Biogas to Biomethane: Engineering, Production, Sustainability

By Abu Yousuf

Biogas to Biomethane: Engineering, Production, Sustainability addresses the technical, social and economic aspects of biogas upgradation to biomethane. With a focus on the latest advances, the book examines the challenges of biomethane production from technology to safety. Sections discuss the fundamentals of biomethane production, examine the latest technologies for biogas upgradation, provide step-by-step guidance on how to calculate and model mass, energy and emission flows from different process and technological configurations, address alternative and combined approaches to biogas upgradation, such as with mesophilic and thermophilic bacteria, consider the socio-economic implications of the biogas to biomethane production chain, and more.

Additional sections discuss biosafety in relation to the technologies and processes discussed throughout the book and assess the sustainability of biomethane through lifecycle assessments and techno-economic and policy perspectives. This is an invaluable reference for researchers and industry engineers interested in biogas upgradation and the production of biomethane, and will be of interest to multidisciplinary teams working across Renewable Energy, Chemical Engineering, Environmental Science, and Sustainability Science.

• Presents a comprehensive examination of the latest upgradation routes of biogas to biomethane
• Provides step-by-step guidance on the modeling and simulation of biogas upgradation processes and technologies
• Analyzes the sustainability of biomethane production techno-economically, socially and environmentally

• Language: English
• Publisher: Elsevier Science
• Release date: Oct 19, 2023
• ISBN: 9780443184789

Book preview

Section 1

Basic understanding

Outline

Chapter 1 An introduction to biogas and biomethane

Chapter 2 Recent advances of biogas production

Chapter 1

An introduction to biogas and biomethane

Grzegorz Piechota¹, Natalia Generowicz², Ammar Ali Abd³, ⁴, Mohd Roslee Othman³, Alina Kowalczyk-Juśko⁵, Gopalakrishnan Kumar⁶ and Ashokkumar Veeremuthu⁷,    ¹GPCHEM, Laboratory of Biogas Research and Analysis, Toruń, Poland,    ²Mineral and Energy Economy Research Institute Polish Academy of Sciences, Cracow, Poland,    ³School of Chemical Engineering, Universiti Sains Malaysia, Nibong Tebal, Pulau Pinang, Malaysia,    ⁴College of Engineering, Al-Qasim Green University, Babylon, Iraq,    ⁵University of Life Sciences in Lublin, Lublin, Poland,    ⁶Institute of Chemistry, Bioscience, and Environmental Engineering, Faculty of Science and Technology, University of Stavanger, Stavanger, Norway,    ⁷Biorefineries for Biofuels & Bioproducts Laboratory, Center for Transdisciplinary Research, Department of Pharmacology, SDC, Saveetha Institute of Medical and Technical Sciences, Saveetha University, Chennai, India

Abstract

Biomethane is a pure form of biogas and in this pure form can be used a natural gas substitute. The biomethane is a natural gas formed in an anaerobic process which is the decomposition of organic matter without oxygen access. This chapter briefly introduces the biogas technology overview, technological aspects of biogas production such as anaerobic fermentation requirements and conditions of the fermentation processes. Moreover, the chapter introduces one of the most challenging aspects of biomethane feed-in: Biogas purification and upgrading technologies, climate changing aspects, biomethane contaminants, and VOSviewer literature overview.

Keywords

Biomethane; biogas; biomethane technology; biogas upgrading

1.1 Basics of biogas production

Biogas is a combustible mixture of gases produced by the natural wet fermentation of biomass under anaerobic conditions (anaerobic digestion). The main combustible component, methane, makes up about 50%–75% of the volume [1]. Hydrogen sulfide, oxygen, water vapor, carbon dioxide, and hydrogen are additional substances found in biogas (Table 1.1) [2]. Biogas is formed naturally in wetlands when organic matter is broken down by anaerobic bacteria into what is known as marsh gas [3].

Table 1.1

1.1.1 Feedstock preparation for biogas plants

The methane fermentation of biomass leads biogas production, and this biotechnological process necessitates both the appropriate material preparation and the appropriate environmental conditions inside the fermentation tank. All procedures performed up to the point where the material enters the digester are called substrate handling. This includes the delivery, storage, hygienization (optional), grinding and introduction of the materials into the fermentation process. All these pretreatment elements are key to the efficient operation of the plant. Furthermore, their correct handling is the key to a positive economic balance of a biogas investment [4].

1.1.1.1 Supply of feedstock to biogas plants

The first step in the general pretreatment of biogas substrates is their delivery to the plant. This is particularly important when using material from outside the company or farm.

It is then necessary to control the delivered substrates. Data concerning, among other things, the weight of the materials are required for billing and documentation purposes. The task of the person carrying out the reception is to perform a visual control.

This allows an initial assessment of the suitability of the substrate for biogas purposes (e.g., excessive amount of water in the solid substrate and visible mold). In addition, representative samples should be taken periodically to assess the conformity of the basic parameters of the material with the data recorded, among others, in the contract with the supplier [5].

1.1.1.2 Sorting and separation

Depending on the composition and origin of the substrates, foreign matter may need to be sorted and separated. The most common contaminants in the material stream entering the biogas plant are stones, wood, and metal components.

To separate them, pretanks are used, the bottom of which is periodically cleaned. In the case of other contaminants, manual separation on the conveyor belt can be used but is rarely applied in agricultural biogas plants [6].

1.1.1.3 Hygienization

It is crucial to carry out a hygienization process before providing slaughter waste to the fermentation tank because it is increasingly used as a substrate for biogas systems. The main purpose of substrate hygienization is to create an environment in which pathogenic and disease-causing microorganisms cannot function. Proper thermal treatment of waste occurs when the temperature in the entire content of the material is a minimum of 70°C for a period (without breaks) of at least 60 minutes [7].

1. The composition of the gas is affected by the kind of feedstock and the production process. The following technologies are used in biogas generation: Biodigesters: These are hermetic systems, such as containers or tanks, where naturally occurring microbes digest organic material diluted in water. Typically, moisture and contaminants are eliminated before using the biogas.

2. Landfill gas recovery systems: Municipal solid waste (MSW) breakdown occurs anaerobically in landfills, producing biogas. Pipes, extraction wells, and compressors that push the flow to a central collection point can all be used to collect this. Wastewater treatment plants: These treatment facilities are capable of recovering nutrients like nitrogen and phosphorus as well as organic matter and sediments from sewage sludge [8].

1.1.2 Conditions for the fermentation process in a biogas plant

Methane fermentation is a process in which anaerobic microorganisms decompose organic matter to produce methane and carbon dioxide. It can take place in both natural and anthropogenic ecosystems, but only in an environment completely oxygen-free and light-free and within a certain temperature range. The entire procedure is categorized into four stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Each stage calls for a certain set of environmental factors as well as the involvement of a particular kind of microorganisms. Bacterial enzymes break down organic polymers including proteins, lipids, and carbohydrates into soluble dimers and monomers called monosaccharides, fatty acids, and amino acids during the hydrolysis process [9].

The hydrolysis byproducts and compounds dissolved in water are transformed into short-chain organic acids, alcohols, aldehydes, carbon dioxide, and hydrogen during the acidogenesis phase, also known as acidification. Acidogenic bacteria are mainly responsible for this process. A characteristic side effect of this phase is a very unpleasant and intense odor of the resulting products.

In the acetogenesis step, higher acids are converted to acetic acid, carbon dioxide, and hydrogen. Thermodynamically, this is one of the most difficult stages. This is because what is required here is the syntrophy of acetanogens with methanogens, literally speaking eating together. This statement refers to the coexistence of organisms where one produces and the other consumes hydrogen [10].

Because all organic acid conversions are the source of around 25% of the acetate and 11% of the hydrogen, this stage is crucial for the generation of biogas. During methanogenesis, methane (CH4) is produced in association with methanogenic bacteria and CO2, and to a minimal extent hydrogen sulfide (H2S), ammonia (NH4), and water (H2O).

The hydrolysis and acidogenesis stages have the common name acid fermentation because the main products are H2, CO2, valerian acid, acetic acid, lactic acid, butyric acid, propionic acid, and ethyl alcohol. On the other hand, the processes of acetogenesis and methanogenesis are referred to as methanogenic fermentation because they are the processes that are responsible for methane production.

The rate at which intermediate products of one phase are synthesized is proportional to their distribution in the following phase in the transitions of methane fermentation. When the rates of transformation that take place in acidic and methanogenic fermentation are equal, the most beneficial process for the overall degradation of biomass is obtained [11].

Any modifications that slow down acidogenesis and hydrolysis have a detrimental impact on the progress of the next phases. This is due to the fact that it results in fewer intermediate materials being consumed by the bacteria throughout the succeeding phases of acetogenesis and methanogenesis. However, it does not have a tremendous impact on the inhibition of methanogenic fermentation. The action does occur, but with a negative effect, which is a smaller amount of methane in the final stage [12].

1.1.3 Conditions for the fermentation process

In particular climatic conditions, the methane fermentation process is aided by microbes, which significantly affect the activity and rate of transformation. At the same time, the pH of the environment, temperature, particle size, salinity, nutrient concentration, and moisture content are the conditions for the proper course of the process, which will increase the speed of the process and the quality and composition of the biogas produced.

1.1.3.1 Microorganisms

The basic microbial communities involved in the anaerobic breakdown of organic materials are as follows: acidogenic bacteria, acetate bacteria, and methanogenic bacteria.

Bacteria of the genera Bacillus, Pseudomonas, Clostridium, Bifidobacterium, and to a lesser extent Streptococcus, Enterobacterium, are the principal actors during the stage of hydrolysis and acidogenesis, or during acid fermentation. Acidic fermentation byproducts are converted into acetate and hydrogen by acetate bacteria like Syntrophomonas sp. and Syntrophobacter sp., which served as the precursor to the methanogenic bacteria. Both acetate and acidogenic bacteria are highly sensitive to environmental changes and have lengthy growth times. The morphology of methanogenic bacteria varies greatly; they might be found as rods, spirals, or granules. They are experts in ingesting and processing particular ingredients. These can be generated in between 15 and 18 hours. Their growth rate is heavily influenced by temperature, and it rises as the temperature does. Temperature between 35°C and 45°C and a pH of around 7 are the ideal conditions for methanogenesis. The amount and quality of biogas are adversely affected by sudden temperature changes of up to 2°C. Effective temperature regulation is essential to ensure the stability of each process, aiming for a consistent temperature level with a maximum daily temperature fluctuation of 1°C. For fermentation to proceed properly, the kinetic balance must be preserved during each of its several phases. Methanogenic bacteria are in charge of stopping the transformation process, and they require a favorable environment to do so. The action of the methanogenic bacteria may be hampered if any of the preceding three steps are disturbed, which would significantly reduce the amount of biogas produced [13].

1.1.3.2 Environmental conditions

Process pH

Depending upon the phase of methane fermentation, the process requires a pH between 4.5 and 7.5. Weak bases (ammonium hydroxide) and weak acids (volatile organic acids, carbonic acid, phosphoric acid, hydrogen sulfide) influence the system’s buffer capacity and pH level.

The presence and concentration of the buffering system components are influenced by the chemical makeup of the input substrates and the total load of organic matter in the chamber. Loading the chamber with waste rich in organic nitrogen compounds generates a large amount of ammonia, which affects the buffer capacity of the system. Maintaining a proper and constant pH value is possible due to the carbonate balance.

The entire fermentation process is controlled by the pH value. Since even minor changes can disrupt the proliferation of methanogenic bacteria that affects the pH, it is important to account for their growth. Depending on the phase, a different pH value is required, so 5.2–6.3 is suitable for sour fermentation and 6.8–7.2 for methanogenic fermentation. If the pH is too low, it can be raised by adding quicklime, sodium carbonate, calcium carbonate, or caustic soda. The best solution is to prevent the accumulation of volatile organic acids. Proper design of the process will significantly reduce unwanted acidification of the environment.

Nutrient compounds

The essential nutrients for preserving the correct viability of microorganisms are nitrogen, carbon compounds, phosphorus, sulfur, and other components. Nitrogen is an essential component for the synthesis of amino acids, proteins, or nucleic acids. Unfortunately, along with phosphorus and sulfur, it is one of the scarce components of the ecosystem. In an appropriate phase of conversion to ammonia, it neutralizes acids produced by fermentation microorganisms and contributes to maintaining the pH within 7.

The C:N ratio is another crucial factor. The ideal ratio is between 10:1 and 25:1, but some works in the literature suggest that the highest possible ratio of C:N is 100:3. The amount of biogas produced will decrease if the maximum value is exceeded because the methanogenic fermentation bacteria will use the excess nitrogen. Nitrogen will be released as ammonia and speed up the environment’s reaction if the quotient drops below the lowest value. The nitrogen balance will be disturbed, and the methanogenic bacteria will become poisonous. Ammonia (NH3) is more than 20 times more hazardous than ammonium ions, according to scientific evidence.

When the N:P:S ratio is 7:1:1, the COD:N quotient is between 400:7 and 1000:7, and C:N:P:S is between 600:15:5:1, these values are regarded as ideal. Together with the elements listed above that serve as the foundation for bacterial growth, the metabolism of bacteria also requires the trace elements copper, zinc, selenium, nickel, molybdenum, manganese, iron, magnesium, and calcium. Therefore, the quantity of agricultural waste with insufficient levels of these components does not need to change the composition of the