Implication of pellet quality on combustion performance

Abstract: Quality aspects of wood pellets and their use have been of utmost importance sincethe development of the pellet market in the early 1980s. Research and hard-earned knowledge have resulted in advancements in the field, but there are still uncertainties in the pellet industry about how different quality aspects affect combustion performance.  The focus of this work has been on pellet quality, with investigations divided into three main topics: (1) the effect of physical properties of pellets on their combustion properties, (2) the effect on particle emissions due to manipulation of the ash chemistry by means of additives and, (3) how radio frequency identification (RFID) technology can be used to achieve traceability of bulk pellets.   In the literature it is not clear how physical pellet quality parameters affect the combustion behavior. To gain knowledge regarding the perception of pellet quality in practice, interviews with pellet industry representatives were made initially. This was followed by detailed combustion experiments to investigate how the key quality parameters moisture content, density, and durability affect the ignition behavior and the conversion time. A large number of well-defined pellet samples produced from four different raw material mixes were used. The results showed that during stable combustion conditions, i.e., high temperature and sufficient air supply in a fully functioning combustion system, these parameters have little practical influence on the combustion performance. However, the results from the detailed laboratory experiments indicated that the choice of raw material can have a more profound effect on both ignition behavior and conversion rates, although the full-scale tests indicated that this was of little practical importance.  Fuel design, i.e., choosing fuels or making adjustments to the fuel based on ash composition, can be used to lower particle emissions. This concept was demonstrated during combustion tests that were performed in three individual campaigns in medium- to large-scale boilers, 200 kWth, 2 MWth and 4 MWth respectively. In the campaigns, peat was utilized to alter the ash transformation reactions, reducing the emission of particulate matter less than 1 micron (PM1) during combustion of woody biomass, while keeping the slag formation at a manageable level. This was achieved by designing fuel blends of woody biomass with carefully selected Scandinavian peats rich in Si, Ca, and S. In one of the campaigns, softwood-based stemwood pellets were co-pelletized with different additions of peat (5 and 15 wt %) before combustion. In the other campaigns, peat was added as a separate fuel feed to Salix chips (15 wt % peat) and softwood-based stemwood pellets (10 and 20 wt % peat). The results showed that, no matter how peat was added to the fuel, the fuel design approach provided PM1 reductions of between 30 - 50 % for all fuel blends. The PM1 reduction could be achieved without causing operational problems due to slagging in any of the three commercial boilers used, although an expected increased slagging tendency was observed.   RFID systems are used today for the tracking of well-defined entities; i.e., the RFID tag is linked to an object - a container, a person etc. While RFID technology has been used in this way in the energy sector to monitor biofuel transport to and from transshipment sites and energy plants, it is not known to have been used for tracing bulk biofuels, i.e. a fuel that cannot be seen as a stand-alone entity. To demonstrate the potential of using RFID technology to trace bulk fuel transportation, three tests were performed. RFID tags were placed together with biomass pellets before being conveyed through a distribution chain, from pellet producer to combustion plant. The two first tests were large-scale trials to investigate if specific RFID tags could be correlated to a specific fuel when fed into the furnace. The third test was performed to see how RFID tags distributed over time in a logistics chain. The results showed that it is possible to trace a bulk biofuel flow using RFID technology, from production site to furnace, although care must be taken to optimize the method, such as using an appropriate number of tags.       

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