Fluidized Bed Torrefaction of Biomass Pellets: Process Performance and Product Quality

Biomass is considered the renewable energy source with the highest potential for replacing fossil fuels in the short and medium term. Biomass is available almost everywhere on earth in the form of both renewable raw materials and biogenic wastes generated by human activities, e.g., agricultural and livestock activities, food processing and timber industry. Biomass is the only renewable energy, which is based on sustainable carbon. Therefore, while other renewable resources produce heat and power only, biomass has the further advantage of its possible conversion into chemicals, commodities and liquid fuels for transportation [1, 2], thus competing with fossil fuels in all their applications. Unlike other renewable energy resources, such as solar and wind energy, which are intermittent and, therefore, can meet only a portion of primary energy demand, biomass can ensure continuous power and heat generation. Furthermore, it can also be easily transported and stored to accommodate changes in the output demand; it can be used to generate electric power with the same equipment or power plants that are now burning fossil fuels. On the other hand, there are currently a number of challenges related to the quality of biomass resources that still prevent them from being used on a large scale for heat and power production. Biomass has, in fact, a lower energy content compared with fossil fuels, which means that a higher load of feedstock is required in the case of biomass-fed plant in order get the same amount of energy when compared to fossil fuels. Moreover, biomass is a relatively bulky material. Typically, biomass bulk density ranges from 80-100 kg/m3 for agricultural straws and grasses to 150-200 kg/m3 for woody resources like wood chips and sawdust [3], while conversely the bulk density of coal is about 700 kg/m3 [4]. As a consequence of this, the volume of feedstock to be handled increases enormously, when biomass is used as a fuel instead of coal, with all the consequent problems that this can bring for logistics, in particular storage and transportation, which are factors that greatly affect profit margins and thus the convenience of a biomass-fed plant. This is the reason why, commonly, it is only economically feasible to transport unprocessed biomass over a distance less than about 200 km [4]. In addition, biomass cannot be stored outdoor without careful protection since it is prone to natural decomposition over time and breakdown with exposure to moisture and pests (e.g., flies and mosquitos), with consequent loss of quality and off-gas emissions; the high moisture content of some kinds of biomass also accelerate the decomposition process. It is worth noting that drying biomass has little benefit for the improvement of biomass storage behavior. In fact, because of its hydrophilic nature, biomass can re-absorb moisture and start to decompose again. Recent developments in mechanical densification technologies, including pelletization and briquetting, have substantially improved the economics of moving biomass around the globe [4, 5]. Typically, these technologies increase the biomass energy density (MJ/m3) through the increase of its bulk density (kg/m3), but have a very little benefit for the improvement of properties such as the high oxygen content or low calorific values (MJ/kg) and the remarkable hydrophobic behavior. The lower heating value (LHV) of currently marketed wood pellets is about 15-18 MJ/kg, which still limits the mixing ratio that can be used in co-firing with coal typically having a lower heating value (HHV) of about 23-28 MJ/kg [6]. Fuel pellets with a very high bulk energy density in the range from 15 to 18 GJ/m3 and a lower heating value as high as 20-24 MJ/kg can be obtained when torrefaction, a relatively new