Environmental Engineering Reference
In-Depth Information
3.2.3. Mechanical/physical processes
Mechanical/physical processes are processes which do not change the state or the
composition of biomass, but only perform a size reduction or a separation of feedstock
components. In a biorefinery pathway, they are usually applied first, because the following
biomass utilization requires reduction of the material size within specific ranges, depending
on feedstock specie, handling and further conversion processes. Biomass size reduction is a
mechanical treatment process that significantly refers to either cutting or commuting
processes that significantly change the particles size, shape and bulk density of biomass.
Separation processes involve the separation of the substrate into its components, while with
extraction methods valuable compounds are extracted and concentrated from a bulk and
inhomogeneous substrates (Huang et al., 2008). Lignocellulosic pre-treatment methods, (e.g.
the split of lignocellusic biomass into cellulose, hemicellulose and lignin) fall within this
category, even if some hemicelluse is also hydrolized to single sugars (Cadoche and Lopez,
1989; Lasser et al., 2002, Sung and Cheng, 2002).
3.2.4. Chemical processes
Chemical processes are those processes which carry a change in the chemical structure of
the molecule by reacting with other substances. The most common chemical processes in
biomass conversion are hydrolysis and transesterification, but this group also includes the
wide class of chemical reactions where a change in the molecular formula occurs. Hydrolysis
uses acids, alkalis or enzymes to depolymerise polysaccharides and proteins into their
component sugars (e.g. glucose from cellulose) or derivate chemicals (e.g. levulinic acid from
glucose) (Lynd, 1996; Sung and Cheng, 2002). Transesterification is the most common
method to produce biodiesel today and is a chemical process by which vegetable oils can be
converted to methyl or ethyl esters of fatty acids, also called biodiesel. This process
implicates the coproduction of glycerine, a chemical compound with diverse commercial uses
(Crabbe et al., 2001; Demirabas, 2003; Marchetti et al., 2007). Other important chemical
reactions in biorefining are Fisher-Tropsch synthesis, methanisation, steam reforming,
catalytic synthesis or reactions, hydrogenation, oxygenation and so on.
3.3. Platforms
In biorefinery, the platforms are the intermediates which constitute the link between
feedstock and final products. This concept is similar to the petrochemical industry, where the
refinery starts with a massive distillation to separate the crude oil into a large number of
intermediates that are further manipulated into the desired products.
Among the several possible alternatives, the chemicals (or mix of chemicals) which are
individuated as the most important biorefinery platforms are the following: biogas, syngas,
hydrogen, C6 sugars, C5 sugars, levulinic acid, furfural, pyrolytic oil, oil and organic juice.
3.3.1. Biogas
Biogas is a biomass derived gas made of mainly CH
4
and CO
2
. It can be produced either
by anaerobic digestion of biological materials or by methanisation of the syngas coming out
from gasification. Biogas can be used as such for electricity and heat generation or can be
