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Central Eastern Europe GRIP 2017 |
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5.4.3 EMISSIONS OF NO
X
AND PARTICULATEMATTER
When compared to diesel engines, the average combustion temperature of NG is
almost 260 °C lower at around 1,890 °C, which results in considerably smaller
amounts of NO
x
being emitted into the environment. The principle mechanism of
NO
x
formation is thermal dissociation and the subsequent reaction of nitrogen with
oxygen molecules in the combustion air. Three major factors play a crucial role:
oxygen concentration, combustion temperature, and time of exposure at the
combustion temperature. As these factors increase, NO
x
emission levels increase
accordingly. Since natural gas has much lower combustion temperatures and does
not require as much excess air, the NO
x
emissions of CNG-powered vehicles are
significantly lower than those from diesel.
The same is true for the emission of particulate matter (PM). NGVs are known to
emit very little or no particulate matter. PM emission is closely associated with the
residual combustion of fuel, which is a typical process for high molecular hydro
carbons, such as those in diesel fuel. Thus, the pollutant emissions of CNG vehicles
have always easily met EURO 6 standards, even in former times when only EURO 1
and EURO 2 were in force.
Since the EURO 5 and EURO 6 emission standards came into force, the manufac-
turers of diesel vehicles must compensate for these handicaps by using costly after-
treatment of the exhaust gas in order to reach the high standards shown in Figure
5.10. Among these treatments are the Selective Catalytic Reduction of NO
x
(SCR)
and the Diesel Particulate Filter (DPF).
50
PM (mg/km)
500
NO
x
(mg/km)
250
180
80
25
5
EURO 3 (2000)
EURO 4 (2005)
EURO 6 (2014) EURO 5 (2009)
CNG vehicles emissions
from EURO 3 to EURO 6
Figure 5.10:
Development of EURO emissions limits of NO
x
and PM pollutants for diesel
passenger cars in the course of time from EURO 3 to EURO 6