Effect of CO₂ on the lower flammability limit of acetylene in O₂/CO₂ atmosphere

Oxy-fuel combustion is one of the effective means to reduce greenhouse gases. To grasp the combustion characteristics of the clean fuel acetylene in O₂/CO₂ atmosphere and to investigate the effect of different CO₂ volume fraction on the lower flammable limit of acetylene, the lower flammable limit of acetylene was experimentally measured in a 5 L cylindrical explosive reaction device. With the increase of CO₂ volume fraction from 14% to 85%, the experimental value of the lower flammable limit of acetylene increased from 2.64% to 3.93%, which was linearly increased in a small range. Compared with hydrocarbon fuels such as ethylene, ethane, and propylene, the lower flammability limit of alkanes, olefins, alkynes decrease sequentially, indicating that alkynes have a larger combustion range and a higher hazard factor. Based on the calculation model of limiting laminar burning velocity method, a prediction model applicable to the lower flammability limit of acetylene was established. Through the verification of experimental data, the average absolute error of this prediction model using the USC Ⅱ combustion reaction mechanism is at 0.52%, and the model is accurate and reliable. To explain the reason for the existence of the lower flammability limit from the perspective of the competition between the temperature rise of the heat generation from fuel consumption and the temperature drop of the heat dissipation from the expansion of the fuel body, this study examines the thermodynamic, chemical, and transport effects of CO₂ on the lower flammability limit. The combustion reaction mechanism of USC Ⅱ is modified to incorporate the virtual substances FCO₂, TCO₂, and MCO₂, and comparing the flammability limits of the three virtual substances as well as those of the five atmospheres of N₂ and CO₂. The thermodynamic, chemical and transport effects of CO₂ on the lower flammability limit were discussed. The results show that the average proportion of thermodynamic effect is 64%, chemical effect is 35% and transportation effect is 1%.