**3. Feedstock pretreatment**

Physical properties like particle size, particle shape and density influence the material handling and flowability of particles that are leading properties for an uninterrupted feeding system [62]. Chemical properties like elemental and proximate analyses influence the pyrolysis products' distribution and properties. Although biomass has inherent heterogeneous properties [1], various physical, chemical and thermal pretreatments contribute to achieving homogeneity in properties of biomass. Homogeneous feedstock with uniform physical and chemical properties plays a key role in a pyrolysis process and quality of its products. Although pretreatments create an additional feedstock cost, they facilitate feeding a wide range of biomass species with broad properties and are beneficial due to producing uniform and homogeneous feedstock for power plants.

### **3.1 Moisture reduction**

Biomass particles are dried for a more-efficient thermal combustion in pyrolysis reactors to produce bio-oil [63, 64]. A fresh biomass has a high moisture content of up to 80% [34]. High moisture content reduces the heating value of the fuel and shifts the ignition point to higher temperatures [6], and inhibits the rise of temperature inside the particles and conversion reactor [65, 66]. Reduced particle and reactor temperatures diminish the liquid yield at the expense of a larger fraction of biochar and non-condensable gases [22, 66]. Water has a catalytic effect on volatile cracking. Di Blasi [67] showed that moisture content conducts the pyrolysis reaction to a low activation energy path that promotes the formation of char, non-condensable gases and more water. Biomass should be dried down to less than 10% moisture content to improve the quality of produced fuel [15]. Pyrolysis of dry material produces less water compared to wet biomass. Bio-oil with a lower amount of water has higher heating value, a lower ignition point, higher combustion rate and a lower potential of phase separation [6].

Rezaei [68] stated that size of particles [69], initial moisture content [70], drying temperature [69, 71], relative humidity of drying gas [72, 73] and particle heating rate [74] influence the rate of moisture loss. Rezaei et al. [75] showed that biomass shrinks during the moisture loss that influences the drying rate. Dehydration of fresh wood causes a reduction in the dimension of wood in a direction normal to the microfibril orientation, whereas the longitudinal shrinkage is usually negligible [76]. Mazzanti et al. [77] showed that the longitudinal shrinkage for poplar wood is negligible. Taylor et al. [78] showed that the radial shrinkage of beech wood is about 70 times of longitudinal shrinkage.

#### **3.2 Size reduction**

Grinding reduces the dimensions of a particle and increases the particle's specific surface area (ratio of the particle's surface area to its mass). The same relationship holds for a bulk of particles where the surface area of the solids increases in a given volume of bulk particles. Therefore, the smaller particles have a more exposed surface to the raised-temperature environment to boost the rate of heat and mass transfer between the particles and surrounding. Rezaei et al. [79, 80] showed that larger particles (from 1 to 5 mm) have a delay in heat and mass transfer and reduced rate of pyrolysis (**Figure 2**). On the other hand, grinding the biomass to smaller particles requires more energy input. Rezaei et al. [81] reported that grinding wood chips to produce particles with average sizes of 1, 1.8, 3 and 5 mm requires about 124.0, 85.7, 52.6 and 28.2 kJ/kg, respectively. Van Der Stelt et al. [82] measured

**65**

*Woody Feedstock Pretreatments to Enhance Pyrolysis Bio-oil Quality and Produce…*

the power required for size reduction of coal, torrefied woodcutting, willow and demolition wood. He showed that to have particles smaller than 0.6 mm, the grinding power consumption increases sharply. The balance of the conversion rate and energy consumption identifies the optimum particle size appropriate for a fast

*Effect of particle size on the pyrolysis rate of ground wood particles at 500°C [80].*

**Figure 3** shows the published data on yields of three phases of solid, liquid and gas in pyrolysis of a range of particle size. In the range of particle size up to 2 mm, only the apricot stone pyrolyzed at 800°C showed a yield that was sensitive to particles size. An increase in particle size increased the char yield at the expense of less liquid yield [15, 22, 26, 83]. Some researchers did not observe any effect of particle size on the yield of products. For example, Şensöz et al. [84, 85] did not observe any meaningful influence of particle size on the pyrolysis products of rapeseed (*Brassica napus* L.) in the range of 0.22–0.85 mm [84] and of debarked pine in the range of 2–5 mm [85]. Encinar et al. [86] reported that liquid yield was independent of biomass particle size during pyrolysis of 0.4–2.0 mm grape residue and olive residue particles pyrolyzed at 500°C. On the other hand, some researchers showed that larger particles in pyrolysis decrease liquid yield and increase biochar yield [22, 83, 87]. NikAzar et al. [22] found out that size increase from 53 to 63 μm to 270–500 μm declined liquid yield from 53 to 38%. He claimed the particle's core temperature diminished by particle size and caused the change in the yield of products. This result was confirmed by some other researchers

Most of the published data recommend that particles in the range of 1–2 mm are appropriate for fast pyrolysis [17, 52, 82, 89]. However, the effect of particle size on the pyrolysis process needs more work. The range of particle sizes tested in various studies is seldom comparable and a study on a wider range of particle size seems necessary. In addition, the challenges associated with commercial grinding and feeding the bulky lightweight biomass in the reactor should be taken into account

The mineral elements in biomass may be listed mostly as potassium, chlorine, sulfur, silicon, calcium and magnesium [45]. The mineral content of biomass exists on the particle surface because of contact with soil during harvest and/or transpor-

*DOI: http://dx.doi.org/10.5772/intechopen.81818*

pyrolysis process.

**Figure 2.**

[15–17, 26, 29, 88].

**3.3 De-mineralization**

that is out of the scope of the current chapter.

tation, or within the material as biogenic characteristics.

*Woody Feedstock Pretreatments to Enhance Pyrolysis Bio-oil Quality and Produce… DOI: http://dx.doi.org/10.5772/intechopen.81818*

**Figure 2.** *Effect of particle size on the pyrolysis rate of ground wood particles at 500°C [80].*

the power required for size reduction of coal, torrefied woodcutting, willow and demolition wood. He showed that to have particles smaller than 0.6 mm, the grinding power consumption increases sharply. The balance of the conversion rate and energy consumption identifies the optimum particle size appropriate for a fast pyrolysis process.

**Figure 3** shows the published data on yields of three phases of solid, liquid and gas in pyrolysis of a range of particle size. In the range of particle size up to 2 mm, only the apricot stone pyrolyzed at 800°C showed a yield that was sensitive to particles size. An increase in particle size increased the char yield at the expense of less liquid yield [15, 22, 26, 83]. Some researchers did not observe any effect of particle size on the yield of products. For example, Şensöz et al. [84, 85] did not observe any meaningful influence of particle size on the pyrolysis products of rapeseed (*Brassica napus* L.) in the range of 0.22–0.85 mm [84] and of debarked pine in the range of 2–5 mm [85]. Encinar et al. [86] reported that liquid yield was independent of biomass particle size during pyrolysis of 0.4–2.0 mm grape residue and olive residue particles pyrolyzed at 500°C. On the other hand, some researchers showed that larger particles in pyrolysis decrease liquid yield and increase biochar yield [22, 83, 87]. NikAzar et al. [22] found out that size increase from 53 to 63 μm to 270–500 μm declined liquid yield from 53 to 38%. He claimed the particle's core temperature diminished by particle size and caused the change in the yield of products. This result was confirmed by some other researchers [15–17, 26, 29, 88].

Most of the published data recommend that particles in the range of 1–2 mm are appropriate for fast pyrolysis [17, 52, 82, 89]. However, the effect of particle size on the pyrolysis process needs more work. The range of particle sizes tested in various studies is seldom comparable and a study on a wider range of particle size seems necessary. In addition, the challenges associated with commercial grinding and feeding the bulky lightweight biomass in the reactor should be taken into account that is out of the scope of the current chapter.

#### **3.3 De-mineralization**

The mineral elements in biomass may be listed mostly as potassium, chlorine, sulfur, silicon, calcium and magnesium [45]. The mineral content of biomass exists on the particle surface because of contact with soil during harvest and/or transportation, or within the material as biogenic characteristics.

*Biomass for Bioenergy - Recent Trends and Future Challenges*

and homogeneous feedstock for power plants.

and a lower potential of phase separation [6].

beech wood is about 70 times of longitudinal shrinkage.

Physical properties like particle size, particle shape and density influence the material handling and flowability of particles that are leading properties for an uninterrupted feeding system [62]. Chemical properties like elemental and proximate analyses influence the pyrolysis products' distribution and properties. Although biomass has inherent heterogeneous properties [1], various physical, chemical and thermal pretreatments contribute to achieving homogeneity in properties of biomass. Homogeneous feedstock with uniform physical and chemical properties plays a key role in a pyrolysis process and quality of its products. Although pretreatments create an additional feedstock cost, they facilitate feeding a wide range of biomass species with broad properties and are beneficial due to producing uniform

Biomass particles are dried for a more-efficient thermal combustion in pyrolysis reactors to produce bio-oil [63, 64]. A fresh biomass has a high moisture content of up to 80% [34]. High moisture content reduces the heating value of the fuel and shifts the ignition point to higher temperatures [6], and inhibits the rise of temperature inside the particles and conversion reactor [65, 66]. Reduced particle and reactor temperatures diminish the liquid yield at the expense of a larger fraction of biochar and non-condensable gases [22, 66]. Water has a catalytic effect on volatile cracking. Di Blasi [67] showed that moisture content conducts the pyrolysis reaction to a low activation energy path that promotes the formation of char, non-condensable gases and more water. Biomass should be dried down to less than 10% moisture content to improve the quality of produced fuel [15]. Pyrolysis of dry material produces less water compared to wet biomass. Bio-oil with a lower amount of water has higher heating value, a lower ignition point, higher combustion rate

Rezaei [68] stated that size of particles [69], initial moisture content [70], drying temperature [69, 71], relative humidity of drying gas [72, 73] and particle heating rate [74] influence the rate of moisture loss. Rezaei et al. [75] showed that biomass shrinks during the moisture loss that influences the drying rate. Dehydration of fresh wood causes a reduction in the dimension of wood in a direction normal to the microfibril orientation, whereas the longitudinal shrinkage is usually negligible [76]. Mazzanti et al. [77] showed that the longitudinal shrinkage for poplar wood is negligible. Taylor et al. [78] showed that the radial shrinkage of

Grinding reduces the dimensions of a particle and increases the particle's specific surface area (ratio of the particle's surface area to its mass). The same relationship holds for a bulk of particles where the surface area of the solids increases in a given volume of bulk particles. Therefore, the smaller particles have a more exposed surface to the raised-temperature environment to boost the rate of heat and mass transfer between the particles and surrounding. Rezaei et al. [79, 80] showed that larger particles (from 1 to 5 mm) have a delay in heat and mass transfer and reduced rate of pyrolysis (**Figure 2**). On the other hand, grinding the biomass to smaller particles requires more energy input. Rezaei et al. [81] reported that grinding wood chips to produce particles with average sizes of 1, 1.8, 3 and 5 mm requires about 124.0, 85.7, 52.6 and 28.2 kJ/kg, respectively. Van Der Stelt et al. [82] measured

**3. Feedstock pretreatment**

**3.1 Moisture reduction**

**64**

**3.2 Size reduction**

**Figure 3.**

*Effect of biomass particle size on the yield of products in a fast pyrolysis process.*

The minerals of the biomass present as ash or fouling in conversion vessels. In a pyrolysis process, the biochar typically contains up to 90% of the biomass minerals [46] that changes the physical properties of char. Ash shifts the size distribution of the char to smaller sizes that hardens its full separation from produced volatile. A partial char separation results in a high solid content bio-oil that cannot be used as turbine fuel [49]. Furthermore, minerals act as a catalyst and cause continuous secondary reactions in liquid phase [16, 47, 48] at the expense of char formation [51, 90]. More secondary reactions promote an increases in bio-oil viscosity with time and accelerate aging phenomenon [50]. Mineral contamination accelerates the catalytic breaking of levoglucosan into unwanted hydroxyl acetaldehyde compounds [51, 91]. Former is desired part of volatiles and latter is an undesirable portion of volatiles. Addition of 0.05 wt.% NaCl to an ash-free cellulose decreases the levoglucosan formation yield by a factor of 6 [92].

One efficient pretreatment to reduce the mineral content is washing the biomass with water, acidic and/or alkaline solutions. Washing with dilute acid and hot water results in a slight decomposition of hemicellulose [93]. Washing with dilute alkali disrupts the lignin structure and solubilize the hemicellulose [94]. Washing biomass prior to pyrolysis takes away a huge amount of minerals from the biomass, up to 70% of the initial minerals [1, 17].

Das et al. [95] studied the effect of various washing solutions, concentrations and the time of washing on ash content and products' yields of sugarcane bagasse pyrolysis (**Figure 4**). The ash content decreased from 1.83% before washing to 0.03%. The only washing solution that had a reverse effect was 5 M HCl. It must have been due to the adding chlorine element into the biomass. The important point is the effect of washing on yield of total liquid versus the bio-oil. The total liquid contains bio-oil and aqueous solutions. All washing solutions reduced the total liquid yield but boosted the bio-oil yield.

**Table 4** lists different washing solutions based on demineralization yield, ash content, char and liquid yield and maximum decomposition rate [96]. Washing demineralized the biomass up to 98%, enhanced liquid yield, lowered the char yield, shifted up the decomposition rate and reduced the low molecular weight compounds

**67**

**3.4 Torrefaction**

**Figure 4.**

*1*

*2*

*3*

**Table 4.**

*pyrolysis [95].*

**Washing solution**

*Hydrochloric acid.*

*Hydrofluoric acid.*

*Woody Feedstock Pretreatments to Enhance Pyrolysis Bio-oil Quality and Produce…*

in pyrolysis products. Solution temperature also influences the demineralization yield. Deng et al. [7] showed that demineralization of candlenut wood raised from 8% at 30°C to 35% to 90°C. Mineral removal increased the higher heating value from 16.53 to 17.82 MJ/kg and the rate of devolatilization increased too. Natural rain and

*Effect of de-mineralization pretreatment on biomass ash content and products' yield of sugarcane bagasse* 

**Char yield (%)**

No washing – 14.9 85.1 0.96 362 HCl1 69.3 11.8 88.2 1.55 366 HF2 97.3 10.2 89.8 1.15 368 Deionized water 97.7 11.0 89.0 1.23 372 Tap water 98.2 11.4 88.6 1.19 376

**Volatiles yield (wt.%)**

**Max. decomposition rate (wt.%/°C)**

**Tp3 (°C)**

**Demineralization yield (%)**

*Temperature at which maximum decomposition rate happens.*

*Specifications for pyrolysis of demineralized poplar wood at 550˚C [96].*

Torrefaction is a mild thermal treatment that modifies the structure and chemi-

cal composition of biomass by removing hemicelluloses [98], dehydrating and partially reducing cellulose and lignin [99]. Li et al. [13] described torrefaction as a mild heat treatment of biomass at a temperature range of 200–300°C, prolonging 15–30 minutes. Westover et al. [6] divided torrefaction into three stages of non-reactive drying (50–150°C), reactive drying (150–200°C) and destructive stage (200–300°C). During thermal treatment, biomass releases moisture up to the temperature of 150–170°C [11]. In 180–270°C, exothermic hemicellulose degradation happens and biomass turns to brown color. At this stage, torrefaction reaction

season of raining change the composition of minerals in biomass [97].

*DOI: http://dx.doi.org/10.5772/intechopen.81818*

*Woody Feedstock Pretreatments to Enhance Pyrolysis Bio-oil Quality and Produce… DOI: http://dx.doi.org/10.5772/intechopen.81818*

#### **Figure 4.**

*Biomass for Bioenergy - Recent Trends and Future Challenges*

the levoglucosan formation yield by a factor of 6 [92].

*Effect of biomass particle size on the yield of products in a fast pyrolysis process.*

70% of the initial minerals [1, 17].

liquid yield but boosted the bio-oil yield.

The minerals of the biomass present as ash or fouling in conversion vessels. In a pyrolysis process, the biochar typically contains up to 90% of the biomass minerals [46] that changes the physical properties of char. Ash shifts the size distribution of the char to smaller sizes that hardens its full separation from produced volatile. A partial char separation results in a high solid content bio-oil that cannot be used as turbine fuel [49]. Furthermore, minerals act as a catalyst and cause continuous secondary reactions in liquid phase [16, 47, 48] at the expense of char formation [51, 90]. More secondary reactions promote an increases in bio-oil viscosity with time and accelerate aging phenomenon [50]. Mineral contamination accelerates the catalytic breaking of levoglucosan into unwanted hydroxyl acetaldehyde compounds [51, 91]. Former is desired part of volatiles and latter is an undesirable portion of volatiles. Addition of 0.05 wt.% NaCl to an ash-free cellulose decreases

One efficient pretreatment to reduce the mineral content is washing the biomass with water, acidic and/or alkaline solutions. Washing with dilute acid and hot water results in a slight decomposition of hemicellulose [93]. Washing with dilute alkali disrupts the lignin structure and solubilize the hemicellulose [94]. Washing biomass prior to pyrolysis takes away a huge amount of minerals from the biomass, up to

Das et al. [95] studied the effect of various washing solutions, concentrations and the time of washing on ash content and products' yields of sugarcane bagasse pyrolysis (**Figure 4**). The ash content decreased from 1.83% before washing to 0.03%. The only washing solution that had a reverse effect was 5 M HCl. It must have been due to the adding chlorine element into the biomass. The important point is the effect of washing on yield of total liquid versus the bio-oil. The total liquid contains bio-oil and aqueous solutions. All washing solutions reduced the total

**Table 4** lists different washing solutions based on demineralization yield, ash content, char and liquid yield and maximum decomposition rate [96]. Washing demineralized the biomass up to 98%, enhanced liquid yield, lowered the char yield, shifted up the decomposition rate and reduced the low molecular weight compounds

**66**

**Figure 3.**

*Effect of de-mineralization pretreatment on biomass ash content and products' yield of sugarcane bagasse pyrolysis [95].*


*2 Hydrofluoric acid.*

*3 Temperature at which maximum decomposition rate happens.*

#### **Table 4.**

*Specifications for pyrolysis of demineralized poplar wood at 550˚C [96].*

in pyrolysis products. Solution temperature also influences the demineralization yield. Deng et al. [7] showed that demineralization of candlenut wood raised from 8% at 30°C to 35% to 90°C. Mineral removal increased the higher heating value from 16.53 to 17.82 MJ/kg and the rate of devolatilization increased too. Natural rain and season of raining change the composition of minerals in biomass [97].

#### **3.4 Torrefaction**

Torrefaction is a mild thermal treatment that modifies the structure and chemical composition of biomass by removing hemicelluloses [98], dehydrating and partially reducing cellulose and lignin [99]. Li et al. [13] described torrefaction as a mild heat treatment of biomass at a temperature range of 200–300°C, prolonging 15–30 minutes. Westover et al. [6] divided torrefaction into three stages of non-reactive drying (50–150°C), reactive drying (150–200°C) and destructive stage (200–300°C). During thermal treatment, biomass releases moisture up to the temperature of 150–170°C [11]. In 180–270°C, exothermic hemicellulose degradation happens and biomass turns to brown color. At this stage, torrefaction reaction

produces more water, CO2, acetic acid and phenols [12–14, 35]. The released gases are combustible and may be used to provide a portion of process energy. Beyond 270°C, the reactions are more exothermic and produce CO and some other heavier products such as CH4 and C2H6. Torrefaction continues to a temperature of 300°C, where pyrolysis starts.

Torrefied biomass contains about 80% of the mass and 90% of the energy of the initial biomass [82, 99]. Similar to other thermal processes, mass and energy yield of torrefaction depends on biomass species [99], particle size [14, 26], operating temperature [6, 13, 33, 100, 101] and residence time [13, 33, 101]. Biomass torrefaction has been recognized as a feasible technique to convert raw biomass to a high energy density, hydrophobic, grindable, homogeneous, low moisture content (<5%) [6], better storage stability [34] and low-oxygen-content fuel that is a suitable feedstock for pyrolysis [102].

Torrefaction changes the stiffness and glass transition temperature (Tg) of biomass [6] that contributes to the grindability of the material. Ground torrefied biomass particles are smaller, drier and more uniform in size and physical and chemical properties [13, 14]. **Table 5** lists the specific grinding energy of raw and torrefied pine and logging residue. Higher torrefaction temperature makes the biomass structure more brittle and reduces the specific grinding energy. Specific grinding energy consumption reduced by 90% from 240 kWh/t for raw pine chips to 24 kWh/t for torrefied pine chips at 300°C.

Using the torrefied biomass as a feedstock for fast pyrolysis has various benefits. The level of these modifications depends on torrefaction temperature and residence time. First of all, torrefaction removes the hemicellulose and enriches the biomass into the cellulose and lignin (**Figure 5**). Presence of hemicellulose intensively decreases the yield of levoglucosan and promotes the formation of hydroxyl acetaldehyde. The torrefied feedstock may eliminate or reduce this interaction as it removes hemicelluloses.

Bio-oil of torrefied biomass has a lower water content that increases bio-oil's stability (less phase separation) [100, 104, 105], lower oxygen/carbon ratio and higher calorific value [104] compared to the bio-oil produced from non-torrefied biomass [5, 89, 99, 101]. Because torrefaction extracts contain acidic condensable volatiles such as acetic acid, furfural, formic acid, methanol, lactic acid and phenol from the biomass [106], the bio-oil has a lower acidity and aldehydes content [17, 105].


*2*

*Torrefied pine chips. 3*

*Logging residues.*

*4 Torrefied logging residues.*

#### **Table 5.**

*Specific energy consumption for grinding of untreated and torrefied biomass with a residence time of 30 minutes [98].*

**69**

**Table 6.**

*torrefied corncob at 240 and 300°C for 20 minutes [104].*

fied corncob.

**Figure 5.**

*Woody Feedstock Pretreatments to Enhance Pyrolysis Bio-oil Quality and Produce…*

Klinger [105] pyrolyzed the torrefied material and observed 27% water reduction, 36% CO reduction, 55% CO2 reduction and 67% acetic acid reduction in the produced volatiles compared to volatiles obtained from non-torrefied biomass. Despite all benefits, pyrolysis of torrefied biomass has a lower yield of total liquid. Liaw et al. [100] reported that pyrolysis total liquid yields of raw Douglas fir and torrefied Douglas fir at 280, 320 and 370°C was 59, 55, 48 and 30%, respectively [100]. Boateng [99] and Zheng [5, 104] reported that more sever torrefaction decreases the total liquid yield and increases the yield of biochar and permanent gases. **Tables 6** and **7** list the results of Zheng's work for pyrolysis of raw and torre-

Water content reduced from 35% in bio-oil produced from raw corncob to 21% in bio-oil produced from torrefied corncob at 300°C. The pH increased in the bio-oil prepared from torrefied corncob. The viscosity of bio-oil also increased probably due to a reduction in water content. The acetic acid content decreased moderately with increasing torrefaction temperature and residence time. The furfural content also decreased gradually with torrefaction temperature and residence time. Acetic acid and furfural are mainly derived from hemicellulose and cellulose. Ren et al. [101, 107] conducted a research on Douglas fir in two sequent

**Properties and compounds Raw corncob Torrefied (240°C) Torrefied (300°C)** Water content (wt.%) 35.00 30.00 21.00 High heating value (MJ/kg) 14.85 16.49 17.21 pH 2.68 3.30 3.34 Kinematic viscosity @ 20°C (cSt) 3.42 7.27 12.62 Acids (95% acetic acid) (%wb) 7.16 5.65 4.75 Ketones (wt.%) 8.16 7.42 6.02 Furans (wt.%) 0.98 0.8 0.76 Phenols (wt.%) 2.45 4.51 5.27

*Physical properties and a few main chemical contents of bio-oil produced from pyrolysis (at 500°C) of raw and* 

*DOI: http://dx.doi.org/10.5772/intechopen.81818*

*Composition of raw and torrefied pine at 240 and 280°C [103].*

*Woody Feedstock Pretreatments to Enhance Pyrolysis Bio-oil Quality and Produce… DOI: http://dx.doi.org/10.5772/intechopen.81818*

#### **Figure 5.**

*Biomass for Bioenergy - Recent Trends and Future Challenges*

where pyrolysis starts.

able feedstock for pyrolysis [102].

removes hemicelluloses.

**Sample Specific** 

**grinding energy (kWh/t)**


to 24 kWh/t for torrefied pine chips at 300°C.

produces more water, CO2, acetic acid and phenols [12–14, 35]. The released gases are combustible and may be used to provide a portion of process energy. Beyond 270°C, the reactions are more exothermic and produce CO and some other heavier products such as CH4 and C2H6. Torrefaction continues to a temperature of 300°C,

Torrefied biomass contains about 80% of the mass and 90% of the energy of the initial biomass [82, 99]. Similar to other thermal processes, mass and energy yield of torrefaction depends on biomass species [99], particle size [14, 26], operating temperature [6, 13, 33, 100, 101] and residence time [13, 33, 101]. Biomass torrefaction has been recognized as a feasible technique to convert raw biomass to a high energy density, hydrophobic, grindable, homogeneous, low moisture content (<5%) [6], better storage stability [34] and low-oxygen-content fuel that is a suit-

Torrefaction changes the stiffness and glass transition temperature (Tg) of biomass [6] that contributes to the grindability of the material. Ground torrefied biomass particles are smaller, drier and more uniform in size and physical and chemical properties [13, 14]. **Table 5** lists the specific grinding energy of raw and torrefied pine and logging residue. Higher torrefaction temperature makes the biomass structure more brittle and reduces the specific grinding energy. Specific grinding energy consumption reduced by 90% from 240 kWh/t for raw pine chips

Using the torrefied biomass as a feedstock for fast pyrolysis has various benefits. The level of these modifications depends on torrefaction temperature and residence time. First of all, torrefaction removes the hemicellulose and enriches the biomass into the cellulose and lignin (**Figure 5**). Presence of hemicellulose intensively decreases the yield of levoglucosan and promotes the formation of hydroxyl acetaldehyde. The torrefied feedstock may eliminate or reduce this interaction as it

Bio-oil of torrefied biomass has a lower water content that increases bio-oil's stability (less phase separation) [100, 104, 105], lower oxygen/carbon ratio and higher calorific value [104] compared to the bio-oil produced from non-torrefied biomass [5, 89, 99, 101]. Because torrefaction extracts contain acidic condensable volatiles such as acetic acid, furfural, formic acid, methanol, lactic acid and phenol from the biomass [106], the bio-oil has a lower acidity and aldehydes content [17, 105].

Untreated-PC1 237.7 15.19 Untreated-LR3 236.7 14.77

TPC-250°C 71.4 6.94 TLR-250°C 110.4 5.87 TPC-275°C 52.0 0.99 TLR-275°C 78.0 5.23 TPC-300°C 23.9 0.56 TLR-300°C 37.6 1.04

*Specific energy consumption for grinding of untreated and torrefied biomass with a residence time of* 

**Sample Specific** 

**grinding energy (kWh/t)**


**Hemicel. (wt.%)**

**Hemicel. (wt.%)**

**68**

*1 Pine chips. 2*

*3*

*4*

**Table 5.**

TPC2

*Torrefied pine chips.*

*Torrefied logging residues.*

*Logging residues.*

*30 minutes [98].*

*Composition of raw and torrefied pine at 240 and 280°C [103].*

Klinger [105] pyrolyzed the torrefied material and observed 27% water reduction, 36% CO reduction, 55% CO2 reduction and 67% acetic acid reduction in the produced volatiles compared to volatiles obtained from non-torrefied biomass.

Despite all benefits, pyrolysis of torrefied biomass has a lower yield of total liquid. Liaw et al. [100] reported that pyrolysis total liquid yields of raw Douglas fir and torrefied Douglas fir at 280, 320 and 370°C was 59, 55, 48 and 30%, respectively [100]. Boateng [99] and Zheng [5, 104] reported that more sever torrefaction decreases the total liquid yield and increases the yield of biochar and permanent gases. **Tables 6** and **7** list the results of Zheng's work for pyrolysis of raw and torrefied corncob.

Water content reduced from 35% in bio-oil produced from raw corncob to 21% in bio-oil produced from torrefied corncob at 300°C. The pH increased in the bio-oil prepared from torrefied corncob. The viscosity of bio-oil also increased probably due to a reduction in water content. The acetic acid content decreased moderately with increasing torrefaction temperature and residence time. The furfural content also decreased gradually with torrefaction temperature and residence time. Acetic acid and furfural are mainly derived from hemicellulose and cellulose. Ren et al. [101, 107] conducted a research on Douglas fir in two sequent


#### **Table 6.**

*Physical properties and a few main chemical contents of bio-oil produced from pyrolysis (at 500°C) of raw and torrefied corncob at 240 and 300°C for 20 minutes [104].*


**Table 7.**

*Effect of temperature and residence time of corncob torrefaction on ultimate analysis of feedstock, the yield of pyrolysis products (at 500°C) and physical properties of bio-oil [5].*

articles. They determined the effects of pyrolysis temperature and torrefaction time, as a pretreatment, on the characterization of produced total bio-oil and syngas. Total bio-oil is the sum of condensed liquids from torrefaction and pyrolysis. **Figure 6** shows that bio-oil yield from raw (untreated) biomass ranged from 31 [107] to 53% [101] for pyrolysis at 400 and 450°C, respectively. The bio-oil yield from a 450°C pyrolysis decreased to 51 and 46% for 8 and 15 minutes torrefaction pretreatment at 275°C was carried out. Longer torrefaction reduced slightly the yield of bio-oil. Torrefaction also altered the compositions of syngas by reducing CO2 and increasing H2 and CH4. The syngas produced from pyrolysis step was rich in H2, CH4 and CO implying that the syngas quality was significantly improved by torrefaction process [105]. The quantity of syngas increased with the severity of the torrefaction.

**Table 8** summarizes a qualitative analysis of benefits of feedstock thermal pretreatment on logistical and quality of bio-oil properties. Column 1 lists major properties that a feedstock would gain as a result of thermal pretreatment. The degree of change in properties depends upon the severity of thermal treatment. Column 2 lists the benefits of these properties on improving conversion efficiency or the quality of produced bio-oil when the pretreated feedstock is pyrolyzed. Column 3 outlines the benefits of feedstock properties from a logistical perspective, that is, handling, transport, storage, unit operations like blending and feeding the feedstock to the pyrolysis reactor. Finally, column 4 lists the potential monetary benefits of thermal pretreatments mostly due to improvements in logistics.

**71**

**Table 8.**

**Figure 6.**

*Woody Feedstock Pretreatments to Enhance Pyrolysis Bio-oil Quality and Produce…*

*Bio-oil yield from raw and torrefied Douglas fir at the various duration and temperature [101, 107].*

**Logistics benefit Economic benefit**

Lower investment and operating cost in grinder and grinder operation

Lower storage cost, lower transport cost

Expensive drying is not

Lower cost (\$/GJ) No need to design a new combustion chamber

Lower management

Reduced cost of shipping and storage

Higher \$/GJ

Reduced overall feedstock cost (\$/t)

required

cost

Torrefied particles are dry and a lower tendency for electrostatic charged

Store unprotected, exposure to rain, long

Reduced mass to handle, stable in storage

Can be blended with coal and other high heat value products

Reduced quality control, may easily become a commodity

(GJ/m3 )

Less chlorine and ash Access to low-quality

Improved flowability, low off-gas emissions

feedstock (e.g. bark)

shelf life

**quality of bio-oil benefit**

particles for high heat transfer in the reactor

to a decrease in OH and COOH groups

higher heat value due to increased C and low

performance in the pyrolysis reactor

Can be controlled to a precise particle size and

Thermal degradation Lower acidity Higher energy density

*DOI: http://dx.doi.org/10.5772/intechopen.81818*

**Feedstock properties Conversion and** 

Grindability Uniform small size

Hydrophobicity Less water content due

Low moisture content Reduced water in the

High heat value Bio-oil will have a

Homogeneity Predictable conversion

High density (after grinding)

Wet fractionation (wet torrefaction)

bio-oil

O/C ratio

density

*These improvements are in comparison with the untreated feedstock.*

*Overview of benefits of torrefied feedstock for bio-oil production.*

*Woody Feedstock Pretreatments to Enhance Pyrolysis Bio-oil Quality and Produce… DOI: http://dx.doi.org/10.5772/intechopen.81818*

#### **Figure 6.**

*Biomass for Bioenergy - Recent Trends and Future Challenges*

*pyrolysis products (at 500°C) and physical properties of bio-oil [5].*

**corncob**

C 43.87 45.10 48.54 57.86 47.64 57.33 H 6.06 6.20 6.65 6.75 6.10 5.93 O 49.50 48.01 44.06 34.43 45.61 35.92 N 0.53 0.61 0.70 0.86 0.60 0.75 S 0.04 0.08 0.05 0.11 0.04 0.06 O/C 1.13 1.06 0.91 0.60 0.96 0.63

Bio-oil 57.20 55.15 47.60 40.74 50.36 39.35 Biochar 21.14 24.57 30.21 38.19 26.31 40.70 Non-condensable gas 21.66 20.28 22.19 21.07 23.33 19.95

Water content 35.0 33.0 25.0 21.0 30.0 22.0 Higher heating value (MJ/kg) 14.85 15.12 16.49 17.21 15.58 17.09 pH 2.68 2.97 3.30 3.34 2.88 3.35 Kinematic viscosity @ 20°C (cSt) 3.42 3.96 7.27 12.62 4.23 12.48 Crystallinity (%) 20.06 25.74 34.82 27.35 23.26 22.83

**Residence time: 20 minutes Temperature: 275°C 250°C 275°C 300°C 10 minutes 60 minutes**

**Properties Raw** 

*Ultimate analysis of torrefied corncob (wt.%)*

*Yield of pyrolysis products (wt.%)*

*Physical properties of bio-oil*

**Table 7.**

articles. They determined the effects of pyrolysis temperature and torrefaction time, as a pretreatment, on the characterization of produced total bio-oil and syngas. Total bio-oil is the sum of condensed liquids from torrefaction and pyrolysis. **Figure 6** shows that bio-oil yield from raw (untreated) biomass ranged from 31 [107] to 53% [101] for pyrolysis at 400 and 450°C, respectively. The bio-oil yield from a 450°C pyrolysis decreased to 51 and 46% for 8 and 15 minutes torrefaction pretreatment at 275°C was carried out. Longer torrefaction reduced slightly the yield of bio-oil. Torrefaction also altered the compositions of syngas by reducing CO2 and increasing H2 and CH4. The syngas produced from pyrolysis step was rich in H2, CH4 and CO implying that the syngas quality was significantly improved by torrefaction process [105]. The quantity of syngas increased with the severity of

*Effect of temperature and residence time of corncob torrefaction on ultimate analysis of feedstock, the yield of* 

**Table 8** summarizes a qualitative analysis of benefits of feedstock thermal pretreatment on logistical and quality of bio-oil properties. Column 1 lists major properties that a feedstock would gain as a result of thermal pretreatment. The degree of change in properties depends upon the severity of thermal treatment. Column 2 lists the benefits of these properties on improving conversion efficiency or the quality of produced bio-oil when the pretreated feedstock is pyrolyzed. Column 3 outlines the benefits of feedstock properties from a logistical perspective, that is, handling, transport, storage, unit operations like blending and feeding the feedstock to the pyrolysis reactor. Finally, column 4 lists the potential monetary benefits of thermal pretreatments mostly due to

**70**

the torrefaction.

improvements in logistics.

*Bio-oil yield from raw and torrefied Douglas fir at the various duration and temperature [101, 107].*


#### **Table 8.**

*Overview of benefits of torrefied feedstock for bio-oil production.*
