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Industrial reforming
Steam reforming of natural gas, 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.[1] 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
Additional hydrogen can be recovered by a lower-temperature gas-shift reaction with the carbon monoxide produced. The reaction is summarised 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.[2]
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.
A great deal of ethylene is produced by a non-catalytic process called "steam cracking" which cracks (i.e., reforms) large hydrocarbon molecules into smaller molecules). In the year 2003, 97,000,000 metric tons of ethylene (used to produce polyethylene and a host of other petrochemicals) were manufactured worldwide by the steam cracking of various hydrocarbons (methane, ethane, LPG, naphtha, and fuel oils.
Fueling fuel cells
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.
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 cells) do not have this problem.
- 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 membrane is poisoned by the carbon monoxide and its performance degrades.
- The catalyst is frequently very expensive.
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. The overall cost of making, transporting and storing the hydrogen fuel is also a key issue.
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
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).
References
- ^ George W. Crabtree, Mildred S. Dresselhaus, and Michelle V. Buchanan, The Hydrogen Economy, Physics Today, December, 2004 [1]
- ^ United States Geological Survey publication
See also
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