Evaluating the combustion of various biomass pellets in a small heat source with underfeed pellet burner: Heat output, gas emission and ash melting behavior

Abstract

The increasing demand for renewable energy sources has increased interest in utilizing agricultural residues as a sustainable fuel alternative for small-scale heating systems. This study examined the combustion of various agricultural pellets — wheat straw, hay, alfalfa, as well as spruce pellets — in a household underfeed 18 kW burner designed for wood pellets. The aim was to evaluate emissions and ash behavior under different air supply settings to identify the most efficient combustion conditions in terms of performance, emissions, and sintered ash formation while minimizing environmental impacts. While adjustments to the air supply could not fully prevent ash sintering, they played a crucial role in optimizing emission and performance parameters. Measurements of CO, NO_x, and SO₂ emissions, and temperature-related parameters such as burning bed and flame temperatures, were conducted. Altering the air supply setting led to a substantial decrease in device performance, particularly for spruce pellets, where a reduction of up to 2.6 kW was observed at the measured settings. Hay pellets exhibited a significant increase in average CO production from 7944 to 12764 mg/Nm³ and NO_x from 307 to 851 mg/Nm³ with varying fan settings. For alfalfa pellets, changes in air supply resulted in higher SO₂ emissions, ranging from 1479 to 1683 mg/Nm³. Upon analysis of ashes, Si, K, Ca, and Mg were identified as the predominant elements in the ash from the fuels, along with Cl, S, and P. Among the tested ashes from fuels, wheat straw had the lowest ash deformation temperature at 797.0 ± 9.9 °C, in contrast to alfalfa, which had the highest at 1115.0 ± 19.2 °C. These results highlight the operational challenges of burning agricultural pellets in underfeed pellet burners, particularly given that the average measured burning bed temperature exceeded 1000 °C in some cases.