In the Power industry it is not uncommon for a combined cycle plant to operate less than 30% of the time, meaning the vast majority of the time the boiler is idle. The rise of renewable power assets that have intermittent and sometimes unpredictable availability (wind, and solar) coupled with variable demand for electricity makes plant “cycling” between operation and standby a business necessity. While plant cycling was formerly reserved primarily for natural gas plants, more and more coal plants are being asked to run intermittently as well. To further complicate things, most power purchase agreements are written such that the power plant must be at “full load” in a matter of hours, so additional attention must be given to ensure that this can be time hurdle can be met. Since most boiler make-up systems do not have the capacity to fill the boiler system quickly enough, most cycling plants must idle while the boiler is full of water. This presents some technical challenges as plant asset protection can conflict with business realities that often results in some compromises. This article will explore some of the options that are available to the boiler operator and weigh the risk factors associated with each.
Having removed heat from the system and lowered the scaling tendency of the boiler cycle water, the primary concern with off-line boilers is metal loss, or corrosion. This can occur in several ways. The first and most predominant is dissolved oxygen corrosion. As the water temperature decreases, the dissolved oxygen content in that water will increase provided the water is exposed to oxygen or ambient air. Once oxygenated water contacts mild steel, corrosion will initiate. The other common corrosion mechanism is acidic corrosion caused by stratification or concentration cells that develop due to stagnant conditions. Corrosion of the boiler internal surfaces not only weakens the integrity of the system, but also releases iron oxides into the boiler water. While not a concern when the boiler system is off line, iron scaling can be a concern after start-up as these iron oxides produced during layup (corrosion products) can deposit on steam generating tubes.
The preferred choice for boiler lay-up is dry lay-up. Removing the water from the system is the best way to prevent off-line corrosion. This accomplished by draining the water from the boiler while it is still hot and under 25 – 50 psi of system pressure. This is done so that the residual heat can dry the internal surfaces and ensure that all “dead legs” are drained. In geographic areas that commonly have relative humidity readings of >30%, a desiccant may be used to prevent dew from collecting inside the boiler, or dehumidified air can be circulated throughout the waterside and fireside areas of the boiler. If a desiccant is used, care must be taken so that the desiccant can be completely removed prior to start-up. The general rule of thumb for silica based desiccants are 10 pounds of silica gel per 1000 gallons of boiler capacity. Even if the waterside lay-up choice is wet lay-up, dry lay-up is needed for the fireside surfaces. Unlike the relatively clean waterside areas, a boiler’s fireside can contain deposits of sulfur or other fuel containing contaminants that, when wet, can be very corrosive. Once the water is removed from the system, the primary concern is to keep the boiler surface temperatures above the ambient dew point. One scenario that can develop involves cool tube surfaces and the influx of warm air that can be a result of rapidly changing ambient environments in the spring or fall. In this case, condensation can occur on the cool boiler surfaces and provide the moisture needed to initiate corrosion. So removing the boiler water/moisture and preventing new sources of moisture are essential to a good dry boiler layup. Most times this requires some level of isolation from ambient air, so stack dampers or balloons are employed to minimize the influx of rain and humid air on the fireside. In practice, dry layup while the most resistant to off-line damage is rarely employed on the waterside of the boiler because it delays the start-up of the plant.
The goals of wet lay-up are to prevent oxygen and acidic corrosion and have the boiler internal chemistry ready to operate the boiler within operating control limits when the boiler is called upon for service. To prevent acidic corrosion, the internal pH should be elevated to between 10.0 and 11.0 prior to shut down so that the water is well mixed. Once off-line, the boiler water should be re-circulated to prevent pH stratification and acidic concentration cells from developing. pH elevation should be accomplished with a suitable chemical that is compatible with your cycle chemistry and metallurgy.
To prevent oxygen ingress, there are two basic methods; hot lay-up and cold lay-up. Hot lay-up is preferred if practical because it reduces the thermal stresses on the system metallurgy but, of course, requires a source of heat. The idea is to keep the system under positive pressure so that oxygen cannot enter the boiler. If the boiler is only planning on being out of service for several hours, the residual system heat may be sufficient to keep the system under pressure. If an extended lay-up is anticipated, other methods must be considered.
If there is an adjacent boiler that is operating or an auxiliary boiler, the blowdown from that boiler can be sent to the off-line boiler, termed cascading blowdown. Depending upon the amount of blowdown, this may be enough heat to do the job. The optimum entry point to the boiler in lay-up is in the lower header or mud drum so that natural circulation can be achieved. The primary concerns with cascading blowdown involve dealing with the pressure differential between two boilers and controlling the water levels in the off-line boiler. Also, if the operating boiler is much smaller than the off-line boiler (or has too little blowdown), cascading blowdown may not provide enough heat to maintain a positive pressure on the system.
Another method is to install a heating coil in the lower header (or mud drum). Either steam or electric coils can be used here. This is a very effective method of hot layup and alleviates some of the concerns with cascading blowdowns. If enough heat can be supplied to the boiler, positive pressure and natural circulation can be achieved.
One last method of hot lay-up that has been used is the injection of steam (called steam sparging) into the off-line boiler. While this may be simpler than installing a heating coil, there are several disadvantages of direct steam sparging. First, the injected steam will condense and add liquid to the boiler so the boiler water levels must me managed (as with cascading blowdown). Second, the steam will dilute the boiler chemistry so chemistry monitoring and chemical additions will be needed in order to maintain the system pH. Finally, direct steam injection can cause some damage from steam impingement if care is not taken to the selection of the location of the injection point.
If there is not a ready heat source for hot (wet) lay-up, the injection of inert nitrogen gas can be utilized to keep a positive pressure on the boiler. As with hot layup, the system pH should be elevated (10.0 – 11.0). A source of nitrogen (purchased tanks or an on-site nitrogen generator) is required and only about 5 psi of pressure is needed, controlled by a pressure regulator. Usually the regulator is set to inject the nitrogen at 3 psi and stop at 5 psi. If the regulator is set above 5 psi, the nitrogen requirement will be much higher as experience has shown that the nitrogen will find a way out of the system. The main disadvantage of cold layup is that natural circulation cannot be achieved. It is recommended that a small, low pressure circulating pump be installed to provide the needed circulation. This pump must be isolated prior to start-up as is will most likely not be designed for the normal system operating pressures.
The final widely used (and not recommended) method of cold (wet) lay-up is to allow the system to drop to ambient pressure and attempt to chemically scavenge the dissolved oxygen from the boiler water. While better than doing nothing, the levels of oxygen scavenger required to chemically scavenge ambient water is very high and at the lower temperatures, the reaction is very slow. In addition, most chemical oxygen scavengers are acidic, so additional pH adjustments are usually required. Finally, EPRI research has shown that reducing conditions can increase the solubility of the protective magnetite layer which could lead to unprotected internal surfaces.