Investigation on the flammability limit and limiting oxygen concentration of N₂-diluted H₂/CO/air mixtures at high temperature and pressure

H₂/CO binary mixtures are common, and their combustion characteristics and reaction mechanism are important. Currently, there are few studies on the combustion characteristics of H₂/CO mixtures at high temperature and pressure conditions, and they are mainly limited by the test conditions. In this work, a flammability limit test device was used to determine the flammability limit and limiting oxygen concentration of a combustible mixture with a composition of H₂/CO = 1:1 at high initial temperature (∼80 °C) and high initial pressure (∼10 MPa) conditions. The effects of temperature and pressure on the flammability limit and limiting oxygen concentration were obtained, and the results showed that the initial temperature and initial pressure expanded the range and reduced the limiting oxygen concentration, which significantly increased the risk of combustion. As the pressure increased, the effect of pressure on the flammability limit and limiting oxygen concentration decreased. In addition, an obvious coupling effect of initial temperature and pressure on the upper flammability limit and the limiting oxygen concentration (<5 MPa) was found. Moreover, kinetic simulations of the CO/H₂ mixture were carried out to verify the synergistic oxidation effect, which was explained using chain reaction theory. We found that the chain initiation reactions caused by H₂ generated OH products and the heat required for CO oxidation. Lastly, we also observed oxygen competition between CO and H₂, which decreased the CO conversion rate as H₂ increased.