[Terrapreta] Feedstock for charcoal making

[Sean K. Barry]

Hi Kurt,

Making charcoal from biomass does not require a significant external source of heat.  A pyrolysis reaction starts out endothermic (i.e. requires an external energy input) up to about 250-400 degrees C, depending on the biomass source material and it's moisture content.  Above that ignition temperature, pyrolysis reactions are exothermic (i.e. they produce excess heat).  So, once ignition has occurred, then a pyrolysis reaction in biomass, will convert the carbohydrates into charcoal, drive off exhaust gases, and produce enough heat to ignite the other present un-carbonized biomass.   With only the addition of more air (oxygen) and more biomass feed, and WITHOUT the addition of any external heat source, a pyrolysis reaction can be sustained.

Pyrolysis reactors and biomass gasification reactors are really the same thing.  They both make some charcoal from biomass and they both vent off gases produced from the "destructive distillation" (decomposition) of the biomass.  Air flow/oxygen flow, pressure, and temperature make the difference in how much charcoal, bio-oils, and off gases are produced by the reaction.

The off gas, sometimes called producer gas for air-blown gasification (or synthesis gas from oxygen blown gasification) contains combustible (fuel) gases; mostly hydrogen (H2), carbon monoxide (CO), and methane (CH4), but small amounts of acetylene (C2H2) and ethylene (C2H4) (i.e. longer hydrocarbon strings).  Producer gas also contains incombustible (inert)exhaust gases; mostly carbon dioxide (CO2), molecular nitrogen (N2), and water vapor (H2O).  The producer gas exiting a pyrolysis reactor is HOT, carrying away a lot of that exothermic energy from the pyrolysis reaction.  Also in the off gas are suspended particles of charcoal and vaporized liquids, made up of longer string hydrocarbons (technically called polycyclic aromatic hydrocarbons, figuratively called bio-oils, soot, wood tars, or simply tars).

If there is sufficient BTU content in the off gas (i.e. high density of combustible gases in the total volume of gas), then they can be cleaned (of tars) and used to burn inside internal combustion engines (or turbines) and drive electric generators.
If the gas is mostly exhaust (CO2, N2, H2O) from the partial combustion of the pyrolisys reaction, then it cannot conatin a sufficient fuel charge to burn effectively inside combustion engines.  The heat energy in any exhaust gas can be made useful, though, if the use is physically close to the exhaust port.  Making use of the energy outputs from a pyrolysis reactor can making charcoal provide other benefits before the charcoal is actually working in the soil "Terra Preta" style.

A pyrolysis reactor which produces a high yield of charcoal from a biomass feed will produce a low BTU gas stream.  The gas may not be useful for further combustion to drive an electric generator, for instance.  The off gases do present a problem then, I think.  Simply venting non-combusted methane (CH4) from landfills, for instance, is illegal, and considered a strong pollutant.  Methane is a much more potent greenhouse gas than CO2.  The gases and tars coming out of a pyrolysis reactor will need to be flared (burned), even if the heat which that combustion produces is not put to a useful purpose.  

On the other hand, a biomass gasifier (pyrolysis reactor) can be operated to produce a richer gas stream, but consequently will produce less charcoal.  The higher BTU producer gas can be used to power electric generation, successfully converting biomass energy into electric power.  This can generate immediate revenues in a grid-tied application.

The choice, then, for biomass conversion is to make it carbon negative; charcoal sequestering in soil, or carbon neutral; gasification and conversion of the chemical energy the biomass contains into heat and electric power.  An attempt to do both may be possible, but in that case I do not think either would be done very well.  Either you harvest the energy (rich gases) or you leave it in the charcoal.

Regards,

Sean K. Barry
Principal Engineer/Owner
Troposphere Energy, LLC
11170 142nd St. N.
Stillwater, MN 55082
(651) 351-0711 (Home/Fax)
(651) 285-0904 (Cell)
sean.barry at juno.com<mailto:sean.barry at juno.com>
  ----- Original Message ----- 
  From: rukurt at westnet.com.au<mailto:rukurt at westnet.com.au> 
  To: terrapreta<mailto:terrapreta at bioenergylists.org> 
  Sent: Wednesday, March 28, 2007 4:44 PM
  Subject: [Terrapreta] Feedstock for charcoal making


  Hi folkes,

  To make the best charcoal for terrapreta you need woody biomass. 
  Hardwood for preference, though in Japan bamboo is considered very good. 
  Where do we get it?

  Bamboo grows well in the tropics and also in temperate zones. The 
  running variety does well in temperate areas, many of them can take snow 
  with no problems at all. It grows like grass, because that is what it is.

  But what about other hardwoods? It seems that in England, back in the 
  Victorian days, most forests were actually coppiced, with a 10 to 11 
  year cycle. At the end of a growing period the woods were cut, leaving a 
  stump, which then re-sprouted and was ready to cut in another 10 odd 
  years time. Today of course, these coppiced woods of before are mature 
  forests. I can just see the greenies permitting them to be cut down. NOT.

  Charcoaling grass clippings and food wastes is probably not a good idea. 
  It is in parts of India, because there the problem is charcoal for 
  cooking fuel, not charcoal full of vesicles to make terrapreta. Much 
  better to turn that sort of thing into compost to be mixed with charcoal 
  and then applied to the soil.

  In parts of Tasmania (that little island down at the bottom of 
  Australia) plantations of a particular eucalypt are to be found all over 
  the place. These were planted to feed an intended chipmill and are the 
  subject of a lot of green anger. Many eucalypts coppice very nicely-- 
  they might be useful for charcoaling.

  In the northern hemisphere, coppiced willows have been tried, but don't 
  seem to be very successful for a number of reasons. Other coppices might 
  be useful, but would probably need to be established.

  To make terrpreta we can't just denude the earth of forests in order to 
  put carbon into the soil. We need to start further back and establish 
  suitable plantings. Charcoaling crop residues will doubtlessly work, but 
  we need to come up with equipment that individual farmers can acquire to 
  feed their residues through. Probably not all that hard.

  Mow and bale a field of wheat stubble and feed the bales into a mobile 
  charcoal retort. Much of the heat needed to charcoal it will come from 
  the boiled off tars and gases themselves. Some of the bales could be 
  burnt to supply the extra heat that would be required. Same same with 
  corn stover and other crop residues. Then there's switch grass etc.

  Kurt

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