8 replies to this topic

[chemicooool]

-We operate NG based steam reformer for production of H2 and CO. The unit was started after taking one month shutdown and is being operated at turndown load.

-During visual inspection of reformer tubes, it was observed that the appearance of bottom half of some of the tubes (not in particular order) was bright orange (photos are attached). Such bright appearance was not noticed before. The color remains same throughout the bottom half portion of these tubes. Top half portion appearance is normal and according to early observations.

-The tube wall temperature measured with cyclops 390 L pyrometer indicates high TWT (tube wall temperatures) in these tubes as compared to the rest of the tubes. The high TWT observed are within the design limits. Our main concern is the appearance of these tubes, especially is bottom half portion.

 

Can anyone share their views on why there is such change in appearance?

 

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[PingPong]

The photos are not clear.

Moreover it is also not clear what kind of steam reformer furnace this is, designed by which licensor; number, location and type of burners, type of fuel(s), et cetera.

Some licensor designs use burners at the top firing down, some  use side wall burners, some use floor and terrace burners.

 

In any case: start with a check of present operation against original design with respect to throughput, feedstock composition, conversion, COT, COP, S/C-ratio, fired duty, kind of fuel(s), excess air, et cetera.

 

High TMT is likely caused by too high heat flux.

That can be caused by uneven distribution of fuel over the various burners, resulting in release of too much duty by certain burners resulting in too high TMT at parts of adjacent coils.

Wrong shape of flames (length, width) may also be a cause in furnaces with burners firing upward or downward.

 

Operating at a higher-than-design coil outlet temperature (COT) will also increase TMT.

 

Operating at a higher-than-design throughput and/or higher-than-design S/C-ratio will increase fired duty for a given COT,

and consequently result in higher TMT due to higher heat flux.

Edited by PingPong, 23 July 2021 - 06:29 AM.

[Pilesar]

A calibrated infrared pyrometer is better at measuring temperature than the human eye. I have wondered also at how the appearance of reformer tubes change and can only speculate. Under turndown conditions, the residence time in the radiant section of the firebox is higher. I don't know how that affects tube color for sure, but heat removal is slower at lower flowrate. The endothermic reaction inside the tubes perhaps is different also. I have witnessed tubes seeming to darken as velocity is increased with throughput. Another possible cause of the color difference might be on the combustion side with the burners having different operation characteristics at reduced firing. I noticed during startup that the firebox always seems so different inside. The atmosphere inside the firebox is not air and I know the firebox appearance also changes depending on the fuel used. Continue observations and maybe you can figure out the cause and how to confirm it with some sort of proof.

[Pilesar]

If you replaced reformer catalyst during your last turnaround, you may have gotten a bad batch. I don't see in the picture that the tubes have a mottled appearance, but poor catalyst performance is a possibility.

[Pronab]

If you replaced reformer catalyst during your last turnaround, you may have gotten a bad batch. I don't see in the picture that the tubes have a mottled appearance, but poor catalyst performance is a possibility.

Should you change Catalysts in last TA, I believe you already checked individual tube differential pressure after catalyst loading and ideally they should be same. Verify the pressure drop across the reformer. If there is a restriction of flow, tube wall temperature will increase, or there might be carbon deposit. Maintain higher side of Steam:carbon ratio and observe pressure drop across reformer. Make sure you use pyrometer after calibration.

[chemicooool]

The photos are not clear.

Moreover it is also not clear what kind of steam reformer furnace this is, designed by which licensor; number, location and type of burners, type of fuel(s), et cetera.

Some licensor designs use burners at the top firing down, some  use side wall burners, some use floor and terrace burners.

 

In any case: start with a check of present operation against original design with respect to throughput, feedstock composition, conversion, COT, COP, S/C-ratio, fired duty, kind of fuel(s), excess air, et cetera.

 

High TMT is likely caused by too high heat flux.

That can be caused by uneven distribution of fuel over the various burners, resulting in release of too much duty by certain burners resulting in too high TMT at parts of adjacent coils.

Wrong shape of flames (length, width) may also be a cause in furnaces with burners firing upward or downward.

 

Operating at a higher-than-design coil outlet temperature (COT) will also increase TMT.

 

Operating at a higher-than-design throughput and/or higher-than-design S/C-ratio will increase fired duty for a given COT,

and consequently result in higher TMT due to higher heat flux.

-The reformer is a top fired reformer with burners on roof. There are 2 rows of tubes and 3 rows of burners with all burners in service.

 

-The fuel used in mix of NG (98% CH4) and waste hydrogen gas (75% H2 and 25% CH4) coming from hydrogen purification unit. The plant is operated at turn down load.

 

-The reformer is being operated as per the recommended procedures by the licensor without any deviations since past 8 years. All burners are in service and firing is adjusted by adjusting fuel header pressure. The heat input is same as there are no change in firebox temperature parameters or any convection zone parameters.

 

- Fresh catalyst replacement was done in 2020. The plant has run for a year and then was shutdown for planned annual maintenance of whole complex. 

 

- There are no restriction in fuel path that gets distributed to burners. Only isolation valves of burners are there to isolate individual burners. The flame behavior is also normal. 

 

- Let me reiterate that the tube wall temperature on these tubes are within the design range. The reformed gas outlet temperature and the transition zone (flue gas temperature from radiant box going to convection box)  temperature are as per the turndown conditions. The methane slip also maintained at 3.8%v/v.

 

-The baffling thing is the different color of some of these reformer tubes even when all other conditions are within operating range

[chemicooool]

If you replaced reformer catalyst during your last turnaround, you may have gotten a bad batch. I don't see in the picture that the tubes have a mottled appearance, but poor catalyst performance is a possibility.

- Fresh catalyst replacement was done in 2020. The plant has run for a year and then was shutdown for planned annual maintenance of whole complex. 

 

-The reformed gas outlet temperature and the transition zone (flue gas temperature from radiant box going to convection box)  temperature are as per the turndown conditions. The methane slip also maintained at 3.8%v/v.

 

-The baffling thing is the different color of some of these reformer tubes even when all other conditions are within operating range

[Pilesar]

I can't tell from your posts whether you consider the red tubes a real problem or just an artistic preference. If a real problem, then I suspect the reaction is not as active as it needs to be to draw off the heat. Is there a chance the catalyst has sulfur poisoning?

[MHS9]

In a top firing arrangement, TWT inherently higher at the bottom as temperature increase from top to bottom. However, I’m assuming your concern is that TWT is higher than normal.
Considering the higher TWT observed at the bottom only with, you might want to check the flue gas path. Check the flue gas coffin and tunnels slut opening with the design as if not done correctly (ex. Some sluts were closed during TA), this might creat a ”high traffic” of flow pattern near certain area.

In addition look if firebox refractory was changed during 2020 TA, if yes then review the material, conductivity. Are the tubes with the high bottom TWT in the outer rows? Radiation from refractory to tubes is possible.

Also consider what mention by other regarding operating at turndown condition. Is the fuel to air ratio on target? Does operating at turndown condition increase the waste hydrogen gas from PSA in the fuel mixture which skew the fuel calorific value?

If you check all the above, you might want to ask your licenser to help and perform firing/flue gas dynamic modeling which MIGHT help to identify the root cause.

Edited by MHS9, 16 August 2021 - 12:00 PM.