Experimental study on the effect of different CO2 concentrations on soot and gas products from ethylene thermal decomposition

This research work reports a laboratory study of the influence of environments with different CO₂ levels, representative of conditions in which exhaust gas recirculation is used in combustion systems, on soot and gas products formed in the thermal decomposition of ethylene–CO₂ mixtures. The investigation includes experiments, in a flow reactor, with 30,000 ppm of ethylene at different experimental conditions of temperature (975–1475 K) and CO₂ concentrations (25%, 50% and 78.5%), using nitrogen as bulk gas. The analysis is performed by comparison with the data obtained during the pyrolysis of ethylene in a N₂ atmosphere.The present results highlight the importance of the CO₂ level in the system, since the presence of 25% CO₂ tends to promote the formation of soot, whereas an increased CO₂ addition of 78.5% leads to a diminution in the production of soot, compared to the pyrolysis of pure ethylene in N₂. The different evolution in soot formation tendencies can be attributed to competing reactions that gain importance depending on the different CO₂ levels, boosting or suppressing soot formation as function of the composition of both the O/H radical pool and the reacting species. The outlet concentrations of H₂, CO and C₂H₂, as well as the formation of H₂O, are directly related to the different soot-forming tendency found as function of different CO₂ environments.