(209f) Experimental Study on NO Reduction By Iron with Methane in Wet Simualted Flue Gas

 Nitrogen oxide (NO) is a serious pollutant emission from coal-fired boilers and other industrial plants. In the current SCR and SNCR technologies, the reducing agent is ammonia, which is expensive and requires special handling and storage as well as a sophisticated metering system to avoid NH₃ slip. There is a great incentive to use natural gas as the reductant in stationary SCR/SNCR units rather than NH₃. A novel and effective method to reduce NO by methane over iron and/or iron oxides was recently proposed by the authors and experimental results in dry simulated flue gas proved this method is very promising.

However, there is water vapor in the flue gas in real conditions. In the SCR-NH₃ technology, many of the catalysts will lost their activity when there is water vapor.  The resistance of this method to vapor need to be demonstrated.

In this paper, the effect of water vapor on NO reduction by methane over iron was experimentally investigated at 300~1100°C in a ceramic tubular flow reactor. The iron samples after reaction were characterized by XRD, SEM and XPS. Results demonstrated that water vapor had a small effect on NO reduction by methane over iron. In N₂ atmosphere, water vapor involved in the oxidation of iron and caused decreased NO reduction efficiencies when 2.5~7% water vapor was added into the reactor, as compared to that when vapor was not added. When the water vapor increased from 2.5% to 7%, however, NO reduction efficiency increased with the increase of water vapor due to the porous surface of the iron sample caused by the oxidizing effect of water vapor on iron. Methane was involved in the reduction of the iron oxides leading to the formation of a dense layer of Fe₃O₄ and FeO, which inhabited the interaction of NO and metallic iron and caused the decreased NO reduction as compared to that without methane. In simulated flue gas atmosphere, water vapor increased the NO reduction by methane over iron. When the excess air ratios in reaction zone and burnout zone were SR₁=0.7 and SR₂=1.2 respectively, the NO reduction efficiencies were found to be 96.7% and 90.6% respectively at 1050 °C when there were 7% and 0% water vapor. SO₂ caused a slight decrease of NO reduction. Experimental data showed that more than 90% of NO was reduced by 1.14% methane over iron at 1050°C in a durable test over 50 hours in simulated flue gas atmosphere containing 7% H₂O and 0.02% SO₂.

   The results proved that the present method for NO reduction by iron with methane is very effective and is not influenced by vapor.