Three steps to a strategy
Armed with this understanding of the basic chemistry of SO3 formation and behavior in coal-fired plants, owners and operators can take a three-step approach to planning an appropriate SO3 control strategy.
Step 1 is understanding SO3 behavior in the context of existing plant design and operating conditions. Targeted testing may be needed to determine how much SO3 is actually being added or removed by downstream systems, as described in the previous section. Planners would be wise to check the consistency of this targeted data against the contents of the plant’s information historian.
Step 2 is assessing the impact that proposed changes in plant equipment and operation would have on SO3 levels. This entails developing—at a minimum—a “most-likely profile” of SO3 behavior based on projected plant conditions and experience at similarly configured plants. To increase the chances of arriving at an optimal strategy, consider using analysis tools such as process or computational fluid dynamics (CFD) models to develop multiple scenarios and profiles.
Step 3 is choosing from among available SO3 control technologies. Options include changing to a lower-conversion SCR catalyst and/or sorbent injection after the SCR. Common sorbents used are hydrated lime, a magnesium-hydroxide slurry, sodium bisulfite (the solution selected by Cinergy’s Gibson Station), or Trona (sodium sesquircarbonate)—the choice at General Gavin Station. CFD modeling (Figure 4) can compare the performance risks of the different control options to help inform decision-making. For example, CFD modeling of the lower and upper furnace under different load conditions can simulate slurry injection performance and provide conceptual design considerations for SO3 control technologies. Specific design and operating considerations such as injector locations and slurry flow rates can be modeled and designed in advance.
4. Predictive tools. A computational fluid dynamics model of slurry droplet injection in an upper furnace. Source: Reaction Engineering International
Final considerations
If a plant is considering installing or upgrading an ESP unit, planners also should attempt to predict the impact of that change on SO3 levels. Assessments should seek to:
- Estimate the performance and reliability of wet-ESP technologies.
- Determine the impact of switching from a hot-side ESP to a cold-side ESP on the plant’s SCR system.
- Evaluate the use of a wet-ESP for controlling mercury and PM2.5 emissions (as opposed to use of a dry ESP plus another SO3 control technology).
Once technical assessments have identified one or more SO3 control technology options appropriate for a given plant, the next step is to determine the extent to which the options comply with current regulations (EPA Method 9). Following this regulatory assessment, an economic and commercial feasibility assessment can be performed to quantify and compare and contrast the options’ lifecycle costs under different projected load and operating conditions.
—Dr. Bradley Adams is president of Reaction Engineering International(Salt Lake City, Utah); he can be reached at 801-364-6925, ext. 18 or adams@reaction-eng.com. Dr. Constance Senior is manager of R&D; she can be reached at 801-364-6925, ext. 37 or senior@reaction-eng.com.
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