What Types of Heat Recovery Heat Exchangers Are Used in Biogas Generators?

For biogas generators, the heat recovery heat exchanger (HRHE) needs to handle hot exhaust gases that often contain moisture, CO₂, and H₂S. Different designs are used depending on the application, space, and heat transfer requirements. The main types are:

1. Shell-and-Tube Heat Exchangers

• Structure: Hot exhaust gases pass through tubes, while the cooling medium (water or thermal oil) flows in the shell or vice versa.

• Advantages:

• Handles high temperatures and high pressure.

• Durable and robust.

• Easy to maintain and clean.

• Applications: Commonly used for engine coolant preheating, process water heating, or thermal oil systems.

2. Finned-Tube Heat Exchangers

• Structure: Tubes carry the cooling fluid, while fins attached to the tubes increase surface area for heat transfer. Exhaust gases flow over the fins.

• Advantages:

• High heat transfer efficiency.

• Compact design for limited space.

• Can handle lower exhaust flow rates efficiently.

• Applications: Preheating water, heating digesters, or generating low-to-medium temperature process heat.

3. Plate Heat Exchangers

• Structure: Multiple thin plates form channels for hot and cold fluids, separated by a narrow gap.

• Advantages:

• Compact and lightweight.

• High efficiency due to large surface area.

• Easy to expand by adding plates.

• Applications: Mostly liquid-to-liquid heat recovery, such as engine coolant to water circuits.

4. Recuperative Heat Exchangers

• Structure: Similar to plate or finned designs but optimized for gas-to-gas or gas-to-liquid heat recovery.

• Advantages:

• Can preheat incoming air or gas using exhaust gases.

• Reduces energy loss in combined heat and power (CHP) setups.

• Applications: Biogas CHP systems for preheating combustion air or drying processes.

5. Heat Pipe Heat Exchangers

• Structure: Uses sealed heat pipes to transfer heat from hot exhaust to a working fluid.

• Advantages:

• Excellent for compact systems.

• Works well when temperature differences are small.

• Applications: Niche applications, like small-scale biogas generators or portable units.

Key Selection Factors:

• Exhaust temperature and composition (H₂S, moisture).

• Space constraints in the generator room.

• Required heat recovery temperature.

• Maintenance accessibility.