POWER February 2015 - 56

FUNDAMENTALS
Moisture Coals " in the November 2014 issue or at powermag.com.)
Unburned Combustible Loss. Unburned combustible losses are
efficiency losses from incomplete combustion of fuel in the boiler.
This is primarily measured in the form of carbon residue in the ash,
but it also includes carbon monoxide (CO) production. These losses
are generally influenced by both fuel properties (fuel volatility) and
operations practices (excess air level, fuel fineness, and the like). It
is important to note that unburned combustible loss is not the same
as loss-on-ignition (LOI), as unburned combustible loss is an energy
loss, whereas LOI is calculated on a mass basis in the ash.
Radiation and Convection Loss. Utility boilers are enormous
equipment systems, with numerous penetrations for tubes and instruments,
and a very large surface area exposed to the environment. As a
result, no matter how well-designed the insulation is and how diligent
plant personnel are in fixing air leaks, energy will still be lost via
radiation and convection.
Does it Make Economic Sense?
It's all very well to propose numerous capital and operations
upgrades at your power plant. But which improvements make
the most economic sense to the power plant owner?
Some plant improvements can be a metaphorical no-brainer,
whereas other improvements may require an external market factor,
such as a carbon emissions tax, in order to become cost-effective.
Table 1 provides a very general ranking of improvements
that can be made to pulverized coal-fired power plants, a range
of potential heat rate improvements, and their relative economic
payback periods. Note that this listing does not include many
specific maintenance items that may be found at some power
plants, and which may provide large improvements in efficiency
when repaired or upgraded.
Table 1. Many options to choose from. Every power
plant has unique opportunities for, and challenges to, improving its
heat rate. The values shown in this table are only general ones based
on research from energy efficiency studies. Source: Una Nowling
Improvement
Improving combustion
controls and monitoring
Increased condenser cleaning
and repair of air leaks
Turbine seal improvements
Increased feedwater
heater monitoring, maintenance,
and repair
Air heater seal repair or
upgrade
Preheating combustion air
with waste heat
Increased cleaning of
turbine deposits
Low-pressure turbine
blade upgrade
Replacement of main
fan motors with variable
frequency drives
Solar combustion air heating
Solar feed water heating
56
Range of heat rate benefit Payback period
0.25%-1.00%
<1 year
NTHR =
0.30%-2.00%
0.50%-2.30%
0.20%-1.00%
0.10%-0.50%
0.10%-0.30%
0.25%-3.50%
1.00%-2.00%
0.20%-0.50%
0.25%-0.75%
3.00%-7.00%
<1 year
Where:
1-3 years
1-3 years
2-3 years
2-3 years
2-4 years
2-4 years
3-5 years
>5 years
>5 years
NTHR = net turbine heat rate, Btu/kWh
HMSOUT
Btu/hr
HFWIN
hr
HHRH = enthalpy of the hot reheat steam exiting the boiler envelope,
Btu/hr
HCRH
= enthalpy of the cold reheat steam entering the boiler envelope,
Btu/hr
PowerBFP
= boiler feed pump power consumption, kW
Improving Turbine Cycle Efficiency. Under ideal conditions, an
ultra-supercritical turbine cycle system can convert steam into rotational
energy at 54% or higher efficiency, supercritical turbine cycles
can achieve 50% efficiency, and subcritical turbine cycles can
achieve 46% efficiency. However, the turbine cycle system of your
power plant is at least as complex as your boiler system, and there are
numerous places for efficiency to be lost.
Bucket tip and packing leakage can constitute 40% of total effiwww.powermag.com
POWER
| February 2015
= enthalpy of the main steam exiting the boiler envelope,
= enthalpy of the feedwater entering the boiler envelope, Btu/
Main steam flow x (HMSOUT - HFWIN) + Reheat steam flow x (HHRH - HCRH)
Gross electrical generation - PowerBFP
Margin and Unknown Losses. Due to the large size and complexity
of the boiler, it is often not practical to measure every single
possible source of energy loss from the power plant. As a result, a
" margin " or " unknown loss " value is typically used to estimate these
losses. Typical values range from 0.5% to 2.0%.
When all of these efficiency losses are taken into account, a typical
utility boiler can utilize fuel energy with an efficiency ranging from
83% to 91%.
Improving Boiler Efficiency. Sensible heat losses can be reduced
by installing improved combustion controls to allow fine-tuning the
excess air level in the furnace operators to reduce the excess oxygen
level in the furnace. Preheating combustion air with waste heat from
the plant will also increase efficiency, and some plants are considering
schemes to use solar thermal collectors as air preheaters during
daylight hours.
As latent heat losses are strongly tied to fuel quality, and current
boiler designs do not allow for condensing air heaters, outside of
switching to a dryer fuel, there is little that can practically be done to
reduce latent heat losses.
Unburned combustible losses can be reduced by improved boiler
and burner tuning, with some plants able to gain more than 1% in
net efficiency as a result of a minor amount of tuning or capital
investment.
Turbine Efficiency
Your turbine efficiency is essentially the efficiency of the turbine to
convert steam from the boiler into usable rotational energy. A simplified
way of viewing your net turbine heat rate (NTHR) is to sum
the enthalpy increases of the feedwater and the cold reheat steam
across the boiler boundary and divide this by the gross electrical
generation.
Determining Turbine Efficiency. As in the case of the overall
plant, the turbine cycle heat rate can be expressed on a " gross " or
" net " basis. Here the terminology becomes a little tricky, as the gross
and net efficiency both utilize the gross output of the generator in their
calculations. However, if the power plant has an electric boiler feed
pump, then the net turbine heat rate must also subtract out the power
consumed by the feed pump; otherwise, that power consumption may
skew your NTHR value to appear overly efficient. As a result, our
simplified NTHR equation for a single-reheat cycle resembles this:
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POWER February 2015

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