The ideal fuel for optimum fuel cell operation is hydrogen. Although that can undergo an oxidisation reaction, it can be used as fuel for fuel cell systems. Hydrogen can be produced by various methods such as steam reforming, ,water electolysis, photoconversion, photoelectrochemical production, and pyrolysis of biomass. The fuel cells are ideally suited to using other fuels including ethanol, methanol and biodiesel to name a few. Followed by some energy properties of fuels.
Ethanol is presently
viewed the perfect fuel for portable fuel cell. Methanol and ethanol presently
can be made from either natural gas or biomass.
Also some other fuels can be used such as natural gas, biogas, naphtha and propane.
In
this process hydrogen is derived from fossil fuels such as natural gas. During
this process, hydrogen is separated from carbon component of the fuel. During
the first step fuel decomposes to hydrogen and carbon monoxide by steaming on
catalytic surfaces. In the second step, which is called shift reaction, CO and
water convert to CO2 and hydrogen. The temperature of reaction is higher than
200 C.
Hydrogen can be produced by dissociation 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 separated and collected at the cathode whereas oxygen is separated and collected at the anode.
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:
Dissolved
metal complexes are used as a catalyst in the second process. The soluble
metal complex absorbs energy and creates an electric charge separation,
which causes water-splitting reaction.
Other fuels to power fuel cells
Ethanol is an alcohol fuel. As with methanol, ethanol is mixed with gasoline as a primary alternative fuel. Ethanol is produced mainly by cooking, fermentation and distillation process using grain crops. Cellulose feed stocks, such as wood and agricultural wastes are considered excellent future alternative for ethanol production.
Compared to methanol, ethanol has approximately 30-50 % less smog forming emissions than gasoline. Air toxins are also reduced by about 50 % compared to gasoline. Ethanol also offers significant greenhouse gas benefits, particularly when produced from renewable, high cellulose feed stocks. However, ethanol is responsible for emitting aldehydes. Ethanol is often denatures to prevent any misuse from ingestion. Ethanol can be used to run vehicles, with Brazil currently running around 4million cars on ethanol from sugar cane. Ethanol has about two-thirds the energy density of gasoline, and has the same limitations as alcohol vehicles.
Methanol is an alcohol fuel. The primary alternative methanol fuel in use is called M85, derived from 85 % methanol and 15 % gasoline. Methanol is created from a synthesis gas of hydrogen and carbon monoxide, which is reacted in the presence of a catalyst. In general, a process using natural gas as the feedstock produces most of the world’s methanol. Methanol can also be produced from non-petroleum feedstocks such as coal and biomass.
Emissions from M*% are slightly lower than gasoline fuel. Smog forming emissions are generally 30-50% lower including nitric oxides and hydrocarbons emissions. Total toxic air pollutants that are suspected or known to cause cancer are 50 % less with Methanol. However, carbon monoxide emissions maybe slightly higher than in gasoline. Methanol is highly toxic, with safety concerns with ingestion, eye or skin contact, and inhalation. Methanol is not a carcinogen. Low-flame luminosity makes methanol difficult to detect in the daylight. By adding the gasoline to methanol, gives the flame a colour and adding a detectable smell. If ignited it burns in more controlled manner with less heat than transportation fuels. Methanol fires can be extinguished by water easily.
Diesel Fuel
Diesel fuel is a complex mixture of hydrocarbons, with a higher number of carbon atoms than gasoline. The components in diesel fuel fall into higher boiling ranges between 188-343°C. When diesel is burned it emits low amounts of reactive hydrocarbons and carbon monoxide. Since diesel engines operate at low temperatures. They also emit lower amounts of Nitric Oxides. However, diesel fuel has heavier hydrocarbons, which do not burn as easily and are emitted as particulates.
Diesel fuel has a very high flash point, implying a higher safety margin than gasoline when handling. By most fire-safety standards, diesel is a safer fuel than gasoline and other fuels. Additionally, if spilled, diesel is not as likely to readily spread on water. Diesel fuel is commonly used in transportation. Diesel is a more favourable power source, due to the fuel efficiency of the engines, around 25 % more and the durability of the engines themselves.
Liquefied Petroleum Gas (commonly called propane)
Liquefied Petroleum Gas (LPG) consists of various hydrocarbons, mainly of propane, propylene, and butane in various mixtures. The mixtures is mainly propane though, a relatively simple molecule, C3H8, which undergoes complete combustion when used as fuel. LPG is a by-product of natural gas processing and petroleum refining.
Propane can have lower emissions of reactive hydrocarbons 33%, nitric oxides 20%, and carbon monoxides 60%. If LPG leaks or spilled, it will remain on the ground or will enter the water system. Propane is used in all major end-use sectors as a heating fuel, engine fuel, cooking fuel, and as a chemical feedstock. The components of LPG are gases at normal temperatures and pressures. With moderate pressures (100 to 300 psi), they condense into liquids, making them easy to transport and store. LPG or propane has been used as a transportation fuel for several decades, and was in relatively widespread automotive use mainly in fleet vans.
Biodiesel is a fuel made from oils and fats of plants. Although it can be used as a straightforward replacement for diesel, the blend of biodiesel with diesel is often used as 20% biodiesel and 80% diesel. Biodiesel can be produced through a transesterfication process, forming fatty esters. One of the by-products is glycerol, which can then be sold as an independent product.
The emissions from using biodiesel are much lower than those of diesel fuel. Biodiesel has no aromatic content and only trace amounts of sulphur. It has low emissions of carbon monoxide, and polycyclic aromatic hydrocarbons compared to conventional diesel. Nitric oxide emissions can be high, but can be reduced with some adjustments. Biodiesel is biodegradable and non-toxic, making it an excellent fuel for marine applications. Additionally, biodiesel has a high flash point and it does not produce explosive air/fuel vapours.
Gasoline (Petrol)
Gasoline is a mixture of several hundred types of complex hydrocarbons and most of the hydrocarbons contain four to twelve carbon atoms. It has a boiling point between 27-225°C. Gasoline is produced from the refining process of crude oil. Emissions from the gasoline are widespread and include CO, CO2, NOx, and SOx. Some of these emissions are known as human carcinogens. Gasoline can also have major impact on the environment when spilled since it spreads on the water surfaces and can quickly penetrate porous soils and groundwater easily.
Gasoline is extremely flammable, is highly volatile and has a corresponding a large risk of fire and explosion in exposed circumstances. Gasoline evaporates quickly and leaves little residue when spilled. The complex chemical nature of gasoline requires a complex production process, but also allows for significant flexibility in adjusting fuel specifications to meet different performance and emissions standards.
Energy properties
|
Hydrogen |
Methane |
Propane
|
|
|
LHV kJ/kg kWh/kg MJ/Nm3 kWh/Nm3
|
119,972 33.33 10.783 2.995
|
50,020 13.9 35.882 9.968
|
46,350 12.88 93.215 25.893
|
|
HHV kJ/kg kWh/kg MJ/Nm3 kWh/Nm3
|
141,890 39.41 12.745 3.509
|
55,530 15.42 39.819 11.061
|
50,410 14.0 101.242 28.123
|
|
Density kg/m3
|
0.08988
|
0.7175 |
2.011 |
|
Gas constant J/kg K
|
4124 | 518.8 | 188.5 |
|
Ignition temperature in air °C
|
530 |
645 |
510 |
|
Ignition limit in air °C
|
4.21-72.5
|
5.1-13.5
|
2.5-9.3 |
|
max. flame velocity cm/s
|
346 | 43 | 47 |
Combustion related properties of fuels
|
Energy Carrier |
Form of Storage |
|
|
||
|
Hydrogen |
Gas (20MPa) Gas (24.8MPa) Gas (30MPa) Liquid (-273°C) Metal hydride
|
33.3 33.3 33.3 33.3 0.58
|
0.53 0.64 0.75 2.36 3.18
|
||
| Natural Gas |
Gas (20MPa) Gas (24.8MPa) Gas (30MPa) Liquid (-162°C)
|
13.9 13.9 13.9 13.9
|
2.58 3.01 3.38 5.8
|
||
| LPG | Liquid | 12.9 | 7.5 | ||
| Methanol | Liquid | 5.6 | 4.42 | ||
| Gasoline | Liquid | 12.7 | 8.76 | ||
| Diesel | Liquid | 11.6 | 9.7 | ||
| Lead Acid Battery | Chemical | 0.03 | 0.09 |
|
3.18 |
|
Natural
gas |