[Terrapreta] Optimum size ( was torrefaction vs. Carbonization )

[Greg and April]

That's what I was talking about, the mass acting as one single large piece rather than many smaller pieces.    You see this happen with bales of straw or hay if they catch fire - it may take days ( if not longer ) for them to burn out.    A large bundle of news papers or a phonebook tossed into the fireplace act's much the same way.

When that happens, it would then take longer to char a given amount of material if it chars all the way through at all.

I have been bouncing it around in the back of my head and I would think that optimum particle size, depends on a number of factors, including size of retort/kiln, rate of feed, material type ( hard wood vs. soft woods / high VM content vs. low VM content ), so on and so forth.

A dry softwood of low VM content, ( like aspen / willow / cottonwood ) it might be ok to have in large chunks in a large retort/kiln but something like oak, you probably want smaller pieces in a smaller retort/kiln for a given amount of time.    A wood like pine might be somewhere in between.    

A continuous fed retort/kiln would probably require a smaller particle size regardless of the material, while a batch retort/kiln could make do with larger pieces.

Is my thinking flawed in this regard?


Greg H. 


  ----- Original Message ----- 
  From: Larry Williams 
  To: Greg and April 
  Sent: Wednesday, December 19, 2007 1:30
  Subject: Re: [Terrapreta] torrefaction vs. Carbonization---------CharHydrophobic / Hydrophilic characters

  Greg-------My pyrolyser has a capacity of less than 3 cu. ft.. In the last burn which I ended because of another commitment I stuffed the stalks of a stiff grass (I can get the name later). Because it was a partial burn some of the charcoal in the bottom of the container was completely burned wood, i.e. charcoal. That left  6" of space for the grass when the container was restack. It was well packed.


  I would think that the pressure of the gasses would not be trapped in this situation. My suggestion---with the use of different materials (i.e. grass, branches or split wood) which influence the empty spaces in the container it would be possible to make charcoal in the container. This means that my container plugged with a grass material may not be charred in the center of the mass.


  After writing the last paragraph several hours ago, the retort has been opened and the stiff grass stems did char on the out side and the top of the container and not in the heat did not char the center of mass in the stuffed grass. The grass was to dense.


  For your consideration as it is late and rainy outside-------Larry




  ------------------------- 
  On Dec 17, 2007, at 9:11 AM, Greg and April wrote:
  This brings to mind something that I have had bouncing around in the back of my head for a while.




  It's one thing if the size is small to facilitate movement ( and speed of reaction ) through a faster pyrolysis reactor, but it is something else if someone is trying to accomplish the same thing in a low tech setting with a retort / kiln.    A small wood partial size, in such a method, might actually increase the time needed to char the biomass, and actually cause a issue with the ability for the VM to leave the biomass - as it might pack together acting as a single large piece rather than many smaller pieces.


  If this is the case, would it not be better to use a larger partial size that would have more space between them ( for heat and gasses to move about more easily?    If so, what would be an optimum size?    2 - 4 cm+?




  Greg H.


  ----- Original Message -----
  From: Sean K. Barry
  To: Greg and April
  Sent: Friday, December 14, 2007 22:15
  Subject: Re: [Terrapreta] torrefaction vs. Carbonization---------Char Hydrophobic / Hydrophilic characters


  Hi Greg,


  Yes.  The particle size of the feed material in the reactor is very critical, as if effects the core and surface temperatures of the feedstock particle and the differences between these.  Tom Reed at the Biomass Energy Foundation (http://woodgas.com) studied this with a pyrolysis reactor built of transparent glass, with gold lining as thermal insulation.


  I think Tom showed that 1 cm x 1 cm x 0.3 cm particle size of wood "chip" had some a more ideal size characteristics for producing non-condensable and condensable fuel and combustion product gases.  Particle size can control the ratios between these gases; concentrating on more useful fuel gases and less combustion gases in the reaction products.  These "larger" sizes particles create a "fixed bed"  of charcoal (or "slowly moving bed").  It also can yield more solid charcoal, with variable "volatile matter" %, depending on maximum particle temperature, and the residence time in the reactor, before the charcoal is dumped and solid feedstock is replenished.


  "Fast pyrolysis" reactors produce more liquid and gases than soild by-products.  I think the particle size in "Fast pyrolysis" feeds are much smaller; 1-3 mm x 1-3 mm x 1-3 mm (like sawdust).  The pyrolysis reaction proceeds more quickly because all of the fuel particles, being smaller, are consumed to quenching more quickly.  The solid biomass fuel is converted to product materials in liquid and gas phases, the heat generating reaction is over, the products exit the reactor, and then new solid feed is introduced into the "fluidizied bed".


  Regards,


  SKB
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