The conundrum of waste cooking oil: Transforming hazard into energy

Highlights

• Disposal of WCO caused deterioration of the environment and health issues.

• WCO has the potential to generate electricity, hydrogen, biodiesel and biogas.

• Anaerobic co-digestion of WCO produces satisfying yield of biogas.

• Utilization of WCO as the substrate for hydrogen gas production shows viability.

• WCO for bioenergy can reduce the reliance on second-generation raw materials.

Abstract

Waste cooking oil (WCO) is considered as one of the hazardous wastes because improper disposal of WCO can cause significant environmental problems such as blockages of drains and sewers as well as water or soil pollution. In this review, the physical and chemical properties of WCO are evaluated along with its regulations and policies in different countries to promote WCO refined biofuels. Blended WCO can be an auxiliary fuel for municipal solid waste incinerators while the heat produced is able to form superheated steam and subsequently generate electricity via combined heat and power system. Also, WCO contains high ratio of hydrogen atoms compared to carbon and oxygen atoms, making it able to be catalytically cracked, synthesizing hydrogen gas. WCO-based biodiesel has been traditionally produced by transesterification in order to substitute petroleum-based diesel which has non-degradability as well as non-renewable features. Hence, the potentials of hazardous WCO as a green alternative energy source for electricity generation, hydrogen gas as well as biofuels production (e.g. biodiesel, biogas, biojet fuel) are critically discussed due to its attractive psychochemical properties as well as its economic feasibility. Challenges of the WCO utilization as a source of energy are also reported while highlighting its future prospects.

Introduction

Currently, global waste generation throughout every year is increasingly worse due to consumerism and the growing disposability of waste. In 2016, global waste generation was up to 2.017 billion tons/year and in 2050, the global waste generated is expected to be increased by 70% which is up to 3.4 billion tons/year (Ellis, 2018; Tiseo, 2020). There is no sign for this trend to slow down due to the factors such as population, economic growth as well as urbanization. For instance, 2.5 kg of waste per citizen per day are produced in the United States and Norway. In Italy, on the other hand, 89,000 tons of waste are produced in the urban area alone (César et al., 2017). Waste cooking oil (WCO), or so called “gutter oil”, generally, can be defined as kitchen waste, gutters and animal fat as well as the recycled cooking oil or the practice of reusing the cooking oil which has been fried before for some of the restaurants (Ye and Pei, 2012). WCO is considered as one of the hazards to the environment and human health if dispose improperly. Typically, WCO is collected from households and restaurants which is required to be processed and utilized in various segments of production in order to reduce the environmental impacts due to the disposal of waste oil. Thus, the utilization of the recycled WCO or even other waste materials can be beneficial for the future energy supply due to its high heating value and high energy potential (Ortner et al., 2016).

However, up to date, the waste recovery is still low as there is a lack of regulations and laws regarding proper management of waste oils for many countries, besides unconscious social action. In fact, César et al. (2017) reported that, only 2% of the waste material was recycled in Brazil, which in reality there were 40% of urban waste made up of recyclable materials (César et al., 2017). Tsai et al. (2007) also pointed out that the total energy supply in Taiwan from 1990 to 2003 increased from 58.57 million kiloliters of oil equivalent (KLOE) to 121.22 KLOE (Ministry of Economic Affairs, 2004; Tsai et al., 2007). From the waste-to-energy perspective, WCO is the best choice of being the feedstock for energy generation in the future as the demand of vegetable oil is increased in order to cater for the large increment of inhabitants every year (Ortner et al., 2016). As a result, subsidies and tax cuts as well as low interest funding should be initiated to promote the collection of WCOs which originate from hospitality and industrial sectors in order to increase the waste recovery instead of elimination.

Regarding utilization of the WCO, the most famous and generally accepted application is the production of fatty acid methyl ester (FAME) which is usually referred to biodiesel (Sadaf et al., 2018). This is due to the ability of biodiesel to be blended with diesel fuel to produce different characteristics of biodiesel and applied to any engine without modification (Nisar et al., 2018). For instance, Costa Neto et al. (2000) found that the biodiesel produced has a higher viscosity and density than diesel oil and this difference is reduced to B20 mixture (blending mixture of 20% biodiesel with 80% diesel) (Costa Neto et al., 2000). In addition, WCO can become an alternative source of energy applied in external combustion steam engine due to its high heating value for generation of electricity (Wang, 2002). Furthermore, hydrogen gas can also be produced from WCO via gasification and steam reforming, as WCO contains large fraction of hydrogen atoms (Gardy et al., 2018). Ortner et al. (2016) had examined three different utilization paths of WCO from households which are conversion of WCO to biodiesel, direct combustion in a cogeneration plant and production of biogas within an agricultural biogas plant. Remarkably, it was found that all three strategies contribute to a saving of greenhouse gases (GHG) emissions when assessing their environmental performance (Ortner et al., 2016).

This review critically evaluates the valorization of WCO as a potential substitute source of energy to generate electricity, hydrogen gas and biofuel. Various methods of generating electricity, synthesizing hydrogen gas as well as biofuel from WCO are analyzed and summarized accordingly. Also, physical and chemical properties of WCO as well as regulations of WCO in different countries are presented. Comparison between WCO and different feedstock particularly for hydrogen and biodiesel production are presented in this review. Also, challenges and future prospects have been highlighted to present the way forward for this old problem at the global level.