What is Evaporative Cooling and How Can it Help Emission Reduction

Evaporative gas cooling is a complex process, one that has experienced improvements and innovation over the years. Ultimately, a proper gas cooling system is a competitive advantage for plants looking to increase efficiency, reduce fuel costs, and emission reduction. In the content below, we explore the importance of evaporative gas cooling, the evolution of cooling systems, and how to apply proper fluid application inside of a downcomer duct.

The Importance of Evaporative Gas Cooling

Plants that prioritize efficiency, monitor fuel costs, and reduce emissions continually search for methods and processes to capture and re-use energy.  In many instances, all of the energy (heat) generated cannot be recovered and a reduction in gas temperature is needed to protect downstream equipment.  If the gas temperature is too high, it could ignite the filtration media in the baghouse that is used for removing the particulate from the flue gas.  In addition, process gas must be cleaned of dust and contaminants. To properly reduce the gas temperature, the flue gas typically enters into a gas conditioning tower (GCT) or downcomer duct where a complex cooling process takes place.  A properly engineered evaporative cooling system prevents wetting inside the duct/tower or downstream equipment.  If the evaporative cooling system is not engineered correctly or properly maintained, wetting of the duct/tower will occur that ultimately results in costly downtime and clean-up of the wet material.  It also contributes to corrosion of the metal surfaces.

Atomization Technologies

Two primary atomization technologies are utilized in the evaporative cooling process today: high-pressure atomization and twin-fluid atomization. What are the primary differences?

High-Pressure Atomization

As the name implies, high-pressure atomization utilizes high pump pressure to mechanically atomize the water into small droplets. Typically, pump outlet discharges pressures range between 500-600 psig.

Twin-Fluid Atomization

The twin-fluid process employs compressed air to atomize water into droplets, also utilizing a spray injection nozzle. Both fluids, the twin fluids, are combined inside the nozzle and gets injected into the flue gas stream. Twin-fluid atomization typically operates in the 60-100 psig range.

Although both technologies are readily used throughout the world, most installations utilize twin-fluid technology. Why might a plant choose one atomization technology over the other? Simply put, twin-fluid systems produce smaller droplets at lower operating pressures. Both characteristics allow for shortened evaporation distances and reduced construction costs. However, reduced construction costs are often made up in increased operation and maintenance expenses as twin-fluid systems require a constant supply of compressed air capacity.

Timely Improvements in Evaporative Cooling Systems

The Problem

Over time, plants have needed improved performance out of their evaporative cooling systems, often encouraged by new regulations in the market. One specific example involves an outlet temperature that needed to be reduced.  Previous attempts to reduce outlet temperatures resulted in wetting of the material and ductwork, ultimately forcing outages for the plant.

The Solution

A few plant modifications were suggested: The first included conversion from a high pressure to a twin-fluid atomization system.  This would require the addition of air compressors which would be expensive to purchase and operate.  The second was pump modifications to increase operating supply pressure.  This would require the purchase of new pumps, another costly expense.  Finally, overall improvement of the current system, utilizing as much current equipment as possible.  Upon review of the choices, the most cost-effective change involved reviewing and improving the water injection system.

Selecting the proper nozzle system involves a plethora of considerations, including material of construction, flowrate, spray angle, operating pressure, placement, droplet size, maximum free passage, etc.  All of these factors needed to be evaluated to come up with a solution that met the customer’s needs.  Ultimately, a review of the current water injection system being used revealed a maldistribution of water in the flue gas.

In response, new lances were designed and installed to improve the distribution of the water – extra care was taken to ensure that additional coverage did not create oversaturation in one area that would result in wetting or corrosion. The plant was able to modify the existing ports to accept the new lances and extending the length of the lances resulted in uniform placement of water in the tower.

The Results

The improved water injection resulted in better cooling capability without wetting or corrosion. No costly changes in the existing pump system were required. The evaporative cooling system’s improvements helped the plant achieve their temperature target which contributed to a reduction in emissions.

Downcomer Duct

The downcomer duct serves as an alternative or addition to the GCT. The gas can be cooled down after the preheating tower in the downcomer duct. The decision to use a downcomer duct is primarily implemented in new plants and plants with long, straight duct. If the downcomer duct is being installed in an already active plant, the system is used to optimize existing processes, such as increasing production or aiding in alternative fuel usage. With the use of a downcomer duct in an existing plant, temperature peaks are compensated and, when necessary, additionally cooling in the gas cooling tower is provided!

The importance of a properly design evaporative cooling to increase efficiency, decrease fuel costs, and reduce emissions cannot be overstated. Thus, plants should consider employing Lechler’s expertise in improving their current downcomer duct cooling system.

Construction

Unlike a gas cooling tower, downcomer ducts are built with smaller cross-sections and higher velocity, reducing evaporation time.  The amount of water required and necessary droplet size for complete evaporation is determined by two important factors: required outlet temperature and available evaporation distance. These two factors are crucial. Why? Because complete evaporation is essential to ensure the durability of the downstream fan and prevent material build-up. So, what droplet size is necessary? Typically, finer droplets are required to achieve total evaporation. Influenced by higher gas velocity and a decrease in evaporation time, fine droplets perform best to achieve optimal results.

Evaporative Cooling Systems Redesigned Using Lechler’s Expertise

Are you unsure of which solution is perfect for your specific application? As industry experts, our team at Lechler comprehends exactly how our systems can complement your plant – you never need to wonder if you are making the right choice for your application. We’re more than just manufacturers.

Serving a plethora of markets, we provide custom engineered spray solutions. At Lechler, we configure our systems according to your unique process data and duct dimensions, creating the perfect situation for your situation.

For more information about our company, products, or primary markets, please do not hesitate to reach out at (800) 777-2926 or via our online contact form!