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

[Greg and April]

Good questions - all of them.

I'm certainly interested in hearing the answers.

Greg H.


----- Original Message ----- 
From: <info at biorealis.com>
To: <terrapreta at bioenergylists.org>; <saffechina at gmail.com>
Sent: Friday, February 15, 2008 18:12
Subject: Re: [Terrapreta] torrified wood vs. charcoal (Robert Flanagan)


> 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!
>
>
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