HOW TO REMOVE CARBON DIOXIDE FROM BIOGAS

HOW TO REMOVE CARBON DIOXIDE FROM BIOGAS

Anaerobic technology for the fermentation of organic matter, the result of which is the production of biogas, is becoming more widespread in the world as an alternative source of energy (biogas, electricity and heat). The advantages of this technology are obvious:

• we get renewable fuels, which are used instead of natural gas and other minerals,

• reduce greenhouse gas emissions,

• we get in the form of a secondary product a high-quality digestate (it is used in agriculture as an environmentally friendly organic fertilizer).

The substrate for anaerobic fermentation is almost any organic waste, for example: sludge from sewage treatment plants, plant and animal waste (manure), organic fractions of household and industrial waste. The production and processing of biogas is stimulated at the legislative level in most European countries, including Ukraine.

The biogas mixture, in turn, consists of methane (CH₄) and CO₂ (carbon dioxide) with a small amount of impurities (oxygen, hydrogen, hydrogen sulfide, ammonia and others).

The concentration of CH₄, as the main energy carrier in the biogas mixture, directly affects the efficiency of the units. Carbon dioxide, by contrast, reduces the efficiency of biogas as an energy source, so its amount should be minimized. Our material is devoted to the question of how to improve the quality of a biogas mixture.

Producing biogas and its characteristics

The increase in oil and natural gas prices, as well as favorable green tariffs for renewable energy sources attract attention to how to get gas and the features of this process. An important aspect in this case is to improve the quality of the biogas mixture by purifying it from impurities or increasing the concentration of CH именно, since the calorific value of biogas depends on the amount of methane. The better the produced gas is purified, the more opportunities there are for its use. However, cleaning increases its cost; therefore, a balance must be struck between the energy consumption of the process and its efficiency. Processing biogas to increase its CH₄ content is an important stage in the operation of modern biogas plants.

The production of biogas by anaerobic (oxygen-free) technology we considered in another article. Recall that the resulting composition is very dependent on the feedstock and compliance with the process. Biogas obtained from solid waste may contain nitrogen, hydrogen sulfide, and other elements, including water and solid particles. These impurities, in addition to reducing heat transfer, lead to corrosion and premature wear of equipment. The table below shows some indicators.

Comparative composition of biogas, landfill gas and natural gas.

BiogasLandfill gasNatural gas (Denmark)Natural gas (Netherlands)
CH₄ (%)60 –7035 – 658981
other hydrocarbons (%)009,43,5
carbon dioxide (%)30-4015-500,671
nitrogen (%)0,25-400,2814
oxygen (%)00,500
hydrogen sulfide (ppm)0-40000-1002,9
ammonia (ppm)100500
calorific value (kWh / nm3), minimum6,54,4118,8

Some countries have introduced standardized requirements for the content of impurities in biogas for supplying it to a common gas transmission network or for use as automobile fuel.

How is extracted gas cleaned

Gas production without carbon dioxide implies the ability to remove carbon dioxide if necessary (increase the concentration of methane). This process consists of several stages: first remove water, hydrogen sulfide, nitrogen and other impurities, and then remove CO₂.

Water removal

Upon exiting the anaerobic plant, the gas mixture is saturated with water vapor due to the natural evaporation of water from the substrate. This moisture can condense in gas pipelines and cause corrosion. Another problem with a high concentration of water in a biogas mixture is a decrease in the volume concentration of methane. In this case, side problems arise with the operation of the cogenerator, the difficulty of starting and burning in general.

Water removal methods can be divided based on the technology used into the following:

• cooling,

• compression

• absorption

• adsorption.

With increasing pressure or lowering the temperature of the gas mixture, water condenses (steam becomes liquid) and is easily removed. For cooling, the gas line is equipped with a special steam trap.

Water can also be removed by adsorption using silica, activated carbon or molecular sieves. Other water removal technologies are absorption in glycol solutions or the use of hygroscopic salts.

How to clean biogas from hydrogen sulfide

The amount of hydrogen sulfide in biogas can be reduced by precipitation directly in the reactor by adding reagents and precipitation of insoluble sulfur compounds directly in the liquid phase of the substrate. An alternative way is to clean the gas mixture itself from hydrogen sulfide.

For precipitation, iron ions in the form of chloride or iron sulfate are added. The resulting iron sulfide precipitates and is removed along with solid residues.

Activated carbon adsorption is used to remove H₂S. In this case, oxygen is added, which oxidizes hydrogen sulfide to sulfur, and it, in turn, is adsorbed by the surface of activated carbon. Chemical absorption is also used to remove hydrogen sulfide by passing the gas mixture through a porous material impregnated with a reagent; washing with sodium hydroxide solution or using industrial catalysts.

How to clean biogas from oxygen and nitrogen

The above elements are not present in every case. Their availability depends on how the waste is processed – which substrate and technology are used. If there is air in the fermentation tank, then nitrogen will enter biogas in any case.

It is most characteristic for producing landfill gas under pressure.

Removal of these elements to a minimum level is necessary if the final product is intended for fuel systems of vehicles. To this end, adsorption by activated carbon, molecular sieves or membranes is used.

Removal of ammonia, siloxanes and particulate matter

Due to its high volatility and instability, ammonia is usually removed at the previous stages of purification and does not require additional measures.

Siloxanes may be present in the case of wastewater sludge treatment as a result of detergents entering the effluent. Most often, they are removed during adsorption on activated carbon during the release of the gas mixture from hydrogen sulfide or during cooling.

Solids are separated by mechanical filters.

Biogas processing and its enrichment

Carbon dioxide, which is contained in biogas at the outlet of the digester, reduces the energy efficiency of the gas mixture. To increase the level of CH₄ (enrichment) and, accordingly, to increase heat transfer, carbon dioxide should be removed from the raw biogas mixture.

Before you receive gas suitable for supply to the common gas transmission network, it is passed through special enrichment plants. Often use adsorption or absorption techniques, membranes, chemicals.

Differential pressure adsorption (PSA). Using this technique, carbon dioxide is separated by adsorption on the surface under elevated pressure. Activated carbon or zeolites are used as adsorbents.

In an enrichment plant using an absorption technique, crude biogas is encountered with an oncoming liquid stream in a column that is filled with plastic filler to increase the contact area between the gaseous and liquid phases. Carbon dioxide is more soluble than methane. The liquid leaving the column contains a high concentration of carbon dioxide, while the gas mixture at the outlet has a high concentration of CH₄.

Dry enrichment membranes are made of materials that are permeable to carbon dioxide, water and ammonia. Hydrogen sulfide and oxygen to some extent penetrate the membrane, while nitrogen and methane pass to a very small extent.

A new direction should be considered cryogenic purification, which is based on the temperature difference of the boiling / sublimation of various substances.

Impurity-free biogas production: conclusions

Recycling waste to produce biogas is a complex process and requires knowledge of microbiology, chemistry and related sciences. Of great importance for the quality of the final product is its degree of purification and the level of methane content. Carbon dioxide and other impurities should be removed. In general, we examined the methods of how to extract the extracted gas.

The specificity of the topic does not allow one article to reveal all the modern possibilities of how to get gas without impurities. Ecodevelop specialists in each specific project offer the most suitable ways to resolve this issue.

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