High-yield, low-cost biogas decarbonization and purification technology – Sichuan Honghu Technology Group

Thread Content

High-Yield, Low-Cost Biogas Decarbonization and Purification Technology Wang Bin, Huang Jiahu (Sichuan Honghu Technology Group Co., Ltd., Chengdu 610036, Sichuan) Abstract: This paper introduces the current status of biogas purification technologies. Through analysis and comparison, it is shown that the use of pressure swing adsorption decarbonization technology with a high recovery rate of 99% can significantly reduce the costs associated with biogas purification, increase output and profit margins, and reduce greenhouse gas emissions; thus, it represents a reasonable choice for the advanced and comprehensive utilization of biogas. Keywords: biogas, purification, decarbonization, pressure swing adsorption, high recovery rate, low cost 1. Introduction Energy is an essential material foundation for economic and social development. Apart from hydropower, biomass energy is the most widely used renewable energy source, with biogas being a key area for development within it. It is estimated that by 2020, China’s annual utilization of biogas will reach 44 billion cubic meters. Due to factors such as economics and technology, small-scale biogas production and low-end applications account for a large proportion in the domestic market at present. However, the transformation and upgrading of biogas production, namely towards larger scales and higher-end applications, is an inevitable trend for the development of this industry. The composition of biogas is complex; varying raw materials and fermentation processes result in different gas compositions. The main component, CH4, accounts for approximately 50% to 70% of the total volume, while CO2 makes up about 25% to 40%. Other gases such as N2, O2, H2S, H2O, CO, H2, NH3, halogenated hydrocarbons, and siloxanes account for around 5%【1】. The impurity gases present can have more or less adverse effects during the utilization of biogas; therefore, it is difficult to make use of biogas in advanced and comprehensive ways without first purifying it. Since CO2 accounts for the majority of impurity gases, the cost of decarburization largely determines the cost of biogas purification; therefore, biogas decarburization technologies will be discussed in detail below. 2. Introduction to technologies for removing impurities from low-concentration biogas 2.1 The saturated water vapor contained in dehydrated biogas can be removed using condensation, adsorption, or water-absorbing substances【2】. 2.2 Sulfides in desulfurized biogas pose significant hazards; H2S is toxic and, when combined with H2O, it can corrode equipment, while the SOX produced upon combustion contaminates the atmosphere. Biogas desulfurization technologies are divided into two main categories: in-situ desulfurization and biogas desulfurization. Wet rough desulfurization is required when the biogas volume is large or its sulfur content is high. This method has a complex process and high operating costs, and generally cannot remove organic sulfur【2】. The dry method is suitable for situations where the volume of biogas is low or the sulfur content is low. It is simple to operate, requires minimal equipment investment, and can remove various types of organic sulfur to ppm levels. In addition, there are also new desulfurization methods such as membrane separation, pressure swing adsorption, and biological desulfurization. 2.3 Removal of other impurities: The content of other impurities is low, and they can usually be removed in existing purification or refining units【2】. If certain industries have specific requirements, such as when used in gas turbines, the siloxanes present can be absorbed by hydrocarbon mixtures, while halogenated hydrocarbons can be removed through temperature-controlled adsorption using specialized activated carbon【3】. 3. Introduction to CO2 removal technologies: Biogas decarbonization technologies originate mainly from the chemical industry, including physical absorption, chemical absorption, pressure swing adsorption, membrane separation, etc. Since the volume of biogas to be processed is small, more emphasis should be placed on miniaturization and energy efficiency. Physical absorption utilizes the property that acidic gases have a higher solubility in solutions than CH4 to remove CO2. The solution is regenerated through methods such as vacuum reduction, flashing, and stripping. The high-pressure water washing method is the most commonly used【2,3】. Chemical absorption uses the principle of reversible reactions between chemical substances and acidic gases in a solution to remove CO2. The solution is absorbed at low temperatures and regenerated at high temperatures; the alcohol amine process is a typical example of this approach [1,2]. The biogas production process does not generate steam as a by-product; steam is produced specifically for solution regeneration, which is cumbersome and consumes biogas, reducing output. Membrane separation takes advantage of the different dissolution and diffusion rates of various gas components in polymer materials; under the effect of the partial pressure difference across the membrane, these components permeate through the membrane wall at different rates, thereby enabling their separation【2】. Membrane separation methods include high-pressure gas phase separation and low-pressure gas-liquid phase absorption membrane separation. Pressure swing adsorption utilizes the principle of different adsorption capacities of the adsorbent surface for various components of biogas to remove CO2. This method features a high degree of automation, stable operation, a long device lifespan, and low purification costs【1,2,4】. However, traditional pressure swing adsorption units suffer from significant CH4 losses. When the CH4 concentration is between 90% and 95%, the recovery rate can reach at most 90%【8,10,11】, while it is as low as below 60% in other cases【9】. This drawback limits the application of pressure swing adsorption in biogas purification. The key to improving the CH4 recovery rate lies in efficient adsorbents, with the aim of increasing the separation coefficient of the CH4/CO2 mixture and enhancing the CO2 adsorption capacity. By drawing on the latest domestic and international technologies [6,7], Honghu Technology has developed a specialized adsorbent for biogas purification that enables a CH4 recovery rate of 99%, with a CH4 content in the exhaust gas below 1%, and a product gas purity of 97%–99% (adjustable). This adsorbent has a high adsorption capacity and high separation coefficient, effectively reducing the size of the equipment, lowering the total investment cost, and decreasing operating expenses. 4. Comparison of biogas decarbonization and purification technologies 4.1 Raw gas specifications Biogas with the specifications listed in Table 1 was used as the raw gas for comparing various technologies. Table 1: Parameters of raw gas
Parameter | Flow rate (Nm3/d) | Pressure (kPaG) | Temperature (℃) | CH4 (V%) | CO2 (V%) | Other components (V%) | Value
--------|-------------------|-----------------|------------------|----------|----------|------------------------|-------
| 20000 | 5 | 40 | 57 | 41 | 2 | 4.2

Investment cost: The investment required for Honghu Technology’s biogas decarbonization and purification system is lower than that of the traditional pressure swing adsorption method; it is also lower than that of MDEA and membrane separation methods, though slightly higher than that of the high-pressure water washing method [1,2,3,4,5,8,9]. 4.3 Methane recovery rate: The methane recovery rate of Honghu Technology’s biogas decarbonization and purification device reaches 99%, which is higher than that of other purification devices [1,2,3,4,5,8,9]. 4.4 Operating costs file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtmlclip1/01/clip_image004.png Note: The output pressure of the product gas is uniformly set at 0.8 MPa. The operating cost of Honghu Technology’s biogas decarbonization and purification device is significantly lower than that of other technologies【1】. 5. Conclusion The pressure swing adsorption technology for biogas decarbonization and purification developed by Sichuan Honghu Technology Group Co., Ltd. boasts significant advantages, including a high degree of automation, simple operation, safety, and environmental friendliness, as well as lower investment costs and operational expenses; moreover, its CH4 recovery rate of 99% is higher than both international and domestic standards. References: Hao Wei. Process technology and economic viability of converting biogas into natural gas (CNG). Jiang Hao et al. Techniques and applications for purifying biogas to produce biomethane. China Biogas, 2012, 30(2): 6–11. Song Canhui et al. Current status of biogas purification technologies. China Biogas, 2007, 25(4): 23–27. Zhen Feng et al. High-value utilization of biogas and purification technologies. Environmental Science and Technology, 2012, 11: 103–108. Li Dong et al. Current status and application prospects of biogas resources in China. Modern Chemical Industry, 2009, 4: 1–5. Yu Xinjiang et al. Preparation of porous carbon using a template method and its application in the adsorption separation of CH4/N2/CO2. Petrochemical Engineering, 2016, 2: 221–226. Li Tong et al. Modification of activated carbon and its impact on CO2/CH4 adsorption performance. Acta Petrolei Sinica, 2011, 12: 2012–2017. Chen Xiang et al. Current development of techniques for purifying biogas to produce biomethane. Agricultural Engineering, 2012, 7: 30–34. Li Sheng et al. Advances in technologies for producing high-purity biomass methane from purified biogas. Modern Chemical Industry, 2014, 11: 19–23. Sichuan Yalian Technology Co., Ltd. A method for extracting methane from biogas. Chinese Patent: CN200910060286, September 1, 2010. Sichuan Yalian High-Tech Co., Ltd. A method for recovering methane from landfill gas. Chinese Patent: CN200810045223, July 16, 2008.