AbS and 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.
Add to this selective catalytic reduction technology (SCR) for nitrogen oxide control the potential to create Ammonium Bisulphate (AbS) is created.
First of all the catalyst used in the SCR oxidizes a small portion of the SO2 in the flue gases to create additional SO3. Ammonia (NH3) is typically injected in the front of the catalyst to reduce the nitrogen oxide in presence of the catalyst to Nitrogen and water. Over time as the catalyst ages, flows and temperatures vary some of the unreacted NH3 slips past the catalyst. This portion of NH3 known as Ammonia slip reacts with the SO3 and forms Ammonium Bisulphate and Ammonium Sulphate salts.
The Challenge – Control Of Sulfuric Acid & AbS
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.
AbS is a problem as these sticky liquid salts melt around 296°F and condense at higher temperatures as they pass through the air heaters and are a major cause of fouling of air air heaters. Fouled air heaters result in higher pressure drop & increased outages for offline cleaning decreasing plant availability.
Successful control of sulfuric acid and AbS levels can have a significant impact on the overall performance of any electric-generating unit in terms of improved operations and maintenance costs.
Direct Measurement of Condensable species
The AbSensor – AbS/SO3 condensables probe from Breen Energy Solutions is an industry proven instrument for measuring the condensable species in utility flue gas streams. If AbS is present the probe will measure it since AbS has a higher dew point than SO3. In the absence of AbS SO3 will be measured. It is effective in measuring condensables & 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.
AbSensor – AbS/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
Optimize Plant Processes with AbSensor-AbS/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)
Breen Thermap Air Heater model which uses the formation and evaporation temperature data to predict if deposition will occur in the air heater, the depth & location in real time and the effect of process variables on deposition. Operators can then take this data to make necessary process adjustments &/or apply mitigation control measures to minimize fouling issues. Learn more.
DySC – Dynamic Speed Controlled Soot Blower System
In many power plants the soot blower systems tend to have problems with AbS fouling & other tenacious deposits in rotary air heaters. Cleaning is fairly effective on the inner baskets but ineffective on the outer baskets, This is because of higher tangential velocities as the soot blower traverses from the hub to the perimeter. Breen’s patented DySC system uses a Vector drive to dynamically change the speed of the air heater rotation to ensure full penetration of soot blower media from innermost to outermost baskets. This results in lesser outages and higher availability of the unit. Learn more.
Breen offers portable (transportable) Dry Sorbent injection systems for SO3 mitigation using Lime, Trona and other sorbents. Using feedback from the AbSensor probe these injection systems can be used to optimize sorbent selection, injection location and usage and generate long term operating cost savings for the plant. Learn more.