1. Introduction

Figure 1. Comparison of first-, second-, third-, and fourth-generation biofuels, and petroleum fuels (adopted and modified from Naik et al. ^[13] and Dragone et al. ^[25]).

(i)

First-generation biofuel is primarily derived from parts of edible plants (i.e., grains and oilseeds). These types of fuel have derived from sugar, starch, vegetable oil, and fats. Examples of most popular first-generation biofuels are biodiesel, ethanol, biofuel gasoline, biogas, etc. ^{[7][13][24][26]}. Presently, first-generation biofuel (biodiesel and bioethanol) is mainly produced by using agricultural feedstock such as sugarcane, corn, sugar beets, etc. ^[23]. Economic feasibility of biofuel production using crops (such as oilseed crops) as feedstock is not cost effective presently, therefore, a more efficient approach is needed to enhance the biofuel production and convert it to an economically feasible stage. Additionally, more research work is needed to increase the biodiesel production using first-generation feedstock such as oil ^[23].

(ii)

Second-generation biofuel is a comparatively advanced biofuel which is derived from various non-food biomass of plant/or animal. Second-generation (lignocellulosic) biofuel is derived from non-edible plants or non-edible parts of the plants. It is well known that non-edible lignocellulosic biomass (such as vegetable grasses, forest residues, agricultural waste, etc.) is present abundantly in the natural ecosystem, therefore, it could be used as a feedstock for biofuel production. Examples of second-generation biofuels are lignocellulosic ethanol, butanol, mixed alcohols, etc. ^{[4][13][24][27]}.

(iii)

Third-generation biofuel is derived from photosynthetic microbes, e.g., microalgae. They derived from autotrophic organism. Here, carbon dioxide, light, and other nutrient sources are used in the synthesis of feedstock (biomass) which is further used in biofuel production ^{[8][24][25][28]}. Biofuels obtained from third-generation sources (such as microalgae) might be a better energy substitute as compared to previous generation biofuels, due to their short life cycle and less requirement of valuable agricultural land and resources for their growth ^[25]. Algae have rapid growth and higher rate of the photosynthesis compared to terrestrial plants used in first- and second-generation biofuel production. Due to their use in biofuel production, photosynthetic microbes (such as algae/microalgae) have recently received more attention from researchers worldwide ^[12].

(iv)

Fourth-generation biofuel is not common and at an under developmental stage since a few years ago. Here, genetically altered photosynthetic microbes (such as cyanobacteria, algae, fungi) are used as feedstock. Photosynthetic microbes have the ability to convert atmospheric CO₂ to biofuel ^[24]. Some studies reported that carbon capturing is undertaken by some crops, taken from the atmosphere and further stored in their leaves, stems, etc., which is further converted into fuel using second-generation techniques ^[12]. Alalwan et al. ^[24] reported that, in the fourth-generation biofuels, genetically modified microorganisms are used to obtain more carbon (HC) yield and reduced carbon emissions ^[24].

2. Second-Generation Lignocellulosic Biofuels

Researchers and companies have now shifted their attention towards second generation biofuel production, in response to the limitations found in the production and supply of first-generation biofuels. Second-generation lignocellulosic biofuels are produced by employing non-edible biomass (e.g., cellulose, lignin, and hemi-celluloses) in a more sustainable manner rather than first-generation biofuel. Examples of second-generation biofuels are Fischer–Tropsch fuels and cellulosic ethanol. Such fuels are either carbon neutral or negative when it comes to CO₂ concentration ^{[13][29][30][31]}.

Raw plant biomass material employed in the production of second-generation biofuels are generally referred to as lignocellulosic material and other non-food material of plants ^{[4][20][32][33][34][35]}. Such lignocellulosic raw material includes: (1) the by-products of plants (i.e., sugar cane bagasse, forest residues, and cereal straw); (2) the organic constituents of domestic waste; and (3) other forms of feedstock (i.e., crops, grasses, and short duration forests) (Figure 2).

Figure 2. Schematic representations of second-generation biofuel (adopted and modified from Naik et al. ^[13]).

Plant biomass is a widely and easily available biological resource for the raw materials for fuel ^{[13][33][36][37]}. There is considerable use of plant biomass in liquid biofuel production, due to these comprising cell walls composed largely of polysaccharides ^[13][37][38]. Badawy et al. ^[37] aimed to determine the most suitable biodiesel source among various sources such as Jatropha, rice straw, sugarcane, algae, etc. During their study, results showed that Jatropha was the most suitable biodiesel source ^[37]. Additionally, Arefin et al. ^[36] described biofuel production by floating aquatic plants, and discussed the methods related to biofuel production by aquatic plants (such as Azolla, duckweed, and water fern). Their observations showed that Azolla and water fern play a much better role in biofuel production as compared to other plants.

In addition to its direct use, second-generation biofuel can also be mixed with petroleum-based fuels in existing engines or used in slightly adapted vehicles with a compatible engine (e.g., vehicles for DME) ^[30].

It should be noted that the current production of such biofuels is not yet cost effective, due to various technical restrictions that require improvements in the methods employed ^[39].

3. Feedstock for Second-Generation Lignocellulosic Biofuels

Feedstock for second-generation lignocellulosic biofuels primarily consists of forms of biomass that are unfit for human consumption, hence, it does not compete with the production of food. Potential raw materials for second-generation biofuels consist of: (1) crop residue biomass; (2) non-food energy crops; (3) Jatropha; (4) wood residues; and (5) bacteria ^{[7][22][31][40][41][42][43]}.

Second-generation biofuels production can also be enhanced by the growth of bio-energy crops in locations unsuitable for the farming of food crops, leading to maximum utilization of marginal land for second-generation biofuel production. Improving current methods will also enable the efficient creation of biofuels from the inedible parts of crops and forest trees. In addition, there is a potential for using waste-products for processing second-generation biofuels.

Firouzi et al. ^[41] used a hybrid Multi-Criteria Decision Making (MCDM) approach for the screening of biomass for the biofuel production. They noticed that wastes from municipal sewage, forest, and poultry were the most important resources for biofuel production. Narwane et al. ^[42] also discussed the integrated MCDM approach in the biofuel industry.

This entry is adapted from the peer-reviewed paper 10.3390/fermentation8040161