Torrefaction subsequent to pelletization: Characterization and analysis of furfural residue and sawdust pellets
Introduction
Growing energy demands, depletion of fossil fuels and environmental problems have increased the world’s interests towards finding new green energy resources. Biomass is one of the renewable energy sources and has potential to replace fossil fuels and produce valuable commodity fuels due to the wide range of availability and physicochemical characteristics of biomass (Bevan et al., 2015, Kleinschmidt, 2011, Liu et al., 2014, Liu and Han, 2015).
Two kinds of biomass feedstocks such as furfural residue (FR) and sawdust were used in the current study. FR is a waste generated from production of furfural using agricultural residues such as corncob, cornstalk, rice husk and vegetable fiber (Mao et al., 2019). China is the largest country for furfural production and on an average 7.8 million tonnes of FR are produced every year (Yadong et al., 2008). Each tonne of furfural production yields 12–13 tonnes of FRs (Mao et al., 2012, Qin et al., 2017). FR in huge quantity are gathered and stored in the open environment, occupying large area of lands and causing serious damage to the ecological environment in furfural plant surroundings (Xing et al., 2015, Yu et al., 2014). FR is rich in lignin and cellulose with small amount of hemicellulose (Wang et al., 2017, Xu et al., 2013), and has potential to make value-added products (e.g. pellets) by means of pelletization in combination with torrefaction. Landfill, composting and incineration are the current technologies available for disposing of FR (Wenyi et al., 2017). However, traditional technologies has limitations and may cause secondary pollutants (Mao et al., 2019). Sawdust on the other hand is a widely available source from forest industry with various intrinsic characteristics.
Pelletization is a promising technology that can overcome various drawbacks of raw biomass such as low bulk density, high moisture content (MC), wide size distribution, irregular shape, potential risks of degradation during transportation and storage (Rentizelas et al., 2009). Hence, the fuel quality is improved and the costs of transportation and storage can be reduced (Li et al., 2012, Liu et al., 2014). However, there are still certain problems related to biomass pellets (e.g. energy density and hydrophobicity) (Wang et al., 2013), limiting the applications of biomass pellets on industrial scales. Previous literature (Ghiasi et al., 2014) reported that HHV (higher heating value) of commercial wood pellets is approximately 19 MJ/kg, lower than 28–30 MJ/kg of coal.
Torrefaction is a low temperature (200–300 °C) thermochemical conversion process of biomass carried out under in an inert gas atmosphere. Torrefaction can improve the energy density, grindability, hydrophobicity, stability and uniformity of biomass due to release of the volatile matter and disruption of lignocellulosic structure of biomass (e.g. breakage of hydroxyl groups) (Artiukhina and Grammelis, 2015, Chen et al., 2016, Chew and Doshi, 2011, Cremers et al., 2015). Some researchers (Manatura et al., 2017, Stelte et al., 2013, Wang et al., 2013) reported that torrefied pellet (TOPs) had higher HHV than un-torrefied pellet, and the overall cost, including production, transportation and logistics costs, for TOPs can be cheaper than regular pellets (Bergman, 2005, Chen et al., 2015a, Peng et al., 2010, van der Stelt et al., 2011).
Recently, combined pelletization and torrefaction has been the subject of researchers and many studies focused on pelletization of torrefied biomass. However, pelletization of torrefied biomass is difficult due mainly to elimination of hydroxyl groups and breakage of lignin components during torrefaction (Chew and Doshi, 2011). Some studies suggested that binding agents (e.g. untreated sawdust, starch, lignin, calcium hydroxide and sodium hydroxide, etc.) can be used for improving the pelletizing characteristics of torrefied biomass and minimizing energy consumption. However, this can lead to burden of additional costs for binders (Ghiasi et al., 2014, Hu et al., 2015, Peng et al., 2015). Torrefaction of pellets offered advantages in terms of overall energy and material balance over pelletization of torrefied biomass as depicted by Ghiasi et al. (2014). Nobre et al. (2015) also found that biomass pellet could be further improved by torrefaction. Even though the pellets lose some strength, become brittle, and shatter more easily after torrefaction, there is always an optimal balance between the advantages and disadvantages that can be achieved (Spîrchez et al., 2017).
Torrefaction subsequent to pelletization has rarely been reported (Abedi and Dalai, 2017, Brachi et al., 2019, Brachi et al., 2018, Manouchehrinejad and Mani, 2018). Manouchehrinejad and Mani (2018) performed torrefaction of wood pellets by employing batch scale reactor. Torrefaction improved the heating value and resistance to water uptake of torrefied wood pellet. However, it reduced the pellet density, hardness and durability. Volumetric energy density did not improve and remained same as that of raw wood pellet until torrefaction temperature of 270 °C, above which it dropped drastically. Similar trend was observed in our study for heating value, hydrophobicity, particle density and strength of both types of TOPs. For volumetric energy densities, TSPs followed the similar trend to the above study. However, volumetric energy densities of TFRPs correspondingly increased with increasing torrefaction temperature, contrary to wood pellets. This could be due to difference in chemical compositions of woody biomass and FR. Abedi and Dalai (2017) found similar results for oat hull torrefied pellets. For example, heating value and hydrophobicity of torrefied pellets improved while density, hardness and durability decreased. Lower heating value (LHV) for commercial wood pellets increased from 18.66 to 23.02 MJ/kg with increasing torrefaction temperature as reported by Brachi et al. (2018). The authors found a slight decrease in bulk and apparent pellet densities due to release of volatiles under mild torrefaction conditions. Volumetric energy density slightly decreased (i.e. 2–3%). In addition to this, we found no breakage of pellets upon the torrefaction of pellets under the temperature of 200, 250 and 300 °C and residence time of 15, 30, 45, 60 and 120 min as shown in Supplementary Fig. 1. This showed that bonds formed during pelletization endured the torrefaction process more effectively; suggesting that torrefaction subsequent to pelletization can be a feasible approach for producing torrefied pellets. This finding was in line with previous study (Brachi et al., 2019). Hence, our findings were supported by these studies in terms of heating value, mass and energy yields, hydrophobicity, pellet density and mechanical properties etc.
Literature on torrefaction of furfural residue pellets (FRPs) in comparison with torrefaction of sawdust pellets (SPs) has not been reported. This study aimed to introduce new material (i.e. FR) rich in lignin content (29.81%) for producing clean energy fuels (e.g. pellets) of higher quality. Influence of process variables (i.e. temperature and residence time) on quality parameters (e.g. LHV, energy density ratio, strength, particle and true density, hydrophobicity etc.) of TOPs were researched, and quality of two types of TOPs were compared based on detailed characterization and thorough analysis. Torrefaction kinetics models were also developed for understanding torrefaction performance. The results obtained are beneficial to the quality improvement and practical application of TOPs.
Section snippets
Materials preparation
FR and sawdust were used as the raw materials. FR was collected from a local furfural production plant located in Hebei Province, China. Sawdust was obtained from a furniture plant located in the suburb of Beijing. Prior to pelletization, FR and sawdust were air dried at room temperature until their moisture reached equilibrium (MCs of FR and sawdust were ~5 wt%). Biomass materials were then sieved for 15 min to obtain the desired particle size (i.e. 0.25–0.5 mm). Proximate, ultimate and
Appearance of TOPs
Appearance of TOPs was greatly affected by torrefaction temperature and residence time. Increasing torrefaction temperature resulted in change of color from light brown to black for TSPs and from brown black to dark black for TFRPs as shown in Supplementary Fig. 1. It seemed that TFRPs remained unchanged in shape and size under different torrefaction conditions, indicating the highly stable structure of corresponding pellets. During pelletization, softening of lignin at certain temperature
Conclusion
Torrefaction subsequent to pelletization improved the characteristics of pellets and made pellets suitable for combustion, gasification, and pyrolysis, and has greater importance in the research and industry fields. Mass and energy yields decreased largely with increasing torrefaction temperature, while the LHV and energy density ratio increased with temperature. Volumetric energy density of TFRPs increased slightly with increasing torrefaction temperature, while for TSPs it significantly
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by Ministry of Science and Technology of the People's Republic of China (2017YFE0124800). The authors declare that there are no conflicts of interest.
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