Tannin-based desulfurization process

I. Reaction principle? The desulfurization method used in our factory is the tannin wet oxidation method. The main components of the tannins employed in this method are hydrolyzed tannins. Although the molecular structures of various tannins are very complex, they are generally polycarboxylic compounds with phenolic structures. When tannins are converted into an alkaline aqueous solution and then heated while exposed to air, most of their phenolic structures are oxidized by air to quinone structures. Due to their high electrical potential, these quinone structures can oxidize vanadium in its lower oxidation state to a higher oxidation state, thereby oxidizing the hydrogen sulfide ions in the solution and resulting in the precipitation of elemental sulfur. The desulfurization process proceeds as follows: 1. The alkaline solution absorbs H2S to form HS-. Na2CO3 + H2S = NaHS + NaHCO3. 2. NaHS reacts with NaVO3 to produce Na2V4O9 along with the precipitation of elemental sulfur: 2NaHS + 4NaVO3 + H2O = Na2V4O9 + 4NaOH + 2S↓. 3. Some of the quinone structures present in the tannins oxidize Na2V4O9 back to NaVO3; at the same time, the quinone structure T(OH)O2 is reduced to the T(OH)3 structure. Na2V4O9 + 2T(OH)O2 + H2O + 2NaOH = 4NaVO3 + 2T(OH)3. 4. The reduced phenolic structures are oxidized by air back to quinone structures: 2T(OH)3 + O2 = 2T(OH)O2 + 2H2O. 5. The NaOH produced reacts with NaHCO3 in the solution to form Na2CO3: NaHCO3 + NaOH = Na2CO3 + H2O. 6. Since the flue gas contains CO2 and O2, the following side reactions also occur during the desulfurization and air regeneration stages: Na2CO3 + CO2 + H2O = 2NaHCO3; 2NaHS + 2O2 = Na2S2O3 + H2O. The Na2S2O3 formed is further oxidized to Na2SO4, and part of the Na2CO3 is lost as a result of these side reactions. ? The carboxyl groups in the gum component can form soluble complexes with tetravalent vanadium ions, thereby preventing the precipitation of tetravalent vanadium (VSO). ? As can be seen from the above reactions, the purity of the converted gas is primarily determined by the concentration of the alkaline solution. An appropriate amount of NaVO3 is added to the solution because NaVO3 reacts very rapidly with NaHS; this **increases the sulfur capacity of the solution and reduces the rate of formation of Na2S2O3. Tannin plays almost no role during the absorption process; it functions merely as an oxygen carrier throughout the absorption and regeneration process. However, the addition of sodium metavanadate can accelerate the entire absorption process. The pH value is important for controlling the reduction process in this process. Regarding the absorption process, a high pH value is favorable for the absorption of H2S, while a low pH value is advantageous for solution regeneration. To ensure efficient desulfurization, the pH value is generally maintained between 8.5 and 9.0, and it can be controlled by adjusting the equivalent amount of sodium carbonate. ? In the presence of oxygen, the reaction between reduced sodium perrhovanate and quinone-form tannin at the same oxidation state occurs rapidly; similarly, the re-oxidation of reduced phenol-form tannin is also fast. However, reduced sodium perrhovanate cannot be regenerated through direct oxidation by air, and must rely on the third reaction mentioned above, in which quinone-form tannin oxidizes it to restore its activity. The carboxyl groups in the tannin components can form soluble complexes with tetravalent vanadium ions, thereby preventing the precipitation of tetravalent vanadium (VSO). Theoretically, during the entire desulfurization and solution regeneration process, only side reactions consume some of the sodium carbonate, with no loss of the other chemicals. Nevertheless, due to mechanical losses and other types of losses during sulfur recovery, it is necessary to regularly add a certain amount of these chemicals to maintain the stability of the desulfurization solution components.

Composition of the tannin desulfurization solution, values in g/L: sodium bicarbonate 20, sodium carbonate 4.5, total alkalinity 21.2, tannin 1.0–1.5, sodium thiosulfate <150, sodium sulfate <150, suspended sulfur