Re: GAS-L: Re: Vedr.: Optimizing Fuel Volume Density... for straw and grass

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

On Wed, 14 Feb 2001 14:30:59 -0600, Alan fractional@willmar.com wrote:

>
>
>  All the literature gives varying reports of energy usage between rams, rotary
>disk extrusion and screws, swapping about with each author.  Perhaps for a test,

I imagine a lot of research is being directed at reducing these
compression energy costs, however it will mean increased capital costs
which tend to mean more centralisation, biomass production tends to be
too distributed (here at least).

>using existing flywheel components of hay balers to make 'Rammed' 60mm pucks
>(possibly quartered) would be a good starting point for those interested in
>densifing switchgrass.  The dies can be made very easily, important if the dies
>only last 300 hours.

The cost of dies is also significant in pelleting with ring dies, at
GBP2000 a piece it relates to ~GBP0.5/tonne with clean sawdust, I
would worry seriously if sander dust got into the system. I know this
can also be a problem with the Shimada system. Also reclaimed wood
pellets seem to be larger (11mm as opposed to 6mm) and from the
slagging my colleagues have experienced (not to mention traces of
chromium :-)) wear on the die, from a contaminated rawstock, may be
one reason for the change in size.

Again water seems to play an integral part here, not only does it help
with some of the bonding, it appears to control the temperature of the
compression and lubricate the dies as escaping steam. The idea of
"conditioning" previously dried sawdust with a water mist seems odd.

Whilst I posted that wood pellets were made by collapsing the cell
structure and the heat generated plasticising lignin, which then , on
cooling, bound the material this does not occur throughout the pellet.
This can be demonstrated by wetting a pellet, it soon breaks down to a
mush of sawdust. Just like pottery I would suggest the art is to
produce a pellet sufficiently stable. In the case of then pellet
enough lignin must flow to bind the material, in a pot the firing is
just sufficient to fuse the particles at points of contact, any
further firing increases the level of vitrification and shrinks the
pot. I wonder just how dense a pellet could be made?


>
>  Again my question is why the Grover report claims rammed briquettes are
>unsuitable for gasification?

I think Peter provided the answer. When paper or straw burns in a heap
the char and ash expands and deflects air from reaching the unburnt
interior as well as preventing conduction of heat, if this happens
with "wafers" (thanks for that it was the term I was looking for when
I said "flakers") because the initial heat is enough to break down the
bonding but not to overcome the inate springiness of the material then
it will be a problem. When a plastic expands on burning/heating it is
described as intumescence, can this term be applied to biomass?

No one seems to wish to discuss the use of pellets in a gasifier, on
the surface there would seem to be good advantages as:

1) The rawstock is homogeneous and dependable
2) The combustion chamber should be smaller for given power rating (we
know wood pellets do not disintegrate or expand on burning)
3) As the mass flow of air will remain the same its velocity can go
up, this should improve the producer gas reaction
4) Heat losses will be lower

Apart from a personal interest I do not see gasification of biomass
for production of motive power to be relevant in UK conditions.
However I do see it as being a means to cleanly burn biomass in small
furnaces with particular relevance to the production of particulates,
which appears to be a very serious problem in my community.

AJH