**2. Pyrolysis**

Pyrolysis is a thermal process to convert biomass particles to secondary solid, liquid and/or gaseous fuels [10]. The liquid bio-oil obtained from pyrolysis of biomass shows promising properties to substitute the conventional fossil fuels. The breakdown of biomass particles takes place in a pyrolysis reactor by exposing them to an oxygen-free or deficit environment. As the temperature of the particle increases, different stages of thermal treatment occur. Drying happens first when a moist particle is heated up to temperatures of about 150°C [11]. At temperatures of 200–300°C and in the absence of oxygen/air, various degrees of torrefaction happen and release volatile [12–14]. At temperatures higher than 300°C, more severe decomposition of biomass happens and structural changes such as a change in chemical composition and porosity of particles occur in the solid matrix.

Pyrolysis processes are generally categorized as "slow" and "fast" according to the time taken for processing the feed into pyrolysis products. Slow pyrolysis utilizes low temperatures of 300–400°C over a residence time of 30 minutes to hours to maximize char formation. Fast pyrolysis is a rapid thermal decomposition process to maximize the liquid fraction of products. In industrial fast pyrolysis, biomass particles would be exposed suddenly to an environment of 400–600°C to heat up rapidly. The combined effects of moisture content, particle size [15–17] and reactor temperature [18, 19] make a non-linear temperature profile inside the particle which affects the rate of conversion. The residence time of the volatile fraction in the pyrolysis reactor and its contact with solid particles should be less than 2–3 seconds to prevent the secondary reactions. Pyrolysis takes place in a complete absence of oxygen. The volatiles are quickly removed and quenched to maximize liquid yield. **Figure 1** displays a schematic diagram of an industrial pyrolysis process. In the following sections, a comprehensive literature review is presented to discuss the effects of various parameters related to feedstock and pre-treatments on the efficiency of the pyrolysis process.

#### **2.1 Feedstock composition**

A plant-based dry biomass has the main fractions of cellulose, hemicelluloses, lignin, extractives and minerals. Wood extractives, or wood extracts, are low molecular weight molecules that are extracted from wood using solvents or other extraction methods. The extractives are the waxes, fatty acids, resin acids and terpenes of a tree. **Table 1** lists the composition of a broad range of biomass species. The composition of biomass changes the chemical composition and heating value of pyrolysis products [33]. Higher lignin content reduces the bio-oil yield at

**61**

*1 Cellulose. 2*

*3 Lignin.*

**Table 1.**

*Hemicellulose.*

**Figure 1.**

*condenser.*

**Biomass species**

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

*Schematic diagram of the pyrolysis process, including drying, grinding, pyrolyzer, separation stages and* 

Pine wood 42.1 27.7 25.0 [20] 45.9 5.3 48.2 0.35 18.98 [21] Beech wood 41.7 37.1 18.9 [22] 46.7 5.7 47.6 1.04 – [22] Douglas fir 50.0 17.8 28.3 [23] 50.7 6.0 42.5 0.50 20.07 [24] Spruce wood 41.1 20.9 28.0 [20] 48.9 6.0 44.6 0.30 19.26 [20] Poplar aspen 49.9 22.4 18.1 [25] – – – – – – Poplar wood 42.2 16.6 25.6 [26] 48.4 5.8 43.7 1.43 19.71 [26] Birch wood 35.7 25.1 19.3 [20] 49.0 6.3 44.1 0.30 18.40 [27] Corn stover 36.4 22.6 16.6 [28] – – – – – – Corn cob 52.0 32.5 15.5 [29] 49.0 5.6 43.8 1.10 – [29] Wheat straw 38.2 24.7 23.4 [28] 40.7 5.8 52.9 10.58 16.24 [30] Rice straw 34.2 24.5 23.4 [31] 36.9 5.0 37.9 11.70 16.78 [31]

**(wt.%)**

**Ash (wt.%)** **HHV (MJ/ kg)**

**Refs.**

**Composition (wt.%) Refs. Ultimate analysis** 

**Cel.1 Hem.2 Lig.3 C H O**

the expense of higher biochar yield in the pyrolysis process [29]. The extractives have the highest heating value in the woody biomass. The heating values of biomass components are 17–18, 16–17, 25–26 and 33–38 GJ/tonne for cellulose, hemicellu-

*Composition and ultimate analysis, ash content and calorific value of various biomass feedstock.*

Olive husk 25.2 24.2 50.6 [29] 50.2 6.4 38.4 4.10 – [29] Tea waste 33.2 23.3 43.5 [29] 48.2 5.5 44.3 1.50 – [29]

Each sub-component of the biomass has a specific thermal reactivity. For example, cellulose decomposes at 300–400°C; hemicellulose decomposes at 200–300°C and lignin decompose continuously in the wide temperature range of

lose, lignin and extractives, respectively [34].

Rice straw 32.1 26.5 12.5 [32]

*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 1.**

*Biomass for Bioenergy - Recent Trends and Future Challenges*

the post-treatment fuel upgrading.

**2. Pyrolysis**

solid matrix.

the efficiency of the pyrolysis process.

**2.1 Feedstock composition**

heterogeneous in physical, chemical and thermal properties; high in moisture [6], mineral [7] and oxygen contents [8]; highly hygroscopic [9] and difficult to handle [6]. Converting biomass to secondary liquid or gaseous fuels through thermal conversion processes is a way of increasing the energy density and transportability. Mechanical, chemical and thermal pretreatments are able to modify the biomass properties in order to produce a more homogeneous fuel and minimize

The current article reviews the feedstock properties that are important for the fast pyrolysis process and the potential pretreatments to modify the biomass properties are explained. Specifically, drying, grinding, washing and torrefaction processes and their influence on the final bio-oil product are thoroughly explained.

Pyrolysis is a thermal process to convert biomass particles to secondary solid, liquid and/or gaseous fuels [10]. The liquid bio-oil obtained from pyrolysis of biomass shows promising properties to substitute the conventional fossil fuels. The breakdown of biomass particles takes place in a pyrolysis reactor by exposing them to an oxygen-free or deficit environment. As the temperature of the particle increases, different stages of thermal treatment occur. Drying happens first when a moist particle is heated up to temperatures of about 150°C [11]. At temperatures of 200–300°C and in the absence of oxygen/air, various degrees of torrefaction happen and release volatile [12–14]. At temperatures higher than 300°C, more severe decomposition of biomass happens and structural changes such as a change in chemical composition and porosity of particles occur in the

Pyrolysis processes are generally categorized as "slow" and "fast" according to the time taken for processing the feed into pyrolysis products. Slow pyrolysis utilizes low temperatures of 300–400°C over a residence time of 30 minutes to hours to maximize char formation. Fast pyrolysis is a rapid thermal decomposition process to maximize the liquid fraction of products. In industrial fast pyrolysis, biomass particles would be exposed suddenly to an environment of 400–600°C to heat up rapidly. The combined effects of moisture content, particle size [15–17] and reactor temperature [18, 19] make a non-linear temperature profile inside the particle which affects the rate of conversion. The residence time of the volatile fraction in the pyrolysis reactor and its contact with solid particles should be less than 2–3 seconds to prevent the secondary reactions. Pyrolysis takes place in a complete absence of oxygen. The volatiles are quickly removed and quenched to maximize liquid yield. **Figure 1** displays a schematic diagram of an industrial pyrolysis process. In the following sections, a comprehensive literature review is presented to discuss the effects of various parameters related to feedstock and pre-treatments on

A plant-based dry biomass has the main fractions of cellulose, hemicelluloses,

lignin, extractives and minerals. Wood extractives, or wood extracts, are low molecular weight molecules that are extracted from wood using solvents or other extraction methods. The extractives are the waxes, fatty acids, resin acids and terpenes of a tree. **Table 1** lists the composition of a broad range of biomass species. The composition of biomass changes the chemical composition and heating value of pyrolysis products [33]. Higher lignin content reduces the bio-oil yield at

**60**

*Schematic diagram of the pyrolysis process, including drying, grinding, pyrolyzer, separation stages and condenser.*


#### **Table 1.**

*3 Lignin.*

*Composition and ultimate analysis, ash content and calorific value of various biomass feedstock.*

the expense of higher biochar yield in the pyrolysis process [29]. The extractives have the highest heating value in the woody biomass. The heating values of biomass components are 17–18, 16–17, 25–26 and 33–38 GJ/tonne for cellulose, hemicellulose, lignin and extractives, respectively [34].

Each sub-component of the biomass has a specific thermal reactivity. For example, cellulose decomposes at 300–400°C; hemicellulose decomposes at 200–300°C and lignin decompose continuously in the wide temperature range of 180–600°C [35]. This specification directly influences the temperature range at which the material thermally decomposes and consequently the optimum pyrolysis temperature at which the maximum bio-oil yield is obtained. **Table 2** lists pyrolysis liquid yield of various biomass species at tested temperatures and depicts the variability of produced bio-oil yield among the pyrolysis of different feedstocks. Apart from variability in conversion rate among species, the conversion rate increases with the reaction temperature.

In addition to the chemical composition, the elemental and proximate analysis of the biomass feedstock changes the properties of the produced fuel. From the elemental point of view, the biomass is mainly contained of carbon, hydrogen and oxygen (**Table 1**). The composition of elements affects the storage properties of the liquid fuel. Compared to conventional fossil fuels, biomass has a high amount of ~40–50% oxygen content. High oxygen content makes the produced liquid fuel unstable, corrosive and consequently not qualified for transportation and storage [16].

The minor elements present in the biomass material are minerals such as potassium, chlorine, sulfur, silicon, calcium and magnesium [45]. Minerals are present in all biomass species, in a much lower amount than carbon, hydrogen and oxygen elements. Agricultural biomass has much more mineral contents than woody biomass. In a thermal process, minerals turn to ash (**Table 1**). The pyrolysis biochar typically contains up to 90% of the biomass minerals [46]. Ash shifts the size distribution of the char to smaller sizes that make their recovery from the gas stream challenging. An incomplete separation of char and volatiles causes continuous secondary reactions in the liquid phase [16, 47, 48] that accelerates the aging phenomenon and contribute to its instability [49, 50]. Aging phenomena is defined as a slow increase in viscosity bio-oil resulting from secondary reactions [16]. Minerals have a catalytic effect on the rate of secondary reactions [51].
