[Gasification] [biochar] Pine char gasification

Tue Dec 24 15:06:08 CST 2013

Ron,

When I read the literature generated by Woods, Lehmann, Glaser, Steiner and
the many anthropologists it paints a picture of smoldering slash and char
like Christoph Steiner describes in his thesis by that title. In it you will
have a mix of pyrolysis, gasification and combustion which are all part of
the combustion process. 

When wood is heated, with or without air, about 80% of the carbon converts
to volatile gases which then burns when you add air. The remaining 20%
carbon converts to charcoal which burns in direct contact with air. In
gasification we add about 30% of the air that you would ordinarily use for
combustion (stoichiometric air) which burns about a third of the fuel which
heats the remainder and converts it to combustible gases. A little char
(<5%) often remains. If you burn the hot raw gas you have the heat from the
third of the fuel that burned plus the heat from the chemical energy (CO,
H2) in the gas so your only loses are generally in heat transfer. If you
have 5% heat loss through refractory then you have 95% of the heat, in the
form of sensible heat and chemical energy, delivered to a boiler. If you
don't burn part of the fuel to create heat to convert the mass to gas then
you have to add it externally. That's pyrolysis. When you use part of the
gases from pyrolysis to provide that heat for pyrolysis then you lose that
energy. So from 3 tons of dry wood you get one ton of charcoal and one ton
equivalent of heat (16 MMBtu). The other energy is consumed to make heat for
the pyrolysis. When you burn both the gas and the char that's combustion. A
device that is externally heated to make gas and char is a pyrolyzer. A
device that uses air to convert the solid to a gas is a gasifier. A gasifier
produces some char through inefficient conversion. If you add air to convert
the fuel to char and gas and and burn the all the char and the gas then
that's a combustor.  

If you add air to a mix of char and volatile gases the gases will consume
most of the oxygen first. You can also see the glowing combustion of the
char but when air is limited the char tends to remain. The volatile gases
burn at about 815 C to 980 C while the char burns at about 1350 C. Char
combustion is important to updraft gasifiers. You blow air and steam into
the bed of char to burn it which provides heat to drive off the volatile
gases from wood that is above the char layer. With char you can develop
enough energy to drive the water gas shift reactions. 

>From 1985 to about 1998 there was a US tax credit for devices that converted
solid biomass to gas. A gasifier could get a tax credit if it converted
solid biomass to a gas and sold it to a boiler owned by another entity. An
enterprising tax attorney was able to get that tax code interpreted so that
if you reduced the amount of air coming through the grate of a boiler, that
would promote mostly pyrolysis and gasification, you could call the furnace
and grate a gasifier. Since you burned the gas above the grate the boiler
assembly could be the buyer or consumer of the gas. So what we would call
staged combustion received tax credits as if it was a gasifier making gas to
sell to a boiler. I called those "taxifiers" at the time. Boiler
manufacturers renamed their staging combustors as gasifiers and their
customers received the tax credits. Hundreds of boilers were sold in this
way. That tuning to stage the combustion is what Alex English does on his
grate (Without the tax credit)..        

In downdraft gasifiers you tend to burn the volatile gases to create the
heat to convert the solid wood togas. In a TLUD you establish the flame by
introducing air to burn the volatile gases. Heat from the flame and the
flaming pyrolysis convert the solid wood fuel to gas which rises up through
the pyrolysis zone to the burner. Partial oxidation takes place in the
pyrolysis flame front by consume oxygen from the primary air. The flame
however is just burning combustible gas so you have a gasifier. All
gasifiers have pyrolysis zones in which the fuel is mostly heated and
converted to gases. . A TLUD is a gasifier with a descending flaming
pyrolysis zone. 

Adding CO2 to char is designed to create combustible gas from the char. It
is interesting to note that you can burn just a small part of the char to
change the quality of the remaining char. You get a char that has improved
nutrient capture. At te expense of some of the char. If your intent is to
make synthesis gases for liquid fuel then you objective is to convert all of
the char togas.    

It is easy to make 15% to 25% char in a gasifier. You just run the gasifier
inefficiently so that a larger part of the char is not consumed in
gasification.  The Ankur Scientific gasifier makes this easy by providing a
motorized grate that allows you to remove char faster than it is consumed.
In a large downdraft you get poor air penetration in the oxidation zone so
you will find that the core is cooler than the perimeter. The core of the
fuel passes through as carbonized but not gasified.         

It's Christmas time. Happy Holidays

Tom

From: biochar at yahoogroups.com [mailto:biochar at yahoogroups.com] On Behalf Of
Ronal W. Larson
Sent: Tuesday, December 24, 2013 11:52 AM
To: Biochar; Tom Miles
Cc: Crispin Pemberton-Pigott; Discussion of biomass pyrolysis and
gasification
Subject: Re: [biochar] Pine char gasification

Tom  - see notes below.  I have little time for a few more weeks, but will
try to get back to this, if others haven't already supplied enough of a
response.

On Dec 23, 2013, at 4:36 PM, Tom Miles <tmiles at trmiles.com> wrote:

Ron,

I didn't mean anything quite so personal. :-/

     [RWL1:  See next response to Mark Ludlow.  I was mostly trying to get
some humor injected - about my own "cult".

Most of the biochar research has focused on pyrolitic char and not on
combustion or gasification char. There is a clear bias toward pyrolysis, or
low temperature char. Can anyone really say this is the way that the
Amazonians, or anyone else, created the charcoal that we find in the terra
preta soils? Or was it smoldering combustion, staged combustion (a la Alex
English), or a combination of pyrolysis, gasification and combustion? I know
that I have had a lot of bad slash and straw burns that have left a lot more
char on the ground than ash. Are there "signatures" in the terra preta char
that point specifically to pyrolysis, gasification or combustion?  

      [RWL2:  I just spent half an hour trying to find something definitive.
I found one Ppt by three friends on this topic,  but not enough words to go
with the pictures.  I will check after Xmas.   

    I hope someone on this list has looked at efforts to mimic the Terra
Preta soils.  It seems clear they did much more than just put out ash -
which seems to have been what the vast majority of aboriginal slash and burn
cultures did.  I favor an argument that the char came from what happened
during and after cooking (If wood is easy to come by, you can make a lot of
char in a 3-stone arrangement.  I have seen one argument for an approach
like HTC.

I see biochar production growing in stages. For the time being a large
quantity of char that is sold as Biochar is actually char from gasification.
As biochar markets grow we might expect to find more pyrolytic char made
"for purpose" but now we have some pyrolitic char and byproducts of
gasification (including TLUDs) and combustion.

    [RWL3:  I wouldn't couple the words "TLUDs" and "gasification".
TLUDs look like pyrolysis to me.

The "high temperature" gasifier char performs very well and in some
applications better than pyrolytic char. Several studies (and some
commercial producers) have found that conditioning the char through
partially oxidation (to higher temperature) enhances nutrient retention.
These products are for improving soil fertility , not necessarily to replace
activated carbon. So why not consider CO2 gasification as a possible process
step?     

   [RWL4:   I need help on this.  I am assuming that adding CO2 to hot char
is designed to leave little char.   Doesn't sound like a major help for
producing a biochar.

One major producer of char in California uses a downdraft gasifier. In a
downdraft gasifier wood devolatilizes at or above the oxidation zone.
Volatile carbon is oxidized by the air injected from nozzles to make CO2.
The hot CO2 reacts with the char to form CO and H2. This occurs in the
"reduction zone". The reduction zone is often shown as a deep bed of carbon
but in fact it is usually only a couple of inches thick. Large chips reduce
to powdered char in less than 2 inches where gas temperatures are 800-900C.
The resultant producer gas is a mixture of this CO from reducing char and
the devolatilized gas. Taking CO2 and reacting it with charcoal at 800-900C
as Purdue has done is not a lot different so the qualities of the char
should be similar.

    [RWL5:   Still need help  (not knowing enough about the term
"gasification").  In downdraft gasifiers, I have been assuming that the
injected air was reacting mostly with the char, not with the already
produced gases.  The intent was to get rid of as much char as possible (and
I assume the same for the Purdue researchers).   I understand that Purdue is
inputting CO2 and not air (in a second stage), but the intent in both cases
is (I presume) to leave as little char as possible.   I just don't see how
that fits into this list - interested in getting a lot of char.   I
understand that part of the processing is to maximize CO and H2.    I'll try
to get back to this.

I think we need to explore all avenues of producing char and energy

1.      Slow pyrolysis - 25%-30% char; 30% oil+gas

2.      Fast pyrolysis - 15% char; 60% oil

3.      Gasification - 5%-25% char; 75%-95% energy

4.      Combustion - 1-5% char; 95% heat

    [RWL:  Tom - the bottom two total near 100%, but not the top two; can
you add some more components?.  I am surprised also to see gasification char
as high as 25%;  who is getting this high - and how?.   

    Adding to this list might be the work of Mike Antal (and Mantria) with
added pressure.  Also Cool Planet uses pressure and catalysts with the term
"fractionator".   Retort char (zero oxygen) could be a little different from
your four - all of which involve some O2?.  Maybe same for char made with
microwaves (heating from the inside of particles being different)?
Certainly HTC  (hydrothermal carbonization) is very different.  Is the
approach by Alex English different from any of these  (I think it is close
to slow pyrolysis).  Nat Mulcahy with World Stove has a different approach
with no oxygen flowing through the fuel bed.  Jim Mason's BEK will be called
gasification?

    I heat my home partly with wood (mostly solar (except when cold and
cloudy), no gas) - and have pulled copious amounts of char out of my (open
front) stove - a lot more than 5%.  I believe that has to be called
interrupted combustion - just the same as the whiskey maker Jack Daniels
does - combustion interrupted at the end of the pyrolysis stage and before
much gasification can have occurred.  The difference seems to be whether an
O2 molecule can reach a hot char surface or not - because of still-exiting
pyrolysis gases getting oxidized first (mainly to CO and H2O).

     All in all I think it great that there are so many carbonization
approaches - hopefully enough for every combination of soil and plant
species.    The big divider will be process temperature, it seems.    Ron

Tom

From:  <mailto:biochar at yahoogroups.com> biochar at yahoogroups.com [
<mailto:biochar at yahoogroups.com> mailto:biochar at yahoogroups.com] On Behalf
Of Ronal W. Larson
Sent: Monday, December 23, 2013 2:53 PM
To: Biochar; Tom Miles
Cc: Crispin Pemberton-Pigott; Gasification-Request
Subject: Re: [biochar] Pine char gasification

Tom etal:

   1.   I'm not sure I want to accept the "philia" part of this message
("philia" goes with "abnormal" and pedophilia at one google site).  I found
the word agape - but that sounds presumptuous.  But I do admit to being at
the non-sensical end of the char spectrum.  Maybe charphilia is apt.

  2.  I know close to zero about any part of gasification, but I can
understand why one would promote the idea of recycling the CO2 to get more
gas (eventually the Purdue group wants liquid, it seems).  But that has to
result in less char - and apparently leaves much higher temperature char.
Eventually it is almost all CO2, for gasification, but I worry that the char
produced this (high temperature) way might only be suited to replace AC =
activated carbon.

  3.  Since Alex English name came up today, we should note that he also
recycles CO2.

  4.  The dogma of the cult I am in says more char beats more heat, gas or
liquid, so I will look forward to some proof that is not correct. 

 Good luck to the Purdue folk.

Ron

On Dec 23, 2013, at 12:58 PM, Tom Miles < <mailto:tmiles at trmiles.com>
tmiles at trmiles.com> wrote:

Ron,

This work is very important for both the biochar and gasification lists.
Biochar will be produced at the large, or even small, scale as a co-product
of energy (liquid fuels and/or power). The most efficient way to generate
power from the gases and vapors from slow pyrolysis (50% of the energy) is
probably through charcoal gasification (e.g. run the pyrolysis gases through
a charcoal gasifier). There are commercial systems under development to make
char and power in this way. There are also commercial systems under
development to make liquid fuels through combinations of pyrolysis and
gasification. The char products from these and fast pyrolysis processes run
from 0% to about 15% of fuel input. I don't know the fuel or char yield for
Cool Planet.

This particular study prepared the char with high temperature (826 C)
nitrogen.  Wood particles (chips, sawdust) and resultant char particles in
this study are larger than for other char studies. Obs

ervations about BET surface area, particle size and the char morphology are
very interesting. The char morphology looks different than the SEM images
that we typically see. From gasification and pyrolysis we know that pine
carbonizes differently than hardwood so it is interesting to see the
shredded fibrous appearance of the pine char in this study compared to the
neat geometric structures that we often see, which is probably from hardwood
chars. The authors observe that the macropore volume is significantly
greater than the mesopore or micropore volume of the char. They observe
"numerous wide tunnel protruding into the char particles. . . [that] may
provide pathways for bulk transport of CO2 into the particle."

Char conversion numbers are interesting. Only 10-12% of the char was
gasified at 726 C (BET 391 m3/g) while 98-100% was converted at 896 C.
Surface area increased with conversion but not much greater than the 35-47%
conversion at 776 C so CO2 gasification could be used to increase surface
area at the expense of half of char (660 m3/g). Meso and micro pore volume
doubles at the higher rate but stays pretty constant above 776 C.
Researchers conclude that a significant proportion of the pore volume is
within macro pores although the majority of the internal surface area is
within micro pores.  They point out that the mass loss with surface
gasification occurs within the smaller pores leading to pore widening.

Researchers explain that the char gasification process involves three steps:
(1) adsorption of the gas-phase species to the char surface, (2) surface
reactions, and (3) desorption of the gasification products from the surface.
The latter is the rate limiting process.

Recycling CO2 from gasification to gasify the char is an interesting concept
that may apply to modifying char properties (e.g. increase surface area)
from pyrolysis or recovering energy (heat, power, syngas) in an industrial
setting.   

There is very little information about gasification or combustion chars.
Sometimes it helps to step back from our char-philia (and gaso-phobia) to
see what products combined pyrolysis and gasification can produce.  

Tom

RL> don't see any relevance to the biochar list.  (Except if this work shows
that char is more valuable in the ground and/or that an approach like Cool
Planet's is more efficient.)  On the biochar list, we should want BOTH high
value fuels and charcoal.   

     This Purdue work is all about gasification of char - not pyrolysis.   I
am not sure whether the topic is appropriate for "gasification" either,
since that list seems to want gases for engines, not liquids.

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