**4. Upgrading of bio-oil in pyrolysis and hydrothermal liquefaction**

The bio-oil obtained from the HTL and pyrolysis process is considered a best-suited alternative to petroleum if and only if the quality of the bio-oil is enhanced. The biooil extracted after the thermochemical conversion process contains phenols, acids, aldehydes, N, and O heteroatoms which confer thermal stability and corrosion. The use of bio-oils is restricted due to the high oxygen content, strong acidity, and high calorific value of bio-oil. Due to these reasons, the up-gradation of bio-oil is essential, which involves enhancing the quality of bio-oil to use in transportation.

#### **4.1 Emulsification**

The simple upgrading method involves the emulsification of bio-oil with other fuels. However, bio-oil is immiscible with petroleum-based fuels and can be emulsified with biodiesel using surfactants. As a liquid fuel, upgrading bio-oil by emulsifying it with diesel oil reduces viscosity and enhances the calorific value and cetane number [45, 46].

Therefore, the use of a cheap and appropriate emulsifying agent is essential in biooil upgrading through emulsification. A study said emulsions of bio-oil with biodiesel and showed that the production of the most stable emulsion was acquired using the surfactant class polyethylene glycol-di-polyhydroxy stearate (PEG-DPHS), having an HLB number of 4.75 and a mass ratio of 32:8:1 diesel: bio-oil: surfactant. Even while using the co-surfactant SPAN80 in addition to the surfactant showed that the ability to solubilize bio-oil in diesel increases with increasing cosurfactant/surfactant ratio

[46]. When compared to the original bio-oil, in case of diesel emulsions possessed more fuel properties. These are very simple and rapid upgrading methods but expensive due to the addition of surfactant and high energy costs.

#### **4.2 Esterification**

Esterification or otherwise called alcoholysis, is the process of conversion of free fatty acids into their respective alkyl esters. The bio-oil produced contains organic acids, which contributes to acidity, instability, and a high degree of unsaturation and can be reduced by the process of esterification. The reaction between the fatty acids and alcohol at atmospheric pressure with the help of catalysts gives rise to the formation of alkyl ester or biodiesel. Bio-oil also consists of aldehydes possessing challenges for bio-oil upgrading through esterification [40, 41]. In some study, ozone oxidation technology is used to pretreat bio-oil for the conversion of aldehydes into acids. And another through the experiment demonstration the two-step esterification-hydrogenation process showed better performance in bio-oil upgrading than the one-step esterification-hydrogenation process, and it provides higher alcohol and more stable compounds [42, 43].

### **4.3 Hydrogenation**

Bio-oil derived after the thermochemical conversion process contains high oxygen content, which can be removed using high-pressure hydrogen, known as hydrogenation. The hydrogenation reaction is carried out during hydrotreating which increases the hydrogen content, thereby increasing the quality of bio-oil. Hydrotreating is a refinery process that aims to reduce bio-oil's N, O, and S contents. Using a catalytic process with high-pressure hydrogen, it eliminates oxygen as water. In a similar way, high consistency of pure nitrogen enhances to form ammonia synthesis. The energy and heat basically utilized here are recirculated easily and recovers it for power generation [44, 47]. Whereas in another case, two-step esterification hydrogenation even helps in upgrading the bio-oil. It basically helps to degrade the active compounds mostly acids and ketones and rather helps in raising the contents of alcohols and esters [48].
