汽车专业毕业设计翻译p001360.doc

INTRODUCTION
Most of the oil daily extracted is consumed in transportation (approximately 66.6% in America) http://www.bts.gov/publications/national_transportation_statistics/html/table_04_03.html
. Of the energy used in this sector, approximately 65% is consumed by gasoline-powered vehicles. Diesel-powered transport (trains, merchant ships, heavy trucks, etc.) consumes about 20%, and air traffic consumes most of the remaining 15% US Dept. of Energy, "Annual Energy Outlook" (February 2006), Table A2
. In Italy, civil mobility accounts for about 74% of total consumption, half of which corresponding to about 30% of the national total consumption on transportation, inside urban and suburban zones. In these environment therefore, car emissions represents the main pollution source. Hydrogen cars, by eliminating CO2 emissions, could drastically reduce in short times pollution, especially in our living areas. The main obstacle, hindering the introduction into the market of hydrogen cars, is represented by the low efficiency of hydrogen storage system.

Figure 1. Energy density, both volumetric and gravimetric, of various systems for hydrogen storage. The 2010 and 2015 DOE targets correspond to 6.0 and 9.0 % wt.

We can assume that a fuel cell equipped car would run about 100 km per kg of hydrogen burned. Consequently a standard of 400 km would be fulfilled by 4 kg of hydrogen. Fig.1 shows various storage systems dimensioned to deliver 4 kg of hydrogen.
Among the different storage systems, the hydrolysis of hydride seems the best promising one (Fig.2) .
The reaction can be represented as:

2 MeHn+ nH2O →Me2On +2nH2 (1)

The hydride should necessarily be regenerated off-board.
Among the different metal hydride NaBH4 shows a hydrogen content higher than 10 % wt. The high hydrogen content and the simplicity of synthesis makes the salt a perfect candidate to store hydrogen for mobile application. As a fuel NaBH4 is less flammable and less volatile than gasoline. It is relatively environmentally friendly because it will quickly degrade into inert salts when released into the environment.

Figure 2. Weight percent of hydrogen evolved from hydrolysis of various complex metal hydrides. The hydrogen weight was referred only to the weight of the metal hydride and it does not take into account the weight of water used for the hydrolysis.

The hydrogen is generated by catalytic hydrolysis of borohydride:

NaBH4 + 2H2O → NaBO2 + 4H2 (2)

The sodium borate can be reversely turned into sodium borohydride by well know methods. The weight percentage of hydrogen over the hydride of the hydrolysis reaction is incredibly high, 20 %. The compound was discovered in the 1940's by H. I. Schlessinger, who led a team that developed metal borohydrides for wartime applications H.I. Schlesinger, H. C. Brown, B. Abraham, A.C. Bond, N. Davidson, A. E. Finholt,J.R. Gilbreath, H. Hoekstra, L. Horvitz, E.K. Hyde, J. J. Katz, J. Knight, R.A. Lad, D.L. Mayfield, L. Rapp, D.M. Ritter, A. M. Schwartz, I. Sheft, L.D. Tuck, A. O. Walker, “New developments in the chemistry of diborane and the borohydrides. General summary” Journal of the American Chemical Society 1953, volume 75, pages 186-90
. In 1939 Anton B. H. Burg and H. C. Brown became research assistants to Professor Schlesinger. In the Fall of 1940 he was requested to undertake for the National Defense Research Committee a search for new volatile compounds of uranium of low molecular weight., without the corrosive properties of uranium hexafluoride. By using diborane the synthesis of U(BH4)4, a compound with adequate volatility, was successful. To effectively confirm the result they were requested to supply relatively large amounts of the material. The bottle-neck was the preparation of diborane. By this time they would not be able to supply sufficient diborane so they attempted to find a more practical route. They discovered that the reaction of lithium hydride with boron trifluoride in ethyl ether solution provided such a route. Unfortunately, lithium hydride was in very short supply and could not be spared for this synthesis. Instead, there was a large amount of sodium hydride. Unfortunately, with the solvents then available, the direct use of sodium hydride was not successful. However, a new compound, sodium trimethoxyborohydride H.C. Brown, H. I. Schlesinger, I., Sheft, D. M. Ritter, J. Am. Chem. Soc. 75, 192 (1953).
, readily synthesized from sodium hydride and methyl borate, solved the problem. It proved to be very active to and provided the desired transformations previously achieved with lithium hydride. Thy also discovered that the addition of methyl borate to sodium hydride maintained at 250° provided a mixture of sodium borohydride and sodium methoxide H. I. Schlesinger Schlesinger, H.C. Brown, A.E. Finholt, J. Am. Chem. Soc. 75, 205 (1953).
. This also provides the basis for the present industrial process for the manufacture of sodium borohydride. At this stage they were informed that the problems of handling uranium hexafluoride had been overcome and there was no longer any need for uranium borohydride. They were on the point of disbanding the group when the Army Signal Corps informed that the new chemical, sodium borohydride appeared very promising for the generation of hydrogen http://nobelprize.org/nobel_prizes/chemistry/laureates/1979/brown-lecture.pdf
to inflate signal balloon. At that time, there was no doubt about the fact that NaBH4 should react with water to liberate hydrogen. Some concerns remained about the safety of the reaction. When they try for the first time to liberate hydrogen by hydrolysis the borohydride was placed in a flask and the entire assembly was put behind an explosion screen since it was not know how violent the reaction could be. Incredibly, the borohydride dissolved in the water without explosion and with slow hydrogen evolution, and it was discovered that sodium borohydride possesses an unusual stability in alkaline water C. Herbert, H.C. Brown, R&D Innovator Volume 2, Number 8, August 1993
. Several study have been then conducted on hydrolysis of sodium borohydride. It was found that it is very fast at low pH and that high pH values, prevent water hydrolysis S.C. Amendola, et al., Int. J. Hydrogen Energy 25 (2000) 969–975.
. In presence of catalyser (cobalt, nichel, iron) the reaction is quite fast and complete also at high pH. Army Signal also proposed to study the new compound for rocket engine H.I. Schlesinger, Final Report to Signal Corps Ground Signal Agency on Contract No. W3434-SC-174, PB-6331, 1944.
but the project was stopped few years later. At that age technicians told that sodium borohydride would have a possibility for civil use at the end of the century. More than 60 year have been passed and now it could be the right time for sodium borohydride. Several car makers announced their intention to power hybrid hydrogen car with sodium borohydride. DaymlerChrysler built a prototype applying the technology that use that salt for fuelling a fuel cell (the Natrium, over a Chrysler Town & Country model). Even PSA-Peugeot built a prototype with rutenium as catalyser, the H2O model. A liquid solution half weight sodium borohydride half weight water, stabilized with sodium hydroxide, delivers hydrogen (through a Ru catalyser) with an energetic ratio, in volume, similar to gasoline. Millennium Cell (which has patents on catalyser and systems) built different prototype with FORD. Millennium Cell announced that it has been awarded a Phase I Small Business Innovation Research Program ("SBIR") contract by the Air Force Research Laboratory ("AFRL") to develop a sodium borohydride based fuel cartridge design that has the flexibility to operate with either ready-to-use, premixed solutions of sodium borohydride, or alternatively, with solid fuel packets that a war fighter can combine with available field water or bodily fluids Air-Attack_com News - USAF Continues Fuel Cell Research.htm
. The ability to ship cartridges containing dry sodium borohydride fuel which can be mixed with an available water supply at point of use would result in a significant reduction in weight to be handled.

EXPERIMENTAL
The hydrolysis in the vapor phase of solid NaBH4 was conducted in a oven. About 1.0 gram of catalyzed solid NaBH4 was placed in the oven in presence of water contained in a separate container. The NaBH4 was catalyzed with various metal salts (Co, Ni or Fe) at a concentration of about 2 % mol. The outlet of the oven was connected to a graduate cylinder filled with water. The evolution of hydrogen was followed by water displacement. The temperature of the oven was raised from room temperature up to 95°C. after The product of hydrolysis was characterized by X-ray powder diffraction using a Miniflex Rigaku (Cuk-a radiation). Cartridges were prepared by using the same mixture (NaBH4 + catalyst). Blotting paper was used to prepare the cartridges. The cartridges were filled with NaBH4 with a weight ranging from 30 up to 150 g. The cartridge were inserted in a metallic reactor. The reactor is a 6cm diameter 6cm high cylinder. Liquid or vaporized water was introduced by pumping it with a controlled pump into a thermo controlled heater. The amount of water was recorded by using a liquid flow meter. The hydrogen evolved was collected, filtered to remove water vapor and measured by using a gas flow meter.

ENEA ACTIVITIES
The new hydrogen releasing system developing in ENEA is based on solid NaBH4 which is hydrolyzed with water or steam P.P. Prosini, C. Cento and P. Gislon, Italian patent RM2006A000221
. To make the reaction very fast a catalyst was used. Figure 3 show the amount of hydrogen evolved when 1.0 gram of catalyzed NaBH4 is put in presence of water vapor in equilibrium with liquid water at various temperatures, ranging from 40 to 95 °C.

Figure 3. Volume of hydrogen evolved as a function of time from 1.0 g of NaBH4 in presence of water vapor at various temperature. The catalyst was nickel acetate 2 % mol.
The presence of catalyst is so effective that hydrolysis can be conducted also at room temperature. By increasing the temperature obviously the rate of hydrogen production increases, and the reaction time decreases. Independently from temperature about the theoretical amount of hydrogen was recovered. For temperature lower than 70°C and for time lower than 100 min, the reaction seems independent from temperature. For longer times it is evident a progressive increase of the kinetic of the reaction as the temperature increases. A second reaction takes place and the activation energy of this reaction is so high that, for temperature higher than 70°C, the reaction becomes the faster one. Figure 4 shows the volume of hydrogen evolved related only to the second reaction. It was obtained by subtracting from the curves plotted in figure 3 the constant contribute due to the first reaction. In the same figure it is plotted the half-life time as a function of the temperature. From these data a new graph (Figure 5) was obtained by plotting the natural logarithm of the inverse of the half-life time as a function of the inverse of the temperature (in Kelvin).

Figure 4. Two reactions concurred to NaBH4 hydrolysis. The volume of hydrogen evolved related only to the second process is plotted as a function of time (left). In the same graph it was reported the temperature as a function of the half-time (right).

Figure 5. Plot of the natural logarithm of the inverse of the half-life time as a function of the inverse of the temperature (in Kelvin). Data were obtained from figure 4.
A typical Arrhenius behavior was observed. The activation constant was found to be 29 kJ/mol. After hydrolysis the sample phase purity was analyzed by X-Ray powder diffraction. Figure 6, 7, and 8 show the diffractogram of three representative samples.

Figure 6. X-Ray diffractogram for the product obtained at room temperature. It was identified as NaBO2*4H2O (JPCS 06-0122).

Figure 7. X-Ray diffractogram for the product obtained at 70°C. It was identified as NaBO2*2H2O (JPCS 06-0122).

Figure 8. X-Ray diffractogram for the product obtained at 90°C. It was identified as Na2B2O4*H2O (JPCS 20-1078).
The product obtained in the range 30-80°C was mainly NaBO2*2H2O. In this range the total reaction can be described as:
NaBH4 + 4H2O → NaBO2*2H2O + 4H2 (3)
While at higher temperature we have:
2NaBH4 + H2O → Na2B2O4*H2O + 4H2 (4)

Figure 9. A 30 g. NaBH4 filled cartridge.

Figure 10. Pulse test for high hydrogen releasing rate. The hydrogen flow (red line) is plotted versus time. The total volume of hydrogen (blue line) was obtained by numerical integration. The experiment was conducted by stopping and restarting the water flow with a 30 g NaBH4 cartridge. The water was heated at 60°C.
Then we tried to increase the size of the reactor to obtain large hydrogen production. The first step was the development of a reactor able to contain up to 150 grams of NaBH4. Figure 9 show a typical cartridge containing 30 grams of NaBH4. The cartridge has a cylindrical shape and it can release more than 50 liters of hydrogen. To store the same amount of hydrogen in the same cartridge volume by compression we should reach the impressive pressure of 2000 atm.! Figure 10 shows the variation of hydrogen flow as a function of time. There was a good response of the system to sustain very high rate for few minutes. The system was also able to easily restart after prolonged pauses. At the end of the reaction about 54 liters of hydrogen was released. Figure 11 shows the facility of the system to release various hydrogen flow by simply changing the water flow. The reactor was filled with 73 g cartridge. The amount of water varied during the run from 0.2 up 1.0 ml/min. Consequently the hydrogen flow changed up to 0.6 liters per min We are now engaged to increase the size of the cartridge up to 1.5 kg. The reactor is very similar to the previous used reactor and it was obtained by repeating the single unit about ten times. It has a tubular shape of about 6 cm of diameter and 60 cm high. Tests are running just now.
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Figure 11. Constant flow hydrogen releasing test. The amount of water was progressively increased from 0.2 to 1.0 ml/min, step 0.2 (blue line). In correspondence the amount of hydrogen increased proportionally (red line).
THE NEXT STEP
In brief time it will possible to scale-up the NaBH4 cartridge up to automotive purpose. The aim is to develop a 10 liters rechargeable cartridges, filled with NaBH4. The cartridge weight is about 8 kg; each cartridge is able to deliver about 1.6 kg of hydrogen. The cartridges can be collected on pallets and moved by train or truck. The same infrastructures used for transportation could then be used to transport the exhausted cartridge for regeneration. The car fue