Nitrogen Generators in Power Plants

Submitted by Jon Newton, featured in Product Design & Development Magazine
on Fri, 05/09/2014
Proper treatment of boiler systems when they are off line is a constant concern for those managing and overseeing the operations of a power plant. Many power plants are known in the industry as peaker plants, meaning they are not running constantly. Making sure that the Heat Recovery Steam Generator (HSRG) is properly prepped for this period of shutdown is critical. Improper lay-up can lead to metal loss and corrosion within the boiler. Corrosion is a real threat as it has the potential to weaken the entire HSRG, leading to a number of safety concerns. Corrosion from ineffective lay-up may also generate potentially harmful iron oxide rust particulates impacting overall readiness.
How is this problem addressed? There are two general ways to layup a HSRG when a power plant is shutoff for a specific period of time: dry and wet lay-up. The decision on which to implement is made based on specific layup protocols determined by each individual power plant. If the shutdown is going to be relatively short, often a wet lay-up is preferred. This is a process where water is left in the HSRG and treated with various chemicals in order to prevent the metal loss and corrosion mentioned earlier.
In a dry lay-up, the water is drained out of the boiler and replaced by an inert gas. The main concern with a dry lay-up is that boiler metal surfaces are now exposed to oxygen in normal air resulting in significantly increased potential for oxidative processes and pitting.
In a dry lay-up, the headspace above any remaining water is filled with an inert gas. Slight positive pressure is maintained to overcome any pressure loss the system may experience. During a drawdown, the nitrogen is fed to the vent and fills as the water is drained. The rate of water removal determines the replacement nitrogen gas flow rate so the system is not subject to any vacuum stress.
Nitrogen is a popular inert gas in dry layups due to its characteristics. Nitrogen is a clean, safe gas that is both convenient and reliable. This gas has been used for many years to protect against corrosion in power plant equipment and is considered the perfect green solution. The biggest concern in the past was the added cost of transporting and renting the cylinders of nitrogen. Managers looked for ways to eliminate this expense.
A trend that we’re seeing more and more in the power generation industry is the concept of power plants generating their own nitrogen to use in the dry lay-up as a way to save on costs. The equipment is safe and a more reliable alternative, as a manager can eliminate missed deliveries, rising costs and contracts. On site nitrogen tanks, while offering the lowest delivered nitrogen cost, occupy real estate, require concrete pads and permitting and still rely on deliveries. Additional non-nitrogen costs, such as demurrage, contribute to the total cost of the nitrogen, not just the per CCF (hundred cubic feet) cost . Furthermore, costs from the supplier typically increase over time.
A nitrogen generator is not only environmentally friendly, but it doesn’t require very much maintenance after it is properly installed. Nitrogen generators don’t typically take up too much space in a plant; the units are generally the size of a large file cabinet. To determine what kind of generator will best fit the needs of a particular power plant, several factors must be taken into account. The first of the three main elements that must be considered is the purity level of the nitrogen your power plant requires, based on protocols. The other two elements that need to be taken into consideration is the dry lay-up head space in terms of cubic feet, and the desired time in which to achieve the lay-up.
There are two basic types of nitrogen generators: membrane and pressure swing absorption (PSA). Both require protection of the membrane or the media from oil contamination generated from the air compressor and air receiver. A nitrogen receiver assists in managing cycle times and maintaining the desired positive pressure for extended lay-ups.
In selecting a membrane system, consider the total cost of ownership. If the membrane system uses heat to enhance the membrane performance, that adds cost. A highly permeable membrane is desirable in an efficient lower cost operating system.
With a PSA system, we see both mono-bed and dual-bed systems. As the compressed air at high pressure (100-110 psig) passes through the media, it selectively retains the oxygen and allows the nitrogen to pass until the media bed is saturated. The next step is depressurization. When the mono or dual bed rapidly depressurizes it releases the oxygen to the atmosphere. A mono-bed system is appropriate with lower flow rates. A dual-bed system achieves higher flow rates through alternating between the two tanks. A nitrogen storage tank is used to minimize pressure fluctuations and to achieve a continuous flow.
The flow rate is determined by the volume of water to be drawn-down, the rate at which draw-down is performed and the acceptable time frame to accomplish the draw down and lay-up. Nitrogen flow rates are typically expressed in cubic feet per hour. Knowing the required nitrogen flow rate, nitrogen purity and time frame aids in selecting the appropriate nitrogen generator system. When evaluating ROI of self-generated versus delivered nitrogen, project your current nitrogen budget for 5-10 years. Compare that to the cost of a nitrogen generator, with maintenance factored in. Plants may see an ROI in less than 2-3 years with a 10-year savings exceeding $50,000.

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