Conversion characteristics of aromatic hydrocarbons in simulated gaseous atmospheres in reducing section of two-stage entrained-flow coal gasifier in air- and O₂/CO₂-blown modes

Conversion of refractory aromatic hydrocarbons was studied with an atmospheric flow reactor that simulated the reducing section of a two-stage entrained-flow coal gasifier in air-blown and O₂/CO₂-blown (CO₂ recycling) modes at temperatures of 1100-1400 C. Mixed vapors of benzene and naphthalene (7/3 on a carbon basis) were fed into the reactor at total concentration in a range from 3.7 to 37 g·Nm^-3 together with a CO-CO₂-H₂-H₂O mixture in the O ₂/CO₂-blown mode or CO-CO₂-H₂-H ₂O-N₂ mixture in the air-blown mode. Soot was the major fate of the aromatics at the inlet benzene/naphthalene concentration of 35-37 g·Nm^-3, and its yield was not influenced significantly either by the mode of gasification or temperature at 1200-1400 C. The contribution of gas-phase reforming to the conversion of the aromatics became more important as their inlet concentration decreased. At the inlet concentration of 3.7-7.5 g·Nm^-3, the O₂/CO₂-blown mode was clearly more effective in reducing the soot yield than the air-blown mode and increasing the gas yield. It was explained on the basis of a detailed chemical kinetic model that the increased partial pressure of CO₂ induced a higher concentration of key active species such as hydroxyl radicals that initiated the oxidative decomposition of the aromatics.