Observation: Biogas Purification and Upgrading – Applications of Extracted Biomethane as Natural Gas and Vehicle Fuel

I. Biogas Purification and Upgrading Technologies

Producing biomethane from biogas is an effective way to achieve high-value utilization. It mainly involves two steps: purification and upgrading. Purification refers to the removal of trace harmful components from biogas. For example, sulfur in biogas primarily exists as H₂S, with a content of 500-5000 mg/L, which must be removed. Upgrading primarily involves the removal of CO₂ from biogas to reduce its content and increase the purity of CH₄. The purification and upgrading process aims to retain the combustible and combustion-supporting components, including CH₄, H₂, O₂, and CO, while removing CO₂, H₂S, H₂O, and other impurities.

Desulfurization is necessary to prevent H₂S from corroding compressors, gas storage tanks, and engines, and to avoid H₂S poisoning. Its combustion produces SO₂ and SO₃, which are even more harmful, and SO₂ lowers the dew point.

CO₂ Removal is carried out because CO₂ reduces the calorific value, energy density, and combustion speed of biogas, and increases its ignition temperature. If biogas needs to meet biomethane standards after purification and upgrading, CO₂ must be removed.

Dehydration is essential because H₂O reacts with H₂S, CO₂, and NH₃, causing corrosion in compressors, gas storage tanks, and engines. Furthermore, when biogas is stored under pressure, water must be removed to prevent condensation or freezing under high pressure.

1 Overview of Biogas Desulfurization Technologies

H₂S is formed by sulfur compounds during microbial reduction processes, and its content varies depending on the fermentation feedstock and process. Due to the numerous problems caused by H₂S, it should be removed as early as possible in the biogas purification process. The most commonly used methods include wet desulfurization, dry desulfurization, and biological desulfurization.

Wet desulfurization is categorized into physical absorption, chemical absorption, and oxidation methods based on the absorption and regeneration properties of the solution.

Dry desulfurization is categorized into chemical adsorption, chemical absorption, and catalytic hydrogenation methods based on principles and techniques.

Biological desulfurization is a new process developed in the 1980s to replace traditional desulfurization methods. Under suitable conditions of temperature, humidity, pH, nutrients, and micro-oxygen, it utilizes desulfurizing bacteria (such as photosynthetic sulfur bacteria, Thiobacillus, and colorless sulfur bacteria) to convert H₂S into elemental sulfur or sulfuric acid.

2 Overview of Biogas Decarbonization (CO₂ Removal) Technologies

Currently, various methods are available for removing carbon dioxide from biogas, including Pressure Swing Adsorption (PSA), pressure water scrubbing, physical absorption, chemical absorption, amine scrubbing, membrane separation, and cryogenic separation. Among these, PSA, pressure water scrubbing, chemical absorption, and membrane separation are the four most commonly applied methods in biogas purification and upgrading.

3 Overview of Biogas Dehydration Technologies

Dehydration is crucial because accumulated water in gas pipelines can dissolve hydrogen sulfide, leading to pipeline corrosion. Additionally, when biogas is stored under pressure, water must be removed to prevent damage to the gas holder due to condensate freezing. Common dehydration methods include cold separation, solid physical adsorption, and solvent absorption.

II. Applications in Natural Gas and Vehicle Fuel

Biogas can be upgraded to produce biomethane. Biomethane, also known as bio-natural gas or renewable natural gas (RNG), is biogas that has been purified and upgraded to meet natural gas standards (GB 17820-1999). It can be injected into the natural gas grid to substitute fossil natural gas or used as vehicle fuel. Upgrading biogas to achieve a methane content of 95%-97% for use as vehicle fuel or for injection into the natural gas pipeline network represents a key direction for the high-value utilization of biogas, unlocking greater economic value.

III. Development of the Biogas Purification and Upgrading Industry Domestically and Internationally

1 Development of the Biogas Purification and Upgrading Industry Abroad

Sweden is the world's most advanced country in using purified and upgraded biogas for vehicle fuel. As early as 1996, Sweden was already purifying biogas to a methane content exceeding 95% for use as automotive fuel and had established relevant standards.

In 2010, Sweden's total annual biogas production was equivalent to 130 million cubic meters of natural gas, accounting for 13% of total gas consumption. In 2010, Sweden had 70,000 vehicles running on compressed natural gas and 500 filling stations. Sweden plans that by 2020, 50% of its natural gas will be replaced by biogas, and by 2060, natural gas will be completely replaced by biogas.

Furthermore, the Swedish government offers a series of preferential policies for biogas used as vehicle fuel, including exemptions from energy tax, H₂S emission tax, and CO₂ emission tax, as well as reduced vehicle use tax for gas-fueled vehicles. Additionally, government purchase subsidies are available for buying biogas vehicles, along with some regional incentives such as exemption from congestion charges and parking fees.

Germany holds a world-leading position in biogas engineering technology. In recent years, Germany's utilization of biogas has shifted from direct use in combined heat and power (CHP) generation units towards the production of pipeline-quality natural gas and vehicle fuel.

Starting in 2006, the first two biomethane plants in Germany began operating and injecting gas into the national natural gas grid. By 2011, approximately 83 biogas plants were upgrading biogas to biomethane during operation, with a total annual biomethane upgrading capacity of about 460 million cubic meters.

Moreover, Germany has set targets for biomethane production to reach 6 billion cubic meters by 2020 and 10 billion cubic meters by 2030, planning for biomethane to replace 6% of the country's total natural gas consumption by 2020.

In addition, countries such as Switzerland, Denmark, and Canada are also progressing rapidly in the development of biogas purification and upgrading technologies internationally, possessing advanced purification and upgrading equipment.

2 Development of the Biogas Purification and Upgrading Industry in China

Bio-natural gas is a significant potential resource to substitute fossil natural gas, as it can directly utilize existing natural gas pipelines or storage tanks for transport and storage. During the "12th Five-Year Plan" period, China accelerated the construction of natural gas supporting infrastructure and initiated "distributed energy utilization projects." These developments provide a foundation and facilitate conditions for the industrial application of bio-natural gas.

In recent years, through the introduction of efficient biogas engineering technologies and key equipment for combined heat, power, and fertilizer production from developed European countries like Germany and Denmark, China has built a number of biogas CHP projects and large-scale grid-connected power generation biogas plants. However, as a new industry, China still has a long way to go in producing biomethane through biogas purification and upgrading.

Conclusion

After years of development, a series of mature technologies for biogas purification and upgrading have been established. In terms of energy applications, injection into the natural gas grid or use as vehicle fuel represents a crucial direction for the high-value utilization of biogas. European countries such as Sweden and Germany hold a leading position in the technology of producing biomethane via biogas purification and upgrading. Consequently, China has introduced relevant advanced technologies in this field to ensure the efficient and stable development of its biogas purification and upgrading industry.