[Stoves] Calc. void space and pellet particle density

[Richard Stanley]

Hi Frank / Tom / Andrew/ Crispin et al,

My own observations about densities achieved with high compression, are limited to 2" --80 mm dia x 8" long sawdust fuel log making machinery of UK, Tiawan and Danish origins over the mid 90"s and early 2000's in east Africa.  To achieve the kind of density you are talking about in a fuel log, involves a lot more energy (motor dirces were 8 to 10 kw at least plus a few more Kws were needed  for the necessary pre drying of the bulk fuel. The product was bound by melting the lignin in the sawdust creating a kind of chipboard cylinder. It was hot enough to char the surface of the cylinder and often involved added heat jackets to achieve the 400 deg f cylinder temperatures. The augur drives I have seen were operating at 6000 to 9000 psig.  400 - 600 grams per cm^2…MAybe my math is off but its a good deal higher than what your numbers indicate. Its also one heck of al ot more expensive toe capitalize and maintain the augurs had to be resurfaced and hardened every few weeks of operation..

Add binders and the pressure drops dramatically. 

Add water and a fiber binder and you are down to 125 to 150 psi/  12 - 15 grams per cm ^2 . 

Of course, you are densifying the inherent material above while we are attempting to densify the matrix with finer and more dense in fillers. we have gotten to ––(and apologies for my former screw up of units ), 0.6 grams per cc,  but normally run at 0.3 to 0.4 grams per cc


Its about different fuels for different markets  industrial central-produced generally for industrial or dense population centers may justify the cost of the first option, esp with centralized management in the inventional business model. That, versus diversified locally produced for local markets which requires a lower tech higher training and initial management  input in the attempt to adapt the hardware for local production, the training necessary to assure a good product and the network of the local micro-entrepreneurs to assure local self sustaining support internal to their own realities.

The confusion often arises, when one try to enter the other market without that end point in mind.

Hasta,
Richard



On Mar 9, 2014, at 5:22 AM, ajheggie at gmail.com wrote:

[Default] On Sat, 8 Mar 2014 14:53:30 -0800,"Frank Shields"
<frank at compostlab.com> wrote:

> This is a sample from a local US pellet source. The bulk density is 0.638 g/cc and the particle density of the pellets is 1.338 g/cc
> 
> Void space for air movement in a TLUD filled with these pellets is 52.3% 

I haven't checked your maths but the numbers seem good, I wasn't
expecting the pellets to be quite so dense, perceived wisdom is that
the density of wood with no pores is 1.5g/cc so the press has got
quite close to the theoretical maximum.

If we consider a 6mm cylinder of dry wood 10mm long then to achieve
this density it will have needed to be squeezed down to 3mm.

Of course we are starting with sawdust which is 1/3 the bulk density
of dry wood so in fact we have to squeeze a 30mm column of sawdust
down to 3mm the average force times 27mm is the energy necessary to do
that, neglecting friction, or are my numbers too far out? Tom Miles
said the pressure necessary is 2000psi so that seems to be an average
weight on the 6mm column of only 40kg (~400N) moved through 0.027m ??
I make that ~11Joules. The pellet has a mass of 0.378 gramme and a
calorific value of 7036 Joules so the which seems to give a very low
figure of 0.16% of the heat energy  is required in motive energy to
compress the sawdust. Unless my suppositions are wildly out the losses
in the pellet making system are high as we worked on 2.5%.

Also the air space being greater than the solid space is more than I
expected.

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