An overview of fast pyrolysis of biomass
Introduction
Renewable energy is of growing importance in satisfying environmental concerns over fossil fuel usage and its contribution to the Greenhouse Effect. Wood and other forms of biomass are some of the main renewable energy resources available and provide the only source of renewable liquid, gaseous and solid fuels. Wood and biomass can be used in a variety of ways to provide energy:
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by direct combustion to provide heat for use in heating, for steam production and hence electricity generation;
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by gasification to provide a fuel gas for combustion for heat, or in an engine or turbine for electricity generation;
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by fast pyrolysis to provide a liquid fuel that can substitute for fuel oil in any static heating or electricity generation application. The liquid can also be used to produce a range of speciality and commodity chemicals
Fast pyrolysis can directly produce a liquid fuel from biomass which can be readily stored or transported.
Section snippets
Fast pyrolysis principles
Biomass is a mixture of hemicellulose, cellulose, lignin and minor amounts of other organics which each pyrolyse or degrade at different rates and by different mechanisms and pathways. Lignin decomposes over a wider temperature range compared to cellulose and hemicellulose which rapidly degrade over narrower temperature ranges, hence the apparent thermal stability of lignin during pyrolysis. The rate and extent of decomposition of each of these components depends on the process parameters of
Process characteristics and technology requirements
Although fast pyrolysis of biomass has achieved commercial status, there are still many aspects of the process which are largely empirical and require further study to improve reliability, performance, product consistency, product characteristics and scale-up. This section summarises these topics.
Product quality
The elemental and chemical composition of pyrolysis oils is very much dependent on the pyrolysis conditions under which they are produced. Maximum yields are obtained at temperatures in the range 450–550°C and residence times of 0.5–5 s depending on the particular process; these being typical conditions of fast pyrolysis. In this case the oil is highly oxygenated, indeed being not very different in composition from the feedstock (see Table 3).
Typical characteristics of fast pyrolysis liquid are
Applications
A summary of the opportunities is shown in Fig. 5. Within Europe the most promising application is seen as electricity production due to the anticipated ability to use raw bio-oil as produced in an engine or turbine without the need for extensive upgrading as well as the ability to de-couple fuel production from electricity generation with storage and/or transport of the liquid fuel which is not possible for gasifier products and IGCC (Integrated Gasifier Combined Cycle) systems. The much
Fast pyrolysis of contaminated wood
As an example of the application of fast pyrolysis, some work on fluid bed fast pyrolysis of wastewood is included to show how fast pyrolysis can be applied to both derive a useful liquid fuel product and also fix the contaminants in the residual char. This work was carried out at the Institute of Wood Chemistry in Hamburg, Germany.
Conclusions
Fast pyrolysis is now accepted as a technology for producing high yields of liquid fuels that can be used in many applications as direct substitutes for conventional fuels or as a source of chemicals. There are some interesting challenges to be faced in developing and modifying fast pyrolysis technology, in upgrading the liquids and adapting applications to accept the unusual behaviour and characteristics of the liquid product. Higher added value products than fuels offer the most challenging
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