Validating the model
Test data from more than 50 full-scale gas cleaning systems have already been interpreted by MercuRator. Each individual test was simulated independently with readily available test-specific coal properties and gas cleaning conditions. The predicted Hg emissions rates are directly compared with test data (Figure 5) for systems with PCDs alone and SCR/PCD/scrubber combinations, and for systems that use sorbent injection for Hg control. The measurement uncertainties usually fall within 10% to 15% of the total Hg inventory. The standard deviation (SD) of the predictions (Table 2) are well within this tolerance for SCR/PCD and SCR/PCD/scrubber combinations, very close to it for sorbent injection applications and systems with PCDs only, and somewhat greater for PCD/scrubber combinations.
5. Educated guesses. A comparison of measured (green bars) vs. predicted (blue bars) Hg emission rates for flue gas cleaning systems with particulate control devices only (top), SCR/ESP/scrubber combinations (middle), and activated carbon injection (bottom). Uncontrolled emission rates also are shown, as orange bars. The rates were predicted for individual tests to make test-to-test variations apparent. Source: Niksa Energy Associates LLC
Table 2. MercuRator prediction variance. Source: Niksa Energy Associates LLC
Since its predictions have proven accurate for the most popular gas cleaning configurations, MercuRator now is being used by several utilities to support their Hg compliance activities. Researchers at Tennessee Valley Authority (TVA), for example, wanted to predict the impact on Hg reduction of adding SCRs to the utility's large baseload plants that have been or are being equipped with wet scrubbers. They also wanted to assess any changes in the inherent Hg control when the SCR system is taken out of service.
At TVA, baseline data from an SCR/ESP/scrubber combination with and without the SCR system in service were used to ensure that the predictions were accurate for the coals and cleaning conditions of interest. Then, a library of MercuRator simulations compiled the Hg removal rates for broad ranges of coal-Cl, LOI, furnace load, and SCR conditions (including SCR bypasses).
The estimated impact of variations in coal chlorine content (Figure 6) is qualitatively different with and without an SCR system in service. When the system is on-line, the impact of chlorine becomes stronger when the variable rises above 0.05% by weight, but then it weakens considerably as the level passes 0.12%. Since the TVA baseline levels were 0.154%, excursions into the Cl-sensitive operating regime will probably be infrequent if a supply of similar coal is secured. This insensitivity arises because the SCR simulated is overdesigned for Hg0 oxidation. By contrast, when the SCR system is off-line, Hg removal increases in direct proportion to coal chlorine content over its full range.
6. Off-line and off the hook. Variations in a coal's chlorine content and LOI have a much greater impact on Hg removal effectiveness when the plant's SCR system is out of service. Source: Niksa Energy Associates LLC
At TVA, there were also markedly different impacts of LOI variations with and without the SCR system in service. When the system was on-line, LOI variations were negligible due to compensation by Hg0 oxidation across the SCR system. For example, the Hg2+ levels at the SCR inlet increased from 39% to 93% when LOI was increased from a negligible level to 3%. That's because the higher LOI accelerated the in-flight Hg0 oxidation on UBC. However, the levels at the SCR outlet increased by less than 10% over the same range of LOI. When the SCR system was off-line, the Hg removal effectiveness increased from 30% to 99% as LOI increased to 3%, matching the performance of the baseline system with SCR at 3% LOI. (Don't expect the numeric values in this paragraph to apply to other gas cleaning conditions.)
Other companies are using MercuRator predictions to identify the optimal injection rate of activated carbon. When the carbon is injected upstream of a cold-side ESP, the optimal rate is elusive because it depends on the inherent levels of chlorine and LOI in the system, as well as on the temperature and sorbent transit time available for Hg capture.
For low chlorine levels, Hg removal is limited by the total pool of chlorinated sites that can form on the injected carbon (Figure 7). As a result, adding more carbon redistributes the chlorine over a larger population of carbon particles, but it does not expand the pool of active sites for Hg removal. Conversely, when chlorine is plentiful, the performance of the sorbent is enhanced by Hg absorption on suspended UBC in flyash upstream of the sorbent injection point. In other words, carbon sorbents perform better at higher LOI levels, provided that sufficient chlorine is present to use all the active sites on the UBC and the carbon sorbent.
7. Impact on ACI of chlorine and LOI. These graphs show the Hg removal effectiveness of activated carbon injection at various coal-Cl levels for a fixed LOI (left), and at various LOI levels for a fixed coal-Cl content (right). Additional LOI enhances sorbent performance, provided that sufficient Cl is available. Source: Niksa Energy Associates LLC
The information from sensitivity studies like these is impossible to obtain from field testing because all of the important variables can never be regulated in a full-scale cleaning system. And, perhaps more importantly, DRA-based simulations can be run for a fraction of the costs of testing.
—Dr. Stephen Niksa (neasteve@pacbell.net) is president and David P. Bour (dbour@sbcglobal.net) is marketing consultant for Niksa Energy Associates LLC. Thomas A. Burnett (taburnett@tva.gov) is senior specialist and Dr. Naresh B. Handagama PE (nbhandagama@tva.gov) is an engineering specialist for Tennessee Valley Authority.
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