Natural gas and other alternative fuels for transportation purposes

Natural gas has been used as fuel for transportation for decades and, currently, about half a million vehicles of different types are running either on compressed natural gas (CNG) or on liquefied natural gas (LNG) in a number of countries, including Italy, New Zealand, the U.S.A., Australia, Iran and France. The country with the most developed program on utilization of natural gas as a transportation fuel is Italy, where almost 300,000 vehicles are running on CNG. Several other countries such as Brazil, Egypt, Canada, Bangladesh and the Soviet Union are implementing programs to use natural gas for domestic transportation needs. Natural gas is composed essentially of methane, which can be obtained also through anaerobic fermentation of different organic products yielding biogas (60% methane). The role of methane as a fuel has shown increasing importance due to the growth in digester construction all over the world, and especially in developing countries. Natural gas can also be used as raw material for the production of liquid fuels suitable for transportation purpose such as methanol, ammonia, gasoline, diesel oils, methyl esters of vegetable oils, and MTBE, a high-octane component of gasoline. In addition, natural gas liquids (propane/butane) can also be used as automotive fuels. This paper covers the technical-economic aspects of natural gas and methanol as fuels for transportation and presents a summary of worldwide experience with emphasis on the existing experience in a developing country such as Brazil, including its commercial large-scale experience with ethanol fuels.     

Leveling the playing field of transportation fuels: Accounting for indirect emissions of natural gas

Natural gas transportation fuels are credited in prior studies with greenhouse gas emissions savings relative to petroleum-based fuels and relative to the total emissions of biofuels. These analyses, however, overlook a source of potentially large indirect emissions from natural gas transportation fuels, namely the emissions from incremental coal-fired generation caused by price-induced substitutions away from natural-gas-fired electricity generation. Because coal-fired generation emits substantially more greenhouse gases and criteria air pollutants than natural-gas-fired generation, this indirect coal-use change effect diminishes potential emissions savings from natural gas transportation fuels. Estimates from a parameterized multi-market model suggest the indirect coal-use change effect rivals in magnitude the indirect land-use change effect of biofuels and renders natural gas fuels as carbon intensive as petroleum fuels.     

Refinement of motivity factor in comparison of transportation fuels

Hydrogen is potentially the fuel of the future, with the transition to a hydrogen economy driven by the concerns about the climate change and the depletion of fossil fuel resources. Hydrogen is better than almost all other fuels when the fuels are evaluated on the basis of several metrics, including safety and versatility of use. Hydrogen is also presumed to have a superior motivity factor, a performance measure arising out of a combination of acceleration and drag forces. The motivity factor is quantified on the basis of the gravimetric and volumetric energy densities of a fuel. The present paper offers a refinement of the motivity factor which incorporates the effects of the mass of the storage matrix in these energy densities. It is shown that other fuels may have higher motivity factors than hydrogen when quantified on the basis of these refinements.     

Nanometallic fuels for transportation: a well‐to‐wheels analysis

Nanometallic iron and aluminium, along with hydrogen and electricity, are among the proposed alternatives to the petroleum‐based fuels for future transportation. The advantages of the metallic fuels appear to be high volumetric energy densities and zero greenhouse gas emissions during the operation of the vehicle. However, nanometallic fuels do not exist in nature, and a well‐to‐wheel analysis of the fuel manufacture‐utilization system is required to quantify the energy consumption and assess the true environmental impact of the proposed alternative. The three‐component nanometallic fuel system consisting of a metal production process, a nanoparticle formulation process and the metal combustion process is analysed in this paper. The energy balance and the environmental impact are estimated for nanometallic iron and aluminium based systems. The sustainability of once‐through systems that do not involve recycle of combustion products is questionable because of resource limitations. A viable system for satisfying the transportation fuel demands will involve the reduction and recycle of the combustion products. A comparison of these nanometallic fuels with gasoline and hydrogen indicates that nanometallic fuels are the least efficient, with primary energy consumption greater than 11 MJ km^?1 compared to 0.625 MJ km^?1 for gasoline and 8.6 MJ km^?1 for hydrogen. The nanometallic fuels will also have the most severe impact of the three, with CO₂‐equivalent emissions of 13.44 billion tons year^?1 for iron and 21.1 billion tons year^?1 for aluminium as compared to approximately 0.8 billion tons year^?1 for gasoline. These emissions from nanometallic fuels are at least an order‐of‐magnitude higher than those for gasoline and hydrogen. The results of the analysis emphasize the need for well‐to‐wheel assessment for determining the true impact of technologies proposed as replacements for the current technologies. Copyright © 2006 John Wiley & Sons, Ltd.     

On-site hydrogen production from transportation fuels: An overview and techno-economic assessment

Hydrogen production for future transportation applications have received increased interest due to its inherent environmental and efficiency benefits. Currently, hydrogen is produced from natural gas and naphtha for its use in refineries for clean fuel production along with its use in ammonia production. The hydrogen demand will grow in future for hydrogen based fuel cell vehicles. Significant research is underway to produce hydrogen from renewable and fossil fuel sources. However, on-site hydrogen production using existing fuel and gas station infrastructure to support future hydrogen based fuel cell vehicles has advantages over other approaches. In this context, this study is focused on a techno-economic assessment of hydrogen production from transportation fuels using different conversion technologies. In addition, detailed economics with higher capacity and volume of the hydrogen stations are also discussed. Finally, a detailed roadmap is presented to produce on-site hydrogen at commercial scale.     

A temporal cross section specification of the demand for gasoline using a random coefficient regression model

This paper uses a random coefficient regression approach to estimate the demand for gasoline by pooling cross-sectional (state level) and time series data. The analysis proceeds by estimating two alternative models, namely a stock adjustment model and a flow adjustment model. The two models are estimated using state level data on gasoline consumption, gasoline price, income and the stock of automobiles. The random coefficient specification of each demand model is estimated assuming heteroskedastic disturbances across states, autocorrelated disturbances over time and variable intercept and slope coefficients across states. The resultant price and income elasticities are compared and inferences concerning the ability of the flow adjustment model to approximate the underlying demand function are made.     

Multi-objective regulations on transportation fuels: Comparing renewable fuel mandates and emission standards

We compare two types of fuel market regulations — a renewable fuel mandate and a fuel emission standard — that could be employed to simultaneously achieve multiple outcomes such as reduction in fuel prices, fuel imports and greenhouse gas (GHG) emissions. We compare these two types of regulations in a global context taking into account heterogeneity in carbon content of both fossil fuels and renewable fuels. We find that although neither the ethanol mandate nor the emission standard is certain to reduce emissions relative to a business-as-usual baseline, at any given level of biofuel consumption in the policy region, a mandate, relative to an emission standard, results in higher GHG emissions, smaller expenditure on fuel imports, lower price of ethanol-blended gasoline and higher domestic fuel market surplus. This result holds over a wide range of values of model parameters. We also discuss the implications of this result to a regulation such as the US Renewable Fuel Standard given recent developments within the US such as increase in shale and tight oil production and large increase in average vehicle fuel economy of the automotive fleet.     

Methanol,natural gas,and the development of alternative transportation fuels

The potential for methanol as a motor fuel, particularly when it is produced from natural gas, is examined. Diverse information related to methanol fuel development is gathered together and the process by which such a new fuel market would evolve is considered. It is concluded that methanol has the capacity to be a significant alternative fuel, but that the realization of that capacity is not yet imminent.     

A review on conversion of triglycerides to on‐specification diesel fuels without additional inputs

Dependence on fossil fuels for global energy supply has continued to generate concerns about climate change and sustainable development. It has motivated the search for carbon‐neutral alternative resources for the production of transportation fuels to replace crude oil. Although biodiesels have recently emerged as a close substitute to petrol diesel, their use in compression ignition engines designed to run on petro‐diesel fuels are linked to adverse effects on the engines' performance and life span. This informed efforts at upgrading biodiesel or direct conversion of triglycerides to hydrocarbon mixtures that are identical or similar to that of petro‐diesel through hydrodeoxygenation. Moreover, it seems that commercial methods for the conversion of triglycerides to diesel fuels depends on inputs (methanol and hydrogen) derived from fossil fuels. However, it will be desirable to do so without inputs from fossil fuels. Hence, reaction paths for direct conversion of triglycerides to on‐specification hydrocarbons fuels without hydrogen gas input are discussed and suggested strategies are in cognisance of green chemistry principles. Copyright © 2012 John Wiley & Sons, Ltd.     

Life cycle analysis of energy use and greenhouse gas emissions for road transportation fuels in China

Life cycle analysis is considered to be a valuable tool for decision making towards sustainability. Life cycle energy and environmental impact analysis for conventional transportation fuels and alternatives such as biofuels has become an active domain of research in recent years. The present study attempts to identify the most reliable results to date and possible ranges of life cycle fossil fuel use, petroleum use and greenhouse gas emissions for various road transportation fuels in China through a comprehensive review of recently published life cycle studies and review articles. Fuels reviewed include conventional gasoline, conventional diesel, liquefied petroleum gas, compressed natural gas, wheat-derived ethanol, corn-derived ethanol, cassava-derived ethanol, sugarcane-derived ethanol, rapeseed-derived biodiesel and soybean-derived biodiesel. Recommendations for future work are also discussed.     

Magma: A potential source of fuels

Recent calculations and measurements indicate that basaltic magma is a new, extensive source for fuels (hydrogen, carbon monoxide, and methane). The fuel production processes have been found to occur in nature as well as the laboratory and as a result, our work indicates that current concepts of geothermal energy can be broadened beyond producing only steam and heat. When magma is considered as a geothermal resource, its use for the direct production of fuels should be included. It is possible to generate several mole percent hydrogen when water-rich fluid is equilibrated with the ferrous and ferric iron in magma. This paper describes the basis of the fuel production processes, the fuel yields for injected water and water plus natural organic matter (biomass), and the increased geothermal resources that would be made available by these processes.     

Applying life-cycle assessment to low carbon fuel standards—How allocation choices influence carbon intensity for renewable transportation fuels

The Energy Independence and Security Act (EISA) of 2007 requires life-cycle assessment (LCA) for quantifying greenhouse gas emissions (GHGs) from expanded U.S. biofuel production. To qualify under the Renewable Fuel Standard, cellulosic ethanol and new corn ethanol must demonstrate 60% and 20% lower emissions than petroleum fuels, respectively. A combined corn-grain and corn-stover ethanol system could potentially satisfy a major portion of renewable fuel production goals. This work examines multiple LCA allocation procedures for a hypothetical system producing ethanol from both corn grain and corn stover. Allocation choice is known to strongly influence GHG emission results for corn-ethanol. Stover-derived ethanol production further complicates allocation practices because additional products result from the same corn production system. This study measures the carbon intensity of ethanol fuels against EISA limits using multiple allocation approaches. Allocation decisions are shown to be paramount. Under varying approaches, carbon intensity for corn ethanol was 36–79% that of gasoline, while carbon intensity for stover-derived ethanol was −10% to 44% that of gasoline. Producing corn-stover ethanol dramatically reduced carbon intensity for corn-grain ethanol, because substantially more ethanol is produced with only minor increases in emissions. Regulatory considerations for applying LCA are discussed.     

Transport fuels for the post-oil era

The fact that natural oil supplies are not limitless is now widely appreciated, but the debate continues as to what the most likely alternatives might be. This debate applies particularly to transport fuels, since transport is almost totally dependent on oil for its supply of fuel. Recent work at the Open University has indicated that the most likely alternative fuels for road transport, when natural oil is unavailable, are electricity or liquid fuels derived from coal. This article discusses this choice of possible fuels systems for future road transport and points out some unexpected systems effects.     

Mechanism reduction for multicomponent surrogates: A case study using toluene reference fuels

Strategies and recommendations for performing skeletal reductions of multicomponent surrogate fuels are presented, through the generation and validation of skeletal mechanisms for a three-component toluene reference fuel. Using the directed relation graph with error propagation and sensitivity analysis method followed by a further unimportant reaction elimination stage, skeletal mechanisms valid over comprehensive and high-temperature ranges of conditions were developed at varying levels of detail. These skeletal mechanisms were generated based on autoignition simulations, and validation using ignition delay predictions showed good agreement with the detailed mechanism in the target range of conditions. When validated using phenomena other than autoignition, such as perfectly stirred reactor and laminar flame propagation, tight error control or more restrictions on the reduction during the sensitivity analysis stage were needed to ensure good agreement. In addition, tight error limits were needed for close prediction of ignition delay when varying the mixture composition away from that used for the reduction. In homogeneous compression-ignition engine simulations, the skeletal mechanisms closely matched the point of ignition and accurately predicted species profiles for lean to stoichiometric conditions. Furthermore, the efficacy of generating a multicomponent skeletal mechanism was compared to combining skeletal mechanisms produced separately for neat fuel components; using the same error limits, the latter resulted in a larger skeletal mechanism size that also lacked important cross reactions between fuel components. Based on the present results, general guidelines for reducing detailed mechanisms for multicomponent fuels are discussed.     

Combustion of droplets of liquid fuels: A review

A review is presented of recent developments of the combustion of single droplets with the object of stimulating discussion and future work in the field. Among the areas covered are the combustion of stationary and moving droplets in an oxidising atmosphere, the combustion of monopropellants, the influence of high pressures and the ignition of droplets. The application of droplet theories to the modelling of various combustion systems is also outlined.     

Production of hydrogen-rich fuels for pre-combustion carbon capture in power plants: A thermodynamic assessment

Hydrogen-fueled plants can play an important role in the field of carbon capture and storage, because they facilitate the mitigation of harmful emissions. In this paper, two combined-cycle power plants with pre-combustion CO₂ capture are examined, in which natural gas is converted into a hydrogen-rich fuel through reforming. The first plant considered operates with a hydrogen-separating membrane and the second with an autothermal reformer. The two plants are compared to a reference plant without CO₂ capture and briefly to alternative oxy-fuel and post-combustion capture technologies. It is found that both plants suffer high penalties caused by the high energy requirements of the reforming components and the CO₂ compression units. Additionally, both plants appear inferior to alternative capture technologies. When comparing the two reforming plants, the plant with the hydrogen-separating membrane operates somewhat more efficiently. However, in order to make these technologies more attractive, their thermodynamic efficiency must be enhanced. The potential for improving the efficiencies of these plants is revealed by an exergetic analysis.     

A computation scheme for the asymptotic. Nusselt number in ducts of arbitrary cross-section

A method is presented to approximate the asymptotic Nusselt number in long ducts with parallel walls and arbitrary cross-sections : The flow in the ducts is laminar and fully developed. The temperature of the ducts' walls changes in the form of a step. The Nusselt number is obtained for large distances from the location of the temperature step. The method shows how to obtain both upper and lower bounds to the Nusselt number and how to improve the approximation to any desired degree. Two examples are given: the circular duct (which is just the Graez problem, solved in ¹) and the square rectangular duct. An extension is made to cases where only numerical solutions are possible.     

Automotive engines and fuels: A review of future options

This paper reviews the current status and development potential of automotive heat engines: various types of internal combustion engines, the gas turbine, Stirling and Rankine cycle engines, and compound engine systems. Expected changes in transportation fuels from natural petroleum, and the likely impact of alternative fuels are also examined. Emphasis is placed on the general structure of the issues to be faced in making choices about where to put research and development resources, rather than on technical detail, to provide an overview of this broad area.     

The demand for transport fuels in Turkey

In this paper, we estimate the demand for transport fuels in Turkey. Specifically, using four different models, namely a partial adjustment model, a distributed lag model, an autoregressive distributed lag model, and an error correction model, we estimate gasoline and diesel demand functions with quarterly data covering the period 2003:Q1–2014:Q3. We find a stable long-run relationship only for diesel demand, income and price. Our results imply that gasoline demand does not respond to income and price in the long run, reflecting a shift from gasoline towards diesel induced by differential tax policies. Furthermore, we find that transport fuel demand is price inelastic, making tax on fuel a perfect tool for raising budget revenues. In addition, our results suggest that fuel demand responds to negative and positive price changes symmetrically.     

A four-zone model for saturated flow boiling in a microchannel of rectangular cross-section

A four-zone flow boiling model is presented to describe saturated flow boiling heat transfer mechanisms in a microchannel of rectangular cross-section. The boiling process in the microchannel is assumed to be a cyclic passage of four zones: (i) liquid-slug zone, (ii) elongated bubble zone, (iii) partially-dryout zone, and (iv) fully-dryout zone. The existence of the partially-dryout zone in this model is proposed to take into consideration of corner effects on boiling heat transfer in the microchannel. To verify this new model, an experimental study was carried out to investigate flow boiling heat transfer of water in a microchannel having a rectangular cross-section with a hydraulic diameter of 137 μm (202 μm in width and 104 μm in depth) with a length of 30 mm under three-side heating condition. The data for bubble nucleation frequency was correlated in terms of the Boiling number, which was used to determine the heat transfer coefficient. It is found that the present four-zone flow boiling model successfully predicts trends of boiling heat transfer data in a microchannel with a rectangular cross-section, having a sharp peak at low vapor quality depending on the mass flow rate. The predictions of flow boiling heat transfer coefficient in the microchannel are found in good agreement with experimental data with a MAE of 13.9%.