THERMOCHEMICAL CONVERSION: AN INTRODUCTION TO FAST PYROLYSIS - Biomass as a Sustainable Energy Source for the Future - page 373

by GC-MS/flame ionization detector (FID) analysis. The sugars present in biomass
originate from the hemicellulose and cellulose and are in the dehydrated form (anhy-
dro). Besides the monosugars, also, large sugars (oligomers) are present in the oil. The
water-insoluble fraction, often referred to as pyrolytic lignin, is the solid residue
obtained after diluting the oil with cold water. It is generally accepted that this residue
exists mainly of large lignin-derived molecules and is therefore rich in aromatics.
A relatively large fraction of the organics in the oil cannot be identified with current
analytical techniques. This fraction is soluble in water and has a relatively high boiling
point. It is argued that this fraction consists mainly of cross-linked oligomer sugars
(see, e.g., Kersten and Garcia-Perez, 2013).
Analysis of identical oil samples by different laboratories resulted in significant
differences in the oil compositions (see Oasmaa et al., 2005 and Elliott et al.,
2012); therefore, careful interpretation of the absolute values reported in the literature
is required.
Since the chemical composition of pyrolysis oil varies significantly compared to
that of fossil fuels, the properties differ from those of standard fuels. The most impor-
tant differences are mentioned hereafter. The high oxygen and water contents result in
a heating value of 14

kg -1 , which is only

40% of the heating value of petro-
leum-derived fuels. Pyrolysis oil contains substantial amounts of organic acids, such
as formic and acetic acid, resulting in a pH of 2

- 19 MJ

3. This low pH causes corrosion to
common construction materials used in, e.g., engines and turbines. The compounds in
pyrolysis oil contain reactive functional groups, which react further via polymeriza-
tion reactions producing water, heavier compounds, and eventually char. This phe-
nomenon occurs already at low temperatures (e.g., at room temperature during
storage, called aging) or more rapidly when heated (e.g., excessive coke formation
during distillation, referred to as coking). Typical pyrolysis oil is a single-phase mix-
ture, but at high water content (

-

> 35 wt%), phase separation of the oil can occur
affecting further processing of the oil. Phase separation of a single-phase oil can also
occur due to aging during storage.
The earlier reported adverse properties of pyrolysis oil negatively influence the
introduction of pyrolysis oil as a substitute for fossil fuels. Oil improvements can
be achieved through actions before, during, or after the pyrolysis step. At present,
research is focusing on oil quality improvement. It seems that the conditions for opti-
mal oil quality differ from those for maximum oil yield. Obviously, the required oil
quality depends on the targeted applications. Oil improvements might be lowering the
acid content to reduce the burden in any downstream processing, increasing the con-
tent of fermentable sugars for biotechnology applications, and lowering the oxygen
content and stability for corefining in crude oil refineries. These applications will
be discussed later in this chapter.
More details on the analysis of the bio-oil can be found in Oasmaa and Meier
(2005), Elliott et al. (2012), and Garcia-Perez et al. (2007). Concerning the bio-oil
properties and its applications, more can be read in the works of Czernik and
Bridgwater (2004), Elliott (2007), Jarboe et al. (2011), de Miguel Mercader
et al. (2010), Oasmaa et al. (2005), Venderbosch et al. (2010), and Westerhof
et al. (2010).