A system using pressurized liquid nitrogen to remove stubborn deposits on power plant heaters and boilers offers a fast and effective alternative to traditional chemical and water treatments, without creating torrents of effluent.

Al Hovland, Conco Systems, USA

Air heater and recovery boiler cleaning in power plants is typically carried out with high-pressure water, chemicals or steam. These techniques, while effective on moderate air-side fouling of heat exchange surfaces, are usually unable to remove the more tenacious deposits that can build up from fly ash, dust and oil in coal fired plants.

Without regular cleaning the heat transfer efficiency reduces, which in turn lowers boiler efficiency and increases a unit’s heat rate. Severely dirty air preheaters and air heaters can even reduce the unit’s power output. However, PPL Generation’s Brunner Island plant has found a highly effective way around these problems by using pressurized liquid nitrogen.

Function and performance

Plant air heaters improve boiler efficiency by using exhaust flue gases to preheat combustion air. This accelerates combustion by producing more rapid ignition, while also allowing low-grade fuels to be used1.Each 10 ºC (18 ºF) increase in boiler inlet air temperature increases boiler efficiency by 1 per cent, up to a maximum of 10 per cent2.

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Figure 1: The NitroLance process flow

Configurations can vary widely, but plants typically use either a regenerative Ljungstrom-type rotary heater or a fixed recuperative tubular device. Depending on the configuration, it may also contain a finned tube air preheater immediately after the forced draft fan.

This keeps the primary heater warmer than the dew point, or acid saturation point, in the primary air heater baskets or coils. If this drops below the acid saturation temperature – usually between 87.8 °C and 110 °C, or sometimes at temperatures as high as 126.7 °C – there is a considerable risk of dew point corrosion damage3.

Without preheated air, flue gas products will corrode the heat transfer surfaces, partially block airflow and, in extreme cases, limit the power capacity of the unit. While playing a critical role in increasing boiler efficiency, preheating is only effective if the air heater is maintained and kept clean of air-side fouling.

Liquid nitrogen cleaning

Conco Systems of Pennsylvania, USA developed a system using pressurized liquid nitrogen to clean heat exchangers and industrial surfaces. The NitroLanceTM cleans tube internals with small rotating insertable jets, or with larger specialized manifolds for external heat exchanger surfaces, including delicate finned heat exchanger tubes.

High-purity liquid nitrogen is pressurized to between 345 bar and 3793 bar, depending on the part being cleaned, passed through a controller that regulates the temperature to between -106.7 °C and -156.7 °C, and then directed out of an extendable nozzle with a reach of over 90 metres onto the surface to be cleaned. The entire system is mounted on a mobile platform to allow it to be positioned in the most convenient place.

As the high-density vapour penetrates the cracks and crevices of the fouling deposit, it rapidly converts to a gas and expands in volume nearly 700 times. The cold temperature also makes the deposit more brittle, which in combination with the high delivery pressure, breaks down the fouling deposit (see Figure 2).

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Figure 2: The liquid nitrogen removes the deposit by volumetric expansion, cold temperature and pressure

No damage to parts

An evaluation of the effects of liquid nitrogen cleaning on the base metal was performed to examine possible changes to the grain boundaries and structure, as well as the potential for shrinkage or modifications of mechanical properties that might have resulted from the cold temperatures. Micro-structural observation and micro-hardness measurements were performed on the material before and after liquid nitrogen cleaning.

Using pressurized liquid nitrogen to clean basic carbon steel induced no micro-structural modifications or changes in the surface superficial micro-hardness of the base metal.

Shrinkage or modifications of the base metal’s mechanical properties is avoided due to the relatively fast speed of the cleaning process. It typically covers more than three metres per minute.

The fast cleaning speed and brief contact with the liquid nitrogen means the base metal only drops in temperature between 5 °C and 10 °C on average. This small temperature drop induced no notable metal shrinkage and had no significant influence on mechanical properties of the base metal.

Efficiency savings

Traditional water-based cleaning methods can produce hundreds of thousands of litres of effluent and in the United States will now require expensive Environmental Protection Agency-mandated disposal, clean-up and processing. These include air heater cleaning and boiler tube cleaning, boiler fireside cleaning and many other plant cleaning processes.

Another benefit of the liquid nitrogen cleaning is that it produces no secondary waste streams. The fouling deposits removed by the cleaning process can be easily vacuumed up or blown out of the stack during subsequent normal operations. This lack of effluent represents a significant cost savings to power plants over traditional water-based methods.

As well as the cost savings from this lack of secondary waste, another advantage comes in the area of critical path maintenance activities. Liquid nitrogen cleaning required a significantly shorter window during a fixed-length outage at PPL Generation’s Brunner Island power plant than water-based methods tried without success in the past.

The water-based cleaning methods also required extended clean-up and disposal activities afterwards, while the liquid nitrogen cleaning method saved several days’ time in the critical path maintenance schedule. That represents a significant saving to PPL Generation.

Case study

Liquid nitrogen cleaning was first conducted at PPL Generation’s Montour power plant in Washingtonville, Pennsylvania, and then at Brunner Island power plant in York Haven, Pennsylvania. These use rotary regenerative Ljungstrom air preheaters with finned tube air preheaters, heated with hot water, upstream of the rotary air heater (on the air side).

The finned tube air preheaters often get fouled after a few years of plant operation. Whenever the rotary regenerative air preheater located above it was cleaned, debris would fall down onto the steam coil surface, requiring it also to be cleaned.

At Brunner Island Unit 3, a 760 MW coal fired unit, the fouling was due to fly ash deposits, dust and debris, and lube oil that had formed a thick shell over the fin coil surfaces. Since this is typically a baseload unit, it can only come down for maintenance at scheduled outages.

Previous attempts to clean the air preheater called on water, chemicals and detergents, but these all proved to be ineffective at removing the tenacious deposit of fly ash and oil. The oil portion of the deposit was only on the B-side and was caused from a bearing seal leak in the unit’s forced draft fan. In operation, this fouling restricted combustion airflow on both sides of the fan.

Operators attempted to force more air to the A-side by the cross-connect air ducting. This partially worked, but still created an imbalanced airflow. This situation often reduced the power output during certain weather and operating conditions.

As an outage approached for the unit in spring 2010, the PPL engineers carried out an evaluation of Conco’s NitroLance system and scheduled the fouled B-side air preheater coils for cleaning.

Cleaning results

Conco’s engineers evaluated the site and concluded it would be beneficial to install a new access door in the ducting adjacent to the air heater. Scaffolding was placed directly under the horizontal air preheater, allowing access to the area to be cleaned.

Although only one mobile unit is required in most cases, due to minimal time available in this case, Conco brought in two mobile units to clean the air heater in only a single 12-hour shift.

The technicians were equipped with full safety gear, including overalls, full-face respirators (with an external air supply), rubber boots and cryogenic insulated gloves. At the start of the unit’s outage, the Conco technicians went into the air preheater coil ducts and quickly cleaned a portion of the fouled tubes. The system’s delivery pressure was carefully adjusted to ensure the heat exchanger’s delicate tube fins (0.3 mm thick) were not damaged.

The externally finned heat exchanger was cleaned almost immediately. The fly ash and oil deposits were removed in seconds and most of the B-side air preheater was cleaned in the 12-hour window. The external tube fins are now functional and they suffered no damage from the liquid nitrogen cleaning process (see Figure 3).

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Figure 3: NitroLance cleaning on Ljungstrom baskets – before (left) and after cleaning (right)

Performance results

After the unit was brought back in service, the airflow blockage on the B-side was immediately found to have been significantly reduced, so that the A- and B-sides had become airflow balanced. In fact, the pressure drop across the air heater was reduced by half, from 2.0 kPa to 1 kPa. Although still higher than the original design air flow pressure drop (0.4 kPA), the improvement was significant considering only a portion of the air heater was cleaned. The potential exists for additional improvements by going back in at a future outage and cleaning all of the tubes to reach the design pressure drop across the preheater coils.

Because of the significant improvement in the air heater’s pressure drop, to bring it closer to original design parameters, the plant is considering delaying its replacement, saving PPL Generation an estimated $2.5 million to $3 million. Further benefits of this new cleaning approach include additional savings such as increased boiler efficiency, improved unit heat rate, a higher MW output, reduced house load, and lower CO2 emissions.

These benefits were not specifically calculated for this unit due to the many other acts of maintenance performed during the outage that also improved the unit’s performance. But the heat exchanger cleaning with nitrogen resulted in a significant improvement in unit operation by allowing increased airflow through the unit and balancing the air distribution within the boiler. Additionally, this work was accomplished without increasing the duration of the planned outage.

Other applications

The cleaning of the steam coil air preheater was so effective that Conco also worked with PPL Generation to test clean sample regenerative air heater baskets. The results from cleaning baskets taken from the Montour plant’s Ljungstrom unit showed that they had been restored to a very high level of cleanliness.

It would also be possible to clean the rotary air heaters’ lower end baskets in-place during an outage, due to acceptable access heights between the steam coil air heater and most primary air heaters. Soot blowing and other water-blasting techniques may now feasibly be replaced or supplemented with liquid nitrogen cleaning.

The deposits are easily vacuumed up and there is no need to remove wastewater since none is produced as a by-product of the cleaning. This avoids the EPA-mandated expense of collection, removal and wastewater processing of potentially thousands of gallons of effluent.

This technique was also recently tested on boiler superheater tubes at PPL Generation’s J E Corette power plant in Billings, Montana. The results indicate efficient cleaning on the deposits of ash and slag, without damaging the parent tube material.

Overall, nitrogen cleaning with a NitroLance provides a new and significant technique for cleaning various types of deposits that accumulate in power plants and industrial facilities without damaging the parent metal. The types of deposits that are easily removable include fly ash, slag, carbon, soot, sulphur and oils.

Liquid Nitrogen: an optimal choice

Air preheaters provide a critical role in boosting boiler efficiency but when compromised with air-side fouling can actually reduce plant output. Historically, air preheater cleaning has been accomplished with water, steam or chemicals, which produce a significant volume of effluent that requires handling and disposal at considerable cost. The recent introduction of new technology using pressurized liquid nitrogen not only speeds the cleaning process, but does it without producing secondary waste streams, saving plants time and money.

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Figure 4: Air heater performance on Brunner Island Unit 3. Red dots show delta pressure on the air heater’s B-side and blue dots show delta pressure on the air heater’s A-side (note improvement in April)

In addition to air preheaters, liquid nitrogen also appears to be an optimal choice for many other applications within power generation and industrial facilities where its speed of deposit removal and zero-waste is a distinct advantage.

References

1. Everett B. Woodruff, Steam Plant Operation, (New York, NY: McGraw-Hill, 2005) 73-74.

2. Steam, Its Generation and Use, Babcock & Wilcox Co. (2005), (41st edition).

3. Air Preheaters, Wikipedia, 22 July, 2010.

4. Air Liquide Corporation, Gas Encyclopedia (encyclopedia.airliquide.com/encyclopedia.asp).

5. Environmental Protection Agency (EPA), Code of Federal Regulations CRFe, Title 40, Part 423, Steam Electric Power Generating Point Source Category, 3 August, 2010.

The author would like to thank Frank G. Lyter of PPL Generation and Charles Truchot of Air Liquide for their invaluable contribution to this article.

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