*Carbon (C), Oxygen (O), Hydrogen (H), Sulfur (S), Nitrogen (N), Volatile matter (VM), Fixed carbon (FC), moisture (M), and ash (A) content.
Biomass is recognized as renewable energy carriers in terms of electrical or heat energy and transportation of fuels to replace the need of fossil fuels so that the harmful effects of environment can be limited but it all depends on quality and quantity of biomass feedstocks. Moreover, several characteristics should be kept in mind to evaluate the performance of any biomass feed which are listed below:
1 (i) Moisture content,
2 (ii) Calorific value,
3 (iii) Ash content,
4 (iv) Proportion of fixed carbon and volatile substances,
5 (v) Alkali metal content,
6 (vi) Cellulose/lignin ratio,
7 (vii) pH, etc.
1.2.2 Biomass Valorization Techniques
Biomass is converted into useful energy after applying various technologies into some valuable products like sugars, lignin, glycerol, etc. Generally, biomass is just decomposed/burned directly to generate energy in industries but in this process, fewer benefits can be achieved. Therefore, some important techniques which valorize the biomass utilization are employed such as pyrolysis, gasification, liquefaction, and biochemical processes [5]. These non-photocatalytic biomass valorization techniques can upgrade the quality and provide variety of products with maximum utilization of biomass feedstocks, if the participation of catalysts is possible.
Table 1.2 Biomass valorization techniques with important aspects [5].
| Biomass valorization techniques | Feedstocks | Temperature | Products |
|---|---|---|---|
| Pyrolysis | Cellulose, hemicelluloses, and lignin | 500 °C | Charcoal, bio-oil and gaseous fuels |
| Gasification | Combustible gas mixture | 800 °C | Syngas |
| Liquefaction | Agro-residue and algal biomass | 150–420 °C | Liquid fuels such char, fertilizers, biofuels |
| Biochemical methods | Organic biomass wastes, agricultural wastes | Low temperature | Biogas, bioethanol |
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