Introduction to Biogas Equipment:

As an emerging energy source, biogas is increasingly being utilized in a wide range of applications. International environmental protection standards strictly stipulate that when utilizing biogas energy, the H2S content in biogas must not exceed 20mg/m3. Whether in industrial or civilian gas applications, it is crucial to remove H2S as much as possible.

When biogas is produced from anaerobic digestion units, especially during mesophilic or thermophilic fermentation, it carries a significant amount of H2S. Due to the large amount of water vapor present in biogas, the combined action of water and H2S accelerates the corrosion and clogging of metal pipelines, valves, and flowmeters. Additionally, after combustion, H2S is converted into SO2, which combines with water vapor in the combustion products to form sulfurous acid, causing corrosion on the metal surfaces of equipment and contributing to atmospheric pollution that affects human health. Therefore, it is imperative to remove H2S from biogas before use.

Commonly used methods for biogas desulfurization include dry desulfurization, wet desulfurization, and biological desulfurization.

Main Methods: Dry Desulfurization and Wet Desulfurization.

Desulfurization Principles:

Dry Desulfurization:
Dry desulfurization is a simple and relatively low-cost desulfurization method, generally suitable for biogas with small volumes and low concentrations of H2S. The basic principle of dry desulfurization equipment for removing hydrogen sulfide (H2S) from biogas is a method that uses O2 to oxidize H2S into sulfur or sulfur oxides, also known as the dry oxidation method. The composition of dry desulfurization equipment includes a container filled with packing material such as activated carbon and iron oxide. The gas passes through the packing layer inside the container at a low velocity, and H2S is oxidized into sulfur or sulfur oxides, which remain in the packing layer, while the purified gas is discharged from the other end of the container.

Dry desulfurization mainly includes components such as the main steel structure, desulfurization agent packing, observation window, pressure gauge, and thermometer. The desulfurization tower is usually designed with one in use and one on standby, alternating between them for desulfurization and regeneration. Biogas containing hydrogen sulfide (H2S) enters the bottom of the desulfurization tower and reacts with the desulfurization agent during its passage through the packing layer to the top. The following chemical reactions occur between H2S and the desulfurization agent:

Step 1: Fe2O3 • H2O + 3 H2S = Fe2S3 + 4 H2O (Desulfurization)
Step 2: Fe2S3 + 3/2 O2 + 3 H2O = Fe2O3 • H2O + 2 H2O + 3 S (Regeneration)

Biogas containing H2S first reacts with the desulfurization agent with a relatively high load at the bottom entrance. The upper part of the reactor contains a layer of desulfurization agent with a low load. Through well-designed biogas space velocity and linear velocity, dry desulfurization can achieve good fine desulfurization results.

Before entering the dry desulfurization tower, the biogas should be equipped with a condensed water tank or a biogas particle filter. This device can eliminate particulate impurities mixed in the biogas and ensure that the biogas contains a certain humidity before entering the desulfurization process.

When the desulfurization agent changes color or the system pressure loss becomes excessive, the other desulfurization tower should be used alternately. After venting the biogas from the current desulfurization tower, natural ventilation is conducted to regenerate the desulfurization agent. When the regeneration effect is not good, the waste desulfurization agent should be removed from the bottom of the tower, and at the same time, the same volume of fresh desulfurization packing should be added into the reactor.

Wet Desulfurization:
Wet desulfurization can be classified into physical absorption, chemical absorption, and oxidation methods. Physical and chemical methods involve the reprocessing of hydrogen sulfide. The oxidation method uses an alkaline solution as an absorbent and adds an oxygen carrier as a catalyst to absorb H2S and oxidize it into elemental sulfur. The wet oxidation method dissolves the desulfurization agent in water, and the liquid enters the equipment and mixes with biogas. The hydrogen sulfide (H2S) in the biogas reacts oxidatively with the liquid to generate elemental sulfur. Liquids that absorb hydrogen sulfide include sodium hydroxide, calcium hydroxide, sodium carbonate, and ferrous sulfate. Mature oxidation desulfurization methods can achieve desulfurization efficiency of over 99.5%.

In large-scale desulfurization projects, wet desulfurization is generally used for coarse desulfurization first, followed by dry desulfurization for fine desulfurization.

The main components of the wet desulfurization tower include a scrubbing tower, hydrogen sulfide sampling and monitoring system, alkaline solution preparation tank, water softening device, liquid level control system, support members, and connecting parts. The desulfurization system operates automatically through monitoring of the outlet hydrogen sulfide concentration and pH value.

During operation, biogas passes through the desulfurization tower from bottom to top, and Na2CO3 solution (or NaOH solution) is sprayed from the top down, allowing H2S gas to fully react chemically with the alkaline solution.

The alkaline solution is stored below the desulfurization tower and automatically added through a metering pump. The metering pump's addition control operates automatically through monitoring of the outlet H2S concentration.

When sodium carbonate (Na2CO3) reagent is used for desulfurization, the following reactions mainly occur:
H2S + Na2CO3 = NaHS + NaHCO3 (1)
CO2 + Na2CO3 + H2O = 2 NaHCO3 (2)

Due to the large amount of CO2 present in biogas, it also consumes alkaline solution. The system should control reaction conditions (including reaction temperature and pH value) and set optimal reaction conditions to minimize alkaline solution consumption.

Biological Desulfurization:
Biological desulfurization technologies include biofiltration, biosorption, and biotrickling filtration. All three systems are open systems, and their microbial populations change with the environment. During biological desulfurization, oxidized sulfur pollutants are first reduced biologically to produce sulfides or H2S, which are then oxidized biologically to elemental sulfur for removal. In most bioreactors, bacterial species dominate, followed by fungi, with yeast being rare. Commonly used bacteria are Thiobacillus ferrooxidans, Thiobacillus denitrificans, and Thiobacillus thioparus. A successful representative is Thiobacillus ferrooxidans, which grows at a pH of 2.0 to 2.2.