## 4 Results and Discussion
### Final Decision Schemes
In other words, the energy and environmental benefits of the polygeneration system can reduce the energy consumption, SO\({}_{2}\) emission, NO\({}_{x}\) emission, and PM emission of the iron and steel plant by 25.3%, 17.5%, 27.1%, and 15.9%, respectively. With the expansion of iron and steel plants' capacity, the energy, environmental, and economic benefits of polygeneration systems are also improved. For the iron and steel plant with 9 Mt/a capacity, the energy conservation, SO\({}_{2}\) reduction, NO\({}_{x}\) reduction, PM reduction, and economic benefit are 2124.49 ktce, 159.99 t, 206.67 t, 100.67 t, and CNY 1088.93 million, respectively, and the energy and environmental benefits of the polygeneration system are equivalent to reducing the energy consumption, SO\({}_{2}\) emission, NO\({}_{x}\) emission, and PM emission of the iron and steel plant by 42.6%, 24.1%, 26.7%, and 16.3%, respectively. The difference in the performance of the polygeneration system caused by the scale of iron and steel plants can be explained from two aspects. One reason is that the output of by-products or waste of iron and steel plants does not change linearly with their capacity. The output of coke oven gas and blast furnace gas in the iron and steel plant with 9 Mt/a capacity is more than three times that of the iron and steel plant with 3 Mt/a capacity, while the output of blast furnace slag and converter slag of the former is three times lower than that of the latter. The other reason is that the output of by-products or waste determines \begin{table}
\begin{tabular}{c c c} \hline \hline
**Polygeneration Path** & **Iron and Steel Plant with 3 Mt/a Capacity** & **Iron and Steel Plant with 9 Mt/a Capacity** \\ \hline Path 1 & T1, T2, T3, T4, T5, and T8 & T1, T2, T3, T5, and T8 \\ Path 2 & T10 and T11 & T10 \\ Path 3 & T14 and T15 & T12 and T15 \\ Path 4 & T16, T17, and T19 & T16, T17, T19, and T20 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Polygeneration technology selection under the final decision scheme. the optimal technology selection scheme of the polygeneration system. For instance, the utilization of residual heat and pressure is different for two iron and steel plants with different scales (T14 and T15 for the iron and steel plant with 3 Mt/a capacity, while T12 and T15 for the iron and steel plant with 9 Mt/a capacity). In addition, there are also differences in the products of the polygeneration system. For example, aggregate and cement will be generated at the same time by using converter slag if the capacity of the iron and steel plant is 9 Mt/a, while only cement will be generated if the capacity is 3 Mt/a. At present, China's iron and steel industry is continuing to carry out mergers and acquisitions, and the industrial concentration is further improved. Initially, these activities are affected by the national environmental protection goals; simultaneously, we believe that it is conducive to improving the energy conservation and emission reduction potential of the polygeneration system in the future based on the results of this study. Another fact reflected by the above results is that polygeneration systems are an important approach to broadening the space for energy conservation and pollutant emission reduction in industrial sectors, which is of great significance for the industrial sectors to achieve the carbon emission control target and pollutant emission reduction target. In 2023, the crude steel production in China will have reached about 1.02 billion tons [57]. Based on the energy and environmental benefits of an iron and steel plant with 3 Mt/a capacity, in the case of widely promoted polygeneration systems in the iron and steel industry in China, it can save energy over 139 million tee (accounting for approximately 2.4% of China's total energy consumption in 2023) per year and reduce SO\({}_{2}\), NO\({}_{\times}\), and PM with 12.86 kt/a, 23.22 kt/a, and 10.83 kt/a, respectively.