Fire is essentially a high-speed exothermic oxidation reaction, and there is a great deal of energy stored in wood’s chemical bonds.
When wood burns, there are two major constituents that decompose to provide the light and heat energy. Those are called cellulose, a type of sugar, and lignin, three types of crosslinked alcohols (paracoumaryl, coniferyl, and sinapyl).
Both cellulose (C14H26O11) and lignin (C81H92O28) are composed of hydrogen and carbon atoms; as a result, they possess properties similar to hydrocarbons. In fact, lignin is considered a unique hydrocarbon, despite the presence of oxygen, but both of these materials lend themselves to the generation of more sophisticated synthetic hydrocarbons.
Those can be used a biofuel, or as the base stock for building or developing polymers such as bioplastic without necessitating the use of fossil-sourced petroleum materials.
Here is a infographic of the wood burning process:
Burning process
- Drying: As the wood is heated, the moisture in the wood evaporates. The heat causes the water molecules to transition from liquid to vapor, which then escape from the wood.
- Pyrolysis: As the temperature increases, the wood begins to break down through a process called pyrolysis. This process involves the thermal decomposition of the wood’s organic compounds into smaller molecules such as gases, liquids, and solid char. The gases and liquids released during pyrolysis include water vapor, carbon dioxide, carbon monoxide, methane, and various volatile organic compounds (VOCs). These gases and vapors are collectively known as wood smoke.
- Ignition: When the temperature of the wood and released gases reaches a certain point, known as the ignition temperature, the gases ignite and produce a visible flame. This is the stage of combustion that most people associate with burning wood.
- Combustion: During the combustion stage, the ignited gases react with oxygen in the air. The primary reactions involve the oxidation of carbon, hydrogen, and other elements in the wood, producing water vapor, carbon dioxide, and other gases. These exothermic reactions release heat and light, which sustain the combustion process.
- Char burning and ash formation: After the volatile gases have been burned off, the remaining solid char, which is mostly composed of carbon, continues to burn slowly in the presence of oxygen. This process is called char burning, and it results in the formation of ash, which is the residue of inorganic compounds that do not burn, such as minerals and metals.
The chemical equation for the combustion of wood:
C_xH_yO_z + O2 → CO2 + H2O + heat
Usually lignin and cellulose are tightly bound together and make wood very strong and resilient. Many trees can resist hurricane force winds without breaking or toppling.
When wood gets hot, however, in the vicinity of 260º Celsius (500º Fahrenheit), the cellulose and lignin molecules disintegrate and separate. This allows the carbon to mix with oxygen in the air to make carbon dioxide, and for the hydrogen to mix with the oxygen to make water.
These energetic reactions are what we call “burning”. Flames generally consist primarily of CO2, H2O, oxygen, and nitrogen.
When heated sufficiently, the gases may become ionized and move into the fourth state of matter, plasma. A candle flame is not hot enough to have any plasma aspect to it. Something must be very hot indeed to disassociate electrons from the nucleus of an atom so that the two swim around independently.
Both CO2 and H2O vapor are gases that are invisible to our eyes. What has happened is that the solid wood has simply altered its state to become heat, light, and two types of gas. That vapor is carried away by the air, explaining where the mass seemingly disappears to…
As Antoine Lavoisier observed in 1785, “matter is neither created nor destroyed”, which applies equally to energy, too, since matter and energy are simply two states of the same thing. His Law of the Conservation of Mass thus became the Law of the Conservation of Mass-Energy that we know today, aka: the First Law of Thermodynamics.
Natural (Dirty) Wood
Now, of course, a wood fire is inherently smoky, since it is rife with products of incomplete combustion. In a perfect world the hydrogen and carbon would combine with their own oxygen atoms and more from the air to make water and carbon dioxide, but that never happens in an open environment.
Instead, partial burning results in carbon monoxide, nitrogen oxide, nitrogen dioxide, as well as water and CO2. You also get particulate carbon, which is what makes up the smoky component from your fire.
Where to get and how much it costs.
Exploiting the Volatiles
Nowadays we deliberately taint food with selected types of wood smoke during the cooking process, such as hickory and mesquite because we like the flavor. It is also popular to cook salmon on a cedar plank, but cedar should never be burned during cooking because it releases Polycyclic Aromatic Hydrocarbons (PAHs) which have been linked to stomach, liver, and skin cancer in laboratory test animals.
The truth is that all woods, petrochemical products, tobacco, and low quality or altered charcoal do, too. It’s made worse when fat drips into open flames and creates smoke (full of PAHs) that adhere to the fat in the meat. Using leaner cuts that don’t drip, smaller cuts that cook faster, or turning frequently to avoid charring, all helps to make barbequed food safer.
More Efficient Wood
Charcoal, however, is actually a very good product. It is simply a form of wood that has undergone pyrolysis, thermal decomposition in a low reactivity atmosphere, which rids it of water and other assorted volatiles. Once that has occurred it provides a very clean, smoke free flame.
Charcoal burns much hotter than natural wood. It has a pale blue, almost invisible flame, releasing carbon monoxide which burns to carbon dioxide, with no appreciable smoke, flavor, or odor. It reaches temperatures in the neighborhood of 1,100º C (~2,000º F), but with the addition of an airflow, the temperature can rise to 1,260º C (~2,300º F) sufficient to soften or melt iron, so it is still often used by blacksmiths.
Common charcoal “cooked” at 300º C ignites at 380° C (~715 °F). Prepared at a higher temperature it becomes hard and brittle, and won’t inflame until it reaches 700º C (~1,300º F). In the production of briquettes it is often powdered and mechanically reformed with binders, accelerants, and additives to help it light faster, burn longer, and to whiten on the surface to indicate it is at “cooking temperature” for the consumer.
Tips how to make a campfire with a ring.
How We Used to Make Charcoal
Originally, for large production, colliers (the proper title for both coal miners and charcoal makers) would build a chimney of three or four posts, driven into the ground, around which was built a large pile of hewn wood. Piles could vary in size depending on wood availability. The one on this old Post Card is about 10 meters across and 3 meters high.
After it was constructed, it would be covered with sod or soil so that air could not penetrate the pile easily. A flame would be introduced through the chimney and within the low oxygen environment part of the wood was consumed to make heat while the remainder was converted to charcoal. The efficiency ranged between 50 and 90%, depending on the size of the starting pile (bigger being better).
There were small vents at the bottom to admit limited amounts of air. Consequently, it would take several days for the pile to finish burning. The covering had to be tended constantly so that any crack that would allow air in was quickly patched, often with mud or wet clay.
Modern Production
Now we used sealed pressure vessels called retorts to manufacture charcoal. This also provides tar, pitch, methanol, turpentine, and other volatiles that are leeched away in a flameless, dry heat process. Some are used to flavor alcohol or candies, to treat dandruff, or as medical balms.
The Zwoyer Fuel Company invented the briquette in 1897 but it was further advanced by Henry Ford who had a great deal of wood scrap and sawdust from manufacturing his Model T cars. Originally called Ford Charcoal, it eventually became the well-known Kingsford Company.
Modern charcoal is also made from coconut shells, grains, and bamboo. In most cases it has an accelerant (sawdust) added to aid lighting, gypsum to make the coals white, and real coal for extra heat. This can cause more PAHs than generated by pure wood charcoal.
Single-cell organisms evolved 3.5 billion years ago. It took until 600 million years ago to arrive at multicellular organisms. The first land plants appeared 470 million years ago, but they were all fern-like, with very few taller than a full grown adult human (who earliest recognizable ancestor was still 467 million years beyond that date, in the far future, and humans only about 300,000 years ago).
The structural components that allowed the strength for “wood” to exist didn’t come along until 420 million years ago. 1.3 billion years after life arose, Earth would finally have trees!
Boil water on campfire. Make sure it is out.
Being a Tree Was Not a Bed of Roses
The limited oxygen of the early atmosphere was not a problem… Trees and plants gobbled up the carbon dioxide (CO2) and drew up water (H2O) to make sugars so they could grow rapidly and profusely; they released oxygen as a waste gas (contributing to making life possible for us eventually). Taller was better the trees learned in their quest for sunlight to power their chemical reactions. Plants started using a bit of the oxygen they liberated from the CO2 and water while making sugar to begin making stronger construction materials.
Unfortunately, there was also a lot of meteorological violence going on at the time. The atmosphere was much more active than today, and lightning storms were a very frequent occurrence. Lightning strikes could blast trees into splinters, or set fires and burn hundreds of thousands of hectares for weeks or months at a time, as happened in the western U.S. in 2020. This releases the sequestered carbon for use by other trees; this is why wood is regarded as carbon-neutral.
Friendlier Weather
Eventually things settled down into a pattern. Trees would grow for a few decades but because the decomposers (like fungi) were still evolving and only beginning to flourish, the forest floors would get clogged with deadwood and underbrush that didn’t properly decay (yet). Lightning would come along, trigger a fire, and clean everything up, returning nutrients to the soil, and a new forest would grow. Balance was achieved.
Humans
Actor Ron Perlman (et al) gave us the 1981 movie Quest For Fire (a very watchable movie after you get past the first few odd minutes and begin to root for the characters to succeed—it won 6 César Awards, 5 Genie awards, and the Academy Award for best Makeup). Even if it is not strictly accurate, scientifically speaking, it does show us just how important fire was to early human culture ~80,000 years ago.
The Takeaway
Burning wood to cook could pass on a lot of undesirable characteristics. Until more sophisticated technology came along, charcoal was the answer for most people. Not incidentally, its manufacture also provided other useful substances like tar and pitch for ship building and better weatherization of roofs, carbon black for art, and the so-called Russian Oil (from birch) to protect leather.
Wood can be a flammable solid (wood or charcoal); rendered as a flammable liquid (e.g. turpentine); produced as flammable gas (primarily carbon monoxide, hydrogen, and some methane) in a wood gas generator to power cars and buses during fuel shortages; it can even yield plasma when heated to sufficiently high temperatures. The only state it completely avoids is that of the artificially created Bose-Einstein Condensates, since that requires extreme cold and human interference rather than heat.
Breaking and making electron bonds is essential for all chemistry and fire is nothing if not chemistry in action. We observe that the mass of the wood is mostly carried away as gases, leaving just a remnant of non-flammable mineral ash.
Burning wood in considered carbon-neutral—the amount released being exactly equivalent to the amount required to replace the same quantity of wood. Only a small portion is converted to radiant heat energy and light—most heat is convected away by air while the carbon moves on to feed other trees that are still growing.
Thus ends the story of burning trees for heat, light, safety, and useful chemicals.
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