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.