Tar Production and Destruction - Biomass Gasification, Pyrolysis and Torrefaction - page 182

TABLE 6.3 Typical Composition of Tar

Component Weight (%)

Benzene 37.9

Toluene 14.3

Other 1-ring aromatic hydrocarbons 13.9

Naphthalene 9.6

Other 2-ring aromatic hydrocarbons 7.8

3-ring aromatic hydrocarbons 3.6

4-ring aromatic hydrocarbons 0.8

Phenolic compounds 4.6

Heterocyclic compounds 6.5

Others 1.0

Source: Adapted from Milne et al. (1998).

temperature, the type of reactor, and the feedstock. Table 6.3 shows that ben-
zene is the largest component of a typical tar.
Tar may be classified into four major product groups: primary, secondary,
alkyl tertiary, and condensed tertiary (Evans and Milne, 1997). Short descrip-
tions of these follow.

6.2.3.1 Primary Tar
Primary tar is produced during primary pyrolysis. It comprises oxygenated,
primary organic, condensable molecules. Primary products come directly
from the breakdown of the cellulose, hemicellulose, and lignin components
of biomass. Milne and Evans (1998) listed a large number of compounds of
acids, sugars, alcohols, ketones, aldehydes, phenols, guaiacols, syringols, fur-
ans, and mixed oxygenates in this group.

6.2.3.2 Secondary Tar
As the gasifier's temperature rises above 500 C, primary tar begins to rear-
range, forming more noncondensable gases and some heavier molecules
called secondary tar, of which phenols and olefins are important constitu-
ents. As such, one notes a rise in secondary product at the expense of pri-
mary product ( Figure 6.2 ).