**1. Introduction**

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162 Biofuels - Status and Perspective

#### **1.1. Biofuels and feed stocks**

The limitation on the availability of petroleum based fuels and environmental concerns have made humans look at alternative fuels such as biofuels, wind energy, solar energy, hydro‐ electric power, hydrogen, nuclear energy and so on. Among all these alternative energy sources, biofuels are one of the potential alternative energy sources that can fulfill a part of the current energy demand. The concept of biofuels dates back to the 19th century when researchers tried to use vegetable oil as a fuel for diesel engines. But the problems associated with the usage of oils directly as fuel and the availability of fossil fuels made the concept of vegetable oil fuels uneconomical. Due to rapid depletion of petroleum based fuels, ever increasing consumption of fuels in developing nations and environmental concerns led scientists and governments to develop fuels based on oils or lipids. These oils or lipids cannot be used directly as liquid fuels but can be processed and/or upgraded to be used as a blend or as a direct substitute for the regular fuel. Biodiesel, green diesel, bio-jet fuel or green jet fuel and green gasoline are examples of renewable transportation fuels. Vegetable and plant oils, animal fats, and waste cooking oil from food processing industries have been used as a primary feedstock for the production of renewable transportation fuels. Along with these sources, micro algae have also attained significant attention as a source of oil/lipids, due to high productivity of oil compared to plants in a given period of time and area [1]. Apart from the higher lipids, algal biomass is being used as a source for the production of proteins, amino acids and carbohydrates [2, 3]. Cultivation of algal biomass in waste water treatment plants which contribute to the cleaning of water and production of fuels. In spite of having more oil yield than plant crops, algae

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feedstock has its own problems of processing the biomass such as harvesting, drying and extraction of oil to produce biofuels. Various processing technologies are available to produce these bio fuels from different feed stock materials. This chapter focuses on the production of biodiesel from oils/fats and wet algal biomass through supercritical alcohol transesterification, novel methods for extraction of oil/lipids from wet algal biomass, liquefaction of whole algal biomass through hydrothermal extraction and liquefaction (HT E&L) and catalytic hydro‐ thermolysis to produce regular hydrocarbon fuels from oils using supercritical water. Before discussing the biofuels production, the sub and super critical technologies will be discussed.

### **1.2. Sub and supercritical conditions**

As shown in figure 1, the four phases of a pure material or compound can be observed at different temperature and pressure conditions. When a compound is heated above its boiling point and below its critical point under pressure, it is called a subcritical fluid and when a compound is heated above its critical point is called as supercritical fluid. The sub critical and supercritical fluids possess different physical-chemical properties compared to their proper‐ ties at normal conditions. They are compressibility (like gases) due to reduced densities, increased polarity due to reduced dielectric constants and they have catalytic properties attained by variations in ion dissociation constants. Above the critical point the particular material obtains gas like densities, liquid like solvating properties and intermediate mass transfer kinetics [4]. By varying temperature and pressure, these enhanced capabilities of the sub and supercritical fluids are being used for environmentally benign selective separations, catalytic reactions for production or purification of various products. The commonly known supercritical fluids are water, CO2, ethanol, methanol, ethane, methane etc. In this chapter the utilization of water, methanol, and ethanol to produce various kinds of biofuels or fuel intermediates will be discussed.

**Figure 1.** Phase diagram of a pure compound
