Reducing PAHs emissions at the source in a pyrolysis furnace under gas carburizing conditions

The hydrocarbon pyrolysis is widely used in many engineering applications like synthesis of carbon nanotubes in a chemical vapour deposition, pyrocarbon by depositing on carbon fibers and vacuum gas carburizing of steel. However, during these processes, polycyclic aromatic hydrocarbons (PAHs) are generated because the pyrolysis of hydrocarbon is incomplete. Several PAH species have been classified into probable or possible human carcinogens by International Agency for Research on Cancer – IARC. In the case of the vacuum gas carburizing, the operating conditions (T=1273 K – P=10 Pa) are such as PAHs are gas into the furnace. They become liquid or solid by condensation output of the process by contacting with cold zones. And during the maintenance of furnaces, the workers' health risks posed by PAHs exposures are important via both routes of inhalation and dermal contact (Tsai et al., 2001). The hydrocarbon pyrolysis leads to the formation of a large amount of hydrogen and various organic compounds such benzene by propagation reactions. However benzene, added to acetylene, is a precursor chemical for the formation of PAHs; it participates in chemical reactions that produce PAHs (Norinaga et al., 2009). Therefore the evolution of benzene concentration has been followed because it assesses the amount of PAHs formed. Vacuum carburizing can be carried out in a more environmentally friendly way. It can be carried out by a plurality of successive carbon enrichment and diffusion phases. During enrichment phases an injection of limited duration of pure acetylene or a mix acetylene/nitrogen (in this work) leads. Quick saturation in carbon atoms of the steel surface layer occurs thanks to an adsorption and a gas-solid chemical reaction. Diffusion phases allow to reach the superficial carbon content necessary with pure nitrogen, a pressure maintained at a low value and greater duration. This method with intermittent periods allows to reduce the amount of generated PAHs but it can be improved. Therefore, the chemical vapour deposition was numerically simulated using a simplified mechanism which uses nine chemical reactions only (Khan et al., 2007) and mass balances in gas and solid phases to calculate the concentrations and the mass flow rate of each compound (MatLab® software was used). Based on the simulations performed in gas phase near the surface, concentration profiles analysis shows that concentration gradients are negligible. This is due to the mass diffusivity for every compound which is a great value because it is an increasing function of temperature and inversely proportional to the pressure. Moreover, the concentration of benzene increases with the duration of enrichment phase independently of the saturation in carbon atoms of the steel surface layer. Furthermore the mass carbon flow rate into the steel decreases over time. This means that the enrichment duration have to be increased to achieve the austenite saturation. In conclusion, the enrichment duration should be reduced without reaching saturation in carbon atoms of the steel surface systematically. The number of successive sequences - enrichment and diffusion - has to be increased and optimized.