Table 3.

Major biomass combustion phases

Precombustion phases		Ignition	Combustion phase
Distillation	Rectification	Torrefaction	Flaming	Smouldering
T < 200 °C	230 < T < 250 °C	250 < T < 300 °C	500 < T < 1600 °C	T < 600 °C
Endothermic	Endothermic	Exothermic	Exothermic	Exothermic
The wood becomes dehydrated and evolves water vapour with perhaps traces of carbon dioxide, formic and acetic acids, glyoxal BVOCs stored in the storage compartments of the plant are evolved. The gases produced by very slow pyrolysis are not ignitable.	Wood pyrolysis remains slow. Water vapour, carbon dioxide, formic and acetic acids, glyoxal and possibly a little carbon monoxide are evolved together with substantial amounts of vapours of BVOCs. Thus far, the reactions are endothermic and the gaseous products are still not ignitable.	This phase occurs suddenly and exothermically when the mixture of gases copiously evolved in the hot zones of wood becomes combustible. Active pyrolysis of wood begins swiftly and the temperature mounts rapidly unless the heat evolved is dissipated. Combustible gases and vapours are mainly carbon monoxide, methane, formaldehyde, formic and acetic acids, methanol, and hydrogen. They are emitted with carbon dioxide and water vapour carrying with them BVOCs and droplets of highly inflammable tars that appear as smoke. The primary pyrolysis products formed inside the wood undergo further pyrolysis and react one with another before they escape. They are produced by glowing combustion between the hot charred, formed by torrefaction and partial carbonization of wood, with encapsulated oxygen.	Flaming combustion occurs entirely in the gas phase outside the wood because the rapidly emerging gases lack the necessary oxygen until they have sufficiently mixed with air in proportions between the lower and upper limits of flammability. Indeed, under suitable conditions flaming combustion may occur at a considerable distance from the wood. Self-sustaining diffusion flames from organic fuels burn at 1100 °C or above. One-half to two-thirds of the heat of wood combustion is liberated in flaming, the rest by glowing combustion of charred wood. In this phase most of the pyrogenic VOCs, such as alkenes, arenes and PAHs are also emitted together with BVOCs and pyrolysis gases.	It is analogous to the glowing phase as it occurs by the direct reaction of oxygen with the surface of carbon. It takes place on the surface of the fragmented piece of charcoal formed by combustion, but is sustained by the emission of pyrolysis gases formed inside the material. This stage continues until the temperature drops below the combustion threshold value, or until only non-combustible ash remains. It is the phase where CO reaches it maximum value. It produces substantial amounts of pyrogenic VOCs, such as alkenes, arenes and PAHs.

BVOC, biogenic volatile organic compound; PAH, polyaromatic hydrocarbon; VOC, volatile organic compound.