## 2 Materials and Methods
### Equipment Description and the Working Principle  
The scene graph of the drying system, heat exchanger and flue-gas valve are depicted in Figure 1a-c, respectively. As can be seen in Figure 1a, the system mainly consisted of two symmetrical drying towers with a loading capacity of 25 t each. During the drying process, the threshed corn kernels were conveyed to a hoist (HST) by a conveyor belt (CB) and continuously fed into the drying chamber (DC); at this period (90 min), the discharging device (DD) remained closed. After the DC was fully filled, the induced draft fan (IDF) and DD were successively opened, and the grain flux began the cycle under gravity as well as with the help of HST. Meanwhile, the flue gas flux and the air flux flowed into the heat exchanger (HE) and the DC driven by the IDF, respectively. Moreover, the fuel of the CC was hard coal, and the details of the electrical components are tabulated in Table 1.  
Figure 1: The scene graph of the drying system (**a**), the heat exchanger (**b**) and the flue-gas valve (**c**).  
During the drying process, the combustion in the combustion chamber (CC) was adjusted by the flue-gas valve in the middle of the flue gas pipeline (FGP), and based on the scene graph of the flue gas valve, it can obviously be seen that there was a baffle inserted into the FGP and six card slots which were used to adjust the baffle angles (15\({}^{\circ}\), 30\({}^{\circ}\), 45\({}^{\circ}\), 60\({}^{\circ}\), 75\({}^{\circ}\) and 90\({}^{\circ}\)). Additionally, in order to simplify the drying system, a schematic diagram of the flows for different fluxes is depicted in Figure 2. As can be seen in detail from the figure, the air volumes of the flue gas and the hot air flux were controlled by the flue-gas valve openings (\(\alpha\)) during the drying process; the combustion of hard coal in the CC was affected by the fresh air volume rate inlet into the CC, i.e., the larger the fresh air volume rate was, the more sufficient the combustion of the coal in the CC and the higher the \(T_{fg,in}\) and \(v_{fg}\) would be. Meanwhile, under the same total pressure of the IDF, the larger the fresh air-volume rate inlet into the CC was, the larger the flue gas volume rate would be, while the smaller the fresh air-volume entering into the HE would be. In other words, the heat-exchanger performance of the HE and the drying performance of the whole drying system were controlled by the flue-gas valve openings (\(\alpha\)); the characteristics of the control may be valuable for the application of a specific drying system in different areas with different ambient conditions. The key physical parameters of the dryer and the HE are tabulated in Table 2.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Components** & **Power (kW)** \\ \hline Data acquisition (DA) & 0.75 \\ Hoist (HST) & 5.5 \\ Conveyor belt (CB) & 1.5 \\ Induced draft fan (IDF) & 30 \\ Discharging device (DD) & 1.5 \\ \hline \hline \end{tabular}
\end{table}
Table 1: The details of the electrical components adopted in this work.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Parameters** & **Value/Method** & **Units** \\ \hline \multirow{5}{*}{Dryer} & Loading capacity & 50 & \(t\) \\  & Drying capacity & 5.5 & \(t\)/h \\  & Cycling time of the dryer & 90 & min \\  & Total air volume of the induced & 17,585–30,774 & m\({}^{3}\)/h \\  & draft fan & 1500–2149 & Pa \\  & Diameter of the flue gas pipeline & 200 & mm \\  & Heat exchanger type & Shell and tube type & - \\  & Material & Stainless steel & - \\ Heat exchanger & Number of pipes & 253 (11 columns and 23 rows) & - \\  & Pipe arrangement & Staggered & - \\  & Pipe space & 120 & mm \\  & Size of pipe (length \(\times\) diameter & 1800 \(\times\) 50 \(\times\) 1.5 & mm \\  & \(\times\) thickness) & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: The key parameters of the dryer and the heat exchanger.