Life cycle emissions and cost of transportation systems: Case study on diesel and natural gas for light duty trucks in municipal fleet operations

• Natural gas (CNG) enables GHG emission reductions up to 34% compared to diesel.
• Conventional natural gas is preferable, while shale gas is acceptable.
• The life cycle fuel cost of a CNG truck is much lower than that of a diesel truck.
• However, CNG combustion may lead to increases in other emissions such as CO and NOx.

The green economy mandate seeks to reduce transportation emissions by replacing traditional fossil fuels with alternative low-carbon fuels. The present article provides a comparative “cradle-to-grave” life cycle assessment (LCA) of light duty commercial vehicles (LDCVs) powered by compressed natural gas (CNG) and diesel. The LCA scope is based on conditions for municipal vehicle fleet operation in a semi-urban Canadian city, suitable for adoption of alternative fuels such as CNG. An in-house application is developed to conduct the LCA with a combination of real-time vehicle fleet data and Natural Resources Canada's GHGenius as the inventory database. The results indicate that the net energy consumed is comparable for both fuels. Mixed results are obtained for criteria air contaminant (CAC) emissions, indicating the need for exhaust treatment for both vehicles. More importantly, a 34% reduction of GHG emissions is predicted by replacing the diesel vehicles with CNG vehicles. The analysis of fugitive methane emissions caused by newly developed hydraulic fracturing methods shows that CNG vehicles are still favorable. The lifetime fuel cost can also be reduced by about $30,000 per vehicle by switching to CNG. Therefore, fleet operators and policy developers are recommended to consider adoption of CNG-powered LDCVs.

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