Fast pyrolysis is the thermal decomposition of organic materials
- in absence of oxygen,
- at temperatures between 400 and 600 oC, and
- at atmospheric pressure,
followed by immediate condensation of the vapour phase produced. While the conditions are optimized for a maximal production of liquid fuel (“bio-oil”), some charcoal and non-condensable gases are left as useful by-products. Fast heating of the biomass particles, and rapid quenching of the produced gases and vapours, is essential to achieve high liquid yields. This is ensured by a high heat transfer rate to small feed particles inside the reactor, and a minimal transfer time of the produced vapours from the reactor to the condenser. Moreover, the char needs to be separated quickly as the minerals left in the char are known to catalyze the cracking of vapours to non-condensable gases. In practice, the feed particles should have a moisture content well below 10 wt % and be smaller than approximately 5 mm. Typical product yields of dry wood are: 70 wt.% bio-oil, 15 wt.% char and 15 wt.% gas. For other types of feedstock materials the maximum bio-oil yield may be lower, especially in case of high ash contents. Phase separation of the liquid product, e.g. as a consequence of a high water content, is usually considered to be unfavourable.
Liquefaction of biomass by fast pyrolysis is often desirable to reduce its volume and increase the energy density. Depending on the type of feedstock, the volumetric energy density is increased by a factor 4 to 10. Apart from any advantages in storage and transport, the liquid properties further facilitate the combustion in boilers, engines and turbines, co-combustion in power stations, and possibly the large scale production of synthesis gas in existing mineral-oil gasifiers. Fast pyrolysis offers the advantages of de-coupling the production and utilization regarding time, place and scale. Bio-oil could for instance be produced at places where the biomass feedstock is cheap and abundantly available -at a scale matching the existing collection infrastructure- to be transported to central sites for large scale processing. Practically all the minerals are left in the char; they can be recovered locally and returned to the soil as a fertilizer.