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# Effect on the Environment

One major claim for the introduction of hydrogen based technologies is the promising environmental benefits of such technologies. Hydrogen is considered a clean fuel capable to avoid the greenhouse effect caused by carbon dioxide releases using fossil fuels. A supporting argument is as follows: Hydrogen may be produced by water electrolysis and under usage it is oxidized back to water without producing any pollutants. However, this is to a certain extent an idealistic view as the major amount of hydrogen is currently produced based on fossil fuels, though in the future hydrogen to a much higher degree may be produced using sustainable energy sources as e.g. wind or water power. The environmentally friendly production of hydrogen depends therefore strongly on the technology applied. The usage of fossil fuels like natural gas conversion to produce hydrogen may result in an increase of greenhouse gas emissions and other pollutants compared with the present situation. More fossil fuels are needed to produce enough “hydrogen energy” for the customers. A solution to avoid such pollution could be technologies of carbon sequestration and other cleaning measures to capture the pollutants to be considered for the specific fuel used, which also would favor central, large-scale hydrogen production and distribution facilities.

Hydrogen is a permanent gas and any diffusive, targeted or accidental release will emit predominantly into the atmosphere. Presently, the major sources for hydrogen emissions are biomass burning, e.g., forest fires, emissions from the traffic using ICE’s. It is also produced by photochemical atmospheric reactions of formaldehyde. It is resulting, e.g., from methane oxidation cycle in the atmosphere as shown in the following reaction equations where one photolysis path for formaldehyde (CH2O) is generating hydrogen.

 $${ CH_4 + OH\cdot {{\scriptscriptstyle{-H_2O}} \atop {\longrightarrow}} CH_3\cdot }$$ $${ CH_3\cdot {{\scriptscriptstyle{+O_2,M}} \atop {\longrightarrow}} CH_3O_2\cdot }$$ $${ CH_3O_2\cdot {{\scriptscriptstyle{+NO,-NO_2}} \atop {\longrightarrow}} CH_3O\cdot {{\scriptscriptstyle{+O_2,-HO_2\cdot}} \atop {\longrightarrow}} CH_2O}$$ $${ CH_2O {{\scriptscriptstyle{h ν}} \atop {\longrightarrow}} CO + H_2 }$$

The hydrogen content of the atmosphere is compared to other atmospheric gases in Table 1 and is found to be on the trace gas level. The atmosphere is divided into different spheres as shown in the figure based on an average temperature profile, which is very important concerning the gas exchange between these spheres.

Table 1: Average relative composition of the troposphere at mid latitudes (HeicklenJ:1976)

 Gas Concentration (ppm) Nitrogen, N2 780840 Oxygen, O2 209460 Argon, Ar 9340 Carbon dioxide, CO2 325 Sum of nobel gases (He, Ne, Kr, Xe) 24.6 Methane, CH4 1.4 Hydrogen, H2 0.5 Nitrous oxide, N2O 0.25 Carbon monoxide, CO 0.08 Ozone, O3 0.025 Nitroxides, NO + NO2 0.006

The hydrogen concentration profile with altitude is found constant at a level of about 0.5 ppm in the troposphere and stratosphere as shown in Table 2. Only in the very high atmosphere, there is an increase of hydrogen.

Table 2: Average relative hydrogen concentration in different altitudes (HeicklenJ:1976)

 Altitude (km) Hydrogen (ppm) 0 0.5 20 0.5 40 0.5 60 0.5 80 4.3 100 1.0

Hydrogen is the only gas capable to escape into space, but it is found that nearly 100 % of the hydrogen is degraded by the photochemical atmospheric processes or deposited back to the biosphere. The atmospheric degradation accounts for about 25 %, while the dry deposition is about 73 % (SchultzMG:2004a). The current hydrogen releases are estimated as follows: 40 million t/yr from methane, VOC releases and their photochemical degradation, 16 million t/yr from biomass burning, 15 million t/yr from traffic and industries, and 6 million t/yr from soils and ocean. The sinks for hydrogen are: 19 million t/yr degraded by reaction with OH free radicals, and 56 million t/yr soil uptake. The numbers are uncertain and are estimated to be within ± 10 million t/yr for traffic and industry emissions and ± 15 million t/yr for soil deposition. Therefore the biological processes to degrade hydrogen are very important for its overall atmospheric balance. These processes are not well understood presently as, e.g., the capacity for hydrogen degradation is unknown. The biosphere could be acting as a large buffer keeping the atmospheric hydrogen concentration constant even though the releases are increased due to the activities from a hydrogen economy.

Recently some concerns on possible adverse environmental effects caused by large increased atmospheric hydrogen concentrations have been described in the literature ((LoganJA:1981); (SchultzMG:2003); (TrompTK:2003); (PratherMJ:2003); (SchultzMG:2004); (WennbergPO:1998)). In the following, the present knowledge is presented.

Hydrogen is not a greenhouse gas, as it does not absorb electromagnetic radiation within the infrared spectrum. Therefore higher atmospheric hydrogen concentrations will not directly contribute to the climate forcing. The consumption of hydrogen will result in water regardless its usage in fuel cells, being burned under controlled or uncontrolled conditions like under accidental fires or explosions. The water will be released into the troposphere with its normally huge content of water. So the additional water will not raise considerably the overall content of water vapor in the troposphere. An adverse effect is identified when hydrogen is atmospherically degraded in the lower stratosphere, where the persistent ice (from the water) may have a cooling effect that again influences the temperature dependent ozone depletion mechanism.

This has a direct effect on the above mentioned possible cooling effect for the lower stratosphere, as it has been predicted through model calculations. It has been shown (TrompTK:2003) that, e.g., a fourfold increase of atmospheric hydrogen will have considerable effects on the ozone depletion. For atmospheric photochemical degradation of hydrogen, the initiating and rate determining step is by an OH free radical reaction with hydrogen.

 $${ H_2 + OH\cdot {{\scriptscriptstyle{}} \atop {\longrightarrow}} H\cdot + H_2O }$$ $${ H\cdot + O_2 {{\scriptscriptstyle{M}} \atop {\longrightarrow}} HO_2\cdot }$$

For atmospheric photochemical degradation of carbon monoxide or other hydrocarbons, the initiating and rate determining step is by an OH free radical reaction with the CO or the hydrocarbon leading to stable products and other free radicals. The latter (blue coloured equations) are reacting with other atmospheric components e.g. NO to give ozone and to regenerate OH free radicals in chemical and photolytic reaction steps. By that the atmosphere contains a stable OH concentration of about 106 molecules per cm3 that is sufficient to remove pollutants.

 $${ CO + OH\cdot {{\scriptscriptstyle{O_2}} \atop {\longrightarrow}} HO_2\cdot + CO_2 }$$ $${ NO + HO_2\cdot {{\scriptscriptstyle{}} \atop {\longrightarrow}} OH\cdot + NO_2 }$$ $${ NO_2 {{\scriptscriptstyle{h ν}} \atop {\longrightarrow}} NO + O(^3P) }$$ $${ O(^3P) + O_2 {{\scriptscriptstyle{M}} \atop {\longrightarrow}} O_3 }$$ $${ O_3 {{\scriptscriptstyle{h ν}} \atop {\longrightarrow}} O_2 + O(^1S) }$$ $${ O(^1S) + H_2O {{\scriptscriptstyle{}} \atop {\longrightarrow}} 2 OH\cdot }$$

The emissions of hydrocarbons and nitrogen oxides lead to the generation of ozone. While this is a very important protection against short wave ultraviolet radiation in the stratosphere, ozone is to be considered a pollutant in the troposphere. It is part of the effects that lead to “forest dead” by acid rain as ozone destroys the protecting wax layer on leaves, and it has adverse health effects to people. In summertime, dangerous ozone levels can build up leading to restrictions for traffic, e.g., in Germany to mitigate high ozone levels. Therefore the establishment of the hydrogen economy will improve the air quality in the cities as the hydrocarbon and nitrogen oxide emissions are reduced. On the other hand nitrogen oxide is an important intermediate for the generation of the constant background concentration of OH free radicals. Model calculations have shown that the OH concentration is slightly decreasing when the nitrogen oxide concentration is substantially decreased. By that the oxidative capacity of the atmosphere is slightly reduced that would give a bit longer lifetime for, e.g., the green house gas methane thus supporting the climate forcing effect (SchultzMG:2003). On the other hand, the emission of carbon dioxide will be reduced avoiding climate forcing.

Nevertheless, less environmental pollution using hydrogen is only to be achieved using sustainable primary energy sources for hydrogen production. Using fossil fuels without carbon dioxide sequestration will result in total in even more pollutant emissions. Therefore the overall environmental benefits need to be assessed using live cycle assessments for the larger-scale production systems.

### Conclusions

The environmental problems with hydrogen are connected with a strong increase of the emissions into the atmosphere. Most of the hydrogen is degraded in the biosphere. More precise knowledge on the mechanisms and capacities are needed to make better model calculations on the possible increase of hydrogen due to the new technologies based on hydrogen and fuel cells.

At the moment, a precaution may be to reduce the emissions as much as possible, which is not only related to the environment, but also to safety, as hydrogen fires and gas phase explosions are very prominent concerns for hydrogen applications. There may also be some economic benefits as production, distribution and storage of hydrogen need substantial amounts of energy making the hydrogen rather costly, but more detailed analyses are needed here. Therefore it would be a win-win situation to minimize leakages.

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