Effect of peat addition to woody biomass pellets on combustion characteristics in residential appliances

Abstract: Increased wood pellets demand in Scandinavia along with limited availability of wood assortments e.g. sawdust are pushing the market towards potentially more problematic raw materials with higher ash content, e.g. peat. The objective of the present work was to determine the effect of peat addition to woody biomass pellets on combustion characteristics (i.e. slagging tendencies/operational maintenance, gaseous- and particulate emissions) in residential appliances. Two peat assortments were studied: peat A with high ash and high Si content and peat B with low ash and high Ca content. These were co-pelletized in three different contents (low, medium, high) with sawdust and energy wood respectively. A commercial underfed pellet burner (20 kW) installed in a reference boiler was used for the combustion experiments. Measurements of gaseous O2, CO, NO, HCl and SO2 as well as particle mass size distribution were made in the exhaust gas directly after the boiler. X-ray diffraction (XRD) and scanning electron microscopy combined with energy dispersive X-ray analysis (SEM/EDS) were used to characterize the collected slag deposit, the corresponding deposited bottom ash in the boiler and flue gas particulate matter regarding morphology and chemical composition. The slagging tendency generally increased when peat were added to the non-slagging wood assortments. The increment was moderate for peat B compared to when peat A were added. The sawdust fuel was generally stronger affected by the peat mixings compared to the energy wood fuel. The slag from fuels containing medium and high amounts of peat A had higher concentrations of Si, Al and Fe and lower concentrations of Ca compared with fuels containing the low admixture of peat A. No major differences in the elemental compositions were shown when comparing the slag from fuels containing different amounts of peat B. Further, the results showed a considerable reduction of the emitted mass of fine (<1 µm) particles when mixing peat A into the woody biomasses fuels.