Torrefaction

Torrefaction is an upgrading technology under development that aims to enhance the fuel quality by addressing issues such as energy density, grindability, and storability.

From: Perennial Grasses for Bioenergy and Bioproducts, 2018

Chapters and Articles

Torrefaction

Wei-Hsin Chen, in Pretreatment of Biomass, 2015

10.5 Oxidative Torrefaction

Oxidative torrefaction provides a different approach from conventional torrefaction to upgrade solid biomass. Oxidative torrefaction refers to when biomass is torrefied in oxidative environments at temperatures of 200–300 °C, which is at the same temperature range of nonoxidative torrefaction. Although nonoxidative torrefaction is able to upgrade biomass, additional operating costs are incurred due to heating and the supply of an inert gas (usually nitrogen). If biomass torrefaction is carried out in oxidative atmospheres, it would thus be possible to reduce costs, for the following three reasons. To start with, if air or a combustion flue gas is utilized as a carrier gas, the process of gas separation to extract nitrogen is no longer needed. Next, oxidative torrefaction not only involves the devolatilization and thermal degradation of biomass [54], which also occur in nonoxidative torrefaction, but also involves oxidative reactions [55–58]. The oxidative reactions in oxidative torrefaction are usually exothermic, implying that heat is generated from the torrefaction. This also reduces the heating demand for endothermic biomass torrefaction reactions. Finally, the reaction rates of oxidative torrefaction are generally faster than those of thermal pyrolysis, which will shorten the torrefaction duration and thus the operating costs.

In oxidative torrefaction, a higher decomposition rate of biomass can be obtained if a higher O2 concentration is used in a torrefaction environment [58–62], and thus a shorter torrefaction time or a lower torrefaction temperature can be desired to achieve a target weight loss. However, if light torrefaction or lower torrefaction temperatures are performed, the influence of increasing the O2 concentration on reducing the solid yield and changing the composition of the resulting material is not significant [63]. In short, oxidative torrefaction is a feasible way to upgrade ligneous biomass such as eucalyptus, but is not suitable for fibrous biomass, such as oil palm fiber, as a consequence of low solid and energy yields [10]. Overall, an increase in the severity of nonoxidative torrefaction increases the enhancement factor of HHV, but decreases the solid yield, as shown in Figure 10.9. In contrast, increasing the severity of oxidative torrefaction decreases both the enhancement factor and solid yield, due to the oxidative reactions in the biomass.

FIGURE 10.9. The trends of the enhancement factor of HHV versus solid yield in nonoxidative and oxidative torrefaction.

Read full chapter
URL: https://www.sciencedirect.com/science/article/pii/B9780128000809000104

Biomass carbonization and torrefaction

Prabir Basu, Priyanka Kaushal, in Biomass Gasification, Pyrolysis, and Torrefaction (Fourth Edition), 2024

5.4.1.4 Torrefaction

The torrefaction stage is key to the whole process as the bulk of the depolymerization of the biomass takes place in this stage. A certain amount of time is needed to allow the desired degree of depolymerization of the biomass to occur. The degree of torrefaction depends on the torrefaction temperature as well as on the time the biomass is subjected to torrefaction. This time is also called reactor residence time or torrefaction time. The torrefaction time should be measured from the instant the biomass reaches the temperature for the onset of torrefaction (200°C) because the degradation of biomass below this temperature is negligible.

The torrefaction process is mildly exothermic (Prins, 2005) over the temperature range of 250°C–300°C. So, the torrefaction stage should require very little energy (Fig. 5.3), but in practice, it could require some heat to make up for the heat loss from the torrefaction section of the reactor. So, the total heat required for this step, Qtor is:

(5.4)Qtor=Hloss+Mf(1M)Xt

Here, Xt is a parameter (kJ/kg product) that determines the amount of heat absorbed during torrefaction. It is positive for endothermic and negative for exothermic torrefaction reactions. The amount of heat loss, Hloss, to the ambiance from the torrefaction section is a function of reactor design.

Read full chapter
URL: https://www.sciencedirect.com/science/article/pii/B9780443137846000214

Conversion of Microalgae Biomass to Biofuels

Min-Yee Choo, ... Joon Ching Juan, in Microalgae Cultivation for Biofuels Production, 2020

Torrefaction

Torrefaction is the thermochemical conversion method to produce coal fuel (biochar) from biomass. The main product of the torrefaction is solid coal fuel, while liquids and gases are produced as by-products [72]. Biochar is a carbon-rich material from a biomass, which is produced by thermal composition of organic feedstock in the oxygen-free condition [73]. As mentioned in the previous section, biochar can be obtained via slow pyrolysis of biomass. The biochar produced by pyrolysis has high surface area, which is more suitable in the soil fertility improvement and waste treatment. However, as compared with the torrefaction, the increase in the calorific value is minimal. Therefore, high-quality solid fuel can be obtained via torrefaction as compared with pyrolysis [74].

Torrefaction can be divided into two groups, namely, (1) dry torrefaction and (2) wet torrefaction. Dry torrefaction is operated at temperature range between 200 and 300°C, under atmospheric pressure and inert nitrogen gas atmosphere with absence of oxygen. Dry torrefaction is also known as mild pyrolysis or low temperature pyrolysis, as lower reaction temperature is used as compared with conventional pyrolysis (>400°C) [74]. In this torrefaction, nitrogen gas is commonly used as a torrefaction gas to prevent the oxidation of the biochar during the reactions. Recently, there are some studies that used noninert gas (combustion gas or CO2) to reduce the energy and the cost [75,76]. However, noninert promotes the oxidative reactions. As a result, CO and CO2 gas are present in the gas phase, while water, phenol, and acetic acid are present in the liquid phase of end product. Besides, the solid yield using noninert gas is lower compared with that of inert nitrogen gas, which may be due to the Boudouard reaction [77]. Torrefaction temperature is a critical factor in the torrefaction process. For example, in the torrefaction of Chlamydomonas sp. JSC4, the solid yield has decreased from 93.9% to 51.3% when the torrefaction temperature increased from 200 to 300°C [78]. Similar trend was observed in another study where solid yield decreased from 86.37% to 63.23% when temperature increased from 200 to 300°C in torrefaction of S. obliquus CNW-N [79]. The main challenge of the dry torrefaction is that the predrying step is required to reduce the moisture content to less than 10 wt% [80]. Large amount of energy (3–5 MJ) is required to reduce the moisture the content from 50–60 wt% to 10–15 wt%, which in turn largely increased the operating cost and reduced the overall performance of dry torrefaction [81].

Wet torrefaction is an attractive technique to produce coal fuel from biomass without predrying step. Similar to dry torrefaction, the process is operated in the inert condition but with a temperature of 180–260°C, which is relatively lower than that of dry torrefaction [80]. In wet torrefaction, subcritical water is used a reaction medium. Its low dielectric constant enhances the ionic reaction and thus effectively solubilizes the biomass elements [82]. In terms of reaction mechanism, presence of subcritical water hydrolyzed the C-O bond in the ether and ester bonds between monomeric sugar, which reduce the activation energy; thus, degradation of hemicellulose in the wet torrefaction is more efficient compared with dry torrefaction [83]. As discussed by Bach and Skreiberg [80], wet torrefaction is more efficient than dry torrefaction as it requires relatively low temperature and holding due to the highly reactive reaction media (hydrothermal treatment). Similar solid yield (∼52%) can be achieved via wet torrefaction at 180°C for 10 min or 170°C for 30 min as compared with dry torrefaction (300°C for 60 min) [76,84].

Read full chapter
URL: https://www.sciencedirect.com/science/article/pii/B9780128175361000102

Exploitation and Biorefinery of Microalgae

Revathy Sankaran, ... Jo-Shu Chang, in Waste Biorefinery, 2018

3.1.4 Torrefaction

Torrefaction is an upgrading process of biomass for other energy generation processes like direct combustion on an industrial scale. Torrefaction is the thermal degradation of organic biomass in an inert or nitrogen atmosphere, one atmosphere pressure and temperature in the range of 200–300°C, for several hours depending on the biomass [24]. Torrefaction enhances the properties of feedstock in a number of ways to enable its use as a direct fuel: reduction in moisture, increase in energy density, reduction in the O/C ratio, increase in heating value, and improved ignitability and reactivity of the processed fuel [51]. During torrefaction, microalgae undergo dehydration; proteins and carbohydrates are thermally decomposed achieving partial carbonization. The torrefaction efficiency determined the temperature used and the holding time. For microalgae, torrefaction can be light (200–235°C), mild (235–275°C), and severe (275–300°C) depending on their temperature range [31]. Torrefaction of microalgal biomass has been performed with Chlamydomonas sp. and Chlorella sorokiniana [48], Scenedesmus obliquus [52], and Spirulina platensis [53].

Read full chapter
URL: https://www.sciencedirect.com/science/article/pii/B9780444639929000197

Introduction

Leonel Jorge Ribeiro Nunes, ... João Paulo Da Silva Catalão, in Torrefaction of Biomass for Energy Applications, 2018

1.5 Origin of Torrefaction Process

Torrefaction is a process of slow pyrolysis at low temperature, not much different from that used in charcoal production piles, used as a reducing agent in the early days 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 19th century, as documented in the first patents by Thiel (1897) and Offrion (1900) [115].

Some research on torrefaction, still in the 1930s, dedicated to the production of gaseous fuels. During the first half and mid-20th century, works dedicated to the torrefaction of biomass for energy only sporadically arise. However, more information and fundamental data on thermal lignocellulosic materials treatments can be found from this period, especially on high-temperature drying, dry distillation, thermal degradation, pyrolysis, thermal stabilization, and wood preservation [116].

The development of modern works in the field of torrefaction can be divided in the French pioneering work documented by Armines and Bourgois et al., during the years 1981–89, and the extensive efforts made by a large number of scientists and engineers, initiated by the work at the Eindhoven University of Technology and the Dutch Center for Energy (ECN) [117,118].

In the late 1980s, the French early work led to a demonstration unit in France, where torrefaction was used to produce a reducing agent for the metal industry. The unit was built by the Pechiney company and operated for a few years until being decommissioned for economic reasons [119]. It should be mentioned and recognized that other scientific works were carried out in this period in addition to French and Dutch works [120–125].

The most recent efforts and current research and development (R&D) carried out on torrefaction are extensive, with a large number of R&D groups working on torrefaction, as evidenced by the growing number of publications. Moreover, there are also a growing number of technical reports and published conference presentations. However, reports on torrefaction represent only 5% of produced studies compared to studies on biomass pyrolysis [126].

Surprisingly, much effort has gone into compiling and reviewing all existing information and processes in several specific areas. For example, introductions were published to the topic of torrefaction, which includes a brief review of the history of torrefaction and much of the data gathered by the Dutch authors, which catalyzed the latest developments on the torrefaction [127,128].

Review articles on the torrefaction process emphasized the constituents of the biomass and their reactions (depolymerization, volatilization, and carbonization). Reviews were conducted with extensive compilation of data on biomass torrefaction [129]. Moreover, the technologies proposed and used for torrefaction were compiled and shown in several articles [130,131].

The work of the Swedish program for the study of torrefaction was also compiled [132]. Other studies on torrefaction and similar processes, such as wood preservation by heat treatment at low temperature and long residence times, were reviewed and published in several studies [133–136]. Important aspects of the effects of torrefaction in the biomass supply-chain economics and the importance of torrefaction for the thermochemical conversion systems have also been compiled [137,138].

Read full chapter
URL: https://www.sciencedirect.com/science/article/pii/B9780128094624000018

Chemical, biochemical, and thermochemical conversion of microalgal biomass into biofuel generation

Priya Rai, Anjana Pandey, in Microalgal Biomass for Bioenergy Applications, 2024

7.3.5 Torrefaction of algal feedstock

Torrefaction is a potential technology of solid fuel production from the thermochemical conversion of biomass at a high temperature under atmospheric pressure and inert nitrogen in an oxygen-free environment (Zhang et al., 2018). The promising application of pretreated solid fuel made through the torrefaction process has improved its commercialization in the industry such as in gasification, combustion, and pyrolysis for power generation. A raw feedstock has a high moisture content, low calorific value, high O/C, and H/C ratio that make the feedstock unsuitable to be used as fuel (Gan et al., 2018). The process of torrefaction can easily separate the moisture from the feedstock and convert it into solid fuel closed to the coal. Technically, torrefaction can thermally degrade the lignin, cellulosic, and hemicellulosic fraction of the lignocellulosic feedstock (Chen et al., 2012) as well as protein, carbohydrate, and lipids fraction of the third-generation feedstock (algae feedstock) (Kumar et al., 2017). Torrefaction can be subdivided into dry torrefaction and wet torrefaction. Dry torrefaction is also known as mild pyrolysis. A high calorific solid fuel (biochar) can be achieved in the absence of oxygen through dry torrefaction of raw feedstock at a temperature of around 200–300°C under atmospheric pressure. The high moisture content of the feedstock severely affects the energy density and calorific value, and carbon content of the fuel so it should be dehydrated before going into the process of torrefaction (Wilk et al., 2015). The moisture content must be less than 10 wt% for better fortification process (Bach et al., 2016) but if it is found in the range of 50–60 wt%, it should be reduced to 10–15 wt%. Biochar is the main fuel product of dry torrefaction while gases as well as liquids are obtained as co-products. Wet torrefaction is a process of producing high-quality biochar from the wet feedstock at a temperature of 180–260°C, and at a shorter residence time of torrefaction in an inert environment. Water is used as a solvent (reaction medium) for wet torrefaction. The water used in wet torrefaction is also known as subcritical water. Wet feedstock with solvent (subcritical water) is kept in the pressurize reactor chamber to remove the moisture content from the feedstock as well as to upgrade the quality of fuel. The physical and chemical properties of subcritical water make it highly favorable for the hydrolysis process in wet torrefaction.

Hydrolysis in wet torrefaction is a process of reacting subcritical water with feedstock to break the ester and ether bonds between the sugars (Libra et al., 2011). Thus, the degradation of feedstock in wet torrefaction is higher than the dry torrefaction. The process of dry torrefaction involves a series of chemical reactions (decarbonylation, dehydration, decarboxylation, condensation, and aromatization) that are not included in the mechanism of wet torrefaction. Thus, wet torrefaction is cost-effective as compared to dry torrefaction. Phusunti et al. (2018) tested the temperature range from 150°C to 300°C and the holding period from 15 min to 60 min for the torrefication of C. vulgaris and analyzed that the temperature (200°C), holding time (30 min) was optimum for upgrading the calorific value of the fuel. Similarly, Mokhtar et al. (2019) studied the effect of temperature ranging from 200°C to 300°C and residence time ranging from 30 min to 90 min on the torrefication of Ulvaintestinalis and found that the torrefication temperature has an effective factor in intensifying the calorific value, and carbon content of the torrefied algal biochar in comparison to residence time. Rivera et al. (2019) analyzed the life cycle assessment of torrefied algal fuel production starting from the cultivation, harvesting, dewatering, drying, and lipid extraction of algae and found that there is huge energy consumption in torrefied algae fuel formation. So, they suggested that to find a new technology or new way to reduce the excessive consumption of the input energy during torrefaction of the algal solid fuel production. Bach et al. (2017) used a microwave-assisted heating system in wet torrefaction of C. vulgaris ESP-31 and concluded that the calorific value of the fuel was made from the respective alga can be raised to 21%. Yu et al. (2020) did microwave-assisted acid hydrolysis pretreatment of two microalgae, Chlorella sp. GD, and C. vulgaris ESP-31 using wet torrefaction to find out the final product (solid biochar) yield. He found a solid fuel yield 74.6% from Chlorella sp. GD and 54.5% from C. vulgaris ESP-31 to form his study.

Read full chapter
URL: https://www.sciencedirect.com/science/article/pii/B9780443139277000189

Introduction to Thermochemical Conversion Processes

Stephen Gent, ... Evan Almberg, in Theoretical and Applied Aspects of Biomass Torrefaction, 2017

1.4.4 Torrefaction

Torrefaction, also known as mild pyrolysis, roasting, or high-temperature drying, is a relatively recent thermochemical technology in the biorenewables field. The aim of torrefaction is to upgrade the fuel characteristics of biomass such that it can be cocombusted with coal or used as an independent fuel by being pelletized and stored with little to no microbial degradation [21,22]. Dry torrefaction is the accepted method for commercialization [23], but research on wet torrefaction using hot compressed water has also been published [24]. Torrefaction is achieved by slowly heating biomass at relatively low temperatures (200–300°C) in a limited oxygen environment, causing devolatilization and degradation reactions to occur. Important chemical and physical transformations occur under these conditions that produce a more hydrophobic, homogeneous, and energy dense solid fuel source [21]. A typical mass and energy yield resulting from torrefaction is approximately 70%–80% and 80%–90%, respectively.

The main difference between pyrolysis, gasification, and torrefaction is derived from their product motivation [23]. Conventional pyrolysis and torrefaction share similar operating conditions, however, significant differences exist in the application and composition of the products. Torrefaction is similar to carbonization in that they share the same product motivation—aiming to maximize the production of energy-dense solids. However, torrefaction also aims to maximize the energy and mass yields by minimizing oxygen to carbon (O/C) and hydrogen to carbon (H/C) ratios. The process parameters of torrefaction also differ from carbonization. Torrefaction employs low heating rates and relatively low temperatures to drive away only the low energy dense volatiles and chemically bound water, avoiding carbonization reactions [25].

Torrefaction is an attractive conversion pathway for several reasons. First of all, it allows for the conversion of biodegradable biomass into a hydrophobic product which is not prone to biological decomposition. This conversion allows for long-term preservation of the processed biomass that would otherwise suffer from degradation in a storage environment that is exposed to the weather. The resulting torrefied product can be utilized as a biobased fuel that can be stored long-term without degradation. Alternatively, the torrefied product can be utilized as a decomposition-resistance biobased product. The torrefied product can also be utilized as a biobased product due to the merits of its own unique chemical and physical properties.

Secondly, torrefaction is an attractive conversion pathway because it requires the lowest conversion temperature compared to other thermochemical conversion pathways, making it a less energy intensive conversion process. Finally, torrefaction can be accomplished with less specialized processes compared to other thermochemical conversion pathways. This opens the door for utilizing less expensive equipment and materials, which aids in producing more economical conversion systems.

Read full chapter
URL: https://www.sciencedirect.com/science/article/pii/B9780128094839000014

Large-scale biomass combustion plants: an overview

S. Caillat, E. Vakkilainen, in Biomass Combustion Science, Technology and Engineering, 2013

Torrefied biomass

Torrefaction is the treatment of biomass using pyrolysis. The process occurs at a relatively low temperature of 225–300 °C. During torrefaction hemicelluloses and lignin present in wood are decomposed to some extent. Some dehydrogenation (chemical elimination of water) occurs. Torrefaction increases calorific value and if the result is pelletised then the density can be even higher than standard pellets. There is some cracking of the organic structures in the wood resulting in some of the heat energy in the wood being released as gases during torrefaction. The main advantage of torrefied wood (biocoal) is that it can be burned in PF boilers without major modifications to the equipment (van der Stelt et al., 2011). In FBs, the high alkali content may lead to bed sintering and deposits.

Read full chapter
URL: https://www.sciencedirect.com/science/article/pii/B9780857091314500091

Solid biofuel production, environmental impact, and technoeconomic analysis

Elsa Cherian, ... K.A. Anju, in Biofuels and Bioenergy, 2022

33.4.3 Torrefaction

Torrefaction is the procedure of depolymerization of hard biomass under specific temperature. The effectiveness of the process depends on the temperature and time of the reaction. The process is an exothermic reaction and is carried mostly at a temperature between 200°C and 300°C (Bergman et al., 2005). Torrefaction reaction mainly cause the decomposition of fibers in the biomass such as hemicellulose for the release of compounds like moisture, carbon monoxide, and carbon dioxide (van der Stelt et al., 2011). The torrefied product generally has a brown or black color, reduced volatile content, and elevated density of energy. The main carbon-enriched material formed as a result of torrefaction is the biochar. Biochar is produced even through pyrolysis reaction, but the calorific value is minimal when compared to torrefaction.

Read full chapter
URL: https://www.sciencedirect.com/science/article/pii/B9780323900409000102

Future Developments and Derived Fuels

Leonel Jorge Ribeiro Nunes, ... João Paulo Da Silva Catalão, in Torrefaction of Biomass for Energy Applications, 2018

13.4 Advantages of Torrefied Biomass

Torrefaction has a great effect on the physical and chemical properties of biomass, so this process brings some benefits, such as:

Agricultural waste does not have favorable heating properties, such as very high humidity and low energy density. Thus, the torrefaction process is a promising method for the pretreatment of residues since it removes moisture from the agricultural residues and increases the energy density of the residues [35].

The grindability is facilitated and, as a consequence, the energy consumption for milling is three to seven times lower than that of the raw material, which will not undergo the torrefaction process [36].

The porosity increases, so the torrefied biomass becomes more reactive during combustion and gasification [37].

The torrefaction process makes the transport and storage logistics of biomass more efficient, since the cost with transportation, storage, and transshipments are mainly based on the volume of the material. Torrefaction reduces the volume of the biomass transported as an economic propellant prior to transport [38].

Read full chapter
URL: https://www.sciencedirect.com/science/article/pii/B9780128094624000134