The heat rate of a unit is a measure of the efficiency of the plant in units that are useful to
the power industry. It can be used to compare to design and look for causes of loss; it may
be used in cost of electricity estimates and may be used with fuel cost to set dispatch order.
Heat Rate Units
The heat rate is a value reported in Btus/KW, or how many fuel Btus does it take to make
one kilowatt of electric energy. Low values mean a more efficient plant, with some coal
plants in the <9,000 Btu/KW, and other plants in the >10,000 range. These are Gross heat
rate values and do not include the auxiliary power to run the plant.
Net verse Gross Heat Rates
It is useful to show examples of the calculations rather than trying to describe
them. Basically the Gross Unit Heat Rate is the heat needed to make the electricity at the
generator. The only power we can sell is the Net Load or the amount of electricity that hits
the grid. Plant power needs for equipment like fans, mills, heating, and the coal yard are
subtracted from the generator load to calculate the grid or net power rating. This load can
vary depending on what equipment is in service. There may be some flexibility in what
equipment is in service, so items like 5 verses 6 mill operation will impact the net heat rate,
rather than the gross heat rate.
Example: Messaging
Search
 Unit Gross Load = 600, 000 KW
 Unit Aux Power = 36,000 KW
 Unit Net Load = 600-36 = 564,000 Kw
 Unit Gross Heat Rate = 10,000 Btu/KW
The Net Unit Heat Rate can be calculated by looking at the total Btus and divide them by the
Net load (564,000 verse 600,000 KW)
 Heat Input (Btus) = 10,000 Btu/KW x 600,000 KW =
 6,000,000,000 Btus / 564,000 KW =
 Net Unit Heat Rate = 10,638 Btu/KW
If the Aux power was reduced to 32,000 KW the net unit heat rate would improve to a
lower:
 6,000,000,000 / 568,000 = 10,563 Btu/KW
This is valuable to recognize as there could be some discretion in what equipment must be
run and/or included in the Aux power, and what reported Net Unit Heat Rate is reported.
Calculating Gross Unit Heat Rate
There are several ways to calculate the gross unit heat rate and they may not provide the
same answer.
Method 1 – Using Turbine Heat Rate and Boiler Efficiency
A good turbine heat rate might be estimated fairly accurately as steam conditions can be
measured. Other factors like condenser performance can also be measured. The only flow is
steam flow, which could have some inaccuracies, but the performance diagram may help us
verify steam flows.
If you divide the turbine heat rate by the boiler efficiency you get the gross unit heat
rate. Using our example:
 Turbine Heat Rate = 8,500 Btu/KW
 Boiler Efficiency = 85% or 0.85
 Gross Unit Heat Rate = 8,500/.85 = 10,000 Btu/KW
If we improve the boiler efficiency 1% to 86% we can calculate the new lower gross unit
heat rate.
Messaging
Search
 Turbine Heat Rate = 8,500 Btu/KW
Boiler Efficiency = 86% or 0.86
 Gross Unit Heat Rate = 8,500/.86 = 9,884 Btu/KW
If the turbine heat rate is a solid value and the boiler efficiency is sound, this result has
merit. You could use this method or at least compare to values using other methods.
Method 2 – Input / Output Method
In this method the total Btu input is found by multiplying the coal flow in Lbs/hr by the coal
Btu/lb. Both of these values can be variable as discussed below. This total Btu input is then
divided by the KW generated to correspond to the coal flow measurement.
Example:
Unit Gross Load = 600,000 KW
Coal Flow = 682,000 Lbs/hr
Coal Btu/lb = 8,800 Btu/lb
Gross Unit Heat Rate = 682,000 x 8,800 / 600,000 = 10,003
Issues – Btu/lb
Where and how the coal sample is taken can influence the Btu/lb value reported from the
lab. Mine samples are almost always sampled using mechanical coal samplers. Mechanical
samplers and even hand sampling involves a crushing step that can dry the sample. ASTM
does not address this potential moisture bias, they indicate that a good mechanically sampled
coal will be within ,+/- 10% of the actual dry ash value at a 95% confidence level. This is
about +/- 0.7% dry ash for PRB coals. If this same standard is applied to the moisture in
PRB coal it indicates about +/- 3% on a 30% moisture coal.
These sampling errors along with any laboratory errors (typically +/- 100 Btu/lb) indicate
that any coal sample could be +/- 500 Btu/lb at the 95% confidence levels. If the coal flows
are accurate this would indicate that the Btu/lb level of a coal being over estimated due to
drying could significantly impact the Btu input portion of the calculation. The example
below shows the impact on gross heat rate if the coal Btu/lb is actually 8,650 rather than
8,800 Btu/lb.
Example:
Unit Gross Load = 600,000 KW
Coal Flow = 682,000 Lbs/hr
Coal Btu/lb = 8,650 Btu/lb Messaging
Search
Gross Unit Heat Rate = 682,000 x 8,650 / 600,000= 9,833 Btu/KW
This is an improvement of 170 Btu/KW over the calculation using 8,800 Btu/lb.
Burn Rate
The burn rate of a unit is the total Lbs of coal flow divided by the KW being generated. This
would be equivalent to KLbs/Hr/MW.
In either case it is a useful value because gross heat rate divided by burn rate is the Btu/lb
value of the coal being burned. For example:
Unit Gross Load = 600,000
Unit Coal Flow = 620,000 Lbs/he
Burn Rate = 620/600 = 1.033
If the Gross Heat Rate is 10,000 Btu/KW
Then the Btu/lb of the coal would be
Btu/lb = 10,000/1.033 = 9,680 Btu/lb
If the coal was PRB, (8,400-8,900 Btu/lb) coal I would suspect that there could be issues
with the heat rate or the coal flow measurement as the calculated Btu/lb is too high for PRB
coals.
Method 3 - Heat Rate Models
If the unit is equipped with a heat rate model, these can be useful as both a valid value and
another method for comparison.
These models can be based on design or other values and then a difference in heat rate is
calculated using plant data compared to design values. This is useful is seeing the impacts of
high air flows, low steam temperatures, condenser back pressure, and other items that can
impact unit heat rate.
Conclusion
In a competitive market place it could be useful to report heat rate values that are more
competitive by using different calculation methods. A good review of the method now used
could offer ways to report better values. Using mine or hand samples for Btu input offers
significant room for improvement. Sometimes it is useful and more valid to start with a good
heat rate and calculate what the coal Btu/lb must be.