Technology Status of Hydrogen Road Vehicles

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7. Environmental Impacts of Hydrogen Vehicles
The great attraction of hydrogen is pollution-free combustion...... Hydrogen vehicles would not produce,
either directly or indirectly, significant amounts of CO, HCs, particulates, SO x, sulfur-acid deposition, ozone
and other oxidants, benzene and other carcinogenic aromatic compounds, formaldehyde and other aldehydes,
lead and other toxic metals, smoke, or CO 2 and other Greenhouse gases. The only pollutant of concern would
be NO x. If hydrogen is made from water using a clean power source, then hydrogen production and
distribution will be pollution-free. Hydrogen thus is a truly clean fuel.
In this section, the environmental impacts of hydrogen production, distribution and end-use are reviewed with
particular emphasis on the environmental impacts of using coal as a feedstock for hydrogen, NO x emissions
from vehicles, and the Greenhouse effect of substituting hydrogen for gasoline and diesel fuel.
Concerning hydrogen production and distribution, ..using clean solar power or other forms of renewable
energy is essentially pollution-free.
The manufacture of some solar energy-converting technologies produces small amounts of undesirable byproducts....
Although solar-hydrogen plants are likely to be land-intensive, land requirements per se are not likely to be
an important restriction on the development of solar hydrogen production.
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The use of coal to produce hydrogen would not be a desirable long-term option from an environmental
standpoint: first, coal mining is environmentally damaging and dangerous; second, emissions from coal-tohydrogen
plants may be significant and harmful; third, and most importantly, the use of coal inevitably
releases large amounts of CO 2 and other trace Greenhouse gases.
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Concerning vehicular emissions, NO x is the only significant pollutant from hydrogen vehicles....(and) is
undesirable for several reasons. First, it can cause acute and perhaps long-term respiratory ailments. Second,
reactive hydrocarbons and NO x are involved in a complex series of chemical reactions that form ozone, a potent
oxidant that damages plants and materials and also causes respiratory problems. Third, emissions of NO x form
particulate nitrates, which reduce atmospheric visibility and, again, may have serious respiratory effects.
Nitrates are the principal constituent of acid deposition in the Western U.S. Finally, NO x forms other toxic,
mutagenic and carcinogenic compounds......
Formation of NO x in any internal combustion engine is primarily a function of reaction temperature, and
duration and available oxygen. Emissions of NO x increase with the combustion temperature, the length of the
high-temperature combustion period, and the availability of oxygen, up to a point. There are several ways to
control NO x in a hydrogen engine: run the engine very lean, which lowers the temperature, or very rich, which
reduces the oxygen supply; decrease the burn time or lower the engine rpm (which allows for better heat
dissipation); or cool the combustion environment by adding water or exhaust gases or using cryogenic fuel.
From the (available) data, and theoretical discussions, several conclusions can be drawn:
! optimized hydrogen vehicles can emit much less NO x than do comparable, optimized gasoline vehicles;
! an optimized hydrogen vehicle could meet the current US NO x standard, and perhaps have lower lifetime
average NO x emissions than a current-model catalyst-equipped gasoline vehicle, without after-treatment
of the exhaust gas;
! the NO x emissions deterioration rate probably would be lower for hydrogen vehicles than for gasoline
vehicles;
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