Optimization of an existing plant - WtE Rothensee, Germany

Waste-to-Energy (WtE) plant Rothensee GmbH operates a modern waste management and energy concept at the Magdeburg Rothensee site.

ete.a

The WtE plant Rothensee is divided into two units, each with two lines, with a total annual capacity of about 650,000 tons of industrial and other waste. ete.a GmbH has been commissioned to optimize the flue gas treatment process.
 
 
Figure 3: Waste-to-Energy plant Rothensee

Description of the exhaust gas purification system before the conversion
The flue gas treatment system for all lines consisted of a SNCR system for NOx emission control. In a spray absorber with lime slurry (suspension of lime and water) and HOK injection and downstream fabric filter, the separation of the acid pollutant gases (HCl, HF and SOx), the heavy metals (such as mercury and its compounds as well as PCDD/F) was ensured.

Reasons for the rebuild measures
  • The consumption of operating fluids, especially quicklime (CaO) and hydrated lime (Ca(OH)2), could not be satisfactorily adjusted (the stoichiometry was 3.3 on an annual average).
  • The increasing raw gas loads of the plant due to the constantly changing waste composition. Depending on the origin, the chlorine and sulfur loads in the plant's input also increased as a result.
  • The corrosion damage to the fabric filter of lines 1 and 2.

Description of the optimization measures
The new flue gas treatment system stays to be a conditioned dry sorption process (Figure 4).
 

 
Figure 4: Process flow diagram of the optimized flue gas treatment system

To extend the reaction times and improve mixing, a deflection reactor is connected between the spray absorber (SA) and the fabric filter.
In the case of graded hydrated lime dosage , only a small proportion of the total hydrated lime is added to the system via the rotary atomizer of the spray absorber (in the past, the lime milk proportion was around 95 %, today it is around 20 %). The major part (80 %) is injected in powder form into the deflection reactor upstream of the fabric filter.

The following major optimizations were carried out:
  • Increase of the recirculated amount of residual material to 25 t/h
  • Installation of a humidification mixer to moisten the recirculated material
  • Installation of a deflection reactor with separate dosing of lignite coke (HOK), reagent and recirculated material
  • Modification of the temperature profiles (inlet and outlet of spray absorber, inlet of fabric filter)
  • Optimization of the control system for the lime milk and hydrated lime dosing
  • Changeover from highly reactive to normal hydrated lime
  • Reduction of the steam generator outlet temperature by 10 K (to 190 °C)
  • Installation of a dry extinguisher per block for the own production of hydrated lime

Result of the optimization measures
The most important plant parameters before and after the rebuild have been summarized in Table 2.

Table 2: Plant parameters before and after conversion of the exhaust gas cleaning system
 
In addition to the positive effects on the environment (by reducing emissions and produced waste volumes), the additives (especially CaO) are also conserved.
The optimization potentials drawn up by ete.a GmbH and the associated cost savings were fully achieved after the rebuild and amount to € 1 million per year for the four lines with an investment volume of approximately € 5 million.

Summary
Whether it is a new plant or a retrofit of an existing plant, a carefully thought-out solution adapted according to the specifics of each plant must be developed. The synergy between the plant operators and the experts in the emission control industry (such as consultants, suppliers) allows innovative emission control systems to be developed for each site under specific requirements. This ensures that very efficient flue gas treatment systems can be realized even for the lowest emission levels while maximizing energy efficiency.
 
Lücker Guido. Optimization of the spray absorption of the MHKW Rothensee. One Challenge?
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