[Terrapreta] torrified wood vs. charcoal (Robert Flanagan)

[info at biorealis.com]

Hi Rob,

Thanks for the great explanation of pyrolysis v gasification.  You 
wrote "why waste all the energy taking biomass to just under it's 
exothermic curve just so you can burn it for heat?"  -- which 
prompted a further question:  What if I already have a source of 
readily available heat that is presently being wasted? How would that 
change the equation? Or would it?

I have a source of "waste" energy available, with temperatures 
ranging between 370C and 400C.  Could these temperatures be used for 
pyrolysis (or gasification? or torrification?) of biomass?  If 
feasible at all, what type of biomass (or moisture content) should I 
be considering?  Ideally, I'd like to be able to extract usable fuel 
gas (H2 and CO) plus biochar.

You also wrote:

"Now with gasification, the lowest possible jump is from around 280C 
to 600C (Depending on air flow), due to primary air (Fresh oxygen) 
flowing through the reactor the whole time (enough to strip the gases 
off, but not enough to reduce the carbon to ash). So with 
gasification stoves we exploit this law and use the excess energy to 
crack the gases and water as they pass through this hot carbon zone."

What if it is *not* a gasification stove, but a completely closed 
(except for a vent to release the gases generated within) vessel 
filled with biomass and heated to 400 degC? What could I expect to 
get out of such a reactor?

Regards,

Bob Crosby


>So to answer your question, or ask the question "What happens to the
>pollutants while it's been turned from wood to torrified wood?" Again I say
>why waste all the energy taking biomass to just under it's exothermic curve
>just so you can burn it for heat?
>
>A little back ground on pyrolysis "V" gasification,
>
>In pyrolysis your feedstock is typically high in moisture (50%, called green
>waste) and little or no oxygen is added during the process (depending on the
>process time and desired charcoal) so typically you have a very wet off gas.
>There is a direct relationship between temperature, duration, carbon content
>and charcoal yield. With slow low temperature pyrolysis (400C) it's possible
>to have charcoal yields of around 33% but not much higher with a low carbon
>content. When you reach higher temperatures, your charcoal yield can drop to
>10-15% (the typical yield for traditional charcoal kilns used in Brazil is
>about 15%). The highest possible charcoal conversion obtainable today is via
>flash carbonization (http://www.hnei.hawaii.edu/flash_carb_biomass.pdf).
>
>Gasification on the other hand uses biomass with much lower moisture content
>(Optimum 20%). The biggest difference between the two is the natural
>exothermic jump in the flaming pyrolysis zone. These figures change a bit
>for different biomass streams but to avoid confusion, I will keep it simple
>and not get too bogged down with the finer details. When you heat up biomass
>to around 280C it goes into an exothermic reaction (begins to give off heat)
>and the temperature jumps to around 400C. Now this energy jump causes the
>next layer of biomass to heat up and also go into the exothermic zone and so
>on and so on, until you're left with a pile of char. Now with gasification,
>the lowest possible jump is from around 280C to 600C (Depending on air
>flow), due to primary air (Fresh oxygen) flowing through the reactor the
>whole time (enough to strip the gases off, but not enough to reduce the
>carbon to ash). So with gasification stoves we exploit this law and use the
>excess energy to crack the gases and water as they pass through this hot
>carbon zone. This then provides fuel that we can burn directly whilst also
>producing charcoal (wood gas (CH4) + water + high temperature carbon, reacts
>as follows H20+C=H2+CO; end gas= (CH4+H2+CO))
>
>  Where gasification is concerned, a 20% charcoal yield is considered high
>but you have to remember the higher the temperature of the charcoal the
>higher the carbon content, so although you might have less charcoal, your
>overall fixed carbon yield might be very close.
>Better to get all the energy from the biomass plus the energy from the
>hydrogen in the water using a Top Lit Up Draft (TLUD) stove!