Biogas Resource Utilization And Purification Pretreatment Technology

1, The nature and use of methane

Biogas is organic matter (such as straw, weeds, human and animal feces, garbage and sewage sludge and industrial organic wastewater, etc.) in anaerobic environment and certain conditions, after various kinds and great number, the function of different kinds of anaerobic microbe catabolism, the final form is given priority to with CH4 and CO2 gas mixture, is a biochemical process.The main component of biogas is methane (CH4).Usually accounts for the total volume of 50%~70%;The second is carbon dioxide, which accounts for 25%~40% of the total volume.Other gases, such as hydrogen sulfide, nitrogen, oxygen, hydrogen and carbon monoxide, make up about 5% of the total volume.Methane, carbon monoxide, hydrogen and hydrogen sulfide in biogas are flammable gases, oxygen is combustion-supporting gas, carbon dioxide and nitrogen are inert gases.Unburned biogas is a colorless, smelly, toxic (hydrogen sulfide), lighter than air, easy to diffuse, difficult to tolerate water flammable gas mixture.Biogas becomes a non-toxic, odorless, smokeless gas after full combustion.

As a kind of biomass energy, biogas has been used in China for nearly a hundred years.It is a valuable renewable source of energy that can be used directly as fuel or converted to produce syngas.After purification and purification, biogas is biomethane. As a biogas, it has four characteristics: clean, efficient, safe and renewable. The efficient preparation and comprehensive utilization of biogas is a very representative two-way cleaning process, and as a new type of energy, it will occupy a place in the energy pattern.

As a by-product of anaerobic reaction, biogas has been ignored, random emissions, causing safety risks.With the progress of technology, as well as the national encouragement in the level of energy conservation and emission reduction policies, more and more biogas after purification is connected to the grid, used as vehicle gas (CNG), through boilers, power generation (cogeneration) and so on are used up.

Based on China's national conditions, biogas power generation is difficult to access the Internet, generally only for self-use;The technology for fuel cells or wastewater purification is not mature;As a chemical raw material, due to the general biogas production is small, does not have a certain scale effect.With the national energy shortage and the high oil price, the global gas price, including China's soaring, CNG price has reached 4~5 yuan /m3, therefore, the use of biogas to produce alternative natural gas or vehicle gas, will have better economic and social benefits.

2. Biogas purification technology

From the analysis of the utilization direction of biogas to make CNG, other components except CH4 are harmful components or impurities, which need to be removed separately.The main impurity is CO2. Due to its high content, the more it exists, the lower the calorific value of the gas will be. According to the technical requirements of the first and second class gas in China's natural gas standard GB 17820-1999, it should be removed to less than 3.0%.Sulphur content is too high will corrode equipment and pipelines, reduce their service life, so different standards have different requirements for sulfur content;Because the biogas is produced in an anaerobic environment, the O2 content will not exceed the standard, but it should be noted that in the CNG standard, the O2 content after purification shall not be higher than 0.5%;In order to prevent ice from clogging pipes, CNG and LNG production also requires deep water removal;Other trace N2, H2, etc. can not be removed if it does not affect the calorific value required by natural gas standard.

3. Methane desulfurization

The small amount of sulfur contained in biogas will cause environmental pollution and corrosion of equipment, pipelines, instruments, etc., so whether it is directly used for combustion or biogas power generation, the necessary desulfurization treatment of biogas must be conducted.

Sulfur in biogas mainly exists in the form of hydrogen sulfide, and there are many removal methods. At present, the commonly used industrial desulfurization methods include wet desulfurization and dry desulfurization.

3.1 Wet desulfurization

Wet desulfurization can be divided into physical absorption method, chemical absorption method, oxidation method and biological method.Physical and chemical methods have the problem of retreating hydrogen sulfide and have been gradually phased out.Oxidation method takes alkaline solution as absorbent and adds oxygen carrier as catalyst to absorb H2S and oxidize it to elemental sulfur, and the desulfurization efficiency can reach more than 99.5%.The process flow is as follows: first, the desulfurizer is dissolved in water to prepare the desulfurization liquid, the liquid enters the equipment and mixes with the biogas, and the hydrogen sulfide (H2S) in the biogas reacts with the desulfurization liquid to generate elemental sulfur.

The basic principle of biological absorption method is that sulfur containing pollutants in oxidation state first generate sulfide or H2S through biological reduction, and then generate elemental sulfur through biological oxidation process and then remove it.In most bioreactors, the microbial species are dominated by bacteria, with fungi as the minor species, and yeasts are rare.Commonly used bacteria are thiobacillus ferrooxidans, thiobacillus denitrifying and thiobacillus.The most successful example is Thiobacillus ferrooxidans, which grows at an optimal pH of 2.0 to 2.2.

Wet desulfurization of biogas usually refers to wet oxidation method, which can be divided into tannin extract method, PDS method, COS method and 888 method according to the different catalysts selected. It is suitable for the condition of large gas volume and high sulfur content, and maintains desulfurization efficiency with pH value, and the operation cost is relatively low.The desulfurization accuracy is low, generally suitable for crude desulfurization occasions, the device is relatively complex, there are many equipment need regular maintenance.In large-scale desulfurization projects, the combination of dry and wet desulfurization process is generally used to purify and pretreatment biogas.

3.2 Dry desulfurization

The basic principle of dry desulfurization equipment is a method of oxidizing corrosive gas hydrogen sulfide into elemental sulfur or sulfide, also known as dry oxidation method.The dry desulfurization equipment is composed of: a desulfurizer is placed in the desulfurizer, gas passes through the desulfurizer bed at a low flow rate from one end, hydrogen sulfide (H2S) is oxidized into sulfur or sulfide and remains in the bed, and the purified gas is discharged from the other end of the container.

Dry desulphurization has the advantages of simple structure, convenient operation, small number of equipment (usually 2 desulphurizers, one for use and one for standby, running alternatively), small footprint, high desulphurization accuracy and low labor intensity.No personnel need to be on duty during the work.It increases linearly with the increase of sulfur content in the raw gas. When the upper limit of desulfurizer is reached, the desulfurizer only needs to be regenerated and can be recycled for use. The main characteristics are high desulfurization efficiency, simple and convenient operation, and generally used in precision desulfurization occasions.

4．the conclusion

Biogas is a kind of recyclable green energy, but its content in addition to the main methane also contain other gases, especially methane ingredients contain strong causticity and highly poisonous hydrogen sulfide gas, in order to be able to make biogas to health, safety, stability, no secondary pollutants produced by green, using effect,Before use, it must be desulfurized and purified. Our company (Jingcheng Weiye Environmental Protection) has professional technical support and a number of successful projects in biogas purification and biogas recovery and utilization.Many achievements of technology is mainly used in livestock breeding, slaughtering industry and agricultural organic waste industry organic waste treatment industry, food, eat hutch waste treatment, alcohol industry, papermaking industry organic waste such as sewage treatment industry, equipment in the running process achieved good economic benefits, ecological benefits and social benefits.

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