Fluidized bed torrefaction of biomass pellets: advantages and drawbacks in terms of 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. Whereas 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 meglio riportare densità energetica come in seguito 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 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 thermal pretreatment of biomass, is combined with pellettization [6]. Note that wood pellets, which are known to be a very energy-dense biomass fuel, have an energy density that typically range from 7.5 to 10 GJ/m3 while that of coal is about 18.4-23.8 GJ/m3. Secondo me questa frase si può anche togliere so che qui si parla di densità energetica mentre prima di potere calorifico però forse è ridondante. Basically, torrefaction is a thermo-chemical treatment where biomass is heated in an inert environment up to a temperature ranging between 200 and 300 °C. It is traditionally characterized by low particle heating rate (typically less than 50 °C/min) and a relatively long reactor residence time that typically ranges from 30 to120 minutes depending on the specific feedstock, technology and temperature. The benefits accomplished by torrefaction include the possibility to convert any hydrophilic lignocellulosic raw materials into a hydrophobic solid, which can be stored outdoors. The bulk density of the torrefied material, however, is generally lower than that of the raw biomass, making transport and storage economically challenging. Therefore, combining torrefaction and pelletization has great potential to upgrade raw biomass to a standard commodity fuel. So far, two potential pathways, either an upstream (torrefaction before pelletization) or downstream (torrefaction after pelletization) configuration, have been assessed for retrofitting torrefaction within pellet production facilities. In particular, outcomes from a recent study by Kumar et al. (2017) have pointed out that downstream integration could be the better solution since it avoids many of the operational and safety issues associated with the size reduction of torrefied material and its densification [7]. Moreover, torrefaction of wood pellets could also led to a 3- to 4-fold increase in torrefaction reactor throughput when compared to untreated wood chips. As regards the quality of torrefied pellets, instead, findings from literature are rather inconsistent. According to some research works [8, 9], it is difficult to bind torrefied biomass particles without a binding agent, while conversely “wood pellets” torrefied at different temperatures exhibit good durability and no disintegration was observed upon the thermal treatment. This suggests that the bonds formed during pelletization outlast the torrefaction quite well. Other research works [10], instead, report that torrefaction after pelletization has a negative impact on the quality of pellets. In particular, it decreases the mechanical durability and the bulk density of pellets, thus affecting their handling and combustion performances. In this context, an experiment study on the torrefaction of wood pellets has been performed and the results are presented in this work. In particular, torrefaction tests were performed in a lab-scale fluidized bed reactor consisting of a tubular glass column (100 mm inner diameter and 75 mm height) equipped with a 4-mm thick, sintered-glass gas distributor. In detail, experimental batch runs were performed at three different temperatures (i.e., 200, 250 and 280 °C) and holding times (i.e., 5, 15 and 30 min) in order to investigate the effect of the main process variables (temperature and time) on both the quality of torrefied wood pellets (i.e., elemental composition, bulk and energy densities, sizes and shape, hydrophobic behavior, mechanical durability and fine particles content) and process performances (i.e., mass and energy yields). To the best of the Authors’ knowledge, the torrefaction treatment of wood pellets in fluidized bed reactor has not been investigated so far. Therefore, findings of this work can also useful to highlight potential advantages and drawbacks related to use of this technology in this specific application. Un confronto con pellet da torrefatto andrebbe comunque riportato. Se non vogliamo impegnarci forse ci sarebbero dati di letteratura da citare (F. Pinto Eucaliptus?). Reference [1] L. Wu, T. Moteki, A.A. Gokhale, .D. W.Flaherty, F. D. Toste. Production of Fuels and Chemicals from Biomass: Condensation Reactions and Beyond. Chem. 1 (2016): 32-58. [2] L. Tock, M. Gassner, F. Maréchal. Thermochemical production of liquid fuels from biomass: Thermo-economic modeling, process design and process integration analysis. Biomass Bioenerg. 34 (2010): 1838-1854. [3] J.S. Tumuluru, C. T. Wright, J. R. Hess, K. L. Kenny. A Review on Biomass Densification Technologies for Energy Application. Biofuels Bioprod. Biorefin, 5 (2011):683-707. [4] S. Clarke, F. Preto. Biomass Densification for Energy Production. Ontario Ministry of Agriculture, Food and Rural Affairs (2011). [5] A. Uslu, A.P.C. Faaij, P.C.A. Bergman. Pre-treatment technologies, and their effect on international bioenergy supply chain logistics. Techno-economic evaluation of torrefaction, fast pyrolysis and pellettization. Energy 33 (2008): 1206-1223. [6] J. Koppejan, S. Sokhansanj, S. Melin, S. Madrali. Status overview of torrefaction technologies. In: IEA Bioenergy Task 32 report FINAL REPORT (2012). [7 ] L. Kumar, A. A. Koukoulas, S. Mani, J. Satyavolu. Integrating Torrefaction in the Wood Pellet Industry: A Critical Review. Energy Fuels 31 (2017): 37-54. [8] B. Ghiasi, L. Kumar, T. Furubayashi, C. J. Lim, X. Bi, C.S. Kim, S. Sokhansanj. Densified biocoal from woodchips: Is it better to do torrefaction before or after densification? Applied Energy 134 (2014): 133–142. [9] N. Doassans-Carrère, S. Muller, M. Mitzka. REVE: Versatile Continuous Pre/Post-Torrefaction Unit for Pellets Production. In book: World Sustainable Energy Days Next 2014, Chapter: Part II, Publisher: Springer Fachmedien Wiesbaden, Editors: Gerhard Dell, Christiane Egger, pp.163-170. [10] C. Nobre, M. Gonçalves, B. Mendes, C. Vilarinho, J. Teixeira. Torrefaction effects on composition and quality of biomass wastes pellets. In book: WASTES: Solutions, Treatments and Opportunities, Publisher: CRC Press, Taylor & Francis Group, Editors: Vilarinho C, Castro F, Russo M, pp.171-177