Detail of a 4-pass steam boiler

The innovation:

Hybrid boilers or self-fired waste heat boilers

The hybrid boiler is often a four-pass boiler. The extremely well thought-out combination of steam boiler and waste heat boiler has, in addition to the design known from the flame-tube boiler, an additional fourth flue for waste heat utilisation.

The conventionally fired boiler section generates heating or process heat while simultaneously utilising the heat potential from waste heat sources (waste heat section).

Hybrid boilers with burners enable continuous and efficient steam generation from process heat when waste heat is not continuously available. This 2- to 4-flue boiler design is mainly used in combination with combined heat and power plants (CHP), gas turbines or TNV/RNV air pollution control systems.

In the waste heat section, the hot flue gases from the upstream combustion process are used to support the generation of heating or process heat. This was also the starting point for the development of the hybrid boilers: biogas plants with their special requirements:

  • If the biogas CHP unit fails, the steam output is to be provided by a "normal" burner. With natural gas or biogas firing.
  • For peak loads that cannot be met with the CHP, the missing energy is generated with an industrial burner (CHP only runs base load).
  • General power increase through hybrid power.

If the flue gas flow fails or is insufficient at peak load, the part fired by oil or gas automatically takes over steam generation or controls the peak load.

The main advantages of hybrid boilers

More efficiency:

  • Only one boiler required - this saves space and installation costs.
  • The system is cheaper, has a longer service life and the need for spare parts is lower.
  • Maintenance by the operator and the manufacturer is less time-consuming.
  • Boiler sequencing is not required. The hybrid boiler "organises itself".
  • Increased efficiency and environmental friendliness through the use of waste heat sources. If the flue gas flow fails, the fossil-fired part automatically takes over the steam supply. As soon as exhaust gas is available again, it can be used to generate steam again in a matter of seconds.

More safety and durability:

  • Thanks to its hybrid function, the four-pass boiler is constantly at operating temperature. Dangerous, corrosion-causing undershooting of the dew point is minimised.
  • If a cold start becomes necessary, you can start the boiler with fuel. The flue gas flow, which can cause greater corrosion when cold, is switched on as soon as the operating temperature is reached.

Higher overall efficiency:

  • Installation of the feedwater preheating for both power units only in the more productive waste heat stream. This improves the overall efficiency of the system. The feed water can be heated more than with two units. Feedwater preheating of both power units only into the waste gas stream of the combined heat and power (CHP) improves the efficiency in the CHP area.
  • There is also a better overall utilisation of the storage heat in the boiler and feedwater tank than with two separate units.

Achenbach hybrid boiler: Robust, reliable, durable and efficient

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