Embed Size (px)
Citation preview
Prepared by N. Byrava Moorthi
TECHNICAL DESCRIPTION
OF BIOGAS
DEVELOPMENT
Introduction Everyone is talking about biogas - politicians and ecologists, technicians and economists,
laymen and experts. Biogas has become fashionable. The energy crisis of the next few years is the shortage of fuel for the daily needs of
millions of people. Simple biogas plants are intended to help solve this problem. It is time to set about this task in a "professional" manner in the best sense of this word.
Simple biogas plants are complicated enough to require total involvement with their specific technology. After all, a biogas plant can only help to solve the problems of the future if it works! But many plants work badly. They are operated wrongly, are deficient in detail and are often incorrectly scaled.
Simple biogas plants have been constructed in Third World countries for about thirty years. We have been able to learn from the biogas pioneers for thirty years. But good and bad solutions are featured side by side without comment in articles and books. The same mistakes are repeated over and over again. This need not be the case. The designer of a biogas plant must be able to distinguish between valid and invalid solutions. This little book is intended to help him in this respect.
The figures and tables reproduced here constitute practical guides. They have been assembled from external and internal sources and simplified or modified in accordance with the author's own experience. They should not be confused with laboratory values.
History of Biogas TechnologyEurope/Germany:
1770 The Italian Volta collected marsh gas and investigated its burning behavior.1821 Avogadro identified methane (CH4 ).1875 Propoff states that biogas is produced under anaerobic conditions.1884 Pasteur researched on biogas from animal residues. He proposed the utilization
ofhorse litter to produce biogas for street-lighting.
1906 First anaerobic wastewater-treatment plant in Germany.1913 First anaerobic digester with heating facility.1920 First German sewage plant to feed the collected biogas into the public gas
supplysystem.
1940 Addition of organic residues (fat) to increase sewage gas production.1947 Research demonstrates that the dung of one cow can give a hundred times more
gas than the feces of one urban inhabitant.And Establishment of the first working group on biogas in Germany.
1950 Installation of the first larger agricultural biogas plant.1950 Nearly 50 biogas plants are built, fed by litter mixed with water and dung. Low
oil prices and technical problems lead to the shutdown of all but two plants.1974 After the first ’energy crisis’, increased promotion of research on and
implementation of agricultural biogas technology by the EC and federal departments.1985 75 biogas plants are listed (built or planned). Biogas slurry is increasingly used
as liquid manure.1990 Progress due to guaranteed prices for biogas-generated electricity. Progress in
optimizing the mixture of substrates, the use of biogas for different purposes and
technology details.1992 Foundation of the German biogas association ‘Fachver band Biogas’1997 More than 400 agricultural biogas plants exist in Germany.
China and India:The history of biogas exploration and utilization in China covers a period of more than 50years. First biogas plants were build in the 1940s by prosperous families. Since the 1970sbiogas research and technology were developed at a high speed and biogas technology waspromoted vigorously by the Chinese government. In rural areas, more than 5 million smallbiogas digesters have been constructed and currently, over 20 million persons use biogascurrently as a fuel.
In India, the development of simple biogas plants for rural households started in the 1950s. A massive increase in the number of biogas plants took place in the 1970s through stronggovernment backing. Meanwhile, more than one million biogas plants exist in India.The historical experiences in Germany, China and India demonstrate clearly, how biogasdevelopment responds to favorable frame conditions. In Germany, biogas disseminationgained momentum through the need for alternative energy sources in a war-torn economyand during an energy crisis or later by the change of electricity pricing. In India and ChinaIt was a strong government program that furthered the mass dissemination of biogastechnology.
Biogas
Biogas is a non-toxic, colorless, odorless,inflammable gas, produced by organic waste andbiomass decomposition (Fermentation/Anaerobic/Digestion/Biomethanation).
Biogas and Global Carbon Cycle
Each year some 590-880 million tons of methane are releasedworldwide into the atmosphere through microbial activity. About90% of the emitted derives from biogenic sources, i.e. from thedecomposition of biomass. The remainder is of fossil origin (e.g. petrochemical processes). In the northern hemisphere, the presenttropospheric methane concentration amounts to about 1.65 ppm.
Biomethanation Process
Biomethanation is a process in which biogas, along withbiocompost is produced by activity of anaerobic bacteria onorganic matter prevalent in biomass/waste. Anaerobic bacteriaoccur naturally in organic environments where oxygen is limited.
General Bio Chemical Reaction
Organic Matter + Combined Oxygen Anaerobes Anaerobes New Cells + Energy for Cells + CH4 + CO2 +
Other End Products
Specific Reaction of Biomethanation
Reaction:1C6H12O6 + 2H2O 2CH3COOH + 4H2 + 2CO2
Reaction:2CH3COOH CH 4+ CO 2
Reaction:3CO 2 + 4 H 2 CH 4 + 2H 2O
Stages involved in Biomethanation Process
Symbiotic groups of bacteria perform different functions at different stages ofthe digestion process. There are four basic types of microorganisms involved.
Stage:1Hydrolytic bacteria break down complex organic wastes into sugars andamino acids. Stage:2Fermentative bacteria then convert those products into organic acids.Stage:3Acetogenic microorganisms convert the acids into hydrogen, carbon dioxideand acetate. Stage:4Finally, the Methanogenic bacteria produce biogas from acetic acid,hydrogenand carbon dioxide.
Methanogenic Bacteria Photo View
Anaerobic Digesters
Anaerobic digesters generate biogas by biomethanation in order to use it for different energy generation purposes. While designing the anaerobic digesters,care should be taken to ensure the safety aspects of the digesters as the biogasgenerated is combustible in nature. Adequate ventilation is provided around thedigester by positioning it in an open area, preferably outdoors. As the preferablelocation of digester is outdoors, hence it should be weatherproof as well. Allpossible ignition sources should be kept away from the anaerobic digesters. Theconditions in the anaerobic digesters are to be kept warm to enable effectivebiomethanation through anaerobic microbes.
Anaerobic Digestion System
Anaerobic Condition MechanismSoluble Organics
Bacterial Cells Volatile Acids+CO2+H2 Other Products
*Methane Producing Bacteria
CH4 + CO2 + Bacteria Cells
Enzymes involved in Anaerobic Digestion
Protolytic (acts in Protein), Lipolytic (acts in Fat),Cellulolytic (acts in Cellulose & Starch) are importantenzymes which is on involved in anaerobic digestion.
Benefits of Anaerobic Digesters
Anaerobic Digesters have the following benefits: Odor and fly control: - Anaerobic digesters consume odor-causing compounds in
organic waste as it moves through the digester, reducing odor problems. Studies show that anaerobic digestion reduce odor by 97 % over fresh manure. Fly propagation is also extremely limited in digested manure compared to fresh manure.
Renewable energy production: - The biogas generated is most often used to produce electricity, and the waste heat used to produce hot water for heating the digester and other applications.
Potential increase in value as a fertilizer: - Anaerobic digestion increases the value of the waste as a fertilizer. The digestion process converts the organic nitrogen into a mineralized form that can be taken up more quickly by plants as manure then organic nitrogen. Also, research suggests that the micro flora present in digested manure may lead to increase in crop yields.
Green house gas reduction: - Methane is a green house gas 23 times more potent then carbon dioxide in causing global warming. By capturing and burning the methane produced from organic waste, anaerobic digesters help in slowing down the rate of global warming.
Reduction in Total Oxygen Demand: - Total Oxygen demand is a measure of how much oxygen could potentially be consumed by breaking down organic matter. If too much oxygen is used to break down the waste, natural stream life will suffer.
No external power requirement unlike aerobic treatment.
Key Factors Impacting Anaerobic Process(1) Raw Materials Storage & ProcessingRaw materials may be obtained from a variety of sources - livestock and poultry wastes, industrialwastes, agricultural produce/ waste, etc. Different problems are encountered with each of these organicmaterials with regard to collection, transportation, processing & storage. Agricultural produce (energycrops) and residues such as spent straw, cane trash, corn and plant stubble/stalks, need to be shredded inorder to facilitate their flow into the digester reactor as well as to increase the efficiency of bacterialaction.
(2) LoadingThe size of the digester depends upon the loading, which is determined by the influent solids content,retention time, and the digester temperature. Optimum loading rates vary with different digesters andtheir sites of location. Higher loading rates have been used when the ambient temperature is high. Inpractice, the loading rate should be an expression of either
a) the weight of total volatile solids (TVS) added per day per unit volume of the digester, (or)b) the weight of TVS added per day per unit weight of TVS in the digester.
(3) SeedingCommon practice involves seeding with an adequate population of both the acid forming and methane forming bacteria. Actively pre processed digesting material from a raw material preparation unit constitutes ideal "seed" material. As a general guideline, the seed material should be twice the volume of the fresh manure slurry during the start-up phase, with a gradual decrease in amount added over a three-week period. If the digester accumulates volatile acids as a result of overloading, the situation can be remedied by reseeding, or by the addition of lime or other alkali.
(4) pHLow pH inhibits the growth of the methanogenic bacteria and gas generation and is often the result of overloading. A successful pH range for anaerobic digestion is 6.0 - 8.0; efficient digestion occurs at a pH near neutrality. A slightly alkaline state is an indication that pH fluctuations are not too drastic. Low pH may be remedied by dilution or by the addition of lime.
(5) TemperatureWith a mesophillic flora, digestion proceeds best at 30 - 40 C; with thermopiles, the optimum range is 50 - 60 C. The choice of the temperature to be used is influenced by climatic considerations In general, there is no rule of thumb, but for optimum process stability, the temperature should be carefully regulated within a narrow range of the operating temperature. In warm climates, with no freezing temperatures, digesters may be operated without added heat. *(Refer the following Graph)
(6) C:N RatioThe maintenance of optimum microbiological activity in the digester is crucial to gas generation and consequently is related to nutrient availability. Two of the most important nutrients are carbon and nitrogen and a critical factor for raw material choice is the overall C/N ratio. In practice, it is important to maintain, by weight, a C/N ratio close to 30:1 for achieving an optimum rate of digestion.
C : N Ratio TableMaterial C:N Ratio Material C:N Ratio
Fruit Waste 35:1 Vegetable Waste 12-25:1
Grass Cuttings 12-25:1 Food Waste 25-30:1
Tree Leaves 30-80:1 Fresh Sewage 11:1
Cow Manure 20-25:1 Tree Wood Chips 500-700:1
Paper 170-200:1 Fat & Oil 90:1
Poultry Manure 15:1 Rice Straw 50:1
(7) Toxic MaterialsWastes and biodegradable residue are often accompanied by a variety of pollutants that could inhibit anaerobic digestion. Potential toxicity due to ammonia can be corrected by remedying the C/N ratio of manure through the addition of shredded bagasse or straw, or by dilution. Common toxic substances are the soluble salts of copper, zinc, nickel, mercury, and chromium. On the other hand, salts of sodium, potassium, calcium, and magnesium may be stimulatory or toxic in action, both manifestations being associated with the cation rather than the anionic portion of the salt. Pesticides and synthetic detergents may also be troublesome to the process.
(8) MixingWhen solid materials not well shredded are present in the digester, gas generation may be impeded by the formation of a scum that is comprised of these low-density solids that are enmeshed in a filamentous matrix. In time the scum hardens, disrupting the digestion process and causing stratification. Agitation can be done either mechanically with a plunger or by means of rotational spraying of fresh influent. Agitation, normally required for bath digesters, ensures exposure of new surfaces to bacterial action, prevents viscid stratification and slow-down of bacterial activity, and promotes uniform dispersion of the influent materials throughout the fermentation liquor, thereby accelerating digestion.
(9) Retention TimeOther factors such as temperature, dilution, loading rate, etc., influence retention time. At high temperature bio-digestion occurs faster, reducing the time requirement. A normal period for the digestion of dung would be two to four weeks.
Biogas Generation
Depending on the raw material and digester efficiency, wecan obtain from 60 to 600 Cu.m biogas for each ton oforganic matter. With an efficient process, the bacteria canconvert about 90 % of the feedstock energy content intobiogas (containing 55% - 75 % methane).
Biogas Composition
Matter Content %Methane, CH4 55 – 75
Carbon dioxide, CO2 25 - 45Nitrogen, N2 1 - 5Hydrogen, H2 0 - 3
Hydrogen Sulphide, H2S 0.1 - 4.0Oxygen, O2 Traces
Biogas Properties Biogas is about 20 percent lighter than air. Biogas has an approximate specific gravity of 0.86 and flame
speed factor of 11.1 It has an ignition temperature range of 650 ºC to 750 ºC. It is an odorless and colorless gas that burns with clear blue flame
similar to that of Liquefied Petroleum Gas (LPG). Biogas firing efficiency is 1.94 times more than fresh cow dung
cake. A 1 m3 biogas will generate 4500 – 5500 kcal/m3( 500 – 700
BTU/ft3) and When burned in efficient burners can produce 5.0 KWh/m3.
Biogas Uses
Electricity Heating Cooking Lighting Vehicle Fuel (after purified & compressed)
Digester Residue
The effluent from a anaerobic digestion system can be either sludge,supernatant, or slurry depending on the design and operation of thesystem. Most of the systems have slurry as their output. The mainadvantage of anaerobic digestion is that conserves nitrogen if theslurry is handled properly. Though approximately 20 percent of thetotal solids contained in the organic material are lost during thedigestion process, the nitrogen content remains largely unchanged.The nitrogen is in the form of ammonia, which makes it moreaccessible when the effluent is used as fertilizer.
Benefits of Digester Residue Contains micro and macronutrients, such as Nitrogen
Phosphorus and Potassium and a fair amount of Manganese, Zinc, Copper and Iron. These are simultaneously added to the soil. Besides, it increases the microbial activity.
Carbon as an energy source and other nutrients are provided to soil microbes, which result in augmentation of their population. This helps in organic matter decomposition, biological nitrogen fixation, solubilization of insoluble phosphates and availability of plant micro nutrients.
Various Feedstock for Biogas GenerationFeed Stock Category Few Examples
Animal Waste Cow Manure, Poultry WasteIndustrial Organic Waste Molasses, Paper & Pulp Mill Effluent, Biodiesel
Plant Reject, Edible and Non Edible Oil Reject and other organic industry waste
Municipal Waste Municipal Solid Waste, Slaughter House Waste, Human Excrements
Agricultural Waste/Energy Plantations Rice Straw, Corn Waste, Tiger Grass, Jetropha Seeds, Pongamia Seeds, Sorghum
Marine Waste Dead Fish , Fish Oil Industry Organic Secondary Waste
Biogas Production/Ton Feedstock
Feedstock Biogas Quantity (M3/Ton)Cow Manure 60Sugar Cane Crop Residue 450Rice Bran De-Oiled Cake 450Mustard Oil Cake 500Vegetable Processing Waste 450Distillery Residue 550Fats from Skimming Tank 1000Brewery Waste 500Slaughter House Waste 500Fish Oil Industry Waste 400Market Waste 180
Biogas Plant Block Diagram
Feedstock Preparatio
n AreaMixer Digester
Digester Reject
Storage
Bio Fertilizer
Unit
Gas Holder
Engine, Gas Stove
Type of the Biogas Plants
Type IBalloon Plants
Type IIFixed – Dome Plants
Type IIIFloating Drum Plants
Balloon Plants
The balloon plant consists of a digester bag (e.g. PVC) in the upperpart of which the gas is stored. The inlet and outlet are attacheddirectly to the plastic skin of the balloon. The gas pressure isachieved through the elasticity of the balloon and by added weightsplaced on the balloon.
Advantages :-Balloon Plants are low cost, ease of transportation, low constructionsophistication, high digester temperatures, uncomplicated cleaning,emptying and maintenance.
Disadvantages:-It can be the relatively short life span, high susceptibility to damage,little creation of local employment and, therefore, limited self-helppotential.
Fixed – Dome Plants
The fixed-dome plant consists of a digester with a fixed, nonmovable gas holder, which sits on top of the digester. When gasproduction starts, the slurry is displaced into the compensationtank. Gas pressure increases with the volume of gas stored and theheight difference between the slurry level in the digester and theslurry level in the compensation tank.
Advantages :-Fixed – Dome Plants are the relatively low construction costs, the absence of movingparts and rusting steel parts. If well constructed, fixed dome plants have a long lifespan. The underground construction saves space and protects the digester fromtemperature changes. The construction provides opportunities for skilled localemployment.
Disadvantages:-These are mainly the frequent problems with the gas-tightness of the brickwork gasholder (a small crack in the upper brickwork can cause heavy losses of biogas). Fixeddome plants are, therefore, recommended only where construction can be supervised byexperienced biogas technicians. The gas pressure fluctuates substantially depending onthe volume of the stored gas. Even though the underground construction bufferstemperature extremes, digester temperatures are generally low.
Floating Drum Plants
Floating-drum plants consist of an underground digester and a movinggas-holder. The gasholder floats either directly on the fermentationslurry or in a water jacket of its own. The gas is collected in the gasdrum, which rises or moves down, according to the amount of gasstored. The gas drum is prevented from tilting by a guiding frame. Ifthe drum floats in a water jacket, it cannot get stuck, even in substratewith high solid content.
Advantages :-Floating Drum Plants are the simple, easily understood operation – the volume ofstored gas is directly visible. The gas pressure is constant, determined by theweight of the gas holder. The construction is relatively easy, construction
mistakesdo not lead to major problems in operation and gas yield.
Disadvantages:-These Plants are high material costs of the steel drum, the susceptibility of steelparts to corrosion. Because of this, floating drum plants have a shorter life spanthan fixed-dome plants and regular maintenance costs for the painting of the drum.
Taking Care of Biogas Plant
Always be very careful when you are near a biogas unit because gas may be leaking.
Never build a fire near the unit, smoke, or even light a match near the unit, because If gas la leaking It may explode.
It biogas is leaking and you breathe in too much of it can make you very sick.
Check your biogas unit and gas lines often to be sure that there are no leaks. Once a year you should take the unit apart and clean and paint the metal gas
holder and all other metal parts. You can use paint which is used to protect metal or coat the metal parts with
tar.
Anaerobic Digester Design Parameters
Inputs:-
o Feed (Slurry) Volume per day xx m3 o Anaerobic Digester Diameter xx mo Ambient Temperature (Feed) xx ºCo Specific Heat of Feed xx KJ/Kg/ºCo Ambient Temperature (Air) xx ºCo Thermal Conductivity of Insulation xx Wm/ºCo Insulation Thickness xx mmo Calorific Value of Methane xx MJ/m3
o Heat Efficiency xx %
Outputs:-
o Anaerobic Digesters Volume xx m3 o Anaerobic Digesters Height xx mo Gross Gas Production xx m3/dayo Power to Heat Effluent xx kWo Power Lost through Walls xx kWo Net Gas Volume xx m3/day
Costing
Actual Digester (INR xxx per m3) INR xxxxxx Heater (INR xxx per kW) INR xxxxxx Insulation (INR xxx per m2) INR xxxxxx Equipment (INR xxx) INR xxxxxx
Total INR xxxxxx
Expected Life of Digesters xx Years
Biogas Comparison with other Fuels Fuel Unit U Calorific Value
Kwh/UApplication Efficiency
%
Cow Dung Kg 2.5 Cooking 12
Wood Kg 5 Cooking 12
Charcoal Kg 8 Cooking 25
Hard Coal Kg 9 Cooking 25
Butane Kg 13.6 Cooking 60
Propane Kg 13.6 Cooking 60
Diesel Kg 12 Cooking 50
Diesel Kg 12 Engine 30
Electricity Kwh 1 Motor 80
Biogas m3 6 Cooking 55
Biogas m3 8 Engine 24
Calorific Value Unit Conversions
Unit Conversions1 KCal 4.18 KJ
1 KCal 1000 Cal
1000 J 1 KJ
1000 KJ 1 MJ
1000 MJ 1 GJ
1000 GJ 1 TJ
1000 W 1 KW
1000 Kwh 1 MW
1 KJ 1.0551 Btu
1 Kwh 3600 KJ
1 Kwh 860 Kcal
THANK YOUPresented by
N. Byrava Moorthi B.EChemical Engineer – Alternate Fuels
Tel: +91 9972880622Email: [email protected]
India
