Environmental Engineering Reference
In-Depth Information
this liquid product, after sufficient upgrading to eliminate phase separation and tar
formation, may serve better as a fuel as opposed to a source of chemicals [116].
Formation of syngas (a mixture of CO and H
2
in varying proportions) via
gasification thermal processing involves the controlled partial combustion of
biomass, which however necessitates considerable product upgrading due to the
diversity of organic components in the feedstock. The syngas platform is reviewed
in greater depth later in Section 4.8.1 as a best-in-class example from the thermal
processing of biomass. The generation of hydrogen from biomass is another
important thermal process, especially as the reduction of highly oxygenated
platform molecules becomes a prevalent conversion route to higher-value
chemicals (see Section 4.6). The method of heating in thermal treatment can be
varied and can include fast or slow heating via conventional means or the use of
emerging technologies such as microwaves (see Chapter 3). In some cases,
thermal processing also involves the addition of chemical agents to influence the
product selectivity and yields, and therefore some biomass to platform molecule
processes sit between chemical-catalytic and thermal (i.e. both high temperatures
and chemical agents required). Examples include cases where an acid, base, or
metal catalyst has been added, hydrothermal treatments or the use of solvents
such as ionic liquids or sulfolane [117, 118]. Catalysts and solvents can
dramatically alter the product profiles, altering gas/liquid/char ratios, increasing
rates of reactions and lowering the temperatures needed for some reactions, while
hydrothermal treatments can increase levels of hydrolysis.
Chemical-catalytic processes are those where a chemical agent(s) has been
included in the process. These reactions proceed at moderate temperatures, generally
much lower than those required for thermal treatments, though as mentioned above
the differentiation between thermal and chemical-catalytic processes can in some
instances be unclear. Examples of chemical-catalytic processes include reduction or
oxidation of saccharides, acid treatment of saccharides to form HMF or CMF,
transesterification or hydrolysis of triglycerides, deacetylation and depolymerisa-
tion of chitin and hydrolytic hydrogenation of lignin.
Biological processes are those involving the use of isolated enzymes and
microorganisms [119]. Biological processes may form a useful pretreatment prior
to other processing methods. For example, developments in the biological
degradation of cellulose to glucose could be vital in reducing the cost of producing
reduced sugars such as sorbitol and xylitol from biomass. A key consideration of
biological processing of biomass is the requirement for various pretreatments,
especially of lignocellulose. Recovery of products following fermentation can also
add to costs; diacids such as succinates and itaconates require acidification while
alcohols are isolated via distillations from the broth. Options for biological pre-
treatments and current methodologies are described in greater detail in Chapter 3.
Extraction processes are those that isolate platform molecules or platform
molecule precursors direct from biomass via the use of physical operations (e.g.
pressing) or solvents (e.g. ethanol, super-heated water and supercritical CO
2
).
