BIOMASS COMPOSITION, PROPERTIES, AND CHARACTERIZATION - Biomass as a Sustainable Energy Source for the Future - page 51

important role in many essential processes, including binding form, cell wall stabiliza-
tion, secretor processes, membrane stabilization, and regulation of the osmotic pressure.
Sulfur is a macronutrient and plants need it for growth; it is increasingly being
recognized as the fourth major plant nutrient after nitrogen, phosphorus, and potas-
sium. S is present in both organic and inorganic forms (Marschner, 1993). Sulfur is
contained in many organic structures that include amino acids and proteins (such as
enzymes). The element is absorbed specifically by the plants roots in the form of

the sulfate ion. The sulfur content for optimal plant growth is between 0.1 and 0.5
wt% (db) and varies from plant to plant. The ratio of organic to inorganic S species
may differ between plant species and may depend on local soil and growth conditions.
Chlorine in plants is available mainly as a free anion or is loosely bound to
exchange sites. In addition, a number of chlorinated organic compounds have been
found in plants, but the functional requirement for plant growth of most of these com-
pounds is not known (Marschner, 1993). Similar to potassium, chlorine has a high
mobility within the plant, and the average chlorine content in plants ranges from
0.2 to 2.0 wt% (db).
Silicon has a number of valuable effects in many plant species. It increases leaf
erectness. In addition, a high content of silicon in leaves increases the resistance of
the tissue against fungal attacks, blast infection, and insect pests. In the case of woody
biomass, additional inorganic species may be present, originating from the soil
(

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materials, such as quartz sand or clay) or from
other sources, such as paints or coatings added during manufacturing in the case of
waste wood.
The distribution of inorganic matter within different biomass fuels varies enor-
mously between diverse samples taken, even when, e.g., crushed and sieved. It can
occur as discrete minerals, amorphous phases, and organically associated or simple
salts, possibly dissolved in pore water. See Figure 2.2 for more details on the forma-
tion of the phases (Vassilev et al., 2010, 2013). In biomass, nearly all of the inorganic
elements are found organically associated or present as simple inorganic salts. The
inorganic material can be divided into two groups, one of which is authigenic or nat-
urally occurring in the fuel and the other is detrital material, which has been added to
the fuel through geologic or processing steps (Baxter, 1993).
The main ash-forming components embedded in the woody biomass fuels are cal-
cium, magnesium, and potassium. A large fraction of the inherent inorganic material
in lignites, but also probably the dominant fraction in many biomass fuels, is associ-
ated with oxygen-containing functional groups (carboxylic acids, hydroxyls, ethers,
and ketones). These functional groups provide sites for inorganic material to become
incorporated in the fuel matrix as, e.g., cations (sodium, potassium, magnesium, and
calcium). The proportion of the inorganic compounds that is organically associated
increases with decreasing fuel rank, due to the higher oxygen content in low-rank
fuels. In the lowest-rank coals, more resembling biomass, the organically associated
inorganic elements can include up to 60 wt% of the total amount of inorganics
(Benson and Holm, 1985).
The second class of inorganic material in solid fuels consists of compounds that are
added to the fuel from extraneous sources, mainly during harvesting. This additional

“ detrital, ” sometimes called “ adherent ”