since they can run for a longer time on electricity. Automakers expect that plug-in hybrids
will become available around 2010.
Fuel cellsare another technology that is in development (Figure22.13). Hydrogen fuel
cells harness the energy released when hydrogen and oxygen come together to create water.
Fuel cells are extremely efficient and they produce no pollutants. But developing fuel cell
technology has its problems. The oxygen the fuel cell uses comes from the atmosphere,
but there is no easy source of hydrogen. Natural gas is a source, but converting it into
usable hydrogen decreases the efficiency of the fuel cells and increases pollution, including
greenhouse gases. Natural gas also has other important uses. A few fuel cell cars are now
being produced as models. Right now these cars are extremely expensive and fueling stations
are rare. Some automakers say that for fuel cell vehicles to become widespread the cost of
production must decrease to 1% of its current price.
Figure 22.13: A hydrogen fuel cell car looks like a gasoline-powered car. ( 15 )Reducing Industrial Air Pollution
Pollutants are removed from the exhaust streams of power plants and industrial plants before
they enter the atmosphere. Particulates can be filtered out, while sulfur and nitric oxides
are broken down by catalysts. Removing these oxides reduces the pollutants that cause acid
rain.
Particles are relatively easy to remove from emissions. Baghouses work like a giant vacuum
cleanerbag, filteringdustasitstreamspast. Baghousescollectabout98%ofdryparticulates.
Cyclones are air streams that rotate quickly through a container shaped like a cylinder or
a cone. Large particles are forced toward the edges of the air stream. When they hit the
outside wall of the container, they fall to the bottom and are swept up. Smaller particles
can be picked up as the radius of the cyclone is reduced. Particles can also be collected
and removed by static electricity. These electrostatic precipitators are useful for removing
materials from very hot gases.