A multidimensional assessment of natural gas as an alternative transportation fuel

Abstract

Natural gas vehicles (NGVs) have seen increased attention as a way to reduce reliance on petroleum (diesel/gasoline) for transportation, but adoption rates lag behind conventional vehicles. There are multiple challenges connected to a globally large scale expansion of natural gas as an alternative transportation fuel including refueling infrastructure, safety, vehicle range, natural gas supply chain emissions, engine performance, engine durability, vehicle cost, public acceptability and policy options. Therefore to maximize the benefits of natural gas as an alternative transportation fuel, these factors must be considered. The aforementioned challenges are the focus of this thesis. The overall aim of the thesis is to identify and analyze the technological, strategic and environmental aspects of compressed natural gas (CNG) as a viable alternative transportation fuel option in the international as well as in Pakistani context. The thesis links twelve journal papers to present a cohesive body of research with the aid of 6 brief contextual chapters. The findings from the papers address three core themes in natural gas as an alternative transportation fuel. The first theme supported with five papers introduces and discusses the technical aspects of CNG technology with the particular focus on the environmental competitiveness of natural gas vehicles (NGVs), safety issues with NGVs, performance of natural gas fueled engines, durability of NGV’s engine parts and current research progress in using natural gas as an alternative transportation fuel. The second theme supported with five papers discusses the strategic framework and transitions to NGVs in four particular respects: addressing the barriers towards natural gas vehicles, policy options for the development of natural gas vehicles, strategies ranking for NGVs’ market and analysis of a specific CNG market by using Pakistan as a case study. The final theme supported with two paper presents a comparative life cycle (Well-to-Wheel) assessment of energy use and greenhouse gas (GHG) emission of natural gas vehicles. The research associated with the first theme identifies comparative drawbacks and advantages of NGV technology. Some of the identified drawbacks include 15–20% loss in brake horsepower, dependence of NGVs safety on the materials, operating conditions and maintenance of CNG system, the build-up of high combustion deposit between engine’s valve face and seat and lack of empirical work which examines the comparative maintenance cost-effectiveness of NGVs over total vehicle lifespan. On the other hand, the identified advantages include relatively lower tailpipe emission of CO2 (20-25%), CO (50-68%), PM (10-75%) and NMHC (50-70%). The research findings associated with the second theme suggest that the principle driving force in the successful removal of barriers to the growth of NGVs program is the user economics. Therefore setting CNG retail prices of 30–50% below the price of gasoline and diesel prices can play a key role in wide adoption of NGVs. In addition the findings suggest that there is no universal policy for the implementation of natural gas as a transportation fuel and it is identified that a combination of the various policy measures is required to remove the existing barriers to NGVs. The findings also suggest that, although due to the Government’s consumer friendly policy and the price differential between CNG and gasoline/ diesel, the NGVs sector in Pakistan has shown significant growth but due to lack of sustainable natural gas supply plans, Pakistan's gas supplies cannot support the high demand of the CNG sector. The findings of the third theme suggest that on Well-to-Wheel (WtW) basis 80 - 89% energy consumption and 73 - 86% greenhouse gas (GHG) emissions for all fuel pathways comes from the Tank-to-Wheel (TtW) phase. Moreover on WtW basis, the GHG emissions for dedicated NGVs may be reduced by 20% and 12% against gasoline and diesel fuel.