[Stoves] Lowering emissions

Mon Jan 8 23:39:36 CST 2007

Dear Tom, Dean and All:

There has been a great flurry of interest in emission testing at the 
stove site recently.  Good and bad.  Let's keep things in balance here.  
I have a simple fan hood over my stove test balance.  It has a CO (<$50) 
meter in the throat and tells me my CO levels and occasionally even our 
WoodGas stove makes CO.  (usually after I have burned all the volatiles 
and only charcoal remains with insufficient air to gasify it.  We're 
working on widening the ratio of combustion air/gasification air to cure 
this.)

I approve of simple emission measurement, but would put ten times the 
effort into improving the stove design, since low emissions and high 
efficiency go hand in hand and people are starving for want of a fuel 
efficient stove in many parts of the world.

Here in the US we have become obsessed with low emissions and are 
willing to spend any $$$$ to drop the emissions farther after they are 
already much lower than any other country in the world. 

Bryan Willson has focused on designing a two stroke engine that can drop 
emissions a factor of ten while doubling efficiency.  He is retrofitting 
his system in Manila and soon to be other parts of the world.  We should 
do a lot more of  improving the stove design with good mixing and 
control of combustion air while keeping a general eye on emissions which 
will go down dramatically.

Keep up the good works...

TOM REED             BEF STOVEWORKS

Dean Still wrote:
> 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
>
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> -----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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