Pyrolysis and Hydrothermal liquefaction
Biocrude is produced by pyrolysis or hydrothermal liquefaction.
PYROLYSIS – The pyrolysis process uses thermal decomposition occurring in the absence of oxygen. It is the first step in combustion and gasification processes where it is followed by total or partial oxidation of the primary products. Lower process temperature and longer vapour residence times favour the production of charcoal. High temperature and longer residence time increase the biomass conversion to gas and moderate temperature and short vapour residence time are optimum for producing liquids.
Typical product yields (dry wood basis) obtained by different modes of pyrolysis of wood:
||~500oC, short hot vapour residence time ~1 s
||~500oC, hot vapour residence time ~10-30 s
|Slow – Torrefaction
||~290oC, solids residence time ~30 mins
|Slow – Carbonisation
||~400oC, long vapour residence time hrs -> days
In fast pyrolysis biomass decomposes to generate mostly vapours and aerosols and some charcoal. After cooling and condensation, a dark brown mobile liquid is formed which has a heating value about half that of conventional fuel oil. While it is related to the traditional pyrolysis processes for making charcoal, fast pyrolysis is an advanced process, with carefully controlled parameters to give high yields of liquid.
The essential features of a fast pyrolysis process for producing liquids are:
• Very high heating and heat transfer rates at the reaction interface, which usually requires a finely ground biomass feed;
• Carefully controlled pyrolysis reaction temperature of around 500oC and vapour phase temperature of 400-450oC;
• Short vapour residence times of typically less than 2 seconds;
• Rapid cooling of the pyrolysis vapours to give the bio-oil product.
Virtually any form of biomass can be considered for fast pyrolysis. While most work has been carried out on wood due to its consistency, and comparability between tests, nearly 100 different biomass types have been tested by many laboratories ranging from agricultural wastes such as straw, olive pits and nut shells to energy crops such as miscanthus and sorghum, forestry wastes such as bark and solid wastes such as sewage sludge and leather wastes.
Source:IEA Task 34
HYDROTHERMAL LIQUEFACTION — Biocrude can also be produced using another thermochemical conversion process, hydrothermal liquefaction (HTL), which is the thermal depolymerization of wet biomass under moderate temperature and high pressure. These conditions, over period of minutes, mimic geologic processes that have converted biogenic materials into carbonaceous energy in the earth’s crust over millennia. HTL produces a liquid oil directly, whereas pyrolysis produces a syngas which is converted into liquid oil in the presence of a catalyst.
Biomass feedstocks for HTL include biowaste (manure and food processing waste), industrial processes wastes (e.g. wastewater treatment sludge), lignocellulose (crop residue), and algae.