunderstandings are cleared up at het same time, so much the better.

 

Last point first:

 

Sustainablility of biomass is defined by the UNFCCC and the current rating
for all of Indonesia is that biomass is sustainable. I don't create ratings,
I just read them. There is no CDM funding available for projects 'conserving
biomass' in Indonesia because the supply is considered to be sustainable. It
happens that in Central Java there is a fully sustainable supply of biomass
available for domestic cooking and industries. In fact large quantities of
biomass are burned (Kaliandra for example) just to get rid of it.

 

Energy equations: It will be helpful for the casual reader to know 'what you
are up to' so to speak with this char making stove thing so from my
perspective, if I may, I will state it as follows: There is a desire to have
the energy remaining of the form of unburned char resulting from a fire
credited in some manner so as to reduce the rated fuel consumption of the
stove. 

 

There is no dispute between us whatsoever as to the energy consumption: the
energy remaining in the char represents energy not liberated from the fuel
consumed. 

 

The important question is not what we want, but what the customer of the
test result wants. They are not asking how much energy was used when
cooking, they asked how much fuel was consumed. The answer is of course
different if there is char remaining and that char is not 'fuel' to the same
stove for the next fire.

 

With a stick burning stove, sticks left over can be used in the next fire
and they are counted as 'not consumed'. No problem, and they are fuel sticks
that can be used 'next time'. Char is not, when it comes to TLUD-char making
gasifiers or pyrolysers. 

 

Energy balance with respect to the fuel and the fire:

 

A - Energy in the fuel put into the stove

B - Energy liberated by the fire

C - Energy lost due to incomplete combustion of gases and particles

D - Energy remaining in char that is recoverable

E - Energy remaining in char and ash that is not recoverable. 

F - Energy in fuel remaining that can be burned in the same stove during the
next fire.

 

A is calculated on the basis of the fuel 'as received (AR) which means
considering the heat content factored for moisture.

B is calculated by deduction as it is not directly measurable. In a clean
burning stove it will be more than 99% of the heat available from the
combusted material.

C is determined by gas mass and composition and particle mass and its
composition). In a clean stove the CO will dominate the value and the total
value will be small.

D is determined by mass and bomb calorimeter or guessing a value. The heat
content of char is highly variable and guessing has not been shown to give
meaningfully accurate numbers. 

 

E can be determined by bomb calorimeter.

F can be determined by bomb calorimeter though the CSI-WBT avoids needing to
measure it each time by recycling the leftover fuel into the next test. All
reported tests are started using fuel remaining from a previous identical
test, where remaining fuel is usable in that stove. For some stoves this is
not possible and the matter does not arise. Stoves in this category (not
able to re-use the fuel remaining) include most TLUD char-making pyrolysers
and some gasifiers, though the matter remains less than fully explored.

 

The fuel consumption (the measurement we are trying to provide to the
programme manager) is determined by measuring the fuel mass consumed per
replication of a task. It is similar in principle to a KPT in that one
observes fuel disappearing, but instead of disappearing in to a homestead,
it is measured disappearing into the stove each time it is used. It is a
very non-negotiable measurement. Either the fuel is going into the stove or
it is not. Credit is given for fuel remaining that can be used next time: 

 

Energy consumption in the form of raw fuel per cycle = A-F [Joules]

 

The result can be expressed in Joules or in unit of mass. Programme managers
want to hear the 'savings' of a stove in % terms. Thus it is possible to use
the calculated energy-equivalent of the mass of fuel needed for each
replication, and the reduction in that mass with a new candidate stove.

 

Thus results can be presented as "This stoves uses 3 kg of fuel and that one
uses 2 kg to complete the assigned task." Or it can be presented as the
second stoves saves 1/3 of the fuel. It is not desirably to present this as
an 'efficiency' unless the efficiency in question is carefully explained as
an efficiency is always a ratio.

 

The major misunderstanding that has dogged the stove community for so many
years, embedded as it was in the UCB-WBT, is the idea that the heat transfer
efficiency is the same as the fuel efficiency. This is a hangover from the
days of power station measurements and when the calculation was first
adopted by the stove community, power station calculations were employed.
They were quickly adapted to be more realistic (as power stations are not
measured when starting and stopping). By 1984 the Eindhoven Group had a
pretty good handle on this calculation and the term 'percentage heat
utilized' was widely adopted. The problem with PHU was the defective
understanding of the heat transferred during simmering so results are
inconsistent, that is to say, highly variable, even if the stove itself was
not. This was dealt with (by Eindhoven) by assuming fuel remaining was
thrown away. That is, char remaining was not useful for the next fire. 

 

Efficiencies: 

 

Here is a slide from a presentation on measuring stove performance:

 



 

The calculation of the "heat transfer efficiency" has been used in place of
the "overall thermal efficiency" in many calculations. Heat energy
potentially available in fuel consumed minus the heat energy theoretically
available in the char remaining is not the heat energy delivered by the
fire. There are so many things that are not the ratio we want I can't list
them all. 

 

Upon looking closely, you will see from the above slide that the heat
theoretically released by the fire based on the missing mass of fuel is not
really the heat available to the pot - it has to be factored for combustion
efficiency and mechanical losses like dropped fuel and unusable fuel
remaining.  

 

It is not the heat released by the fire either. That is why I call that
number (as you have calculated it and as the UCB-WBT calculated it) a 'proxy
for the heat transfer efficiency'. It does not use the heat actually
available from the fire nor does it use the heat transferred to the pot, nor
does it even use the heat that changes the temperature of the pot and what
is in it (sensible heat). It is an 'efficiency' because it is a ratio but it
is not a useful number for rating stove performance, or even designing a
stove. 

 

Let's call the energy that could have been released from the burned fuel
'Fuel Energy". Let's call the energy in the char remaining, "Char Energy".
Let's call the difference between them, 'Fire Energy".

 

Fuel energy - char energy = fire energy

 

Agreed?

 

Fuel energy can be expressed as a mass of fuel.

Char energy can be expressed as a mass of fuel equivalent (which is how the
GACC-WBT works)

Fire energy can also be expressed as a mass of fuel equivalent.

 

Let's call the heat getting into the pot, "Pot Energy".

Let's call the heat accumulated in the pot, "Net heat gained by the pot"
which is the same as 'sensible heat' or 'detectable heat' as the term
sensible implies.

 

"Pot energy" and "Net heat gained by the pot" are not equal because some
heat that gets into the pot is lost from the sides, especially by convection
and the amount as a % of the total, varies (a lot) with firepower.

 

Back to Efficiency:

 

"Net heat gained by the pot"

"Fuel energy - char energy"

 

Is not the energy efficiency, nor is it the fuel efficiency, nor is it the
heat transfer efficiency, not is it the mass equivalent ratio of any
efficiency. It is just a number. It is used as a proxy for the heat transfer
efficiency. It is not the heat transfer efficiency, but it is close enough
for government work, apparently, because the UNFCCC uses a variation of it
to determine 'fuel savings' which clearly cannot be the case if the stove
produces a lot of char. I believe it is the same as your Eff 3.

 

It is not the answer programme managers are asking for. They are asking what
the reduction in fuel consumption is, which is easily determined by
measuring the fuel demand of the stoves during each replication of the
cooking task. It does not involve any jiggery-pokery with energy in some
fuel fractions and not others.

 

 

Regarding the 85% efficient stoves:

 

A stove that has a very high combustion efficiency, burns all the fuel
placed in it, and having a high heat transfer efficiency can put 85% of the
energy theoretically available in the fuel consumed into the pot in the form
of sensible heat. I have a water heater in my house that is 88% efficient
calculated on this basis. I have a furnace that is 92% calculated on this
basis. It is therefore possible to accomplish.

 

If the challenge is to provide a pellet burning, char-making stove with a
rated fuel efficiency of 45% (also called the overall thermal efficiency),
it would be a requirement to have a heat transfer efficiency that is above
45% in order to have char remaining as unburned fuel. If the heat transfer
efficiency is 85% and the rated fuel efficiency is 45%, the difference of
40% can be delivered in the form of char remaining from the fuel consumed.

 

I don't think you need to me put numbers on it to show this. 

 

Of the original energy in the fuel consumed, 45% would get into the pot as
sensible heat, and 40% would emerge as unburned char. To achieve this would
require a fire heat (factored for combustion efficiency) to pot heat (net
heat gained) efficiency of 85%. The system efficiency is 45% (because for
each unit of fuel consumed, 45% of the energy available is doing something
useful in the pot). The char production efficiency would be the mass of char
yielded divided by the mass of char potentially yieldable (I suppose) or the
mass of char divided by the dry mass of fuel used.

 

The question most often posed by the programme managers is the relative fuel
consumption of candidate stoves relative to the baseline products in common
use. So far no one has asked for a char production ratio or a heat transfer
efficiency or a heat transfer efficiency proxy. 

 

Regards

Crispin

 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20140224/3264cbb0/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: image001.jpg
Type: image/jpeg
Size: 43195 bytes
Desc: not available
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20140224/3264cbb0/attachment.jpg>


More information about the Stoves mailing list