Computational Investigation of Fuel-Derived Nitrogen Oxides Emissions in Hydrothermal Liquefaction Biofuel Combustion Using Pyridine-Based Surrogates

This study investigates the role of fuel-bound nitrogen in hydrothermal liquefaction (HTL) biofuels on Nitrogen Oxides (NOx) emissions by evaluating pyridine as a representative nitrogen-containing species within a surrogate fuel formulation composed of n-decane and pyridine, designed to mimic key chemical features of HTL biofuels. A Stochastic Reactor Model (SRM) framework was used to simulate the combustion and emission behavior of HTL-diesel blends under engine-relevant conditions. The numerical model was calibrated and validated against experimental data from an optically accessible compression ignition chamber (OACIC), focusing on in-cylinder pressure traces, apparent heat release rates (HRR), and ignition delay for different fuel blends. A simplified surrogate formulation was developed using n-decane to represent the alkane-rich fraction of HTL fuels and pyridine to model nitrogen-containing species. A detailed chemical kinetic mechanism was composed by merging the CRECK mechanism with a validated pyridine sub-mechanism, enabling accurate simulation of nitrogen chemistry, including hydrogen cyanide (HCN) formation pathways. The enhanced mechanism was validated using flow and jet-stirred reactor data to ensure predictive accuracy for pyridine decomposition and HCN production. SRM simulations demonstrated that the proposed surrogate effectively captures combustion behavior and emissions trends observed in experiments. The analysis quantified the contribution of fuel-derived NOx relative to thermal NOx, showing that the increase in NOx with higher HTL blending ratios is largely attributable to nitrogen originating from the fuel itself. Importantly, while HCN is produced during combustion, the simulations indicate it is fully oxidized before exhaust, suggesting minimal environmental or health risks from HCN in practical applications. This study provides a validated, numerically efficient framework for analyzing HTL biofuel combustion and emissions, particularly for understanding the impact of nitrogen-containing compounds. Pyridine is shown to be a promising surrogate for modeling fuel-bound nitrogen in chemical kinetic simulations.