## 1 Introduction  
The extensive use of conventional fuel has not only induced economic problems, but also ecological damages. Limited fossil fuel reserves have served as a challenge, globally, to look for alternative sources of energy. Thus, the apprehension for greenhouse gas emissions (GHG), energy security, and sustainability has caught worldwide attention to look for renewable and alternative energy resources. Therefore, to curtail these issues, an appropriate replacement for renewable energy is necessary. Korea is dependent for its energy demands and imports almost 97% of its energy. It is vital that the country generates ideas for the production of renewable energy. One of the materials that can be used for the production of energy is sewage sludge which is generated from wastewater treatment plants. According to a study by Environment Department, Water and Wastewater Policy office of South Korea, 2012 [1], an annual increase of 5.9% was observed in the case of sewage sludge production, with a daily production of 9799 tons, as per the 2012 statistics. This projects that the amount of sewage sludge produced is sufficient for its usage in energy generation. Sewage sludge disposal and management can be one of the most complex and challenging environmental issues. In today's time, a great deal of work has been centralized for proper management of sewage sludge, for instance, applications of sewage sludge to agricultural land or thermochemical treatments, like combustion, gasification, and pyrolysis [2; 3; 4]. Due to the high amount of organic content present in sewage sludge, it can be utilized as a biomass resource for generating energy. A study by [PERSON] et al. [5] reflects the potentiality sewage sludge holds for energy recovery. The author also states that the UK water industry generated approximately 800 GWh/year of electrical energy from sewage sludge. In addition, [PERSON] et al. [6] also highlight the conversion of waste sludge to energy-densified solid fuels for energy generation. Although the concept of utilizing waste for generating energy through thermochemical treatment is not recent, the properties of biomass, such as low heating value, high moisture content, and hygroscopic behavior, have impeded this sector [7]. Torrefaction, a thermal pretreatment process, can, therefore, be utilized to overcome these barriers that biomass possess and can produce low-value biomass feedstocks applicable for use in energy-related applications, like pyrolysis, combustion, or gasification. Some of the principal influences of pretreatment by torrefaction include: (i) low transportation cost; (ii) high grindability; (iii) reduction in size of biomass; (iv) increase in calorific value; (v) hydrophobic product, making storage easier; (vi) slowing of biological and thermal degradation improving shelf life and storage; and (vii) the combustibility of biomass [8; 9; 10; 11; 12]. Torrefaction is a pretreatment process where the feedstock (biomass) is heated to a temperature of 200-300 \({}^{\circ}\)C in an inert environment [13]. One of the major advantages of this pretreatment process is the energy densification of the torrefied biomass as projected in Figure 1[14]. During this process, decomposition of biomass takes place by emitting various kinds of volatiles. [PERSON] et al. [15] exhibited in his research that the torrefied product or yield retained the energy constituent remarkably. In addition, the torrefied product showed enhanced calorific value and energy density, and higher carbon percentage with a lower hydrogen to carbon ratio. Furthermore, a study by [PERSON] et al. (2008) [16] testifies to the improvement in biomass properties through torrefaction which, in turn, expedited its co-firing with coal. Moreover, [PERSON] et al.