Alternative Fuels

W. Addy Majewski, Peter Ahlvik

Abstract: The development of alternative fuels has been driven by climate change and energy security concerns. A number of alternative fuel options have been considered and developed, including alcohols, biomass-based diesel, methane, hydrogen, and synthetic fuels. The choice of future fuel/powertrain combinations, ideally based on well-to-wheel energy efficiency and emission analysis, is limited by such factors as alternative fuel resources and distribution infrastructure.

Introduction
Alternative Fuel Options
Energy Efficiency and Emissions

Introduction

The world’s transportation sector has been predominantly powered by liquid hydrocarbon fuels—diesel and gasoline. Efforts to develop alternative fuels have continued for decades, albeit justified by evolving motivations. In the 1990s, the development of alternative fuels in North America and Europe was largely justified by reduced exhaust emissions. Current developments are justified by a reduced dependence of the transportation sector on fossil fuels, particularly on oil, to mitigate climate change emissions, to reduce the dependence on petroleum imports and improve energy security, and ultimately to address the depletion of crude oil resources—which are finite and nonrenewable (see also Energy Alternatives).

The objectives that motivate alternative fuel development are not always complementary. For instance, some alternative fuels can increase energy security, while producing more GHG emissions than the petroleum they replace—an example is the use of coal-derived methanol as a motor fuel in China. The development of alternative fuels has also been driven by business interest and supported by industrial lobbying—the same mechanisms that have long played a role in the fossil fuel market.

While an adequate, technically and commercially feasible replacement for diesel and gasoline has yet to be found, a few alternative fuels have made inroads in the fuel markets. These include ethanol (from corn in North America or from sugar cane in Brazil), biodiesel, or natural gas. A number of other alternative fuels can be described as experimental or niche market fuels. Further developments—for instance, in the area of synthetic electrofuels—can be expected in the future.

To present a viable replacement option for petroleum fuels, an alternative fuel has to meet certain criteria, which include:

Resource base & energy density—To substitute a significant fraction of conventional fuel, the alternative fuel must have large resource (feedstock) base and high energy density (in W/m²). Biofuels, as well as wind and solar energy, are often characterized by low energy density and very high demand for land.
Energy return on investment (EROI)—The alternative fuel should be produced with the least possible input of (fossil) energy. As EROI drops to low single digits or even approaches one (as is the case with some biofuels), the production process becomes self-defeating and increasingly resembles a wasteful conversion of petroleum into an alternative form of energy, as opposed to the extraction of additional energy from the environment. One of the revealing signs of low EROI fuels can be their high cost, reflecting the high energy input during production—such fuels cannot compete with petroleum and must be supported in the marketplace by government subsidies or mandates.
Renewable character & low carbon emissions—To reduce climate impacts, the fuel should be made from a natural resource that is never used up or that can be replaced by new growth. Respective examples include wind/solar energy and biomass energy. In the case of biomass and other resources that depend on regrowth, their renewable character depends on the rate of harvesting—they can be depleted beyond the point of renewability. For instance, considering the ongoing deforestation of the planet, wood is not necessarily a renewable resource.

The main alternative fuel resources that have been considered promising include biomass, natural gas and, in the longer term, electrofuels (a.k.a. e-fuels or eFuels) produced using renewable electricity. It may be noted that these fuel options fall short of meeting all of the above criteria—natural gas is not renewable, while synthetic e-fuels tend to have low EROI and low life cycle energy efficiency.

Standard Specifications. Before an alternative fuel can be adopted on a wider scale for use in existing powertrains, its compatibility with engines—be it the standard diesel/gasoline engine or a modified engine design—must be assured, including short- and long-term effects. In particular, standard specifications for all of the relevant fuel properties must be developed and adopted. The standard specifications need to cover and/or safeguard several aspects, including:

Engine performance
Impact on emissions, regulated and unregulated
Impact on engine materials, including metals, plastics, rubber, etc.
Impact on handling, operability, safety, toxicity, etc.
The need for any fuel additives (e.g., lubrication)
Impact of fuel impurities on additives
Availability of standardized test methods

Distribution Infrastructure. Fuel distribution is a very important factor for an alternative fuel that is intended for a high market penetration as widespread use is necessary to significantly influence GHG emissions and energy security. A finely branched, low cost distribution network to make a fuel available everywhere would require liquid fuels that can be easily blended with and handled by the existing gasoline and diesel infrastructure—so called “drop-in” alternative fuels.

Figure 1. Gasoline stations in the United States, 1972-2019

The declining number of stations reflects their increasing capacity. In 2019, gasoline volume dispensed per station was at a record high. Data: Oak Ridge Transportation Energy Data Book [4615]

In the United States, the distribution network for liquid hydrocarbon fuels—developed over more than 100 years at a very substantial cost—includes nearly 150,000 gasoline stations, Figure 1. Duplication of such a network for handling liquefied gases under pressure or cryogenic liquids is difficult to justify due to cost reasons. A shift to a “hydrogen economy” would require such an effort if current transportation patterns were to be maintained. However, the problems in hydrogen distribution are so significant that some have instead proposed a “methanol economy” [950][5226]. Fuels that require a radically different and costly distribution network will be likely limited to niche applications.

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