Auto-thermal steam reforming

[Ujjwal Thakur]

Hello Everyone!!
I got a small question. So for the purpose of testing my experiment, I am building a small industrial scale reactor for steam reforming of pyrolysis gas in combination with partial oxidation with air. This test would be done in small scale with 25 m3/h of pygas, 30 m3/h of steam and around 10 m3/h of air . The steam and pygas is premixed at 400°C and would be entered in a spouted bed reactor and air will be entered from the bottom for getting a proper H2/CO ratio and for partial oxidation. My question is that, for auto-thermal reforming, is it necessary to have a burner, or the required temperature can be reached without a burner?

Thanks!

 

[georgeverghese]

No need for spark ignition if that is what you mean. You may use the burner as a mixing nozzle though for good mixing of the air - pygas/steam streams. Reaction may be slow at 400degC, but it should be faster as temperature picks up. Also depends on whether there is some catalyst action with the reactor bed material. The catalyst would then lower the activation temp to kickstart the reaction.

 

[Ujjwal Thakur]

Thank you for the response.
Yes, I meant the spark ignition.
Yes, I am using a dolomite catalyst for the tar conversion and syngas production. Since steam reforming is a endothermic process, I am also planning to partially oxidize the pygas to reach the desired temperature of 900°C. But I am ambigious that without any external heat source if I would be able to reach temperature of 900°C. I have not done any energy balance to support this. Do you have any suggestion about this problem, or is it possible to reach the desired temperature by POX?
Thank you!!!

 

[georgeverghese]

Agreed, since the steam reforming reaction is endothermic, and your feed temp is only 400degC, it does look you'll need to combust the reactants to get it up to 900degC. Temp control may be maintained by control of steam or air.

 

[Ujjwal Thakur]

Does that mean, I have to ignite the reactants?
I am providing air as a oxygen source ( in desired amount) to partially oxidize it.

 

[georgeverghese]

Yes. Though in theory, only part of the reactants need to be ignited to get the whole stream up to temp, it is not safe, since the rest of the stream will autoignite as the stream flammables concentration is most likely well above LFL. I am not familiar with this type of reactor, but you can see info on this on Google also. My experience has been with primary steam- NG nickel catalysed reformer ( where the stream is kept at >900degC by external heating), and a downstream shift gas reactor, which does not require any more heat. But your feed stream temp is too low to get good conversion. You wont need this air or ignition if you do external heating in a furnace.

 

[Ujjwal Thakur]

Thank you. Since its a test in pilot scale I have no external heat, the temperature is achieved by applying air in the dolomite bed. Spouted bed reactors are type of fluidized bed reactor and conical bottom for spout. Providing air to syngas+steam mixture without igniting would be safe or not?
Could there be chances of explosion?
The products gases are entered into cyclone, then cooled and entered into combustion chamber of pyrolysis reactor.
The air is entered in the catalyst bed upto a level where it can reach temperature upto 900°C as steam reforming is exothermic reaction. But the question can it react with the product gas in the outlet with the unreacted oxygen causing temperature increase?

 

[georgeverghese]

From info I see on the internet, autothermal reforming involves partial oxidation by combustion of the the entire feedstream to get the stream temp up to 900degC or so. Steam reforming reaction is endothermic, while POX is exothermic. Shift gas reaction is only mildly exothermic. It is essential that ALL the O2 in the air component in the mix stream is consumed by use of a good burner and good mixing is achieved so that localised CO2 formation / soot formation is minimised. Air is fed through a dedicated nozzle on the burner assembly.

Agreed, air should not be fed into the catalyst bed downstream of the flame.

 

[Ujjwal Thakur]

Since the temperature would be 900°C and yes the mixture will be above above LFL as you said, it will autoignite above the catalyst bed. The reactor is run under vacuum condition, does this auto-ignition can cause explosion?
Or if the oxygen is kept below the LOC, it would be safe.
In the spouted bed reactor the entry point for the reacting gases are from the bottom. So the pygas + steam is entered from the inner pipe and air is entered from the out pipe (as in burner) in the catalyst bed.

 

[Compositepro]

The energy released in the reaction is limited by the amount of oxygen present.

 

[georgeverghese]

Yes, if you have O2 concentration below the limit for combustion, that may be possible, but I wouldn't try this approach. In any case, the O2 level in this case may most likely be less than the amount required for the POX reaction to get the entire mix stream up to 900degC.
The entire stream has to ignited at the burner assembly, with O2 flowrate adequate for formation of CO only.

 

[Ujjwal Thakur]

I am in doubt about the chance of explosion of mixture.
What you said is absolutely correct. However, I have read researches about the catalytic partial oxidation where no burner is used. The problem here is I am using a spouted bed reactor( which is conical silo) the feed strea will enter the bottom of the conical part and produce a spout in the dolomit bed. So here the catalytic reaction of steam+pygas will take place which are initially at temperature about 300°C, but I need temperature upto 900°C to crack the tar and reform the long chain hydrocarbons to CO +H2. If I combust the gases below the reactor, it is not possible while there is dolomite bed in the conical bottom. The feed stream (Pygas+Steam) which is initially at 300°C has to be reformed to CO+H2 and tar cracking must take place. But the external heat requirement is very expensive for such a reactor. So when limited amount of air is provided the partial oxidation in the bed and above the bed will provide the temperature for the steam reforming. In addition, the partial oxidation is mildly exothermic. However, the problem is that if the autoignition at 900°C of the pygas+air+steam mixture might lead to an explosion or not?
The outlet gas enters a cyclone then condensor and then enters into a combustion chamber. The sample is taken before the combustion chamber for analysis.

 

[georgeverghese]

If you dont combust this stream, even if you get it up to 900degC some other way, the reactor contents will detonate, given there is O2 in the stream.

 

[Ujjwal Thakur]

Okay. Even if the feed air is below the sub-stoichoimetric level for partial oxidation?

 

[georgeverghese]

Yes, as long as the mix stream is within the LFL to UFL limits.

 

[Ujjwal Thakur]

Correct. It will lie within the limits and will partially oxidize, incomplete combustion generating heat (mildly exothermic). When this is coupled with highly endothermic tar cracking and steam reforming. The incomplete combustion must be so regulated just to reach upto 800-900°C, if I am not wrong. I read couple of publications where they have coupled steam reforming with catalytic partial oxidation to make the process energy efficient. But I am not sure if it works on a bigger scale.

 

[georgeverghese]

If you want to enable catalytic POX (without combustion), then there must be a catalyst that can lower the activation energy required for this reaction down to your 400degC or even lower ( and also considerably increase the kinetics of the forward reaction), else you wont be able to kickstart the net heat production to get the equilibrium reaction temp of 900degC safely. I have doubts if there is anything in this dolomite to enable this.