[Stoves] stoves and credits again

Andrew Heggie aj.heggie at gmail.com
Thu Sep 21 10:47:09 CDT 2017


I've changed the title to move back to the idea of what value can be
added to TLUD use:

On 18 September 2017 at 20:21, Crispin Pemberton-Pigott
<crispinpigott at outlook.com> wrote:

> As pointed out here years ago, the challenge for pyrolytic stoves to have a
> heat transfer efficiency that more than makes up for the additional fuel
> requirement. If you look back far enough I provided a formula for
> calculating the requirement.

Which to me is an impossible challenge so not worthy of discussion,
the simple fact is that if 50% of the energy remains bound up in the
char then it is not available for cooking. The heat exchange
efficiency will not differ much between a TLUD flame and any other
stove using a flame. There might be a slight case for saying a
gasifier stove can achieve  a lower massflow (particularly lower N2)
because the primary combustion doesn't go to completion so less
primary air is used,  the corollary may be that the secondary flame
also can be burned with less excess air because the offgas has a
higher calorific value but not enough to make up for using 50% less
energy..
>
>
>
> As whole-wood burning cooking stoves have reached the 33-35% efficiency
> range, the efficiency of a gasifier has to be high enough to compensate for
> the additional fuel, if the fuel savings matters to the project, which in
> the case of CDM and GS it does. If the char retention is 25% of dry mass,
> and that fraction contains 50% of the original energy in the fuel (at
> least), then the stove will have to be twice as efficient as the wood
> burner, i.e. 66-70% heat transfer efficiency. In theory it is possible, in
> practice I haven’t seen it. All pyrolytic gasifiers consume more fuel than
> the best wood burner available at the time.

I agree all that but cannot see why the heat transfer efficiency
between the two types should differ.

> Your stoves might compare favourably with an open fire or a declared 10 or
> 15% efficient baseline, but they will not be as fuel efficient as a stove
> that burns wood completely.

In my view this is only true if you are going to argue that the
remaining char is a heat loss to the system, I argue that it is a co
product  which contains unused potential chemical energy. If Kevin and
Paul get their spreadsheet terms right the value of carbon credit can
be calculated. Apart from being put off examining Kevin's spreadsheet,
because it uses imperial units and thus not checking the calculation,
he is entirely right that the carbon credit payable to the stove user
must exceed the fuel value remaining in the char.
>
>
>
> A stove that burns wood completely paralleled with a small charcoal making
> operation in the same community might use less total wood and produce more
> total char because both technologies can be optimised to their function.

I really cannot see this, see above. To make charcoal in a dedicated
device still requires that the offgas is used for it to be efficient
as there is an excess over that necessary to raise the raw material to
pyrolysis temperature, unless it is exceptionally wet.


>If
> the purpose is to create the most char and the most cooking from a given
> source of biomass, so at least, a pyrolytic gasifier is not the best option.


...and this depends on what sort of char you require. For smoke free
cooking it needs to be made at sufficiently high a temperature that it
burns without evolving a tarry offgas. As a soil amendment the lower
temperature char will contain the same minerals plus some
hydrocarbons which bugs will feed on for a while but it won't have the
higher cation exchange capacity which growers want nor the level of
adsorptivity to hold organic compounds (in order to prevent leaching
or oxidation of soil organic matter) which growers desire. As a result
the fixed carbon retention is also less than char made above 600C.


> It is an option but it is not yet out-competing other technology
> combinations. The cleanest wood burning stoves are as clean-burning as an
> LPG stove, or there is not enough between them to find a meaningful
> difference.

Good it's nice to see that reiterated, it means it remains a goal, to
aim for the dissemination of better solid fuel cook stoves.
>
>
>
> If one can sift through a biomass source and take out everything ideal for a
> wood burner, and pyrolyse the rest into char, that is a reasonable thing to
> do if the char has a use or value.

Of course that is what we did in UK but in that instance the logs and
firewood were luxury goods and not necessities.
>
>
>
> I proposed two years ago that in Hebei, which has a serious problem with air
> pollution caused by the in-field burning of crop residue, that they put a
> small price on the material making it worth collecting it to a central
> point. This could be charred while making wood gas that can be piped into
> the local distribution network. The remaining char would go into the input
> line of local fertiliser factories that are making organic fertiliser, of
> which char is one component. There are multiple factories making these
> products, almost of all of which is sold outside the province to others
> which subsidise organic fertiliser products (Hebei doesn’t).

There appears to be a win win situation here and I gather there is
still a vast part of equatorial Africa where annual burning  takes
place. However it brings me to another reason I like the idea, though
not the practicalities, of a householder-subsistance farmer being paid
a subsidy funded by the developed world. The trouble is I have a
parochial view and not a good worldview of what types of persons
depend on biomass fuelled stoves. Are they also predominantly growers?

Please bear with this preamble:

When biomass burning  came to the fore in UK it was largely from
forestry residues which typically had mineral ash of about 1% of the
dry matter, less if left to lose their needles prior to harvesting.
Then a firm had a simple  steam turbine design , taken from a naval
design, and their raw material was agricultural waste including the
litter from chicken houses. The chicken litter was for free, apart
from haulage, but straw from OSR, wheat, barley and oats had to be
bought and it wasn't long before farmers realised the price being
offered was less than the cost of the mineral fertilisers that had to
be replaced before another cereal crop could be grown. Combine
harvesters were modified to chop the straw rather than bale it for
sale, so the straw could be incorporated into the soil for the next
season's growth.

Nothing is ever black or white so there remain both baling and carting
and straw chopping and incorporation depending on local variations in
costs and returns.

A bit long-winded but to my point: we are, on this list, addressing
relative poverty. If it were not so everyone could have gas and
electricity for cooking. So I am happy to see these carbon credits and
CDM?? neither of which I am familiar with, used to subsidise improved
stoves. I like the idea of paying a near subsistence farmer to put a
char  soil amendment in the soil because it becomes a cash crop that
she/he does not have to go to the trouble of exporting away from the
locality with the aforesaid loss of mineral fertility which I suspect
in many cases is not being replaced.

Here in UK we have a very benign, if cold and occasionally miserable,
climate but we do have a history of soil deterioration from
overgrazing and export of minerals on the hoof in some lighter soils
which were the ones initially cleared from the wild-wood that covered
much of southern Britain. We also know when European farming practises
were exported to the American midwest that the climate there was less
forgiving of old world practices.

So my contention is that apart from the carbon credit there is a value
to the land in not having to export a cash crop.


Andrew




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