Figure 1. Hydrogen Life Cycle
West Virginia University's
National Alternative Fuel Training Program
Hydrogen production in U.S is 5 billion cubic meters (178 billion cubic feet) (1993) [1]. Most of the production is consumed in ammonia production and in the stripping of sulfur from petroleum during the refining process. The only significant application of hydrogen as a fuel is in space program. Liquid hydrogen and liquid oxygen are reacted for propulsion of space shuttle and other rockets. Besides, most of the electic consumption of the shuttle is provided by using onboard fuel cells.
The aim of the hydrogen program is to expand the role of hydrogen as a fuel for surface transportation. The using of hydrogen initially will be as an additive to conventional fuels. The combination of hydrogen with fossil fuels such as gasoline, natural gas, ethanol, methanol for fueling IC engines provides less emision, and increase performance.
Although this advantage of hydrogen, there are some impediments in using hydrogen in vehicles.
Figure 1. Hydrogen Life Cycle
Source: [1]
Flame speed is one of the fundemental characteristics in combustion process. It identifies the relative motion of flame front respect the unburt mixture.The high flame speed of hydrogen provides efficient cycle similar to constant volume process. On the other hand, higher thermal and mechanical loads along with increased combustion temperatures are the reasons of thermal losses, combustion noise and nitric oxide emissions.
The wide flammability limit of hydrogen in air provides lean operation which brings low NOx emission along with higher thermal efficiencies.
Diffusion coefficient of hydrogen which means better homogeneous charge also affect lean operation positively.
High quenching distanceprovides the flame front to penetrate into smallest crevice without being quenched.With respect to this characteristic, hydrogen flame can reach the unburnt mixtures than flames of fossil fuels.
Low ignition energy of hydrogen causes preignition and backfiring of hydrogen engines using externally formed hydrogen-air mixtures.The reason is probably hot oil deposits on other hotspots in cylinder. Although,several approches have been applied in order to solve this problem, None of them could not provide satisfactory results. Cooled exhaust gas recirculation, cold gaseous hydrogen, water injection, four-valve schemes are among those methods.
Energy density is related to storage. As a gas, hydrogen has a very low energy density by volume. This causes large fuel tank size even with high pressure storage and short vehicle range. For example, to store the equivalent of only five gallons of gasoline in compressed hydrogen requires a heavy tank at least the size of a 55 gallon drum. Besides, the low energy density of gaseous hydrogen causes a 20 % power reduction compared to gasoline. Because a stoichometric hydrogen-air mixture contains 20 % less energy than same volume of a gasoline-air mixture. Exhaust heat is not sufficient for turbocharging application, but supercharger with some efficiency loss can be used in order to compansate power loss. The energy density by volume of liquid hydrogen is also low (one fourth that of gasoline). Energy density by mass of hydrogen is 3 times that of gasoline which means that liquid hydrogen system will not suffer a weight penalty compared to gasoline.
Direct high pressure injection is the most efficient application in IC engines. High pressure liquid which is pressurized by a high pressure pump is converted to cold high pressure gas by using a heat exchanger. Injection is provided after the closing of valves which is similar to diesel engine. Ignition is done with a spark or glow plug. Preignition decreased by using this application. Since hydrogen does not displace air in the cylinder, it provides better volumetric efficiency which means higher power.
| Property | Hydrogen | Methane | Propane | Gasoline |
| Specific Gravity at NTP Relative to air | 0.07 | 0.55 | 1.52 | ~ 4.0 |
| Normal Boiling Point (K) | 20.3 | 111.6 | 231 | 310-478 |
| Critical Pressure (atm) | 12.8 | 45.4 | 41.9 | 24.5-27 |
| Density of Liquid at NTP (kg/L) | 0.0708 | 0.4225 | 0.5077 | ~ 0.70 |
| Density of Gas at NTP (kg/m3) | 0.838 | 0.6512 | 1.96 | ~ 4.40 |
| Density Ratio, NTP Liquid/NTP Gas | 845 | 649 | 259 | ~ 150 |
| Diffusion Coefficients in NTP air (cm2/s) | 0.61 | 0.16 | 0.1 | ~ 0.05 |
| Diffusion Velocity in NTP air (cm/s) | ~ 2 | ~ 0.51 | ~ 0.34 | ~ 0.34 |
| Quenching Gap in NTP Air (mm) | 0.64 | 2.03 | 1.78 | 2 |
| Limits of Flammability in Vol (%) | 4-75 | 5.3-15 | 2.1-10.4 | 1-7.6 |
| Limits of Detonation in Air Vol (%) | 18.3-59 | 6.3-13.5 | 3.4-35 | 1.1-3.3 |
| Minimum Energy for Ignition in Air (mJ) | 0.02 | 0.29 | 0.305 | 0.24 |
| Autoignition Temperature (K) | 858 | 813 | 740 | 501-744 |
| Flame Temperature in Air (K) | 2318 | 2148 | 2243 | 2470 |
| Maximum Burning Velocity in NTP Air (cm/s) | 278 | 37-45 | 43-52 | 37-43 |
| Energy of Stoichometric Mixture (MJ/m3) | 3.58 | 3.58 | 3.79 | 3.91 |
In the fuel cell, the cathode terminal is positively charged and the anode terminal is negatively charged. A membrane is used in order to divide terminals in the proton exchange membrane type of fuell cell. At the anode, hydrogen is split into its electrons and protons (positive hydrogen ions). When protons pass through the membrane, it creates a flow of direct current electricity between the terminals. The only byproduct of this process is water which is formed by electrons, hydrogen ions and oxygen at the cathode.
The efficiency of a fuel cell is as high as 75 %. There is no N0x, CO, HC emissions, because hydrogen is not burned in air. Pure hydrogen is used in lower temperature fuel cells. Platinum catalyst is used in the structure. For example a 40 KW fuell cell which is suitable for automotive applications needs at least 80 grams of platinum catalyst, which is 30-60 times the platinum in today`s catalytic convertors. It needs time to produce cost-effective, practical fuel cell for vehicles.
Figure 2. Hydrogen Fuel Cell
Source: [1]
Current on-board storage methods for hydrogen are too expensive and there are some problems in terms of performance requirements for applications due to low energy density of hydrogen. In other word, energy content per unit of space and lightweight mobile storage are important points for a transportation fuel. At normal temperature and pressure conditions,energy density of hydrogen is about 1/1330 that of gasoline [1] , which is extremely low. There are some points which affects hydrogen storage:
Research is focused on phsyical storage in a compressed gas or liquefied state, and solid-state storage using gas- on-solid adsportion in materials such as high surface area carbon, or absorption in the intersices of a metal hydride.
Source:[4]
5 Gallon Gasoline Reference Liquid Hydrogen (20 K) Hydride FeTi (1.2 %) Compressed Hydrogen (207-690 bar) Btu 629,500 629,500 629,500 629,500 Fuel wt (lb) 30.8 10.3 10.3 10.3 Tank Weight (lb) 14 41 1210 140-190 Total fuel system wt (lb) 45 51 1220 150-200 Volume (gal) 5 47 50 60-108 PHYSICAL STORAGE SYSTEMS
It is possible the storage of hydrogen in the form of compressed gas and cyrogenic liquid which is as low as 20 K. Energy consumption is significant for liquefaciton process, and it costs expensive. Compressed gas is stored at 2000-2500 psi (14-17 mPa) and requires large, heavy containers.. Lightweight graphite composite materials which resist high pressure up to 6000 psi (41 mPa) will bring promising results in on-board storage area. This new technology requires additional research for developing practical, safe, reliable storage systems using these materials.
Figure 3. Metal Hydride Storage
Source:[1]
The key factors in developing practical, economic hydride systems :
Long life under repeated loading and unloading cycles without significant loss in storage capacity is expected from hydride systems. Impurities in hydrogen can affect capacity negatively. Polyhdride complexes using cobalt and other transition materials have higher storage capacity and faster recyling times than other hydride system tested. [1]
The hydride storage system is approximately has 800 lbs weigh penalty compared to gasoline storage. This affects acceleration times and fuel economy negatively. But optimized hydrogen engines which can be run on high compression ratios (up to 14:1) and at equivalance ratio of 0,6 (67 % excess air) provide high thermal efficiencies and (less NOx emission). The result is that optimized hydride vehicle would consume between 1 % less and 8 % more energy per mile compared to gasoline vehicle.
The hydrogen powered vehicle which uses compressed hydrogen gas fueled vehicle has smaller weight penalty although it occupies larger volume. This storage method provides less consumption which is between 6-14 % than gasoline powered vehicle.
The low weight penalty of liquid hydrogen fueled vehicle combined with the highest performane ability among storage systems results in 11-18 % lower consumption compared to gasoline powered vehicle.[2]
| Baseline | H2 IC Engine | Baseline | H2 Fuel Cell EV | ||||
| Gasoline | Natural Gas | Hydride (800 lb) | Compressed H2 | Liquid H2 | Battery EV(800lb NaS) | ||
| Weight (lb) | 2440 | 2700 | 3200 | 2700 | 2500 | 2941 | 2941 |
| 0-60 mph (sec) | 13 | 14 | 17-20 | 14-17 | 13-15 | 16 | 16 |
| Relative Engine Efficiency | 1 | 1.1 | 1.15-1.25 | 1.15-1.25 | 1.15-125 | - | - |
| Weight Penalty | 1 | 1.08 | 1.24 | 1.08 | 1.02 | - | - |
| On-board Fuel Consumption(Btu/mi) | 3703 | 3644 | 3670-3990 | 3200-3480 | 3020-3280 | 944 | 944 Elec 2098 H2 |
| Range (mi) | 340 | 200-300 | 110 | 200-300 | 200-300 | 113 | 200-300 |
Currently,there are two widespread applications for production of hydrogen. Hydrogen is also byproduct of petroleum refining and chemical process.
Hydrogen can be produced by dissosication of water. Water splits into its two basic species, oxygen and hydrogen by using electrolysis which is the only practical method currently. In this method an electric current passes through water. The current enters the electrolysis device through the cathode which is charged negatively and leaves through the anode which is charged positively. During the process, hydrogen is seperated and collected at the cathode whereas oxygen is seperated and collected at the anode. With this method , hydrogen is very pure and can be used in electronics, food industries and space program. The production cost is as high as $ 28 per gigajoule.
High cost in production of hydrogen and the selection of the best feedstock and production process are important points in hydrogen production. Production technology needs more improvement compared with technologies in storage and utilization systems. Research in production is focused on technologies for overcoming cost and energy source barriers. There are several technologies which are in the early research and development stages. These technologies which use renewable sources of energy have strong potential for being cost-effective production systems.
Cell-free system which isolates and only uses hydrogen-producing enyzmes has potential for more long-term technologies that provides efficiency nearly 25 %.
PHOTOELECTROCHEMICAL PRODUCTION
The aim of this process is to convert optical energy into chemical energy by using semiconducting electrodes in a photochemical cell.There are two systems available:
has dual purpose. It absorb solar energy and act as an electrode for dissociation of water. The efficiency in this process is more than 8 % currently. Operating lifetimes of these systems are limited due to the light-induced corrosion of semiconducters and other chemical effects.
Hydrogen along with different gasses are obtained by applying heat to coal, municipal solid waste, and biomass-wood, grasses and agricultural waste. The composition of the gases depends on the type of the feedstock, the availability of oxygen,the temperature of the reaction and other parameters. There is no CO2 contribution to the enviroment during the process. Because the same amount of CO2 which is consumed by the biomass while growing returns to enviroment during conversion process. Some research needs to be done in this technology in order to have a cost-effective method. Techniques by using selective membranes or catalytic for seperating and purifying the hydrogen must be improved to remove of tars and oils from hydrogen.
An alternative method of producing hydrogen from biomass is a combination of pyrolysis and steam reforming process. First step in pyrolysis is to use heat to dissociate complex molecules into simple units. After first step, reactive vapors which occurs during first step convert to hydrogen. Although this process is at an early stage, it has potential for being one of the least expensive production method. Research is focused on improving efficient catalyst and for the steam-reforming step, and evaluating economic feasibility amd enviromental sustainability of total process.
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