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

Sammanfattning: The biomass pellet consumption, both in the industry and in residential appliances, has increased during the latest years and this is expected to continue. In order to handle the demand, more ash rich (>0,5 wt-%) raw materials have been introduced as energy wood and other forest based fuels. In connection with this, ash related problems as fouling, slagging and corrosion have occurred. However introduction of peat as a co-combustion fuel has turned out to have positive effects on these problems. Earlier research shows that introduction of peat into biomass results in reduced bed agglomeration, fouling and corrosion. Nevertheless the previous works has mainly focused on fluidized bed boilers with bed agglomeration and deposit formation in focus. Experimental work in small and residential combustion appliances is still scarce (e.g. in grate firing boilers and burner’s). In this work the influence of peat addition to biomass pellets on combustion characteristics here defined in terms of slagging characteristics, operational maintenance, gaseous- and particulate emissions in residential appliances, were determined. Six peat samples representing a broad variation in ash forming matter of Scandinavian peats were chosen in this study. They were co-pelletized with sawdust, energy wood, forest residue as well as wheat straw and combusted in a P-labeled underfed commercial pellet burner (15 kW) installed in a reference boiler. The NO and SO2 emissions were generally higher when introducing peat. However these emissions are both fuel and combustion specific and the increment has not been further investigated to conclude its origin. The particle emissions were in all cases totally dominated by fine (<1 μm) particles and contained high concentrations of K. When adding peat containing high amounts of Si and/or clay minerals into the wood derived fuels a clear reduction of emitted fine particles were shown. The likely mechanism is that the reactive Si and/or the clay minerals from the peat react with K vapor from the biomass forming K-silicates that will stay in the coarse ash fractions, hence capturing the K. The reduction was most obvious for the biomasses short in reactive Si as sawdust and forest residue. In fact a K reduction of up to 70 % was shown when adding a Si- or clay-rich peat to forest residue. However, when adding a peat with low ash content and high Ca content the reduction was diminished. It was further shown that addition of peat to the K and Si rich wheat straw did not generally affect the reduction of fine particle- and deposit forming K i.e. only a dilution effect of ash forming matter occurred. The slagging tendency was increased in all cases when peat was introduced hence also the operational maintenance was disturbed. However, great differences between different peat assortments existed in this aspect. High Si together with low Ca contents in the peat resulted in the most severe slagging, whereas a peat with higher Ca/Si ratio gave a significant lower slagging tendency. The sawdust was generally more affected by the peat addition than the other studied biofuels. In order to receive a considerable particle reduction in parallel with a manageable slagging tendency when using "problematic" biomass fuels in small scale burners and grate boilers, co-combustion with a carex based peat with a high ash content and relatively high Ca/Si ratio is suggested.