In situ FT-IR absorption spectroscopy was used with a laboratory-scale reactor and thermogravimetric analysis to describe the release of nitrogen-containing compounds from a fixed bed of fibreboard residues. Such in situ detection of NH3 requires knowledge of the absorption coefficients at high temperatures. These coefficients were obtained from calibrations using a laboratory-scale reactor at temperatures up to 800 degreesC, as well as from calculations using a database. On the basis of laboratory experiments, the rates of release of N species, of the major C species (CO, CO2, and CH4), and of H2O have been derived. It is concluded that the release of nitrogen from the fuel can be modelled if such parameters as the superficial velocity and the flow rate of oxygen are known. This method allows profiles of the nitrogenous species to be generated and used as input data for CFD calculations based on thermodynamic combustion models for a fixed bed on a grate. (C) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.