[Stoves] Lowering emissions

Mon Jan 1 20:48:12 CST 2007

Dear Tom,

This has been a busy year and I have not done a great job describing in
STOVES what we have experienced here at the Aprovecho lab and in our stove
projects. I've got the day off and it's a quiet night at home so here's my
attempt at an emissions update.

If I remember correctly, Nordica told me a couple of weeks ago that we have
done over 100 Water Boiling Tests this year under the emissions hood. UCB
has refined the WBT a bit this year to include a smaller pot that is used if
the low powered stove cannot bring the 5 liters of water (in the 7 liter
pot) to boil. Nordica does most of the testing these days because she can
get about 10% coefficient of variation between tests so that 3 tests (each
test is: boil from cold start, boil from hot start, simmer for 45 minutes)
are statistically significant. 

Stove Camp was amazing! So many folks built good fan stoves! The mixing of
gases, air and flame created by low volume, high velocity jets of air
reduced CO and PM dramatically. The good fan stoves were something like 9
times cleaner compared to a Rocket stove. Even blowing air (not preheated)
up into the fire from underneath got close to accomplishing the same results
as fancier techniques like preheating air and having air blow both into and
above the fire. A five to ten dollar fan stove powered by the grid is
definitely possible. Burning wood almost as cleanly as a liquid fuel is a
reality. Unfortunately, it is easier to do this in a batch stove but I'm
sure that with a bit more experimentation someone will learn how to do about
the same thing when burning long sticks.

It seems that it is possible to 'scrub' PM by having the flue gases go over
surfaces, by extending dwell time inside the stove. PM is lower from griddle
and other chimney stoves even when the combustion chamber is not optimized. 

IN MY OPINION, lowering emissions is a compromise between making the
channels near the pot small (which decreases wood use and speeds up time to
boil) and increasing air into the fire (which decreases CO and PM). Excess
air is needed to lower emissions.

On the other hand, the 'charcoal making' stove as designed by Ron Larson,
Tom Reed, Dr. Karve reduces air to the fire so that little flame is made.
For some really weird reason that I don't understand, this technique burns
very cleanly producing low levels of CO and PM while creating the end
product of charcoal. This simple approach fascinates me and seems very
promising.

CO rises when charcoal is being made in the fire with lots of air and when
flame above charcoal in this kind of (more active) fire diminishes. But when
flame goes down, PM goes down. It is a balancing act to reduce fuel used, CO
and PM. In a Rocket type stove, the lighter the insulation around the fire,
the better. It takes something like 800C to burn up CO and the better
insulation creates a hotter fire. Also lighter insulation absorbs less heat
which results in faster time to boil and lower fuel used which equals lower
emissions.

It took 10 minutes or so for temperatures to rise to around 800C when we
burned charcoal in a highly insulative Rocket combustion chamber 16" high,
4" in diameter open at the bottom. When temperatures went above 800C or so
the CO2 stayed the same but CO went down close to zero.

The lowest emission stove is a system which includes a pot that is the best
possible heat exchanger. An optimized pot dramatically improves heat
transfer. This pot is big on the bottom and small on the top. The big bottom
has more surface area to absorb heat. The small top exposes the least
necessary surface area of water. Reducing the water surface area reduces
evaporation which is how pots loose the most energy. When the lid is used
very little energy is needed to maintain simmering 3 to 6 degrees below full
boiling temps. Dr. Alan Berick has shown that cooking oil can also be
floated on the water which forms a barrier reducing evaporation to near zero
during simmering if bubbles do not break the oil barrier.

Emissions are lowered by about 1/3 if a pot skirt is used that forces hot
flue gases to scrape against the sides as well as the bottom of the pot in a
very narrow channel. Channel size increases with firepower. The relationship
is shown in our Design Principles booklet.

As mentioned, very little power is needed to simmer if a pot lid is used.
Most wood stoves cannot keep the tiny fire going at this very low level. So
energy is wasted during simmering because the fire is larger than necessary.
I won't steal his thunder but Dr. Alan Berick will display one solution at
ETHOS.

So, the combustion chamber, the pot and the heat delivery techniques to the
pot all contribute to lowered emissions. Efficient heat transfer to the pot
is just as important as combustion efficiency for clean cooking with wood.
(Institutional sized stoves have much lower CO and PM per liter of food
cooked compared to family sized stoves/pots.)

This just about covers what I think that I have seen this year. Of course,
what I've written are just my impressions offered as hypotheses. Happy New
Year to everyone! See some of you at ETHOS!

Best,

Dean

-----Original Message-----
From: stoves-bounces at listserv.repp.org
[mailto:stoves-bounces at listserv.repp.org] On Behalf Of Tom Miles
Sent: Sunday, December 31, 2006 9:48 AM
To: 'Discussion of biomass cooking stoves'; 'Crispin Pemberton-Pigott'
Cc: 'Marlis Kees'; 'David Hancock'
Subject: Re: [Stoves] Emissions
fromResidentialWoodCombustion:EffectofMoisture on Emissions

Thanks Dean. 

As a general comment we would all benefit if everyone included more detail
about the fuels they burn, species, moisture content, size, etc. As to fuel
species, there are differences in the way that hardwoods, softwoods and
woods of different densities decompose during gasification and combustion
that affect the design and efficiency of the stove. More information will
help us understand a particular local application or test.

Thanks

Tom 

-----Original Message-----
From: stoves-bounces at listserv.repp.org
[mailto:stoves-bounces at listserv.repp.org] On Behalf Of Dean Still
Sent: Saturday, December 30, 2006 11:40 PM
To: 'Discussion of biomass cooking stoves'; 'Crispin Pemberton-Pigott'
Cc: 'Marlis Kees'; 'David Hancock'
Subject: Re: [Stoves] Emissions from
ResidentialWoodCombustion:EffectofMoisture on Emissions

Dear Tom,

I hope to get the ok from the publisher to have an 'early release' of
results for ETHOS but we'll see. 

All Best,

Dean

-----Original Message-----
From: stoves-bounces at listserv.repp.org
[mailto:stoves-bounces at listserv.repp.org] On Behalf Of Tom Miles
Sent: Saturday, December 30, 2006 6:26 PM
To: 'Discussion of biomass cooking stoves'; 'Crispin Pemberton-Pigott'
Cc: 'Marlis Kees'; 'David Hancock'
Subject: Re: [Stoves] Emissions from Residential
WoodCombustion:EffectofMoisture on Emissions

Dean,

You presented some charts of the results of those tests in your ETHOS
presentation last year but we haven't seen complete reports with data tables
that show the different stoves, tests, fuels, conditions, etc. Will you be
putting more complete information from your benchmark tests on the Aprovecho
website?

Thanks

Tom   

-----Original Message-----
From: stoves-bounces at listserv.repp.org
[mailto:stoves-bounces at listserv.repp.org] On Behalf Of Dean Still
Sent: Saturday, December 30, 2006 5:51 PM
To: 'Crispin Pemberton-Pigott'; 'Discussion of biomass cooking stoves'
Cc: 'Marlis Kees'; 'David Hancock'
Subject: Re: [Stoves] Emissions from Residential Wood
Combustion:EffectofMoisture on Emissions

Dear Crispin,

The wood used in the Aprovecho tests is Douglas fir which is a hard
softwood. The UCB WBT accounts for wood type. 

The proposed benchmarks were not based on Rocket stoves. They were based
instead on testing many different types of stoves (more than 30 now) ranging
from the three stone fire to very clean burning stoves. Rocket types are in
the middle range.

Emissions do change depending on the moisture of the wood. So does fuel
used. The University of California at Berkeley revised Water Boiling Test
Excel spreadsheet accounts for moisture content in wood used. The wood we
have used averages around 10% moisture content in the winter. 

As I pointed out yesterday the WBT is designed to minimize confounding
variables to highlight the performance of the stove. The UCB revised
Controlled Cooking Test which follows the WBT uses all local cooks, pots,
fuels, food, techniques. The Kitchen Performance Test is the third test in
this series, a larger survey of stoves in use in kitchens. 

The proposed benchmarks are based on the WBT because Aprovecho has a
'library' of WBT tests (done under an emissions hood). One fine day we hope
to have a similar 'library' of CCT and KPT results (with emissions) from
which to evolve definitions of improved performance. 

All best,

Dean

-----Original Message-----
From: stoves-bounces at listserv.repp.org
[mailto:stoves-bounces at listserv.repp.org] On Behalf Of Crispin
Pemberton-Pigott
Sent: Saturday, December 30, 2006 2:14 PM
To: Stoves
Cc: Marlis Kees; David Hancock
Subject: Re: [Stoves] Emissions from Residential Wood Combustion:
EffectofMoisture on Emissions

Dear Testers

I reviewed the paper by Fernando Preto and noticed interesting and important

things throughout the document.

For example, testing done with softwood gives consistently higher CO than 
hardwood. Higher moisture content gives consistently lower CO, all the way 
to 35% moisture (dry weight basis I believe).

This means that benchmarks proposed for emissions which have been based on 
low moisture softwoods (as is the case in the draft proposal forwarded for 
discussion in Bonn for WHO, could be quite misleading. We could set a lower 
permissible level for CO.

The reasons for this is that the proposed benckmarks were based on Rocket 
Stoves mostly optimised (dimensions and operator technique) for low CO in 
ppm.  The emissions may have been significantly lower had the wood contained

15 to 35% moisture, and the PM 2.5's would have been affected either up or 
down.

The vast majority of particulate emissions from wood fires are PM 2.5's 
(something over 85%) and they seem to follow a path inverse to the CO 
production.

It is clear that testing of stoves has to be done with wood that is 
reasonably moist and comparable with field conditions, and then the stove 
dimensions optimised to that fuel. Also, the operator of the stove has to be

familiar with how to operate it with that particular fuel.  Reproducing the 
exact method as used with very dry fuel will not perhaps give the best 
result.

Both the USA and Canada seem to be leaning towards using a dry weight basis 
for testing and certification.  This is inconvenient for us with our simple 
scales, but it is only a matter of mathematics and can be resolved on the 
spreadsheet.

One must again remember that these tests by Preto are 'device dependent' and

do not constitute the expected emission of cooking stoves, only standard 
(three sets or types) of box-stoves for space heating.  Surprise still might

be lurking.  I would not be surprised to find improved cooking stoves are 
cleaner than space heating stoves.

If a stove is going to be primarily used with hardwood, it should not be 
tested with softwood because the emissions are quite different.  The biggest

difference of all is between dry softwood, as used in APROVECHO standardised

testing, and moist hardwood, especially in CO production.  As make cook in 
fact use moist hardwood of many species (softwood being a lousy fuel) the 
relevance of APROVECHO's standard fuel test results is suspect.

It is likely that the target emissions or benchmarks for CO and particulates

can be reduced if Preto's work is borne out for improved cooking stoves.

A very interesting set of developments!

Regards
Crispin

----- Original Message ----- 
From: "Tom Miles" <tmiles at trmiles.com>
To: "'Discussion of biomass cooking stoves'" <stoves at listserv.repp.org>
Sent: Saturday, December 30, 2006 2:24 PM
Subject: [Stoves] Emissions from Residential Wood Combustion: Effect 
ofMoisture on Emissions

Emissions from Residential Wood Combustion: Effect of Moisture on Emissions
Fernando Preto, Canmet Energy Technology Center, Canada, Paris, October 21,
2005
 <http://www.ieabcc.nl/meetings/task32_Paris_ssc/Preto.pdf>
http://www.ieabcc.nl/meetings/task32_Paris_ssc/Preto.pdf (1.3 MB pdf)
http://stoves.bioenergylists.org/en/canmetiea32paris
<http://stoves.bioenergylists.org/en/canmetiea32>

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