[Stoves] Torrified Pellets

Sun May 24 14:05:51 CDT 2015

Dear Dean

I am somewhat surprised by this comment:

“With biomass the preparation including recipe, drying, pellet size, etc. makes a big difference in emissions when trying to get down to the very low levels needed to protect health.”

Public health is protected by complete combustion. Full stop.

‘What is smoke?’ is discussed by Prof Harold Annegarn when teaching about PM at university. He says it is unburned PM or more precisely, it is composed of condensed volatiles and dry particulate matter, all the result of incomplete combustion. Combustion is the responsibility of the combustor, not the fuel.

Wood doesn’t make ‘smoke’, incomplete combustion does. If a combustor burns the ‘smoke’ then there isn’t and ‘smoke’ emitted. All fires make ‘smoke’ then burn some portion of it. The better suited the stove is to the fuel, the better the combustion efficiency and the less smoke there is.

It worries me that someone plans to introduce something that people will call a ‘clean fuel’ based on a demonstration that under certain conditions it doesn’t produce ‘smoke’. 

There is a whole set of thinking about outdoor PM that is based on the idea that some fuels are ‘clean’.  What they really mean is that they are easier to burn cleanly (given what they know about available combustors or ones that might be made).  There is no such thing as a clean fuel. Even hydrogen can be criticised for producing 100% a GHG – water vapour – as the only combustion product. 

Coke (which is just about pure carbon+ash) can produce lots of particulate matter if the combustion conditions are not correct.  Wood pellets can be burn extraordinarily cleanly – see Austrian conference papers from May 2008.  Torrefied pellets, whether made before or after heating, might be ‘cleaner to burn’ in a stove not designed for the unprocessed fuel, but that has no impact on the underlying principle that a stove+fuel combination is what produces smoke, not a fuel on its own. ‘Smokiness’ is not a fuel property.

Look at the conceptual bias in the report here <http://siteresources.worldbank.org/INTMONGOLIA/Resources/3HelmutVierathPresentaionRevisedApril10200812pVersion.pdf> , slide 4. Imagine what this costs (slide 10 <http://siteresources.worldbank.org/INTMONGOLIA/Resources/3HelmutVierathPresentaionRevisedApril10200812pVersion.pdf> ). The UK even has a ‘smokeless fuel’ description in their Clean Air Act! There is no mention whatsoever of stove+fuel combinations that produce virtually zero smoke – because they didn’t know such a thing was possible.  It was ruled out before even trying.  It shows how deeply ingrained the misunderstanding is about ‘smoke’.

During tests conducted by Prof Günter Baumbach <http://www.wtert.eu/default.asp?Experte=202618>  in Mongolia in March 2008 (for the European Bank for Reconstruction and Development) he obtained some emission rates. On a different topic, article here <http://www.wtert.eu/default.asp?Menue=1&ArtikelPPV=17691> , he concludes, “Based on the correlation coefficients it can be concluded that in the investigated residential site about 36% to 64% of ambient PM 10 concentrations can be attributed to residential wood combustion.”

This is not exactly true if we really get into it. The real cause is that the residential combustor failed to burn the wood smoke. The report directly blames the fuel!  Thus a stove that fails to burn wood, or torrefied pellets of wood, or kerosene, or anything else, is caused by the suitability of the stove design, not the fuel.

Thus the entire concept of ‘clean fuels’ and ‘clean stoves’ is flawed. We have two groups of people doing exactly the same thing, in two separate rooms, one group proclaiming that ‘the fuel is clean’ and in the other claiming that ‘it is a clean stove’.  Only a combination of factors gives rise to ‘smoke’.  Attributing to one of the factors the result (an emergent property) of the combination of all contributing factors is a ‘category error’.

Prof Baumbach’s experiment showed that for both traditional stoves and the improved stoves available at the time, using semi-coked coal briquettes produces at least as much and up to 35% more total PM10 than raw coal burned in a traditional stove in a traditional way.  That was a very useful result, though it was not included in their final report. Why? You’d think that was of primal importance to the City of Ulaanbaatar.

In this report <https://urbanhealthupdates.wordpress.com/2010/03/23/mongolia-urban-smog-caused-by-coal-stoves/>  it mentions ‘high quality semi-coked’ coal, as if removing volatiles was a way to ‘improve the quality’.  No stove burning semi-coked coal produces as little PM2.5 as a stove designed to burn Nalaikh (the traditional fuel). The reason is simple: getting semi-coke lit and refuelling it produces more PM that the whole operation of a raw coal + improved stove. Plus it is three times the price. Why burn money when you can burn raw coal with lower emissions?

Torrefaction is based on the same principle. Semi-coked wood is no guarantee that the result will be as good as an improved stove perfected to burn ‘raw’ wood pellets. The volatiles are very helpful in reducing emissions because they make it far easier to speed the ignition (a big problem with semi-coked products) and to maintain the flame. The fact that many current stoves don’t do a good job is not the fault of the fuel.

All Turbofan jet aircraft run on kerosene (Jet A <http://www.csgnetwork.com/jetfuel.html> ) and they don’t make any smoke at all yet thousands of people have been led to believe that ‘kerosene is smoky <http://newscenter.berkeley.edu/2012/11/28/kerosene-lamps-black-carbon/> ’. This ‘clean fuels’ meme is clouding the real issue.

Regards

Crispin

Dear Ronald,

Thanks for your helpful comments. Charcoal without remaining wood in it doesn't make smoke but, of course, wood really likes to smoke. With biomass the preparation including recipe, drying, pellet size, etc. makes a big difference in emissions when trying to get down to the very low levels needed to protect health. I'm making some torrified pellets and will report back after testing under the hood.

All Best,

Dean

On Sat, May 23, 2015 at 3:52 PM, Ronald Hongsermeier <rwhongser at web.de <mailto:rwhongser at web.de> > wrote:

Dear Dean,
I may seem like I'm harping here, but now that you've said a bit more, you've mentioned a couple of factors that I have noticed in my limited experience without a hood and therefore without concrete parametric analysis, but for which I can give some postulates.

1. The wood-gas stove in design mode depends on a uniform fuel "chunk-size" which promotes a level "pyrolysis" front(planar) migrating towards the bottom of the stove. If that plane is broken by dis-uniformity in the fuel or overly large gaps between the fuel pieces, you will get a spot drop in temperature along with glowing fuel which will migrate unevenly towards the bottom of the stove, breaking the pyrolysis front and sometimes dropping the mean temp inside the burning chamber such that the rising wood gas will no longer be close enough to critical temp that the onset of 2nd-ary air will ignite it.

2. 1. would be consistent with the bark observation. Bark generally has lots more minerals and less energy per unit mass. Did you/they assure dryness level of the bark? Were the pieces between (cross-section/10-20) of the stoves throat? Do you mean pellets from eucalyptus wood or when eucalyptus oil is poured on the fuel? Off the shelf kerosin burners here in germany are strictly regulated as to what you may or may not burn in them. If it is smoking it is either wrong fuel, design flaw or user error.

3. Pellets pack closely, so if they are smoking, usually the problem can be fixed by either turning up the fan a bit or putting some chimney length between the secondary air and the pan to increase the draw. (i.e., the primary air is probably not of sufficient pressure to deliver enough O2 to the pyrolysis front to keep it hot enough such that it will ignite upon contact with the secondary air.)

4. Because torrefied pellets are partially reacted, they may be somewhat less dense and especially because the ends will be cleaner, they may not resist airflow as much, which may help somewhat with 3. above. I would expect them to act very similar to dry pellets, except for the difference in density.

regards,
Ronald von Aftermidnighttimeforantibiotics (and bed)

On 23.05.2015 23:49, Dean Still wrote:

Dear Ron, 

We had three women interns here at the research center for a summer who found cleaner burning recipes for the TLUDs gathered from the surrounding forest. Some things like bark make smoke, etc.

Many pellets smoke when using eucalyptus, etc. Different mixtures of kerosene make more or less smoke in off the shelf stoves.

I'm wondering if torrified pellets will burn cleaner than normal pellets.

Best,

Dean

On Sat, May 23, 2015 at 1:28 PM, Ronald Hongsermeier <rwhongser at web.de <mailto:rwhongser at web.de> > wrote:

Hi Dean,
I remember that there was a proposed university program (W. Virginia or the Carolinas??) In view of the deaths(not _many_ but tragic) that have taken place due to off-loading of pellets at some European ports (CO in the hold) The torrefaction could lead to a more inert fuel. That's hopeful speculation on my part however. You would be transporting some less bound water and the energy density is better than regular pellets in addition to the lack of liquid or vapor H2O absorption issue mentioned before-- these would lend efficiencies in the logistics end of things and an even more uniform fuel than regular pellets, which tend to absorb quite a  bit of water here in the damp winter weather...
I have to admit that I'm a bit puzzled as to what you mean by cleaner fuel recipes. What specific pollutant are you interested in or are you talking about particulate?
If you aren't choking the burn by putting the pot too close, you should be getting a very clean burn with a TLUD?!?

regards,
Ron

On 23.05.2015 22:01, Dean Still wrote:

Hi Ronald, 

I'll keep looking. I think that we shouldn't forget cleaner fuel recipes as part of the solution. 

Best,

Dean

On Sat, May 23, 2015 at 12:33 PM, Ronald Hongsermeier <rwhongser at web.de <mailto:rwhongser at web.de> > wrote:

Hi Dean,
I did some searching around on the internet several years ago on this topic.
I'd like to try torrefied stuff as well, but I struck out at that time.
No idea where to find them.
The main advantage that I picked up on at that time was that they should be relatively inert as to picking up extra water after they were pelletized.
Cleaner burning than what?
clean burning is a control issue, not a fuel issue as far as I understand. If I understand the concept correctly, it's like using part of the large molecules initially broken out (cracked) by the heat in the reaction vessel to coat the surfaces of the remaining unpyrolized material. This should burn quite okay in a TLUD.

regards,
Ronald von Nasennebenhöhlenhölle (but I'm coming back)

On 23.05.2015 20:56, Dean Still wrote:

Hi All, 

I'd like to try torrified pellets in a TLUD under the emissions hood.

Any ideas on where to find them?

Or if it should be cleaner burning?

Best,

Dean

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