Energy and Supercritical Fluids - Topical Themes in Energy and Resources - page 87

Fig. 5.9 Treatment of bagasse with supercritical water allows separation of the various fractions
and and produces a cellulose-rich phase that is high in purity. (Adapted from Arai et al. ( 2009 ) with
permission Elsevier ©)
Torrefied biomass is also referred to as bio-coal and its pellets are often co-burned
with coal to reduce costs and improve environmental aspects. Pyrolysis applies
moderate heat (300-500 °C) to biomass in the absence of oxygen to produce gases
such as hydrogen or methane and solid bio-char. Gasification applies high heat (ca.
700 °C) to biomass in the presence of oxygen or water to produce gases such as
carbon monoxide, carbon dioxide and hydrogen. The gas mixture, carbon monox-
ide and hydrogen along with some carbon dioxide is referred to as synthesis gas or
syngas , since it can be used for synthesis of ammonia or methanol.
Biochemical methods use enzymes or genetic engineering methods to transform
biomass into chemical products. For example, fermentation can be used to produce
ethanol from grains with yeast. More interestingly, enzymatic transformations of
biomass have the possibility of transforming wood into edible starch, which would
help solve the world's global food demand (You et al. 2013 ).
Chemical methods use additional substances and solvents to convert biomass
into chemical products. Two methods can be introduced:
(1) water as the solvent with added heat, which is called hydrothermal;
(2) ionic liquids with added heat, which is called solvothermal.
Hydrothermal methods take advantage of the hydrolytic properties of water with
biomass, whereas solvothermal methods take advantage of specific interactions of
the solvent with biomass.
In hydrothermal methods, water is heated (100-400 °C) and mixed with bio-
mass to promote reactive separation usually without the addition of other chemicals.
When water above its critical point ( T c : 374 °C, P c : 22.1 MPa) is used and rapidly
mixed with biomass, a cellulose-rich phase and a lignin-rich phase (Fig. 5.9 .) is
formed (Arai et.al. 2009 ) or if heat is applied in steps then the biomass can be
fractionated (Sasaki 2003 ). Thus, hydrothermal treatment of biomass can provide
a method to convert biomass to chemicals or as a pretreatment step in a chemical
process.