There are at least two (2) distinct processes VERDANT employs to produce
Bio-Coal, depending on what heat content is sought, namely 'Slow Pyrolysis' and 'Torrefaction'.
Slow Pyrolysis:
Pyrolysis is a process for thermal conversion of solid biomass fuels in the complete absence of oxidizing agent (air/oxygen), or with such limited supply that gasification does not occur to any appreciable extent.
During pyrolysis, which takes place at temperatures in the range 400-800°C, most of the cellulose and hemicellulose and part of the lignin will disintegrate to form smaller and lighter molecules which are gasified at the pyrolysis temperature. The VERDANT process captures these gases (called Volatile Organic Compounds, or simply “VOC’s”), and recycles these Hyrdocarbons by combustion, thereby supplying the primary source of heat for the Slow Pyrolysis process. The remaining part of the biomass, mainly parts of the lignin, is left as a solid fuel (i.e. Bio-Coal). The typical heat content of the Slow Pyrolysis product is in the 11,000 BTU/Lb. – 14,000 Btu/Lb. range (~ 25MJ/Kg – 32MJ/Kg).
Torrefaction:
Torrefaction is a thermo-chemical treatment method carried out at atmospheric pressure in the absensce of oxygen. It was originally developed in the 1930’s to roast Coffee beans.
During torrefaction, which occurs at temperatures between 200-300°C, the biomass is partially decomposed giving off various types of volatiles. The process is characterized by relatively long reactor residence times, with the final product being a solid uniform product called char. The typical heat content of the Torrefied product is in the 9,000 BTU/Lb. – 11,000 BTU/Lb. range
(~ 20MJ/Kg – 25MJ/Kg).
