Experimental Investigation of Physicochemical Properties of the Produced Biodiesel from Waste Frying Oil and Its Blend with Diesel Fuel

Abstract

The imperative of utilising alternative fuels for the operation of internal combustion engines stems from the requirements to reduce the emissions of greenhouse gases and other contaminants, the substantial demand for fuels, and the diminishing reserves of natural resources. The global inclination towards sustainable development necessitates the employment of biofuels as a substitute for fossil fuels. Nonetheless, the expenditures on raw materials for the manufacture of biodiesel remain substantial, thus underlining the importance of exploring solutions for reducing them. An instance of this could be the utilisation of plant and animal by-products, such as used frying oils and slaughterhouse waste, as feedstock for biodiesel production. Not only will this facilitate the creation of less costly biofuel, but it will also provide an effective solution for the management of post-production waste. The objective of the research delineated in this paper was to ascertain select physicochemical attributes of second-generation biodiesel, derived from spent frying oil, as well as mixtures of this biodiesel with diesel and biodiesel concentrations of 10, 20, and 30% (v/v). The biodiesel produced is the waste frying oil methyl esters WFOME. The proprietary GW-201 reactor was employed in the production of biodiesel. For WFOME biodiesel, DF diesel, and their blends—B10, B20, and B30—properties that influence the formation process of the combustible mixture, autoignition, and combustion of fuel–air mixtures in self-ignition engines were determined. The conducted research has established that “B” type fuels prepared from WFOME and DF present a viable alternative to fossil fuels. Pure biodiesel exhibited a marginally reduced lower heating value, however, in the case of fuel mixtures comprising up to 30% (v/v) biodiesel and diesel, the lower heating values approximated that of diesel. An elevated cetane number alongside an increased flash point of pure B100 biodiesel have been noted. The values of cetane number for WFOME and DF mixtures were found to be either comparable or marginally higher than those of pure DF diesel fuel.

1. Introduction

The imperative to seek out alternative fuels for powering internal combustion engines arises from the need to mitigate the detrimental environmental impact of transport, the increasing demand for fuel, and the depletion of primary energy sources. The production and application of bio-components as energy sources is not a novel idea, and there has been a clearly visible surge in interest towards this approach of utilising agricultural resources for a considerable number of years. The primary agricultural raw materials used for biofuel production are oil plants as well as those that contain sugars or starches. Biofuels represent a category of fuels that are derived from biomass, intended for the operation of internal combustion engines. Currently, around 60% of ethanol is produced from maize, 23% from sugar cane, 7% from molasses, 3% from wheat, and the rest from other crops [1]. Approximately 70% of biodiesel is obtained from vegetable oils, out of which 14% comes from rapeseed oil, 23% from soybean oil, and 29% from palm oil. About 25% of biodiesel comes from used vegetable oils [1]. Biofuels derived from the treatment of agricultural crop residues or timber do not constitute a significant proportion of the overall biofuel production. Esters of fatty acids derived from rapeseed and sunflower oil are predominantly utilised in Europe, soybean oil in the United States and South America, and palm oil in Southeast Asia [2]. In contrast, the primary raw materials for ethanol production in Europe are cereals (wheat, maize) and sugar beet, in the United States corn, and in South America sugar cane [2]. Presently, the leading producer of biodiesel across the globe is the European Union, contributing 32.2%, followed by the United States with a share of 18.3%, and Indonesia at 17.6% [1]. In Poland, rapeseed oil is the main raw material for biodiesel production. Currently, increasing attention is being paid to the use of used vegetable oils to produce biodiesel, then classed as a second-generation biofuel. Policy has a significant influence on the international biofuel sector. Its main goals are: support measures for farmers, greenhouse gas emission reduction, and/or enhancement of energy independence. As projected by the International Energy Agency, global production of biofuels will continue to be predominantly influenced by traditional raw materials such as vegetable oils for biodiesel production, and sugar cane and corn for ethanol production [1]. Biodiesel, derived from utilised culinary oil, will assume a pivotal function in the European Union, Canada, the United States, and China. For this reason, in this work it was decided to present the possibility of producing biodiesel from waste frying oil and to test the properties of the resulting fuel.

In contemporary society, the quest for alternative energy sources has become indispensable, with biodiesel being perceived as a potential successor to diesel [3]. Biodiesel is sustainable, non-toxic, generates reduced quantities of detrimental exhaust constituents, and represents a more environmentally friendly substitute for fossil diesel [4,5]. The disadvantage of using biodiesel to power compression–ignition piston engines is the increase in nitrogen oxide emissions. Systems that limit their emissions are still necessary. Potential raw materials for biodiesel production encompass a variety of renewable sources, including vegetable oils, animal fats, waste cooking oils, and algae [6]. Biofuels procured from inedible vegetable oils and residual organic materials are referred to as second-generation biofuels. They are manufactured from inedible plants like jatropha, babassu, neem, tobacco, and so on [7,8,9,10,11]. Non-edible plants do not directly compete with crops cultivated for consumption. Nevertheless, they might contribute to a reduction in the surface area dedicated to edible plant cultivation, and thus, they might influence the formation of food prices. The European Union is therefore paying particular attention to ensuring that the increase in biodiesel production is not linked to deforestation. More attention has been paid to the use of waste organic matter for the production of biofuels including biodiesel. Therefore, the aim of this work is to demonstrate the possibility of producing biodiesel from waste frying oils, using the GW-201 reactor, patented by one of the co-authors. Furthermore, biodiesel produced from biologically derived waste, such as plant-based organic material and animal-derived waste, is also classified within second-generation biofuels. The distinguishing characteristic of this waste is its low procurement cost coupled with the mitigation of issues related to waste disposal. Hence, they can be utilised for the production of renewable fuel.

Legal regulations in Poland impose on entrepreneurs the obligation to properly dispose of waste. The paper proposes a method of using used frying oils, i.e., a waste management method. In order to increase the amount of biodiesel produced from waste oils, a collection system for this type of waste should be built. Incentives should be introduced for the collection of waste fats, especially in households. Such a system exists for business entities in Poland. By law, they are obliged to manage the waste they produce in such a way that it does not pose a threat to the environment. In Poland, there are specialized companies that not only offer special containers for collecting and storing oil, but also collect it themselves. Producing fuel from waste vegetable oils or waste animal fats is a method of removing the problem of waste management. This is especially true in companies where this type of waste is generated in large quantities. The main problem of the Polish waste oil management system is the lack of a sufficiently developed system for collecting waste oils from households. There are no legal regulations in this area. Recently, however, a project has appeared on the market that allows for the transfer of frying oils from households to special containers located in some regions of Poland for further processing. In comparison, Belgium collects 64% of its waste from used cooking oils (UCO) from households and almost all from the professional sector, leading to a collection efficiency of over 80% [12]. Around 98% of the UCO collected is processed into biodiesel. Unlike Belgium, where the collection of UCO from households is regulated by law, collection in the Netherlands is a voluntary initiative. It is organised by an association that coordinates several partners from the entire supply chain: collectors, biodiesel producers, local authorities, and wastewater treatment companies. The collected UCO is used to produce biodiesel.

According to the National Center for Agricultural Support, Poland in 2023 produced: 339,390 tonnes of bioethanol, 971,170 thousand tonnes of methyl esters and 5.33 thousand tonnes of liquid biohydrocarbons [13]. The production of esters mainly used vegetable oils (90% of the total raw materials used); used cooking and frying oils (9%); and animal fats, fatty acids, and synthesized oils (1%). The main feedstocks used for bioethanol production were corn (59% of the total raw materials used) and waste starch slurry (30%), starch production residues from wheat processing (5%), waste and beet molasses (3%), distillates from beet molasses, corn, waste and residues (2%), and post-rectification (1%). Liquid biocarbons were produced from vegetable oil (58% of the total raw materials used) and alcohols and waste distillates (42%).

The main objective of the investigations delineated in this paper was to ascertain specified physicochemical attributes of second-generation WFOME biodiesel procured from recycled frying oil and mixtures of WFOME biodiesel with diesel oil. The characteristics of the investigated fuels influencing the formation, auto-ignition, and burning of fuel-air mixtures in spontaneous ignition engines were identified. In addition, the aim of the work was also to produce biodiesel itself from waste frying oils using the GW-201 reactor. This reactor can be used for biodiesel production in small-scale local applications.

2. State of the Knowledge Review of Biodiesels Produced from Waste Oils

Research on the operating parameters of self-ignition engines powered by vegetable oil esters or their mixtures with diesel fuel have been the focus of many studies [14,15,16,17,18,19]. Rajam et al. [20] examined the impact of incorporating biodiesel derived from waste cooking oil into diesel fuel at a proportion of 10–40%. The escalation of biodiesel content within the diesel blend instigated an augmentation in both density and viscosity, contrastingly, there was a decrement in the lower heating value and cetane number in correlation to diesel.

In the research conducted by Zhang et al. [21] utilised commercial diesel fuel and mixtures of diesel with biodiesel derived from spent B10 and B20 cooking oil. The test results showed that augmenting the proportion of biodiesel in the diesel-biodiesel mixture led to increased cetane number, viscosity and density values, whilst the lower heating value underwent a reduction compared to diesel. There was also a rise in the cetane number, contrary to the findings in paper [20].

Kukana et al. [22] manufactured biodiesel from mixtures of waste edible oil (WCO) with oil derived from Ambadi seeds (AO). Biodiesels derived from the mixtures of W50A50 (50% WCO and 50% AO) and W75A25 (75% WCO and 25% AO) oils exhibited lower viscosities and heating values in contrast with biodiesel produced from waste oil. Furthermore, it has been demonstrated that the prepared biodiesel–diesel blends in various ratios indicate an increase in the biodiesel content in the mixture with diesel fuel leads to a rise in density and a reduction in lower heating value. Furthermore, a minor decrement in the lower heating value was noted for the biodiesel mixtures derived from WCO + AO oils and diesel fuel.

Thompson W. et al. [23] demonstrated that the influence of employing plant-derived raw materials for the production of second-generation biofuels on the food prices is not unequivocal and apparent. They do not subscribe to the perspective that second-generation biofuels are occasionally portrayed as a universal solution for all the issues related to producing food and fuel from plants. The cultivation of inedible plants may vie for soil with plants cultivated for consumption purposes. This may influence the increase in food prices. However, if waste from edible crop cultivation is utilised in the production of second-generation biofuels, it will not influence an increase in food prices. Contrarily, it may even influence a decrease in prices if the cultivation area of edible plants is expanded, with the aim of acquiring waste as raw material for fuel production, for instance, straw.

The work of Man X.J. et al. [24] conducted a study on the impact of biodiesel derived from waste cooking oil on the emissions of hydrocarbons, carbon monoxide, nitrogen oxides and particulate mass concentrations, as well as unregulated emissions, inclusive of two carbonyl compounds (formaldehyde and acetaldehyde), three unsaturated hydrocarbons (1,3-butadiene, propene and ethene), and three aromatic compounds (benzene, toluene and xylene). The findings indicate that with the escalation of biodiesel composition in diesel, there is a decrease in HC, CO, and solid particle mass concentrations yet an augmentation in NO_x. In the case of unregulated emissions, the emissions of formaldehyde and acetaldehyde augment with an enhancement in biodiesel content. An identical pattern can be observed for 1,3-butadiene, propene, and ethane. In the context of aromatic compound emissions, incorporating biodiesel results in a surge in benzene emissions; however, emissions of toluene and xylene diminish. The findings also indicate that the total emissions are influenced by the engine’s operating conditions, particularly its load.

Kurczyński et al. [8] produced esters in residual animal fats derived from the industry engaged in the preparation of animal hides for footwear and clothing production. Utilising biodiesel as a fuel source for the engine resulted in a decrease in the smoke opacity, in addition to a reduction in the concentration of carbohydrates, particulates, and carbon monoxide. The concentrations of carbon dioxide were analogous when it comes to biodiesel and diesel fuel. Simultaneously, a marginal increase in the concentration of nitrogen oxides and fuel usage was observed when utilising biodiesel.

The impact of biodiesel mixtures from waste cooking oil (B0, B10, B20, B30) on the combustion process and emission of harmful exhaust components of a compression ignition engine with a common rail supply system was examined by Meng et al. [25]. The findings of the study indicated that the incorporation of biodiesel derived from utilised culinary oil prolongs the duration of the combustion process. This causes an increase in NO_x and acetaldehyde emissions. The proportion of biodiesel in the fuel concoction had no discernible impact on the emission of particulate matter.

The work of C. Adhikesavan et al. [26] examined the impact of biodiesel derived from waste cooking oil (WCO) on the physicochemical properties of biodiesel and the emission of exhaust gases from a self-ignition engine. Furthermore, biodiesels derived from fresh sunflower and palm oils were utilised for comparative purposes. The findings of the research indicated that the overall content of polar substances in waste cooking oil significantly affects the kinematic viscosity of biodiesel. An engine powered by biodiesel derived from waste cooking oil (WCO) exhibited a slightly elevated carbon monoxide emission compared to biodiesel produced from virgin oils. Conversely, the emissions of nitrogen oxide and the smoke opacity from an engine powered by biodiesel derived from both used cooking oils and fresh oils exhibited comparable levels. The researchers did not observe any substantial disparity in the engine’s operating parameters during its examination and when powered by biodiesel generated from virgin oils versus biodiesel produced from used kitchen oils. As per their findings, the application of biodiesel obtained from waste cooking oil as a biofuel didn’t adversely affect the motor efficiency and exhaust emissions.

Balasubramanian et al. [27] examined the influence of WCO-based biodiesel and its combinations with diesel on the performance metrics of compression-ignition engines. The researchers observed a reduction in the thermal efficiency of the engine (BTE) fuelled by biodiesel compared to that powered by diesel oil. An engine fuelled by biodiesel produced from waste cooking oil and its combination with diesel characterised itself with higher CO₂ and NO_x emissions than diesel. Conversely, the researchers noted a decrease in CO and HC emissions from biofuel and its mixtures with DF in comparison to DF pure. The smoke opacity was more excellent for diesel fuel compared to biodiesel and its mixtures with diesel fuel.

Liu et. al. [28] conducted a study on the influence of biodiesel derived from waste cooking oil and its combinations with diesel fuel on the functioning of a 4-cylinder self-ignition engine at varying crankshaft velocities. The research demonstrated an augmentation in NO_x emissions when utilising biodiesel derived from waste cooking oil and its combinations with DF as opposed to diesel across all velocities. The minimal CO concentration was noted for B20 across all rotational speeds of the engine crankshaft. B50 was characterised by a higher CO₂ concentration compared with other fuels at all engine rotational speeds, except at 2000 rpm, where B20 achieved the peak CO₂ concentration. The fuels B100, B80, and DF demonstrated higher HC emissions compared to other fuels.

Air pollution has been acknowledged as a global issue. The emission of particulate matter, predominantly derived from the combustion of fuels in compression-ignition engines within the transport sector, is progressively being curtailed. The work of Sathaporn Chuepeng et al. [29] utilised biodiesel extracted from waste cooking oil (WCOME) and waste chicken oil (CKOME) for the comparative analysis of the combustion process, engine efficacy, and exhaust emissions. A biodiesel derived from palm oil (POME) has also been formulated, serving as a reference point. Operating the engine on WCOME biodiesel resulted in a decrease in CO, HC, and smoke emissions when compared to biodiesel derived from POME. Powering the engine with biodiesel obtained from CKOME resulted in similar CO, HC and exhaust smoke emissions, and lower NO_x emissions compared to biodiesel derived from POME palm oil. Hence, biodiesel obtained from waste cooking oil and residual chicken oil may be viewed as potential alternatives to supplant biodiesel derived from palm oil, serving as the principal raw material for biodiesel production in Thailand, devoid of any penalties pertaining to emission.

In the work of Gad M.S. et al. [30] biodiesel was derived from waste cooking oil and blended with nanoparticles: TiO₂, Al₂O₃, and CNTs at different concentrations 25, 50 i 100 mg/L. The incorporation of nanoparticles into the biodiesel-diesel amalgamation induced a minor escalation in both the flash point and cetane number relative to standard diesel. The lower heating value of the biodiesel-diesel blend with diesel and nano-additives also increased. However, the values achieved were lower to the lower heating value of diesel oil.

Sharma V. et al. [31] investigated the properties of biodiesel from waste cooking oil mixed with the addition of multilayer graphene, graphite nanoparticles and also butanol. The inclusion of nanoparticles marginally escalated the viscosity of the fuel relative to pure biodiesel. For instance, the viscosity of a fuel mixture: 40% biodiesel from used oil, 40% diesel, 20% butanol, and 100 ppm graphene was higher than the blended fuel: 40% bodiel, 40% diesel, 20% butanol. Biodiesel derived from used oil (B100) obtained the highest flash point, registering at 165 °C. The incorporation of butanol reduced flash point the entirety of the fuel mixtures. Moreover, it was discerned that alterations in the percentage composition of graphene and graphite additive marginally influenced the flash point of the concocted mixtures. Nevertheless, as per the authors, the fluctuations in the flash point primarily stem from the incorporation of butanol. The incorporation of nano-additives into fuel mixtures has resulted in an enhancement of their lower heating value. For instance, the lower heating value for a fuel comprised of: The composition of 40% biodiesel, 40% diesel oil, 20% butanol, and 100 ppm graphene exhibited a higher value compared to the mixed fuel alternative: 40% biodiesel, 40% diesel oil, 20% butanol, and 50 ppm graphene. Similar results were achieved when using a fuel blend: 40% of biodiesel, 40% of diesel oil, 20% of butanol, and 100 ppm of graphite, for which the lower heating value surpassed that of the mixture: 40% biodiesel, 40% diesel oil, 20% butanol, and 50 ppm graphite.

The European Union’s biofuel policy is causing an increasing interest in the production of biodiesel from waste organic matter. This means that new possibilities and solutions in this area will be sought in the near future. In this work, biodiesel was produced from used frying oil, using technology and a reactor patented by one of the co-authors. The produced biodiesel meets the normative requirements specified in the PN-EN 14214+A2:2019-05 standard, applicable in Europe.

3. Materials and Methods

3.1. Production of Biodiesel from Waste Frying Oil

The biofuel employed was biodiesel WFOME from our own production. The raw material for second-generation biodiesel was post-frying vegetable oil. The biodiesel was manufactured utilising our exclusive patented technology (P.218554) and a unique design of the GW-201 reactor, depicted in Figure 1. The fundamental technical specifications of this reactor are cited in

Table 1. Technical data of the GW-200 reactor.

Measured Parameter	Unit	Value
Supply voltage	V	230
Rated power	kWh	5.15
Production time per cycle	h/cycle	1.5
Efficiency per cycle	dm3/cycle	50
Type of catalyst	-	Basic/acidic
Process	-	Periodic or semi-continuous
Type of process	-	Single-stage or two-stage

. The studies used VERVA diesel sourced from an ORLEN fuel station as the benchmark fuel. Blends categorised as B10, B20, and B30, incorporating 10, 20, and 30% (v/v) WFOME into diesel fuel, were also formulated. Qualitative research on WFOME biodiesel, DF diesel oil, and “B” type fuels was conducted according to PN-EN 590:2022 [32] and PN-EN 14214+A2:2019-05 [33] standards. Research samples were manually collected following the stipulations of EN ISO 3170 [34].

The transesterification process was used to produce biodiesel from used frying oil, which involves the reaction of an alcohol—in this case, methanol—with triglycerides found in vegetable oils or animal fats in the presence of a catalyst, particularly potassium hydroxide. In the process of transesterification, higher fatty acid esters (biodiesel) are obtained as the principal product, with glycerine being the resultant by-product. In this investigation, considering the model and the composition of the fatty acids present in the fat, a proportion was employed for biodiesel production, wherein for every 10 kg of oil, a catalytic blend of 84 g KOH dissolved in 1.28 dm³ of methyl alcohol CH₃OH was utilised. The catalytic blend was synthesised at a station furnished with an IKA C-MAG HS 7 magnetic stirrer. The interaction of methyl alcohol with KOH yields a transitional compound (i.e., potassium methanolate CH₃OK), which serves as a catalytic mixture in the transesterification process. The strategies implemented and the optimisation of the process facilitated the production of biodiesel of exceedingly high purity within a span of 90 min. The production process of WFOME was conducted at a temperature of 60 °C. The calibre of the biodiesel discharged directly from the reactor conforms to the stipulations of EN 14214+A2:2019-05 [33] concerning the proportion of esters: that is, a minimum of 96.5% (w/w). Upon completion of the process, the glycerine fraction as well as the remnants of the catalyst and alcohol were eliminated from the reactor. Upon completion of the pertinent biodiesel production stage, the residual alcohol was evaporated by heating the ester mixture to temperatures surpassing the boiling point of methyl alcohol, namely 75 °C. The evaporation procedure persisted for 20 min. The fatty acid esters obtained in this manner were neutralised through a single rinse with acidified water that contained 2% acetic acid. The concluding phase entailed the segregation of water from the esters, followed by their desiccation with anhydrous magnesium sulphate. Moreover, fuels were obtained by blending the biodiesel produced from used cooking oil with Verva B0 diesel, procured from an Orlen station, in the subsequent volumetric ratios: B10 (90 DF and 10% WFOME biodiesel), B20 (containing 80% DF and 20% WFOME biodiesel) and B30 (containing 70% DF and 30% WFOME biodiesel).

3.2. Research Equipment

In the subsequent phase, parameters such as density at 15 °C, higher heating value and lower heating value, cetane number, fractional composition, flash point, and kinematic, and dynamic viscosity were ascertained. The influence of temperature on dynamic viscosity was ascertained within the temperature spectrum of −20 to +50 °C a station furnished with a ReolabQC rheometer from Anton Paar GmbH (Graz, Austria), accompanied by a GRANT thermostatic bath. The density of the fuels was ascertained at a station furnished with a DA-100M device from Mettler Toledo (Columbus, OH, USA). The fractional composition of the examined fuels was ascertained utilising a Herzog HDA 620/1 (Lauda-Königshofen, Germany) distillation apparatus. The flash point was ascertained utilising a semi-automatic Herzog HFP 380 apparatus. The lower heating value was ascertained utilising a KL-10 calorimeter (PRECYZJA-BIT Sp. z o.o., Bydgoszcz, Poland). The determination of the cetane number was accomplished using an alternative method, employing the Irox Diesel analyser (Grabner Instruments Messtechnik GmbH, Vienna, Austria). This process utilised an analysis of characteristic radiation coupled with the application of mathematical models grounded in linear regression. Figure 2 shows the equipment used to carry out the aforementioned tests.

To investigate the correlations between the chosen quality parameters of the examined biofuels, the derived results were subjected to statistical analysis utilising the Statistica 13.3 software. Pearson correlation was used, with a significance level for differences of p < 0.05.

4. Results and Discussion

Among the characteristics of biodiesel fuel, those of paramount significance are the ones influencing the values of engine performance indicators, specifically the emission of exhaust gases. They encompass density, kinematic viscosity, cetane number, and lower heating value [35]. Figure 3 presents the density values of: biodiesel generated from spent WFOME frying oil, DF diesel, and combinations of WFOME and DF, with the volumetric content of biodiesel in the diesel mixture being 10, 20, and 30%. The density of WFOME biodiesel exhibits an increase of approximately 6.5% when juxtaposed with DF. Incorporating WFOME into DF enhances the density of fuels constituted of a blend of WFOME and DF. For the B30WFOME fuel, there was an approximate increase in density by 1.9% in comparison to diesel.

The kinematic viscosity possesses significant importance in the properties of biodiesel, given that an appropriate value facilitates fuel fluidity, thereby impacting the functionality of fuel injection systems, predominantly under lower temperatures [36]. Excessive viscosity may result in the development of deposits and soot due to inadequate fuel atomisation [37]. In this work, the measurement of dynamic viscosity was conducted as it is a superior parameter in evaluating the resistance to flow posed by the fuel in the injection system under actual dynamic scenarios—specifically, fuel flow at elevated pressures and velocities. This parameter serves as an indicator of the resistance to flow or deformation of the liquid, influencing the process of injection, fuel atomisation, and the reach of the jet within the engine’s combustion chamber. It also influences the lubricative characteristics. It is of particular significance when utilising rotary injection pumps or in contemporary common rail injection systems, where the actual dynamic injection pressure ascends to as much as 2500 bar [38]. The measurement results of dynamic viscosity presented in Figure 4 indicate that the B0 diesel oil had the lowest viscosity, as anticipated, whereas the B10 WFOME and B20 WFOME fuel exhibited a slightly greater viscosity. In contrast, the highest viscosity value was recorded for the B100 WFOME fuel. The escalation in the volumetric participation of WFOME biodiesel in DF fuel led to a viscosity increment. For pure B100 biodiesel within the temperature range exceeding 10 °C, the changes in biodiesel viscosity values exhibited a fundamentally linear character. Conversely, from approximately 10 °C to −20 °C, they exhibited a parabolic trait, thereby allowing for the assertion that the application of pure biodiesel (sans suitable additives) during the summer or spring–autumn period should not result in a notable rise in fuel flow resistance within the fuel system, nor a considerable degradation of injection, atomisation, and combustion conditions. Conversely, during the winter, utilising pure biodiesel inhibits the initiation of the engine. These findings concur with the data acquired by the authors Wcisło et al. [39]. Incorporating additives into biodiesel facilitates a reduction in its viscosity. Girardi et al. [40] modified the impact of natural compounds of camphor, limonene and isoamyl alcohol on the properties of biodiesel derived from babassu oil. An enhancement in all scrutinised properties of the fuel was discerned, most prominently in the blend incorporating 0% isoamyl alcohol, 7% camphor, and 7% limonene. An increased presence of biodiesel, limonene, and camphor in the mixture led to a reduction in viscosity. In specimens possessing an elevated concentration of isoamyl alcohol, an augmentation in viscosity was observed. Lapuerta M. et al. [41] demonstrated that the application of alcohol mixtures (butyl and ethanol) in biodiesels exhibits a tendency towards reducing the viscosity and density of the biofuel.

Kinematic viscosity tests of the tested fuels were also carried out. Figure 5 presents the results of kinematic viscosity determinations for DF, WOFME, and their mixtures: B10, B20, and B30. The tests were carried out in accordance with the requirements of the PN-EN ISO 3104 standard at a temperature of 40 °C. Note that all tested fuels meet the requirements of the PN-EN-590 and PN-EN 14214 standards for diesel oil and biodiesel. The kinematic viscosity of biodiesel B100 WOFME is 1.6 units higher, which shows that biodiesel at a temperature of 40 °C is characterized by slightly higher flow resistance than pure B0 diesel oil.

Establishing the points of the distillation curve is significant in evaluating the initiation (auto-ignition) and operational characteristics of the fuel. Vegetable oils exhibit inferior distillation properties, subsequently affecting their starting characteristics when compared to DF diesel [42,43,44]. The fractional composition of the examined fuels is illustrated in Figure 6, while selected distinctive parameters of the distillation process are delineated in

Table 2. Distinctive parameters of the distillation process of biodiesel produced from WFOME frying oil and DF diesel oil.

Distillation Process Parameter	WFOME	DF	WFOME-DF
Temperature of onset of distillation TD; °C	274	163	111
Distillation temperature of 10% of the fuel sample volume T10; °C	286	192	94
Distillation temperature of 50% of the fuel sample volume T50; °C	307	277	30
Distillation temperature of 90% of the fuel sample volume T90; °C	334	327	7
Distillation temperature of 95% of the fuel sample volume T95; °C	342	334	8
Fuel distillation end temperature (T97); °C	358	348	10

. The conducted research demonstrated that WFOME biofuel commences distillation at temperatures exceeding 270 °C in contrast to DF, which begins at 163 °C. This is disadvantageous from the perspective of engine ignition. In an engine, the auto-ignition temperature of the fuel is significantly influenced by the initial distillation temperature and the quantity of fuel that evaporates in the early stages of distillation. The greater the concentration of light fractions, the more optimal are the self-ignition properties, thereby resulting in a gentler engine start-up [45]. 10% (T10) of WFOME evaporated at 286 °C, while 10% (T10) of DF evaporated at 192 °C, 94 °C lower than biodiesel. The difference between WFOME biodiesel and diesel at 50% evaporation (T50) of the fuel was 30 °C. At temperatures above 327 °C, both fuels evaporated 90% (T90). The temperature differential between the two fuel types was 7 °C. Conversely, 97% of the WFOME fuel underwent evaporation at a temperature of 358 °C, while 97% of the DF did so at a temperature of 348 °C, which is 8 °C lower. Nevertheless, in accordance with the EN 14214 standard for FAME vegetable biofuels, the entirety of the WFOME biofuel has evaporated at a temperature of 360 °C. A similar study was carried out by Kurczynski et al. [46]. They showed that AFME (animal fats methyl esters) and AFBE (animal fats butyl esters)biodiesels from animal fat waste have inferior distillation properties compared to commercial DF. The initial temperature for distillation of AFBE biodiesel exceeded that of diesel by 102 °C, whilst the same parameter for AFME biodiesel was found to be 111 °C higher as compared to DF. Nevertheless, both of the manufactured biodiesels complied with the EN 14214 standard, which stipulates that the entirety of the fuel ought to evaporate up to a temperature of 360 °C. In the article by Wcisło G. [47], research demonstrated that both the commencement and progression of distillation are contingent upon the quantity of RME rapeseed oil biodiesel present in diesel fuel. The higher the proportion of biodiesel RME in the biofuel, the later the distillation process initiates. The work of Chuepeng S. et al. [29] established the distillation temperatures of biodiesels originating from waste cooking oil (WCOME), waste chicken oil (CKOME), and palm oil (POME). They demonstrated that the evaporation temperature at 90% (v/v) for WCOME and CKOME was inferior to that of palm oil biodiesel and, unsurprisingly, DF diesel.

As can be seen from Figure 7, the temperatures at the beginning of distillation and at the end of distillation of B10, B20, and B30 biofuels are identical. This seems logical because when WOFME Biodiesel is added to diesel oil, which differs in temperature, especially at the beginning of distillation, only the more volatile fractions are distilled at the beginning, and the least volatile ones at the end. Diesel oil contains more volatile fractions and therefore, when distilling biofuels from B10 to B30 to a temperature of 274 °C, they distil only fractions from diesel oil. However, after evaporating a large part of the diesel oil from the biofuels, approximately half of the diesel oil and Biodiesel B100 WOFME remain in the remaining mixture. In such a case, the fractions originating from WOFME also influence the distillation temperature of the appropriate volume of fuel, i.e., more than 50% (v/v) of the remaining amount. A similar situation can be observed with temperatures from 90% (v/v) distillations to the end. In this case, the distillation temperatures depend to a greater extent on the fractions coming from WOFME. However, at the end of distillation, only the heavier fractions from WOFME are affected. Analysing the impact of the addition of 10 to 30% (v/v) WOFME to diesel oil on the parameters responsible for fuel evaporation, the onset of spontaneous ignition, as well as self-ignition and combustion, it can be concluded that it is small because in the first, most important phase, the oil itself evaporates. In such a case, ignition will take place similarly to the case when the engine is fed with pure B0 diesel oil. Moreover, because B100 WOFME evaporates up to 360 °C, there is no need to worry that this fuel might not burn out during combustion in the engine’s combustion chamber. Analysing the numerical data contained in

Table 3. Distinctive parameters of the distillation process of mixtures WFOME biodiesel and DF diesel oil.

Distillation Process Parameter	B10 WFOME	B20 WFOME	B30 WFOME
Temperature of onset of distillation TD; °C	163	163	163
Distillation temperature of 10% of the fuel sample volume T10; °C	197	199	201
Distillation temperature of 50% of the fuel sample volume T50; °C	273	278	284
Distillation temperature of 90% of the fuel sample volume T90; °C	329	331	334
Distillation temperature of 95% of the fuel sample volume T95; °C	343	345	347
Fuel distillation end temperature (T97); °C	358	358	358

regarding differences in characteristic distillation temperatures, we can notice that in the range of temperatures at the beginning and end of distillation they are identical. However, by analysing the T50 temperatures (distillation of 50% (v/v) of the fuel), it can be concluded that the difference between B10 and B30 is only 11 °C, so it is relatively small. In the case of temperatures T90 and T95, the limit differences are even smaller and amount to 5 and 4 °C, respectively.

The flash point is defined as the minimum temperature at which a mixture of fuel vapour and air can be ignited by an external ignition source during the heating process of the fuel. It does not exert a direct influence on the combustion process. However, it enhances the safety of biodiesel during storage and transportation [48]. It influences the simplicity of cold engine commencement and eradicates the potential for fuel evaporation in fuel pipelines, subsequently preventing the formation of vapour blockages [49]. The flash point of biodiesel is considerably higher than that of diesel fuel and even notably surpasses the minimum threshold delineated in biodiesel standards [50]. As per the EN 14214 standard, the biodiesel’s flash point should be no less than 101 °C. The flash point of WFOME biodiesel, its mixtures with diesel fuel, and DF diesel fuel are depicted in Figure 8. The pure DF diesel fuel and a mixture of diesel with a 10% supplement of B10WFOME fryer oil biodiesel were characterised by the lowest flash point, standing at 58 °C. A further increase in the proportion of biodiesel in the mixture with diesel fuel to 30% (v/v) WFOME resulted in a slight increase in the flash point. This phenomenon is attributable to the fact that the DF fuel molecules undergo evaporation first. Indeed, in biofuels comprising up to 30% WFOME additive, only the hydrocarbons present in the diesel fuel are observed to evaporate. Similar conclusions have also been drawn by other authors [51,52]. The fatty acid methyl esters, obtained from the post-frying vegetable oil known as WFOME, achieved a flash point of 158 °C, which is over 2.5 times higher in comparison to diesel fuel. Conversely, the authors of [53] derived a flash point of 171 °C for biodiesel produced from used frying oil. In the study [54], the findings of flash point assessments for diesel fuel, soy-based biodiesel, and their mixtures with n-butyl and n-pentanol alcohols were showcased. The authors noted a substantial decrease in this value, especially for mixtures possessing an alcohol concentration between 0–20%. Mattos et al. [55] conducted a study of biodiesels based on methyl esters from vegetable oils, including soya, maize, rapeseed and sunflower oils, and swine lard. They were mixed with mineral diesel to yield mixtures ranging from 5% (v/v) to 50% (v/v) biodiesel, denoted as B5, B10, B20, and B50. A correlation has been observed between the rise in flash point and the increase in biodiesel content for blends ranging from B0 to B20. Foroutan et al. [53] synthesised biodiesel from waste materials including goat fat, chicken fat, spent oil, and palm kernel oil via a transesterification procedure. The biodiesels that were manufactured were amalgamated in diverse ratios with diesel fuel. The flash points of pure biodiesels produced from recycled edible oil, chicken fat, palm kernel oil, and goat fat were observed to be 171, 174, 176, and 180 °C, respectively. The flash points of the biodiesel were found to be superior to that of diesel oil. With the incremental addition of biodiesel to the diesel blend, there was a corresponding increase in the value of the flash points. The significantly higher ignition temperature of pure biodiesel compared to diesel can be disadvantageous from a combustion process point of view, especially when the engine is cold. The higher ignition temperature is due to the fact that the fuel components evaporate and form a fuel-air mixture at higher temperatures. This will affect the combustion process, especially at low temperatures in engine cylinder.

The lower heating value of a fuel is the amount of heat given off during the complete combustion of a unit mass of fuel in an oxygen atmosphere, the products of combustion being carbon dioxide and water in a vapour state. Biodiesel typically has a lower heating value than diesel (approximately 42 MJ·kg⁻¹) due to a higher proportion of oxygen in its elemental composition [48,56]. This value escalates with an increase in the quantity of carbon atoms and diminishes with the degree of unsaturation [57]. The higher heating value of a fuel is the amount of heat given off during the complete combustion of a unit mass of fuel in an oxygen atmosphere, with the products of combustion being carbon dioxide and liquid water. Figure 9 and Figure 10 illustrate the higher heating value and lower heating value generated by WFOME biodiesel and its mixtures with diesel oil, in contrast to DF. The lower heating value, which greatly influences the individual fuel consumption, showed similarity to DF when considering mixtures of WFOME biodiesel with diesel oil, particularly B10 WFOME and B20 WFOME. Contrarily, the lower heating value for WFOME’s B100 biodiesel was found to be 41.3 MJ/kg, whereas it was 48.4 MJ/kg for diesel fuel. In the research document [58], an investigation was conducted incorporating jatropha oil biodiesel into diesel fuel. The researchers noted comparable declines in the lower heating value as the ratio of fatty acid methyl esters in the diesel mixture increased. The inferior lower heating values of biodiesels derived from used frying oil, chicken fat, and palm oil, equating to, respectively, 38.15 MJ/g, 39.38 MJ/g, and 39.98 MJ/g, as presented by Chuepeng S. et al. [29]. Szabados et al. [59] conducted comparative tests of three fuels (fossil diesel, biodiesel from rapeseed oil (FAME) and biodiesel from triglycerides with modified structure (TBK)). The lower heating values of biodiesels were found to be inferior compared to diesel fuel (42.12 MJ/kg), specifically for FAME 36.29 MJ/kg and for TBK 34.81 MJ/kg. In the study [60], the authors examined the lower heating value of biodiesel derived from beeswax, registering at 38.5 MJ/kg. The authors of the study [61] examined among other aspects, the lower heating value of biodiesel derived from waste cooking oil through the process of pyrolysis, which was found to be 46.62 MJ/kg, surpassing the lower heating value of diesel at 45.62 MJ/kg. The authors posit that the augmentation in the content of aromatic and unsaturated hydrocarbons, generated during the pyrolysis process, resulted in a marginal enhancement in the lower heating value of biodiesel.

The cetane number serves as a fundamental parameter in evaluating the self-ignition capability of a diesel fuel. The magnitude of this variable can potentially influence the engine performance, the byproduct emissions, and the generated noise levels [62]. It influences the diesel auto-ignition delay value during injection into the combustion chamber [63,64]. It depends on the length of the carbon chain and the degree of unsaturation of the fatty acids [65,66,67,68]. Low cetane number (CN) values amplify the auto-ignition delay, thereby escalating the probability of the occurrence of knocking combustion in a diesel engine [63,64,69,70]. High cetane number values contribute to the reduction of exhaust smoke opacity and the enhancement of cold-start performance [71]. In the case of biodiesels, the cetane number is high because there is atomic oxygen in the fuel structure which, when heated, causes the fraction to break down and form radicals which, in the next stage, accelerate auto-ignition. The cetane number of biodiesel typically exceeds that of diesel fuel, as corroborated by scientific literature [61,72,73]. Conversely, in the study [29], the cetane number of biodiesel derived from palm oil, spent frying oil, and waste chicken oil were found to be lower than that of diesel oil. Heywood [74] suggested that the cetane number of biodiesel fuel should not exceed 65. A fuel characterised by an exceptionally high cetane number combusts proximate to the injector, consequently leading to its overheating. Consequently, the injector nozzles may become obstructed. The EN 14214 standard stipulates that the minimum cetane number for biodiesel should be no less than 51 [33]. As illustrated by the research findings depicted in Figure 11, incorporating methyl esters derived from spent edible oil into diesel fuel precipitated a linear augmentation in the cetane number. The cetane number (CN) of the diesel-biodiesel mixture escalated corresponding to the amplified biofuel content in the concoction, culminating in a value of 53.1 for the B30 blend. The data align with the literature findings [52]. In the study [75] the mechanism of the influence of different proportions of biodiesel blends derived from waste cooking oil containing 10%, 20%, and 30% (v/v) n-pentanol were investigated and designated as BP10, BP20, and BP30 on the combustion parameters and exhaust emissions of a diesel engine with a common rail fuel system. The primary fuel, namely biodiesel derived from waste cooking oil, has achieved a cetane number value of 56, exceeding the CN value of diesel, which stands at 54. Additionally, the results obtained show that the cetane number decreases as the alcohol in the fuel mixture increases. For the BP20 and BP30 blends, it even falls below the cetane number of the DF reference fuel.

The study also included a correlation analysis between the quality indicators of the biofuels studied.

Table 4. Correlation coefficients of the qualitative characteristics of the fuels: Pearson’s correlation coefficient r(X,Y), coefficient of determination r², value of the t statistic testing the significance of the correlation coefficient, significance level p.

Parameter	Standard Deviation	r(X,Y)	r2	t	p
FP CN	3.3166 0.7745	0.9342	0.8727	3.7033	0.06580
FP HHV	3.3166 2.2794	−0.9634	0.9281	−5.0824	0.03660
FP LHV	3.3166 0.7632	−0.9415	0.8865	−3.9524	0.05846
CN HHV	0.7745 2.2794	−0.9005	0.8109	−2.9289	0.09948
CN LHV	0.7745 0.7632	−0.9980	0.9960	−22.2999	0.00201
HHV LHV	2.2794 0.7632	0.9230	0.8520	3.3934	0.07695

and Figure 12 and Figure 13 present the results of the statistical analysis carried out. The red dashed lines in the figures indicate the confidence interval. Statistically significant relationships for which the significance level is p < 0.05 are highlighted in red. A negative correlation was observed between flash point and higher heating value (Figure 12) and cetane number and lower heating value (Figure 13). The correlation coefficient (r) for the fuels tested between cetane number and higher heating value was r = −0.9980, and that between flash point and the lower heating value was r = −0.9634, indicating a very strong linear relationship between the variables. It was observed that the higher the flash point of the test fuel, the lower the higher heating value, and lower values of the cetane number correspond to higher values of the lower heating value.

5. Conclusions

In light of the necessity to diminish fossil fuel consumption and air pollution, while concurrently seeking out novel, environmentally benign alternative fuels, this study employs post-frying vegetable oil as a waste feedstock for biodiesel production. This does not result in an increase in demand for vegetable oil; rather, it facilitates the management and utilisation of waste products generated by the food industry. The selected physicochemical properties of biodiesel were determined. The analysis permits the following conclusions to be drawn:

• The utilisation of proprietary technology and a transesterification reactor and model has permitted the production of biodiesel that meets the specifications set forth in EN-14214.
• The initial distillation temperature of subsequent quantities of WFOME biodiesel is demonstrably higher than that of diesel. This can result in a deterioration of the engine’s cold starting capabilities, particularly at low ambient temperatures. Concurrently, WFOME distilled completely to 360 °C, signifying that the most substantial fuel fractions should be incinerated within the engine and will not accumulate as carbon deposits on combustion chamber components. Furthermore, the diesel fully evaporated, reaching a temperature of 360 °C.
• The course of the distillation process of mixtures of WFOME biodiesel and DF diesel oil, with volumes of biodiesel content in the mixtures of 10, 20, and 30% (v/v), does not differ significantly from the distillation process of pure diesel oil. The use of B10, B20, and B30 fuels will not have a significant impact on engine starting and auto-ignition delay.
• The cetane number of diesel oil was determined to be 51.3 units. The addition of WFOME resulted in an enhancement of the cetane number. Increasing the content to 30% (v/v) WFOME to diesel resulted in a 1.8-unit increase in CN value, or 3.5%.
• The flash point of WFOME biodiesel is demonstrably higher than that of DF, which is a favourable feature from the point of view of the safety of storage, transport, and use of the fuel but is a disadvantageous feature from the point of view of the combustion process.
• The higher heating value and lower heating value of WFOME biodiesel exhibited slightly lower values than those determined for diesel. However, this was to be anticipated, given that oxygen is present in the structure of vegetable oils and FAME.
• An increase in the volume proportion of WFOME fuel in DF diesel resulted in an increase in dynamic viscosity.
• The kinematic viscosity of fuels: DF, B10, B20, and B30 meets the requirements of the PN-EN-590 standard, while the kinematic viscosity of WFOME biodiesel meets the requirements of the PN-EN 14214 standard.
• A negative correlation was identified between flash point and higher heating value, lower heating value and cetane number. The correlation coefficient (r) for the fuels tested between cetane number and lower heating value was r = −0.9980, and that between flash point and the higher heating value was r = −0.9634, indicating a very strong linear relationship between the variables.

As a renewable energy source, biodiesel produced from WFOME used frying oil can be blended with diesel and used to power compression-ignition engines, which can reduce gas emissions including CO₂ to some extent. The WFOME biodiesel produced is a viable alternative fuel to diesel. The production of WFOME represents an effective method for the management of waste in the form of used vegetable oils derived from frying. Nevertheless, the production process must be conducted in a manner that ensures the production of high-quality products. In contrast, the use of pure B100 fuel, especially at temperatures close to 0 °C and below, necessitates the use of additives that lower the fuel’s crystallisation temperature. Otherwise, the absence of these additives can lead to fuel crystallisation in the fuel supply system. Nevertheless, in order to conduct a comprehensive investigation of the biofuel in question, further research in this area, including in-use testing of engines, appears to be necessary.