The price of fossil Diesel is soaring in these two years,
and it will be exhausted some day. Thus, looking for an
alterative way to develop a substitute for Diesel (biodiesel)
is an imperious task for humans. Biodiesel, defined as ‘‘a
substitute for, or an additive to Diesel fuel that is derived
from the oils and fats of plants and animals’’ [1] or monoalkyl
esters of long chain fatty acids derived from a renewable
lipid feedstock, such as vegetable oil or animal fat
(ASTM), is becoming popular in the markets of developing
countries as well as developed ones. The European Union
has set an objective to secure a market share for motor biofuels
of 20% of the total motor fuel consumption by 2020.
Fossil Diesel blended with 20% of biodiesel produced from
soybean oil is available in the US market now [2]. Developing
renewable energy is the national strategy of China,
which does not have plentiful fossil oil deposits. This biggest
developing country with rapid economic growth needs
more energy than before [3].
The main advantages of using this alternative fuel are its
renewability, better quality of exhaust gas emissions, its
biodegradability and, given that all the organic carbon
present is photosynthetic in origin, it does not contribute
to a net rise in the level of carbon dioxide in the atmosphere
and, consequently, to the greenhouse effect [4].
The most common way to produce biodiesel is by
transesterification, which refers to a catalyzed chemical
reaction involving vegetable oil and an alcohol to yield
fatty acid alkyl esters (i.e., biodiesel) and glycerol. Methanol
is the most commonly used alcohol because of its low
cost [1,5].
0196-8904/$ - see front matter _ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.enconman.2006.04.016
* Corresponding author. Tel.: +86 20 85226635; fax: +86 20 85226630.
E-mail address: twyong@jnu.edu.cn (Y. Wang).
www.elsevier.com/locate/enconman
Energy Conversion and Management 48 (2007) 184–188
From the viewpoint of chemical reaction, refined vegetable
oil is the best starting material to produce biodiesel
because the conversion of pure TG to FAME (fatty acid
methyl ester) is high, and the reaction time is relatively
short. Nevertheless, in China, the largest developing country
with a population of over 1.3 billion, the limited refined
edible oil must meet the need of consumers first. Waste
cooking oil (WCO), if no suitable treatment is available,
would be discharged and cause environmental pollution,
but now, WCO is collected by the environmental protection
agency authorized by the local government in the main
cities of China. It is estimated that the WCO collected in
Guangzhou, the third largest city in China, is over 20 thousand
tons every year. This collected material is a good commercial
choice to produce biodiesel due to its low cost.
Biodiesel produced by transesterification reactions can
be alkali catalyzed, acid catalyzed or enzyme catalyzed,
but the first two types have received more attention because
of the short reaction times and low cost compared with the
third one. Some studies show that the lipase is very sensitive
to methanol in an enzyme catalyzed process, so the
methanol in the reaction system should be kept at quite
low concentration to ensure the activity of the enzyme
[6–8]. It took 34 h to convert 97.3% of the TG of refined
vegetable oil to FAME in a two step, batch, enzyme catalyzed
process. When waste cooking oil was used as the
reactant, the conversion of the oil was only 90.4% by a
three step catalyzed process in 48 h [6].
Chemically catalyzed processes, including alkali catalyzed
and acid catalyzed ones, have proved to be more
practical nowadays. An alkali catalyzed process can
achieve high purity and yield of biodiesel product in a short
time (30–60 min) [9–11]; however, it is very sensitive to the
purity of the reactants. Only well refined vegetable oil with
less than 0.5 wt% free fatty acid (FFA) can be used as the
reactant in this process [12]. The high cost of raw material
is the major obstacle to its commercialization. When waste
cooking oil (WCO) with more than 10 wt% FFA is used, an
acid catalyzed process is preferred, but it requires more
excess of methanol, high pressure (170–180 kPa) and high
cost stainless steel equipment. In addition, the yield of
product is low (82% of mass conversion with 200% excess
of ethanol) when the most common sulfuric acid is used
[12–14].
Hence, a combined process with acid catalyzed pretreatment
was developed to synthesize biodiesel from waste
cooking oil. The first step is to esterify the FFA with methanol
by acid catalysis. When the FFA content is lower than
0.5 wt%, the sulfuric acid is drained, and the solid alkali is
introduced into the system to complete the transesterification
[12,15]. However, the drawbacks of this combined process
include: effluent of sulfuric acid, no recovery of
catalyst and high cost of reaction equipment [16].
To overcome the disadvantages of the acid catalyzed
process or pretreatment, the homogeneous Lewis acid catalyst
(carboxylic salts) was used to synthesize biodiesel
from waste cooking oil. However, the reaction temperature
was high (T = 200 _C), but the conversion was relatively
low (90%) [17].
In this work, a new two step catalysis process was first
adopted for the production of biodiesel (Fig. 1). At the first
step, ferric sulfate was introduced to catalyze the esterification
reaction in which the FFA in the waste cooking oil
reacted with methanol. The ferric sulfate that was insoluble
in the oil was centrifuged from the liquid after the methanol
recovery and could be reused in the next term. At the
second step, potassium hydroxide was added to catalyze
the transesterification reaction in which the triglyceride
(TG) reacted with methanol. Without waste water, reusing
catalysis and low cost reaction tank, this two step catalysis
process exhibits potential application in the biodiesel fuel
industry.