## 3 System Description  
### Reference Waste-to-Energy Unit  
The WTEU locates in eastern China, which operates continuously 24 h a day, and its annual operating hour is not less than 7000 h. It equips with a 500 t/d mechanical grate incinerator and a 10 MW condensing steam turbine generator set and is depicted in Figure 1. The feed fuel is MSW, including household garbage and urban road cleaning garbage. The composition of MSW and the main operating parameters of WTEU are recorded in Tables 1 and 2, respectively. The flue gas in combustion chamber needs to be kept above 850 \({}^{\circ}\)C for more than two seconds to inhibit the production of dioxins and other harmful substances [37]. Then flue gas flows through each heat exchanger to heat feedwater or steam. At last, cleaning devices would treat exhaust flue gas. However, the exhaust flue gas temperature behind ECO should not be too low because of the possible low-temperature corrosion caused by complex flue gas composition. Hence, the exhaust flue gas temperature generally demands more than 190 \({}^{\circ}\)C. High exhaust flue gas temperature would cause large energy loss and decrease the boiler's thermal efficiency. Besides, because of low live steam parameters, high exhaust steam parameters and high auxiliary power rate, the net power generation efficiency of reference WTEU is less than 20%.  
In the WTEU, a regenerative heating system consisting of two regenerative heaters (RHs) is adopted to raise the feedwater temperature of WTE boiler. To intensify the waste combustion in WTE boiler, the air involved in the ignition and combustion of waste is preheated by the extraction steam from boiler drum and steam turbine (ST). The significant parameters of air preheaters (APHs) are displayed in Table 3. The APHs comprise a primary air heater (PAH) and a secondary air heater (SAH). The PAH is split into three sections, and the first section of PAH (PAH1) connects with the third section of PAH (PAH3). The boiler drum provides the saturated steam for PAH3 to heat the outlet air, and the condensate from PAH3 flows into PAH1 to heat the inlet air. The ST's first stage extraction steam supplies heat to the second section of PAH (PAH2) and the SAH. The primary air is heated from 15.0 \({}^{\circ}\)C to 220.0 \({}^{\circ}\)C in PAH, and the secondary air temperature is enhanced from 15.0 \({}^{\circ}\)C to 166.0 \({}^{\circ}\)C in SAH. A larger logarithmic mean temperature difference (LMTD) could decrease the area of APHs while it might cause greater exergy destruction in such components.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Fuel** & **Proximate Analysis (wt\%)** & \multicolumn{5}{c}{**Ultimate Analysis (wt\%)**} & \multicolumn{5}{c}{**LHV (kJ/kg)**} \\ \hline MSW & A & M & C & H & O & N & S & Cl & 7000 \\  & 41.75 & 20.59 & 21.97 & 1.91 & 12.78 & 1.91 & 0.20 & 0.30 & 7000 \\ \hline \hline \end{tabular}  
A: ash; M: moisture, LHV: lower heating value.  
\end{table}
Table 1: Properties of the fuel used by WTEU (as received basis).  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Unit** & **Value** \\ \hline Fuel consumption rate (MSW) & kg/s & 5.79 \\ \hline \multirow{3}{*}{Live steam} & Pressure & MPa & 4.10 \\  & Temperature & \({}^{\circ}\)C & 400 \\  & Mass flow & kg/s & 13.50 \\ \hline \multirow{3}{*}{Exhaust steam} & Pressure & MPa & 0.007 \\  & Temperature & \({}^{\circ}\)C & 39.0 \\  & Mass flow & kg/s & 9.95 \\ \hline \multicolumn{2}{c}{Boiler efficiency} & \% & 78.55 \\ \multicolumn{2}{c}{Total power generation} & MW & 9.75 \\ \multicolumn{2}{c}{Net power generation} & MW & 8.00 \\ \multicolumn{2}{c}{Net WTE efficiency} & \% & 19.73 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Operating parameters of the WTEU.  
Figure 1: Diagram of the reference WTEU.