Coaling & wet wood

[7acres]

Last year somebody asked why they had a a firebox full of coals and not much in the way of flames or heat. I remember the consensus seemed to be their wood wasn't dry enough.

I've been wondering since then how the physics works. By the time you are at the coaling stage all the volatiles (including H2O) will have been driven off. So regardless of how wet or dry your wood was to start with, the moisture situation won't be in play anymore when you have a firebox full of glowing coals.

So does moisture in the wood promote a huge coal bed situation? If so, how?

 

[edyit]

for the how of it i can't answer that. but i can say from experience that burning a load of 1 year seasoned oak produces a mountain of coals, where burning a load of 2 year + seasoned oak produces amazing secondaries and 1/2 the coals.

 

[Nick Mystic]

I'm not an expert on this, but I have read some on how wood burns and turns into hot coals. Initially, as wood heats up, it gives off volatile gasses, which is what produces the flames that appear to be on the surface of the wood. Later as these gasses burn off you reach pyrolisis, which is a thermochemical decomposition of organic material at elevated temperatures in the absence of oxygen. This is what you see when the wood is glowing orange with no flame.

As to why wet wood would produce more unburned coals at the end of the burn cycle, here is one hypothesis: Perhaps wet wood keeps the temperature too low for the coals to finish burning as they would in a hotter stove. The wood reaches this stage and there is not enough residual heat left in the stove to finish burning off the coals. This is why it is often recommended to increase your primary air and even add some smaller pieces of kindling to such coals to finish burning them off.

 

[7acres]

I was just reading the wiki article about Pyrolisis. It states, "Pyrolysis differs from other processes like combustion and hydrolisis in that it usually does not involve reactions with oxygen, water, or any other reagents."

So that confirms there is no water involved by the time Pyrolysis occurs. I wonder if what's happening is this. Because of the higher MC the load isn't as engulfed in flames combusting completely. So kind of like Nick said, the lower temps are enough to turn the wood into charred coals, pyrolysis occurring between 390–570 °F. But since the load isn't rip roaring sucking lots of oxygen into the stove these coals just sit there with a dim glow.

If the wet wood spends the majority of its time at boiling point it makes sense that by the time the water has been driven out the volatiles have gone up the flue too. All you're left with is a pile of charred wood and no volatiles to finish the job.

 

[Jags]

I like Nick's hypothesis. I'll take a stab at it from a more simplistic direction. Your flame to coal ratio is all wonky. Flame helps burn down coals (tis a hotter fire). By the time wet wood gets dry to the core the volatiles that make flame have already been driven off leaving piles of slower, cooler burning coals.

(Ha - you beat me to coming to the same conclusion.)

 

[7acres]

I like Nick's hypothesis. I'll take a stab at it from a more simplistic direction. Your flame to coal ratio is all wonky. Flame helps burn down coals (tis a hotter fire). By the time wet wood gets dry to the core the volatiles that make flame have already been driven off leaving piles of slower, cooler burning coals.

(Ha - you beat me to coming to the same conclusion.)

I like Nick's hypothesis. I'll take a stab at it from a more simplistic direction. Your flame to coal ratio is all wonky. Flame helps burn down coals (tis a hotter fire). By the time wet wood gets dry to the core the volatiles that make flame have already been driven off leaving piles of slower, cooler burning coals.

(Ha - you beat me to coming to the same conclusion.)

Bingo! I'm happy with this line of reasoning. Glad I asked! So Pyrolysis is Stage 1; it gets the organics charred. Ideally there are plenty of volatiles that remain to mix with oxygen and heat to take the reaction to Stage 2, Combustion aka flame. Stage 2 is where the real heat comes from, BTW.

If the wood is good and dry you've got volatiles being released in lock step with the char for a very complete consumption of the fuel.

If the wood is wet those uncombusted volatiles are wasted right up the flue and you are left with a heap of carbon. Which ultimately gets shoveled up and removed from the stove. Such a waste!

Harvested firewood is just like a company 401k match. Until it has been in the stacks for a couple years it isn't vested. And you don't get the full BTU value in return if you try to prematurely burn it.

Okay, I think I've beat this dead horse enough now.