Increasing the Overall Plant Efficiency of WtE Plants by Means of Combustion Air Humidification

Initial operating experience from the plant in Vilnius

Steinmüller by Hitachi Zosen INOVA

UAB Vilniaus kogeneracine jegaine


UAB Vilniaus kogeneracine jegaine

Substitute fuel, household waste & sewage sludge

1 x 28 t/h
9000 kJ/kg

Grate system
Moving grate



SBE - Scope of delivery within an EPC consortium

Furnace, steam generator,flue gas cleaning, flue gas condensation



Location and concept


The plant is located in Vilnius and is operated by VKJ operated.

The provisional takeover (PAC) took place on 04.10.2021.

The Vilnius plant generates annually 610 GWh divided into:

District heating: 480 GWh

Electricity generation: 130 GWh


The highlights of the cogeneration plant

The solution from HZI Steinmüller includes:

  • the single-line firing and boiler plant,
  • the waste throughput of 31.5 t/h or 70 MW th
  • the air-cooled moving grate,
  • boiler with horizontal superheater and vertical economizer
  • an SNCR system with <80 mg/Nm³ NOx and <10 mg/Nm³ slip (at 11 vol.--% O2, tr.)
  • the water steam cycle with district heat extraction, feed water supply for the two-line biomass power plant,
  • the gas cleaning system,
  • the auxiliary equipment (compressed air system, cooling water supply, TGA)
  • the switchgear, and the I&C system.

The motivation for combustion air humidification

  • Reduction of exhaust gas losses by improving heat recovery.
  • Increase of the overall plant efficiency.
  • Increasing the flexibility of energy utilization with regard to its distribution in district heat or electrical power.
  • Improvement of the energy yield in thermal waste treatment.
  • Compensation of the reduced heat output of the flue gas condenser at low outside airlow outside air temperatures.

About the design and process chain
Challenges for design and engineering
When integrating combustion air humidification into the overall process, the conventional plant concept has been optimized with regard to the following points:
Risk of frost when cold ambient air is drawn in during winter,
Necessity of chemical dosing to limit biofouling in the humidifier circuit
Necessity of Waste Water treatment to use the condensate from the exhaust gas condenser for water treatment of the whole plant
Optimization of the duct routing to connect the heat recovery from the exhaust gas condensation with the combustion air supply
Design of the combustion air system with a total air intake and subsequent division into primary and secondary air.
Technical equipment details
Bunker air intake (1),
Boiler house air intake (2)
Total combustion air fan (3),
air humidifier including condensate circulation with bypass (4)
single stage steam heated air heater (5),
two-stage steam-heated air preheater with bypass (6),
primary air distribution to the combustion zones of the grate firing (7)
booster fan for secondary air (8) and
secondary air distribution to the afterburner zone (9)
The equipment on the flue gas side
Duct behind flue gas treatment (1)
Quencher (2)
Flue gas condenser (3)
Humidifier (4)
Flue gas duct chimney (5)
The equipment on the Condensate side
Circulation pump (1)
condensate cleaning (2)
upper spray level (3)
Condensate outlet (4)
Retrofitting of existing plants
The knowledge and concepts gained can also be used for retrofitting existing plants.
The prerequisite for combustion air humidification is a flue gas condensation system
Combined unit consisting of two packed scrubbers for flue gas condensation and air humidification rather for new construction projects.
Spatial separation of the condensation part in the area of the flue gas condensation plant and the air humidifier part in the area of the combustion air intake for concepts in existing plants.
In both cases, the heat is exchanged via a condensate circuit.
Summary and outlook
  • In summary, we would like to claim that a reliable and safe plant was reliable plant has been built and handed over.
  • The operational experience is good.
  • Due to the fully automatic operation, an easy operation and observation of the process is realized.
  • The condensate discharged from the combustion air humidifier can be reused in the water treatment plant after cleaning. can be reused in the water treatment system.
  • The energy efficiency of the entire plant is increased by using the
  • humidification as a condensing technology, the energy efficiency of the entire plant is further optimized by approx. 3 to 4 %. optimized.
  • Another positive aspect is that this plant technology can be integrated into existing plants or retrofitted. or retrofitted in existing plants.