Biogas Plant Classifications

Biogas Plant Classifications

• Batch type and continuous type plants are the main classifications for biogas plants.
• Continuous type plants are further classified as fixed dome and floating drum designs.
• Floating drum and fixed dome type biogas plants schematics exist.
• Numerous designs exist worldwide, including the KVIC model developed in India.
• The family-type KVIC model is designed for bio digestion.
• The steel construction acts as a gas holder in the KVIC model.
• Construction of the KVIC model is partially underground.
• A partition wall bifurcates the digester to maintain ideal conditions for bacteria and residence time.
• The partition wall helps maintain the residence time for organic matter digestion leading to good gas yield.
• Slurry enters the digester and spends time where acid-forming bacteria act.
• The slurry enters a second chamber where methanogenic bacteria act.
• Gases accumulate inside the drum, causing it to float due to pressure.
• Digested slurry exits into an overflow tank

Fixed Dome Type Biogas Plant

• These plants are more economical due to machinery construction.
• Gas pressure varies based on consumption and production.
• The dome structure provides strength against outside pressure but weakness against inside pressure.
• Skilled workmanship is crucial to avoid cracks that can inhibit anaerobic fermentation.
• Slurry is fed into the digester in fixed dome biogas plants.
• The absence of a separating wall results in lower gas yield compared to floating drum types.
• The gas yield is relatively less in the fixed dome type biogas plant.
• Variations of basic designs exist to reduce material costs using local materials.
• Digested slurry is displaced into a displacement tank.
• Gas accumulates inside the dome, accounting for 10% of the digester's volume.
• High pressure can cause cracks in the dome.
• Underground construction helps maintain constant temperature.
• Digester temperature can be higher than ambient, especially in winter.

Comparison of Floating Drum and Fixed Dome Biogas Plants

• The construction cost for floating drum plants is higher due to the steel drum requirement.
• Corrosion is more likely in floating drum plants due to the steel drum.
• Floating drum plants require more maintenance, including painting and gas pipe replacement.
• Fixed dome plants offer good thermal insulation due to underground construction.
• Scum formation is typically less in floating drum plants.
• Gas production is higher in floating drum plants due to digester bifurcation.
• Floating drum plants can break scum through drum rotation, eliminating the need for a stirrer.
• Fixed dome plants often require an external stirrer.
• The danger of oxygen mixing is negligible in floating drum plants.
• The likelihood of oxygen mixing is high in fixed dome plants due to potential leakage, caused by pressure that may create cracks or leaks on the dome.
• Gas pressure is constant in floating drum plants.
• The gas pressure is variable in fixed dome plants.
• Average skill in construction is needed for floating drum plants.
• Specialized, skilled masonry workmanship is needed for fixed dome plant construction.

Factors Affecting Biogas Yield - Temperature

• Optimizing factors is crucial for better gas yield.
• Temperature is a very important factor for efficient bio digestion of organic matter.
• Gas production reaches its maximum at 35°C.
• The optimal temperature for methane production is 35 to 40°C.
• Digestion is more rapid at higher temperatures.
• In cold climates, the external drum may need heating to maintain 35°C.
• A fall in gas production starts at 20°C and ceases at lower temperatures.
• Supplying gas from the same unit for heating is uneconomical.
• Methanogenic bacteria thrive in the 20 to 55°C range.
• Mesophilic and thermophilic microorganisms are responsible for digestion at different temperatures.
• The optimum mesophilic temperature is about 35°C, and the thermophilic temperature is about 55°C.
• The temperature range for mesophilic bacteria is 35 to 40°C, while it's approximately 50 to 60°C for thermophilic bacteria.
• Gas production stops when the temperature exceeds 60°C, inhibiting bacterial growth.
• Optimum temperature must be maintained for efficient biogas production.
• Raising the temperature increases gas production but may decrease methane content; therefore, an optimum temperature is vital.
• Temperature variation can lead to unsatisfactory digester performance.

Factor Affecting Biogas Yield - Pressure

• A minimum pressure of 6 to 10 cm of the water column is essential, or 1.2 bar.
• Pressure should not exceed 40 to 50 cm of the water column.
• Excess pressure prevents gas release from slurry.
• Excess pressure may lead to equipment leakage and risks explosive mixtures.

Factors Affecting Biogas Yield - Total Solid Content

• Water is essential for microorganism survival and activity.
• Water helps hydrolyze organic matter, which enhances digestion.
• It aids in enzyme activity.
• A uniform mixture is a result of proper water levels.
• Good water content aids bacterial movements.
• Optimum dry matter levels are around 8 to 10%.
• 20% dry matter may save on digester volume but lead to souring and reduced biogas output.
• Cow manure is mixed in a 1:1 proportion to achieve 8 to 10% solid content.
• Raw cow manure contains 80 to 82% moisture, so water is added to reduce solid content to the optimum range.

Factors Affecting Biogas Yield - pH

• pH is important due to the different requirements of acid and methanogenic bacteria.
• pH changes at various digestion stages.
• In the initial acid-forming stage, the pH may be 6 or less, and much CO2 is given off.
• During methane formation, a pH of 6.5 to 7.5 should be maintained.
• Methane-forming bacteria are sensitive to acidity.
• A constant gas supply needs a suitable pH range.
• The digester is buffered when the pH is maintained between 6.5 and 7.5.
• A pH range of 6.5 to 7.5 results in high microorganism activity.
• Acidity corresponds to a pH of 4 to 6, and alkalinity to a pH of 9 to 10.
• Deviation from the ideal pH can harm methanogenic bacteria and affect digestion.

Factors Affecting Biogas Yield - Loading Rate

• Loading rate is an important factor in the anaerobic digestion process.
• Loading rate is the amount of raw material fed to the digester per day per unit volume.
• Units include chemical oxygen demand (COD) per cubic meter per day or volatile solid matter (VS) per cubic meter per day.
• An overload of material can lead to acid accumulation, halting the digestion process.
• An optimized loading rate maintains proper conditions inside the digester.
• High loading rates cause less retention time, resulting in undigested slurry and reduced gas yield.
• Low organic loading rates also reduce gas production and make the process uneconomical.
• The recommended organic loading rate without medium is 1 to 4 kg of volatile solid matter per meter cube per day.

Hydraulic Retention Time (HRT)

• HRT refers to the time the substrates stay inside the digester.
• HRT is calculated by dividing the volume of the digester by the volume of slurry added per day.
• HRT is strongly dependent on the type of feedstock and temperature.
• HRT typically varies between 30 to 45 days but can extend to 60 days.
• 30 days is a typical retention time for non-stirring digesters.
• Digesters with high decomposition rates can reduce retention time to 10 to 20 days.
• HRT is optimized to achieve 70 to 80% of complete digestion.
• Cow manure requires a retention time of around 50 days.
• Rice straw, sugarcane tops, and water hyacinth have slightly different retention times.

Seeding

• Seeding accelerates the start of the anaerobic digestion process.
• Cow manure contains the bacteria needed for digestion, but in low numbers.
• Seeding is done to reduce the retention time of the slurry inside the digester.
• Acid formers proliferate fast, while methane formation bacteria reproduce and multiply slowly.
• Seeding increases the number of methane formers through the addition of digested slurry.
• The solid content of digested slurry should not exceed 5 to 10% of the new material feed.
• Digested slurry is rich in methane formers.

Carbon to Nitrogen (C/N) Ratio

• Carbon and nitrogen are nutrients for bacteria, essential for their growth and survival inside the digester.
• An improper C/N ratio leads to unsatisfactory performance and harmful effects on anaerobic bacteria.
• Maintaining the required nutrients in the correct proportion, chemical form, and concentration is essential.
• Carbon, in the form of carbohydrates, supplies energy.
• Nitrogen, such as proteins and ammonium nitrates, is needed for building cell structure.
• Bacteria use up carbon 30 times faster than nitrogen.
• The optimal C/N ratio inside the digester is considered to be around 25 to 30:1 for maximum microbiological activity.
• Any deviation from this ratio slows down the bio digestion process.
• Low C/N ratios generate more ammonia, inhibiting microbial growth and lowering gas yield.

Mixing or Stirring

• In fixed dome and floating drum biogas plants, scum forms on the slurry's top layer.
• Mixing or stirring helps maintain uniformity in substrate concentration, temperature, and other environmental factors.
• Mixing avoids the formation of scum on the surface of the slurry.
• Mixing minimizes the deposition of solids at the bottom of the digester.
• Without stirring, the substrate stratifies into three layers: a floating layer, a fermentation solution, and a sediment slurry.
• Stratification results in uneven contact of bacteria with the substrate.
• Stirring ensures even contact of the bacteria and enhances the decomposition process.
• Slight mixing improves fermentation.
• Violent agitation may retard the digestion process.
• The floating layer contains light materials.
• The sediment slurry consists of sedimented solids

Toxic Substances

• The source of toxic substances depends on the type of organic material used for bio digestion.
• High concentrations of ammonia, antibiotics, pesticides, and heavy metals are potentially toxic to microorganisms.
• Low C/N ratios lead to high ammonia concentrations.
• Traces of pesticides and disinfectants in farm products may be toxic.
• Synthetic materials are toxic to microorganisms.
• Heavy metals from wastewater sewage are toxic to microorganisms.
• The digested slurry becomes toxic after a certain time, inhibiting the growth of microorganisms.
• Allowable limits of these substances inside the digested slurry should be maintained.

Raw Material Availability and Gas Yield

• Most organic matters containing protein, fats, and carbohydrates can be transformed into gas.
• These materials include animal waste, land-based resources, and water-based resources like sewage sludge.
• Gas yield provides information for the effective digester design.
• Lignocellulosic materials need to be pretreated to break down crystalline cellulose into simple sugars for effective digestion.
• Pretreatment techniques include physical, chemical, biological, and thermal processes.
• The solid content of digested slurry, when fed inside the digester, should be maintained in the ratio of 5 to 10%.

End Uses of Biogas

• Crude biogas can be upgraded to increase methane content to 70 to 90%.
• Upgraded biogas can reach normalized natural gas composition.
• Normalized natural gas can be compressed and used as fuel in vehicles or injected into an electricity grid.
• Crude biogas can be purified using a scrubber technique to remove unwanted gases and increase methane percentage.
• Biogas with a methane percentage of 65-70% or higher can be used as a boiler feed.
• Heat generated from the boiler can be used directly or in heat applications.
• Biogas can be co-generated or used in a generator to produce electricity.
• Electricity produced from biogas can be used in households, commercial outlets, or farms.
• Biogas has applications in heating and electricity production.
• Biogas can be used as fuel in vehicles after processing.
• Biogas is considered an efficient renewable fuel with wide applications.