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

Implementation of a slagging prediction tool to lignite blend fired boilers VGB PowerTech 10 l 2020
Tab. 1. Ash composition in % of the reference coal samples.
Case Ash* S* SiO 2 Al 2 O 3 TiO 2 Fe 2 O 3 CaO MgO K 2 O Na 2 O SO 3 P 2 O 5
1 6.05 0.85 44.00 11.1 0.72 13.1 11.3 2.79 0.77 0.09 14.7 0.11
2 7.86 1.38 49.10 10.3 0.82 11.50 10.3 2.35 0.64 0.09 12.5 0.06
3 10.10 3.45 50.4 14.5 0.84 8.17 10.1 2.16 0.88 0.10 11.00 0.06
Note: *) percentage in the fuel on as received basis
––
Case 1: date 15.03.2017, low slagging
operation
––
Case 2: date 16.03.2017, medium slagging
operation
––
Case 3: date 21.03.2017, high slagging
operation
The quality and ash content of lignite blends
fired during the selected cases are compared
in Ta b l e 1 . These blends show a difference
mostly in sulphur and ash contents
in the fuel as well as in the Fe 2 O 3 content
in the ash. The highest ash and sulphur
amounts were found in the blends for
case 3 (high slagging) followed by case 2
(medium slagging) and then case 1 (low
slagging).
The field slagging observations for these
cases differ from the predictions obtained
based on conventional indices. The calculated
conventional slagging index B/A predicted
medium (case 1) or low risk (case 2
and 3). Based on the Fe 2 O 3 /CaO index, the
predicted risk was ambiguous. Moreover,
the first two ash fusion temperatures (IDT
and ST) determined for the three investigated
blends under oxidizing conditions
were on a similar level of around 1,190 °C
(please see Ta b l e 2 and Ta b l e 3 ). Overall,
the conventional indices could not give
a clear indication about the potential slagging
severity or even showed different slagging
risks as observed during the measurement
campaign. However, based on the
fuel data and data collected from the monitoring
tools it was agreed that the ash and
sulphur contents (mostly associated with
pyrites) as well as reducing conditions inside
the furnace play an important role in
furnace slagging for Nochten and Reichwalde
lignite blends.
Evaluation of the slagging
predictor results
The developed slagging predictor enables
an advanced thermal analysis of a boiler
for investigating the impact of fuel switching
on boiler performance including the
ash deposition effects. The flue gas temperature
distribution in the furnace, its
maximum value in the most intense heat
release zones as well as at the furnace outlet
mostly determine the slagging conditions
inside the boiler. There are equally
important conditions on the furnace walls,
in terms of the wall/deposit outer temperature
and the presence of reducing atmospheres
along with the furnace height. The
one-dimensional flue gas temperature profiles
predicted for the Unit Q for the three
investigated cases (100 % boiler load) and
Tab. 2. Slagging risk assessment based on conventional indices.
Case B/A* Risk Fe 2 O 3 /CaO** Risk
1 0.503 Medium 1.16 Medium-Severe
2 0.413 Low 1.12 Medium-Severe
3 0.326 Low 0.81 Medium-Severe
Note: *) Base to Acid ratio (B/A), where A = XSiO2 + XAl2O3 + XTiO2, B = XFe2O3 + XCaO + XMgO +
XNa2O + XK2O , and Xi is the mass ratio of i component in the ash; **) Fe2O3/CaO – Iron-calcium ratio,
where Fe2O3 = XFe2O3, CaO = XCaO , and Xi is the mass ratio of i component in the ash
Tab. 3. Ash fusion temperatures.
Case 1 2 3
Initial deformation temperature (IDT), o C 1,150 1,160 1,160
Softening temperature (ST), o C 1,200 1,190 1,180
Hemispherical temperature (HT), o C 1,250 1,270 1,320
Flow temperature (FT), o C 1,280 1,430 1,360
additional medium 80 % load case are
shown in F i g u r e 5a. The corresponding
temperature distribution on the furnace
walls for case 3 and 100 % boiler load is
presented in F i g u r e 5 b .
The peak temperature in the most intense
heat release flue gas zone located just
above the burner rows (around 42 m) varied
between 1,315 o C to 1,330 o C for the full
load boiler operation and dropped down to
around 1,280 o C for 80 % load. In the entrance
to the convective section (elevation
ca. 94 m), the predicted furnace exit gas
temperature (FEGT) for low slagging operation
case (case 1) was around 967 o C.
Because of the increased slagging formation
rate for the case 2 and case 3 (see F i g -
u r e 6 b ), assessed based on the integrated
thermochemical ash deposition model,
the predicted FEGT temperatures were adequately
higher and equal to 971 o C and
a) b)
991 o C, respectively. The increase of FEGT
temperature, and thus drop in the furnace
thermal efficiency, are related to
the higher thermal resistivity of the ash
deposits that may be formed at an elevated
rate on the furnace walls. The ash deposits
formed on the furnace wall affect
the temperature profile in the furnace
wall zones. For the medium slagging operation
case (case 2), the maximum wall/
deposit temperature was predicted at the
elevation of 42 m and was equal to around
900 o C as shown in F i g u r e 5 b ). Such
temperature conditions along with reducing
atmospheres may be critical for the increased
formation of slag on the furnace
walls.
In order to assess the furnace slagging/
fouling risk according to the furnace height
for the investigated cases, the molten ash %
profiles were determined for the obtained
Fig. 5. a) Predicted average flue gas temperature distributions throughout the boiler;
b) Predicted temperature distribution on the furnace wall for 100 % boiler load.
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