Effect of peat addition to woody biomass pellets on slagging characteristics during combustion in residential pellet burners

Upgraded biofuels as pellets, briquettes and powder are today commonly used in small as well as large scale appliances. In order to cover a great fuel demand new materials as bark, whole tree assortments and peat are introduced. These materials have higher ash content why they are potentially more problematic compared with stem wood. Ash related problems in the combustion device, e.g. fouling, slagging and corrosion may occur from the incombustible leftover i.e. the fuel ash. The objective of the present work was to determine the effect on slagging characteristics when co-pelletizing peat into woody biomass. In this project woody biomass (pine and spruce 50/50) has been co-pelletized with three different mixtures (low, medium and high) of peat; peat A with high ash and high Si content and peat B with low ash and a relatively high Ca/Si content. There were totally 6 mixed assortments in which softwood sawdust and energy wood each represented the biomass in 3 samples respectively. The experiments were carried out in a commercial underfed pellet burner (20 kW) installed in a reference boiler. X-ray diffraction (XRD) was used to characterize the phase composition and scanning electron microscopy combined with energy dispersive X-ray analysis (SEM-EDS) were utilized in order to characterize the elemental distribution and morphology of the collected slag deposits and bottom ashe. The slagging tendency showed an obvious increment when adding peat into the woody biomasses. Especially the ash- and relatively Ca-poor sawdust fuel was generally more sensitive for the different peat assortments. Co-firing of peat A (high ash- and Si content) resulted in the most severe slagging tendency, where four out of six experiments led to a total shut down of the burner due to the slagging. The slagging tendency was significantly lower when adding peat B (ash poor and relatively high (Ca/Si ratio) into the woody biomass fuels. A significant increment of the Si, Al and Fe content and a significant decrement of the Ca content could be seen when increasing the mixing content of peat A in both woody biomasses. The slag from the peat B mixtures had a slightly higher Ca content compared with the Si content and a clearly higher Ca content compared with the peat A mixtures. When introducing the relatively silicon rich Peat A to the woody biomasses the slagging tendency increased because high temperature melting Ca-Mg oxides reacts to form more low temperature melting Ca-Al-K silicates. In the case of the more ash poor peat B, with relatively high Ca/Si ratio, there were still Ca-Mg oxides left in the bottom ash i.e. a less amount of sticky low temperature melting Ca-Al-K silicates were formed. Godkänd; 2009; 20091104 (nysida)