Environmental Engineering Reference
In-Depth Information
also been used to produce solvents (
para
-cymene) and monomers [109]. Natural
rubber is also classified as a terpene and can be used to produce platform molecules
via pyrolysis. However, instead of harvesting expensive natural rubber for platform
molecule production, it would probably be more economical to convert spent
rubber into chemicals such as monomers and limonene ethers [113, 114].
Other notable platform molecules obtained from the extraction from biomass
include reduced sugars such as d-mannitol, readily isolated from brown seaweed
(Phaeophyceae), and lipids (wax esters, sterols, fatty alcohols, etc.) found on the
exterior and within terrestrial plants. Extraction of waxes and sterols from the
surfaces of plants is often practised as a pretreatment of lignocellulosic wastes for
the removal of the hydrophobic layers, which improves penetration of chemical or
biological agents into the cellular structure. However the wax content in the
biomass is low and the wax comprises many compounds, however; it is therefore
more likely that waxes and sterols will instead continue to be used directly in
higher-value applications such as cosmetics.
Hydroxy acids derived from polyhydroxyalkanoates (PHAs) also represent an
interesting opportunity for platform molecules. PHAs are produced in bacteria
and therefore require the use of carbohydrates (starch, sugars) in a biological
process and subsequent recovery of the lipophilic polymer via extraction. Hydroxy
acids can be produced from the hydrolysis of the extracted PHAs allowing access
to a range of potentially useful platform molecules.
4.5 Process Technologies: Biomass to Platform Molecules
Biomass can be converted into platform molecules via several processing
technologies, the most common and practical being thermal (sometime referred to
as thermochemical), chemical-catalytic (sometimes referred to as chemocatalytic)
[55], biological or extraction (see Chapter 3 for more information).
Thermal treatment involves the application of elevated temperatures (>200°C)
to biomass, and can be in the absence (e.g. pyrolysis) or presence (e.g. partial
combustion) of oxygen [115]. Slow pyrolysis of biomass has been practised by
humans for centuries, the goal primarily being formation of charcoal for use as
fuel. During the last century a link between the rate of heating and yield of liquid
from the pyrolysis process was inferred, and this led to a rise in the interest of fast
pyrolysis via the rapid heating of biomass in a controlled manner. Yields of liquid
from fast pyrolysis can reach 70 wt%, and it is from this liquid that a range of
platform molecules can be derived. Conditions for pyrolysis can be altered to
favour higher gas (gasification) or liquid (liquefaction) yields, while keeping solid
yields below 10 wt%. From pyrolysis it is the liquid fraction that has the greatest
scope for platform molecule isolation, but the complex mixture of up to hundreds
of molecules means expensive separation could prevent fast pyrolysis from
becoming economical for platform molecule production. Instead, fast pyrolysis
liquefaction is seen as a means of producing an intermediate bio-oil platform, and
