1. General

[GLOSS]Pyrolysis[/GLOSS] is the transformation of a compound or material into one or more substances by heat alone (without oxidation); in other words thermal decomposition. [1]

Pyrolysis is somewhat similar to vaporization, however, it is a relatively slow chemical process compared to the vaporization. The temperature at which pyrolysis occurs depends on the [GLOSS]fuel[/GLOSS] type and the heating rate. Typically, bituminous coal pyrolises at about 420 °C (1% mass loss) for heating rates < 100 K/s.
This temperature is at or below the spontaneous ignition temperature ([GLOSS]SIT[/GLOSS]) for hydrocarbons. At high heating rates (>10 000 K/s), pyrolysis is presumed to start around 1220
°C.

Pyrolysis products consist of volatile gases, liquids (tar), and [GLOSS]char[/GLOSS] generally. Products ranges from lighter volatiles (CH4, C2H4, C2H6, CO, CO2, H2, H2O, etc.) to heavier tars. The composition of the
[GLOSS]volatile matter[/GLOSS]
(gases), products of pyrolysis, depends also on the fuel. The composition of the volatiles from lignite (low rank) coals at 1000K is 3% CH4 and 38% CO and CO2.

2. Pyrolysis of Biomass

Pyrolysis of Biomass is the thermal degradation of the material in the absence of reacting gases, and occurs prior to or simultaneously with gasification reactions in a gasifier. The liquid fraction of pyrolisised biomass consists of an insoluble viscous tar, and pyroligneous acids (acetic acid, methanol, acetone, esters, aldehydes, and furfural). The distribution of pyrolysis products varies depending on the feedstock composition, heating rate, temperature, and pressure, see Table 1. Pyrolysis units cannot operate independently without an external supply of heat. As examples, two technologies [3] for biomass pyrolysis and their properties are listed below:

• Slow pyrolysis (e.g. Dutch Gibros-Pec technology):a) size reduction, drying

  b) pyrolysis (atmospheric pressure, 450 °C)

  c) produced char is mixed with coal, followed by conventional coal treatment

  d) pyrolysis gas is used – to drive the pyrolysis or to combust in gas burner (low temperature gas clean-up may be installed)

• Fast pyrolysis (e.g. Pyrovas technology):a) size reduction, drying

  b) pyrolysis (0.15 bar, 500 °C)

  c) char, oil combusted in the coal boiler

  d) gas is used to drive the pyrolysis ( most of the contaminants are in the oil and char, therefore it is applicable for relatively clean fuels)

Table 1. Different types of pyrolysis. [3]

Keywords:

Biomass, combustion, pyrolysis, thermal decomposition.

Source:

[1] Consumer energy information. Energy Efficiency and Renewable Energy Network. U.S. Department of Energy. Source:
http://www.eren.doe.gov/consumerinfo/glossary.html
[2] Sami, M., Annamalai, K. & Wooldridge, M. (2001): Co-firing of coal and biomass fuel blends. Progress in Energy and Combustion Science, Vol 27., pp. 171-214.
[3] Ree, R.van, Korbee, R., Meijer, R, Konings, T., Aart F. van (2001): Operational experiences of (in)direct co-combustion in coal and gas fired power plants in Europe. Power-Gen Conference, Brussels, February.