Page 95 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 3 Biomass Characteristics 73

gives. Ignition temperature, however, is not necessarily a unique property
of a fuel because it depends on several other factors like oxygen, partial
pressure, particle size, rate of heating, and a particle’s thermal surroundings.

3.6 COMPOSITION OF BIOMASS
Biomass contains a large number of complex organic compounds, moisture
(M), and a small amount of inorganic impurities known as ash (ASH). The
organic compounds comprise four principal elements: carbon (C), hydrogen
(H), oxygen (O), and nitrogen (N). Biomass (e.g., MSW and animal waste)
may also have small amounts of chlorine (Cl) and sulfur (S). The latter is
rarely present in biomass except for secondary sources like demolition wood,
which comes from torn-down buildings and structures.
Thermal design of a biomass utilization system, whether it is a gasifier or
a combustor, necessarily needs the composition of the fuel as well as its
energy content. In the context of thermal conversion like combustion, fol-
lowing two types of compositions are mostly used:
1. Ultimate or elemental composition
2. Proximate composition.
Besides these, there is also the polymeric composition of biomass, which
is important for chemical conversions like torrefaction, pyrolysis, and
gasification.

3.6.1 Ultimate Analysis
Here, the composition of the hydrocarbon fuel is expressed in terms of its
basic elements except for its moisture, M, and inorganic constituents, ASH.
A typical ultimate analysis is:
C 1 H 1 O 1 N 1 S 1 ASH 1 M 5 100% (3.17)
Here, C, H, O, N, and S are the mass percentages of carbon, hydrogen,
oxygen, nitrogen, and sulfur, respectively, in the fuel. Not all fuels contain
all of these elements. For example, the vast majority of biomass may not
contain any sulfur (S). The moisture or water in the fuel is expressed sepa-
rately as M. Thus, hydrogen or oxygen in the ultimate analysis does not
include the hydrogen and oxygen in the moisture, but only the hydrogen and
oxygen present in the organic components of the fuel. Table 3.10 compares
the ultimate analysis of several biomass materials with that of some fossil
fuels.
The atomic ratios (H/C) and (O/C) plotted in Figure 3.10 is derived from
the ultimate analysis of different fuels. This figure shows that biomass, cellu-
lose in particular, has very high relative amounts of oxygen and hydrogen,
which results in relatively low heating values.