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How to produce syngas

Hydrogen production

Ammonia production

Methanol production

DME production

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Aid gas removal

Metal dusting

Reference:

Hirotani, K & K. Futamura: “Potential of Metal Dusting in Syngas for Methanol Production from Natural Gas”, Asian Nitrogen & Syngas International Conference, pp173-186, Kuala Lumpur, 9-11 October (2012)

 

Metal Dusting is a catastrophic phenomenon disintegrating structured metals and alloys into dust of fine particles composed of such metals and alloys. It typically occurs at temperatures of 400-850°C in syngas with certain conditions. In particular, recent concept on steam saving

Reference:

Hrivnák, I., L. Caprovic, G. Bakay and A. Bitter: “Metal Dusting of Inlet Tube Made of Alloy 800”, Kovove Mater.(Metallic Materials) 43, 2005, pp290–299

Pitting corrosion by metal dusting
Photo supplied courtesy by Dr. Ivan Hrivnák

methanol or GTL processes with oxygen reformer, which is most effectively installed to aim at syngas production with the H2/CO molar ratio of 2 for GTL plant, favors the gas mixture to be enriched with CO. Such Syngas with reduced S/C ratio may finally cause metal dusting.

MECHANISM OF METAL DUSTING AND CARBON FORMATION REACTIONS


Metal dusting corrosion appears in the form of pits as shown in the above figure and general metal wastage. There are two substantial issues in the phenomenon of metal dusting. The first one is carbon formation and subsequent carbon deposition on metal surface. The second one is the initiation of the metal dusting degradation of the alloy.

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Jones and Baumert (Reference: Jones, RT. and KL. Baumert: “Metal Dusting – An Overview of Current Literature,” Corrosion 2001, Paper No.
0132) reviewed literatures on metal dusting and are explaining very well the metal dusting mechanism consisting of the following micro-processes in order.

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  1. Carbon is formed from Syngas of higher carbon activity ac and supersaturates the metal at the metal surface.

  2. With alloys of Fe (and Co to a lesser degree), absorbed carbon forms carbides (Fe3C in the case of Fe -base alloys), which reduce the rate of carbon ingress into metal when it is formed.

  3. The continued charging of carbon into the metal supersaturates the carbide at the gas–metal interface. This causes instability and decomposition of the carbides back to the small pure-metal particles and carbon. Dissociated metal attaches to the basal planes of carbon (graphite) that are growing into un-decomposed metal carbide.

  4. The metal diffuses through the graphite lattice structure away from the arbide and agglomerates into nano-sized particles, which serve as catalyst site for further carbon deposition.

  5. In the case of Ni and Ni -base alloys, dissociation of Ni3C is immediate, because of its instability and graphite is formed directly from the carbon-saturated Ni.

  6. Transport of Fe and Ni atoms through the graphite is faster when the lattice planes of the graphite formed in the carbide-dissociation reaction are perpendicularly aligned to the metal surface as opposed to parallel alignment.

 

The following three reactions, (8) (9) and (10), are possibly leading to carbon deposition potentially causing metal dusting when carburization progresses with the deposited carbon on the metal surface as above.

Reaction-8
Reaction-9
Reaction-10

 

Boudouard reaction

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2 CO =  CO2  + C                                   ΔH =  -172.8 MJ/kmol                (8)

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Reduction of CO

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CO + H2  =  H2O  + C                            ΔH =   - 31.3 MJ/kmol                (9)

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Thermal cracking of CH4

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CH4 = 2 H2  + C                                      ΔH =     74.4 MJ/kmol              (10)

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The both reactions (8) and (9) are exothermic favorable at lower temperature and the reaction (10) is endothermic favorable at higher temperature which is important for carbon formation in the oxidation zone of ATR or POx reactor as well as at the stream of methane rich feedstock to or inside reforming reactor.

CARBON ACTIVITY

Defining Kp,b as the equilibrium constant of the reaction (8) at the temperature T and pi as partial pressure of the component i, if

then there is potential of carbon formation existing. In another word, when the carbon activity ac,b of the reaction (8) is defined as

ac,b,>1 is an absolutely necessary requirement for carbon deposition. Similarly,

are also defined for the reactions (9) and (10).

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The table below lists-up the equilibrium temperatures of the reactions (8), (9) and (10) respectively, having the compositions in the table for reforming configuration, i.e., at which temperatures each carbon activity sets across the unity of its axis. The figure illustrates an example for the carbon activities ac,b, ac,c, and ac,t respectively for the syngas produced by the SMR Alone configuration for methanol production calculated based on the composition in the said table which the cited reference shall be referred to on the data for  Kp,b, Kp,c, and Kp,t respectively.

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The Boudouard reaction (8) has a potential of carbon formation at the temperature less than 671°C and the reaction (9) has at the temperature less than 743°C for the case of SMR MeOH. Since the carbon activity is, on the contrary, always less than the unity for the reaction (10) in the temperature range below the exit of SMR, carbon is not formed by this reaction, the thermal cracking of residue methane. The equilibrium temperature of the reaction j (= b, c, t), Tj,e,  in the above table is corresponding to this temperature for each process configuration. 

 

ac,i  < 1 (i=b or c) truly an absolute indicator not to cause carbon formation in syngas stream. However, it would be practically impossible to design all the heat recovery equipment in the effluent streamsof reformer with ac,i  < 1. On the other hand, most of ammonia, methanol and hydrogen plants are no doubt operated under the condition with ac,i  > 1 in certan parts of the heat recovery equipment Even if there are some potentials of carbon formation in accordance with chemical equilibrium, metal dusting does not occur unless super-saturated condition of the deposited carbon toward forming metal carbide on the surface of metal material is maintained continuously. Therefore the authors of the referred paper believe that it must be possible to set up a certain criteria to be able to indicate the potential for metal dusting quantitatively. For more details on such criteria, namely RELATIVE RATE OF CARBON FORMATION, please read the original paper cited. The following figures are examples of their result.

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