The Low Down on Low Loads in Wet Flue Gas Desulfurization Systems

Components and systems within power plants are typically designed to run effectively at particular, steady-state energy production loads. But what happens when a power plant runs at low loads? With many power generators forced to run for longer periods of time at low load conditions, system components can often face unintentional operational challenges.

And that's where we step in.

Modeling and analyzing process systems is one of the ways we help our power industry clients optimize low-loaded systems for better cost-effectiveness, efficiency, reliability, and regulatory compliance.

For the sake of this article, we are going to focus on Wet Flue Gas Desulfurization (WFGD) units. WFGD is a proven technology that can be used effectively to remove sulfur dioxide (SO2) from the flue gas of a power plant. A properly functioning WFGD system can achieve SO2 removal efficiencies of >99%.

In 1970, The Clean Air Act (CAA) was passed into law, effectively giving the Environmental Protection Agency (EPA) the power to fight environmental pollution. It established standards to minimize pollutants which pose a threat to public health, including sulfur dioxide, a contaminant that is released into the atmosphere when fossil fuels are burned. When SO2 combines in the atmosphere with water and air, it forms acid rain—a pollutant that can cause respiratory diseases in people and is harmful to the environment.

By using WFGD units, power generators can effectively clean up effluent and reduce emissions as they continue to work toward clean energy production, including clean coal initiatives. In addition, using wet flue gas desulfurization technology produces a useful and saleable byproduct called gypsum, a material used for wall board or as a cement additive. 

Essentially, the WFGD process involves injecting a low cost reagent, aqueous limestone slurry, into the flue gas to react with and convert the SO2 into gympsum. 

The chemical reaction of WFGD looks like this:

SO2(g) + CaCO3(s) + 2H2O(l) + ½O2(g)CaSO4 ·2H2O(s) + CO2(g)

The system itself looks like this:

wet flue gas desulfurization system diagram
Image credit: www.babcock.com

Unintended Operational Issues with WFGD

While the wet flue gas desulfurization process relies on a set process formula for the slurry mixture to scrub sulfur from the flue gas, any change in the original design parameters (like less optimal loading) can produce unplanned results.

When our clients come to us, we help them figure out how to optimize their unique system for process variations to mitigate the unintended operational issues. These conditions can appear as spray coverage problems, flue gas sneakage, or poorly distributed flow profiles through the mist eliminator sections.

Yet, one of the bigger operational issues inside a low-loaded WFGD unit is inlet duct wetting and solids buildup. That relatively inexpensive and effective limestone reagent can form walls of cement under certain conditions.

Inlet duct wetting typically occurs at low load caused because the slower moving flue gas has less energy to push the slurry sprays away from the inlet duct and keep the droplets from depositing in the inlet duct work. When the wet/dry interface moves back into the inlet duct, the droplets can land and dry, leaving behind solid deposits that, over time, can produce a substantial buildup. Shown below is a photograph of solids deposition in a WFGD inlet duct.

photo of WFGD inlet duct with more that 1ft solids buildup

The solution is fairly simple: design an inlet awning to keep the wet/dry interface away from the inlet duct at low flow conditions. A properly designed inlet awning will protect the upstream ducting from problematic liquid pullback which results in the aforementioned solids buildup. Solids buildup is of particular importance because in increases operating costs and requires more frequent maintenance cycles. And if it is left unchecked over time, the solids can build up to a level in which they can cause blockages that can alter flow patterns and system performance.

Inlet Awning Case Study

A client came to Alden with a problem of solids buildup in the inlet ducting of their WFGD. The solids buildup was so severe that the plant was forced to go offline every other month to get inside the duct work to clear away the buildup.

Working with the client, process information was shared, along with pictures of the buildup. From those discussions, the Alden team noted that the system did not have an inlet awning, and recommended using CFD simulations at various loads to gain an understanding of the gas-to-spray interactions. These results would then be correlated to the plant's operating experience in order to confirm the model accuracy before evaluating any design changes.

The baseline “as-is” model (figure 1), with no inlet awning, showed liquid sprays depositing on the inlet ducts during both full and low load operation, with the slurry deposition being more extreme during low load operation. The model indicated a distance of wetting back into the inlet duct that corresponded with plant observations, thus providing confidence in the models.

WFGD-CFD-Model-No-AwningWFGD-CFD-Model-No-Awning-detail

Figure 1: CFD with simulated slurry particles, indicated by particle pathlines. Note the wetting and slurry buildup in the duct work

 

Once we were satisfied that the models were producing realistic results, Alden developed design solutions to solve the problem. Each modification was discussed with the client to develop a prioritization list based on the impact, likelihood of success, and feasibility to install.

Recommendations included adding an inlet awning, changing some nozzles to wider angle sprays, or changing some spray nozzles to full cone from hollow. The largest impact would come from adding an inlet awning, and was the ultimate selection.

A new inlet awning was designed by Alden and evaluated in the models. The final awning design eliminated the inlet duct wetting, and thereby the inlet buildup problem. Figure 2 shows the model results with the recommended awning. Figure 3 provides a side by side comparison. 

WFGD-CFD-Model-with-AwningWFGD-CFD-Model-with-Awning-detail

 
Figure 2: CFD simulation with the suggested awning design. Note the absence of wetting and solids buildup in the duct work

 

WFGD-contours-droplet-concentration-no-awningWFGD-contours-droplet-concentration-with-awning
 
Figure 3: Contours of droplet concentrations—without an awning (left) and with the proposed awning design (right)

 

After the plant implemented the suggested awning design in their WFGD unit, they reported no issues with solids buildup or the need for continuous maintenance, indicating that the duct wetting solution did indeed work. 

Conclusions

Modeling and analyzing process systems is one of the ways we help our power industry clients optimize low-loaded systems for better cost-effectiveness, efficiency, reliability, and regulatory compliance. A properly designed WFGD inlet awning is an important aspect for the successful operation of a wet flue gas desulfurization system.  CFD modeling can be used to optimize the designs of WFGD inlet awnings. Further, CFD modeling can also be used to evaluate and optimize the gas and liquid distributions throughout the WFGD, optimize spray coverage, minimize gas sneakage, and improve flow profiles through the mist eliminator sections of a wet flue gas desulfurization system.

As other processes and industry begin adopting scrubber technology, similar issues could arise. And if that's the case, we can help.  Contact us to get started. 

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