NEWS RELEASE                                                                                                    MARCH 2015

SO₃ Poses New Challenges and Opportunities for Power Plants

SO₃ is a pollutant but it is injected into flue gas to improve particulate control. However, SO₃ interferes with mercury capture.  It is unintentionally created by SCR catalysts. It is the major component of condensibles in total particulate.  SO₃ control is key to a new approach to power plant efficiency improvement.

Power plant decisions need to consider all the economic, operational and environmental impacts for each potential approach involving SO₃. A number of decision trees need to be navigated and then revisited. The best route can be likened to GPS guidance. McIlvaine has created a system for making all power plant air quality decisions: 44I Power Plant Air Quality Decisions.  A fast track decision tree on SO₃ has just been posted in the system.

Decades ago sulfur trioxide (SO₃) was of interest to power plants and regulators for only one reason. Injection of small quantities into the gas stream improved precipitator efficiency.  In the 1990s the U.S installed many SCR units for NO_x reduction.  SO₃ suddenly took center stage among all the pollutants. It was identified as the cause of acidification so severe that a small town in Ohio had to be permanently vacated and the buildings demolished.

In the last decade, the pollutant took center stage again on the basis of its very large contribution to total particulate emissions (discrete particles plus condensibles).  SO₃ is back in the news as part of a program to increase boiler efficiency through greater heat extraction. Its role in mercury capture is also now in the spotlight.

The U.S. experience with SO₃ is going to be useful to China which now is in the same position as the U.S. was in the 1990s.  China has just installed a large number of SCR systems to reduce NO_x.  These systems have the unintended consequence of converting SO₂ to SO₃.  The SO₃ converts to H₂SO₄ in the stack and causes a blue plume and potential damage to the surroundings.

Power plants SO₃ decisions affect many others, therefore, it is necessary to consider all the economic, operational and environmental impacts for each potential approach involving SO₃.

Economics:  The remaining plant life can determine whether it is worth considering high capital cost approaches for SO₃ and particulate control e.g., wet precipitators  The potential to inject sorbents ahead of the air heater and improve the capture of exhaust heat can add one percent to boiler efficiency. The cost of SCR catalyst is a function of the SO₂/SO₃ conversion activity.  The amount of activated carbon for mercury control can be affected by SO₃.  If the plant must meet total particulate rather than discrete particulate limits, SO₃ is a big consideration.

Operations:  Does the plant want to purchase high sulfur coal? SO₃ mitigation choices can determine the maximum coal sulfur content. How often will NO_x catalyst be replaced? Should flue gas conditioning be employed?

Maintenance:Sulfuric acid corrosion can be a problem in the air heater and ductwork.  With ammonia injection, ammonium bisulfate builds up on surfaces of the air heater and reduces heat transfer. If you utilize a fabric filter, will there be bag plugging problems?

Air Emissions:  Are there limits on total particulate emissions including condensibles? The new federal air toxic rules were revised to eliminate condensibles but states may have or are planning total particulate limits.  If so, SO₃ can create 80 percent of the total particulate.  Just 10 ppm of SO₃ will cause particulate emissions of 0.03 lbs/MMBtu.  A blue plume will also be an opacity problem.

Each pollutant has to be viewed separately and all the ramifications evaluated. It is best to view all the ramifications relative to each pollutant separately and then determine a route which provides the best holistic approach. 

Power Plant Air Quality Decisions has an organized approach to making these decisions.  McIlvaine has added individual pollutant tracks to simplify the process. The SO₃ fast track GDPS has now been posted. It allows the reader to quickly view all the ramifications of SO₃ decisions.

Power plant operators can subscribe to this system at no charge:  Power Plant Systems and Components

Others can subscribe through: 44I Power Plant Air Quality Decisions