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

[fractional@willmar.com]

Hello Andrew,

   I recently read a report by the Food and Agriculture Program of the UN, P.D.
Grover called:  Biomass Briquetting: Technology and Practices.

Here is the link:

http://www.rwedp.org/acrobat/fd46.pdf

  It reviews briquetting technology through the years and there use in India.  It
mentions that mechanical ram presses and screws can both make dense briquettes,
but that the pressed 60mm pucks are not suitable for gasification, while the screw
made briquettes were.  They did not mention if it was because they were fragile in
the gasification process due to residence time and humidity, or because of the
weight of the fuel column.

  Another article,
http://www.fao.org/docrep/T0275E/T0275E03.htm   (good overview)
does mention this problem with briquettes and gasification but does not
necessarily attribute it to just a problem with rammed briquettes.

  The feed PELLETIZERS and waferers are said by Vernon Lundell in his patents on
wafering machines to have a problem with binding ag residues into wafers due to
varying crop conditions, water content.....  With higher density screw machines it
seams the only limiting factor is moisture in excess of 15%.

  No doubt the shearing action in screw machines 'fiberizes' the material as the
strawboard people would say to make for a good mechanical binding whereas the
'stomped' pucks naturally can break on many layers.  My concern lies with whether
this claim of unsuitability of rammed briquettes  is due to them falling apart in
the gassifier under long residencies and humidity, or some other problem.  My
interest lies with making rather smaller briquettes from switchgrass, moderately
chopped, which might have less internal stress than a larger puck. Apparently
where they sell these pucks for residential use they break them before burning,
perhaps quartering the pucks after extrusion and slicing to width would be an
inexpensive way to produce a switchgrass pellet for personal consumption or local
sale.
  To quarter the disks would require sufficiently finely ground feedstock so as to
not interfere with breaking them, which might make them weaker also.

  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,
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.

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

Thanks,

Alan




Andrew Heggie wrote:

> On Sat, 10 Feb 2001 09:08:22 EST, you wrote:
>
> >Dear Jens:
> >
> >I have never been a great fan of straw and switchgrass as fuels, but I don't
> >live in Denmark either.  Your point is well taken for this particular form of
> >biomass.
>
> It's horses for courses, if pellets made from switchgrass meet the
> same standards as wood pellets then I see no problem with burning
> them.
>
> I suspect they will have a higher ash content which may have
> implications in the crop potentially depleting nutrient. On a local
> scale this would be recycled, on a large scale costs of re
> fertilisation would go up.
>
> The big advantages I see are, unless I have mis-interpreted something
> (as these conditions will not pertain in UK):
>
> 1) The crop is dried standing and in Canadian conditions can be
> harvested dry throughout the winter, this optimises harvesting
> equipment and reduces costs of drying.
>
> 2) Advantage is taken of the significant reduction in harvesting costs
> by agri-business compared with wood harvesting.
>
> 3) Comminution costs are reduced, having said that most wood pellets
> are made from industrial waste and are comminuted in the earlier
> process at no additional cost.
>
> The worry would be that, with allegedly 10 days food in the global
> supply chain at any one time, can we do without the grain displaced?
> >
> >However, baling is a form of densification and may be sufficient for straw,
> >since you clever Danes have invented a number of "bale burners".
>
> I wonder about the performance of these at the domestic scale. The
> simple water jacket burners are not clean or efficient. Later ones
> with a fast burn like a masonry stove are better.
>
> Some while back a UK firm made a straw flaker which punched out discs
> of straw in the field, I have not seen or heard of it recently. I
> think the same firm made a header which stripped whole wheat heads and
> left the straw standing. This always struck me as a more holistic
> approach as the weed seeds were not re deposited on the ground and the
> straw was kept dry prior to baling. Essentially a cheap machine
> harvested the heads quickly and the threshing was done in the drying
> shed over a longer period. Obviously this has implications for large
> buildings to offset against the savings on combines.
> >
> >What isn't economical today may be a necessity some day, so we need to
> >investigate all possibilities.
> >
> >Wood pellets cost ~ $50/ton, made using unimproved technology.  Yet
> >homeownders are willing to pay ~$120/ton in the form of 20 kg bags of pellets
> >for the convenience of using pellet stoves.
>
> In terms of direct costs I suspect the bagging operation is the same
> cost as the pelleting!
> >
> >So, keep your straw options open.  You may have to eat your words.
>
> I read reports that there is a reduction of North American wheat
> growing, words may be the only thing on the menu.
>
> AJH
>
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