[Terrapreta] Life cycle of forest fire char?

[Greg and April]

I'll take a stab at that, as I've spent time discussing forest fires with my father-in-law who used to lead fire crews, but I can't exactly point you towards specific articles.

What some people forget, is the fact that forest fires come in 2 different flavors.

Hot burn and Cool burn.

Cool burn fires which may only reach a few hundred degrees is basically low level vegetation and forest litter that burns, and it usually results in allot of fine partials that can easily be washed/blown away, but no real levels of char.

Hot burn fires which can exceed 1000* tend to turn all the small and fine stuff to ash, and while larger pieces may or may not be charred all the way through by the time that the flame front has left an area.    If a larger piece is charred all the way through, there is a good possibility that the it will continue to smolder until almost nothing is left.    Remember that the biggest idea in forest fire fighting, is to contain it and let it burn out ( with the exception of the otherwise few hot spots that may cause a flair up ).  

Burning experiments in my fireplace, has shown me that even a very thick piece of wood, when exposed to hot burn conditions, will (once it is almost completely charred ), continue to smolder until it is to cold to continue supporting the self combustion and all that is left, is a few pieces of char, a few inches in size is left - and if this any indication then the ash to char ratio would be very high, if not extreme.    

Now you might be asking "how do I know that I have reached hot burn conditions in my fireplace?" 

Let's put it this way - I can see that the metal grate, is not just a dull red from the heat, but at times is bright red to light red - and maybe pushing into orange, and becomes soft enough that I end up replacing the grate about once every 2-3 yrs, because it has sagged to the point that it's almost touching the fireplace floor - needless to say, the firebrick at the back of the fireplace, is not covered in soot.   

Wood type is another factor.    Some types of wood are more likely to burn until it is totally consumed.    Pine and cottonwood is of that type, choke cherry, is a little harder to do so, but, once it is completely charred it too, will continue to burn until almost all of it has turned to ash, with just a few lumps of charcoal left.

When I burn in my fire place, I might put in more than 25 lbs of seasoned wood, but there is usually less than 1 lb of charcoal that is left, and that is usually lumps that ended up being buried in ash and thus being smothered.

Greg H.

  ----- Original Message ----- 
  From: Green Waste Recycle Yard 
  To: terrapreta at bioenergylists.org 
  Sent: Tuesday, January 29, 2008 17:57
  Subject: [Terrapreta] Life cycle of forest fire char?


  I was trying to explain pyrolysis yesterday to some well-meaning old school environmentalists who have the classic anti-burning bias. I was trying to explain the difference between black carbon and carbon in compost/mulch, and that the BC was basically unavailable as a direct plant nutrient, that its function in Terra Preta is not as a nutrient, and that it could last for hundreds of years without degradation. They had a hard time wrapping their heads around the idea of low degradation soil-sequestered char, stuck in their idea that the best use for green waste MUST be compost or mulch.

  As their counterargument, they said that all the char from a forest fire was essentially gone a few years later, therefore (in their view) it must have been biodegraded as a nutrient. If true (that it is gone), I suggested that most of it was removed by water run-off, but I actually don't know. So I'm asking here. What happens to char produced by a forest fire?

  1) Is it mostly not pure charcoal, and thus does get utilized as a nutrient?
  2) Does it wash/blow away because it is on the surface?
  3) Does it enter into the soil?
  4) Does it actually "go away" as they claimed? Or is this just simplistic observation on their part?
  5) Some combination of the above, or other explanation?

  No need to go into a detailed response if you can simply point me to appropriate articles, etc instead.

  There is going to be a lot of resistance from old school anti-burners, so it is important to know how to answer their questions and help them reframe their thinking.

  TIA,
  Bernie
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