Hydrogen as a Transportation Fuel

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Hydrogen (H2) has been long considered as a possible fuel for use in transportation and distributed generation, primarily because of its ability to generate electricity with high-energy conversion efficiencies while producing almost nothing other than water as the process byproduct. From a climate perspective, it is important to recognize that H2, like electricity, requires significant inputs of other forms of energy to produce. Most H2 is currently produced from natural gas, although other methods of production from fossil fuels include thermal cracking, partial oxidation, and gasification, all of which generate large amounts of CO2 [94, 95]. One of the primary by-products of steam-methane reforming (SMR) is CO2 which is presently vented to the atmosphere. In order to achieve mitigation from using hydrogen as a transportation fuel, this CO2 by-product must be captured and sequestered. Depending on the efficiency of the reformer, carbon monoxide may be another emission with environmental concern. Hydrogen from the SMR process is at low pressure, and thus energy is required to compress the gas to a pressure required for refueling vehicles. Generally, there is about 25% more energy required to produce hydrogen as there is to produce gasoline [96]. Both of these energy demands will mean more fossil-fuel use and associated emissions.

Other concerns are associated with the methane feedstock. Use of natural gas as a feedstock for hydrogen production will compete with natural gas usage as a fuel in electricity production. The potential for fuel switching to cheaper coal and oil in electric generation would lead to increased pollutant emissions associated with these fuel sources [97]. Increased demands for methane can lead to increased fugitive emissions from methane operations. In addition, demand will lead to greater import quantities and thus the potential for ecosystem impacts as LNG facilities are constructed to receive the imports [98].

Use of renewable energy sources (solar, wind, and biomass) for H2 production has been tested, but these are currently not economically viable, particularly given the need for development of a transport, distribution, and storage infrastructure for H2. Emerging approaches include photobiological, photolysis, and enzymatic methods for H2 production, but these processes remain in the concept stage [99]. In general, increased H2 production requires an increased use of natural gas, with the associated environmental impacts associated with its production, transport, and processing [95]. Life cycle emissions of GHGs may be reduced if H2 is used in high-efficiency fuel cells, but these are not likely to be available in the near term.

Production of H2 fuel cells will require significant increases in the total amount of platinum (Pt) consumed worldwide, with a similar increase in mining and the environmental impacts associated with mining, processing, and transport. Although it has been estimated that the world reserves of Pt are sufficient to meet projected demand [100], most Pt is produced by South Africa and Russia and imported to the U.S. and other industrial nations. This has led to concern over Pt supply bottlenecks [101], and even to evaluation of the potential for production of Pt from waste generated from nuclear power reactors [102]. Although it is unlikely that Pt will be derived from spent nuclear fuel, these proposals do highlight the fact that a switch to a new technology will result in shifts in the critical infrastructure needed to support that technology, and that development and maintenance of that infrastructure may lead in unforeseen directions.

Fundamentally, it must be recognized that H2 is not a primary energy source but is instead a means of energy storage and transport. Thus, the net environmental impacts associated with use of H2 will necessarily include the impacts associated with the production, storage, and transport in addition to the impacts associated with H2 use. It is crucial, then, to evaluate H2 energy systems over the entire life cycle, including the sources of energy used to generate H2, to fully understand the environmental impacts associated with H2 use.

If H2 is used in a significant way as a transportation fuel, consideration must be given to the impacts of H2 emissions from leaks during production, fueling, and operation. Tromp et al. estimated that anthropogenic H2 emissions could increase by a factor of 4-8, and total emissions by a factor of 2-3, if it were used to completely replace petroleum combustion [103]. They modeled the impacts on atmospheric chemistry and estimated that such increased H2 could result in stratospheric ozone depletion by up to 20%, affecting the atmospheric lifetimes of CO and CH4 due to changes in OH- radical concentration, and increase noctilucent clouds (with subsequent impacts on albedo). They also point out that H2 interacts with soils, and changes in atmospheric H2 concentrations could have unforeseen impacts on soil microbial populations.

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