Biomass torrefaction: properties, applications, challenges, and economy

Renewable and Sustainable Energy Reviews 115 (2019) 109395

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Biomass torrefaction: properties, applications, challenges, and economy ∗

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Yanqing Niu , Yuan Lv, Yu Lei, Siqi Liu, Yang Liang, Denghui Wang, Shi'en Hui State Key Laboratory of Multiphase Flow in Power Engineering, Department of Thermal Engineering, School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an, 710049, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Biomass Torrefaction Combustion Gasification Ash Economy

Biomass accounts for the largest renewable energy in the world, whereas its inherent drawbacks, such as low energy and mass density, hydrophilicity, poor grindability and severe ash-related issues, inhibit its extensive utilization. Torrefaction, conducted at 200–300 °C in an inert atmosphere, successfully overcomes the abovementioned drawbacks and improves the biomass applications. Thus, a critical review is performed for the new insight into further study, involving the properties improvement of torrefied biomass, applications on combustion and gasification, as well as the intractable challenges of ash-related issues during thermal conversion and economy viability. Compared to torrefaction duration and the moisture and size of biomass, the torrefaction temperature has the strongest impact on the biomass properties improvement. Respecting physical properties (energy density and grindabilty) and chemical thermal conversion characteristics, there exists an optimal torrefaction temperature at approximate 250 °C. Biomass torrefaction is strongly dependent on the degradation of hemicellulose. Herbaceous residues possess a higher degradation ratio compared to ligneous biomass; Besides, deciduous trees mainly containing xylan in hemicellulose are more active than coniferous trees which mainly contain glucomannan in hemicellulose. The torrefied biomass possesses increased carbon content, decreased H/C and O/C ratios, increased mass energy density, similar chemical compositions with coal, and the availability for gasification and co-firing. Moreover, large amount of Cl, S, and K release during torrefaction, which bring considerable fringe benefits by reduction or elimination of the intractable ash-related issues during thermal conversion, such as slagging, agglomeration and corrosion. At present, the cost of biomass torrefaction is higher than coal. However, it can be significantly reduced by the implementation of carbon credits market, increasing torrefaction plant equipment size, and empirical cumulation. Later, more attention should be focused on application demonstration and systematic economic optimization.

1. Introduction To date, biomass energy accounts for the largest fraction among the developed (including biomass, wind, solar, geothermal, and tidal energies) and developing renewable energies (including ocean thermal gradient, wave and marine current energy) in the world [1]. It is also the most promising energy source in the short to medium term [2,3]. In the EU, 30% or more of the total transportation fuels are stipulated to be derived from biofuels in 2040 [4]; In the USA, most of renewable fuel is prescribed to be derived from cellulosic feedstock after 2016 [2]; And in China, the biomass power installed capacity will reach 3 × 104 MW in 2020, which accounts for 3% of the total installed capacity [5]. However, the inherent drawbacks of biomass inhibit its extensive utilization. Low energy and mass densities as well as large volume have

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negative effect on long-distance transportation [2,6–8]; High moisture content increases the costs of thermo-chemical conversion due to vaporization [9,10]; Hydrophilic nature doesn't conduce to long-term storage [6,8,11]; Fibrous nature increases energy consumption for grinding [12–14]; Poor spherical shape leads to low flowability and poor fluidization behavior [15,16]; Heterogeneous nature makes process design and control more complicated [17]. Moreover, the severe smoke generated during combustion [6,17,18], seasonal and decay [13] also need to be considered. All of these restrict the application of biomass directly as fuel in the current combustion and gasification system [8,19]. In order to improve biomass properties, some pretreatment technologies have been developed, including drying, briquette/pelletization, and torrefaction. Drying can reduce the moisture in biomass effectively, but the dried biomass will re-adsorb water to decompose

Corresponding author. E-mail address: [email protected] (Y. Niu).

https://doi.org/10.1016/j.rser.2019.109395 Received 26 November 2018; Received in revised form 11 August 2019; Accepted 12 September 2019 1364-0321/ © 2019 Elsevier Ltd. All rights reserved.

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ash-related issues are discussed in detailed. 4) Economy viability. As a key restriction factor for industrial application, the economic analysis of biomass torrefaction is performed.

[20]. As a mass and energy densification technology for the fuels with low bulk density [17], pelletization can not only reduce the transportation cost and facilitate handling and feeding of the biomass, but also contribute to storage and off-season utilization [20–22]. However, due to its hydrophilic nature, the pelletized fuels fail to be stored in the open for a long time without decay. Compared to drying and pelletization, low-temperature pyrolysis at 200–300 °C in an inert atmosphere, so-called torrefaction, has become the focus of biomass pretreatment in recent years. After torrefaction, the properties of biomass are significantly improved such as the mass and energy density, hydrophobicity, ignitability, combustion and gasification reactivity, and grindability [11]. The torrefied biomass can be blended with coal as pulverized fuel during combustion [23] or as feedstock into the entrained-flow gasifier [16]. In summary, the advantages of torrefied biomass contain:

2. Properties of torrefied biomass 2.1. General knowledge of biomass torrefaction Torrefaction, also named roasting or high-temperature drying, which can be defined as a thermal treatment of biomass in an inert environment at atmospheric pressure and temperatures of 200–300 °C, is a promising pretreatment method to convert biomass into energy densified solid fuel with improved grindability and increased heating value. During torrefaction, biomass partly (especially the hemicellulose) decomposes, giving off various type of volatiles, which can be classified into condensable volatiles (mainly containing water and acetic acid) and permanent gases (mainly containing carbon dioxide and carbon monoxide). The torrefied biomass (solid) has approximately 30% more energy density than the parent biomass [26]. With the increase of torrefaction temperature and residence time (torrefaction duration), the solid yield decreases, while the yields of volatiles increase [17,38]. Currently, torrefaction in lab-scale is mainly conducted by utilizing TGA [11,24,47,53], small scale tube reactor [12,38,42,44] and oven [6]. In TGA, the sample is heated at a constant heating rate from ambient temperature to target temperature (200–300 °C) in an inert atmosphere (nitrogen), and then kept at the object temperature for a certain residence time. Subsequently, the sample can be heated continually in nitrogen or oxygen for pyrolysis or combustion tests [11]. In the process of torrefaction, the TGA can be combined with a mass spectrum analyzer (MS) to measure the non-condensable volatile. Compared to the less solid product in TGA, torrefaction in lab-scale or bench-scale tube furnace and oven can produce sufficient torrefied biomass for subsequent tests of grindability and hydrophobic behavior as well as chemical analysis. Accompanied by the delicate lab-scale setups which are used to optimize the torrefaction process parameters for later commercial application, some demonstration-scale torrefaction reactors including rotary drum, screw conveyor, multiple hearth furnace, fluidized bed reactor, moving bed reactor, and microwave reactor have began to operate around the world [54,55]. However, the commercialization is few, and more efforts on the development, scaling up, and introduction to market of the mentioned various torrefaction technologies are needed. Besides common dry torrefaction conducted in an inert environment, there exists another torrefaction technology, named wet torrefaction, which may be defined as a treatment of biomass in hydrothermal media or hot compressed water at 180–260 °C [56]. In either dry or wet torrefaction, compared to residence time, fuel particle size and moisture content in parent biomass, temperature is the primary variable [2,22,30,43]. High temperature causes a significant reduction in both mass and energy yields as well as an increase in energy density [43]. After torrefaction, the solid residues have higher carbon content and lower oxygen content [19,32,57]. But wet torrefaction changes the carbon and oxygen more dramatically than dry torrefaction, and hence the biomass undergoing wet torrefaction yields higher energy density than that of dry torrefaction [43,58]. In wet torrefaction, the hemicellulose is almost completely removed at 200 °C, and cellulose and lignin partly react with hot water. Also the products of wet torrefaction are more hydrophobic than that of dry torrefaction under the same torrefaction conditions [43]. However, although the biomass decomposition in dry torrefaction is slower even at high temperature [59], dry torrefaction is still widely investigated. That may be attributed to the lower costs, easier operation and potential availability in practical application. Moreover, the biomass undergoing wet torrefaction shows the more rapid and concentrated combustion, while the biomass undergoing dry torrefaction tends to the similar combustion behavior with

1) Lower moisture [24]. Limited moisture content affects the water gas shift reaction and further increases the hydrogen content in syngas, which is beneficial to gasification [25]; 2) Higher energy density, mass density, and heating value [11,13,19,26–29]. These reduce the volume of biomass and thus extend the transport distance for use or further processing [26]; 3) Good hydrophobicity [24,29,30]. Attributing to the loss of hydroxyl groups during torrefaction [19,28], the torrefied biomass can be stored in the open for long period, with low risk of moist, decomposition and decay [11,26]; 4) Improved grindability [28]. The promotion on brittleness and the disappearance of fibrous structure result in reduced particle size of the torrefied biomass [13,19], which also reduce the energy consumption for grinding when co-firing with coal in current pulverized coal (PC) fired furnace [24,31,32]; 5) Reduced O/C ratio [24,29,33,34]. Resulting in high cold gasification efficiency [32,35] and less smoke in combustion [17,18,36]; 6) Reduced seasonal effects [13], reduced costs of storage [2], improved powder flowability [37], and so on. Since torrefaction was firstly performed in French in the 1930s and re-pioneered by Bourgeois and Dot in the 1980s (mentioned in Refs. [38,39]), biomass torrefaction has been investigated intensively. Whereas torrefaction is still far away from commercialization and mainly preformed in thermogravimetric analyzer (TGA) and lab- or bench-scale tube furnace as well as oven. Attentions are mainly focused on torrefaction temperature [2,12,40], torrefaction duration [2,12,40], grindability [13,41,42], hydrophobicity [25,43], combustion behaviors [6,8], and gasification characteristics [16,20,44], etc. Besides, some researchers pay attention on mass and energy balance [45], moisture [25], kinetics [46–49] and economic analysis [4], [50–52]. Based on current research progress and application challenges, we dedicate to a comprehensive and in-depth review on biomass torrefaction to provide new insight for further study. This critical review mainly includes four key aspects: 1) Properties of torrefied biomass. It mainly focuses on the torrefaction properties of three basic constituents of biomass (hemicellulose, cellulose, and lignin) and the effects of torrefaction temperature and residence time (duration) on the yields of solid and volatiles. Meanwhile, the different torrefaction properties of herbaceous biomass, deciduous and coniferous wood are compared, and the properties of torrefied solid product including morphology, mass yield, energy yield, energy density, chemical compounds, grindability, and hydrophobic behavior are discussed. 2) Applications. Mainly focus on the combustion behaviors and gasification characteristics. 3) Challenges of ash-related issues. The ash-related issues during biomass thermal conversion and the improvements of torrefaction on the primary troublesome elements (Cl, S and K) responsible for the 2

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energy density. Thus, the torrefaction provides a guarantee to promote the transportation and utilization of agricultural residues as a feedstock for combustion and gasification.

coal [58]. Here, we also concern on the dry torrefaction in the following review.

2.2. Torrefaction properties of hemicellulose, cellulose, and lignin

2.3. Effect of pretreatment parameters

Biomass is composed of three primary compounds, namely hemicellulose, cellulose and lignin [60]. Therefore, the torrefaction of biomass is heavily dependent on the degradation of the three constituents. Generally, the degradation of agricultural residues is higher than that of ligneous plants because of higher volatile and hemicellulose in herbaceous residues [20]. Meanwhile, deciduous trees are more active than coniferous trees because the hemicellulose in deciduous trees mainly contains xylan, which is more reactive than the glucomannan existed in coniferous woods [35]. Chen et al. [11] performed the torrefaction testing of hemicellulose, cellulose and lignin using TGA at 230, 260 and 290 °C. Results showed that hemicellulose was significantly affected by torrefaction procedure even at the temperature as low as 230 °C (mass loss was 2.74%, 37.98% and 58.33% corresponding to 230, 260 and 290 °C, respectively), cellulose was influenced markedly only at torrefaction temperature as high as 290 °C (mass loss were 1.05%, 4.43% and 44.82% corresponding to 230, 260 and 290 °C, respectively), while lignin was hardly affected in the temperature range (mass loss were only 1.45%, 3.12% and 6.97% corresponding to 230, 260 and 290 °C, respectively), as shown in Fig. 1. That is basically consistent with the decomposition temperatures of hemicellulose at 200–250 °C, cellulose at 240–350 °C, and lignin at 280–500 °C, respectively [61]. Therefore, the weight loss of torrefied biomass is mainly dominated by the decomposition of hemicellulose and a small part of cellulose and lignin (mainly short chain lignin compounds [31]) [8,13]. Meanwhile, a study on the nonsynergistic effect of the three compounds by co-torrefaction showed that the weight loss of biomass during torrefaction could be predicted by linear superposition of the weight losses of individual constituents [11]. The decomposition of hemicellulose is also affected by its existing forms. Xylan is the predominant existing form of hemicellulose in the deciduous woods and herbaceous crop residues, while glucomannan is the primary existing form in coniferous woods [39]. Due to different chemical structures, xylan is more reactive and easier to break down quickly at lower temperature compared to glucomannan [62]. Consequently, the weight loss of deciduous woods and herbaceous biomass with higher xylan content are easier to happen than coniferous woods during torrefaction. In current, woody biomass has been widely used for power generation and gasification. Whereas, the utilization of agricultural residues is not ideal maybe attributing to its low mass and

All the products in the torrefaction process can be classified into three categories: solid char, liquid condensable volatiles and permanent gaseous volatiles. With increased temperature and residence time, solid yield decreases, while permanent gas and condensable volatiles increase [2,16,32]. Mass loss is more pronounced at high temperature due to the higher reactivity and more intensive devolatilization and decarbonization of hemicellulose, as well as the decomposition of cellulose and lignin [2,11]. The main condensable volatiles are water, acetic acid, less acetic anhydride and furfural etc. The main non-condensable gases are carbon dioxide and carbon monoxide [12,17]. During the torrefaction of woody biomass, over 50 wt% of consumed wood is transformed into liquid, regardless of the torrefaction temperature and duration [22]. Similar results were also reported for the torrefaction of corn stover [2]. Temperature (rather than particle size, residence time and moisture content) is a significant variable in torrefaction. High temperature causes a considerable reduction in mass and energy yields as well as an increase in energy density [17,22,43]. Pimchuai et al. [6] studied five agricultural residues in a bench-scale torrefied unit (at 250, 270 and 300 °C, with residence time of 1, 1.5 and 2 h) and found that temperature had a more profound effect on mass and energy yields as well as energy density compared to residence time. Furthermore, Bridgeman et al. [42] compared the effect of temperature, residence time and particle size on mass yield and grindability by a multifactor method. The results showed that temperature was the most important parameter followed by residence time, and biomass particle size had the least impact. Similar results were also reported by Nimlos et al. [63], Sadaka et al. [64], Huang et al. [65] and Chen et al. [22]. Medic et al. [2] compared the effects of temperature and initial moisture content on mass loss and energy yield, and revealed that temperature had a stronger impact on energy and mass loss than moisture content, and the quantitative relationship were showed as Eq. (1) and Eq. (2).

Mass loss = 95.68 − 1.0396*T + 0.2491*Moisture + 0.00284*T 2

(1)

Energy yield = 10.0379 − 0.0144 ∗ Moisture + 0.9278 ∗ T − 0.00721 ∗ Moisture 2 − 0.002461 ∗ T 2

(2)

Wang et al. [32] proposed that the optimum torrefaction temperature was between 230 and 250 °C with the residence time of 30 min for cotton stalk and wheat straw respecting of the energy yield, grindability and other factors. Fonseca et al. [66] studied the torrefaction of wood briquettes at 220–270 °C and reported that the most suitable temperature was 250–270 °C according to the energy yield. Chen [22] found that the grindability of the wood biomass can be improved obviously by torrefaction at 250 °C for more than 1 h. It seems that herbaceous biomass has lower optimum torrefaction temperature and shorter residence time than woody biomass taking into account the mass yield, energy density, and grindability. This may be due to the higher hemicellulose content in herbaceous biomass than that in woody biomass [20] and the excessive consumption of cellulose and lignin at high temperature [53]. Anyway, temperature is the domination parameter during torrefaction, and an optimum torrefaction temperature can be identified at approximate 250 °C after taking mass yield, energy density, and grindability fully into account. 2.4. Yield of solid product Numbers of researches indicate that increased temperature and residence time lead to a reduction on solid yield [6,17,19,22,38,43]. Fig. 2 shows the effect of torrefaction temperature and residence time

Fig. 1. Torrefaction of hemicellulose, cellulose, and lignin at 230, 260 and 290 °C using TGA, residence time 1 h [11]. 3

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torrefied fuels is higher than that of parent biomass [20]. High porosity (i.e., low mass density with constant particle size) facilitates the diffusion of reactants and products in the carbon matrix, thus improving the combustion and gasification characteristics [67,68]. 2.4.2. Mass and energy density With increased temperatures and residence time, the solid yield decreases, while the volatile yields increase, resulting in a solid with increased carbon content, decreased oxygen content and volatile, as well as an increase in energy density [2,6,16,19,32,43]. Phanphanich and Mani [19] reported that the mass yield, energy yield and energy density of woody biomass (pine chips and logging residues, torrefaction temperature 225/250/275/300 °C, residence time 0.5 h) fell in the ranges of 52–89%, 71–94% and 1.05–1.39, respectively. Pimchuai et al. [6] found that the mass yield, energy yield and energy density of agricultural residues (rice husk, peanut husk, bagasse and water hyacinth, torrefaction temperature 250/270/300 °C, residence time 1/1.5/2 h) were in the ranges of 41–79%, 55–98% and 1.08–1.66, respectively. The agricultural residues appear to have higher mass loss and energy density due to its relatively higher volatile content and hemicellulose [20]. However, regardless of mass yield, energy yield or energy density, they are all dependent on the raw biomass, torrefaction temperature, residence time and reactor type etc. Based on the wide definition of mass yield, energy yield and energy density (Eq. (3) －(5)) [2,6,32,42,43], the relationships of HHV, energy yield, mass energy density and mass yield have been plotted in Fig. 4.

Fig. 2. Solid yields in torrefaction at different conditions; 250-30 means torrefied at 250 °C for 30 min, the others are similar [38].

on solid yields of four biomass reported by Prins et al. [38], where beech and willow are deciduous wood, larch and straw belongs to coniferous wood and herbaceous biomass, respectively. It can be seen that with an increase of temperature the solid yield decreased for both woody biomass and herbaceous biomass, and residence time showed a similar effect. This may be explained by the truth that the weight loss of torrefied biomass is dominated by the decomposition of hemicellulose, which has higher weight loss at higher temperature and longer residence time [13,19]. Meanwhile, from a comprehensively comparison of the effect of temperature and residence time on the solid yields for three woody biomass, the torrefaction temperature had a dominant influence, and the efficiency of residence time decreasing half was still absolutely inferior to that of the temperature increasing of 20 °C. Meanwhile, Fig. 2 shows the different effects of torrefaction temperature and duration on deciduous wood, coniferous wood and herbaceous biomass. Coniferous has the smallest weight loss, and deciduous wood and herbaceous biomass have a similar weight loss. In deciduous wood and herbaceous biomass, xylan is the predominant existing form of hemicellulose, while in coniferous wood, glucomannan is the dominant form [39]. Xylan has higher active and tends to break down more quickly than glucomannan in the torrefaction temperature range [35]. Therefore, the weigh loss of deciduous wood and herbaceous biomass are higher than that of coniferous wood during torrefaction. After torrefaction, the torrefied biomass possesses improved flowability enabling as fuel in entrained-flow gasifier, high energy density and heat value comparable with coal, comparable H/C and O/C ratios with peat, well grindability favoring co-mill with coal, and well hydrophobic behavior enabling long-term storage in the open. Therefore, the properties of the solid product reviewed here mainly consist of morphology, mass yield and energy density, H/C ratio and O/C ratio, grindability and hydrophobicity.

Mass yield =

mtorrefied × 100 mraw

Energy yield = Mass yiels ×

Energy density =

HHVtorrefied HHVraw

Energy yield Mass yield

(3)

(4)

(5)

where m denotes biomass mass, HHV denotes high heat value (MJ/kg), the subscript torrefied means torrefied biomass or roasted biomass, and the subscript raw means raw biomass or parent biomass. Seen from Fig. 4, with an increase in mass loss (decreased solid yield), the HHV and mass energy density increase, while the energy yield decreases. In the process of torrefaction, with increased torrefaction temperature and residence time, the yields of volatiles taking away energy stored in the raw biomass increase, resulting in reduced energy yield in the solid residues. Increased carbon content and decreased hydrogen and oxygen content result in increased HHV because the energy contained in C–C bond is higher than that in C–H or C–O bonds [19]. On the other hand, the decrease of energy is inferior to the decrease of mass, resulting in increased energy density and HHV. Comparing Fig. 4a and b, it can be seen that the energy yield of herbaceous biomass is lower than that of woody biomass with lower hemicellulose content [16]. In the range of 80–60% of solid yield the torrfied herbaceous biomass has a high energy density, and woody biomass also undergoes a relatively high and stable energy density zone. Therefore, the optimum torrefaction condition of biomass may be to maintain the solid yield in the range of 80–60%, so as to obtain relatively high HHV, energy yield and mass energy density. That indicates that in the future research, more attention can be paid to the relationship of solid yield and energy density to select the optimum torrefaction condition, instead of solely dependence on the torrefaction temperature and/or duration.

2.4.1. Morphology It has been widely reported that the color of torrefied products is more brown than the parent biomass [6,16,19,22,38,42,57]. Fig. 3a shows a group of pictures of wood chips torrefied at different temperatures. Due to continuously intensive carbonization, the torrefied chips become darker with increasing temperature. Meanwhile, with the increasing torrefation temperature, the biomass experiences different structure changes as integrated surface, cell structure and tubular-shape structure (as shown in Fig. 3b) [22]. These clearly indicate that the biomass becomes brittle after torrefaction due to the breakdown of filaments in biomass. In addition, due to the release of gaseous and volatile products, the total pore volume of

2.4.3. Van Krevelen diagram (H/C vs. O/C) As the torrefaction temperature increases, the elemental carbon content of torrefied biomass increases, while hydrogen and oxygen contents decrease due to the release of volatile being rich in hydrogen and oxygen, such as water and CO2 [2], which results in decreased H/C 4

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Fig. 3. The physical and SEM images of biomass at different torrefaction temperatures.

From Fig. 7, it can also be seen that the temperature had the predominated effect on torrefaction compared to residence time. Similar results were given in the other references [19,22], and the specified energy consumption of torrefied biomass had a linearly relationship with the torrefaction temperature [19]. Energy consumption of grinding depends on initial particle size, moisture content, material properties, feed rate of material and machine variable etc. [41]. Multifactor calculation employed by Bridgeman et al. [42] showed that torrefaction temperature had the largest effect on grindability followed by residence time, while particle size had the least effect. Otherwise, with an increase in moisture, the shear strength of the material increased, thus more energy was needed for grinding [41]. However, Wang et al. [32] studied the grindability of cotton stalk and wheat straw and found that if the torrefaction temperature was higher than 250 °C, it almost had no impact on the grindability. Meanwhile, the small sized particles of torrefied race straw and rape straw were the most intensive at the torrefaction temperature of 250 °C [16]. Again, it indicates that 250 °C is an optimal temperature for biomass torrefaction.

and O/C ratios [19,20,32,33,57] and makes the fuel properties shift towards coal [42]. Fig. 5 shows the Van Krevelen diagram for coal, torrefied and untorrefied biomass. Torrefaction changes the chemical compositions of biomass significantly and shifts the fuel properties away from biomass towards coal. Under some conditions (high temperature and long residence time), torrefied biomass, especially torrefied woody biomass, has the similar chemical compositions with peat, lignite and even subbituminous, and can be used for gasification and co-firing [10,17,19,29]. High temperature and long residence time make the chemical compositions of torrefied biomass more accessible to coal (Fig. 5b). 2.4.4. Grindability After torrefaction, the fibers between biomass particles are broken, and the particles become shorter and spherical, which improve the flowability of the biomass [13,32,39]. Torrefied biomass is therefore brittle, which improves the grindability with lower energy consumption and results in a smaller particle size distribution [15,16,19,32]. Generally, the energy requirement for grinding torrefied biomass is 10–20% of that for parent biomass, and can be comparable with that of coal [19,31,39]. Torrefaction improves the flowability and grindability of biomass remarkably. Currently, the studies on grindability of torrrefied biomass are mainly conducted in lab by testing the energy consumption and particle size distribution [15,19]. Repellin et al. [15] studied the energy consumption for the grinding of torrefied spruce and beech by using ultra centrifugal mill (Retsch ZM1) equipped with 0.5 mm grid. They reported that the energy consumption of grinding plunged after torrefaction (Fig. 6), and the particle size distribution skew towards smaller size due to the intense brittleness of torrefied biomass being similar to coal (Fig. 7). Meanwhile, grinding energy and particle size decreased significantly with increasing torrefaction duration upto 20 min, whereas a further increase of the torrefaction duration (such as 40 and 60 min) showed slight effects (Figs. 6 and 7b). Manouchehrinejad et al. [69], who studied the grindability of both torrefied wood pellets and chips using a knife mill, reproted the similar results that the specific grinding energy lineratly decreased with increased torrefaction temperature, and the Hardgrove Grindability Index (HGI) increased with an increased in torrefaction temperature.

2.4.5. Hydrophobicity The hydrophobicity of biomass is closely related to the contents of hemicellulose, cellulose and lignin in the fuels. In general, hemicellulose has the greatest capacity for water sorption, while lignin has little [70]. However, the degradation of hemicellulose as well as the decrease of H/C and O/C ratios in torrefied biomass reduce the capacity of hydroxyl groups to bond water through hydrogen bonds [19,28]. The enhanced hydrophobicity favors the long term storage of torrefied biomass in the open without worrying about the hydrophilic behavior and biological deterioration or decay occurred to raw biomass [43]. Biomass with higher torrefaction temperature absorbs less moisture, i.e., possessing higher hydrophobicity [6]. The hydrophobicity mechanisms are summarized by Ciolkosz and Wallace as following [39]: 1) The breakdown of hemicellulose unbinds the cellulose and lignin, which leads to the release of water molecules stored at the cell level; 2) The deconstruction of hemicellulose leads to a greater brittleness for cellulose and lignin; 3) The removal of OH groups reduces the formation of hydrogen bonds with water; 4) The breakdown of hemicellulose tends to form more non-polar molecules. Yan and Acharjee et al. [25,43] proposed a method named 5

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Fig. 5. Van Krevelen diagrams (In Fig. 5a, data of torrefied herbaceous biomass are from Refs. [2,42], data of torrefied woody biomass are from Refs. [12,13,19,20,22,42,44], data of untorrefied biomass are from China Guodian Cooperation, Fig. 5b is from Ref. [31]).

Fig. 4. Relationships of HHV, energy yield, mass energy density and mass yield for both herbaceous and woody biomass.

Equilibrium Moisture Content (EMC) which can be used as an indicator of hydrophobicity. The EMC is measured on the basis of static desiccator technology [71], where the solid sample is exposed to an environment with constant humidity and temperature over a long period until the moisture in the solid sample reaches an equilibrium value. A solid with lower EMC can be stored over long-time with low risk of biological deterioration, i.e., possessing high hydrophobicity. A detailed comparison of the effect of torrefaction temperature on the hydrophobicity under different humidity has been illustrated in Fig. 8 [43]. It can be seen from Fig. 8 that at low humidity the hydrophobicity (EMC value) was not affected by torrefaction temperature and kept unchanged. Whereas at high humidity the hydrophobicity became inferior with a high EMC value, and the hydrophobicity enhanced with increased torrefaction temperature and then kept constant. This can be explained that the adsorbed water consists of bound water and free water (non-bound water). At low humidity, the total amount of moisture in the fuels was almost equivalent to the amount of bound moisture, which decreased with an increase in torrefaction temperature and fully saturated above 20% humidity. However, the free water was little related to torrefaction temperature, which was very little at low

Fig. 6. Effect of torrefaction on the energy consumption for grinding, Retsch ZM1 ultra centrifugal mill equipped with a 500 μm grid, initial particle size 2–4 mm [15].

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Fig. 7. Particle size distribution of fine powder of beech wood, Retsch ZM1 ultra centrifugal mill equipped with a 500 μm grid, initial particle size 2–4 mm [15].

humidity and increased with an increase in the humidity [25]. Therefore, torrefaction enhances the hydrophobicity and favors the long period storage of torrefied biomass in the open with low risk of moist, decomposition and decay. While high ambient humidity can weaken the hydrophobicity, which should be avoided. 2.5. Yield of condensable and non-condensable volatiles Water and acetic acid are dominant condensable volatiles during torrefaction, followed by furfural, methane and hydroxyacetone in much less quantities [2,16,17,32,35,38,45]. Water is released in the form of evaporation followed by dehydration and depolymerization [16,32]. In the process of polymer dehydration, the hydroxyl groups are released to form water [2]. Acetic acid and methanol are derived from acetoxy and methoxy groups attached to hemicellulose sugar monomers and lignin [2,38]. Other compounds are related to the thermal decomposition of plant polymer monomers [2]. Fig. 9 shows the condensable volatiles analyzed with HPLC (High Performance Liquid Chromatography) by Prins et al. [38], and the results are similar with the reports by Deng et al. [16] and Medic et al. [2]. In the torrefaction of willow (deciduous wood) and straw (herbaceous biomass), water is the dominant products followed by a significant amount of acetic acid, smaller quantities of methanol, formic acid, lactic acid, furfural, hydroxyl acetone and traces of phenol. While

Fig. 8. Effect of relative humidity on the EMC of torrefied loblolly pine, ambient temperature at 30 °C [43].

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Fig. 10. Yields of non-condensable volatiles in torrefaction at different conditions; 250-30 means torrefaction at 250 °C for 30min, and the others are similar [38].

By comprehensively considering the yields of volatiles illustrated in Figs. 9 and 10, the yields of condensable volatiles accounted for more than a half of the decomposed biomass regardless of the torrefaction temperature and residence time, which is consistent with the results reported by Chen et al. [22]. Therefore, concerning the mass and energy balance as well as the economic performance of torrefaction, condensable volatiles should be emphasized in torrefaction process. 3. Applications on combustion and gasification 3.1. Combustion In the current co-firing scheme, a separated biomass feed system is needed because the biomass cannot be ground small enough by coal mill. While torrefied biomass has excellent grindability and combustibility, and can be co-milled and co-fired with coal [26,42]. Compared to parent biomass, the torrefied biomass possesses higher particle surface and smaller particle size, which are the desired properties of biomass for efficient combustion and co-firing application [13,19]. In the 1980s, Pentananunt et al. [18] did the smoke experiment in a bucket stove and found that the torrefied wood experienced shorter smoke and flame period and longer incandescent period compared to the parent wood. In the 2010s, Pimchuai et al. [6] conducted the combustion experiments of both raw and torrefied rice husks in a labscale spout-fluid bed combustor and reported that the torrefied rice husks raised the furnace temperature, which should be granted because of higher heat value, higher fixed carbon and lower moisture in the torrefied biomass. On basis of the numerous research conducted in TGA and drop tube furnace, it has been concluded that attributed to the decreased volatile content [73], increased fixed carbon content and density [73,74], and hemicellulose molecular alteration [75] during torrefaction, the torrefied biomass show higher activation energy [73], higher devolatilization, ignition, peak, and burnout temperatures [75], lower burnout ratio [73], lower reactivity [75–77], and longer burnout time [36,74,78] compared to raw biomass. However, Pohlmann et al. [79] reported that low torrefaction temperature yielded high reactivity; Hu et al. [33] found that the torrefied biomass had lower peak combustion temperature and burnout temperature, and higher activation energy; Meanwhile, the arguments on the easy ignition and improved combustibility are existed [30,36,77]. Recently, Magalhaes et al. [78] compared the combustion behaviors of three biomass (raw and torrefied) and two lignite coals in a drop tube furnace using pyrometry techniques, and found that biomass particles

Fig. 9. Yields of condensable volatiles in the torrefaction process at different conditions for willow, larch and straw; Vertical axis is mass percentage of parent biomass on dry and ash free basis (wt, daf %) [38].

in the torrefaction of larch (coniferous wood), formic acid substitutes for acetic acid as the second largest product, attributing to the different hemicellulose compositions [39]. In deciduous wood, the acetoxy- and methoxy-groups attached to the poly sugar form acetic acid and methanol when heated, while the coniferous wood is different [38]. Carbon monoxide and carbon dioxide are main permanent gas products (non-condensable volatiles), and the yield increases with both torrefaction temperature and residence time [12,16,17,38]. As trace compositions, methane and hydrogen present at high temperature [2,16,32,38]. Torrefaction temperature and volatile content in raw biomass show positive effects on the gas concentration and cumulative amount of the gas products [16,38]. The compositions of permanent gas change little with moisture content in raw feedstock [2]. Meanwhile, carbon dioxide as the byproduct of decarboxylation of acid groups is hardly affected by particle size when it is less than 5 mm [72]. Fig. 10 shows the non-condensable volatiles analyzed by Prins et al. [38]. With an increase in temperature both carbon dioxide and carbon monoxide increased, but the yield of carbon dioxide was markedly higher than that of carbon monoxide. Besides, coniferous wood (Larch) had lower yields due to inferior reactive of glucomannan existed in hemicellulose than that of xylan existed in the hemicellulose of deciduous wood (Willow) and herbaceous biomass (Straw). 8

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various properties of torrefied biomass, which may be used as co-firing fuel in an existing PC-fired boiler (such as the TL-fired furnace), while cannot burn out completely in another PC-fired boiler (such as the SLfired furnace). Besides these, other researches focusing on the combustion behaviors and application of torrefied biomass are scarce, possibly, due to the challenges of ash-related issues during thermal conversion [80–82] and the economy [52,83]. Therefore, the direct combustion or co-firing of torrefied biomass has great research potential, which needs more application demonstrations.

3.2. Gasification Compared to parent biomass, torrefied biomass has lower moisture content and smaller particle size. Therefore, the dried biomass with dimension of several hundreds of micrometer can be rapidly gasified in several seconds in an entrained-flow reactor at the high temperature of 1400 °C [44]. Compared to two-stage pyrolysis-gasification, torrefaction-gasification saves more energy because of the lower heat required for torrefaction than that for pyrolysis at higher temperature [35,44]. Couhert et al. [44] accomplished the gasification experiment of torrefied beech wood with dimension of 80–200 μm in a 18 kW entrained-flow reactor at 1200/1400 °C with 20 vol % of stream in nitrogen, and results showed that the torrefied wood produced the same amount of CO2, but 7% more H2 and 20% more CO in comparisons with the parent wood. Moreover, Chen et al. [20] employed cold gasification efficiency and gasification efficiency to compare the effect of torrefaction on gasification (Fig. 12). The results showed that the gasification efficiency decreased with increased torrefaction temperatures due to the energy loss caused by the release of volatiles. While torrefaction improved the syngas quality and cold gasification efficiency which reached the highest at torrefaction temperature of 250 °C. The torrefied sawdust at 250 °C also had the largest specific surface area and smallest pore size. Therefore, 250 °C was recommended as the optimum torrefaction temperature in the authors’ opinion. More specifically, Prins et al. [35] studied the possibility of more efficient biomass gasification via different systems including air-blown circulating fluidized bed (CFB) gasification of wood, torrefaction and CFB gasification of torrefied wood, and torrefaction integrated with entrained flow gasification of torrefied wood. Fig. 13a shows the conventional gasification of biomass in CFB below 1000 °C, Fig. 13b shows the torrefied biomass in CFB gasifier without the utilization of volatile

Fig. 11. Flame and char temperatures and burnout time from pyrometry measurements for Olive residue (OR), Olive residue -Torrefied (OR-T), Almond shell (AS), Almond shell-torrefied (AS-T), Hazelnut shell (HS), Hazelnut shelltorrefied (HS-T), Tuncbilek lignite (TL), and Soma lignite (SL) [78].

and Tuncbilek lignite ignited homogeneously forming volatiles envelope flame and a distinguishable char combustion, while Soma lignite exhibited simultaneous volatile and char combustion due to fragmentation. As shown in Fig. 11a, the torrefied biomass showed higher flame temperature compared to raw biomass and Tuncbilek lignite due to the different pyrolyzates. However, the char burning temperatures of raw and torrefied biomass were comparable, and both of which were higher than that of Tuncbilek lignite and lower than that of Soma lignite due to the different physical structures (such as specific surface area) and ash compositions (catalytic effect of Ca and K). Meanwhile, Fig. 11b shows the flame and char burnout time. It can be seen that both the volatiles burning time and char burning time of torrefied biomass was longer than that of raw biomass, especially the latter, resulting in longer burnout time of torrefied biomass particles in comparison with raw biomass. The volatiles burning time of biomass was comparable to that of Tuncbilek lignite, but because of the various fixed carbon content in the fuels the char burning time followed the order: Soma lignite < biomass < torrefied biomass < Tuncbilek lignite. Overall burnout time of raw and torrefied biomass was shorter than that of Tunçbilek lignite, but much longer than that of Soma lignite. Lu et al. [74] pointed out that the char burnout time depended linearly on the char particle mass, and the increasing torrefaction severity led to higher char yield. These results suggests that due to the

Fig. 12. Gasification efficiency and cold gasification efficiency [20]. 9

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Fig. 13. Gasification process schemes: (a) CFB gasification of wood (air-blown); (b) wood torrefaction and CFB gasification of torrefied wood (air-blown) and (c) wood torrefaction integrated with pulverized entrained-flow gasification of torrefied wood (oxygen-blown) [35].

troublesome ash-related issues (especially the slagging) [80,81] and economy viability [52,83]. As shown in Fig. 14, severe ash-related issues including alkali-induced slagging, silicate melt-induced slagging, and corrosion occur in biomass-fired furnaces frequently [84].

products in torrefaction process, and Fig. 13c shows the combined biomass torrefaction and entrained-flow gasification, where the volatile products from torrefaction are used as fuel in the gasifier, which avoids mass and energy losses from torrefaction reactor and all original fuels are used in the gasifier for syngas. Simulation results showed that the overall gasification efficiency of torrefied biomass in CFB gasifier was lower than that of untorrefied biomass due to the considerable loss of energy carried by discharged volatiles. The higher the torrefaction temperature was, the lower the overall gasification efficiency of torrefied biomass was. However, the overall gasification efficiency of integrated entrained-flow gasification of torrefied biomass was comparable with that of the CFB gasification of untorrefied biomass at high torrefaction temperature. Thermal process efficiency can be increased by using torrefaction gases and liquids as an energy source for process heat [17]. The gaseous and liquid products of torrefaction, containing 15–20% of the original energy of the feedstock (parent biomass), can be used to provide process heat to control reaction temperature or drying heat. All the aforementioned results are from lab-scale experience or simulation, and the differences between them and real industrial applications are inevitable. Although these provide considerable information for gasification of torrefied biomass presently, more practical verifications and demonstrations are required.

4.1. Ash-related issues during biomass combustion During biomass combustion, alkali-induced slagging [85–88] and silicate melt-induced slagging [86,89] happen frequently due to the high concentrations of chlorine (Cl) and alkali metals (K and Na) in the fuels. The unmanageable deposits reduce both heat transfer and boiler efficiency [90] and result in unscheduled shutdown of the entire power plant in serious cases [91–93]. Furthermore, the deposits containing high content of Cl lead to tube corrosion [94]. The alkali-induced slagging is mainly caused from the triple effects of alkali metals [84]: 1) the direct condensation and adhesion of alkali metal aerosols on the heating surfaces, such as KOH, KCl, K2SO4, and NaCl; 2) the condensation of alkali metal aerosols on fly ash particles and then react with SiO2 in the fly ash particles to generate low-melting substances; 3) the formation of low-temperature eutectic substances by coexisting with other substances, such as Na2SO4 + NiSO4 (melting point at 671–886 °C [91]) and KCl + K2SO4 (melting point at 550 °C [95]). Once condensed on the heating surface, the sticky layers will be formed to capture coarse ash particles, promoting the growth of slagging [87]. In addition, attributing to the high contents of Cl and K in biomass, the corrosions associated with Cl2, alkali chlorides (gaseous, solid/deposited, and molten), molten alkali sulfates and carbonates, and the sulfation/silication of alkali chlorides happen inevitably during combustion [84]. KCl, NaCl, Na2SO4 and K2SO4 are the dominant alkali-containing substances that cause biomass ash-related issues [84,86]. During biomass combustion, the alkali metals are first released as solid particles (mainly as alkali silicates and alkali aluminosilicates) and vapor species (mainly in the forms of M(g), MCl(g), (MCl)2(g), M2SO4(g) and MOH

4. Challenges of ash-related issues The pronouncedly improved properties of biomass after torrefaction have been well-known, such as the high heat value comparable with coal, comparable H/C and O/C ratios with peat, well grindability favoring co-mill with coal, high energy density and hydrophobicity, as well as the high-energy-generation potential and carbon dioxide neutrality. All of these promote the rapid development of biomass-fired power plants. However, the combustion of torrefied biomass remains challenging for several reasons. These mainly include problems in 10

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Fig. 14. Images of various ash-related issues in biomass-fired furnaces [84]. Upper left: alkali-induced slagging in a 12 MW grate furnace; upper middle: alkaliinduced slagging in a commercial fluidized bed; upper right: agglomeration in a lab-scale bubbling fluidized bed; bottom left: silicate melt-induced slagging (ash fusion) in a 12 MW grate furnace; bottom middle: silicate melt-induced slagging (ash fusion) in a lab-scale furnace; bottom right: corrosion in a commercial bubbling fluidized bed.

(g), M represents Na and K) [96]; Then, several possible gas-phase chemical reactions occur, including sulfation (R1-R3), chlorination (R4 and R5), and carbonation (R6) [97,98]. Consequently, alkali sulfates, chlorides, carbonates, silicates, and aluminosilicates coexist and influence the ash-related issues. Also, there are some additional reactions, such as the condensation, silication and alumina-silication shown as R7R14.

2MCl(g) + (2SiO2 + Al2 O3)(s,l; Oxides or compounds)+ H2

2MOH(g)+ SO2 (g)+0.5O2 (g) ↔ M2 SO4 (g)+ H2 O(g)

R1

......

2MCl(g)+ SO2 (g)+0.5O2 (g)+ H2 O(g) ↔ M2 SO4 (g) + 2HCl(g)

R2

M2 SO4 (g) + (2SiO2 + Al2O3 )( s,l; Oxides or compounds)

M2 CO3 (g)+ SO2 (g)+ 0.5O2 (g) ↔ M2 SO4 (g)+ CO2 (g)

R3

O(g) → 2MAlSiO4 (s,l) + 2HCl(g) 2MCl(g) + (4SiO2 + Al2 O3)(s,l; Oxides or compounds)+ H2 O(g) → 2MAlSi2 O6 (s,l) + 2HCl(g) 2MCl(g) + (6SiO2 + Al2 O3 )( s,l; Oxides or compounds)+ H2 O(g) → 2MAlSi3O8 (s,l) + 2HCl(g)

R12

→ 2MAlSiO4 (s,l)+ SO3 (g) M2 SO4 (g) + (4SiO2 + Al2O3 )( s,l; Oxides or compounds)

Chlorination reactions:

→ 2MAlSi2 O6 (s,l)+ SO3 (g)

MOH(g) + HCl(g) ↔ MCl(g)+ H2 O(g)

R4

M2 SO4 (g) + (6SiO2 + Al2O3 )( s,l; Oxides or compounds)

M2 CO3 (g) + 2HCl(g) ↔ 2MCl(g)+ CO2 (g)+ H2 O(g)

R5

......

→ 2MAlSi3O8 (s,l)+ SO3 (g)

R13

Carbonation reactions:

2MOH(g)+ CO2 (g) ↔ M2 CO3 (g)+ H2 O(g)

2MOH(g) + (2SiO2 + Al2 O3 )( s,l; Oxides or compounds)

R6

→ 2MAlSiO4 (s,l)++H2 O(g)

Condensation of alkali sulfates

2MOH(g) + (4SiO2 + Al2 O3 )( s,l; Oxides or compounds)

M2 SO4 (g) → M2 SO4 (s,l)

→ 2MAlSi2 O6 (s,l)++H2 O(g)

R7

2MOH(g) + (6SiO2 + Al2 O3 )( s,l; Oxides or compounds)

Condensation of alkali chlorides:

MCl(g) → MCl(s,l)

......

R8

R9

M2 SO4 (g) + nSiO2 (s,l) → M2 O⋅nSiO2 (s,l) + SO3 (g)

R10

2MOH(g) + nSiO2 (s,l) → K2O⋅nSiO2 (s,l) + H2 O(g)

R11

R14

Thus, the nature and origin of ash-related issures during biomass combustion are mainly associated with the Cl, S and K in biomass as well as the formation of KCl, NaCl, Na2SO4 and K2SO4 in high-temperature combustion. To solve the ash-related issues during biomass combustion, a number of studies concerning additives [99–101], cofiring [102,103], chemical pretreatment [104–107] and alloying [108,109], all of which can change the generation and transformation of alkali chlorides and sulfates, have been conducted. A good review of the formation and contermeasures of ash-related issues during biomass combustion has been documented by Niu et al. [84].

Silication reactions of alkali chlorides, sulfates, and hydroxides:

2MCl(g) + nSiO2 (s,l) + H2O(g) → K2O⋅nSiO2 (s,l) + 2HCl(g)

→ 2MAlSi3O8 (s,l)++H2 O(g)

When n is 1, 2, and 4, the melting points of K2O·nSiO2 are lower than 1073 K [72,73]. Alumina-silication reactions of alkali chlorides, sulfates, and hydroxides: 11

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4.2. Effect of torrefaction on intractable Cl, S, and K elements in biomass Recently, Shoulaifar et al. [110,111] investigated the torrefaction characteristics of varied of biomass, and found that in the range of torrefaction temperature, there existed significantly releases of Cl, S, and K and the change of water-soluble K content. Similar phenomena were observed by Chen et al. [112]. These finds give new lights on biomass combustion to solve the serious ash-related issues. Besides the inherent properties of torrefied biomass such as high energy density, well grindability and reduced biological decay, etc, the reduced contents of Cl, S and K as well as K-conversion by torrefaction bring significant additional benefits for the combustion of torrefied biomass, particular in alleviating the ash-related issues including slagging, corrosion and agglomeration. 4.2.1. Chlorine — Cl As a common consensus, chlorine plays a role of shuttle and facilitates the transfer of potassium from fuel to gas phase [98,111,113], and subsequently the formation of potassium salts induced slagging [84,86] and corrosion [84,114]. Numbers of studies indicate that the releasing ratio of Cl during biomass torrefaction increases with an increase in torrefaction temperature and residence time, and a decrease in particle size, sample weight, and initial Cl content in the raw biomass [110–112,115]. Thus, the reduced Cl content in torrefied biomass facilitates the alleviation of biomass ash-related issues during combustion. As shown in Fig. 15a [112], with increasing torrefaction temperature from 200 °C to 350 °C, the release of Cl showed remarkable increase for all four residence times, especially in the temperature range of 250–300 °C. Meanwhile, it can be seen that in comparisons with nonisothermal heating stage where the sample was put into the heating furnace at the beginning and the torrefaction stopped as soon as reaching the desired temperature (labeled as NIHT in Fig. 15), the release ratio of Cl increased significantly with increased residence time in isothermal heating stage at 250 °C and 300 °C. However, the effect of residence time almost disappeared at 350 °C. Meanwhile, the release ratio of Cl increased linearly with the increase of weight loss by devolatilization (Fig. 15b). In addition, when the particle sizes were reduced, the release ratio of Cl increased obviously (Fig. 15c). Similar results were reported by Saleh et al. [111]. The explanation for this result can be the capability of char to recapture the released Cl species, which can react with relatively stable basic functional groups on the char surface or potassium to generate KCl [116]. Small particle size means that the Cl species leaving the biomass particles will be exposed to fewer surfaces during the release process, thus reducing the capture amount. Similarly, increasing sample weight led to reduced release of Cl due to the increased char capability [111]. Fig. 16 shows the effects of initial Cl and K contents in raw biomass (two herbaceous biomasses (Danish wheat straw and miscanthus) and four woody biomasses (spruce chips, bark, waste wood and short rotation coppice (SRC) poplar with a particle size of less than 4 mm)) on the release of Cl conducted by Saleh et al. [111]. On the whole, the release ratio of Cl decreased with an increase in initial Cl and K contents. For woody biomass (bark, waste wood and SRC poplar), about 40–65% of Cl was released at 250 °C, and it increased to about 85–95% at 350 °C. Doping KCl into the low-Cl content of spruce resulted in a lower chlorine release. Thus, the release ratio of Cl decreases with the increase of Cl and K contents in the biomass, regardless of inherited or enthetic. This result agrees well with the observations by Knudsen et al. [115] and Björkman and Strömberg [117]. Meanwhile, it can be seen from Fig. 16 that compared to woody biomass, herbaceous biomass (such as miscanthus and straw) shows lower Cl release ratio because of higher Cl and K contents in the fuels. A possible explanation is that the availability of methyl groups in fuel limits the release of Cl from the biomass with high Cl content

Fig. 15. Release behaviors of Cl during torrefaction and pyrolysis of straw. (a) Release ratio of Cl at different temperatures and residence times, (b) release ratio of Cl along with the weight loss and (c) release ratio of Cl with different raw biomass mass and particle sizes with the residence time of 60 min (NIHT: non-isothermal heat treatment) [112].

[111,115]. By adding inorganic salts into wood, the chlorine atoms have a possibility to interact with the organic material [117]. Knudsen et al. [118] studied the ability of biomass char to capture HCl from a gas stream in the temperature range of 400–800 °C. They observed that with an increase in the addition of HCl from 0.16 to 0.52 mmol/g straw, the capture efficiency decreased from 87% to 67% because the reactive sites in the char were gradually occupied with the increasing Cl load. Meanwhile, they found that the capture of HCl was entirely governed by the inorganic metal species (particularly in potassium) in 12

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Fig. 16. Release ratio of Cl in biomass as a function of (a) chlorine content and (b) potassium content [111].

the biomass. Chen et al. [112] conducted a detailed research on straw torrefaction with different temperature, residence time, sample weight, particle size and heating rate. They noted that the secondary reactions of once released Cl species and pyrolytic char (especially the AAEMs and carbon active sites) was essential in the process of volatile diffusion. The release of Cl was not equal to the fractions of K transformed into ion-exchanged species and released to the gas phase due to the different intensities of their secondary reaction. Hydrochloric acid (HCl) is widely reported as the major Cl species released in the pyrolysis temperature range of 250–500 °C [115,117]. Zintl et al. [119] performed the experiments by using wood doped with KCl at 200–700 °C and proposed that the initial low-temperature release of Cl was originated from the reaction between KCl and carboxylic groups (R15a). Similarly, Saleh et al. [111] found that the release of Cl was controlled by the reaction between KCl and organic constituents.

KCl + CM − COOH ↔ CM − COOK + HCl↑

R15a Fig. 17. Schematic diagram of the releases and transformations of K and Cl during torrefaction [112].

However, Hamilton et al. [120] reported that the volatilization of chloride as methyl chloride (CH3Cl) occurred during woody biomass pyrolysis at temperatures below 350 °C, and CH3Cl observed at the initial temperature of 150 °C significantly increased as the temperature increased to 300 °C. They also proposed that pectin, a major component of the primary cell wall, could promote the release of CH3Cl by acting as a CH3 donor. A further study performed by Saleh et al. [111] on basis of gas phase measurements during biomass torrefaction showed that all the Cl released at 250 °C appeared as CH3Cl; while at 350 and 500 °C, although the release of Cl was dominated by CH3Cl, other Cl containing species such as HCl may also contribute to the release of Cl [111]. However, the pectin concentration in biomass could not be the ratelimiting step and other organic species also acted as CH3 donors. More detailed research is required to clarify the controlling mechanisms of the release of Cl at torrefaction temperatures, particularly in the organic decomposition reactions that result in the release of reactive methyl groups. Fig. 17 shows a schematic diagram of the releases and transformations of K and Cl during biomass torrefaction [112]. In biomass (such as straw), 98% of Cl exists as alkali chlorides (KCl and NaCl). At the torrefaction temperature, KCl reacts with oxygen containing functional groups (such as –COOH and methylesterified carboxyl group), expressed as R15b [110,112], and results in the release of Cl in combination with free radicals like (·H) and (·CH3). As reaction products, HCl (from R15a) and CH3Cl (from R15b) move from inner structure of the fuel particle to external surface and then release to gas phase. However, part of once-released HCl maybe reacted with inherent AAEMs and active char through secondary reactions, and the latter resulted in the formation of organic chloride (or C–Cl). As torrefaction intensified, the free radicals, most likely (·H) from subsequent volatiles decomposition,

would probably react with char matrix and result in the release of K (R16) [112,121,122]. Consequently, because of the re-condensation of Cl caught by the inherent AAEMs and carbon active sites, the release of Cl decreases with increased sample weight and particle size. Meanwhile, the slight bicarbonate and carbonate existed in biomass can transform into carboxylate by reaction R17 [112,123,124], which facilitates the reaction of R15. The secondary reaction has been widely recognized [112,118].

KCl+CM−COOCH3 ↔ CM −COOK+CH3 Cl CM-K+%H→CM-H+K↑

R15b R16

2KHCO3 → K2CO3 + CO2 + H2 O

R17a

K2CO3 → (−CO2 K) + (−COX)

R17b

4.2.2. Sulfur — S On basis of the major existing forms of S in biomass, a two-step releasing mechanism for S has been proposed. The organically associated S is released at low temperature, while inorganic S is retained in the ash until the combustion temperature is up to 900 °C [115,125,126]. With increased torrefaction temperature, the release ratio of S increases [126]. From Fig. 18, it can be seen that the release of S increased gradually with increasing torrefaction temperature, and approximately 60% of the sulfur in the straw was released to the gas phase at 500 °C; Even at typical torrefaction temperature of 250 °C, the release 13

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larger sample weight (such as 10g compared to 1g) impeded the release of K due to the low release of Cl and the recapture capability of char [111,116]. Although the sample particle size showed certain effect on both water-soluble and ion-exchanged K, the improvement effect on the release of K to gas phase remained slight. KCl reacted with the carboxyl or methylesterified carboxyl groups leading to a transformation from KCl to ion-exchangeable K (R15) [110,112]. The release of K was mainly because of the emission and decomposition of K-containing organic functional groups and the substituted reaction by free radical (R16) [112,121,122]. The influence of sample weight and heating rate on the release of K and transformation behavior is mainly caused by secondary reactions of volatiles (include K, Cl and free radicals) and pyrolysis char. During the volatiles diffusion process, the released K would react with Cl and the carbon active sites leading to the re-condensation of K on char surface. However, compared to the rapid increase of Cl release with the decreased sample weight and particle size, the sample weight and particle size by secondary reactions have limited influence on the release and transformation behaviors of K. Trace Na in biomass shows similar behaviors as K in either ashrelated issues or during torrefaction. During torrefaction, some Na releases into gas phase, and some part of water-soluble Na transforms into ion-exchangeable CH3COONH4–Na [123]. Recently, Jagodzińska et al. [80] studied the slagging and fouling propensities of torrefied biomass during combustion in a 140 t/h PC boiler through ash deposition indices and its impact on heat exchange in the boiler. Results showed that torrefaction slightly decreased ash deposition tendency by affecting ash melting properties, and higher torrefaction temperature disfavored fouling and slagging. Although numerous theoretical mechanism research and certain demonstration have been carried out, more and deeper practice research in lab-scale or commercial-scale boiler to test the ash-related issues of torrefied biomass and the effects of torrefaction parameters are essential.

Fig. 18. Influence of temperature on the release of S from straw torrefaction [111].

ratio of S accessed to 20% [111]. However, different from the release of Cl, the initial sulfur content in the biomass did not influence the releasing fraction of sulfur during torrefaction [111]. Yet, there existed the recapture of S by char during torrefaction [127]. In comparison with wood biomass which shows higher sulfur releasing ratio (20–35% at 250 °C, and 40–70% at 350 °C), the releasing ratio of S in herbaceous biomass is lower (about 20% at 250 °C, and approximately 50% at 350 °C) [111]. The release of S at low torrefaction temperatures arises from the organic bound sulfur (such as protein) [128], thus the higher fraction of organic-S to total sulfur in wood than that in herbaceous biomass caused the relative higher release of sulfur from the former. Knudsen et al. [128] made a comparison of the release of sulfur from two straws with different contents of organic and inorganic sulfurs. Results showed that the straw with the highest inorganic sulfate content had a lower sulfur releasing ratio during pyrolysis, supporting the hypothesis that the initial sulfur release during torrefaction was from the decomposition of organic sulfur [115,125,126].

5. Economy viability Despite lots of scientific knowledge on the performance of different torrefaction technologies are reported, available information on the economy of biomass torrefaction is rare. Biomass torrefaction increases the energy content per unit weight (mass), and subsequent pelletization markedly improves the energy density per unit volume, thereby facilitating logistics throughout the supply chain [83,135,136]. Thus, speaking of the economy, a combined torrefaction–pelletization technology in a commercial scale is developed [52,83]. With the increase of both torrefaction temperature and residence time, the pelleted torrefied biomass becomes more similar to coal respecting heating value, grindability, powder flowability and bulk density [19,136]. Therefore, we also focus on torrefied biomass pellet in this section. As presented in Fig. 20, an industrial-scale biomass torrefaction–pelletization factory was schematized by Adrian Pirraglia et al. [50]. Before torrefaction, the biomass undergoes chipping or comminution and screening; after torrefaction, they are milled for further particle size reduction and then pelleted with an addition of binding agent to aid in pellets formation and durability improvement. In torrefaction unit (Fig. 20b), the torrefaction is an autothermal operation without the addition of external heat except for initial ignition (i.e., the energy content of the torrefaction gases matches exactly the overall heat input requirements of the entire process [137]). Then, considering critical production parameters in the process, including biomass delivered costs, capital expenditure, labor, energy consumption, etc., biomass delivery costs and depreciation were the dominant factors influencing production with capital expenditures (CAPEX). More interestingly, the cost of torrefied biomass pellet ($ 282/metric ton) is comparable with that of pelleted biomass ($ 280/metric ton) [50]. The combined torrefaction–pelletization technology can even further reduce the costs and thus improves the international economy of biomass.

4.2.3. Potassium — K As a root cause of slagging, agglomeration and corrosion during biomass combustion, potassium and chloride show negative effects on the reduction of above ash-related issues [84,129,130]. Together with available Cl and S during devolatilization, the generated KCl(g) and K2SO4(g) cause alkali-induced slagging and corrosion [87,131]. K also react with SiO2 in the residual ash to form silicates with very low melting points (< 800 °C) [132], which can cause agglomeration or low-temperature silicate melt-induced slagging. Fortunately, alkali release was clearly detected at 300–400 °C due to the decomposition of organic structure in biomass pyrolysis experiments conducted by Olsson et al. [133] and Davidsson et al. [134]. Torrefaction of birch wood at 280 °C resulted in a transformation from water-soluble K to ion-exchangeable because of the destruction of carboxylic acid groups [110]. Fig. 19 shows the effects of temperature, sample weight, sample particle size and heating method on the transformation of K during straw torrefaction [112]. Although the release of K to gas phase only accounted for about 10% at 300–350 °C, the releasing ratio increased with the increased temperature. Meanwhile, a transformation of watersoluble K to ion-exchangeable K existed. As the torrefaction temperature increased to 300 °C, the fraction of water-soluble K decreased continuously from 85.84% to 68.40%, and the fraction of ion-exchangeable K increased, which improved the release of K by R16. A 14

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Fig. 19. Release and transform behavior of K during straw torrefaction and mild pyrolysis (P1: 250–420 lm, P2: 178–250 lm, P3: 124–178 lm, P4: 74–124 lm) [112].

subsystems, biomass premium, drying and train transport presents the highest weightiness, respectively. In addition, the amount of available biomass, biomass premium, logistics equipment, biomass moisture content, drying technology, torrefaction mass yield and torrefaction plant CAPEX are important sensitivity parameters affecting the total costs. Adrian Pirraglia et al. [50] reported the similar results. The costs of biomass torrefaction are higher than coal at present. However, the torrefied pellets business becomes much more attractive, considering the proposed implementation of carbon credits market, the increasing torrefaction plant equipment size and the empirical cumulative that decreases the cost significantly [50,52,136], as well as the human concern on climate and environment. Carbon credits can bring an additional income of $ 36–72 per metric ton of torrefied wood and thus result in a significant increase in Internal Rates of Return and Net Present Value [50]. Because of the relatively less material requirement for a larger capacity torrefaction plant, scaling up the torrefaction reactor is expected to reduce the investment costs in a power of 0.7 [50,136]. In the long term, a combination of scaling up and technological learning can cut down 50% of the total costs, thus reducing the production costs of woody biomass to 2.1–5.1 $/GJLHV [136]. Anyway, the further concern should be intensified on the transport-

When importing biomass from Africa to Europe, torrefaction combined with pelletization can save 4–16% of the cost in comparison with sole pelletization [138]. However, the application costs of the torrefied biomass not only relates to the biomass supply cost (including biomass purchase, storage, comminution and transportation), processing cost (including investment in torrefaction reactor and milling, maintenance cost, and energy consumption, etc), transport cost and labor salary, but also includes the cost for integration biomass torrefaction and application system [52,139]. Recently, after considering the biomass supply, the energy and mass balance during drying, torrefaction and densification, the investment and operating costs, and the distribution of end user, Martin Svanberg et al. [52] developed a techno-economic system model to evaluate the effects of logistics and production parameters on the costs of torrefaction supply chain under Swedish conditions. They reported that a torrefaction supply plant with a size of about 150–200 kilotons dry substance per year produced the optimal economies. Fig. 21 shows a detailed cost statistic of varied of possible activities. The biomass supply system accounts for the largest fraction of the total costs, followed by production costs and distribution system costs. Among the three

Fig. 20. Torrefaction–pelletization process for an industrial-scale factory [50]. 15

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Fig. 21. Costs for different activities in a torrefaction plant with a production capacity of 200 kiloton dry substance per year [52].

drying and train transport are primary in the supply chain of biomass torrefaction. Presently, the cost of biomass torrefaction is higher than coal. However, the cost can be significantly decreased by the implementation of carbon credits market, increasing torrefaction plant equipment size and empirical cumulation. Later, more attention should be paid on the application demonstration and economic optimization of the system, such as integrated torrefaction-combustion and torrefaction-gasification, as well as the extended fuel supply to application system.

mode and biomass collection-distance to increase the transport efficiency [140], processing technology to improve product properties and economy, and the gap between supplier and demander, etc.

6. Conclusions Biomass torrefaction is a kind of low-temperature pyrolysis or hightemperature drying technology conducted at 200–300 °C in an inert atmosphere. It has been studied intensively to improve the biomass properties pronouncedly, such as energy density, hydrophobicity, ignitability, reactivity, grindability, and combustion and gasification characteristics. Thus, as a critical review to provide new insight for further study, the key properties of torrefaction biomass, potential applications on combustion and gasification, as well as the intractable challenges on ash-related issues during thermal conversion and the economy are discussed in detailed. In term of the properties improvement, torrefaction temperature has the greatest impact, followed by residence time and moisture content in the parent biomass, and its particle size has the smallest impact. For either physical properties (energy density and grindabilty) or gasification characteristics, approximate 250 °C can be an optimal torrefaction temperature option. Alternatively, maintaining the solid mass yield in the range of 80–60%, which causes relatively high HHV, energy yield, and mass energy density, is another method for the determination of optimum torrefaction condition. Biomass torrefaction strongly depends on the remarkable degradation of hemicellulose and a less degradation degree of cellulose and lignin. The degradation of agricultural residues during torrefaction is higher than ligneous plants because of the higher volatile and hemicellulose in herbaceous residues. Meanwhile, deciduous trees mainly containing xylan in hemicellulose are more active than coniferous trees which mainly contain glucomannan in hemicellulose. The torrefied biomass, especially torrefied woody biomass, can be used for gasification and co-firing. Torrefaction improves the syngas quality and cold gasification efficiency significantly. Moreover, in the torrefaction temperature range the release of Cl, S, and K are significant, which bring great additional benefits for the reduction or elimination of ash-related issues (such as slagging, corrosion, and agglomeration) during biomass thermal conversion. During torrefaction, the release ratio of Cl increases with the increased temperature and residence time, as well with the decreased particle size, sample weight and initial Cl content in the parent biomass. Higher sample weight has an adverse influence on the release of K, while particle size shows an ignorable effect. The release of S mainly originates from the decomposition of organic sulfur in the parent biomass. The economy analysis shows that the costs of biomass premium,

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