Measurements of fuel burn rate, emissions and thermal efficiency from a domestic two-stage wood-fired hydronic heater

An experimental study is conducted of a two-stage wood-fired hydronic heater (WFHH). The WFHH contains two combustion chambers. The first is associated primarily with wood pyrolysis while the second, located down stream, is designed for secondary burning of undesirable emissions. A piezo-electric load cell based apparatus is developed to obtain direct measurements of fuel burn rate (FBR) - avoiding possible inaccuracies of standardized (full appliance weighing) approaches used for certification. To check the internal consistency of the experimental measurement, a theoretical mass loss relation is developed and used for reducing data and to explain the physical mechanism responsible for the existence of the experimentally observed global maximum burn rate. A system level numerical model is also developed based on a combination of well-stirred reactor theory and chemical equilibrium to provide estimates of flue exhaust products and temperature. Overall agreement between experiments and model predictions are reasonable for temperature and major combustion species. Experimental emissions maps within a temperature/equivalence ratio state space are used to demonstrate the current operating path for this WFHH. Average thermal efficiencies are measured in the range of 48–55%. These measurements are found to be internally self-consistent and provide guidance for more complete theoretical studies.