Life cycle assessment of hydrogen production via thermochemical water splitting using multi-step Cu–Cl cycles

Abstract

A comparative environmental impact study of the three-, four- and five-step copper–chlorine (Cu–Cl) thermochemical water splitting cycles is undertaken through life cycle assessment (LCA). This analytical tool is used to identify and quantify environmentally critical phases during the life cycle of a system or a product and/or to evaluate and decrease the overall environmental impact of the system or product. The LCA results for the hydrogen production processes indicate that the four-step Cu–Cl cycle has lower environmental impacts than the three- and five-step Cu–Cl cycles due to its lower thermal energy requirement. The global warming, acidification and eutrophication potentials of the system using the four-step Cu–Cl cycle are 0.56 kg CO₂-eq, 0.00284 kg SO₂-eq and 0.000232 kg Phosphate-eq, respectively.

Introduction

Global energy demand tends to increase with an increasing population. Today's energy systems, which are based mainly on fossil fuels, cannot be considered as sustainable. Concerns about energy supply security are increasing due to declines in the fossil fuel resources leading to increases in prices of energy carriers, local air pollution and global climate change. Petroleum is a central concern, with a share of more than one third of global primary energy consumption and more than 95% of the energy consumption in the transport sector. The negative environmental impacts of coal mining combined with the large contribution of coal usage to global carbon dioxide emissions, as well as the risks of the dwindling reserves of natural gas, are other factors of concern (Urbaniec et al., 2010). Hence, alternatives to fossil fuels have been sought. Increases in energy demand will likely lead to growth in nuclear and renewable energy utilization, partly to meet the objective of sustainability. The shift from fossil fuels to nuclear and renewable resources is expected due to increases in energy demand as well as concerns over environmental issues like global warming. Jaber (2009) states that in the future all energy systems are expected to be hybrid systems, which combine various energy resources and energy conversion methods to maximize efficiency while reducing environmental impacts and wastes. Hydrogen is a promising candidate as an energy carrier (not an energy source) that helps expand markets for renewable and nuclear energy resources and contributes to sustainability and environmental stewardship, and that can act as a link between these technologies when they are utilized in hybrid systems.

The energy carrier hydrogen is expected by many to become an important fuel that will help solve several energy challenges. Since its oxidation does not emit GHGs, its use does not contribute to climate change, provided it is derived from clean energy sources. Numerous researchers anticipate that hydrogen will replace petroleum products for fuelling of transportation vehicles, in turn decreasing dependence on petroleum. Industrial sectors are also interested in hydrogen energy. Hydrogen complements the energy carrier electricity, which can be generated from a variety of primary energy sources and is widely used in a broad range of applications. These two energy carriers are expected to have complementary roles in the future, in part since hydrogen adds the capability of storage (Urbaniec and Ahrer, 2010). Hydrogen exists in abundance in nature in the form of water. But pure hydrogen needs to be produced and there are several ways this can be accomplished including steam reforming of natural gas, coal gasification, water electrolysis and thermochemical water decomposition. Dufour et al. (2009) indicate that 96% of the world's hydrogen is produced using fossil fuels, and steam reforming is the most commonly used method.

H₂ production using thermochemical water splitting cycles has the potential to be cleaner and more cost-effective than other production methods. Although H₂ production systems using thermochemical cycles have not yet been commercialized, studies have shown that such systems can be expected to compete with the conventional H₂ production methods including steam methane reforming (Orhan, 2008; Lewis et al., 2009; Pilavachi et al., 2009). Water can be directly split in one step, but the required process temperature is too high to be practical. However, a series of selected chemical reactions can be utilized to achieve same result at much lower temperatures (Funk, 2001). A variety of thermochemical water decomposition cycles have been identified (Funk, 2001), but a few have progressed beyond theoretical calculations to working experimental demonstrations. Considerations of several factors including availability and abundance of materials, simplicity, chemical viability, thermodynamic feasibility and safety, the following cycles have been identified as of possible commercial significance: sulphur–iodine (S–I), copper–chlorine (Cu–Cl), cerium–chlorine (Ce–Cl), iron–chlorine (Fe–Cl), magnesium–iodine (Mg–I), vanadium–chlorine (V–Cl), copper–sulphate (Cu–SO₄), Ni–Ferrite (NiFe₂O₄), cerium-oxide (CeO₂/Ce₂O₃), ZnO/Zn and Fe₃O₄/FeO redox reactions and hybrid chlorine (Naterer et al., 2008; Steinfeld, 2002; Ishihara et al., 2008; Abanades and Flamant, 2006). Most of these cycles require process heat at temperatures as high as 800 °C and above. Due to its lower temperature requirements (around 530 °C), the Cu–Cl thermochemical water decomposition cycle has some advantages over other cycles (Naterer et al., 2008), including reduced material and maintenance costs. Moreover, the Cu–Cl cycle has some advantages over other existing H₂ production methods, and it can utilize low-grade/waste heat to improve its efficiency (Naterer et al., 2009).

Fossil fuels, nuclear energy and renewables can be used as energy sources for H₂ production. Renewables are usually considered the most environmentally benign alternative. But, an important challenge is to obtain sustainable large-scale H₂ production, although daily production capacity of ∼38,000 kg H₂ can be obtained using solar power towers and electrolyzers (Kolb et al., 2007). Furthermore, fossil fuels negatively affect the environmental significantly. Using nuclear energy for H₂ production is consequently advantageous for two main reasons. First, nuclear plants do not emit GHGs during operation. Second, nuclear energy can contribute to large-scale H₂ production (Orhan, 2008). For these reasons, thermochemical water decomposition linked with nuclear plants is seen as a promising alternative for H₂ production. Despite rising concerns regarding uranium resources, the International Atomic Energy Agency (IAEA) and the Nuclear Energy Agency (OECD/NEA) project uranium reserves as 5.47 million tons and the reserve-production ratio as more than 100 years. Moreover, the reserve-production ratio becomes over 3000 years if a Fast-Breeder reactor becomes commercial (Saito, 2010). The Generation IV SCWR (super-critical water cooled reactor) is viewed as a particularly suitable option for pairing with the Cu–Cl thermochemical cycle.

Although hydrogen is a clean energy carrier since its oxidation mainly emits water, negative environmental impacts can arise during its production. Hence, the environmental impact of H₂ production methods should be investigated. H₂ production using thermochemical water splitting driven by clean energy sources has lower environmental impacts than conventional methods. A comprehensive study has not been performed of the environmental impacts of the Cu–Cl cycle, although related research is available for the other H₂ production methods.

Life cycle assessment (LCA) is essentially a cradle to grave analysis to investigate environmental impacts of a system or process or product. LCA provides an analytical tool for evaluating and decreasing environmental impact. Heikkila (2004) points out that LCA can also be used to identify environmentally critical phases in the life cycle of a system or a product. A life cycle assessment is required for the Cu–Cl thermochemical hydrogen production.

The objective of this study is to investigate the environmental impacts of nuclear-based hydrogen production via thermochemical water splitting using the Cu–Cl cycle by performing a life cycle assessment. Since a detailed assessment has not yet been performed of the environmental performance of the Cu–Cl cycle, this work fills an important need. Beyond the environmental performance of H₂ production, other factors (such as economical, safety, instrumental, chemical challenges) must be taken into consideration in decision making. However, this paper only deals with the issues related to emissions. The specific objectives are as follows:

• To conduct an LCA of nuclear-based hydrogen production using the three-, four-, and five- step Cu–Cl cycle for four different scenarios, which are defined as follows:

  • All processes use power from the electrical grid.

  • Electricity needed for electrolysis in the hydrogen production plant is provided by a nuclear power plant, while the remaining electricity needs are met using power from the electrical grid.

  • Electricity needed for electrolysis and heavy water production is provided by a nuclear power plant, and electricity required for fuel processing is provided by the electrical grid.

  • All electricity needs are met using power from a nuclear power plant.

• To determine based on the findings from the scenarios employed the corresponding measures for the processes of the following CML 2001 environmental impact categories: abiotic resource depletion potential (ADP), acidification potential (AP), eutrophication potential (EP), global warming potential (GWP), ozone depletion potential (ODP), photochemical ozone creation potential (POCP) and radiation (RAD).