Current Status and Perspectives of Biogas Upgrading and Utilization

Introduction

Anaerobic digestion (AD) of animal wastes, organic plants, food wastes, sewage wastewater, and industrial wastes generates biogas. Anaerobic digestion is a biochemical way to covert wastes into value. Biodegradable waste alone can process through this technique (Dahiya 2015). The process of converting biodegradable wastes into biogas in an anaerobic digester is clearly shown in Figure 11.1. The anaerobic digestion process is a step-by-step processes in which bacterial breakdown of complex heterogenous organic substances occurs in the absence of oxygen (Kadam and Pamvar 2017). The formation of biogas from biodegradable wastes has four major steps (Ghavinati and Tabatabaei 2018):

  • • Hydrolysis: The first step is the breakdown of organic substances into long- chain compounds by microorganisms. For example, carbohydrates and proteins are decomposed into sugars and amino acids.
  • • Acidogenesis: In the second step, microorganisms break long-chain compounds into single molecules. In this step, sugars and amino acids are converted into fatty acids (long-chain hydrocarbon), ethanol, H2S, and C02.
  • • Acetogenesis: Long-chain fatty acids are broken into short-chain acids. Here ethanol and fatty acids are decomposed into CO,, CH,COOH, and H2.
  • • Methanogenesis: Finally acetic acid and hydrogen combine together and form CH4 and CO,.

Environmental benefits of biogas production:

a. Biogas is one source of renewable energy.

b. Reduces landfill and methane liberation to the atmosphere.

c. Upgraded biogas will be utilized in transport vehicles instead of petroleum products (Ullah Khan et al. 2017).

d. Anaerobic digester sludge is a good fertilizer for agriculture purposes, and it simultaneously produces biogas (Sahota, Shah, et al. 2018).

Conversion of biodegradable wastes into biogas in an anaerobic digester

FIGURE 11.1 Conversion of biodegradable wastes into biogas in an anaerobic digester.

Biogas production is a conventional way to generate renewable energy while safely disposing of biodegradable wastes. Biogas is recognized as the best way to reduce the present energy demand and environmental impacts facing India (Starr et al. 2012).

The components present in the conventionally produced biogas are as follows:

  • • Main components: Methane (around 65%) and carbon dioxide (around
  • 35%)
  • • Trace components: H2S, water vapor, ammonia, oxygen, nitrogen, and halogenated volatile organic compounds.

Note: The percentage of trace components is always less than 2%. Percentages of trace components varies with respect to the source of biodegradable wastes fed into the anaerobic digester. The quantity of these trace compounds is very low compared to the major components (Kapoor et al. 2019). Traditionally generated biogas can be used for heating, cooking, power production, and lighting applications. Biogas is the best alternate to natural gas. The scope of biogas should be broadened to include facets such as transport, developing a substitute for natural gas network, and acting as the substrate for the production of chemicals and fuel cells. Hence, various countries are concentrating on upgrading biogas quality so that it is equal to natural gas. The main drawback of biogas is the presence of H2S, C02, and other impurities. It must be treated before being used as transport fuel (Angelidaki et al. 2018).

Many upgrading technologies are being developed by researchers. Without purification, biomethane availability in raw' plant biogas is only around 60%. Biogas purification techniques are mandatory for producing biomethane of more than 90% purity (Ullah Khan et at. 2017).

The operating principles involved in biogas upgradation can be categorized as follows:

i) Physicochemical processes

a. Adsorption

b. Absorption

c. Cryogenic

d. Membrane separations

ii) Biological processes

a. In situ

b. Ex situ

c. Hybrid process (Angel idaki et al. 2018)

Each upgradation technology should be reviewed with respect to its operations, methane purity energy requirements, and cost savings. Deep analysis was conducted on technology gaps and implementation barriers, and detailed comparisons are given in this chapter. For a broad and successful implementation of biogas upgrading technology, research and development (R&D) trends such as the development of efficient biogas upgrading technologies, adsorbents, cost reduction, and methane loss were carefully evaluated.

This chapter provides a comprehensive description of the main principles of the various methodologies for biogas upgrading, the scientific and technical results related to their biomethanization efficiency, the challenges to further development, and the incentives and feasibility of valuation concepts.

The core objective of biogas upgrading technologies is to reduce CO, presence in the biogas and to enhance the biogas quality to be equivalent to natural gas. The volumetric energy density of upgraded biogas was high compared to that of raw biogas. That upgraded biogas can be used for vehicle’s application in dual fuel mode. A comparison of the compositions of biogas, natural gas, and landfill gas is given in Table 11.1.

Need for Biogas Upgradation

i) Biogas contains CH4, along with CO,, H,S, nitrogen, water vapor, oxygen, and hydrogen. Heating value is an appropriate tool to measure the energy density and quality of a particular fuel. Traditional raw biogas has a volumetric heating value of 21.5 MJ/m w'hile the volumetric heating value of natural gas is 35.8 MJ/m3.

ii) Comparing raw biogas and natural gas heating values, raw biogas has poor energy density. To replace natural gas with biogas, we need to enhance energy content. Biogas contains a high volume of an incombustible constituent (CO,). Eliminating CO, from raw' biogas helps to increase the heating value of the gas and decrease the cost of compression and transport. With

TABLE 11.1

Various Constituents Present in Biogas, Natural Gas, and Landfill Gas (Eriksson et al. 2016)

Component

Unit

Biogas

Natural

gas

Landfill gas

Methane (CH4)

Volumetric %

Around 65

89

Around 45

Carbon dioxide (CO,)

Volumetric %

Around 35

0.9

Around 35

Hydrogen sulfide (H,S)

ppm

Around 3000

1-8

0-100

Hydrogen (H,)

Volumetric %

0

0

0-3

Nitrogen (N,)

Volumetric %

0.2

0.3

5-40

Oxygen (O,)

Volumetric %

0

0

0-5

Lower calorific value

MJ/kg

20.2

48

12.3

supporting commercial and technical evidence, biogas has been suggested for electricity generation and transport applications (Awe et al. 2017).

iii) Other contaminants present in the biogas have negative impacts (corrosion and contamination) on IC engine components, steel chimneys, and boilers. Removing impurities from biogas allows its use without negative effects on downstream components.

iv) Greenhouse gas emissions were drastically reduced when upgraded biogas was used for combustion. Improved biogas combustion gives a lower quantity of carbon dioxide and nitrogen oxide than does combustion of gasoline or diesel (Song, Liu, Ji, Deng, Zhao, and Kitamura 2017).

v) The presence of C02 drops the energy yield from the burning of biogas (Ghatak and Mahanta 2016).

vi) During upgradation, separated C02 can be stored and utilized for other purposes (Morero, Groppelli, and Campanella 2017).

Technologies Involved in Biogas Upgrading

Biogas upgrading is defined as removal of other gases except methane in biogas. The upgradation process of biogas consists of the following steps:

  • • Drying or condensation process: Removal of water vapor from raw biogas.
  • • Cleaning process: Removal of trace elements like H2S, N2, 02, etc. (Ryckebosch, Drouillon, and Vervaeren 2011).

Recovery process: Improves the heat value (Pellegrini, De Guido, and Lange 2018).

Developed technologies for purification of biogas include absorption, adsorption, cryogenic, and membrane separation. The major objective of these technologies major objective is to separate C02 from raw biogas. Before CO, separation, some pre-processing is required to remove H20, H2S, and siloxanes (Sahota, Vijay, et al. 2018).

Absorption

Absorption is the process in which constituents (absorbate) are dissolved by absorbent. Atoms or molecules cross the surface and enter the bulk volume of the material. During the absorption gas phase, impurities (mostly C02) present in biogas are dissolved by the liquid phase absorbent. Based on the absorbent usage, absorption is classified into physical and chemical absorption. Absorbent selection is the most challenging task for researchers.

11.2.1.1 Physical absorption

In the physical absorption process, there are no chemical reactions. Atoms or molecules only dissolve in absorbent, and there is no chemical reaction between absorbate and absorbent (Rotunno, Lanzini, and Leone 2017).

  • • Water scrubbing system - Water used as absorbent
  • • Organic absorbent - Organic solvent used as absorbent
  • 11.2.1.2 Chemical absorption

In the chemical absorption process, chemical reactions take place between absorbate and absorbent. A chemical change exists after complete absorption.

• Amine solutions are mostly used as chemical absorbents

Physical Absorption Method Using Water Scrubbing System

Water scrubbing is most commonly used technique for biogas upgradation. The basic principle for physical absorption is Henry’s law. Henry’s law states that the mass of a dissolved gas in a given volume of solvent at equilibrium is proportional to the partial pressure of the gas (Sahota, Shah, et al. 2018). C02 and H2S absorbed from raw biogas based on the dissolvability of carbon dioxide and hydrogen sulfide in water but these gases are more soluble in methane (Cozma et al. 2013). A water scrubbing system will remove more than 90% of the CO, from raw biogas.

In a high-pressure water scrubbing system, the raw biogas is sent from the bottom of a high-pressure chamber as in Figure 11.2. At the same time, fresh water is

High-pressure water scrubbing system

FIGURE 11.2 High-pressure water scrubbing system.

TABLE 11.2

Advantages and Disadvantages of Water Scrubbing System (Rotunno, Lanzini, and Leone 2017)

Advantages:

Disadvantages:

  • • Simple principle and process
  • • High-purity biomethane produced with minimum methane loss
  • • Chemicals are not required in entire process
  • • Low operating and maintenance costs
  • • Huge amount of fresh water is used in the water scrubbing process
  • • Requires external heat and high energy input for spraying water

showered from the top of the chamber. Raw biogas and water are allowed to flow in opposite directions, and the contact surface area between liquid and gases element is thus increased enormously. Several column arrangements are created inside the high-pressure chamber with packing materials. Biomethane of a maximum purity of 96% can be obtained using a water scrubbing system. Using a counter-flow water scrubbing system, up to 99% of the impurities from raw biogas will be removed, and this can also be done for landfill gas by altering the necessary parameters by using a suitable optimization tool (Cozma et al. 2014). The merits and demerits of water scrubbing system are given in Table 11.2.

 
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