## 4 Results and Discussion
### Final Decision Schemes  
As for the optimal variables, the average utilization ratio results of 20 technologies in the polygeneration system are shown in Figure 6. In general, the average utilization ratio of about half of the technologies in two steel plants with different production is similar. On the contrary, there exist some differences in the utilization of several by-products, such as coke oven gas (T2-T7), blast furnace gas (T12 and T13), and blast furnace slag (T16-T18), due to the scale effects of the technologies. This result demonstrates the necessity of considering the plant's scale in technical selection. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{5}{c}{**Iron and Steel Plant with 3 Mt/a Capacity**} \\ \cline{2-6}  & **Energy Conservation** & **SO\({}_{2}\) Reduction** & **NO\({}_{x}\) Reduction** & **PM Reduction** & **Economic Benefit** \\  & **(ktce)** & **(t)** & **(t)** & **(t)** & **(CNY 10\({}^{8}\))** \\ \hline Maximum & 638.88 & 51.39 & 84.10 & 54.03 & 4.20 \\ Minimum & 557.31 & 39.70 & 48.32 & 39.94 & 2.59 \\ Average & 588.98 & 45.86 & 61.53 & 47.03 & 3.740 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The average optimization results of energy conservation and emission reduction in the iron and steel plants. Figure 6: The utilization ratio of by-products/waste of 20 technologies in the polygeneration system for the iron and steel plants with 3 Mt/a and 9 Mt/a capacity. The polygeneration system's decision-making scheme is discrete, which means the above optimal results cannot be used directly to guide business decisions. Therefore, it is necessary to transfer the solution set to obtain the final decision scheme in order to select the polygeneration technologies from the perspective of iron and steel enterprises. When considering the scale economy, 13 technologies should be adopted by the iron and steel plant with 3 Mt/a capacity and 12 technologies should be adopted by the iron and steel plant with 9 Mt/a capacity, and the technology selection under various polygeneration paths is shown in Table 5.  
Based on the scheme and the technical scheme proposed in Table 5, various types of products can be produced by using the by-products or wastes from the iron and steel plant. To be specific, for the iron and steel plant with 3 Mt/a of capacity, 407 GWh of power, 31.5 kt of hydrogen (H\({}_{2}\)), 935.9 kt of cement, 218.4 kt of devitrified glass, 42.1 kt of sulfuric acid (H\({}_{2}\)SO\({}_{4}\)), 47.3 kt of coal tar, and 263.0 kt of MMA will be output with the application of the above 12 technologies in the polygeneration system. It should be noted that methanol is also produced in the above process, but it is used as a raw material for MMA production. However, for the iron and steel plant with 9 Mt/a capacity, the technical scheme of its polygeneration system is different from that of the iron and steel plant with 3 Mt/a capacity, which also leads to differences in their final products. Under the optimized technical scheme, 1308 GWh of power, 338.8 kt of methanol, 1.5 million tons of cement, 300 m\({}^{3}\) of liquified natural gas (LNG), 1.3 million tons of devitrified glass, 129.1 kt of H\({}_{2}\)SO\({}_{4}\), 145.3 kt of coal tar, 807.1 kt of MMA, and 147.4 kt of aggregate can be produced through the polygeneration system with an iron and steel plant with 9 Mt/a capacity as its core. Polygeneration systems can bring significant energy, environmental, and economic benefits by providing a large number of electricity and other industrial products. According to the final technical scheme, the iron and steel plant with 3 Mt/a capacity can achieve energy conservation with 410.53 ktce, SO\({}_{2}\) reduction with 37.85 t, NO\({}_{x}\) reduction with 68.34 t, PM reduction with 31.89 t, and the economic benefit is CNY 244.92 million.