POWER February 2015 - 54

FUNDAMENTALS
Understanding Coal Power Plant
Heat Rate and Efficiency
Proposed U.S. standards for reducing carbon emissions from existing coalfired
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.
Una Nowling, PE
T
he 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.
54
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%).
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
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POWER | February 2015
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.
1. Coal feeders are important. Often
ignored until something breaks, inaccurate
coal feeders can make it difficult to determine
your plant heat rate. Courtesy: Una Nowling
http://www.powermag.com
POWER February 2015

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