By Erwin Platvoet, Chief Technology Officer
Part 3 up next: Burner fouling!
If you didn’t read Part 1, catch up here!
Exterior Fouling of Radiant Process Tubes
The main fouling mechanism on the fireside is corrosion fouling. According to the National Board of Boiler and Pressure Vessel Inspectors, all fossil fuels, with the possible exception of natural gas, contain constituents that promote corrosion on the fireside of boiler components. The bad actors are compounds of sulfur, vanadium, and sodium; but in the case of municipal-refuse boilers, chlorine is an increasing concern. In fired process heaters the main problem is hot fuel ash corrosion that occurs when firing heavy fuel oil.
Hot Fuel Ash Corrosion
Hot fuel ash corrosion is an accelerated form of corrosion in which molten sulfate salts form a film that destroys the normal protective oxide layer. The problems occur with fuel oils containing sodium and/or vanadium and sulfur (0.6 – 3.6 wt%). Salts like Na2SO4 form at high temperature and condense on the tubes, causing rapid metal consumption at about 550 °C. Reducing conditions will exacerbate fuel-ash corrosion. The presence of carbon monoxide and/or unburned carbon and hydrogen sulfide promote the formation of metallic sulfides. Iron sulfide, for example, is inherently less protective than iron oxide. Sulfides tend to be less protective because they are porous and less firmly attached to the steel.
Figure 6 – Hot fuel ash deposition and corrosion
Corrosion rates can be exceedingly high. In boilers, where this type of corrosion is more common, carbon steel wastage rates of about 1/2 inch per year (failure in less than 2,000 hours of operation) are known.(source: https://www.nationalboard.org/index.aspx?pageID=164&ID=196)
High Temperature Oxidation
High temperature oxidation is a corrosion process that occurs when tubes are operated above their limits. The reasons for overheating can be flame impingement, overfiring of the heater or incorrect selection of tube metallurgy for the service. The resulting oxidation layer (“scale”) will result in tube wall thinning and can reduce the local absorbed heat flux by half:
- the lower emissivity of the scale reduces the radiant absorbed heat flux
- the thermal conductivity of the scale is of the same order of magnitude as coke and therefore has a comparable effect on the overall thermal resistance
- the problems become exponentially worse if the oxide layer loses contact with the tube and prevents any heat transfer by convection, conduction, or radiation
Each cycle of scale formation and removal reduces the tube wall thickness until the tube is too thin to contain the fluid pressure, and failure occurs. The problem can be detected in several ways:
- visual tube inspection during outage. When inspecting the radiant tubes, metal oxides are sometimes confused with combustion deposits. Since oxide scales are magnetic, they can be distinguished from combustion deposits with a magnet.
- IR thermography when the heater is in operation. It can be used to compose a tube temperature map, where areas of high temperature can be the result of internal/external fouling, an oxidation layer or a combination of these.
- CFD can be used to model the temperature patterns inside the heater and demonstrate high local fluxes that will result in high metal and film temperature.
- Skin thermocouples can identify a scaling trend, but only if they are installed in the correct location. Scaling can be a very local problem depending on the root cause of the high temperatures.
Figure 8 – flame impingement, predicted by CFD (left) and actual operation (right)
Figure 9 – IR maps of radiant coil Figure 10 – Scale on radiant tube
(https://integratedglobal.com/industries/oil-gas/cru-catalytic-reformer-heaters/)
Mitigation / Prevention
High temperature oxidation can be prevented by reducing the hot spots on the coils. Flame interactions that result in impingement and hot spots on the tubes can be prevented by changing burner design or burner layout inside the firebox, or by adding the patented Xceed technology developed by XRG.
Tube metallurgy can be upgraded to higher chrome and nickel alloys to better withstand high temperature oxidation. While a chromium-rich oxide layer mainly accounts for the corrosion resistance of stainless steels, the addition of nickel improves the creep resistance of austenitic steels. Nickel also promotes the stability of the protective oxide film and reduces spalling during thermal cycling.
While cleaning tubes during outages is easiest and most effective, severe scaling and fouling may have to be removed while the furnace remains online. Specialized companies have developed methods to clean radiant tubes, as shown in Figure 11.
Figure 11 – Online chemical blasting of radiant tubes (https://www.furnace-solutions.com)
Ceramic coatings can be applied to the exterior surface of the tubes to prevent oxidation. For example, Cetek Ceramic Coatings provide a durable, protective, thin-film layer on the surfaces of process tubes which prevents oxidation, corrosion, and carburization of the metal and maintains the tube thermal conductivity coefficient close to new tube conditions.
Convection Fouling
To maximize the heat transfer in the convection bank of a heater, the tubes are fitted with radial fins of up to 1 inch high, spaced at maximum 5 fins per inch. The dense spacing makes the fins great trapping sites for refractory fiber, ash from oil combustion, dust and sand.
Figure 12 – fouled fins
Even with a small layer of deposits, the heat transfer efficiency is severely reduced (Figure 13).
Convection section fouling can be detected as follows:
- High convection pressure drop due to the accumulation of fouling on the fins
- The heater can become draft limited when the convection section pressure drop exceeds the draft capacity of the stack
- Low firebox oxygen, even with the stack damper full open
- Low crossover temperature due to the loss of heat transfer in the convection section
- High convection section exit temperature
- High stack temperature
Figure 13 – Finned tube efficiency vs deposit thickness (M. Watson – AIChE, 2015)
The risks of excessive convection fouling are:
- Running the burners out of oxygen, accumulation of unburned hydrocarbons in the firebox
- Increased flue gas temperature can overheat tube supports, fins, and any downstream component that is not rated for such high temperatures
- Low fuel efficiency, high fuel consumption
- Heater throughput becomes limited
Convection Bank Cleaning
Various methods are available to clean the convection bank tubes. Their efficiency depends on the ability to reach every part of the fouled surface.
Figure 15 – Convection tubes before and after cleaning
Water Wash
Water washing can damage refractory and produce cement like fouling deposits on and between fins. Tarp systems are required to catch the water. Cleaning progress can be measured by sampling the water and checking its color after each cycle. Detergent can be added to improve cleaning efficiency.
Figure 16 – Convection section water wash
CO2 (Dry Ice) Pellets
For efficient contamination removal, the pellets system shoots small dry ice pieces at the cleaning target. This system is designed to clean by applying thermo-mechanical impact shock.
Sand Blasting
Like water washing, requires collection and disposal. Abrasive methods can damage fins and refractory.
Robotic Cleaning
Self-propelled robots move along the tubes to clean the surface.
Figure 17 – robotic cleaning (www.TubeTech.com)
Soot Blowers
Fixed or retractable system of lances that periodically clean the tubes by injecting steam or high-pressure air from opposing nozzles while the furnace remains online. Typically, soot blowing is only effective on softer types of fouling (i.e.,“soot”).
Figure 18 – Soot blower module
The appropriate cleaning method is selected based on these considerations:
- Access
To allow proper access to the tubes, be sure to provide cleaning lanes in the convection section. The cleaning lane height should be at least 2 feet. The maximum vertical coverage of a lane should not exceed three tube rows. Make sure they cover all rows, including shield and future rows.
- Efficacy / Time
While the online sootblowers require much less time than off-line cleaning methods, they are not nearly as capable of removing anything other than soft types of fouling like ash and soot. Since robotic cleaning is less dependent on the number of access doors, its cleaning efficiency is typically the best of all methods.
- Disposal
Compared to water washing and sand blasting, soot blowing, and dry ice pellets do not require any tarps to catch and dispose of the effluents.
- Erosion
Abrasive cleaning methods such as dry ice and sand blasting may damage refractory and fins.
- Safety
Some end users have concerns with accumulation of CO2 and asphyxiation risks when dry ice pellets are used.
- Cost
While it is the most efficient, robotic cleaning is often the most expensive cleaning method.
Sulfuric Acid Corrosion
Another potential source of convection fouling is sulfuric acid corrosion. Any sulfur in the fuel forms sulfur oxides in the flame. For example, dihydrogen sulfide reacts with oxygen to form sulfur dioxide as follows:
H2S + 1.5 O2 -> SO2 + H2O
The sulfur dioxide then reacts with excess oxygen to make sulfur trioxide:
SO2 + 0.5 O2 <- -> SO3
The SO3 then condenses as sulfuric acid:
SO3 + H2O ->H2SO4
Even at low sulfur concentrations, the sulfuric acid dewpoint is well over 110°C / 230°F, as shown in Figure 18:
Figure 19 – General relationship between sulfur content and acid dewpoint (source: API 560 – Annex)
The effects of sulfuric acid corrosion can be dramatic:
Figure 20
Disposing of the cleaning effluent poses another problem, since water washing will result in a potentially strong sulfuric acid.
Studded tubes are used in applications that require frequent cleaning such as oil firing. They are more resistant to corrosion, such as sulfuric acid. While they are more resistance to aggressive and frequent cleaning methods, opinions differ whether they are easier to clean than solid fins when the studs are arranged in staggered fashion.
Figure 14 – Fouling in studded tubes
SCR / APH Fouling
The deNOx reactor and air preheater are located downstream of the convection section and suffer from the same types of fouling as the finned tubes:
- Particle deposition of ash, soot, refractory fibers, and dust
- Sulfuric acid corrosion
- Fouling by ammonia salts
Since the air preheater is usually much cooler than the convection tubes, sulfuric acid corrosion is a bigger problem. It can be challenging to design the system so that the heat exchange surface is well above the acid dewpoint (ADP). The problem can be severe in plate type exchangers where plate thickness is in 1 – 2 mm and corrosion quickly leads to holes and leakage between the air and flue gas side. It is important to monitor the cold corners using thermocouples or IR cameras. Use cold air bypass or hot air recirculation to maintain the cold corner above the ADP. If steam or water is available, a small heat exchanger (‘calorifier’) can be used to preheat the air before entering the APH. All these methods come with an efficiency penalty, so they should be used in moderation.
Figure 21 – Sulfuric acid corrosion in plate type APH
If an SCR is used to reduce the NOx in the flue gas, a fouling problem may occur. Sulfur in the fuel is converted to ammonia salts as follows:
First, SO2 to SO3 conversion is catalyzed by V2O5 catalyst. From the previous discussion we know that SO3 reacts with H2O to form the sulfuric acid. The sulfur trioxide reacts with ammonia to form ammonium salts:
Ammonium sulphate is a powdery substance:
2NH3 + SO3 + H2O <–> (NH4)2SO4 (s)
Ammonium bisulphate is sticky and hard to remove:
NH3 + SO3 + H2O <–> NH4HSO4 (l)
The rate of salt formation depends on the ammonia concentration (slip) and flue gas temperature. The formation rate becomes very significant below 500°F.
Risks & Mitigation
The fouling results in SCR catalyst masking and eventual deactivation. It fouls downstream equipment like air preheaters. Cleaning with water can create sulfuric acid and ammonia and could cause severe corrosion.
Since the formation rate depends on the flue gas composition and temperature it may be extremely difficult to control in cycling services.
It is important to monitor the ammonia slip and keep it as low as possible. Monitor the APH metal temperature in the coldest corner and maintain it well above the ADP temperature. Use soot blowers on the cold APH passes. Use cold air bypass, preheat combustion air with an external fluid, or recirculate hot air.
Figure 22 – Fouling by ammonia salts
Conclusion
Exterior fouling of the radiant process tubes, convection section, and the SCR/APH reduces heater efficiency and reliability. The best mitigation strategy is to prevent fouling from occurring by using design and operation best practices, including CFD. When fouling cannot be prevented, choosing the right maintenance strategy can have a large impact on long-term equipment health.
Reach out to us with fouling related questions at info@xrgtechnologies.com
Watch out for Part 3 on Burner Fouling!