[Stoves] Dry air dilution

[Crispin Pemberton-Pigott]

Dear Friends and testers

 

I have just returned from Yogyakarta where we were finalising the
construction of the lab for testing stoves participating in the
CSI-Indonesia Pilot programme. The system for measuring particles is worth
looking at because it has to work in a hot, humid place. This might be the
situation you face.  The problem is that the humidity is so high, and the
wood so damp, that with a boiling pot added to the mix, there is a lot of
steam in the exhaust.

 

This steam condenses into water droplets and this is 'seen' by the particle
counters if they are of the optical type. To get real time performance (in
order to know when a stove is working well and when it is not) it is
necessary to use an optical PM counter or a TEOM which measure real time
mass, but is expensive.

 

So here is a lovely shot of the key ingredient that makes it possible to
measure PM with an optical instrument even if there is water boiling, wood
drying, hydrogen burning and it is already really humid:

 



 

The black thing has an air inlet on the left and an outlet on the right
(little brass fitting). There are two black columns. Inside each is an
adsorbent material which strips the incoming pressurized air of CO, CO2 and
water vapour. This it can do for 30 seconds. Then it is mechanically
switched out of the gas line and the other is switched in. Some air from the
compressed side is washed over the relaxing column while the other does its
work. The CO, CO2 and water vapour are released by the adsorbent, no longer
pressurized, and the material can be re-used.

 

So every 30 seconds this swapping is repeated.

 

The output is 28 litres per minute of dry air with no CO2 in it. This is fed
into the diluter at a much lower rate (about 2-5 L/min as required) to
dehydrate the smoke from the stove while it is still hot. In doing so it
prevents the condensation of water vapour into droplets and the particle
counter only sees dry particles and condensed volatiles that are not too
diffuse to remain as gases. 

 

The filter set to the left of the adsorber is the pre-treatment for it, and
the two sets on the right are for the stack gases sample and the diluter gas
sample.

 

The little pistol looking things to the right of the top of the adsorber are
vortex tubes that create 'cold' for condensing the water vapour in gas
samples. They are cheap and reliable.

 

The CO2 in the Stack gases is determined, and so is the CO2 in the diluter.
Any CO2 in the diluter had to come from the stack, not the diluter air which
has had it removed. Thus a calculation of the actual dilution can be made,
and the dilution can be changed in real time as appropriate to suit the
stove emissions.

 

Using this approach the SEET lab has measured up to 17 g of smoke per cubic
metre, and as low as 1 microgram. 

 

The YDD Lab has been able to measure the emissions of the baseline Keren
stove and a Philips stove burning pellets. Those are the two extremes seen
so far. The system will also be tried in the National Lab which is an even
hotter and more humid place. 

 

The reason to go to all this trouble is that TSI (which makes the Dusttrak)
says in one of their technical notes that above a humidity of 70% the number
of water droplets forming in the air can exceed the number of particles! In
other words, it care is not take the PM count can be over-reported by more
than 100%. 

 

The diluter that the air is fed to looks something like this:

 



 

A metal pipe about a metre long fits into the right side. The two red lines
show the 'closed' position. The carrier is rolled back a few degrees to
create a gap for the smoke to enter the dilution tunnel, pulled in by the
treated air entering from the left.  The opening by the big hex nut is
conical and polished. The gap is about 0.8mm.

 

Details and drawings are available. There is one at LBNL, SeTAR, SEET and
YDD at this time. 

 

Regards

Crispin

 

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