Pyrolysis converts wood biomass into solid (charcoal), liquid (bio-oil) and gaseous products by using fast pyrolysis mostly in the absence of oxygen at 500 °C. This technology is used in many industries on a wide scale [6, 7]. Gasification, on the other hand, is used to convert combustible gas mixture by partial oxidation process at high temperature approximately 800 °C. It produces an important fuel as syngas which is used in the production of methanol [8]. Liquefaction is a little different than the previous two techniques because of the conversion of biomass into valuable liquid fuels at low temperature within the range of 150–420 °C and high pressures as 1–240 bars [9, 10]. This valorization technology is more useful to improve yield through homogeneous/heterogeneous catalysts. Pyrolysis, gasification and liquefaction processes are considered as thermo-chemical conversion methods.
The biochemical route consists of anaerobic digestion and fermentation processes at low temperature which converts animal wastes, sewage sludge, agricultural residues into bio fuels such as biogas and bioethanol. It is widely used to treat organic wastes because of its large numbers of benefits such as increasing nutritional recovery of fertilizers [11, 12].
Except these technologies, there many more physico-chemical conversion processes such as Esterification/Transesterfication of vegetable oils, animal fats, and waste oil products which provides glycerol and liquid fuels as products. The obtained products with operating conditions of aforementioned techniques are given in Table 1.2.
1.2.3 Economic Aspects of Biomass Utilization
Biomass may be utilized either directly or indirectly by industries and it participates definitely in the economy of all nations because the energy generated due to biomass is used in cooking, space heating, and many industrial processes, etc. According to “The Biomass Energy Resource Centre (BERC)”, the utilization of biomass as natural and wasted can be beneficial in terms of economic through many ways to the communities, schools and colleges, central, state and local governments, small and large businesses, and other utilities. Some of the economics related benefits are given below:
1 (i) Generation of many new jobs in the region’s economy because of biomass processes’ utilization at local level.
2 (ii) It can boost the economy because it replaces dependency of conventional fuels.
3 (iii) It can remove the import of fuels from outside countries and minimizes extra expenses.
4 (iv) It can overcome the harmful effects on health and environment generated due to use convention fuels and provides a platform for the scope of a new green revolution.
5 (v) Initial investment is high but in long terms, the valorization technologies of biomass are sustainable and profitable.
6 (vi) It can minimize the generated wastes of biomass and promotes conversion of wastes into useful energy and energy ids the key fact in any country’s economy directly and indirectly.
7 (vii) Many industries can boost their profits after utilizing wastes through direct use of biomass by applying these valorization techniques. If properly managed these things, industrial growth will be coming in the picture.
1.3 Photocatalysis & Photocatalyst
As a subtype of catalysis, photocatalytic reactions are carried out under ambient conditions of temperature and pressure to produce costly chemical products. IUPAC defined photocatalytic reactions as a “change in the rate of chemical reaction under the action of ultraviolet, visible or infrared radiation in the presence of a substance—named as “Photocatalyst” with its main function which is to absorb light radiation for the transformation of reactants [5].
For simple example, Chlorophyll is an essential photocatalyst which exists in plants as a natural substance. In photocatalysis process, the chlorophyll allows plants to absorb energy from light which is used to convert carbon dioxide and water into glucose together with nutrients. In brevity, photocatalysis in plants is a photochemical reaction which provides oxygen with the help of a photocatalyst, released by plants or trees into air. A schematic diagram for this process in which facilitation of chemical reaction from solar light radiation due to photocatalyst is given is shown in Figure 1.3 [13].
In short, photocatalysis is the activity in which light radiation intersects on the surface of a specific substance to carry out chemical reactions such as oxidation and reduction reactions.
Here, specific substance is known as “Photocatalyst” which is quite responsible to attain enough energy level to absorb those incident heat waves for modifying the state of reacting molecules into valuable chemical products. Photocatalysis has two types:
1 (A) Homogeneous photocatalysis/photochemical reactions
2 (B) Heterogeneous photocatalysis/photochemical reactions.
Figure 1.3 Photosynthesis process in plants [13].
In homogeneous photocatalysis, the reactants and photocatalysts are available in the same phase. Acid catalysis, organo-metallic catalysis, and enzymatic catalysis are more common homogeneous photocatalysis. Ozone and photo-Fenton systems are prominent homogeneous photocatalysts in nature.
In heterogeneous photocatalytic reaction, the reactant and photocatalyst are present in different phases. These reactions include dehydrogenation, metal depositing, removal of gaseous pollutants, water detoxification, oxidation hydrogen atom transfer, etc. Most commonly heterogeneous photocatalysts are semiconductors and transition metal oxides. In heterogeneous photocatalysis, titanium dioxide TiO2 is the most recognized and studied photocatalyst. It is highly efficient for removing severely toxic and non-biodegradable organic contaminates of air or water. TiO2 is well-known as superior photocatalysts compared to others due to its versatile characteristics, such as cost effectivity, safety in use, highly stability, high photocatalytic activity at ambient conditions, i.e. temperature.
1.3.1 Mechanism for Photocatalytic Conversion of Biomass
Usually, photocatalysis depends on the substrate type present on the surface of semiconductor. In primary steps of photocatalysis, several observations are important such as generation of charge carrier, solar incident photon radiation or its absorption, charge separation and charge trapping. Many researchers reported that the separation of product and less stability of photons in homogeneous systems are tough to analyze. Although the predictability about the interaction between reactants and heterogeneous photocatalysts is easy than interaction of the heterogeneous photocatalysts with reactants the efficiency
