**2.3 Pyrolysis**

Pyrolysis defines as thermal decomposition in the absence of oxygen to break biomass chemical bonds in high temperatures to produce biofuels [46]. Depending on process requirement and desired product the process temperatures vary between 280 to 1000°C [47, 48]. During the process, generally three-step mechanisms including de-hydrogenation, de-polymerization and fragmentation occur to transfer biomass to biofuel [49]. The percentage of main products including bio-oil and bio-char and bio-syngas as the byproduct differ depending on heating rate, solid residence time, and temperature as the main operational parameters in the process [50]. Lower pyrolysis temperatures and longer residence times (Slow pyrolysis) tend to produce more bio-char while high temperatures and longer residence times increase the production of gas. Moderate or high temperatures and short residence times (Fast and Flash pyrolysis) resulted in more bio-oil [51]. Several technologies and reactors with the semi-continuous or continuous process have been developed on a laboratory scale and considered as suitable reactors for commercialization of pyrolysis including Bubbling Fluidized Bed (BFB), Circulating Fluidized Bed (CFB), Circulating Spouted Bed (CSB), Rotary Cone (RC), Ablative reactor and Screw/ auger reactor [52]. In addition, plasma pyrolysis reactor configuration, Vacuum pyrolysis, Microwave-assisted, and solar-assisted pyrolysis have been extensively investigated as the state-of-the-art technologies related to biomass pyrolysis which demonstrates their advantages over conventional electrical-heating-assisted biomass pyrolysis [53, 54].

The higher heating value (HHV) of the bio-oils normally ranges between 15 and 20 MJ/kg which is only 40–50% of the conventional petroleum fuels with HHV between 42 to 45 MJ/kg. The HHV of the bio-oils can be approximately calculated through some empirical equations formulated by elemental analysis of the bio-oil (CHNOS analysis plus ash content**)** as represented in (Eq. (1)) [49].

*Biomass and Energy Production: Thermochemical Methods DOI: http://dx.doi.org/10.5772/intechopen.102526*

$$\text{HHV} \left( \frac{\text{MJ}}{\text{kg}} \right) = 0.3491 \times \text{C} \star \text{1.1783} \times \text{H} \star 0.1005 \tag{1}$$
 
$$\times \text{S-0.1034} \times \text{O-0.0151} \times \text{N-0.0211} \times \text{Ash}$$

Since the liquid bio-fuel which contains oxygenated compounds such as acids, alcohols, phenols, ketones, and esters is commonly considered as poor quality, thus, it requires upgrading into a higher value-added product through promising methods such as catalytic steam reforming. The process of bio-oil quality upgrading and the water gas shift (WGS) reaction is presented in (Eq. (2)) and (Eq. (3)) respectively [55].

$$\mathrm{C\_nH\_mO\_k + (n-k)H\_2O \oplus nCO + \left(n + \left(\frac{m}{2}\right) \cdot k\right)H\_2} \tag{2}$$

$$\text{CO} + \text{H}\_2\text{O} \rightleftharpoons \text{CO}\_2 + \text{H}\_2\tag{3}$$

#### **2.4 Gasification**

In the condition in which production of biogas fuel is required, the gasification process under a reduced oxygen atmosphere applies to convert solid biomass to a gaseous fuel known as synthesis gas [56]. The biomass gasification process is conducted in four main stages including drying of the biomass particles followed by pyrolysis of the dried biomass particles(de-volatilization), in the next step partial oxidation of the pyrolysis gases and/or char occurred and finally char gasification happened (reduction). In contrast to pyrolysis, the feed is brought into contact with a gasifying agent (air) to ease the reaction between oxygen and biomass content in higher temperatures between 600°C and 1500°C. The produced gas contains various percentages of CO, H2, CH4, CO2, H2O, N2, and eleven other gases depending on the quality of the biomass used and the way gasification is conducted [57, 58]. Fixed bed, fluidized bed, entrained flow, rotary kiln reactor, and plasma reactor can be utilized based on the operational conditions in gasification [59]. Briefly, biomass feedstock type and composition, particle size, moisture and ash content (higher ash content cause ash agglomeration during the process especially in high temperature), operational temperature, pressure and residence time, gasifying media, equivalence ratio (actual air-to-biomass ratio), steam-to-biomass ratio (S/B) and finally catalyst type and amount are the most prominent factors during the gasification process [60].

According to the **Figure 2** Biomass pass their steps of drying, pyrolysis, and partial oxidation before reach to the gasification point. Each stage is accrued in a specific range of temperature [61]. After the drying step, biomass is decomposed to solid char and pyrolysis which will be faced with the second decomposition stage and conversion into decomposes gases (non-condensable) and volatile hydrocarbons. Then, these products react with the oxidizing agent to produce syngas and smaller amounts of lower hydrocarbon gases (C1–C4) [62]. The global reaction inside the gasifier (except for unconverted solid carbon) can be described as (Eq. (4)) while for simplicity only the amount of hydrogen, carbon, nitrogen, oxygen, and sulfur of the biomass are considered in the model [58]:

**Figure 2.** *Gasification steps and the temperature zones.*

$$\begin{aligned} \text{CH}\_{\text{a}}\text{O}\_{\text{b}}N\_{\text{c}}S\_{\text{d}} + \text{wH}\_{2}\text{O}\_{(l)} + \text{sH}\_{2}\text{O}\_{(g)} + \text{e}\text{O}\_{2} + \text{p}\text{e}N\_{\text{2}} &\leftrightarrow (1-\alpha)\text{C}\_{\text{s}} \\ + \text{n}\_{\text{co}}\text{CO} + \text{n}\_{\text{CO}\_{2}}\text{CO}\_{2} + \text{n}\_{\text{O}\_{2}}\text{O}\_{2} + \text{n}\_{\text{H}\_{2}}\text{H}\_{2} + \text{n}\_{\text{H}\_{2}\text{O}}\text{H}\_{2}\text{O} + \text{n}\_{\text{CO}\_{2}}\text{CO}\_{2} \\ + \text{n}\_{\text{N}\_{\text{1}}}\text{N}\_{\text{2}} + \text{n}\_{\text{NO}}\text{NO} + \text{n}\_{\text{NO}\_{2}}\text{NO}\_{2} + \text{n}\_{\text{NH}\_{3}}\text{NH}\_{3} + \text{n}\_{\text{HCN}}\text{HCN} \\ + \text{n}\_{\text{H}\_{2}}\text{H}\_{2}\text{S} + \text{n}\_{\text{SO}\_{2}}\text{SO}\_{2} + \text{n}\_{\text{SO}\_{3}}\text{SO}\_{3} + \text{n}\_{\text{co}\text{CO}} \end{aligned} \tag{4}$$

One of the important issues during gasification is the removal of tar which is formed during the pyrolysis stage (as a transition step toward the gasification). Various tars components are released which can condense and form sticky deposits by quenching downstream when they contact cold points of the gasification system [63]. Tar roots severe damage to gas engines or turbines through fouling and coking in the system. Therefore, it is very important to reduce the tar content and particulate matter, in the syngas below the level of 100 mg/m3 and 50 mg/m3 respectively to apply for gas engine consumption [64]. Therefore, even though gasification is a relatively well-known technology, the share of gasification in overall energy demand is insignificant due to barriers concerning biomass harvesting and storage, biomass pre-treatment (drying, grinding, and densification), gas cleaning (physical, thermal or catalytic), process efficiency and syngas quality issues [65].
