Definition of pyrolysis & why you should care:
This article defines pyrolysis - the chemical change that occurs in wood and other materials, lowering the material combustion point in response to repeated heating at even relatively low temperatures such as around 200F.
An understanding of the meaning and process of pyrolysis forms the basis of standards & requirements for clearances from combustibles, such as the required distances between a flue vent connector or chimney or woodstove and nearby wood framing or other potentially flammable materials. In our page top photo our inspection client is pointing to inadequate clearance between a flue vent connector and wood framing.
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The reason that building codes specify a healthy distance between wood materials (or other combustibles) and flue vent connectors is not just that the heat from the flue will immediately set the wood on fire. Rather it is also that wood that has been heated over time, even to the relatively low temperature of 200 to 300F, will be chemically affected to become more readily combustible.
Our photo at left shows how wood-to-combustible clearance hazards can be tricky to spot - only when this wood door was left open was it in contact with a heating flue.
[Click to enlarge any image]
Pyrolysis is defined as the chemical or thermal decomposition of a material when it is exposed to heat.
Watch out: Actual fire also "decomposes" combustible material, but in a fire the material is burned or decomposed both visibly and rapidly - dramatically. In contrast, pyrolysis can decompose material also lowering its combustion or ignition point to begin an actual fire, with no visible change in the external appearance of the material, particularly in the case of wood framing.
Wood exposed to heat, such as wood framing too close to a metal chimney in a building, is chemically transformed in an important way: its ignition point or combustion point is lowered - the wood can actually catch fire at a lower temperature.
This means that by pyrolysis, wood and some other combustibles found in buildings are chemically changed by exposure to even relatively low but warm temperatures over time.
The chemical change of pyrolysis lowers the temperature at which a substance will catch fire. What may be surprising is that wood exposed to temperatures as low as 200 degF. can over time be changed such that its ignition point or combustion point is significantly lowered.
In our photo (above/left) I'm hold ing a ruler showing that we have only 4 3/4" between an oil fired heating appliance flue vent connector and nearby wood framing.
Considering that in at un-tuned or dirty oil fired boiler I've measured stack temperatures at a terrifying 1000 degF., it should not be a surprise that pyrolysis is a real hazard in cases such as this one.
Because that chemical change occurs over time as a function of the frequency and duration of exposure of the wood to high temperatures, we can understand that a building owner may use an improperly-installed fireplace, woodstove or other heating device (lacking adequate fire clearance from combustibles) for quite some time before a fire occurs.
Or a fire may not occur after prolonged pyrolysis until a new user happens to build a hotter fire in the woodstove or some other similar change in conditions occurs.
In sum, a fireplace, wood stove, other heating appliance, chimney, or a flue vent connector may have been too close to wood framing for decades before unexpectedly causing the nearby combustible building components to catch fire.
The process of pyrolysis discussed above occurs at relatively low temperatures (200 F) and without attempting to restrict the level of oxygen, in processes that are well-studied and well defined in sources we cite below in this article at References on the chemistry of pyrolysis particularly Shafizadeh (1984) who points out that pyrolysis of wood includes an initial low-temperature process:
The first pathway, which dominates at temperatures below 300 °C, involves reduction in the degree of polymerization by bond scission; elimination of water; formation of free radicals, carbonyl, carboxyl, and hydroperoxide groups; evolution of CO and CO2; and, finally, production of a highly reactive carbonaceous char. 
That oil fired heating systems easily reach temperatures of 700F and higher is demonstrated by our thermometer measuring flue gas temperature at a modern oil fired heater that needed service. Soot in the heat exchanger slows heat transfer into the hot water (or warm air if it's a furnace) thus sending more heat up the chimney and subjecting nearby building surfaces to high temperatures as well.
But pyrolysis is often defined differently or perhaps more narrowly in special applications. The production of charcoal by heating wood to high temperatures under conditions of very low oxygen is a process that has been used by humans for many centuries. A portion of a Wikipedia definition of pyrolysis is of interest and needs further comment:
Pyrolysis differs from other high-temperature processes like combustion and hydrolysis in that it usually does not involve reactions with oxygen, water, or any other reagents. In practice, it is not possible to achieve a completely oxygen-free atmosphere. Because some oxygen is present in any pyrolysis system, a small amount of oxidation occurs.
Key in understanding the relation between this definition and the changes in wood that we described earlier is the portion of this statement that includes "... does not involve reactions with oxygen ...". The absence of a reaction with oxygen does not have to be due only to the unavailability of oxygen (as in the case of production of charcoal or biofuels).
The absence of a significant reaction with oxygen also may be due to the much lower temperature at which wood pyrolysis is occurring. It is also helpful to understand that low-temperature pyrolysis of wood is almost certainly an endothermic process - that is, a reaction in which the wood absorbs energy from its surroundings in the form of heat. 
More recently the U.S. Department of Agriculture's Agricultural Research Service, in discussing the production of biofuels from biomass, defines a higher-temperature use of pyrolysis in the absence of (much) oxygen as follows:
Pyrolysis is the heating of an organic material, such as biomass, in the absence of oxygen. Because no oxygen is present the material does not combust but the chemical compounds (i.e. cellulose, hemicellulose and lignin) that make up that material thermally decompose into combustible gases and charcoal.
Most of these combustible gases can be condensed into a combustible liquid, called pyrolysis oil (bio-oil), though there are some permanent gases (CO2, CO, H2, light hydrocarbons). Thus pyrolysis of biomass produces three products: one liquid, bio-oil, one solid, bio-char and one gaseous (syngas).
Thus pyrolysis of biomass produces ... liquid, bio-oil, solid, bio-char and ... gaseous (syngas). ... all things being equal, the yield of bio-oil is optimized when the pyrolysis temperature is around 500 °C and the heating rate is high (i.e. 1000 °C/s) i.e. fast pyrolysis conditions. 
Our FD recently went to a "chimney fire", which turned out to be in the basement, where someone had connected a wood-fired boiler using horizontal single-wall pipe to the chimney.
Being less than 4 inches from bare wood, the gap stuffed with pink fiberglass, the intermittent and long-term pyrolysis on the adjacent structure caused the spontaneous structure fire one cold night when the wood reached its (reduced) ignition temperature.
Had they installed the proper pipe, or proper shielding and a 9-inch clearance, it would have been safe and approved, assuming they requested an inspection.
Indeed FireBear I've seen similar hazards. Sometimes a fire occurs by the ignition of combustible material that has undergone pyrolysis over a period of years and when there is a change in building ownership or occupancy. The old owner knew that there was a fire hazard - say from inadequate clearance from combustibles - and as one owner told me "Of course I knew it was dangerous, that's why I never built a big fire in the woodstove".
Then the home is sold, a new owner moves in and is excited to fire up that old wood-stove on the first very cold night. It's cold. They build a really hot fire, and boom! Or I should say "snap crackle and pop" as the nearby wall begins to burn.
Harriet and I experienced this first hand in 1969 when a wood block exposed to heat in an improperly-built masonry fireplace caught fire, burned-through, and in turn acted as a chimney to set the wall above the hearth on fire.
It was New Years Eve. New home, cold outside, hot fire in the fireplace. Dan Martin called our attention to an orange spot glowing through the drywall above the stone fireplace mantel.
"Is your wall ... supposed to burn like that?" Dan asked as he passed me a fire extinguisher.
We got everyone out of the house, called the Red Oaks Mill fire department, and then hooked up a garden hose to our water heater - the only convenient spot - and began squirting out the fire in the fireplace - which did not do much to discourage the fire in the wall cavity.
The fire department was having their New Years' Eve party. So lots of fellows stopped by to help chop out walls and douse the house fire.
When the fire was out I asked the fire chief if he could tell me what caused the fire. He leaned down and looked up into the fireplace throat with his flashlight.
"Aaah, it's another Phil Hansee Fireplace" he said.
They knew this guy's work, which maybe tells us something worth remembering.
Poughkeepsie's Phil Hansee built all of his fireplaces the same way. It's lucky there were not more house fires where Phil's fireplaces were installed. Phil liked to set the outer lip of the cast iron fireplace damper on a cross-over 2x4 during fireplace construction. His design left about an inch wide strip of wood exposed in the fireplace throat. In homes with a wood framed wall above the stone fireplace hearth, the framers rested the wall studs atop this very convenient horizontal 2x. Even if the wood had not been exposed to view (making it impossible to discover) the heat from the fireplace and pyrolysis would risk an eventual building fire from this design feature.
All of Phil's fireplaces were built using that same feature.
I found one last fireplace of the Phil design more than 20 years later during a home inspection. The realtor and home buyer were admiring the stone work. I lay on the floor and looked up into the fireplace throat.
Realtor: "Think of how beautiful and cozy it will be for you next winter when you build your first fire in this new home."
Buyer: gushes. "Yes yes yes. Ummmm. I can just imagine the warmth."
Me: "There's just one thing though..." like an amateur Columbo I hesitated, trying to think of how to break the news that the wall needed to be torn out.
Buyer and realtor in chorus: "What's that?"
Me (still lying on my back on the floor shining my flashlight into the fireplace): "Well, you only get one fire, or maybe a few, then the house burns down. But what I can't understand is how this could be another Phil Hansee fireplace. Surely he's retired or even dead by now."
Buyer and realtor lay down on the floor on either side of me and we all looked up at the new 2x4 crossing the interior of the fireplace throat. The owner-builder of the home came into the room and stopped dead, staring at the three of us lying on the living room floor.
"What the hey?"
It emerged that Phil had come out of retirement and as a special favor had built the fireplace for the new owner. The stonework was beautiful though.
At THERMAL TRACKING BRIDGING GHOSTING we pointed out that some theorists suggest that some fabric fibers are burned by pyrolysis in clothes dryers. Indeed, pyrolysis is a chemical process that through exposure to heat causes organic material decomposition that in turn lowers the combustion point of a material. And as pyrolysis changes the properties of a material it can indeed release gases, liquids (tar) and char (solid residue).
This view can be a bit confusing since pyrolysis is not quite classic combustion as it is commonly understood to involve fuel and flame, for example in consuming of oil, gas, or wood fuels.
Pyrolysis of wood, for example, can occur when wood is exposed to temperatures in the 200-300 degF range [some forensic sources give a lower range of 120-150 degF ] . Pyrolysis of wood does not immediately consume that material nor even change its physical appearance (at those comparatively low temperatures) so much as change it chemically so that its combustion point is lowered. Wood (other than decayed wood) may ignite at 180-220 degF.
At these lower temperatures the ignition point of wood varies considerably by species, moisture level, exposure time to heat at different temperatures and other conditions.
Pyrolysis at a level sufficient to actually blacken or char material is more accurately called carbonization.
But be sure to also read our notes about other types of carbon like or soot like deposits, discussed at THERMOPHORESIS.
We were stunned to receive email and a few website comments from readers who had trouble believing that wood can be chemically changed, lowering its combustion point, by repeated heating. To aid readers in understanding the process of pyrolysis we include the following citations:
Continue reading at THERMOPHORESIS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
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