High catalytic performance of magnesium cations-added limonite in the decomposition of ammonia in a simulated syngas-rich fuel gas

Abstract

Catalytic decomposition of 2000ppm NH3 with an Australian limonite ore, which is composed mostly of goethite (α-FeOOH), has been studied from a viewpoint of hot gas cleanup with a vertical, cylindrical quartz reactor at 750–850°C under a high space velocity of 45,000h−1. It has already been reported that the limonite achieves the almost complete decomposition of NH3 in inert He at 500°C and shows very stable performance in the reaction at 750°C in the coexistence of 50–500ppm H2S or at 850°C in the presence of fuel gas components produced in an air-blown coal gasification process. In the coexistence of a high concentration of syngas (50% CO/25% H2) produced with an O2-blown coal gasifier, the limonite is deactivated almost completely because of the remarkable occurrence of carbon deposition from the CO. On the other hand, the addition of small amounts of CO2 and H2O, which are always included in actual coal-derived fuel gas, to the syngas improves the activity of the limonite, and conversions of NH3–N2 at 750 and 850°C become about 65% and almost 100% without carbon deposition, respectively. When several limonite-based catalysts with alkali metal and alkaline earth metal cations are prepared by the impregnation method and then used in the NH3 decomposition at 750°C in 50% CO/25% H2/5% CO2/3% H2O, fine particles of MgO derived from Mg cations can work more effectively as the promoter, and the 3 mass% Mg-added limonite maintains the high and stable NH3 conversion of almost 100% for 25h. In this case, no significant carbon deposition takes place. It is probable that MgO with strong basicity suppresses the carbon formation from CO, and that the limonite-based composite catalyst thus shows the superior performance in the decomposition of NH3 in syngas-rich fuel gas that simulates product gas in O2-blown coal gasification.