Dry desulfurization: Dry desulfurization is a simple and relatively low-cost desulfurization method, generally suitable for biogas desulfurization with small biogas volume and low hydrogen sulfide concentration. The basic principle of the equipment for dry removal of hydrogen sulfide (H2S) from biogas gas is a method of using O2 to oxidize H2S into sulfur or sulfur oxides, also known as dry oxidation method. The composition of dry process equipment is to place fillers in a container, and the filler layer includes activated carbon, iron oxide, etc. The gas passes through the packing layer inside the container at a low flow rate, and hydrogen sulfide (H2S) is oxidized to sulfur or sulfur oxides, remaining in the packing layer. After purification, the gas is discharged from the other end of the container.
Dry desulfurization mainly includes components such as main steel structure, desulfurizer filler, observation window, pressure gauge, temperature gauge, etc. Desulfurization towers are usually designed as one for use and one for backup, alternating between one for desulfurization and one for regeneration. Biogas containing hydrogen sulfide (H2S) enters the bottom of the desulfurization tower and undergoes the following chemical reactions with the desulfurizer during the process of passing through the desulfurization packing layer and reaching the top: ******Step: Fe2O3 • H2O+3 H2S=Fe2S3+4 H2O (desulfurization) ******Step: Fe2S3+3/2 O2+3 H2O=Fe2O3 • H2O+2 H2O+3 S (regeneration)
Biogas containing hydrogen sulfide first reacts with a desulfurizer with a relatively high load at the bottom inlet. The upper part of the reactor is a desulfurizer layer with a low load. By designing good biogas airspeed and line speed, dry desulfurization can achieve good precision desulfurization effect.
Before biogas enters the dry desulfurization tower, a condensate tank or biogas particle filter should be installed. This device can eliminate particle impurities in biogas and make biogas contain a certain humidity before entering desulfurization.
When discoloration of the desulfurizer or excessive pressure loss in the system is observed, another desulfurization tower should be used alternately. The current desulfurization tower undergoes natural ventilation after releasing biogas to regenerate the desulfurizer. When the regeneration effect is poor, the waste desulfurizer should be discharged from the bottom of the tower. At the same time as the waste packing is discharged at the bottom, fresh desulfurized packing of the same volume should be added to the reactor.
Wet desulfurization: Wet desulfurization can be classified into three types: physical absorption method, chemical absorption method, and oxidation method. There is a problem of hydrogen sulfide reprocessing in physical and chemical methods. The oxidation method uses an alkaline solution as an absorbent and an oxygen carrier as a catalyst to absorb H2S and oxidize it into elemental sulfur. The wet oxidation method dissolves the desulfurizer in water, and the liquid enters the equipment and mixes with biogas. Hydrogen sulfide (H2S) in biogas reacts with the liquid to form elemental sulfur. The liquids that absorb hydrogen sulfide include sodium hydroxide, calcium hydroxide, sodium carbonate, ferrous sulfate, etc. The mature oxidation desulfurization method can achieve a desulfurization efficiency of over 99.5%.
In large-scale desulfurization projects, wet desulfurization is generally used first for coarse desulfurization, and then dry desulfurization is used for fine desulfurization.
The main body of the wet desulfurization tower includes a washing tower, a hydrogen sulfide sampling and monitoring system, an alkaline solution preparation tank, a water supply softening device, a liquid level control system, and supporting components
When using sodium carbonate (Na2CO3) reagent 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 contained in biogas, it also consumes alkaline solution. The system should be able to control reaction conditions (including reaction temperature, pH value), set optimal reaction conditions, and minimize the consumption of alkaline solution as much as possible.
Biological desulfurization Biological desulfurization technology includes biological filtration, biological adsorption, and biological drip filtration, all of which belong to open systems, and their microbial populations change with environmental changes. In the process of biological desulfurization, oxidized sulfur-containing pollutants first undergo biological reduction to form sulfides or H2S, and then undergo biological oxidation to generate elemental sulfur before being removed. In most bioreactors, the microbial species are mainly bacteria, followed by fungi, and there are very few yeast. The commonly used bacteria are Thiobacillus ferrooxidans, Thiobacillus denitrifying, and Thiobacillus excreting. The representative of success is Thiobacillus ferrooxidans, which grows at a pH value of 2.0-2.2.
