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Steam reforming

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See also: Methanation

Steam reforming is a method for producing hydrogen, carbon monoxide, 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, but other fuels are also being considered such as propane, gasoline, autogas, diesel fuel, and ethanol.[2][3]

Industrial reforming

Main article: Methane reformer

Steam reforming of natural gas - sometimes referred to as steam methane reforming (SMR) - is the most common method of producing commercial bulk hydrogen at about 95% of the world production[4][5] of 500 billion m3 in 1998.[6] Hydrogen is used in the industrial synthesis of ammonia and other chemicals.[7] 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

Catalysts with high surface area to volume ratio are preferred because of diffusion limitations due to high operating temperature. Examples of catalyst shapes used are spoked wheels, gear wheels, and rings with holes. Additionally, these shapes have a low pressure drop which is advantageous for this application.[8]

Additional hydrogen can be recovered by a lower-temperature gas-shift reaction with the carbon monoxide produced, in the presence of a copper or iron catalyst. The reaction is summarized by:

CO + H2O ⇌ CO2 + H2

The first reaction is strongly endothermic (consumes heat, ΔHr= 206 kJ/mol), the second reaction is mildly exothermic (produces heat, ΔHr= -41 kJ/mol).

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 144 million metric tonnes in 2014.[9]

This steam reforming 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.

Steam reforming of natural gas is approximately 65–75% efficient.[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

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;[14] however the major market players discarded the approach of on-board reforming as impractical.[citation needed] (At high temperatures see above).

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 and nearly eliminates carbon monoxide emissions as compared to the burning of conventional fuels due to increased efficiency and fuel cell characteristics.[15] 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.[16]

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.

See also

References

  1. ^ "Fossil fuel processor".
  2. ^ Wyszynski, Miroslaw L.; Megaritis, Thanos; Lehrle, Roy S. (2001). Hydrogen from Exhaust Gas Fuel Reforming: Greener, Leaner and Smoother Engines (PDF) (Technical report). Future Power Systems Group, The University of Birmingham.
  3. ^ "Commonly used fuel reforming today".
  4. ^ Ogden, J.M. (1999). "Prospects for building a hydrogen energy infrastructure". Annual Review of Energy and the Environment. 24: 227–279. doi:10.1146/annurev.energy.24.1.227.
  5. ^ "Hydrogen Production: Natural Gas Reforming". Department of Energy. Retrieved 6 April 2017.
  6. ^ Rostrup-Nielsen. "Large-scale Hydrogen Production" (PDF). Haldor Topsøe. p. 3. The total hydrogen market was in 1998 390·10^9 Nm3/y + 110·10^9 Nm3/y co-production.
  7. ^ Crabtree, George W.; Dresselhaus, Mildred S.; Buchanan, Michelle V. (2004). The Hydrogen Economy (PDF) (Technical report).
  8. ^ Reimert, Rainer; Marschner, Friedemann; Renner, Hans-Joachim; Boll, Walter; Supp, Emil; Brejc, Miron; Liebner, Waldemar; Schaub, Georg (15 October 2011). Ullman's Encyclopedia of Industrial Chemistry. Gas Production, 2. Processes: Wiley. {{cite book}}: |access-date= requires |url= (help)CS1 maint: location (link)
  9. ^ Nitrogen (Fixed)—Ammonia (PDF) (Report). United States Geological Survey. January 2016.
  10. ^ "Hydrogen Production – Steam Methane Reforming (SMR)" (PDF), Hydrogen Fact Sheet, archived from the original (PDF) on 4 February 2006, retrieved 28 August 2014
  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".
  14. ^ Advantage of fossil fuel reforming
  15. ^ Fossil fuel reforming not eliminating any carbon dioxides
  16. ^ A realistic look at hydrogen price projections
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