Re: GAS-L: Re: Pellet stove and Pelletizing Switch Grass

[Andrew Heggie <andrew.heggie@dtn.ntl.com>]

On Sun, 28 Jan 2001 09:08:07 -0500, Alex wrote:

>I am unsure what you mean by "a second generation stove". Perhaps 
>Phase Two EPA.

I was not trying to make a definite rule, I am not familiar with Phase
Two EPA, I was looking on the first generation as being the early
1980s domestic stoves and the next generation being essentially better
engineered versions of the same. The upcoming generation seem to have
been thought out better in terms of combustion technology especially
in the use of flue gas sampling to control air. On the domestic
boiler/water heater front the Danes seem to have some efficient
offerings.  The domestic visible flame stoves seem to be compromised
by the dispensation of not requiring testing if the air:fuel ratio is
>32:1. In fact this requirement (which I guess small manufacturers
would use as cost of compliance testing is high) is not necessarily a
good means of guaranteeing cleanliness.

I do not think it likely these EPA exempt room heaters can achieve
better than 70% conversion. Have you looked at the Dell Point test
results? They appear to achieve 80+% conversion with 200% excess air.
Now this worries me a little as I cannot get my Lambda probe to give a
meaningful output at this level. Further the efficiency is inferred
from the flue gas temperature and knowledge of the declared calorific
value of the fuel. With these hot air stoves this can be enhanced by
increasing cooling of the heat exchanger to a level that would be
uncomfortable in a domestic setting.

Is anybody able to comment on these all purpose flue gas analysers
that indicate efficiency figures and potential drawbacks? I had always
thought that an attribute of gasification was enabling of better
combustion conditions, I had assumed low excess air was such a
benefit.
> I have yet to actually see one burning. The people who 
>have, say that its flame is very different from other pellet stoves. 

Now this I find interesting, one of the features of these room heaters
seems to be that most secondary combustion is with a diffuse flame, it
is quite easy to provoke soot deposits even with high excess air.
Having played around a little  I would think premixing secondary air
will change the flame colour and appearance. I have been experimenting
a little in this area.

>I think, but I am not sure, that the top fed version, may be the 
>first stove to have no under fuel air supply. All the air comes in 

Well may be first commercial stove ;-)

>above the fuel. That doesn't necessarily mean that there is no air 
>contacting the fuel as the air could drop down. What exactly 
>happens, I don't know, however it appears as though this is how ash 
>slagging and fly ash is avoided. 


>
>> I am intrigued by the differences in particulates in the flue gases
>> between understoked and over stoked pellet stoves
>
>Could you point me to this reference. I looked for it, but did not 
>find it.

And to think you referred someone to the source quite recently, I am a
bit unsure about quoting copy write material directly but look in:
LOW EMISSION AND HIGH EFFICIENCY RESIDENTIAL PELLET-FIRED
HEATERS
James E. Houck and Andrew T. Scott
OMNI Environmental Services, Inc.
5465 SW Western Ave., Beaverton, OR 97005
 a file on their website called pellet2.pdf

page 5:
The carbon analysis revealed a
dichotomy between PM emissions from under-feed and top-feed pellet
stove models
(Figure 2) and between pellet stoves in general and cordwood stoves. A
large fraction of
the PM emitted from the top-feed model was elemental carbon.

Now what I find interesting here is that this is what occurs in
burning over-dry wood. I am fairly confident this soot comes from
cracking of pyrolysis products in the secondary flame. Whilst there
may be a lot of air passing up the flue I contend that this air has
never really been involved in the combustion process. As such
conditions in the diffuse (but attractive yellow) flame are hot enough
but not turbulent or well mixed to give complete combustion.

This ties in with what I think can be inferred by Mike Antal's work on
high yield charcoal, that is that conditions can be such that
pyrolysis products in the offgas can be cracked to form smaller
molecules and elemental carbon, in the high yield charcoal this "soot"
is held within the char matrix.

>
>What's in a name? It could be called a close-coupled gasifier based 
>on the conceptually air starved fuel. It is supposed to operate with 
>a very shallow fuel bed.

I agree there is no need to be picky over semantics but the language
is there to enable communication of ideas, in this case I wonder if it
misleads. I have asked Tom Reed about this, so far no response, I
accept pyrolysis is a form of gasification with a high char residue.
The next stage to my mind involves reacting the char via the producer
gas and water gas reactions to combine with the pyrolysis products to
give a combustible offgas. To burn the char to CO2 and use the hot CO2
and N to pyrolise the biomass above is a bonfire :-).
>
>> I would be interested to know if anyone
>> has monitored the CO:CO2 ratio in the offgas from the top down stove
>> prior to the secondary flame. 
>
>I could do this with a little dilution to bring the CO into the
>instrument's range  and a lot of filtering of condensibles.

I have some ideas here more related to the workings of the idd stove.
It relates to the CO2 to CO reduction and the range of temperatures
over which the equilibrium shifts. If there is no interest on this
list I would be happy to pursue this via e-mail.
>
>> And a major attribute of a pellet system to my mind is that it can be
>> switched off to conserve fuel. A problem with conventional stoves
>> seems to be that being batch loaded fuel is wasted after cooking is
>> finished 
>
>Not necessarily. I have stopped the top down process before 
>completion buy shutting down primary air. After letting it cool there 
>was an untouched layer of fresh pellets in the bottom, covered of 
>course with charcoal pellets with only a thin layer (<2 cm) of brown 
>pellets inbetween.

Point taken, I was however thinking along the lines of a metered
fuel+air delivery system in a low tech cook stove I have in mind.
>
>> 
>> Joacim made a very relevant point on the energy account, in general it
>> is not yet feasible to use biomass as the source for generating the
>> motive power to make the pellet.
>
>I understand what he is getting at. We need an affordable efficient 
>one hundred horsepower pellet fired heat engine and use for the waste 
>heat. Then its full steam ahead.

I still live in hope of a wood gasifier in this size range which is
affordable and reliable, unlike Peter S I *do* consider fuel
preparation worth investigating in this respect, however I do not see
it yet. I think there are always more than one way to skin a cat,
again it may pay to revisit the attributes of pellets used in the
western world and decide which bits are worthwhile in poorer regions.
>
>I think REAP is claiming that the pelleting can be done with biomass 
>energy using the  currently under utilized steam power capacity from 
>bagasse  at existing sugar mills. 

Well I am a strong advocate of small scale co generation, I have been
involved with micro chp so I am keen on this. Some years back on one
of the CREST lists we debated Jim Arcate's proposal to pyrolyse
bagasse and ship it in to a centralised power plant. With the price
structure of energy T&D I think reaching a final consumer may be more
worthwhile. With the bagasse being low value at source and the pellets
high value at retail there must be a lot of scope here.
>
>I wonder if pellets could be a heat  battery helping to store any 
>unused power production during fluctuations supply and demand. 
>How fast can these machines start and stop.

Now you've lost me :-(.
>
>What I am hearing now is that pellet demand is very close to out  
>pacing supply in North America thanks to  current oil and gas heating 
>cost. They seem to be holding the price steady, but quality is less 
>consistent.  It has been their mantra that pellets are the only fuel 
>which hasn't seen a price rise in ten years. 


I could see the same happening here if pellets take off. 
AJH
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