## 1 Introduction  
Biomass energy exploitation has been developed in recent years, according to the indications of the energetic policy plans of the European Union and the plan of renewable energies [1]. These plans set a target of taking renewable energy as a primary source, which has been fitted as an objective for the year 2020. A huge number of resources have been devoted to the energy exploitation of biomass in the latter years [2]. The use of biomass residues as non-pollutant fuels in combustion processes has an economical benefit and a social development of rural zones [3]. In addition, the use of these residues decreases the environmental impact by eliminating the remnants and decreasing the emissions of pollutants to the atmosphere. The contents of Cl, S and N\({}_{2}\) in biomass are much lower than those corresponding to the fossil fuels [4]. The use of biomass has a high benefit to the atmosphere that leads to a favorable CO\({}_{2}\) balance, since the organic matter is able to retain more CO\({}_{2}\) than that released by its combustion. The biomass can be obtained in a sustainable and renewable manner, where the consumption speeds of these residues are not faster than their producing speeds, giving a favorable CO\({}_{2}\) balance [5]. The biomass should undergo a series of transformation processes before it is used as an energy source. These processes are: physical processes, such as grinding and drying; biochemical processes, such as alcoholic fermentation and anaerobic digestion; and thermochemical processes that yield carbon, liquid hydrocarbons and gases [6]. The treatment of biomass residues and their conversion technologies could be presented under four basic categories: direct combustion processes, thermochemical processes, biochemical processes and agrochemical processes. Thermochemical conversion processes can be subdivided into gasification, pyrolysis, supercritical fluid extraction and direct liquefaction [7]. To increase energy efficiency, improve energy production quality and reduce emissions during energy conversion, biomass residues must commonly be dried in industry by using rotary dryers and screw dryers [8]. The biomass is introduced in the dryer inlet point in parallel flow with hot gas or air, and is dried along the whole length of the dryer up to its exit point. The residence time of biomass is controlled by changing a rotational velocity of a screw. Therefore, the drying time can be suggested according to the final moisture content for each type of wet biomass [9]. There are a variety of dryer types in industry. One of them is a contact-type dryer that consists of a hollow cylinder: inside it, the products that need to be dried are mixed with the drying hot gas or air [10]. In direct contact dryers, the heat and the mass transfer between the products and the hot gas occur. These processes are more effective by using a series of pallets installed on the inner surface of a dryer cylinder [11]. On the other hand, the benefit of using pallets inside the dryer is that a uniform moisture content distribution of the dried product can be achieved [12]. [PERSON] et al. developed experimental studies based on a drying process using a screw conveyor dryer, and studied mean residence time distribution in the screw conveyor dryer [13]. They found that by increasing the screw speed, the degree of mixing was increased, but the mean residence time was decreased. The flow speed in a screw conveyor dryer approached the speed of the plug flow as the feed rate was increased, whereas an increase in the screw speed resulted in a mixed flow [13]. The drying time can be controlled by varying the rotational velocity of the screw, while the discharge uniformity can be controlled by both the screw velocity and the product flow rate [14]. An experimental analysis of a biomass plant with a maximum power of 2.3 MW was studied by [PERSON] et al. [15].