Researchers at the US Department of Agriculture (USDA) Agricultural Research Service (ARS) have developed and have filed a patent on a new fast pyrolysis process called Tail Gas Reactive Pyrolysis (TGRP), which removes much of the oxygen from bio-oils without the need for added catalysts.

Fast pyrolysis is the process of rapidly heating biomass from wood, plants and other carbon-based materials at high temperatures without oxygen. Using pyrolysis to break down tough feedstocks produces three things: biochar, a gas, and bio-oils that can be refined to end-products such as green gasoline.

The bio-oils are high in oxygen, making them acidic and unstable, but the oxygen can be removed by adding catalysts during pyrolysis. Although this adds to production costs and complicates the process, the resulting bio-oil is more suitable for use in existing energy infrastructure systems as a “drop-in” transportation fuel that can be used as a substitute for conventional fuels.

The ARS researchers modified the standard pyrolysis process by gradually replacing nitrogen gas in the processing chamber with the gases produced during pyrolysis. The TGRP process was very effective in lowering oxygen levels and acidity, and no additional catalysts were needed.

The team conducted a pilot-scale study using three types of biofeedstock with different characteristics: oak, switchgrass, and pressed pennycress seeds.

Bio-oils produced from oak and switchgrass by the new process had considerably higher energy content than those produced by conventional fast pyrolysis. The energy content of the oak bio-oil was 33.3% higher and contained about two-thirds of the energy contained in gasoline. The energy content for switchgrass was 42% higher, slightly less than three-fourths of the energy content of gasoline.