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Friday, January 14, 2011

Charcoal Production

Traditional methods of charcoal production are messy, often operate in batches,  produce variable yields and  only use the volatile fraction of the pyrolysis process  to create the heat to char the wood.  A higher yield, continuous system is suggested which utilizes the combustion of the volatile fraction of the pyrolysis process to protect already pyrolyzed wood (charcoal)  from further oxidation.  In a commercial operation based on this system, considerable heat energy will be available for drying the feed stock or for whatever other purpose is required.  Continuous production should be easily to mechanize.  

Over the last few years biochar/charcoal has become a hot new research topic. A number of factors, some old and some new have led to this situation.

Archaeologists have long known that Charcoal is refractory (doesn't break down easily) since they often find charcoal in ancient sites where fire has been used.  This is fortunate for them since at a push, charcoal can be used for carbon dating extending back 50,000 years.

Global warming has come upon us with the villain in the piece being our burning of sequestered carbon in the form of coal and oil and gas.  The resulting CO2 is the main suspect.

Some countries, Notably New Zealand, have rushed to sign up to Kyoto and take on a financial obligation for her production of green house gases.  This will cost the tax payers of New Zealand considerable money for no gain what so ever.  If we can use biochar to sequester carbon, this will reduce this hemorrhage of money.

All of the above were necessary but not sufficient reasons to spark the present interest in biochar.  The critical final factor was the discovery of Terra Preta in jungle locations.  In an area of  very poor soils, these charcoal rich soils are very productive.

Research efforts are underway all over the world to understand biochar.  The efforts are concentrating on the effect of different production methods (mainly the temperature at which the charcoal is produced) on its value as a soil enhancer and on its longevity in the soil.  In the Appendix, some information is given on the questions being asked.

However, the use of biochar as a soil enhancer will never become commercial if it is expensive to produce.  A commercial system should be inexpensive enough to locate at each source of raw material such as lumber mills with their offcuts and sawdust,  at forests with large supplies of prunings and forest litter or at an abattoir with a supply of bones.  It should be a continuous system rather than a batch system and it should effectively char a wide variety of material from fine sawdust and leaves to large pieces of wood and bark.The advantage of producing biochar on site is that it is reduced in volume and weight and hence is less expensive to transport.

The Learning Curve
As soon as Terra Preta was heard of, we started experiments  to produce charcoal.  It was thought that if charcoal is a valuable addition to tropical soils which are too warm to retain humus, it couldn't hurt to add it to temperate soils, many of which are humus poor.  The hope is that biochar will have the same water retaining and ion exchange properties as humus.

Charcoal Mark 1 consisted of simply making a fire, using material from the branch pile (about 2 meters high) and covering it with dirt once the flames had died down.  Anywhere a smoker showed through the dirt, more dirt was added.  After a dozen tries, discouragement set in.  The morning after the charcoal making exercise, the fire would more often than not still be hot and there were sections of ash where the charcoal had been consumed.  The system was laborious, dirty, batch rather than continuous and ineffective.  A huge quantity of branches resulted in very little charcoal.

Carcoal Mark 2 consisted of stuffing a 45gal drum with prunings from the branch pile and lighting it.  When the flames had died down, the barrel was gently tipped on its side and then upended, open side down.   Some dirt was kicked around the rim to seal it.  Next morning (many next mornings) we had some charcoal, the material was cold but there was much unburnt material from the bottom of the barrel.  However this led to Mark 3.

Charcoal Mark 3 used the same 45 gal drum but this time, a flame was lit in the bottom of the barrel using shavings from the woodwork shop and then branches were fed in to the barrel from the ever growing branch pile.  The system worked best if a large branch was propped under one edge of the drum to tilt it a little.  Air flowed down the lower side and flame up the upper side. Branches were added until the drum was about half full of charcoal and then for a few minutes, only very fine material was added to to give lots of flame which died down quickly but kept the barrel very hot.  This was done to ensure that any large pieces at the top of the charcoal were fully charred.  The barrel was then upended as described above.

During the production of one batch, large branches were pushed down into the charcoal layer to avoid them toppling  the drum.  When the Charcoal was examined next morning, uncharred wood was  found.  The butts of the branches had been protected from the heat of the fire and from oxygen.   It is critical that  new material is introduced on or above the surface of the growing layer of charcoal.

In all batches, the next morning the charcoal was cool, no ash was to be seen and everything from leaves to 5cm diameter branches were charred. The only batch with uncharred material was the above one where the branches were pushed into the charcoal layer.  An easily identified gum leaf, placed in the palm and rubbed with the thumb disintegrated into powdered charcoal while large chunks of wood, rapped on the edge of the drum to break them were completely charred all the way through.  A modest supply of branches gave a good yield of charcoal.

Why Does It Work
What is apparently happening is that as new material is put in the burning drum, it pyrolyzes and give out flammable gases.  Nothing new in that.  The burning gases use up the oxygen, protecting the charcoal from further combustion.  As long as there is a reasonable amount of visible flame, charcoal is produced rather than being consumed.

A Comercial Unit
As a first pilot plant, one could start with a cast iron or steel cylinder with the same proportions as a 45 gal drum.  For ease of fabrication it could probably be octagonal, hexagonal or even square.  A conveyor belt would bring feed stock to a feed in trough sticking out of the side (like the old trash burners had).  The critical part, though, is to turn this into a continuous rather than a batch system.  This could be done by having an augur at the bottom to extract the charcoal.  The charcoal extraction system would have to be  air tight to ensure that air did not enter the charcoal bed.  The charcoal would be dumped into steel carts with air tight tops and left sealed overnight to ensure that the charcoal was extinguished.

Combustion air could come from the top as in the simple home system or could be introduced through vents in the side of the retort, above the surface of the charcoal.  Having these vents adjustable would give an added measure of control to the operator.  The extraction of charcoal from the bottom of the retort would ensure that the top of the charcoal bed was always below the vents.  The air could  be introduced tangentially to ensure a whirling, well mixed flame.

For the use of biochar to catch on, charcoal production must be inexpensive.  It is best if it can be carried out where the feed material is available since turning wood into charcoal greatly reduces its shipping weight and somewhat reduces its volume.  Any system which is continuous will be far more productive per retort than a batch system of the same configuration and size and hence more cost effective.  Having to cool and harvest a system takes considerable time and greatly reduces the production of a system of a given size.  Considerable heat will be produced which can be utilized for whatever purpose needed.

appendix and pictures to come
pilot plant under construction

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