Hydrogen has been considered as a promising alternative for fossil fuel in recent years because it is very "clean". Fossil fuel generates CO2, CO, SOx, unburned hydrocarbon and particles during combustion, while hydrogen only yields NOx. In this study, a medium-pressure boiler with 130 ton/h boiler loading in a full-scale plant was studied with two inlet hydrogen-rich refinery gas (RG)/fuel oil (FO) volumetric flow rate ratios (inlet RG/FO ratio) and two residual 02 concentration (vol.%) in flue gases (2%, 4%) to evaluate their influence on the emissions of NOx and CO2, flue gas temperatures and boiler efficiencies. The result shows significant improvements in both boiler efficiencies and emissions of air pollutants. By increasing the inlet RG/FO ratio from 1:5 to 1:1.5, the fuel cost was reduced by 11%, NO, emission down by 12%, and the CO2 emission 20,200 ton lower per year was achieved. Thus, better economic operating conditions for the boiler are suggested at inlet RG/FO ratio = 1: 1.5 with the residual O-2 concentration in flue gases = 2%. (c) 2006 Elsevier Ltd. All rights reserved.