Biogas Basics: Simple Design of Biogas Plant

In this, you will learn about biogas, biogas models, and how to design biogas with an interactive design sheet. You will get the required design and drawings.

What is Biogas?

Biogas is a gas generated through the anaerobic decomposition of organic waste by bacteria. Typically comprising 60% methane and 40% carbon dioxide, biogas also contains trace amounts of water vapor, hydrogen sulfide, ammonia, and other gases.

The feed is not only limited to cow manure but can be of any type that can be:

• Cow/buffalo/Pig/Chicken manure
• Human Feces
• Invasive plants like Water Hyacinth
• Decomposable Solid Waste (municipal waste, fruits, vegetables, pulp, molasses, etc.)

Types of Biogas Plants

• Fixed Dome (Chinese origin, with known leakage issues, later modified by India as Janata and Deenbandhu models)
• Floating Drum (Indian, specifically the KVIC model)
• Bag Type (Taiwanese)

Simple small-scale plants can be constructed using plastic drums or inflatable tanks. In simple terms, the production of biogas just needs an oxygen-free environment for bacteria and sufficient gas storage volume. However, for safety, durability, and ease of use, permanent constructions are typically preferred.

Mixing Methods

• Unmixed (conventional): Slow process
• Mixed: Faster process
• Batch type: Completely emptied and refilled after a retention time

Biogas Production Stages

Biogas production occurs in the bio-digester through the following stages:

1. Hydrolysis
2. Acidogenesis
3. Acetogenesis
4. Methanogenesis (resulting in biogas production)

Various factors affect biogas production, with temperature being a key determinant of the rate of gas production. Biogas plants are generally designed to operate within the mesophilic temperature range due to the challenges associated with maintaining consistent thermophilic temperatures.

Factors affecting Biogas production

• Temperature
• pH
• Seeding with digested sludge
• Presence of toxic compounds and inhibitors like copper, fluorides, and heavy metals.
• The moisture content of the feed
• Percentage of Volatile Suspended Solids (VSS) in feed
• Carbon to nitrogen ratio
• Retention time of feed

Temperature Ranges

• Mesophilic Range digestion (25-40 degree Celsius)
• Thermophilic Range digestion (40-60 degree Celsius)

Hydraulic Retention Time (HRT)

Maintaining a specific hydraulic retention time (HRT) is crucial in biogas plant design and operation. HRT is the duration the feed mixture stays in the tank, influencing biogas production. Optimization involves finding the balance between maximizing biogas production and minimizing the biogas tank’s size.

Optimization Factors

• Providing more HRT yields more biogas but increases the tank size.
• Reducing HRT decreases the tank size but produces less gas.

Design of Biogas Plant (General)

1. Identify the organic content and quantity of the feed.
2. Achieve the desired moisture content.
3. Determine the designed retention time.
4. Collect the generated biogas.
5. Dispose of the sludge.

Online Design of Biogas Plant (GGC 2047 Nepal)

• Input: Select animal type and quantity.
• Output: Obtain a comprehensive design and drawing for the biogas plant.

Frequently Asked Questions (FAQ)

1. Is the biogas design completely accurate?

The biogas production process is biological, introducing variable parameters like temperature fluctuations, bacteria type and content, carbon-to-nitrogen ratio, and system pH. The design is an engineering simulation, integrating experience and calculations to model the biological intricacies.

2. What is the difference between Biogas and Bio-CNG?

Biogas generally comprises 60% methane and 40% carbon dioxide. For commercial applications, purification methods remove water vapor, carbon dioxide, and trace gases, resulting in mostly methane. This purified methane product is known as Bio-CNG (Compressed Natural Gas).

3. Is gas production linear? Is gas produced the same every day?

Gas production in the biogas plant follows a parabolic nature, as illustrated by the mesophilic digestion curve. This leads to variable daily gas production, contrary to conservative linear digestion assumptions in some designs.

4. What should be done with the digested sludge from the biodigester?

Digested sludge, produced post-biogas production, exits from the digestor tank outlet. Stabilization involves further digestion in open pits by bacteria (sludge digestion). Afterward, the sludge is dried on sludge drying beds and can be utilized as a soil conditioner (partially as fertilizer). Else, mechanical screw dewatering can also be done for a fast process.

5. How much volume of the feed is reduced in digestion?

Approximately 1/3 (33%) of the volume (kg or m3) of the input feed becomes digested sludge. For instance, 100kg of input feed transforms into 33kg of final digested sludge, while the remainder converts to biogas, with additional supernatant liquid.

6. Why do mesophilic and thermophilic digestion differ?

Thermophilic conditions (40-60 degrees Celsius) involve different bacteria that accelerate biogas production.

7. What is the optimum temperature for biogas production?

The optimal temperature for biogas production is 29 degrees Celsius, as indicated by the digestion curve. At this temperature, 90% of digestion can be completed within 30 days.

8. How is mesophilic temperature maintained in the biodigester?

Anaerobic digestion generates heat, ensuring the maintenance of mesophilic temperature even when atmospheric temperature decreases.

9. How can I know which feed is good for biogas?

The feed which has high volatile content and low lignin content is the best feed for the high yield of biogas. For example, food has high volatile solids when compared to feces. Feces have high volatile solids when compared to straws, leaves, and branches of plants (not suitable for biogas production).

10. What happens if the moisture content is not maintained (feed has high organic loading)?

The acidification will occur by acetogenic bacteria which will inhibit the methanogenic bacteria causing less biogas to produce. The methanogenic bacteria are pH sensitive and prefer a neutral pH. Lime can be a solution to increase the pH if the acidity increases in the digestor.