Accurate knowledge of the combustion gas composition is necessary for practical combustion systems where emissions reduction is a major concern. Industries use a variety of sensor technologies, many of which can be expensive. In industries that use plant biomass for heat production, a constant fuel composition is often assumed for estimation of unmeasured exhaust gas species and for the calculation of thermal efficiency. To address the fact that biomass decomposes non-homogeneously, a new analysis is developed where a 2:1 ratio of hydrogen to oxygen atoms in the fuel is assumed, thereby relaxing the constant fuel composition constraint. This analysis provides a reliable and accurate estimate of exhaust H2O and CO2 given commonly monitored air and fuel rates along with O-2, CO, and NO measurements. Validation of the analysis is investigated using tunable diode laser absorption spectroscopy (TDLAS) measurements of the flue gas. Results show good agreement between the new analysis and TDLAS measurements, verifying the validity of the approach. The new analysis also provides time accurate elemental species information for interpreting the non-homogeneous fuel decomposition. (C) 2017 Elsevier Ltd. All rights reserved.