Composite Liquid Biofuels for Power Plants and Engines: Review

Abstract

The problems of environmental pollution caused by the operation of power plants and engines motivate researchers to develop new biofuels. The environmental aspect of composite biofuels appears to have great potential because of the carbon neutrality of plant raw materials. This study analyzes recent advances in the production of biofuels and their application. The research findings on the properties of promising plant raw materials and their derivatives have been systematized. The most important stages (spraying, ignition, and combustion) of using biofuels and mixtures based on them in internal combustion engines have been analyzed. A separate section reviews the findings on the environmental aspect of using new fuel compositions. Most studies show great prospects for involving bio-components in the development of composite fuels. The real issue is to adjust existing engines and plants to non-conventional fuel mixtures. Another big problem is the increased viscosity and density of biofuels and oils, as well as the ambiguous effect of additives on burnout completeness and emissions. The impact of the new kinds of fuels on the condition of components and parts of engines, corrosion, and wear remains understudied. The interrelation of industrial process stages (from feedstock to an engine and a plant) has not been closely examined for composite liquid fuels. It is important to organize the available data and develop unified and adaptive technologies. Within the framework of this review work, scientific approaches to solving the above problems were considered and systematized.

1. Introduction

There is currently a steady trend towards increased consumption of liquid hydrocarbon fuels in internal combustion engines of cars and propulsion systems of air transport [1]. The most widely used types of liquid hydrocarbons in the fuel sector are kerosene, Diesel fuel, and gasoline. Kerosene is primarily used as a jet fuel for planes and rockets (aviation kerosene) and as an additive (up to 20%) to Diesel fuel at low operating temperatures to prevent freezing without significant worsening of performance characteristics [2]. Aviation kerosene in aircraft is not only a motor fuel in turboprops and turbojets but also serves as a refrigerant in heat exchangers (fuel-to-air heat exchangers) and as a lubricant in fuel and engine systems.

Similar to kerosene, Diesel fuel also has a wide range of applications [3,4,5]. It is used as a motor fuel for different kinds of land and water transport, as well as for special-purpose machines. Rail transport (diesel locomotives and diesel multiple units), maritime transport (mainly motor ships), road transport (buses, trucks, and amphibious all-terrain vehicles), and different special-purpose machines (tractors, combines, etc.) rely heavily on Diesel fuel. Apart from its application in transport, Diesel fuel is also used in electric generators.

Compared to kerosene and Diesel fuel, gasoline has fewer application areas as a fuel. However, the market for gasoline as a motor fuel for road transport is very big. Gasoline vehicles (together with hybrid vehicles) account for more than 70% of the total number of vehicles, while only 15% of vehicles are diesel-powered [6,7]. Apart being used in cars, gasoline is also used in the aviation industry. Special high-octane aviation gasoline with a low content of tetraethyl lead is used in bush planes all over the world.

However, despite the widespread use of liquid hydrocarbon fuels, all of them have a serious drawback: a high level of CO₂ emissions during their life cycle (Figure 1). Electric motor units can provide an alternative to internal combustion engines, yet they have their limitations as well. The central problem of electric motors is that they are powered by electricity produced primarily through the combustion of fossil fuels, such as coal, fuel oil, and natural gas. According to IEA World Energy Outlook [8], over 80% of global power is generated by burning fossil fuels. Significantly, electrical power production and transportation to charging stations entail some losses associated with the efficiency of power facilities (boilers, turbines, etc.), electric generators, transmission lines, and charging stations themselves. Moreover, the growth of the electric transport industry makes it necessary to recycle dead batteries. However, there are no solutions addressing this issue so far. As reported in [9,10], only 5% of all lithium-ion batteries are now recycled. The remaining 95% go to landfill sites for long storage.

It is also important to consider the current level of infrastructure for the widespread adoption of electric vehicles into everyday life. Although the network of charging stations for electric vehicles is growing around the world, a significant proportion of cars, especially trucks, in the largest automotive markets (China, India, and South America) still rely on internal combustion engines [7,11], as they do not require a substantial upgrade of the whole national infrastructure or considerable government expenditure.

An environmentally friendly alternative to conventional types of motor fuels can be offered by biofuel produced from plant-derived components [12]. Such fuels are carbon-neutral because raw materials (trees, shrubs, etc.) used to produce them absorb CO₂ during growth, which compensates for carbon oxide emissions into the atmosphere during their combustion (Figure 1).

Apart from carbon neutrality, these fuels have other benefits. For instance, no significant upgrade of fuel feeding systems at existing filling stations is required for using these fuels. Another advantage is the high speed of filling these fuels into a vehicle tank, whereas the full charge of electric motors may take from half an hour to several hours.

In recent years, there has been an increasing interest in studying biofuels and developing technologies for their widespread adoption in different applications [13,14]. Such research is underway, though not without impediments. The introduction of new composite fuels implies that a wide range of factors should be analyzed that directly affect the operation of engines and power facilities. Overall, this is a very diverse area with great potential for study and development.

The purpose of this review paper is to systematize the main trends in the production and application of composite liquid fuels derived from plant components, identify the advantages and disadvantages of these technologies, and find promising development paths for this field. The difference of this work from other previously published reviews is that the emphasis here is on the diversity of the feedstock used and the types of biofuels obtained from it, suitable for a wide range of applications.

2. Liquid Biofuels: Feedstocks, Production Methods and Properties

There is a growing body of literature that recognizes the importance of using liquid biofuels. Preferable components for such fuels are vegetable oils or products of their thermochemical conversion [15,16]. This is because the feedstock is almost carbon-neutral [17]. Apart from vegetable oils, acyclic hydrocarbons with a general formula C_nH_2n+2 or their derivatives [18,19,20], alcohols [19,21], waste oils [22,23], biomass [24,25], municipal waste [26], etc. can also be used as feedstock. Not only liquid components but also solid waste from different industries (agriculture, MSW, and others) can be used for the synthesis of biofuels. These components are virgin raw materials for the preparation of pyrolysis or gasification products that can be later used as additives to biofuels or as a component for the synthesis of biofuels in pyrolytic cracking and hydrocracking units.

Table 1. Promising solid components for biofuel synthesis.

Component	Moisture Content, %	Ash, %	Vdaf, %	Higher Heating Value, MJ/kg	Cdaf, %	Hdaf, %	Ndaf, %	Std, %	Odaf, %
Cedar nut shells	13.0	1.0	69.7	21.0	51.81	6.39	0.24	traces	41.56
Sunflower seed shells [27]	10.10	1.95–4.2	77.72	17.37	50.26	5.98	1.28	0.25	42.23
Chlorella [28]	<1	12.3	78.9	-	48.6	7.3	9.0	0.9	34.2
Liquid product of pine pyrolysis [29]	23.6	-	-	18.5	42.50	7.10	0.06	traces	50.34
Artichokes [30]	5.7	7.5	74.5	14.9	45.4	6.6	2.6	0.2	45.2
Rice husk [31,32,33]	5.62–6.79	14.77–17.82	62.61–65.65	16.02	38.23–49.4	4.88–6.20	0.40–1.02	0.16–0.30	33.29–34.15
Peanut shells [31]	7.88	1.60	68.10	21.42	54.9	6.10	1.37	0.10	7.47
Bamboo stems [34]	15.62	0.92	75.24	17.41	44.2	5.15	0.49	0.22	39.40
Corn straw [35]	2.16	2.68	77.64	-	40.6	5.51	0.79	0.09	52.94
Palm kernel [36]	2.88	5.30	75.83		49.0	5.93	34.10	2.46	0.29
Spirulina algae [37]	-	-	-	33.2	68.9	8.9	6.5	14.9	0.86
Anaerobic sludge [37]	-	-	-	32.0	66.6	9.2	4.3	18.9	0.97
Aspen wood [38]	3.8	0.48	-	34.3	75.2	8.2	0.05	15.8	0.3
Spent coffee grounds [39]	-	-	-	31.0	71.2	7.1	3.0	18.7	-
Viburnum odoratissimum [40]	-	-	-	32.5	71.7	8.1	1.2	19.0	0.01
Salix alba [40]	-	-	-	23.1	73.7	9.2	3.1	14.1	0.01
Garbage [41]	-	36	-	21–36	73.6	9.1	4.6	-	12.7
Beech wood [41]	34.9	16.1	-	21	76.7	7.1	0.1	-	16.1
Phytomass [41]	-	-	-	5–25	76.6	7.6	2.1	0.1	13.6
Cyanobacteria [42]	-	-	-	36.51	76.0	9.1	6.29	7.44	1.15
Seaweed [43]	-	-	-	35.97	75.5	9.16	3.65	11.66	0.62
Coffee husk	8.44	7.4	-	16.79	43.1	5.02	1.55	32.78	0.67
Peanut shell [44]	7.98	12.8	–	16.52	41.5	7.43	2.12	27.96	0.6
Rice husk [44]	8.19	29.53	–	15.39	31.4	6.67	1.04	23.03	0.5
Pine sawdust [44]	6.9	4.71	–	17.03	45.9	7.47	0.32	34.32	0.57
Spirulina [45]	-	-	-	-	48.1	6.97	10.14	34.13	0.66
Chlorella [45]	-	-	-	-	51.3	7.9	9.8	30.38	0.59

presents promising solid waste that can be used for the synthesis of biofuels.

The most common method of biofuel production is the transesterification of initial components to produce certain esters. Thermochemical conversion and transesterification are used to produce new components. Therefore, their subsequent application requires research into the properties, environmental, energy, and performance parameters.

Such characteristics as ash content, the specific heat of combustion, ignition and flash temperature, as well as elemental composition, are enormously important for fuel applications. Density, viscosity, and crystallization temperature are also important, as these properties make it possible to predict the behavior of fuel mixtures during transportation, storage, and actual combustion in a furnace.

Most studies on biofuels focus on the preparation of components. The synthesis often involves esterification and transesterification [46], pyrolytic cracking [47], hydrocracking [48], and hydrothermal liquefaction [49] (Figure 2). Additional steps or catalysts are often used to improve the production of biofuels. For example, a number of works [50,51] report that ionic liquids can reduce energy consumption throughout the process of biofuel production. For example, in the research [50], 3 of 5 investigated ionic liquids displayed a good catalytic activity and resulted in conversion higher than 77%. The chosen ionic liquid, 1-methylimidazolium hydrogen sulfate, led to the highest conversion in the screening step.

The pretreatment of dry and ensiled hemp with steam for the production of ethanol was studied in [52]. The process efficiency was assessed in terms of sugar recovery and polysaccharide conversion when using enzymatic hydrolysis. It was established that impregnation with 2% SO₂ followed by steam pretreatment at 210 °C for 5 min resulted in the highest yield of glucose.

Another effective method of biofuel synthesis is pyrolytic cracking using different catalysts [53,54,55]. Amini et al. [54] utilized waste cooking oil (WCO) consisting of triglycerides and contaminated derivatives from the frying process. Dolomite was used as a catalyst that accelerated the production of aromatic hydrocarbons during chemical reactions. Amini et al. [54] used two-step pyrolytic cracking to obtain products such as gasoline, kerosene, and Diesel fuel. The resulting biofuels met the standards of the certified fuel [54].

Le-Phuc et al. [55] studied the conversion of high acid value (AV) waste cooking oils (WCOs) into biofuels through cracking over spent fluid catalytic cracking (SFCC) catalysts. WCOs were processed in a fluid catalytic cracking lab-scale unit at 450–520 °C. The authors note that this approach potentially minimizes the costs associated with buying catalysts and managing spent catalysts while maximizing the conversion of used vegetable oil into a clean fuel with almost zero AV. In addition, rare earth elements from spent FCC catalysts, after being used for WCO processing, can be recovered as a high-purity RE mixture. Therefore, this procedure has a high potential as a practical alternative that is both economically and environmentally beneficial.

Thermochemical conversion of solid agricultural waste is the most promising method in regions with limited availability of fossil fuels. These methods have a number of benefits: a positive economic effect (reduced dependence on conventional energy sources, competitive advantage on the global market, variety of components and fuels produced); lower environmental impact (reduced emissions of greenhouse gases when using biofuel, recycling of stockpiled waste); energy independence (availability of fuel, a wider scope of raw materials).

Hydrocarbons can also be produced from coal. It is possible to obtain ultra-clean transportation fuels from coal in four sequential steps: coal gasification, syngas cleanup, F-T (Fischer-Tropsch) synthesis, and F-T product workup. A Fischer-Tropsch (F-T) sub-system is incorporated into an IGCC (integrated gasification combined cycle) complex to produce an ultra-clean CTL (coal-to-liquids) fuel together with electric power, chemicals, and steam [56].

The most widespread methods of conversion are pyrolysis, gasification, and hydrothermal liquefaction. Pyrolysis results in 3 types of products that can be used in the energy sector (biochar, bio-oil, and syngas) [57]. Bio-alcohols are obtained using a hybrid method of fermentation with subsequent gasification. This combination of biological and thermochemical processes produces pure ethanol, methyl, or butyl alcohol. Preliminary fermentation of solid waste of biomass catalyzes the reaction of further gasification of the feedstock since this method simultaneously uses different types of biomass [58].

Table 2. Operation parameters of equipment.

Synthesis Method	Parameters	Ref.
Transesterification	V ≈ 3.8 mL T ≈ 260–320 °C P ≈ 1.2–5.9 MPa	[46]
Pyrolysis + cracking	T ≈ 450–500 °C Catalyst: MgCO3 + CaCO3; in relation to 0/100, 10/90, 20/80, 30/70	[47]
Hydrothermal liquefaction	T ≈ 280–370 °C P ≈ 10–25 MPa	[49]
Pyrolysis	T ≈ 550–600 °C Catalysts: zeolite, AAEMs	[59]
Digestion gasification process	T ≈ 300–350 °C P ≈ 3–8 MPa Catalyst: Ferrite	[58]

sets out the operating conditions of the equipment used for the synthesis of biofuels from plant raw materials and waste.

The synthesis methods shown in Table 1 can be used to produce different biofuels.

Table 3. Characteristics of liquid raw materials and synthesized fuel compositions.

Fuel/Feedstock	Density, g/cm3	Viscosity, mm2/s	Heating Value, MJ/kg	Cetane Number	References
RBOFAD	0.88	5	41.7	–	[46]
RBOFAD	0.89	3.8	-	–
RRBO	0.87	4.14	-	–
RRBO	0.89	8	-	–
Comm. Biodiesel	0.88	4.4	39.03	–
Bio-gasoline	–	0.65	43.49	–	[47]
Bio-diesel	–	3.25	40.58	–
Bio-kerosene	–	0.84	42.61	–
Bio-crude (pyrolysis)	0.93	71.05	22.6	–	[57]
Bio-crude (HLF)	0.95	15.306	35.7	–
Palm oil	–	3.62	–	59.11	[60]
Papaya oil	–	3.69	–	56.27
Rambutan oil	–	3.95	–	61.17
Vegetable oil	0.904	5.4	38.4	49	[61]
Waste cooking oil	0.86	3.8	196.2	–	[62]
Hybrid oil	0.8831	7.83	38.63	–	[63]
Hybrid oil	0.88	7.85	39.51	–	[63]
Cyanobacterium	0.8	2.9	35.5	56	[64]
Sweet basil	0.87	4.26	39.72	-	[54]
Pistacia chinensis seed	0.88	4.15	39.8	49	[65]
Waste cooking oil	0.892	9.12	–	68	[66]
Rubber seed oil	0.920	58.11	39.32	47.6	[67]
Neem oil	0.941	124.43	40.82	57.71	[67]
Castor oil	0.965	6.6	40.83	-	[68]
Jatropha oil	0.92	37.28	38.96	21	[69]
Palm oil and sesame oil (biodiesel)	0.88	4.63	41.24	55.37	[69]
Rubber seed and neem oils (bio-diesel)	0.897	5.94	39.66	56.53	[67]
Waste cooking oil (bio-diesel)	0.88	4.89	-	-	[67]

illustrates the key characteristics of typical feedstock and groups of synthesized fuels.

3. Atomization

One of the most crucial factors for the efficiency of combustion of any fuel is its spraying. It is a relevant task in this science and technology area to identify the impact of a group of different factors on the spraying of biofuels. These factors include the physicochemical properties of biofuels, pressure and time of injection, nozzle shape, conditions of the ambient gas medium, etc. At present, there are a lot of experimental and theoretical findings on the spraying of biofuels with different compositions. However, many of the spraying characteristics and factors affecting them have not yet been determined for these fuel types.

It is necessary to investigate the spraying of liquid fuels of bio-origin because their properties are distinctly different from those of conventional fuels. For instance, the high viscosity of biodiesel obtained through the transesterification of vegetable oils with monohydric alcohols worsens the spraying characteristics of droplets, and the fuel does not mix well with the air in the engine during ignition [70,71]. It is possible to improve the spraying of biodiesel by reducing the kinematic viscosity and surface tension by adding alcohol [72]. Suraj et al. [73] found that biodiesel stored for one year had greater kinematic viscosity and a lower mass flow rate compared to fresh biodiesel. Park et al. [74] used a spray visualization system to investigate the spraying characteristics of biodiesel mixed with ethanol. The spraying of composite fuel [74] was characterized by a smaller size of droplets and faster evaporation (because of the high volatility of ethanol) compared to the spraying of pure biodiesel. Gao et al. [75] determined the spraying parameters of biodiesel derived from inedible oil (in different proportions) using high-speed video recording: spray penetration, spray cone angle, and spray tip speed. An increase in the proportion of biodiesel in the fuel composition led to greater spray penetration and a faster spray tip speed, yet to a smaller spray cone angle [75]. Chaudhari et al. [76] utilized the high-speed shadowgraphy technique to examine the spraying properties of the following fuels: Diesel fuel, biodiesel Azadirachta Indica (Neem), and biodiesel mixed with anhydrous ethanol (50 vol%). The effect of fuel properties and injection pressure on the spraying and mixing of the fuel with the air was identified [76]. The most efficient spraying, i.e., the biggest cone angle, was recorded [76] for diesel mixed with ethanol with a volume fraction of 50% at an injection pressure of 50 MPa. This type of fuel is the preferred option for modern internal combustion engines for improved mixing of the fuel with the air. When a nozzle with an equilateral triangular orifice was used [77], the spray width of biodiesel was bigger than that of Diesel fuel at different injection pressures (50–90 MPa).

The visualization of spraying in [76] revealed the occurrence of cavitation that improved the efficiency of spraying and hence, combustion. The collapse of cavitating droplets destroyed the spray jet, thus increasing the spray cone angle of the fuel [78]. Droplet cavitation parameters depend on the injection conditions and the nozzle geometry. Cavitation shedding was normally observed beside the lower wall of the nozzle at high injection pressure [79]. The sprayed fuel is disintegrated not only through cavitation but also by a transverse flow of gas under pressure, which is of great interest for internal combustion engines. Jagadale et al. [80] conducted experiments on the fragmentation of levitating droplets of ethanol, rapeseed methyl ester, and their emulsions induced by a laser pulse. They used an acoustic levitator for droplet levitation and its non-contact manipulation [80]. Three droplet breakup regimes were observed [80]: droplet rupture and air entrapment, sheet breakup, and prompt/catastrophic fragmentation. Jagadale et al. [80] established that emulsion droplets (ethanol and rapeseed methyl ester) broke up not so easily as rapeseed methyl ester droplets or ethanol droplets did, even at lower laser energy.

The problem of environmental pollution is becoming more acute each year. There is a growing body of literature on devising solutions for reducing emissions by varying the spraying characteristics. For instance, Palash et al. [81] found that NO_x emissions during the combustion of biodiesel are significantly affected not only by the physicochemical characteristics, flame temperature, and engine load but also by the injection time. They established experimentally [81] that NO_x emissions can be effectively reduced (5–25%) by retarding biodiesel injection [81]. Park et al. [74] reported that early injection of biodiesel with ethanol contributed to lower emissions of exhaust gases. Evans et al. [82] studied how the methods of supplying biofuel into a hydrogen flame (prevaporization or direct spraying) affected soot formation. They showed [82] that in the direct spraying of toluene with a hydrogen-nitrogen fuel mixture, there was slightly more soot than in the case of spraying toluene as a vapor. As in [82], the prevaporization of palm methyl ester droplets resulted in little soot, hence, lower NO_x and CO emissions [83].

Studies on biodiesel produced from coconut oil, waste coffee grounds, tomato seeds and microalgae [84] revealed that compared to the traditional fuel, using mixtures of 20% and 50% biodiesel reduced nitrogen oxides (NO_x) by 4.6 and 1.2%, respectively. Using sustainable aviation fuel, namely biofuel derived from Euglena [85] and biofuel from algae [86], can also cut greenhouse gas emissions compared to the conventional fuel. In contrast, a mixture of biodiesel produced from Scenedesmus obliquus produced increased emissions of carbon dioxide and nitrogen oxides [87]. Lapuerta et al. [88] also reported higher levels of emissions and soot in exhaust gases as a result of increasing the content of glycerol in the biodiesel composition.

An overview of the recent research into biofuels indicates that their spraying has been at the forefront over the past few decades in an attempt to find optimal spray parameters by varying their composition and hence physicochemical properties. The general theory in the field of rational spraying of biofuels has not yet been built. One of the reasons for this is the variety of compositions of fuels under study and their properties.

4. Ignition and Combustion Performance of Liquid Biofuels

Ignition and combustion behavior is crucial for the sustainable operation of different engines, turbine plants, and boilers. Being intended for use in internal combustion engines, composite liquid fuels have certain specific features. Despite a great deal of research on this subject, numerous aspects still remain understudied. Ignition and combustion are complex processes in terms of chemical transformations and heat and mass transfer. Another important issue is the relationship between spraying, ignition, combustion, and emissions.

The main area of application of biofuels is the diesel engine. Oo et al. [89] compared the ignition and combustion of Diesel fuel and several biodiesels (jatropha methyl ester, palm methyl ester, soybean methyl ester, and coconut methyl ester) using an experimental setup at different pressures and temperatures. They discovered [89] that the ignition delay times of biodiesels were lower than those of Diesel fuel over the temperature range under study (350–950 °C). Of biodiesels, coconut methyl ester demonstrated the fastest evaporation and ignition. Hidegh et al. [90] explored the properties and characteristics of the combustion of biodiesels produced from palm, coconut, and waste cooking oils. Biofuels were mixed with diesel and subjected to temperature-controlled combustion in a swirl burner. The fuels were burned in a test rig where fuel was supplied together with air, pre-heated to 150–350 °C. Hidegh et al. [90] established that the properties of mixtures with 25% biofuel were very similar to those of Diesel fuel. For instance, the initial boiling point and the flash point did not differ significantly. With a further increase in the content of biodiesel, the properties started changing. Thus, with the proportion of palm methyl ether increased from 25% to 100%, the initial boiling point and the flash point grew to 345 °C and 190 °C, respectively. Of the three biodiesels under consideration, coconut methyl ether in a wide range of its proportions differed the least from commercial diesel in the above temperature characteristics [90]. The visualization of combustion revealed three flame shapes: straight, V-shaped, and distributed. The burner performance was consistent even under highly lean conditions. High atomization pressure and low air temperature caused distributed combustion.

The ignition and combustion behavior of single droplets of crude and pure glycerol, petroleum diesel, ethanol, and biodiesel was explored by Setyawan at al. [91]. The fuel droplets were heated in an electric furnace in the temperature range of 675–775 °C. Setyawan et al. [91] pointed out that glycerol is overproduced. According to [92], every kilogram of biodiesel produced comes with approximately 0.1 kg of glycerol. This signifies that glycerol can be used as an alternative fuel for boilers. Depending on the production technology, crude glycerol may contain different proportions of impurities such as water, soap, methanol, fatty acid methyl esters, etc. In [91], crude glycerol contained at least 31% impurities. These impurities inevitably affect the dynamics of ignition and combustion of glycerol droplets. The experiments showed [91] that the size of a pure glycerol droplet varied greatly during heating, unlike the size of the other fuel droplets. Impurities caused micro-boiling during heating. It resulted in bubbles and variations in the shape and size of fuel droplets. Setyawan at al. [91] found that the combustion rate of pure glycerol was lower than that of Diesel fuel, biodiesel, and ethanol. This result is largely attributed to higher density and a rather low Spalding number of pure glycerol. Crude glycerol, on the contrary, was characterized by the highest burnout rate, which is due to impurities [91]. Methanol and water reduced the total combustion time of crude glycerol droplets by increasing the combustion rate, which was due to decreased latent heat and boiling temperature, increased vapor pressure, and a micro-explosion. As for the ignition rate, within the whole range of heating temperatures, the fastest ignition was typical of pure glycerol; the longest one was observed with Diesel fuel. The greatest difference in the recorded ignition delay times was 0.3–1.8 s. Setyawan at al. [91] determined the flame standoff ratio (the flame radius to the droplet radius) for different liquid fuels. For droplets of ethanol and pure glycerol, this ratio remained constant throughout combustion, whereas for crude glycerol, Diesel fuel, and biodiesel it tended to increase.

Tariq and Saleh [93] analyzed the possibility of using heavy petroleum fuel in a diesel engine. Such fuels have a low cetane number, which complicates their ignition in an internal combustion engine. Overall, this fuel is almost unsuitable for a standard engine. Therefore, a one-cylinder test rig in the experiments was adjusted to heat the fuel to 70 °C before its injection into a combustion chamber. The authors tested heavy fuel oil and its blends with light fuel oil. The heating of the fuel mixture (80% light fuel, 20% heavy fuel) improved the burnout yet increased the exhaust gas temperature by approx. 30 °C. Tariq and Saleh [93] demonstrated good prospects for recovering cheap feedstock (fuel oil) and showed that the satisfactory performance of engines is possible with some adaptation of the fuel composition and operating conditions.

The prospects of combining hydrocarbon fuel with components of plant or animal origin have recently gained momentum [94]. Significantly, using vegetable oils for combustion in engines has serious drawbacks. Because of the specific properties of oils (high density and viscosity), their ignition is more complicated than that of petroleum fuels. Thus, the viscosity of oils is 10–15 times as high as that of Diesel fuel [95]. Operating problems might occur because of the gradual formation of deposits, filter clogging, low-efficient spraying, injector coking, and increased production of CO. High viscosity is a limitation both for heavy fuels [93] and for vegetable oils [96]. Uddin et al. [96] proposed mixing kerosene with mustard oil to make the fuel suitable for internal combustion engines. A 4-stroke single-cylinder diesel engine was used in the experiments. It was installed on a hydraulic dynamometer bed [96]. Adding 20–30% mustard oil to kerosene was found to produce a fuel with a viscosity comparable to that of Diesel fuel. This proportion provided a brake-specific fuel consumption of 258–270 gm/kw-h, which is comparable to the equivalent parameter of diesel oil (233.51 gm/kw-h) burned in this engine.

Chivu et al. [97] outlined good prospects for using blends of conventional Diesel fuel with turpentine oil. Unlike vegetable oils, its density is higher than that of regular Diesel fuel. Chivu et al. [97] found that the performance indicators changed only slightly with a switch to composite fuel (5–30% turpentine). Thermal efficiency differed by no more than 1.5%. However, the emissions of hydrocarbon and nitrogen oxides were higher than during the combustion of common diesel [97].

Fuel mixtures provide adequate heat of combustion [98] and other characteristics (e.g., density and brake-specific fuel consumption). Hossain et al. [98] prepared stable emulsions of “water-rapeseed oil-Diesel fuel” and “water-rapeseed oil” to be sprayed and burned in a 2-cylinder engine. The proportions of water and surfactant were 2.5–5% and 2%, respectively. Compared to Diesel fuel, emulsions based on rapeseed oil have a 10–15 °C higher flash temperature. The higher the percentage of water and rapeseed oil, the higher the flash temperature. The highest flash temperature (118 °C) was recorded for the mixture with 95.5% rapeseed oil, 2.5% Diesel fuel, and 2% surfactant [98]. Hossain et al. [98] estimated that at full load, the thermal efficiency during the combustion of the emulsion derived from vegetable oil was 12% higher than that of Diesel fuel.

A review of literature shows that compromise solutions are necessary when using liquid biofuels to achieve satisfactory indicators of ignition and combustion, as well as additional operating parameters of equipment. Thus, for instance, adjusting straight vegetable oils to power facilities is rather problematic. Therefore, despite the availability and diversity of vegetable oils, they can only be used in blends with a conventional fuel in relatively low proportions (5–15%) or can be processed into biodiesel. Vegetable oils are characterized not only by high viscosity and density but also by a negative impact on some metals (copper and its alloys, zinc, lead, and iron) because of free fatty acids in their composition [95]. These factors affect the durability and smoothness of operation of units and engines over a period of time. In summary, to ensure the viability of an engine running on a non-conventional fuel, strategies are being developed nowadays on the basis of microemulsification, preliminary heating of the fuel, and the creation of blends with fossil Diesel fuel.

5. Emission Performance

In line with the general trend towards a reduction in anthropogenic emissions from transport and power-generating facilities, much attention is given to the environmental and social aspects of the application of alternative fuels in their development (Figure 3). The main pollutants from the combustion of different types of fuels are carbon oxides (CO₂ and CO), nitrogen oxides (primarily NO and NO₂), sulfur oxides (SO₂), and polycyclic aromatic hydrocarbons (PAH) [99]. The combustion of fuels also produces particulate matter (PM) of different sizes. The greatest health hazard for humans and animals is volatile micro-particles with a size of less than 2.5 µm (PM2.5), as they can penetrate through biological barriers of living beings [100]. From 1990 to 2010, about 3.1 mln deaths were caused by PM2.5 particles. Moreover, PM2.5 particles were found to reduce life expectancy by 8.6 months on average. PM2.5 is responsible for a total of 3% of deaths from cardiovascular and respiratory diseases and 5% of deaths from lung cancer [101].

Carbon dioxide emissions are not hazardous for human respiration. However, in an excess of carbon dioxide, it acts as thermal insulation for the planet. Carbon monoxide (CO), on the contrary, is poisonous to humans and animals [102,103]. CO enters the lungs and binds to hemoglobin to form carboxyhemoglobin, thus preventing the transportation of oxygen by the blood and leading to hypoxia [103]. At a high concentration, it has a toxic effect; namely, it inhibits cellular respiration in the cerebral cortex [103].

Nitrogen oxides (NO_x) are potential irritants that can increase the risk of chronic lung diseases. Nitrogen oxides volatilizing to the atmosphere pose a serious threat to the environment. They combine with the air to form HNO₂ and HNO₃, which are the basic components of acid rain [104]. They are also toxic themselves and cause irritation to the mucous membrane. Nitrogen dioxide (NO₂) mainly affects the airways and lungs. It also alters the blood composition, or more specifically, reduces the content of hemoglobin in the blood [105,106]. Sulfur oxides entering the atmosphere can travel hundreds of kilometers before converting to H₂SO₄ to precipitate with rain. People living near sources of sulfur oxide emissions often suffer from wheezing, coughing, and mucous membrane irritation [104] (Figure 3).

5.1. Diesel-Biofuel Emissions

Hamza et al. [107] investigated the emissions of particulate matter (PM) during the combustion of composite fuels derived from common Diesel fuel, biodiesel, and kerosene. To this end, an experimental rig was constructed on the basis of a 4-stroke, 4-cylinder Diesel engine. Biofuel was produced from sunflower oil through transesterification. Four fuel blends were produced to compare them with conventional Diesel fuel: Diesel fuel with a volume fraction of biodiesel of 10% and 20% (marked BD10 and BD20, respectively) and kerosene with 10% and 20% biodiesel (marked KB10 and KB20, respectively). Hamza et al. [107] established that using composite fuels (namely, BD10, BD20, KB10, and KB20) reduced the emissions of PM1.0 by up to 12.3%, 36.65%, 60.92%, and 81%, respectively, compared to conventional Diesel fuel. The emissions of PM2.5 fell by up to 21.29%, 25%, 41.43%, and 51.85% for the fuel mixtures BD10, BD20, KB10, and KB20. Two factors account for this [107]. First, biodiesel has a high content of oxygen, which causes complete combustion and contributes to the oxidation of soot. The second reason is the low content of sulfur in the composite fuel, especially when kerosene, rather than Diesel fuel, is used as a base. Another study on the formation of PM during biodiesel combustion is the review paper by Mohankumar et al. [108]. The authors note that the use of biodiesel can significantly reduce PM emissions because of low sulfur, aromatic components, and high oxygen content.

Arias et al. [109] studied the PAH emissions of a diesel engine run on different fuel types. The experiments were conducted using a Diesel fuel, to which a road load simulation system was connected. It simulated the operation of the gearbox, tires, and other powertrain-dependent parts of a Nissan Qashqai [109]. The tested fuels were conventional Diesel fuel, commercial fuel derived from hydrotreated vegetable oils (HVO), and four experimental fuel types: hydrogenated turpentine and hydrogenated orange oil at 20 vol% were blended with Diesel fuel (HT20 and HO20, respectively); polyoxymethylene dimethyl ethers (OME) at 20 vol% were blended with Diesel fuel (OME20); biofuel derived from glycerol (consisting of a mixture of fatty acid methyl esters (FAME, 70 vol%), fatty acid glycerol esters (FAGE, 27 vol%) and acetals (3 vol%)), was blended at 20 vol% with 80 vol% HVO (the resulting fuel was given a commercial name of SLB100). Arias et al. [109] reported that the blend of Diesel fuel with hydrogenated turpentine had the highest PAH emissions in the engine tests. The lowest PAH emissions in all the engine tests were typical of HVO. This is explained by its specific thermophysical characteristics. HVO has the highest lower heating value, so it exhibited the lowest fuel consumption. Moreover, with no oxygen in its composition, neat HVO takes more time to burn than the other biofuels and blends. It has the best thermal efficiency at low and high loads, as combustion is more centered around the top dead center of the piston unit. Similar findings were obtained in [110,111,112] when using HVO.

5.2. Kerosene-Biofuel Emission

Gas-turbine power units are another promising area for using biofuels with a high priority on the environmental aspect. Some studies demonstrate the benefit of biofuels in terms of nitrogen oxide emissions [113,114]. López Juste and Salvá Monfort [114] examined the combustion behavior of a composite fuel produced from a commercial biofuel B-EUO4-B with a volume fraction of 80% and ethanol with a volume fraction of 20%. The obtained characteristics were compared with those of the traditional aviation fuel JP-4. The experiments were carried out in a gas turbine combustor equipped with a swirl atomizer. López Juste and Salvá Monfort [114] established that at a fuel/air ratio of 0.09, which corresponds to the energy contribution by a unit of mass of gases about 1.36 MJ/kg, the emissions of nitrogen oxides in the combustion of JP-4 were four times as high. At a fuel/air ratio of 0.06 and energy contribution by a unit of mass of gases 1 MJ/kg, the emissions of NO_x in the combustion of JP-4 were comparable to those for the bio-oil/ethanol blend. This result is accounted for by a higher temperature of the flame during the combustion of JP-4, which causes additional formation of nitrogen oxides [114].

Boomadevi et al. [115] performed tests using a small experimental jet engine. The operation parameters of the engine and the level of anthropogenic emissions were determined for the fuel blends produced from Jet-A and Spirulina algae biofuel. The following fuels were investigated: B20% (20% biofuel with 80% Jet-A); B40% (40% biofuel with 60% Jet-A); B60% (60% biofuel with 40% Jet-A); B80% (80% biofuel with 20% Jet-A); biofuel B100%. The authors found [115] that using biofuel as an additive lowered the emissions of CO₂ at any turbine engine speed. Thus, the emissions of CO₂ when using 100% Jet-A were 3025 g/kg of fuel at a speed of 30,000 rpm and 3095 g/kg of fuel at 80,000 rpm. At the same time, the emissions of CO₂ when using B20%, B40%, and B60% were 3000–3050 g/kg of fuel, 2950–3010 g/kg of fuel, and 2920–2090 g/kg of fuel, respectively. It was also established that an increase in engine speed reduced the emissions of CO. As with CO₂ emissions, the highest CO emissions were also recorded for the 100% Jet-A. An increase in the rotation speed from 30,000 rpm to 80,000 rpm when using the 100% Jet-A reduced the production of carbon monoxide from 138 g/kg of fuel to 68 g/kg of fuel. When using B20%, the emissions of CO fell from 104 g/kg of fuel to 48 g/kg of fuel. Nitrogen oxide emissions increased with the higher speed of the turbine engine. For instance, with the 100% Jet-A, the emissions of NO_x rose from 0.2 g/kg of fuel to 1.2 g/kg. In the case of B20%, this indicator varied in the range of 0.18–1.15 g/kg. However, a further increase in the proportion of biofuel in the blend composition increased the emissions of NO_x. The reason for this is a higher concentration of oxygen in the blends, which led to additional oxidation of nitrogen in the combustion zone.

5.3. Gasoline-Biofuel Emissions

A promising area for using biofuels is gasoline powertrains. In [116], the combustion and emission characteristics of a gasoline engine were investigated. It was fueled by a blend based on gasoline and hydrogenated catalytic biodiesel (HCB) derived from waste cooking oils through one-step catalyzed hydrogenation processes. The concentration of biodiesel in the fuel blend varied in the range of 20–40 vol%. Zhang et al. [116] established that an increase in the proportion of biodiesel in the fuel blend composition reduced the emissions of NO_x, CO, and HC. However, the emissions of PM increased. This occurred because the high content of HCB reduced the ignition delay time and worsened the mixing of the fuel with the air, thus leading to more emissions of solid particles.

The ignition and combustion behavior of composite fuels based on gasoline and lemon peel oil was examined in [117]. The experimental setup was a twin-cylinder gasoline engine with a combustion endoscopic window fitted in it. The fuel blends under study consisted of gasoline with an octane number 87 and lemon peel oil with a volume fraction of 10–30% (named Lp10–Lp30). Velavan et al. [117] reported findings on the concentrations of anthropogenic emissions when varying the proportion of lemon peel oil in the composite fuel composition. Thus, the lowest emissions of CO were recorded for the fuel with 10% lemon peel oil. The highest emission level was for the fuel with 20–30% lemon peel oil. This result is explained by a longer diffusion combustion phase compared to gasoline and Lp10. Diffusion combustion has a higher localized temperature zone, which leads to the destruction of fuel droplets with soot formation and their incomplete burnout in the gas phase. Higher viscosity and slower evaporation of lemon peel oil leads to incomplete evaporation of the fuel, which contributes to the diffusion combustion. The CO emissions in the combustion of Lp10 were 5–7% lower than in the combustion of gasoline. The opposite is true for the emissions of CO₂: the highest content of carbon dioxide in the composition of combustion products was observed in Lp10 and gasoline. This is accounted for by more complete burnout of these fuels. Predictably enough, Lp20 and Lp30 were characterized by lower CO₂ emissions because of diffusion combustion. The content of CO₂ in the composition of combustion products of Lp20 and Lp30 was 2.8% and 4.6% lower than that for gasoline. Velavan et al. [117] obtained interesting findings on the emissions of nitrogen oxides. The emissions of NO_x were maximum for Lp20 and Lp30 and minimum for Lp10. Compared to gasoline, the combustion of Lp10 at full load proceeded with 5% lower emissions of nitrogen oxides. This can be explained by the fact that the slightly higher latent heat of vaporization of the lemon peel oil reduces the temperature in the combustion zone, which, in turn, reduces the emissions of NO_x. However, the emissions for Lp20 and Lp30 were 4% and 9% higher than those for gasoline. This result is due to diffusion flames. The blending of gasoline with lemon peel oil reduces the evaporation rate of the fuel, thus enhancing the diffusion combustion flames. The diffusion flame mechanism, on the one hand, provides a more stoichiometrically correct air/fuel ratio in the combustion zone. However, the air/fuel ratio in the other part of the combustion chamber may be lower than the stoichiometrical one. An increase in the adiabatic temperature of the flame promotes localized formation of NO_x in the combustion zone.

A similar experimental setup was employed in the study by Manoj Babu et al. [118]. They examined gasoline and its blends with pine oil with a volume fraction of 10–30% (named Pn10–Pn30). They established [118] that the lowest emission of nitrogen oxides was recorded for the fuel blend Pn30 over the whole range of engine loads. Lower emissions stem from a decrease in the combustion temperature because of the high density of pine oil, which impairs the formation of the mixture with the air. Moreover, the calorific value of pine oil is lower than that of gasoline. Therefore, an increase in the proportion of pine oil in the blend lowers the calorific value of the resulting fuel, thus reducing the heat release. At a minimum engine load, the highest emissions of nitrogen oxides were for gasoline. With an increase in the engine load, the maximum emissions of NO_x were typical of the fuel blends Pn10 and Pn20. Increased emissions for Pn10 and Pn20 at high loads were caused by a significant amount of oxygen in the combustion zone and a high temperature in the cylinders. As reported by Manoj Babu et al. [118], the heat release for the two blends at higher loads was higher than for gasoline. Therefore, the temperature in the cylinders was high enough for the formation of NO_x. The maximum emissions of CO were recorded for the fuel blend with 40 vol% pine oil. CO emissions are mainly produced because of oxygen shortage in the combustion zone or at a low temperature in the cylinder. Manoj Babu et al. [118] concluded that the cause of the high content of carbon monoxide, in this case, was low heat release during the combustion of Pn30. An interesting trend is observed for fuels Pn10 and Pn20. At low engine loads, when there is not enough oxygen in the combustion zone, these composite fuels have higher emissions of CO than pure gasoline. However, with an increase in the load and, thus, a greater amount of oxygen supplied to the combustion zone and greater heat release in the cylinders, the emissions of CO for Pn10 and Pn20 are lower than those for gasoline.

This leads to the conclusion that anthropogenic emissions of all conventional liquid hydrocarbon fuels can be reduced using biofuels or different oil types as additives. However, it should be taken into account that the level of emissions is determined not only by the fuel composition and characteristics but also by the parameters of the efficient operation of a power unit used with these fuels.

Table 4. Overview of key findings on the emissions when testing composite fuels in different engines.

Ref.	Fuel	Engine	Main Results
[107]	Kerosene + 10–20% biodiesel; Diesel fuel + 10–20% biodiesel		PM1 emissions fell by up to 81% compared to Diesel fuel; PM2.5 emissions fell by up to 51% compared to Diesel fuel
[109]	Commercial biofuel on the basis of hydrogenated vegetable oils (HVO); Diesel fuel + 20 vol% hydrogenated turpentine (HT20); Diesel fuel + 20 vol% hydrogenated orange oil (HO20); Diesel fuel + 20 vol% polyoxymethylene dimethyl ethers (OME20); 80% HVO + 20% glycerol-based biofuel	Diesel engine	The lowest PAH emissions in all the engine tests were typical of HVO.
[119]	Sunflower oil methyl ester/diesel blends	A stationary diesel engine	Emissions of NOx, CO, HC, and particulate matter decreased when using blends based on Diesel fuel and biofuel compared to neat Diesel fuel
[120]	Methyl ester of Karanja and 20–80% blends with diesel	Direct injection diesel engine with a single cylinder	Lower emissions of CO and NOx when using composite fuels rather than Diesel fuel
[114]	80% biofuel B-EUO4-B + 20% ethanol;	Gas turbine combustor with a swirl atomizer	Nitrogen oxide emissions decreased almost 4-fold compared to conventional jet fuel JP-4
[115]	Fuel blends on the basis of Jet-A and Spirulina algae biofuel: B20% (20% biofuel with 80% Jet-A); B40% (40% biofuel with 60% Jet-A); B60% (60% biofuel with 40% Jet-A); B80% (80% biofuel with 20% Jet-A); biofuel B100%.	Experimental jet engine	Emissions of CO2 decreased by up to 11% compared to Jet-A; Emissions of CO decreased by up to 35% compared to Jet-A; Emissions of NOx decreased by up to 5% compared to Jet-A;
[121]	Jet A + 20%/40% biodiesel (B20/B40)	Turbofan engine (CFM56-7B)	Emissions of NOx decreased for B20 and B40 by 29% and 23%, respectively, against Jet A.
[122]	Diesel fuel + 10–50% biodiesel based on palm oil (B10–B50)	MGT (30 kW)	Lower emissions of CO and NOx when using composite fuels rather than Diesel fuel
[116]	Gasoline + 20–40% hydrogenated catalytic biodiesel (HCB)	Gasoline engine	Emissions of NOx, CO, and HC decrease when using HCB; PM emissions increase when using HCB
[117]	Gasoline + 10–30% lemon peel oil (Lp10–Lp30)	Gasoline engine	CO emissions in the combustion of Lp10 were 5–7% lower than in the combustion of gasoline; NO emissions in the combustion of Lp10 were 5% lower than in the combustion of gasoline; CO2 emissions in the combustion of Lp30 were 4% lower than in the combustion of gasoline;
[118]	Gasoline + 10–30% pine oil (Pn10–Pn30)	Gasoline engine	Maximum emissions of CO and minimum emissions of NOx were recorded for the fuel blend with 30 vol% pine oil.

presents studies on anthropogenic emissions when using biofuels in powertrains. Taken together, these findings suggest that the optimal choice of composition of fuels and operation of powertrains can bring about a significant environmental effect.

6. Conclusions

(i)

Many of the energy industry aspects are crying out for revision and innovation. In this respect, biofuels are attracting more and more interest. The conducted review has revealed that this area provides tremendous opportunities for replacing fossil fuels of petroleum origin. However, the diversity of raw materials and their processing methods determine the respective variation ranges of end-product properties. Moreover, only some technologies provide high-quality biofuel (transesterification, some types of thermal conversion). More complex technologies (cracking and hydrocracking) serve to create biofuels whose performance is comparable to that of conventional hydrocarbon fuels.

(ii)

Spraying, being a practical aspect of using liquid fuels, is an important stage. Data from numerous studies suggest that this stage, in many ways, determines the efficiency of combustion of aerosol, thus, the thermal efficiency of a unit and emissions. Increased viscosity of composite liquid fuels containing biofuel is the main constraint on efficient spraying in a combustion chamber. To solve this problem, researchers have proposed using additives (e.g., ethanol), preliminary heating of the fuel, or transverse injection of gas to provide disruption.

(iii)

The carbon neutrality of plant raw materials makes composite biofuels very environmentally attractive. Particular performance characteristics of the engine may vary, and in some cases, the combustion of composite fuels is inferior to that of conventional Diesel fuel, kerosene, and gasoline in emissions. There is a clear trend towards a decrease in the emissions of particulate matter (PM) with an increase in the proportion of biodiesel or bio-kerosene in the liquid fuel composition. At the same time, the yield of CO depends greatly on the operating conditions of the equipment. Generally, the review indicates that CO emissions tend to rise with an increase in the proportion of oils or biofuel to over 15–20%. This is largely attributed to a lower combustion temperature.

(iv)

The fuel properties, chemical composition, and presence of impurities affect the spraying, mixing with the air, rates of evaporation, and kinetics of ignition and burnout. The studies conducted with different composite fuels reveal that satisfactory combustion is possible by controlling the fuel density and viscosity, as well as by selecting the composition with a high evaporation rate. The impact of the new kinds of fuels on the condition of components and parts of engines, corrosion, and wear is understudied. The most promising development paths are the reduction of the cost of cracking raw materials, the search for affordable low-viscosity components for fuel blends, upgrade of plants and fuel feeding systems for optimal atomization, evaporation, ignition, and burnout of fuel mixtures.