Page 157 - New Trends In Coal Conversion

120 New Trends in Coal Conversion

Biomass differs significantly from coal in terms of physical and chemical proper-
ties, as well as composition and energy content. Biomass usually contains less carbon,
more hydrogen and oxygen, less sulfur and nitrogen, more volatile matter (VM), lower
heating value, and lower bulk density. These properties affect the design, operation,
and performance of cofiring systems.

5.2.1 Chemical properties
As mentioned above, the moisture content in biomass is high compared with coal.
Biomass usually has less fixed carbon (FC) and more VM than coal, and it also has
a lower ash content. Biomass typically has a VM/FC ratio of >4.0, whereas the
VM/FC ratio for coal is almost always <1.0 (Tillman, 2000). Thus, for biomass fuels,
the predominant form of combustion will take place via the gas phase oxidation of the
volatile species (Wang et al., 2009).
The nitrogen and sulfur contents in the biomass are low, and therefore, emissions of
nitrogen and sulfur oxides from these elements will probably be minimal, which favors
clean combustion conditions (Vamvuka et al., 2003a).
In comparison with coal, biomass contains a higher proportion of oxygen and
hydrogen and less carbon, which reduces its heating value because the amount of en-
ergy contained in CeO and CeH bonds is lower than in CeC bonds (Munir et al.,
2009). However, the higher oxygen content in the biomass indicates that it will
have a higher thermal reactivity than coal (Haykiri-Acma and Yaman, 2008).
The ash analysis is very important to predict the behavior of a solid fuel during com-
bustion because it provides information of the inorganic elements present, which are
responsible for many of the drawbacks of biomass combustion. The ash fusion temper-
atures can offer an approximate prediction of the slagging tendency of the fuel because
fusion of ash is the basis of slag formation. Other slagging indices have also been
defined, such as the proportion of basic oxides to acidic oxides in the ash, or other
ones that also incorporate the sulfur content, although these indices have all been defined
for coal. Because the ash resulting from biomass and coal combustion varies signifi-
cantly in composition, these indices may not be reliable in predicting slagging/fouling
in biomass. Biomass ash deposition depends primarily on sodium and potassium chlo-
rides, sulfates, hydroxides, etc., whereas coal ash consists of an aluminosilicate system.
However, they could be useful in predicting the ash behavior of biomass/coal blends,
which contain low percentages of biomass (Madanayake et al., 2017).

5.2.2 Thermochemical properties
When compared with fossil fuels, biomass fuels usually contain less heating value. The
heating value or calorific value of a solid fuel is usually considered as the principal
thermochemical property for comparing solid fuels because it represents the amount
of energy released during the combustion of the fuel. However, it cannot be used to
predict the efficiency of the combustion system. The maximum efficiency that can
be attained is a function of the flame temperature, which in turn depends on both
the calorific value and the chemical composition of the fuel. The chemical composition