Coal Power Plant Heat Rate And Efficiency Part 1

Proposed U.S. standards for reducing carbon emissions from existing coal-fired power plants rely heavily upon generation-side efficiency improvements. Fuel, operations, and plant design all affect the overall efficiency of a plant, as well as its carbon emissions. This review of the fundamentals of coal plant efficiency, frequent problems that reduce efficiency, and some solutions for improving operation and reducing generation costs should be valuable to plants wherever they are located.

The scene: Twenty years ago, a young engineer stands in front of a group of plaques and awards in the lobby of a large coal-fired power plant. She notes with interest that several of them refer to “best heat rate” awards, and she also notes that the last award is more than three years old. A grizzled station engineer, looking like a coal-dusted Sam Elliott, joins her in front of the display.

“Why did this plant stop winning the heat rate award?” she asks.

“Well ma’am, since we added the scrubbers, there isn’t much point any more. And the other plants went to Powder River Basin (PRB) coal, so they took a heat rate hit too. So, someone just reckoned since we had to give up heat rate to meet emissions limits, there wasn’t much point in having the award anymore.”

Fast-forward to 2014, and the scene is radically different. Advanced coal plant emissions controls are the norm, and PRB coal is in use to some extent at most power plants in the U.S., and the Environmental Protection Agency (EPA) has proposed standards for reducing carbon emissions from existing power plants under Section 111(d) of the Clean Air Act. Comprising a variety of possible methods for reducing carbon emissions, one building block of the EPA plan is improving net plant heat rate (NPHR) by 6% or greater. Although this may sound like a small number to the layperson, power plant engineers know that a 6% heat rate improvement would require a serious commitment on many different levels within their utility.

This article outlines the basics of plant efficiency and heat rate, such that one can quickly understand where the best opportunity for improvement is for a specific generating asset. It then examines ways in which the 6% NPHR goal might be achieved.



Heat Rate Fundamentals

The term “heat rate” simply refers to energy conversion efficiency, in terms of “how much energy must be expended in order to obtain a unit of useful work.” In a combustion power plant, the fuel is the energy source, and the useful work is the electrical power supplied to the grid, the steam heat supplied to an industrial customer or used for heating, or both. Because “useful work” is typically defined as the electricity and steam that is delivered to the final customers, engineers tend to work with the net plant heat rate (NPHR).

In the U.S., heat rate is typically expressed using the mixed English and SI units of Btu/kWh. Though confusing at first, this merely indicates how many Btu/hr of energy are required to produce 1 kW of useful work. Other countries commonly use kJ/kWh, kCal/kWh, or other measures. This article uses the U.S. format.

Because approximately 3,412 Btu/hr equals 1 kW, we can easily determine the thermodynamic efficiency of a power plant by dividing 3,412 by the heat rate. For example, a coal power plant with a heat rate of 10,000 Btu/kWh has a thermal efficiency of 3,412/10,000, or 0.3412 (34.12%).

From Power Magazine