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                This review studied various hydrocarbon decomposition
    methods reported in the literature for reducing automotive CO2
    emissions as part of carbon management. The ‘value-added’ carbon
    economy reduces H2 costs and motivates H2 economy progress.
    The objective of hydrocarbon decomposition is to eliminate or
    drastically reduce the amount of CO2 emitted from primary fuel
    resources. A wide variety of decomposition methods and
    techniques have been reported in the literature for the generation
    of carbon-free H2 from gaseous or liquid hydrocarbon fuels [1].
    There is steadily increasing awareness of the need to reduce CO2
    emissions to limit global warming. Many scientists now believe
    that an increase in the concentration of CO2 and other green house
    gas (GHG) such as methane and nitrous oxide, will increase the
    mean global temperature of the earth. In general, CO2 comprises
    85–95% of the total GHG emissions. Transportation is certainly the
    sector that has the largest potential impact for reducing GHG
    emissions. Motor vehicles are a major source of CO2. Globally,
    transport related emissions of CO2 are growing rapidly accounting
    for 20–25% of the CO2 release in the atmosphere [2].
    Fuel consumption and CO2 emission of a vehicle are two
    indissociable parameters. According to the U.S. Environmental
    Protection Agency (EPA) estimates [3], the average annual amount
    of CO2 emitted by a passenger car is about 4.6 ton (equivalent to 1.3
    ton of elemental carbon) assuming a total annual consumption of
    about 2000 l of gasoline. In other words, the combustion of one
    gallon of conventional gasoline produces about 8.9 kg of CO2 and
    the average car CO2 emission is about 230 g/km. Table 1 presents
    the emission rates for hydrocarbons, CO, CO2, oxides of nitrogen
    (NOx) and particulate matter. The calculations for ‘annual emission’
    and ‘fuel consumption’ were based on an average annual mileage of
    20,000 km (12,500 miles) and a fuel economy of 10 km/l or 23.8
    miles per gallon (mpg). These emission factors and fuel consumption
    rates are for gasoline-fuelled passenger cars and light-duty
    trucks only.
    2. Onboard hydrogen generation

    The development of an onboard system capable of converting
    liquid hydrocarbon fuels, such as gasoline, into a stream of H2-rich
    gas and carbon would make it possible to power internal
    combustion engine (ICE) vehicles using standard fuels with H2.
    Capturing and storing carbon onboard the vehicle could help
    mitigate climate change [4]. The advantage of onboard fuel
    processing for H2 and carbon production is clear: the utilization of
    conventional fuels at improved efficiency and lower pollution
    levels. The advantage of using liquid hydrocarbons as a H2 storage
    medium is that the current fuel distribution infrastructure can be
    used for the transportation, storage, and dispersal of the liquid
    hydrocarbon. Moreover, the development of fuel-efficient engines
    that produce fewer pollutants has been a major objective for many
    years. The response of engine manufacturers, oil refiners, academic
    researchers and catalyst industries to this public policy pressure
    has been remarkable [5]. A steady increase in fuel economy and a
    decrease in unwanted emissions have been achieved. However, it
    is now believed that a new approach to reduce CO2 emissions is

    2.1. Hydrogen generation by catalytic routes (SMR, POX and ATR)

    Turning gasoline into H2-rich gas for automobiles has been the
    object of much R&D. Jamal and Wyszynski [6] reviewed the use of
    H2 and H2-enriched gasoline as a fuel for SI-engines and the
    techniques used to generate H2 from liquid fuels such as gasoline
    and methanol, onboard the vehicle. Processes such as thermal
    decomposition, SMR, POX, and exhaust gas reforming were
    discussed. The authors discussed onboard generation of H2 for
    use as an alternative or supplemental fuel for spark ignition
    engines. A considerable amount of both theoretical and experimental
    work has been done in this field. Predictive and
    experimental results of the various investigators were reviewed
    and summarized. The authors claimed that the difficulty of
    gasifying or handling the solid carbon makes hydrocarbon
    decomposition not suitable for onboard H2 generation.
    There are various strategies to provide H2 to the engine. If H2 is
    stored onboard in a tank, the mixture will consist of pure H2 and
    gasoline. If H2 is obtained by processing a certain fraction of
    gasoline in an onboard POX reformer, two different reformer gases
    can be obtained. The first has a typical composition (by volume):
    21% H2, 24% CO and 55% N2 (ignoring trace components). Since
    such reaction is exothermic, the lower heating value (LHV) of the
    reformer gas is roughly 15% lower than that of the incoming
    gasoline [7]. The second gas is obtained water-gas shift reaction
    and has a typical composition of 45% H2, 20% CO2 and 35% N2.
    Allgeier et al. [8] presented a brief comparison of the various
    types of onboard reformers (Table 2). A storage for reformate gas is
    needed to cover the time during the reformer is being warmed up
    to its operation temperature and as a buffer during high dynamic
    transients. A wide variety of processes are available for onboard H2
    generation from gaseous or liquid fuels [9]. The three main
    onboard processes differ according to the nature of the primary
    fuel used (ammonia, methanol, ethanol, gaseous or liquid
    hydrocarbons) and to the chemical reactions involved (decomposition,
    steam reforming or POX). Onboard generation of H2 is
    certainly feasible from a technical standpoint but it is far from
    evident that it could simply replace pure H2, stored in compressed
    tanks or liquefied. Trimm et al. [5] reviewed the onboard
    conversion of methanol, methane, propane, and octane to H2. A
    combination of oxidation and steam reforming (indirect POX) or
    direct POX is the most promising processes for H2 fuel cell. Indirect
    POX involves combustion of part of the fuel to produce sufficient
    heat to drive the endothermic steam reforming reaction. Direct
    POX is favored only at high temperatures and short residence times
    but is highly selective. However, indirect partial oxidation is shown
    to be the preferred process for all fuels.
    Other researchers investigated the utilization of auto-thermal
    reforming (ATR) of gasoline for onboard generation of H2 [10,11].
    ATR has been widely accepted as the most promising route to meet
    efficiency, weight and volume, durability, and cost goals for
    onboard fuel processors for automotive fuel cell systems.
    Reforming catalysts are being developed to meet the unique
    operating requirements for reforming complex fuel mixtures, such
    as gasoline. Huffman and co-workers [12] and Goodman and coworkers
    [13] used dehydrogenation of liquid hydrocarbons (such
    as cycloalkanes) to produce CO-free H2. The advantage of using
    liquid hydrocarbons as a H2 storage medium is that the current
    distribution infrastructure can be used for the transportation,
    storage, and dispersal of the liquid hydrocarbon. Supportedbimetallic
    catalysts based on Fe, Ni and Pt are among the most used

    2.2. Hydrogen generation by plasma route

    Several different plasma reactors have been developed by
    different research groups for onboard H2 generation from gasoline
    using POX and ATR. Only a few plasma reactors have however been
    developed for SMR of hydrocarbons. Several researchers [14–18]
    have developed a non-thermal plasma reactor based on gliding arc
    technology for gasoline reforming. These technologies are claimed
    to be the most relevant techniques for onboard H2 generation
    suitable for a large range of fuel flow rate. The initial version of a 2-l
    plasmatron device required as much as 2 kWof electrical energy to
    operate. The unit was developed to use an average of less than
    100 W. Early systems took many seconds to produce H2 from cold
    exhaust, an important disadvantage in real-world use, as emissions
    are highest at this time. The latest versions were running in less
    than a second. Moreover, the first prototypes produced H2 at just
    one flow rate, however, recent prototypes managed transient or
    varying flow demands equally well [14].
    2.3. Onboard hydrogen supplementation to gasoline
    The concept of H2 supplementation to gasoline in SI-engines
    relies on the improvement of the thermal efficiency of engines
    using conventional hydrocarbon fuels by supplementing them
    with relatively small quantities of H2. The addition of H2 to the
    cylinder charge can extend the lean limit equivalence ratio while
    maintaining a sufficiently high flame speed. This will eliminate the
    need to treat NOx emissions altogether. In this way the main fuel is
    used more efficiently and only a small quantity of H2 is needed.
    Jamal and Wyszynski [6] concluded that the operation of the
    engine with 5–10 wt.% of H2 fuel makes it possible to operate the
    engine in the very lean regime, which would not have been
    possible without the presence of H2. A small amount of H2 added to
    the air intake of a gasoline engine would enhance the flame
    velocity and thus permit the engine to operate with leaner air to
    gasoline mixture than otherwise possible. Although combustion
    engines are expected to remain the dominant form of propulsion
    for the next 20–30 years, there will be a wider range of vehicle
    technologies and fuel types to address the economic, social and
    environmental challenges of increased mobility.
    The introduction of H2 as a supplemental automotive fuel
    could be hindered by serious logistic problems. Worldwide, no
    distribution system exists for H2, and its storage as a highpressure
    gas or cryogenic liquid requires vehicle capabilities
    which do not exist commercially. These potential difficulties,
    however, can be avoided by generating H2 in an onboard gas
    generator using gasoline or other liquid hydrcarbons as feedstock.
    Conte and Boulouchos [7] suggested that combustion of mixtures
    of H2 and gasoline appears to be a good opportunity to join the
    major advantages given by both fuels, avoiding many problems,
    especially if small amounts of H2 are produced onboard directly
    from gasoline. Use of H2 and gasoline blends seems to be
    especially suitable for part load operation and reduction of
    emissions during cold start. Some researchers have suggested that
    a move toward a mix of transportation fuels and H2 may offer a
    potential solution for reducing CO2 emissions. Because of the
    lower carbon content of the fuel mix (gasoline and H2), it will be
    possible to achieve carbon emission reduction. The technology of
    usingH2 as a combustion enhancement in ICE has been researched
    and proven for many years. The benefits are factual and well
    Cohn [17] summarized the benefits of H2 addition as follows:
    i. H2 addition provides a large increase in fuel octane number.
    ii. High octane fuel allows higher performance engines (turbocharging,
    high compression ratio).
    iii. Engines can be smaller and more efficient.
    iv. H2 addition also facilitates ultra-lean burn.
    v. Engine efficiency can be increased by up to 30%.
    Beister and Smaling [18] reported on the progress made with
    the Hydrogen-Enhanced Combustion Engine (HECE) concept, as
    applied to an SUV-class 3.2-l V6 test engine. The promise of HECE is
    that the addition of a small amount of H2 to the cylinder charge can
    allow homogeneous charge ultra-lean burn combustion engines to
    operate much leaner than otherwise possible. Enhancing gasoline
    combustion with a small H2 gas stream pointed toward a potential
    estimated improvement in gasoline fuel economy of 20–30%,
    depending upon the baseline engine.
    In another application, Cohn [17] installed and tested the
    plasmatron fuel converter on a commercial diesel car engine
    thereby reducing NOx emissions by 80%. The goal of diesel
    reformation by the plasmatron was the conversion of the heavy
    diesel compounds into H2, CO, and light hydrocarbons for use in
    after-treatment applications. The onboard H2 gas was used as a
    low-cost, highly efficient, clean-burning, fast-starting regenerator
    of particulate matter and NOx filters, which are used in diesel
    exhaust gas after-treatment. For NOx catalyst regeneration
    applications, the plasmatron may be turned on for a few seconds
    every half minute or so. For diesel particulate filter applications,
    the plasmatron could well be operated for a few minutes every few
    hours resulting in much smaller duty cycles [15].
    The addition of H2-rich gas to gasoline in an ICE seems to be
    particularly suitable for achieving a near-zero emission Otto
    engine, which would be able to easily meet the most stringent
    regulations [7]. A bottled blend of CO, H2, and N2 (24%, 21%, 55% by
    volume) was chosen to simulate the most likely output of a POX
    reformer suitable for ICE applications. Experiments were carried
    out on a Lombardini 4-stroke, 2-cylinder, 0.5-l engine, model LGW
    523 OHC. The engine was equipped with a water-cooled EGR line
    and water trap. Investigation on flame propagation was carried out
    using ion-detection probes on the cylinder head surface and an
    optical spark plug for light emission detection in the ignition
    The results of measurements included fuel consumption,
    engine efficiency, exhaust emissions, analysis of the heat release
    rates and combustion duration, for both pure gasoline and blends
    with H2-reformer gas. Simulations were performed to better
    understand the engine behavior and NOx formation. The results
    showed a significant decrease of unburned hydrocarbons (UHC)
    and NOx emissions to near-zero. Light increase in CO emissions was
    detected and CO2 emissions could be reduced by 3.5% in the FTP
    cycle. Moreover, a significant increase of engine efficiency was
    measured, which seems to be enough to compensate and
    overcome the losses due to POX of gasoline in the onboard
    reformer [7].
    A conceptual design for the reduction of CO2 emissions from
    transportation vehicles was proposed for producing hydrogen and
    carbon using known hydrocarbon decomposition technology
    onboard a vehicle. The produced hydrogen is then used in ICE
    thus reducing CO2 emissions [4]. One of the advantages of the
    proposed conceptual onboard fuel decomposition system is that
    the vehicle will still be fuelled by liquid hydrocarbons (gasoline or
    diesel), thus avoiding many of the direct infrastructure problems
    associated with H2. Part of the gasoline (25 vol.%) is decomposed
    onboard, and converted into H2 and carbon. H2 is then burned in
    the engine and carbon is stored onboard for further sale, depending
    on its quality. The decomposition of gasoline does not introduce
    any energy losses since exhaust energy is used for the process.
    However, the onboard fuel decomposer and carbon storage may
    introduce some weight penalty.
    Allgeier et al. [8] reported on the advanced emission and fuel
    economy concept by using combined injection of gasoline and H2
    in SI-engines. The authors tested concept for an SI-engine
    consisting of combined injection of gasoline and H2. An H2-
    enriched gas mixture was injected additionally to gasoline into the
    engine manifold. The gas composition represents the output of an
    onboard gasoline reformer. The simulations and measurements
    showed substantial improvements in the combustion process
    resulting in reduced cold start and warm up emissions and
    optimized part load operation. The replacement of gasoline by H2-
    rich gas during engine start led to zero hydrocarbons in the exhaust
    gas. The mixed fuel operation enabled high EGR rates up to 50% or
    extended lean burn limits resulting in reduced pumping losses and
    increased effective engine efficiency.

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