From the outside towards the centre of the wood:
- Gas zone - just outside of the wood, a mix of air and products of the drying and burning of the wood
- Char zone - top layer of wood which has burned, containing tars and char
- Pyrolysing zone - the burning wood
- Dehydration zone - water evaporating
- Wet zone - wet wood, made wetter by entry of evaporated water from the dehydration zone
Free water begins to evaporate as temperatures approach 100C, in a dehydrating zone. Most will leave by the surface, and there is a dry zone between the dehydrating zone and the surface; some will penetrate deeper into the wood and condense back to water, forming a wet zone.
At low heat fluxes, dehydration and pyrolysis occur independently; at higher fluxes they occur together, in which case moisture slows the temperature rise until about 115C, and water passing through the pyrolysing zone cools it.
Bound water is released at about 240C.
When ignition occurs, the products of pyrolysis (in the presence of oxygen) undergo a rapid, exothermic reeaction when they burn.
Ignition can be classified as follows:
- flaming (which I concentrate on) or smouldering
- piloted (started with a spark or flame) or unpiloted (ignition starts just from heat)
The point at which ignition occurs is given by the critical heat flux and/or critical temperature. Values given can vary significantly, particularly the ignition temperature.
Smouldering ignition can occur with a critical heat flux of 5-10kW/m2 with a surface temperature of about 200C.
For piloted ignition, ignition can occur with a heat flux of 10-13kW/m2
For unpiloted ignition, ignition can occur with a heat flux of 25-33kW/m2
For piloted ignition, ignition can occur with a surface temperature of 200-400C
For unpiloted ignition, ignition can occur with a surface temperature of 200-600C
With piloted ignition, a flame impinging on the wood means that it will ignite at much lower heat fluxes.
Wood covered in char requires a higher surface temperature than virgin wood, due to the insulation of the char.
Damp wood takes longer to ignite and requires a higher heat flux, with dry wood igniting in half the time of wood with a 12% moisture content.
This is the thermal decomposition of the wood.
To burn, polymers must first decompose into smaller molecules that can exist in the gas phase at ambient conditions. To create a self-sustaining reaction, the combustion of these gases must generate sufficient heat to perpetuate the production of volatiles. Upon heating, the constituent natural polymers present in timber will degrade, producing inert and combustible gases (the nature and composition of which will depend on the char yield), liquid tars, a solid carbonaceous char (typically around 20% the density of virgin wood) and inorganic ash. This can occur before dehydration is completed if the heating rate is fast enough, but will be faster after the sample has dried. Under sustained heating conditions, these pyrolysis products can then undergo further pyrolysis themselves. This process is further complicated due to charring and material variability, and the chemical processes occurring are numerous and interdependent. It is also necessary to distinguish between pyrolysis and combustion. Pyrolysis refers to the thermal decomposition of a substance, is endothermic, and can occur without an oxidiser.
Wood typically undergoes three main stages of pyrolysis due to its relatively low thermal conductivity and density and relatively high specific heat: dehydration and very slow pyrolysis below 200°C, onset of pyrolysis up to 300°C, and rapid pyrolysis above 300°C.
Slow mass loss occurs, giving off non-combustible gases, e.g. CO2, formic and acetic acids.
Prolonged heating at low temperatures (95-120C) can convert hemicellulose and lignin (bit not cellulose) into carbonaceous char.
Cellulose type materials as in wood don't have a liquid state, but may lose their structure and go into a rubbery state, with reduced strength. For ligning, this occurs around 55-170C.
Above 200C the wood will discolour, and eventually form char, though emissions are still non-combustible.
Because of high heat loss from the surface, inner depths of wood can remain cool, e.g. 180C 6mm below the surface, and heat not penetrating more than 35mm into the wood.
The main pyrolysis begins at 225-275C
Hemicellulose starts termal decomposition at 120-315C, depending on heating rate, species, density and moisture content.
Cellulose is next, at 240-400C. It may decompose in two ways:
- at low temperatures the main route is producing char plus CO, CO2 and H2O, and emitting heat
- from 250-300C it breaks down to form the tar levoglucosan, which breaks down to create flammable gases or, if cooled, repolymerises to form char
The char yield from cellulose is heavily dependent on organic impurities, with pure α-cellulose yielding only 5% char upon prolonged heating at 300°C, but viscose rayon (having a relatively high concentration of impurities) giving up to 40%.
Lignin goes at 110-500C, but if melted at low temperature it re-hardens until 210C. It produces aromatic products, and creates more char than cellulose - at 400-450C half of lignin remains as char. As softwoods have higher lignin than hardwoods, they create greater quantities of char.
Between 300 and 500C, pyrolysis rates increase rapidly, further aided by exothermic reactions from the flammable gases emitted. The gases also contain highly flammable tars, appearing as smoke, favouring the production of levoglucosan. This all results in residual char, which is less readily volatilised than the virgin wood.
As the layer of char builds up, it affects the decomposition of the virgin wood. The char is much more porous than wood, and also cracks form. Heat is transferred much more quickly by radiation along the cracks and pores in the char, as opposed to convection of virgin wood. Cracks also allow volatile emissions to escape to the surface much more readily.
Once ignition has occured, the products produced by pyrolysis mix with air and generate heat, which causes more pyrolysis, and so combustion is established. If the pyrolysis products are gases then we get flaming pyrolysis; if solids, we get smouldering.
The heat of combustion of wood is about 15-20MJ/kg. Half to 2/3 of this is through flaming (primarily due to cellulose, as it produces more volatiles) and the balance due to smouldering.
In a flaming environment, most of the oxygen in the air will be consumed by the flames, so ongoing pyrolysis will occur in reduction / vitiated environment. The rate of combustion is determined by the rate of pyrolysis.
Because char is a better insulator than wood, the flaming may initially be strong, then reduce as the char builds up, and only grow again once the deeper wood is pyrolised.
Under a standard test fire (see below), the average charring rate is 0.6mm/min, rising to 0.8mm/min for light, dry softwoods, and falling to 0.4mm/hr for dense, wet hardwoods. There is quite a spread, but the volumetric burn of char is faster for low density softwoods than for high density hardwoods (how does this compare if using weight instead of volume, though?).
Moisture of the wood has an impact, e.g. fully dried wood charring 25% faster than 10% moisture content, and 50% faster than 20% moisture content. High moisture increases the time to ignition, and reduces the heat output after ignition if the wood is not fully dry at that point, due to water being evaporated off. Note that test results show quite high variability, due to the effect of other factors, including the species and the density of the wood. Naturally dried wood has a MC of 18-19% in winter, 13-15% in summer in the UK.
Increasing permeability of the wood results in a higher charring rate, as higher permeability allows a faster flow of volatiles. Permeability along the grain is typically 10 000 times higher than cross grain.
There is a dependency on the species being burnt, beyond factors such as density and permeability. This takes into account the structure of the wood, and its chemical composition (primarily lignin content - high lignin gives a higher char rate).
One test gave the following variation of pyrolysis temperature (lowest first):
chestnut -> Douglas fir -> redwood -> pine -> beech
and for char yields (lowest first):
beech -> pine -> Douglas fir -> redwood -> chestnut
Chestnut has high hemicellulose, giving a low pyrolysis temperature. Also, lower lignin gives less CO2.
The rate of heating has a significant effect, e.g. in one test a slow rate of heating resulted in char forming, and then ignition at about 410C, whereas fast heating gives ignition at about 365C and minimal char forming before ignition. Charring rate is also directly proportional to the incident heat flux, resulting in greater heat output.
The lower the oxygen content, the slower the rate of oxidation of the char. This then builds up to form greater insulation, reducing the charring rate of virgin wood as it is slower to heat up. Mass loss rates reduce by about 20% for a 50% reduction in oxygen, and 35-50% for a 100% reduction.
Most tests are in compliance with ISO 834-1. Here the fire is taken to 538C after 5 minutes, 927C after 1 hour, and then a steady climb rate to 1093C after 4 hours.