The former lead to a reduction in PM emissions of
more than 95%, whereas the latter are more permeable and less effective, but need no maintenance. In order to prevent the filter from
blocking the particles captured by the filter must
be periodically removed (a process referred to as
“regeneration”). The regeneration may be induced
using, for example, a heater, fuel burner, or a
reactant (such as NO 2 ) generated in situ. DPF
technologies are also suitable for use on rail and
marine engines [124]. However, they can be very
bulky and for rail locomotives there are questions
concerning the feasibility of retrofitting. Moreover, they appear to be a less cost-effective measure than re-engining [123].
SCR will be the main technology for enabling
compliance with future NO x emission standards.
The method works by injecting ammonia or urea
into an exhaust stream to chemically reduce NO x
to nitrogen. Again, SCR systems are very bulky;
the catalyst itself can be considerably larger than
the engine. Space is also required for oxidation
catalysts and the tank for storing urea or ammonia.
This puts severe restrictions on its potential for
use as a retrofit item to rail vehicles – where spare
space and weight availability is limited – and
again re-engining appears to be a more costeffective option [123]. SCR is in use in a few
ships and is still being developed. As well as
being an effective NO x -reduction measure, one
study has also shown that SCR can reduce ship
PM emissions by around 50% [35]. No limitations
are thought to have been encountered regarding
the fitting of SCR to different types of ship, and it
can be integrated with other aftertreatment
devices [96]. However, sufficiently high temperatures in the exhaust are required for the SCR
process to function efficiently. Consequently,
large NO x reductions are mainly achieved at
higher engine loads. Other drawbacks include
the space and additional weight requirements,
and the potential for “ammonia slip.” This occurs
when excess urea or ammonia is injected into the
exhaust stream, and any unused reagent is emitted
from the exhaust. The risk of ammonia slip is
higher for transient load changes (i.e.,
maneuvering) [96].
Another shipping technology is seawater
scrubbing, in which the exhaust gas is passed
through water. This is effective at reducing emissions of SO 2 and probably has benefits for other
pollutants.
Alternative Fuel and Vehicle and Technologies The development of technology in the
longer term will undoubtedly be driven more by
the need to reduce CO 2 emissions than by the need
to improve air quality. In many cases, a “win-win”
outcome is possible, although there can be some
conflicts. For example, some studies have indicated that NO x emissions can increase as a result
of biofuel use [125].
Alternative fuels include liquefied petroleum
gas (LPG), compressed natural gas (CNG), and
various biofuels (biomethane, ethanol, esterified
vegetable oils, etc.). For road transport, limited
supplies mean that LPG could never fully replace
petrol or diesel, and the scale of uptake of CNG
has been impaired by the cost of converting car
engines [126]. The availability of suitable engines
for rail applications is uncertain; available natural
gas engines can also only cover the lower end of
the power range needed for railway applications,
and there appears to be no use of LPG
[123]. Around 50 seagoing ships are currently
powered by natural gas, with a reduction in emissions of PM and NO x of at least 70%
[127]. Biofuels can be used in low concentration
blends (5–10%) with petrol or diesel without
engine modification, and in high concentrations
(85–100%) with specially adapted vehicles. The
main benefit of biofuels is their contribution to the
reduction of CO2 emissions; its effects on NO x
and PM are less clear. Barriers to increased biofuel
use include higher costs, compatibility with conventional fuels, and availability of technology for
more advanced biofuels [15]. In addition, there
have been warnings of hidden costs and adverse
impacts on the environment and society, which
could arise from the large-scale production of
biofuels [128]. It has also been noted that the use
of alternative fuels is context-specific; a fuel that
is appropriate to one location may not be appropriate to another [23].
Automobile propulsion technologies that are
currently under development include “mild” and
“full” hybrid vehicles, “plug-in” hybrid vehicles,
battery electric vehicles, hydrogen ICE vehicles,
Air Quality, Surface Transportation Impacts on
71
more than 95%, whereas the latter are more permeable and less effective, but need no maintenance. In order to prevent the filter from
blocking the particles captured by the filter must
be periodically removed (a process referred to as
“regeneration”). The regeneration may be induced
using, for example, a heater, fuel burner, or a
reactant (such as NO 2 ) generated in situ. DPF
technologies are also suitable for use on rail and
marine engines [124]. However, they can be very
bulky and for rail locomotives there are questions
concerning the feasibility of retrofitting. Moreover, they appear to be a less cost-effective measure than re-engining [123].
SCR will be the main technology for enabling
compliance with future NO x emission standards.
The method works by injecting ammonia or urea
into an exhaust stream to chemically reduce NO x
to nitrogen. Again, SCR systems are very bulky;
the catalyst itself can be considerably larger than
the engine. Space is also required for oxidation
catalysts and the tank for storing urea or ammonia.
This puts severe restrictions on its potential for
use as a retrofit item to rail vehicles – where spare
space and weight availability is limited – and
again re-engining appears to be a more costeffective option [123]. SCR is in use in a few
ships and is still being developed. As well as
being an effective NO x -reduction measure, one
study has also shown that SCR can reduce ship
PM emissions by around 50% [35]. No limitations
are thought to have been encountered regarding
the fitting of SCR to different types of ship, and it
can be integrated with other aftertreatment
devices [96]. However, sufficiently high temperatures in the exhaust are required for the SCR
process to function efficiently. Consequently,
large NO x reductions are mainly achieved at
higher engine loads. Other drawbacks include
the space and additional weight requirements,
and the potential for “ammonia slip.” This occurs
when excess urea or ammonia is injected into the
exhaust stream, and any unused reagent is emitted
from the exhaust. The risk of ammonia slip is
higher for transient load changes (i.e.,
maneuvering) [96].
Another shipping technology is seawater
scrubbing, in which the exhaust gas is passed
through water. This is effective at reducing emissions of SO 2 and probably has benefits for other
pollutants.
Alternative Fuel and Vehicle and Technologies The development of technology in the
longer term will undoubtedly be driven more by
the need to reduce CO 2 emissions than by the need
to improve air quality. In many cases, a “win-win”
outcome is possible, although there can be some
conflicts. For example, some studies have indicated that NO x emissions can increase as a result
of biofuel use [125].
Alternative fuels include liquefied petroleum
gas (LPG), compressed natural gas (CNG), and
various biofuels (biomethane, ethanol, esterified
vegetable oils, etc.). For road transport, limited
supplies mean that LPG could never fully replace
petrol or diesel, and the scale of uptake of CNG
has been impaired by the cost of converting car
engines [126]. The availability of suitable engines
for rail applications is uncertain; available natural
gas engines can also only cover the lower end of
the power range needed for railway applications,
and there appears to be no use of LPG
[123]. Around 50 seagoing ships are currently
powered by natural gas, with a reduction in emissions of PM and NO x of at least 70%
[127]. Biofuels can be used in low concentration
blends (5–10%) with petrol or diesel without
engine modification, and in high concentrations
(85–100%) with specially adapted vehicles. The
main benefit of biofuels is their contribution to the
reduction of CO2 emissions; its effects on NO x
and PM are less clear. Barriers to increased biofuel
use include higher costs, compatibility with conventional fuels, and availability of technology for
more advanced biofuels [15]. In addition, there
have been warnings of hidden costs and adverse
impacts on the environment and society, which
could arise from the large-scale production of
biofuels [128]. It has also been noted that the use
of alternative fuels is context-specific; a fuel that
is appropriate to one location may not be appropriate to another [23].
Automobile propulsion technologies that are
currently under development include “mild” and
“full” hybrid vehicles, “plug-in” hybrid vehicles,
battery electric vehicles, hydrogen ICE vehicles,
Air Quality, Surface Transportation Impacts on
71