Selective Catalytic Reduction (SCR)
The Complete SCR Cleaning System
Coal-fired SCR reactors have been plagued by buildup issues since entering the market. Though the severity of build up problems can vary greatly between boilers, the causes of, and solutions to build up problems are remarkably predictable. IGS is the first company to combine various cleaning products to offer SCR owners a comprehensive solution. This document will briefly discuss how each cleaning product, namely LPA Screens, Air Cannons, Sonic Horns, and Secondary Screens, work in unison to maintain a high standard of SCR cleanliness.
LPA Screens: The Foundation
The first and most important step for SCR cleanliness is to prevent Large Particle Ash (LPA) from ever reaching the reactor. LPA is accepted as the #1 and most severe cause of SCR buildup problems. If LPA is present and allowed to reach the SCR reactor, fly ash accumulation will occur. LPA is generally defined as particles that are 4-7 mm in characteristic dimensions, of adequate size to lodge within the opening of installed catalyst. LPA is often larger than the catalyst opening. If this is the case, the particle will sit on top of the catalyst and prevent fly ash from passing through. Additional fly ash will collect on top of the blocked area and form ash piles. LPA can also be small enough to enter the catalyst, but too large to pass through the catalyst. If this is the case, the particle will create a blockage in the catalyst channel, and generate buildup on top of the catalyst.
It would be a great relief to SCR owners if a company could eliminate the formation of LPA. Unfortunately, several studies concluded that the formation of LPA is difficult to predict and virtually impossible to prevent. The best-known defense against LPA is the installation of a LPA screen, which is why all new high dust reactors in the United States and Europe install LPA Screens as original equipment. Several examples can be cited in which massive, LPA-related buildup problems have been solved by the installation of a reliable LPA Screen.
The LPA Screen is typically installed in the economizer hoppers to prevent LPA from reaching the SCR. If the LPA screen fails and allows LPA to enter the reactor, high operational and maintenance costs are certain to follow. A reliable, long-life LPA Screen is critical to the health of all SCRs and is the first step toward a clean SCR.
Other Causes of Buildup
LPA is not the only cause of SCR buildup. Generally, all other buildup problems are caused by poor gas and ash flow distribution. A combination of regions of low gas velocity and high ash loading overwhelm the catalyst and cause pluggage. As the ash covers more and more of the catalyst, the severity of the problem increases. This can lead to some, or all, of the following problems:
- Forced outages for off line cleaning
- A derating of boiler operation
- Shortened catalyst life
- High pressure drop
- Excessive ammonia slip
- Direct negative effect on catalyst performance
In order to address problems related to poor gas and ash distribution, SCR owners should install online cleaning systems specifically designed to address these factors. Data proves that assuming LPA is kept out of the reactor, the combined cleaning efficiency of Air Cannons and Sonic Horns is sufficient to tackle some of the most severe buildup problems, when the cleaning system is designed properly. Secondary LPA Screens are specifically designed to enhance the cleaning efficiency of Air Cannons and Sonic Horns, ensuring the maximum effectiveness of the cleaning system.
Air Cannons: The Corrective Agent
When gas and ash distribution are poor, the buildup generally starts along the walls of the SCR (the area with the lowest gas velocity). As the ash piles grow, they reach an angle of repose causing it to spill out toward the middle of the reactor. This process repeats itself until a small ash pile grows into a sprawling mass of fly ash. The key to eliminating this problem is attacking its source: the small ash pile.
Air Cannons are specifically designed to eliminate ash accumulation along the walls of a SCR by addressing poor flow distribution. Each Air Cannon is a reservoir tank filled with compressed air that is held behind an actuator valve. When the valve is triggered, the stored air erupts through a specially designed outlet nozzle (mounted in the walls of a vessel). The Air Cannons redistribute the fly ash that accumulates in the high ash loading regions and poor flow zones, sending it back into the higher flow zone in the middle of the reactor. By doing so, the Air Cannons also re-entrain the ash particles into the flue gas stream, never allowing them to settle, pack in, or solidify. This cleaning action serves as a corrective agent for poor gas and ash distribution problems. Air cannons eliminate large buildup problems by not allowing small deposits to form and propagate.
Air cannons have successfully eliminated buildup in some of the most problematic SCR installations. They are the only cleaning device that directly addresses poor flow problems. All SCRs would benefit from an Air Cannon installation.
Sonic Horns: The Perfect Complement
The corrective agent of air cannons is critical to the health of the SCR. However, their cleaning range is limited to the poor flow zones (about 20 feet from the reactor wall). In order to guarantee cleanliness throughout the reactor, an additional cleaning device should complement the Air Cannon. The Sonic Horn is the perfect complement.
Sonic Horns use high-energy, low frequency sound to energize airborne particles and reduce their deposition on layers of catalyst. In other words, the sound waves of a Sonic Horn vibrate ash particles. Gravity and/or gas flow then filter the accumulation through the catalyst. Although Sonic Horns do not address poor ash distribution and will become overwhelmed with excessive ash loading, they are very effective at helping a reasonable amount of ash pass through the catalyst. Sonic Horns clean a large area and work very well addressing buildup beyond the cleaning range of the air cannons. Tests display remarkable cleaning ability even at a distance of 50 feet.
LPA Screens, Air Cannons, and Sonic Horns work together to eliminate problems of poor gas and ash distribution. The LPA Screen blocks particles too large to pass through the catalyst from reaching the reactor. Air Cannons address poor ash flow distribution and do not allow ash to settle. Sonic Horns then help move the remainder of the ash through the reactor and address the hard to reach areas. This complementary cleaning system ensures full coverage for a SCR.
Secondary LPA Screens: The Enhancement
Metal screens have been placed on top of SCR catalyst for decades. The primary purpose of these screens is to enable workers to walk in the reactor during outages. While these screens generally accomplish that purpose, they offer little protection to the catalyst and present a turbulent cleaning surface that can even hinder online cleaning devices. The Secondary LPA Screen is the first technological advancement to those metal screens. It is specifically designed to offer superior protection to the catalyst and enhance the cleaning action of Sonic Horns and Air Cannons.
Secondary LPA Screens offer superior protection to the catalyst by virtue of its material and design. The screen itself is more rugged and better able to withstand the harsh environment of the SCR. The average perforation size of the Secondary LPA Screen is much smaller than that of typical metal screens, allowing for maximum filtration while still having a high open area ratio. This prevents chunks of bisect ash from bypassing, reaching and plugging the catalyst. Whereas most screens only protect the catalyst from being walked upon, the Secondary LPA Screen offers much more protection.
The Secondary LPA Screen is specifically designed to enhance the cleaning efficiency of Air Cannons and Sonic Horns by using a technology that is well known throughout the power industry: a smooth surface. This improves the cleaning efficiency of Air Cannons by keeping the air blast on the screen longer and requiring less force to move the ash. Test studies by Air Flow Science suggest that this can improve the cleaning distance by as much as 66%, while simultaneously keeping the blast away from the fragile catalyst. The smooth surface enables the ash on the screen to move more freely when vibrated by the Sonic Horn, thereby improving its cleaning efficiency.
LPA Screen + Air Cannons + Sonic Horns + Secondary LPA Screens = Clean SCR Reactor
As previously stated, the causes and solutions to SCR buildup are remarkably predictable, once the root causes are identified and understood. When present, Large Particle Ash is primary root cause of ash accumulation in the SCR. IGS eliminates that problem with the most effective and reliable LPA screen in the world. Once the primary root cause is addressed, the secondary root causes of buildup, poor ash and gas flow distribution, can be resolved utilizing the remainder of the online cleaning package.
Primary LPA Screens, Air Cannons, Sonic Horns, and Secondary LPA Screens therefore work in unison to provide a high standard of SCR cleanliness. The combined cleaning efficiency of these products is why IGS is able to offer customers an unmatched 95% Clean SCR Guarantee.