Chemical composition of biomass - Biomass as Energy Source: Resources, Systems and Applications - page 36

Table 2.2. Global availability of biomass suitable for energy conversion according
to Woolf et al . (2010).

Maximum sustainable

Biomass technical potential* [Pg/y]

Agroforestry 1.28

Biomass crops 1.25

Rice 0.67

Manure 0.59

Cereals excluding rice 0.42

Forestry residues 0.29

Sugar cane 0.27

Harvested wood 0.21

Green waste 0.07

Total 5.10

∗ 1Pg = 1000 Tg = 1000 million tonnes.

multiple utilization of existing agro- and forest-based crops is likely to be the preferred source of
biomass for energy conversion in most cases.

2.2 MAJOR COMPONENTS OF BIOMASS

The major components of biomass are shown in Figure 2.1. The composition of biomass is more
complex than other energy sources such as oil or coal. For example, the water content of natural
biomass can be more than 50% on a wet basis, and both its organic and inorganic components
are highly variable. Biomass from different plant species (and from different structural elements
of individual species) can have very different relative contents of cellulose, hemicellulose and
lignin, as well as extractives (fats, oils etc.), sugars, starch, and proteins. This results in differences
in the relative abundance of carbon (C), hydrogen (H), oxygen (O), nitrogen (N) and sulfur (S),
which affect the resulting fuel's energy value. Volatile matter accounts for a significant fraction
of all biomass. The amount of volatile matter in a sample is determined using a standardized
method involving measuring the amount of organic material lost when the dry sample is briefly
subjected to a high temperature (typically 900 ◦ C for seven minutes). The organic material that
remains following such treatment is referred to as fixed carbon.
The biochemical composition of biomass is expressed in terms of the percentage of the weight
of the dry material that is attributable to each constituent. However, the amount of water in a
sample is often expressed in terms of the percentage of the mass of the wet material that is due to
water.
The inorganic content of biomass is usually defined in terms of the mass of ash remaining after
its combustion. This is typically lowest for pure wood (accounting for 0.3-0.5% of the original
sample mass) but may exceed 10% for bark or other structural tissues. At a more detailed level,
ash is defined by its elemental composition. One problem when determining the ash content of
biomass is that biomass often is contaminated, e.g. by soil particles.
There are two major groups of analytical procedures used when studying biomass: proximate
and ultimate analyses. Proximate analyses are used to determine the sample's moisture content,
the weight percentage of volatile matter and fixed carbon in dry biomass (wt% d.b.), the heating
value (gross calorific value), and the ash content. Ultimate analyses are those that determine the
wt% of carbon (C), hydrogen (H), oxygen (O), nitrogen (N) and sulfur (S) in dry biomass, as well
as those of chlorine (Cl), fluorine (F) and/or bromine (Br) in some cases.