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

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

On Sun, 21 Jan 2001 16:50:15 -0500, Alex wrote:

>Dear Tom,
>The stove you mention below  is not quite a "conventional" pellet 
>burner. Aside from being clean burning and efficient, it has been 
>designed to handle high ash pellets as well as the regular pellets. 
>That's what  makes it even more  unique and relevant. 

Where do you think it differs from a second generation stove then?
>From my cursory look it seems the same layout as the top fed unit in
my front room with some sort of feedback loop, probably a lambda
sensor in the flue and flue gas temperature monitoring. The big
difference is probably to do with US regulatory compliance testing.
Early stoves appear to have been exempted if the air fuel ratio
exceeded 32:1. At this level of excess air both the efficiency is
dropped and you are outside the range of a Lambda sensor.

I am intrigued by the differences in particulates in the flue gases
between understoked and over stoked pellet stoves (I see mention of
understoking in the report) CRE in britain developed self de-ashisng
understoked devices in the post war years. I thought the difference
was relevant to the previous thread on "too dry".

What about the talk of controlling pyrolysis temperature in the lower
part of the combustion chamber (I hardly think is deserves the label
of gasifier, it looks like an updraught device not far removed from a
bonfire)? If the temperature is kept too low the bottom part of the
fire merely pyrolises the material falling onto it in the combustion
product of the charcoal. If the charcoal layer is deep enough (20
particle diameters using Tom Reeds rule of thumb) and hot enough (and
it looks hot enough in mine) then the pyrolysis takes place in a flow
of largely nitrogen and CO. 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. To prevent slag forming I would have
thought the temperature in the combustion box needs to remain below
the ash slagging temperature and air flow low enough not to fluidise
ash formed.

<snipped discussion on trade off of a high capital cost refined fuel
over low cost and inefficiently converted refined fuel from agri
residues>
>wood resources. If REAP is right the 7% energy invested in pelleting 
>could possibly double the net energy yield for cooking not to mention 
>all the other higher efficiency applications which start to become 
>possible when biomass is burned almost  as effectively as fossil 
>fuels. 

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 (I saw this was also posted to stoves but I cannot post to
that list from this address)

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. It looks like there is a good chance
that this energy requirement may be reduced by:

1) Pre treating the rawstock
2) Redesign of the die 
3) Deciding which attributes of pellets are appropriate for the
environments that will consume them

>
>I grant you that not all the pieces are in place in terms of 
>appropriate technologies, but I know your working on it, and so am I 
>for what it is worth.  I look forward to REAP's further exploration 
>of the economies of pelleting and I hope that the folks at Chardust 
>and ARTI will have a look at REAP's work and comment.
>See www.reap.ca 

I am in there too! In this country it looks likely there is excess
pelleting capacity as farming shrinks, I imagine a number of people
are looking into using this capacity once the legal matters can be
sorted out (cross contamination of feedstocks is obviously a big
issue).
>
>If all previous attempts have failed, why? and what might be 
>different now?

Here it is driven by high fuel prices, high disposal costs of biomass
residues and loss of industrial markets.
AJH
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