**3. Various processes of thermochemical conversion from algal biomass**

The thermochemical conversion process is known to be an efficient method for the conversion of algal biomass into biofuel. It involves the thermal degradation of the biomass structure. From the evidence of many journals, chemical and biochemical methods are utilized in conversion to biofuel, whereas these days thermochemical conversion is also commonly used as it provides a more straightforward route to synthesize biofuel. The following thermochemical conversion processes are into Gasification, Pyrolysis, Direct combustion, Hydrothermal process, and Torrefaction. These processes also consist of demerits followed by merit points where the differences are shown in **Table 1**.

#### **3.1 Gasification**

As mentioned earlier, the gasification process is the partial oxidation of algal biomass that prefers to work only at high temperatures along with the combustible fuel. The syngas, basically produced by the gasification process, has a low calorific value of 4–6 MJ/m3 and can be used as a fuel for gas engines or gas turbines. The gasification process also produces hydrocarbon compounds which can be further converted into methanol *via.* The Fischer Tropsch conversion pathway. To effectively perform the gasification process, the moisture content of the biomass should be less than 14% [3]. In a study, it had pointed out that 40% of the moisture content in the algal biomass can be tolerated by the gasifier through the comparative performance analysis. It was also shown that this moisture content of the biomass is considered to be an important factor influencing the heating value of gas and even the high moisture content seriously affecting the performance of the gasifier. At 5% moisture content, the high heating value and the cold gas efficiency of the syngas produced are 5.138 MJ/kg and 73.81%. At 30% moisture content, it would be 3.338 MJ/kg and 44.24% [29].


#### **Table 1.**

*Comparison among various thermochemical processes.*

#### **3.2 Pyrolysis**

Through the process of pyrolysis, the algal biomass is converted into bio-oil, syngas, and charcoal in the absence of air. It is an anaerobic heating process, and heating can be done at a moderate temperature range of 350–700°C. The pyrolysis can be categorized into fast, flash, and slow pyrolysis on the basis of operating conditions. The production of bio-oil and biochar can be achieved by performing fast pyrolysis, and slow pyrolysis results in the production of pyrolytic gas and biochar. Slow pyrolysis having a heating rate range of 0.1 and 1°C/S with the sample particle size ranging between 5 and 50 mm,

allows the production of solid, liquid and gaseous products. Fast pyrolysis gives rise to liquid and gaseous products. Since having a high heating rate, flash pyrolysis gives liquid products [30]. Microwave-induced pyrolysis is carried out from the microalga *Scenedesmus almeriensis* in an electric furnace and showed that the microwave-induced pyrolysis gives rise to higher syngas and H2 production [22, 23].

### **3.3 Direct combustion**

The combustion process is said to be the easiest among all thermochemical processes. Both microalgae and macroalgae residues while heating follows into lipid extraction which is termed as an effective method [31–33]. Combustion is usually carried out at a temperature. But the capacity to carry out a temperature is around 800°C in the boiler, that furnaces or steam turbines and used to generate electricity. The major products generated after the combustion processes include CO2, H2O, and heat. The major disadvantage of this process involves that it requires pretreatment processes like chopping, drying, and grinding, which utilizes more energy and leads to high cost. Also the presence of impurities in biomass such as sodium, potassium, sulfur and nitrogen leads to problems with fouling and corrosion [34].

Various studies have been done in the combustion of microalgae. Among the study used *Haematococcus pluvialis* microalgae (M) and the chemical extraction residue (MR). A couple of TG-MS systems were used to investigate the combustion and emission properties of M and MR and the results revealed that the combustion of M and MR took place in three stages i.e. the decomposition of proteins, carbohydrates, lipids, and char was the first stage, followed by the volatilization of free water and a tiny amount of volatiles, and finally the decomposition of minerals. Whereas cocombustion of *C. vulgaris*, industrial waste of textile dyeing sludge (TDS) and their blends were also included in few of the studies [24, 26].
