Spectroscopic emission measurements and system level modeling of a two-stage wood-fired hydronic heater: Effects of non-homogeneous fuel decomposition

This study investigates the time-dependent non-homogeneous decomposition of solid biomass fuels in a wood-fired heating appliance. Throughout a run (heating cycle), flue exhaust gases are monitored using a Bosch oxygen sensor and a tunable diode laser absorption spectroscopy system. The data is used to derive an experimental pyrolysis model to calculate the instantaneous fuel composition (carbon-normalized H, O, and N). The fuel H=O atomic ratio remains nearly constant until all the H and O have been released. The results from the experimental pyrolysis model are used as input to a simplified three-zone (chamber) system level model to calculate time-accurate bulk emissions and temperatures in a domestic two-stage woodfired hydronic heater. Pyrolysis gas and carbon char fuel streams are separately defined in the three chambers. Wood pyrolysis products and char are determined via a multi-phase chemical equilibrium solver at a constant pressure and specified pyrolysis temperature. Comparison of experimentally measured emissions to the system level model predictions show good agreement, indicating that coupling a non-homogeneous description of the decomposing fuel with a simplified model can provide accurate emissions predictions from a boiler run in batch-mode.