**1. Introduction**

Fossil fuels, such as oil, gas and coal, are the world's primary energy sources. However, these resources have limited reserves that will only be sufficient for the next 50 years [1,2]. Fossil fuels also make a significant contribution to the environmental impact of carbon dioxide emissions. Reductions in CO2 emissions through the use of renewable energy aim to reduce greenhouse gas emissions from 1990 to 2030 by 40% and reduce greenhouse gas emissions by 80%–95% by 2050 [3–6]. It is imperative that we use natural resources to achieve the goals of the 2030 agenda so that the needs of the present situation and the satisfaction of future ones will be covered [7].

The use of renewable energy sources and particularly, of biomass, is important due to the economic factor, since the use of cheaper energy resources is more selective, enhancing the conservation of clean environment, as natural, abundant and reusable means of producing thermal energy are mostly in use [8]. Biomass is becoming more promising due to a set of features that allow fossil fuel substitution, thereby reducing greenhouse gas emissions [9,10]. Biomass is one of the major sources of renewable energy, accounting for about 10% of total primary energy and 78% of total renewable energy [11]. Thus, the need to utilize non-wood lignocellulosic biomass as a promising raw material for future renewable fuels is widely recognized, since the latter is in abundance [12,13]. Lignocellulosic biomass, while presenting several positive features, is, however, associated with various deficiencies, such as structural heterogeneity, non-uniform physical properties, low energy density, hygroscopic nature

and low bulk density. All of these features create di fficulties in transport, handling, storage, and conversion [14–18]. These features impede the use of biomass in the replacement of fossil fuels for energy production. Therefore, biomass must be pre-treated before it can be used in any thermochemical process. The torrefaction process is an appropriate such pretreatment method that removes many of the above limitations associated with crude biomass. The torrefaction process is the partial pyrolysis of the biomass which is carried out usually under atmospheric pressure over a small temperature range of 200–300 ◦C and under an inert environment [19–21]. The process is usually performed at a low heating rate, which gives a higher yield of solid product [22]. A grea<sup>t</sup> motivation for torrefaction is the maximization of solid performance, which is not achieved with pyrolysis. During the torrefaction process, three major phases, namely decomposition, rehabilitation and depolymerization, occur. The process releases concentrated hydrocarbons, hydrogen, oxygen and some of the carbon content of the biomass as carbon monoxide and carbon dioxide [23]. During the torrefaction process, drying is considered to be the most destructive between the intramolecular hydrogen bonds, C–O and C–H [24]. This results in the significant emissions of hydrophilic and oxygenated pollutants, and hydrophilic and oxygenated compounds, forming a black hydrophobic energy-dense product.

The main motivation of torrefaction is to improve the quality of biomass fuels and make it more suitable for energy use. The torrefied biomass can be applied in briquetting, pelletizing, gasification and thermal energy cogeneration [5,25,26]. Biomass torrefaction destroys biomass strength and fibrous structure and also increases energy density. Many studies have concluded that torrefied biomass can avoid many constraints associated with crude biomass because it produces moisture-free hydrophobic solid products [27], reduces the O/C ratio [13], decreases milling energy [15,28], increases energy density [29], increases bulk density and simplifies storage and transport [30]. It also improves particle size distribution [15], strengthens burning with less smoke [31], shifts the combustion zone to the high temperature zone in a gasifier [32] and increases resistance to biological decomposition [33]. Therefore, the torrefied biomass is more appropriate than the raw biomass for co-firing in the conventional coal power plants due to many of these improvements, as mentioned above. In addition, torrefied biomass is more appropriate than crude biomass for eligible fuel in conventional coal-fired power plants [20]. The removal of volatiles during torrefaction process leads to a decrease of the O/C ratio, and to an increase the energy density of the biomass [34].

In the present study, the process of torrefaction caused by mu ffle furnace on barley straw under di fferent experimental conditions was studied, aiming at increasing the energy content of barley straw. Barley straw was placed in a porcelain capsule and was heated using a mu ffle furnace for various experiments with di fferent sets of temperatures and residence times, allowing the critical parameters of the combustion process to be identified and a ffecting the energy content of the material. Furthermore, innovative kinetic models were applied to fit the experimental data using the severity factor ( *R*0), which combines the e ffect of temperature and time on the torrefaction process in a single reaction operator.
