SulfoChar as a Novel Nano-Biochar Additive for B20 Biodiesel Blends: Enhancing Combustion and Reducing Exhaust Emissions

Abstract

While biodiesel blends offer renewable alternatives to conventional diesel, mitigating their nitrogen oxide (NOx) penalties and optimizing combustion typically requires synthetic or metallic additives. This study introduces SulfoChar, a sulfonated walnut-shell nanobiochar functionalized with sulfonic acid (–SO3H) groups, as a novel, surfactant-free fuel-borne additive for B20 (20% biodiesel/80% diesel) blends. Synthesized via pyrolysis and subsequent functionalization, the nanobiochar was characterized using FTIR, SEM, and EDS, exhibiting excellent colloidal dispersion in the fuel matrix for over 30 days without chemical stabilizers. Engine evaluations were conducted at additive concentrations of 30, 60, and 90 mg/L under full-load conditions across 2900–3300 rpm. Experimental data revealed a distinct decoupling of optimal emission and performance dosages. The maximum concentration (90 mg/L) provided the highest environmental benefits, yielding peak reductions of 48.4% in carbon monoxide (CO) and 17.4% in NOx relative to the baseline B20. Conversely, mechanical output peaked at the lowest dosage (30 mg/L), which increased brake power by up to 19.3% at 2900 rpm. Surface map analysis indicated a balanced operational regime at 40–50 mg/L and 3000–3100 rpm. The simultaneous improvements in physical stability and engine characteristics suggest that -SO3H functionalization potentially facilitates better fuel micro-dispersion and favorable surface-level interactions during combustion. Ultimately, under the tested conditions, SulfoChar demonstrates promising potential as a sustainable, metal-free additive to concurrently mitigate exhaust emissions and enhance engine performance.