Biological Conversion Technologies Anaerobic Digestion

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Biological Conversion Technologies Anaerobic Digestion. Training on Technologies for Converting Waste Agricultural Biomass into Energy Organized by United Nations Environment Programme (UNEP DTIE IETC) 23-25 September, 2013 San Jose, Costa Rica. Surya Prakash Chandak - PowerPoint PPT Presentation


<ul><li><p>Biological Conversion Technologies Anaerobic DigestionTraining onTechnologies for Converting Waste Agricultural Biomass into EnergyOrganized by United Nations Environment Programme (UNEP DTIE IETC)</p><p>23-25 September, 2013San Jose, Costa Rica</p><p>Surya Prakash ChandakSenior Programme OfficerInternational environmental Technology CentreDivision of Technology, Industry and EconomicsOsaka, Japan</p></li><li><p>CONTENTIntroductionFermentationAnaerobic DigestionBasic ProcessFactors Influencing Biogas ProductionTypes of Biogas DigestersBiogas Yield*</p></li><li><p>INTRODUCTIONBiological ConversionConversion of the biomass to fuel by exposing biomass to certain microorganisms is called biological conversion. The secondary fuels are produced as a result of metabolic activity of the microorganisms. Fermentation and anaerobic digestions are the two most common biological conversion processes and products of these processes are ethanol and biogas.Today, ethanol is widely used as an alternative source of liquid fuels for the transport sector in countries and regions like USA, Brail, EU and China. Ethanol as a fuel offers many advantages such as high octane number (99) than petrol (80100), low emission. </p></li><li><p>INTRODUCTIONBiogas Biogas originates from the bio-degradation of organic material under anaerobic conditions. Today biogas has several applications such as industrial and household cooking, lighting, radiant heaters and incubators for agricultural purposes and absorption refrigerators. Biogas system provides a whole range of benefits for their users, the society and the environment in general.</p></li><li><p>INTRODUCTIONBiogas Benefits:Production of energy (heat, light, electricity),Transformation of organic waste into high quality fertilizer,Improvement of hygienic conditions via reduction of pathogens,worm eggs and flies,Increase of productivity, mainly for women, in firewood collection and cooking,Environmental advantages through protection of soil, water, air and woody vegetation,Micro-economical benefits through energy and fertilizer substitution, Additional income sources and increasing yields of animal husbandry and agriculture,Macro-economical benefits through decentralized energy generation, import substitution and environmental protection.</p></li><li><p>FERMENTATIONOverview Fermentation is a natural process initiated by microorganisms, similar to common yeast cultures, under anaerobic conditions. Ethanol can derive from any material which contains sugar. In the Fermentation process, sugar elements such as glucose, fructose and sucrose are converted into ethanol and carbon dioxide as metabolic waste products. The net chemical equation for the production of ethanol from glucose is:</p></li><li><p>FERMENTATIONBio-ethanolThe raw materials used in the production of ethanol via fermentation are mainly classified into three types as sugars, starches, and cellulose materials. Sugars (extracted from from sugarcane, sugar beets, molasses, and fruits) can be converted into ethanol directly. But starches (from corn, cassava, potatoes, and root crops) and cellulose (from wood, agricultural residues, waste sulfite liquor from pulp, and paper mills) are needed to pre-treat prior the fermentation. The final product, ethanol (C2H5OH) mixed with small amounts of methanol or higher alcohols and diluted in water.</p></li><li><p>FERMENTATIONBio-ethanol</p></li><li><p>ANAEROBIC DIGESTIONBasic ProcessA simplified stoichiometry for anaerobic digestion of biomass is;</p><p>The whole process of biogas production from organic wastes occurred in main three steps namely hydrolysis, acidification, and methane formation. Three types of bacteria namely fermentative, acetogenic and methanogenic are involved in hydrolysis, acidification and methane formation, respectively. </p></li><li><p>Basic ProcessThe three stage anerobic fermentaton of biomassANAEROBIC DIGESTION</p></li><li><p>ANAEROBIC DIGESTIONHydrolysisIn hydrolysis aerobic micro-organisms convert complex organic compounds (carbohydrates, proteins and lipids) into simple forms which are soluble and can be consumed by the micro-organisms. As an example, Polysaccharides are converted into monosaccharides, lipids to fatty acids, proteins to amino acids and peptides.AcidificationIn the second step, acid-producing bacteria (acetogenic bacteria), convert the intermediates of fermenting bacteria into mixture of acetic acid (CH3COOH), H2, CO2, alcohols, organic acids, amino acids and hydrogen sulphide. </p></li><li><p>ANAEROBIC DIGESTIONAcidificationThe oxygen requirement for producing acetic acid is fulfilled by the oxygen solved in the solution or bounded-oxygen. The acid-producing bacteria create an anaerobic condition which is essential for the methane producing microorganisms. Methane formationIn the third step, methane-producing bacteria utilize hydrogen, carbon dioxide and acetic acid formed in acidification process to form methane and carbon dioxide. </p></li><li><p>ANAEROBIC DIGESTIONFactors influencing the biogas productionSubstrate temperatureOptimal temperature range for biogas production: 20-28C. This can be achieved in a satisfactory level only where mean annual temperatures are around 20C or above or where the average daily temperature is at least 18C. If the temperature is below 15C, gas production will be so low that the biogas plant is no longer economically feasible.Changes in temperatureThe process is very sensitive to changes in temperature. Most of the biogas plants are builds in underground in order to overcome this issue. The temperature fluctuations between day and night are no great problem for plants built underground, since the temperature of the earth below a depth of one meter is practically constant.</p></li><li><p>ANAEROBIC DIGESTIONFactors influencing the biogas productionAvailable nutrientIn order to grow, bacteria need organic substances as a source of carbon and energy. In addition to carbon, oxygen and hydrogen, the generation of bio-mass requires an adequate supply of nitrogen, sulfur, phosphorous, potassium, calcium, magnesium and a number of trace elements such as iron, manganese, molybdenum, zinc, cobalt, selenium, tungsten, nickel etc. "Normal" substrates such as agricultural residues or municipal sewage usually contain adequate amounts of the mentioned elements. Higher concentration of any individual substance usually has an inhibitory effect, so that analyses are recommended on a case-to-case basis to determine which amount of which nutrients, if any, still needs to be added.</p></li><li><p>ANAEROBIC DIGESTIONFactors influencing the biogas productionRetention timeThe effective retention time may vary widely for the individual substrate constituents depending on the vessel geometry, the means of mixing, etc. Selection of a suitable retention time depends on process temperature as well as on the type of substrate used. For liquid manure undergoing fermentation, the following approximate values apply:liquid cow manure: 20-30 daysliquid pig manure: 15-25 daysliquid chicken manure: 20-40 daysanimal manure mixed with plant material: 50-80 daysIf the retention time is not maintained properly and it is too short, the bacteria in the digester are "washed out" faster than they can reproduce, and fermentation practically comes to a standstill. This problem rarely occurs in WAB systems.</p></li><li><p>ANAEROBIC DIGESTIONFactors influencing the biogas productionpH ValueThe best condition for the methane-producing bacteria is neutral to slightly alkaline conditions. Once the process of fermentation has stabilized under anaerobic conditions, the pH will normally take on a value of between 7 and 8.5. If the pH value drops below 6.2, the medium will have a toxic effect on the methanogenic bacteria.Nitrogen inhibitionNitrogen in the substrate inhibits the process of fermentation. Noticeable inhibition occurs at a nitrogen concentration of roughly 1700 mg ammonium-nitrogen (NH4-N) per liter substrate. The main prerequisite is that the ammonia level does not exceed 200-300 mg NH3-N per liter substrate. </p></li><li><p>ANAEROBIC DIGESTIONFactors influencing the biogas productionC/N ratioMicroorganisms required both nitrogen and carbon for assimilation into their cell structures. Various experiments have shown that the metabolic activity of methanogenic bacteria can be optimized at a C/N ratio of approximately 8-20, whereby the optimum point varies depending on the nature of the substrate.Substrate solid contentSolid content of the substrate impaired the mobility of the methanogens within the substrate. Therefore the biogas yield decreases with the increase of solids content. No generally valid guidelines can be offered with regard to specific biogas production for any particular solids percentage. </p></li><li><p>ANAEROBIC DIGESTIONTypes of Biogas PlantsThere are various types of biogas plants available in the world and they are classified mainly based on feeding method and construction. Based on the feed method, three different forms can be distinguished:Batch plants,Continuous plants,Semi-batch plants.Based on the construction, two main types of simple biogas plants can be distinguished:Fixed-dome plants,Floating-drum plants.</p></li><li><p>ANAEROBIC DIGESTIONTypes of Biogas PlantsBatch type digesters. In batch type plants, materials fed into the digester at a time and sealed only allowing the gas to exit and then emptied completely after a fixed retention time. Each design and each fermentation material is suitable for batch filling, but batch plants require high labor input. The major disadvantage of this type is unsteady gas-output. </p></li><li><p>ANAEROBIC DIGESTIONTypes of Biogas PlantsContinuous type digesters. Once the process started, regular quantity of waste are fed and regular quantity of material discharged, continuously. They empty automatically through the overflow whenever new material is filled in. Therefore, the substrate must be fluid and homogeneous. This technology is suitable for both medium and large scale waste treatment and large scale biogas production. Advantages of this type are constant and higher gas production</p></li><li><p>ANAEROBIC DIGESTIONTypes of Biogas PlantsSemi-batch type digesters. If the two materials which have completely different digestion rates (such as straw and dung) are to be digested together, a biogas plant can be operated on a semibatch basis. The slowly digested straw-type material is fed in about twice a year as a batch load. The dung is added and removed regularly.</p></li><li><p>ANAEROBIC DIGESTIONTypes of Biogas PlantsFixed Dome type digesters. </p><p>A fixed-dome plant comprises of a closed, dome-shaped digester with an immovable, rigid gas-holder and a displacement pit. </p></li><li><p>ANAEROBIC DIGESTIONTypes of Biogas PlantsFixed Dome type digesters. The gas is stored in the upper part of the digester. When gas production commences, the slurry is displaced into the displacement tank. Gas pressure increases with the volume of gas stored, i.e. with the height difference between the two slurry levels. If there is little gas in the gasholder, the gas pressure is low.The digesters of fixed-dome plants are usually masonry structures, structures of cement and ferro-cement exist. Main parameters for the choice of material are technical suitability (stability, gas- and liquid tightness), cost-effectiveness, availability in the region and transport costs and availability of local skills for working with the particular building material.</p></li><li><p>ANAEROBIC DIGESTIONTypes of Biogas PlantsFixed Dome type digesters. Currently, various types of fixed dome plants are available such as Chinese fixed dome plant, Janata model, Deenbandhu and CAMARTEC model. </p></li><li><p>ANAEROBIC DIGESTIONTypes of Biogas PlantsFixed Dome type digesters. 1. Mixing tank with inlet pipe and sand trap.2. Digester. 3. Compensation and removal tank. 4. Gasholder. 5. Gas pipe. 6. Entry hatch, with gastight seal. 7. Accumulation of thick sludge. 8. Outlet pipe. 9. Reference level. 10. Supernatant scum</p></li><li><p>ANAEROBIC DIGESTIONTypes of Biogas PlantsFloating drum type digesters. </p><p>Major difference between fixed dome and floating drum type plant is, a floating-drum plant consists of a floating gas-holder, or drum. </p></li><li><p>ANAEROBIC DIGESTIONTypes of Biogas PlantsFloating drum type digesters. This floats either directly in the fermenting slurry or in a separate water jacket. The drum in which the biogas collects has an internal and/or external guide frame that provides stability and keeps the drum upright. If biogas is produced, the drum moves up, if gas is consumed, the gasholder sinks back.Floating-drum plants are used mainly in continuous feed mode of operation. They are used most frequently by small- to middle-sized farms (digester size: 5-15m3) or in institutions and larger agro-industrial estates (digester size: 20-100m3) </p></li><li><p>ANAEROBIC DIGESTIONTypes of Biogas PlantsAdvantages of fixed dome type. Produce just as much gas as floating-drum plants, if they are gas-tight.Low cost operationSimple designLong life of the plant (20 years or more)Disadvantages of fixed dome type. Utilization of the gas is less effective as the gas pressure fluctuates substantially.Labor-intensive designNot easy to build. Difficult to achieve gas tightness.</p></li><li><p>ANAEROBIC DIGESTIONTypes of Biogas PlantsAdvantages of floating drum type. Simple operation Provide gas at a constant pressureStored gas-volume is immediately recognizable by the position of theDisadvantages of floating drum type. The steel drum is relatively expensive and maintenance-intensive.Removing rust and painting has to be carried out regularly. The life-time of the drum is short (up to 15 years; in tropical coastal regions about five years). </p></li><li><p>ANAEROBIC DIGESTIONBiogas YieldBiogas yield of a biomass material depends on the organic fraction of dry matter in the material and the waste management system associated with it. The dry matter (DM) of the waste is the matter left after removal of its moisture content. It may be obtained as the weight loss on heating to a temperature of 105 C. Whereas, Volatile Solids (VS) are defined as the organic fraction of dry matter in waste. Around 50-60% of the initial energy content in the organic material can be converted to biogas in a properly operated digester. </p></li><li><p>ANAEROBIC DIGESTIONBiogas YieldBiogas production after addition of substrate</p></li><li><p>ANAEROBIC DIGESTIONBiogas YieldThe resultant gas mixture consists of about 50 - 70%, CH4 and the rest is CO2 with small amounts of water vapours, H2S, NH3, and some organics that give bad odour.Methodology of Estimation:Amount of biogas that can be potentially produced from recoverable wastes= Amount of dry matter recoverable (kg DM/year) Volatile solids fraction in dry matter (kg VS/kg DM) Biogas yield (m3/kg VS)</p><p>Energy potential of the biogas recoverable (MJ /year) =Amount of biogas recoverable (m3/year) Heating value of biogas (MJ /m3)</p></li><li><p>ANAEROBIC DIGESTIONBiogas YieldThe heating value of the biogas depends on its composition, especially the amount of methane. HHV of methane is about 35.8 MJ/m3 and therefore biogas with 60% methane, HHV could be taken as 20 MJ/m3.Selected values for waste characteristics</p><p>Animal TypeFraction of Volatile Solid (VS/DM)Biogas Yield(m3/kg of VS)Cattle0.80.20 0...</p></li></ul>