Optical insights into low-temperature lean combustion characteristics of pilot biodiesel-ignited premixed natural gas

Using renewable biodiesel as an ignition source is an effective strategy for enabling efficient and clean combustion of natural gas (NG). This study explores the low-temperature lean combustion characteristics of premixed NG ignited by pilot biodiesel using a Constant Volume Chamber (CVC). The combustion characteristics of premixed NG ignited by diesel and biodiesel were first compared. The results demonstrate that, compared to diesel-ignited NG, biodiesel-ignited NG exhibits a larger spatially integrated natural flame (SINL), a higher percent flame area (PFA), faster combustion speed, as well as higher chamber pressure and heat release rate (HRR). Subsequently, the effects of the premixed NG equivalence ratio (ΦNG = 0∼0.9) and the pilot biodiesel energy ratio (EB = 10 %–25 %) on the combustion behavior of biodiesel-ignited premixed NG were systematically investigated. As ΦNG increases, the ignition delay extends, the SINL and PFA grow, and the OH∗ generation reduces. Meanwhile, the combustion of premixed NG is enhanced, leading to higher chamber pressure and HRR. The HRR curve also transitions from a double-peak to a single-peak profile, signifying a shift in the combustion process from two stages (biodiesel-dominated diffusion combustion and NG-dominated premixed combustion) to a unified stage of NG/biodiesel premixed combustion. The rise in EB enlarges the ignition region and increases OH∗ production, leading to elevated pressure and HRR, as well as a shift in the HRR curve from a double-peak to a single-peak pattern, but excessive biodiesel injection mass will significantly prolong ignition delay. In conclusion, as ΦNG increases, it facilitates the propagation of the premixed NG flame, but it inhibits the ignition performance of biodiesel, by appropriately increasing EB under high ΦNG conditions, the combustion performance can be effectively improved, achieving stable ignition and combustion.