Experimental study on the flammability limits of natural gas/air mixtures at elevated pressures and temperatures

•A novel apparatus for measuring the flammability limits of natural gas at high pressures and temperatures was developed.•Effects of temperatures and pressures upon the flammability limits of natural gas were investigated.•The results showed that both temperature and pressure had a marked effect upon the UFL.•The effect of temperature and pressure on LFL was slight compared with the effect on UFL.•The fitting equations of flammability limits as a function of pressure or temperature were obtained. While injecting air into an oil reservoir for enhanced oil recovery, there may be explosion risks at reservoir conditions including elevated pressures and temperatures. A high-pressure experimental apparatus was developed to investigate the flammability limits of natural gas/air mixtures at reservoir conditions with pressure ranging from 1 MPa to 15 MPa and temperature ranging from 40 °C to 120 °C. The results showed that an increase in initial pressure or temperature leads to a wider flammability limit range. Specifically, the upper flammability limit (UFL) shows a logarithmic pressure dependence, and the lower flammability limit (LFL) is linearly dependent on pressure. The UFL shows a linear dependence on temperature, whereas the LFL is slightly affected by temperature. Moreover, it was also found that when the pressure is between 1 MPa and 5 MPa, the effect of temperature on the UFL is affected by pressure. Temperature has little influence on the relationship between pressure and UFL. These results improve our understanding of the fire and explosion hazards of natural gas in oil recovery processes.