Climate change has emerged as one of the most critical environmental challenges of the modern era, with global warming recognized as a pressing climate emergency. Mitigating greenhouse gas emissions is therefore an urgent global priority. According to the EIA Energy Outlook (2020),³⁵ coal consumption has declined sharply, while the adoption of renewable fuels has increased worldwide. Among these, biodiesel has gained considerable attention as a renewable, biodegradable, and cleaner burning alternative to fossil diesel, and its global demand is expected to continue rising in the coming years.^29,36

Biodiesel combustion generally produces fewer hazardous emissions owing to its high oxygen content, resulting in reduced carbon monoxide (CO) and unburnt hydrocarbons (UBHCs) compared to conventional petro-diesel. However, the increased oxygen availability often leads to higher nitrogen oxides (NO_x) emissions, one of the key challenges associated with biodiesel utilization. A practical strategy to mitigate NO_x emissions involves the incorporation of small quantities of antioxidants into biodiesel blends. Synthetic antioxidants such as PG, BHA, and BHT have demonstrated the ability to lower NO_x emissions by reducing oxygen availability during combustion, albeit with a slight trade-off in the form of increased CO and UBHC emissions. Moreover, many plant-based extracts possess inherent antioxidant activity and can similarly reduce NO_x emissions due to their high phenolic content. Consequently, in this study, cost-effective and locally available natural antioxidants were employed to develop an environmentally benign, i.e., green fuel with performance characteristics comparable to conventional petro-diesel.

Emission studies were carried out using biodiesel–diesel blends of varying compositions, including B5 (5% biodiesel + 95% petro-diesel), B20 (20% biodiesel + 80% petro-diesel), B40 (40% biodiesel + 60% petro-diesel), B60 (60% biodiesel + 40% petro-diesel), and B100 (pure biodiesel). Subsequently, B20 blends of each biodiesel were fortified with different concentrations of three synthetic antioxidants (PG, BHA, BHT) and three plant-derived antioxidant extracts (Moringa oleifera, olive pomace, and citrus fruit residues). The emission profiles of all fuel samples are presented in Fig. 1, with pure petro-diesel (D) included for reference.

Comparative analysis of the D, B5, B20, B40, B60, and B100 blends revealed that petro-diesel combustion produced higher CO and UBHC emissions than biodiesel blends. As the biodiesel fraction increased, CO and UBHC emissions decreased, whereas NO_x emissions showed a proportional increase. This emission behavior was consistent across all tested biodiesels. Specifically, B20 blends of hemp, karanja, castor, and amla biodiesels exhibited 25%, 18%, 21%, and 29% higher NO_x emissions, respectively, compared to petro-diesel. These findings align with previous studies reporting that NO_x generation is influenced by combustion chamber temperature, fuel oxygen concentration, and enthalpy of vaporization.^37,38 The elevated NO_x emissions observed with higher biodiesel content can thus be attributed to the increased oxygen availability in biodiesel, which promotes more complete combustion resulting in lower CO and UBHC emissions but higher flame temperatures, ultimately enhancing thermal NO_x formation.^38,39

The influence of varying concentrations of synthetic and natural antioxidants on the emission characteristics of B20 biodiesel blends is illustrated in Fig. 2–5. The emission analysis revealed a distinct reduction in NO_x emissions upon the addition of synthetic antioxidants, with the extent of reduction increasing proportionally with antioxidant concentration. The maximum NO_x reduction (20.56%) was observed for B20 blends of HOMEs containing 1000 ppm of PG, followed by COMEs (19.91%), AOMEs (19.79%), and KOMEs (19.16%). BHA and BHT exhibited comparable trends, though BHA performed slightly better in several cases. Specifically, BHA reduced NO_x by 20.00% in HOMEs, 18.57% in KOMEs, 19.30% in COMEs, and 19.23% in AOMEs, while BHT achieved reductions of 17.78%, 19.16%, 18.14%, and 17.03%, respectively.

Although the inclusion of synthetic antioxidants effectively mitigated NO_x emissions, a concurrent increase in CO and UBHC emissions was observed. Among the synthetic antioxidants, BHA caused the smallest rise in CO emissions, with increases of 13.48%, 16.94%, 18.09%, and 12.63% for HOMEs, KOMEs, COMEs, and AOMEs, respectively, at 1000 ppm. For UBHC emissions, BHA again demonstrated the most favorable performance, showing increases of 19.05% in HOMEs, 21.05% in KOMEs, 22.54% in COMEs, and 34.48% in AOMEs. In comparison, BHT at 1000 ppm caused higher CO emissions (19.73% in HOMEs, 11.73% in KOMEs, 12.55% in COMEs, and 18.17% in AOMEs) and substantially greater increases in UBHCs, particularly in HOMEs (30.16%) and AOMEs (41.38%). PG at 1000 ppm produced moderate rises in CO and UBHC emissions, averaging 17.75% and 21.75%, respectively, across all biodiesels. Overall, BHA was identified as the most effective synthetic antioxidant in minimizing CO and UBHC increases while achieving considerable NO_x reduction. Importantly, CO emissions from all biodiesel blends, except karanja-based biodiesel, remained lower than those of petro-diesel. It is suggested that optimizing antioxidant concentration or selecting a milder antioxidant for karanja biodiesel may alleviate this issue. Collectively, the findings indicate that the synthetic antioxidants exhibit NO_x reduction efficiency in the order of PG > BHA > BHT, accompanied by CO and UBHC increases following the trend BHA < PG < BHT.

The impact of plant-derived antioxidants on the emission behavior of biodiesel blends is summarized in Table 2. In general, natural extracts exhibited lower NO_x reduction efficacy than synthetic antioxidants. At a concentration of 500 ppm, Moringa oleifera leaf extract, olive pomace extract, and citrus residue extract reduced NO_x emissions by 5.33%, 2.83%, and 2.39%, respectively, averaged across all four biodiesels. However, significant improvements were observed at higher concentrations (700–1000 ppm). Moringa oleifera extract demonstrated the highest NO_x reduction efficiency among natural antioxidants, with decreases of 16.67%, 14.97%, 15.18%, and 15.93% for HOMEs, KOMEs, COMEs, and AOMEs, respectively, at 1000 ppm. Olive pomace extract at 1000 ppm achieved NO_x reductions between 7.69% and 9.36%, while citrus residue extract produced more modest reductions ranging from 6.59% to 7.22%.

Regarding CO emissions, the average increase across the four biodiesels was highest for olive pomace extract (4.29%), followed by Moringa oleifera extract (3.80%) and citrus residue extract (3.56%). The smallest CO increase (2.22%) was recorded with citrus extract in AOMEs. For UBHC emissions, the average increment was highest with olive pomace extract (12.50%), followed by citrus extract (9.98%), while Moringa oleifera extract resulted in the lowest average increase (6.97%). Notably, Moringa oleifera extract in HOMEs led to only a 3.17% increase in UBHCs, indicating its superior stabilizing ability. Although all three natural extracts caused some elevation in CO and UBHC emissions, the magnitude of increase was considerably lower than that observed with synthetic antioxidants.

These findings highlight the promising potential of plant-derived antioxidants in emission control. The results suggest that Moringa oleifera extract offers the most efficient NO_x mitigation, followed by olive pomace and citrus extracts. Moreover, optimization of extraction methods (e.g., employing green extraction technologies) and concentration levels could further enhance the antioxidative and emission-reducing capabilities of such bioactive compounds. The present trends are consistent with the observations of previously reported literature. For instance, Afzal et al. reported the significant reductions in NO_x emissions upon the addition of both synthetic and plant-based antioxidants to biodiesel fuels.³⁷ Rajamohan et al., reported the antioxidant potential of synthetic antioxidants (BHT, PG, TBHQ and Pyrogallol) to improve the oxidation stability for storage, combustion characteristics and engine performance of Prosopis juliflora biodiesel/diesel fuel blends. However, the use of synthetic antioxidants increased the emissions of CO and UBHC in the combustion studies.⁴⁰ Jeyakumar et al., reported the antioxidant potential of sesame, horse gram, sweet basil, coffee, and peas to improve the shelf life of binary biodiesel blend of Jatropha biodiesel and lemongrass oil. They reported the substantial improvement in the oxidation stability of biodiesel in the following order: sesame > horse gram > sweet basil > coffee > peas.⁴¹ Similarly, Jeyakumar et al., also reported the improvement in the combustion properties of Pithecellobium Dulce seed-derived biodiesel by the addition of Groundnut shell nanoparticles.⁴² All these findings demonstrated the beneficial effect of natural antioxidants on the oxidation stability of the biodiesel, as reported by our findings.