Physical Absorption Method Using Organic Solvents

Physical absorption is based on the solubility of CO, in a chemical reaction-free solution, which in turn is based on Henry’s law; therefore, high partial pressures of CO, and low temperatures are strongly recommended for its application. Performance of the physical absorption process is optimized in terms of absorption rates and solubility equilibrium of CO,. Then, the rich (CO,-loaded) solvent is regenerated in a desorption chamber. The working principles and system arrangements are very similar to those of the water scrubbing system. The performance of a physical solvent can be predicted. The solubility of an absorbate in the absorbent is directly proportional to the pre-processed raw biogas partial pressure. Hence, the performance of physical solvent processes are enhanced with rising gas pressure (Vega et al. 2018).

Some of the organic solvents commonly used for physical absorption are the following:

  • • N-methyl-2 pyrolidone
  • • Dimethyl ethers of polyethylene glycol
  • • Propylene carbonate
  • • Methanol

The merits and demerits of physical absorption using organic solvents are given in Table 11.3.

TABLE 11.3

Advantages and Disadvantages of Physical Absorption Using Organic Solvents

Advantages:

Disadvantages:

  • • Highly recommended to separate C02 in pre-combustion processes
  • • Noticeable selectivity for hydrogen sulfide over carbon dioxide
  • • High capital and operating costs
  • • Chance of biological contamination

Chemical Absorption Method Using Amine Solutions

This section discusses biogas purification systems that utilize amine solution or soda lime to remove carbon dioxide from biogas by absorbing in it through chemical reaction (Ghatak and Mahanta 2016). Raw biogas from the biogas plant is sent into three- stage gas compressors with intercooling arrangements as shown in Figure 11.3. Inter air coolers help to reduce the compressed raw biogas temperature. Pressurized (above 20 bar) raw biogas passes into the absorption chamber from the bottom. Amine, soda lime, or NaOH solution passes into the absorption chamber from the top. Amine and pressurized raw biogas flow in opposite directions. This counter-current in the direction of the flow creates a large area of contact, and a chemical reaction takes place between the amine solution and raw biogas impurity (CO,). Rich methane biogas was liberated at the top of the chamber after complete chemical absorption. For absorbent regeneration, biogas (20 bar, 48°C) enters into the pre-heater, and then the amine solution temperature is increased up to 100°C. Again, it is sent into the desorption chamber. Absorbent impurities are purified by desorbing agents. In the desorption chamber, water is used as desorption agent and vaporized, and it leaves along with H2S and carbon dioxide. In the desorption chamber, a small quantity of

Three-stage gas compressors

FIGURE 11.3 Three-stage gas compressors.

TABLE 11.4

Advantages and Disadvantages of Chemical Absorption Using Amine Solution

dvantages:

Disadvantages:

  • • Simple and easy to use in rural areas.
  • • High degree of biogas purity
  • • Preprocessing to remove hydrogen sulfide from raw biogas is not essential
  • • To maintain high methane concentration at the output, frequent replacement of absorbent is needed, which increases the operating cost
  • • Chemical absorbents are challenging to handle because of their corrosive nature

water loss occurs in every cycle, and this will be replaced with externally provided fresh water (Cozma et al. 2014). The merits and demerits of physical absorption using organic solvents are given in Table 11.4 (Adnan et al. 2019).

Some of the chemical absorbents commonly used for biogas purification (Ramaraj and Dussadee 2015) are as follows:

  • • Diisopropanolamine (DIPA)
  • • Methyl diethanolamine (MDEA)
  • • Diglycol amine
  • • Diethanol amine
  • • Monoethanol amine
  • 11.2.4.1 Adsorption

Adsorption is totally different from absorption. It is a surface deposition process, in which gaseous or liquid impurities are deposited on the surface of the adsorbing medium, which may be solid or liquid (Cozma et al. 2014). A surface attractive force acts between the adsorbent and the adsorbates.

• Physeorption:

Physical binding (usually Van der Waals) surface attractive force between absorbent and adsorbate.

• Chemical absorption:

Chemical binding (usually covalent bonds) involved between absorbate and absorbent.

Based on the process of adsorbent regeneration, biogas upgradation technologies can be classified into three different categories (Morero, Groppelli, and Campanella 2017)

  • • Vacuum swing adsorption
  • • Pressure swing adsorption
  • • Temperature swing adsorption (30-120°C) (Sahota, Vijay, et al. 2018).

The most commonly and commercially used adsorption technique is pressure swing adsorption, because of lower input energy needs, design flexibility, environmental safety, and reasonable performance output compared with other adsorption techniques (Cozma et al. 2014).

Pressure Swing Adsorption (PSA)

The surface deposition of adhesive elements on an adsorbing medium is known as adsorption. Raw biogas from an anaerobic digester is sent to a compressor to increase the pressure above atmospheric pressure. Then, H2S and water vapors are removed before it is sent into an adsorption column. Pre-processed and pressurized biogas is sent into adsorption columns. In the adsorption column, CO, molecules are attracted by the absorbent surfaces. Absorbed molecules are considered as a contaminant on the adsorbent surfaces.

In some rare cases, chemical reactions will take place between adsorbent and contaminants. In the case of such a chemical reaction, some non-hazardous components will be produced; these can be removed and utilized for secondary purposes. The adsorbing medium’s efficiency is measured in terms of surface porosity. Because porous materials have large surface areas, porous adsorbent materials ultimately give the best-quality output (Ryckebosch, Drouillon, and Vervaeren 2011).

Various adsorbent materials commonly used in pressure swing adsorption systems are the following:

  • • Zeolites
  • • Molecular sieves
  • • Alkaline solids
  • • Iron sponge
  • • Silica gel
  • • Activated carbon

Membrane Separation

Membrane separation technology is a separation technology in which undesired constituents of the raw biogas are filtered by using various membranes. Membrane separation is most suitable for biomethane production from biogas. Membrane material selection is very challenging. The membrane separation technique has additional benefits that can be combined with other biogas upgrading techniques. Hybrid processes help to reduce the investment and operating costs compared with conventional processes (Song, Liu, Ji, Deng, Zhao, Li, et al. 2017). Organic, inorganic, and metal matrix membranes are commercially available in market. Advantages and drawbacks of each type of membrane are discussed in Table 11.5 (Ryckebosch, Drouillon, and Vervaeren 2011).

Cryogenic Separation Process

The basic principle for the cryogenic separation process is liquifying the gas state component into liquid phase species by reducing the temperature to as low as -250°C (Hosseinipour and Mehrpooya 2019). Cryogenic separation also called

TABLE 11.5

Organic Polymers and Inorganic Membrane Materials (Xia, Cheng, and Murphy 2016)

Organic polymers

Inorganic materials

Mixed matrix materials

Merits:

  • • Nowadays, most commonly used membrane materials for membrane gas separation
  • • Process ability good and surface area enhancing also possible
  • • Good choice compared with inorganic materials.

Merits:

  • • Physical and chemical stability
  • • Quality of purified biogas was good while purifying through inorganic material membrane

Merits:

• Introduced to overcome the demerits of inorganic membranes and organic membranes

Demerits:

  • • Costly and poor availability
  • • Need to change simultaneously

Demerits:

  • • Worst machining properties and difficult to process
  • • Manufacturing cost is expensive and surface area enhancing is very complicated

Some organic membrane polymer materials:

Polysulfone, polyethersulfone, cellulose acetate, polyimide, polyetherimide, polycarbonate (brominated), polyphenylene oxide, polymethylpentene, polydimethylsiloxane, polyvinyltrimethylsilane

Some inorganic membrane materials:

Nanoporous carbon, zeolites, alumina, cobalt, copper, iron, palladium, platinum, tantalum, vanadium, nickel, niobium

Some mixed matrix membrane materials:

Mixed conducting perovskites, metal organic frameworks, palladium alloys, ultra-microporous amorphous silica

low-temperature upgradation technology. Liquid biogas (LBG) can be directly utilized as vehicular transport fuel. Liquid biogas production is only possible by cryogenic separation technique (Baena-Moreno et al. 2019). LPG is a fossil fuel which is derived from petroleum, while LBG is a renewable fuel which is derived from biogas. LBG is also known as a carbon-neutral fuel. The heating value of LBG is 2 times more than compressed natural gas (CNG).

The carbon dioxide liquification temperature differs from the methane liquifi- cation point. Therefore, if C02 is separated from raw biogas, pure liquid biomethane will be produced. Sometimes liquid biomethane also called liquid biogas. The energy requirement for the cryogenic separation technique is comparatively less than that for adsorption using amine solution. Liquid biomethane purity is very high compared with biomethane produced by other techniques. Low-temperature separation techniques also give liquid CO, as a by-product. Transportation and storage of liquid fuel are comparatively easier than compressed gaseous fuel storage. Transportation and storage space requirements are reduced significantly. The biogas impurity CO, is also collected in this technique; hence we can eliminate greenhouse gas emissions and global warming (Pellegrini, De Guido, and Lange 2018; Baena-Moreno et al. 2019).

Chemical Hydrogenation Process

Before describing the chemical hydrogenation process, we need to know the definition of hydrogenation. Hydrogenation means conversion of unsaturated organic components into saturated organic components with the help of hydrogen molecules in the presence of a catalyst. Catalysts are substances that are used to speed up the rate of a reaction without being consumed during the process. Catalysts used for hydrogenation reactions are metals such as nickel, ruthenium, and platinum. We can also convert double- or triple-bond hydrocarbon to single-bond hydrocarbon by adding hydrogen atoms (Ramaraj and Dussadee 2015). In this process, hydrogen is added to carbon dioxide in the presence of a catalyst. The most commonly used catalysts for hydrogenation are nickel and ruthenium. The optimum condition for the hydrogenation reaction is 473 К temperature and 50-200 bar pressure. (Xia, Cheng, and Murphy 2016).

Biogas purification through biological methods are listed below:

  • • Chemoautotrophic method
  • • Photoautotroph ic method
  • • Fermentation method
  • • Microbial electrochemical method

Chemoautotrophic Methods

In the chemoautotrophic method, hydrogenotrophic methanogen microorganisms (bacteria) help to produce methane gas from carbon dioxide using hydrogen. Microorganisms are used for biogas upgradation, so it is eco-friendly and renewable in nature. It needs a continuous hydrogen gas supply that may be produced from renewable sources. Hydrolyzation of water is done with solar/wind plant electricity and can give hydrogen continuously. The investment costs of this technique are lower than those of other purification methods. The major merits of the chemoautotrophic method is that C02 is not liberated but rather is converted into methane (Wang et al. 2020). The process of hydrogenation of CO, to CH4 with nickel catalyst is shown by the Sabatier reaction.

Selecting the catalyst for hydrogenation is one of the challenging issues in enhancing the carbon dioxide conversion efficiency and purity of methane. While developing catalysts for hydrogenation, we need to focus on the operating temperature and the life of the catalyst. If catalyst reactivity is reduced, then methanol is produced from carbon dioxide. The formation of methanol, an exothermic reaction, is shown in Equation 11.2:

It can be further classified into three types:

  • In situ biogas upgrading
  • Ex situ biogas upgrading
  • • Biogas upgrading systems using microbial communities

An overview of in situ and ex situ upgradation techniques is shown in Figure 11.4.

In situ Biological Biogas Upgrading

In situ systems directly send the hydrogen into an anaerobic digester. In an anaerobic digester, hydrogen combines with the carbon dioxide, and double bonds are broken into single-bond organic structures. At this stage, methane is formed from carbon dioxide by losing its double bonds by the action of microorganisms like autochthonous methanogenic archaea. There are two different types of methane formation from carbon dioxide: Wood-Ljungdahl and hydrogenotrophic methanogenesis (Voelklein, Rusmanis, and Murphy 2019).

• In hydrogenotrophic methanogenesis, the microorganism is directly added to the anaerobic digester unit; this causes interaction between carbon dioxide and hydrogen molecules to directly form pure saturated single-bond hydrocarbon (methane) in a single-step reaction as in Equation 11.1.

In situ and ex situ upgradation techniques

FIGURE 11.4 In situ and ex situ upgradation techniques.

• At the same time Wood-Ljungdahl does not directly convert carbon dioxide into methane in a single step. This microorganism also converts carbon dioxide into methane indirectly. In the first step, homoacetogenic bacteria combine carbon dioxide with hydrogen to form acetic acid. In the second step, acetic acid converts into methane by acetoclastic methanogenic archaea bacteria. The first and second steps of the reaction are shown in Equations 11.3 and 11.4, respectively.

Ex situ Biological Biogas Upgrading

Another biogas upgradation process is ex situ biological upgrading, in which a digester’s externally provided H2 and C02 are biologically transformed to CH4 by the help of hydrogenotrophic methanogens. The CH4 concentration in the product gas is more than 98%, permitting its utilization as a substitute for natural gas (Kougias et al. 2017). The ex situ biological upgradation system layout is shown in Figure 11.4.

Microbial Communities in Biological Biogas Upgrading Systems

Upgradation and purification of the biogas are essential because cleaned biogas offers reductions in harmful gas emissions and several other environmental advantages when it is utilized as a transportation fuel. Reducing carbon dioxide and hydrogen sulfide concentrations will considerably enhance the biogas quality. Several technologies are being developed and implemented for removing biogas impurities; these include physical absorption, absorption by chemical solvents, membrane separation, cryogenic separation, and chemical or biological methods. While the physicochemical method of removal is costly and environmentally harmful, the biological processes are considered as feasible and environmentally friendly. Moreover, algae biomass is plentiful and universal. Purification of biogas using algae involves the use of the photosynthetic capability of micro- or macroalgae to remove impurities existing in the biogas. Biological methods help to purify the biogas, improve the calorific value percentage of the gas, and make biogas with characteristics as close as possible to those of natural gas (Ramaraj and Dussadee 2015).

Photoautotrophic Methods

To produce biogas rich in methane concentration, the photoautotrophic technique is the most appropriate, since it gives the highest level of carbon dioxide sequestration. Apart from this, hydrogen sulfide contamination can also be eliminated in this method. Approximately 97% of CH4 is recovered from this, with the recovery level based on the selected algae types. Algae are extensively used in the alteration process and are grown in mass in open ponds. Photosynthetic effectiveness is higher when the process is commenced in a closed pond. At the time of the recovery process, biogas is passed through the photo reactor for the effective conversion of gas to CH4. The biggest downside is the high cost investment. This gas-processing technology has been utilized to instantaneously condense and disperse water and HC from the natural gas. Progress to allow for the high reduction of carbon dioxide and H2S is presently underway.

Biogas Upgrading through Other Fermentation Processes

In this process, biogas is upgraded through the biological conversion of CO, and transformed into valued liquid products such as acetate, ethanol, and butyrate (Omar et al. 2019). Different organisms are proficient in transforming carbon dioxide and hydrogen into liquid products. Most of these organisms are acetogens, which effectively ferment C, compounds using hydrogen as the electron donor and generate valuable chemicals and biofuels. However, different fermentation investigations have been studied using pure culture, which faces numerous limitations. Limited availability of adaptation measures for several substrates (e.g., toxic components) and high costs of operation to sustain the conversion conditions make these techniques less appropriate for large industries.

In general, H, needed for the fermentation process can be produced from large- scale sources such as petroleum refinery, soda manufacture, coal gasification, and petrochemical plants. The renewable electricity concept for the utilization of hydrogen generation is receiving great attention nowadays. This sustainable process, which also called conversion of power to biogas technology, depends on water electrolysis that utilizes surplus electricity generation by renewable energy resources, such as solar panels and windmills.

The upgradation of biogas of volatile fatty acids via mixed-values fermentation of carbon dioxide and hydrogen was described in an earlier study. This area of research is still in its infancy, and much progress concerning organisms and processing is necessary. Hence, a novel gas upgradation technique, involving the fermentation of carbon dioxide into valuable chemicals using various diverse culture acetogenic groups as biocatalysts and outside added hydrogen as a source of energy under various temperatures, has been suggested.

Biogas Upgrading through Microbial Electrochemical Methods

A modern electrochemical separation cell was designed to in situ adsorb and regenerate CO, via alkali and acid regeneration alkali (Angelidaki et al. 2018). The electrochemical process is viewed to be the greatest and most cost-effective technique of biogas restoration for methane generation and CO, removal. One example is the electrochemical technique in the microbial electrolysis cell. Here, the oxidation of compounds by the help of bacteria issues electrons into the anode chamber, where they mix with protons in the cathode chamber to produce hydrogen that will be used for the recovery of biogas. Using biocathode in an electrolysis cell, CH4 can be generated by reducing CO,, for an attainment of 80% energy efficiency. The CO, reduction to CH4 depends on the transfer of electrons and the H, generated. Based on the potential of the cathode, the reduction process happens. In situ and ex situ methods of biogas recovery using the electrolysis cell have been investigated experimentally. The results show that the performance of the in situ gas recovery technique is much better, having the greatest capacity to remove C02. Apart from this, it has been found that the removal of CO, is linked with both CH4 production and CO, ionization. This ionization is because of the production of alkalinity from the cathode.

 
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