Overview of the production of biodiesel from Waste cooking oil

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Abstract

In recent years, biodiesel has attracted significant attention from researchers, governments, and industries as a renewable, biodegradable, and non-toxic fuel. However, several feedstocks have been proven impractical or infeasible because of their extremely high cost due to their usage primarily as food resources. Waste cooking oil (WCO) is considered the most promising biodiesel feedstock despite its drawbacks, such as its high free fatty acid (FFA) and water contents. This review paper provides a comprehensive overview of the pre-treatment and the usage of WCO for the production of biodiesel using several methods, different types of reactors, and various types and amounts of alcohol and catalysts. The most common process in the production of biodiesel is transesterification, and using a methanol–ethanol mixture will combine the advantages of both alcohols in biodiesel production. In addition, this paper highlights the purification and analysis of the produced biodiesel, operating parameters that highly affect the biodiesel yield, and several economic studies. This review suggests that WCO is a promising feedstock in biodiesel production.

Introduction

Biodiesel from renewable resources is one of the most attractive alternative fuels currently being developed because of its low emissions and its desirable chemical characteristics, such as being non-toxic, biodegradable [1], and carbon neutral [2]. Additionally, biodiesel fuel can be used in any conventional diesel engine, unlike non-renewable fuel (petroleum or diesel). Non-renewable fuel also lasts for a limited time and emits pollutants, such as nitrogen, sulphur, and carbon oxides, lead, hydrocarbons, etc. [3].

Cooking oil sources differ across the globe. Their base materials are plant-based lipids, such as corn oil, margarine, coconut oil, palm oil, olive oil, soybean oil, grape seed oil and canola oil, or animal-based lipids, such as butter, ghee, kermanshahi oil and fish oil. In Malaysia, the most common cooking oil is made from oil palm because of its low cost relative to other sources, such as coconut, corn or soybean plants. Biodiesel is produced by the transesterification of these lipids. Previous studies have shown that biodiesel can be produced from various types of vegetable oil, such as sunflower oil, palm oil and soybean oil [4], [5], [6].

However, the use of a food source (edible oil) to produce biodiesel at the expense of the millions of people facing hunger and starvation around the world has received harsh criticism from several non-governmental organisations (NGOs) worldwide [2] due to the resultant increase in the demand for vegetable or edible oil and unnecessary clearing of forests for plantation. Deforestation will disturb animal and plant ecosystems. The use of WCO as a biodiesel feedstock could reduce such problems as water pollution and blockages in water drainage systems, which require extra work to clean. However, there is also a growing concern regarding the environmental impact of an increase in the production of WCO in homes and restaurants [7].

WCO can be freely collected from restaurants and houses using a special “recycle bin” placed in each restaurant or house, which may require public awareness campaigns preceding the collection process. In Malaysia, NGO volunteers conducted an awareness campaign on the environmental impact caused by the direct discharge of WCO into the drainage system. These volunteers inform the community that disposing of WCO via drainage or a landfill could cause water and soil pollution and disturb the aquatic ecosystem in addition to being a human health concern. They also alerted the community to the negative effects of using recycled WCO as cooking media in food preparation. The NGO will establish a collection centre and arrange for the community to appoint a representative to collect the WCO. The WCO will be collected monthly, and payment will be made to the community fund. Finally, the collected WCO will be sent to the diesel manufacturer and factory.

Although state-of-the-art biodiesel production from WCO is less profitable than the use of fossil fuels, research is still on-going to improve the yield and quality of the fuel. Again, because the more fossil fuels are used each year than are produced, WCO is an excellent alternative. Additionally, this practice could prevent the recycling of WCO for cooking, which is being performed by some companies. Cooking oil recycled from WCO is believed to cause cancer because of the toxic contents produced when the oil is oxidised. However, obtaining WCO in large amounts remains a concern. Future studies should compare the methods used by other countries: their pros and cons, implementation, and economic impact. In London, Uptown Oil supplies fresh cooking oil to selected Western restaurants and pubs with a current average return of approximately 60% of the fresh oil. Their supplier has been trained to maximise the quality of the obtained WCO to reduce the impurities and ease the transformation process to biodiesel. The company collects and processes the WCO and produces recycled biodiesel directly in their plant. The biodiesel is then sold on the premises, especially to London black taxis. A strong relationship with local authorities offers direct contact with public facilities and institutions, such as hospitals, airports, schools, and catering services. Uptown also provides an eco-friendly window sticker to restaurants that supply the WCO in recognition of their co-operation [8].

Section snippets

Biodiesel

“Biodiesel” has been defined by the American Society for Testing and Materials (ASTM) as a monoalkyl ester of fatty acids or fatty acid (m)ethyl ester [3] derived from renewable feedstocks, such as vegetable oils. The term “bio” indicates the biological source of biodiesel, in contrast with conventional diesel [9]. Biodiesel is a clear liquid with a light- to dark-yellow colour. It has a boiling point of over 200 °C, a flash point between 145–175 °C, a distillation range of 195–325 °C, and a

Properties of WCO and its feedstock potential

WCO is obtained after using edible vegetable oils [5], such as palm, sunflower, and corn oils, several times for frying. The chemical and physical properties of WCO are slightly different from those of fresh oils because of the changes that occur during frying [18]. Table 2 shows the chemical and physical properties of a WCO sample collected by Wen et al. [19].

The amounts of WCO generated by homes and restaurants are increasing rapidly due to the tremendous growth in human population [7].

Pretreatment of WCO

One of the main drawbacks of using WCO as feedstock in biodiesel production is the presence of unwanted contents, such as FFA, water [2], and other solid impurities [28]. The presence of water in the oil sample often leads to hydrolysis, and a high FFA content leads to saponification. Both reactions result in low biodiesel yield [1] and catalyst consumption [2]. To reduce the high FFA content in the oil, several techniques have been proposed, such as acid esterification with methanol and

Transesterification

The most practical and common way of producing biodiesel is by transesterification (also called methanolysis) [31], which is a catalysed reaction of vegetable oil in the presence of alcohol to yield biodiesel and glycerol [32], [33]. In transesterification, glycerol in triglycerides is replaced with a short-chain alcohol [34]. The process starts with a sequence of three consecutive reversible reactions, wherein triglycerides are converted to diglycerides, diglycerides are converted to

Alcohol type

As indicated in the previous section, alcohol is one of the main reactants in transesterification reactions. Primary and secondary monohydric aliphatic alcohols (with 1–8 carbon atoms) are mainly used in this reaction. However, methyl alcohol (methanol) and ethyl alcohol (ethanol) are also widely used for this purpose [27]. Previous reports have proposed the use of methanol for biodiesel production because of its wide availability, low cost [11], and high reactivity compared with ethanol, which

Reactor type

Transesterification can be carried out discontinuously (batch) or continuously [84]. Batch transesterification usually needs a larger reactor (container) and requires longer reaction and separation time relative to continuous transesterification because all of the reaction stages are performed in the same reactor [45]. Several researchers have reported on the use of various types of reactors for the production of biodiesel from edible and non-edible oils with varying degrees of success.

Downstream processing of biodiesel

The output products of the transesterification reaction are crude ester and glycerol [27]. Crude ester may consist of excess methanol, unreacted oil, catalyst residue, soap, and glycerol [28]. The purification of the final product of transesterification is one of the most important steps in the overall process [30]. The most commonly used methods in separating glycerol from crude ester are gravitational settling, centrifugation [84], and decantation funnelling [7], [27]. Crude ester can then be

Analysis of biodiesel produced from WCO

Biodiesel analysis is performed after purification. Several analytical methods have been practiced in recent years, such as gas chromatography (GC), high-performance liquid chromatography (HPLC), thin-layer chromatography (TLC), and proton nuclear magnetic resonance (1H NMR). GC is most widely used for this purpose. In this method, either helium [2], [30], [92], [93] or nitrogen [28], [33] is used as the carrier gas. Methyl heptadecanoate is also used as an internal standard (reference) [93].

Parametric study

The transesterification reaction depends on several parameters, such as reaction temperature, reaction time, catalyst loading, and alcohol-to-oil ratio [7], [13], [27], [28], [29], [96]. In the following sub-sections, the effects of several parameters on biodiesel yield are studied with the aid of graphs based on previous research.

Cost estimation and feasibility studies

In the past few years, several studies have been carried out to produce biodiesel at a minimum total production cost to compete with conventional diesel [2]. These studies found that the cost of feedstock (raw oil) is more than 70% of the total cost [13], [44]. Thus, research has been intensified to find a substitute to expensive virgin vegetable oils. WCO is one of the most suitable alternatives (Table 11) [97].

Van Kasteren and Nisworo [9] carried out feasibility studies on a biodiesel

Conclusion

Regarding the production of biodiesel from waste cooking oil (WCO), the following important key points have been highlighted:

  • 1.

    Biodiesel has been proven the best substitute to fossil fuel. It is superior to petroleum-based fuels because it is renewable, biodegradable, and non-toxic.

  • 2.

    Several edible oils have been use as WCO feedstock in biodiesel production, such as sunflower oil, palm oil, soybean oil and olive oil.

  • 3.

    Transesterification is more common in the production of biodiesel than other

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