[Stoves] Looking for stove designþ

[vetle.w.cappelen at biomass-stove.no]

Dear Crispin

It become a large mail. But hope you find some of it interesting.


C:Is 1 litre large enough to interest a family? How much to people cook at a
time?

V: they use about 1,3 liter of water to make Nshima for a household of two
adult and 3 children.

C:The answer might be ‘a little in the morning and a lot in the
evening’.

V:True

C: It is quite reasonable for there to be two stove sizes or for one to be
used only for certain tasks. If the fit is good, people will adopt them.

V: Most of the people we meet are cooking with only one energy unit
 Both
for cooking and to heat water for other use. Charcoal stoves can be
purchased for as little as 3 $ but still they buy and use only one, so I
don’t think they need more than one size stove! As I said an investment
issue.



C: If I estimate 600 g @ 15% moisture (air dry in summer) I get about 7.8
MJ of
potential energy. Subtracting for 20% char (125 g) I get 4.1 MJ of heat over
that 30 minutes. That is 2.3 kW average (with perhaps a peak of 3 kW?).
Does that sound reasonable? Is the heat fairly constant during that time?
If
so, it is going to have a pretty constant thermal efficiency.

V: The Moisture in the fuel used are less than 10% I get about 8,5 MJ of
potential energy
And average of 25% Char (150 g) Assuming your calculation of Net calorific
value (dry fuel)of Maize are 9600 kj/kg? If I Now have done it correctly I
get 3,2 KW?.
Pls show me the formula you are using.

C: If the fire is low for 2.5 minutes then the pot goes on, and the
boiling is
6.5 minutes per litre, that is an overall efficiency of about (0.335 MJ heat
absorbed / 0.975 MJ heat applied) = 34%.  If the power is not up to that 2.5
kW average at the time, perhaps it is higher. If the fire is really large at
that time, perhaps the efficiency is In the high 20’s. Does any of this
sound like what you are seeing?

V: The 2.5 minutes before putting on the pot is to make sure gasification
have started. The fire don’t get ”low” before the ”ending phase”. and
flame are about constant during this time (10 - 18cm high)
The temperature are pretty constant as well (550 to 650C) until 30 minutes
when it’s dropping into end phase.

C: 30% is pretty good.

C: So the information I will be looking for relates to the cooking power
in the
first phase (pyrolysis), the duration of the burn and the heating power that
is available if one wanted to continue to simmer on the remaining char.
There is a test which would be very valuable and it is a water heating
(not
boiling) test.

C: Put on a large pot with a lot of water, large enough that it will not boil
for at least 40 minutes. Record the temperature every 20 seconds or so (be
regular to make it easier to analyse) using a thermometer that gives 0.1
Degrees.

V: Using the PekoPe or an traditional charcoal stove? when putting the
remaining "charcoal " into a traditional stove would be allot more
efficient. Caused of the lack of sufficient air flow in the PP.

C: As the thermal efficiency will probably not change a great deal during the
cooking session, the temperature rise rate is an indicator of power output.
It can be calculated for various intervals based on time or temperature. I
have attached a typical result of such a test for an old Panda paraffin
stove using a heterogeneous test protocol. The top blue line is the mass
of
water remaining. You can see when the stove was changed from High to Medium
power as the boiling rate (water mass loss) changes. The thermal
efficiency
line (green) detects a change in efficiency with the turn-down.

C: If the power level changes the slope of the water heating line will
change.
This can tell you what the power change is as the stove goes from ignition
to a fully developed fire to a late-paralysis burn. This information is
very
valuable for you. It would be very helpful if you share the result. Using
this approach, you can tune the performance to a cooking task in a given
community.

C: If the pot boils during this type of test the result is harder to get
because it has to be done on a scale but it can be done if you have to.

C: For South Africa and vicinity it would take 45 minutes after boiling to
cook
‘pap’ which is much longer.

V: This is to be taking into consideration for every place/country when
stoves are tested. My answers for this are that Maize cobs may not be
perfect with a 6 liter stove at all sites. but to be able to use other
kind of fuel like Chopped wood would give a cooking flame for 70 minutes.
So the difference of fuel availability can in some cases compromise the
stove size.


V:>Cooking Nshima need less than 10 minutes so usually they can prepare most
of their additional food with one filling.

C: That is a very good fit then, especially if adding some chopped wood will
extend it by a few minutes if needed. Do people object to
chipping/chopping
the wood? Zimbabwe (rural) had difficult with splitting wood for cooking
because the trees are so hard. That problem is avoided it no wood or
little
wood is involved. As the maize cobs collect around the house it should be
used first I guess. Hopefully.

V: This is, as I said, mainly Biochar from Maize project. But when there
is no Maize around we have been teaching them to find, test and use other
Biomass fuel as long as it is dry.

V: It’s true that the trees are a bit hard to splitter. But we did try
branches from different trees. and packed the Peko Pe and the result was
1,1kg burned at 700C for about 1 hour before temperature was dropping.  
People have axes to cut down threes for charcoal production or maybe to
make a canoe, this wood chips will burn for more than one hour.

V: Grass is also a really good result 700g will burn with a lower temp.
(500C) but for 40 minutes cooking time for a litre are about 9 minutes
after the pot is put on.


V:>We do feel that 6 liter Peko Pe stove is a suitable size for
unprocessed
BIOmass wich are most available around the farms.

C: Agreed, by the sound of it. You have a good match between the stove, fuel
and the cooking task.

V:>Since they already are cooking with one pot at the time, we don’t think
they need to have more than one stove,.

C:Good point. How do they prepare the ‘relish’ to go with the nshima? I
have seen it prepared in a smaller separate pot.

V: They start with the relish and cook the Nshime after. Yes use smaller
pots for the relish.

C:Is the burn rate (mass per minute) the same with all the fuels or do see
a noticeable difference?

V: The results is not confirmed but as we have tested some different
fuels. More will be tested. And there are slightly different rate about 5%
compared maize to wood

600g Maize big 13,6g/minute 550C 30 minutes
800g Maize small 13,3g/minute 550C 45 minutes
1200g Wood brunches 12,8 g/minute 650C 70 minutes
700 g Grass/straw 13,1 g/minute 500C 40 minutes



V:So far it seems that the farmers are satisfied with both the size and
cooking time. Time will show if adjustment of stove size are required.

Agreed.

V:>Since the farmers in this project are targeted to collect BIOchar from
maize cobs used for household energy purpose, they will refill the stove
when needed to get sufficient cooking/energy time.

C: They will probably do that until they run low on fuel and there is
nothing wrong with that.

V:>The BIOchar will be stored for later to be used as soil improvement
instead
of using it as glowing charcoal.

C: I understand that process. The soils of Zambia are already famously
productive so any effect will be important to document.

V: In the areas like Mongu and Kaoma that we are visiting there is very
sandy soil the growth of Maize are increased up to 4 times the control
plant by using BioChar as soil improvement. Mkushi have better soil so the
Bio char have less, no or even negative impact. The final report will be
published during the next year after harvest by Norwegian Geotechnical
Institute (NGI).


Best regards
Vetle Wendelbo Cappelen

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> Dear Vetle
>
>
>
> This is exactly the sort of information that builds the case for a new
stove
> or a new approach to fuelling and operation.
>
>
>
> Use acceptance is a major issue and the follow-up is important – meaning do
> the new stoves remain in use. When they do, it is often for reasons that
have to do with fuel availability in my limited experience. Not have to
go
> as far or spend as much for fuel is a strong incentive. The only thing that
> mitigates hard against it is if the stove is inconvenient to use or
won’t
> cook properly. African women tend to like a high heat (high firepower)
to
> ‘cook fast’ so speed to boil is important.
>
>
>
>>The 6 liter Peko Pe stove can be filled with 600 grams of Maize cobs,
>> with
> the sizes of 40-60 mm, it will burn for about 30 minutes and boil one
liter of water within 9 minutes after ignition (6,5 minutes after the
pot
> is put on the "flame").
>
> Is 1 litre large enough to interest a family? How much to people cook at
a
> time? The answer might be ‘a little in the morning and a lot in the
evening’. It is quite reasonable for there to be two stove sizes or for
one
> to be used only for certain tasks. If the fit is good, people will adopt
them.
>
> If I estimate 600 g @ 15% moisture (air dry in summer) I get about 7.8
MJ
> of
> potential energy. Subtracting for 20% char (125 g) I get 4.1 MJ of heat
over
> that 30 minutes. That is 2.3 kW average (with perhaps a peak of 3 kW?).
Does that sound reasonable? Is the heat fairly constant during that
time?
> If
> so, it is going to have a pretty constant thermal efficiency.
>
> If the fire is low for 2.5 minutes then the pot goes on, and the boiling is
> 6.5 minutes per litre, that is an overall efficiency of about (0.335 MJ
heat
> absorbed / 0.975 MJ heat applied) = 34%.  If the power is not up to that
2.5
> kW average at the time, perhaps it is higher. If the fire is really
large
> at
> that time, perhaps the efficiency is In the high 20’s. Does any of this
sound like what you are seeing?
>
> 30% is pretty good.
>
> So the information I will be looking for relates to the cooking power in
the
> first phase (pyrolysis), the duration of the burn and the heating power
that
> is available if one wanted to continue to simmer on the remaining char.
>
> There is a test which would be very valuable and it is a water heating (not
> boiling) test.
>
> Put on a large pot with a lot of water, large enough that it will not
boil
> for at least 40 minutes. Record the temperature every 20 seconds or so
(be
> regular to make it easier to analyse) using a thermometer that gives 0.1
Degrees.
>
> As the thermal efficiency will probably not change a great deal during
the
> cooking session, the temperature rise rate is an indicator of power output.
> It can be calculated for various intervals based on time or temperature.
I
> have attached a typical result of such a test for an old Panda paraffin
stove using a heterogeneous test protocol. The top blue line is the mass
of
> water remaining. You can see when the stove was changed from High to Medium
> power as the boiling rate (water mass loss) changes. The thermal efficiency
> line (green) detects a change in efficiency with the turn-down.
>
> If the power level changes the slope of the water heating line will change.
> This can tell you what the power change is as the stove goes from
ignition
> to a fully developed fire to a late-pyrolysis burn. This information is
very
> valuable for you. It would be very helpful if you share the result.
Using
> this approach, you can tune the performance to a cooking task in a given
community.
>
> If the pot boils during this type of test the result is harder to get
because it has to be done on a scale but it can be done if you have to.
>
>>For normal cooking of Nshima and similar food
> preparing, 30 minutes would be enough for each meal in everyday use.
>
> For South Africa and vicinity it would take 45 minutes after boiling to
cook
> ‘pap’ which is much longer.
>
>>Cooking Nshima need less than 10 minutes so usually they can prepare
most
> of their additional food with one filling.
>
> That is a very good fit then, especially if adding some chopped wood
will
> extend it by a few minutes if needed. Do people object to
> chipping/chopping
> the wood? Zimbabwe (rural) had difficult with splitting wood for cooking
because the trees are so hard. That problem is avoided it no wood or
little
> wood is involved. As the maize cobs collect around the house it should
be
> used first I guess. Hopefully.
>
>
>>We do feel that 6 liter Peko Pe stove is a suitable size for unprocessed
> BIOmass wich are most available around the farms.
>
> Agreed, by the sound of it. You have a good match between the stove,
fuel
> and the cooking task.
>
>>Since they already are cooking with one pot at the time, we don’t think
> they need to have more than one stove,.
>
> Good point. How do they prepare the ‘relish’ to go with the nshima? I
have
> seen it prepared in a smaller separate pot.
>
>>This is off course an investment cost up for discussion for every
> household. When less cooking time are needed they can
> just use less fuel.
>
> Exactly.
>
>>With beans, the farmers will spend more time for cooking. This can
either
> be done by empty the stove and refilling it with maize or use smaller
cobs
> (>10 - 20mm) to get more mass into the stove (800g = <40 min "flame
time")
> or also by
> using other kind of denser BIOmass. Chopped wood will burn for more than
1
> hour.
>
> Is the burn rate (mass per minute) the same with all the fuels or do see
a
> noticeable difference?
>
> So far it seems that the farmers are satisfied with both the size and
cooking time. Time will show if adjustment of stove size are required.
>
> Agreed.
>
>>Since the farmers in this project are targeted to collect BIOchar from
> maize cobs used for household energy purpose, they will refill the stove
when needed to get sufficient cooking/energy time.
>
> They will probably do that until they run low on fuel and there is
nothing
> wrong with that.
>
>>The BIOchar will be stored for later to be used as soil improvement
>> instead
> of using it as glowing charcoal.
>
> I understand that process. The soils of Zambia are already famously
productive so any effect will be important to document.
>
> I really appreciate you taking the time to provide hard data for us who are
> disconnected from the work on the ground. I would have shown one to the
Chinese rural energy delegation Roger Samson is bring around this week
if
> I
> had a working sample. We are going to demonstrate several stoves that
can
> burn pelletized (and chunked) switchgrass to try to generate interest in
TLUD technologies.
>
> Thanks
>
> Crispin
>
>
>
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>
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http://www.bioenergylists.org/
>
>