A novel hot gas cleanup method to decompose a low concentration of NH3 to N-2 with metal cations present inherently in low rank coals has been studied with a quartz reactor under the conditions of 750-850 degrees C, 0.1 MPa and high space velocity of 45,000 h(-1). Each coal is pyrolyzed at 900 degrees C to prepare the char, which is subjected to the decomposition of 2000 ppm NH3 after pretreatment with H-2. All of five chars examined promote NH3 decomposition in inert gas, but the promotion effect depends strongly on the kind of char and can correlate more closely with the Fe content than with the Ca content. This result may indicate that the Fe plays a crucial role in the reaction. A commercial activated carbon with a very low Fe content of < 0.05 wt% exhibits lower conversion of NH3 to N-2 than five chars. The TEM pictures reveal the formation of nanoscale particles of Fe and Ca components on a brown coal char that provides the largest catalytic performance. The char maintains the high conversion level of 80% during 25 h reaction at 750 degrees C and achieves the complete decomposition of NH3 at 850 degrees C. The co-feeding of a mixture of H-2, CO, and CO2 does not change significantly the catalytic activity of the char at a steady state, whereas the coexistence of 2000 ppm H2S lowers it in the whole range of time on stream. It is proposed by combining the XRD and TPD observations with our previous results that the catalytic decomposition of NH3 in inert gas with the chars derived from low rank coals proceeds through two cycle mechanisms involving iron metal, iron nitrides, CaO and CaCN2. (c) 2005 Elsevier Ltd. All rights reserved.