Fundamentally, they're both carbohydrates, although the cellulose in wood is essentially polymerized glucose, which combined with its isomer fructose forms sucrose.

So why does wood readily burn while table sugar chars?

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    $\begingroup$ Related: Sugar burning with the assistance of ash $\endgroup$ – Loong Nov 3 '18 at 9:35
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    $\begingroup$ Sugar burns very well. It does take more effort to ignite it, I would hazard guessing that is due to higher thermal conductivity and lower specific surface area. If you powder and disperse it, it can even explode. $\endgroup$ – Stian Yttervik Nov 3 '18 at 18:13
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    $\begingroup$ Sugar burns easily, it flames then forms black lumps. $\endgroup$ – matt_rule Nov 3 '18 at 18:43
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    $\begingroup$ Ever heard of sugar rockets? $\endgroup$ – PlasmaHH Nov 3 '18 at 20:14
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    $\begingroup$ Sugar will burn fine if you give it enough oxygen. Put sugar in a deep saucepan (it will splash a bit), and add a tiny bit of water to make it a slurry. Add in KNO3 in a 13:7 ratio, and start heating. Slowly add in just enough water to completely dissolve both sugar and KNO3, bring to a boil and evaporate the water until you get a smooth creamy color and texture. Quickly pour it in an old can and let it cool. Put the pan back on the fire and add a lot of water and boil to clean it. Ignite the can with a few magnesium sparklers. All of this should be done outside with an extinguisher on hand. $\endgroup$ – Drunken Code Monkey Nov 4 '18 at 5:39

Combustion is a gas phase reaction. The heat of the flame vapourises the substrate and it's the vapour that reacts with the air. That's why heat is needed to get combustion started.

Anyhow, wood contains lots of relatively volatile compounds so it's not too hard to get combustion started. Once combustion has started the heat of the flame keeps the reaction going.

However sugar dehydrates and emits water when you heat it. Water isn't flammable (obviously) so there's no way to get combustion started. Dehydration leaves behind pure carbon and that is non-volatile so again there's no way to get this to burn. Carbon will burn of course, but you need a high temperature to get it going.

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    $\begingroup$ If there is no way to get a combustion reaction started for sugar, then why are the TGA thermograms for wood and sugar similar? $\endgroup$ – Agriculturist Nov 4 '18 at 3:23
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    $\begingroup$ @Agriculturist of course sugar will burn. Some of the ways to make it burn are discussed in the comments to the question (I would be cautious about mixing it with oxidising agents though as it might be a wee bit more exothermic than you expect! :-). The point is that unlike wood it doesn't burn when you put a match to it. $\endgroup$ – John Rennie Nov 4 '18 at 9:10
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    $\begingroup$ I'm trying to lead you to the correct line of thinking. If you put a match to an oak log does the oak log burn? In the 2008 imperial sugar plant explosion, why did the sugar dust explode? $\endgroup$ – Agriculturist Nov 4 '18 at 13:15
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    $\begingroup$ @Agriculturist Read again - it's not that the water extinguishes the flame. It just doesn't combust in air, unlike the volatiles in wood (some of which are very volatile indeed). To get a flame, you first need to get burnable volatiles out of the target material - that's really what a flame is, volatiles violently reacting with ambient air and heating to high temperatures. John didn't talk about water preventing combustion at all - just that it doesn't facilitate the combustion. $\endgroup$ – Luaan Nov 5 '18 at 6:31
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    $\begingroup$ It might be useful to point out that water vapor carries away heat. IIRC, the thermal dissociation of sugar is endothermic, and then you need the high temperature to burn pure carbon. $\endgroup$ – MSalters Nov 5 '18 at 12:29

With hydrocarbons a certain amount of oxygen (n) and a certain amount of heat energy (Q) are required for complete combustion. In complete combustion the byproducts are carbon dioxide and water in the form of steam.

$$\ce{C_xH_yO_z + nO_2 + Q -> xCO2 + \frac{y}{2}H2O}$$

Hypothetically, if wood and sugar were assumed to have an identical chemical composition and supplied the exact same amount of heat energy, then any difference in the reaction would need to be derived from the amount of oxygen.

A lack of oxygen is why sugar does not readily burn. This reaction was demonstrated back in the day by Mr. Wizard. He added some oxygen in a chemical form which made table sugar go up in flames quickly.

If oxygen is not readily available then another reaction, called pyrolysis, occurs. This reaction often gives off some combination of methane, hydrogen gas, and carbon monoxide, all of which are flammable, and often leaves behind carbon. This carbon can be observed as char. $$\ce{C_xH_yO_z + Q -> aH2 + bCO + cCH4 + dCO2 + eH2O + fC }$$

The answer to the question is oxygen cannot readily get to the pile of sugar, which is why it does not burn well and it displays the byproducts of pyrolysis. In this situation the wood is undergoing pyrolysis as well; however, enough oxygen is available such that all of the pyrolytic byproducts burn leading to complete combustion.

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    $\begingroup$ Why does oxygen have better access to wood rather than sugar? Is it due to wood's porosity? $\endgroup$ – SwagMcMuffins Nov 4 '18 at 6:29
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    $\begingroup$ The matchstick vs. lump of sugar scenario is a specific case. Matchsticks are indeed made from porous woods such as pine and aspen. When these woods burn the cellulose in the wood burns before the lignin. Lignin gives wood its structure as it burns because it burns at a much higher temperature than cellulose. Because of this wood will hold its porous structure as it burns thus giving it access to oxygen. A lump of sugar has little structure as it burns so it goes out due to lack of oxygen. $\endgroup$ – Agriculturist Nov 4 '18 at 13:52
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    $\begingroup$ @SwagMcMuffins is it written around in answers and comments. It is about how much flammable volatiles are released as compared to non flammable and heat carriers are formed (carbon dioxide, water). Look at the reaction in gas phase,or at the interface, and what is happening there. Not in bulk. If sugar is finely suspended in air it will burn and even explode. Same should do wood, of course. $\endgroup$ – Alchimista Feb 17 at 11:39
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    $\begingroup$ Looking back at this assumptions can be made (such a match being the ignition source) to generate some very plausible sounding answers through inductive reasoning. Using deductive reasoning it is known burning things require oxygen availability, charring things requires reduced oxygen availability, therefore something is different about the oxygen availability. The oxygen availability is affected by some mechanism which cannot be deduced from the question the way it is stated. $\endgroup$ – Agriculturist Feb 20 at 5:44
  • $\begingroup$ Mr. Wizard's point is that with enough oxygen even sugar burns. You are reasoning by making analogies from there, and come to wrong and irrelevant conclusions. $\endgroup$ – Karl Apr 14 at 18:02

It's a matter of physics more than of chemistry. If you apply a lit match to a log, it won't catch fire. Doing the same to a (headless) matchstick will set it alight immediately. This is because the matchstick is surrounded by flame and heats up quickly as it is so thin. If you apply a match similarly to a stick of sugar as thin as a matchstick, then it will burn too. However, the sugar will melt as it heats up, and the burning sugar will tend drip off the remaining stick so that the combustion will not propagate as well.

There is also a chemical reason why cellulose inflames more readily than sucrose (ignoring the lignin content of wood for the present). Because of the additional cross-linkages, cellulose is more stable, and will start to decompose at higher temperatures than does sucrose. At the lower temperature of sucrose decomposition, the main reaction is loss of water vapour, which is incombustible, leaving behind a carbon-rich char which does not ignite so readily at moderate temperatures. While the same process happens as cellulose heats up, at the higher temperature of decomposition, volatile organic compounds, which are combustible, are produced in addition. As these ignite, the flame is enhanced and propagates by positive feedback. The same effects apply a fortiori to lignin in comparison with sucrose.

  • $\begingroup$ Thanks for this answer. I was trying to figure out a unique case of why a cotton ball (which is nearly all cellulose) ignites more readily than cotton candy (which is nearly all sucrose.) In this particular case, the air supply is nearly identical, but there is a noticeable difference in the way they combust if the flame is applied slowly. $\endgroup$ – Agriculturist Nov 6 '18 at 0:09

As noted in other answers, commercial table sugar is typically crystalline and the bulk of it must undergo a phase change into a form accessible to oxygen in the air. Melting the sugar consumes energy and may slow down its reaction (the melting point is reported to be ~186 $^o$C). According to more recent studies sugar undergoes thermal decomposition rather than melting. This suggests that the process of combustion of sugar is complex and can involve both thermal decomposition and oxidation by air. Presumably the formation of the molten caramelized mass reduces access to oxygen unless very high temperatures encourage further reaction with air.

Cellulose within dried wood also contains crystalline domains, as well as amorphous ones, but their size is smaller and the energy required to induce phase changes that provide access to air is presumably less. The polymeric structure probably interferes with decomposition and the porous structure of wood aids access to air.

As an interesting corollary to this problem and some of the answers and comments, the generation of coke from raw coal (containing impurities such as sulfur compounds) via heating in the absence of oxygen generates a cleaner burning carbon energy source.

EDIT: added consideration of thermal decomposition of sugar.

  • $\begingroup$ The heat of melting is totally insignificant compared to the heat of combustion. $\endgroup$ – Karl Apr 14 at 18:35
  • $\begingroup$ @Karl I think your answer addresses many aspects more convincingly, but I am a little surprised after reading your answer that you bring this point up, since you make a very similar one (that the structure of sugar -molten or not - impedes access to oxygen). Perhaps you just think it's of secondary importance? $\endgroup$ – Night Writer Apr 15 at 19:06
  • $\begingroup$ Perhaps I was a bit unclear. Molten sugar has a compact, closed surface, very much like a large crystal. The surface of a heap of table sugar is much more open, but it melts long before it can ignite. It seems powdered sugar keeps so much air even when it melts that its heat conductivity stays low, and it can actually burn by itself. (never tried that myself) $\endgroup$ – Karl Apr 15 at 19:13

This is an experimental answer to a slightly different question. The way the question is asked, it is confounded by the substances wood and sugar. If wood and sugar are removed from the question it will read...

Why does substance A burn but not substance B?

Substance A and B are both carbohydrates of similar composition. So why does substance A readily burn but substance B chars?

The fact that A burns and B chars is the key piece of information to discovering the correct answer. Imagine substance A and B are both granulated cellulose. (Cellulose happens to be both sugar and the dominant component of wood.)

crucible of cellulose

This is a picture of what is left when substance A is heated to 550°C in the presence of air.

enter image description here

This is a picture of what is left when substance B is heated to 550°C in the absence of air.

enter image description here

Because the question says substance A burned and substance B charred the right answer is most likely substance A had enough oxygen for combustion while substance B did not.

In the original question, too many unknowns have the potential to affect the right answer. The heat energy of the ignition flame is an unknown. The type and size of wood are unknown. The granularity of the sugar is unknown. The size and shape of the sugar pile are unknown. The presence or absence of any controlled conditions is an unknown. The presence or absence of a flame is an unknown.

Because wood burns it is clearly evident that both heat and oxygen are present. Because the sugar chars it is clearly evident that heat was present but oxygen was deficient. Any explanation beyond this involves some level of speculation as to the conditions implied by the question.

-- cautionary note about speculation --

Once speculation starts, weird answers can become true. For example, If someone posted an answer says that the sugar was buried under a campfire while the wood was in a campfire. The question mentions no campfire nor does it say anything about burying sugar, it cannot be said for certain that this answer is right or wrong.

If the explanation is embellished with interesting facts it could sound awesome, but be completely wrong if one thing is off. For example, someone could point out sugar makes caramel at 186°C and that wood contains a very flammable group of compounds called terpenes. The question mentions no formation of caramel so there is no telling if it is present or not. It is not known the extent to which the wood is dried so terpenes may not be significant. It is not possible to say for certain if these explanations are right or wrong.

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    $\begingroup$ "The sugar/cellulose is identical in both reactions; however, in one situation it burned and the other it did not burn." Just to clarify, you mean the crucibles after experiments with either sugar or cellulose were identical, but in the presence of oxygen both combust to near completion? $\endgroup$ – Night Writer Nov 5 '18 at 15:56
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    $\begingroup$ The crucibles were identical. The temperature profile was identical. The initial amount of cellulose was identical. The only difference was the presence of oxygen. One burned, one charred. The cellulose can be switched with table sugar or ground up wood and the same results will occur. One will burn, the other will char. This is presented as direct evidence that the correct answer to this question involves the presence of oxygen in the reaction. The wood had sufficient oxygen to burn, the sugar did not so it charred. $\endgroup$ – Agriculturist Nov 5 '18 at 23:40
  • $\begingroup$ Without oxygen, neither wood nor table salt burn. So, logically, with enough oxygen, table salt will burn. $\endgroup$ – Karl Apr 14 at 18:34
  • $\begingroup$ @karl no that is flawed logic. A duck is not a stone. You are not a stone. Therefore, you are a duck. $\endgroup$ – Stian Yttervik Apr 15 at 8:16
  • $\begingroup$ @StianYttervik I have just one more thing to say: Quack! ;-) $\endgroup$ – Karl Apr 15 at 19:20

The oxygen (chemically reduced oxygen; that is, essentially, water) content of sugar is very high. It's already highly oxidised.

$$ \begin{array}{llr} \hline \text{Compound} & \text{Formula} & \text{Oxygen}/\text{wt %} \\ \hline \text{Glucose} & \ce{C6H12O6} & 53 \\ \text{Saccarose} & \ce{C12H22O11} & 51 \\ \text{Cellulose} & \ce{C12H20O10} & 49 \\ \text{Dry wood} & & \sim 35 \\ \hline \end{array} $$

The oligo- and polysaccharides have lost water after polymerisation, but not very much. Cotton is not very flammable. That's why labwear is required to be made of cotton (plus it doesn't melt and then stick to your skin).

Wood however is not just a carbohydrate, but has a high content of lignin, which contains much less oxygen and has a sum formula in the range of $\ce{C3H3O}$.

Very simply put, you have to boil a lot of water out of sugar before anything can actually burn. If you look at the sum formulas above, about two thirds of the mass and all the hydrogen are gone at that point. Pyrolysis of sugar gives water + carbon, and very little flameable volatiles. Which are what you see on burning wood. When all the volatiles are pyrolysed out of the wood, the remaining charcoal is hot enough that it keeps burning.

Sugar has the additional problem that it melts, which makes it burn even worse, because the grainy or powdery structure fuses into a closed surface with good internal heat conductivity. There is an old trick, mixing sugar with ash, which makes it burn a lot better. Cellulose doesn't melt, that's why paper burns much better than sugar. (And because it's thin, a complete book is not so easy to burn completely.)

Btw. the autoignition temperature of sugar (probably saccharose) is ~410°C, wood 280-340°C. (from de:wp, without reference unfortunately)

  • $\begingroup$ Cotton is flammable! The reason cotton is a popular choice for lab coats is because cotton does not adhere to the skin when it burns. $\endgroup$ – Agriculturist Feb 20 at 3:45
  • $\begingroup$ @Agriculturist That too, because it doesn't melt, but I said not very flammable. The flame smokes and stinks and often goes out when the ignition source is removed. $\endgroup$ – Karl Feb 20 at 5:40
  • $\begingroup$ The cotton in treated lab coats resist flames. Traditional cotton lab coats are not recommended for use around flames. $\endgroup$ – Agriculturist Feb 20 at 5:54
  • $\begingroup$ This is completely wrong. Not least because sugar does burn well once the reaction starts (cigarette ash is a good catalyst to kick it off). The key issue is activation energy not whether the overall reaction is favourable. And that barrier is as much about the physical properties as about the inherent chemistry. $\endgroup$ – matt_black Apr 14 at 12:10
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    $\begingroup$ downvote - because this line of logic could get a lab worker seriously injured. $\endgroup$ – Agriculturist Apr 15 at 19:41

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