## 2 Materials and Methods  
The energy equation of a thermal drying process can be written as a function of enthalpy change of the drying hot air or gas. The heat transfer rate \(dQ\) (\(kW\)) from hot gas to the product or the biomass residue can be obtained as:  
\[dQ=\frac{dm_{hotgas}}{dt}\times(h_{inlet}-h_{outlet}) \tag{1}\]  
where \(dm_{hotgas}/dt\) (kg/s) is the mass flow rate of drying hot gas, and \(h_{inlet}\) and \(h_{inlet}\) are the enthalpies of the drying gas at the inlet and the outlet of the dryer, respectively. The same heat transfer rate can be written in terms of drying gas temperatures (T\({}_{hotgas}\)) at inlet and outlet, as shown in Equation (2):  
\[dQ=\frac{dm_{hotgas}}{dt}\times Cp_{hotgas}\times\Delta T_{mlg} \tag{2}\]  
where \(Cp_{hotgas}\) (kJ/kg \({}^{\circ}\)C) is the medium heat capacity and \(\Delta T_{mlg}\) (\({}^{\circ}\)C) is the log mean temperature difference across the dryer. Finally, from Equation (2), the necessary heat of the drying processes can be obtained. In this study, the biomass has been firstly crushed and milled to size of 2-5 mm. The chemical characterization of the samples was performed in order to determine the elemental composition (C, H, N and O) according to norm EN-15407:2011 [34]. The elemental analysis of these residues demonstrated the low contents of S and N.  
The determination of volatile matter, moisture, and ashes was performed following Spanish norms UNE-32019:84 [35], UNE-32001:81 [36], and UNE-32004:84 norms [37], respectively. The HHV was determined with a Parr 1351 bomb calorimeter. The combustion parameters (i.e., CO, NO, CO\({}_{2}\), NOx, and HC contents in the fumes and temperature of the fumes) were analyzed by means of a Testo 300 M-I compact flue gas analyzer. Temperatures in different points of the installation were measured with multiple thermometers. The results of the analysis were shown on a dry basis of 0% S.  
A drying of biomass has been carried out. In each drying experiment, moisture content of the dried residues has been measured using a gravimetric moisture analyzer equipment. In addition, the volatile compound, ash and fixed carbon amounts were analyzed in order to determine whether any emissions were releasing. It was found that 79.5% volatile compound, 18.9% fixed carbon and 1.6% ash were measured on a dry basis. According to these data, all samples including original ones had the same amount of the volatile organic compound. Therefore, it can be concluded that gas emissions during a drying process at an average of 180 \({}^{\circ}\)C were negligible. This is due to the fact that when hot gas met wet biomass, the temperature inside the dryer doped immediately, and thus, no emissions were detected. The drying efficiency has been determined as a function of the temperature and the flow rate between the hot gas and solid residues inside the dryer. In these experiments, we used exhaust gases for the drying of biomass with an average temperature of 180 \({}^{\circ}\)C. The cylindrical housing of the dryer was made of a drawn steel pipe, with an inner diameter of 20 cm and a length of 100 cm (distance between inlet and outlet hot air pipes of the dryer). The outer diameter of the screw was 18 cm with 4 cm in pitch and 100 cm in length. A schematic of an experimental setup is shown in Figure 2. In these drying experiments, the process type was parallel flow. Figure 3 shows that a direct-contact thermal screw dryer installation was used. Several experiments were carried out for the drying of biomass residues of 50% moisture content. There were two points to measure the temperatures of the inlet and outlet of the drying medium (hot gas). Other points were installed to measure the temperatures inside the dryer at different positions. Figure 3: Laboratory experimental facility.