Acoustic Cleaning Combats PRB Ash Deposits on Regenerative Air Heaters

Issue 9 and Volume 106.

By: Mel Freund, Advanced Acoustic Technologies LLC and
Randy Vann, Xcel Energy

Acoustic cleaning of fireside deposits has been practiced for more than 20 years. Experience has demonstrated infrasound’s effectiveness in removing ash deposits in applications where large heat exchanger elements are used. It has been particularly effective in removing the aggressive deposits formed by Powder River Basin (PRB) ash from regenerative air heaters. Xcel Energy’s Roy Tolk Station has been using infrasound since 1996 to clean Ljungstrom air heaters subjected to PRB deposits. This article discusses the effectiveness of infrasound versus sootblowing as well as various issues related to the design, operation and optimization of acoustic cleaning systems.

One of the infrasonic generators installed at Xcel Energy’s Roy Tolk Station.
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Xcel’s Roy Tolk Station lies in the panhandle of Texas, about 80 miles northwest of Lubbock. There are two CE 565 MW units firing low-sulfur PRB coal from Wyoming. Unit 1 came on-line in 1982, with Unit 2 following in 1985.

First Attempt

The four size 311/2 Ljungstrom-type air heaters (two on each unit) were the first in North America to be installed with infrasonic cleaners in the original plant construction. The infrasonic cleaners (one per air heater) were installed on the gas inlet ducting, and were operated along with retractable sootblowers to provide maximum cleaning. Each horn operated at 20 Hz and provided 800 W of acoustic energy.

Pressure drop across the air heaters remained relatively constant for the first several years of operation. Tolk decommissioned the original infrasonic cleaners in 1989, however, because plant personnel were not convinced the cleaners provided any additional benefit beyond that provided by the sootblowers. As a result, when the infrasonic cleaners stopped working, they weren’t repaired.

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Within a couple of years, the pressure drop across the air heaters started to climb. Because this is typical of general industry experience with PRB ash in air heaters, it is not possible to know whether the removal of the sonic cleaners from service contributed to the degradation in cleanliness.

Second Attempt

Tolk installed a newer, more powerful infrasonic cleaner on the gas outlet of the Unit 1 West air heater, which started service in May 1996. This cleaner operated at 22 Hz, with acoustic power output of up to 5000 W. Based on experience from the latter 1980s, which showed that infrasound was extremely effective in cleaning regenerative airheaters with PRB ash when used in conjunction with conventional sootblowers, Tolk personnel believed this would be a much-improved combination. In one instance from the late 1980s, a plant documented a case where an air heater laden with initial deposits cleaned itself over time through the use of infrasound and sootblower in tandem. Previously only sootblowing had been used, which resulted in frequent washes and occasional load limits due to extreme pressure drops.

Because the air heaters at Tolk are in parallel on the gas side, pressure drop data is not capable of determining the difference in cleanliness between the two. Tolk performed Pitot tube tests to evaluate the performance of the infrasonic cleaner by showing the relative difference in gas flow between the air heaters. Each test comprised a 54-point traverse of the inlet gas duct of both the East and West air heaters. Figure 1, which plots the flow differential between the air heaters, shows how the cleanliness of the West air heater steadily increased when compared to that of the East air heater.

In November 1996, Tolk installed new hot and intermediate layer elements in the Unit 1 air heaters. At that time, the plant also decided to switch the West air heater sootblower off to get a feel for how much of the cleaning was being done by the infrasonic cleaner. The trend continued as shown in Figure 1. Tolk experienced the highest measured flow difference in August 1998, when the West air heater was passing over 22 percent more flow than the East one.

During this period, plant personnel decided to install more infrasonic cleaners to take advantage of the cleaning improvements. New infrasonic cleaners were installed on both air heaters of Unit 2, one cleaner per air heater, in the spring of 1999, along with replacement of all three element layers of the air heaters. The infrasonic cleaners were the only cleaning used on these air heaters.

The pressure differential began to climb across both air heaters of Unit 1 (because the gas flues are in parallel, the pressure drop across both air heaters is necessarily the same), so Tolk conducted a high-pressure wash in February 2000. By December 2000 the flow difference was back up to almost 10 percent.

To see if any measurable change would register, Tolk brought the sootblowers on the Unit 2 air heaters back into service. Since a reduction in pressure drop of about 1/2 inch resulted, the plant returned the sootblowers to service along with the infrasonic cleaners. That is how things remain on Unit 2 to the time of this writing. There has been no measurable increase in pressure drop since.

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Because of the results on Unit 2, Tolk placed the West air heater sootblower on Unit 1 back into service as well, and the fourth infrasonic cleaner entered service in February 2002. This will complete the complement of cleaning equipment.

Pressure Drop

The cost of increased pressure drop through a regenerative air heater is substantial. Fan power requirements increase through both the ID and FD circuits. Depending on the criteria used for cost evaluations, the actual cost of increased pressure drop can vary substantially. Fuel costs have historically been used for such evaluations, but the cost of replacement power has been used by some in recent years due to the competitive nature of the market.

The point where 17.3 percent more gas flow was going through the West air heater on Unit 1 equates to about 11/2 inches of increased pressure drop through the air heaters, if neither air heater had infrasonic cleaning versus both having it. On the Tolk boilers, this pressure drop corresponds to a 700 kW increase in fan power consumption. In the worst case, this could be an annual loss of margin of $140,000, based on a gross margin of 3.5 cents/kWh.

Water Washing

Improved air heater cleaning is also beneficial in minimizing the frequency of expensive water washes, particularly for PRB ash. Forced outages to recover lost generating capacity because of excessive pressure drop through regenerative air heaters are significantly more costly. Now that there is more experience with PRB ash in regenerative air heaters, air heater washing has become an accepted maintenance expense. This doesn’t necessarily have to be the case.

PRB ash contains a high percentage of CaO, which makes gypsum after reacting with sulfur in the flu gas, mixing with water and then being allowed to dry in place. This happens to a certain degree every time a regenerative air heater is washed that has PRB ash in it. Experience with inadequate washings has led to practices where extremely conservative washing schedules are used. Unfortunately, even these practices cannot remove all deposits. Channeling occurs that takes water in the intermediate elements through open paths and leaves deposits behind. Without being able to force water through all points of all passages, this cannot be avoided. This is especially true for some of the high thermal efficiency element designs currently in use.

Even when water washes are avoided at all costs, if there is PRB ash in the elements, it will eventually turn hard during an outage. As the air heater cools, water condenses out that mixes with the ash. This dries and hardens into the same cement. The only way to avoid these deposits, which eventually force replacement of the elements, is to keep the deposits from forming in the first place. If the PRB ash is not allowed to accumulate, the cement cannot form.

Tolk has never had to wash the air heaters when infrasonic cleaning and sootblowers were used in tandem. Whether you consider the first seven years or so of operation, or Unit 2 since installation of the infrasonic cleaners in 1999, there has not been a degradation of air heater pressure drop that required water washing.

Notably, Tolk does not look at the infrasonic cleaners as a replacement for the sootblowers or as an opportunity to extend the time between sootblowing cycles. The goal is to prevent the need for water washing, not reduce sootblowing. Availability and pressure drop are much more valuable than a reduction in sootblowing.

Air Heater Element Replacement

With fuels other than PRB, air heater element replacement has generally occurred when the metal has been eroded by sootblowing, or when the temperature cycles on the elements are numerous enough to cause failure. With PRB ash, however, the elements become unusable when they have accumulated too much “cement.”

The rate at which the pressure drop grows after a water washing provides an indication of when replacement is needed (Figure 2). After a number of run-wash cycles, the expected duration for a run becomes too short. For instance, if cleaning occurs in the early fall and the prediced duration for the next cycle is only a few months, then the next cleaning cycle would be in mid winter, when power demand for heating would be greatest.

Elements have been redesigned to allow the plates to shift, sometimes referred to as “loose pack,” to allow scale to break up and be removed. These improve the situation, but not allowing the ash to form in the first place is a preferred solution.

Authors –

Mel Freund is Vice President of Advanced Acoustic Technologies, LLC. He has been technically involved with the fireside cleaning of fossil-fired boiler systems and associated sootblowing equipment since 1975. This includes direct experience with infrasound cleaning since its introduction to this hemisphere in 1981.

Randy Vann is Plant Engineer at Xcel Energy’s Tolk Station. He has extensive experience with maintenance and operational projects relating to performance and reliability improvements, including turbine steam path, boiler slagging/fouling, air preheater cleaning, and baghouse/cooling tower optimization.

Infrasound Basics

Infrasound has properties that separate it from higher frequencies. The most significant of these is its ability to fill large enclosures with resonant sound. This allows one infrasonic generator to provide enough sound intensity to clean large areas. The technology has matured and application is now based on three-dimensional acoustic modeling to determine operating frequency and optimum location of the infrasonic cleaner. Physical obstructions (such as the heating element) and structures within the volume are considered, along with temperature, gas velocity, and the location of the sound sources.

The acoustic modeling has not only provided more consistent results in infrasonic cleaning applications, it has also explained why past installations did not work. In some instances, infrasonic cleaners were changed to produce different frequencies, and/or moved to a more preferred location.

Infrasound cleaners operate at low frequencies, in the 15-35 Hz range. To generate these lower tones efficiently, the equipment must get longer, like musical wind instruments. Infrasonic generators are about 14 feet long and can have outlet openings about 40 inches in diameter. Only one device is required per air heater, and since low-pressure air is adequate, a positive displacement blower can supply the air. This significantly reduces the electric energy required, eliminates the need for a receiver tank, and reduces the length of piping runs.