[Stoves] Between PM 2.5 and PM 10

[Crispin Pemberton-Pigott]

Dear Julien

 

Thanks for the explanations. As it is clearly a subject at the early stage
of investigation we will expect to hear a great deal more on the subject.

 

>The cookstoves they tested were (1) a 3-stone fire, and (2) the “Chulika”
rocket stove (distributed by iSquareD, Bangalore, India), both burning
sticks of hemlock; and (3) the fan-powered  “Oorja,” TLUD stove (First
Energy Ltd., Bangalore, India) burning commercial wood pellets

 

The fan stoves have the ability to loft a lot of ash so it is not a surprise
to see the emission profile change. So far I have found that there is very
little ash (evidenced by larger particles) in natural draft stoves, even
those with 3 metres of chimney. Wood and coal ash can be quite fine so it is
easy to get airborne. When there is no stirring of the fire which shakes ash
into the gas stream, the particle size is usually 99% <1.0 µm.

 

>> Something that is not clear from the abstract is whether the 

>> 'increase' in particles is an increase in the number of particles or 

 

>Number of particles.  Over a range of 15 to 660 nm particle diameter, they
measure the number of particles per unit volume of air, as a proportion of
the mass of carbon emitted 


 

I see two potential problems with this. The first is that stove often make
char these days and the amount of carbon detected is not a true indication
of the amount of fuel burned. It is one of the reasons the SeTAR HTP uses a
chemically balanced approach to calculations rather than a Carbon Balance
one. The second problem is whether it has been shown that the number of
particles is more important than the mass, epidemiologically speaking. They
have to report something but it is interesting to watch both counts, as it
were.

 

>
but there was a decrease in the size of particles emitted.

 

The most likely reason is that the stove is producing gas and burning it –
in other words it is staged combustion. This is well known and it not
restricted to TLUDs or any particular stove type, though it is clear that
open fires and sheltered fires (like the Rocket) are basically the same in
their combustion method. Where there is a clear separation within the
combustion zone(s) between solid fuels and burnable gases, there are far
fewer particles.  This is really easy to demonstrate with coal stoves. All
coal fires are gas fires. The only way to get significant PM from it is to
evaporate something liquid and then cool it without combustion. It is a PIC
(particle of incomplete combustion) not a product of the gas fire. A very
simple measure to prevent the PIC’s from ‘getting away’ in a Mongolia
traditional stove is to put in a ‘flame tube’ where all flame and smoke and
CO are bought together at the rear of the stove. This simple addition
(taught as a retro-fit) reduced PM about 80% and cost only $1 for a
stainless steel tube (from a milk pipe supply shop).

 

>The increase in efficiency is good, because there is less fuel used (fewer
trees cut, less time gathering wood, less time cooking and attending the
stove), and a lower total mass of particles emitted.

 

We have to be really clear what is meant by ‘increased efficiency. The
reason, at this time, for such caution is that the CDM credit system
attributes fuels savings indirectly, on the basis of a change in the heat
transfer efficiency. With char making stoves – now gaining popularity at
least among enthusiasts, there can be a large increase in heat transfer
efficiency (because of burning gas in close proximity to the pot) while not
reducing the fuel consumption at all. We are very clear at the CSI project
in Indonesia that the meaning of ‘efficiency’ is ‘system efficiency’. System
efficiency is defined in terms of the potential energy available in the fuel
provided, and the % of that which gets into one or more pots. 

 

This does not raise the question of combustion efficiency v.s. heat transfer
efficiency. One can make more PM (in a different spectrum) while increasing
the heat transfer efficiency.

 

>
When all trade-offs are considered, a better nutrition and quality of life
may come from using improved cookstoves, even though they may pose a new set
of health risks down the line.

 

Using a chimney or hood almost always makes a big improvement to the kitchen
air quality.

 

>That is an interesting point that there is a natural background of
untrafine particles.  

 

Burning grass and forests create vast amounts of PM of all sizes. Cool fires
with damp grass are particularly ‘productive’. I think the brown cloud of
India is more because of biomass burning in the fields than from cooking
fires.

 

Huge areas of Africa are burned each year as part of the grazing cycle. Many
plants require fire each year to survive. We may have a remarkable tolerance
to UFP’s but I see no guarantee of it. Perhaps we are and have been sick for
centuries! At least we can eliminate the ones we produce deliberately.

 

Regards

Crispin

 

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