General Question

AdventureElephants's avatar

When wood burns, where does all of the physical matter go?

Asked by AdventureElephants (1397points) January 9th, 2016 from iPhone

I’ve burned at least 60 logs in my fireplace so far this winter, and there’s barely a pile of ash. What’s going on?

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16 Answers

ragingloli's avatar

Most of it gets turned into gas. That is the smoke that the fire produces.

zenvelo's avatar

Particulate air pollution is dangerous to health. A fair amount of it has gone in your lungs.

Coloma's avatar

It went up in smoke. haha
The harder the wood the hotter it butrns and the hotter it burns the less ash remains.

Love_my_doggie's avatar

The law of conservation of matter states that nothing is created or destroyed, it merely changes form. Burning wood undergoes a chemical change. Most of your 60 logs were converted to light and heat energy, which was your very reason for burning the wood.

Strauss's avatar

Most of it goes up in smoke, which is a combination of gasses (a lot of it is CO2), unburned particulates (including creosote, which is flammable and can deposit onn the inside wall of the flue) and water vapor. @Coloma is correct about hard wood. It burns at a higher temperature, and more particulates are burned.

CWOTUS's avatar

While it’s true that “it goes up in smoke”, not all of what people perceive as “smoke” from a chimney (or power plant stack, or car exhaust, etc.) is really smoke.

By weight, even well-seasoned and ‘dried’ firewood contains about 14–15% moisture when it’s left inside the house (where the ambient conditions will generally be >50% relative humidity even in a dry climate). So much of what seems to be smoke is actually steam. In that case, 14% of the “wood” weight is actually water.

The quantity of ash will be a factor of the type of wood and the heat of the combustion process. A well-oxygenated fire in a well-designed firebox will generally produce the least ash for the type of wood that is being burned, but not all wood species’ ash content is the same. And a lot of that ash escapes via the flue and the smoke. That’s flyash, as opposed to the “bottom ash”, the heavier ash particles that are left in the bottom of the combustion chamber.

Some of the escaping flue gas condenses on the walls of the flue in the form of creosote. Again, this is dependent upon the condition and dryness of the wood being burned, the species of wood, and the conditions of the combustion process and chamber.

The flue gas that escapes through the end of the flue (top of the chimney or smokestack), combined with the steam, is smoke; generally carbon dioxide, carbon monoxide, sulphur compounds (plus whatever else was in the wood), flyash (as noted above) and unburned particulate matter. Because the combustion process is not 100% efficient.

LuckyGuy's avatar

Ooo! Ooo! I know this one!!!

Start with nice, dry wood. In winter it can be 4–6% water. In spring it can be as high as 10%.
Wood is a combination of cellulose, hemi-cellulose and lignin. If you look up the equations you will see all 3 constituents are made of carbon and hydrogen and oxygen. When you burn wood you are converting it into CO2 and water vapor (steam).
Wood is not quite that simple. There are some trace element that get pulled up from the soil as the tree grows. They account for only about 1% of the mass of the tree. That is the stuff that is left over as ash. About half of the ash is Calcium Carbonate.
If you burn 100 pounds of wood you can expect to see about 1 pound of ash in your stove. As far as smoke and particulate new stoves meet particulate standards that are as low as 2 grams per hour. Nothing.
I have a high efficiency stove that preheats the combustion air so hot it begins to pyrolyze the wood and turn it into a gas. Those gasses are sent through secondary air combustors which burn it at high temperature and convert it all to CO2 and H2O.
Tis is not your grand-daddy’s wood burning stove. Those are fossils.

Here are some numbers for Wood ash . The numbers are similar to my rule of thumb estimates.

“Typically between 0.43 and 1.82 percent of the mass of burned wood (dry basis) results in ash.[5] Also the conditions of the combustion affect the composition and amount of the residue ash, thus higher temperature will reduce ash yield.[3]

Much wood ash contains calcium carbonate as its major component, representing 25[6] or even 45 percent[1] Less than 10 percent is potash, and less than 1 percent phosphate; there are trace elements of iron, manganese, zinc, copper and some heavy metals.[6] However, these numbers vary, as combustion temperature is an important variable in determining wood ash composition.[5] All of these are, primarily, in the form of oxides.[5]

Coloma's avatar

^^^ “If you burn 100 lbs. of wood you can expect to see about 1 pound of ash in your stove.”
That’s close to the average cremation conversion. haha

stanleybmanly's avatar

Hand Lucky Guy the trophy. You won’t get a better summation nor technical description.

LostInParadise's avatar

Burning is in essence the same as our respiration process, which turns glucose and oxygen into water, carbon dioxide and energy. We speak of it, somewhat inaccurately, as burning energy. More mind blowing to me is the reverse process, photosynthesis, which builds carbon compounds from carbon dioxide, water and sunlight.

LuckyGuy's avatar

@LostInParadise Brings up a good point and reminds me of a corollary question.

Where did the stuff come from that makes that giant tree I see out standing in its field? That tree is huge! It’s at least 2 full cords of fire wood. If that tree comes out of the ground why isn’t there a hole or huge depression in the field?
Answer: It didn’t come out of the soil! As soon as the plant came out of its seed and reached the surface it started mixing rain water and CO2 from the air and using solar power to convert them to the lignin and sugars that form the tree you see. Certainly some small quantity of minerals are taken up from the soil but they account for less than ½ % of the mass of the tree. Peanuts.

You know that scientists and engineers are always trying to improve things. People are working on these:
1) Improved, higher efficiency photosynthesis. Photosynthesis is notoriously inefficient, only about 2% by area. Solar cells are 20% . Can you invent a different chemical reaction that is more efficient but just as cheap as nature?
2) Reduced ash content crops. Crops, trees, plants all leave ash when burned. Can you develop a plant with lower ash content so it might be used in industrial power facilities (CHP) yet not need frequent ash removal?

stanleybmanly's avatar

But wouldn’t a reduction in ash mean a corresponding increase in carbon, sulfur, etc. compounds polluting the atmosphere?

LuckyGuy's avatar

Not necessarily. The ash comes from minerals the tree/plant pulls out of the ground. I know of an energy crop specifically designed and genetically modified to pull fewer minerals from the soil .

cazzie's avatar

It gets turned into light and heat, mostly. ‘Fire’ is just a form of oxygenation. You could think of it as really efficient ‘rusting’. When something is lit on fire, any energy in the matter gets released in the form of heat and light. The material that is left is what is left over from oxidisation. The ash and smoke are the minority content of the wood. The light and heat is the majority. Your chimney is designed to take up the smoke and gasses. Nothing wrong with a good modern wood stove. I’d be lost without mine at the moment.

Hard wood ashes are good for making potash to make soap. It contains enough potassium hydroxide that when made into a slurry and sieved, it is a strong enough alkali to create a soap when added to fats.

ragingloli's avatar

light and heat is not actually the majority of the energy content of the wood when burnt.
If it was, you would see a multi-gigaton nuclear explosion.

cazzie's avatar

@ragingloli just as well it doesn’t all get released at once, eh?

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