## 4 Case Analysis
### Analysis of the Optimization Results
After optimization on the basis of the original equipment, the BFG, LDG, and COG are fully utilized. The amount of emission is reduced to zero. When new energy conversion technologies are added, the COG is used to produce urea, and the BFG and LDG are fully used for power generation. Overall, through optimization of the model, the utilization rates of energy source can be improved, and zero emission of gas can be realized. As shown in Table 5, prior to optimization of the energy source, the economic benefits of the energy system products for sale in the S plant are -28.16 CNY/t steel, indicating that the S plant needs to spend 24.16 CNY for the outsourcing of energy for each ton of steel the plant produces and 4 CNY to pay for the environmental costs. In the first kind of optimized scheme, under the current equipment and technological conditions, after optimization, the economic benefits of the energy system are -12.39 CNY/t steel; namely, the costs of the energy systems (including pollution costs) are 12.39 CNY/t steel. Compared with the actual situation of the S plant, the economic benefits of the energy system increased by 15.77 CNY/t steel. This result is significantly higher than that before the optimization, indicating that, in the case of a stable production, the model provides the optimal distribution plans of the different energy sources with significant economic advantages in a practical case. \begin{table}
\begin{tabular}{l c c c} \hline \hline \multicolumn{1}{c}{**Variable**} & \multicolumn{1}{c}{**Practical**} & \multicolumn{1}{c}{**Optimal Results of**} & \multicolumn{1}{c}{**Optimal Results of**} \\  & **Conditions** & **the Original Process** & **the New Process** \\ \hline Electricity generated by pulverized coal—gas & & & \\ mixed combustion boiler power generators & 492.77 & 454.64 & 436.47 \\ (kWh/t steel) & - & - & 120.00 \\ Electricity generated by the CHP system & - & - & 32.10 \\ (kWh/t steel) & 2.20 & 0.00 & 0.00 \\ Electricity consumed by the air separation & 94.67 & 90.03 & 90.03. \\ Electricity consumed by the air compressor & 29.25 & 25.56 & 25.56 \\ Electricity consumed by urea production & - & - & 293.12 \\ (kWh/t steel) & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison between the model optimization results and practical conditions of electric power production. In the second kind of scheme, the model is used to optimize the secondary energy conversion device configurations of the S steel plant. As shown in Table 4, the energy sources are optimally distributed after the process modification. Meanwhile, the BOG, LDG, and COG can be utilized more efficiently, and their emissions are reduced by 53.41, 5.66, and 7.95 m\({}^{3}\)/t steel, respectively. The economic benefits of the energy system of the S plant are 152.10 CNY/t steel. Compared with the actual situation of the S plant, the economic benefits of the energy system increased by 180.26 CNY/t steel. This result is significantly superior to that before the optimization. Compared with the S plant process before the optimization, the new techniques introduced in the optimized process include the CCPP and urea production. This result also indicates that the CCPP performs better than the CHP system in electric power production and that the COG will be utilized more efficiently to produce urea but not hydrogen or methanol. When urea, hydrogen, and methanol are under the price conditions shown in Table 1, considering the energy conversion rates and economic benefits of the energy system and other factors, the selection of COG to produce urea is more conducive to the optimization of the entire energy system.