Experimental investigation of natural gas reactivity using a fuel substitution method

Abstract

Fuel substitution tests using reference fuels of methane/hydrogen and methane/propane were performed for the pure test fuels hydrogen, ethane, ethylene, propane, n-butane, n-heptane, and isooctane using HCCI combustion. The reactivity of each test fuel relative to the reference fuels was quantified with a newly defined hydrogen index (HI) and propane index (PI). It was observed that the common hydrocarbon fuels tested show a strong sensitivity of HI and PI to temperature and pressure, and that the reactivity relative to the reference fuels can change substantially at different operating conditions. The fuels that have similar low temperature chemistry (n-butane, n-heptane, and isocotane) exhibited PI that were independent of operating condition. Ethane, ethylene, and hydrogen exhibited PI that were strongly dependent on operating condition, with the relative reactivities increasing with increasing temperature. Fuel substitution tests performed with two multi-component methane number (MN) gas mixtures followed trends similar to those observed for the pure hydrocarbon fuel testing, exhibiting similar relative reactivity trends as for propane relative to the reference fuels for the different operating conditions. A linear energy-based blending rule was used to predict the HI of each fuel mixture from the individual fuel components and seen to match experimental trends; however, predicted HI were lower than experimental HI across all operating conditions. The MN was also used to predict the HI of each fuel mixture and was seen to match experimental results only at low pressure/high temperature conditions but not at high pressure/low temperature conditions.