Extended Testing of the Auto-Ignition Characteristics of Hydrogen-Natural Gas Mixtures for the Safety of Power Plants

Abstract

Abstract A successful transition to global clean energy hinges on meeting the world’s growing demand for power, while at the same time reducing greenhouse gas (GHG) emissions. Achieving this will require significant growth in electricity generation from clean and carbon-free energy sources. Several energy providers [1,2] have already begun the transition from traditional carbon-based fuels to cleaner alternatives, such as hydrogen (H2) and hydrogen-enriched natural gas (HENG). However, there are still many technical questions/challenges that must be addressed when applying these fuels in gas turbines. The application of H2 or H2/natural gas (NG) blends to advanced class gas turbines, which have higher operating pressures and temperatures, has raised concerns about the potential for leakages or fuel sequencing operations where flammable mixtures of fuel and air could auto-ignite. Public information on the auto-ignition of H2 in the air at atmospheric pressure shows an autoignition temperature (AIT) between 520–585 °C [3–5]. Such data shows AIT of H2 is ∼100°C lower than that of methane (CH4) which has a minimum AIT of around 600 °C [6]. Studies also show that as pressure increases, methane’s AIT decreases significantly to around 390 °C [7]. However, there was insufficient information in the published literature to characterize the influence of pressure on the AIT of H2 and HENG fuels. At atmospheric conditions, H2 has a wider flammability range of equivalence ratios that ignition can occur compared to methane. H2’s flammability ranges from 4% to 75% molar (volume) fuel concentration, which is an equivalence ratio range of 0.137–2.57. Methane’s flammability limit ranges from 5% to 15% molar (volume) or an equivalence ratio between 0.53–1.58 [8]. Previous research has also been done to determine the effect of longer hydrocarbons present in natural gas mixtures. The presence of ethane (C2H6) and propane (C3H8) has been shown to reduce the AIT of natural gas, especially at elevated pressures [9]. These longer hydrocarbons also tend to promote ignition in richer conditions whereas methane tends to ignite easier in slightly lean conditions [8]. Numerous variables besides the pressure, fuel, and equivalence ratio can affect the AIT including chamber volume size, chamber materials, presence of diluents, and other factors [10]. This study describes the test methodology used to evaluate conditions where auto-ignition occurs for various fuel-air mixtures operating at different pressures (10–30 atm) and temperatures. Testing was completed with 100% H2 and multiple H2/NG blends at various equivalence ratios (ER) between 0.2–2.5. Testing was similarly performed for 100% NG to validate the test and data collection methods cited in prior published literature. Results indicate that, at atmospheric pressures, an increase in H2 concentration results in a reduced AIT. However, at 30 atm, the increased presence of H2 increased the AIT. At elevated pressures above 10 atm, increased equivalence ratio resulted in reduced AIT for all mixtures with NG having the greatest sensitivity to equivalence ratio. Variations of auto-ignition delay times (AIDT) were also observed during the testing and are compared to modeling predictions, providing insight into auto-ignition characteristics.