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Is there an energy related reason why, in the production of ammonia, air is not introduced in the primary reformer? In this case we don't need the second reformer. I could think of a few reasons like the fact that in the secondary reformer hydrogen gas is needed to react with the oxygen to water, which can react with $\text{CH}_4$.

But this is not really energy related?

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  • $\begingroup$ You're more likely to get answers if you write down the chemical reaction steps to explain what you think is the added value of what you propose. $\endgroup$ – Han-Kwang Nienhuys May 31 '16 at 19:07
  • $\begingroup$ Are you talking about the Bosch Haber process? If so you don't want air (oxygen) in the later stage. It would burn the hydrogen. You need the second stage, nitrogen is added then, and this nitrogen is separated from air. $\endgroup$ – Andy Jun 1 '16 at 10:42
  • $\begingroup$ I am not talking about the partial oxidation route, but I am talking about the steam reforming route. What I mean is: why can't you add the air in the primary reformer? $\endgroup$ – Thomas Jun 1 '16 at 21:02
  • $\begingroup$ I would say you can't add air in the steam reforming process simply because you'll burn the methane. (the reaction takes place at high temperatures from what I remember.) I don't understand why you would want to add air anyway... $\endgroup$ – Andy Jun 2 '16 at 7:46
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This report from Rice University covers design and economics of synthesis gas reformers.

Reaction 1: $CH_4+H_2O<->CO+3H_2$ (Steam reforming)

Reaction 2: $CO+H_2O<->CO_2+H_2$ (Water gas shift)

Reaction 3: $CH_4+3/2O_2<->2H_2O+CO$ (Combustion)

Reactions 1 & 2 are endothermic and occur in the primary reformer. Reaction 3, the combustion reaction, is exothermic and occurs along with reactions 1 and 2 in the secondary reformer.

Among many other reasons, air is not introduced in the primary reformer so that we get the most methane conversion to hydrogen with the smallest reformer possible. The link above explains sizing considerations between primary and secondary reformers.

The secondary reformer is a much simpler vessel and has the primary purpose to burn up extra methane that has slipped out of the primary reformer. The heat from the exothermic combustion reaction is recovered and used for the primary reforming.

To answer the air question from the comments, there are two ways to get nitrogen into the process. One method is using an air separation unit. The other is to put air in the secondary reformer, combust the methane and oxygen, and keep the left over nitrogen in the syngas for ammonia conversion.

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  • $\begingroup$ Ok thank you, I know all the rest. But we thought the professor spoke of a energy related reason. But apparently it is a matter of sizing. $\endgroup$ – Thomas Jun 5 '16 at 10:42

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