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12/2/11 Four Methods of Fly Ash Sampling
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Four Methods of Fly Ash SamplingOne of the best indications of
performance in the furnace is the quality of ash coming out of a
power plants boiler.
Ideally, all of the carbon is completely burned in the furnace
before the products of combustion are quenched in the
superheater. Perfect combustion remains the goal, but one that
can never be achieved. However, we can come
close.
Fly ash should be measured periodically (at least weekly) to
determine its carbon content or loss on ignition (LOI).
Low-NOx burners make it ever more difficult to reach low levels
of carbon content in fly ash. However, when a furnace
is performing well, the coal pulverizers are tuned for best
fineness, and air/fuel ratios are optimum, then a realistic
goal for carbon burnout is to reduce fly ash LOI to 3% to 5% or
lower for eastern coals. With western Powder River
Basin and lignite fuels, due to their high reactivity, fly ash
LOI should be below 0.5%.
Obtaining representative samples of fly ash from very large flue
gas ducts remains the largest challenge. For
example, a typical 500-MW coal unit firing 8% ash content fuel
will be passing about 40,000 pounds of fly ash per
hour. The challenge is to obtain two representative 1-gram
samples for LOI determination.
Why Doesnt All of the Carbon Burn in the Furnace?From our
experience, there are three major factors that prevent complete
burnout of carbon before it reaches the
furnace exit:
Insufficient furnace oxygen.
Particle size that is too large (coal fineness is too
coarse).
Poor fuel and air distribution in the burner belt.
If there is insufficient well-distributed oxygen in the furnace
to mix proportional amounts of fuel, it doesnt matter what
the furnace temperature is, the carbon will not burn. Its
important to remember the fundamentals such as ignition
temperature, fuel, and oxygen that are required for combustion.
Lack of oxygen in the furnace can be the root cause
for one of three reasons:
Insufficient total airflow.
Fuel and air imbalance in the furnace (there may be an adequate
oxygen level on an average basis, but some
areas may have pockets of low oxygen levels).
Falsely high oxygen indications from air in-leakage.
Side-to-side imbalances may be determined by taking fly ash
samples from both the left and right sides of the boiler.
One side of the furnace may show a carbon content of say 3%, but
on the other side the LOI may be high, say 15%.
This would indicate an imbalance that can lead back to the
burners or secondary air. This type of information is
valuable when tuning a boiler.
Larger fuel particle size contributes to high unburned carbon in
two ways:
More time is required for the complete combustion of the carbon
char. Insufficient residence time, due to the
furnace arrangement, is a big factor contributing to high
LOI.
Poor fuel fineness further contributes to high LOI from the
resulting poor fuel distribution. Poor fuel distribution
nearly always accompanies poor fineness.
Figure 1 illustrates how better coal fineness creates much
greater surface area for optimizing the short residence
time in the furnace. Finer pulverization also yields improved
distribution through each coal pipe, as shown in Figure
2.
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12/2/11 Four Methods of Fly Ash Sampling
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12/2/11 Four Methods of Fly Ash Sampling
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3. The high-volume in-situ fly ash sampler. Source: Storm
Technologies Inc.
Remember that the intent of fly ash sampling is to obtain a
representative ash sample from very large ducts. To do
this requires that many points be sampled. For boiler acceptance
testing, as many as one point sample every 9
square feet of duct area is usually collected by experienced
test engineers. This is equivalent to one sample every 3
feet. Of course, this is impractical for weekly tests, but at
least a reasonable number of sample points must be
sampled. One sample every 25 square feet of duct area (one point
every 5 feet) is typical.
Fl\ Ash Sampling Method 2. Using an isokinetic fly ash probe is
the most accurate method, although this
approach requires a minimum of two people, more time, and
attention to detail during testing. A fecheimer probe is
built into the head of the fly ash sampler, which measures the
velocity in the duct. Once the velocity is measured,
the sampling rate is matched with a manometer attached across an
in-line orifice. The weight of the sample
collected over a defined period of time can be used to calculate
the ash flow rate in the duct. Other data taken during
the test are the duct gas velocity, temperature, static
pressure, and gas density. This level of precision is not
needed
for periodic performance testing.
Perhaps the biggest challenge is to obtain a representative
sample. Our experience with the sampler shown in
Figure 4 with a nozzle sized for near isokinetic sampling shows
that it is quite sufficient for weekly sampling.
However, we feel it is our obligation to state that the best and
most accurate fly ash sampling method is the
isokinetic sampler used for a representative number of sample
points.
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4. The isokinetic fly ash sampler. Source: Storm Technologies
Inc.
Fl\ Ash Sampling Method 3. The multipoint fly ash sampler system
in Figure 5 uses a probe that is essentially an
arrangement of sample tubes installed in the flue gas duct to
extract fly ash samples simultaneously from multiple
points. By so doing, reasonably representative ash samples can
be collected by one operator with minimal effort.
Incorporated into this arrangement are thermocouples and flue
gas sampling tubes so that representative
temperatures and flue gas analyses can also be collected. This
system was designed by Storm Technologies Inc.,
and a patent application has been filed.
5. The integral in-situ fly ash sampler. Source: Storm
Technologies Inc.
Fl\ Ash Sampling Method 4. The fourth and least accurate, but
most common, method is taking grab samples
from the ash hoppers. The three-part fly ash sample analysis
method can determine whether the root cause of high
fly ash LOI is due to the pulverizers or other combustion
issues, such as poor airflow distribution, unbalanced fuel
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flows, or postcombustion duct air in-leakage. Because these ash
samples can be very biased, they are not
recommended for determination of furnace performance or
combustion efficiency.
Fly ash samples are first burned to determine their carbon
content either in a furnace or by the hot foil method. Both
operate under the same principle: Heat the sample to about 200F
to drive off the moisture, and then weigh the
difference (WO). Then heat the sample to at least 1,600F and
weigh the difference again (WF). The weight difference,
(WO-WF)/WO will give the percent moisture and the percent
unburned carbon, or LOI, in percentage of weight loss.
When calculating the LOI it is assumed that all of the remaining
combustibles are carbon.
If there is a problem with high LOI in the fly ash, a
representative sample can be further analyzed to ascertain the
root cause of the problem. The fly ash can be sieved through a
200 mesh screen, and the portion that passes the
200 mesh screen (fine particles) can be burned for LOI (Figure
6). If the fine particle LOI is higher than required, then
the unburned carbon can be attributed to a combustion
problem.
. The three-part fly ash test separates coarse and fine particle
samples (left) and a composite sample (right) that
are weighed and compared to determine LOI. Source: Storm
Technologies Inc.
If the fine particles are above 2% LOI for bituminous coal, this
indicates that, although the particles are fine enough,
there was not enough oxygen to complete combustion. If the LOI
of the coarse particles is high, the problem can be
traced back to the pulverizers. Thats because if the fine
particle LOI is low, then the fine particles had enough
oxygen and residence time in the furnace to burn. The unburned
carbon in the larger particles indicates that the
particles are too large to burn in the residence time
allotted.
Richard F. (Dick) Storm, PE is senior consultant for Storm
Technologies Inc.
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