Fast pyrolysis of biomass is one of the most recent renewable energy processes to have been introduced. It offers the advantages of a liquid product, bio-oil that can be readily stored and transported. Bio-oil is a renewable liquid fuel and can also be used for production of chemicals. Fast pyrolysis has now achieved a commercial success for production of chemicals and is being actively developed for producing liquid fuels. Bio-oils have been successfully tested in engines, turbines, and boilers, and have been upgraded to high-quality hydrocarbon fuels, although at a presently unacceptable energetic and financial cost. The paper critically reviews scientific and technical developments in applications of bio-oil to date and concludes with some suggestions for research and strategic developments.

Biomass pyrolysis oils have potential to be used as a fuel oil substitute. Combustion tests have shown that the oils burn efficiently in standard or slightly modified boilers and engines with rates similar to those for commercial fuels. However, these tests also identified several challenges in bio-oils applications resulting from their properties. The oils have heating values of only 40−50% of that for hydrocarbon fuels. They have a high water content that is detrimental for ignition. Organic acids in the oils are corrosive to common construction materials. Solids (char) can block injectors or erode turbine blades. Over time, reactivity of some components in the oils leads to formation of larger molecules that results in high viscosity and in slower combustion. This paper discusses physical and chemical characteristics of bio-oils relevant to fuel applications as well as some low-cost methods for improvement of these properties. It also provides bio-oil specifications proposed by some industrial users and recommendations for storage and handling.

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The initial development of additives to stabilize the viscosity of biocrude during long-term storage has produced dramatic results. The additives investigated were ethyl acetate, methyl isobutyl ketone and methanol, acetone, methanol, acetone and methanol, and ethanol. These additives represent three chemical families, which all demonstrated the ability to drastically reduce the aging rate of biocrude, as defined by the increase in viscosity with time. Accelerated aging tests were run at 90 °C to screen the additives. The additives not only lowered the initial viscosity at 40 °C by half but also reduced the aging rate of a hot gas filtered pyrolysis oil made from hybrid poplar (NREL run 175) by factors of 1−18 compared to the original pure oil. With the best additive, methanol, at a 10 wt % level in the pyrolysis oil, the modified biocrude was still a single-phase liquid and still met the ASTM No. 4 diesel fuel specification for viscosity even after 96 h exposure to 90 °C. Based on the aging rate at 90 °C recently determined for pure biocrude without additives, the pure biocrude tested would have exceeded the allowable ASTM No. 4 viscosity after only 2.6 h. In addition, the unmodified biocrude formed a waxy precipitate that floated on top of the liquid phase after 8 h exposure to 90 °C. Use of methanol with previously aged oils greatly reduces the resultant viscosity, but not quite as effectively as the use of the methanol shortly after the pyrolysis oil is produced. The cost of the additive, e.g., methanol, may be offset by the heating value it adds to the pyrolysis oil, depending on the local cost of each.

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