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

Origin and composition

Since all kind of biowaste contains hydrocarbonaceous material it can also be processed via pyrolysis. However, the composition of the feedstock has an impact on the pyrolysis process and therewith on the products which can be obtained. Usually wood or herbaceous are processed via pyrolysis which contain cellulose, hemicellulose, and lignin in different ratios.

	Corn stover	Switchgrass	Wood
Proximate analysis wt [%]
Moisture	8.0	9.8	42.0
Ash	6.9	8.1	2.3
Volatile matter	69.7	69.1	47.8
Fixed carbon	15.4	12.9	7.9
Elemental analysis wt [%]
Carbon	49.7	50.7	51.5
Hydrogen	5.91	6.32	4.71
Oxygen	42.6	41.0	40.9
Nitrogen	0.97	0.83	1.06
Sulphur	0.11	0.21	0.12
Chlorine	0.28	0.22	0.02
Structural organics
Cellulose	36.3	44.8	38.3
Hemicellulose	23.5	35.3	33.4
Lignin	17.5	11.9	25.2

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 :

• Physical pre-treatments
  • Sizing (e.g. chipping, grinding)
  • Densification (e.g. pressure-densification)
• Chemical pre-treatment
  • Acid and alkali pre-treatment (e.g for the removal of minerals)
  • Hydrothermal pre-treatment (e.g. torrefaction)
  • 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. Depending on the feedstock origin and composition as well as the pre-treatment and process the yield as well as the chemical and physical properties of the products can vary.

Char

As mentioned the functional properties of char may vary which includes carbon content, functional groups, heating value, surface area, and pore-size distribution. The application possibilities are versatile, the char can be used as soil amendment for carbon sequestration, soil fertility improvement, and pollution remediation. Furthermore the char can be used for catalytic purposes, energy storage, or sorbent for pollutant removal from water or flue-gas.

Pyrolysis oil

Produced pyrolysis oil is a multiphase emulsion composed of water and and hundrets of organic molecules such as acids, alcohols, ketones, furans, phenols, ethers, esters, sugars, aldehydes, alkenes, nitrogen- and oxygen- containing molecules. A longer storage or exposure to higher temperature increases the viscosity due to possible chemical reactions of the compounds in the oil which leads to the formation of larger molecules. The presence of oligomeric species with a molecular weight >5000 decreases the stability of the oil, furthermore the formation of aerosols from volatile substances accelerates the aging process in which the water content and phase separation increases. The application as fuel in standard equipment for petroleum fuels (e.g. boilers, engines, turbines) may be limited due to poor volatility, high viscosity, coking, and corrosiveness of the oil. To overcome these problems the pyrolysis oil has to be upgraded in a post-treatment to be used as fuel and/or the equipment for the end-application has to be adapted.

Pyrolysis gas

Syngas can be obtained from the pyrolysis gas which is composed of different gases such as carbon dioxide, carbon monoxide, hydrogen, methane, ethane, ethylene, propane, suphur oxides, nitrogen oxides, and ammonia. The different gases can be fractionated from each other in the post-treatment to utilise them for different applications such as the production of chemicals, cosmetics, food, polymers or the utilisation as fuel or technical gas.

Post-treatment

• Fischer-Tropsch-Synthesis

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