1. Introduction
In today's energy demanding life style, there is a need for exploring and exploiting new sources of energy that are renewable, as well as, eco-friendly. The renewable energy resource systems such as solar, wind, hydro wave, geothermal and biomass, offer attractive prospects because they are unlimited and cheap [1]. Biomass, which is made up of a wide variety of agricultural residues, is found in large quantity and it is the major contributor to renewable energy (occupying approximately about 10% of the total energy) [2]. It is considered as a worldwide valuable energy alternative to fossil fuels, because it may be converted to a variety of usable forms of energy such as biogas, and liquid transportation biofuels [3]. In developing countries, large quantities of cellulosic biomass such as rice husks and other forms of agricultural residues (straws, nut shells, fruit shells, fruit seeds, plant stovers, green leaves, and molasses) are produced annually but are vastly under-utilized [4]. Thus, agricultural biomass abounds in the rural and sub-urban areas and contributes to major environmental pollution [5]. In Nigeria alone, it has been estimated that about 227,500 tons of fresh animal wastes is produced daily. Since 1 kg of fresh animal wastes produces about 0.03 m3 gas, then Nigeria can produce 6.8 million m3 of biogas every day. In addition to these, 20 kg of municipal solid waste per capita has been estimated to be generated in the country annually [6]. With increasing urbanization and industrialization, the annual municipal solid waste generated will continue to increase [7]. Therefore, developing biomass-to-biofuel technology could be significantly beneficial for enhancing energy security, reducing greenhouse gas emissions, and utilizing the renewable resources [8]. However, developing cost effective approaches for biomass-to-biofuel production is still identified as the key bottleneck preventing its widespread commercialization [9]. Anaerobic digestion (AD) of agricultural residues and other biodegradable wastes is widely used as the best treatment options because it yields methane and CO2 rich biogas which is suitable for energy production [10] and [11]. Anaerobic digestion of organic waste and residues, therefore, combines both sustainable treatment and renewable energy production [12] and [13]. The by-product, or nutrient-rich digestate, from the AD process can be used as a fertilizer for agriculture.