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Gases

Page history last edited by Philip Small 11 years, 10 months ago
My appreciation to Sean K. Barry for posting this to Yahoo! Tech Group: Biochar on October 12, 2008
 
Pyrolysis of biomass using limited oxygen supplied from air emits a gas called "producer gas".  This gas is really a mixture on both non-condensable and condensable gases.  The difference between condensable versus non-condensable gases is that condensable gases will phase-shift and can form liquids or solids when they come into contact with "cooler" surfaces.  The "producer gas" is primarily made up of {non-condensable gases: Hydrogen gas-H2:~20%, Carbon Monoxide-CO:~20%, Carbon dioxide-CO2:~10-15%, Methane-CH4:2-3%, Oxygen-O2:<1%, and Nitrogen gas-N2:~40-45%, and condensable Water vapor-H2O:~5% plus other condensable vapors of some liquid hydrocarbons called poly-cyclic aromatic hydrocarbons (PAHs), substances like pyroligneous acid, acetic acid, turpentine, and some alcohols}.  Entrained along with and in the flow of gases and liquids exiting out of a pyrolysis reactor, there may also be particles of solid matter which are mostly unburned carbon particles blown explosively off the biomass when it is in the "flaming pyrolysis zone", which a a point usually nearest the entry of "oxidant" into the reactor.
 
The non-condensable gases H2, CO, & CH4, are all flammable.  The other non-condensable gases: N2, O2, and CO2, are just hot when they exit the reactor, but they are not flammable.  When pure Oxygen gas-O2 is used as the oxidant, rather than "air" (which contains ~19% Oxygen gas-O2 and 78% Nitrogen gas-N2), then the gas which is emitted is called "synthesis gas".  The "producer gas" coming from an "air-blown" pyrolysis reaction is very similar to "synthesis gas", except that it is diluted with Nitrogen-N2 gas.  Condensing vapors mix with condensed liquids and particles to form what can be semi-solid or even solid "tars" on the cooler surfaces that are impinged with the exiting gas mixture.
 
The ONLY Green House Gases (GHGs) emitted from a pyrolysis reaction are Carbon dioxide-CO2, and potentially Methane-CH4.  If sufficient secondary air is introduced and mixed into the exhaust from the reactor and there is continuous ignition in case the exhaust stopa burning, then all of the flammable non-condensable gases: H2, CO, CH4 and some of the flammable vaporized liquids and particles will all be consumed in the "Flare".  This will create heat (usually lost heat) and will emit only CO2 and H2O in the exhaust from that flame.  An effective "flare" has sufficient oxygen/air, good mixing of oxygen/air with the exhaust, and does not go out.
 
Pyrolytic reactions do not emit Sulfur Oxides-SOx, Nitrogen Oxides-NOx (specifically NO, NO2, NO2), nor Nitrous Oxide-N2O.  The only one of these which is a GHG is N2O.  The temperatures of pyrolysis, 400C to 600C, are just not hot enough to form these compounds, nor is there much sulfur in most biomass.
 
The most significant problems creating "non-clean" GHG emissions from a pyrolysis reaction done in a low cost/low tech reactor/kiln would be to NOT BURN (or FLARE) any Methane-CH4 exiting from the reactor or to be so inefficient as to release more carbon in the "producer gas" (as CO, CO2, or CH4) than is left in the charcoal by-product.  The main reason not releasing Methane-CH4 is that it is such a potent GHG.  It is 23 times more potent than CO2 in the atmosphere.  This means it can absorb and re-radiate infrared radiation (heat) that is rising from the Earth's surface 23 times more effectively than the same volume of CO2.  When biomass is converted into charcoal via pyrolysis with air and with some emitted exhaust, then there will necessarily be emission of some CO2.
 
In the best, most efficient pyrolysis reactions, only ~60-65%% of the original carbon in the biomass is retained in the charcoal.  The rest is emitted as CO, CO2, and Methane-CH4.  Emitting large amounts of CO2, CO, or worst of all Methane-CH4 means that the amount of carbon that is left in the charcoal to be sequestered into the soil is less than the amount of carbon emitted.  There will be no gain in combating increases in atmospheric carbon if the kiln/pyrolysis reactor emits the 2% Methane in the producer gas without FLARING.  If 3% Methane-CH4 is emitted, then there is more damage done to the atmosphere by increasing the Methane-CH4 content than could ever be recovered, even if all of the carbon from the original biomass could be captured and sequestered.

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