Keeping an Eye on SO3

Issue 4 and Volume 110.

Injective additives can control SO3 emissions at coal-fired power plants, but without a way to continuously monitor SO3, it is impossible to optimize additive use. EPRI is studying technologies that can be used to monitor SO3 emissions, making it possible to lower additive use and costs.

By Richard Himes, EPRI

When used to reduce nitrogen oxide (NOx) emissions from coal-fired power plants, selective catalytic reduction (SCR) technology can result in a fraction of the sulfur dioxide (SO2) in the flue gas being oxidized into sulfur trioxide (SO3). Injected additives can control SO3, but no method is currently available to continuously monitor SO3 and make it possible to optimize additive use. Recently, an EPRI sponsored research team demonstrated the feasibility of the first technique developed to reliably monitor SO3 emissions from coal-fired boilers on a continuous basis.

Effects of SO3

SO3 can adversely affect plant operations, primarily due to sulfuric acid (H2SO4) formation, which can lead to air heater and/or ductwork surface corrosion. The growing implementation of SCR and selective noncatalytic reduction (SNCR) NOx control technologies also can lead to a reaction between SO3 and residual ammonia slip. This reaction forms ammonium bisulfate, which can lead to potential air heater fouling.

Other SO3-related environmental issues that power generators should consider include the formation of visible plumes, which can be a precursor to particulate-level PM2.5. In addition, U.S. utilities must estimate H2SO4 emissions because utilities are required to report their annual toxic chemical releases to the Toxic Release Inventory, a national compilation of chemical releases designated toxic by the U.S. Environmental Protection Agency (EPA).

Control of Additives

SO3 control often entails injecting an additive into the flue gas. Additive feed rate control, however, is typically based on single point, manually collected wet-chemistry SO3 measurements, which are labor and time intensive and may not be totally representative of the flue gas concentration.

A continuous SO3 monitoring method could provide significant cost savings by minimizing the amount of additives used to control SO3 emissions. This minimization could be achieved by reducing SO3 concentrations enough to avoid a visible plume, while still keeping them high enough to condition the fly ash for high-efficiency collection by the electrostatic precipitator (ESP).

An example of possible cost savings can be seen at a 500-MW medium-sulfur coal-fired plant where additives needed to achieve a clear stack with an SCR cost between $500,000 and $1 million per year. If more accurate measurement allowed a 20 percent reduction in reagent use, the savings would be $100,000 to $200,000 per year.

Even when additives are not used, accurate SO3 emissions measurements help monitor and detect an increase in SO3 levels that result from a change in fuel specifications, combustion considerations, SCR operation, and so on, allowing appropriate corrective actions to be taken. A continuous SO3 monitor would also help plants that burn low-sulfur coal minimize the amount of SO3 they inject to condition fly ash for high ESP removal efficiencies.

Monitoring SO3 with Spectroscopy

In 2004, the EPRI research team conducted an initial study to review available SO3 and H2SO4 monitoring technologies and methods. The team studied EPA Method 8 (Determination of Sulfuric Acid Mist and Sulfur Emissions from Stationary Sources), the modified EPA Method 8, the controlled condensation method, the Severn Science analyzer and corrosion probes.

The team found that the methods studied do not take into consideration all pertinent site-specific factors and have limited accuracy. The methods are based on extractive techniques, which only represent point measurements of the local SO3 in the flue gas duct, requiring an assumption of homogenous distribution that is often not representative of most boilers. In addition, because the methods require adherence to proper procedures, they have been shown to have a significant variability from tester to tester, leading to testing inaccuracies.

Figure 2: Merom Station SCR and SCR outlet duct. Photo courtesy of EPRI.
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The study also revealed that an effective SO3 monitor would provide better accuracy if it could provide in-situ measurements, obviating the need for sample extraction and the problems associated with that type of sampling. In-situ SO3 measurements are preferable over extractive approaches because with in-situ measurements sample reactivity is limited and there is less potential for measurement bias. In-situ measurement offers a fast system response to process changes, and the line-of-sight measurements are more representative of the flue gas flow field than are the single-point batch samples.

The project team determined that for in-situ measurements, optical spectroscopic techniques are reliable. Spectroscopy can be defined as the investigation of the interaction between a defined source of electromagnetic radiation and a target sample. In most boilers, opacity meters are included in the emission monitoring equipment. Some coal-fired power plants have incorporated tunable diode laser spectroscopy to measure ammonia (NH3) slip. Spectroscopy is used extensively to investigate SO3 and H2SO4 in the infrared regions where both can be monitored concurrently with the least amount of interference.

The research team conducted a literature search of two spectroscopic technologies: Fourier transform infrared (FTIR) spectroscopy and differential optical absorption spectroscopy (DOAS). While DOAS has promise for future development, currently available systems measure only SO2 and have not yet been configured for in-situ measurements of SO3. As a result, the investigation focused on FTIR as a monitoring technology.

FTIR began to come into its own in the early 1950s when experimental groups first built and tested high-resolution spectrometers. Today commercial FTIR spectrometers are widely available. Aided by fast computers that perform Fourier transforms quickly, FTIR spectrometers are used to make spectroscopic measurements in many diverse disciplines.

A study conducted prior to the EPRI project by Industrial Monitors and Control Corp. (IMACC), Austin, Texas, found that FTIR might be feasible for monitoring SO3 and H2SO4. “However, our results also showed that the best detection is obtained when the spectral regions used for FTIR analysis are as free as possible from interferences from other compounds,” says Dr. Robert Spellicy, IMACC’s president. The key issue is that dust loadings in power plants would make FTIR cross duct measurements difficult because the signal is significantly attenuated. With expected dust loadings in power plant ducts of nominally five grains per cubic foot, in-situ FTIR measurements would likely not work without being interfaced to some dust-attenuating probe.

Phase Discrimination Probe

To develop such a probe, EPRI teamed with the University of California, Riverside, (UCR) to create a phase discrimination probe. “The probe aerodynamically separates the flue gas from a majority of the fly ash on a continuous basis,” says John Pisano, Senior Development Engineer at the Center for Environmental Research & Technology – UCR’s Bourns College of Engineering. The interior of the probe sees representative flue gas with reduced fly ash content; therefore, the probe allows optical measurements over path lengths that were unattainable before.

The EPRI-UCR team believes the combination of the FTIR analyzer and the phase discrimination probe can enable SO3 in-situ measurement over a line-of-sight within the flue gas duct anywhere downstream of the economizer outlet to regions where the flue gas temperature approaches the acid condensation temperature. The line-of-sight measurement will provide a more representative sample volume than the single-point wet-chemistry batch measurements that are typically collected.

Plant Demonstration

In 2005, the EPRI-UCR monitoring system was field tested at Hoosier Energy’s Merom Station in Sullivan County, Ind. The equipment was installed in Unit 1, a 500-MW Riley Turbo unit, burning a nominal 3 percent sulfur coal.

The cross-duct probes were installed in retrofit four-inch ports in the SCR outlet duct, about six feet from the top of the duct, which has SO3 emissions of 25-40 ppm. For demonstration purposes, the FTIR was mounted on an optical table to isolate it from plant vibration. An optical periscope was used to translate the beam from the FTIR to the probe port entrance.

Several FTIR measurement approaches were possible for field testing. The preferred standard monostatic transmission approach was attempted, but the probe warped when it was heated, making this approach impossible. The next best approach, passive bistatic transmission, was tested. Its disadvantage is that the source is no longer modulated, so all background radiation from the gas, the probe or the duct must be separately measured and corrections made.


Over several days in August 2005 the monitoring system tests were conducted and data was collected. The data was then compared against three sets of measurements using the standard controlled condensate method. Controlled condensate measurements were taken from 10 existing sample ports across the top of the duct. Each port provided an average value of SO3 and H2SO4 over the sample collection extraction time (Figure 1). As noted in Table 1, an overall average value of 31.1 ppm was attained over three days of testing, with a nominal standard deviation of +/-10 percent associated with all 56 of the spatially resolved measurements obtained from each of the 10 ports.

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The FTIR measurements fell within the controlled condensate measurements, attaining an average value of 37.8 ppm. The FTIR data standard deviations were larger, but this was to be expected due to the large background correction required with the bi-static measurement approach. In summary, the results provided a proof of concept for FTIR spectroscopy in an in-situ measurement configuration for the continuous measurement of SO3 and H2SO4 in a coal-fired boiler flue gas stream.

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Next Steps

Future applications will focus on cross-duct phase discrimination probe installation procedures and requirements, so the FTIR measurements can be conducted in a monostatic configuration. In addition, the research team will assess the monitor’s reliability and maintenance needs over time. The team will consider a cooling system for stabilizing the instruments’ temperature and their sensitivity to the vibration and dust in the plant. With ever increasing requirements being placed on plant instrumentation and control resources, for FTIR to be a viable process control option, it must be capable of operating with minimal attention from plant staff.

The controlled condensate SO3 measurement system. Photo courtesy of EPRI.
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EPRI also will continue to work on developing a portable monostatic approach that will eliminate the need for dual, optically aligned ports. Because one set of optics is positioned within the flue gas stream, it must be capable of operating at 750 F, while maintaining sufficient cleanliness. The cleanliness is necessary to reflect the infrared source with sufficient power and signal-to-noise ratio to provide a clear signal.


Richard Himes has been an EPRI Senior Project Manager specializing in boiler performance and NOx control for the past seven years. He is responsible for research and development concerning emerging NOx control technologies as well as balance of plant technologies impacted by NOx control. Prior to joining EPRI, Mr. Himes was a consultant to the electric utility industry for 17 years. He specializes in combustion sciences and works in emissions measurement and control, boiler performance and compliance planning. Mr. Himes received a bachelor’s degree in chemistry and biology, and a master’s degree in mechanical engineering from the University of California at Irvine, as well as an M.B.A. from the University of Southern California.