(234v) Investigation of the Emission of Nitrogen-Sulfur Compounds As Persistent COD and Nitrogen Components from Flue Gas Desulfurization Wastewater

Some thermal power plants experience problems due to the increasing chemical oxygen demand (COD) and total nitrogen concentration (T-N) in the flue gas desulfurization (FGD) wastewater. Nitrogen-sulfur (NS) compounds present in FGD wastewater are speculated as the cause of increasing COD and T-N. However, the emissions of NS compounds are not yet fully understood. In this study, we present the method and results of analysis of NS compounds in real FGD wastewater from four units of coal-fired power plants. In addition, the emissions of NS compounds from FGD wastewater from two units of coal-fired power plants were investigated for a longer period of time.

Hydroxylamine-N, N-disulfonate (HADS), formed from the reaction between nitrite and bisulfite in FGD wastewater, is the precursor of NS compounds. HADS can be converted to various NS compounds by hydrolysis, oxidation and reduction, depending on the wastewater conditions. All 8 types of NS compounds contribute to T-N, while five contribute to COD. For the analysis, standard reagents of six different NS compounds, which are not commercially available, were synthesized. The conditions necessary for the measurement of NS compounds using ion-pair chromatography (IPC) and ion chromatography (IC) were examined. An analytical procedure was established for seven different NS compounds using a combination of two IPC and one or two IC measurement conditions, respectively.

NS compounds present in the real FGD wastewater from four units of coal-fired power plants were analyzed. When the COD or the T-N was high, hydroxylaminetrisulfonate (HATS), imidodisulfate (IDS), or amidosulfonate (AS) was generated in the FGD wastewater. On the other hand, when the COD or the T-N was low, these NS compounds were either not detected, or were detected in low concentrations.

The dependence of the generation of NS compounds on wastewater conditions was investigated by taking measurements over a long period of time, in two units of coal-fired power plant. The main type of NS compound was HATS in both the case. FGD wastewater of the first unit showed a strong negative correlation between the HATS concentration and the total organic carbon (TOC), while it showed a weak negative correlation between the HATS concentration and the oxidation-reduction potential (ORP). The FGD wastewater of the second unit showed a strong negative correlation between the HATS concentration and the ORP, while it showed a strong positive correlation between the HATS concentration and the nitric acid concentration. Therefore, maintaining the ORP of the absorbing solution for the FGD at a higher level can be effective in managing the COD and the T-N of FGD wastewater.