Greenhouse gas emission

Natural gas offers the lowest "carbon content" per energy unit of all mainstream fuels, about 25 % lower than diesel fuel. This advantage is however partly compromised by bad fuel economy (11 to 28 percent worse than diesel) which is shown by test runs with CNG and diesel busses (urban driving cycle comparable to railways). Compression or liquefaction reduces the carbon dioxide advantage even more. Emissions of natural gas itself, e.g. from leakage and refilling losses, contribute to greenhouse effect.

According to a study by IFEU, the maximal carbon dioxide benefit can only be achieved if fuel economy for natural gas is as good as for diesel traction, which is doubtful. In the worst case there is a disadvantage for CNG traction; in the best case only a slight advantage (cf. Figure 1). If LNG was applied, the emissions from the prechain would be about equal.

Figure 1: Greenhouse gases in CO₂ equivalents

Source: IFEU 2000
				Other environmental impacts: positive
					Pollutants The toxic emissions are much lower than for diesel propulsion. In contrast to the diesel engine, the combustion for the natural gas engine is soot-free, whereby the use of a three-way catalytic converter is possible. Comparison of relative emissions from natural gas and diesel engine: MAN diesel engine      D 2866 MAN gas engine        E 2866 Hydrocarbons       100 % 32 % CO 100 % 16 % NOX 100 % 3 % Particulates 100 % < 5 % Source: Althammer, Hattensperger 1998, IZT calculations. Noise Natural gas engines emit less noise than diesel engines. At 1 meter distance, the gas version of the MAN 2866 shows a noise emission of 97 dB(A) compared to 105 dB(A) for the diesel version. Resource availability According to current assessments, natural gas reserves will last much longer than oil reserves: Natural gas: 60 years based on current exploitation rates and the assumption that no additional reserves are exploited. ~ 170 years based on current exploitation rates and estimates of reserves exploitable in the future. The gas reserves would be even much higher than this if the huge gas hydrate reservoirs could technically be exploited. According to the Intergovernmental Panel on Climate Change (ZEIT 2001), the gas hydrate reservoirs are approx. 12.000 billions of carbon tons (as opposed to < 4000 billions for coal, and less than 1000 for both natural gas and oil). Diesel: 40 years based on current exploitation rates and the assumption that no additional reserves are exploited. Up to 80 years based on current exploitation rates and estimates of reserves exploitable in the future.
	Economic criteria
	Application outside railway sector
	Overall rating

References / Links:  DB AG 1999;  Althammer, Hattensperger 1998;  Chabas et al. 2001;  Xin et al. 1997;  Schweiger et al. 1997;  ZEIT 2001;  DB AG 2001;  b-verband gas und wasser.de

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Related projects:  Energy chains of alternative fuels;  Natural gas propelled railcar;  Shunting locomotive with LNG propulsion;  Railcar with CNG propulsion

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date created: 2002-10-09