Electrical Efficiency
For the generator system we are not so concerned about the conversion efficiency of rotational energy to electrical energy, as modern-day generators tend to convert between the two energy types with 98% or greater efficiency. However, a significant portion of the inefficiency seen within this box involves the station service or auxiliary power consumption of the power plant itself.
As most large power-consuming systems at a power plant are needed, little can be gained by eliminating or turning off major equipment systems. Even sacrificing ancillary electrical consumption can have unintended consequences. One scorchingly hot June, I was stationed at a power plant in its engineering office, when a young man from the corporate office had the clever idea of turning off the office lights, bumping the air conditioning to 85F, and unplugging the coffee makers, water fountains, and soda machines. The reasoning was that power prices were more than $1,000/MWh, so he wanted to be able to sell every last Watt possible. What the gentleman had not considered were the potential ramifications of placing a group of plant engineers in a dark, hot office with no cold drinks or coffee. It was not a pretty sight.
As more than 80% of the electrical usage at a power plant is via electrical motors, these should be the primary focus when improving your electrical efficiency. Just the main power plant fans (primary air, forced draft, and induced draft) can consume as much as 2% to 3% of the plant’s gross output. One option for reducing fan power consumption is to use variable-frequency AC drives, especially if the plant tends to operate at lower loads for extended periods of time. Switching all of your main plant fans from conventional to variable-frequency drives could improve your NPHR by more than 0.5%.
Air and gas leakage can account for up to 25% of fan power consumption, so reducing leakage in the air heaters and ductwork can result in a significant fan power savings. Reducing your boiler excess air will reduce fan demand as well. Electrostatic precipitator optimization programs can both increase electrical efficiency and improve particulate collection.
Creative Heat Rate Improvement
Other opportunities that may not appear to affect heat rate may in fact result in significant efficiency improvements.
For example, at one power plant I was told of an improved reclaim hopper design in the coal yard that reduced the time to fill coal bunkers by 2 hours per day. A rough cost-benefit analysis determined that the new hopper design to prevent wet coal from sticking saved a net of $1,700 per year over a five-year period due to reduced coal-handling system operation time. Though that sounds like small potatoes, metaphorically speaking, it also greatly reduced coal yard operator effort during the reclaim process, resulting in a human factors improvement.
Staff at another power plant determined via a fuel quality impact analysis study that the only obstacle preventing them from switching to a higher-heat-content and lower-moisture coal was a sootblower upgrade. Costing a net of $1.3 million, the upgrade resulted in a net improvement in heat rate of more than 2% by allowing use of more-efficient but higher-slagging coals, as well as having a coincident benefit of preventing catastrophic slag falls due to insufficient sootblower coverage. The payback period of this investment was determined to be 18 to 24 months.
Final Thoughts
I have never visited a power plant where significant improvements in energy efficiency could not be made. In my lengthy experience, power plant engineers and operators are smart, motivated people who take pride in their job and their plant, and who understand what needs to be done to improve plant efficiency. A century of relatively cheap coal and a focus on plant emissions controls has, unfortunately, taken the focus away from maintaining and improving plant heat rate.
Although some folks in the industry view the proposed EPA carbon emissions standards as an impossible task, many plant engineers and operators I’ve spoken with have been optimistic that they may be given the funding and tools to start winning those heat rate awards once again.
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