Figure 6. The variations of O2 content of PPS-based bag
filter materials with different loading units.
The variations of material structure and properties is bound up with the
working conditions. The major duty parameters including temperature of
flue gas, the content of
O2,
the content of NOx, the content of SOx and so on could
cause damage to the PPS fibers in the coal-fired power plants. The
effect of NOx can be ignored because all samples were
collected from the system containing denitrification system with regular
service. The O2 content should be lower than 8% for the
safety operation of PPS-based bag filter in the flue gas of coal-fired
power plants based on the previous report [36]. The
O2 content for all samples is lower than 8% from Fig.6,
making it negligible to influence the structure of PPS fibers.
Therefore, the failure of PPS-based bag filter must exist closely
relationships with the content of SOx and temperature of
flus gas.
\begin{equation}
SO_{2}+O_{2}\leftrightharpoons SO_{3}\ \ \ \ \ \ \ \ \ \ (1)\nonumber \\
\end{equation}\begin{equation}
SO_{3}+H_{2}O\leftrightharpoons H_{2}SO_{4}\ \ \ \ \ \ \ \ \ \ (2)\nonumber \\
\end{equation}\begin{equation}
2H_{2}SO_{4}\rightleftharpoons H_{2}SO_{4}\bullet H_{2}O+SO_{3}\text{\ \ }\ \ \ \ \ \ \ \ (3)\nonumber \\
\end{equation}The major component in the SOx of flue gas is
SO2, which could be oxidized into SO3under the assistance of O2 at high temperature (eq (1)).
The conversion rate from SO2 into
SO3is between 0.1% and 3% based on literature. SO3 would
combine with water in the flue gas to form vapor of
H2SO4 (eq (2)), and the acid gas dew
point (condensing temperature) would increase with the addition of
concentration of
H2SO4vapor. When the temperature of flue gas reduces to the acid gas dew
point, H2SO4 vapor would condense into
liquid concentrated H2SO4. Then, liquid
concentrated H2SO4 combines with flying
ash in the flue gas and co-adheres to the three-dimensional network of
bag filters. After that,
dehydration
reaction (eq (3)) occurs between molecules of
H2SO4 at high concentration to produce
SO3 with powerful oxidizing properties in the above
network, resulting in the oxidation or even
sulfonation
of PPS-based bag filter. Therefore, the key point leading to the failure
of PPS-based bag filters is the relationship between the temperature of
flue gas and acid gas dew point.
\begin{equation}
T_{a}=116.5515+16.06329*\lg V_{SO_{3}}+1.05377*(lgV_{SO_{3}})^{2}\ \ \ \ \ \ \ \ \ \ (4)\nonumber \\
\end{equation}Where, VSO3 is the volume fraction of
SO3 in the flue gas (ppm).
Based on above conclusion, the eq (4) [37] was introduced to obtain
the temperatures of acid gas dew point (Ta) of
coal-fired power plants which was fitted from Müller curve [38].
Considering the fluctuation of flue gas parameters and the diversity of
influential factors of Ta, Tas of
fifteen coal-fired power plants were calculated on the basis of eq (4)
with 0.1%, 1.5% and 3.0% of SO2 conversion rates and
marked as T0.1, T1.5 and
T3.0, respectively. The relationship between temperature
of flue gas and Ta was displayed in Fig. 7. It’s easy to
observe that running temperatures (T) of 1#, 6# and 11# are higher
than relative Ta, while running temperatures of 12 other
coal-fired power plants are between T0.1 and
T1.5, hinting that T is often passing through
Ta. When T is higher than Ta,
H2SO4vapor produced from SOx couldn’t condense on the surface
of PPS fibers, and the oxidation of PPS fibers largely reduces which is
beneficial to prolong the service life of bag filters in coal-fired
power plants. Conversely, PPS fibers could be oxidized gradually with
the increase of adhesion amount of liquid concentrated
H2SO4 when T is lower than
Ta. Ta increases with the addition of
SO2 content in the flue gas, and the oxidation degree of
PPS-based bag filters becomes deeper at the same time. Thus, above
phenomenon can be illustrated. Besides, the SO2 content
in the flue gas is lower than 1500 mg/m3 for 1#, 6#
and 11#, which is a critical concentration of SO2 that
needs to be concerned especially for safety and long-time running of
PPS-based bag filters in coal-fired power plants.