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

VGB PowerTech 10 l 2020
Implementation of a slagging prediction tool to lignite blend fired boilers
a) b)
Fig. 6. a) Predicted molten ash profiles for the reference cases;
b) Predicted slag formation rates for the reference cases.
Tab. 4. Predicted furnace slagging risk against the furnace exit-gas temperature (FEGT) and
slagging observations.
Case Boiler Load, % FEGT (SP), o C FEGT (TDM), o C Slagging risk
(FSI)
temperature profiles under reducing conditions
in the furnace and an oxidizing atmosphere
in the convective section of the
boiler. In the furnace, these profiles were
calculated for the conditions existing on
the furnace walls and for the average temperatures
in the flue gas zones (F i g -
u r e 6 a ). In addition, the slag formation
rate (kg/s) (F i g u r e 6 b ) was calculated
to assess the risk of ash deposition.
Analysing the molten ash profiles obtained
for the furnace and different reference cases,
it can be seen that in the most intense
heat release flue gas zones (between 40 m
to 45 m) for all cases the ashes are completely
molten. The main and critical difference
is in the molten ash % distributions
on the furnace walls and slag formation
rates calculated for the furnace.
In order to rank the slagging risk in the furnace
for different cases, the furnace slagging
index (FSI) has been developed. It
considers the maximum slag formation
rate (kg/s) predicted in the furnace as well
as the molten ash distribution profile on
the furnace walls.
The highest furnace slagging risk for 100 %
boiler load was determined for case 3,
whereas, the lowest is for case 1. This was
due to a relatively high slag percent predicted
(above 35 % for case 3) over the
relatively high elevation range according to
the height of the furnace walls and very
high, almost doubled as compared to case 1
slag formation rate. The medium slagging
risk in the furnace was predicted for case 2,
100 %, and case 3, for 80 % boiler load.
Changing the boiler load from 100 % to
80 % for case 3 led to reducing the slagging
Slagging observations
(InfraScan)
1 100 967 937 Low Low
2 100 971 947 Medium Medium
3 100 991 963 High High
3 80 909 n.a. Medium (LF) n.a.
Deposited mass, g
45
40
35
30
25
20
15
10
5
36FA 15.03.17 (Case 1)
36BA 16.03.17 (Case 2)
48BA 16.03.17 (Case 2)
48BA 21.03.17 (Case 3)
Fig. 7. IR pictures taken by the slagging
monitoring system (SMART InfraScan).
formation rate by around 5 kg/s and obtaining
similar slagging risk or even lower
for elevations above 55 m as compared to
case 2, and full load operation. Regarding
the high temperature fouling risk in the
convective section (above 95 m elevation),
this was assessed to be relatively low for
every reference case investigated. It was
found, that the predicted furnace slagging
risk (FSI) correlates well with the corresponding
InfraScan pictures, FEGT evaluated
by the slagging monitoring and TDM
system, and ash deposition rates measured
for the reference days during
the measurement campaign, as shown
in Ta b l e 4 . The increase of FEGT indicates
a decreased heat exchange in the furnace
due to ongoing slagging and deposit
build-up. The higher FEGT calculated
by the Slagging Predictor (SP) as compared
to FEGT (TDM), can be a result of
not including the effects of furnace wall
cleaning by soot blowers for FEGT (SP) assessment.
Analyzing results from the slagging monitoring
system, during the case 1 – reference
day, a relatively low slagging was observed
on the furnace walls, FEGT (TDM) was of
around 937 o C, which corresponded very
well with a low slag formation rate and a
short molten ash profile predicted on the
furnace wall by the Slagging Predictor. The
next day (case 2), the quality of the coal
blend slightly decreased and a medium
slagging was observed and predicted by
FSI. The prediction for the case 2 high
molten ash percent were distributed over a
0
0 25 50 75 100 125 150 175 200
Time, min
Fig. 8. Ash deposition rate measured with the online probe in 36 m and 48 m elevations.
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