SCR Optimization FAQ

Is your SCR performing optimally? When was the last time you checked?

What is selective catalytic reduction?

Selective catalytic reduction, or SCR, is a process of removing NOx. NOx consists of NO and NO2 from the flue gas stream by injecting ammonia. This ammonia and flue gas mixture passes over a fixed bed of catalysts, and the ammonia reacts with the NOx to create nitrogen and water. And you can see the two reactions on the lower right. The first one is the predominant reaction, and this one reduces NO. And there is some NO2 in the flue gas, and there’s a smaller reduction of NO2 and NO2 nitrogen and water.

What are the typical performance parameters of an SCR system?

Typical SCR System Performance Parameters vary state by state and utility by utility. The end user or the HRSG supplier usually specifies the system design requirements. Usually, CO ranges from 2 to 10 ppm. NOx can range from 2 to 5. Ammonia slip can range from 2 to 10 ppm if it’s a permit parameter. And the CO catalyst, which is upstream of the SCR, could range from 1 to 2 inches of pressure drop, and the SCR catalyst pressure drop can range from 2 to 4 inches.

The ammonia NOx distribution can range from 5% to 10% RMS. RMS is the root mean squared, a statistical evaluation of the scatter of data points. Percent RMS of zero is perfect distribution, which we won’t see. So, some tolerance has to be provided. The catalyst supplier usually specifies the ammonia NOx distribution based on their design.

Now, ammonia slip is one of the critical performance parameters. In most systems, NOx and CO can be reasonably straightforward to meet, but ammonia slip can be challenging to maintain. Ammonia slip is either periodically tested or continuously monitored. Overfeeding ammonia can usually drive a NOx down to the required limits, but ammonia slip will go up. And high ammonia slip can result in, of course, ammonia emissions exceedances, wasted ammonia usage, or elevated ammonia usage and downstream HRSG fouling.

And the three typical causes of high ammonia slip are poor ammonia NOx distribution, gas bypass, or deactivated catalyst. Deactivated catalysts are probably not as common in gas units because there aren’t any poisons in the flue gas that would cause the deactivation of the catalyst, but it does happen over time.

The HRSG typically contains the catalyst where the temperature is between 550 to 750 degrees Fahrenheit. This temperature is the sweet spot for SCR system design and combined cycle units.

What is an ammonia injection grid?

Now, the ammonia injection grid (AIG) distributes the ammonia air mixture into the exhaust flue upstream of the SCR catalyst. If there is one, the AIG is typically located downstream of the CO catalyst. And the AIG should be designed so that someone can tune it. Now, tuning should not be necessary if the system is appropriately designed and modeled, but you want that flexibility just in case.

How are ammonia injection grids kept in tune?

In terms of tuning, the ammonia injection grid should have some controllability, and that’s what we talk about with control zones. And the number of zones would depend on the flue size, of course. The bigger the flue, the more zones you would need to control. Now the valves can either be centrally located.

Now, tuning itself can’t be done effectively if there isn’t some way to measure the distribution at the catalyst. Of course, you can tune in and balance flow to each zone and look at stack readings such as NOx and ammonia slip, but that’s a complicated way to do the tuning and could be more effective. So, having a permanent test grid downstream of the catalyst provides you individual points of NOx and O2 and possibly ammonia slip. Unfortunately, if the unit is not equipped with this from day one, a retrofit of these grids can be very costly.

Why does an AIG need good ammonia distribution?

Why are we so interested in this distribution? The distribution can directly impact ammonia slip, which is a big concern. The better the ammonia to NOx distribution, the lower the percent RMS, and the lower the overall ammonia slip. If we look at the 90% dNOx case, we can see that if we had a 2% RMS, the ammonia slip could be less than one ppm. However, if we go up 5%, 9%, and 12%, the ammonia slip goes up. We want to see the typical range between 5% and 9%. So, you can see that between those numbers, the ammonia slip can range between 1 to 4 ppm. As you go up above 12%, the ammonia slip starts going up. So, these are essential parameters you need to look out for when designing a system.

Why are SCR Inspections important?

Inspections are one of the first things we’ll recommend to identify performance issues. SCR inspections are an excellent method for identifying performance problems. We look at various things in the SCR, the mechanical integrity of the AIG, any potential pluggage from scale or foreign debris that would get into the ammonia injection grid and cause pluggage in some of the holes, and of course, catalyst buildup or pluggage, which is usually due to insulation liberation. And then the big one is sealed. Seals can have a direct impact on performance.

Why is CFD modeling important?

I need to emphasize the need for and the advantages of CFD modeling for these types of systems. Many systems may be considered straightforward or duplicates of a previous system. An actual duplicate doesn’t exist unless it’s the same plant. So, there’s a need to make sure that the ammonia distribution and the flow distribution have been carefully detailed in the modeling. And, you know, poor flow, ammonia flow, and ammonia distribution or temperature distribution can affect the CO performance and, of course, the SCR performance in terms of NOx reduction and ammonia slip.

If you are having emissions issues and need support to implement a solution to keep your plant reliable and within performance targets, Bremco can help support anything form a catalyst change out to a complete AIG retrofit.