[Stoves] Charcoal stove design

Tue Sep 17 13:56:10 CDT 2013

Dear Crispin,

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of
Crispin Pemberton-Pigott
Sent: Tuesday, September 17, 2013 11:37 AM
To: 'Discussion of biomass cooking stoves'
Subject: Re: [Stoves] Charcoal stove design

Dear Frank

Charcoal largely burns C > CO2 and converts some of it back to CO in the hot
zone above the hot coals. [Frank >] Above the hot coals or in the hot coals?
To convert back from CO2 > CO we need more carbon. 

If the excess air level is high it can create a lot of CO directly and not
burn it. [Frank >] Is it because the gas is too cool? Molecules are spread
too far apart to continue the complete combustion? Not enough hydrogen to
aid in supporting continued complete combustion? The high CO detected in
nearly all charcoal stoves is an indication of poor combustion and is not an
inherent property of the fuel.  Most charcoal stoves are of a brazier type
which is basically not a stove at all.

When wood burns to completion it is burning charcoal somewhere in there, but
under better conditions. If you put wood in a 'charcoal stove' and lit it
there would be pretty bad emissions.

Most people building charcoal stoves try to get the secondary air through
the grate from below. Under ideal conditions this can work well, which is to
say maybe 3% of the time. The rest of the time, and particularly at low
power, the EA is far too high, the fire is cool and CO is produced.  We
think it makes 'a lot' of CO because a) the majority of what could burn is
Carbon, and b) we don't measure H2  so assume it is not there.  Duh! There
is plenty of both when combustion is poor.

[Frank >] I find the hydrogen is less than one percent in char. But moisture
can increase the hydrogen is not tested directly after drying. Wonder if
this water hydrogen is any help in these small stoves?

[Frank >] 

Thanks

Frank

There is nothing 'special' about burning a high carbon fuel. It requires the
same attention to details with respect to the air-fuel ratio and the
combustion conditions. Obviously on average there will be more CO2 that H2O
in the exhaust because the ratio is different, and it requires more air per
kg because there is much less O2 inside the fuel.

Regards

Crispin

+++++++

Paul and Stovers,

I've never seen a char stove and, from what I have seen here in the lab
working with biomass to char I am surprised it works at all. So there is a
lot I don't understand about this process. You suggest the char is heated
when combined with oxygen to produce C > CO then moved to another point and
heated again to produce CO > CO2. Seems to me both would create a challenge.
Does the char need a little biomass mixed in with it to keep reacting? Or a
fan? Does this work the same as burning coal?

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: Paul Anderson [mailto:psanders at ilstu.edu] 
Sent: Monday, September 16, 2013 11:17 PM
To: Discussion of biomass cooking stoves
Cc: Frank Shields
Subject: Re: [Stoves] Charcoal stove design

Frank and all Stovers,

When combustible gases are produced (and CO is produced in the charcoal
stoves), the air for combusting those gases would be the secondary air.

In typical charcoal stoves that air comes in over the top rim of the stoves,
and is poorly mixed, and the result is massive CO into the room.   And very
little air can enter when a pot is in place, especially if there is also a
pot skirt.

Work on introducing secondary air via specific openings can be seen some
nice work done by GIZ:

https://energypedia.info/wiki/File:ECLAIR_Benin_stove-factsheet_eng.pdf 

More work needs to be done on this topic.

About reflective shinny surfaces, does anyone have any info that is related
to stoves comparing metals (different status of shinny) and
clay/mud/ceramic?   For example, how much more heat-to-the-pot is gained by
using metal versus ceramic, and is that difference sufficient to justify (in
part?) the extra cost of metal?   High mass ceramic/clay takes up more heat
than does the metal, but is it really significantly more?

Paul

Doc  /  Dr TLUD  /  Prof. Paul S. Anderson, PhD  
Email:  psanders at ilstu.edu   
Skype: paultlud      Phone: +1-309-452-7072
Website:  www.drtlud.com

On 9/16/2013 1:00 PM, Frank Shields wrote:

Stovers and Paul Anderson, 

Since good char burns with little smoke it seems there could be some design
improvements from the stove in Paul's attachment:

1)      We can use shinny surfaces, perhaps like a bowl under the charcoal,
to reflect the heat up to the pot.

2)      Best to heat a pot we need a blast of hot air directed at the pot
-that is not available with radiant heat. I believe to get char to burn we
need the same; a blast of air directed at the char. Perhaps because when the
carbon goes to CO or CO2 it releases enough heat to start another reaction
IF there is oxygen stuck to that carbon allowing a reaction to take place.
If not the heat dissipates and does little.  

Wondering: if a basket of char fitted with a plate at the bottom set on a
spring pressing char up against a top grate. The container sealed such that
primary air being drawn is directed through a pipe blasting 2cm from the top
of the char bed (vertical pipe through the center of the char).  The
reflective surface directing heat produced to the pot. Something like that?

Also; For char stoves is there ever a secondary? 

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
Paul Anderson
Sent: Monday, September 16, 2013 7:24 AM
To: Discussion of biomass cooking stoves; James S. Schoner
Subject: [Stoves] Amount of charcoal for simmering

Stovers,     (this will also be a "post" at my  www.drtlud.com   website,
BUT all replies and discussions should be directed via the Stoves Listserv.
Dr TLUD does NOT carry on separate discussions via comments to his website.)

I am looking for a calculated answer that could give guidelines about the
sizes of charcoal-burning stoves, referring to how much charcoal must be
consumed for the cooking tasks.

Question:   How much charcoal  is needed (per minute, or per hour, or ??) to
maintain a pot (5 liters?) of already-boiling water at the simmer
temperature of about 96 deg C?

So many variables, but the replies can state whatever "givens" or
assumptions you wish to make, such as:

1.  Heat transfer efficiency of the stove-pot configuration:    Is 30% about
typical?   And does that vary between clay-mud charcoal stoves vs metal
charcoal stoves?  Please specify, if your response takes that into account.

Reference:   See     Experimental study for improving energy
<http://search.mywebsearch.com/mywebsearch/redirect.jhtml?searchfor=heat+tra
nsfer+efficiency+charcoal+stoves&cb=XQ&p2=%5EXQ%5Exdm002%5ES03729%5Eus&qid=4
99b92ca6697461da7047b820086bd7d&n=77DE8857&ptb=F7EBB78D-F7A7-4077-B858-0D10F
0C7CB0E&si=COjP4cCp0rMCFao7MgodRQUALQ&pg=GGmain&action=pick&ss=sub&pn=1&st=h
p&qs=&pr=GG&tpr=hpsb&redirect=mPWsrdz9heamc8iHEhldEVRIIbml7k2%2FvmB8U5crkZmb
OTdLROAzdl%2F4ZQYYskvUwjkty6%2BVZ%2BNQiIr863cMN8jdRHpxuJvwsN2u6HPpdnPNB%2BBy
mTTjJMbwcgCi5H8Q&ord=1&ct=AR&> efficiency of charcoal stove 
http://nopr.niscair.res.in/bitstream/123456789/3794/1/JSIR%2068(5)%20412-416
.pdf
<http://nopr.niscair.res.in/bitstream/123456789/3794/1/JSIR%2068%285%29%2041
2-416.pdf> 
with reported efficiencies between 21% and 62%.

2.  Still air, no wind.

3.  Comments on types of pots (Stainless vs aluminum vs other).

4.  Lid or no lid.   Probably no lid because so much testing is done with no
lid on pot.

5.  Ambient temperature:   probably 22 to 27 deg C.

6.  Any other variables.?

Comments:   The question arises because the TLUD stoves  (and any other
charcoal producing stoves) can be a convenient and low cost source of
charcoal for a household.   That household has some cooking needs for LOW
heat (simmer-type), and a charcoal-burning stove.

Example from the world of stove testing:  a pot with 5 liters brought to
boil can be placed onto a charcoal stove for the remaining 45 minutes of the
standard water boiling test (WBT).   The charcoal could be already red-hot,
having been placed in the charcoal stove directly from the TLUD stove that
made the charcoal.   How much charcoal is needed for that simmering task?

When we know the amount of charcoal, we can calculate backwards to determine
how much raw fuel was needed to make that charcoal.   (That amount of raw
fuel is approximately 5 X of the charcoal weight.)   Example:   100 g of
char is produced from 500 g of woody biomass.   

Therefore, about 70% of the energy content of the raw fuel would be
available for INITIALLY putting heat into the pot.  I think we should assume
better than 40% heat transfer efficiency because that is the target for Tier
4 stoves and can be achieved by gas-burning stoves.    40% of 70% is 28%, so
round it to 30%.    And 30% of the energy content of the raw fuel is how
much?     1 kg of wood has 16 MJ, so 500 g has 8 MJ, and 30% would be 2.4
MJ.   

Somebody please complete the calculations:   

A.     2.4 MJ will raise 5 liters (5000 g or ml) of water how many degrees?

B.      How many g or ml of water can be brought to 100 C if starting temp
of water is 20 C?

In the end, I hope we have some understanding about the amount of raw fuel
and resultant charcoal needed to accomplish the WBT with minimal waste.

Paul

-- 
Doc  /  Dr TLUD  /  Prof. Paul S. Anderson, PhD  
Email:  psanders at ilstu.edu   
Skype: paultlud      Phone: +1-309-452-7072
Website:  www.drtlud.com

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