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May/June 2009

Sulfur Oxides

The measurement of acid mist is becoming increasingly important due to recent actions taken by the U.S. Environmental Protection Agency on fine particles (2.5 micrometers in diameter and smaller) that are unhealthy to breathe. ASTM International Committee D22 on Air Quality is currently working on a proposed standard for measuring acid mists that come from combustion sources.

The proposed new standard, WK22846, Test Method for Determination of Sulfur Oxides Including Sulfur Dioxide, Sulfur Trioxide, and Sulfuric Acid Vapor and Mist, from Stationary Sources Using the Controlled Condensation Sampling System, is under the jurisdiction of Subcommittee D22.03 on Ambient Atmospheres and Source Emissions. WK22846 has its origins in an ASTM standard that was withdrawn in 1978, D3226, Method of Test for Sulfur Oxides in Flue Gases (Barium Chloranilate Controlled Condensation Method).

“During the intervening years, many variations of the method have arisen, leading to questions about comparability of the data,” says Scott Evans, senior vice president, consulting services, Clean Air Engineering, and a D22 member. “This new effort is an attempt to once again have a single, recognized condensation method.” Coal-fired power plants and refineries would be the leading users of WK22846.

According to Evans, measuring sulfur trioxide and its reaction product, sulfuric acid, poses one of the more difficult challenges in emission measurement.

“Typically, when measuring pollutants in flue gases, the pollutant is measured in a single phase, either as a gas, liquid or solid,” says Evans. “The reaction of sulfur trioxide with water vapor to form gaseous sulfuric acid is a dynamic process dependent upon the concentration of the reaction flue gas temperature and, to a lesser extent, the water vapor present in the gas stream. Once formed, sulfuric acid can condense to form an aerosol.”

Evans says that direct measurement of sulfuric acid emissions are not available at many facilities, so emission rates are often based on engineering estimates or EPA guidelines. WK22846 will give air emission test bodies the ability to accurately sample and analyze sulfuric acid concentrations.

Subcommittee D22.03 invites all interested parties who measure acid mist or have a need to measure or regulate acid mist, to join in D22.03’s work on WK22846.


Technical Information: Scott Evans, Clean Air Engineering, Palatine, Ill.

Phone: 847-654-4569

ASTM Staff: George Luciw

Phone: 610-832-9710