Pyrolysis, a thermochemical conversion process involving rapid heating of feedstock under oxygen-absent condition to moderate temperature and rapid quenching of intermediate products, is an attractive way for bio-oil production. Various efforts have been made to improve pyrolysis process towards higher yield and quality of liquid biofuels and better energy efficiency.
Microwave-assisted pyrolysis is one of the promising attempts, mainly due to efficient heating of feedstock by "microwave dielectric heating"
Thermal decomposition in the complete absence of an oxidizing agent (air or oxygen), or with such a limited supplies that combustion or gasification do not occur to any appreciable extent.
Pyro lytic cracking of biomass like algae yields mainly liquids, together with a solid residue (char) and gas. In comparison to gasification, pyrolysis occurs at relatively low temperatures (673 to 873 K). High temperatures and long residence times promote the formation of gas, while low temperatures and long residence times promote the formation of char. Optimum to produce oils is medium or slow pyrolysis.
Bio crude from conversion at 300 °C or above had 71–77% elemental
carbon, and 0.6–11.6% elemental oxygen and viscosities in the range 40–68 cP.
The high viscosity one of the problems in separating different fractions combined with inhabitation.
Fossil fuel having the same origin as bio crude however is low on elemental oxygen.
Slow pyrolysis (heated to 450 °C at a rate of 50 °C/min), were used to produce bio-oils from Scenedesmus (raw and defatted) and Spirulina biomass that were compared against Illinois shale oil.
Chemical compounds rich in oxygen and chelated metals are much more reactive than the classic fossil based products. Photosynthetic produced chemical compounds by plants and seaweeds differ and the process only possible due to iodine which acts as catalyst. C/MS of bio crude reported higher hydrocarbons (C16–C17), phenolic, carboxylic acids, esters, aldehydes, amines, and amides.
Sharp differences were observed in the mean bio-oil molecular weight (pyrolysis 280-360 Da; HTL 700-1330 Da) and the percentage of low boiling compounds (bp<400 °C) (pyrolysis 62-66%; HTL 45-54%).
Microwave-assisted pyrolysis of biomass improved conversion and output even further. First, conventional fast pyrolysis and improvement because of microwave dielectric heating.
Two thermo chemical processes, hydrothermal liquefaction (HTL) and slow pyrolysis an alternative to fermentative utilisation.
First is the fact that most bio-oil-associated sugars are present in the anhydrous form.
Metabolic engineering has enabled utilization of the main anhydrosugar, evoglucosan, in workhorse biocatalysts. The second challenge is the fact that it is rich in microbial inhibitors.