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Boiler
& Plant Operations Part 1:
Energy Survey
by William G. Acker
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The efficient use of energy is fast becoming a top concern for industry. As business managers become more conscientious about their energy use, they are realizing that effective energy conservation can result in significant returns to their companies. Energy surveys are key to helping them achieve their goals.
In today's world—as the cost of fuel
steadily increases while the supply decreases—energy conservation is a
must. For industry to conserve energy without lowering production, it must find ways to use resources more efficiently.
Energy management is the judicious control of energy to accomplish a purpose: the production of a product, the completion of a process, or heating and cooling a building. Energy management is required to get the most out of every Btu
of fuel and every KWH of electricity. Energy conservation is becoming an important issue along with reliability, fuel flexibility, and pollution control.
Organizing an energy conservation program is
not an easy task. To be effective, the program needs a reasonable target and
timetable, adequate technical
information and resources for implementation,
a firm commitment on the
part of management and all members of
the organization, and a sound, consistent
plan of action. |
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To begin an energy conservation effort,
managers need to pinpoint areas of
high fuel and electric power consumption.
This requires an energy audit detailing
all the energy inputs and distribution
throughout the plant. Preparing a
plan requires a detailed survey. In addition,
it is necessary to visually inspect the
equipment and components, and to
conduct various tests at critical points to
determine energy consumption. This article
aims to illustrate the different boiler
energy losses and present an accurate
picture of energy use that is easy to follow
and understand.
To understand the flow of energy
(Btu) through a system, you need to
understand the first law of
thermodynamics; simply stated it says
that energy (Btu) can neither be created
nor destroyed. Energy can only be
transformed from one form to another.
For the end-user then, we can diagram a
complete energy analysis (heat balance)
of all energy inputs into the system, and
all energy discharges out of the system.
Since energy can't be created or destroyed,
the summation of total energy
inputs must equal the summation of the
total energy discharges. To put it an-other way, the sum of the energy
(potential, kinetic, thermal, chemical,
and electrical) entering a process
must equal the sum of the energy
leaving.
A properly prepared energy balance
sheet analysis of a system or an
entire plant is an excellent way to
present your energy survey results to
management. Terms such as combustion
efficiency, boiler efficiency,
and plant efficiency should be included
in the energy survey and the
losses, but these terms can be confusing.
The confusion results from
the many different ways to calculate
these terms. Therefore, it is important
to show the calculation procedures
and to provide all information
on losses.
Combustion Efficiency
Combustion efficiency is a
measure of the percent of available
energy in the fuel and air, which is
released in the combustion process.
Stated another way, it is the ratio of
available heat (after losses) divided
by the fuel input. Table 1 shows the
combustion efficiency equation and
the associated losses.
Heat loss (due to dry gas) is heat
carried away (out of the stack) by the
hot flue gases. This loss is usually
called "dry flue gas" loss. Hotter
stack temperatures and large
quantities of excess air increase the
loss. Excess air beyond stoichiometric
conditions is required to ensure
complete combustion and to allow
for proper control. The sensible heat
loss in the dry chimney gases is one
of the losses. For complete
combustion, solid fuels require the
greatest excess air and dry flue gas
losses, and gaseous fuels the least. A
flue gas temperature well above the
acid dew point temperature is a good
indicator of excessive dry gas losses.
The ratio of hydrogen atoms to
carbon atoms in a given fuel greatly
affects the efficiency of a boiler. In
complete combustion, hydrogen
atoms in the fuel combine with oxygen
in the combustion
air to
form water,
which consumes
the heat of vaporization
to become
water vapor
in the flue
gas. The latent
heat of vaporization
is lost when
water vapor
leaves the boiler
stack because
the vapor cannot
be condensed
due to an inherent
corrosion
problem due to
the presence of
SO3 in the flue
gas. Some commercial
furnaces
do offer secondary stainless-steel
heat exchangers so that this latent
heat can be recovered. See Table 3
for typical values for water vapor
formation from hydrogen in the
fuel. The table shows that natural
gas produces much more water vapor
than coal due to its higher hydrogen
content. Natural gas also has
much higher losses in this category
than coal.
Moisture in fuel reduces the efficiency
of the boiler by discharging
heat up the stack in the form of
highly superheated vapor. The water
present in the fuel consumes latent
heat of vaporization (from the
available energy) to become water
vapor in the flue gas. There is also a
sensible heat loss due to superheating
the vapor. The total loss for this
category can be a significant loss for
solid fuels, but tends to be small for
gaseous fuels.
If insufficient oxygen (air) is
supplied, the mixture is "rich" and
the fire is "reducing," which results
in a flame that tends to be longer
and sometimes smoky. This is usually
called incomplete combustion, which
occurs when the fuel particles combine with some oxygen but
cannot get enough to burn
completely. Incomplete combustion
results in the formation of carbon
monoxide gas, which is another loss
in the combustion process, because
the fuel introduced into the boiler is
not completely burned.
With perfect combustion of
hydrocarbon fuels, the carbon is
heated in the presence of air;
oxygen in the air combines with the
carbon to form carbon dioxide. The
hydrogen in the fuel combines with
the oxygen in the air to form water
vapor. For greatest efficiency, the
fuel-and-air mixture should be
adjusted so there is little to no
carbon monoxide in the flue gas.
To maintain high boiler efficiency,
the carbon monoxide concentration
in the flue gas must be kept below
15 to 20 ppm (wet basis).
The last combustion efficiency
loss is unburned carbon. This can
be unburned carbon or partially
burned carbon found in the ash pit
or the flue gas (fly ash). This is a
loss that occurs in the burning of
solid fuels such as coal. A low value
for coal is 2 lb of carbon per 100 lb of refuse (carbon and ash), or less.
High-end values can be 6 to 10 lb
per lb. The retrofitting of low NOx
burners can increase the amount of
unburned carbon in the refuse.
Boiler Efficiency
The equations used for this article
may be found in Table 1. Boiler
efficiency loss is a combination of
combustion efficiency losses plus
some additional losses found in Table
2. Boiler efficiency as presented here
is the amount of energy transferred
from the combustion side to the
water side (of a steam boiler) after
losses and divided by the fuel input.
The boiler efficiency losses (for
non-cycling boilers) include the
combustion efficiency losses plus:
• Radiant heat loss
• Blowdown losses
• Unaccounted losses
Radiant heat loss from a boiler is
the radiant and convection heat loss
from the hot surfaces of the boiler.
The American Boiler Manufacturers
Association (ABMA) provides a chart for estimating this heat loss. It is
important to remember that the
radiant loss (Btuh) does not change
with boiler load, therefore a 50,000
lb of steam per hr boiler with
a radiation loss of 1 percent
(or 500 lb per hr) will still
have the same radiation loss when the
boiler runs at 10,000 lb of steam per
hr. Therefore, at the lower boiler load
of 10,000 lb of steam per hr, our
radiation loss is now 5 percent of the
boiler load. If the boiler cycles on and
off, the radiant loss as a percentage of
boiler output gets worse.
Boiler blowdown losses can be
reduced with the installation of a
heat recovery system. Boiler
feedwater introduces soluble salts,
silt, and other solids into the boiler
system. Build-up of the solids will
lead to scaling and/or corrosion on
the boiler's internal surfaces. Buildup
can be controlled with chemical
treatment and blowdown of the
boiler water. Blowdown of the
boiler drum bleeds-off a portion of
water containing a high concentration of solids and replaces
it with treated low concentration
make-up water.
The percent
blowdown equation:
The last boiler efficiency loss is
unaccounted losses. These are
customarily included in a heat
balance to provide a margin of
safety or tolerance in the calculated
efficiency. The typical value for
these losses is around 1.5 percent.
The losses discussed so far have
been for non-cycling boilers. Cyclic
losses occur when the plant demand
(Btuh) is lower than the low-fire
limit (Btuh) of the boiler. The
oversized boiler must then cycle off
and on to maintain the plant
demand. Cyclic losses are subtracted
from the boiler efficiency. It is not
unusual to have a boiler efficiency of
80 percent or greater and to end up
with an overall efficiency below 50
percent due to high cyclic losses.
There are four types of cyclic losses: those resulting from prepurge
and post-purge drafts, shell
radiant losses, and natural draft
losses. Pre- and post-purge procedures
can result in draft losses because
they involve forcing air
through the boiler to remove unburned
combustibles before start-up
and after shutdown. While this eliminates
the possibility of a boiler explosion
due to too much fuel in the
combustion chamber, unfortunately,
the air traveling through the boiler
removes heat at the same time. Preand
post-purge draft losses can be
determined by testing the air flow,
inlet, and outlet temperatures, and
the amount of operating time.
Shell losses are radiant losses that
continue to occur during the off cycle.
Natural draft losses are losses
that occur during the boiler off cycle,
and are associated with the air
flow through the boiler created by
the difference in temperature between
the boiler and outside ambient
conditions. Natural draft loss
can also be tested during the boiler
off cycle.
Commercial building heating systems
typically have a wide range of
heating demands throughout the
year. To prevent cycle load losses,
many buildings use multiple small
boilers.
Energy-Survey Example
Figure 1 shows an example of a
mass balance energy survey that
accounts for all the energy flow Btu
and all mass flows into and out of the
entire plant system. The energy
analysis involves a combination
psychrometrics and thermodynamics
computer program, a boiler
efficiency program, and a flue gas
Btu analysis program, that an
associate and I developed for these
surveys. This boiler and plant analysis
procedure provides the business
owner with an easy-to-understand
energy analysis, provides a greater
degree of accuracy in the Btu energy
flows, and reduces the engineering
time because the calculations are computerized.
In this particular survey, the combustion
efficiency is 88.1 percent,
the boiler efficiency is 85.5 percent,
and the overall plant efficiency is
81.9 percent. Overall plant efficiency
for use in this article is the Btu of energy
consumed by the plant processes
divided by the Btu of fuel
consumed to produce the steam.
Plants with many steam leaks, little
condensate return, poor pipe insulation,
and poor maintenance of steam
traps tend to have a plant efficiency
that is well below the boiler efficiency.
The computer program provides
nine data sheets for each boiler
covering all the losses, flue gas analysis
including SO2, SO3, acid dew
point, Btu, and particulate emissions.
The flue gas program allows the engineer
to change the leaving flue gas
temperature (for the same mass flow)
to check for potential heat recovery.
Through the reduction of excess air,
the program can also calculate the
energy savings associated with the
combustion air reduction.
To develop an energy balancing
procedure, it was necessary to
choose enthalpy-calculating procedures
with the same zero-energy
starting point for dry air, water vapor,
and dry flue gases. Table 4
shows some of the procedures reviewed
for the combustion air. The
gas table procedure by Keenan,
Chao, and Kaye is used in a number of gas turbine energy analysis programs.
This procedure eliminates
negative enthalpies because the zero
enthalpy level is set really low. I
chose the Zimmerman and Lavine
procedure, which also allowed the
use of the Keenan and Keyes steam
tables. This was so engineers could
see enthalpies they are most familiar
with. Also, Zimmerman and Lavine
utilize procedures that allow for
analysis of air and water vapor up to
very high temperatures.
Special thanks to Nels Strand, for his assistance
in the development of the computer
programs.
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References
1) 1997 ASHRAE Handbook of
Fundamentals, Chapter 17: Combustion
and Fuels.
2) Ganapathy, V., Steam Plant
Calculations Manual, 2nd edition.
3) Ganapathy, V., Waste Heat
Boiler Desk Book.
Boiler & Plant Operations Part 2: Energy Analysis
by William G. Acker
Boiler Energy Balance - Btu's Into Equal Btu's Out & Boiler Efficiency
A. Energy Entering the Boiler |
1. Combustion Air |
3,508,176 Btu/hour |
2. Fuel |
134,469,741 Btu/hour |
3. Boiler Feedwater |
24,991,642 Btu/hour |
4. Total Energy Entering |
162,969,559 Btu/hour |
B. Energy Leaving the Boiler |
1. Flue Gas Flow |
19,465,094 Btu/hour |
2. Steam Output |
137,898,800 Btu/hour |
3. Radiant Loss |
591,667 Btu/hour |
4. Unaccounted Loss |
2,017,046 Btu/hour |
5. Blowdown |
2,890,586 Btu/hour |
6. Unburned Carbon |
106,366 Btu/hour |
7. Total Energy Leaving |
162,969,559 Btu/hour |
C. Boiler Losses |
1. Dry Gas Heat Loss |
9,085,245 Btu/hour |
2. Evaporation off Hydrogen Formed Water |
5,388,834 Btu/hour |
3. Evaporation off Fuel Moisture |
1,028,209 Btu/hour |
4. Water Vapor In Combustion Air |
217,152 Btu/hour |
5. Carbon Monoxide |
0 Btu/hour |
6. Unburned Carbon in Flue Gas |
106,366 Btu/hour |
7. Radiant Loss |
591,667 Btu/hour |
8. Unaccounted Loss |
2,017,046 Btu/hour |
9. Blowdown Loss |
1,707,766 Btu/hour |
10. Total Losses |
20,141,985 Btu/hour |
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