Coal Power Plant Heat Rate And Efficiency Part 2

The Input/Output Method

One of the simplest ways to calculate your NPHR is to divide the Btu/hr of fuel heat input by your net generation (electricity and steam to the customers) in terms of kW. However, determining the heat input can be quite difficult.

In my experience, a minority of combustion power plants have a good measure of their actual fuel burn rate at each unit. An industry rule of thumb is that volumetric feeders are accurate to within +/–5% at best, and gravimetric feeders are accurate to +/–2% at best. In practice, I find that the actual error in fuel burn rate measurement can be from 5% to 10%.

At one power plant I worked at, the only capability for estimating the coal burn rate was to rely on photographs of the coal yard taken by a spritely lady from her Cessna aircraft, and by comparing the estimated stockpile size with train receipts for the month to determine how much coal was burned overall. The potential error for this method could easily be greater than 25%.

Another important factor in heat input measurement is the fuel quality analysis, especially the fuel’s heating value. (See “Primer on Fuel Quality Analysis” in the January 2015 issue for more detail.) Generally speaking, the error in a fuel burn rate calculation cannot be less than the error in the fuel analysis, so choosing one’s sampling methods and frequency carefully will provide greater certainty when calculating the fuel burn rate.

In short, the input/output method is not an ideal method to track the difference in efficiency at your coal-fired power plant unless you have accurate coal feeders (Figure 1) plus an accurate and regular determination of your fuel heating value.




The Heat Loss Method and the Three Efficiency Boxes

A significant problem with using the input/output method to determine your heat rate is that, should your heat rate change from one situation to the next, you have no idea of what led to the change. Was the boiler less efficient at burning the fuel? Is turbine efficiency reduced due to high condenser backpressure? Has station service power increased? Because the input/output method treats the power plant as a black box, the engineer must rely on a more accurate method of determining heat rate.

The heat loss method for determining your heat rate essentially breaks the power plant into three subsystems where an energy conversion process occurs:

■ The boiler, where fuel heat is converted to steam energy.

■ The turbine, where steam heat is converted to mechanical rotational energy.

■ The generator, where rotational energy is converted into gross and net electric power.

The heat loss method for calculating heat rate essentially draws a box around each of these subsystems and determines the efficiency of each energy conversion process. The product of all of these conversion efficiency values results in the total net plant heat rate for the power plant:



As we can see from this equation, to reduce the NPHR, we need to increase the boiler efficiency, reduce the net turbine heat rate, or increase the net generation relative to the gross generation.