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Dewpoint and other temperature-related corrosion of the tubes in a heat exchanger cold-end flue-gas outlet is a common problem with all tubular heat exchangers, especially when burning biomass. Dewpoint corrosion can occur when gas reaches a temperature at which the evaporating and condensing rates of its moisture content are similar. Moisture in a heat exchanger’s flue gas can react with airborne gases that contain sulfur-forming acids that attack metallic surfaces. Because it is an extremely strong oxidizer, chlorine in the gas can attack as hydrochloric acid (HCl) or directly on the metal as chlorine.

Managing the severity of this corrosion process is a juggling act of boiler efficiency, fuel chemistry and the steam-generating unit’s load profile. The heat exchanger can be designed to operate above the dewpoint at full load, assuming a specific fuel blend, and down to any selected lower load by the choices made by the designer/owner (if the air heater was designed properly). Most of the corrosives are sulfuric, sulfurous and chlorine — well above the dewpoint of HCl at higher temperatures. Hydrochloric acid forms at a lower temperature that normally does not occur within the heat exchanger. (In addition, there are other corrosives, but those are the predominant concerns.) These compounds are present in the wood or wood waste products, coal or any additives that may be in the emissions-reduction or freeze-protection processes such as adding brine on the fuel or handling equipment to prevent fuel feed problems in winter.

Generally, the boiler and auxiliaries are designed to meet efficiency goals. In doing so, consideration must be given to the heat exchanger metallurgies and fuel chemistry based on the projected load profile planned for steam or power generation.

One major factor that is often not well understood — or possibly ignored — is that the presumed dewpoint temperature normally is estimated by measuring the flue-gas exit temperature. It is a mistake to use flue-gas temperature as an indicator for the tube-metal temperature. The tube-metal temperature at the heat exchanger’s coldest operating zone is one of the most important process parameters — and one that is almost never measured. When the tube-metal temperature is measured, the metal sensor must be isolated from the influence of air or gas: It is important to detect the metal surface temperature only. This is a unique challenge because the sensors must be installed within the gas or air paths; however, it is possible using specific thermocouple installation procedures (not covered in the scope of this article).

Another consideration beyond the lack of monitoring the metal temperature is that designers must acknowledge the reality that tube leaks are eventually going to occur. Planning for this should be included in the design for long-term reliability. Leaks are inevitable due to the life-cycle processes of a heat exchanger and fuel chemistry, but the reality of the resulting damage is rarely accounted for in the design margin of the heat exchanger. A better understanding of the corrosion processes by the owner/operator is a first step to avoiding this problem and specifying the design operating margins.

Once a tube leak develops, the entire destructive corrosion process is accelerated. Initiation of a broader spread area of corrosion begins due to cold air diluting the flue gas in a larger zone than the isolated single-tube leak. In this scenario, the high pressure combustion air increases the total flue-gas flow to the stack. This leads to the cold air creating a cold zone below the dewpoint within the combustion gas that contains all the suspended corrosives and high moisture. The result is condensation saturated with corrosives and erosive ash. This sticky ash also can plug tubes or the gas path.

Secondary Consequences of Tube Leaks

Another failure mode that is frequently overlooked is what happens to the flue gas flowing inside the tubes when a leak develops. This condition allows the high pressure cold air to enter the leaking tube and flow in both directions — toward the gas inlet and the gas outlet of the tube — at the same time.

The result is a cold zone at the gas inlet of the bundle. This can be visualized as a group of tubes at the heat exchanger gas inlet under an umbrella of mixed cold air and flue gas surrounding the leaking tube (figure 1). This group of tubes will now operate out of the design temperature range due to cold air mixing with and diluting flue gas. In most cases, the tubes around the leaking tube are cooled below the dewpoint. A monitoring program focused on leak detection and frequent inspections (visual and nondestructive) will help prevent the failure of surrounding tubes and extend the service life of the heat exchanger.

FIGURE 1. An umbrella of mixed cold air and flue gas surrounding the leaking tube creates a cold zone at the top gas inlet of the tube bundle and the strong potential for dewpoint corrosion. Photos credit: CMS (Click on the image to enlarge.)

Another source of secondary damage comes from seemingly small air leaks that result in a stream of high volume and high velocity air blowing into the gas path. Over time, the high velocity air drives ash particulate just as a sandblaster functions: cutting tubes, tubesheets, ducts, structural supports and anything else in its path (figure 2).

FIGURE 2.  Tube air leaks drive ash into air heater components, causing erosion damage as shown here to the casing. Photos credit: CMS (Click on the image to enlarge.)

If the heat exchanger is fabricated with stainless steel tubes for corrosion protection, this abrasive ash can remove the chromic oxide protective layer from the tubes’ exposed surface. This leaves the metal stripped and susceptible to attack by the corrosives. The protective layer is continually removed by the abrasive ash, and the tube will eventually fail.

There is an additional important factor that often leads to a decline in performance. When the combustion airflow to the boiler is limited due to the air leakage bypassing the boiler and going directly to the stack, the boiler control system will drive the supply fans at higher rates to satisfy the air requirements for combustion. This results in the use of additional auxiliary fan power. The boiler control system must maintain the steam flow or net generation target by increasing the fuel rate to maintain maximum continuous rating (MCR). This increased fuel consumption can amount to an additional 1 to 5 percent demand. The extra airflow and gas flow from combustion increase gas velocities in the system. These result in increased erosion rates from the higher velocity of ash-laden gas passing through the same volume of ducts and ancillary operating equipment (including the heat exchanger itself). Operating costs will increase due to fan power, fuel consumption, maintenance costs and emissions treatment chemicals. Eventually, a point may be reached where attaining MCR steam flow or power generation is no longer possible.

Many heat exchangers recover too much heat from the flue gas to attain the designed thermal performance goals. This often lowers the tube-metal temperature below the dewpoint. Flue gas conditions, including pressure, moisture content and chemistry, are all factors that impact the actual tube-metal dewpoint temperature value. This overcooling often occurs because the operating load profile and cycling of steam or power production will not approach the boiler design conditions for the flue gas outlet temperature.

Some So-Called Solutions May Create Problems

Reducing the surface area of the heat exchanger typically is not an acceptable solution for several reasons. If you remove heating surface by blocking sections of the tubes, the fan losses impact the unit performance by limiting the air that can be supplied to the boiler and the gas that must be removed after combustion by the induced-draft fans. In this case, the heat exchanger becomes an obstruction in the air and gas paths. In addition, the current load profile for generation may not be the same in the future because the load profiles are driven by economics outside the control of the operators.

Replacing tubes with corrosion-resistive metallurgies is another potential solution that sounds viable on the surface. But, higher metallurgies present other problems. They can diminish the performance of a heat exchanger and, eventually, create other problems such as ash plugging.

Using a Heat Transfer Thermal Barrier to Reduce Corrosion

One potential method of solving the dewpoint corrosion problem involves incorporating thermal-barrier-lined tubes into the heat exchanger. Though developed more than 30 years ago, this method has been recently enhanced and functions as a heat transfer thermal barrier. It can be accomplished without reducing the heat exchanger size by installing the thermal barriers within selected overcooled areas of a heat exchanger.

A heat exchanger with a dewpoint corrosion problem can be equipped with a thermal-barrier-lined tube modification. Usually, this is located near the combustion air inlet, where the tube-metal temperature is operating below the dewpoint. The elimination of corrosion in this section of the tube bundle can be accomplished by reducing the heat transfer — only within this area — to maintain the tube-metal temperature above the dewpoint.

This modification does not limit airflow or gas flow to the boiler system. The modified area does not represent a large percentage of the heat exchange surface area; therefore, the overall heat reduction in recovery impact to efficiency is minimal (figure 3).

FIGURE 3 BEFORE. The method of installing a thermal barrier tube has successfully been implemented for many years. The corrosion-affected heat exchanger tube is shown before repair (left) and a tubesheet is shown five years after it was repaired (right). Photos credit: CMS (Click on the image to enlarge.)

FIGURE 3 AFTER. The method of installing a thermal barrier tube has successfully been implemented for many years. The corrosion-affected heat exchanger tube is shown before repair (left) and a tubesheet is shown five years after it was repaired (right). Photos credit: CMS (Click on the image to enlarge.)

By controlling specific dimensional and heat transfer parameters of the gap between the liner and the parent tube, the heat transfer can be controlled precisely. Other parameters that must be considered are tube metallurgies; the balance of airflow and gas flow rates; air and gas pressures; the unit operating load range; and, most importantly, the fuel chemistry. The tubesheet material, thickness and the gas or air pressures entering the heat exchanger also are considered in the design.

Due to its large mass, the tubesheet in a heat exchanger is a controllable heat sink. As cold combustion air hits its front face, heat is conducted from the tube metal into the tubesheet. Under corrosive conditions, this can result in severe dewpoint corrosion at the tube-to-tubesheet interface. Because the tubes are expanded to the tubesheet by a mechanical rolling process and have metal-to-metal contact, conducting heat into the tubesheet can subcool the first few inches of the tube. This causes dewpoint corrosion (figure 4).

FIGURE 4. A heat exchanger contains a significant amount of metal that can act as a heat sink. The heat sink pulls heat from the tubes, into the tubesheet, after the tubes are mechanically rolled into metal-to-metal contact. Photos credit: CMS (Click on the image to enlarge.)

The heat transfer between the tube and tubesheet is at a much higher rate than the convective heat transfer by gas or air heating or cooling the metal (figures 5, 6 and 7). The result is that the tubesheet can drive the tube below the dewpoint due to the tubesheet mass of metal exposed to cold combustion air.

FIGURE 5. This single tube heat exchanger has air entering horizontally in the lower cold-end section and flowing over the outer diameter of the tube. The air leaving the cold-end sections turns and enters the top section, flowing over the outer diameter of the hot end of the heat exchanger. Photos credit: CMS (Click on the image to enlarge.)

FIGURE 6. This single-tube heat exchanger has the same flue gas and airflow pattern as figure 5, but the thermal barrier is installed in the cold-end lower section. The color shading depicts the heat-affected areas. Photos credit: CMS (Click on the image to enlarge.)

FIGURE 7. Five types of heat transfer related to a standard heat exchanger modified with a thermal barrier are shown. In simple terms, the heat transfer by convection is extremely slow between gas and metal. Once the heat energy is inside the metal, the heat transfer by conduction is extremely fast by comparison. The thermal barrier reduces the heat transfer efficiency and keeps the inside metal temperature above the dewpoint. Photos credit: CMS (Click on the image to enlarge.)

The result of the modification is that with the tube surfaces exposed to the combustion gas flow, whether the gas is on the tube outside diameter or the inside diameter, the metal temperature is no longer cooled below the dewpoint. The rapid heat transfer through the metal of a single wall tube can be altered and controlled by the low heat conduction between the thermal-barrier liner and the parent tube because of the gap between them.

In designing the modification, there may be process limitations that govern heat exchanger design decisions. For example, there may be limitations to increasing the exit gas temperature to the emissions equipment and the potential reduction of combustion air temperature to the boiler processes for drying fuel. In most installations, no measurable changes in boiler efficiency were detectable.

Sometimes, depending on the geographic location of the unit, more heat is needed for fuel drying and combustion than is practical to obtain by the thermal-barrier method. This opens the possibility of installing steam preheating coils to heat the combustion air before it enters the heater. Steam coil heaters can be combined with the thermal-barrier tube modification if more heat is needed to maintain the tube metal above the dewpoint.

Steam heating coils may not be acceptable in some cases, however. The unit steam capacity requirements may be critical for generation or other processes. The steam heating systems require high maintenance. Another consideration involves steam leaks from the coils, which can result in speeding up the corrosion damage when the water or vapor is sprayed into the heat exchanger. Cooling the heat exchanger metal surfaces with water vapor from leaks in the preheating coil results in continuous evaporation (heat of vaporization) and cooling of the metal surface at the air inlet.

There are cases when it is not desirable to remove a significant amount of heat from a specific corrosion zone of the heat exchanger. One example is when the amount of heat input from primary air required to dry the fuel is marginal. Installing the thermal barrier may stop the corrosion but prevent the proper application of the required heat. In this case, the solution may be to install the thermal-barrier tubes in the secondary air section and move heat to the primary air section to provide corrosion protection and sufficient heat input. In figure 8, the shaded blue area in the hot end illustrates how this can be accomplished. Moving additional heat to the primary air section below prevents the cold-end tube-metal temperature from going below the dewpoint.

FIGURE 8. The thermal barrier liner is positioned in a hot area to keep the flue gas hot as it transitions into a cold area of the heat exchanger. Photos credit: CMS (Click on the image to enlarge.)

In conclusion, left unchecked, dewpoint corrosion can cause significant damage to heat exchanger tubes, tubesheets and structures. Heat exchanger problems cascade upstream and downstream from seemingly minor cracks and holes, causing damage to equipment like boilers, fans and emissions equipment. In worst-case scenarios, it can lead to equipment failure and unplanned shutdowns. When dewpoint corrosion is identified at an early stage, there are methods that can stop further damage and lengthen the life of a heat exchanger. Thermal barrier-lined tubes have a long track record of minimizing extreme corrosion cases. They are also effective in reducing fly-ash plugging due to condensation of the flue gas. This approach also results in reduced fuel consumption due to the fan power reduction and improved thermal efficiency of the boiler. These modifications can be accomplished during tight turnarounds and restore better airflow and gas flow immediately. While most units will eventually need replacing, there are also opportunities to extend heat exchanger operating lives with smarter repairs.