Review

Wet desulfurization can be broadly categorized into three methods: physical absorption, chemical absorption, and oxidation. Both physical and chemical methods face the challenge of hydrogen sulfide (H2S) reprocessing. The oxidation method employs an alkaline solution as the absorbent, to which an oxygen carrier is added as a catalyst, to absorb H2S and oxidize it into elemental sulfur. In wet oxidation, the desulfurizing agent is dissolved in water, and the liquid enters the equipment where it mixes with biogas. The H2S in the biogas reacts oxidatively with the liquid, generating elemental sulfur. The liquids used to absorb H2S include sodium hydroxide, calcium hydroxide, sodium carbonate, ferrous sulfate, etc. Mature oxidation desulfurization methods can achieve a desulfurization efficiency of over 99.5%. Generally, wet desulfurization is used for rough desulfurization, followed by dry desulfurization for fine desulfurization.

1. Reaction principle of wet desulfurization

The main components of the wet desulfurization tower include the scrubber, hydrogen sulfide sampling and monitoring system, alkali solution preparation tank, water supply and softening device, liquid level control system, supports, and connectors. The desulfurization system achieves fully automated operation through monitoring the concentration of hydrogen sulfide in the exhaust gas and the pH value.

During operation, biogas passes through the desulfurization tower from bottom to top, while Na2CO3 solution (or NaOH solution) is sprayed down from the top, enabling sufficient chemical reaction between H2S gas and the alkali solution.

Due to the high content of CO2 in biogas, it will also consume alkali solution. The system should be able to control the reaction conditions (including reaction temperature, pH value), and set appropriate reaction conditions to minimize the consumption of alkali solution.

Wet catalytic desulfurization generally employs phthalocyanine cobalt sulfonate organic polymer compounds as catalysts, and the reaction principle is as follows:

Cobalt phthalocyanine sulfonate is an organic polymer compound, which is a coordination compound centered around various metal ions. The fundamental difference between it and other liquid-phase catalytic oxidation desulfurization methods is that both desulfurization and oxidative regeneration processes exhibit catalytic effects. It changes the control step of the general liquid-phase catalytic oxidation regeneration process to the desulfurization process as the control step of the entire process.

 Chemical absorption and catalytic conversion of inorganic sulfur

H2S + Na2CO3 = NaHS + NaHCO3     (1)

NaHS + NaHCO3 + (X – 1)S = Na2SX + CO2 + H2O   (2)

NaHS + H2O = NaOH + H2S    (3)

Na2SX + H2O = NaOH + H2SX   (4)

Reaction (2) is the reaction of the cobalt phthalocyanine sulfonate method, and the so-called catalytic chemical absorption refers to this reaction.

 The removal reaction of organic sulfur

RSH + Na2CO3 = RSNa + NaSCO3   (5)

COS + 2 Na2CO3 + H2O = Na2CO2S + 2NaHCO3  (6)

CS2 + Na2CO3 + H2O = 2NaHCO3  (7)

 Catalytic oxidation (regeneration) of sulfides

Na2S + 1/2O2 + H2O = NaOH + S↓  (8)

NaHS + 1/2O2 = NaOH + S↓   (9)

4RSNa + O2 + H2O = 2RSSR + 4NaOH   (10)

Na2CO2S + 1/2O2 = Na2CO3 + S↓   (11)

2NaHS + 2O2 = Na2S2O3 + H2O  (12)

2. Wet desulfurization process flow