Table of Contents:
Results and Discussion
Base case analysis
To investigate the base case condition of activities in the MRW, various environmental interventions from each activity are plotted in Figure 3. Thermoelectric activity shows the most dominant contribution to many environmental interventions. Thermoelectric power plants are responsible for 69.6% of water consumption, 82.4% of GHG emissions, 68.4% of NOv emissions, and 97.1% of SO, emissions in the MRW. These results align well with the U.S. national average (Maupin et al., 2014; EPA, 2018a, 2014), although the contribution from thermoelectric activity in the MRW to those interventions is much larger than the national average. This is because the MRW is an intensive area in terms of thermoelectric activity. The MRW is a region where 17.4% of Ohio's electricity is generated (EIA. 2018a), although the population in the MRW is only 1.1% of the state’s population. For some air emissions, such as PM10 and CO. transportation activity is the dominant contributor. Also, most of the water nutrient emissions, such as total N and P loads, are attributed to agricultural activity. For electricity consumption, industrial activity is the most dominant activity.
Conventional sustainability assessment approaches only account for environmental interventions, such as emissions and resource uses, which correspond to ecosystem demands. As described in section 2.5, however, all ecosystem demands need to be compared with their corresponding ecosystem supplies when assessing the sustainability of watershed activ ities. Figure 4 represents base case analysis results in terms of the demands and supplies of electricity and ecosystem goods and services. The TES sustainability metrics are calculated for each ecosystem good and sen-ice and are shown in Figure 5.
As shown in Figure 4, net electricity generation in the MRW is calculated to be 9.6 TJ/day. Most ecosystem supplies are smaller than their corresponding ecosystem demands. Only the water demand from activities in the MRW does not exceed the water supply from the ecosystem in the MRW. This indicates that the MRW is not a water-scarce region but has a scarcity of other ecosystem sen-ices. As shown in Figure 3, the amount of water withdrawn is significantly large, but it is not considered as the ecosystem demand for water provision since most of the withdrawn water returns to the water body. Rather, the amount of water consumption is considered as the ecosystem demand. In terms of the natural
Figure 3. Various environmental interventions from each activity in the MRW.
Figure 4. Base case analysis results in terms of the demands and supplies of electricity and ecosystem goods and sendees. ONG provisioning mcludes the amount of accumulated NG by allowing for 2 °C global wanmng smee the pre-mdustnal
Figure 5. TES sustainability metrics calculated for each ecosystem good and service.
gas supply, approximately 2.5 x 106 m3/day of accumulated NG is considered as the supply of NG by allowing for 2 °C of global warming, as described in section 2.5. The TES metric for NG is then calculated for this adjusted NG supply.
To identify ecological overshoots for activities in the MRW, various TES metrics are calculated for the base case. As shown in Figure 5, most TES metrics are negative, which indicates that environmental interventions overshoot the capacity of ecosystems in the MRW. Most of those interventions are attributed to the thermoelectric activity, as shown in Figure 3. Activities in the MRW are environmentally sustainable only with respect to water consumption since the TES metric for water is positive.
To address the nexus of multiple flows in the analysis, multiple objectives that represent these flows need to be considered. In this work, the following objectives are included: Net electricity generation, TES metrics for greenhouse gases and air pollutants (Ff0y Fvo^ Vso^, Vf^b, Vpu10- and Vco), TES metrics for water nutrient runoffs (Vm and VTP), and TES metrics for ecosystem goods (VH 0 and KVG). Various scenarios about technological alternatives are examined in order to improve the sustainability of activities in the MRW. As a functional unit for the comparison between alternatives, power plants in the MRW are assumed to generate 230 TJ/day of electricity, regardless of which alternatives are adopted. This corresponds to the amount of electricity generated from the five fossil plants in the MRW in 2014. To identify the interactions between objectives for each scenario, all TES metric values (Vk) are converted to the positive values by adding one (+1) to the original TES metric values, then normalized by the maximum value (Vimax) in each scenario, as shown below:
S/Dk may be interpreted as an inverse of scarcity index (Aitkeu et al., 2016). Overbar notation refers to the normalized value. Similarly, net electricity generation (£), which corresponds to total electricity generation minus total electricity consumption, is also normalized by the maximum value (£J, as shown below:
The results are plotted in radar diagrams, as shown in Figure 6. Hie normalized values (E and Vk) range from 0 to 1. Larger values are preferred for each objective.
Figure 6. Radar diagrams for sustainability of technological alternatives. All indicators are normalized by the maximum value. Larger values represent better sustainability of corresponding ecosystem service, (a) Fuel and electricity generation technology options, (b) Cooling technology options, (c) CO, conversion technology options, (d) Renewable power generation
Technology> options for fuel and power generation
Figure 6(a) shows eleven normalized indicators for the scenario where one type of fuel and electricity generation technology are adopted for all fir e fossil power plants in the MRW. Three kinds of fuel and generation technology options include coal-fired steam turbine power plants (Coal ST), conventional NG-fired combined cycle power plants (Com' NG CC), and shale NG-ftred combined cycle power plants (Shale NG CC). All options are assumed to employ recirculating cooling technology (RE).
For most indicators, except Vvg, the coal-fired steam turbine option is the worst of the fuel and generation technology options. This is not only because burning coal causes more air emissions than burning NG but also because coal-fired steam turbine plants have lower generation efficiency than the NGCC plants. In particular, VSQ indicator can be implor ed significantly by employing NGCC plants since NGCC plants har e negligible SO, emissions compared to coal-fired plants. Also, coal-fired steam turbine plants consume more water than NGCC plants per kWh of electricity generated. Moreover, coal mining and coal-fired steam turbine plants require more electricity than NG extraction and NGCC plants. Only the Vya indicator shows that the coal-fired steam turbine option is better than two NG options because coal is selected to be burned as fuel instead of NG. In terms of the comparison between conventional NG and shale NG options, there are very minor differences. For instance, VH 0 value for the conventional NG option is only 4.5% larger than the shale gas option. This is because power generation technology is the same between these two NG options and the impacts of mining activity are negligible compared to the impacts of the thermoelectric activity, which is the most dominant activity for most interventions. Moreover, the slight decrease in VH 0 indicator for the shale option compared to the conventional NG option does not imply a meaningful difference since water resource in the MRW is abundant. Rather, we should take account of the availability of NG reserves because the production rate of shale NG in Ohio is expected to keep increasing (EIA, 2018c).
Cooling technology’ options
Two types of wet cooling technologies, once-througli cooling (ОТ) and recirculating cooling (RE), and dry cooling technology are compared for shale NG-fired combined cycle power plants, as shown in Figure 6(b). For VR 0 indicator, the dry cooling option is better than wet cooling options since it does not require the use of water. However, since dry cooling is 1-3% less efficient than wet cooling methods (Loew et al., 2016), dry cooling requires more electricity and fuel inputs to produce the same amount of electricity. Accordingly, net electricity generation of diy cooling is smaller than that of the wet cooling options. Also, dry cooling technology is much more expensive than wet cooling technologies. It is estimated that the average cost to plant operator for converting the existing power plants in the MRW to the power plants with diy cooling is approximately S340,000/day (Loew et ah, 2016).
The once-througli cooling option shows a larger VH 0 value than the recirculating cooling option. However, once-through cooling technology withdraws a huge amount of water and returns most of it at a wanner temperature, which results in significant thermal water pollution. According to the records for the existing power plants in the MRW, coal-fired steam turbine plant with once-through cooling system causes 19 tunes larger thermal water pollution than that with recirculating cooling system (EIA, 2018a). Therefore, once-through cooling technology is not advisable.
CO, conversion technology options
Although V co indicator can be improved by employing NG instead of coal, its TES metric value is still negative (Vco = -0.87). Figure 6(c) exhibits the results for several CO, conversion technology scenarios to mitigate CO, emissions in the MRW. In this work, it is assumed that 1,000 t/day of CO, are converted to CH4, synthetic gas, and formic acid. The results exhibit the increase in V co indicator and the reduction in E indicator for CO, conversion options compared to the base case.
Table 2 shows the interventions from CO, conversion processes and displacement credits from conventional processes. The CO, conversion processes include CO, capture, compression, and conversion to products. Total CO, emissions from the conversion processes are -745 t/day when 1,000 t/day of CO,
Table 2. Interventions from CO, conversion processes and displacement credits from conventional processes. ‘CO, conversion includes CO, capture, compression, and conversion to products. Stoichiometric reactions are assumed for CO,
is captured and converted to products since approximately 255 t/day of CO, is emitted from the CO, capture process. The conventional coal gasification process to produce syngas has high greenhouse gas emissions. Since these GHG emissions can be avoided as a displacement credit for CO, conversion to syngas, the syngas option shows the best Vco value among the conversion options. On the other hand, in terms of E indicator, the formic acid option shows a higher value than the other two conversion options. This is because the total electricity requirement for CO, conversion process to formic acid is relatively smaller than that for other conversion processes.
In this work, hydrogen is assumed to be provided from water resource. As shown in Figure 6(c), VH 0 values are decreased when CO, is captured and converted to the products. This is because a CO, capture system increases water consumption for cooling (Magneschi et al., 2017). Even though CO, conversion scenarios intensify water consumption, the sustainability of watershed is affected very little because the MRW is not a water-scarce region, as shown in Figure 5. VH 0 values for CO, conversion scenarios are approximately 1.20, which is still positive.
As the results show, in choosing the best option between CO, conversion alternatives, it is crucial to consider the impacts that are avoided from conventional processes as well as technological advances of CO, conversion processes because the displacement credits from the conventional processes can be significant. Also, the results indicate that the increased water consumption from the CO, conversion options is not of much concern since the MRW has abundant renewable water to offset total water consumption. This emphasizes the needs for holistic assessment and TES assessment when addressing the sustainability of watershed.
Using CO, as a source of carbon is an energy-intensive process. In most cases, therefore, CO, conversion technologies are economically expensive. According to the study, the net present value of CO, conversion to formic acid is negative at least for 10 years due to the capital investment cost (Aganval et al., 2011). However, the profitability of CO, conversion process to formic acid can be enhanced if the cost of consumable chemicals is reduced and the life time of catalyst is increased. Also, if the carbon price is included, the additional revenues can be earned through the CO, conversion process.
The limitation of market capacity for formic acid from CO, needs to be considered, especially if the cost for converting CO, to formic acid can be cheaper than that for conventional formic acid. Formic acid is generally used as a preservative and antibacterial agent in animal feed. The global production capacity of formic acid in 2009 was roughly 720,000 t/y (Sankaranarayanan and Srinivasan, 2012).
1,000 t/day of CO, conversion to formic acid in this study corresponds to more than half of the worldwide formic acid production. However, the demand for formic acid could be increased if its production cost is decreased significantly. Formic acid can also be used to remove impurities on the metal surface if its price is competitive enough to replace HC1 and H,SOa (Aganval et al.. 2011). Nonetheless, other CO, conversion pathways, such as syngas production, must be utilized as well to maximize the opportunity for converting CO, to valuable products.
Renewable power generation technology options
CO, conversion options have a considerable electricity requirement, as shown in Figure 6(c). This could make CO, conversion technologies infeasible to be implemented if electricity is provided from fossil fuel power plants which have significant environmental impacts. To offset the impacts of generating electricity for CO, conversion technologies, solar and wind power generation technology options are considered. Figure 6(d) shows the results for CO, conversion to formic acid with different power sources. If 1.000 t/ day of CO, is converted to formic acid, 7.64 TJ/day of electricity is required for the conversion process. Given the barren laud area in the MRW. energy potentials for solar and wind power generation in the MRW are 158 and 56 TJ/day, respectively. Therefore, there is enough land area in the MRW for solar and wind power generation for CO, conversion to formic acid.
If renewable power generation technology is employed for CO, conversion to formic acid instead of fossil fuel-based technology, the impacts from thermoelectric power generation can be avoided. Figure 6(d) exhibits the increase in Vco and VH Q indicators by employing renewable power generation options. Wind power option shows a higher VH 0 value than the solar power option since some water resource is still required for solar power generation technology, as described in section 2.7. VKG indicator can also be improved since renewable power generation technologies do not require the use of NG to generate electricity. Hie scale of those changes is very small because only a tiny portion of fossil power plants (7.64 TJ/day out of 230 TJ/day) are replaced by renewable power generation technologies for
1,000 t/day of CO, conversion.
Forreuewable power generation technologies to be economically feasible, they need to be cheaper than conventional fossil power generation. According to the U.S. EIA report, levelized costs of electricity (LCOE) for NGCC, solar (PV), and wind (onshore) power generation technologies, respectively, are estimated to be 42.8, 48.8, and 42.8 S/kW if new generation facilities are introduced in 2023 (EIA, 2019). The report also describes that the LCOE for solar and wind power generation technologies can be cheaper if federal tax credits are included for the renewable technologies: 37.6 S/kW for solar (PY) and 36.6 S/kW for wind (onshore). In this context, appropriate tax incentives can accelerate the use of renewable generation technologies. Economy models, such as general or partial equilibrium models, could address these tax incentives in the analysis (Golosov et al., 2014).
Although technological alternatives can improve many sustainability indicators, as described above, they only help reduce the environmental interventions from activities, which correspond to the ecosystem demands. According to the TES framework, watershed sustainability can also be enhanced by increasing the supply of ecosystem sendees (Bakslii et ah, 2015). Also, none of the technological alternatives har e significant impacts on water quality indicators, such as Vm and VTp. As shown in Figure 3, most water nutrient runoffs (total N and P loads) are attributed to agricultural activity. In this section, we discuss potential agroecological alternative options that include various tillage practice and available laud use change options.
Tillage practice options
Figure 7(a) shows the results of sustainability indicators for the base case and various tillage practice options. Unlike most technological alternatives addressed in section 3.2, tillage practice options affect water quality-related indicators. No-till option is most sustainable with respect to KJV and VTP indicators. Although uo-till option does help improve Vn. and VTP indicators, changes in these indicators are insignificant. In fact, the prime mover for nutrient runoffs is precipitation rather than the implementation of certain agricultural practices. In this static analysis, however, the amount of precipitation is fixed and only the impacts of agricultural practices on nutrient runoffs are examined.
Figure 7. Radar diagrams for sustainability of agroecological alternatives. All indicators are normalized by the maximum value. Larger values represent better sustainability of corresponding ecosystem service, (a) Tillage practice options.
(b) Available land use change options.
No-till fanning also improves soil carbon sequestration. According to the previous study, the conversion of intensive tillage to no-till for the com and soybean fanning in Ohio can increase carbon sequestration rate by 82.5 gCm“2y_1 (West and Post, 2002). If reduced tillage practice is converted to no-till, additional 69.3 gCm“}y_1 can be sequestered in soil. As a result, the increase in Vcc. is obtained from the no-till option. However. The uo-till practice may reduce food production yield, which is not included in this analysis.
Available land use change options
Figure 7(b) represents the results for available land use change alternatives. If the barren laud is converted to forest, VTS and V Jp indicators are improved since tree cover helps reduce nutrient runoff through soil infiltration. The impacts of reforestation on other indicators, such as Vco^ and VH 0, are negligible although reforestation certainly increases the amount of carbon sequestration service. Since GHG emissions are much larger than the supply of carbon sequestration sen-ice, and therefore, the reforestation does not improve the TES metric for CO, considerably.
Constructed wetlands can be built in the available laud as well. The supply of water regulation service is provided from the wetland. As shown in Figure 7(b), the positive impacts of constructed wetland on VTV and Vrp indicators outweigh the reforestation option.
In addition to two ecological alternatives to land use change, wind fanning option is considered as a technological land use change alternative. Contrary to ecological laud use change options, the installation of wind fanns improves Vco , VH^0, and Vm indicators, instead of water quality-related indicators. The increase in those indicators is attributed to the displacement credits from thermoelectric activity, which is replaced by wind power generation.
Solutions to improve watershed sustainability
In sections 3.2 and 3.3, various technological and agroecological alternatives are investigated as means to improve the overall sustainability of watershed. While technological alternatives mainly help reduce air emissions and water consumption, which correspond to the ecosystem demands, agroecological alternatives improve water quality indicators by enhancing the supply of water quality regulation sendee and reducing water nutrient runoffs. As best case scenarios, the following technological alternatives are selected as technological solutions: Shale NG-fired combined cycle power plants with recirculating cooling system and 1,000 t/day of CO, conversion to formic acid with wind power generation. Also, an agroecological solution is determined as follows: The implementation of uo-till practice and the construction of wetlands on available land.
Figure 8. Radar diagram for sustainability of best case scenarios. Technological solutions include shale NG-fired combined cycle power plants with recirculating coolmg system and 1,000 t/day of CO, conversion to formic acid with wind power generation. Agroecological solutions include the implementation of no-till practice and the construction of wetlands on available land. The synergistic solution combines both technological and agroecological solutions.
Figure 8 shows sustainability indicators for the two solutions described above. As compared to the base case results, VCOl indicator is greatly improved for the technological solution. The reduction in VNQ indicator for the technological solution is inevitable since NG is used instead of coal for thermoelectric power generation. On the other hand, the agroecological solution enhances and VJp indicators. Since each solution recommends alternatives to different groups of activities, both solutions can be combined to obtain synergies between the solutions. Synergistic solution includes alternatives that are selected for both technological and agroecologial solutions addressed above. As shown in Figure 8, the synergistic solution can give a “win-win” situation between objectives.