|A nozzle directs the pellets directly on deposits. When the CO2 pellet changes phase from solid to a gas, the deposit breaks free.
Credit: Environmental Alternatives, Inc.
Deposits and corrosion on heat recovery steam generator (HRSG) tubes and other surfaces are inevitable problems and are a common cause of reduced steam production, sinking steam temperatures and degraded combustion turbine efficiency. The effects of a fouled HRSG are also tied to reduced electricity production and lost revenue.
A number of factors contribute to deposit formation on HRSG tubes, including fuel sulfur content, tube leaks, insulation failures, ammonia injection for NOx control, and condensation due to low stack temperatures. Corrosion also becomes a major problem in plants operating in locations with high humidity, particularly when cycling plants originally designed for baseload operation. HRSGs equipped with oil supplemental firing also experience a higher rate of tube fouling than when burning only natural gas.
Over time, fouling of finned tubes can ‘bridge’ the gap between adjacent tube fins or other heat transfer surfaces, further disrupting heat transfer and increasing the gas-side pressure drop. Increased HRSG gas-side pressure drop will degrade the efficiency of the combustion turbine (CT) and thus the heat rate of the entire combined-cycle plant. In cases where the HRSG performance is severely compromised, the entire plant may require an extended forced outage to repair corrosion-induced tube leaks, clean tubes of deposits, or even replace an entire module.
Mitigating deposits and corrosion
Removing HRSG gas-side deposits should be a part of every plant’s annual maintenance program. Effective maintenance planning can be improved by closely monitoring specific operating parameters, such as CT backpressure, steam production and temperature (for each pressure level) and stack temperature, and comparing the data against corrected plant design conditions. In addition, plant heat rate and output should be tracked. Carefully scrutinizing the data can provide advance warning about the location, amount of fouling present, and the rate of deposit formation within the HRSG. This information allows the owner to determine precisely when an outage for tube cleaning is economically justified. In general, HRSG cleaning is required when the gas path pressure drop across the HRSG reaches 3-4 inches (8-10 cm) WC over ‘new and clean’ condition.
Once the need for cleaning has been established and an outage date determined, the next step is to select the best cleaning technology. The standard options for cleaning an HRSG are high pressure water blasting, grit blasting, acoustic cleaning, and carbon dioxide (CO2) blast cleaning. The plant owner should carefully consider the pros and cons associated with each cleaning option before making a final selection.
High pressure water blasting can be effective but may also have the undesirable side effect of a water-deposit interaction that creates an acidic environment and accelerates tube corrosion. Also, it may turn the water-deposit mixture into a concrete-like substance when the plant is restarted. Further, this form of cleaning is limited to line-of-sight deposits, and the high-pressure water may push removed deposits further back into inaccessible regions of the HRSG. Unless carefully performed, high pressure water blasting can also quickly damage insulation that is extremely difficult to access for repairs, or may erode some tubes or damage tube fins. Contaminated water from the blasting is also difficult to contain and may require expensive waste disposal, if determined to be a hazardous waste.
Grit blasting, also limited to line-of-sight cleaning, can quickly thin the metal tubes or damage tube fins if not carefully performed by experienced technicians. Unfortunately for the plant owner, thinning of tube walls is not obvious during cleaning but will become apparent when the rate of tube leaks increases in the future. Like high pressure water blasting, large amounts of waste material are generated, some of which may be classified as a hazardous waste requiring special (and expensive) handling and disposal.
Users report mixed results when using sonic horns for dust removal from tubes, particularly in the cold end of the HRSG. Sonic blasting is ineffective in removing ammonia salts and baked on deposits.
CO2 blast cleaning
The remaining option for HRSG cleaning is CO2 pellet blasting, the only option that is non-destructive and produces no secondary waste products. CO2 blasting is a dry process that avoids future heat transfer surface corrosion and eliminates the risk of erosion of tube metal surfaces. Just as important to the owner, deep cleaning between tubes can be performed. CO2 blasting penetrates and completely cleans modules located deep within the HRSG, eliminating the time and expense of mechanically spreading tubes to obtain access to tubes not within the technician’s line of sight. CO2 blasting has been proven by over 20 years of industry experience and has been recognized by HRSG manufacturers as a cleaning best practice (see Figure 1).
|Figure 1: CO2 blast cleaning uses small cylindrical dry ice pellets to remove fouling, rust and scale from tube and fin surfaces. The process involves conversion of liquid carbon dioxide to solid dry ice pellets.|
The general cleaning process is illustrated in Figure 2. CO2 pellets are fed into a portable machine that is connected to a high-pressure compressor. The pellets are educted into the air stream and propelled through a hose to a specially designed nozzle that propels the pellets at speeds up to 1000 feet (305 metres)/second. The pellets exit the nozzle and penetrate the debris layer on the surface being cleaned (see Figure 3).
|Figure 2: The CO2 pellet blast cleaning process is illustrated.|
The CO2 sublimates once the pellets penetrate the deposit. During sublimation at atmospheric conditions, the CO2 pellets undergo a transformation from a solid directly to a gas, unlike ice that must first melt into liquid water before evaporating into vapour form. When CO2 sublimates from a solid to a vapour, it expands 750 times in volume creating a ‘mushroom’ effect inside the deposit that lifts and removes deposits from metal surfaces. A HEPA vacuum system is then used to collect the deposits which are removed from the boiler tube surface and fall on the floor. Two typical HRSG tube banks shown prior to and following CO2 blast cleaning, Figures 4 and 5 respectively, illustrate the cleaning effectiveness of the process.
|Figure 4: Typical fouling and bridging of HRSG tubes shown before cleaning.|
|Figure 5: The same tubes shown in Figure 4 have been restored to ‘new and clean’ condition after CO2 blast cleaning.|
Making Carbon Dioxide Pellets
High-density CO2 pellet production is the cornerstone of the cleaning process. Sufficient pellets are manufactured on-site to guarantee the quality and density for maximum cleaning effectiveness. Pre-made pellets from an off-site dry ice vendor are usually 24-48 hours old before they are used and the pellets will have already experienced a loss in density. Lower density pellets will begin to sublimate in the hose and the remaining softer pellets will have reduced cleaning efficiency due to their inability to penetrate the interior of the bundle.
On-site production of high density CO2 pellets is possible by using a completely self-contained mobile support trailer (see Figure 6). The trailer houses a 350 psi high-pressure air compressor, air dryer/after-cooler (for clean instrument grade air with low moisture content), a liquid CO2 storage tank, a pellet conversion unit, and all necessary support systems for direct connection to site power. The trailer also carries all the necessary tools, personal protection equipment, and other safety gear to the site.
The true effectiveness of high density CO2 pellet blast cleaning becomes evident when comparing pre- and post-cleaning plant performance data, such as pinch points, steam flow, heat rate, fuel consumption, pressure drop and unit power. In the first case study, the performance restoration experienced after cleaning is presented. In the second case study, the value of a CO2 blast cleaning allowed the owner to cancel a scheduled major outage and avoid paying for a replacement economizer module.
Case Study 1: Regaining lost performance
Monitoring important performance data points at a nominal 500 MW combined cycle plant located in the northeastern US is part of the plant’s ongoing HRSG maintenance and cleaning program. The data collected is used to develop performance trends and an estimate of the power output that can be restored by cleaning. A simple economic analysis compares the value of lost power sales revenue when running with a fouled HRSG, with the lost revenue incurred for an outage and the cost of an HRSG cleaning. This analysis quickly informs the plant owners when a cleaning should be scheduled.
Data collected from the plant historian before and after an HRSG cleaning is shown in Table 1. The plant power output restored as a direct result of the cleaning was 1120 kW. Also, the plant normally operates at a 90 per cent capacity factor and sells power into the market at US3.5 cents/kWh off-peak, a very conservative sell price for this analysis. Assuming the plant can sell the additional power generated, the gross savings resulting from the restored power is around $309,000 per year. The owner’s payback for the HRSG cleaning is a matter of weeks.
Another approach to calculating the value of an HRSG cleaning is to calculate the fuel savings that occur when a plant runs at a fixed power output. In that situation the fuel savings are a function of the plant’s improved plant heat rate. If the gas-side HRSG pressure drop increases by four inches (10 cm) WC due to fouling, the resulting heat rate increase can be determined from plant-specific design data. For the purposes of this case study, the heat rate improvement is approximated as proportional to the power restored (1120/500,000) or 0.22 per cent. As the typical 500 MW combined cycle plant has a gross heat rate of around 7000 Btu/kWh, the heat rate restoration is around 16 Btu/kWh. If fuel is purchased at $3.50/million Btu then the annual fuel savings for the improved heat rate is around $220,000 per year.
Case Study 2: Avoiding unexpected cost
The second case study involves a combined-cycle/cogeneration plant located in the UK that produces steam and electricity for two paperboard mills. The plant uses a General Electric LM6000 and a Siemens steam turbine.
Sticky combustion products were condensing out on the HRSG economizer tubes as a tar-like substance because the flue gas temperature had dipped below the dew point. In addition, ceramic fibre insulation blocks used in the HRSG combustion zone were deteriorating, with fibre strands coming loose into the gas flow and sticking on the economizer fin tubes. The combined effect was a loss of heat transfer in the economizer and a rise in the HRSG gas-side pressure drop that severely decreased steam production.
The plant owner’s initial diagnosis was to replace the entire economizer module with one that is equipped with an economizer recirculation system.
An economizer recirculation system takes a portion of the hotter economizer outlet water and returns it to the inlet to ensure the economizer tube metal temperature remains above the dew point temperature, thereby avoiding condensation of sticky combustion productions.
However, procuring an expensive new economizer module was going to require at least 40 days, putting the plant owners at commercial risk for failing to supply the contracted amount of steam.
As an alternative approach, the plant owner investigated cryogenic cleaning of the economizer even though, at the time, there was no large boiler experience with the technology within the UK, only cleaning of small equipment, such as motors or generator windings.
The plant owner sent representatives to the US to observe the cleaning process in action and the decision was made to bring the process to the UK for the first time. The CO2 pellet blasting equipment was shipped to the UK for a planned HRSG outage. Figures 7 and 8 show the state of economizer tube fouling before and after cleaning. Figure 9 shows the debris removed from the HRSG after the cleaning was completed.
|Figure 7: Fouling in this economizer was reducing customer process steam flow and increasing HRSG pressure drop to unacceptable levels. A replacement economizer was thought to be the only solution.|
|Figure 8: Economizer fouling was eliminated during a short maintenance outage and the plant was quickly able to resume full process steam supply to its customer. In addition, the reduced gas side pressure drop improved the combustion turbine operating efficiency.|
The cleaning process was very successful and at the close of the outage the plant resumed supply of the contracted amounts of steam to the customer. By selecting CO2 pellet cleaning, the plant owner avoided an unnecessary replacement economizer expense, sidestepped an extended outage for the economizer replacement, and avoided an unpleasant contract discussion.
Chris Norton is President and Randy Martin Vice-President at Environmental Alternatives, Inc, a US-based company providing solutions for nuclear decommissioning and industrial cleaning applications. www.eai-inc.com
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