**1. Introduction**

Biomass has been widely recognized as an important source of renewable energy due to its inherent properties such as availability, abundant supply, carbon stability, organic nature, etc. Unlike non-renewable resources such as oil and coal, biomass is a renewable natural resource and organic material mostly derived from animals and plants for the production of fuels at local and commercial scales, which is the dream of many biofuel producers and energy experts over the years. Interest in biomass as a renewable resource is increasing with time thanks to its ability to be burned directly for heat or converted to renewable fuels via several thermal decomposition methods including torrefaction, gasification, hydrotreating, carbonization, and pyrolysis [1–4].

It is important to know that biomass which can be crops, wood, landfill gas, alcohol fuels, and garbage contribute the largest percentage of energy used in many sectors including the electric power, commercial, residential, commercial, transportation, and industrial. For instance, wastes derived from biomass and woods are used to produce electricity in electric power sector, renewable natural gas derived from municipal solid waste (MSW) are consumed and sold in commercial sector whereas the wood pellets and firewood are mostly consumed in residential sector. In addition, the plant and animal-based biomass are used to generate liquid biofuels including biomass-based diesel and ethanol which finds major application in most transportation and industrial sectors.

Biomass can be converted to energy via several methods such as biological conversion for the production of fuels (gaseous and liquid), chemical conversion for producing liquid fuels, thermochemical conversion for the production of liquid, gaseous, and solid fuels; and direct combustion to generate heat. In biological conversion process, biomass can be converted into renewable natural gas or biogas [5] through anaerobic digestion method or ethanol through fermentation process. Meanwhile, in chemical conversion process, greases, animal fats, and plant-based biomass such as vegetable oils can be converted into fatty acid methyl esters (FAME) which are mostly utilized for the production of biodiesel through transesterification method. On the other hand, through thermal decomposition methods including torrefaction, gasification and pyrolysis, biomass can be thermochemically converted to produce bio-oil (hydrotreating), hydrogen, methane, renewable diesel (pyrolysis), or synthesis gas and carbon monoxide (CO) (gasification) [1, 2, 4]. Furthermore, biomass can be converted to energy in direct combustion method such as the generation of electricity in steam turbines, industrial process heat, and heating buildings.

Generally, biomass sources for energy include but not limited to human sewage and animal manure, biogenic materials in MSW (wood wastes, yard, food, wool products, cotton, paper), waste materials and agricultural crops (woody plants, switchgrass, sugar cane, soybeans, corn, etc.), and wood processing wastes (paper mills, sawdust, wood chips, wood pellets, firewood) [6–11]. For easy storage and transportation, biomass can be interestingly made or densified into briquettes for producing fuels and biogas [5]. Briquettes are the solid biofuels with compact shape and size for producing renewable energy which can be made with binders [6, 8–11] or without the use of binders [7]. Biomass has been densified into solid briquettes in the past using the same or different materials including corncob, rice bran [8–10], mesocarp fiber (MF), fruit fresh bunches (FFB), palm kernel shell (PKS) [8–11], bagasse, tea waste, cotton stalk, sugarcane bagasse (SCB), empty fruit bunches (EFB), etc. Most of the performance related problems such as low yield and energy content are linked to the effectiveness of the binder materials, type and compositions [6].

As a form of thermal decomposition processes of converting biomass to energy, torrefaction is presently receiving wide attentions for producing high grade solid biofuels with greater energy density/high heating value (HHV). Torrefaction process is capable of significantly improving the quality and properties of solid biofuels. It improves the physicochemical properties of biomass for a long-term storage. In addition, torrefied biomass can serve as a good replacement for coal in the generation of heat and electricity. The three common types of torrefaction types are the dry torrefaction (DT), wet torrefaction (WT), and ionic-liquid assisted torrefaction (ILA) [6]. The DT is usually carried out in a gas-phase environment, while the WT is often performed under pressure of a liquid-phase environment. Meanwhile, the ILA is a combination of a typical torrefaction and a pretreatment process, which is aimed at improving the reaction rate. The ionic-liquid assisted torrefaction (ILA) usually involves the use of ionic liquids which is often referred to as green solvents due to their ability to dissolve lignocellulosic biomass under normal conditions due to their special properties such as recyclability and high thermal stability [4].

**597**

with original feedstock.

*Economics, Sustainability, and Reaction Kinetics of Biomass Torrefaction*

of biomass torrefaction is presented in the next section of this chapter.

However, an in-depth understanding on the economics and sustainability of the torrefaction process is lacking. With the aim of giving new insight into further study, a comprehensive overview on the reactor design for commercialization purposes, reaction kinetics and mechanism, economics, as well as the sustainability

Biomass torrefaction is the process of producing high-quality and attractive solid biofuels from several sources of ordinary agro residues or woody biomass, with the sole aim of improving biomass properties and performance for gasification [1–3] and combustion applications via thermal decomposition at temperature ranging from 200–300°C [12] under atmospheric pressure. Through torrefaction, a coal-like material can be generated from biomass with superior fuel properties and quality when compared with the parent materials. Torrefaction is a mild pyrolysis process where biomass is thermally treated in a controlled environment (in a nonfluidized bed reactor or fluidized bed) with low or no traces of air or oxygen [13] resulting in the production of torrefied biomass which is water resistant, brittle and stable with less energy intensive and easy grindability. During torrefaction, drying of biomass and partial devolatilization occurs leading to mass reduction without losing or decreasing the energy content. Heating biomass at typical temperatures between 200°C and 300°C often lead to the evaporation of moisture or unbound water (H2O) through thermo-condensation process (at temperature above 160°C) and the removal of volatiles (low-calorific parts), resulting in the decomposition of hemicellulose in the biomass hence the transformation of biomass from a lowquality fuel into an excellent high-quality fuel. In a bid to improve the biochemical, chemical, and physical properties of biomass, the basic principle behind the

biomass torrefaction process can thus be summarized as the removal of volatiles via several decomposition reactions. With torrefaction, there is no biological activity, hydrophobicity and higher durability can be obtained, excellent grindability and higher bulk density can be achieved. In addition, more homogenous product and a fuel comparable to coal can be produced with higher calorific value as compared

It should be noted that the torrefaction processing parameters such as the residence time and torrefaction temperature have significant effects on the overall properties and performance of torrefied biomass. In other words, there is a direct relationship between the torrefaction processing parameters and the physicochemical properties of torrefied biomass. For instance, high torrefaction temperature and short residence time tend to optimize the material flow via the torrefaction reactors thereby producing a cost-efficient torrefied biomass on a large scale. By increasing the torrefaction temperature, the fixed carbon and ash contents in biomass can be markedly increased with a decrease in volatile contents. This can lead to a decrease in atomic ratios of oxygen-carbon (O/C) and hydrogen-Carbon (H/C), as well as decreasing the oxygen content resulting in improved calorific value, which ultimately enhance the overall fuel features and performance of the biomass products. Moreover, the acid content in biomass materials can be significantly reduced with increasing the torrefaction temperature. The decrease in the acidity of the biomass say bio-oil for example [14] can be attributed to the fact that the acetic acid solely originates from the deacetylation reaction and decomposition of hemicellulose component of the biomass. In addition, increasing the torrefaction temperature can reduce the moisture content of the biomass, hence improving the quality of the biomass. By this, it can be said that torrefaction

*DOI: http://dx.doi.org/10.5772/intechopen.94400*

**2. Biomass torrefaction: A general overview**

However, an in-depth understanding on the economics and sustainability of the torrefaction process is lacking. With the aim of giving new insight into further study, a comprehensive overview on the reactor design for commercialization purposes, reaction kinetics and mechanism, economics, as well as the sustainability of biomass torrefaction is presented in the next section of this chapter.
