Automotive Fuels & Emissions Alternative Fuels Chapter-3

Hydrogen and Hythane

Dr. S.S. ThipseAssistant Director, EDL, ARAI

Copyright © 2005 ARAI PGA

ABSTRACT

This chapter describes properties and applications of a promising alternative fuel namely Hydrogen. It essentially a carbon free fuel and is very environmentally friendly. Hydrogen is a renewable fuel which can be produced by several methods. Hydrogen (H2) is one of the lightest element in the periodic table. As early as in 1850, researchers tried to use hydrogen as an engine fuel.

INTRODUCTION

Otto the German scientist, in 1870 used hydrogen gas as a fuel for his IC engine. Since then several researchers have tried to use hydrogen as an automotive fuel. However till date all attempts to commercialize IC engines running on pure hydrogen have failed. This is due to the unresolved technical problems of production, storage and transportation.

One of the most attractive features of Hydrogen as an energy source is that it can be produced from water which is abundantly available in nature. Also Hydrogen is a very clean burning fuel with emissions lower than those produced by CNG or LPG. Thus vehicle running on Hydrogen can meet the most stringent emission norms. That is why it is termed as a “Freedom fuel”

Even today several researchers are trying to develop pure hydrogen engines. Hydrogen can also be used as fuel for fuel cell vehicles. IIT Delhi is at the forefront of R&D work on H2 Engines in India. Researchers are also trying to use hydrogen in from of blends with other fuels as a first step in its use. Hythane which is a blend of Hydrogen and CNG is thus seen as a first step in introducing hydrogen in the fuel market.

PROPERTIES OF HYDROGEN

The combustion reaction for Hydrogen is H2 + 1/2O2 H2O + EnergyMolecular Weight = 2.02Density = 0.0013 kg/litreSpecific Gravity = 0.07

Freezing Point = -275°CSpecific Heat = 14.2 kJ/kgKCalorific value = 12.1 MJ/m3

Viscosity = 0.009 mPa-s @ 20°CLatent Heat of Vaporization = 448 kJ/kgFlammability Limits (% Vol) = 49 to 75Stoichiometric Air-Fuel Ratio = 34.3Storage Pressure = 350 barFlame Speed = 50 cm/s

PRODUCTION OF HYDROGEN

Hydrogen can be produced by several methods. Considerable debate is going on in the industry as to which method is economically viable.

1. Electrolysis of water.2. Catalytic steam reforming of natural gas.3. Partial oxidation of heavy oil.4. Coal gasification.5. Steam iron process. 6. Thermal cracking of natural gas.

We shall briefly acquaint ourselves with these methods.

1. Electrolysis of Water

This method does not rely on fossil energy. Hydrogen produced by this method is pure in nature. Water electrolysis involves splitting water molecule into hydrogen and oxygen atoms by passing a direct current through it. Water is made conductive by adding hydroxyl ions to it.

In the electrolysis process, the distilled water enters a deionizer module where DC current is passed through it. The DC current is obtained by converting the AC current using an AC to DC Converter. Upon electrolysis the Hydrogen & Oxygen atoms separate out of the water molecule. The residual H2O molecules are removed using separators. The hydrogen and oxygen produced is then regulated and stored. Water electrolysis is a modular process. The rate of H2 generation is related to current density. The capital cost of an electrolysis plant is very high. Thus cost of hydrogen produced by this method is on the higher side.

2. Catalytic Steam Reforming of Natural Gas

This is an economical method to produce H2 . The process involves catalytic conversion of hydrocarbons using steam to form hydrogen and carbon oxides. The feedstock used for this method is Naptha. The process consists of three steps.

1) Synthesis gas generation.2) Water-gas shift.3) Gas purification.

The sulfur content of the naptha feedstock is first removed and then it is mixed with process steam. The mixture is then passed through steel tubes, which contain nickel catalysts. The reactions taking place are as follows.

Cn Hm + n H2O nCO + (n + m/2) H2

CO + H2O CO2 + H2

The Hydrogen gas generated in this first step contains some methane and carbon monoxide. To remove CO and CH4, the gas passes through a heat recovery step. The gas is fed to a water gas shift reactor where additional Hydrogen is produced. The gas finally passes through a gas purifier where trace impurities like CO2 are removed. The purification step involves wet scrubbing with an amine solution. The purity of H 2 gas produced by this process is 98%.

3. Partial Oxidation of Heavy Oil

Heavy oils are hydrocarbon rich fuels. These hydrocarbons are oxidized to produce CO & H2. The CO produced is removed from the mixture to obtain pure Hydrogen. The reaction occurring in this process is as follows

CnHm + n/2 O2 nCO + m/2 H2 + Heat.

The reaction is exothermic in nature and proceeds at a temperature in the range of 1150°C to 1315°C. Catalysts are required to accelerate the reaction. Compared to steam reforming process, this method requires additional facilities like air separation plant to provide oxygen. This method of producing hydrogen is expensive as it is capital intensive. Heavy oil prices are also rising daily adding to the overall cost of the process.

4. Coal Gasification In this process the coal is fed to a gasification plant. Air is separated into its constituents O2 and N2 by an air separation plant. The O2 stream enters the gasification plant, along with process steam. The reaction of coal with steam and oxygen, produces a raw gas and some ash. The ash is removed and the raw gas is passed through a desulfarization unit to remove sulfur. The sulfur amount in coal is high and varies from place to place. The cleaned gas is known as synthesis gas (Syn gas). Syn gas is then compressed and subjected to the shift conversion process. Here CO and CH4 impurities are removed from Hydrogen. Finally in the methanation process, CO2 is removed and pure hydrogen is produced.

5. Steam-Iron Process

This process involves reforming steam by reaction with hot iron. The product of the process is rich H2 gas and iron oxide. The steam undergoes decomposition during the process to produce H2 . The reactions occurring are as follows:

3 FeO + H2O Fe3 O4 + H2

FeO + CO Fe + CO2

The hydrogen produced contains impurities like CO and N2. These impurities are removed by methanation reaction. Acid scrubbing is not required for purification of Hydrogen. The cost of this process is dependent on capital investment and price of coal.

6. Thermal Cracking of Natural Gas

The thermal decomposition of natural gas was carried out in earlier times to obtain Carbon black. The decomposition requires a lot of energy some of which comes from the produced hydrogen.

CH4 + Heat C + 2H2

The process is carried out in a furnace at near atmosphere pressure. A methane air flame at 14000C is used to heat the furnace. The methane decomposition produces carbon particles which are collected in filter bags. Catalysts are used to achieve faster decomposition. This very economic method of producing Hydrogen as prices of natural gas are low as compared to other fuels.

ON BOARD STORAGE OF H2

One of the biggest challenge in developing a Hydrogen fuelled vehicle is the designing of a satisfactory on-board storage system for the fuel. The principal problem with storing hydrogen is its low density. Following are the different ways in which H2 is stored.

1. Liquid Hydrogen (LH2)

Hydrogen can be stored as a liquid under cryogenic conditions of temperature 20K and pressure 200 kPa. To maintain these conditions, liquid H2 is typically stored in a double-walled super insulating vessel (Dewar vessel)

2. Metal Hydrides

The basic principle of metal hydride formation is that gaseous hydrogen gets absorbed on the metal surface by weak bonding. The metal hydrides are in a granular form. The metal hydride is heated to release the gaseous hydrogen absorbed on its surface. Heat energy of the exhaust gases from the engine is sometimes used for heating the metal hydrides. Metal hydrides have low energy density and are heavy. However this is the safest method of storing H2.

3. Compressed Hydrogen Gas

Hydrogen is stored at 350 bar in a high pressure vessel. These vessels are typically aluminium cylinders wrapped with fibreglass. The compressed hydrogen storage weighs more than liquid Hydrogen storage and occupies more volume. Gas leakages are possible, so safety is a major concern for this method of storage.

4. Activated Carbon Storage This technique involves adsorbing Hydrogen on to a carbon surface. The amount of adsorption can be increased as the temperature is lowered, but cryogenic temperatures are not required. The activated carbon can be heated on-board to release the Hydrogen gas.

5. Glass Microspheres

Hydrogen can be stored under high pressure in small glass spheres. Hydrogen can be released by heating the microspheres, which increases the rate of diffusion of Hydrogen through the glass surface.

MATERIAL COMPATIBILITY FOR HYDROGEN

Hydrogen has lower molecular weight. This property is useful for storing hydrogen as a gas or a cryogenic liquid. Long-term exposure of hydrogen to carbon steel can cause embrittlement of steel. The hydrogen molecules migrate within the steel structure promoting stress corrosion cracks and cause failures. Bends in piping or welded joints are susceptible to failure as well. Non-ferrous metals are not impacted by Hydrogen. Aluminium, Copper and alloys like Bronze and Brass are compatible with Hydrogen. Hydrogen is compatible with all elastomers and does not pose material or compatibility concerns. Hoses for Hydrogen should be non-porous to prevent leakage of Hydrogen.

PIPING AND DISPENSERS FOR HYDROGEN

Stainless steel piping is preferred for Hydrogen. Liquid Hydrogen requires use of vacuum jacketed piping to minimize heat gain. Compression fittings and flanged joints are preferred on pipes instead of threaded joints to minimise leaks.

Only a few experimental dispensers have been built for liquid Hydrogen. The Hydrogen dispensers should control the cooling of the fuel transfer lines. The fuel transfer lines are jacketed to minimize heat gain from the surrounding. The hydrogen dispensers use Programmable Logic Controllers or PC based control systems. Liquid H2 and compressed H2 dispensers are required to maintain pressure and temperature of fuel, monitor opening and closing of valves, switch on pumps and interface with fire emergency shutdown systems.

FIG 1 HYDROGEN FILLING AND STORAGE

SAFETY SYSTEMS FOR HYDROGEN

1. Leak Detection Systems Hydrogen is a colourless and odourless gas, hence it will require leak detection systems. Hydrogen detectors are required to be placed strategically over the refuelling area at the appropriate locations. The Hydrogen detectors should produce an audible alarm when the concentration of Hydrogen reaches 1% by volume in air, which is 25% of the lower flammability limit for Hydrogen. If H2 concentration continues to rise further, the detectors should have the capability to shut down the refuelling systems until the ambient H2 concentration is reduced.

2. Fire Suppression Systems

All refuelling systems of significant size should incorporate a fire suppression system. The standard system of using dry chemical powders is effective for extinguishing Hydrogen flames. Electronic fire suppression systems incorporating special sensors are available, however they are very costly.

STANDARDS FOR HYDROGEN

Currently no Indian standard is available for Hydrogen as an automotive fuel. A draft ISO standard is available for Hydrogen. In the US, NFPA 52 and NFPA 57 are the regulatory standards for storage and dispensing of compressed Hydrogen gas and liquefied hydrogen respectively.

DIFFICULTIES IN USING HYDROGEN AS A FUEL

1. High flammability lends an explosive quality to the fuel-air mixture.2. Leak detection of H2 is difficult as it is odourless.3. Pre-ignition occurs when the cylinder charge ignites before the spark.4. Backfire occurs when the flame travels back to the fuel induction system.

USE OF HYDROGEN IN SI ENGINES

Four fuel delivery systems are possible. (1) Carburetion (2) Inlet manifold injection (3) Inlet port injection (4) Direct cylinder injection.

Carburetion is an easy technique to convert a gasoline engine to dual-fuel gasoline and hydrogen engine. However carburettors are susceptible to preginition and backfire problems. Power output is also very low. In inlet manifold injection, fuel is injected directly into the intake manifold. Air is injected separately and it dilutes the fuel charge and prevents formation of hot spots. In port injection, fuel and air are injected from separate ports into the combustion chamber during intake stroke. Preignition problems

are avoided by this method. Hydrogen can also be directly injected in the engine cylinder, however this requires costly gas injectors. Researchers at IIT Delhi have developed a timed manifold injection technique for Hydrogen engines and have obtained good power and efficiency along with reduced preignition. The hydrogen is injected directly into the cylinder. Drawback of this method is the possibility of incomplete combustion. Exhaust Gas Recirculation (EGR) and Water Injection are two additional techniques used to reduce preignition in H2 engines. In the EGR technique a fraction of the exhaust gases is recirculated back to the intake manifold. In water injection technique, water is injected in the hydrogen stream before mixing with air.

Usually Hydrogen fuelled SI engines use disk shaped combustion chamber to reduce turbulence and swirl. Large bore to stoke ratio is used for Hydrogen engines. Two spark plugs are used to accommodate the wide flammability limits of Hydrogen. The coolant system has to be redesigned for providing sufficient cooling. Two smaller exhaust valves are used instead of a single large exhaust valve to reduce the possibility of preignition. The scavenging system needs to be improved as well.

USE OF HYDROGEN IN CI ENGINES Direct injection diesel engines have been used to experiment on Hydrogen fuel. The CI engines produce more power and are thermally efficient and durable. The auto ignition temperature of H2 is higher than that of diesel. Thus ordinary CI engine compression is not sufficient for H2. Thus auto ignition is not possible even at high compression ratios of the order of 29 :1. Some researchers have used glow plugs to obtain ignition of H2 in CI engines. Ceramic parts are known to retain heat. CI engine with ceramic parts was used with direct injection of Hydrogen fuel in the cylinder. Recycling of hot residual gases from previous cycles can also serve as an ignition source.

EMISSIONS FROM HYDROGEN

Hydrogen is clean burning fuel. HC, CO, CO2, SOx emissions are practically absent. NOx

emissions are the only significant emissions from the H2 engine. Using lean burn technique and DeNOx catalysts the NOx emissions can be reduced. Thus there is considerable interest in using hydrogen as a commercial automotive fuel due to its ability to meet stringent pollution norms.

FIG 2 HYDROGEN STORAGE IN A PASSENGER CAR

BLEND OF H2 WITH CNG (HYTHANE)

Hythane is a blend of Hydrogen and CNG where the Hydrogen content is limited to 10% by energy content i.e. approx 10-30% by volume. Hythane is a trade mark of Hydrogen Consultants Inc, USA. Hythane combines the advantages of both Hydrogen and CNG. Safe combustion, lower flammability, reduced emissions, lower preignition and backfire tendencies are some benefits of using Hythane.

Hythane can be burned by using (1) lean burn approach or (2) stoichiometric approach. In lean burn approach, excess air is used for combustion (λ>1). The NOx and THC emissions are lower for this method of combustion. The exhaust temperatures are also lower for this method. Oxidation catalysts are used to lower emissions for a lean burn engine. In stoichiometric approach an exact air quantity is used to burn the fuel (λ=1). The CO and NOx emissions are higher in this type combustion method. Precise control over the air fuel ratio is required for this method. Three way catalysts are used for this combustion method. The exhaust temperatures are higher than those obtained in lean burn combustion method.

In India efforts are underway by IOC to experiment on Hythane as a fuel.

CONCLUSION

Hydrogen is a promising alternative fuel due to its clean burning characteristics. Several technical issues have to be resolved for Hydrogen to be used as an automotive fuel. The on board storage and safety issues for Hydrogen fuelled vehicles are being actively investigated. Hydrogen has a bright future as an automotive fuel for conventional IC Engine driven vehicles as well as hybrid and fuel cell vehicles. CONTACT

Dr. S.S. Thipse,Assistant Director,Engine Development Laboratory, ARAI.Phone : 25437180 (x 265)E-Mail : thipse.edl@araiindia.com

REFERENCES

1. Non Conventional Energy Sources - G. D. Rai.

2. Alternative Fuel Guidebook- R. Bechtold. 3. Hydrogen fuel for surface transportation (SAE) – Norbeck et.al.

4. Proceedings for SIAT –2005 – ARAI, Pune.

QUESTIONS

1. Why is Hydrogen termed as a freedom fuel?2. Describe any two methods of Hydrogen production in detail. 3. What are the different methods for storage of Hydrogen?4. Explain the material compatibility for Hydrogen.5. Describe the safety systems for Hydrogen.6. What are type of fuel delivery systems used for Hydrogen in SI engines ?7. What are the problems in using Hydrogen in CI engines ?8. What is Hythane? What are its advantages ?

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