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).