Comprehensive review of algae biodiesel production engine performance and emission characteristics w

Abstract

Keywords: Biodiesel Feedstocks; Algae; Processing Methods; Additives; Compression Ignition Engine Combustion; Performance and Emissions

Introduction

A drastic transformation in global environmental pollution has recently occurred due to increased emissions from different industries, such as agriculture, transportation, power plants, and chemical processing. These industries directly or indirectly rely on using petro-diesel-derived fossil fuels, which are considered harmful nowadays. The main reason for the concern was the high consumption of petro-diesel-derived fossil fuels, which are the primary energy resources and supply 88% of the world’s energy needs [1].For more than a century, the diesel engine has profoundly impacted the industrial economy, as it is used in various applications that require mechanical motive power. With the growth of the population worldwide [2], demand for fossil fuel supplies is on the edge of devastation [3],and environmental air pollution, especially the toxic emissions from the combustion of these fossil fuels, is considered one of the highest concerning factors, in which diesel engine plays a predominant role. Internal combustion engines exert harmful pollutants such as CO2, CO, NOx, PM, and smoke, significantly impacting human respiratory disease [4]. Scenario of Global and National Energy Consumption The two major contributing factors prevailing the surge in energy consumption are the world’s population and theexpanding economy at an astounding rate. However, switching energy systems from fossil fuels to low-carbon sources was challengingin mitigatinggreenhouse gas emissions [5]. The change in global energy[6] consumption annually states that the world’s energy consumption is intensifying, with an average annual increase of 1 % to 2% [7].India meets nearly 80 – 85 % of its total petro-diesel requirements through imports. As tabulated in Table 1, the energy consumption of hydrocarbon fuel shows an increased trend year to year, which plays a significant contributor in developing the nation’s economic growth [7]. With this, the proportion of fossil fuel supply of energy, the portion of imported energy, is expected to surpass 90% by 2030 [8].Thus, an extensive search for options to reduce energy consumption is the

primary option to minimize the environmental air, soil, and water pollution in the nation and worldwide. Hence, searching for alternative fuels like biodiesels and mixing additives like alcohol, ethanol, methanol, diethyl ether, butanol, n-pentanol, decanol, hexanol fuels, etc., [9] [10]plays a predominant role in diesel engines and is much more recommended in developing countries like India. Scenario of Environmental Air Pollution Air pollution is considered the most alarming environmental situation, and the daily estimated economic cost of air pollution is 3% -4% of the global gross product. According to the World Air Quality Report 2023 [11]. India has been identified as the world’s third most polluted country, with an average annual PM2.5 concentration of 54.4 micrograms per cubic meter. As per records, around 96% of the Indian population faces PM2.5 concentrations higher than the WHO-recommended level (5 micrograms per cubic meter). Therefore, the PM2.5 concentration increases the risk of developing health problems such as asthma, cancer, stroke, and mental health complications [11]. The baseline levels of PM2.5 and PM10 infew cities like Bengaluru, Chennai, Hyderabad, Thiruvananthapuram, and Vishakhapatnam are estimated to be in the range of 5–15 μg/m3 and 15–43 μg/m3, whereas their background levels are 18–37 μg/m3 and 46–70 μg/m3, respectively [12]. India stood in 8th place with 53.3 μg/m3 in 2022, PM 2.5 concentration, and Delhi ranked 4th out of 50 of the world’s most polluted cities [11].According to WHO statistics from 2017 – 2021, India ranked 11 of the 15 most polluted cities 2021. In India, the major cities contributing to air pollution - PM2.5 [13] are presented in Figure 1. It is evident from Figure 1 that 48% of India’s air pollution reached 50 μg/m3 in 2021, and it observed that all the cities are beyond the limit of WHO guidelines [13]. Therefore, India’s significant causes of air pollution include vehicle emissions, industrial waste, biomass combustion for cooking, power generation, crop burning, plastic burning, and the construction sector.Among these, vehicle emissions exerted from internal combustion engines are one of the prevailing causes of concern for air pollution. However,

Table 1. Share of the major energy resource supply of the fuels in the World and India (%) No.

Figure 1. Annual average (µg/m³) of PM 2.5 in Indian major cities (2017 – 2021).

the practical applications of internal combustion engines are growing gradually due to their high-power output and marginally low fuel consumption. In addition, they found almost all industrial applications, from small to large scales, in today’s world. The primary concern of these combustion engines is their toxic exhaust emissions[14]. In the present situation, thedangerous pollutants exerted by automobile engines, including carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxide (NOx), particulate matter (PM), unburned hydrocarbons (UHC), and smoke, are considered as a severe threat to humankind. Different research groups have made various proposals to regulate this issue. Replacing internal combustion engines with electric motors and replacing existing fossil fuels with renewable and sustainable fuels are attractive proposals that are gaining wide attention. In addition, the entry of 17 sustainable development goals into the force has motivated the researchers to achieve Affordable and Clean Energy by 2030. Within this limited time frame, burning and using sustainable alternative fuels to replace fossil fuels are considered feasible solutions compared to replacing existing engines with other technology like electric motors.However, the fossil-derived petro-diesel fuels presently used in compression ignition (CI) engines are at

the exhaustion phase, which led to a focus on a sustainable energy source known as biodiesel.

Emergence Of Biodiesels

Biodiesels are widespread for their clean burning, high oxygen levels, and low carbon content[15] and are famous for producing less emissions than conventional diesel fuels [16]. Biodieselsare used in the CI engines as neat biodiesel orblended. They have beenproven to be one of the emerging sources for replacing fossil fuels.Raman et al. [17]used rapeseed biodiesel to estimate the engine behavior of a conventional DI diesel engine. The investigation concluded that the B25 biodiesel blends enhanced engine performance, resulting in high BTE and reduced engine exhaust emissions. Similarly,Chozhavendhan et al.[18] discussed various parameters influencing biodiesel production, like lipid/fat content, catalyst, molar ratio, and purification process, to reduce the unit and operation cost to produce biodiesel. The study reported that biodiesel from lignocellulose, edible, non-edible oils, and micro and macroalgae emits less pollution than fossil fuels. Prasada Rao et al.[19] conducted an experimental investigation using palmyra biodiesel blends of POME10, POME20, POME30, and

POME40 to estimate the engine performance and emissions. The findings reported that the exhaust emissions for the blend POME20 are reduced for CO by 15.86 %, UHC by 18.5%, and smoke emissions by 14.28 % compared to diesel.

Biodiesel Generations

Biodiesels are extensively categorized into first-generation (IG), second-generation (IIG), third-generation (IIIG), and fourth-generation (IVG). A detailed overview of the four-generation biofuels and their latest developments were highlighted[20], and various feedstocks, oil, and biodiesel yield percentages,and characterization of biodiesel fuel properties for the production of biodiesels were reviewed [21].The first-generation biodiesels (IG) come under edible oils; these crops produce biodiesel and are used for food consumption. Gradually, the utilization of IG biodiesels has raised concerns about food vs. fuel. Hence, to overcome the drawbacks associated with biodiesel, IIG biodiesels exist; these are linked to the non-edible oil category, such as seed oil, waste vegetable oils, and animal fats [22]. Rezania et al.[23]reviewed different non-edible sources (IIG) for biodiesel production using the transesterification method as these are economical, and there is no issue or debate on food vs. fuel, likewise in edible sources. However, various second-generation biodiesel feedstocks are tested on the CI engine to observe the performance characteristics and emission levels. Correspondingly, experimental studies are conducted by mixing additives [2426]into the biodiesel blends. Aruna Kumari etal.[27] tested the engine emissions using lemon peel biodiesel blends of 10%, 20 and 30% vol.% and trailed on the CI engine. The test observations revealed that CO, HC, and smoke emissions were recorded at 20% of lemon peel oil blend by 25%, 25.6%, and 15.44%, and an increase in NOx was observed when compared with diesel. With this motivation,Enweremadu etal. [28] used canola and sunflower biodiesel feedstocks and simulated the engine behavior using C++ software on a Mercedes Benz DI engine. The output indicated that biodiesel possesses higher Specific fuel consumption (SFC), brake power (BP), and thermal efficiency than diesel. Further, high NOx and low smoke emissions were recorded compared to diesel. Thus, several researchers extended their investigations to mitigate emissions by employing additives in IIG biodiesels [29 - 31]. Accordingly, Ahmad S et al. [10]mixed oxygenated additives, methanol or diethyl ether in Neem biodiesel, a second-generation biodiesel, due to the rise in NOx emissions. The engine experimental test showed that BSFC was reduced by 10%, CO and HC were decreased by 25%, and NOx emissions lowered when Neem biodiesel was tested with diethyl ether. In a similar form, the addition of heptane butanol and diethyl ether was mixed in Jatropha biodiesel, and the output results showed that there is an enhancement in BTE and an increase in BSFC by 5-20% and a reduction in engine emissions by 3-12%. Similarly,Calophyllum inophyllum biodiesel enhanced engine efficiency and lowered

emissions by adding oxygenated additives butanol, pentanol, decanol, and hexanol. Thus,overcominga few shortcomings in biodiesels,such as higher density, kinematic viscosity, lower calorific value, and rise in NOx, has instigated researchers to add oxygenated additives in different biodiesel generations. Based on the investigations contributed by enormous numbers of researchers, it is understood that biodiesels can replace fossil fuels. Though IIG biodiesels such as waste cooking oil gained positive attention as this feedstock is obtained at lowcost and readily available, the main set back observed over WCO biodiesel was the filtering and processing of WCO[32]. Thus, the hitches raised in IG and IIG biodiesels have directed the approach towards sustainable, clean, and green fuels, known as third-generation algae biodiesels (IIIG). Algae are ubiquitous photosynthetic organisms on the earth, identified as the fastest-growing biomass. Algae are known for their abundant availability, require less land space, rapid growth, use of natural resources such as sunlight CO2, and use as food supplements [33]. The processing of algae is a crucial agent in the preparation of algae oil for biodiesel[33]. In particular, algae cultivation is significant for growing and harvesting algae. However, a slight increase in economic viability is a setting concern during harvesting [34], andseparating algae layer by layer [35] consumes more time in algae processing.Hence, algae are the best source of biodiesel, producing clean-burning renewable fuel. In recent years, several researchers[36-38] have turned their interest toward fuel production from one of the oldest living creatures on the earth, known as algae, serving as a biofuel source. With this motivation, an experimental investigation was performed on one cylinder, 4-stroke, ata speed of 1500rpm, for the algae biodiesel blends of 5%, 10%, 20%, and 30% [39] to estimate the engine combustion performance and emission characteristics.The tested results reported that at blend up to5%, the BTE decreased, and the reduction in CO and UHC was observed up to 28%. Finally, a gradual increase in NOx emission (up to 13%) was shown in the experimental tests. And a volume fraction of 20% exhibited better results compared to other blends. However, additive mixing in biodiesel plays a significant role in overcoming the rising NOx emissions. Based on this,Sekharraj K et al. [40]conducted an experimental study on green microalgae biodiesel blends for 80-20 (80% diesel and 20%biodiesel) mixed with Bi2O3 nanoadditives to determine the performance and emission characteristics. The engine test results reported that using nano additives enhanced the engine’s performance and reduced the engine emissions.The schematic representation of the biodiesel production based on different generations is represented in Figure 2 [22]. The fourth-generation biodiesel (IVG) deals with the genetic modification of microalgae, fungi, yeast, and cyanobacteria[1],[22]. The primary benefit of employing IVG biodiesel was its ability to grow microalgae and collect significant

Figure 2. Different generations and their process of production of biodiesel. amounts of CO2 by boosting productivity and availability by incorporating modern technologies. However, this technology cannot be employed with all microalgae species due to a lack of gene and biological understanding [41]. Therefore, third-generation algae are recognized as the most significant potential to produce algae biodiesel and are considered one of the emerging sources of fuel to run and test on existing diesel to replace fossil fuels. The observations from the biodiesel generations revealed that the IG edible oils created a debate on food vs. fuel, which hinders using fuel for transportation. Though IIG biodiesel (non-edible oils) play a vital role in improving efficiency and reducing emissions, the limiting factors, such as the availability of land to cultivate the non-edible oil crops and high fatty acid profiles, led to opt for IIIG algae biodiesel as one of the better options for replacing conventional fuels in CI engines since the land area required to cultivate the algae [36] is significantly less and reported high oil yields compared to the IG and IIG. However,the rise in NOx emissions in second-generation [27-28]and third-generation biodiesel [29] confirmed an increase in NOx emissions. Thus, to mitigate emissions, several researchers extended their investigations by employing additives in IIG biodiesels [29 -31] and IIIG biodiesels[41],resulting in improvements like complete combustion in ignition chambers and reduction in temperature,

leading to enhanced engine performance and lowers engine emissions. Sustainable fuels like biodiesel are compact with existing engines, and their application doesn’t require any major modifications in the existing engine designs, which made them attractive and popularly investigated by different research groups [42]. Therefore, in this review article, a clean burning and sustainable biodiesel from the 3rd generation oils is critically reviewed,highlighting its significance at different stages, such as its production process, significant properties, and applications in diesel engines in terms of performance and emissions. In addition, various additives that improve the overall properties with their addition to biodiesels are also reviewed. To further strengthen the readership of this review article, a bibliometric analysis [44] is carried out to understand the past and current research trends on this topic. For this purpose, the keywords “Algae Oil” AND “ Production” are taken from the Scopus database, and their results as network graphs are highlighted in Figure 3, which clearly indicates that research on IIIG oils is emerging.

Third-Generation Algae OIL

Algae are living organisms primarily found in different breeds of aquatic environments and can reproduce faster than land-grown plants. These can survive in freshwater resources, ponds, rivers, lakes, rocks, marine, brackish,

Figure 3. Bibliometric analysis for algae oil and its production.

municipal, industrial wastewaters, and moist soil surfaces. The significant features of algae are ease of cultivation, utilization of natural sunlight and CO2, less land area for growth, more effective than terrestrial seed crops [26], higher growth rate, use as a food supplement, and mitigation of environmental pollution. The invention of algae is not new in the 21st century; algae as biodiesel originated inthe 19th century.These are classified as microalgae and macroalgae. The microalgae primarily consist of unicellular structures measured in micrometers. These usually grow in open ponds such as freshwater tanks, reservoirs, etc., and closed ponds (photobioreactors),whereas macroalgae are multicellular in structure, measured in inches, and grow large. These are found in marine seawater, wastewater, and ponds [36]. The main characteristics of microalgae growth depend on productivity, low production and maintenance cost, temperature control, pH, oxygen, and reliability. Among these, microalgae species have higher oil content and can extract oil in large amounts [44]. Thus, microalgae are considered the best form of green energy for supplying the world’s demand for transportation fuels. OVERVIEW OF ALGAE: TYPES OF ALGAE Algae species are one of the oldest living organisms. Worldwide, over 60,000 microalgae species were spotted,

out of which 35000 species were recognized as the potential for the generation of biodiesel [44]. These are the most exciting future solutions for the energy crisis, especially that of transportation fuel. Based on their growth and climatic conditions, microalgae species were grouped into Bacillariophyta (diatoms), Charophyta (Stoneworts), red (Rhodophyta), green (Chlorophyta), brown (Phaeophyta), blue-green (Cyanobacteria), red-green, golden-algae (Chrysophyta), phylankton, seaweeds and other algae strains[20], [26]. Among these[45],greenmicroalgae (Chlorophyta) bagged with more essential benefits than other algae species since these are unicellular and aquatic. In India, green microalgae species grow abundantly, and the climate conditions are well-suited for growing green algae in large quantities. Thus, the most prolific strains are Chlorophyceae (green algae) [44], which can grow well in open ponds, accumulate lipids at low temperatures like 10°C, and have a better tolerance to CO2. Some green-microalgae species,Chlorella, Spirogyra, and Spirulina,are cultivated and grown in an open atmosphere. In particularalgae species, the oil content ranges between 20 to 40% of its dry weight, whereas, in some specific algae strains, nearly 80 to 85% of the oil content was identified. Thus,if suitable species were identified, more than 85% of the oil could be extracted [38]. The oil yield and land area

requirement of various biodiesel feedstocks are presented in Table 2 and Figure 4. Thus, it is observed from Table 2 that algae species are recorded with high oil yields. Figure 4 shows less land area required to cultivate the algae than second-generation biodiesel.

Algae Processing Methods

The selection of the sitefor algae cultivation is one of the basic requirementsforalgae biodiesel production. The four main processing methods of algae are cultivation, harvesting, oil extraction, and biodiesel preparation. Chowdhury etal.[46] demonstrated the importance of microalgae site selection for algae, reviewed different microalgae cultivation, harvesting, oil extraction, and biodiesel production methods, and reported that green algae possess a high growth rate. In an ideal environment, green algae can double their biomass in less than a day and possess massive lipid content (more than 50%), an excellent biodiesel production source.

Algae Cultivation

The essential factors required to absorb and convert natural sunlight into chemical energy are CO2 and H2O. The Algae can be cultured in two processes: natural cultivation and artificial cultivation process [26]. In the natural cultivation process, the algae are grown in open ponds,

Table 2. Oil yield produced by various biodiesel feedstocks No. Feedstock

raceway ponds, and freshwater tanks, whereas in artificial cultivation,algae growth is initiated in closed reactors

(photobioreactors). Subsequently, the naturally-grown algaecan be cultivated in freshwater tanks, directly collected from lakes, rivers, and ponds, and found as vats [45]. Kalyani et al.[35] cultivated the naturally grown green algae in the freshwater tank without chemical fertilizers; only sunlight, CO2, and temperature are required to produce the algae. The basicparameters needed to maintain the algae growth and reproduction are temperature (20 °C to 30 °C), salinity, light intensity (33 μmol m-2s-2 to 40 μmol m-2s-2), and pH value (5.5 to 7.5). Another researcher [47] collected the naturally grown green algae from open ponds and reservoirs without constructing a water tank for cultivation. Further,algae processing methods such as harvesting, oil extraction, and biodiesel production were processed using algae collected from open ponds.

Harvesting

After algae growth, the wet algae biomass is harvested using various techniques [48],such as centrifugation, sedimentation, flocculation, and filtration. V. Ananthi etal.[48] studied the merits and demerits of several harvesting techniques and suggested that magnetic separation and chemical coprecipitation are the most commonly used methods. Thus, based on the above harvesting methods, the amount of processed dried algae biomass acquired can be further processed for algae oil extraction.J. Sen Tan et al.[49] provided information on microalgae culturing, harvesting, and extraction methods using advanced ionic liquids. Thus, using various algae harvesting methods, the algae is ready to extract the algae oil and determine the fuel properties.

OIL Extraction Process

The extraction process involves transferring the solid dried algae powder into a liquid phase. The oil extraction techniques were classified into chemical and mechanical methods [50]. Chemical methods include solvent extraction, Bligh and Dyer’s, Ionic liquid extraction, Supercritical CO2 (S-CO2) techniques, and mechanical methods include oil expeller process, Osmotic shock, microwave-assisted and

ultrasonic assisted process[51],[52]. The oil yield is estimated using Equation 1. (1) Studies from various methods have reported that supercritical CO2, solvent extraction, and oil expeller techniques are viable methods to extract. Among these, the solvent extraction method is most efficient and recovers the oil up to 40 – 78%[45],[52],[53].This method is economical, suitable for small scale, and has high extraction efficiency. However, to extract maximum oil, this method requires more time and a massive amount of algae powder [50]. Transesterification Process Transesterification is the most widely adopted method to produce biodiesel at the micro and macro levels. The raw oil recovered from the crop seed and algae oil extraction process is converted into biodiesel using various preparation methods such as mixing raw oil, pyrolysis, dilution, microemulsions, and transesterification process[23],[51]. Among these methods, the transesterification method is the most appropriate method, as it reduces the high viscosity of the raw oil [53], and 98% of the methyl ester can be produced using this method [54]. The influence of different types of catalysts, such as homogeneous and heterogeneous catalysts used in the transesterification process, yielded high-purity biodiesel and glycerol, which depends on the long-chain fatty acids. Kalyani et al. [35] used homogeneous catalyst sodium hydroxide (NaOH) and methanol (CH3OH) to initiate the transesterification reaction process for the algae biodiesel, and the saturated FFA was 73.95 wt.%. [55] used a waste chicken eggshell as a heterogeneous catalyst and obtained

Biodiesel Blending

Biodiesel of various feedstock obtained using a transesterification process was blended with diesel using a highspeed stirring device and whisked continuously for half an hour (30 mins.) from 10% to 90% with a 10% volumetric

Figure 8. Flow chart representing the algae processing methods and engine experimentation.

increment of B10 to B100 [35]. The pictorial representation of the preparation of blends is shown in Figure 9.Biodiesels have significant benefits, including the technical viability of blending in any ratio and proportion with petroleum diesel fuels. Several investigators continued their research by testing both IIG and IIIG biodiesels for B10 and B20 [48], and

the test results were compared with the established international standards [58], [59], [60]. Several studies on the fuel refinement policies and experimental investigations [19], [61,62,63,64] were conducted on various biodiesel blends (i.e., B5 to B100) to determine the vital fuel properties and fatty acid compositions to

improve the engine performance characteristics. Hence, these draw a significant interest in mitigating the environmental problems to meet the future energy requirements in CI engines.

Biodiesel FUEL Properties

One of the significant features of biodiesel is its ability to compete with diesel. These biodiesels are vital in storing, handling, transporting fuel, and commercialization. Studies revealed that fuel properties, such as flash and cloud points [65], are more excellent than diesel, representing a favorable sign to store the fuel. The literature suggests that engine performance depends upon the physicochemical fuel properties, and the vital fuel property values were compared with internationally recognized biodiesel fuel standards(ASTM). Several researchers[18],[65],[66]tested the biodiesel fuel properties ofa few IIG and IIIG feedstocks and compared them with diesel are presented in Table 3. Kinematic Viscosity(mm2/sec) The kinematic viscosity (KV) of the working fluid plays a crucial part throughout the fuel injection process. Fuels with less viscosity were suggested to achieve improved fuel atomization during combustion and avoid blockage problems associated with fuel injectors. The kinematic viscosity was determined using a Redwood Viscometer according to ASTMD445 for the biodiesel blends. It is evident from Table 3 that the kinematic viscosity value of Spirogyra green algae biodiesel [36] is less than algae, canola, olive,

and cotton seed biodiesel. However, the KV values are higher than diesel KV values and fall within the range of ASTM D6751 standards. This change is due to the large size of triglyceride molecules in oils and the low volatility of biodiesels. Therefore, raw oils with high viscosity are not recommended to run on CI engines as they will cause incomplete combustion and could observe a high rise in engine exhaust emissions. In addition, some researchers tested using neat biodiesel on the CI engine running at a high compression ratio, and the results showed that the engine efficiency was enhanced [21].Thus, increasing the diesel percentage can reduce the kinematic viscosity of the biodiesel. Density (kg/m3) The fuel density (D)is one of the key characteristics that mark its effects on the fuel performance, atomization quality, and combustion. The density was measured using a relative density meter, and ASTMD1298 standards were followed. Like kinematic viscosity, the density is higher for neat biodiesel fuels than diesel. This variation is due to the high molecular weight; however, the limit range is within the standard range. Similar to kinematic viscosity, the density of the neat biodiesel was reduced by addinga high percentage of diesel. The fuel’s density influences combustion, atomization quality, and fuel performance, as the CI engines employ the fuel injection pump system to allow the fuel into the combustion chamber. The presence of FFA, H2O content, and molar mass are the prime determinants of density in methyl or ethyl esters[47], [67].

…... …... …... 168 178 …... …... 153 Cleveland Apparatus Fire Point (oC)

180 130 147 152 170 140 115 146 Pensky Marten Closed Cup Flash Point (oC)

58 52 38050 - 38540 55 38.08 …... 37000 41000 41,243 Bomb Calorimeter Calorific Value (kJ/kg)

39455 38293 38559 58-65 52 59 37 - 72 53.49 Ignition Quality Tester Cetane Number

864 873 878 860 - 880 880 881 850 to 870 887 Relative Density Meter Density (kg/m3)

Evolution Of Algae Transportation

Algae-derived biofuels can improve the world’s transportation fuel and lower global GHG emissions. This renewable energy is expected to switch the world’s energy supply by 2070 [74]. The control of emissions plays a pivotal role

when selecting the fuel/biodiesel. Due to increased global environmental pollution, stringent rules were imposed on the transportation industry, and several emission reduction measures such as exhaust gas recirculation [19], mixing of metallic, oxygenated additives [63], [75] mixing of nanoparticles [31], etc., were implemented.

Engine Emissions The rising concerns about internal combustion engine exhaust emissions are due to their toxic nature. The main byproducts from the combustion of petro-diesel fuels are Unburnt hydrocarbons (UHC), carbon dioxide (CO2), carbon monoxide (CO), sulfur oxide (SOx), nitrogen oxide (NOx), particulate matter (PM), soot and black smoke, which are dangerous to human health and also causes environment air pollution. Some of the human health issues like respiratory problems, cardiovascular diseases, cancer risks, neurological effects, irritation, and allergic reactions are witnessed in patents when they are exposed for a prolonged period to these exhaust gases [63]. Different factors like fuel properties, air-fuel ratio, injection timing, engine speed, engine load, turbulence, and mixing play a significant role in the combustion of injected fuel and its byproducts of exhaust emissions. For instance, incomplete combustion of fuel will lead to the formation of carbon monoxide emissions, and this incomplete combustion is due to the unavailability of oxygen, poor mixing of air-fuel mixtures, and dissociation effects [76].The chemical reaction, as shown in Equation 2, is a clear indication that a lower quantity of oxygen participation during the combustion reaction led to CO formation. It is also known that diesel engine combustion is heterogeneous combustion [77] and involves both liquid and gas phases. The hydrocarbon molecules react with available oxygen during the combustion process as a result of the formation of CO2 and water (H2O), which is also considered complete combustion, and the chemical reaction is shown in Equation 3 & 4. The unburnt hydrocarbons are formed due to the incomplete oxidation process. Sometimes, the fuel does not receive enough oxygen to oxidize completely into CO2 and H2O during the combustion process; as a result, the unanticipated fuel remains unburnt hydrocarbons [78]. This phenomenon occurs particularly in fuel-rich areas where less air is available. Another significant contributing factor to harmful exhaust emissions is the combustion temperatures. High combustion temperatures result in high thermal NOx formation, one of the most dangerous exhaust emissions. The NOx formation during the combustion process is governed by the Zeldovich mechanism [63], which describes the reaction of nitrogen with oxygen at elevated temperatures. At high temperatures (above 1200oC) the nitrogen molecule (N2) in the air reacts with the available oxygen (O2), and the chemical reactions are shown in Equations 5 to 7.Different studies [79] show that biodiesel combustion increases NOx emissions, which may be true due to the inherent property of available oxygen in biodiesel being more than any other fuel, which helps increase combustion temperatures during its combustion. However, this case is applicable to neat biodiesel combustions. To control the NOx formation from biodiesel combustion, different techniques like low-temperature combustion, such as injecting the alcohols to maintain controlled

combustion temperatures and also with the implementation of different additives like oxygenated additives, nanoparticlesregulate this NOx formation to a greater extent [31],[64],[76],[81]. The Table 5 reveals the trends of exhaust emissions from biodiesels investigated by different research groups. (2) (3) (4) (5) (6) (7) From Table 5, the above reference studies observed a positive incline toward the increase in engine performance and reduction in emissions. Interestingly, a drastic improvement in reducing emissions was observed using third-generation algae biodiesels (IIIG). However, an upsurge in NOx emissions was reported as the signifying issue in most of the experimental tests [47,85,88,89]. Hence, to mitigate this raising concern, mixing additives in biodiesel feedstock is considered an alternate option to mitigate NOx emissions.

Introduction To Additives

Mixing additives in biodiesels is crucial in increasing the fuel’s durability and decreasing the hazardous pollutants produced during fuel combustion. Also, the option to substantiate these issues was additive to avoid limitations of biofuel properties like auto-ignition, flash point, temperature, and cetane number. Additives primarily depend upon the selection, concentration ratio, fuel solubility, and physicochemical properties such as viscosity, density, toxicity, ignition quality, etc. Various types of additives, such as metallic-based additives, oxygenated additives [25], metal-based additives, cold flow improver additives, cetane number improver additives, water, antioxidants, lubricity improvers, polymeric-based additives, and nanoparticles additives [24,26], are employed to blend with fuels used in diesel engines to achieve total fuel combustion, which further improves the performance of CI engines during combustion and reduce the engine emissions. Therefore, mixing additives in diesel - biodieselblends result in improvements like complete combustion and temperature reduction, enhancing engine performance and reducing engine emissions [75], [89]. Many researchers have investigated various additives mixed in some IIG and IIIG biodiesel blends and tested them in

Marine dinoflagellate Crypthecodinium cohnii and waste cooking oil (WCO) biodiesel

Nodularia Diesel engine Spumigena microalgae (A), Karanja (K), rice bran and castor oil

Kirloskar TV – I, Bore & stroke: 87.5 mm& 110 mm Power: 5.2 kW,

↑ Decrease in ↑ Increase in BTE by 10% for BSFC for all B20 blends

↓ Decrease in ↑ Increase in CO for CEME all CO2 for B20 blend blends

↓ Decrease in ↓ Decrease in NOx by 20% for CO by35% for B20 B20

Performance and emission characteristics compared with diesel

Table 5. Review on engine performance and emission characteristics of different second and third generation biodiesel feedstocks

↓ Decrease ↓ Decrease in in UHC for smoke for CEME CEME for all blends

↓ Decrease in ↓ Decrease in UHC by 18% smoke by 33% for B20 for B20

1898 J Ther Eng, Vol. 11, No. 6, pp. 1883−1909, November, 2025

Chlorella emersonni methyl ester (CEME) Blends: B10, B20, B30 B100 and D100

Stationary singleCompared to diesel, B30 exhibited better results than other blends. cylinder diesel engine. ↑ Increase in ↓ Decrease in ↑ Increase in ↓ Decrease in BTE BSFC NOx for B100, CO by 50% B30, B20

↓ Decrease in ↓ Decrease in ↓ Decrease in CO At 100% load UHC by smoke at 100% 50% reduction 40.57% load by 38.18% is observed POBD, for POBD, for POBD and 10.14% 63.63% for WCBD WCBD WCBD and and a 25% and 37.68% 58.18% for reduction in AFBD at AFBD AFBD recorded 100% load compared with compared to diesel fuel diesel fuel

Compared to Compared to diesel, B20 emits diesel, B20 emits Compared to diesel, B20 8.33% less 0.86% less emits 5.55% less.

↓ Decrease in Water-cooled 1-cylinder, 4-stroke CI BTE at 100% load engine; Loads: 0, 20, 40, 60, 80 and 100 N= 1500 rpm and CR: ↓ Decrease in BTE by 26.87% 17.5. at B25 than B20.

↓ Decrease in NOx emission levels 1282 for B20, which is 5.78% higher than diesel.

Eureka Sativa (Taramira) seeds oil Blends: B10, B15, B20, and B25.

↓ Decrease in BSFC by 0.29 kg/kw at B20 blend than all blends

↑ Increase up -----------to 20% at high load, low speed (N<1500 rpm)

-----------↓ Slight reduction (57%) of torque at high speed, full load

Lombardini, 4-cylinder, 4-stroke, naturally aspirated IDI engine, CR: 22.8:1 Bore: 75 mm Stroke: 77.6 mm

↑ Increase in CO2 by 11.23% for WCBD, 13.28% for AFBD, and decrease in CO2 for POBD

BSFC ↑ Increase in NOx by 2.52% for POBD, 17.16% for WCBD, and 8.09% for AFBD compared to diesel fuel at 100% load. And observed even at 75% and 50%

Performance and emission characteristics compared with diesel

↑ Increase ↑ An increase in BSFC by in BTE was achieved for 9.72% for POBD, 5.48% WCBD of (32.22%) AFBD & 4.90% followed by for WCBD AFBD (31.99%) compared to diesel fuel. and POBD (30.93%) at 75% load.

Bd15t, Bd20t Bd25t

↓ Decrease in ↓ Decrease in CO2 21% for CO by 50% for 3%DOEE 3%DOEE in PKME at 100% in PKME at 100% operating operating load load compared compared to to diesel diesel

↓ A decrease in BSFC was observed for 3%DOEE in PKME compared with other additives

↑ Maximum efficiency of 33.79% is achieved for 3%DOEE in PKME at 100% operating load compared with other additives

BSFC, BTE

↑ Increase in BTE for BD100 compared to diesel. ↓ Decrease in IMEP, BMEP, Friction Power

6-cylinder turbocharged diesel engine Power 162 kW Speed 2000 rpm CR17.3:1

Ali Zare et Waste cooking al.,[92] biodiesel + Triacetin Blends: D100, B100, T100, T4B96, T8B92 T10B90 D60B35T5

↓ Decrease in NOx with the addition of butanol for all the blends

↑ Increase in BSFC about 10.9%, 17.1% and 29.7% for D80B20 D70B20But10 and D60B20But20 compared with diesel fuel

Mitsubishi Canter 4D34-2A, direct injection, 4stroke, 4-cylinder diesel engine. Power 89 kW 1200 - 2800 rpm at full-load

Microalgae + Butanol Blends: Diesel D80B20 D70B20But10 D60B20But20

Algae Botryococcus Braunii + Acetylene Induced at a mass flow rate of 100, 150, 200, and 300 g/hr.

4-stroke (Kirloskar AV1), dual fuel CI engine with enhanced IT

Table 6. Review on engine performance and emission characteristics of mixing additives in differentsecond and third generationbiodiesel feedstocks (continued)

↓ Decrease in PM by 37.58% ↓ Decrease in HC by 33 ppm ↓ Decrease in NOx ↓ Decrease in CO ----------by 846 ppm by 0.02% ↑ Increase in BTE ↑ Increase in BSFC

Diesel engine Injection pressures of 200, 225, and 250 bar Illipilla M TiO2 nano particles et al., [95] + dispersant (QPAN80) Blends: BD20+ TiO2 at 50, 75, 100 mg/ L+ dispersant (QPAN80) 1:1 ratio.

The experimental reports revealed that BD20+ TiO2 at 75 mg/L+ dispersant (QPAN80) @ 250 bar pressure exhibited superior stability compared to the other combinations.

↓ Decrease in smoke by 27.5% reductions were observed at 2% additive at maximum engine-operated load ↑ Increase in CO2 with the increase in additive percentage. ↓ Decrease in CO by 14% reductions was observed at 2% additive at maximum engine-operated load. ↓ Decrease in BSFC by 2.63% at2% additive for 2.7kW load IDI engine test rig, Bajaj RE100 diesel engine with an engine displacement of 447.3 cc operated at varying loads

↑ Increase in BTE by 4.3% at 2% additive at maximum engine-operated load

↓ Decrease in NOx by 36.5% reductions were observed at 2% additive at maximum engineoperated load

Talamala V Rice bran et al., [94] methyl ester (RBME) + Isopropanol as an additive. Blends: 2/98%, 3/97%, 4/96% and 5/95%, by volume

↓ Decrease in HC for 2% additive at maximum engine-operated load

Table 6. Review on engine performance and emission characteristics of mixing additives in differentsecond and third generationbiodiesel feedstocks (continued)

diesel engines to analyze the engine performance and emission characteristics. In this approach, [28] tested the engine using waste oil biodiesel mixed with various volume proportions of 1-pentanol additive. The ternary blends prepared for testing were DB, D75B20Pen5, D70B20Pen10, and D60B20Pen20, and conducted experiments at 0,1.5,3 kW load conditions running at 2000 rpm. Therefore, the results suggested that the increase in additive ratio resulted in the rise of brake-specific fuel consumption (BSFC), exhaust gas temperature (EGT), carbon monoxide (CO), unburned hydrocarbons (UHC), and nitrogen oxides (NOx) emissions, where a slight reduction in brake thermal efficiency (BTE) and combustion efficiency was observed. Thus, a minimum quantity of additive mixing in biodiesel is preferable. In a similar case [30], the sunflower biodiesel was mixed with pentanol to estimate the engine characteristics. The blends prepared for testing are B20, B20P, B20P10, B20P15, and B20P20, and they are operated on four loads at a3000rpm fixed speed. The results showed a decrease in BTE and an increase in BSFC of 13.90% compared to diesel. Also, the increase in pentanol addition observed a reduction in NOx and an increase in CO2 emissions, and the heat release rate and ignition delay were lowered by 15%. Therefore, the study summarized that if the alcohol percentage (pentanol) increases, there can be an improvement in engine behavior. Another researcher [80] mixed Al2O3 nano additive 50ppm and 100ppm in waste cooking oil and neem biodiesel with diesel and tested for the blends B10 and B20. The experimental investigation was carried out on a single-cylinder, four-stroke diesel engine at a fixed speed of 1500rpm. The observed results confirmed that 50ppm in the B10 blend improves the combustion with high exergetic efficiency and low exergy destruction. Also, at 100% load, the blend B20 with 50 ppm of useful work increases by 33.1% more than other blends. Also, the study quoted that at 100% load, the B10+ 50 ppm Al2O3 nano additiveobserved high exergetic efficiency of 38.69%, enhanced engine combustion performance, and engine emissions such as UHC, CO, and NOx reduced after the addition of nano additives. Inspite of all the changes with nano additives, a similar approach was observed with the oxygenated additive. Some researchers investigated oxygenated additives in IIG and IIIG biodiesel, which is continued in the discussion below. An experimental investigation was conducted using the oxygenated additive Triacetin (T) mixed in coconut oil methyl ester (COME) [75] for the determination of the engine characteristics. Diesel, COME, and COME mixed with Triacetin in the vol.% of 5 %, 10%, 15 %, 20%, and 25% were tested on the DI engine without any change. The output results reported that exhaust emissions lowered positively, near the blend of 10% triacetin mixed with 90% COME. Correspondingly, better engine performance results of BSFC and BTE were achieved with the oxygenated additive mixed in the biodiesel blend. Therefore, the

study concluded that adding oxygenated additives to biodiesel resulted in a favorable agreement to replace with conventional fuels. In a similar approach, Kalyani et al. [63]mixed oxygenated additive triacetin in an algae biodiesel blend (ABD40) to estimate the enginecombustion, performance,and emission characteristics. The oxygenated additive Triacetin was mixed in ABD40 at various percentages of 2%, 4%, 6%, and 8% by volume and conducted experimental tests on a cylinder, water-cooled, four-stroke diesel engine operated at constant speed and varying loads. The observations confirmed that at full operating load (100%), it was revealed that at 4%Triacetin in ABD40 achieved high combustion pressure compared to diesel and other blends. and a high BTE of 30.39%.Also,at 100% load, lower engine exhaust emissions of CO (25.51 %), NOx (12.81 %), UHC (70.53 %), and smoke (14.85 %) were recorded for the 4 % triacetin in ABD40,recorded with low exhaust emissions compared to diesel. Thus, the investigation proved that the addition of oxygenated additive Triacetin in algae biodiesel enhanced the engine performance and emissions. A detailed literature review conducted by enormous researchers on the IIG and IIIG biodiesel mixed with additives is presented in Table 6 below.

Conclusion

The studies contributed by the enormous number of researchers on varioussecond and third-generation biodiesel feedstocks, fuel properties, fatty acid composition, andmixing of additives in diesel-biodiesel of various blendssuggested that third-generation green microalgae biodieselis one of the best sources of green fuel to enhance engine performance and mitigate the emissions in compression ignition engine. The findings revealed that: • Third-generationgreen microalgae biodiesel is an emerging renewable energy source, as these species can be cultivated in open and closed ponds. • Besides natural and artificial cultivation, the green algae can be collected from rivers, lakes, and reservoirs as these species doubles every 3-4 hours. • The microalgae species have higher oil content and can extract algae in large quantities. • The solvent oil extraction process is the best-suited method to extract algae oil compared to other extraction techniques. Though the extraction process of the algae cellular components consumes more time, it can recover 40 – 50% of the algae oil. • The cultivation of algae requires less land space compared to second-generation biodiesel. Though second-generation biodiesels play a significant role in improving efficiency and reducing emissions, the prevailing limiting factors, such as the availability of land to cultivate the non-edible oil crops and high fatty acid profiles, led to opt forthird-generation algae biodiesel

as one of the better options for replacing conventional fuels in compression ignition engines. Fatty acid composition in green algae biodiesel was reportedwith high saturated fatty acid composition percentage of 74%wt.% followed by second-generation palm oil biodiesel with 46%wt.%.Thus, this observation indicated that third-generation biodiesel plays a significant role in controlling the generation of NOx emissions during combustion. The engine experimentations using second- and third-generation biodiesel stated that the third-generation algae biodiesel operated in diesel engines and observed enhancement in engine combustion, performance, and efficiency.These improvements reduced engine emissions such as carbon monoxide, carbon dioxide, unburnt hydrocarbons, smoke, and particulate matter, and a slight increase in NOx emissions was observed. Mixing of additives in second and third-generation biodiesel improved the vital fuel properties, regulated the combustion issues formed inside the engine cylinder,and controlledthe escalationin nitrogen oxide emissions. Reduction in maintenance cost is the foremost importantaspect of microalgae biodiesel, making them commercially viable. Finally, the review studies elucidated that third-generation algae biodiesel is one of the emerging sources of fuel to run and test on existing diesel engines. Thus, algae biodiesel is considered one of the best alternatives to replace fossil fuels,and it holds great potential as a feedstock for future economically sustainable production.

Nomenclature

IG First-Generation Biodiesel IIG Second-Generation Biodiesel IIIG Third-Generation Biodiesel IVG Fourth-Generation Biodiesel BD Biodiesel BSFC Brake Specific Fuel Consumption BMEP Brake Mean Effective Pressure CI Compression Ignition CR Compression ratio DI Direct Injection FAC Fatty Acid Composition GHG Green House Gas IDI Indirect Direct Injection IT Injection Timing IMEP Indicated Mean Effective Pressure VCR Variable Compression Ignition

Data Availability Statement

The authors confirm that the data that supports the findings of this study are available within the article. Raw data that support the finding of this study are available from the corresponding author, upon reasonable request.

Conflict Of Interest

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethics

There are no ethical issues with the publication of this manuscript.

Statement On The Use Of Artificial Intelligence

Artificial intelligence was not used in the preparation of the article.

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