BIOMASS COMPOSITION, PROPERTIES, AND CHARACTERIZATION - Biomass as a Sustainable Energy Source for the Future - page 52

material is of particulate nature, in contrast to the atomically dispersed material. It is
one of the major contributors (next to, e.g., additives supplied to improve combustion
processes or some sticky ash agglomerates) to fly ash particles with sizes larger than
about 10 μ m (Baxter, 1993).

2.5 PROXIMATE AND ULTIMATE ANALYSIS

It is common practice to characterize the composition of a given fuel by means of two
standard analyses, the proximate and the ultimate analysis. These techniques are dis-
cussed in the following sections.

2.5.1 Proximate Analysis

The proximate analysis is used to determine the following constituent classes in
the fuel:

￿ Moisture content

￿ VM content

￿ FC content

￿ Ash content

One of the main constituents of biomass is moisture, and this also forms a main
difference with coals, and in particular high-rank (older) coals. Water is an essential
molecule to sustain life as it acts as major cell liquid (cytoplasm) and plays a
fundamental role in photosynthesis, as explained in Chapter 1. High moisture contents
may severely impact thermal conversion, and either drying is needed (see Chapter 8)
or conversion techniques using the water should be applied (see, e.g., Chapters 10, 11,
12, 13, and 14). Moisture consists of free (or surface) and inherent moisture. The
moisture content is determined by placing a biomass sample on a balance in an oven
filled with inert gas. The weight loss is determined after 24 h at 105 C, a temperature
at which one can assume all moisture to have been evaporated, so by heating to 105 C
also the inherent moisture is removed. Water that is chemically bound by hydrates is
not removed in this procedure. The weight fraction of moisture measured on an “ as

received ” basis, abbreviated as “ ar, ” can be determined as

Y a moist = wet weight

ð − dry weight Þ

ð Eq : 2 : 1 Þ

wet weight

Sometimes, moisture content is presented on a db:

Y d moist = wet weight

ð − dry weight Þ

ð Eq : 2 : 2 Þ

dry weight

The effect of the moisture content on the heating value is explained in Section 2.7.
The amount of heat that can be recovered from the biomass drops dramatically with