April 2024

Special Focus: Maintenance and Reliability

Case study: Causes and countermeasures of a wall tube leak accident by an SNCR solution of a CO boiler in an RFCC process

Residue fluid catalytic cracking (RFCC) is a process that cracks heavy oil into light oil, such as gasoline, with zeolite catalysts.

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Residue fluid catalytic cracking (RFCC) is a process that cracks heavy oil into light oil, such as gasoline, with zeolite catalysts. Treated atmospheric residue (AR) [low-sulfur bunker-C from the residue hydrodesulfurization (RHDS) process] and untreated AR are the primary feedstocks. The main equipment of the RFCC process consists of a reactor, a regenerator that reactivates catalysts and a carbon monoxide (CO) boiler that produces steam utilizing heat from regenerator flue gas. Here, a case study details an accident in the boiler by a pollution reduction apparatus.

Catalysts become less active in the reactor flow and reactivated in the regenerator by burning hydrocarbons surrounding the surface of catalysts. The spent (less-active) catalysts are exposed to combustion air for reactivation. At this moment, large amounts of CO are produced in the high-temperature flue gas before being introduced into the CO boiler. In the boiler, CO-rich flue gas is converted to carbon dioxide (CO₂) and an exothermic reaction occurs. The heat from the reaction is used to produce high-pressure steam; therefore, it is called a CO boiler.

Note: In accordance with the strengthened hazardous substances discharge regulation, nitrogen oxides (NO_x) generated by combustion in boilers or fired heaters must be reduced as low as possible. Ultra-low NO_x burners (ULNBs), selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) are widely installed for this purpose.

In the CO boiler discussed in the case study, SNCR was adopted. Unfortunately, a boiler water tube leak occurred after 1 yr of operation and caused the whole RFCC process to shut down. An SNCR system is an auxiliary system used to reduce NO_x in a boiler. An aqueous solution (ammonia or urea) is sprayed inside a boiler through nozzles (in this case, urea). Sprayed ammonia or urea converts NO_x into nitrogen (N₂) and water (H₂O).

Since it is in a high-temperature combustion chamber, the aqueous solution should evaporate immediately after spraying, so the risk of corrosion by the aqueous solution itself can be overlooked. If, for some reason, the sprayed solution remains wet without immediate evaporation, it will cause corrosion problems—this should be carefully assessed.

As government regulations on NO_x produced in boilers, fired heaters and furnaces intensify, further burner modifications, and SCR and SNCR installations will be required to meet environmental regulations. This case study provides implications to plants in which SNCR facilities are in operation or plan to install.

Accident summary.

In July 2009, a sudden decrease in steam production in the CO boiler was detected. As a result of an operational data investigation, it was confirmed that the metal temperature and flue gas temperature of the boiler superheater dropped sharply. This is a typical sign of a boiler water tube leak. The boiler was shut down to locate the leaking tube and conduct maintenance work. The impact was significant. The boiler system could not be isolated or bypassed for maintenance, so RFCC production was reduced by 50% for ~6 d, resulting in a couple of $MM lost.

On a positive note, it was possible to discharge the flue gas through an emergency stack to maintain partial operation of the RFCCU. If this happened today, the entire process would be shut down due to tighter emissions regulations.

The specifications of the CO boiler are shown in TABLE 1.

 

For reference, the operation scheme of the RFCC process and the process configuration of the CO boiler are detailed in FIG. 1. As previously mentioned, the RFCC reactor converts heavy oil to light oil by reacting with fluidized catalysts. The reaction creates high-pressure steam. This is shown in Eq. 1:

CO + 1/2 O₂   -->   CO₂ + heat (5,663 kcal/kg carbon)           (1)

 

NO_x are also generated by the reaction (combustion). To release combustion substances—including NO_x—into the atmosphere, they must be treated until they fall below regulated concentrations. An SNCR system was installed in the boiler of the case study because it requires less facility modification than ULNB and SCR systems and can be applied during operation. An ammonia solution and urea solution are used within SNCR systems as a reduction agent. A urea solution is advantageous in terms of storage and handling, so it was used in the boiler.

The steps for removing NO_x using a urea solution are discussed below. A urea solution is ideally hydrolyzed to ammonia (NH₃) and CO₂, as shown in Eq. 2:

CO(NH₂ )₂ + H₂O   -->   2NH₃ + CO₂           (2)

However, the hydrolyzation is not immediate—it occurs stepwise and produces intermediate compounds like isocyanic acid (HNCO) and isocyanate (NCO) (Eq. 3):

CO(NH₂ )₂   -->   HNCO + NH₃, HNCO + H₂O   -->   NH₃ + CO₂           (3)

Then, ammonia reacts with NO_x in the flue gas (Eq. 4):

6NO + 4NH₃   -->   5N₂ + 6H₂O           (4)

Overall, the NO_x reduction reaction using the urea solution is shown in Eq. 5:

6NO + 2CO(NH₂ )₂   -->   5N₂ + 4H₂O + 2CO₂           (5)

Here, attention must be paid to CO₂ generation as a corrosion factor.

CO boiler inspection results. 

The CO boiler was equipped with 10 SNCR spray nozzles inside the combustion chamber. The leak point was found on a wall tube under the No. 7 SNCR nozzle. The through-wall SNCR nozzle was in an area covered by refractory. Damaged refractory surface near the nozzle and a penetration hole on the wall tube were observed. The hole on the tube appeared as if corrosion penetrated from the outside to the inside of the tube, and significant thickness reduction was also found around the hole (FIG. 2)

FIG. 2. A corrosion hole on the wall tube under the No. 7 SNCR nozzle. Corrosion from outside the wall tube and severe localized corrosion around the hole are shown.

 

All areas around other nozzles were also inspected. After removing the refractory, corrosion was also found in the wall tubes near the No. 2, 3, 4 and 6 nozzles (TABLE 2).

 

 

The time for repairs were limited as a temporary shutdown is costly, especially due to the complexity of the configuration. The leaking tube was removed and replaced with a straight-wall tube. As a result, the No. 7 nozzle was not restored afterward. Build-up welding was applied to the rest of the tubes on the thinned area (FIG. 3).

 

FIG. 3. The before and after for each damaged tube.

 

Additionally, the chemical composition and microstructure of the corroded tube metal were examined, and the properties met the specification requirements of the tubes (carbon steel), shown in FIG. 4.

 

FIG. 4. The analysis result of corroded tubes. As a result, the normal carbon-steel fabricated in the structure of ferrite and pearlite phase. No oxidation defect was found.

ROOT CAUSE ANALYSIS

Damage mechanism. 

During the 2009 accident, the carbon-steel wall tube corroded from the outside, resulting in a through-hole and leaking pressurized water into the boiler chamber.

As a result of the investigation, the main corrosion mechanism of the tube was wet CO₂ corrosion. The analysis revealed that the urea solution sprayed from the SNCR nozzles hit the refractory and formed liquid droplets, and this aqueous solution penetrated the refractory under the nozzle and continuously wet the tube under the refractory, resulting in corrosion.

As explained earlier, urea solution generates NH₃ and CO₂ during hydrolysis, and the carbon-steel material is susceptible to CO₂ corrosion. The urea solution is sprayed in small droplets and should evaporate immediately inside the boiler combustion chamber at 900°C, so it seems it is unnecessary to consider wet CO₂ corrosion. However, CO₂ corrosion may occur when the sprayed urea interferes with an adjacent structure (as in this case) to create a corrosion-susceptible environment in which solution droplets are in continuous contact with the carbon steel.

American Petroleum Institute (API) Standard 571 specifies CO₂ corrosion that is consistent with this case:

“Carbon dioxide (CO₂) corrosion results when CO₂ dissolves in water to form carbonic acid (H₂CO₃). The acid may lower the pH and sufficient quantities may promote general corrosion and/or pitting corrosion of carbon steel.”

To assess this, the author’s company performed the injection test before adjusting the nozzle protrusion distance. As expected, it was confirmed that the sprayed solution hit the refractory around the nozzle tip and flowed down in an aqueous solution state. The protrusion distance of the nozzle that caused corrosion was too short and insufficient to avoid interference with the refractory—even the refractory was thicker than the drawing for construction. The tube leak scenario is shown in FIG. 5.

FIG. 5. Tube leak scenario.

 

To summarize, the sprayed urea solution interfered with the refractory and flowed down; the solution then penetrated the refractory and accumulated between the refractory and tubes, resulting in the corrosion of the carbon-steel tubes under the refractory.

Facility management issues (non-technical cause). 

The cause of the insufficient protrusion distance of the urea nozzle and the excessive refractory thickness were investigated.

First, it was determined that the nozzle length was designed poorly by the nozzle manufacturer. The manufacturer only considered high-temperature oxidation of the nozzle and designed the length to be as short as possible, and the refractory construction subcontractor misunderstood the construction drawings.

Second, the engineering contractor overlooked the risk of corrosion by an urea solution during design. As mentioned earlier, it was assumed that the urea solution would evaporate immediately after injection; therefore, CO₂ corrosion in a wet environment was not considered. Additionally, the SNCR was regarded as an auxiliary facility and, unfortunately, was excluded from the list of vital equipment for regular corrosion review after operation.

Actions taken after the accident. 

Following the accident, the leak tube was replaced, and the thinned tubes were repaired by weld build-up. Subsequently, the protrusion distance of the urea nozzles was rearranged to avoid interference, and the refractory at the bottom side of the nozzles were reconstructed according to the drawing. Finally, performance tests were conducted on each nozzle to check the spray pattern and interference (FIG. 6).

FIG. 6. Spraying test after repair.

Takeaways. 

NO_x removal facilities are considered auxiliary facilities and may not be closely reviewed in the engineering phase. To prevent CO₂ corrosion within the boiler and on fired heater tubes, it is vital to review the corrosion mechanism of the sprayed aqueous solution and identify the risk of it flowing down in a liquid state due to interference by any surrounding structures, such as the refractory. It is recommended to perform a spray test before operation to confirm that there is no interference.

This case study provides best practices to companies planning to install SNCR or SCR facilities to remove NO_x from boilers and heating furnaces, including those already in operation. HP

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