Onboard hydrogen generation by methanol decomposition for the cold start of neat methanol engines

0360-3199/91 $3.00+ 0.00 PergamonPress pie. © 1991 InternationalAssociationfor HydrogenEnergy.

Int. J. Hydrogen Energy, Vol. 16, No. 10, pp. 671-676, 1991. Printed in Great Britain.

ONBOARD HYDROGEN GENERATION BY METHANOL DECOMPOSITION FOR THE COLD START OF NEAT METHANOL ENGINES L. PETTERSSONand K. SJ6STR6M Royal Institute of Technology, Department of Chemical Technology, Teknikringen 42, S-100 44 Stockholm, Sweden (Received for publication 8 July 1991) Al~tract--Hydrogen is excellent for use as a cold starting aid for methanol vehicles at low ambient temperatures. Hydrogen can be produced by partial decomposition of methanol at the moment of start. This enables the spark-ignition engine to utilize only one fuel and there is no need for fuel additives or a separate starting fuel. An experimental investigation indicates that it is possible to start a neat methanol engine down to at least - 30°C using the product gas from the cold start reactor described in this paper. The engine ignited on the mixture, but additional methanol was needed to establish proper idling. The dfivability was good directly after cold start by the addition of the hydrogen-rich gas. At room temperature it was possible to both start the engine and reach an acceptable engine speed at idling, with the use of only the gas from the cold start reactor.

1. I N T R O D U C T I O N Hydrogen has many attractive features as an automotive fuel. It has a wide range of combustibility, high flame speed and creates a much more homogeneous air-fuel mixture than liquid fuels. The throttle valve can be eliminated in hydrogen engines because of the quality regulation procedure. This enables a higher efficiency compared to gasoline engines thanks to decreased pumping losses. Furthermore, hydrogen engines do not need enrichment at start, at idle or during acceleration [1]. Hydrogen can be stored in metal hydrides, as liquid at extremely low temperatures or as compressed gas at high pressures [2]. It can also be stored as methanol, which is superior to the methods described above in terms of energy density [3]. Hydrogen generation by methanol decomposition yields two moles of hydrogen and one mole of carbon monoxide for every mole of methanol reacted: CH3OH -~ 2H 2 + CO.

(1)

The use of decomposed methanol as a fuel for sparkignition engines has been investigated by several authors [4-11]. The main advantage compared to gasoline operation is higher efficiency and considerably lower emissions of nitrogen oxides and hydrocarbons I11]. The cold start of a decomposed methanol engine depends, however, on neat methanol fuel. Due to low vapor pressure and high heat of vaporization it is difficult to cold start a methanol engine at temperatures below +5°C [12]. The cold starting of a gasoline engine in Nordic climates is made possible by the use of a special winter

gasoline, which contains a larger fraction of volatile compounds than summer gasoline. The method of adding highly volatile compounds to neat methanol is, however, undesirable since fuel additives reduce the environmental advantages with methanol fuel [13]. Special starting fuels such as propane or winter gasoline have been utilized by the methanol test fleets [14,15]. Dual-fuel systems are not desirable for consumer-oriented reasons, however. Moreover, the emissions are negatively influenced by the introduction of a special starting agent. It is also possible to vaporize the fuel electrically but it is a power-consuming method since the air also has to be heated in order to prevent recondensation of methanol. Bruetsch [l 6] has described a method where long-duration spark ignition is used to facilitate cold start. The facts above show that a non-condensable gaseous fuel produced from methanol would be preferable. Hydrogen is an excellent cold start fuel, which can be generated at the moment of start by methanol decomposition [17]. This method has been studied by different researchers [8, 18-27]. All these studies use different approaches to the problem. Some use thermal decomposition, while others use catalytic decomposition, where the latter takes place at a considerably lower temperature. A review of the different studies can be found in an earlier paper by Pettersson and Sj6str6m [28]. The addition of a hydrogen-rich gas during the warmup period may decrease the emissions of unburnt fuel [29]. Furthermore, the drivability will be improved and acceleration directly after start is facilitated.

671

672

L. PETTERSSON and K. SJOSTROM

I Methanol ~[~ ~ Hydrogen generator , ~ ~ - - ' ~ /~ | ~X~eroductl I{ l \ k, / L !urner~ I ~ A i r II By-passel-'~ J'l z'-'z'-jU Exhaust ,,) Fig. l. A schematic representation of the experimental setup. 2. E X P E R I M E N T A L P R O C E D U R E

2.1. Objective The objective of this study was to verify whether it is possible to start a spark-ignition engine at - 3 0 ° C on partially decomposed methanol. The fuel was produced by catalytic decomposition of methanol, where the heat of reaction was supplied through combustion of a part of the methanol feed to the reactor. Instead of using a well defined mixture of pure gases, a laboratory reactor was constructed. The theoretical considerations have been described earlier by Pettersson and Sj6str6m [17] and Sj6str6m [24]. 2.2. Experimental setup The experimental setup is shown in Fig. 1. The reactor consists of a burner unit and a hydrogen generating unit. The burner works stoichiometrically during the warmup period and all methanol is consumed for heating the catalyst bed. When the bed has reached operating temperature, the air/fuel ratio in the burner is decreased. The part of the methanol, which is not combusted, is vaporized and decomposed catalytically to hydrogen and carbon monoxide in the bed. The ideal composition of the product gas from the reactor is shown in Table 1. The direct heat exchange procedure enables a faster warm-up than indirect heat exchange. 2.3. Testing of experimental equipment The reactor was filled with 400 g catalyst pellets of a cylindrical shape, with a diameter of 3.2 mm. The inner Table 1. Ideal composition of product gas from cold start reactor Concentration (% by volume) CO CO 2 H20 N2 30 15 6 13 36 Assumptions: The air/fuel equivalence ratio, 2 (actual air/fuel ratio divided by stoichiometric air/fuel ratio), is 0.3, all oxygen is converted to carbon dioxide and water in the burner, and the decomposition of methanol to H 2 and CO is complete in the reactor with no side reactions occurring. H2

diameter of the reactor is 57 mm and the catalyst volume 400 cm 3. The catalyst consisted of 0.5% platinum by weight on 7-alumina, type 72 from Johnson Matthey Chemicals. This catalyst was chosen because of its high resistance to thermal degradation, high hydrogen yield and good oxidation characteristics. The weight hourly space velocity (WHSV), defined as mass flow of methanol divided by mass of catalyst, was varied between 2.9 and 4.6 in the cold start experiments. The burner contains an electric motor with impellers, which supply the combustion chamber with air. The air is sucked in through the inlet of the fan. On the lower shaft end of the electric motor, a rotating fuel disperser is mounted. At start, the electricity is supplied to the fan motor, the fuel pump and the glow spark plug. The methanol is injected into the rotating cup and is then atomized into small drops by the air flow. The air/fuel mixture is ignited by the glow spark plug, which is shut off when the flame is sufficiently developed. The equipment is more extensively described in Pettersson and Sj6str6m [17]. The air flow was regulated by a mass flow controller and the methanol flow was supplied by a piston pump. This pump was not well suited for methanol fuel operation, since the sealings swelled after prolonged contact with methanol. The pump flow was more or less independent of the operating pressure. 2.4. Preliminary engine experiments at room temperature The first part of the experiments was performed at room temperature (15-18°C) in order to investigate whether it was possible to start an engine on the product gas from the cold start reactor. The purpose was also to develop efficient experimental routines for the subsequent cold room tests. The warm-up of the reactor took a rather long time with high methanol flows, because the mass flow controller that was used was not dimensioned for the air flows needed for stoichiometric combustion. Since the warm-up time of the reactor was not an experimental parameter, this was of secondary importance. The experimental engine was a 1.6 dm 3 in-line fourcylinder four-stroke Volkswagen spark-ignition engine

COLD START OF METHANOL ENGINES with an overhead camshaft. The carburetor was from Weber Brasilia and it was designed for ethanol fuel. This means that the combustion was leaner than if the carburetor had been designed for methanol. The compression was raised from 8.2 to around 10. The lubricating oil was a mineral oil, SAE 10W30, intended for all-year use. The spark plugs were N G K B8 EGV, which have a rather low heat range index. The reactor was connected to the intake manifold by a metal hose, downstream of the throttle and carburetor. A three-way valve was mounted between the reactor and the engine in order to be able to vent the exhaust gas during the warm-up period of the reactor. This valve was also necessary for safety reasons, to ensure a fast shut-off of the feed of product gas at potential starting failures. Since the sealings of the piston pump used were not resistant to methanol, it was replaced by an available fuel-resistant membrane pump. The flow was influenced by the pressure in the intake manifold, however. The higher the intake manifold vacuum, the higher the methanol flow. It was therefore difficult to set the desired operational air/fuel ratio during the partial oxidation. This explains the low 2 (actual air/fuel ratio divided by stoichiometric air/fuel ratio) figures in Table 2. The additional methanol, when used, was added by a fuel injection valve mounted in the connection between the reactor and the engine intake manifold. The method was not very successful, partly because the injection angle was too small and the vaporization was insufficient. This resulted in some methanol pouring into the intake manifold and causing operational disturbances. The advantage with this method was the possibility of controlling the fuel flow exactly. In other experiments, the carburetor was used for the addition of supplemental fuel. This allowed good running characteristics but the disadvantage was the Table 2. Preliminary cold start experiments, with a 1.6 dm3 Volkswagen spark-ignition engine at room temperature (15-18°C), showing the engine speed at different parameters. Engine: oil--all-season mineral oil, SAE 10W30; spark plug-NGK B8 EGV No. 1¶ 2**

Time* (min)

2 part. ox.t

1.7 4.3 10.3 4.0 13.8 17.3 22.0

0.15 0. I 1 0.08 0.15 0.13 0.13 0.13

M reac.:~ M engine~ Speedll (gs -I) (gs i) (rpm) 0.36 0.36 0.36 0.35 0.35 0.35 0.35

0 0 0.12 0.13 0 0.12 0

552 552 940 840 675 897 788

Notes: Additional methanol from an injection valve mounted in the connection between reactor and inlet manifold. * Total time of system operation including reactor warm-up. ? Partial oxidation. :~Neat methanol flow to the reactor. § Additional neat methanol flow to the engine. II Engine speed. ¶ Instant engine start at 1.7 rain. ** Instant engine start at 1.5 rain.

673

problem of measuring the flow in any satisfactory way. An integral measurement of the flow over the whole experiment was the only possibility and this was not so interesting from our point of view. 2.5. Preliminary experiments with the cold start reactor at room temperature Different pumps were tested in these experiments in order to establish a stable methanol flow to the reactor, not influenced by the intake manifold vacuum. A pump which is part of the Bosch L-jetronic system was chosen in conjunction with a constriction in order to limit the flow to a suitable level. This pump is connected to a pressure regulator, which is a membrane-controlled overflow valve. The valve is connected via a vacuum hose to the engine intake manifold behind the throttle. This results in the pressure in the fuel system being influenced by the absolute pressure in the intake manifold, which means that the pressure drop over the injection valves is constant at every positioning of the throttle. In our case, a flow restriction device was used instead of an injection valve to establish a proper flow. The device yielded a satisfactory result and a steady flow. By changing the restriction devices, the flow could be varied. 2.6. Engine cold start experiments The time had now come for performing cold start experiments at -30°C. This low temperature was obtained by placing the test engine in a cooling container. The container, manufactured by Thermo King, could be operated on both diesel fuel and electricity. However, due to some operational problems with this cooling unit, not all experiments were carried out at -30°C. Except for the ambient temperature, experimental setup was the same as described above. The engine was conditioned overnight at maximum capacity of the cold room before every experiment. During the warm-up phase of the reactor, the exhaust gas was vented out from the container through a separate tube and it was not connected to the engine tailpipe (see Fig. 1). In the experiments, the preheater of the air/fuel mixture supplied with the gasoline version of the engine, the so called "Igel", was used. The preheater is placed under the carburetor and consists of a number of spikes, which are electrically heated. All experiments were made with the battery at room temperature. The air was supplied to the reactor by cylinders with compressed air, which were placed in the cold room. 3. RESULTS A N D DISCUSSION The results of the preliminary cold start experiments at room temperature, which are presented in Table 2, show that the engine started solely on product gas from the reactor and established idling at 600-700rpm, depending on which operation parameters were chosen in the reactor. The engine started momentarily in these tests when the hydrogen-rich gas was fed to the engine. Tests were also made during the warm-up period of

674

L. PETTERSSON and K. SJOSTROM

using different loads in order to register how the engine reacted to different operating conditions. The engine showed better drivability in these loading tests with reactor gas and additional methanol, than with neat methanol only. On one occasion, the engine was started at a load of 44 N m in order to simulate high friction. Low ambient temperatures mean a high engine friction and the result indicated that a cold start at - 3 0 ° C was attainable. The results of the cold r o o m experiments show that it is possible to cold start a neat methanol engine at - 3 0 ° C , with the aid of catalytically decomposed methanol, provided that extra methanol is added to obtain proper idling (see Tables 3-5). At - 1 3 ° C , the engine started immediately once it was fed with product gas. This is an interesting result, since 85% of passenger cars in Sweden are registered in areas where average temperatures below - 1 3 ° C seldom occur in wintertime [30]. Experiments were made to try to start the engine on the reactor gas alone. The previously theoretically estimated methanol flow of 0.3 g s -L [17] seems to be sufficient to perform cold starts. This energy amount is, however, too small to attain stable idling at - 3 0 ° C . Generally, there was an uncertainty about the air/fuel ratio in the cylinders at start, since it was impossible, for experimental reasons, to control the air and the methanol flow to the carburetor exactly. After a period of time of experimentation it was observed that the starter speed was continuously decreasing after every experiment. This was caused by a dilution of the lubricating oil by methanol and water from the cylinders. The grayish viscous oil was replaced by a synthetic oil, Shell T M O SAE 5W30. This oil was far less viscous at - 3 0 ° C than the mineral oil above. It allowed better lubrication, decreased friction and a

Table 4. Cold start experiments. Engine: oil--synthetic oil, SHELL TMO SAE 5W30; spark plug--Bosch Super W7DC. Experimental conditions and symbols otherwise as in Table 3 No.

2 part. ox.*

MeOH reac.t (gs ')

5 6 7 8 9 10t

0.30 0.31 0.30 0.29* 0.26 0.31

0.35 0.34 0.42 0.44* 0.32 0.40

No.

2 part. ox.*

MeOH reac.t (g s-l )

I 2¶ 3 4**

0.3011 0.3011 0.31 0.31

0.35 0.35 0.42 0.51

-26.5 -20 -20 -25 - 27 -24

85 85 15 110 85 25

* Uncertain value. t Total reactor operating time, before cranking: 105 s. higher starting speed. The spark plugs were exchanged for a kind with a higher heat range index than earlier, Bosch Super W7DC. The initial test procedure was to shut off the engine a short time after start, in order to achieve fast cooling. F r o m experiment 5 on, the engine was operated until it was warm before it was shut down. In order to test the cold starting limit of the engine, a few experiments were performed with a synthetic gas mixture simulating the product gas from the cold start reactor described above. It consisted of 30% hydrogen by volume, 16% carbon monoxide, 10% carbon dioxide and 44% nitrogen. The experimental series was too small to draw any far-reaching conclusions from the turn-out of these tests, since it was difficult to control the air/fuel ratio in the cylinders at start. The high frequency pulses from the engine influenced the mass flow controller and induced a fluctuating gas flow. This problem did not occur in the reactor experiments, since the catalyst bed functioned as a pressure compensation device. N o signs of carbon precipitation was observed on the catalyst surface emanating from either the Boudouard reaction 2 C 0 --* C(s) +

Table 3. Cold start experiments with a 1.6 dm 3 Volkswagen spark-ignition engine in a cold room. Engine: oil--all-season mineral oil, SAE 10W30; spark plug--NGK B8 EGV

Temp:~ Start time~ (°C) (s)

(2)

CO 2

or from decomposition of methane C H 4 --~ C(S) + 2H 2 .

(3)

Temp:~ Start time§ (°C) (s) - 13 -28 -31 -25

1 150 20 180tt~

Notes: Additional methanol from the carburetor. During reactor warm-up, the air/fuel ratio was not set at 2 = l due to a limited mass flow controller. * Partial oxidation. t Neat methanol flow to the reactor. ~,Engine and air temperature. § Total time before idling was achieved, after the first start attempt. Cranking was begun after 120 s of reactor operation, including warm-up. ][ Uncertain value. ¶ Total reactor operating time, before cranking: 135 s. ** Total reactor operating time, before cranking: 165 s. t t No additional methanol. :~:~The engine started and went for 15 s before it stopped.

Table 5. Cold start experiments with synthetic gas mixture

No.

Gas flow (Ndm 3min i )

Equiv. MeOH flow* (g s- i )

I1

59

0.30

12 13

47 35

0.24 0.18

Temp. (°C)

Start time (s)

30 - 33t -23t

40 85 180

-

Notes: Gas mixture: 30% H2, 16% CO, 10% CO 2 and 44% N 2 by volume. * The equivalent methanol flow was calculated as the gas flow which a defined methanol flow would yield. When calculating, it was assumed that 2 was 0.3 in the burner, total conversion to H E and CO in the catalyst bed, and no side reactions occurring. t Cold room temperature. The engine was not completely cool. Experimental conditions and symbols otherwise as in Table 4.

COLD START OF METHANOL ENGINES The activity of the platinum catalyst and the selectivity for hydrogen formation was high throughout the experimental series. The concentrations of methane and dimethyl-ether were typically around 0.5% and 0.1% by volume, respectively. A quicker light-off catalyst would be desirable to reach high conversion as fast as possible. Earlier catalyst research indicates that there are good possibilities for finding a less expensive non-noble catalyst [24]. On the other hand, it is important that the catalyst can withstand short periods of high temperatures without thermal degradation. The problems related to operation on solely decomposed methanol were probably partly due to an insufficient amount of air, i.e. oxygen, in the cylinders. The engine was running excessively rich and this influenced the smoothness of operation. The engine was started on the hydrogen-rich fuel alone and such a high fuel flow rate as 0.5 g s-~ will fill up a large part of the cylinder (see Table 6). It is therefore essential with high cranking speeds at high flows of product gas. If an engine with four valves for every cylinder is used, one valve can be used for the induction of air. When the air valve is closed at the end of the induction stroke, the hydrogen-rich gas is injected at the beginning of the compression stroke. According to Table 6, the product gas flow will constitute about one half of the total engine feed. Since the density of a hot gas is lower than the density of a cool gas, it is important to cool the gas before it enters the cylinder. This is preferably accomplished by methanol injection into the gas stream. The high heat of vaporization of methanol (1.1 MJ kg 1) lowers the temperature substantially and a higher energy density in the cylinder is obtained. Through this arrangement, the additional methanol is used for both gas cooling and energy contribution. To overcome the greater resistance caused by friction, a cold engine has a greater fuel consumption. Engines with liquid fuels usually utilize a higher engine speed at idling to enhance stability. Hydrogen engines do not suffer from fuel condensation on the cold cylinder walls and can therefore operate smoothly at low idling speeds. Table 6. Flows needed for stoichiometric (2 = 1) engine operation at different methanol flows to the cold start reactor Reac. MeOH flow* Gas flowt Air flow:~ Total flow (g s - i ) (Ndm 3min- i )~ (Ndm3min- t )~ (Ndm 3min- t )~ 0.3 59 63 122 0.4 79 84 163 0.5 99 105 204 Total inducted air/fuel amount by the engine at 200 rpm and e~ll = 0.8:128 dm 3min -I * Methanol flow to the reactor. t Assumptions: 2 = 0.3 in the burner and total conversion to H 2 and CO in the reactor. :~2 =1. § At NTP: 273.15 K and 101.3 kPa. IIVolumetric efficiency.

675

A lower idling speed decreases the energy consumption and this indicates that a feed of gaseous fuel during warm-up is energy efficient. The design of the reactor is essential for achieving short heating times. Since the objective of this study was to investigate the feasibility of the concept, the cold start reactor was not optimized concerning heating time. A compact automotive design is possible, but fundamental studies of the chemical reaction needs to be carried out to reduce starting time. Most of the problems described in this study are related to the inability to control all the experimental parameters. The concept is well suited for an engine with electronic fuel injection. In such an engine it is possible to control the air and methanol flows properly and at the same time control the flow of hydrogen-rich gas. The flammability limit for hydrogen in air is 4.0-75% by volume [31]. This means that even if the air/fuel mixture is rich, while cranking at low starter speeds, it is easily ignited. At the time of ignition the engine speed increases and thereby a greater amount of air is inducted, which eliminates the problem. A more efficient utilization of the heat during warmup of the catalyst would be to perform the combustion of methanol on the catalyst surface. This procedure would eliminate the use of a burner. The risk of superheating the catalyst has to be considered, however, and through this the loss of active metal surface area. 4. CONCLUSIONS The results prove that the concept is practically feasible. It is possible to start a spark-ignition engine at - 3 0 ° C with the hydrogen-rich gas from the presented cold start reactor, but extra methanol has to be added in order to establish proper idling, 80(01000 rpm. The drivability was good directly after cold start with the engine running on both product gas and neat methanol. This was not the fact when using only neat methanol. This indicates that the philosophy of feeding the product gas until the engine has reached the temperature of operation is correct. The hazardous emissions ought to decrease substantially with the addition of decomposed methanol during the warm-up period of the engine. At room temperature it was possible to both start the engine and reach an acceptable engine speed at idling, with the use of only the gas from the cold start reactor. The total heat capacity of the reactor must be decreased if the theoretically possible starting times are to be realized. Fundamental studies of the chemical reaction is essential in order to investigate the reaction characteristics and design an efficient reactor with a short start-up time. The distance between the intake manifold and the reactor must be minimized in order to prevent heat loss. The heat content of the product gas can be utilized to vaporize additional methanol and thereby facilitate start. On the other hand, if no additional methanol is used during cranking, the hydrogenrich gas will have to be cooled in order to establish an acceptable volumetric efficiency in the cylinders.

676

L. PETTERSSON and K. SJOSTROM

The choice of lubricating oil is important at very low ambient temperatures, around - 3 0 ° C . An ordinary all-season mineral oil is highly viscous at these temperatures and will strongly affect the starter speed, which in turn makes start difficult. Cold starting an engine at very low ambient temperatures, without the use of oils suitable for cold climates, will also increase engine wear and thereby shorten engine life. Acknowledgements--The financial support of this work provided by the Swedish National Board for Technical Development is gratefully acknowledged. We would also like to thank Benny Lindbrandt and Kurt Jansson at BLT Autolab AB for experimental assistance, and Volvo Flygmotor AB for providing the burner. The authors are indebted to Professor Olle Lindstr6m and Professor Sven J/irAs for valuable discussions and suggestions.

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