沼气提纯技术

　　Biogas purification technology

　　根据分离原理不同，目前常见的沼气提纯技术有：吸附法、高压水洗法（物理吸收法）、化学吸收法（胺洗法）、膜分离法等。

　　According to different separation principles, common biogas purification technologies currently include adsorption method, high-pressure water washing method (physical absorption method), chemical absorption method (amine washing method), membrane separation method, etc.

　　1.1吸附法

　　1.1 Adsorption method

　　吸附法是一种物理提纯方法，包括变压吸附法（PSA）和变温吸附法（TSA），其中用于沼气提纯的主要是PSA法。

　　Adsorption method is a physical purification method, including pressure swing adsorption (PSA) and temperature swing adsorption (TSA), among which PSA method is mainly used for biogas purification.

　　变压吸附法（PSA）主要是利用吸附剂（如活性炭、硅胶、氧化铝和沸石等）根据不同气体的吸附量不同或气体分子大小不同的特点，来脱除沼气中的杂质气体，实现气体的分离（如图1）。在变压吸附过程中，沼气经过过滤、脱水、脱氨、脱硫和压缩等预处理后，沼气在变压吸附装置（吸附塔）内分别处于吸附、减压、脱附、加压的一个循环状态，通过加压将沼气中的CO2和N2等吸附在吸附塔内，而CH4为轻组分由变压吸附塔顶部排出，并作为产品进行收集，直吸附塔达到饱和状态，进行减压将吸附柱减压甚抽成真空，这时被吸附的CO2和N2等杂质气体就会被脱附出来，由塔底排出。如此循环往复去除沼气中的杂质气体来提纯沼气。

　　The pressure swing adsorption (PSA) method mainly uses adsorbents (such as activated carbon, silica gel, alumina, zeolite, etc.) to remove impurity gases from biogas according to the characteristics of different gas adsorption capacities or gas molecule sizes, and achieve gas separation (as shown in Figure 1). In the process of pressure swing adsorption, after pre-treatment such as filtration, dehydration, deamination, desulfurization, and compression, the biogas is in a cycle of adsorption, depressurization, desorption, and pressurization in the pressure swing adsorption device (adsorption tower). CO2 and N2 in the biogas are adsorbed in the adsorption tower by pressurization, while CH4, as a light component, is discharged from the top of the pressure swing adsorption tower and collected as a product until the adsorption tower reaches saturation. The adsorption column is depressurized or even evacuated by depressurization. At this time, the adsorbed impurity gases such as CO2 and N2 will be desorbed and discharged from the bottom of the tower. Repeat this process to remove impurities from biogas and purify it.

　　变压吸附法（PSA）占地面积小，一般为集成装置，运输、安装方便，甲烷回收率为97%左右，但需要一定的压力，对原料中H2S的含量有一定的要求（一般＜50ppm），且对控制元件及阀门的精度要求较高。相比于水洗法和化学吸收法，PSA法更为灵活，适用于中、小规模的沼气提纯工程项目。

　　The pressure swing adsorption (PSA) method has a small footprint, is generally an integrated device, is easy to transport and install, and has a methane recovery rate of about 97%. However, it requires a certain pressure and has certain requirements for the H2S content in the raw materials (generally<50ppm), and has high precision requirements for control components and valves. Compared to water washing and chemical absorption methods, PSA method is more flexible and suitable for medium and small-scale biogas purification projects.

　　1.2高压水洗法

　　1.2 High pressure water washing method

　　高压水洗法主要利用CO2和CH4在水中溶解度的差异，通过物理吸收将CO2和CH4进行分离。在外部条件相同的情况下，CO2在水中的溶解度是CH4的30倍左右，当沼气通过水体后，CO2被水吸收从而达到提纯沼气的目的。同时，经过试验证明，CH4与CO2在水中的溶解度随着压力的加大，沼气提纯的效果更加显著。

　　The high-pressure water washing method mainly utilizes the difference in solubility between CO2 and CH4 in water to separate them through physical absorption. Under the same external conditions, the solubility of CO2 in water is about 30 times that of CH4. When biogas passes through the water, CO2 is absorbed by the water to achieve the goal of purifying biogas. Meanwhile, it has been experimentally proven that the solubility of CH4 and CO2 in water increases with increasing pressure, resulting in a more significant purification effect of biogas.

　　图2为高压水洗法工艺流程，通常沼气通过压缩后从吸收塔底部进入，水从顶部进入实现错流吸收。为提高CO2在水中的溶解度，水洗工艺一般采用较高的压力，CO2在水中的溶解度随着压力的升高逐渐增大，甲烷损失少。但是，在CO2的吸收过程中需要大量工业用水，同时产生的废液需通过CO2再生装置进行回收处理，净化后的CH4也需通过脱水装置进行干燥处理。

　　Figure 2 shows the process flow of high-pressure water washing method. Usually, biogas enters from the bottom of the absorption tower after compression, and water enters from the top to achieve cross flow absorption. To improve the solubility of CO2 in water, the water washing process generally uses higher pressure, and the solubility of CO2 in water gradually increases with the increase of pressure, resulting in less methane loss. However, a large amount of industrial water is required in the absorption process of CO2, and the waste liquid generated at the same time needs to be recovered and treated through a CO2 regeneration device. The purified CH4 also needs to be dried through a dehydration device.

　　高压水洗法主要适用于处理规模较大的气体，提纯纯度高（＞97%CH4），甲烷损失小（0.05%~6%）；该方法所使用的溶剂是可再生的，并且可以耐受一定的杂质；主要通过改变装置的压力和温度来调整

　　The high-pressure water washing method is mainly suitable for processing large-scale gases, with high purification purity (>97% CH4) and low methane loss (0.05%~6%); The solvent used in this method is renewable and can tolerate certain impurities; Mainly adjusted by changing the pressure and temperature of the device

　　胺洗法适用于规模较大的沼气提纯工程（沼气处理量达到每小时万立方米），规模越大，经济性越好，提纯纯度高（＞99%CH4），甲烷损失小（＜0.1%）。在提纯过程中具有设备成本低、操作简便、净化效果好以及操作压力低等优点，其操作压力一般为0.1MPa，操作压力低于水洗法沼气提纯工艺，该方法所使用的溶剂是可再生的。但同时，使用该方法能源消耗高，且由于有机胺存在一定的毒性，回收后的CO2不适合再利用，不适用于含O2的沼气提纯。

　　The amine washing method is suitable for large-scale biogas purification projects (with a biogas processing capacity of up to 10000 cubic meters per hour). The larger the scale, the better the economy, the higher the purification purity (>99% CH4), and the smaller the methane loss (<0.1%). In the purification process, it has the advantages of low equipment cost, easy operation, good purification effect, and low operating pressure. The operating pressure is generally 0.1 MPa, which is lower than the water washing method for biogas purification. The solvent used in this method is renewable. However, at the same time, using this method consumes high energy, and due to the toxicity of organic amines, the recovered CO2 is not suitable for reuse and purification of biogas containing O2.

　　1.4膜分离法

　　1.4 Membrane Separation Method

　　膜分离法的基本原理是依靠气体在膜表面的吸附能力不同、溶解度不同和扩散速率差异，来选择“过滤”气体中的各组分，即利用薄膜材料对各种气体的渗透率不同来达到分离的目的，常用的分离膜材料有：高分子材料、无机材料和金属材料，但是在沼气的CO2和CH4的分离中常用的是中空纤维膜。在膜两侧分压差的作用下，大部分的CO2以及少量的CH4作为快气通过膜壁渗透分离排出，剩下大部分的CH4以透余气形式获得提纯，为提高CH4的浓度，通常采用多级膜分离工艺。膜分离法工艺流程如图4所示。

　　The basic principle of membrane separation method relies on the different adsorption capacity, solubility, and diffusion rate of gases on the membrane surface to select the various components in the "filtered" gas, that is, to achieve separation by using the different permeability of membrane materials to various gases. Commonly used separation membrane materials include polymer materials, inorganic materials, and metal materials, but hollow fiber membranes are commonly used in the separation of CO2 and CH4 in biogas. Under the action of the pressure difference on both sides of the membrane, most of the CO2 and a small amount of CH4 are separated and discharged through the membrane wall as fast gas, while the remaining CH4 is purified in the form of permeate gas. To increase the concentration of CH4, a multi-stage membrane separation process is usually used. The process flow of membrane separation is shown in Figure 4.

　　常见的膜分离法主要有高压气相分离和气相-液相吸收膜分离两种。采用高压气相分离方法时，由于膜的两侧都是气相，所需的操作压力较大，CH4的纯度只有92%，经多级膜分离，CH4的纯度可达99.5%。气相-液相吸收膜分离一侧为气相，一侧为液相，不需要较高压力，沼气从膜的一侧流过，其中的H2S和CO2分子能够扩散穿过膜，在另一侧被相反方向流过的液体吸收，吸收膜的工作压力仅为0.1MPa，温度为25~35℃，液相的吸收剂可以用化学吸收法中提到的胺溶液和碱溶液。

　　The common membrane separation methods mainly include high-pressure gas phase separation and gas liquid phase absorption membrane separation. When using high-pressure gas phase separation method, due to the fact that both sides of the membrane are in the gas phase, a high operating pressure is required, and the highest purity of CH4 is only 92%. After multi-stage membrane separation, the purity of CH4 can reach 99.5%. The gas-liquid absorption membrane separates the gas phase on one side and the liquid phase on the other side, without requiring high pressure. Biogas flows through one side of the membrane, and H2S and CO2 molecules can diffuse through the membrane and be absorbed by the liquid flowing in the opposite direction on the other side. The working pressure of the absorption membrane is only 0.1 MPa and the temperature is 25-35 ℃. The absorbent for the liquid phase can be amine solution and alkali solution mentioned in chemical absorption method.

　　膜法装置工艺简单、操作简便，设备占地面积小、自动化程度较高，安装方便，较适用于规模较小和中等规模的沼气提纯工程，对其扩容也较易操作。但操作压力高，投资运行费用较高，且对原料中H2S的含量有较高的要求（一般＜10ppm）。

　　The membrane method device has a simple process, easy operation, small equipment footprint, high degree of automation, easy installation, and is suitable for small-scale and medium-sized biogas purification projects. Its expansion is also easy to operate. But the operating pressure is high, the investment and operating costs are high, and there are high requirements for the content of H2S in the raw materials (generally<10ppm).

　　从上述论述可以看出，各种提纯技术各有优缺点，在实际应用中需根据建设项目自身的情况，如对回收率的要求、占地面积、对产品中甲烷含量的要求等来选择相应的技术，以达到的效果。

　　From the above discussion, it can be seen that various purification technologies have their own advantages and disadvantages. In practical applications, the corresponding technology should be selected according to the situation of the construction project itself, such as the requirements for recovery rate, land area, and methane content in the product, in order to achieve the best effect.

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