Glycerol is a material which has numerous uses mainly because
of its physical and chemical properties. Table 1 shows typical the
elemental analysis results of crude glycerol produced in biodiesel
industries, indicating that C, H, and O are the main elemental
contents of this material [10]. Glycerol is a good renewable energy
source for various applications which might be due to its high
carbon content (52.8%). Furthermore, its high oxygen content
(36.2%) to indicate that it is a valuable compound [
Glycerol can be classified into three main categories: crude,
purified/refined, and commercially synthesized. Table 2 shows the
major differences between these three types of glycerol from
biodiesel industries. This table shows that the differences between
purified and commercial glycerol are insignificant, while considerable
differences can be observed between crude and purified
glycerol. Actually, purified or refined glycerol is often prepared
with qualities nearly equivalent to that of commercially synthesized
glycerol because of its applications in sensitive fields, such as
medicine, food, and cosmetics. Furthermore, Table 2 shows that
Fig. 1. Glycerol as a by-product of the methanolysis of vegetable oils.
Table 1
Typical elemental analysis results of crude glycerol
from biodiesel industries [10].
Element Weight %
Carbon (C) 52.8
Hydrogen (H) 11.1
Nitrogen (N) o0.0001
Sulfur (S) –
Balance oxygen (O) 36.2
Table 2
Differences between types of glycerol [12].
Parameter Crude
glycerol
Purified
glycerol
Refined/commercial
glycerol
Glycerol content (%) 60–80 99.1–99.8 99.20–99.98
Moisture content (%) 1.5–6.5 0.11–0.8 0.14–0.29
Ash (%) 1.5–2.5 0.054 o0.002
Soap (%) 3.0–5.0 0.1–0.16 0.04–0.07
Acidity (pH) 0.7–1.3 0.10–0.16 0.04–0.07
Chloride (ppm) ND 1.0 0.6–9.5
Color (APHA) Dark 34–45 1.8–10.3
Fig. 2. Changes in glycerol supply drivers from 1999 to 2009 [14].
328 Z. Gholami et al. / Renewable and Sustainable Energy Reviews 39 (2014) 327–341
crude glycerol has about 60–80% purity, whereas purified or
synthesized glycerol is generally almost 100% pure [12]. Likewise,
ash, soap, and moisture could present at high quantities in crude
glycerol. The acidic value of crude glycerol is slightly higher than
the acidic value of the others. Its color is also darker, which may be
due to the aforementioned attribute, along with some other minor
impurities.