Trona Injection Reduces SO3 Emissions

By Dr. Robert Peltier, PE

Emissions of SO₃ (or its hydrated form, H₂SO₄) have created a nagging problem for some coal-fired power generators after they’ve installed a selective catalytic reduction (SCR) system. If your plant is in that unfortunate group, here’s a summary of the state of our understanding of the problem—and its solutions.

Essential Reading

SO₃ emissions are formed by oxidizing SO₂ in the catalyst. Beyond the periodic “blue plume” that appears under the right combination of atmospheric and coal combustion conditions, the acidic nature of the emissions can also adversely affect equipment life and performance.

POWER published a series of three articles (October 2006, February 2007, and April 2007) written by Robert E. Moser of Codan Development LLC that we consider to be one of the most comprehensive treatises ever written on the subject. Your first step in understanding various SO₃ formation mechanisms and their effects on power plant operations is to read those three articles.

The editors also dedicated the March/April 2007 issue of COAL POWER to methods of controlling SO₃ emissions by focusing on successful case studies. Of particular interest are case studies about the methods used by AEP and Dominion Generation plants that eliminated SO₃ emissions and recent work by NETL on why coal-fired plants with wet scrubbers are particularly prone to acid-related stack opacity problems. To find these and related articles in our archives, just type “SO3” in the COAL POWER home page search box. If you have experienced the dreaded “blue plume” or are considering an SCR upgrade at your plant, both series of articles are must-reads.

Where Does SO₃ Originate?

Part I of Moser’s series of articles presents a cogent discussion of SO₃ formation mechanisms and SO₃’s effect on equipment life and maintenance concerns. In brief, SO₂ is formed when sulfur in coal is burned and further oxidizes perhaps 1% to 2% of the SO₂ to SO₃ during the combustion process. The amounts are dependent on many variables but principally are determined by the type of boiler and its combustion stoichiometry and the volume of SCR catalyst present. Moser notes that the amount of SO₃ produced can vary from a few ppm with low-sulfur coals to 30 ppm to 40 ppm or more for very high sulfur coals. SO₃ generation can also increase when the boiler is operated at low loads, because reducing the temperature of the flue gas leaving the air heater increases SO₃ fouling and corrosion.

Moser goes on to note that SO₂ is also oxidized in the SCR reactor, forming SO₃ at a rate of 0.3% to around 2%, with the norm about 1%. The increase in the amount of SO₃ leaving the SCR system is a function of the oxidation level of its catalyst: Very low oxidation catalysts raise the level only slightly, but high-oxidation catalysts can double the amount. Further downstream the flue gas will continue to cool and the amount of vaporous sulfuric acid formed is a function of dew point of the gas or the amount of moisture in the flue gas.

By the time the gas has exited the air heater, at a temperature around 300F, most of the SO₃ from both sources has taken the form of vaporous sulfuric acid. When further cooled in the flue gas desulfurization (FGD) system, the vaporous sulfuric acid undergoes what Moser calls “shock condensation,” which produces very fine aerosol particles that, for the most part, are too small to be captured in the FGD system, so they enter the atmosphere as a sulfuric acid aerosol mist. At best, conventional wet FGD systems may achieve anywhere from marginal to perhaps 30% SO₃ removal. However, depending on the arrangement particulars of the plant, some SO₃ will condense on flyash particles and end up in the air heater or electrostatic precipitator (ESP). Figure 1 illustrates the forms that SO₃ takes at various points in the flue gas stream.

1. Where SO₃ is created and the forms it takes. Source: Codan Development LLC

How Is SO₃ Eliminated?

One of the most popular procedures for eliminating SO₃, and the one used by AEP and Dominion in the referenced case studies, is injecting a sorbent into the fuel stream, into the furnace, or into the flue gas stream. In each case, the sorbent reacts with SO₃ in the flue gas to form a solid compound that is easily removed in the plant’s particulate collection device—an electrostatic precipitator or fabric filter.

AEP pioneered the use of trona for SO₃ mitigation at its General Gavin plant and then introduced its proprietary process across AEP’s entire bituminous coal-fired fleet where both SCR and FGD systems are in place. Similarly, Dominion Generation implemented a limestone sorbent injection system at its Chesterfield Station using the CleanStack technology jointly developed by the University of North Dakota’s Energy & Environmental Research Center (www.eerc.und.nodak.edu), Marsulex Inc. (www.marsulex.com), and Alstom Power’s Air Preheater Co. (www.airpreheatercompany.com).

The common thread between the AEP and Dominion Generation projects, and in many other successful SO₃ mitigation projects across the industry, is finding the right sorbent to optimize SO₃ reduction given the specific boiler and air-quality control system components in place at a plant.

Chances are you aren’t familiar with trona (a high-reactivity sodium sorbent), its properties, or where it comes from. One of the leading suppliers of sodium bicarbonate, or trona, to the power industry is SOLVAir Products, part of Solvay Chemicals Inc.

Dr. Yougen Kong and his team presented a comprehensive presentation at the May 2008 ELECTRIC POWER conference on the sources and uses of trona in the power generation industry and its advantage in SO₃ mitigation. Download a PDF of their PowerPoint presentation, “Dry Sorbent Injection of Trona or Sodium Bicarbonate for Air Pollution Control and Corrosion Prevention,” as part of your research into your SO₃ mitigation options.