Biogas and Biofuel Production: Key Factors and Technologies

Posted on Dec 17, 2024 in Technology

Factors Affecting Biogas Production

Solid-to-Water Ratio:

  • Optimal: 9% solids for anaerobic fermentation (1:1 ratio of dung to water).
  • Adjustment: Add crop residues or weed plants if needed.

Volumetric Loading Rate:

  • Optimal: 1.0–1.5 kg/m³ per day.
  • Overloading or underloading reduces gas yield.

Temperature:

  • Optimal Range: 35°C–38°C (mesophilic) or 55°C–60°C (thermophilic).
  • Production decreases below 20°C and stops at 8°C.

Seeding:

Adding digested slurry rich in methane-forming bacteria accelerates fermentation.

pH Value:

  • Optimal: 6.8–7.8.
  • Adjust with alkali if pH drops below 6.6.

Carbon-to-Nitrogen (C/N) Ratio:

  • Optimal: 30:1.
  • Adjust with materials like sawdust (high C/N) or poultry waste (low C/N).

Retention Time:

Time biomass stays in the digester; calculated as digester volume divided by daily input.

  • Typical: Around 24–30 days.

Stirring:

Enhances microbial contact with the substrate, improving gas production by up to 15%.

Biomass to Ethanol Production

1. Feedstock Selection

Biomass feedstocks are chosen based on their carbohydrate content. Common feedstocks are:

  • Sugars: Sugarcane, sugar beet, molasses, sweet sorghum, and grapes.
  • Starches: Maize, wheat, barley, potatoes, cassava, and rice.
  • Cellulose: Wood, agricultural residues (straw, grasses), sugarcane bagasse, and bamboo.

2. Pretreatment

For starch and cellulose feedstocks, the biomass must undergo pretreatment to release fermentable sugars.

  • Starch Hydrolysis: Starch is broken down into simple sugars (e.g., glucose) using enzymes like amylase.
  • Cellulose Hydrolysis: Cellulose is broken down into glucose using acids or cellulase enzymes after removing lignin.

3. Fermentation

The fermentable sugars (mainly glucose) are converted into ethanol and carbon dioxide through alcoholic fermentation:

Chemical Reaction: C6H12O6 → Yeast Enzymes 2C2H5OH + 2CO2

Microorganisms like Saccharomyces cerevisiae (yeast) are commonly used.

Conditions:

  • Anaerobic environment (absence of oxygen).
  • pH: 4–5.
  • Temperature: 30°C–35°C.

4. Distillation

After fermentation, the ethanol concentration in the mixture is low (5-10%).

The fermented mixture undergoes distillation to separate ethanol from water and other impurities. Ethanol purity can reach 95% through this process.

5. Dehydration

To achieve absolute ethanol (99% purity) for fuel applications, the distilled ethanol is dehydrated using molecular sieves or azeotropic distillation to remove residual water.

6. By-products

Carbon dioxide (CO2) is a by-product during fermentation. It can be captured for industrial use.

Residual biomass, like spent grain or lignin, can be utilized as animal feed or for energy generation.

Applications

Ethanol produced from biomass is a renewable biofuel used as:

  • A blend with gasoline (e.g., E10 or E85 fuel).
  • A substitute for fossil fuels in internal combustion engines.
  • Feedstock for the chemical industry.

Fixed Dome Type Biogas Plant

Construction

Digester:

  • A large underground chamber constructed of brick and cement masonry.
  • Dome-shaped gas holder integrated with the digester.

Inlet:

A pipe through which the feedstock (e.g., slurry of cattle dung and water) is added.

Outlet:

An overflow chamber for the spent slurry.

Gas Outlet Pipe:

Located at the top of the dome for biogas extraction.

Working

The biomass slurry is fed into the digester through the inlet.

Inside the digester, anaerobic bacteria break down the organic matter into biogas (methane and carbon dioxide) under anaerobic conditions.

The gas accumulates in the dome, creating pressure, which pushes the spent slurry into the overflow chamber.

The biogas is extracted through the gas outlet pipe and used for cooking, lighting.

Advantages

  • Durable with no moving parts, ensuring minimal maintenance.
  • Cost-effective construction.

Disadvantages

  • Variable gas pressure depending on gas volume.
  • Difficult to construct and maintain a gas-tight dome.

Floating Dome Type Biogas Plant

Construction

Digester:

Similar to the fixed dome type, made of masonry.

Gas Holder:

  • A steel drum that floats over the slurry inside the digester.
  • Moves up and down depending on the gas volume.

Inlet and Outlet:

Similar to the fixed dome model for adding feedstock and removing spent slurry.

Working

Feedstock is added to the digester, where anaerobic digestion occurs.

The biogas generated collects in the steel drum, which rises and falls based on the gas volume.

The gas holder’s movement maintains constant gas pressure, enabling consistent usage.

Advantages

  • Provides constant gas pressure.
  • Easier to construct compared to fixed dome models.

Disadvantages

  • The steel gas holder is prone to rusting and requires regular maintenance.
  • Higher initial cost due to the steel drum.

Comparison

  • Gas Pressure: Floating dome ensures constant pressure, while fixed dome varies.
  • Cost: Fixed dome is more economical.
  • Durability: Fixed dome is more durable due to no moving parts.
  • Maintenance: Floating dome requires frequent maintenance of the steel drum.

Urban Waste-to-Energy (WtE) Plant

1. Types of Waste Considered:

  • Municipal Solid Waste (MSW): Organic waste, plastics, paper, and other biodegradable/combustible materials.
  • Sewage Waste: Treatment of sewage sludge.
  • Industrial Waste: Non-hazardous, energy-rich industrial waste.

2. Key Waste-to-Energy Conversion Technologies:

  • Incineration: Direct combustion of waste at high temperatures to produce heat, which can be used to generate steam for driving turbines.
  • Anaerobic Digestion: Decomposition of organic waste by bacteria in an oxygen-free environment, producing biogas (primarily methane) that can be used to generate electricity.
  • Gasification and Pyrolysis: Thermal processes that convert waste into syngas (a mixture of hydrogen, carbon monoxide, and methane), which is used to drive gas turbines or internal combustion engines.
  • Landfill Gas Recovery: Collection of methane gas produced by the anaerobic decomposition of organic matter in landfills, which can then be used to generate electricity.

3. Steps of the Scheme:

  • Collection and Segregation: Urban waste should be properly segregated at source into recyclable, organic, and non-recyclable waste. This improves the efficiency of the WtE process.
  • Pre-processing: The collected waste undergoes shredding, drying, and screening to remove non-combustible materials (like metals and glass).
  • Energy Conversion: Based on the type of waste:
    • Organic waste (food waste, green waste) goes to anaerobic digestion plants.
    • Non-recyclable waste (plastics, paper) is fed into incinerators, gasifiers, or pyrolysis plants.
    • Landfill gas is captured and combusted in engines or turbines for power generation.
  • Power Generation: The heat generated from incineration or the combustion of biogas/syngas is used to produce steam, which drives a steam turbine connected to an electricity generator.
  • Energy Storage and Grid Integration: The electricity produced can be fed directly into the grid or stored in batteries for later use, ensuring a reliable supply of power to urban areas.

4. Environmental Impact and Benefits:

  • Reduction in Landfill Use: Diverts waste away from landfills, reducing methane emissions and conserving land.
  • Renewable Energy Source: Waste-to-energy is considered renewable as it uses sustainable sources (biodegradable waste, etc.).
  • Lower Emissions: Modern WtE plants are equipped with pollution control technologies (scrubbers, filters) to minimize emissions.
  • Reduced Dependency on Fossil Fuels: Helps reduce urban dependency on fossil fuel-based power plants.

5. Challenges and Considerations:

  • Initial Cost: High capital investment required for setting up WtE plants and waste collection infrastructure.
  • Public Awareness: Need for strong public participation in waste segregation and reduction at the source.
  • Emission Control: Incineration plants must meet strict emissions standards to prevent air pollution.

KVIC Model (Floating Drum Biogas Plant)

Design:

  • The KVIC model is a floating drum type biogas plant.
  • It consists of a cylindrical underground digester made of bricks or concrete, and a floating gas holder or drum made of mild steel or plastic.
  • The gas holder rises and falls based on the amount of gas produced and consumed.
  • The gas produced (primarily methane and carbon dioxide) is stored in the drum and is directly piped for use.

Working Principle:

  • Organic waste (cow dung, plant material, etc.) is mixed with water to create slurry and fed into the digester.
  • Anaerobic bacteria break down the organic matter in the absence of oxygen, producing biogas.
  • The gas holder floats over the slurry and collects the produced biogas. The pressure of the gas causes the drum to rise.
  • When gas is used, the drum sinks back into the digester.

Advantages:

  • Constant gas pressure: Since the gas holder rises and falls, it maintains consistent pressure, making the gas flow easier for end-use.
  • Easier gas collection: The floating drum design allows for easy collection and measurement of gas.
  • Visual indication: The rise and fall of the drum provide a visual indication of the amount of gas available.

Disadvantages:

  • Cost: The steel drum makes the KVIC model more expensive to construct and maintain.
  • Corrosion: The steel drum is prone to rusting, requiring regular maintenance or replacement.
  • Complex design: Slightly more complex to construct compared to fixed-dome models like the Janata plant.

Janata Model (Fixed Dome Biogas Plant)

Design:

  • The Janata model is a fixed dome type biogas plant, where both the digester and gas holder are built underground as part of the same structure.
  • It has a fixed dome (gas holder) that does not move, made of bricks or concrete.
  • As gas is produced, it accumulates under the fixed dome, creating pressure to push the gas out through a pipe.

Working Principle:

  • Organic waste mixed with water is fed into the digester.
  • Anaerobic digestion takes place, producing biogas.
  • The biogas collects under the fixed dome and builds pressure, which forces the gas out through a gas outlet pipe.
  • The slurry moves to an overflow chamber as more waste is added.

Advantages:

  • Lower cost: As there is no steel drum, the construction cost is lower, making the Janata model more affordable for rural communities.
  • Durability: The underground dome, made of bricks or concrete, has a longer lifespan and is resistant to corrosion.
  • Low maintenance: The absence of moving parts reduces maintenance needs.

Disadvantages:

  • Variable gas pressure: The gas pressure fluctuates depending on the amount of gas inside the dome. This can cause irregular gas flow.
  • Difficulty in monitoring gas: Since the dome is fixed, it’s harder to visually assess the amount of gas produced compared to the KVIC model.
  • Complexity in construction: Although cheaper, constructing the dome requires skilled labor and careful design to avoid gas leakage.