SO3 formation in Flue Gas Streams
There are many boiler operational parameters that influence the degree of total SO3 formation. Among these are:
- Fuel sulfur content,
- Ash content and composition,
- Convective pass surface area,
- Gas and tube surface temperature distributions,
- Excess air level, and
- Coal fineness.
As a result, the same coal burned in two different boilers, or in the same boiler at different operating conditions, can produce substantially different levels of SO3. Assumption of the level of total SO3 based on any one variable is clearly inadequate.
The Challenge – Control Of Sulfuric Acid
High sulfuric acid levels produce multiple detrimental effects including:
- Corrosion and fouling of heat exchangers and ductwork,
- Increased carbon emissions through elevated air heater outlet temperature, and
- Formation of acid mist in the stack plume.
Conversely, sulfuric acid has the beneficial effect of promoting fly ash collection in cold-side electrostatic precipitators. Successful control of sulfuric acid levels can have a significant impact on the overall performance of any electric-generating unit.
Direct Measurement of Sulfuric Acid Vapor
The AbSensor – SO3 condensables probe from Breen Energy Solutions is an industry proven instrument for measuring the concentration of sulfuric acid vapor in utility flue gas streams. It is effective in measuring acid vapor levels from the SCR outlet to the scrubber inlet and, by combination with gas moisture level, the corresponding levels of SO3.
The system works by controlling the boundary layer temperature between the flue gas and the sensor tip of the instrument. Precisely controlled cycling between preset high and low temperatures results in accurate determination of the vapor dewpoint. Combining this information with gas moisture levels using proven mathematical analysis provides real time information on SO3 concentration.
Acid Vapor / SO3 Relationship
For applications related to acid condensation (Blue Plume and back end corrosion), it is actually more important to know the dewpoint than the SO3 concentration. This is because the dewpoint provides the real-time summation of gas moisture, gas pressure and SO3 level. Where the SO3 concentration must be expressed in parts per million, addition of a moisture level variable is required.
Optimize SO3 Processes with AbSensor-SO3
There are a great many processes within the power plant environment that can be optimized by controlling sulfuric acid vapor concentration including:
- Heat rate improvement through control of the air heater cold-end temperature,
- ESP back-end and duct corrosion avoidance through control of ESP outlet temperature,
- ESP performance improvement through control of acid dewpoint/ESP inlet temperature relationship, and
- Blue Plume (acid mist) mitigation.
Breen Energy Solutions offers full turn-key, closed-loop control solutions for each of these applications. Closed loop solutions may include:
- Process optimization using our multi-variable predictive controller (QuickStudy),
- DCS logic changes to accommodate the optimization strategy,
- DCS graphic interface modifications to allow user monitoring and control of the process,
- Application specific PLC code biasing, and
- Site-specific process consulting to coordinate Breen optimization with existing plant processes (e.g., combustion, sootblowing)
AbSensor – SO3 Technology
The AbSensor – SO3 condensables probe measures conduction across a uniquely constructed probe surface resulting from condensed sulfuric acid below its dew point. The condensables measurement technique and probe designs are described in United States Patent No. 6,677,765 and 8,256,267 and other foreign patents.
The detection process consists of cooling the initially hot detector surface by controlled application of cooling air. The descent rate is tightly controlled to allow continuous monitoring of condensate conditions on the probe tip. The presence of a condensed liquid phase is determined by the resistance between two electrodes. When current is detected, the kinetic dew-point (or formation) temperature has been reached.
Following detection of condensate, the cooling gas is removed and the probe is allowed to return to localized gas temperature. As the probe heats, the instantaneous current is measured and reported back to the controller. When the liquid evaporation temperature is reached (detected by a rapid decrease in probe surface current) the process has completed and a new measurement cycle is initiated.
The instrument reports a multitude of information variables to the plant control room via 4-20 mA loop, Modbus or OPC link. Examples of reported data:
- Formation Temperature
- Evaporation Temperature
- SO3 Concentration