The only renewable energy sources that can directly replace fossil fuels for current and future energy shortages are biofuel and biomass. These sources are environmentally favorable and renewable [18]. Generally, there are three generations of biofuels categorized based on their sources. Biofuels which are obtained directly from the food source are termed as first generation biofuels such as those that have been manufactured from the biomass comprising sugar, starch, and vegetable fats and oils [19]. The biofuels which fall under the category of the second generation are those that are produced from the plant biomass, which is mostly comprised of lignocellulosic materials, as this builds up most of the economical and ample nonfood compounds accessible from plants. But, in the present situation it is not economical to produce these fuels as there is a large number of mechanical obstacles that need to be avoided before their perspectives can be considered [20]. Second-generation biofuels, for instance, ethanol and methanol created from woody biomass, are more energy productive and more adaptable concerning their feedstock. The likelihood to utilize cellulosic and heterogeneous biomass recommends lower costs [21]. In any case, the ecological effect raised from biofuel combustion extraordinarily affects the carbon cycle (carbon balance), which is connected with the ignition of petroleum derivatives. Furthermore, the weariness of various existing biomass without suitable compensation brought about colossal biomass shortage, arising ecological issues like deforestation and biodiversity loss [22,23,24]. In an inquiry for feasible and practical options in contrast to non-renewable energy sources, past investigations have detailed the predominant abilities of green algae inferred biomass for the development of a better form: the third-age biofuels [25].
Available processes that carry out the transformation of algae-derived biomass to diverse energy resources are categorized as thermochemical conversion, biochemical conversion, chemical pathways, and direct combustion (Fig. 2). Algal biomass can be converted into biofuels with the help of thermochemical methods including pyrolysis, gasification, and superficial liquid extraction, or by carrying out hydrothermal liquefaction. Apart from the sugars and lipids, all of the algae-derived biomass can be converted into biofuels using these techniques. After cultivation, if the further processing of algal biomass involves the use of the thermochemical method, there is no need of employing special surroundings such as nitrogen scarcity during the cultivation process in hope of achieving maximum content of lipid [100]. The process of gasification involves the algae-based biomass reaction in a gasifier under the partial oxidation of air. This process is carried out with any kind of combustion and in the presence of oxygen, air, or steam. A number of other downstream methods also accompany this traditional method. The gasification process along with some other downstream processing techniques eventually results in the production of carbon monoxide, methane, and hydrogen, combined with definite undesired by-products that are solid [101].
fuels and combustion by sharma and chander mohan pdf 98
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