Life cycle (well-to-wheel) energy and environmental assessment of natural gas as transportation fuel in Pakistan
Graphical abstract
Introduction
The Paris Agreement – the first-ever universal, legally binding global climate deal – was adopted by 195 countries at the Paris Climate Conference (COP21) in December 2015. The Paris Agreement requires all Parties to put forward their best efforts through “nationally determined contributions” (NDCs) to greatly reduce greenhouse gas (GHG) emissions. Being the world’s 6th most populated nation, its energy requirement establishes Pakistan as a major contributor of GHG emissions; therefore, the reduction of the GHG emissions in Pakistan has attracted substantial local attention. The energy consumption of the road transportation sector accounts for 33% of the total energy consumption in Pakistan [1] and is responsible for a significant share (around 25%) of GHG emissions nationwide [2]. Therefore the reduction of GHG emissions in the transportation sector is a top priority of the government [3].
Emissions and energy consumption are often measured at the point of use. This does not, however, account for the overall emissions and energy consumption. To evaluate the impact of fuels and energy carriers the whole supply chain has to be considered [4]. To evaluate and assess the energy consumption, emissions, and economic effects of automotive fuels and vehicle technologies, a holistic or comprehensive approach has to be considered. The approach, often referred to as life cycle approach, or life cycle assessment (LCA), which must include all the steps required to produce a fuel, to manufacture a vehicle, and to operate and maintain the vehicle throughout its lifetime including disposal and recycling at the conclusion of its life cycle. A lifecycle analysis of energy consumed and emissions generated is especially important for technologies that employ fuels with different primary energy sources and fuel production processes. A typical life cycle of a vehicle technology is shown in Fig. 1. The life cycle can be classified into two major categories: the fuel cycle and the vehicle cycle. The fuel lifecycle analysis, also known as well-to-wheel analysis is vital for selecting vehicle fuels and technologies for the future.
The well-to-wheel analysis indicates the study of the energy use and GHG emissions in the production of the fuel and its use in the vehicle or engine, hereinafter called WtW analysis. Compared to Life Cycle Assessment (LCA) a WtW analysis can have the same system boundaries but does not consider energy or emissions involved in the construction of the facilities, the vehicles, consumption of other materials, water, and end of life disposal [5]. The whole WtW cycle is comprised of two independent stages, as shown in Fig. 2. These include (i) a Well-to-Tank (WtT) stage, which includes the recovery or production of the feedstock for the fuel, transportation and storage of the energy source through conversion of the feedstock to the fuel and the subsequent transportation, storage, and distribution of the fuel to the vehicle tank, and (ii) a Tank-to-Wheel (TtW) stage, which refers to the vehicle in utilizing the fuel for traveling purposes throughout its lifetime.
The rest of this paper is structured as follows: Section 2 reviews the existing literature. Section 3 defines the key assumptions and parameters used in well-to-wheel analysis including functional unit, GHG coefficients, fuel pathways, and methane slip/leakage and vehicle technologies. Section 4 describes the research methodology and data. The results and discussion are reported in Section 5. A comparative analysis of this study with previous studies is presented in Section 6. Section 7 concludes the outcomes of the study.
Section snippets
Review of the state-of-the-art
Many variations of WtW studies [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18] have been proposed in the literature to capture different aspects of the fuel life-cycle of transportation fuels for various propulsion in different regions of the world. However WtW studies on CNG vehicles haven’t got much academic interest and only a few analyses have been conducted targeting the CNG fuel, with often varied and even contrasting results. In this section, we have presented a
Methodology and data
As mentioned above the WtT cycle consist of two stages i.e (i) WtT stage, and (ii) TtW stage. The WtT stage of study has been covered in part-1 [33] of this two-part study. In this paper, the Well-to-Tank (WtT) results observed in part-1 [33] are combined with the TtW (Tank-to-Wheel) results reported in this present paper to provide the comprehensive WtW (Well-to-Wheel) results for the operation of conventional and CNG passenger vehicle drivetrains specific to Pakistan.
Tank-to-Wheel phase of
Key assumptions and parameters
Following are key parameters and assumptions used in this study:
Results and discussion
In this study, a WtW analysis on 25 combinations of automotive fuel and matching powertrain systems available in Pakistan was conducted.
Comparison with other studies
The WtW energy use and GHG emissions results of various studies and the present study are represented in Fig. 12. Generally speaking, detailed comparisons cannot be made among the findings of WtW analysis of similar fuel due to different methods of modeling, types of input data used, system boundaries, engine parameters etc. Different methodologies and assumptions in different studies make scenario comparison difficult or impossible. Therefore the comparison of absolute results from these
Conclusions
The present study has been conducted to provide detailed WtW assessment of energy consumptions and GHG emissions of natural gas, gasoline and diesel fuel pathways at the Pakistan and energy importing developing countries levels. The results of the present study can be used as an input to the strategic decision-making process for future transport energy policy and also to identify key areas of interest for further technology research and development of the Pakistan transport system. Furthermore,
Acknowledgements
The authors are thankful to AVL List GmbH for providing licenses to AVL CRUISE under AST-University Partnership Program with University of Engineering & Technology, Peshawar, Pakistan.
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