Steam reforming

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Steam reforming, sometimes called Fossil fuel reforming is a method for producing hydrogen or other useful products from hydrocarbon fuels such as natural gas. This is achieved in a processing device called a reformer which reacts steam at high temperature with the fossil fuel. The steam methane reformer is widely used in industry to make hydrogen. There is also interest in the development of much smaller units based on similar technology to produce hydrogen as a feedstock for fuel cells.[1] Small-scale steam reforming units to supply fuel cells are currently the subject of research and development, typically involving the reforming of methanol or natural gas[2] but other fuels are also being considered such as propane, gasoline, autogas, diesel fuel, and ethanol.[3]

History

Industrial 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.[4] 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.

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 summarized by:

CO + H2O ⇌ CO2 + H2

The first reaction is strongly endothermic (consumes heat), the second reaction is mildly exothermic (produces heat).

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.[5]

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%.[6]

Reforming for fuel cells

Advantages of reforming for supplying fuel cells

Steam reforming of gaseous hydrocarbons is seen as a potential way to provide fuel for fuel cells. The basic idea for vehicle on-board reforming 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] however the major market players discarded the approach of on-board reforming as impractical.[citation needed]

Disadvantages of reforming for supplying fuel cells

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. However, there is an active debate about whether using these fuels to make hydrogen is beneficial while global warming is an issue. Fossil fuel reforming does not eliminate carbon dioxide release into the atmosphere but reduces the carbon dioxide emissions as compared to the burning of conventional fuels due to increased efficiency.[8] However, by turning the release of carbon dioxide into a point source rather than distributed release, carbon capture and storage becomes a possibility, which would prevent the carbon dioxide's release to the atmosphere, while adding to the cost of the process.

The cost of hydrogen production by reforming fossil fuels depends on the scale at which it is done, the capital cost of the reformer and the efficiency of the unit, so that whilst it may cost only a few dollars per kilogram of hydrogen at industrial scale, it could be more expensive at the smaller scale needed for fuel cells.[9] Recently, a Polish company Bioleux Polska has been advertising renewable hydrogen (RH2) plasma reformers, producing RH2 at under $2 per kilogram ,[citation needed] and available for lightweight mobile applications using vegetable oil or glycerol as feedstock.

Current challenges with reformers supplying fuel cells

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 in the fuel will poison certain catalysts, making it difficult to run this type of system from ordinary gasoline. Some new technologies have overcome this challenge 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.

A recent development that employs a new combination of gold and iron oxide can reduce carbon monoxide levels to 20 parts per million in the presence of hydrogen and produce byproducts of carbon dioxide and water at much lower temperatures than conventional methods.[10]

Reforming for combustion engines

Flared gas and vented VOCs are known problems in the offshore industry and in the on-shore oil and gas industry, since both emit unnecessary greenhouse gases into the atmosphere.[11] Reforming for combustion engines utilizes steam reforming technology for converting waste gases into a source of energy.[12]

Reforming for combustion engines is based on steam reforming, where non-methane hydrocarbons (NMHCs) of low quality gases are converted to synthesis gas (H2 + CO) and finally to methane (CH4), carbon dioxide (CO2) and hydrogen (H2) - thereby improving the fuel gas quality (methane number).[13]

In contrast to conventional steam reforming, the process is operated at lower temperatures and with lower steam supply, allowing a high content of methane (CH4) in the produced fuel gas. The main reactions are:

Steam reforming:

CnHm + n H2O ↔ (n + m/2) H2 + n CO

Methanation:

CO + 3 H2 ↔ CH4 + H2O

Water-gas shift:

CO + H2O ↔ H2 + CO2

References

  1. ↑ Fossil fuel processor
  2. ↑ Commonly used fuel reforming today
  3. ↑ 2001-Fuels usable for fuel reforming
  4. ↑ 2004-George W. Crabtree, Mildred S. Dresselhaus, and Michelle V. Buchanan, The Hydrogen Economy
  5. ↑ 2005-United States Geological Survey publication
  6. ↑
  7. ↑ Advantage of fossil fuel reforming
  8. ↑ Fossil fuel reforming not eliminating any carbon dioxides
  9. ↑ A realistic look at hydrogen price projections
  10. ↑ Shodiya, T.; Schmidt, O.; Peng, W.; Hotz, N. (2013). "Novel nano-scale Au/α-Fe2O3 catalyst for the preferential oxidation of CO in biofuel reformate gas". Journal of Catalysis 300: 63. doi:10.1016/j.jcat.2012.12.027. 
  11. ↑ "Atmospheric Emissions". 
  12. ↑ "Wärtsilä Launches GasReformer Product For Turning Oil Production Gas Into Energy". Marine Insight. 18 March 2013. 
  13. ↑ "Method of operating a gas engine plant and fuel feeding system of a gas engine". 

External links

See also

Pollution
Renewable energy
Conservation

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