Design Changes in a Fixed-Bed Pellet Combustion Device: Effects of Temperature and Residence Time on Emission Performance

Abstract

The use of wood fuel pellets has proven to be well-suited for the small-scale market, enabling controlled and efficient combustion with low emission of products of incomplete combustion (PIC). Still, a potential for further emission reduction exists, and a thorough understanding of the influence of combustion conditions on the emission characteristics of air pollutants, such as polycyclic aromatic hydrocarbons (PAHs) and particulate matter (PM), is important. The objective of the present work was to determine the effect of design changes, i.e., increasing the temperature and/or residence time, on the emission performance and characteristics for a pellet combustion device using a laboratory fixed-bed reactor (<5 kW). The temperature and residence time after the bed section were varied according to statistical experimental designs (650−950 °C and 0.5−3.0 s) with the emission responses: CO, organic gaseous carbon (OGC), NO, volatile organic compounds (VOCs, 20 compounds), PAHs (43 compounds), PMtot mass concentration, fine particle mass/count median diameter (MMD and CMD), and number concentration. The temperature was negatively correlated with the emissions of all studied PIC, with limited effects of the residence time. The PMtot emissions of 15−20 mg/MJ were in all cases dominated by fine (<1 μm) particles of K, Na, S, Cl, C, O, and Zn. An increased residence time resulted in increased fine particle sizes (i.e., MMD and CMD) and decreased number concentrations. The importance of a high temperature (>850 °C) in the bed zone with intensive, air-rich, and well-mixed conditions was illustrated for wood pellets combustion with almost a total depletion of all studied PIC. The importance of the residence time was shown to be limited, and the results emphasize the need for further verification studies and technology development work.