Application of Exhaust Gas Fuel Reforming in Compression Ignition

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Energy & Fuels 2003, 17, 1464-1473

Application of Exhaust Gas Fuel Reforming in Compression Ignition Engines Fueled by Diesel and Biodiesel Fuel Mixtures A. Tsolakis, A. Megaritis,* and M. L. Wyszynski School of Engineering, Mechanical and Manufacturing Engineering, The University of Birmingham, Birmingham B15 2TT, U.K. Received April 2, 2003. Revised Manuscript Received July 23, 2003

In this study, the application of exhaust gas-assisted fuel reforming in compression ignition engines (CI) has been investigated. Experiments were conducted in a single-cylinder directinjection (DI) diesel engine fueled by conventional diesel and also by a biodiesel mixture. First, the effects of exhaust gas recirculation (EGR) and addition of small amounts of hydrogen on the combustion and exhaust emissions were explored. With the addition of hydrogen, the flow of the main fuel (diesel or biodiesel) was reduced to maintain constant indicated mean effective pressure (IMEP). Thus, in effect the tests involved fuel replacement by hydrogen rather than hydrogen addition. Second, the feasibility of producing hydrogen “on-board” by catalytic exhaust gas fuel reforming was examined by incorporating a laboratory reforming mini reactor in the engine exhaust system. Prototype catalysts and different reaction conditions were examined. The results from the first part of the study showed that partial replacement of the hydrocarbon fuel by hydrogen combined with EGR resulted in simultaneous reductions of smoke and nitrogen oxides emissions (NOx) without significant changes to engine efficiency. In the second part of the study, it was shown that the amount of hydrogen required to achieve these beneficial effects potentially can be produced by exhaust gas-assisted reforming of the hydrocarbon fuel.

Introduction Diesel engines offer higher efficiency, better fuel economy, and lower CO2 emissions than conventional gasoline engines, and they are gaining a larger share of the market for high-speed passenger cars in addition to their traditional use in heavy- and medium-duty applications. On the other hand, diesel engines need to overcome problems related to emission regulations. They generally emit high nitrogen oxide (NOx) emissions and are associated with much higher particulate matter (PM) emissions than spark ignition engines. The problems associated with the so-called particulate-NOx tradeoff are well-known. Extensive work has been carried out on engine development including the engine inlet system, exhaust, combustion chamber geometry, and fuel injection system such as high-pressure injection in combination with multiple injections. These systems have given some reductions of NOx and particulate emissions.1-3 The homogeneous diesel combustion combined with multiple fuel injections and use of EGR has also been investigated. The technique provides very attractive reduction of the NOx and soot simultaneously; however, the operating range is extremely limited.4 The use of the well-known EGR technology has shown * Corresponding author. E-mail: [email protected]. (1) Montgomery, D. T.; Ritz, R. D. SAE Tech. Pap. Ser. 1996, No. 960316. (2) Pierpont, D. A.; Montgomery, D. T.; Reitz, R. D. SAE Tech. Pap. Ser. 1995, No. 950217. (3) Su, T. F.; Patterson, M. A.; Reitz, R. D.; Farrell P. V. SAE Tech. Pap. Ser. 1996, No. 960861. (4) Gray, A. W.; Ryan, T. W. SAE Tech. Pap. Ser. 1997, No. 971676.

effective reduction of the NOx emissions but it is always associated with an increase of exhaust smoke and particulates as well as of fuel consumption.5,6 To achieve future more stringent emissions requirements, there is a need to look for new technologies beyond the engine system developments and modifications. The use of “clean” and renewable fuels in combination with the above techniques may be the key to overcoming emission regulations without significant changes to engine efficiency and fuel economy. In recent years, ester-based oxygenated fuels have been used in compression ignition engines in pure form or as an addition to diesel fuel. The most common biodiesels are the Soyate Methyl Ester (SME) and Rapeseed Methyl Ester (RME). Vegetable oils are produced from a wide range of oilbearing plants, for example, rapeseed and soybean. Pure vegetable oil is unsuitable as a fuel, primarily due to its high viscosity. Esterification of the rapeseed or soybean oil with methanol produces the RME and SME, respectively, which are more suitable for use in diesel engines, fuel cells, and fuel reformers. Biodiesel can be used as pure 100% or as a blend with conventional diesel fuel. Using biodiesel in internal combustion (IC) engines results in lower hydrocarbon (HC), carbon monoxide (CO), and particulate emissions and higher NOx emis(5) Ladommatos, N.; Balian, R.; Horrocks, R.; Cooper, L. SAE Tech. Pap. Ser. 1996, No. 960841. (6) Ladommatos, N.; Balian, R.; Horrocks, R.; Cooper, L. SAE Tech. Pap. Ser. 1996, No. 960840.

10.1021/ef0300693 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/19/2003

Application of Exhaust Gas Fuel Reforming in CI Engines

sions.7,8 The increase of the percentage of biodiesel into the fuel mixture generally results in small but measurable losses of engine power and increases in fuel consumption.9,10 Hydrogen has been long believed to be one of the most promising alternative fuels for IC engines in terms of emission control and engine performance. A great number of publications related to research work on hydrogen-fueled engines is available in the literature and a few representatives are briefly presented in the following paragraphs. Apostolescu and Chiriac11 investigated the influence of small amounts of hydrogen added to gasoline-air mixtures on combustion characteristics. The authors carried out the experiments using a single-cylinder spark ignition (SI) engine. They found that the brake thermal efficiency of the engine was 10% higher when the engine was fueled with gasoline and hydrogen addition compared to gasoline only. The HC emissions were reduced, but the NOx emissions were higher, except for very lean mixtures. Welch and Wallace12 conducted experiments in a single-cylinder Lister ST-1 direct-injection diesel engine converted to operate on hydrogen. Their test results showed that the hydrogen-fueled diesel engine could produce higher power and less NOx emissions than the ordinary diesel engine. Shrestha et al.13 added small quantities of hydrogen and oxygen to a diesel engine and obtained reduced emissions of PM, CO, and NOx in comparison to the corresponding baseline diesel operation. A hydrogengenerating system using power from the vehicle’s battery/alternator circuit to conduct water electrolysis was employed to produce the hydrogen and oxygen gases. Several techniques are already available for hydrogen production from hydrocarbon fuels. In hydrocarbon steam reforming (SR), high-temperature steam separates hydrogen from carbon atoms. This process is extremely productive, but it poses a number of problemss mainly an exceedingly high loss of energy due to the reaction being largely endothermic.14 The reaction for a typical diesel fuel is

C12.31H22.17 + 12.31H2O f 12.31CO + 23.4H2

(1)

The water-gas shift reaction can also take place as a secondary reaction to produce additional hydrogen gas due to the presence of CO and excess steam:

CO + H2O f CO2 + H2

(1a)

The unwanted methanation reaction, which consumes (7) Graboski, M. S.; Ross, J. D.; McCormick, R. L. SAE Tech. Pap. Ser. 1996, No. 961166. (8) Marshall, W.; Schumacher, G. L.; Howell S. SAE Tech. Pap. Ser. 1995, No. 952363. (9) Sharp, C. A.; Howell, S. A.; Jobe, J. SAE Tech. Pap. Ser. 2000, No. 2000-01-1967. (10) Friis, H. K.; Grouleff, J. M. SAE Tech. Pap. Ser. 1997, No. 971689. (11) Apostolescu, N.; Chiric, R. SAE Tech. Pap. Ser. 1996, No. 960603. (12) Walch, A. B.; Wallace, J. S. SAE Tech. Pap. Ser. 1990, No. 902070. (13) Shrestha, S. O. B.; LeBlanc, G.; Balan, G.; De Souza, M. SAE Tech. Pap. Ser. 2000, No. 2000-01-2791. (14) Steban, R. F.; Parks, F. B. SAE Tech. Pap. Ser. 1974, No. 740184.

Energy & Fuels, Vol. 17, No. 6, 2003 1465

hydrogen, may also take place:

CO + 3H2 f CH4 + H2O

(1b)

Partial oxidation (POx) is another commercially available method for deriving hydrogen from hydrocarbon fuels. The process is not usually considered to be attractive in terms of efficiency because it is an exothermic process and the resulting hydrogen-containing fuel gas has a lower calorific value than that of the original feedstock.15 The partial oxidation reaction for typical diesel fuel is

C12.31H22.17 + 6.15O2 f 12.31CO + 11.08H2 (2) Depending on the reactor conditions, complete oxidation may also take place, which obviously is undesirable in terms of hydrogen production:

C12.31H22.17 + 17.85O2 f 12.31CO2 + 11.08H2O

(3)

In autothermal reforming (ATR), the POx and SR take place in one reactor. In ATR a hydrocarbon feed is reacted with both steam and air (oxygen) to produce a hydrogen-rich gas16 as shown below:

C12.31H22.17 + 5.65O2 + H2O f 12.31CO + 12.08H2 (4) Thermal decomposition of the hydrocarbon fuel can also result in production of hydrogen. A number of investigators have presented various benefits in terms of performance and emissions realized by using reformed fuels in IC engines. Some representative publications are presented briefly below. Kopasz et al.17conducted tests using an Argonne prototype autothermal reforming catalyst to reform conventional and alternative fuels to hydrogen-rich product gases. They reported that alcohols are reformed at temperatures lower than 600 °C, and alkanes and unsaturated hydrocarbons at slightly higher temperatures; fuels such as gasoline and diesel require temperatures higher than 700 °C for maximum hydrogen production. Sogaard et al.18 converted a four-stroke BUK DV diesel engine to spark ignition operation. The engine was operated using natural and reformed natural gas. The reformed gas composition was about 66.5% methane, 23% hydrogen, and 9% carbon dioxide. In comparison to natural gas, the reformed gas-fueled engine showed an increase of engine power and thermal efficiency and a decrease of HC and CO emissions. On the other hand the NO emissions were higher. A 1986 Pontiac 4-cylinder in-line turbocharged, sparkignition engine was fueled with air-reformed natural gas (natural gas broken down into hydrogen and carbon (15) Houseman, J.; Hoehn, F. W. SAE Tech. Pap. Ser. 1974, No. 741169. (16) Ogden, M. J. Review of Small Stationary Reformers for Hydrogen Production. Report to the International Energy Agency, Center for Energy and Environmental Studies, Princeton University, Princeton, NJ, 2001. (17) Kopasz, J. P.; Wilkenhoener, R.; Ahmed, S.; Carter, J. D.; Krumpelt, M. Fuel- Flexible Partial Oxidation Reforming of Hydrocarbons for Automotive Applications. 218th American Chemical Society National Meeting, New Orleans, LA, Aug 22-26, 1999. (18) Sogaard, C.; Schramm, J.; Jensen, T. K. SAE Tech. Pap. Ser. 2000, No. 2000-01-2823.

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monoxide) by Andreatta and Dibble.19 For this study the authors used simulated fuel (using bottles of industrial gas). The tests showed that hydrogen in the reformed fuel allowed the engine to run leaner than when running on natural gas. At the leaner equivalence ratios, low levels of NOx were observed, frequently below 10 ppm. Carbon monoxide and hydrocarbons were generally reduced. It is important to note that at the high values of reformed natural gas there was a considerable amount of CO going into the engine, but most of the CO was oxidized during the combustion. The authors also reported that the efficiencies were higher with reformed fuel in some ranges of operation, and about the same in other ranges of operation compared to natural gas engine operation. Kirwan et al.20 used a single cylinder from a 1996 GM engine with overhead cams and 4 valves per cylinder to determine the emissions of HC and NOx during operation on POx gas and POx gas blended with gasoline. Tests were run with simulated bottled gas (composition similar to that of the product of a POx reformer). The tests showed that, for the POx gas-fueled engine, HC were reduced by more than an order of magnitude and the NOx were 50 times lower compared to gasoline fueling. The authors also found that blends of gasoline with up to 30% POx gas extended the dilute EGR limit compared to operation with gasoline alone. NOx emissions were between 55% and 85% lower with gasoline/POx gas blends than with gasoline alone. A method for production of hydrogen enriched gas in IC engines is the exhaust gas fuel reforming process. It has been extensively investigated at the University of Birmingham in spark ignition engines, and benefits in terms of combustion quality and emissions have been reported in the literature.21,22 In the present study, the application of exhaust gas fuel reforming in diesel engines has been studied and the effects on efficiency, emissions, and combustion quality have been investigated. Exhaust gas fuel reforming is a combination of all the basic reforming processes, and involves hydrogen generation by direct catalytic interaction of the fuel with engine exhaust gases. The resulting balance and choice between the different processes depends on the exhaust gas temperature and composition, i.e., steam and oxygen content. There are several reactions taking part but the main ones are the steam reforming and partial oxidation processes. The diesel engine steam and oxygen contents vary with the engine operating conditions. At high engine loads (higher fuel consumption), the exhaust gas steam content is higher and the oxygen content lower, than at low loads. The percentages of EGR and hydrogen addition also affect the steam and oxygen content of the exhaust gas. EGR reduces the percentage of oxygen in the exhaust gas, while the addition of hydrogen increases the percentage of steam. (19) Andreatta, D.; Dibble, R. W. SAE Tech. Pap. Ser. 1996, No. 960852. (20) Kirwan, J. E.; Quader, A. A.; Grieve M. J. SAE Tech. Pap. Ser. 1999, No. 1999-01-2927. (21) Allenby, S.; Chang, W. C.; Megaritis, A.; Wyszynski, M. L. Proc. Instn. Mech. Engrs. 2001, 215 (part D), 405-418. (22) Jamal, Y.; Wyszynski, M. L. Int. J. Hydrogen Energy 1994, 19 (7), 557-572.

Tsolakis et al. Table 1. Fuel Properties

fuel analysis

method

cetane number density at 15 °C (kg/m3) viscosity at 40 °C (cSt) 90% distillation (°C) sulfur (mg/kg)

ASTM D613 ASTM D4052 ASTM D445 ASTM D86 ASTM D2622

Ultralow Rapeseed Sulfur Diesel Methyl Ester (ULSD) (RME) 53.9 827.1 2.467 329 46

54.7 883.7 4.478 342 5

Experimental Section Test Engine. The experiments were carried out in a Lister Petter TR1 engine. The engine is a naturally aspirated, aircooled, single-cylinder, direct-injection diesel engine. The engine specifications are as follow: bore, 98.4 mm; stroke, 101.6 mm; conrod length, 165.0 mm; displacement volume, 773 cm3; compression ratio, 15.5; maximum power, 8.6 kW at 2500 rpm; and maximum torque, 39.2 Nm at 1800 rpm. The IMEP at maximum power and maximum torque is 6.5 and 7.5 bar, respectively. The maximum obtainable IMEP at 1500 rpm (engine speed used in this study) is 6.8 bar. The injection timing was kept at 22 °CA (degree crank angle) BTDC (before top dead center) as set by the manufacturer. Engine Instrumentation. A Thrige Titan DC electric dynamometer with a load cell and a thyristor-controlled Shackleton System Drive was used to load and motor the engine. A positive displacement Romet rotary gas flow meter fitted with a shaft encoder and a tachometer was used to measure the air flow into the engine. A large reservoir was fitted upstream of the flow meter to dampen pulsations. A calibrated glass bulb, connected in parallel with the fuel tank, was used to determine the liquid fuel flow by timing the consumption of 100 mL of fuel. Hydrogen was added through the engine inlet, and the flow rate was measured by a calibrated Platton rotameter. The EGR flow was controlled manually by a valve, and the EGR level was determined volumetrically as the percentage reduction in volume flow rate of air at a fixed engine operating point. When hydrogen was added, the level of EGR was reduced accordingly so that the mass flow rate of fresh air remained the same. A KISTLER 6125B pressure transducer, mounted flush at the cylinder head and connected via a KISTLER 5011 charge amplifier to a National Instruments data acquisition board, was used to record the cylinder pressure. The crankshaft position was measured using a digital shaft encoder. The test rig included other standard engine instrumentation such as thermocouples to measure oil, air, inlet manifold and exhaust temperatures, and pressure gauges mounted at relevant points. Normal engine test bed safety features were also included. Atmospheric conditions (humidity, temperature, pressure) were monitored during the tests. Data acquisition and combustion analysis were carried out using an in-house developed LabVIEW-based software. Output from the analysis of consecutive engine cycles included peak engine cylinder pressure, values of IMEP, percentage coefficient of variation (% COV) of IMEP, average values and percentage COV of peak cylinder pressures, average crank angle for ignition delay, burn duration, 50% burn point, etc. The COVs of IMEP and peak cylinder pressure were used as criteria for combustion quality, or more accurately, combustion stability (cyclic variability). COVs below 5% indicated good combustion stability. Fuels. The fuels used were Ultralow Sulfur Diesel (ULSD) and Rapeseed Methyl Ester (RME) provided by Shell Global Solutions UK. The fuel properties are given in Table 1. The tests presented in this study were performed using pure ULSD and a blend of 20% RME with 80% ULSD (by volume). The latter will be referred to as B20 in the following sections. Exhaust Emissions Monitoring. A HORIBA analyzer model MEXA-547GE was used to measure CO, CO2, and unburned hydrocarbons (hexane equivalent) by NDIR (non-



Figure 1. Schematic of the liquid fuel reforming system. dispersive infrared gas analysis), and oxygen concentrations in the exhaust (electrochemical method), together with a Beckman 951A chemiluminescence NO/ NOx analyzer and a Beckman 864 infrared CO2 analyzer. Smoke was measured using a Bosch smoke meter where a constant volume of exhaust gas is drawn through a white filter paper and the darkening of the paper is taken as a measure of the smoke density, with a scale of descending opacity from 0 to 10 Bosch Smoke Number (BSN). Reforming Mini Reactor. The mini reactor system was designed by Allenby et al.21 and used in a natural gas-fueled engine as a two-phase system: solid phase (catalyst) and fluid phase (exhaust gas and natural gas). The modified reactor system used in this study, shown in Figure 1, involved a threephase operation: solid (catalyst), liquid (diesel, biodiesel), and gas (exhaust gas). The reactor was placed in a tubular furnace, and the temperature was controlled by means of a temperature controller. The engine exhaust gas was fed to the reactor through a 1/4-in. stainless steel tube. The exhaust gas flow was controlled and measured by a rotameter. The exhaust gas temperature and pressure were monitored by a K-type thermocouple and a pressure gauge, respectively. A syringe pump fitted with a glass syringe was used to supply the liquid fuel and control the fuel flow rate. The fuel was mixed with the exhaust gas in the mixing volumes, Figure 1, and it was passed through a preheating coil. In this coil the fuel was completely mixed with the exhaust gas, and the mixture was then passed into the catalyst bed where the reactions occurred. A K-type thermocouple was used to record the reactor temperature profile. The thermocouple was placed inside a stainless steel tube (o.d., 2.95 mm) fitted in the center of the reactor. The arrangement allowed vertical movement of the thermocouple and thus monitoring of the reactor temperature profile. Downstream of the reactor the excess of steam was condensed and then the dry gas was analyzed. The prototype precious metal catalysts used in this study were provided by Johnson Matthey. The catalyst was in the form of granules of approximately 1 mm diameter, and it was loaded into the mini reactor tube (i.d., 10 mm) in three different layers as shown in Figure 2. The total catalyst bed size employed in the reactor was 80 mm long. Quartz wool and granular alumina were used to support the catalyst layers. The Gas Hourly Space Velocity (G.H.S.V.; volumetric flow rate of gas per hour divided by the volume of the catalyst bed) was kept constant at about 90000 1/h, which is typical of automotive three way catalytic converters.

Figure 2. Minireactor configuration. Hydrogen Analysis. A Hewlett-Packard (HP) gas chromatograph equipped with a thermal conductivity detector (TCD) was used for measurement of the hydrogen content of the reactor product. A double-column arrangement was used for the hydrogen analysis. The first column was a 1 m long 1/8-in. diameter Haysep Q, 80-100 mesh. The second column was a 2 m long 1/8-in. diameter Molesieve 5Å (MS5A). Higher TCD sensitivity to hydrogen was achieved by using argon as the carrier gas (as argon’s thermal conductivity is less similar to that of hydrogen compared with other typical carrier gases such as helium and nitrogen). The H2 chromatogram area was measured using a HP 3395 integrator. The apparatus was calibrated using certified gases (10% H2 in N2, and 30% H2 in N2).

Results and Discussion The results presented in this section are divided in to two parts. First the effects of biodiesel, hydrogen addition, and EGR on the engine NOx and smoke emissions and efficiency are presented and discussed. Then, the exhaust gas fuel reforming results, obtained in the mini reformer, are presented and the effects of different reactor operating parameters on the reforming performance (hydrogen production) are discussed. Effects of Biodiesel, Hydrogen Addition, and EGR. The results presented in this part were obtained at 1500 rpm engine speed at two different engine loads, 4 and 5.5 bar IMEP. Different levels of EGR and hydrogen addition were used. The EGR and hydrogen levels for each test mode are given in Table 2 as percentages by volume of the engine intake charge; hydrogen levels are also given as percentage of EGR.


Tsolakis et al.

Figure 3. Effects of EGR with hydrogen addition (10% and 20% in EGR) to ULSD (dark lines) and B20 (light lines) operation. Engine condition: speed, 1500 rpm; IMEP, 4 bar.

Figure 4. Effects of EGR with hydrogen addition (10% and 20% in EGR) to ULSD (dark lines) and B20 (light lines) operation. Engine condition: speed, 1500 rpm; IMEP, 5.5 bar. Table 2. Engine Test Points with Different Levels of EGR and H2 Enrichment EGR + H2 mode (%) by volume in no. engine intake charge 0a 0b 0c 0d 0e 0f 1a 1b 1c 2a 2b 2c

0 5 10 15 20 25 0.5 5 10 1 5 10

H2 % in EGR

H2 % of intake charge

0 0 0 0 0 0 100 (pure H2) 10 10 100 (pure H2) 20 20

0 0 0 0 0 0 0.5 0.5 1 1 1 2

The NOx and BSN for all the test modes are shown in Figures 3 and 4 for 4 bar and 5.5 bar IMEP, respectively. NOx emissions were higher with B20 compared to ULSD. This may be a result of the increased oxygen concentration in the combustion cham-

ber with B20 due to the biodiesel oxygen content.23 RME contains a number of different methyl esters (CH3OOCR). For simplicity, RME is generally assumed to consist of 40 wt % CH3OOC18H35 and 60 wt % CH3OOC22H43. In the case of 20% (by volume) RME in the fuel (B20), the fuel oxygen percentage is about 2 wt % of the total fuel. Wang et al.24 reported that the higher NOx emissions are mainly due to the shorter ignition delay time when biodiesel is used. The shorter ignition delay, which advances the combustion timing, increases the peak pressure and temperature, and enhances NOx formation, may be attributed to the higher cetane number of biodiesel. In the present experiments the difference of cetane number of the fuels used was less than one and probably did not affect significantly the engine emissions. However, when B20 was used, the (23) Graboski, M. S.; McCormick, R. L. Prog. Energy Combust. Sci. 1998, 24, 125-164. (24) Wang, W. G.; Lyons, D. W.; Clark, N. N.; Gautam, M.; Norton P. M. Environ. Sci. Technol. 2000, 34 (6), 933-939.



Figure 5. Effects of EGR on average peak cylinder pressure and NOx emissions.

ignition delay was shorter by 1 °CA (degree crank angle) and the combustion was faster by about 5% compared to engine operation with ULSD. Senatore et al.25 suggested that the higher concentrations of NOx detected when the engine is fueled with biodiesel are primarily due to the operation of the injection system and not due to the fuel composition (e.g., oxygen content, etc.). The use of biodiesel results in advance of the injection timing, because of the different densities and bulk modulus of compressibility of diesel and biodiesel and the different quantities of injected fuel mass per cycle. The advance in fuel injection timing with biodiesel was also confirmed in a detailed experimental study by Szybist and Boehman.26 The BSN for the low load engine operating condition (4 bar IMEP) is very small for both fuels and the effect of B20 on the BSN compared to ULSD is not clear. The BSN seems to be about the same for both fuels. For the high load (5.5 bar IMEP) condition, Figure 5, the BSN was decreased by about 0.5 with the use of the B20 compared to ULSD. The use of EGR resulted in decreased NOx and increased BSN for both fuels. The expected tradeoff between NOx and BSN with the use of EGR is evident. For the same EGR level, the reduction of NOx emissions was higher when the engine was fueled with B20 compared to ULSD. The EGR, when not cooled, is usually hotter than the inlet air it displaces, and this may result in a rise of the combustion temperature which tends to increase NOx production.27 On the other hand, the greater inlet charge heat capacity accompanied by oxygen displacement when EGR is used decreases the flame temperature and reduces NOx emissions. The greater NOx reduction for B20 than for ULSD (when using the same EGR level) is probably due to the different exhaust gas composition for the two fuels. The exhaust gas oxygen content with B20 is lower by approximately 2%, while the CO2 content is higher by (25) Senatore, A.; Cardone, M.; Rocco, V.; Prati, M. V. SAE Tech. Pap. Ser. 2000, No. 2000-01-0691. (26) Szybist, J. P.; Boehman, A. L. SAE Tech. Pap. Ser. 2003, No. 2003-01-1039. (27) Ladommatos, N.; Abdelhalim, S. M.; Zhao, H.; Hu, Z. SAE Tech. Pap. Ser. 1998, 980184.

approximately 1.5% compared to ULSD. Clearly the effects of EGR on the inlet charge heat capacity and oxygen displacement are different for the two fuels, resulting in different levels of NOx reduction. The increase of the exhaust smoke and PM with the use of EGR occurs due to lower oxygen availability in the combustion chamber and thus reduced soot oxidation. Another reason is the reduction of gas temperature, which reduces the soot oxidation rate. When hydrogen was added to the engine intake, either alone (test modes 1a and 2a) or with EGR (modes 1b, 2b, 1c, and 2c), the NOx versus BSN curves shifted toward lower values of NOx and BSN. This is evident in Figures 3 and 4 for both diesel and B20 engine operation at both low and high engine load. This simultaneous reduction of NOx and BSN is of crucial importance since it shows that the combination of EGR and hydrogen may be a way of breaking the NOx-smoke tradeoff in diesel engines. This is attributed to the oxygen displacement in the combustion chamber resulting from the use of EGR and the hydrocarbon fuel replacement by hydrogen resulting from the addition of hydrogen (and the subsequent reduction of the hydrocarbon fuel flow into the engine in order to keep the same engine IMEP at the same speed). At the same time the combustion stability, which normally deteriorates with increased levels of EGR, was maintained when hydrogen was also added. The coefficients of variation (COV) of IMEP and peak cylinder pressure were below 5% in all cases, even when high EGR levels were used. Hydrogen burns faster compared to hydrocarbon fuels, has lower activation energy, and incurs more molecular collisions than heavier molecules.13 The use of a small amount of hydrogen gave longer ignition delay (retard) by 2 °CA, but the combustion duration was faster by about 5% when compared to ULSD and B20 operation without hydrogen. The high laminar burning velocity and the wide flammability limits of hydrogen shift the engine operation to leaner fuel-air mixtures.28 Hydrogen combustion may also reduce hot spots that are one of the major contributors to NOx (28) Swain, M. R.; Yusuf, M. J.; Dulger, Z.; Swain, M. N. SAE Tech. Pap. Ser. 1993, No. 932775.


Tsolakis et al.

Figure 6. Effects of hydrogen addition and EGR combined with hydrogen addition (“low” 10% of the EGR) on average peak pressure and NOx.

Figure 7. Effects of hydrogen addition and EGR combined with hydrogen addition (“high” 20% of the EGR) on average peak pressure and NOx.

emissions in IC engines.29 The combination of the hydrogen burning characteristics with the use of EGR is shown to be highly beneficial in terms of reducing diesel engine NOx and smoke emissions. The effects of biodiesel, EGR, and hydrogen on the NOx emissions can be better understood when the corresponding measured cylinder pressures are considered. The NOx levels increase at high loads with higher peak cylinder pressures, higher temperatures, and larger regions of close-to-stoichiometric cylinder charge.24 Figures 5-7 show that at the high engine load operating condition (thick lines) the NOx emissions were higher compared to low load (thin lines). The higher engine load resulted in higher peak cylinder pressure and temperature. When the engine was fueled with B20, the peak cylinder pressure was higher compared to ULSD and subsequently the NOx emissions were higher. This is a result of increased oxygen concentration in the combustion chamber. Figure 5 shows that as the percentage of the EGR increased, the peak pressure decreased and (29) Heywood, J. B. Internal Combustion Engine Fundamentals; McGraw-Hill: New York, 1988. ISBN 0-07-100499-8.

the NOx emissions also decreased for both high- and lowload engine operation. A 5% increase of EGR resulted in a decrease of the peak cylinder pressure by about 0.5 to 1 bar for both B20 and ULSD at high and low engine load. A small addition of hydrogen either alone (0.5% and 1% of the engine intake charge) or combined with EGR (10% and 20% of the EGR) resulted in a decrease of the peak cylinder pressure and NOx emissions (Figures 6 and 7). Apart from the exhaust emissions, the engine efficiency (ratio of engine power output over the fuel power supplied) was also affected by the use of biodiesel, EGR, and hydrogen addition. These effects are shown in Figure 8. EGR levels up to 15% (modes 0a-0d) did not seem to affect significantly the engine efficiency. However, higher levels of EGR resulted in lower efficiency, and the effect was more pronounced at the high engine load operating condition. Figure 8 also shows that the engine efficiency was decreased with the use of 20% RME (B20). At high load (thick lines), the decrease of the efficiency was not as



Figure 8. Effects of EGR and hydrogen addition on engine efficiency.

evident as at low load, where the decrease was about 1 to 1.5%. A small addition of hydrogen (modes 1a-2b) did not change significantly the engine efficiency for either ULSD or B20. For higher hydrogen addition (mode 2c), the fuel conversion efficiency was reduced slightly for all the tests. As already has been mentioned, the high laminar burning velocity of hydrogen and the wide flammability limits shift the engine operation to leaner fuel-air mixtures, which affects favorably the thermodynamic efficiency. The thermodynamic effects are also coupled with the effect of shortening the combustion duration.11 Exhaust Gas Fuel Reforming. All the reforming tests were conducted using real engine exhaust gas, and in all cases the engine was fueled with ULSD. As already has been mentioned, the reactor temperature was controlled by the furnace temperature controller. In this way, the reforming temperature was not the actual exhaust gas temperature. However, the controlled reactor temperature allowed a better assessment of the reforming process. Obviously, in a real enginereformer system, the process would have to be conducted at the available actual exhaust gas temperature without the need for an external heat source. The hydrogen contents of the reformer product gas produced at different reactor conditions are shown in Figure 9. Two sets of data presented areobtained at two different reforming temperatures, 530 °C and 470 °C. The term reforming temperature refers to the temperature of the mixture of exhaust gas and vapor fuel measured at the “entry” of the catalyst bed, shown in Figure 2. In all these reforming tests, the engine operated at 1500 rpm speed and 5.5 bar IMEP and was fueled with ULSD. Three different reformer fuels were examined: ULSD, B20, and pure RME (this refers to fuel fed to the reformer only; the engine was always fueled with ULSD). The reformed gas hydrogen content was monitored over 3 h at about every three minutes, and the hydrogen percentages shown in Figure 9 are the average values. No deterioration of the catalyst was observed during the relatively short period of 3 h. At

Figure 9. Hydrogen production for two different reactor temperatures. GHSV, 90000 1/h; reactor pressure drop, 0.3 bar(g); engine exhaust temperature, 460 °C.

the higher reforming temperature (530 °C), the hydrogen content of the reformer product gas was about 18% for ULSD, 20% for B20, and 16% for pure RME. As expected at the lower temperature (470 °C), the reformer product hydrogen content was lower compared to that obtained at the higher reforming temperature. For all the three fuels, the hydrogen content was about 13% - 15%. One important aspect here was that the chosen reformer temperature of 470 °C was very close to the actual engine exhaust gas temperature, which at this condition was 460 °C. The compositions of the reforming reactor product are presented in more detail in Figure 10 for the three different reactor fuels tested and different fuel flows into the reactor. The modes shown in Figure 10 are described in Table 3 (modes 1-5). The reforming temperature for these test modes was 470 °C. For comparison, the engine exhaust gas composition is also shown in Figure 10. As already has been mentioned, in all the reforming tests the engine operating condition was kept the same (1500 rpm and 5.5 bar IMEP, fueled with ULSD). The results show that one reaction taking place in the minireactor was hydrocarbon oxidation partial or complete (eqs 2 and 3). The oxygen percentage of about 13% at the inlet of the reactor (oxygen content of the engine exhaust gas only; fuel oxygen content in the case


Tsolakis et al.

Figure 10. Engine exhaust and minireactor product gas composition. GHSV, 90000 1/h; reactor pressure drop, 0.3 bar(g); engine exhaust temperature, 460 °C. Table 3. Reforming Test Points mode no.

engine fuel

reactor fuel and flow rate

1 2 3 4 5

ULSD ULSD ULSD ULSD ULSD

ULSD, 30 mL/h B20, 30 mL/h B20, 34 mL/h RME, 30 mL/h RME, 34 mL/h

of biodiesel is not accounted for) was reduced to about 3% at the outlet of the mini reactor. The presence of the oxidation reactions is also confirmed by the increase of CO2 from 4% to about 10%. The second main reaction taking place in the mini reactor was, as expected, steam reforming of the hydrocarbon fuel. This is evident from the increase of CO (from 0 at the reactor inlet to 7% at the outlet) and hydrogen (from 0 to 14%). The H2/CO ratio in the reactor products is about 2 in all cases. According to the steam reforming reaction equation of diesel (eq 1) the ratio of the H2/CO is predicted to be about 2. The overall catalytic reforming can be exothermic or endothermic. The main factor determining the heat balance for the reaction is the steam and oxygen content of the exhaust gas. The most important parameter affecting the steam and oxygen concentration of the exhaust gas is the engine load; i.e., at high loads the oxygen content is low, and at low loads it is high. The un-reactive hydrocarbons in the reactor products show that the fuel conversion in the reformer was not 100%. For modes 3 and 5 (Figure 10), the liquid fuel flow rate into the minireactor was increased; this resulted in increased hydrogen production but also increased un-reacted hydrocarbon (HC) production. However, the increased un-reacted HC is not a problem since HCs and also CO are combustible fuels and would be oxidized during the combustion in the case of an engine-reformer closed-loop system. The performance of the prototype reforming catalyst under these conditions, when compared with results presented earlier for the engine hydrogen demand, demonstrates that current prototype catalysts have the potential to provide a sufficiently hydrogen-rich reformed gas. The effects of carbon formation, sulfur, and

catalyst sintering need further investigation to assess the effect on hydrogen yield from diesel and biodiesel fuel mixtures. In this paper the mini reactor product composition from exhaust gas fuel reforming of diesel and biodiesel mixtures was examined as a function of two relatively high exhaust gas temperatures. Further investigation of the reactor product composition needs to be done at lower exhaust gas temperatures associated with the engine part load operation. The range of the exhaust gas temperatures of the engine used in the present study is between 130 °C at idle and 600-700 °C at high engine load. At low exhaust temperatures, the exhaust gas oxygen can drive the exothermic partial (eq 2) or complete (eq 3) oxidation by using part of the fuel to increase the reactor temperature so that the endothermic steam reforming (eq 1) can take place using the rest of the fuel to produce hydrogen-rich gas. Different engine conditions (speed, load) and different fuel mixtures (diesel-biodiesel) with or without the use of (EGR + H2) need to be tested so that the effects of different exhaust gas oxygen and steam contents on the minireactor product composition will be assessed. This will allow a detailed investigation of the reactor product composition as a function of different steam/fuel and oxygen/fuel ratios. Further research and development of suitable reforming catalysts is also required. Finally the closed loop operation of a diesel enginefuel reformer system must be studied for different system configurations. It is envisaged that a reformer incorporated in the existing EGR system of diesel engines will be able to provide the required amounts of hydrogen needed for reduced engine emissions. In this case the engine will be fed with “reformed EGR” which will be rich in hydrogen and will allow realization of the benefits of the application of combined EGR and hydrogen addition. Conclusions The use of 20% (by volume) RME mixed with ULSD decreased smoke emissions (BSN) but increased NOx emissions and reduced the engine efficiency.


As expected, EGR reduced NOx but increased smoke emissions. The engine efficiency was also reduced with EGR. However, when the EGR was combined with addition of relatively small quantities of hydrogen to the engine intake (10% and 20% of EGR), a simultaneous reduction of both NOx and BSN was achieved. At the same time, the engine efficiency and the combustion quality were not adversely affected. Using prototype catalysts, developed for this study by Johnson Matthey, a reformed fuel with up to about 20% hydrogen content was produced at a reforming temperature of 530 °C, even at the relatively high GHSV of 90000 1/h. At the lower reforming temperature of 470 °C, the reformer product contained up to 15% hydrogen. These levels of hydrogen in the reformer product are comparable with those required in the engine intake charge in order to reduce the engine NOx and smoke emissions. Overall, it has been shown that the application of the exhaust gas fuel reforming process for production of


hydrogen-rich gas in compression ignition engines fueled by diesel or biodiesel mixtures is feasible and can provide a solution to the diesel engine exhaust emissions problems. Further investigation is needed to optimize the production of reformed gas with the required hydrogen content at temperatures equal to the actual engine exhaust gas temperatures. This will eventually allow the design of a closed-coupled reformer incorporated in the EGR system of the engine. Acknowledgment. Support for this work by The UK Engineering and Physical Science Research Council (EPSRC), Lister-Petter Ltd., Johnson Matthey Plc, and Shell Global Solutions UK is gratefully acknowledged. The authors express their thanks to Miss L. K. S. Teo for her participation in the test rig instrumentation and Mr. S. Rowan for technical support. EF0300693

Application of Exhaust Gas Fuel Reforming in Compression Ignition

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