Driving off water requires a large amount of energy.
Ideally biomass should have a moisture content of ~15% when it enters the pyrolysis kiln, to ensure high yield, quality biochar and low emissions.
Heat flows into (and out of) biomass and char slowly.
Tip: It takes about 1 hour for charring to penetrate 30mm (~1in) into wood.
Charring proceeds at about 0.5mm/min (the “charring rate” in fire science).
Biochar can be made at different temperatures in seconds, or over many days depending on size and type of feedstock
Slow pyrolysisis is carried out in an oxygen-starved environment in kilns or retorts, in a batch process or with a slow auger feed.
Peak temperatures are relatively low, heating rates are relatively slow, and residence times of the char in the reaction are long.
The term “biochar” was originally associated with this type of production.
Fast pyrolysis converts finely ground feedstock into bio-oils, gas and char, in seconds. It is likely there will be higher condensed volatiles present in the char, which could affect its performance and desirability as biochar for soil amendment.
Fast pyrolysis tends to be used by commercial biochar/bio-oil producers.
Gasification includes a combustion and reduction stage after drying and pyrolysis. It is designed to produce a gaseous fuel mostly of H2, CH4, CO. Gasification processes happen over a wide range of temperatures, with combustion and tar-cracking often occurring above 1000°C.
The gas can be used in heating, or (after cleaning) to run engines, or as a feedstock for conversion to liquid fuels, chemicals and fertilizers.
1. External Heating of the Biomass
2. Internal heating of biomass by flaming, often known as “Flaming Pyrolysis”
3. External Heating of the Biomass
Mass of wood – mass char = Mass pyrogas + mass bulk water
Energy in moist biomass
Energy in Biochar
Energy for Drying Bulk Moisture
Energy in the Exhaust Gas
Heat energy lost from the kiln
HHV is the higher heating value, a measure of energy content, including the latent heat of vaporisation of water in the biomass.
Every 100kg wood (20% MC) produced 20kg of steam and 60kg of Pyrogas (plus 20kg of BC).
To burn the gas cleanly, with low emissions, requires:
Kilns need to be designed to meet limits for emissions of nitrogen oxides and unburned carbon compounds such as CO.
Flameless combustion: The pyrogas is introduced on the outside of the air jets, and air and fuel are premixed with hot exhaust gases. Conditioned fuel and air burn cleanly, at lower temperature, without a flame.
FLOX (Flameless Oxidation) Burner
Conventional flame (with no air preheat)
Flameless Oxidation (with air preheat)
Drying and torrefaction processes require external energy.