Fossil fuel reforming
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Fossil fuel reforming, hydrogen reforming or catalytic oxidation, is a method of producing useful products, such as hydrogen or ethylene from fossil fuels. Fossil fuel reforming is done through a fossil fuel processor or reformer.[1] At present, the most common fossil fuel processor is a steam reformer. This conversion is possible as hydrocarbons contain much hydrogen. The most commonly used fossil fuels for reforming today are methanol and natural gas[2], yet others as propane, gasoline, autogas, diesel fuel, methanol and ethanol are also being looked into for reforming.[3] During the conversion, the leftover carbon is released into the atmosphere.[4] On an industrial scale, it is the dominant method for producing hydrogen. Small-scale steam reforming units are currently subject to scientific research, as way to provide hydrogen to fuel cells.
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[edit] History
[edit] Industrial reforming
[edit] Steam methane reforming
- Main article: Methane reformer
Steam reforming of natural gas or syngas sometimes referred to as steam methane reforming (SMR) is the most common method of producing commercial bulk hydrogen as well as the hydrogen used in the industrial synthesis of ammonia. It is also the least expensive method.[5] At high temperatures (700 – 1100 °C) and in the presence of a metal-based catalyst (nickel), steam reacts with methane to yield carbon monoxide and hydrogen. These two reactions are reversible in nature.
Additional hydrogen can be recovered by a lower-temperature gas-shift reaction with the carbon monoxide produced. The reaction is summarized by:
The United States produces nine million tons of hydrogen per year, mostly with steam reforming of natural gas. The worldwide ammonia production, using hydrogen derived from steam reforming, was 109 million metric tonnes in 2004.[6]
This SMR process is quite different from and not to be confused with catalytic reforming of naphtha, an oil refinery process that also produces significant amounts of hydrogen along with high octane gasoline.
The efficiency of the process is approximately 65% to 75%.
[edit] Advantages
Steam reforming of liquid hydrocarbons is seen as a potential way to provide fuel for fuel cells. The basic idea is that for example a methanol tank and a steam reforming unit would replace the bulky pressurized hydrogen tanks that would otherwise be necessary. This might mitigate the distribution problems associated with hydrogen vehicles.[7]
Another advantage is that hydrogen fuel stations are still not widespread. Fossil fuel reforming allows the use of existing fuels.
[edit] Disadvantages
The reformer–fuel-cell system is still being researched but in the near term, systems would continue to run on existing fuels, such as natural gas or gasoline or diesel, but there is an active debate about whether using these fuels to make hydrogen is beneficial, when global warming is such an issue. Fossil fuel reforming does not eliminate any carbon dioxides of being released into the atmosphere when compared to the burning of conventional fuels. [8] The overall cost of making, transporting and storing the hydrogen fuel is also a key issue.
[edit] Current problems
However, there are several challenges associated with this technology:
- The reforming reaction takes place at high temperatures, making it slow to start up and requiring costly high temperature materials.
- Sulfur compounds present in the fuel poison certain catalysts, making it difficult to run this type of system from ordinary gasoline. Some new technologies have overcome this challenge, however, with sulfur-tolerant catalysts.
- Low temperature polymer fuel cell membranes can be poisoned by the carbon monoxide (CO) produced by the reactor, making it necessary to include complex CO-removal systems. Solid oxide fuel cells (SOFC) and Molten carbonate fuel cells (MCFC) do not have this problem, but operate at higher temperatures, slowing start-up time, and requiring costly materials and bulky insulation.
- The thermodynamic efficiency of the process is between 70% and 85% (LHV basis) depending on the purity of the hydrogen product.
- The biggest problem for reformer based systems remains the fuel cell itself, in terms of both cost and durability. The catalyst used in the common polymer-electrolyte-membrane fuel cell, the device most likely to be used in transportation roles, is very sensitive to any leftover carbon monoxide in the fuel, which some reformers do not completely remove. The anode catalyst is poisoned by the carbon monoxide and the fuel cells performance degrades.
- The catalyst in low temperature fuel cells is based on platinum, and is hence very expensive. A typical automotive fuel cell stack prototype (100kW) contains 20-30g of platinum metal in the form of nano-particles supported on carbon black.
[edit] The process
Some of the chemical reactions that can take place are:
- CnHm + n H2O → n CO + (m/2 + n) H2
- CO + H2O → CO2 + H2 (water gas shift reaction)
The produced carbon monoxide can combine with more steam to produce further hydrogen via the water gas shift reaction. Other reactions (some undesirable, like coke formation) can take place if local conditions are favorable.
The first reaction is endothermic (consumes heat), the second reaction is exothermic (produces heat).
The process produces 2.51 times as much CO2 by mass as it does H2.
[edit] References
- ^ Fossil fuel processor
- ^ Commonly used fuel reforming today
- ^ Fuels usable for fuel reforming
- ^ Leftover carbon released into the atmosphere
- ^ George W. Crabtree, Mildred S. Dresselhaus, and Michelle V. Buchanan, The Hydrogen Economy, Physics Today, December, 2004 [1]
- ^ United States Geological Survey publication
- ^ Advantage of fossil fuel reforming
- ^ Fossil fuel reforming not eliminating any carbon dioxides
[edit] External links
- "New catalyst boosts hydrogen as transport fuel". By Alok Jha. August 21, 2008. The Guardian.
- "Hydrogen Production - Steam Methane Reforming (SMR)"
[edit] See also
- Biogas
- Cracking (chemistry)
- Hydrogen technologies
- Lane hydrogen producer
- PROX
- Reformer sponge iron cycle
- Timeline of hydrogen technologies
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