The typical pulverized coal power plant in the U.S. is about 35 years old, yet the fleet is expected to continue operating for many years to come. New coal-fired plants continue to enter service but at a very slow rate. Coal use for power generation has many opponents, but our use of coal well into the future is undeniable—even the Energy Information Administration predicts that coal will continue to be our principal fuel source for decades. Why? Because coal is low in cost and is a domestic fuel that cannot be held hostage by those hostile to the U.S.
The New Source Review (NSR) is defined in a single sentence by the Environmental Protection Agency (EPA) on its website as “A Clean Air Act (CAA) requirement that State Implementation Plans must include a permit review that applies to the construction and operation of new and modified stationary sources in nonattainment areas to ensure attainment of national ambient air quality standards.” There’s no question that new plants must comply with NSR, but what about existing plants that wish to replace their aging steam turbine rotor with a new, more efficient design?
The historic and current definition of a major modification to an exiting plant is, according to the EPA: “Any physical change in or change in the method of operation of an existing major source that would result in a significant net emissions increase of any pollutant subject to regulation under the CAA.” The NSR rule has provided a disincentive for utility plant efficiency improvements and made the term “upgrade” a dirty word in the industry.
Physical and operational changes are excluded from NSR under the routine maintenance, repair, and replacement (RMR&R) exclusion. Consequently, is the repaired steam turbine (and perhaps the only available upgrade physically possible) a major upgrade or an RMR&R? Who knows? But it may cause the plant to increase its hours of operation or perhaps the quantities of emissions such that it may trigger an NSR review. The WEPCO rule adopted in 1992 did give plants a longer past actual emission baseline period and other exclusions, but it didn’t firm up the RMR&R definition. The Bush administration also tried to quantify RMR&R and failed.
Today, the EPA has maintained that what constitutes RMR&R is determined on a case-by-case basis and that EPA staff will “weigh the nature and extent, purpose, frequency and cost of a project as well as other relevant factors, with no one factor necessarily conclusive.” The end result is that few utilities will make any plant modifications or upgrades that might possibly be construed as a major modification and trigger an NSR.
In the European Union (EU), in contrast, utilities are encouraged to improve the efficiency of their coal-fired plants, and additional carbon allowances have been awarded under the EU Emissions Trading Scheme to the owners of those upgraded plants. To their credit, the EU emissions regulators understand that their responsibilities are to reduce their overall system’s carbon emissions rather than focus exclusively on the emissions from single units.
There will come a time in the life of every plant when its existing equipment cannot perform as well as equipment in a new plant. Concerned about NSR enforcement, utilities have consciously focused on like-kind replacements and repairs to avoid any possible misunderstandings with regulators.
However, since the establishment of the NSR, there have been many technical improvements in power generation theory and practice. The upgraded steam path components mentioned earlier are an example that is well-documented and proven in practice. Other improvements include advanced heat transfer alloys, digital controls, and automated and more efficient fire-side cleaning options. Technology advances continue, and so do efficiency improvement opportunities.
Over the life of the existing coal fleet, fuel costs have escalated by a factor of 10 in cost per million Btus, making efficiency improvements much more attractive now than at any time in the past. New equipment designs, such as larger and more efficient air heaters for reducing boiler exit gas temperatures to a lower level and reducing air leakage rates, are now available. Easily understood and documented improvements also are available for steam cycle upgrades, such as installing more advanced and larger condensers or cooling towers for improved turbine performance.
The question must now be asked: What if NSR were modified to allow plant efficiency improvements that would put downward pressure on electricity costs and carbon emissions?
Specific Example of Efficiency Improvement
Here are some examples of significant improvements that could be implemented for less cost than the equivalent installed cost of new generation capacity, which is around $2,000/kW. The examples are based on three actual coal plants that represent a large segment of the nation’s existing coal-fired fleet of more than 1,400 individual units:
- Plant A: 600 MW pulverized coal 2,400 psi/1,000F main steam/1,000F reheat steam, corner-fired unit burning western Powder River Basin (PRB) coal.
- Plant B: 500 MW pulverized coal 2,400 psi/1,000F/1,000F, wall-fired unit burning PRB coal.
- Plant C: 650 MW pulverized coal 2,400 psi/1,000F/1,000F wall-fired unit, burning eastern bituminous coal.
Given these operating conditions, a set of candidate physical upgrades to the boilers were developed, unconstrained by the limits imposed by an NSR, including these:
- Install new regenerative air heaters and replace aging ductwork from the boiler to the induced draft (ID) fans.
- Change the superheater and reheater surfaces to permit the furnace exit gas temperatures (FEGT) to be combustion-tuned to be consistent with new fuel source requirements. Some boilers have insufficient superheater or reheater surface to produce design steam temperatures with a furnace-side best possible FEGT. The insufficient superheater (SH) and reheater (RH) surface requires the FEGT higher than optimum, which reduces combustion efficiency.
- The higher-than-optimum FEGT required for best steam-side thermal performance is not compatible with the best fire-side slagging and fouling performance. The elevated upper furnace temperatures contribute to accelerated slagging and fouling, which is mitigated by aggressive sootblowing.
- Upgrade the alloy of the existing superheaters and reheaters.
- Replace existing feedwater heaters with upgraded alloy and improved heaters.
- Redesign and upgrade the furnace waterwalls and add water-cooled platens.
- Install new and larger condensers and/or cooling towers for reduced condenser back pressure.
- Install hybrid air-cooled/water-cooled condensers to reduce cooling water usage.
- Install new, more efficient steam turbine rotors to upgrade and uprate capacity and efficiency.
- Other changes as required to “debottleneck” both the combustion process and the steam cycle.
- Upgrade coal pulverizers for less auxiliary power consumption, larger capacity, and better fineness.
Plant A Improvement Potential
This unit was originally designed for a higher-quality fuel than what is currently fired. PRB subbituminous fuel is the typical fuel today because of its lower sulfur, lower price, and lower NOx production. PRB fuel operates best when the FEGT is about 2,150F for reduced slagging and fouling. For both reasons of the fuel change and the changing firing conditions of low-NOx operation, the FEGT now tends to operate at about 2,400F rather than the desired 2,150F. The reduced FEGT is desirable for reduced slagging and less-aggressive sootblowing. When the FEGT is reduced for more favorable fire-side slagging and fouling conditions, then the superheater and reheater temperatures cannot achieve the design and required 1,000F (Figure 1).
1. Upgrade options for Plant A configuration. Source: Storm Technologies Inc.
Specific upgrades to the plant, in a non-NSR world, include:
- Redesigned superheater and reheater surfaces and upgraded metals
- New and upgraded cooling towers, possibly a hybrid air-cooled conventional to reduce water evaporation losses
- Upgraded turbine rotors
- New feedwater heaters
- New and larger boiler feed pumps
- New and larger coal pulverizers
- Condenser metals upgrades
Another possible improvement and upgrade is a complete redesign of the superheater and reheater to add more tube surface and upgrading the alloy for increased reliability and life. These boiler upgrades are expected to cost about $5 million. The turbine rotors and steam path improvements are also feasible. Between the combination of steam path improvements and boiler surface changes, an expected 50 MW in power output could be added to the plant, plus a plant efficiency increase of 300 to 500 Btu/kWh, not to mention approximately 3% to 5% less carbon emissions and lower NOx and SO2 emissions on an hourly basis.
How do these improvements translate into savings? For an average-performing midsize coal-fired plant, an extra 50 MW of additional power sales at $20/MWh translates into perhaps another $2 million of net power sales revenue each year. Also, a 500-Btu/kWh improvement in plant heat rate for a typical 500-MW coal plant operating at an 80% capacity factor burning PRB coal will reduce fuel consumption by about 10,000 tons each year. At $40/ton delivered, that’s $400,000 saved each year. Overall, this efficiency upgrade has a simple payback on its $5 million cost of only two years.
Plant B Improvement Potential
This wall-fired boiler has a similar steam-side, fire-side incompatibility (Figure 2). The FEGT must be increased to over 2,300F average bulk gas temperature in order to reach the design steam temperature. Here, too, the redesign of the superheater and reheater on this boiler to match the heat transfer surfaces with today’s fuels and steam demand will yield significant overall heat rate improvement.
2. Upgrade options for Plant B configuration. Source: Storm Technologies Inc.
Combining the boiler improvements with uprated and upgraded steam turbine rotors and controls could conceivably increase output 35 MW or more and also improve the overall heat rate by 500 Btu/kWh, for about a 5% increase in plant efficiency.
Plant C Improvement Potential
The improvement potential for this boiler mainly involves the boiler exit gas ductwork and air heater replacement (Figure 3). The existing air heaters are an unusual design and tend to have leakage rates well over 15%. Also, the exit gas temperature corrected to no leakage can be reduced at least 35F. The combination of replacing the air heaters with the latest and most advanced regenerative ones, increasing the boiler heat transfer surface area, and reducing the total leakage can improve the heat rate by about 200 Btu/kWh.
3. Upgrade options for Plant C configuration. Source: Storm Technologies Inc.
Combining the improvements to the combustion process with advanced steam turbine rotors and steam path improvements could result in a 50-MW increase in capacity and an estimated overall heat rate improvement of about 500 Btu/kWh or better.
We cannot name the plants used as examples for obvious reasons. However, they demonstrate the huge incentives in both CO2 reduction as well as fuel cost savings and capacity increases that exist in the power generation industry.
Finally, and perhaps most important, these upgrades could be completed for far less cost than the very high costs of building a new coal plant.
—Dr. Robert Peltier, PE is COAL POWER’s editor-in-chief. Dick Storm (rfstormsea @aol.com) is the senior consultant for Storm Technologies Inc.