Pyrolysis oil, sometimes also known as biocrude or bio-oil, is a synthetic fuel with few industrial applications and under investigation as substitute for petroleum. It is obtained by heating dried biomass without oxygen in a reactor at a temperature of about 500 °C (900 °F) with subsequent cooling, separation from the aqueous phase and other processes. Pyrolysis oil is a kind of tar and normally contains levels of oxygen too high to be considered a pure hydrocarbon. This high oxygen content results in non-volatility, corrosiveness, partial miscibility with fossil fuels, thermal instability, and a tendency to polymerize when exposed to air. As such, it is distinctly different from petroleum products. Removing oxygen from bio-oil or nitrogen from algal bio-oil is known as upgrading.
Standards There are few standards for pyrolysis oil because of few efforts to produce it. One is from ASTM.
Feedstock decomposition Pyrolysis is a well established technique for decomposition of organic material at elevated temperatures in the absence of oxygen into oil and other constituents. In second-generation biofuel applications—forest and agricultural residues, waste wood, yard waste, and energy crops can be used as feedstock.
Wood
Historical wood gasification
Syngas, historically called "manufactured gas" or simply "gas" was made by the gasification of combustible materials, usually coal—but also wood—by heating the material in retorts or enclosed ovens with an oxygen-poor atmosphere. In the modern context of creating pyrolysis oil from wood feedstock, the syngas produced during the process is primarily used as fuel to provide the heat required for the pyrolysis reactor.
Modern pyrolysis plants Process Energy: The energy required to run a "fast" pyrolyzer (which maximizes liquid oil yield) is approximately 15% of the total energy it outputs. Self-Sustaining Operation: Modern plants recycle the non-condensable syngas—a mixture of hydrogen, carbon monoxide, and methane—to be burned as a heat source. This allows the system to output 3 to 9 times the amount of energy required to operate. By-product Utility: During fast pyrolysis, wood is typically converted into roughly 60% bio-oil, 20% biochar, and 20% syngas. While the bio-oil is the primary liquid product sought for fuel or chemical use, the syngas is essential for the thermal decomposition stage, where wood is heated to around 500 °C (932 °F) in an oxygen-poor atmosphere.
Wood decomposition When wood is heated above 270 °C (518 °F) it begins a process of decomposition called carbonization. In the absence of oxygen, the final product is charcoal. If sufficient oxygen is present, the wood will burn when it reaches a temperature of about 400–500 °C (752–932 °F) leaving wood ash behind. If wood is heated away from air, the moisture is first driven off and until this is complete, the wood temperature remains at about 100–110 °C (212–230 °F). When the wood is dry its temperature rises, and at about 270 °C (518 °F) it begins to spontaneously decompose and generate heat. This is the well known exothermic reaction which takes place in the burning of charcoal. At this stage evolution of carbonization by-products starts. These substances are given off gradually as the temperature rises and at about 450 °C (842 °F) the evolution is complete. The solid residue, charcoal, is mainly carbon (about 70%), with the remainder being tar-like substances which can be driven off or decomposed completely only by raising the temperature to above about 600 °C to produce biochar, a high-carbon, fine-grained residue that today is produced through modern pyrolysis processes, which is the direct thermal decomposition of biomass in the absence of oxygen, which prevents combustion, to obtain an array of solid (biochar), liquid—Pyrolysis oil (bio-oil/pyrolysis-oil), and gas (syngas) products. The specific yield from the pyrolysis is dependent on process conditions. such as temperature, and can be optimized to produce either energy or biochar. Temperatures of 400–500 °C (752–932 °F) produce more char, while temperatures above 700 °C (1,292 °F) favor the yield of liquid and gaseous fuel components. Pyrolysis occurs more quickly at higher temperatures, typically requiring seconds instead of hours. High temperature pyrolysis is also known as gasification, and produces primarily syngas. Typical yields are 60% bio-oil, 20% biochar, and 20% syngas. By comparison, slow pyrolysis can produce substantially more char (~50%). For typical inputs, the energy required to run a “fast” pyrolyzer is approximately 15% of the energy that it outputs. Modern pyrolysis plants can use the syngas created by the pyrolysis process and output 3–9 times the amount of energy required to run.
Algae Algae may be subjected to high temperatures (~500 °C) and normal atmospheric pressures. The resultant products include oil and nutrients such as nitrogen, phosphorus, and potassium.
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