Development of a novel pellet feeder for emissions reduction in wood pellet stove

The combustion of wood pellet in stoves is a viable, carbon neutral option for residential heating. In these appliances, a screw conveyor is used to feed the pellet to the burner pot. Although the screw rotates steadily, the actual mass flow rate can be quite discontinuous, as easily observable; therefore, repeated deviations from optimal Air/Fuel ratio occur, leading to temporary peaks in the emissions. The aim of this work is to assess whether a more uniform pellet feed flow rate, provided by an innovative feeding device previously developed, effectively results in lower emissions. First, the stability and uniformity of the mass flow rate generated by the conventional (screw) and the new feeding devices were analyzed; the new one proved to deliver a dramatically steadier mass flow rate (CV, coefficient of variability = from 161 to 46 %). Then, the stove performance using the two conveyors was compared at both nominal and reduced stove power. Continuously monitored emissions include carbon monoxide (CO), nitrogen oxides (NOx), along with residual oxygen (O2) concentrations in the flue gases. Particulate matter (PM) was sampled, as well. Results indicate that the new feeder reduces significantly the CO concentration in the flue gas, both at nominal (from 186 to 100 ppm, -46 %) and partial load operation (from 232 to 145 ppm, -38%). PM concentration almost halves with the new feeder, both at nominal (from 29 to 14 mg/Nm3, -52%) and partial load operation (from 43 to 24 mg/Nm3, -44%), as well. The average concentration of NOx in the flue gas is not significantly affected by the feeder, being mainly determined by the nitrogen content in the biomass, and the flame temperature does not significantly change with the feeder, to trigger the thermal NOx production. The innovative feeding device represents an appealing option to effectively limit the air pollution from domestic combustion of wood pellet.