190
Phytotechnology with Biomass Production
(250°C–350°C), cellulose (325°C–400°C), and lignin (300°C–550°C) (Isahak
et al., 2012; Kan et al., 2016). After fast pyrolysis, bio-oil, syngas, and biochar typically account for 60%–70%, 13%–25%, and 12%–15%, respectively
(Isahak et al., 2012).
The yield and quality of bio-oil and syngas during pyrolysis or gasification
can be determined by parent biomass (organic composition, inorganic impurities, harvesting time, senescence time, and genotype) and operational (pretreatment, temperature, size reduction, feed rate, gas flow rate, and catalytic type)
factors (Banks et al., 2014; Bok et al., 2013; Dickerson & Soria, 2013; Greenhalf
et al., 2013; Heo et al., 2010; Kim et al., 2016; Kim et al., 2014; Melligan et al., 2011;
Mos et al., 2013; Yorgun & Şimşek, 2008, 2003). Although biomass inorganics
remain predominantly in the biochar, the fraction ejected as well as fine biochar particles entrained can have drastic impacts on the bio-oil properties and
product yields (Liu et al., 2017). Inorganic impurities create specific challenges
ranging from corrosion and fouling of surfaces to rapid and permanent deactivation of catalysts (Liu et al., 2017). Moreover, the chemical interaction between
hemicellulose and lignin induces the generation of lignin-derived phenols,
limiting hydrocarbon formation (Wang et al., 2011), and the cross-link between
lignin and cellulose has a negative influence on the formation and distribution
of pyrolysis products (Hosoya et al., 2007).
In order to make the biomass amenable, mechanical refining, thermal
(torrefaction, steam explosion, ultrasound, and microwave irradiation),
chemical (acid, alkali, and ionic liquid), and biological (microbial consortium and enzymes) pretreatments have been widely proposed to enhance
biomass conversion efficiency and quality of bio-oil and syngas (Kan et al.,
2016; Wang et al., 2017). However, these are not reviewed in this chapter
due to the limited available studies. Regardless of which pretreatment is
used, the intrinsic mechanism is mainly to (i) modify structural characteristics and alter chemical composition by decomposing hemicellulose and
disrupting lignin; (ii) increase energy density; (iii) eliminate mineral substance (ash content). From the intrinsic mechanism, CO 2 and CO mainly
derive from the degradation and recombination of carbonyl (C=O) and
carboxyl (COO) groups (Qu et al., 2011), while CH 4 is primarily ascribed
to methoxyl (–O–CH 3 ) and methylene (–CH 2 –) groups and H 2 results from
aromatic C=C and C–H groups (Liu et al., 2008). Therefore, changes in cellulose, hemicellulose, and lignin content of biomass would cause elemental variation, thus influencing the final composition and quality of bio-oil
and syngas during pyrolysis.
The application of bio-oil obtained from biomass pyrolysis as candidate
combustion fuels for electricity and heat production has been extensively
investigated (Kan et al., 2016). However, due to the poor quality (weak volatility, high viscosity, and high corrosiveness), its long-term operation in a
diesel test engine is still unfeasible (Bridgwater, 1999). Therefore, further biorefining and upgrading of bio-oil using catalytic cracking technologies, highpressure hydroprocessing, steam reforming, and gasification are needed to