ASG- Biogas

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    Assignment -3 Biogas by Waqar Ahmad (FA-10-RO7-009) Page 0

    Env504 Energy and Environment

    Assignment No-3

    BIOGAS

    By

    WAQAR AHMAD FA10-R07-009

    FACULTY: Dr Iftikhar Raja

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    Assignment -3 Biogas by Waqar Ahmad (FA-10-RO7-009) Page 1

    BIOGAS

    1. Introduction. There is an increased recognition, in both developing and

    industrial countries, of the need for technical and economic efficiency in the

    allocation and exploitation of resources. Systems for the recovery and utilization ofhousehold and community wastes are gaining a more prominent place in the world

    community. A new environmental agenda is emerging, which is now forcing itself on

    the attention of policy-makers and the public at large.One of the main environmentalproblems of todays society is the continuously increasing production of organic

    wastes. In many countries, sustainable waste management as well as waste

    prevention and reduction have become major political priorities, representing an

    important share of the common efforts to reduce pollution and greenhouse gasemissions and to mitigate global climate changes. Uncontrolled waste dumping is no

    longer acceptable today and even controlled landfill disposal and incineration of

    organic wastes are not considered optimal practices, as environmental standards

    thereof are increasingly stricter and energy recovery and recycling of nutrients and

    organic matter is aimed.Production of biogas through anaerobic digestion (AD) ofanimal manure and slurries as well as of a wide range of digestible organic wastes,

    converts these substrates into renewable energy and offers a natural fertilizer for

    agriculture. At the same time, it removes the organic fraction from the overall waste

    streams, increasing this way the efficiency of energy conversion by incineration of

    the remaining wastes and the biochemical stability of landfill sites. AD is a

    microbiological process of decomposition of organic matter, in the absence of

    oxygen, common to many natural environments and largely applied today to produce

    biogas in airproof reactor tanks, commonly named digesters. A wide range of micro-

    organisms are involved in the anaerobic process which has two main end products,

    biogas and digestate. Biogas is a combustible gas consisting of methane, carbon

    dioxide and small amounts of other gases and trace elements. Digestate is the

    decomposed substrate, rich in macro- and micro nutrients and therefore suitable to

    be used as plant fertilizer.1.1 The production and collection of biogas from a biological process was

    documented for the first time in United Kingdom in 1895. Since then, the process

    was further developed and broadly applied for wastewater treatment and sludge

    stabilisation. The energy crisis in the early 70s brought new awareness about the

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    use of renewable fuels, including biogas from AD. The interest in biogas has further

    increased today due to global efforts of displacing the fossil fuels used for energy

    production and the necessity of finding environmentally sustainable solutions for the

    treatment and recycling of animal manure and organic wastes.Biogas installations,processing agricultural substrates, are some of the most important applications of

    AD today. In Asia alone, millions of family owned, small scale digesters are in

    operation in countries like China, India, Nepal and Vietnam, producing biogas for

    cooking and lighting. Thousands of agricultural biogas plants are in operation in

    Europe and North America, many of them using the newest technologies within this

    area, and their number is continuously increasing.

    2. The Biochemical Process of AD. AD is a microbiological process ofdecomposition of organic matter in absence of oxygen. The main products of this

    process are biogas and digestate. Biogas is a combustible gas, consisting primarily

    of methane and carbon dioxide. Digestate is the decomposed substrate, resulted

    from the production of biogas. During AD, very little heat is generated in contrast to

    aerobic decomposition (in presence of oxygen), like it is the case of composting. The

    energy, which is chemically bounded in the substrate, remains mainly in the

    produced biogas, in form of methane. The process of biogas formation is a result oflinked process steps, in which the initial material is continuously broken down into

    smaller units. Specific groups of micro-organisms are involved in each individual

    step. These organisms successively decompose the products of the previous steps.

    The simplified diagram of the AD process highlights the four main process steps:

    Hydrolysis

    Acidogenesis

    Acetogenesis

    MethanogenesisThe process steps quoted run parallel in time and space, in the digester tank. The

    speed of the total decomposition process is determined by the slowest reaction of

    the chain. In the case of biogas plants, processing vegetable substrates containing

    cellulose, hemi-cellulose and lignin, hydrolysis is the speed determining process.

    During hydrolysis, relatively small amounts of biogas are produced. Biogas

    production reaches its peak during methanogenesis.

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    2.1 Hydrolysis. Hydrolysis is theoretically the first step of AD, during which the

    complex organic matter (polymers) is decomposed into smaller units (mono- and

    oligomers). During hydrolysis,polymers like carbohydrates, lipids, nucleic acids and

    proteins are converted into glucose, glycerol, purines and pyridines. Hydrolytic

    microorganisms excrete hydrolytic enzymes,converting biopolymers into simpler and

    soluble compounds. A variety of microorganisms is involved in hydrolysis, which is

    carried out by exoenzymes, produced by those microorganisms which decompose

    the undissolved particulate material. The products resulted from hydrolysis are

    further decomposed by the microorganisms involved and used for their own

    metabolic processes.

    2.2 Acidogenesis. During acidogenesis, the products of hydrolysis are

    converted by acidogenic (fermentative) bacteria into methanogenic substrates.

    Simple sugars, amino acids and fatty acids are degraded into acetate, carbon

    dioxide and hydrogen (70%) as well as into volatile fatty acids (VFA) and alcohols

    (30%).

    2.3 Acetogenesis. Products from acidogenesis, which cannot be directly

    converted to methane bymethanogenic bacteria, are converted into methanogenic

    substrates during acetogenesis. VFA and alcohols are oxidised into methanogenicsubstrates like acetate, hydrogen and carbon dioxide. VFA, with carbon chains

    longer than two units and alcohols, with carbon chains longer than one unit, are

    oxidized into acetate and hydrogen. The production of hydrogen increases the

    hydrogen partial pressure. This can be regarded as a waste product of

    acetogenesis and inhibits the metabolism of the acetogenic bacteria. During

    methanogenesis, hydrogen is converted into methane. Acetogenesis and

    methanogenesis usually run parallel,as symbiosis of two groups of organisms.

    2.4 Methanogenesis. The production of methane and carbon dioxide from

    intermediate products is carried out bymethanogenic bacteria. 70% of the formed

    methane originates from acetate, while the remaining 30% is produced from

    conversion of hydrogen (H) and carbon dioxide (CO2).Methanogenesis is a critical

    step in the entire anaerobic digestion process, as it is the slowest biochemical

    reaction of the process. Methanogenesis is severely influenced by operation

    conditions. Composition of feedstock, feeding rate, temperature, and pH are

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    examples of factors influencing the methanogenesis process. Digester overloading,

    temperature changesor large entry of oxygen can result in termination of methane

    production.

    3. AD Parameters. The efficiency of AD is influenced by some critical parameters,

    thus it is crucial that appropriate conditions for anaerobic microorganisms are

    provided. The growth and activity of anaerobic microorganisms is significantly

    influenced by conditions such as exclusion of oxygen, constant temperature, pH-

    value, nutrient supply, stirring intensity as well as presence and amount of inhibitors

    (e.g. ammonia). The methane bacteria are fastidious anaerobes, so that the

    presence of oxygen into the digestion process must be strictly avoided.

    3.1 Temperature. The AD process can take place at different temperatures, divided

    into three temperatureranges: psychrophilic (below 25 C), mesophilic (25C 45C),

    and thermophilic (45 C 70 C). There is a direct relation between the process

    temperature and the Hydraulic retention time (HRT). The temperature stability is

    decisive for AD. In practice, the operation temperature is chosen with consideration

    PROTEINS CARBOHYDRATES LIPIDS

    Hydrolysis

    AMINO ACIDS, SUGARS FATTY ACIDS, ALCOHOLS

    Fermentation INTERMEDIARY PRODUCTS(Propionate, Butyrate, etc.)

    Anaerobic oxidation

    ACETATE HYDROGENCARBON DIOXIDE

    Reductive methanogenesis

    Homoacetogenesis

    Aceticlastic methanogenesis

    METHANECARBON DIOXIDE

  • 8/2/2019 ASG- B

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