Biorefinery platforms – Pyrolysis oil / Biobased oil / Biobased crude
Concept
- Pyrolysis oil, also known as bio-oil or biobased oil, results from the thermochemical decomposition of solid biomass in the absence of oxygen in order to avoid combustion.
- The composition and characteristics of the pyrolysis oil may vary depending on several factors: feedstock, type of reactor, operating conditions… But, it is possible to highlight some common features: dark red-brown to dark green appearance and characteristic acrid smoke odor; low viscosity; highly corrosive due to its low pH; unstable due to the polymerization of various compounds (ageing phenomenon that causes an increase in viscosity and a decrease in volatility); miscible with polar solvents but totally immiscible with fossil oil due to its polar nature; low content in cetanes (between 10 and 25).
- As it is the direct result of the thermal decomposition of biomass, its elemental analysis is much more similar to it than to fossil oil. It is composed of complex mixtures that can involve several hundreds of compounds (carboxylic acids: 4-6 w%; aldehydes, ketones, furans, pyrans, monomeric phenols: 15-20 w%; sugars: 25-35 w%; water: 20-30 w%; pyrolytic lignin, extractives, solids, polymerization products: 20-25 w%).
- When the liquid is produced by biomass hydrothermal liquefaction (a specific pyrolytic process that takes place in an aqueous medium), it is usually named as bio-crude or biobased crude. It is closer in properties to petroleum than biobased oils but it is not a direct analogous to crude petroleum.
Applications
- Direct heat and power generation. The direct combustion of biobased oil for heat or electricity is its most obvious use and has been extensively tested in boilers, engines, and turbines. Compared to fossil fuels, it has much lower emissions of sulphur compounds. However, due to the heterogeneity, viscosity and corrosive nature of the biobased oils, most of the equipment requires some modifications to improve its efficiency. In the ASTM D7544 standard, you can find indications on the specifications that the grades of biofuel from biomass pyrolysis for industrial use must meet.
- Production of biobased fuels. As biobased oil is immiscible with fossil fuels and its direct use as a liquid fuel is not feasible due to its high-water content and low pH, different physical and chemical processes can be performed to upgrade it as a fuel for transportation: ash content reduction; addition of polar solvents; addition of surfactants; distillation; hydrotreatment.
- Production of biobased chemicals and food ingredients. After pyrolysis, the oil can easily be fractionated into three product streams: pyrolytic lignin (potential substituent for fossil phenol in phenol/formaldehyde resins and a precursor of biobased bitumen), pyrolytic sugars (renewable source for fermentation) and a watery phase containing smaller organic components (for instance, acetic acid can be produced by means of an extraction step followed by simple distillation). Also, specific chemicals and food ingredients can be extracted.
