**1. Introduction**

Due to industrialization, population growth, and urbanization, there has been a rapid increase in global energy demand and consumption over the past few decades. Currently, more than 80% of the world energy demand is supplied by fossil fuels. Global attention on alternative resources has increased due to the high prices and limited reserves of fossil fuels, environmental pollution and climate change. As a result, there is renewed interest in the production and use of renewable energy resources such as biomass, solar, wind, geothermal and tidal. However, biomass is the only renewable energy resource to produce transport fuels. Various biomass components, including sugars, starches and lignocellulosic materials can be used to produce transport fuels and potentially limit the use of fossil fuels. On the other hand, biomass residues and wastes (such as agricultural residue, food waste, animal manure, municipal solid waste) as resources for bioenergy production are promising alternatives to reduce environmental issues concerning waste management and disposal. If improperly managed, these residues and wastes could lead to greenhouse gas emissions, pests breeding, insects, foul odor, etc. Converting these residues and

wastes, that would otherwise have been left to decompose, into useful bioenergy is environmentally beneficial for waste management as well as to reduce the dependence on fossil fuel [1–3]. Lignocellulosic biomass (LCB) such as agricultural crop residues, wood and forestry residues are readily available, inexpensive and promising resource for biofuels. Biomass can be considered as the best option for meeting future energy demand sustainably. The efficient utilization of biomass resources is essential by having more efficient biomass production and conversion [4].

Biomass is an organic matter of plant or animal origin that is living or was living in the recent past [5]. Biomass is perhaps the oldest source of energy used by humans. People have used biomass as an energy source for cooking, heating, and lighting for thousands of years. The energy contained in biomass originally came from the sun. Plants get their energy from the sun and convert water (H2O) and carbon dioxide (CO2) into oxygen (O2) and sugars (carbohydrates) in the process of photosynthesis. The energy of sunlight is stored in chemical bonds of plant organic matter. When different conversion processes break the bonds between adjacent carbon, hydrogen and oxygen molecules of carbohydrates, these substances release their stored chemical energy [6]. Biomass is a renewable energy source as it can be produced year after year and, therefore, is valuable as a fuel source. Compared to petroleum-based products, the use of biomass represents the potential to reduce CO2 emissions, resulting in negligible or zero SOX emissions and lower NOX emissions [7, 8].

Biomasses include agricultural crop residues, forestry and wood residues, purposely grown bioenergy crops, aquatic biomass, sewage sludge, digestate, animal, industrial, municipal and food waste. Various types of wastes such as cow manure, poultry litter, wastepaper, sewage sludge, municipal and many industrial wastes are treated as biomass because these are a mixture of organic (and non-organic) compounds.
