Heat rate, as any power plant engineer knows, is the amount of energy a plant has to expend in order to generate a unit of work or electricity. It’s expressed in Btu/kWh, or how many Btu/hr of energy are needed to generate one kW.
For decades, the gold standard of coal power plant optimization was improving the heat rate. In the early days of modern coal plant technology, leaps and bounds were made in improving heat rate and thermal efficiency (3,412 divided by the heat rate, expressed as a percentage).
Now, however, engineers have to work harder and be more innovative to grind out those last few percentage points of efficiency. That’s assuming the plant even uses an accurate, modern way to calculate heat rate; many still use the flawed input/output method, which can have error rates of anywhere from 5-10 percent or higher.
Assuming your plant has an accurate read on its heat rate, and that your plant is more or less in line with standard configurations and common-use technology, what can be done to further improve heat rate and thermal efficiency?
Examining Three Key Areas in Improving Power Plant Efficiency
A standard coal-fired plant can be broken down into three distinct areas that all contribute to producing heat:
- Boiler system
Improving heat rate, and therefore thermal efficiency, means interacting with each of these three systems. Engineers want to increase the efficiency of the boiler, reduce the net heat rate of the turbine system, and boost net generation compared to gross generation.
One of the areas that is most prone to heat rate losses is the boiler system, where the heat from the fuel is converted into steam energy that feeds into the turbine. Within a boiler system, there are a few sources of loss of heat energy, including:
- Sensible heat loss
- Latent heat loss
- Radiation/convection loss
- Unburned combustible loss
- Margin/unknown loss
By examining each source, you can figure out how to improve heat rate by manipulating variables through the application of engineering principles and new equipment. For example, combustion controls that allow for more precise control over excess air levels in the plant’s furnace can reduce sensible heat loss (i.e. loss that can be detected via temperature differentials). Lower excess air levels mean the temperature of the exhaust gas is as close as possible to the ambient temperature when the air for combustion enters the plant, therefore resulting in lower rates of loss.
Sensible heat loss can be further reduced by installing air preheaters to raise the ambient temperature of combustion air before it enters the plant.
Tuned boilers – and upgraded boilers in general – can contribute to lower unburned combustible loss rates that can result in a net gain of over 1% in efficiency, which is a significant improvement.
There are ways to further improve efficiency by working on the turbine system. Leakage at the bucket tip and packing level can account for as much as 40% of all the efficiency lost in the turbine. Turbine deposits are another common culprit that can result in significant losses. Other common areas for improvement include nozzles (particularly keeping them from eroding and wearing away) and buckets (also vulnerable to erosion and roughness).
One major change to a turbine system that can result in rapid efficiency improvement is in the turbine blades themselves. Regular maintenance of condensers, heaters, pumps, piping, and cooling towers can reduce efficiency losses, as can upgrading blades with low-pressure variants.
Additionally, replacing the plant’s main fan motors with VFD versions (variable frequency drives) can result in heat rate benefit of up to 0.5%. Industrial fan upgrades in general can contribute to better efficiency and constitute one of the most overlooked areas for heat rate improvement.
Typical Payback Periods for Heat Rate Upgrades
There are many things a coal power plant can do to improve heat rate and upgrade efficiency, but does it make economic sense?
Some power plants are so advanced that it may not make sense to expend a large amount of capital for 0.5% of heat rate improvement. Most plants, though, have considerable room for improvement, especially considering payback periods.
For most minor and major upgrades, the payback period is no greater than five years. Only when building solar-powered air and water heating systems does the payback period climb to over five years. VFD fan upgrades pay for themselves in 3-5 years; preheating combustion air (particularly with plant waste heat) can pay for itself in 2-3 years. Some upgrades, like better combustion controls, have payback periods of less than a year. One fan system upgrade at a New Mexico power plant resulted in payback in fewer than 12 months.
A full engineering evaluation can determine what can be done to boost a coal power plant’s heat rate and thermal efficiency. Engineers can then decide what can be done in line with the plant’s resources and timeline.
In today’s age, efficiency will continue to become even more important, particularly as the environment for coal power becomes more and more competitive with other energy sources. The best thing a coal plant can do is order a full efficiency evaluation and create an action plan for making sensible, ROI-focused upgrades using custom engineering and an emphasis on industry best practices that make use of the latest technology.