1. Characteristics of biogas

沼气是一种混合气体，其组成不仅取决于发酵原料的种类及其相对含量，而且随发酵条件及发酵阶段的不同而变化。当沼气厌氧反应器处于正常稳定发酵阶段时，沼气的体积组成大致为：甲烷（CH4）50%～75%，二氧化碳（CO2）25%～45%，水（H2O，20～40℃下）2%～7%，氮气（N2）0～2%，少量的氧气（O2），以及少于1%的氢气（H2）和硫化氢（H2S）。

Biogas is a kind of mixed gas. Its composition depends not only on the type and relative content of fermentation materials, but also on the fermentation conditions and fermentation stages. When the biogas anaerobic reactor is in the normal and stable fermentation stage, the volume composition of biogas is roughly 50% - 75% of methane (CH4), 25% - 45% of carbon dioxide (CO2), 2% - 7% of water (H2O, at 20 - 40 ℃), 0 - 2% of nitrogen (N2), a small amount of oxygen (O2), and less than 1% of hydrogen (H2) and hydrogen sulfide (H2S).

与其它可燃气体相比，沼气具有抗爆性良好和燃烧产物清洁等特点。目前，沼气主要应用在发电、供热和炊事方面，沼气中的CO2降低了沼气的能量密度和热值，限制了沼气的利用范围，要去除沼气中的CO2、H2S 和水蒸气等将沼气提纯为生物天然气（BNG）。生物天然气可压缩用于车用燃料（CNG）、热电联产（CHP）、并入天然气管网、燃料电池以及化工原料等领域。汽车使用生物天然气不仅可以降低尾气排放造成的空气污染，而且温室气体的净排放量减少75%～200%，生物天然气可混入现有的天然气管网，降低对石化能源的依赖。

Compared with other combustible gases, biogas has the characteristics of good explosion resistance and clean combustion products. At present, biogas is mainly used in power generation, heating and cooking. The CO2 in biogas reduces the energy density and calorific value of biogas, and limits the utilization range of biogas. It is necessary to remove CO2, H2S and water vapor in biogas to purify biogas into biological natural gas (BNG). Biogas can be compressed and used in the fields of vehicle fuel (CNG), cogeneration (CHP), integration into natural gas pipeline network, fuel cell and chemical raw materials. The use of biological natural gas in automobiles can not only reduce the air pollution caused by tail gas emissions, but also reduce the net emissions of greenhouse gases by 75% to 200%. Biological natural gas can be mixed into the existing natural gas pipeline network, reducing the dependence on petrochemical energy.

2、沼气提纯技术

2. Biogas purification technology

目前填埋气提纯工艺有变压吸附法（PSA）、水洗法、化学吸收法、膜分离法、选择分离法等，在目前世界范围内工艺较为成熟、应用相对较多的方法是变压吸附法（PSA）、化学吸收法（胺法净化）、膜分离法。

At present, the purification processes of landfill gas include pressure swing adsorption (PSA), water washing, chemical absorption, membrane separation, selective separation, etc. At present, the more mature and widely used methods in the world are pressure swing adsorption (PSA), chemical absorption (amine purification), and membrane separation.

2.1变压吸附法（PSA）

2.1 Pressure swing adsorption (PSA)

变压吸附法（PSA）是在加压条件下，利用沼气中的CH4、CO2以及N2在吸附剂表面被吸附的能力不同而实现分离气体成分的一种方法。组分的吸附量受压力及温度的影响，压力升高时吸附量增加，压力降低时吸附量减少；当温度升高时吸附量减小，温度降低时吸附量增加。变压吸附对气体来源的要求非常严格，H2S的存在会导致吸附剂永久性中毒，并且变压吸附要求气体干燥，所以变压吸附前要先脱除H2S和H2。

Pressure swing adsorption (PSA) is a method to separate gas components under pressure by using the different adsorption capacities of CH4, CO2 and N2 in biogas on the adsorbent surface. The adsorption capacity of components is affected by pressure and temperature. The adsorption capacity increases when the pressure increases, and decreases when the pressure decreases; The adsorption capacity decreases when the temperature increases, and increases when the temperature decreases. PSA has very strict requirements on the gas source. The presence of H2S will cause permanent poisoning of the adsorbent, and PSA requires gas drying, so H2S and H2 must be removed before PSA.

吸附材料在该技术中起到关键的作用，一般采用不同类型的活性炭、沸石、硅胶、氧化铝和分子筛作为吸附材料。不同的吸附材料对沼气的纯化效果各不相同。目前，以活性炭和分子筛为主的碳基吸附剂，在研究沼气提纯方面经常被使用。近年来出现的一些新型吸附材料，如有序介孔材料、胺修饰吸附剂和金属框架物（MOFs）对CO2具有很高吸附选择性，应用前景广阔，而且MOFs被认为是在CO2分离方面最具潜力。

Adsorption materials play a key role in this technology. Generally, different types of activated carbon, zeolite, silica gel, alumina and molecular sieve are used as adsorption materials. Different adsorption materials have different purification effects on biogas. At present, carbon-based adsorbents, mainly activated carbon and molecular sieve, are often used in the research of biogas purification. In recent years, some new adsorption materials, such as ordered mesoporous materials, amine modified adsorbents and metal frameworks (MOFs), have high adsorption selectivity for CO2, and have broad application prospects, and MOFs are considered to have the most potential in CO2 separation.

2.2化学吸收法

2.2 Chemical absorption method

化学吸收法是利用胺溶液将CO2从CH4中分离的方法，分离过程中CO2被吸收后进一步与胺溶液发生化学反应，通过加热完成胺溶液的再生。由于化学反应具有很强的选择性，而CH4被胺溶液吸收的量又非常低，所以这种方法CH4的损失率低于0.1%。该技术操作压力一般为1atm。

Chemical absorption method is a method that uses amine solution to separate CO2 from CH4. During the separation process, CO2 is absorbed and then chemically reacts with amine solution to complete the regeneration of amine solution by heating. Because the chemical reaction has strong selectivity and the amount of CH4 absorbed by amine solution is very low, the loss rate of CH4 in this method is less than 0.1%. The operating pressure of this technology is generally 1 atm.

常用的胺溶液主要有乙醇胺（MEA）、二乙醇胺（DEA）和甲基二乙醇胺（MDEA）[13]。由于CO2被吸收后与胺溶液发生了化学反应，因此吸收过程可以在较低的压力条件下进行，一般情况下只需在沼气已有压力的基础上稍微提高一些压力即可。胺溶液的再生过程比较困难，需要160℃的温度条件，因此运行过程需要消耗大量的工艺用热，存在运行能耗高的弊端。此外，由于存在蒸发损失，运行过程需要经常补充胺溶液。

The commonly used amine solutions mainly include ethanolamine (MEA), diethanolamine (DEA) and methyldiethanolamine (MDEA) [13]. Since CO2 is absorbed and chemically reacts with amine solution, the absorption process can be carried out at a lower pressure. Generally, it is only necessary to slightly increase the pressure on the basis of the existing pressure of biogas. The regeneration process of amine solution is relatively difficult, requiring 160 ℃ temperature conditions, so the operation process needs to consume a large amount of process heat, which has the disadvantage of high operation energy consumption. In addition, due to evaporation loss, amine solution needs to be supplemented frequently during operation.

2.3膜分离法

2.3 Membrane separation method

膜技术被认为是21世纪工业技术改造中的一项极为重要的技术，有专家指出：谁掌握了膜技术谁就掌握了化学工业的明天。膜分离法原理是利用各气体组分在膜表面的吸附能力不同，溶解、扩散速率不同，在膜两侧分压差的推动下，大部分CO2等组分和少量的CH4透过膜壁进入渗透侧分离出去，大部分CH4在高压侧作为生物天然气输出。

Membrane technology is considered to be an extremely important technology in the industrial technological transformation in the 21st century. Some experts pointed out that whoever masters membrane technology will master the future of chemical industry. The principle of membrane separation method is to make use of the different adsorption capacity of each gas component on the membrane surface and the different dissolution and diffusion rates. Under the promotion of the partial pressure difference on both sides of the membrane, most of the components such as CO2 and a small amount of CH4 enter the permeation side through the membrane wall for separation, and most of CH4 is exported as biological natural gas at the high pressure side.