Originally Posted By delorean:


I think you're going to like this.....

The surface area is a common misconception because folks don't realize that nearly all of the surface area is not visible.  Lump that's carbonized to 80%+ can have a surface area (AKA porosity) of 200-300 m2/g.  That means a piece of charcoal the size of a golf ball splayed out has the surface area the size of a tennis court.  It absolutely crazy how porous it is!!  So it doesn't matter what you're seeing on the outside, the oxygen is going inside of it.  I'll try to most a pic of what charcoal looks like under a microscope.  That's why charcoal is used in filters, detoxifiers, soil amendments, etc.  When they activate charcoals with super high temp steam and/or chemicals, the porosity can be as high as 2000 m2/g.....that's the medical grade stuff they give patients that overdose or are poisoned (usually a bismuth or anthracite coal for those though......we carried it on the ambulance.)

Back to the BBQ, when you see those glowing red, nearly translucent coals in the firebox, fireplace, or wood stove, that's the true charcoal that's nearly all carbon at that point.  100% of it is burning--inside and out--tons of heat.  It's not like a log, briquette, etc that has to burned from the outside in.  With those, you have to burn through all the other "stuff" to get to the carbon.

A piece of "green" wood can have a moisture content equal to or over the weight of the wood itself and many months of seasoning brings it down to under 20%.  That's getting most of the water, tars and liquors out of it.  When you start burning it, you'll see the blue flames which is the rest of those volatiles be burned out of it (that's where the creosote comes from).  Then the wood fiber starts to burn.....that's where the yellow flames (that's where most of the ash come from.)  Finally, when that log breaks apart into a bunch of glowing red coals, that's the carbon--that's when the fire is starved from O2 in the charcoal making process.  If done right, it results in a lump charcoal that is close to 12000 BTU/#.  It produces very little smoke or ash because the things that produce smoke are ash are already gone.  It can burn hot and fast or low and slow--just depends how much O2 you let get to it.  It also can get wet, and dry quickly because it adsorbs (not absorbs) moisture.  That's why the cave man invented it--it was a lightweight, transportable fuel he could get wet yet still have a hot fire in short amount of time.  Fun fact: Lump charcoal is the oldest manufactured product known to man.  

Most US manufactured charcoals come from "slabs" which are all the offcuts and scrap from the milling industry.  A tree that has had an injury from someone nailing something into it, shooting it, wrapping barbed wire around it, etc, will have a knot or deformity thus causing that piece to get rejected at the mill.  It goes to the charcoal plant with all the other pieces that are too knotty, warped, short, etc to be made into hardwood lumber.  A piece with a nail, bullet, or whatever in it may make it through a kiln with that still in there, too heavy to be grabbed by a magnet, which you don't find until the charcoal burns away.  There were some brands using "construction scrap" and you should stay away from that junk if you see plywood, molding, subfloor, etc because of the glues and such (the crappy thing was they still called these "all natural" because resin come from trees.)  But a couple right angles on a piece of charcoal in the bag doesn't mean construction scrap, a 2x4, or anything bad.  It's mill scrap, which makes awesome charcoal because it's most likely virgin white or red Oak, Hickory, Maple, Cherry, etc from the Mark Twain and Ozark National Forests.  This is what you use to smoke anyway.

South and Central American charcoals come from very dense woods you cannot pronounce--from species similar to cashew and walnuts.  Very bitter woods, super dense, thus hard to kiln.  Despite what they claim, these countries do not have the rules that the US does about harvesting, labor, nor emissions.  They can clear cut the rainforest, use slave labor to do it, and pump as much particulate into the atmosphere as they want.  That's how they can bring it 5000 miles into the US for basically the cost of freight--far less than what a US manufacture will spend to produce and comply here.  That story is for another day though.

Lastly, yes, you don't want a ton of "fines" (1" minus), because it will choke the airflow.  The dust or small pieces that fall into an ash pan will burn on hotter cooks so you don't necessarily lose those BTU's.  On the other side, if you trying to get a piece of charcoal the size of a cantalope lit--first, it's probably full of water and wood fiber and that's why it's still that big (so you're spending time and BTUs [$] to complete the kilning process that the manufacturer should have done!)  Second, with a low and slow fire use a very small amount of charcoal is actually burning depending on the smoker.  A big chunk like that is acting as a heat sink, so getting a small portion of it to burn is tough when the rest of it is radiating heat is tougher than a couple smaller pieces burning.  Plus the neighboring huge chunk might be too far away for the heat to get to ignite it--that's where the little piece come in.  Faster lighting, and providing a "fire bridge".

The third part of the fire triangle is heat, better known as a kindling temp.  Lump charcoal is about 670F.  So the area that's burning is can be very small to result in a temp of 200F-something in the cooking chamber.  If you're using a BGE or kamado type cooker, that's why things goes "nuclear" when the dome temp hits about 450F or so......down in the firebox, it just hit about 700F and ALL the charcoal lit through "flashover".  That's why that temp needle will wrap all the way around in less than a minute.  Otherwise you're using the draft door and/or chimney to starve the fire of O2--it will burn out-of-control since their nothing else holding it back.

Hope this makes sense......wrote a lot more than I planned to, but science behind fire fascinates me.

EDIT:

Here ya go.....normally all these crannies are clogged with water, tars, and liquors.  The kilning process burns those out and leaves behind a very porous piece of carbon that the O2 can flow through.

https://media.sciencephoto.com/image/c0403815/800wm/C0403815-Charcoal,_SEM.jpg

https://www.youtube.com/watch?v=kF10i39VQ4w