Torrefaction is a slow and low temperature pyrolysis process that is not very different from the one used in charcoal production cells, which were used as a reducing agent in the earlier stages of metallurgical processes at the beginning of the industrial revolution. However, the development of the torrefaction process only began with the production of coffee in the late nineteenth century, as documented in the first patents of Thiel (1897) [67
] and Offrion (1900) [68
]. Other patents in the area of torrefaction were patented in the following years and can be seen in Table 2
Some research on torrefaction, still in the 1930s, was devoted to the production of gaseous fuels. During the first half and the middle of the twentieth century, only a few works sporadically appeared that were dedicated to the torrefaction of biomass for energy. However, more information and fundamental data on heat treatments of lignocellulosic materials can be found from this period, mainly on high temperature drying, dry distillation, thermal degradation, pyrolysis, thermal stabilization, and preservation of wood.
5.1. Research Focused on Torrefaction
The development of modern works in the area of torrefaction can be divided into the pioneering French publications documented by ARMINES Assoc pour Recherche et Dev des Methodes et Process Ind
] and Bourgois et al. [70
], from 1981 to 1989, and the recent and extensive efforts of a large number of groups initiated by the work of scientists and engineers at the Eindhoven University of Technology and the Dutch Center for Energy (ECN) [72
]. In the late 1980s, initial research implemented in France resulted in a specialized unit in France, where torrefaction was utilized to create a reducing agent for the metallurgical industry. The unit was built by the Pechiney enterprise and operated for a few years until it was dismantled for economic reasons. It must be mentioned and acknowledged that other scientific research was carried out during this period, in parallel with the French and Dutch works [73
The torrefaction of biomass has been attracting a considerable amount of attention in the research community in the last few years [27
]. Thus, the authors proceeded to the quantification of this interest. By examining all publications that mention biomass torrefaction for this study the authors sought to gather the current research. The aim of the analysis made in this study was to highlight further advances in the use of torrefaction. In this review, journals, conference proceedings, and book chapters were all considered to make this study as broad as possible. In the case of this study, the most significant databases were utilized in search of related papers. The databases used for this search are: Elsevier, Springer, MDPI, IEEE Xplore, Taylor & Francis, Wiley, Emerald Insight, Nature, and Inderscience Online. These databases were chosen as they comprise almost all the publications on this subject.
After compiling and categorizing all found publications, more than a few conclusions can be reached. The first finding is that 2304 publications were found in the above mentioned databases. The second finding is that the majority of the publications concerning the torrefaction of biomass have been published relatively recently, as can be seen in Figure 5
. Also, by observing this figure, it is possible to deduce that the number of publications has been increasing almost exponentially and academic interest in this topic has been steadily growing. In Figure 6
, the distribution of publication by database can be seen. By carefully observing Figure 6
it can be concluded that the majority of biomass torrefaction publications can be found in the Elsevier database.
5.2. Future Perspectives and Research Developments
The increasing interest of the industry in the use of fuels also causes an increase in the studies that involve this subject to improve the production of biomass while reducing the costs of this process [25
As previously mentioned, torrefaction parameters affect the final properties of biomass, thus, studies are required that investigate such parameters to find the best set to obtain an ideal torrefied biomass sample [16
To investigate the influence of temperature and residence time on the final biomass quality, Grigiante et al. analyzed the effect of different temperature pathways versus time to obtain mass yield rate values for samples of pine biomass. The results allowed the conclusion that, regardless of the route used, for the same temperature and time values, the TYR is the same. These parameters affected, to a large extent, the final results of the TYR, however, considering the low temperature range chosen, the temperature oscillation did not provoke oscillations in the final biomass energy parameters [79
]. Chen et al. studied cellulose, lignin, and hemicellulose at a range of torrefaction temperatures based on the properties of their three-phase products, and a substantial difference in torrefaction characteristics was found due to their different molecular structures [75
]. In [77
], the characterization of biomass waste torrefaction under conventional and microwave heating was studied and the conclusions indicated that microwave torrefaction is more efficient for biomass upgrading and densification than conventional torrefaction.
Little attention has been directed to the definition of a numerical representation of the quality of biomass torrefaction. In addition to not having a complex database for biomass torrefaction, there is also an index that quantifies the torrefaction degree and shows the effect of the type of biomass used, as well as the effect of torrefaction parameters [80
]. Almeida et al. noticed a linear relationship between mass loss and energy yield and carbon fixation of torrential biomass for a range of temperatures [81
]. Li et al. observed a linear relationship between mass yield and energy yield, whereas Peng et al. used the mass loss as an indicator of the torrefaction state and developed a linear relationship between energy density and mass loss [54
]. However, these parameters would have to be measured separate from the torrefaction process. Thus, Basu et al. attempted to develop a quantitative parameter to measure the degree of torrefaction by specifically targeting its goal for the energy industry. Three torrefaction regimes were then defined: Mild, medium, and severe, according to the temperatures used and the torrefaction rates obtained [80
]. In [78
], different kinetic, volatile release, and solid composition models were analyzed through numerical simulations and optimized different biomass.
Hence, torrefaction is a technique that exploits biomass characteristics to the maximum extent and it is important to consider efficient conversion techniques. With increasing knowledge about the torrefaction of plants, such as bamboo or sugar cane, it is necessary to make the most of the properties of these plants, since they have high growth rates, low ash formation, low alkaline index, and low heating rates. Therefore, Rousset et al. tested different properties of torrefied bamboo, comparing them with the properties of other solid fuels. The results allowed the conclusion that this type of biomass has much improved energy properties after its torrefaction [83
]. The torrefaction of many different plant volatile organic compounds are starting to attract the attention of the research community. Poudel et al. conducted a comparative study of the torrefaction of empty fruit bunches and palm kernel shell performed in a horizontal tubular reactor at a temperature ranging from 150 to 600 °C [84
]. They concluded that 290–320 °C is the required temperature range for optimum torrefaction of empty fruit bunches and 300–320 °C is the optimum range for palm kernel shell. In [85
], the energy densification of sugarcane bagasse through torrefaction under minimized oxidative atmosphere was studied. The results indicated that torrefaction improved several fuel characteristics, making the sugarcane bagasse suitable for both domestic and industrial applications. Other possibilities of torrefaction have been applied and studied, such as for microalgae [86
], Black Lilac (Sambucus nigra
], Prosopis juliflora
], corncob [89
], almond and walnut shells [90
], bamboo sawdust [91
], rice husk [92
], and cotton stalk [93
], among many others [94
Brachi et al. have studied an innovative torrefaction process based on fluidized bed technology at various temperatures and with different residence times, using tomato peels as the raw material. This shows that it is possible to take advantage of low quality raw materials and, through torrefaction, improve them, thus, reducing the costs of obtaining raw material. The results of this study showed that it is possible to increase the energetic quality of the starting material by maintaining its TYR. This study also showed that this technology has numerous possibilities for its use in torrefaction when it comes to non-wood raw materials, allowing a uniform and consistent quality of the final torrefied products [95
Álvarez et al. addressed the non-oxidative torrefaction of biomass to enhance its fuel properties in which both the hydrophobicity and the fixed carbon were increased [76
]. Chen et al. researched the effect of torrefaction pretreatment on the pyrolysis of rubber wood sawdust through pyrolysis-gas chromatography/mass spectrometry and concluded that the contents of oxy-compounds, such as acids and aldehydes, decreased with rising torrefaction temperature [74
]. Finally, in [86
], the effects of torrefaction on physical properties, chemical composition, and reactivity of microalgae were studied. For the development of an ideal torrefaction protocol, Rodrigues et al. analyzed the effects of torrefaction undergoing normal conditions of temperature (265 °C) and residence time (15 min) in an N atmosphere and during a total 1 h 45 min heating period on a set of sixteen woody biomasses provenient from poplar short rotation coppice (SRC) and other Portuguese roundwoods [53
5.3. The Current State of the Built Production Units
Of the more than 60 announced torrefaction initiatives and the 15 large-scale units scheduled to start by 2011, very few were built and hardly ever achieved a steady total industrial production and commercial status. The assumptions and expectations for the start-up were initially very high. Most equipment suppliers tend to overstate their capabilities and underestimate the time and effort required. Technological entrepreneurs with limited biomass experience have also encountered difficulties in the face of simple challenges, such as food, transport, storage, and quality of raw materials. Another issue is the relatively high total costs. “Drying and re-drying a little more”, seems very simple, and attracted a huge group of serious entrepreneurs, but also the so-called “fortune hunters”. However, torrefaction is a more complex process than initially anticipated.
Torrefaction must be conducted intelligently, with controlled costs, and is entirely directed towards the progress and success of the marketing. There are currently a number of challenges in systems and processes that require careful research and development (R&D) and intelligent solutions, such as: Processing and control of the atmosphere; gas production for inertization; heat transfer; control and moderation of exothermic reactions; cooling of products; behavior of the torrefaction gases, their deposition, and use; integration of systems and processes; energy optimization and exergetics; densification; and the optimization of the entire process supply chain. Perhaps, the most important part is the diagnosis and control of the process. Due to the close relationship between temperature and residence time with the product quality and standardization, careful control of these process variables is critical.
The material produced should be completely homogeneous, with respect to the degree of torrefaction, and preferably dark brown (not over-torrefied), to allow sufficient yield and to facilitate densification. There are few initiatives that are paving the way for the torrefaction industry. There are currently five industrial torrefaction units constructed and functioning in Europe [33
] and at least eight units of torrefaction are planned and ready to begin functioning in the near future. These torrefaction industrial units are presented in Table 3