Catalytic decomposition of 2000 ppm NH3 in different atmospheres with an Australian alpha-FeOOH-rich limonite ore at 750-950 degrees C under a high space velocity of 45000 h(-1) has been studied with a cylindrical quartz reactor to develop a novel hot gas cleanup method of removing NH3 from fuel gas produced in an air-blown coal gasification process for an integrated gasification combined cycle (IGCC) technology. The limonite shows very high catalytic activity for the decomposition of NH3 diluted with inert gas at 750 degrees C, regardless of whether the catalyst material is subjected to H-2 reduction before the reaction or not. Conversion of NH3 to N-2 over the reduced limonite reaches >= 99% at 750-950 degrees C, and the catalyst maintains the high performance for about 40 h at 750 degrees C. When the decomposition reaction is carried out in the presence of fuel gas components, the coexistence of syngas (20% CO/10% H-2) causes not only the serious deactivation of the limonite catalyst but also the appreciable formation of deposited carbon and CO2. On the other hand, the addition of 10% CO2 or 3% H2O to the syngas improves the catalytic performance and concurrently suppresses the carbon deposition almost completely, and the NH3 conversion in the 3% H2O-containing syngas reaches about 90% and almost 100% at 750 and 850 degrees C, respectively. Influential factors controlling the catalytic activity of the limonite ore in the coexistence of fuel gas components are discussed on the basis of the results of the powder X-ray diffraction measurements, thermodynamic calculations, and some model experiments.