Why can wood burn all the way to charcoal instead of just creating a passivating charcoal layer?

Question

I know burning to charcoal is actually incomplete combustion because charcoal can also burn. When a strike anywhere match burns, it burns all the way to charcoal but the charcoal doesn't further burn. How can all of it burn to charcoal? According to https://en.wikipedia.org/wiki/Pyrolysis, the heat causes pyrolysis which undergoes flaming combustion generating more heat. Once some charcoal has already formed, why doesn't it just passivating the wood blocking further endothermic pyrolysis. I know wood is porous but at the small scale, should't it's surface become passivated by charcoal blocking further pyrolysis? Does it occur for a similar reason that iron can continue rusting after it already formed a rust layer by producing a porous rust layer? Could nonporous single crystal cellulose also burn to charcoal? If so, can you give me a detailed mathematical description of how it would continue burning to create a thicker charcoal layer and how iron can just keep on rusting?

Answer 1

Charcoal doesn't passivate and the results you see depend on the physical structure of the fire

The difference between a match and a larger wood fire has little to do with the formation of charcoal and more to do with the circumstances of the reaction. And the charcoal isn't really passivating in the reaction on a match.

Wood consists of a complex mix of chemicals as well as the core cellulose, which is a carbohydrate. Some of the ingredients, like the sap and other volatile components, burn easily. Cellulose burns in multiple steps one of which involves pyrolysis where some components are driven off quickly leaving mostly charcoal. Charcoal itself will burn but it takes more energy to ignite and therefore more time to get going. It isn't so much that charcoal passivates, it is just that it is the last component to burn.

The difference between a match and a larger wood fire isn't about passivation but about the scale of the fire. There isn't enough energy in a match head to drive combustion all the way so only the easy-to-burn components ignite (unless you hold the match upside down so the flame spreads up the matchstick when the heat from the tip is enough to cause much more extensive combustion). In a larger wood fire there is enough energy in the flame to cause more complete combustion. So the volatile components burn quickly but leave enough heat for the charcoal to burn as well. Since heat penetrates charcoal very well, the volatile components under the charcoal will tend to ignite before the charcoal itself (it isn't doing much to passivate the wood underneath). In a badly organised fire the overall heat may never be enough to set off the charcoal.

So the issue isn't passivation just a reflection of the order the components of wood burn and the physical structure of the fire. Charcoal isn't passivating, just the last component to burn and, if the fire isn't well constructed, there may never be enough heat to burn it. Much more of a match will burn if you drive the flame along the stalk by holding it upside down and a well organised fire will burn the whole way to ash.

Also controlled pyrolysis (which is used to create charcoal deliberately) relies on controlling the amount of air available to the work being pyrolysed. If there were enough air, the charcoal would burn but the conditions for pyrolysis involve restricting the air so the end-product cannot burn. Charcoal doesn't passivate anything when there is enough air (which is why you can cook on a barbecue).

Answer 2

Charcoal is nearly pure carbon, and its vapor pressure is miniscule. So, rather than pyrolysis, it burns slowly, by converting at the surface to CO (or maybe CO2, somewhat less quickly); the CO then burns to CO2 rapidly, above the solid carbon surface. Because the oxides are gasses, the carbon surface remains clean, and nearly pure carbon. It burns by corrosion in the presence of hot O2.

As for iron, iron can form black 'scale' which adheres and protects against further corrosion (but not very well). It can also be 'blued' to form a hematite or magnetite thin film, of somewhat better purity and adhesion than the black scale. Either of these coatings can protect against further rust, but they are brittle and do not always re-form after damage. The more usual rust is brown rust, which is a combination of oxides and hydroxides (and chlorides as well). This can be converted to black oxides with mild acid treatment. Again, these oxides are brittle and can be damaged, after which environmental water (even water vapor) can be expected, with oxygen, to penetrate any cracks and crevices, so the metal underneath is continually under attack.

Some alloys of iron DO form a tight surface of 'rust', notably Cor-Ten steel CorTen.com and wrought iron. There is an iron pillar in Delhi that has attracted some attention Ancient iron for its longevity (over a millennium outdoors).

Answer 3

The quandary of why burning wood doesn't burn completely is the heat at which the wood is burning. Wood isn't a pure chemical, but rather has all sorts of chemicals within it. So some of the more volatile chemicals burn off at lower temperatures. In fact it is the volatile chemicals burning in the air that give the "flame" above the match.

If you put the match residue in a crucible and heated the crucible red hot with a Bunsen burner, then you could burn the match residue completely to a powdery ash.