[Terrapreta] energy balance and emissions

[Sean K. Barry]

Hi Mariska,

Go to the Biomass Energy Foundation website, http://woodgas.com<http://woodgas.com/> site and review the library for materials you can buy, which will give you more information.  Write to Tom Reed and/or Agua Das and ask them to refer you to the texts which will answer your questions.

There is chemical energy content in all biomass, but it is finite.  Gasification of biomass and the production of charcoal from biomass go hand in hand (i.e. the pyrolysis reactors are gasifiers).  The energy contained in biomass can produce produce fuel gases, usable (sensible) heat, and charcoal, but diluting the energy contained therein to so many outputs will generally require using more biomass than would be needed if just one of these were harvested.

For instance, just burning (complete combustion of) biomass for the heat, allows the production of steam (to carry the heat to other processes) and/or generation of electricity.  The sugar industry does this with bagasse.  But, there is little or no char left and only complete combustion gases.

Biomass can be decomposed by pyrolysis into mostly gases (fuel gases; H2, CO, CH4, and "hot" complete combustion gases; CO2 and H2O).  This process can be done until there is nothing left of the original feedstock but mineral ASH, which will no longer produce these gases.  The production of gases occurs from and while the biomass is in the form of a charcoal "bed".  There is little or no charcoal left here either and less "sensible" heat, because it occurs at a lower temperature then complete combustion of the entire feedstock load.

If charcoal is prematurely removed from the reactor and kept in an oxygen free environment, then when it cools, it no longer produces the gases.  Removing charcoal this way also takes away energy.  A charcoal yield of 25-30% by weight can contain as muach as 60% of the original total chemical energy of the biomass feedstock.  So, producing charcoal at a high yield (25-30% is high) can severely limit the amount of energy which can otherwise be harvested.  The gases produced will be low BTU (not much fuel, mostly complete combustion gases) and the heat will be cooler (and hence, less "sensible" or usable)

It is not required that external heat be used to bring a pyrolysis reaction from the endothermic stage into the exothermic stage.
If the flow of oxygen into the reaction is sufficient, some of the biomass will completely combust.  This can be the source of heat that brings the reaction in other parts of the feedstock (the charcoal "bed" part) out of the endothermic phase and into the exothermic phase.

Emission from well controlled pyrolysis reactions (oxygen supply and temperature are controlled) are predominantly gaseous (H2, CO, CH4, CO2, H2O, some trace others).  There can be particulates (<1%) and tars too (15-2000 ppm).  CO2 is a green house gas (GHG).  Avoiding CO2 emissions is done by keeping the temperature and the oxygen supply low enough that complete combustion is not occurring.  Methane-CH4 is a more potent GHG (~26 times CO2) and it can be released (un-burned) at 3% levels.  Releasing 3% CH4 at 26 times more potent than CO2 would be like releasing 78% of the CO2 (78% of the carbon) from the biomass into the atmosphere.  This is why it is important to BURN the Methane-CH4 (releasing CO2 and H2O instead) and use that heat energy if possible.

What reduces the energy efficiency of the pyrolysis reaction more than anything is lost heat.  Unless the temperature of that heat is not significantly above the ambient temperature and exit temperature of the heat engines it could be used in, then it will not produce much usable work and will be lost to the environment.  A steam engine and a steam turbine are examples of "heat engines".  Both can turn electric generators.  Every time energy is converted from one form to another energy is lost.  From biomass molecular bond energy (chemical) to heat and smaller chemical compound bonds (in fuel gases H2, CO, CH4, etc.), from heat via steam into kinetic energy (turning spindle), from turning spindle to generated electricity, all these processes LOSE some of the input energy, which does not go to the OUTPUT energy.  It is most always lost as heat.

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: mariska evelein<mailto:mariska_evelein at hotmail.com> 
  To: terrapreta at bioenergylists.org<mailto:terrapreta at bioenergylists.org> 
  Sent: Tuesday, November 13, 2007 8:35 AM
  Subject: [Terrapreta] energy balance and emissions


  Hello

  Sorry if this has come up before, I'm quite new on this list and haven't seen this discussed before.

  I'm doing some research into the benefits and draw backs of biochar production on a large scale,
  which has led me into looking at the energy use and emissions of the production process.

  Has anyone found any publications or other information covering this, I have pretty much exhausted the www and available literature on pyrolysis - but they are all evading this issue.

  I'm assuming that the energy balance of the production is positive - ie some energy will be produced in the exothermic stage which will make up for the energy used to set the process off - but I want to find some hard numbers.


  Any info is helpful
  Thanks
  Mariska


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