By Brian Robertson, Vice President, Paragon Air Heater Technologies
New multi-pollutant regulation and a heightened focus on greenhouse gas emissions have forced power plants to focus on certain operating parameters, some of which have not historically been given a high priority. Consequently, power plants are increasingly focused on improvements in stack emissions and unit operating efficiency to meet new regulatory requirements.
Many power plants are improving their stack emissions by installing sulfur dioxide (SO2) scrubbers, DeNOX systems and/or fabric filters to treat boiler flue gases upstream of the stacks. Each of these add-on systems will add pressure-drop for the existing (or new) boiler fans, which has to be overcome to produce the rated boiler output.
Optimizing the performance of the rotary regenerative air heater is one way to recover from the impact of added pressure drop due to the back-end systems and achieve desired operating efficiency. If the air heater is experiencing excessive seal leakage or high pressure-drop due to plugged heating elements, unplanned extra load is placed on the fans. This condition will, at a minimum, result in higher fan power consumption. In the worst case, it could cause a power output restriction on the unit. It becomes essential, therefore, to improve the performance of the air heaters so that some capacity recovery may be available.
Five Critical Areas
SO2 ReductionIn order to meet required SO2 reduction levels, some units will switch to low-sulfur coal as their compliance strategy. This approach places an extra burden on the air heater given that this low-rank fuel requires a higher tonnage feed rate to achieve the same boiler capacity due to its high equilibrium moisture content and lower heating value. The increased volume results in significantly higher flue gas flow rate (combustion gases plus the evaporated fuel moisture) in the air heater. Also, the higher volume increases the gas velocity through the air heater and thus the pressure-drop increases even when there is no pluggage in the heating elements. In coal-fired boilers, this could cause a decrease in firing system capacity due to insufficient amount of hot air to the pulverizers for grinding, drying and transporting the coal to the furnace, causing boiler output restriction.
When flue gas scrubber systems are added, total boiler system resistance is increased. This requires existing fans to be upgraded or replaced with fans capable of the same volume capacity at higher static pressures. This increased air/flue gas flow capability requires higher horsepower, thus reducing net unit capacity. “Dry” scrubber systems will add ≈ 4” to 6” H2O more system resistance while “wet” scrubber designs will typically require twice as much pressure drop capability, requiring an even greater increase in fan capacity.
NOX ReductionMany existing coal-fired units, most of which already have low NOX burners, are now installing selective catalytic reduction (SCR) systems in order to meet the next level of nitrous oxide compliance. Again, these systems will require the fans to overcome an additional pressure drop of ≈ 6” H2O. This is required to cause the flue gas to flow through multiple layers of catalyst which is added to the exhaust gas path.
Fine Particulate RemovalIn order to achieve compliance for the new PM 2.5 particulate standard, many units are adding fabric filter systems (baghouses) as part of their multipollutant reduction strategy. Here again, depending on the air-to-cloth ratio and whether a “pulse-jet” or “reverse-air” design is selected, the additional pressure drop capability requirement could be in excess of 10” H2O.
Boiler Efficiency Improvement Unfortunately, each of the measures taken at the back end of the boiler system to limit stack emissions results in an efficiency decrease due to the extra fan power requirements as well as any added power to operate various auxiliary equipment, which is integral with the gas cleaning systems. While in reality this is not the fault of the air heater, it is important the air heater performance does not impose another, unplanned efficiency loss. The new emphasis on unit efficiency stems directly from the growing concern over greenhouse gas emissions, especially carbon dioxide. Each decrease in unit efficiency results in a corresponding increase in the amount of fossil fuel required to generate the rated capacity. Thus, higher CO2 emissions per net kilowatt generated would be expected for plants retrofitted with multi-pollutant reduction systems.
Seal Leakage MeasurementAir heater functioning can affect power plant performance. Even though many plants record air heater leakage through their performance monitoring systems, the recorded leakage rates are usually grossly underestimated. The nature of seal leakage means that most leakage occurs close to the ductwork walls, a place where O2 monitors and flow and temperature sensors are rarely located. These instruments, although used for assessing air heater leakage, are generally installed in the central areas of the ductwork where leakage will not be detected. (Unless the instruments are located six to 10 duct diameters in a straight run downstream from the source of the leak, which is the standard engineering “good practice” distance for complete mixing.)
The Importance of Air Heaters
Air preheaters are used in all large gas, oil, bark and coal-fired power boilers. The most common type of air preheater used in power plants today is the rotary regenerative air preheater. The rotary regenerative air preheater is essentially a rotating heat exchanger that captures heat from flue gases exhausted from the boiler, then rotates the heated portion of the heat exchanger into the inlet (cold) side of the boiler air stream where the captured heat is released to warm (preheat) the incoming combustion air. This process increases the overall efficiency of the power plant by about 10 percent.
An often neglected part of air heater performance is gas leakage that occurs through the sealing system dividing the hot and cold sides as the air heater rotates at 1 to 3 rpm. The rotating wheels on air heaters are subject to differential temperatures from hot side to cold side, in the range of 400 F end-to-end. Differential temperature causes the wheel to actually turndown during the cold condition while operating. Considering that air heaters in modern boilers can exceed 60 feet in diameter, these large differential temperatures can cause the wheel to deflect as much as two inches toward the cold end of the air heater as the rotor heats up to operating temperature.
Many plants have learned to live with leakage rates greater than 20 percent of the total air moved by the fans. Optimal air preheater performance requires that the hot and cold sides of the air heater be properly sealed to prevent cross-air leakage that negatively affects plant performance. The existence of the thermal deflection that occurs in rotary air heaters, however, makes sealing especially difficult.
Power plant operators tend to focus on the deterioration and plugging of the heat exchange element in air preheaters, which leads to increased pressure drop and increased demand on forced draft and ID fans. In many cases, the pressure drop can become so great (worsened by the addition of baghouses, electrostatic precipitators ESPsand scrubbers, all of which increase overall pressure drop through the system) that the fans cannot move enough air through the preheater to sustain full load. This is especially common during summer months when warm ambient temperatures reduce the density of the incoming air, placing a greater demand on boiler fans.
In addition to emission control equipment, issues that impair preheater performance include fuel switching, blending of lower grade fuels (such as Powder River Basin coal) and the demands of today’s power marketplace that force units to run in different modes than for what they were originally designed. For units that burn higher sulfur coals, less cold air leaking into the exhaust stream keeps the exhaust ductwork and the cold end layer of the air heater element from cooling below the sulfur dew point, thus keeping this part of the plant out of the “corrosion zone”. Western coals demand greater primary airflow. Burning them in a plant designed for eastern coal can exceed the existing fan capacity.
Performance & Optimization
Going forward, it is clear that setting-up and maintaining the air heater so that it operates at peak performance can help offset some of the inefficiencies being added to the typical steam generation systems for backend cleanup. Given the extreme thermal expansion design conditions required to accommodate the 400 F temperature variation between offline state to full-load state of the air heater rotor, it is critical that the seal designs be upgraded for flexibility and material properties in order to maximize performance and endurance against leakage. The basket heating elements must also be designed from steel with optimal heat transfer and resistance to corrosion and plugging. These critical objectives have driven Paragon Air Heater Technologies to develop and bring to market the products required to reduce the leakage of the typical air heater by one-half.
An example of the dramatic efficiency increase was recently reported for a power boiler burning western sub-bituminous coal. The plant installed Paragon’s seals to maximize air heater performance. Critical boiler details are provided in Table 1. The cost of the new seals was just over $35,000.
After the outage, an immediate reduction in fan power consumption was observed as evidenced by a reduction from 560 amps(avg.) to 520 amps(avg.) on the ID fan motor. The impact from the reduced fan horsepower as well as the theoretical improvement in air heater thermal performance due to reduced air and flue gas leakage was dramatic in terms of fuel consumption. A reduction of 25 tons a day of coal (from the pre-outage average of 1,250 tons a day) was measured, indicating an overall unit efficiency increase of around 2 percent.
The thermal efficiency improvement components of the leakage reductions (radial and circumferential) cannot be specifically calculated until detailed performance data including velocity pressure, temperature and O2 content on both the hot and cold ends of the flue gas side of the air heater are taken. However, the annual fuel cost savings can be calculated:
25 Tons/day x 365 days/yr. x 99 (percent) x $55/Ton = $ 496,856 or, $1,361.25/ day.
The return on investment for seals is compelling and should be considered as “low-hanging fruit” in all power plants as emphasis on improved efficiency gains focus. The payback period for the investment in new design seals is calculated to be less than one month: [$ 35,100 divided by $1,361.25/day] = 26 days.
Author: Brian Robertson is vice president of sales and marketing at Paragon Air Heater Technologies, a supplier of air heater performance solutions, including high performance seals, basketed elements and standard replacement parts for rotary regenerative air heaters.