CataStak SCR 101 - Nationwide Boiler Inc.
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The Nationwide CataStak™ is a portable, selective catalytic reduction (SCR) system capable of reducing NOx by up to 99% and is a cost-saving alternative to ultra low NOx burners. Specific expertise and success has been achieved in the more difficult low-temperature zone for SCR’s. For first time users, we have compiled a detailed description of the system and all components that make up the CataStak™ SCR system.

System Overview

The CataStak™ SCR system can be installed either vertically or horizontally and has been installed in applications ranging from boilers, gas turbines, and fired heaters.  It consists of a single-reactor housing with NOx reducing catalyst and/or CO oxidizing catalyst, and a standard reagent delivery system (pure anhydrous ammonia, aqueous ammonia, or urea). Nationwide Environmental Solution’s NOx reduction control system for the precise control flow of ammonia and an ammonia injection grid is also included. Based on design and operating conditions, the reagent can be directly injected or supplied by a vaporizer unit.

Catalyst Features

Nationwide Boiler offers multiple options for catalyst, honeycomb, plate type, or pelletized, based on the specific application and equipment type. The unique design of the catalyst provides high NOx or CO removal activity, low pressure drop, low SO2 oxidation rate and excellent durability. The catalyst features provide:

  • High NOx or CO removal level with minimum ammonia slip
  • Low activity towards SO2 oxidation, minimizing the risk of fouling downstream equipment
  • High poison resistance ensuring a long and stable service life 

Structural Steel Reactor Housing

Catalyst is installed in a single structural carbon steel housing with custom inlet and outlet transitions (square to round) flanged to match the stack connections. The transitions are designed to have access doors for inspection. The entire housing with transitions is insulated and covered with aluminum jacketing, and is provided with structural steel supports designed in accordance with local standards and structural calculations.

Ammonia Delivery System

Nationwide's standard ammonia delivery system consists of the anhydrous ammonia storage, ammonia flow control unit with a mass ammonia control valve, and ammonia injection grid. Anhydrous ammonia is typically stored in standard 150 lb. gas cylinders that connect to a manifold which accommodates up to three cylinders, allowing each cylinder to be isolated and switched over on-line.  Anhydrous ammonia gas cylinders are the most cost-effective and practical storage method for most SCR systems, however alternate delivery systems, including tanks and forwarding skids can also be supplied.

Ammonia Flow Control Unit

Ammonia flow control unit (AFCU) components are skid mounted in a compact space-saving arrangement where all operating devices are within close proximity of each other and within easy access for the operator. The NOx reduction control system for anhydrous ammonia includes a vapor ammonia header with safety valves, switches, a pressure regulator, and a “smart” flow control valve. For an aqueous ammonia reagent, the system also includes strainers/filters and a vaporizer.  The skid operation is managed by using a stainless steel panel with PID loop controllers, indicating lights, on/off switches, and an alarm bell. Two dilution air blowers are provided (one for 100% redundancy), and one stationary gas monitor for continuous measurement of NOx for monitoring alarm and trim capabilities is also included.

The flow rate of the ammonia is controlled based on the actual fuel flow measurements and pre-determined uncontrolled NOx emissions.  This feed forward control ensures compliance is maintained throughout the modulating range of the unit.

Ammonia Injection Grid

The Ammonia Injection Grid (AIG) consists of a header feeding a group of injection lances distributed across the flue gas duct.  The AIG provides uniform injection of the ammonia/air mixture over the duct cross section.

Ammonia-Free Option

For some users, an ammonia based SCR system is a major hindrance to the widespread acceptance of SCR technology.  Until recently, the only option available to customers adverse to the use of ammonia was complex external urea to ammonia processes that are very energy intensive and problematic to operate.

 Nationwide Environmental Solutions can now offer a simple and cost effective urea based CataStak™ SCR system to reduce NOx emissions through direct injection, which does not require external urea decomposition and their associated problems of energy consumption.

Researching urea based options led Nationwide Environmental Solutions to work closely with a leading edge company that had developed proven urea injection technology.  Working closely with their engineers and performing actual fire testing on a 27,600 lb/hr package boiler at Nationwide’s test facility in Fremont, CA, a new breakthrough technology enabled Nationwide to offer a cost effective ammonia free alternative for reducing NOx; the Ammonia-Free CataStak™ SCR system. This advanced system couples urea injection technology with programmable electronic controls to achieve up to 99% NOx reduction at a very low capital and operating cost. 

The operation of the urea injection system is similar to Nationwide’s well proven ammonia control systems. A 4-20a feed forward load or fuel flow signal precisely meters small amounts of safe urea-based reagent into a boiler outlet duct through patented injectors. The unit operates at temperatures down to 370F, and the urea decomposes to ammonia within the duct and is intimately mixed into the flue gas stream.  It is then transported over the catalyst bed where the catalytic process occurs. This process converts the NOx into harmless nitrogen and water vapor.  This process has been designed and tested both in the field and by CFD and cold flow modeling. 

Now boiler owners and operators adverse to ammonia can economically obtain the superior performance of SCR technology and not be limited to the poor operating and energy intensive performance of 9 ppm burners.  Eliminating NOx in the stack does not impede combustion or burner operation, nor is flame stability, load response, and turndown compromised.  The Ammonia-Free CataStak™ offers the best of both worlds: safe and reliable SCR performance without ammonia.

Ammonia-Free Option: Urea Injection Control Skids

The urea injection skid consists of a NEMA 12 control panel with a PLC based controller, touch screen display, variable speed injection pump, flow meters, 17 gallon day tank, level sensor, and solenoid valve. A 32.5% aqueous urea solution is utilized as the system’s reagent.  Type K thermocouples for system “ON” indication is also included.

Ammonia-Free Option: Urea Injectors

Low capacity stainless steel patented electronic urea injectors are supplied as part of the Ammonia-Free system. For boiler applications, they will be mounted in each boiler’s outlet duct closest to the furnace exit where the temperature is hottest, in order for the system to utilize the internal temperature of the boiler to induce the chemical reaction. The innovative design and features of this pre-packaged injection system assures precise metered urea flow for maximum NOx reduction and minimum ammonia slip.