The gaseous emissions of primary concern from biomass combustion are nitrogen oxides (NOX), carbon monoxide, and various unburned gaseous components. Detailed characterization of the gas in the hot reaction zones is necessary to study the release, formation, and evolution of the gas components. In the present study, gas temperature and concentration were measured in a 12-MWth biomass-fired reciprocating-grate boiler operated with over-fire air and flue-gas recirculation. Temperature measurement was combined with flue gas quenching and sample gas extraction using two water-cooled stainless-steel suction pyrometers. The concentration profiles of O-2, NO, and CO were experimentally determined throughout the furnace, and the profile gas temperature was measured in several positions inside the furnace for the two types of woody biomass studied. For both fuels, the gas temperature varied between approximately 450 degrees C (average primary chamber temperature) and 1200 degrees C (average secondary chamber temperature). The concentration profiles of CO and O-2 suggested no conclusive difference between the two types of biomass. However, the local mean concentrations of NO and NOX emission factors (measured in the stack) were higher for Greenery fuel due to its higher nitrogen content than that of Standard fuel. (C) 2014 Elsevier Ltd. All rights reserved.