To investigate why the trade-off curve shift to different directions, we pick the points at -24%, 23% and 42% of GHG emissions reduction, under which we investigate their economic breakdowns (Figure ). We can find carbon tax has dual effects on this CCU system. On the one hand, the process cost increases with carbon tax. This is because the CCU plant is still associated with emissions from raw materials, utilities and unreacted CO2 emissions, so the cost of these emissions is consequently increased. On the other hand, the revenue from fuel products rises with the growth of carbon tax, as the carbon tax increases the price of fuel products, which brings in extra credits to the CCU system. MS can reduce more GHG emissions and the credit for methanol is larger than FT products. Hence, the revenue increase in methanol is much more significant than that in FT products, which reflects that raising carbon tax brings in more revenue at 42% GHG reduction than that at -24% GHG emissions reduction.