India
South Africa
Japan
Reforming configurations
SMR Alone Configuration
Reactions (1), (3) and (4) take place in numbers of tubular reactor ("catalyst tube") containing reforming catalyst(s) and installed inside a radiation box of steam reformer furnace where fuel is combusted to supply reaction heat through external surface of the catalyst tubes. There are several convection coils which are equipped to recover the useful heat energy of the high temperature flue gas leaving the radiation box as shown in the figure below. They are steam-feed mixed gas heater, steam superheater, feedstock preheater for desulfurizer and combustion air preheater and so on.
Combined Reforming ("CR") Configuration
It is combination of SMR as a primary reformer and ATR as a secondary reformer in series or in a way where a part of feedstock is bypassing SMR to ATR depicted as follows. Theoretically, it is possible to take any combination of heat duty contributions for steam reforming and O2 reforming, and the contributions are determined considering economical viability with investment cost vs. operating cost.
Autothermal Reforming ("ATR") Configuration
All the Reactions (1), (2), (3) and (4) take place under fixed bed containing reforming catalyst(s) where reaction heat for the Reaction (1) and (3) is supplied by Reaction (2) and Reaction (4). No external heat is directly supplied to catalyst bed. Note that a feed gas preheater for desulfurizer and a steam - feed mixed gas heater for ATR have to be provided in order to obtain reasonable composition of syngas product. Usually direct fired heaters are used as such heating devices, and similar systems to steam reformer are applied for the corresponding flue gas heat recovery from the direct heater(s) illustrated as below.
Syngas Composition
Natural gas is catalytically converted to syngas consisting of H2 and CO in accordance with the reforming reaction (1) and (3). The reaction (1) and (3) are endothermic. In the bed of nickel catalyst for SMR, the syngas composition is also converging with the equilibrium of the CO shift reaction (4), an exothermic reaction, at the same time with the reaction (3). Syngas is also generated by POx in accordance with the reaction (2) , but m=1 and n=4 for CH4, in a noncatalytic POx reactor or in the oxidation zone of a catalytic ATR.
The above reaction (2) seems ideal if one considers methanol synthesis reactions (M1) and (M2) in the downstream as a user of the generated syngas. However, the generated H2 in the reaction (2) does react with O2 rapidly in accordance with a competing combustion reaction (5) below relatively against the successive POx reaction (2). On the other hand, the CO combustion reaction (6) below is said, in comparison, extremely slow and does not consume O2 so rapidly. The overall oxidation reaction in the POx system, which is addition of the reaction (2) and the reaction (5), is therefore described as the reaction (7) below.
H2 Combustion (rapid reaction)
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H2 + 1/2 O2 → H2O ΔH = -241.8 MJ/kmol (5)
CO Combustion (relatively slow reaction)
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CO + 1/2 O2 → CO2 ΔH = -283.0 MJ/kmol (6)
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CH4 Imcomplete Combustion (relatively rapid reaction)
CH4 + 3/2 O2 → CO +2 H2O ΔH = -549.8 MJ/kmol (7)
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Only after O2 is completely consumed by methane in accordance with the reaction (7), then the residual methane is steam-reformed in accordance with the reaction (1). The final Syngas composition leaving the POx system is determined by the equilibriums of the reaction (3) and the reaction (4).
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SMR is a most common practice to generate Syngas for methanol production and a 5,400 t/d plant is running as a today’s maximum capacity facility of a single box reformer furnace in Trinidad. Non-catalytic POx reactor does not usually use steam as a reforming agent and has not been used for methanol production until today, but Shell is operating it for GTL production in the SMDS project in Malaysia and the Pearl project in Qatar. The conventional CR process for methanol plant is a series integration of SMR with a secondary reformer using O2 as a reforming agent same as that for ammonia plant except for the use of pure O2 instead of air in ammonia plant. What one worries about is higher adiabatic flame temperature of pure O2 which may cause some mechanical trouble in internals of the pressure vessel while the reaction system is exactly same as ammonia. The CR-Bypass process was invented by Lurgi for saving process steam in SMR, and Air Liquide (ex Lurgi ) has a number of licensing experiences.
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The following table lists process technologies for syngas generation from natural gas currently seemed available for technology license in the world. According to each technology, the employed reactions and the reaction
mechanism are considerably different.
The mechanisms of oxidation (or combustion) reactions in the conventional CR process and in the CR-Bypass process are essentially different. The conventional CR process must ensure that all of O2 is consumed by the reaction (6) with H2. However, the CR-Bypass process uses more O2 than the amount which burns out H2 completely. As a result the reaction (7) will further take place to consume the residual O2 completely. JM’s LCM process is a sort of conventional CR process in view of the configuration of reforming reactions as
well as TOYO’s improved CR process (Reference: US Patent 60100303) integrated with a reformer exchanger in parallel with SMR. On the other hand, in KBR’s KRES configuration, the reactions proceed differently from the conventional CR process because of parallel running of ATR and exchanger reformer. Since there is in principle no H2 included in the feed gas, only the reaction (7) occurs in the combustion zone in ATR to consume O2. ATR is essentially a catalytic partial oxidation, and it is necessary to mix the feed gas with certain amount of steam to avoid carbon formation in the catalyst bed. According to Haldor Topøe, S/C ratio of 0.6 is sufficient to avoid this incident. JFE reported to have its successful operation of ATR with S/C ratio of 0.1 but CO2 addition for Syngas generation of [H2/CO] = 1 for DME production in a 100 t/d demonstration plant (Reference: Ohno, Y., “Update of JFE’s DME Activities”, EFI Members Conference “GAS-TO-MARKET”, Vancouver, B.C.,Canada, 20-23 September 2005).
Typpical syngas composition produced by each configulation is as follows (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). The compositions influence the rate of carbon formation and/or metal dusting in the heat exchangers equipped for waste heat recovery of process gas in the down stream of reformer.
