On-board generation of hydrogen-rich gaseous fuels—a review

Int. J. Hydrogen Energy, Vol. 19. No. 7, pp. 557 572, 1994 International Association for Hydrogen Energy Elsevier Science Ltd Printed in Great Britain 036(~3199/94 $7.00 + 0.00

Pergamon

ON-BOARD GENERATION OF HYDROGEN-RICH GASEOUS F U E L S - A REVIEW Y. JAMAL and M. L. WYSZYNSKI* School of Manufacturing and Mechanical Engineering, University of Birmingham, Birmingham B15 2TT, U.K. (Received Jor publication 1 September 19931

Abstract Hydrogen has a good potential as an alternative fuel for spark ignition engines. It can extend the lean flammability limit of conventional fuels in order to achieve higher thermal efficiency and lower exhaust emissions. This paper reviews the use of hydrogen and hydrogen-enriched gasoline as a fuel for SI engines and the techniques used to generate hydrogen from liquid fuels such as gasoline and methanol, on-board the vehicle. The processes of thermal decomposition, steam reforming, partial oxidation and exhaust gas reforming are evaluated. A considerable amount of both theoretical and experimental work has been done in this field. Predictive and experimental results of the various investigators are reviewed and summarized.

INTRODUCTION The scarcity of fossil fuels and the associated pollution problems have attracted the attention of researchers towards the search for alternative fuels. With any alternative fuel, the availability of the source, as well as the emissions of pollutants are most important from the aspect of energy preservation and environment, respectively, while its potential effects on engine performance and the form of storage will be significant issues from the stand point of the engine and vehicle technologies. An alternative energy carrier that has great environmental advantages is hydrogen. It is a clean fuel, when burned in air, produces non-toxic exhaust emissions except at some equivalence ratios, where its high flame temperature results in significant N O x levels in the exhaust products. The use of gaseous fuels inducted with air does, however, limit the total power output of the engine due to the reduction of volume of air aspirated. The current approach for the reduction of emissions relies on three-way catalytic converters. An alternative and more basic approach to the emissions problem is to modify the initial combustion process in the engine by using lean mixtures. The primary advantage of lean burn is that it increasingly reduces N O x and CO. The problem with it is that engine power declines rapidly, while unburned hydrocarbon emissions increase because of misfire [1]. Unfortunately, an engine still produces unacceptably high N O x exhaust pollutants near the lean flammability limit of gasoline. In a practical sense, lean-burning engines are limited by the onset of engine misfiring as the lean flammability limit of any fuel is approached. For this reason emission standards for N O x

*Author to whom correspondence should be addressed.

could not be achieved in the engine by operating lean, using only gasoline. Mixtures of hydrogen and gasoline, on the other hand, can burn lean enough to meet this requirement [2]. Hydrogen may be used to extend the lean limit of conventional fuels in order to achieve higher efficiency and lower pollutant emissions. Because of its wide flammability limits and high flame speeds the hydrogenrich fuel lends itself readily to ultra lean combustion and should allow the use of higher compression ratios. Combining the increase in heating value, the recovery of waste energy from the engine exhaust, lean operation and higher compression ratios provides a potentially high increase in thermal efficiency for the hydrogen-enriched fuels over that of the conventional fuels. However, there are significant problems associated with the use of hydrogen, especially concerning production and storage. Hydrogen is commercially produced by electrolysis of water and by coal gasification. These methods are not widely used because they are more expensive than steam reforming of natural gas or partial oxidation of heavy oils [3]. Most of the world's hydrogen is currently derived from hydrocarbons such as oil or natural gas, via the catalytic steam reformation [4,5]. There are generally three ways to store hydrogen in an automobile: as a gas "dissolved" in a metal (metal hydride), as a cryogenic liquid or as a compressed gas. Hydride storage is the simplest and the safest, but it increases vehicle weight and results in a severe fuel economy penalty. Liquid hydrogen is light, but due to its low energy density occupies three times as much volume as gasoline. Storage as a compressed gas is inexpensive and provides for ease of operation but its weight and bulk are the main problem. One solution to the storage problem is the on-board hydrogen generation from a suitable high energy density patent fuel such as gasoline or methanol. With this form 557

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of storage, the technical problem is how to generate the hydrogen. There are a number of methods to generate hydrogen from such fuels. The most common amongst those are partial oxidation, steam reforming, thermal decomposition and exhaust-gas reforming. Partial oxidation is not usually considered to be attractive in terms ofefficiency because it is an exothermic process, and the resulting hydrogen containing fuel gas has a lower calorific value than that of original feed stock. However, it is an interesting means of generating free hydrogen gas for use as a charge supplement in ultra lean combustion exercises [6]. On the other hand, steam reforming is an endothermic process, the fuel gas thus produced has higher calorific value than the feed stock, and the efficiency of the process is quite favourable, particularly if the heat energy requirement is supplied from a source which would otherwise be wasted, like hot engine exhaust gases [7]. Thermal decomposition of hydrocarbons results in the formation of hydrogen and carbon. The difficulty of gasifying or handling the solid carbon makes hydrocarbon decomposition not suitable for on-board hydrogen generation. Methanol can be catalytically decomposed into hydrogen and carbon monoxide at temperatures of the order of 250°C [8]. The reaction is endotherrnic and requires a heat source to provide energy. This energy can usually be supplied by the engine exhaust gas. In exhaust gas reforming fuels are reformed catalytically by direct contact with a portion of hot products of combustion utilizing the fact that exhaust gases contain a certain quantity of steam. The fuel gas thus generated contain quantities of hydrogen, carbon monoxide and nitrogen, thus providing a potential for lean combustion leading to lower emissions and higher engine thermal efficiency than conventional fuel [9]. The objective of this paper is to review the use of hydrogen as a fuel for gasoline engine, in particular as a supplementary fuel, and to discuss the different methods of onboard hydrogen generation.

USE OF HYDROGEN AS A FUEL There is a considerable interest (both short and long term) in hydrogen as a fuel. Not only it will help to eliminate the present-day problem of dependence on petroleum fuels, but it also has a potential to reduce vehicular pollution as it is a clean burning fuel. It offers the unique advantage of being a fuel, the basic resource of which (water) is recyclable. Basically hydrogen can be commercially produced either by coal gasification or by electrolysis of water using electricity generated from coal, nuclear fission, solar energy or possibly nuclear fusion. Coal gasification process would be the most economical but a comparison of total energy per mile for current technology vehicles puts hydrogen at a disadvantage compared to other coal derived fuels such as methanol, reformed methanol and synthesized petrol. Several other production methods such as thermochemical decomposition of water and solar

photo-electrolysis hold promise, but presently are laboratory rather than commercial techniques [3]. Hydrogen is a unique potential automotive fuel with significant drawbacks in its storage properties. Generally hydrogen may be stored in a vehicle in three ways: as a gas dissolved in the form of metal hydrides, cryogenic store of liquid hydrogen, and as a compressed gas. Metal hydride systems have been widely developed and provide improved safety over the other two methods of storage. Hydrogen is absorbed into the metal structure under pressure which is accompanied by the release of heat. Exhaust heat is normally used to release the hydrogen stored in the hydride, but hydrides that arc the most effective storage media require a temperature higher than available from typical exhaust gas under light-load operating conditions [10]. The most serious shortcoming of hydrides are their low mass and volumetric energy density. Low mass energy density means that hydride vehicles are much heavier and less efficient than comparable gasoline vehicles [11]. The cryogenic storage of liquid hydrogen for automotive use is both complex and expensive, in addition, loss rates of 1 2% per day have to be countered [12]. Moreover, 10 25% of the fuel boils off during refuelling [13]. The cost and energy associated with the liquefaction process must also be considered. To store a practical quantity for vehicles in high pressure vessels would result in a very large and heavy storage system. In addition, the storage of any high pressure gas presents a safety hazard in the event of vehicle collision. Hydrogen storage in a chemical compound such as gasoline is convenient, but requires the development of a small on-board chemical plant (reformer) capable of highly transient performance to convert the chemical back into hydrogen. In the on-board hydrogen generation from liquid fuels two distinctly different cases can be identified, namely conversion of all of the fuel and conversion of only a fraction of the fuel. In the second case the hydrogen-rich gas is mixed with liquid fuel in the engine induction system. The resulting hydrogen/ gasoline mixture is then burned under lean conditions. The lean limit of the mixture of hydrogen and gasoline lies between the lean limits of the two components. However, a small amount of hydrogen will allow a considerable reduction in the lean limit of the mixture. This concept is often called hydrogen enrichment. The use of hydrogen in automotive vehicles is not a recent event, it was utilized during the 2nd World War in gazogene composition (50% hydrogen in average) resulting from coal gasification [14]. More recently it has been experimented in a great number of vehicles in different countries, principally U.S.A., Japan and Germany. Mercedes-Benz, BMW, and Mazda prototypes are using gasoline engines that have been modified for hydrogen, although not yet optimized. The key question is how the fuel should be stored. Mercedes has opted for gaseous hydrogen that bonds in the fuel tank with powdered metals; Mazda plans a tank that stores hydrogen in metal alloy spheres; BMW uses liquid hydrogen [15]. BMW sees the development of the infrastructure for the production and distribution of hydrogen as one

ON-BOARD GENERATION OF HYDROGEN-RICH GASEOUS FUELS of the greatest challenges to its widespread use. BMW is studying the use of hydrogen in reciprocating engines as a means of retaining high performance on non petroleum fuels. Cryogenic hydrogen is injected directly into the combustion chamber, where it mixes with air. A lean fuel/air mixture results in high efficiency, minimizes nitric oxide emissions, and alleviates premature ignition and flame backfire into the intake manifold. Direct injection optimizes the cylinder charge and prevents flame backfiring. Injecting cryogenic hydrogen under high engine load also cools the charge in the cylinder in order to substantially reduce nitrous oxide emissions. A spark is needed for ignition as it is in a gasoline engine. BMW uses a mechanically driven centrifugal supercharger to make up for the hydrogen engine's lower power output. The company recently introduced the world's first engine test stand designed especially for testing hydrogen engines [163.

Properties of hydrogen Oll Earth, free hydrogen occurs only in negligible quantities, therefore it cannot be considered as a primary energy source. It is chemically very active and can readily form compounds with many other elements. As an energy carrier it has to be produced. The most abundant and accessible hydrogen-dense feed stocks are water, ammonia, oil and natural gas. Table 1 compares the combustion related properties of hydrogen to those of gasoline. Table 1. Comparative properties of hydrogen (derived from [I8 20]) Property Lean limit equivalence ratio in air Maximum flame speed in air (ms ~) Ignition limits in air (vol%) Minimum ignition energy in air (mJ) Quenching distance (mm) Spontaneous ignition temp. (° C) Lower calorific value (MJ kg-l) Net energy density (MJ m 3) (at 15°C and 101.3 kPa)

Gasoline

Hydrogen

0.58 0.5 10 7.6

0.1 3.5 4.0-75.0

0.24 2.84 280400 44 202

0.02 0.6 574 120 10.3

The low lean-limit equivalence ratio of hydrogen permits the use of very lean operating mixtures thus paving the way for high thermal efficiencies, while the resulting low peak cycle temperatures help to reduce NO x emissions substantially. The high thermal efficiency arises from an increase in the ratio of specific heats (7) of the charge becoming closer to that of air (7 = 1.4), lower heat loss to the engine coolant and reduced dissociation. Hydrogen has a very high flame speed and good ignitibility, thus it enables extremely lean operation, so lean that engines have been run down to 30% of full load without any throttling [17]. A stoichiometric hydrogen air mixture contains about 30% of hydrogen by volume. The lower flammability limit of hydrogen is 4% by

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volume and the upper limit is about 75 %. This property of hydrogen is best suited for adopting qualitative governing in which the air intake need not be throttled. The higher flame speed of hydrogen helps to maintain combustion as near to constant volume as possible when operating at lean mixtures, while the low minimum ignition energy of hydrogen helps in the rapid initiation of stable, self-sustaining flame kernels thereby reducing cyclic variations during lean combustion. In addition, the low quenching distance of hydrogen minimizes flame quenching in proximity to the combustion chamber walls. The high spontaneous ignition temperature of hydrogen helps to resist the knock and also to reduce the risk of fire following vehicle collision when escaping fuel may come into contact with the hot engine parts. Hydrogen is energetic in terms of the energy per unit mass of fuel burned but on a volume basis it is less attractive because of its low density. Thus, the problem is to find a means of densifying hydrogen for storage. The low energy density of gaseous hydrogen is detrimental in that the consequential low engine volumetric efficiency leads to poor maximum power. However, at extra cost, this could be overcome by adopting direct injection of the hydrogen into the cylinder after inlet valve closure. Summing up, hydrogen has great potential in engine performance. Its high flame speed and wide flammability limits permit efficient use of a wide band of fuel air equivalence ratios (q~) without the necessity of intake throttling in SI engines with its attendant volumetric efficiency loss and increase of pumping losses.

Spark ignition engine performance of hydrogen Hydrogen has been of interest as a fuel for spark ignition engines since the turn of the century. It has ability to burn efficiently at relatively weak mixtures, giving good fuel economy. The maximum efficiency from a hydrogen engine is obtained at an equivalence ratio of around 0.4 vs 0.9 for a gasoline engine [21]. However, all early attempts to burn either hydrogen-enriched fuels or hydrogen itself were plagued with abnormal combustion in the form of pre-ignition, back-firing and knocking, particularly when the mixture strength approached the region of stoichiometric and maximum power. The readiness for abnormal combustion in its various forms is no doubt due to the very low requirement of ignition energy, the high flame speed, and the very small quenching distance which allows flames to pass through the narrow openings of inlet valves causing backfire. Knock and backfire problems can be reduced by operating at a lean fuel-to-air mixture or by diluting the charge with exhaust gas recirculation or water injection [22]. Use of direct hydrogen injection into cylinder in a spark ignition engine precludes the possibility of backfiring and, since the rate of pressure rise can be controlled by the rate of hydrogen injection, it is possible to eliminate knock. High specific power is obtained since the volumetric efficiency for air is not impaired by the inclusion of hydrogen in the inlet stream. The disadvantage is the

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added cost and complexity of the high pressure injection system. Mathur and Das [23] carried out engine performance tests with a specifically designed timed manifold injection (TM[) system to achieve ultra lean operation with H 2, at six different compression ratios (6 11). At each compression ratio, constant speed tests were performed for six different speeds (1000, 1200, 1400, 1600, 1800, 2000 rpm). During these tests, the throttle was kept wide open and the load was varied from no load to full load. At each load setting the spark timing was adjusted to the MBT (minimum values for best torque). It was found that the indicated mean effective pressure (IMEP) increased with the increase in fuel air equivalence ratio within the range used, indicating the complete combustion of hydrogen at all equivalence ratios employed. The highest | M E P values were achieved at the values of equivalence ratios beyond the stoichiometric with all the compression ratios employed. An interesting observation was made in that the firing of hydrogen-operated engine was possible using TMI at an equivalence ratio as low as 0.218. However, at that low value, continued smooth operation of the engine could not be sustained for long. An indicated thermal efficiency as high as 56.2% was obtained using TMI. Rotary engines are particularly suited to hydrogen power because, unlike in a normal car engine, the fuel intake and combustion chambers are separate. This eliminates the risk of backfires, to which hydrogen is prone. Mazda is today the only car company manufacturing rotary engines for vehicle use [24]. The possibility of using pure hydrogen as an automotive fuel has received attention because of the increased importance of reducing exhaust emissions. When an internal combustion engine uses pure hydrogen, the equivalence ratio q5 can be extended to very low values, where exhaust emissions are reduced by several orders of magnitude from those achievable by lean operations with conventional hydrocarbon fuels [2]. One significant advantage of the hydrogen engine is that it does not generate carbon dioxide, carbon monoxide, hydrocarbons (except for the small amount associated with the lubricating oil), lead or sulphur pollutants. In addition, when a gaseous fuel is used, problems of the fuel atomization and vaporization are eliminated, and mixing with air can be more effective. When the engine is operating with atmospheric air, however, NO~ are formed. Emissions of NO~ increase with the combustion temperature, the length of the high-temperature combustion period, and the availability of oxygen, up to a point. There are several ways to control NOx formation in a hydrogen engine: run the engine very lean, which lowers the temperature, or very rich, which reduces the oxygen supply; decrease the burn-time or lower the engine rpm (which allows for better heat dissipation); or cool the combustion environment by adding water or exhaust gases or by using cryogenic fuel. Finegold and yon Vorst [25] used a 1973 Chevrolet, 350 in 3 (5736 cm3), V-8 engine to run on compressed hydrogen gas at 1500 rpm. Spark advance was set at

MBT for each operating point which varied from 66°BTDC to --2°BTDC for a range of equivalence ratios from 0.15 to 1.0. MBT spark timing yields the lowest NO x emission at the highest thermal efficiency. Two methods of operation with hydrogen were employed. In the first case, hydrogen and air were used in stoichiometric ratio and water was injected into the intake stream. Governing was accomplished by throttling the intake, This method resulted in lower efficiency. In the second case, neither water was injected nor the intake stream was throttled. The highest efficiencies were obtained with this quality governing method. At lower loads use of hydrogen showed an efficiency almost 100% greater compared to gasoline. Emission of NO x was dramatically less for the hydrogen fuelled operation than with gasoline, and even at higher power loads reduction was of the order of 90%. However, at higher power output dilution of the charge was necessary. Stebar and Parks [21] carried out tests on a single cylinder engine operated with 100% isooctane and then with 100% hydrogen as fuels. Operation on pure hydrogen fuel extended the lean limit equivalence ratio from 0.89 to 0.18. The maximum NO~ emission measured with H 2 was almost twice that obtained with isooctane. However, ultra lean operation consistent with hydrogen's wide flammability limits permitted dramatic reduction in NO x. Emissions of NO x were extremely low for equivalence ratios less than 0.55. All NO~ emissions would be eliminated by adoption of oxygen alone as the oxidant, hut this brings added complications of storage of oxygen in the liquid, pressurised gas, or metallic oxide form. Hydrogen fuel does not foul internal engine components with carbon deposits. Unlike gasoline, hydrogen does not dilute engine oil, and is less apt to form acids which reduce oil life time. On the other hand, hydrogen may be more explosive in the combustion chamber than gasoline, thus putting more stress on the engine. The high proportion of unburned mixture normally found in the blow-by gases traversing the piston~zylinder gap and entering the crankcase draws attention to the need either for re-design of piston rings, or inert-gas purging of the crankcase, when using hydrogen as fuel, otherwise violent explosions might result from an overheated bearing or other hot spot. Hydrogen can be considerably more thermally efficient than gasoline, primarily because it burns better in excess air, and permits the use of a higher compression ratio. On average, hydrogen vehicles are 22% more efficient than gasoline vehicles [18]. As hydrogen can burn in lean fuel/air mixtures, as well as, in rich mixtures, it can cause large improvements in fuel use effciencies in the stop start regime common in city driving. Performance gains reported for hydrogen relative to gasoline resulted from: the broad range of fuel air equivalence ratios (0.3 _< q5 < 1.1) that can be used without throttling of the air mixture (which increases pumping losses and reduces mechanical efficiency); the increased heat energy in the cylinder charge resulting from direct cylinder injection; and the rapid and efficient combustion over a relatively small crank angle [3].

ON-BOARD GENERATION OF HYDROGEN-RICH GASEOUS FUELS A naturally aspirated engine with a given displacement and compression ratio using stoichiometric hydrogen-air mixture as fuel will have a comparable thermal efficiency, but will have 20% reduced power compared to the same engine using gasoline as the fuel. A stoichiometric mixture also leads to higher NO~ emissions. Lean mixtures solve the emission problem and lead to higher thermal effciency but yield further power reductions [3]. Hydrogen would be about 15 50% more thermally efficient than gasoline in a fully optimized engine, depending on the various factors. Three factors favour operating the engine in the lean range: lower NO~, lower fuel costs and less tendency to backfire; and only one factor, reduced power, stands against lean operation [11]. In summary, the hydrogen spark-ignition piston engine, when compared with its conventional gasoline counterpart, appears to offer the following: 1. Wider range of operating mixture strength. 2. High thermal efficiency but low power at weak mixtures. 3. Large NO~ emissions at stoichiometric operation, but much less if timed fuel injection and unthrottled air flow leading to very lean operation are adopted. 4. Strong tendencies to pre-ignition and back-firing unless timed fuel injection after inlet valve closure is adopted, together with elimination of hot spots and carbon particles in the cylinder, and ignition timing retarded to near tdc. 5. Fairly strong tendencies to knock unless weak mixtures and cool operation are adopted, with preignition eliminated, in which case high compression ratios are possible [26]. Although the lean-mixture capability of hydrogen is beneficial at light loads, in order to approach the power capability of gasoline at full loads, the mixture has to be enriched.

SI engine performance qf hydrogen enriched gasoline The concept of hydrogen supplementation relies on the improvement of the thermal efficiency of engines using conventional hydrocarbon fuels by supplementing them with relatively small quantities of hydrogen. The addition of hydrogen can extend the lean-limit equivalence ratio while maintaining a sufficiently high flame speed. In this way the main fuel is used more efficiently and only a small quantity of hydrogen is needed. The operation of the engine with 5 to 10% by mass of hydrogen fuel makes it possible to operate the engine in the very lean regime, which would not have been possible without the presence of hydrogen [27]. Hydrogen supplementation is applicable to a wide variety of fuels. The reason of considering gasoline as the main fuel for this study is that it involves fewer modifications to the engine. Hydrogen and gasoline can be burned together in an internal combustion engine, in a wide range of mixtures, and generally with good results. The addition of small quantities of hydrogen to the combustion process increases the burn-rate of the mixture with better acceler-

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ation response while enabling more efficient operation at lower equivalence ratio. The backfire problems associated with hydrogen-only operation are avoided by limiting the level of supplementation and this in turn implies a requirement for a relatively compact and light weight hydrogen storage unit. Adding hydrogen to gasoline significantly increases the flame speed. This increase occurs at all equivalence ratios but especially at lean regime. Cassidy [28] found that at an equivalence ratio of 0.66, which is close to lean-limit of gasoline, the apparent flame speed was 61% faster with hydrogen enrichment (mass fraction of hydrogen -- 0.068). Yu et al. [29] have developed a model to predict the laminar flame speeds of various mixtures of hydrocarbons with hydrogen and air. Tests indicated that the flame speed of hydrocarbon/air mixtures was substantially increased by the addition of small quantities of hydrogen, and that the resulting speed could be correlated with that for the hydrocarbon fuel and a parameter indicating the extent of hydrogen addition. Milton and Keck [30] carried out combustion bomb experiments to evaluate flame speeds of hydrogen, acetylene, propane and methane, and mixtures of hydrogen with each of the other gases. The experiments were conducted over a range of temperatures and pressures from 0.5 to 7 bar and 300 to 550 K under stoichiometric conditions. The hydrogen/acetylene burning velocities approximated to the values indicated by proportional averaging of the values for the individual gases. Hydrogen/methane and hydrogen/propane mixtures exhibited a peak in burning velocity shortly after flame initiation, followed by a reduction, and then a second increase in flame speed. No explanation is offered for the doublepeak behaviour. It may be due to the propagation of two flames. The first peak in combustion rate may be due to the propagation of the hydrogen/oxygen flame, while the second peak could be due to the propagation of the flame for the second fuel [20]. The concept of hydrogen enrichment was first proposed by Bresheas et al. at Jet Propulsion Laboratory (JPL) to allow lean operation of the engine to produce low N O , emissions [31]. In experiments on a Cooperative Fuel Research (CFR) engine they showed that NO~ emissions from gasoline could be reduced slightly by lean operation. Levels equivalent to the EPA 1977 standard could not be achieved because misfire limited the minimum equivalence ratio to about 0.63. With hydrogen, however, the engine was operated down to equivalence ratios of 0.1 where the NO x emissions were less than 1/100 of the EPA standard. Thermal effciency was increased by 20 50%. In order to evaluate the full potential of the hydrogen enrichment concept, other reported experiments were conducted with increasing levels of complexity. The preliminary feasibility demonstration work was accomplished using hydrogen supplied from bottled storage, first using an engine dynamometer and then in a passenger car. The adoption of that method of hydrogen delivery to multicylinder gasoline engines was reported by Stebar and Parks [21], and Hoehn and Dowdy [2].

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Stebar and Parks [21] carried out tests on a CFR engine at the General Motors Research Laboratories operating with a compression ratio of 8:1 at 1200 rpm and M BT ignition timing. They found that supplementing petrol with a 10% mass fraction of bottled hydrogen (H 2 energy fraction,/~ = 23%) extended the lean-limit equivalence ratio from 0.89 to 0.55 and reduced NO:, emissions from 20 g kW -I h -~ to 0.27 g kW -1 h -1. With a 20% mass fraction of hydrogen lean-limit equivalence ratio was extended up to 0.4. Engine power decreased by 30% but thermal efficiency increased from 33% for an equivalence ratio of 1.00 to about 37% at 0.55 equivalence ratio. There was minor effect on CO emission. In contrast hydrocarbon emissions were adversely effected, at 0.55 equivalence ratio HC emission increased by about 100 % from the minimum level. HC emission increased with very lean mixtures reflecting the increased quench layer thickness and reduced post-flame oxidation of the quenched hydrocarbons. Since the hydrogen/oxygen combustion reactions are reported to be independent of the combustion reactions for the other fuel [20], it is possible that the hydrogen combustion proceeded independently. It also supports the two flame phenomenon of Milton and Keck [30], already discussed above. Hoehn and Dowdy [2] found that at equivalence ratios of 0.53 or less, very low NO., and CO emissions were produced, and that engine thermal efficiency was substantially increased over the normal gasoline operation. HC emissions were again somewhat higher. The hydrogen requirement to avoid engine misfire at the ultra lean conditions was about 15% by weight of the gasoline. Parks [32] carried out experiments on a CFR engine at 1200 r/rain and operating with a compression ratio of 8 : 1. MBT ignition timing was employed. The equivalence ratio was decreased from 0.9 in steps of 0.1 and jq values of 0%, 13 %, 23 %, 48 % and 100 % were selected to study the effects of hydrogen enrichment of hydrocarbon and NO~, emissions. For all [J values other than 100%, the minimum HC emission level occurred at approximately = 0.8 and then increased until the lean-limit for that [] was reached. At constant qS,the HC emissions decreased as ,q increased. For all values of/~, NO~, emissions peaked at approximately ~b = 0.85 and then fell sharply as ~b decreased towards the lean-limit. At constant qS, NO:~ actually increased slightly as [] increased. Rauckis and McLean [33] carried out work on a CFR engine at 1000 r/min with ignition timing set at 10 ° BTDC and running with a compression ratio of 8:1. The equivalence ratio was varied between 1.12 and 0.57 and t3 was varied from 0 to 28 %. Cylinder pressure data were collected and used in a zero-dimensional combustion model to calculate (~2%, 2 10% and 10-90% mass fraction burn durations. Indicated thermal efficiency and cycle-to-cycle cylinder pressure variation data were also calculated. It was found that the ignition delay period, as characterized by the 0~2% mass fraction burn duration, was significantly reduced by the addition of hydrogen, especially at lean equivalence ratios. Cycleto-cycle cylinder pressure variations were significantly reduced by hydrogen addition.

Lucas and Richards [34] conducted tests on a 1275 cm 3 4-cylinder engine at compression ratios of 8.9 : 1 and 11.7 : 1. The hydrogen flow-rate was set at 69.5 mg s- ~ to maintain the engine at idle (850 rpm, no load), with a fully open throttle and no petrol. Wide open throttle operation with hydrogen supplementation was found to increase part-load thermal efficiency and reduce the sfc. by up to 30%. Higher thermal efficiency was attributed to reduced heat loss to coolant and to the reduced pumping losses. Maximum power was found to be slightly penalized during hydrogen supplementation because of the drop in air volumetric efficiency due to the displacement of air by the hydrogen. This could be overcome by direct cylinder injection of the hydrogen but would be costly. When the hydrogen flow-rate was increased to 89 mg s 1 the part-load thermal efficiency was increased but the maximum power was further reduced. It was suggested that partial throttling might improve the thermal efficiency by increasing the equivalence ratio which would result in higher flame speeds. No attempt was made to study the joint effects of equivalence ratio and hydrogen energy fraction on the performance of the engine. Hydrogen supplementation resulted in a reduction in NO.~ and CO emissions but HC emissions were found to be high at very low loads. The reason for the increase in unburned hydrocarbons at weak mixture strengths is reported to be because the lean limit of operation had been approached. Sher and Hacohen [35] carried out work on a 2310 cm 3 4-cylinder engine with gasoline and hydrogen-enriched gasoline. Significant reduction in the bsfc of the order of 10 20% was achieved with hydrogen-enriched gasoline for a hydrogen to fuel mass ratio of 2 to 6%. Above 6% of hydrogen enrichment, the decrease in the bsfc was marginal throughout the experimental range. Cycle-to-cycle variation was reduced when a small amount of hydrogen was introduced. As a result of maintaining a near stoichiometric equivalence ratio, the HC and CO emissions were low but the NO~ emissions were high because of high combustion temperatures. Sfinteanu and Apostolescu [36] carried out experiments on test beds and in a vehicle. In the first set of tests a single-cylinderengine, derived from a conventional four cylinder car engine, was tested. Hydrogen was supplied to the engine from a high pressure cylinder. In the second set of experiments a four cylinder car engine was fuelled with hydrogen and gasoline. Hydrogen was supplied from a metallic hydride tank (Fe Ti). In the last set of experiments the fuelling system was tested on a DACIA 1300 car, with hydrogen percentage varying from 100% to 0%, according to the engine load. The general trends of the experimental program were: decrease of CO and HC concentration with the increase of hydrogen/gasoline ratio for a nearly constant fuel/air ratio; an increase of the NOx concentration with the increase of hydrogen due to higher temperatures of the burned gas during combustion; extension of lean limit to q~ = 0.83 to 0.71, which also increased thermal efficiency. The CO concentration decreased as the fuel/air ratio increased for all the operating regimes. Experiments with the car

ON-BOARD GENERATION OF HYDROGEN-RICH GASEOUS FUELS operating on a test bed yielded the same trends with dual-fuelling. The CO emission decreased by 25 %, compared with full gasoline fuelling. NO x concentrations were almost the same with the two fuelling systems. Gasoline consumption decreased with dual fuelling (7.75 litre per 100 km, compared with 9.37 litre per 100 km for full gasoline fuellingl. In summary, the addition of hydrogen to the gasoline engine appears to have the following effects: 1. Extension of lean-limit equivalence ratio resulting in reduced NO X and CO emission levels and improved engine thermal efficiency. 2. Reduction in cycle-to-cycle cylinder pressure variation. 3. Reduction in maximum power output. 4. Increase in the flame speed of mixture. 5. Increase in HC emission with the decrease of equivalence ratio. 6. Decrease in HC emission and increase in NO, emission with the increase of hydrogen energy fraction at constant equivalence ratio. To sum up the requirements for hydrogen/gasoline operation, hydrogen energy fraction ([~),equivalence ratio (~b), and ignition timing need to be optimized to give the maximum thermal efficiency within the constraints of the legislative requirements for HC and NO x exhaust emissions for a given engine speed/torque operating point. FUEL REFORMING TECHNIQUES The introduction of hydrogen as a supplemental automotive fuel could be hindered by serious logistic problems. No nationwide distribution system exists for hydrogen, and its storage as a high-pressure gas or cryogenic liquid requires vehicle capabilities which do not now exist commercially. These potential difficulties, however, can be avoided by generating the hydrogen in an on-board gas generator using one or more of the fuel reforming techniques discussed below. Thermal decomposition In a thermal decomposition process heat is added at suitable temperatures to dissociate the fuel. The thermal decomposition of hydrocarbon results in the formation of hydrogen and carbon, as shown in the following idealized equation: CH1.8o -* C s + 0.93H 2. In practice other hydrocarbon products are also formed, like methane, ethylene, etc., up to and including aromatic compounds [8]. The higher the temperature of decomposition, the more hydrogen the product gas contains. The difficulty of gasifying or handling the solid carbon makes hydrocarbon decomposition not suitable for onboard hydrogen generation.

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Methanol can be thermally decomposed into a 2 to 1 molar mixture of hydrogen and carbon monoxide. Hydrogen and carbon monoxide have several chemical and physical similarities from the view point of combustion properties. The velocity of their flames is simply proportional to the initial hydrogen content in the mixed fuel. When hydrogen is added in quantities of 2 molar% or more, stable combustion of carbon monoxide is promoted [37]. Methanol decomposition reaction may be represented as: CH3OH --~ 2H 2 + CO. The reaction is endothermic and requires a heat source to provide an amount of energy equivalent to 20% of the lower heating value of the methanol [8]. This reaction can occur under ambient temperature. At 200°C, 1 atm, more than 90% of methanol is dissociated [38], but the reaction rate is slow and a catalyst must be introduced, which inhibits the formation of carbon and facilitates the decomposition reaction at relatively low temperatures. The temperature at which the dissociated reaction occurs depend upon the type of catalyst used. Heat energy can usually be supplied by the engine exhaust gas. Voecks et al. [39] presented thermodynamic equilibrium calculations which predict formation of carbon solids and very little hydrogen and carbon monoxide production in the normal engine exhaust temperature range. Ultimately, in an engine which would be combusting H2/CO, the engine would probably operate at leaner equivalence ratios. This would lower the engine exhaust gas temperatures even further demanding the use of an active catalyst and an effective exchange of heat from the exhaust gas into the methanol feed streams. If the engine exhaust can supply all of the decomposition energy, the heating value of the product gases will be 20% greater than that of original methanol. The use of decomposed methanol as a fuel for spark ignition engine has been investigated by several authors [40-48]. Anthonissen and Wallace [47] carried out tests on an ASTM/CFR single-cylinder engine to compare the performance of liquid methanol and gasoline with that of simulated decomposed methanol (H z CO mixture). The results indicated that engines fuelled with H2--CO mixture had good efficiency at low imep, but could not achieve the same maximum imep as gasoline or liquid methanol fuelled engines. However, addition of liquid methanol to supplement the H z CO fuel increased the usable range of power outputs. In addition, it resulted in higher efficiency and lower nitric oxide (NO) emissions than if the engine were operated on H2-CO mixture alone. CO emissions were higher with H2-CO alone or with its blends indicating the incomplete combustion of CO. Improvements in brake efficiency with H2-CO mixture were up to 22% over gasoline at light loads, but the maximum brake power output was only 74% of that attained using gasoline. It was noted that increasing the intake manifold temperature resulted in a sharp decrease in efficiency at successively lower loads due to the occurrence of abnormal combustion. These results stress

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the importance of cooling the fuel gas after it leaves the reactor. Bechtold and Timbario [46] suggested that decomposed methanol can be capable of providing very low exhaust emission if operation is confined to equivalence ratios below 0.6. Yamaguchi et al. [45] of Japan Automobile Research Institute (JARI) conducted research on the dissociated methanol engine system for passenger cars. They suggested that the space velocity of methanol must be small in order to decompose it efficiently into hydrogen and carbon monoxide. The use of dissociated methanol gas as fuel under heavy load conditions resulted in preignition combustion because of a high fraction of hydrogen in the mixture. Therefore they employed a two-fuel system using dissociated methanol and liquid methanol. The cooling of the engine due to evaporation of methanol prevented the flash-back. They compared the performance of dissociated methanol gas engine with that of a liquid methanol and a gasoline engine. Dissociated methanol operation at q~ = 0.67 gave a relative efficiency improvement of 8%, compared to methanol operation, and of 20% compared to gasoline operation, both at ~0 = 1. Dissociated methanol gas engine could be operated in leaner region than the liquid methanol engine but it's power output was smaller than that of liquid methanol. NO x and CO emissions of the dissociated methanol gas engine met the Japanese 10-mode regulations of 1978 but UBF emission exceeded the regulation. Brinkman and Steber of General Motors Research Laboratories [44] found that the indicated thermal efficiency of gasoline running on dissociated methanol was only 0.89~0.95 of that obtained with methanol when compared at the same equivalence ratio and compression ratio. The authors were not interested in effects resulting from the decomposition process. They were concerned with engine efficiency and not with the system efficiency. Hence, the figures above represent engine efficiency values only. Thermodynamic analyses were made to isolate the factors responsible for the lower values of efficiency. The most important factors found were the lower molecular mass of dissociated methanol, which led to increase compression work, and the higher heating value, which caused increased heat losses. Sato et al. [42] described performance of spark ignition engine fuelled with both gasoline and dissociated methanol. They found that increasing the ratio of reformed gas to gasoline resulted in improved indicated thermal efficiency as well as in the extended lean limit of air/fuel ratio, owing to the reduction of burning duration and to the recycle of exhaust heat. The optimum engine operating conditions were obtained with the high ratio of reformed gas to gasoline and with the lean operation for medium to low load conditions, and with the use of gasoline only for high load conditions. They also conducted studies of noble metal and base metal catalysts and found that a base metal material containing quantities of copper, NiO and Cr20 3 provided good methanol conversion at temperatures as low as 200°C. The performance of catalyst remained unaffected during a 100 h durability test.

It can be concluded that hydrocarbon decomposition is not suitable for on-board hydrogen generation whereas methanol decomposition can be suitable. The use of decomposed methanol in an engine results in lower pollutant emissions and higher thermal efficiency. On the other hand, maximum power output is reduced. Therefore, for low load conditions the use of decomposed methanol alone is most suitable at very lean equivalence ratios, where unthrottled running and hence reduced pumping losses are possible, whereas for high load conditions a blend of liquid methanol/gasoline should be used. Steam reforming

Hydrogen can be produced by steam reforming of hydrocarbons or alcohols with or without the presence of a catalyst. The use of catalyst may result in low enough temperatures and short reaction times. The objective of a catalytic steam reforming process is to liberate the maximum quantity of hydrogen held in water and the feed stock fuel [-49]. Steam reforming of gasoline may be represented by the following idealised chemical reaction: CH1.86 + H20 ---* 1.93H 2 + CO. Carbon in the fuel is converted into CO by oxidation with oxygen supplied in the steam, and hydrogen in the fuel, together with hydrogen in the steam, is released as free hydrogen. In other words, the resulting hydrogen comes from fuel as well as from the steam. The reaction is endothermic, i.e. it requires external heat input through a heat exchanger surface. The amount of energy required is typically of the order of 22% of the (lower) heat of combustion of the liquid fuel. The heat of combustion of the products is thus 22% higher than the heat of combustion of the liquid hydrocarbon input. Houseman and Voecks [8] suggested that the reaction requires a temperature of 75(~850°C, a nickel catalyst, and excess steam. If the product gas is cooled down to 20(~300°C and more steam is added, carbon monoxide can be converted into carbon dioxide and more hydrogen can be liberated from steam by means of the following catalytic reaction: CO + H20 --~ CO 2 + H 2. This reaction is referred to as the shift conversion reaction and is responsible for producing some carbon dioxide in the reaction product. By utilizing the steam reforming and shift conversion reactions, it is possible to obtain product gas consisting of approximately 75% H 2 and 25% CO 2. Two important parameters which need to be considered, with respect to the desired hydrogen yield, are the required ratio of water to gasoline, and the required operating temperature. There is a trade-off between these two parameters to reach a certain hydrogen yield in relation to gasoline. It is important to avoid soot formation. A higher steam to gasoline fuel ratio is used to increase soot suppression, Houseman and Cerini [50] recommend a mass ratio of steam to gasoline of 3.5 to 4.0.

ON-BOARD GENERATION OF HYDROGEN-RICH GASEOUS FUELS One of the first efforts in on-board hydrogen generation was reported by Newkirk and Abel [51]. In the Boston Reformed Fuel Car project they decided to employ the steam reforming reactions without the use of a catalyst on the grounds that catalyst would introduce unnecessary cost and complexity. In their system water condensed from engine exhaust gases was fed to the reactor where it was vaporized and superheated to a temperature near 1000°C. Preheated gasoline was injected into this steam flow to produce gaseous fuel containing hydrogen and carbon monoxide. To provide the heat required by the reaction, a portion of the produced gaseous fuel was recirculated and burned in the reactor in the presence of air. The remaining fuel was stored in a small tank for engine operation. The theoretical reformer thermal efficiency was 59.7% without heat exchange. Problems with carbon solid deposition in the reformer were reported. Although their reforming process was not efficient, they achieved the objective of minimizing emissions. Bresheas et al. [31] presented a concept of using small amounts of hydrogen to allow burning of gasoline at ultra-lean conditions. By heating gasoline and water to 1500 2000°F (815 1094°C) they were producing hydrogen, carbon monoxide plus various hydrocarbons diluents. The reaction took place in a thermal reactor in the presence of catalysts. Heat was supplied by pumping air into the reactor and burning a portion of the gasoline. The maximum H 2 yield was 29%. It was possible to establish operating conditions at which soot was not produced. When the same generator was operated without water, larger quantities of soot were produced. The generator had conversion efficiency of approximately 67%. This work was performed at the Jet Propulsion Laboratory (JPL). It was later continued by Houseman and Cerini [6], Houseman and Hoehn [52], Hoehn and Dowdy [2] and Hoehn et al. [53]. Engine test results showed that 10% reduction in brake specific fuel consumption (bsfc) was obtained over the entire road load speed range. NO x emissions were reduced to below the 1977 EPA Standards. The work is summarized in a report published by the JPL [54]. In the JPL experiments described above, hydrogen was generated on demand by steam reforming of a portion of the fuel. Martin [27] at the University of Arizona carried this process one step further and described catalytic steam reforming of entire liquid fuel at a reaction temperature of I I50°F (621°C) into gaseous products which include 48 vol% Hz, 7.7 vol% CH4, 9 vol% CO, 11 vol% CO 2 and 24.3 vol% H20. He attempted to utilize as much of the engine's waste heat as possible, from both exhaust gas and engine coolant, in order to promote the endothermic reforming reactions. It was predicted that such a reformed fuel would raise the brake thermal efficiency of a test engine from a gasoline-fuelled level of 18.6% to a value of 26%. Martin predicted that methanol would be more suitable as a fuel for use in a reactor of the type described in his paper. In the case of steam reforming of methanol, the external heat source requirement disappears. As the reaction can be carried out at a much lower temperature of 20(~250°C,

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hot exhaust gas from the engine can be used as the only heat source. In addition, carbon solids formation problem can be easily avoided. At 250°C, the shift conversion reaction favours carbon dioxide rather than carbon monoxide [8]. Steam reforming of methanol can be represented as: CH3OH + H20 -* 3 H 2 + CO 2. Gaseous product of the reaction have a net calorific value 8% higher than that for gaseous methanol, thus reactor thermal efficiency of 108% is obtained. When product gases are compared with the liquid tanked methanol, thermal efficiency of the reaction becomes 114%, with the 6% increase being attributed to the heat of vaporization of the methanol. Hence, a fuel gas having a calorific value improved by 14% could be produced [8]. The reaction presented above is only an idealized presentation and the actual composition of the gas leaving the reformer is determined by the residence time, temperature, pressure, feed gas composition and catalyst used. In actual reformer the product gas would be composed of H z, CO, CO2, H/O, and unreacted methanol, as well as dimethyl ether, methane, and other organic compounds [39]. Kester et al. the Institute of Gas Technology [55] have investigated on-board steam reforming of methanol. At atmospheric pressure and temperature of 450°F (232°C), using engine waste heat, an enrichment of fuel combustion value of 8% (LHV) was obtained when reactor effluents were compared with tanked fuel. Operation at high pressure was investigated theoretically, without performing experimental tests, to predict production of highpressure gas for direct cylinder injection at maximum power conditions. The calculated system and generator efficiencies were similar to these at low pressure, i.e. 32.4% and 108% respectively. Finegold [56] reported a 15% increase in the LHV which was corresponding to calculation of the LHV of the methanol water mixture based solely upon the methanol in the mixture. McCall et al. [43] compared experimentally the performance of simulated dissociated methanol (2H 2 + CO), simulated steam reformed methanol (3H 2 + CO2) and liquid methanol in an unmodified SI engine. Operation on dissociated and steam reformed methanol was limited by flash-back to lean equivalence ratios, thus limiting the maximum power output of the engine as compared to liquid methanol at the same engine speed. Maximum power at 2000 rpm was lower by about 50% for dissociated methanol, and by 65% for steam reformed methanol compared to liquid methanol at the same speed. Greater reduction in maximum power for steam reformed methanol than for dissociated methanol was caused by the presence of an inert component (CO2) in the gaseous fuel mixture. There was little or no measurable difference of brake thermal efficiency between dissociated and steam reformed methanol over the range for which equal power was produced with each fuel. Under the ideal conditions investigated, approximately 25% greater brake thermal efficiency was obtained with reformed methanol compared to liquid methanol. In general the exhaust

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emissions were lower when using the reformed gaseous fuel than those obtained with liquid methanol. At lean equivalence ratios HC, NO x and aldehyde emission from dissociated methanol and steam reformed methanol were about the same but CO emissions were high from dissociated methanol while absent from steam reformed methanol. It was suggested that dissociated methanol is a better engine fuel than steam reformed methanol. It can be concluded that steam reformed gaseous fuel yields higher efficiency, lower pollutant emissions and lower bsfc values as compared to liquid fuel, when burned in an SI engine. Methanol can be steam reformed at lower temperatures as compared to gasoline, although dissociated methanol seems an even better fuel.

Partial oxidation Partial oxidation technique involves the exothermic reaction of feed fuel in the presence of a small amount of air, such that incomplete combustion should occur. Partial oxidation process has the following advantages: • It provides a simplified system designed by elimination of the external water and heat supply required for steam reforming. • Potentially better transient response characteristics since there are no heat transfer processes involved. • There is little or no soot production, depending upon type of the fuel. • The reduced propensity for soot production allows the use of liquid fuels from naphthas to heavy fuel oils: whereas, steam reforming feed stocks are limited to naphtha or light hydrocarbons [54]. The generation of hydrogen by partial oxidation from gasoline may be represented as: CHl.s6 + 1/2(O 2 + 79/21 N2) -~ 0.93 H 2 + CO + 79/42 N 2, where the oxygen is supplied in air. Carbon in the fuel is converted into carbon monoxide, and hydrogen in the fuel is released as free hydrogen. The product gas now contains the residual nitrogen from air, Partial oxidation is an exothermic reaction and 17% of the (lower) heat of combustion of the gasoline is released. This raises the temperature of the product gases to approximately 870°C [8]. No external heat source is required, in fact this released heat has to be removed before the product gas can be used in the engine otherwise volumetric efficiency would be low. If this heat is not recovered, the system efficiency suffers a 17% penalty. The above reaction represents a goal of converting all the carbon into carbon monoxide and liberating all the hydrogen as a free hydrogen. In practice this objective can be approached very closely. Houseman ant Cerini [50] have suggested an optimum condition just short of soot formation at a gravimetric air/fuel ratio of 5.2 with 80% thermal efficiency of the fuel conversion process. Hoehn and Dowdy [2] at the JPk obtained acceptable hydrogen yields from partial oxidation of gasoline with

reaction temperatures of about 1500°F (815°C) in the presence of nickel catalyst and at temperatures of 2000°F (1094°C) and higher without catalyst. In the further reported work [53, 54], the JPL system was evaluated in terms of fuel consumption and engine exhaust emission through a multicylinder (V-8) automotive engine/hydrogen generator test, single cylinder research (CFR) engine tests, and hydrogen generator characteristics tests. Equilibrium hydrogen output was achieved in approximately 60 s with sufficient energy content of the output stream to achieve a V-8 engine start after 20 s. A mixture of hydrogen and gasoline burned in the CFR engine gave very low NO x emission in the ultra-lean region. CO emission was also below EPA1978 standards. HC emissions were above the EPA1978 standard. The CFR studies further indicated that engine thermal efficiency was inversely related to the equivalence ratio (~b) and was increased approximately up to 40% (from 0.23 for conventional system to about 0,33 for hydrogen and gasoline mixtures). Federal Driving Cycle tests on a dynamometer showed that 0.4 grams per mile of NO x emissions could be obtained with a 10% increase in overall system efficiency. However, a catalyst converter was needed to control the HC and CO emissions [54]. Jenkins of Johnson Matthey [57] patented a small scale reactor to generate hydrogen by partial oxidation of hydrocarbons in the presence of a noble metal catalyst using the so-called "Hot Spot" technology. The technique is still under development and it is not yet possible to produce enough quantity of fuel gas to run an automotive engine using this technique. Partial oxidation of methanol is similar to gasoline but it yields lower temperatures than for gasoline and as a process has a maximum thermal efficiency of around 82.5%. Houseman and Cerini [50] at JPL evaluated different methods of hydrogen generation for automobiles. They concluded that hydrogen-rich gaseous fuels can be burned under ultra-lean conditions to yield very low NO.,. emissions without running into lean flammability limit problems. They predicted that methanol would be a suitable hydrogen source instead of gasoline. They developed a reactor, the operation of which indicated a good approximation of equilibrium over the full range of air to fuel ratio from stoichiometric at gravimetric A/F = 6.4 to A/F = 1.0. No soot was formed at any test conditions. At A/F of 1.0, the product contained 28.6% H 2 by volume. Partial oxidation and steam reforming processes can be combined together where exothermic partial oxidation reaction heat can be utilized by the endothermic steam reforming reactions. Partial oxidation steam reforming can be carried out with or without a catalyst. The use of a catalyst is always accompanied by a potential catalyst poisoning problem by sulphur or lead components in the gasoline or by carbon formation. Houseman and Cerini at JPL [6] developed a compact reactor (150 mm dia x 250 mm long) to produce a hydrogen-rich gas by partial oxidation/steam reforming process using air, gasoline and water. Air was preheated

ON-BOARD GENERATION OF HYDROGEN-RICH GASEOUS FUELS to 150°C and mixed with vaporized gasoline. In their first step they reformed the fuel without catalyst but they could not obtain chemical equilibrium even at 2400°F (1315°C) due to slow reaction speed. A nickel catalyst was then used to speed up the chemical reaction. Yields approximating the predicted equilibrium compositions were obtained at 1800°F (982°C). Under conditions where soot would be found in the non-catalytic thermal reactor, not a trace of carbon was found with the use of the catalyst. The product gas contained 22% H 2, 24% CO, 1% CH 4, 0.1% C2H4, 1% CO2, 1% H20, and 50.9% nitrogen by volume. Over a 100 h test the product gas composition did not change indicating the absence of any carbon formation and possibly the absence of sulphur poisoning. Optimum performance occurred at the A/F of around 5.2 lowest possible without formation of the soot. The generator efficiency was 78.5%, which corresponds to 96% of the maximum theoretical value. Houseman and Hoehn [52] carried out tests on a V-8 IC engine and Parks [32] on a single cylinder CFR engine using reformed fuel produced by on-board hydrogen generator developed by Houseman and Cerini [6]. In the Houseman and Hoehn's case, a reduction of approximately 10% of bsfc was measured over the entire level road load speed range relative to the engine fuelled with stock liquid fuel. Maximum engine thermal efficiency was 39.7% which represents a 22.1% improvement relative to the stock engine. NO x emissions were reduced to below the equilibrium 1977 EPA Standards. But HC and CO emissions could not be reduced to the corresponding level. In Park's study the equivalence ratio was varied from near stoichiometric to the lean operating limit of 0.34. Optimal equivalence ratio for emission control was 0.55. At that equivalence ratio, NO x and HC emissions were near their minimum values and CO was slightly above compared with gasoline. This improvement in emissions resulted, however, in power and fuel economy penalties. Houseman and Voecks [8] described different hydrogen generation processes and discussed the inherent advantages and disadvantages. They pointed out that it is impossible to produce an absolutely soot-free hydrogen-rich gas from gasoline by partial oxidation without a catalyst. it can be concluded that the use of reformed fuel in a spark ignition engine exhibits lower NO x emission and higher thermal efficiency with leaner mixtures. Partial oxidation of methanol yields lower temperatures than for gasoline without any soot formation problem.

Exhaust-gas re[brining In the exhaust-gas reforming technique, fuel is reformed catalytically by direct contact with a portion of the hot products of combustion leaving the engine. The ideal reforming equations representing the combustion, reforming and reformed-fuel combustion for gasoline feed stock may be written as:

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(i) Normal combustion:

CHI.86 + 1.465 tO 2 + 3.76 N2) -~ CO 2 + 0.93 H20 + 5.5 N 2. 0it Exhaust-gas reforming, using portion of exhaust gases from (il above: CHI.8~, + 0.52 (CO 2 + 0.93 H2O + 5.5 N2) -, 1.52 [CO + 0.93 H 2 + 1.88 N2}. (iii) Combustion of reformed fuel produced in (ii) above: 1.52 (CO + 0.93 H 2 + 1.88 N2) + 1.465 (O 2 + 3.76 N2) --, 1.52 (CO z + 0.93C H2O + 5.5 N2). It can be noted that the exhaust gas composition indicated in equations (i) and (iii) are the same, and the exhaust gases from (iii) are used to reform gasoline (CHI.8o) in accordance with equation (ii). Thus the cyclic process involves equations (ii) and (iii). Lindstr6m [58] of the Swedish Royal Institute of Technology appears to have pioneered the exhaust-gas reforming technique, leading to a U.S.A. Patent in 1975. He invented a catalytic reactor in which part of the fuel, water (as steam), air, and a part of the engine exhaust-gas were contacted to produce a gaseous fuel containing hydrogen and carbon monoxide as combustibles. Reforming reaction temperature depended upon the type of catalyst and fuel, and the structural form and dimensions of the system depended upon the type and size of the combustion engine and other design considerations. Part of the heat was supplied to the reactor indirectly by hot exhaust gases from the combustion engine which were not recirculated to the engine via the reforming reactor. A significant advantage of the invention is that the system can utilize various types of fuels and their mixtures. The generated reformed fuel gas was used to supplement the fuel/air charge fed to the engine, thus permitting operation at ultra-lean air to fuel ratios. HC, CO, and NO~ emissions were claimed to be relatively low. Jones et al. [7] at the University of Birmingham studied the thermodynamic feasibility of the exhaust-gas reforming of the J P-5 fuel (CH1.92) as representative of gasoline. Software based on the Gibbs function minimization was developed to predict equilibrium compositions of reformed fuels producible by the direct contact of engine exhaust gases with the fuel. A two zone thermodynamic model of the engine cycle was used to predict the effect of reformed fuels on engine performance. Predictive studies of the equilibria and energy requirement of the exhaust-gas reforming with unmodified as well as with exhaust gas modified with 100% additional steam or with 3% excess oxygen in air or with both additions simultaneously were conducted. With unmodified exhaust gas, at the final product temperature of 1000 K and excess

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oxidant factor (2r = 1.5), just above the carbon solid formation limit, hydrogen and carbon monoxide yields of around 18% were predicted, with a reactor thermal efficiency of 125.4%, representing a 25.4% increase in fuel calorific value across the reactor. Exhaust gas temperature requirement of 1350 K was predicted. At the final product temperature of 900 K, with modified exhaust gas containing 100% additional steam and 3% excess oxygen in air, a product fuel containing 20% hydrogen at 2r = 2 was predicted with reactor thermal efficiency of I 11.5%. Prediction of exhaust gas temperature requirement was 1120 K, which was at a more realistic level. Engine cycle simulation runs were conducted for a number of reformed fuels. 7.5 to 19.8 % improvements in imep over base line were predicted. Reduced NO., emissions level were also predicted. The worst-case peak NO x level predicted for the reformed fuels still represented an improvement of some 74% below baseline. The ideal exhaust-gas reforming equations for methanol may be written as: (i) Normal combustion: CH3OH + 1.5 (0 2 + 3.76 N2) CO 2 -~- 2 H 2 0 + 5.64 N 2. (ii) Exhaust-gas reforming, using portion of exhaust gases from (i) above: CH3OH + 0.5 (CO 2 + 2 H20 + 5.64 N2) ---* 1.5 ( C O 2 ~- 2 H 2 + 1.88 N2). (iii) Combustion of reformed fuel produced in (ii) above: 1.5 (CO 2 + 2 H 2 + 1.88 N2) + 1.5 ( 0 2 + 3.76 N2) -~ 1.5 (CO 2 + 2 H20 + 5.64 N2). Improvements in lower heating value resulting from exhaust-gas reforming of methanol are 13.6% [9]. Sj6str6m et al. [59] of the Royal Institute of Technology Sweden demonstrated the exhaust-gas reforming of methanol. The conversion of methanol in the reactor was higher than 98%. The reformed fuel gas composition was almost in the equilibrium and average volume percentages of combustibles were 37% H 2, 17% CO and 3% CH 4. It is claimed that the system have high potential for running on ICE with low pollutant emissions and high energy efficiency. Although the maximum efficiency and minimum emission did not appear under the same running conditions, it was possible to run the engine with low NO x suffering a small penalty in efficiency. An air/fuel ratio (2) 1.3 1.4, about 5% recirculated exhaust gas and about 30~40 vol% methanol of the total fuel were reported to be the optimum running conditions. In further work at the University of Birmingham, Jones [9] and Jones and Wyszynski [60] studied the thermodynamic feasibility of the exhaust-gas reforming of n-heptane, gasoline and methanol feed stocks. A proto-

type test rig, comprising a propane-fuelled exhaustgas generator and a prototype catalytic reforming reactor, was designed, constructed and tested to establish optimum operating conditions for the reforming process. Experiments were conducted to produce reformed fuels using n-heptane and European reference-grade unleaded gasoline, RF-08 feed stocks by exhaust-gas reforming process. Good conversion of n-heptane feed stock at close to equilibrium levels was achieved. Conversion of ULG, however, was reported to be more difficult than that of n-heptane. Peak hydrogen and carbon monoxide levels for n-heptane were 32.2% and 20.9% respectively, whereas for ULG these were 19.8% and 12% respectively. However, reactor gas inlet temperature was high (1232 K) and gas hourly space velocities in the catalytic reactor, based on the volume of catalyst bed (GHSV's) were low (1000 h 1). Low levels of GHSV resulted in low rates of generation of reformed fuels which were inadequate in terms of fuelling a typical automotive power unit. High gas inlet temperature requirement would be difficult to achieve over the wide range of engine operating conditions experienced in the current passenger car. The highest reactor thermal efficiency reported was 128 % for n-heptane feed stock. However, generally it was between 95 and 115%. On the other hand, for ULG reactor thermal efficiency was in the range of 70 and 90% that was explained to be due to heavy carbon solids deposition and presence of unreacted feed stock components in the product gases which could not be measured. Theoretical predictive studies of the exhaust-gas reforming of methanol indicated that the process was far less prone to carbon solids formation than that applied to the hydrocarbons, whilst temperature requirements were considerably lower. At 900 K final product temperature and )~r = 1, H 2 and CO yields were predicted to be 27.2 and 9.7% respectively, with reactor thermal efficiency of 115.8% and an exhaust gas temperature requirement at inlet to the reactor of around 1060 K. Cycle analysis simulations, based on the computed equilibrium compositions of reformed fuels, predicted improvements in overall engine efficiency as compared with standard fuels. Predictive reductions in peak equilibrium and rate-controlled levels of NO x were in excess of around 48.5 and 89.7% respectively, in the case of n-heptane fuels, and 30.3 and 51.0% for the methanol derived fuels, when reformed fuel levels were compared to standard fuel baseline at the same level of cycle work output. It was concluded that reforming at 2r between 1 and 2 which coincides with the practical soot formation limit, and at a GHSV of I000 h 1, would give the best compromise in terms of efficiency and hydrogen yield in the reactor used. The use of an exhaust-gas composition containing a small amount of excess air and steam would also be beneficial. The high reformer inlet temperature requirement problem is under continuing investigation at the University of Birmingham. Bradley and Sheppard [61] at the University of Leeds selected a typical composition of reformed fuel produced by Jones [9] and tested that reformed fuel in a simulated

ON-BOARD GENERATION OF HYDROGEN-RICH GASEOUS FUELS (bottled) form for lean combustion limit and flame speeds using fan-stirred turbulent combustion vessel. A high turbulence intensity is required to minimize the burn duration in an engine because minimizing the burn duration maximizes the work done by the engine [62]. The fuel contained 22.84% H2, 12.07% CO, 3.95% CH 4, 5.19% ethylene, 1.21% ethane, 1.27% propylene, 9.07% CO z and 44.40% nitrogen by volume. It was reported that for equivalence ratios above 0.5, reliable ignition was achieved at different turbulence intensities (O-15 ms 1). At low turbulence levels, reliable ignition was obtained with mixtures of equivalence ratio as low as 0.35. At a given equivalence ratio, the laminar burning velocity for that fuel was approximately twice than that for iso-octane air mixtures. The wider burning limits and higher laminar burning velocity of the reformed fuel was reported to be associated with the high concentrations of H z in the mixture. Sher and Ozdor [63] have shown that the addition of 5 % H 2 can result in a 35 % increase in laminar burning velocity for n-butane air mixtures. West [64] at the University of Birmingham carried out experiments to study the performance and emissions of a single cylinder Ricardo E6 engine fuelled with simulated reformed fuel and gasoline. The composition of simulated reformed fuel was based on one of the reformed fuels produced by Jones [9] and was very close to that used by Bradley and Sheppard [61]. All the tests were conducted at compression ratio of 8 and at an ignition timing of 25°BTDC. Test results indicated the trend for a decrease in the overall equivalence ratio needed to maintain constant speed and load as the reformed fuel input was increased. Emissions of CO, NO x and total hydrocarbons were reduced as the reformed fuel intake was increased. Overall fuel conversion efficiency increased as the flow of reformed fuel increased and then decreased as the mixture became leaner. In some of the tests, decrease in hydrocarbon occurred rapidly as the first 8 % (energy proportion) of reformed fuel was added, then levelled out to give no further reduction as the proportion of reformed fuel increased. This reduction indicates that the quality of combustion improves as the proportion of reformed fuel increases. It may prove that 8% is the optimum reformed fuel energy proportion to be used. The benefits of using more gaseous fuel will then be small. The main benefit obtained from the addition of reformed fuel was a very large reduction (60--70%) in emissions of aromatic hydrocarbons, (such as benzene, toluene, etc.), as measured by on-line mass spectrometry. It can be concluded that exhaust-gas reforming of methanol is more feasible than that of the hydrocarbons, as the reactions occur at lower temperatures and are less prone to carbon solids formation. Use of reformed fuel as an additive have substantial benefits in terms of both exhaust emission and fuel consumption. DISCUSSION From the standpoint of helping to solve the pollution problems, hydrogen is a good alternative fuel, as it burns clean and it can be burned at a wide range of air/fuel

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ratios. By burning it in the lean region the thermal efficiency of an engine is improved and pollutant emissions are reduced. However, there are certain problems involved in running the engine using hydrogen alone. Due to its high flame speed and low ignition energy requirement it may cause abnormal combustion particularly at high load conditions. Due to its low density it occupies a large volume in the engine cylinder, reducing the volumetric efficiency for air of the engine and hence, the maximum power output. It is also necessary to find a means of densifying the hydrogen for storage. To avoid such problems it is more practical to use small amounts of hydrogen to extend the operating range for gasoline down into the ultra-lean region. In this paper work of many authors has been discussed and it has been found that the engine thermal efficiency increases as equivalence ratio (~b) decreases from unity until some peak value just short of the lean-misfire limit. Increasing hydrogen energy fraction (/~) results in an extension of the lean-misfire limit thus allowing leaner operation and raising the value of peak thermal efficiency. In theory, peak thermal efficiency should continue to rise until [/ reaches 100% but in practice it has been found that a relatively low optimum value of [/exists; the reason for this being attributed to the quenching of the gasoline oxidation process as the equivalent gasoline only leanmisfire equivalence ratio is reached For the lean mixtures, the effects of hydrogen-enrichment on the already negligible CO emission can be ignored. Wirth respect to HC and NO.~ emissions, hydrogen enrichment is a compromise situation. Each level of H 2 enrichment to reduce HC emission must be accompanied by leaning out for NO., control [32]. The practical implementation of operating an IC engine on hydrogen-enriched gasoline depends on the availability of an on-board hydrogen generator. It is desirable to limit the amount of hydrogen needed to minimize the hydrogen generator size and reduce the effect of generator efficiency on overall heat economy. The energy requirements for hydrogen generation are balanced by increases in engine efficiency under lean operating conditons. The trade-offs are the extra complexity of the hydrogen generator and a loss in maximum power capacity. Four methods of on-board hydrogen generation, namely thermal decomposition, steam reforming, partial oxidation and exhaust gas reforming of hydrocarbons and methanol have been discussed in this paper. It is apparent that there are many trade-offs in on-board hydrogen generation, both in the choice of fuels as well as in the choice of a chemical process. Thermal decomposition method is not suitable for hydrocarbon fuels due to the difficulty of gasifying or handling the carbon solids. The methanol decomposition reaction yielding carbon monoxide can provide good overall engine efficiency at a given operating condition as it is possible in this process to recover the waste exhaust heat and store it as fuel energy. However, operation during engine transients is unknown. During deceleration and low power operation it is likely that the reformer

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performance will fall below optimum and less than 100% decomposition will occur. This defeats the purpose of the unit and likely to cause driveability problems [46]. In terms of energy increase, the methanol decomposition reaction yielding CO is more attractive than the steam reforming reaction yielding CO 2. It also offers the advantage of a simplified system since no water is required. In steam reforming, the fuel reacts with water on a catalytic surface to produce a mixture of hydrogen and carbon monoxide. The reaction is endothermic so heat musl be supplied to the catalyst bed. In partial oxidation, the fuel reacts with air, either on a catalytic surface or in a flame front, to yield a mixture of hydrogen and carbon monoxide. This reaction is exothermic, and heat is evolved. Steam reforming requires a source of on-board water, whereas partial oxidation does not. On the other hand, the hydrogen-rich gas produced by partial oxidation contains nitrogen, whereas, there is no nitrogen in the steam-reformed fuel gas [50]. Thermal efficiency for the steam reforming process is greater than unity whereas that of partial oxidation is less than unity. Since the partial oxidation reaction is an exothermic reaction, therefore the heat release results in a temperature rise in the hydrogen product gas. Unfortunately, the IC engine cannot derive any mechanical energy from the sensible heat in the hydrogen product gas. The steam reforming process produces a hydrogen-rich product gas from gasoline for combustion in the engine. The reforming reaction occurs at high temperature, and because it is endothermic, it requires a large heat input supplied indirectly by heat exchange through the reactor wall. The fact that all the gasoline passes through the reformer makes it necessary that the reformer have a high thermal efficiency, wide dynamic range, and large capacity. Losses in the reformer can quickly offset any improvement in engine thermal efficiency gained by lean operation [2]. By combining the partial oxidation and steam reforming processes together the heat of exothermic partial oxidation reaction can be utilized in the endothermic steam reforming reactions. Oxygen hydrocarbon reaction will take place first (as oxygen is a much stronger oxidizer for hydrocarbons than water), which is then followed by secondary reaction of steam with the partial oxidation products. The only way to obtain a higher overall system efficiency would be to carry out the steam reforming and the partial oxidation reactions in separate containers with good mutual thermal contact for heat transfer. Obviously such a combined system is several times more complicated than a single reactor, particularly with respect to controls [50]. In exhaust gas reforming hydrogen-containinggaseous fuels are produced efficiently in a reactor which is simpler than a steam reforming reactor since no water is normally required. Exhaust gases contain sufficient amount of steam to promote the reaction. However, in exhaust-gas reforming of gasoline exhaust gas temperature requirements are too high to be achieved over a wide range of engine operating conditions, and the quantities of fuel

produced by existing reactor are not sufficient to power an automotive engine. It is apparent that methanol is the easiest fuel for producing a hydrogen-rich gas, either by thermal decomposition, by steam reforming, by partial oxidation or by exhaust-gas reforming. The main reasons for this are the low temperature requirement, the ease of avoiding carbon formation, and the absence of sulphur. But methanol vehicles are not in widespread use, so a lot of modifications to the existing gasoline engine are required to develop a methanol reforming reactor. On the other hand, the use of reforming reactor with gasoline involves fewer modifications. It can be concluded that an automobile could not be operated solely on reformed fuels over the entire required power range. A supplementary fuel system or power source would be necessary to attain the higher power levels. The use of reformed fuels compared to liquid fuel, may result in a small improvement in thermal efficiency in the low power range and may result in lower exhaust emissions. At the University of Birmingham work is under progress to optimize the exhaust-gas reforming of gasoline and hopefully in future we will be able to enjoy some of the benefits of the hydrogen engine by storing hydrogen on-board in the form of gasoline. On-board hydrogen generation from gasoline is technically feasible and yields substantial improvements in fuel economy and emissions. CONCLUSIONS 1. Spark ignition engine can be operated efficiently at light loads using hydrogen fuel alone. 2. Some charge dilution to avoid knock is essential to permit operation with hydrogen at higher power output. 3. Hydrogen supplementation of gasoline combustion has been shown to yield reduction in fuel consumption. 4. Hydrogen-rich gaseous fuels can be burned under ultra lean conditions to yield very low NO x emissions without running into lean flammability limit problems. 5. The lean burning conditions give possibilities for very low CO emissions. 6. Enrichment by pure hydrogen does not appear to be a suffcient means of reducing HC emission as measured by total HC methods. 7. Consideration of the hydrogen/gasoline/air combustion process, coupled with the observation of steady state test-bed performance, suggested the possibility that the hydrogen and gasoline oxidation processes are independent and may result in two flames. 8. On-board hydrogen generation from liquid fuels, either hydrocarbons or alcohols, is technically feasible. 9. The economy benefit from running lean almost compensates for the energy requirements for making hydrogen from gasoline in an atmospheric reactor. 10. Some of the waste heat from the vehicle exhaust gas can be reclaimed by converting it to chemical energy in the fuel. 11. Optimum performance of a reforming reactor occurs at the lowest possible excess oxidant factor just short of the soot formation limit.

ON-BOARD GENERATION OF HYDROGEN-RICH GASEOUS FUELS 12. The use of a catalyst in the reforming reactor allows a closer a p p r o a c h to equilibrium H 2 yields. 13. T h e use of reformed fuel ( c o m p a r e d to liquid raw fuel) may result in higher engine thermal efficiency in the low power range, with the i m p r o v e m e n t due to the increase in the heating value of the gaseous fuel resulting from the reforming reaction. 14. Use of reformed fuel ( c o m p a r e d to liquid fuell in a spark ignition engine m a y result in lower exhaust emissions, particularly of the heavier and a r o m a t i c hyd r o c a r b o n s and NO~. 15. In all cases considered, a considerable loss in m a x i m u m power capacity of the engine occurs as a result of the use of gaseous fuels a n d by operating u n d e r lean conditions. Effective utilisation of o n - b o a r d h y d r o g e n generation calls for lightweight high displacement engines to o v e r c o m e this disadvantage. 16. An a u t o m o b i l e could not be o p e r a t e d over the required power range when it was fed exclusively with reformed fuel. A s u p p l e m e n t a r y fuel supply would be required to reach the higher loads.

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