Better mousetrap
What is needed is the means to generalize the results of a few tests to the operational multitude of Hg conversion conditions—in other words, a way to accurately forecast how changing fuels will affect Hg emissions, whether the Hg removal effectiveness of existing and planned gas cleaning systems will be enough to meet pending regulations, and where to add external Hg controls to get the biggest bang for the buck. Models are normally used to satisfy these imperatives, but statistical models have so far failed to extrapolate from their correlation databases to new situations. That's because statistical approaches work best in static situations but are poorly suited for dynamic environments like the ones that determine Hg emissions.
The modeling tools developed by Niksa Energy Associates solve that problem. Because they are based on dynamic reaction analysis (DRA), their equations account for the rates of all chemical reactions in a gas cleaning system that affect Hg emissions (Figure 2). As the flue gas cools along a gas cleaning system, the reaction rates in a DRA-based model automatically slow down at different rates, and high-temperature bottlenecks give way to new bottlenecks at cooler temperatures. Different fuel qualities and cleaning conditions also affect these rates, as does the injection of chlorine or other halogens and sorbents into the flue gas stream. The rates used for catalytic Hg oxidation along SCR systems are faster than the rates for the in-flight Hg chemistry.
2. Predicting Hg levels. Users enter fuel properties and gas cleaning conditions into simulations, based on dynamic reaction analysis, of Hg speciation along the flue gas cleaning system. Once the predictions have been validated with baseline Hg test data, the simulations can predict how emissions will vary as fuels, cleaning conditions, and the configuration of pollution control systems change. Source: Niksa Energy Associates LLC
Once the essential chemical information has been introduced into the rate equations, they can be solved to predict the complete Hg speciation at all points along a gas cleaning system (Figure 3). In a system with an SCR/ESP/scrubber combination, Hg0 begins to oxidize as it moves out of the economizer into the SCR system. The oxidation rate accelerates along the SCR catalyst and then slows down as the flue gas moves through the air preheater. Hg-P begins to form at the back end of the air preheater and reaches a level determined by the LOI level. At the ESP outlet, the high level of Hg2+ paves the way for substantial Hg retention in the scrubber.
3. Predicted Hg speciation from the furnace exit through the electrostatic precipitator. In a plant with an SCR system (left), Hg0 begins to oxidize upstream of the SCR inlet and will be converted much faster along the SCR catalyst, forming Hg-P downstream of the air preheater inlet. In a plant without an SCR system (right), the Hg0 oxidation rate upstream of the air preheater is much slower, but activated carbon injected upstream of the precipitator collects most of the Hg0 and Hg2+. Source: Niksa Energy Associates LLC
At plants without an SCR system, the Hg2+ level upstream of the ESP will be lower for similar levels of coal-Cl and LOI. But activated carbon injection upstream of the ESP can still collect most of the Hg as Hg-P in the ESP flyash.
If a scrubber is downstream of the PCD, the predicted Hg speciation at the PCD's outlet is used as the input for an equilibrium analysis of Hg retention in a wet scrubber or spray dryer-absorber. Because this analysis can accurately predict even test-to-test variations in Hg2+ retention by wet scrubbers (Figure 4), it is not necessary to specify the chemical reaction rates in scrubber solutions. However, the analysis does not predict Hg0 re-emission from dissolved Hg2+, although this contribution rarely exceeds the measurement uncertainties for the vast majority of scrubbers that have already been tested for Hg retention.
4. Achieving equilibrium. An equilibrium analysis of Hg2+ retention in commercial wet scrubbers accurately describes the wide range of Hg scrubbing efficiencies recorded at different sites and even on different days in the same test campaign. Source: Niksa Energy Associates LLC
DRA is usually the province of chemists and chemical engineers working in the petrochemical, specialty chemicals, automotive, and electronics industries. Utilities developed an appreciation of the value of mathematical models based on DRA during the development of in-furnace NOx abatement strategies and in the management of fuel quality impacts on furnace operations.
EPRI took a major step forward in developing DRA into a predictive capability for Hg emissions in early 2005 when it authorized a program to validate and evaluate the predictive capabilities of two DRA-based models, as a way of overcoming the shortcomings of statistical models. The ProMerc model from Reaction Engineering International Inc. (www.reaction-eng.com) combines DRA with statistical models for some air pollution control devices. Niksa Energy Associates' MercuRator model applies DRA up to the scrubber inlet and then uses a slightly simpler approach to predict Hg2+ retention in the scrubber. The remainder of this article uses predictions from MercuRator to illustrate the performance and benefits of this modeling approach.
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