VGB POWERTECH 10 (2020) - International Journal for Generation and Storage of Electricity and Heat

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VGB PowerTech 10 l 2020
Implementation of a slagging prediction tool to lignite blend fired boilers
Signal
amplifier
Signal
converter
Data
recorder
W
R
Boiler wall
Fig. 3. Scheme of the online ash deposition monitoring probe.
tangential firing is the long spiral burnout
path. The boiler layout integrated with the
tool interface is shown in F i g u r e 2 .
Boiler measurement campaign and
data collection
The performance of the online deposition
probe was tested and operational data was
collected during a two-week boiler measurement
campaign carried out at the Boxberg
power plant Unit Q. The IFK on-line
deposition monitoring probe is a simple
and robust measuring system for the local,
quantitative determination of slagging/
fouling rates in boilers as schematically
shown in F i g u r e 3 . The mass of the ash
deposited on the sensor is recorded as a
function of time.
The online deposition probe can be placed
in different boiler locations to detect and
quantify the impact of a fuel quality switch
and boiler load changes, as well as effects of
a soot blower activity on deposit growth
rate and/or shedding. During the measurement
campaign, the probe was inserted in
the furnace at different elevations. The signal
was recorded for a minimum of two
hours. Overall, the online deposition rates
were determined for several operational
cases, including changes in fuel blend quality
and mill configurations. The boiler load
during measurements was stable and close
to 100 %. Other measurements included
the flue gas composition and temperature
profiles measured in selected available
openings as well the collection of coal samples,
deposit, fly ash, electrostatic filter and
bottom ash samples. In addition, the data
and pictures derived from the Clyde Bergemann
smart deposit cleaning and monitoring
system (SMART InfraScan and TDM)
were collected. SMART InfraScan measures
the surface temperature of the boiler wall
in the furnace using infrared sensors to de-
Furnace Exit Temperature (TDM), o C
Deposit
mass
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960
950
940
930
920
910
900
Flue gas flow
Sensor
tect areas with the increased deposition
and to optimise furnace cleaning procedures.
The convective section of the boiler
is monitored with the use of a thermodynamic
model (TDM), which utilises the
steam temperatures, pressure, and flow
rates data to measure the effectiveness of
heat transfer and to optimise soot blowing
operations for different heating surfaces.
The difference in steam production rates between
theoretical (clean surface) and the
actual value is attributed to the build-up of
deposits on the surface of the steam-generating
section. In this way, the heat transfer
in the various convection passages of a boiler
can be monitored for use in determining
specific cleaning patterns. Moreover, with
the TDM approach, the furnace exit flue gas
temperature (FEGT) can be estimated,
which indicates the heat transfer conditions
in the furnace affected by ash deposition.
FEGT
Ash % (as received)
5 % (water free)
Results and discussion
In this section, the main findings and example
results from the implementation
and validation of the developed engineering
tool at the Boxberg power plant, unit Q
are presented and discussed.
Fuel quality fluctuations and case
selection
The quality of the lignite blend varied during
the measurement campaign although
the blend was composed of around 50 wt%
of Nochten and 50 wt% of Reichwalde
coals. The highest fluctuations were observed
for the ash content 4.48 % to
10.10 % (as received basis) and sulphur
content in the blend 1.72 % to 2.93 % (water-free
basis). In the one coal sample
(21.03.2017) the extreme sulphur content
of 7.26 % (wt) was identified which was
above a typical, normal variation of sulphur
in supplied coal blends. The moisture content
in the blend varied between 53.4 % to
56.1 % (ar), whereas, the lower heating
value was in the range of 8.202 MJ/kg to
9.047 MJ/kg (ar). It was found that the ash
and sulphur contents correlate well with
the changes of the furnace exit flue gas temperatures
(FEGT) assessed by the TDM
Clyde Bergemann online monitoring system
as shown in F i g u r e 4 . The increase
of FEGT indicates a decrease in the heat
exchange in the furnace and, thus, a lower
furnace efficiency due to ongoing slagging
and deposit build-up.
Three different scenarios were selected for
performing more detailed analyses. This
selection was based on the identified differences
in the fuel quality fired, available
data from the online ash deposition rate
monitoring probe as well as corresponding
pictures and data gathered from the Clyde
Bergemann slagging monitoring system
operated during a two-week measurement
campaign, are as follows:
Fig. 4. Furnace exit-gas temperature (FEGT), ash and sulphur contents variations during
measurement campaign.
12
10
8
6
4
2
0
Content in fuel, %
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