Efficiency measures

A sewage sludge dewatering plant is required as the first step in fuel preparation for sewage sludge incineration. This is usually installed at ground level on the plant premises. Such a plant is already in place at the Mainz sewage treatment plant. However, this would require the dewatered sludge to be further conveyed using energy-intensive thick material pumps. For reasons of energy efficiency, a new, more energy-efficient sewage sludge dewatering plant was planned on the upper floor of the incineration plant to replace the old dewatering plant, so that the dewatered sludge can fall into the sludge bunker without consuming large amounts of energy. This will save approximately 1,763 MWh el/a or 940 tCO2/a.

Usually, the delivered dewatered sewage sludge is pumped from the delivery bunker to the silo and then from the silo to the sewage sludge dryer using energy-intensive thick material pumps. A more energy-efficient solution involving a grab crane system is planned for the new incineration plant in Mainz. This will save approx. 2,624 MWh el/a or 1,400 tCO2/a.

Some of the existing sewage sludge incineration plants burn undried sewage sludge in a fluidized bed furnace and, due to the high water content in the sewage sludge, cannot enable autothermal combustion. For this purpose, fossil fuels must be fed into the furnace. The planned plant in Mainz will feature two energy-efficient thin-film dryers heated by exhaust steam from the turbine, eliminating the need for a continuous supply of fossil fuels to the furnace. Approximately 952 MWh el/a of electricity and approximately 2,094 MWh th/a of thermal energy, or a total of 1,630 tCO2/a, will be saved by the two dryers. Furthermore, the purchase and installation of the thin-film dryer will enable district heating to be supplied to at least 2,000 households in Mainz. This will save approximately 36,900 MWh th/a and a further 7,450 tCO2/a.

Almost all sewage sludge incineration plants burn sewage sludge in a single-stage fluidized bed furnace. In Mainz, sewage sludge dried to 45% is then incinerated autothermally in a specially developed, energy-efficient, two-stage fluidized bed furnace. This makes it possible to use more energy-efficient blowers and at the same time reduce emissions of nitrogen oxide and greenhouse gases in the form of nitrous oxide by approx. 16.7 tN2O/a and 5,190 tCo2/a respectively.

The planned plant includes an energy-efficient high-pressure steam boiler developed specifically for the fluidized bed furnace, which guarantees a uniquely high steam pressure and more efficient steam yield for sewage sludge incineration.

This regeneratively generated steam is converted into electrical energy in a particularly energy-efficient turbo generator, thanks to its design. 80% of the electrical energy is used as internal power for the incineration plant. The 20% surplus energy is supplied as renewable energy to the Mainz sewage treatment plant, which results in energy neutrality there. The fluidized bed furnace saves 426 MWh el/a of electricity and 1,148 MWh th/a of heat. The high-pressure steam boiler and the turbo generator make it possible to generate renewable electricity in the order of approx. 11,480 MWh el/a. The furnace, boiler, and turbine save at least 6,600 tCO2/a.

Most sewage sludge incineration plants use a wet scrubbing process for flue gas cleaning after the electrostatic precipitator, in which more than 99.9% of the ash has already been separated. A wide variety of wet chemicals are used in this process, which are fed into a sewage treatment plant as contaminated wastewater and must be cleaned there in an energy-intensive process. In Mainz, the more energy-efficient quasi-dry flue gas cleaning process will be used in the future. This will save at least 492 MWh el/a in dry flue gas cleaning, as well as further large amounts of electricity in the sewage treatment plant. This corresponds to a further CO2 saving of 260 tCO2/a.

All of the energy-efficient mechanical systems mentioned above must be enclosed and connected via short routes without energy losses. This requires a complex structure with the appropriate technical building equipment (lighting, sanitation, etc.), which provides the plant with a weatherproof, soundproof, and odor-reducing shell. The structural part of the incineration plant does not specifically serve the purpose of energy efficiency and was not included in the list of eligible costs.

It should be emphasized that only by considering the plant as a whole is it possible to achieve CO2 savings of 2,160 tCO2/a in sewage sludge transport and generate renewable district heating for approximately 4,000 households in Mainz.