1. Introduction
With the rapid growth of the population and the economy, the worsening
environmental pollution and the growing energy deficit have become a
major concern in the world. The existing petrodiesel is always limited
by its diminishing reserves and environmental hazards [1]. Among the
proposed alternatives, biodiesel for diesel engines has attracted the
increasing attention of scholars over the past decade because of the
advantages of renewability, biodegradability, nonflammability,
nontoxicity, and environmental friendliness [2, 3]. One of the major
technical obstacles that hinders the developments and application of
biodiesel fuels is their poor low-temperature flow properties, which may
cause blockage of the oil pipelines and filter because of the
crystallization of the saturated fatty acid methyl esters with high
melting points in biodiesel fuels [4-7]. Nevertheless, many methods
can be used to mitigate this problem, such as modifying structures [8,
9], winterization [10, 11], adding pour point depressants
[12-14], and blending with diesel [15, 16].
In all these methods, blending biodiesel with fossil diesel is the
simplest, most efficient, and indispensable technique used by
manufacturers and researchers to enhance the low-temperature flow
properties of biodiesel, and maximize the profits from various fuel
products. In China, the coals are the major source of energy fuels, and
occupy approximately 92.4% of the total fossil energy reserves of the
country [17, 18]. Diesel from direct coal liquefaction (DDCL) is a
value-added diesel fuel that is directly transformed from solid coal via
hydrogenation liquefaction reaction under high temperature, high
pressure, and suitable catalyst [15, 18]. In comparison, DDCL has
more excellent low-temperature flow properties than petroleum diesel
(PD), and it is a good substitute for enhancing the low-temperature
performance of biodiesel in coal-rich countries like China [15, 19,
20].
In our previous study [15], 0# PD and DDCL blended together with
the waste cooking oil biodiesel (BWCO), and exhibited positive effects
on improving the pour point (PP), cold filter plugging point (CFPP), and
cloud point (CP) of BWCO. For ternary blends of BWCO-PD-DDCL containing
20 vol.% BWCO and 10 vol.% to 40 vol.% PD, the CFPP was relatively
lower than those of binary biodiesel blends (20 vol.% BWCO).
Nevertheless, the limited petroleum resources and the environmental
pollution always restrict the concoctions of petrodiesel and biodiesel.
Alcohols, such as ethanol (ET) and 1-butanol (BT), are primarily
bio-based renewable energy because they are derived from the
fermentation of renewable biomass [21-23]. Many researchers have
investigated the effect of blending alcohols with biodiesel or diesel on
the cold flow properties because of their relatively lower freezing
points [14, 24]. Hence, replacing the petrodiesel with bio-based
alcohols in ternary blends with biodiesel and DDCL to enhance the
low-temperature flow properties of biodiesel is extremely possible.
However, there is a paucity of technical data in previous reports
regarding the effect of ternary bends of biodiesel with bio-based
alcohols and DDCL on the low-temperature flow properties of BWCO.
Waste cooking oil (WCO) is one of the potential raw materials for
producing biodiesel because of its low-cost, extensive source and
environmental friendliness [25-27]. In this work, BWCO was produced
and compared with ASTM D6751 standard [28] and EN 14214. The
bio-based ET and BT were first introduced into BWCO together with DDCL
to improve the low-temperature flow properties of biodiesel. New data
presenting the blending effect of those three components on the PP, CFPP
and CP of BWCO-ET-DDCL and BWCO-BT-DDCL ternary blends were
comparatively reported by ternary phase diagrams. In particular crystal
morphology and crystallization behavior were explored by using polarized
optical microscope (POM).