**1. Introduction**

The finite nature and rapid depletion of fossil fuels due to growing global energy demands have negatively impacted the environment as their combustion entails have led to the search for alternative ways of producing fuels from sustainable, renewable, green, and economical energy sources [1–5]. One source of renewable energy production is biomass, which can be transformed into biofuels, which provide many advantages over fossil fuels in terms of mitigating residues generation and greenhouse gas (GHG), increasing energy independence, and improving the economy of agriculturalists [1,2,6,7].

The term biofuel refers to any liquid, gas, or solid fuel predominantly produced from a renewable biomass feedstock. Biofuel examples are bioethanol, biomethanol, biosynthetic gas (biosyngas), biodiesel, biogas (biomethane), biochar, bio-oil, biohydrogen, and Fischer\_Tropsch produced liquids [1,8]. Biofuels serve as a bridge between the agricultural and energy markets as agricultural commodities are the significant feedstocks in biofuel production [9].

Recently, crop residues as a potential source of feedstock to produce bioenergy have been gaining importance as they are not in competition with food production for human consumption. They have high availability, wide distribution, and almost zero cost [1,10,11]. Crop residues are generally the waste generated in the harvesting of any agricultural crop. These are typically the parts of the products that are often of no value, including the stem, empty fruit bunches, leaves, and stalks. The amount of agricultural waste is variable and reaches 50% for specific crops [12]. In the production of commodities like sugar, rice, flour, starch, and oil, only a fraction of the main crop is recovered and used. The leftover fraction of the processed products are referred to as agro-industrial crop residue. Generally, these residues are either bagasse, molasses, the spent fibrous pulp left behind from stalks of crops, or seed coats, shells, and husks [1,12]. The optimal use of agricultural wastes has excellent economic and ecological advantages due to the possibility of recycling and producing materials with added value [2]. Wheat straw, rice straw, corn straw, and sugarcane bagasse are the agricultural residues available in the approximate amounts of 354.34, 731.30, 128.02, and 180.73 million tons, respectively. The primary composition of these residues is cellulose (20 to 60%), hemicellulose (20 to 40%), and lignin (10 to 25%) [1,13].

Production and use of biofuels made from crop residues could prevent the over-exploitation of fossil fuel and low waste management problems in the field. However, this potential can only be unlocked if they are cost-competitive to petroleum and starch and sucrose-based biofuels. To reduce capital and processing costs, simplifying the process scheme, and integrating as many unit operations as possible is necessary. Therefore, the consolidated bioprocessing CBP is a strategy where all four steps occur in a single reactor. A single microorganism or microbial consortium converts pretreated biomass to a commodity product such as ethanol without adding saccharolytic enzymes, which would represent an innovative breakthrough for low-cost biomass processing. In principle, a CBP strategy can be applied to produce a broad range of chemicals from natural biomass. It requires degrading recalcitrant biomass substrates into solubilized sugars and metabolic intervention to direct metabolic flux toward desired products at high yield and titer [14–16].

The increase in population brought a greater demand for food and energy; this requires an increase in agricultural production and an increase in agricultural waste, which presents enormous challenges for its proper management. Although crop residues represent a potential source for the generation of biofuels, the environmental impacts that their use could cause must also be critically evaluated [16]. Therefore, this work aims to briefly describe the current situation of biofuels, available raw materials, and their production strategies, especially the CBP approach, a strategy that is aimed at the best option for the sustainable production of biofuels.
