The presence of sulfur-bearing compounds in refinery fuel gas heaters, power generation boilers and other combustion processes can lead to the formation of SO3 and subsequently, sulfuric acid. Additionally, the operation of Selective Catalytic Reduction (SCR)NOx control technologies introduce ammonia into the process and furthers the conversion of SO2 to SO3 leading to more severe sulfur condensables (AbS). The effects can lead to:
- Downstream Air Pre-heater (APH) Fouling
- Cold-end Corrosion
- Visible Emissions (Blue Plume or Acid Mist)
In order to control the effects, it’s imperative that the plant measure the severity and type of these condensables.
Flue Gas Dew Point Monitoring
Dew point testing can be helpful for Air Pre heater (APH) systems with the capacity for reducing stack temperatures below the flue gas acid dew point. These determinations can be used to adjust the APH’s cold end temperature. If large variations occur, consider real-time measurement such as The Breen Probe, which will allow the plant to have better control over the fluctuations
Also, downstream equipment is exposed to the risk of condensing acid, resulting in corrosion in ductwork and equipment as well as corrosion and/or fouling of equipment such as tube bundles and air preheaters. Some of the upstream processes include:
- Refinery Fuel Gas and Oil Fired Heaters
- FCCU CO Boilers
- Steam Methane Reformers
- Power and process steam boilers
APH System Operating Considerations
The heater’s operating conditions will alter the APH operating temperatures:
- Lower Firing Rates – yield lower flue gas temperature; move cold end temperatures closer to FGADP Temperature
- Lower Excess Air Level – yield lower flue gas temperature; move cold end temperatures closer to FGADP Temperature
- Lower Ambient Air Temperature – move cold end temperatures closer to flue gas acid dew point (FGADP) Temperature
The primary effect of the above changes is to reduce the APH exchanger’s operating temperature. It is important to maintain the preheater’s cold end temperatures above the FGADP. This is best achieved measuring in real-time. If the cold end temperatures result in a flue gas discharge temperature higher than the design discharge temperature, dew point corrosion avoidance is achieved.
Maintaining an air heater’s cold end surface temperature above dew point will prevent unwanted buildup on the surface. This buildup can cause corrosion, reduce the amount of heat transfer, and increase gas-side pressure drop. Once an air heater becomes fouled, the fouling material acts as a magnet for further accumulation of particles, making it extremely important to keep the air heater clean.
The measurement point for the flue gas acid dew point depends on the equipment that needs to be protected. If protecting the air pre-heater, the measurement should be taken upstream of the air heater so that the dew point of the gas would reflect the remaining acid levels in the flue gas. Consequently, the air heater would not be protected if the temperature of the upstream equipment is kept above the downstream dew point temperature.
The Breen Probe Solution
The Breen Probe provides real-time measurement of the actual condition impacting the equipment by placing FGADP temperature instrumentation upstream of the equipment to be protected.