Multiobjective optimisation results

Figure 6.12 shows the optimal solutions for waste-to-energy and chemicals process on MSW 1 by optimising between the economic values (maximising NPV) and the environmental impact (minimising GW) at the same time. The solutions are plotted in the form of a Pareto curve, where X-axis represents global warming (GW) and Y-axis represents net present value (NPV). The area under the Pareto front is the feasible area, whereas the area above is the infeasible region. The results show that SOFC is always the optimal process in the optimal range (GW 28 to 162 kton CO2 eq. per year) for the case of using MSW 1 as feedstock. It can be also seen that if NPV is higher, GW is also higher and if NPV is lower, GW is also lower as shown in figure 6.12. The minimum GW (28.38 kton CO2 eq. per year) is obtained when NPV is $35.4 million (point A) corresponding to the capacity of 84 ktpa. The ratio between NPV and GW at this point is $1.25 per kg CO2 eq. per annum. and the capital cost (CAPEX) per electricity generated is equal to $114.63 per kW/y. The electricity generated at this point is 11.08 MW.

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Figure 6.12 Pareto curve between NPV and GW for SOFC process on MSW 1.

The maximum NPV ($320.72 million) is obtained when GW equals to 162 kton CO2 eq. per year at point B in figure 6.12, which equals to the capacity of 481 ktpa. The ratio between NPV and GW at this point is $1.98 per kg CO2 eq. per annum. and the capital cost (CAPEX) per electricity generated equals to $67.96 per kW/y. The electricity generated at this point is 63.3 MW. It can be seen that CAPEX per electricity generated from at the maximum NPV (point B, $67.96 per kW/y) is much lower than CAPEX per electricity generated from point A ($114.63 per kW/y). This is caused by the scaling factor from equation (6.92). Therefore, the ratio between NPV and GW from point B ($1.98 per kg CO2 eq. per annum) increases from point A ($1.25 per kg CO2 eq. per annum) for around 58.6%.

Figure 6.13 shows the optimal solutions for waste-to-energy and chemicals process on MSW 2 from maximising NPV and minimising GW simultaneously. The solutions are also represented as a Pareto front, where X-axis is GW and Y-axis is NPV. The results demonstrate that SOFC is still always the optimal process in the optimal range (GW 40 to 265 kton CO2 eq. per year) for this case. It also has the same trend with MSW 1 that if NPV is higher, GW is also higher and vice versa, as shown in figure 6.13. At point C, the minimum GW (40 kton CO2 eq. per year) is obtained when NPV is $80.7 million, which corresponds to a capacity of 92 ktpa. The ratio

A

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between NPV and GW at this point is $2.01 per kg CO2 eq. per annum. and CAPEX per electricity generated is $95.5 per kW/y. The electricity generated in this case is 20.37 MW

Figure 6.13 Pareto curve between NPV and GW for SOFC process on MSW 2.

When considering point D, where the maximum NPV ($750 million) is obtained and GW equals to 263 kton CO2 eq. per year. The capacity in this case is 602 ktpa. The ratio between NPV and GW at this point is $2.85 per kg CO2 eq. per annum. The capital cost (CAPEX) per electricity generated in one year equals to $54.32 per kW/y and the power generated is 134 MW. It can be noticed that the NPV per GW ratio ($2.85 per kg CO2 eq. per year) at point D is also higher than for point C ($2.01 per kg CO2 eq. per year), since from lower CAPEX per electricity generated (point D $54.32 kW/y and point C $95.5/kW/y respectively). Moreover, when compared to the previous case where MSW 1 was used as feedstock, it can be seen that this case (using MSW 2) has a wider optimal range of GW (MSW 2: 40 to 265 kton CO2 eq. per year, MSW 1: 28 to 162 kton CO2 eq. per year). All NPV from both point C and D ($81 million and $750 million respectively) are also higher than NPV from point A and B ($35 million and $321 million). This is because MSW 2 has lower moisture and ash than MSW 1, so it has more LHV than LHV from MSW 1 as discussed earlier. Therefore, it can produce more electricity than MSW 1 when compared on the same waste feed rate.

C

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Figure 6.14 shows the optimal solutions for waste-to-energy and chemicals process on MSW 3 based on the same approach as in the previous 2 cases by maximising NPV and minimising GW simultaneously. The solutions are also represented as Pareto curve, where X-axis is GW and Y- axis is NPV. The results show that SOFC is the optimal process just in the range of GW between 36-103 kton CO2 eq. per year as shown in red line from point E to F in figure 6.14. After that, direct DME synthesis becomes the optimal process from GW range between 214 to 678 kton CO2 eq. per year as shown in blue line from point G to H in figure 6.14.

At point E, the minimum GW (36 kton CO2 eq. per year) is obtained and NPV is $60 million. The capacity at this point is 59 ktpa and the ratio between NPV and GW at this point is $1.65 per kg CO2 eq. per annum. The CAPEX per power generated is to $102.2 per kW/y and the power generated is 16 MW. For point F, GW at this point is 103 kton CO2 eq. per year and NPV is $221 million. The capacity at this point is 168 ktpa. The ratio between NPV and GW at this point is $2.15 per kg CO2 eq. per annum, which is higher than at point E. The CAPEX per power generated is to $74.72 per kW/y, which is lower than point E. The power generated is 46 MW.

Figure 6.14 Pareto curve between NPV and GW for SOFC and direct DME process on MSW 3.

E F

G

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When consider in the range between 214 to 678 kton CO2 eq. per year, which the optimal process is the direct DME synthesis, NPV obtained at point G is $316 million at the capacity of 218 ktpa. The NPV per GW ratio is $1.48 per kg CO2 eq. per year and CAPEX per total energy recovery is $122 per kW/y. The total energy recovery in this case is 52 MW (electricity 16.3 MW and DME 36 MW). The maximum NPV for the MSW 3 case is obtained ($1197 millions) at point H in figure 6.14. GW and the waste capacity at this point are 678 kton CO2 eq. per year and 691 ktpa respectively, which are higher than the both MSW 1 and MSW 2 cases. NPV per GW ratio is $1.76 per kg CO2 eq. per year. The CAPEX per electricity generated is $86 per kW/y and the total energy recovery in this case is 166 MW (electricity 51.6 MW and DME 113.9 MW). All these data are summarised in table 6.6.

Table 6.6 Optimal value from Pareto curve for waste-to-energy and chemicals process on MSW1, MSW 2 and MSW 3.

MSW 1

Point Technology GW

(kton CO2 eq./y)

NPV (M$) NPV/GW ($/kg CO2/y) CAPEX/energy ($/kW/y) A SOFC 28 35 1.25 115 B SOFC 162 321 1.98 68 MSW 2 Point Technology GW

(kton CO2 eq./y)

NPV (M$) NPV/GW ($/kg CO2/y) CAPEX/energy ($/kW/y) C SOFC 40 81 2.01 96 D SOFC 263 750 2.85 54 MSW 3 Point Technology GW

(kton CO2 eq./y)

NPV (M$) NPV/GW ($/kg CO2/y) CAPEX/energy ($/kW/y) E SOFC 36 60 1.65 102 F SOFC 103 221 2.15 75 G D.DME 214 316 1.48 122 H D.DME 678 1197 1.76 86

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From the above results, it can be seen that the optimal range for MSW 3 (36 to 678 kton CO2 eq. per year) is longer than the rest of waste compositions, as shown in table 6.6. This means in case of MSW 3 as a feedstock, there are more feasible values (the area under the graph in figure 6.14) than the case of MSW 1 and MSW 2. Moreover, the maximum optimal NPV ($1197 million) from MSW 3 (point H) and the NPV at the minimum GW point (point E) are also higher than the maximum NPV values and NPV at the minimum GW from MSW 1 and MSW 2 as shown in table 6.6. This is due to carbon content in MSW 3 (66%) which is higher than MSW 1 (49.6%) and MSW 2 (49.4%) as described before, therefore, there are more products or energy recovered from the waste and more revenues, which also increases NPV when compared to other cases at the same capacity. However, increased carbon content in the MSW 3 also increases more the GW as shown in table 6.6, so some of the ratio values between NPV and GW in case of MSW 3 are still lower than some points in case of MSW 1 and MSW 2 when compare on the close amount of NPV case, for example, NPV per GW at point E ($1.65 per kg CO2 eq./y) is lower than point C ($2.01 per kg CO2 eq./y) and NPV per GW from point G ( $1.48 per kg CO2 eq./y) is also lower than from point B ($1.98 per kg CO2 eq./y). Finally, it can be concluded that the waste composition is still a key parameter for waste-to-energy and chemicals optimisation since it can increase more optimal range (GW) and also exhibits enhanced optimal NPV, as shown in the case of MSW 3. It can have this effect even the technology is changed from SOFC to be the direct DME synthesis, in the case of MSW 3 at higher range (GW > 214 kton CO2 eq. per year), as shown in figure 6.14, which is different from the waste with the lower carbon content that always have only one optimal technology, which is SOFC because its high efficiency on generating power, as discussed before.

6.5 Conclusions

The economic analysis on seven waste-to-energy and chemicals processes was developed using NPV as an indicator. Every process is based on the same waste compositions (MSW 1, MSW 2 and MSW 3) and process parameters as described in chapter 5, as well as same operating time, project period (8000 hours per year, 20 years) and discount rate (6%).The results show that based on just NPV and capacity of all waste-to-energy and chemical processes, the incineration process has always the highest NPV, in case of low waste capacity (16 ktpa) for every type of waste., but

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SOFC exhibits the highest NPV in the case of higher capacity (160 ktpa and 800 ktpa) when MSW 1 and MSW 2 are used as feedstock. However, when MSW 3 is employed, the direct DME synthesis provides the highest NPV for both 160 ktpa and 800 ktpa capacities. On the other hand, in the case GW is also considered alongside NPV, SOFC seems to be the best process to process MSW 1 and MSW 2 since it delivers the highest NPV and the lowest GW at the same time. Nevertheless, when MSW 3 is employed as the feedstock, it appears that the direct DME synthesis exhibits higher NPV than SOFC when compare at the same capacity, but it also has higher GW than SOFC for all capacities. As discussed, this is because the direct DME synthesis produces more DME when the waste has more carbon content, so it delivers more revenues and increased NPV compared to SOFC which generates only the electricity. As a result, it is necessary to several decision criteria and optimise the system using multiple objective functions, which are maximising NPV and minimising GW in this particular case. 𝜖𝜖-constraint method is employed for multi-objective optimisation by converting the GW objective to a constraint then maximising NPV as a single objective function. The optimisation is carried out by GAMS software, using BARON as an optimiser. The optimisation results confirm that in the case of MSW 1 and MSW 2 as feedstock, SOFC is the optimal process for waste-to-energy and chemicals, since it can provide the highest NPV value and the least GW score. The optimal ranges for SOFC process on MSW 1 and MSW 2 are 28-162 kton CO2 eq. per year (capacity 84- 481 kton MSW per year) and 40-263 kton CO2 eq. per year (capacity 92-602 kton MSW per year). In case MSW 3 is employed as feedstock, it is shown that the optimal range is between 36- 103 kton CO2 eq. per year (capacity 59-168 kton MSW per year), SOFC is the optimal process. However, when the optimal range is between 214-678 kton CO2 eq. per year (capacity 218-691 kton MSW per year), direct DME synthesis becomes the optimal process. This is because there is more carbon content in this kind of waste and as such is more DME can be produced, which has a higher market value than electricity, which explains both increased revenues and NPV of the process compared to the case of the SOFC. Although SOFC provides lower GW than the direct DME process, the increased NPV is enough to make direct DME synthesis to be more advantageous than SOFC at the higher capacity (218 to 691 kton MSW per year). The overall conclusion and recommendation for the future works are described the chapter 7.

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Chapter 7 Conclusion and Recommendation for future work