[Stoves] Wattle cuttings

Sun May 7 10:01:11 CDT 2017

Andrew:  cc List

   	See more below.

> On May 7, 2017, at 5:28 AM, Andrew Heggie <aj.heggie at gmail.com> wrote:
> 
> Ronal you seem to be responding to original post from elsewhere? I
> don’t see a copy on stoves.

	 Apologies (to the full list - maybe also Dave).   You are right.  I just assumed that the two messages I was responding to were from the list. At the end I re-append the two photos that I thought everyone would have seen.  (Not sure if they will be stripped or not because of size limits.)
> 
> David also has an article on the stoves website
> 
> http://stoves.bioenergylists.org/content/elsa-stove

	Different Dave.
> 
> I've looked at some of the videos and there seems to be different
> iterations of the stove dating from 2014.

	Yup.  Dave Lello has done a good job of documenting via videos.  This message was also on a new stove.
> 
> Just two comments at this stage:
> 
> On 7 May 2017 at 02:28, Ronal W. Larson <rongretlarson at comcast.net> wrote:
> 
>> It's weight is about 1kg and at about 15 MJ for air dried wood should
>> produce about 4kw of heating power. (Of which we use 70 % in the gasifier).
>> 
>> [RWL2:  I see numbers above 15 MJ/kg for wood (and you use 18 MJ/kg for
>> pellets).
> 
> I believe softwood pellets can exceed 20MJ/kg because of their higher
> lignin content.

	I don’t recall ever seeing such a high number - but I like that since it better justifies the number 18 MJ/kg, which  is so nice to work with (being exactly 5 kWh/kg)

	In googling I found some references that could be helpful:

		a.  FAO  http://www.fao.org/3/a-i4441e.pdf <http://www.fao.org/3/a-i4441e.pdf>   uses kWh/kg.  That value close to 4 (not 5) with moisture content of 20%.  Lots on densities.
		b.  http://www.forestbioenergy.net/training-materials/fact-sheets/module-5-fact-sheets/fact-sheet-5-8-energy-basics/ <http://www.forestbioenergy.net/training-materials/fact-sheets/module-5-fact-sheets/fact-sheet-5-8-energy-basics/>   Found a value of 22.5 MJ/kg for 0 moisture resinous woods!   And below 10 for 50% moisture.
		c.   http://www.biomasstradecentre2.eu/available-literature/#CmsC34E1 <http://www.biomasstradecentre2.eu/available-literature/#CmsC34E1>  (and then choose the handbook)  This seemed the most thorough.  From the EU.
		d.  Wiki  https://en.wikipedia.org/wiki/Energy_content_of_biofuel <https://en.wikipedia.org/wiki/Energy_content_of_biofuel>  was interesting because of the many leads toothed wiki sites

	My conclusions:  I will try to switch to kWh units rather than MJ.  I will try to do computations with both 4 and 5 kWh/kg.  Might also stat thinking also about kWh in volume rather than weight units.

> 
> 
>> 
>> [RWL5:   Dave is here saying he is recommending a larger diameter for the
>> less dense wood stick fuel.
> 
> This makes logical sense in that the power varies with the amount of
> offgas being produced, as pellets are denser for a given amount of
> primary air the same offgas will be produced from the smaller area
> with the denser fuel. One of the reasons a pellet stove burns well is
> that the combustion takes place in a more compact volume.
	RWL:   There is also the issue of using fans/blowers.  Dave has two speeds plus a rotating micro-controller

> 
> Crispin made a comment in the "List of woods for TLUDs?" thread
> regarding denser fuels needing to be more finely  comminuted in order
> that more surface area is initially presented to the descending
> pyrolysis front.

	[RWL:   I’ll give this more thought, but we also know that the power level is directly related to the air (oxygen) flow.  Again the fans can overcome a lot of size considerations.
> 
> Actually this bears a bit more thought in itself. The less dense fuels
> are so because there is more air between the fibres and air is an
> insulator. So that the surface of a  "light" wood like willow or
> poplar will reach pyrolysis temperature faster than a dense wood like
> beech, which conducts heat into it's interior and this heat loss means
> the surface stays cooler for longer. Way back Tom Reed introduced us
> to the concept of Biot number  which is a measure of the ratio between
> heat received at the surface to the heat conducted to the interior of
> a particle.
	[RWL:  The air and thermal flows in wood is decidedly different in branches and chips (not pellets) with and cross grain.  Heat can travel longitudinally inside the branch than across, so one sees conical changes of color if one stops the pyrolysis process.

> 
> So for a given air flow and with woods of the same moisture content
> and size does the pyrolysis front migrate migrate downward faster?

	[RWL:  I haven’t got the question.  Faster than what?   I guess maybe the question is beech vs willow?   I think there are list members who have the equipment to test this.  

> Because more heat should initially be available to radiate and conduct
> downward.

	[RWL:  One important point may be that most/much of the heating in advance of the falling pyrolysis front is via radiation. This has to be a function of packing for branches stacked vertically for instance vs horizontally.
> 
> Does the net effect mean the pyrolysis zone is deeper with one or
> other densities? At end of burn the result is much the same as all
> particles are charred.

	RWL:  We had a dialog on this list (or was in the biochar list) about watching the pyrolysis from move downward using a pyrex window.  The surprise then was periodic views of downward-flowing gases.  But I think a visual test on pyrolysis zone thickness might be possible.  I’d wager there is a big difference in the height of the region from which visible radiation is being radiated for different packing densities.  As you say we are fortunate that this does not seem to be a critical parameter.  They all work.

Ron  (being sent without a final check.  Apologies for typos)
> 
> Andrew
> 
> _______________________________________________ 
> Stoves mailing list
> 

Two photos that Dave sent to Paul and I:

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