Water is essential to maintaining life on Earth, and it is also a key resource for electricity generation. It is expected that worldwide water demand will double by 2050 due to population growth and the improvement of living standards [1
nexus has a significant influence on a country’s energy development. The amount of water required for electricity generation depends on the capacity of the plant, the power generation technology, and the availability and location of water sources. Electricity generation has a potential impact on the quantity and quality of available water [2
In Mexico, according to the National Water Commission [3
], by the end of 2016, the volume of water allocated for offstream use was 85,664 Mm3
, of which 4149 Mm3
was used for thermoelectric plants.
Energy consumption is expected to grow at an average annual rate of 3.1% between 2018 and 2032, that is, from 320,629 GWh (2018) to 492,165 GWh (2032), which represents an increase of 53.5% (171,536 GWh). This future demand will be satisfied with new plants—an added capacity of 66,912 MW, of which 42% (28,105 MW) will be generated by combined-cycle technology [4
Combined-cycle gas turbine (CCGT) plants use two technologies: a steam turbine cycle (conventional thermoelectric) and a gas turbine cycle (turbo gas), resulting in higher efficiency—close to 59% at full load compared to conventional systems, whose efficiencies range between 32% and 39%. For this reason, the amount of CO2
emitted per kWh of electricity generated is nearly 40% lower than that emitted by other types of power stations that use fossil fuels. Emissions of sulfur dioxide (SO2
), nitrogen oxides (NOx
), and solid particles are very low and, in some cases, negligible. Therefore, CCGT plants have lower electricity generation costs and cause less environmental pollution compared to plants that use only one technology [5
]. Furthermore, their construction time is relatively short because they can be installed by modules—the gas turbine first and the steam turbine later—increasing their operational flexibility. Due to their characteristics, these technologies are becoming widespread in the Mexican electricity sector [6
On the other hand, CCGT plants use chemicals (some of them toxic) throughout the different stages of the electricity generation process. These materials are released into the environment as atmospheric emissions or wastewater discharges, with impacts on natural resources, air, soil, and water [5
Chemical contamination of water resources has serious implications on aquatic ecosystems. These effects range from eutrophication—which can lead to oxygen depletion in water, killing fish and other aquatic populations—to health problems such as cancers, reproductive disorders, damage to cellular structures and DNA, and malformations. Furthermore, adverse effects on aquatic microorganisms can have a ripple effect throughout all levels of the food chain. Chemicals that accumulate in aquatic organisms bioconcentrate in other animals that feed on these organisms [7
In the United States of America, some environmental studies on steam turbine plants identified large amounts of toxic pollutants discharged into surface water sources. Most of these toxic pollutants—which include arsenic, lead, mercury, selenium, chrome, and cadmium—remain in the environment for many years. This concern led the Environmental Protection Agency (EPA) to establish federal limits on the levels of toxic metals in wastewater discharged from power plants to preserve environmental and public health conditions. These regulations were included in the EPA 2015 Final Rule [8
]. In Mexico, the Mexican Official Norm NOM-001-SEMARNAT-1996 [9
] establishes the maximum permissible limits of pollutants in wastewater discharged into national waters and resources.
However, there are chemicals—known as emerging pollutants—that are not currently regulated and could have a negative effect on health. Their occurrence on the environment, risk contribution, and ecotoxicological effects are often unknown. Emerging pollutants may be candidates for future regulation, depending on the results of ongoing studies on their potential effects on health and the incidence monitoring data [10
A powerful tool for evaluating environmental impacts is the life cycle assessment (LCA) methodology. This technique identifies and quantifies the energy and material inputs as well as environmental releases associated with all the stages of the life cycle of a product or service—from raw material extraction to disposal or recycling [11
In our review of the literature on LCAs of electricity generation systems, we found that most of the studies evaluate the environmental impact by focusing on the use of different types of fossil fuels and/or by comparing alternative technologies [12
Some of these studies have only focused on natural gas [17
]. Others such as that of Spath and Mann [18
] are more comprehensive. They conducted an LCA of a natural gas combined-cycle system to identify its environmental advantages and disadvantages, and their scope included the fuel life cycle, from extraction and transportation to plant construction, operation, and dismantling.
Agrawal et al. [15
] carried out an LCA on a CCGT plant to identify the midpoint (problem-oriented) and endpoint (damage-oriented) impacts. The LCA included the processes within the power plant boundary (electricity, water usage, natural gas combustion, wastewater disposal, etc.), and it also considered upstream processes, such as extraction, natural gas processing, and transportation.
All of these studies focused on fossil fuel consumption and considered upstream and downstream processes. Furthermore, the electricity generation process—which involves water conditioning, the water–steam cycle, and the cooling system—is seen as a whole, without getting into details about its internal functioning.
Mertens et al. [19
], on the other hand, determined the water footprint of CCGTs and, by analyzing different cooling systems, looked at each stage of the electricity generation process separately. The authors applied three different methodologies and took as criteria the type of fuels, the water used in the demineralization process, and the cooling technologies employed—aero condensation, cooling towers with surface raw water and seawater, and open circuit.
The results reported in most of the articles analyzed suggest that environmental impacts are caused mainly by the high consumption of fossil fuels and the effects of their combustion and upstream processes—extraction, processing, and transportation.
However, to our knowledge, no previous study has investigated the effects of the chemicals used for water conditioning in a CCGT plant.
To determine the environmental impact of the chemicals used for water conditioning in a CCGT plant, an LCA of the electricity generation process was carried out. The process was broken down into stages, which were analyzed separately. An analysis of the fuel cycle and the materials used for maintenance works was also carried out to complete the study.
Our results showed that the most affected categories were related to air, water, and toxicity to the environment and living organisms. These impacts were mainly caused by the amount of natural gas, the volume of water extracted, and the manufacture and consumption of the chemicals used in the process.
The rest of this paper is organized into three sections. Section 2
describes the methodological approach taken in this study; this section also includes the criteria that was used to interpret the results. Section 3
presents the results of the LCA, focusing on contribution and sensitivity analyses. Finally, Section 4
provides a summary of the findings, comments on the limitations of the study, and suggests areas for further research. A list of the acronyms, initialisms, and chemical symbols used throughout this paper is included in Appendix A
An LCA of the electricity generation process was carried out to determine the environmental impacts of the chemicals used for water conditioning in a CCGT plant. The process was broken down into stages, which were analyzed separately. In each stage, the materials/processes that had the highest contribution to the impact categories were identified.
Our results showed that most affected categories were related to water and toxicity to the environment and living organisms. Water depletion (9.77 × 10−1 m3/MWh) was caused mainly by the high volume of water consumption in the cooling systems and the reverse osmosis process; ecotoxicity—freshwater (8.49 × 10−3 kg 1,4-DB eq/MWh), marine (7.35 × 10−3 kg 1,4-DB eq/MWh), and terrestrial (4.92 × 10−5 kg 1,4-DB eq/MWh)—and human toxicity (1.1 × 10−1 kg 1,4-DB eq/MWh) were due to the manufacture and consumption of the chemicals used.
Not surprisingly, the high contribution of the fuel cycle overshadowed the contributions of the rest of the stages to the impact categories. However, these are the stages to which chemicals are added. Some of these substances are toxic to aquatic organisms. The effects of the likely presence of d-Limonene, a bio-dispersant, in the wastewater discharge were identified through a sensitivity analysis. The results showed that this chemical increases the already high contributions to human toxicity, freshwater and marine ecotoxicity. Unlike hydrazine, which is known for its adverse effects on the environment and has been identified as a pollutant, the rest of the organic compounds used for water conditioning could be considered as emerging pollutants. Further research should be done to investigate their effects as they could potentially leave traces in wastewater discharges.
It is essential to identify these toxic compounds and quantify their presence in the environment and living organisms to create models for assessing their impact. The lack of models for some chemicals made it difficult to obtain better results as the use of limited data provides only a rough estimate of the impacts. However, these first approximations can yield some insight into the potential effect of the compounds under study. By creating more robust databases, the results obtained by applying the LCA methodology would provide a more complete picture of the impact of these substances on the environment. Hence, the importance of this study is twofold: first, the data that was generated could contribute to the expansion of environmental datasets as the study was carried out in a Mexican power plant; and, second, this study identifies the effects of commonly-used chemicals in power plants—effects that, to our knowledge, have never been reported before.
This methodology approach, in conjunction with the block diagram proposed, could be applied to evaluate other electricity generation technologies and find better production alternatives that use less harmful chemicals for water conditioning, reduce environmental impacts, lower electricity generation costs, and make the processes more efficient. By fostering a closer link with academia, public and private sectors could work with researchers to align their work with these goals.
In addition, government authorities could use these results to define guidelines with stricter discharge parameters to be included in environmental assessments.
Furthermore, it is important to let consumers know about the environmental impacts that the electricity generation process causes and the close link between water and energy. In this way, a culture of electricity optimization and consumption can be encouraged to protect our environment, reducing the impact on climate change, water consumption, and toxicity to humans and other living organisms.
Water is essential for supporting life, and it is a key resource for economic development. The threat of water scarcity highlights the need for a more efficient and responsible water resource management to preserve the health of aquatic ecosystems, within a sustainability framework.
By 2032, the demand in Mexico for water and energy will increase, and the construction of CCGT plants will continue to grow because of their high efficiency. CCGTs are expected to generate 246,990 GWh of electricity, which would require, at least, a yearly consumption of approximately 46 t of cyclohexylamine for water conditioning. This is just one of the many toxic chemicals used for this process by this type of technology.
For this reason, it is imperative to protect our water resources and implement new public policies aimed at achieving energy efficiency in the production and consumption of electricity and the development of innovative water-treatment technologies.
LCA, without a doubt, is a powerful tool. Even though it contains a particular value of uncertainty, this methodology helps to identify the potential environmental impacts, providing information that can be used as input for further research.