Page 82 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

60 Biomass Gasification, Pyrolysis and Torrefaction

(Figure 3.9). It is one of the most abundant organic polymers on Earth
(exceeded only by cellulose). It is the third important constituent of the cell
walls of woody biomass.
Lignin is the cementing agent for cellulose fibers holding adjacent cells
together. The dominant monomeric units in the polymers are benzene rings.
It is similar to the glue in a cardboard box, which is made by gluing together
papers in a special fashion. The middle lamella (Figure 3.5), which is com-
posed primarily of lignin, glues together adjacent cells or tracheids.
Lignin is highly insoluble, even in sulfuric acid (Klass, 1998, p. 84).
A typical hardwood contains about 18 25% by dry weight of lignin, while
softwood contains 25 35%.

3.4 GENERAL CLASSIFICATION OF FUELS
Classification is an important means of assessing the properties of a fuel.
Fuels belonging to a particular group have similar behavior irrespective of
their type or origin. Thus, when a new biomass is considered for gasification
or other thermochemical conversion, we can check its classification, and
then from the known properties of a biomass of that group, we can infer its
conversion potential.
There are three methods of classifying and ranking fuels using their
chemical constituents: atomic ratios, the ratio of lignocellulose constituents,
and the ternary diagram. All hydrocarbon fuels may be classified or ranked
according to their atomic ratios, but the second classification is limited to
lignocellulose biomass.

3.4.1 Atomic Ratio
Classification based on the atomic ratio helps us to understand the heating
value of a fuel, among other things. For example, the higher heating value
(HHV) of a biomass correlates well with the oxygen-to-carbon (O/C) ratio,
reducing from 20.5 to about 15 MJ/kg, while the O/C ratio increases from
0.86 to 1.03. When the hydrogen-to-carbon (H/C) ratio increases, the effec-
tive heating value of the fuel reduces.
The atomic ratio is based on the hydrogen, oxygen, and carbon content
of the fuel. Figure 3.10 plots the atomic ratios (H/C) against (O/C) on a dry
ash-free (daf) basis for all fuels, from carbon-rich anthracite to carbon-deficient

CH O
3
HO C C C HO C C C HO C C C

CH O CH O
3
3
FIGURE 3.9 Some structural units of lignin.