[Stoves] cook stoves for Cameroon

[Andrew Heggie]

On 18 September 2014 03:27, Huck Rorick <huckrorick at groundwork.org> wrote:
> Hi All,
>
>
>
> I found myself a little confused by the discussion.

I am not surprised, the basics are simple but once you have been on
the list a while you  begin to understand the "second order"
complications.

>
> There is a certain amount of energy per kilogram of wood (I’m going to stick
> with wood for the moment rather than all biomass).
> When burned, some of that energy is realized and some is not, i.e. there is
> not complete combustion.  How complete is the combustion?  How much energy
> is released?  That would be the first measure.

As a rule if you see no smoke you have near complete combustion but
there will be some CO and particulates present., if the smoke is blue
then you have some sort of flaming combustion and probably a dangerous
amount of CO as well as obvious soot., white and you are driving off a
lot of the products of pyrolysis. The first two scenarios  mean you
have near complete combustion the latter you are probably wasting 50%
of the fuel value of the wood. One of the quirks about the combustion
of wood is that the flaming part of combustion can be very clean,
especially in that it doesn't create the particulates that are the
main worry in indoor pollution to occupants health. However as
combustion progresses and the flame dies as the products of burning
the remaining char dilute the offgas the production of CO increases.
The major helath attribute of burning wood Top Lit Up Draught is that
if the combustion is stopped at the end of the flaming phase then the
burn can have very low pollution.


> How much of the released energy goes into cooking?

This would be the heat exchange efficency, it is  limited by the
flame-pot-contnets interface and is often in the 15-30% range. It can
be enhanced, often by simple skirts around the pot but is generally
culturally unacceptable. We have similar poor heat trnafer in
developed countries but then we tend to be profligate with our enrgy
use, it is a small proportion of our living costs.


> That would be my next
> measure.  That should tell me what weight of wood people have to collect to
> cook their food.  It is worth noting that the amount of energy that goes
> into cooking is also affected by the pots and lids used as well as how they
> fit onto the stove.

Yes lids reduce a lot of convection and reduce vapour loss during
simmering, it takes a lot more energy to turn water to steam than to
raise it to boiling point.

> It is also important to know how much energy was expended to get the fuel
> and prepare it for use.  Some of that energy is human energy so it gets
> treated a bit differently and has a different impact.  For example, it
> doesn’t convert simply to climate impact (are humans low global warming gas
> emitters?).  If you cut up the fuel a lot and process it a lot there is a
> cost there.  I don’t know how that stacks up for gasifiers vs other stoves.

Yes transport and comminution costs are a necessary part of the equation.


> Regarding charcoal.  I am presuming you can still use the charcoal.  I was,
> apparently erroneously, under the impression that gasifier stoves could
> continue to receive primary air and therefore burn the charcoal.  I actually
> liked that idea because it was simple and used most of the energy in the
> stove.

A traditional 3 stone fire burns all the wood out to a white ash, I'm
not sure what sort of gasifier stove other than the TLUD type you are
referring to, downdraught gasifiers tend to produce a high char ash
because of the nature in which the ash has to be continulaly
discarded. TLUDs can burn out the car but as I have pointed out
burning out the char requires different air proportions plus the stove
walls are subject to much higher temperatures whereas in the flaming
mode the hot flame is mostly above the stove.


> If you take the charcoal out of the stove you then have a couple of
> options for using it.  You can burn it in another stove, which has some
> appeal as you can do a different kind of cooking with it (e.g. BBQ, or ?).
> But also seems like quite a bit of work and complication for a small amount
> of charcoal.  Or, you can use it in the soil. So another question:
> Is a gasifier stove with charcoal (biochar) buried actually carbon negative?
> Then the other important measure: what are the emissions?

As I said they are linked, the use or efficacy of biochar tends to be
a contntious subject on this list so is best discussed on one of the
biochar lists, the economics of what you do with char as a residue
from cooking with a TLUD is pertinent to fuel costs and again the
subject of contention. With indoor air pollution being a major health
issue having a clean flame has a big value.
>
>
>
> And, a kind of crude question: with the ins and outs of this discussion is
> it the case that rocket stoves or some other stove is more efficient than
> the gasifiers?  In my question by efficiency I mean kg of wood required for
> a Cameroonian to cook their meals?
>
> Which stove do they have to carry more wood for and do more fuel preparation
> for?  (I’m not sure how you measure the combined work for those tasks).

If the wood is sticks I would say the rocket needs less fuel
preparation and converts more of the available energy, it is in
essence 1/3 of a 3 stone fire with some air control. Excess air and
moisture in the fuel are probably the biggest factors in reducing
combustion efficiency, the low temperatures from poor combustion
efficiency then have a deleterious effect on heat transfer to the pot.

AJH