Pyrolysis

Pyrolysis (from greek pyr, "fire" and lysis, "loosing/unbind") is a thermochemical process which is able to convert organic compounds in presence of heat and absence of oxygen into valuable products which can be solid, liquid or gaseous. The chemical transformations of substances are generally accompanied by the breaking of chemical bonds which leads to the conversion of more complex molecules into simpler molecules which may also combine with each other to build up larger molecules again. The products of pyrolysis are usually not the actual building blocks of the decomposed substance, but are structurally modified (e.g. by cyclization and aromatisation or rearrangement).

Feedstock

Pre-treatment

The pre-treatment of the feedstock has an impact on the pyrolysis process, its efficiency, and the yield of certain products. The following pre-treatments may be considered :

• Sizing (e.g. chipping)
• Densification
• Chemical pre-treatment
  • Acid and alkali pre-treatment
  • Hydrothermal pre-treatment
  • Steam explosion
  • Ammonia fibre expansion
  • Thermal pre-treatment (e.g. drying)
  • Biochemical pre-treatment

Process

The pyrolysis is an endothermal process which requires the input of energy in form of heat which can either be directly (direct pyrolysis) applied via hot gases or indirectly (indirect pyrolysis) via external heating of the reactor. Compared to gasification, the process takes place in an atmosphere without oxygen or at least under a limitation of oxygen.

In general, pyrolysis can be divided into different steps which includes:

1. Evaporation and vapourisation of water and other volatile molecules which is induced at temperatures > 100 °C
2. Thermal excitation and dissociation of the molecules induced at temperatures between 100-600 °C, which also may involve the production of free radicals as intermediate stage
3. Reaction and recombination of the molecules, and triggering of chain reactions through free radicals

The pyrolysis process and the formation of products can be controlled to a certain extend via different temperature ranges and reaction times as well as by utilising reactive gases, liquids, catalysts, alternative forms of heat application (e.g. via microwaves or plasma), and a variety of reactor designs.

Reactions

A range of different reactions occur during the process such as dehydration, depolymerisation, isomerisation, aromatisation, decarboxylation, and charring.

Product

A range of solid, liquid, and gaseous products can be obtained from the pyrolysis process including char, pyrolysis oil, and pyrolysis gas.

Char

Depending on the feedstock origin and composition as well as the pre-treatment and process the chemical and physical properties (e.g. carbon content, functional groups, heating value, surface area, pore-size distribution) of the produced char can vary.

References

Al Arni, S. 2018: Comparison of slow and fast pyrolysis for converting biomass into fuel. Renewable Energy, Vol. 124 197-201.  doi:https://doi.org/10.1016/j.renene.2017.04.060

Czajczyńska, D., Anguilano, L., Ghazal, H., Krzyżyńska, R., Reynolds, A. J., Spencer, N. and Jouhara, H. 2017: Potential of pyrolysis processes in the waste management sector. Thermal Science and Engineering Progress, Vol. 3 171-197.  doi:https://doi.org/10.1016/j.tsep.2017.06.003

Speight, J. 2019: Handbook of Industrial Hydrocarbon Processes. Gulf Professional Publishing, Oxford, United Kingdom.

Tan, H., Lee, C. T., Ong, P. Y., Wong, K. Y., Bong, C. P. C., Li, C. and Gao, Y. 2021: A Review On The Comparison Between Slow Pyrolysis And Fast Pyrolysis On The Quality Of Lignocellulosic And Lignin-Based Biochar. IOP Conference Series: Materials Science and Engineering, Vol. 1051  doi:10.1088/1757-899X/1051/1/012075

Waheed, Q. M. K., Nahil, M. A. and Williams, P. T. 2013: Pyrolysis of waste biomass: investigation of fast pyrolysis and slow pyrolysis process conditions on product yield and gas composition. Journal of the Energy Institute, Vol. 86 (4), 233-241.  doi:10.1179/1743967113Z.00000000067

Zaman, C. Z., Pal, K., Yehye, W. A., Sagadevan, S., Shah, S. T., Adebisi, G. A., Marliana, E., Rafique, R. F. and Johan, R. B. 2017: Pyrolysis: A Sustainable Way to Generate Energy from Waste. IntechOpen