Pyrolysis

Pyrolysis is thermal decomposition occurring in the absence of oxygen, and it is also the first step in combustion and gasification processes where it is followed by total or partial oxidation of the primary products. The goal of pyrolysis is to produce a liquid fuel, termed bio-oil or pyrolysis oil, which can be used as a fuel for heating or power generation. The produced oil is composed of a very complex mixture of oxygenated hydrocarbons, and like crude fossil oil can be used in refining to produce a range of chemicals, fuels and fertilisers. The main benefit of the pyrolysis process, when compared to combustion and gasification, is that a liquid fuel is easier to transport then either solid or gaseous fuels. This also means that the pyrolysis plant doesn’t have to be located near the end-use point of the bio-oil, but can instead be located near the biomass resource supply, which results in considerably lowering the fuel transportation costs. High transport costs are one of the limiting factors for the construction of large scale biomass power plants, which have higher efficiencies and lower emissions the small plants.

The pyrolysis reaction is relatively complex and results in non-equilibrium products, making their properties hard to predict. The properties are dependent on the process temperature, the period of heating, ambient conditions, the presence of oxygen, water and other gases, and the nature of the feedstock. In general, lower process temperature and longer heating periods result in the production of charcoal, high temperature and longer heating periods increase the biomass conversion to gas, and moderate temperature and short heating periods are optimum for producing liquids.

Pyrolysis technology, in comparison with combustion and gasification, is in the early state of development and thus the development costs are still very high and not well established, but this also means that there is considerable scope for cost reduction. Current trends in the research and development of pyrolysis are also focused on the so-called fast pyrolysis.

NREL's fast-pyrolysis reactor, which rapidly converts wood waste into complex pyrolysis oils, is at the heart of an industrial consortium working to refine the oils into adhesives and plastics. Credit: Gretz, Warren – NREL/DOE		Solid Biomass (wood) chips can be converted to a dark brown viscous liquid (shown here) by a process known as pyrolysis. Credit: Gretz, Warren – NREL/DOE