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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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%.     

A novel solution for pressure drop in singly connected microchannels of arbitrary cross-section

This paper outlines a novel approximate solution for determining the pressure drop of fully developed, laminar, single-phase flow in singly connected microchannels of arbitrary cross-section. Using a “bottom-up” approach, it is shown that for constant fluid properties and flow rate in fixed cross-section channels, the Poiseuille number is only a function of geometrical parameters of the cross-section, i.e., perimeter, area, and polar moment of inertia. The proposed model is validated with experimental data for rectangular, trapezoidal, and triangular microchannels. The model is also compared against numerical results for a wide variety of channel cross-sections including: hyperellipse, trapezoid, sine, square duct with two adjacent round corners, rhombic, circular sector, circular segment, annular sector, rectangular with semi-circular ends, and moon-shaped channels. The model predicts the pressure drop for the cross-sections listed within 8% of the values published.     

A reaction mechanism for gasoline surrogate fuels for large polycyclic aromatic hydrocarbons

This work aims to develop a reaction mechanism for gasoline surrogate fuels (n-heptane, iso-octane and toluene) with an emphasis on the formation of large polycyclic aromatic hydrocarbons (PAHs). Starting from an existing base mechanism for gasoline surrogate fuels with the largest chemical species being pyrene (C₁₆H₁₀), this new mechanism is generated by adding PAH sub-mechanisms to account for the formation and growth of PAHs up to coronene (C₂₄H₁₂). The density functional theory (DFT) and the transition state theory (TST) have been adopted to evaluate the rate constants for several PAH reactions. The mechanism is validated in the premixed laminar flames of n-heptane, iso-octane, benzene and ethylene. The characteristics of PAH formation in the counterflow diffusion flames of iso-octane/toluene and n-heptane/toluene mixtures have also been tested for both the soot formation and soot formation/oxidation flame conditions. The predictions of the concentrations of large PAHs in the premixed flames having available experimental data are significantly improved with the new mechanism as compared to the base mechanism. The major pathways for the formation of large PAHs are identified. The test of the counterflow diffusion flames successfully predicts the PAH behavior exhibiting a synergistic effect observed experimentally for the mixture fuels, irrespective of the type of flame (soot formation flame or soot formation/oxidation flame). The reactions that lead to this synergistic effect in PAH formation are identified through the rate-of-production analysis.     

Plant oils as fuels for compression ignition engines: A technical review and life-cycle analysis

As an alternative fuel for compression ignition engines, plant oils are in principle renewable and carbon-neutral. However, their use raises technical, economic and environmental issues. A comprehensive and up-to-date technical review of using both edible and non-edible plant oils (either pure or as blends with fossil diesel) in CI engines, based on comparisons with standard diesel fuel, has been carried out. The properties of several plant oils, and the results of engine tests using them, are reviewed based on the literature. Findings regarding engine performance, exhaust emissions and engine durability are collated. The causes of technical problems arising from the use of various oils are discussed, as are the modifications to oil and engine employed to alleviate these problems. The review shows that a number of plant oils can be used satisfactorily in CI engines, without transesterification, by preheating the oil and/or modifying the engine parameters and the maintenance schedule. As regards life-cycle energy and greenhouse gas emission analyses, these reveal considerable advantages of raw plant oils over fossil diesel and biodiesel. Typical results show that the life-cycle output-to-input energy ratio of raw plant oil is around 6 times higher than fossil diesel. Depending on either primary energy or fossil energy requirements, the life-cycle energy ratio of raw plant oil is in the range of 2–6 times higher than corresponding biodiesel. Moreover, raw plant oil has the highest potential of reducing life-cycle GHG emissions as compared to biodiesel and fossil diesel.     

The demand for gasoline: a comment on the cross-section analysis by Drollas

A recent article [1] used data for 1977 from different countries to estimate the long-run price elasticity of demand for gasoline. Some aspects can be criticized, particularly the interpretation of the results and the model used. An alternative is developed which accepts that the key factor influencing gasoline consumption is the stock of cars. However, energy use per car will vary depending on the size of the country and its average income and price of fuel. This model gives a non-linear equation which is estimated using the same data. All the coefficients are significant with the price elasticity being inelastic.     

Clean fuel options with hydrogen for sea transportation: A life cycle approach

In this study, two potential fuels, namely hydrogen and ammonia, are alternatively proposed to replace heavy fuel oils in the engines of sea transportation vehicles. A comparative life cycle assessments of different types of sea transportation vehicles are performed to investigate the impacts of fuel switching on the environment. The entire transport life cycle is considered in the life cycle analyses consisting of production of freight ship and tanker; operation of freight ship and tanker; construction and land use of port; operation, maintenance and disposal of port; production and transportation of these clean fuels. Various environmental impact categories, such as global warming, marine sediment ecotoxicity, marine aquatic ecotoxicity, acidification and ozone layer depletion are selected in order to examine the diverse effects of switching to clean fuels in maritime transportation. As a carbon-free fuel for marine vehicle engines, ammonia and hydrogen, yield considerably lower global warming impact during the operation. Furthermore, numerous production methods of alternative fuels are evaluated to comparatively show environmentally benign options. The results of this study demonstrate that if ammonia is even partially utilized in the engines of ocean tankers as dual fuel (with heavy fuel oils), overall life cycle greenhouse gas emissions per tonne-kilometer can be decreased about 27% whereas it can be decreased by about 40% when hydrogen is used as dual fuel.