Characteristics and deflagration conditions of acetylene for thermal spraying

       Acetylene (C2H2) is an unsaturated hydrocarbon. It is a colorless gas at normal temperature and pressure. Industrial acetylene has a special odor because it contains impurities such as hydrogen sulfide (H2S) and phosphine (PH3). In the standard state, 1m3 of acetylene weighs 1.17kg, which is slightly lighter than air.



The auto-ignition point of acetonitrile is 480°C, the ignition temperature in air is 428°C, the temperature of combustion when it is mixed with air is 2350°C, and the flame temperature when it is mixed with oxygen is 3100°C - 3300°C. The maximum speed of acetylene combustion flame propagation in air is 2.37m/s. The maximum speed of propagation in oxygen is 13.5m/s.



The decomposition and explosion of pure acetylene depends first on the pressure and temperature of acetylene, as well as on the contact medium, impurities in acetylene and the size of the container.



When the temperature exceeds 400°C, the acetylene molecules begin to polymerize, forming other more complex compounds, while releasing heat. The heat released can promote the strengthening and acceleration of the polymerization. Finally, when the temperature is higher than 500 ° C, the polymerized acetylene can be exploded and decomposed, and all the heat absorbed by it during the formation can be released. The decomposition products are fine-grained solid carbon and hydrogen. If the decomposition is carried out in a closed container (such as an acetylene generator), the pressure will suddenly increase by 10-13 times due to the increase in temperature, causing an explosion. Increased pressure can also promote and accelerate the polymerization and decomposition of acetylene.



Acetylene can easily explode when mixed with air or oxygen. The auto-ignition temperature of acetylene-air mixture is 305°C, and at this temperature, it can explode even under atmospheric pressure. Mixtures of acetylene and oxygen have wide explosion limits.



For the acetylene-air mixture containing 7-13% acetylene and the acetylene-oxygen mixture containing 30% acetylene, the propagation speed of the explosion wave can reach 300m/s, and the explosion pressure can exceed 350 atmospheric pressure (gauge pressure). In individual cases, it can reach 600 atmospheres (gauge pressure).



When acetylene gas copper, silver, mercury and other metals or other salts are in contact for a long time, explosive compounds such as acetylene copper (Cu2C2H2) and acetylene silver (Ag2C2H2) will be formed. These compounds will explode when subjected to friction or impact.