Environmental Engineering Reference
In-Depth Information
ics of such places. If it becomes necessary to transport the CO
2
through pipes over
long distances, costs will climb upward.
An increase in the use of cars seems to be inevitable, but excessive use is a prob-
lem—and not a solution—for urban mobility.
There are now almost 1 billion automobiles circulating in the world, but vehicle
ownership per 1,000 persons varies widely. In the United States there are almost
800 cars per 1,000 people, while in the OECD, the number is approximately 500
cars per 1,000 people. If the use of cars in developing countries, currently around
100 cars per 1,000 people, reaches OECD or US levels, problems related to the en-
vironment, infrastructure, and land use will become insoluble.
From the view point of social organization the best solution to urban mobility
is public transportation. Bus corridors and traffic management are relatively cheap
and fast solutions. Nonmotorized transportation can be stimulated within neighbor-
hoods and communities.
From the view point of technology the future of transportation hinges on im-
provements in the efficiency of current motors and a shift toward electric motors,
with a transitional period of hybrids. Biofuels also offer a possible—albeit par-
tial—solution to the problem of replacing fossil fuels.
Natural gas is one of the fuel alternatives and, compared to gasoline, its use may
achieve up to a 70% reduction in CO
2
emissions and eliminate particulate matter
and SO
x
emissions. Both liquefied petroleum gas (LPG) and compressed natural
gas (CNG) have a higher hydrogen-carbon ratio than gasoline and, therefore, emit
less CO
2
per energy unit. Moreover, their higher octane rates allow their use under
higher compression. Although significant changes in engine design are not needed
to allow the use of LPG or CNG, it is necessary to take precautions to prevent NO
x
and hydrocarbon emissions from increasing considerably due to inadequate con-
versions.
Hydrogen is also an important energy carrier and can be used for ultra-low-
emission vehicles. Hydrogen storage is a problem due to its low-energy density.
The use of compressed hydrogen is the most viable form, although it is also pos-
