[Stoves] Calc. void space and pellet particle density

[ajheggie at gmail.com]

[Default] On Mon, 10 Mar 2014 15:11:12 -0400,Crispin Pembert-Pigott
<crispinpigott at outlook.com> wrote:

>
>The rest of the energy goes into heat, I suspect. In order to get that
>density isn't it necessary to crush the cell structure of the wood,
>releasing the lignin? 


I actually suspect some of my assumptions for my calculations were
wrong. We installed a Sprout Matador pellet machine and IIR it was
300kW(e) installed and maximum quoted output was 5 tonnes/hr. Of it
neither ran at peak installed power not the quoted output but I was
off the project before I got any decent figures.

Yes the power must be consumed to crush the cells down but one assumes
that if the particles are small enough they don't need much crushing
before being close enough for hydrogen bonding. Look at the
papermaking analogy again, here the fibres are broken down
sufficiently by pulping that they form these bonds as they dry.

The lignin is a different matter. it's a polyphenol, that is a huge
molecule based on lots of rings of 6 carbon atoms joined together, it
is the glue that holds the fibres together, warm it up and it
plasticises  and flows. It's the same process by which the ash backs
of windsor chairs are bent as well as the handles of chestnut walking
sticks. Once the system gets running the friction of the sawdust
squeezed through the die heats it up enough  for it to flow such that
the pellets emerge hot and flexible until they cool.

Pelleting is a bit of a black art and the shape of the hole in the die
controls the pressure. The pellets are made just well enough to hold
together without too much wasted power and not allowed to get so hot
that they begin to pyrolyse. Lower pressure devices do add a form of
lignin powder, I suspect this is a calcium  compound that is a waste
from the papermaking industry. The give away is that these pellets
burn to give more ash.

AJH