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Biogas Technology Notes describes basics of biomethanation, digestors for rural & wastewater treatment applications and mentions Indian text and references.

Biogas Technology Notes describes basics of biomethanation, digestors for rural & wastewater treatment applications and mentions Indian text and references.

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Biogas notes Biogas notes Document Transcript

  • NOTES ON BIOGAS TECHNOLOGYIntroductionProperties of biogasFeedstock for biogas: Aqueous wastes containing biodegradable organic matter,animal & agro - residuesMicrobial and biochemical aspects,Operating parameters for biogas production.Kinetics and mechanismDry and wet fermentation.Digesters for rural applicationHigh rate digesters for industrial waste water treatment. TEXT BOOKS AND REFERENCES 1. Biotechnology Volume 8, H.J. Rehm and G. Reed, 1986, Chapter 5, „Biomethanation Processes.‟ Pp 207-267 2. K. M. Mital, Non-conventional Energy Systems, (1997), A P H Wheeler Publishing, N. Delhi. 3. K. M. Mital, Biogas Systems: Principles and Applications, (1996) New Age International Publishers (p) Ltd, N. Delhi. 4. Nijaguna, B.T., Biogas Technology, New Age International publishers (P) Ltd., 2002, Reprinted in 2009References:1. Effluent Treatment & Disposal: I Ch. E, U.K., Symposium Series No 96, 1986,P 137-147, Application of anaerobic biotechnology to waste treatment and energyproduction, Anderson & Saw.2. „Anaerobic Rotating Biological Drum Contactor for the Treatment of Dairy Wastes‟,S. Satyanarayana, K. Thackar, S. N. Kaul, S.D. Badrinath and N.G. Swarnkar, IndianChemical Engineer, vol 29, No 3, July-Sept, 19873. Energy Environment Monitor, 12(1), 45-51,„Biomethanation Technologies inIndustrial Water Pollution Control‟ A. Gangagni Rao, Pune.4. „Biogas production from sugar mill sludge by anaerobic digestion and evaluation ofbio-kinetic coefficients‟, Tharamani. P, and Elangovan. R. Indian journal ofEnvironmental protection, 20, (10), 745-748, 2001. 1
  • 5. „Biogas Production Technology: An Indian Perspective‟, B. Nagamani and K.Ramasamy (TNAU), Current Science, Vol7, No1, pp 44-55 10th July, 19996. Khandelwal K. C. and Mahdi, “Bio-gas Technology”, Tata McGraw-Hill publ. Co.Ltd., New Delhi, 1986.7. State-of-the-art of anaerobic digestion technology for industrial wastewater treatment -KV Rajeshwari, M Balakrishnan, A Kansal, K Lata, … - Renewable and SustainableEnergy Reviews, 2000 – Elsevier8. Anaerobic digestion technologies for energy recovery from industrial wastewater - astudy in Indian context, Arun Kansal, K V Rajeshwari, Malini Balakrishnan, KusumLata, V V N Kishore, TERI Information Monitor on Environmental Science 3(2): 67–75,9. Biogas Purification and Bottling into CNG Cylinders: Producing Bio-CNG from Biomass for Rural Automotive Applications, Virendra K. Vijay1,*, Ram Chandra1, Parchuri M. V. Subbarao2 and Shyam S. Kapdi3 1Centre for Rural Development and Technology, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi – 110 016, The 2nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)” C-003 (O) 21-23 November 2006, Bangkok, Thailand10. Biogas scrubbing, compression and storage: perspective and prospectus in Indian Context, S.S. Kapdi, V.K. Vijay*, S.K. Rajesh, Rajendra Prasad, Centre for Rural Development and Technology, Indian Institute of Technology, New Delhi 110 016GTZ project Information and Advisory Service on Appropriate Technology (ISAT)for the ISAT Website in a collaborative effort of the following institution:Information and Advisory Service on Appropriate Technology (ISAT)GATE in Deutsche Gesellschaft für Technische Zusammenarbeit (GTZ), GmbH(German Agency for Technical Cooperation)Post Box 5180D-65726 EschbornFederal Republic of GermanyTelephone: +49 6196/79-0Fax: +49 6196/797352E-mail: gate-isat@gtz.de 2
  • K. M. Mital, Biogas Systems: Principles and Applications, (1996) New Age InternationalPublishers (p) Ltd, N. Delhi.Contents:1. An Overview of Biogas Technology2. Microbiology of Anaerobic Digestion3. Properties of Biogas and Methods For Its Purification4. A Compendium of Biogas Plant Design5. Design, Construction, Operation and Maintenance of Biogas Plants6. Analysis of Factors Affecting Biogas Yield7. Biogas Yield from Different Organic Wastes8. Biogas Yield from Water Weeds9. Biogas Generation from Industrial Wastes10. Biogas Recovery from Sanitary Landfills11. Applications and Usage Of Biogas12. Potential of Biogas Plant Effluent As Enriched Fertilizer.13. Approaches For Implementing Biogas Program Areas For Further Research AndConcluding ObservationsBiogas Technology by B. T. NijagunaContentsIntroduction,Materials for Biomethanation andProducts of Methanation,Kinetics and Physico-Chemical Factors Affecting Biogasification,Bio-reactors, Design, Selection, Construction and operation of Biogas Plants,Purification, Scrubbing, Compression and Storage of Biogas,Utilization Systems of Biogas,Ethanol. 3
  • INTRODUCTION: Production of a combustible gas by anaerobic digestion of aqueous organic matterby mixed bacterial culture involving methane producers is called „biomethanation‟ andthe product is called „biogas‟PROPERTIES OF BIOGAS:Composition: 60 to 70 per cent Methane, 30 to 40 per cent carbon dioxide, traces ofhydrogen sulfide, ammonia and water vapor.It is about 20% lighter than air (density is about 1.2 gm/liter).Ignition temperature is between 650 and 750 C.Calorific value is 18.7 to 26 MJ/ m3 (500 to 700 Btu/ ft3.)Calorific value without CO2: is between 33.5 to35.3 MJ/ m3Explosion limit: 5 to 14 % in air.Removal of CO2: Scrubbing with limewater or ethanol amine solution.Removal of H2S: Adsorption on a bed of iron sponge and wood shavings.Air to Methane ratio for complete combustion is 10 to 1 by volume.One cubic meter of biogas is equivalent to 1.613 liter of kerosene or 2.309 kg of LPG or0.213 kW of electricity. 4
  • WHY Biomethanation in villages?  COOKING  LIGHTING  FUEL FOR DUNG BIOGAS KILN PLANT BIOGAS  FURNACEWATER ETC. PURIFY  I. C. ENGINE + PUMP OR TO I. C. ENGINE + COMPOST PIT GENERATOR (MANURE)  ENERGY RECOVERY, CLEAN BURNING  SUBSTITUTES FUELWOOD & DUNG CAKE AS RURAL FUEL  HYGIENIC DISPOSAL OF ANIMAL WASTE  CONSERVATION OF MANURE VALUE  MILD CONDITIONS: 30o C, pH 6.8-7.2, FEED ONCE A DAY  BURNER, MANTLE LAMP AVAILABLE; EASY GAS PURIFICATION FEASIBLE  SUBSIDY IS AVAILABLE FOR RURAL FARM OR FAMILY SIZE PLANT  DUAL FUEL ENGINE CAN PUMP WATER, GENERATE POWER  BIOGAS TECHNOLOGY: SIMPLE & INDIGENOUS 5
  • WET ORGANIC WASTE AS FEED FOR BIOGAS PLANTANIMAL WASTES: Excreta of cow, pig, chicken etcMANURE, SLUDGE: Canteen and food processing waste, sewageMUNICIPAL SOLID WASTE: After separation of non-degradableWASTE STARCH & SUGAR SOLUTIONS: Fruit processing, brewery, press mudfrom sugar factory etcOTHER INDUSTRIAL EFFLUENTS (B O D): pulp factory waste liquor,leather industry waste, coal washery wastewater etc.Commonly Used Feed for Biomethanation:Animal Wastes,Crop Residues,Urban Wastes,Food and Agro - Industry Wastes. (Mital, Ch. 7 To 10) 6
  • MICROBIOLOGIAL ASPECTS OF BIOMETHANATION The biomethanation of organic matter in water is carried out in absence of dissolvedoxygen and oxygenated compounds like nitrate and sulphate. The mixed groups ofbacteria are naturally occurring in the cow dung slurry and decomposition in three stagesfinally produces a gas mixture of methane and carbon dioxide. Initially larger moleculesare hydrolysed to simpler molecules which in turn are decomposed to volatile fatty acidslike acetic acid, propionic acid etc. by a second set of bacteria. Methane forming bacteriacan convert acetic acid, hydrogen and carbon dioxide and produce methane. HYDRLYSIS OF BIOPOLYMERS TO MONOMERS CONVERSION OF SUGARS, AMINO ACIDS, FATTY ACIDS TO HYDROGEN, CO2, AMMONIA AND ACETIC, PROPIONIC AND BUTIRIC ACIDS CONVERSION OF H2, CO2, ACETIC ACID TO CH4 AND CO2 MIXTUREFigure 1: The process of methanogenesis (After GTZ, 1999) 7
  • Methanogenesis is a microbial process, involving many complex, and differentlyinteracting species, but most notably, the methane-producing bacteria. The biogas processis shown below in figure 1, and consists of three stages; hydrolysis, acidification andmethane formation.In the first stage of enzymatic hydrolysis, the extracellular enzymes of microbes, such ascellulase, protease, amylase and lipase externally enzymolize organic material. Bacteriadecompose the complex carbohydrates, lipids and proteins in cellulosic biomass intomore simple compounds. During the second stage, acid-producing bacteria convert thesimplified compounds into acetic acid (CH3COOH), hydrogen (H2), and carbon dioxide(CO2). In the process of acidification, the facultatively anaerobic bacteria utilize oxygenand carbon, thereby creating the necessary anaerobic conditions necessary formethanogenesis. In the final stage, the obligatory anaerobes that are involved in methaneformation decompose compounds with a low molecular weight, (CH3COOH, H2, CO2), toform methane (CH4) and CO2 .The resulting biogas, sometimes referred to as gobar gas, consists of methane and carbondioxide, and perhaps some traces of other gases, notably hydrogen sulphide (H2S). Itsexact composition will vary, according to the substrate used in the methanogenesisprocess, but as an approximate guide, when cattle dung is a major constituent offermentation, the resulting gas will be between 55-66% CH4, 40-45% CO2, plus anegligible amount of H2S and H2 (KVIC, 1993). Biogas has the advantage of a potentialthermal efficiency, given proper equipment and aeration, of 60%, compared to wood anddung that have a very low thermal efficiency of 17% and 11% respectively (KVIC,1993).Methanogenesis or more particularly, the bacteria involved in the fermentation processare sensitive to a range of variables that ultimately determine gas production, and it isworth briefly outlining these factors. Temperature is perhaps the most criticalconsideration. Gasification is found to be maximized at about 35oC, and below thistemperature, the digestion process is slowed, until little gas is produced at 15oC andunder. Therefore in areas of temperature changes, such as mountainous regions, or winterconditions that may be more accentuated inland, mitigating factors need to be taken into 8
  • account, such as increased insulation (Kalia, 1988), or the addition of solar heaters tomaintain temperatures (Lichtman, 1983).Loading rate and retention period of material are also important considerations. In theKVIC model, retention ranges between 30-55 days, depending upon climatic conditions,and will decrease if loaded with more than its rated capacity (which may result inimperfectly digested slurry). KVIC state that maximum gas production occurs during thefirst four weeks, before tapering off, therefore a plant should be designed for a retentionthat exploits this feature. Retention period is found to reduce if temperatures are raised, ormore nutrients are added to the digester. Human excreta, due to its high nutrient content,needs no more than 30 days retention in biogas plants (KVIC, 1983).Various factors such as biogas potential of feedstock, design of digester, inoculum, natureof substrate, pH, temperature, loading rate, hydraulic retention time (HRT), C : N ratio,volatile fatty acids (VFA), etc. influence the biogas production.Meher et al. reported that the performance of floating dome biogas plant was better thanthe fixed dome biogas plant, showing an increase in biogas production by 11.3 per cent,which was statistically significant. Furthermore, the observed reduction in biogas yieldwas due to the loss of gas from the slurry-balancing chambers of fixed dome plant.Dhevagi et al. used different feedstocks like cow dung, buffalo dung, dry animal waste,stray cattle dung, goat waste, and poultry droppings for their biomethanation potentialand observed that poultry droppings showed higher gas production. Earlier Yeole andRanade compared the rates of biogas yield from pig dung-fed and cattle dung-feddigesters and reported that the biogas yield was higher in the former. They attributed thishigher biogas yield to the presence of native microflora in the dung. Shivraj andSeenayya reported that digesters fed with 8 per cent TS of poultry waste gave betterbiogas yield, and attributed the lower yield of biogas at higher TS levels to high ammoniacontent of the slurry. 9
  • BIOLOGICAL MODELING 10
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  • The modeling and its simulation referred to is from the following paper: 13
  • Operating parameters affecting the biogas production: 1. Temperature is an important parameter. Mesophilic methane producing bacteria grow at an optimum temperature of 35oC the gas production rate drops very much when temperture is less than 10oC. 2. pH range of the waste water should be in the range of 6.8 to 7.8 as excess acid state hampers the methane producing bacteria and the balance of nutrients is disturbed. 3. Ratio of carbon to nitrogen in the waste water influent or C/N ratio is 30:1 and if nitrogen content in ammoniacal form is less the bacterial growth is affected and the process slows down. 4. Proportion of solids to water: This is found to be not more than 10 per cent for optimum operation of digester to ensure sufficient decomposition of „volatile solids‟ and rate of production of gas. 5. Retention time: The ratio of volume of slurry in the digester to the volume fed into and removed from it per day is called retention time. Thus a 20 liter digester is fed at 4 liters per day so that the volume of digester is constant the retention time is 5 days. The required retention time is normally 30 days for mesophilic (25-35oC) conditions. 6. Volumetric organic loading rate: This can be expressed as kg Vs per volume per day based on the % weight of organic matter added each day to the digester volume. Digester loading rate % = (Per cent of organic matter in feed)/(Retention Time) Loading rate range is 0.7 to25 kg VS/ m3 / Day 14
  • Kinetics of anaerobic fermentationSeveral kinetic models have been developed to describe the anaerobic fermentationprocess. Monod101 showed a hyperbolic relationship between the exponential microbialgrowth rate and substrate concentration. In this model, the two kinetic parameters,namely, microorganisms growth rate and half velocity constant are deterministic innature, and these predict the conditions of timing of maximum biological activity and itscessation. This model can be used to determine the rate of substrate utilization (rS) by theequation:rS = qmax ´ Sx/K + S,where S is limiting substrate concentration, K is half constant, x is concentration ofbacterial cells, and qmax is maximum substrate utilization rate.The above equation is applicable for low substrate concentration.However for high substrate concentration, the equation is re-written as:rS = qmax · x.The Monod model suffers from the drawback that one set of kinetic parameters are notsufficient to describe biological process both for short- and long-retention times, and thatkinetic parameters cannot be obtained for some complex substrates. To alleviatelimitations of the Monod model while retaining its advantages, Hashimoto102 developedan alternative equation, which attempts to describe kinetics of methane fermentation interms of several parameters. According to this equation, given below, for a given loadingrate So/q daily volume of methane per volume of digester depended on thebiodegradability of the material (Bo) and kinetic parameters µm and K.rV = (Bo ´ So/q ) · {1– (K/q µm – 1 + K)}where,rv is volumetric methane production rate, l CH4 l– 1digester d– 1So is influent total volatile solids (VS) concentration,g l– 1Bo is ultimate methane yield, l CH4 g– 1 VS added as q 15
  • q is hydraulic retention time d– 1µm is maximum specific growth of microorganism d– 1K is kinetic parameter, dimensionless. ******KINETICS OF ANAEROBIC FERMENTATION (Reference: Mital, pp 36-39):Rate of substrate Utilization, rs = Qmax * (Sx) / (K+S) ---(1)Where S is limiting substrate concentration K is half life constant X is concentration of bacterial cells Qmax is maximum substrate utilization rateFor low substrate concentration, this equation is valid. For high substrate concentration, itbecomes as follows: rs = Qmax*x ----(2)The above model known as Monod model has limitations. For complex substrates, kineticparameters cannot be obtained for the entire concentration range.Chen and Hashimoto, Biotechnology Bio-engineering Symposium 8, (1978) p 269-282 and Biotechnology Bioengineering (1982) 24: 9-23Volumetric methane rate in cubic meter gas per cubic meter of digester volume V = (Bo So / HRT)[1- K / (HRT*m-1+K)]Bo = Ultimate methane yield in cubic meters methane (Varies from 0.2 to 0.5)So = Influent volatile solids concentration in kgVS/m3 (Loading rate range = 0.7 to 25 kg VS/m3 d)HRT = Hydraulic retention time in days 0.06 SoK = Dimensionless kinetic parameter, for cattle dung, K= 0.8+ 0.0016e 16
  • m = Maximum specific growth rate of the microorganism in day-1 Different types of biogas plant recognized by MNES (Ministry of Non- Conventional Energy Sources). After Gate, 1999. 1. Floating-drum plant with a cylinder digester (KVIC model). 2. Fixed-dome plant with a brick reinforced, moulded dome (Janata model). 3. Floating-drum plant with a hemisphere digester (Pragati model). 4. Fixed-dome plant with a hemisphere digester (Deenbandhu model). 5. Floating-drum plant made of angular steel and plastic foil (Ganesh model). 6. Floating-drum plant made of pre-fabricated reinforced concrete compound units. 7. Floating-drum plant made of fibreglass reinforced polyester. 17
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  • RURAL DIGESTERS ACCEPTED BY MNES: (Digesters for rural application)1 KVIC (FLOATING DOME)  MASONRY CYLINDRICAL TANK  ON ONE SIDE INLET FOR SLURRY  OTHER SIDE OUTLET FOR SPENT SLURRY  GAS COLLECTS IN INVERTED ‘DRUM’ GAS HOLDER OVER SLURRY  GAS HOLDER MOVES UP & DOWN DEPENDING ON ACCUMULATION OF GAS /DISCHARGE OF GAS, GUIDED BY CENTRAL GUIDE PIPE  GAS HOLDER (MILD STEEL): PAINTED ONCE A YEAR.  K V I C Mumbai  MEDIUM FAMILY SIZE BIOGAS PLANT HAVING GAS DELIVERY OF 3 M3 /DAY REQUIRES 12 HEAD OF CATTLE AND CAN SERVE A FAMILY OF 12 PERSONS TECHNICAL DETAILS OF A 3 M3 /DAY BIOGAS PLANT OF FLOATING DRUM DESIGNName of the model KVIC Model 3Size for 3m / day gas delivery 4.15m high, 1.6m dia, Volume 8.34m3 Inlet pipe 0.1m dia, 4m long Inlet tank 0.75m dia, 1m high Outlet pipe 0.1m dia, 1.1 m longRetention period 30 to 50 daysGas Holder 1.5 m dia, 1m highConstruction of gas holder MS sheet & angles, fabricated.Constr. & layout, digester Brick, cement, digester below G. level 19
  • 2. JANATHA (FIXED DOME) inlet BIOGAS outlet  DIGESTER WELL BELOW GROUND LEVEL  FIXED DOME GAS HOLDER BUILT WITH BRICK & CEMENT  BIOGAS FORMED RISES PUSHES SLURRY DOWN  DISPLACED SLURRY LEVEL PROVIDES PRESSURE-UPTO THE POINT OF ITS DISCHARGE/ USE3 DEENABANDU (FIXED DOME, MINIMISES SURFACE AREA)  FIXED DOME PLANT, MINIMISES SURFACE AREA BY JOINING THE SEGMENTS OF TWO SPHERES OF DIFFERENT DIAMETERS AT THEIR BASES  FIXED MASONRY DOME REQUIRES SKILLED WORKMANSHIP AND QUALITYMATERIALS TO ELIMINATE CHANCE OF LEAKAGE OF GAS  AFPRO, 25/1A, Institutional Area, D block, Panka Rd, Janakpury, N.Delhi. 20
  • 4 PRAGATI  COMBINES FEATURES OF KVIC & DEENABANDU, MAHARASHSTRA  LOWER PART: SEMI-SPHERICAL IN SHAPE WITH A CONICAL BOTTOM  UPPER PART: FLOATING GAS HOLDER  POPULARISED IN MAHARASHTRA, UNDARP, PUNE5 FERROCEMENT DIGESTER:  CAST SECTIONS, MADE FROM A REINFORCED (MORTAR+WIRE MESH)- COATED WITH WATER PROOFING TAR  S E R I, ROORKEE6 FRP DIGESTER:  FIBER REINFORCED PLASTIC MADE BY CONTACT MOULDING PROCESS7 UTKAL / KONARK DIGESTERReference: „Konark biogas plant-A user friendly model‟ Mohanty, P.K., and Choudury,A. K, (Orissa Energy Dev. Agency), Journal of Environmental Policy and Studies 2(1);15-21Konark Biogas plant:  SPHERICAL IN SHAPE WITH GAS STORAGE CAPACITY OF 50%  CONSTRUCTION COST IS REDUCED AS IT MINIMIZES SURFACE AREA  BRICK MASONRY OR FERROCEMENT TECHNOLOGY  A PERFORATED BAFFLE WALL AT THE INLET PREVENTS SHORT CIRCUITING PATH OF SLURRY (OPTIONAL) 21
  • 8 FLEXIBLE PORTABLE NEOPRENE RUBBER MODEL:  FOR HILLY AREAS, MINIMIZES TRANSPORT COST OF MATERIALS  BALLOON TYPE, INSTALLED ABOVE GL, MADE OF NEOPRENE RUBBER  FOR FLOOD PRONE AREAS, UNDERGROUND MODELS NOT SUITABLE SWASTHIK COMPANY OF PUNE DESIGN 22
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  • HIGH RATE DIGESTERS FOR WASTE WATER TREATMENT: 1. ANAEROBIC FILTER (UPFLOW and DOWNFLOW) 2. UPFLOW ANAEROBIC SLUDGE BLANKET DIGESTER( UASB) 3. ANAEROBIC LIQUID FLUIDISED/ EXPANDED BED DIGESTER 4. ANAEROBIC ROTATING DISC CONTACTING DIGESTER 5. ANAEROBIC MEMBRANE DIGESTER 6. ANAEROBIC CONTACT DIGESTEREffluent Treatment & Disposal: I Ch. E, U.K., Symposium Series No96,1986., P 137-147, Application of anaerobic biotechnology to wastetreatment and energy production Anderson & Saw.Energy & Environment Monitor, 12(1) 45- 51, „BiomethanationTechnologies in industrial water pollution Control‟ A.Gangagni Rao, Pune.Techniques for enhancing biogas productionDifferent methods used to enhance biogas production can be classified intothe following categories:(i) Use of additives.(ii) Recycling of slurry and slurry filtrate.(iii) Variation in operational parameters like temperature, Hydraulicretention time (HRT) and particle size of the substrate.(iv) Use of fixed film / biofilters. 25
  • HIGH RATE DIGESTERS FOR WASTE WATER TREATMENT:1 ANAEROBIC FILTER (UPFLOW and DOWNFLOW)  ANAEROBIC FILTER CONTAINS A SOLID SUPPORT OR PACKING MATERIAL IT WAS DEVELOPED BY YOUNG & MC CARTHY IN 1967  WASTEWATER FLOWS FROM BOTTOM UPWARDS THROUGH THE PACKING, GAS SEPARATES, BACTERIA ARE RETAINED MOSTLY IN SUSPENDED FORM,HRT RANGE OF 0.5 TO12 DAYS IS OBTAINED  SINCE SUSPENDED GROWTH TENDS TO COLLECT NEAR THE BOTTOM OF THE REACTOR, ACTIVITY IS HIGHER THERE.  TYPICAL ORGANIC LOADING RATE OF 1 TO 40 KG COD/ M3/DAYAND A SRT OF 20 DAYS IS ACHIEVED.  AVOIDANCE OF PLUGGING DUE TO ACCUMULATION OF SOLIDS IN THE PACKING MATERIAL AND ENSURING AN ADEQUATE FLOW DISTRIBUTION IN THE BOTTOM OF THE REACTOR ARE THE LIMITATIONS OF THIS. 26
  • 2 UPFLOW ANAEROBIC SLUDGE BLANKET DIGESTER (UASB)  UASB REACTOR IS BASED ON SUPERIOR SETTLING PROPERTIES OF THE SLUDE  INFLUENT FED INTO THE REACTOR FROM BELOW LEAVES AT THE TOP VIA AN INTERNAL BAFFLE SYSTEM FOR SEPARATION OF THE GAS, SLUDGE AND THE LIQUID  GAS SEPARATED FROM SLUDGE, COLLECTED BENEATH PLATES  IN QUIET SETTLING ZONE, SLUDGE SEPARATES, SETTLES BACK TOWRDS DIGESTION ZONE.  ORGANIC LOADING RATES OF 10 TO 30 KG COD /M3 DAY  REACTOR MIXING SHOULD BE ONLY BY THE GAS PRODUCTION  HRTRANGE OF 0.5 TO 7 DAYSS IS FEASIBLE WITH EXCEL. SETTLING SLUDGE AND A SRT OF 20 DAYS(AT 35 0 C)  REF: TIDE, VOL9, NO4, DEC.1999, PAGE 232 27
  • 3 ANAEROBIC LIQUID FLUIDIZED/ EXPANDED BED DIGESTER  ACTIVE BIOMASS IS ATTACHED TO SURFACE OF SAND PARTICLES THAT ARE KEPT IN SUSPENSION BY UPWARD VELOCITY OF LIQUID FLOW  DEGREE OF BED EXPANSION IN EXPANDED BED IS 10-20% AND IN FLUIDIZED BED IT IS 30-100%  BIOMASS RETENTION IN THE REACTOR IS EFFICIENT ,SRT OF 30 DAYS  PARTICLES PROVIDE LARGE SURFACE AREA FOR MICROBIAL GROWTH AND BETTER MIXING COMPARED TO PACKED BED, HRT RANGE OF 0.2 TO 5.0 DAY ACIEIVED.  TYPICAL RANGE OF LOADING RATE OF 1 TO 100 KG COD/M3 /DAY  REF: COMPREHENSIVE BIOTECHNOLOGY-MURRAY MOO YOUNG, VOL 4, PAGES 1017-1027. 28
  • 4. ANAEROBIC ROTATING BIOLOGICAL DISC CONTACTOR ANAEROBIC ROTATING BIOLOGICAL CONTACTOR CONSISTS OF ASERIES OF DISCS OR MEDIA BLOCKS MOUNTED ON A SHAFT WHICH ISDRIVEN SO THAT THE MEDIA ROTATES AT RIGHT ANGLES TO THEFLOW OF SEWAGE. THE DISCS OR MEDIA BLOCKS ARE NORMALLYMADE OF PLASTIC (POLYTHENE, PVC, EXPANDED POLYSTYRENE) ANDARE CONTAINED IN A TROUGH OR TANK SO THAT ABOUT 40% OFTHEIR AREA IS IMMERSED.Reference Article:„Anaerobic Rotating Biological Drum Contactor for the Treatment of DairyWastes‟, S. Satyanarayana, K. Thackar, S.N.Kaul, S.D.Badrinath and N.G.Swarnkar, (NEERI) Indian Chemical Engineer, vol 29, No 3, July-Sept,1987. 29
  • 5. ANAEROBIC MEMBRANE DIGESTER  SUSPENDED GROWTH REACTOR, COMBINED WITH A SEPARATOR  EXTERNAL ULTRA / MICROFILTRATION MEMBRANE UNIT FOR SOLID-LIQUID SEPARATION  PERMEATE BECOMES THE EFFLUENT AND THE BIOMASS IS RETURNED TO THE REACTOR  MEMBRANE UNIT ROVIDES POSITIVE BIOMASS RETENTION AND PARTICULATE FREE EFFLUENT 30
  • 6 ANAEROBIC CONTACT DIGESTER  BIOMASS SETTLED IN A SECOND TANK, RECYCLED TO THE DIGESTER.  RECYCLE GIVES HIGHER SRT AND EFFICIENCY  MIXING IN THE FIRST TANK AND EFFICIENCY OF SETTLING IN THE SECOND TANK IMPROVES PERFORMANCE.  REQUIRE HRT OF 10 DAYS OR MORE. 31
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  • Comments on Rural biogas plants:Biogas has shown to be a useful component in the rural economy in India,though its application is logistically difficult. Ill-co-ordinated disseminationhas led to high rates of non-functioning plants, and may endanger furtheruptake, as such, its status as a fuel remains marginal.Participation in biogas technology varies across socio-economic groups, andacross regions. Despite a well-intentioned attempt to cater for the energyneeds of rural India, and particularly the poor, as defined by scheduled casteand scheduled tribe, the biogas programme has not appeared to meet theseneeds on any meaningful scale, through insurmountable constraintsassociated with their very marginality, paradoxically. Limited success hasoccurred in other agricultural groups.Further, the essential commodification of dung, which has occurred sincethe introduction of biogas systems may impact detrimentally upon thepoorest families, who may experience a scarcity of the fuel once gathered forfree. The need to provide rural India with a viable and sustainable source offuel has perhaps never been more urgent, yet curiously, this is not reflectedin current literature, as biogas seemingly drops out of journals in the 1990s,as a subject to be written about. Therefore, the very current situationregarding the status of biogas technology in India is unknown, thoughdissemination is still being undertaken. Bapus (Gandhis) dream thereforeremains largely unrealized, though small steps may have been achieved. 34
  • 1. What properties of biogas have to be improved before it is used as an engine fuel?2. Write short notes on (i) Feedstock for biogas, (ii) Dry and wet fermentation, (iii) Microbial and biochemical aspects.3. Discuss the operating parameters for biogas production by anaerobic digestion.4. What criteria are applied in selecting a rural biogas plant of a small family size?5. Why biogas is not supplied in cylinders like LPG? Can we use same stove for both?6. Explain hydraulic and solid retention time for a fixed film biogas digester.7. In a flood prone area, what type of small biogas plant would you use? 35
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  • Reduction of the greenhouse effectLast but not least, biogas technology takes part in the global struggle againstthe greenhouse effect. It reduces the release of CO2 from burning fossil fuelsin two ways. First, biogas is a direct substitute for gas or coal for cooking,heating, electricity generation and lighting. Additionally, the reduction in theconsumption of artificial fertilizer avoids carbon dioxide emissions thatwould otherwise come from the fertilizer producing industries. By helping tocounter deforestation and degradation caused by overusing ecosystems assources of firewood and by melioration of soil conditions biogas technologyreduces CO2 releases from these processes and sustains the capability offorests and woodlands to act as a carbon sink.Methane, the main component of biogas is itself a greenhouse gas with amuch higher “Greenhouse potential" than CO2. Converting methane tocarbon dioxide through combustion is another contribution of biogastechnology to the mitigation of global warming. However, this holds trueonly for the case, that the material used for biogas generation wouldotherwise undergo anaerobic decomposition releasing methane to theatmosphere. Methane leaking from biogas plants without being burnedcontributes to the greenhouse effect! Of course, burning biogas also releasesCO2. But this, similar to the sustainable use of firewood, does only returncarbon dioxide which has been assimilated from the atmosphere by growingplants maybe one year before. There is no net intake of carbon dioxide in theatmosphere from biogas burning as it is the case when burning fossil fuels. 38
  • Different Purification Processes:-1) Removal of H2S -The gas coming out of system is heated to 150 degree Cand over ZnO bed, maintained at 1800 C leaving process gas free ofH2S.ZnO + H2S = ZnS + H2O.ZnSO4 + 2NaOH = Zn (OH) 2 + Na2SO42) Removal of CO2 –CO2 is high corrosive when wet and it has no combustionvalue so its removal is must to improve the biogas quality.The processes to remove CO2 are as follows –a) Caustic solution, NAOH – 40%NAOH + CO2 = NAHCO3b) Renfield process – K2CO3 - 30 %K2CO3 + CO2 = 2KCO33) Removal of NH3:-The chemical reaction is as:NH3 + HCL =NH4Cl4) Removal of H2O:-For the removal of moisture, pass the gas from abovereaction, through the crystals of white silica gel. 39
  • BIOGAS in INTERNAL COMBUSTION ENGINE1. S. I. EnginesThe only adoption for a spark ignition engine is a gas (notgasoline!) carburetor to work at the supply pressure (just like an LPGconversion, but an evaporator would not be needed as the storage pressure islow). It is also a good idea to scrub the H2S (as it causes corrosion) and toderate the engine (unless you want to replace it each year if operatingcontinuously).Modification of S.I. Engine -S.I. engines can run completely on biogas, however, the engines are requiredto be started on petrol at the beginning, conversion of S.I. engine for theentry of biogas, throttling of intake air & advancing the ignition timing.Biogas can be admitted to S.I. engine through the intake manifold & airflow control valve can be provided on the air cleaner pipe connecting aircleaner & carburetor for throttling the intake air.2. C.I. Engine:-Diesel engines also need a gas carburetor and scrubbing, but require atleast 10% diesel via the injectors for ignition (and cooling). The initialstarting of diesel engine is done on pure diesel.Modification of C.I. Engine:–C.I. engine can operate on dual fuel & the necessary engine modificationinclude provision for the entry of biogas with intake air, provision ofcarburetor & system to reduce diesel supply, advanced injection timing. Theentry of biogas and mixing of gas with intake air can be achieved byproviding the mixing chamber below the air cleaner which facilitate through 40
  • mixing of biogas with air before entering into the cylinder. The arrangementis largely used in stationary engine commercially available in India. Thecapacity of mixing chamber may be kept equal to the engine displacementvolume. The pilot injection of cycle is required to be advanced for smoothand efficient running of engine on dual fuel. The admittance of biogas intothe engine at the initial stage increases engine speed and therefore a suitablesystem reduces the diesel supply by actuating the control rack needs to beincorporated.There is a wide range of thoughts on what treatments should these biogasesbe subjected to before being used as fuel. Most operators simply remove thewater present in the biogas, leaving it to the engine manufacturers todesign engines which will cope with the impurities inevitably included in thebiogas (significant maintenance costs); other Operators are seriouslyevaluating maintenance costs against initial investments in biogas clean uptechnologies such as has been developed by Acrion Technologies (althoughAcrions technologies are mainly aimed at biogas contaminant removal andseparation into methane and carbon dioxide as feed stocks for a variety ofcommercial applications). 41
  • 38% HHV Caterpillar Biogas Engine Fitted to Long Reach Sewage WorksA Caterpillar bio-gas engine was fitted to Long Reach Sewage Works, operated byThames Water Utilities. This is a V16 engine running at 1500 rpm, on biogas which istypically 60 % methane. Output about 1150 kWe electrical and 1.4 MW thermal energywhich heats the digesters to 37 deg C . Electrical efficiency is about 38% HHV, thermalefficiency. Life cycle maintenance costs about 0.9 /kWh. Caterpillar makes about 50units per day of this basic engine in either gas or diesel (1.8MW) format at its US factoryin Lafayette Indiana equivalent to some 23 GWe of capacity per annum. The fullyinstalled cost of this kind of plant, ie the engine, heat exchangers, generator, enclosure,silencer, cooling system, controls, gas supply, commissioning is around £400/kWe.Further info – Claverton Contact Form 42
  • See also http://www.claverton-energy.com/for-sale-66mwe-chp-station.html if you wantto buy a gas engine power station,andhttp://www.claverton-energy.com/for-sale-complete-9mwe-power-station.htmlhttp://www.claverton-energy.com/first-energy-offer-excellent-condition-complete-gas-engined-chp-system-for-sale-and-installation.htmlhttp://www.claverton-energy.com/complete-wartsila-9-mwe-gas-engine-power-station-for-sale.html 43