When hydrogen is slower than methane to ignite

Hydrogen (H 2 ) is known to be the fastest fuel to ignite among all practical combustion fuels. In this study, for the first time, longer ignition delay times (IDTs) for the H 2 and H 2 blended CH 4 mixtures were measured compared to those for pure CH 4 . This work investigates the ignition characteristics of H 2 , CH 4 , and 50% CH 4 /50% H 2 mixtures using a rapid compression machine at pressures ranging from 20 to 50 bar and at equivalence ratios ( & phi;) from 0.5 to 2.0 in air in the temperature range 858-1080 K. The experimental IDTs are simulated using a newly updated kinetic mechanism, NUIGMech1.3, and good agreement is observed. At lower temperatures the IDTs of H 2 , CH 4 , and the 50% CH 4 /50% H 2 mixtures are similar to one another, and the IDTs of the 50% CH 4 /50% H 2 mixtures are longer than those for pure CH 4 at temperatures below 930 K. At temperatures below 890-925 K, depending on the operating pressure and equivalence ratio, the hydrogen mixtures are the slowest to ignite, with IDTs being 2.5 times longer than those recorded for CH 4 at a pressure of 40 bar at 890 K for & phi; = 1.0, and at 875 K for & phi; = 2.0. At low temperatures alkyl ( R = CH 3 and H) radicals add to O 2 producing R O 2 radicals, which then react with H O 2 radicals forming ROOH (H 2 O 2 and CH 3 OOH) and O 2 . For H 2 , the self-recombination of H O 2 radicals leads to chain propagation which inhibits reactivity, whereas for CH 4 , the reaction between R O 2 (CH 3 O O) and H O 2 leads to chain branching, increasing reactivity. Furthermore, CH 3 OOH decomposes more easily to produce CH 3 O and OH radicals than does H 2 O 2 to produce two OH radicals. Thus, mixtures containing higher H 2 concentrations are slower to ignite compared to those with higher CH 4 concentrations at low temperatures. & COPY; 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.