**2. HRDD as a renewable fuel**

HDRD, otherwise called renewable diesel, green diesel, and hydrotreated vegetable oil, is a second-generation liquid biofuel. HDRD is chemically identical to PBD fuel but not the same as biodiesel. Biodiesel, also referred to as Fatty Acid Methyl Ester (FAME), is a mono-alkyl ester mostly generated by the catalytic transesterification process, HDRD is a blend of straight-chain and branched paraffin hydrocarbons within the C15–C18 range. The similarities in properties of petroleum diesel and HDRD allow it to meet the automotive fuel specifications, seamless application of HDRD in CI engines, and use of the same transport infrastructure [18, 19]. The global production of HDRD grew from 1.5 billion liters in 2011 to 9.5 billion liters in 2017 and is projected to become 13 billion liters in 2024 [20, 21]. Also, due to attractive properties and advantageous utilization of HDRD, the production capacity and the share of

**Figure 2.** *Global HDRD production capacity and share in biodiesel production 2019–2022. Adapted from [22].*

*Performance and Emission Characteristics of Hydrogenation Derived Renewable Diesel… DOI: http://dx.doi.org/10.5772/intechopen.104820*

biofuel production have been increasing since 2019, globally (**Figure 2**). This trend is expected to continue.

To meet up with the growing demand and utilization of HDRD, many commercial production plants have been installed and commissioned using advanced technologies (**Table 1**). **Figure 3** shows the producer, capacity/year, and country of location of HDRD plants, worldwide. The HDRD is usually produced through catalytic


#### **Table 1.**

*HDRD production plants [20, 21, 23].*

#### **Figure 3.** *Locations, company, and capacity/year of major HDRD plants [21].*

#### **Figure 4.**

*Schematic diagram of HDRD production by hydroprocessing. Adapted from [25].*

hydroprocessing, decarboxylation, and/or decarbonylation of triacylglycerol. During hydroprocessing, hydrogen is applied for the removal of oxygen from the triglyceride molecules through decarboxylation and hydrodeoxygenation, depending on the catalyst selection and process conditions [24]. This can be accomplished either through a co-processing arrangement of a distillate hydroprocessing unit or by building a standalone unit as shown in **Figure 4**. **Figure 5** shows the reaction pathways for HDRD production.

$$\begin{array}{ll} \text{O}\_2\text{-C}\text{O}\cdot\text{R} \\ \text{H}\_2\text{-C}\text{O}\cdot\text{R} \\ \text{H}\_2\text{-C}\text{O}\cdot\text{R} + 3\text{H}\_2 & \xrightarrow{\text{Dearbopropion}} 3\text{CO}\_2 + 3\text{R-H} + \text{C}\_3\text{H}\_6 \\ \text{H}\_2\text{-C}\text{O}\cdot\text{R} + \text{C}\_2\text{H} & \\ \text{H}\_2\text{-C}\text{O}\cdot\text{R} & \\ \text{H}\_2\text{-C}\text{O}\cdot\text{R} + \text{H}\_2 & \\ \text{H}\_2\text{-C}\text{O}\cdot\text{R} + \text{OH}\_2 & \\ \text{H}\_2\text{-C}\text{O}\cdot\text{R} & \\ \text{H}\_2\text{-C}\text{O}\cdot\text{R} & \\ \end{array} \\ \begin{array}{ll} \text{O}\text{O}\text{ }+\text{3}\text{H}\_2\text{O} + 3\text{R-H} + \text{C}\_3\text{H}\_6 \\ \text{O}\text{ }+\text{3}\text{H}\_2\text{O} + 3\text{R-H} + \text{C}\_3\text{H}\_6 \\ \text{H}\_2\text{-C}\text{O}\cdot\text{R} \\ \text{H}\_2\text{-C}\text{O}\cdot\text{R} \\ \text{H}\_2\text{-C}\text{O}\cdot\text{R} + \text{H}\_2\text{O} \\ \text{H}\_2\text{-C}\text{O}\cdot\text{R} + \text{H}\_2\text{O} \\ \text{H}\_2\text{-C}\text{O}\cdot\text{R} \end{array}$$

**Figure 5.**

*Reaction pathways for HDRD production [26].*

*Performance and Emission Characteristics of Hydrogenation Derived Renewable Diesel… DOI: http://dx.doi.org/10.5772/intechopen.104820*


#### **Table 2.**

*Performance of some renewable feedstocks for HDRD production [12].*

Generally, HDRD can be synthesized from feedstocks such as sugar, starch, or cellulosic materials through various techniques like catalytic conversion, Biomass to Liquid, and pyrolysis. Also, vegetable oil, waste cooking oil, waste animal fats, recovered fats, and other triglycerides-bearing oils are converted into HDRD by pyrolysis and hydroprocessing. The outcome of the use of some renewable feedstocks such as waste cooking oil, animal fats, algae oil, jatropha oil, and Karanja oil have shown high product yield under moderate production conditions (**Table 2**). The conversion of triglycerides to HDRD through hydroprocessing entails chemical reactions such as hydrogenation, decarboxylation, decarbonylation, and hydrodeoxygenation reactions [12]. HDRD is produced in line with the methods and specifications of the American Society for Testing and Materials (ASTM) D975 and the European Committee for Standardization EN 590 [27]. **Table 3** shows the specifications and International Standards Organizations (ISO) test method for HDRD.

The cetane number of HDRD, a measure of the ignition quality of diesel fuel in CI engines, is usually between 820 Kg/m3 and 845 Kg/m3 and higher than PBD fuel and biodiesel. The high value of cetane number allows a CI engine fueled with HDRD to operate with higher thermal efficiency and at a lower fuel consumption [12]. The lower value of density, compared with biodiesel or PBD fuel indicates reduced volumetric heating value and increased fuel consumption. The high lubricity of HDRD ensures minimum engine wear, noiseless running, and smooth engine operation [12, 20].

CI engines are a form of an internal combustion engine. As heat engines, CI engines convert the chemical energy in the fuel into mechanical work [30]. The diesel fuel is passed into the engine through a fuel injector into the cylinder and mixed with preheated air where the mixture auto ignites due to the movement of the piston.


#### **Table 3.**

*Specifications and testing methods of HDRD [28, 29].*

The piston reciprocates between the Bottom Dead Center (BDC) and the Top Dead Center (TDC). The application of HDRD in CI engines makes the engine behave in a certain way and the efficacy of the fuel is measured in line with some set performance criteria and emission characteristics.
