Coal, Instrumentation & Controls, O&M

Factors to Consider When Selecting a Compressor and Dryer System

Issue 12 and Volume 121.

Compressed air is an essential utility in manufacturing processes for all types of power plants ranging from traditional coal to nuclear- or gas-fired power plants, and new-generation solar and geothermal power plants. It powers applications like pneumatic conveying for fly ash, limestone and mill rejects, or dust suppression for coal-handling plants. Because of its critical function, compressed air is often referred to as the fourth utility behind water, gas and electricity.

Factors to consider when selecting your system

With the latest technology innovations, today’s compressor and dryer systems are increasingly more efficient. It’s important to find a system that meets the demands of your power plant so you can take advantage of additional energy savings. As you’re looking at different options, here are some factors to consider:

Quality of Instrument Air

ANSI /ISA – 7.0.01 – 1996 is a globally-recognized standard for instrument air as defined by the Instrument Society of America. As recommended by the Standard, lubricant content in compressed air should be as close to zero as possible because any introduction of oil to the system could cause serious issues like oil carryover in power plant applications.

An increase in flow & temperature also increases oil carryover through downstream filters. At close to 105°F, oil carryover from the oil-injected compressors and high-efficiency filters increases from 0.05 ppm to almost 0.3 ppm. The higher oil carryover contaminates the desiccant of the downstream dryers, leading to a decrease in the dryers’ performance. Water and oil then have the ability to enter the compressed air system where solenoid pilot valves and I/P converters may stick together and potentially trip the entire power plant.

Efficiency

Machine operating efficiency can be significantly affected when oil is introduced to the compressed air system. Oil can force power plants to start and stop their compressor systems, resulting in downtime, increased energy consumption, penalties and lost profits. In worst case scenarios, oil contamination can also force a complete plant shutdown.

In gas-based power plants, gas turbines are particularly sensitive to machine starts and stops because each start and stop reduces the life of the gas turbine. Not only is there machine degradation, but each trip in the system and shutdown results in losses due to potential penalties and lost profits. These penalties are often levied for non-dispatch of power by the transmission companies, which could cost up to $250 per MW. Not to mention, each stoppage could last anywhere from 2-4 hours depending on how long it takes to locate the fault and restart the gas plant.

“Oil-free compressors can maintain a consistent 100 psig without using more energy, which can save plants an estimated 10 to 12 percent on power costs.”

Let’s take a look at some examples of how much it can cost for an average penalty and its effects on a plant’s profit.

A 750 MW gas-power plant tripping for 4 hours with a penalty of $250 per MW:

$250 X 750 X 4 = $750,000 per stoppage on account of penalties

A 750 MW gas-power plant with a 20 percent profit on the sale of power at an average power cost of 10 cents per MW and a stoppage of 4 hours

$0.1 X 0.2 X 750 x 1,000 x 4 = $ 600,000 on account of lost profits

These costs and penalties are even higher for coal-based plants because it can take up to 16 hours to restart a machine, and large quantities of heavy-fuel oil are required to atomize air and heat boilers to temperatures that auto ignite pulverized coal. It’s important that coal-based power plants consider the additional cost of using residual oil and atomizing air while calculating the risk of oil contaminating the plant’s compressor system.

Power Costs

With oil-injected compressors, pressure loss is a big concern that can greatly increase power costs. Most instruments and actuating valves in an oil-injected compressor require a compressed air pressure of 30-75 psig at its point of use. Once the air has passed through the compressor, dryer and air receiver, the pressure required increases to 90 psig, which means that the compressor must work at 100 psig downstream of the oil-removal filters to maintain this pressure level. Pressure drops are common when air is filtered, which means the pressure at the outlet of the screw element must be 107 psig. As the machine runs around the clock, the separator and oil-removal filters can choke, causing the outlet pressure to increase to 120 psig as the number of operating hours increase.

Unlike oil-injected compressors, oil-free systems do not experience pressure drops as air passes through separators and downstream filters. Oil-free compressors can maintain a consistent 100 psig without using more energy, which can save plants an estimated 10 to 12 percent on power costs. Since these compressors do not require filters or the addition of oil to the machine, regular maintenance checks and services are also less frequent.

Pressure Dew Point

According to the ANSI /ISA – 7.0.01 – 1996, the pressure dew point as measured at the dew point outlet should be at least 18°F below the minimum temperature to which any part of the instrument air system is exposed. Moreover, the pressure dew point should never exceed 39°F.

Ambient temperatures vary widely across the U.S. and throughout the year with low ambient temperatures ranging from -10°F to 50°F and high ambient temperatures from 40°F to 110°F.

Because ambient temperatures are not fixed, a variable dew point of -30°F to 39°F is used based on ambient conditions.

This variable dew point requirement can be met using heat of compression desiccant dryers, which are dew point suppression dryers. Heat of compression dryers require very little additional energy in the form of compressed air or electrical heating. As these dryers use the heat of compression energy to regenerate the desiccant, they can save up to 20 percent of compressed air energy costs when compared to guaranteed dew point dryers.

Conclusion

Power plants can save up to 30 percent on the power costs of compressed air when they select an oil-free compressor with a heat of compression dryer as opposed to an oil-injected compressor with a heatless desiccant dryer. In a typical combined-cycle plant of 750 MW with 3 large, 160 KW compressors, this would save approximately 100 kW. If the system runs for 8,600 hours at a power cost of 10 cents per unit, savings could equal:

100 x 8,600 x 0.1= $ 86,000 per year

 

Deepak Vetal is product marketing manager (Oil free Screw and Centrifugal Compressors) at Atlas Copco Compressors LLC.