沼氣的主要成分是CH4和CO2，將CO2從混合氣中分離得到的高純度甲烷氣被稱為生物天然氣。生物天然氣可以直接作為石化天然氣的替代燃料，發(fā)展沼氣已成為增加天然氣供應(yīng)量的一個(gè)重要方向。人類對(duì)天然氣需求量的增加推動(dòng)了生物天然氣技術(shù)的發(fā)展。沼氣提純生物天然氣可高值化利用沼氣，有效減少沼氣工程因沼氣利用不充分而排空造成的溫室效應(yīng)，具有環(huán)保和能源雙重效益。本文概述了沼氣提純技術(shù)及國(guó)內(nèi)外發(fā)展現(xiàn)狀，以期為沼氣提純技術(shù)的發(fā)展提供參考。
The main components of biogas are CH4 and CO2. The high-purity methane gas obtained by separating CO2 from the mixture is called biogas. Biogas can be directly used as an alternative fuel for Petrochemical Natural gas. The development of biogas has become an important direction to increase the supply of natural gas. The increase of human demand for natural gas has promoted the development of biogas technology. Biogas purification of biogas can make high-value use of biogas, effectively reduce the greenhouse effect caused by the emptying of biogas projects due to insufficient utilization of biogas, and have dual benefits of environmental protection and energy. This paper summarizes the biogas purification technology and its development status at home and abroad, in order to provide reference for the development of biogas purification technology.
1、沼氣特性
1. Biogas characteristics
沼氣是一種混合氣體，其組成不僅取決于發(fā)酵原料的種類及其相對(duì)含量，而且隨發(fā)酵條件及發(fā)酵階段的不同而變化。當(dāng)沼氣厭氧反應(yīng)器處于正常穩(wěn)定發(fā)酵階段時(shí)，沼氣的體積組成大致為：甲烷（CH4）50%～75%，二氧化碳（CO2）25%～45%，水（H2O，20～40℃下）2%～7%，氮?dú)猓∟2）0～2%，少量的氧氣（O2），以及少于1%的氫氣（H2）和硫化氫（H2S）。
Biogas is a kind of mixed gas. Its composition depends not only on the type and relative content of fermentation raw materials, but also on the different fermentation conditions and fermentation stages. When the biogas anaerobic reactor is in the normal stable fermentation stage, the volume composition of biogas is roughly: methane (CH4) 50% ~ 75%, carbon dioxide (CO2) 25% ~ 45%, water (H2O, at 20 ~ 40 ℃) 2% ~ 7%, nitrogen (N2) 0 ~ 2%, a small amount of oxygen (O2), and less than 1% of hydrogen (H2) and hydrogen sulfide (H2S).
與其它可燃?xì)怏w相比，沼氣具有抗爆性良好和燃燒產(chǎn)物清潔等特點(diǎn)。目前，沼氣主要應(yīng)用在發(fā)電、供熱和炊事方面，沼氣中的CO2降低了沼氣的能量密度和熱值，限制了沼氣的利用范圍，要去除沼氣中的CO2、H2S 和水蒸氣等將沼氣提純?yōu)樯锾烊粴猓˙NG）。生物天然氣可壓縮用于車用燃料（CNG）、熱電聯(lián)產(chǎn)（CHP）、并入天然氣管網(wǎng)、燃料電池以及化工原料等領(lǐng)域。汽車使用生物天然氣不僅可以降低尾氣排放造成的空氣污染，而且溫室氣體的凈排放量減少75%～200%，生物天然氣可混入現(xiàn)有的天然氣管網(wǎng)，降低對(duì)石化能源的依賴。
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 scope of its utilization. It is necessary to remove CO2, H2S and water vapor in biogas and purify biogas into biological natural gas (bng). Biogas can be compressed for vehicle fuel (CNG), cogeneration (CHP), incorporation into natural gas pipelines, fuel cells, chemical raw materials and other fields. The use of biogas in automobiles can not only reduce the air pollution caused by exhaust emissions, but also reduce the net emission of greenhouse gases by 75% - 200%. Biogas can be mixed into the existing natural gas pipeline network, reducing the dependence on petrochemical energy.
2、沼氣提純技術(shù)
2. Biogas purification technology
目前填埋氣提純工藝有變壓吸附法（PSA）、水洗法、化學(xué)吸收法、膜分離法、選擇分離法等，在目前世界范圍內(nèi)工藝較為成熟、應(yīng)用相對(duì)較多的方法是變壓吸附法（PSA）、化學(xué)吸收法（胺法凈化）、膜分離法。
At present, landfill gas purification processes 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變壓吸附法（PSA）
2.1 pressure swing adsorption (PSA)
變壓吸附法（PSA）是在加壓條件下，利用沼氣中的CH4、CO2以及N2在吸附劑表面被吸附的能力不同而實(shí)現(xiàn)分離氣體成分的一種方法。組分的吸附量受壓力及溫度的影響，壓力升高時(shí)吸附量增加，壓力降低時(shí)吸附量減少；當(dāng)溫度升高時(shí)吸附量減小，溫度降低時(shí)吸附量增加。變壓吸附對(duì)氣體來源的要求非常嚴(yán)格，H2S的存在會(huì)導(dǎo)致吸附劑永久性中毒，并且變壓吸附要求氣體干燥，所以變壓吸附前要先脫除H2S和H2。
Pressure swing adsorption (PSA) is a method to separate gas components under pressurized conditions by using the different adsorption capacities of CH4, CO2 and N2 in biogas on the surface of adsorbent. The adsorption capacity of components is affected by pressure and temperature. When the pressure increases, the adsorption capacity increases, and when the pressure decreases, the adsorption capacity decreases; When the temperature increases, the adsorption capacity decreases, and when the temperature decreases, the adsorption capacity increases. PSA has very strict requirements on gas source. The presence of H2S will cause permanent poisoning of adsorbent, and PSA requires gas drying, so H2S and H2 should be removed before PSA.
吸附材料在該技術(shù)中起到關(guān)鍵的作用，一般采用不同類型的活性炭、沸石、硅膠、氧化鋁和分子篩作為吸附材料。不同的吸附材料對(duì)沼氣的純化效果各不相同。目前，以活性炭和分子篩為主的碳基吸附劑，在研究沼氣提純方面經(jīng)常被使用。近年來出現(xiàn)的一些新型吸附材料，如有序介孔材料、胺修飾吸附劑和金屬框架物（MOFs）對(duì)CO2具有很高吸附選擇性，應(yīng)用前景廣闊，而且MOFs被認(rèn)為是在CO2分離方面具潛力。
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 based on activated carbon and molecular sieve are often used in the study 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 broad application prospects, and MOFs are considered to have the most potential in CO2 separation.
2.2化學(xué)吸收法
2.2 chemical absorption method
化學(xué)吸收法是利用胺溶液將CO2從CH4中分離的方法，分離過程中CO2被吸收后進(jìn)一步與胺溶液發(fā)生化學(xué)反應(yīng)，通過加熱完成胺溶液的再生。由于化學(xué)反應(yīng)具有很強(qiáng)的選擇性，而CH4被胺溶液吸收的量又非常低，所以這種方法CH4的損失率低于0.1%。該技術(shù)操作壓力一般為1atm。
Chemical absorption method uses amine solution to separate CO2 from CH4. After CO2 is absorbed in the separation process, it further reacts with amine solution and completes 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 1atm.
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被吸收后與胺溶液發(fā)生了化學(xué)反應(yīng)，因此吸收過程可以在較低的壓力條件下進(jìn)行，一般情況下只需在沼氣已有壓力的基礎(chǔ)上稍微提高一些壓力即可。胺溶液的再生過程比較困難，需要160℃的溫度條件，因此運(yùn)行過程需要消耗大量的工藝用熱，存在運(yùn)行能耗高的弊端。此外，由于存在蒸發(fā)損失，運(yùn)行過程需要經(jīng)常補(bǔ)充胺溶液。
Commonly used amine solutions mainly include ethanolamine (MEA), diethanolamine (DEA) and methyl diethanolamine (MDEA) [13]. Since CO2 reacts with amine solution after being absorbed, the absorption process can be carried out under low 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 a temperature of 160 ℃, so the operation process needs to consume a lot 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膜分離法
2.3 membrane separation method
膜技術(shù)被認(rèn)為是21世紀(jì)工業(yè)技術(shù)改造中的一項(xiàng)極為重要的技術(shù)，有專家指出：誰掌握了膜技術(shù)誰就掌握了化學(xué)工業(yè)的明天。膜分離法原理是利用各氣體組分在膜表面的吸附能力不同，溶解、擴(kuò)散速率不同，在膜兩側(cè)分壓差的推動(dòng)下，大部分CO2等組分和少量的CH4透過膜壁進(jìn)入滲透?jìng)?cè)分離出去，大部分CH4在高壓側(cè)作為生物天然氣輸出。
Membrane technology is considered to be a very 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 use the different adsorption capacity of each gas component on the membrane surface and the different dissolution and diffusion rates. Driven by the differential pressure 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 and are separated out, and most of CH4 is output as biological natural gas on the high-pressure side.
適合沼氣提純的有聚酰亞胺膜、聚砜膜和醋酸纖維素膜，后者的耐水性不佳，使用過程中需要嚴(yán)格脫水，沼氣提純膜元件使用中空纖維、螺旋卷類型的較多，封套式較少。沼氣提純膜分離技術(shù)有兩套基本的膜分離系統(tǒng)：氣體滲透模塊系統(tǒng)和氣液膜分離系統(tǒng)。對(duì)于氣體滲透模塊系統(tǒng)來說，在階段，CH4含量高可達(dá)到92%。但若經(jīng)過多級(jí)膜處理后，CH4含量高會(huì)超過96%。沼氣氣液膜分離提純技術(shù)是近年才發(fā)展起來的，對(duì)于CO2的去除非常有效，特別是采用堿性溶液的膜系統(tǒng)，能夠在階段中將55%CH4含量的沼氣純化到CH4含量96%。
Polyimide membrane, polysulfone membrane and cellulose acetate membrane are suitable for biogas purification. The latter has poor water resistance and requires strict dehydration in the process of use. Biogas purification membrane components use more hollow fibers and spiral rolls, and less envelopes. There are two basic membrane separation systems for biogas purification membrane separation technology: gas permeation module system and gas-liquid membrane separation system. For the gas permeation module system, the maximum CH4 content can reach 92% in the first stage. However, after multi-stage membrane treatment, the maximum CH4 content will exceed 96%. Biogas gas-liquid membrane separation and purification technology has only been developed in recent years, which is very effective for CO2 removal. Especially, the membrane system using alkaline solution can purify biogas with 55% CH4 content to 96% CH4 content in the first stage.
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