An experimental study of the in-furnace processes in a full-scale liquid-fuelled utility boiler is presented, based on measurements of mean gas temperature, stable gas-species concentrations (CO, CO2, O2 and NO(x)) and wall incident heat fluxes within the radiation chamber up to the entrance of the superheaters. The experiments, which were carried out at full and part loads with different excess-air levels, included analysis of the influence of modifications in the burners, with the objective of overcoming limitations on the production of superheated steam imposed by the degradation of fuel quality. The original burners include two-fluid atomisers of the ＇internal-mixing chamber＇ type operating with steam and a perforated bluff -body for flame stabilisation; the modified burners include Y-type fuel atomisers, and the flame was stabilised through swirl vanes in the primary air duct. Analysis of the results is aimed at improving the understanding of combustion and heat-transfer processes in full-scale boilers, and at having the necessary detail to provide the means of validating mathematical models of these processes. Current flame emissivity and tube-wall temperatures are estimated from overall energy balances, based on the experimental results; they allow us to improve our knowledge of the fundamental mechanisms giving rise to the improvements achieved with the modified burners.