Re: GAS-L: Densification vs Briquetting

["Chris Downing" <c.downing@sri.org.au>]

Tom,

 

In regard to your discussion of pelleting and cell wall behaviour, you may be interested in the attached scanning electron microscope image of sugar cane fibre restrained by resin at high compaction (compression ratio := 3, void ratio = vol void/vol solid = 1.8), from Downing (1999).

 

"...A sample of spent bagasse contaminated with clay loam was oven dried and clamped to a thickness of 25 mm between a double-bridge (“thumbscrew”) consisting of two pieces of square tubing and threaded rod.  Once the specimen had been trimmed it was dried in an atmosphere of acetone for 6 weeks, impregnated with low viscosity polyester resin under vacuum over an additional 6 week period (FitzPatrick and Gudmundsson 1978;  England et al. 1997), sectioned, polished with 3 mm diamond, and observed using the JEOL microprobe.  Backscattered electron images (BEI) were produced using a 15kV accelerating voltage at a working distance of 25 mm and a current less than three nano-Amperes.  Energy dispersive spectroscopy (EDS) was used to characterise features of interest. "

 

"...a backscattered electron image illustrating the dispersion of inorganic particles within the space between fibrovascular bundles.  The bright areas in the image are dense materials, such as quartz, which reflect large numbers of electrons;  grey areas are residual calcium and potassium salts, an artefact of drying residual fibre moisture which consequently produces an image of the cellulose;  dark regions are low density or low atomic weight material, ie. the polyester resin used to stabilise the compressed specimen;  and, black regions indicate voids/cracks in the resin."

 

After studying this SEM image a simple hexagonal honeycomb model was applied to the fibrovascular cell structure, per Gibson and Ashby (1997).  Two figures from my thesis showing this application are included in the attached Word 2000 document.  One can draw some interesting conclusions about what is happening inside a sugar cane mill (or pelleter) from this simple theory and "frozen" image.

 

With regard to forming pellets from bagasse, the main obstacle is the high moisture content and the need for a finite time period of compression for inter-fibre bonding.  Moisture must be reduced from 50% wet basis to somewhere in the teens for successful pelleting.  The quality of the pellet improves with hold time at a finite pressure, and the use of a small amount of molasses as a binder (however, the binder may have implications for boiler slagging...).

 

The following references should be observed in quoting this work:

Downing, C.M. (1999).  Investigation of the effects of soil contamination on the crushing of comminuted sugar cane.  PhD thesis, James Cook University, Australia.

Gibson, L.J. and Ashby, M.F. (1997).  Cellular Solids: Structure and Properties.  Second Ed.  Cambridge University Press.

 

Regards,
Chris Downing, PhD

Sugar Research Institute
239-255 Nebo Rd
PO Box 5611
Mackay MC, 4741
Australia
International: +61 7 4952 7600
Switch: 07 4952 7600
Direct: 07 4952 7647
Fax: 07 4952 7699
Email: c.downing@sri.org.au
http://www.sri.org.au

 
"Thomas Reed" <tombreed@home.com>
10/09/2001 15:03 CST

To: "gasification" <gasification@crest.org>, "Stoves" <Stoves@crest.org>
cc:
bcc:
Subject: GAS-L: Densification vs Briquetting

Dear Stovers and Gasifiers:

I have had a 23 year interest in densification and briquetting of biomass,
but continue to come up with new insights and questions.

1)  Wood has a VERY strong cellular structure and so requires VERY high
pressure to make pellets/cubes/logs.  This is DENSIFICATION and uses 100
hp-hr/ton and machines like the California Pellet Mill, the Taiga extruder,
the JD cuber etc.

2) Charcoal is very weak and the cell structure is mostly broken down.
Therefore it is only necessary to paste the particles together to get a
dense fuel.  This is BRIQUETTING and uses a pillow or hand briquetter and
handpower-hrs/kg.

                                                   ~~~~~~
Now comes an interesting question.  Sugar manufacture requires intense
pressure to squeeze out the juice.  Can the resulting bagasse be dried and
briquetted or does it require densification?

Nut hulls are often very dense.  Can they be briquetted or do they require
densification?

Paper is quite dense, having had the cell structure of the wood broken down.

There should be a scale of power requirements vs type of biomass, and I
presume there are many forms of biomass that do NOT require high pressure to
get a reasonable fuel density.

COMMENTS?

Yours truly,                    TOM REED

        Dr. Thomas Reed
 The Biomass Energy Foundation
1810 Smith Rd., Golden, CO 80401
303 278 0558;
tombreed@home.com; www.woodgas.com


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