Chemical composition of biomass - Biomass as Energy Source: Resources, Systems and Applications - page 41

Another well-known example is sugar beet, whose sugar content ranges from 12 to 17% of its dry
weight (Zhou et al ., 2011).
2.3.6 Starch
Starch is often found in all the living tissues of a plant. High amounts of starch are found in many
fruit tissues e.g. in grains of maize (corn), wheat and rice and also in underground tubers such
as potato and cassava. Starch generally contains 20 to 25% amylose and 75 to 80% amylopectin
(Brown and Poon, 2004).
2.3.7 Proteins
In contrast to other major compounds present in biomass, proteins contain high amounts of
nitrogen (N); their average ratio of N to C is 0.25 (Perrett, 2007). Some plant species (especially
legumes) are known for their high protein contents. In such cases the highest protein contents are
generally found in fruit tissues, as with beans and lentils. However, all living plant cells contain
proteins that play crucial roles in various life-supporting processes. In dead cells, proteins are
found in the cell walls and the remnants of the protoplast. Wood has extremely low nitrogen to
carbon (N/C) molar ratios of 0.003 or below.
2.4 INORGANIC SUBSTANCES
The reason for studying ash elements in biomass is that they can cause significant operational
problems during energy conversion (for instance by combustion), including deposition, corrosion,
and particle emissions.
The term biomass encompasses a wide range of very heterogeneous substances with widely
varying properties. This is also true of the content and abundance of ash-forming matter, which
both vary depending on the species from which the biomass was obtained and the specific tissue
harvested. In general, woody biomass contains relatively little inorganic matter. For instance,
ash-forming materials (i.e. non-combustible inorganic matter) account for only 0.3-0.4% of the
dry mass of stem wood from pines, spruces, birches and aspen. Other parts of the tree, such as
the bark, branches, twigs, shoots and leaves/needles, contain higher levels of inorganic matter,
rising to approximately 7 wt% for aspen shoots.
Plants take up macronutrients, micronutrients and trace elements from the soil. The inorganic
content of biomass comprises many metals as well as some non-metals. Specifically, plants tend
to contain at least some amount of Si, Ca, Mg, K, Na, P, S, Cl, Al, Fe, Mn, N Cu, Zn, Co, Mo, As,
Ni, Cr, Pb, Cd, V, and Hg. Most of these elements act as nutrients and have essential biological
functions in the living plant.
Some plant species and varieties may hyper-accumulate metals. At present, there are about
ten species that are known to hyper-accumulate Mn, most of which are woody trees and shrubs.
Plants that accumulate above-normal metal concentrations in their leaves do so mainly by primary
sequestration in the non-photosynthetic part of the leaves. This is the case for Salix , which is an
important crop for biofuel production in some countries. It has been shown that the pectin-
rich layers of the collenchyma cell walls of leaf veins are an important Cd sink in Cd-tolerant
Salix viminalis , and it has been suggested that some varieties of this species could be used as soil
photoremediators. Reed canary grass is a non-food biomass crop that accumulates Si when grown
on soils containing clays but has a lowSi content when grown on organogenic soils (Burvall, 1997).
Another important problem is the in situ contamination of biomass crops with inorganic mate-
rials during growth, harvesting, handling, or processing. This problem is particularly severe
when using low-value and residual biomass components such as straw, branches, tops, stumps,