The production process

Electricity production×

Two turbines are installed in the Zabalgarbi plant: a 56.5 MWh turbo-generator and a 43 MWh gas turbine which, coupled in a single integrated process, generate 99.5 MWh gross power.

The net energy produced by the plant is conveyed to an electrical substation for export.

This energy is exported to the substation via buried cables under the plant access road in order to avoid the environmental impact of overhead power lines.

1Electrical substation

2Steam turbine

3Gas turbine

4Heat recovery boiler

Gas scrubbing×

MSW is incinerated in the Furnace-boiler at a temperature above 850 ºC for two seconds. Under these conditions combustion is controlled in accordance with the regulations, and any dioxins and furans which enter along with the waste are destroyed.

Not only are these organochlorine molecules destroyed by nitrogen oxides are reduced in the furnace-boiler by injecting ammonia solution (23%).

Subsequently, two flue gas cleaning processes take place in the absorption towers. On the one hand, the gases are scrubbed to neutralise acidity by injecting lime slurry, and on the other heavy metals, dioxins and furans which have been newly created after the combustion process are removed by adsorption through injection of micronised active carbon.

The gases are again neutralised and cleared of particles by bag filters during their final passage through the tower.

Before leaving the stack 20% of the scrubbed gases generated are recirculated to be used as secondary air in the combustion process.

1Emission monitoring


3Lime slurry injection

4Bag house filter

5Active carbon injection

6Heat recovery boiler


8Gas scrubbing


Oven feed: MSW is removed from the pit by a grapple crane and transferred to a hopper. Provided the oven is operational - at a temperature above 850 ºC - the waste is gravity-fed onto a slide rack for the energy recovery process.

Waste combustion: During the incineration process waste combustion is carried out in the presence of oxygen-rich air extracted from the pit. Recovery of this air for combustion along with the underground location of the pit itself prevents odours from escaping into the atmosphere.

Steam production: The heat generated during the combustion process in the oven is used to produce steam through heat exchangers located in the furnace. Around 100 t/h of steam at 310 - 330 ºC and 100 bar is produced.

1Steam generation

2Ammonia injection

3Combustion control

4Ash extraction

5Slag extraction

6Scrap extraction

7Incinerator grate

Incoming storage zone×

Every day around one hundred trucks enter the plant transporting between 700 and 720 t. of municipal waste.

Incoming loads are scanned for radioactive material as a preventive measure. The detection gate is located at the entry to the weighbridge.

If the truck is allowed to pass it goes to the tipping hall. This large building is fully enclosed. The trucks deposit waste through ten dumping chutes into a pit with capacity for about ten thousand tons of waste. It is constructed below ground level to prevent odour and dust from escaping to the exterior of the plant. It is also sealed with concrete and various layers of impermeable materials to prevent leaks into the soil and ground-water.

1MSW feed into boiler

2MSW pit

1 - Electrical substation

2 - Steam turbine

3 - Gas turbine

4 - Heat recovery boiler

1 - Emission monitoring

2 - Stack

3 - Lime slurry injection

4 - Bag house filter

5 - Active carbon injection

6 - Heat recovery boiler

7 - Scrubber waste extraction

8 - Gas scrubbing

1 - Steam generation

2 - Ammonia injection

3 - Combustion control

4 - Ash extraction

5 - Slag extraction

6 - Scrap extraction

7 - Incinerator grate

1 - MSW feed into boiler

2 - MSW pit

Follow the entire process from the moment waste is delivered to the Zabalgarbi plant through to its power generation, or choose the part of the process that most interests you.

Combustion residues

The residues remaining after the combustion of the waste products – once their energy value has been removed- are as follows: slag, scrap and ash.


Slag is collected at the bottom of the furnace. It is classified as non-hazardous waste and accounts for 18.31% by weight and 8% by volume of the MSW entering the plant.

It is composed of inert materials such as glass, rubble and ceramics. It will subsequently be reused as aggregate material in civil engineering and public works.


Ferrous scrap accounts for 2% by weight of the waste delivered to the plant. It is reused in steelworks.


The material classed as ash is composed of fly ash and waste treatment residues from gas scrubbing. It represents 3.60% by weight and slightly less than 1% by volume of the MSW delivered.

Lime makes up from 50 to 60% of its composition by weight. There are also small metal fractions mixed with the waste, such as aluminium, iron, magnesium and potassium and trace amounts of Zn (0.75%), Pb (0.17%), Ni (0.0076%), Cd (0.0065%), Cr (0.0139), Hg (<0.001%) and As (0.0013%).

  • The ash, classified by an approved technological centre, is free of harmful, toxic or mutagenic substances.
  • Following storage in a sealed silo, the ash is currently collected and transported by truck for inertisation by an authorised waste manager.
  • It is transformed into an inert residue suitable for landfill disposal by a physical-chemical fixing and stabilisation process.