Hydrogen (H-2) is produced by burning and reforming ammonia (NH3) in a microreforming system. A micro-combustor that burns H-2-added NH3-air mixtures as a heat source is a cylinder with an expanded exhaust outlet that facilitates ignition and an annular-type shield that adopts a heat-recirculation concept. A micro-reformer that converts NH3 to H-2 using ruthenium as a catalyst surrounds the micro-combustor, which facilitates heat transfer between the micro-combustor and micro-reformer. Performance of the microreforming system is affected by the fuel-equivalence ratio and micro-combustor inlet velocity of NH3-H-2-air mixtures and the concentration of the substituted H-2 in the fuel gas. The production rate of H-2, the conversion rate of NH3, the overall efficiency of the microreforming system and the NOx concentration in the exhaust gas from the micro-combustor for optimized operating conditions are 5.4W (based on lower heating value), 97.0%, 10.4% and 158 ppm, respectively. This supports the potential of using NH3 as a clean, carbon-free fuel for both burning and reforming in microreforming systems. (C) 2011 Elsevier Ltd. All rights reserved.