Influence of Fuel Oxygenation on Regulated Pollutants and Unregulated Aromatic Compounds with Biodiesel and n-Pentanol Blends

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Introduction
Government regulations and increased competition in the automotive industry have driven the development of new engine technologies with a focus on fuel economy and emissions reduction [1].The high thermal efficiency of diesel engines is due to their high expansion ratio and lean combustion, which enables heat dissipation by the excess air [2].Although these performance characteristics make diesel engines attractive for widespread adoption, they also produce higher emissions of CO, NO X , particulate matter (PM), unburned HC, and life cycle carbon dioxide (CO 2 ).
It is believed that diesel engines are among the greatest sources of NO x and smoke pollution [3].Although all engine manufacturers regulate such emissions, petroleum fuels frequently face supply issues and see a resulting increase in cost.Finding better renewable energy sources is becoming more important as alternative fuels from those renewable sources can be used as an attractive method to reduce NO x , smoke, and CO 2 emissions [4].
For nearly half a century, research has been continuing to find sustainable alternative fossil-based fuels for automotive and aerospace applications [5].The focal point of research to find fossil fuel alternatives has been oxygenated fuels and bioalcohols [6,7].Biodiesel has potential as a suitable replacement for fossil-based fuels because it is biodegradable, sustainable, and environmentally friendly.Due to these attributes, it is becoming widely used and researched around the world.Neat biodiesel is commonly used, or it is mixed with diesel fuel at specific blend ratios.The oxygen content of biodiesel lowers the in-cylinder temperature and reduces emissions, which greatly affects engine combustion performance [5].More importantly, biodiesel has fewer of the harmful aromatic compounds usually present in fossil fuels and has almost no sulfur.However, biodiesel production from feedstocks presents risks to food security.Therefore, biodiesel manufactured using inedible or waste oils has become an attractive option for use in diesel engines.When using biodiesel straight, or in a mixture with diesel, negative effects such as freezing and high NO x emission can occur due to its properties.The integration of bioalcohols into biodiesel fuel blends has gained popularity because it compensates for these disadvantages and has the potential to lead to a greater adoption rate of biofuels.Properties such as density and viscosity of biodiesel fuel blends can be ameliorated with the addition of alcohol [8,9].
In determining the type of alcohol for use with compression ignition engines, a high carbon number is key to the stability of the mixture and combustion performance.One of the high-carbon alcohols, n-pentanol, has been at the forefront of associated research due to its heat of combustion, flame speed, higher cetane number, viscosity, lower ignition temperature, and reduced corrosion risk [10][11][12].In all studies in the literature regarding biodiesel and n-pentanol mixtures, it is shown that n-pentanol absorbs more heat during evaporation since it has a lower heat of combustion and higher LHE.The premixed and diffusion combustion stages are improved through the use of an oxygen-rich alcohol such as n-pentanol [13].Thus, mixing n-pentanol with biodiesel or diesel improves fuel performance, while decreasing regulated emission species [14,15].Such benefits show promise for reducing dependance on diesel fuel and help protect the environment.Unlike regulated emissions, unregulated emissions or polycyclic aromatic hydrocarbons (PAHs) require more rigorous analytical chemistry techniques for analysis, detection, and quantification.The International Agency for Research on Cancer (IARC) and the United States Environmental Protection Agency (EPA) identified 17 compounds as carcinogens; these are the PAHs discussed in this study [16][17][18].In addition, the formation of PAHs causes excessive harm to the operation of the engine, such as wetstacking.PAHs are pervasive environmental pollutants created, for the most part, by poor combustion in diesel engines.PAH formation is associated with cold burning as a result of low load operation, as noted in the literature [19].Yilmaz and Donaldson [20] reported that elevated PAH emissions were observed as the equivalence ratio decreased during low load operations and greater PAH emissions were anticipated as the fuel/air ratio increased in the fuel-rich region.In another study, high PAH production was found in studying the combustion of coal and tire in a steady flow reactor at various temperatures [21].Such studies have concluded that higher air consumption or reactor temperatures lead to decreased PAH generation at the same equivalence ratio.Therefore, for engine applications, wetstacking is observed during low load operations and cold temperatures [22,23].For these reasons, unregulated diesel emissions must be investigated, especially while also assessing the viability of alternative fuels.
In the literature, there are several studies investigating the production of PAHs from biodiesel.Tsai et al. [24] investigated the particle matter (PM) and PAH emissions and showed that 20% biodiesel fuel blends reduced total-PAHs, total-BaPeq, and PM, compared to diesel, irrespective of engine load.Correa and Arbilla [25] conducted a PAH emission study focusing on neat diesel and diesel/biodiesel and stated that diesel/biodiesel reduced some PAHs.However, some other PAH compounds increased with biodiesel addition, such as trimethyl benzene, phenanthrene, and ethyl benzene.The evaluation of sustainably sourced, longchain, ether-oxygen additives in terms of regulated and unregulated engine emissions was performed by Wu et al. [26].Long-chain ethers reduced gaseous PAH emissions by 25%-44%, particulate PAH emissions by 39%-67%, and PAH toxicity was reduced by 32%-55%.The corpus of research to date shows that alternative biofuels containing oxygen in their molecule (such as FAMEs) can reduce PAHs and PM emissions [27].As previously mentioned, biodiesel can be mixed with alcohols to eliminate their poor fuel properties and potentially reduce PAH emissions.However, studies in the literature on the emissions from blended alternative fuels have primarily examined the regulated emissions of such blends, while only a limited number of studies have investigated the PAH formation which results from use of biodiesel-alcohol mixtures.Guan et al. [28] showed that diesel/biodiesel and diesel/biodiesel/ethanol can decrease PM emissions and the total PAH concentration at high and low engine loads.Furthermore, in the study of Choi et al. [29], it was emphasized that as the use of oxy fuels increases, they may increase the precursors of PAH formation (e.g., benzene or toluene) in diesel combustion.Yilmaz and Davis studied PAH generation from biodiesel fuel blends containing n-butanol, biodiesel, and diesel and reported significant reduction in PAH emissions for biodiesel and blended fuels.It was also noted that the addition of 10% n-butanol (by volume) to biodiesel decreased PAHs; however, rates above 10% led to increasing levels of PAHs [30].In another study, Yang et al. [31] found that fuel blends containing a larger pentanol concentrations had less particulate phase PAH compounds and benzo[a]pyrene equivalent in comparison to biodiesel.
In the current literature, there is insufficient information on PAH emissions from combustion of biodiesel/alcohol fuel blends, and it is necessary to fill the research gap on the generation of PAHs and associated toxicity levels resulting from the utilization of oxy-fuels in diesel engines.The purpose of this research is to examine aromatic-free biodiesel/n-pentanol fuel blends and evaluate their effects on HC, CO, and NO x and distribution of PAH, PAH, concentration in the exhaust and toxicity.Emphasis is placed on the formation of PAHs, high and low-ring PAH distributions and toxicity.The motivations for this focus are the significant 2 International Journal of Energy Research repercussions on public health, nature, and engine failure (i.e., wetstacking) from PAHs.

Engine Test Conditions and Test
Fuels.An Onan DJC compression ignition engine was operated at 1800 rpm (±1 rpm) with an electric load ranging from idle to 9 kW (±0.005 kW).Details of the engine setup and specifications can be found in Figure 1 and Table 1.
Table 2 lists the common properties of the test fuels.An emission analyzer (model EMS 5002-5) was used to quantify the NOx, CO, and HC content in the engine exhaust gas.
Measurement ranges and resolution for these gas constituents can be found in Table 3.For each experimental sample, tests were conducted in triplets for higher accuracy and experimental confidence.

Sample Collection and Analysis of PAH Emissions.
The rate of PAH sampling was 10 liters per minute (no dilution) for three hours at the idle mode.Filtering of exhaust samples utilized a multistep method; first, a 2 μm pore diameter polytetrafluoroethylene (PTFE) filter paper was used, followed by a 50 mg/100 mg Amberlite XAD-2 resin cartridge which was  3 International Journal of Energy Research targeted at the collection of more volatile species.An internal standard containing 2 μg anthracene (D10, 98%) was added to each sample for isotope dilution mass spectrometry (IDMS).High-performance liquid chromatography (HPLC) grade hexane (5 mL) was used for extraction of PAHs.Washing of samples was performed with the addition of 5 mL n-hexane and filtering through a 500 mg Na2SO4/ 500 mg alumina column.A 1 : 1 mixture of HPLC grade benzene and acetonitrile was used to recover the washed samples, which were then concentrated using nitrogen blowdown.Each PAH was then identified and measured using GC-MS according to a selected ion monitoring (SIM) process (Table 4).

Results and Discussion
3.1.Regulated Emissions 3.1.1.NO x Emissions.The thermal (Zel'dovich) NO X mechanism provides the most important effect on total NO X for-mation in diesel engines [32].In this context, the causes of NO X emission examined in this study are based on the thermal (Zel'dovich) NO X mechanism.As shown in Figure 2, NO X emissions increased with higher engine loads.On average, neat biodiesel produced 2.87% fewer NO X emissions than neat diesel.n-Pentanol generates a cooling effect in the combustion chamber because of its latent heat of evaporation (LHE) and lower heating value (LHV) values, which lead to lower NOx emissions [33][34][35].Biodiesel blends containing n-pentanol at concentration of 5%, 20%, and 35% resulted in NO X emission reductions of 13.71%, 26.3%, and 11.86%, respectively, when compared to neat biodiesel.The ignition delay time is increased at higher loads, leading to premixed combustion of which n-pentanol improves, resulting in a reduction of the cooling effect [36,37].Although the ignition delay time is increased at higher loads, the residence time is reduced at high temperatures, producing less NO X [6].In addition, blends with higher oxygen contents result in lower flame temperatures in oxygen-rich zones [22].Overall, the biodiesel test fuel containing 20% n-pentanol was the most successful at reducing NO X emissions.These results concur with comparable testing found in literature [38][39][40].

HC Emissions.
It is seen in Figure 3 that neat biodiesel increased HC emissions by 25.37% compared to diesel.The dominating factors that contribute to HC emissions are fuel properties, air-fuel ratio (AFR), fuel injection parameters, and engine operating conditions [41].Higher viscosity and density cause the formation of lean zones during the premix combustion phase.The biodiesel/n-pentanol fuel blends with n-pentanol concentrations of 5%, 20%, and 35% increased average HC emissions by 5.42%, 6.22%, and 2.86%, respectively.The increase in HC emission results is caused by incomplete combustion and higher oxygen  [42,43].Of the test fuels, the 20% n-pentanol concentration blend produced the highest HC emissions.An elevation in temperature in the last phase of combustion may be caused by greater n-pentanol concentrations, leading to decreased HC emissions.Lower cetane number and higher LHE of n-pentanol create a quenching effect [44].In addition, a long burning time can result in higher HC emissions because it creates a weak outer flame zone within the combustion chamber [45,46].

CO Emissions.
Multiple factors contribute to the production of CO emissions including AFR and temperature [30,[47][48][49].Results shown in Figure 4 indicate that poor air-fuel mixtures at low engine loads contribute to the production of CO while lower production of CO at medium loads can be attributed to more complete combustion.However, increased CO production at high loads is due to richer fuel mixture zones [50].Compared to diesel, neat biodiesel produced an average of 39.63% high CO emissions.

International Journal of Energy Research
Increased CO emissions for biodiesel are caused by poor atomization due to biodiesel's high viscosity and density [30,48,51].The low rate of evaporation of biodiesel/n-pentanol fuel blends with concentrations of 5% and 20% leads to a high rate of CO oxidation to CO 2 at high loads, causing a cooling effect, as evidenced by the 17.97% and 3.49% decrease in CO emissions, respectively.Similar tend CO emission trends have been reported in other studies investigating higher alcohol fuel blends [47,48,52].However, in contrast to the 5% and 20% n-pentanol fuel blends, the 35% n-pentanol fuel blend increased the formation of CO by 19.55%.Particularly at higher loads, CO formation increases because of the lower in-cylinder temperature [22].5, PAHs from biodiesel/ n-pentanol fuel blends, biodiesel, and diesel primarily comprised of two-ring, three-ring, and four-ring aromatic structures.Biodiesel/n-pentanol fuel blends significantly decreased the amount of the three-and four-ring aromatic compounds compared to neat biodiesel, indicating that n-pentanol addition decreases both the total PAHs and the percentage of high ring aromatic compounds [22].Diesel was the only fuel that produced a 5-ring PAH compound (see Table 5).Naphthalene, acenaphthylene, and acenaphthene, all low molecular PAHs, were common to all test fuels.Phenanthrene and fluoranthene, both medium molecular weight PAHs, were secondary compounds for diesel and biodiesel.Biodiesel/npentanol blends produced only small amounts of fluoranthene and phenanthrene.Incorporation of engine oil to the combustion process may contribute to the formation of these PAHs, as well as boiling points and incomplete combustion [53][54][55].

Unregulated Emissions
Diesel was the only test fuel to produce benxo[a]pyrene, which is a heavy PAH and is of concern because heavy PAHs have been linked to carcinogenic and teratogenic effects.It has been noted in the literature that higher PAHs contribute to engine failure [56].The Type of PAHs produced and reduction of higher ring PAHs should be prioritized along with total PAH formation when investigating modes of diesel engine failure.n-Pentanol fuel blends decreased the generation of high ring PAHs, meaning that these fuel blends exhibited optimal performance characteristics compared to diesel or biodiesel in terms of their effects on engine lifecycle.Heavier PAHs can condensate and form deposits on tighttolerance engine components, causing engine failure [30].

PAH Concentration in Exhaust
Gas.Total PAH emissions of the fuels are given in Figure 6, and their concentrations are given in Table 5.According to Figure 6, the maximum total PAH generation was observed in diesel blends with a concentration of 4.73 μg/m 3 .
Lower molecular weight PAHs (naphthalene, acenaphthylene, etc.) are typically found in diesel fuel as a result of diesel's chemical structure; however, reactions that occur during combustion produce higher-ring PAHs [19,27,28].The formation of unsubstituted compounds and the generation of particles with greater aromatic content are promoted  7 International Journal of Energy Research at high temperatures [57][58][59].Total PAH emissions are heavily influenced by PAHs from the engine oil and fuel participating in the combustion process and pyrosynthesis of PAHs during combustion [60,61].Combustion is the primary source for the formation of higher molecular weight PAHs.Lower molecular weight PAHs originate principally from unburned fuel.Compared to diesel, biodiesel showed a 48.02% reduction in total PAH emissions.Among 17 priority PAHs (Table 5), particularly pyrene, fluoranthene, phenanthrene, fluorene, and naphthalene were noticeable in this decrease.The excess oxygen and aromatic-free chemical structure of biodiesel is the likely reason for the decrease in PAH emissions.The chemical structure of biodiesel is such that it does not innately contain aromatic components; however, the combustion reactions can initiate PAH formation mechanisms, including Diels-Alder [57,62].Polyunsaturated fatty esters are easily affected by chemical reactions related to the generation of PAHs [63].In comparison to biodiesel, BPen5 decreased the total PAH emission by 21.26%.Like biodiesel, n-pentanol lacks aromatic content, but the aforementioned factors caused the formation of PAHs in biodiesel-n-pentanol blends.However, with higher n-pentanol concentrations, an increase of 7.93% and 17.03% was recorded in total PAH emissions for BPen20 (2.66 μg/ m 3 ) and BPen35 (2.88 μg/m 3 ) fuels, respectively, compared to biodiesel.Additional PAH accumulation may be caused by engine lubricant residues from the prior cycle.This accumulation is associated with the cooling of the in-cylinder temperature resulting from the addition of alcohol with higher LHE to the fuel blend.Additionally, low cetane number fuels with slow combustion and decreased adiabatic flame temperature generate greater quantities of PAHs compared to fuels that exhibit quicker combustion and larger cetane numbers [64].In view of various pathways for PAH formation, exceedingly oxygen-rich conditions may have a negative effect on combustion and lead to decreased chemical reactivity, which causes reordering of radicals in the ignition zone and optimal radical formation for PAH occurrence [53,55].Thus, parameters including oxygen content, cetane number, and LHE significantly affect combustion performance and formation of PAHs, as well as regulated emissions.Overall, although blend ratios of n-pentanol exceeding 20% elevated total PAHs in comparison to biodiesel, biodiesel-n-pentanol significantly reduced PAH emissions and higher-ring PAHs as compared to diesel.The optimal fuel that minimized total PAH production was 95% biodiesel and 5% n-pentanol.8 International Journal of Energy Research on the TEF [53,64,65].The toxicity distribution of PAH emissions is shown in Figure 7, and toxicity BaPeq (ng/m 3 ) concentration in exhaust is given in Table 6.Acenaphthylene, fluorene, phenanthrene, pyrene, and chrysene are measurable PAHs amongst all fuel blends.The BaPeq toxicity due to using diesel fuel was determined to be 16.20 ng/m 3 .This result means that diesel has the highest toxicity, measured in the five aromatic rings of Benzo[a]pyrene (13.37 ng/m 3 ), which forms as a result of diesel fuel between 300 °C and 600 °C and listed as a Group 1 carcinogen by IARC [16,18].
It is shown in Table 6 that most species measured in large quantities (two and three aromatic rings) have minimal BaPeq contribution.Because of the quantity and ordering of highring PAHs, diesel is more hazardous than biodiesel in terms of toxicity.The total BaP eq due to biodiesel (2.67 ng/m 3 ) was 83.49% less than that due to diesel.Biodiesel is a renewable fuel, but the combination of reactants that form at high temperatures in the diesel engine combustion process can cause toxicity [19,30].In comparison to biodiesel, toxicity was reduced with BPen5 (1.09 ng/m 3 ), BPen20 (1.12 ng/m 3 ), and BPen35 (1.38 ng/m 3 ) fuels, 59.15%, 57.89, and 48.33%, respectively.Overall, the total BaP eq was minimized by BPen5, while all of the biodiesel-n-pentanol blends significantly reduced the toxicity as compared to biodiesel and diesel.

Conclusions
For wide-range adoption of biodiesel and alcohol mixed fuels, it is essential to examine such fuel combinations for regulated emissions, as well as unregulated emissions for low and high concentrations of n-pentanol.This investigation filled gaps in existing studies and reports the impact of biodiesel-n-pentanol fuel mixtures on regulated pollutants, total PAHs, toxicity, and the formation of specific rings that cause wetstacking or engine failure.Biodiesel/n-pentanol fuel mixtures decreased NO X emissions as compared to biodiesel due to a cooling effect caused by lower LHV and higher LHE.With regard to unregulated emissions (i.e., PAHs), the following noteworthy outcomes have been observed: (i) The Diels-Alder PAH formation mechanism may be caused by reactions that occur during combustion despite the fact that biodiesel does not contain aromatic components due to its chemical structure.However, biodiesel and biodiesel-n-pentanol blends, which are free of aromatic compounds, significantly reduced the formation of PAHs, pointing out the importance of the aromatic content of the fuel (ii) Of the 17 analyzed PAHs, three were below the detection limit.BPen5 consisted of 80% two, 16% three, and 4% four aromatic rings, BPen20 had 98% two and 2% three aromatic rings, and BPen35 had 98% two, 1% three, and 1% four aromatic rings (iii) Compared to diesel fuel, biodiesel showed a 48.02% reduction with total PAH emissions of 2.46 μg/m 3 .
The biodiesel/n-pentanol fuel blend with a 5% npentanol concentration further decreased the total PAH emission by 21.26%, making BPen5 the best blended fuel in terms of minimizing total PAHs.However, higher n-pentanol concentrations caused 7.93% and 17.03% increase in total PAH emissions for BPen20 (2.66 μg/m 3 ) and BPen35 (2.88 μg/m 3 ) fuels, respectively, as compared to biodiesel (iv) There was a substantial decrease in toxicity for BPen5 (1.09 ng/m 3 ), BPen20 (1.12 ng/m 3 ), and BPen35 (1.38 ng/m 3 ) fuels, 59.15%, 57.89, and 48.33%, respectively.BPen5 showed the best reduction among the mixtures not only in total PAH emissions but also in toxicity as compared to all fuels Overall, biodiesel/n-pentanol blends significantly reduced total PAHs, toxicity, and the higher-ring PAHs, making such blends advantageous over diesel for human health, the environment, and engine operation at low load or cold operating conditions.

Figure 1 :
Figure 1: Engine test facility and PAH sampling.

Figure 2 :
Figure 2: Correlation between NO x emissions and engine load for fuel blends.

Figure 3 :
Figure 3: Correlation between HC emissions and engine load for fuel blends.

3. 2 . 1 .
Distribution of PAHs.The 17 types of PAHs that this study is centered on are those recommended by the EPA[15].Of the 17 PAHs, seven were below the detection limit, acenaphthene, anthracene-D10, anthracene, benzo[b]fluoranthene,

Figure 4 :Figure 5 :
Figure 4: Correlation between CO emissions and engine load for fuel blends.

3. 2 . 3 .
Toxicity in PAH Emissions.The carcinogenicity of PAH emissions is defined as toxic equivalent factor (TEF) values.The weighted BaPeq (benzo[a]pyrene equivalent (ng/m 3 )) of each aromatic compound is calculated based

Figure 7 :
Figure 7: The toxicity distribution of test fuels.

Table 3 :
Engine test facility ranges and resolutions.