Environmental Engineering Reference
In-Depth Information
to lower temperatures the 'floor' tilts towards longer wavelengths at lower temperatures. As the
moisture content increases, the binding energy for the interaction between water and biomass
decreases, allowing the water molecules to vibrate more freely and shifting the peak to shorter
wavelengths. In pure water, this peak occurs at around 1190 nm at room temperature.
The presence of water in biomass is also important because it influences its susceptibility to
microbial colonization followed by the consumption of its nutrients. At moisture contents below
the fiber saturation point, there is little scope for microbial degradation, which is completely
inhibited at lower moisture contents.
Biomass is hygroscopic, and dried biomass will therefore equilibrate to the humidity of the
ambient air. This is an exothermic process. The adsorption of water vapor from the surrounding air
thus releases heat that can potentially set off other self-heating processes such as auto-oxidation,
leading to the emission of toxic and hazardous substances. If the process continues and thematerial
heats up even further, it may catch fire.
2.2.2 Dry matter content
The dry matter content of biomass is usually defined using oven-based methods involving drying
at 105 C. At this temperature, volatile organic compounds (VOCs) may vaporize, causing the
real dry matter content to be underestimated. In such cases, lower drying temperatures can be
used. The dry substance consists of both organic matter and inorganic substances. If the biomass
is contaminated by inorganic dust or soil particles, these will be included in the measured dry
matter content.
The relative abundance of different organic and inorganic compounds in biomass varies between
plant genera, species, age, site of harvesting (including genotype
environment interactions),
management (e.g. liming and fertilization regimes), season, structural origin (tissue type), cell
type, and location in cell wall. This makes it difficult to provide a full and detailed review of the
composition of biomass. The brief discussion in this chapter provides only a rough overview of
the organic and inorganic contents of dry plant biomass. The cells of most plant species have both
a primary and secondary cell wall. While the cell is growing, it has only a primary wall, but when
growth stops, the secondary wall is formed inside the boundaries of the first, i.e. between the
primary cell wall and plasma membrane. The primary wall consists of densely packed cellulose
chains that are bound to one another by hydrogen bonds and arranged in micro fibrils, which are
arranged in a network to increase the strength of the cell wall. Conceptually these micro fibrils are
like a cross-linked network of girders, with the crosslinking units being embedded hemicelluloses
and/or pectin, as well as proteins in some cases. The cell walls are laid down in stages. First, the
primary wall and the middle lamella that occupies the space between cells are laid down. These
are rich in pectic material. When the primary cell wall has been established, a secondary wall
is formed inside the cell. This secondary cell wall consists mainly of cellulose, hemicellulose
and lignin; it includes no proteins but pectin may also be absent. The cellulose micro fibrils of
the secondary wall are cross-linked by hemicellulose. The secondary cell wall strengthens the
cell against dynamic forces and static forces such as gravity, and helps to hold the plant up. It is
therefore thicker and less pliant than the primary wall. It often has three distinct layers that are
known as s1, s2 and s3. Typically, s2 is the thickest and accounts for about 90% of the fibrous
mass of plants (Batchelor et al. , 2000). In subsequent phases of cellular development, lignin is
deposited as a filler material around the primary wall, in the cell corners and the intercellular
space, and also in the secondary wall.
×
2.3 ORGANIC MATTER
The main organic components of biomass are cellulose (the most abundant organic polymer
on earth), hemicelluloses, lignin, pectin, extractives, starches, sugars, and proteins. Proteins
Search WWH ::




Custom Search