Energy recovery

  • Photo 1: Example of a steam turbine

Types of energy recuperation for incinerators

The heat recovery of a waste incinerator we call just heat recovery system. If we can produce electricity from the heat recovery system, we have an electrical power generation system. The combination of heat recovery and power recovery is COGENERATION system.

For example:

A waste incinerator produces steam by a steam boiler. The steam goes to a back pressure turbine and produces electrical power. So, after the turbine, there is steam of 3 barG available. This steam contains evaporation power. This steam we use it to evaporate water for a dryer. In this case we have :

  • Electrical power from the turbine
  • Evaporation power for the dryer

This is a combined energy recovery system, electricity together with evaporation, this is a cogeneration system. A "Waste-to-cogeneration" system.

The limit of energy recovery is the minimum requirements needed for a correct flue gas treatment. This is classically about 150 - 200 degr.C. A flue gas of a low temperature would make for example condensation of dust on the bag house sleeves.

Steam boiler, thermal oil boiler or air-air heat exchanger?

A waste incinerator is producing hot flue gases. The flue gases need to be cooled, otherwise the filter installation will burn out, and the heat can be used as an energy source. The different parameters which need to be taken into account by choosing the boiler are :

  1. Operation costs vs. the energy recovery profit
  2. Is electricity needed or guaranteed on site
  3. Safety
  4. Type of energy needed for further processes
  5. If there is a wet scrubbing system in the flue gas, does the flue gas need to be heated up in the chimney.

 

1. Operation costs vs. the energy recovery profit - ROI

A steam turbine is actractive as an energy recuperation system. Only the turbine, generator, synchronization panel, demineralization water treatment plant and cooling tower are creating quickly high maintenance costs. This is why the min. capacity of a turbine needs to be high enough to have a profitable energy recuperation system.

2. is electricity needed or guaranteed on site?

If the installation is not always guaranteed with electrical power from the grid, it is possible to make the installation working on the energy produced from waste in island mode of the turbine. Extra equipment and energy calculations (about the load) need to be made for making smoothly switch from island to parallel mode of the turbine. So, in case the grid fails off, the island mode of the turbine deliver the electrical power. And if the grid switch on again, the turbine will run again parallel to the grid.

3. Safety

Safety with boiler pressures is mainly good regulated by the technical authorities. However if we take a thermal oil boiler, we know that there is no pressure on the boiler, but in case of a leakage, the oil fires up in the boiler. A steam boiler doesn't have this disadvantage. The safety problem is also taken in consideration further in the process, for example in a dryer.

4. Type of energy needed for further processes?

If next to the incinerator there is an other process, which requires heat, the energy recuperated from the boiler can be used in this process. The choose of this energy medium can define the type of boiler (steam, thermal oil or air air).

If there is a wet scrubbing system in the flue gas and does the flue gas need to be heated up in the chimney?

Wet flue gas purification systems create a white smoke at the chimney. This is from the flue gas, which is saturated with water. To have this effect away, it will be necessary to overheat the flue gas. The heat from the boiler can be used for this. For example: The steam from the boiler goes to the turbine, after the turbine the steam is still a little bit superheated and can be used to heat up the flue gas in the chimney. This is a cogeneration system, a combination of electricity and heat is recovered from the boiler.

Conclusion

The energy recuperation system is the system which fits personally to the business of the investor and his view on the market. That's why it is needed to be very creative in energy recovery systems.