Steam Methane Reforming & Methane Reformer

Steam Methane Reforming

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Steam Methane Reforming

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Steam Methane Reforming & Methane Reformer

Steam reforming, hydrogen reforming or catalytic oxidation, is a method of producing hydrogen from hydrocarbons. 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.

A methane refomer is a device used in chemical engineering, which can produce pure hydrogen gas from natural gas using a catalyst. There are two natural gas reformer technologies — autothermal reforming (ATR) and steam methane reforming (SMR). Both methods work by exposing natural gas to a catalyst (usually nickel) at high temperature and pressure.

Steam Methane Reforming

1 Industrial Reforming
2 Fueling fuel cells
3 The process
4 References
5 See also

Methane Reformer

1 Steam reforming
2 Autothermal reforming
3 Advantages and disadvantages
4 See also
5 References
6 External links

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.

CH₄ + H₂O → CO + 3 H₂

Additional hydrogen can be recovered by a lower-temperature gas-shift reaction with the carbon monoxide produced. The reaction is summarised by:

CO + H₂O → CO₂ + H₂

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, there was 97,000,000 metric tons of ethylene (used to produce polyethylene and a host of other petrochemicals) 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.
The carbon monoxide (CO) produced by the reactor poisons the fuel cell, making it necessary to include complex CO-removal systems.
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

The chemical reactions that take place are:

C_nH_m + 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.

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

Steam reforming

SMR uses an external source of hot gas to heat tubes in which a catalytic reaction takes place that converts steam and lighter hydrocarbons such as natural gas (methane) or refinery feedstock into hydrogen and carbon monoxide (syngas). Syngas reacts further to give more hydrogen and carbon dioxide in the reactor. The carbon oxides are removed before use by means of pressure swing adsorption (PSA) with molecular sieves for the final purification. The PSA works by absorbing all impurities from the syngas stream to leave a pure hydrogen gas.

Autothermal reforming

Autothermal reforming (ATR) uses oxygen and carbon dioxide or steam in a reaction with methane to form syngas. The reaction takes place in a single chamber where the methane is partially oxidized. The reaction is exothermic due to the oxidation. When the ATR uses carbon dioxide the H₂:CO ratio produced is 1:1; when the ATR uses steam the H₂:CO ratio produced is 2.5:1

The reactions can be described in the following equations, using CO₂:

2CH₄ + O₂ + CO₂ $\rightarrow$ 3H₂ + 3CO + H₂O + Heat

And using steam:

2CH₄ + O₂ + H₂O $\rightarrow$ 5H₂ + 2CO

The outlet temperature of the syngas is between 950-1100 C and outlet pressure can be as high as 100 bar.^[1]

The main difference between SMR and ATR is that SMR uses no oxygen. The advantage of ATR is that the H₂:CO can be varied, this is particularly useful for producing certain second generation biofuels, such as DME which requires a 1:1 H₂:CO ratio.

Advantages and disadvantages

The capital cost of steam reforming plants is prohibitive for small to medium size applications because the technology does not scale down well. Conventional steam reforming plants operate at pressures between 200 and 600 psi with outlet temperatures in the range of 815 to 925 C. However, analyses have shown that even though it is more costly to construct, a well-designed SMR can produce hydrogen more cost-effectively than an ATR.^[2]